Spark plug resistance element with increased ZrSiO.SUB.4 .phase fraction

Abstract

A method for manufacturing a spark plug. The method includes: furnishing an insulator; introducing into the insulator a material mixture that is configured to constitute a resistor paste, the material mixture containing ZrO.sub.2 and SiO.sub.2; heating the insulator and the material mixture present therein to a temperature T of at least 870° C., so that ZrO.sub.2 and SiO.sub.2 in the material mixture react at least partly to ZrSiO.sub.4.

Claims

1. A spark plug, comprising: a housing; an insulator disposed in the housing; a center electrode disposed in the insulator; a terminal pin disposed in the insulator; a resistance element disposed in the insulator, the resistance element being disposed physically between the center electrode and the terminal pin, the resistance element electrically connecting the center electrode to the terminal pin, the resistance element including a resistor paste, the resistor paste being made up of a material mixture that contains ZrO.sub.2, SiO.sub.2, and ZrSiO.sub.4; and a ground electrode that is disposed on a combustion-chamber-side end face of the housing and, together with the center electrode, forms a spark gap; wherein at least one of the following: (I) the material mixture of the resistor paste is such that (1) in the material mixture, $\frac{w\left({\mathrm{ZrSiO}}_4\right)}{\left(w\left({\mathrm{ZrSiO}}_4\right)+w\left({\mathrm{ZrO}}_2\right)\right)}$ is at least 40%, (2) w(ZrSiO.sub.4) is a proportion, by weight percent, of ZrSiO.sub.4 in the resistor paste, and (3) w(ZrO.sub.2) is a proportion, by weight percent, of ZrO.sub.2 in the resistor paste; and (II) the resistor paste is structured so that a subjection of the resistor paste in the spark plug to 300 hours of operation of the spark plug would not cause in the resistor paste a relative resistance change as high as 50%.

2. The spark plug as recited in claim 1, wherein: in the material mixture, $\frac{w\left({\mathrm{ZrSiO}}_4\right)}{\left(w\left({\mathrm{ZrSiO}}_4\right)+w\left({\mathrm{ZrO}}_2\right)\right)}$ is at least 40%; w(ZrSiO.sub.4) is the proportion, by weight percent, of ZrSiO.sub.4 in the resistor paste; and w(ZrO.sub.2) is a proportion, by weight percent, of ZrO.sub.2 in the resistor paste.

3. The spark plug as recited in claim 1, wherein the resistor paste is structured so that the subjection of the resistor paste in the spark plug to 300 hours of operation of the spark plug would not cause in the resistor paste a relative resistance change as high as 50%.

4. The spark plug as recited in claim 1, wherein: in the material mixture, $\frac{w\left({\mathrm{ZrSiO}}_4\right)}{\left(w\left({\mathrm{ZrSiO}}_4\right)+w\left({\mathrm{ZrO}}_2\right)\right)}$ is equal to 40%; w(ZrSiO.sub.4) is the proportion, by weight percent, of ZrSiO.sub.4 in the resistor paste; and w(ZrO.sub.2) is a proportion, by weight percent, of ZrO.sub.2 in the resistor paste.

5. The spark plug as recited in claim 1, wherein: in the material mixture, $\frac{w\left({\mathrm{ZrSiO}}_4\right)}{\left(w\left({\mathrm{ZrSiO}}_4\right)+w\left({\mathrm{ZrO}}_2\right)\right)}$ is at least 50%; w(ZrSiO.sub.4) is the proportion, by weight percent, of ZrSiO.sub.4 in the resistor paste; and w(ZrO.sub.2) is a proportion, by weight percent, of ZrO.sub.2 in the resistor paste.

6. The spark plug as recited in claim 1, wherein: in the material mixture, $\frac{w\left({\mathrm{ZrSiO}}_4\right)}{\left(w\left({\mathrm{ZrSiO}}_4\right)+w\left({\mathrm{ZrO}}_2\right)\right)}$ is in a range of 50-55%; w(ZrSiO.sub.4) is the proportion, by weight percent, of ZrSiO.sub.4 in the resistor paste; and w(ZrO.sub.2) is a proportion, by weight percent, of ZrO.sub.2 in the resistor paste.

