Thermal conductivity of neutron-irradiated MgO

The thermal conductivity of pure and neutron-irradiated MgO has been measured in the temperature range 0.4–80 K and for neutron doses up to 2×10 ¹⁹ n·cm^–2 . Resonance dips in the thermal conductivity vs. temperature curves are observed at 1 and 20 K. It is suggested that the high-temperature dip, which becomes more pronounced with increasing radiation dose, may be a quasilocalized mode resulting from the production ofF-type centers. The low-temperature dip, which fades with increasing neutron dose, may result from the presence of small aggregate centers which give rise to another set of quasilocalized modes. The experimental data for the pure and irradiated crystals are in agreement with calculated curves based on the Debye model of solids. A combined relaxation time for the calculated curves of the irradiated specimens includes a term for the defect scattering rate consisting of two resonance expressions of the Lorentzian form.Supported by the U. S. Atomic Energy Commission.     

热电材料β-Cu2-xSe热传导性能模拟研究

“声子液体”热电材料β-Cu2-xSe具有优异的热电转换效率,采用分子动力学模拟的方法研究其热传导性能,分析了类“液态”离子的扩散能力和材料导热性能的相关性,并探究了材料加工处理手段(掺杂和空位)对材料热传导性能的影响。结果表明:类“液态”离子扩散能力和导热系数存在极强的相关性,β-Cu2-xSe中Cu^+移动能力的增强会增加晶格的非简谐振动,从而强化了声子散射,导致材料导热系数的降低。掺杂和空位对“声子液体”热传导性能有不同的影响:材料内部存在空位时,Cu^+倾向于在晶格缺陷中移动,降低了与Se构成的固定亚晶格碰撞概率,造成声学支声子频率的下降,有效地降低了导热系数,提高了材料的热电转换效率;相比空位,掺杂对导热系数的影响不明显。     

Low temperature thermal conductivity of aluminum alloy 1200

The thermal conductivity of aluminum alloy 1200 was determined from 4.2 K to 160 K using a thermal conductivity integral method. This steady state method has been implemented in a cryostat having a cold finger cooled with liquid helium and nitrogen. These materials were considered to create thermal link for the Planck research satellite. Two samples are studied; the “as fabricated” 1200 alloy and the 1200 H19 (cold-drawn). As expected, the evolution of the thermal conductivity with temperature of both alloys follows the electronic thermal conductivity theory with a good accuracy below 60 K. At higher temperature, the thermal conductivity reaches a maximum then decreases as T^?n and finally remains constant due to the electron–phonon scattering. As expected, the thermal conductivity of the cold-drawn alloy, 1200 H19, is reduced compared to that of the 1200 alloy due to a higher concentration of defects in the metal.     

Low-Temperature Thermal Conductivity and Specific Heat of Plastically Deformed High-Purity Tantalum Single Crystals

The thermal conductivity and the specific heat of plastically deformed, high-purity tantalum single crystals have been measured together with an amorphous SiO₂ specimen in the temperature range between 50 mK and about 2 K. After plastic deformation, the thermal conductivity was reduced by a factor of more than 100 and had a magnitude comparable to that of the amorphous SiO₂ specimen. However, the specific heat measurements revealed a T³-relationship for the phonon contribution down to the lowest temperatures with a magnitude as in the case of undeformed crystalline solids. Thus, it must be concluded that the scattering of thermal phonons introduced by the plastic deformation has to be attributed to intrinsic properties of dislocations rather than to the interaction of phonons with tunneling systems. In the present paper the scattering mechanism is related to oscillations of geometrical kinks in non-screw dislocations.     

Thermal conductivity and electrical resistivity of pure polycrystalline cobalt in the temperature range 2.5–30 K

Thermal conductivity and electrical resistivity of 99.99% pure Co sample were measured in the temperature range 2.5–30 K. The annealing, procedure of the sample (either above or below Curie temperature), followed by cooling it down to room temperature at a slow cooling rate, caused an unexpected increase in its thermal resistivity and residual electrical resistivity, contrary to the results obtained for most pure metals. Co samples either not thermally treated or annealed consist only of a HI phase as proved by X-ray and electron diffraction analyses. The result, led to the conclusion that changes of grain structure and physical defects appearing in the Co at Curie temperature and at 690 K, when phase transitions take place, should be taken into account. The electron-magnon scattering, is significant in electrical conductivity but the electron-physical defect and impurity scattering plays a dominant role in thermal conductivity. The electron-physical defect and impurity scattering is elastic (validity of the Wiedemann Franz law)) as demonstrated by the value of _{th el} = 1.0, obtained in this work.     

