MGB ₂ Study On Superconducting Single Crystal Growth

Since Kamerlingh Onnes discovered superconductivity in Hg with 4.2 K and won Nobel Prize in 1911, many scientists have focused their minds on this field. W. Meissner and R. Ochsenfeld discovered Meissner effect which is the theory basis of modern suspended superconductive train in the superconductors in 1933. J. Bardeen, L. Cooper and J. Schrieffer joint interpretation the microcsomic theory of superconduct transition in 1957, which is the famous B.C.S. theory, and thus won Nobel Physical Prize in 1972. Brian D. Josephson, is the doctor candidate at University of Cambridge, proposed that some direct tunnel current can get across the thin insulating layer between two superconductors, this is called "Josephson Effect", and therefore won Nobel Physical Prize in 1973. G. Bednorz, German physical scientist, and A. Muller, Swiss physical scientist, discovered La₂BaCuO₄ with 30 K superconductivity, which resulted from important development in the superconductor field, and thus won Nobel Physical Prize in 1987. T_c (HgBa₂Ca₂Cu₃O₈) has been reach to 135 K (the highest T_c is 160 K under high pressure) with a few years research development. The Nobel Physical Prize was awarded to A. Abrikosov, V. Ginzburg A. and Leggett in 2003, in order to honor their full developments about superconductor and superfluid. Superconductive materials have been widely used in magnetic resonance imaging (MRI) and particle accelerator, and supercfluid makes deep clear matter manifestation under low temperature. Up to 2004, there have been nine gainer of the Nobel Prize with relation to superconductivity or superconductor, which proved that scientific community pay attention to this field.The discovery of high-T_c superconductivity in MgB₂ at beginning of 2001 was a kind of shock for researchers in the field ofsuperconductivity. Later, several experiments predicted phonon-mediated s-wave BCS superconductivity with a double energy gap a and n bands, Specific heat, spectroscopy, scanning tunneling spectroscopy and high-resolution angle-resolved photoemission spectroscopy (APRES) gave evidences of this predication. However, it is necessary to grow high quality MgBi single crystal in order to make clear the superconductive mechanism of MgBi. Until now, some key superconducting parameters with relation to the superconductive mechanism are still not affirmed. So it is important to grow single crystal. Although there are several reports about single crystal superconductivity, most crystals were synthesized under high pressure with high pressure facilities. Their shapes were mostly irregular, so that it is almost impossible to recognize the correspondence between the crystal shapes and crystallographic axes, which causes experimental difficulties for measurements on anisotropic properties, the relation between structure and superconductive mechanism, and investigations on mechanism of crystal growth.As the situation stands about MgB2 now, I adopt a method of low melting alloy flux to investigate MgB2 single crystal growth thoroughly under ambient pressure. This research contents mainly include several parts in the following text.Firstly, Single crystals of MgBi have been grown at ambient pressure by using Mg-self-flux method. We chose the marketed MgB2 powder as the raw material and Mg as the flux. Although large size crystal is about 10 ^m, they have well shape with hexagonal structure. It is the first time to introduce self-flux to grow MgBi single crystal. It is possible to grow large MgB2 using flux method as long as the flux is modified with low melting point alloys. The X-ray powder diffraction (XRD) confirmed the MgB2 phase, and at the same time, results of high resolution transmission electron microscope (HRTEM) and scanningelectron microscope (SEM) showed no grain boundaries in these crystals and confirmed that these small single crystals were of high quality. A new mechanism of liquid-assisted solid-state re-crystallization for growth of MgEb single crystal is proposed according to observation of start materials and product morphology. This mechanism is helpful for 100 um single crystal growth of MgE$2 using evaporated flux.Secondly, Microstructure and defects were carried on an investigation in MgE$2 hexagonal plate crystals. Edge dislocation with Burgers vector b = - a, and self-ordering superstructure with 5aohave been directly detected by HRTEM along [001] direction in the (001)face. Frank partial dislocation (Burgers vector is—(0001)) is detectedbeing perpendicular to the (001) face. The strain value 0.7% can be estimated from the slope in a plot of the calculated full width at half maximum (FHWM) xcos# againstsin^ and the strain is isotropic. The as-prepared sample showed an onset of superconductivity at about 37 K with large superconducting transitions width (ATC 22 K) by measurement of superconducting quantum interference device (SQUID) magnetometer. The unconventionality of superconductivity may be caused by these defects in the crystals.Thirdly, the single crystal growth is performed under ambient pressure in order to investigate the mechanism of crystal growth and explore the difficulty of growing under natural conditions. Hexagonal MgB2 crystals about 100 um are obtained by evaporating flux slowly at low temperature. The reason is that vapor pressure