Showing papers on "Thermal expansion published in 2001"

Thermally conducting aluminum nitride polymer-matrix composites

Abstract: Thermally conducting, but electrically insulating, polymer-matrix composites that exhibit low values of the dielectric constant and the coefficient of thermal expansion (CTE) are needed for electronic packaging. For developing such composites, this work used aluminum nitride whiskers (and/or particles) and/or silicon carbide whiskers as fillers(s) and polyvinylidene fluoride (PVDF) or epoxy as matrix. The highest thermal conductivity of 11.5 W/(m K) was attained by using PVDF, AlN whiskers and AlN particles (7 μm), such that the total filler volume fraction was 60% and the AlN whisker–particle ratio was 1:25.7. When AlN particles were used as the sole filler, the thermal conductivity was highest for the largest AlN particle size (115 μm), but the porosity increased with increasing AlN particle size. The thermal conductivity of AlN particle epoxy-matrix composite was increased by up to 97% by silane surface treatment of the particles prior to composite fabrication. The increase in thermal conductivity is due to decrease in the filler–matrix thermal contact resistance through the improvement of the interface between matrix and particles. At 60 vol.% silane-treated AlN particles only, the thermal conductivity of epoxy-matrix composite reached 11.0 W/(m K). The dielectric constant was quite high (up to 10 at 2 MHz) for the PVDF composites. The change of the filler from AlN to SiC greatly increased the dielectric constant. Combined use of whiskers and particles in an appropriate ratio gave composites with higher thermal conductivity and low CTE than the use of whiskers alone or particles alone. However, AlN addition caused the tensile strength, modulus and ductility to decrease from the values of the neat polymer, and caused degradation after water immersion.

Spontaneous thermal expansion of nematic elastomers

Abstract: We study the monodomain (single-crystal) nematic elastomer materials, all side-chain siloxane polymers with the same mesogenic groups and crosslinking density, but differing in the type of crosslinking. Increasing the proportion of long di-functional segments of main-chain nematic polymer, acting as network crosslinking, results in dramatic changes in the uniaxial equilibrium thermal expansion on cooling from the isotropic phase. At higher concentration of main chains their behaviour dominates the elastomer properties. At low concentration of main-chain material, we detect two distinct transitions at different temperatures, one attributed to the main-chain, the other to the side-chain component. The effective uniaxial anisotropy of nematic rubber, r(T) = / proportional to the effective nematic order parameter Q(T), is given by an average of the two components and thus reflects the two-transition nature of thermal expansion. The experimental data is compared with the theoretical model of ideal nematic elastomers; applications in high-amplitude thermal actuators are discussed in the end.

A review of measurement techniques for the thermal expansion coefficient of metals and alloys at elevated temperatures

Abstract: Metallurgical operations at elevated temperatures, such as those that involve solidification and/or mechanical deformation, can be critically influenced by the thermal stresses and strains that result from expansion and contraction of the material as a function of temperature. With the increasing use of computer-based process models for these operations, there arises a greater need for quantitative data on the thermal expansion coefficient of the relevant alloy at the temperatures involved. After briefly reviewing some existing sources of data for this property, the various techniques for its measurement at elevated temperatures are then described. These include mechanical dilatometry, optical imaging and interference systems, x-ray diffraction methods and electrical pulse heating techniques. Finally the implications, for process modelling, of the available data and measurement techniques are discussed.

Isobaric thermal expansivity and thermophysical characterization of liquids and liquid mixtures

Abstract: A methodology for the thermophysical characterization of liquids and liquid mixtures based on measurements of density, sound speed and isobaric heat capacity per unit volume at atmospheric pressure as a function of temperature is proposed. Density and sound speed data are used to determine the isentropic compressibility from the Laplace equation. The precision in density measurements allows one to obtain the isobaric thermal expansivity at different temperatures using an incremental procedure with quite acceptable accuracy and precision. The isothermal compressibility and isochoric molar heat capacity are both obtained from the previous properties, using well-known thermodynamic relations. The accuracy of the proposed methodology was checked by determining the above-mentioned properties for liquid n-hexane, n-heptane, n-octane, n-dodecane, n-hexadecane, cyclohexane, and toluene over the temperature range (288.15–333.15) K and comparing the results with selected reported data. The average absolute deviations from the latter showed data obtained with the proposed methodology to be reasonably accurate. The excess quantities for nine binary mixtures of the cyclohexane + n-dodecane system were also determined with a view to assessing precision, which was found to be quite good as regards the dependence on both composition and temperature.

