| 1. |
- Andersson, Ove, et al.
(author)
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Chirality and the thermophysical properties of molecular solid phases under pressure: (+)- and (±)-camphor
- 1990
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In: Molecular Physics. - : Taylor & Francis Group. - 0026-8976 .- 1362-3028. ; 70:6, s. 1065-1083
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Journal article (peer-reviewed)abstract
- Thermal conductivity λ, heat capacity per unit volume pcP and P-T phase diagrams are investigated for both (+)- and (±)-camphor. The transient hotwire method is used at temperatures in the range 100–430 K and at pressures up to 1·75 GPa. We detect solid phases I-IV for (+)-camphor and I′-V′ for (±)-camphor. We conclude that λ is independent of enantiomeric composition for plastic-crystal phase pairs I-I′, II-II′ and IV-IV′. Phase V′ also appears to be a plastic-crystal phase. Non-plastic phases III and III′ show a variation of λ with T that differ considerably from that expected for a simple crystal, with the greatest such difference being observed for the racemic compound III′. An unannealed specimen of the racemic compound III′ is probably chirally disordered and shows a variation of λ with T for T < 130 K, which may indicate phonon-assisted fracton hopping.
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| 2. |
- Andersson, Ove, et al.
(author)
-
Phase behavior and thermal conductivity of urea at pressures up to 1 GPa and at temperatures in the range 50–370 K
- 1994
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In: International journal of thermophysics. - : Plenum Publishing. - 0195-928X .- 1572-9567. ; 15:3, s. 513-524
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Journal article (peer-reviewed)abstract
- The thermal conductivity of the solid phases I and III of urea was measured at temperatures in the range 50-370 K for pressures up to 1 GPa. Phase III, previously detected only at pressures above 0.5 GPa, was observed here at low pressures (< 0.07 GPa) below about 230 K. Extrapolation of the I-III phase line indicates that phase III might be obtained at 218 K at atmospheric pressure and, consequently, that urea might exhibit two solid phases at atmospheric pressure. The temperature dependence of the thermal conductivity of both phase I and phase III could be described by the Debye model for thermal conductivity assuming phonon scattering by three phonon umklapp processes only. Despite a volume decrease at the I --> III transition, the thermal conductivity decreased by about 20%. Normally, thermal conductivity increases at a phase transition at which volume decreases. This rather unusual behavior of urea might be due to an increase in the nearest-neighbor distance at the I --> III transition.
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| 3. |
- Andersson, Ove, et al.
(author)
-
Thermal conductivity, heat capacity and phase diagram of cyclooctanol in liquid, solid and glassy crystal states under high pressure
- 1990
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In: Molecular Physics. - : Taylor & Francis Group. - 0026-8976 .- 1362-3028. ; 71:3, s. 523-539
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Journal article (peer-reviewed)abstract
- Using the transient hot-wire method, thermal conductivity and heat capacity per unit volume are measured for solid and liquid phases and glassy crystal states of cyclooctanol, and information is provided on the phase diagram under high pressure. A new solid phase (III) is detected and characterized as a normal crystal phase, whereas all other solid phases (I, II, IV, V) are characterized as plastic crystal phases. We find evidence that the plastic crystal phases I, II and IV could each be the source for a distinct glassy crystal state. It is argued for phase II that its possession of both a low thermal conductivity and a low dielectric permittivity could be accounted for by assuming restricted reorientational motion of the molecules. The unusual (although small) decrease of thermal conductivity observed through the glassy to plastic crystal transitions may indicate that phonons can couple to reorientational motion in the plastic crystal phases I, II and IV.
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| 4. |
- Andersson, Ove, et al.
(author)
-
Thermal conductivity of (+)- and (±)-camphor at pressures up to 0·5 GPa and temperatures down to 40 K
- 1992
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In: Molecular Physics. - : Taylor & Francis. - 0026-8976 .- 1362-3028. ; 76:2, s. 433-444
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Journal article (peer-reviewed)abstract
- The transient hot-wire method was used to investigate the thermal conductivity of (+)- and (±)-camphor in the temperature range 40–300 K and for pressures up to 0·5 GPa. Results for the thermal conductivity of crystalline (+)- and (±)-camphor exhibited maximum values at temperatures of 60 and 70 K, respectively, independent of pressure. Above the maxima, the thermal conductivity λ exhibited a less pronounced temperature dependence than the variation λ ∼T-1 which is predicted theoretically for perfect crystals. We showed that the Debye theory can describe the thermal conductivity of molecular crystals like (+)- and (±)-camphor when structural disorder is assumed to provide a significant contribution to the phonon scattering process. We investigated two samples of (±)-camphor of estimated purity 99·3% and 97% and both samples exhibited the same thermal conductivity within experimental inaccuracy.
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| 5. |
- Andersson, S. Peter, et al.
(author)
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Thermal conductivity and heat capacity per unit volume of poly(methyl methacrylate) under high pressure
- 1994
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In: International journal of thermophysics. - : Springer. - 0195-928X .- 1572-9567. ; 15:5, s. 949-962
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Journal article (peer-reviewed)abstract
- The thermal conductivity and heat capacity per unit volume of poly(methyl methacrylate) (25 and 350 kg · mol−1 in molecular weight) have been measured in the temperature range 155–358 K at pressures up to 2 GPa using the transient hot-wire method. The bulk modulus has been measured up to 1.0 GPa at 294 K and yielded a constant value g = 3.4 ± 0.3 for the Bridgman parameter. No dependence on molecular weight could be detected in the properties we measured.
