Structure and Properties of Piezoelectric Strontium Fresnoite Glass-Ceramics Belonging to the Sr–Ti–Si–Al–K–O System

Article Structure and Properties of Piezoelectric Strontium Fresnoite Glass-Ceramics Belonging to the Sr–Ti–Si–Al–K–O System

Marie-Sophie Renoirt, Nathalie Maury, Florian Dupla and Maurice Gonon *

University of Mons, Materials Institute, Rue de l’Epargne 56, 7000 Mons, Belgium; [email protected] (M.-S.R.); [email protected] (N.M.); ﬂ[email protected] (F.D.) * Correspondence: [email protected]; Tel.: +32-6537-4422

Received: 3 December 2018; Accepted: 23 January 2019; Published: 26 January 2019

Abstract: Crystallization of strontium fresnoite Sr2TiSi2O8 piezoelectric crystals in Sr–Ti–Si–K–Al–O parent glasses is investigated with the aim of showing the influence of composition and crystallization conditions on the microstructure and piezoelectric properties of the resulting glass-ceramic. All the investigated conditions lead to a surface crystallization mechanism that induces a preferential orientation of crystal growth in the glasses. Near the surface, all the glass-ceramics obtained exhibit (002) planes preferentially oriented parallel to their faces. Deeper in the specimens, this preferential orientation is either kept or tilted to (201) after a depth of about 300 µm. The measurement of the charge coefficient d33 of the glass-ceramic highlights that surface crystallization induces mirror symmetry in the polarization. It reaches 11 to 12 pC/N and is not significantly influenced by the preferential orientation (002) or (201). High temperature XRD shows the stability of the fresnoite phase in the glass-ceramics up to 1000 ◦C. Mechanical characterization of the glass-ceramics by impulse excitation technique (IET) highlights that the softening of the residual glass leads to a progressive decrease of Young’s modulus in the temperature range 600–800 ◦C. Damping associated to the viscoplastic transition become severe only over 800 ◦C.

Keywords: piezoelectric; fresnoite; glass-ceramic; surface crystallization; high temperature

1. Introduction Most piezoelectric sensors and actuators used today are based on ferroelectric polycrystalline ceramics. Due to initially randomly distributed polar domains within the ceramic grains, polarization under a high-strength electric ﬁeld is required to confer macroscopic piezoelectric properties. These ceramics exhibit high piezoelectric performances, but their main drawback is the depolarization occurring over time and with increasing temperature. For high temperature applications, pyroelectric non-ferroelectric phases can be used. The non-ferroelectric behavior is the result of the single domain conﬁguration of the crystals. Consequently, in the case of polycrystalline ceramics, a preferential orientation of the crystallite’s polar direction needs to be induced during the elaboration process to obtain a macroscopic piezoelectric material. Highly preferentially oriented microstructures are not easily achievable through conventional ceramic powder processing, so these phases are usually obtained in single crystals. In the present paper, the glass-ceramic process is investigated as an alternative route to produce highly textured polycrystalline non-ferroelectric piezoelectric materials by controlling the crystallization of a piezoelectric phase within a suitable glass composition. The piezoelectric phase selected in this work is strontium fresnoite Sr2TiSi2O8. This phase is pyroelectric but

Ceramics 2019, 2, 86–97; doi:10.3390/ceramics2010008 www.mdpi.com/journal/ceramics Ceramics 20192018, 21, x FOR PEER REVIEW 2 of 1387

[1–3], belongs to the space group P4bm, and its lattice parameters are a = 8.3218 Å and c = 5.0292 Å [4].non-ferroelectric The dipole moment [1–3], belongsof the unit to thecell spaceis oriented group along P4bm, the and [00l] its direction lattice parameters [5]. are a = 8.3218 Å and cThe = 5.0292 scientific Å [4 ].literature The dipole highlights moment that of the variou units cell processing is oriented routes along have the [00l]been direction experimented [5]. to induceThe a scientificpreferentially literature oriented highlights crystallization that various of fresnoite processing crystals routes in have a parent been experimented glass [6–7]: (i) to applicationinduce a preferentially of a thermal oriented gradient crystallization between the of two fresnoite opposite crystals faces inof a the parent glass glass [8], [(ii)6,7 electrochemical]: (i) application nucleationof a thermal of gradient the glass between melt [5,9–11], the two (iii) opposite and ul facestrasonic of thetreatment glass [8 [12].], (ii) Ho electrochemicalwever, these techniques nucleation areof the not glasseasy to melt operate [5,9– 11and], (iii)limit and the specimens' ultrasonic treatmentgeometries [ 12[13].]. However,An extensive these review techniques by Wisniewski are not eteasy al. tocovered operate different and limit aspects the specimens’ of the crystallization geometries of [13 fresnoite]. An extensive in glasses review [14]. byIn previous Wisniewski papers, et al. wecovered have differentshown that aspects a (002)-oriented of the crystallization surface crysta of fresnoitellization in glassescan be obtained [14]. In previous through papers, laser treatment we have [15]shown and that conventional a (002)-oriented thermal surface treatment crystallization of parent canglasses be obtained belonging through to the Sr–Ti–Si–K–B–O laser treatment [ 15system] and [16–18].conventional We also thermal highlighted treatment that of the parent evolution glasses of belonging the preferential to the Sr–Ti–Si–K–B–Oorientation in depth system is strongly [16–18]. Wedependent also highlighted on the residual that the glass evolution characteristics, of the preferential and specifically orientation the in amount depth is of strongly K and dependentB. However, on masteringthe residual the glass K and characteristics, B quantities and was specifically difficult due the amountto the formation of K and B.of However,volatile KBO mastering2 during the the K meltingand B quantities process. was difficult due to the formation of volatile KBO2 during the melting process. To overcome this problem, fresnoite-based glass-ceramicsglass-ceramics have been prepared from parent glass compositions belonging to the Sr–Ti–Si–K–Al–O system.system. The The goal goal of of K K is to increase the thermal expansion of the residual glass and then to reduce thethe dilatometricdilatometric mismatch. The The effect of the Al amount onon thethe crystallizationcrystallization and and on on the the microstructure microstructure is investigatedis investigated with with the aimthe aim of obtaining of obtaining high preferentialhigh preferential orientation orientation of the of crystallites the crystallites polar direction.polar direction. The piezoelectric The piezoelectric properties properties of the selected of the selectedsamples aresamples characterized. are characterized. Eventually, Eventually, high temperature high temperature stability of stability the fresnoite of the crystals fresnoite and crystals of the andglass-ceramics of the glass-ceramics mechanical mechan propertiesical properties is evaluated. is evaluated.

