J. P. Harrison

Review paper: Heat transfer between liquid helium and solids below 100 mK

The many experimental determinations of heat transfer between liquid helium and solids are reviewed and compared with the existing theories. Generally heat transfer is a complex process at low temperatures, involving parallel heat paths with several resistances in series in each path. The standard theories for the individual resistances are reviewed and as far as possible a definite value of the expected resistance obtained. The experiments have been considered in five groups: cerium magnesium nitrate in liquid3He, cerium magnesium nitrate in dilute3He in liquid4He, metals in liquid3He, metals in dilute3He in liquid4He, and miscellaneous experiments. Where appropriate, the experimental results have been reevaluated in terms of standard models for heat transfer and presented as resistance versus temperature on diagrams showing the theoretical predictions. Between 20 and 100 mK many measurements show good agreement with acoustic mismatch theory for thermal boundary resistance. Below 20 mK most experiments have been made with finely divided solids (sintered metals or powdered paramagnetic salts); invariably the resistance has been anomalously low, with the metal/3He systems showingR ∫ T−1 and the metal/dilute3He in4He systems showingR ∫ T−3. The cerium magnesium nitrate/helium experiments have shown a temperature—independent resistance and sometimes anR ∫ T~1 dependence. These resistances have been attributed to the spin-lattice resistance in cerium magnesium nitrate. Evidence for heat transfer by magnetic coupling has been reviewed and it is concluded that the positive evidence has other explanations, while the lack of dependence upon helium pressure,3He phase, and large magnetic fields is strong negative evidence. If the disagreement of experimental results with standard theory is not to be attributed to magnetic coupling, then several theoretical questions remain to be answered; these questions are posed. Basically, at low temperatures the excitations in solids and liquid helium have wavelengths and mean free paths much larger than the size of the finely divided particles or pores between the particles. Thus theories for bulk solids and bulk liquid helium are not appropriate for describing excitations and their interactions at these low temperatures.

Wind Turbine Noise

Following an introduction to noise and noise regulation of wind turbines, the problem of adverse health effects of turbine noise is discussed. This is attributed to the characteristics of turbine noise and deficiencies in the regulation of this noise. Both onshore and offshore wind farms are discussed.

Critical temperature and critical current of thin-film superfluid3He

Superfluid3He film flow over the rim of a copper beaker has been measured. The flow rate was measured as a function of temperature and as a function of depth of3He below the rim or film thickness at the rim. The critical current, calculated from the flow rate, varied as (1 −T/Tcp)3/2 as expected for pair-breaking;Tcp is a film-thickness-dependent critical temperature. However, the magnitude of the current was an order of magnitude smaller than expected for pair-breaking, in agreement with other experiments that have demonstrated a lower dissipation mechanism in superfluid3He. The suppression of the critical temperatureTcp/Tcb, whereTcb=0.93 mK is the bulk3He transition temperature, varied from 0.93 to 0.7 as the film thickness at the rim varied from 120 to 90 nm. These ratios are larger than expected from Ginzburg-Landau or microscopic theory of superfluid3He-B.

The vibrating wire viscometer as a magnetic field-independent3He thermometer

An analysis of the semicircular loop, vibrating-wire3He viscometer has been made. Emphasis was placed upon the magnetic field dependence of the resonance frequency and frequency width (damping) in order to study the suitability of the device for a field-independent thermometer. The analysis was complemented by experiments with Ta, Nb-Ti, Cu, Ag, Au, and Cu-Ni wires. It was found that the various wires, whether superconducting or pure or impure normal metals, have advantages and disadvantages. The superconducting wires show a large field-dependent frequency shift due to induced magnetization and a field-dependent damping due to flux motion. Pure normal metals also show induced magnetization (eddy-current) frequency shift and damping, and impure metals and alloys give Joule heating. Nevertheless, it is shown that the device can be used in the range 1–10 mK and for fields from 20 mT to 1 T.

Specific heat, electric susceptibility and thermal conductivity of praseodymium ethyl sulphate: A one-dimensional electric dipolar XY system

Specific heat and electric susceptibility measurements on praseodymium ethyl sulphate have shown that down to 0.06K it is well described by the one-dimensional XY model with the interaction between nearest neighbour pairs of praseodymium ions arising from electric dipole-dipole coupling. A comparison of the experimental data and the theoretical expression for electric susceptibility gives the nearest neighbour pair interaction parameter, J, equal to kB*0.76K. The electric dipole moment of the Pr3+ ion was found to be confined to the XY plane and was of magnitude gamma =7.8*10-31 C m; the consequent electric dipole-dipole interaction parameter between nearest neighbours was kB*1.12K, in reasonably good agreement with the J-value determined from electric susceptibility.

