Title of Invention

SPARK PLUG

Abstract A glaze layer 2d of the spark plug has the composition comprising 1 mol% or less of a Pb component in terms of PbO; 25 to 45 mol% of a Si component in terms of SiO2; 20 to 40 mol% of a B component in terms of B2O3; 5 to 25 mol% of a Zn component in terms of ZnO; 0.5 to 15 mol% of Ba and/or Sr components in terms of BaO or SrO; 5 to10 mol% in total of at least one alkaline metal component of of Na, K and Li in terms of Na20, K20, and Liz, respectively, where K is essential; and. further, 0.5 to 5 mol% in total of one or two kinds or more of Mo, W, Ni, Co,Fe and Mn in terms of MOO3, WO3, Ni304, CO304, Fe2O3, and MnO2, respectively.
Full Text

SPARK PLUG
Background of the Invention
1. Field of the Invention
This invention relates to a spark plug.
2. Description of the Related Art
A spark plug used for ignition of an internal engine of such as automobiles generally comprises a metal shell to which a ground electrode is fixed, an insulator made of alumina ceramics, and a center electrode which is disposed inside the insulator. The insulator projects from the rear opening of themetal shell in the axial direction* A terminal metal fixture (terminal) is inserted into the projecting part of the insulator and is connected to the center electrode via a conductive glass seal layer which is formed by a glass sealing procedure or a resistor A high voltage is applied to the terminal metal fixture to cause a spark over the gap between the ground electrode and the center electrode♦
Under ' some codified conditions, for example, at an increased spark plug temperature and an increased environmental humidity, it may happen that high voltage application fails to cause a spark over the gap but, instead, a discharge called as a flashover occurs between the terminal metal fixture and the metal shell, going around the projecting insulator. Primarily for the purpose of avoiding flashover, most of commonly

used sparkplugs have a glassy layer on the surface of the insulator The glaze layer also serves to smoothen the insulator surface thereby preventing contamination and to enhance the chemical or mechanical strength of the insulator.
In the case of the alumina insulator for the spark plug’ such a glaze of lead silicate glass has conventionally been used where silicate glass is mixed with a relatively large amount of PbO to lower a softening point- In recent years, however, with a globally increasing concern about environmental conservation, glazes containing Pb have been losing acceptance. In the automobile industry, for instance, where spark plugs find a huge demand, it has been a subject of study to phase out Pb glazes in a future, taking into consideration the adverse influences of waste spark plugs on the environment.
Leadless borosilicate glass- or alkaline borosilicate glass-based glazes have been studied as substitutes, for the conventional Pb glazes, but they inevitably have inconveniences such as a high glass transition or an insufficient insulation resistance. • To address this problem, JP-'A-lobs proposes a leadless glaze composition having an adjusted Zn component to improve glass stability without increasing viscosity, and JP’A-11-106234 discloses a composition of leadless glaze for improving the insulation resistance by effects of joint addition of alkaline component.
Incidentally, since the glazes for spark plugs are used

attaching to engines/ they are apt to rise in temperature than cases of general insulating porcelains* Further, in recent years the voltage applied to spark plugs has been increasing together with advancing predominance of engines- For these, the glaze for this use has been required to have insulation performance withstanding severer conditions of use. However, the glaze composition disclosed in JP-A-11--106234 is not always satisfactory in insulating performance at high temperatures/ particularly the performance as evaluated as a glaze layer formed on an insulator in a spark plug (e.g., anti-flashover properties) *
JP-A-11-106234 refers to the improvement of the insulationresistanceby few feet s of joint addition of an alkaline component of the glaze containing Si or B as the glass skeleton component, but it could hardly recognized that a satisfactory attention is paid to a cancellation of differential thermal expansion coefficient in relation with the alumina based ceramics as composing ceramics of the insulator, and an improving level of -the insulation resistance is not always satisfied.
Summary of the Invention
It is a first object of the invention to provide such a spark plug having a glaze layer which has a reduced Pb content, is capable of being baked at relatively low temperatures, exhibits excellent insulation properties, and is easy to get a baked smooth surface.

It is a second object of the invention to provide such a spark plug where reduced is the differential thermal expansion coefficient in relation with the alumina based ceramics as composing the insulator by adjusting an alkaline metal component in the glaze, thereby to make less to cause defects as cracks or crazing in the glaze layer and farther heighten the insulation resistance*
Brief Description of the Drawings Fig, 1 is a whole front and cross sectional view showing the spark plug according to the invention.
Fig, 2 is a front view showing an external appearance of the insulator together with the glaze layer.
Figs * 3A and 3B are vertical cross sectional views showing some examples of the insulator.
Fig, 4 is a whole front view showing another example of the spark plug according to the invention. . ,
Fig. 5 is a whole front view showing a further; example of the spark plug according to the invention.
Fig. 6 is an explanatory view showing the measuring method of the insulation resistant value of the spark plug*
Fig* 7 is an explanatory view of the forming step of coating the slurry of the glaze.
Figs* 8A to 8D are explanatory views of the gas sealing step.
Figs* 9A and 9B are explanatory views continuing from

Figs. 8A to 8D,
The reference numerals and sign are set forth below*
1 : Metal shell;
2 : Insulator; 2d : Glaze layer;
2d' : Blaze slurry coated layer;
3 : Center electrode;
4 : Ground electrode; and
5 : Glaze slurry
Detailed Description of the Invention
The sparkplug according to the invention comprises
an alumina based ceramic insulator disposed between a center
electrode and the metal shell/ where at least part of the surface
of the insulator is covered with a glaze layer comprising oxides.
A first composition thereof is characterized in that the glaze layer comprises 1 inol% or less of Pb component in terms of PbO; 25 to 45 mol% of Si component in terms of SiO’; 20 to 40mol% of B component in terms of B2O3; 5 to 25mol% of Zn component in terms of'ZnO; 0,5 to 15 mol% of Ba and/or rS components in terms of Bao or SrO;
at least one alkaline metal components of 5 to 10 mol% intotalof Na/Kindles intermsof NaSO/ K2O/ candles, respectively/ where K is essential;
and further, one or two kinds or more of Mo, W, Ni, Co, Fe and Mn 0.5 to 5 mol% in total in terms of M0O3, WO3, NiaO’,

CO304, FesOs, and MnOs/ respectively-
Reference will be hereafter made to effects of the first composition of the inventive spark plug, (Work & Effect A)
For aiming at the adaptability to the environruental problems, it is a premise that the glaze to be used contains the Pb component 1.0 mol% or less in terms of PbO (hereafter called the glaze containing the Pb component reduced to this level as 'leadless glaze") . When the Pb component is present in the gla2:e in the form of an ion of lower valency (e.g./ Pb’"‘), it is oxidized to an ion of higher valency (e.g.’ Pb’*) by a corona discharge. If this happens, the insulating properties of the glaze layer are reduced/ which probably spoils an anti-flashover. From this viewpoint, too/ the limited Pb content is beneficial, A preferred Pb content is 0.1 mol% or less. It is most preferred for the glaze to contain substantially no Pb (except a trace amount of lead unavoidably incorporated from raw materials of the glaze). (Effect B) •
While reducing the Pb content, the glaze used in the invention has a specifically designed composition for securing the insulating properties, optimizing the glaze baking temperature/ and improving the finish of the baked glaze face. The Pb component in conventional glazes has played an important role in adjusting a softening point (practically/ moderately

lowering the softening point of the glaze to secure a fluidity when baking the glaze), and in the leadless glaze, a B component (B2O3) and the alkaline metal component have strong relationship with adjustment of the softening point. Inventors have found that there is a specific range of the B component in relation with a content of the Si component/ which is suited to improving of the baking finish/ and being based on the premise-of this containing range, if one or two kinds or more of Mo, W, Ni/ Co, Fe, admen are added, it is possible to provide such a spark plug having a glaze layer which can secure the fluidity when baking the glaze, is capable of being fired at relatively low temperatures, exhibits excellent insulation properties, and is easy to get a smooth surface, and thus accomplished this invention* That is, the first problem is solved.

