Negative thermal expansion structures constructed from positive thermal expansion trusses

The negative thermal expansivity of a type of space frame structure is investigated herein. On the basis of space frame structure consisting of tetrahedral representative volume elements, a volumetric thermal strain, and a volumetric coefficient of thermal expansion (CTE) models are developed in this article for a special category of tetrahedron that is made from two types of materials, each for the three apex and the three base rods. Based on these models, the conditions for attaining negative volumetric thermal strain and negative coefficient of volumetric thermal expansion are established. Plotted results reveal a trend in which the extent of negative expansivity is increased for lower apex-to-base rod length and CTE ratios, and higher base rod CTE.     

Interconnect thermal expansion matching to solid oxide fuel cells

A method for the rapid fabrication of homogeneous potential interconnect alloys from metal oxide precursors and evaluation of their thermal expansion mismatch to solid oxide fuel cells is described. Pastes of metal oxide powders were extruded into honeycomb geometries and sintered in hydrogen. Thermal expansion mismatch was evaluated based on heating with a zero-mismatch at room temperature, and on cooling with zero-mismatch temperatures at 600 or 1100∘C. The non-linear expansion behavior of Fe-Ni invar alloys resulted in different compositions being optimum, based on the assumed zero-mismatch temperature.     

Linear thermal expansion coefficients for an epoxy/glass matte-insulated solid cast transformer

Glass matte/epoxy-reinforced composites provide high-quality electrical insulation, structural integrity, and environmental protection in solid cast transformers. The thermal expansion characteristics of those composites are very important; the thermal expansion must be compatible with the conductor of the transformer in order to minimize stresses and prevent decohesion between the composite and the copper. The glass matte orientation and loading greatly influence the thermal expansion characteristics of the composite. A section was removed from a glass matte/bisphenol A epoxy-insulated, copper conductor wound cylindrical transformer coil. The linear expansion coefficients of the glass matte/epoxy composite were determined by differential dilatometry for three mutually perpendicular orientations with respect to the cylindrical coil. The expected reduction in thermal expansion of the epoxy in the tangential and axial directions due to the glass matte, which produced improved thermal expansion compatibility with the copper windings, was demonstrated. The measured linear thermal expansion coefficients were compared with theoretical values derived from a model for thermal expansion of a two-dimensional isotropic composite filled with fibers randomly oriented in a plane. An alternate composite system used for solid cast coil transformers, consisting of a cycloaliphatic resin filled with silica flour, was also investigated for comparison.Paper presented at the Ninth International Thermal Expansion Symposium, December 8–10, 1986, Pittsburgh, Pennsylvania, U.S.A.     

An investigation of the mechanical fatigue behavior of low thermal expansion lattice structures

A study of the mechanical fatigue behavior of a Ti–6Al–4V lattice structure designed to exhibit controlled thermal expansion has been performed. Comparison of S–N curves generated under both zero-tension and fully reversed cyclic loading has determined that the fatigue resistance of the lattice is substantially poorer than that of the constituent Ti–6Al–4V material for the same remote applied (macroscopic) stress. In addition, the effect of beta annealing the as-received mill-annealed alloy was also to reduce fatigue life in both the lattices and parent material. This effect is due to significant microstructural changes that occurred during heat treatment. Increasing the stress ratio (σ_min/σ_max) of the cyclic waveform from −1 to 0 had a similar effect. An analytical model has been developed to predict the fatigue life of the lattice structures from the S–N curves of the parent material, by determining the relationship between the macroscopic stresses acting on the lattice structure and the local stresses. The local stresses were then used in a multiaxial fatigue model to determine the fatigue life. The analytical model is able to predict the fatigue life with reasonable accuracy and minimal cost. The Findley multiaxial fatigue parameter for the parent material and lattice structures can be fitted with a power law equation and appears to fall onto a single curve, suggesting the local behavior within the lattice material is similar to the parent material. The analytical tools developed in this study can be hugely beneficial to the design of these lattice structures in the aerospace and communications industries.     

