James G. Fagan

The effects of impurity oxides on YBa2Cu3O7−x microstructure development

Abstract Diffusion-couple experiments in conjunction with scanning electron (SEM) and energy dispersive X-ray analysis were used to determine the effect of various oxides on the microstructural development of YBa 2 Cu 3 O 7− x ceramic superconductors. The interaction of a number of impurity oxides including SiO 2 , Bi 2 O 3 , Pr 6 O 11 , TiO 2 , SnO 2 , In 2 O 3 and Sb 2 O 3 resulted in exaggerated grain growth and domain formation in bulk specimens through a liquid-phase wetting mechanism. Reaction sequencing was determined by thermal analysis (DTA) and real-time dynamic X-ray diffraction. The ability and extent of domain formation for samples coupled with SiO 2 , Bi 2 O 3 , Pr 6 O 11 and TiO 2 was determined to be a function of oxygen partial pressure during sintering as well as of the sintering temperature in the range 940°C–980°C. The use of Bi 2 O 3 coupled to pre-oriented samples (i.e. hot forged) resulted in extensive domain growth at 980°C. The presence of SiO 2 in conjunction with hot forging resulted in a lower magnetization response, due to the presence of larger 211 particles and or extensive cracking during sintering.

Influence of excess yttrium and platinum on the growth behavior of YBa 2 Cu 3 O 7−σ and Y 2 BaCuO 5

The high temperature (1100°C) coarsening of Y2BaCuO5 (211) and subsequent YBa2Cu3O7−δ (123) growth kinetics using melt quenched and 123 precursor powders were examined via quenching. Fine scale excess yttrium addition by sol gel coating was employed up to 20 mol percent. X-ray diffraction identified 211 fraction between 123 (∼30 wt.%) and melt quenched (∼10 wt.%) in the precursor powders. The addition of yttrium was seen to shift the 211 weight fraction to higher levels for the MQ powders. Refinement of the 211 particle size was seen in the presence of Pt but not with yttrium addition. The coarsening behavior of 211 in either powder did not appear to significantly change with excess yttrium addition at 1100°C. Differential thermal analysis showed that the 123 phase solidification temperature increased in the presence of Pt and reduced with yttrium addition up to 10 mol %. Dilatometric measurements showed the influence of yttrium addition on the densification behavior of 123 due to the presence of low temperature liquid phase formation. Directly inserting samples at soak teperatures were seen to significantly alter the 211 weight fraction for 123 powder in contrast to slower thermal heating to temperature. However, this effect was not seen in the case of the melt quenched precursor powder.

The universal magnetic field dependence of the critical current density in high‐Tc ceramics

We present a new contactless method for determining the local critical current density Jc(B) and the effective magnetic permeability μ of ceramic superconductors. Using this method we have carried out systematic investigation of these parameters in ceramic samples with different microstructures. We find that the dependence of Jc(B) can be described by a single universal function over a wide region of magnetic field. At low fields this function has a plateau and at higher field it varies as B−3/2. The universal behavior of Jc(B) breaks down as the field exceeds the first critical magnetic field of grains.

Enhanced anisotropic grain growth of YBa2Cu3O7−x via TiO2 sol‐gel coating

The superconduction current of bulk high Tc ceramics has been shown to be greatly influenced by anisotropic grain growth (melt texturing). Melt texturing usually occurs during the sintering process in the presence of a liquid phase. This paper describes the effect of TiO2 additions incorporated on YBa2Cu3O7−x powder via sol‐gel coating. TiO2 additions upto 5 mole% resulted in anisotropic grain growth. Dilatometric measurements showed a lowering of the sintering temperature by ∼200 °C indicating the formation of a liquidus phase below that normally reported for YBa2Cu3O7−x with TiO2 addition. TiO2 additions significantly changed the resistivity and DC susceptibility response of YBa2Cu3O7−x superconductors. Microstructural changes were evaluated using SEM/TEM and EDS analysis.

Enhancement of grain orientation in YBa2Cu3O7−x using selective oxide diffusion couples/hot forging

The sintering behavior and critical current of high Tc ceramic superconductors can be greatly influenced by the presence of liquid phases, ambient processing atmosphere, and the application of pressure. This paper examines the use of a number of oxides, Bi2O3, Pr6O11, SiO2, and TiO2, in diffusion couples with YBa2Cu3O7−x, which were seen to form liquid phases during sintering resulting in texture development and/or grain growth. Increasing oxygen partial pressure was seen to deter texture formation at temperatures between 940 °C to 980 °C. The use of certain oxides resulted in bulk texture development at temperatures of 960 °C and 980 °C. YBa2Cu3O7−x samples sol‐gel coated with TiO2 showed increased grain orientation on hot forging compared to uncoated YBa2Cu3O7−x. However hot forging did not result in significant texture formation at the pressures and temperatures employed in this study.

Characterization of the texturing of YBa 2 Cu 3 O 7−σ in the presence of liquid phase additives

Grain growth and texturing of YBa2Cu3O7−δ is greatly influenced by the presence of liquid phase additives during sintering. Oxides such as TiO2, SiO2, Bi2O3, and Pr6O11 were incorporated into the liquid phase during the sintering of YBa2Cu3O7−δ (123) by use of grain boundary diffusion couples and the microstructure was analyzed using scanning electron microscopy/electron dispersive spectroscopy. Exaggerated grain growth and domain formation was observed in bulk specimens. Differential thermal analysis and real time dynamic x-ray diffraction were used to determine reaction sequencing. The ability and extent of domain formation was determined for 123 samples coupled with impurity oxides to be a function of sintering temperature (940–980°) and oxygen partial pressure. Enhanced texturing was observed at low PO2 atmospheres. The addition of Bi2O3 and TiO2 was shown to degrade dc magnetic susceptibility of 123 whereas SiO2 and Pr6O11 enhanced it. The domain formation and texturing takes place in the bulk for 123 at temperatures of 980°C or below (i.e. well below the peritectic decomposition temperature) by the interaction of an impurity doped liquid phase followed by a precipitation and exaggerated grain growth.