domain: chim phys spi

text:
Dr Pierre Gibot
email: pierre.gibot@isl.eu

Loic Vidal

Lydia Laffont

Julien Mory

Zirconia nanopowder synthesis via detonation of trinitrotoluene

Keywords: A. Powders: Detonation synthesis, D. ZrO2, Transition metal oxides, B. Nanoparticles A. Powders: Detonation synthesis, D. ZrO 2, Transition metal oxides, B. Nanoparticles

Zirconium (IV) oxide nanopowder was successfully synthesized through the detonation of a mixture composed of 2,4,6 trinitrotoluene (TNT, C7H5N3O6) and zirconium sulphate tetrahydrate (Zr(SO4)2.4H2O) as the energetic material and ceramic precursor, respectively. TNT, one of the most popular explosives, is a secondary energetic molecule and exhibits high stability and low sensitivity toward external stresses, making its handling safe. After detonation of the energetic material/ceramic precursor mixture and purification of the detonation soot, a crystallized zirconium oxide (ZrO2) powder composed of nanosized particles with a spherical morphology was produced and analysed by the usual characterization techniques (X-ray diffraction, Fourier transform infrared spectroscopy, transmission electron microscopy and nitrogen physisorption). The reaction mechanism, considering the thermochemical aspect of the explosive, is offered. This approach could provide promising opportunities for the synthesis of various nano-sized oxide ceramic powders.

Introduction

Zirconium (IV) oxide is an outstanding refractory ceramic material since it exhibits very interesting physicochemical properties, such as high melting (2700 °C) and boiling points (> 4500 °C) and high thermal and chemical inertias, particularly under alkali conditions [1]. The crystallographic structure of zirconium oxide material can be described as transitioning between three well-known crystallographic phases [2]: the monoclinic phase (P2 1 /c space group) up to 1170°C, the intermediate tetragonal phase (P4 2 /nmc space group) from 1170 -2370 °C and the cubic phase (Fm3m space group) from 2370 °C to the melting point. These last two crystal structures can be stabilized at room temperature by the addition of a weak molar percent of calcium oxide (CaO), cerium (IV) oxide (CeO 2 ), magnesium oxide (MgO) or yttrium oxide (Y 2 O 3 ) [3]. The high diversity of the physicochemical properties and crystalline structures of this ceramic material makes it promising for a large number of applications in many areas, such as fuel cells [4], sensors [START_REF] Ghom | Oxygen sensing with mesoporous ceriazirconia solid solutions[END_REF], bioceramics [START_REF] Piconi | Zirconia as a ceramic biomaterial[END_REF] and catalysts [START_REF] Li | Effect of calcium salts on isosynthesis over ZrO 2 catalysts[END_REF].

To satisfy these numerous applications, synthesizing a tunable zirconium (IV) oxide material that exhibits distinct physical surface properties (i.e., particle size, specific surface area, and porosity) or particles with various morphologies has become paramount. Thus, as a large number of these properties strongly depend of the synthesis conditions, many processes have been developed, including sol-gel without ultrasound assistance [START_REF] Yan | Facile synthesis of titania-zirconia monodisperse microspheres and application for phosphopeptides enrichment[END_REF][START_REF] Judes | Sol-gel preparation and characterization of ceria stabilized zirconia minispheres[END_REF], coprecipitation [START_REF] Chen | Sodium hydroxide-sodium oxalate-assisted co-precipitation of highly active and stable Cu/ZrO 2 catalyst in the partial oxidation of methanol to hydrogen[END_REF][START_REF] Dechamps | Structure of zirconia prepared by homogeneous precipitation[END_REF], hydrothermal [START_REF] Arantes | Stable colloidal suspensions of nanostructured zirconium oxide synthesized by hydrothermal process[END_REF], mechanochemical [START_REF] Dodd | Synthesis of nanocrystalline ZrO 2 powders by mechanochemical reaction of ZrCl 4 with LiOH[END_REF], microwave [START_REF] Moon | Preparation of monodisperse ZrO 2 by the microwave heating of zirconyl chloride solutions[END_REF], Pechini [START_REF] Lin | Phase transformation and photoluminescence properties of nanocrystalline ZrO 2 powders prepared via the Pechini-type sol-gel process[END_REF] and combustion methods [START_REF] Fabregas | Parametric study of the gel-combustion synthesis of nanocrystalline ZrO 2 -based powders[END_REF]. More recently, an unconventional dynamic method based on the detonation of highly energetic materials (EM) or explosives to generate precursors for powder preparation has been published in the literature. This method combines high temperatures (> 1000 °C) and pressures (> GPa) for short times, thus producing (metastable) materials at nanoscales. The nanometric dimensions of the materials are assured by a cooling bath surrounding the energetic charge. The first example of successful detonation synthesis was reported for nanodiamonds using a mixture of two common secondary explosives, 2,4,6 trinitrotoluene (TNT) and 1,3,5 trinitro-1,3,5-triazine (RDX) [START_REF] Mochalin | The properties and applications of nanodiamonds[END_REF]. From these initial results, some researchers have used RDX and ammonium nitrate (NH 4 NO 3 ), an inorganic energetic molecule; or a mixture of both as the detonation source for producing single (Al 2 O 3 , ZrO 2 , B 2 O 3 , TiO 2 , ZnO, CeO 2 ) [START_REF] Cudzilo | Detonative synthesis of inorganic compounds[END_REF][START_REF] Bukaemskii | Explosive synthesis of ultra-disperse aluminium oxide in an oxygen-containing medium[END_REF][START_REF] Li | Experimental Investigations of the controlled explosive synthesis of ultrafine Al 2 O 3[END_REF][START_REF] Qu | Detonation synthesis of nanosized titanium oxide powders[END_REF][START_REF] Qu | Selective synthesis of TiO 2 nanopowders[END_REF][START_REF] Yan | Slurry explosive detonation synthesis and characterization of 10 nm TiO 2[END_REF][START_REF] Xie | Detonation synthesis of zinc oxide nanometer powders[END_REF][START_REF] Han | Study on the application of emulsion explosives in synthesizing nanostructured ceria[END_REF] or mixed oxide ceramics (Gd 2 O 3 /CeO 2 , LiMn 2 O 4 , MnFe 2 O 4 , SrAl 2 O 4 ) [START_REF] Vasylkiv | Nanoexplosion synthesis of multimetal oxide ceramic nanopowders[END_REF][START_REF] Xie | Growth and morphology of nanometer LiMn 2 O 4 powder[END_REF][START_REF] Wang | Nano-MnFe 2 O 4 powder synthesis by detonation of emulsion explosive[END_REF][START_REF] Li | Preparation of SrAl 2 O 4 : Eu 2+ , Dy 3+ nanometer phosphors by detonation method[END_REF]. The ammonium nitrate-based explosive emulsion approach was patented by [START_REF] Silva | Nanometric-sized ceramic materials, process for their synthesis and uses thereof[END_REF][START_REF] Santos | Process for nanomaterial synthesis from the preparation and detonation of an emulsion, products and emulsions thereof[END_REF], and Innovnano markets an as-detonated yttria-stabilized zirconia material [START_REF] Silva | Nanometric-sized ceramic materials, process for their synthesis and uses thereof[END_REF][START_REF] Santos | Process for nanomaterial synthesis from the preparation and detonation of an emulsion, products and emulsions thereof[END_REF][32]. These authors claim many advantages regarding the detonation method, such the synthesis of a pure material consisting of particles with a spherical morphology and of nanometric size. For example, Bukaemskii synthesized a nanosized zirconia powder from a mixture of zirconium hydroxide (Zr(OH) x ) and RDX as the zirconium precursor and explosive molecule, respectively [START_REF] Bukaemskii | Nanosize powder of zirconia. Explosive method of production and properties[END_REF]. Cubic 5-6 nm-sized ZrO 2 particles were successfully produced without the assistance of a stabilizing compound, as used in previous studies. Recently, non-oxide ceramics have also be prepared via the explosive route [START_REF] Ornek | Observations of explosion phase boron nitride formed by emulsion detonation synthesis[END_REF][START_REF] Ornek | Formation of metastable wurtzite phase boron nitride by emulsion detonation synthesis[END_REF][START_REF] Langenderfer | Detonation synthesis of silicon carbide nanoparticles[END_REF]. For example, Örnek et al. have successfully demonstrated the synthesis of nanoscale metastable boron nitride phases (w-BN and e-BN) from the detonation of an hexagonal boron nitride phase (h-BN) mixed with an explosive emulsion [START_REF] Ornek | Observations of explosion phase boron nitride formed by emulsion detonation synthesis[END_REF][START_REF] Ornek | Formation of metastable wurtzite phase boron nitride by emulsion detonation synthesis[END_REF]. Langenderfer and co-workers have synthesized silicon carbide nanoparticles by submitted a pre-ceramic polymer (polycarbosilane) to the high pressures and temperatures generated by the detonation of a 2,4,6-trinitrotoluene and 1,3,5-trinitro-1,3,5-triazine energetic mixture [START_REF] Langenderfer | Detonation synthesis of silicon carbide nanoparticles[END_REF]. Succinctly, the literature cited above evidences the high potential of this approach given the possibility of producing nano-sized materials, of stabilizing metastable phases and that for different kinds of materials.

