Planes, and Grains

It is surprisingly easy to make a superconductor out of one of the new ceramic materials. Don Gubser of the Naval Research Laboratory says somewhat facetiously  

You simply go down to your local chemist s supply store and buy some copper oxide, some barium carbonate, and some yttrium oxide, all readily available in white powders. You go home and you do a little calculation so you ll get the ratios right, so that the yttrium, barium, and copper are in a ratio of 1-2-3. If you ve had any chemistry at all, that s very easy to do. Then you simply stir the mixture thoroughly in a pan and stick it in your microwave oven and you bake it at 950° F for about four hours—and when you pull it out, you ve got this black powder. You compress it into a little plate, and you put it back in the oven to sinter it and make it one solid piece. And if you know a little more about what you re doing, you blow some oxygen on it and 

Next you drop that little plate into liquid nitrogen. You can get that in any small town in the country, it s that plentiful. In fact, I was giving a talk in Iowa one day and I said I couldn t finish the demonstration without liquid nitrogen and I couldn t bring any with me. So someone ran out to the vet store and got some. The store had been using it to store bull sperm for artificial insemination. 

you drop that little disc of ceramic into the liquid nitrogen—you can put it all in a styrofoam cup—and put a little magnet on top. If the magnet floats, you ve got a superconductor, the Meissner effect. 

Don Gubser s experience with the vagaries of processing came early on, just after the first reports of the new ceramics surfaced. 

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The lattice defects are classified as (i) point defects, such as vacancies, interstitial atoms, substitutional impurity atoms, and interstitial impurity atoms, (ii) line defects, such as edge, screw, and mixed dislocations, and (iii) planar defects, such as stacking faults, twin planes, and grain boundaries. 

Page 83. You ve just got to distinguish..Personal interview. CHAPTER 7 CHAINS, PLANES, AND GRAINS... 

In marked contrast. Lynch and Trevena [98] for pnre Mg in a NaCl -I- K2Cr04 solution, reported that the fracture surfaces consisted of flutes containing equiaxed dimples on 10IX planes and grain boundaries as well as cleavage-like features on 0001 planes, with the flutes and dimples being smaller and shallower than those occurring in air. 

The test operates at a potential above 2-00 V (vs. S.H.E.), and the ditch structure obtained with sensitised alloys must be due, therefore, to the high rate of dissolution of the sensitised areas as compared with the matrix. The step structure is due to the different rates of dissolution of different crystal planes, and the dual structure is obtained when chromium carbides are present at grain boundaries, but not as a continuous network. 

H-BN is produced by hot-pressing the powder or by CVD. The processes impart different properties. The hot-pressed material shows less anisotropy than the CVD BN, since the powder grains are randomly oriented. CVD BN is usually a turbostratic boron nitride with warped basal planes and lattice defects. It is also known as pyrolytic boron nitride or PBN.1 11 " ]... 

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