Low temperature sintering and properties of piezoelectric PZT-PFW-PMN ceramics with YMnO3 addition

Low temperature sintering of Pb(Zr,Ti)O₃-Pb(Fe_2/3W_1/3)O₃-Pb(Mn_1/3Nb_2/3)O₃ (PZT-PFW-PMN) quaternary piezoelectric ceramics were studied with the use of YMnO₃ as sintering aid. The sintering aid improved the sinterability of PZT-PFW-PMN ceramics due to the effect of YMnO₃ liquid phase. The effects of YMnO₃ contents and sintering temperature on the phase structure, density, dielectric and piezoelectric properties were investigated. The results show that the sintering temperature can be decreased and the electrical properties can be maintained by the YMnO₃ addition. The optimized properties were obtained by doping 0.30 wt.% YMnO₃ and sintering at 1020 °C, which are listed as follows: d₃₃ = 341 pC/N, K_p = 0.57, Q_m = 1393, tan δ = 0.0053, T_c = 304 °C, P_r = 17.13 μC/cm² and E_c = 11.15 kV/cm, which make this system be a promising material for multilayer piezoelectric actuator and transformer applications.     

Microwave dielectric properties and microstructures of the 11Li2O-3Nb2O5-12TiO2 ceramics with B2O3 addition

The effects of B₂O₃ addition, as a sintering agent, on the sintering behavior, microstructure and microwave dielectric properties of the 11Li₂O-3Nb₂O₅-12TiO₂ (LNT) ceramics have been investigated. With the low-level doping of B₂O₃ (≤2 wt.%), the sintering temperature of the LNT ceramic could be effectively reduced to 900 °C. The B₂O₃-doped LNT ceramics are also composed of Li₂TiO₃ss and “M-phase” phases. No other phase could be observed in the 0.5-2 wt.% B₂O₃-doped ceramics sintered at 840-920 °C. The addition of B₂O₃ induced no obvious degradation in the microwave dielectric properties but increased the τ_f values. Typically, the 0.5 wt.% B₂O₃-doped ceramics sintered at 900 °C have better microwave dielectric properties of ?_r = 49.2, Q × f = 8839 GHz, τ_f = 57.6 ppm/°C, which suggest that the ceramics could be applied in multilayer microwave devices requiring low sintering temperatures.     

Low-temperature firing and microwave dielectric properties of ZnO-Nb2O5-TiO2-SnO2 ceramics with CuO-V2O5

The effects of CuO-V₂O₅ addition on the sintering temperature and microwave dielectric properties of ZnO-Nb₂O₅-TiO₂-SnO₂ were investigated. The CuO-V₂O₅ addition lowered the sintering temperature of ZnO-Nb₂O₅-TiO₂-SnO₂ ceramics effectively from 1150 to 860 °C due to the liquid-phase effect of Cu₂V₂O₇ and Cu₃(VO₄)₂, as observed by XRD. The microwave dielectric properties were found to strongly correlate with the sintering temperature and the amount of CuO-V₂O₅ addition. The maximum Qf values decreased with increasing CuO-V₂O₅ content, due to the formation of the second phase, Cu₃(VO₄)₂ and CuNbO₃. Zero τ_f value can be obtained by properly adjusting the sintering temperature. At 860 °C, ZnO-Nb₂O₅-TiO₂-SnO₂ ceramics with 1.5 wt.% CuO-V₂O₅ gave excellent microwave dielectric properties: ?_r = 42.3, Qf = 9000 GHz and τ_f = 8 ppm/°C.     

Microstructure and mechanical properties of silicon nitride-titanium nitride composites prepared by spark plasma sintering

Si₃N₄-TiN composites were prepared by spark plasma sintering (conventional sintering (SPS1) and in situ reaction sintering (SPS2)). Homogeneous distribution of equiaxed TiN grains in Si₃N₄ matrix results in the highest microhardness (21.7 GPa) and bending strength (621 MPa) of sample SPS1 sintered at 1550 °C. Dispersion of elongated TiN grains in Si₃N₄ matrix results in the highest fracture toughness (8.39 MPa m^1/2) of sample SPS2 sintered at 1300 °C.     

