MgB2超导材料的薄膜制备及其超导器件的实现

报道了采用电子束蒸发技术实现MgB₂薄膜制备的实验工作。通过蒸镀B膜和Mg/B多层膜两种前驱体,分别经高温区(～900℃)和中温区(～700℃)退火处理后成功获得了高质量的MgB₂薄膜样品。对样品的超导性能、晶体结构和表面形貌进行了详细的测量和表征,并对两类样品性质上所表现出的差异进行了分析。此外,还报道了在薄膜制备基础上利用微加工技术实现的超导微波谐振器的结果。     

脉冲激光沉积法原位后退火生长MgB2薄膜的研究

采用脉冲激光沉积的方法在 Al₂O₃（0001）基片上先生成 Mg-B 混和薄膜,然后采用原位后退火的方法生成 MgB₂ 超导薄膜,采用磁测量（M-T）、X射线衍射、扫描电子显微镜技术分析了各种沉积及退火条件对 MgB₂ 超导薄膜表面形貌、晶体结构、超导电性的影响。在沉积温度为 200 ℃,退火时间 5min 时,改变退火温度得到一组薄膜,研究退火温度对超导薄膜性质的影响,得到了转变温度-退火温度曲线。在退火温度为 670℃、720℃ 时,得到了最高的临界转变温度 T_c=33K,X射线衍射结果表明此时的薄膜有 c 轴取向。同时比较了在 200℃ 下沉积,在 670℃ 下分别退火不同时间的薄膜的超导性质。还比较了分别在不同温度下沉积,然后在 670℃ 下退火 5min 的薄膜的超导性质。结果表明,沉积温度和退火温度、退火时间极大的影响了薄膜的各种性质。     

MgB2超导线材的原位热处理研究

通过原位SPS热处理和管式炉退火获得SPS-AN MgB₂线材样品.XRD结果表明热处理后的各样品中均有MgB₂相生成.在SPS热处理阶段，样品出现许多MgB₂片状颗粒.由于在SPS阶段样品分解程度不同，退火后样品的形貌出现明显差异.SPS-AN-800℃样品保持了较大的片状形貌，在颗粒边缘有杂相堆积，导致样品的J_c性能最差.SPS-AN-850℃和SPS-AN-880℃样品在退火过程中片状颗粒分解成纳米颗粒，产生了更多的边界钉扎作用.由于SPS-AN-880℃样品分解生成的杂相小颗粒更多，其J_c性能较SPS-AN-850℃样品更好.     

混合物理化学气相沉积法制备Mg衬底MgB2薄膜

用混合物理化学气相沉积（Hybrid physical-chemical vapor deposition 简称为 HPCVD）法在Mg衬底上原位制备出MgB₂超导薄膜样品。超导起始转变温度T_c(onset)＝39.5K,SEM(Scanning Electron Microscopy)图显示晶型结构为六方形,晶粒生长较为致密。样品的X射线衍射分析表明,MgB₂薄膜是多晶的,没有择优方向。晶粒大小约为400nm～1μm。结果表明HPCVD技术在Mg表面直接沉积MgB₂薄膜是可行的,对Mg原料的节约和MgB₂线材的制备具有一定优势。     

退火处理对Dy4(Co21Cu79)96颗粒膜结构和巨磁电阻效应的影响

采用磁控溅射法制备Dy₄(Co₂₁Cu₇₉)₉₆颗粒膜,研究薄膜的巨磁电阻（GMR）效应及磁性能.应用X射线衍射仪（XRD）对薄膜微观结构随退火温度的变化进行分析,采用四探针及振动样品磁强计（VSM）测量薄膜的磁电阻和磁性能.X射线衍射实验结果表明:制备态的薄膜形成了单相亚稳态面心合金结构,退火处理将促进Cu和Co的相分离.磁电阻测试发现:所有不同成分的Dy_x(Co₂₁Cu₇₉)_100-x（x=0,4,8,9,12,14）薄膜样品均随着退火温度的升高,颗粒膜巨磁电阻（GMR）效应不断增大,当达到最佳退火温度之后,GMR值又随退火温度的升高而降低.当退火温度为425℃时,Dy₄(Co₂₁Cu₇₉)₉₆薄膜的巨磁电阻效应达到最大,GMR值为-4.68%.退火前后样品磁滞回线的变化表明薄膜中发生了从超顺磁性到铁磁性的转变,矫顽力H_c随退火温度的升高逐渐增大.     

