BET Analysis with Krypton Gas

Instrumentation

Krypton BET analysis necessitates a volumetric or manometric gas adsorption gas analyzer to have the capability of krypton gas as well as to hold sporting temperatures. Its main elements are a sample cell, a cold bath (liquid nitrogen), pressure transducers, and accurate gas dosing valves. Krypton BET systems measure very small, adsorbed gas quantities and can therefore analyze thin films, foils, or metal powders of small quantity in comparison with nitrogen-based systems, which require relatively large sample masses. The system should be very leak-tight and be able to measure very low pressures since adsorption of krypton takes place at much smaller partial pressures relative to nitrogen.

Principle and Methodology

The isotherm of BET with krypton obeys the same core principles as the BET with nitrogen, but is optimized to the low range of adsorption volumes. To eliminate moisture and contaminants, the sample is degassed to start with. A liquid nitrogen Dewar is then used to cool it to 77 K. The sample chamber is gradually filled with krypton gas, and the adsorption of krypton gas onto the sample surface is measured as a function of the equilibrium gas pressure readings after each increment. The adsorption isotherm is put into the BET equation within the relative pressure range of approximately 0.05-0.3 to arrive at the monolayer capacity. Since the saturation pressure of krypton is low, the corresponding low volumes of gas required to provide monolayer coverage make it sensitive and precise to small areas. The last surface area is obtained as a product of the previous volume of monolayers and the well-known area of the cross-section of the molecules of Krypton (0.202 nm²).

Strengths and Limitations

The strength of BET analysis using krypton is that it is sensitive to small areas of surface. It enables the accurate determination of the surface area of materials. The method does not destroy the structure, and a low quantity of sample is needed.  Additionally, Krypton provides more straightforward and understandable isotherms at a low range of surface areas. There are shortcomings, however, to the use of krypton-based BET analysis as well. 

Krypton is much more costly than nitrogen and is limited in availability as well. It also needs more specialized gear that can work at very low speeds. The rates of adsorption may be low, and the equilibrium time may be longer than normal measurements. Also, krypton is not as appropriate for materials whose surface areas are large, as it might take longer to analyze and consume more gas.

Importance

The BET determination based on krypton gas has been developed into a vital tool used in the characterization of materials having small specific surface areas, the metallurgy industry, the electronics industry, the catalysis industries, and the advanced coating industries, to mention but a few. It gives reliable, repeatable results that assist with quality management, material development, and performance testing. An example is the question of catalyst development, where small variations in surface area intimately affect the reactivity. Krypton BET allows the researcher to measure these variations with accuracy. With the increasing demand for surface-sensitive material, there is a possible increase in the value of the krypton-based BET analysis, especially in research and industrial laboratories working on high-performance materials with limited exposure to the surfaces.