Influence of Bi2O3 and CuO addition on low-temperature sintering and dielectric properties of Ba0.6Sr0.4TiO3 ceramics

In this study, we tried to lower the sintering temperature of Ba_0.6Sr_0.4TiO₃ (BST) ceramics by several kinds of adding methods of Bi₂O₃, CuO and CuBi₂O₄ additives. The effects of different adding methods on the microstructures and the dielectric properties of BST ceramics have been studied. In the all additive systems, the single addition of CuBi₂O₄ was the most effective way for lowering the sintering temperature of BST. When CuBi₂O₄ of 0.6 mol% was mixed with starting BST powders and sintered at 1100 °C, the derived ceramics demonstrated dense microstructure with a low dielectric constant (? = 4240), low dielectric loss (tan δ = 0.0058), high tunability (Tun = 38.3%) and high Q value (Q = 251). It was noteworthy that the sintering temperature was significantly lowered by 350 °C compared with no-additive system, and the derived ceramics maintained the excellent microwave dielectric properties corresponding to pure BST.     

Synthesis and microwave dielectric properties of CaO-MgO-SiO2 submicron powders doped with Li2O-Bi2O3 by sol-gel method

Using Ca(NO₃)₂·4H₂O, Mg(NO₃)₂·6H₂O, Si(OC₂H₅)₄, LiNO₃ and Bi(NO₃)₃·5H₂O as raw materials, CaO-MgO-SiO₂ submicron powders were prepared at low temperature by sol-gel method. The crystallization temperature was decreased enormously by the introduction of Li-Bi liquid phase sintering aids into Ca-Mg-Si sol, and the powders with average particle sizes of 80-100 nm and 200-400 nm were obtained at the calcining temperature of 750 °C and 800 °C, respectively. The sintering characteristic and dielectric properties of powders calcined at 750 °C with different content of powders calcined at 800 °C were studied. When the content of powders calcined at 800 °C was 10 wt%, the dielectric ceramic sintered at 890 °C had compact structure, and possessed excellent microwave dielectric properties: ?_r = 7.16, Q × f = 25630 GHz, τ_f = −69.26 ppm/°C.     

Synthesis of CuCr and CuCrAg alloy with nano-ceramic dispersion by mechanical alloying and consolidation by laser assisted sintering

Cu-4.5Cr and Cu-4.5Cr-3Ag (in wt%) alloys without or with 10 wt% nanocrystalline Al₂O₃ and ZrO₂ dispersion have been synthesized by mechanical alloying or milling and consolidated by laser assisted sintering in Ar atmosphere. Microstructural characterization by scanning and transmission electron microscopy and phase analysis by X-ray diffraction suggest that the alloyed matrix undergoes significant grain growth after sintering while the dispersoids retain their ultrafine size and uniform distribution in the matrix. The dispersion of nano-Al₂O₃ is more effective than that of nano-ZrO₂ in enhancing the mechanical properties due to the smaller initial particle size of Al₂O₃ than that of ZrO₂. In general, laser sintering of mechanically alloyed Cu-4.5Cr and Cu-4.5Cr-3Ag alloys with 10 wt% nanocrystalline Al₂O₃ at 100 W laser power and 1-2 mm s⁻¹ scan speed yields the optimum combination of high density (7.1-7.5 mg m⁻³), hardness (165-225 VHN), wear resistance and electrical conductivity (13-20% IACS).     

Effect of sintering temperature on dielectric properties of Ba0.6Sr0.4TiO3–MgO composite ceramics prepared from fine constituent powders

Composite ceramics of Ba_0.6Sr_0.4TiO₃ + 60 wt.% MgO were prepared from fine constituent powders by sintering at 1200–1280 °C. The composite specimens sintered at the relatively low temperatures showed satisfactory densification due to fine morphology of the constituent powders. The elevation of sintering temperature promoted the incorporation of Mg²⁺ into the lattice of the Ba_0.6Sr_0.4TiO₃ phase and grain growth of the two constituent phases. The dependence of the dielectric properties on sintering temperature was explained in relation to the structural evolution. Controlling the sintering temperature of the composite was found to be important to achieve the desired nonlinear dielectric properties. Sintering at 1230 °C was determined to be preferred for the composite in terms of the nonlinear dielectric properties. The specimen sintered at the temperature attained a tunability of 17.3% and a figure of merit of 127 at 10 kHz and 20 kV/cm.     

