James R. Gaines

Stress induced electrical noise in stainless steel

Stainless steel wires under tensile stress displayed narrow band peaks in the low frequency region of the power spectrum that were ∼104 times the background Johnson noise. The electrical response to stress was analyzed in terms of the root mean squared (rms) voltage (Vrms) of the noise signal over a 13 Hz bandwidth. The information obtained from such measurements is independent of other nondestructive testing methods and is therefore a useful probe of the stress-strain dynamics in metals and alloys. The rms voltage initially increases exponentially with the applied stress but subsequently decreases near the point of failure. This voltage is also approximately proportional to the square root of the sample resistance (Rs1/2). It is proposed that this noise signal is due to the fluctuations in the piezoresistivity as a result of stress induced reorientation of defects, similar to the Zener relaxation of solute atoms in a solid solution.

1/f NOISE IN Y1Ba2Cu3O7−δ SINGLE CRYSTALS

1/f noise experiments performed on Y1Ba2Cu3O7−δ single crystals indicate that (1) the normalized noise spectral density Sυ/V2 is much larger than that of metals; (2) the temperature dependence of Sυ/V2 differs from that of the resistivity; and (3) in the superconducting transition region, the behavior of 1/f noise is distinct from that observed in bulk and thin film copper oxide systems. Possible explanations for these observations including the important role played by the crystal morphology are discussed.

Nuclear relaxation of deuterium-tritium adsorbed onto silica aerogel

The longitudinal nuclear relaxation times of tritons in equilibrium deuterium–tritium (eD–T, actually D2–DT–T2 ) adsorbed onto amorphous silica aerogel has been measured from 4.2 to 23 K from 0.5 to 6.2 monolayers of coverage. Below 7 K, the relaxation time dramatically increases with decreasing temperature, especially for low coverages. A value of 16 s for 0.5 monolayer at 4.2 K may be compared with the usual electric quadrupole–quadrupole (EQQ) determined relaxation time of 0.1 s. It is shown that absorption of 10% of the tritium beta particle energy occurs in aerogel fully loaded with D–T. This decreases the concentration of J=1 T2 and increases the nuclear relaxation time. Similar energy absorption calculations in the thin layers shows that long relaxation times are expected, and that the measured values must be caused by spin–lattice relaxation with the aerogel surface. Both (temperature)−7 and exponential mechanisms are considered with no final decision being possible. Electric field gradients from ...

Ultrasonic measurements on silver(Ag)‐high Tc superconductor (YBa2Cu3O7−δ) composites at high pressures up to 0.4 GPa

The longitudinal and transverse velocities of various Ag‐YBa2Cu3O7−δ composite samples have been measured under high pressure in nitrogen gas (up to 0.4 GPa) using ultrasonic techniques. A systematic relationship between the elastic properties and the composition of these composites is observed. The sound velocities of pure YBa2Cu3O7−δ derived from our experimental data are in good agreement with previous results obtained on single crystalline and oriented polycrystalline samples