US 2806005 A
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Sept. 10, 1957 3, WHITE 2,896,005
SPARK GAP SEMI-CONDUCTORS Filed Sept. 28, 1951 INVENTOR ATTORNEY United States Patent Qfiice Zfiiihfiifi Patented Sept. 10, 1957 SPARK GAP SEl /H-fiNDUCT@r Robert C. White, Sidney Center, N. Y., assignor to Bendix Aviation Corporation, New York, N. 1., a eerporaticn of Delaware This invention relates to ceramics and particularly to the particular type of ceramic which is employed in spark plugs as a semi-conductor for ionizing the spark gap.
There has recently been described in application Ser. No. 221,435, for instance, filed April 17, 1951 a new type of spark plug having a circular center electrode and a circumscribing electrode spaced therefrom which produces ignition by a fiash across the gap between the electrodes and in which this flash is induced by a ceramic semi-conductor which overlies the gap. This construction is shown in the accompanying drawing wherein the outer barrel of a spark plug terminates in an electrode 11 which is annular. Within this electrode 11, and spaced therefrom, is a center electrode 12 which is possessed of a circular head 13 which is spaced from the electrode 11 by a distance adequate to a good spark. Overlying the gap between the electrodes 11 and 13 is a ceramic semi-conductor 14 which operates to initiate the spark, theoretically by ionizing the air, or the gas, which is found between the two electrodes. In this way the spark is caused at lower tension than was possible with plugs of prior art type.
These new spark plugs have introduced an improvement in the operation of motors, but they have also introduced problems which never existed before. It is an object of this invention to recognize and to correct the problems which exist in these new spark plugs.
The word semi-conductor or spark gap semi-conductor will be employed in this specification and in the claims to describe the piece of semi-conductive ceramic which is employed to bridge the gap. Some semi-conductors have been used heretofore, but they have been accompanied by difficulties in operation or in construction. For example, it has been difiicult to obtain a spark gap semi-conductor of a proper conductivity which will also have those characteristics of durability and resistance to conditions existing within engines, which is essential to satisfactory life. It is important that the spark gap semi-conductor shall have excellent resistance to erosion under the conditions of sparking and of ignition which exist in modern engines, but this has been diflicult to obtain with known ceramics Furthermore, it has been necessary to find ceramics which are capable not only of withstanding the high temperatures which exist within an engine during the time of ignition of the charge, but the great variation in temperatures which exists between the time of firing and the time of discharge and admittance of the cold charge. Therefore, it is not only necessary that these resistors have proper conductivity, proper resistance to erosion, but that they have proper resistance to heat and to rapid changes in temperature. Furthermore, it is necessary that these spark gap semi-conductors shall have the strength to withstand high pressures, not only gaseous but also mechanical and that, being somewhat porous, they may withstand internal high pressures as Well as compressive pressures without disruption. It is necessary that they shall have excellent resistance to thermal shock, which arises by reason of repeated rapid changes in temperature. It is also desirable that the porosity of the pieces shall be low in order that the pressures existing on the outside of the piece may not enter within it and act disruptively. Furthermore, it is necessary that the porosity be low in order that the piece shall not act as an accumulator of carbon which, being a good conductor, is able by presence in sufiicient amount to change the conductivity of the semi-conductor and, consequently, its operating characteristics Within the engine.
The applicant has had great experience with all the semi-conductors employed in the prior art and has found that they have material failings in one or all of the categories hereinbefore discussed. It is consequently an object of the invention to make a dense, relatively nonporous semi-conductor of proper conductivity, of good resistance to erosion, to high temperature, of strength to withstand high pressures, both gaseous and mechanical, and of good resistance to thermal shock. The objects of the invention are accomplished generally speaking by a ceramic spark gap semi-conductor having as its essential ingredient a major proportion of silicon carbide and as minor ingredients, cobalt oxide and aluminum oxide.
The silicon carbide employed is preferably of fine grade. A silicon carbide which has been satisfactorily employed has analyzed free carbon about .5%, silicon dioxide about 1.4%, iron oxide about .5 aluminum oxide about 1% and silicon carbide about 96%. This silicon carbide should be of a particle size such that a minimum of 98% passes through a screen having a minimum of 325 mesh, which means holes per square inch. In general it is preferred that the silicon carbide shall pass through a 400 mesh sieve.
The second material employed in the ceramic mixture is an oxide from metals of the iron and silver groups, numbers 24-28 of the iron group and 42 of the silver group being preferred. Such metals are, respectively, chromium, manganese, iron, cobalt, nickel, molybdenum. Black cobalt oxide is excellent, and has been employed analyzing a minimum of cobalt and of a particle size such that a minimum of about passes through a 325 mesh screen. Cobalt oxide may be replaced by equivalent amounts of the other oxides of the class. An excess should be avoided as it tends to run off during firing. About 20% of the cobalt oxide produces optimum electrical properties in the finished piece. As little as 4% has been used but it is better to use a minimum of 5%.
The third ingredient of the new ceramic is alumina, which preferably is of high grade and has a particle size of about ten microns or less.
