ceramics

Article Effect of High Pressure Spark Plasma Sintering on the Densification of a Nb-Doped TiO2 Nanopowder

Alexandre Verchère 1, Sandrine Cottrino 2,*, Gilbert Fantozzi 2, Shashank Mishra 1 , Thomas Gaudisson 3, Nicholas Blanchard 3, Stéphane Pailhès 3 , Stéphane Daniele 4 and Sylvie Le Floch 3,* 1 IRCELYON, Université Lyon1-CNRS, UMR 5256, 2 Avenue Albert Einstein, 69626 Villeurbanne, France; [email protected] (A.V.); [email protected] (S.M.) 2 MATEIS, INSA-Université Lyon1-CNRS, UMR 5510, INSA de Lyon, 69621 Villeurbanne, France; [email protected] 3 Institut Lumière Matière, Université Lyon1-CNRS, UMR 5306, Université de Lyon, 69622 Villeurbanne, France; [email protected] (T.G.); [email protected] (N.B.); [email protected] (S.P.) 4 C2P2, Université Lyon 1-CPE Lyon-CNRS, UMR 5265, 43 Bvd du 11 Novembre 1918, 69616 Villeurbanne, France; [email protected] * Correspondence: [email protected] (S.C.); sylvie.le-fl[email protected] (S.L.F.); Tel.: +33-472-431-402 (S.L.F.)


Received: 23 October 2020; Accepted: 17 November 2020; Published: 4 December 2020


Abstract: Sintering under pressure by means of the spark plasma sintering (SPS) technique is a common route to reduce the sintering temperature and to achieve ceramics with a fine-grained microstructure. In this work, high-density bulk TiO2 was sintered by high pressure SPS. It is shown that by applying high pressure during the SPS process (76 to 400 MPa), densification and phase transition start at lower temperature and are accelerated. Thus, it is possible to dissociate the two densification steps (anatase then rutile) and the transition phase during the sintering cycle. Regardless of the applied pressure, grain growth occurs during the final stage of the sintering process. However, twinning of the grains induced by the phase transition is enhanced under high pressure resulting in a reduction in the crystallite size.

Keywords: nanopowder TiO2; high-pressure spark plasma sintering; grain growth; densification rate

1. Introduction

Nb-doped Titanium dioxide (Nb-TiO2) is a promising cheap, chemically stable and nontoxic transition metal oxide for high temperature thermoelectric (TE) devices working in oxidizing environments. A good TE efficiency requires materials with low thermal conductivity and high electrical conductivity. While a small amount of Nb (2–4 at.%) increases the electrical conductivity of TiO2 by several orders of magnitude, it has been recently shown that the thermal conductivity of bulk nano-structured Nb-TiO2 obtained by spark plasma sintering (SPS) is significantly lowered by decreasing the average grain size [1]. A value of 2.5 W/mK has been observed at 900 K for an average grain size of 170 nm, while the value in the single crystal is of about 4 W/mK. SPS has proven its efficiency in fast densification with limiting grain growth [2,3]. However, the role of the applied pressure remains unclear [4–6]. This work provides a detailed investigation of the sintering mechanism of Nb-TiO2 during the SPS process and of the effect of the high pressure with the aim of identifying the parameters that control the average grain size in the nano-structured materials. Our aim is to investigate whether high-pressure SPS can produce dense ceramics with finer nanostructures.

Ceramics 2020, 3, 507–520; doi:10.3390/ceramics3040041 www.mdpi.com/journal/ceramics CeramicsCeramics 20202020,, 33 FOR PEER REVIEW 5082

2. Materials and Methods 2. Materials and Methods

2.1. Synthesis of Niobium-Doped TiO2 Nanoparticles. 2.1. Synthesis of Niobium-Doped TiO2 Nanoparticles. AllAll synthesessyntheses werewere carriedcarried outout underunder argonargon usingusing standardstandard air-freeair-free techniques.techniques. The reagents, [Ti(OEt)4]3 and [Nb(OEt)5]2(Strem Chemicals, 99.9%) were distilled prior to use. Anhydrous pentane [Ti(OEt)4]3 and [Nb(OEt)5]2(Strem Chemicals, 99.9%) were distilled prior to use. Anhydrous pentane waswas obtainedobtained usingusing aa solventsolvent drier,drier, MBMB SPS-800SPS-800 (solvent(solvent purificationpurification system),system), fromfrom MBRAUNMBRAUN andand storedstored underunder argonargon overover 44 ÅÅ molecularmolecular sieves.sieves. In a typical sol–gel synthesis procedure forfor thethe preparation of a TiO2 nanopowder doped with 2 mol% Nb: 17.00 g (24.84 mmol) of [Ti(OEt)4]3 taken preparation of a TiO2 nanopowder doped with 2 mol% Nb: 17.00 g (24.84 mmol) of [Ti(OEt)4]3 taken in 15 mL of pentane were mixed with 0.47 g (0.75 mmol) of [Nb(OEt)5]2 under stirring at room in 15 mL of pentane were mixed with 0.47 g (0.75 mmol) of [Nb(OEt)5]2 under stirring at room temperature.temperature. ThisThis solutionsolution was was added added dropwise dropwise to to 150 150 mL mL of of water water with with 2.40 2.40 g (7.45g (7.45 mmol, mmol, 0.1 0.1 eq /eq/Ti)Ti) of of t(NtBu4)Br at 100 °C, and the medium was stirred for 1 h at 150 °C. The suspension was then (N Bu4)Br at 100 ◦C, and the medium was stirred for 1 h at 150 ◦C. The suspension was then centrifuged tocentrifuged yield a white to yield solid. a white The solid. as-prepared The as-prepared precipitate precipitate was washed was bywashed 3 50 by mL 3 × of50 deionizedmL of deionized water water and 50 mL of ethanol, and dried at 80 °C for 12 h. Then, to remove× all the organic compounds, and 50 mL of ethanol, and dried at 80 ◦C for 12 h. Then, to remove all the organic compounds, the the powder was calcinated at 500 °C for 4 h under air. After the calcination, the powder crystallized powder was calcinated at 500 ◦C for 4 h under air. After the calcination, the powder crystallized under under the anatase phase (JCPDS no. 00-021-1272) with a brookite fraction of 5%. The average the anatase phase (JCPDS no. 00-021-1272) with a brookite fraction of 5%. The average crystallite crystallite size, determined by Rietveld refinement, was around 10 nm. This crystallite size size, determined by Rietveld refinement, was around 10 nm. This crystallite size corresponds to the corresponds to the particle size observed by transmission electronic microscopy (TEM) (Figure 1). particle size observed by transmission electronic microscopy (TEM) (Figure1). These crystallites are These crystallites are agglomerated in aggregates smaller than 1µm, as measured by laser agglomerated in aggregates smaller than 1µm, as measured by laser granulometry. granulometry.

