Effect of Thermal Expansion Mismatch on the Thermal Diffusivity of Glass-Ni Composites

The effect of interfacial decohesion, due to the thermal expansion mismatch, on the thermal diffusivity of a hot-pressed glass matrix with a dispersed phase of nickel was investigated by the laser-flash technique at 25° to 600°C. The interfacial gap formed on cooling acts as a barrier to heat flow and lowers the thermal diffusivity to values below those predicted from composite theory and also creates a strongly positive temperature dependence of the thermdl diffusivity. Preoxidation of the Ni spheres promotes interfacial bonding and yields values of thermal diffusivity higher than those for nonoxidized spheres and a thermal diffusivity which is relatively temperature-independent. The results of the present study also confirm the criteria for the effective thermal diffusivity of composites established by Lee and Taylor.     

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.     

Low-Temperature Fine-Grained Alumina–Aluminum Titanate Composites Produced from a Titania-Coated Alumina Precursor

A method for the preparation of alumina–aluminum titanate (AT) composites, which can be sintered to high density with a fine-grained microstructure at <1450°C, is reported. The composite precursor is alumina particles coated by sol–gel-derived titania, which reacts during sintering to form AT in situ at temperature as low as 1300°C. The composite can be sintered at 1350°C to 98% density with 1.5–2.0 μm grain size. Other composites containing 5–50 wt% AT are also investigated.     

Effect of Grain Contiguity on the Thermal Diffusivity of Aluminum Nitride

Thermal diffusivity of AlN-based ceramics was studied as a function of second-phase amount and heat-treatment time. The Y₂O₃·Al₂O₃ contents varied over the range of 13-31 vol%. The thermal diffusivity decreased as the amount of second phase increased. After sintering at 1850°C, the AlN ceramics consisted of rounded, largely isolated grains. Heat treatment of these samples for 5-50 h at 1800°C resulted in microstructures that consisted of largely contiguous AlN grains. There was a substantial increase in the thermal diffusivity after the heat-treatment step, and the incremental improvement was essentially constant for the three compositions that have been studied. The amount of second phase was unchanged during heat treatment; therefore, the increase in thermal diffusivity is assumed to be a direct result of the enhanced contiguity of AlN grains.     

Microstructural Development and Creep Deformation in an Alumina–5% Yttrium Aluminum Garnet Composite

An alumina–5 vol% yttrium aluminum garnet (YAG) composite was obtained through in situ reaction of alumina and yttria during sintering. Analysis of creep experiments together with microstructural data indicated that both pure alumina and alumina–5 vol% YAG composite deform by a Coble grain boundary diffusion creep process. Comparison with other data suggests that at temperatures greater than ∼1650 K, an isolated or interconnected fine-grained YAG phase does not significantly affect creep in alumina. However, an isolated YAG phase retards both static and dynamic grain growth in the composite.     

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.     

Effect of Silicon Dioxide on the Thermal Diffusivity of Aluminum Nitride Ceramics

The thermal diffusivity of AlN ceramics was significantly decreased by the addition of SiO₂. The AlN ceramics with 4 wt% SiO₂ could not be densified by pressureless sintering in the temperature range 1400° to 1800°C. The thermal diffusivity of these samples was very low because of their porous structure. The AlN ceramics containing 2, 4, and 8 wt% SiO₂ were densified by hot-pressing and also had low thermal diffusivity. In these samples, the grains of the 27R polytype that resulted from the reaction between AlN and SiO₂ were dispersed, obstructing the conduction of heat. The relation between the amount of 27R polytype and the thermal diffusivity of the AlN ceramics was determined.     

Electric-Field Effects on Sintering and Reaction to Form Aluminum Titanate from Binary Alumina–Titania Sol–Gel Powders

The field-activated sintering technique (FAST) was applied to simultaneously sinter and react sol–gel amorphous powders to form Al₂TiO₅. Densities close to theoretical and conversion to Al₂TiO₅ (to 92.5%) have been achieved using FAST at 1050°–1200°C for 10 min. Conventional sintering of the same powders at 1300°C for 2 h resulted in 88.9% Al₂TiO₅ and ∼75% of theoretical density. The enhanced sintering and compound formation using FAST have been explained by the synergistic effects of precursor reactivity, nanosized powders, and electric-field effects.     

