Preparation of polycrystalline BaTi2O5 by pressureless sintering

Polycrystalline BaTi₂O₅ (BT₂) was prepared by pressureless sintering in air using BaCO₃ and TiO₂ as starting materials. XRD results of the calcined powder showed BaCO₃ and TiO₂ reacted completely after being calcined above 950 °C, showing a mixture of BaTiO₃ (BT), BT₂, BaTi₄O₉ and Ba₄Ti₁₃O₃₀. A small amount of ZrO₂ (less than 0.1 wt%) was effective to prepare BT₂ in a single phase and the second phase of BT and B₆T₁₇ increased with ZrO₂ content. BT₂ sintered body in a single phase was obtained at 1175-1300 °C when ZrO₂ content was 0.08 wt%. The maximum permittivity of BT₂ sintered body was 340 at the Curie temperature (T_c) of 463 °C and the frequency of 100 kHz.     

Transparent polycrystalline ruby ceramic by spark plasma sintering

Fine-grained and transparent polycrystalline ruby ceramics (Cr₂O₃-doped Al₂O₃) were successfully prepared by spark plasma sintering (SPS). The effect of Cr₂O₃ concentration on the grain size, hardness, fracture toughness and thermal conductivity of ruby ceramics was investigated systematically. For 0.05 wt.% Cr₂O₃, high in-line transmittance of 85% at 2000 nm can be reached, further increase of Cr₂O₃ concentration leads to the decrease in transmittance. High hardness of 23.95-25.05 GPa can be achieved due to the fine grain size in all ruby ceramics. The fracture toughness of 1.9-2.29 MPa m^1/2 indicates that no improvement in fracture toughness over pure Al₂O₃ can be obtained by Cr₂O₃ doping in these submicron grained ruby ceramics. High thermal conductivity of 28-29.8 W/(m K) at room temperature, close to that of single crystal sapphire, can be achieved. The change in grain size for different Cr₂O₃ concentrations is the major reason for the change in mechanical and thermal properties, but not for the change in optical properties.     

Aluminum doped zirconia nanopowders: Wet-chemical synthesis and structural analysis by Rietveld refinement

Alumina/zirconia nanopowders, with up to 20 mol% Al₂O₃, were prepared by wet-chemical synthesis technique, using controlled hydrolysis of alkoxides. The as-synthesized powders are amorphous, have very high specific surface area and the corresponding particle size smaller than 4 nm. Amorphous powders with 0, 10 and 20 mol% Al₂O₃ crystallize at 460, 692 and 749 °C, respectively, as a single-phase tetragonal zirconia, without any traces of alumina phases. Rietvled refinement of X-ray diffraction data, used for the detailed structural analysis of annealed nanopowders, showed that the high-temperature zirconia phase is stabilized due to the formation of ZrO₂/Al₂O₃ solid solutions. High solubility of alumina in the tetragonal zirconia (up to 28.6 at% Al³⁺) and stabilization of tetragonal zirconia solid solution up to high temperature (as high as 1150 °C) were also confirmed.     

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.     

Characterization of Al2O3/ZrO2 nano multilayer thin films prepared by pulsed laser deposition

Microstructural characterization of pulsed laser deposited Al₂O₃/ZrO₂ multilayers on Si (1 0 0) substrates at an optimized oxygen partial pressure of 3 × 10⁻² mbar and at room temperature (298 K) has been carried out. A nanolaminate structure consisting of alternate layers of ZrO₂ and Al₂O₃ with 40 bi-layers was fabricated at different zirconia layer thicknesses (20, 15 and 10 nm). The objective of the work is to study the effect of ZrO₂ layer thickness on the stabilization of tetragonal ZrO₂ phase for a constant Al₂O₃ layer thickness of 5 nm. The Al₂O₃/ZrO₂ multilayer films were characterized using high temperature X-ray diffraction (HTXRD) in the temperature range 298–1473 K. The studies showed that the thickness of the zirconia layer has a profound influence on the crystallization temperature for the formation of tetragonal zirconia phase. The tetragonal phase content increased with the decrease of ZrO₂ layer thickness. The cross-sectional transmission electron microscope (XTEM) investigations were carried out on a multilayer thin films deposited at room temperature. The XTEM studies showed the formation of uniform thickness layers with higher fraction of monoclinic and small fraction of tetragonal phases of zirconia and amorphous alumina.     

