Investigation of ferroelectric,piezoelectric and mechanically coupled properties of lead-free (Ba0.85Ca0.15) (Zr0.1Ti0.9)O3 ceramics

Lead-free piezoelectric ceramic (Ba_0.85Ca_0.15)(Zr_0.1Ti_0.9)O₃ (BCZT) has been synthesised by solid-state sintering method and the effect of grain size on ferroelectric, piezoelectric, dielectric, mechanical and piezo-electro-mechanical properties was systematically studied. The compacted powders were sintered at three temperatures i.e. 1450°°C, 1500°C and 1550°C for an optimised duration of 5?h and they have exhibited well resolved morphotropic phase boundary with an average grain size ranging from 10 to 25?µm. Enhanced piezoelectric charge, d₃₃ ～ 560 pC/N and voltage, g₃₃ ～ 14.3?mV?m/N coefficients were obtained for the 1450°C sintered BCZT sample. A maximum strain of around ～0.14% was obtained which is comparable to that of lead-based piezoelectrics. Variation of relative permittivity with temperature revealed that the curves are independent of frequency, indicating the typical relaxor ferroelectric nature of the samples. A systematic study on cyclic loading was performed to evaluate piezo-electro-mechanical coefficients at different loads which showed hysteresis behaviour. High value of elastic modulus (E) and hardness (H) i.e. 262.7?±?38.1?GPa and 13.7?±?1.7?GPa were exhibited by samples sintered at 1450°C, which is higher than that of BCZT synthesised by wet-chemical methods. The results are discussed.     

(Ba,Ca)(Ti,Zr)O3–CeO2 translucent and opaque ceramics — Optical and electromechanical characteristics

This work demonstrates promising approaches to synthesize multifunctional-translucent BCZT-CeO₂ ceramic for electro-mechanical/optical conversion. Nearly fully dense BCZT-CeO₂ electroceramics were fabricated by spark plasma sintering (SPS) showing high dielectric permittivity of 5875 and low dielectric loss of 2.1%. Altogether, an extraordinary optical transmittance was attained as ～ 40% at 750 nm wavelength and ～ 46% in the infrared region. The fine grains restricted piezoelectric constant, d₃₃, to a moderate value of ～ 145 pC/N but with an enhanced Curie temperature, T_C ～ 135 °C. Post rapid pressure-less sintering on SPSed samples led to a fast grain growth within an extra hour of sintering and a major improvement of piezoelectric properties (d₃₃ = 532 pC/N and k_p = 43%) was achieved. A substantial amount of energy can be saved by both approaches owing to rapid heating/cooling rates and short dwell-times, at least when compared to conventional sintering. This success highlights ingenious avenues for further studies on BCZT-CeO₂ electro-optical ceramics.     

Bending strength and microstructure of Al2O3 ceramics densified by spark plasma sintering

Al₂O₃ ceramics were superfast densified using spark plasma sintering (SPS) by heating to a sintering temperature between 1350 and 1700°C at a heating rate of 600°C/min, without holding time, and then fast cooling to 600°C within 3 min. High-density Al₂O₃ ceramics could be achieved at lower sintering temperatures by SPS, as compared with that by conventional pressureless sintering (PLS). The bending strength of Al₂O₃ superfast densified by SPS in the range of sintering temperature between 1400 and 1550°C reached values as high as 800 MPa, almost twice that obtained by the PLS. SEM observations indicated that intragranular fracture was the preponderant fracture mode in these samples, resulting in these excellent bending strength values.     

Mechanical Properties and Microstructure of Nano-SiC–Al2O3 Composites Densified by Spark Plasma Sintering

Heterogeneous precipitation method has been used to produce 5 vol% SiC–Al₂O₃ powder, from aqueous suspension of nano-SiC, aqueous solution of aluminium chloride and ammonia. The resulting gel was calcined at 700°C. Nano-SiC–Al₂O₃ composites were densified using spark plasma sintering (SPS) process by heating to a sintering temperature at 1350, 1400, 1450, 1500 and 1550°C, at a heating rate of 600 °/min, with no holding time, and then fast cooling to 600°C within 2–3 min. High density composites could be achieved at lower sintering temperatures by SPS, as compared with that by hot-press sintering process. Bending strength of 5 vol% SiC–Al₂O₃ densified by SPS at 1450°C reached as high as 1000 MPa. Microstructure studies found that the nano-SiC particles were mainly located within the Al₂O₃ grains and the fracture mode of the nanocomposites was mainly transgranular fracture.     

