Lead-free (Ba0.85Ca0.15)(Ti0.9Zr0.1)O3–CeO2 ceramics with high piezoelectric coefficient obtained by low-temperature sintering

Lead-free (Ba_0.85Ca_0.15)(Ti_0.9Zr_0.1)O₃–x%CeO₂(BCZT–xCe) piezoelectric ceramics have been prepared by the traditional ceramic process and the effects of CeO₂ addition on their phase structure and piezoelectric properties have been studied. The addition of CeO₂ significantly improves the sinterability of BCZT ceramics which results in a reduction of sintering temperature from 1540 °C to 1350 °C without sacrificing the high piezoelectric properties. X-ray diffraction data show that CeO₂ diffuses into the lattice of BCZT and a pure perovskite phase is formed. SEM images indicate that a small addition of CeO₂ greatly affects the microstructure. Main piezoelectric parameters are optimized at around x = 0.04 wt% with a high piezoelectric coefficient (d₃₃ = 600 pC/N), a planar electromechanical coefficient (k_p = 51%), a high dielectric constant (?_r = 4843) and a low dissipation factor (tan δ = 0.012) at 1 kHz, which indicates that the BCZT–xCe ceramics are promising for lead-free practical applications.     

Dielectric properties and relaxor behavior of high Curie temperature (Ba0.85Ca0.15)(Zr0.1Ti0.9)O3–Bi(Mg0.5Ti0.5)O3 Lead-free ceramics

The (1?x)(Ba_0.85Ca_0.15)(Zr_0.1Ti_0.9)O₃?xBi(Mg_0.5Ti_0.5)O₃ [(1?x)BCZT–xBMT, x=0.1–0.7] lead-free solid solution ceramics were prepared by the conventional mixed oxide method. The phase structure was investigated by X-ray diffraction and results show that a single perovskite phase was obtained in all of these samples, suggesting that the added BMT diffused into BCZT to form a solid solution. Dense ceramics with relative densities >95% were obtained, and a small amount of BMT (≤50 mol%) acted as grain growth promoter, had an evident effect on grain size growth. Further increase of the BMT content inhibited the grain growth of BCZT samples. Temperature dependence of the dielectric properties showed that all the BCZT–BMT solid solution ceramics exhibited relaxor-like characteristics. With increasing BMT content, the Curie temperature was first increased and then decreased, giving a maximum value of 218 °C for the 0.4BCZT–0.6BMT composition. Furthermore, stable dielectric constants and low losses were obtained with 0.5≤x≤0.7 in the wide temperature range, indicating that the system possess potential for high-temperature application.     

Microstructure and electrical properties of (Ba0.85Ca0.15)1−xLix(Ti0.90Zr0.10)1−xNbxO3 ceramics with a low dielectric loss and a low sintering temperature

A low sintering temperature is demonstrated for (Ba_0.85Ca_0.15)_1?xLi_x(Ti_0.90Zr_0.10)_1?xNb_xO₃ (BCLTZN-x) piezoelectric ceramics, where BCLTZN-x lead-free piezoelectric ceramics were prepared by the normal sintering. Effects of Li and Nb on the microstructure and electrical properties of these ceramics were investigated. The sintering temperature of BCLTZN-x ceramics was decreased greatly by introducing Li and Nb, and the grain size of these ceramics decreases with increasing x. These ceramics with a small amount of Li and Nb maintain good piezoelectric properties, together with a low sintering temperature and a lower dielectric loss. These ceramics with x = 0.01 demonstrate optimum electrical properties: d₃₃ ～ 353 pC/N, k_p ～ 41.1%, T_c ～ 86 °C, ?_r ～ 4236, and tan δ ～ 0.75%.     

