Factors Controlling the Thermal Stability of Aluminum Titanate Ceramics in Vacuum

Aluminum titanate (Al₂TiO₅) is a promising engineering material because of its low thermal expansion coefficient, excellent thermal shock resistance, good refractoriness, and nonwetting with most metals. However, it is susceptible to thermal dissociation in the temperature range of 800°–1280°C, which degrades its desirable properties. In this work, the various factors controlling the thermal instability of Al₂TiO₅ in the temperature range of 20°–1100°C have been characterized by neutron diffraction to study the temperature- and time-dependence structural changes in real time during the process of thermal decomposition. Results show that the thermal stability of Al₂TiO₅ is strongly influenced by temperature, dwell time, heating rate, phase purity, grain size, atmosphere, and additives. Possible mechanisms of structure stabilization and instability in Al₂TiO₅ under different conditions are discussed.     

Aluminum Titanate Powder Synthesis via Thermal Decomposition of Transparent Gels

Aluminum titanate powder has been prepared through thermal decomposition of a transparent gel formed from a mixture of titanium butoxide and boehmite sol in acetic acid—butanol medium. The thermogravimetric curve of aluminum titanate precursor gel is characterized by a decomposition temperature extending up to 800°C while the constituent gels of boehmite and hydrous titania independently decompose at lower temperature. The removal of water entrapped in the gel structure heated at the above temperature is further made clear by infrared spectral data. The gel stays in the amorphous state up to 800°C, as revealed from XRD. The sintered aluminum titanate shows that grains with sizes above 2 μm are cracked while smaller ones are intact. When heated for a period of 2 h, the gel decomposes to powders with average particle sizes of 2.7 μm at 900°C and 6.5 μm at 1400°C.     

Formation of Lead Zirconate-Lead Titanate Solid Solutions

Formation of the Pb(Zr,Ti)O₃ solid solution in the system PbO-TiO₂-ZrO₂ was studied by chemical analysis and X-ray powder diffrac-tometry. Only PbTiO₃ and Pb(Zr,Ti)O₃ were found as reaction products. The following three elementary reactions seemed to be reasonable for the formation of the Pb(Zr,Ti)O₃ phase: PbO + TiO₂→ PbTiO₃, PbTiO₃+ PbO + ZrO₂→Pb(Zr_1-λTi_λ)O₃, and Pb(Zr_l-λTi_λ)O₃+ PbTiO₃→ Pb(Zr_1-λ'Ti_λ')O₃ (λ<λ').     

Hydrothermal Precipitation of Lead Zirconate Titanate Solid Solutions: Thermodynamic Modeling and Experimental Synthesis

A previously developed thermodynamic model of hydro-thermal synthesis of ceramic powders has been extended to include cases when solid solutions are formed. The model has been applied to the synthesis of a series of lead titanate zirconate solid solutions PbZr _x Ti_{1– x} O₃ (PZT, 0.46 < x ≤ 0.75). It predicts the optimum conditions (i.e., reagent, concentration, pH, and temperature) for the precipitation of phase-pure homogeneous PZT, provided that the reactants are well mixed. The predictions have been experimentally corroborated using coprecipitated hydrous oxide Zr _x Ti_{1– x} C₂ n H₂O (0.46 < x ≤ 0.75), as a precursor for Ti and Zr and water-soluble lead acetate or nitrate salts as a source for Pb. When mixtures of hydrous oxides ZrO₂· n H₂O and TiO₂· n H₂O were employed as Ti and Zr precursors, independent PbTiO₃ and PbZrO₃ precipitates rather than the PZT solid solutions formed. These results can be rationalized on the basis of reaction kinetics where thermodynamic modeling includes or excludes the possibility of solid-solution formation.     

Dielectric Aging in Tetragonal Solid Solutions of Calcium Titanate in Barium Titanate

The dielectric aging phenomenon was observed in tetragonal polycrystalline solid solutions of calcium titanate in barium titanate between 30° and 90°C, and the 90° ferroelectric domain microstructures were analyzed using replica electron microscopy and statistical procedures. The results show a correlation between aging rate and 90° domains that may be satisfactorily interpreted in terms of the internal residual stress relaxation theory of aging.     

低温非水解溶胶-凝胶法制备钛酸铝

应用TG、DTA、XRD和F1-1R和TEM等测试手段研究了凝胶热处理过程中的相变化以及非水解溶胶-凝胶的反应过程和生成粉末的微观结构.结果表明:无水乙醇作氧供体制备的凝胶可在750℃直接合成钛酸铝,比固相法合成大大降低了温度.随着热处理温度的升高,钛酸铝粒子的尺寸逐渐变大,形貌也更加完整.     