(Effect C)
In the conventional glazes, the Pb component plays an important role as to the fluidity when baking the’ glaze, but in the leadless glaze of the invention, while containing the alkaline metal component for securing the fluidity when baking the glaze, the high insulating resistance can be provided by determining the containing range of the Si component as above mentioned. That is, the alkaline metal component in the glaze lowers the softening point of the glaze and serves to secure the fluidity when baking the glaze. If containing the alkaline metal component in the above mentioned range, such effects are

exhibited which can form the glaze layer difficult to generate pinholes or glaze crimping in an outer appearance.
If the content of the alkaline metal component is less than the above mentioned range/ the fluidity when baking the glaze is probably decreased. However, if selecting the total containing amount as above mentioned of the alkaline metal component/ it is assumed that such a glaze layer may be provided which is uniform in thickness and is less to cause glaze crimping or pinholes in the appearance owing to air bubbles involved as glaze slurry. (Effect D)
Further, the first composition of the invention has a characteristic also in containing essentially K as the alkaline metal component While securing the fluidity when baking the glaze and in turn improving a smoothness in the glaze layer to be formed’ it is possible to largely heighten the insulating performance. The reason therefor is assumed that since the K component has a larger atomic weight than other alkaline metal components of Na and Li in spite of the same mol containing amount and the same cation number, it occupies a larger weight ratio. For more heightening this effect, it is desirable to determine a component of the highest content to be K in the alkaline, metal components in the glaze layer.
A second composition of the spark plug according to the invention is characterized in that the glaze layer comprises

1 loll or less of the Pta component in terms of Pi)0; 25 to 45 moll of the Si component in terms of Si02; 20 to 40 mol% of the 3 component in terms of B2O3; 5 to 25 moll of the Zn component in terms of ZnO; 0.5 to 15 mol% of the Ba and/or Sr components in terms of BaO or SrO;
5 to 10 knoll% in total of at least one alkaline metal components of Na, K and Li in terms of NaSO, K2O, ‘nd hiZf respectively;
0,5 to 5 mol% in total of one or two kinds or more of Ti/ 2r and Hf in terms of Ti02, ZrOa and Hoff, respectively, and
0,5 to 5 mol% in total of one or two kinds or more of Mo, W/ Ni, Co, Fe and Mn in terms of M0O3, WO3, Ni304, C03O4, Fe203, and Hn02/ respectively.
The second structure is the same as the first one in other glaze compositions excepting that the -glaze layer does not necessarily take the alkaline metal component K a’s essential and one or two kinds or more of Ti, Zr and Hf are contained in the above mentioned range. Accordingly/ 'the Effects A to C are similarly accomplished* On the other hand, if containing one or two kinds or more of Ti, Zr and Hf/ new effects can be exhibited as follows. (Effect E)
By addition of Ti, Rohr, a water resistance is improved. As to the Zr or Hf components/ the improved effect of the water

resistance of the glaze layer is more noticeable- By the way, "the water resistance is good" is meant that if, for example, a powder like raw material of the glaze is mixed together with a solvent as water and is left as a glaze slurry for a long time, such inconvenience is difficult to occur as increasing a viscosity of the glaze slurry owing to elusion of the component ■ As a result/ in case of coating the glaze slurry to the insulator, optimization of a coating thickness is easy and unevenness in thickness is reduced. Subsequently, saidoptimizationandsaid reduction can be effectively attained. If the addition amount of these components is less than 0,5 mol%, the effect of the optimization is short, probably resulting in lowering of the insulating resistance of the glaze layer by increase of the film thickness.
For the glaze layer, it is possible to select a composition corresponding to the combination of the above first and second ones* Thereby, the Effects A to E can be accomplished at the same time-
A third composition of the spark plug according to the invention is characterized in that the glaze layer comprises 1 mol% or less of the Pb component in terms of PbO; and contains either or both of the Si and B components as a glass skeleton structure, and the glaze layer comprises three components of Li, Na and K as the alkaline metal components/ and has a composition which satisfies the relationship of

NNazO The alkaline metal component is inherently high in an ion conductivity, and serves to lower the insulating properties in a vitreous glaze layer* On the other hand, these or B components form the glass skeleton, and if their contents are appropriately determined, dimensions of skeletal meshes are made convenient for blocking the ion conductivity the alkaline metal, and the favorable insulating properties can be secured. As the Si or B components easily form the skeleton, they act to reduce the fluidity when baking the glaze/ but if containing the alkaline metal component in the above mentioned range, the fluidity when baking the glaze is increased by lowering of the melting point owing to eutectic reaction and avoidance of complex

anion owing to interaction of S ion and 0 ion*
Herein, since th’ K component has a larger atomic weight than Na and Li as mentioned above, in case of setting a total containing amount of the calone metal components in the same mol?/ the K component does not exhibit the improved effect of the fluidity as the Na and Li components do, but comparing with Na and Li (in particular Li) , since an ionic mobility in the vitreous glaze layer is relatively small/ the K component has a property difficult to decrease the insulating properties of the glaze layer though increasing the containing amount* On the other hand, since the Li component is small in the atomic weight, the improved effect of the fluidity is larger than that of the K component, but as the ionicmobility is high, an excessive addition is apt to cause the insulating properties of the glaze
layer to decrease* However, being different from 'the K component, the Li component has a property to reduce the thermal expansion coefficient*
Accordingly, the insulating property of the glazing layer
I
can be effectively prevented from decreasing by making the most amount of the K component, and the fluidity when baking the glaze can be secured by mixing the Li component with a containing amount next to that of the K component/ and at the same time it is possible to suppress the increase of the thermal expansion coefficient of the glaze layer by mixing the K component, enabling to agree with the thermal expansion coefficient of

a substrate alumina. A trend of decreasing the insulating propertybyaddingtheL component can be effectively restrained by an effect of joint addition (later mentioned) of the three components where the Na component is less than K and Li. As a result/ an ideal composition of the glaze is r-3alized which is high in the insulating property, rich in the fluidity when baking the glaze/ and small in the difference of the-thermal expansion coefficient from that of alumina as the insulator composing ceramics. That is, the second problem of the invention is solved-
The glaze layer to be used with the third composition may have a composition corresponding to the gale composition of the above first and/or second glaze-
Explanation will be made to the critical meaning of the containing range of each glaze layer in the above mentioned sparkplug compositions- If the total amount in terms of oxides of one or two kinds or more of Mo, W, Ni, Co, Fe Mn (called as "fluidity improving transition metal component" hereafter) is less than 0.5'mol%/ there will be probably a case of not always providing an effect of improving the fluidity when baking the glaze for easily obtaining a smooth glaze layer- On the other hand, if exceeding 5 mol%, there will be probably a case of being difficult or impossible to bake the glaze owing to too much heightening of the softening point of the glaze-
As a problem when the containing amount of the fluidity

improving transition metal component Is excessive’ such a case may be taken up that not intentioned coloring appears in the glaze layer. For example, visual information such as letters/ figures or product numbers are printed with color glazes on external appearances of the insulators for specifying producers and others, and if the colors of the glaze layer is too thick, it might be difficult to readout the printed visual information-As another realistic problem, there is a case that tint changing resulted from alternation in the glaze composition is seen to purchasers as "unreasonable alternation in familiar colors in external appearance", so that an inconvenience occurs that products could not always be quickly accepted because of a resistant feeling thereto.
The insulator forming a substrate of the glaze layer
comprises alumina based ceramics taking white/ and in view of preventing or restraining coloration, it is desirable’ that the coloration in an observed external appearance of the glaze layer formed in the insulator is adjusted to be 0 to 6 in chroma Cs and 7.5 to 10 in lightness Vs, for example, the amount of the above transition metal component is adjusted. If the chroma exceeds 6, the gray or blackish coloration is easily distinguished- In either way, there appears a problem that an impression of "apparent coloration" cannot be wiped out. The chroma Cs is preferably 8 to 10, more preferably 9 to 10, In the present specification, ameasuringmethodof the lightness