Pretreatment and sintering of Si3N4 powder synthesized by the high-temperature self-propagation method

Self-propagation high-temperature synthesis (SHS) was applied for the synthesis of low-cost Si₃N₄ powder. The powder was purified and ground until its particle size reached submicron levels and its purity reached 98%. Using this pretreated powder, with α/β = 60/40 content, fully dense Si₃N₄ ceramics, having improved mechanical properties, were obtained by liquid-phase sintering in the presence of (Y, La)₂O₃-AlN. The mechanical properties achieved finally were as follows: strength, 784 MPa; hardness, 15.1 GPa; and fracture toughness, 5.2 MPa m^0.5. The behaviors of the SHS-Si₃N₄ powders before and after the pretreatment were compared. The relation between microstructure and mechanical properties of the sintered specimens and the effect of different β content in the powder on the sintering process of Si₃N₄ were also studied.     

Study on properties of CaO-SiO2-B2O3 system glass-ceramic

Low temperature co-fired ceramic (LTCC) is prepared by sintering a glass selected from CaO-SiO₂-B₂O₃ system, and its sintered bodies are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). It is found that the optimal sintering temperature for this glass-ceramic is 820 °C for 15 min, and the major phases of this material are CaSiO₃, CaB₂O₄ and SiO₂. The glass-ceramic possesses excellent dielectric properties: ?_r = 6.5, tan δ < 2 × 10⁻³ at 10 MHz, temperature coefficient of dielectric constant about −51 × 10⁻⁶ °C⁻¹ and coefficient of thermal expansion about 8 × 10⁻⁶ °C⁻¹ at 20-400 °C. Thus, this material is supposed to be suitable for the tape casting process and be compatible with Ag electrode, which could be used as the LTCC materials for the application in wireless communications.     

(5 − x)BaO-xMgO-2Nb2O5 ceramics prepared using a reaction-sintering process

(5 − x)BaO-xMgO-2Nb₂O₅ (x = 0.5 and 1; 5MBN and 10MBN) microwave ceramics prepared using a reaction-sintering process were investigated. Without any calcinations involved, the mixture of BaCO₃, MgO, and Nb₂O₅ was pressed and sintered directly. MBN ceramics were produced after 2-6 h of sintering at 1350-1500 °C. The formation of (BaMg)₅Nb₄O₁₅ was a major phase in producing 5MBN ceramics, and the formation of Ba(Mg_1/3Nb_2/3)O₃ was a major phase in producing 10MBN ceramics. As CuO (1 wt%) was added, the sintering temperature dropped by more than 150 °C. We produced 5MBN ceramics with these dielectric properties: ?_r = 36.69, Qf = 20,097 GHz, and τ_f = 61.1 ppm/°C, and 10MBN ceramics with these dielectric properties: ?_r = 39.2, Qf = 43,878 GHz, and τ_f = 37.6 ppm/°C. The reaction-sintering process is a simple and effective method for producing (5 − x)BaO-xMgO-2Nb₂O₅ ceramics for applications in microwave dielectric resonators.     

Spark plasma sintering of BiFeO3

Dense BiFeO₃ ceramics were prepared by a novel spark plasma sintering (SPS) technique. The sintering was conducted at temperatures ranging from 675 to 750 °C under 70 MPa pressure. A bulk density value up to 96% of theoretical density was achieved in the process. This contrast to around 90% of the theoretical density achieved by conventional sintering at around 830 °C. It was found that the tendency to form unwanted Bi₂Fe₄O₉ phase is higher at a high sintering temperature for SPS. The dielectric and ferroelectric properties also improved (with respect to conventionally sintered sample) for spark plasma-sintered samples.     

