Robert W. Reed

Fluxoid pinning by vanadium carbide precipitates in superconducting vanadium

Vanadium carbide precipitates were formed in pure, annealed vanadium foils by the introduction of carbon in the specimens. Thin, disk-shaped precipitates resulted with mean diameters in the range 100–2600 Å and with number densities from 3 × 1015 to 4 × 1017 particles/cm3. The macroscopic, pinning-force density for magnetic fluxoids was measured at temperatures from 2 to 5 K and for magnetic fields from 0 to Hc2(T). Peak pinning-force densities in the range of 3 × 10 to 3 × 106 dyn/cm3 for T=0K were realized in the 30 specimens studied. The pinning force density was found to obey a scaling law for specimens meeting certain requirements with respect to precipitate particle size and number density. These requirements correlate with the temperature-dependent, superconducting coherence length ξ(T). Many specimens obeyed the scaling law at temperatures T < Tc except near Tc, where ξ(T) is large in comparison with the precipitate size.

The scattering of ultrasonic third sound from substrate surface defects

The interaction of third sound with isolated surface defects is examined. Phase-inverted reflections are observed to occur at surface defects even when the wavelength of the third sound is much greater than the dimensions of the defect. The origin of the scattering mechanism is believed to be due to the change in the helium film thickness in the vicinity of the topological variations introduced by the presence of the surface defects. Data are obtained for ultrasonic frequencies in the 20–200 kHz range.

The attenuation and phase velocity of ultrasonic third sound for optically smooth and roughened substrates

The attenuation and phase velocity of third sound have been measured for propagation on optically smooth and roughened substrates and for ultrasonic frequencies in the 20–200 kHz range. The attenuation for both kinds of substrates is found to be proportional to the frequency and have a magnitude that increases with the amount of roughening. The data are found to be inconsistent with a number of proposed attenuation mechanisms. The phase velocity is nearly independent of the frequency. Pulse wave shapes change substantially as the third sound propagates along a substrate surface.

Torsional velocity measurements in wire, with application to metal‐matrix composites

A technique is described for measuring torsional wave velocity in (nonferromagnetic) wire with diameters of less than 1 mm. Transduction is noncontacting, via Lorentz forces acting across a gap between transducer and wire, so that velocity can be ’’scanned’’ along the length of the wire. A double‐receiver approach permits the scan to be made in a ’’point‐by‐point’’ fashion. The wires are a metal‐matrix composite of 6061 aluminum and pitch mesophase graphite. Some data are presented, together with a discussion of the effects of metal‐matrix‐composite parameters, such as graphite content and porosity, on torsional velocity. An extrapolation of our metal‐matrix‐ composite data yields a value for c44, of the graphite fiber, that was within 1% of a value determined from data of Fischbach and Scrinivasagopalan. Good predictions of experimental results were obtained through use of theory of Hashin and of Achenbach and Herrmann. Some differences, and their implications, between torsional and longitudinal waves pr...

Control and sampling circuitry for the synchronous detection and measurement of ultrasonic pulse echoes

Synchronous detection of ultrasonic pulse echoes with a lock‐in amplifier can be used to obtain improved signal‐to‐noise ratio in the measurement of ultrasonic attenuation. It is our purpose to present circuitry that can be used for the generation of the necessary timing, trigger, and reference signals. In addition, a high‐speed sample and hold circuit, used to convert low‐duty‐cycle pulse echoes into the square‐wave signals needed for operation of a lock‐in amplifier, is described. The apparatus described has been used for attenuation measurements over the 10–1400‐MHz range in both single‐ and double‐ended sonic sample‐transducer configurations.

Ultrasonic attenuation in normal and superconducting indium

Measurements have been made of the ultrasonic attenuation in the normal and superconducting states of pure In single crystals. The measurements spanned the frequency range of 6–66 MHz. The data, taken in the amplitude-independent regime, displayed the expected deviations from the BCS-type attenuation which are generally attributed to dislocation attenuation as described by Granato and Lücke. Earlier attempts to use this theory to study the amplitude-independent dislocation attenuation of ultrasound in superconductors (e.g., Mason) were generally limited to very narrow frequency ranges. These earlier results were in general agreement with the Granato and Lücke theory. However, the present multiple-frequency measurements are shown to be inconsistent with the Granato and Lücke theory.

Ultrasonic determination of the superconducting energy gap in high-purity tantalum

Ultrasonic attenuation measurements as a function of temperature were made for sound propagating along the [100], [110], and [111] crystal directions of Ta single-crystal specimens. Sound frequencies from 180 to 500 MHz were used with single crystals having residual resistance ratios up to 3866. For the three propagation directions used, the superconducting energy gap had an average value of 1.77kTc, which was within the experimental error for the three separate values of the gap parameter.

An Amplifier Gain Control for Ultrasonic Pulse‐Echo Measurements

An automatic gain control (AGC) for maintaining constant gain of the i.f. amplifiers in an ultrasonic pulse‐echo system is described. The AGC stabilizes the receiver against long‐term drifts. It is applicable if the duty cycle of the sonic‐echo train is small (5% or less) and the rest of the cycle is available for introduction of a reference signal. The AGC samples the amplified reference signal and keeps its level constant, thereby maintaining constant gain in the i.f. amplifiers without the necessity for using an additional gated integrating amplifier. The AGC improved the reliability of measurements of sonic‐echo voltage from ±3% without the AGC to ±0.5% with the AGC over a long period of time. The total ultrasonic system for attenuation measurements in the range 20–500 MHz, including the AGC, is described.

Quantitative Attenuation and Velocity Measurements in Metal Matrix Precursor Wires

The nondestructive evaluation of metal matrix composite precursor wires is being pursued by measurements of the attenuation and velocity of both torsional and longitudinal ultrasonic pulses propagating along the wire axis. The sound waves are generated by non-contacting electromagnetic transducers. Continuous scans of the attenuation and velocity are made along wires of any length by use of a four-transducer arrangement. The attenuation and velocity have been related to physical wire properties which are important for nondestructive evaluation and characterization.

Ultrasonic Measurements of Elastic Moduli of Thermally Cycled Metal Matrix Composite Precursor Wires

Metal matrix composite structures are often made by the consolidation of precursor wires typically on the order of 1 mm in diameter. The elastic and mechanical properties of the consolidated parts depend critically on the elastic and mechanical properties of the precursor wires. The consolidation process involves hot pressing at elevated temperatures. The final end-item products may also be subjected to elevated temperatures in their normal use cycles.