2. Materials and Methods Parent glasses of compositions 2SrO 1TiO 3.3SiO xK O yAl O (ST1.3S + xK O + yAl O with Parent glasses of compositions 2SrO 1TiO2 2 3.3SiO22 xK22O yAl22O33 (ST1.3S + xK22O + yAl22O3 with the new nomenclature [14]) [14]) are prepared from reagent grade SrCO3 (Alfa(Alfa Aesar, Aesar, 99.99%), 99.99%), TiO TiO2 (VWR,(VWR, 99.99%), SiO2 (Sigrano, 97–99.98%),97–99.98%), AlAl22O3 (Almatis, +99%) and KK2CO33 (VWR,(VWR, +99%). Based Based on on a previous work [16], [16], x is is fixed ﬁxed to to 0.2 0.2 and and y y = = 0 0;; 0.02 0.02; ; 0.05 0.05; ; 0.10 0.10; ; and 0.15. Wet Wet processing in isopropanol is used to mixmix thethe powderspowders andand obtainobtain aa goodgood homogeneity.homogeneity. After completecomplete evaporation of the isopropanol, the dried powderpowder mixturemixture isis pressedpressed inin pellets.pellets. MeltingMelting isis realizedrealized inin aa Pt/Au Pt/Au 95/595/5 ◦ crucible atat 15001500 °CC forfor 22 hours.hours. TheThe melt melt is is cast cast in in 70 70× ×70 70× × 66 mmmm platesplates in in a a steel steel mold mold (Figure (Figures1a,b). 1a ◦ Toand release 1b). To internal release stressesinternal andstresses avoid and cracks, avoid these cracks, plates these are plates annealed are annealed for 2 hours for at 2 650hoursC at (close 650 °C to (closethe glass to the transition glass transition temperature temperature Tg) before T slowg) before cooling slow inside cooling the inside turned the off turned furnace. off Each furnace. glass plateEach glassis surface plate polished is surface and polished cut in 30and× cut30 ×in6 30 mm × 30 parallelepipeds × 6 mm parallelepipeds (Figure1c). (Figure 1c).

FigureFigure 1. 1. CastingCasting of of the the parent parent glass glass (a; (a, bb);); specimen beforebefore ((c)c) and after ((d)d) crystallization treatment. The samples are placed on an alumina-silica substrate and crystallized (Figure1d) through The samples are placed on an alumina-silica substrate and crystallized (Figure◦ 1d) through isothermal treatment at different temperatures, Tc, in the range of 850–950 C according to the isothermal treatment at different temperatures, Tc, in the range of 850–950 °C according to the following schedule: following schedule:

Ceramics 2019, 2 88

◦ • 300 C/h from RT to Tc • CeramicsDwell 2018 time, 1, x ofFOR tc PEERhours REVIEW at Tc 3 of 13 • CeramicsNatural 2018 cooling, 1, x FOR inPEER switched REVIEW off furnace 3 of 13 • 300 °C/h from RT to Tc • After the crystallization• Dwell300 °C/h time from treatment,of t RTc hours to T cat qualitative Tc crystalline phase analyses are carried out by XRD • with a Siemens• D5000 NaturalDwellθ-2 time θcoolingdiffractometer of tc inhours switched at T usingc off furnace a CoKα radiation source. Preferential orientation of • the c-polarAfter axis theof crystallization fresnoiteNatural cooling crystals treatment, in isswitched characterized qualitative off furnace cr byystalline calculating phase the analyses ratio R are(002) carried: out by XRD with aAfter Siemens the crystallization D5000 θ-2θ diffractometer treatment, qualitative using a CoKcrystallineα radiation phase source. analyses Preferential are carried orientation out by XRD of thewith c-polar a Siemens axis of D5000 fresnoite θ-2θ crystals diffractometer Ris(002) characterized= using I(002) /[Ia CoK (002)by calculatingα +radiation I(211)] source.the ratio Preferential R(002): orientation of (1) the c-polar axis of fresnoite crystals is characterized by calculating the ratio R(002): R(002) = I(002)/[I(002) + I(211)] (1) I(002) and I(211) are the diffraction peak intensities of the (002) and (211) planes respectively. R(002) = I(002)/[I(002) + I(211)] (1) AccordingI(002) toand the I(211) Sr 2areTiSi the2O 8diffractionJCPDS card peak (no. intensities 39-0228), of (211)the (002) planes andgive (211) the planes strongest respectively. peak for a non-texturedAccordingI(002) andto the fresnoite I(211) Sr 2areTiSi 2 andtheO8 JCPDSdiffraction (002) diffractioncard peak(no. 39-0228),intensities peak has (211) of a theplanes relative (002) give intensityand the (211) strongest ofplanes 22%. peak respectively. Consequently, for a non- preferentialtexturedAccording fresnoite orientation to the Sr and2TiSi of 2 (002)O(002)8 JCPDS planediffraction card parallel (no. peak 39-0228), to thehas analyzed (211)a relative planes surface intensitygive isthe evidenced strongest of 22%. peak ifConsequently,0.18 for < a R non-(002) < 1. Inpreferential ordertextured to followfresnoite orientation changes and of (002) (002) in preferential diffractionplane parallel orientationpeak to thehas analyzed a with relative depth, surface intensity XRD is evidenced analysesof 22%. if are Consequently,0.18 performed < R(002) < 1. at eachInpreferential order 100 µ mto follow step orientation of changes a step-by-step of in(002) preferential plane grinding parallel orientat (Figure to ionthe2 withanalyzed). For depth, some surface XRD samples, analysesis evidenced pole are ﬁgures performedif 0.18 are< R(002) collected at each< 1. using100In order theµm samestep to follow of XRD a step-by-step changes diffractometer. in preferential grinding Note (Figure orientat that the2).ion For signiﬁcant with some depth, samples, information XRD poleanalyses figures depth are performedare of thecollected X-rays at eachusing in the glass-ceramicthe100 same µm step XRD is of aboutdiffractometer. a step-by-step 10 µm, as grindingNote calculated that (Figure the by signific the 2). AbsorbForant some information program samples, fromdepthpole figures theof the Siemens–Bruker areX-rays collected in the using glass- Diffrac the same XRD diffractometer. Note that the significant information depth of the X-rays in the glass- ATceramic software. is about 10 µm, as calculated by the Absorb program from the Siemens–Bruker Diffrac AT software.ceramic is about 10 µm, as calculated by the Absorb program from the Siemens–Bruker Diffrac AT software.

