Residual Stresses in Fiber-Reinforced Ceramics due to Thermal Expansion Mismatch

Principles for the calculation of residual stresses due to thermal expansion mismatch and previous solutions are reviewed. A general model for this subject is proposed. Considering the effective thermoelastic properties of a unidirectional composite in axial and transverse directions allows a more comprehensive treatment of the residual stress situation in fiber-reinforced composites. Parameter variations show the important influence of the thermomechanical properties of an interfacial layer, which is disregarded in most of the conventional calculations.     

Germanium-Modified Cordierite Ceramics with Low Thermal Expansion

Magnesium cordierite ceramics modified by ionic substitution of Ge for Si were synthesized, fabricated, and characterized. The effect of the amount of ionic substitution on thermal expansion was investigated. Ionic substitution of ∼20% germanium for silicon reduces thermal expansion to zero at room temperature. A processing schedule to achieve nearly theoretical densities in germanium cordierite specimens was developed, and the microstructures of sintered samples were examined by scanning electron microscopy. Other thermal properties such as specific heat and thermal conductivity were also measured for the germanium-modified cordierites.     

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.     

Linear Thermal Expansion Coefficients of Mullite-Matrix Aluminosilicate Refractory Bodies

Twenty-seven mullite-matrix aluminosilicate refractory bodies in three different refractory systems were synthesized and their thermal expansions were measured and then correlated with their corundum and mullite contents by regression analyses. Thermal expansion coefficients were also calculated from the physical properties of the individual constituent phases. It is concluded that the best estimator of the thermal expansion of these materials is the regression analysis based on the volume percentage mullite content and that those bodies based on tabular alumina aggregates are the most predictable. For the best predictions, only a single raw material system, or very similar raw material systems can be considered.     

Thermal Conductivity and Thermal Expansion Coefficients of the Lanthanum Rare-Earth-Element Zirconate System

To enhance the insulating properties of a thermal barrier coating, one has to focus on new materials with lower intrinsic thermal conductivity than established yttria-stabilized zirconia. Substances with pyrochlore structure were investigated. Starting from lanthanum zirconate, substitutions of the lanthanum by other trivalent rare-earth elements were made, and the thermal conductivity and the thermal expansion coefficient of the manufactured materials were measured. A complete substitution of the lanthanum led to increased thermal expansion coefficients, whereas the partial substitution did not show an appreciable effect. The thermal conductivities of the modified materials were lower than that of the pure lanthanum zirconate for temperatures <1000°C for all amounts and elements of substitution. A comparison of the observed values with calculated values of the thermal conductivities showed a relatively good agreement.     

Thermal Conductivity Coefficients of Unidirectional Fiber Composites Defined by the Concept of Interphase

In this paper, the thermal conductivity coefficients of unidirectional composites have been estimated. In the present analysis, a model which takes into account the influence of the boundary interphase has been proposed. The thermal conductivity coefficients of the composite were calculated by using this concept to determine the role of the interphase which mainly depends on the quality of adhesion between fibers and matrix. The interphase matter is the part of the polymer matrix lying on the close vicinity of the fiber bounding surface. It has been primarily assumed that the interphase is inhomogeneous with thermophysical properties varying from the fiber surface to the matrix. Five laws of variation have been taken into consideration in order to derive closed form expressions for the thermal conductivity coefficients in which the role of the interphase layer occurs, possessing properties different from those of the constituent phases. A thermal analysis method known in the literature has been adopted in order to find the extent of the interphase layer. According to this viewpoint, theoretical expressions in order to calculate the thermal conductivity coefficients of the glass fiber reinforced composite materials were derived taking into consideration the interphase layer.     

Effect of Grain Thermal Expansion Mismatch on Thermal Conductivity of Porous Ceramics

Many ceramics contain microcracks, which are often situated between sintered grains. These microcracks constitute thermal resistances, which may affect heat transfer through the material and its effective thermophysical properties. The thicknesses and the contact areas of the microcracks change with temperature as a result of the thermal expansion mismatch between the grains on opposite sides of the microcracks. This physical mechanism affects changes of the material's thermal conductivity, k , with temperature. The above mechanism usually plays a minor role at atmospheric pressure, where heat may flow via the gas filling the cracks. Hence, the temperature-induced changes of the crack geometry have little effect on heat transfer. However, at low gas pressures, where the heat flow between the grains occurs mainly via the contact areas, the grains' thermal expansion mismatch causes unusual temperature behavior of the material's thermal conductivity observed for several industrial refractories. In this paper, the influence of the above physical mechanism is discussed relative to other heat transfer mechanisms described in the literature. A simple physical model of the thermal expansion of grains bonded by an agent, having different thermal expansion coefficients, is developed. This model allows calculation of the contact area and the average microcrack opening between the grains as functions of the temperature, the characteristic grains sizes and their thermal expansion coefficients, and the permanent crack area. These parameters are evaluated and used to calculate the effective thermal conductivity of ceramic materials containing microcracks that appear as a result of thermal contraction of grains. The calculated thermal conductivity satisfactorily correlates with the experimental data collected for several chrome-magnesite refractories over a wide range of temperatures and gas pressures.     

ZrW2O8负膨胀陶瓷材料进展

叙述了各向同性负膨胀ZrW2O8陶瓷材料的制备、结构、负膨胀机理和应用.     

低热膨胀钛酸铝陶瓷的制备及应用现状

文章综述了钛酸铝陶瓷的制备方法及其应用领域,并就钛酸铝陶瓷制品的研究开发方向作了展望.     