白云石炼镁收尘灰制备碱式碳酸镁的碳化-热解法工艺研究

以白云石炼镁收尘灰作为原料，采用碳化-热解法制备碱式碳酸镁，通过单因素试验研究了碳化过程中温度、时间、CO₂流速对碱式碳酸镁产率的影响及热解过程中温度对碳酸氢镁水溶液(重镁水)分解率的影响，确定了碳化-热解法制备碱式碳酸镁的最佳工艺条件，并通过XRD、SEM和EDS对制备的样品进行表征.结果表明，在碳化时间为40 min、碳化温度为25～30℃、CO₂流速为0.4 L/min、碳化终止pH为8.0、碳酸氢镁水溶液热分解温度为90℃的条件下，采用碳化-热解法制备得到的碱式碳酸镁产率最高为84.72%.在最佳制备工艺条件下制备得到的片状碱式碳酸镁(4MgCO₃·Mg(OH)₂·4H₂O)杂质含量少、纯度高，达到了冶镁原料标准要求，实现了灰渣废弃物的高效回收利用.     

SnO2纳米线的制备及结构表征

以SnO₂粉末和碳粉的混合物为源,高纯氮气为载气,利用化学气相沉积法在1 000 ℃下,在溅有Au的单晶Si衬底上制备了SnO₂纳米线。用SEM、XRD测试技术对样品进行了结构、形貌的表征,利用PL技术分析了样品的发光特性。由分析可知,样品均为四方金红石结构,退火时间对样品形貌具有一定的影响,但不影响其结构。所制备的SnO₂纳米线结晶质量较高,其生长遵循VLS机制。     

a—Si_(1-X)C_X∶H 钝化膜热稳定性的研究

采用外耦合电容式射频辉光放电等离子体 CVD 系统,在低于300℃的衬底温度下,制备出了 a—Si(1-x)C_x∶H 钝化膜.对含碳量不同的3种样品作了热稳定性实验(即高温存放、温度循环和退火).另外,还对同一号样品做了在不同温度下退火30min 的实验.结果表明,此钝化膜的释氢温度高于600℃.具有很好的热稳定性.     

SrFeO3-δ纳/微米纤维的制备及其磁性

利用静电纺丝法与溶胶 凝胶技术, 制备PVP/Sr(OOCCH₃)₂·0.5H₂OC₁₅H₂₁FeO₆复合纳米纤维, 热处理后得到了SrFeO_3-δ纳/微米纤维, 并采用X射线衍射（XRD）、 扫描电镜（SEM）、 透射电镜（TEM）和超导量子干涉仪磁强计（SQUID）研究纤维样品的晶体结构、 微观形貌及其磁性. 实验结果表明： PVP/Sr(OOCCH₃)₂·0.5H₂OC₁₅H₂₁FeO₆复合纤维表面光滑, 长直连续, 平均直径约为500 nm; 在空气气氛下, 经800 ℃焙烧2 h后得到的SrFeO_3-δ纳/微米纤维均为较纯的立方钙钛矿型结构, 平均晶粒尺寸约为32 nm, 纤维直径为260~480 nm, 平均直径约为400 nm, 具有较大长径比; 当温度为175 ℃时, SrFeO_3-δ纤维样品的磁化率有极大值, 磁性纳米效应的影响及SrFeO_3-δ中螺旋反铁磁有序性、 顺磁性和铁磁性的共同作用使得纤维样品与块体样品的磁学性能不同, 其Neel温度与Curie温度均升高.     

退火工艺对SiN膜及SiO2/SiN双层膜钝化特性的影响

针对硅太阳电池的背表面钝化,研究了在多晶硅P型衬底上SiN膜及SiO2/SiN双层膜的热稳定性及在不同温度下两种膜的退火特性.通过准稳态光电导衰减法测试其少数载流子寿命,发现SiN膜比较适合于在500℃以内的温度下进行常规炉热退火,而SiO2/SiN双层膜则比较适合在600~700℃之间的温度下进行常规炉热退火.对SiN膜进行链式炉热退火实验表明,低温(700~850℃)条件下表面钝化效果最好.     