of melting Mg is increasing intensively along with temperature increment. It is useful for MgB2 crystal growth used high pressure method if vapor pressure is controlled. It is complicated to grow crystal under high pressure, and obtained crystals are irregular. Therefore, in my research, single crystalgrowth confirms the mechanism of liquid-assisted solid-state re-crystallization is very fit for crystal growth of matter having no melting points or very low solubility in the flux.The as-prepared sample including single crystals showed an onset of superconductivity at 33.67 K. That the difference between the field cool (FC) and the zero field cool (ZFC) data is very large shows that the flux pinning is very strong in these crystals, which suggesting that there are many defects in the sample. Defects of a single crystal are not carried on investigation constraint of experiment method.Fourthly, That ( 111) > ( 101 ), ( 011) crystal faces disappeared after single crystal growing large with observations of SEM andmetallographical microscope (MM) shows that these crystal facesgrowing velocity is faster than (110K (100), (010) . However, the largest crystal size is about 100 fim among these as-prepared crystals. That faces of fast growing velocity disappeared in such small crystals shows it is very difficult for MgB2 crystals to grow large. Crystal growing velocity along aorb-axes is obviously increased and growing velocity along c-axes is very slowly or unchanged along with crystal growth. They are different in size along a-axes, but almost no vary in thickness along c-axes, that is to say, the size of (001) face is directlyv proportional to the — ( v , v is the velocity along a -axes andc-axes, respectively). When the size of (001) face grows large, difference in size is increased between crystal size along aorï¿¡-axes and thickness, and finally form a hexagonal plate shape. The reasons that the single crystal cannot grow large, and very thin thickness, and a few crystals more than 100 um in every as-prepared product are that very low solubility of MgB2 in the flux causes local supersaturation quite large (insoluble) and the size of (001) face directly proportionalv to the — has an extremum. It is very difficult to make clear about thisVconly by means of current experimental measurement.Fifthly, Cleavage research was performed using hexagonal plate MgB2 crystal growing by evaporating flux method. Cleavage happened along {0001} layer direction similar to graphite. It is very disadvantageous for MgB2 crystal growth because of cleavage in such little crystals. Little negative crystals of Different orientations are observed on the crystal surface and edge.Results of MgB2 crystals microstructure research using HRTEM show that there are many cleavages in microstructure because of inner strain resulting from crystal growth system and environmental disturbance. These microstructure defects stunt the single crystal continuous growth, and it is occasional for the large crystal.There are three morphologies on the crystal surface: firstly, (001) smooth crystal face containing strip steps; secondly, (001) rough face containing convex cylindrical; thirdly, (001) face containing local growth steps with height of 6-17 nm, which are bunch steps. It shows that crystal along [001] direction grows with non-single atomic or molecular layer. Steps all expand breadthwise along growth face and form discontinuous growth layers causing crystal face processing along normal direction. This kind of step is usual, and finally forms hexagonal plate shape.Sixthly, MgB2 crystals are also grown with low melting alloy flux. The sample with superconductivity at 38.3 K is prepared in the Zn-Mg flux and magnetization curve above the zero line. The sample with superconductivity at 34.11 K is prepared in the Cd-Mg flux and magnetization curve below the zero line. MgB4 was found in both samples, and MgB45s magnetization curve being above zero line was not superconductivity. No other component was observed by XRD and EDS.Up to now, the reason of magnetization curve difference between Zn-Mg flux and Cd-Mg flux is not made clear. MgB2 prepared by these two methods is all flaky and irregular shape and low quality of single crystal; it may be the result of defect of crystal growth system, or the fit technic is still not searched for because of being restricted of time. However, these two metals must be the flux for MgEh crystal growth.Seventhly, new discovery is in the experiment. Although starting materials and stainless steel tube is detached by Ta foil, high magnetism acerose matter is often observed in the as-prepared MgB2 using by Cd-Mg alloy flux. The result of EDS confirmed it is a kind of iron alloy. High magnetism of iron alloy is still needed deep research.Finally, Lio.4Ni1.6O2 with cubic phase has been synthesized based on layer structure of superconductors such as high temperature Cu-0 superconductors, organic metal superconductors, MgB2 and Nao.35CoO2-l.3H2O. Magnetic susceptibility measured by SQUID weakened with a drop in temperature (200-140 K), and magnetization curve reached zero line at about 140 K. When temperature keeps on dropping, magnetization curve will cross the zero line, that is to say, Meissner effect increases gradually. Lio.4Ni1602 and elements doped or structure modified will be researched and reported.