Low‐Temperature Sintering of Bi0.5(Na,K)0.5TiO3 for Multilayer Ceramic Actuators

We investigated the influence of CuO amount (0.5–3.0 mol%), sintering temperature (900°C–1000°C), and sintering time (2–6 h) on the low‐temperature sintering behavior of CuO‐added Bi_0.5(Na_0.78K_0.22)_0.5TiO₃ (BNKT22) ceramics. Normalized strain (S_max/E_max), piezoelectric coefficient (d₃₃), and remanent polarization (P_r) of 1.0 mol% CuO‐added BNKT22 ceramics sintered at 950°C for 4 h was 280 pm/V, 180 pC/N, and 28 μC/cm², respectively. These values are similar to those of pure BNKT22 ceramics sintered at 1150°C. In addition, we investigated the performance of multilayer ceramic actuators made from CuO‐added BNKT22 in acoustic sound speaker devices. A prototype sound speaker device showed similar output sound pressure levels as a Pb(Zr,Ti)O₃‐based device in the frequency range 0.66–20 kHz. This result highlights the feasibility of using low‐cost multilayer ceramic devices made of lead‐free BNKT‐based piezoelectric materials in sound speaker devices.     

Microstructure,Ferroelectric, Piezoelectric,and Ferromagnetic Properties of Sc‐Modified BiFeO3–BaTiO3 Multiferroic Ceramics with MnO2 Addition

0.725BiFe_1?xSc_xO₃–0.275BaTiO₃ + y mol% MnO₂ multiferroic ceramics were fabricated by a conventional ceramic technique and the effects of Sc doping and sintering temperature on microstructure, multiferroic, and piezoelectric properties of the ceramics were studied. The ceramics can be well sintered at the wide low sintering temperature range 930°C–990°C and possess a pure perovskite structure. The ceramics with x/y = 0.01–0.02/1.0 sintered at 960°C possess high resistivity (~2 × 10⁹ Ω·cm), strong ferroelectricity (P_r = 19.1–20.4 μm/cm²), good piezoelectric properties (d₃₃ = 127–128 pC/N, k_p = 36.6%–36.9%), and very high Curie temperature (618°C–636°C). The increase in sintering temperature improves the densification, electric insulation, ferroelectric, and piezoelectric properties of the ceramics. A small amount of Sc doping (x ≤ 0.04) and the increase in the sintering temperature significantly enhance the ferromagnetic properties of the ceramics. Improved ferromagnetism with remnant magnetization M_r of 0.059 and 0.10 emu/g and coercive field H_c of 2.51 and 2.76 kOe are obtained in the ceramics with x/y = 0.04/1.0 (sintered at 960°C) and 0.02/1.0 (sintered at 1050°C), respectively. Because of the high T_C (636°C), the ceramic with x/y = 0.02/1.0 shows good temperature stability of piezoelectric properties. Our results also show that the addition of MnO₂ is essential to obtain the ceramics with good electrical properties and electric insulation.     

Novel Temperature Stable Li2MnO3 Dielectric Ceramics with High Q for LTCC Applications

Novel microwave dielectric ceramics in the Li₂MnO₃ system with high Q prepared through a conventional solid‐state route had been investigated. All the specimens exhibited single phase ceramics sintered in the temperature range 1140°C–1230°C. The microwave dielectric properties of Li₂MnO₃ ceramics were strongly correlated with sintering temperature and density. The best microwave dielectric properties of ε_r = 13.6, Q × f = 97 000 (GHz), and τ_f = ?5.2 ppm/°C could be obtained as sintered at 1200°C for 4 h. BaCu(B₂O₅) (BCB) could effectively lower the sintering temperature from 1200°C to 930°C and slightly induced degradation of the microwave dielectric properties. The Li₂MnO₃ ceramics doped with 2 wt% BaCu(B₂O₅) had excellent dielectric properties of ε_r = 11.9, Q × f = 80 600 (GHz), and τ_f = 0 ppm/°C. With low sintering temperature and good dielectric properties, the BCB added Li₂MnO₃ ceramics are suitable candidates for LTCC applications in wireless communication system.     

