Thermal Conductivity Of Niobium Research Articles

Thermal conductivity of niobium and thermally sprayed copper at cryogenic temperature

We have developed a test facility dedicated to thermal conductivity k(T) measurements at low temperature. These experimental devices allow the test of four samples simultaneously during each run. The measurements are performed using steady-state axial heat flow method with a careful control of heat leaks to the surrounding. The apparatus was successfully used for measuring k(T) (Temperature T in the range: 1.5 K - 10 K) of niobium and other materials used for the fabrication of Superconducting Radio Frequency (SRF) cavities. We have tested the following samples: 1) niobium samples from different supplier (as received or/and after various high temperature Heat Treatments (HT)), 2) thermally sprayed copper coatings. The resulting experimental data are presented and compared to the experimental results previously reported by other groups and theoretical values. As expected heat treatment have a strong impact on k(T) and in particular on the phonons peak at T∼2K. More precisely, HT @ 1200°C with Ti gettering improves the Nb Residual Resistance Ratio (RRR) by a factor of 3 and consequently k(T). Finally, the correlation between the niobium RRR and the thermal conductivity. at T=4.2 K is confirmed in good agreement with the Wiedemann-Franz law

A non-contact RRR estimation using planar inductor sensing element

The Residual Resistivity Ratio (RRR) is an important parameter which is used to determine the purity and thermal conductivity of Niobium (Nb) utilized for fabrication of Superconducting Radio Frequency (SRF) cavities. The usual 4-probe electrical resistance measurement method is both destructive in nature and produces a non-local (average) measurement of the electrical conductivity. The present work utilizes Printed Circuit Board (PCB) based planar inductor as sensing element. The impedance variation of the sensing element in the presence of a conducting metal (Nb), is used to determine the conductivity of the Nb target. A generalized equation for RRR is proposed in terms of parameters ‘a’ and ‘b’ associated with the sensing coil. The measurement method is experimentally verified by utilizing samples of Nb of known RRR values. The sensor parameters (‘a’ and ‘b’) are also established for cryogenic conditions. Errors in measurement, accuracy and repeatability of measurements down to 4.2 K are reported.

An Eddy-current based RRR measurement technique for SRF cavities

Residual Resistivity Ratio (RRR) is an important parameter which is used to determine the purity and thermal conductivity of Niobium (Nb) utilized for fabrication of Superconducting Radio Frequency (SRF) cavities. The usual 4-wire electrical resistance measurement method is destructive in nature and produces a non-local (average) measurement of the electrical conductivity. The present work utilizes Printed Circuit Board (PCB) based planar inductor as sensing element. The impedance variation of the sensing element in the presence of a conducting metal (Nb), is used to determine the conductivity of the Nb target. A generalized equation for RRR is proposed in terms of parameters ‘a’ and ‘b’ associated with the sensing coil. The measurement method is experimentally verified by utilizing samples of Nb of known RRR values. The sensor parameters (‘a’ and ‘b’ ) are also established for cryogenic conditions. Errors in measurement, accuracy and repeatability of measurements down to 4.2 K are reported.

Выбор метода исследования теплопроводности ниобия в сверхпроводящем состоянии

Выбор метода исследования теплопроводности ниобия в сверхпроводящем состоянии

Kapitza resistance cooling of single crystal (111) niobium for superconducting rf cavities

The use of large grains or single crystal niobium to improve the $Q$ factor of superconducting rf cavities for particle accelerators, is presently under study. Heat extraction which plays a decisive role in the thermomagnetic stability of these devices depends on the thermal conductivity of niobium $K$ and the thermal boundary (Kapitza) resistance ${R}_{K}$ at the niobium/superfluid helium interface. Here we present the first measurements of ${R}_{K}$ performed between 1.5--2.1 K with single crystal (111) niobium, having two different surface morphologies, namely, a surface with a damage layer and a chemically polished surface. The thermal conductivity of the single crystal Nb samples is also simultaneously determined. For monocrystalline niobium we demonstrate that ${R}_{K}$ is an increasing primary limiting factor with temperature, contrary to the behavior found for polycrystalline cavities. The present investigation reveals for the first time that the presence of impurities (metallic particles and oxygen) within the damage layer leads to a stronger ${R}_{K}$, although the effective heat exchange area to the superfluid is increased. We further show the importance of dislocations in the thermal conductivity of monocrystalline niobium.

