Youssef El Jabbar
email: ysf.eljabbar@gmail.com

Hind Lakhlifi

Rachida El Ouatib

Lahcen Er-Rakho

Sophie Guillemet-Fritsch

Bernard Durand

Green nanoceramic pigments based on cobalt chromite doped with Al: synthesis, characterisation and colour performance

Keywords: Chromite spinel, Nanoparticles, Chemical route, Ceramics, Optical proprieties

Nano-sized spinel cobalt chromite oxide CoCr2O4 and its substituted forms CoCr2-xAlxO4 (0≤x≤2) have been successfully synthesised by wet chemical route. The prepared powders were characterised by means of thermal analysis (DTA/TGA), X-ray diffraction (XRD), Field emission scanning electron microscopy (FE-SEM), Brunauer-Emmett-Teller analysis (BET), UV-visible techniques (UV-Vis) and colorimetric analyses (L* a* b*). The results show that the CoCr2-xAlxO4 (0≤x≤2) samples are in the crystalline phase at 900°C with a particle size range of 20-200 nm. The colouration of the synthesised pigments is sensitive to the distribution of doping trivalent ions in the spinel lattice. The Al-enrichment leads to a colour shift from green to blue and the grains become more and more spherical and less agglomerated. Green cobalt chromite powders were substituted with 50% aluminium (CoCrAlO4), allowing the specific surface area of green nanometric powders to be increased from 7.4 to 12.4 m 2 /g.

Introduction

In the ceramics industry, the choice of pigments used for colouring or surface treating has become vital to improving the quality of products. In order to have a wide variety of colours, the manufacturer in this area is always looking for new stable colours while optimising manufacturing costs. Mixtures of certain oxides, such as CoO, ZnO, Al2O3 and Cr2O3, are traditionally the most used pigments. Various shades of colours can be obtained, in stoichiometric quantities, from these reagents calcined at high temperatures [START_REF] Bao | Synthesis and characterization of Fe 3+ doped Co0.5Mg0.5Al2O4 inorganic pigments with high near-infrared reflectance[END_REF][START_REF] Verger | Synthesis, properties and uses of chromium-based pigments from the Manufacture de Sèvres[END_REF][START_REF] Verger | Spectroscopic properties of Cr 3+ in the spinel solid solution ZnAl2-xCrxO4[END_REF][START_REF] Cornu | Luminescence properties and pigment properties of A-doped (Zn,Mg)MoO4 triclinic oxides (with A = Co, Ni, Cu or Mn)[END_REF][START_REF] Zhang | Pigments based on Er2O3-Al2O3: Preparation and colouring performance in zirconia ceramics[END_REF][START_REF] Zhou | Synthesis and characterization of Zn1-xCoxO green pigments with low content cobalt oxide[END_REF]. Quite recently, considerable attention has been paid to inorganic pigments of oxides with the formula AB2O4, in which A 2+ are accommodated in tetrahedral positions, whereas B 3+ are in octahedral positions. This material is a double oxide of a spinel-type structure with cubic symmetry and a space group of Fd-3m, and it has very important mechanical and optical properties. The substitution of chromophore ions (mainly transition metals) within these oxides makes it possible to obtain a wide variety of colours [START_REF] Anand | Microwave combustion synthesis, structural, optical and magnetic properties of Zn1-xCoxAl2O4 (0≤x≤0.5) spinel nanostructures[END_REF][START_REF] Desouza | Blue pigments based on CoxZn1-xAl2O4 spinels synthesized by the polymeric precursor method[END_REF][START_REF] Fernández-Osorio | Luminescent ceramic nanopigments based on terbium-doped zinc aluminate: Synthesis, properties and performance[END_REF][START_REF] Gaudon | Cobalt and nickel aluminate spinels: Blue and cyan pigments[END_REF][START_REF] Kanwal | Effect of Co 2+ Ions Doping on the Structural and Optical Properties of Magnesium Aluminate[END_REF][START_REF] De Sousa Santos | Synthesis and characterization of colourful aluminates based on nickel and zinc[END_REF][START_REF] Menon | Thermal effects on rapid microwave synthesized Co: ZnAl2O4 spinel nanoparticles[END_REF][START_REF] Yan | Investigation of the microstructure, cation distribution and optical properties of nanoscale NixMg1-xAl2O4 spinel pigments[END_REF][START_REF] De | Synthesis and characterization of pigments based on copper-zinc aluminate (CuxZn1-xAl2O4) by combustion[END_REF]. Zhang et al. [START_REF] Zhang | The structural evolution and optical properties of Mg1-xZnxAl1.8Cr0.2O4 pink ceramic pigments[END_REF] prepared the spinel solutions Mg1-xZnxAl1.8Cr0.2O4 and studied the relationship between cation distribution and optical absorption performance in order to explain the difference in colours.

In this paper, we focused on the green ceramic pigments based on CoCr2O4 doped with aluminium. Previous studies have presented certain materials with shades of green colours obtained by Cr-doped Al2O3, ZnAl2O4, MgAl2O4 or MgAl2Si3O12 oxides [START_REF] Verger | Synthesis, properties and uses of chromium-based pigments from the Manufacture de Sèvres[END_REF][START_REF] Verger | Interaction between Cr-bearing pigments and transparent glaze: A transmission electron microscopy study[END_REF][START_REF] Martos | Towards more ecological ceramic pigments: Study of the influence of glass composition on the colour stability of a pink chromium-doped ceramic pigment[END_REF]. The conventional method of synthesis of these types of oxides is via solid-state reaction of mechanical mixtures of metal oxides at a high temperature. Although this process is relatively inexpensive, it generally results in inhomogeneity of the powders and poor control of the stoichiometry. Furthermore, the sintering caused by calcination at a high temperature leads to materials with a low specific surface [START_REF] Prim | Pink ceramic pigments based on chromium doped M(Al2-xCrx)O4 , M=Mg, Zn, normal spinel[END_REF]. The focus of recent research has been on chemical synthesis, in which the powders are synthesised in liquid systems. Thus, several authors have proposed different synthesis routes, such as hydrothermal processes [START_REF] Chang | Controlled synthesis of blue spherical CoAl2O4 pigment powder in Pickering emulsion assisted with a hydrothermal process[END_REF], coprecipitation [START_REF] Ramachandran | Synthesis of nanostructured pirochromite magnesium chromate with egg shell membrane template[END_REF], microemulsion [START_REF] Giannakas | Surface properties, textural features and catalytic performance for NO + CO abatement of spinels MAl2O4 (M = Mg, Co and Zn) developed by reverse and bicontinuous microemulsion method[END_REF], combustion [START_REF] Zhou | Combustion synthesis of ZnFe2-xCrxO4 nanocrystallites for ceramic digital decoration[END_REF][START_REF] Kassem | Structure, optical and varying magnetic properties of insulating MCr2O4 (M= Co, Zn, Mg and Cd) nanospinels[END_REF][START_REF] Tatarchuk | Inversion degree, morphology and colorimetric parameters of cobalt aluminate nanopigments depending on reductant type in solution combustion synthesis[END_REF] and sol-gel [START_REF] Hedayati | Synthesis and characterization of Co1-xZnxCr2-yAlyO4 as a near-infrared reflective color tunable nano-pigment[END_REF][START_REF] Grazenaite | Sol-gel and sonochemically derived transition metal (Co, Ni, Cu, and Zn) chromites as pigments: A comparative study[END_REF][START_REF] Habibi | Low-temperature synthesis of mesoporous nanocrystalline magnesium aluminate (MgAl2O4) spinel with high surface area using a novel modified sol-gel method[END_REF]. Amongst these methods is the polymerisable complex process using the polymeric precursor, in which the metal cations are homogeneously distributed within the polymer chain. This process has the major advantage of preparing nano-sized oxide powders with high chemical and morphological homogeneities [START_REF] Jabbar | Influence of temperature and pH on the morphology and the color of the CoAl2O4 prepared by sol gel method[END_REF][START_REF] Jabbar | Photocatalytic degradation of navy blue textile dye by nanoscale cobalt aluminate prepared by polymeric precursor method[END_REF].

In this work, cobalt chromites (CoCr2-xAlxO4, 0≤x≤2) nanoparticles were prepared by polymerisable complex process. The calcination of the precursors obtained leads to the formation of the mixed oxides at a relatively low temperature. Therefore, the Al-substituted cobalt chromites obtained yield shades of green and blue. This can be related to the electronic transitions of cobalt and chromium [START_REF] Nath | CaAl2Cr2O7: Formation, synthesis, and characterization of a new Cr(III) compound under air atmosphere in the Al2O3-CaO-Cr2O3 system[END_REF][START_REF] Jabbar | Structure, microstructure, optical and magnetic properties of cobalt aluminate nanopowders obtained by sol-gel process[END_REF]. The structural, morphological and optical properties are presented and discussed.

Experimental

Preparation of CoCr2-xAlxO4, 0≤x≤2 nanoparticles

Nanocrystalline cobalt chromite-doped aluminium (CoCr2-xAlxO4, where 0≤x≤2) nanoparticles were prepared using analytical grade cobalt nitrate (Co(NO3)2•6H2O; 99.0%, Sigma Aldrich), aluminium nitrate (Al(NO3)3•9H2O; 99.0%, Sigma Aldrich) and chromium nitrate (Cr(NO3)3•9H2O; 99.0%, Sigma Aldrich) as the starting materials. The total metal ion concentration was 0.2 M, and the stoichiometric molar ratio was fixed (Co/(Cr2-xAlx)= 1/2).

Polyacrylic acid was used in excess as the chelating agent. The pH of the mixture was maintained at 7.0 by adding a small amount of pure ammonia. Then, evaporation of the aqueous solution at 80°C led to the formation of a viscous gel. After drying at 120°C over 24h, the obtained xerogel was pre-calcined at 300°C over 12h (pyrolysis) under air to remove the organic matter. The formed precursors (Pr0(x)) were heat-treated at 900°C for 5h. The produced powders (PR(x)) were of different shades indicating that they were strongly affected by the substituted amount x.

Characterization

The precursors Pr0(x) were characterised by thermogravimetric analysis (TGA) and differential thermal analysis (DTA) using a simultaneous DTA-TG apparatus (DTG-60H, Shimadzu).The heating rate was set at 2.5°C/min. The x-ray diffraction patterns were recorded with a Bruker D8 Advance x-ray diffractometer equipped with a LynxEye detector.

Results and Discussion

Thermal analysis

The thermal decomposition of the precursors (Pr0 (x being the aluminium amount)) was followed by thermogravimetric (TGA) and differential thermal (DTA) analyses. The temperature was gradually increased from room temperature to 1000 °C. Figs. 1 (a-c) represent TGA and DTA curves corresponding to the samples Pr0 (x=0), Pr0 (x=1) and Pr0 (x=2), respectively. The TGA thermograms show similar thermal behaviours, with total weight losses of around 16% for the three samples analysed. These weight losses were accompanied by two DTA effects. The first (endothermic) effect is due to the elimination of water adsorbed on the surface of the powder, whereas the second (exothermic) effect is attributable to the removal of the organic residues and further crystallisation. Beyond 900°C, no discernible weight loss was observed by the TGA curves. DTA curves showed slight endothermic inflections at around 925 °C, which can be attributed to the reduction of Co 3+ to Co 2+ . This transformation can be explained by the reaction of the mixed valence spinel structure with the amorphous alumina. According to the previous work, the equation ( 1) can be proposed for the composition x=0 [START_REF] Jabbar | Structure, microstructure, optical and magnetic properties of cobalt aluminate nanopowders obtained by sol-gel process[END_REF]:

Co 2+ [ Al 3+ 2(1-y) Co 3+ 2y]O4 + 3y Al2O3  (1+2y) Co 2+ [ Al 3+ 2]O4 + ½ O2 (1)

Structural analysis

In previous report based on the same spinel-type composition synthesis protocol performed on the composition x=0, it was shown that blue powder which represents the direct spinel was obtained from T≥900°C [START_REF] Jabbar | Structure, microstructure, optical and magnetic properties of cobalt aluminate nanopowders obtained by sol-gel process[END_REF]. Therefore, in the present work, the calcination temperature was taken at 900 ° C for the synthesis of the substituted compositions. (Fig. 3). Thus, the results were compatible with the ionic radii of Cr 3+ (r6-coord=0.615Å) and Al 3+ (r6-coord=0.535Å) in octahedral coordination, in accordance with the classification by Shannon and Prewitt [START_REF] Finley | Deciphering the loss of persistent red luminescence in ZnGa2O4:Cr 3+ upon Al 3+ substitution[END_REF]. It was also noted that the intensity of the (222) reflection decreased and the intensity of the (331) reflection increased with the amount of aluminium inserted (x).