FigureFigure 2. 2.XRD XRD and andpiezoelectric piezoelectric characterization of of the the specimens. specimens. Figure 2. XRD and piezoelectric characterization of the specimens. HighHigh temperature temperature stability stability of of the the fresnoite fresnoite crystals crystals in inthe the glass-ceramicsglass-ceramics is investigated by by using using a High temperature stability of the fresnoite crystals in the glass-ceramics is investigated by using Brukera Bruker D8 DISCOVERD8 DISCOVERθ-θ θdiffractometer-θ diffractometer equipped equipped with with a high-temperaturea high-temperature chamber. chamber. a Bruker D8 DISCOVER θ-θ diffractometer equipped with a high-temperature chamber. MicrostructuresMicrostructures are are observed observed by by means means ofof aa JEOLJEOL GSM 6100 SEM. SEM. Microstructures are observed by means of a JEOL GSM 6100 SEM. GlassGlass transition transition temperatures temperatures (Tg (T) ofg) parentof parent glasses glasses and and residual residu glassesal glasses in the in ﬁnal the glass-ceramicsfinal glass- Glass transition temperatures (Tg) of parent glasses and residual glasses in the final glass- areceramics obtained are by obtained dilatometry by dilatometry analyses performedanalyses performed with a horizontal with a horizontal dilatometer dilatometer at a heating at a heating rate of ceramics are obtained by dilatometry analyses performed with a horizontal dilatometer at a heating 10 rate◦C/min of 10 (Figure°C/min3 (Figure). The accuracy 3). The accuracy of the measurement of the measurement is about is about± 2 ◦ C.± 2 °C. rate of 10 °C/min (Figure 3). The accuracy of the measurement is about ± 2 °C.

FigureFigureFigure 3. 3. 3.Example Example Example of of measurement measurement ofofof glass-transitionglass-transition glass-transition temperaturetemperature temperature TTgg ofof theTtheg ofresidual residual the residual glass glass in in a glassa glassglass- in a glass-ceramicceramicceramic from from from the the slope theslope slope breakbreak break inin thethe in dilatometrydilatometry the dilatometry curve. curve.

Ceramics 2018, 1, x FOR PEER REVIEW 4 of 13

Ceramics 2019, 2 89 Parent glasses and glass-ceramics densities are calculated by Archimedes method in water, with specimens of about 10 g and using a 0.001g accurate scale. Each measurement is repeated three times on threeParent specimens glasses and of the glass-ceramics same type. The densities precision are calculatedof the measurement by Archimedes is about method ± 0.01 in g/cm water,3. with specimensThe piezoelectric of about 10 gcharge and using coefficient a 0.001g d33accurate is measured scale. with Each PIEZOTEST measurement PM is300 repeated after each three grinding times 3 onstep three in order specimens to draw of the samed33 curves type. versus The precision the remaining of the measurementthickness. is about ± 0.01 g/cm . −1 TheThe piezoelectricevolution of charge elastic coefﬁcient modulus d33 Eis and measured internal with friction PIEZOTEST (Q ) PMof the 300 afterglass-ceramic each grinding with steptemperature in order tois drawfollowed the by d33 Impulsecurves versus Excitation the remaining Technique thickness. (IET) using an IMCE equipment [19]. The evolution of elastic modulus E and internal friction (Q−1) of the glass-ceramic with temperature3. Results is followed by Impulse Excitation Technique (IET) using an IMCE equipment [19].

3.3.1. Results Crystallization

3.1. CrystallizationGlass transition temperature and density of the parent glasses 2SrO 1TiO2 3.3SiO2 0.2K2O yAl2O3 are given in Table 1. There is a weak influence of the alumina content, probably due to the low Al/Si ratiosGlass (Table transition 1). temperature and density of the parent glasses 2SrO 1TiO2 3.3SiO2 0.2K2O yAl2O3 are given in Table1. There is a weak inﬂuence of the alumina content, probably due to the low Al/Si ratios (Table1). Table 1. Density and glass transition temperature of the parent glasses

Table 1. Density and glass transition temperature of the parent glasses 3 ST1.3S Al/Si ρpg (Archi.) ± 0.01 g/cm Tg ± 2 °C 3 ◦ ST1.3S Al/Si ρpg (Archi.) ± 0.01 g/cm Tg ± 2 C + 0.2K2O 0.0000 3.41 712 + 0.2K2O 0.0000 3.41 712 + 0.2K2O + 0.02Al2O3 0.0061 3.41 716 + 0.2K2O + 0.02Al2O3 0.0061 3.41 716 + 0.2K2O + 0.05Al2O3 0.0152 3.41 713 + 0.2K2O + 0.05Al2O3 0.0152 3.41 713 0.0303 + 0.2K+2 O0.2K + 0.10Al2O + 0.10Al2O3 2O3 0.0303 3.403.40 712 712 + 0.2K+2 O0.2K + 0.15Al2O + 0.15Al2O3 2O3 0.04550.0455 3.403.40 720 720

CrystallizationCrystallization treatmentstreatments werewere performedperformed forfor variousvarious dwelldwell timestimes atat temperaturestemperatures ofof 850850 ◦°C,C, 900900◦ °CC andand 950950 ◦°C.C. ForFor all temperatures andand compositions,compositions, crystallizationcrystallization startsstarts fromfrom thethe surfacessurfaces andand propagatespropagates in in depth depth over over time time (Figure (Figure4). 4). The The speed spee ofd theof the crystallization crystallization front front is mainly is mainly inﬂuenced influenced by temperatureby temperature (Table (Table2). However, 2). However, an increase an increase in alumina in alumina content content tends tends to slow to slow it down. it down.

(a) (b)

FigureFigure 4.4. ((a)a) ImageImage ofof aa polishedpolished crosscross sectionsection ofof specimen specimen ST1.3S ST1.3S + + 0.2K 0.2K2O2O ++ 0.1Al0.1Al2O2O33 afterafter crystallizationcrystallization treatmenttreatment for 2h 2h at at 950 950 °C;◦C; (b) (b schemati) schematizationzation of the of themotion motion of the of crystallization the crystallization front frontover overtime. time. Table 2. Speed of the crystallization front. Table 2. Speed of the crystallization front µ CrystallizationCrystallization Speed speed (mm/h) (mm/h) Cry Crystallizationstallization speed Speed (µ/s) ( /s) ST1.3S ST1.3S 850 ◦C850 °C 900 900◦C °C 950 950◦C °C 850 850 °C◦C 900 900 °C ◦C 950 °C 950 ◦C + 0.2K O 0.06 0.36 2.6 0.017 0.010 0.72 + 0.2K2O2 0.06 0.36 2.6 0.017 0.010 0.72 + 0.2K2+O 0.2K + 0.02Al2O +2O 0.02Al3 2O0.053 0.05 0.33 0.33 2.1 2.1 0.014 0.014 0.092 0.092 0.58 0.58 + 0.2K2+O 0.2K + 0.05Al2O +2O 0.05Al3 2O0.063 0.06 0.30 0.30 1.4 1.4 0.017 0.017 0.083 0.083 0.39 0.39 + 0.2K2+O 0.2K + 0.10Al2O +2O 0.10Al3 2O0.043 0.04 0.23 0.23 1.3 1.3 0.011 0.011 0.064 0.064 0.36 0.36 + 0.2K2+O 0.2K + 0.15Al2O +2O 0.15Al3 2O0.043 0.04 0.19 0.19 1.2 1.2 0.011 0.011 0.053 0.053 0.33 0.33