Vs and the chroma Cs adopts the method specified in "4.3 A Measuring Method of Reflected Objects" of "4. Spectral Calorimetry '* in the "AMeasuringMethod of Colors" of JIS-Z8721. As a simple method, the lightness and the chroma can be known through visual comparisons with standard color chart prepared according to JTS-28721.
That the effect of improving the fluidity when acing the glaze is especially remarkable is exhibited by W next to Mo And Fe, For example, it is possible that all the essential transition metal components are made Mo, Fe or W. For more heightening the effect of improving the fluidity when baking the glaze/ it is preferable that Mo is 50 mol% or more of the essential transition metals-
Next/ desirably/ the total amount of the alkaline metal components is 5 to 10 mol%. In case of being less than 5 mol%/ the softening point of the glaze goes up/ baking of’ the glaze might be probably impossible. In case of being more than 10 mol%, the insulating property probably goes down, and an anti-flashover might be spoiled- The containing amount of the alkaline metal components is preferably S to 8 moll- With respect to the alkaline metal components, not depending on one kind/ but adding in joint two kinds or more selected from Na, K and Li, the insulating property of the glaze layer is more effectively restrained from lowering. As a result/ the amount of the alkaline metal components can be increased without

decreasing the insulatingproperty/ consequently it is possible to concurrently attain the two purposes of securing the fluidity when baking the glaze and the anti-flashover (so-called alkaline joint addition effect).
Of the alkaline components of Na/ K and Li, it is desirable to determine the rate of the K component in ter;:,s of oxide to be 0*4 Thereby, the effect of increasing the insulating property is more heightened. But if the value of K/(Na + K + Li) is less than 0.4, this effect is probably insufficient.
On the other hand, a reason for the value of K/ (Na + K + Li) to be 0 • 8 or less is for securing the fluidity when baking the glaze, which means that the other alkaline metal components than K is added in joint in a range of the rest balance being 0,2 or more (0. 6 or less) » It is more preferable that the value of K/(Na + K -H Li) is adjusted to be 0.5 to C.7, .,
Further, in the alkaline metal components,’preferably
the Li component is contained if feasible for exhibiting the
f joint-addition of alkaline components so as tc improve the
insulating property, adjusting the thermal expansion
coefficient of the glaze layer, securing the fluiditywhen baking
the glaze, and heightening mechanical strength-
It is desirable that the Li component in mol % in terms of the oxide to be determined to be
0,2 S Li/(Na + K + Li)
If Li is less than 0.2/ the thermal expansion coefficient is too large in comparison with that of the substrate alumina, and consequently defects such as crazing easily occur, so that it might be insufficient to secure a finish of the baked glaze surface. In contrast/ if Li is more than 0*5/ as an Li ion is relatively high in mobility among the alkaline metal ions, bad influences are probably given to the insulating property-It isbetter that values of Li/ (Na + K + Li) are desirably adjusted to range 0.3 to 0*45. For more heightening the insulating property by the joint addition of the alkalinemetal components, it is possible to mix other alkaline metal components following the third component as Na in a range where the electric conductivity is not spoiled by excessive joint-addition of the total amount of the alkaline metal components* In particular desirably, it is good to contain all the three components of Na’ K and Li-
With respect to the Si component, being less than25mol%/ it is often difficult to secure a sufficient insulating performance. Being more than 45 mol%/ it is often difficult to bake the glaze. The Si containing amount should be more preferably 30 to 40 moll-
If the B containing amount is less than 20 mol%, the softening point of the glaze goes up, and the baking of the glaze will be difficult- On the other hand, being more than 40 mol%/ a glaze crimping is easily caused. Depending on

containing amounts of other components/ such apprehensions might occur as a devitrification the glaze layer, the lowering of the insulating property/ or inconsequence of the thermal expansion coefficient in relation with the substrate* It is good to determine the B containing amount to range 25 to 35 molS if possible.
If the Zn containing amount is less than5mol%, the’thermal expansion coefficient of the glaze layer is too large> defects such as crazing are easily occur in the glaze layer* As the Zn component acts to lower the softening point of the glaze’ if it is shorty the baking of the glaze will be difficult. Being more than 25 mol%, opacity easily occurs in the glaze layer due to the devitrification. It is good that the Zn containing amount to determine 10 to 20 mol%.
The Ba and Sr components contribute to heightening of the insulating property of the glaze layer and is >effective to increasing of the strength. If the total amount is less than 0.5 mol%, the insulating property of the glaze layer goes down, and the anti-flashover might be spoiled/ Being more than 20 mol%, the thermal expansion coefficient of the glaze layer is too highr defects such as crazing are easily occur in the glaze layer. In addition, the opacity easily occurs in the glaze layer. From the viewpoint of heightening the insulating property and adjusting the thermal expansion coefficient, the total amount of 3a and Sr is desirably determined to be 0-5

to 10 mol%- Either gr both of the Ba and Sr component may be contained, but the Ba component is advantageously cheaper in a cost of a raw material.
The Ba and Sr components may exist in forms other than oxides in the glaze depending on raw materials to be used* For example;- BaS04 is used as a source of the Ba component, an S component might be residual in the glaze layer* This sulfur component is concentrated nearly to the surface of the glaze layer when baking the glaze to lower the surface expansion of a melted glaze and to heighten a smoothness of a glaze layer to be obtained-
The total amount of the Zn and Ba and/or Sr components is desirably 8 to 30 mol% in terms of the above mentioned oxides * Being more than 30 mol%, the opacity will occur in the glaze layer. For example/ the visual information such as letters/ figures or product numbers are printed with color glazes on external appearances of the insulators for specifying producers and others, it might be difficult to read out the printed visual information owing to such as the opacity* Being less than 8 mol%/ the softening point extremely goes up/ the glaze baking is difficult and a bad external appearance is caused-Preferably, the total amount is 10 to 20 mol%.
The one or two kinds or more of the Al component of 1 to 10 mol% in terms of AI2O3/ the Ca component of 1 to 10 mol% in terms of CaO, and the Mg component of 0.1 to 10 mol% in terms

of MgO may be contained 1 to 15 mol% in total, The Al component is effective to restraining the devitrification, while the Ca and Mg components contribute to heightening of the insulating property of the glaze layer. In particular, th’ Ca component is next to the Ba or Zn components to be useful I'or improving the insulating property of the glase layer. If Lhe addition amount is less than each of the lower limitS/ the effect is insufficient, and if being more than the upper limit of each component or more than the upper limit of the total amount/ it is difficult or impossible to bake the glaze by the extreme increase of the softening point of the glaze layer*
In the viewpoint of the thermal expansion coefficient, it is preferable that in case B is in terms of B2.O2 and 2n is in terms of ZnO, the total mol containing amount is N(B203 + ZnO) / and in case the alkaline earth metal component RE (RE is one or two kinds or more selected from Ea, Ng, Ga and Sr) is in terms of composition formula of REO and the alkaline metal component R (R is one or two kinds or more selecttriid from Na, K and Li) is in terms of composition formula of R2C, the total mol containing amount is N(RE0+R20), and preferable is to be
1.5
thermal expansion coefficient in relation with the substrate of alumina. As a result’ the glass layer can foe prevented from appearances of defects such as erasing, cracking or peeling. If the above ranges are less than 1,5, the thermal expansion coefficient is too large in comparison with that of the substrate aluiaina/ and consequently defects such as crazing easily occur, so that it xaight be insufficient to secure the finish of the baked glaze surface. In contrast/ being more than 3*0, the thermal expansion coefficient is too small in comparison with that of the substrate alumina, resulting in easily causing cracking, peeling or crimping in the glaze layer. For making these effects more remarkable, preferable is to be
1.7 ‘ N(B203+ZnO)/N(REO+R20) £ 2.5.
Auxiliary components of one or two kinds or more of Bi, Sn, Sb, P/ Cu, Ce and Cr may be contained 5 mol% or less in total as Bi in terms of BiaO’, Sn in terms of Sn02, Sh in terms of SbsOs, P in terms of P2O5, Cu in terms of CuO/ Ce in terms of CeOa, and Cr in terms of Cr203. These components may be positively added in response to purposes or often inevitably included as rawmaterials of the glaze (otherwise latermentioned clay minerals to be mixed when preparing a gla2e slurry) or impurities (otherwise contaminants) from refractory materials in the melting procedure for producing glaze frit. Each of them heightens the fluidity when baking the glaze, restrains bubble formation in the glaze layer, or wraps adhered materials

on the baked glaze surface so as to prevent abnormal proj actions. Bi and Sb are especially effective.
In the composition of the spark plug of the invention, the respective components in the glaze are contained in the forms of oxides, and owing to factors forming amorphous and vitreous phases, existing forms as oxides cannot be often identified. In such cases, if the containing amounts of components at values in terms of oxides fall in the above mentioned ranges, it is regarded that they belong to the ranges of the invention.
The containing amounts of the respective components in the glaze layer formed on the insulator can be identified by use of known micro-analyzing methods such as Epi’m (electronic probe micro-analysis) or XPS (X-ray photoelectron spectro scopy) - For example, if using EPMA, either of a wavelength dispersion system and an energy dispersion system is sufficient for measuring characteristic X-ray* Further, there Is amethod where the glaze layer is peeled from the insulator and is subjected ‘ to a chemical analysis or a gas analysis for identifying the composition.
The spark plug having the glaze layer of the invention maybe composedby furnishing, in a through hole of the insulator, an axially shaped terminal metal fixture as one body with the center electrode or holding a conductive binding layer in relation therewith, said metal fixture being separate from a