Excellent piezoelectric properties of PNN-PHT ceramics sintered at low temperature with CuO addition

Multilayer components with excellent piezoelectric performance have been developed for fulfilling the requirement of new-generation electromechanical devices and systems. Multilayering of the piezoelectric ceramic requires good sinterability preferentially at lower temperature. In this study, copper (II) oxide (CuO) was utilized as the sintering additive to increase the sinterability of 0.49Pb(Ni_1/3Nb_2/3)O₃–0.51Pb(Hf_0.3Ti_0.7)O₃ (PNN-PHT) ceramics at low temperature, and simultaneously enhance the piezoelectric and dielectric properties of the ceramics. The results demonstrated that the addition of CuO influenced the sintering behavior, grain growth, and piezoelectric properties of the PNN-PHT ceramics. A ternary high performance piezoelectric PNN-PHT ceramic sintered at 1050°C with 0.5 mol% CuO exhibited excellent properties as follows: d₃₃ = 912 pC N⁻¹ and ε_r = 6665.     

Low‐Temperature‐Fired ReVO4 (Re = La,Ce) Microwave Dielectric Ceramics

We report a series of ReVO₄ (Re = La, Ce) microwave dielectric ceramics fabricated by a standard solid‐state reaction method. X‐ray diffraction and scanning electron microscopy measurements were performed to explore the phase purity, sintering behavior, and microstructure. The analysis revealed that pure and dense monoclinic LaVO₄ ceramics with a monazite structure and tetragonal CeVO₄ ceramics with a zircon structure could be obtained in their respective sintering temperature range. Furthermore, LaVO₄ and CeVO₄ ceramics sintered at 850°C and 950°C for 4 h possessed out‐bound microwave dielectric properties: ε_r = 14.2, Q × f = 48197 GHz, τ_f = ?37.9 ppm/°C, and ε_r = 12.3, Q × f = 41 460 GHz, τ_f = ?34.4 ppm/°C, respectively. The overall results suggest that the ReVO₄ ceramics could be promising materials for low‐temperature‐cofired ceramic technology.     

Effect of CuO Addition on the Microstructure and Electric Properties of Low‐Temperature Sintered 0.25PMN–0.40PT–0.35PZ Ceramics

Low‐temperature sintering of 0.25PMN–0.40PT–0.35PZ ceramics was investigated using CuO as a sintering aid. Effect of CuO on the sinterability, microstructure, and electric properties of 0.25PMN–0.40PT–0.35PZ system was systematically studied. The CuO addition significantly reduced the sintering temperature of 0.25PMN–0.40PT–0.35PZ from 1260°C to 950°C. SEM results indicated that a dense microstructure without any second phase was obtained when the amount of CuO was 0.25 wt%, which gave rise to high values of d₃₃ = 532 pC/N and k_p = 58.4%. A large field‐induced longitudinal strain ~2.28% (at 30 kV/cm) can also be obtained for 0.25 wt% CuO‐added specimens, which shows a great promise for multilayer actuator applications.     

Influences of B2O3/CuO additions on the sintering behavior,microstructure and microwave dielectric properties of 6Nd[(Zn0.7Co0.3)0.5Ti0.5]O3–4(Na0.5Nd0.5)TiO3 ceramics

B₂O₃ and CuO were codoped into 6Nd[(Zn_0.7Co_0.3)_0.5Ti_0.5]O₃–4(Na_0.5Nd_0.5)TiO₃ (abbreviated as 6NZCT–4NNT) ceramics as sintering aids. The influences of the sintering aids on the sintering characteristics, microstructure and microwave dielectric properties of the 6NZCT–4NNT ceramics were systematically investigated as a function of the proportion of B₂O₃ and CuO. Codoping could greatly reduce the sintering temperature from 1410 °C to 1150 °C, indicating that B₂O₃/CuO are good sintering aids for 6NZCT–4NNT ceramics. The B₂O₃/CuO sintering aids had no significant impact on the phase purity of the investigated ceramics, even though a solid solution was formed due to Cu²⁺ ion substitution. However, they had evident influences on the surface morphology and grain size. The average grain size was enlarged with increasing amounts of CuO in the B₂O₃/CuO sintering aids. Remarkable deterioration of the microwave dielectric properties for 6NZCT-4NNT ceramics was not observed when codoping an appropriate amount of B₂O₃ and CuO. The 6NZCT–4NNT ceramics codoped with 2.0 mol% B₂O₃ and 2.0 mol% CuO sintered at 1150 °C for 3 h exhibited a homogeneous microstructure and promising microwave dielectric properties: an appropriate dielectric constant (ε_r = 49.37), a high quality factor (QF = 47,295 GHz), and a near-zero temperature coefficient of resonant frequency (TCF = +0.9 ppm/^°C).     