Measurements of Thermal Conductivity and Kapitza Conductance of Niobium for SRF Cavities for Various Treatments

Niobium is the material of choice for making the superconducting radio frequency cavities used in present-day accelerators for the acceleration of charged particles. In order to achieve high accelerating gradients for future accelerators such as the linear collider, thermal limitations in Nb cavities play an important role. The effects of plastic deformations due to applied strains on thermal conductivity of Nb in phonon transmission regime as well as on its Kapitza conductance have been studied. The study reveals absence of the phonon peak due to applied strains beyond the elastic limits of the Nb metal as well as reduced Kapitza conductance. This resulted in almost 80% reduction in thermal conductivity of the niobium at 2 K. Low temperature annealing did not recover the phonon peak as was seen before the application of plastic deformations.

Impurities analysis of high purity niobium in industrial production

There are two suppliers in the world, providing high purity niobium with RRR > 300, for application in superconducting accelerators. One of the high purity niobium producers, Tokyo Denkai, ships niobium mill products totalling several tons per year. To assure the quality of its products, impurity analysis is essential. The impurities of niobium are categorised in two types: metallic impurities and interstitial impurities. Usually the metallic impurities are analysed by ICP-AES and the interstitial impurities are analysed by a gas analyser. Additionally, residual resistivity ratio (RRR) indicates total amount of trace elements and thermal conductivity of niobium. In this paper, the present status of such a quality control of niobium production in Tokyo Denkai will be introduced.

Improving the Thermal Conductivity of Niobium in Nb3Sn/Nb/Cu Structures by Solid-State Refining

Experimental data are presented which demonstrate that the thermal conductivity of Nb in Nb3Sn/Nb/Cu structures can be raised by solid-state refining in the range 1200–1500 K, using titanium and zirconium electrolytic coatings, plates, or powders as impurity sinks. The removal of oxygen from the Nb layer increases its thermal conductivity by a factor of 3–4, without impairing the performance parameters of the superconducting Nb3Sn layer.

Long-range internal stresses and low temperature phonon scattering in plastically deformed niobium and tantalum single crystals

The thermal conductivity of niobium and tantalum single crystals plastically deformed in steps at intermediate temperatures has been measured in the temperature range 0.3–20 K. From these measurements the thermal resistivity W d induced by the plastic deformation was evaluated in the temperature range 0.3–5 K. After each deformation step a thermal resistivity W d proportional to T −2 was found, indicating that phonons are scattered strongly by static strain fields rather than by dynamic phonon-dislocation processes. X-ray topography and transmission electron microscopy studies revealed that the dislocation arrangement is heterogeneous, consisting of kink walls and dislocation sheets. Since classical dislocation pile-ups have not been observed by the lattice thermal conductivity measurements nor by the dislocation arrangement studies, the long-range internal stresses causing phonon scattering have to be attributed to the heterogeneous dislocation arrangement as proposed according to Mughrabi's composite model.

Recent Progress in RF Superconductivity

The wide spread activities to prepare major size superconducting accelerating systems mainly for the large storage rings presently under construction in Europe and Japan are based on the continuous progress in rf superconductivity achieved in the past years. The thermal stability of s. c. cavities has been increased significantly by improving the thermal conductivity of niobium with different techniques. The promising attempts to sputter high purity niobium onto 500 MHz copper cavities have shown encouraging results. Multicell cavities at 3 GHz coated with Nb3Sn achieved high Q values already at 4.2 K. Field emission studies using modern surface analytical tools are shedding more light on anomalous electric surface breakdown fields. This report summarizes briefly the current activities in different laboratories and comments on recent achievements with superconducting cavities and in materials research. It concentrates on work performed prior to the preparation of accelerator experiments. These are discussed by R. Sundelin 1 and in other contributions to this conference.

