Method and Instrumentation for Nondestructive Characterization of Surface Area and Pore Size Distribution of Thin Films
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TPL has developed a method and instrumentation system for characterizing the surface area and pore size distribution of a thin-film or coating in its as-deposited state. Errors associated with destructive removal of the film are avoided and small sample sizes can be utilized with high-fidelity results. INTRODUCTION Frye, Martin and Ricco [1] have demonstrated the use of ultrasonic waves to measure mass uptake in thin-film solids. By using an ultrasonic signal as a highly sensitive mass sensor, it is possible to accurately measure the minute quantities of gas adsorbed by I pores in a small sample of thin-film material. This data, when acquired as a function of gas partial pressure and Mkrxo~os temperature, can be used to determine r the material's sufarce area and pore size distribution using the BET and
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BJH models respectively.
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BACKGROUND
IV
Gas Adsorption
"An adsorption isotherm is the relationship, at constant temperature,
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between the amount of gas adsorbed and the corresponding pressure..." [2]. The term adsorption,originally introduced by Kayser [3], connotes the
condensation of gases onto a surface, as opposed to into a surface (absorption).
This term is now used generally to describe both adsorption and desorption. The adsorbate in an adsorption experiment refers to the material actually being adsorbed onto the solid surface (or absorbent). There are two distinguishable
Relative Pressure, PT* Figure 1. The characteristic Isotherms (volume
adsorbed as a function of partial pressure). 231
Mat. Re0. Soc. Syrmp. Proc. Vol. 591 0 2000 Materials Research Society
types of adsorption. In physisorption, the forces are of a physical nature, and relatively weak. These types of forces were described by Van der Waals, who termed them dispersion forces [4]. The heat of adsorption of these forces is usually small, less than 20 kJ/mol. In chemisorption, the adsorbed molecules are held to the surface by covalent forces. Langmuir [5,6] first described these forces. The heat of adsorption for this process is usually comparable to that for chemical bonding, 300-500 kJ/mol. These attractive forces between the gas and solids give rise to distinctive adsorption isotherms, which have been classified by Brunauer, Demming, Demming and Teller [7]. Although it is not always possible to mathematically predict the type of isotherm based on properties of the solid and gas, the idealized models do reinforce the nature of the adsorptive process, as described by Lowell and Shields [8]. The Type I isotherm is the typical adsorption curve for a microporous solid with a relatively small external surface. The initial rapid uptake at low pressures, is attributed to micropore filling. The plateau indicates the relatively small additional adsorption after the pores are filled. Langmuir gave the first theoretical model to
describe the Type I isotherm, detailed in the next section. The Type 1H isotherm is the typical adsorption curve for a nonporous solid. The knee of the curve is related to the completion of
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