Thermal Treatment In Vacuum Research Articles

Preparation of Carbon‐Free GaAs Surfaces: AES and RHEED Analysis

Chemical polishing using an system and thermal treatments in vacuum to obtain clean surfaces have been systematically studied for {100} and {111} surfaces. The behavior of these surfaces as far as oxidation and carbon adsorption are concerned has been found to depend on both conductivity type and crystal orientation. The results presented for {100} n‐ and p‐type and semi‐insulating material show that it is possible to obtain carbon‐free surfaces, covered by a thin oxide layer which protects the semiconductor surface against contamination. A subsequent thermal treatment around 550°C under vacuum permits elimination of the oxide and achieves atomically clean, crystalline and ordered surfaces, which has been confirmed by AES and RHEED analysis.

ESR and XPS investigation of a molybdenum-.eta. aluminum oxide model catalyst system and its interaction with adsorbed aromatics

A systematic study was performed by ESR and XPS of calcined, evacuated, or hydrogen-reduced 9% molybdenum on alumina, a catalyst that often serves as model for industrial catalysts. No Mo(V) or Mo(IV) was detected by ESR in unsupported MoO/sub 3/ or MoO/sub 2/, but Mo(IV) was detected by XPS in unsupported MoO/sub 2/. In the supported molybdenum oxide, neither ESR nor XPS detected Mo(V) before heat treatment; ESR always detected Mo(V) after thermal treatment in air, hydrogen, or vacuum; for the calcined and evacuated catalysts, the Mo(V) concentration depended on the length of the treatment (it decreased with increasing time) and the probe molecule (it was a maximum of 1.3% with adsorbed xanthene); it was nearly constant in reduced catalysts. The XPS method did not detect differences caused by length of calcination or evacuation treatment, but deconvolution of the XPS Mo 3d envelopes yielded large differences between reduced and oxidized or evacuated samples.

Combined EPR and TG techniques : comparison of the thermal reduction of chromium (VI) ions in some zinc chromates and chromate-oxalate mixtures

Lower oxydation state chromium species which are successively formed and stabilized upon thermal reduction of chromate (VI) ions, are studied by combined EPR and TG-techniques. The spontaneous oxygen release occuring during the vacuum thermal treatment of some zinc chromates leads to the formation of both Cr(V) and Cr(III) species, the latter being either dispersed in ZnO, clustered in a Cr 2O 3-like phase or ordered in a ZnCr 2O 4-spinel phase. The Cr/Zn ratio in the initial compound as well as the presence of some reducing OH − ions are critical for the stability of such intermediate Cr species. When solid mixtures of ZnCrO 4 and ZnC 2O 4 are calcined, the CrO 4 = reduction is enhanced by the in-situ genesis of reducing gaseous or adsorbed carbon monoxyde stemming from the oxalate. Surprisingly such a reducing system favours the high temperature formation of some other very stable Cr(V) species.

Hydrogen activation on silica gels containing transition-metal ions

1. Silica gels containing Co2+ and Ni2+ ions will catalyze the H2-D2 exchange reaction after thermal treatment in vacuum, differing in this respect from similar V5+ ion systems. 2. The activation of H2 molecules in longitudinal (model I) and transverse (model T) configurations in the coordination spheres of Co2+, Ni2+, and V5+ ions has been treated through semiempirical MO LCAO calculations. This work has led to an interpretation of the experimental results and to a mechanism for the H2−D2 reaction through type-I complexes.

Carbon and sulphur on the [111] and [001] faces of nickel during thermal treatment in ultra-high vacuum and in an oxygen atmosphere

The behaviour of carbon and sulphur on [111] and [001] nickel faces on heating in a vacuum and in oxygen (10 −7 –10 −6 torr) has been investigated by means of Auger electron spectroscopy (AES). Carbon disappeared from the surface under investigation in vacuo on heating up to 500 °C for Ni[111] and up to 800 °C for Ni[001]. The amount of sulphur was increased as the samples under investigation were heated in vacuo to higher temperatures, reaching its maximum at 800–900 °C and above 1000 °C it was reduced considerably. These effects were explained as due to the reversible segregation of sulphur to the nickel surface. On prolonged heating of the sample under investigation in oxygen (10 −7 –10 −6 torr, 800 °C) sulphur was removed from the surface and the amount of sulphur inside the sample was reduced. Thus, new superficial contamination of the sample with sulphur could have been removed by heating for several minutes up to 1100 °C. After sulphur removal from the surface, further heating of the sample was found to lead to other admixtures being segregated to the surface.

Kinetics of the thermal dehydration of variscite and specific surface area of the solid decomposition products

The surface properties of active solids resulting from decomposition of AlPO4 · 2 H2O by thermal treatment in vacuum and in air are studied in connection with the parameter governing the kinetics of the dehydration process. The values obtained for the activation energies of decomposition under isothermal conditions agree acceptably well with those computed from TG curves by the Freeman and Carroll method. Kinetic results, which are analysed in terms of the Mampel theory, strongly suggest that the activation energy of the nucleation process determines the rate of surface formation, and justify the observed fact that the specific surface areas of samples treated in vacuum are higher than those of samples treated in air.