This finding shows that these peaks were sensitive to the nature of the atoms at the octahedral sites of the spinel structure, which can be explained by the difference in the diffusion factors of Al and Cr [START_REF] Prince | C: Mathematical, physical and chemical tables[END_REF].

The Rietveld method was used to carry out structural refinements on the CoCr2-xAlxO4 (x=0, 1.0, 2.0) samples. The corresponding data given in Table 1 show that the refined cation occupancies at the 8a sites and 16d sites confirms the spinel-type structure of the samples with a very low degree of inversion. Fig. 4a depicts the graphical illustration of the experimental, calculated and difference signals for the CoCrAlO4 (x=1) sample. The Fig. 4b shows the ball and stick representation of the crystal structure in which the Co 2+ and Cr 3+ /Al 3+ ions are located at the center of tetrahedral and octahedral O 2-cages, respectively. The average crystallite sizes of the samples were determined from the most intense peak (311) using Scherrer's formula [START_REF] Hoda | Influence of synthesis parameters on the physical properties of Cu3Se2 nanostructures using the sonochemical method[END_REF]. As can be seen in Fig. 3, the crystallite size (D) decreased with Alenrichment from 48 nm (for x=0) to 22 nm (for x=2). This decrease almost stabilised at around 22 nm for the samples with greater than 50% Al-enrichment (x≥1).

Morphological properties

The morphologies of the powders PR (x) (CoCr2-xAlxO4 where x=0, 1.0 and 2.0) were examined via FE-SEM analysis. The obtained micrographs are given in Fig. 5(a-c). As shown in Fig. 5a, powder PR (x=0) is composed of grains of variable shapes exhibiting a high state of agglomeration and an average particle size of approximately 75-200 nm. For powder PR (x=1), the SEM micrograph shows grains that are quasi-spherical in shape, more or less agglomerated and sized between 35 and 100 nm (Fig. 5b). Fig. 5c shows that powder PR (x=2) is constituted of ultrafine grains 20-40 nm in size and quasi-spherical in shape. The SSAs of the synthesised powders measured by the BET method are provided in Fig. 6. The experimental values gradually increase with aluminium substitution, from 7.4 to 12.4 m 2 /g for the green powders PR (x=0) and PR (x=1), respectively. For the aluminium-enriched powders (x> 1), the SSA continues to increase and is accompanied by a change in colour from green to blue. The particle sizes were determined from the SSAs, considering the hypothesis that the grains are spherical and mono-dispersed. These calculations were carried out using the following formula [START_REF] Akbari | Particle size characterization of nanoparticles-a practical approach[END_REF]:

𝑑 = 6 𝜌×𝑆 10 3 (2)
d: particle sizes (nm).

ρ: theoretical density (cm 3 /g) obtained by Rietveld refinement.

S: specific surface area (m 2 /g).

Table 2 summarizes the results; notably, these values have the same orders of magnitude as those observed in the SEM analysis.

The colour evolution and UV-Vis analysis

The powders prepared at 900°C (CoCr2-xAlxO4, 0≤x≤2) showed varying shades of blue and green colours. As shown in Fig. 7, this colour shift was related to the content of the inserted Al 3+ cations. CIE Lab colorimetric measurements and UV-Vis spectroscopy analysis were used to perform the optical characterisation of the synthesised powders.

Tristimulus colourimetry (CIELAB system) was used to measure the colour of the PR(x) powders. The ratios (-b*/-a*) and (-a*/-b*) were studied as the function of the amount (x) of aluminium inserted in order to provide a better representation of the evolution of the colour shades. These ratios can aid in determining the dominance of blue or green colours on powders. Fig. 8 illustrates this evolution using the following three domains of the shades:

 The powders with predominantly green shades had the composition x≤1, where in the parameter -a*, which was responsible for the green colour, increased and the parameter -b*, which was responsible for the blue colour, was stable.

 The powders with blue-green shades had compositions corresponding to 1<x≤1.4.

 The powders with a dominant blue shade had the composition x >1.4.

The UV-Vis absorption properties of the CoCr2-xAlxO4 (x=0, 1.0, 2.0) samples were measured by diffuse reflection spectroscopy. Fig. 9 shows that the spectra presented large bands in the visible region 400-950 nm. These absorptions are attributed to the spin-allowed 4 A2 (F) → 4 T1(P) transition of the Co 2+ ions in the tetrahedral sites and the spin-forbidden electronic transitions ascribed to the 4 A2g→ 2 T1g and 4 A2g→ 2 Eg transitions caused by the chromium (Cr 3+ ) in the octahedral site [START_REF] Jabbar | Photocatalytic degradation of navy blue textile dye by nanoscale cobalt aluminate prepared by polymeric precursor method[END_REF][START_REF] Gao | Optical properties and magnetic properties of antisite-disordered Ni1-xCoxCr2O4 spinels[END_REF]. Using the spectra, it is possible to deduce the optical band gap energies (Eg) of the compounds. The Eg was calculated using Tauc relation [START_REF] Anand | Microwave combustion synthesis, structural, optical and magnetic properties of Zn1-xCoxAl2O4 (0≤x≤0.5) spinel nanostructures[END_REF][START_REF] Sameera | Nanostructured zinc aluminates: A promising material for cool roof coating[END_REF]. The Kubelka-Munk (K-M) reemission function was generally applied to convert the diffused reflectance into equivalent absorption coefficient. The function K-M was given by the following equation [START_REF] Bao | Synthesis and characterization of Fe 3+ doped Co0.5Mg0.5Al2O4 inorganic pigments with high near-infrared reflectance[END_REF]:

F(R) = α = (1-R) 2 2R (3)
where F(R) is Kubulka-Munk function, α is the absorption coefficient, R is the reflectance of samples. Thus the Tauc relation becomes,

𝐹(𝑅)ℎ𝑣 = 𝛼ℎ𝑣 = 𝐴(ℎ𝑣 -𝐸 𝑔 ) 𝑛/2 (4)
hʋ: energy of the incident photons (eV).

A: constant.

n: factor associated with the type of transition (n = 1 for a direct transition, and n = 4 for an indirect transition).

The energies of the forbidden bands were evaluated by considering the transition is to be direct (n = 1). Fig. 10 shows the variation in the plot of (αhʋ) 2 versus hʋ. The band gap energies (Eg) of the analysed compounds were estimated by extrapolating the linear part of the curve (hʋα) 2 as a function of hʋ((hʋα) 2 =0). The estimated values of band gap of CoCr2-xAlxO4 (x=0, 1.0, 2.0) are 3.5, 2.68 and 2.42 eV respectively (Fig. 10). These results

were consistent with the results found in the literature [START_REF] Choudhary | Structural and optical studies of nanocrystalline ZnCr2O4 and CoCr2O4 spinel[END_REF][START_REF] Torkian | Effects of β-alanine on morphology and optical properties of CoAl2O4 nanopowders as a blue pigment[END_REF][START_REF] Rahnamaeiyan | Novel sol-gel method for synthesis of cobalt aluminate and its photocatalyst application[END_REF]. The change in the band gap values would explain the colour variation of metal aluminates. Furthermore, this shift may be attributed to the difference in particle size and size distribution which can cause a variation of the optical spectra [START_REF] Tatarchuk | Inversion degree, morphology and colorimetric parameters of cobalt aluminate nanopigments depending on reductant type in solution combustion synthesis[END_REF].

Conclusion

In this study, aluminium-substituted cobalt chromite nanoparticles were obtained by chemical route synthesis. The powders obtained at 900°C over 5h exhibited shades of green and blue colours. X-ray diffraction analysis showed pure phases of spinel-type structures of Reflectance (%)

  The X-ray generator(40 kV, 40 mA) is a copper anticathode tube that uses the CuKα line. A graphite monochromator eliminates the Kβ lines. The wavelength of the Kα1/Kα2 lines of copper is 0.15406/0.15443 nm. The phases obtained are analyzed using Eva PLUS software by comparing the positions and intensities of the different diffraction lines observed with those available in the PDF-4-2010 Database established by the ICDD (International Center for Diffraction Data). The FullProf software was used for Rietveld refinement of diagram patterns. Diffuse reflectance spectra were measured using an ultraviolet-visible spectrophotometer (Lambda 35, Perkin Elmer). The specific surface area (SSA) of the powder was measured using the Brunauer-Emmett-Teller (BET) method (Micrometrics Flowsorb II 2300 instrument). The surface morphology was examined by field emission scanning electron microscopy (JEOL JSM 6700F). The colourimetric parameters (L*a* b*) were measured in the system Commission International de L'Eclairage (CIE) using a colourimeter (Chroma Meter CR-400/410, Konica Minolta).

Fig. 2

 2 presents the XRD patterns of the samples heat-treated at a temperature of 900°C (PR(x)). The diffraction peaks of the samples with x=0 and x=2 followed the standard JCPDS card No. 080-1668 and JCPDS card No. 070-0753 for CoCr2O4 and CoAl2O4 spinels, respectively. In addition, the peaks of the Al-enriched spinels shifted towards the high angles. Therefore, the experimental values of lattice parameter (a), determined from the XRD patterns, decreased linearly with the enrichment of Al from 8.32Å (for x=0) to 8.09 Å (for x=2), which follows Vegard's law

the

  CoCr2-xAlxO4 (0≤x≤2) samples. The colorimetric measurements revealed that these solid solutions possessed three colour domains:  x≤1, where in the green shade was dominant,  1<x≤1.4, where in a blue-green shade was observed,  x>1.4, wherein the compositions rich in aluminium a blue shade was dominant.The CoCrAlO4 (x=1) powder was green in colour, had quasi-spherical grains, and was more or less agglomerated. The grain size ranged between 35 and 100 nm, and the SSA was approximately 12.4 m 2 /g. This composition exhibited a better morphology than the green powders rich in chromium CoCr2-xAlxO4 (x<1).