Dielectric Properties of Strontium Titanate Solid Solutions Containing Niobia

SrTiO₃ solid solutions containing Sr_0.5NbO₃ were studied by X-ray diffraction and dielectric measurements. Single-phase perovskite solid solutions were formed up to the composition containing 30 mole % Sr_0.5 NbO₃. The addition of niobium resulted in a symmetry change from cubic (SrTiO₃) to tetragonal (solid solution) and the c/a ratio increased with increasing niobium content. The dielectric constant for the solid solutions was lower than that for SrTiO₃. Pronounced dielectric relaxation peaks and dispersion in permittivity were observed for the composition 7SrTiO₃:3Sr_0.5NbO₃. The activation energy for the relaxation process is about 0.42 ev. It is suggested that the existence of the A-site vacancy in the perovskite ABO₃ lattice introduced by the niobium substitution distorts the oxygen octahedra, producing more than one possible "noncentral" site for the Ti⁴⁺ ion. The relaxation arises from the thermal motion over potential barriers separating these alternative sites.     

Synthesis of Aluminum Nitride–Silicon Carbide Solid Solutions by Combustion Nitridation

AlN–SiC solid solutions were synthesized via a combustion nitridation process. Reactions between powder mixtures of aluminum, silicon, and carbon or aluminum with β-SiC and gaseous nitrogen under pressures of 0.1–8.0 MPa are self-sustaining once they have been initiated. Investigations were made with reactant ratios of Al:Si:C = 7:3:3, 6:4:4, and 5:5:5 and Al:SiC = 7:3, 6:4, and 5:5. For the Al-Si-C system (molar ratio of 6:4:4), the maximum combustion temperature was dependent on the nitrogen pressure, increasing from 2300°C to 2480°C with an increase in pressure, from 0.1 MPa to 6.0 MPa. In all cases, the product contained the solid solution as the primary phase, with minor amounts of silicon. The amount of unreacted silicon decreased as the nitrogen pressure increased; the presence and dependence of unreacted silicon on pressure has been explained in terms of the volatilization of aluminum. The full width at half maximum for the (110) peak of the AlN–SiC solid solution decreased as the nitrogen pressure increased, which indicated the formation of a more homogeneous product.     

原材料与钛酸铝陶瓷性能之间关系的研究

用不同纯度的二氧化钛粉体和不同晶型的氧化铝粉体为原料进行钛酸铝合成研究。结果表明:引入稳定剂和烧结助剂,在一定的烧结温度条件下,这些原材料均可生成具有良好力学性能的钛酸铝陶瓷;在相同的烧成条件下,钛黄粉反应生成的钛酸铝陶瓷比钛白粉反应生成的陶瓷具有更好的力学性能;采用钛黄粉与工业氧化铝粉合成的钛酸铝陶瓷,由于生产成本低,在耐火材料领域具有广泛的应用潜力。     

Synthesis of Lead Nickel Niobate–Lead Zirconate Titanate Solid Solutions by a B-site Precursor Method

A modified processing method for lead nickel niobate–lead zirconate titanate (Pb(Ni_1/3Nb_2/3)O₃–Pb(Zr,Ti)O₃, PNN–PZT) solid solutions is presented. This method is based on the high-temperature synthesis of a precursor that contains all the B-site cations (Ti, Zr, Ni, and Nb). This synthesis yields a diphasic mixture that contains a ZrTiO₄-like phase and a rutile-like phase. Both phases exhibit a cationic valence of 4; thus, it is concluded that the mixing of Ni and Nb cations is adequate for the preparation of PNN–PZT solid solutions. Indeed, a pure perovskite phase has been obtained after calcination with lead oxide for compositions that contain 40 and 50 mol% PNN. Moreover, their electromechanical properties have been shown to be superior to values reported for standard columbite routes. This conclusion has been interpreted in terms of enhanced chemical homogeneity.     

Correlation between Grain Size and Thermal Expansion for Aluminum Titanate Materials

Al₂TiO₅ materials were sintered under different conditions to produce different grain sizes. The resultant microcracked materials exhibited a range of bulk thermal expansions which showed a strong correlation with average grain size. An Al₂TiO₅ average grain size of 3 to 4 μm was the minimum at which the size and population of microcracks effectively reduced the apparent thermal expansion of the polycrystalline material. Further increases in grain size resulted in a rapid drop in the bulk thermal expansion, followed by diminishing decreases with further increases in grain size. Small amounts of phase stabilizers (<2.1 wt% MgO or Fe₂O₃) or limited mullite additions in mullite—aluminum titanate composites had no significant effect on this correlation.     

