Showing papers on "Thermal expansion published in 1996"

Negative Thermal Expansion from 0.3 to 1050 Kelvin in ZrW2O8

Abstract: Negative thermal expansion was found for ZrW 2 O 8 from 0.3 kelvin to its decomposition temperature of about 1050 kelvin. Both neutron and x-ray diffraction data were used to solve and refine the structure of this compound at various temperatures. Cubic symmetry persists for ZrW 2 O 8 over its entire stability range. Thus, the negative thermal expansion behavior is isotropic. Essentially the same behavior was found for isostructural HfW 2 O 8 . No other materials are known to exhibit such behavior over such a broad temperature range. These materials are finding applications as components in composites in order to reduce the composites9 overall thermal expansion to near zero.

Effect of temperature

Abstract: Methods for measuring the effects of temperature on rubber, both in the short and long term, are described. This includes thermal expansion, glass transition, tests for low temperature resistance and thermal ageing.

Effect of strongly favorable substrate interactions on the thermal properties of ultrathin polymer films.

Abstract: The thermal behavior of ultrathin films of poly-(2)-vinylpyridine spin-cast on acid-cleaned silicon oxide substrates is considered. The interaction between the polymer and the substrate is polar in nature and very favorable. As a means of examining the thermal properties of the films, x-ray reflectivity is used to measure the temperature dependence of the film thickness. This experimentally measured thickness-temperature data is used to determine transition temperatures and thermal expansivities. Significantly increased transition temperatures (20-50 \ifmmode^\circ\else\textdegree\fi{}C above the measured bulk glass transition temperature) are observed in ultrathin polymer films. The transition temperature increases with decreasing film thickness, while the degree of thermal expansion below the transition temperature decreases with decreasing film thickness. If one assumes that a region of reduced chain mobility exists near the solid substrate-polymer interface, an analysis of the measured thermal expansion behavior below the transition temperature indicates that the length scale of substrate interactions is on the order of the macromolecular size.

The influence of fibre length and concentration on the properties of glass fibre reinforced polypropylene: 2. Thermal properties

Abstract: In this report we present the results from the second part of a study on the influence of fibre length and concentration on the properties of glass reinforced polypropylene laminates. The heat deflection temperature of these laminates is dependent on both fibre length and concentration. A maximum plateau level close to the polypropylene melting point was observed, longer fibres require a lower concentration to attain this plateau value. Elevated temperature stiffness retention was also enhanced by higher fibre concentration and longer fibres. The Cox—Krenchel equations gave a good prediction of the laminate stiffness over the temperature range —50 to 100°C. Both the in-plane and out-of-plane linear coefficients of thermal expansion were strongly dependent on fibre concentration but relatively insensitive to the fibre length. We obtained excellent correlation between experimental values of the in-plane linear coefficients of thermal expansion and theoretical predictions based on the shear lag theory. Out-of-plane linear coefficients of thermal expansion were found to be much larger than predicted by the equations used for continuous fibre reinforced composites. An approach based on the in-plane compression of the matrix due to the restriction of the matrix expansion by the reinforcing fibres was found to give good agreement with the experimental data. Good correlation of the experimental and predicted data was only obtained when the effect of voids was included in the calculations

Composites with extremal thermal expansion coefficients

Abstract: We design three‐phase composites having maximum thermal expansion, zero thermal expansion, or negative thermal expansion using a numerical topology optimization method. It is shown that composites with effective negative thermal expansion can be obtained by mixing two phases of positive thermal expansions with a void phase. We also show that there is no mechanistic relationship between negative thermal expansion and negative Poisson’s ratio.

