October, 1927
INDUSTRIAL A N D ENGINEERING CHEMIBTRY
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Spark-Plug Insulation By Arthur S. Watts THEOHIO
STATE UNIVERSITY, COLUMBUS, OHIO
0 DATE the only materials offering promise for sparkplug insulation are porcelains and special types of glass. The latter have been exploited very little as, in general, they lack resistance to mechanical shock and are objectionable in other respects. Fused quartz, which has been proposed for this service, has other physical faults which must be overcome before it can be considered. The molded organic mixtures all suffer deterioration in service and are therefore not acceptable. Chemistry of Porcelain
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This leaves porcelain as the only important consideration in spark-plug insulation today. What is porcelain? The most comprehensive definition is "A ceramic product, consisting essentially of alumina, silica, and alkaline bases, impervious to moisture, normally white in color, and possessing a glassy to stony fracture." Within the limits of this definition, however, there are many types of porcelain, and from a study of the different types and the influence of composition and constituents as well as details of manufacture upon the physical properties of the product, we have learned what we know regarding porcelains. The chemistry of porcelains and all ceramic products is essentially pyrometric chemistry-i. e., the chemistry of fusion, solution, and selective crystallization a t high temperatures. The field of ordinary chemistry in general covers temperatures from zero to about 300' F. (149" C.) while the major field of ceramic chemistry covers the range 1000" to 3000" F. (538' to 1649" C.), and the field of porcelain chemistry is all above 2300" F. (1260"C.). Below this temperature the constituents remain inert and the mass is merely a mixture or at most a conglomerate of the original constituents, Above this temperature a process of solution develops, the constituents dissolving one another to form new compounds and attacking the more resistant constituents with increased temperature and time. General chemistry was for years supposed to have the same possibilities of service in ceramics as in other fields of chemistry, and chemical analysis was considered the logical mbde of attack when a ceramic material or a ceramic product was investigated. It was soon found that chemical analysis was inadequate for this field of research because two materials may have almost identical chemical compositions but possess widely different properties in the ceramic field. The microscope was called in and showed that the processes of solution were extremely involved. The constituents of a porcelain body undergo many changes during the firing treatment which constitutes a real chemical process. The study of the progressive development of porcelains during the firing process is being studied and we are just now beginning to learn something of ceramic chemistry through the microscopic study of ceramic mixtures a t temperatures a t which the pyrochemical changes are taking place. By means of the x-ray we are obtaining additional information which microscopic study did not disclose. Development of Porcelain Suitable for Spark Plugs
Porcelain is not a homogeneous compound in which the constituents are completely dissolved in one another. The porcelain of commerce is an intimate mixture of feldspar, clay, snd quartz which has been heated to a temperature a t
which the feldspar fuses and attacks the clay and quartz, forming a dense mass, but the major portion of the clay can be discerned through a high-power microscope as independent particles and the particles of quartz imbedded in a matrix of feldspathic glass are equally apparent. The product is really a conglomerate. One constituent, however, has undergone a very important change. The clay which was introduced as an amorphous earth has undergone a pyrochemical change and becomes a mixture of crystalline needles of mullite (72 per cent AlzOsand 28 per cent SiOz)and a glass consisting of about 6 per cent AlzOa and 94 per cent SiOn. This glass is readily attacked and dissolved in the molten feldspar, but the mullite resists attack, and any specimen of true porcelain appears under the microscope as a mass of needle crystals imbedded in a mixture of feldspar, glass, and quartz. This normal porcelain was soon found to possess faults which rendered it unsuited for spark-plug insulation. The quartz particles owing to their many inversions, rendered it sensitive to sudden temperature change and this porcelain frequently cracked, causing the spark plug to short-circuit. A search was made for a substitute for the quartz. The chemical porcelains of Europe have been made for many years from a mixture of only feldspar and clay, a portion of the clay having been previously fired a t a high temperature in order to develop its maximum mullite content. It was completely pulverized and introduced in place of quartz. This porcelain was a distinct improvement as regards cracking, but if the porcelain became overheated its electrical resistance decreased very rapidly and it had little value as an electrical insulator in a modern motor, which operates at a much higher temperature than the internal-combustion engines made prior to 1916. A search for the cause of this fault resulted in the condemnation of feldspar as a vitrifying agent. Feldspar is normally a potassium-aluminum silicate and investigators decided that the potassium was the objectionable constituent. The mineral talc (steatite), a magnesium silicate, has been recognized for years as an excellent electrical insulator a t high temperatures, but it is porous and hence quickly becomes charged with carbon in an internalcombustion engine, and of no value. Attempts to coat the talc insulator with an artificial glaze were unsuccessful and research along this line was abandoned. Furthermore, talc is a natural rock mineral and possesses a variable composition and also laminations which caused insulators to fail. Attempts to use talc as a vitrifying agent, wholly or partly replacing the feldspar of the normal porcelain, resulted in the first really successful spark plugs in the air-cooled engines of airplanes. A new difficulty was encountered, however. The natural talc is extremely non-uniform in composition and also has a very limited vitrification range, so that in an ordinary kiln, such as is commonly employed for firing porcelains, many of the products are slightly overfired or slightly underfired, and in either case their value as spark-plug insulators is greatly reduced. Research proved that magnesium was far superior to potassium as a vitrifying agent but must receive a much more definite treatment than the potassium feldspar porcelains demanded. The h t step was to produce an artificial magnesiumaluminum silicate which would have a definite composition and definite properties. This work was undertaken by the
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INDUSTRIAL A N D ENGINEERING CHEMISTRY
