The electrical resistivity of evaporated films of titanium and erbium and the study of the resistance changes of erbium films owing to the sorption of gases
This thesis has been written in two parts in order that the reader will be able to follow the different types of experiment and the results obtained in two different vacuum systems.
In the first part of the thesis are described the results on the electrical resistivity and resistance-temperature characteristics of titanium and erbium films, evaporated onto soda glass microscope slides at room temperature in a glass belljar vacuum system at about 5 x 10-8 torr. The films of both the metals varied in their thickness from 40 to 1100 ?, and the resistivity was very high for the thinnest films but for the thickest ones it approached approximately double the bulk value. The measured resitivities for the continuous freshly prepared films of both the metals are too high to be explained on the basis of the size-effect theory 3,4 for diffuse scattering, and are attributed to the gaseous impurities taken down during and after their formation, and to small grain size, porosity, lattice defects etc. The temperature coefficient of resistance was negative for films less than 50-60 ? thick but positive for thicker ones. A bulk mean free path of 285 ? in titanium and l85±l5 ? in erbium was calculated at room temperature.
It was very difficult to prepare erbium films for study= in the electron microscope, and reasons are given for considering it unlikely that their structure was the same as for the normal films in vacuum. A mass spectrometric analysis of the residual gases showed that in residual vacua of 10-6 to 10-8 torr, films of both the metals had a large gettering effect on hydrogen and a small effect on other gases. The extremely large gettering effect of erbium on hydrogen (partial pressure decreased from 10-8 to 10-11 torr in a few minutes) and very small effect on oxygen, raised the author's curiosity to study the effect of individual pure gases on erbium films from the point of view of their resistance.
The second part of this thesis describes the new apparatus designed for studying resistance changes of erbium films due to sorption of controlled amount of pure gases; representative of which are hydrogen and oxygen. A special adsorption vessel was constructed from pyrex glass with two platinum electrodes sealed into its wall to make electrical contact with the film (deposited on the inside wall of it), and this vessel was connected to a stainless steel ultra-high vacuum (10-9 torr or less) line with steel flanges and copper gaskets. The mass of the film was known from the original metal put in the filament, and was checked by quantitative volumetric analysis of the film after the experiment had been completed. The surface area of the films was measured from the physical adsorption of krypton at liquid nitrogen temperature, using the BET method. The total number of surface sites was calculated from a relationship suggested by Brennan et al. 109
for polycrystalline films. The quantity of gas sorbed by the film was calculated from the initial and final pressures of the gas in the reservoir and adsorption vessel and from their previously known volumes (calibrated by expanding helium in them). The measured surface area of the films increased linearly with increase of film thickness, and a mean specific surface area of 71±18 square metre per gram of erbium was calculated.
At room temperature, the surface interaction of hydrogen with erbium was followed by its bulk sorption, and both types of interaction were accompanied by different types of resistance change. Experiments on hydrogen interaction with erbium at temperatures lower than 295?K were made to find out if the bulk sorption could be restricted. From the experiments carried out at 200?K, approximately 130-160?K and 78?K, it is concluded that a temperature as low as 78?K would be needed to restrict the bulk hydride formation. However, the resistance changes at 78?K were not reproducible, most probably due to a magnetic transition (AFM-PM) of the bulk metal, which takes place around this temperature (78-86?K). At all the temperatures of study (132-145?K, 200?K and 295?K), the oxygen interaction is possibly confined to the measured surface, and for most of the adsorption, the resistance increased monotonically with the amount of oxygen adsorbed by the film. Oxygen is more strongly adsorbed on the film surface than hydrogen i.e. it blocks the surface and makes it completely inactive with respect to both molecular and atomized hydrogen. On the other hand, a film that has previously interacted with hydrogen is still capable of interacting with molecular oxygen.
The different likely mechanisms, which could change the resistance on adsorption, are discussed with simple mathematical expressions. A simple model 79 suggests that both these gases are adsorbed as atoms, and that each gas atom forms one chemical bond with a surface atom of the metal. The magnitudes of the resistance changes on adsorption of both the gases (15-30% due to hydrogen and 25-40% due to oxygen) are too large to be explained by one predominating mechanism or even by all the mechanisms operating simultaneously. The large magnitude of the resistance changes is attributed to the porous structure of the films and diffusion/adsorption on open capillaries, grain boundaries and dislocation lines reaching the film surface.
|Publication Date||Jul 1, 1971|