Microstructure induced ultra-high energy storage density coupled with rapid discharge properties in lead-free Ba0.9Ca0.1Ti0.9Zr0.1O3–SrNb2O6 ceramics

Novel lead-free (1-x)Ba_0·9Ca_0·1Ti_0·9Zr_0·1O₃-xSrNb₂O₆ ceramics were synthesized via a two-step high energy ball milling process. The evolution of microstructural properties, phase transformation, and energy storage characteristics was comprehensively investigated to assess the applicability of material in multi-layered ceramic capacitors. The substitution of SrNb₂O₆ (SNO) in Ba_0·9Ca_0·1Ti_0·9Zr_0·1O₃ (BTCZ) has resulted in substantial improvement in materials density along with a small increase in the grain size of the synthesized ceramic. A thorough microstructural investigation indicates an excellent dispersibility and compatibility between BTCZ and SNO phases. With an increase in SNO substitution, a transition from typical ferroelectric to relaxor ferroelectric has been observed, which has led to a significantly slimmer ferroelectric loop along with frequency dispersive dielectric properties. The optimized composition (i.e., x = 0.10) exhibits an ultra-high recoverable energy density of 2.68 J/cm³ along with a moderately high energy efficiency of 83.4%. Further, SNO substituted samples have also shown an enhancement in breakdown strength. The improvement in energy storage performance and breakdown strength of SNO substituted BTCZ composites are mainly attributed to relatively homogeneous grain morphology, optimum grain size, microstructural density, and improved grain boundary interface.     

Crystal structure and microwave dielectric properties of Zn0.9Ti0.8−xSnxNb2.2O8 ceramics

The crystal structure and microwave dielectric properties of Zn_0.9Ti_0.8?xSn_xNb_2.2O₈ (x = 0.00, 0.05, 0.10, 0.15) ceramics sintered at temperatures ranging from 1100 °C to 1140 °C for 6 h were investigated. A single phase with ixiolite structure was obtained. With the increase of Sn content, the dielectric constant decreased attributed to the decrease of dielectric polarizability. The Qf value decreased with the decrease of packing fraction and grain size. The temperature coefficient of resonant frequency (τ_f) increased due to the increase of the bond valence of Zn_0.9Ti_0.8?xSn_xNb_2.2O₈ ceramics. The excellent microwave dielectric properties of ? = 35.05, Qf = 49,100 GHz, τ_f = ?27.6 × 10^?6/°C were obtained for Zn_0.9Ti_0.8?xSn_xNb_2.2O₈ (x = 0.05) specimens sintered at 1120 °C for 6 h.     

Low temperature sintering and microwave dielectric properties of (Mg3 − xZnx)(VO4)2 ceramics

This paper describes the effects of Zn substitution for Mg on the microwave dielectric properties of (Mg_3 − xZn_x)(VO₄)₂ ceramics. As for the XRPD patterns of (Mg_3 − xZn_x)(VO₄)₂ ceramics sintered at the sintering temperature of 750 °C, no secondary phase was detected over the whole composition range. However, in the case of the sample sintered at 850 °C, the Zn₄V₂O₉ and Zn₂V₂O₇ compounds were identified by using XRPD; this result was attributed to the decomposition of Zn₃(VO₄)₂ phase. From crystal structure analysis, it was found that the atomic distances of M(1)O (M = Mg and Zn) and M(2)O in MO₆ octahedra increased, though that of VO in VO₄ tetrahedron decreased. Moreover, the slight tilting of M(2)O₆ octahedron was observed by the Zn substitution. As for the covalency of cation–oxygen bond, the covalency of MO bond in M(1)O₆ and M(2)O₆ octahedra decreased because of the increase in the atomic distance of MO, whereas that of VO increased with increasing the Zn addition. However, as a result, the slight decrease in the covalency of cation–oxygen bond was recognized because the variation in the covalency of MO bond is predominant in this crystal structure. The dielectric constants of the samples range from 4.4 to 11.1. The decrease in the covalency may be related to the difference in the dielectric constant of each composition. The maximum Q · f value of bulk densities is effected by varying the chemical composition of (Mg_3 − xZn_x)(VO₄)₂ ceramics and it shifts toward lower sintering temperature with an increase in x within the temperature region of 800–1050 °C. The temperature coefficient of resonant frequency (τ_f) of the samples decreased with increasing of Zn, and then a variation in τ_f value was attributed to the tilting of M(2)O₆ octahedron caused by Zn substitution for Mg.     