Figure 1. TEM image of TiO2: 2 mol% Nb nanopowder calcined at 500 ◦C for 4 h. Figure 1. TEM image of TiO2: 2 mol% Nb nanopowder calcined at 500 °C for 4 h. 2.2. SPS Densification 2.2. SPS Densification. The titanium dioxide powders, after the calcination, were sintered by using spark plasma equipmentThe titanium (SPS) HPD dioxide 25 (FCT powders, Systeme after GmbH, the Germany).calcination, The were powdered sintered samples by using were spark loaded plasma in a tungstenequipment cemented (SPS) HPD carbide 25 (FCT (WC-Co) Systeme die GmbH, (10 mm German inner diameter)y). The powdered lined with samples a graphite were foil loaded (0.35 mmin a thick).tungsten The cemented sintering carbide temperature (WC-Co was) regulateddie (10 mm by inner a K-type diameter) thermocouple lined with (1 mm a graphite diameter) foil introduced (0.35 mm inthick). the WC The die. sintering The pulse temperature patterned consisted was regulate of twod pulses by a lastingK-type 10 thermocouple ms followed by (1 one mm pause diameter) period ofintroduced 5 ms. in the WC die. The pulse patterned consisted of two pulses lasting 10 ms followed by one pauseFour period experiments of 5 ms. were performed in order to observe the effect of pressure on the sintering behaviorFour ofexperiments the anatase were powder. performed The powder in order was to first observe pressurized the effect up of to thepressure targeted on pressurethe sintering (76, 200,behavior 300 and of the 400 anatase MPa). Then,powder. the The same powder heating was cycle firs wast pressurized applied for up each to the experiment targeted (heatingpressure rate(76, 200, 300 and 400 MPa). Then, the same heating cycle was applied for each experiment (heating rate n◦1-heating rate n◦2-sintering temperature-dwell time): 100 ◦C/min up to 680 ◦C–20 ◦C/min up to 700 n°1-heating rate n°2-sintering temperature-dwell time): 100 °C/min up to 680 °C–20 °C/min up to 700 ◦C-10 min. After the dwell time, the heating power was shut off and the pressure was released. °C-10 min. After the dwell time, the heating power was shut off and the pressure was released. Subsequently, for a better understanding of phenomena, two samples were sintered under 400 MPa following the same heating rates up to a lower sintering temperature without dwell time: 450 °C and 600 °C.

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Subsequently, for a better understanding of phenomena, two samples were sintered under 400 MPa following the same heating rates up to a lower sintering temperature without dwell time: 450 ◦C and 600 ◦C. Two samples sintered at 76 MPa by heating 10 min at 850 and 960 ◦C with heating rates 10 and 100 ◦C/min, respectively, are also referred for comparison.

2.3. Characterization Techniques The density of the sintered samples was measured using Archimedes method with distilled water as a liquid medium. The values for the relative densities were calculated assuming a theoretical density of titanium dioxide doped with 2 mol% of niobium. The calculated theoretical density is 3.925 g cm 3 · − for the anatase phase and 4.267 g cm 3 for the rutile phase. X-ray diffraction (XRD) was used to · − analyse the crystallographic phase of the powder and the sintered sample. A Bruker D8 Advance A25 diffractometer (Cu kα radiation at 0.154184 nm) equipped with a Ni filter and 1-D fast multistrip detector (LynxEye, 192 channels on 2.95◦) was used. The diffractograms were collected at 2Θ with steps of 0.02◦ from 15 to 85◦ (2Θ) for a total acquisition time of 120 min. The data analyses were carried out by Rietveld refinement (Fullprof Suite package) [7] using Thompson Cox Hastings (modified pseudo-Voigt) and instrumental resolution functions. The observation of microstructures and the determination of grain size were performed using scanning electron microscopy (SEM) and image analysis software (ImageJ). The average size was obtained by measuring about 100 grains. Transmission electronic microscopy (TEM) of prepared thinned blades was performed using a MET JEOL2100.

3. Results and Discussion

3.1. Effect of Pressure on Densification and Phase Transition To study the influence of pressure on the densification and the phase transition phenomena, four TiO2:2%Nb pellets were sintered by SPS at 76, 200, 300 and 400 MPa at a constant temperature of 700 ◦C according to the thermal cycle described in the experimental part. Sintering of the pre-calcined powders resulted in pellets (density > 85%) of 10 mm diameter and 2 mm thick, with a metallic blue-black color. The characteristics of the pellets are presented in Table1. It can be observed from XRD profiles (Figure2) that the phase transition shifted to lower temperatures as the pressure increased. Indeed, at 76 MPa—700 ◦C, the pellets were mainly composed of anatase phase, while at 200 MPa—700 ◦C, they mainly consisted of rutile phase (Figure2). For pure TiO 2 nanoparticles, the anatase to rutile transformation occurred within 600–850 ◦C[8]. The transformation from anatase to rutile involved a volume decrease of 8.7%, which was promoted by the application of pressure [9]. On the other hand, according to our previous work [10], Nb doping (2% mol.) delays the transition temperature from 700 to 800 ◦C during the SPS sintering at 76 MPa.

Table 1. Characteristics of the different pellets produced in this study. The heating rate is 100 ◦C/min for all samples, except for the sample “850 ◦C–76 MPa” which was heated at 10 ◦C/min. The mean grain size and the corresponding standard deviation, D90 (90% of the particles are below this size), D50 (median) are evaluated from the SEM images. The crystallite sizes are extracted from the XRD patterns fitted using Fullprof Suite package [7].