Thermal conductivity of niobium in the purely superconducting and normal states

The thermal conductivity λ of four niobium samples has been measured between 1 and 10 K, both in the superconducting and normal states. The specimens differed in their crystal defect structures due to annealing at different temperatures (dislocations, grain boundaries) and, in one case, to subsequent fast neutron irradiation (dislocation loops). A procedure has been developed with which the electron and phonon contributions to the thermal conductivity can be separated with an accuracy not hitherto obtainable. All the samples proved to have the same energy gap at 0K:δ(0)=(1.95±0.02)kT _c . The phonon conductivity in the superconducting stateλ _p ^s has been compared with the formula of Bardeen, Rickayzen, and Tewordt extended for scattering mechanisms other than phonon-electron interaction. For the unirradiated samples at \({\text{T}} \lesssim 0.15T_{\text{c}} \) , λ _p ^s is proportional toT ², showing that dislocations are mainly responsible for the phonon scattering. The results are qualitatively in agreement with the theory of Klemens, giving a rough indication that the grain boundaries may be considered as arrays of line dislocations. Dislocation loops introduced by the neutron irradiation turn out to behave like clusters of point defects. A second consequence of the irradiation is an enhancement of the original dislocation scattering term.     

Thermal Conductivity and Viscosity in Dilute 3He-4He Mixtures Below 0.6 K

The effects of three-phonon processes on the phonon thermal conductivity and phonon viscosity in dilute ³He-⁴He mixtures below 0.6 K are investigated. The characteristic scattering times for phonon-phonon interaction-limited thermal conductivity and viscosity are calculated from a collision matrix which contains information on the three-phonon processes. We find that due to phonon-³He interactions the phonon momentum range in which three-phonon processes occur is reduced in comparison with that of the pure ⁴He case. Results for thermal conductivity and viscosity show good agreement with experimental data without having to adjust the rates of phonon absorption by ³He and the rates of elastic scattering of phonons with ³He.     

Phonon scattering and thermal conduction mechanisms of sintered aluminium nitride ceramics

From thermal diffusivity measurements of sintered AIN at temperatures ranging from 100 to 1000 K, the phonon mean free path of AIN was calculated in order to investigate phonon scattering mechanisms. The calculated mean phonon scattering distance was increased with decreasing temperature. The mean phonon-defect scattering distances were respectively limited to about 50 nm at temperatures ranging from 100 to 270 K and about 30 nm at temperatures ranging from 100 to 700 K, for AIN specimens with a room-temperature thermal conductivity of 220 and 121 Wm^–1 K^–1 containing 0.1 and 1.4 wt % oxygen, respectively. These short phonon-defect scattering distances were considered to correspond to the separation of oxygen-related internal defects in AIN grains. Calculation of the mean phonon scattering frequencies indicated that the phonon scattering is dominated by phonon-defect scattering at temperatures below 270 K for an AIN specimen with an oxygen content of 0.1 wt %, and at temperatures below 350 K for an AIN specimen with an oxygen content of 1.4 wt %.     

Thermal Conductivity of a Molecular Crystal with Rotational Degrees of Freedom: Orientational Defect Scattering

Measurements of thermal conductivity of solid methane-deuteromethane solutions at equilibrium vapor pressure in the temperature range 1.2÷20 K are reported. The obtained dependences of thermal conductivity on temperature and concentration can be explained qualitatively assuming that the dominant mechanism of phonon scattering is connected with the interaction of phonons with the rotational motion of the molecules in all of the three orientational phases of the CH₄-CD₄ system. The contribution of the orientational defect scattering to the thermal conductivity is discussed in frame of the model of local changes in the moments of inertia of molecules.      

Transport properties of Cu5SmSe4

The electrical conductivity, Hall coefficient, thermoelectric power, and thermal conductivity of Cu₅SmSe₄ have been measured at temperatures from 80 to 400 K, and its linear thermal expansion coefficient has been determined from X-ray diffraction data. The observed negative a-axis linear thermal expansion coefficient and anomalies in temperature-dependent transport properties of this compound are tentatively attributed to a transition of the samarium atoms from a divalent to a trivalent state.     