Fig. 1

 1 Fig.1 Thermal analysis (TGA-DTA) curves of dried gel precalcined at 300°C for 12h

Fig. 2 X

 2 Fig.2 X-ray powder diffraction patterns of the solid solution CoCr2-xAlxO4, 0≤x≤2.

Fig. 3 Fig. 4

 34 Fig.3 Lattice parameter and crystallite size of CoCr2-xAlxO4 compounds as a function of the Al amount (x)..

Fig. 5 Fig. 6

 56 Fig.5 SEM micrographs of CoCr2-xAlxO4 (PR(x)) powders as a function of the Al amount (x)

Fig. 7 Fig. 8 a

 78 Fig.7 Powders obtained at 900°C for 5hours (PR(x)).

Fig. 9

 9 Fig.9 UV-Vis diffuse reflectance spectra of (a) CoCr2O4, (b) CoCrAlO4 and (c) CoAl2O4 compounds.

Table 1

 1 Rietveld refinement for PR (x=0), PR (x=1) and PR (x=2) samples

			Wyckoff			Refinement
	Sample	Atom	Positions	Occupancy	Biso	quality
			x=y=z			parameters
	PR (x=0) CoCr2O4	Co Cr Co Cr	0.125 0.125 0.500 0.500	0.97 0.03 0.03 0.97	0.727 (4) 0.727 (4) 0.122 (3) 0.122 (3)	Rp=1.35% Rwp=1.68% χ 2 =1.10%
		O	0.261(4)	1.00	0.270 (6)
	PR (x=1) CoCrAlO4	Co Cr/Al Al Cr	0.125 0.125 0.500 0.500	0.96 0.02 0.46 0.44	0.785 (5) 0.785 (5) Rp=0.70% Rwp=0.87% 0.353 (7) χ 2 =1.05% 0.353 (7)
		Co	0.500	0.04	0.353 (7)
		O	0.262 (3)	1.00	0.818 (5)
		Co	0.125	0.88	0.899 (3)	Rp=0.87%
	PR (x=2) CoAl2O4	Al Co Al	0.125 0.500 0.500	0.02 0.10 0.84	0.899 (3) 0.642 (7) 0.642 (7)	Rwp=1.08% χ 2 =1.05%
		O	0.263(3)	1.00	0.744 (5)

Table 2

 2 Particle sizes and the specific surface areas of the CoCr2-xAlxO4 (x=0, 1.0, 2.0) samples

			Specific	Average
	Samples	Particle size range	surface area	particle size
		SEM (nm)	BET (m 2 /g)	BET (nm)
	PR (x=0) (Green powder)	75-200	7.4	145.04
	PR (x=1) (Green powder)	35-100	12.4	91.25
	PR (x=2) (Blue powder)	20-40	34.26	35.72

Fig. [START_REF] Gaudon | Cobalt and nickel aluminate spinels: Blue and cyan pigments[END_REF] The plot of (αhν) 2 versus hν for (a) CoCr2O4, (b) CoAlCrO4, (c) CoAl2O4 coumpounds.


domain: spi chim

text:
Hind Lakhlifi
email: lakhlifihind21@gmail.com

Youssef El Jabbar

Sophie Guillemet-Fritsch

Bernard Durand

Lahcen Er-Rakho

Rachida El Ouatib

R El Ouatib

Purple nanometrics pigments based on cobalt-doped manganese molybdate: Synthesis, characterization, structural, thermal, optical, colorimetric and chemical properties

Keywords: , characterization, structural, thermal, optical, colorimetric and chemical properties Co-precipitation, Sol-Gel, Nanometrics materials, FTIR, Raman, CIE-L* a* b * system

ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

The divalent element molybdates (AMoO 4 A= Co 2+ , Zn 2+ , Cu 2+ ...) have attracted plenty of attention in recent years because of their thermochromic, piezochromic, and tribochromic applications [START_REF] Huerta-Flores | Synthesis of AMoO4 (A = Ca, Sr, Ba) photocatalysts and their potential application for hydrogen evolution and the degradation of tetracycline in water[END_REF][START_REF] Jiang | Thermal annealing effects on the luminescence and scintillation properties of CaMoO4 single crystal grown by Bridgman method[END_REF][START_REF] Raghunath | Solvent assisted synthesis and characterization of AMoO4 (A = Ca, Sr & Ba) nanomaterials[END_REF][START_REF] Wang | Rhombic dodecahedron-like CoMoO4/C composite: A facile construction and excellent electrochemical performances for lithium-ion batteries[END_REF][START_REF] Najafvandzadeh | First-principles study of elastic and thermal properties of scheelite-type molybdates and tungstates[END_REF]. Indeed, studies on tungsten-doped copper molybdate CuMo 1-x W x O 4 have shown that the tungsten content can control the temperature or pressure of the α γ phase transition in this molybdate. As these two polymorphic varieties are of different colors (green and ocher, respectively), considering the use of this material as an indicator of temperature or pressure is thus possible [START_REF] Benchikhi | Investigation of structural transition in molybdates CuMo1-xWxO4 prepared by polymeric precursor method[END_REF][START_REF] Pudza | Study of the thermochromic phase transition in CuMo1-xWxO4 solid solutions at the W L3-edge by resonant X-ray emission spectroscopy[END_REF][START_REF] Jezierski | Electronic and magnetic properties of Cu2MgV2O8 and CuMg2V2O8ab initio study[END_REF][START_REF] Zhang | Cyclic voltammetry analysis of copper electrode performance in Na2WO4 solution and optical property of electrochemical synthesized CuWO4 nanoparticles[END_REF]. Recently, the work carried out in our laboratory by H. Lakhlifi et al. [START_REF] Lakhlifi | Synthesis of molybdates Zn1-xCoxMoO4 (0 ≤ x ≤ 1), by decomposition of the precursors developed by the glycine-nitrate process (GNP), and their characterization[END_REF][START_REF] Lakhlifi | Synthesis and physicochemical characterization of pigments based on molybdenum[END_REF][START_REF] Lakhlifi | Structural, morphological and optical properties of cobalt-substituted MgMoO4 ceramics prepared by pyrolysis of citric acid precursors[END_REF] showed that the partial substitution of cation A in the matrix of A 1-x Co x MoO 4 (A=Zn and Mg) by the chromophoric element cobalt allows the synthesis of new colored materials. Indeed, Co 2+ ions can induce different colorations depending on the environment they occupy. For example, in the tetrahedral site, they are at the origin of a blue color [START_REF] Lakhlifi | Synthesis of molybdates Zn1-xCoxMoO4 (0 ≤ x ≤ 1), by decomposition of the precursors developed by the glycine-nitrate process (GNP), and their characterization[END_REF], while in regular octahedral coordination, a pink color is observed, and a purple hue is obtained when the latter is deformed [START_REF] Lakhlifi | Structural, morphological and optical properties of cobalt-substituted MgMoO4 ceramics prepared by pyrolysis of citric acid precursors[END_REF].

The Mn 1-x Co x MoO 4 composite is likely to be formed, due to the size and electronegativity of the metal (Mn) such as X Mn <X Co . The CoMoO 4 molybdate is likely to have a more compact structure, leading to the formation of octahedral MoO 6 (α-phase) [START_REF] Yu | Controllable synthesis of cobalt molybdate nanoarrays on nickel foam as the advanced electrodes of alkaline battery-supercapacitor hybrid devices[END_REF], while the crystal cell, with a higher volume containing tetrahedral MoO 4 (β-phase) [START_REF] Zhang | Redispersion effects of citric acid on CoMo/γ-Al2O3 hydrodesulfurization catalysts[END_REF], is preferred to occur in MnMoO 4 [START_REF] Sheng | Solid-state synthesis and superior electrochemical performance of MnMoO4 nanorods for asymmetric supercapacitor[END_REF][START_REF] Xu | Fabrication of hierarchical MnMoO4•H2O@MnO2 core-shell nanosheet arrays on nickel foam as an advanced electrode for asymmetric supercapacitors[END_REF][START_REF] Thangappan | Synthesis, structural and electrochemical properties of Mn-MoO4/graphene nanocomposite electrode material with improved performance for supercapacitor application[END_REF]. However, a transition from α phase to a β phase has been reported experimentally in the CoMoO 4 system [START_REF] Costa | Influence of europium doping on the structural phase-transition temperature of βand α-CoMoO4 polymorphs[END_REF]. Several synthesis methods have been used to synthesize the molybdate AMoO 4 (A= Co, Zn, Ni, Cu…) such as the solvothermal method [START_REF] Keereeta | Effect of medium solvent ratios on morphologies and optical properties of α-ZnMoO4, β-ZnMoO4 and ZnMoO4•0.8H2O crystals synthesized by microwave-hydrothermal/solvothermal method[END_REF], the Pechini method [START_REF] Maczka | Vibrational and luminescence studies of MIIn(MoO4)2 (MI=K, Rb) and MIAl(MoO4)2 (MI=K, Na) molybdates doped with chromium(III) prepared via the Pechini method[END_REF], the sol-gel route [START_REF] Kausar | Synthesis and characterization of zirconium(IV) molybdosulphosalicylate, multi-walled carbon nanotubes and polypyrrole based ternary nanocomposites: Adsorption and lead sensing studies[END_REF][START_REF] Zhang | Preparation and photoluminescence properties of MMoO4 (M = Cu, Ni, Zn) nano-particles synthesized via electrolysis[END_REF][START_REF] Hcini | Thermal, microstructural, optical, magnetic and magnetocaloric studies for Ni0.5Mn0.5Cr2O4 chromite spinel prepared using sol-gel method[END_REF][START_REF] Kesmez | Preparation of hybrid nanocomposite coatings via sol-gel method for hydrophobic and self-cleaning properties[END_REF], and the solid-solid route [START_REF] Lakhlifi | Synthesis of molybdates Zn1-xCoxMoO4 (0 ≤ x ≤ 1), by decomposition of the precursors developed by the glycine-nitrate process (GNP), and their characterization[END_REF]. In the present work, we proposed to elaborate on and study molybdates by substituting the manganese in MnMoO 4 with cobalt. Indeed, with this matrix being beige, its doping by a chromophore element such as cobalt can highlight a piezochromic phenomenon for this material [START_REF] Prabhu | Synthesis of petal-like CoMoO4/r-GO composites for high performances hybrid supercapacitor[END_REF]. To evaluate the synthesis method's effect on the molybdates' quality, as well as to compare the results with those obtained in the synthesis by the polymerizable complex route (sol-gel), we opted to elaborate on the solid solutions Mn 1-x Co x MoO 4 (0≤x≤1) by coprecipitation. X-ray diffraction, FTIR, Raman spectroscopy, particle size, morphology, surface specific area, colorimetric parameters, and thermal stability were discussed in detail.

Methods and materials

Synthesis

The synthesis by the sol gel route of the solid solution Mn 1-x Co x MoO 4 (0≤ x ≤ 1) consists of instilling in the solution the salts of divalent cations: manganese nitrate Mn(NO 3 ) 2 ,6H 2 O (Aldrich,98%), cobalt nitrate Co(NO 3 ) 2 , 6H 2 O (Aldrich,98%) in the presence of ammonium heptamolybdate (NH4) 6 Mo 7 O 24 ,4H 2 O (Acros,99%) in a stoichiometric amount. To this mixture, an excess of citric acid (Acros,98%) was added to complex the different cations, with the pH being controlled. We thus obtained a matrix that insured the homogeneous presence of the various cations. The evaporation of the solution at 80°C led to the formation of gels. These gels were dried at 120°C for 24 hours and then calcined at 300°C under air for 12 hours. A heat treatment in the air at an adequate temperature allows for the isolating of powders of different colors.

The synthesis of Mn 1-x Co x MoO 4 (0≤ x ≤ 1) molybdates by the co-precipitation method consists of making precipitated in aqueous medium in hydrated form. To do this, mixtures of solutions of Mn 2+ and Co 2+ cations such that [Mn 2+ ]+[Co 2+ ]=1, were added to aqueous solutions of sodium molybdate Na 2 MoO 4 , under continuous magnetic stirring. Sodium hydroxide solution NaOH (1M) was added dropwise to the mixture to maintain an optimal pH for precipitation. The temperature was fixed at about 80°C. After one hour of stirring, the reaction mixture obtained was washed with distilled water and ethanol several times. This washing allowed to removal of the different residues from the product and to obtain hydrated precursors, as proposed in the following reaction.

(1-x)Mn 2+ + xCo 2+ + MnO 4 2-+ nH 2 O Mn 1-x Co x MoO 4 , nH 2 O (1)
The obtained precursors were dried at 120°C for 12 hours to remove volatile impurities, then ground to fine powders and then calcined in the air for 2 hours, allowing crystallization of the Mn 1-x Co x MoO 4 samples.

