Effects of terbium sulfide addition on magnetic properties,microstructure and thermal stability of sintered Nd-Fe--B magnets

To increase coercivity and thermal stability of sintered Nd–Fe–B magnets for high-temperature applications, a novel terbium sulfide powder is added into(Pr_(0.25)Nd_(0.75))_(30.6)Cu_(0.15)Fe_(bal)B_1(wt.%) basic magnets. The effects of the addition of terbium sulfide on magnetic properties, microstructure, and thermal stability of sintered Nd–Fe–B magnets are investigated.The experimental results show that by adding 3 wt.% Tb_2S_3, the coercivity of the magnet is remarkably increased by about 54% without a considerable reduction in remanence and maximum energy product. By means of the electron probe microanalyzer(EPMA) technology, it is observed that Tb is mainly present in the outer region of 2:14:1 matrix grains and forms a well-developed Tb-shell phase, resulting in enhancement of HA, which accounts for the coercivity enhancement.Moreover, compared with Tb_2S_3-free magnets, the reversible temperature coefficients of remanence(α) and coercivity(β) and the irreversible flux loss of magnetic flow(hirr) values of Tb_2S_3-added magnets are improved, indicating that the thermal stability of the magnets is also effectively improved.     

High-pressure elastic anisotropy and superconductivity of hafnium:A first-principles calculation

The elastic anisotropy and superconductivity upon hydrostatic compression of α, ω, and β Hf are investigated using first-principle methods. The results of elastic anisotropies show that they increase with increasing pressure for α andω phases, while decrease upon compression for β phase. The calculated superconducting transition temperatures are in excellent agreement with experiments. Electron–phonon coupling constants(λ) are increasing with pressure for α and ωphases, while decreasing for β phase. For β phase, the large values of λ are mainly due to the obvious TA1 soft mode.Under further compression, the TA1 soft vibrational mode will disappear gradually.     

Direct measurement of the AC loss of an ITER relevant coil

In the ENEA Frascati Laboratory, an ITER relevant coil is being tested in pulsed regime. One item of the testing program is the search for possible ramp rate limitations. The range explored (up to 3.2 T/s) covers amply the ITER Central Solenoid operating regime (0.5–1 T/s). An interesting outcome of these tests is that repeated run sets, performed in similar conditions, show an increasing value of the ramp rate that produces a quench. This behaviour is attributed to a continuous decrease of the value of the AC loss constant nτ, due to the increase of the cable transversal resistivity. An explanation on the basis of a ‘classical' training was also considered, but for a number of reasons reported in the paper, it was discarded. Direct measurements of the AC loss on the whole coil confirm that the original value of nτ (140 ms), as measured on a ‘virgin' piece of conductor, has gone below 40 ms. The measurements are made difficult by the presence of a heavy mechanical structure and of a background coil. The techniques applied and the results are reported in detail.     

Three-dimensional pictures of industrial scenes applying an optical radar

For small scale ranging, a light beam, modulated with a wavelength typical for radio waves, is a suitable means of scanning. The ranging beam can be focussed to illuminate only a small spot. The diffuse reflection on the spot ensures that an echo appears. By applying phase comparison techniques to the ranging and echo beam the optical path length and thus the height of the illuminated surface element can be determined. By systematically deflecting the beam and thereby scanning the scene, a discrete height map is obtained. The realisation of such a small scale ‘optical radar’ is described, the main elements of which are a semiconductor diode laser for emission, an avalanche photo diode for detection, and a two-dimensional beam deflection mechanics for scanning. The typical performance is taking a picture of a volume of about 0·5 m × 0·5 m × 0·5 m within 1s, consisting of about 64 000 pixels.     