Phases Relative Mean Standard Crystallite Size (nm) Pellets (A + R = 100%) Density Grain Size Deviation D90 (nm) D50 (nm) Anatase Rutile (%) (nm) (nm) 700 C–76 MPa 69.6% A 84.8 39 12 31 66 57 2 38 1 ◦ ± ± 700 C–200 MPa 98% R 96.2 260 53 86 319 7 258 2 ◦ ± ± 700 C–300 MPa 100% R 95.3 287 101 93 455 7 306 3 ◦ ± ± Ceramics 2020, 3 510

Table 1. Cont.

Phases Relative Mean Standard Crystallite Size (nm) Pellets (A + R = 100%) Density Grain Size Deviation D90 (nm) D50 (nm) Anatase Rutile (%) (nm) (nm) 700 C–400 MPa 100% R 96.0 322 98 62 428 7 272 2 ◦ ± ± 600 C–400 MPa 98.9% R 94.2 168 73 92 277 6 165 6 ◦ ± ± 450 C–400 MPa 95.4% A 89.7 22 5 17 28 2 22 1 ◦ ± ± 850 C–76 MPa 100% R 89.0 159 70 136 225 3 152 2 ◦ ± ± 960 C–76 MPa 100% R 96.3 252 158 92 448 4 200 4 Ceramics 2020◦ , 3 FOR PEER REVIEW ± ± 4

Figure 2. XRD patterns of the materials obtained after spark plasma sintering (SPS) cycle up to 700 ◦C Figure 2. XRD patterns of the materials obtained after spark plasma sintering (SPS) cycle up to 700 °C at the different pressures. The peaks are identified as the anatase phase (a) and the rutile phase (r) in at the different pressures. The peaks are identified as the anatase phase (a) and the rutile phase (r) in the samples. (g) Corresponding to graphite residue (originating from the graphite foil placed between thethe samples. WC mold (g) and Corresponding the sample during to graphite sintering) residue present (originating on the surfacefrom the of graphite a sample. foil placed between the WC mold and the sample during sintering) present on the surface of a sample. The SPS dilatograms for these four samples are shown in Figure3a–d. The displacement rate of theThe piston SPS dilatograms (related to thefor these sample four shrinkage samples rate)are sh showsown in that Figure for the3a–d. low The pressure displacement (76 MPa), rate theof the piston (related to the sample shrinkage rate) shows that for the low pressure (76 MPa), the densification was slow and extended over a wide temperature range (from 350 to 700 ◦C). A small part densificationof the sample was was slow transformed and extended into rutile. over a For wide an intermediatetemperature pressurerange (from (200 350 MPa), to 700 the °C). densification A small part of the sample was transformed into rutile. For an intermediate pressure (200 MPa), the started near 280 ◦C and extended up to 700 ◦C. A large part of the sample was transformed into rutile. densificationAs the pressure started increased, near 280 the first°C and stage extended of densification up to 700 took °C. place A large at a lower part temperatureof the sample and was the transformedtransformation into of rutile. the anatase As the phasepressure to theincreased, rutile phase the first was stage favored, of densification but no phase took transition place at signature a lower temperaturewas observed and in the the transformation dilatogram during of the the anatase temperature phase to rise. the rutile phase was favored, but no phase transition signature was observed in the dilatogram during the temperature rise. For the higher pressures (300 and 400 MPa), the dilatograms exhibited two stages of densification around 380 and 650 °C. For the sample sintered at 300 MPa, the maximum displacement rate of the first densification stage was observed at 385 °C. At 400 MPa, three phenomena could be distinguished. The first and the second densification stages show the maximum displacement rates at 365 and 620 °C, respectively. An interesting phenomenon can be noted for the sintering at 400 MPa. At 546 °C, we observed a sharp jump with a piston displacement of 0.15 mm which may correspond to the transition from the anatase phase (theoretical density = 3.925 g·cm−3) to the rutile phase (theoretical density = 4.267 g·cm−3)—i.e., ΔV/V = −8 %. Indeed, we placed about 1 g of the powder in the matrix; a phase change caused a variation of the volume of about 0.02 cm3. This implies a piston displacement of 0.26 mm. This variation is purely indicative because we are uncertain about the exact mass of the powder, the density of the pellets, and we did not take into account the thermal expansion of the lattice. The phase transition rate at 400 MPa was very fast and certainly occurred at a lower temperature as compared to sintering at 300 MPa where this peak was not observed. At 200 and 300 MPa, the transition phase occurred but was more gradual. In fact, the anatase–rutile transition was reconstructive, and the total transformation took place slowly between 600 and 700 °C under moderate pressure [8].

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6 3,0 5,5 1,6 76 MPa a) 200 MPa b) 1,4 5 2,5 5,0 1,2 2,0 4 4,5 1,0

1,5 0,8 3 4,0 1,0 0,6 2 3,5 0,4 0,5 Rate (mm/min) Rate (mm/min)

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0 -0,5 2,5 -0,2 100 200 300 400 500 600 700 800 100 200 300 400 500 600 700 800 Temperature (°C) Temperature (°C)

6,0 1,2 6,2 1,2 300 MPa c) 400 MPa d) 1,0 6,0 1,0 5,5 0,8 5,8 0,8

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0,2 5,2 0,2 Rate (mm/min) Rate (mm/min) Rate Displacement (mm) Displacement 4,0 (mm) Displacement 0,0 5,0 0,0

3,5 -0,2 4,8 -0,2 100 200 300 400 500 600 700 800 100 200 300 400 500 600 700 800 Temperature (°C) Temperature (°C)

