US2944910A

US2944910A - Ceramic spark plug insulator

Info

Publication number: US2944910A
Application number: US747354A
Authority: US
Inventors: Harry G Schurecht
Original assignee: Champion Spark Plug Co
Current assignee: Federal Mogul Ignition LLC
Priority date: 1958-07-09
Filing date: 1958-07-09
Publication date: 1960-07-12
Anticipated expiration: 1977-07-12

Description

r A 2,944,910 Patented July 12, 196i) :I fiPARK PLUG INSULATOR Harry G. Schurecht, Detroit,l\/i'ich., assignor to Champion Spark Plug Klompauy, Toledo, Ohio, 21 corporation of Delaware This invention relates to a ceramic spark plug insulator, and specifically to such an insulator where at lea-st the surface at the firing end, is produced by sintering a particular blend of certain refractory oxides, and a specific flux composition, within relatively narrow limits of proportions.

A recent trend in the automotivefield is in the direction of extremely high torque engines. Such engines are necessary to meet What seems to be a public demand for engines capable of causing extremely rapid acceleration of an automobile, under substantially allconditions of operation. 4

S'uch'high output engines impose severe requirements upon spark plugs used therein. Such spark plugs must be capable ofwithstanding the extremely high temperatures developed within the engine during periods of high torque performance. During high torque performance, the plugs must not be subject to cracking and must not become overheated to an extent such that they cause premature ignition of a combustible mixture in the engine cylinder. In addition, electrically conducting deposits must not form on the insulators of such plugs during periods of low torque engine operation, when cylinder temperatures are several hundred degrees lower than during, sustained periods of high output.

-Changes in spark plug insulator design can be made to decreasethe tendency of a particular spark plug to cause premature ignition of a combustible mixture because of overheating of the insulator. Such changes, however, increase the likelihood that electrically conducting deposits will form on the spark plug insulator during periods of low torque engine operation. There is a substantial need, therefore, for a ceramic spark plug insulator for use in high torque engines, which insulator is capable of operation during periods of high engine torque without overheating to an extent such that it causes p-re-ignition in the engine, but on which electrically conducting deposits do not form during periods of low torque engine performance.-

In many respects, insulators consisting essentially of beryllium oxide would be ideal for such spark plug use. Such insulators have high thermal conductivities, and electrically conducting deposits are not readily formed thereon under most engine operating conditions. Berylliur'n oxide, however, is an expensive raw material, and can be sintered into a matured ceramic body only by means of a high temperature firing, forexample to about Orton Cone 35. In addition, beryllium oxide is a toxic 7 compound, so that the production of such insulators requl-rescertain precautions to be taken. Considerations of; cost and ease of handling have, therefore, militated against the adoption of spark plug. insulators consisting essentially of beryllium oxide. I y

In most instances, ceramic insulators produced from a blend of beryllium oxide and another ceramic raw ma-. terial do not show the high thermal conductivity and resis'ta'nce to formation of electrically conducting deposits fired under conditions intermediate between Cone 16 and under engine conditions which is characteristic of extremely high BeO insulators. As a consequence, relatrvely few ceramic insulators are known which have es sentially the properties of insulators consisting essentially of BeO, but which contain a sufliciently small proportion thereof to be safe and economically feasible as spark plug insulators.

The present invention is based upon the discovery that ceramic insulators produced by sintering a blend of raw materials consisting essentially of beryllium oxide, a tin oxide and a particular flux composition within certain relatively'narrow ranges of proportions, have properties which suit-them admirably for use as spark plug insulators in high torque internal combustion engines. The blend may be used to form the entire insulator or only a portion thereof, as the firing tip or the surface of the A ceramic sparkplug insulator is provided according to the invention. Such insulator is produced by sintering a blend consisting essentially of from about 30 parts to 60 parts of a tin oxide, from 35 parts to 60 parts of BeO, and from l'part to 5 parts of a flux comprising a magnesium silicate and an alkaline earth compound which, after firing, is in the form of the oxide. It has been found that considerable amounts of alumina can be added to a suitable blend of BeO, Sn0 and a flux, within the propontions indicated above, without appreciably changing the properties of insulators which are produced, in particular, the high thermal conductivity and high electrical resistance at elevated temperatures. Because the use of alumina further reduces the proportion of the hazardous and expensive BeO in the ceramic batch, A1 0 is advantageously employed, and, most desirably, inproportions of from 10 percent to 20 percent of the ceramic batch or blend. A suitable plasticizer or binder is also preferably employed in the blend. The plasticizer or binder does not appreciably change the properties of the insulator, but is added principally to facilitate processing of the unfired blend. From .1 to 2 parts of bentonite can advantageously be used in the above and other blends as a plasticizer or hinder.

