| High temperature superconductor （HTS） YBCO possesses high critical current density（Jc）, high critical transition temperature （Tc）, low surface resistance （Rs）, and goodsuperconducting properties under magnetic fields. So it is very popular in superconductingapplication. For low current applications, YBCO films are usually grown on apropriatecrystaline wafers. These YBCO epitaxial films can be ultilipd for microwave filters, SQUID,MRI, etc.. For high current applications, YBCO films are usually coated on opportune metalsubstrate. These coated conductors can be used in power transmission, superconductingcurrent limiter, superconducting magnetic energy storage （SMES）, etc. Various filmpreparation techniques, such as laser ablation, magnetron sputtering, co-evaporation, andmetalorganic chemical vapor deposition （MOCVD）, have been utilized for epitaxial growthof high quality YBCO films for coated conductors. Among them, MOCVD has certainunique advantages over others, such as uniform growth of films over large areas, thecapability to grow high-quality YBCO films with high growth rate and beingsingle-crystal-like. So it is attractive for industrial applications. For preparation of a c-axisYBa2Cu3O6.9film by photo-assisted MOCVD, there are two characteristics because of photoactivation. Firstly, photo-assisted MOCVD often can have much higher growth rate （>500nm/min） than traditional MOCVD technique （<200nm/min）; secondly, the c-axis YBCOfilms grown by photo-assisted MOCVD often can have very high crystalline quality. Mythesis work is done with three core issues that in the preparation of YBCO films byphoto-assisted MOCVD. Firstly, morphology and superconducting properties of photo-assisted MOCVD processed YBCO films as variation of precursors’ proportion wasstudied. Then we discussed the effects of photo-assisted and thermal annealing on oxidationand de-oxidation of YBCO films. Finally, the preparation of YBCO film on biaxiallytextured Ni substrates and the effects of thermal conductivity of different substrates, onwhich YBCO films grown by photo-assisted MOCVD were studied.Firstly, crystalline quality and superconducting properties of photo-assisted MOCVDprocessed YBCO films as variation of sublimation of the Cu-based precursor was studied. Itwas found that by decreasing appropriately the sublimated gas ratio of the Cu-based precursor,as specified by themole ratio of （Cu/Y）mol, and practically performed by gradually loweringthe sublimation temperature Tsubof the Cu-based precursor, surface growth particles on c-axisoriented YBCO epi-films prepared by photo-assisted MOCVD could then be effectivelysubdued. But the holes and other defects would be happened in YBCO film if the （Cu/Y）molvalue was too small. Finally, it is found that with the condition that Ba precursor in excessamount, YBCO film sample A03has it （Cu/Y）molas2.3, with essentially no outgrowths on itssurface.From the Jc analysis, it also found that as the （Cu/Y）molvalue decreasing, a trend of Jcreduce can be found. The main causes of the jc lower trend is possibility of formation ofholes in quantity and size with the film becomes higher with the （Cu/Y）molvalue decreasing.The holes will cause lower Jcto the YBCO film obviously. Overall, the sample, preparedunder condition that （Cu/Y）mol=2.3, with some bits of small size Cu-O outgrowths onsurface （sample A03）, has a Jc value as2.6MA/cm2（0T，77K）. Moreover for sampleA04and A05prepared under much lower （Cu/Y）mol, there is no outgrowths on top but many holes,have the Jc value of1.8MA/cm2. For the5YBCO film samples used in this study, sampleA01has the highest Jc, which value is3.4MA/cm2. However it also has the outgrowths withmost types, maximum quantity, and larger size. Thus, how to optimize the YBCO filmsepitaxial parameters should be guisted by the need of applications.Then effects of photo-assisted and thermal annealing on oxidation and de-oxidation ofYBCO films were studied. In this study, the effects of photo-assisted annealing and thermal annealing on oxidation and de-oxidation rates and the equilibrium oxygen content of YBCOfilm were discussed.The YBCO film samples were annealed at460oC and550oC photo-assisted andthermally for oxidation and de-oxidation. Then effects of these two annealing processes onoxidation/de-oxidation rate YBCO were analyzed. It is found that the photo-activation isclearly more effective for oxidation of YBCO films than thermal activation. The oxidationrate of the YBCO film is obviously higher by photo-assisted annealing than thermalannealing. However, the photo-assisted annealing is less effective for de-oxidation of YBCOfilms than thermal annealing. It indicates that the de-oxidation rate of YBCO film isobviously lower by photo-assisted annealing than thermal annealing. These two results canbe explained by the fact that in oxidation and de-oxidation processing of YBCO films, O2molecules adsorbed on surface can be dissociated into oxygen atoms more easily byphoto-assisted process than thermally. We think that the reaction rate of O2moleculesdissociating into oxygen atoms can be speed up by high energy photons provided by thehigh power halogen tungsten lamps. From this the experimental results cited above can beexplained. On the other hands, from the comparison study of equilibrium oxygen content ofYBCO film processed at different annealing temperature and oxygen partial pressure, it isfound that