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oai:www.ideals.illinois.edu:2142/23740 2023-07-10 col_2142_5131 col_2142_8859 com_2142_5130 com_2142_8858 com_2142_234

Simmons, R.O. Thorpe, John Clifton 2011-05-07T14:25:19Z 2011-05-07T14:25:19Z 10000-01-01 1991 Activation energies of thermal vacancies have been determined in hcp $\sp4$He 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 cm$\sp3$/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 $\sp4$He. The lower bounds on the activation energies from the relative lattice parameter measurements are consistent with those determined from the pressure. Below 20 cm$\sp3$/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. Made available in DSpace on 2011-05-07T14:25:19Z (GMT). No. of bitstreams: 2 license.txt: 4922 bytes, checksum: 910b249b4beec47e7ab768910c8f966f (MD5) 9124500.pdf: 5502854 bytes, checksum: b1fb5958d00316f576197e07092f93f8 (MD5) Previous issue date: 1991 Item marked as restricted to the 'UIUC Users [automated]' Group (id=2) by Howard Ding (hding2@illinois.edu) on 2011-05-07T15:06:31Z Item is restricted indefinitely. Restriction data tranferred 2014-07-01T11:31:57-05:00 Original Data Group with Access UIUC Users [automated] Release Date: none Reason: ETDs are only available to UIUC Users without author permission ETDs are only available to UIUC Users without author permission U of I Only AAI9124500 (UMI)AAI9124500 http://hdl.handle.net/2142/23740 eng Copyright 1991 Thorpe, John Clifton Physics, Condensed Matter Thermal vacancies in HCP helium-4 text Physics, Condensed Matter Physics, Condensed Matter University of Illinois at Urbana-Champaign Dissertation Ph.D.