Herein, a zirconium (IV) oxide nanopowder was detonation-synthesized using a mixture of 2,4,6 trinitrotoluene (TNT) and zirconium (IV) sulfate tetrahydrate as the explosive substance and zirconium source, respectively. TNT has the advantage of being less sensitive to mechanical stimuli than RDX, thus contributing to operator safety during the handling of these explosive compositions [START_REF] Meyer | Explosives[END_REF]. For example, the impact and friction sensitivities of TNT are 15 J and 353 N, respectively, compared to 7.4 J and 120 N for RDX. Finally, the as-detonated product was submitted to a series of analyses for characterization. (99.65/0.35 wt. %) was provided by Titanobel. Estane 5703, a polyester type thermoplastic polyurethane, was obtained from Lubrizol Advanced Materials, Inc. Hydrochloric acid (37%) was obtained from Prolabo.

Experimental section

Preparation of the zirconium-based energetic charge

The zirconium sulfate hydrate material was covered by an Estane 5703 polymer layer using a dissolution/precipitation synthesis method. Using vigorous stirring (600 rpm), 1.54 g of Estane 5703 was dissolved in 200 mL of acetone. Then, 29.64 g of zirconium salt was dispersed in the solution, and the mixture was stirred to obtain a homogeneous solution. Then, 460 mL of heptane was added (the first 280 mL added drop-by-drop) to the previous solution and maintained under stirring conditions to precipitate the polymer. Acetone was removed using a rotating evaporator (50 °C, 100 mbars), and the Zr(SO 4 ) 2 .4H 2 O/Estane 5703 granules (95/5 weight ratio) were separated by filtration followed by washing with heptane. The Zr(SO 4 ) 2 . A booster (graphite/wax/1,3,5-trinitro-1,3,5-triazinane secondary explosive 0.5/5/94.5 wt%, 55 g, Ø = 28.4 mm, L = 57.5 mm) and an electric initiator (SA4000, Davey Bickford) were used to complete the energetic train. After detonation, the soot was filtered to remove plastic and metal fragments, washed with 2 mol/L hydrochloric acid to dissolve the rust from the detonation chamber and calcinated at 700°C for 1 hour to oxidize the carbonaceous products of the TNT detonation and polypropylene tube combustion. The oxide ceramic yield was ~ 35%.

Characterization techniques

X-ray powder diffraction (XRD) was recorded on a D8 Advance diffractometer (Bruker, Billerica, MA, USA) using Cu-Kradiation ( = 1.54056 Å) equipped with a Lynxeye detector and operating at 40 kV and 40 mA in the range 2= 20-80 ° with a 2step size of 0.02 °. Fourier Transform Infra-Red (FT-IR) spectroscopy analysis of the sample was conducted using a Bruker Tensor 27 spectrometer.

The spectrum was recorded in the transmission mode (KBr pellets) with a wavenumber range of 4000-450 cm -1 and by superposing 16 scans. The resolution was fixed at 4 cm -1 . The structure and microstructure of the product was determined using a JEM 2100F transmission electron microscope (JEOL, Tokyo, Japan) at an acceleration voltage of 200 kV and equipped with an energy dispersive spectrometer. The diffraction pattern of the investigated material was obtained using the selected area electron diffraction mode or by Fast Fourier transform of the high-resolution transmission electron microscopy images. Nitrogen sorption measurements at -196 °C were carried out on an ASAP2020 surface area analyser (Micromeritics, Norcross, GA, USA) with the sample outgassed at 200 °C under vacuum for 6 hours. The specific surface area was determined according to the Brunauer-Emmett-Teller method within the 0.05-0.20 relative pressure domain. The particle size distribution of the powder, which as previously dispersed in chloroform (0.1 g/L), was determined by dynamic light scattering using a Zetasizer nanoZS apparatus (Malvern Instrument, Worcestershire, United Kingdom).