Slow dynamics near glass transitions in thin polymer films.

Abstract: The alpha process (segmental motion) of thin polystyrene films supported on glass substrate has been investigated in a wider frequency range from 10(-3) Hz to 10(4) Hz using dielectric relaxation spectroscopy and thermal expansion spectroscopy The relaxation rate of the alpha process increases with decreasing film thickness at a given temperature above the glass transition This increase in the relaxation rate with decreasing film thickness is much more enhanced near the glass transition temperature The glass transition temperature determined as the temperature at which the relaxation time of the alpha process becomes a macroscopic time scale shows a distinct molecular weight dependence It is also found that the Vogel temperature has a thickness dependence, ie, the Vogel temperature decreases with decreasing film thickness The expansion coefficient of the free volume alpha(f) is extracted from the temperature dependence of the relaxation time within the free volume theory The fragility index m is also evaluated as a function of thickness Both alpha(f) and m are found to decrease with decreasing film thickness

Electrical properties and thermal expansion of cordierite and cordierite-mullite materials

Abstract: Commercially available cordierite and mullite powders were used to obtain cordierite and composite materials with mullite content up to 65 wt.% by attrition milling, uniaxial pressing and sintering. The employed cordierite powders were the coarse, medium and fine single granulometric fractions and the binary mixtures of them with 30, 50 and 70 wt.% of the smaller size component. Mullite powder employed in composites was a 7 h-attrition milled one. The dielectric constant (e), dielectric loss tangent (tan δ), resistivity (ρ) and thermal expansion coefficient (α) were measured. The influence of the porosity, mullite and glassy phase contents and grain size in the electrical parameters was analyzed. The thermal expansion coefficient as a function of the composition was studied.

Novel Ionic Liquid Thermal Storage for Solar Thermal Electric Power Systems

Abstract: Feasibility of ionic liquids as liquid thermal storage media and heat transfer fluids in a solar thermal power plant was investigated. Many ionic liquids such as [C4min][PF6], [C8mim][PF6], [C4min][bistrifluromethane sulflonimide], [C4min][BF4], [C 8mim][BF4], and [C4min][ bistrifluromethane sulflonimide] were synthesized and characterized using thermogravimetric analysis (TGA), differential scanning calorimeter (DSC), nuclear magnetic resonance (NMR), viscometry, and some other methods. Properties such as decomposition temperature, melting point, viscosity, density, heat capacity, and thermal expansion coefficient were measured. The calculated storage density for [C8mim][PF6] is 378 MJ/m 3 when the inlet and outlet field temperatures are 210 o C and 390 o C. For a single ionic liquid, [C4mim][BF4], the liquid temperature range is from –75 o C to 459 o C. It is found that ionic liquids have advantages of high density, wide liquid temperature range, low viscosity, high chemical stability, non-volatility, high heat capacity, and high storage density. Based on our experimental results, it is concluded that ionic liquids could be excellent liquid thermal storage media and heat transfer fluids in solar thermal power plant.

Low thermal conductivity thermal barrier coating system and method therefor

Abstract: A coating system having a low thermal conductivity, and a method by which the low thermal conductivity of the coating system is maintained through the development of cracks within a thermal-insulating layer of the coating system. The thermal-insulating layer is a mixture of two or more materials with different coefficients of thermal expansion (CTE). The materials of the thermal-insulating layer are selected and combined so that a low thermal conductivity is maintained for the coating system as the result of cracks developing and propagating from interfaces between the materials when the coating system is subjected to heating and cooling cycles.

The effects of thermal distortions on the diameter and cylindricity of dry drilled holes

Abstract: A model is developed to predict the effects of thermal distortion of the drill and workpiece on the diameter and cylindricity of dry drilled holes. Experiments using embedded thermocouples verify that the model predicts the flow of heat into the workpiece and into the drill reasonably well. The model predicts that thermal expansion of the drill is the dominant effect and leads to oversized holes with diameters that increase with depth.