ForFor allall compositions,compositions, glass-ceramicsglass-ceramics fullyfully crystallizedcrystallized fromfrom thethe surfacessurfaces toto thethe centercenter werewere preparedprepared byby applying dwell dwell times times of of 50 50 h, h,15 15 h and h and 3 h 3for h temperatures for temperatures of 850 of °C, 850 900◦C, °C 900 and◦C 950 and °C

Ceramics 2019, 2 90 Ceramics 2018, 1, x FOR PEER REVIEW 5 of 13 Ceramics 2018, 1, x FOR PEER REVIEW 5 of 13 respectively.◦ XRD analyses performed on powder specimens of the obtained glass-ceramics evidence respectively.950 C respectively. XRD analyses XRD analyses performed performed on powder on sp powderecimens specimens of the obtained of the glass-ceramics obtained glass-ceramics evidence the presence of Sr-fresnoite as the only crystalline phase (Figure 5). theevidence presence the of presence Sr-fresnoite of Sr-fresnoite as the only as crystalline the only crystalline phase (Figure phase 5). (Figure 5).

FigureFigure 5. 5.ExampleExample of of XRD patternpattern collected collected on on a powder-milled a powder-milled glass-ceramic glass-ceramic ST1.3S ST1.3S + 0.2K +O 0.2K + 0.1Al2O +O . Figure 5. Example of XRD pattern collected on a powder-milled glass-ceramic ST1.3S2 + 0.2K2O2 +3 0.1Al2O3. 0.1Al2O3. SEM cross sections of glass-ceramics ST1.3S + 0.2K2O + 0.02Al2O3 and ST1.3S + 0.2K2O + 0.1Al2O3 ◦ ◦ bothSEM crystallized cross sections at 850 of glass-ceramicsC and 950 C areST1.3S shown + 0.2K in2 FigureO + 0.02Al6. Fresnoite2O3 and ST1.3S crystals, + 0.2K which2O + contain 0.1Al2O the3 SEM cross sections of glass-ceramics ST1.3S + 0.2K2O + 0.02Al2O3 and ST1.3S + 0.2K2O + 0.1Al2O3 both crystallized at 850 °C and 950 °C are shown in Figure 6. Fresnoite crystals, which contain the bothheavier crystallized elements at (Sr, 850 Ti), °C can and be associated950 °C are toshown the bright in Figure phase 6. whereas Fresnoite the crystals, residual which glass appears contain dark. the heavier elements (Sr, Ti), can be associated to the bright phase whereas the residual glass appears heavierA rise in elements temperature (Sr, leadsTi), can to abe coarser associated microstructure. to the bright On phase the contrary, whereas an the increasing residual alumina glass appears content dark. A rise in temperature leads to a coarser microstructure. On the contrary, an increasing alumina dark.reﬁnes A therise microstructure. in temperature leads to a coarser microstructure. On the contrary, an increasing alumina content refines the microstructure. content refines the microstructure.

FigureFigure 6. SEM 6. SEM images images in inBSE BSE mode mode of ofglass-ceramic glass-ceramic cross cross section. section. Figure 6. SEM images in BSE mode of glass-ceramic cross section. MassMass and and volume volume fraction fraction of ofthe the residual residual glass glass ha haveve been been evaluated. evaluated. Assuming Assuming all all the the Sr Srand and Ti Ti Mass and volume fraction of the residual glass have been evaluated. Assuming all the Sr and Ti atomsatoms are are in inthe the fresnoite fresnoite crystals, crystals, the the residual residual glass glass composition composition is is 1.3SiO 1.3SiO2 20.2K0.2K2O2O yAl yAl2O2O3. 3In. Inthat that atoms are in the fresnoite crystals, the residual glass composition is 1.3SiO2 0.2K2O yAl2O3. In that case,case, the the weight weight fraction fraction of ofthis this residual residual glass, glass, m mrg, rgincreases, increases from from 19.2 19.2 wt wt%% to to21.6 21.6 wt% wt% with with the the case, the weight fraction of this residual glass, mrg, increases from 19.2 wt% to 21.6 wt% with the increasingincreasing amount amount of ofAl Al2O23O (Table3 (Table 3).3 ). increasing amount of Al2O3 (Table 3). CalculationCalculation of ofthe the volume volume fractions fractions requires requires the the knowledge knowledge of ofthe the residual residual glasses glasses densities. densities. Calculation of the volume fractions requires the knowledge of the residual glasses densities. TheoreticalTheoretical values values are are calculated calculated using using A. A. Fluege Fluegel’sl’s model, model, with with a aclaimed claimed precision precision of of ±2± 2g/cm g/cm3 [20].3 [20 ]. Theoretical values are calculated using A. Fluegel’s model, with a claimed precision of ±2 g/cm3 [20]. ToTo validate validate theses theses theoretical theoretical values, values, a aglass glass composed composed of of 1.3SiO 1.3SiO2 20.2K0.2K2O2O 0.1Al 0.1Al2O23O, corresponding3, corresponding to to To validate theses theoretical values, a glass composed of 1.3SiO2 0.2K2O 0.1Al2O3, corresponding to thethe residual residual glass glass of ofglass-ceramics glass-ceramics ST1.3S ST1.3S + 0.2K + 0.2K2O 2+O 0.1Al + 0.1Al2O3,2 hasO3, been has been synthesized. synthesized. The density The density of the residual glass of glass-ceramics ST1.3S + 0.2K2O + 0.1Al2O3, has been synthesized. The density of thisof thisglass, glass, experimentally experimentally measured measured by Archimedes’ by Archimedes’ method, method, is 2.34 is 2.34 ± ±0.010.01 g/cm g/cm3, which3, which is isin in this glass, experimentally measured by Archimedes’ method, is 2.34 ± 0.01 g/cm3, which is in agreement with the theoretical value (2.35 ± 0.02 g/cm3). Using the calculated data, volume fractions agreement with the theoretical value (2.35 ± 0.02 g/cm3). Using the calculated data, volume fractions of all residual glasses vrg range between 28.4 ± 0.3 and 31.1 ± 0.3 vol% depending on the amount of of all residual glasses vrg range between 28.4 ± 0.3 and 31.1 ± 0.3 vol% depending on the amount of

Ceramics 2019, 2 91 agreement with the theoretical value (2.35 ± 0.02 g/cm3). Using the calculated data, volume fractions of all residual glasses vrg range between 28.4 ± 0.3 and 31.1 ± 0.3 vol% depending on the amount of alumina. Finally, glass-ceramics densities, ρgc, experimentally measured by Archimedes method, are compared to theoretical densities, ρth, calculated by the relation:

ρth = vrg·ρrg + (1 − vrg)·ρfresnoite (2)

Theoretical and experimental densities are in very good agreement (Table3).