Measurement of iron characteristics under ramp compression

Laser-driven ramp compression was used to investigate iron characteristics along the isentropic path. The iterative Lagrangian analysis method was employed to analyze the free surface velocity profiles in iron stepped target measured with two VISARs. The onset stress for the α to ε phase transformation was determined from the sudden change in the sound velocity and was found over-pressurized compared to the static and shock results. The derived stress(26 GPa) and strain rate(up to 10~8 s~(-1)) are consistent with our previous experimental results. The stress-density relations were compared with those from previous ramp experiments and good agreements were found, which experimentally confirms the simulations,showing that iterative Lagrangian analysis can be applied to the ramp-compression data with weak shock.     

轴对称高场超导磁体电磁应力有限元分析方法

核磁共振成像和核磁共振谱仪是高场超导磁体的主要应用领域.高场超导磁体通常具有较高的磁场和运行电流,在运行过程中超导线会产生较高的电磁应力,其临界特性将发生退化,影响磁体的稳定性.开展高场超导磁体的电磁应力精确分析显得尤为必要.本文发展了一种快速有效的有限元分析方法,第一步,为整个超导磁体系统建立平均有限元模型,采用传统的电磁-结构耦合方法求解电磁应力,获得最大应力位置;第二步,对最大应力所在的超导线圈建立详细有限元模型,采用单积分-结构分析方法精确求解每一组分中电磁应力.基于该模型研究了500 MHz NMR超导磁体的电磁应力.该分析方法也可以用于超导磁体冷却过程中的热应力分析.为高场超导磁体设计和建造提供有益的理论依据.     

Improvement in coercivity,thermal stability,and corrosion resistance of sintered Nd-Fe-B magnets with Dy_(80)Ga_(20) intergranular addition

To investigate the coercivity,corrosion resistance,and thermal stability of Nd-Fe-B magnets,their properties were investigated at room and high temperature before and after doping with Dy_(80)Ga_(20)(at.%) powder.The coercivity of the magnets increased from the undoped value of 12.72 kOe to a doped value of 21.44 kOe.A micro-structural analysis indicates that a well-developed core-shell structure forms in the magnets doped with Dy_(80)Ga_(20) powder.The improvement in magnetic properties is believed to be related to the refined and uniform matrix grains,continuous grain boundaries,and a hardened(Nd,Dy)_2Fe_(14)B shell surrounding the matrix grains.Additionally,the doped magnets exhibit an obvious improvement in thermal stability.For the magnets with added Dy_(80)Ga_(20) powder,the temperature coefficients of remanence(α) and coercivity(β) increased to-0.106%℃~(-1) and-0.60%℃~(-1) over the range 20-100 ℃,compared to temperature coefficients of-0.117%℃~(-1)(α) and-0.74%℃~(-1)(β) in the regular magnets without Dy_(80)Ga_(20) powder.The irreversible loss of magnetic flux(Hirr) was investigated at different temperatures.After being exposed to 150 ℃ for 2 h,the Hirr of magnets with 4 wt.%Dy_(80)Ga_(20) decreased by ~95%compared to that of the undoped magnets.The enhanced temperature coefficients and Hirr indicate improved thermal stability in the doped Nd-Fe-B magnets.The intergranular addition of Dy_(80)Ga_(20) also improved the corrosion resistance of the magnets because of the enhanced intergranular phase.In a corrosive atmosphere for 96 h,the mass loss of the sintered magnets with 4 wt.%Dy_(80)Ga_(20) was 2.68 mg/cm~2,less than 10%of that suffered by the undoped magnets(28.1 mg/cm~2).     

Segregation of copper in the ITER cable and ramp rate limitation

The quench of a superconducting cable containing a defect and subjected to a current increase is studied. The cable is represented by a model which consists of a strand with a local decrease in the critical current I_c and the rest of the cable lumped together. It is assumed that the quench takes place when the electric field in the defect reaches the instability level. An important feature of the process is the noticeable portion of current expelled from the defect to the rest of the cable prior to the quench. It is shown that given the strong inequality in the strand currents during the ramp, the influence of the defect on the quench current is proportional to I_c⁴. This criterion is applied to the problem of copper separation in the triplets of the ITER cables.     