the stabilized on equilibrium state oxygen-content x of a c-axis oriented YBCOfilm can be specified by thermodynamic variables such as annealing temperature T, oxygenpartial pressure PO2, irrespective of whether annealed by photo-assisted or thermalactivation process. This means that in case of photo-assisted processing the photo activationcan have effect on reaction rates, but no effect on thermodynamic equilibrium conditions forthe material.At the same time the feasibility of the technique to evaluate the oxygen content x in aYBCO film indirectly from values of the c-axis lattice parameter determined by XRD2θ-scan patterns was discussed in this part. Adoption of this indirect method is based on a“linear relationship” between c-axis parameter determined by XRD2θ-scan data andoxygen content of powder samples of YBCO obtained by the standard iodometric titration. However, comparing with the powder samples of YBCO, effects such as stresses andgradients as well as interfacial layers happened in films of YBCO must be concerned.Certainly, these effects will cause differences in c-axis values resulted between powder andfilm samples of YBCO. Thus the c-axis parameter values will be different for the YBCOfilms and powder that have the same oxygen content. So the linear relationship betweenc-axis parameter and oxygen content will also be different for the film sample and powdersample of YBCO. In this part the all small pieces of YBCO film samples used for thiscomparison study （about2×5mm2） were cut from a2" c-axis oriented epitaxial YBCO film.So the effects from stresses and gradients as well as interfacial layers are similar for all ofthose YBCO film samples. Then a similar line was drawn based on the c-axis parameters ofYBCO film samples that with oxygen content of6.0and6.9, just as the “linear relationship”found between c-axis value and oxygen content x of powder YBCO samples as cited earlier.Then based on this line, all the relative changes of oxygen content values at various c-axisparameters of YBCO film samples used in this study can be estimated by the relativechanges of c-axis values. Thus, we think that the relative changes of oxygen content x of afilm sample can be estimated reliably from the changes of the respective c-axis parametersof these YBCO film samples.Finally, a preliminary research on preparation of YBCO film on biaxially textured Nisubstrates by photo-assisted MOCVD was presented. In this issue, the effects ofphoto-assisted on substrate temperature of biaxially textured Ni were discussed. The effect ofthermal conductivity of substrates on YBCO film growth rate was analyzed from this study.At the same time, the reason of c-axis and a-axis orientation growth of YBCO film wasanalyzed from the point of thermodynamics.From the experiments, it is found that the growth temperature is about770oC for c-axisYBCO film on biaxially textured Ni substrates, and about800oC for that grown on LAOsubstrates. There are two reasons for this difference between different substrates. The firstreason is that, because the light transmittance is different for Ni substrate and LAO substrate,so the heat transfer behavior between the susceptor and these two substrates is different. For substrates as LAO, the substrate is heated by susceptor as heat conducted from susceptor tosubstrate. However, for substrate as Ni tape, the substrate is heated by tungsten halogen lamp.More over it should keep well contact between substrate and susceptor to dissipate heatenergy equally from the substrate. For this case, the actual temperature of Ni substrate will bemuch higher than that of LAO substrate at the same set temperature. The second reason isthat, because Ni is a good conductor, but LAO is a dielectric material, so the thermalconductivity of Ni substrate is higher than that of LAO substrate. Thus the heat can beconducted more easily from Ni to reactant atoms than from LAO to reactant atoms.At the same time, it is found the growth rate of YBCO films on Ni substrates is higherthan that on LAO. The reason of this different of growth rate can be got form the differenceof thermal conductivity between these tow substrates. Because the thermal conductivity isdifferent from Ni to LAO, so the mobility and reaction rate of the molecule on surfaces ofsubstrates are different. The heat can be conducted more easily from Ni to reactant atomsthan from LAO to reactant atoms. Thus the mobility and reaction rate of the molecule onsurfaces of Ni substrate are higher than that on LAO substrate. And then the growth rate ofYBCO film on Ni substrate is higher than that on LAO substrate.On the other hand, the reason of c-axis and a-axis orientation growth of YBCO film wasanalyzed from the point of thermodynamics. We think that, they are two differentthermodynamical equilibrium states for c-axis and a-axis orientated YBCO films. It isknown, the growth temperature of a-axis YBCO film is lower than that of c-axis YBCO film.This means the activation energy for growth of a-axis YBCO film is lower than c-axisYBCO film. We also find from the experiment of annealing a-axis YBCO at highertemperature, the a-axis YBCO film can change to be c-axis at a high anneal temperature forlong time, but the c-axis YBCO film can not change to be a-axis. This means the free energyof the c-axis YBCO film is lower than that of a-axis YBCO film. |
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