Results and Discussion

The XRD pattern of the product synthesized by the detonation of a TNT/Zr(SO 4 ) 2 . 4H 2 O (90/10 wt%) energetic charge is displayed in Figure 1 (top). All the diffraction peaks could be assigned to the single zirconium (IV) oxide phase (ZrO 2 ) except for the two thick and low intensity peaks located at ~21° and ~26-27°, which were assigned to an impurity from the white paint covering the inner wall of the detonation tank (Figure 1 top, symbol star). The impurity was identified as silica in quartz form (JCPDS 01-087-2096). Note that a sand blasting operation is planned in the near future to remove this impurity. Regarding the zirconia phase, both polymorphs were detected, including the monoclinic and tetragonal structures described as P2 1 /a and P4 2 /nmc space groups, and were identified by the JCPDS card No 37-1484 and No 72-7115, respectively. Taking into account the relative intensity of the diffraction peaks described in both crystallographic reference cards, a preferential orientation/growth of the crystallite was observed at 2Ɵ = 35-36°, which may correspond to the (002) and (110) Miller planes of the monoclinic and tetragonal phases, respectively. To date, the reaction mechanism is not yet well-understood, and an investigation is in progress.

The volume percent (V m ) of the monoclinic structure was determined using formula (1) by Schmid et al. [START_REF] Schmid | Quantitative analysis of polymorphic mixes of zirconia by X-ray diffraction[END_REF] as given below:

(1)

where X m , the ratio of integrated intensities (noted I) of specific (hkl) diffraction peaks, was specified by formula ( 2):

(

A volume fraction of ~52% monoclinic phase was calculated (45% of X m weight fraction), making this structure the majority in the ZrO 2 powder.

Figure 2 shows the FTIR spectrum of the product synthesized by the detonation of the TNT/Zr(SO 4 ) 2 . 4H 2 O (90/10 wt. %) energetic charge, magnified on the 500-1000 cm -1 domain characterizing the metal-oxygen bonds vibration. The wavenumbers identifying each infrared vibration band are reported on the spectrum. The intense vibration band at 505 cm -1 was assigned to the Zr-O bond vibration in the tetragonal structure, while the bands at 578 and 744 cm -1 were correlated to zirconia material in the monoclinic phase [START_REF] Aflaki | Synthesis, luminescence and photocatalyst properties of zirconia nanosheets by modified Pechini method[END_REF]. This result supports the previous structural result (XRD) that showed the coexistence of both zirconia phases. An overview of the spectrum; i.e. over 1000 cm -1 is shown on Supporting information. The different visible vibration bands were associated with the vibrational modes of hydroxyl groups (-OH) or water molecules, both physisorbed on the surface of the material. Traces of the silica impurity were also observed. The as-detonated zirconia powder was imaged by transmission electron microscopy (TEM) and high resolution (HR)-TEM analysis in order to reveal the morphology, structure and crystalline degree of the particles. Representative views are displayed in Figure 3. At low magnification, the TEM brightfield images clearly showed highly agglomerated particles exhibiting a nanometric dimension (Figure 3A). At an intermediate magnification, as shown in Figure 3B, the nanoparticles adopted a spherical morphology with a shape factor close to one. The particle dimensions were definitively at the nanoscale, and the particle size distribution appeared to be narrow with the main size range of approximately 25-30 nm. In the insert in Figure 3B, a selected area electron diffraction pattern (SAED) obtained on the nanoparticles confirmed the crystallinity of the powder due to the presence of rings. The HRTEM image shown in Figure 3C highlights the diffraction planes present in a particle.

The Fast Fourier transform (insert of Figure 3C) of the HRTEM image of one nanoparticle is indexed using the monoclinic structure with a space group P2 1 /a corresponding to ZrO 2 zirconia. Nitrogen adsorption analysis provided textural properties data for the as-detonated ZrO 2 sample. In particular, the specific surface area (S), taking into account the Brunauer-Emmett-Teller (BET) model, was determined to be ~32 m 2 /g. Assuming the zirconia powder was composed of monodisperse dense spherical particles and knowing its density ( = 5890 kg/m 3 ), the average particle size (∅) was estimated using the following formula (5) [START_REF] Lowell | Characterization of porous solids and powders: surface area, pore size and density[END_REF]:

(5)

The average particle size was thus calculated to be approximately 32 nm, which is in good agreement with the trend observed by the microscopic analysis.

Based on the results determined above, a discussion is attempted in order to elucidate some points regarding the present synthesis method, namely the composition of the detonation soot, the particles size as well as the structure of the detonation products.

The detonation soot's composition is dependent on the reaction mechanism. The chemical decomposition mechanism taking place during the zirconia ceramic detonation synthesis was quite complex since the initial explosive mixture consisted of several ingredients, including explosive substances, such as 2,4,6 trinitrotoluene (TNT, C 7 H 5 N 3 O 6 ) and hexanitrostilbene (HNS, C 14 H 6 N 6 O 12 ), and the inert zirconium salt (Zr(SO 4 ) 2 .4H 2 O). Estane polymer was voluntary discarded to simplify the reactive system. To investigate the detonation process, the decomposition reaction of the mixture may consider the sum of the decomposition chemical reactions of each ingredient independently. Based on the chemistry of the explosives, including the thermochemical aspect [START_REF] Akhavan | Zirconia nanopowder synthesis via detonation of trinitrotoluene P. Gibot 1* , Loic Vidal 2[END_REF], the decomposition reaction of both explosive molecules, TNT and HNS, considering the Kistiakowsky-Wilson rules (the oxygen balance of TNT and HNS are much lower; i.e., -74.00 and -67.60 wt%, respectively), could be described by reactions ( 6) and ( 7) below:

2C 7 H 5 N 3 O 6(s)  3N 2(g) + 5H 2 O (g) + 7CO (g) + 7C (s) (6)
C 14 H 6 N 6 O 12(s)  3N 2(g) + 3H 2 O (g) + 9CO (g) + 5C (s) [START_REF] Li | Effect of calcium salts on isosynthesis over ZrO 2 catalysts[END_REF] Note that negative oxygen balances will lead to the formation of a carbon solid phase since the amount of oxygen within the molecules was not enough to completely oxidize the compounds considered [START_REF] Akhavan | Zirconia nanopowder synthesis via detonation of trinitrotoluene P. Gibot 1* , Loic Vidal 2[END_REF].

The thermal decomposition reaction of the Zr-salt inert compound could be represented by equation [START_REF] Yan | Facile synthesis of titania-zirconia monodisperse microspheres and application for phosphopeptides enrichment[END_REF]:

Zr(SO 4 ) 2 . 4H 2 O (s)  ZrO 2(s) + 4SO 2(g) + 3O 2(g) + 4H 2 O (g) (8)
Finally, the global detonation reaction could be summarized by equation [START_REF] Judes | Sol-gel preparation and characterization of ceria stabilized zirconia minispheres[END_REF]

Note that only carbon and zirconia solid phases were formed during the detonation, in line with the results obtained from the different characterization techniques shown previously. These two species were found in the detonation soot and were accompanied by impurities, such as iron oxide (Fe 2 O 3 ) and silicium oxide (SiO 2 ), originating from the roost and paint covering the inner wall of the detonation chamber. Most of the impurities were discarded during the purification process.