High thermal stability of electric field-induced strain in (1−x)(Bi0.5Na0.5)TiO3-xBa0.85Ca0.15Ti0.9Zr0.1O3 lead-free ferroelectrics

In ferroelectric materials high electric field-induced strain (EFIS) with good thermal stability is of much interest from both fundamental research and potential applications. Here we propose a strategy to achieve high thermally stable EFIS based on electrostrictive effect and thermal stability of polarization. According to this strategy, we synthesized (1−x)(Bi_0.5Na_0.5)TiO₃-xBa_0.85Ca_0.15Ti_0.9Zr_0.1O₃ (BNT-xBCZT) ferroelectric ceramics in order to tailor the thermal stability of dielectric permittivity, polarization and EFIS. A dielectric platform with a wide temperature region is induced by increasing x from 0.24 to 0.36 gradually. From 30 °C to 150 °C, a variation of 20% polarization results in a change of 36% EFIS, suggesting a good thermal stability as expect. Temperature-insensitive electrostrictive coefficient Q₃₃ ranges from 0.0264 m⁴/C² to 0.0314 m⁴/C². These results not only prove the effectiveness of this strategy, but also suggest that this strategy can be applied to other ferroelectric materials to improve the thermal stability.     

Structure and microwave dielectric properties of Ba1−xSrxMg2V2O8 ceramics

The Ba_1−xSr_xMg₂V₂O₈ (0≤x≤0.4) microwave dielectric ceramics were fabricated by a standard solid-state reaction method. The formation of a continuous solid solution within the whole composition range was identified. The ceramic samples could be well densified in the temperature range of 885–975 °C in air for 4 h. The permittivity ε_r was found to increase with increasing ionic polarizabilities. The Q×f values were believed to be closely related with packing fraction and grain refinement. The Sr²⁺ substitution contributed to a monotonous increase of the A-site bond valence, such that the τ_f value experienced a considerable variation from negative to positive values. The optimum microwave dielectric properties of an ε_r of 13.3, a high Qxf of 86,640 GHz (9.6 Hz) and a near-zero τ_f of −6 ppm/°C could be yielded in the x=0.15 sample when sintered at 915 °C for 4 h.     

Microwave dielectric properties of (Ba1−xSrx)(Mg0.5W0.5)O3 ceramics

The densification, microstructural evolution and microwave dielectric properties of (Ba_1−xSr_x)(Mg_0.5W_0.5)O₃ ceramics with x=0, 0.25, 0.5 and 0.75 are investigated in this study. The sintering temperature of the (Ba_1−xSr_x)(Mg_0.5W_0.5)O₃ is significantly reduced from 1575 °C to 1400 °C as the x value increases from 0 to 0.25 and 0.50; this result is accompanied by the formation of the (Ba_1−ySr_y)WO₄ phase and a small quantity of second phase surrounding the grains. The grain size of the (Ba_1−xSr_x)(Mg_0.5W_0.5)O₃ ceramics is increased by raising the Sr²⁺ content, which significantly lowers the sintering temperature. The microstructure of the (Ba_0.75Sr_0.25)(Mg_0.5W_0.5)O₃ ceramic displays the smallest average grain size of approximately 0.8 μm, with a narrow grain size distribution. Without long annealing time, very high Q×f values are obtained for the (Ba_1−xSr_x)(Mg_0.5W_0.5)O₃ ceramics sintered at 1400–1575 °C for a duration of only 2 h. The (Ba_0.75Sr_0.25)(Mg_0.5W_0.5)O₃ ceramic sintered at 1400 °C results in the best microwave dielectric properties, including ε_r of 20.6, Q×f of 152,600 GHz and τ_f of +24.0 ppm/°C.     