Lateral thermal expansion of chitin crystals

Abstract: Measurements of the thermal expansion coefficients (TECs) of chitin crystals in the lateral direction are reported. We investigated highly crystalline a chitin from the Paralithodes tendon and an anhydrous form of β chitin from a Lamellibrachia tube from room temperature to 250 °C, using X-ray diffraction at selected temperatures in the heating process. For a chitin, the TECs of the a and b axes were α a = 6.0 × 10 -5 °C -1 and α b = 5.7 × 10 -5 °C -1 , indicating an isotropic thermal expansion in the lateral direction. However, the anhydrous β chitin exhibited an anisotropic thermal expansion in the lateral direction. The TEC of the a axis was constant at α a = 4.0 x 10 -5 °C -1 , but the TEC of the b axis increased linearly from room temperature to 250 °C, with α b = 3.0-14.6 × 10 -5 °C -1 . These differences in the lateral thermal expansion behaviors of the a chitin and the anhydrous β chitin are due to their different intermolecular hydrogen bonding systems.

Thermophysical Properties of MOX and UO2 Fuels Including the Effects of Irradiation

Abstract: Available open literature on thermophysical properties of both MOX and UO{sub 2} fuels has been reviewed, and the best set of thermal properties has been selected. The properties reviewed are solidus and liquidus temperatures of the uranium-plutonium dioxide system (melting temperature), thermal expansion, density, heat of fusion, enthalpy, specific heat, and thermal conductivity. Only fuel properties are studied in this report. The selected properties are used in thermal-hydraulic codes to study design basis accidents. The majority of the properties presented are for solid fuel.

High Temperature Elastic Constant Prediction of Some Group III-Nitrides

Abstract: Thermoelastic properties are important for modeling thermal residual stresses and for optimizing the growth conditions of semiconductor thin films. Thermal expansions of AlN and GaN have been evaluated and predicted by us earlier. Here, high temperature elastic constants are estimated empirically from corresponding state relationships and data from other hexagonal Grimm-Sommerfeld compounds. This information together with our earlier thermal expansion data will further improve capabilities for calculating thermal residual stresses in various semiconductor thin films.

Room-temperature wafer bonding of Si to LiNbO3, LiTaO3 and Gd3Ga5O12 by Ar-beam surface activation

Abstract: A room-temperature wafer bonding method using surface activation by Ar-beam sputter etching were applied to the bonding between dissimilar materials. LiNbO3, LiTaO3 and Gd3Ga5O12 wafers were successfully bonded to Si wafers without any heat treatment. This method is free from the various problems caused by the large thermal expansion mismatch between these materials during heat treatment in the conventional wafer bonding processes. The bond prepared by the Ar-beam treatment is so strong that fracture from inside the bulk materials is observed after the tensile test. The results of the bonding of Si wafers to both 128° Y-cut and Z-cut LiNbO3 wafers indicate that the influence of the crystal orientation on the bonding strength is negligible in this method. This method provides a very low damage bonding process for various material combinations regardless of any thermal expansion mismatch or crystal lattice mismatch.

Thermal expansion and elastic properties of InN

Abstract: The thermal expansion coefficients of wurtzite structure InN are evaluated within the constraints of a basic model and predicted for an extended temperature range. Together with the elastic constants provided earlier, this information gives a basis for optimizing thin-film growth conditions and thereby reducing the residual stresses in group-III-nitride thin-film devices.

Molecular Dynamics Simulations of Liquid Nitromethane

Abstract: A classical potential consisting of both intramolecular and intermolecular (Buckingham and Coulombic) terms that was developed for the simulation of crystalline nitromethane has been used to investigate the dynamics of liquid nitromethane at various temperatures and pressures. The validation of the proposed potential model was done for a large number of static and dynamic properties including the heat of vaporization, the variation of density with temperature and pressure, the thermal expansion coefficient, the self-diffusion coefficients, the viscosity coefficient, the dielectric constant, the bulk modulus, and the variation of vibrational frequencies with pressure. The analyses performed using constant pressure and temperature and constant volume and temperature molecular dynamics simulations show that the potential accurately reproduces the structural properties of liquid nitromethane at ambient pressure in the temperature range 260−374 K as well as the compression effects up to 14.2 GPa.

