Thermal vacancies in hcp helium-4

Activation energies of thermal vacancies have been determined in hcp 4He as a function of molar volume. A sensitive strain gauge was used to measure the temperature dependence of the pressure in twenty crystals between 18.17 and 20.9 cm3/mole. Absolute lattice parameters at low temperatures were measured for many of these crystals. The temperature dependence of the relative lattice parameter was measured in selected crystals. This provided lower bounds on the vacancy activation energy. The activation energies determined here agree at all reported molar volumes with those from previous measurements of pressure in hcp 4He. The lower bounds on the activation energies from the relative lattice parameter measurements are consistent with those determined from the pressure. Below 20 cm3/mole the activation energies reported here are in disagreement with those from ion mobility, NMR, and plastic deformation experiments. The determination of vacancy activation energies from the measured pressure requires knowledge of the phonon pressure. Difficulties with the present understanding of the phonon pressure are discussed.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Physics

Year dc:date

2011

Author dc:creator

• Thorpe, John Clifton

Contributors dc:contributor

• Simmons, R.O.

• thermal vacancies
• hcp helium
• phonon pressure

Statement dc:rights

• 1991 John Clifton Thrope

Language dc:language

en

Identifier

3476222

OAI identifier oai:identifier

oai:www.ideals.illinois.edu:2142/23861