Table 3. Properties of residual glasses and glass-ceramics.

Residual Glass Glass-Ceramic 1.3SiO2-0.2K2O-yAl2O3 m ρ (Fluegel) ρ (Archi.) v T ρ ρ (Archi.) ST1.3S Al/Si rg rg rg rg g th gc wt% ±0.02 g/cm3 ±0.01 g/cm3 ±0.3 vol% ±2 ◦C ±0.02 g/cm3 ±0.01 g/cm3

+ 0.2K2O 0 19.2 2.33 - 28.4 605 3.44 3.43 + 0.2K2O + 0.02Al2O3 0.0154 19.6 2.33 - 28.8 610 3.43 3.43 + 0.2K2O + 0.05Al2O3 0.0385 20.0 2.34 - 29.4 624 3.43 3.42 + 0.2K2O + 0.10Al2O3 0.0769 20.8 2.35 2.34 30.3 640 3.42 3.40 + 0.2K2O + 0.15Al2O3 0.1154 21.6 2.37 - 31.1 652 3.41 3.39

Eventually, the glass transition temperature of the residual glasses was measured by dilatometry on the glass-ceramics (Table3). Values of T g are lower than those of the corresponding parent glasses due to the higher K2O/SiO2 ratio, and they clearly increase with the alumina content which acts as a glass former.

3.2. Preferential Orientation XRD patterns have been collected on the surface after a 0.3-mm grinding for all glass-ceramics. From these patterns, preferential orientation of (00l) planes is highlighted by the calculation of the R(002) coefﬁcient (Table4). All parent glass compositions and crystallization temperatures lead to specimens showing a strong (00l) orientation at their surfaces (R(002) close to 1). Only a few specimens keep this orientation in depth. This is the case for compositions ST1.3S + 0.2K2O + 0.1Al2O3 and ◦ ◦ ST1.3S + 0.2K2O + 0.15Al2O3 crystallized either at 850 C or 900 C. This phenomenon is also observed in References [21] and [22].

Table 4. Orientation factor R(002) at specimens’ surface and after a 0.3-mm grinding.

850 ◦C 900 ◦C 950 ◦C ST1.3S Surf −0.3 mm Surf −0.3 mm Surf −0.3 mm

+ 0.2K2O 0.97 0.05 1 0.02 0.99 0 + 0.2K2O + 0.02 Al2O3 0.98 0.1 1 0.01 0.98 0.01 + 0.2K2O + 0.05 Al2O3 1 0.1 0.99 0.02 1 0.05 + 0.2K2O + 0.10 Al2O3 1 1 0.99 0.96 0.99 0.3 + 0.2K2O + 0.15 Al2O3 1 1 0.97 1 1 0.24

Next to these results, several specimens of ST1.3S + 0.2K2O + 0.1Al2O3 glass-ceramics, crystallized at 900 ◦C, were analyzed by XRD at each step of a step-by-step grinding from the surface to the center. This composition is chosen for further analyses because it shows the best orientation factor and a sufﬁcient crystallization speed. These analyses highlight two different cases:

• Case A (Figure7a), the samples show a very strong (002) peak over the whole thickness. • Case B (Figure7b), the (002) peak intensity decreases in favor of (201) peaks. Ceramics 2018, 1, x FOR PEER REVIEW 7 of 13

For case B, pole figures of (002) and (201) planes were collected after each grinding step. They show a progressive tilt of the (002) planes over the first 300 µm (Figure 8). After 300 µm, (201) planes seem to become parallel to the surfaces and no more changes are observed deeper in the samples. Ceramics 2019, 2 92 Patschger et al. also observed this tilt in glass-ceramics obtained from parent glass composition ST0.75SCeramics 2018[23]., 1However,, x FOR PEER careful REVIEW analysis of orientation changes must be further realized by EBSD.7 of 13

For case B, pole figures of (002) and (201) planes were collected after each grinding step. They show a progressive tilt of the (002) planes over the first 300 µm (Figure 8). After 300 µm, (201) planes seem to become parallel to the surfaces and no more changes are observed deeper in the samples. Patschger et al. also observed this tilt in glass-ceramics obtained from parent glass composition ST0.75S [23]. However, careful analysis of orientation changes must be further realized by EBSD.

(a) (b)

FigureFigure 7. 7. XRDXRD patterns patterns vs. vs. depth: depth: (a) (a) case case A A and and (b) (b )case case B B (0 (0 and and 6 6 mm mm corresponding corresponding to to the the opposite opposite facesfaces of of the the specimens) specimens)

For case B, pole ﬁgures of (002) and (201) planes were collected after each grinding step. They show a progressive tilt of the (002) planes over the ﬁrst 300 µm (Figure8). After 300 µm, (201) planes seem to become parallel to the surfaces and no(a) more changes are observed deeper (b) in the samples. Patschger et al. also observedFigure 7. XRD this tiltpatterns in glass-ceramics vs. depth: (a) case obtained A and from (b) case parent B (0 and glass 6 mm composition corresponding ST0.75S to the [23 opposite]. However, carefulfaces analysis of the specimens) of orientation changes must be further realized by EBSD.

Figure 8. Pole figures of (002) and (201) lattice planes over depth, case B.

3.3. Piezoelectric Properties

The piezoelectric coefficient d33 was measured on the previous ST1.3S + 0.2K2O + 0.1Al2O3 specimens after each step of grinding. Both cases of orientation lead to the same variation of d33 versus remaining thickness (Figure 9). Before grinding, samples exhibit a d33 close to zero. The value of d33 Figure 8. Pole figures of (002) and (201) lattice planes over depth, case B. increasesFigure progressively 8. Pole figures during of (002) the and grinding (201) lattice until planes it reaches over depth, a plateau case B. at ±12 pC/N when half of the initial thickness is removed (more or less 3 mm). This result can be explained by assuming that the 3.3.3.3. PiezoelectricPiezoelectric PropertiesProperties symmetry of the crystallization induces a mirror symmetry in the orientation of the dipolar moments The piezoelectric coefficient d33 was measured on the previous ST1.3S + 0.2K2O + 0.1Al2O3 (FigureThe 10) piezoelectric [24]. In order coefficient to confirm d 33this, was piezoelectric measured oncharge the coefficientprevious ST1.3S d33 has + been 0.2K measured2O + 0.1Al for2O 3 specimens after each step of grinding. Both cases of orientation lead to the same variation of d versus bothspecimens halves afterof a sampleeach step cut of following grinding. Bothits middle cases ofplane. orientation The two lead half-samples to the same exhibit variation near of thed3333 versussame remaining thickness (Figure9). Before grinding, samples exhibit a d 33 close to zero. The value of d33 dremaining33 in absolute thickness value but (Figure with 9).the Before opposite grinding, sign. samples exhibit a d33 close to zero. The value of d33 ± increasesincreases progressivelyprogressively duringduring thethe grinding grinding until until it it reaches reaches a a plateau plateau at at ±1212pC/N pC/N whenwhen halfhalf ofof thethe initialinitial thicknessthickness isis removedremoved (more(more oror lessless 33 mm).mm). ThisThis resultresult cancan bebe explainedexplained byby assumingassuming thatthat thethe symmetrysymmetry ofof thethe crystallizationcrystallization inducesinduces aa mirrormirror symmetrysymmetry inin thethe orientationorientation ofof thethe dipolardipolar momentsmoments (Figure 10)[24]. In order to confirm this, piezoelectric charge coefficient d has been measured for (Figure 10) [24]. In order to confirm this, piezoelectric charge coefficient d3333 has been measured for bothboth halveshalves ofof aa samplesample cutcut followingfollowing itsits middlemiddle plane.plane. TheThe twotwo half-sampleshalf-samples exhibitexhibit nearnear thethe samesame d in absolute value but with the opposite sign. d3333 in absolute value but with the opposite sign.