7. A method for manufacturing a spark plug, comprising the following steps: furnishing an insulator; introducing into the insulator a material mixture that is configured to constitute a resistor paste, the material mixture containing ZrO.sub.2 and SiO.sub.2; and heating the insulator and the material mixture in the insulator to a temperature of at least 870° C., so that ZrO.sub.2 and SiO.sub.2 in the material mixture react at least partly to ZrSiO.sub.4 thereby forming a resulting material mixture, wherein at least one of: (I) the resulting material mixture is such that (1) in the resulting material mixture, $\frac{w\left({\mathrm{ZrSiO}}_4\right)}{\left(w\left({\mathrm{ZrSiO}}_4\right)+w\left({\mathrm{ZrO}}_2\right)\right)}$ is at least 40%, (2) w(ZrSiO.sub.4) is a proportion, by weight percent, of ZrSiO.sub.4 in the resistor paste, and (3) w(ZrO.sub.2) is a proportion, by weight percent, of ZrO.sub.2 in the resistor paste; and (II) the resulting material mixture is structured so that a subjection of the resulting material mixture in the spark plug to 300 hours of operation of the spark plug would not cause in the resulting material mixture a relative resistance change as high as 50%.

8. The method as recited in claim 7, wherein the resulting material mixture is such that: in the resulting material mixture, $\frac{w\left({\mathrm{ZrSiO}}_4\right)}{\left(w\left({\mathrm{ZrSiO}}_4\right)+w\left({\mathrm{ZrO}}_2\right)\right)}$ is at least 40%; w(ZrSiO.sub.4) is the proportion, by weight percent, of ZrSiO.sub.4 in the resistor paste; and w(ZrO.sub.2) is a proportion, by weight percent, of ZrO.sub.2 in the resistor paste.

9. The method as recited in claim 7, wherein the temperature is in a range from 870° C. to 965° C.

10. The method as recited in claim 7, wherein the temperature is maintained for a time, the time being with a range from and including 15 minutes to and including 60 minutes, such that the higher the temperature of the range that is used, the shorter the time can be.

11. The method as recited in claim 7, wherein the resulting material mixture is structured so that the subjection of the resulting material mixture in the spark plug to 300 hours of operation of the spark plug would not cause in the resulting material mixture a relative resistance change as high as 50%.

12. The method as recited in claim 7, wherein the resulting material mixture is such that: in the resulting material mixture, $\frac{w\left({\mathrm{ZrSiO}}_4\right)}{\left(w\left({\mathrm{ZrSiO}}_4\right)+w\left({\mathrm{ZrO}}_2\right)\right)}$ is equal to 40%; w(ZrSiO.sub.4) is the proportion, by weight percent, of ZrSiO.sub.4 in the resistor paste; and w(ZrO.sub.2) is a proportion, by weight percent, of ZrO.sub.2 in the resistor paste.

13. The method as recited in claim 7, wherein the resulting material mixture is such that: in the resulting material mixture, $\frac{w\left({\mathrm{ZrSiO}}_4\right)}{\left(w\left({\mathrm{ZrSiO}}_4\right)+w\left({\mathrm{ZrO}}_2\right)\right)}$ is at least 50%; w(ZrSiO.sub.4) is the proportion, by weight percent, of ZrSiO.sub.4 in the resistor paste; and w(ZrO.sub.2) is a proportion, by weight percent, of ZrO.sub.2 in the resistor paste.

14. The method as recited in claim 7, wherein the resulting material mixture is such that: in the resulting material mixture, $\frac{w\left({\mathrm{ZrSiO}}_4\right)}{\left(w\left({\mathrm{ZrSiO}}_4\right)+w\left({\mathrm{ZrO}}_2\right)\right)}$ is in a range of 50-55%; w(ZrSiO.sub.4) is the proportion, by weight percent, of ZrSiO.sub.4 in the resistor paste; and w(ZrO.sub.2) is a proportion, by weight percent, of ZrO.sub.2 in the resistor paste.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows an example of a spark plug.

