Powder Structure Characterization by Gas Adsorption and Other Experimental Methods

10 Powder Structure Characterization by Gas Adsorption and Other Experimental Methods 

The basic concept employed in gas adsorption studies of the structure of powders is to study the way in which gas molecules are adsorbed onto the surface of the powder under specified conditions so one can deduce the surface area of the powder. The technology of this method comes from the late 1930 s. A very good review of the basic concepts has been written by Orr [1]. Although the basic principles of the method are simple, the interpretation of the data when dealing with complex powder systems can be very difficult. In this chapter we will outline the basic methodology and direct the reader to more comprehensive literature which can be consulted if the detailed structure of systems from a research perspective is of interest. Excellent books have been written describing the research methodology of gas adsorption for complex powder systems see for example references 2, 3, 4, and 5. 

Routine gas adsorption studies can often be used when the aim of the investigation is quality control of powder systems. In the late 1950 s I was involved in measuring the surface area of uranium dioxide powders using Krypton adsorption as a quality control method for the fabrication of nuclear reactor fuel rods. 

During this type of measurement, the volume of gas adsorbed by a unit mass of solid denoted by VyW depends on the equilibrium pressure, the absolute temperature, the solid, and the gas. The adsorbable gas will be below it s critical temperature and the pressure inside the equipment as measured by the manometer shown is expressed as a fraction of the saturation vapour pressure P. The resulting ratio P/P is called relative pressure. The amount of gas adsorbed can be calculated using the gas law relationships, the measured pressures, and known volumes of the equipment. 

Within the group of isotherms which have been studied, by far the largest number are known to be Type 2 isotherms. In this discussion we will restrict ourselves to the interpretation of Type 2 isotherms using a theory known as the B.E.T. theory based on the work of S. Brunauer, B. Emmett, and E. Teller [5]. For a discussion ofType 1 isotherms, see reference 1. Type 1 is usually found in solids which have a porous structure. 

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