E. Fromm

Thermodynamische beschreibung der festen lösung von kohlenstoff, stickstoff und sauerstoff in niob und tantal

Zusammenfassung Unter Verwendung neuerer Arbeiten werden die zur thermodynamischen Behandlung der α-Mischkristallphase notwendigen Daten fur die Systeme Nb-C, Nb-N, Nb-O, Nb-C-O, Ta-C, Ta-N, Ta-O und Ta-C-O zusammengestellt und besprochen. Weiterhin werden die Gleichgewichtsdrucke von N 2 , CO, O 2 , O, NbO, NbO 2 , TaO und TaO 2 uber dem Mischkristallbereich von Niob und Tantal in Abhangigkeit von der Temperatur und der Konzentration angegeben. Dadurch kann abgeschatzt werden, welche Rolle die einzelnen Begasungs- und Entgasungsreaktionen in den verschiedenen Druck- und Temperaturbereichen spielen.

Effect of oxygen surface contamination on the hydrogen absorption kinetics of metal films studied by a modified volumetric Wagener method

Abstract The volumetric method originally developed for the determination of the sticking probabilities of adsorption or absorption reactions of hydrogen, nitrogen and oxygen with pure metals can also be used to determine the kinetics of more complex metal-gas reactions if appropriate changes in the experimental technique and in formulae for data evaluation are made. The modifications necessary are discussed and some typical results are presented. These are the hydrogen absorption of titanium and FeTi films with and without oxygen precoverage and of a Ti-Pd sandwich with oxygen precoverage, and the reaction of H2O with an iron film.

Effect of oxygen sorption layers on the kinetics of hydrogen absorption by tantalum at 77– 700 K

Abstract Hydrogen absorption from the gas phase by tantalum is strongly impeded by oxygen sorption layers at temperatures below 700 K. The removal of these layers increases the absorption rate by several orders of magnitude. Experiments performed between 77 and 700 K using.specimens with and without such oxygen surface layers enabled us to identify the processes which determine the rate of hydrogen absorption at low temperature.

Surface phenomena in hydrogen absorption kinetics of metals and intermetallic compounds

Abstract Oxide layers on metal surfaces impede or prevent hydrogen absorption at temperatures below 400 °C. Passivation can be caused by slow dissociation of hydrogen molecules on the surface, slow transfer of chemisorbed hydrogen atoms into the oxide or slow permeation of hydrogen atoms through the oxide layer. Experiments on the hydrogen absorption kinetics of film samples with known oxygen precoverage yield direct information on reaction models if relevant results of other techniques are taken into account together with inherent thermochemical and atomistic limits. Typical examples are discussed to elucidate this approach for the investigation of complex reaction mechanisms in hydrogen absorption kinetics at room temperature. They indicate that hydrogen dissociation on the surface is a very important partial step in the reaction and plays a substantial role in activation and passivation of hydrogen absorption reactions at room temperature.

Kinetics of hydrogen absorption by titanium, tantalum, tungsten, iron and palladium films with and without oxygen preabsorption at 300 K

The hydrogen absorption rates of titanium, tantalum, tungsten, iron and palladium films 15–20 nm thick were measured at room temperature and hydrogen pressures between 10−9 and 10−4 mbar using the volumetric method. Films with clean surfaces absorb gas amounts in the concentration ranges corresponding to hydride formation for titanium and tantalum and equivalent to about one monolayer of hydrogen for tungsten, iron and palladium with reaction probabilities in the range 1–10−4. The reaction rates are strongly reduced if the film surface is coated with oxygen at thicknesses equivalent to several monolayers. Iron and palladium films precoated with oxygen show an H2O partial pressure peak which indicates that the oxygen sorption layer is reduced by the formation of H2O during exposure to hydrogen.