The complex evaluation of functional properties of nearly dense BCZT ceramics and their dependence on the grain size

In this article, the effect of dwell time (2–48?h) during isothermal sintering at 1520?°C on the grain size, density and crystal structure of (Ba_0.85Ca_0.15)(Zr_0.1Ti_0.9)O₃ is demonstrated. All ceramics exhibited the phase transition from orthorhombic to mixed orthorhombic and tetragonal phases between 12 and 24?h dwell time. The piezoelectric, dielectric and ferroelectric properties of these ceramics show a strong correlation with grain size in the range of 16.5–44.5?µm. Nearly dense microstructure (93.1–94.3% t.d.) was maintained during entire dwell time range without sacrificing much of its functional properties. Increasing grain size caused a little decrease in d₃₃* piezoelectric constant from 480 pC/N to 460 pC/N while mechanical quality factor (Q_m) increased from 62 to 136. Diffusivity coefficient (γ) ranges between 1.46 and 1.55 which indicates a normal ferroelectric behavior.     

Composite microstructures and piezoelectric properties in tantalum substituted lead-free K0.5Na0.5Nb1-xTaxO3 ceramics

K_0.5Na_0.5Nb_1-xTa_xO₃ (KNNT) (with x?=?0.00, 0.05, 0.10, 0.20, 0.30, 0.50 and 1) ceramics are prepared by ball milling and two calcinations at 830?°C for 5?h. Subsequent sintering of centimeter size pellets, 1–2?mm thick, is studied using conventional and spark plasma sintering techniques with various conditions. X-Ray diffraction and Raman spectroscopy phase identification reveal orthorhombic to tetragonal phase transitions occurring at about x?=?0.50, associated to chemical disorder. Scanning electron microscope observations and associated energy dispersive X-ray spectroscopy analysis reveal some composite aspect of the ceramics. Substitution of niobium by tantalum, corresponding to x increase, decreases significantly the grain size but also the densification of the ceramics sintered by conventional sintering, while, enhancement of the piezoelectric properties is observed for both sintering techniques. Thanks to parameters optimization of the spark plasma sintering process, temperature-time-pressure, significant improvement of the relative density over 96%, is obtained for all the compositions sintered between 920 and 960?°C, under 50?MPa, for 5–10?min with heating rates of 100?°C/min. High relative permittivity (ε_r =?1027), piezoelectric charge coefficient (d₃₃ =?160 pC/N) and piezoelectric coupling factor (k_p =?46%) are obtained in spark plasma sintered K_0.5Na_0.5Nb_1-xTa_xO₃ composite ceramics, for x ranging between 0.10 and 0.30 and for some specific spark plasma sintering conditions. Thus, tantalum single element substitution on niobium site, combined with spark plasma sintering, is revealed to be a powerful combination for the optimization and the reliability of piezoelectric properties in KNN system.     

Influence of microwave sintering on electrical properties of BCTZ lead free piezoelectric ceramics

Barium titanate doped with calcium and zirconium (BCTZ) could be used at low temperature to replace lead based piezoelectric ceramics (PZT). The classical way to obtain BCTZ is the solid-state route coupled with conventional sintering, but this step is time-consuming. To reduce the duration of this process, microwave heating was used for sintering. It is a fast sintering method and the heating rate was around 200 °C/min in this study. Slightly better electrical properties with finer microstructures (d₃₃* = 706 pm/V, grain size about 42.1 ± 14.2 μm) were obtained for samples sintered by microwave heating during 50 min compared to the conventional sintering (d₃₃* = 622 pm/V, 22.6 ± 4.4 μm). The main result of this study is that by using microwave heating, the sintering step duration (including heating, dwell time and cooling) was drastically reduced: 1.5 h for microwave sintering against 12.5 h for conventional sintering.     

Spark plasma sintering of ultra-porous γ-Al2O3

Nanocrystalline gamma alumina (γ-Al₂O₃) powder with a crystallite size of ~10 nm was synthesized by oxidation of high purity aluminium plate in a humid atmosphere followed by annealing in air. Spark plasma sintering (SPS) at different sintering parameters (temperature, dwell time, heating rate, pressure) were studied for this highly porous γ-Al₂O₃ in correlation with the evolution in microstructure and density of the ceramics. SPS sintering cycles using different heating rates were carried out at 1050–1550 °C with dwell times of 3 min and 20 min under uniaxial pressure of 80 MPa. Alumina sintered at 1550 °C for 20 min reached 99% of the theoretical density and average grain size of 8.5 µm. Significant grain growth was observed in ceramics sintered at temperatures above 1250 °C.     