Characterization

The decomposition of pre-calcined powders was investigated through differential thermal analysis (DTA) and thermogravimetric analysis (TGA) using a simultaneous DTA/TGA analyzer (DTA-TG-60H Shimadzu). The resulting powders were characterized using an X-ray diffractometer (Bruker D8 Advance), equipped with a LynxEye detector. The X-ray generator (40kV, 40mA) is a copper anticathode tube that uses the CuK line. A graphite monochromator eliminates the K  lines. The wavelength of the K 1 /K 2 lines of copper is 0.15406 / 0.15443 nm. The phases obtained were analyzed using Eva PLUS software by comparing the positions and intensities of the different diffraction lines observed with those available in the PDF-4-2010 database, established by the ICDD (International Center for Diffraction Data). The infrared spectra were taken using a Fourier-transform infrared spectrometer (IR Affinity-1S Shimadzu). The Raman spectra were recorded with a LabRAM HR 800 (Horiba Jobin-Yvon) spectrometer. The morphologies of the powders were examined using scanning electronic microscopy (JEOL JSM 6400), and the specific surface areas were determined using the Brunauer-Emmett-Teller (BET) method (Micrometrics Flowsorb II 2300). The color parameters (L*a*b*) were measured using the CIE Lab system colorimeter (CR-400/410, KONICA MINOLTA).

Results and discussion

Thermal analysis

To determine the optimal calcination temperatures of the Mn 1-x Co x MoO 4 molybdate powders, thermogravimetric analyses under air with a temperature rise of 5°C/min were carried out in the temperature range of 25-900°C on the precursors corresponding to the composition of Mn 0.4 Co 0.6 MoO 4 from each synthesis mode. In the case of the precursor obtained from the solgel route, the thermogravimetric analysis showed a global loss of 60% distributed across several steps (Fig. 1a):

-A first loss, of about 5% starting around 105°C and was accompanied by a weak endothermic effect that could correspond to the dehydration of the sample.

-A second loss, of about 34%, occurred between 340°C and 550°C accompanied by weak exothermic effects that likely due to the combustion of organic species with the release of carbon monoxide and nitrogen.

-A final loss, of about 21%, was accompanied by a strong exothermic effect observed between 580°C and 620°C. This process probably corresponds to the oxidation of the carbon produced during the combustion of the xerogel and to the crystallization of a solid phase.

Beyond 650°C, no phenomena were observed on the thermal analysis curve, which indicated that the decomposition of the precursor was complete. The X-ray diffraction analysis (XRD) of the residue of TGA showed the crystallization of the solid Mn 0.4 Co 0.6 MoO 4 . Based on this result, we could choose the temperature of 700°C as the optimal temperature for the calcination of xerogels obtained from sol gel synthesis.

In the case of the precursor obtained by co-precipitation, the thermal analysis curve indicated a global mass loss of about 8% (Fig. 1b). The successive losses accompanied by endothermic and exothermic effects, recorded between 50°C and 350°C, could correspond to the departure of water and to the beginning of crystallization in the corresponding solid phase. For temperatures above 380°C, no phenomena were recorded on the thermal analysis curve, which meant that the decomposition of the precursor was complete. An XRD analysis of the TGA residue confirmed the crystallization of Mn 0.4 Co 0.6 MoO 4 molybdate. Following this observation, we could adopt 500°C as the optimal temperature for the calcination of the precursors obtained by Co precipitation.

2. X-ray diffraction

A room temperature XRD analysis was conducted to identify the phases formed after treatment at 500°C and 700°C, respectively, of the precursors from co-precipitation and pyrolysis of the gels. The obtained diffractograms highlight, regardless of the synthesis method used, the formation of a continuous solid partition of nominal formula: Mn 1-

x Co x MoO 4 (0≤x≤1) of monoclinic symmetry, isotype to -MnMoO 4 (PDF: 01-072-028) and to β-CoMoO 4 (PDF: 00-021-0868) (Figs. 2 and3). For compositions (0≤x≤0.8), no transition was observed after grinding. The DRX spectrum of the ground and unground composition x=0.6 serves as an example to illustrate this observation (Fig. 3b). This allows us to suggest that the β polymorph could be stabilized by manganese doping.

The Enlarged view of XRD peaks of Mn 1-x Co x MoO 4 (x=0, 0.3, 0.6, 1) compounds obtained by co-precipitation are given in Figure 4. All peaks are isotopically indexed to the monoclinic β-CoMoO 4 phase (JCPDS 00-021-0868). This confirms the formation of a continuous β-CoMoO 4 -β-MnMoO 4 solid solution.

The calculation of crystallite size using the Scherrer equation from width at mid-height measurements of X-ray diffraction lines made in the angular range 19.6°-30.5°, for Mn 1-

x Co x MoO 4 (0≤ x ≤ 1) molybdates prepared by sol-gel route, is given in Figure 5. We note that the crystallite size decreases significantly upon the insertion of 30% cobalt, beyond the composition x=0.6, stable crystallite sizes ≈ 53 nm are noted.

Infrared spectroscopy

To confirm the X-ray diffraction results, infrared spectroscopic analyses were performed on the composition (x=0.3 and x=0.6) of the solid solution Mn 1-x Co x MoO 4 elaborated by the solgel route (Fig. 6a) and by the co-precipitation method (Fig. 6b). The infrared spectra show a similar appearance regardless of the synthesis mode adopted; the stretching vibration bands of Mo-O-Mo are observed at 887, 830, 893, and 832 cm -1 [START_REF] Rabbani | Sonochemical assisted synthesis of manganese-nickel molybdate/reduced graphene oxide nanohybrid for energy storage[END_REF][START_REF] Purushothaman | Supercapacitor behavior of α-MnMoO4 nanorods on different electrolytes[END_REF][START_REF]Synthesis of MnMoO4 Nanorods by a Simple Co-Precipitation Method in Presence of Polyethylene Glycol for Pseudocapacitor Application[END_REF]. The bands located at 754 and 760 cm -1 are characteristic of the tetrahedral [MoO 4 ] group, and their presence justifies the existence of the tetrahedral coordination of Mo [START_REF] Yesuraj | A facile sonochemical assisted synthesis of α-MnMoO4/PANI nanocomposite electrode for supercapacitor applications[END_REF][START_REF] Sekar | Combustion synthesis and characterization of spherical α-MnMoO4 nanoparticles[END_REF]. The band located at 856 cm -1 is associated with the vibration mode of the Mo-O band [START_REF] Jeseentharani | Nanocrystalline composites of transition metal molybdate (Ni1-xCoxMoO4[END_REF], those located at 499 and 480 cm -1 are specific to the vibrations of Co-O-Mo bands [START_REF] Lakhlifi | Structural, morphological and optical properties of cobalt-substituted MgMoO4 ceramics prepared by pyrolysis of citric acid precursors[END_REF][START_REF] Jeseentharani | Nanocrystalline composites of transition metal molybdate (Ni1-xCoxMoO4[END_REF].

Raman spectroscopy

The observed vibrational modes are divided into two characteristics, internal modes; which are generated from the oscillation inside the molecular ionic groups with an immobile center of mass, and external modes; which are vibrations of phonons of the lattice due to the movement of cations. To confirm the results obtained previously, Raman spectroscopic analyses were performed on Mn 1-x Co x MoO 4 (x=0.3 and x=0.6) powders elaborated by the solgel process (Fig. 7a) and by co-precipitation (Fig. 7b). The high intensity bands observed at 926 and 924 cm -1 correspond to the Mo(1)O(2) symmetric stretching vibration (ʋ 1 (Ag)) [START_REF] Sekar | Combustion synthesis and characterization of spherical α-MnMoO4 nanoparticles[END_REF][START_REF] Veerasubramani | Sonochemical synthesis, characterization, and electrochemical properties of MnMoO4 nanorods for supercapacitor applications[END_REF]; the bands located at 874 and 872 cm -1 are assigned to the Mo(1)O(1) symmetric stretching vibrations [START_REF] Gao | High-performance symmetric supercapacitor based on flower-like zinc molybdate[END_REF]. The peaks at 813 and 810 cm -1 are assigned to the anti-symmetric stretching ʋ 3 (Ag/C 2 h) and ʋ 3 (Eg) vibration modes of the Mo-O-Mo of German entartet (e.g.),

respectively [START_REF] Chen | Facilesynthesized NiCo2O4@MnMoO4 with novel and functional structure for superior performance supercapacitors[END_REF][START_REF] Tian | Needle-like CoMoO with multi-modal porosity for pseudocapacitors[END_REF]. The bands observed at 353 and 350 cm -1 correspond to the antisymmetric ʋ 4 (Bg/D 2 h) and ʋ 2 (Ag/C 2 h) bending modes, respectively [START_REF] Meng | Facile preparation of CoMoO4 nanorods at macroporous carbon hybrid electrocatalyst for non-enzymatic glucose detection[END_REF][START_REF] Liu | Phosphide-oxide honeycomb-like heterostructure CoP@CoMoO4/CC for enhanced hydrogen evolution reaction in alkaline solution[END_REF][START_REF] Yang | Enhanced electrochemical performance of CoMoO4 nanorods/reduced graphene oxide as anode material for lithium-ion batteries[END_REF]. Those located at 320 and 318 cm -1 correspond to the stretching vibrations of the Mo-O-Co bands [START_REF] Herrera | Loss of single-walled carbon nanotubes selectivity by disruption of the Co-Mo interaction in the catalyst[END_REF]. The free rotation mode was observed at 276 cm -1 [START_REF] Luz-Lima | Temperature-dependent Raman spectroscopy studies of phase transformations in the K2WO4 and the MgMoO4 crystals[END_REF].

Microstructural characterizations

The powders of the Mn 1-x Co x MoO 4 molybdates with x=0.3 and x=0.6 obtained via the solgel route, and by co-precipitation, were characterized by scanning electron microscopy (SEM).

The SEM micrographs (Fig. 8a) show, for the product obtained via the sol-gel route, that powders formed agglomerates consisting of particles of varied morphology with sizes ranging from 1 to 9 μm. The micrograph of the composition x=0.6 (Fig. 8b) shows an advanced presintering state, which explains the low values of the specific surface areas determined for these powders.

The powders obtained by co-precipitation present various morphologies. In fact, the powders of compositions x=0 and x=0.3 (Fig. 9a and Fig. 9c) are formed by particles with a rectangular parallelepiped shape, which come from agglomerated clusters of heterogeneous morphology.

The powders of fractions x=0.6 and x=1 (Fig. 9b and Fig. 9d) are composed by particles in the form of more dispersed nano-rods; the average size of the particles of these powders is less than 200 nm.

Specific surface area measurements were performed on the Mn 1-x Co x MoO 4 compositions (x=0.3 and x=0.6) (Table 1). It was noted that the powders obtained by the sol-gel process presented lower specific surface areas than those obtained by co-precipitation, irrespective of the cobalt content. This can be explained by the effects of exposure to high temperature. The particles of the powders obtained by the sol-gel process appeared spherical and individualized. The average sizes of these particles were deduced by the above formula [START_REF] Gao | Optical properties and magnetic properties of antisite-disordered Ni1-xCoxCr2O4 spinels[END_REF].

They were 9571 nm for the x=0.3 composition and 5963 nm for x=0.6, respectively.

(2)

d: particle sizes (nm).

ρ: theoretical density (cm 3 /g) obtained by Rietveld refinement.

S: specific surface area (m 2 /g).

Evaluation of application properties of the synthesized pigments

The colorimetric parameters (L*a*b*) of the Mn 1-x Co x MoO 4 0≤x≤1 powder, obtained at 700°C by the sol-gel method and at 500°C by co-precipitation, were measured in the CIE Lab system (Fig. 10a and10b). It was noted that the component (-b*) that characterized the purple color, changed faintly with the cobalt incorporation rate, regardless of the synthesis method used. The different colors obtained are shown in Figure 11.

