A. T. Santhanam

Microstructural evaluation of multicomponent ZnO ceramics

A microstructural evaluation was made of experimental non‐Ohmic ZnO ceramics by optical, scanning, and transmission electron microscopy complemented with electron beam microprobe and x‐ray analyses. Four crystalline phases were identified: a major phase of ZnO grains containing Co and Mn in solid solution and three minor phases of bismuth silicate (12 Bi2O3⋅2SiO2), pyrochlore (Bi2[Zn4/3Sb2/3]O6) and spinel (Zn7Sb2O12) in the intergranular region. Transmission electron microscopy revealed that the ZnO grains are mostly in direct contact with each other and that the intergranular phase is located primarily in the multiple grain junctions. In those regions where ZnO grains are in contact with each other, the measured grain‐boundary width was ?25 A. It is suggested that the barrier to conduction in ZnO varistors resides in the vicinity of the grain boundary within the ZnO grains and not in the intergranular insulating layer.

The nucleation of high‐Tc Nb3Ge in the presence of impurities

Analyses of high‐Tc Nb3Ge films show that they all have a peak in oxygen concentration near the substrate‐film interface and that their lattice parameters in that region are abnormally large. It is proposed that high‐Tc Nb3Ge is a metastable phase which is formed via a homoepitaxial process from a large lattice parameter A15 Nb‐Ge phase. This phase near the interface is believed stable due to an expanded lattice resulting from the presence of impurities.

Nucleation of high-T/sub c/ Nb/sub 3/Ge in the presence of impurities

Analyses of high‐Tc Nb3Ge films show that they all have a peak in oxygen concentration near the substrate‐film interface and that their lattice parameters in that region are abnormally large. It is proposed that high‐Tc Nb3Ge is a metastable phase which is formed via a homoepitaxial process from a large lattice parameter A15 Nb‐Ge phase. This phase near the interface is believed stable due to an expanded lattice resulting from the presence of impurities.

Impurity doping of chemical‐vapor‐deposited Nb3Ge and its effect on critical‐current density

In an earlier work, we demonstrated that high self‐field and low‐field critical‐current densitites Jc of the order of 106 A cm−2 can be attributed to flux pinning on a dispersed Nb5Ge3 tetragonal phase present in Nb3Ge3 layers grown by chemical‐vapor deposition (CVD). In this study, we examined the effect of impurity gas additions on Jc and the critical temperature Tc of the A15 superconducting phase. The gas impurities were N2, C2H6, and CO2. The impurity concentration in the gas phase was varied over three to four orders of magnitude to establish tradeoffs between Tc deterioration and Jc enhancement. The x‐ray phase analysis of samples containing the highest impurity concentrations indicated by the presence of niobium nitrides and carbides, respectively. The Tc was affected least by N2 and most by CO2 additions. Doping by N2 or C2H6 resulted in A15 deposits free of the tetragonal phase and having Jc’s of the order of 106 A cm−2, comparable to the best Nb5Ge3‐containing samples. The grain size of deposit...

Mass‐spectrographic analysis of high‐Tc Nb‐Ge sputtered films

Secondary ion mass spectroscopy (SIMS) has been used to determine the concentration profiles of niobium and germanium in high‐Tc (22 K) Nb‐Ge films which were deposited onto polished sapphire substrates by a low‐energy sputtering technique. X‐ray lattice parameter data on such films indicate that in some cases their average composition is Nb‐rich nonstoichiometric Nb3Ge. The SIMS analysis reveals a nonuniform composition within these sputtered films. The Ge concentration increases with film thickness but at least a portion of the film has the stoichiometric Nb3Ge composition. The occurrence of high critical temperatures in such films is thus still compatible with the hypothesis that the highest Tc’s in the Nb‐Ge system are due to the presence of the stoichiometric or near‐stoichiometric Nb3Ge compound.

Identification of the tetragonal Nb5Ge3 phase in two‐phase Nb‐Ge superconducting films by electron diffraction

Magnetization studies of CVD Nb3Ge superconducting films had previously shown that the critical current density (Jc) significantly improved in the presence of a few volume percent of the tetragonal Nb5Ge3 phase. The presence of this tetragonal phase was previously determined by x‐ray analysis. In the present study, Nb‐Ge films containing the A‐15 Nb3Ge phase and the tetragonal Nb5Ge3 phase were examined by transmission electron microscopy (TEM), with a view to determine the size and distribution of the tetragonal phase in the microstructure. This paper describes a procedure by which the Nb5Ge3 phase could be identified in the microstructure using electron diffraction. The Nb5Ge3 particles were present at the grain boundaries as well as within the grains of the A‐15 matrix.

THIN FILMS AND METASTABLE PHASES

Publisher Summary It is believed that all high-temperature superconducting phases are inherently unstable. This may then explain the current interest in growth techniques that might be capable of forming nonequilibrium, metastable phases of potentially high-T c superconductors. Among T c superconductors, attention is centered on techniques that involve growth from the vapor phase, such as sputtering, evaporation, and chemical vapor deposition (CVD). The result of those methods is the preparation of a compound called Nb 3 Ge. At present, that compound remains the highest temperature superconductor known with a maximum onset temperature of 23.6 K. This chapter reviews the mechanisms by which nonequilibrium phases are formed in thin films. The ability of sputtering, evaporation, and CVD to form phases is generally attributed to one or a combination of the following factors: (1) sputtering, evaporation, and CVD can all form phases at much lower temperatures than is possible by growing from the melt; (2) in each of these processes the impurities are introduced into the forming material; (3) sputtering and evaporation, in particular, are often considered to be very high-rate quenching methods by which high-temperature phases can be “frozen in” at lower temperatures.

Microstructure and flux pinning in commercial Nb‐25% Zr superconducting wires

A detailed structural examination was carried out on commerical Nb‐25% Zr conductors which had been heavily cold worked and annealed in the temperature range 600–700 °C for times ranging from 1/4 to 10 h. The magnetization of some of these wires had previously been measured (Mathur et al.) as a function of applied magnetic field at temperatures ranging from 4.2 °K to the transition temperature and the results were interpreted in terms of a flux‐pinning model proposed by Kramer. Additional magnetization measurments on larger‐diameter wires were made to aid in the interpretation of the flux‐pinning mechanisms. Transmission electron microscopy, x‐ray analysis, and hardness measurements revealed that the predominant structural change in the annealed wires was recovery of dislocation structure. No βZr or αZr precipitates were detected. It is concluded that the optimum flux‐pinning properties in Nb‐25% Zr conductors for the processing schedules studied are associated with a highly recovered and small dislocatio...

Nb3Ge as a potential candidate material for 15- to 25-T magnets

The Nb3Ge compound is a promising candidate material for superconducting magnets in the 15- to 25-T range at 4.2 K. Preliminary work [1] on thin films of sputtered material revealed that this A15 superconductor possessed an upper critical field (H c2) around 37 T at 4.2 K and a critical temperature, T c , of 22 to 23 K. Measurements of the critical current density, J c , gave a value of 1 GA m−2 at 21 T and 4.2 K, the highest recorded value at this field. J c values in the range 0.5 to 1 GA m−2 at the operating field density and temperature are considered adequate for constructing a magnet. Therefore, Nb3Ge is indeed a promising material for very high field applications. Growth of Nb3Ge by chemical vapor deposition (CVD) lends itself readily to the manufacture of lengths of conductor in tape form. This work concentrates on the CVD material. Other fabrication techniques, such as electron beam evaporation or high-rate magnetron sputtering, could also be amenable to the production of lengths of tape conductor.