United States Bureau of Standards during the World War under the guidance of A. V. Bleininger. The best results were obtained with a mixture containing about 10 per cent magnesium oxide, 27 per cent alumina, and 63 per cent silica. This was fired a t about 2600" F. (1427" C.) and pulverized. It was then intimately mixed with pure clays and pulverized artificial mullite. The difficulty from variable firing treatment was, however, B barrier to commercial success and the continuous tunnel kiln was used to correct this difficulty, as it has a relatively small cross-sectional area and can be maintained at a much more uniform temperature than the larger peiiodic kiln. Furthermore, the ware passes through the kiln a t a definite rate and a practically definite heat treatment is assured for all wares passing through the kiln. The product is our present-day spark plug which is so successfully meeting the demands of the automotive engineer. Requirements of Ideal Spark-Plug Insulator The successful spark-plug insulator must have satisfactory electrical insulating properties a t the temperatures at which it operates. It must withstand sudden temperature change, because a cold motor must reach its temperature of maximum efficiency in a minimum time. It must have a low coefficient of expansion, because it extends from the hottest part of the motor to the outside, where it is scarcely above atmospheric temperature. It must withstand a reasonable amount of mechanical shock because it is likely to receive rather rough treatment in installation. Probably as many sparkplug porcelains are broken by careless handling while being installed in motors as by any other accident to which they are exposed. All these demands point toward a dense conglomerate as the ideal insulator, but the conglomerate must be designed and developed with due consideration for the following: (1) The raw mixture must be capable of being molded into the deslred shape without developing internal strains. ( 2 ) It must dry without sufficient shrinkage to cause sepa-
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ration of the plastic portions from the faces of the non-plastic portions which do not shrink. (3) I t must mature a t a uniform rate throughout, without developing segregation or causing the glassy constituents to sweat toward the surface, causing a structural difference between the inner and the outer portions of the insulator, (4) I t must be coated with a glaze that will mature a t the maturing temperature of the body of the insulator and form a perfectly smooth surface free from microscopic pits or craters which can harbor carbon development and cause dirty plugs. This glaze must be in complete physical harmony with the body of the insulator a t all temperatures, and not craze even though subjected to abrupt temperature shock. (5) The various constituents of the conglomerate mass constituting the body of the insulator must be in complete harmony after it has been fired. hTo strain can exist between the glassy matrix and the crystalline material with which it is loaded. (6) A perfect bond must exist between matrix and crystallite and there can be no cavities of any sort if the porcelain is t o possess its maximum value. The amount of glassy matrix must be just sufficient t o enclose and bond the crystalline material and the dielectric strength of the matrix must be as high as possible. The distribution of crystallite in the matrix must be uniform, as any dielectric strength possessed by the mass depends on the dielectric strength of the glassy portion, which is the only constituent that is continuous. Electrical resistivity of such materials is roughly proportional to the distance and, assuming that the crystallite has a higher dielectric value than the glass bond, the increased length of the path of the electric current due to the introduction of the needle crystals must raise the dielectric strength of the insulator proportionately. This is only true if no electrical charge develops on the surface of the crystals, hence the necessity for perfect union between matrix and crystallite.
Finally, we have developed in the United States with the aid of government bureaus our present spark-plug insulator, which when ideally produced is undoubtedly superior to any other in the world. If a change in service demands develops, further development in spark-plug insulation may be asked. Much research with materials other than those here mentioned has been conducted and new developments are always possible. There is little danger that the field of sparkplug insulation will ever again constitute a barrier to progress in automotive engineering.
The Automotive Storage Battery By W. L. Reinhardt WILLARD STORAGE BATTERYCOMPANY, CLRVELAND, OHIO
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HE lead-acid storage battery had its beginning in 1859 with the pioneer experiments of Plant6. This original Plant6 battery consisted of sheets of pure lead which were electrolyzed in dilute sulfuric acid, the surface of the anode being converted into lead peroxide and that of the cathode to finely divided metallic lead. The development of the industry was retarded for several years by the slow and expensive methods available for forming Plant6 plates. I n order to secure satisfactory capacity it was necessary to charge and discharge the battery a large number of times, and it often required as long as two years to produce a highcapacity storage battery. At this time the only available method for charging batteries was by means of the primary cell. The next important advance in the industry was the development of the paste plate battery in 1881. This differed from the Plant6 battery in that the active materials were formed from lead oxides, the oxides having been applied to a framework or grid of lead-antimony alloy in the form of a paste, which was later converted into the active materials electrolytically. This paste plate battery was an improvement over the Plant6 battery in that it gave much higher
ampere-hour capacity per unit weight and could be manu'factured in less time. At about this time the industry was given impetus by the invention of the electric dynamo, which furnished a more economical and satisfactory means of charging the batteries. By 1900 storage batteries were finding a variety of uses in the industry, such as telephone service, electric vehicles, railwaytrain lighting, and stand-by service in power stations. Possibly the greatest development in the storage battery industry has been the application of the storage battery to starting and lighting service on the automobile. The first commercial applications of starting and lighting service were made in 1911. From this time the growth has been rapid, until there are now approximately twenty million automobiles in this country with electrical starting and lighting equipment. Construct ion In the manufacture of these twenty million batteries the following materials were required : approximately 350 million pounds of lead, 28 million pounds of antimony, 140 million pounds of container material, 165 million pounds of sulfuric acid electrolyte, 200 million square feet of insulator or sepa-
