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

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