T Franck

E Njamkepo

J Rauzier

M.-L Quilici

Keywords: SARS-CoV-2 Phylogenetic Analysis, Lazio Region, Italy, February-

3 These authors were joint principal investigators.

Four cholera outbreaks were reported in the Central African Republic during 1997-2016. We show that the outbreak isolates were Vibrio cholerae O1 serotype Inaba from 3 seventh pandemic El Tor sublineages originating from West Africa (sublineages T7 and T9) or the African Great Lakes Region (T10).

C holera is a life-threatening diarrheal disease caused by the bacterium Vibrio cholerae, which produces cholera toxin. The seventh cholera pandemic, caused by V. cholerae O1 biotype El Tor, began in Indonesia in 1961 and reached Africa in 1970 [START_REF] Barua | Cholera[END_REF]. Fifty years later, >100,000 cases of cholera are reported annually in sub-Saharan Africa [START_REF] Lessler | Mapping the burden of cholera in sub-Saharan Africa and implications for control: an analysis of data across geographical scales[END_REF].

The Central African Republic (CAR) is a landlocked and resource-limited country in central Africa; it was ranked 188/189 on the United Nations Human Development Index in 2018 (http://hdr.undp.org). CAR is relatively large but has a low population density; 2019 data estimate ≈4.75 million inhabitants, or 7.75 persons/km 2 (Macrotrends LLC, https://www. macrotrends.net). CAR largely has been spared by the cholera epidemic; only 4 outbreaks had been reported by 2020 (Table 1; Figure 1). The first 2 cholera outbreaks occurred during the same month in 1997 [START_REF] Germani | Emergence of cholera in the Central African Republic[END_REF]. In the first, 443 cases and 88 deaths were reported in southern CAR, along the Oubangui River, close to the border with the Democratic Republic of the Congo (DRC). In the second, 113 cases and 19 deaths were reported in northern CAR, close to the borders with Cameroon and Chad, after which cholera cases continued to be detected in southern CAR, along the Oubangui River. According to reports from the International Federation of Red Cross and Red Crescent Societies, 172 cholera cases and 16 deaths were reported in the region in 2011 (https://reliefweb.int/ sites/reliefweb.int/files/resources/MDRCF009finrep.pdf) and 265 cases and 20 deaths were reported in 2016 (https://reliefweb.int/report/central-africanrepublic/central-africa-republic-cholera-epidemicoutbreak-dref-operation).

The Institut Pasteur de Bangui in CAR performed microbial analyses to confirm the causal agent of these outbreaks and identified 30 V. cholerae O1 serotype Inaba isolates collected during 1997-2016 (Appendix 1 Table 1, https://wwwnc. cdc.gov/EID/article/27/1/20-0375-App1.x1sx). We used whole-genome sequencing to fully characterize all 30 V. cholerae O1 isolates in terms of virulence and antimicrobial resistance determinants. We also placed these genomes within a broader phylogenetic context to elucidate their origins and evolutionary history.

The Study

The 30 V. cholerae O1 isolates were received at the Institut Pasteur, Paris, France. We performed antimicrobial susceptibility testing, whole-genome sequencing, comparative genomics, and phylogenetic analyses by using methods previously described [START_REF] Weill | Genomic history of the seventh pandemic of cholera in Africa[END_REF][START_REF] Abubakar | Cholera epidemic in south Sudan and Uganda and need for international collaboration in cholera control[END_REF][START_REF] Weill | Genomic insights into the 2016-2017 cholera epidemic in Yemen[END_REF][START_REF] Irenge | Genomic analysis of pathogenic isolates of Vibrio cholerae from eastern Democratic Republic of the Congo (2014-2017)[END_REF][START_REF] Bankevich | SPAdes: a new genome assembly algorithm and its applications to single-cell sequencing[END_REF][START_REF] Larsen | Multilocus sequence typing of totalgenome-sequenced bacteria[END_REF][START_REF] Croucher | Rapid phylogenetic analysis of large samples of recombinant bacterial whole genome sequences using Gubbins[END_REF][START_REF] Stamatakis | RAxML-VI-HPC: maximum likelihoodbased phylogenetic analyses with thousands of taxa and mixed models[END_REF] article/27/1/20-0375-App2.pdf). We then contextualized these 30 V. cholerae O1 isolates within a global collection of 1,185 seventh pandemic V. cholerae El Tor (7PET) genomic sequences and constructed a maximum-likelihood phylogeny of 1,215 genomes by using 9,964 single-nucleotide variants (SNVs) evenly distributed over the nonrepetitive, nonrecombinant core genome (Figure 2, panel A). Our phylogenomic analysis (Appendix 1 Tables 234) showed that all CAR isolates belonged to the 7PET lineage [START_REF] Domman | Integrated view of Vibrio cholerae in the Americas[END_REF][START_REF] Mutreja | Evidence for several waves of global transmission in the seventh cholera pandemic[END_REF]. Previous genomic studies described 12 introductions of the 7PET lineage from Southern Asia into Africa during 1970-2016 [START_REF] Weill | Genomic history of the seventh pandemic of cholera in Africa[END_REF][START_REF] Weill | Genomic insights into the 2016-2017 cholera epidemic in Yemen[END_REF]. The introduced sublineages were called T1 and T3-T13. The 2 cholera outbreaks in CAR in 1997 were caused by sublineage T7, which had been introduced into West Africa during the early 1980s, and T9, which was introduced in the late 1980s, according to Weill et al. [START_REF] Weill | Genomic history of the seventh pandemic of cholera in Africa[END_REF]. T9 isolates were identified in neighboring countries such as Chad and Cameroon (particularly northern Cameroon) before they were detected in northwest CAR, but T7 isolates were identified in Gabon and the western part of DRC, along the Congo River in Kinshasa, before being identified in CAR along the Oubangui River, a tributary of the Congo River (Figure 2, panel B; Appendix 2 Figures 1,2).

The 2011 and 2016 outbreaks were caused by closely related bacterial populations from the same sublineage, T10, introduced into East Africa during the 1990s and later detected in the African Great Lakes Region (AGLR) (3) (Figure 2, panel B; Appen-dix 2 Figure 3). The prevalent T10 sublineage has several clades, and the 2011 and 2016 CAR isolates are characterized by multilocus sequence type (ST) 515, a single-locus variant of ST69; ST69 is the predominant ST among 7PET isolates. In addition, these isolates have an alteration to the wbeT gene, a 4-nucleotide deletion called B01 (Tables 1,2), that underly the Inaba serotype. Phylogenetic data showed that the 7PET strains causing the 2011 and 2016 cholera outbreaks in CAR spread from AGLR to the western part of the DRC and CAR (Figure 2, panel B; Appendix 2 Figure 3). These genomic data are consistent with results from a recent epidemiologic study showing the spatiotemporal distribution of cholera cases during the 2011-2012 and 2015-2017 outbreaks in DRC, suggesting a spread from cholera hotspots in AGLR to major cities in the upstream section of the Congo River, followed by a downstream spread toward the densely populated CAR capital, Kinshasa, and then to the mouth of the Congo River, which opens into the Gulf of Guinea [START_REF] Ingelbeen | Recurrent cholera outbreaks, Democratic Republic of the Congo, 2008-2017[END_REF]. Except during the outbreak in northwest CAR in 1997, all cholera cases were reported along or close to the Oubangui River, suggesting that 7PET strains probably moved from area to area along the river and with the displacement of human populations. The risk factors in these remote areas are unknown, but the prevailing conditions, such as poor hygiene and sanitary conditions, overcrowding, lack of latrines, and drinking water from the Oubangui River, likely would increase the risk for transmission via the fecal-oral route, as evidenced by the high attack rates observed at several sites when the 2011 outbreak began [START_REF] Penguele | Cholera outbreak investigation in the Central African Republic[END_REF]. Nevertheless, since the declaration of the first case in 1997, the small number (<1,000) of cholera cases in CAR contrasts with the much larger numbers in central Africa [START_REF] Lessler | Mapping the burden of cholera in sub-Saharan Africa and implications for control: an analysis of data across geographical scales[END_REF]. The low population density of CAR, its poor transport infrastructure, and poor trading links are probably key factors limiting disease spread [START_REF] Penguele | Cholera outbreak investigation in the Central African Republic[END_REF]. Phylogenetic analyses showed no other isolates from Africa were derived from CAR isolates in the aftermath of the 4 outbreaks, which also suggests that the transmission of cholera is impeded in this country. Of note, all 4 outbreaks were caused by serotype Inaba 7PET strains. This serotype has a nonmethylated form of lipopolysaccharide caused by an alteration to the wbeT gene (3) (Table 2). The implication of this serotype in all 4 outbreaks suggests that these 7PET sublineages circulated regionally for some time, long enough to acquire this alteration to the wbeT gene, before reaching CAR (Appendix 2 Figures 123).

All CAR isolates in this study displayed resistance to polymyxin B, consistent with the susceptibility pattern reported for the El Tor biotype until recently [START_REF] Weill | Genomic insights into the 2016-2017 cholera epidemic in Yemen[END_REF]. All but 1 of the isolates collected along the Oubangui River in 1997 were susceptible to all other antimicrobial drugs tested; the outlying isolate contained the extended-spectrum β-lactamase bla SHV-2a gene on an IncA/C2 plasmid (Table 1; Appendix 1 Table 3). No susceptible isolates have been collected in CAR since. All the other isolates display mutations of the VC_0715 and VC_A0637 genes, conferring nitrofuran resistance, and carry an SXT/R391 genomic element called ICEVchInd5, encoding resistance to streptomycin (strAB), sulfonamides (sul2), trimethoprim, the O/129 vibriostatic agent (dfrA1), and trimethoprimsulfamethoxazole (sul2 and dfrA1). The 2011 and 2016 CAR isolates also recently acquired gyrA mutations (Table 1), resulting in resistance to nalidixic acid (Appendix 1 Table 3).

