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Adaptation of commercial instrument which measures nitrogen and oxygen in steel gave results in less than four minutes. Sample is heated in helium atmosphere in single-use graphite crucible. Platinum flux facilitates melting of sample. Released gases are separated chromatographically and measured in thermal-conductivity cell.
Results from tests of the first variable-gap diminiode at an initial interelectrode spacing of 0.23 millimeter indicate sharply defined, relatively low ultimate power points. This characteristic supports the value of the diminiode as a well-controlled tool for thermionic-conversion research and development.
A search was made for new cationic conducting phases in alkali-tantalate and niobate systems. The phase equilibrium diagrams were constructed for the six binary systems Nb2O5-LiNbO3, Nb2O5-NaNbO3, Nb2O5-KNbO3, Ta2O5-NaTaO3, Ta2O5-LiTaO3, and Ta2O5-KTaO3. Various other binary and ternary systems were also examined. Pellets of nineteen phases were evaluated (by the sponsoring agency) by dielectric loss measurements. Attempts were made to grow large crystals of eight different phases. The system Ta2O5-KTaO3 contains at least three phases which showed peaks in dielectric loss vs. temperature. All three contain structures related to the tungsten bronzes with alkali ions in non-stoichiometric crystallographic positions.
In the method proposed for fabricating multifilamentary Nb3Sn tape, filamentary superconducting paths are produced in standard commercial superconductor tape by chemical milling of separator slots through the Nb3Sn layer. The multifilament configuration features a matrix of ten 1.2 mm wide parallel helical superconducting paths along the length of the tape. The paths are spaced 0.4 mm apart. Tapes tested as small pancake coils demonstrated the integrity and continuity of the matrix, and showed that critical current was sustained in direct proportion to retained superconductor.
Three exceptionally high toughness Fe-12Ni alloys designed for cryogenic service were welded by using the gas tungsten arc welding process. Evaluation of their weldability included equivalent energy fracture toughness tests, transverse weld tensile tests at -196 and 25 C, and weld crack sensitivity tests. The Fe-12Ni-0.25Ti alloy proved extremely weldable for cryogenic applications, having weld and heat affected zone properties comparable to those of the wrought base alloy. The Fe-12Ni-0.5Al alloy had good weld properties only after the weld joint was heat treated. The Fe-12Ni-0.25Nb alloy was not considered weldable for cryogenic use because of its poor weld joint properties at -196 C and its susceptibility to hot cracking.
A process of fabricating superconducting Nb3Ge tapes by chemical vapor deposition (CVD) has been developed and tapes up to 10 meters long fabricated. The typical properties achieved were: critical temperature T sub c = 20 K, upper critical field H sub c2 = 29 tesla at 4.2 K, and J sub c = 3 to 4 x 10 to the 8th power A m(-2) at 4.2 K, 18 tesla. The relative depression of T sub c and H sub c2 compared with the best thin film samples sputtered on sapphire was due to the presence of Nb5Ge3 second-phase particles used as flux pinning centers and to strains induced by thermal mixmatch with Hastelloy B tape substrates. A peculiar field dependence of flux pinning force that was observed in both CVD and sputtered Nb3Ge indicated a premature pin-breaking mechanism or a phase inhomogeneity. Directions of further optimization work were defined.
Preliminary tests were conducted to determine if interstitial element transport through a circulating helium working fluid was a potential problem in Brayton and Stirling space power systems. Test specimens exposed to a thermal gradient for up to 3000-hr included Nb-1%Zr, a Sm-Co alloy (referred to as SmCo in this paper), Hiperco 50 steel, and alumina to simulate various engine components of the Brayton and Stirling systems. Results indicate that helium transport of interstitial contaminants can be minimized over a 7-yr life with a monometallic Nb-1%Zr design. Exposure with other materials indicated a potential for interstitial contaminant transport. Determination of contamination kinetics and the effects on structural integrity will require additional testing.
The high vacuum creep and creep-rupture behavior of a Nb-1Zr-.1C alloy (PWC 11) was investigated at 1350 and 1400 K with an applied stress of 40 MPa. The material was tested in the following four conditions: annealed (1 hr 1755 K/2 hr 1475 K); annealed plus EB welded; annealed plus aged for 1000 hr at 1350 or 1400 K; and annealed, welded, and aged. It was found that the material in the annealed state was the most creep-resistant condition tested, and that aging the alloy for 1000 hr without an applied stress greatly reduced that strength; however, it was still approximately three times as creep resistant as Nb-1Zr. Additionally, the EB weld region was stronger than the base metal in each condition tested, and phase extraction of the dispersed precipitate revealed the presence of a 70%ZrC-30%NbC cubic monocarbide phase.
Transport of gaseous contaminants through the working fluid to or from sensitive refractory alloys is theoretically possible during long time operation of Brayton and Stirling space power generation systems which use a gas as the working fluid. A test was designed which could give an answer to whether transport of contaminants through the working fluid was a potential major problem. The findings of that preliminary study are summarized.
The undercoolings of pure Nb and Nb-Ge alloys are examined using the 100-m drop tube of the Marshall Space Flight Center. The temperatures of the samples prior to release were measured, and IR detectors were utilized to monitor recalescence from solidification. It is observed that the Nb and Nb-Ge samples undercooled to the homogeneous nucleation limit. The data reveal that all the samples displayed a dendritic primary phase, except for alloys nucleating in the beta-phase field, which had a cellular phase. The composition of these phases are studied using EDXA; it is detected that the composition of the phases corresponds to the Jorda (1978) phase diagram.
