Nb_3Al超导材料及磁体制备技术的研究进展

Nb3Al超导材料在高场下具有非常高的临界电流密度以及优异的应变容许特性,被认为是高场磁体应用的理想材料.但由于Nb3Al超导材料成相反应过程的特殊性导致其制备难度极大,成为目前实用化低温超导材料研究领域的热点和难点.本文对目前国内外Nb3Al超导材料及磁体制备技术的研究进展进行了系统分析,对未来发展的趋势进行了探讨.     

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.     

绿色荧光粉Gd2Ba3B3O12:Tb3+的发光性能研究

采用高温同相法制备了一系列Gd<,2>Ba<,3>B<,3>O<,12>:Tb<'3+>绿色荧光粉,借助X射线粉末衍射仪(XRD)、真空紫外光谱和荧光光谱仪(VUV-UV)对样品的物相、发光性能进行了表征.结果表明,Tb<'3+>作为发光中心全部进入到基质Gd<,2>Ba<,3>B<,3>O<,12>的品格中并占据Gd...     

Al2O3:Fe3+体系晶格局域结构的EPR理论研究

本文通过分析Al2O3∶Fe3+体系中Fe3+离子的EPR谱,研究Fe3+的局域晶体结构结果表明Al2O3∶Fe3+的局域结构存在各向异性膨胀.用拟合EPR谱的低对称参量D和(a-F)实验值的方法,求得两个三棱锥的棱与C3轴的夹角分别为θ1=46.54°和θ2=61.26°,相对于原Al2O3结构的畸变角分别是Δθ1=-1.1°±0.1°,Δθ2=-1.8°.两畸变角同时均小于0说明Al2O3∶Fe3+体系中含Fe3+离子的晶格主要产生沿C3轴的伸长畸变.     

产生蓝色相干辐射和实现Nd:YAG 1.0642μm倍频90°非临界相位匹配的Nb:KTiPO4

本文报道了用熔盐顶部籽晶法生长Nb浓度(0～13)mol%的Nb:KTP晶体的倍频的Ⅱ型相位匹配的截止波长和Nd:YAG1.0642μm及Nd:YA1O₃1.0795μm激光在这些晶体中倍频的最佳相位匹配角的测量结果.从中可看出,由于Nb⁵⁺的引入使KTP晶体倍频的Ⅱ相位匹配的截止波长有效蓝移,目前已使截止波长蓝移至937nm且有效产生468.5nm的倍频蓝光.同时Nb⁵⁺的引入使Nd:YAG1.0642μm激光和Nd:YA1O₃倍频的最佳相位匹配方向产生很大的变化,目前已使Nb:KTP晶体倍频的最佳相位匹配方向为θ=88.32°、Φ=0°,非常接近于90°非临界相位匹配方向.     

铝、碳共掺提高MgB_2临界电流密度

本文研究了在MgBq超导体中Al,C及两种元素共同掺杂时对样品的超导转变温度,不可逆场和临界电流的影响.研究发现,少量的铝掺杂可以提高MgB2超导材料在低场下的临界电流密度,但由于对Tc的抑制而降低其在高场下的载流能力,而碳掺杂则可以有效提高MgB2超导材料的上临界场和不可逆场,从而达到改善其在高场下的载流能力的目的.研究发现,在0-7T外磁场范围内,1%铝和1%碳共同掺杂的样品表现出最佳的载流性能.钉扎力曲线的分析也应证了这一结果.     

新型六方晶型磁载TiO2纳米粒子的合成及其光降解性能

采用溶胶-凝胶法,在磁性Fe<,3>O<,4>表面包覆TiO<,2>,制备了一种新型六方晶型纳米TiO<,2>/Fe<,3>O<,4>光催化复合材料.X射线衍射(XRD),透射电镜(TEM)对材料形态结构及包覆情况的分析,显示TiO<,2>包覆在Fe<,3>O<,4>表面,对染料废水光催化降解的模拟研究表明,该复合材料...     

