Gain in the stability of NbTi composite superconductor doped by large-heat-capacity substance Gd<Subscript>2</Subscript>O<Subscript>2</Subscript>S

The effect of internal doping of a NbTi composite wire by a large-heat-capacity substance (Gd₂O₂S ceramics) on the critical currents and stability against short thermal disturbances (with a typical time on the order of 1 ms) is studied experimentally and theoretically. The composite wire studied in this work is similar in design to conductors used in the international thermonuclear experimental reactor (ITER). The additive introduced into the wire in an amount of 5 vol % raises its specific heat ninefold at 4.2 K. It is found that the critical current of the (NbTi + Gd₂O₂S) wire increases by 8–11% in comparison with a reference NbTi wire depending on the external magnetic field varying between 5 and 7 T. Although the potential of high specific heat is not utilized completely, the critical thermal energies of the doped wires are three to four times higher than those of the undoped (reference) wires at near-critical currents.     

Considerable rise in the stability of combined superconductors doped by intermetallic compounds with an extremely high low-temperature specific heat

The influence of doping intermetallics based on rare-earth elements and ceramics with an extremely high low-temperature specific heat on the stability of combined NbTi superconductors against electromagnetic disturbances is studied experimentally and theoretically. The objects of investigation are standard NbTi conductors (0.85 mm in diameter) in a copper matrix that are soldered to copper wires containing high-specific-heat dopants. CeCu₆, HoCu₂, CeAl₂, and PrB₆ intermetallics (at 4.2 K, their specific heat is on average 150 times that of copper) or Cd₂O₂S ceramics (at 4.2 K, its specific heat is higher than that of copper by 600 times) are introduced into combined superconductor samples in the form of fine powder. The powder is placed into the copper wire either as one thick (0.6 mm in diameter) strand or as 19 thin (0.1 mm in diameter) strands. An undoped reference sample contains a solid conductor. Samples with a transport current placed in an external transverse magnetic field are subjected to longitudinal magnetic disturbances causing pulsed heating of the samples by eddy currents. The disturbance time is varied in a wide range: from 50 μs to 1.2 ms. To compare the critical energy densities of the disturbances in differently shaped samples, a mathematical technique is developed that is based on analytical solution of the equation of electromagnetic diffusion into the sample. It is found that the critical energy density of doped samples is much higher (by several times) than that of the reference sample. Upon direct cooling of the samples by liquid helium in a vertical channel (the most intense heat removal under steady-state conditions), the effect of stability improvement for the doped samples persists. Moreover, it is shown that the high-specific-heat dopants, raising the heat needed to warm the sample, restrict the heat flux into the liquid and thereby increase the energy removed by the coolant during unsteady heat transfer.     

Effect of large-heat-capacity substances doping of superconducting coils on their dynamic stability

The dependence of the quench currents on the ramp rate was studied for four small NbTi coils. Two pairs of superconducting coils were tested. In one pair the SC 0.85-mm-dia wire with 2970 filaments was used, in the other two coils the SC wire contained 8910 filaments of smaller size. Two coils (with different number of filaments) contained 4.9 vol % of Large-Heat-Capacity Substance (LHCS) in the form of tiny powder evenly distributed over the winding volume, therefore their heat capacity at 4.2 K was an order of magnitude larger than that for coils without LHCS. The LHCS was introduced into the winding in a mixture with epoxy resin (“wet” winding). When the self-magnetic field varied with a rate of ≥5 T/s, premature quenches were observed in the central turns of the undoped coil made of a wire with 2970 filaments. These transitions are likely to be caused by magnetic flux jumps. In the LHCS-doped coil made of the same wire, the quenches took place at currents two to three times higher, since the sample was heated up to a critical temperature because of electrical losses (as confirmed by calculations). Thus, the improved stability of the LHCS-containing coils not only against long-term (0.1–1.0 s) disturbances but also against much shorter (10–100 μs) jumps of the magnetic flux is demonstrated.     

Focussing and defocussing of ballistic phonons in diamond and Nb3Sn

The ballistic propagation of phonons in diamond and Nb₃Sn at 40 and at 4.2 K is examined. The nature of variation of the phonon magnification factor has been analysed both in the wave vector as well as group velocity spaces. Using the Polar Schmidt-net with Pole at (π/2,π/2), the mappings of phonon focussing and defocussing are made. It is shown that the mapping for theFTA mode for Nb₃Sn exhibits islands acting as impenetrable barriers for phonon propagation.     

