Self-propagating High-temperature Synthesis (SHS) Method MgB₂ Superconductor And SiC, C Doping Phase Transition Process

MgB₂ superconductor with critical temperature of about 39K has attracted much attention in condensed matter physics,chemistry and material science fields,and so on, because of its many unique properties,Since it has been found.MgB₂ has many excellent properties,such as higher critical current density,higher upper critical field,et al.All of these indicate that the superconductor of MgB₂ is a good candidate in the many applications fields.In recent years,many laboratories and groups in the world have done a great deal of research in the MgB₂ superconductor,but the study on the phase transition process of MgB₂ is very much less.This thesis aims at the above mentioned to research the characteristics of a chemical reaction of the Mg-B system,to probe preparation of MgB₂ superconductor by Self-propagating High-temperature Synthesis, and to study phase transition process with doped SiC and C by the method of the Combustion Front Quenching,in order to provide reference for theoretical and experimental in the practical application of MgB₂.In this paper,the reaction of the Mg-B system is systematically analysed with the methods of thermodynamics and kinetics in theoretical and experimental aspects.The analytic result shows that:When the preheating temperature goes up,the adiabatic temperature of the Mg-B system will increase,too.When the adiabatic temperature of the system is 1614K,preheating process is need before igniting the reaction of the Mg-B system,and preheating temperature can not be less than 501K.When the temperature ranges from 298K to 2000K,Gibbs free energy change values of MgB₂, Mg₂Si,CB₄ and MgB₄ in the formation reaction are negative,and Gibbs free energy change values of MgB₄ is biggest among them.Which indicates that the formation reaction of MgB₄ occurs more easily,so the synthesis conditions should be controlled to ensure that the amount of Mg in the process of synthes is sufficient to prevent the non-superconducting MgB₄.At the same time,experiment and experimental results indicated that solid-liquid reaction mechanism is the best for MgB₂ superconductivity. There are many ways to synthesize the superconductor of MgB₂.However,in general, the processes of these ways are very complex.The temperature in synthesis and times are in a range of 600℃-900℃,and 1-48 hours respectively.In this thesis,we introduce that prepared the superconductor of by Self-propagating high-temperature synthesis(SHS) method.The SHS method is simpler and more economical than other methods.The experimental results show that the sample of MgB₂ contains a little MgO, and the main phase for sample is MgB₂,the compact degree of sample is low,the sample has the layered structure with lots of holes between the layers made of granular crystallites with a size of 2μm-5μm,the critical temperature of samples is about 38.45K, Jc=1.60×10⁶ A/cm²(10K,0.5T)which is similar to those prepared by the conventional ways.In this paper,the factors influencing phase component of MgB₂ superconductor by SHS method are researched on.The experimental results are as following:When the size of Mg powder becomes small,the reaction temperature of the Mg-B system will have a trend to rise,and synthesis product seems to be sintered.The preheating rate is irrelevant to the reaction temperature of the Mg-B system,but it can affect on the compact degree of the product,and the slower the preheating rate,the more compact the product is prepared.The holding temperature time has little effect on the MgB₂. superconductivity.Phase transition process were studied by Combustion Front Quenching(CFQ) experiments doped SiC and C with self-propagating high-temperature synthesis(SHS) method.The experimental results are as following:When the temperature reaches the range from 527℃to 650℃,the reaction of Mg and B in solid state will occur by diffusing,When the temperature reaches the range from 650℃to 700℃,the Mg will be melted,and the melted Mg will rapidly get across the MgB₂ layer and react to B in liquid by diffusing and infiltrating;the phase interface of MgB₂ will not expand towards B atom until they completely come into being MgB₂.when the range of temperature is from 700℃to 750℃,the mostly SiC will react to MgB₂,and synthesize Mg₂Si;At the same time,the fraction of C atoms come into MgB₂ crystal lattice and substitute for B atoms;On the other hand,the slight fraction of MgB₂ was decompounded into MgB₄ and Mg.Owing to the short time of the reaction and the higher temperature grads,Mg has been solidified and crystallized again before some Mg diffuse.The(101) peaks of XRD patterns of the superconductor bulk of MgB_1.95C_0.05 are separated.The experimental results are as following:When the temperature of the system is 650℃,the(101) peak of product MgB₂ shift to the right direction,and the offset of 2θangle is about 0.2°.On the other hand,a part of doping carbon enters MgB₂ crystal lattice and the residue carbon is at grain boundary of MgB₂,which can improve flux pinning,and the lattice constants of the c-axis does not change.