Reactive and dense sintering of reinforced-toughened B4C matrix composites

Reactive hot-press (1800-1880 °C, 30 MPa, vacuum) is used to fabricate relatively dense B₄C matrix light composites with the sintering additive of (Al₂O₃ +Y₂O₃). Phase composition, microstructure and mechanical properties are determined by methods of XRD, SEM and SENB, etc. These results show that reactions among original powders B₄C, Si₃N₄ and TiC occur during sintering and new phases as SiC, TiB₂ and BN are produced. The sandwich SiC and claviform TiB₂ play an important role in improving the properties. The composites are ultimately and compactly sintered owing to higher temperature, fine grains and liquid phase sintering, with the highest relative density of 95.6%. The composite sintered at 1880 °C possesses the best general properties with bending strength of 540 MPa and fracture toughness of 5.6 MPa m^1/2, 29 and 80% higher than that of monolithic B₄C, respectively. The fracture mode is the combination of transgranular fracture and intergranular fracture. The toughening mechanism is certified to consist of crack deflection, crack bridging and pulling-out effects of the grains.     

Reaction sintering fabrication of (AlN, TiN)-Al2O3 composite

The (AlN, TiN)-Al₂O₃ composites were fabricated by reaction sintering powder mixtures containing 10-30 wt.% (Al, Ti)-Al₂O₃ at 1420-1520°C in nitrogen. It was found that the densification and mechanical properties of the sintered composites depended strongly on the Al, Ti contents of the starting powder and hot pressing parameters. Reaction sintering 20 wt.% (Al, Ti)-Al₂O₃ powder in nitrogen in 1520°C for 30 min yields (AlN, TiN)-Al₂O₃ composites with the best mechanical properties, with a hardness HRA of 94.1, bending strength of 687 MPa, and fracture toughness of 6.5 MPa m^1/2. Microstructure analysis indicated that TiN is present as well dispersed particulates within a matrix of Al₂O₃. The AlN identified by XRD was not directly observed, but probably resides at the Al₂O₃ grain boundary. The fracture mode of these composites was observed to be transgranular.     

Low-temperature sintering of temperature-stable LaNbO4 microwave dielectric ceramics

We demonstrate the correlation between sintering behavior and microstructural observations in low-temperature sintered, LaNbO₄ microwave ceramics. Small CuO additions to LaNbO₄ significantly lowered the sintering temperature from 1250 to 950 °C. To elucidate the sintering mechanism, the internal microstructure of the sample manipulated by a focused ion beam (FIB) was investigated using transmission electron microscopy (TEM) and energy-dispersive spectroscopy (EDS). LaNbO₄ with 3 wt% CuO sintered at 950 °C for 2 h possessed the following excellent microwave dielectric properties: a quality factor (Qxf) of 49,000 GHz, relative dielectric constant (?_r) of 19.5, and temperature coefficient of resonant frequency (τ_f) of 1 ppm/°C. The ferroelastic phase transformation was also investigated using in situ X-ray diffraction (XRD) to explain the variation of τ_f in low-temperature sintered LaNbO₄ as a function of CuO content.     

Effect of sintering process on the microstructures of Bi2O3-doped yttria stabilized zirconia

Variations of microstructures in Bi₂O₃-doped yttria stabilized zirconia (YSZ) with conventional furnace and microwave sintering were investigated in this work. The results demonstrated that a small amount of addition of Bi₂O₃ was effective in reducing the sintering temperature of YSZ from 1500 °C to 1200 °C and promoting the densification rate of the ceramics. It is interesting that microwave sintering is found to suppress the evaporation rate of Bi₂O₃ and formation of the monoclinic-ZrO₂ or other amorphous phases. Compared to conventional furnace sintering, significant improvement in density of Bi₂O₃-doped YSZ at lower sintering temperatures with microwave sintering was observed. Rapid heating rate and short sintering time for restricting serious segregation at grain boundary were observed as well. Employing microwave sintering at the same sintered condition, the density of a specimen was evidently increased by 4.59% in comparison to the specimen sintered with a conventional furnace sintering.     