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.     

Effect of liquid-forming additives on the sintering and mechanical properties of Al2O3/3Y-TZP (30 vol.%) composite

Al₂O₃/3Y-TZP (30 vol.%) composite was pressurelessly sintered with addition of TiO₂MnO₂ and/or CaOAl₂O₃SiO₂ glass. It was found that TiO₂MnO₂ addition greatly enhanced the densification of the composite by the formation of a low-viscosity liquid at sintering temperature. In contrast, the high-viscosity liquid formed by CaOAl₂O₃SiO₂ glass improved mechanical properties because of its repressing effect on grain growth. The composite could be obtained at a temperature as low as 1400°C by co-doping with TiO₂MnO₂ and CAS glass. Bending strength of 552±64 MPa and fracture toughness of 6.03±0.22 MPa m^1/2 were obtained with a doping level of 2 wt.% TiO₂MnO₂ and 2 wt.% CAS glass.     

In situ fabrication of (α-Al2O3 + Al3Zr)/Al composites in an Al–ZrO2 system

Synthesis of a (α-Al₂O₃ + Al₃Zr)/Al in situ composite via exothermic dispersion synthesis (XD) process in an Al–ZrO₂ system has been investigated. Thermodynamic analysis indicates that the reaction between Al and ZrO₂ can spontaneously occur due to its negative Gibbs free energy of the Al–ZrO₂ system. When the reinforcement volume fraction is 30 vol.%, there is only one exothermic peak observed in the DSC curve. The final combustion products are α-Al₂O₃ particles and Al₃Zr blocks, which distribute uniformly in the aluminum matrix. The tensile strength, elongation rate and micro-hardness are 215.2 MPa, 3.0% and 304 MPa, respectively. However, if the reinforcement volume fraction increases to 100 vol.%, two exothermic peaks are observed in its corresponding DSC curve and the peak intensities are significantly increased. When the heating rates are 10 °C/min, 20 °C/min and 30 °C/min, the initiation temperatures are 744 °C, 776 °C and 782 °C, respectively. The reactions occurred in the Al–ZrO₂ system have two steps: the first is that the Al atom reacts with ZrO₂ to produce the α-Al₂O₃ and active Zr atoms, and the second is that the active Zr atoms disperse into the matrix and react with Al to form Al₃Zr. The values of activation energy of the two reactions are around 315.8 kJ/mol and 191.1 kJ/mol, respectively.     

Sintering and dielectric properties of 0.88Al2O3-0.12TiO2 microwave ceramics by glass addition

The microwave characteristics and the microstructures of 0.88Al₂O₃-0.12TiO₂ with various amounts of MgO-CaO-SiO₂-Al₂O₃ (MCAS) glass sintered at different temperatures have been investigated. The sintering temperature can be lowered to 1300 °C by the addition of MCAS glass. The densities, dielectric constants (ε_r) and quality values (Q×f) of the MCAS-added 0.88Al₂O₃-0.12TiO₂ ceramics decrease with the increase of MCAS glass content. The temperature coefficients of the resonant frequency (τ_f) are shifted to more negative values as the MCAS content or the sintering temperatures increase. The change of the crystalline phases of Al₂TiO₅ phase and rutile-TiO₂ phase has profound effects on the microwave dielectric properties of the MCAS-added Al₂O₃-TiO₂ ceramics. As sintered at 1250 °C, 0.88Al₂O₃-0.12TiO₂ ceramics with 2 wt.% MCAS glass addition exists a ε_r value of 8.63, a Q×f value of 9578 and a τ_f value of +5 ppm/°C.     

