DE1590215C3 - Process for the production of an electrically conductive surface on an insulator consisting essentially of aluminum oxide - Google Patents

Description

The invention relates to a method for producing an electrically conductive surface on an im essential insulator made of aluminum oxide, especially for jet engine igniters where on the desired surface area a copper-containing coating is applied and this is subjected to a temperature treatment. It is common practice to place a semiconducting material between and in contact with the electrodes of a Provide jet engine ignition device to reduce the response or threshold voltage that is necessary to induce a spark between the electrodes. Used in some cases one is a disk of semiconducting material that is inserted between and in abutment with the electrodes was, while in still other cases a thin coating, also referred to as a cast coating, on one electrical insulator body of the type was used that a thin semiconducting layer of the coating meets the gap between the electrodes of the ignition device and bridges it. The essentially insulator bodies used in all cases contained approximately 85 to 95% aluminum oxide and were made Fired to hard, erosion-resistant bodies, which, however, had a few percent defects. In this In connection with this, it should be pointed out that here and everywhere where percentages or parts are spoken of is, percentages by weight or parts by weight are meant.

The cast coatings previously used had an oxide of either iron or copper as an electrically conductive component. These ingredients have usually been mixed with one or more other oxides. Examples of such cast coatings are described in U.S. Patent 2,294,756. In the known method, the Begußüberzüge be sintered an oxidizing medium within a temperature range from 1000 to 1500 ⁰ C in the presence of. This creates an electrically conductive coating only on the surface of the insulator body in the vicinity of the ignition device. This is soft and erodes quickly, so that a sufficient service life is not ensured. One of the most commonly used materials contained copper oxides mixed with a minor percentage of either chromium oxide or iron oxide or a mixture of both. In some cases copper powder was mixed with chromium oxide or iron oxide and processed into a suspension and applied as such to the insulator body and fired in an oxidizing atmosphere to produce copper oxide in situ.

3 4

If you use a semiconducting material at the nose end of the insulator 15 and is with the

Bridging the spark gap on an ignition pre-reference numeral 18 denotes.

direction, then this material naturally erodes. The electrical semiconductor layer 18 provides a das borrowed away. Discs of noticeable thickness have so much nose end 19 of center electrode 17 and a ground material on that it takes a comparatively long time, 5 electrode 20 to the sleeve 12 connecting electrical until they are eroded, however, it has been shown that orbit. Will be a comparatively low voltage the thickness of the charge on the center electrode of the igniter, which is necessary for a long service life Do not disconnect the flow of electricity onto the upper device 11, for example from a condensation surface of the semiconductor is limited, so that too high a gate is applied, then a high-energy current flow occurs before ignition is required. There was 10 tables of electrical discharge along the surface of the also found that the previously used semiconducting layer 18. The electrical resistance Cast coatings begin to soften so that the layer 18 depends generally on its thickness erode faster without a usable one and the material from which it is made. Operating time results. The resistance decreases when the layer thickness increases.

The invention is based on the task of a 15 takes. The thickness of the layer 18 can be determined by the

Control methods of making an electrically conductive type of manufacture.

The following is a suitable method for specifying the surface on an insulator

Ignition erosion is as resistant as possible. Making this layer on the basis of the following

According to the invention, this object is explained in more detail by way of example,

solves that the time as well as the temperature of the touch 20. .

between the coating and the surface of the ISO Example I

lators are controlled in such a way that even under its A preferred embodiment of an ignition

Surface an electrically conductive phase is created. direction was determined using a previous

This is the case when the temperature is roughly between burned ceramic insulator 95 from approximately

1315 and 1485 ° C. In a preferred embodiment, 92% Al ₂ O ₃ made up to approximately 95% of the

Approximate form of the invention is the surface of the theoretical density had been fired. A