Dielectric Properties of Solid Solutions of Sodium Niobate-Lead Zirconate and Sodium Niobate-Lead Titanate

Solid solutions based on sodium niobate with limited amounts of lead and zirconium or lead and titanium solute ions have been investigated with respect to their dielectric constant versus temperature at 1 kc. per second from −175° to 300°C. Lead zirconate additions to sodium niobate up to 15 mole % cause an increase in dielectric constant, with a value of 1345 at 6°C. being the maximum recorded. Lead titanate additions to sodium niobate cause a general increase in dielectric constant up to 25 mole %, with 675 at 30°C. and 4670 at 154°C. being the maximum values recorded. Polarization versus applied field measurements up to 30,000 volts per cm. show nonreversible polarity for the lead zirconate compositions, whereas the compositions containing from 10 to 50 mole lead titanate are ferroelectric. An induced ferroelectricity phenomenon occurs for the composition containing 5 mole % lead titanate. A remanent polarization of 15 microcoulombs per sq. cm. at a coercive force of 7800 volts per cm. was obtained at room temperature for the composition 0.85 NaNbO₃–0.15 PbTiO₃.     

Effect of Thermal History on the Thermal Diffusivity and Thermal Expansion of an Alumina–Aluminum Titanate Composite

A study was conducted of the temperature dependence of the thermal diffusivity and thermal expansion of an alumina–aluminum titanate composite heated to a range of maximum temperatures followed by cooling to room temperature. Heating to temperatures above about 600°C resulted in a hysteresis behavior in which the data on cooling differed from the data obtained during heating. For both the thermal diffusivity and thermal expansion, the degree of hysteresis increased with increasing maximum temperature. On return to room temperature, following heating to a temperature of about 1200°C, the thermal diffusivity exhibited a significant decrease, with a corresponding increase in specimen size. This effect was attributed to an increase in microcrack density over the corresponding value prior to heating. On subsequent cycles of heating and cooling for a maximum temperature of 1200°C this decrease in thermal diffusivity was partially recovered, indicative of the structural integrity of the alumina–aluminum titanate composite of this study in practical applications involving temperatures of at least 1200°C.     

Positions of the Morphotropic Phase Boundaries in Solid Solutions of Lead Titanate

The morphotropic phase boundary, which, separates the tetragonal and rhombohedral ferroelectric solid solutions in the (1 - x)PbTiO₃ +xPbBO₃ and (1 - x)PbTiO₃ + xPb(B', B')O₃ systems, is almost always close to x = 0.5, when the relation of numbers of the smaller Ti and larger B or (B', B') ions is near to 1:1. Then, the smaller and larger ions can be ordered chequerwise. As the main cause of the morphotropic boundary existence, the paper considers growth of the B, B', B' ion sizes at increase of x. Nevertheless, it is assumed that its appearance can be triggered by the ion ordering, expected at x ≈ 0.5. Possibly, such an ordering could assist also to formation of some intermediate monoclinic ferroelectric phase.     

Grain-Boundary Microcracking Due to Thermal Expansion Anisotropy in Aluminum Titanate Ceramics

The relation between grain size and grain-boundary microcracking during cooling in aluminum titanate ceramics was studied. Microcracking temperature was determined by the measurement of thermal contraction and expansion, which was accompanied by acoustic emission. When the ceramics were cooled at a rate of 6°C/min, stress relaxation did not occur below the sintering temperature of 1500°C. The relation between the temperature difference from the sintering temperature to the microcracking temperature and the grain size showed good agreement with the prediction based on the energy criterion of grain-boundary microcracking.     

双基系固体推进剂热安定性及安定机理研究进展

从化学安定剂的结构、性能、应用及其作用机理等方面综述了化学安定剂在双基系固体推进剂热安定性中的作用及安定机理的研究进展,分析了近年来研究中存在的问题,包括缺乏化学安定剂本构关系的研究、化学安定剂安定机理研究不足、热安定性检测手段有待改进等。提出了今后研究的重点主要是加强化学安定剂本构关系研究、深入研究安定剂作用机理和反应动力学、拓展新方法和新技术的应用等。附参考文献74篇。     

Thermal Expansion Anisotropy of Oxides and Oxide Solid Solutions

The thermal expansion properties of the oxides stishovite (the rutile form of SiO₂), tetragonal vanadium dioxide, and zincite (ZnO) were measured. Solid solutions of rutile-, corundum-, and zincite-structure oxides were prepared, and the thermal expansion properties were measured in an effort to find compositions with more isotropic thermal expansion coefficients. The changes in the observed lattice constants and the changes expected on the basis of the cation radii were compared for both pure oxides and the oxide solid solutions. The difference in these changes indicates the variations to be expected in the thermal expansion anisotropy. A composition range containing the thermally isotropic composition in the system A1₂O₂,-Cr₂O₃ was determined, and the isotropic composition was estimated to be 75% Al₂O₃-25% Cr₂O₃.     