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.}     

Tb0.3Dy0.7(Fe1-xAlx)1.95合金的磁致伸缩、自旋重取向和穆斯堡尔谱研究

系统研究了室温下Tb0.3Dy0.7(Fe1-xAlx)1.95(x=0,0.05,0.1,0.15,0.2,0.25,0.3,0.35)合金中金属Al替代Fe对晶体结构、磁致伸缩、各向异性和自旋重取向的影响.结果发现,x<0.4时,Tb0.3Dy0.7(Fe1-xAlx)1.95完全保持MgCl2立方Laves相结构,晶格常量a随Al含量x的增加而增大.磁致伸缩测量发现,随着替代量x的增多磁致伸缩减小,x>0.15时超磁致伸缩效应消失;x<0.15时磁致伸缩在低场下(H≤4kA/m)有小幅增加,高场下迅速减小,而且易趋于饱和,说明添加少量Al有助于减小磁晶各向异性.内禀磁致伸缩λ111随Al替代量x的增加大幅度降低.对于0≤x<0.15,穆斯堡尔效应表明,随Al含量的增加Tb0.3Dy0.7(Fe1-xAlx)1.95合金中发生了自旋重取向,易磁化方向经历了[111]→[u v 0]→[u v w]的转变.由相对磁化率随温度的变化关系可以看出,Al替代Fe使自旋重取向温度降低.当x=0.15时,Tb0.3Dy0.7(Fe1-xAlx)1.95合金中出现了少量非磁性相;x>0.15时,合金在室温下呈现顺磁性.     

Nb-Ti-Al合金高温氧化机理电子理论研究

采用递归法计算了Nb合金的电子态密度、原子镶嵌能、亲和能和团簇能等电子结构参数,研究Nb合金高温氧化机理.研究表明,氧在Nb合金表面的吸附能较低,易在合金表面吸附,并逐渐扩散到Nb合金的基体中.氧在合金基体中镶嵌能为负值,氧的态密度和Nb相似,在Nb中具有很高的溶解度.Ti,Al在合金晶内的镶嵌能均高于各自在合金表面的镶嵌能,Ti,Al从合金内部向合金表面扩散,最终在Nb合金表面偏聚,形成富Ti,Al的表层.团簇能计算结果表明Nb合金表面的Ti,Al原子各自均有聚集倾向,分别形成Ti和Al原子团.氧与合金     

Site preference of Ti and Nb in L12-ordered Co-Al-W phase and their effect on the properties of the alloy: first-principles study

In this work,the equilibrium structure,electronic and elastic properties of L1₂-ordered Co-Al-W and Co-Al-W-X(X=Ti and Nb)phase were calculated,using first-principles calculations.Among six nonequivalent sites(Al₁,Al₂,Co₃,Co₄,W₅,W₆),Ti and Nb prefer to occupy the W₆site,since the formation enthalpy of the system is lowest when Ti and Nb occupy the W。site.Both Ti and Nb most affect the density of states of Al atoms.Compared with the Al₂ site,which is the sub-preference site of Ti and Nb,the density of states of Al atoms is higher with the addition of Ti and Nb in the W₆site,which means that the latter system is more stable.According to the bulk modulus B,shear modulus G,Young's modulus E,hardness Hv and Poisson's ratioσ,for Co₃(AI,W)alloy,the addition of Ti and Nb in the W₆site decreases its hardness hut increases its duetility.This work cnnfirms that Ti and Nh can stahilize the Cag(Al,W)alloy and have a positive effect in solving the relatively poor ductility of this alloy,which has important implications for the development of cobalt.based alloys.     

Al/Nb interface study based on the analysis of the energy spectra of reflected electrons

The interface between a Nb layer deposited on an Al layer is studied via comparison of measured and calculated reflected electron energy loss spectra. The interface region analysis is performed after layer-by-layer sputtering of the sample combined with Auger monitoring of its component composition. The elaborated spectrum calculation technique makes it possible to characterize Al and Nb distributions in the interface region with a nanometer resolution. The possibility of determining the number of phases at the Al/Nb interface is discussed.     