Regarding the particles size of the as-detonated zirconia powder, the detonation approach is known to generate high pressures (GPa) and temperatures (> 1000 °C) that are suitable to the gasification of the precursors and the formation of crystalline phases during the subsequent condensation step [19, 30-33, 37, 41]. More, the extremely high cooling rate of the detonation gaseous products, estimated as high as 10 9 °C/sec. due to the rapid expansion of the chemical species, impedes the growth of particles produced and consequently leads to the formation of materials of nanoscale dimension [START_REF] Qu | Detonation synthesis of nanosized titanium oxide powders[END_REF][START_REF] Qu | Selective synthesis of TiO 2 nanopowders[END_REF][START_REF] Yan | Slurry explosive detonation synthesis and characterization of 10 nm TiO 2[END_REF][START_REF] Xie | Detonation synthesis of zinc oxide nanometer powders[END_REF][START_REF] Han | Study on the application of emulsion explosives in synthesizing nanostructured ceria[END_REF][START_REF] Vasylkiv | Nanoexplosion synthesis of multimetal oxide ceramic nanopowders[END_REF][START_REF] Xie | Growth and morphology of nanometer LiMn 2 O 4 powder[END_REF][START_REF] Wang | Nano-MnFe 2 O 4 powder synthesis by detonation of emulsion explosive[END_REF][START_REF] Li | Preparation of SrAl 2 O 4 : Eu 2+ , Dy 3+ nanometer phosphors by detonation method[END_REF][START_REF] Silva | Nanometric-sized ceramic materials, process for their synthesis and uses thereof[END_REF][START_REF] Santos | Process for nanomaterial synthesis from the preparation and detonation of an emulsion, products and emulsions thereof[END_REF][32][START_REF] Bukaemskii | Nanosize powder of zirconia. Explosive method of production and properties[END_REF][START_REF] Ornek | Observations of explosion phase boron nitride formed by emulsion detonation synthesis[END_REF][START_REF] Ornek | Formation of metastable wurtzite phase boron nitride by emulsion detonation synthesis[END_REF][START_REF] Langenderfer | Detonation synthesis of silicon carbide nanoparticles[END_REF]. The use of water to cool down the detonation products (see Experimental section) efficiently also acts to stop the growth of condensed species and thus controls the particles size in the nanoscale.

Concerning the structure of the as-detonated zirconia powder, monoclinic and tetragonal phases were prepared. The metastable cubic structure was not identified. Based on the work of Bukaemskii [START_REF] Bukaemskii | Nanosize powder of zirconia. Explosive method of production and properties[END_REF],

the cubic phase is favoured for the smallest particles due to a layer-by-layer growth model where surface layers are preferentially made of high-temperature phases. Particles smaller than ~10 nm would adopt a cubic structure moving gradually towards the tetragonal and finally the monoclinic phases as the particle growths. As, in the present work, the as-detonated ZrO 2 particles exhibit a particle size ranging from 25 to 30 nm, the cubic structure was not favoured. Now, regarding the formation of both monoclinic and tetragonal phases, it can be explain by a mitigation of the shock wave during its dissipation in all the volume of the detonation chamber [START_REF] Bukaemskii | Nanosize powder of zirconia. Explosive method of production and properties[END_REF]. It can be speculate that this mitigation leads to the coexistence of reaction zones where pressures and temperatures are different. Highest pressures and highest temperatures domains (close to the central axis of the cylindrical charge) would promote the formation of the metastable tetragonal phase at the expense of the monoclinic one which might rather originate from the zone located at the edge of the charge [START_REF] Bukaemskii | Nanosize powder of zirconia. Explosive method of production and properties[END_REF][START_REF] Ornek | Observations of explosion phase boron nitride formed by emulsion detonation synthesis[END_REF][START_REF] Ornek | Formation of metastable wurtzite phase boron nitride by emulsion detonation synthesis[END_REF]. Currently, it has not be possible to distinguish the crystalline structure of the ZrO 2 material as function of the particles size.

Conclusion

In summary, a zirconia nanopowder (ZrO 2 ) was synthesized via a detonation process using 2,4,6 trinitrotoluene (TNT) as the high energetic molecule mixed with zirconium sulfate. TNT is one of the energetic materials with the lowest sensitivity compared to other high explosives that can be used in such a process, and therefore, it is seen as a promising compound for future development. In addition, as the melting point of TNT is low (80.8 °C), it can be used in melting or casting processes to lead to more homogeneous explosive/ceramic precursor mixtures. In the present work, purified soot was composed of ZrO 2 particles exhibiting well-defined microstructural properties (spherical morphology, nanoscale particles size, narrow particles size distribution). However, the simultaneous synthesis of two zirconia polymorphs could be a disadvantage for many applications, which may be able to be overcome by adding stabilizing ingredients, such as yttrium, cerium, or calcium oxides, in the initial detonative mixture. Successful synthesis of mixed oxides has been demonstrated in the previous literature. Finally, the present investigation confirmed that explosive synthesis routes are efficient and suitable for producing nanosized ceramics powders.

  4H 2 O/Estane 5703 powder was mixed with the 2,4,6 trinitrotoluene/1,3,5-trinitro-2-[2-(2,4,6-trinitrophenyl)-ethenyl] benzene explosive substance at a 10:90 weight ratio using a Turbula mixer (model T2A -W.A. Bachofen) at 23 rpm for 20 min. The mixture was preheated at 70 °C (30 min.) and pressed at 765 bars for 10 min. Cooling was performed at 230 bars to 8 °C before releasing the mould. Cylindrical pellets (L = 50 mm, Ø = 45 mm) with a density of 1.703 ± 0.002, corresponding to 98% of the theoretical maximum density (TMD), were thus obtained. Two pellets were glued, placed in a plastic bag containing demineralized water and hung in the centre of a detonation chamber. Water was used to cool the detonation products extremely rapidly.

Figure 1 :

 1 Figure 1: XRD patterns of the zirconium (IV) oxide sample (ZrO 2 ) synthesized via detonation of a 2,4,6 trinitrotoluene/zirconium-salt (90/10 wt. %) explosive charge. Monoclinic and tetragonal structures are given for the XRD peak attribution.

Figure 2 :

 2 Figure 2: FTIR spectrum of the ZrO 2 powder synthesized via detonation of a 2,4,6 trinitrotoluene/zirconium-salt (90/10 wt. %) explosive charge.

