US3546369A

US3546369A - Spark plug having composite insulator

Info

Publication number: US3546369A
Authority: US
Inventors: Masami Ouki; Masatoshi Suzuki
Original assignee: NipponDenso Co Ltd
Current assignee: Denso Corp
Priority date: 1970-01-16
Filing date: 1970-01-16
Publication date: 1970-12-08
Anticipated expiration: 1987-12-08

Description

United States Patent 174/70 .1 ux 2,645,819 7/1953 Peters...........................l74/l70(.l)UX

MasarniOuki 2,635,975 4/1953 Pouzet................. Nagoya; 1 A 1 No gagztoshisuzukiokauki'shihpan Primary Examiner-Laramie E. Askin pp g 16 1970 Attorney-Cushman, Darby and Cushman Continuation-impart of Ser. No. 730,305,

at: 13? abandoned ABSTRACT: A spark plug having a composite insulator com- Nlppolldem xabushmxakha prising a leg portion and a head portion formed of ceramic Kafiybshi chum! Japan materials of different properties and compositions. in the insuacorponuon hp; later, the leg portion is formed of the first ceramic material consisting essentially of beryllium oxide'which has a high heat conductivity and the head portion is formed of the second ceramic material consisting essentially of aluminum oxide which has a high mechanical strength, these portions being firmly chemically bonded to each other through at least one intermediate layer having a composition of 1-55 percent by weight of beryllium oxide, 89 percent by weight of alu- [72] Inventors [22] Filed Patented [73] Assignee [54] SPARK PLUG HAVING COMPOSITE INSULATOR 9 Claims, 4 Drawing Figs.

minum oxide and 5-25 percent by weight of an additive, provided that, in the compositions of the respective layers, the sum of BeO and M 0 is larger than 75 percent by weight and smaller than percent by weight. The compositions change gradually from said first ceramic material to said second [5 6] References Cited UNITED STATES PATENTS ceramic material. Said additive generally consists of at least one member selected from the group consisting of MgO, SiO CaO, BaO, TiO ZrO Fe O MO and MnO and preferably consists of about 5 25 percent by weight of two to three members selected from the group consisting of MgO, SK), and CaO.

313/137 313/137 l74/209X 174/209X m m m "uni n n "r. r e fi w .Dme f wu ac u B818 376 0 35 9999 iii] ///l 6694 2794 3964 ,33 2038 3354 72 0 5 22 SPARK PLUG HAVING COMPOSITE INSULATOR CROSS-REFERENCES TO RELATED APPLICATION This is a Continuation-ln-Part Application of US. application Ser. No. 730,305, filed on May 20, 1968, now abandoned.

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to the manufacture of a composite insulator having excellent thermal characteristics for spark plugs mounted on internal combustion engines for automotive vehicles, said insulator being divided into a leg portion consisting of a beryllia ceramic material having a high heat conductivity and a head-portionconsisting of an alumina ceramic mated material having high mechanical strength. These two portions are not connected with each other by any generally known cementation process or mechanical method, but are formed together as a unit with a gradually changing composi- U)".

2. Description of the Prior Art Previously known electrical insulators for spark plugs of this kind have been madeof an alumina ceramic material consisting essentially of aluminum oxide and have a uniform composition throughout the insulator body. As is well known, the aluminum ceramic material has a high mechanical strength, but the heat conductivity is relatively low compared with that of metals. Therefore, when such an insulator having a relatively large length of the legportion is fitted in a spark plug, a large amount of heat is applied to the insulator and the temperature at the exposed end of the leg portion tends to increase, resulting in a tendency to give rise to preignition. On the other hand, an attempt to prevent undesirable preignition by shortening the length of the leg portion of the insulator results in'a defect that the insulation is deteriorated by carbon and the like deposited during the running of the vehicles at a low speed. Especially, for mounting on a racing car engine a spark plug must have a satisfactory degree of heat dissipation, and for this purpose it has been common practice to employ an insulator having a length of its leg portion of about 3 to 4 millimeters. In the near future, the length of the leg portion will be further shortened and will possibly be reduced to zero in an extreme case. In such a case, the problem of insulation deterioration will become more outstanding.

