Showing papers on "Thermal expansion published in 2003"

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

Abstract: 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.

Influence of processing conditions on the thermal and mechanical properties of SU8 negative photoresist coatings

Abstract: The thermal and mechanical properties of a new negative photoresist, SU8, were characterized. The influence of curing conditions, such as baking temperature, baking time and UV dosage, on the thermal and mechanical properties of the resultant coatings was studied in detail. It was found that the glass-transition temperature (Tg) of the coatings was coincident with the baking temperature over the temperature range of 25 °C–220 °C for coatings being baked for just 20 min. However, the Tg reached a limiting value (about 240 °C) once the cross-linking reaction was complete, and would not increase further with the baking temperature. The peak temperature of the dimension versus temperature plots, where heat shrinkage occurred, was about a factor of 1.16 times higher than the baking temperature for the temperature range studied. Both the Tg and the shrinkage temperature were affected by the baking time. The thermal expansion coefficients (TEC), including the volumetric TEC (αv), the in-plane TEC (α1) and the out-of-plane TEC (α2), were measured by a pressure–volume–temperature (PVT) apparatus and thermal–mechanical analyzer (TMA). Great residual stress could be generated during the process, and the change in residual stress with the environmental humidity was investigated using vibrational holographic interferometry.

Nanomechanics of carbon nanotubes and composites

Abstract: Computer simulation and modeling results for the nanomechanics of carbon nanotubes and carbon nanotube-polyethylene composite materials are described and compared with experimental observations Young’s modulus of individual single-wall nanotubes is found to be in the range of 1 TPa within the elastic limit At room temperature and experimentally realizable strain rates, the tubes typically yield at about 5–10% axial strain; bending and torsional stiffness and different mechanisms of plastic yielding of individual single-wall nanotubes are discussed in detail For nanotube-polyethylene composites, we find that thermal expansion and diffusion coefficients increase significantly, over their bulk polyethylene values, above glass transition temperature, and Young’s modulus of the composite is found to increase through van der Waals interaction This review article cites 54 references @DOI: 101115/11538625#

Effect of temperature on thermal properties of high-strength concrete

Abstract: For use in fire resistance calculations, the relevant thermal properties of high-strength concrete (HSC) were determined as a function of temperature. These properties included the thermal conductivity, specific heat, thermal expansion, and mass loss of plain and steel fibre-reinforced concrete made of siliceous and carbonate aggregate. The thermal properties are presented in equations that express the values of these properties as a function of temperature in the temperature range between 0 and 1,000°C. The effect of temperature on thermal conductivity, thermal expansion, specific heat, and mass loss of HSC is discussed. Test data indicate that the type of aggregate has a significant influence on the thermal properties of HSC, while the presence of steel fiber reinforcement has very little influence on the thermal properties of HSC.

Zero thermal expansion in YbGaGe due to an electronic valence transition

Abstract: Most materials expand upon heating. Although rare, some materials expand on cooling, and are said to exhibit negative thermal expansion (NTE); but the property is exhibited in only one crystallographic direction. Such materials include silicon and germanium1 at very low temperature (<100 K) and, at room temperature, glasses in the titania–silica family2, Kevlar, carbon fibres, anisotropic Invar Fe-Ni alloys3, ZrW2O3 (ref. 4) and certain molecular networks5. NTE materials can be combined with materials demonstrating a positive thermal expansion coefficient to fabricate composites exhibiting an overall zero thermal expansion (ZTE). ZTE materials are useful because they do not undergo thermal shock on rapid heating or cooling. The need for such composites could be avoided if ZTE materials were available in a pure form. Here we show that an electrically conductive intermetallic compound, YbGaGe, can exhibit nearly ZTE—that is, negligible volume change between 100 and 400 K. We suggest that this response is due to a temperature-induced valence transition in the Yb atoms. ZTE materials are desirable to prevent or reduce resulting strain or internal stresses in systems subject to large temperature fluctuations, such as in space applications and thermomechanical actuators.