Figure 3 :

 3 Figure 3: (A, B) TEM and (C) high resolution (HR) TEM images of the ZrO 2 powder synthesized via detonation of a 2,4,6 trinitrotoluene/zirconium-salt (90/10 wt%) explosive charge.

Acknowledgements

The French National Centre for Scientific Research (CNRS), French German Research Institute of Saint-Louis (ISL, Saint-Louis, France) and University of Strasbourg (UNISTRA, Strasbourg, France) are acknowledged by the authors for funding. The authors thank E. Fousson (ISL, Saint-Louis) and D. Spitzer (director of NS3E laboratory, Saint-Louis) for the help in the explosion experiments and the incentives to develop this research topic, respectively.

Supporting Information

As mentioned in the main text, the active domain located at 500-800 cm -1 corresponds to Zr-O bonds vibrations modes. The second ones observed at 1000-1300 cm -1 is attributed to silicon-oxygen bonds (impurity from the set-up). The IR vibration bands at 1020, 1097 and 1207 cm -1 were associated with the asymmetrical stretching vibration of the Si-O-Si functional entity (impurity inherent to the detonation chamber) following longitudinal and transverse optical components [1]. Finally, the bands observed at 1640 cm -1 and approximately 3400 cm -1 were attributed to the vibrational modes of hydroxyl groups (-OH) or water molecules, both physisorbed on the surface of the material.


text:
Pierre Gibot
email: pierre.gibot@isl.eu

Centimetric-sized chromium (III) oxide object synthesized by means of the carbon template replication

Keywords: Ceramic pieces, Cr 2 O 3, Hollow morphology, Carbon fibers, Multiscale architectures

A simple, efficient synthesis approach to designing large ceramic pieces, herein termed chromium (III) oxide (Cr 2 O 3 ) material, is provided. The process can be called the replica technique or replication. The elaboration of the material with a unique morphology is a result of a ceramic salt coating, previously dissolved in ethylene glycol as the solvent, performed on the carbon material surface selected as a template. The carbon template was carbon fiber. After a heat treatment to convert the ceramic precursor to the corresponding ceramic oxide, followed by the removal of the template, hollow ceramic oxide wires were thus obtained. The resulting material was characterized by X-ray diffraction, Raman and Fourier transform infrared spectroscopies, and scanning electron microscopy. The material exhibits a multiscale architecture, associating nanosized nodules to form micron-sized tubes, which assemble themselves into a centimetric structure.

Objects with such tailored architectures can be used in a large variety of applications, in fields as diverse as pyrotechnics, adsorption and catalysis.

Introduction

Research in designing inorganic materials with well-defined and controlled multi-scale architectures is ubiquitous in the literature [START_REF] Patzke | Oxide nanomaterials: Synthetic developments, mechanistic studies, and technological innovations[END_REF]. These shapings largely contribute to improving the existing physicochemical properties of the material, or sometimes, reveal unexpected properties of the raw material [START_REF] Garcia | Introduction to the themed issue on Advanced Complex Inorganic Nanomaterials[END_REF]. These tailoring fabricated architectures are able to revolutionize the application of numerous materials in unsuspected areas and are of interest to a large majority of materials scientists' community. Chromium (III) oxide exhibits many unique properties, making it a candidate for several domains of application such as, for instance, protective material in front of thermal and abrasive phenomena [START_REF] Berdhal | Pigments to reflect the infrared radiation from fire[END_REF][START_REF] Kitsunai | Transitions of microscopic wear mechanism for Cr2O3 ceramic coatings during repeated sliding observed in a scanning electron microscope tribosystem[END_REF]; heterogeneous catalysis, specifically for dehydrogenating alkanes such as propane ((CH3)2CH2) [START_REF] Cherian | Oxidative dehydrogenation of propane over Cr2O3/Al2O3 and Cr2O3 catalysts: effects of loading, precursor and surface area[END_REF]; sensors for different species, like alcohol (ethanol, CH3CH2OH), ketone (acetone, (CH3)2CO), alkane (toluene, C6H5(CH3)) and toxic gases like ammonia (NH3) and dichloride (Cl2) [START_REF] Zhang | Preparation of porous chromium oxide nanotubes using carbon nanotubes as templates and their application as an ethanol sensor[END_REF][START_REF] Wang | Mesoporous single-crystal Cr2O3: Synthesis, characterization, and its activity in toluene removal[END_REF][START_REF] Suryawanshi | Fe2O3-activated Cr2O3 thick films as temperature dependent gas sensors[END_REF]; and pyrotechnic applications with the elaboration of energetic composites-where mixed with a fuel-like aluminum (Al), they react according to a redox reaction, releasing a large amount of heat (10.9 kJ/cm 3 ) tied to a high adiabatic temperature (2054 °C) [START_REF] Fischer | Theoretical energy release of thermites, intermetallics, and combustible metals[END_REF]. Chromium (III) oxide's global technic capabilities can be enhanced by nanostructuring the matter, allowing the development of materials with large specific surface areas where the reactive area number is considerably improved, synonymous with optimized performance. To satisfy this permanent requirement of nanostructuring and/or developing complex architectured materials, in particular, for the Cr2O3 chromium (III) oxide, many processes were developed. For instance, Cr2O3 material can be synthesized in the form of spherical nanoparticles [START_REF] Gibot | Highly insensitive/reactive thermite prepared from Cr2O3 nanoparticles[END_REF][START_REF] Pei | Preparation of Cr2O3 nanoparticles via C2H5OH hydrothermal reduction[END_REF][START_REF] Kim | Preparation of chromia nanoparticles by precipitation-gelation reaction[END_REF][START_REF] Gibot | Ca3(PO4)2 biomaterial: a nontoxic template to prepare highly porous Cr2O3[END_REF], nanotubes [START_REF] Zhang | Preparation of porous chromium oxide nanotubes using carbon nanotubes as templates and their application as an ethanol sensor[END_REF], films [START_REF] Suryawanshi | Fe2O3-activated Cr2O3 thick films as temperature dependent gas sensors[END_REF] and porous matrices by means of soft (surfactants) and hard (mesoporous carbon and silica materials, polymeric building) templates [START_REF] Dickinson | Formation Mechanism of Porous Single-Crystal Cr2O3 and Co3O4 Templated by Mesoporous Silica[END_REF][START_REF] Tuÿsuz | A crystal structure analysis and magnetic investigation on highly ordered mesoporous Cr2O3[END_REF]. To the best of our knowledge, a nanostructured chromium (III) oxide material designed at the macroscale (centimeter) has not yet been proposed in the literature. These kind of architectures are potentially easier to handle than architectures exhibiting smaller dimensions. Herein, the work is in the same vein as works published in the literature regarding macroscale materials synthesis, whose large number spotlights a real interest of the scientific community. For example, oxide ceramic objects (TiO2, ZnO, HfO2, Fe2O3, NiO, SnO2, ZrO2 (stabilized with yttria or not) and SiO2) in the shape of macro-and micron-sized tubes and hollow fibers have already been prepared. They are candidates for applications relating to the lithium ion battery, bacterial filtration, photocatalysis, and ultravisible microlasers. Some of them are synthesized by means of direct methods, such as electrospinning [START_REF] Chen | Nanowire-in-microtube structured core/shell fibers via multifluidic coaxial electrospinning[END_REF][START_REF] Lang | Controllable synthesis of porous Fe2O3 microtube and tube-in-tube by non-coaxial electrospinning[END_REF], microwaves [START_REF] Cheng | Zinc oxide single-crystal microtubes[END_REF][START_REF] Jiang | Microwave-assisted synthesis of hexagonal structure ZnO microtubes[END_REF], sol-gel method combined with centrifugal spinning [START_REF] Bao | Si-doped mesoporous TiO2 continuous fibers: preparation by centrifugal spinning and photocatalytic properties[END_REF], extrusion [START_REF] Kroll | Highly efficient enzymefunctionalized porous zirconia microtubes for bacteria filtration[END_REF], self-rolling of gel films [START_REF] Jarvekulg | A sol-gel approach to self-formation of microtubular structures from metal alkoxide gel films[END_REF] or a vapor diffusion route [START_REF] Li | Synthesis and properties of aligned ZnO microtube arrays[END_REF]. In all cases, tubes of 0.1 to 100 μm in diameter with a length of several micrometers were designed. The second approach mentioned in the literature uses the well-known hard-template route. For that, different kinds of templates were implemented, such as biomineral ones, like cotton fibers [START_REF] Huang | Tubular structured hierarchical mesoporous titania material derived from natural cellulosic substances and application as photocatalyst for degradation of methylene blue[END_REF][START_REF] Xie | Preparation and characterization of biomorphic nickel oxide microtubes templated from cotton[END_REF][START_REF] Zhou | Cotton-templated fabrication of hierarchical SnO2 mesoporous microtubes as the anode material of lithium ion battery[END_REF], human hairs [START_REF] Liu | Facile fabrication of porous titania microtube arrays by replication of human hair[END_REF], vegetal fibers (Platanus acerifolia, Ceiba pentrada, Kapok) [START_REF] Yang | Natural fiber templated TiO2 microtubes via a double soaking solgel route and their photocatalytic performance[END_REF][START_REF] Hwang | Synthesis and characterization of hollow metal oxide micro-tubes using a biomaterial template[END_REF] and calcite [START_REF] Liu | Fabrication of size-tunable TiO2 tubes using rod-shaped calcite template[END_REF], and synthetic ones like carbon fibers [START_REF] Motojima | Preparation of TiO2 microcoils from carbon microcoil templates using a sol-gel process[END_REF][START_REF] Liu | Visible-light photoresponse in a hollow microtubenanowire structure made of carbon-doped ZnO[END_REF][START_REF] Valtchev | Preparation and characterization of hollow fibers of silicalite-l[END_REF], carbon cloth [START_REF] Indra | Boosting Electrochemical Water Oxidation with Metal Hydroxide Carbonate Templated Prussian blue Analogues[END_REF] and glass fibers [START_REF] Liu | Glass fibers templated preparation of TiO2 microtubes assembled from nano/micro hierarchical TiO2 crystals[END_REF]. These processes allow the building of materials with tube-like morphologies, exhibiting a diameter ranging from 10 to 20 μm, always with lengths of a couple of micrometers. All these investigations evidenced that the synthesis of such architectures, with well-defined textural properties (porosity, surface areas, pore size…), significantly enhanced the performance of the material for the considered application.

