Method for making polycrystalline alumina arc tubes

Claim 1

THE METHOD OF PREPARING A TUBULAR POLYCRYSTALLINE ALUMINA MEMBER SUITABLE FOR USE AS AN ARC-CONTAINING TUBE FOR AN ARC-DISCHARGE DEVICE, WHICH METHOD COMPRISES: A. ADDING TO VERY FINELY DIVIDED HIGH PURITY ALUMINA POWDER FROM ABOUT 0.15 TO 0.7% BY WEIGHT OF SAID ALUMINA OF MAGNESIUM ALUMINATE SPINEL CONSISTING OF IRREGULAR SHAPED PARTICLES OF AVERAGE DIAMETER LESS THAN ABOUT 100 NM, AND THOROUGHLY MIXING SAID SPINEL AND SAID ALUMINA TO ACHIEVE A UNIFORM DISTRIBUTION OF SAID SPINEL THROUGHOUT SAID ALUMINA; B. FILLING SAID MIXTURE AS A DRIED POWDER INTO AN ISOSTATIC PRESSING MOLD HAVING TUBULAR-SHAPED MOLD CAVITY DIMENSIONS A PREDETERMINED AMOUNT LARGER THAN THE DESIRED FINAL DIMENSIONS FOR SAID COMPLETED ARC TUBE TO COMPENSATE FOR SHRINKAGE ENCOUNTERED DURING PRESSING AND SINTERING THEREOF; C. ISOSTATICALLY PRESSING THE MOLD-CONTAINED POWDER AT A PRESSURE OF AT LEAST 15,000 PSI TO PRODUCE A GREEN TUBE; D. PRESINTERING THE PRESSED GREEN TUBE AT A TEMPERATURE OF AT LEAST 800*C FOR AT LEAST 1 HOUR TO PROVIDE THE PRESSED GREEN TUBE WITH ADDITIONAL STRENGTH TO FACILITATE HANDLING; AND E. FINAL SINTERING THE TUBE AT A TEMPERATURE OF FROM 1700*C TO 1950*C IN OXYGEN, HYDROGEN OR VACUUM FOR A SUFFICIENT PERIOD OF TIME TO OBTAIN A GRAIN SIZE IN SAID SINTERED TUBE OF AT LEAST 10 MICRONS AS MEASURED BY THE AVERAGE PLANAR INTERCEPT METHOD.

Claim 2

THE METHOD OF PREPARING A TUBULAR POLYCRYSTALLINE ALUMINA MEMBER SUITABLE FOR USE AS AN ARC-CONTAINING TUBE FOR AN ARC-DISCHARGE DEVICE, WHICH METHOD COMPRISES: A. ADDING TO VERY FINELY DIVIDED HIGH PURITY ALUMINA POWDER FROM ABOUT 0.15 TO 0.7% BY WEIGHT OF SAID ALUMINA OF MAGNESIUM ALUMINATE SPINEL CONSISTING OF IRREGULAR SHAPED PARTICLES OF AVERAGE DIAMETER LESS THAN ABOUT 100 NM, AND THOROUGHLY MIXING SAID SPINEL AND SAID ALUMINA TO ACHIEVE A UNIFORM DISTRIBUTION OF SAID SPINEL THROUGHOUT SAID ALUMINA; B. FILLING SAID MIXTURE AS A DRIED POWDER INTO AN ISOSTATIC PRESSING MOLD HAVING TUBULAR-SHAPED MOLD CAVITY DIMENSIONS A PREDETERMINED AMOUNT LARGER THAN THE DESIRED FINAL DIMENSIONS FOR SAID COMPLETED ARC TUBE TO COMPENSATE FOR SHRINKAGE ENCOUNTERED DURING PRESSING AND SINTERING THEREOF; C. ISOSTATICALLY PRESSING THE MOLD-CONTAINED POWDER AT A PRESSURE OF AT LEAST 15,000 PSI TO PRODUCE A GREEN TUBE; D. PRESINTERING THE PRESSED GREEN TUBE AT A TEMPERATURE OF AT LEAST 800*C FOR AT LEAST 1 HOUR TO PROVIDE THE PRESSED GREEN TUBE WITH ADDITIONAL STRENGTH TO FACILITATE HANDLING; AND E. FINAL SINTERING THE TUBE AT A TEMPERATURE OF FROM 1700*C TO 1950*C IN OXYGEN, HYDROGEN OR VACUUM FOR A SUFFICIENT PERIOD OF TIME TO OBTAIN A GRAIN SIZE IN SAID SINTERED TUBE OF AT LEAST 10 MICRONS AS MEASURED BY THE AVERAGE PLANAR INTERCEPT METHOD.