Ca含量对(Ba1-xCax)(Ti0.9Zr0.1)O3无铅压电陶瓷性能的影响

采用传统的固相法制备(Ba1-xCax)(Ti0.9Zr0.1)O3(简称BCTZ)无铅压电陶瓷.借助扫描电镜(SEM)和X射线衍射仪(XRD)等测试方法研究了Ca含量对所制备BCTZ无铅压电陶瓷显微结构和压电介电性能的影响.结果表明:随着Ca含量的增加,(Ba1-xCax)(Ti0.9Zr0.1)O3无铅压电陶瓷的晶粒尺寸先增大后减小,所制备的BCTZ陶瓷的物相都是钙钛矿结构,没有杂相;随着Ca含量的增加,BCTZ陶瓷压电常数(d33),介电常数(εr),机电耦合系数(kp)分别先增加后降低,而介质损耗(tanδ)先减小后增大.当Ca含量(x)为0.15mol时,在1450℃烧结制得的(Ba1-xCax)(Ti0.9Zr0.1)O3无铅压电陶瓷性能最佳:压电常数(d33)为363pC/N,机电耦合系数(kp)为39.63％,介电常数(εr)为4184,介质损耗(tanδ)为1.08％.     

Microwave-assisted sintering and improved dielectric,ferroelectric properties of 0.5[(Ba0.7Ca0.3)TiO3]–0.5[Ba(Zr0.2Ti0.8)O3] lead-free ceramics

0.5[(Ba_0.7Ca_0.3)TiO₃]–0.5[Ba(Zr_0.2Ti_0.8)O₃] lead-free ceramics were synthesised by coprecipitation method and sintered by fast microwave sintering (MWS) and by conventional sintering (CS) at 1200°C. After being sintered with the two different methods, the materials were characterised for structural, microstructural, frequency and temperature-dependent dielectric properties, Raman spectroscopy, and ferroelectric measurements. Results are compared and discussed in the present paper. X-ray diffraction confirms the presence of the tetragonal and rhombohedral phases in the composites sintered by both methods. The ferroelectric to paraelectric transition temperature (Tc) is increased in microwave-sintered composite. Diffuse constant (γ) values show BCT–BZT ceramics to be neither normal ferroelectrics nor relaxor ferroelectrics. Raman spectra confirm phase transition in the ceramic samples. Saturation polarisation (Ps) values are 7.62 and 4.28?µC?cm^?2 and nearly equal remanant polarisation (Pr) values were observed for BCT–BZT composite sintered with MWS and CS, respectively.     

(Ba0.85Ca0.15)(Ti0.90Zr0.06Sn0.04)O3-Fe2O3无铅压电陶瓷的性能研究

采用固相法制备了(Ba0.85Ca0.15) (Ti0.90Zr0.06Sn0.04)O3-xmol％Fe2O3(简写为BCTZS-xFe)无铅压电陶瓷.研究了不同掺杂量对该陶瓷的显微结构、介电、铁电及压电性能的影响.结果表明,所有样品均具有单一的钙钛矿结构,少量掺杂能使晶粒长大,提高电性能.在x=0.025时,具有最佳的综合电性能,压电常数d33 =515 pC/N,机电耦合系数kp=48.2％,机械品质因数Qm =182,2Pr=18.2 μC/cm2,2Ec =4.3 kV/cm,介电常数εr=5175.     

Low temperature synthesis of novel SrCe0.9Yb0.1O3−α-chlorides composite electrolytes for intermediate temperature protonic ceramics fuel cells

In this study, novel SrCe_0.9Yb_0.1O_3−α-(Na/K)Cl and SrCe_0.9Yb_0.1O_3−α-NaCl-BaCl₂ composite electrolytes were synthesized at low temperature (750 °C). The structural properties of the composite electrolytes based on mixtures of SrCe_0.9Yb_0.1O_3−α (SCY) and chlorides ((Na/K)Cl, NaCl-BaCl₂) are characterized. The microstructure of the composite electrolytes are observed by scanning electron microscopy (SEM). The electrical conductivity is determined by impedance spectroscopy. The ionic conductivities of the composite electrolytes are higher than that of SCY. Finally, high-performance intermediate temperature fuel cells of the composite electrolytes are obtained.     