Nb3Sn正常-超导转变时的比热反常

在16.0°K—20.3°K之间测量了Nb₃Sn样品的热容量。Nb₃Sn在临界温度附近的比热跳跃值ΔC=2.21(±5％)焦耳/克分子·度。样品的临界温度T_c=17.88°K,转变宽度ΔT_c≈0.2°K。ΔC值利用热力学关系式确定了Nb₃Sn在0°K时的热力学临界场H₀=5300奥斯特。利用本文的结果和文献上关于热膨胀系数的跳跃值Δα及?T/?P值验证了热力学关系式。扼要地描述了比热测量装置.     

Upper critical fields of cubic and tetragonal single crystal and polycrystalline Nb3Sn in DC fields to 30 tesla

Measurements of the upper critical field, H_c2(T), in single crystal Nb₃Sn were extended to 30 tesla (300 kG) with dc fields produced by a Hybrid magnet. Observations of H_c2(T) were made for materials which remain in the cubic (c) phase and those which show a martensitic transformation at the tetragonal (t) phase. H_c2(T) measurements of Nb₃Sn for a pure crystal for which the de Haasvan Alphen (DHVA) effect was observed and for polycrystalline (t) phase and (c) phase materials are also reported. Measured values of H_c2(4.2 K) and calculated values of H_c2(0) are: 1) along the [100] direction for our earlier Nb₃Sn, H_c2(4.2 K) = 26 T for the (c) phase and 21.5 T for the (t) phase; H_c2(0) = 29T for the (c) phase and 24 T for the (t) phase; 2) along the [100] direction for the DHVA material H_c2(4.2 K) = 18 T and H_c2(0) = 20 T; 3) for polycrystalline Nb₃Sn (t) phase material H_c2(4.2 K) = 23 T and H_c2(0) = 25 T and for (c) phase material H_c2(4.2 K) = 26 T and H_c2(0) = 29 T. The values of (dH_c2/dT)_T=Tc vary from 2.4T/K for the highest H_c2(T) material to 1.6T/K for the DHVA material. The anisotropy for various Nb₃Sn single crystal materials is small and independent of temperature from T_c to 0.1 T_c. δT_c between the (c) and (t) phase is <0.3 K. Within experimental error excellent fits of H_c2(T) with theory are obtained assuming a dirty or clean Type II superconductor with no Pauli paramagnetic limiting. Experimental details and strong-coupling effects are discussed. When strong-coupling is included, the effects of any paramagnetic limiting would be small and not detectable within our present experimental error. Brief comments also are made concerning H_c2 of V₃Si.     

Valence transition and magnetic ordering in Sn doped EuPd2Si2

The sharp, temperature induced, continuous valence transition in EuPd₂Si₂ is drastically changed by doping with Sn at the Si site upto 5 at. %. Only a first order valence transition occurs for a 3% Sn doped sample and the 2⁺ component which survives the valence transition orders magnetically at 4.2 K. No valence transition at all occurs for a 5% Sn doped sample right up to 1.9 K and magnetic ordering sets in around 30 K.     

The relation betweenT c and anharmonicity in Sn-base A15 superconductors

The absolute value of the recoil-free fraction was measured accurately at three reference temperatures in the range 300 K down to 4 K, in two Sn-baseA 15 superconductors with disparate superconducting properties. The higher-T _c compound Nb₃Sn (T _c≈18K) exhibits low-temperature anharmonicity; this is in contrast to the lattice-dynamics of the low-T _c isomorph V₃Sn (T _c≈4K) in which it is observed that harmonic binding of the Sn atoms is prevalent down to low temperatures. The difference in the superconducting properties of the two compounds is shown to correlate with the considerable difference in their lattice-dynamics.     