Conclusions

Strains from 3 7PET sublineages caused 4 cholera outbreaks identified in CAR during 1997-2016. The southern and southeastern parts of CAR are higher risk areas for cholera outbreaks, particularly when cases are reported in the western part of DRC. These findings highlight the need for an effective surveillance system, and for coordinated communication actions on cholera that target healthcare professionals and the populations living along the Oubangui River, to prevent and control cholera outbreaks in CAR.

About the Author

Dr. Breurec is a medical microbiologist on the faculty of medicine and the University Medical Center of Pointe-à-Pitre, Guadeloupe, and at the Institut Pasteur International Network. His research interests include genomic epidemiology, resistance, and the virulence determinants of bacterial pathogens.

Figure 1 .

 1 Figure 1. Geographic location of cholera cases during the 4 outbreaks reported in the Central African Republic, 1997-2016. Inset shows the location of Central African Republic in the continent of Africa. Numbers correspond to outbreaks during 1) June-October 1997; 2) June-August 1997; 3) September-October 2011; and 4) July-December 2016. Arrows show movement of outbreaks corresponding to colors for each outbreak.

264

  Emerging Infectious Diseases • www.cdc.gov/eid • Vol. 27, No. 1, January 2021

Figure 2 .

 2 Figure 2. Phylogenomics of the Vibrio cholerae O1 El Tor isolates from CAR, 1997-2016. A) Maximum-likelihood phylogeny for 1,215 7PET genomic sequences. A6 was used as an outgroup. The last 9 sublineages introduced into Africa (T5-T13) are indicated. On inner ring, color scale denotes geographic locations of the V. cholerae isolates. On outer ring, brown denotes isolates from CAR. Tree branches containing isolates from CAR are shown in red. Scale bar indicates substitutions per variable site. B) Locations on the African continent in which T7, T9, and T10 V. cholerae O1 serotype Inaba isolates were detected before their identification in CAR. CAR, Central African Republic.

  This work was supported by the Institut Pasteur, Santé Publique France and the French Government's Investissement d'Avenir program, Laboratoire d'Excellence Integrative Biology of Emerging Infectious Diseases (grant no. ANR-10-LABX-62-IBEID).

Seventh Pandemic Vibrio cholerae O1 Sublineages, Central African Republic

  (Appendix 2, https://wwwnc.cdc.gov/EID/

	Sebastien Breurec, 1 Thierry Franck, 1,2 Elisabeth Njamkepo, Jean-Robert Mbecko, Jean Rauzier, Hugues
		Sanke-Waïgana, Guyguy Kamwiziku, Renaud Piarroux, Marie-Laure Quilici, 3 François-Xavier Weill 3
	262	Emerging Infectious Diseases • www.cdc.gov/eid • Vol. 27, No. 1, January 2021

Table 1 .

 1 Characteristics of the Vibrio cholerae O1 isolates associated with outbreaks of cholera, Central African Republic* †Data from the International Federation of Red Cross and Red Crescent Societies 2011 (https://reliefweb.int/sites/reliefweb.int/files/resources/MDRCF009finrep.pdf) and 2016 (https://reliefweb.int/report/central-african-republic/central-africa-

		Outbreak no. 1,	Outbreak no. 2,	Outbreak no. 3,	Outbreak no. 4,
	Characteristics	1997 Jun-Oct	1997 Jun-Aug	2011 Sept-Oct	2016 Jul-Dec
	No. deaths/no. cases †	88/443	19/113	16/172	20/265
	No. isolates	9	6	7	8
	7PET sublineage	T7	T9	T10	T10
	Sequence type	69	69	515	515
	ctxB	B3	B1	B1	B1
	wbeT ‡	A03	C19	B01	B01
	AMR phenotypes (no. isolates) §	R1 (8), R2 (1)	R3 (6)	R3 (7)	R3 (8)
	AMR determinants
	Plasmid	IncA/C ¶	NT	NT	NT
	VC_0715	WT	R169C	R169C	R169C
	VC_A0637	WT	Q5Stop	Q5Stop	Q5Stop
	SXT/R391 element	NT	ICEVchInd5	ICEVchInd5	ICEVchInd5
	gyrA	WT	WT	S83I	S83I

Emerging Infectious Diseases • www.cdc.gov/eid • Vol. 27, No. 1, January 2021 263 *7PET, seventh pandemic Vibrio cholerae O1 El Tor; AMR, antimicrobial resistance; NT, not detected; WT, wild type. republic-cholera-epidemic-outbreak-dref-operation). ‡Nomenclature according to Weill et al. (3) (

Table 2). §R1, resistance to polymyxin B and colistin; R2, resistance to polymyxin B, colistin, ampicillin, streptomycin, sulfonamides, vibriostatic agent, trimethoprim/sulfamethoxazole, chloramphenicol; R3, resistance to polymyxin B, colistin, streptomycin, sulfonamides, vibriostatic agent, trimethoprim/sulfamethoxazole, furazolidone, and resistance or intermediate resistance to chloramphenicol. ¶Isolate (CNRVC970079) with the R2 AMR type.

Table 2 .

 2 Alterations of the wbeT (formerly rfbT) gene in Vibrio cholerae El Tor isolates from the Central African Republic*

	Alteration no.	Alteration type	Genetic alteration	Protein consequence †	7PET sublineage
	A03	Premature stop codon	G133T	G45-to-STOP	T7
	B01	Premature stop codon	Deletion TGTAC (nt 24-28)	Frameshift after N7; then STOP	T10
	C19	Amino acid substitution	G674A	C225Y	T9

*7PET, seventh pandemic Vibrio cholerae O1 El Tor. †The codon number and single-letter amino acid abbreviation are shown (STOP corresponds to a stop codon).

®

Parasitic Infections

To revisit the August 2020 issue, go to: https://wwwnc.cdc.gov/eid/articles/issue/26/8/table-of-contents


E Gastaldi

F Touchaleaume

G Cesar

C Jourdan

P Coll

C Rodrigues

Agronomic performances of biodegradable films as an alternative to polyethylene mulches in vineyards

Keywords: Vineyard, mulch, biodegradable films, vegetative growth, fruiting yield, rootstock, soil properties

The use of polyethylene (PE) mulch at planting ensures the successful and homogeneous establishment of vineyards and also leads them to produce one year earlier. However, as they are rarely removed from the vineyards, the aesthetic and environmental impact of those films is devastating. In addition, the long-term stability of conventional plastics appears disproportionate with respect to the material lifespan required in mulching practices. To face up this issue biodegradable films already used in horticulture would be an environmentally friendly alternative provided that they meet the requirements of the perennial crop and associated farming practices i.e. remain intact in the field long enough to provide its expected positive effects on the crop and then, biodegrade on the soil surface without additional ploughing. The performance and environmental impact of a commercially available biodegradable mulch films displaying different thicknesses were studied through two consecutive fullscale field experiments conducted in the south of France. The impact of mulching on soil properties and agronomic performance was measured during the three first crop cycles by monitoring vine growth and fruiting yield concomitantly with film ageing and fate in field. In parallel, respirometric tests were undertaken to evaluate the impact of field ageing on the biodegradation rate. As a result, the use of biodegradable films for vine mulching provide similar agronomical performances as PE; grapevine growth and harvest yield were found to be significantly higher for the mulched modalities than for the bare-soil. This positive mulch-induced effect appeared from the first growing season and was maintained three years later even if the biodegradable mulch started deteriorating 5 months after planting. Thus, in spite of the early loss of integrity of the studied biodegradable mulch films, a significant positive effect on the vine growth, the rootstock development, the earliness of the first harvest and the yield (and quality) of the two first harvests have been evidenced. This leads to reconsider the required life expectancy of mulch film in the case of vineyard and constitutes encouraging results towards the possibility of substituting PE films with biodegradable ones.

INTRODUCTION

The successful establishment of a new vineyard strongly depends on the first growing years, grapevines generally starting producing during the third season. The use of polyethylene (PE) mulch films in the vine row is sometimes used in newly planted vineyards, not only for a herbicidal effect and to ensure their successful and homogeneous establishment, but also to deliver earlier production. Indeed, mulched vines may produce a crop one year earlier than unmulched vines [START_REF] Godden | Comparison between grapevine response to polyethylene mulch and herbicide control of weeds[END_REF][START_REF] Moore | Plastic mulch aids growth of young grape vines and cuttings[END_REF][START_REF] Sasser | Compost mulch effects on soil fertility, nutritional status and performance of grapevine[END_REF][START_REF] Van Der Westhuizen | The effect of Black Plastic Mulch on Growth, Production and Root Development of Chenin blanc Vines under dryland conditions[END_REF]. Such films lead to a better preservation of soil moisture, more uniform soil temperatures, less leaching of fertilizers, lower soil compaction, weed control, an increased growth and a higher survival rate [START_REF] Van Der Westhuizen | The effect of Black Plastic Mulch on Growth, Production and Root Development of Chenin blanc Vines under dryland conditions[END_REF]. Moreover, an increase in vegetative growth and yield were observed up to three years after planting when the film was removed after the first season [START_REF] Moore | Plastic mulch aids growth of young grape vines and cuttings[END_REF], or up to five years after planting when it was maintained 18 months [START_REF] Van Der Westhuizen | The effect of Black Plastic Mulch on Growth, Production and Root Development of Chenin blanc Vines under dryland conditions[END_REF].