(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.     

Effects of BaO–SiO2 glass particle size on the microstructures and dielectric properties of Mn-doped Ba(Ti,Zr)O3 ceramics

The sintering behavior, microstructures and dielectric properties of Mn-doped Ba(Ti, Zr)O₃ (BTZ) ceramics with different particle sizes of BaO–SiO₂ glass (D50 ranging between 185 and 1200 nm) were investigated. From the metallographic observation, adding finer glass frit revealed more homogeneous compositional distribution. It was found that better spreading of the glass phase could be achieved by adding finer glass particles that could penetrate the BTZ ceramic interface more easily, thus enhancing the grain growth. The extent of the incorporation between glass and ceramic increased with smaller glass particles, and the Curie temperature was altered accordingly. Microstructural evaluation conducted by using X-ray diffraction (XRD) and transmission electron microscopy (TEM) with an energy-dispersive X-ray spectrometer (EDS) indicated the glass particle size has a dramatic influence on the sintering behavior and microstructure of Mn-doped BTZ ceramics. The relationship between microstructures and dielectric properties was also discussed in this study.     

Electrical properties of (La0.9Ca0.1)(Co1−xNix)O3−δ cathode materials for SOFCs

Modified perovskite ceramics (La_0.9Ca_0.1)(Co_1?xNi_x)O_3?δ (x = 0–0.3) cathodes for solid oxide fuel cells (SOFCs) were synthesized by solid state reaction. The lattice parameters, electrical conductivity, activation energy, and microstructures of these specimens were investigated systematically in this study. The results exhibited that all specimens are rhombohedron structures and their tolerance factors were greater than 0.97, indicating that the perovskite was not distorted by Ni²⁺ cation substitution for the B site of (La_0.9Ca_0.1)CoO_3?δ. The microstructures of the (La_0.9Ca_0.1)(Co_1?xNi_x)O_3?δ specimens showed good densification, and were well-sintered, with few pores. The electrical conductivity behavior conformed to the nature of a semiconductor, for all specimens. As x = 0.1, the electrical conductivity reached the maximum value of 750.3 S/cm at 800 °C, and the activation energy calculated from the Arrhenius plot of the electrical conductivity versus the reciprocal of temperature is 7.1 kJ/mol.The novelty of this study is its introduction of the concept of defect chemistry to explain the relationship between compensation mechanisms and electrical conductivity. The information gleaned regarding charge compensation mechanisms and defect formation may be valuable for a better understanding of the cathode of (La_0.9Ca_0.1)(Co_1?xNi_x)O_3?δ ceramics used for SOFCs. Moreover, the information about oxygen content versus temperature is useful for expressing the relationship between electrical conductivity and composition. Therefore, we also used thermogravimetric analysis combined with the room-temperature oxygen content which was determined by iodometric titration to investigate the oxygen content from room temperature to high temperature, in air. Based on the experimental results, the (La_0.9Ca_0.1)(Co_0.9Ni_0.1)O_3?δ specimen shows high electrical conductivity. Consequently, it is identified as a promising candidate for cathode SOFC applications.     

High temperature thermoelectric properties of Ca3Co4O9+δ by auto-combustion synthesis and spark plasma sintering

A rapid method for the synthesis of Ca₃Co₄O_9+δ powder is introduced. The procedure is a modification of the conventional citric-nitrate sol–gel method where an auto-combustion process is initiated by a controlled thermal oxidation–reduction reaction. The resulting powders inherit the advantages of a wet chemical synthesis, such as morphological and compositional homogeneity, and fine, well-defined particle sizes coming from the controlled nature of the auto-combustion. Optimized spark plasma sintering (SPS) processing conditions were determined and used to fabricate dense and highly c-axis oriented samples. The microstructure and thermoelectric transport properties were determined both parallel (||) and perpendicular (⊥) to the SPS pressure axis in order to investigate any possible anisotropy variations in the transport properties. At 800 °C, power factors of 506 μW/m K² (⊥) and 147 μW/m K² (||), thermal conductivities values of 2.53 W/m K (⊥) and 1.25 W/m K (||), and resulting figures-of-merit, ZT, of 0.21 (⊥) and 0.13 (||) were observed.     

