Factors for Thermal Conductivity Units

The most common units for thermal conductivity are cal/cm.s.°C and Btu.in/ft. h.°F. The SI unit for conductivity is W/mK. Since a variety of units has been in practice for thermal properties, the conversion factors are given in Table 12.27. 

A variety of units are used in the literature for thermal properties, and this can be a nuisance when different sets of results have to be compared, or when values from an older publication are being used for ealculations. Prior to the adoption of the SI system, the two most eommon units for thermal conductivity were the cal. cm s C and the BTU in ft h F. There are two units of length in the imperial unit, because area is measured in square feet and thickness in inches, and this inconsistency is a potential pitfall for the unw ary. A self-consistent conductivity unit, the BTU ft h F, is obtained if the temperature gradient is measured in F ft instead of F in. but this is not as common. For diffusivity the e.g.s, unit is the cm s and the imperial unit is the ft h. The SI unit for conductivity is the W niK. and the unit for diffusivity is the m" s. For polymers it is more convenient to use a submultiple of the diffusivity unit, the mm" s. because this eliminates a factor of 10 Conversion factors arc given in Table 1. 

Table 1 Conversion Factors for Some Thermal Conductivity Units...

Additional assays may also be conducted on this material to more comprehensively characterize its thermal properties. The following may be considered variable temperature XRD (VT-XRD) studies may be conducted to observe changes in the powder diffraction patterns of the material as a function of temperature. Selection of appropriate temperatures depends on several factors the DSC transition temperatures, corroborating TGA and hot-stage evidence, and the accuracy of the heating controller for the XRD unit. It should also be noted that although XRD data may be collected at several different temperatures to simulate a variable temperature experiment, the follow-up VT-XRD assay is essentially a series of isothermal experiments, and the DSC is a dynamic assay. It is therefore reasonable to expect some variation in transition temperatures. 

Conversion factors for mass, density, pressure, energy, specific energy, specific heat, thermal conductivity, dynamic viscosity, and kinematic viscosity in different systems of units are also given in Chap. 2 (Tables 2.1-2.9). 

TABLE 2.7 Conversion Factors for Units of Thermal Conductivity... 

The proportionality factor u, is a transport property, like thermal conductivity or diffusivity, called the mobility because it measures how mobile the charged particles are in an electric field. The mobility may be interpreted as the average velocity of a charged particle in solution when acted upon by a force of 1 N mol . The units of mobility are therefore mol N ms or mol s kg . The concept of mobility is quite a general one, since it can be used for any force that determines the drift velocity of a particle (a magnetic force, centrifugal force, etc.). The flux relation can also be expressed in terms of mass by... 

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