However, despite the described benefits from the use of mulch films, they are not widely used by most European winegrowers. Among the cited reasons, a clear disadvantage of PE films is that they are not easily removed from vineyards, leading to significant waste disposal issues often resulting in an environmental pollution. Indeed, there is a discrepancy between the long-term stability of PE and the material lifespan required in mulch applications (Martin-Closas and Pelacho, 2011). In the case of long-term perennial crops such as grapevines, the mulch lifetime needs to be further studied, considering that the reported effects mainly focus on the early crop cycles. In this context, the performances of biodegradable mulch films, which constitute an environmentally friendly alternative to the currently used polyethylene film [START_REF] Briassoulis | Mechanical behaviour of biodegradable agricultural films under real field conditions[END_REF], have to be assessed in vineyard field experiments. [START_REF] Minuto | Weed Control with Biodegradable Mulch in Vegetable Crops[END_REF] reported a preliminary study on the use of Mater-Bibased mulch films in vineyards. According to the authors, film thickness and formulation are key parameters for the efficiency of the film. They report a reduction of weeds growth over a 12 month period improved vine establishment with mulching vineyards compared to an unmulched treatment. In a vine nursery situation, a comparison between mechanical tillage, PE and Mater-Bi mulch films was conducted by [START_REF] Tarricone | Amendolagine AM Evaluation of the effects of biodegradable mulching on grapevine nursery soil management[END_REF] who concluded that mulching should not be recommended due to the impact of water status on grafting. However, the Mater-Bi mulch film proved to have positive effects on the vine growth (both on aerial parts and roots).

Despite the low number of available studies on the use of biodegradable mulch films in vineyards, there is a strong need for further research activities on the topic. The present work is part of the AGROBIOFILM European project, which aimed at developing biodegradable mulch films fitting specific regions and crops requirements. The impact of mulching on soil properties and agronomic performance was measured during the three first crop cycles by monitoring vine growth and fruiting yield concomitantly with film ageing and fate in field. In parallel, respirometric tests were undertaken to evaluate the impact of field ageing on the biodegradation rate.

MATERIALS AND METHODS

Experimental design.

Two full-scale field experiments were set up with an interval of one year both in a vineyard located in Southern France near Carcassonne (43°12 N-02°36 E). The first experimental field test was initiated in 2010 (April the 1 st ) and was conducted over 3 years with Chardonnay cultivars grafted on SO4 rootstocks. The second experimental field test consisted of Cot cultivars (red grapevines) grafted on SO4 rootstocks was launched in 2011 (February the 25 th ) and was conducted over two years. In both vineyards, the inter-vines and inter-rows distances were of 0.8 and 3 m, respectively. Vines were mechanically mulched 10 days after planting and each treatment tested was applied through a complete randomized block design with three replicates with five guard vines in the row between each modality to prevent potential border effects. Each treatment consisted of an average of 115 vines, grown in 5 consecutive rows, with guard rows and vines between each treatment for the first experiment, and an average of 165 vines grown in 8 consecutive rows, with 5 guard vines between each treatment for the second experiment. The soil contained 19% clay, 42% silt and 39% sand and had a pH of 8.46. The biodegradable mulch film tested (BF) was made of Mater-Bi® CF04P grade (Novamont, Italy) with different thicknesses (BioBag, Askim, Norway). For comparison, PE film (Agripolyane, Saint Chamond, France) and bare soil (BS) were used. All the mulch films were 110 cm wide, of which 80 cm was exposed at the soil surface. In the case of BF, three thicknesses were tested: 40 µm, 25 and 70 µm whereas for PE only film of 40 µm thick were tested.

Viticultural practices and parameters.

Vineyard management practices were typical of those used in the region. In the two experimental field tests, vines were pruned from the end of the first season with a single cane as to a Guyot system. In both cases, only vigorous vines were pruned in this manner whereas for the weakest ones, the main cane was shortened to two buds. In order to quantify the canopy development, the pruning woods of a representative number of vines (30 per treatment) were yearly collected and weighed. To quantify the fruit production the vines (30 per treatment) were manually harvested 17.5 and 29 months after planting. Fruit quality parameters were evaluated by measuring the total solids content of the grapes using a Convex refractometer (Medline Scientific Limited, Oxfordshire, UK) and pH using a CONSORT C833 pH meter (Consort. Turnhout, BE). The development of the vine rootstock was monitored for the 25 months-old vines of the first field experiment. For this purpose, the roots of three representative vines per treatment were excavated and manually collected in the first 60cm-thick soil horizon from a quarter of the Voronoi polygon (Figure 1), which is the elementary space defined by the half distances between the sampled vine and its neighbours on the row or inter-row [START_REF] Snowdon | Protocol for sampling tree and stand biomass[END_REF]. The roots were washed and their morphological parameters (length and diameter) were characterized using WinRhizo software (Regent Inc. Canada).

Soil analysis.

Soil samples were taken from the superficial layer of the soil (0-20cm) and analysed for their physical, chemical and biological properties. They were collected at planting date (t0) and 24 months later (t24 months). The apparent density was evaluated according to the cylinder method (ISO 11272,1998). Organic carbon and nitrogen contents were evaluated according to NF ISO 13878 and NF ISO 13878. Organic matter content was deduced from the organic carbon, using a ratio of 1.72. The microbial biomass was evaluated using the Fumigation-extraction method [START_REF] Wu | Measurement of soil microbial biomass C by fumigation extraction -an automated procedure[END_REF]). The nematofauna was analysed according to [START_REF] Oostenbrink | Estimating nematode populations by some selected methods[END_REF] and NF ISO 23611-4 as reported in [START_REF] Villenave | Influence of long-term organic and mineral fertilization on soil nematofauna when growing Sorghum bicolor in Burkina Faso[END_REF]. Nematode taxa were then assigned to trophic groups [START_REF] Yeates | Feeding habits in soil nematode families and genera -an outline for soil ecologists[END_REF].

Film ageing

Tensile properties of mulch films were monitored on a tensile machine (ZWICK Roell type BZ 2.5/TN1S) using a 5kN force sensor. Samples (13x70mm) were razor-blade cut along the parallel direction. Tests were conducted according to EN ISO 527-3 standard at 500 mm/min crosshead speed.

Biodegradability assessment

Respirometric tests in standardized aerobic conditions were conducted firstly at 28°C to evaluate the biodegradability of initial BF film, and then, in compost medium at 58 °C to evaluate the rate of biodegradation of mulch films before and after 18 months spent in field. The methods were adapted from the US standard ASTM D5988-96 to determine the aerobic biodegradation of plastic materials in Soil and in compost medium. The released CO2 being proportional to the percentage of biodegraded substrate, CO2 evolution quantifies the ultimate degradation (i.e. mineralization) during which a substance is broken down to its final products. Beforehand, materials were cut into 1-2 mm bits and their carbon contents were measured with an elementary analyser (ThermoQuest NA 2500) in order to precisely set the plastic dose to either 2 mg of carbon per g of soil and 16 mg of carbon per g of compost. Biodegradation tests were carried out in a soil collected at the location of the field experiments and in real and mature compost. Distilled water was initially added to the solid media and then regularly added all along the experiment to maintain the moisture content of the medium around 80-100%.

Data analysis.

Data were subjected to analysis of variance using Statgraphics Plus 5.0 software. If not mentioned, a confident level of 95% was considered. In all the tables, mean values followed by mean standard error are given.

RESULTS AND DISCUSSION

Overview

The performance and environmental impact of a commercially available biodegradable mulch films displaying different thicknesses have been assessed through two full-scale field experiments set up with an interval of one year in the south of France. The impact of mulching on soil properties and agronomic performance was measured during the three first crop cycles by monitoring vine growth, roots development and fruiting yield concomitantly with film ageing and fate in the field. In parallel, respirometric tests were undertaken to evaluate the impact of field ageing on the biodegradation rate.

Research Data Ageing of mulch film under field conditions

The biodegradable mulch film (BF) started deteriorating about 5 months after the films installation with the apparition of small holes and perforations. This early degradation of the biodegradable films occurred whatever the thickness of the film and was mainly attributed to a decrease in elongation at break (εB) as evidenced by the evolution of the mechanical properties of the films 4 months after planting (Table 1). This embrittlement of the biodegradable material (-90% for εB) was much marked than these of the PE film (-25% for εB). Indeed, as already reported by [START_REF] Touchaleaume | How performance and fate of biodegradable mulch films are impacted by field ageing[END_REF], UV-light can be considered as the major ageing factor responsible for the mulch film degradation on top of the soil since in two field experiments, successively launched at one-year interval, the first signs of deterioration of the biodegradable mulch films occurred for approximately the same value of cumulated solar radiations. As a result of the field exposure (rain, animals, tractor, etc.), one year after mulch installation, when comparing the aspect of the films in the raw, it could be noticed that all the films appeared very degraded (Figure 2). BF degradation was such that the film did not cover totally the soil after 12 months, whereas the PE film remained still intact, the BF*40 being the most damaged and the BF70 the less damaged (Figure 2). For a same thickness, the biodegradable film that contains 10% scraps (BF*40*) would be more prone to degrade on the top of the soil than the one which does not contain any (BF40). After apparition of the first signs of film deterioration, it seemed that the rate of damaging would be higher in the case of the BF*40 than for the BF40. In contrast, the introduction of 10% recycled material in the formulation had no effect on its degradation once buried in soil. The use of thinner film (25 instead of 40 µm) would have no significant influence on its degradation on the top of the soil. Paradoxically, the use of a thicker film (70 µm) would delay the film deterioration on the top of the soil whereas it would not prevent it to degrade once buried in soil. Nevertheless, this fast damaging of the films (that is expected to increase the surface area in contact with soil) combined with the fact that they are all partially recovered by the soil might be seen as positive when considering their further end of life and expected biodegradation in vineyard.

