1. Field of the Invention
The invention relates to relatively low cost pressureless sintered silicon carbide ceramic composites having relatively high electrical conductivity, relatively high density and relatively great mechanical strength, in comparison to those known to persons skilled in the art, and a process for making such composites.
2. Background of the Invention
For many devices in the electrical power, electrochemical and gas appliance industries, there is an increasing need for conductive ceramics with light weight, good thermal conductivity, high mechanical strength and good oxidation resistance, especially at elevated temperatures. Such ceramics can serve to conduct electrical current, absorb power surges, generate heat for many purposes and/or serve as elevated temperature structural elements. For reasons of economy, it is usually highly preferable that such articles and devices be made from relatively inexpensive raw materials, but that such articles and devices also last for a substantial length of time in service. Additionally, it is often very desirable that the materials be readily formable into complex shapes at relatively low cost; for ceramic materials, this often requires pressureless sintering and mass production shaping methods, such as injection molding and dry pressing.
Conventionally, for electrically conductive heating elements, recrystallized silicon carbide material is used to form the body. The reasons often stated for using a recrystallization process include the driving off of impurities, including without limitation excess, uncombined silicon, and to otherwise react as much free silicon as possible with carbon to produce additional silicon carbide. Further, recrystallization is employed by some in an attempt to ensure that all of the structure is crystalline, and that the crystals are all of the .alpha.-phase, to ensure more uniform electrical resistivity. Also, recrystallization is believed by some to serve to bond the crystalline structure together by reformation of the crystals so as to produce crystalline grain growth.
A recrystallized silicon carbide body usually has the required electrical conductivity [between about 1 and 100 (.OMEGA.-cm).sup.-1 ], however, such bodies, e.g., in the form of heating elements, tend to be of relatively low density (typically less than about 2.5 gram/cm.sup.3), with a relatively high degree of porosity and relatively quite low strength. Furthermore, it is often difficult, if not impossible, to achieve sufficient control, in the manufacture of such recrystallized silicon carbide heating element bodies, to obtain precise electrical and mechanical properties, due to the very high furnacing temperatures required for formation, e.g. in the range of 2300.degree. C. to 2500.degree. C.; furnace temperatures in such temperature ranges are inherently difficult to control, given the present state of the art in high temperature furnace design combined with the degradation of the furnace interiors and seals, from run-to-run, through repeated heat-up and cool-down.
Within a temperature range of 2300.degree. C. to 2500.degree. C., silicon carbide tends to sublimate, resulting in random porosity which reduces density, weakens the body thus produced, and results in somewhat unpredictable and uncontrollable variations in electrical properties. Further, the nature of the process attempts to achieve a precisely balanced and complete chemical reaction, between silicon and carbon, to form silicon carbide from the free carbon impurities contained in the beginning silicon carbide material, during the period of heat-up to recrystallization temperature while in the furnace.
The reason for attempting to eliminate free carbon is in an effort to control electrical conductivity within the body produced, as carbon is an electrical conductor, and discrete randomly placed and randomly sized pockets of trapped free carbon within the final body can tend to result in widely varying and unpredictable results in respect to electrical conductivity, the antithesis of manufacturing precision. Further, the entrapped free carbon pockets tend to reduce the physical strength of the final body and to produce porosity by gasification of that carbon near the surface which is not completely entrapped.
It seems, however, that an excess of free silicon metal must be added in order to convert substantially all of the free carbon; this process is sometimes referred to as "siliconizing" or "reaction bonding" of the silicon carbide, and it is sometimes performed with the addition of a controlled amount of free carbon over and above the free carbon impurities which are contained in silicon carbide starting materials. However, some of the excess free silicon tends to remain trapped within the heating element body at random, discrete locations. Alternatively, the excess silicon, especially that near the surface of the heating element body, tends to be driven off as a gas to leave random porosity which varies in size and location in a manner which is virtually impossible to precisely control given the required economic parameters for profitable manufacture. The random variations in product composition, coupled with the random porosity, tend to produce varying, imprecise degrees of product properties, including imprecise electrical conductivity and reduced mechanical strength. The result is that final product quality is quite often less than desirable, and usable product yield tends to be quite low, for example, only 60% to 70% or less. Thus, the market price for those products which are deemed marketable is driven up to cover the cost of the scrap.
In an attempt to overcome these problems, and also to produce articles for other end uses, hot press sintered silicon carbide bodies have, in the past, been made available to the market, and pressureless sintered silicon carbide structural bodies are presently available. Some of these bodies may contain up to a 15% addition of free carbon (beyond that in the original particulate silicon carbide) derived from carbon black, graphite or carbonized organic compositions. Such silicon carbide bodies may well have the required electrical conductivity and improved strength for use at room temperature. However, the excess carbon contained in the final product is not suitable for many high temperature applications, specifically for use in high temperature oxidizing atmospheres, e.g., air and hot steam. Moreover, hot pressing is well known to be a decidedly un-economically viable method for the manufacture of products with complex shapes or for mass production, and the pressureless sintering of such bodies is known to require very exacting sintering temperature range control, so as to produce sufficiently high enough temperatures to produce complete sintering, but not so high as to generate sublimation, e.g, a rather narrow sintering temperature range of about 2150.degree. C. to about 2190.degree. C., for a narrowly specified time range.
Several variations of pressureless sintered silicon carbide ceramics are known to those skilled in the art. For example, some known silicon carbide/graphite/carbon ceramic composite bodies are known to exhibit very low electrical resistivity (i.e. high conductivity). However, graphite/carbon containing materials do not have suitable oxidation resistance for applications as, for example, heating elements. Further, the inclusion of graphite and carbon tends to significantly reduce structural strength.
Other known pressureless sintered silicon carbides, with very low free carbon, have very low electrical conductivity (i.e. high electrical resistivity). Such silicon carbide materials can be used for structural application, as they have very good high temperature strength and oxidation resistance, however, when it comes to electrical applications, they can only be used as an electrical insulating barrier, i.e., as an insulator, such as may be required of an electronic semiconductor substrate.
Pressureless sintered silicon carbide/titanium composites are known; such tend to have quite low electrical resistivity (less than 1.0 .OMEGA.-cm and in some cases less than 0.2 .OMEGA.-cm). Such materials may be produced by adding a conductive titanium compound, such as titanium carbide or titanium nitride, up to about 3 weight percent (hereinafter "Wt.%"), to the silicon carbide particulate before sintering. A variation of such a material may be produced by adding a distinct, electrically conductive titanium diboride second phase, in an amount of about 5 to 20 Wt.%, to the sintered silicon carbide phase so as to develop quite low electrical resistivity. It is well known, however, that titanium compounds are readily oxidized in air at elevated temperatures. Moreover, the inclusion of titanium compounds tends to reduce structural integrity of the sintered silicon carbide at elevated temperatures. Thus, these material systems are not considered suitable for high temperature applications.
None of these predominantly silicon carbide composite material appear to be capable of manufacture with specific, precise electrical conductivity, with sufficient mechanical strength and with sufficient oxidation resistance, for use as long-life heating elements, as electrodes or as igniters, as currently being demanded, respectively, by furnace manufacturers, the electrochemical industries and the gas appliance industry. Thus, there remains a need for a material having all of the above mentioned properties, which is capable of being prepared from low cost non-toxic ingredients, and which can be cost effectively manufactured into the complex shapes desired by the producers of furnaces and gas burning appliances.