Special Refractories for Use above 1700"C. 0. J. WHITTEMORE, JR. Norton Co., Worcester, Mass.
New extreme temperature processes have spurred development of special refractories. Since World War IT, commercial pure oxide refractories have been manufactured of alumina, magnesia, and stabilized zirconia. Alumina is used most because of its excellent stability and lower cost. Magnesia is utilized for its high electrical resistivity, slag resistance, and high melting point. Stabilized zirconia has an extremely high melting point and very low thermal conductivity. Small quantities of lime, thoria, urania, beryllia, and spinel refractories have also been made. The properties, applications, and limitations of pure oxide refractories are discussed. Special refractory development is also concentrating on the refractory carbides, borides, and nitrides. Although not highly resistant to oxidation, these materials include the highest melting substances and also have unusual electrical and hardness properties. Silicon carbide is the most important of this group because of its high oxidation resistance, high thermal shock resistance, and lower cost.
T
H E general field of ceramics has experienced great expansion and diversification since World War 11. This growth has been stimulated by the increased American economy, by new and more stringent requirements by other industries using ceramic products, and, not t o say the least, by research resulting in radically different ceramic materials. The refractories division of the ceramics field has undergone much diversification due to the push to higher operating temperatures. Most of this diversification has taken place in the special refractories industry. The impetus came from two sources. One source was the discovery made by Russell (9)and Kistler (6) while surveying German industry in 1945 that ceramic materials were being tested in gas turbine blades. Since this discovery until the present time, much government sponsored research has attacked this problem. Finding that ceramic materials are too brittle and sensitive t o thermal shock to be substituted for metals directly, ceramic research scientists have developed "cermets," a new family of materials. Cermets combine refractory ceramics and metal attaining the high temperature rigidity of ceramics with, to a limited extent, the ductility of metals. Both fundamental and applied ceramic research studies in the future should develop many high strength materials useful a t high temperatures. However, appreciable application has not yet resulted. The other impetus came from more severe operating conditions in both existing and new industrial processes. Much of these high temperature problems originated in the chemical industry for gas syntheses (2)such as, the Wulff process for making acetylene from natural gas and the Wisconsin Research Foundation process for converting atmospheric nitrogen directly to nitric oxide at about 2200' C. The atomic energy program also has and will demand refractories which require added purity and service specifications based on nuclear properties. The metallurgical industries have difficult refractory problems as a result of refining and melting metals such as zirconium, titanium, and special steels. One unsolved problem is the development of a container for molten titanium that contributes no contamination.
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Even the ceramics field itself requires refractories for higher temperature kiln linings and for supports on which to fire the new ceramics. The special refractories to be considered will be divided in two groups 1. Oxides 2. Other refractory compounds-carbides, borides, and nitrides PURE OXIDE REFRACTORIES
Study of oxide phase diagrams quickly leads one to conclude that an oxide refractory must be very pure to achieve maximum refractoriness since most oxide mixtures have much lower melting points than the single components. Minimum content of the glass forming oxide, silica ( SiOz), is particularly desirable despite the common occurrence of silica in minerals. Table I lists properties of the seven important refractory oxides and also the spinel, magnesium aluminate. These properties are for nonporous bodies thus presenting comparison but not the properties of most commercial products. Thermal conductivities, from Kingery (6),are expressed in cal./sec./cm.z, cm./' C. a t 1000' C. Thermal expansion values are mean from room temperature t o 1500" C. Elasticities and strengths are of hot-molded bodies a t room temperature. Melting points of thoria and urania are from Lambertson ( 7 , 8), and the value for zirconia is adjusted for the presence of the stabilizer, calcium oxide. Notable extremes are the high densities of thoria and urania, the melting point of thoria, the thermal conductivities of beryllia and zirconia, the thermal expansion coefficients of magnesia and lime, and the strength of alumina. These extremes, together with cost, delineate most of the uses and limitations of these materials. The high costs of beryllia, thoria, and urania limit their use to small applications such as laboratory crucibles. Hydration limits the use of lime refractories. The remaining three oxides, alumina, magnesia, and zirconia are sufficiently reasonable in cost so that large furnaces have been constructed using them. Pure oxides are normally prepared by calcination of some
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 47, No. 12
ABRASIVES AND REFRACTORIES
Table I.
Properties of Oxides Thermal at
~
1
Be0 CaO
~
0
MgO MgO. AlzOa ThOz UO?
