MgB2超导线材的机械性能

尽管MgB2超导线材的临界转变温度相对较低,但MgB2没有晶界弱连接,因而具有低的加工成本,相对于BSCCO和YBCO表现出明显的应用优势.目前,粉末装管技术(PIT)广泛用于制备MgB2带材和线材.利用高能球磨工艺的超细粉末制备的MgB2线材临界电流密度达到3.0×105 A/cm2(20 K,自场).为了了解MgB2线材的实际使用条件和提高使用条件下的性能,很有必要对其进行机械性能方面的研究.     

提高MgB2超导体临界电流密度的研究进展

临界电流是超导体载流能力大小的重要表征,论述了制备工艺、元素掺杂和物理场对MgB2超导体临界电流密度的影响.使用非晶态高纯硼粉制备致密的MgB2试样有利于样品临界电流密度的提高,添加SiC或者Ti也能起到提高临界电流密度的作用,质子照射使MgB2试样在高磁场条件下的临界电流密度增加,强磁场下烧结则有利于提高MgB2样品临界的传输电流密度.     

烧结方法对MgB2超导体结构和性能的影响

以Mg粉和B粉为主要原料,采用两种烧结方法制备出MgB2超导体.利用x射线衍射仪、扫描电子显微镜、阿基米德法和标准的直流四引线法等,分别研究了样品的物相组成、显微结构、密度和临界电流密度等.研究发现,MgB2块材的密度随Mg粉粒度的增加而降低.与常规烧结相比,快速烧结时MgB2块材的密度较低且含有更多的非超导相,如Mg和MgBx(x=4,6或12).快速烧结MgB2线材的临界电流密度低于常规烧结线材的.结果表明,快速烧结不利于获得高性能的MgB2超导体.     

NbTi超导线材的研究与发展

讨论了NbTi/Cu超导线材的应用范围及其特性,介绍了NbTi/Cu超导线材的传统制备工艺和人工钉扎中心制备工艺,以及这两种工艺生产的NbTi/Cu超导线材的临界电流密度和商业化生产NbTi/Cu超导线材对加工工艺的要求.NbTi/Cu超导线材是目前应用最广的一类超导材料,为了满足超导磁体工作的安全性要求,大铜比NbTi/Cu超导线材的制备工艺和6.5K下NbTi/Cu超导线材的性能将是NbTi/Cu超导线材今后的研究方向之一.     

(Ba,K)Fe2As2铁基超导多芯线材的晶间耦合强化与性能提高(英文)

铁基超导材料在高场超导磁体领域具有很大的应用潜力,而研究具有高传输临界电流密度的多芯复合包套线材对于其实际应用至关重要.本研究基于粉末装管法,结合孔型轧制工艺和热等静压烧结工艺制备了多芯Cu/Ag复合包套(Ba,K)Fe₂As₂ (Ba-122)超导线材.通过对不同直径线材中Ba-122超导芯的质量密度、晶粒取向、晶粒尺寸、元素分布与线材的电流传输性能之间的关系进行系统研究,我们发现线径较小的样品中超导晶粒间的耦合得到了显著增强,其传输临界电流密度在4.2 K, 10 T下达到3.3×10⁴A cm^-2,同时由于晶粒尺寸和超导芯丝的均匀性得到改善,其电流传输的一致性(n值)也获得有效提高.本研究为制备高性能、高度均匀的铁基超导多芯线材提供了一种可规模化的工艺路线,对促进铁基超导材料的高场强电应用有重要意义.     

分步反应法制备SiC掺杂MgB2超导线材的研究

以干燥的B粉和Mg粉为原料,采用分步反应法制备了SiC(40～45μm)掺杂的MgB2/Nb/Cu超导线材.通过X射线衍射仪(XRD)、扫描电子显微镜(SEM)研究了在650℃热处理温度下不同掺杂量对所制备样品的相组成、微观结构的影响.采用标准四引线法测定了样品的电阻温度曲线,结果表明,当该工艺第二步热处理温度为650℃时,反应不完全,应适当升高温度;同时高SiC掺杂量有利于在MgB2线材中引入更多B位C掺杂,表现为Tc略有降低,从而提高了其磁通钉扎特性.     

机械合金化工艺制备大块MgB2超导体

由德国德累斯顿金属材料研究所的约根·意科特领导的研究小组采用一种机械合金化工艺制备了具有高临界电流密度的大块MgB2超导体,这种超导体在20K,1T磁场下临界电流密度为10000A/cm2,并具有较高的不可逆场.他们认为,由于在这种材料中含有直径大约为40nm～100nm的球形晶粒,并存在大量的晶界,因此改善了磁通钉扎特性,提高了临界电流密度.     

