JOHN T.
884
CLARKE
Surface Area Measurement of Graphite' Using the 7-Radiation of Krs5
by John T. Clarke Brookhacen National Laboratorg, U p t o n , New York
(Received November 14, 1963)
A method has been developed to measure the surface area of small samples of low surface area materials using the 7-radiation of Krs5-traced Kr to determine the amount of Kr adsorbed. Since the count rate observed is directly proportional to the Kr adsorbed, the method is much more rapid and the calculations much less time-consuming than other methods of surface area measurement. The method is applicable over a wide range of surface areas (0.001 to 10 m.2/g.) and to sample sizes of 0.5 g. or less. The long half-life of Krs5 (10.4 years) minimizes the corrections necessary for radioactive decay. The usual advantages of Kr over nitrogen (inertness, low vapor phase corrections, and spherical shape) are utilized in this method. Using this new method, it was found that the surface area of TSX nuclear graphite increased by a factor of four as the particle size was reduced from 3-mm. cubes to 80-mesh. This is interpreted as indicating that the increased surface area results primarily from the opening of closed pores; this allows one to estimate the surface area of graphite samples having different sizes and shapes.
Introduction The properties of graphite can be markedly changed by varying the coke, binder, and graphitization temperature and atmospherr They ran also be modified by impregnation and degassing at high temperature. As a consequence, nuclear grade graphites are generally characterized2" in terms of the starting materials and the conditions of their manufacture rather than by their observable properties. Surface area IS an important property in investigating chemical reaction rates between a solid and a gas; therefore, a simple, rapid method of measuring it is of corisjderable importance. The s p ~ c i f i rsurface of a graphite arid its variation with degassing temppratiires and with particle size, can be used to help characterize it in terms of its present properties rather than by specifying its method of manufacture. In studying the reaction of graphite with a gas, the results are found to vary depending on the size and shape of the graphite sample. Thus the change of surface area with particle size is particularly important as it allows one t o estimate the shape factor. Before developing the present method, the surface area of graphite samples was determined using a standard nitrogen adsorption apparatus. It was necessary to use large samples and minimize the gaseous volume T h e Jorirnal of Physical Chemistr:y
in the sample cell in order to obtain the desired 5% accuracy. Various additional modifications of the st,andard B.E.T. apparatus were considered. The substitut'ion of Kr for Nz would m.inimize the gaseous volumc correction since the vapor pressure of Kr is only 2.3 mm. a t liquid NQ t'cmperature. This method however., rcquires a n entirely different appuratus. The use of nonradioactive Kr for surface area measu~emerits is reported in detail by Rosenberg2b arid Tomlinson. This method does allow accurate measiiremeiit of small surface areas with gram-size samples, but it is as slow and tedious as the nitrogen method. In either method, the amount of gas advorbed on the siirfacc: at liquid nitrogen temperature is determined by accurately mcasuring the amount of gas in an exterior system before and after t,he adsorption, and involves many measurements and arithmetical calculations. Aylmor and Jepson4 have simplified the Kr method (1) This work was performed under the auspices of the U. S. Atomic Energy Commission. ( 2 ) (a) R. E. Nightingale, Ed., "Nuclear Graphite," Academic Press, New York, N . Y.. 1962, pp. 182--185;(b) A. J. Rosenberg, .I. Am. Chem. SOC..78, 2929 (1956). (3) L. Tomlinson, United Kingdom Atomic Energy Authority, Industrial Group Report 1032, 1959, Her Majesty's Stationery Office, London, England.