Effect of mulching on physical, chemical and biological soil properties

Soil analyses were undertaken to describe the impact of mulching on soil physical, chemical and biological characteristics (Tables 2 and3). Actually, the effects of soil management on the soil characteristics are expected to partially explain the mulch-induced effect on viticultural parameters. However, because the chemical properties of a soil are characterized by a great inertia, they are not expected to significantly change in a short time scale. Moreover, the different additions of foliar fertilizers made in the field are supposed to render difficult the interpretation of the results. As a consequence, only the most relevant mulching-induced changes observed two years after vine planting are presented (Tables 2 and3). The initial chemical properties of the soil are representative of those of a vineyard soil of this region. No significant changes in bulk density were observed suggesting that the texture of the soil was not influenced by the presence of mulch film during the two first years following vine planting. In contrast, the soil organic matter content and its C/N ratio slightly increased with time, especially for both mulching treatments (BF and PE) as compared to bare soil. Given that the same amount of fertilizers was supplied whatever the treatment considered, those changes may be attributed to a higher biological activity in mulched soils. This was supported by recent findings by [START_REF] Liu | Response of nitrogen use efficiency and soil nitrate dynamics to soil mulching in dryland maize (Zea mays L.) fields[END_REF], who showed that the carbon stocks decreased only in the top soil (<20 cm depth), but increased in the rooting zone (20-40 cm depth) after four years of plastic mulching. This could be due to higher water retention in the mulched modalities, especially in the case of PE film whose water vapour permeability is 10 fold lower than these of BF. The soil organic matter content and its C/N ratio slightly increased with time, especially for BF and PE treatments.

The values of the biological parameters i.e. the microbial biomass and the nematode abundance at planting time (t0) revealed a rather low biological activity (Tables 2 and3, indicative of a disturbed soil, which is also poor in nutrients as typically observed in the location of the present experimental field. A significant increase in the microbial biomass (+220%) was observed two years after vines planting but without link with mulching since recorded for all the treatments. Actually, according to [START_REF] Kapanen | Performance and environmental impact of biodegradable films in agriculture: a field study on protected cultivation[END_REF] and [START_REF] Chen | Mulching practices altered soil bacterial community structure and improved orchard productivity and apple quality after five growing seasons[END_REF], plastic mulching are known to exert no significant impact on the total microbial diversity compared to bare soil. This was expected to favour the ultimate bio-assimilation of the biodegradable film by the soil at the end of the crop, the soil microbial biomass content being a key indicator of its biodegradation ability.

Regarding the nematofauna two years after planting, it could be noted that the abundance of total nematodes increased in all treatments (Table 3). This rise was notably higher in the case of BF treatment due largely to an increase in the plant-feeding nematode group, mainly Tylenchidae et Pratylenchidae, both known to be harmless against vinegrape. These results suggest a more diversified soil biological activity with the BF treatment, possibly a result of the progressive decomposition of BF film leading to an enrichment of the soil with carbon components potentially available to the nematodes [START_REF] Salome | The soil quality concept as a framework to assess management practices in vulnerable agroecosystems : a case study in Mediterranean vineyards[END_REF]).

Table 3. Evolution of the abundance of the nematode according to trophic groups for unmulched soil (BS) and soil under PE and BF during the two first years after vine planting

Bacterialfeeders

Fungalfeeders

Omnivores and carnivores

Plantfeeding

Free-living nematodes

Effect of mulching on vine growth and crop performance

At the end of the first growing season, Vines were pruned with a single cane as to a Guyot system. However, only vigorous vines were pruned in this manner in the first season, whereas for the weakest ones, the main cane was shortened to two buds. The first pruning is an important step for the vineyard since closely related to its capacity to be harvested. As a general way, vines in the mulched treatments had similar vigour, while the vines grown on BS were weak and exhibited variable growth, as previously reported [START_REF] Moore | Plastic mulch aids growth of young grape vines and cuttings[END_REF][START_REF] Van Der Westhuizen | The effect of Black Plastic Mulch on Growth, Production and Root Development of Chenin blanc Vines under dryland conditions[END_REF]). These observations on growth were confirmed by the amount of pruning wood, which was signficantly higher for PE-and BF-mulched treatments compared to the BS treatment (Table 4). It is worth noting that differences in pruning weight between the PE and BF treatments were not statistically significant despite the early degradation of the BF mulch film. As a result 97% and 94% of the vines mulched with PE and BF respectively had sufficient vigour to be pruned with a cane (i.e. vines reached bearing size) while only 13% of the unmulched vines met this requirement. The yields at the first harvest (2011) confirmed this trend (Table 4). It is important to underline that vines grown under PE and BF produced significant quantities of grapes 17 months after planting while it was necessary to wait for an extra year in the case of BS treatment. Although yield differences were not significantly different between the mulching treatments, the BF treatment produced higher must soluble solids than the PE treatment (20.4°B and 18.5°B respectively) and had a lower pH (3.3 and 3.5 respectively). The yields of the second harvest ( 2012) also indicated no significant difference in terms of fruiting yield and quality of the must between the BF and PE treatments (Table 4). In contrast, the unmulched vines (BS), for which it was the first harvest, led to a significantly higher yield than the two mulched treatments. The decrease in fruiting yield observed for the two-mulched treatments is probably due to their exceptionally high production yield the year before while it was the first time they bore grapevines. However, the fruiting yield of the un-mulched vines is in the same range of order than these obtained for the first harvest of the mulched vines. Thus, despite the early loss of BF film integrity, which occurred before the end of the first crop cycle, it is worth noting that the mulch-enhanced effect on vine growth was maintained during the second and third crop cycles. This suggested that the presence of a mulch film during the 5 months following planting would be sufficient to bring the vines to fruiting age during the second season and too lead them to higher production than un-mulched vines.

In order to attest the durability of the films in the climate conditions of the experimental area and verify if BF films with different thicknesses or containing a certain percentage of recycling material (10wt%) could lead to similar results, a second field test was launched in 2011. Six months later, all the mulched vines had similar vigour and no difference in vine growth between the different mulch film treatments was observed at this stage. In contrast, vines grown on bare soil had a lower development. At the end of the first growing season (March 2012), vines were pruned as previously mentioned and the pruning woods of a representative number of vines were collected and weighed (table 5). Results indicated a positive effect of mulching on the wood production during the first crop cycle confirming the observations previously made even if significant differences between PE and BF mulch films were observed. However, it is worth noting that the thickness of the film had no significant effect on the pruning yield (Table 5), the thinnest one (BF25) leading to a yield comparable with the three others BF films tested (BF40, BF*40 and BF70).

Regarding the first harvest of this second trial, the fruiting production of the mulched vines (whatever the treatment considered) was significantly higher than these of the un-mulched vines (Table 5). No significant difference was also observed between the different film thicknesses of the biodegradable films with reference to the quality of the must in terms of soluble solids, pH and acidity. The lower soluble solids recorded for the BS treatment was directly related to its lower fruiting production. Thus, it can be assumed that the film thickness of BF films had no impact on the fruiting yield leading to question the use of a thicker film that necessarily would induce an unjustified over-cost.

Effect of mulching on rootstock development

To better understand the impact of mulching on vine growth during the first growing season and confirm the positive effect observed on the aerial parts, the rootstock development of 25-months old vines was studied after excavating and manually collecting the vine roots in the first 60cm-thick soil horizon from a quarter of the Voronoï polygon. The morphological parameters (length and diameter) of the collected roots were then characterized and their dry weight determined (Table 6 and Figure 3). As evidenced by the total dry weight of roots developed between the soil surface and 60 cm depth, vines mulched with the BF film developed significantly twice more roots than the unmulched treatment (Table 6) and this repartition appeared quite well distributed in the space when considering the three horizons and two squares (Figure 3). This behaviour specifically observed the BF treatment was also confirmed by the total root length that was higher for BF despite a rather weak statistical reliability of the results due to the low number of replicates. The root distribution according to size (Table 6) indicated that mulching treatments (PE and BF) produced more fine roots (diameter < 2mm), which control water and nutrient uptake from soil than for un-mulched treatment. In the first horizon, which is the closest to topsoil, the PE treated vines developed more fine roots than the BF ones (results not shown) probably because the PE mulch film was still intact 25 months after vine planting whereas the BF film had already lost its integrity. Regarding the medium size roots (2mm < diameter < 10mm), which control not only water and nutrients transport but also soil exploration, the root development of the BF treatment appeared higher than these of both PE or BS treatments in most of the depths and squares considered (results not shown). It is worth being underlined that even in the farthest horizon, the root development of the un-mulched vines appeared lower.

Hence, in spite of the high variability of the values obtained, the development of vine rootstock appeared to be favoured with the BF treatment and this specific behaviour might be related to the progressive degradation of the mulch film that occurred at the end of the first growing season. This result is in agreement with Van der Westhuizen (1980) who reported a higher root mass of Chenin Blanc vines mulched with a plastic film disintegrating 18 months after planting as compared to un-mulched vines. This could be explained by an adaptation of the root system consecutive to the loss of mulch integrity and the resulting changes in the soil water content. In the case of permanently mulched vines, it is well known that the rootstock develops first close to the soil surface, which is a drawback to the newly planted vines in terms of nutrient and water uptake by roots. In contrast, with the BF treatment, it is likely that there would be loss of water after the film degradation with a reduction in the water content of the soil close to the surface, favouring root development deeper in the soil profile in search of water. To conclude, the vines treated with BF mulch film would be able to firstly adopt the behaviour of the PE-treated vines at the soil surface, and then, the behaviour of the un-mulched vines in depth. As a result, the rooting of vines grown under BF would be more vigorous and better distributed as shown by the roots distribution in the different horizon studied (Figure 3). This assumption is also in agreement with a higher turnover of the fine roots and the specific development of plantfeeding nematodes two years after vine planting with the BF treatment

End of life and biodegradability assessment in standardized conditions

The sustainable requirement expected for the BF film were quantified as the biodegradation level through two respirometric test conducted in a vineyard soil at 28°C (Figure 4) and in a compost medium at 58°C (Figure 5).

