Air and other gases are generally good insulators, in the absence of convection. The effect of temperature, pressure, and chemical species on the thermal conductivity of a gas may be explained in terms of the kinetic theory of gases. In gases, the thermal conduction is caused by diffusion of molecules from higher energy level to the lower level. In liquids, the thermal conduction is caused by atomic or molecular diffusion. The thermal conductivity of gases and liquids is therefore generally smaller than that of solids. Because the intermolecular spacing is much larger and the motion of the molecules is more random for the fluid state than for the solid state, thermal energy transport is less effective. Fluids are a subset of the phases of matter and include liquids, gases, plasmas and, to some extent, plastic solids. In physics, a fluid is a substance that continually deforms (flows) under an applied shear stress. Thermal Conductivity of Liquids and Gases In particular, diamond has the highest hardness and thermal conductivity (k = 1000 W/m.K) of any bulk material. In fact, for crystalline, nonmetallic solids such as diamond, k ph can be quite large, exceeding values of k associated with good conductors, such as aluminum. Phonons play a major role in many of the physical properties of condensed matter, like thermal conductivity and electrical conductivity. The quanta of the crystal vibrational field are referred to as ‘‘ phonons.’’ A phonon is a collective excitation in a periodic, elastic arrangement of atoms or molecules in condensed matter, like solids and some liquids. At sufficiently high temperatures k ph ∝ 1/T. The regularity of the lattice arrangement has an important effect on k ph, with crystalline (well-ordered) materials like quartz having a higher thermal conductivity than amorphous materials like glass. The vibrations of atoms are not independent of each other, but are rather strongly coupled with neighboring atoms. In solids, atoms vibrate about their equilibrium positions (crystal lattice). In fact, lattice thermal conduction is the dominant thermal conduction mechanism in nonmetals, if not the only one. Thermal Conductivity of Nonmetalsįor nonmetallic solids, k is determined primarily by k ph, which increases as the frequency of interactions between the atoms and the lattice decreases. In contrast, for alloys, the contribution of k ph to k is no longer negligible. In fact, in pure metals such as gold, silver, copper, and aluminum, the heat current associated with the flow of electrons by far exceeds a small contribution due to the flow of phonons. Their contribution to the thermal conductivity is referred to as the electronic thermal conductivity, k e. The electrical and thermal conductivities of metals originate from the fact that their outer electrons are delocalized. The unique feature of metals as far as their structure is concerned is the presence of charge carriers, specifically electrons. Accordingly, transport of thermal energy may be due to two effects: Metals in general have high electrical conductivity, high thermal conductivity, and high density. Metals are solids and as such they possess crystalline structure where the ions (nuclei with their surrounding shells of core electrons) occupy translationally equivalent positions in the crystal lattice. When electrons and phonons carry thermal energy leading to conduction heat transfer in a solid, the thermal conductivity may be expressed as:
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