Lattice thermal conductivity and phonon-dislocation interaction in niobium and tantalum single crystals plastically deformed at intermediate temperatures

Abstract The thermal conductivity of niobium and tantalum single crystals plastically deformed in steps at intermediate temperatures has been measured in the temperature range 0.3 to 20 K. From these measurements the thermal resistivity W d induced by the plastic deformation was calculated in the temperature range 0.3 K < T < 5 K. After each deformation step a thermal resistivity W d proportional to T −2 was found. The dislocation arrangement was studied by X-ray topography and transmission electron microscopy. The results are consistent with a model of phonon scattering by the static distortion field of dislocations.

A new purification technique for improving the thermal conductivity of superconducting Nb microwave cavities

Improvement factors up to 4 in the thermal conductivity of Niobium between 4 and 9K are obtainted by a new purification technique which removes the dominant interstitial impurity (oxygen) found in commercial Nb. In the process, surfaces of the cavity are brought into proximity with Yttrium foil at -10-5torr for several hours. A vapor deposited film of several μm thickness traps oxygen diffusing rapidly from the bulk to the surface. A single cell, 1500 MHz, elliptically shaped cavity with thermal conductivity improved by this method shows amelioration of thermal breakdown, as predicted by thermal model calculations.

Electron–Phonon Scattering and Low‐Temperature Thermal Conductivity of Niobium and Tantalum Single Crystals

AbstractThe thermal conduction of niobium and tantalum single crystals of different impurity contents is measured below 20 K. The investigation of the lattice thermal conductivity as limited by the interaction of phonons with electrons demonstrates that both, longitudinal and transverse phonon modes are scattered strongly.

Thermal conductivity of niobium and molybdenum single crystals at low temperatures

The thermal conductivity of Nb and Mo single crystals was measured in the temperature range of 0.05–20 K and 2–71 K, respectively.

Investigation of the thermal and electrical transport properties of niobium and niobium films at temperatures between 9 and 50 K

The thermal conductivity and the electrical conductivity of niobium crystals and niobium films have been investigated in the normal state from 9 to 50 K. The deviations from the Matthiessen rule, W=..beta../T+..cap alpha..T/sup 2/, have been studied in detail for the thermal case. The investigation shows a slight dependence of the electron--phonon scattering coefficient ..cap alpha.. upon the impurity content of the sample. With the specific electrical residual resistivity rho/sub 0/ as the measure for the impurity content, the following correlation can be formulated: ..cap alpha..=1.2 x 10/sup -3/(rho/sub 0//(..cap omega..-cm))/sup 0.04/, ..cap alpha.. being obtained in cm/WK. Above 20 K an additional scattering mechanism occurs. The temperature dependence of the additional resistance ..delta..W between 20 and 50 K is proportional to T/sup 5.5/xxxT/sup 4/. Possible causes of this phenomenon are discussed. For the discussion, all the data available in the literature on the thermal conductivity of niobium in this temperature region are used.

Thermal conductivity of niobium in the mixed state

The magnetic field dependence of the thermal conductivity of four niobium samples in the mixed state has been investigated at several temperatures and three different field orientations. The samples are of moderate purity, that is, l e ≈ ξ0. At low temperatures, just above H c1, the phonon conductivity can be easily separated. Good agreement is obtained with qualitative predictions of Canel and Vinen for the scattering of phonons by flux lines. At high temperatures both the phonons and the electrons are scattered by the flux lines. Though it is evident that the scattering mechanism is more effective in the case that H ⊥ ▽T than for H ∥ ▽T, only a qualitative discussion can be given. Near H c2 the finite slopes of the λ(H) curves are in qualitative agreement with the gapless superconductivity theory of Caroli and Cyrot, but the dependence on the electron mean free path can be better understood from the theory of pure type II superconductors of Houghton and Maki. Some aspects of pure type II superconductivity are also recognized in the anisotropic behavior near H c2.

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)kTc. The phonon conductivity in the superconducting stateλps 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\({\text{T}} \lesssim 0.15T_{\text{c}} \), λps is proportional toT2, 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 of Niobium in Magnetic Fields NearHc2

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The effect of radiation damage on the thermal conductivity of superconductors

The effects of neutron irradiation at 30 deg C on the thermal conductivity of niobium and vanadium single crystals were investigated at low temperatures. Measurements were carried out in both superconducting and normal states where conduction is due predominantly to phonons and electrons, respectively. The observed effect on the phonon conductivity indicates an increase in the dislocation density whereas changes in electron conduction revealed the presence of vacancies. (auth)