Normal sintering of (K, Na)NbO3-based lead-free piezoelectric ceramics

Lead-free piezoelectric ceramics have received more attention due to the environmental protection of the earth. (K, Na)NbO₃-based ceramics are one of the most promising candidates. Normal sintering of un-doped and Li/Ta co-doped (K, Na)NbO₃ ceramics was investigated to clarify the optimal sintering condition for densification, microstructure and electrical properties. It was found that density increased greatly within a narrow temperature range but turned to decrease when the sintering temperature slightly exceeded the optimal one. Piezoelectric properties also showed similar relationship between the density and sintering temperature, but the highest piezoelectric strain coefficients were obtained at the temperatures lower than that for the highest density. The grain growth and property change as a function of sintering temperature were discussed on basis of the formation of liquid-phase and the composition deviation caused by the volatilization of alkali components during sintering.     

Preparation,microstructure, and mechanical properties of ZrB2-based ceramics with layered Zr2Al4C5 inclusions

ZrB₂/Zr₂Al₄C₅ composite ceramics with different volume contents of Zr₂Al₄C₅ formed in situ were fabricated by the spark plasma sintering technique at 1800 ^°C. The content of Zr₂Al₄C₅ was found to have an evident effect on the preparation, phase constitution, microstructure as well as the mechanical properties of ZrB₂/Zr₂Al₄C₅ ceramics. The results indicated that sinterability of the composites was remarkably improved by the addition of Zr₂Al₄C₅ compared to the single-phase ZrB₂ ceramic. The microstructure of the resulting composites was fine and homogeneous, the average grain size of the ZrB₂ decreased, and the average aspect ratio of the Zr₂Al₄C₅ increased with the increase in the amount of Zr₂Al₄C₅. As the content of Zr₂Al₄C₅ increased, both the Vickers hardness and Young's modulus of the composites first increased and then decreased. The fracture toughness of the ZrB₂–40 vol% Zr₂Al₄C₅ composite was 4.25 MPa m^1/2, which increased by approximately 70% compared to the monolithic ZrB₂ ceramic. The improvement was mainly attributed to the toughening mechanisms such as the layered structure toughening, crack deflection and crack bridging, caused by the in situ formed layered Zr₂Al₄C₅ inclusions.     

Sintering behavior and microwave dielectric properties of Bi2O3–ZnO–Nb2O5-based ceramics sintered under air and N2 atmosphere

The sintering behavior and dielectric properties of the monoclinic zirconolite-like structure compound Bi₂(Zn_1/3Nb_2/3)₂O₇ (BZN) and Bi₂(Zn_1/3Nb_2/3−xV_x)₂O₇ (BZNV, x = 0.001) sintered under air and N₂ atmosphere were investigated. The pure phase were obtained between 810 and 990 °C both for BZN and BZNV ceramics. The substitution of V₂O₅ and N₂ atmosphere accelerated the densification of ceramics slightly. The influences on microwave dielectric properties from different atmosphere were discussed in this work. The best microwave properties of BZN ceramics were obtained at 900 °C under N₂ atmosphere with _r = 76.1, Q = 850 and Q_f = 3260 GHz while the best properties of BZNV ceramics were got at 930 °C under air atmosphere with _r = 76.7, Q = 890 and Q_f = 3580 GHz. The temperature coefficient of resonant frequency τ_f was not obviously influenced by the different atmospheres. For BZN ceramics the τ_f was −79.8 ppm/°C while τ_f is −87.5 ppm/°C for BZNV ceramics.     

Electrical and mechanical properties of ferroelectric lead zirconate titanate/tungsten oxide ceramics

Ferroelectric PZT/xWO₃ ceramics (when x = 0, 0.5, 1, 3 and 5 vol%) were fabricated from PZT and nano-sized WO₃ powders by a solid-state mixed-oxide method. Phase characterization suggested that the reaction between PZT and WO₃ occurred during the sintering. This reaction seemed more pronounced with increasing the content of WO₃. The maximum density at approximately 97% of the theoretical value was achieved at 1 vol% of WO₃ addition. The grain size was reduced with an addition of WO₃ particles from 7.8 μm for PZT to 1.8 μm for 0.5 vol% WO₃ and 0.8 μm for 1–5 vol% WO₃. Mechanical properties of PZT could be improved with an addition of WO₃ nano-particulates. The addition of 0.5 vol% WO₃ could maintain good electrical properties while increasing WO₃ significantly reduced dielectric and piezoelectric constants of the PZT.     