center electrode. In this case, the whole of the 3park plug is kept at around 500*C, and an electric conductivity is made between the terminal metal fixture and a metal shelly enabling to measure the insulating resistant value. For securing an insulating endurance at high temperatures, it is desirable that the insulating resistant value is secured 200 MQ or higher, desirably 400 MQ or higher so as to prevent the flashover.
Figs • 8A to 8D show one example of measuring system. That iS/ DC constant voltage source {e-g., source voltage 1000 V) is connected to the side of a terminal metal 13 of the spark plug 100, while at the same time/ the side of the metal shell 1 is grounded, and a current is passed under a condition where the spark plug 100 disposed in a heating oven is heated at SOO'‘C.
For example, imagining that a current value Im is measured by
»■
use of a current measuring resistance (resistance value Bm) at the voltage VS/ an insulation resistance value Rx to be measured can be obtained as (VS/Im}-Rm (in the drawing, the current value Im is measured by output of a differential amplifier for amplifying voltage difference at both ends of the current measuring resistance)•
The insulator may comprise the alumina insulating material containing the Al component 85 to 98 mol% in terms of A1203 * Preferably, the glaze has an average thermal expansion coefficient of 5 x 10"V**C to 8.5 x 10*V**C at the temperature ranging 20 to 350*C. Being less than this lower limit, defects

such as cracking or graze skipping easily happen in the graze layer. On the other hand, being more than the upper limit/ defects such as crazing are easy to happen .n the graze layer. The thermal expansion coefficient more prcjrerai-ly ranges 6 x 10-Vc to 8 X lO’V’C-
The thermal expansion coefficient o: ::he glaze layer is assumed in such ways that samples are cut cut from a'vitreous glaze bulk body prepared by mixing and melting raw materials such that almost the same composition as the glaze layer is realized/ and values measured by a known dilatometer method-The thermal expansion coefficient of the glaze layer on the insulator can be measured by use of, e.g., a laser inter-ferometer or an interatomic force nicroscope.
The insulator is formed with a projection part in an outer circumferential direction at an axially central position thereof* Taking, as a front side, a side directing'toward the front end of the center electrode in the axial direction, a cylindrical face is shaped in the outer circumferential face at the base portion of the insulator mainbody in the neighborhood of a rear side opposite the projection part. In this case/ the outer circumferential face at the base portion is covered with the glaze layer formed with the film thickness ranging 7 to 50 ijm-
In automobile engines/ such a practice is broadly adopted that the spark plug is attached to engine electric equipment

system by means of rubber caps, and for heightening the anti-
flashover, important is the adherence between the insulator
and the inside of the rubber cap. The inventors made earnest
studies and found that’ in the leadless glaze of borosilicate
glass or alkaline borosilicate/ it is important to adjust
thickness of the glaze layer for obtaining a smooth surface
of the baked glaze, and as the outer circumference of ihe base
portion of the insulator main body particularly requires the
adherence with the rubber cap/ unless appropriate adjustment
ismadeto the film thickness, a sufficient anti-’flashover cannot
be secured. Therefore, in the insulator having the leadless
glaze layer of the above mentioned composition of the spark
plug according to the third invention, if the film thickness
of the glaze layer covering the outer circumference of the base
portion of the insulator is set in the range of the above numerical
values, the adherence with the baked glaze face and the rubber
cap may be heightened, and in turn the anti-flashover may be
improved without lowering the insulating property of the glaze
layer. ' V
If the thickness of the glaze layer at said base portion of the insulator is less than 7 )am, rhe leadless glaze of the above mentioned composition is difficult to form the smooth baked surface, so that the adherence with the baked glaze face and the rubber cap is spoiled and the anti-flashover is made insufficient. But if the thickness of the glaze layer is more

than 50 vjxi, a cross sectional area of the electric conductivity increases, the leadless glaze of the above mentioned composition is difficult to secure the insulating property, probably resulting in lowering of the anti-flashc’’er.
For uniforming the thickness of .i'e -jlaae layer or controlling excessively (or partially) thick qiaze layers, it is useful to add Ti, Zr or Hf as inentior.';ed above. ■ ‘
The spark plug of the invention c’n be produced by a production method comprising
a step of preparing glaze powders in which the rawmaterial powders are mixed at a predetermined ratio, the mixture is heated 1000 to iSOC'Candmelted/ theiuelted material is rapidly cooled, vitrified and ground into powder;
a step of piling the glase powder on the surface of an insulator to form a glaze powder layer; and
a step of heating the insulator, thereby tc bake the glaze powder layer on the surface of the insulator. -
The powdered rawmaterial of each component includes not only an'oxide thereof (sufficient with ccmplex oxide) but also other inorganic materials such as hydroxide, carbonate, chloride, sulfate, nitrate, or phosphate. These inorganic materials should be those of capable of being converted to corresponding oxides by heating and melting. The rapidly cooling can be carried out by throwing the melt into a water or atomizing the melt onto the surface of a cooling roll for

obtaining flakes.
The glaze powder is dispersed into the water or solvent, so that it can be used as a glaze slurry. For example, if coating the glase slurry onto the insulator surface to dr y it, the piled layer of the glase powder can be formed a* a -oated layer of the glaze slurry. By the way, as the lueihod ‘f coating the glaze slurry on the insulator surface, if adopting a merhod of spraying from an atomizing nozzle onto th-3 insulator surface/ the piled layer in uniform thickness of the glaze powder can be easily formed and an adjustment of the coated thickness is easy.
The glaze slurry can contain an adequate Mtiount of a clay mineral or an organic binder for heightening a .'shape retention of the piled layer of the glaze powder. As the clay mineral, those composed of mainly aluminosolicate hydrates can be applied/ for example, those composed of mainly one or two kinds or more of allophane/ imogolite, hisingerite, sinectite’ kaolinite, halloysite, montmorillonite, vermiculite, ano dolomite (or mixtures' thereof) can be used. In relation v;ith the oxide components, in addition to SiOs andAI2O3, those mainly containing one or two kinds or more of Fe203, TiOz, CaO, MgO, Na20 and KzO can be used*
The spark plug of the invention is constructed of an insulator having a thx-ough-hole formed in the axial direction thereof, a terminal metal fixture fitted in one end of the

through-hole, and a center electrode fitted in rhe other end. The terminal metal fixture and the center electrode are electrically connected via an electrically conductive sintered body mainly comprising a mixture of a gl-’ss and a conductive material (e.g., a conductive glass seal ..r a resistor). The spark plug having such a structure can ba made by a process including the following steps-
An assembly step: a step of assembling a structure comprising the insulator having the through-hole, the terminal metal fixture fitted in one end of the through-hole, the center electrode fitted in the other end, and a filled layer formed between the terminal metal fixture and the cent'=sr electrode, which filled layer comprises the glass powder and the conductive material powder.
A glaze baking step: a step of heating the assembled structure formed with the piled layer of the glaze powder on the surface of the insulator at temperature ranging.800 to 950*’0 to bake the piled layer of the glaze powder on the surface of the insulfeitor so as to form a glaze layer, and at the same time softening the glass powder in the filled layer.
A pressing step: a step of bringing the center electrode and the terminal metal fixture relatively close within the through-’hole, thereby pressing the filled layer between the center electrode and the terminal metal fixture into the electrically conductive sintered body*

In this case, the terminal metal fixture and the center electrode are electrically connected by the electrically conductive sintered body to concurrently seal the gap between the inside of the through-hole and the terminal metal fixture and the center electrode* Therefore/ the glaze baking step also serves as a glass sealing step. This process is efficient in that the glass sealing and the glaze baking are performed simultaneously. Since the above mentioned glaze allows the baking temperature to be lower to 800 to 950*C, the center electrode and the terminal metal fixture hardly suffer from bad production owing to oxidation so that the yield of the spark plug is heightened* It is also sufficient that the baking gla5:e step is preceded to the glass sealing step.
The softening point of the glase layer is preferably
ft
adjusted to range, e*g., 520to700*c. When the softening point is higher than 700*’0, the baking temperature above-SSO'‘C will be required to carry out both baking and glass sealing, which may accelerate oxidation of the center electrode and the terminal metal fixture. When the softening point is lower than 520*’C, the glaze baking temperature should be set lower than 800*c, In this case/ the glass used in the conductive sintered body must have a low softening point in order to secure a satisfactory glass seal- As a result, when an accomplished spark plug is used for a long time in a relatively high temperature environment, the glass in the conductive sintered body is liable to

