Thermal Conductivity Of Crystals Research Articles

Light-driven dynamical tuning of the thermal conductivity in ferroelectrics.

Dynamical tuning of the thermal conductivity in crystals, κ, is critical for thermal management applications, as well as for energy harvesting and the development of novel phononic devices able to perform logic operations with phonons. Such a desired κ control can be achieved in functional materials that experience large structural and phonon variations as a result of field-induced phase transformations. However, this approach is only practical within reduced temperature intervals containing zero-bias phase transition points, since otherwise the necessary driving fields become excessively large and the materials' performances are detrimentally affected. Here, based on first-principles calculations, we propose an alternative strategy for dynamically tuning κ that is operative over broad temperature conditions and realizable in a wide class of materials. By shining light on the archetypal perovskite oxide KNbO3, we predict that ultrafast and reversible ferroelectric-to-paraelectric phase transitions are induced, yielding large and anisotropic κ variations (up to ≈30% at T = 300 K). These light-induced thermal transport shifts can take place at temperatures spanning several hundreds of kelvin and are essentially the result of anharmonic effects affecting the phonon lifetimes.

Phonon thermophoresis of crystalline nanoparticles in liquids.

Our nonequilibrium thermodynamic model of thermodiffusion in molecular liquid systems is used to examine the role of thermal phonons in the thermophoresis of liquid suspensions of crystalline nanoparticles, which tend to have high thermal conductivity. The Soret coefficient used to characterize stationary thermodiffusion is related to differences in entropy between a particle and the body of liquid that it displaces. Calculated phonon Soret coefficients for graphite and diamond nanoparticles in three polar solvents are used to establish parameters where the phonon mechanism is expected to dominate particle thermophoresis compared to slip-flow caused by forces induced in the surface layer by the temperature gradient. Because the active mode of thermal conductivity in crystals varies with particle size, phonon thermophoresis is expected to dominate within a specific range of particle size, which varies with the properties of the particle and suspending liquid. For graphite and diamond particles in polar solvents the model estimates a size range of around 10-100 nm. Finally, thermophoretic particle accumulation is ultimately limited by the increasing concentration of particles having high thermal conductivity because the zone of particle concentration decreases the local temperature gradient that drives thermophoresis. The respective nonperturbing concentration is evaluated as the function of the size of a given material.

Unified theory of thermal transport in crystals and glasses

Crystals and glasses exhibit fundamentally different heat conduction mechanisms: the periodicity of crystals allows for the excitation of propagating vibrational waves that carry heat, as first discussed by Peierls; in glasses, the lack of periodicity breaks Peierls' picture and heat is mainly carried by the coupling of vibrational modes, often described by a harmonic theory introduced by Allen and Feldman. Anharmonicity or disorder are thus the limiting factors for thermal conductivity in crystals or glasses; hitherto, no transport equation has been able to account for both. Here, we derive such equation, resulting in a thermal conductivity that reduces to the Peierls and Allen-Feldman limits, respectively, in anharmonic-and-ordered or harmonic-and-disordered solids, while also covering the intermediate regimes where both effects are relevant. This approach also solves the long-standing problem of accurately predicting the thermal properties of crystals with ultralow or glass-like thermal conductivity, as we show with an application to a thermoelectric material representative of this class.

Unified theory of thermal transport in crystals and disordered solids

Crystals and glasses exhibit fundamentally different heat conduction mechanisms: the periodicity of crystals allows for the excitation of propagating vibrational waves that carry heat, as first discussed by Peierls; in glasses, the lack of periodicity breaks Peierls' picture and heat is mainly carried by the coupling of vibrational modes, often described by a harmonic theory introduced by Allen and Feldman. Anharmonicity or disorder are thus the limiting factors for thermal conductivity in crystals or glasses; hitherto, no transport equation has been able to account for both. In the paper https://arxiv.org/abs/1901.01964, we derive such equation, resulting in a thermal conductivity that reduces to the Peierls and Allen-Feldman limits, respectively, in anharmonic-and-ordered or harmonic-and-disordered solids, while also covering the intermediate regimes where both effects are relevant. This approach also solves the long-standing problem of accurately predicting the thermal properties of crystals with ultralow or glass-like thermal conductivity, as we show with an application to a thermoelectric material representative of this class. This database contains the raw data related to the images reported in the paper https://arxiv.org/abs/1901.01964.