An apparatus for the measurement of internal stress and thermal expansion coefficient of metal oxide films

Abstract: A measuring apparatus based on a phase shifting interferometry technique to determine the mechanical properties of metal oxide films was presented. Thin films were prepared by ion-beam sputter deposition at low substrate temperature. Quantitative determination of the mechanical properties such as the internal stress, biaxial elastic modulus, and thermal expansion coefficient of metal oxide films were investigated. A phase shifting Twyman–Green interferometer with the phase reduction algorithm was setup to measure the temperature-dependent stress in thin films. Two types of circular glass plates, with known Young’s moduli, Poisson’s ratios, and thermal expansion coefficients, were used as coating substrate. The temperature-dependent stress behavior of the metal oxide films was obtained by heating samples in the range from room temperature to 70 °C. The stresses of thin films deposited on two different substrates were plotted against the stress measurement temperature, showing a linear dependence. Four oxide films were reported for their film stresses and thermal expansion coefficients.

Pressure-induced invar effect in Fe-Ni alloys

Abstract: We have measured the pressure-volume (P-V) relations for cubic iron-nickel alloys for three different compositions: Fe 0.64Ni (0.36), Fe 0.55Ni (0.45), and Fe 0.20Ni (0.80). It is observed that for a certain pressure range the bulk modulus does not change or can even decrease to some minimum value, after which it begins to increase under still higher pressure. In our experiment, we observe for the first time a new effect, namely, that the Fe-Ni alloys with high Ni concentrations, which show positive thermal expansion at ambient pressure, become Invar system upon compression over a certain pressure range.

Thermal effects on coated resonant microcantilevers

Abstract: High sensitivity detectors can be made using microcantilevers and measuring shifts in the resonance frequency of the cantilever motion arising from changes in mass loading or surface stress. However, undesirable shifts in the resonance frequency can also be caused by changes of temperature, particularly for microcantilevers coated with thin films. Two problems concerning such thermal effects are addressed. One problem is the thermal mismatch-induced frequency shift due to the different thermal expansion coefficients of a coating and substrate materials. The other is the thermal drift introduced by the thermal expansion and temperature-dependent material properties. For the first problem, the equation of motion for a circular arch can be used to calculate the resonance frequency shift due to the bending caused by thermal mismatch. For the second problem, two cases are investigated, namely uniform temperature distribution and linear temperature distribution along the length of the microcantilever. It is found that the thermal frequency drift in the resonance frequency is dominated by the temperature-dependent material properties. For a typical microcantilever made from silicon, the drift is of order ∼30 ppm/°C. The thermal drift does not depend on the lever section for both uniform and linear temperature distribution cases. For coatings of different thickness, the relationship between thermal frequency drift and the ratio of coating-to-substrate thickness is nearly linear. For a cantilever coated only partly along the length, the frequency drift increases approximately linearly as a function of length of cantilever coated. The additional thermal frequency drifts arising from a thin gold coating on a typical silicon microcantilever are of order 10–30 ppm/°C. The frequency shift due to thermal mismatch bending is negligible if the temperature change is small (∼10 °C).

Extreme thermal expansion, piezoelectricity, and other coupled field properties in composites with a negative stiffness phase

Abstract: Particulate composites with negative stiffness inclusions in a viscoelastic matrix are shown to have higher thermal expansion than that of either constituent and exceeding conventional bounds. It is also shown theoretically that other extreme linear coupled field properties including piezoelectricity and pyroelectricity occur in layer- and fiber-type piezoelectric composites, due to negative inclusion stiffness effects. The causal mechanism is a greater deformation in and near the inclusions than the composite as a whole. A block of negative stiffness material is unstable, but negative stiffness inclusions in a composite can be stabilized by the surrounding matrix and can give rise to extreme viscoelastic effects in lumped and distributed composites. In contrast to prior proposed composites with unbounded thermal expansion, neither the assumptions of void spaces nor slip interfaces are required in the present analysis.

The Thermophysical Properties (Heat Capacity and Thermal Expansion) of Single-Crystal Silicon

Abstract: Fragmentary investigations of the heat capacity and of the thermal expansion coefficient of single crystals of high-purity silicon are reported. The results of these investigations are compared with the entire body of data on these properties available to date. Generalized equations expressing the heat capacity and thermal expansion coefficient of silicon as functions of temperature are obtained for the temperature ranges of 298–1690 and 100–1400 K, respectively. The Debye temperature of crystalline silicon and the root-mean-square dynamic displacement of atoms from the equilibrium position in its crystal lattice are calculated using the available data on thermal expansion.