Showing papers on "Thermal expansion published in 2002"

Thermal Expansion and Diffusion Coefficients of Carbon Nanotube-Polymer Composites

Abstract: Classical molecular dynamics (MD) simulations employing Brenner potential for intra-nanotube interactions and van der Waals forces for polymer-nanotube interface have been used to investigate thermal expansion and diffusion characteristics of carbon nanotube-polyethylene composites. Addition of carbon nanotubes to polymer matrix is found to significantly increase the glass transition temperature Tg, and thermal expansion and diffusion coefficients in the composite above Tg. The increase has been attributed to the temperature dependent increase of the excluded volume for the polymer chains, and the findings could have implications in the composite processing, coating and painting applications.

Lattice parameters and anisotropic thermal expansion of hexagonal boron nitride in the 10–297.5 K temperature range

Abstract: Lattice parameters for hexagonal boron nitride (hBN) were determined using X-ray powder diffraction at a synchrotron-radiation source (beamline B2, Hasylab/DESY, Hamburg) in the previously uninvestigated temperature range from 10 K up to 297.5 K. The relative change of a and c with rise of temperature in the studied range is anisotropic and amounts to about -0.05% and +0.82%, respectively. The corresponding increase of the unit-cell volume is 0.73%. The evaluated values of thermal expansion coefficients derived from the lattice-parameter dependences on temperature are generally consistent with earlier data determined by interferometryfor T≥82 K.

Evaluation of yttrium-doped SrTiO3 as an anode for solid oxide fuel cells

Abstract: Yttrium-doped SrTiO3 (SYT) was assessed as an anode material for solid oxide fuel cells in terms of electrical conductivity, phase stability, redox behavior, chemical compatibility with yttria-stabilized zirconia (YSZ) and La0.8Sr0.2Ga0.8Mg0.2O2.8 (LSGM), thermal expansion coefficient, and fuel cell performance. With the optimized composition Sr0.86Y0.08TiO3−δ, the electrical conductivity was as high as 82 S/cm at 800 °C and oxygen partial pressure of 10−19 atm. A reversible change of conductivity was observed upon oxidation and reduction. The resistance to oxidation was enhanced by partially replacing Ti with transition metals such as cobalt. This material has high structural stability over a broad range of temperature (up to 1400 °C) and oxygen partial pressure (1–10−20 atm). No phase change was found for mixtures of SYT with YSZ or LSGM sintered at 1400 °C for 10 h. The thermal expansion of doped-SrTiO3 was determined to be compatible with that of YSZ and LSGM. A maximum power density of 58 mW/cm2 at 900 °C was obtained for single cells with the new anode.

Impact of thermal strain on the dielectric constant of sputtered barium strontium titanate thin films

Abstract: Barium strontium titanate thin films were deposited by sputtering on Pt/SiO2 structures using five different host substrates: magnesium oxide, strontium titanate, sapphire, silicon, and vycor glass. These substrates were chosen to provide a systematic change in thermal strain while maintaining the same film microstructure. All films have a weakly textured microstructure. Temperature dependent dielectric measurements from 100–500 K determined that decreasing thermal expansion coefficient of the host substrate (i.e., larger tensile thermal strain) reduced the film dielectric permittivity. The experimentally determined Curie–Weiss temperature decreased with increasing tensile thermal strain and the Curie–Weiss constant increased with tensile strain as predicted by Pertsev et al. [J. Appl. Phys. 85, 1698 (1999)].

Coherent vibrational motion in metal particles: Determination of the vibrational amplitude and excitation mechanism

Abstract: Ultrafast laser excitation of metal particles coherently excites the symmetric breathing mode. This changes the electron density in the particle, which produces a periodic redshift in the position of the plasmon band. In this paper transient absorption data recorded over a range of wavelengths are analyzed to determine the amplitude of the breathing motion for 24.2 nm radius Au particles. The results are compared to a model calculation where the expansion coordinate is treated as a damped harmonic oscillator and the driving force is thermal expansion due to lattice heating (the temperature rise is determined from the energy absorbed by the sample). The only adjustable parameters in these calculations are the dephasing time of the oscillations and the time scale for energy transfer to the solvent. The experimental and calculated vibrational amplitudes are in excellent agreement, which shows that all the absorbed energy goes into expansion. However, the phases of the calculated and experimental traces do not match. The calculations can be brought into almost perfect agreement with the experimental results by including hot-electron pressure effects in the coefficient for thermal expansion of the particles. This contribution is significant in our experiments because laser excitation initially creates a very high electronic temperature. A simple expression for the time dependence of the transient absorption signal is also derived that explicitly accounts for sample polydispersity. In this expression the beat period is related to the mean radius, and the damping time to the mean radius and the width of the size distribution. Thus, time-resolved laser experiments can be used to obtain accurate information about the size distribution of metal particle samples.