Influence of impurities on the low-temperature deformation of nanocrystalline copper

The influence of ZrO₂ particles on the low-temperature deformation of nanocrystalline copper produced by strong plastic deformation is investigated using equichannel angular pressing. A comparison is made between the deformation characteristics in tension and compression in the temperature range 4.2–400 K, measured for copper and the composite Cu:0.3 vol. % ZrO₂. It is shown that within 4.2–200 K the yield point σ _sm of the composite is higher than that for copper, attaining 680 MPa at 4.2 K, then the yield points are close in value up to room temperature, and diverge again as the temperature is raised. Possible causes of the dissimilar influence of an impurity on the strength and plasticity characteristics of nanocrystalline copper in various temperature intervals are discussed. Fiz. Tverd. Tela (St. Petersburg) 40, 1639–1641 (September 1998)     

Specific heat of A-15 Nb3Si produced by explosive compression

The specific heat from 1.2 to 23 K has been measured on a new high T_c superconductor, A-15 Nb₃Si. The sample was prepared by explosive compression and has an onset of bulk superconductivity at 18.0 K, with a transition width of 0.7 K. The density of states for pure A-15 Nb₃Si implied from the specific heat data is 0.94 ± 0.20 states/eV-atom, ΔC/γ T_c is 2.0 ± 0.2.     

Specific heat of high Tc,A-15 Nb0.75Ga0.25

The specific heat from 1.4 to 27 K has been measured on a NbGa sample that was approximately 85% A-15 Nb_0.76Ga_0.24 with a T_c of 19.8 K and a transition width of 1 K. The data imply that the density of states for A-15 Nb_0.75Ga_0.25 is 1.7 ± 0.25 states/eV-atom, in good agreement with recent band structure calculations. The Debye temperature is found to increase approximately 1 K per degree from T_c down to 4 K, with θ_D(0) = 280 K.     

Ultrasonic attenuation of surface acoustic waves in a thin film of superconducting Nb3Sn

The attenuation of 660 MHz surface acoustic waves propagating in a thin film of Nb₃Sn 5000 Å thick has been measured as a function of temperature from 4.2 K to 16 K. The A 15 Nb₃Sn, electron-beam codeposited on YZ lithium niobate and annealed at 700°C, was studied using 5.1 μm wavelength interdigital electrodes. The film revealed a transition temperature of 14.2 ± 0.1 K and using the BCS theory, an energy gap 2Δ(0) = 3.5 k_BT_c.     

Specific heat of single phase Nb3Ge

For the first time, the specific heat of single phase, stoichiometric, high transition temperature (21.8 K) A-15 Nb₃Ge has been measured. From the data between 4 and 29 K, the linear term coefficient, γ, of the specific heat is found to be 30.3±1. mJ/mole-K² and the Debye temperature, ?_D, is 302±2 K. The bulk energy gap parameter, 2Δ/kT_c, is found to be 4.2±0.2, in agreement with tunneling measurements.     

Investigate the effect of anisotropic order parameter on the specific heat of anisotropic two-band superconductors

The effect of anisotropic order parameter on the specific heat of anisotropic two-band superconductors in BCS weak-coupling limit is investigated. An analytical specific heat jump and the numerical specific heat are shown by using anisotropic order parameters, and the electron–phonon interaction and non-electron–phonon interaction. The two models of anisotropic order parameters are used for numerical calculation that we find little effect on the numerical results. The specific heat jump of MgB₂, Lu₂Fe₃Si₅ and Nb₃Sn superconductors can fit well with both of them. By comparing the experimental data with overall range of temperature, the best fit is Nb₃Sn, MgB₂, and Lu₂Fe₃Si₅ superconductors.     

Low-temperature anharmonicity,strong electron-phonon interactions and heavy-fermion behaviour,in the A15 superconductor Nb3Sn

Absolute values of the Sn recoil-free fraction in the A15 superconductor Nb₃Sn were accurately measured in the range 300 K down to 50 K. Low—temperature anharmonicity is evident in this compound and no further lattice—dynamical anomalies were observed in the measured temperature range. The data has been used to estimate the dimensions of the highly anharmonic atomic potential-wells of the Sn atoms at low temperatures.     

