A. J. Bevolo

Auger study of grain boundaries in large‐grained YBa2Cu3Ox

A scanning Auger microscopy study has been carried out on fractured surfaces of YBa2Cu3Ox samples prepared by standard sintering procedures from powders. It is concluded that in the large‐grained samples examined, a majority of the grain boundaries are sufficiently ‘‘open’’ that standard metallographic preparation causes them to become contaminated with C. This suggests that an actual loss of contact at the grain boundaries during sample preparation is responsible for the low Jc values observed in bulk‐sized large‐grained YBa2Cu3Ox.

Specific heat of 2H NbSe2

The specific heats of four samples of 2H NbSe2 have been measured in the temperature range from 1 to 54 K. No anomalous behavior was found near 40 K where x‐ray, NMR, and Hall‐effect measurements have indicated the presence of a phase transition. An upper limit of 0.3 J/mole K has been placed on the magnitude of any anomaly in the heat capacity associated with this transition. Measurements of high‐purity stoichiometric 2H NbSe2 sample gave a superconducting transition temperature of 7.23 K, a specific heat jump (ΔC) in zero field of 240 mJ/mole K, and a Debye temperature of 204 K. A deviation from the usual T3 behavior for the lattice contribution to the heat capacity was observed and assumed to be due to the two‐dimensional layer structure of the compound. Our best estimate for the electronic coefficient of specific heat (γ) is about 10±5 mJ/mole K2. Based on the values for Tc and ΔC, the value for γ calculated from BCS theory is 23 mJ/mole K2 for 2H NbSe2 which is in disagreement with our estimated valu...

Studies of the initial oxidation of Fe-Si alloys by AES, XPS and EELS

Thin oxide layers on polycrystalline Fe-8.75at%Si alloys were investigated by various surface spectroscopies, i.e., AES (including Ar+ depth profiling), EELS, and XPS during the initial oxidation stage at room temperature under very low oxygen pressure. It was observed that a very thin “SiO2”-rich external layer is formed, as predicted by others, and established preferentially at the first stage of oxidation. A Si-depletion zone ∼5 A deep was found in the alloy with about 25% Si-depletion at the alloy/oxide interface. The formation of an Fe silicate-like structure just benath the Si oxide-rich top layer down to the alloy/oxide interface was also observed. Although this Fe-silicate layer was determined to be predominantly in the form of Fe2SiO4, gradual changes of the oxidation state of Fe from its highest oxidation state (“Fe2O3”-like) at the top to the lowest (“FeO”-like) at the bottom are also observed. The rates of oxydation of these alloys were determined to be retarded to approximately half the rate of pure Fe after exposure to 200 L of O2. The retardation involves the blocking of outward diffusion of Fe, reduction in the inward arrival rate of O by the top two network-forming oxide layers (the Si oxide-rich layer and the underlying mixed layer of trivalent Fe oxide and Fe-silicate), and, work of other, blocking of the alloy grain boundaries by the entire Fe-silicate layer with grains of very small size. Various comparative experiments were performed to support the interpretations.

Effectiveness of SiO2 for preventing silicon ‐metal reactions at high temperatures

Silicon dioxide layers, deposited by rf sputtering and measuring less than 1000 A thick, were found to prevent silicide formation between evaporated silicon films and molybdenum substrates after annealing for 1 h at 1250 °C. Silicon grain size ranged from 0.2 to 2.5 μ in cross section. In situ Auger electron spectroscopy (AES) was used in the silicon deposition chamber to monitor the heat treated Si/SiO2/Mo samples for silicide formation. Composition depth profiling by ion beam sputtering and AES analysis was used to measure oxygen diffusion into the silicon and metal phases. This technique established that the breakdown of SiO2 as a reaction barrier between the molybdenum and silicon occurred because of oxygen diffusion from the SiO2 layer into the silicon. A similar behavior is expected between SiO2 and tungsten. However, when SiO2, deposited on tantalum, was heated to 1100 °C, the oxygen was found to have migrated into the metal resulting in the breakdown of the oxide layer.

Magnetic and crystallization behavior of Be‐substituted Fe82B18 metallic glasses

Substitution of Be for B in the amorphous binary alloy Fe82B18 was observed to cause an initial increase in saturation magnetization (Ms) to a maximum of 200 emu/g at 4.2K, followed by a decrease for alloys with more than 4 at.% Be. Meanwhile, the Curie temperature of the Fe82B18−y Bey alloys decreased progressively with the Be content. These changes in Ms and TC differ from those observed in the Fe‐B‐M′ metallic glasses, where M′ is another metalloid (P, C, Si or Ge). Results from Auger electron spectroscopy and Mossbauer experiments confirmed the reversal trend detected in the magnetization measurements. Combining all these results enables us to attribute the initial increase in Ms to a smaller charge transfer from the Be atoms than that from the B atoms to Fe and the subsequent decrease in Ms to a possible clustering of the Be atoms at higher concentrations. The annealing behavior for the present alloys is described in terms of crystallization stages and the phases formed.

