Development of high purity niobium used in SRF accelerating cavity

Niobium is widely used in SRF(Superconducting Radio Frequency)cavities due to its excellent superconductivity and workability.With the continuous development of technology,higher demands of material are raised.One of the key issues is that RRR(Residual Resistance Ratio)of the Nb material should be more than 300.which requires that the Nb ingot have even higher RRR.This article introduces the development and the experimental results of high purity niobium in OTIC in Ningxia(Ningxia Orient Tantalum Industry Co.Ltd.),and the test results of the single cell TESLA(Tera Electron volt energy Superconducting Linear Accelerator)shaped cavity manufactured by Peking University using Nb material from OTIC.     

用于射频超导腔制造的大晶粒铌材热导研究（英）

大晶粒铌材由于声子峰效应在2 K温度具有良好的热导,这有助于提高超导腔的热稳定性。对宁夏东方钽业股份有限公司（OTIC）生产的大晶粒铌材在不同热处理条件下的热导和晶格缺陷开展了研究,结果表明经过超过800 ℃的热处理,OTIC的大晶粒铌材能够恢复声子峰,这与之前DESY的测量结果不同。认为用其加工的超导腔有望具备较好热稳定性。     

The First Nine-Cell TESLA Cavity Made in China

A totally home-made 9-cell TESLA type superconducting cavity is mane at Peking University. The cavity abrication is according to DESY specification. The cavity is made of high purity niobium from OTIC, Ningxia. The electron beam welding is carried out at Harbin Institute of Technology, Harbin. By the cooperation, the cavity is tested at Thomas Jefferson National Accelerator Facility, USA. The preliminary result shows the acceleration gradient Eacc is 23 MV/m without quench and has potential for improvement.     

1.3GHz单cell高梯度低损耗超导腔设计研究

计划中的国际直线对撞机(简称 ILC), 已确定采用低温超导技术. 研究表明, ILC的整体造价与超导腔的加速梯度密切相关, 因此新型高梯度超导腔的研制, 已成为目前ILC的热门课题. 目前超导腔加速梯度的提高受限于超导体射频临界磁场(H_c^rf), 希望通过腔形的优化设计, 来减小超导腔表面峰值磁场与加速梯度的比值(H_p/E_acc), 进而提高超导腔的加速梯度, 以满足ILC的要求. 目前, 具有代表性的高梯度超导腔新腔形有两种, 即低损耗超导腔(LLSCC)和重入式超导腔(RESCC). LLSCC相对于RESCC而言, 具有较低的H_p/E_acc, 因此在理论上LLSCC更易于达到较高的梯度. 作者对1.3GHz单cell高梯度低损耗超导腔的腔形进行了详细的研究, 给出了相应低损耗超导腔腔形, 并对这一设计结果进行了分析比较.     

锐钛矿相和金红石相Nb:TiO_2电学性质的GGA(+U)法研究

采用基于密度泛函理论第一性原理GGA和GGA+U相结合的方法研究了不同掺杂浓度下锐钛矿相和金红石相Nb:TiO2的晶体结构、电子结构以及稳定性.结果表明:锐钛矿相Nb:TiO2能带结构与简并半导体类似,呈类金属导电机理.金红石相Nb:TiO2呈半导体导电机理.Nb原子比Ti原子电离产生出更多的电子.锐钛矿相Nb:TiO2中Nb原子的电离率比金红石相Nb:TiO2的大.以上结果说明锐钛矿相Nb:TiO2比金红石相Nb:TiO2更适宜用作TCO材料;掺杂浓度对其杂质能级,费米能级和有效质量都有影响.Nb原子掺杂浓度越高,材料电离率呈降低趋势;形成能计算结果显示:在富钛条件下不利于Nb原子的掺杂,而在富氧条件下有利于Nb原子的掺杂.对于金红石相和锐钛矿相Nb:TiO2,不论是在贫氧或富氧条件下,随着Nb原子掺杂浓度的提高,形成能均增大.     

Role of the external surfaces on the superconducting properties of superconductor/normal metal trilayers

Superconducting proximity effect is studied in superconductor/normal metal trilayers. The dependences of the superconducting transition temperature _Tc$T_c$ versus Nb thickness in Cu/Nb/Cu systems and versus Cu thickness in Nb/Cu/Nb ones are described by different values of the microscopical parameters. We attribute this difference to the influence of the external surfaces of the Nb/Cu/Nb hybrids on the superconducting properties of the system.     