Negative thermal expansion materials: technological key for control of thermal expansion

Most materials expand upon heating. However, although rare, some materials contract upon heating. Such negative thermal expansion (NTE) materials have enormous industrial merit because they can control the thermal expansion of materials. Recent progress in materials research enables us to obtain materials exhibiting negative coefficients of linear thermal expansion over −30 ppm K⁻¹. Such giant NTE is opening a new phase of control of thermal expansion in composites. Specifically examining practical aspects, this review briefly summarizes materials and mechanisms of NTE as well as composites containing NTE materials, based mainly on activities of the last decade.     

Negative thermal expansion materials: technological key for control of thermal expansion

Abstract

Most materials expand upon heating. However, although rare, some materials contract upon heating. Such negative thermal expansion (NTE) materials have enormous industrial merit because they can control the thermal expansion of materials. Recent progress in materials research enables us to obtain materials exhibiting negative coefficients of linear thermal expansion over ?30 ppm K^?1. Such giant NTE is opening a new phase of control of thermal expansion in composites. Specifically examining practical aspects, this review briefly summarizes materials and mechanisms of NTE as well as composites containing NTE materials, based mainly on activities of the last decade.     

Negative thermal expansion materials

Materials exhibiting negative thermal expansion through room temperature have a variety of applications, mainly in controlling the overall thermal expansion of various composites. Several materials showing negative thermal expansion have recently been identified. The most dramatic of such behavior is exhibited by ZrW₂O₈, which shows strong isotropic thermal expansion from 20 to 425K.     

Crystal structure and negative thermal expansion of solid solution Yb2W3-xMoxO12

Abstract

A new series of solid solutions Yb₂W_3-xMo_xO₁₂ (0.5 ≤ x ≤ 2.5) were successfully synthesized by the high-temperature solid state method. Their crystal structure and negative thermal expansion properties were studied using high-temperature X-ray powder diffraction and the Rietveld method. All samples of rare earth tungstates and molybdates were found to crystallize in the same orthorhombic structure with space group Pnca, and show the negative thermal expansion phenomena related to transverse vibration of bridging oxygen atoms in the structure. Thermal expansion coefficients (TEC) of Yb₂W_3-xMo_xO₁₂ were determined as ?18.3×10^?6 K^?1 for x = 0.5 and ?19.6×10^?6 K^?1 for x = 2.5 comparing to ?19.1×10^?6 K^?1 and ?18.3×10^?6 K^?1 for unsubstituted Yb₂W₃O₁₂ and Yb₂Mo₃O₁₂ in the identical temperature range of 200–800°C. High temperature X-ray diffraction data and bond length analysis suggest that the difference between W—O and Mo—O bond is responsible for the change of TECs after the element substitution in this series of solid solutions.     

Nonlinear convection with variable coefficient of thermal expansion

Summary The problem of nonlinear convection in a horizontal layer of fluid with variable coefficient of thermal expansion is considered. It is found that square cells transport always more heat than two-dimensional rolls and that rolls are always unstable. Variable coefficient of thermal expansion can strongly affect the critical Rayleigh number and the horizontal scale of the motion. Subcritical instabilities can exist which are associated with either hexagon pattern convection or square pattern convection. Particular forms of the coefficient thermal expansion are essential to determine the preferred flow patterns which could be of the forms of squares or hexagons with either upward of downward motion at the cells' centers.     

On the thermal expansion of composite materials and cross-property connection between thermal expansion and thermal conductivity

The paper focuses on the quantitative characterization of heterogeneous microstructures from the point of view of the material’s thermal expansion. First, we derive expression for the second rank thermal expansion contribution tensor of an inhomogeneity and specify it for various inhomogeneity shapes. Case of a spheroidal inhomogeneity in an isotropic material is discussed in detail. Thermal expansion contribution tensor is used as a basic building block to calculate effective thermal expansion of a heterogeneous material and to derive explicit cross-property connection between thermal expansion and thermal resistivity of a composite. We compare our results with experimental data available in literature and with other approaches.     