Field and Frequency Dependence of the Dynamic Hysteresis in Lead Zirconate Titanate Solid Solutions

Dynamic hysteresis of Nb‐doped Pb(Zr_1?xTi_x)O₃ (PZT, 0.20 ≤ x ≤ 0.60) ceramics were studied systemically at different field (E) and frequency (f). The hysteresis loops were strongly dependent on E and f. The measured coercive fields (E_c0) were far lower than the calculated values based on the Landau–Ginzburg theory, and increased dramatically from the rhombohedral phase to the tetragonal phase while had less variation with composition in the same phase. With increasing E or f for each composition, three types of loops were observed: linear, minor, and saturated loops. The cross fields (E₀), cross polarizations (P₀), and hysteresis areas (<A>) showed different variation regularities with f. Similar varying curves were observed for all PZT ceramic samples by normalizing E₀, P₀, and <A> with E/E_c0, which indicated the same polarization switching for different domain structures. Further analyses revealed that the switching processes can be divided into three stages: space charge polarization, domain switching, and steady state. The first and second stage occurred at ~0.5 and ~1.5 E/E_c0, respectively. These results would be very helpful to further understand the polarization switching of ferroelectric ceramics.     

High‐Temperature Flexural Strength and Thermal Stability of Near Zero Expanding doped Aluminum Titanate Ceramics for Diesel Particulate Filters Applications

A substantial increase in sinterability, high‐temperature flexural strength, thermal stability in combination with an average thermal expansion of 0.42 × 10^?6/^°C (30–1000°C) is achieved through magnesium silicate (Mg₃Si₄O₁₀(OH)₂) doping of Aluminum Titanate (Al₂TiO₅) ceramics. Doped specimens exhibited the sintered density of 99% of theoretical density at 1550°C and a maximum enhancement of 169.23% (70 MPa) in flexural strength at 1200°C as compared with 26 MPa measured at 30°C. Enhancement of flexural strength at elevated temperature can be attributed to the increasing extent of thermally activated crack blunting with increasing temperature, which is further evident from the dilatometric hysteresis curve recorded for these samples. XRD investigations of undoped (Al₂TiO₅, AT) samples annealed at 1100°C for 5 and 10 h have shown clear evidence of decomposition to precursor oxides by 7% and 21.13%, respectively. However, the samples of magnesium silicate–doped Al₂TiO₅ (TAT) under identical conditions have shown no sign of decomposition, indicating significantly high thermal stability. TAT formulations were also extrusion processed to investigate the suitability of forming cellular honeycomb structures. TAT formulation with superior thermo‐mechanical properties and excellent adaptability for extrusion processing can be explored for the development of next generation diesel particulate filters (DPF).     

纳米Fe2O3对钛酸铝陶瓷热稳定性能的影响

以溶胶–凝胶法制备的钛酸铝前驱体和固相合成法制备的纳米Fe2O3为原料,在不同温度煅烧保温2h制备出钛酸铝固溶体[Al2(1–x)Fe2xTiO5]。通过X射线衍射、电子探针分析并计算晶格常数和热分解率,研究了纳米Fe2O3含量以及烧成温度对烧后试样热分解性能的影响。结果表明:纳米Fe2O3很容易与Al2TiO5反应,形成固溶体,抑制钛酸铝陶瓷的热分解;随着纳米Fe2O3加入量的增加,Al2(1–x)Fe2xTiO5的晶格常数变大,热分解率降低,但当加入量超过10%时,Al2(1–x)Fe2xTiO5的晶格常数不变甚至减小,热分解率反而会增大;纳米Fe2O3作为添加剂可改善钛酸铝陶瓷的热分解性能。煅烧温度对钛酸铝的热分解率有很大影响,随着温度升高,热分解率降低,当温度大于1350℃时,钛酸铝陶瓷晶格常数保持不变,钛酸铝陶瓷的热分解率变化不大。