The tin oxide employed can be in the form of stannic oxide, stannous om'de or in another form which reverts to an oxide during firing, since substantially the same properties result in the completed insulator.

The terms percent and part, as used herein and in the appended claims, refer to percent and parts by Weight, unless otherwise indicated.

Tin oxides, either alone or in admixture with various fluxing materials, can be pressed into a desired shape and Cone 31 to produce a matured ceramic body. Such body, however, is completely unsuited for spark plug use, because it is an electrically semi-conducting material at elevated temperatures, having a resistance of less than 0.1 megohm at 1000 F., and a correspondingly lower resistance at higher temperatures. It is unexpected therefore, that ceramic insulators having dielectric properties which suit-them for spark plug use can be produced from araw batch of materials containing from 30 percent to 60 percent of a tin oxide. Insulators according to the invention-however, produced from batches containing such amounts of a tin oxide and specified amounts of BeO and a particular flux, with or without alumina, have the dielec-, tric properties necessary for spark plug use.

A preferred species of insulators according to the invention is produced by sintering a blend consisting essen the blend consists essentially of from 36 parts to 40 parts properties and thermal conductivities.

Another preferred species of insulators according to the invention is produced by sintering a blend which consists essentially of, in addition to the indicated flux in the stated proportions, from 40 parts to 50 parts of a-tin oxide, and from 45 parts to 55 parts of BeO. Such insulators have thermal conductivities and dielectric properties almost equivalent with those of insulators in the first preferred group, and can be produced with greater ease. y Still another preferred species of insulators according to the invention is produced by sintering a blend which consists of from about 30 parts to 60 parts of tin oxide, from 35 parts to 60 parts of BeO, from 1 part to 5 parts of a flux having the indicated identity, and from 10 percent to 20 percent of A1 based upon the total weight of the ceramic batch or blend. Such insulators have thermal conductivities and dielectric properties at high temperatures which are substantially equivalent to those produced from a ceramic batch or blend which is identical except that A1 0 is omitted. Such species is preferred because the A1 0 minimizes the extent to which the hazardous and expensive BeO is used. 7

A final preferred species of insulators according to the invention is produced by sintering a blend consisting essentially of from 50 parts to 60 parts of tin oxide, from 35 to 45 parts of BeO, and from 1 part to parts of a flux, as defined above. Such insulators combine excellent thermal conductivity and dielectric properties with a minimum of ditliculty in production.

It has been found that a flux of the indicated type is essential for the production of an insulator according to the invention. If, for purposes of comparison, but not in accordance with the invention, insulatorsare produced from a ceramic batch consisting of beryllium oxide and a tin oxide, in the proportions stated above, or from a batch consisting of beryllium oxide, a tin oxide and whit-. ing, in the indicated proportions, it is found that such insulators lack the dielectric properties requisite for spark plug use. It will be appreciated, therefore, that there is an unexpected cooperation among the various constituents of the batch, i.e., the tin oxide, the beryllium oxide and the flux composition. The mechanism of such cooperation is not understood, but the results are readily apparent from the properties of a finished insulator.

As has been indicated above, the flux composition must include a magnesium silicate and an alkaline earth compound which is converted, during firing, to the oxide. Calcium carbonate, or Whiting, is a convenient alkaline earth compound which is soconverted during firing. Barium carbonate or strontium carbonate can also be so employed, as can other such known alkaline earth compounds. The corresponding oxides might be used, if desired, but are likely to give rise to processing problems during production of the insulator. Talc is a convenient magnesium silicate for use as a part of the flux composition. 'Bentonite has been found to be somewhat less desirable than talc, but can be used, if desired, as well as other magnesium silicates.

