Sintering of bulk high- Tc superconductors: Y-Ba-Cu-O Page: 3 of 12
9 pagesView a full description of this article.
Extracted Text
The following text was automatically extracted from the image on this page using optical character recognition software:
Many 123 sintering studies have been published. Effects of processing
atmosphere [14-17], second phases [18-21], and method of heating [22] have been
examined. The studies have been conducted on powders of differing phase purities
and particle size, shape, and distribution. As a consequence, it can be very
difficult to compare results directly. This work will include a summary of our
previous work on sintering of 123, for which we have consistent powders, and new
results on direct control of grain growth.
The key features of this work are the control of Po, and selective use of
transient liquids. In the P02 range of this study, 2 x 102 to 16 Pa, the melting point
of 123 decreases 65-70 C with the P02 decrease [14]. Our work was restricted to
temperatures at which 123 was solid. It has been well documented that eutectics
of 123-CuO-BaCuO2 melt at temperatures 80-130 C lower than 123 itself melts
[8,14]. Thus, the presence of CuO and BaCuO2 can lead to rapid liquid-phase
sintering at relatively low temperatures [20,23,24]. In addition, sintering, creep,
and diffusional data indicate that solid-state diffusional kinetics in 123 increase
with decreasing PO2 [4,5,14,25,26]. As a final consideration, if the P02 is too low,
the 123 phase will begin to decompose [27]. It will be shown that this
decomposition can be used to one's advantage.
EXPERIMENTS
The 123 powder used in most of the experiments was synthesized from
reagent-grade Y203, BaCO3, and CuO. The starting powders were weighed to the
correct cation stoichiometry and placed in a polyethylene jar containing methyl
alcohol and ZrO2 grinding media. The mixture was ball milled for 16 h and dried.
The dry powder was screened through a 30-mesh sieve and calcined at 800 C for
4 h in flowing 02 at a pressure of 2.6 x 102 Pa. Final grinding yielded an average
particle size of 4 m. The powder was phase pure by X-ray diffraction and
differential thermal analysis [28].
For a few of the older studies, the 123 powder was heat treated differently. The
dry powder was pressed at a pressure of 140 MPa into a series of 80-g,
44-mm-diameter pellets that were placed on MgO single crystals and heated in
air to 890 C for 24 h. The pellets were then cooled, crushed, and ground in a
tungsten carbide rotary mill. The resultant powder was pressed into pellets, fired
in air at 890 C for 24 h, and ground two additional times. Final grinding in a jet
mill yielded an average particle size of 2.5 pm. Slight traces of BaCuO2 and
Y2BaCuO5 were present [14,19].
The powders were pressed into a series of small pellets and heat treated
under a variety of conditions. The temperature range was 850-1010 C, the P02
range was 2 x 102-105 Pa, and the time range was 1-250 h. The densities of the
resultant pellets were measured geometrically. The microstructures and phase
contents were examined by X-ray diffraction, differential thermal analysis (DTA),
optical microscopy, and scanning electron microscopy (SEM).
RESULTS
Either pure or impure powders have been used in all 123 sintering studies.
Many powders contain appreciable concentrations of impurity phases such as
CuO, BaCuO2, and Y2BaCuO5. Higher quality powders contain little or none of
these phases [28]. As shown in Fig. 1, the more-pure powders melt at about
1025*C in 02 and the less-pure powders contain phases that melt at lower7-
Upcoming Pages
Here’s what’s next.
Search Inside
This article can be searched. Note: Results may vary based on the legibility of text within the document.
Tools / Downloads
Get a copy of this page or view the extracted text.
Citing and Sharing
Basic information for referencing this web page. We also provide extended guidance on usage rights, references, copying or embedding.
Reference the current page of this Article.
Goretta, K.C.; Chen, N.; Lanagan, M.T.; Wu, W.; Singh, J.P.; Olson, R.A. et al. Sintering of bulk high- Tc superconductors: Y-Ba-Cu-O, article, May 1, 1992; Illinois. (https://digital.library.unt.edu/ark:/67531/metadc1055481/m1/3/: accessed July 16, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.