On Some Mechanical Properties of Superconductors
Measurements have been made of the Young’s modulus E and the internal friction A of polycrystalline Pb and Sn at liquid helium temperatures. Use was made of a 45 kc/sec composite oscillator consisting of an X-cut quartz crystal to which was cemented an appropriately matched length of the specimen under investigation. Two measurements of the electric impedance of such a vibrating system, one at resonance and the other in the neighborhood of resonance, are sufficient to determine E and Δ for the metal. It was found for both Pb and Sn that the value of Δ increases with the vibrational strain amplitude ε, while E decreases. Moreover, at moderate vibration amplitudes, both E and Δ increase when the specimen makes the transition from super-conducting to normal states. In the case of well-annealed Pb, the changes in E and Δ, that occur as a result of the transition, are independent of ε for ε <1.0 × 10–6. As ε → 0, the value of Δ is small enough that the fractional change approaches 30%. For unannealed Pb, the values of Δ are much larger. However, the absolute value of the changes in E and Δ are approximately the same as in an annealed specimen.