多晶MgB2相形成中纳米尺寸硼粉的作用

原料硼粉在MgB2相形成中的作用已经利用原位高温电阻率(HT-ρT)的测量技术进行了研究.MgB2相形成中的起始转变温度Tonset和相转变完成温度TPF在完成了的热处理过程中已直接确定了.用纳米硼粉和镁粉制备MgB2样品(简称为NanoB-MgB2)时的Tonset和TPF这2个温度值分别是440和 490 ℃,类似于用微米硼粉和纳米镁粉制备MgB2样品(简称为MicroB-MgB2)的情况.这表明MgB2相形成的温度并不特别依赖于硼粉的尺寸.另外,烧结温度的上极限TN是750 ℃,在此温度下NanoB-MgB2样品的超导电性将丧失.此温度远低于用微米硼粉和毫米尺寸的镁粉制备的MgB2样品(简称为DM-MgB2)的TN= 980 ℃.把直接在650 ℃＜T N,烧结的样品与先在750 ℃下烧结,之后又再次重磨和处理后,并于富镁的环境中在650 ℃下再烧结处理的样品相比较,观察到了MgB2的不可逆的晶体结构转变.     

The origin of the anomalous superconducting properties of MgB(2)

Magnesium diboride differs from ordinary metallic superconductors in several important ways, including the failure of conventional models to predict accurately its unusually high transition temperature, the effects of isotope substitution on the critical transition temperature, and its anomalous specific heat. A detailed examination of the energy associated with the formation of charge-carrying pairs, referred to as the 'superconducting energy gap', should clarify why MgB(2) is different. Some early experimental studies have indicated that MgB(2) has multiple gaps, but past theoretical studies have not explained from first principles the origin of these gaps and their effects. Here we report an ab initio calculation of the superconducting gaps in MgB(2) and their effects on measurable quantities. An important feature is that the electronic states dominated by orbitals in the boron plane couple strongly to specific phonon modes, making pair formation favourable. This explains the high transition temperature, the anomalous structure in the specific heat, and the existence of multiple gaps in this material. Our analysis suggests comparable or higher transition temperatures may result in layered materials based on B, C and N with partially filled planar orbitals.     

The origin of multiple superconducting gaps in MgB2

Magnesium diboride, MgB2, has the highest transition temperature (T(c) = 39 K) of the known metallic superconductors. Whether the anomalously high T(c) can be described within the conventional BCS (Bardeen-Cooper-Schrieffer) framework has been debated. The key to understanding superconductivity lies with the 'superconducting energy gap' associated with the formation of the superconducting pairs. Recently, the existence of two kinds of superconducting gaps in MgB2 has been suggested by several experiments; this is in contrast to both conventional and high-T(c) superconductors. A clear demonstration of two gaps has not yet been made because the previous experiments lacked the ability to resolve the momentum of the superconducting electrons. Here we report direct experimental evidence for the two-band superconductivity in MgB2, by separately observing the superconducting gaps of the sigma and pi bands (as well as a surface band). The gaps have distinctly different sizes, which unambiguously establishes MgB2 as a two-gap superconductor.     

Superconductivity in MgB2

In January of 2001 the superconductivity of the compound MgB2 with a critical temperature Tc of up to 39 K was discovered. This Tc is the highest in all intermetallic compound and alloy superconductors. MgB2 has a simple structure and its manufacturing capital cost is lower, therefore it could become a practical superconductor in the future. The recent progress is reviewed here which covers the progress in electronic structure, high Tc mechanism, superconducting parameters (Debye temperature, specific heat coefficient of electron, critical fields, coherent length, penetration depth, energy gap, critical current and relaxation rate of flux). Moreover the issue on power transmission is discussed.     

