The thermal conductivity, k, of a substance is defined as the quantity of heat, Q, transmitted due to a unit temperature gradient, in a direction normal to a surface of unit area in unit time under steady–state conditions and where heat transfer is dependent only on the temperature gradient...
In several earlier issues ofElectronics Cooling, I discussed the thermal conductivity of air as a function of temperature and pressure. Therein, it was stated that the temperature dependence cannot be neglected, but that the pressure dependence, under ‘normal’ conditions, is virtually absent...
however, the thermal conductivity is not a pure material constant, but depends on the temperature. At large temperature differences, the thermal conductivity can therefore change relatively strongly over the thickness of the material. In these cases...
The calculator below can be used to calculate the air thermal conductivity at given temperatures and pressure. The output conductivity is given as mW/(m K), Btu(IT)/(h ft °F) and kcal(IT)/(h m K). Temperature Choose the actual unit of temperature:°C°FK°R ...
air-mediated losses. We find the reduction of the thermal conductivity and its temperature dependence closely correlated with the structure feature size. On the basis of two-phonon Raman spectra, we attribute this behaviour to diffuse (incoherent) phonon-boundary scattering. Furthermore, we ...
where B is a constant and ΘD is Debye's temperature. This result suggests four criteria for choosing high thermal conductivity materials: (i) low atomic mass, (ii) strong interatomic bonding, (iii) simple crystal structure, and (iv) low anharmonicity. Conditions (i) and (ii) help increase...
Sapphire is a notable example of variable thermal conductivity based on orientation and temperature, with 35 W/(m·K) along the c-axis and 32 W/(m·K) along the a-axis.[13] Air and other gases are generally good insulators, in the absence of convection. Therefore, many insulating ...
Reliable thermal insulation under extreme conditions requires an unusual combination of mechanical flexibility, high thermal stability and low thermal conductivity across a wide temperature range. Some of these critical material parameters exhibit intrinsic trade-offs with each other for most ceramic aerogels...
The Wiedemann-Franz law which establishes an intimate relationship between thermal conductivity and electric conductivity of a given metal, states that the Lorentz number is equal to a universal constant: (7)κelσT=π23kBe=L0 where kB is the Boltzmann constant and L0 is the Sommerfeld number...
Thermal conductivity (k) is a physical property of a material, which indicates the material's ability to transfer heat. It is mathematically defined as: (1)k=Q/AdT/dx where Q is the amount of heat passing through a cross-section A that causes a temperature difference, dT/dx is the the...