Effect of thermal coarsening on the thermal conductivity of nanoporous gold

The thermal conductivities of nanoporous gold (NPG) microwires annealed at different temperatures have been measured in the temperature range from 100 to 320 K. Considering the electron-surface scattering, the thermal conductivity is expected to increase with the increase of ligament diameter. However, the thermal conductivity of NPG microwire is found to decrease after thermal coarsening, and has a maximum value at around 250 K for the as-dealloyed sample. We suggest that the defects accumulating at a relatively high temperature and the reduction in defect spacing may cause these temperature behaviors of thermal conductivity. Taking into account the electron scattering on ligament surfaces and defects, a modified theoretical model for the thermal conductivity of nanoporous metal is proposed to agree with our experimental results.     

How big is the linear region of irreversible thermodynamics?

The driving forces of diffusion reach their largest values in thin films. Temperature and voltage gradients may be steep simply because the film is thin.The matter and energy fluxes of diffusion contain, in principle, a whole series of cross terms. Commonly, we take only first order terms and ignore those of second and higher order; we restrict ourselves to “small” temperature and voltage gradients, to “the linear region of irreversible thermodynamics”.But how big is “small”? The present paper proposes that the linear region of irreversible thermodynamics extends from complete equilibrium to limits in which the energy change in the carrier system (e.g. phonons or plasmons), over a distance of one mean free path along the energy gradient, is comparable with the total energy in the carrier system. If the energy is E and the mean free path is λ, the limit occurs when

$\text{λ▽E}\text{E→1}$

Within this limit, experiments in thermomigration and electromigration measure equilibrium properties. The “reduced heat of transport” of a crystal defect is rigorously identified as the thermal energy of the defect structure.     

Heat conduction in bismuth-antimony alloy single crystals between 4.2 and 300 K

Thermal conductivity measurements in the temperature range 4.2–300 K on three single crystals of bismuth containing 1.77, 3.2, and 9.33 at % antimony, respectively, are reported. In the low-temperature region, a change from purely phonon-phonon scattering process to a predominantly impurity scattering process was observed with the increase of antimony concentration. In the high-temperature region, the analysis of the results shows the dominance of the electronic contribution over the phonon contribution to the total thermal conductivity, though the latter is not negligible. Individual contributions to total electronic thermal conductivity were calculated, using the equation formulated by Price, for the sample containing 9.33 at % antimony. The agreement between (K _E)_calc and (K _E)_exp = K ? K_L seems to be quite satisfactory up to about 200 K.     

Fluorine Passivation Inhibits “Particle Talking” Behaviors under Thermal and Electrical Conditions of Pure Blue Mixed Halide Perovskite Nanocrystals

Owing to outstanding optoelectronic properties, lead halide perovskite nanocrystals (PNCs) are considered promising emitters for next-generation displays. However, the development of pure blue (460-470 nm) perovskite nanocrystal light-emitting diodes (PNC-LEDs), which correspond to the requirements of Rec. 2020 standard, lag far behind that of their green and red counterparts. Here, pure blue CsPb(Br/Cl)₃ nanocrystals with remarkable optical performance are demonstrated by a facile fluorine passivation strategy. Prominently, the fluorine passivation on halide vacancies and strong bonding of Pb–F intensely enhance crystal structure stability and inhibit “particle talking” behaviors under both thermal and electrical conditions. Fluorine-based PNCs with high resistance of luminescence thermal quenching retain 70% of photoluminescent intensity when heated to 343 K, which can be attributed to the elevated activation energy for carrier trapping and unchanged grain size. Fluorine-based PNC-LEDs also exhibit stable pure blue electroluminescence (EL) emission with sevenfold promoted luminance and external quantum efficiencies (EQEs), where the suppression of ion migration is further evidenced by a lateral structure device with applied polarizing potential.     

Electrical conductivity of polycrystalline tin dioxide and its solid solution with ZnO

Electrical conductivity (σ) of “pure” and ZnO doped SnO₂ has been measured at different temperatures and oxygen partial pressures ( \(p_{{\text{O}}_{\text{2}} } \) )- From the variation of electrical conductivity of these materials three partial pressure ranges have been identifieD. In the high partial pressure rangeσ increases with decreasing \(p_{{\text{O}}_{\text{2}} } \) followed by a \(p_{{\text{O}}_{\text{2}} } \) independent region at lower \(p_{{\text{O}}_{\text{2}} } \) ´s and finally increases once again with a further decrease of \(p_{{\text{O}}_{\text{2}} } \) . These variations have been explained on the basis of an anti-Frenkel type defect structure and an interstitial solid solution of ZnO in SnO₂. The activation energy for the conduction process has been estimated and the values are found to differ in two different temperature ranges. In the low temperature range the conductivity is attributed mainly to the chemisorption of oxygen on the surface of the specimen.     