Results obtained in soil measurements indicated that both biodegradable films (BF and BF*) reached a mineralisation threshold higher than 60% after two years (Figure 4). This indicated that they were both were able to biodegrade in the standardized conditions of temperature and humidity claimed by one of the NF U52001 standard requirements the despite the content in recycled material. However, it is worth noting that, even if the tests have been undertaken using the vineyard soil as incubation medium, known to be poor in organic matter and microbial biomass, the kinetics of biodegradation would be even slower in real field conditions. Indeed, temperature and humidity in use certainly differ from the standardized ones. In spite of that, based on the lifespan of a vineyard (several decades), the biodegradation process is expected to be complete before a new crop would be planted even without burying step. To complete this study, a BF film sample collected from the field was subjected to a biodegradation test conducted in compost medium with the objective to evaluate how its rate of biodegradation might be affected by a 18 months field-weathering. According to [START_REF] Touchaleaume | How performance and fate of biodegradable mulch films are impacted by field ageing[END_REF], this ageing duration corresponded to a cumulated solar radiations received by the film of 953 kJ/cm 2 . The biodegradation curves (Figure 5) indicated that field ageing had few impact on the biodegradation pattern of the biodegradation of BF despite a 10% decrease in the percentage of mineralization was noticed for the aged film compared to the un-aged one at the end of the experiment, Hence, the changes in the biodegradation rate induced by field ageing were closely linked with the level of crosslinking undergone by each material [START_REF] Touchaleaume | How performance and fate of biodegradable mulch films are impacted by field ageing[END_REF]. However, the biodegradable character of BF could not be questioned even after undergoing photo ageing in field conditions during 18 months.

CONCLUSION

Mulching has a noticeable effect onto the vine growth, starting from the first growing season and enabling one year earlier production than unmulched vines. During the three growing seasons after planting, vines initially grown on BF and PE mulches were both more vigorous and productive than BS ones, with a similar behaviour in terms of vegetative growth and production. In addition, root development was favoured in the case of the BF treatment compared to PE and BS treatments.

Thus, despite the early loss of integrity of the BF film, which occurred before the end of the first crop cycle, it is worth noting that the mulch-enhanced effect on vine growth was still noticeable during the second and third crop cycles. This suggested that the presence of mulch film at the critical early stage of growth would be as efficient as permanently mulching, i.e. it allowed the plant to grow fast enough to reach economically significant fruiting production. Based on these results, the long-term stability of PE appeared over-estimated, the material lifespan required for mulching application in the case of vineyard being probably around 4-5 months from the planting of the vines. The biodegradable mulch film tested fulfilled the requirements of vineyard mulching and its correlated farming practices i.e. remain intact in the field long enough to provide its expected positive effects on the crop, and be able to be biodegraded afterwards.

Figure 1 .

 1 Figure 1. Description of the experimental design where the vine roots were sampled

Figure 2 .

 2 Figure 2. Comparison of the mulch films degradation 12 months after vine planting; a) BF 40 µm; b) BF 40*µm; c) BF 25 µm; d) BF 70 µm; e) PE 40 µm

Figure 3 .

 3 Figure 3. Effect of mulching treatment (BF, PE and BS) on the dry weight of roots collected at different depth and distance from the vine 24 months after planting (Values with different subscripts are considered as significantly different at p=0.05)

Figure 4 .

 4 Figure 4. Biodegradation in soil (collected from vineyard) at 28°C of BF and BF*(10% of scraps included in the formulation)

Figure 5 .

 5 Figure 5. Kinetics of biodegradation in compost medium at 58°Cof BF film before and after 18 months field-ageing.

Table 1 .

 1 Evolution of the mechanical properties of the mulching films (PE and BF) after 5 months field exposure.

		Film	σB 1 (MPa)	εB 2 (%)
		thickness
		µm	t0	t4 months	t0	t4 months
	PE	40	18.5 ± 0.6	14.3 ± 0.4	409 ± 23	311 ± 25
	BF	40	20.4 ± 0.8	11.7 ± 0.8	487 ± 18	159 ± 65
	BF*	40	16.9 ± 1.2	11.2 ± 0.7	428 ± 27	51 ± 17
	BF	25	16.9 ± 0.1	12.8 ± 0.5	299 ± 10	57 ± 9
	BF	70	14.8 ± 0.5	10.4 ± 0.4	574 ± 50	113 ± 7

1 εB: elongation at break in parallel direction 2 σB: stress at break in parallel direction

Table 2 .

 2 Evolution of soil physical (bulk density), chemical parameters (organic matter and C/N ratio) and biological (microbial biomass) parameters for unmulched soil (BS) and soil under PE and BF during the two first years after vine planting

			Bulk density	Organic matter	C/N	Microbial biomass
			(g/cm 3 )	(g/kg)		(mgC/kg dry soil)
	t0		1.46 a ± 0.05	10.38 a ± 0.49	11.97 a ± 0.60	55.2± 9.3
		BS	1.39 a ± 0.06	11.90 aba ± 0.89	14.07 ab ± 1.46	171.0± 7.2
	t24m	PE	1.38 a ± 0.03	12.30 aba ± 0.99	14.90 ba ± 0.28	175.7± 28.4
		BF	1.42 a ± 0.07	12.90 ba ±1.32	16.00 ba ± 1.40	171.6± 32.4
	Values with different subscripts are considered as significantly different at p=0.05

Table 4 .

 4 Effect of vine mulching on the viticultural performances (pruning and fruiting production and quality) during the three first growing seasons 1 (planting in)

			Pruning wood	Fruiting production	Soluble solids	pH
			(g/vine)	(kg/vine)	(°Brix)
		BS	23.1 a ± 17.2	0.39 a ± 0.03	--	--
	2011	PE	102.3 b ± 73.0	5.06 b ± 0.87	18.5 a ± 1.5	3.46 a ± 0.04
		BF	83.9 b ± 64.2	4.24 b ± 1.23	20.4 b ± 0.6	3.33 b ± 0.03
		BS	225.7 a ± 144.3	4.34 b ± 1.57	21.5 a ± 0.1	3.30 a ± 0.09
	2012	PE	427.6 b ± 136.5	3.34 a ± 1.49	21.6 a ± 1.6	3.44 a ± 0.03
		BF	411.5 b ± 206.1	3.06 a ± 1.35	22.0 a ± 1.5	3.42 a ± 0.13
	Values with different subscripts are considered as significantly different at p=0.05
	1 vine planted in April 2010; first pruning in March 2011 and second in 2012; first harvest in August 2011 and
	second in September 2012

Table 5 .

 5 Effect of vine mulching on the viticultural performances (pruning production, fruiting production and grapes quality) during the two first growing seasons 1

	Thickness (µm)	Pruning wood (g/vine)	Fruiting production (kg/vine)	Soluble solids (°Brix)	pH	Acidity (g tartric acid/l)
		25	94 a ± 85	3.28 a ± 1.51	20.3 a ± 1.6	3.20 a ± 0.13	6.08 ab ± 0.75
		40	88 a ± 45	3.16 a ± 1.35	19.8 a ± 0.7	3.21 a ± 0.06	6.30 ab ± 0.62
	BF
	2012	40 *	103 a ± 40	3.60 a ± 1.17	19.8 a ± 1.6	3.18 a ± 0.14	6.24 ab ± 0.50
		75	106 a ± 76	4.51 a ± 1.54	19.9 a ± 1.5	3.16 a ± 0.05	6.14 ab ± 0.31
	PE	40	141 b ± 81	4.10 a ± 0.99	19.8 a ± 0.7	3.15 a ± 0.05	6.63 a ± 0.47
	BS	-	35 c ± 49	1.13 c ± 1.68	21.3 b ± 1.5	3.25 a ± 0.12	5.85 b ± 0.45
	Values with different subscripts are considered as significantly different at p=0.05
	1 vine planted in March 2011; first pruning in March 2012 and first harvest in September 2012

Table 6 .

 6 Root growth of mulched(PE and BF) or unmulched (BS) 2 years-old vines. Cumulated root dry weight and root length for a total soil volume of 96 dm 3 corresponding to the first 60cm thick soil horizon. 2 Fine root were ascribed to roots having a diameter d<2cm 3 Medium root were ascribed to roots having a diameter ranging between 2mm<d<10mm Values with different subscripts are considered as significantly different at p=0.05

			Total root 1	Total root 1	Fine root 2	Medium root 3
			dry weight	length	length	length
			(g)	(cm)	(cm)	(cm)
		BS	40.93 a ± 5.87	3416 a ± 457	3111 ab ± 409	305 a ± 104
	t24m	PE	47.05 a ± 3.48	4479 a ± 359	4062 ab ± 506	417 a ± 173
		BF	78.23 b ± 7.89	4683 a ± 1425	4168 b ± 761	515 b ± 187

1

ACKNOWLEDGEMENTS

This study has been carried out in the frame of the AGROBIOFILM project with financial support from the European Union 7th Framework Program (FP7/2007-2013) under the grant agreement number 262257. The authors gratefully thank Olivier Mandeville (Château Vaissiere) for kindly providing vineyard, Anne Chevillard, Fanny Berger and Marie Toussaint for valuable contribution in discussions about vine growing, and Maet Le Lan for technical support regarding mulching film characterization


Iñaki Cornu
email: inaki.cornu@unilim.fr

Maxime Cheype

Marion Baudier-Pons

Jean-François Léon

Samuel Couillaud

Fabrice Rossignol

Jean-Marc Heintz

Pixelated sintering of α-Al 2 O 3

Keywords:

The sintering of α-alumina by a brand new and innovative technique, called pixelated sintering (PS), is here studied. Densification and grain growth by PS of perfectly controlled granular compacts are analysed and compared to results obtained using Spark Plasma Sintering (SPS) and Pressure-Less SPS (PL-SPS). Materials are exposed to the same temperature profiles whatever the sintering technique used in order to assess the potential of PS in terms of microstructure control. It is shown that PS can be used as an alternative technique to SPS for fast sintering with the advantages of a much simpler and cost-effective set-up, as well as a better control of the localised heat input. PS also appears to be a very modular technology in the way it controls the temperature gradients allowing its implementation for multi-step sintering approaches, as well as for the fabrication of large and complex parts.