Low‐Temperature Sintering and Piezoelectric Properties of CuO‐Added KNbO3 Ceramics

Dielectric and piezoelectric properties of CuO‐added KNbO₃ (KN) ceramics were investigated. The CuO reacted with the Nb₂O₅, formed a CuO–Nb₂O₅‐related liquid phase during the sintering, and assisted the densification of the KN ceramics at low temperatures. Moreover, some of the Cu²⁺ ions replaced the Nb⁵⁺ ions in the matrix and behaved as a hardener. The dielectric and piezoelectric properties of the KN ceramics were considerably influenced by the relative density. The 1.0 mol% CuO‐added KN ceramic sintered at 960°C for 1.0 h, which showed a maximum relative density, exhibited a high phase angle of 86.9°, P_r of 14.8 μC/cm², and E_c of 1.8 kV/mm. This specimen also exhibited good dielectric and piezoelectric properties: ε^T₃₃/ε_o of 364, d₃₃ of 122 pC/N, k_p of 0.29, and Q_m of 611.     

Structural,dielectric and ferroelectric properties of lead-free Ba0.85Ca0.15Zr0.10Ti0.90O3 ceramics prepared by Plasma Activated Sintering

Lead-free Ba_0.85Ca_0.15Zr_0.10Ti_0.90O₃ (BCZT) ceramics were prepared by Plasma Activated Sintering (PAS). The influence of PAS sintering temperature on the crystalline phase, microstructure, and, dielectric and ferroelectric properties of BCZT ceramics were studied. The phase structure of BCZT ceramics first changed from rhombohedral phase to the coexistence of rhombohedral and tetragonal phases and then to tetragonal phase as the sintering temperature increased. Microstructural characterization of BCZT ceramics indicated that PAS can obtain a compact microstructure at lower temperatures of 1150–1300 °C compared with that from common pressureless sintering. The BCZT ceramics showed different degrees of diffuseness with increased temperature, and the diffuseness exponents C are all approximately on the order of 10⁵ °C. The dielectric and ferroelectric properties of BCZT ceramics were enhanced with increased sintering temperature. BCZT ceramics sintered at 1250 °C exhibited optimum properties of room-temperature ε_r=2863, ε_m=6650, and 2P_r=25.24 μC/cm², resulting from the relatively higher tetragonal phase content of the MPB between tetragonal and rhombohedral phases together with a compact microstructure.     

Liquid‐phase sintering,microstructural evolution,and microwave dielectric properties of Li2Mg3SnO6–LiF ceramics

The liquid‐phase sintering behavior and microstructural evolution of x wt% LiF aided Li₂Mg₃SnO₆ ceramics (x = 1‐7) were investigated for the purpose to prepare dense phase‐pure ceramic samples. The grain and pore morphology, density variation, and phase structures were especially correlated with the subsequent microwave dielectric properties. The experimental results demonstrate a typical liquid‐phase sintering in LiF–Li₂Mg₃SnO₆ ceramics, in which LiF proves to be an effective sintering aid for the Li₂Mg₃SnO₆ ceramic and obviously reduces its optimum sintering temperature from ~1200°C to ~850°C. The actual sample density and microstructure (grain and pores) strongly depended on both the amount of LiF additive and the sintering temperature. Higher sintering temperature tended to cause the formation of closed pores in Li₂Mg₃SnO₆‐x wt% LiF ceramics owing to the increase in the migration ability of grain boundary. An obvious transition of fracture modes from transgranular to intergranular ones was observed approximately at x = 4. A single‐phase dense Li₂Mg₃SnO₆ ceramic could be obtained in the temperature range of 875°C‐1100°C, beyond which the secondary phase Li₄MgSn₂O₇ (<850°C) and Mg₂SnO₄ (>1100°C) appeared. Excellent microwave dielectric properties of Q × f = 230 000‐330 000 GHz, ε_r = ~10.5 and τ_f = ~?40 ppm/°C were obtained for Li₂Mg₃SnO₆ ceramics with x = 2‐5 as sintered at ~1150°C. For LTCC applications, a desirable Q × f value of ~133 000 GHz could be achieved in samples with x = 3‐4 as sintered at 875°C.     