Finite-size and surface effects on the glass transition of liquid toluene confined in cylindrical mesopores

Abstract: Some of the most regular porous silicates (MCM-41 and SBA-15), with several different pore diameters from 2.4 to 8.7 nm, are used to study the van der Waals fragile liquid toluene in confined geometry. We measure two major macroscopic signatures of a glass transition, i.e., a discontinuous change in the heat capacity and in the thermal expansion, by adiabatic calorimetry and neutron scattering experiments. A nontrivial size dependence of the glass transition features, most notably a nonmonotonic variation of the mean glass transition temperature, is observed. The range of the glass transition is found extremely broad. This supports the notion of competition between surface boundary conditions and cutoff or finite-size effects.

Stable sealing glass for planar solid oxide fuel cell

Abstract: The thermal and chemical stability of glasses in the BaO–Al2O3–La2O3–B2O3–SiO2 system were investigated as well as bonding characteristics and wetting behavior to yttria stabilized zirconia (YSZ) electrolyte, to develop a suitable sealing glass for planar solid oxide fuel cell operating at 800–850 °C. The thermal properties such as glass transition temperature, softening temperature and thermal expansion coefficient were found to depend on the B2O3:SiO2 ratio in glass composition; thus the bonding characteristics of the glass to YSZ were also influenced by this ratio. The glass having a minimum thermal expansion mismatch with YSZ showed an excellent endurance during thermal cycling. No interface reaction was observed for all the glass/YSZ specimens heat-treated at 800–850 °C up to 100 h.

Thermal and electrical properties of Nb2AlC, (Ti, Nb)(2)AlC and Ti2AlC

Abstract: The heat capacities, thermal-expansion coefficients, thermal and electrical conductivities of Nb2AlC (actual Nb:Al:C mole fractions: 0.525±0.005, 0.240±0.002, and 0.235±0.005, respectively), Ti2AlC and (Ti, Nb)2AlC (actual Ti:Nb:Al:C mole fractions: 0.244±0.005, 0.273±0.005, 0.240±0.003, and 0.244±0.005, respectively) were measured as a function of temperature. These ternaries are good electrical conductors, with a resistivity that increases linearly with increasing temperatures. The resistivity of (Ti, Nb)2AlC is higher than the other members, indicating a solid-solution scattering effect. The thermal-expansion coefficients, in the 25 °C to 1000 °C temperature range, are comparable and fall in the narrow range of 8.7 to 8.9 × 10−6 K−1, with that of the solid solution being the highest. They are all good conductors of heat, with thermal conductivities in the range between 15 to 45 W/m K at room temperature. The electronic component of the thermal conductivity is the dominant mechanism at all temperatures for Nb2AlC and (Ti, Nb)2AlC. The conductivity of Ti2AlC, on the other hand, is high because the phonon contribution to the conductivity is nonnegligible.

The mechanical and thermophysical properties of ZrC/W composites at elevated temperature

Abstract: In order to improve the elevated temperature mechanical properties and modify the thermophysical properties of tungsten, tungsten matrix composites containing 30 vol.% zirconium carbide particles (ZrC/W) were prepared by vacuum hot-pressing at 2000 °C. Flexural strength of ZrC/W composites increases gradually with increasing temperature and reaches a maximum value at 1000 °C. The observation of the fracture characteristics of ZrC/W composites at various temperatures reveals that the excellent elevated temperature strength is attributed to the brittle–ductile transition in the tungsten matrix, which allows a more effective strengthening effect from the zirconium carbide particles. The elastic modulus decreases slightly with increasing temperature from room temperature to 1200 °C, and as a result of the addition of zirconium carbide particles the modulus of ZrC/W is higher than that of monolithic tungsten at all testing temperatures. The addition of zirconium carbide remarkably decreases the thermal conductivity of the composites. The unique combination of high melting point, good high temperature strength, high elastic modulus, and low thermal conductivity is very beneficial to the high temperature application of ZrC/W composites.

