P. Stachowiak

LOW TEMPERATURE THERMAL CONDUCTIVITY OF CARBON MONOXIDE

Thermal conductivity of solid CO was investigated over the temperature range 1.3–37 K by steady state flow method. The thermal conductivity coefficient reaches its maximum value 28 mW/cm K at about 6 K. Problem of the dipolar ordering at low temperatures has been discussed.

Low-temperature thermal conductivity of solid carbon dioxide

Preliminary measurements of the thermal conductivity of pure carbon dioxide in the temperature range 1.5–35 K are reported. The first data below 25 K have been obtained. The thermal conductivity reaches very high values, about 700 W/(m⋅K), which is unusual for simple molecular crystals. A straightforward analysis of the data shows a coarse-grained sample.

The peculiarities of heat transfer in CO2 and N2O solids at low temperatures

The thermal conductivities of CO2 and N2O solids are investigated in the low-temperature range 1–40K. The thermal conductivities of CO2 and N2O are large compared with those of simple molecular crystals such as N2, CO, or O2 in the whole investigated temperature range. Analysis of the experimental data by the Callaway method shows that a relatively large size of the crystalline grains, low density of dislocations, and weak phonon–phonon interaction might be the reasons for the good thermal conduction in these crystals at temperatures near the maxima. A comparison between calculated values of the intensity of normal phonon scattering processes and experiment gives evidence that in N2O there is an additional (in comparison with CO2) giant scattering of phonons. This scattering is described in the frameworks of the soft potential model by the resonance phonon scattering on tunneling states and low-energy vibrations.

Thermal conductivity of bulk GaN single crystals

Abstract We have measured thermal conductivity, κ, in the wide temperature range 4– 300 K of GaN bulk single crystals grown by high-pressure, high-temperature synthesis. Obtained results ( 1600 W/Km at 45 K ) are the highest κ values reported on GaN material. At the room temperature κ is about 210 W/Km . The contributions to the GaN thermal resistance of Umklapp process, mass point defects as well as phonon scattering on dislocations and sample boundary are discussed.

Observation of relaxation of molecular spins in CH4 and CD4 crystals in thermal conductivity experiment

Preliminary results are reported on the kinetics of the molecular spin conversion in solid methane (CH4) and deuterated methane (CD4), obtained through thermal conductivity measurements in the temperature range 2–10K.

Temperature dependence of the magnetic susceptibility of solid oxygen

The temperature dependence of the magnetic susceptibility of polycrystalline oxygen samples is measured in the range from 1to54K. The measurements are performed using our home-designed SQUID-based gradiometer. The sensitivity of the instrument with respect to susceptibility reaches values of the order of 10−13. The results are compared with data in the literature.

Thermal Characterization, Crystal Field Analysis and In-Band Pumped Laser Performance of Er Doped NaY(WO4)2 Disordered Laser Crystals

Undoped and Er-doped NaY(WO4)2 disordered single crystals have been grown by the Czochralski technique. The specific heat and thermal conductivity (κ) of these crystals have been characterized from T = 4 K to 700 K and 360 K, respectively. It is shown that κ exhibits anisotropy characteristic of single crystals as well as a κ(T) behavior observed in glasses, with a saturation mean free phonon path of 3.6 Å and 4.5 Å for propagation along a and c crystal axes, respectively. The relative energy positions and irreducible representations of Stark Er3+ levels up to 4G7/2 multiplet have been determined by the combination of experimental low (<10 K) temperature optical absorption and photoluminescence measurements and simulations with a single-electron Hamiltonian including both free-ion and crystal field interactions. Absorption, emission and gain cross sections of the 4I13/2↔4I15/2 laser related transition have been determined at 77 K. The 4I13/2 Er3+ lifetime (τ) was measured in the temperature range of 77–300 K, and was found to change from τ (77K) ≈ 4.5 ms to τ (300K) ≈ 3.5 ms. Laser operation is demonstrated at 77 K and 300 K by resonantly pumping the 4I13/2 multiplet at λ≈1500 nm with a broadband (FWHM≈20 nm) diode laser source perfectly matching the 77 K crystal 4I15/2 → 4I13/2 absorption profile. At 77 K as much as 5.5 W of output power were obtained in π-polarized configuration with a slope efficiency versus absorbed pump power of 57%, the free running laser wavelength in air was λ≈1611 nm with the laser output bandwidth of 3.5 nm. The laser emission was tunable over 30.7 nm, from 1590.7 nm to 1621.4 nm, for the same π-polarized configuration.

Thermal conductivity of argon-SiO2 cryocrystal nanocomposite

The effective thermal conductivity of samples of cryocrystal nanocomposite obtained from argon and SiO2 nanopowder was determined in the temperature interval 2–35 K using the steady-state method. The thermal conductivity of crystalline argon with nanoparticles of amorphous silica oxide embedded in its structure shows a weak dependence on particle linear dimension in the interval 5–42 nm. The temperature dependence of the thermal conductivity of the nanocomposites can be well approximated by taking into account only the two mechanisms of heat carrier scattering: phonon-phonon interaction in U-processes and scattering of phonons by dislocations.

Investigations of thermal conductivity of simple van der Waals crystal-based nanocomposites

The experimental setup for obtaining and determination of the thermal conductivity of simple van der Waals crystal-based nanocomposites is described. Preliminary thermal conductivity results of measurements carried out in the temperature range 1–40 K on two samples of methane crystals containing nanoparticles of hydroxyapatite are presented. These results confirm usability of the setup and its suitability as a proper experimental method for investigations of the thermal conductivity of the nanocomposites.

Structure and thermal conduction in solid O2 with nonmagnetic impurities

Thermal conductivities of solid oxygen doped with nonmagnetic impurities of nitrogen and argon were measured in the 1–35 K. The antiferromagnetic α-O2 doped with this impurities exhibits a strong thermal conductivity anomaly, which is assumed to be a result of supperpression of the magnon contribution to the heat transport by impurities. At the same time, the jump in the thermal conductivity at the α-β transition and th thermal conductivity in the β-O2 are insensitive to the doping effect, which we ascribe to strong magnetostriction effects. The halfwidth of the x-ray reflexes indicates that doping with Ar and Kr enhanced magnetostriction effects while doping with N2 weakens.