Low‐Temperature Sintering of β‐Spodumene Ceramics Using Li2O–GeO2 as a Sintering Additive

Low‐temperature sintering of β‐spodumene ceramics with low coefficient of thermal expansion (CTE) was attained using Li₂O–GeO₂ sintering additive. Single‐phase β‐spodumene ceramics could be synthesized by heat treatment at 1000°C using highly pure and fine amorphous silica, α‐alumina, and lithium carbonate powders mixture via the solid‐state reaction route. The mixture was calcined at 950°C, finely pulverized, compacted, and finally sintered with or without the sintering additive at 800°C–1400°C for 2 h. The relative density reached 98% for the sample sintered with 3 mass% Li₂O–GeO₂ additive at 1000°C. Its Young's modulus was 167 GPa and flexural strength was 115 MPa. Its CTE (from R.T. to 800°C) was 0.7 × 10^?6 K^?1 and dielectric constant was 6.8 with loss tangent of 0.9% at 5 MHz. These properties were excellent or comparative compared with those previously reported for the samples sintered at around 1300°C–1400°C via melt‐quenching routes. As a result, β‐spodumene ceramics with single phase and sufficient properties were obtained at about 300°C lower sintering temperature by adding Li₂O–GeO₂ sintering additive via the conventional solid‐state reaction route. These results suggest that β‐spodumene ceramics sintered with Li₂O–GeO₂ sintering additive has a potential use as LTCC for multichip modules.     

Fabrication of a transversal multilayer thermoelectric generator with substituted calcium manganite

The sintering behavior and thermoelectric performance of Ca_0.99Gd_0.01Mn_0.99W_0.01O₃ was studied, and a multilayer thermoelectric generator was fabricated. The addition of CuO as sintering additive was found to be effective for the reduction in the sintering temperature from 1300°C to about 1000°C‐1050°C. Dense samples were obtained after firing at 1050°C, whereas some porosity remained after firing at 1000°C. Samples sintered at reduced temperature exhibit lower electrical conductivity, whereas the Seebeck coefficient S = ?150 μV/K at 100°C is not affected by lowering the sintering temperature. The figure of merit is ZT = 0.12 at 700°C for samples sintered at 1300°C; ZT = 0.08 and 0.03 were obtained for multilayer laminates sintered at 1050°C and 1000°C, respectively. A transversal multilayer thermoelectric generator (TMLTEG) was built by stacking layers of substituted CaMnO₃ green tapes, and printing AgPd conductor stripes onto the thermoelectric layers at an angle of 30° relative to the direction of the heat flow. The multilayer stack was co‐fired at 1000°C. The TMLTEG has a power output of 2.5 mW at ?T= 200 K in the temperature interval of 25°C‐300°C. A meander‐like generator with larger power output comprising six TMTEGs is also presented.     

Low‐firing and temperature stable microwave dielectric ceramics: Ba2LnV3O11 (Ln = Nd,Sm)

Low‐firing and temperature stable microwave dielectric ceramics of Ba₂LnV₃O₁₁ (Ln = Nd, Sm) were prepared by solid‐state reaction. X‐ray diffraction (XRD) and scanning electron microscopy (SEM) were used to investigate the phase purity, crystal structure, sintering behavior, and microstructure. The XRD patterns indicated that Ba₂LnV₃O₁₁ (Ln = Nd, Sm) ceramics belong to monoclinic crystal system with P2₁/c space group in the whole sintering temperature range (800°C ‐900°C). Both ceramics could be well densified at 880°C for 4 hours with relative densities higher than 96%. The Ba₂LnV₃O₁₁ (Ln = Nd, Sm) samples sintered at 880°C for 4 hours exhibited excellent microwave dielectric properties: ε_r = 12.05, Q × f = 23 010 GHz, τ_f = ?7.7 ppm/°C, and ε_r = 12.19, Q × f = 27 120 GHz, τ_f = ?16.2 ppm/°C, respectively. Besides, Ba₂LnV₃O₁₁ (Ln = Nd, Sm) ceramics could be well co‐fired with the silver electrode at 880°C.     