Origin of the negative thermal expansion in and

Abstract: The negative thermal expansion recently observed over a wide range of temperatures in may be attributed to the existence of low-frequency phonon modes which can propagate with no distortions of the tetrahedra and octahedra, the so-called `rigid unit modes' Using methods developed for the study of similar modes in silicates we have located the surfaces of these modes in wave-vector space The rigid-unit mode interpretation accounts for the weak effect of the 430 K structural phase transition on the negative thermal expansion, provided that the disordered phase does not involve formation of and complexes On the other hand, the crystal structure of the related material is cross braced by pairs of linked tetrahedra and is therefore significantly less flexible In this case a qualitatively different mechanism may be responsible for the negative thermal expansion observed in

Cellular solid structures with unbounded thermal expansion

Abstract: Thermal expansion of materials is of considerable practical interest since materials in service may experience a considerable range of operating temperature. Materials with microstructure, including composites, honeycombs, and foams (which are composites with one void phase) are finding increasing application for many purposes. It is therefore of interest to explore the range of thermal expansion coefficient attainable in materials with microstructure. The constitutive equation for a linear isotropic thermoelastic continuum is [1]:

Measurements of stress during vapor deposition of copper and silver thin films and multilayers

Abstract: Copper and silver single layer and multilayered thin films were thermal vapor deposited onto cantilevered substrates [Si(100) with native oxide] near room temperature in ultrahigh vacuum. The total force per unit width (F/w) in the film during and after deposition was determined from the change in substrate curvature measured in situ by a laser scanning technique. The intrinsic component of F/w was obtained by subtraction of the thermal component, which was obtained by measuring the product of the biaxial modulus of the film (Yf) and the difference in coefficients of thermal expansion of the substrate and the film (Δαs−f) while each sample was still in the ultra‐high vacuum deposition chamber. For all samples, the measured value of YfΔαs−f was substantially lower than the calculated value based on the {111} biaxial modulus and the coefficients of thermal expansion of the bulk materials, even though x‐ray diffraction indicated strong {111} film texture. During deposition, a general trend in F/w was found r...

Thermal equation of state of CaSiO3 perovskite

Abstract: A comprehensive pressure-volume-temperature data set has been obtained for CaSiO3 perovskite up to 13 GPa and 1600 K, using synchrotron X ray diffraction with a cubic-anvil, DIA-6 type apparatus (SAM-85). For each volume measurement, nonhydrostatic stress is determined from the relative shift in the diffraction lines of NaCl, within which the sample was embedded. Heating to above 973 K greatly reduced the strength of NaCl (to below 0.05 GPa), making the measurements hydrostatic. At room temperature the cubic perovskite structure remains metastable at pressures as low as 1 GPa, below which the sample transforms into an amorphous phase as indicated by a large background, a marked decrease in diffraction signals, and an anomalous volume decrease of the remaining crystalline phase. Because our experimental uncertainties are significantly smaller than those in previous measurements, the new data provide a tighter constraint on the zero pressure bulk modulus for CaSiO3 perovskite. A new set of room temperature equation of state parameters are identified so that both our data and the diamond cell data of Mao et al. [1989] are compatible [KT0 = 232(8) GPa, K′T0 = 4.8(3), and V0 = 45.58(4) A3]. Volume measurements along several isotherms under both stable and metastable pressure conditions allow isochoric and isobaric interpolations within the range of experimental pressure and temperature conditions. Analyses using various approaches yielded consistent results for (∂KT/∂T)P of −0.033(8) GPa K−1, and (∂α/∂P)T of −6.3 × 10−7 GPa−1 K−1, with a zero-pressure thermal expansion α0 of 3.0 × 10−5 K−1. The thermal pressure is found to be virtually independent of volume, and thus the Anderson-Gruneisen parameter δT = K′T0 = 4.8. These results are used to predict the bulk modulus and density of CaSiO3 perovskite under lower mantle conditions. Along an adiabat with the foot temperature of 2000 K, the density of the perovskite agrees with that of the preliminary reference Earth model (PREM) within 1% throughout the lower mantle. The bulk modulus shows a smaller pressure dependence along the adiabat; it matches that of PREM at the top of the lower mantle but is about 10% too low near the core-mantle boundary.