The degree of bluing of the compounds prepared by the sol-gel route was higher than that obtained from the powders synthesized by co-precipitation. In order to optimize the processing temperature required for the industrial use of a dye pigment, we processed the samples (x= 0, 0.3, 0.6, and 1) obtained by both synthesis methods, at a temperature of 1100°C (Fig. 12). We noted that the parameter (-b*) presented a maximum in the vicinity of the x=0.6 fraction, irrespective of the method of synthesis used. The relatively high value of this parameter might encourage the use of the composition x= 0.6 as a pigment in ceramic staining. This result was compatible with that obtained in the case of the Mg 1-x Co x MoO 4 system (0 ≤x≤ 1) [START_REF] Lakhlifi | Structural, morphological and optical properties of cobalt-substituted MgMoO4 ceramics prepared by pyrolysis of citric acid precursors[END_REF].

The thermal and colorimetric stability of these pigments was studied in order to evaluate their application as ceramic colorants. The thermal stability was verified by differential thermal analysis (DTA) performed between 25°C and 1100°C with a 10°C/min rise. The curves obtained for the compositions Mn 0.7 Co 0.3 MoO 4 and Mn 0.4 Co 0.6 MoO 4 synthesized by coprecipitation and the sol-gel processes, are shown in Figure 13. No thermal phenomena were observed which indicated that these pigments had a good thermal stability.

The stability of the color of the Mn 0.7 Co 0.3 MoO 4 and Mn 0.4 Co 0.6 MoO 4 compositions treated at 1100°C for two hours, was evaluated by tests of attacks by chemical agents on the pigments in different media (acid (HNO 3 ) and basic (NaOH)) titrated at 10%. After two hours of attacks a filtration, washing and drying, colorimetric analyses were performed. The values obtained showed that the colorimetric parameters of these compositions remained unchanged (Table 2).

This indicated that these pigments had good stability and resistance to an acid-base test.

Conclusion

The elaboration of the Mn 1-x Co x MoO 4 (0≤x≤1) molybdates by the sol-gel and co-precipitation methods led to powders with distinct microstructural characteristics. The X-ray diffraction result showed the formation of continuous systems of the solid solution Mn 1-x Co x MoO 4 isotype at -MnMoO 4 , regardless of the adopted synthesis method. The grinding of the powders with compositions 0≤x≤0.8 seemed to have no influence on the obtained pure phase.

The powders prepared by co-precipitation have an average particle size below 200 nm. These powders also presented larger specific surface areas (6.39 m 2 /g and 7.52 m 2 /g) than those obtained by the sol-gel process (0.15 m 2 /g and 0.22 m 2 /g). This was due to the high calcination temperature (700°C) which led to an advanced pre-sintering phenomenon in the case of powders prepared by the sol-gel process. The SEM images confirm these results. An infrared and Raman spectroscopy showed that the cobalt was well inserted into the -MnMoO 4 system regardless of the synthesis method employed. Measurements of the

Fig. 1 .

 1 Fig. 1. Thermal analysis (TGA-DTA) curves of Mn 0.4 Co 0.6 MoO 4 prepared by sol-gel route (a) and by co-precipitation (b).

Fig. 2 .

 2 Fig.2. X-ray powder diffraction patterns of Mn1-X Co x MoO 4 (0 ≤ x ≤ 1) prepared by sol-gel at

Fig. 3 . 4 (

 34 Fig.3. (a) X-ray powder diffraction patterns of Mn 1-X Co x MoO 4 (0 ≤ x ≤ 1) prepared by

Fig. 4 .

 4 Fig.4. Enlarged view of XRD peaks of Mn 1-x Co x MoO 4 (x=0, 0.3, 0.6, 1) compounds in the 2θ range of 20-30°.

Fig. 5 .

 5 Fig.5. Evolution and growth of crystallite size of Mn 1-x Co x MoO 4 compounds prepared by sol

Fig. 6 .Fig. 7 .

 67 Fig.6. FT-IR spectra of Mn 1-x Co x MoO 4 (x=0.3 and x=0.6) powders obtained by sol-gel route

Fig. 8

 8 Fig. 8. SEM micrographs of Mn

Fig. 9 .

 9 Fig. 9. SEM micrographs of Mn 1-x Co x MoO 4 powders (a): x = 0, (b): x = 1, (c): x = 0.3 and (d): x = 0.6 prepared by co-precipitation.

Fig. 10 .

 10 Fig. 10. Evolution of the color parameters of Mn 1-x Co x MoO 4 (0 ≤x≤ 1) powders obtained by sol-gel route at 700°C (a) and by co-precipitation at 500°C (b) as a function of x.

Fig. 11 .Fig. 12 .

 1112 Fig.11. The colors of Mn 1-x Co x MoO 4 (0 ≤x≤ 1) powders synthesized by sol-gel at 700°C (a) and by co-precipitation at 500°C (b).

Fig. 13 .

 13 Fig.13. DTA analysis of Mn 0.7 Co 0.3 MoO 4 and Mn 0.4 Co 0.6 MoO 4 powders obtained by sol-gel route at 700°C (S-G) and by co-precipitation at 500°C (Cop).

4 : 01-072-0285

	(001)	(-201)	(021)			(-112) (-202)	(-311)	(-131)	(022)	(-222)	(400)	(040)	(003)	(222)	(-422)	(113)	-CoMoO 4 00-021-0868 x=1 (440) (-441) (-204) (241)
																x=0.8
																x=0.6
																x=0.4
																x=0.35
																x=0.3
																x=0.1
	(110)	(-201)	(110)	(201)	(220)	(-221)	(-311)	(112)	(-311)	(400)	(040)	(222)		(241)	(440)	x=0
																-MnMoO

Table 1

 1 Specific surface areas of Mn 0.7 Co 0.3 MoO 4 and Mn 0.4 Co 0.6 MoO 4 powders prepared by sol-gel and by co-precipitation.

Table 2

 2 Colorimetric coordinates of Mn 0.7 Co 0.3 MoO 4 and Mn 0.4 Co 0.6 MoO 4 powders measured before and after tests by chemical agents. Mn 0.7 Co 0.3 MoO 4 Mn 0.4 Co 0.6 MoO 4

	Method of		Incorporation rate	Specific surface
	synthesis			of Co 2+		area BET (m 2 /g)
	Sol-gel			0.3			0.15
	(700°C)			0.6			0.22
	Co-precipitation		0.3			6.39
	(500°C)			0.6			7.52
	Sample
	Colorimetric coordinates	L*	a*	b*	L*	a*	b*
	Before tests		54.48	0.83	-10.15	48.64	1.75	-21.69
	After tests	HNO 3 54.46	0.81	-10.13	48.63	1.72	-21.67
	10% Acid /Base	NaOH 54.47	0.82	-10.14	48.62	1.73	-21.68

colorimetric parameters (L*a*b*) of the Mn 1-x Co x MoO 4 (0≤x≤1) system, obtained by both synthesis methods, at a temperature of 1100°C, showed that the component (-b*), which characterized the purple color, presented a maximum in the vicinity of the fraction x=0.6.

Thermal and colorimetric stability studies of the Mn 0.7 Co 0.3 MoO 4 and Mn 0.4 Co 0.6 MoO 4 pigments confirmed that they showed good stability and resistance under temperature and chemical tests.


P M Radingoana

S Guillemet-Fritsch

J Noudem

P A Olubambi

G Chevallier

C Estournès

Sophie Guillemet
email: sophie.guillemet@univ-tlse3.fr

Microstructure and Thermoelectric Properties of Al-doped ZnO ceramic prepared by Spark Plasma Sintering

Keywords: ZnO, Aluminum, Thermoelectric properties, Spark plasma sintering, Ceramics

The high thermal and low electrical conductivity of ZnO ceramics have hindered their thermoelectric applications. The doping of ZnO with group 3 elements can enhance the thermoelectric properties. In this work, Al (2at %) doped ZnO powder was sintered using spark plasma sintering at varying parameters (such as temperature (550-700 ℃), pressure (250-500 MPa) and the point of pressure application (Room temperature (RT) and holding time (HT))). Maximum relative density of 98.9% was achieved at a temperature and pressure of 650°C and 250 MPa, respectively. The Al-doped ZnO ceramics improved in electrical conductivity which caused a decrease in the Seebeck coefficient because of increased carrier concentration. The reduction in the grain size due to inhibiting growth effects of aluminum lead to a decrease in the thermal conductivity through phonon scattering at the grain boundaries. Hence, ZT of 1.5x10 -3 at 500°C was obtained. This study indicated that Al-doped ZnO ceramics can be sintered at very low temperature of 650°C, this helps to retain the nanostructure which is beneficial in improving the thermoelectric properties.

Introduction

Over the years, there has been a great progress in the thermoelectric performance of n-type ZnO based ceramics (such as Group 3 elements doping, polymer insertion, morphology modification etc.…) [START_REF] Fergus | Oxide materials for high temperature thermoelectric energy conversion[END_REF][START_REF] Feng | Metal oxides for thermoelectric power generation and beyond[END_REF][START_REF] Zhu | Multiple doped ZnO with enhanced thermoelectric properties[END_REF][START_REF] Mitra | Facile synthesis and thermoelectric properties of aluminum doped zinc oxide/polyaniline (AZO/PANI) hybrid[END_REF][START_REF] Wu | Enhanced thermoelectric figure of merit in nanostructured ZnO by nanojunction effect[END_REF][START_REF] Wu | Enhanced thermoelectric properties of ZnO: C doping and band gap tuning[END_REF][START_REF] Fedotov | Electron transport and thermoelectric properties of ZnO ceramics doped with Fe[END_REF][START_REF] Baghdadi | The effect of morphological modification on the thermoelectric properties of ZnO nanomaterials[END_REF]. We recently reported on the thermoelectric properties of pure zinc oxide (ZnO) [START_REF] Radingoana | Thermoelectric properties of ZnO ceramics densified through spark plasma sintering[END_REF]. According to the literature, doping ZnO with group 3 elements (such as aluminum (Al), gallium (Ga), indium (In)) can lead to enhanced electrical properties because dopants can modify the band gap of the oxide. Furthermore, their ionic radius are close to that of Zn (Al 3+ =0.054 nm, In 3+ = 0.08 nm, Ga 3+ = 0.062 nm, Zn 2+ = 0.074 nm) [START_REF] Jayaraman | Effect of co-doping concentration on structural, morphological, optical and electrical properties of aluminium and indium co-doped ZnO thin films deposited by ultrasonic spray pyrolysis[END_REF][START_REF] Look | Progress in ZnO materials and devices[END_REF]. Therefore, the ZnO structure will remain the same and the doping may generate free electrons in the conduction band.

Thus far, the highest ZT reported for doped ZnO ceramics is from dual doping of Al and Ga; ZT = 0.45@1000 K and ZT = 0.65@1247 K obtained using conventional sintering [START_REF] Ohtaki | High thermoelectric performance of dually doped ZnO ceramics[END_REF]. Aldoped ZnO ceramics were extensively investigated with ZTs of ~ 0.17 to 0.47 at 1000 K from ceramics prepared through hot press [START_REF] Ohtaki | High thermoelectric performance of dually doped ZnO ceramics[END_REF][START_REF] Tsubota | Thermoelectric properties of Al-doped ZnO as a promising oxide material for high-temperature thermoelectric conversion[END_REF][START_REF] Ohtaki | High-temperature thermoelectric properties of (Zn1-x Al x) O[END_REF][START_REF] Fujishiro | Effect of Microstructural Control on Thermoelectric Properties of Hot-Pressed Aluminum-Doped Zinc Oxide[END_REF], microwave [START_REF] Jood | Al-doped zinc oxide nanocomposites with enhanced thermoelectric properties[END_REF] and spark plasma sintering [START_REF] Gautam | Thermoelectric properties of pulsed current sintered nanocrystalline Aldoped ZnO by chemical vapour synthesis[END_REF][START_REF] Han | Effects of morphology on the thermoelectric properties of Al-doped ZnO[END_REF][START_REF] Nam | High-temperature charge transport and thermoelectric properties of a degenerately Al-doped ZnO nanocomposite[END_REF][START_REF] Han | High Temperature Thermoelectric Properties of ZnO Based Materials[END_REF][START_REF] Han | The influence of α-and γ-Al2O3 phases on the thermoelectric properties of Aldoped ZnO[END_REF][START_REF] Zhang | Hybrid-structured ZnO thermoelectric materials with high carrier mobility and reduced thermal conductivity[END_REF]. The substitution of Al on the zinc site of ZnO helps to improve thermoelectric properties through enhanced electrical conductivity and reduced thermal conductivity caused by nanostructuring. Aluminum is known to be a grain growth inhibitor [START_REF] Zhang | The Solubility and Temperature Dependence of Resistivity for Aluminum-Doped Zinc Oxide Ceramic[END_REF][START_REF] Teranishi | Thermoelectric Property of Polycrystalline Aluminum-Doped Zinc Oxide Enhanced by Micropore Foaming[END_REF].