Oxidation of iron, aluminium and titanium films in the temperature range 50–200 °C

Abstract The oxidation kinetics of iron, aluminium and titanium films have been studied in the temperature range 50–200 °C at a pressure of 0.7 Pa with a piezoelectric quartz microbalance. For titanium, additional measurements in the pressure range 7 × 10 −3 – 70 Pa have been carried out. All three metals under investigation showed qualitatively common features in their oxidation behaviour: a very fast initial oxygen absorption step, completed in less than 1 min, followed by a slow, temperature-independent oxide growth with logarithmic time law at lower temperatures, and an enhancement of the oxidation rate at higher temperatures. The oxidation curves are compared with the results of model calculations based on the Cabrera-Mott theory.

Oxidation kinetics of metal films at 300 K studied by the piezoelectric quartz crystal microbalance technique

Abstract The oxidation kinetics of metal films of lanthanum, titanium, hafnium, vanaidum, tantalum, chromium, manganese, iron, nickel and aluminium were studied with a quartz crystal microbalance at 35°C and O 2 pressures between 10 −3 and 10 5 Pa for exposure times ranging from 1 to 10 4 min. In the initial stage of the reaction (less than 2 min) a large amount of oxygen is absorbed in a very fast process and oxide scales are formed with a thickness between 1 and 10 nm. This quantity depends on the metal under consideration and in some cases (lanthanum, titanium, vanadium, tantalum, aluminium) strongly also on O 2 pressure. In the subsequent oxidation process (10 to 10 3 min) smaller amounts of O 2 are absorbed at a logarithmic growth rate of differing magnitude for various metals.

Kinetics of hydrogen absorption of tantalum coated with thin films of palladium, iron, nickel, copper and silver

Abstract The effects of coatings of palladium, nickel, iron, copper and silver on the kinetics of hydrogen atom absorption of tantalum wires were measured in the temperature range 300–700 K. Thin palladium coatings were the most effective and yielded absorption rates that were near the theoretical value for a diffusion-controlled reaction mechanism. This corresponds to an enhancement factor of 104 or more at room temperature. Nickel and iron films increased the reaction rate by one to two orders of magnitude but copper and very thin silver films yielded no pronounced improvement of the small absorption rates of hydrogen on tantalum below 500 K.

Mechanical properties and adhesion strength of TiN and Al coatings on HSS, steel, aluminium and copper characterized by four testing Methods

Abstract TiN and Al coatings on substrates of high-speed steel, steel, aluminium and copper have been used to study mechanical properties of coating systems, especially the adhesion of the coating. The quantities measured are internal stress of the coating, determined by X-ray diffraction, the critical load of the scratch test, the microhardness obtained by the indenter technique, and the interface fracture energy, determined by a three-point bend test developed recently by the authors. The fracture energy, G c , is a measure for the adhesion strength of a coating system. The effect of bias voltage, sputter cleaning and contamination of the substrates on the adhesion strength and other mechanical properties are investigated with the four methods mentioned. Each of the testing methods reveal only specific aspects of the behaviour of the coating systems. The data obtained depend on bulk properties of the film and the substrate material and on properties of the interface. Variation of the bias voltage can change them in quite different ways. In addition, the inter-relations between the adhesion strength of the coating and the failure behaviour of the three-point bend test samples are discussed.

Kinetics of aluminium film oxidation measured by a modified quartz crystal microbalance

Abstract The oxidation kinetics of aluminium films have been studied in the temperature range 330–400 K at a pressure of 0.66 Pa. A special type of quartz crystal microbalance has been developed in order to extend the temperature range of experiments up to 500 K. The vibrating quartz crystal was heated by IR light and the quartz temperature was measured by a thermocouple fixed to the quartz itself. The drift of the quartz frequency could be stabilized to values below 0.5 Hz within 1 h which is equivalent to about 0.1 monolayer (ML) of O 2 . The oxidation process on clean film surfaces starts with a very fast initial oxygen absorption step which is completed in less than 1 min. The thickness of the oxide layer after this first stage corresponds to 2–4 ML of oxygen. At temperatures below 350 K further oxidation becomes very slow. At higher temperatures a remarkable enhancement of the oxidation rate occurs and the time law becomes logarithmic. After long exposure times oxide layers with a thickness up to 16 ML are observed. The oxidation curves are compared with results of model calculations based on the Cabrera-Mott theory.