Modelling of the thermal dependence of structural and elastic properties of calcite, caco3

Abstract: A computational method, based on the quasiharmonic approximation, has been computer-coded to calculate the temperature dependence of elastic constants and structural features of crystals. The model is applied to calcite, CaCO3; an interatomic potential based on a C-O Morse function and Ca-O and O-O Borntype interactions, including a shell model for O, has been used. Equilibrations in the range 300–800 K reproduce the experimental unit-cell edges and bond lengths within 1%. The simulated thermal expansion coefficients are 22.3 (//c) and 2.6 (⊥ c), against 25.5 and-3.7×10−6K−1 experimental values, respectively. The thermal coefficients of elastic constants tend to be underestimated; for the bulk modulus, -2.3 against-3.7×10−4K−1 is obtained.

A noncontact measurement technique for the density and thermal expansion coefficient of solid and liquid materials

Abstract: A noncontact measurement technique for the density and the thermal expansion coefficient of refractory materials in their molten as well as solid phases is presented. This technique is based on the video image processing of a levitated sample. Experiments were performed using the high‐temperature electrostatic levitator (HTESL) at the Jet Propulsion Laboratory in which 2–3 mm diam samples can be levitated, melted, and radiatively cooled in vacuum. Due to the axisymmetric nature of the molten samples when levitated in the HTESL, a rather simple digital image analysis can be employed to accurately measure the volumetric change as a function of temperature. Density and the thermal expansion coefficient measurements were made on a pure nickel sample to test the accuracy of the technique in the temperature range of 1045–1565 °C. The result for the liquid phase density can be expressed by ρ=8.848+(6.730×10−4)×T (°C) g/cm3 within 0.8% accuracy, and the corresponding thermal expansion coefficient can be expressed by β=(9.419×10−5) −(7.165×10−9)×T (°C) K−1 within 0.2% accuracy.

Thermal expansion of forsterite up to the melting point

Abstract: As determined from powder X-ray diffraction experiments with synchrotron radiation, the thermal expansion coefficient of forsterite increases smoothly from 2.8 to 4.5 K−1 from 400 K to 2160 K. No anomalous increases of the cell parameters are observed near the melting point. The consistency between the observed and calculated value of the initial slope of the melting curve of forsterite suggests that defects do not make a large contribution to thermal expansion near the melting point. Along with previous results, the new data confirm the influence of anharmonicity on the high-temperature heat capacity of forsterite and indicate that both the Gruneisen parameter and αKT (α = thermal expansion coefficient, KT = bulk modulus) have nearly constant values at high temperatures.

Thermal stresses and effective thermal expansion coefficient of a functionally gradient sphere

Abstract: We present the exact solution to the problem of uniform heating of a spherical body whose elastic moduli and thermal expansion coefficient vary linearly with the radius. The Frobenius series method is used to find exact expressions for the displacements and stresses. Both the radial and hoop stresses are largest in magnitude at the center of the sphere. The radial stress decays to zero at the outer edge, whereas the hoop stresses always change sign at some intermediate value of the radius. We also find an exact expression for the effective thermal expansion coefficient. For the special case where the thermal expansion coefficient varies with the radius but the elastic moduli are uniform, the effective thermal expansion coefficient of the sphere is equal to the volumetric average of the local thermal expansion coefficient. In the more general case, where the moduli also vary, the moduli variations have very little influence on the effective thermal expansion coefficient.

Thermal expansion, molar volume and specific heat of diamond from 0 to 3000K

Abstract: Thermal expansion is of both practical and theoretical importance and it together with specific heat is essential for predicting a thermodynamic equation of state. We utilize a semi-empirical quasi-harmonic model to evaluate available data for diamond. The model allows us to predict the thermal properties of the metastable diamond polymorph to 3000K. The approach consisting of a simplified frequency spectrum with several Einstein terms provides a convenient mathematical method where a minimum of empirical parameters represent the thermal property.