However, these properties can be easily affected by the preparation methods of the doped powders. In most reported works, the authors have prepared them by solid state reactions of ZnO and Al 2 O 3 raw powders which can cause inhomogeneity in the material [START_REF] Cai | Preparation and thermoelectric properties of Al-doped ZnO ceramics[END_REF]. The most reliable technique is the synthesis of the powders by wet chemical methods that lead to a homogenous mixture [START_REF] Beynet | ZnO-based varistors prepared by spark plasma sintering[END_REF][START_REF] Guy | Elaboration and characterization of powders and varistors based on zinc oxide dope[END_REF]. Nowadays, consolidation is being conducted using spark plasma sintering (SPS) technique; resulting in full densification which is attained in a short period which minimizes grain growth [START_REF] Chaim | Sintering and densification of nanocrystalline ceramic oxide powders: a review[END_REF][START_REF] Guillon | Field-assisted sintering technology/spark plasma sintering: mechanisms, materials, and technology developments[END_REF]. Smaller grains are effective for reducing thermal conductivity through phonon scattering [START_REF] Holland | Phonon scattering in semiconductors from thermal conductivity studies[END_REF][START_REF] Giovannelli | Thermal conductivity and stability of Al-doped ZnO nanostructured ceramics[END_REF] and increasing the Seebeck Coefficient [START_REF] Jood | Al-doped zinc oxide nanocomposites with enhanced thermoelectric properties[END_REF][START_REF] Li | High-performance nanostructured thermoelectric materials[END_REF]. So, bulk materials with smaller grains provide the most promising thermoelectric properties. Thus, it is important to develop various grain size manipulation techniques to obtain improved thermoelectric properties.

Recently, cold sintering process (CSP) [START_REF] Funahashi | Demonstration of the cold sintering process study for the densification and grain growth of ZnO ceramics[END_REF][START_REF] Gonzalez-Julian | Unveiling the mechanisms of cold sintering of ZnO at 250° C by varying applied stress and characterizing grain boundaries by Kelvin Probe Force Microscopy[END_REF][START_REF] Guo | Cold sintering process: a novel technique for low-temperature ceramic processing of ferroelectrics[END_REF][START_REF] Jiang | Preparation of high density ZnO ceramics by the Cold Sintering Process[END_REF] and functional graded material (FGM) [START_REF] Cramer | Continuous functionally graded material to improve the thermoelectric properties of ZnO[END_REF][START_REF] Cramer | Testing and Modeling of Functionally Graded Aluminum-Doped Zinc Oxide Using Spark Plasma Sintering and Discrete Powder Layers of Varying Composition[END_REF] are reported to control the grain size of bulk materials. Cold sintering process adds small fraction of aqueous solution into powder to assist with densification of ceramics at very low temperatures of ~200℃, as a result, the nanostructure of the material is maintained [START_REF] Jiang | Preparation of high density ZnO ceramics by the Cold Sintering Process[END_REF]. Whereas FGM improves the efficiency and temperature range of TE materials through continuous grain size gradient [START_REF] Cramer | Continuous functionally graded material to improve the thermoelectric properties of ZnO[END_REF]. FGM Al-doped ZnO (0-5 wt% Al) layered ceramic prepared using spark plasma sintering at 1000℃ indicated that the current density could be widened which provides temperature range of useful energy conversion [START_REF] Cramer | Testing and Modeling of Functionally Graded Aluminum-Doped Zinc Oxide Using Spark Plasma Sintering and Discrete Powder Layers of Varying Composition[END_REF]. However, impurity phase, ZnAl 2 O 4 , was observed in the ceramic which has a huge impact on the resistivity. This is in accordance with other reported work on Al-doped ZnO ceramics prepared by spark plasma sintering, the presence of spinel phase is problematic [START_REF] Gautam | Thermoelectric properties of pulsed current sintered nanocrystalline Aldoped ZnO by chemical vapour synthesis[END_REF][START_REF] Nam | High-temperature charge transport and thermoelectric properties of a degenerately Al-doped ZnO nanocomposite[END_REF][START_REF] Han | High Temperature Thermoelectric Properties of ZnO Based Materials[END_REF][START_REF] Zhang | Hybrid-structured ZnO thermoelectric materials with high carrier mobility and reduced thermal conductivity[END_REF]. So, obtaining lesser spinel phase is desirable for better electrical conductivity.

During spark plasma sintering the thermal distribution is determined by the current density that has a great influence on the grain size, concentration of impurity phases (e.g ZnAl 2 O 4 ) and dislocation defects [START_REF] Yang | Influence of electric current on microstructure and electrical property of Aldoped ZnO ceramic consolidated by spark plasma sintering[END_REF][START_REF] Bagheri | Numerical modeling of heat transfer during spark plasma sintering of titanium carbide[END_REF][START_REF] Yang | Microstructure and electrical property of aluminum doped zinc oxide ceramics by isolating current under spark plasma sintering[END_REF]. For instance, sintering with high current density in Al-doped ZnO ceramics contributes to refined grain size and enhanced concentration of impurity phase, ZnAl 2 O 4 [START_REF] Yang | Influence of electric current on microstructure and electrical property of Aldoped ZnO ceramic consolidated by spark plasma sintering[END_REF]. Chen et al. [START_REF] Chen | Defects and microstructure of highly conducting Al-doped ZnO ceramics obtained via spark plasma sintering[END_REF] reported that sintering in low oxygen partial pressure leads to better substitution of Al ions into the ZnO lattice as compared to high oxygen partial pressure where it is transformed into spinel phase. Therefore, great caution needs to be taken when setting sintering conditions (such as temperature, current density, pressure, atmosphere etc.) for spark plasma sintering of Al-doped ZnO ceramics. This will assist to have a better understanding of the influence of spark plasma sintering parameters on the microstructure and thermoelectric properties of these ceramics.

In this present work, ZnO was doped with 2 mol% Al because of the previously reported high ZT of between 0.3 -0.45 at temperatures of about 1000K due to high electrical conductivity and reduced thermal conductivity [START_REF] Ohtaki | High thermoelectric performance of dually doped ZnO ceramics[END_REF][START_REF] Nam | High-temperature charge transport and thermoelectric properties of a degenerately Al-doped ZnO nanocomposite[END_REF][START_REF] Zhang | Hybrid-structured ZnO thermoelectric materials with high carrier mobility and reduced thermal conductivity[END_REF]. The Zn 0.98 Al 0.02 O powder was synthesized through co-precipitation followed by calcination. The sintering was performed using Spark plasma Sintering (SPS) at varying temperatures, pressures and point of pressure application to find full densification and suitable microstructure (i.e. fully densified with as much as possible low grain size). This discussion includes characterization of bulk samples, densification characteristics and thermoelectric properties of the Al-doped ZnO polycrystalline materials.

Materials and methodology 2.1 Materials

Al (2at%) doped ZnO powder was synthesized through the wet chemical method. Basically, the co-precipitation techniques followed by calcination, similarly to the procedures previously developed by Guy et al. [START_REF] Guy | Elaboration et caractérisation de poudres et de varistances à base d'oxyde de zinc dopé[END_REF][START_REF] Mory | Poudre d'oxyde de zinc dope, procede de fabrication et ceramique obtenue a partir de ladite poudre[END_REF].

Spark plasma sintering of the Zn 0.98 Al 0.02 O powder

Al-doped ZnO powders were densified under air atmosphere with a Dr. Sinter 2080 unit (SPS Syntex Inc., Japan) available at the Plateforme Nationale de Frittage Flash (PNF2) located at the Université Toulouse 3 Paul Sabatier using tungsten carbide molds with 8 and 20 mm inner diameters. Prior to their filling with the powder, the molds were lined with 0.2 mm thick graphite foil (PERMAFOIL®Toyo Tanso). A pre-compaction step at 25 MPa for 1 min was systematically applied before the thermal cycle to ensure the same initial green state for all the samples. The influence of sintering temperature (550-700°C), pressure (250-500 MPa) and point of applied pressure (room temperature (RT) or at the setpoint temperature (HT-Holding Temperature) was investigated. The sintered ceramics were polished by silicon carbide discs (P320 and P600) to remove graphite foil for further characterizations.

Characterization techniques

Dense ceramics

The particle size and the morphology of the starting powders and the microstructure of the dense ceramics were investigated using Scanning Electron Microscopy (MEB JEOL JSM65 10LV). X-ray diffractometer (XRD Bruker D4) was used to analyse the phase structure. The surface area and particle size distribution were conducted using Brunauer-Emmett-Teller technique (BET) (Micromeritics desorb 2300A) and the mastersizer (Mastersizer 2000 (CRISMAT) and Mastersizer 3000 (CIRIMAT)), respectively.

Electrical properties

The electrical measurements were conducted at the Laboratoire de Cristallographie et Sciences des Matériaux (CRISMAT, UMR 6508 CNRS, CAEN France). Seebeck coefficient and resistivity of the dense ceramics were simultaneously measured using ZEM-3 (Ulvac-Riko) in argon atmosphere. The dense ceramics were cut into 9x3x3 mm bars and coated with gold to ensure electrical ohmic contact. Graphite foil was inserted on both sides of heaters to have a good electrical and thermal contact with the sample. The thermal conductivity was determined by measuring separately thermal diffusivity (α), ceramic density (d) and heat capacity (Cp) (Eq.01). The samples were cut to dimensions of 6x6x1 mm. [START_REF] Fergus | Oxide materials for high temperature thermoelectric energy conversion[END_REF] The band gap was determined on ceramics, of diameter 20 mm and thickness 0.5 mm, through the photovoltaic route using Bentham PVE 300 PV. The Hall Effect measurements were performed on 3x3 mm square samples using a commercial physical property measurement system (Model 7100 AC transport controller, Quantum Design).

Results and discussion

Spark plasma sintering of Al-doped ZnO powder

The Al-doped ZnO powder (Zn 0.98 Al 0.02 O) was sintered by SPS in air at various parameters such as temperature, pressure and point of pressure application in order to get fully dense ceramics. Indeed, it has been previously shown that the use of WC tools allows to process oxide materials by SPS under air atmosphere and to overcome the problem of reduction due to the low oxygen partial pressure (neutral atmosphere, graphitic environment or dynamic vacuum) commonly encountered [START_REF] Noudem | Thermoelectric properties of Ca0. 9Yb0. 1MnO3-x prepared by spark plasma sintering in air atmosphere[END_REF][START_REF] Philippot | Local distortions in nanostructured ferroelectric ceramics through strain tuning[END_REF]. As a consequence, the annealing treatment, usually performed around 800°C, is not necessary to re-oxidized the specimen which can induce grain growth. The summary of the ceramics characteristics for different sintering conditions are given in Table 1.