Mechanisms of periodic pinning in superconducting thin films

We studied the changes in the superconducting properties of Nb films due to an array of Ni dots used as collective pinning sites. To determine the pinning mechanism, thin Ag layers of varying thicknesses were deposited on the Ni dots prior to the Nb film deposition. The Ag deposited on the pinning dots has little effect on the collective pinning phenomena, which implies that the main pinning mechanism is of magnetic origin.Received: 3 February 2004, Published online: 31 August 2004PACS: 74.25.Qt Vortex lattices, flux pinning, flux creep - 74.78.-w Superconducting films and low-dimensional structures     

Inhibiting radiative recombination rate to enhance quantum yields in a quantum photocell

Inhibiting the radiative radiation is an efficient approach to enhance quantum yields in a solar sell. This work carries out the inhibition of radiative recombination rate(RRR) in a quantum photocell with two coupled donors. We perform explicit calculations of the transition rates, energy gaps and the absorbed solar wavelength-dependent RRR, and find that two different regimes play the crucial roles in inhibiting RRR. One is the quantum coherence generated from two different transition channels, the other includes the absorbed photon wavelength and gaps between the donor and acceptor in this proposed photocell model. The results imply that there may be some efficient ways to enhance the photoelectron conversion compared to the classic solar cell.     

Fabrication of Al air-bridge on coplanar waveguide

Superconducting coplanar waveguide(CPW) can be widely used as two-dimensional(2 D) resonator, transmission line or feedline, providing an important component for superconducting quantum circuit which is a promising candidate for quantum information processing. Due to the discontinuities and asymmetries in the ground planes, CPW usually exhibits the spurious resonance, which is a common source of decoherence in circuit quantum electrodynamics experiments. To mitigate the spurious resonance, we fabricated superconducting aluminum air-bridges on Nb CPW. The fabricated airbridges are approximately 3 m high and up to 120 m long. Compared with other methods, the fabrication procedures of our air-bridges are simpler, and the air-bridge can withstand strong ultrasound.     

具有Washer型输入线圈的超导量子干涉放大器的制备与表征

超导量子干涉仪(SQUID)放大器具有低输入阻抗、低噪声、低功耗等优点, 目前被广泛用于微弱信号的检测领域. 与其他工艺相比, Nb/Al-AlO_x/Nb结构的约瑟夫森结具有相对较高的转变温度(T_c)、高的磁通电压调制系数以及良好的热循环能力、较宽的临界电流范围, 因此是制备SQUID放大器的很好选择. 设计并制作了欠阻尼、过阻尼约瑟夫森结以及具有Washer型输入线圈的单SQUID放大器, 通过在He³制冷机3 K温区下对器件电流-电压特性进行测量, 得到良好的结I-V特性曲线、SQUID调制特性, 初步实现利用SQUID进行放大作用, 并计算了SQUID的电流分辨率. 此项工作对于超导转变边沿传感器读出电路的实现具有重要的意义.     

Atomistic simulations to study crack tip behaviour in single crystal of bcc niobium and hcp zirconium

In this article, molecular dynamics based simulations were carried out to study the fracture toughness of single crystals of niobium (Nb) and zirconium (Zr). Separate set of simulations were performed with different orientations of crack plane in Nb and Zr. In each case, efforts were made to align the crack front with principal planes of corresponding crystal structure, that was bcc for Nb and hcp for Zr. Deformation in single crystal of Nb and Zr was governed either by twinning or emergence of dislocations from the crack tip and surfaces. The deformation mechanism in single crystal of Nb and Zr also helps in governing the overall toughness of the material, while deforming via twinning leads to higher change in the crack length, whereas dislocations emerging from the crack tip blunts the opening and improves the toughness.     

Rectified vortex motion in an Nb film with a spacing-graded array of holes

Superconducting Nb thin films with a spacing-graded array of holes were prepared by electron beam lithography. Two films with different hole gradients were fabricated. The ac-driven vortices were investigated in Nb superconductors with a spacing-graded array of holes. The measurements revealed pronounced rectified voltage when the vortex lattice is driven by an ac injected current. The rectified voltage is mainly caused by the strength of the vortex–vortex interaction. The rectified motion of a vortex is affected by the pinning potential of the spacing-graded array and the applied magnetic field. The vortex–vortex interaction strength changes the effective pinning landscape of the vortices and an asymmetric potential is formed. Vortices depin easily from high concentration to low concentration of pinning sites. In both samples, the ac-driven vortices exhibit a variety of dynamical responses and the rectified voltage is tunable with the applied magnetic field.     

超导腔的热击穿和超导铌材料RRR值的无损测量

介绍超导腔热击穿原理 ;讨论了超导体热击穿磁场与超导材料 RRR值之间的定量关系 ;同时介绍一种非常实用的超导腔体 RRR值的测量方法涡流法无损测量原理及其实现方法 ;最后给出了国内外两种典型超导铌材料 RRR值的比较测量结果     

Submm heterodyne mixing using NbCN/Nb sis tunnel junctions

We describe heterodyne mixing experiments with NbCN/Nb quasi-particle tunnel junctions at submillimeter wavelengths. In this wavelength range junctions with niobium nitride as superconducting material are promising because of the high gap voltage, about 5.7 mV, as compared to 3 mV for the more commonly used niobium. As a first step towards all-NbN junctions we investigate the development of junctions with a NbN base electrode and a Nb upper electrode. We present results from samples with two different insulating barriers: aluminum oxide and magnesium oxide. Measured noise temperatures range from 500 K at 345 GHz and 1600 K at 482 GHz to 8500 K at 644 GHz. These results are about one order of magnitude worse than the best results obtained with all-Nb junctions. The difference can be attributed partly to the relatively high radio-frequency (rf) losses in NbN films as compared to Nb. Also sensitivity is reduced because of the relatively high leakage current in the sub-gap region of the I-V curve of the junction.     