The relative proportions of the magnesium silicate and the alkaline earth compound can be varied within relatively broad limits. It has been found that at least an appreciable amount, for example at least about 0.5 part, of each should be present. Excellent results have been achieved using approximately 1 part of whiting per part of talc. Most desirably, the 1 to 5 parts of flux composition, as defined, includes at least 1 part of whiting or other alkaline earth compound and at least ,1 part of talc or other magnesium silicate. V

" A suitable method for producing a spark plug insulator according to the invention is set forth in the following example, which is presented solely for the purpose of further illustrating and disclosing the invention, and is in no way to be construed as a limitation thereon.

of 10,000 pounds.

4 EXAMPLE Sintered ceramic electrical insulators having properties suitable for spark plug insulator use were prepared from various compositions including BeO, SnO and a flux composition comprising a magnesium silicate and an 'alka line earth compound converted during firing to the oxide. In all cases, a blend of the named ingredients was wet milled to substantial uniformity (usually for 18 hours), was then dried and mixed with 4 percent of paraiun wax, based upon the weight of dry ingredients. The wax was added as an approximately saturated solution in carbon tetrachloride. The Wax was then thoroughly mixed with the dry ingredients, and the carbon tetrachloride volatilized, leaving the wax uniformly dispersed throughout the composition. -Test cylinders about /2 inch in diameter and approximately /2 inch in length were then pressed from the mixed composition using a total pressure Cylinders of each of various compositions were then fired to Cone 16 and to Cone 31.

The bodies so prepared were tested to determine their suitability for use in spark plug insulators. The electrical resistance of the /2 inch cylinders was determined at various temperatures. Although the resistance required of insulators for spark plugs depends to a certain extent upon the design of the plug, such an insulator cylinder having a resistance of at least 1 megohm at 1200 F., or at a higher temperature, is a satisfactory spark plug insulator ceramic, provided that the cylinder has a thermal diifusivity, measured as described below, of not great 7 er than 10 seconds.

Thermal dilfusivities of the various insulators were also measured by another standard test: The test involved placing a crystal of citric acid on one end of the sintered inch cylinder, and immersing the other end of the cylinder in a 600? F. metal bath to a depth of about /s inch. The number of seconds required for heat con: ducted through the insulator to melt the citric acid crystal, which melts at 307.4 F., is reported in the table herein as thermal diffusivity, and is an inverse function of thermal conductivity. While insulators having thermal diifusivities, so measured, as high as about 50 seconds can be used in light duty engines, insulators according to the invention have thermal diffusivities not greater than 10 seconds. The thermal diffusivity of most present-day spark plug insulators is about 15 seconds or higher.

Typcial test results for bodies produced from compositionsconsisting essentially of the indicated ingredients are presented in the table.

Several important facts concerning bodies 1-10 are readily apparent from the data presented in the foregoing table. Each of the bodies is admirably suited for use as a spark plug insulator in a high torque engine because of its high thermal conductivity, as indicated by a thermal diffusivity time ranging from 6.2. seconds to 9.6 seconds. Each of the bodies has an electrical resistance sufiiciently high, at elevated temperatures, for use as a spark plug insulator. Bodies 2, 3, 5, 6 and 10 have been demonstrated to be particularly advantageous for use as spark plug insulators by virtue of their having resistances,

measured as indicated, of at least 3.4 megohms at 1500 F. In this connection, it has been observed that the resistance of ceramic insulators is an inverse function oftemperature, usually tending to decrease rapidly above some relatively high temperature which varies from insulator to insulator, but which temperature is usually in the range of from about 1200' F.-to 1300 F. Since the indicated bodies all have resistances of at least 3.4 megohms at 15 00 F., they are all admirably suited for spark plug use from the standpoint of electrical resistance. Bodies 1, 4, 7. 8 and 9 have suflicient resistance for such use, as indicated by the values given at 1200 F. Blanks infthetable for resistances .of various bodies at specified temperatures indicate only that data are not presently Table Body No. 1 2 3 4 5 6 7 8 9 10 Composition in Parts:

SnO 38. 6 38. 6 38. 6 48. 3 48.3 48 3 57. 9 57. 9 57. 9 28. 5 eQ 57. 9 57. 9 57. 9 48. 2 48. 2 48. 2 38. 6 38. 6 38. 6 51. 7 Whrtmg. 1. 5 1. 5 1. 5 1. 5 1. 5 1. 5 1. 5 1. 5 1. 5 1. 5 ale 2. 0 2.0 2.0 2.0 2.0 2. 0 2.0 2.0 2. 0 2. 0 Bentonite 1. 5 1. 5 1. 5 1. 5 1. 5 1. 5 1. 5 Mg(0H) 1. 5 v 1. 5 1. 5

,0. I 14.8 Thermal Difiusivity (seconds after firing Gone 16 6.4 7.6 7. 0 8. 1 7. 5 7.6 8. 6 8. 1 8.2 O0ne31 6.2 6.4 6.9 7.7 7.2 8.1 9.6 8.5 9.5 9.0 Electrical Resistance. (megohms):

After firing to Gone 16- 1,200 F 100+ 100+ 48 85 1,450 F 90 100+ 1,500 F 32 48 After firing to Gene 31- 1,50 F 33 3. 4 11 5.2 7 Condition of insulator after firing to:

one l SUF SUF SUF SUF SUF BUF SUF SUF SUF SUF Gone 31 1 Mat. SUF SUF SUF SUF SUF SUF 3 OF SUF Mat.

1 SUF indicates thatthe insulator was slightly underfireda somewhat higher temperature or longer time should be used to fire the specific composition, or slightly more of the flux should be employed under the indicated conditions.

1 Mat. indicates that the insulator was properly fired.

3 OF indicates that the insulator was overfireda lower temperature, a shorter time, or less flux should be employed for optimum properties.

available at such temperatures. Blanks in the table in the line appropriate for a particular ingredient of the composition indicate that that ingredient was not employed in the body.

If, for purposes of comparison, but not in accordance with the invention, a body is produced from 57.9 parts of SnO and 38.6 parts of BeO, with no flux, such body is found to have, after firing to Cone 16, a resistance of only 1 megohm at 1000 F., and, after firing to Cone 31, a resistance of only 0.1 megohm at 1000 F. Such body, therefore, has insuificient electrical resistance to be used as a spark plug insulator. Similarly, a Cone 16 body produced from 57.9 parts of SnO 38.6 parts of BeO and 1.5 parts of whiting has a resistance of only 0.9

megohm at 1000 F., and a Cone 31 body from the' same composition has a resistance of only 0.6 megohm at 1000 F. Such bodies, therefore, are unsuited for spark plug use because of their low electrical resistance.

This application is a continuation-in-part of application Serial No. 666,763, filed June 19, 1957, and, also, of application Serial No. 371,073, filed July 29, 1953, both now abandoned.

It will be appreciated that various changes and modifications can be made from the specific details disclosed herein without departing from.the spirit and scope of the following claims.

What I claim is:

1. A ceramic spark plug insulator, at least the surface of which, at the firing end, is produced by sintering a blend consisting essentially of from about 30 parts to 60 parts of SnO from 35 parts to 60 parts of BeO, and from 1 part to 5 parts of a flux comprising at least about 0.5 part of a magnesium silicate and at least about 05 part of an alkaline earth compound which, after firing, is in the oxide form.

2. A ceramic spark plug insulator, at least the surface of which, at the firing end, is produced by sintering a blend consisting essentially of from 35 parts to 40 parts of SnO from 55 parts to 60 parts of BeO, and from 1 part to 5 parts of a flux comprising at least about 0.5 part of a magnesium silicate and at least about 0.5 part oflan alkaline earth compound which, after firing, is in the oxide form.

3. A ceramic spark plug insulator, at least the surface of which, at the firing end, is produced by sintering a blend consisting essentially of from 38 parts to 40 parts of SnO from 57 parts to 59 parts of BeO, and from 1 part to 5 parts of a flux comprising at least about 0.5 part of a magnesium silicate and at least about 0.5 part of an alkaline earth compound which, after firing, is in the oxide form.

4. A ceramic spark plug insulator, at least the surface of which, at the firing end, is produced by sintering a blend consisting essentially of from 45 parts to 50 parts of SnO from 45 parts to 50 parts of BeO, and from 1 part to 5 parts of a flux comprising at least about 0.5 part of a magnesium silicate and at least about 0.5 part of an alkaline earth compound which, after firing, is in the oxide form.