' In order to eliminate these vital defects, at present various kinds of insulators for spark plugs having varying lengths of the leg portion thereof are manufactured to suit various operating conditions. For this purpose, it has recently been proposed to make an electrical'insulator for spark plugs of a ceramic material having .an excellent heat conductivity. Among ceramic materials which may be used at high temperatures, a beryllia ceramic material consisting essentially of beryllium oxide has the highest heat conductivity. However, the beryllia ceramic material has a mechanical strength which is only about a half of that of the alumina ceramic material consisting essentially of aluminum oxide and is very expensive. The insulator for spark plugs must have a high mechanical strength at several parts thereof including the part projecting from the outer shell of the spark plug. Furthermore, the insulator occupies a greater part of the cost of a spark plug, and thusthe cost of the insulator is also regarded as an important factor. Therefore, a filler material having a high mechanical strength such as aluminum oxide may be added to the beryllia ceramic material to improve the mechanical strength of the insulator and to reduce the' cost of the insulator compared with an insulator for spark plugs made of beryllia ceramic material alone.

SUMMARY OF THE lNVENTION the leg portion of the insulator, where a high heat conductivity is required, of a beryllia ceramic material consisting essentially of beryllium oxide, forming the head portion of the insuceramic material obtained by adding asmall amount of an additive (oxide) to an appropriate mixture of the above-mentioned two ceramic materials. I I

It is therefore an object of the present invention to providea composite insulator for spark plugs having a high heat conductivity and a high mechanical strength at a low cost. Another object of the invention is to adapt a certain kind of an insulator for spark plugs to a wider range of operating conditions. Still another object of the invention is to provide a composite insulator for spark plugs wherein the leg portion thereof is strongly bonded to the head portion thereof so that the insulator may be sufficiently strong to withstand a large bending force imparted thereto during assembly work. Still another object of the invention is to provide a spark plug which is most suitable for use in a high speed engine for racing cars.

In order to bonddifferent ceramic materials, there has been known a method of mechanical bonding of those materials, a method involving the insertion of an appropriate refractory material between those materials and the mechanical bonding of the materials. A method for bonding different materials by a cementation process is also generally known. Also, it is known that a rubber and a synthetic resin can be chemically bonded by the use of a special technique due to the characteristics of the materials, and by particularly specifying an additive or an intermediate composition. Different ceramic materials have not heretofore been completely bondedwith a gradually changing composition. We have now found that a beryllia ceramic material and a alumina ceramic material can be bonded with a gradually changing composition.

It is impossible to directly bond the ceramic material A consisting essentially of beryllium oxide with the ceramic material B consisting essentially of aluminum oxide which are used in the present invention. The material A and the material B should be equal in 1. thermal expansion coefficient, 2. melting point, 3. sintering velocity, 4. coefficient of contraction, 5. concentration gradient and 6. reaction product in order to strongly bond the materials with a gradually changing composition. Even if the material A is once bonded to the material B, the bonded insulator may be cracked and their bonding is not so simple.

According to the present invention, the material A and the material 18 are bonded by providing at least one intermediate layer between the two materials having intermediate characteristics with regard to 1. thermal expansion coefficient, 2. concentration gradient, 3. sintering velocity, etc. In the intermediate layer a mixing ratio of aluminum oxide and beryllium oxide, the number of such intermediate layers provided and the kind and amount of the additive used may be varied. In this case, the material A and the material B cannot be strongly bonded with a gradually changing composition if an intermediate layer consisting of a mere mixture of aluminum oxide and beryllium oxide is provided. A special composition must be developed in order to obtain an intermediate bonding material of desirable properties.

The present invention will be illustrated by the following examples in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING FIG. l is a vertical section of aspark plug having composite insulator according to the present invention. F l6. 2 is a vertical sectionof a moulding apparatus employed for the manufacture of an insulator for spark plugs according to the present invention. HO. 3 is a graphic illustration of the performance of a spark plug having an insulator according to the present invention compared with the performance of a spark plug having a previously known insulator for spark plugs. FIG. 4 is a graphic illustration showing the relationship between the number of intermediate layers and the bending strength of the bonded insulator.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, a composite insulator for spark plugs according to the present invention is composed of a leg portion la, a head portion lb and an intermediate portion 1c connecting the leg portion In with the head portion lb. The leg portion la is formed of a ceramic material consisting essentially of beryllium oxide which has a very high head conductivity, and the head portion lb is formed of a ceramic material consisting essentially of aluminum oxide which has a high mechanical strength. The intennediate portion 1c is formed of a ceramic material having an intermediate composition between the compositions of said two ceramic materials. At the tip of an outer shell 2 of the spark plug there is formed a grounded electrode 4 opposite to a central electrode 3 which has been inserted into an axial bore in the leg portion of the insulator. The leg portion is shrunk onto the head portion of the insulator lb by well known shrinkage means such as by electrical heating shrinkage. A rod for a high voltage terminal 5 is electrically connected with the central electrode 3. Numeral 6 is a sealing member. The structures of the outer shell 2, central electrode 3, grounded electrode 4, rod for high voltage terminal 5 and sealing member 6 are similar to those of previously known spark plugs.