Figure 3. Sintering cycles of TiO2:2%Nb nanoparticles (pre-calcined at 500 ◦C/4 h) up to 700 ◦C under Figuredifferent 3. Sintering pressures: cycles (a) 76, of ( bTiO) 200,2:2%Nb (c) 300 nanoparticles and (d) 400 MPa.(pre-calcined at 500 °C/4 h) up to 700 °C under different pressures: (a) 76, (b) 200, (c) 300 and (d) 400 MPa. For the higher pressures (300 and 400 MPa), the dilatograms exhibited two stages of densification aroundIn order 380 and to 650understand◦C. For thethe sample significant sintered piston at 300displacement MPa, the maximum rate observed displacement in Figure rate 3d, of two the sinteringfirst densification experiments stage were was observedperformed at 385at 400◦C. MPa. At 400 The MPa, first three sintering phenomena was interrupted could be distinguished. at 450 °C, beforeThe first the and sharp the peak second and densification after the first stages densificat showion the peak. maximum The second displacement sintering rates was at stopped 365 and 620at 600◦C, °Crespectively. immediately An after interesting the sharp phenomenonpeak (Figure 3d). can Th bee notedXRD patterns for the (Figure sintering 4) obtained at 400 MPa. at the At pressure 546 ◦C, ofwe 400 observed MPa could a sharp be identified jump with mainly a piston with displacementthe anatase phase of 0.15 for mm450 °C which and the may rutile correspond phase for to 600 the °C.transition This result from (Figure the anatase 4) clearly phase shows (theoretical that the density sharp =jump3.925 observed g cm 3) around to the rutile 546 °C phase was (theoretical related to · − thedensity sudden= 4.267 transformation g cm 3)—i.e., of ∆theV/ Vanatase= 8 %. to Indeed,the rutile we phase. placed In about consequence, 1 g of the the powder two densification in the matrix; · − − stagesa phase observed change causedduring athe variation sintering of at the 400 volume MPa can ofabout be attributed 0.02 cm 3to. Thisthe sintering implies a of piston the anatase displacement phase aroundof 0.26 mm.365 °C This and variation then of the is purelyrutile phase indicative around because 620 °C. we After are uncertain the first stage, about at the 450 exact °C, the mass relative of the densitypowder, of the the density pellet ofthat the mainly pellets, comprised and we did the not an takeatase into phase account was the equal thermal to 89.7%. expansion After ofthe the second lattice. densificationThe phase transition stage, at rate700 at°C, 400 the MPa relative was verydensity fast of and the certainly pellet reached occurred 96%, at aand lower the temperatureanatase phase as wascompared totally totransformed sintering at to 300 the MPa rutile where phase. this peak was not observed. At 200 and 300 MPa, the transition phase occurred but was more gradual. In fact, the anatase–rutile transition was reconstructive, and the total transformation took place slowly between 600 and 700 ◦C under moderate pressure [8]. a 450 °C r 600 °C In order to understand the significant piston displacement rate observed in Figure3d, two sintering experiments were performed at 400 MPa. The first sintering was interrupted at 450 ◦C, before the sharp Anatase peak and after the first densification peak. The second sintering was stoppedRutile at 600 ◦C immediately r after the sharp peak (Figure3d). The XRD patterns (Figure4) obtained at the pressure of 400 MPa

a could be identified mainly with the anatase phase for 450 ◦C and the rutile phase for 600 ◦C. This r result (Figure4) clearly shows that the sharp jump observed around 546 ◦C was related to the sudden a Intensity (arb. unit) Intensity (arb. unit) r transformation of the anatase to the rutile phase. In consequence,a the two densificationa stages observed during the sintering at 400 MPa can be attributed to the sintering of the anatase phase around 365 ◦C 20 30 40 50 60 70 80 20 30 40 50 60 70 80 and then of the rutile phase around 620 ◦C. After the first stage, at 450 ◦C, the relative density of the 2θ (degree) 2θ (degree) pellet that mainly comprised the anatase phase was equal to 89.7%. After the second densification

Figure 4. XRD patterns of the materials obtained after SPS cycles up to 450 and 600 °C under 400 MPa.

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6 3,0 5,5 1,6 76 MPa a) 200 MPa b) 1,4 5 2,5 5,0 1,2 2,0 4 4,5 1,0

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6,0 1,2 6,2 1,2 300 MPa c) 400 MPa d) 1,0 6,0 1,0 5,5 0,8 5,8 0,8

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3,5 -0,2 4,8 -0,2 100 200 300 400 500 600 700 800 100 200 300 400 500 600 700 800 Temperature (°C) Temperature (°C)

Figure 3. Sintering cycles of TiO2:2%Nb nanoparticles (pre-calcined at 500 °C/4 h) up to 700 °C under different pressures: (a) 76, (b) 200, (c) 300 and (d) 400 MPa.

In order to understand the significant piston displacement rate observed in Figure 3d, two sintering experiments were performed at 400 MPa. The first sintering was interrupted at 450 °C, before the sharp peak and after the first densification peak. The second sintering was stopped at 600 °C immediately after the sharp peak (Figure 3d). The XRD patterns (Figure 4) obtained at the pressure of 400 MPa could be identified mainly with the anatase phase for 450 °C and the rutile phase for 600 °C. This result (Figure 4) clearly shows that the sharp jump observed around 546 °C was related to the sudden transformation of the anatase to the rutile phase. In consequence, the two densification stages observed during the sintering at 400 MPa can be attributed to the sintering of the anatase phase aroundCeramics 3652020 ,°C3 and then of the rutile phase around 620 °C. After the first stage, at 450 °C, the relative512 density of the pellet that mainly comprised the anatase phase was equal to 89.7%. After the second densification stage, at 700 °C, the relative density of the pellet reached 96%, and the anatase phase stage, at 700 ◦C, the relative density of the pellet reached 96%, and the anatase phase was totally was totally transformed to the rutile phase. transformed to the rutile phase.

a 450 °C r 600 °C

Anatase Rutile r

a r

a Intensity (arb. unit) Intensity (arb. unit) r a a

20 30 40 50 60 70 80 20 30 40 50 60 70 80 2θ (degree) 2θ (degree)

CeramicsFigure 2020 4., XRD3 FOR patterns PEER REVIEW of the materials obtained after SPS cycles up to 450 and 600 ◦C under 400 MPa. 6 Figure 4. XRD patterns of the materials obtained after SPS cycles up to 450 and 600 °C under 400 MPa. TheThe Figure Figure5 presents 5 presents the evolutionthe evolution of the of temperature the temperature corresponding corresponding to the maximum to the maximum of the piston of the displacementpiston displacement speed, observed speed, onobserved the dilatograms, on the dilatogr as a functionams, as of a thefunction applied of pressure.the applied A decrease pressure. of A aboutdecrease 200 ◦ Cof wasabout induced 200 °C when was induced the pressure when increased the pressu fromre 76increased to 400 MPa. from The 76 to increase 400 MPa. in the The pressure increase acceleratedin the pressure the rates accelerated of the densification the rates of the and densific the phaseation transformation and the phase (Figuretransformation3) and lowered (Figure their 3) and occurrencelowered their temperature occurrence (Figure temperature5), which (Figure led to dissociate5), which theled dito ffdissociateerent densification the different steps densification (anatase thensteps rutile) (anatase and tothen distinguish rutile) and the to phase distinguish transition. the phase transition.

