Search

Metallic reactive multilayers are known as high energy-density storage systems. Conventionally, these multilayers are tailored for high reaction rates with the purpose to achieve high maximum reaction temperatures and explosive-like behavior upon mixing. However, in some instances such as neutralization of biological hazards or chemical energy-storage systems, a low heat flow rate is desired. In the present work, we show that Nb-alloying presents an efficient approach to stabilize the asdeposited state and to form a diffusion barrier in situ, effectively reducing the heat flow rate by more than 50%. The validation of the concept is carried out by a comparative study of thermally induced phase reactions in Ni/Al and (Nb-Ni)/Al reactive multilayers. Kinetics of the phase reactions in these systems were followed by differential scanning calorimetry, analytical scanning transmission electron microscopy, and in situ electron diffraction analysis. The results confirm alloying as a design strategy for tailoring reaction kinetics of reactive multilayers. [ABSTRACT FROM AUTHOR]

The effect of Nb substitution for Ni in Ni45Co5Mn40Sn10 magnetic shape memory alloys on their magnetic properties, martensitic transformation characteristics, transformation hysteresis, and magnetocaloric properties was studied using wavelength-dispersive X-ray spectroscopy, differential scanning calorimetry, and the temperature and field dependence of the magnetization. Ni45Co5Mn40Sn10 alloy has a very low transformation hysteresis; however, the martensitic transformation temperatures are notably above room temperature, which is not desirable for magnetic refrigeration applications. In this study, small quantities of Nb substitution were shown to drastically shift the transformation temperatures to lower temperatures, at a rate of 68 K/at.% Nb, which is needed for household refrigeration. The austenite Curie temperature also decreased with increasing Nb content. However, a decrease in the latent heat of the martensitic transition was observed, which negatively affects the magnetic field-induced adiabatic temperature change capability. Still, the relatively large transformation entropy and the low transformation hysteresis make the Nb-doped Ni45Co5Mn40Sn10 alloys potential candidates for solid state refrigeration near room temperature. [ABSTRACT FROM AUTHOR]

The quantum efficiency of NbN and NbTiN superconducting single photon detectors drops with decreasing photon energy. A lower gap material would enable single photon detection deeper in the infrared. We have fabricated a NbSi detector and compare its characteristics with a NbTiN device. NbSi (TC≃2 K) has a smaller superconducting gap than NbTiN or NbN (TC≃15 K). We measure the detection efficiency for a wavelength range from 1100 to 1900 nm. In this range the NbSi detector shows a 10-fold increase in relative efficiency with respect to the NbTiN detector. [ABSTRACT FROM AUTHOR]

Single- and multiphase samples of the n-type half-Heusler NbCoSn were prepared by directional solidification using the optical floating zone melting method, and the thermoelectric properties of these samples were evaluated. NbCoSn has an excellent thermoelectric power which exceeds -250 μV K-1 at around 900 K and a relatively high carrier concentration, 4.82×1026 m-3. A metalliclike temperature dependence of the electrical resistivity indicates that NbCoSn is a degenerate semiconductor. NbCoSn also shows an excellent power factor, 2.5 mW m-1 K-2 at about 650 K, even without any tuning of the electrical properties which are susceptible to coexisting metallic phases. [ABSTRACT FROM AUTHOR]

Dielectric relaxation and ultrasonic attenuation spectra of Sr4La2Ti4Nb6O30 tungsten bronze ceramics were investigated over a broad temperature range. A broad permittivity peak with strong frequency dispersion was observed around 200 K, which followed well the Vogel-Fulcher relationship, and is attributed to relaxor ferroelectric behavior with spin glass nature. The ultrasonic investigation provided further evidence on this thermally activated process, and both the dielectric relaxation and the freezing of polar clusters were clearly observed in the ultrasonic velocity shift and ultrasonic attenuation spectra. [ABSTRACT FROM AUTHOR]

Develops a low-noise superconductor-insulator-superconductor quasiparticle mixer with niobium titanium nitride (NbTiN) thin-film microstrip tuning circuits and hybrid Nb/AlN/NbTiN tunnel junctions. Use of a quasioptical configuration with a planar twin-slow antenna feeding a two-junction tuning circuit; Uncorrected double-sideband receiver noise temperature.

We have fabricated and characterized the noise performance of well-coupled Nb-Pb alloy edge junction dc superconducting quantum interference devices (SQUIDs) with a resistively shunted inductance. The extra damping resistor removes the resonances in the current-voltage characteristics resulting in a very low value of the extrinsic energy sensitivity in flux-locked-loop operation. Numerical simulations, with the junction capacitance included, indicate that the damping resistance does not degrade noise performance until it is significantly less than the junction-shunting resistance. We also tested different SQUID designs and demonstrated that a large SQUID inductance, proposed earlier to be desirable when resistively shunted, can seriously affect the low-frequency flux noise. [ABSTRACT FROM AUTHOR]

Nb-Al alloys have been prepared by mechanical alloying of elemental crystalline powders and investigated with x-ray diffraction and transmission electron microscopy. Crystalline equilibrium phases form for Al-rich alloys. Amorphous phase formation is observed for the Nb50Al50 composition. Mechanically alloyed powders with more than 70 at. % Nb exhibit the metastable bcc (A2) solid solution which transforms after thermal annealing into the A15 phase. Formation of the different phases can be understood by considering the free-energy curves of the amorphous and bcc phases as calculated from the phase diagram and additional thermodynamic data. [ABSTRACT FROM AUTHOR]

Superconducting accelerating cavities made of Nb-coated copper were produced. Niobium films of a thickness ranging from 1.4 to 4 μm were deposited onto the inside of 3-GHz cavities and 500-MHz frequency by bias diode sputtering. A maximum accelerating field of 8.6 MV m-1 was reached without quench which is attributed to the large thermal conductivity of copper at liquid helium temperatures. [ABSTRACT FROM AUTHOR]

Experiments were conducted to determine the transverse stress sensitivity of the critical current (Ic) of jelly-roll multifilamentary Nb3Al wires at 8 and 12 T. For comparison, similar experiments were conducted on bronze-process (NbTi)3Sn wires. At 12 T and under a transverse compressive stress of 150 MPa-conditions expected in fusion magnets-the Ic degradation of the Nb3Al specimen was ∼20% vs ∼65% for the (NbTi)3Sn specimen. The intrinsic superiority of Nb3Al over (NbTi)3Sn with regard to transverse compressive stress is thus clear. There is optimism that Nb3Al will eventually become a useful superconductor for large-scale, high-field applications. [ABSTRACT FROM AUTHOR]

Nb3Al multifilamentary wire has been fabricated by a newly developed process, in which a composite consisting of a niobium matrix and a large number of Al-Mg alloy cores is cold drawn into a wire with ultrafine Al-Mg alloy filaments (about 0.1 μm), and then heat treated to form Nb3Al filaments by the diffusion reaction between the Al-Mg alloy cores and the Nb matrix. The wire shows excellent high-field superconducting properties comparable to those of the practical Nb3Sn multifilamentary wire. [ABSTRACT FROM AUTHOR]

Utilizing the containerless electrostatic levitation facility at NASA/MSFC, we were able to undercool the Ni[sub 59]Nb[sub 41] (atomic) alloy by 210 K which was 160° farther than the results of previous flight experiments. Undercoolings were clustered around 200 K during the repeated melting-freezing cycles on a single sample. Prior to this work, a metastable liquid separation had been presumed to limit the undercooling of this alloy. However, microstructural observations have revealed that undercooling was limited by crystal nucleation. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]