The phase structure and electric properties of low-temperature sintered (K,Na)NbO3-based piezoceramics modified by CuO

0.25 wt% CuO-doped (Li,K,Na)(Nb,Ta)O₃–AgSbO₃ lead-free piezoceramics with pure perovskite structure were successfully prepared at a sintering temperature below 1000 °C. The sintering temperature of KNN-based piezoceramics was effectively reduced by about 100 °C due to the enhanced densification process induced by the addition of CuO. Besides, the acceptable sintering temperature window was broadened by the addition of CuO. It is found that the CuO-doped samples show slightly higher tetragonal–orthorhombic phase transition point (T_T−O) but a lower Curie point (T_c), compared to undoped ones. The KNN-based piezoceramics became “hard” as CuO was added, supported by an increase of Q_m. Fairly good electrical properties of d₃₃^*=383 pm/V, ε_r=860, Q_m=188 and T_c=215 °C could be obtained in dense CuO-modified KNN-based piezoceramics sintered at 970 °C, demonstrating promising potential in practical applications.     

Microstructure and electrical properties of Sc2O3-doped ZnO-Bi2O3-based varistor ceramics

The microstructure and electrical properties of ZnO-Bi₂O₃-based varistor ceramics doped with different Sc₂O₃ content sintered at 1100 °C were investigated. The results showed that the nonlinear coefficient of the varistor ceramics with Sc₂O₃ were in the range of 18-54, the threshold voltage in the range of 250-332 V/mm, the leakage current in the range of 0.1-23.0 μA, with addition of 0-1.00 mol% Sc₂O₃. The ZnO-Bi₂O₃-based varistor ceramics doped with Sc₂O₃ content of 0.12 mol% exhibited the highest nonlinearity, in which the nonlinear coefficient is 54, the threshold voltage and the leakage current is 278 V/mm and 2.9 μA, respectively. The results confirmed that doping with Sc₂O₃ was a very promising route for the production of the higher nonlinear coefficient of ZnO-Bi₂O₃-based varistor ceramics, and determining the proper amounts of addition of Sc₂O₃ was of great importance.     

Processing of Bi1.5ZnNb1.5O7 ceramics for LTCC applications: Comparison of synthesis and sintering methods

Bismuth zinc niobate posses a cubic pyrochlore structure and normally is obtained by the conventional solid-state reaction. The great disadvantage of this method is the lack of chemical homogeneity, requiring high synthesis and sintering temperatures (higher than 1000 °C), which is an impeditive for BZN application in LTCC with silver as the internal electrode. The aim of this paper is to compare, from synthesis to sintering, BZN ceramics, derived either from chemically or conventionally synthesized powders, sintered either in both conventional oven for 2 h or microwave oven for 15 min. The results showed that chemically synthesized BZN ceramics sintered in microwave oven at 900 °C for 15 min presented a relative density of 97%, while those obtained by conventional method required 1000 °C to reach the same density. Despite the short period for thermal treatment in microwave oven, the electrical properties of BZN ceramics are compatible with those sintered in conventional oven for 2 h.     

