L. E. Toth

Composition‐vs‐depth profiles obtained with Auger electron spectroscopy of air‐oxidized stainless‐steel surfaces

Auger electron spectroscopy (AES) combined with simultaneous sputter removal of material from the sample (profiling) was used to determine changes in composition within surface layers (∼1000 A) of stainless steels which were air oxidized at various temperatures. Oxidation of 304 stainless steel at temperatures up to 600°C results in the formation of an Fe‐oxide layer with Cr and Ni appearing only deeper below the surface at the oxide‐bulk interface. After treatment at 700°C the first signs of Cr‐oxide formation near the surface begin to appear. At 900°C the Fe‐oxide layer is replaced by a Cr‐oxide film and becomes the dominant feature. Composition profiles for air‐oxidized (at 900°C) 321 stainless steel and Inconel are also reported.

Electron microscopy of crystalline and amorphous Ni-P electrodeposited films: In-situ crystallization of an amorphous solid

Electron microscopy has been used to study the structure of Ni-P electrodeposited thin films with 7, 12, 20 and 22 at. pct P. For the crystalline as-deposited films with 7 and 12 at. pct P (low P films), the crystal structure is fcc and the grains are a supersaturated solid solution of P in Ni. Grain size decreases with increasing P content; the present findings agree with previous ones. For “amorphous” as-deposited films with 20 and 22 at. pet P (high P content films) the amorphous phase is not completely homogeneous and there are regions in which small microcrystals exist. Electron beam heating a low P con-tent film causes the crystalline array of supersaturated Ni grains to decompose to an equilibrium mixture of Ni and Ni3P; both types of grains are randomly oriented. Electron beam heating a high P content film causes the amorphous regions to undergo several complex transformations. The first reaction to occur is: Amorphous (Ni-P) -NixPy + Ni (random) where NixPy is a newly discovered phase with a variable composition. Further beam heating causes a second transformation to equilibrium phases: NixPy + Ni → Ni3P + Ni (random). The microstructure resulting from the above transformations depends on variations in composition of the as-deposited specimens, rates of heating and temperature gradients. The mode of phase transformation in microcrystalline regions and amorphous regions is distinctly different. Crystallization in amorphous regions occurs by a nucleation and growth of NixPy, and Ni; a crystallization front is seen to advance into the amorphous re-gion. Crystallization in microcrystalline regions occurs by nucleation and growth of the Ni3P phase and grain coarsening of the Ni phase. No distinct crystallization front is ob-served.

Superconducting properties, electrical resistivities, and structure of NbN thin films

Superconducting properties and electrical resistivities of NbN thin films have been studied as a function of sputtering parameters, film structure, and composition. A good correlation exists between superconducting Tcs and film structure and composition regardless of the sputtering conditions used to achieve this structure. The most important parameters affecting Tc are lattice parameter, crystal structure, island structure, and impurity content. Substrate temperature, Ar/N2 ratio, and impurity content are the principal sputtering parameters affecting film structure, and thus Tc.

Low temperature specific heat study of the electron transfer theory in refractory metal borides

Abstract Low temperature heat capacities of eleven 4th, 5th and 6th group transition metal borides were measured from 1.5–18°K to test current theories about bonding and band structure of borides. The electron transfer theory helps to interpret γ values, but the experiments also suggest certain modifications to that theory.

SUPERCONDUCTING CRITICAL TEMPERATURES OF THE CARBIDES AND NITRIDES WITH THE NaCl STRUCTURE: SUPERCONDUCTIVITY OF MoC

Abstract The superconducting transition temperatures of the metallic carbides and nitrides with the NaCl structure are interpreted on the basis of the tight-binding model of BILZ . (1) It is found that total valence electron concentrations from 9 to 10 are required for high critical temperatures in these compounds. The cubic modification (NaCl) of molybdenum carbide has been found to be superconducting at 13.0°K in agreement with the Bilz band model.

Low temperature heat capacities of superconducting niobium and tantalum carbides.

Abstract Low-temperature heat capacities have been measured from 1.5 to 18°K in the Nb-C and Ta-C binary systems to determine the effects of carbon concentration and crystal structure on the coefficient of electronic specific heat, Debye temperature, and the superconducting Tc. Comparison of the present results with other measurements on carbides and nitrides reveals very simple regularities in the variation of γ and θD values with composition and crystal structure. The rapid decrease in Tc with decreasing carbon concentration in the monocarbides results from a decrease in both the electron—phonon interaction parameter and the unrenormalized density of electron states.(1)

Superconducting critical temperatures of nonstoichiometric transition metal carbides and nitrides

Abstract Superconducting critical temperatures have been studied in the TiN 1−x , VN 1−x , YN, VC 1−x and Ta-W-C systems. The present data are reviewed together with other superconducting measurements for monocarbides and mononitrides of transition metals from the III to VI groups of the Periodic Table. Data for both stoichiometric and nonstoichiometric phases are correlated with the electron concentration. This correlation is found to be consistent with theoretical band structures and experimental data on magnetic susceptibilities, Hall coefficients and thermoelectric powers. The interesting variation of the T c s of nonstoichiometric phases is discussed in terms of Denkers (1) theoretical model.

Low temperature heat capacities of superconducting molybdenum carbides

Abstract Low-temperature heat capacities from 1.5 to 20°K have been measured for several molybdenum carbides with closely related crystal structures. These carbides are found to have very similar thermodynamic and superconducting properties. This similarity, as well as their unusually high Tcs, can be explained by their related crystal structures. An experimental evaluation of the applicability of the GLAG theory(1) to these carbides shows that the upper critical field, Hc2, is limited by the Ginzburg-Landau κl parameter, in which the coherence length is limited only by the electron mean free path.

On the elastic and thermodynamic properties of transition metal carbides

Values of the Debye temperature,θE, for a number of refractory carbides were calculated from elastic constant data and then compared with those of the Debye temperature,θD, as derived from low-temperatureCP data. It is found that an apparent discrepancy existed in the literature between the values ofθE andθD for these carbides. However, when the same units are used, agreement of 10 pct or better between the values ofθE andθD is found for all the carbides with the exception of MoC1/2. A discrepancy of 10 pct is reasonable based on an analysis of both the experimental and theoretical difficulties associated with the evaluation ofθE and θD for this class of materials. All the elastic constant data used were taken from the literature. Values ofCP for TiC, HfC, and WC were measured in the present study from 1.5° to 15°K. These values in addition to the literature data were used in derivingθD values. A correlation of the bulk modulus with interatomic spacing is presented for these refractory carbides. Based on this correlation, the bonding forces responsible for the cohesion of this class of materials are discussed.

Superconducting Hc‐Jc and Tc Measurements in the Nb–Ti–N, Nb–Hf–N, and Nb–V–N Ternary Systems

Superconducting critical fields, current densities and temperatures have been measured as a function of composition in the Nb–Ti–N, Nb–Hf–N and Nb–V–N ternary systems. In the Nb–Ti–N system the upper critical fields increase sharply and monotonically with increasing NbN concentration to over 130 kOe at about 90 mole % NbN. In the Nb–Hf–N system the highest upper critical‐field value observed was 130 kOe for the composition Nb0.92Hf0.08N. In the Nb–Hf–N system the Tc increases monotonically with NbN concentration, in the Nb–Ti–N system a maximum in Tc exists at about 65 mole % NbN and in the Nb–V–N system a broad minimum exists between 30 and 50 mole % VN.