Effect of Thermal History on Glass Expansion Characteristics

Thermal expansion data are presented for two glasses for both heating and cooling at constant rates. Heating data were taken on samples with varying thermal histories brought about by either constant-rate cooling or by constant-temperature stabilization before the expansion test. The fictive temperature concept is discussed in regard to these data. Expansion data are correlated with density and refractive index at room temperature for a variety of heat-treatments.     

Thermal Expansion Characteristics of Alumina–Magnesia and Alumina–Spinel Castables in the Temperature Range 800°–1650°C

The thermal expansion of Al₂O₃–MgO castables containing 5.5 wt% MgO and 1.36 wt% CaO and Al₂O₃–spinel castables containing 20 wt% spinel having 95 wt% Al₂O₃ and 1.7 wt% CaO was measured in the temperature range of 800–1650°C by dilatometry. A sharp increase in expansion from around 1425° to 1525°C, followed by a sharp decrease with further increasing temperature, is characteristic of Al₂O₃–MgO castables. The sharp increase in expansion is believed to be caused by the bond linkage between the CA₆ and spinel grains in the bonding matrix, while the sharp decrease is apparently related to liquid-phase sintering. The sharp increase and decrease in expansion were not observed in Al₂O₃–spinel castables because of the much lower MgO (around 1 wt% MgO) and impurity contents. The magnitude of thermal expansion of calcium aluminate bonded castables containing self-forming or preforming spinels or both is dictated by the MgO content of the castables.     

Specimen Size Effect of the Thermal Diffusivity/Conductivity of Aluminum Nitride

It has been generally found that the thermal diffusivity/conductivity of AIN measured by the laser-flash technique decreases with decreasing specimen size. The results of this study indicate that much of this effect can be attributed to the relatively large temperature rises for thin specimens, especially for high-energy laser pulses, and results from the combination of the noninearity of the commonly used type of IR detector and the strongly negative temperature dependence of the thermal diffusivity of AIN. The experimental results indicate that for reliable data, the specimen temperature rise at thermal equilirium should be kept to less than 1°C by using specimens with a thickness near 3 mm in combination with keeping the pulse energy to a reasonable minimum by attenuation of the laser beam by passing it through an aqueous solution of CuSO₄.     

钛酸铝陶瓷热膨胀机理研究

本文运用了电镜观察、显微结构分析、再加热条件下的热膨胀实验等方法，对钛酸铝陶瓷材料的热膨胀机理进行分析、研究，指出升温时由于微裂纹的弥合导致Ａｌ２ＴｉＯ５陶瓷有较小的热膨胀系数。并着重研究了钛酸铝陶瓷降温时的收缩机理，指出Ａｌ２ＴｉＯ５陶瓷的降温曲线有一最低点，在最低点右边，由于降温收缩造成的内应力还不足以造成大多数晶界和晶粒上发生微裂纹，Ａｌ２ＴｉＯ５陶瓷随着温度的下降而收缩，在最低点左边，由于     

氧化铈和氧化镁复合改性钛酸铝复相陶瓷性能的研究

钛酸铝是目前所知唯一集低膨胀和耐高温于一体的结构材料.但它在850～1280 ℃时会分解成氧化铝和氧化钛,并且其强度也很低,这两个弱点严重制约了它的使用范围.以CeO2和MgO为添加剂,对钛酸铝陶瓷进行复相改性.用万能材料试验机和扫描电镜等研究了材料的体积密度、热膨胀系数、机械性能和显微结构等.结果表明CeO2和MgO可以有效地改善钛酸铝复相陶瓷的各种性质,且添加(4～6)%CeO2+9%MgO的钛酸铝复相陶瓷经 1450 ℃烧结2 h就可以获得较好的综合性能.     

