Determination of thermal conductivity of CVD diamond films via photoacoustic measurements

A novel method of determination of polycrystalline diamond films’ thermoconductive properties using the photoacoustic effect is proposed. By this method, we studied the diamond films grown on silicon substrates using chemical vapor deposition technique. A value of thermal conductivity obtained for the films was less than half that for diamond single crystal. The decrease of thermal conductivity, as well as characteristic features of optical properties of the films, is explained by the presence of a large amount of intercrystallite boundaries and other structure defects and admixures detected using Raman and photoluminescent spectroscopies. Received: 22 October 1998 / Accepted: 27 January 1999 / Published online: 28 April 1999     

On the role of atomic motion soft modes in acoustic phonon broadening below the boson peak in glasses

This Letter presents a mechanism of acoustic phonon broadening for frequencies lower than the boson peak frequency in glasses exhibiting a high-frequency sound above the boson peak. The mechanism is based on a resonant interaction of an acoustic phonon with harmonic vibrational excitations of soft modes in such glasses. The related width of the phonon is found to be independent of temperature and characterized by a power-law frequency dependence ν?${\nu }^{?}$, with the exponent ?   varying from ?≈2$?\approx 2$ below the boson peak to ?≈4$?\approx 4$ at lower frequencies. The dependencies do not appear to contradict some recent experimental data, for the glasses under discussion.     

Thermal conductivity of pressure polymerized C60

60 polymerization in the temperature interval at pressures below by measurements of the time dependence of the thermal conductivity. It has been found at that the polymerization process at is slower than the reverse transformation from “polymeric” to “monomeric” phase at . The thermal conductivity of polymerized C₆₀ was measured in the temperature range and found to increase with increasing temperature, which reflects strong phonon scattering. Both the presence of non-bonded C₆₀ molecules and a high degree of structural disorder in the crystalline lattice of the polymeric phase might be responsible for the behaviour of . The results for are qualitatively similar to those reported previously for C₆₀ polymerized at higher , but an order of magnitude smaller. Received: 20 September 1996/Accepted: 11 November 1996     

Phonon Transport and Thermal Conductivity in an Acoustic Filter

We investigate the phonon ballistic transmission and the thermal conductivity in a dielectric quantum structure. It is found that these observable quantities sensitively depend on geometric parameters, and are of quantum character. The total transmission coetfficient as a function of the reduced waveguide-length exhibits periodical behaviour and the reduced thermal conductance decreases below the ideal universal value for the low temperature. Our results show that one can control the thermal conductivity of the structure and make all kinds of acoustic filters to match practical requirements in devices by adjusting the geometric parameters.     

A model for compression dependence of thermal expansivity

In the present study, we propose a suitable model to compute volume dependence of thermal expansivity which is applicable up to infinite pressure or compression. The newly developed model satisfies the constraints of infinite pressure as suggested by high-pressure thermodynamics. The compression dependence of thermal expansivity for lower mantle of the Earth is evaluated with the help of the proposed model. A close agreement between theory and the results predicted with the seismic data is found, which in turn reveals the validity of the present work.     

Thermal conductivity of selenium doped with indium and iodine in the solid and liquid states

We have measured the thermal conductivity in both the solid and liquid states for two amorphous samples. The first was selenium doped with indium and the second was selenium doped with iodine. The concentration of In and I in both samples was 50000 ppm in weight, which is equal to 3.33% for indium and 3.02% for iodine additives in atomic percent. The measurements were taken in the temperature range from 100 to 470°C and were carried out using the concentric cylindrical wall. It was found that the thermal conductivity of both samples is of the phonon type; its temperature dependence follows the relation K _ph T ^–1, and can be explained by the influence of thermal effects on the lattice structure.     