Aluminum insulator body with a slurry coating with the following materials was made by

drew that made a smaller proportion of aluminum and mixing it with an equal amount of water

contains a larger proportion of an oxide which is a and on the lower end surface of the insulator 15

electrically conductive aluminate brushed with aluminum 30,

of the insulator body. The coated body “ _f . ", _{7ft1t /}

is then heated so that in the insulator under copper metal powder 70.2 / 0

its surface is a dense, electrically conductive aluminum oxide. /, »/ 0

minate layer forms. Aluminum oxide 20.0%

Further refinements of the invention result from 35 '

from the subclaims. The insulator with the coating from the top

The layer according to the invention is essentially grain-described layer was passed through an oven,

more compact and stable than the known ones whose internal temperature is 137O ⁰ C in a way

The method obtained, outside of the insulator body was kept that the insulator in the hot zone of the

lying layer. 40 oven could be held for 10 minutes. The

The method according to the invention can be advantageously coated insulator then left on

It was also liable to cool room temperature when making spark plugs, another brushed

for engines of motor vehicles as well as for jet coating on the material described above

use engines for aircraft. and burned the recoated insulator for the

In the latter, the electrically conductive material 45 is at the same time and at the same temperature. The heavy mechanical and thermal shock-coated insulator was allowed to cool down again to room demands as well as corrosive atmospheres and temperature, and a third alternating reducing and oxidizing reference was made from this suspension. Also exposed to these third conditions. For a better understanding, the coating was fired in the same way as the two of the invention, this is first fired in connection with the 50 previously described coatings,
a jet engine ignition device described In a development program will be a large one, followed by the description of some changes in the number of test items on its ignitability of the invention. tests, with the standard test item and a test

The jet drive order shown in the drawing for the ignition test of each insulator body Factory ignition device is provided with the reference number 11 55 with a semiconducting layer. The use denotes and has a thin electrically semiconducting th test specimens were tubular bodies with a Layer on which bridges the two electrodes, length of 31.75 mm and an outer diameter around a path for the discharge of a high energy of about 9 mm and an inner diameter of see spark deliver. The ignition device 11 is approximately 2.7 mm. These insulator bodies were made from the holds a metal sleeve 12 with a screw thread 13 60 made of the same material as the insulator body 15 for insertion into the combustion chamber, for example, and contained approximately 92% alumina. It In the manner of a jet engine and with a screw, strips of various materials were applied to the unthread 14 for attaching an ignition cable. An an tere ring surface of the test insulators brushed on, the a second ceramic insulator 16 was then attached at a predetermined temperature Ceramic insulator 15 is burned into metal sleeve 12. Each isolator was at room temperature sealed, while in the insulator 15 a central electrode cooled, and one has a second and third strip auftrode 17 is sealed. One brought according to the invention as described above. The test items with The electrically semiconducting layer produced is located on the applied and fired strips

then into a spark plug holder with a dust electrode used, the head under spring tension against the end having the fired strip was pressed. This assembly was then inserted into a test case, with a shoulder: extending itself placed on the outer edge of the fired strip of the sample. There was a gap of approximately 1.2 mm between the head of the. Center electrode, the spark plug attachment and the edge of the shoulder of the coated body and the test samples were installed in a blasting machine with the burned coated end facing up. JP4 jet fuel was dripped every minute on the end of the test piece with the burned strip and. produced two ignitions per second over this end with the burned strip between the electrodes. These ignitions were made by discharging a 10 microfarad capacitor, which was charged with 2000 volts, so that 20 joules of energy were consumed during each ignition.

Five specimens made using the same strip and fired at 1370 ° C gave an average life of 57.7 hours. The above-mentioned strip was burned in an oxidizing atmosphere, so that the copper metal was converted into a copper oxide which, at approximately 1093 ^° C., is primarily formed from copper oxide. Above this firing temperature, copper oxide evidently reacts with chromium oxide and aluminum oxide to form aluminates and chromates. An examination of the test specimens after firing shows that very little remains of the application and that a black penetration tape made of copper oxide has penetrated the aluminum oxide insulator body to different depths, which in some cases are up to 0.8 mm. There is little swelling of the insulator body, and a spectrographic analysis shows that a large amount of the copper oxide is present in the surface layer of the insulator in a form combined with aluminum oxide and chromium sesquioxide. The electrical resistance across the electrodes on the device under test varies between 30 and 40,000 ohms, as initially generated. The samples were ignited in the test machine described above for 7 hours at room temperature; the samples were then ^{heated to 538 °} C. in order to remove the carbon deposits. New electrodes were then applied to these specimens, and the response voltage was determined. If the response voltage required to initiate ignition exceeded 1800VoIt with a capacity of 0.1 mF, the test series was terminated and the defective sample was discarded.