Low‐Temperature Sintering and Microwave Dielectric Properties of (Mg0.95Zn0.05)2(Ti0.8Sn0.2)O4–(Ca0.8Sr0.2)TiO3 Composite Ceramics

0.9(Mg_0.95Zn_0.05)₂(Ti_0.8Sn_0.2)O₄–0.1(Ca_0.8Sr_0.2)TiO₃ (MZTS–CST) ceramics were prepared by a conventional solid‐state route. The MZTS–CST ceramics sintered at 1325°C exhibited ε_r = 18.2, Q × f = 49 120 GHz (at 8.1 GHz), and τ_f = 15 ppm/°C. The effects of LiF–Fe₂O₃–V₂O₅ (LFV) addition on the sinterability, phase composition, microstructure, and microwave dielectric properties of MZTS–CST were investigated. Eutectic liquid phases 0.12CaF₂/0.28MgF₂/0.6LiF and MgV₂O₆ were developed, which lowered the sintering temperature of MZTS–CST ceramics from 1325°C to 950°C. X‐ray powder diffraction (XRPD) and energy dispersive spectroscopy (EDS) analysis revealed that MZTS and CST coexisted in the sintered ceramics. Secondary phase Ca₅Mg₄(VO₄)₆ as well as residual liquid phase affected the microwave dielectric properties of MZTS–CST composite ceramics. Typically, the MZTS–CST–5.3LFV composite ceramics sintered at 950°C showed excellent microwave dielectric properties: ε_r = 16.3, Q × f = 30 790 GHz (at 8.3 GHz), and τ_f = ?10 ppm/°C.     

Microwave Dielectric Properties of Novel Glass‐Free Low‐Firing Li2CeO3 Ceramics

Novel glass–free low temperature firing microwave dielectric ceramics Li₂CeO₃ with high Q prepared through a conventional solid‐state reaction method had been investigated. All the specimens in this paper have sintering temperature lower than 750°C. XRD studies revealed single cubic phase. The microwave dielectric properties were correlated with the sintering conditions. At 720°C/4 h, Li₂CeO₃ ceramics possessed the excellent microwave dielectric properties of ε_r = 15.8, Q × f = 143 700 (GHz), and τ_f  = ?123 ppm/°C. Li₂CeO₃ ceramics could be excellent candidates for glass‐free low‐temperature co‐fired ceramics substrates.     

Dielectric Properties and Low‐Temperature Sintering of the Ba0.6Sr0.4TiO3 Ceramics with B2O3/CuO Additions

The sintering behaviors and dielectric properties of Ba_0.6Sr_0.4TiO₃ ceramics were investigated as a function of B₂O₃ and CuO content. The addition of both B₂O₃ and CuO reduced the sintering temperature of Ba_0.6Sr_0.4TiO₃ about 500°C. It was suggested that a liquid phase BaCu(B₂O₅) was formed and assisted the densification of Ba_0.6Sr_0.4TiO₃ ceramics. Ba_0.6Sr_0.4TiO₃ ceramics co‐doped with 3.0 mol% B₂O₃, and 2.0 mol% CuO, sintered at 950°C for 5 h, had a dense microstructure and showed good microwave dielectric properties of a moderate dielectric constant (ε = 1048), low dielectric loss (0.0090) and high tunability (42.2%) at dc electric field of 30 kV/cm.     

Synthesis of a low-firing BaSi2O5 microwave dielectric ceramics with low dielectric constant

In this study, BaSi₂O₅ ceramics with an orthorhombic structure were synthesized by using a traditional solid-state method at a low temperature by doping with Li₂O–B₂O₃–CaO–CuO (LBCC) glass. The phase composition, mechanism of low-temperature sintering, microwave dielectric properties, and changes in the mesophase during the heating of low-temperature sintered BaSi₂O₅ ceramics were examined by performing an X-ray diffraction analysis. A compact matrix of BaSi₂O₅ can be wetted by the liquid phase of the formed LBCC glass. Therefore, LBCC glass with different doping percentages can effectively reduce the sintering temperature of BaSi₂O₅. The microwave dielectric properties of BaSi₂O₅ ceramics sintered at 900 °C at 4 wt% of LBCC glass were determined: ε_r = 7.32, Q × f = 19,002 GHz, and τ_f = ?35.8 ppm/°C. The chemical compatibility of the samples with Ag was studied at 4 wt% doping with LBCC glass, and the samples were fired for 4 h at 900 °C.     