denaturalization’ and where, for example, the conductive sintered body comprises a resistor, the denaturalization of the glass tends to result in deterioration of the performance such as a life under load. Incidentally, the softening point of the glaze is adjusted at temperature range of 520 to 620The softening point of the glaze layer is a value measured by performing a differential thermal analysis on the glaze layer peeled off from the insulator and heated, and it is obtained as a temperature of a peak appearing next to a first endothermic peak (that the second endothermic peak} which is indicative of a sag point. The softening point of the glaze layer formed in the surface of the insulator can be also estimated from a value obtained with a glass sample which is prepared- by compounding raw materials so as to give substantially the sane composition as the glaze layer under analysis, melting the composition and rapidly cooling.
Modes for carrying out the invention will be explained with reference to the accompanying drawings. Fig. 1 shows an example of the spark plug of the first structure according to the invention. The sparkplug 100 has a cylindrical metal shell 1, an insulator 2 fitted in the inside of the metal shell 1 with its tip 21 projecting from the front end of the metal shell 1, a center electrode 3 disposed inside the insulator 2 with its ignition part 31 formed at the tip thereof, and a ground electrode 4 with its one end welded to the metal shell 1 and

the other end bent inward such that a side of this end may face the tip of the center electrode 3. The ground electrode 4 has an ignition part 32 which faces the ignition part 31 to make a spark gap jgf between the facing ignition parts •
The metal shell 1 is formed to be cylindrical of such as a low carbon steel» It has a thread 7 therearound for screwing the spark plug 100 into an engine block {not shown)... Symbol le is a hexagonal nut portion over which a tool such as a spanner or wrench fits to fasten the metal shell !•
The insulator 2 has a through-hole 6 penetrating in the axial direction- A terminal fixture 13 is fixed in one end of the through-hole 6, and the center electrode 3 is fixed in the other end- A resistor 15 is disposed in the through-hole 6 between the terminal metal fixture 13 and the center electrode 3. The resistor 15 is connected at both ends thereof to the center electrode 3 and the terminal metal fixture.13 via the conductive glass seal layers 16 and 17/ respectively. The resistor 15 and the conductive glass seal layers 16, 17 constitute the conductive sintered body* The resistor IS is formed by heating and pressing a mixed powder of the glass powder and the conductive material powder (and, if desired, ceramic powder other than the glass) in a later mentioned glass sealing step. ‘ The resistor 15 may be omitted, and the terminal metal fixture 13 and the center electrode 3 may be directly connected by one seal layer of the conductive glass seal*

The insulator 2 has the through-hole 6 in its axial direction for fitting the center electrode 3/ and is formed as a whole with an insulating material as follows. That is, the insulating material is mainly composed of an alumina ceramic sintered body having an Al content of 85 to 98 mol% (preferably 90 to 98 mol%) in terms of AI2O3.
The specific components other than Al are exemplified as follows-
Si component: 1-50 to 5*00 mol% in terms of SiOa; Ca component: 1*20 to 4.00 mol% in terms of CaO; Mg component: 0,05 to 0-17 mol% in terms of MgO; Ba component: 0*15 to 0.50 mol% in terms of BaO; and B component : 0,15 to 0*50 mol% in terms of B203, The insulator 2 has a projection 2e projecting outwardly, e.g., flange-like on its periphery at the middle part in the axial direction, a rear portion 2b whose outer diameter is smaller than the projecting portion 2e/ a first front portion 2g in front of the projecting portion 2e/ whose outer diameter is smaller than the projecting portion 2e, and a second front portion 2i in front of the first front portion 2g, whose outer diameter is smaller than the first front portion 2g. The rear end part of the rear portion 2b has its periphery corrugated to form corrugations 2c. The first front portion 2g is almost cylindrical, while the second front portion 2i is tapered toward the tip 21.

On the other hand, the center electrode 3 has a smaller diameter than that of the resistor 15* The through-hole 6 of the insulator 2 is divided into a firstportir;: 6a (front portion) having a circular cross section in which t:.e c’wt&r electrode 3 is fitted and a second portion 6b (re’i: portion) having a circular czoss section with a larger diam’rar thian that of the firstportion 6a. The terminal metal fixture 13 ar.d the resistor IS are disposed in the second portion 6o, and the center electrode 3 is inserted in the first portion 6a. The center electrode 3 has an outward projection 3c around its periphery near the rear end thereof r with which it is fixed to the electrode . A first portion 6a and a second portion ‘b of thoi through-hole € are connected each other in the first front portion 2g in Fig. 3A, and at the connecting part/ a projection receiving face 6c is tapered or rounded for receiving the projection 3c for fixing the center electrode 3-
The first front portion 2g and the second front portion 2i of the insulator 2 connect at a connecting ps’rt 2h, where a level difference is formed on theouter surface OJ:'the insulator 2. The metal shell 1 has a projection ic on its inner wall at the position meeting the connecting part 2h so that the connecting part 2h fits the projection Ic via a gasket ring 63 thereby to prevent slipping in the axial direction. A gasket ring 62 is disposed between the inner wall of the metal shell 1 and the outer side of the insulator 2 at the' rear of the

flange-like projecting portion 2e, and a gasket ring 60 is
provided in the rear of the gasket ring 62. The space between
the two gaskets 60 and 62 is filled with a filler 61 $uch as
talc. The insulator 2 is inserted into the metal shell 1 toward
the front end thereof, andunder this condition, the rear opening
edge of the metal shell lis pressed inward the gasket 60 to
form a sealing lip Id/ and the metal shell 1 is secured to the
insulator 2.
Figs. 3A and 3B show practical examples of the insulator
2 • The ranges of dimensions of these insulators are as follows.
Total length LI: 30 to 75 mm;
Length L2 of the first front portion 2g; 0 to 30 mm (exclusive
of the connecting part 2f to the projecting portion 2e and
inclusive of the connecting part 2h to the second front portion
2i);
Length L3 of the second front portion 21: 2 to 27 mm;
Outer diameter Dl of the rear portion 2b; 9 to 13' mm;
Outer diameter D2 of the projecting portion 2e: 11 to 16 mm;
■ Outer diameter D3 of the first front portion 2g: 5 to 11 mm;
Outer base diameter D4 of the second front portion 2i: 3 to
8 mm;
Outer tip diameter D5 of the second front portion 2i (where
the outer circumference at the tip is rounded or beveled, the
outer diameter is measured at the base of the rounded or beveled
part in a cross section containing the center axial line 0) ;

2.5 to 7 mm;
Inner diameter D6 of the $econd portion 6b of the through-hole
6: 2 to 5 mm;
Inner diameter D7 of the first portion 6a of the through’-hole
6: 1 to 3-5 mm;
Thickness tl of the first front portion 2g: 0.5 to 4.5 mm;
Thickness t2 at the base of the second front portion,2i (the
thickness in the direction perpendicular to the center axial
line 0): 0,3 to 3,5 mm;
Thickness t3 at the tip of the second front portion 2i {the
thickness in the direction perpendicular to the center axial
line 0; where the outer circumference at the tip is rounded
or beveled, the thickness is measured at the base of the rounded
or beveled part in a cross section containing the center axial
line 0): 0-2 to 3 mm; and
Average thickness tA ( (t2-f-t3)/2) of the second front portion
2i: 0.25 to 3.25 mm.
In Fig* 1/ a length LQ of the portion 2k of the insulator 2 which projects over the rear end of the metal shell 1, is 23 to 27 mm' (e.g., about 25 mm) . In a vertical cross section containing the center axial line 0 of the insulator 2 on the outer contour of the projecting portion 2k of the insulator 2, the length LP of the portion 2k as measured along the profile of the insulator 2 is 26 to 32 mm (e.g., about 29 mm) starting from a position corresponding to the rear end of the metal shell

