HIGH TEMPERATURE REFRACTORIES1 0. J. WHITTEMORE, JR. Norton Company, Worcester, Massachusetts
REFRACTORIESare
materials capable of withstanding high temperatures, say 1000°C. or better. The refractories that will come to your minds will be firebrick in furnaces and crucibles for melting substances. I shall tell you of many unusual refractories, including one used in your kitchen, of another as radioactive as uranium and with the density of lead, of others capable of being heated to high temperatures in a few seconds, and of a refractory which has been heated to 2500°C. commercially, has twice the density of firebrick, yet is a better insulator.
"bentone." A new group of materials called "cermets" are combinations of ceramics and metals. CERAMIC ENGINEERING
As scientific educators, you will be interested in curricula available in ceramics. Ceramic engineering degrees have been granted for over 50 years, the first school being Ohio State, which is quite logical since Ohio ranks first among the states in value of ceramic products. There are now about 15 schools offering studies leading to degrees in ceramic engineering or ceramic technology. Unfortunately, there are no CERAMICS New England colleges that offer a full curriculum, but Before getting into my subject too deeply, however, graduate study can be pursued a t the Massachusetts I should like to tell about the ceramics field, of which Institute of Technology, where some of the most adrefractories are one part. Probably the word ceramics vanced work is being carried on under F. H. Norton. means art pottery to most of you. Pottery is but one At the University of New Hampshire, ceramic art small part of the p a c k a g e t h e gilt wrapping. The courses are offered. Many other colleges, schools, and china on your table are ceramics. The porcelain other groups throughout the country are giving art enamel coatings on your refrigerators and bathtubs, pottery studies. Ceramic engineering courses include basic engineerthe glass in your windows, the brick in your houses, perhaps even the teeth in your mouth are ceramics. ing studies with two tothree years of chemistry, geolThe filaments in your radio tubes may be ceramic- ogy, and mineralogy, and the ceramic studies such as coated and the envelope may be a glass sheath. Your properties of ceramic materials, pyrometry, and studies radio circuits are insulated with ceramics, and new of particular fields. The jobs that ceramic engiueers do are quite similar ceramic capacitors-the titanates-are undoubtedly present. The major classifications are: abrasives, to those of chemical engineers-research, developrefractories, glass, whitewares, porcelain enamels, ment, control, manufacturing supervision, and salescements and plasters, and structural clay products. except that, of course, the jobs are in the ceramics The ceramics industry was once defined as the clay- rather than the chemicals industry. At the present working field, but this definition is inadequate today and ever since World War 11, there has been a serious because many ceramics contain no clay. Ceramics shortage of ceramic engineers, as there has been in all might be defined as inorganic materials converted by scientific pursuits. The deficiency is probably greater heat to usefulness. This difficulty of definition is in ceramics because the field is not so well known. I n caused partly by the growing interrelation of fields. one excellent college this year, not one freshman enThe organic chemists have developed silicones, which rolled for ceramic engineering. Many manufacturers are close to silicates, and have also attached organic have had to employ graduates in other engineering radicals to the clay bentonite, and call the product fields because ceramic engineers were not available, had to supplement their education. ' Presented at the 273rd Meeting of the New England Associa- andI have can recommend ceramic engineering highly as a tion of Chemistry Teachers, Brandeis University, Waltham, field in which students with scientific and engineering Mssmehusetts, December 5, 1953.
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About this same time, another industry was developing-the abrasives industry-which was interested in hard materials. I n general, the very hard materials are also refractory. Fused alumina and silicon carbide are the two common abrasives today but they also furnish the backbone of the special refractories industry. Silicon carbide has very high thermal conductivity (a silicon carbide brick conducts heat seven times better than a fireclay brick). I t also is very resistant to thermal shock, because of its high conductivity and relatively low thermal expansion. Of all the carbides it is the most resistant to oxidation. Silicon carbide bricks are strong a t high temperatures and are reHIGH TEMPERATURE REFRACTORIES sistant to coal ash slags. They are used to construct Besides the business expansion since World War 11, muffle walls where high heat transfer is necessary and several other factors have contributed to the current to line water-gas generators and boiler furnaces where growth and importance of ceramics. Radar and tele- heat shock resistance and slag resistance are required. vision required better insulators and capacitors. I n Silicon carbide plates are used in the whitewares inaddition, emphasis began to be placed on higher and dustry for supporting the ware during firing. higher temperatures and other severe operating condiSilicon carbide conducts electricity, and rods are tions. The atomic energy program required containers made from i t to heat furnaces to 1450°C. in air atmosto melt and refine the new metals and construction phere, or several hundred degrees above the use temmaterials for nuclear reactors. Jet propulsion will perature of wire-wound furnaces. One spectacular use achieve much higher efficiency a t higher temperatures of silicon carbide refractories is as nozzles for experithan now possible with metals, so much work is being mental rockets. This is a refractory that can be devoted to ceramics. So now I return to my subjectheated to high temperatures in a few seconds. Fused alumina has great refractoriness (melting high temperature refractories. The first refractories were rocks and bricks that man point 2015"C.), has good chemical stability, is an exused to refine metals and to build fireplaces. As time cellent electrical insulator, and has relatively high vent on he found by experience that certain rocks were thermal conductivity. As chemists, you are familiar more refractory, that certain clays made better fire- with fused alumina filtering crucibles and cores for brick. He found that quartz (SiOJ is a good refractory wire-wound furnaces. The chemical industry has been if heated carefully and that kaolin (A120a9Si02.