堇青石基低膨胀陶瓷材料研究

堇青石是一种优异的抗热震性原料,天然矿物原料很少,一般采用人工合成方法制取。纯堇青石烧成范围非常窄,难以获得致密烧结体。将堇青石原料与磷酸盐,锂铝酸盐复合烧结。可以降低材料的烧结温度,提高其密度,同时仍具有低的膨胀系数。     

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     

Microcracking in Ceramics Induced by Thermal Expansion or Elastic Anisotropy

The effect of crystal anisotropy on the formation of grain-boundary microcracks is analyzed, by considering a planar array of hexagonal grains as a model of a polycrystalline ceramic. The stress singularities at triple-grain junctions are analyzed by an asymptotic method as well as by a numerical solution, and the critical size of a grain-boundary defect is investigated by a crack analysis. It is found that elastic anisot-ropies can significantly increase the stress levels near triple points, which results in a smaller critical grain size for microcracking.     

Development of Textured Microstructures in Ceramics with Large Thermal Expansion Anisotropy

Fe₂TiO₅ exhibits a high degree of anisotropy in both thermal expansion and paramagnetic susceptibility. Anisotropy in paramagnetic susceptibility allows textured microstructures to be produced using magnetically-assisted grain alignment during sample fabrication. The resulting solid-state sintered aligned samples had a b -axis orientation coefficient of 3.33 (vs 6.60 for an aligned powder–epoxy suspension) where an orientation coefficient of 1 represents a randomly-oriented sample. This texturing reduced the residual stresses generated from the large thermal expansion anisotropy of the control samples, as evidenced by reduced microcracking and morphological texturing of the aligned microstructure.     

堇青石-莫来石复相低膨胀耐热瓷的制备研究

以合成堇青石、合成莫来石、贵州高岭土、樟州土、弋阳瓷石、烧滑石、锂云母为主要原料,添加适量滑石,制备了堇青石-莫来石复相低膨胀耐热瓷,通过正交试验确定了耐热瓷的基础配方,并运用XRD、热膨胀仪等测试手段对样品进行表征,分析了滑石含量对坯体性能的影响,在优化配方的基础上,研究了烧成制度与坯体热膨胀性能、抗折强度及吸水率的关系。研究结果表明:当滑石加入量为20%时,烧成温度1270℃,保温时间30min,坯体热膨胀系数为2.75×10-6/℃,吸水率为8.19%,抗折强度为85.08MPa。     

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.     

Micromechanical Analysis of Short Cracks in Fiber-Reinforced Ceramics

The finite-element method is used to perform a micromechanical analysis of a crack in a fiber-reinforced ceramic, when the crack length is the same order of magnitude as the fiber spacing. The stress intensity factor is calculated under mode-I loading for various crack lengths and fiber volume fractions. Cracks with and without fibers bridging the faces of the crack are analyzed.     

Bend Strength versus Tensile Strength of Fiber-Reinforced Ceramics

The bending strength of fiber-reinforced glasses and ceramics is often observed to be higher than their tensile strength; the difference varies, however, from one material to another. To gain an understanding of the relationship between these two measure of strength, we have carried out an analysis of bending which accounts for the deviations from linearity that occur on the tensile side of the beam. The results of this analysis indicate that the strength ratio (bending strength/tensile strength) depends most sensitively on the rate at which the stress drops after the ultimate tensile strength. In particular, composites failing gracefully (with a gradual decay in stress) tend to have comparatively higher strengths in bending. A method of inferring the: tensile strength from simply the load-deflection curve in bending is proposed. In addition, by accounting for the weakness in interlaminar shear, we can predict the variation in bend strength with beam aspect ratio. The various theories are compared with experimental data.     

低膨胀陶瓷材料

本文阐述了晶体的热膨胀与化学键及晶体结构之间的关系，以及晶体热膨胀的各向异性对多晶陶瓷体热膨胀的影响，微裂纹对降低陶瓷材料的平均膨胀系数有一定作用，但同时又降低了材料的机械强度。     

石墨化温度对针状焦热膨胀系数的影响

测定了三种不同类型针状焦(石油针状焦,煤沥青针状焦以及加氢煤沥青针状焦)的热膨胀系数,同时测定了不同石墨化温度下针状焦的CET值,指出了在2500℃～3000℃石墨化温度范围内,不同种类的针状焦CET值都呈现出线性增长,并且指出了由于加氢煤沥青针状焦具有特殊的结构而导致其石墨化以后具有最小的CET值。     

Experimental Observations of Frictional Heating in Fiber-Reinforced Ceramics

The influence of fatigue loading history and microstructural damage on the magnitude of frictional heating and interfacial shear stress in a unidirectional SiC fiber/calcium aluminosilicate matrix composite was investigated. The extent of frictional heating was found to depend upon loading frequency, stress range, and average matrix crack spacing. The temperature rise attained during fatigue can be significant. For example, the temperature rise exceeded 100 K during fatigue at 75 Hz between stress limits of 220 and 10 MPa. Analysis of the frictional heating data indicates that the interfacial shear stress undergoes an initially rapid decrease during the initial stages of fatigue loading: from an initial value over 20 MPa, to approximately 5 MPa after 25 000 cycles. Over the range of 5 to 25 Hz, the interfacial shear stress was not significantly influenced by loading frequency. The implications of frictional heating in fiber-reinforced ceramics are also discussed.