Thermal expansion of solid methane

Relative length changes in solid methane at its approximate vapor pressure have been measured between 4 and 26 K. The linear-expansion coefficient calculated from these data is found to have a maximum at 19.8 K, 0.6 K lower than the peak found in specific-heat measurements. A negative expansion coefficient has been observed below 8 K. The possibility of nuclear spin-species conversion is discussed in terms of the long equilibration times and the observed length changes below 8 K.Research supported by National Science Foundation.     

原位反应固相法合成低热膨胀ZrW_2O_8/ZrO_2复合材料

以分析纯ZrO2、WO3为原料,采用分步焙烧-原位反应固相法成功合成了低热膨胀ZrW2、WO8/ZrO2复合材料,着重研究了不同烧结温度和烧结时间对产物的影响.通过X射线衍射仪(XRD)、扫描电镜(SEM)、能谱仪(EDS)和热膨胀仪对合成的ZrW2O8/ZrO2复合材料的晶体结构、断面形貌和热膨胀性能进行表征.研究结果表明,在1125、1150、1200℃烧结6h都可以合成高纯度的ZrW2O8/ZrO2复合材料,其组元为α-ZrW2O8和m-ZrO2相;随烧结时间的延长,衍射半峰宽逐渐减小,晶粒在不断长大,致密度相对提高;随着ZrW2O3质量分数增加,复合材料的热膨胀系数减小,其中25%(质量分数)ZrW2O8/ZrO2复合材料在30～600℃的平均热膨胀系数为0.2153×10-6K-1.     

Low thermal expansion electrical insulators

Vitrification, thermal expansion and electrical resistivity of some mixtures within the system: MgO-Al₂O₃-SiO₂-ZrO₂ were studied in the light of their phase composition. The effect of adding 1 to 2% Li₂O on the phase composition and properties of these mixtures was also investigated. All of the investigated mixtures vitrify between 1200 and 1400‡C. The vitrified bodies show low thermal expansion and reasonable electrical resistivity. The increase of cordierite, at the expense of cristobalite and mullite phases, decreases the thermal expansion and increases the electrical resistivity. In mixtures containing Li₂O, an LiMg-aluminosilicate phase crystallizes leading to a further decrease in thermal expansion and to a slight deterioration of electrical resistivity.     

Anisotropic thermal expansion of arsenic

The lattice parameters of arsenic have been determined at various temperatures between 28 and 286° C, using 19 cm diameter, high temperature camera. The a-parameter is observed to increase between 28 and 200° C and thereafter it decreases up to 286° C. The c-parameter is found to increase parabolically throughout the range of temperature. The mean values of thermal expansion coefficients along, and perpendicular to, the principal axis between 28 and 200° C are 44.3×10⁻⁶/° C and 3.1×10⁻⁶/° C respectively.     

聚合物的热膨胀

热膨胀既是聚合物的基本属性,同时也与聚合物结构与性能密切相关。本文在提出聚合物热膨胀产生机制的基础上,介绍了测量聚合物热膨胀的方法,讨论了热膨胀在聚合物基础研究中的应用,着重评述了热膨胀在PTC材料为代表的聚合物基复合材料设计中的重要价值。     

Negative thermal expansion of laminates

Measurements have been carried out on the in-plane and through the thickness thermal expansion coefficients of glass polypropylene fibre composites of 50% volume fraction between room temperature and 120°C. Only in the temperature range 20° to 75°C are reproducible results obtained. It is confirmed that in-plane negative values may be obtained in specific directions. The physical reason for this and its connection with the necessary appearance of a large Poisson ratio is pointed out. The expansivity of the matrix material depends strongly on temperature. Very good agreement between the experimental values and those predicted from the properties of the two constituents is found provided that the value for the expansivity of the polyoropylene is that within the temperature range considered. It is shown that the polypropylene matrix does not provide a matrix which is stable enough in its properties to enable the system to yield consistent negative values of expansivity. An alternative system is proposed and a single experiment confirms that a negative value of the thermal expansivity of as large as –50 × 10^–6 K^–1 may be obtained.