Limitations of rf field by thermal breakdown in superconducting niobium cavity

The thermal breakdown is unacceptable for high power use of superconducting rf cavities. Limitations of rf field both in Hel and Hell baths were estimated and discussed on C-band TE011 mode niobium cavities. Calculations were performed by a simulation program under the different assumptions of Kapitza resistance, thermal conductivity and wall thickness, taking lateral heat flow and curvature of the wall into account.     

The lattice thermal conductivity of normal and superconducting niobium

The lattice thermal conductivity of superconducting and normal Nb as limited by the interaction of phonons with electrons has been deduced from measurements in the superconducting state. The results indicate that the mean free paths of transverse and longitudinal phonons are similar, ～4×10^?5 T ^?1 (cm K), in the normal state. Comparison is made with measurements on other metals. A compilation is included of the ratio of lattice conduction in the normal state to that in the superconducting state, based on the BCS theory of superconductivity.     

The thermal conductivity of niobium—zirconium alloys at low temperatures

The thermal conductivities of three samples of heat-treated niobium—zirconium alloy have been measured at low temperatures, and analysed into components due to phonon and electron carriers. The phonon carrier component is found to be dominant at most temperatures, particularly below the superconducting critical temperature. The observed variations in the magnitude of the conductivity are correlated with the metallurgical structures.     

Influence of nonstoichiometry on the thermal conductivity of boron carbide

It is shown that the deviation of the boron carbide composition from stoichiometry results in a change in its thermal conductivity.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 32, No. 3, pp. 414–417, March, 1977.     

Influence of thermal expansion on the conductivity of polymer conducting composites

The temperature dependence of conductivity in systems that undergo a metal-dielectric phase transition is investigated. A quantitative theory is developed that makes it possible to calculate electrical conductivity as a function of temperature for these systems with allowance made for thermal expansion of a polymer and a filler.Deceased.Tashkent State University. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 66, No. 3, pp. 335–337, March, 1994.     

Influence of interfacial modification on the thermal conductivity of polymer composites

In this study, the influence of modifying ZnO filler with surface-treating agents on the thermal conductivity of the ethylene-vinyl acetate copolymer (EVA) composites was reported. Fourier transform infrared (FTIR) spectroscopy and scanning electron microscopy (SEM) were applied to analyze the surface modification of ZnO filler and the fractured surface morphology of the EVA–ZnO composites. The test results indicated that surface-treating ZnO filler with the valid modifying agents (stearic acid, OL-AT16, KH-560, or NDZ-132) at right dosage and treating temperature, the thermal conductivity of the EVA–ZnO composites can be enhanced effectively on account of reducing the interfacial phonon scattering; while the dosage of modifying agents exceed a right amount, the thermal conductivity of EVA–ZnO composites modified with the small-molecule treating agents (F-1, n-Octylic acid, or stearic acid) descend more evidently with the dosage increase.     

Influence of the Kapitza resistance on the thermal conductivity of filled epoxies

The thermal conductivity of epoxy resins filled with copper powder was measured as a function of grain size and filler concentration between 1.5 and 20 K. In addition, the thermal boundary layer resistance (Kapitza resistance) between epoxy resin and copper was measured. As a consequence of this resistance the thermal conductivity is strongly dependent on grain size in the lower temperature range. Below a characteristic temperature dependent on grain size, thermal conductivity is reduced by adding filler. A simple formula is presented for calculation of the thermal conductivity of filled resins.     

Influence of polymeric residue on the thermal conductivity of suspended bilayer graphene

The thermal conductivity (κ) of two bilayer graphene samples each suspended between two microresistance thermometers was measured to be 620 ± 80 and 560 ± 70 W m(-1) K(-1) at room temperature and exhibits a κ ∝ T(1.5) behavior at temperatures (T) between 50 and 125 K. The lower κ than that calculated for suspended graphene along with the temperature dependence is attributed to scattering of phonons in the bilayer graphene by a residual polymeric layer that was clearly observed by transmission electron microscopy.     

Influence of magnetic breakdown on low-frequency conductivity and weak damping electromagnetic waves in metals

