Ceramics for Industrial Heating
|Fig. 3. Manufactured in virtually any shape or size|
Alumina ceramics and steatites have primarily been used in the furnace industry for electrical insulators with advantageous structural properties at elevated temperatures. Steatites are better heat insulators but do not have nearly the strength or the suitability for high-temperature applications that alumina formulations have. Traditionally, steatites had a significant cost advantage. However, that has narrowed in recent years as the processing costs have decreased. Alumina ceramics offer a high-strength, low-cost advantage over any other technical ceramic. As a result, the markets for applications of alumina continue to expand as engineers gain further positive experience with these materials.
Why are there so many ceramic formulations, and how does one choose the most appropriate for the application?
Remembering where the ceramics industry originated will give you a better understanding. Potters, tile makers and brick makers all used local sources of clay to produce their products. Like other artisans, they developed many unique techniques and methods that still are prevalent today, even by sophisticated industrial ceramic producers. Because each operation had their own formulations, their products would differ in physical characteristics even if they looked similar and met the same specifications. In the early 1970s, great strides were made to develop standards by which to specify and measure the typical physical properties of common ceramic formulations used in industry. These developments established common nomenclature and common quality measurement techniques. Today, for instance, it is pretty well established what to expect from a 94% alumina body versus 96% or 98% alumina. Many manufacturers offer several different formulations for the varied applications in which they are used (Fig. 1).
In the 1980s, the rapid development and application of engineered or technical ceramics came about. Solid-state electronic components had been growing in volume, and ceramics were at the very center of that development. In the late 1950s, in order to make pure silicon wafers that could be sliced and diced for circuit boards, a clean environment with abundant, clean water was needed. There were few locales that met the qualifications. Brevard, N.C., where KEIR is headquartered, is one of them. As early as 1960, improved water-purification techniques as well as clean-room technologies made it possible to locate almost anywhere. Hence, Silicon Valley bloomed out of the desert in north-central California near where much of the electronic research was taking place. By the 1980s, high-purity technical ceramics (originally developed for electronics) were finding new applications as mechanical parts, in the cores of nuclear reactors, in space, in the automotive industry and for military applications.
Technical ceramics (mostly high-purity oxides) compared to common refractories or porcelains and steatites, have enormous advantages in strength, uniformity, the ability to hold much tighter dimensional tolerances, and crack or fracture toughness. They are dense and, therefore, not good thermal insulators but have outstanding mechanical properties even at very high temperatures. Also, being nonporous, they do not absorb moisture or gas. In short, these high-purity oxide ceramics make terrific mechanical components for furnaces.
Because of their predictable physical characteristics, technical ceramics have several processing advantages that make them more cost-effective to manufacture. The leading cause of scrap in a ceramic processing plant is lack of uniformity. Great pains are taken within the ceramic industry to control materials and processes to reduce scrap losses and increase final yields. Technical ceramics are far more predictable in many areas of processing, which leads to higher yields and better tolerances. Although the raw materials are more expensive by sometimes a factor of 10, the final price to the customer is not nearly as high because of higher yields.
Why are older ceramic compositions still made today?
There are several good reasons why old formulas for ceramic compositions are still commonly produced and marketed today. One is that they perform reasonably well in many applications and are somewhat less expensive to produce. Another less-understood reason is that ceramic processing is very specialized and often capital intensive. Typically, expensive tools are designed and produced for each part. Processing equipment, as well as furnaces and kilns, are specially produced for specific cycles and temperatures. Since each formulation has a different shrinkage when fired and fire on different cycle times and temperatures, it is not possible to use the same tooling or often the same process to make parts from a newer or different ceramic formulation. The cost of making a wholesale change to a more modern formulation is prohibitive. The cost investment in the tooling alone could well keep companies bound to outdated but still reasonably good formulas.
|Fig. 2. Furnace components made from 99.8%-pure alumina ceramics|
Newer Production Methods Lead to Advances in Quality, Delivery and Cost
Like most industries, ceramic producers have experienced revolutionary change over the past 25 years with the computerization of machine tools, statistical process controls and lean manufacturing techniques. This has eliminated much of the waste while improving quality and delivery performance. In the mid-1980s, several ceramic companies made deliberate moves away from traditional forming methods that rely on permanent steel or carbide tools to make specific parts. They moved toward several flexible methods of manufacturing that employ computer-controlled machine tools and far-fewer dedicated tools. This revolutionized their ability to quickly deliver parts designed and made for specific applications that would later be modified. This also made short production runs affordable, leading to greater innovation by the purchaser who could, for the first time, cost-effectively experiment with different designs.
Design engineers today have many material options open to them that were not available even a generation ago. High-purity alumina ceramics have become an intelligent choice in many situations where high strength, toughness, nonporosity and reasonable cost are desirable. This is what makes high-alumina ceramics a real value today. IH
For more information: Contact Gordon Murray, sales & marketing specialist, KEIR Manufacturing Inc., 133 McLean Rd., Brevard, NC 28712; tel: 828-885-8444; fax: 828-884-7494; U.S. toll free: 800-992-2402; e-mail: Murray@KEIRmfg.com; web: www.keirmfg.com