Development of rapid heating and cooling systems for injection molding applications

Abstract: The injection molding process has several inherent problems associated with the constant temperature mold. A basic solution is the rapid thermal response molding process that facilitates rapid temperature change at the mold surface thereby improving quality of molded parts without increasing cycle time. Rapid heating and cooling systems consisting of one metallic heating layer and one oxide insulation layer were investigated in this paper. Design issues towards developing a mold capable of raising temperature from 25°C to 250°C in 2 seconds and cooling to 50°C within 10 seconds were discussed. To reduce thermal stresses in the layers during heating and cooling, materials with closely matched low thermal expansion coefficient were used for both layers. Effects of various design parameters, such as layer thickness, power density and material properties, on the performance of the insert were studied in detail with the aid of heat transfer simulation and thermal stress simulation. Several rapid thermal response mold inserts were constructed on the basis of the simulation results. The experimental heating and cooling response agrees with the simulation and also satisfies the target heating and cooling requirement.

The characterization of thermal and elastic constants for an epoxy photoresist SU8 coating

Abstract: All of the thermal and elastic constants of the high-aspect ratio, negative, UV resist, SU8 coating were carried out, and the compliance matrix of the coating was obtained DSC, TGA, TMA, and DMTA techniques were utilized to study the thermal properties of the material The in-plane thermal expansion coefficient (TEC) (α1) was determined by TMA, and the glass transition behavior was studied by DMTA The TGA study provided information about the thermal stability of the material, and DSC was applied to study the thermal calorimetric properties of the material The in-plane Young's modulus (E 1) was measured by tensile tests The residual stresses of a 1D stretched ribbon sample and a 2D stretched membrane sample were measured by vibrational holography tests, and the in-plane Poisson's ratio (ν1) was also determined by holography The out-of-plane Poisson's ratio (ν2) was obtained by high pressure gas dilatometry measurements The bulk compressibility (κ) and the volumetric TEC (α v ) of the material were measured by a pressure-volume-temperature (PVT) apparatus Finally, the out-of-plane properties, including the out-of-plane TEC (α2) and the out-of-plane Young's modulus (E 2), were calculated from the measured in-plane properties and the volumetric properties Therefore, the compliance matrix of the studied SU8 coating could be obtained

Thermal expansion and crystal structure of FeSi between 4 and 1173 K determined by time-of-flight neutron powder diffraction

Abstract: The thermal expansion and crystal structure of FeSi has been determined by neutron powder diffraction between 4 and 1173 K. No evidence was seen of any structural or magnetic transitions at low temperatures. The average volumetric thermal expansion coefficient above room temperature was found to be 4.85(5) × 10−5 K−1. The cell volume was fitted over the complete temperature range using Gruneisen approximations to the zero pressure equation of state, with the internal energy calculated via a Debye model; a Gruneisen second-order approximation gave the following parameters: θD=445(11) K, V0=89.596(8) A3, K0′=4.4(4) and γ′=2.33(3), where θD is the Debye temperature, V0 is V at T=0 K, K0′ is the first derivative with respect to pressure of the incompressibility and γ′ is a Gruneisen parameter. The thermodynamic Gruneisen parameter, γth, has been calculated from experimental data in the range 4–400 K. The crystal structure was found to be almost invariant with temperature. The thermal vibrations of the Fe atoms are almost isotropic at all temperatures; those of the Si atoms become more anisotropic as the temperature increases.

Compressibility and thermal expansion of cubic silicon nitride

Abstract: The compressibility and thermal expansion of the cubic silicon nitride (c-Si3N4) phase have been investigated by performing in situ x-ray powder-diffraction measurements using synchrotron radiation, complemented with computer simulations by means of first-principles calculations. The bulk compressibility of the c-Si3N4 phase originates from the average of both Si-N tetrahedral and octahedral compressibilities where the octahedral polyhedra are less compressible than the tetrahedral ones. The origin of the unit cell expansion is revealed to be due to the increase of the octahedral Si-N and N-N bond lengths with temperature, while the lengths for the tetrahedral Si-N and N-N bonds remain almost unchanged in the temperature range 295-1075 K. (Less)

Systems and methods for epitaxially depositing films on a semiconductor substrate

Abstract: Systems and methods for epitaxial deposition. The reactor includes a hot wall process cavity enclosed by a heater system, a thermal insulation system, and chamber walls. The walls of the process cavity may comprises a material having a substantially similar coefficient thermal expansion as the semiconductor substrate, such as quartz and silicon carbide, and may include an isothermal or near isothermal cavity that may be heated to temperatures as high as 1200 degrees C. Process gases may be injected through a plurality of ports, and are capable of achieving a fine level of distribution control of the gas components, including the film source gas, dopant source gas, and carrier gas. The gas supply system includes additional methods of delivering gas to the process cavity, such as through temperature measurement devices, and through a showerhead.