Thermal Expansion Properties of Lanthanum-Substituted Lead Titanate Ceramics

The Pb_{1− x} La _x TiO₃ powders were prepared by a sol–gel route in the composition range from x =0.0 to 0.3 in 0.05 increments. The lattice parameters of Pb_{1− x} La _x TiO₃ ( x =0.0, 0.05, 0.10, 0.15, 0.20, 0.30) were determined by a high temperature X-ray diffraction, and the intrinsic thermal expansion coefficients were obtained in the temperature range from room temperature to 900°C. The Pb_{1− x} La _x TiO₃ compounds ( x =0.10, 0.15, 0.20) in the tetragonal system exhibit low thermal expansion behaviors. As the La content is increased, the phase transition in the Pb_{1− x} La _x TiO₃ changes continuously from conventional to diffuse phase transition.     

Synthesis and Thermal Stability of Aluminum Titanate Solid Solutions

Aluminum titanate solid solutions with empirical formulas of Al₂Ti_1-xZr_xO₅, Al_6(2-x)(6+x)Si_6x/(6+x)□_6x/(6+x)TiO₅, and Al_2(1-x)Mg_xTi_1+xO₅ were synthesized by reaction sintering and annealed at 900° to 1300°C in air to evaluate the thermal stability. Substitution of Al in Al₂TiO₅ by Si and 2Al by Mg and Ti ions to form solid solutions such as AI_{6(2-x)/(6+x)l-Si}6x/(6+x)□_6x/(6+x)TiO₅, and Al_2(1-x)Mg_xTi_1+xO₅ was effective in controlling the thermal decomposition, but substitution of Ti by Zr had little effect.     

Temperature Dependence of the Thermal Diffusivity/Conductivity of Aluminum Nitride

The temperature dependence of the thermal conductivity of a polycrystalline AIN was measured using the flash-diffusivity technique over a range of experimental conditions. Thermal conductivity data from room temperature to 300°C, obtained with attenuated laser pulse energies to minimize the specimen temperature increase, were found to be inversely proportional to the absolute temperature, as expected from theory. For high pulse energies, the experimental data deviated significantly from expected behavior. This latter effect is offered as an explanation for the anomalously low temperature dependence for the thermal conductivity reported in the literature.     

木塑复合材料配方组成对其线性热膨胀系数的影响

以PP、HDPE、POE、EPDM或其共混物作为塑料基体,以木粉作为填充料,用挤出成型法制备了PP/PE基和PP基木塑复合材料(WPC),研究了配方中各组分的热膨胀特性、配方中塑料基体组成变化、以及配方中木粉含量变化等因素对所制得的WPC线性热膨胀系数的影响.结果表明:制备WPC的主要原料线性热膨胀系数的大小顺序为:木粉＜聚丙烯＜聚乙烯＜增韧剂,其中各塑料成分的线性热膨胀系数均远大于木粉,随着WPC中塑料含量增加,WPC的线性热膨胀系数增加;随着配方中PP/PE比值增加,WPC的线性热膨胀系数减小;随着配方中增韧剂EPDM或POE用量增加,WPC的线性热膨胀系数增大.     

Near-Surface Deformation in an Alumina–Silicon Carbide-Whisker Composite due to Surface Machining

Deformation due to two different surface-machining conditions—grinding (126 μm diamond) and polishing (3 μm diamond)—in an uniaxial hot-pressed Al₂O₃–30%-SiC-whisker composite has been investigated. A Warren–Averbach analysis of grazing incidence X-ray diffractometry data shows that the deformation is localized to the very top surface zone. The cell size and the root mean square of the strain show a gradient in the deformed layer. Transmission electron microscopy studies, in cross-sectional view, also show a near-surface deformation zone containing dislocations, twins, and cracks. This is seen for both machining procedures, but the depth of the zone and the degree of deformation, in terms of dislocation density and number of cracks, is much higher in the roughly ground specimen than in the polished one. For comparison, a monolithic Al₂O₃ sample also has been studied after grinding. The deformation zone is very similar to the Al₂O₃–SiC sample with the same grinding condition, but cracks and dislocations are present at a slightly larger depth. The deformation depth for the polished Al₂O₃–SiC sample is ∼50 nm. In the ground Al₂O₃–SiC sample, the deformation depth is 1–1.5 μm and corresponds to the grain size. The deformation zone in the ground monolithic Al₂O₃ sample is 1.5–2 μm deep. The observed grain-boundary cracks are almost parallel to the surface and may originate from nonaccommodated plastic flow between grains.