The raw materials are mixed in suitable proportions for instance, in proportions of 8.5 parts by weight of the silicon carbide, 1 part by weight of cobalt oxide, 2 parts by weight of aluminum oxide, and ball milled in water for about 4 hours using about 2 parts of distilled water for each 1.6 parts of the mixture. Ball milling may be extended or shortened depending upon the ingredients employed and the time necessary to. reduce them to a state of perfect intermixture.
After the mixing has been satisfactorily completed, the slurry may be poured into a drying pan and the mill should be rinsed with a small amount of distilled water and the water added to the slurry. The drying pan is oven dried at ZOO-300 F. until the moisture content is less than 1% as determined by Dietert moisture determinator on 15 minute exposure. The slurry after drying is passed through a 100 mesh screen to remove lumps and mill particles.
The ceramic mix as thus prepared may be molded and fired, but molded is facilitated by incorporating into the mix a suitable binder of the type which has heretofore been employed in molding ceramics, for example, flour,
may be added to the ceramic mix in the second preceding paragraph. .In other cases the binder is satisfactorily composed of up to about 30% of the weight of the ceramic mix, half of that weight being the binder'itself, and half being water. After thorough mixing with the binder, the ceramic mix is partially dried at 200 plus or minus F. in an'oven for to 'minutes oruntil the material will ,granulate through a 20 mesh screen, after which it is oven dried for about a half hour at 200 plus or minus 10 F. or until the granules will flow freely;with no tendency to cling together and the moisture content, as determined by a Dietert computer after 10 minutes ex posure, is 1.2,to 2%.
The foregoing composition is molded-in any of the ways which have heretofore been satisfactory for the molding of ceramic, pieces, butpreferablydry, under heavy pressure on the order of 30to 40 thousand pounds per square inch. Drying of the molded piece is not essential, it can be fired immediately, for instance, in an electric kiln at a temperature sufficient to down Ortons pyrometric cone 19. A satisfactory firing schedule commences at 0 hours at room temperature, at 1 hour has obtained 1200 plus or minus 50 F., at 2 hours has obtained 1900 plus or minus 50 F., at 3 hours has obtained 2400 plus or minus 50 F. and at 4 hours, plus or minus hour, cone 19 down, which indicates approximately 2780 F The cone is considered down when the tip touches the plaque. In general the peak temperature should not exceed that at which the peak of cone 20 reaches 3 oclock. Air or some other oxygen-containing gas should be admitted to the furnace during firing. It is difficult to explain the invention, but present theory supposes a partial oxidation of the silicon carbide and a partial reduction of 'the cobalt oxide producing a piece of improved conductivity.
After cooling, the piece may be machined as desired, then may be washed in distilled water, and dried to constant weight in .a 200 F. oven. This sometimes requires four hours or more of drying.
The mix according to this invention may contain silicon carbide Within the range 40% to 91%, cobalt oxide in the range 4% to 40% and alumina in the range 5% to The:semi-conductors which are made according to this process are believed tobe superior in all respect to those which have previously been employed for the same purpose. They have a resistance within the best range of semi-conductivity; they resisterosion better than prior .art semi-conductors; they resist sparking erosion and their resistance to heat and high temperature is very good; they have the strength to withstand both mechanical and gaseous high pressures; they resist the thermalshock which comes from rapid changes 'in .temperature accompanied by rapid changes in pressure; they have low porosity, and they do not absorb sufiicientcarbon to materially alter.
their performance characteristics during use. Theyare inert at operating temperature and do' not form' deposits on the electrodes. 7
Although only a limited number of embodiments and modifications of the invention have been illustrated in the drawing and mentioned or described in the foregoing specification it is to be expressly understood that the invention is not limited thereto.
What is claimed is:
1. A ceramic spark gap semi-conductor having as its essential constituents about 8.5 parts by weight silicon carbide, about 1 part cobalt oxide, and about 2 parts aluminum oxide.
2. A spark gap semi-conductor consisting essentially of the following ingredients in about the weight percentages indicated:
Fine grade silicon carbide, analyzing about .5% free carbon, 1.4% silica, .5 iron oxide, 96% silicon carbide in a particle size about 98% of which will pass a 400 mesh screen 40-91% Black cobalt oxide analyzing a minimum of cobalt, in a particle size of which a minimum of about High grade alumina in a particle size of about 10 microns or less 5-25% Silicon carbide in particle size about 98% of which will pass a 325 mesh screen 40-91% Cobalt oxide in a particle size of which a minimum of about 90% will pass a 325 mesh screen -440% Alumina less than about 10 microns in size c 525% 5. The semi-conductor of claim 4 in proportions are about asfollows:
which the weight Silicon carbide -5. 8.5 Cobalt oxide 1 Alumina 2 6. A spark gap semi-conductor having as its essential constituents from 4091% silicon carbide, from 440% cobalt oxide, and from 525% alumina, all percentages being by weight. I
7. A spark 'gap semi-conductor consisting essentially of from 409l% silicon carbide, from 440% of an oxide of a heavy metal selected from the group consisting of chromium, manganese, iron, cobalt, nickel, and molybdenum, andfrom 5-25% alumina, all percentages being by weight. 7
References Cited in the file of this patent,
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