Structure and electrical properties of Ca0.28Ba0.72Nb2O6 ceramics with addition of rare earth oxides (CeO2, La2O3)

Ca_0.28Ba_0.72Nb₂O₆ (CBN28) ceramics with addition of CeO₂ and La₂O₃, were prepared by the conventional ceramic fabrication technique. XRD results showed that the single tungsten bronze structure of CBN28 was not changed by adding CeO₂ or La₂O₃. SEM results indicated that both CeO₂ and La₂O₃ dopants were effective in inhibiting the grain growth and suppressing the anisotropic growth behavior in tungsten bronze structure. It was also found that both two kinds of dopants had remarkable effects on the dielectric and ferroelectric properties of CBN28 ceramics. Compared with CBN28 ceramics, the dielectric constant around room temperature ε_r, dielectric loss tan δ, the degree of diffuseness γ and coercive field E_c were all ameliorated when doping proper amount of CeO₂ or La₂O₃. The comprehensive electric performance was obtained in CBN28–0.3 wt% CeO₂ and CBN28–0.4 wt% La₂O₃ ceramics. Besides, the underlying mechanism for variations of the electrical properties due to different dopants was explained in this work.     

Low-temperature sintering of 0.85CaWO4–0.15LaNbO4 ceramics

Microwave dielectric properties of 0.85CaWO₄–0.15LaNbO₄ (CWLN) ceramics were investigated as a function of H₃BO₃, Li₂CO₃ content and sintering temperature. With the co-addition of 3.0 wt.% H₃BO₃–1.0 wt.% Li₂CO₃, the sintering temperature could be effectively reduced from 1150 °C for pure CWLN ceramics to 900 °C without any degradation of dielectric properties. These results are due to the enhancement of the sinterability of CWLN by liquid phase sintering. For the specimens with H₃BO₃–Li₂CO₃ sintered at 900 °C for 3 h, the dielectric constant (K) did not changed remarkably. However, the quality factor (Qf) and the temperature coefficient of resonant frequency (TCF) increased up to y = 1.0 of 3.0 wt.% H₃BO₃–y wt.% Li₂CO₃, and then decreased due to the formation of the secondary phases. Typically, K of 11.8, Qf of 45,200 GHz and TCF of −23.1 ppm/°C were obtained for the specimens of CWLN with 3.0 wt.% H₃BO₃–1.0 wt.% Li₂CO₃ sintered at 900 °C for 3 h.     

TaB2-based ceramics: Microstructure, mechanical properties and oxidation resistance

TaB₂-based ceramics were hot pressed in low vacuum with addition of 5-10 vol% MoSi₂. Temperatures in the range of 1680-1780 °C led to relative density around 90-95%. The hardness was about 18 GPa, the fracture toughness 4.6 MPa m^1/2 and the room temperature flexural strength was around 630 MPa, but abruptly decreased above 1200 °C to 220 MPa. The composite containing 10 vol% of MoSi₂ was tested in a bottom-up furnace in the temperature range 1200-1700 °C for 30 min. The microstructure appeared covered by a SiO₂ layer, whose thickness increased with the temperature, but the bulk remained unaltered up to 1600 °C. At 1700 °C the specimen vaporized. Nanoindentation was employed on the oxidized cross sections in order to detect eventual mechanical properties modification associated to chemical/microstructural change, like formation of Ta-B-O solid solutions.     

Microwave dielectric properties and co-firing with copper of (Bi1−xCux)(Nb1−xWx)O4 ceramics

Effect of substitution of CuO and WO₃ on the microwave dielectric properties of BiNbO₄ ceramics and the co-firing between ceramics and copper electrode were investigated. The (Bi_1−xCu_x)(Nb_1−xW_x)O₄ (x = 0.005, 0.01, 0.015, 0.02) composition can be densified between 900 and 990 °C. The microwave dielectric constants lie between 36 and 45 and the pores in ceramics were found to be the main influence. The Q values changes between 1400 and 2900 with different x values and sintering temperatures while Q_f values lie between 6000 and 16,000 GHz. The microwave dielectric losses, mainly affected by the grain size, pores, and the secondary phase, are discussed. The (Bi_1−xCu_x)(Nb_1−xW_x)O₄ ceramics and copper electrode was co-fired under N₂ atmosphere at 850 °C and the EDS analysis showed no reaction between the dielectrics and copper electrodes. This result presented the (Bi_1−xCu_x)(Nb_1−xW_x)O₄ dielectric materials to be good candidates for LTCC applications with copper electrode.     