Ceramics 2018, 1, x FOR PEER REVIEW 8 of 13

Ceramics 2019, 2 93 Ceramics 2018, 1, x FOR PEER REVIEW 8 of 13

(a) (b)

Figure 9. Evolution of piezoelectric(a) charge coefficient d 33 vs. depth (b) for both case A (a) and case B (b).

33 FigureFigure 9. 9.EvolutionEvolution ofof piezoelectric piezoelectric charge charge coefficient coefﬁcient d d 33vs.vs. depth depth for for both both case case A A(a) ( aand) and case case B B(b). (b ).

Figure 10. Schematization of permanent dipolar moment symmetry in fresnoite crystals across the sample. Figure 10. Schematization of permanent dipolar moment symmetry in fresnoite crystals across the Figuresample. 10. Schematization of permanent dipolar moment symmetry in fresnoite crystals across the 3.4. Stability at High Temperatures of ST1.3S + 0.2K2O + 0.1Al2O3 Glass-Ceramic sample. 3.4. StabilityAs mentioned at High Temp in theeratures introduction, of ST1.3S the + 0.2K interest2O + of0.1Al using2O3 non-ferroelectricGlass-Ceramic piezoelectric materials is mainly due to their stability at high temperatures. In order to evaluate the potential of the 3.4. StabilityAs mentioned at High Temp in theeratures introduction, of ST1.3S the interest + 0.2K of2O using + 0.1Al non-ferroelectric2O3 Glass-Ceramic piezoelectric materials ST1.3S + 0.2K2O + 0.1Al2O3 glass-ceramics for high temperature applications, their mechanical and is mainly due to their stability at high temperatures. In order to evaluate the potential of the ST1.3S + Asstructural mentioned stabilities in the have introduction, been investigated the byinterest HT-XRD of and using IET. non-ferroelectric piezoelectric materials 0.2K2O + 0.1Al2O3 glass-ceramics for high temperature applications, their mechanical and structural HT-XRD patterns have been collected every 100 ◦C from room temperature up to 1000 ◦C is mainlystabilities due have to their been stabilityinvestigated at highby HT-XRD temperatures. and IET. In order to evaluate the potential of the ST1.3S + (Figure 11). They show the stability of fresnoite in the glass-ceramic over this temperature range. Ceramics 20182 , 1HT-XRD, x FOR2 PEER patterns3 REVIEW have been collected every 100 °C from room temperature up to 1000 °C (Figure 9 of 13 0.2K OA + heating 0.1Al treatmentO glass-ceramics realized at 1200 for◦ Chigh resulted temperature in a partial thermalapplications, decomposition their mechanical and then a ﬂowing and structural 11). They show the stability of fresnoite in the glass-ceramic over this temperature range. A heating stabilitiesof the have sample been due investigated to the increase by of theHT-XRD silica amount. and IET. HT-XRDtreatment realizedpatterns at have1200°C been resulted collected in a partial every thermal 100 °C decomposition from room temperatureand then a flowing up to of 1000 the °C (Figure sample due to the increase of the silica amount. 11). They show the stability of fresnoite in the glass-ceramic over this temperature range. A heating The softening of the 30 vol% of residual glass above its glass transition temperature Tg, may treatmentaffect therealized use of theat 1200°Cglass-ceramics resulted at high in tempa partialerature. thermal IET measurements decomposition highlight and the then beginning a flowing of the sampleof the due decrease to the inincrease Young’s ofmodulus the silica E around amount. 600–650 °C (Figure 12). This agrees with the value of TheTg obtained softening by dilatometryof the 30 vol%(Table of3). residual The decre glaseass in above Young’s its modulusglass transition remains slow temperature in the Tg, may affecttemperature the use of range the glass-ceramicsof 600–800 °C. Above at high 800 °C temp it declineserature. severely IET andmeasurements simultaneously highlight the damping the beginning coefficient strongly increases, which is due to the strong softening of the residual glass. of the decrease in Young’s modulus E around 600–650 °C (Figure 12). This agrees with the value of Tg obtained by dilatometry (Table 3). The decrease in Young’s modulus remains slow in the temperature range of 600–800 °C. Above 800 °C it declines severely and simultaneously the damping coefficient strongly increases, which is due to the strong softening of the residual glass.

Figure 11. High temperature XRD patterns collected on glass-ceramic ST1.3S + 0.2K2O + 0.1Al2O3 oriented (201) from RT to 1000 ◦C by 100 ◦C steps. Observed peaks correspond to (201) and (402) planes. Figure 11. High temperature XRD patterns collected on glass-ceramic ST1.3S + 0.2K2O + 0.1Al2O3 oriented (201) from RT to 1000 °C by 100 °C steps. Observed peaks correspond to (201) and (402) planes.

Figure 12. Evolution of Young’s modulus E and damping over temperature for glass-ceramic ST1.3S

+ 0.2K2O + 0.1Al2O3.