(2) FIG. 2 shows the transformation rate q as a function of temperature T for a constant sample heating time t=30 minutes.

(3) FIG. 3 shows the transformation rate q as a function of heating time t for comparative temperatures T=890° C. and T=950° C.

(4) FIG. 4 shows the resistance of various samples after various long-term testing times.

(5) FIG. 5 schematically shows the manufacturing method for the spark plug according to the present invention.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

(6) FIG. 1 is a semi-section view of a spark plug 1. Spark plug 1 encompasses a housing 2. An insulator 3 is inserted into housing 2. Housing 2 and insulator 3 each have a bore along their longitudinal axis X. The longitudinal axis of housing 2, the longitudinal axis of insulator 3, and the longitudinal axis of spark plug 1 are coincident. A center electrode 4 is inserted into insulator 3. A terminal pin 8 also extends in insulator 3. Disposed on terminal pin 8 is a terminal nut 9 by way of which spark plug 1 is electrically contactable to a voltage source (not depicted here). Terminal nut 9 constitutes the combustion-chamber-remote end of spark plug 1.

(7) Located between center electrode 4 and terminal pin 8, in insulator 3, is a resistance element 7, also called a “paste.” Resistance element 7 electrically conductively connects center electrode 4 to terminal pin 8. Resistance element 7 is constructed, for example, as a layer system made up of a first contact paste 72a, a resistor paste 71, and a second contact paste 72b. The layers of resistance element 7 differ in terms of their material composition and the electrical resistance resulting therefrom. First contact paste 72a and second contact paste 72b can have different electrical resistances or an identical electrical resistance. Resistance element 7 can also have only one layer of resistor paste or several different layers of resistor paste having different material compositions and resistances.

(8) Insulator 3 rests with a shoulder on a housing seat embodied on the inner housing side. Disposed between the insulator shoulder and the housing seat, in order to seal the air gap between the inner housing side and insulator 3, is an internal seal 10 that plastically deforms as insulator 3 is tightened in housing 2, and thereby seals the air gap.

(9) A ground electrode 5 is disposed electrically conductively on housing 2 on the latter's combustion-chamber-side end face. Ground electrode 5 and center electrode 4 are disposed with respect to one another in such a way that a spark gap, at which the ignition spark is generated, forms between them.

(10) Housing 2 has a shank. A polygon 21, a shrinkage undercut, and a thread 22 are embodied on that shank. Thread 22 serves for threading spark plug 1 into an internal combustion engine. An external sealing element 6 is disposed between thread 22 and polygon 21. In this exemplifying embodiment, external sealing element 6 is embodied as a bellows seal. Alternatively, external sealing element 6 can also be a solid seal.

(11) Table 1summarizes the results of the experiments. Each sample corresponds to a spark plug. Before heating, the resistor pastes of all the samples had the same material composition. It would also have been possible to carry out the transformation experiments on samples that correspond only to the resistance element or resistor paste, i.e. that have the corresponding material composition. In the initial state, i.e. before heating and thus before any possible transformation, the resistor paste contains a ZrO.sub.2— and SiO.sub.2-containing glass. Further constituents of the glass are, for example, B.sub.2O.sub.3, CaO, and Li.sub.2O. Further constituents of the material composition of the resistor paste are, for example, carbons or ceramic particles such as Al.sub.2O.sub.3 or TiO.sub.2.

(12) Each sample was heated for a specific heating time t to a specific temperature T. The proportions of the various materials (ZrO.sub.2 and ZrSiO.sub.4) were then determined by XRD Rietveld analysis. The transformation rate q was determined in turn from that. The transformation rate q is calculated using the following formula:

(13) $q=\frac{w\left({\mathrm{ZrSiO}}_4\right)}{\left(w\left({\mathrm{ZrSiO}}_4\right)+w\left({\mathrm{ZrO}}_2\right)\right)},$

(14) where w(X) is the proportion of material X, in wt %, after transformation, where X is ZrSiO.sub.4 or ZrO.sub.2.

