'n ISS studie van die (110), (111) en (100)-enkel-kristalvlakke van NiAI

[摘要] English: In this study the [110]-, the [111]- and the [1OO]-single crystal surfaces of Ni Al were investigated. The aim was to firstly harmonize the existing knowledge of thesesurfaces and secondly to create a clear and coherent overview of their properties.The essence of a problem of this kind is the determination of the structure andcomposition of the surface of a material. In this study, low energy ion scattering(LEISS) was used as primary investigative technique. Other techniques such as ICISS(Impact Collision ISS, Auger electron spectroscopy (AES) and low energyelectron diffraction (LEED) were used in conjunction with LEISS.Several intensive investigations of the NiAI(IIO)-surface were launched in recenttimes. The results of these investigations were largely similar and are widelyaccepted. In this study the results of several previous investigations are discussed. It isshown that the surface is largely a simple truncation of the bulk in the (110)-direction,but that the surface exhibits a ripple effect due to the particular way in which the Aland Ni atoms in the surface and second layer relaxes.In contradiction with the NiAI(ll O)-surface, there is still a measure of uncertaintyregarding the properties of the NiAI(ll I)-surface. As part of this study, theNiAI(111)-surface was investigated using LEISS. The results of this investigation aregiven and is compared with the results of previous studies of the surface from theliterature. It is shown that the surface consists of small Al-areas on top of a Ni-layer.One of the studies showed that the Al-areas could be removed by heating the surfaceto 1300 K. From other studies however, including this one, it seems that the removalof the Al-atoms from the surface may be inhibited or prevented by the presence ofoxygen.As in the case of the NiAI(110)- and NiAI(l1 l j-surfaces, a large number ofinvestigations were done on the NiAI( 1OO)-surface. Contradictory results werehowever a commonplace occurrence. In this study, the NiAI(lOO)-surface wassubjected to a comprehensive LEED, LEISS, ICISS and AES investigation with the aim of clarifying the structure and composition of the surface. An overview of resultsof previous, as well as this investigation is given.It is shown that the NiAI(100)-surface is very sensitive to changes in temperature andthat the surface composition and sometimes also the surface structure undergo severalchanges during heating. At low temperatures (approximately 500 K), a largely AIterminationcan be found, but with heating from 500 K to 873 K the surfacecomposition changes to 65 at. % Al and 35 at. % Ni and voids. At this temperature thesurface undergoes a reconstruction process so that it exhibits a c (Ji x 3.J2)R45°-LEED pattern. Further heating leads to another change in composition to 75 at. % Aland 25 at. % Ni and voids at 1073 K. At this temperature the surface exhibits ap(1 x 1)-LEED pattern. At temperatures above 1073 K the Al concentration in thesurface starts to decline until the Al atoms are complete removed at approximately1300 K. After this point the surface consists of only Ni atoms.

[发布机构] University of the Free State