Thermal conductivity of Er<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Sn<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Se (<Emphasis Type="Italic">x</Emphasis> ≤ 0.025) solid solutions

The thermal conductivity of Er _x Sn_{1 ? x} Se solid solutions has been measured at temperatures from 80 to 360 K. The results have been used to evaluate the electronic and lattice components of thermal conductivity for elastic carrier scattering, parabolic bands, and arbitrary degeneracy. With increasing erbium content and temperature, both the electronic and lattice components decrease considerably. Long-term annealing increases both components. It follows from the present experimental data that heat conduction in Er _x Sn_{1 ? x} Se is mainly due to phonons and that the observed rise in thermal resistance with Er content is due to phonon-phonon and paramagnetic-ion scattering.     

Effect of mechanical milling on thermoelectric properties of half-Heusler ZrNiSn0.98Sb0.02 intermetallic compound

Thermoelectric properties of half-Heusler ZrNiSn intermetallic compound were investigated. The partial substitution for Sn site in ZrNiSn by Sb is effective for the reduction of the electrical resistivity, which leads to increase the power factor of the compound. The thermal conductivity for ZrNiSn_0.98Sb_0.02 can be reduced by the mechanical milling process maintaining the moderate Seebeck coefficient and electrical resistivity. The reduction of the thermal conductivity is ascribed to the enhancement of the phonon scattering accompanied with the minute crystal grains in the milled compounds. As a result, ZrNiSn_0.98Sb_0.02 with a milling time of 3 h shows a maximum ZT of 0.67 at 573 K.     

Influence of Structure Defects on Thermal Conductivity in Solid p-H2 and p-H2 - o-D2 Solid Solutions

The formation of structure defects induced by thermal stress in pure H₂ and H₂-D₂ solutions has been investigated. The thermal conductivity of p-H₂ crystals and p-H₂ - o-D₂ solutions is measured by the steady-state method from 1.5 K to the melting point. Crystals with different numbers of structure defects were prepared by varying the growth rate and parameters of subsequent annealing, and thermal shock. The value of thermal conductivity and the character of the temperature dependence are observed to change, depending on the number of defects present. The experimental results are analyzed within the Callaway model taking into account the phonon scattering processes such as phononphonon scattering, boundary scattering, scattering on D₂ impurities, and scattering on structure defects (dislocations and low-angle boundaries). The contribution of isolated dislocations into the total relaxation rate is distinct only for the pure parahydrogen sample subjected to thermal shock. In the p-H₂ - o-D₂ solutions this contribution is not, detectable against the background of the strong frequency-independent scattering by low-angle boundaries. It is shown that the density of the dislocations that form low-angle grain boundaries is proportional to the concentration of impurity molecules.     

Low temperature thermal conductivity of aluminum alloy 5056

The thermal conductivity of 5056 aluminum alloy was determined from 4.2 K to 120 K using a differential steady-state method. This method has been implemented in a low temperature cryostat using a Gifford–McMahon cryocooler as heat sink. The thermal conductivity of the 5056 H39 aluminum alloy was determined since it was under consideration as a part of a thermal link for the Planck research satellite. As expected, below 10 K the thermal conductivity is exclusively given by the electron-defect scattering term. At higher temperature, the other terms from the electronic and the lattice contributions come into play but the electronic thermal conductivity term is still dominant. A workable fit, based on theory, is presented and can be used up to 300 K. Our measurements are compared with data at lower temperature and available fits from the literature.     

Thermal conductivity of pure monoisotopic silicon

The thermal conductivity of pure monoisotopic silicon is estimated by two methods, which give similar results. One estimate, based on the observed thermal conductivity of monoisotopic germanium, yields a maximum of 66 W · cm^–1 · K^–1 at 22 K. The other estimate, based on measurements of natural silicon and on the theoretical isotope scattering rate, yields 75 W · cm^–1 · K^–1 at 22 K, an increase of only 45% over the natural crystal. These values are for crystals of approximately 0.5 cm diameter; smaller crystals yield lower values of the maximum conductivity and smaller isotope effects. Silicon cooled to liquid hydrogen temperature seems promising for high-irradiance laser mirrors. The small gain obtained by using monoisotopic silicon would be substantially greater in cases when the generated phonon distribution is athermal and weighted to higher frequencies. The effective heat transport could then be increased by as much as a factor 60 through the use of monoisotopic silicon.