1/ through the surface of the corrugations 2C/ to the rear end of the insulator 2.
The insulator 2 shown in Fig. 3A has the following dimensions. LI = ca- 60 wun’ L2 - ca* 10 mm, L3 = ca. 14 mm, Dl « ca, 11 mm, D2 = ca. 13 mm, D3 = ca* 7.3 mm, D4 = 5*3 mm, D5 == 4,3 mm, D6 - 3.9 mm, D7 = 2.6 mm, tl - 3,3 mm, t2 = 1-4 mm, t3 = 0.9 mm, and tA =1,15 mm.
The insulator 2 shown in Fig. 3B is designed to have slightly larger outer diameters in its first and second front portions 2g and 2i than in the example shown in Fig. 3A* It has the following dimensions- LI « ca* 60 mm, L2 = ca. 10 mm, L3 = ca. 14 mm, Dl = ca* 11 mm, D2 = ca. 13 mm, D3 = ca* 9.2 mm, D4 « 6.9 mm, D5 = 5,1 mm, D6 * 3-9 mm, D7 « 2,7 mm, tl = 3.3 mm, t2 = 2*1 mm, t3 = 1-2.mm, and tA » 1.65 mm.
As shown in Fig. 2, the glaze layer 2d is formed on’ the outer surface of the insulator 2, more specifically, on the outer peripheral surface of the rear portion 2b inclusive of the corrugated part 2c, The glaae layer 2d has a thickness of 7 to 150 pm, preferably 10 to 50 ym* As -shown in Fig* 1, the glaze layer 2d formed on the rear portion 2b extends in the front direction farther from the rear end of the metal shell 1 to a predetermined length, while the rear side extends till the rear end edge of the rear portion 2b.
The glaze layer 2dhas anyone of the compositions explained in the columns of the means for solving the problems, works

and effects* As the critical meaning in the composition range of each component has been referred to in detail, no repetition will be made herein. The thickness tg (average value) of the glaze layer 2d on the outer circumference of thr’ base of the rear portion 2b (the cylindrical and non-rorr’agated outer circumference part 2c projecting downward from thf-j metal shell 1) is 7 to So pm. The corrugations 2c may be omitted* -, In this case, the average thickness of the glaze layer 2d on the area from the rear end of the metal shell 1 up to 50% of the projecting length LQ of the main part lb is taken as tg.
The ground electrode 4 and the core 3a of the center electrode are made of an Ni alloy. The core 3a of the center electrode 3 is buried inside with a core 3b comprising Cu or Cu alloy for accelerating heat dissipation. An ignition part 31 and an opposite ignition part 32 are mainly made of a noble metal alloy based on one or two kinds or more of Ix, Pt and Rh- The core 3a of the center electrode 3 is reduced in diameter at a front end and is formed to be flat at the front face, to which a disk made of the alloy composing the ignition part is superposed, and the periphery of the joint is weldi’d by a laser welding, electron beam welding, or resistance welding to form a welded part W, thereby constructing the ignition part 31. The opposite ignition part 32 positions a tip to the ground electrode 4 at the position facing the ignition part 31, and the periphery of the joint is welded to form a similar welded

part W along an outer edge part* The tips are prepared by a molten metal comprising alloying components at a predetermined ratio or forming and sintering an alloy powder or a mixed powder of metals having a predetermined ratio. At least one of the ignitionpart 31 and the opposite ignitionparr 32 L.aybe omitted. The spark plug 100 can be produced as follows- In preparing the insulator 2, an alumina powder is mixed with raw material powders of a Si component/ Ca component/ Mg component/ Ba component/ and B component in such a mixing ratio as to give the aforementioned composition after sintering, Mnd the mixed powder is mixed with a prescribed amount of a tinder (e.g./ PVA) and a water to prepare a slurry- The raw material powders include, for example, SiOa powder as the Si component/ CaCOs powder as the Ca component/ MgO powder as the Mg component/ BaC05 as the Ba component/ and H3PO3 as to the B component. 'H3BO3 may be added in the form of a solution. ■
A slurry is spray-dried into granules for forraing a base, and the base forming granules are rubber-pressed into a pressed body a prototype of the insulator. The formed body is processed on an outer side by grinding to the contour of the insulator 2 shown in Fig. 1, and then baked 1400 to 1600*C to obtain the insulator 2*
The glaze slurry is prepared as follows.
Raw material powders as sources of Si, B, Zn, Ba/ and alkaline components (Na, K, Li) (for example/ Si02 powder for

the Si component/ H3PO3 powder for the B component, Zno powder for the Zn component/ BaCOs powder for the Ba component, Na2C03 powder for the Na component, K2CO3 powder for the K component/ and Li2C03 powder for the hi component) are mixed for obtaining a predetermined composition* The mixed powder :s heated and melted 1000 to 1500*’0/ and thrown into the water to rapidly cool for vitrification/ followed by grinding to prepare a glaze fritz. The glaze fritz is mixed with appropriate amoiints of claymineral, such as kaolin or gairome clay/ andorganicbinder/ and the water is added thereto to prepare the glaze slurry.
As shown in Fig. 7, the glaze slurry S is sprayed from a nozzle N to coat a requisite surface of the insulator 2/ thereby to form a coated layer 2d' of the glaze slurry as* the piled layer of the glaze powder.
The center electrode 3 and l:he terminal metal fixture 13 are fitted in the insulator 2 formed with the glaze slurry coated layer 2d' as well as the resistor IS and the electrically conductive glass seal layers 16/ 17 are formed as follows. As shown in Fig. BA, the center electrode 3 is inserted into the first portion 6a of the through-hole 6, Then a conductive glass powder H is filled in the through-hole 6 as shown in Fig, 8B. The powder H is preliminary compressed by pressing a press bar 28 into the through-hole 6 to form a first conductive glass powder layer 26. A raw material powder for a resistor composition is filled and preliminary compressed in the same

manner, so that/ as shown in Fig. 8D, the first conductive glass powder 26, the resistor composition powder layer 25 and a second conductive glass powder layer 27 are laminated from the center electrode 3 (lower side) into the through-holo:: 6.
An assembled structure PA is formed where the terminal metal fixture 13 is disposed from the upper part into the through-hole 6 as shown in Fig* 9A- The assenibled structure PA is put into a heating oven and heated at a predetermined temperature of 800 to gsCc being above the glass softening point, and then the terminal metal fixture 13 is pressed into the through’hole 6 from a $ide opposite to the center electrode 3 so as to press the superposed layers 25 to 27 in the axial direction. Thereby, as seen in Fig. 9B/ the layers are each compressed and sintered to become a conductive glass seal layer 16, a resistor 15, and a conductive glass seal layer 17 (the above is the glass sealing step) -
If the softening point of the glaze powder contained in the glaze slurry coated layer 2d' is set to be 600 to 700*'C,
r
the layer 2d' can be baked as shown in Figs, 9A .’nd 93, at the same time as the heating in the above glass sealing steP/ into the glaze layer 2d- Since the heating temperature of the glass sealing step is selected from the relatively low temperature of 800 to.9S0’C, oxidation to surfaces of the center electrode 3 and the terminal metal fixture 13 can be made less*
If a burner type gas furnace is used as the heating oven

(which also serves as the gla’e baking oven), a heating atmosphere contains relatively much steam as a combustion product. If the glaze composition containing the B component 40 mol% or less is used, the fluidity when baking the glaze can be secured even in such an atmosphere, and it is possible to form the glaze layer of smooth and homogeneous substance and excellent in the insulation.
After the glass sealing step, the metal shell 1, the ground electrode 4 and others are fitted on the structure PA to complete spark plug 100 shown in Fig, 1. The spark plug 100 is screwed into an engine block using the thread 7 thereof and used as a spark source to ignite an air/fuel mixture supplied to a combustion chamber. A high-tension cable or an ignition coil is connected to the spark plug 100 by means of a rubber cap

RC (comprising, e.g*, silicone rubber) : The rubber cap RC has a smaller hole diameter than the outer diameter Dl (Fig. 3) of the rear portion 2b by about 0*5 to 1,0 mm. The rear portion 2b is pressed into the rubber cap while elastically expanding the hole until it is covered therewith to its base.
As a result, the rubber cap RC comes into close contact with the outer surface of the rear portion 2b to function as an insulating cover for preventing flashover.
By the way, the spark plug of the invention is not limited to the type shown in Fig. 1, but for example as shown in Fig. 4, the tip of the ground electrode 4 is made to face the side

of the center electrode 3 to form an ignition gap £, Further, as shown in Fig. 5, a semi’-pl’nar discharge type spark plug is also useful where the front end of the insulator 2 is advanced between the side of the center electrode 3 and the front end of the ground electrode 4*
Examples For conf iriaation of the effects according to the invention, the following experiments were carried out. (Experiment 1)
The insulator 2 was made as follows. Alumina powder (alumina content: 95mol%; Na content (asNasO) ; O,lmol%; average particle size: 3.0 ym) was mixed at a predetermined mixing ratio with Si02 (purity; 99’5%; average particle size: 1,5 urn), CaCOs (purity; 99*9%; average particle size: 2.0 ym) , MgO (purity: 99*5%; average particle size: 2pm) BaC03 (purity: 99,5%; average particle size: 1*5 pm), HaBOa (purity; 99.0%; average particle size 1-5 ]m) r and ZnO (purity: 99.5%, average particle size: 2.0 ym) . To 100 parts by weight of the resulting mixed powder were added 3 parts by weight of PVA as a hydrophilic binder and 103 parts by weight of water, and the mixture was kneaded to prepare a slurry*
The resulting slurry was spray’dried into spherical granules, which were sieved to obtain fraction of 50 to 100 pm. The granules were formed under a pressure of 50 MPa by a known rubber-pressingmethod. The outer surface of the formed