The similarities in loss creation in LTS and HTS cables

The coupling current losses represent an essential contribution to AC losses in most practical superconducting conductors. The origin of this loss type is well known, being caused by induced currents in different loops consisting of superconducting and non-superconducting parts. However, the ‘current pattern' in different conductor types (strands, flat or round cables, more complicated cable structures, CICC) varies appreciably. These differences are mainly due to geometrical effects (size and shape of filaments and/or strands, their spatial distribution, conductor aspect ratio, demagnetization effects). Although the general knowledge about AC losses in low temperature cable structures is by far not complete (mainly at higher frequencies, in inhomogeneous fields and for inhomogeneous cable structure), an attempt is made to summarize those results which can be adopted to high T_c conductors and some remarks are made about new features of AC losses in these conductor types.     

镁基合金自由枝晶生长的相场模拟研究

利用定量相场模型, 以Mg-0.5 wt.%Al合金为例模拟了基面((0001)面)内镁基合金的等温自由枝晶生长过程. 通过研究该合金体系数值模拟的收敛性, 获得了最优化值耦合参数λ = 5.5及网格宽度Δx/W₀ = 0.4, 并在该参数下系统研究了各向异性强度和过饱和度对枝晶尖端生长速度、尖端曲率半径、Péclet数及稳定性常数σ^* 的影响. 结果表明, 由微观可解性理论得到的稳定性系数σ^* 与ε₆ 拟合值σ^*≅ ε₆ ^1.81905, 更接近理想值σ ^* (ε₆) ≅ε₆ ^1.75. 此外, 当过饱和度Ω < 0.6时, 稳定性系数σ ^* 不随ε₆ 的变化而变化, 而当Ω > 0.6时, 稳定性系数σ ^* 随着ε₆ 的增加而减小. 这反映了枝晶的生长由扩散控制向动力学控制的转变. 随着过饱和度的增加, 枝晶形貌由雪花状枝晶向圆状枝晶转变.     

Cu NMR study of the superconducting thiospinel Cu1.5Rh1.5S4

The ⁶³Cu NMR Knight shift K and spin-lattice relaxation rate 1/T₁ have been measured to study the thiospinel superconductor Cu_1.5Rh_1.5S₄ from a microscopic viewpoint. K is negative and has a weak dependence on temperature, and the hyperfine coupling constant H_hf^d is estimated to be −52.4 kOe/μ_B. 1/T₁ is proportional to the temperature in the normal state. In the superconducting state, 1/T₁ takes a coherence peak just below T_c, and decreases exponentially well below T_c, from whose temperature dependence the superconducting energy gap has been proved to be close to 2Δ = 3.52k_BT_c given by the BCS theory.     

Control schemes for synchronizing two subnetworks with weak couplings

In this paper, we propose a well-designed network model with a parameter and study full and partial synchronization of the network model based on the stability analysis. The network model is composed of a star-coupled subnetwork and a globally coupled subnetwork. By analyzing the special coupling configuration, three control schemes are obtained for synchronizing the network model. Further analysis indicates that even if the inner couplings in each subnetwork are very weak, two of the control schemes are still valid. In particular, if the outer coupling weight parameter θ is larger than (n2 2n)/4, or the subnetwork size n is larger than θ2, the two subnetworks with weak inner couplings can achieve synchronization. In addition, the synchronizability is independent of the network size in case of 0 < θ < n/(n + 1). Finally, we carry out some numerical simulations to confirm the validity of the obtained control schemes. It is worth noting that the main idea of this paper also applies to any network consisting of a dense subnetwork and a sparse network.     