Thermal expansion of carbon structures

Abstract: Using an empirical bond-order potential, molecular dynamics (MD) simulations, and lattice dynamics calculations, we study the thermal expansion of diamond, graphite and single-walled carbon nanotubes. MD simulations demonstrate that, while the C-C bond length increases at a similar rate with increasing temperature in all structures, the thermal expansion coefficient varies greatly in a manner consistent with experiment. An analysis of the mode-dependent Gr\"uneisen parameters provides a detailed picture of how structure influences the competition between various vibrational modes associated with negative and positive Gr\"uneisen parameters in determining the overall thermal expansion coefficient.

Thermal and chemical induced expansion of La0.3Sr0.7(Fe,Ga)O3−δ ceramics

Abstract: The linear thermal expansion coefficients (TECs) of perovskite-type La 0.3 Sr 0.7 Fe 1− x Ga x O 3− δ ( x =0–0.4), determined by dilatometric and high-temperature X-ray diffraction techniques, are in the range (19–41)×10 −6 K −1 at 770–1170 K, decreasing when the oxygen partial pressure or gallium concentration increases. At oxygen pressures from 10 −4 to 1 atm, the isothermal chemically induced expansion of La 0.3 Sr 0.7 Fe(Ga)O 3− δ ceramics is a linear function of the oxygen nonstoichiometry. The magnitude of changes in δ and, thus, chemical expansion both are reduced by gallium doping. The ratio between isothermal chemical strain and nonstoichiometry variations, ( e C /Δ δ ), follows an Arrhenius-type dependence on temperature and varies in the range (1.7–5.9)×10 −2 . The drastic increase in the thermal expansion at temperatures above 700 K, typical for ferrite-based ceramics, was shown to be mainly apparent, resulting from the chemically-induced expansion of the lattice due to oxygen losses. The TEC values, corrected for the chemical strain on heating, are close to the TECs at low temperatures and increase with gallium content. The observed correlations between the thermal and chemical expansion and ionic conductivity of La 0.3 Sr 0.7 Fe 1− x Ga x O 3− δ are discussed in terms of their relationships with the oxygen deficiency and cation composition.

Temperature-dependent dispersion equations for KTiOPO4 and KTiOAsO4.

Abstract: We have measured the thermal expansion and the temperature dependence of z and y components of the refractive index for KTiOPO4 and KTiOAsO4, in the wavelength range 532-1585 nm and temperature range 25-200 degrees C, using an interferometric technique. The measurements were used to derive temperature-dependent Sellmeier equations for the two materials. These equations predict with good agreement the temperature dependence of quasi-phase-matched nonlinear frequency converters.

Temperature dependence of thermal expansion and elastic constants of single crystals of ZrB2 and the suitability of ZrB2 as a substrate for GaN film

Abstract: Coefficients of thermal expansion (CTE) and elastic constants of single crystals of ZrB2 have been determined in the temperature ranges from room temperature to 1073 K and from room temperature to 1373 K, respectively. The elastic constants of ZrB2 are best characterized by the large value of the Young modulus (as high as 500 GPa) and the small values of the Poisson ratio (0.13–0.15), indicating the high stiffness and hardness and the brittleness, respectively. The values of CTE along the a- and c-axis directions are 6.66×10−6 and 6.93×10−6 K−1, respectively, when averaged over the temperature range from room temperature to 1073 K. The CTE value along the a-axis direction of ZrB2 is only moderately larger than the corresponding value for GaN. This together with the small lattice mismatch along the a-axis direction between ZrB2 and GaN in the heteroepitaxial orientation relationship of (0001)GaN//(0001)ZrB2 and 〈1120〉GaN//〈1120〉ZrB2 indicate that only a small compressive stress develops in the GaN thin-film crystal grown on the (0001) surface of the ZrB2 substrate. The stresses developed in the GaN thin-film crystal are evaluated with the values of CTE and elastic constants of ZrB2 determined in the present study. The evaluation verifies the suitability of ZrB2 as a substrate for heteroepitaxial growth of GaN.

