The effect of B2O3 impurity in the original B powder on the phase formation of MgB2 during the sintering process

Combined with the thermal analysis and phase identification, the influence of the B₂O₃ impurity in the original B powder on the phase formation of MgB₂ during the sintering process was investigated. It is found that how the B₂O₃ impurity in the original B powder affects the sintering process of MgB₂ depends on the reactivity of Mg particles in the original powder. If the Mg particles in the original powder are small and reactive enough, the B₂O₃ impurity could react with them before Mg melting during the sintering process and accelerate the solid–solid reaction between Mg and B. Otherwise, the B₂O₃ impurity could not react with Mg particles before Mg melting and depress the solid–solid reaction between Mg and B.     

超导体MgB2成相过程研究

根据镁和硼的基本化学性质、杂化轨道理论、前线轨道理论和粉末反应理论,分析了粉末反应中MgB<,2>晶核的形成及生长过程.这一过程可分三步完成:(1)两种粉粒碰撞接触,做反相微幅受追振动,产生MgB<,2>成相区;(2)两个硼原子相遇,价轨道经sp<'2>杂化生成B<,2>,镁原子的两个3s价电子自旋相反成对填入B<,2...     

压强对二硼化镁超导电性的影响

基于强耦合理论的 Mc Millan公式 ,研究了压强对 Mg B2 超导电性的影响 ,得到了与实验趋于一致的结果 ,进一步表明了 Mg B2 的超导机制应在 BCS理论框架下讨论。并同时探讨了压强对 Mg B2 超导电性影响的物理机制     

原位法MgB_2-B_4C复相超导体合成及相含量调控研究

以B4C和Mg为原料合成的MgB2-B4C复相超导体具有高的临界电流密度(Jc)和高的超导转变温度(Tc),是一种有潜力的实用MgB2超导材料,其成相机理对复相MgB2超导体的相含量调控和磁通钉扎研究具有重要意义。结合经典烧结理论,研究了B4C-Mg真空固相烧结制备MgB2-B4C复相超导体的超导相形成和晶粒生长过程,给出了B4C-Mg的金斯特林格扩散模型和MgB2晶粒生长过程。通过选择B4C原料粒径,MgB2-B4C复相超导体超导相体积相含量在18%-88%范围可控。相含量88%的MgB2-B4C复相超导体临界转变温度达33.5K,转变宽度1.5K。10 K环境6T外场下电流密度可以达到1×104A/cm2,表明MgB2-B4C复相超导体具有良好的磁通钉扎行为。     

Near-Tc second-harmonic emission in high-density bulk MgB2 at microwave frequency

We discuss the microwave second-harmonic generation in high-density bulk MgB₂, prepared by the reactive liquid Mg infiltration technology. The intensity of the harmonic signal has been investigated as a function of temperature and amplitudes of the DC and microwave magnetic fields. The results are discussed in the framework of a phenomenological theory, based on the two-fluid model, which assumes that both the microwave and static magnetic fields, penetrating in the surface layers of the sample, weakly perturb the partial concentrations of the normal and superconducting fluids. We show that, in order to account for the experimental results, it is essential to suppose that in MgB₂ the densities of the normal and condensed fluids linearly depend on the temperature.     

High-density MgB2 superconducting bulk samples prepared at ambient pressure

High-density MgB₂ (HD-MgB₂) superconducting samples (D ⩾ 2.2 g/cm³), using different sources of magnesium powder as raw material, were synthesized in ambient pressure in a rich Mg environment. The magnesium powders used in the fabrication process include nanometer-sized magnesium particles, powders from Alfa Aesar, ordinary off-the-shelf powder, and magnesium chip. The fabrication procedure involved a double-sintering process in a rich-Mg environment. A transition temperature T _c of 39 K was observed. Samples with the equally high density and matching superconducting properties were obtained as well by a triple sintering process of the MgB₂ powder directly from Alfa Aesar.      

The effect of SiC nanoparticle addition on the flux pinning properties of MgB2

The nanoparticle–MgB₂ composite superconducting sample, (SiC)_4wt.%(MgB₂)_96wt.% ((SiC)₄–MgB₂), was prepared, and the effect of nanoparticle additions on the magnetic flux pinning was investigated. The measurement and comparison of isothermal magnetization M(H), for pure-MgB₂ powder and sintered pellets of (SiC)₄–MgB₂ and pure-MgB₂ were carried out at temperatures between 5 and 50 K in fields up to 8.5 T. The magnetic irreversibility ΔM(H) curves of the (SiC)₄–MgB₂ follow almost identical lines of both pure-MgB₂ powder and sintered bulk MgB₂ in the region above a specific magnetic field (called a merged field), which gradually decreases as the temperature increases. The (SiC)₄–MgB₂ composite superconductor has exhibited the flux pinning effect which comes from both the grain boundaries and the point defect weak pinning centers in the region below the merged field line. This is different from the case of pure-MgB₂ powder and sintered bulk MgB₂ which showed mostly the grain boundaries pinning.     

