Thermal properties of graphite

The physical properties reviewed in the previous section are essentially unaffected by the size and orientation of the crystallites in the aggregate. As a result, they can be considered valid for all forms of graphite. This is no longer true for some of the properties listed in this and the following sections, and these properties may vary considerably depending on crystallite size and orientation and other factors related to the processing conditions.

The thermal properties are summarized in Table 3.4. Whenever possible, a range of property values is given. More detailed values are given in subsequent chapters.

Table 3.4. Theoretical thermal properties of graphite

Heat of combustion △Hco @ 25C and                      393.13

Constant pressure to form CO2 gas, kJ/mol

Standard entropy S° at 25C, J/mol.K                     5.697-5.743

Entropy △S298, J/mol.K                                152.3

Enthalpy △H298, kJ/mol                               716.88

Specific heat @ 25C, kJ/kg.K (see bellow)                  0.690-0.719

Thermal conductivity @ 25C, W/m.K  ab directions          398

C direction           2.2

specific heat of graphite vs. temperature at one atmosphere

Fig.3.8-specific heat of graphite vs. temperature at one atmosphere

Thermal expansion: see bellow

Heat capacity (Specific heat): The molar heat capacity of graphite is reported as 8.033-8.635 J/mol.K at 25C. As with all elements, it increases with temperature, with the following relationship (T in deg K).

Cp=4.03+(1.14 ×10-3)T – (2.04 ×105)/T2

The specific heat increases rapidly with temperature, up to 1500K where it levels off at approximately 2.2 kj/kg.K

as shown in Fig.3.8. It is believed to be relatively insensitive to the differences between the various grades of synthetic graphite, and the spread of values found in the literature may be attributed to experimental variations. The average value is compared to that of selected other elements in Table 3.5. As can be seen, it is higher than most metals.

Table 3.5. Specific heat of selected elements

Cp at 25C and 1 atm.

(kJ/kg.K)

Graphite             0.690-0.719

Diamond             0.502-0.519

Boron                          1.025

Aluminum                   0.900

Tianium                       0.523

Copper                        0.385

Niobium                       0.263

Rhenium                      0.242

Tungsten                      0.130

Water                          4.186

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