NbSe3 exhibits superior characteristics such as high capacity, high volumetric and gravimetric energy densities, and high discharge rate capability, as compared to other intercalating cathodes. This paper reports the preparation, characterization, and performance of NbSe3. Several electrochemical techniques, such as cyclic voltammetry, constant-current/constant-potential discharges, dc potentiodynamic scans, ac impedance, and ac voltammetry, have been used to give insight to the mechanisms of intercalation of three lithiums with NbSe3 and also into the rate determining process in the reduction of NbSe3.
The electrochemical behavior of NbSe3 in the battery electrolyte 1.5M LiAsF6/2Me-THF is reported. A detailed study has been carried out using various ac and dc electrochemical techniques to establish the mechanism of intercalation of three equivalents of Li with NbSe3 as well as the rate governing processes in the reduction of NbSe3. An equivalent circuit has been formulated to represent the NbSe3-solution interface. The kinetic parameters for the reduction of NbSe3 were evaluated from the ac and dc measurements. The structural change in NbSe3 on lithiation during initial discharge which results in higher cell voltages and different electrochemical response as compared to virgin NbSe3 was identified to be a loss of crystallographic order.
A study is being conducted at NASA Lewis Research Center to determine the feasibility of using a carbide particle strengthened Nb-1 percent Zr base alloy to meet the anticipated temperature and creep resistance requirements of proposed near term space power systems. In order to provide information to aid in the determination of the suitability of the PWC-11 alloy as an alternative to Nb-1 percent Zr in space power systems this study investigated: (1) the long-time high-vacuum creep behavior of the PWC-11 material and the Nb-1 percent Zr alloy, (2) the effect of prior stress-free thermal aging on this creep behavior, (3) the effect of electron beam (EB) welding on this creep behavior, and (4) the stability of creep strengthening carbide particles.
Solidification mechanisms had a dominant influence on microfissuring behavior of the test group. Carbon modified the Laves formation significantly and showed that one approach to alloy design would be balancing carbide formers against Laves formers. Boron's strong effect on microfissuring can be traced to its potency as a Laves former. Boron's segregation to grain boundaries plays at best a secondary role in microfissuring.
Containerless processing and rapid solidification techniques were used to process Nb-Si alloys in the Nb-rich eutectic range. Electromagnetically levitated drops were melted and subsequently splat-quenched from different temperatures. A variety of eutectic morphologies was obtained as a function of the degree of superheating or undercooling of the drops prior to splatting. Metallic glass was observed only in drops quenched from above the melting temperature. Microstructures of splats deeply undercooled prior to quenching were very fine and uniform. These results are discussed in terms of classic nucleation theory concepts and the expected heat evolution at different regions of the splat during the rapid quenching process. The locations of the coupled-zone boundaries for the alpha-Nb + Nb3Si eutectic are also suggested.
We report on our ongoing work using Nb/Al/AlO(x)/Nb junctions for the detection of X-rays. Detectors based on superconducting tunneling junctions offer the prospect of resolution over an order of magnitude higher than is obtainable with the current generation of semiconductor-based detectors. Results of measurements taken at 1.85 K (a temperature achievable with current space flight technology) include the current-voltage (I-V) curve, subgap current vs temperature, the dependence of the superconducting current on the applied magnetic field (Fraunhofer pattern), X-ray pulses, and the spectra from a 6 keV X-ray source which gave an intrinsic device resolution of approximately 700 eV. The collection of more than 10 exp 5 electrons per 6 keV photon is established.
A reduction in the mixer noise is expected when using superconductors with a lower transition temperature (T(sub c)) since the thermal noise components of the mixer noise should scale with T(sub c). Also, the local oscillator (LO) power required for a diffusion-cooled device should decrease as T(sub c) when T(sub bath) << T(sub c). We previously studied mixing in aluminum based hot-electron bolometers (HEBs) at microwave frequencies (approximately 30 GHz), and observed a significant improvement in noise performance, and a reduction in LO power as predicted. However, the bias voltage range over which good mixer performance was observed was approximately 5 - 10 microV. These devices are thus susceptible to saturation effects, in particular output saturation. In the present work, we have investigated Nb HEBs whose T(sub c) is lowered by applying a magnetic field. The goal is to study a case intermediate between Nb and Al, and hopefully to find properties that will allow use in practical receivers. A 15 kOe perpendicular magnetic field was applied to a Nb HEB (L = 0.16 micrometers, W = 0.08 micrometers, R(sub N) = 90 ohms) to reduce T(sub c) from 5.2 K to 2.4 K. The mixer noise, as inferred from the output noise and the conversion efficiency, decreased from 390 K, DSB to 171 K, DSB. The LO power required for near optimum mixer conversion efficiency (eta(sub mixer) = -9 dB in this device) was 8 nW in zero field, and approximately 2 nW when T(sub c) was reduced to 2.4 K. T(sub bath) = 0.22 K. The conversion bandwidth was previously measured to be 2.4 GHz and the same bandwidth was observed in the presence of a magnetic field. By lowering T(sub c), the voltage range over which good mixing was observed also decreased. However, even with T(sub c) reduced to 2.4 K, the conversion efficiency dropped by 3 dB from its maximum value only when the bias voltage was changed by approximately 90 microV. Saturation effects should thus be much less of a concern in these devices than in Al HEBS. In situations where the application of a large magnetic field is not feasible, we suggest using Ta based HEBS. Ta HEBs should have T(sub c) = 3 - 3.5 K and material properties very similar to Nb.
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