The manufacture of a composite insulator 1 according to the present invention will be described with regard to a spark plug having the above-mentioned structure.

A moulding apparatus as shown in FIG. 2 was used for the manufacture of the composite insulator for spark plugs. The moulding apparatus comprised a cylindricalrubber mould 7 to which a moulding pressure was applied by the use of oil pressure from the outer periphery toward the center thereof. A core 9 had an integral needle 8 for forming an axial bore for receiving the central electrode and the rod for the high voltage terminal. A lower mould 10 provided a moulding cavity together with said cylindrical mould 7.

The ceramic materials having compositions A, B and C as shown in the following Table l, which were necessary to form the desired composite insulator for spark plugs, were first prepared.

The composition A consisted essentially of beryllium oxide and the composition C consisted essentially of aluminum oxide. In case the the intermediate layer comprises only one layer as shown in Table l, the composition B for the intermediate a layer consists of l0-30 percent by weight of Be0, 55-85 percent by weight of Al fl and 5-15 percent by weight of an additive provided that the sum of Bell and A1 0. I

formed by the cylindrical rubber mould 7 and the lower mould 10 of the moulding apparatus as shown in FIG. 2. Ceramic material 12 having the composition B was then charged in a small amount. Finally, a ceramic material 13 having the composition C was charged as the uppermost layer. After completing the charging of these ceramic materials into the cylindrical rubber mould 7, a suitable moulding pressure of 200 to 1,000 kg/em was applied to the outer periphery of said cylindrical mould 7 to effect compression moulding. The moulded body was then subjected to cutting or grinding into the desired shape and fired at a suitable firing temperature of about l600 C. During the firing step, the ceramic material 1 I having the composition A reacted with the ceramic material 12 having the composition B at the bonded portion. A reaction product of chrysoberyl (Be0**Al O was partly formed and these ceramic materials were firmly bonded to each other. Thus, the leg portion of the insulator a formed of the beryllia ceramic material and the head portion of the insulator lb formed of the alumina ceramic material were firmly bonded by the bonded portion 10 formed of the special ceramic material having an intermediate composition between the compositions of said ceramic materials to form a composite insulator for spark plugs l as'shown in FIG. 1. As seen from FIG. 1, the bonded portion 10 did not show a distinct change in composition owing to the formation of said reaction product and a firm bonding was obtained possessing a gradually changing composition.

Such a composite insulator for spark plugs of such a structure shows high mechanical strength at the upper portion including the portion, where the outer shell of the spark plug is shrunk thereon, and the portion projecting from said outer shell problems relating to mechanical strength. There is no problem relating to mechanical strength. Further, the leg portion of the composite insulator serving as a path of heat dissipation has a high heat conductivity and rapidly removes the large amount of heat which is generated at the portion to effectively prevent local overheating.

A spark plug as shown in FIG. 1 having the composite insulator for spark plugs according to the present invention and a spark plug having a prior art composite insulator for spark plugs formed of the known alumina ceramic material consisting essentially of aluminum oxide were compared with regard to the relationship between their preignition limits and the lengths of the leg portions of the insulators. Referring to FIG. 3, the preignition limit is taken as an ordinate and the length is millimeters of the leg portion of the insulator is taken as the abscissa. FIG. 3 shows that the relationship between the preignition limit and the length of the leg portion of the insulator is represented by a straight line B in the case of the former and by a straight line F in the case of the latter. At the same length of the leg portion of the insulator, the preignition limit of the former was about 1.5 times that of the latter. Also, at the same preignition limit, the length of the leg portion of the composite insulator of the former was about 2-3 mm. longer than that of the latter.

A spark plug having a composite insulator which has a selected length of the leg portion of the insulator according to the present invention may be advantageously used under both driving conditions in one of which a vehicle travels a long distance frequently running uphill or under a heavy load, and in the other of which a vehicle travels a short distance on a flat road in the city. Even under the former driving conditions in which the spark plug tends to be burnt, the very high heat conductivity of the leg portion of the insulator produces such a striking effect that a large amount of heat applied to the leg portion is quickly dissipated thereby to substantially eliminate any undesirable temperature increase at the lower end of the leg portion and to avoid preignition. Also, under the latter driving conditions in which the spark plug is liable to be fouled with carbon and other deposits the considerable length of the tion of carbon and the like materials at the leg portion and to eliminate undesirable deterioration of the insulation resistance. In other'words, a single kind of spark plug may be adapted to a wide range of operating conditions.

