Experiments have been performed to investigate the nature and evolution of the melt phase during melt processing. To this end YBa2Cu3O7-y (Y123) samples with Y2BaCuO5 (Y211) additions were melt processed and quenched at various stages of processing. The microstructure of these samples was investigated by optical microscopy and Scanning Electron Microscopy (SEM). Compositional information was obtained by X-ray Diffractometry (XRD) and Energy Dispersive X-ray Spectroscopy (EDS) and Wavelength Dispersive X-ray Spectroscopy (WDS).
The quenched melt of samples quenched from temperatures greater than 975°C consisted of Y211, BaCuO2 (BC1), BaCu2O2 (BC2) and CuO. Samples quenched from 975°C or lower consisted of Y123, Y211, BC2 and CuO. The melt in bulk samples of Y123 + 20mol% Y211 was not found to be homogeneous for all temperatures greater than the melting temperature of Y123, as is commonly assumed. Rather the melt was homogeneous for temperatures above 1040°C but entered a region of phase separation where BaCuO2 (BC1) crystallised at some point between 995°C and 1040°C. The proportion of BC1 and BC2 in the quenched melt varied with the temperature from which the samples were quenched, with the volume fraction of BC1 increasing with decreasing temperature to a maximum at 995°C. For temperatures below 995°C the amount of BC1 present decreased and no BC1 was observed in the quenched melt of the 975°C sample. The two Ba-Cu oxides had different Yttrium solubilities with BC1 having 4.3at% and BC2 having 5.1at%.
Thin samples (250µm ≥ thickness ≥ 50µm) were processed in a similar fashion, quenched in liquid nitrogen and their compositions and microstructures analysed in order to determine the effect of the quenching rate on the results obtained for thick samples. The composition of the rapidly quenched samples differed from that of the thicker, more slowly quenched samples with samples quenched in liquid nitrogen from temperatures greater than 985°C consisting of Y211, BC2, Ba2Cu3OX (BC1.5) and CuO. Samples quenched from 985°C had less BC1.5 and also contained BC1. BC1.5 appeared to separate into distinct regions of BC1 and BC2 during cooling and was only preserved by very rapid cooling rates.
The effect of PtO2 and CeO2 on the quenched melt of Y123 + Y211 samples was investigated. These additions reduced the Y211 particle size and changed the general morphology of the Y211 particles from blocky to acicular. The Yttrium contents of both the BC1 and BC2 phases in these samples was lower than in samples without additives. Y123 formation temperature was higher in the PtO2 sample. The amount of BC1 in the quenched melt was increased.
The magnetic properties of various regions in melt processed Y123 + Y211 bulk samples were investigated via magneto-optical microscopy. Flux penetration was found to occur firstly in CuO regions, then along Y123 domain boundaries and finally into the Y123 domains along platelet boundaries.