Introduction

The permanent need to improve properties of materials while reducing their processing costs has led to the development of new nonconventional sintering processes over the last years [START_REF] Degroat | One-shot powder metal parts[END_REF][START_REF] Cologna | Flash sintering of nanograin zirconia in < 5 s at 850 ∘ C[END_REF][START_REF] Sutton | Microwave processing of ceramics -an overview[END_REF][START_REF] Riman | Method of hydrothermal liquid phase sintering of ceramic materials and products derived therefrom[END_REF][START_REF] Randall | Cold sintering ceramics and composites[END_REF]. Currently the most developed non-conventional techniques are the Electric Current Activated/Assisted Sintering (ECAS) [START_REF] Grasso | Electric current activated/assis-ted sintering (ECAS): a review of patents 1906-2008[END_REF], and Spark Plasma Sintering (SPS) is likely the most popular of them. SPS allows to sinter, in a short time, a powder compact confined in a die, the whole being heated up by a pulsed DC electric current passage through the die and eventually through the powder itself, the die allowing as well to apply a mechanical pressure in vacuum or inert atmosphere. With SPS it is possible to sinter a wide range of materials like ceramics [START_REF] Zhang | Processing and characterization of fine-grained monolithic SiC ceramic synthesized by spark plasma sintering[END_REF][START_REF] Bellosi | Fast densification of ultra-high-temperature ceramics by spark plasma sintering[END_REF][START_REF] Anselmi-Tamburini | Spark plasma sintering and characterization of bulk nanostructured fully stabilized zirconia: part I. Densification studies[END_REF], metals [START_REF] Kwon | Spark plasma sintering behavior of pure aluminum depending on various sintering temperatures[END_REF][START_REF] Zhang | Microstructure characteristic, mechanical properties and sintering mechanism of nanocrystalline copper obtained by SPS process[END_REF], composites [START_REF] Basu | Development of WC-ZrO 2 nano-composites by spark plasma sintering[END_REF][13][14], or non-equilibrium materials [START_REF] Suffner | Microstructure evolution during spark plasma sintering of meta-stable (ZrO 2 -3 mol% Y 2 O 3 )-20 wt% Al 2 O 3 composite powders[END_REF][START_REF] Herisson De Beauvoir | Synthesis, sintering by Cool-SPS and characterization of A 2 Cu(CO 3 ) 2 (A = K, Na): evidences for multiferroic and magnetoelectric cupricarbonates[END_REF]. Many advantages over conventional sintering can be attributed to SPS. Among them, rapid heating ramps and mechanical pressure allow for a lowest sintering temperature [START_REF] Rajeswari | Comparative evaluation of spark plasma (SPS), microwave (MWS), two stage sintering (TSS) and conventional sintering (CRH) on the densification and micro structural evolution of fully stabilized zirconia ceramics[END_REF][START_REF] Herisson De Beauvoir | Cool-SPS: an opportunity for low temperature sintering of thermodynamically fragile materials[END_REF]. The SPS sintering of pure α-alumina has been extensively studied [START_REF] Makino | Consolidation of ultrafine alumina powders with SPS method[END_REF][START_REF] Demuynck | Densification of alumina by SPS and HP: a comparative study[END_REF][START_REF] Guyot | Hot pressing and spark plasma sintering of alumina: discussion about an analytical modelling used for sintering mechanism determination[END_REF][START_REF] Aman | Spark plasma sintering kinetics of pure α -alumina[END_REF][START_REF] Shen | Spark plasma sintering of alumina[END_REF][START_REF] Santanach | Spark plasma sintering of alumina: study of parameters, formal sintering analysis and hypotheses on the mechanism(s) involved in densification and grain growth[END_REF][START_REF] Manière | Spark-plasma-sintering and finite element method: from the identification of the sintering parameters of a submicronic α -alumina powder to the development of complex shapes[END_REF]. It is shown that the densification and the grain size may be adjusted depending on experimental conditions, but the sintering mechanisms are still comparable to conventional free-sintering technique. A significant grain size increase cannot be avoided when the pore pining effect is over (from 95 % of relative density). The modular aspect of SPS technique gives the opportunity to make SPS setups operates both with a mechanical pressure on the sample and the current flow through, or separately [START_REF] Giuntini | Initial stage of free pressureless spark-plasma sintering of vanadium carbide: determination of surface diffusion parameters[END_REF][START_REF] Lin | Microchannelled hydroxyapatite components by sequential freeze drying and free pressureless spark plasma sintering[END_REF][START_REF] Aman | Pressure-less spark plasma sintering effect on nonconventional necking process during the initial stage of sintering of copper and alumina[END_REF][START_REF] Bertolla | Pressureless spark plasma-sintered Bioglass® 45S5 with enhanced mechanical properties and stress-induced new phase formation[END_REF][START_REF] Bradbury | Production of SiC-C composites by free-pressureless spark plasma sintering (FPSPS)[END_REF][START_REF] Quan | Ti6Al4V foams fabricated by spark plasma sintering with post-heat treatment[END_REF]. In this present work, α-alumina sintering by SPS and pressure-less SPS (PL-SPS) will be compared to α-alumina sintering made by our brand new sintering technique called "pixelated sintering". The pixelated sintering (PS) is a very original modular sintering technique created and developed by Galtenco Solutions in partnership with the Institut de Recherche sur les Céramiques (Limoges, France) and the Institut de Chimie de la Matiére Condensée de Bordeaux (France). Its name alludes to the heating elements (pixels) placed around the sample and which can be controlled independently of each other. Each pixel is composed by a power supply, a resistive heating element and a thermocouple located as close as possible to the sample. The whole sintering cycle is controlled by a proprietary software that drives each heating element independently, using a thermocouple which is paired to it. The energy delivered to the part is optimized and thermal losses are limited. Beyond energy intensification, sintering is free which opens prospects for the consolidation of complex 3D parts and for transfer to large dimensions. This study aims to show that, using PS, it is possible to sinter α-alumina using natural sintering with a high heating rate and a control of the temperature as good as it is with SPS.

Experimental

Materials

Alumina powder

All the experiments performed in this study were carried out with a α-alumina powder (purity > 99,9 %,BMA15,Baikowski, France). Fig. 1 shows the volume and number distributions of the BMA15 powder measured by laser granulometry (Mastersizer 2000, Malvern, UK). The volume distribution exhibits a bimodal repartition with a main peak centred around 150 nm and a second peak around 4 μm. This last population corresponds to some aggregates but in small quantity because the number distribution shows only one population centred around 150 nm.

The SEM (VEGA II SBH, TESCAN, Czech Republic) micrograph presented in Fig. 2a indicates that spherical granules with a diameter of about 100 μm form the BMA15 α-alumina powder. These granules contain a large number of primary particles (Fig. 2b) with mean size of 145 nm confirming the granulometry results. The specific surface area was measured by BET as S SSA = 14 m 2 .g -1 (Tristar II 3020, Micromeritics, USA), which corresponds to an equivalent spherical particle size of G BET = 110 nm.

Preparation of presintered alumina samples

The green BMA15 samples were prepared by a slipcasting method. A slurry was first formulated from a mixture of 70 wt% of BMA15 powder with 0.5 wt% of an electrosteric dispersing agent (Darvan CN) in distilled water. The slurry was firstly homogeneized using a magnetic stirrer for 5 h and secondly placed into a non-contact planetary mixer (ARE-250, THINKY, JAPAN) where the slurry was mixed and degassed at 1000 rmp for 1 min. Then the slurry was casted into a polyoxymethylene cylindric (ϕ = 10 mm) mould placed itself on a plate of Paris plaster. The green bodies were removed from the mould after 48 h of casting and were dried in air during 24 h at room temperature. The final diameter of all samples was 10 mm and the thickness 3 mm. Finally, a pre-sintering treatment at 1000 ∘ C during 1 h under air was performed with a heating ramp of 0.5 ∘ C.min -1 . It led to porous BMA15 ceramics with a relative density measured of 60 % (the theoretical density of BMA15 was taken as 3.94 g.cm -3 measured by He pycnometry). The relative density of alumina presintered and sintered samples was determined by Archimedes'method in deionized water.