Sintering behavior of Al-doped ZnO ceramics

The shrinkage curves of the pure ZnO and the 2 at% Al doped powders are given in Fig. 1. The shrinkage of both samples increased sharply till 550°C. The sintering steps of 2 at% Aldoped ZnO can be divided into three segments; the first stage is the rearrangement of the nanoparticles due to applied pressure. The displacement of 2 at% was slow and reached only 0.7 mm as compared to 1.2 mm for 0 at%, could be related to the aggregates present in the powder. The second stage is related to the progressive breaking of the remaining agglomerates and/or to the neck formations between the particles due to localized heating caused by spark discharge [START_REF] Beynet | ZnO-based varistors prepared by spark plasma sintering[END_REF][START_REF] Chaim | Sintering and densification of nanocrystalline ceramic oxide powders: a review[END_REF][START_REF] Chaim | Densification and preservation of ceramic nanocrystalline character by spark plasma sintering[END_REF][START_REF] Radingoana | Influence of processing parameters on the densification and the microstructure of pure zinc oxide ceramics prepared by spark plasma sintering[END_REF]. The third stage is related to extend of the neck's formation between the particles and to the densification step with progressive pore elimination. After 550°C, the slight increase in displacement of both samples could be due to final densification with elimination of the residual pores [START_REF] Cavaliere | Spark plasma sintering: process fundamentals[END_REF]. The total displacement between these samples differed by 0.5mm. This difference can be related to the influences of grain size on the sintering behavior of the ceramics. The higher density of grain boundaries in the doped sample compared with the pure ZnO could have enhanced the diffusion rate [START_REF] Ewsuk | Analysis of nanocrystalline and microcrystalline ZnO sintering using master sintering curves[END_REF], as visible in zone 2 where the kinetics are higher. As a result, the optimal sintering temperature decreased. The reduction in the sintering temperature has been reported on the sintering of nanoparticles materials such as ZnO [START_REF] Chaim | Sintering and densification of nanocrystalline ceramic oxide powders: a review[END_REF][START_REF] Dargatz | FAST/SPS sintering of nanocrystalline zinc oxide-Part I: Enhanced densification and formation of hydrogen-related defects in presence of adsorbed water[END_REF]. Reduced temperature implies less grain growth and more grains boundaries, which will be beneficial for improving thermoelectric properties. The relative densities of the ceramics are given in Table 4 from sintering temperature of 550℃ to 700℃.

Effect of sintering temperatures

The influence of the sintering temperature between 550-700°C on the structure, the microstructure, and the density of Al-doped ZnO ceramics was studied. Relative density versus temperature given in Fig. 2 showed an increase in the relative density with an increment in the temperature. Maximum relative density of 98.9% was achieved at 650°C, with initial relative green density of 57% at room temperature. Beyond 650°C a decrease in the relative density was observed which could be due to the presence of pores and/or decrease in the ceramic density related to the physical properties such as the density of the individual materials [START_REF] Ma | Microstructure and thermoelectric properties of Zn1-xAlxO ceramics fabricated by spark plasma sintering[END_REF][START_REF] Chaim | Grain growth during spark plasma and flash sintering of ceramic nanoparticles: a review[END_REF]. For instance, at 700°C the concentration of secondary phases increases, see XRD, which have lesser density as compared to that of ZnO. The sum of the total density of the ceramic will drop. Ma et al. [START_REF] Ma | Microstructure and thermoelectric properties of Zn1-xAlxO ceramics fabricated by spark plasma sintering[END_REF] reported higher densification rate (i.e. 99.9%) but such level was obtained after sintering in reduced atmosphere for higher sintering temperature (i.e. 900°C) than the range explored in the present study. While, Nam et al. [START_REF] Nam | High-temperature charge transport and thermoelectric properties of a degenerately Al-doped ZnO nanocomposite[END_REF] and Han et al. [START_REF] Han | Effects of morphology on the thermoelectric properties of Al-doped ZnO[END_REF] found lower relative densities of 94% and 90% at temperatures of 900°C and 950°C, respectively, by sintering in vacuum. Other reports mentioned relative densities of 99% at temperatures between 800-1200°C and only 65-93% sintering between 700-900°C for Al-doped ZnO ceramics [START_REF] Gautam | Thermoelectric properties of pulsed current sintered nanocrystalline Aldoped ZnO by chemical vapour synthesis[END_REF][START_REF] Díaz-Chao | Textured Al-doped ZnO ceramics with isotropic grains[END_REF]. In the present study we want to point out that a high relative density of 98.9% was obtained at a temperature of only 650°C and high pressure of 250 MPa. The achievement of such high relative density at low temperature could be due to the combination of high reactivity of the powders (i.e. small grain size), better rearrangement of the grains, and higher pressure applied. The analysis of the XRD patterns of most of the Al-doped ZnO ceramics (Fig. 3) showed that the main phase corresponds to the ZnO hexagonal Wurzite structure. It was observed that as the sintering temperature increases the XRD peaks of the ceramics are sharpened which is related to the increase in the grain size. The zoomed inserted image indicated the presence of spinel phase (ZnAl 2 O 4 ) with very small peaks at 37°, 35°, 32.5° and 31.5°. This indicates that sintering at low temperatures in air there is efficient incorporation of Al ions in ZnO lattice as compared to at high temperatures where it is transformed into a spinel phase. This is because of the differences in the partial pressure of oxygen, the lower the temperature the lower the partial pressure of oxygen in the air and vice versa, see Fig.4 [START_REF] Chen | Defects and microstructure of highly conducting Al-doped ZnO ceramics obtained via spark plasma sintering[END_REF][START_REF] Imam | Effect of Oxygen Partial Pressure and Temperature on the Oxidation Behavior of SiB6[END_REF]. Low partial pressure of oxygen promotes further substitution of Al into the ZnO structure. Whereas, in high oxygen partial pressures excess Al reacts with ZnO to form ZnAl 2 O 4 . This is in line with reported work on the comparison between conventional and SPS of Al-doped ZnO ceramics sintered at 1200℃ in air and 1000℃ in vacuum, respectively [START_REF] Chen | Defects and microstructure of highly conducting Al-doped ZnO ceramics obtained via spark plasma sintering[END_REF]. It was found that the samples exposed to air had high concentration of spinel phase compared to those in vacuum due to the differences in oxygen partial pressure and rapid dissolution of Al ions into ZnO lattice caused by the high DC current applied in SPS process. Similarly, high concentration of spinel phase was observed in air than in nitrogen atmosphere prepared using conventional route at 1400℃ for Al-doped ZnO ceramic [START_REF] Bérardan | Influence of the preparation conditions on the thermoelectric properties of Al-doped ZnO[END_REF]. Nunes and Bradt [START_REF] Nunes | Grain Growth of ZnO in ZnO-Bl2O3 Ceramics with Ai2O3 Additions[END_REF] report on conventionally sintered ZnO/Bi 2 O 3 /Al 2 O 3 ceramics found that the majority of the activation energy (400 kJ/mol) was related to the transport of the ZnAl 2 O 4 spinel particles which are most probably controlled by the diffusion of 0 2-ions in the ZnAl 2 O 4 spinel structure.

It is possible that this secondary phase present in higher quantity at higher temperature could have contributed to the decrease in the relative density at 700°C, the density of ZnAl 2 O 4 phase is 4.61 g/cm 3 , compared to ZnO (i.e. 5.61g/cm 3 ). The SEM micrograph of 0 at% and 2 at% Al doped ZnO ceramics sintered at 700°C are given in Fig. 5. It is shown that there is a decrease in the grain size when 2 at% Al is doped in the ZnO from 5.8 µm (0 at%) to 0.76 µm (2 at%) (see Fig. 5 (a) and (b)). This could be due to the grain growth inhibiting effects of aluminum as discussed in previous sections. Further studies were done to understand the influence of sintering temperature on the microstructure of 2 at% Al doped ZnO. It was observed that sintering at 550°C and 600°C did not change the grain size of the ceramics. However, at a temperature of 650°C and 700°C the grain size increased from initial grain size of 75 nm to 457 nm and 760 nm, respectively, (see Fig. 5 ((b) and (c)). EDX analysis on the sample sintered at 700°C indicated that the light phase is Al rich and the dark phase is Zn rich. This can be supported by the XRD results because the ZnAl 2 O 4 is Al rich phase. Therefore, the light phase in the SEM analysis is the spinel phase of ZnAl 2 O 4 as determined by the XRD analysis. This is in conformity with the ZnAl 2 O 4 peak observed in XRD by Jood et al. [START_REF] Jood | Al-doped zinc oxide nanocomposites with enhanced thermoelectric properties[END_REF]. It was noticed on their SEM observations that the spinel phase is homogeneously distributed across the sample. The presence of the spinel phase will influence the electrical properties such as resistivity and carrier mobility. Some reports indicated that it could lead to a decrease of the thermal conductivity while on the other hand it could be a hindrance in improving the electrical conductivity due to electron scattering [START_REF] Tsubota | Thermoelectric properties of Al-doped ZnO as a promising oxide material for high-temperature thermoelectric conversion[END_REF][START_REF] Jood | Al-doped zinc oxide nanocomposites with enhanced thermoelectric properties[END_REF][START_REF] Gautam | Thermoelectric properties of pulsed current sintered nanocrystalline Aldoped ZnO by chemical vapour synthesis[END_REF][START_REF] Han | Effects of morphology on the thermoelectric properties of Al-doped ZnO[END_REF][START_REF] Nam | High-temperature charge transport and thermoelectric properties of a degenerately Al-doped ZnO nanocomposite[END_REF][START_REF] Cai | Preparation and thermoelectric properties of Al-doped ZnO ceramics[END_REF]. EDS analysis of the sample sintered at 650°C did not indicate any secondary phases which may be attributed to the low concentration of ZnAl 2 O 4 . Further studies were conducted to reduce the concentration of secondary phase in the ceramics while keeping high densification.

Reduction of the amount of spinel phase in Al-doped ZnO ceramics

In the previous section, it was indicated that sintering at temperature above 650°C with a pressure of 250 MPa applied at room temperature causes the formation of the secondary phases that can be a hindrance in improving thermoelectric properties. One way to avoid the presence of spinel phase in the sintered ceramics is to decrease the setpoint temperature below 650°C while conserving the objective to obtain dense specimen. To do so two strategies have been followed: the first one is to apply the high pressure at the holding point/temperature (HT) and the second one is to double the pressure. At first the influence of the changes in sintering conditions on the densification was determined. For a set point temperature of 600°C, the relative densities of the samples reached only 84% even though a pressure of 250 MPa is applied at room temperature and at the dwell (HT), see Table 1. An increase in the relative density from 84 to 98.5% was observed when the sintering pressure was increased to 500 MPa during heating stage at a temperature of 600°C. However, the sintered ceramic cracked at this pressure due to distribution of residual stresses during cooling of sintered ceramic [START_REF] Tomaszewski | Residual stresses in layered ceramic composites[END_REF]. So, an optimal densification of 98.6% was achieved at 650°C and 250 MPa without cracks. This will be beneficial in improving the electrical conductivity and better thermoelectric properties.

The XRD patterns of the ceramics sintered at different temperatures and point of applied pressure are reported in Fig. 6. For all the samples, the major phase present is ZnO with the same hexagonal structure of the powder. It is presented that applying pressure during the dwell time (HT) for both 600°C and 650°C, the secondary phases were not detected as compared to applying the pressure at room temperature (RT). This could be due to the rapid diffusion of Al atoms which promotes the replacement Al ions into the ZnO lattice [START_REF] Chen | Defects and microstructure of highly conducting Al-doped ZnO ceramics obtained via spark plasma sintering[END_REF].