ZrO2 (stabilized)
~3.97 3.03 3.32 3.58 3.68 10.01 10.96 6.1
2015 2520 2570 2800 2135 3300 2880 2650
10000c
0.27 0.50 0.19 0.29 0.29
147 485 186 167 138 73 82
0 : i7
55
..
salt-e.g., aluminum oxide from aluminum hydroxide. Large shapes fabricated from a calcined oxide are subject to shrinkage when heated to temperatures above that of calcination or firing of the shape. Such shrinkage will not only result in subsidence of a structure but also render the inner shrink zone more susceptible to fracture on repeated heating and cooling. Arc furnace-fused oxides remove this limitation. Though containing normally about 25% porosity, bricks made of prefused oxides have a skeletal nonshrinking internal structure allowing usage to extreme temperatures ( 1 1 ) .
~
Figure 1.
~~~
~~~
x
10s
9.0 9.5 14.5 16.0 9.6 10.4 i:7
x
10-s 54 55
..
x
10-8 413 103
40
1'12
32
220 .. 303
..
..
25
thermal conductivity allow furnace operation a t extreme temperatures with low heat losses. One gas synthesis furnace has been operated for several months a t temperatures above 2200" c. Zirconia is not wetted by most metals. Crucibles are used for melting platinum, palladium, ruthenium, and rhodium. In the Rossi continuous steel casting process, tons of molten steel pass through a stabilized zirconia orifice which shows no erosion. However, slags react severely with stabilized zirconia. Stabilized zirconia is relatively resistant to reducing atmospheres but in extremely reducing conditions, such as in the presence of graphite in a vacuum furnace, it will be completely reduced. Stabilized zirconia is the accepted setting material on which barium and strontium titanates are fired. These titanates are new ceramic compounds used in the electronics industry for condensers and capacitors because of their high dielectric constants.
~
Alumina light refractory shapes
Description of the uses of pure ovide refractories follow. Alumina. Because of excellent stability in oxidizing and reducing atmospheres, alumina is the most used pure oxide. Shapes are made varying from small impervious laboratory crucibles to massive furnace parts. High electrical resistivity allows use to 1900" C. as a core in wire-wound furnaces. Insulating brick of 53% porosity allow thermal insulation where high temperatures and reducing atmospheres prohibit insulating firebrick. Hardest of the oxides, alumina is being used as dense shapes for wear resistance. The Wulff process furnace is constructed of alumina. shapes. Magnesia. Though possessing a melting point of 2800" C., magnesia has high vapor pressure restricting its use to 2200" C. in oxidizing atmospheres. I n reducing atmospheres, much lower temperatures are required down to about 1700' C. in presence of graphite. Its high thermal expansion causes thermal shock sensitivity. The Wisconsin nitrogen fixation process utilizes magnesia brick. Resistance to metals and slags accounts for large use as metallurgical containers. Because of high electrical resistivity, fused magnesia is the accepted insulating medium in sheathed electrical heaters. Zirconia. Produced from the mineral zircon, zirconia has long been known to be available in large quantities and t o be very refractory. However, inversion in crystal form a t about 1000O C. causes volume contraction with accompanying cracking. When stabilized in one crystal form with about 5q;', calcium oxide, zirconia can be used successfully ( 1 2 ) . High melting point and low December 1955
Thermal
Figure 2.
Fused stabilized zirconia heavy refractory shapes
Calcium Oxide. Lime is quite similar to magnesia in refractory properties. Exceptions are low vapor pressure and high hygroscopicity. Hydration of lime can be inhibited by fusion, thus presenting lower surface area, and crucibles have been successfully manufactured. However, these shapes must be protected from the atmosphere or collapse will occur in a few weeks. It may be possible to satisfactorily inhibit hydration by adding other components but not to a low enough addition that the resulting material could be considered a pure oxide refractory. Pure lime crucibles are of interest for melting several metals. The mineral dolomite when heated results in a mixture of calcium oxide and magnesium oxide near equal molecular proportions. Fused dolomite has properties similar to lime and is of metallurgical interest, Magnesia Aluminate (MgO .Alz03). This spinel is of interest when pure oxides are considered because it has a higher melting point than alumina and also is quite hard. Significant usage has not resulted, however. Beryllia. Because of very high thermal conductivity, beryllia
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PRODUCT AND PROCESS DEVELOPMENT ~~
has excellent thermal shock resistance which was the basis for much of the research done on its use in gas turbine blades. The severe health hazard of working with beryllia powders has resulted in discontinuance of most work, however. Beryllia has high electrical resistivity and has been used as a core for wire wound furnaces. It has been used for metal melting crucibles. I t s disadvantages are high cost and volatility in the presence of water vapor above 1350° C. Thoria. Of all the oxides thoria has the highest melting point and is stable under most conditions. Thoria crucibles have been used for melting small quantities of titanium ( 1 ) . Its disadvantages are high cost, radioactivity, and sensitivity to thermal shock. Uranium Dioxide. Of all the oxides uranium dioxide has the second highest melting point. I t n6t only has all of the disadvantages of thoria but it also must be confined in oxygen-free atmospheres or oxidation to U308will result. Uranium dioxide crucibles have been prepared for special applications ( 3 ) . Figures 1, 2, and 3 illustrate t,he variety of shapes niadr from pure oxide refractories.