Achievement of high electric performances for bismuth titanate-based piezoceramics via hot press sintering

Bi₄Ti₃O₁₂ (BIT) ceramic has great potential in high-temperature environment due to its high Curie temperature T_C ~ 675 °C. In this work, hot press sintering (HPS) is applied on the 0.01 mol Ce and Nb/Cr co-doped BIT (BCTNC) ceramics to enhancing the low piezoelectric coefficient (d₃₃ < 7 pC/N) and resistivity. Extremely high d₃₃ (~39 pC/N) together with high T_C (~672 °C) and high dc resistivity ρ (~1.49 ×10⁷ Ω?cm at 500 °C) are obtained in HPS samples. The enhancement of piezoelectricity benefits from high density of domain and high poling electric field. Moreover, outstanding thermal stability of piezoelectric constant (d₃₃ ~ 35 pC/N after annealing at 650 °C) and low dielectric loss (tanδ ~ 3.8% at 500 °C) are observed as well. These findings are instrumental in understanding HPS and provide a possible manipulation of crystallized mechanism and domains growing kinetics to enhance piezoelectric performances of BIT based ceramics.     

Performance enhancement of soft-PZT5 piezoelectric ceramics using poling technique

Pb(Zr_1−xTi_x)O₃ (PZT) ceramics are the most widely used piezoelectric ceramics due to their excellent performance. It has been reported that the direct current poling (DCP) apply on alternating current poling (ACP) relaxor-PbTiO₃ ferroelectric crystals can further improve the piezoelectric properties. Herein, we report the dielectric and piezoelectric properties of soft-PZT5 ceramics under DCP, ACP, and ACP + DCP methods. The piezoelectric coefficient d₃₃ of the soft-PZT5 ceramics was 560 pC/N using ACP+DCP at room temperature (RT), which is 4% higher than the ACP-treated sample (540 pC/N) and 24% higher than the DCP-treated sample (450 pC/N). The ideal poling temperatures of DCP and ACP were found to be 120°C and 60°C, showing optimal d₃₃ values of 540 pC/N and 565 pC/N, respectively. The ACP and ACP+DCP samples show the same aging trend. After 30 days of aging, the d₃₃ values of the DCP, ACP, and ACP+DCP soft-PZT5 ceramics were 415 pC/N, 500 pC/N, and 510 pC/N, respectively, showing decreases of 12%, 8%, and 9%, respectively. This work indicates that the ACP+DCP method is an effective method to improve the piezoelectric properties of soft-PZT5 ceramics.     

Pairing high piezoelectric properties and enhanced thermal stability in grain-oriented BNT-based lead-free piezoceramics

(Bi_0.5Na_0.5)TiO₃ (BNT)-based piezoceramics usually exhibit enhanced piezoelectric coefficient d₃₃ together with the deterioration of depolarization temperature T_d, which is the common drawback limiting their use in practical application. Here, we demonstrate that harnessing the microstructure in BNT-based ceramics will be an efficient way to resolve this obstacle. <00l> oriented piezoelectric ceramics 0.94(Bi_0.5Na_0.5)TiO₃ ?0.06BaTiO₃ was engineered by templated grain growth (TGG) using NaNbO₃ (NN) as templates. The manufactured textured ceramics with the optimized microstructure was characterized by not only approximately 200% enhancement in the magnitude of piezoelectric response (d₃₃~297pC/N) but also improved thermal stability (T_d~57?°C) in comparison to its randomly oriented counterparts (d₃₃~151pC/N and T_d~32?°C). Moreover, the enhanced piezoelectricity in grain oriented specimens primarily originated from a high degree of non-180° domain switching as compared to the randomly axed ones. The current study opens the door to pair high piezoelectric properties and enhanced thermal stability in BNT-related materials though texture technique.     

Electrical properties of lead-free 0.98(Na<Subscript>0.5</Subscript>K<Subscript>0.5</Subscript>)NbO<Subscript>3</Subscript>-0.02Ba(Zr<Subscript>0.52</Subscript>Ti<Subscript>0.48</Subscript>)O<Subscript>3</Subscript> piezoelectric ceramics by optimizing sintering temperature

Lead-free 0.98(Na_0.5K_0.5)NbO₃-0.02Ba(Zr_0.52Ti_0.48)O₃ [0.98NKN-0.02BZT] ceramics were fabricated by the conventional mixed oxide method with sintering temperature at 1,080°C to 1,120°C. The results indicate that the sintering temperature obviously influences the structural and electrical properties of the sample. For the 0.98NKN-0.02BZT ceramics sintered at 1,080°C to 1,120°C, the bulk density increased with increasing sintering temperature and showed a maximum value at a sintering temperature of 1,090°C. The dielectric constant, piezoelectric constant [d ₃₃], electromechanical coupling coefficient [k _p], and remnant polarization [P _r] increased with increasing sintering temperature, which might be related to the increase in the relative density. However, the samples would be deteriorated when they are sintered above the optimum temperature. High piezoelectric properties of d ₃₃ = 217 pC/N, k _p = 41%, dielectric constant = 1,951, and ferroelectric properties of P _r = 10.3 μC/cm² were obtained for the 0.98NKN-0.02BZT ceramics sintered at 1,090°C for 4 h.     