Loss of superconductivity with the addition of Al to MgB2 and a structural transition in Mg1-x AlxB2

The basic magnetic and electronic properties of most binary compounds have been well known for decades. The recent discovery of superconductivity at 39 K in the simple binary ceramic compound magnesium diboride, MgB2, was therefore surprising. Indeed, this material has been known and structurally characterized since the mid 1950s (ref. 2), and is readily available from chemical suppliers (it is commonly used as a starting material for chemical metathesis reactions). Here we show that the addition of electrons to MgB2, through partial substitution of Al for Mg, results in the loss of superconductivity. Associated with the Al substitution is a subtle but distinct structural transition, reflected in the partial collapse of the spacing between boron layers near an Al content of 10 per cent. This indicates that superconducting MgB2 is poised very near a structural instability at slightly higher electron concentrations.     

Superconducting Properties and Microstructure in MgB2 Bulks, Wires and Tapes

We prepared a series of MgB2 bulk samples under different temperatures, holding time and increasing rates in temperature by the solid state reaction. The thermodynamic behavior and phase formation in the Mg-B system were studied by using DTA, XRD and SEM. The results indicate that the formation of the MgB2 phase is very fast and the high increasing rate in temperature is necessary to obtain high quality MgB2. In addition, the effects of the Zr-doping in Mg1-xZrxB2 bulk samples fabricated by the solid state reaction at ambient pressure on phase compositions, microstructure and flux pinning behavior were investigated by using XRD, SQUID magnetometer, SEM and TEM. Critical current density Jc can be significantly enhanced by the Zr-doping and the best data are achieved in Mg0.9Zr0.1B2. For this sample, Jc values are remarkably improved to 1. 83 × 106 A/cm2 in self-field and 5. 51 × 105 A/cm2in 1T at 20K. Also, high quality MgB2/Ta/Cu wires and tapes with and without Ti-doping, MgB2/Fe wires and 18 filamen     

The complex nature of superconductivity in MgB2 as revealed by the reduced total isotope effect

Magnesium diboride, MgB2, was recently observed to become superconducting at 39 K, which is the highest known transition temperature for a non-copper-oxide bulk material. Isotope-effect measurements, in which atoms are substituted by isotopes of different mass to systematically change the phonon frequencies, are one of the fundamental tests of the nature of the superconducting mechanism in a material. In a conventional Bardeen-Cooper-Schrieffer (BCS) superconductor, where the mechanism is mediated by electron-phonon coupling, the total isotope-effect coefficient (in this case, the sum of both the Mg and B coefficients) should be about 0.5. The boron isotope effect was previously shown to be large and that was sufficient to establish that MgB2 is a conventional superconductor, but the Mg effect has not hitherto been measured. Here we report the determination of the Mg isotope effect, which is small but measurable. The total reduced isotope-effect coefficient is 0.32, which is much lower than the value expected for a typical BCS superconductor. The low value could be due to complex materials properties, and would seem to require both a large electron-phonon coupling constant and a value of mu* (the repulsive electron-electron interaction) larger than found for most simple metals.     

Doping effects of ZrC and ZrB2 in the powder-in-tube processed MgB2 tapes

We have investigated the effects of ZrC and ZrB2 doping on the superconducting properties of the powder-in-tube processed MgB2/Fe tapes. Sam- ples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM/EDX), transport and magnetic measurements. We confirmed the fol- lowing quite different roles of ZrC and ZrB2 in MgB2. ZrC doping was found to decrease the transport critical current density (Jc) at 4.2 K, while the critical temperature (Tc) kept constant. In contrast, the Jc values in magnetic fields were enhanced greatly by the ZrB2 addition, which resulted in a decrease in Tc by only 0.5 K. The reason for different effects of two dopants is also discussed.     

High critical current density in powder-in-tube processed MgB2/Ta/Cu wire

The magnetization of dense MgB2/Ta/Cu wires prepared by the powder-in-tube method is measured by a SQUID magnetometer. The results indicate that the critical temperature of MgB2/Ta/Cu is around 38.4 K with a sharp transition width of 0.6 K. The MgB2/Ta/Cu wire shows a strong flux pinning and the critical current density is higher than 105 A/cm2 (5 K, self-field) and 104 A/cm2 (20 K, 1 T). Also, the irreversibility field of the sample reaches 6.6 T at 5 K.