Preparation and properties evaluation of zirconia-based/Al2O3 composites as electrolytes for solid oxide fuel cell systems

Yttria-doped zirconia powders containing 3 to 8 mol% Y₂O₃ and 0 to 20 wt% Al₂O₃ were prepared by both mixing commercial oxides and a coprecipitation method, and the mechanical and electrical properties have been examined as a function of the Al₂O₃ content. The bending strength of the composite at room temperature increased with increasing Al₂O₃ content. In the temperature range 500–1000 °C the bending strength increased with Al₂O₃ content up to 10 wt% and then decreased, the measured value at 1000 °C (200 MPa) being higher than those at lower temperatures for cubic zirconia materials. Fracture toughness (K_IC) decreased with increasing Y₂O₃ content in the Al₂O₃-free zirconia materials. Al₂O₃ additions enhanced the fracture toughness and this was maximum (7 MPa m^1/2) for the composite ZrO₂-3 mol% Y₂O₃/10 wt% Al₂O₃. The electrical conductivity of cubic ZrO₂/Al₂O₃ composites decreased monotonically with Al₂O₃ content, but in tetragonal ZrO₂/Al₂O₃ composites hardly varied or apparently increased up to 10 wt% Al₂O₃. At 1000 °C the highest electrical conductivity was 0.30 S cm^–1 for ZrO₂-8 mol% Y₂O₃, and this decreased up to 0.10 S cm^–1 for the composite ZrO₂-8mol% Y₂O₃/20 wt% Al₂O₃.     

Thermomechanical properties and microstructure of aluinina-zirconia

Al₂O₃-ZrO₂ composites were prepared in two compositional ranges, 15 wt% ZrO₂ and 29wt% ZrO₂ with or without yttria or magnesia stabilizers. While 1.5 wt% Y₂O₃ produced tetragonal ZrO₂ and fine grain microstructure, the 4.5 wt% Y₂O₃ developed cubic and tetragonal ZrO₂ with similar microstructure. Al₂O₃ with 29.5wt% ZrO₂-1.5wt% Y₂O₃ composition had the highest strength (3,300 kg/cm²). The bending strength remained more or less the same after the first thermal shock, and then it decreased gradually, but retained some strength after 20 cycles of quench. The load vs displacement curve became nonlinear after thermal shock possibly because of formation of microcracks which could be seen by microstructural studies.     

Synthesis of submicron-sized spherical Y2O3 powder for transparent YAG ceramics

Spherical monodispersed, submicron-sized Y₂O₃ powder was prepared via a homogeneous precipitation method using nitrate and urea as raw materials. The structure, phase evolution and morphology of Y₂O₃ precursor and the calcined powder were studied by FTIR, TG/DTA, XRD and SEM methods. The sphere size of the precursor was about 250 nm and that of Y₂O₃ powder calcined at 800 °C for 2 h was about 200-210 nm. With the spherical Y₂O₃ powder and a commercial Al₂O₃ ultrafine powder, high transparent YAG ceramics was fabricated by vacuum sintering at 1780 °C for 6 h through a solid-state reaction method. The in-line transmittances of the as-fabricated YAG ceramics at the wavelength of 1064 nm and 400 nm were 82.8% and 79.5%, respectively, which were much higher than that of the YAG ceramics with a commercial Y₂O₃ powder and a commercial Al₂O₃ ultrafine powder directly. The superior properties are attributed to the good morphology, dispersibility and uniform grain size of the as-prepared spherical Y₂O₃ powder, which matches that of the commercial Al₂O₃ powder.     

Sintering and dielectric properties of BaTiO3 prepared by a composite-hydroxide-mediated approach

High purity tetragonal BaTiO₃ powders consisted of uniform particles of ca. 150 nm in diameter were synthesized by a composite-hydroxide-mediated approach at 240 °C using a novel hydrothermal reaction apparatus with a rolling system. The product showed sinterability superior to the commercial powder, i.e., it could be sintered to full theoretical density at 1200 and 1100 °C without an additive and with 0.3 wt% of Li₂CO₃-0.04 wt% of V₂O₅ mixed sintering aid, respectively. The sintered body of the product also showed piezoelectric properties superior to the commercial one.     