body wa$ machined with the grinder into a predetermined figure and baked at 1550’*0 to obtain the insulator 2. The X-ray fluorescence analysis revealed that the insulator 2 had the following composition.
Al component (as AI5O3) ; 94.9 niol%; Si component (as Si02) : 2 A inol%; Ca component (as CaO) : 1,9 iuol%; Mg component (as MgO) : 0.1 mol%; Ba component (as BaO) : 0,4 mol%; and B component (as B203) : 0.3 mol%.
The insulator 2 shown in Fig. 3A has the following dimensions. LI = ca.60 mm, L2 *= ca.S mm, L3 - ca-14 mm/ Dl = ca.lO mm, D2 ** ca*13 mm, D3 = ca-7 mm, D4 = 5.5 mm, D5 = 4.5 mia, D6 = 4 mm, D7 « 2.6 mm, tl = 1.5 mm, t2 = 1.45 mm, t3 = 1.25 um, and tA = 1.35 mm. In Fig. 1, a length LQ of the portion 2k of the insulator 2 which projects over the rear end of the metal shell 1, is 25 mm- In a verci’al cross section containing the center axial line 0 of the insulator 2 on the outer contour of the projecting pcrtiovY 2k of the insulator 2, the length LP of the portion 2k as measured along the profile of the insulator 2 is 29mm/ starting from a position corresponding to the rear end of the metal shell 1, through the surface of the corrugations 2c, to the rear end of the insulator 2.
Si02powder (purity: 99.5%), AlsO’powder (purity: 99.5%),

H3B03 powder (purity; 98,5%), NazCOa powder (purity: 99.5%), K2CO3 powder (purity: 99%), LizCOs powder (purity: 99%), BaSO’ powder (purity: 99,5%), SrCOa powder (purity: 99%), ZnO powder (purity:.99.5%), Mo03powder (purity: 99%)’ FeiO’pov.'der (purity: 99%), WO3powder (purity; 99%), Ni’O’powder (purity: 99%), CosO’ powder (purity: 99%), Mn02 powder (purity: 99%), CaO powder (purity: 99,5%), TiOz powder (purity; 99.5,%), ZrOz powder (purity: 99.5%), HfOa powder (purity: 99§), MgO powder (purity: 99.5%), SbsOs powder (purity: 99%), BiaOs powder (purity; 99%), Sn02powder (purity: 99.5%), P2O5powder (purity: 99%), CuO powder (purity: 99%), Ce02 powder (purity: 99.5%), and Cr203 powder (purity: 99-5%) were mixed. The mxture was melted 1000 to 1500**C, and the melt was poured into the water and rapidly cooled for vitrification/ followed by grinding in
*
an alumina pot mill to powder of 50 pm or smaller. Three parts by weight of New Zealand kaolin and 2 parts by weight of PVA as an organic binder were mixed into 100 parts by weight of the glaze powder, and the mixture was kneaded with 100 parts by weight of the water to prepare the glaze slurry.
The glaze slurry was sprayed on the insulator 2 from the spray nozzle as illustrated in Fig. 7, and dried to form the coated layer 2d' of the glaze slurry having a coated thickness of about 100 Jim. Several kinds of the sparkplug 100 were produced by using the insulator 2 through the process explained with reference to Figs. 11 to 12* The outer diameter of the thread

7 was 14 mm- The resistor 13 was made of the mixed powder consisting of B203-Si02-BaO-LiO2 glass powder, ZxQz powder, carbon black powder, Ti02 powder, and metallic Al powder. The electrically conductive glass seal layers 16, 17 vrere made of the mixed powder consisting of BsOa-SiOa-NasO glas:-. powder, Cu powder/ Fe powder, and Fe-B powder. The heating temperature for the glass sealing, i.e., the glaze baking temperajture was set at 900’C
On the other hand, such glaze samples were produced which were not pulverized but solidified in block. The block-like sample was confirmed by the X’ray diffraction to be a vitrified (amorphous) state.
The experiments were performed as follows.
(1) Chemical composition analysis
•t
The X-ray fluorescence analysis was conducted * ' The analyzed value per each sample (in terms of oxide) w&s shown in Tables 1 to 6. The analytical results obtained by EPMA on the glaze layer 2d formed on the insulator were almost in agreement with the results measured with the block-11 ks samples *
(2) Thermal expansion coefficient
The specimen ofSmmxSmmxSmm was cur out from the block-'like sample, and measured with the known dilatometer method at the temperature ranging 20 to SSO’'C. The same measurement was made at the same size of the specimen cut out from the insulator 2* As a result, the value was 73 x 10"Vc.

(3) Softening point
The powder sample weighing 50 mg was subjected to the differential thermal analysis, and the heating was measured from a room temperature • The second endotherT.ic peek was taken as the softening point•
Withrespect to the respective spark pluq’;, the insulation resistance at 500*’C was evaluated at the applied voltage lOOOV through the process explained with reference to Figs* 8A to 8D. Further, the appearance .of the glaze layer 2d formed on the insulator 2 was visually observed. The film thickness of the glaze layer on the outer circumference of the base edge part of the insulator was measured in the cross section by the SEM observation. In judgements of the outer appearance of the glaze layer, no abnormality seen in luster and transparency is excellent (00) , and slight crimping or devitrification, though being within an allowable range is good (0) . ' Apparent abnormality is specifically shown within the columi; as to kinds of abnormalities- The above mentioned results are shown in Tables 1 'to 6.









According to the results’ depending on the compositions of the glaze of the invention, although no Pb is substantially contained, theglazemaybebakedat relatively low temperatures, sufficient insulating properties are scar’s, rend the outer appearance of the baked glaze faces are almost .satisfied.
The entire disclosure of each and every foreign patent application from which the benefit of foreign priority-has been claimed in the present application is incorporated herein by reference/ as if fully set forth herein-



We claim:
1, A spark plug comprising:
a center electrode;
a metal shell; and
B.T\ alumina ceramic insulator disposed: n the center electrode and the metal shell, wherein at least part of the surface of the insulator is covered with a glaze layer comprising oxides’
wherein the glaze layer comprises:
1 tool% or less of a Pb component in terms of PbO;
25 to 45 mol% of a Si component in terms of SiOz;
20 to 4 0 mol% of a B component in terms of BiOs;
5 to 25 mol% of a 2n component in terms of ZnO;
0 - 5 to 15mol% in total of at least one of Ba and Sr components in terms of BaO and SrO/ respectively;
5 to 10 mol% in total of at least one alkaline metal component of Na’. K and Li, in terms of NaaO, K2O,/,and Li2/ respectively/ wherein K is essential; and
0.5 to 5 mole% in total of at least one of tiOr W, Ni, Cuff Fe and Mn in terms of MoOa WO3, Ni304, C03O4, FezOz, and KnOZf respectively.
2. - The spark plug according to claim 1, wherein K has a highest content in the at least one alkaline metal component in the glaze layer.

3 • The spark plug according to clarion 1, wherein the glaze layer further comprises 0,5 to 5 mol% in total of at least one of Ti/ Zr and Hf in terms of Ti02/ ZrOa and HfCz, respectively.
4- A spark plug comprising:
a center electrode;
a metal shell; and
an alumina ceramic insulator disposed between the center electrode and the metal shell/ wherein at least part of the surface of the insulator is coveredwithaglaze layer comprising oxides/
wherein the glaze layer comprises:
1 mol% or less of a Pb component in terms of PbO;
25 to 45 mol% of a Si component in terms of SiOz;
20 to 40 mol% of a B component in. terns of B2O3;
5 to 25 mol% of a 2n component in terms of ZnO;
0-5tol5mol%intotalof at least one of Bands components in terms of BaO and SrO/ respectively;
5 to 10 mol% in total of at least one alkaline metal component of Na, K and Li, in terms of Na’O, K2O, and Liz, respectively;
0-’.S to 5 mol% in total of at least one of Ti, Zr and Hf in terms of TiOs’ 2rOa and HfOa, respectively; and
0.5 to 5 mole% in total of at least one of Mo, W, Ni,

Co’ Fe and Mn in terms of MoOS/ WO3, Ni30 C03O4, FeaOS/ and Mn02, respectively.
5. The spark plug according to any one of claims 1 to 4, wherein the glaze layer comprises three components of Li, Na and K as the at least one alkaline metal components’ and has a composition which satisfies the relationship of:
NNa20 £ NLizO of LiaO, NNa20 is a mol content of the Na component in terms
of NaaO, and NK2O is a mol content of the K competent in terms
of K2O.
6. A spark plug comprising:
a center electrode;
a metal shell; and
an alumina ceramic insulator disposed between the center electrode and the metal shell, wherein at least part of the surface of the insulator is covered with a glaze layers comprising oxides,
wherein the glaze layer comprises: 1 mol% or less of a Pb component in terms of PbO; at least one of Si ands components as a glass skelton structure; and three components of Li, Na and K as alkaline metal components, and the glaze layer has a composition which satisfies the relationship of:

NNajO 7. The sparkplug according to claim 1, '.herein the glaze layer contains the K component and at least z’-’o alkaline metal components among the Li, Na and K component-’, and satisfies the relationship: 0.4 8 . The spark plug according to claim 4, wherein’ the gla2:e layer contains the K component and at least two alkaline metal components among the Li, Na and K components/ and. satisfies the relationship: 0.4 9* The sparkplug according to claim 6, wherein the glaze

layer contains the K component and at least two alkaline metal components among the Li, Na and K components, and satisfies the relationship: 0.4 10. The spark plug according to clip. 1, wherein the glaze layer contains the Li component and at alkaline metal components among the Li/ Na and K components, and satisfies the relationship: 0.2 11, The spark plug according to claim 4, wherein the glaze layer contains the Li component and at least two alkaline metal components among the Li/ Na and components, and satisfies the relationship: 0.2
12. The spark piling according to claim 6, wherein the glaze layer contains the Li component and at least two alkaline metal components among the Li, Na andKcomponer-::s, and satisfies the relationship: 0,2 13. The spark plug according to claim 1, wherein the glaze layer contains the Zn component and the at legist one of Ba and Sr components in an amount of 10 to 3 0 mol% in total in terms of ZnO, BaO and SrO, respectively.
14. The spark plug according to claim 4, wherein the glaze layer contains the Zn component and the at least one of Ba and Sr components in an amount of 10 to 30 moil% in total in terms of ZnO’, BaO and SrO, respectively.
15. The spark plug according to claim 6, v/herein the glaze layer contains the Zn component and the at least one of Ba and Sr components in an amount of 10 to 30 mol% in total in terms of ZnO, BaO and SrO/ respectively.

16. The spark plug according to claim 1, wherein the glaze layer further comprises 0-1 to 15 mol% in total of at least one of 0*1 to 10 mol% of an Al component in terms of AI2O3, 0.1 to 10 mol% of a Ca component in terms of CaO/ and 0,1 to 10 mol% of a Mg component in terms of MgO,
17. The spark plug according to claim 4/ wherein the glaze layer further comprises 0.1 to 15 mol% in total of at least one of 0.1 to 10 mol% of an Al component in terms of AI2O3/ 0.1 to 10 mol% of a Ca component in terms of CaO, and 0.1 to 10 mol% of a Mg component in terms of MgO.
18, The spark plug according to claim 6, wherein the glaze layer further comprises 0.1 to 15 mol% in total of at least one of O.l to 10 mol% of an Al component in terms of AI2O3/ 0,1 to 10 mol% of a Ca component in terms of GaO, and 0-1 to 10 mol% of a Mg component in terms of MgO, -
19, The spark plug according to claim i, wherein the glade layer further comprises 5 mol? or less in total of at least of Bi/ Sn, Sb, P, Cu, Ce and Cr in terms of BizO’, Sn02/ Sb205, PsOs, CuO/ CeOz and Cr20j, respectively.
20. The spark plug according to claim 4, wherein the gla5:e layer further comprises 5 mol% or less in total of at

least of Bi/ Sufi Sb, P, Cu, Ce and Cr in terms of BisOa, SnOi, SbzOsr PiOi, CuO/ Ce02 and Cr203, respectively.
21, The spark plug according to claim 6, wherein the glaze layer further comprises 5 mol% or less in total of at least of Bi/ Sn, Sb/ P/ Cu/ Ce and Cr in teams of Bi203, SnOZf SbsOs, PsOs/ CuO/ CeOs and CraOa, respectively*
22* The spark plug according to claim 1, wherein the insulator is formed with a projection part in an outer circumferential direction at an axially central position thereof,
taking/ as a front side/ a side directing toward the front end of the center electrode in the axial direction/ a cylindrical face is shaped in the outer circumferential face at the base portion of the insulator main body in the neighborhood of a rear side opposite the projection part, and
the outer circumferential face at the base portion is covered with the glaze layer formed with a film thickness ranging 7 to 50 ]m..
23- The spark plug according to claim 4, wherein the insulator is formed with a projection part in an outer circumferential direction at an axially central position
thereof/

taking’ as a front side, a side directing toward the front end of the center electrode in the axial direction/ a cylindrical face is shaped in the outer circumferential face at the base portion of the insulator main body in the neighborhood of a rear side opposite the projection part/ and
the outer circumferential face at the base portion is covered with the glaze layer formed with a filmthicknessLranging 7 to 50 urn
24. The spark plug according to claim 6’ wherein the insulator is formed with a projection part in an outer circumferential direction at an axially central position thereof,
taking, as a front side, a side directing toward the front end of the center electrode in the axial direction, a cylindrical face is shaped in the outer circumferential face at the base portion of the insulator main body in the neighborhood of a rear side opposite the projection part, and
the outer circumferential face at the base portion is covered with the glaze layer formed with a film thickness ranging 7 to 50 um-
25. The spark plug according to claim 1, which comprises one of: a terminal metal fixture and the center electrode as one body, in a through hole of the insulator; and a terminal

metal fixture and the center electrode provided separately from the center electrode via a conductive bonding layer, and
an insulation resistant value is 400 MCi or more/ which is measured by keeping the whole of the spark plug at about 500and passing a current between the terminal metal fixture and the metal shell via the insulator.
26. The sparkplug according to claim 4, which comprises one of: a terminal metal fixture and the center electrode as one body, in a through hole of the insulator; and a terminal metal fixture and the center electrode provided separately from the center electrode via a conductive bonding layer, and
an insulation resistant value is 400 MO or more/ which
is measured by keeping the whole of the spark plug at about
SOO’C and passing a current between the terminal metal fixture
and the metal shell via the insulator. . ,
27. The spark plug according to claim 6, which comprises one cf: a terminal metal fixture and the center electrode as one body, in a through hole of the insulator; and a terminal metal fixture and the center electrode provided separately from the center electrode via a conductive bonding layer, and
an insulation resistant value is 400 MQ or more/ which is measured by keeping the whole of the spark plug at about 500"c; and passing a current between the terminal metal fixture

and the metal shell via the insulator.
28. The spark plug according to claim 1, wherein the insulator comprises an alumina insulating material containing 85 to 98 mol% of an Al component in terms of Al203’ and the glaze layer has an average thermal expansion coefficient at the temperature ranging 20 to SSO’C of 5 x lO'‘/’'C to 8.5
29, The spark plug according to claim 4, wherein the insulator comprises an alumina insulating material containing 85 to 98 mol% of an Al component in terms of A1203, and the glaze layer has an average thermal expansion coefficient at the temperature ranging 20 to 350’C of 5 x lO'‘/'‘Q to 8*5 x lO’’C,
30. The spark plug according to claim 6, wherein the insulator comprises an alumina insulating material containing 85 to 98 mol% of an Al component in terms of AI2O3, and the glaze layer has an average thermal expansion coefficient at the temperature ranging 20 to 350’C of 5 x 10"V to 8.5 x lO’’C.
31- The spark plug according to claim 1, wherein the glaze layer has a softening point of 520 to 620*’0.
32. The spark plug according to claim 4, wherein the glaze layer has a softening point of 520 to 620'C-

Documents:


Patent Number 211912
Indian Patent Application Number 435/MAS/2001
PG Journal Number 02/2008
Publication Date 11-Jan-2008
Grant Date 13-Nov-2007
Date of Filing 31-May-2001
Name of Patentee M/S. NGK SPARK PLUG CO., LTD
Applicant Address 14-18 TAKATSUJI-CHO, MIZUHO-KU, NAGOYA-SHI, AICHI,
Inventors:
# Inventor's Name Inventor's Address
1 KENICHI NISHIKWA 14-18 TAKATSUJI-CHO, MIZUHO-KU, NAGOYA-SHI, AICHI,
2 YOSHIHIDE KOUGE 14-18 TAKATSUJI-CHO, MIZUHO-KU, NAGOYA-SHI, AICHI,
3 MAKOTO SUGIMOTO 14-18 TAKATSUJI-CHO, MIZUHO-KU, NAGOYA-SHI, AICHI,
PCT International Classification Number H01T 13/00
PCT International Application Number N/A
PCT International Filing date
PCT Conventions:
# PCT Application Number Date of Convention Priority Country
1 2000-163846 2000-05-31 Japan
2 2001-099528 2001-03-30 Japan