Effect of phosphorus addition on the sintering and dielectric properties of Pb(Zr0.52Ti0.48)O3

It has been found that the sintering temperature of piezoelectric Pb(Zr_0.52Ti_0.48)O₃ (PZT) can be reduced by phosphorus addition without compromising the dielectric properties. A sintered density of 98.6% of the theoretical density was obtained for 2 wt.% P₂O₅ addition after sintering at 1050 °C for 4 h. The P₂O₅ addition, either above or below 2 wt.%, showed an inferior densification. Coincidentally, the P₂O₅ addition gave rise to a lower lead loss, and the dielectric constant showed a peak at 1 wt.% P₂O₅ addition.     

Study on properties of BaxSmyTi7O20 ceramics with CaO-SiO2-B2O3 glass

The effect of CaO-SiO₂-B₂O₃ (CSB) glass addition on the sintering temperature and dielectric properties of Ba_xSm_yTi₇O₂₀ ceramics has been investigated using X-ray diffraction, scanning electron microscopy and differential thermal analysis. The CSB glass starts to melt at about 970 °C, and a small amount of CSB glass addition to Ba_xSm_yTi₇O₂₀ ceramics can greatly decrease the sintering temperature from about 1350 to about 1260 °C, which is attributed to the formation of liquid phase. It is found that the dielectric properties of Ba_xSm_yTi₇O₂₀ ceramics are dependent on the amount of CSB glass and the microstructures of sintered samples. The product with 5 wt% CSB glass sintered at 1260 °C is optimal in these samples based on the microstructure and the properties of sintering product, when the major phases of this material are BaSm₂Ti₄O₁₂ and BaTi₄O₉. The material possesses excellent dielectric properties: ?_r = 61, tan δ = 1.5 × 10⁻⁴ at 10 GHz, temperature coefficient of dielectric constant is −75 × 10⁻⁶ °C⁻¹.     

Sintering, microstructure and piezoelectric properties of CuO and SnO2 co-modified sodium potassium niobate ceramics

The CuO and SnO₂ co-modified Na_0.52K_0.48NbO₃ ceramics were prepared by a conventional mixed oxide method. Densification can be further improved but the grain growth is inhibited as a small amount of SnO₂ is added into 1% CuO doped Na_0.52K_0.48NbO₃. The results indicate that the physical and various electrical properties of CuO and SnO₂ doped Na_0.52K_0.48NbO₃ ceramics significantly depend on sintering conditions and the content of dopants. The ceramics doped with 1 mol% CuO and 1 mol% SnO₂ sintered at 1070 °C for 3 h show improved dielectric and piezoelectric properties: d₃₃ = 120 pC/N, k_p = 0.38, Q_m = 1040, ?_r = 710 and tanδ = 0.013 (1 kHz), in comparison with un-doped or CuO doped compositions.     

Dielectric properties of barium strontium titanate thick films prepared by electrophoretic deposition

Ba_xSr_1−xTiO₃ (x = 0.5, 0.6, 0.7) thick films were prepared by electrophoretic deposition (EPD) technique on platinum metallic foils using BaTiO₃ and SrTiO₃ nanoparticles with different molar proportion of 1:1, 3:2 and 7:3, respectively. An isostatic pressure method was used to increase density of the thick films before high temperature sintering. Microstructures of the deposited films were examined with XRD and SEM techniques. Porosity of the thick films decreased after the isostatic pressure process. The Ba_0.5Sr_0.5TiO₃ thick films of 10 μm, 15 μm and 20 μm showed a tunability of 28.8%, 33.3% and 33.9%, respectively, at room temperature and at a biasing field of 2 kV/mm. The dielectric constant was from 2138 to 3446 and dielectric loss was from 0.016 to 0.011 at zero bias field at 10 kHz. The temperature dependence of dielectric constant was also measured and the effect of porosity and thickness on the electrical performance of the thick films was discussed.