The impact of nuclear magnetism on superconductivity

The influence of nuclear magnetism on the superconducting critical field B_c has been measured in the nuclear magnets Al, AuIn₂, and Sn, at ultralow temperatures, 17 μKT1 K. The materials have been chosen in respect to their distinctive Korringa constants κ(Al, AuIn₂, Sn)=(1.8, 0.1, 0.05) Ks. Both in the weakly and in the strongly coupled nuclear magnets, Al and Sn, a reduction of B_c is observed being proportional to the nuclear magnetization at T4 mK. However, using the high resolution of the experiment performed on Sn, the decrease of B_c at cooling down to 70 μK is found to be nonmonotonic. The features of the recently measured B_c(T) curve of Sn which are not understood yet as well as the formerly observed results of the interplay of nuclear ferromagnetism and superconductivity in AuIn₂ have motivated ongoing experiments which try to clear up the pair breaking contribution of nuclear magnetism.     

浅海内波影响下的波导不变量变化特性分析

针对浅海内波引起波导不变量变化的问题,利用声场波导不变量的概率分布并结合声场简正波的理论,研究了内波活动下波导不变量的时变性,给出了波导不变量变化的机理和规律.具体结论是,在负跃层波导中,声场的波导不变量的最大概率取值具有明显的频变特性.内波环境下,当声传播方向与内波波阵面平行时,接收声场简正波的幅度变化不大,但是简正波的相慢度差和群慢度差的变化却能引起波导不变量最大概率取值的变化;而当声传播方向与内波波阵面垂直时,内波引起的简正波耦合同样会导致波导不变量的最大概率取值的明显变化.     

精确提取一维互易有限周期性结构色散特性的宏元胞法

提出了一种可精确提取一维互易有限周期性结构色散特性的宏元胞法. 将多个周期性元胞组合构成单个“宏元胞”, 以严格考量元胞间的互耦效应, 通过Bloch分析分别计算出正向激励和反向激励时Bloch 波的本征传输因子ξ^N和1/ξ^N, 对ξ^N取复对数操作即可解出一维有限周期性结构的衰减常数α, 而复对数函数的多值性使得相移常数β存在多整数分支的选择问题. 通过与解卷绕法获得的β近似值相比较,选择出合理的整数分支, 可精确计算出β. 与传统的本征模式法、解卷绕法及单元胞法相比,宏元胞法对元胞间的耦合强度不作要求, 计算更加精确快速, 可作为刻画一维互易有限周期性结构色散和Bloch阻抗特性的有力工具. 关键词： 宏元胞法 Bloch分析 色散特性     

Enhancement of subgap conductance in a graphene superconductor junction by valley polarization

We theoretically study the differential conductance of a graphene/graphene superconductor junction, where the valley polarization of Dirac electrons is considered in the nonsuperconducting region. It is shown that the subgap conductance will increase monotonically with the valley-polarization strength when the chemical potential μ is near the Dirac point μ≤ 3?(? is the superconducting gap), whereas it will decrease monotonically when μ is far away from the Dirac point, μ≥ 5?.The former case is induced by the specular Andreev reflection while the retro-reflection accounts for the later result. Our findings may shed light on the control of conductance of a graphene superconductor junction by valley polarization.     

Three-party reference frame independent quantum key distribution protocol

We present a three-party reference frame independent quantum key distribution protocol which can be implemented without any alignment of reference frames between the sender and the receiver. The protocol exploits entangled states to establish a secret key among three communicating parties. We derive the asymptotic key rate for the proposed protocol against collective attacks and perform a finite-size key security analysis against general attacks in the presence of statistical fluctuations. We investigate the impact of reference frame misalignment on the stability of our protocol, and we obtain a transmission distance of 180 km, 200 km, and 230 km for rotation of reference frames β=π/6, β=π/8 and β=0, respectively. Remarkably, our results demonstrate that our proposed protocol is not heavily affected by an increase in misalignment of reference frames as the achievable transmission distances are still comparable to the case where there is no misalignment in reference frames (when β=0). We also simulate the performance of our protocol for a fixed number of signals. Our results demonstrate that the protocol can achieve an effective key generation rate over a transmission distance of about 120 km with realistic 10⁷ finite data signals and approximately achieve 195 km with 10⁹ signals. Moreover, our proposed protocol is robust against noise in the quantum channel and achieves a threshold error rate of 22.7%.     