The time and temperature at which an insulator body coated with a coating compound is fired determines the depth to which the copper oxide of the potting compound diffuses into the insulator, and determines the proportion of aluminate that is formed relative to the non-conductive material from which the Isolator body is made. By regulating the time and temperature of the firing process, layers of different resistance can therefore be generated, and if the insulator body coated with the potting compound is fired at a temperature below approximately 1316 ° C, a cast coating is created on the insulator body without noticeable diffusion of the copper oxide into the Insulator body. For most igniter applications it is desirable to have a relatively high concentration of copper oxide in a thin, dense layer adjacent the outer surface of the insulator so that the surface has a fairly high conductivity and is therefore dense and erosion resistant. Apparently there is no noticeable diffusion of the copper oxide into the insulator body with formation of aluminates and chromates below approximately 1316 ^° C. As a result, when firing below this temperature, a coating of casting compound is created on the surface of the insulator body. When burning above this critical temperature, diffusion takes place in the insulator body at a rate that increases with temperature. A satisfactory diffusion rate ER 'are at about 1370 ° C, while too high a diffusion rate is formed above approximately 148o C ^0. If a casting compound coating instead of an aluminate or chromate layer arises within the surface of the insulator body, this results in a significantly reduced service life of the ignition device. The casting compound coatings in the form described above are relatively porous and soft and have a service life that is only a fraction of that of an insulator body ^; a good dense layer of aluminates and chromates near the surface of the insulator body. This was evidenced by the following procedure, which was carried out for the purpose of comparison and not in accordance with the invention.

A casting compound, hereinafter referred to as "casting compound No. 2", made of 93% copper, 5% chromium oxide and 2% kaolin was tested on an alumina test insulator of the type described above Art brushed on that contained 92% alumina.

The insulator was coated with this casting compound and then fired at 1150.degree. then the insulator body was cooled and the process repeated to add two additional coatings of casting compound to apply, which resulted in a total layer thickness of approximately 0.15 mm. An exam Visually indicated that the insulator had a grout coating with essentially no diffusion of the material from which the casting compound consisted in the insulator. This insulator body with the The casting compound thus formed was tested in the manner described above and had a service life of only approximately 6 hours.

An optimal ignition life is obtained when an extremely dense concentration electrical conductive copper chromates dispersed in aluminates near the surface of the insulator j can be generated with minimal diffusion of the insulator material from the surface of the insulator. the The temperature given above must be high enough to produce aluminates and chromates, and the Burning time must be long enough for the oxide applied to the outer surface of the insulator body is exhausted by so far diffusion into the insulator that a porous one is poorer in aluminates and chromates Area remains. This can be shown by the following experiments.

Numerous Al ₂ O ₃ insulators of the type described above are coated with a casting compound of 88.3% copper, 9.8% chromium oxide and 1.9% kaolin, which

later to be referred to as "potting compound no. 3". The coated insulators are used for an in Table 1 shown time and heated to the temperature shown there. Then everyone becomes an insulator chilled. A second casting compound coating is applied and the insulator is again used at the baked at the same temperature for the same time as when the first coating was baked. This method is repeated a third time, and the finished one

The insulator body is then tested in the manner described above. Although there is a considerable difference results in the life between samples fired at some of the higher temperatures, shows a comparison of the average lifespan at different times and temperatures burned insulators, what life expectancy for an insulator body at a certain temperature and time is to be expected.

