Firing Advanced Ceramics In High-temperature Furnaces

On April 12, 1981, when the first space shuttle lifted from its launching pad in Cape Canaveral, Fla., the employees of Johns-Manville Corp. breathed a sigh of relief. This sigh would only be completed upon the successful return of the space shuttle Columbia after a safe landing on Earth 54.5 hours later. The entire bottom of the space shuttle was covered with armor-like tile that is formed from pure silica fibers. This tile was formed and heated to 2,200°F in a furnace, which allowed for a ceramic bond to form. The tile was made from Q-Fiber™, which was placed into production in a white-glove facility in Waterville, Ohio. The fibers were never touched by human hands; computers continually gauged the quality, and X-ray units measured the material’s density from every conceivable angle.

Transforming Ceramics

Traditional ceramics are known to be stable thermally, mechanically and chemically. These properties have been important since early times in products such as cooking vessels, serving dishes and sculpted figures. Clay alone, or mixed with various materials such as silica sand, can be formed into a desired shape, then dried and hardened (sintered) in a high-temperature kiln or furnace. As technology has advanced, so has the perceived utility of ceramics. Today, ceramic products fall into at least four categories:

• Tableware, cookware, wall tile and sanitaryware
• Structural ceramics, such as brick and roof tile
• Refractories, such as furnace and kiln insulation, and metal crucibles
• Technical (advanced) ceramics

High-Temperature Ceramic Applications

Another field that is constantly evolving is the medical field. Many existing and developing applications use high-temperature furnaces. The surgical implant device field is one area that has had a long-standing need for high-temperature ceramics. Many companies use high-temperature furnaces to sinter pure alumina powder at temperatures up to 2,500°F (1,371°C). The sintered powder is then separated from the hard crust that forms on the surface. The fine alumina powder is slurried and then applied to such medical implant devices as hip joints and knees.

Remotely controlled undersea cameras and other manned devices also use technical ceramics that require high-fire furnaces. In addition to certain electronic components inside these cameras, high-temperature ceramics can find applications in buoyancy devices, in which a sphere or part of a sphere made from alumina of precise thickness is heated to 3,000°F (1,650°C). These spheres are then post-process-heated and placed in buoyancy nets. Some of the spheres are made in two halves and then fit together with a ceramic bonding material. They are then heated again to 3,000°F (1,650°C) to complete the ceramic bonding.

The thickness is ultra-critical as it determines the amount of pressure the buoyancy sphere can withstand, which is especially necessary for deep exploratory dives. The ocean bottom is becoming better understood as a result of the confluence of many technical advances, not the least of which is the development of more accurate temperature control devices for these high-temperature furnaces.

Furnace Equipment

Once the equipment is designed and built, a great deal of testing is sometimes needed at the equipment manufacturer’s facility. Temperature uniformity surveys are undertaken to determine the characteristics of the chamber. In many cases, actual product samples are run through exacting process cycles using inert and active gases to simulate actual process conditions. These processes are supplemented and further refined at the user’s facility, with adjustments made until all of the desired properties are obtained in the final product.