合金元素对Nb3Sn生长动力学的影响

研究了合金元素Zr对Cu-Sn/Nb界面上Nb₃Sn生长动力学的影响。实验表明:在单芯复合材料中加入Zr显著提高了Nb₃Sn层的生长速率,层厚与时间的关系显著超过抛物线规律。这些结果不能仅用内氧化生成的ZrO₂颗粒使晶粒细化来解释,还必须考虑ZrO₂颗粒周围过饱和空位使扩散系数增大等因素。在多芯复合材料中热处理时Cu-Sn基体中Sn量的消耗,显著影响Nb₃Sn的生长。考虑了这一因素的Nb₃Sn生长动力学修正公式能对实验结果进行解释。 关键词：     

Low-energy lattice excitations in the decagonal Al-Ni-Fe and icosahedral Al-Cu-Fe quasicrystals and the (Al,Si)-Cu-Fe cubic phase

The specific heat of decagonal Al_71.3Ni_24.0Fe_4.7 and icosahedral Al₆₂Cu_25.5Fe_12.5 quasicrystals and the Al_55.0Si_7.0Cu_25.5Fe_12.5 cubic phase approximating the structure of the icosahedral alloy has been studied in the temperature range 4.2–40.0 K. All the three compounds exhibit low coefficients of the electronic heat capacity and pronounced deviations of the low-temperature lattice heat capacity from a cubic temperature law in the range 5–10 K. The results obtained by the thermodynamic method and inelastic neutron scattering have been compared and analyzed. It has been established that, at energies ɛ < 14 meV, the spectral density of thermal vibrations in the icosahedral quasicrystal is substantially higher than those in the cubic approximant and in decagonal quasicrystal.     

Optimization calculations of polyhelix coils for hybrid magnets

In order to generate very high, steady magnetic fields for research in solid-state physics one can combine a high-power resistive magnet with an outer superconducting coil. If magnetic fields above 30 Tesla have to be produced it is advisable to use a water cooled polyhelix copper coil as an inner part of such a hybrid magnet. An algorithm is described which allows the optimization of the design paramaters of a polyhelix coil in an external magnetic field with respect to dimensions and material for various values of the heat flux. The calculations show that with an outside field of 13 Tesla generated by a Nb₃Sn−NbTi coil and with a 10 MW polyhelix magnet made of Cu (Al₂O₃) and Cu−Ag continuous magnetic fields around 40 Tesla may be generated in a 3 cm room temperature bore.     

Tokamak DEMO-FNS: Concept of magnet system and vacuum chamber

The level of knowledge accumulated to date in the physics and technologies of controlled thermonuclear fusion (CTF) makes it possible to begin designing fusion—fission hybrid systems that would involve a fusion neutron source (FNS) and which would admit employment for the production of fissile materials and for the transmutation of spent nuclear fuel. Modern Russian strategies for CTF development plan the construction to 2023 of tokamak-based demonstration hybrid FNS for implementing steady-state plasma burning, testing hybrid blankets, and evolving nuclear technologies. Work on designing the DEMO-FNS facility is still in its infancy. The Efremov Institute began designing its magnet system and vacuum chamber, while the Kurchatov Institute developed plasma-physics design aspects and determined basic parameters of the facility. The major radius of the plasma in the DEMO-FNS facility is R = 2.75 m, while its minor radius is a = 1 m; the plasma elongation is k ₉₅ = 2. The fusion power is P _FUS = 40 MW. The toroidal magnetic field on the plasma-filament axis is B _t0 = 5 T. The plasma current is I _p = 5 MA. The application of superconductors in the magnet system permits drastically reducing the power consumed by its magnets but requires arranging a thick radiation shield between the plasma and magnet system. The central solenoid, toroidal-field coils, and poloidal-field coils are manufactured from, respectively, Nb₃Sn, NbTi and Nb₃Sn, and NbTi. The vacuum chamber is a double-wall vessel. The space between the walls manufactured from 316L austenitic steel is filled with an iron—water radiation shield (70% of stainless steel and 30% of water).     

Nb3Sn复合超导体晶粒生长机制

本文研究了Nb/固态Cu-Sn和Nb/液态Cu-Sn界面上Nb₃Sn晶粒的生长。实验表明:固态-固态界面上生长的晶粒尺寸虽小(约0.1μm),但Nb₃Sn晶粒的长大仍符合通常的固态晶粒长大规律;固态-液态界面上生长的Nb₃Sn分成两层,靠近Nb的内层晶粒细小,排列致密,外层晶粒粗大,分布零散,后者是前者经过溶解/沉积过程引起的,晶体形貌大多数呈菱形十二面体,部分呈正交平行六面体,说明Nb₃Sn的{110},{100}面