Optimizing the piezoelectric properties of Ba0·85Ca0·15Zr0·10Ti0·90O3·lead-free ceramics via two-step sintering

The two-step sintering of lead-free Ba₀·₈₅Ca₀·₁₅Zr₀·₁Ti_0·9O₃·(BCZT) ceramics was investigated as a way to enhance its piezoelectric properties. The variations in grain size as a function of the calcination and sintering conditions and its effect on performance is discussed. Results indicate that as the calcination and first-step sintering temperatures increased, grain size became large and was independent of the second sintering step. Large grains were responsible for the enhanced piezoelectric properties by causing lattice distortion, larger domains, and easy motion of domain walls. The BCZT ceramic calcined at 1200 °C and sintered at 1540 °C without holding and then cooled to 1400 °C and held at 1400 °C for 4 h exhibited optimal performance with the highest remnant polarization P_r ∼13.5 μC/cm², the largest piezoelectric constant d₃₃ ∼ 529 pC/N at room temperature, and the highest Curie temperature T_c ∼125 °C. Two-step sintering has been turned out to be an effective method to realize high-performance BCZT ceramics by microstructure optimization.     

Phase structure,band gap and microwave dielectric properties of Ba8Ti3Nb4−xSbxO24 ceramics

High dielectric constant and low loss ceramics in the Ba₈Ti₃Nb_4?xSb_xO₂₄ (x=0–2) system were prepared by conventional solid-state ceramic route. As x increased from 0 to 1.5, a single phase with hexagonal 8H perovskite structure was formed and the band gap values increased from 3.38 to 3.47 eV. However, the Sb₂O₃ secondary phase was detected as the x reached 2. The optimum sintering temperature was reduced from 1460 to 1380 °C, the quality factors (Q×f) were effectively enhanced from 22,900 to 38,000 GHz and τ_f was significantly lowered from 110 ppm/°C to 2 ppm/°C, whereas the dielectric constant decreased from 49 to 35. A good combined microwave dielectric properties with ε_r=37.5, Q×f=38,000 GHz, τ_f=15 ppm/°C were obtained for x=1.5.     

Products and microwave dielectric properties of ceramics with nominal composition (Ba0.9Ca0.1)(YxB′1/2)O(3x+4.5)/2 (B′=Nb5+, Ta5+)

The products and microwave dielectric properties of ceramics with nominal composition (Ba_0.9Ca_0.1)(Y_xB′_1/2)O_(3x+4.5)/2 (B′=Nb⁵⁺, Ta⁵⁺) are investigated. When x=0.5, i.e. (Ba_0.9Ca_0.1)(Y_1/2B′_1/2)O₃ (B′=Nb⁵⁺, Ta⁵⁺), the product contains a considerable amount of Y₂O₃ as well as the main perovskite phase. When x=0.3 the product is single phase, equivalent to Ba(Ca_1/9Y_3/9Nb_5/9)O₃ or Ba(Ca_1/9Y_3/9Ta_5/9)O₃. The lattice parameters of these new compounds are smaller than those of Ba(Y_1/2Nb_1/2)O₃ and Ba(Y_1/2Ta_1/2)O₃. The relative permittivities (ε_r) of these new compounds are larger than those of Ba(Y_1/2B′_1/2)O₃ (B′=Nb⁵⁺, Ta⁵⁺). The increase in ε_r of the Nb-system is about 4 times larger than that of the Ta-system. The Q f values of the present ceramics are larger than the Ca-containing perovskite in the (Ba_1−xCa_x)(Mg_1/3Ta_2/3)O₃ system. The sharp increase of ε_r in this study cannot be explained by the Ca²⁺ rattling ion model at the A-site, which applies to the case of the (Ba_1−xCa_x)(Mg_1/3Ta_2/3)O₃ system. A new method to explain the increase in ε_r is discussed.