Dimensional Instability of Doped Lanthanum Chromite

Abstract: Lattice expansion phase stability, and dimensional stability of doped lanthanum chromites have been examined over a wide range of temperatures and oxygen partial pressures. Reduction of doped lanthanum chromite resulted in a linear expansion of the sample that was dependent on the acceptor (Sr, Ca) concentration, temperature, oxygen partial pressure, and oxygen content within the sample. Additional doping with aliovalent B-site additives significantly reduced lattice expansion in reducing environments. The lattice expansion in reducing environments was directly related to the loss of lattice oxygen and the simultaneous reduction of Cr{sup 4+} to Cr{sup 3+} to maintain electroneutrality.

Yttrium SiAlON glasses: structure and mechanical properties — elasticity and viscosity

Abstract: Two series of oxynitride glasses in the YSiAlON system, with varying ratios of Al Si and Al Y , were studied from both structural and mechanical point of view. A Raman scattering study showed that by changing the Al; content, at constant Y cationic equivalent concentration (e/o), aluminum substitutes for silicon without affecting the degree of polymerisation (cross-linking) of the glass network, whereas at constant Si e/o, replacement of yttrium by aluminum results in a higher polymerisation degree. The effect of glass composition on the hardness, Young's modulus, the thermal expansion coefficient and the glass transition temperature range are discussed in light of the corresponding structural changes.

Article comprising a temperature compensated optical fiber refractive index grating

Abstract: Conventional optical gratings are relatively temperature sensitive. This sensitivity is generally undesirable but can be reduced or eliminated by attaching the grating to a support member having a negative coefficient of thermal expansion. Exemplarily the member comprises Zr-tungstate and/or Hf-tungstate. The thermal expansion can be tailored by admixture of positive expansion coefficient material (e.g., Al2 O3, SiO2) to the negative expansion coefficient material (e.g., ZrW2 O8), or by a variety of other techniques.

Thermal expansion of gypsum investigated by neutron powder diffraction

Abstract: The cell parameters of deuterated gypsum have been extracted from Rietveld analysis of powder neutron diffraction data within the temperature range 4.2-320 K. The thermal expansion of gypsum is highly anisotropic along the b axis principally because of the effect of the D2.. .01 hydrogen bond. The high-temperature limits for the expansion coefficients for the cell edges a, b, and care 3.98 x 10-6,4.36 X 10-5, and 2.53 x 10-5 K-l, respectively, and for the cell volume the limit is 6.96 x 10-5 K-l. The (3angle displays oscillatory variation, reflecting a change in the influence of at least two coupled processes. Empirical data analysis, within this temperature range, results in a~ = 1.251 x 10-6 sin(0.0116T + 0.311) K-l.

Resistance‐expansion‐temperature behavior of a disordered conductor–insulator composite

Abstract: I have measured the resistance and linear expansion of a disordered carbon‐black–polymer composite as a function of temperature from 25 to 180 °C. The sample resistance and thickness both increases monotonically as a function of temperature, with precipitous increases at the polymer melt temperature. The resistance versus thickness data show a monotonic increase with no precipitous changes at any point. These results suggest the electrical conduction mechanism is the same below, during, and above the polymer melt temperature. This casts doubt on some earlier models proposed to explain the resistance versus temperature behavior of these systems. These results also suggest a new approach to understanding the resistance versus temperature behavior by first developing a quantitative understanding of the resistance versus thickness behavior.

