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2.2 Preparation of Bulk Ceramic Electrodes

The first step to preparing YBa₂Cu₃O₇ bulk electrodes is to make ceramic pellets of cuprate material.  Stoichiometric amounts, as seen in Table 2.1, of Y₂O₃, BaCO₃, and CuO are mixed together with a small amount of acetone.  The mixture is ground until dry.  At this point a uniform gray mixture is produced.  The mixture is then placed in an alumina crucible and heated in a box furnace to 905^oC at 300^oC/hr and held for ~12 hrs.  The sample is then cooled to room temperature and ground into a fine powder.  The sample is then heated one more time to 915^oC at 300^oC/hr and held for ~12 hrs. Again, the sample is cooled to room temperature and ground into a fine powder.  The sample is then pressed into pellets.  The pellets are placed on the backs of combustion boats and sintered in a box furnace.  Here, the sample is heated to 935^oC at 300^oC/hr. and held for ~12 hrs.  The pellets are then cooled to room temperature and placed in a tube furnace for oxygen annealing.  The tube furnace is then connected to slow flowing oxygen and heated to 450^oC at 300^oC/hr.  The pellets are held at 450^oC for ~6 hrs. Then, the pellets are cooled slowly to room temperature at 30^oC/hr.  This final step adjusts the oxygen content of YBa₂Cu₃O_7-x to its maximum value, with x~0.04. Other substitutions of YBa₂Cu₃O₇ are prepared by similar procedures.

Following previously described methods,³ the superconductor pellets were then broken in half.  As seen in Illustration 2.2, the top portion was painted with silver paint (SPI) and allowed to dry.  Next, a wire was connected to the pellet by silver epoxy or silver paint.  The ceramic electrode assembly was then placed in a plastic tube where it was filled with a mixture of Epon 828 epoxy resin (Shell) and metaphenylenediamine (Aldrich) curing agent (13% by weight).  The mixture was cured overnight at 80^oC.  The electrode was then sanded down to the ceramic superconductor on a grinding wheel.  Then, the electrode was resurfaced using coarse sand paper in an inert atmosphere glovebox just prior to use.

Illustration 2.2:     These are the steps that can be used for making a bulk superconductor electrode, where a) is 1/2 of ceramic pellet, b) is a pellet coated on one end with Ag paint, c) has a wire attached with Ag epoxy, d) has the pellet encapsulated in epoxy, and e) has the encapsulated electrode sanded down to expose the superconductor’s surface.