F. Ogasawara

Temperature Dependence of Sound Velocity in High-Strength Fiber-Reinforced Plastics

Longitudinal sound velocity in unidirectional hybrid composites or high-strength fiber-reinforced plastics (FRPs) was measured along the fiber axis over a wide temperature range (from 77 K to 420 K). We investigated two kinds of high-strength crystalline polymer fibers, polyethylene (Dyneema) and polybenzobisoxazole (Zylon), which are known to have negative thermal expansion coefficients and high thermal conductivities along the fiber axis. Both FRPs had very high sound velocities of about 9000 m/s at low temperatures and their temperature dependences were very strong. Sound velocity monotonically decreased with increasing temperature. The temperature dependence of sound velocity was much stronger in Dyneema-FRP than in Zylon-FRP.

Facet Growth of 4He Crystal Induced by Acoustic Waves

Very fast growth of the c-facet of a 4 He crystal was induced by acoustic waves. The growth velocity was larger at lower temperatures and saturated below about 400 mK. The velocity was proportional to the acoustic wave power. This fast growth cannot be explained by the spiral growth mechanism for the known value of the step mobility. We developed a step multiplication model for high-power acoustic waves and found reasonable agreement with the observed temperature and power dependence of the growth velocity.

A Single Bubble Nucleated by Acoustic Waves in 3He‐4He Mixtures

We report the first study of a bubble nucleation in the dilute 3He phase of phase‐separated 3He‐4He liquid mixtures. When an acoustic wave pulse of 1 msec duration was applied to the dilute phase at saturated vapor pressure, a single bubble was nucleated on an active area of the piezoelectric transducer. We succeeded in observing fast motion of the bubble by using a high‐speed camera. We also observed bubbles nucleated in the 3He rich phase and the pure superfluid 4He by acoustic waves. Bubble shape in the dilute 3He phase was similar to that in 3He rich phase but quite different from that in pure superfluid 4He.

Solid‐Liquid Interface Motion of 4He Induced by Heat Pulse

Displacement of the solid‐liquid interface of 4He induced by a heat pulse of 2 ms duration was measured by a high‐speed camera. Either crystallization or melting was induced at low temperatures depending on whether the heat pulse was applied to the interface from the solid side or the liquid side. The heat pulse had qualitatively the same effect on the interface as acoustic waves reported in R. Nomura et al., Phys. Rev. B 70 054516 (2004). However, the effect was smaller and a larger power was needed to induce an interface motion than acoustic waves. Another difference between them is that the heat pulse induced no interface motion at all above 0.8 K, where acoustic waves induced melting.