Jenifer Taylor

The effect of particle size distribution on the phase composition in YBa2Cu3O7−x as determined by DTA

Abstract The effect of crystallite size on thermal events during heating of YBa2Cu3O7−x was investigated. Differential thermal analysis up to 1100°C in air was performed on seven samples with different particle size distributions. The coarsest powder (davg=62 μm and 100%>45 μm) has one endothermic peak at 1000°C, which is the 211 peritectic melting point of 123. The powders containing fine crystallites showed two peaks, at 940°C and at 1000°C. The 940°C peak disappears when the thermal analysis is done in oxygen, indicating that ambient atmosphere has a strong effect on the thermal decomposition of YBa2Cu3O7−x.

Quality control of thermal barrier coatings using acoustic emission

Thermal barrier coatings (TBCs) are used to protect underlying metal from heat generated during combustion of fuel, especially in truck engines and jet turbines. These coatings are thin, partially stabilized zirconia, separated from the substrate metal by an interface layer, which serves to enhance bonding and reduce the thermal expansion mismatch between the metal and the ceramic. The reliability of these coatings is currently not predictable.The work described in this paper focused on the use of acoustic emission (AE) as a quality control test for TBCs. The test specimens were commercially sprayed straps. The data show that differences in spraying parameters and microstructure are clearly visible in the emissions during thermal cycling. This work indicates that the failure mechanism can be predicted from the AEs during the first thermal cycle.

Quench study investigation of the peritectic reaction as a function of crystallite size in YBa2Cu3O7-x

Abstract A quench study was performed to investigate the effect of the precursor powder size on the microstructure of bulk samples. At 1030°C, the size of 211 in samples produced from coarse powders was larger than that produced from fine powders and more inhomogeneously distributed. On cooling from 1030°C, 123 was not formed until 980°C for samples made from either coarse or fine powder. On quenching from 980°C after being fired to 1030°C, a large difference in the degree of surface orientation between samples prepared from coarse and fine powders was observed. This may be due to the difference of 211 size and its phase distribution in the melt.

Development of surface orientation of YBa2Cu3O7−x as a function of particle size distribution, firing time and temperature

Abstract The degree of grain orientation on the surface of bars melt processed at temperatures from 1010 to 1050°C was observed for two precursor powders having different particle size distributions. For the coarse powder, surface texture increases as a function of temperature to 1030°C and then levels off. For the fine powder, maximum orientation was achieved at 1020°C. The effect of firing time was evaluated by holding specimens at 1030°C for up to five hours. The degree of orientation increased for the first couple of hours and then leveled off. In most cases, the surface grain alignment of the coarse specimens was significantly higher than that of the fine samples. The difference in microstructure is related to particle size distribution after being reproduced to a statistically significant level and are related to particle size distribution.

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.

The development of inert substrates for melt processed YBa2Cu3O7−x

Melt textured YBa2Cu3O7−x can satisfy the high current demands of magnetic and power transmission applications. The purpose of this study was to identify a suitable polycrystalline substrate which does not react with the liquid phase formed during this process. The effects of several novel substrates for melt textured YBa2Cu3O7−x have been studied by X‐ray diffraction and SEM. The materials studied are: barium zirconate, 10wt% CuO doped barium zirconate, and YBa2Cu2.2Co0.8Ox. The least reactive substrate was the cofired cobalt doped YBa2Cu3O7−x. Slightly more reactive was presintered 10wt% CuO/BaZrO3, which produced a two phase mixture of YBa2Cu3O7−x and Y2BaCuO5. Even more reactive was presintered BaZrO3 substrate, which exhibited extensive barium cuprate diffusion at the grain boundaries.

Processing and characterization of cobalt-stabilized YBa2Cu3Oy as a substrate for bulk superconductors

Abstract The physical properties of ceramic YBa2Cu(3−x)CoxOy were investigated to evaluate its usefulness as a substrate material for bulk YBa2Cu3Oy superconductors. YBa2Cu(3−x)CoxOy is thermally and chemically compatible with YBa2Cu3Oy and displays adequate electrical properties for a substrate material. An intensive investigation of materials with the nomial composition of YBa2Cu2.2Co0.8Oy revealed semiconductor-like behavior, with a room-temperature resistivity of 70 mωcm and a 77 K resistivity of 4 × 106 mωcm. Measured mechanical properties YBa2Cu(3−x)CoxOy indicated that its flexural strength is low (60 MPa) and highly dependent on sintering conditions. YBa2Cu(3−xCoxOy was found to be a good substrate material for bulk YBa2Cu3Oy superconductors. Coils with a continuous superconducting path, made from superconductor/substrate composites, were characterized, and Co diffusion into the superconducting layer was investigated. Optimal processing conditions are discussed.

YBa2 Cu(3-x)CoxOy- A Substrate Material for YBCO Superconductors

The physical properties of the ceramic YBa 2 Cu (3-x) Co x O y have been investigated in order to evaluate its usefulness as a substrate material for YBCO superconductors. YBa 2 Cu (3-x) Co x O y has been found to be thermally and chemically compatible with 123 and displays adequate electrical properties for a substrate material. A material with the nominal composition of YBa 2 Cu 2.2 CO 0.8 O 7 was investigated, extensively. The mechanical properties of this material were found to be poor, e.g., tensile strength was only 60 MPa. A semiconductor-like behavior was observed with a room-temperature resistivity of 70 mμ.cm and a resistivity equal to 4 × 106 mn.cm at 77K. [ Key words: YBa 2 Cu 3 O y , cobalt substitution, substrate, electrical properties, thermal properties, processing]

The Fast Firing of the Ferroelectric Relaxor Pb(Mn 1/3 Nb 2/3 ) .02 (Mg 1/2 W 1/2 ) .48 Zr .23 Ti .27 O 3

The possibility of fast firing a relaxor dielectric to suppress grain growth, mitigate the lead loss and reduce the cost of sintering has been investigated. The relaxor Pb(Mn 1/3 Nb 2/3 ). 02 (Mg 1/2 W 1/2 ) .48 Zr .23 Ti .27 O 3 [1] exhibits a highly desirable broad phase transistion range when the prbcessing procedures are carefully controlled and fired conventionally. When fast fired, this materials density, percentage of secondary phases, and dielectric constant were highly dependent on the sintering time. A sample fired for 6 minutes possessed a dielectric constant nearly equal to the same material conventionally fired for 16 hours.

Submicron Particle Sizing by Light Scattering and Sedimentation

Submicron particle sizing is complex, requiring careful interpretation of distribution data. Information gathered from laboratory use of a sedimentometer and a light scattering system are discussed in terms of strengths and weaknesses.