G. Bäckström

Thermal conductivity of solids and liquids under pressure

Thermal conductivity of solids and liquids under pressure is covered in this review. Experimental techniques are critically considered and compared, and an introduction to theory is provided. Results are presented and discussed for ionic crystals, normal molecular crystals, plastic crystal phases, clathrate hydrates, polymers and glass-formers, liquids, covalent and semiconducting crystals, rocks and metals. Special attention is given to isochoric conditions, change of crystal structure and molecular orientational disorder. Available reliable measurements at pressures up to a few GPa indicate the need for theoretical development, especially in connection with molecular crystals and ferromagnetic metals.

Improved hot‐wire procedure for thermophysical measurements under pressure

A new and simplified version of the circuitry for the transient hot‐wire method is presented. The circuitry provides a wide range of currents allowing probe wires of various diameters to be used in order to match the thermal properties of the specimen to be investigated. The analysis of the temperature increase during the heat pulse is based on the exact solution for a heated wire immersed in a medium. Data are corrected for varying power. The method was tested by computer simulations and by measurements of the thermal conductivity (λ) and the heat capacity per unit volume (ρcp ) of glycerol at room temperature and atmospheric pressure, and for CsCl and NaCl at room temperature and at pressures up to 2 GPa. The results on glycerol and CsCl are in excellent agreement with previous works. The inaccuracy in λ and ρcp is estimated as 1%–2% and 3%–5%, respectively, but the standard deviation of the measurements is as low as 0.2% for λ and 1% for ρcp. The improved procedure makes it possible to detect systemati...

Thermal conductivity of solids under pressure by the transient hot wire method

An electronic circuit has been constructed which delivers constant power to a nickel wire, also functioning as a resistance thermometer. The temperature increase versus time, t, is linear in ln t, and the coefficient of slope directly yields the thermal conductivity of the medium surrounding the wire. Systematic errors due to finite time, wire length, and lateral extension of the medium and due to plastic deformation of the wire are analyzed. Temperatures are sampled automatically, and the digital data are reduced by means of an on‐line desk calculator. The thermal conductivities of AgCl and polytetrafluoroethylene have been measured up to 10 kilobar.

Heat capacity and thermal conductivity from pulsed wire probe measurements under pressure

If a metal wire immersed in an electrically insulating medium is heated by a known constant power, the thermal conductivity of the medium may be deduced from the temperature variation of the wire. It is shown that further analysis of this variation also permits the specific heat capacity or the thermal diffusivity to be determined. A nickel wire is used both as a heater and as a temperature sensor. The power is kept constant by electronic means, and the resistance measured by a four-probe technique. Eight temperature values are recorded during the power pulse which has a typical duration of 1 s. This extended method has been tested by studying glycerol in its amorphous and crystalline states in the range 130-300K and at pressures of up to 0.8 GPa.

Thermal conductivity and heat capacity of solid phases of benzene under pressure

Using the transient hot-wire method, simultaneous measurements of the thermal conductivity, λ, and the heat capacity per unit volume, ρc p, were made for solid phases I and II of benzene (C6H6). Data were obtained over the temperature range 105 K to 375 K, and at pressures up to 2·4 GPa. At 300 K, the I → II transition was sluggish, so phase I could be retained over a pressure range where it was metastable with respect to phase II. At about 1·2 GPa and 375 K, λ for phase II is detectably greater than for phase I, but at 300 K the two values are almost equal. The value of c p for phase I was found to decrease by about 10 per cent over the range 0–1 GPa and to remain essentially constant at higher pressures. Values of c p in phases I and II were found to be equal at all temperatures. The temperature and volume dependences of λ are discussed in relation to current theoretical ideas.

Dielectric relaxation of glycerol and n-propyl alcohol at high pressure

The dielectric relaxation of glycerol and n-propyl alcohol in the supercooled liquid state has been investigated at frequencies from 5 Hz to 13 MHz and at pressures up to 1 GPa. The high-pressure dielectric cell and the measurement procedures are described in detail. A significant broadening of the loss peak is observed for both liquids with increasing pressure. The results are discussed in terms of a many-body relaxation theory which accounts for increasing correlation of the relaxing dipoles at high pressure.

A low-temperature high-pressure apparatus with a temperature control system

A low-temperature high-pressure apparatus was designed using commercial cryogenic equipment. Pressures up to 1 GPa and temperatures down to 40 K can be obtained in a volume of up to 30 cm3. The app ...

Thermal conductivity of crystalline and glassy crystal cyclohexanol under pressure

The thermal conductivity λ of phases liquid, I, II, III and the glassy crystal state of cyclohexanol has been measured using the transient hot-wire method at temperatures in the range 100–370K and at pressures up to 0·7 GPa. It was inferred from the observed temperature dependence of the thermal resistivity W (= λ-1) that molecular orientational disorder made a dominant contribution to the resistivity for the glassy crystal state under isobaric conditions. The same was found for plastic crystal phase I under isochoric conditions and also under isobaric conditions especially at the highest pressure. It was suggested that conformational disorder may make an important contribution to W for molecular crystal phases II and III.

Crystallization temperature of amorphous Fe80B20 under pressure

Abstract The resistance of rapidly quenched Fe 80 B 20 was recorded versus T at a rate of 7 K/min and various pressures up to 2.5 GPa. The onset of crystallization, T x , was defined by extrapolation at the sharp change in resistance associated with crystal growth. The resulting slope of T x is 15 K/GPa. Separate measurements at atmospheric pressure yielded T x versus rate of temperature increase, R = d T /d t . crystallization times from the literature were recalculated to yield T x , and the results are compared with our data. Nominally amorphous materials of the same composition show markedly different properties. It is also shown that the variation of T x with R is mostly due to the change in viscosity.

Pressure dependence of the thermal conductivity, thermal diffusivity, and specific heat of polyethylene

The pressure dependence of the specific heat of high‐density polyethylene was determined from simultaneous measurements of thermal conductivity and diffusivity in a cylindrical geometry at 300 K and in the pressure range 0–25 kbar. The thermal conductivity and the diffusivity both increase strongly with pressure, the values at 25 kbar being higher by factors of 2.75 and 2.77, respectively, than those at atmospheric pressure. The specific heat decreases, most strongly so at low pressures, and its value at 25 kbar is 0.80 times that at atmospheric pressure. The thermal conductivity of low‐density polyethylene was determined as a function of pressure. Its value at 25 kbar was found to be 2.19 times that at atmospheric pressure. The difference in conductivity response to pressure between the two varieties is given a simple theoretical interpretation.