5. A ceramic spark plug insulator, at least the surface of which, at the firing end, is produced by sintering a blend consisting essentially of from 50 parts to 60 parts of S from 35 parts to 45 parts of BeO, and from 1 part to 5 parts of a flux comprising at least about 0.5 part of a magnesium silicate and at least about 0.5 part of an alkaline earth compound which, after firing, is in the oxide form. 1

6. A ceramic spark plug insulator, at least the surface of which, at the firing end, is produced by sintering a blend consisting essentially of from 35 parts to 60 parts of SnO from 35 parts to 60 parts of BeO, from 1 to 2 parts of a plasticizer, and from 1 part to 5 parts of a flux comprising at least about 0.5 part of a magnesium silicate and at least about 0.5 part of an alkaline earth compound which, after firing, is in the oxide form.

7. A ceramic spark plug insulator, at least the surface of which, at the firing end, is produced by sintering a blend consisting essentially of from 35 parts to 40 parts of SnO from 55 par-ts to 60 parts or BeO, from 1 to 2 parts of a plasticizer, and from 1 part to 5 parts of a flux comprising from about 0.5 part of a magnesium silicate and from about 0.5 part of an alkaline earth compound which, after firing, is in the oxide form.

8. A ceramic spark plug insulator, at least the surface of which, at the firing end, is produced by sintering a blend consisting essentially of from 38 parts to 40 parts of SnO from 57 parts to 59 parts of BeO, from '1 to 2 parts of a plasticizer, and from 1 part to 5 par-ts of a flux comprising at least about 0.5 part of a magnesium silicate and at least about 0.5 part of an alkaline earth compound which, after firing, is in the oxide form.

9. A ceramic spark plug insulator, at least the surface of which, at the firing end, is produced by sintering a blend consisting essentially of from 45 parts to 50 parts of SnO from 45 parts to 50 parts of BeO, from 1 to 2 parts of a plastizer, and from 1 part to 5 parts of a flux comprising at least about 0.5 part of a magne- "7 '8 sium silicate and at least about 0.5 part of an alkaline Sn,:.from 35 parts to 60 parts of BeO, from 1 part to earth compound which, after firing, is in the oxide-form. parts of a fluxcomprising at least about 0.5 part of 10. A ceramic spark plug insulator; at least the;su1'- a magnesium silicate and at least about 0.5 part of an face of which, at the firing end, is produced by sintering a W alkaline earth compound which, after firing, is in the blend consisting essentially of from 50 parts to 60 parts 5 oxide form, and from 10 percent to "percent of of SnO fromparts to parts of BeOff'rom l't'o 2 A1 0 based upon the total weight of the blend. parts of a plasticizer, and from 1 part to 51 parts of a) 1 1 If?"lijflflii 'fflf' flux comprising at least about 0.5 art of amagnesiuin. i silicate and at least about 0.5 part of an alkaline arthmm if fmqwfiw i sthqfis wf h a a compound which, after firing, is in the oxide form-.- ---10 e UNITED STATES PATENTS 11. A ceramic spark plug insulator, at least the sur- 7 face of which, at the 'firing end, is producedhy sin te ringf C 2,577,369" g 1 1951 a blend consisting of from about 30 parts to parts of 2,643,192 Jonker eta]; ILL Q1. June 23, 1953

Claims

1. A CERAMIC SPARK PLUG INSULATOR, AT LEAST THE SURFACE OF WHICH, AT THE FIRING END, IS PRODUCED BY SINTERING A BLEND CONSISTING ESSENTIALLY OF FROM ABOUT 30 PARTS TO 60 PARTS OF SNO2, FROM 35 PARTS TO 60 PARTS OF BEO, AND FROM 1 PART TO 5 PARTS OF A FLUX COMPRISING AT LEAST ABOUT 0.5 PART OF A MAGNESIUM SILICATE AND AT LEAST ABOUT 0.5 PART OF AN ALKALINE EARTH COMPOUND WHICH, AFTER FIRING, IS IN THE OXIDE FORM.