An X-ray study of gases on solids
The work described in this thesis is concerned with the study of ph-ysisorbed phases by x-ray diffraction using a conventional sealed x-ray tube source. Diffraction data has been collected for a number of adsorption systems using graphite, a montmorillonite clay (Gel White) and zeolite rho as the substrate. It is well known that phases of unique two-dimensional character can be formed on the surface of graphite, and the structure of adsorbed benzene and hexaflurobenzene on graphite have been studied in this thesis. Contrary to current theoretical predictions the >/7x T/7R19" commensurate structure of submonolayer benzene has been confirmed. Submonolayer hexaflurobenzene appears to form a striped domain structure based upon the commensurate x3 lattice in which the molecules are incommensurate with the surface. ' In contrast to the homogeneous surface of graphite, the surface of a clay is microporous and heterogenous, and this gives rise to broad diffraction lines from an adsorbed phase. Despite this, the surface area of Gel White has been deduced from the evolution of the diffraction pattern of a krypton adlayer as a function of krypton loading. The formation of bulk krypton is readily identified and the small size of the three-dimensional crystallites suggests that they are formed within the micropores of the clay. The structure of adlayers of krypton and xenon within the interlayer spacing of Al-pillared Gel White has been studied in order to determine the mean inter-pillar separation. At all the coverages studied, xenon forms a close packed single layer structure whilst krypton appears to form a more complex bilayer phase. A tentative suggestion as to the mean pillar separation from this work is 30X. The adsorption site of krypton, xenon and CH3CI within zeolite rho has been determined using the method of x-ray Rietveld whole profile refinement. The principal site of adsorbed krypton and xenon is at the centre of the octagonal prism. The chlorine atom of CH3CI sits in the centre of the face of the octagonal prism and the methyl group is slightly displaced from the centre of the prism. 37-5354 Hydrodynamics of liquid encapsulation czochralski crystal growth Hicks, T.W. Bristol Ph.D. 1989 Dig. Certain aspects of crystal growth from a melt are investigated. We begin by describing the methods of producing single crystals. Particular emphasis is placed on the need for a better understanding of the hydrodynamics of the encapsulant region of the Liquid Encapsulation Czochralski (LEG) technique. We also introduce the basic physical processes which govern crystal growth. In Chapter 2 we develop a mathematical model of the encapsulant region of the LEC crystal growth system. The equations and boundary conditions that govern the encapsulant flow are formed using a vorticity-stream-function approach, after which the problem is recast in a dimensionless form. In Chapter 3 the equations of motion are represented in a finite difference form and a numerical method for solving the time-marching problem presented by the parabolic equations is developed. The elliptic stream-function equation is solved at each time level using the successive over-relaxation technique. Solutions of the model equations for the growth of GaAs crystals through B3O3 encapsulant are presented in Chapter 4. In all cases considered the flow field tends towards a steady state. For shallow encapsulants, the heat transfer in the encapsulant is conduction dominated, but for deeper encapsulants, advective heat transfer can be significant. In the last chapter we investigate the effect of Soret diffusion on the morphological stability of a freezing interface using linear stability theory. A Soret flux directed towards the interface has a destabilising effect. Over-stable modes of instability exist for very low crystal growth rates, but we are unable to find conditions under which the overstable mode is the most unstable.