The microstructure and the thermal expansion characteristics of Cu/SiCp composites

Abstract: Copper/silicon carbide composites (Cu/SiC p ) were made by the powder metallurgy method. Electroless plating was employed to deposit a copper film on SiC p powder before mixing with Cu powder in order to improve the bonding status between Cu and SiC particles during sintering. Thermal expansion property of as-formed product was measured in the temperature range from 50 to 550 °C. The results showed that copper coating on silicon carbide particles could render uniform distribution of SiC p in the copper matrix. The composites exhibited positive thermal hysteresis behavior when cooled down from the peak temperature to room temperature, which can be explained in terms of the residual stresses and the interfacial bonding between copper and silicon carbide. The magnitude of this strain was a function of the SiC p volume fraction and the number of thermal cycles. The thermal expansion property of composites was measured and compared with those predicted from various theoretical models.

Whole-cell heater for the diamond anvil cell

Abstract: An external heating assemblage allowing diamond anvil cell (DAC) experiments at megabar pressures and temperatures above 1200 K was constructed. The complete high-pressure high-temperature system consists of an anvil assembly made from a special high-temperature alloy, a mechanical loading mechanism, and external resistive heaters placed around the cell. The new system allows fine adjustment of the pressure (within 1 GPa) over the whole temperature range. It maintains constant pressure (within 1 GPa at megabar pressures) and constant temperature (within 5 K at 1000 K) for several hours. Temperature is measured with an external thermocouple. The pressure chamber does not have a measurable temperature gradient. The new heating assemblage is easily coupled with a experimental setup at synchrotron radiation facilities and Raman spectrometers. We tested the performance of the new system by measuring the thermal expansion of Fe0.90Ni0.05 at different pressures and by studying phase transformations in TiO2 and t...

Melting of tantalum at high pressure determined by angle dispersive x-ray diffraction in a double-sided laser-heated diamond-anvil cell

Abstract: The high-pressure and high-temperature phase diagram of Ta has been studied in a laser-heated diamond-anvil cell (DAC) using x-ray diffraction measurements up to 52 GPa and 3800 K. The melting was observed at nine different pressures, the melting temperature being in good agreement with previous laser-heated DAC experiments, but in contradiction with several theoretical calculations and previous piston–cylinder apparatus experiments. A small slope for the melting curve of Ta is estimated ( at 1 bar) and a possible explanation for this behaviour is given. Finally, a P–V –T equation of states is obtained, the temperature dependence of the thermal expansion coefficient and the bulk modulus being estimated.

Thermal and optical properties of glasses of the system Bi2O3 – B2O3

Abstract: In the binary system Bi 2 O 3 - B 2 O 3 glasses were prepared in the composition range 57. 5 mol% - 80 mol% B 2 O 3 by defined slow cooling of large melt samples (about 75 cm 1 , each). Temperatures of crystallization, of melting and of glass transition were determined and density data of the glasses were derived using the hydrostatic weighting method. Thermal expansion coefficients and high precision refractive indices, together with their dispersion, were measured. The measured physical properties indicate subtle discontinuous structural changes of the glasses with glass composition, that match with the ranges of existence of the crystalline compounds of the binary system Bi 2 O 3 - B 2 O 3 . Thermal investigations together with X-ray powder diffraction analyses of crystallized glass samples prove the so far doubtful existence of a borate compound named BiBO 3 in the PDF within the composition range 52. 5 - 57.5 mol% B 2 O 3 .

