Fe Alloy Powders Research Articles

A new generation of porous non-evaporable getters

The application of non-evaporable getters is increasing. In many of these applications the getters have to work at relatively low temperatures to avoid damaging internal components and their excessive degassing. At these low temperatures the bulk diffusion of the sorbed gases is normally very small (except for H 2) and therefore the active surface must be maximized. This means that large specific surfaces and porous structures have to be used. Already existing porous getters normally require activation at high temperatures (700–900°C) but in several cases this is not easy or even possible to achieve. A new generation of porous getters has therefore been prepared and studied to fulfill these requirements. The getters are based on a recently developed ZrVFe getter alloy, which is characterized by a high activity for the residual gases. The new porous getters are obtained via a sintering process between zirconium and ZrVFe alloy powders, which, however, maintains a relatively large surface area and porosity. Therefore, they combine the large specific surface and porous structure necessary for surface sorption of gases at low temperatures with the characteristics of high diffusivity, which makes possible efficient sorption also after activation at relatively low temperature (400–500°C). Besides these already remarkable properties, the getters show particularly attractive mechanical characteristics to meet the special working conditions of some tubes (vibrations, shocks, etc.). The getter performances for the main gases (H 2, CO, H 2O) are presented and discussed; the tests are performed using standard dynamic sorption techniques and the microbalance method (for H 2O). The results obtained for sorption at room temperature confirm the good performances due to the high surface area. Higher sorption temperatures (200°C, 400°C) tested show the large effects of the high diffusivity of the material used, which is responsible for the further noticeable improvement of the sorption characteristics obtained in the lower temperature ranges. These getters appear, therefore, to be a real contribution to particular problems of many vacuum devices such as special electron tubes.

Effect of electrolysis conditions on the formation, structure, and magnetic properties of a finely divided iron-cobalt alloy

An investigation was carried out into the effect of electrolyte concentration and current density on current efficiency in the electrodeposition of Fe, Co, and Fe-Co alloy powders. It was established that raising the electrolyte concentration from 6.65 to 26.62 g/liter of Fe2+ + Co2+ increases current efficiency, whose maximum value is about 90%. The highest current efficiency is attained at ic of 20–30 A/dm2. Changing the electrolyte concentration and current density does not significantly affect the composition of alloy deposits. At an iron-to-cobalt ion content ratio in the electrolyte of 1∶1 the rate of discharge of Fe2+ during alloy formation in the twolayer bath is greater than that of Co2+. X-ray structural analysis revealed that the greatest changes in the internal structure of a very finely divided iron-cobalt alloy take place at low electrolyte concentrations. That raising the electrolyte concentration facilitates alloy formation is confirmed by a decrease in the degree of defectiveness of the particles and a stabilization of the crystal lattice parameter of the alloy; ic does not have such an effect on the structure of the alloy. Magnetometric measurements demonstrated that the coercive force of alloy powders is greater at higher densities of dislocations in their particles.

Carbon deposition and activity changes over FeRu alloys during Fischer-Tropsch synthesis

The interaction of 3H 2 CO at 1 atm and 573 to 673 °K with unsupported Ru, Fe, and RuFe alloy powders was investigated using X-ray photoelectron spectroscopy, secondary ion mass spectrometry, and differential chemical kinetics. Direct surface analysis of fresh, reacted, and regenerated surfaces, without exposure to air, shows a strong dependence of carbon deposition on catalyst composition. Pure Ru and Ru doped with 3 atom% Fe show no carbon buildup, and therefore exhibit steady specific activity and selectivity in Fischer-Tropsch synthesis as a function of time on stream. Increasing the iron content of the alloys results in a thick carbon overlayer being deposited during the first few hours of reaction. The carbon overlayer reduces specific activity by a factor of 2 to 3 and shifts selectivity toward lower-molecular-weight products. Regeneration of these catalysts requires heating in H 2 at 683 °K. Both XPS and SIMS confirm that pure iron first incorporates carbon into the bulk with a resulting temporary increase in activity and selectivity to saturated products. Selectivity then shifts to lighter products as carbon gradually builds up on the iron carbide surface.

Densification and alloy formation in the sintering of molybdenum-iron powders

1. The sintering of a mechanical mixture of molybdenum and iron powders (10 and 40% Fe) in the temperature range 900–1000° C is accompanied by growth of specimens induced by the formation of the intermetallic compound Mo2Fe3. A solid solution of Fe in Mo does not appear to form under these conditions. 2. Mo -10% Fe alloy powders produced by the coreduction of higher oxides are characterized by good sinterability thanks to their small particle size and specific phase composition, enabling relative densities of the order of 90–92% to be obtained in specimens sintered at 1200°C. With Mo-40% Fe alloy powders the factor of active shrinkage is counterbalanced by the factor of specimen growth brought about by intense formation of ɛ-phase. 3. In the sintering of molybdenum-iron alloy powders produced by the coreduction of oxides the concentration of Fe in the Mo grows with rise in sintering temperature.