Figure 5. Effect of the pressure on the temperature showing the maximum densification rate. Figure 5. Effect of the pressure on the temperature showing the maximum densification rate.

At a constant sintering temperature (here 700 °C), as the sintering pressure increases, the anatase phase progressively transforms to the rutile phase (Figure 2). After sintering at 76 MPa, two phases coexisted, and a density of 3.42 g·cm−3 was measured. After sintering at 200 MPa, a small anatase peak could still be seen, but the pellet was mostly rutile with a density of 4.10 g·cm−3. Above this pressure, the densities no longer change and the diffractograms showed a single rutile phase. This result confirms that with increasing pressure, the phase transition is shifted to lower temperatures, as already reported by Liao et al. [6]. They observed by hot pressing that the transformation is lowered to 400 °C under a pressure equal to 1.5 GPa. It can be noticed that Maglia et al. [5] obtained different results by high pressure field assisted rapid sintering of the anatase phase using a different sintering cycle than ours.

3.2. Effect of Pressure on Grain Size In order to measure the grain size, SEM observation was performed on the pellets sintered at 700 °C with a heating rate of 100 °C/min and several pressures of 76, 200, 300 and 400 MPa. The images are shown in Figure 6. The pellet sintered at 76 MPa had a very fine microstructure with a mean grain size of 39 nm. Coarsening of grains was obvious in sintered samples under the higher pressures, leading to a mean grain size higher than 260 nm. Figure 7 shows the grain size distributions (GSD) extracted from the analysis of the SEM images (Figure 6). As the pressure increased, the GSD broadened and deviated increasingly from a Gaussian shape. This suggests that abnormal grain growth (AGG) occurs when pressure is equal to or higher than 300 MPa. AGG is known to lead to GSD broadening, which can ultimately produce a bimodal distribution [11]. In contrast, for samples

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At a constant sintering temperature (here 700 ◦C), as the sintering pressure increases, the anatase phase progressively transforms to the rutile phase (Figure2). After sintering at 76 MPa, two phases coexisted, and a density of 3.42 g cm 3 was measured. After sintering at 200 MPa, a small anatase peak · − could still be seen, but the pellet was mostly rutile with a density of 4.10 g cm 3. Above this pressure, · − the densities no longer change and the diffractograms showed a single rutile phase. This result confirms that with increasing pressure, the phase transition is shifted to lower temperatures, as already reported by Liao et al. [6]. They observed by hot pressing that the transformation is lowered to 400 ◦C under a pressure equal to 1.5 GPa. It can be noticed that Maglia et al. [5] obtained different results by high pressure field assisted rapid sintering of the anatase phase using a different sintering cycle than ours.

3.2. Effect of Pressure on Grain Size In order to measure the grain size, SEM observation was performed on the pellets sintered at 700 ◦C with a heating rate of 100 ◦C/min and several pressures of 76, 200, 300 and 400 MPa. The images are shown in Figure6. The pellet sintered at 76 MPa had a very fine microstructure with a mean grain size of 39 nm. Coarsening of grains was obvious in sintered samples under the higher pressures, leading to a mean grain size higher than 260 nm. Figure7 shows the grain size distributions (GSD) extracted from the analysis of the SEM images (Figure6). As the pressure increased, the GSD broadened and deviated increasingly from a Gaussian shape. This suggests that abnormal grain growth (AGG) occurs when pressure is equal to or higher than 300 MPa. AGG is known to lead to GSD broadening, Ceramics 2020, 3 FOR PEER REVIEW 7 which can ultimately produce a bimodal distribution [11]. In contrast, for samples sintered at 76 and 200 MPa, the GSD shape suggests a normal grain growth (NGG). AGG may occur only at the final sintered at 76 and 200 MPa, the GSD shape suggests a normal grain growth (NGG). AGG may occur sintering stage when the relative density has reached at least 95% [12]. only at the final sintering stage when the relative density has reached at least 95% [12].

a) b)

200 nm 200 nm c) d)

200 nm 200 nm

Figure 6. SEM images of the sintered pellets at 700 ◦C with the different pressures of (a) 76, (b) 200, Figure( c6.) 300SEM and images (d) 400 MPa.of the sintered pellets at 700 °C with the different pressures of (a) 76, (b) 200, (c) 300 and (d) 400 MPa.

Figure 7. Grain size distributions of the samples sintered at 700 °C for 10 min under different pressures. Grain size is the grain diameter measured on the SEM images (50 to 175 values per sample). The data are discretized in twelve 50 nm width intervals. For the clarity of the graphic representation, each interval is represented by its center value. The average grain size, ܦഥ, and the standard deviation, SD, are reported for each sample.

The grain size and relative density of the pellets sintered at 700 °Care plotted in Figure 8 as a function of pressure. It shows that for the same density, the grain size increases with pressure. To

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0.2

0.1

Figure 7. Grain size distributions of the samples sintered at 700 ◦C for 10 min under different pressures. Grain size is the grain diameter measured on the SEM images (50 to 175 values per sample). The data are discretized in twelve 50 nm width intervals. For the clarity of the graphic representation, each interval is represented by its center value. The average grain size, D, and the standard deviation, SD, are reported for each sample.

