The current-voltage characteristics of semiconducting BaTiO3 ceramic have been investigated with electric fields up to 2×104 V/cm. The results obtained are in agreement with Heywang's model for conduction in this material (other published models being unsatisfactory) provided that allowance is made for the Ohmic drop in the bulk material. It is found that most of the current is channeled through small pores (0.1–0.5 μ diam) in the intergranular boundaries.
We report here what we believe is conclusive evidence that the excessive degradation of critical current which occurs in solenoids wound from heattreated Nb-25% Zr wire is due to the simultaneous presence of transport current and flux jumping. Previous unpublished work in our Laboratories had shown that, while the critical current in a 5-cm short sample of Nb-25% Zr wire heated to 70boc for one hour was about 150 A in a transverse field of 20 kG, the same wire wound into solenoids went normal when producing 5 to 10 kG at critical currents less than 10 A. This same type of degradation, in a less severe form, is of course well known for solenoids wound from heavily cold-worked wire.The present experiments were performed with insulated 0.025-cm diam Nb-25% Zr wire which had been heat-treated at 700°C for one hour after the final cold-worked reduction. It has been previously reported that heat treatment of Nb-Zr alloys increases the critical current density of short samples. 2 -4 The short sample critical current vs field for this wire is listed in Table I. The wire was wound into a solenoid of approximate dimensions: 0.3-cm i. d., 1.3-cm o. d., 2.5 cm long (calculated G/ A = 434). The solenoid was cooled to 4.2°K, and a uniform magnetic field was applied to its axis. As this external field was increased smoothly from zero, with no transport current (I = 0) in the solenoid, a sequence of very narrow «1 msec CONTENT ANALYSIS
Nuclear magnetic double resonance has been used to investigate first-order electric quadrupole interactions of Na 23 and CI 35 nuclei near an Ag ion substituted for Na in NaCI. The study has determined the direction of the principal axes, asymmetry parameter rj, and strength of the interaction e 2 Qq for sodium ions at a (2, 0, 0) position, and chlorine ions at (2, 1, 0) position relative to the impurity at (0, 0, 0).
Dipole magnets designed for the SSC have shown anomalous ramp rate dependence, and it is thought that interstrand coupling may contribute to this effect. This article describes measurements made on cable samples which were prepared and measured under circumstances which approximate those which an actual magnet conductor will experience.Subjecting the cable to pressure during heat treatment and during measurement significantly increases the eddy current loss, but it is not altogether clear that the increase results primarily from interstrand coupling.
Flux jumping has been investigated in both cold-worked and heat-treated (1 h at 700°C) 0.025-cm-diam Nb+2S% Zr wire wound into solenoids and noninductive coils of approximate dimensions: i.d. =0.3 em, o.d.= 1.2 em, length=2.6 em. The measurements taken at 4.2°K were: (1) The induced voltage pulses vs applied magnetic field with both I =0 and a preset transport current. (2) The induced voltage pulses vs coil current for H =0 and with a preset uniform external field.For the case with no transport current in the coil (l =0) when the external field was increased smoothly from zero, a squence of very sharp voltage pulses (flux jumps) were induced in the coil, and recorded simultaneously on both an X -Y recorder and an oscilloscope. These flux jumps had the following general characteristics for all the cases discussed in this paper: (1) Starting from well defined initial conditions, the pulse always appeared at about the same place in the field sweeping cycle. (2) The polarity of the pulses is reversed when the direction of field change is reversed. (3) The pattern of these pulses was essentially unchanged when the field sweep rate was increased from 7 to 340 G/sec.As a result of these measurements, we have obtained new insight into both the multilevel transitions of noninductive coils and the phenomenon known as degradation of solenoids. Two successful methods are described for partially overcoming the excessive degradation of a solenoid made from heat treated wire. The flux jumps are interpreted according to an extension of a model proposed by both Bean and London.
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