Nb_3Sn超导电磁铁

本文报导了关于筒形Nb_3Sn磁体的研究结果。观察到磁通跳跃现象。“冻结”场的分布和历史及激发场的分布紧密相关。“冻结”场可以不是中心对称的,这进一步证明了烧结的Nb_3Sn材料是非均匀介质。 我们描述了新的磁体结构,避免了磁通跳跃。除利用铁心来捕获磁通外,在激发和测量磁场的方法上均具有一定的特点。 利用圆筒状Nb_3Sn作为压挤磁通的活塞,在1.5°K时,曾获得的最高场强是28.5KG。     

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.     

第二类超导体的临界磁场

在Гинэбург-Ландау理论中借引入顺磁能导出了第二类超导体的临界磁场公式。理论公式与Ti-V合金系统的实验数据符合。考虑了顺磁能的影响之后,第二类超导体由“混合态”向正常态的转变仍为二级相变;在相变点附近,磁化曲线仍为直线;在强磁场中相变时的比热跳跃与零场中的比热跳跃同数量级;与V₃Ga的比热测量及Nb₃Sn的磁化曲线测量很好地符合。     

添加Nb在快淬NdFeB永磁体中的作用研究

系统研究了Nb元素的添加对快淬(Nd,Dy)_11.5Fe_82.4-nNb_nB_6.1(n=0,0.5,1,1.5,2)永磁体磁性能、温度特性及显微组织的影响. 结果表明：少量Nb元素的添加可以在不显著影响剩磁的情况下较大幅度提高磁体的内禀矫顽力，降低磁通不可逆损失. Nb在NdFeB磁体中的作用是使晶粒细小化、均匀化、规则化，提高交换耦合钉扎场H_p，减小材料内部的散磁场，显著降低磁通不可逆损失，改善NdFeB磁体在高温下的使用性能.     

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     

烧结Nd-Fe-B磁体的微观结构和冲击韧性研究

对烧结Nd₁₅Fe_72-xCo_yNb_xB₈(y=0, 5;x=0, 05, 10, 15, 20, 25)永磁体的微观结构和冲击韧性及二者间的关系进行了研究.结果表明，添加Nb能够改善Nd-Fe-B磁体的微观结构，提高磁体的冲击韧性.当无Co磁体中Nb的原子百分含量为15%时，其冲击韧性达到最大；对于添加了Co的磁体，其冲击韧性的     

氧化还原可逆Nb2TiO7复合阴极电解池高温水蒸气电解研究

系统地研究Nb₂TiO₇与Nb_1.33Ti_0.67O₄材料相互转变的氧化还原循环可逆性能,同时研究Nb2TiO7和Nb_1.33Ti_0.67O₄样品随温度和氧分压变化的电导率,并与复合电极对称电池和电解池的电化学性能相关联. 在830 oC下,对Nb_1.33Ti_0.67O₄复合电极电解池进行水蒸气的电解研究测试. 电流电压曲线和电解池短期性能测试表明在低电压下主要为电极的还原和活化过程;而在高电压下主要为水蒸气的电解. 当3%H₂O/Ar/4%H_{2气体通入阴极时电解池水蒸气电解的法拉第效率为98.9%;而当通入气体转换为3%H2O/Ar时效率为89%.}     

C-S Activation of CS2 by Nb+ in Gas Phase

以Nb⁺与CS₂反应作为第二前过渡金属离子与CS₂反应生成金属硫化物离子和CS的范例体系. 采用密度泛函UB3LYP方法,对于Nb⁺采用Stuttgart赝势基组,对于C和S采用6-311＋G(2d)基组,计算研究了Nb⁺在基态和激发态时与CS_{2气相反应的机理. 全参数优化了反应势能面上各驻点的几何构型,并且用频率分析方法和内禀反应坐标方法对过渡态进行了验证. 结果表明Nb⁺与CS2的反应是插入-消去反应,在反应过程中会发生系间窜越,并且找到了两个势能面的能量最低交叉点.}     

BiFe1-xNbxO3的结构、磁性和光学性质

用溶胶凝胶法制备了Nb掺杂多铁BiFe_1-xNb_xO₃粉晶样品（0 <x <0.05）,研究Nb掺杂对样品的结构、磁学和光学性质的影响。根据XRD图谱和Rietveld精修的结果可知,所有的样品仍保持R3c相,但晶格常数a,c,晶胞体积V和Fe-O-Fe键角发生变化。适当的Nb掺杂使得样品晶粒尺寸减小,导致剩余磁化强度的增强,使得BiFe_1-xNb_xO₃样品的禁带窄化.     

光折变晶体η-2θ曲线峰值位置明显偏移的研究

实验发现在Ce:(K_0.5Na_0.5)_0.2(Sr_0.61Ba_0.39)_0.9Nb₂O₆(Ce:KNSBN)光折变晶体的同一点记录两幅光栅时,其中一幅光栅的衍射效率(η)相对于两光束夹角(2θ)的关系曲线峰值位置由仅存储一幅光栅时的36°变为44°.采用跳跃模型的微扰分析方法解释了这一现象.     

