Seals between Sintered Ceramic Parts

摘要

1,196,899. Joining ceramic articles; shaped alumina articles. THORN LIGHTING Ltd. 18 April, 1969, No. 20031/69. Headings C1A and C1J. The tubular envelope of a discharge lamp is made from a sub-micron, ball-milled alumina powder, containing a small amount of magnesia, by isostatic pressing at 5 tons per square inch and firing at 1100‹ C. to a density of 1À84 gm. cm-3. Some of the same ball-milled magnesiacontaining alumina powder is spray dried from an aqueous suspension, which can contain an organic binder, to produce spheroidal particles having a higher bulk density than the ballmilled powder. End plugs for the envelope are made by compacting the spray-dried powder at 5 tons per square inch and firing the compacts at 1100‹ C. to a density of 2À15 gm. cm-3. The plugs are machined, if necessary, to fit tightly inside the envelope and the envelope and plug assembly is sintered in hydrogen at 1800‹ C. to a density of 3À95 gm. cm-3. The envelope shrinks more than the plugs to achieve a hermetic seal with grain growth across the interface. The plugs may have one or more holes for electrodes and/or exhaust tubes. The differential shrinkage can be controlled by using materials of different compositions; by pressing the plugs at a higher pressure than the envelope or by partially sintering the compacted plugs before machining.