Herein, a templated-synthetic route for the elaboration of chromium (III) oxide (Cr2O3) microtubes of centimetric size is reported. The method, inspired of previous works of C. Vix-Guterl and the author [START_REF] Vix-Guterl | Formation of tubular silicon carbide from a carbon-silica material by using a reactive replica technique: infra-red characterization[END_REF][START_REF] Gibot | TiO2 and [TiO2/SiC] microtubes prepared from an original process[END_REF], uses commercial carbon fibers as a template and coats them with a metal oxide film in order to produce a hollow cylindrical ceramic by carbon template removal. This approach was named (negative) replica technique since it consists to reproduce or replicate an existing model, herein the carbon fibers. The interest of this synthetic method is to give the opportunity of designing large materials that are easy to handle and exhibiting interesting textural properties. The carbon template used is made of numerous uniform and linear filaments, with a 7 μm average diameter. The carbon template was desirable in this study for many reasons: it is commercially available on a large scale, it exhibits uniform shape and homogeneous size (diameter can be adjusted from 5 to 20 μm), and it shows good mechanical properties. These criteria seem to us well-suited to future implementation of a continuous process. Regarding the as-prepared material (Cr2O3) in this investigation, one of a potential applications will be its use as oxidizer, combined with aluminium metal, to formulate energetic composites (thermites) of large dimensions; an emerging research activity in pyrotechnics [START_REF] Wang | Reactive nanoenergetic graphene aerogel synthesized by one-step chemical reduction[END_REF][START_REF] Comet | Nanothermite foams: From nanopowder to object[END_REF][START_REF] Yang | Electrospun nanofiber-based thermite textiles and their reactive properties[END_REF][START_REF] Yang | Hierarchical MnO2/SnO2 heterostructures for a novel free-standing ternary thermite membrane[END_REF]. The template-synthesized oxide ceramic was submitted to a multiscale characterization.

Materials and Methods

Chromium (III) nitrate nonahydrate (Cr(NO3)3.9H2O, 99%) and a chromium (III) oxide nanopowder (Cr2O3, selected as a reference) were purchased from Sigma Aldrich. Ethylene glycol (HOCH2CH2OH, Mw = 62.07 g/mol, d 20 4 = 1.113 g/ml) was obtained from Fluka Chemie AG. Carbon fibers (average diameter = 7.2 ± 0.4 μm) were purchased from Airtech.

Synthesis of the Cr2O3 chromium (III) oxide tubes

Prior to use, the carbon fibers were cleaned of any organic substances by heating at 900 °C (15 hours) under an argon flow (100 mL/min.). Then, the fibers were immersed for one hour in a 2 mol/L chromium (III+)-based ethylene glycol solution before transfer into an alumina boat and heated under air at 500 °C (2 °C/min, 1 hour) and then at 800 °C (1 °C/min) for 2 hours. While the first heating step assures the conversion of the Cr precursor into a Cr2O3 layer at the carbon fibers' surface, the second one removes the carbon template to release the Cr2O3 tubular ceramic. The ethylene glycol solvent was desirable due to its ability to chelate metal ions [START_REF] Dong | Polyol synthesis of nanoparticles: status and options regarding metals, oxides, chalcogenides, and non-metal elements[END_REF]. The chromium (III) oxide phase yield is near 20%.