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| 6. |
- Håkansson, Björn, et al.
(author)
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Effective thermal conductivity of binary dispersed composites over wide ranges of volume fraction, temperature, and pressure
- 1990
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In: Journal of Applied Physics. - : American Institute of Physics (AIP). - 0021-8979 .- 1089-7550. ; 68:7, s. 3285-3292
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Journal article (peer-reviewed)abstract
- The effective thermal conductivity of binary dispersed composites prepared from mixed and compacted powders of AgCl and low‐density polyethylene was measured over the entire range of volume fraction at temperatures in the range 100–330 K together with pressures up to 1.75 GPa. Measurements were made using the transient hot‐wire method. The ratio of the thermal conductivities of AgCl and LDPE was an adjustable parameter in the range 2–8 at the temperatures and pressures employed. The rigorous effective medium approximation (EMA) provided a good description for most of our results although some limits to its applicability were also indicated. An empirical theory due to Nielsen provided a significantly less successful description. Suggestions are made for an improved theoretical approach going beyond the limits of the EMA.
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| 7. |
- Kutcherov, Vladimir, et al.
(author)
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Experimental test of theories for the effective thermal conductivity of a dispersed composite
- 1992
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In: Journal of Applied Physics. - : American Institute of Physics (AIP). - 0021-8979 .- 1089-7550. ; 71:4, s. 1732-1736
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Journal article (peer-reviewed)abstract
- The effective thermal conductivity of binary dispersed composites prepared from mixed and compacted powders of NaCl and low-density polyethylene (LDPE) was measured over the entire range of volume fraction at temperatures in the range 120-320 K together with pressures up to 1.73 GPa. Measurements were made using the transient hotwire method. The ratio of thermal conductivities of NaCl and LDPE varied over the range 10-50 at the temperatures and pressures employed. Using our results (together with previous data for AgCl-LDPE composites), we tested theoretical predictions from both the effective-medium approximation and the real-space renormalization group approximation. The two approximations exhibited a comparable overall level of agreement with experiment and neither was successful in describing our data within experimental inaccuracy over the entire range of volume fraction. For both approximations, the extent to which agreement with experiment was achieved depended significantly on the volume fraction being considered.
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| 8. |
- Lundin, Anders, et al.
(author)
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Apparatus for equation-of-state measurements
- 1994
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In: High Temperatures-High Pressures. - 0018-1544 .- 1472-3441. ; 26:5, s. 477-496
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Journal article (peer-reviewed)abstract
- An apparatus for measurements of the equation of state (EOS) is described. The apparatus, of piston and cylinder type, is equipped with modern measureing techniques and can be operated over a broad temperature range, 150-450 K, and up to a pressure of 1.5 GPa. The apparatus can be used for determination of both isothermal bulk modulus and thermal expansivity. At room temperature, the inaccuracy in bulk modulus is about ±0.4% for a typical solid polymeric material. The inaccuracy in determination of thermal expansivity is about ±1% for the same type of material. EOS measurements on indium, cesium iodide, and urea are presented and it is shown how the apparatus can be used for accurate determination of first-order phase-transition pressures.
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| 9. |
- Lundin, Anders, et al.
(author)
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Glass transition in a polymer under pressure : isothermal bulk modulus and relaxation effects
- 1994
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In: High Temperatures-High Pressures. - 0018-1544 .- 1472-3441. ; 26:4, s. 385-391
-
Journal article (peer-reviewed)abstract
- A piston-cylinder pressure vessel and the piston displacement method have been used to measure relative volume, V(p)/V(0), versus pressure, p, at room temperature for an indium-encapsulated specimen of a polymer electrolyte based on poly(ethylene glycol). The bulk modulus, B, was deduced form the slope of a plot of p versus -ln[V/V(0)]. The quantity B(p) showing a sloping step at the glass transition and the low-pressure end of this step was taken to define the glass transition pressure pg. After a step change in pressure, the volume was observed to relax with a characteristic time τ which depended on pressure. The quantity τ(p) increased strongly as the glass transition was approached with increasing pressure. When the glass transition was traversed in a relatively rapid sequence of measurements a decrease in the value of pg was observed.
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| 10. |
- Ross, Russell G.
(author)
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Predicting the equation of state of condensed materials through use of low-pressure expansion and room-temperature compression data
- 1992
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In: Journal of Physics and Chemistry of Solids. - : Elsevier. - 0022-3697 .- 1879-2553. ; 53:7, s. 857-864
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Journal article (peer-reviewed)abstract
- Empirical prediction of the equation of state (EOS) over an area in pressure-temperature (p-T) space was attempted using primary data for low-pressure (p0) expansion and room-temperature (T0) compression together with various ansatz to effectively restrict the functional form which the EOS could have. Tests against experimental data for the EOS of a variety of substances at p-T co-ordinates which were not on the p0 isobar or T0 isotherm showed that some ansatz could successfully predict the EOS at a general p-T co-ordinate which was within the ranges of the primary data. However, success rate in prediction was only at about the 50% level and furthermore no systematics were detected which would enable prior selection of the appropriate ansatz to be applied to a specific substance. Theoretical models for the EOS which include a general assumption of one of the ansatz under consideration were therefore shown to be lacking in empirical justification. It was concluded that at present reliable knowledge of the EOS over an area in p-T space can only be obtained through direct experimental determination.
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