Summary - Enthalpy and Heat Capacity of Graphite
Recommended Equations
The equations recommended by the Scientific Group Thermodata Europe (SGTE)[29] for
the enthalpy and heat capacity of carbon are recommended because they are in excellent
agreement with recent assessments by JANAF[27] and CODATA[28] as well as data on different
grades of graphite from measurements by McDonald,[24] Cezairliyan and Righini,[25-26] West and
Ishihara,[11] and Buchnev et al.[4] The SGTE recommendations are also in good agreement with
the recommendations for nuclear graphite given by Butland and Maddison.[6]
The SGTE equation for the graphite enthalpy increment relative to the enthalpy in the
standard state at 298.15 K is:
H(T)-H(298.15) = - 17368.441 + 24.3 T + 4.723 10^-4 T^2
+ 5.1252 10^6 T^-1 - 7.929 10^8 T^-2 (1)
+ 4.8 10^10 T^-3
where enthalpy is in J/mol and temperature is in K. The SGTE equation for the heat capacity
of graphite in J/(mol-K) is :
Cp = 24.3 + 9.446 10^-4 T - 5.1252 10^6 T^-2 + (2)
1.5858 10^9 T^-3 - 1.44 10^11 T^-4
where temperature is in K. Values calculated with Eqs (1-2) are given in Table 1. The
SGTE equations for graphite enthalpy increments and heat capacities in kJ/kg and kJ/(kg-K) are:
H(T) - H(298.15) = - 1446.04454 + 2.023145 T +
3.9322 10^-5 T^2 + 4.26709 10^5 T^-1 (3)
- 6.60145 10^7 T^-2 + 3.9963 10^9 T^-3
and
Cp = 2.023145 + 7.8645 10^-5 T - 4.26709 10^5 T^-2 + (4)
1.3203 10^8 T^-3 - 1.199 10^10 T^-4
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| Figure 2 |
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| Figure 3 |
Table 2 gives graphite enthalpy increments and heat capacities in kJ/kg and kJ/(kg-K).
Both experimental measurements by Rasor and McClelland[9] on a number of types of
graphite and those of Sheindlin et al.[3] on six brands of Russian graphite gave enthalpies and heat
capacities which increased at much faster rates than those from measurements on high-purity
graphites (Fig 2 and Fig 3). The observations by Rasor and McClelland that (1) significant
vapor was released at high temperature and (2) white deposits were formed on some graphites
led analysts to conclude that the results of Rasor and McClelland and of Sheindlin et al. may be
due to impurities in the samples or to the breakdown of the graphite at high temperatures (above
2500 K).
Uncertainties
The good agreement of the measurements of McDonald, Cezairliyan and Righini,
West and Ishihara, and Buchnev et al. and the recent recommendations of CODATA, JANAF,
and the SGTE would indicate low uncertainties (within 5%) for the recommended enthalpies and
heat capacities of graphite. However, the lack of data on the grades of graphite used in Russian
reactors leads to uncertainty in the high-temperature properties. Are Russian nuclear graphites
closer to those used in the experiments by Sheindlin et al. or those of Buchnev et al.? Because
the graphite enthalpies and heat capacitics obtained by Sheindlin et al. deviate significantly from
those of Buchnev et al. and from the recommended values, large uncertainties are estimated at
high temperatures.
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| Figure 5 |
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| Figure 6 |
Uncertainties in the enthalpy increments are: 8% below 500 K, 3% from 800 to 2500 K,
and 10% above 3000 K. Between 500 and 800 K, enthalpy increment uncertainties decrease
linearly from 8% to 3%. Between, 2500 and 3000 K, uncertainties increase linearly from 3% to
10%. Uncertainties in the graphite heat capacities are: 10% below 500 K; 5% from 800 through
2500 K. Between 500 and 800 K, the heat capacity uncertainties decrease linearly from 10% to
5%. Above 2500 K, uncertainties increase linearly from 5% at 2500 K to 11% at 3000 K.
Above 3000 K, the negative uncertainty remains at -11% but the positive uncertainty increases
linearly to +50% at 3600 K. So at 3600 K, the uncertainties in the heat capacity values are:
+50%, -11%. Fig 5 and Fig 6 show the recommended values for the enthalpy and heat capacity
of graphite with estimated uncertainties designated by dotted lines.
Assessed 6/95
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