Die-upset Nd11.5Fe72.4Co9Nb1B6.1 magnets with additions of Zn, Al and Sn

The magnetic properties and microstructure were studied for bulk Nd_11.5Fe_72.4Co₉Nb₁B_6.1 magnets synthesized by hot-pressing and subsequent die-upsetting the melt-spun ribbons with additions of three kinds of low-melting-point metal (Zn, Al and Sn). Die-upset Nd_11.5Fe_72.4Co₉Nb₁B_6.1 magnets have low magnetic properties since they have an inhomogeneous microstructure with many coarse grains. The microstructure of die-upset magnets remains almost unchanged with Al and Sn additions, which only have negative effects on the magnetic properties. Different from Al and Sn additions, Zn addition changes the phase composition of the starting melt-spun powers due to the reaction of Zn and Nd₂Fe₁₄B during hot-pressing and hot-deforming and enhances the development of the desired [0 0 1] texture and improves the microstructure of die-upset magnets. As a result, an anisotropic magnet with good maximum energy product (221 kJ/m³) and high coercivity (670 kA/m) is obtained by adding 2 wt% Zn to the Nd_11.5Fe_72.4Co₉Nb₁B_6.1 alloy.     

Nb3Sn超导材料的高场钉扎行为

本文利用线性回归分析法处理扩散和气相沉积Nb₃Sn带材样品以及青铜法多芯Nb₃Sn线材样品在高场(²2T)下的临界电流测试数据,得到J_c-B曲线和钉扎力随磁场变化的经验规律。该规律与Kramer模型的钉扎力公式不一致,有待提出新的理论模型加以说明。 关键词：     

添加Dy在快淬NdFeB永磁体中的作用

在之前研究Nb元素的添加对快淬(Nd,Dy)_11.5Fe_82.4-mNb_mB_6.1永磁体磁性能、温度特性及显微组织影响的基础上,进一步研究了Dy元素在Nd_11.5-nDy_nFe_81.4Nb₁B_6.1 (n＝0,0.5,1,1.5,2)永磁体中 关键词： NdFeB 磁性能 温度特性 显微组织 X-ray absorption fine structure     

Evidence for the surface barrier in a high-κ superconductor

The entry and exit of magnetic flux have been measured for Nb₃Sn in fields up to 9.5 kOe. The results indicate that the barrier is the most likely mechanism for surface currents in this high-κ material.     

Microprocessing of superconducting Nb3Sn tapes by electron-beam microetching

The unsuitable mechanical properties of Nb₃Sn (its brittleness) make the production of a conductor in the required shape and dimensions very difficult. A method has been proposed and experimentally investigated by which the existing superconducting Nb₃Sn tape could be modified to obtain intrinsically stable superconductor. The required filamentation of commercial Nb3Sn conductor is manageable by electron-beam microetching. The achieved results could serve as a starting point for the construction of a special electron-beam machine. Such device may work in connection with the Nb₃Sn producing apparatus.The authors are grateful to I.Hlásnik for the encouraging discussions, to V.ernuko for providing us with the Nb₃Sn vapour deposited tape and to S.Koina for performing the microanalysis.     

Considerable increase in the thermomagnetic stability of multifilament superconductors internally doped by large-heat-capacity substances

It is demonstrated that the thermomagnetic stability of composite superconductors can be considerably increased by introducing into them a small quantity of a material with an extremely high specific heat at low temperatures. Measurements show that the criterion of “adiabatic” stability for a (Nb₃Sn + 7 vol. % PrB₆) wire is 70% higher than for a reference Nb₃Sn wire (at 4.2 K, the specific heat of the doped sample is seven times higher than that of the reference sample). For a (NbTi + 5vol. % Gd₂O₂S) sample, the specific heat of which at 4.2 K is nine times higher than that of a reference NbTi wire, this increase in stability is as small as 10% (because the characteristic thermal time in the transverse direction is much longer than the time of the magnetic flux jump development).     

The effect of compound addition Dy2O3 and Sn on the structure and properties of NdFeNbB magnets

NdFeNbB with the additions of Dy₂O₃ and Sn permanent magnets have been attained by means of powder-blending technique, and their magnetic properties, temperature performance and microstructure were studied in this paper. The addition of just 2.0 wt% Dy₂O₃ or 0.3 wt% Sn proved to be very effective in improving the permanent magnetic properties of NdFeNbB magnets. Dy₂O₃ additions result in the increase in the H_ci and temperature dependence due to the increase of T_c, formation of (NdDy)-rich phase and grain refinement of Φ phase. This improvement of the coercivity stability of the magnets from the addition of Sn is attributed to the smoothing effect of the Sn addition at the grain boundaries. The magnetic properties, the temperature dependence and Curie temperature of NdFeNbB with Dy₂O₃ and Sn combined addition were found to be considerably improved. From the X-ray diffraction, SEM-EDAX studies and the thermo-magnetic study, the improved properties due to the solution of Dy and Sn to the Φ phase, the reduced N_eff and the smaller Φ phase.     

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).     

Anisotropy of Hc2 in single crystal Nb3Sn and V3Si at high magnetic fields: Limitations of linear chain model

The anisotropy of H_c2 for single crystal Nb₃Sn and V₃Si in the low temperature tetragonal phase was measured in high magnetic fields. At ～ 8K, δ = [H_c2[100]-H_c2[110])/H_c2] ≈0.05 for Nb₃Sn and δ ≈ 0.02 for V₃Si. The data do not show evidence of a highly anisotropic Fermi surface for Nb₃Sn or V₃Si.