- developing high temperature gm synthesis processes 2Hz0) was one of the most refractory clays. Much since World War I1 and fused alumina "pure oxide" later, he found that the minerals magnesite, chromite, brick have been used for the furnace linings because of and dolomite were very useful refractories for steel their high refractoriness and resistance to atmospheres melting. About this time (the end of the nineteenth which would reduce silicate-bearing refractories. century), chemistry and physics began to help him to understand his materials, to show him how t o purify PURE OXIDE REFRACTORIES them for greater value. The "pure oxide" class of refractories is relatively tm\- :11d itdudes those wfrutwriru c.s;cntinlly of m e oxide, frcc pnrfiml~rly from gins-forming silira. Phase di~grumsshow in most wsrc that higllwt meltinc, r)oints can l)r ndizrd with no ronr;~minxntor othcr oxide. However, the presence of a small amount of silica not only lowers the melting point of a refractory but also forms glass which reduces the load-bearing capacity to a greater extent than would be expected by a study of the phase diagram. Clay-bonded fused alumina brick increase in thermal conductivity with temperature in comparison with self-bonded or recrystallized fused alumina brick, which decrease in thermal conductivitv with tem~erature. Alumina, A120J, magnesia, MgO, and stabilized zirconia, Zr02, are the three important "pure oxide" refractories. Magnesia has a very high melting point (2800°C.) but has high vapor pressure above 2200°C. and cannot be used in reducing atmospheres above 1700°C. One new aovlication for maenesia is in a rigure1. stabilized zirconis~~~~~t~~ shagas furnace for the fixatidn of atmospheric nitrogen by
ability can find interesting and fruitful employment. I t is a field rapidly becoming more and more complex, yet has many traditions, and is one of the few technical fields in which art has a place. The American Ceramic Society, organized 55 years ago, is our technical society, and holds annual, divisional, and sectional meetings and publishes a Journal, a Bulletin, and Ceramic Abstracts. Two internal groups are the National Institute of Ceramic Engineers and the Ceramic Educational Council. Information can be obtained from The American Ceramic Society, 2525 North High Street, Columbus 25, Ohio.
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merely heating air to 2200°C. Magnesia is a very important refractory for metal-melting or purification because of its great resistance to basic slags. Fused magnesia is the special refractory used in your kitchen in the sheathed electric stove heaters. These contain fused magnesia because of its high electrical resistivity and thermal conductivity. Stabilized zirconia refractories contain about five per cent lime, CaO. Pure ZrOz, when heated, inverts from a monoclinic to a tetragonal crystal form a t about 1000°C. This transition is accompanied by a large volume contraction, which usually cracks up any refractory shape. When about five per cent CaO is fused into solid solution with ZrOl, a new cubic crystal is formed which does not invert when heated. However, by allowing a small residual amount of the monoclinic form to remain present, the over-all thermal expansion is lowered. Stabilized zirconia refractories, therefore, require careful control t o insure the proper crystal content. To my knowledge, this is the first Properties of New Special Refractories
- .... ...-.
.Verne
Fomula
Zirconium carbide Titanium carbide Silicon carbide Boron carbide Titanium boride Zirconium boride Chromium boride Molvbdenum
zrc
Tic Sic B,C Ti&
ZrB2
CrB MoSiz
Decomposes.
Density
conductivity, Melting Cal./see./m.z point., "C. n./'C.
refractory for which such control has been required. Quantitative crystal analyses can be readily made with X-ray equipment. Stabilized zirconia is the refractory I mentioned which has been used commercially in furnaces a t 2500°C. for gas synthesis. A number of stabilized zirconia refractory shapes are shown in Figure 1. This refractory has twice the density of fireclay brick, yet is a much better thermal insulator. It is relatively expensive but makes possible processes for which suitable refractories have not been available before. OTHER SPECIAL REFRACTORIES
Active research and development work is being conducted by the special refractories industry on many other unusual refractories. These include: Boron carbide (BaC); used primarily because of great hardness and wear resistance, this compound has a melting point of 2450%. Its high boron content (70 to 85 per cent) is the reason for its use in nuclear reactors as a neutron absorber. Articles made of boron carbide are shown in Figure 2. Thoria (Thoz); this is the refractory I mentioned that has the density of lead and is as radioactive as uranium. Thoria has the highest melting point of all oxides (3300°C.) and is less reactive at high temperatures than other oxides. Metallic titanium has been melted successfully on a small scale in thoria crucibles. Boron nitride (BN); dissociation temperature is 3000°C. I t is an excellent high temperature lubricant and a thermal and electrical insulator. It has the appearance of graphite except for its white color. Many other refractory compounds are being prepared and tested; several are cited in the table, together with some of their properties.