Fig. 6 XRD patterns of Al-doped ZnO ceramics sintered at 600°C and 650°C with a pressure of 250 MPa using RT and HT mode SEM micrograph of the sample sintered at 650°C using HT mode (Fig. 5 (d)) revealed that the nanostructure of the sample is maintained compared to the sample sintered using RT mode. The differences in heat transfer between the grains could have caused the grain size difference; RT mode had better contact as compared to HT mode [START_REF] Radingoana | Influence of processing parameters on the densification and the microstructure of pure zinc oxide ceramics prepared by spark plasma sintering[END_REF]. This polycrystalline sample was further annealed in air at 600°C for 24 h (not shown) to obtain fully oxidized ceramic to ensure stable measurement and reproducibility of electrical properties. XRD analysis of the annealed sample did not show any changes in the phase structure and the grain size slightly increased from 90 nm to 140 nm.

Thermoelectric properties

The thermoelectric properties were determined for the annealed ceramic sintered at 650°C (HT) as it contains minimal/no secondary phase and exhibit a densification rate of 98.6%. see Fig 7 . The resistivity as a function of temperature is given in Fig. 7 (a); for the samples 0 at% and 2 at%. It is shown that addition of Al in ZnO reduces the resistivity especially at low temperature (< 200°C). At 50°C the resistivity reduced from ~1960 Ω.cm to ~50 Ω.cm for 0 Al at% and 2 Al at%, respectively. This difference is attributed to the substitution of Al 3+ ions on the Zn 2+ site creating extra free electrons [START_REF] Gautam | Thermoelectric properties of pulsed current sintered nanocrystalline Aldoped ZnO by chemical vapour synthesis[END_REF][START_REF] Han | Effects of morphology on the thermoelectric properties of Al-doped ZnO[END_REF][START_REF] Nam | High-temperature charge transport and thermoelectric properties of a degenerately Al-doped ZnO nanocomposite[END_REF][START_REF] Han | High Temperature Thermoelectric Properties of ZnO Based Materials[END_REF][START_REF] Han | The influence of α-and γ-Al2O3 phases on the thermoelectric properties of Aldoped ZnO[END_REF][START_REF] Zhang | Hybrid-structured ZnO thermoelectric materials with high carrier mobility and reduced thermal conductivity[END_REF]. It is observed that the resistivity of both samples decreases with an increase in temperature, which is significant of a semiconducting behavior. At high temperature the valence electrons gain enough energy to reach the conduction band, hence, the lowest resistivity of about 1 Ω.cm at 500°C [START_REF] Pei | Convergence of electronic bands for high performance bulk thermoelectrics[END_REF]. Sondergaard et al. [START_REF] Søndergaard | Sintering and annealing effects on ZnO microstructure and thermoelectric properties[END_REF] reported a lower resistivity compared to our samples for the same Al content, i.e. resistivity values of 0.005 and 0.0015 Ω.cm at room temperature were obtained for samples sintered at 850°C and 900°C, respectively, using spark plasma sintering in vacuum. This difference is probably due to high carrier concentration caused by differences in preparation methods and particularly the annealing treatment after sintering. Sondergaard et al. [START_REF] Søndergaard | Sintering and annealing effects on ZnO microstructure and thermoelectric properties[END_REF] performed an annealing treatment in air for a week at 800 °C while in our case , the treatment was chosen at much lower temperature, to avoid the grain growth, i.e. 600 °C (because of our low sintering temperature) for 24 h in air also. Obviously, the annealing at 800 °C for such a long time leads to the creation of more oxygen vacancies that significantly reduce the resistivity.

The Seebeck Coefficient as a function of temperature is given in Fig. 7 (b). The negative sign indicates and confirms the n-type conductivity. Adding 2 at% Al reduced the Seebeck coefficient value from 800 to 175 µV/K at 50°C. This decrease could be caused by the increase in the carrier concentration. The broadband equation ( 1) indicates inverse relationship between Seebeck coefficient and carrier concentration [START_REF] Walia | Transition metal oxides-Thermoelectric properties[END_REF].

(

The absolute Seebeck Coefficient, ,|S| for the 2 Al at% sample increases with an increase in temperature from 50°C to 500°C; which is related to the scattering of the electrons at high temperature. The behaviour is different from pure ZnO which showed a decrease in the |S| with temperature increment. Sondergaard et al. [START_REF] Søndergaard | Sintering and annealing effects on ZnO microstructure and thermoelectric properties[END_REF] reported similar behaviour to that of 2at% Al-doped ZnO with |S| of about 125 µV/K at 50°C for sintering at 850°C and 900°C.

The temperature dependence of the thermal conductivity of the polycrystalline ceramic presented in Fig. 7 (c) revealed that doping with Al into ZnO ceramic reduces the thermal conductivity from 50 W/m.K (0at%) to 27 W/m.K (2at%) at 50°C. This was caused by phonon scattering at the grain boundaries because of reduced grain size [START_REF] Gautam | Thermoelectric properties of pulsed current sintered nanocrystalline Aldoped ZnO by chemical vapour synthesis[END_REF][START_REF] Han | Effects of morphology on the thermoelectric properties of Al-doped ZnO[END_REF][START_REF] Nam | High-temperature charge transport and thermoelectric properties of a degenerately Al-doped ZnO nanocomposite[END_REF][START_REF] Han | High Temperature Thermoelectric Properties of ZnO Based Materials[END_REF][START_REF] Han | The influence of α-and γ-Al2O3 phases on the thermoelectric properties of Aldoped ZnO[END_REF][START_REF] Zhang | Hybrid-structured ZnO thermoelectric materials with high carrier mobility and reduced thermal conductivity[END_REF]. It is clearly shown that the thermal conductivity determined in this present work is almost similar to that reported by Sondergaard et al. [START_REF] Søndergaard | Sintering and annealing effects on ZnO microstructure and thermoelectric properties[END_REF] probably because of similar grain size. A grain size difference of only 20nm was determined between the present study and that of Sondergaard et al. [START_REF] Søndergaard | Sintering and annealing effects on ZnO microstructure and thermoelectric properties[END_REF].

The performance of the ceramics was determined using figure of merit, ZT, given in Fig. 7 (d). An increase in ZT was observed with temperature increment for all samples. It was observed that 0 at% performs better than the 2 at% ceramic by a ZT value of 1.5x10 -3 at a temperature of 500°C. The improved performance is because of high Seebeck Coefficient shown by 0 at% ceramics. The performance of the 2 at% can be enhanced by improving the solubility of Al in ZnO through process manipulations such as sintering temperature increment and operating in vacuum to increase the electrical conductivity. Also, new materials could be incorporated with the Al-doped ZnO ceramic to enhance the electrical conductivity while maintaining the nanostructure of ceramic.

Fig. 1

 1 Fig.1 Sintering behavior of pure ZnO and Al-doped ZnO using spark plasma sintering

Fig. 2

 2 Fig.2 Relative density versus temperature of Al-doped ZnO ceramics

Fig. 3 4 Fig. 4

 344 Fig.3 XRD patterns of Al-doped ZnO ceramics at varying temperature

Fig. 5

 5 Fig.5 SEM micrographs of 0 at% and 2 at% Al ZnO ceramic sintered at 700°C and 650°C (a) Pure ZnO (700-RT), (b) Al-ZnO (700-RT) (c) Al-ZnO (650-RT) and (d) Al-ZnO (650-HT) Fig.8 SEM micrograph of Al-doped ZnO ceramic at different sintering temperatures using RT mode

Fig. 7

 7 Fig.7 Thermoelectric properties of annealed pure and Al-doped ZnO ceremicc sintered at 650°C (HT): (a) Resistivity (b) Seebeck Coefficient (b) Thermal conductivity (d) Figure of merit (ZT)

Table 1

 1 Summary of pure and Al-doped ZnO ceramics characteristics for varied SPS parameters (Note: RT-Room temperature, HT-Holding time)

	Ref.	Temper ature	Initial Pressure	Final Pressure	Point of application Pressure	Atmosphere	RD	Grain size
		°C	MPa	MPa	°C		%	µm
					ZnO
	1		25	250	RT	Air	99.3±0.1	2.4±0.6
	2		25	250	RT	Air	100	5±2
				Zn 0.98 Al 0.02 O
	3	RT	250	250	RT	Air	57±0.2	-
	4		250	250	RT	Air	79.9±0.5	-
	5		250	250	RT	Air	84.9±0.2	-
	6		250	250	RT	Air	98.9±0.1 0.46±0.01
	7		25	250	RT	Air	98.1±0.1 0.76±0.26
	8		25	250	HT	Air	84.3±0.2	-
	9		25	250	HT	Air	98.6±0.1 0.09±0.02
	10		100	500	HT	Air	98.5±0.1	-
			[

Zn 0.98 Al 0.02 O] Bibliography Ma et al.

	[52]	50	50	-	Vacuum	99.9	-
	Nam et al. [19]	50	50	-	Vacuum	94	0.2
	Han et al.[18]	50	50	-	Vacuum	90	-

Acknowledgements

This project was funded by A European and South African Partnership on Heritage and Past+ (AESOP+, Europe) (551128-EM-1-2014-1-FR-ERA MUNDUS-EMA21) and National Research Fund (NRF, South Africa) (102699). The authors would like to thank J. Lecourt, F. Veillon and C. Bilot from CRISMAT laboratory for their technical assistance.


Antoine Klos

Lucie Bailly

Sabine Rolland

Du Roscoat

Laurent Orgéas

Nathalie Henrich Bernardoni

Ludovic Broche

4D X-ray microtomography of vibrating multiscale structures: numerical optimisation and experimental validation

come

18th European Mechanics of Materials Conference (EMMC18) April 4 -6, 2022, Oxford, UK

4D X-ray microtomography of vibrating multiscale structures: numerical optimisation and experimental validation

Antoine KLOS 1,2 In the last decade, some pioneer studies have been proposed to image the structural motions of biological oscillators using synchrotron X-ray tomography. The first advances have allowed visualising 3D low-frequency tissue motions, notably for lung and cardiac function measurement [START_REF] Lovrić | Vivo Study of Lung Physiology with Dynamic Synchrotron-Based Tomographic Microscopy[END_REF]. More recently, 3D images of fly muscles during wingbeat at 145 Hz were obtained [START_REF] Walker | In Vivo Time-Resolved Microtomography Reveals the Mechanics of the Blowfly Flight Motor[END_REF]. However, breakthroughs are still expected in acquisition and reconstruction procedures to keep improving the spatiotemporal resolution, and study the mechanics of fast-vibrating multiscale structures. Thereby, this work aims at: (i) proposing a numerical tool to optimise scanning parameters for any multiscale oscillator; (ii) assessing its relevance by comparison with 4D X-ray data acquired at various time and spatial scales. First, an optimisation procedure is proposed to minimise the blurring artefacts typically obtained when imaging a moving structure, which strongly depend on the couple formed by the speed of its periodic motion and the exposure time of the detector in use. Based on simulations conducted on a vibrating Shepp-Logan phantom, charts of ideal scanning parameters are computed. Then, a custom vibro-mechanical set-up was designed to subject samples to a large range of vibrating loadings (up to 1500 Hz). Knowing the a priori optimal scanning parameters, 4D X-ray tomographies with retrospectively gated projections were then performed both with laboratory and synchrotron sources. Samples were made of soft silicone matrices reinforced with different fibrous architectures and were subjected to sinusoidal rigid body translation (from 1 to 400 Hz). Finally, comparing real and virtual 4D data, the relevance of the methodology will be discussed according to spatial (macro/micro) and time scales (low/high frequency).

  , Lucie BAILLY 1 , Sabine ROLLAND DU ROSCOAT 1 , Laurent ORGÉAS 1 , Nathalie HENRICH BERNARDONI 2 , Ludovic BROCHE 3

	1 Univ. Grenoble Alpes, CNRS, Grenoble INP, 3SR, 38000 Grenoble, France
	2 Univ. Grenoble Alpes, CNRS, Grenoble INP, GIPSA-lab, 38000 Grenoble, France
	3 ID19 beamline, ESRF, CS40220, 38043 Grenoble, France
	(antoine.klos@3sr-grenoble.fr, lucie.bailly@3sr-grenoble.fr)