carbide has such a high melting point (4000” C.) that it has been used as a crucible for melting carbon (4). Table I1 lists a number of these compounds with some of their properties. Thermal conductivities are given at room temperature. Vasilos (10) has shown the conductivities of several of these cmompoundi decrease rapidly with temperature. Silicon carbide has been used for years as a refractory and as an electrical heating element. It is more resistant to oxidation than the other compounds, has much higher thermal conductivity, and is one of the least dense. It also is available in large quantities, in many types of product compositions and shapes, and is low in cost.
OTHER REFRACTORY COMPOUNDS
The search for high temperature materials has not been restricted to oxides but has dealt with a large number of refractory carbides, nitrides, borides, and sulfides. Among these compounds are the highest melting substances. Tantalum
Figure 4.
Boron carbide shapes
Boron carbide also has been used for years as a wear-resistant material and has lately been used as a neutron absorber or shield. Boron carbide is normally shaped by hot molding under pressure in graphite molds. This process is also adaptable for molding the other compounds listed as well as the oxides and results in many cases in near theoretical densities. A number of boron carbide shapes are shown in Figure 4. Boron nitride is particularly unusual among these compounds in that it has a layered structure like graphite and thus is an excellent lubricating material for high temperature uses. As it is normally available as a low density powder, boron nitride is an excellent thermal insulator. Figure 3.
Thoria Refractories LITERATURE CITED
Table 11.
Material
Density
Sic
ZrC
Tic TaC WC
MoC
NbC
vc
BIC TiBz ZrB2
TaB2 NbBi BN
TiN ZrN
TaN
0
Properties of Other Refractory Compounds
Sublimes.
3.217 6.73 4.93 14.65 15.63 8.78 7.82 5.77 2.50 4.50 6.085 12.38 6.97 2.20 6.43 7.09 16.30
Melting Point, O
C.
2600a 3640 3140 3880 2870 2690 3500 2810 2450 2600 ( ? ) 3000 3000 (?) 2900 (?) 3000n 2950 2980 3090
Thermal Conductivity,
Cal./Sec./Crn.Z, Cm./ at 20’ C. X 104
310
’
Therinal Expansion Coefficient, X 108
..
.. ..
650
4.5
600 580
.. ..
..
.. ..
700 400 ,.
.. ..
..
Brace, P. €1.. iMetal Progr., 5 5 , 2, 196 (1949). Chem. W e e k , 75, 16 (Oct. 9, 1954). Corwin, R. E., and Eyerly, G. B., J . Am. Ceram. Soc., 36, 4 , 137-9 (1953). Finlay, G. R., Chemzstry z n Can., 4 , 3, 41-4 (1952). Kineerv. W. D.. Francl. J . Coble R. L.. andVasilos. T.. J . Am. __. C&k. Soc-, 37, 2 (II), 107-10 (1954). Kistler, S. S., “Refractories in Turbine Blades,” H. ,M.S. Ststionery Office, London, Item N o . 1, 18, 21, 8z 25, File No. XXXI-22, 1945. Lambertson, W. A., and Mueller, 111. H., J. Am. Ce‘eram. SOC., 36, 10, 329-31 (1963). Lambertson, W . A , llueller, M. H., and Guneel. F. H., Zbid., 36, 12, 397-9 (1953). Russell, R.. “Electrical and Technical Ceramic Industry of Germany,” Fiat Final Report No. 617, Mapleton House, p. 26 (December 1945). Vasilos, T., and Kingery, W. D., J . A m . Ceram. SOC., 37, 9, 40914 (1954). Whittemore, 0. J., Jr., Ibzd., 32, 2, 48-53 (1949). Whittemore, 0. J., Jr., and Marshall, D. W., Ibid., 35, 4, 85-9 (1952). RECEIVED for review April 6 , 1955.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
A C C E P T ~August D 17, 1955.
Vol. 47, No. 12