Microstructure and mechanical properties of dental 3Y-TZP ceramics by using CaO-P2O5 glass as additive

This study investigated the effects of glass frits on the sintering and mechanical properties of dental 3Y-TZP ceramics. The glass frits, which consisted of MgO, CaO, Al₂O₃, SiO₂, and P₂O₅, were selected to lower the sintering temperature of zirconia via liquid phase sintering.The results of the experiment showed that these glass frit additives neither destroy the stability of the high temperature t-phase nor induce grain growth. All the mechanical properties and the relative densities were strongly correlated with the addition of glass frits. At lower sintering temperatures, the presence of glass additives resulted in an increase in mechanical properties. At higher sintering temperatures, the presence of glass additives decreased the mechanical properties.     

Preparation of Al2O3 and MgAl2O4-based samples with tailored macroporous structures

In this work we successfully obtained freeze-cast alumina (Al₂O₃) and magnesium aluminate spinel (MgAl₂O₄) samples. Camphene was used as the freezing vehicle in this study. The specimens prepared herein were examined by Archimedes tests, scanning electron microscopy, and X-ray powder diffraction. Cold crushing tests were also carried out at room temperature. It was observed that the pore structure of Al₂O₃ samples can be tailored by changing the solid loading and freezing rate; the higher the solid loading and freezing rate, the finer the pore structure of the freeze-cast sample. MgAl₂O₄-based specimens were fabricated by keeping the solid loading in the starting slurry at 30 vol% and using liquid nitrogen as the cooling agent. The material obtained from a 60 Al₂O₃?40 MgO slurry showed a spinel amount of about 90%, an expressive total porosity (63 ± 3%), and a significant cold crushing strength (58 ± 6 MPa). In addition, this material exhibited the finest pore structure among the composition studied herein, showing a mean pore size of about 4 µm.     

Hydrothermal synthesis and piezoelectric property of Ta-doping K0.5Na0.5NbO3 lead-free piezoelectric ceramic

Ta-doping K_0.5Na_0.5Nb_1−xTa_xO₃ (x = 0.1, 0.2, 0.3, 0.4) powder was synthesized by hydrothermal approach and its ceramics were prepared after sintering and polarizing treatment in this work. The K_0.5Na_0.5Nb_0.7Ta_0.3O₃ ceramics near morphotropic phase boundary (MPB), which exhibited optimum piezoelectric properties of d₃₃ = 210 pC/N and good electromechanical coupling factors of K_p = 0.3. The domain structure has been observed from TEM images which indicates that the K_0.5Na_0.5Nb_0.7Ta_0.3O₃ ceramics have good piezoelectric and ferroelectric properties for it is near the MPB.     

Fabrication and properties of highly transparent Tm3Al5O12 (TmAG) ceramics

Highly transparent Tm₃Al₅O₁₂ (TmAG) ceramics were fabricated by solid-state reaction and vacuum sintering. Densification, microstructure evolution, mechanical, thermal, and optical properties of the TmAG ceramics were investigated. Fully dense TmAG ceramic with average grain size of 15 μm was obtained by sintering at 1780 °C for 20 h. The in-line transmittance was 80.5% at 2000 nm. The absorption coefficients at 682 nm and 785 nm were 8.03 cm⁻¹ and 8.33 cm⁻¹, respectively. The Vickers hardness, the Young modulus, the bending strength, and the fracture toughness values were 15.14 GPa, 343 GPa, 230 MPa, and 2.35 MPa m^1/2, respectively. The thermal conductivity at room temperature was 3.3 W/m K and the average linear thermal expansion coefficient from 20 °C to 1000 °C was 8.915 × 10⁻⁶ K.     

Effect of processing method on physical properties of Nb2O5

Samples of Nb₂O₅ were prepared by laser floating zone (LFZ) technique and by solid-state reaction in order to study some of their physical properties as a function of synthesis conditions. Single crystals fibres were obtained by LFZ, while a structural orthorhombic to monoclinic phase transition was observed in samples sintered at temperature higher than 800 °C. Transmission optical spectroscopy and photoconductivity measurements allowed identifying a ∼3.2 eV bandgap energy for the H-Nb₂O₅ monoclinic crystalline phase. Band gap shrinkage of ∼100 meV was observed from 14 K to RT. For the orthorhombic phase (T-Nb₂O₅), the photoconductivity measurements evidence a higher energy bandgap. The sintered samples have shown a broad recombination luminescence band at the orange/red spectral region while no luminescence was detected from the LFZ grown fibres. A dielectric constant of ∼40 was found for the 800 °C and 1200 °C sintered pellets while that of 1000 °C reached four times that value.     