Nb在Ni3Al中取代行为及合金化效应的第一性原理研究

采用基于第一性原理的平面波赝势方法,研究了Nb原子在Ni₃Al中的格点取代行为及合金化效应.通过对不同原子被置换后体系的形成热、结合能及电子态密度的计算和比较,发现Nb原子倾向于取代Ni₃Al中的Al原子,其取代行为主要由系统的电子结构决定,计算结果与实验相符.为了进一步研究Nb原子的取代行为,对Nb原子占据的格点以松散或紧凑分布下体系的总能、形成热、结合能以及电子态密度进行了计算,结果表明Nb原子占据的格点更倾向于紧凑分布.为了研究Nb对Ni_{3关键词： 第一性原理 3}Al合金')" href="#">Ni₃Al合金 电子结构 合金化效应     

Nb掺杂γ-TiAl金属间化合物的电子结构与力学性能

运用基于密度泛函理论的第一性原理方法计算了Nb掺杂γ-TiAl金属间化合物的结构参数、能带结构、电子态密度及弹性常数.结果表明:Nb替代Ti掺杂相比Nb替代Al掺杂的形成能低,Nb在替位掺杂时更倾向于取代Ti原子形成稳定的结构,Nb替代Ti掺杂能够提高γ-TiAl金属间化合物的抵御塑性变形能力、断裂强度和延展性;与Nb替代Ti掺杂相比,Nb替代Al掺杂同样增强γ-TiAl金属间化合物的断裂强度且其增强延展性的效果更好,但抵御塑性变形的能力有所削弱.     

Influence of the transparency of tunnel barriers in Nb/Al2O3/Al/Al2O3/Nb junctions on transport properties

The transparency of the tunnel barriers in double-barrier junctions influences the critical current density and the form of the current–voltage characteristics (IVC). Moreover, the barrier asymmetry is an important parameter, which has to be controlled in the technological process. We have performed a systematic study of the influence of the barrier transparency on critical current, I_C, and normal resistance, R_N, by preparing SIS and SINIS junctions under identical technological conditions and comparing their transport properties. We have fabricated Nb/Al₂O₃/Nb and Nb/Al₂O₃/Al/Al₂O₃/Nb devices with different current densities using a conventional fabrication process, varying pressure and oxidation time. The thickness of the Al middle electrode in all Nb/Al₂O₃/Al/Al₂O₃/Nb junctions was 6 nm. Patterning of the multilayers was done using conventional photolithography and the selective niobium etching process. The current density of SIS junctions was changed in the range from 0.5 to 10 kA/cm². At the same conditions the current density of SINIS devices revealed 1–100 A/cm² with non-hysteretic IVC and characteristic voltages, I_CR_N, of up to 200 μV. By comparing the experimental and theoretical temperature dependence of the I_CR_N product we estimated the barrier transparency and its asymmetry. The comparison shows a good agreement of experimental data with the theoretical model of tunneling through double-barrier structures in the dirty limit and provides the effective barrier transparency parameter γ_eff≈300. A theoretical framework is developed to study the influence of the barrier asymmetry on the current–phase relationship and it is proposed to determine the asymmetry parameter by measuring the critical current suppression as function of applied microwave power. The theoretical approach to determine the non-stationary properties of double-barrier junctions in the adiabatic regime is formulated and the results of calculations of the I–V characteristics are given in relevant limits. The existence and the magnitude of a current deficit are predicted as function of the barrier asymmetry.     

Deposition of aluminide and silicide based protective coatings on niobium

We compare aluminide and alumino-silicide composite coatings on niobium using halide activated pack cementation (HAPC) technique for improving its oxidation resistance. The coated samples are characterized by SEM, EDS, EPMA and hardness measurements. We observe formation of NbAl₃ in aluminide coating of Nb, though the alumino-silicide coating leads to formation primarily of NbSi₂ in the inner layer and a ternary compound of Nb-Si-Al in the outer layer, as reported earlier (Majumdar et al. [11]). Formation of niobium silicide is preferred over niobium aluminide during alumino-silicide coating experiments, indicating Si is more strongly bonded to Nb than Al, although equivalent quantities of aluminium and silicon powders were used in the pack chemistry. We also employ first-principles density functional pseudopotential-based calculations to calculate the relative stability of these intermediate phases and the adhesion strength of the Al/Nb and Si/Nb interfaces. NbSi₂ exhibits much stronger covalent character as compared to NbAl₃. The ideal work of adhesion for the relaxed Al/Nb and Si/Nb interfaces are calculated to be 3226 mJ/m² and 3545 mJ/m², respectively, indicating stronger Nb-Si bonding across the interface.     