Coloring elimination in Sr1 − x Ce x F2 + x crystals in the visible spectral range during growth from melt

Crystals of the Sr1 − xCexF2 + x compositions close to the congruent one (x ∼ 0.3) are fabricated by the vertical directional crystallization. It is shown that the use of CF4 to form a fluorinating atmosphere during growth leads to additional spurious absorption in the crystals in the range 350–600 nm. The use of PbF2 and ZnF2 for fluorination makes it possible to obtain colorless Sr1 − xCexF2 + x crystals of the desired optical quality from melt. The thermal conductivity of crystal with x ∼ 0.28 in the temperature range 80-500 K lies within 1.50 ± 0.03 W m−1 K−1. High ionic conductivity makes the Sr1 − xCexF2 + x crystals promising for application in solid-state ionics.

Spectral Absorptivity and Thermal Conductivity of BGO and BSO Melts and Single Crystals

The present work describes the results of spectral absorptivity, α, and thermal conductivity, λ, studies for compound oxides Bi4Ge3O12, Bi12GeO20, Bi4Si3O12, and Bi12SiO20 in molten and monocrystalline states. The data for the spectral absorptivity were obtained by placing the sample onto a mirror and using the transmission method. To obtain the data on the thermal conductivity of crystals, the stationary method of two identical samples was used. The data for the thermal conductivity of melts were obtained by a new stationary relative method in which the thermal conductivity of the crystal is used as a reference. Special attention is focused on numerical and experimental error analysis at high temperature. The studies have shown that α in the range of a transmission band strongly depends on crystal purity. It varies from 0.0005 cm−1 to 0.03 cm−1 for Bi4Ge3O12 and reaches 0.15 cm−1 for Bi4Si3O12. It was found that α is significantly greater for melts than for crystals, reaching (150 to 200) cm−1 for the Bi4Ge3O12 melt. The thermal conductivity of the melts under investigation was found to be much smaller than that of the corresponding crystals.

Thermal conductivity of crystalline fullerite C60 in the simple cubic phase

The behavior of the thermal conductivity k(T) of bulk faceted fullerite C60 crystals is investigated at temperatures T=8–220 K. The samples are prepared by the gas-transport method from pure C60, containing less than 0.01% impurities. It is found that as the temperature decreases, the thermal conductivity of the crystal increases, reaches a maximum at T=15–20 K, and drops by a factor of ∼2, proportional to the change in the specific heat, on cooling to 8 K. The effective phonon mean free path λp, estimated from the thermal conductivity and known from the published values of the specific heat of fullerite, is comparable to the lattice constant of the crystal λp∼d=1.4 nm at temperatures T>200 K and reaches values λp∼50d at T<15 K, i.e., the maximum phonon ranges are limited by scattering on defects in the volume of the sample in the simple cubic phase. In the range T=25−75 K the observed temperature dependence k(T) can be described by the expression k(T)∼exp(Θ/bT), characteristic for the behavior of the thermal conductivity of perfect nonconducting crystals at temperatures below the Debye temperature Θ (Θ=80 K in fullerite), where umklapp phonon-phonon scattering processes predominate in the volume of the sample.

Do phonons exist in non-crystalline matter?

The low-frequency properties of glasses are usually explained by phonons. It will be shown here that the motions at these frequencies are vibrational and are not phonons, but localized vibrations of the multifractal cluster structures of the glasses that should be called clustrons. Because of the distinct elements of these structures (clusters) far fewer atoms hold a vibrational degree of freedom than in crystals, thereby explaining the greater density of states in glasses than in the crystal. The extremely strong localization of these clustrons (because the neighbour clusters vibrate at different frequencies) explains the decreased thermal conductivity in glasses apart from the specific heat increase (which would cause a parallel increase in the thermal conductivity in crystals).