Enhanced mechanical properties of W-doped Nb4AlC3 ceramics

The W-doped Nb₄AlC₃ ceramics [(Nb_1-xW_x)₄AlC₃, x?=?0–0.0375] were successfully fabricated by in-situ reactive hot-press-aided method using elemental niobium, aluminum, graphite and tungsten powders. The XRD results suggest that the matrix phase (Nb_1-xW_x)₄AlC₃ and the second phase (Nb_1-xW_x)C were simultaneously formed when W was added. The SEM images show that (Nb_1-xW_x)C is dispersed in the W-doped Nb₄AlC₃ ceramics matrix. The mechanical properties of Nb₄AlC₃ were greatly enhanced by W doping. Typically, the (Nb_0.975W_0.025)₄AlC₃ exhibits the highest flexural strength (483?±?21?MPa), fracture toughness (8.5?±?0.3?MPa?m^1/2) and Young′s modulus (382?±?18?GPa) at room temperature (RT), which are increased by 59%, 15% and 30%, respectively, compared with the present Nb₄AlC₃. The Vickers hardness of (Nb_0.9625W_0.0375)₄AlC₃ (4.8?±?0.2?GPa) is 92% higher than that of Nb₄AlC₃. The (Nb_0.975W_0.025)₄AlC₃ also retains a high flexural strength of 344?±?4?MPa at 1400?°C (71% of RT value), which is much higher than the RT flexural strength (303?±?22?MPa) of the present Nb₄AlC₃. The strengthening effect is attributed to the solid solution of W and the incorporation of the second phase (Nb_1-xW_x)C. The excellent mechanical properties endow the W-doped Nb₄AlC₃ ceramics as promising high-temperature structural materials.     

Processing and properties of Pb(Mg1/3Nb2/3)O3–PbTiO3-based ceramics

Ceramic compositions of a combination between lead magnesium niobate, Pb(Mg_1/3Nb_2/3)O₃, and lead titanate, PbTiO₃, were fabricated using the Mg₄Nb₂O₉ precursor technique. Their electrical properties with respect to temperature and frequency were examined and the effect of sintering conditions on phase formation, densification, microstructure and electrical properties of the ceramics were examined. It has been found that optimisation of sintering conditions can lead to a highly dense and pyrochlore-free PMN–PT ceramics. The gradual decrease of the physical properties of the sintered ceramics was related to the gradual decrease of density and inhomogeneous microstructure. The results also revealed that for the lower concentration of lead titanate, a relaxor behaviour is noticed with a high electrostrictive effect, which was almost hysteretic free. However, higher amount of lead titanate led to a normal ferroelectric behaviour.     

Effect of the excess of bismuth on the morphology and properties of the BaBi2Nb2O9 thin films

In this study, the effect of bismuth content on the crystal structure, morphology and electric properties of barium bismuth niobate (BaBi₂Nb₂O₉) thin films was explored with the aid of X-ray diffraction (XRD), scanning electron microcopy (SEM), atomic force microscopy (AFM) and dielectric properties. BaBi₂Nb₂O₉ (BBN) thin films have been successfully prepared by the polymeric precursor methods and deposited by spin coating on Pt/Ti/SiO₂/Si (1 0 0) substrates. The phase formation, the grain size and morphology of the thin films were influenced by the addition of bismuth in excess. It was observed that the formation of single phase BBN for films was prepared with excess of bismuth up to 2 wt%. The films prepared with excess of the bismuth showed higher grain size and better dielectric properties. The 2 wt% bismuth excess BBN thin film exhibited dielectric constant of about 335 with a loss of 0.049 at a frequency of 100 kHz at room temperature.