Molecular dynamics study of cristobalite silica using a charge transfer three-body potential: Phase transformation and structural disorder

Abstract: Structural and dynamic properties of cristobalite silica have been studied using molecular dynamics simulations based on a charge transfer three-body potential model. In this potential model, the directional covalent bonding of SiO 2 is characterized by a charge transfer function of the interatomic distance between Si and O atoms, and in the form of Si‐O‐Si and O‐Si‐Othree-body interactions. The dynamic properties such as infrared spectra and density of states at room and elevated temperatures are in excellent agreement with experiments, and are also consistent with the recently proposed rigid unit modes model. The a- and b-cristobalite crystallographic structures are well reproduced in this model, and the transition between these modifications occurs reversibly and reproducibly in simulations, both as a result of changes in pressure and temperature. The thermally induced transition results in a significantly more disordered b-cristobalite than the pressure-induced b-cristobalite at room temperature. While simulated a-cristobalite exhibits a positive thermal expansion coefficient, it is almost zero forb-cristobalite up to 2000 K and slightly negative at higher temperatures, confirming results from recent x-ray diffraction experiments and other simulations with potential models based on ab initio calculations. © 2003 American Institute of Physics. @DOI: 10.1063/1.1529684#

Thermal expansion coefficient of hydrogenated amorphous carbon

Abstract: The coefficient of thermal expansion (CTE) of hydrogenated amorphous carbon (a-C:H) was investigated as a function of the concentration of sp2 hybridization. The CTE, determined using the thermally induced bending technique, depends on the concentration of sp2 bonded carbon, increasing to the value of graphite as the sp2 concentration approaches 100%. By using a combination of the thermally induced bending technique and nanohardness measurements, we extract separately the Young’s modulus and Poisson’s ratio of the a-C:H films as function of the sp2 concentration.

Crystalline Structure and Morphology in Nylon-12: A Small- and Wide-Angle X-ray Scattering Study

Abstract: Simultaneous small- and wide-angle X-ray scattering has been employed to monitor the temperature dependence of the crystalline structure and morphology of nylon-12. Wide-angle measurements reveal that the α‘-phase is a stable high-temperature crystalline phase. Upon cooling, a crystalline transition takes place from the α‘-form into the hexagonal γ-phase. The small-angle data indicate at this point a discontinuous change in the density. At low temperatures, two crystal reflections occur at wide angles. This is interpreted as a monoclinic or an orthorhombic structure, which evolves from the hexagonal structure of the γ-form by an anisotropic thermal expansion. The evolution of the crystalline thickness during cooling and heating suggests the occurrences of partial surface crystallization and melting. This effect is superimposed on the transition α‘ ↔ γ and leads to crystal thickening and thinning during cooling and heating, respectively. The transition from γ to α‘ with increasing temperature arises from a...

The thermal expansion and mechanical properties of high reinforcement content SiCp/Al composites fabricated by squeeze casting technology

Abstract: With mixing different sized SiC particles, high reinforcement content SiCp/Al composites ( V p =50, 60 and 70%) for electronic packaging applications were fabricated by squeeze casting technology. The composites were free of porosity and SiC particles distributed uniformly in the composite. The mean linear coefficients of thermal expansion (20–100 °C) of SiCp/Al composites ranged from 8.3 to 10.8×10 −6 /°C and decreased with an increase in volume fraction of SiC content. The experimental coefficients of thermal expansion agreed well with predicted values based on Kerner's model. The Brinell hardness increased from 188.6 to 258.0, and the modulus increased from 148 to 204 GPa for the corresponding composites. The bending strengths were larger than 370 MPa, but no obvious trend between bending strength and SiC content was observed.

Thermodynamical fluctuations in optical mirror coatings

Abstract: Thermodynamical fluctuations of temperature in mirrors may produce surface fluctuations not only through thermal expansion in mirror body but also through thermal expansion in mirror coating. We analyze the last "surface" effect which can be larger than the first "volume" one due to larger thermal expansion coefficient of coating material and smaller effective volume. In particular, these fluctuations may be important in laser interferometric gravitational antennae.