The grain size and relative density of the pellets sintered at 700 ◦Care plotted in Figure8 as a function of pressure. It shows that for the same density, the grain size increases with pressure. To compare with the density and the microstructure of a pellet sintered at a higher temperature (960 ◦C) but at the lower pressure (76 MPa), the density and grain size values are added in the graph. It can be noticed that the relative density (96%) and the mean grain size of this pellet were similar to those of the pellet synthesized at 700 ◦C under 200 MPa. Under the higher pressures (300 and 400 MPa), the sintering at 700 ◦C did not enhance the densification but led to AGG. These results confirm that AGG starts at the final sintering stage, when the relative density has reached about 95%. Figure9 shows the evolution of grain size during the SPS at an equal pressure of 400 MPa, between 450 and 600 ◦C. With a temperature difference of 150 ◦C, the grain size increased from 22 to 168 nm. It seems that the phase transition led to an increase in grain size.

1

Ceramics 2020, 3 FOR PEER REVIEW 8

Ceramicscompare 2020 with, 3 FOR the PEER density REVIEW and the microstructure of a pellet sintered at a higher temperature (960 °C) 8 but at the lower pressure (76 MPa), the density and grain size values are added in the graph. It can comparebe noticed with that the the density relative and density the microstructure(96%) and the me ofan a pelletgrain size sintered of this at pe a llethigher were temperature similar to those (960 °C) butof the at thepellet lower synthesized pressure at (76700 MPa),°C under the 200 density MPa. Underand grain the highersize values pressures are added(300 and in 400 the MPa), graph. the It can besintering noticed at that 700 the°C did relative not enhance density the (96%) densificatio and then mebutan led grain to AGG. size These of this results pellet confirm were similar that AGG to those ofstarts the pelletat the synthesizedfinal sintering at stage, 700 °C when under the 200 relative MPa. density Under has the reachedhigher pressuresabout 95%. (300 and 400 MPa), the sintering at 700 °C did not enhance the densification but led to AGG. These results confirm that AGG Ceramics 2020, 3 515 starts at the final sintering stage, when the relative density has reached about 95%.

Figure 8. Evolution of grain size and relative density as a function of pressure for a thermal cycle up to 700 °C.

Figure 9 shows the evolution of grain size during the SPS at an equal pressure of 400 MPa, betweenFigure 450 8. andEvolution 600 °C. of With grain a sizesize temperature andand relativerelative difference densitydensity as asof a a150 function function °C, the of of pressuregrain pressure size for forincreased a thermala thermal from cycle cycle 22 up upto to 168 nm.to700 700 ◦ItC. °Cseems. that the phase transition led to an increase in grain size.

Figure 9 shows the evolution of grain size during the SPS at an equal pressure of 400 MPa, between 450 and 600 °C. With a temperature difference of 150 °C, the grain size increased from 22 to 450°C 600°C 168 nm. It seems that the phase transition led to an increase in grain size.

450°C 600°C

200 nm Figure 9. SEM images of niobium-doped TiO after SPS at 400 MPa in a WC matrix. The influence of Figure 9. SEM images of niobium-doped TiO2 after2 SPS at 400 MPa in a WC matrix. The influence of phasephase transitiontransition on grain grain size size is is clearly clearly visible visible in inthe the SEM SEM images images..

This effect of the phase transition can be observed in Figure 10, representing the evolution of This effect of the phase transition can be observ ed 200in nFigurem 10, representing the evolution of graingrain sizesize asas aa function of of density density for for the the isobaric isobaric sintering sintering cycles: cycles: 76 and 76 and 400 400MPa. MPa. We have We have shown shown

(Figure Figure4) that 9. SEM at 400 images MPa, of the niobium-doped transition occurred TiO2 after around SPS at 400 500 MPa◦C. in Three a WC pellets matrix. were The influence sintered of under 76 MPaphase at transition three diff onerent grain sintering size is clearly temperatures visible in the (700, SEM 850 images and. 960 ◦C) (Table1). XRD analysis was performed on each pellet, and it appears that transition phase occurred around 700 ◦C. At 850 ◦C, the pelletThis consisted effect onlyof the of phase the rutile transition phase. Itcan appears be observ thated a significantin Figure grain10, representing growth occurred the evolution around the of phasegrain size transition as a function temperature of density range. for the isobaric sintering cycles: 76 and 400 MPa. We have shown

Ceramics 2020, 3 FOR PEER REVIEW 9

(Figure 4) that at 400 MPa, the transition occurred around 500 °C. Three pellets were sintered under 76 MPa at three different sintering temperatures (700, 850 and 960 °C) (Table 1). XRD analysis was performed on each pellet, and it appears that transition phase occurred around 700 °C. At 850 °C, the Ceramicspellet consist2020, 3 ed only of the rutile phase. It appears that a significant grain growth occurred around516 the phase transition temperature range.

FigureFigure 10.10. EvolutionEvolution ofof thethe graingrain sizesize andand thethe densitydensity asas aa functionfunction ofof thethe temperaturetemperature atat 22 constantconstant pressures:pressures:76 76 (∆ () and) and 400 MPa 400 MPa( ). Open ( ). Open and closed and symbols closed symbols indicate if indicate the sample if the is mainly sample crystallized is mainly incrystallized the anatase in phase the an oratase the phase rutile# phase,or the rutile respectively. phase, respectively.