起始粉末对MgB2超导线材成相及微结构的影响

采用原位法粉末装管工艺制备了不同起始粉末MgB2/Nb/Cu单芯线材。通过X射线衍射(XRD)、扫描电镜(EMS)研究了不同起始粉末对线材微观结构及相成分的影响。结果显示,从线材的微观结构和相成分角度来看,B粉影响不大,起主要作用的是Mg粉。     

具有高传输电流的机械强化MgB2超导线

为设计制造MgB2超导线,需要考虑几方面的问题.首先,就标准线圈绕制工艺而论,线材具有的几何形状比带材更合适.第二,用粉末装管工艺(PIT)制备MgB2线时,虽然可以用商品MgB2粉,但缺点是芯丝不能延性变形.如果用镁和硼混合粉制备MgB2线,虽然延性变形性能有所改善,但缺点是粉末粒子较粗,导致MgB2不均匀.     

我国在二硼化镁线带材制备及其性能研究取得新进展

中科院电工所应用超导重点实验室和日本东北大学等合作,在新型超导材料二硼化镁（MgB2）线带材制备及其性能研究中取得系列新进展。通过对掺杂物以及掺杂机理的分析研究,采用多种有机物对MgB2线带材进行掺杂,大幅度提高了MgB2线带材在高磁场下的临界电流密度,研制出了多种具有国际先进水平的高性能MgB2线带材,如临界电流密度高达10800A／cm^2（4.2K,12T）。     

Structural and optical properties of AlxOy/Pt/AlxOy multilayer absorber

We report on the microstructure and optical properties of Al_xO_y–Pt–Al_xO_y interference-type multilayer films, deposited by electron beam (e-beam) deposition onto corning 1737 glass, silicon (1 1 1) and copper substrates. The structural properties were investigated by Rutherford backscattering spectrometry, X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectroscopy and atomic force microscopy. The optical properties were extracted from specular reflection/transmission, diffuse reflectance and emissometer measurements. The stratification of the coatings consists of a semi-transparent middle Pt layer sandwiched between two layers of Al_xO_y. The top and bottom Al_xO_y layers were non-stoichiometric with no crystalline phases present. The Pt layer is in the fcc crystalline phase with a broad size distribution and spheroidal shape in and between the rims of Al_xO_y. The surface roughness of the stack was found to be comparable to the inter-particle distance. The optical calculations confirm a high solar absorptance of ∼0.94 and a low thermal emittance of ∼0.06 for the multilayer stack, which is attributed not only to the optimized nature of the multilayer interference stacks, but also to the specific surface morphology and texture of the coatings. These optical characteristics validate the spectral selectivity of the Al_xO_y–Pt–Al_xO_y interference-type multilayer stack for use in high temperature solar-thermal applications.     

Synthesis of a new perovskite Pb(Li14Nb34)O3

A high-pressure technique was adopted to obtain perovskite-type $\text{Pb(Li}{}_{\mathrm{<mtext>1</mtext><mtext>4</mtext>}}\text{Nb}{}_{\mathrm{<mtext>3</mtext><mtext>4</mtext>}}\text{)}\text{O}{}_3$. A new perovskite $\text{Pb(Li}{}_{\mathrm{<mtext>1</mtext><mtext>4</mtext>}}\text{Nb}{}_{\mathrm{<mtext>3</mtext><mtext>4</mtext>}}\text{)}\text{O}{}_3$ was characterized to have a cubic symmetry with $\text{a}{}_{\mathrm{<mtext>o</mtext>}}\text{= 4.069}\text{A}\text{?}$; Li and Nb ions in the B-site of perovskite lattice may be in a random arrangement.     

Optimization of AlxOy/Pt/AlxOy multilayer spectrally selective coatings for solar-thermal applications

Spectrally selective Al_xO_y/Pt/Al_xO_y multilayer absorber coatings were deposited onto corning 1737 glass, Si (111) and copper substrates using electron beam (e-beam) vacuum evaporator at room temperature. The employment of ellipsometric measurements and optical simulation was proposed as an effective method to optimize and deposit multilayer solar absorber coatings. The optical constants (n and k) measured using spectroscopic ellipsometry, showed that both Al_xO_y layers, which used in the coatings, were dielectric in nature and the Pt layer was semi-transparent. The optimized multilayer coatings exhibited high solar absorptance α ∼ 0.94 ± 0.01 and low thermal emittance ? ∼ 0.06 ± 0.01 at 82 °C. The Rutherford backscattering spectroscopy (RBS) data of Al_xO_y/Pt/Al_xO_y multilayer absorber indicated the Al_xO_y layers present in the coating were nearly stoichiometry. The scanning electron microscope analysis (SEM) result indicated that the average diameter and inter-particles distance of Pt grains were statistically about 146 ± 0.17 nm and 6-10 ± 0.2 nm respectively.     

Bulk crystalline BaPb34Bi14O3: A ceramic superconductor

The preparation conditions of the high T_C ceramic superconductor Ba(Pb,Bi)O₃ is correlated with the superconducting transition. Transition onsets of all materials are similar, but transition widths and transition completeness is strongly dependent on firing temperature. Only materials prepared over a narrow temperature range, resulting in a nearly ideal weight loss, have a complete and narrow transition.     