Molybdenum Carbide by Electrolysis of Sodium Molybdate

Preparation of di‐molybdenum carbide from sodium molybdate dissolved in fused bath of sodium carbonate, sodium tetraborate, sodium fluoride, and potassium fluoride was investigated. Under the optimum conditions established, a maximum electrodeposition rate corresponding to 0.5 g/A‐hr of the carbide was obtained. In the follow‐up investigation the carbide was admixed with monitored amounts of molybdenum dioxide for vacuum thermal sintering treatment to yield molybdenum metal of purity comparable to conventional molybdenum extraction process.

Study of carbon-fiber surfaces using Auger and secondary electron emission spectroscopy

Structure at 3.0, 4.0, 7.5, 8.5, 10.0, 12.0, 14.5, and 17.5 eV in the secondary electron energy distribution of low-energy electrons back-scattered from carbon-fiber surfaces is shown to be similar to that observed from single-crystal graphite, which is due to emission from the high density of conduction-band states located 5 to 20 eV above the Fermi level. The intensity of secondary emission from these states is related to the degree of graphitic order in high- and low-modulus specimens of polyacrylonitrile (PAN-) and pitch-based carbon fibers. The results are discussed in terms of the effect of thermal treatment in high vacuum on the size and orientation relative to the fiber axis of regions of graphitic order in a surface layer of the order of 100 Å and a comparison is made with known models of carbon-fiber micromorphology. Auger electron spectroscopic analysis revealed the nature and degree of surface contamination on the different fiber types.

Influence of the vacuum thermal treatment of selenium on the formation of hetero-junctions in the Se-CdSe system: Yu V Babikov et al, Trudy Kafedry Fiz Mariysk Ped Institute, Yoshkar-Ola 1970, 66–73 ( in Russian)

Influence of the vacuum thermal treatment of selenium on the formation of hetero-junctions in the Se-CdSe system: Yu V Babikov et al, Trudy Kafedry Fiz Mariysk Ped Institute, Yoshkar-Ola 1970, 66–73 ( in Russian)

The crystalline structure of PbTe thin films

The crystalline structure of PbTe thin films was studied both by X-ray diffraction and electron diffraction methods. The samples were obtained by evaporation in vacuum on optically polished glass substrates. In order to establish the optimum conditions for obtaining PbTe thin films with good crystalline structure, the evaporation conditions were very widely varied. It was established that the crystalline structure of PbTe films was improved with substrate temperature increasing up to 300–350°C as well as with decreasing pressure in the evaporation chamber and as a result of thermal treatment in vacuum. With the improvement in the degree of lattice order the crystallites contained in the PbTe thin films receive a preferential orientation relative to the substrate surface and their linear dimensions grow considerably, attaining the magnitude of 0.05-0.2 μm. By X-ray analysis the characteristic diffraction lines of lead telluride cubic crystals were identified in thin films obtained by thermal evaporation in vacuum. The lattice constant of the cubic PbTe cell was computed.

Structure et conditions de formation de la couche d'adsorption du soufre sur le cuivre

Previous work carried out in this laboratory, in the field of reversible chemisorption, has been concerned with the thermodynamics of the systems copper—sulphur and silver-sulphur. We have measured in particular, the maximum quantities of sulphur adsorbed on the close-packed faces of these two metals. However, the values thus obtained could not be related in a simple way to the topography of the different crystallographic orientations studied. Considerations based on the mean distance between atoms of adsorbed sulphur suggest that the saturated adsorbed layer consists not only of atoms from the gas phase but also of atoms from the underlying metal : the layer can be envisaged as a two-dimensional adsorbed compound. In order to define the structure and the growth mechanism of these adsorbed layers we have undertaken a study by low-energy electron diffraction. The simultaneous use of radioactive sulphur has enabled us to determine, with precision, the number of adsorbed atoms per unit surface area at different stages of the reaction. This information is indispensable for the interpretation of the diffraction patterns. In addition, it was possible, by the tracer method, to verify a posteriori that the adsorption stage had not been passed. The most complete part of the investigation has been carried out on the (111), (100) and (110) faces of copper in the presence of hydrogen sulphide marked with sulphur-35. The surfaces were initially prepared by means of bombardment with argon ions followed by thermal treatment in ultrahigh vacuum. The activity of the surface was measured at the end of each experiment, after bringing the specimen into the atmosphere, by means of classical counting methods. The essential characteristics of the mechanism of formation and growth of the adsorbed layer were determined from a large number of experiments carried out over a wide range of temperature and pressures. The development of the reaction on the three faces can be described as follows : A dissociative adsorption occurs with localization of S 2− on sites of maximum coordination imposed by the supposedly rigid arrangement of atoms in the metallic substrate. At a critical coverage of this first, relatively dispersed, phase there appear attractive forces between adsorbed sulphur atoms. We consider this attractive binding to be due to copper atoms which insert themselves in the adsorbed layer: this process is, in fact, the nucleation of a true compound or adsorbed two-dimensional sulphide. The compound then grows from the dispersed phase until the surface becomes completely covered. The saturated state thus obtained corresponds quantitatively to the values previously determined from experiments carried out under conditions of reversible chemisorption. The structures of the adsorbed layers are sharply affected by the topography of the different faces. In the case of (111) and (100) surfaces there exists a coincidence lattice between the network of the two-dimensional adsorbed sulphide and that of the underlying metal. The (110) surface behaves differently in that the epitaxy is characterized by the localization of sulphur atoms in the grooves which constitute the special arrangement of underlying metal atoms. However the minimum sulphur-sulphur distance observed for the saturated states depends only very slightly on the crystalline orientation of the metal and it is only slightly different from the sulphur-sulphur distance in three-dimensional copper sulphide. One may therefore consider the saturated adsorbed layers to be derived from a single twodimensional sulphide for which the structure is determined by the crystalline face of the metal on which it is adsorbed. The proposed mechanism reconciles the classical theories of adsorption in sites with the recent hypotheses concerning the mixed nature of adsorbed layers ; this interpretation may be extended to systems other than copper-sulphur.