Thermal expansion of metals reinforced with ceramic particles and microcellular foams

Abstract: The thermal expansion of three isotropic metal-matrix composites, reinforced with SiC particles or microcellular foam, is measured between 25 °C and 325 °C. All three composites show initial co-efficient of thermal expansion (CTE) values in agreement with the Turner model predictions, and near Schapery’s lower elastic bound for CTE. At higher temperatures, the CTE of foam-reinforced Al decreases, while that of the two particle-reinforced composites increases. These observations are interpreted as resulting from the presence of a very small fraction of microscopic voids within the infiltrated composites. This interpretation is confirmed with finite-element simulations of the influence of voids, cracks, and reinforcement convexity in two-dimensional (2-D) composites featuring an interconnected reinforcement of SiC surrounding isolated Al phase regions, thermally cycled from an elevated processing temperature and deforming in generalized plane strain.

Measurement of Enthalpy Differences in Cryosolutions: Influence of Thermal Expansion

Abstract: Cryosolvents such as liquid nitrogen, argon, krypton, and xenon show a substantial thermal expansion. Its influence on the value of enthalpy differences for simple chemical equilibria in dilute solutions in such solvents, determined using infrared spectroscopy, is discussed. Corrections to account for the thermal expansion are proposed.

Residual stress in WC-Co measured by neutron diffraction

Abstract: Large thermal residual microstresses (TRS) can develop in WC-Co composites owing to the difference of the coefficients of thermal expansion (CTE) of the constituents. The variation with temperature of average stresses in a WC-11 wt.% Co sample were studied between room temperature and 1273 K by measuring the cell parameters of cobalt and WC using neutron diffraction. WC powder was also measured to provide stress free reference standards. At room temperature, a hydrostatic compressive stress of about 500 MPa was measured in the WC. The evolution of TRS shows two temperature domains. The low temperature domain (300 1000 K) is characterized by an increase of residual stress in WC, a rapid increase of Co lattice parameter, and a hysteresis between the heating and cooling cycles. A model, based on Eshelby's equivalent inclusion method, predicts the observed behavior in both domains. In the low temperature domain, the CTE mismatch between WC and Co accounts for the decrease of TRS upon heating. In the high temperature domain, the system is modelled by the solution of a layer of WC in the Co, which increases the Co lattice parameter and leads to an increase of compressive stress in WC. The model indicates that there is 2.09 at.% W in solution in the cobalt. The hysteresis is attributed to a difference in the heating and cooling kinetics of solution-precipation of W from WC and WCo3. The results are compared with the mechanical properties of WC-Co.

Ab initio study of the volume dependence of dynamical and thermodynamical properties of silicon

Abstract: Motivated by the negative thermal expansion observed for silicon between 20 K and 120 K, we present an ab initio study of the volume dependence of interatomic force constants, phonon frequencies of transverse-acoustic TA(X) and TA(L) modes, and of the associated mode Gruneisen parameters. The influence of successive nearest-neighbor shells is analyzed. Analytical formulas, taking into account interactions up to second-nearest neighbors, are developed for phonon frequencies of TA(X) and TA(L) modes and the corresponding mode Gruneisen parameters. We also analyze the volume and pressure dependence of various thermodynamic properties (specific heat, bulk modulus, and thermal expansion), and point out the effect of the negative mode Gruneisen parameters of the acoustic branches on these properties. Finally, we present the evolution of the mean-square atomic displacement and of the atomic temperature factor with the temperature for different volumes, for which the anomalous effects are even greater.

Properties of siliconaluminumyttrium oxynitride glasses

Abstract: The thermal and mechanical properties of two SiAlYON systems, including linear thermal expansion coefficient, glass transition temperature, elastic modulus, hardness and fracture toughness, were characterized. Results are compared with those previously reported in the literature in an attempt to expand our knowledge of silicon oxynitride glasses. It is found that increasing the nitrogen content generally results in decreasing the linear thermal expansion coefficient while increasing the glass transition temperature, the elastic modulus and the hardness. These observations are consistent with previous studies on similar oxynitride glasses. Present work also shows that the Y:Al cation ratio has a stronger effect on the thermal properties of the glasses than does the nitrogen content, a factor not systematically examined before. The outcome of this work adds to the database on oxynitride glasses which is essential in research on silicon nitride ceramics.