Microstructures and properties of Cu-10Sn oil bearings reinforced by Al2O3 nanoparticles

Using Sn and Cu-Al powders as raw materials, three oxide dispersion-strengthened (ODS) Cu-10Sn alloy powders with different Al₂O₃ mass content (0.42%, 0.85% and 1.68%, respectively) were prepared by mechanochemical synthesis combined with diffusion alloying method. The oil bearings were then fabricated by pressing and sintering. The effects of Al₂O₃ on microstructures and properties of the powders and the oil bearings were investigated. The results show that Al₂O₃ nanoparticles with sizes of about 5 nm are uniformly distributed in the ODS Cu powders. The content of Al₂O₃ nanoparticles has no effect on the distribution uniformity of tin during the diffusion process. And three ODS Cu-10Sn alloy powders with homogeneous tin distribution are prepared. However, the temperatures of forming liquid phases in the ODS Cu-10Sn alloys decrease with increasing Al₂O₃ content. This affects the sintering behaviors and mechanical properties of oil bearings. Increasing Al₂O₃ content also has a significant promoting effect on the precipitation of Sn-rich particles. In order to synergize the solid solution strengthening of tin and the dispersion strengthening of Al₂O₃ nanoparticles, the content of Al₂O₃ in the ODS Cu-10Sn alloys will not exceed 0.85%. The ODS Cu-10Sn oil bearings with 0.42% Al₂O₃ sintered at 900 °C have the best comprehensive properties, with uniform radial and axial shrinkage, oil content of 19.1%, radial crushing strength of 284 MPa and micro-hardness of 142 HV.     

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.     

Texture development in 3 mol% yttria-stabilized tetragonal zirconia

We have developed a method of forming textured tetragonal zirconia. A suspension containing 10 vol% solid loading of monoclinic ZrO₂ mixed with 3 mol% Y₂O₃ was prepared, and then a bead-milling process was performed using 50 μm diameter zirconia beads resulting in a well-dispersed suspension. The mixture suspension of monoclinic zirconia and yttria nanoparticles was slip cast under a magnetic field of 12 T to produce oriented monoclinic zirconia with yttria. The reaction sintering between yttria and the oriented monoclinic zirconia produces a final 3 mol% Y₂O₃ doped tetragonal zirconia that remains oriented.     

Properties of lead zirconate-alumina ‘nanocomposites’

Microstructures, Vickers hardness and dielectric properties of PbZrO₃ ceramics with co-additions of 0.5-5 vol% Al₂O₃ nanoparticles have been investigated. The additive inhibited grain growth, with average grain size decreasing from ∼13 μm for PbZrO₃ to ∼1 μm for the nanocomposites. The mode of fracture also changed, from predominantly inter-granular in PbZrO₃ to a mixed-mode of intra- and inter-granular fracture in the composite samples. Vickers hardness values increased from 2.9 GPa for PbZrO₃ to 4.1 GPa for the sample with 1 vol% Al₂O₃, but there was a more gradual increase for higher Al₂O₃ contents. Plots of relative permittivity versus temperature indicated subtle differences which were attributed to a chemical reaction between the additive and matrix during sintering. X-ray powder diffraction showed that lead aluminium oxides were the principal products of this reaction.     

Synthesis of nanocrystalline rare earth oxides by glycothermal method

The reaction of yttrium acetate hydrate in 1,2-propanediol at 300 °C yielded a product containing acetate groups and glycol moieties. From this product, Y₂O₃ was directly crystallized at 400 °C without the formation of a carbonate oxide phase. The thus-obtained Y₂O₃ samples had a small crystallite size (2.2 nm) and significantly large surface area (280 m²/g). Other nanocrystalline rare earth (Gd-Yb) oxides were also obtained by this method.     

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.     

Synthesis of BaTi4O9 ceramics by reaction-sintering process

Synthesis of BaTi₄O₉ ceramics by a reaction-sintering process was investigated. The mixture of raw materials for stoichiometric BaTi₄O₉ were pressed and sintered into ceramics without any calcination stage involved. Pure BaTi₄O₉ phases were obtained at 1150-1280 °C. High-sintered density, 98.2-99.5% of theoretical value (4.533 g/cm³), can be obtained for pellets sintered at 1200-1280 °C for 2-6 h. Some rod-shaped grains 3-7 μm in the longitudinal axis appear in pellets sintered at 1230 °C. Both the size and the amount of these rod-shaped grains increase at higher sintering temperature.