Specific heat spectra of non-interacting fermions in a quasiperiodic ladder sequence

We compute the specific heat spectra of non-interacting fermions whose energy spectrum was obtained from a quasiperiodic ladder sequence (Fibonacci and Rudin-Shapiro type), mimicking a DNA molecule model. The specific heat is calculated from their underlying multi-fractal energy spectrum, considering several values of energy densities. Comparisons are made with a real DNA sequence, namely the human chromosome 22 (Ch22).     

Thermal conductivity of C60 and C70 crystals

We have performed thermal conductivity measurements on C₆₀ and C₇₀ crystals grown by sublimation. For single crystal C₆₀, the thermal conductivity k is 0.4 W/m K at room temperature and is nearly temperature independent down to 260K. We observed a sharp orientational phase transition at 260K, indicated by a 25% jump in k. Below 90K, k is time dependent, which manifests itself as a shoulder-like structure at 85K. The temperature and time dependence of k below 260K can be described by a simple model which accounts for the thermally activated hopping of C₆₀ molecules between two nearly degenerate orientations, separated by an energy barrier of 240 meV. It is found that solvents have a strong influence on the physical properties of C₇₀ crystals. For solvent-free C₇₀ crystal, k is about constant above 300K. There is a broad first-order phase transition in k at 300K with a 25% jump. We associate this transition with the aligning of the fivefold axes of the C₇₀ molecules along the c-axis of the hexagonal lattice. Upon further cooling, k increases and is time independent.     

Photothermal measurement of thermal anisotropy in pyrolytic graphite

We investigated the anisotropic thermal conductivity in pyrolytic graphite by thermoreflectance. A laser-heated circular spot on a surface perpendicular to the planes developed into an elliptical temperature distribution which was recorded by a raster scanning technique at modulation frequencies ranging from 600 Hz to 100 kHz. The ratio of in-plane and perpendicular thermal conductivity was determined by fitting the phase of the temperature data with an analytical model, and was found to decrease with increasing modulation frequency. Highest conductivity values were considerably smaller than previously published data based on steady-state measurements. The frequency dependence and additional features in the phase profiles at high frequencies are discussed in view of sample surface preparation and the local nature of the thermoreflectance measurement.     

Thermal conductivity of isotopically enriched Si: revisited

The thermal conductivity of isotopically enriched ²⁸Si (enrichment better than 99.9%) was redetermined independently in three laboratories by high precision experiments on a total of four samples of different shape and degree of isotope enrichment in the range from 5 to 300 K with particular emphasis on the range near room temperature. The results obtained in the different laboratories are in good agreement with each other. They indicate that at room temperature the thermal conductivity of isotopically enriched ²⁸Si exceeds the thermal conductivity of Si with a natural, unmodified isotope mixture by 10±2%. This finding is in disagreement with an earlier report by Ruf et al. At ∼26 K the thermal conductivity of ²⁸Si reaches a maximum. The maximum value depends on sample shape and the degree of isotope enrichment and exceeds the thermal conductivity of natural Si by a factor of ∼8 for a 99.982% ²⁸Si enriched sample. The thermal conductivity of Si with natural isotope composition is consistently found to be ∼3% lower than the values recommended in the literature.     

The effects of experimental parameters in the thermal diffusivity measurements of oriented polymer films using mirage effect: Computer simulation

Computer simulations have been carried out to study the effects of the experimental parameters when the mirage method has been applied to thermal diffusivity measurements of oriented polymer films. The parameters under study are the thermal diffusivity of the fluid surrounding the sample, the modulation frequency and the radius of the heating beam, the height and the radius of the probe beam, and the sample thickness and thermal diffusivity. Proposals for the optimum parameter values to maximize the measurement sensitivity for the sample diffusivity are made and the difficulties arising from the low diffusivity of the samples are described. It is also concluded that because the thermal properties of the fluid surrounding the sample have a strong contribution to the mirage signals, the signals do not include any simple feature corresponding to the sample diffusivity. Therefore it should be determined from the entire measurement data using regression methods.     