Sintering

Presentation of the new pixelated sintering technique

The BMA15 presintered samples were densified using the simple pixelated sintering system presented in Fig. 3. As the considered geometry was a cylinder (10 mm in diameter and 3 mm in thickness), the heating unit was composed by a single pixel made of a graphite diffuser containing one thermocouple which monitored two graphite heating elements placed on the top and bottom of the sample (Fig. 3). The system was assembled on a mica plate (44 × 26 cm) where fixed steel and copper rods were allowing the current flow to Papyex® flexible graphite and so through the two graphite heating elements. The two heating elements were graphite plate (ET10, Graphitech, France) measuring 6 mm long, 4 mm wide and 1 mm thick. They were connected to two low voltage power supplies (30 V, 100 A) wired in parallel, and were dimensioned and machined to obtain a total electric resistance of 0.6 Ω in operation. It was then possible to heat the resistance up to 2000 ∘ C with ramp higher than 200 ∘ C.min -1 . The temperature was measured by a C thermocouple (W/5%Re-W/26%Re) placed into the graphite diffuser where the sample laid (green point in Fig. 3). Therefore, the measurement of the temperature was done as close as possible to the surface of the sample. In this configuration, the thermocouple controlled the two heat elements at the same time. The upper graphite heat element is placed at 1 cm from the sample and the bottom graphite heat element is placed at 2 cm from the graphite diffuser. Depending on the sintering cycle used and the temperature measured by the thermocouple, the proprietary software (Galtenco Solutions) was able to turn off or on the power supplies connected to the heating elements with a response time of a few milliseconds. The temperature input on the sample surface was then optimized and the risk of thermal gradient minimized. Each component was individually insulated from the heat by alumina insulating whool. This allowed to protect the signal response against electrical interferences but also to avoid a thermal confinement on the sample area, which would have made the temperature measurement biased. The whole set up was finally placed into a vacuum chamber (10 -1 Pa, Inox 316).

An example of a sintering cycle with maximum temperature of 4. The C thermocouple placed under the sample maintains or cuts off the power supplies. This is not a continuous heating but rather a pulsed one giving a much better control on the heat input. The cooling is achieved by simply switching off the graphite heating elements (i.e., free cooling). The temperature drops rapidly to 1000 ∘ C (Fig. 4) then, due to the thermal inertia, decreases slowly until reaching 5 ∘ C.min -1 below 300 ∘ C. So it is difficult to make a cooling quench with the pixelated sintering technique but the cooling step can be controlled with the same approach as that used for heating ramp. It opens the way to a modular sintering where every steps of the cycle are controlled.

Sintering study

The PS of α-alumina was studied and compared to conventional and SPS sintering. The conventional sintering assessment of our BMA15 powder was made using dilatometry (TMA, Setsys evolution, France). At first, this technique allowed us to extrapolate sintering curves of BMA15 with high heating ramps (50 and 150 ∘ C.min -1 ). These curves were then compared to the experimental ones obtained by PS. Secondly the PS and SPS performed on BMA15 samples with non-isothermal sintering cycles were compared in terms of densification and grain growth. Two rapid heating ramps were studied: 50 and 150 ∘ C.min -1 . The maximum temperature reached depended on the sintering techniques. Grain size of sintered ceramics was determined from SEM micrographs by observation of mirror polished samples and thermally etched at 50 ∘ C below the sintering temperature in air. Grain sizes were measured using the Jeffries'planimetric method [START_REF] Allen | Standard Test Methods for Determining Average Grain Size (F112)[END_REF]. For statistical purposes, the number of measured grains was higher than 500 and 3 different micrographs were taken for each samples. The SPS experiments were carried out on the SPS model Dr.Sinter 825 (Fuji Electronics Industrial Co. Ltd, Japan). For SPS, the BMA15 samples were placed into a cylindrical graphite die (grade 2333, Mersen, France) with a inner diameter of 10 mm. Graphite foils (Papyex® N998, Mersen, France), with a thickness of 0.2 mm, were used to cover the internal surface of the die and the T-shaped punches surfaces. Two configurations were studied. The first was a standard setting where the mechanical pressure applied on the sample was 40 MPa during the whole sintering cycle. The second was called Pressure-Less SPS (PL-SPS). In this setting, the upper graphite T-shaped punch was shortened. The gap between the punch and the sample allowed to avoid a direct contact with the sample and the application of a mechanical pressure on it. This configuration was inspired from Bradbury and Olvesky study [START_REF] Bradbury | Production of SiC-C composites by free-pressureless spark plasma sintering (FPSPS)[END_REF]. The non-isothermal SPS and PL-SPS tests were performed under vacuum at (6 < P(Pa) < 14). The temperature measurements were made using an infrared camera focalized on the external surface of the graphite die containing the sample. A standard pulse sequence (12:2) was chosen for the heating by SPS and PL-SPS.

Results and discussion

Sintering mechanisms

In order to compare the sintering behaviour of our BMA15 powder to other works reported in the literature, a sintering study was firstly undertaken, using dilatometry. The shrinkage curves were recorded as a function of temperature in the 20-1550 ∘ C temperature range for different heating ramps (in static air): 2, 10 and 20 ∘ C.min -1 . Analyses of Johnson [START_REF] Su | Master sintering curve: a practical approach to sintering[END_REF], allows the determination of the sintering activation energy assumed to be a unique value linked to a single densification mechanism. When the model is applicable, the relative density of the material can be predicted from the master sintering curve at any time for any sintering cycle. The MSC model is formulated and based on the densification rate equation combining volume diffusion and grain boundary diffusion mechanisms of the Hansel model [START_REF] Hansen | Combined-stage sintering model[END_REF]. The apparent activation energy, E a , corresponds to the value where all experimental data, from all heating profiles, converge to a single curve. The convergence is quantified through the minimum of the sum of residual squares for different values of E a , as shown in Fig. 5 for our α-alumina. From our experimental values, the calculated apparent activation energy is E a = 568 kJ.mol -1 . This obtained value is closed to other values reported in the literature [START_REF] Tatami | Control of shrinkage during sintering of alumina ceramics based on master sintering curve theory[END_REF][START_REF] Zuo | Investigation of the mechanism(s) controlling microwave sintering of α -alumina: influence of the powder parameters on the grain growth, thermodynamics and densification kinetics[END_REF] for α-alumina. For Bernard-Granger et al. [START_REF] Bernard-Granger | Sintering of an ultra pure α -alumina powder: I. Densification, grain growth and sintering path[END_REF],

such an E a can be attributed to a grain boundary diffusion mechanism. Comparison between experimental and calculated sintering behaviours are presented in Fig. 6. The analytic linearization of our MSC is performed by the Blaine et al. equation [START_REF] Blaine | Linearization of master sintering curve[END_REF].

Experimental densities of α-alumina samples, heat-treated at 2, 10 and 20 ∘ C.min -1 in the 20-1550 ∘ C temperature range are represented by solid blue, green and orange lines respectively. The corresponding blue, green and orange dotted lines represent the calculated curves. The good agreement observed between these curves validates the proposed MSC approach. It can be used to predict the density evolution of BMA15 powder using high heating rates, such as 50 and 150 ∘ C.min -1 , as used for our PS, SPS and PL-SPS tests. The density evolution curves are presented, in Fig. 6, in red and black dotted lines for 50 and 150 ∘ C.min -1 , respectively.

Sintering results

Fig. 7 represents the relative density and grain size variations depending on the maximum temperature reached during the different non-isothermal sintering tests performed by PS (Fig. 7a), SPS (Fig. 7b) and PL-SPS (Fig. 7c). Then, the sintering path obtained upon these three techniques is shown in Fig. 7d. Finally all results are also shown in Table 1. For the PS technique (Fig. 7a), seven maximum temperatures were studied: 1100, 1200, 1300, 1400, 1500, 1600 and 1700 ∘ C. The cooling rate was the same as presented earlier in Fig. 4 (i.e., free cooling). A strong increase of relative density is observed between 1300 ∘ C and 1400 ∘ C. At 1400 ∘ C the relative density reached depends on the heating ramp: 90.5 % for 50 ∘ C.min -1 and 80.8 % for 150 ∘ C.min -1 . Then the relative densities reach around 95 % at 1500 ∘ C. The SEM micrographs in Fig. 8 (blue frame) illustrate the transition from open porosities to closed porosities for the PS technique. We observe a densification delay between the two studied heating ramps (Fig. 7a). This delay can be assigned to a kinetic effect due to a different holding time at high temperature (i.e. in between 1400 and 1500 ∘ C). Finally the relative densities progress together reaching about 98 % at 1700 ∘ C. Calculated values of densities for 50 and 150 ∘ C.min -1 (grey and red dotted lines, respectively) obtained from the MSC model are also plotted. We observe for the two heating ramps that the sintering initiation starts in the same temperature range, towards 1300 ∘ C. The experimental sigmoid curves inflexion point, where the densification rate is the highest, is superimposed on corresponding MSC curves. The experimental and calculated curves separate with each other for relative densities around 92 % for 50 ∘ C.min -1 and around 95 % for 150 ∘ C. min -1 . The pixelated sintering follows the MSC predicting behaviour. Thus, it can be considered as a conventional sintering technique. The grain size variation (blue curves in Fig. 7a) shows a constant grain growth from 1200 to 1500 ∘ C. At 1400 ∘ C, the gap between the grain size observed for the two heating ramps is linked to the gap of relative densities. The difference of holding time at high temperature is brought to light at 1600 ∘ C. Indeed for the 50 ∘ C.min -1 heating rate, the pore pinning effect [START_REF] German | Sintering Theory and Practice[END_REF][START_REF] Chinelatto | Mechanisms of microstructure control in conventional sintering[END_REF] is already over. Therefore, we observe a significant increase of the grain size. For the 150 ∘ C.min -1 heating rate, the end of pore-pinning effect takes place between 1600 and 1700 ∘ C. Hence, at 1700 ∘ C, the grain sizes for the two heating ramps are similar. For the SPS standard technique (Fig. 7b), five maximum temperatures were studied: 1000, 1100, 1200, 1300 and 1400 ∘ C. The cooling rate of 25 ∘ C. min -1 was controlled this time to avoid thermal shock and cracks in sintered BMA15 samples. From 1000 ∘ C, a slight increase of relative density is observed for the both ramps. This low enhancement could come from a particles rearrangement due to the application of the mechanical pressure (40 MPa) on the BMA15 samples throughout the sintering cycle. At 1100 ∘ C, a clear relative density difference is observed between both heating rates.