Heat transfer between liquid3He and sintered metal heat exchangers

A new model to explain the unexpectedly large heat transfer between liquid3He and sintered metal heat exchangers is described and evaluated. The heat transfer results from a direct coupling of3He quasiparticles in the pores to vibrational modes of the sintered metal powder. It is proposed that for a range of temperatures below 20 mK the dominant vibrational modes of the sinter are localized oscillations involving a few powder particles with frequencies distributed over a constant density of states. The3He is then treated as a Fermi gas in a set of boxes corresponding to the pores in the sinter. The vibrating or shaking boxes transfer energy to the3He quasiparticles inside the box. The calculated heat transfer between liquid3He and the vibrational modes of sintered metal heat exchangers isQ/(VΔT)∼4×10−15T/d3 W m−3 K−1, where Q is the heat flow for a temperature differenceΔT, V is the volume of the sinter (metal and helium), andd is the powder particle diameter in meters; this has the observed linear temperature dependence, and in magnitude is larger than or comparable to published results from several laboratories. The heat transfer between the vibrational modes and the electron gas in the metal sinter that is needed to complete the heat path can also be described by the same model with the result that the electron-“phonon” coupling is significantly larger than the3He-“phonon” coupling. When the model is applied to heat transfer between liquid3He-4He mixtures and sinter the calculated results are again comparable to or larger than those measured. The postulated localized oscillator modes give a specific heat, linear in temperature, that is in reasonable agreement with measurements for pressed powder by Pohl and Tait.

Demonstration of Temperature Dependent-Damping of a Vibrating System by Superconducting YBa2Cu3O 7-x

A major problem associated with superconducting magnetic bearings is the damping of vibration. A demonstration is given of the use of a dissipation peak due to flux line movement in YBa2Cu3O7-x to provide temperature-dependent damping of vibration of a mechanical oscillator. Scaling of the present results shows that it is possible to decrease the resonance Q of a 1 kg oscillator to 10 with just 0.02 kg of melt-processed YBa2Cu3O7-x . The proposed superconducting damper is distinct from the levitation superconductor in the magnetic bearing.

Thermal resistance between phonon and quasiparticle excitations in dilute mixtures of3He in4He

As part of a Kapitza resistance study, the thermal resistance between the phonon and3He quasiparticle excitations in three dilute3He-4He mixtures (0.03%, 0.1%, and 0.3%3He) has been measured over the temperature range 20–50 mK. The results are compared with a theory based upon the Baym-Ebner interaction between the excitations. The temperature dependences agree, but the measured resistance is smaller than the theoretical resistance by a factor of ∼2 that varies inversely with concentration. This concentration-dependent phonon-quasiparticle coupling discrepancy is consistent with the results of recent heat pulse absorption experiments by Husson et al. An indirect measurement was made of the same thermal resistance in3He-4He mixtures confined within the pores of a submicrometer sintered copper powder, with the results confirming a size-effect model for the resistance.

Phonon reflection at a silicon-3He interface

The reflection of heat pulses at an interface between the (111) surface of silicon and liquid 3He has been measured for heat pulse temperatures from 1.4 to 9K. The measurements were made with the silicon-helium temperature equal to 0.4 or 1.8K. The detectors were cadmium or aluminum superconducting bolometers. Longitudinal and transverse phonons incident nearly perpendicularly on the surface had reflection coefficients of 1 and 0.8, respectively, independent of pulse temperature. Transverse phonons incident at a large angle and diffusely reflected had reflection coefficients dropping from 0.7 at 1.5K to 0.4 at 9K. The results are in broad agreement with previous measurements on solid-4He interfaces obtained from heat pulse and monochromatic phonon experiments.

3He Film flow: 4He substrate coating effect

Abstract Superfluid 3He film flow out of a copper beaker was measured without and then with a 4He coating on the copper surface. The effect of the 4 He was to convert the substrate from a purely diffuse to a purely specular surface for the 3He quasiparticles.