Novel wavelength-division-multiplexed ring network architecture using optical path technology for multiple services and simple media access control procedure

A novel WDM ring network architecture that employs an attractive integrated-optic arrayed-waveguide grating (AWG) add-drop multiplexer (ADM) is proposed that fully uses a variety of optical path technologies. Single-hopped multiwavelength looped-bus optical paths establish multipoint-to-multipoint (M-M) communication, while conventional ringtype optical paths realize point-to-point and broadcast communication. Optical paths for OAM & C can also be established on a multihop basis with no additional wavelengths. All these optical paths can be established by a simple AWG-ADM in a node. A simple MAC procedure for M-M communication is proposed, and its performance is simulated. This procedure switches between the random with minimum slot distance limitation scheme and the slot assignment scheme. Simulation results confirm its fairness as evaluated by the metric of delay variance and its small delay. Network survivability is realized by the optical loop-back (LB) method.     

14T全身超导MRI磁体的技术挑战——大规模应用强场超导磁体未来十年的发展目标之一

本文首先综述了大规模应用的超导磁体,依赖并推动铌三锡Nb3Sn导线技术进步,向更强磁场发展的趋势.着重分析了超高场14 T全身MRI磁体的挑战性技术.选择青铜Nb3Sn导线,采用Nb3Sn线圈和NbTi线圈相结合的混合结构,对14 T全身MRI磁体进行了电磁概念设计和热稳定性及失超保护仿真分析,并简要阐述了14 T全身MRI磁体在应力、接头和匀场方面的关键问题.根据分析结果认为:1)Nb3Sn导线是14 T全身MRI磁体需要面临的首要挑战性问题—作为最佳选择的青铜Nb3Sn导线,其现有产品的性能指标离14 T全身MRI磁体的要求尚存在有一定的差距;2)14 T全身MRI磁体的失超保护涉及线圈的铜超比设计、运行电流同线圈电感的协调配置、被动保护的分段策略和主动保护的失超触发控制以及主动屏蔽结构磁体在失超过程中的逸散磁场限制等多个十分复杂的环节,是最具挑战性的综合性技术.     

The geometry of the SU(2) Kepler problem

The SU(2) Kepler problem is defined and analyzed, which is a Hamiltonian system reduced from the conformal Kepler problem on T^*( ⁸ − {0}) by the use of the symplectic SU(2) action lifted from the SU(2) left action on the SU(2) bundle ⁸ − {0} → ⁵ − {0}. This reduced system has a parameter μ ε su(2) coming from the value of the moment map associated with the symplectic SU(2) action. If μ ≠ 0, the phase space of this system have a bundle structure with base space T^*( ⁵ − {0}) and fibre S². The fibre, a (co)adjoint orbit through μ for SU(2), represents the internal degrees of freedom, called the isospin, of the particle of this system. The SU(2) Kepler problem with μ ≠ 0 is then interpreted as describing the motion of a classical particle with isospin in the Newtonian potential plus a specific repulsive potential together with a Yang-Mills field. This Yang-Mills field is to be referred to as BPST Yang's monopole field in ⁵ − {0};, since it becomes the Belavin-Polyakov-Schwartz-Tyupkin instanton, restricted on S⁴. If μ = 0, the SU(2) Kepler problem reduces to the ordinary Kepler problem. Like the ordinary Kepler problem, the Hamiltonian flows of the SU(2) Kepler problem of negative energy are all closed. It is shown in an explicit manner that the energy manifolds and isoenergetic orbit spaces for the SU(2) Kepler problem of negative energy are both homogeneous manifolds on which SU(4) acts transitively to the right; those homogeneous manifold are classified into two, according as the parameter μ is zero or not. For a certain value of μ, however, they contracts to the manifold which represents the set of all the equilibrium states. The isoenergetic orbit spaces are finally shown to be symplectomorphic to certain Kirillov-Konstant-Souriau coadjoint orbits for U(4), if μ is not the exceptional value mentioned above.