Characterization techniques

X-ray diffraction (XRD) was used to analyze the structure of the as-synthesized sample. The XRD pattern was collected by means of a D8Advance diffractometer (Cu-Kradiation,  = 1.5406 Å, Bruker), working at 40 kV-40 mA and equipped with a Lynxeye detector. The sample was scanned for 2 angle from 20° to 80°, with a 2step size and time of 0.02° and 0.1 s, respectively. Raman spectra were collected on a Renishaw InVia confocal microscope equipped with an Ar+ laser, with a wavelength of 514 nm. The laser was focused on the sample through a x 50 objective and the spectra were acquired on the 200-2000 cm -1 spectral domain, with an exposure time of 10 s and an accumulation of five scans.

The laser power (P = 25 mW) was set at 5%. Prior to analysis, a silicium reference was used to calibrate the device. Fourier transform infrared (FTIR) spectroscopy was used to characterize the surface chemistry and the metal-oxygen bonds of the sample. For that, a Tensor 27 spectrometer (Bruker) was used in the transmission mode and the spectra were recorded in the wavenumber range of 4000 to 520 cm -1 , with a resolution of 4 cm -1 and superposing 16 scans. Prior to the analysis, the sample was mixed with potassium bromide (KBr, 99.99%) and pressed to obtain disks (200 mg, 10 tons, 60 seconds). The textural properties of the sample were determined by nitrogen sorption measurements (77 K) carried out on an Accelerated Surface Area and Porosimetry 2020 analyzer (ASAP 2020, Micromeritics, USA).

The sample was outgassed at 200 °C (6 hours) under vacuum before analysis. The surface area was determined according to the Brunauer-Emmet-Teller (BET) model, applied in the 0.05-0.20 relative pressure domain. The microstructure (morphology, size) of the sample was examined using a Nova Nano-SEM 450 scanning electron microscope (FEI™, USA), operating at 10 kV. Gwyddion free software was used to determine carbon fiber and ceramic sample dimensions (diameter and thickness). The values were averaged over at least twenty measures randomly taken on at least different places of the materials. Macroscopic views of the pristine sample were obtained with a Canon EOS SDR-R camera.

Results and Discussion

Figure 1 shows the X-ray diffraction pattern of the pristine material after removal of the carbon template. The XRD pattern evidences diffraction peaks that were indexed to a rhombohedra Bravais lattice (Corundum structure), with an R-3c space group identifying the Eskolaite chromium (III) oxide phase (Cr2O3), as described in the JCPDS card No. 038-1479 (Figure 1, down). The Miller indices (hkl) characterizing the reticular planes defined by the different diffraction peaks present on the pattern were reported in Figure 1. No additional peaks coming from crystalline secondary phases (impurities) were observed on the investigated angular domain. The broad and very low-intensity peaks located around 23-28° and 40-50° were ascribed to the polymethylmethacrylate sample holder (Supporting Information 1). The crystallite size or coherence length (D) of the Cr2O3 material was determined by using the Scherrer equation, D = 0.9.  / (.cosθ) [START_REF] Patzke | Oxide nanomaterials: Synthetic developments, mechanistic studies, and technological innovations[END_REF] with  the X-ray wavelength of the radiation,  the corrected peak width at half-maximum intensity and θ the diffraction peak position for the (hkl) Miller considered. Taking into account the four most intense peaks, i.e., (012), (104), ( 110) and (116) Miller planes, an average crystallite size of 105 ± 22 nm was thus calculated for the Cr2O3 material derived from the carbon fiber. Additional spectroscopic characterizations were performed to confirm the purity of the assynthesized chromium (III) oxide structure. Figure 2 displays the Raman and the Fourier transform infrared spectra of the as-synthesized Cr2O3 material and the carbon fiber used as a template. The spectrum of the chromium-based material (Figure 2A) presents six active Raman peaks, located at 303 cm -1 , 350 cm -1 , 530 cm -1 , 551 cm -1 , 610 cm -1 , and 1400 cm -1 . Based on the literature and the commercial Cr2O3 nanopowder Raman spectrum taken as a reference (Supporting Information 2)

[43], Cr-O stretching modes in the material were considered. While the bands at 303 and 551 cm -1

were attributed to an A1g symmetry mode, the bands at 351, 397, 530 and 609 cm -1 were indexed to an Eg symmetry mode [START_REF] Mougin | High-pressure study of Cr2O3 obtained by high-temperature oxidation by X-ray diffraction and Raman spectroscopy[END_REF]. The Raman spectrum of the carbon fiber material, given in Figure 2B,

shows two peaks at 1594 cm -1 and 1353 cm -1 , typically attributed to a graphitic (G) carbon material exhibiting some disorders or defects (D) in its structure [START_REF] Ferrari | Interpretation of Raman spectra of disordered and amorphous carbon[END_REF]. The fact that these last peaks were not observed on the templated-Cr2O3 Raman spectrum (Figure 2A) suggests that the chromium oxide material is free of any carbon phase and, accordingly, that the oxidative treatment applied during the experimental procedure to remove the carbon template is efficient. The FTIR spectrum of the templated Cr2O3 material shown in Figure 2C is similar to the one expected for the Eskolaite phase (-Cr2O3), as depicted in the Supporting Information 2. Four vibration bands were observed in the 400-800 cm -1 domain, which correspond to chromium-oxygen single bonds in stretching vibrations, as noted in the literature [START_REF] Musi | Formation of chromia from amorphous chromium hydroxide[END_REF][START_REF] Serna | Infrared surface modes in corundum-type microcrystalline oxides[END_REF]. The bands at 444 cm -1 and 569 cm -1 were ascribed to Eu vibrational modes, while the band at 414 cm -1 was ascribed to A2u modes. The intense band at 653 cm -1 was attributed to a contribution of the spherical morphology of the particles [START_REF] Serna | Infrared surface modes in corundum-type microcrystalline oxides[END_REF]. At higher wavenumbers, two vibration bands were observed at 1642 cm -1 and 3000-3500 cm -1 . Basically, they are attributed, respectively, to the (δ) symmetric/antisymmetric bending mode of hydroxyl groups (-OH) and the (υ) stretching mode of water molecules physisorbed at the investigated particle's surface [48]. Other very small vibration bands, observed at wavenumbers of 1000-1300 cm -1 and 2800-2900 cm -1 , were assigned to functional groups (C-O-H, C-O, O-H, -CH2) and may originate from the decomposition products of the ethylene glycol solvent. According to the resolution limit of the analysis technique, no further vibration bands were observed, confirming that the astemplated Cr2O3 material was free of any chromium-based amorphous impurities. Photographs and a scanning electron microcopy analysis were achieved to collect information relating to the macro-and microstructure of the carbon template and the as-templated Cr2O3 material. The representative pictures of both samples are depicted in Figure 3A and 3B, respectively. Regarding characteristic of the Cr2O3 sample, a macroscopic voluminous tangle of intense green fibers, with large air gaps, was obtained as displayed in Figure 3B1. This result clearly highlights the successful replication of the carbon fibers (Figure 3A1) into a ceramic material, as the morphology of the carbon fibers was preserved. As observed, it is possible to design a flexible complex ceramic structure of large dimensions with chromium (III) oxide filaments, which may exhibit a length of several centimeters coupled to a strong curvature. However, from a magnified image of this photograph (zoom x 5), displayed in Figure 3B2, the Cr2O3 filaments seem to be assembled between themselves and do not any evidence of being isolated from each other, unlike the carbon filaments observed in Figure 3A2. From a scanning electron microscopy analysis, the Cr2O3 fibers are revealed in Figure 3B3.

