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Simmons, R.O. Thorpe, John Clifton 2011-05-10T14:58:46Z 2011-05-10T14:58:46Z 10000-01-01 1991 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. Submitted by Carolyn Mead (cmead2@illinois.edu) on 2011-05-10T14:58:46Z No. of bitstreams: 1 1991_thorpe.pdf: 5929111 bytes, checksum: dc81626d472c121dc925a216a8b19a4c (MD5) Made available in DSpace on 2011-05-10T14:58:46Z (GMT). No. of bitstreams: 1 1991_thorpe.pdf: 5929111 bytes, checksum: dc81626d472c121dc925a216a8b19a4c (MD5) Previous issue date: 1991 Item marked as restricted to the 'UIUC Users [automated]' Group (id=2) by Carolyn Mead (cmead2@illinois.edu) on 2011-05-10T14:58:46Z Item is restricted indefinitely. Restriction data tranferred 2014-07-01T11:12:34-05:00 Original Data Group with Access UIUC Users [automated] Release Date: none Reason: Thesis Thesis U of I Only 3476222 http://hdl.handle.net/2142/23861 en 1991 John Clifton Thrope thermal vacancies hcp helium phonon pressure Thermal vacancies in hcp helium-4 Dissertation / Thesis text Physics Physics University of Illinois at Urbana-Champaign Dissertation Ph.D.