Building submicron crystalline piezoceramics: One-step pressureless sintering partially amorphized nanopowder

The key to miniaturization of piezoelectric devices is to build high-performance fine-grained piezoceramics. Although the preparation of fine-grained ceramics can be achieved by hot press sintering or spark plasma sintering, complex processes, and high costs hinder the popularization of such technologies. In this work, a one-step conventional pressureless sintering technique based on partially amorphized nano-precursors has been proposed for low-cost preparation of submicron-structured ceramics. Using this technology, 0.36BiScO₃–0.64PbTiO₃ ceramics with an average particle size of 170 nm and a relative density of 95% can be prepared at temperature as low as 900°C, while the samples still have excellent piezoelectric properties (d₃₃ = 220 pC/N, g₃₃ = 40 × 10⁻³ Vm/N). In the process of sintering partially amorphized nano-precursors, the initial densification rate is faster than the grain growth rate, which can be attributed to two effects promoting low temperature densification, one is the liquid phase sintering mechanism associated with the amorphous phase, and the other is the filling effect of small particles deposited at the grain boundaries. This work is simple and easy to implement, and it is expected to be extended to the preparation of other types of fine-grained ceramics.     

High piezoelectric performance and domain configurations of (K0.45Na0.55)0.98Li0.02Nb0.76Ta0.18Sb0.06O3 lead-free ceramics prepared by two-step sintering

Research for high-performance lead-free piezoelectric materials has become an urgent issue from the environmental concern. Very limited attempts on two-step sintering had been made so far. In this study, (K_0.45Na_0.55)_0.98Li_0.02Nb_0.76Ta_0.18Sb_0.06O₃ ceramics were prepared by both conventional sintering and two-step sintering. Piezoelectric properties, microstructure and domain structure were found to change significantly with sintering methods and sintering conditions. Two-step sintering was performed in the way that temperature is first quickly raised to 1180 °C, kept for 1 min, then immediately cooled down to 1120 °C and maintained for a desired time length. The effects of dwelling time on piezoelectric performance and microstructure as well as domain structure were investigated. High piezoelectric properties of d₃₃ = 455 pC/N, k_p = 0.54 and k₃₃ = 0.67 were obtained in a ceramic prepared under the dwelling time of 20 h. This ceramic also possesses a very good piezoelectric thermal-ageing stability over −50 °C–150 °C. Further investigation reveals that this ceramic has a quite uniform grain-size distribution with the average grain size of about 12 μm in microstructure and shows domain patterns of simple parallel stripes with a hierarchical nanodomain structure appearing inside some of broad stripes. The observed excellent piezoelectric performance is considered to associate closely with the unique domain structure.     

Low‐temperature sintering and enhanced piezoelectric properties of random and textured PIN–PMN–PT ceramics with Li2CO3

Low‐temperature sintered random and textured 36PIN–30PMN–34PT piezoelectric ceramics were successfully synthesized at a temperature as low as 950°C using Li₂CO₃ as sintering aids. The effects of Li₂CO₃ addition on microstructure, dielectric, ferroelectric, and piezoelectric properties in 36PIN–30PMN–34PT ternary system were systematically investigated. The results showed that the grain size of the specimens increased with the addition of sintering aids. The optimum properties for the random samples were obtained at 0.5 wt% Li₂CO₃ addition, with piezoelectric constant d₃₃ of 450 pC/N, planar electromechanical coupling coefficient k_p of 49%, peak permittivity ε_max of 25 612, remanent polarization P_r of 36.3 μC/cm². Moreover, the low‐temperature‐sintered textured samples at 0.5 wt% Li₂CO₃ addition exhibited a higher piezoelectric constant d₃₃ of 560 pC/N. These results indicated that the low‐temperature‐sintered 36PIN–30PMN–34PT piezoelectric ceramics were very promising candidates for the multilayer piezoelectric applications.     