The filaments are uniform in the long axial direction, with a length of several ten of micrometers and an average diameter in the same order of magnitude as the carbon fibers selected as the templating agent (Figure 3A3). A cross-section of the fibers (Figure 3B4) shows that chromia fibers are, in fact, hollow, with an inner diameter of the tubes ca. 6.6 ± 0.3 μm, validating the negative replica concept expected from this approach. The fact that this last value is slightly below the average diameter of the carbon fibers may be explained by a structural shrinkage coming from the crystallization of the Cr2O3 material [START_REF] Vix-Guterl | Formation of tubular silicon carbide from a carbon-silica material by using a reactive replica technique: infra-red characterization[END_REF][START_REF] Gibot | TiO2 and [TiO2/SiC] microtubes prepared from an original process[END_REF]. By comparison, a representative cross-sectional image of the carbon fibers shows solid fibers (Figure 3A4). In addition, from careful observation of Figures 3A4 and 3B4, the inner surface texture of the Cr2O3 tubes matches well with the outer surface aspect of the carbon fibers, highlighting the good adhesion of the chromium-based coating on the carbon fibers' surface during the dip-coating step. Another microscopic view showing hollow fibers is reported in Supporting Information 3. Figure 4 shows other views of the Cr2O3 hollow fibers where textural aspects are highlighted. For example, Figure 4A shows the thickness of the Cr2O3 walls which was determined to be around 0.160 ± 0.04 μm. Higher-magnification pictures show that the thickness of the Cr2O3 walls was determined to be around 0.160 ± 0.04 μm (Figure 4A). By scrutinizing the wall of the tubes in the long axial direction, an assembly of submicron-sized particles is observed (Figure 4B andC). The particles seem to adopt a sphere-like morphology that is in agreement with the results of the FTIR analysis, exhibiting a strong vibration band with a shoulder at 653 cm -1 . The average particle size is approximately 0.15 μm and the particles seem to be sintered between themselves with a minor interparticular porosity, as seen in Figure 4B. The specific surface area of the corresponding hollow Cr2O3 fibers, determined by means of nitrogen physisorption and taking into consideration the BET model, is 7 m 2 /g. This value is higher than the specific surface area of the pristine carbon fibers, which was measured below 1 m 2 /g. Finally, since SEM images and results from BET evidence larger primary particles than the coherence length determined by means of XRD analysis, defects and polycrystallinity inside the Cr2O3 particles are expected.

Conclusions

Hollow Cr2O3 chromium (III) oxide ceramic fibers were successfully manufactured from carbon fibers as a template agent in a replication method. The ceramic product, a negative replica of the native carbon moulds, shows a centimetric complex architecture made of an assembly of tubes of several thousand micrometrics in length, an inner average diameter of around 6 μm and a wall thickness of approximately 0.1-0.2 μm. The Cr2O3 tubes comprise the assembly of submicron-sized nodules, allowing the development of interesting textural properties such as a specific surface area of 7 m 2 /g. The replica process when implementing such a hard template seems to be efficient and reliable to design ceramic materials (oxides, carbides, nitrides, etc.) exhibiting a specific and controlled morphology at multiscales. The intrinsic properties of the carbon fiber template, as mentioned below, consolidates the idea of a scalable process. Such architectures could potentially be promising in a large variety of applications, for diverse fields such pyrotechnics, adsorption and catalysis. For example, energetic macroscopic objects (thermite) could be performed by associating the oxide ceramic piece with aluminium coating deposited by means of gaseous route.

Figure 1 .

 1 Figure 1. XRD pattern of the as-templated Cr2O3 material (top pattern). For comparative purposes, the diffraction profile of the Cr2O3 Eskolaite phase is shown (bottom pattern) (JCPDS No. 038-1479).

Figure 2 .

 2 Figure 2. (A, C) Raman and Fourier transform infrared (FTIR) spectra, respectively, of the astemplated Cr2O3 material and (B) the Raman spectrum of the carbon fiber used as template. In the FTIR spectrum of Cr2O3, all the vibration bands related to the chromium-oxygen bonds are noted by means of vertical lines, while a star symbol is used to identify solvent traces and/or moisture adsorbed at the particles surface.

Figure 3 .

 3 Figure 3. Photographs (1-2) and SEM views (3-4) of the (A) carbon fibers template and (B) as-templated Cr2O3 material.

Figure 4 .

 4 Figure 4. SEM views of the Cr2O3 hollow fiber material; (A) thickness, (B,C) axial direction views.

Supplementary Materials:

  The following are available online at www.mdpi.com/xxx/s1, Figure S1: XRD pattern of the polymethylmethacrylate sample holder, Figure S2: Raman and FTIR spectra of a commercial Cr2O3 nanopowder (99.9%), Figure S3: SEM pictures of Cr2O3 hollow fibers synthesized by means of a replication method. Micron-sized carbon fibers were selected as a templating agent. Author Contributions: P.G. carried out this research, performed the experimental work, and wrote the manuscript.

Acknowledgments:

The author gratefully acknowledges V. Pichot, F. Schnell (NS3E, Saint-Louis, France) and Y. Boehrer (ISL, Saint-Louis, France) for the assistance in Raman spectroscopy analysis, the scanning electron microscopy analysis and the photographs, respectively.

Funding:

The author gratefully acknowledges the French National Centre for Scientific Research (CNRS), French German Research Institute of Saint-Louis (ISL, Saint-Louis, France) and University of Strasbourg (UNISTRA, Strasbourg, France) for funding.

Conflicts of Interest:

The author declare no conflict of interest.