HighHigh appliedapplied pressurepressure causescauses strongstrong constraintsconstraints onon thethe grainsgrains andand thusthus aa higherhigher densitydensity ofof thethe initialinitial compacts.compacts. As thethe temperaturetemperature increases,increases, thermalthermal energyenergy drivesdrives thethe phasephase transition.transition. ItIt isis clearclear thatthat highly constrained grains grains lead lead to to a atr transitionansition at at lower lower temperature temperature.. AsAs soon soon as the as thetransition transition has hasoccurred occurred and and the the densification densification is is partially partially achieved, achieved, grain grain growth growth accelerates accelerates regardless regardless of thethe pressure.pressure. Pellets Pellets sintered sintered in in the the rutile rutile stability stability region, region, at at400 400 MPa MPa and and 600 600 °C ◦andC and at 76 at MPa 76 MPa and and 850 850°C had◦C hadthe same the same mean mean grain grain size sizeof 160 of nm 160 and nm the and same the same relative relative density density (94%). (94%). However However,, at lower at lowertemperature, temperature, in the inanatase the anatase stability stability region, region, a pressure a pressure as high as as high 400 MPa as 400 could MPa densify could densifythe anatase the anataseup to 90% up with to 90% very with a limited very grain a limited growth grain (22 growth nm). These (22 nm).observations These observations suggest that suggestthe grain that growth the grainis not growthdriven isby not the driven same phenomena by the same at phenomena 76 and 400at MPa 76 and. At 76 400 MPa, MPa. grain At 76 growth MPa, grainwas governed growth was by governeddiffusion phenomena. by diffusion phenomena.At 400 MPa, it At was 400 managed MPa, it was by the managed grain boundary by the grain sliding boundary and grain sliding rotation and graingenerated rotation by generated mechanical by stress. mechanical For pellets stress. with For pellets a density with lower a density than lower 95%, than sintering 95%, atsintering 400 MPa at 400allow MPaed allowedcompacts compacts with finer with microstructures finer microstructures to be obtained to be obtained.. The gain The in gain density in density is due is to due the to very the verydense dense grain grain packing packing obtained obtained by by the the high high initial initial pressure. pressure. They They were were a arrangedrranged as as compactly as as possible.possible. OnceOnce the 95%95% relativerelative densitydensity waswas overcome,overcome, itit waswas observedobserved thatthat thethe compactcompact sinteredsintered atat 400400 MPaMPa had had a a coarser coarser microstructure microstructure for for the the same same density density as compact as compact sintered sinter ated 76 at MPa 76 dueMPa to due AGG. to BelowAGG. 95%Below densification, 95% densification, grain coarsening grain co appearedarsening toappear be a directed to consequence be a direct of conseque phase transformationnce of phase viatransformation nucleation and via fast nucleation grain growth. and The fast transition grain growth. to the rutile The phase transition was accompanied to the rutile by phase significant was grainaccompanied growth resulting by significant in large grain rutile growth grains resulting and small in anatase large rutile ones (Figure grains and12b). small The hypothesis anatase one ofs rutile(Figure nucleation 12b). The at hypothesis the boundary of betweenrutile nucleation anatase particles at the boundary can explain between the formation anatase of particles large rutile can grainsexplain by the the formation coalescence of oflarge two rutile or more grain anatases by the grains coalescence [8]. In Figure of two 11 or, we more report anatase the grain grains size [8] as. I an functionFigure 1 of1, thewe relativereport the density grain for size the as eight a function samples. of Thethe relativerelative densitydensity of for the the powder eight compactedsamples. The at 1000relative MPa density at room of temperaturethe powder iscompact also reported.ed at 1000 This MP wasa at measuredroom temperature in compression is also usingreported. a Zwick This testing machine. Relative density for the compact at 76 and 200, 300, 400 MPa, not reported on the graph, were equal to 29, 35, 39 and 43%, respectively. Ceramics 2020, 3 FOR PEER REVIEW 10

Ceramics 2020, 3 517 was measured in compression using a Zwick testing machine. Relative density for the compact at 76 and 200, 300, 400 MPa, not reported on the graph, were equal to 29, 35, 39 and 43%, respectively.

Figure 11. Grain size and crystallite size versus relative density for all samples.

It isis clearclear fromfrom FigureFigure 11 11 that that the the phase phase transition transition induced induced a jumpa jump in grainin grain size size from from 30 to30 170 to 170 nm regardlessnm regardless of the of appliedthe applied pressure. pressure. Then, Then, grain gr growthain growth accelerated accelerated when when the densitythe density exceeded exceeded 95% independently95% independently of the of process the process pressure. pressure. The higher The higher grain grain growth growth rate observed rate observed for rutile for grainsrutile grains is not inherentis not inherent in the naturein the nature of the phase.of the phase. In fact, In the fact, activation the activation energy energy for grain for growth grain growth is known is known to be five to timesbe five smaller times smaller for anatase for anatase nanoparticles nanoparticles than for than rutile for nanoparticles rutile nanoparticles [13]. The [13]. observed The observed grain growth grain accelerationgrowth acceleration appears appears to be related to be torelated the relative to the density,relative asdensity, shown as by shown Bernard-Granger’s by Bernard-Granger’s study on sparkstudy plasmaon spark and plasma hot-pressure and hot-pressure sintering sintering of zirconia of [zirconia14]. This [14]. could This be could related be to related the pore to the pinning pore epinningffect [15 ].effect In fact, [15]. as In long fact, as theas long sample as containsthe sample dispersed contains pores, dispersed they pin pores, the grain they boundaries pin the grain and preventboundaries their and migration, prevent reducingtheir migration, the rate reducing of the grain the growth.rate of the When grain the growth. porosity When decreases the porosity to less thandecreases 5%, the to less pinning than of5%, the the pores pinning decreases, of the pores which decreases, dramatically which accelerates dramatically the grain accelerates growth. the grain growth.As mentioned above, high pressure sintering increased the densification rate but also the grain growthAs mentioned rate at the above, end of high the sinteringpressure sintering cycle, inducing increased an the AGG. densification This acceleration rate but isalso induced the grain by graingrowth boundary rate at the sliding end of and the sintering grain rotation. cycle, induci This stress-enhancedng an AGG. This dynamic acceleration grain is growthinduced hasby grain been demonstratedboundary sliding in the and work grain of Barak rotation. Ratzker This et al. st [ress-enhanced16] on the sintering dynamic of transparent grain growth alumina. has Sliding been anddemonstrated rotation can in leadthe work to grain of Barak coalescence Ratzker [17 et] foral. high[16] on temperature the sintering deformation of transparent of dense, alumina. fine-grained Sliding materials.and rotation The can rotation lead ofto onegrain grain coalescence relative to [17] the for other high occurs temperature by sliding, deformation so that the crystallographic of dense, fine- atomicgrained planes materials. of the The two rotation neighboring of one grains grain become relative identical. to the other The grain occurs boundary by sliding, is erased, so that and the a singlecrystallographic grain is formed. atomic This planes mechanism of the two contributes neighboring to enhanced grains become grain growthidentical. during The grain sintering boundary at low temperaturesis erased, and a under single high grain pressure. is formed. Large This grains mechanism (400 nm) contributes with angular to enhanced grain-boundary grain growth curvatures during weresintering observed at low by temperatures TEM in the sample under sintered high pres at 700sure.◦C Large under grains 400 MPa (400 (Figure nm) with12a). Theseangular unstable grain- grainsboundary may curvatures be due to were the slow observed diffusion by TEM rates in observedthe sample at sintered relatively at low700 °C temperatures. under 400 MPa As (Figure shown in12a). Figure These 10 ,unstable at 400 MPa, grains AGG may occurred be due into athe temperature slow diffusion range rates of 500 observed to 700 ◦ C,at whichrelatively are low temperaturestemperatures. toAs activate shown diinff Figureusion phenomena.10, at 400 MPa, AGG occurred in a temperature range of 500 to 700 °C, which are low temperatures to activate diffusion phenomena.