The sum of Bel) and M is larger than 75 percent by weight and smaller than 95 percent by weight. The additive is the same as in the case of one intermediate layer.

As shown in FIG. 4, the bending strength of the bonded in- The present invention is not limited to the specific embodi- 5 sulator varies according to the number of intermediate layers. ment illustrated above. The number of the intermediate layers If there is not present at least one intermediate layer, the elec-i may be also two, three, four and five. In each case, the comtrical strength and mechanical strength of the spark plug willi positions of the ceramic materials for forming each portion of be insufficient for the spark plug to be put into practical use. the composite insulator spark plugs according to the present Regarding the respective points plotted on a curve showingf invention are limited as mentioned below. lo the relationship between the bending strength of the bonded In case of two intermediate layers, the compositions of the insulator and the number of intermediate layers in said FIG. 4, respective layers are shown in Table 2. the representative examples of the compositions of the respective layers are shown as follows: TABLE 2 The first ceramic layer consists of 95.0 percent by weight of l Be0, I.0 percent by weight of M 0, and 4.0 percent by weight Comwmon of an additive. The second ceramic material consists of 95.0

percent by weight of A1 0; and 5.0 percent by weight of an ad- Component, percent by Weigh ?;2 mg ditive. The compositions of intermediate layers are shown below for each number of the intermediate layers. The inter- 90-98 25-40 10-25 2 mediate layers are successively numbered from the side {:3 gig 33g closest to the first ceramic layer.

The sum of Bell and A50; is larger than 80 percent by weight 25 and smaller than 95 percent by weight. The additive is the same as in the case of one intermediate layer.

In case of three intermediate layers, the compositions of the respective layers are shown in Table 3.

TABLE 3 Composition Component, percent by wt. First First Second Third Second ceramic layer layer layer ceramic B00 00-45 -00 5-15 Ami"-.. 1-5 40-65 55-80 60-65 90-06 Additive 1-0 5-15 5-15 10-25 2-10 The sum of Bell and ALO, is larger than 75 percent by weight and smaller than 95 percent by weight. The additive is the same as in the case of one intermediate layer.

In case of four intermediate layers, the compositions of the respective layers are shown in Table 4.

TABLE 4 Composition First First Second Third Fourth Second Component, percent by wt. ceramic layer layer layer layer ceramic B00 90-08 35-50 -65 10-20 3-10 Alloi 1-5 -60 50-75 60-80 65-87 00-03 Additive 1-0 5-15 7 5-15 10-20 10-25 2-10 The sum of Be0 and 111.0, 8 larger than 75 percent by weight and smaller than 95 percent by weight. The additive is the same as in the case of one intermediate layer.

In the case of live intermediate layers, the compositions of the respective layers are shown in Table 5.

TABLE 5 Composition First First Second Third Fourth Filth Second Component, percent by wt. ceramic layer layer layer layer layer ceramic (1) In the case of one intermediate layer:

First layer Percent by wt.:

(2) In the case of two intermediate layers:

First layer Second layer 27. 12. 0 63. 0 73. 0 g 0. 5. 5 S 5. 0 5. 0 CaO 4. 5 4. 5

Norm-Bending strength of bonded insulator: 17.8 kgJmm. (3) In the case of three intermediate layers:

First Second Third layer layer layer N01E.Bending strength of bonded insulator: 20.1 kgJmm.

(4) In the case of four intermediate layers:

First Second Third Fourth layer layer layer layer Nora-Bending strength of bonded insulator: 19.9 kgJmm.

(5) In the case of five intermediate layers:

No'rE.-Bending strength of bonded insulator: 20.2 kg/mrn.