4. Discussion All the parent glass compositions investigated show a surface crystallization mechanism, starting from oriented nucleation [22] and leading to a crystallization front propagating in depth over time. No volume crystallization is observed. By comparison to the work of Wisniewski et al. [25] on ST0.75S parent glasses crystallized at 970 °C (Tg + 210 °C), the velocity of the crystallization front is of the same order (around 0.5 µm/h) when crystallization is performed at 950 °C (Tg + 240 °C), but significantly slower for lower temperatures. Wisniewski showed that the degree of crystallization of the microstructure evolves over time so that zones first crystallize near the surface appear opaque (longer crystallization time) and zones later crystallized are translucent. In the present work also, the images taken with an optical microscope on ST1.3S + 0.2K2O + 0.1Al2O3 samples crystallized for 2h, 4h and 6h hours at 900 °C show a more translucent zone roughly corresponding to the last 400 µm crystallized (Figure 13). Considering that for ST1.3S + 0.2K2O + 0.1Al2O3 composition crystallized at a temperature of 900 °C, the velocity of the crystallization front is 0.23 mm/h (Table 3), it can then be concluded that the stability of the crystallization is achieved after around 1.7 hours. Consequently, when applying a long crystallization

Ceramics 2018, 1, x FOR PEER REVIEW 9 of 13

Ceramics 2019, 2 94

The softening of the 30 vol% of residual glass above its glass transition temperature Tg, may affect the use of the glass-ceramics at high temperature. IET measurements highlight the beginning of ◦ the decrease in Young’s modulus E around 600–650 C (Figure 12). This agrees with the value of Tg obtainedFigure by 11. dilatometry High temperature (Table3 ).XRD The patterns decrease collected in Young’s on glass-ceramic modulus remains ST1.3S slow+ 0.2K in2O the + 0.1Al temperature2O3 rangeoriented of 600–800 (201) ◦fromC. Above RT to 8001000◦ C°C itby declines 100 °C step severelys. Observed and simultaneously peaks correspond the to damping (201) and coefﬁcient(402) stronglyplanes. increases, which is due to the strong softening of the residual glass.

Figure 12. Evolution of Young’s modulus E and damping over temperature for glass-ceramic Figure 12. Evolution of Young’s modulus E and damping over temperature for glass-ceramic ST1.3S ST1.3S + 0.2K2O + 0.1Al2O3. + 0.2K2O + 0.1Al2O3. 4. Discussion 4. Discussion All the parent glass compositions investigated show a surface crystallization mechanism, starting fromAll oriented the parent nucleation glass [22compositions] and leading investigated to a crystallization show a front surface propagating crystallization in depth mechanism, over time. startingNo volume from crystallization oriented nucleation is observed. [22] and By leading comparison to a crystallization to the work of Wisniewskifront propagating et al. [25 in] depth on ST0.75S over time.parent No glasses volume crystallized crystallization at 970 is◦C observed. (Tg + 210 By◦C), comparison the velocity to of the the work crystallization of Wisniewski front iset of al. the [25] same on ST0.75Sorder (around parent 0.5glassesµm/h) crystallized when crystallization at 970 °C (Tg is performed+ 210 °C), the at 950velocity◦C (Tg of +the 240 crystallization◦C), but signiﬁcantly front is ofslower the same for lower order temperatures. (around 0.5 µm/h) when crystallization is performed at 950 °C (Tg + 240 °C), but significantlyWisniewski slower showed for lower that temperatures. the degree of crystallization of the microstructure evolves over time so thatWisniewski zones ﬁrst crystallize showed that near the the degree surface of appear crystallization opaque (longerof the micros crystallizationtructure evolves time) and over zones time later so thatcrystallized zones first are crystallize translucent. near In the the surface present appear work opaque also, the (longer images crystallization taken with an time) optical and microscopezones later ◦ crystallizedon ST1.3S + are 0.2K translucent.2O + 0.1Al In2O the3 samples present crystallized work also, the for images 2h, 4h andtaken 6h with hours anat optical 900 Cmicroscope show a more on ST1.3Stranslucent + 0.2K zone2O + roughly 0.1Al2O corresponding3 samples crystallized to the last for 400 2h,µm 4h crystallized and 6h hours (Figure at 90013). °C Considering show a more that ◦ translucentfor ST1.3S +zone 0.2K roughly2O + 0.1Al corresponding2O3 composition to the last crystallized 400 µm crystallized at a temperature (Figure of 13). 900 ConsideringC, the velocity that forof theST1.3S crystallization + 0.2K2O + 0.1Al front2O is3 0.23composition mm/h (Table crystallized3), it can at thena temper be concludedature of 900 that °C, the the stability velocity ofof thethe crystallizationcrystallization isfront achieved is 0.23 after mm/h around (Table 1.7 hours.3), it Consequently,can then be concluded when applying that athe long stability crystallization of the crystallizationtime (e.g. 15h atis 900achieved◦C), the after glass-ceramic around 1.7 ishours. homogeneously Consequently, crystallized when applying from the a surfacelong crystallization to the center (Figure 14).

The microstructures obtained are quite different to those in the work of Wisniewski. SEM pictures of Figure6 show that the residual glass is fully interconnected with itself in the three directions of space and embedded within the fresnoite crystals. This difference is probably due to the volume fraction of glass that is around 30 vol% in our case and only 6.8 vol% in the work of Wisniewski. SEM-EDX analysis of Sr and Si through the crystallization front highlights a transition layer of residual glass between the crystals and the parent glass (Figure 15). Propagation of the crystallization front in depth requires diffusion of Sr and Ti atoms through the residual glass layer which acts as a barrier. Ceramics 2018, 1, x FOR PEER REVIEW 10 of 13

Ceramics 2018, 1, x FOR PEER REVIEW 10 of 13 time (e.g. 15h at 900 °C), the glass-ceramic is homogeneously crystallized from the surface to the Ceramics 2019 2 timecenter (e.g. (Figure 15h, 14).at 900 °C), the glass-ceramic is homogeneously crystallized from the surface to the95 center (Figure 14).

Figure 13. Optical microscope images of ST1.3S + 0.2K2O + 0.1Al2O3 glass-ceramic crystallized at 900 Figure 13. Optical microscope images of ST1.3S + 0.2K O + 0.1Al O glass-ceramic crystallized at °C for 2h, 4h and 6h. 2 2 3 Figure900 ◦C for13. 2h,Optical 4h and microscope 6h. images of ST1.3S + 0.2K2O + 0.1Al2O3 glass-ceramic crystallized at 900 °C for 2h, 4h and 6h.

◦ Figure 14. Optical microscope image of ST1.3S + 0.2K2O + 0.1Al2O3 glass-ceramic crystallized at 900 C CeramicsforFigure 2018 15 hours. ,14. 1, x Optical FOR PEER microscope REVIEW image of ST1.3S + 0.2K2O + 0.1Al2O3 glass-ceramic crystallized at 90011 of 13 °C for 15 hours. Figure 14. Optical microscope image of ST1.3S + 0.2K2O + 0.1Al2O3 glass-ceramic crystallized at 900 °C for 15 hours. The microstructures obtained are quite different to those in the work of Wisniewski. SEM picturesThe ofmicrostructures Figure 6 show obtained that the areresidual quite glassdifferen is tfully to those interconnected in the work with of Wisniewski.itself in the threeSEM picturesdirections of of Figure space and6 show embedded that the within residual the fresnoiteglass is crystals.fully interconnected This difference with is probably itself in duethe tothree the directionsvolume fraction of space of and glass embedded that is aroundwithin the30 fresnoitevol% in crystals.our case This and difference only 6.8 isvol% probably in the due work to the of volumeWisniewski. fraction of glass that is around 30 vol% in our case and only 6.8 vol% in the work of Wisniewski.SEM-EDX analysis of Sr and Si through the crystallization front highlights a transition layer of residualSEM-EDX glass between analysis the of crystalsSr and Si and through the parent the crysta glass llization(Figure 15). front Propagatio highlightsn ofa transitionthe crystallization layer of residualfront in depthglass between requires thediffusion crystals of and Sr andthe parentTi atoms glass through (Figure the 15). residual Propagatio glassn layerof the which crystallization acts as a frontbarrier. in depth requires diffusion of Sr and Ti atoms through the residual glass layer which acts as a barrier.