Ceramics 2020, 3 518 Ceramics 2020, 3 FOR PEER REVIEW 11

FigureFigure 12.12. TEM images of samples sintered atat 700700 ◦°CC underunder 400 MPa. The black arrow shows fractured grainsgrains ((aa)) andand 7676 MPaMPa ((b);); grain comprised ofof twinned crystals,crystals, observedobserved inin thethe sample sinteredsintered underunder 400400 MPa.MPa. The white arrow shows an intragranular dislocationdislocation fold originating from a triple point (c); highhigh resolutionresolution TEMTEM imageimage ofof thethe twintwin boundaryboundary ((dd).).

AA moremore accurateaccurate observationobservation showedshowed thatthat thethe majoritymajority ofof thethe grains consistedconsisted ofof twinned crystals (Figure(Figure 1212c,d).c,d). SomeSome ofof themthem werewere fracturedfractured alongalong thethe twintwin boundariesboundaries (Figure(Figure 1212a).a). TheThe twinningtwinning shouldshould bebe duedue toto anatase–rutile anatase–rutile transformation, transformation, but bu itt it was was clearly clearly enhanced enhanced by by increasing increasing pressure. pressure. In fact,In fact, a high a high stress stress is known is known to promote to promote twinning. twinni Inng. addition, In addition, the higher the pressurehigher pressure (400 MPa) (400 applied MPa) duringapplied sintering during sintering induced aninduced abrupt an phase abrupt transition phase withtransition volume with reduction volume of reduction 8%, which of generated 8%, which a stronggenerated additional a strong stress additional leading to twinningstress leading and fracture to twinning of the rutile and grains.fracture Intragranular of the rutile dislocation grains. foldsIntragranular from triple dislocation points are folds also from visible triple in this points sample are (Figurealso visible 12c). in These this sample kind of (Figure defects 12c). have These been reported on fine-grained Y2O3-tetragonal zirconia ceramic by Bernard-Granger et al. [18]. They explain kind of defects have been reported on fine-grained Y2O3-tetragonal zirconia ceramic by Bernard- thisGranger microstructure et al. [18]. asThey the resultexplain of th grainis microstructure boundary sliding as the during result high of grain temperature boundary creep sliding deformation during underhigh temperature 100 MPa. creep deformation under 100 MPa. InIn ourour case, case, the the twinning twinning and and fracture fracture of grains of grai werens consistentwere consistent with the with evolution the evolution of the crystallite of the sizecrystallite extracted size from extracted the X-ray from di fftheractogram X-ray diffract analysisogram (Table analysis1 and Figure (Table 11 1). and Above Figure a relative 11). Above density a ofrelative 95%, thedensity crystallite of 95%, size the was crys smallertallite size than was the smaller grainsize than observed the grain by size SEM. observed This di byfference SEM. wasThis maximumdifference forwas the maximum sample sinteredfor the sample at 400 MPa–700sintered at◦C, 400 which MPa–700 underwent °C, which colossal underwent stress duecolossal to the stress fast phasedue to transition the fast phase combined transition with the combined high applied with pressure.the high Theapplied stress pressure. generated The can stress induce generated dislocations can andinduce even dislocations rupture of largeand even rutile rupture grains. of These large results rutile aregrains. consistent These withresults those are reportedconsistent by with Maglia those et al.reported [5], who by observed Maglia et a decreaseal. [5], who in theobserved rutile crystallite a decrease size in withthe rutile increasing crystallite sintering size temperature.with increasing 4.sintering Conclusions temperature.

4. ConclusionsThe anatase to rutile transition temperature decreased from 700 to 550 ◦C when the applied pressure varies from 76 to 400 MPa. However, the densification process occurred at a lower temperature The anatase to rutile transition temperature decreased from 700 to 550 °C when the applied under high pressure. Therefore, using high pressure SPS, we were able to densify the anatase powder: pressure varies from 76 to 400 MPa. However, the densification process occurred at a lower temperature under high pressure. Therefore, using high pressure SPS, we were able to densify the anatase powder: (i) At a sintering temperature as low as 450 °C with high applied pressure (400 MPa),

Ceramics 2020, 3 519

(i) At a sintering temperature as low as 450 ◦C with high applied pressure (400 MPa), the anatase phase is retained with a very fine grain size ( 20 nm). A relative density up to 90% was achieved. ≈ (ii) At a higher sintering temperature, a phase transition from anatase to rutile concomitant with a fast grain growth was observed. A twinning of the rutile grains was induced by the phase transition and enhanced by pressure. In general, to limit grain growth, the sintering temperature must be lowered when the high pressure process is used. Low-temperature sintering under high pressure is particularly interesting to sinter temperature-sensitive materials or metastable phases.

Author Contributions: Conceptualization, G.F. and S.L.F.; Formal analysis, S.C., T.G. and S.L.F.; Funding acquisition, S.D. and S.P.; Investigation, A.V., S.C., S.M. and N.B.; Project administration, S.P. and S.D.; Supervision, G.F.; Validation, S.P. and S.D.; Writing—original draft, S.C. and S.L.F.; Writing—review and editing, G.F. and S.L.F. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by the LABEX iMUST (ANR-10-LABX-0064) of Université de Lyon, within the program “Investissements d’Avenir” (ANR-11-IDEX-0007) operated by the French National Research Agency (ANR). Acknowledgments: The authors would particularly like to thank three people from the MATEIS laboratory: Bérangère Lesaint for her help in the preparation of thin foils for the TEM analysis, Laurent Gremillard and Sandrine Cardinal for their support in Rietveld analysis. Conflicts of Interest: The authors declare no conflict of interest.

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