Low-frequency electroconductivity under magnetic breakdown conditions is investigated in the two qualitatively different situations: (a) coherent and (b) stochastic. In the former situation (a) the consideration is fulfilled in terms of the incidental magnetic breakdown spectrum of electrons. In the latter situation (b) it is fulfilled in terms of discrete random walking of classical electron. In these cases the general expressions of the conductivity tensor under conditions of both strong and weak spatial dispersion are derived. It is shown that magnetic breakdown does not violate the conditions of compensation of the volumes of electrons and holes in the case of closed trajectories. In the coherent situation the magnetic breakdown has the most essential influence upon Landau adsorption which is determined by a macroscopic layer of valuesP _z (projection of the momentum of the electrons on the direction of the magnetic field). In the stochastic case it is shown that magnetic breakdown results in the appearance of peculiar ergodic states in which an electron is spread on all bands (which take part in the breakdown) with equal probability. Together with it the probability of breakdown disappears from the low-frequency conductivity in a wide interval of magnetic fields and the conductivity is expressed in terms of the classical motion along an united compound orbit. The results of both coherent and stochastic calculations are used to investigate the weak damping electromagnetic waves. It is shown that the magnetic breakdown essentially changes only the dependence of the helicon damping on the magnetic fieldH. In many situations the dependence is represented by nonmonotonic functions ofH. In the coherent and stochastic cases these functions are qualitatively different.     

Influence of structural defects on the thermal conductivity of polycrystalline and single-crystal PbTe

We have measured the thermal conductivity of unannealed and annealed (800 K, 120 h) polycrystalline and single-crystal PbTe samples at temperatures from 80 to 303 K, evaluated the electronic and lattice components of their thermal conductivity, and determined the thermal resistivity due to structural defects, whose concentration in the unannealed single-crystal samples reaches ∼10¹⁷ cm⁻³. The results demonstrate that the thermal resistivity of the unannealed polycrystalline and single-crystal samples is 9.4 and 1.7 cm K/W, respectively. Annealing eliminates the defects, thereby increasing the lattice thermal conductivity of the material.     

Influence of structural relaxation on the low-temperature thermal conductivity of ancient natural glasses

The thermal conductivities of two natural glasses, amber and obsidian, were measured over the temperature range 0.06–10 K both in the as-received condition and following heat-treatments near and above the glass-transition temperature. The of amber decreased 6% with heat treatment revealing that this glass had undergone configurational relaxation during the 10⁷ years prior to measurement, but with a relaxation time greater than seven years. The of the obsidian did not change with heat treatment, a null result possibly produced by the ratio of oxides comprising the glass.     

Kapitza conductance and thermal conductivity of copper niobium and aluminium in the range from 1.3 to 2.1 K

The Kapitza conductance and thermal conductivity of ofhc-copper, niobium, ultra high purity aluminium, and of the aluminium alloy 6061 A1 have been measured in the temperature range from 1.3 to 2.1 K, yielding both quantities in the same steady state experiment. The temperature dependence of the Kapitza conductance, h_o, was between T^3.3 and T^4.6 for the different samples, which is higher than the most frequently observed T³ dependence. The magnitude of h_o for both ofhc-copper and aluminium agrees well at 1.9 K with an empirical prediction, but for niobium it is a factor of two to four lower than the value predicted. At 1.9 K, h_o is higher by a factor of two for an annealed and chemically polished niobium sample than for an untreated sample. The thermal conductivity measured from ofhc-copper and 6061 A1 as in good agreement with the value calculated from the resistivity of these materials and the Wiedemann-Franz law. The measured thermal conductivity obtained for an annealed niobium sample is a factor of 2.8 higher than the highest published value.     

The effect of magnetic field on the thermal conductivity and electrical resistivity of different materials

In impregnated superconducting coils it is assumed that copper heat sinks extract the heat produced by ac losses.For this type of cooling we study the effect of magnetic field on the Wiedemann—Franz law for copper of different purities. We also look at the effect of magnetic field on the thermal conductivity tensor of resin with 2% silica gel and of a composite interlayer of an impregnated coil.     

Comments on the measurement of thermal conductivity and presentation of a thermal conductivity integral method

A discussion is presented regarding the significance of the spatial temperature gradient approximation normally used in thermal conductivity measurement. Examples are presented illustrating the magnitude of temperature differences allowed for conductivity integral (TCI) method of analysis is presented as an alternative method which totally eliminates the need to impose temperature difference restrictions on the measurement process, so long as other errors, such as radiative heat losses, do not become excessive.     

磁场中磁性纳米流体热导率的研究

进行了提高磁场中含四氧化三铁(Fe3O4)纳米磁性颗粒机油(磁性纳米流体)热导率的研究。在有或无超声波辅助的条件下,采用共沉淀法合成了不同粒度的Fe3O4磁性纳米颗粒,采用一种热常数分析仪对该磁性纳米流体的热导率进行了测定,探讨了纳米磁性颗粒粒度、体积分数以及背景磁场对磁性纳米流体导热性能的影响。实验结果表明,在外磁场中,磁性纳米流体所含Fe3O4纳米颗粒的体积分数越大,磁性颗粒粒度越小,磁性纳米流体的热导率越高;当磁场方向平行于温度梯度时更有利于提高磁性纳米流体的热导率。