| Based on the isotropic round wire structure and high current-carrying capacity in high magnetic field and low temperature,the Bi2Sr2CaCu2Ox(Bi-2212)is one of the preferred superconducting materials for the high-field applications.At present,all of the Bi-2212 wires are fabricated by the powder-in-tube(PIT)process.Efforts should be done to improve the flux pinning and the intergranular weak links with increasing critical current density(Jc).Thus,the precursor powder synthesis,deformation and heat treatment of Bi-2212 wire should be optimized and improved to control the microstructure such as phase purity,second phases,defect,texture of superconducting core and couple between the Bi-2212 grains,etc.The stoichiometry and grain size,homogeneity of precursor,uniform of the filaments and smoothness of interface Ag/superconductor of the wire during drawing all have an effect on the microstructure and Jc of Bi-2212 wire.The reducing of the second phase and bubbles in superconductor core and improvement of the superconducting-grain texture may effectively reduce the weak link between the superconducting grains and increase Jc.In this thesis,many works have been done concerned with three aspects,which are,the precursor powder synthesis,deformation and heat treatment of Bi-2212 wire.First,many efforts have been done to prepare high purity precursor powder with steady chemical stoichiometry,excellent homogeneity and suitable grain size.Second,the drawing process of composite wire has been studied,and the wire deforming technology with low core breaking rate,smoothing of Ag/superconductor interface and uniform of core size is obtained.Next,the research on atmospheric and high-pressure heat treatment technology of wire has been carried out.The(Sr,Ca)xCuyOz(AEC)phase of the Bi-2212 wire is reduced and its texture is improved.The bubble problem of Bi-2212 wire has been solved by overpressure heat treatment and high Jc and uniform Bi-2212 wire has been fabricated in this thesisIn allusion to the problem of inability of stoichiometry maintenance during the traditional oxalate coprecipitation,a new solvent,the mixture of alcohol and water,has been employed at low temperature during precipitating.The residual cation concentration of supernatant is reduced by two orders of magnitude through the new technology,which succeeds to maintain the original stoichiometry of the precursor powder.In order to avoid component segregation from the formation of tiny liquid phases,the low-melting-point phase has been converted into high-melting-point phase by multistep low temperature heat treatments under the formation temperature of Bi-2212 phase.Thus,uniformity of precursor powder has been improved by the modified heat treatment.The second phase wrapped in Bi-2212 phase is fully exposed by three-dimensional grinding techniques and converted into Bi-2212 during the next heat treatment resulting in high Bi-2212 phase content of precursor powder.To achieve uniform deforming of multi-filament Bi-2212 wires,the evolution regularities and formation mechanism of breaking of the core in Bi-2212 wire with a traditional filaments structure of 85 X 7 have been investigated.A new structure with strong centre has been employed resulting in low core breaking rate of the Bi-2212 wire.Because the excessive tensile stress on the centre filaments of the wire is the main reason for the breaking core of the multi-filament wire.Then,it is found that the stress distribution along across section of the wire is more uniform resulting in improvement of its uniformity by using the large pass working rate during drawing process.Large-size precursor powder,with more Bi-O slip face,is very useful for slipping of the powder in the wire and increasing shaping properties of the Bi-2212 wire,leading to the improvements of breaking core and smooth of Ag/superconductor interface of the wire.Part of Bi-O bonds of Bi-2212 grains in powder are broken after a low temperature and oxygen partial pressure annealing resulting in reducing the binding energy of Bi-O surface and slipping energy of the powder.It increases the shaping properties of the precursor powder and the smooth of Ag/superconductor interface and Jc of the wire.In order to reduce the AEC phase and improve the texture of the Bi-2212 wire during the heat treatment,the phase evolutions of Bi-2212 wire during heat treatment have been studied,and we found that the content of AEC phase increasing obviously during initial cooling after melting.By thermodynamic analysis of Bi-2212 grains formatting,the formation model of texture has been established.Bi-2212 phase near sliver precipitates directly from the liquid during cooling below liquidus and the superconducting grains inside the core wire are formed from peritectic reaction and arranged in disorder.Based on this,a new technology of step cooling heat treatment has been put forward.A rapid cooling rate above liquidus can effectively inhibit the nucleation and growth of AEC phase.The slower cooling rate below the liquidus improves the Bi-2212 grain arrangement for the final wire and increases the wire properties.The voids existed in Bi-2212 long-length wire will reduce drastically Jc of the wire.In order to eliminate the voids,the air absorbed mechanism of precursor powder has been studied and the expansion process of the wire and tape has been calculated by the yield model of cylindrical and rectangular bucket pressure vessel.It is found that there are several reasons for the void formation in Bi-2212 wire,mainly including the gases adsorbed on powder and existed in the gaps among the powder particles.On this basis,we have presented the high-temperature vaccum degassing method to reduce the gases.It is discovered that the high-temperature vacuum degassing can eliminate the gases in wire and suppress the void formation.It is an effective method to manufacture the Bi-2212 long wire with high-performance.Moreover,it is beneficial for arranging Bi-2212 grains and avoiding leakage of Bi-2212 wire with high Je.The Je of Bi-2212 wire made by this method reaches to 440 A/mm2 at 4.2 K and 5 T.This property is the best performance of the long Bi-2212 wire sintered under atmosphere in the world at present.For further increasing the core density and the Jc of multi-filament Bi-2212 wire,the heat treatment under overpressure of the wire has been investigated.First of all,it is realized that keeping pressure stability by intelligent pressure control system during the whole heat treatment process,resulting in avoiding crack formation in ceramic core of the Bi-2212 wire from pressure change.The oxygen partial pressure of heat treatment atmosphere is also very stable during the whole process by controlling the ventilation rate.Therefore,a new type of high-pressure heat treatment device,which can meet the demand of technological requirements of Bi-2212 wire,has been set up in this thesis.Subsequent,the microstructure and superconducting property of wire treated under overpressure has been compared with those of wire heat-treated under atmospheric pressure.The results show that under the combined action of the.reducing of the voids and the Bi-2201 phase,and the dendritic structure around filaments of Bi-2212 wires and the effective superconducting area,the current-carrying capacity of wire heat-treated under overpressure has a substantial increase than that of wire treated under atmospheric pressure.The texture of Bi-2212 wire is also improved because the destory of grain arrangement from voids and the dendritic structure is prevented after heat-treating under overpressure.The Je of the wire increases 2.2 times and reaches 800 A mm-2(4.2 K,12 T)under overpressure of 5 Mpa.This performance is comparable to the best performance in the world at the same period.The solenoid coil,Rutherford cable and cable-in-conduit conductor(CICC)have been prepared by self-made wire.It is found that the Jc retention rate of the solenoid coils and the Rutherford cable prepared is 83%and 87%respectively.It shows that the flexural and torsional properties of the Bi-2212 wire produced in this paper can meet the requirements of the solenoid coil and the Rutherford cable.In cooperation with Institute of Plasma Physics,the three class CICC with 2×3 × 7 structure was fabricated.The Ic of the conductor after high pressure heat treatment reaches 26.6 kA(4.2 K,self).The conductor is the first real Bi-2212 CICC conductor in the world.The successful test of the conductor laid a solid foundation for the application of Bi-2212 wire in fusion device. |
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