Low‐Temperature Sintering and Microwave Dielectric Properties of ZnNb2O6–TiO2 Ceramics with BaCu(B2O5) Additions

The influence of BaCu(B₂O₅) (BCB) on densification, phases, microstructure and microwave dielectric properties of ZnNb₂O₆–xTiO₂ (x = 1.70–1.90) composite ceramics have been investigated. Undoped ZnNb₂O₆–1.8TiO₂ ceramics sintered at 1200°C exhibit temperature coefficient of resonant frequency (τ_f) ~9.25 ppm/°C. When BaCu(B₂O₅) was added, the sintering temperature of the ZnNb₂O₆–1.8TiO₂ composite ceramics was effectively reduced to 950°C. The results indicated that the permittivity and Q × f were dependent on the sintering temperature and the amounts of BaCu(B₂O₅). Addition of 3.0 wt% BaCu(B₂O₅) in ZnNb₂O₆–1.8TiO₂ ceramics sintered at 950°C showed excellent dielectric properties of ε_r = 40.9, Q × f = 12,200 GHz (f = 5.015 GHz) and τ_f = +0.3 ppm/°C.     

Structure,Microwave Dielectric Properties,and Low‐Temperature Sintering of Acceptor/Donor Codoped Li2Ti1−x(Al0.5Nb0.5)xO3 Ceramics

The structure, microwave dielectric properties, and low‐temperature sintering behavior of acceptor/donor codoped Li₂TiO₃ ceramics [Li₂Ti_1?x(Al_0.5Nb_0.5)_xO₃, x = 0–0.3] were investigated systematically. The x‐ray diffraction confirmed that a single‐phase solid solution remained within 0 < x ≤ 0.2 and secondary phases started to appear as x > 0.2, accompanied by an order–disorder phase transition in the whole range. Scanning electron microscopy observation indicated that the complex substitution of Al³⁺ and Nb⁵⁺ produced a significant effect on the microstructural morphology. Both microcrack healing and grain growth contributed to the obviously enhanced Q×f values. By comparison, the decrease of ε_r and τ_f values was ascribed to the ionic polarizability and the cell volume, respectively. Excellent microwave dielectric properties of ε_r ~ 21.2, Q×f ~ 181 800 GHz and τ_f  ~ 12.8 ppm/°C were achieved in the x = 0.15 sample when sintered at 1150°C. After 1.5 mol% BaCu(B₂O₅) additive was introduced, it could be well sintered at 950°C and exhibited good microwave dielectric properties of ε_r ~ 20.4, Q×f ~ 53 290 GHz and τ_f ~ 3.6 ppm/°C as well. The cofiring test of the low‐sintering sample with Ag powder proved its good chemical stability during high temperature, which enables it to be a promising middle‐permittivity candidate material for the applications of low‐temperature cofired ceramics.     

Improved sinterability and microwave dielectric properties of [Zn0.5Ti0.5]3+-doped ZnAl2O4 spinel solid solution

Low-permittivity ZnAl_2-x(Zn_0.5Ti_0.5)_xO₄ ceramics were synthesized via conventional solid-state reaction method. A pure ZnAl₂O₄ solid-state solution with an Fd-3m space group was achieved at x ≤ 0.1. Results showed that partial substitution of [Zn_0.5Ti_0.5]³⁺ for Al³⁺ effectively lowered the sintering temperature of the ZnAl₂O₄ ceramics and remarkably increased the quality factor (Q × f) values. Optimum microwave dielectric properties (ε_r = 9.1, Q × f = 115,800 GHz and τ_f = −78 ppm/°C) were obtained in the sample with x = 0.1 sintered at 1400°C in oxygen atmosphere for 10 h. The temperature used for the sample was approximately 250°C lower than the sintering temperature of conventional ZnAl₂O₄ ceramics.