Table I.

Triple application and triple firing
Hour ignition until failure at different firing temperatures

Burn time 1150 "C 1260 · C 1370 C 1400X 1425 "C 1455 ⁰ C 148O'C 10 mins 10.7 12.2 20.6 27.5 35.0 67.4. 34.2 11.3 12.3 24.0 41.9 39.2 71.0 52.1 15.2 12.8 38.1 44.7 42.2 81.6 65.0 17.5 17.6 . 23.0 21.1 average 15.5 15.2 27.6 38.0 38.8 73.3 50.4 20 minutes 14.0 7.0 13.8 23.5 35.5. 15.2 9.8 14.0 12.0 - 20.7 33.1 51.4 42.3 15.0 25.5 11.1 55.7 36.2 61.2 76.0 71.5 average 17.8 10.0 30.1 30.9 49.4 44.5 32.1 50 minutes 7.0 19.6 18.6 15.5 37.6 21.0 14.0 21.3 12.1 7.0 14.0 36.0 77.1 28.0 10.7 12.3 21.0 (42.0) 48.2 57.8 14.0 average 13.0 14.7 15.5 23.8 40.6 52.0 18.7

It has been shown that the consistency of the casting compound affects the furnace atmosphere and other variables affect the service life for samples produced at a given time. To make more precise comparisons, see For the production of a number of test items, it is therefore necessary to run control test items that have been calculated in advance Times and temperatures were burned.

Tables shows the results of other series of tests on insulator bodies carried out in the same manner as mentioned above in connection with the test items according to Table I, but for the Times and temperatures according to Table II were fired. Table II shows that one has improved results obtained when the insulator body is burned for only approximately 7 minutes, but at a temperature of 1482 ° C became.

Table II code temperature Time Slücknummer Average
■ Lifespan 1
2
3 1425 ° C
1425 ⁰ C
1425 ° C 6 min. + 40 sec.
10 min.
20 min. 5
5
5 20.1 hours
29.6 hours
25.0 hours 4th
5
6th 1455 ° C
1455 ° C
1455 ° C 6 min. + 40 sec.
10 min.
20 min. 5
5
5 23.9 hours
34.5 hours
36.5 hours 7th
8th
9 1482 '· C
1482'C
1482 '· C 6 min. + 40 sec.
10 min.
20 min. 5
5
5 48.2 hours
19.1 hours
10.4 hours
409 548/249

Other experiments have shown that a single application and firing a Begußmasse leads to a life, is the approximately _^2/3 of life, which can be achieved if one brings a threefold application and a triple firing the Begußmasse to produce the insulating body for use.

In a number of other attempts it has been found to be beneficial to incorporate the same material from which the insulator is made into the copper-containing material which is applied to the insulator body. Copper oxide is ^{very liquid at about 137O 0} C, and the viscosity of the material applied to the insulator increases considerably with the addition of aluminum oxide. The addition of aluminum oxide and chromium sesquioxide therefore helps to hold the copper oxide in place to promote uniform penetration into the insulator body. In addition, the addition of aluminum oxide additives to the casting compound increases the density of the materials that remain on the surface of the insulator body, and thus their resistance to erosion.

Test specimens are obtained by applying coatings of a suitable casting compound to the insulator body prepared in the manner described above, at the temperatures given in Table III for 10 minutes baked for a long time and repeated the coating and baking steps a total of 3 times. The covers were composed from "Casting Compound No. 3" and "Casting Compound No. 3 +" with different amounts of aluminum oxide. The compositions used and the average lifespan in Hours are given in Table III.

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Table III

40

45

Coating material Brenn
temperature lifespan Casting No. 2 1425 ° C 25.2 Casting No. 2
100 parts + 5 parts
Aluminum oxide 1425 ° C 33.4 Casting No. 2
100 parts + 10 parts
Aluminum oxide 1400 ⁰ C 45.6 Casting No. 2
100 parts + 15 parts
Aluminum oxide 1400 ⁰ C 43.3 Casting No. 2
100 parts + 20 parts
Aluminum oxide 137O ° C 57.7

55

60

Cuprous oxide is stable in air at temperatures up to approximately 1026 ⁰ C, at which temperature it dissociates into the copper state. Copper oxide is stable above 1026 ° C and melts at 1235 ° C. Cuprous oxide combines with Al ₂ O ₃ to form Spinol which is stable below approximately 900 ⁰ C.