Dispersive phonon imaging of InAs

The phonon-focusing patterns of ballistic phonons in InAs are measured in the frequency range 0.1 to 1 THz, in an effort to test the global validity of lattice dynamics models for this semiconductor. Phonon caustic patterns depend sensitively on the shapes of constant frequency surfaces. Several tunnel-junction detectors with sensitivity onsets in this frequency range are used to measure dispersive shifts in the phonon caustics. The measured caustic positions are compared to those predicted by rigid-ion and bond-charge models. Similar to the case of InSb studied by Hebboul and Wolfe, a 6-parameter bond-charge model (BCM)-with force constants determined by neutron, X-ray, and Raman scattering-reproduces the phonon-imaging data both qualitatively and quantitatively. Comparisons of the focusing patterns with an 11-parameter rigid-ion model (RIM) do not show good agreement. New structures are predicted in the phononfocusing patterns at frequencies above about 1.2 THz — presently outside our experimental range-which are highly sensitive to the theoretical modeling.     

Phonon focussing patterns: Calculation of response of finite area detectors to pulsed ballistic beams of dispersive and dispersionless phonons

The phonon images of crystals are described in the frame of the Boltzmann kinetic equation. Monochromatic heat pulses of the dispersive and dispersionless acoustic phonons are considered. Exact expressions for the energy and quasimomentum carried by a pulsed beam of monochromatic dispersionless acoustic phonons falling onto a detector of the finite area are derived. These formulae provide us with a convenient starting point for numerical calculations of phonon images. For the example of long wave-length acoustic phonons and a point as well as extended sources, an algorithm for numerical calculations of phonon images of anisotropic crystalline media is presented. However, it is quite general and can be easily adapted for dispersive phonons and to quasiparticles with an arbitrary dispersion low.     

Effects of titanium addition on the microstructure of carbon/copper composite materials

Carbon(C)/copper(Cu)-based materials with high thermal conductivity and good stability at high temperatures were developed by adding a small amount of titanium, which has a low enthalpy of alloy formation with C and Cu. The isotropic fine-grained nuclear grade graphite and felt type C/C composite, which were impregnated by Cu and titanium, provided 1.3 times higher thermal conductivity at 1200 K than the original carbon materials. Microstructural analysis showed that the increase of thermal conductivity is due to the formation of titanium compounds at the C/Cu interface. These carbon-based materials could be a candidate material for the plasma facing components of fusion devices.     

Photoelastic effect and mirage deflection in anisotropic materials

The collinear mirage technique is widely used to measure the thermal diffusivity of semi-transparent materials. However, in a recent paper [A. Salazar, M. Gateshki and A. Sánchez-Lavega: Appl. Phys. Lett. 76, 2665 (2000)], it was shown that for isotropic materials, because of the influence of photoelastic effect, the method was sensitive to the polarization state of the probe beam. The present paper extends the previous work to include anisotropic materials. In particular, we focus on the experimental conditions under which the thermal diffusivity of each crystal system can be measured using the phase method. Our theoretical model indicates that while the thermal diffusivity of isotropic materials can be measured using an unpolarized probe beam, for anisotropic materials, even the use of an unpolarized probe beam does not guarantee the validity of the method in all crystal systems. Experimental measurements performed on cubic, hexagonal and monoclinic crystals confirm the validity of the model. Received: 17 March 2001 / Accepted: 17 March 2001 / Published online: 25 July 2001     

Composition dependence of thermal stability, micro-hardness and compactness in glassy Se90In10−xGex alloys

In the present work, the effect of Ge incorporation is studied on the thermal stability, micro-hardness and compactness of glassy Se₉₀In₁₀ alloy.Thermal stability of glassy Se₉₀In_10−xGe_x alloys has been studied using differential scanning calorimetric technique. Micro-hardness of glassy Se₉₀In_10−xGe_x alloys is measured at room temperature. The compactness of the structure of Se₉₀In_10−xGe_x alloys is also determined from the measured densities. The variation in micro-hardness and compactness with composition has been discussed.