Electrostriction in Pb (Zn13Nb23)O3

The electrostriction in $\text{Pb (Zn}{}_{\mathrm{<mtext>1</mtext><mtext>3</mtext>}}\text{Nb}{}_{\mathrm{<mtext>2</mtext><mtext>3</mtext>}}\text{)}\text{O}{}_3$ crystals has been investigated using a strain gauge method. In the ferroelectric phase below 140 C, the strain vs the electric field shows a hysteresis, which is ascribed to the effect of ferroelectric domains. A quadratic relation holds between the strain x and the electric polarization P as x = QP² above about 170 C in the paraelectric phase. Values of the electrostrictive Q coefficients are determined from the measurements near 190 C, as Q₁₁ = 1.6·10^?2m⁴/C², Q₁₂ = ?0.86·10^?2m⁴/C², and Q₄₄ = 0.85·10^?2m⁴/C².     

Magnetic,electrical and thermal studies on the V1−xMox02

The monoclinic-to-tetragonal structure transition of oxides V_1?xMo_x0₂ with $\text{0≤}\text{x}\text{≤0.20}$ has been studied by means of DTA, X-ray diffraction, magnetic susceptibility (powder samples) and electrical conductivity (single crystals) measurements within the temperature region 80 K to 400 K. A linear decrease of the transition temperature of 11 K per atom % Mo was observed. The magnetic susceptibility of the low temperature phase was found to be temperature independent paramagnetic for all preparations. Electrical conductivity measurements on the same phase showed crystals with $\text{x}\text{? 0.04}$ to be semiconducting, while a metallic behavior was observed in the region $\text{0.10 ?}\text{x}\text{? 0.14}$.     

n-PbTep+−Pb1−xSnxTe heterojunctions prepared by a modified hot wall technique

$\text{n-PbTe}\text{p}{}^{\mathrm{+}}\text{?}\text{Pb}{}_{\mathrm{1?x}}\text{Sn}{}_{\mathrm{x}}\text{Te}$ heterojunctions with a long wavelength spectral cutoff (λ_c ≈ 6 μm) were prepared using the double-channel hot wall technique. The electrical and photoelectrical properties of the heterojunctions at 77, 197 and 300 K were investigated. Detectors with R_oA equal to 170 Ω cm² and a quantum efficiency of 25–40% were obtained. Reasons for the shift of the long wavelength spectral cutoff of the heterojunctions towards shorter wavelengths are given.     

Reactive plasma deposited SixCyHz films

Si_xC_yH_z films have been prepared at 200°C by reactive plasma deposition from SiH₄ and CH₄ diluted in helium in a tubular reactor. These films have a ratio s (equal to $\text{Si}\text{(}\text{Si+C}\text{))}$ ranging from 0.2 to 0.8, a refractive index ranging from 1.96 to 2.6 and an optical energy band gap in the range 2.7-2.2 eV. The total quantity of hydrogen in the film is 40% when s=0.5. Infrared analysis shows that these films have large fractions of homonuclear bonds and that this material is best described as a polymer. Mass spectrometric measurements of the gaseous products formed in the SiH₄-CH₄-He plasma have been performed and the results are related to the composition of the deposited layers.     

Core-current-enhanced domain-wall pinning in nanocrystalline Fe/sub 73.5/Cu/sub 1/Nb/sub 3/Si/sub 15.5/B/sub 7/ ribbon

We have studied the influence of surface fields H/sub p/ (generated by either direct or alternating core current) on soft magnetic properties of amorphous and nanocrystalline Fe/sub 73.5/Cu/sub 1/Nb/sub 3/Si/sub 15.5/B/sub 7/ ribbon. While in an amorphous ribbon the coercive field H/sub c/ decreases with H/sub p/, in the same optimally annealed ribbon (H/sub c/=1.3 A/m, M/sub m//spl ap/M/sub s/) H/sub c/ increases with H/sub p/ for all the explored types of H/sub p/ (static and dynamic with different phases with respect to that of the magnetizing field H). The unexpected increase of H/sub c/ in nanocrystalline ribbon is associated with the influence of H/sub p/ on the surface and main (inner) domain structure. Here, we develop a model that takes into account this influence and explains the experimental results.     

Interfacial microstructure of NiSix/HfO2/SiOx/Si gate stacks

Integration of NiSi_x based fully silicided metal gates with HfO₂ high-k gate dielectrics offers promise for further scaling of complementary metal-oxide- semiconductor devices. A combination of high resolution transmission electron microscopy and small probe electron energy loss spectroscopy (EELS) and energy dispersive X-ray analysis has been applied to study interfacial reactions in the undoped gate stack. NiSi was found to be polycrystalline with the grain size decreasing from top to bottom of NiSi_x film. Ni content varies near the NiSi/HfO_x interface whereby both Ni-rich and monosilicide phases were observed. Spatially non-uniform distribution of oxygen along NiSi_x/HfO₂ interface was observed by dark field Scanning Transmission Electron Microscopy and EELS. Interfacial roughness of NiSi_x/HfO_x was found higher than that of poly-Si/HfO₂, likely due to compositional non-uniformity of NiSi_x. No intermixing between Hf, Ni and Si beyond interfacial roughness was observed.