Above approximately 900 ⁰ C, a copper aluminate constitutes the formula CuAlO. ₂ It therefore appears that the presence of Al ₂ O ₃ lowers the temperature at which a transition from copper oxide to copper oxide takes place.

At temperatures above approximately 1165 ° C, a liquid is formed that consists mainly of Cu ₂ O. This liquid can be in equilibrium with CuAlO ₂ , depending on the amount of Al ₂ O ₃ in the mixture. Above approximately 1260 ⁰ C decomposes CuAlO ₂ in Al ₂ O ₃ and a melt kupferoxydreiche. It therefore appears that Cu ₂ O flows with the aluminum oxide of the insulator body and quickly migrates under the surface of the insulator body.

It has been found that the presence of chromium sesquiocide, when added to the material applied to the insulator body, stabilizes the conductive phase that is formed, so that the resistance of the body so formed does not noticeably increase when used at a temperature of about 870 ⁰ C changes. It appears that chromium sesquioxide has a strong affinity for copper oxide, presumably with the formation of CuCrO ₂ , and that the large size of the chromium sesquioxide molecules prevents the chromium sesquioxide from noticeably diffusing or migrating into the alumina insulator body and therefore the copper oxide on the surface and in its place holds. It may also be that the chromium sesquioxide lowers the transition temperature from copper oxide to copper oxide.

In any case, it has been found that more than one cast coating is formed when using copper or a copper compound which is in equilibrium. with oxygen at elevated temperatures, applies to an aluminum oxide insulator body and burns at temperatures between approximately 1370 and 1485 ° C in an oxidizing atmosphere for a controlled short period of time, which the copper oxide diffusion on approximately 0.25 mm of the insulator surface and preferably less than 0.125 mm of this Surface lasts. In addition, it has been found that the presence of Chromsesquioxyd remarkably prevents the changes in resistance of the electrically conductive layers at temperatures above 870 ⁰ C and even at temperatures up to 1100 ⁰ C.

To determine the effect of chromium and aluminum, 6 test specimens of the type described above were produced from the compositions according to Table IV. In each case 2% kaolin was incorporated, while the percentages of Cr ₂ O ₃ or Al ₂ O ₃ corresponded to those according to the table and the remainder was a copper powder. Molding compounds made from these compositions were brushed in a tape onto the cylindrical outer surface of the aluminum oxide insulator bodies of the type described above, and the coated bodies were then baked for 10 minutes at the specified temperature. Then a second coating of the same casting compound was applied over the first band and the coated body was fired again at the specified temperature, whereupon a third coating was applied and fired in the same way. Resistance was measured by sandwiching the treated surface of the samples between flat plates and measuring the resistance therebetween with a 500 volt ohm meter. The average resistance measured for the 6 specimens is shown in Table IV.

11 12

Table IV
Resistance after three-time firing of the coating (K-Ohm)

Begüßmassensubst 1095 "C 1120 ° C Firing temperature 1150-C 1205 ° C 1315 ° C 1425 ° C Pure copper 508 236 293 1.188 7,944 25.721 1% Cr ₂ O ₃ 176 158 168 444 10.333 24.610 2% 149 170 111 101 3,000 8,611 3% 81 118 104 119 5.611 28.223 5% 53 61 85 116 231 10.223 10% 17th 18th 9 15th 21 33 20% 8th 9 9 10 20th 18th 30% 4th 4th 4th 11th 6th 9 1% Al ₂ O ₃ 593 204 188 1.959 8.389 30.277 2% 280 224 ' 231 2.131 11,889 26,833 3% 346 208 172 454 6.111 12,167 5% 213 158 143 1.467 7,500 28,705 10% 125 117 91 3,562 9.333 30.555 20% 81 74 74 3.889 12,866 23,333 30% 88 77 70 133 15,000 29.722

*) Less than 6 samples in this average value.