FigureFigure 15. 15.ContentContent of Sr of and Sr and Si at Si the atthe interface interface betw betweeneen a fresnoite a fresnoite crystal crystal and and the the parent parent glass. glass.

We have seen that for a given composition, a rise in temperature increases the speed of the crystallization front (Table 2). This can be explained by the more rapid diffusion of the elements through the residual glass. In the same way, alumina acts as a glass former in the residual glass by increasing its glass transition temperature (Table 1), and thus, its viscosity at a given temperature. Consequently, the crystallization front slows down as the alumina content increases (Table 2). Crystallization temperature also influences the microstructure. This can be discussed with respect to the difference ΔT between the crystallization temperature Tc and the glass transition temperature Tg. Nucleation is promoted at a lower ΔT: This increases the density of nucleation spots at the surface and limits the crystal growth. Numerous literature papers show that fresnoite-based glass-ceramics exhibit (002) planes preferential orientation at the surface and a systematic tilt to (201) planes in the bulk [23,25]. This is often explained by a natural selection due to the faster growing speed of (201) planes compared to (002) planes. Our experimental results show that (002) planes can remain preferentially oriented in the bulk of 6-mm-thick samples although tests of reproducibility have shown that preferential orientation may change for identical conditions. However, polarization direction resulting in (002) or (201) orientation is perpendicular to the specimen’s surface (Figure 16) and the charge coefficient d33 does not seem to be significantly influenced by the orientation. This result is opposed to the theoretical d33 values as a function of the tilt angle calculated by Davis et al. [26].

Figure 16. Dipolar moment p and polarization P direction for (002) and (201) preferential orientation.

Finally, high temperature XRD analyses and mechanical characterization suggest that the piezoelectric glass-ceramic might remain functional up to 600–800 °C. The drastic fall of the mechanical properties above 800 °C may impair its use as a piezoelectric material. Future work will attempt to verify this through high-temperature measurements of piezoelectric performances.

Ceramics 2018, 1, x FOR PEER REVIEW 11 of 13

CeramicsFigure2019, 152 . Content of Sr and Si at the interface between a fresnoite crystal and the parent glass. 96

We have seen that for a given composition, a rise in temperature increases the speed of the crystallizationWe have seenfront that(Table for 2). a givenThis can composition, be explained a rise by inthe temperature more rapid increasesdiffusion theof the speed elements of the throughcrystallization the residual front (Table glass.2). In This the can same be explainedway, alumina by the acts more as rapida glass diffusion former in of thethe elementsresidual throughglass by increasingthe residual its glass. glass In transition the same temperature way, alumina (Table acts as 1), a glassand thus, former its inviscosity the residual at a given glass bytemperature. increasing Consequently,its glass transition the temperaturecrystallization (Table front1), andslows thus, down its viscosity as the atalumina a given content temperature. increases Consequently, (Table 2). Crystallizationthe crystallization temperature front slows also down influences as the the alumina microstructure. content This increases can be (Table discussed2). Crystallization with respect to thetemperature difference also ΔT between influences the the crystallization microstructure. temperature This can beTc discussedand the glass with transition respect totemperature the difference Tg. Δ Nucleation∆T between is the promoted crystallization at a lower temperature T: This T increasesc and the the glass density transition of nucleation temperature spots Tg. at Nucleation the surface is promotedand limits atthe a crystal lower ∆growth.T: This increases the density of nucleation spots at the surface and limits the crystalNumerous growth. literature papers show that fresnoite-based glass-ceramics exhibit (002) planes preferentialNumerous orientation literature at the papers surface show and thata systematic fresnoite-based tilt to (201) glass-ceramics planes in the exhibit bulk [23,25]. (002) This planes is preferentialoften explained orientation by a natural at the selection surface anddue ato systematic the faster tiltgrowing to (201) speed planes of in(201) the planes bulk [23 compared,25]. This to is (002)often explainedplanes. Our by experimental a natural selection results due show to the that faster (002) growing planes speed can remain of (201) preferentially planes compared oriented to (002) in planes.the bulk Our of experimental6-mm-thick samples results show although that (002) tests planes of reproducibility can remain preferentially have shown oriented that preferential in the bulk orientationof 6-mm-thick may samples change although for identical tests conditions. of reproducibility However, have polarization shown that direction preferential resulting orientation in (002) may or (201)change orientation for identical is perpendicular conditions. However, to the specimen’s polarization surface direction (Figure resulting 16) and in the (002) charge or (201) coefficient orientation d33 doesis perpendicular not seem to to thebe significantly specimen’s surface influenced (Figure by 16 the) and orientation. the charge coefficientThis result d 33is doesopposed not seemto the to theoreticalbe significantly d33 values influenced as a function by the orientation. of the tilt angle This calculated result is opposed by Davis to et the al. theoretical [26]. d33 values as a function of the tilt angle calculated by Davis et al. [26].

Figure 16. Dipolar moment p and polarization P direction for (002) and (201) preferential orientation. Figure 16. Dipolar moment p and polarization P direction for (002) and (201) preferential orientation. Finally, high temperature XRD analyses and mechanical characterization suggest that the piezoelectricFinally, glass-ceramichigh temperature might XRD remain analyses functional and up mechanical to 600–800 ◦ C.characterization The drastic fall ofsuggest the mechanical that the propertiespiezoelectric above glass-cerami 800 ◦C mayc might impair remain its use functional as a piezoelectric up to material.600–800 Future°C. The work drastic will fall attempt of the to verifymechanical this through properties high-temperature above 800 °C may measurements impair its use of piezoelectricas a piezoelectric performances. material. Future work will attempt to verify this through high-temperature measurements of piezoelectric performances. Author Contributions: Investigation, M.-S.R., N.M. and F.D.; Methodology, M.-S.R., N.M. and F.D.; Supervision, M.G.; Writing—original draft, M.-S.R. and F.D.; Writing—review & editing, F.D. Funding: This research was partially funded by the INTERREG V program (CUBISM project).

Acknowledgments: Authors would like to thank Professor Jan D’Haen from Hasselt University for the high temperature XRD analyses and Ir Jean-Pierre Erauw from the Belgian Ceramic Research Centre for the IET analyses. Conﬂicts of Interest: The authors declare no conﬂict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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