Modelling the energy gap in transition metal/aluminium bilayers

We present an application of the generalised proximity effect theory. The theory has been used to determine the energy gap (Δ_g) in proximised transition metal/aluminium bilayer structures such as Nb/Al, Ta/Al, V/Al and Mo/Al. These bilayers have different film thicknesses ranging from 5 to 260 nm. For the cases of Nb/Al, Ta/Al and V/Al bilayers, the interface parameters γ and γ_BN (here we define γ as the ratio of the products of normal state resistivity and coherence length in each film of the bilayer while γ_BN is the ratio of the boundary resistance between films 1 and 2 to the product of the resistivity and coherence length in the second film), which were used as input parameters to the model, were inferred experimentally from an existing bilayer of each kind and then suitably modified for different film thicknesses. This experimental assessment is therefore based on a comparison of measurements of the critical temperature and the energy gap at 300 mK with the predictions from the model for various values of γ, γ_BN. The energy gap of the bilayer was experimentally determined by using symmetrical superconducting tunnel junctions (STJs) of the form S–Al–AlO_x–Al–S, where each electrode corresponds to a proximised bilayer. However for the case of Mo/Al bilayers the interface parameters were determined theoretically since currently no STJ data for this configuration are available. The results for the Nb/Al, Ta/Al and V/Al bilayers have also then been compared to experimentally determined energy gaps found for a series of STJs with different film thicknesses. The correspondence between experiment and theory is very good.     

Fabrication,characterization and biocompatibility of TiO2 nanotubes via anodization of Ti6Al7Nb

Ti6Al7Nb has been used as an implant material because of its good corrosion resistance and high mechanical properties. However, the presence of aluminium (Al), which may lead to ostemalacia, anaemia and nervous system disorders, limited its wide clinical use. In this study, a titanium oxide (TiO₂) nanoporous layer was fabricated on a Ti6Al7Nb alloy using an electrochemical anodic oxidation method. The structure of the TiO₂ nanoporous layer was examined by scanning electron microscopy. The chemical compositions of the samples were analysed by X-ray photoelectron spectroscopy (XPS). Biocompatibility was evaluated by culturing rat osteoblast cells. The result showed that TiO₂ nanoporous layers comprise a mixed oxide containing TiO₂ and a small amount of nobium oxides (Nb₂O₅) and almost no elemental aluminium. The outer layer of the TiO₂ nanoporous layer comprises highly ordered nanotubes and the inner layer forms disordered nanopores. The TiO₂ nanoporous layer could support the adhesion, proliferation, differentiation and gene expression of osteoblast cells. Therefore, a TiO₂ nanoporous layer could enhance the biocompatibility of Ti6Al7Nb alloy and is as a promising candidate for Ti6Al7Nb alloy implants.     

Study of the Microstructure,Crystallographic Structure and Thermal Stability of Al–Ti–Nb Alloys Produced by Selective Electron Beam Alloying

This paper aims an investigation of the microstructure and crystallographic structure as well as the thermal stability of Al–Ti–Nb formed by selective electron beam surface alloying. The fabrication of the samples has been carried out using circular sweep mode, as two velocities of the sample movement have been chosen: V₁ = 1 cm/s and V₂ = 0.5 cm/s. The studied microstructure and crystallographic structure have been investigated by X-ray diffraction (XRD) and Scanning electron microscopy (SEM) respectively. The thermal behavior of the obtained surface alloys are evaluated by the coefficient of thermal expansion (CTE) which has been evaluated by neutron diffraction measurements at high temperature. The results show that in the earlier stages of formation, the microstructure of the intermetallic phase is mainly in the form of coarse fractions, but at the following moments they dissolve, forming separated alloyed zone and base Al substrate as the alloyed zone consists of fine (Ti,Nb)Al₃ particles dispersed in the Al matrix with small amount of undissolved intermetallic fractions. Formation of preferred crystallographic orientation as a function of the speed of specimen motion has not been observed. The performed neutron diffraction measurements show that the lattice parameters of the obtained intermetallic (Ti,Nb)Al₃ are less upshifted in comparison to pure Al. It has been found that the aluminium lattice is much more unstable at high temperatures than that of the intermetallic phase. The CTE for the intermetallic phase is 8.70 ppm/K for a axis and 7.75 ppm/K for c axis respectively while considering Al it is 12.95 ppm/K.