SURFACE AREAMEASUREMENT OF GRAPHITE
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by counting the /?-radiation of Kr85-traced Kr in a thermostated constant-volume cell to determine thc pressure ; the pressure is directly proportional to the count rate. They used a constant amount of Kr (adsorbed and unadsorbed) for one isotherm, and varied the external volume to vary the equilibrium pressure. The method works satisfactorily, but the amount of Kr adsorbed is still obtained by subtracting the amount of gas in the external system from that originally present. It is also necessary to control carefully the amount of Kr added to the volumetric system so that a monolayer covers the sample surface; this may vary widely with different samples. 27 ,
Experimental In the present method, the amount of Kr adsorbed by the sample is measured directly by counting the 7-radiation from the Kr85-traced Kr. This simplifies the calculations and allows one to work with small samples having widely different surface areas. We have measured 0.5-1.0-g. samples with surface areas varying from 0.001 to 10.00 m.2/g., but these are not its limits. The time involved in a surface area measurement is about one-third of that using the earlier B.E.T. apparatus. Samples having only 0.01 of the surface area/g. can be determined. KrE5 has a half-life of 10.4 years so radioactive decay is a small correction. A schematic drawing of the apparatus is given in Fig. 1. The essentials of the apparatus consist of a tube containing the sample which is connected to a reservoir of Krs6-traced Kr, The sample tube is held at liquid Nz temperature while the Kr storage tube is held at a series of constant temperatures below liquid nitrogen temperature to keep the vapor pressure of the solid Kr at a desired value. The sample adsorbs Kr a t constant pressure until it equilibrates with Kr in the storage tube. The amount adsorbed is a function of the surface area and is measured by counting the 0.53 MeV. y-radiation of Kr85-traced Kr. This measures directly t h e amount of Kr adsorbed, and only a small correction is needed for the Kr in the gas phase detected by the counter. The equilibrium Kr pressure (100 to 500 1.1) is determined by a thermistor gage28 to an accuracy of 1 p. The Kr storage tube is shut off from the cell for this pressure measurement. The amount of Kr adsorbed as a function of the Kr pressure above the sample allows one to calculate its surface area. The rate of Kr adsorption can also be measured directly with this apparatus. The apparatus for maintaining the solid Kr storage tube a t different temperatures below 77"K., greatly simplifies the surface area measurement, since it supplies the amount of Kr
Figure 1.
needed a t a suitable pressure whether the sample has a small or large surface area. A measure height of the sample (22), whose surface area is to be measured, is placed in a sample tube (24) which has been removed from the apparatus for loading and weighing. The counterweighted assembly containing the dewar and scintillation crystal is then raised so that the sample tube (24) is centrally located and rests on the bottom of the thin-walled dewar (26). A nitrogen thermometer (28) is used to determine the temperature of the liquid nitrogen. A 1.0 X 1.5-in. AI-covered scintillation crystal (30) is mounted directsly below the dewar (26) and above the photomultiplier tube (32); the preamplifier (34) is connected to a discriminator and scaler (38) by coaxial leads (36). The sample tube and detection apparatus are enclosed in a brass tube which is shielded by 8 mm. of lead and counterweighted by weights (48) so that the assumbly may be raised or lowered. KrE5-traced Kr is stored in a Pyrex bulb (Fi2) surrounded by 0.5 in. of lead (54) and is connected to the pumping manifold (56). The assembly (64-88) is used to control the temperature in the Kr storage tube (76) and thus the Kr pressure. A metal dewar (64) ~~
(4) D. W. Aylmor and W. P. Jepson,
J. Sei. Instr., 38, 156 (1961).
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with a stainless steel cover (66) and vacuum sealed with an “0”-ring is connected to a vacuum pump by a valve (78). A bundle of 0.375-in. copper tubes is held in a vertical position by brass rings (74 and 82) to maintain a uniform temperature inside the dewar (66). This dewar is half-filled with liquid XZthrough the port (70) and closed by a rubber stopper which also serves as a safety valve. The pressure is measured with a vacuum gage (SO) and gaseous nitrogen can be added through the valve (88). The Kr storage tube is vacuum sealed to the cover (66) by means of a short piece of rubber tubing, A thermocouple has also been introduced a t this point, By pumping on the nitrogen in the dewar; the temperature can be controlled between 55 and 77’K. This storage tube also serves as a convenient means of keeping the Kr pure; cooling the Kr to below the solid point reduces its pressure to
