Crystal thermal conductivity

Metal Crystal 22° C stmeture 1000° c Melting point, °C Density, g/cm Thermal expansion coefficient at RT, ioV°c Thermal conductivity at RT, W/(m-K)" Young s modulus, GPa "... 

Single-crystal sUicon has also been employed as substrate material, particularly in multichip module (MGM)-Si appUcations. As a substrate, sUicon offers good thermal conductivity and matches the GTE of the devices mounted on it it does, however, have a relatively high dielectric constant and is very britde. 

The lead-base babbitts ate based upon the lead—antimony—tin system, and, like the tin-base, have a stmcture of hard crystals ia a relatively soft matrix. The lead-base ahoys ate, however, mote prone to segregation, have a lower thermal conductivity than the tin-base babbitts, and ate employed genetahy as an iaexpensive substitute for the tin-base ahoys. Properly lined, however, they function satisfactorily as beatings under moderate conditions of load and speed. 

Thermal Conductivity. The value of 2000 W/(m-K) at room temperature for Type Ila natural stones is about five times that of Cu, and recent data on 99.9% isotopicaHy pure Type Ila synthesized crystals ate in the range of 3300—3500 W/(m-K) (35). This property combined with the high electrical resistance makes diamond an attractive material for heat sinks for electronic devices. 

Electronic. Diamonds have been used as thermistors and radiation detectors, but inhomogeneities within the crystals have seriously limited these appHcations where diamond is an active device. This situation is rapidly changing with the availabiHty of mote perfect stones of controUed chemistry from modem synthesis methods. The defect stmcture also affects thermal conductivity, but cost and size are more serious limitations on the use of diamond as a heat sink material for electronic devices. 

By beginning with methane, the diamonds formed have only in them. These tiny diamonds may then be used as the carbon source to form large (5 mm) single crystals by growth from molten catalyst metal in a temperature gradient. The resulting nearly pure crystals have outstanding thermal conductivities suitable for special appHcations as windows and heat sinks (24). 

When a substance is transparent to visible light, such as single crystals of simple inorganic solids which do not contain uansition metal ions, or some glasses, anotlier significant component in the thermal conductivity is the transmission of photons in the infra-red region, which becomes more important with... 

Figure 7.6. A filled. skutterudite antimonide crystal structure. A transition niclal atom (Fc or Co) at the centre of each octahedron is bonded to antimony atoms at each corner. The rare earth atoms (small spheres) are located in cages made by eight octahedra. The large thermal motion of rattling of the rare earth atoms in their cages is believed be responsible for the strikingly low thermal conductivity of these materials (Sales 1997).

As with other properties of solids, the increased relative significance of surface energy in very small (i.e. micrometre-sized) crystals influenced the melting points [2,16,17] and diffusion at this temperature. Quantitative studies of rates of melting of solids are impracticable since superheating is effectively forbidden and the rate of the endothermic phase change is determined by the rate of heat supply and the thermal conductivity of the solid. 

The compact structure of diamond accounts for its outstanding properties. It is the hardest of all materials with the highest thermal conductivity. It is the most perfectly transparent material and has one of the highest electrical resistivities and, when suitably doped, is an outstanding semiconductor material. The properties of CVD and single-crystal diamonds are summarized in Table 7 2.[1][18]-[20]... 

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