退火时间对MgB_2超导薄膜的影响

在多晶A l2O3衬底上,以B2H6作为硼源,化学气相沉积先驱B薄膜,采用Mg扩散方法,在不同退火时间条件下制备了MgB2超导薄膜。通过电阻-温度曲线测量、X射线衍射分析和扫描电子显微镜形貌观测方法,研究了退火时间对MgB2薄膜的超导特性、晶体结构、表面形貌的影响。     

电子性作用对MgB2超导体同位素效应的影响

通过引入电子性作用V_c,在弱耦合条件下导出了两带模型的T_c公式和同位素效应指数公式,计算了超导体MgB_2的超导转变温度和同位素效应,得到了与实验相符的结果。结果表明,带内电子性作用比带间电子性作用对同位素效应指数的影响要大。     

Influence of crystallization process on Ni3Al properties

The paper relates to the preceding works dealing with structural characteristics of Ni₃Al and phases that are generated at crystallization of Al-Ni alloys in application of various melting technologies and thermal processing. Attention was paid to the process of crystallization and cooling at melting and casting, comprehensive chemical analysis in the context of fracture surfaces analyses, grain boundaries and phases with stress to determination of behaviour and occurrence of possible admixtures in Ni₃Al. Such mechanical properties as tensile strength and high-temperature creep were investigated as well. Presented at the 6th Joint Seminar “Development of Materials Science in Research and Education”, Karlštejn, Czech Republic, 17–19 September 1996.     

Impact of Al addition on the formation of Ni germanosilicide layers under different temperature annealing

Solid reactions between Ni and relaxed Si_0.7Ge_0.3 substrate were systematically investigated with different Al interlayer thicknesses. The morphology, composition, and micro-structure of the Ni germanosilicide layers were analyzed with different annealing temperatures in the appearance of Al. The germanosilicide layers were characterized by Rutherford backscattering spectrometry, cross-section transmission electron microscopy, scan transmission electron microscopy, and secondary ion mass spectroscopy. It was shown that the incorporation of Al improved the surface and interface morphology of the germanosilicide layers, enhanced the thermal stabilities, and retarded the Ni-rich germanosilicide phase to mono germanosilicide phase. With increasing annealing temperature, Al atoms distributed from the Ni/Si_0.7Ge_0.3 interface to the total layer of Ni₂Si_0.7Ge_0.3, and finally accumulated at the surface of NiSi_0.7Ge_0.3. We found that under the assistance of Al atoms, the best quality Ni germanosilicide layer was achieved by annealing at 700 ℃ in the case of 3 nm Al.     

带间耦合相互作用对MgB2超导体同位素效应指数的影响

在弱耦合极限下考虑带间相互作用,得到了二带s-波超导体临界温度Tc的公式及同位素效应指数的公式。在此模型中,耦合相互作用在每一带中包括两部分:电子-声子相互作用和非电子-声子相互作用,当Tc≈40K时,在弱耦合机制中得到MgB2的同位素效应指数αB=0.3,并且带间的电子-声子相互作用比非电子-声子作用对同位素效应指数的影响大。     

相结构形成因子统计值对钢中化合物形成的影响

本文根据宏观物理量是微观原子状态统计值的思想,将原子组态最可几值改用统计值代替,利用固体与分子经验电子理论(EET)计算钢中FeS、MnS、TiC(N)、NbC(N)、VC(N)、AlN的相结构形成因子的统计值S^',探讨了各种化合物的S^'对钢中组成原子自发凝聚成各种化合物能力的影响。研究发现S_MnS^'相似文献   

相结构形成因子统计值对钢中化合物形成的影响

本文根据宏观物理量是微观原子状态统计值的思想,将原子组态最可几值改用统计值代替,利用固体与分子经验电子理论(EET)计算钢中FeS、MnS、TiC(N)、NbC(N)、VC(N)、AlN的相结构形成因子的统计值S′,探讨了各种化合物的S′对钢中组成原子自发凝聚成各种化合物能力的影响.研究发现S′McNS′McC,可以从电子层面合理解释钢在平衡状态下由高温向低温转变时,FeS先于MnS析出,合金碳化物先于合金氮化物析出,在非平衡状态下,析出顺序相反的事实.并成功解释了铁碳相图共晶转变和共析转变各个相的形成顺序.从电子层次得出,在平衡条件下,S′较大的相先形成,在非平衡条件下,S′较小的相先形成.在某一适当冷却速度范围内,S′相差较小的相可以共生,这一冷却速度区间将是S′差值的函数.     

CoxTi1-xO2-δ体材中氢退火引起的铁磁性及结构相变

利用固相反应法在700℃-1000℃不同的温度下、空气中烧结Co3O4和TiO2混合物,制备了(Co3O4)x/3(TiO2)1-x(0相似文献   

Solid phase formation of silicon nanocrystals by bulk ultrafast laser-matter interaction

Ultrashort pulse laser interaction with silica-silicon interfaces is presented as a means for all-solid-phase formation of high-purity silicon nanoparticles in the absence of ablation plumes or any substrate intermixing with surfaces in ambient air. Transmission electron microscopy and Raman spectroscopy provide definitive evidence for creation of nanocrystals in the silica host, while compressive stress in the silicon substrate corroborates the formation of optical waveguides parallel to the tracks.     

Effects of pre-carbonization treatment on the sintering process and Jc performance of sugar-doped MgB2 superconductors

The carbonization treatment was employed to the sugar-coated B powders before it mixed with Mg powders. It is found that the pre-carbonization treatment could effectively suppress the formation of MgO and Mg₂C₃ impurity in the sugar-doped samples and thus result in an improved J_c at low fields in the sugar-doped samples. Besides, the sintering process of sugar-doped samples was also investigated combined with the thermal analysis and it is found that the sugar addition could obviously lower the onset temperature of the solid–solid reaction between Mg and B.