VOLUME 54, NUMBER 11 P H Y S I C A L R E V I E W L E T T E R S 18 MARCH 1985

Comment on "Measurement of Thermodynamic Parameters of Graphite by Pulsed-Laser Melt­ing and Ion Channeling"

Venkatesan et all have reported a value of about 4300 K for the melting temperature of graphite ob­tained from pulsed-laser experiments.

Preliminary measurements of the radiance tempera­ture (at 0.65 /jim) of graphite at high temperatures per­formed recently in our laboratory indicate that the melting temperature is considerably higher than 4500 K. The measurements were performed with a subsecond-resolution resistive pulse-heating technique in which the specimen was heated from room tempera­ture to its melting temperature in less than 100 ms by the passage of a high-current pulse (about 1000 A) through it. The specimen was in a pressurized argon environment at about 15 MPa. Temperature of the specimen was measured with a high-speed photoelec­tric pyrometer operating at 0.65 jtim. Data were recorded every 0.1 ms with a full-scale signal resolu­tion of about 1 part in 4000. Melting of graphite was manifested by a plateau in the temperature versus time data. An average of the temperature values at the pla­teau yielded the melting temperature of graphite. The preliminary measurements suggest a value of 4500 K for the radiance temperature at 0.65 jjum. A correction for the normal spectral emissivity must yield an even higher value for the melting temperature. Graphite evaporation at high temperatures can cause serious dif­ficulties in radiance temperature measurements. In our experiments, this problem was eliminated because of the following: (1) By the nature of the experimen­tal technique, the specimen was exposed to high tem­peratures for only a brief period which minimized the

amount of evaporated graphite, and (2) the evaporated graphite was converted to CO and C0 2 (transparent to radiation at 0.65 jam) by the small amount of oxygen present in the argon-gas environment.

The melting temperature obtained by Venkatesan et aL was based on melt-depth measurements and specific-heat data in the range 3000-4000 K. The result obtained by this indirect method can be subject to very large errors which may be due to (1) uncertain­ties in the significance of melt depth and its measure­ment, (2) uncertainty in the absorbed energy, and (3) uncertainty in the specific heat of graphite at high tem­peratures, especially above about 3000 K. Under the most favorable conditions, the total uncertainty of such a determination is not likely to be better than 10%, which corresponds to an uncertainty of over 400 K in the reported melting temperature of graphite. Realistically, much larger uncertainties may be associ­ated with this indirect technique.

Work is under way in our laboratory to determine the melting temperature of graphite with an uncertain­ty of 1% by the use of a new high-speed mul-tiwavelength pyrometer.

Ared Cezairliyan Thermophysics Division National Bureau of Standards Gaithersburg, Maryland 20899

Received 18 October 1984 PACS numbers: 65.50.-fm

1T. Venkatesan, D. C. Jacobson, J. M. Gibson, B. S. El-man, G. Braunstein, M. S. Dresselhaus, and G. Dres-selhaus, Phys. Rev. Lett. 53, 360 (1984).

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