Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

2THE SOLID-GAS INTERFACE 199LampFig. 10. Schematic diagram of a recording microbalance. From L. Cahn and H. R. Schultz, inVacuum Microbalance Techniques, Ed. K. H. Behmdt. 3 (1963) 30.The simplest method of beam microbalance operation is to measure the deflectionof the ends of the beam by optical magnification. This method has the disadvantagethat the maximum weight change that can be observed is limited andthe vacuum system must usually be opened to reset the zero point. However, a beammicrobalance can be readily converted to a null point instrument by electromagneticadjustment. A commercially available instrument which features electromagneticcompensation and automatic recording is shown schematically in Fig. 10.The beam is constructed from aluminium and suspended from an elastic metalribbon. As the beam rotates, the flag on the right hand end uncovers the photocell,increasing the photocurrent. This current is then amplified and applied to the coilto rebalance the beam. Coil voltage is proportional to sample weight change,and this is recorded by means of an accurate potentiometer and a chart recorder.A different arrangement has been described by Cochran'og. He used a variablepermeance transducer to detect the movement of a steel rod attached to the balance.The signal from the transducer was amplified and used to control the current to thecompensating solenoid, which restored the beam- position. Langer"' has describeda more recent construction using a photocell and electromagneticcompensation.Microbalances have been used to measure adsorption kinetics under a widevariety of conditions. Several authors"0i"' have studied adsorption on singlecrystal surfaces of semiconductors un'der ultra high vacuum conditions and utilisingion bombardment to obtain a clean surface. Gulbransen et ~ 1 . " and ~ Walkeret uZ.114*115 have made a special study of microbalance operation at high temperatures(up to 2500" C).References pp. 270-278