Phonon scattering and phase equilibrium in3He and4He crystals containing neon impurities

Results of measurements of the thermal conductivity of crystals of 3He and 4He containing up to 0.1% Ne which have already been reported by Berman and Livesley (1981) are considered in more detail. The way in which the various changes associated with a point defect should be combined to obtain the net phonon scattering strength is considered and a qualitative argument is used to show how the observed results may be explained by a compensation of the effect of mass changes by force constant changes. A form for the equilibrium temperature-concentration phase diagram is suggested and is shown to be consistent with most of the observations, but some results obtained at high concentrations remain puzzling.

Structural characteristics of alkaline earth fluorides and their thermal conductivity: SrF2 with YF3 and LuF3 impurities

The relationship between structural characteristics and thermal conductivity of crystals is examined. Thermal conductivity data are obtained for strontium fluoride crystals, both pure and doped with YF3 and LuF3.

Thermal Conductivity of Gallium Arsenide at Low Temperatures

The thermal conductivities of gallium arsenide single crystals doped with tellurium and chromium have been measured from 2 to 90\ifmmode^\circ\else\textdegree\fi{}K. The tellurium-doped crystal shows normal behavior in agreement with the findings of other workers. At low temperatures, the thermal conductivity of this crystal follows a ${T}^{3}$ behavior indicative of boundary scattering limiting the phonon mean free path. The phonon mean free path in this temperature region, calculated from experimental results, agrees fairly well with that obtained from Casimir's formula. The thermal conductivity of the chromium-doped sample also shows a ${T}^{3}$ behavior at the lowest temperatures. However, the phonon mean free path observed experimentally is more than an order of magnitude less than that given by Casimir's formula. The chromium-doped sample shows, in addition, evidence of an anomalous, possibly a resonant, phonon scattering in the region of temperatures 4-10\ifmmode^\circ\else\textdegree\fi{}K with a peak at 6.5\ifmmode^\circ\else\textdegree\fi{}K. Possible reasons for this are discussed.

The effect of a magnetic field on the thermal conductivity of paramagnetic crystals: cerium ethylsulphate

The thermal conductivity of crystals of concentrated cerium ethylsulphate has been measured in the range 1 to 4·58°K and in magnetic fields of up to 53 kG. In zero field there is a marked anomaly at 2·5°K in the variation of conductivity with temperature. Owing to the anisotropy of the g -values the magnetic field dependence of the thermal resistivity at constant temperature depends on the field direction; with the field parallel to the hexagonal axis, a maximum occurs in the resistivity which moves roughly linearly with temperature, and in very high fields it is always less than in zero field; with the field applied in the perpendicular direction a resistivity maximum is only observed above 2°K, and in the highest available field it is always much greater than in zero field. These results are explained by assuming that direct process phonon-spin interactions scatter certain bands of phonons whose frequency depends on the separation of the energy levels produced by the applied magnetic field. A statistical theory is used to determine the relative populations of the energy levels in the calculation of the thermal resistivity. It is assumed that the spin-phonon absorption lineshape is Gaussian. By fitting the theory to the experimental data, approximate values of the spin-lattice coupling constant, the linewidth of the transitions and the mean free paths for boundary and point-defect scattering are obtained.

Effect of Neutron Irradiation on the Thermal Conductivity of a Quartz Crystal at Low Temperature

THE thermal conductivity of crystals is very structure-sensitive, particularly at low temperatures. In the case of dielectric crystals, this has been demonstrated by de Haas and Biermaszl, who measured the effect of heat treatment on the conductivity of a quartz crystal in the liquid-hydrogen temperature region.

III. On the thermal conductivities of crystals and other bad conductors

The author commences by pointing out the great differences be­tween the results obtained in 1879 by G. Forbes for the conductivities of quartz in different directions and those obtained in 1883 by Tuschmidt. He then refers to Kundt’s discovery, that the metals stand in the same order as conductors, and as to the velocity of propagation of light through them, and mentions that his experiments were originally intended, to furnish data for a similar comparison for crystals, but that their object has been extended.

XVI. On a method of measuring the absolute thermal conductivity of crystals and other rare substances.—Part I

XVI. <i>On a method of measuring the absolute thermal conductivity of crystals and other rare substances</i>.—Part I