On the preparation of Nb3Sn-layers on monocrystalline Nb-substrates

Superconducting Nb₃Sn cavities are prepared, using the standard technique, by exposing polycrystalline Nb cavities to Sn vapour at 1050°C for some hours. The influence of the niobium crystal orientation on the quality of the Nb₃Sn layers is studied by microscope (optical and SEM) and AES. Without preanodization of the niobium singlecrystal substrates all layers are patchy, especially for (111)-and (531)-planes. With preanodization homogeneous dense layers are obtained independent of the niobium orientation. All layers are essentially free from C and O contaminations. This work has been supported under the technological program of the Federal Department of Research and Technology of the FRG.     

地球化学样品中铌、钽的ICP-MS检测技术研究与应用

为给铌、钽资源勘查和开发提供分析测试技术支撑,基于地球化学样品中铌、钽的特殊化学性质,应用电感耦合等离子体质谱技术（ICP-MS）,对地球化学样品中铌、钽的检测方法进行了深入研究。结果表明：样品溶液制备和仪器测定参数对检测效果有重要影响,特别是前者,因此优化样品溶液制备的重要参数是检测技术的关键所在,试验选定条件是：试样重量为50 mg;氢氟酸用量为15 mL;样品溶液中硝酸、酒石酸浓度分别为2%和1.5%;溶液有效测定期限≤1 d。优化并确立的检测方法经国家一级地球化学标准物质验证,其准确度、精密度均达到行业标准的要求。方法检出限分别为Nb：1.05 μg·g-1和Ta：0.13 μg·g-1。实验研究证明：应用特定的样品溶液制备流程,并结合ICP-MS技术适用于地球化学样品中铌、钽的准确快速检测,尤其是样品量较大且铌、钽含量较低的地球化学勘查样品。     

Comparison of laser-induced damage in Ta2O5 and Nb2O5 single-layer films and high reflectors

Ta2O5 and Nb2O5 films are deposited on BK7 glass substrates using an electron beam evaporation method and are annealed at 673 K in the air.In this letter,comparative studies of the optical transmittance,microstructure,chemical composition,optical absorption,and laser-induced damage threshold(LIDT) of the two films are conducted.Findings indicate that the substoichiometric defect is very harmful to the laser damage resistance of Ta2O5 and Nb2O5 films.The decrease of absorption improves the LIDT in films deposited by the same material.However,although the absorption of the Ta2O5 single layer is less than that of the Nb2O5 single layer,the LIDT of the former is lower than that of the latter.High-reflective(HR) coatings have a higher LIDT than single layers due to the thermal dissipation of the SiO2 layers and the decreased electric field intensity(EFI).In addition,the Nb2O5 HR coating achieves the highest LIDT at 25.6 J/cm 2 in both single layers and HR coatings.     

用于电压基准的Nb/NbxSi1-x/Nb约瑟夫森单结的研制

现代可编程约瑟夫森电压基准的核心器件是约瑟夫森结阵.目前最具有优势的约瑟夫森结阵是 Nb/Nb_xSi_1-x/Nb材料的结阵. Nb/Nb_xSi_1-x/Nb材料的约瑟夫森结 具有三层薄膜的制作过程简便, Nb和Nb_xSi_1-x刻蚀工艺相同以及Nb_xSi_1-x 势垒层成分可调等优点.中国计量科学研究院设计制作了Nb/Nb_xSi_1-x/Nb约瑟夫森单结. 通过在4.2 K低温下对所做单结进行直流电流-电压特性测量,观测到了清晰的超导隧穿电流和 从零电压态向电压态的跳变,最后就测量结果进行了分析讨论.此项工作属于国内首个开展 Nb/Nb_xSi_1-x/Nb材料约瑟夫森单结研究的工作.     

A TEM study of α-phase stability in Zr-2.5 Nb pressure tubes following neutron irradiation (A TEM study of α-phase stability)

The α-phase in Zr-2.5 Nb pressure tubes has been analysed after irradiation with neutrons for temperatures between 523 and 573 K. The α-phase contains Nb in supersaturated solid solution (about 0.5−1 wt% Nb) in the as-fabricated condition. Irradiation results in the formation of small discrete precipitates having diameters of about 5 nm. Energy Dispersive X-ray (EDX) analysis using a scanning transmission electron microscope (STEM) was complicated by the presence of a self-generated X-rays in the irradiated material and by the fact that the precipitates were small relative to the foil thickness (typically 100–200 nm). Techniques developed to measure segregation of impurities at interfaces were employed to show that the precipitates were Nb-rich, their formation being consistent with a decrease in Nb concentration in the matrix.