Very basically, ceramics – derived from the Greek word keramos – are significantly comprised of inorganic, nonmetallic materials. Ceramics are important because they encompass a large and basic industry and because their properties are critical for many applications.
Due to their high-temperature stability and ability to not react with metallic materials, alumina (Al2O3 of various purities), mullite (comprised of Al2O3 and silica – SiO2) and silicon carbide (SiC) are used in high-temperature applications within the ferrous and nonferrous industries. All of these materials can be supplied in a variety of shapes in both dense and porous forms.
Depending on the material and required properties (e.g., high density and fine particle size), ceramics can be manufactured using the following processes:
• Dry pressing
• Isostatic pressing
• Injection molding
It is critical that ceramic products provide proper structural support under high temperature as well as good thermal-shock resistance.
Nonferrous Industry Applications
One particular material that has been widely used and extensively accepted is porous mullite. The composition of mullite is approximately 60% Al2O3 and 40% SiO2. Developed years ago for the furnace construction market as a fast-fire roller, requirements included:
• High-temperature strength and stability
• High load-bearing ability
• Excellent thermal-shock resistance
• Economical to produce
Mullite rollers or tubes are extruded, and relative close tolerances can be held, including +/-1.0 mm on the OD and ID. Excellent straightness can also be maintained over the full length within 2.0-4.0 mm TIR range.
A key application for porous-mullite rollers/tubes is supporting coiled electric heating elements over relatively long spans (Fig. 2). Mullite is a good choice because of its economical nature, variety of sizes available, high-temperature strength and thermal-shock resistance.
These ceramic electrical heating-element supports are used in heating equipment and furnace systems for melting, holding and transferring aluminum in the die-cast industry. Because of excellent thermal-shock resistance, rapid heat-up and cooldown of the porous-mullite element supports can occur.
Mullite exhibits application temperatures up to 1350˚C (2462˚F) and refractoriness up to 1250˚C (2282˚F), depending on the load. These porous-mullite element support tubes can be used in virtually any type of electrical heating and furnace application, including aluminum die casting, glass annealing and heat treating.
Another application of porous mullite in the nonferrous industry is molten aluminum heating-element protection tubes, which protect the heating elements from spattering aluminum. These tubes have the proper thermal-shock resistance required, and they are nonreactive with the heating elements.
The versatility of alumina-based products in furnace construction is unmatched. High-purity alumina (99.8%) can be used in the 1700˚C (3092˚F) range, exhibiting excellent mechanical strength, thermal conductivity and electrical resistivity.
Thin-wall shapes may be produced to ensure proper thermal-shock resistance. Alumina is also resistant to many forms of chemical attack, including hydrogen and reducing gases. Examples of applications for alumina include:
• Thermocouple protection sheaths and insulators used in conjunction with platinum for high-temperature sensors (Fig. 3)
• Electrical insulators or spacers in heat-treating furnaces
• Combustion tubes
• Laser tubes
• Special shapes
Alumina furnace components can be produced using all of the aforementioned ceramic production processes.
Once primarily thought of as an abrasive, silicon carbide’s usefulness has expanded in the past few decades. SiC tubes can be used to protect thermocouples that are immersed in molten aluminum. Tubes can be made with thick walls to provide maximum mechanical strength. SiC is well suited for high-temperature applications due to its high thermal conductivity and excellent thermal-shock resistance.
This material has a very low thermal expansion and can be used continuously at temperatures up to 1000˚C (1832˚F). Applications include riser tubes for continuous casting and crucibles for melting that require excellent thermal-shock resistance.
Ceramics in a variety of forms are utilized by the thermal-processing community daily. Without ceramics and regular developments in the industry, high-temperature processing would be much more challenging. From thermocouple and furnace-element protection to riser tubes and crucibles, the properties of ceramics are critical for many of the operations we perform on an ongoing basis. IH
For more information: Contact Bill Bolt, Ceramic Solutions, Inc. 12621 Hwy. 105 W., Suite 301, Conroe Texas; tel: 936-588-2646 x 222; fax: 936-588-2688; e-mail: firstname.lastname@example.org; web: www.ceramicsolutionsconroe.com