Electron Microscope Observations of the Crystallization of Anodically Formed Tantalum and Niobium Oxide Films

Anodic oxide films ranging in thickness from about 200 to 1000Aå were formed on electropolished surfaces of tantalum and niobium by conventional techniques. The films were then stripped from the metal, given a variety of thermal treatments in vacuum, and examined directly in the electron microscope. Crystallization of the amorphous films was induced by high‐temperature annealing or in the microscope itself by increasing the beam intensity. The distinguishing features of each type of crystallization process are discussed and illustrated with appropriate micrographs.

Spectroscopic investigation of polyphenylene and its block copolymers with p-diethylbenzene

1. The predominance of 1,2,4-substitution in the block copolymets as compared with polyphenylene, the insolubility of the block copolymers, and the presence in the ir spectra of an absorption background shows that a three-dimensional structure is formed during block copolymerization. 2. A comparison of the x-ray diagrams of polyphenylenes, the products of their thermal treatment in vacuum, and the block copolymers shows that all the samples studied are amorphous substances.

Spectral Energy Distribution Curves of ZnS : Ag and ZnCdS : Ag after Thermal Vacuum Treatment

Liquid settled screens of and of of thickness comparable to those of cathode ray tubes were subjected to various temperatures in vacuum. The temperatures ranged from 300° to 800°C and exposure times from 5 to 30 min. Spectral energy distribution curves were then obtained for these phosphors under electron bombardment. Intensity changes, color shifts, and the appearance of new emission bands are noted. These seem to be related to the diffusion and evaporation rates of the phosphor components and the effects depend upon screen thickness as well as time and temperature of exposure.

Spectral energy distribution curves of ZnS:Ag and ZnCdS:Ag after thermal vacuum treatment

Spectral energy distribution curves of ZnS:Ag and ZnCdS:Ag after thermal vacuum treatment

眞空燒鈍によつて鏡面仕上面に現われる條紋について

Soft steel machine parts were mirror-polished and annealed in a vacuum furnace at the temperature above recrystalization temperature.There appeared on the surface, 11el white or black lines whose width was 0.3mm or thinner. The origin and reason of these lines were investigated through many experiments with many specimens. Several criticisms were done through experimental results of 8 microphotographs and 6 figures.In conclusion we should say, firstly there happen the minute cracks ⊥ ar to the grinding direct ion on the surface of the specimen, by comparatively severe grinding operation which direction are ⊥ ar to the rolled direction and secondly if the surfaces of these ground and cracked specimens are mirror-polished, the cracks are covered up by the amorphous layer which should be affected later by thermal treatments in vacuum furnace.Lastly the appeared white traces, thermal cracks and other related phenomena were all explained.

Thermal Plasticization of Rubber

Abstract It is unnecessary to discuss the fact that modern views on the plasticization of rubber as well as the fundamental processes and methods involved, are not final and cannot pretend to an exhaustive solution of the problem. Nevertheless, despite the exceptional importance of these problems to the technology of rubber, they have not, in our opinion, attracted sufficient attention in the literature. Even less is found in the literature about the most recent methods of plasticization. This explains why in an investigation of the thermal plasticization of rubber which was begun by the authors in the early part of the current year, there was found to be little information in the literature. Only a few articles and reports indirectly related to the problem were available. Therefore we were compelled to spend considerable time in developing effective methods, and various experiments on the thermal vacuum treatment of rubber were carried out to ascertain the effect of the time, degree of vacuum, and temper...