Grain growth anomaly and piezoelectric properties of liquid phase sintered high TC 0.36BiScO3 - 0.64PbTiO3 ceramics with sillenite Bi12PbO19

The sintering behavior of 0.36BiScO₃-0.64PbTiO₃ (0.36BS-0.64 PT) ceramics was studied to investigate the effect of grain growth by the sillenite Bi₁₂PbO₁₉ (BP) phase on their piezoelectric properties for application in high-temperature piezoelectric devices. The BP phase formed during calcination at temperatures <750 °C led to a grain growth anomaly of the 0.36BS-0.64 PT ceramics sintered at 1000 °C. This phase assisted the grain growth of the 0.36BS-0.64 PT ceramics by liquid phase sintering. In particular, the 0.36BS-0.64 PT ceramic calcined at 700 °C exhibited excellent piezoelectric properties with a d₃₃ of 531 pC/N, g₃₃ of 41×10⁻³ Vm/N, k_p of 61.8%, and Q_m of 16. In addition, the 0.36BS-0.64 PT ceramics exhibited ferroelectric relaxor-like characteristics with an extremely large relaxation coefficient (γ) of 1.94 along with high maximum dielectric permittivity temperature (426 °C).     

Processing of 0.55(Ba0.9Ca0.1)TiO3-0.45Ba(Sn0.2Ti0.8)O3 lead-free ceramics with high piezoelectricity

We report a large piezoelectric constant (d₃₃), 720 pC/N and converse piezoelectric constant (d₃₃^*), 2215 pm/V for 0.55(Ba_0.9Ca_0.1)TiO₃-0.45Ba(Sn_0.2Ti_0.8)O₃ ceramics; the biggest value achieved for lead-free piezoceramics so far. The ceramic powders were calcined between 1050°C-1350°C and sintered at 1480°C. The best properties were obtained at a calcination temperature (CT) of 1350°C. The fitting combination of processing and microstructural parameters for example, initial powder particle size >2 µm, ceramics density ~95%, and grain size ~40 µm led to a formation of orthorhombic-tetragonal-pseudo-cubic (O-T-PC) mixed phase boundary near room temperature, supported by Raman spectra, pointed to the extremely high piezoelectric activity. These conditions significantly increase piezoelectric constants, together with high relative permittivity (ε_r) >5000 and a low loss tangent (tan δ) of 0.029. In addition, the d₃₃ value stabilizes in the range of 400-500 pC/N for all samples calcined between 1050°C and 1250°C. The results entail that the (Ba,Ca)(Sn,Ti)O₃ ceramics are strong contenders to be a substitute for lead-based materials for room temperature applications.     

Influence of processing parameters on thermal conductivity of uranium dioxide pellets prepared by spark plasma sintering

High density uranium dioxide (UO₂) pellets with grain sizes between 0.9 μm and 9 μm were produced by spark plasma sintering (SPS). A systematic study was performed by varying the sintering temperature between 750 °C and 1450 °C and hold time between 0.5 min and 20 min to obtain UO₂ pellets with a range of theoretical densities (TD) and grain sizes. The microstructure development in terms of grain size, density and porosity distribution was investigated. The oxygen/uranium (O/U) ratio of the resulting pellets was found to decrease after SPS. The thermal conductivity of UO₂ pellets increased with the theoretical density but the grain size in the investigated range had no significant influence. The measured thermal conductivity values up to 900 °C were consistent with the reported literature for conventionally sintered UO₂ pellets. The benefits of using SPS over the conventional sintering of UO₂ are summarized.     

Ultra-high-temperature piezoelectric responses and ultra-high thermal stability of piezoelectricity in ceramic PbZr0.54Ti0.46O3

To date, most piezoceramics with a high piezoelectric coefficient (d₃₃ > 500 pC/N) and a high Curie temperature (T_C around 400°C) are BiScO₃-PbTiO₃-based (BS-PT-based) systems, containing the rare-earth element Sc, whose high cost hinders mass production. We investigated the effect of Nd-doping on the morphotropic phase boundary and synthesized low-cost Nd-doped PbZr_0.54Ti_0.46O₃ (PZT) piezoceramics, achieving high piezoelectric performance. At room temperature, the piezoelectric coefficient d₃₃ reached 550 pC/N with a T_C = 375°C and this changed by only 3.6% over a broad temperature range (30–260°C). The d₃₃ value reached an ultra-high value of 941 pC/N at 345°C, which is higher than that of a BS-PT-based ceramic (810 pC/N at 350°C). The developed PZT ceramic material has a superior electrostrictive strain of 0.45% at 40 kV/cm, and a room temperature piezoelectric coefficient d₃₃* of 1312 pm/V at 20 kV/cm. Our research provides a new paradigm for designing piezoceramics that can be used over a wide temperature range.