Thermal contraction of au nanoparticles.

Abstract: A fine Au powder, with a mean particle diameter of 4 nm, has been successfully fabricated. The crystalline structure of the 4 nm Au nanoparticles remains in fcc symmetry. No structural changes were found between 15 and 450 K. A crossover from a positive thermal expansion at low temperatures to a negative thermal expansion at high temperatures was observed in the fcc cell parameter at about 125 K. Anomalies associated with the crossover were also observed in the magnetic response and the heat capacity measurements. The observations can be reasonably well interpreted by accounting for the effects of the valence electron potential on the equilibrium lattice separations, with a weakly temperature dependent level spacing.

Determination of the coefficient of thermal expansion of high performance concrete from initial setting

Abstract: Autogenous shrinkage, which is a consequence of the absolute volume contraction resulting from cement hydration, occurs in any concrete but its effect is particularly amplified in high performance concrete in which it can be as large as drying shrinkage. Autogenous shrinkage can be directly measured in concrete samples under isothermal conditions but from a practical standpoint the experimental procedure is not always possible. On the other hand, it can be evaluated after having taken into account volumetric variations due to the release in heat during cement hydration. To separate the thermal effect from autogenous shrinkage, it is necessary to know at any moment the evolution of the coefficient of thermal expansion of the concrete from initial setting. A simple method to determine the coefficient of thermal expansion at early ages is proposed in this paper. It consists in submitting concrete samples instrumented with vibrating wire extensometers to thermal shocks. The response of the concrete sample to this shock results in a nearly instantaneous deformation, which is measured by the sensor. These deformations, as well as the temperature signal, are used to calculate the coefficient of thermal expansion. By repeating this experiment at various ages, it is possible to follow the variation in the coefficient of thermal expansion of the concrete over time.

Thermal and electrical properties of BaO–B2O3–ZnO glasses

Abstract: Glasses in the BaO–ZnO–B2O3 system were examined as potential replacement for PbO glass frits with low firing temperature (500–600 °C) for the dielectric layer of a plasma display panel (PDP). The glasses were evaluated for glass transition temperature (Tg), thermal expansion coefficient (α) and dielectric constant e. The electrical and the thermal properties were also compared with theoretical data calculated by a known empirical equation. Tg of the glasses varied between 480 and 560 °C, and α was in the range of 7–9×10−6 K−1. The dielectric constant ranges from 14 to 19 and the theoretical data showed lower α and e than the experimental data. The results suggest that BaO–ZnO–B2O3 glasses would be suitable as an alternative to Pb-based dielectric layer in PDPs.

Quantitative evaluation of biaxial strain in epitaxial 3C-SiC layers on Si(100) substrates by Raman spectroscopy

Abstract: We present experimentally determined biaxial strain coefficients for the longitudinal optical (LO) and transversal optical (TO) Raman lines in 3C-SiC. Suspended 3C-SiC membranes with a (100) texture are deflected by a variable pressure load on one side and the strain-induced shifts of the LO and TO Raman lines are measured while the strain is simultaneously calculated from the membrane deflection vs pressure. Using these results we measure the residual strain of 3C-SiC films grown on Si as a function of preparation conditions. The largest residual strain is found in thin samples which relaxes as film thickness increases to a value imposed by the different thermal expansion coefficients of 3C-SiC and Si. As the residual strain decreases and the film thickness increases, the Raman lines narrow indicating an improved crystalline quality. We also find a reduction of the residual strain with increasing growth rate.

Magnetochemical origin for Invar anomalies in iron-nickel alloys

Abstract: Zero- and finite-temperature (T) first-principles calculations versus composition (c) show that magnetochemical effects lead to Invar anomalies in Fe-(Ni, Co, Pt) alloys. Chemical short- or long-range order and negative interatomic exchange interaction of electrons in antibonding majority-spin states force the face-centered-cubic lattice to compete simultaneously for a smaller volume (from antiferromagnetic tendencies) and a larger volume (from Stoner ferromagnetic tendencies). The resulting additional negative lattice anharmonicity is very large for Fe-(Ni, Co) while absent for Fe-Pt. Our results explain the T- and c-dependent behavior of Invar properties, including the lattice softening and thermal expansion of Fe-Ni. In addition, the occurrence of a noncollinear spin structure at $T=0 \mathrm{K}$ near Invar can be understood on the basis of our results.