The Schottky barrier height and Auger studies of yttrium and yttrium silicide on silicon

Schottky barrier diodes of yttrium and yttrium silicide were fabricated on p‐type (111) silicon. The barrier heights of the diodes determined by I‐V measurements were correlated with heat treatments and interface conditions as determined by Auger composition depth profiles. The as‐deposited Y‐Si contact had a barrier height of 0.70 eV. During heat treatment at 300 °C, oxygen diffused into yttrium and formed Y2O3, resulting in a barrier height of 0.65 eV. Yttrium silicide was formed at 410 °C that lead to a Schottky barrier height of 0.60 eV.

The growth of polycrystalline silicon on molybdenum, tantalum, tungsten, and their disilicides

Silicon was electron beam vaporized in an ultralow pressure system on substrates of molybdenum, tantalum, and tungsten between 550–850 °C. Polycrystalline silicon was grown on the molybdenum and tungsten substrates at 550 °C with grains measuring ∼0.3 μm in cross section and having a {110} orientation. This orientation was attributed to the deposition technique and substrate temperature. At 670 °C silicon reacted with the tantalum substrates to form a thin interfacial layer of TaSi2 but grew with a {110} and {111} orientation. At this same sample temperature {111} and {110} polycrystalline silicon was detected on molybdenum and tungsten. By increasing the substrate temperature to 750 °C, silicon reacted with the metals to produce ’’columnar’’ structures composed of {110}, {111}, {100}, {311}, or {331} silicon crystallites that grew on a layer of the respective metal disilicide. Phosphorus coevaporated with the silicon retarded the rate of the silicide growth at 670 and 750 °C. Only the refractory metal di...

Heteroepitaxial silicon growth on polycrystalline MoSi2

Heteroepitaxial silicon was grown on polycrystalline MoSi2 by electron beam evaporation at 800 °C in an ultrahigh vacuum. The silicon films with a thickness between 2 and 12 μm grew with either an (111) or (233) orientation. The growth and morphology of the silicon deposit was studied as a function of temperature and the partial pressure of oxygen. Depositions at substrate temperatures of 950 and 1100 °C exceeded the epitaxial temperatures producing films that had faceted grains measuring 2–8 μm in cross section along with larger grains measuring nearly 200 μm across. The introduction of 10−10 Torr partial pressure of oxygen during deposition at 1100 °C changed the morphology and eliminated the large grains. The presence of oxygen at 800 °C prevented epitaxial growth.

Comparison of initial oxidation of single- and polycrystalline FeSi alloys, and the effects of annealing and crystal faces on the oxidation behavior

Abstract The initial oxidation of Fe-6.85 at % Si (110) and (111) single crystal surfaces, and (110) oriented pure Fe surfaces were investigated by AES (including Ar + depth profiling) and LEED at 300 K under very low oxygen pressure. The best annealing condition for each crystal was determined to provide the cleanest surface for the following initial oxidation experiments. The initial oxidation results were compared with those of the previously studied polycrystal alloy to show a general resemblance in the various initial oxidation features. Gradual changes of “Fe oxide” in the Fe-silicate layer from its highest oxidation state at the top to the lowest at the bottom were also observed as in the polycrystalline case, and the predominance of “FeO” was determined. The retardation in Fe oxidation rate with respect to that of the corresponding pure Fe surface is, however, enhanced by ∼ 33% on (110) compared to the polycrystal case. Comparison between (110) and (111) planes reveals only a minor crystallographic dependence of the initial oxidation of single-crystalline Fe-6.85at%Si. A general increase in the degree of retardation of the Fe oxidation in FeSi (110) was observed as the initial surface Si concentration goes up by segregation to a certain critical value, > 13 at% Si.

Investigation of solid solubility and metastable phase in dilute Th-Co alloys

Abstract The solubility of cobalt in α thorium is described by the equation c s (at.% Co) = 10970 exp(−13844/ T ). The heat of solution is −115.2 kJ mol − Co and the terminal solubility at the eutectic temperature 1100 °C is 0.46 at.% Co. A newly identified metastable phase with a plate morphology is present in thorium-rich alloys on quenching from the high (αTh) region. Upon aging for different times at 500 °C and above, this phase transforms to rod-like particles of Th 7 Co 3 .