As described above, according to the prCLLnt invention, the leg portion of a composite insulator is formed of a beryllia ceramic materialconsisting essentially of beryllium oxide, and the head portion of the composite insulator is formed of an alumina ceramic material consisting essentially of aluminum oxideqThereby, a composite insulator for spark plugs of a high heat conductivity and a high mechanical strength may be provided which is excellent in heat conductivity at its portion requiring a high heat conductivity and it is also excellent in mechanical strength at theportion requiring a high mechanical strength. Furthermore, an effect that a spark plug can be used under a wide range ofoperating conditions is remarkable achieved because the leg portion of the composite insulator is formed'of a beryllia ceramic material as described above. The composite insulator for spark plugs according to the present invention can be produced at a relatively low cost in spite of the fact that the leg portion isformed of an expensive beryllia ceramic material. becausethe leg portion occupies only about one-third or less of the total volume of the insulator. As the leg portion and the head portion are bonded to each other through a ceramic material having a particular intermediate composition between the composition of the beryllia ceramic material and the composition of'the alu'mina ceramic material, the bond between the leg p'oition'andth'e head portion provided by a reaction between thesecraniic materialsat the bonded portion of the layers is astrbnger than the boh obtained by merely bonding the leg portionto the head p tion with glass or a heat resistant cement arty mechanical fabrication. The insulator can sufficiently withst'anda strong bending force applied thereto during the assemblyoperation.

We claim:

1. A spark plug having a composite insulator comprising a leg portion consisting essentially of beryllium oxide and a head portion consisting essentially of aluminum oxide characterized in that said two portions are chemically bonded to each other through at least one intermediate layer consisting of 30 to 89 1 percent by weight of aluminum oxide, l to 55 percent by weight of beryllium oxide and 5 to 25 percent by weight of a ceramic oxide additive, provided that the sum of Be0 and Al 0 is larger than 75 percent by weight and smaller than 95 percent by weight, and that the compositions of the respective intermediate layers gradually change from that of the head portion to that of the. leg portion..-

2. A spark plug according to claim 1, wherein the additive consists of at least one member selected from the group consisting of Mg0, Si0 Ca0, Ba0,-'Ti0 Zr0 Fe 0 M0 and MnO. 3. A spark plug according to. claim 1, wherein the additive consists of about 5 to 25 percent by weight of two to three group consisting of Mg0, Si0 and members selected from the Ca0.

4. A spark plug according to claim 1, wherein the number of said intermediate layers is 3 to 5.

5. A spark plug according to claim 1, which on has one intermediate layer consisting of 10 to 30 percent by weight of Be0, 55 to 85 percent by weight of A1 0 and 5 to 15 percent by weight of the additive, provided that the sum of Be0 and A1 0 is larger than 85 percent by weight and smaller than 95 percent by weight.

6. A spark plug according to claim 1, which has two intermediate layers, the composition of the first layer being 25 to 40 percent by weight of Be0, 45 to 70 percent by weight of M 0 and 5 to 15 percent by weight of the additive and the composition of the second layer being 10 to 25 percent by weight of Bet) and 55 to percent by weight of A1 0 and 10 to 20 percent by weight of the additive, provided that the sum of Bet) and A1 0 is larger than 80 percent by weight and smaller than 95 percent by weight.

7. A spark plug according to claim 1, which has three intermediate layers, the composition of the first layer being 30 to l third layer being 5 to 15 percent by weight of Be0, 60 to .percent by weight of A1 0 and 10 to 25 percent by weight of the additive, provided that the sum of B60 and Al 0 is'larger than 75 percentby weight and smaller than percent by weight. v

8. A spark plug according to claim 1, which has fourintermediate layers, the composition of the first layer being 35 to 50 percent by weight of Bot), 35 to 60 percent by weight of Al 0 and 5 to 15 percent by weight of the additive, the composition of the second layer being 20 to 35 percent by weight of Be0, 50 to 75 percent by weight of M 0 and 5 to 15 percent by weight of the additive, the composition of the third layer being 10 to 20 percent by weight of Be0, 60 to 80 percent by weight of M 0 and 10 to 20 percent by weight of the additive and the composition of the fourth layer being 3 to l0 percent by weight of Be0, 65 to 87 percent by weight of Al 0 and 10 to 25 percent by weight of the additive, provided that the sum of Be and M 0 is larger than 75 percent by weight and smaller than 95 percent by weight.

9. A spark plug according to claim 1, which has five intermediate layers, the composition of the first layer being 40 to 55 percent by weight of 81:0, 30 to 55 percent by weight of A50 and S to 15 percent by weight of the additive, the composition of the second layer being 25 to 40 percent by weight of Be0, 45 to 70 percent by weight of A1 0 and 5 to percent by weight of the additive, the composition of the third layer being 15 to 25 percent by weight of Bel), 55 to 75 perlarger than 75 percent by weight and smaller than percent by weight.