From the above and other experiments it was found that chromium sesquioxide should constitute between 3 and 30% of the solids which are applied to the insulators, the values preferably being between 5 and 20% and, for economic reasons, preferably around 10%. These experiments also show that alumina additions improve the life of the finished article. Improved results are also achieved when the aluminum oxide contains between approximately 10% and 50% by weight of solids. It appears, however, that there is no benefit to using more than approximately 30 percent by weight of alumina, and optimum results appear to be obtained when the alumina is approximately 20 percent of the solids mixture. Lower percentages of other materials can be used without _{adversely shifting the Cu 2} O-Al ₂ O ₃ and Cu ₂ O-Cr ₂ O ₃ phase ratios, and considerable amounts of materials that do not shift the phase ratios can be incorporated, without the synergistic effect being canceled. It has been confirmed that large amounts of alumina can be used in the solid materials applied to the alumina insulator bodies without noticeably reducing the usefulness of the fired insulator. Although alumina is not inert, it is the same material as the material the insulator is made of and acts as a diluent, among other things.

1 sheet of drawings

Claims (10)

Patent claims: 1. A method for producing an electrically conductive surface on a substantially Alumina existing insulator, especially for jet engine ignition devices to which a copper-containing coating is applied to the desired surface area and this is exposed to a temperature treatment, characterized in that that the time as well as the temperature of the contact between the coating and the surface of the insulator are regulated so that also below its surface an electrically conductive phase is created. 2. The method according to claim 1, characterized by the use of copper oxide as Coating material. 3. The method according to claim 1, characterized in that a mixture with a larger proportion of copper oxide and a smaller proportion of aluminum oxide is applied to the insulator and that the surface layer is converted into an electrically conductive layer containing _{CuAlO 2 and aluminum oxide by heat treatment.} 4. The method according to claim 1, characterized in that on the insulator with a mixture a larger proportion of copper oxide and a smaller proportion of chromium sesquioxide applied and by heat treatment into an electrically conductive surface layer of reacted Copper oxide, aluminum oxide and chromium sesquioxide is converted. 5. The method according to claim 4, characterized in that the mixture for changing the insulator surface in a layer of aluminates and chromates of copper oxide contains a small amount of aluminum oxide. 6. The method according to claim 5, characterized by the use of a mixture of 3 to 30 percent by weight chromium sesquioxide, 10 to 30 percent by weight aluminum oxide and residual copper in a form that equilibrates with oxygen at the treatment temperatures results, the treatment time and treatment temperature are chosen so that no noticeable Migration of the aluminum oxide from the insulator body takes place. 7. The method according to claim 6, characterized in that the coating mixture and the insulator body heated for about 10 to 6 minutes to a temperature between 1315 and about 1485 ° C will. 8. The method according to claim 4, characterized in that a mixture of about 90 percent by weight of copper in a form which gives an equilibrium with oxygen at elevated temperatures, and about 10 percent by weight of CrO _{3 is} used as the coating and that the mixture and the insulator about 10 to 6 minutes in an oxidizing atmosphere to a temperature between about 1315 and about 1485 ⁰ C are heated. 9. The method according to any one of claims 4 to 6, characterized in that a mixture of about 75 weight percent copper in a form that is in equilibrium at elevated temperatures with oxygen gives about 16.5 percent by weight alumina and about 8.5 percent by weight Chromosesquioxide is used and kept at a temperature of about 1315 ° C for about 10 minutes is heated. 10. The method according to any one of the preceding claims, characterized in that an aluminum body is fired to about 95% of the theoretical density and has a surface layer between 0.025 and 0.25 mm thick, the combined with the aluminum oxide of the insulator to form CuAlO ₂ Cu ₂ O contains.