(15) TABLE-US-00001 TABLE 1 Sample T.sub.sample t.sub.heat q no. [° C.] [min] [%] H1 860 30 0 H6 890 10 0 H7 890 15 0 H8 890 20 2 H2 890 30 27 H9 890 30 27 H10 890 60 51 H11 890 120 55 H3 920 30 50 H12 950 5 0 H13 950 10 5 H14 950 20 49 H4 950 30 55 H5 965 30 52

(16) The results from this table are the data basis for the diagrams below in FIG. 2 and FIG. 3.

(17) In FIG. 2, the transformation rate q (in %) is plotted against temperature T (in ° C.). Each data point here stands for a sample that was heated to the respective temperature T for 30 minutes. The dashed line is a theoretical curve that is obtained from the measured data using a simple fitting function q(T)=q.sub.sat*(e.sup.(−T+T.sup.0.sup.)).sup.−1, where T is the oven temperature, q.sub.sat the saturation value of the transformation rate, and T.sub.0 is a fitting parameter. It is apparent from this diagram that for a heating time t of 30 minutes, the transformation q goes to a saturation of 50-55% for a temperature above about 920° C.

(18) FIG. 3 is a diagram in which the transformation rate q (in %) is plotted against heating time t for two samples. The first sample (circles) was heated to 950° C. The second sample (crosses) was heated to 890° C. The transformation rate q was determined, as described above, for both samples after various times. The dashed lines are once again theoretical curves that are obtained from the respective measured data using a simple fitting function q(t)=q.sub.sat*(e.sup.(−t+t.sup.0.sup.)).sup.−1, where t is the heating time, q.sub.sat the saturation value of the transformation rate, and t.sub.0 is a fitting parameter. The result evident from this diagram as well is that the transformation rate q goes to saturation at a value of 50-55%. The diagram also shows that this saturation is reached earlier in time for a higher temperature.

(19) FIG. 4 shows comparative resistance values of spark plugs after specific operating times t. Before the long-term test run began, all the spark plugs had an initial resistance value (circle data point) R(t=0)=1.5 kΩ.

(20) The reference spark plug (diamond data point) and its resistance element were treated in accordance with sample H1 in Table 1, and have a transformation rate q=0%. After more than 350 operating hours the reference spark plug had a resistance R(350 h)=13.7 kΩ, which corresponds to a relative resistance change rW of 813%.

(21) The relative resistance change (rW) is calculated as

(22) $rW=\frac{.Math.R\left(t\right)-R\left(t=0\right).Math.}{R\left(t=0\right)},$

(23) where R(t=0) is the resistance of the spark plug before the long-term test run begins, and R(t) is the resistance of the same spark plug after the operating time t.

(24) The spark plugs in accordance with the present invention (triangle data point) and their resistance elements were treated in accordance with sample H3 in Table 1, and have a transformation rate q=50%. Each data point is a spark plug for which the test, and a subsequent determination of the resistance, were performed. The relative resistance change for all these novel spark plugs is less than 50%. The greatest relative resistance change (rW=33%) is found for the novel spark plug that has a resistance of approximately 1 kΩ after almost 450 operating hours.

(25) FIG. 5 schematically depicts manufacturing method 100 for a spark plug; the individual steps for manufacturing the individual components of spark plug 1, such as the manufacture of insulator 3 or the manufacture and mixing of the material for resistance element 7, are not depicted here.

(26) In a first step 103, insulator 103 of the spark plug is furnished. In a second step 104, the center electrode is inserted into the insulator. In a third step 107, the material for resistance element 7 is introduced into the insulator. Optionally, this step 107 can encompass three sub-steps: introducing contact paste 1072b, introducing resistor paste 1071, introducing second contact paste 1072a; a densification can occur as an intermediate step after each introduction. In a following step 108, terminal pin 8 is inserted into the insulator. In the next step 120, terminal pin 8 and insulator 3 are pressed together and simultaneously heated. The material of resistance element 7 is also heated in that context, so that the transformation of ZrO.sub.2 and SiO.sub.2 into ZrSiO.sub.4 in resistance element 7 or resistor paste 71 takes place in this step 108. The temperature is at least 870° C. Further steps 130 then occur, in which the insulator is joined and connected to the housing, and the electrodes are aligned with one another.