Dependence of Heat Transport in Solids on Length-Scale, Pressure, and Temperature: Implications for Mechanisms and Thermodynamics

Accurate laser-flash measurements of thermal diffusivity (D) of diverse bulk solids at moderate temperature (T), with thickness L of ~0.03 to 10 mm, reveal that D(T) = D∞(T)[1 − exp(−bL)]. When L is several mm, D∞(T) = FT−G + HT, where F is constant, G is ~1 or 0, and H (for insulators) is ~0.001. The attenuation parameter b = 6.19D∞−0.477 at 298 K for electrical insulators, elements, and alloys. Dimensional analysis confirms that D → 0 as L → 0, which is consistent with heat diffusion, requiring a medium. Thermal conductivity (κ) behaves similarly, being proportional to D. Attenuation describing heat conduction signifies that light is the diffusing entity in solids. A radiative transfer model with 1 free parameter that represents a simplified absorption coefficient describes the complex form for κ(T) of solids, including its strong peak at cryogenic temperatures. Three parameters describe κ with a secondary peak and/or a high-T increase. The strong length dependence and experimental difficulties in diamond anvil studies have yielded problematic transport properties. Reliable low-pressure data on diverse thick samples reveal a new thermodynamic formula for specific heat (∂ln(cP)/∂P = −linear compressibility), which leads to ∂ln(κ)/∂P = linear compressibility + ∂lnα/∂P, where α is thermal expansivity. These formulae support that heat conduction in solids equals diffusion of light down the thermal gradient, since changing P alters the space occupied by matter, but not by light.

Negative linear compressibility in nanoporous metal-organic frameworks rationalized by the empty channel structural mechanism

Cobalt squarate hydroxide (Co3(C4O4)2(OH)2), zinc squarate tetrahydrate (ZnC4O4·4 H2O) and titanium oxalate trioxide dihydrate (Ti2(C2O4)O3·2 H2O) are nanoporous metal-organic frameworks possessing empty channels in their crystal structures. The crystal structures...

Сжимаемость и электронные свойства цианидов металлов

The compressibility and electronic properties of metal cyanides are investigated within the density functional theory taking into account the dispersion van der Waals interaction. It was shown that gold cyanide has a low linear compressibility (less than 0.1% at a pressure of 1 GPa) and a high linear modulus (~ 1200 GPa) along the -Au-CN-Au-CN- chains. Silver cyanide exhibits negative linear compressibility, which correlates with the compressibility of Ag-N coordination bonds. For sodium cyanide, the linear compressibility along the C - N covalent bonds is greater than for gold and silver cyanides, while the elastic anisotropy is less. Unlike sodium cyanide, for gold and silver cyanides, cation-anionic bonds (Au-N, Au-C and Ag-N, Ag-C) are partially covalent in nature, and the upper valence states correspond mainly to the states of cations. The band gap of gold cyanide is smaller than that of silver and sodium cyanides. The band gap widths of gold and silver cyanides significantly decrease with increasing pressure, which indicates the possibility of metallization at sufficiently high pressures.

The Effect of Pressure on the Structure and Electronic Properties of Hydrated Calcium Carbonates

This paper studies the effect of pressure on the structure and electronic properties of CaCO3-H2O and CaCO3-6H2O crystalline hydrates. The study is based on the density functional theory (DFT) and the linear combination of atomic orbitals (LCAO) method. Calculations are performed using the CRYSTAL17 software package and the PBE gradient functional. The calculated lattice parameters of hydrated calcium carbonates and their dependence on external hydrostatic pressure are shown to be in good agreement with the available experimental measurements. Dependencies of linear compressibility on the direction are obtained using the calculated pressure dependencies of the structural parameters. It is demonstrated that the linear compressibility of calcium carbonate hexahydrate, in contrast to calcium carbonate monohydrate, is highly anisotropic (the smallest and largest compressibility values are correlated as K max /K min ~ 4). In this case, the maximum compressibility is located between crystallographic axes (between the axes a and c) and not along the crystallographic axes. The bulk modulus for the monohydrate (CaCO3-6H2O) is greater than for the hexahydrate (CaCO3-6H2O). Total and partial densities of electronic states for CaCO3-H2O and CaCO3-6H2O are calculated. Also, dependencies of the band gap width on pressure for hydrated calcium carbonates are established. It is shown that, with increasing pressure, the increase of the band gap is greater for CaCO3-6H2O than for CaCO3-H2O.