Indeed 86 % and 77.5 % relative densities of SPS samples are obtained for 50 and 150 ∘ C.min -1 respectively. Thus, the sintering initiation is already started at this temperature. Once more, a different temperature holding time could explain this discrepancy. At 1200 ∘ C, the relative densities are around 96 %. The transition from open porosities to closed porosities for the SPS technique is occurring between 1100 ∘ C and 1200 ∘ C. As well as pixelated sintering, this transition is illustrated by SEM micrographs in Fig. 8 (green frame). The rapid densification observed for 150 ∘ C.min -1 (77.5-96.3 % from 1300 to 1400 ∘ C) is also shown in Santanach et al. study [START_REF] Santanach | Spark plasma sintering of alumina: study of parameters, formal sintering analysis and hypotheses on the mechanism(s) involved in densification and grain growth[END_REF]. Finally, relative densities close to 99 % are obtained for the both heating ramps at higher temperatures (1300 and 1400 ∘ C). The grain size variation (green curves in Fig. 7b) is the same as pixelated sintering. At 1300 ∘ C, we have, once more, a difference between the both heating ramps due to the end of pore-pining effect. Finally at 1400 ∘ C, the grain sizes heating rates for both join each other and are similar. We can notice the interest of SPS sintering to obtain dense materials with a temperature 200 ∘ C lower than PS and without isothermal holding. However, even if the contribution of mechanical application is clear, it does not allow to avoid, in this SPS conditions, the significant grain growth phenomenon when the sample relative density reaches 95 %. For the PL-SPS technique (Fig. 7c), six maximum temperatures were studied: 1100, 1200, 1300, 1400, 1500 and 1600 ∘ C. The cooling rate was controlled at 100 ∘ C.min -1 . From 1100-1200 ∘ C the relative densities slightly increase. This is at 1300 ∘ C that we observe a significant difference between both heating ramps. Indeed, for the 50 ∘ C.min -1 ramp, the densification is already at an advanced stage with a relative density of 87.4 %. For the 150 ∘ C.min -1 ramp, the relative density is only 73.3 %. The experimental sintering curves join each other at 1400 ∘ C with a relative density around 95 %. The SEM micrographs in Fig. 8 (orange frame) illustrate the pore-closing transition for the PL-SPS technique. Finally relative densities slowly evolve up to 97 % from 1500 to 1600 ∘ C. The PL-SPS is a technique where the configuration is studied to avoid direct mechanical contact with sample and thus the application of a mechanical pressure on it. We note a shift between experimental and calculated values at high heating rates (grey and red dotted lines for 50 et 150 ∘ C.min -1 respectively) obtained from the MSC model. If we add 75 ∘ C to each point of the experimental curves, it seems to follow back the MSC curves behaviour until relative density reaches around 90 ~ 92 %. This temperature difference could be explained by the temperature measurement method used for the SPS and PL-SPS setups, namely an infrared optical camera. The temperature measured is the temperature of the graphite die external surface and not the real temperature of the sample that is here probably higher. This observation is already reported in the literature [START_REF] Chakravarty | Direct characterizing of densification mechanisms during spark plasma sintering[END_REF][START_REF] Anselmi-Tamburini | Fundamental investigations on the spark plasma sintering/synthesis process[END_REF][START_REF] Song | Heating mechanism of spark plasma sintering[END_REF][START_REF] Tiwari | Simulation of thermal and electric field evolution during spark plasma sintering[END_REF][START_REF] Muñoz | Temperature and stress fields evolution during spark plasma sintering processes[END_REF][START_REF] Räthel | Temperature distribution for electrically conductive and non-conductive materials during field assisted sintering (FAST)[END_REF]. It is shown that the inside and outside temperature difference is exacerbated in the case of the sintering of insulated materials. Moreover a FEM analysis of SPS sintering of alpha alumina has been carried out by Wang et al. [47] considering a large range of die sizes, heating rates and uniaxial stresses. It shows that, for a SPS die with inner diameter of 12 mm, radial and axial thermal gradients occur. In our study, these gradients are notably exacerbated in our PL-SPS setup due to the air film presence between the non-touching upper graphite punch and the sample. That is why such a gap around 75 ∘ C is observed between experimental and calculated curves. A further study of our PL-SPS must be made if we want to optimize PL-SPS. A higher grain size for PL-SPS in comparison with PS and SPS should be noted. Once more, the error in the internal temperature measurement could be the origin. Finally a comparison between the different sintering paths obtained from the three sintering techniques is reported in Fig. 7d. The graph shows the grain size obtained depending on the relative density. It confirms that the sintering behaviour is the same between SPS, PL-SPS and PS. In the same sintering conditions, PS allows to access to the same results as the sintering of α-alumina by SPS in terms of densification and grain growth behaviour with high heating ramps. Thus PS can be classified among so-called fast sintering techniques.

Conclusion

This study aimed to show the ability of PS sintering technique to obtain results close to those obtained with other well-known fast sintering techniques: SPS and PL-SPS. Here we used a high purity submicron size α-alumina to compare SPS, PL-SPS and PS in terms of densification and grain growth. We showed as expected that SPS has its own densification behaviour because of the application of pressure. We succeeded anyway to get densification and grain size with PS comparable to SPS and PL-SPS. Furthermore, we demonstrated the same sintering behaviour between the three techniques. The process undertaken with our novel PS technique is not really to compete with SPS or other fast sintering techniques but rather to offer an alternative sintering technique with many advantages: free and fast sintering into controlledatmospheres (under air, vacuum or inert gas) with a fine and localised heat input for energy intensification. This totally modular technique can also represent a new way in modular sintering like multi-step sintering, or else the large size materials and complex architecture sintering. This last very important point for industrial leveraging of PS will be described in a future article.

Fig. 1 .

 1 Fig. 1. Particle size distribution of α-alumina.

Fig. 2 .

 2 Fig. 2. SEM micrographs (a) spherical granule (b) primary particles.

Fig. 3 .Fig. 4 .Fig. 5 . 6 .

 3456 Fig. 3. Pixelated sintering system for densification experiments on BMA15 pre-sintered specimens.

Fig. 7 .

 7 Fig. 7. Relative density and grain size depending on the maximum temperature reached for (a) pixelated sintering, (b) SPS and (c) Pressure-less SPS. (d) Sintering path of α-alumina BMA15 obtained upon all the studied sintering techniques. The heating ramps are represented by closed squares for 50 ∘ C.min -1 and empty squares for 150 ∘ C.min -1 . In the specific case of high heating rates: 50 and 150 ∘ C.min -1 (grey and red dotted lines, respectively) the values were calculated from the MSC model.

Fig. 8 .

 8 Fig. 8. SEM micrographs show α-alumina microstructures sintered through different temperatures (T max ) and heat ramps (50 and 150 ∘ C.min -1 ) by (blue frame) pixelated sintering, (green frame) SPS and (orange frame) pressure-less SPS. ρ corresponds to the sample relative density.

Table 1

 1 Relative density and grain size obtained for each sintering technique.

		Pixelated sintering
	Temperature ( ∘ C)	Relative density (%)	Grain size (μm)
		50 ∘ C.min -1	150 ∘ C.min -1	50 ∘ C.min -1	150 ∘ C.min -1
	1200	58	57	0.16 ± 0.01	0.15 ± 0.01
	1300	68	65	0.17 ± 0.01	0.16 ± 0.01
	1400	90.5	80.8	0.32 ± 0.05	0.19 ± 0.06
	1500	95.7	94.5	0.57 ± 0.08	0.46 ± 0.06
	1600	97.4	96	1.14 ± 0.08	0.65 ± 0.07
	1700	98.5	98.1	1.8 ± 0.1	1.71 ± 0.12
		Spark Plasma Sintering
	Temperature ( ∘ C)	Relative density (%) 50 ∘ C. min -1 150 ∘ C. min -1	Grain size (μm) 50 ∘ C. min -1 150 ∘ C. min -1
	1000	69.8	64.4	0.16 ± 0.04	0.15 ± 0.04
	1100	86	77.5	0.33 ± 0.02	0.27 ± 0.05
	1200	96.7	96.3	0.48 ± 0.1	0.4 ± 0.1
	1300	99.5	98.1	1.21 ± 0.07	0.83 ± 0.1
	1400	99.9	98.3	1.45 ± 0.06	1.4 ± 0.08
		Pressure-less Spark Plasma Sintering
	Temperature ( ∘ C)	Relative density (%) 50 ∘ C. min -1 150 ∘ C. min -1	Grain size (μm) 50 ∘ C. min -1 150 ∘ C. min -1
	1200	59.4	57	0.13 ± 0.05	0.14 ± 0.03
	1300	87.4	65	0.29 ± 0.07	0.17 ± 0.05
	1400	95.4	80.8	1.12 ± 0.08	1.11 ± 0.1
	1500	96.4	94.5	2.05 ± 0.2	1.48 ± 0.08
	1600	97.2	96	2.34 ± 0.2	2.4 ± 0.1

Acknowledgements

The authors want to acknowledge Mrs. Marion Vandenhende for her technical support in carrying out the SPS experiments.

Funding

This work was supported by an institutional grant of the French National Research Agency (ANR-17-CE08-0033 PIXUFAST project).

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.