GB2092050A

GB2092050A - Method for connecting a silicon carbide part to a metal part, and a flame-monitoring electrode comprising such parts

Info

Publication number: GB2092050A
Application number: GB8202648A
Authority: GB
Original assignee: Danfoss AS
Current assignee: Danfoss AS
Priority date: 1981-01-29
Filing date: 1982-01-29
Publication date: 1982-08-11
Also published as: DE3200738C2; FR2498503B1; DE3200738A1; GB2092050B; JPS621598B2; IT1155458B; FR2498503A1; DK38981A; JPS57149876A; JPS57182160A; IT8267090A0

Abstract

This invention relates to a method for connecting a high density (for example, a reaction sintered) silicon carbide part (1) to a metal part (2), the method comprising fastening the two parts together using a binder (3) comprising silicon and metal, the binder being heated to effect the fastening. The invention also relates to a flame- monitoring electrode comprising parts which have been fastened together in this way. <IMAGE>

Description

SPECIFICATION Improvements in and relating to a method for connecting a silicon carbide part with a metal part, and to an electrode comprising such parts.

This invention relates to a method for connecting high-density silicon carbide parts with metal (especially iron) parts.

Making sound mechanical and/or electrical connections to ceramic semiconductor parts of the silicon carbide type has always given problems. Among the already known methods for making electrical and mechanical contact can be mentioned flame-sprayed metal connections or shrunk-on metal connections.

When high temperatures are applied, e.g.

temperatures of 600 C-700 C., the flamesprayed metal may be burnt away, and other connections, such as shrunk-on metal connections, may become mechanically unstable because of the heating stresses caused by the high temperatures.

The present invention relates to a method for establishing a reliable and heat-resistant connection between metallic parts and silicon carbide parts.

The present invention provides a method for connecting a high density (for example, a reaction-sintered) silicon carbide part with a metal part (especially iron), the method comprising fastening the two parts together with a binder of silicon and metal, the binder being heated to effect the fastening.

It is known, for the purpose of obtaining a very dense silicon carbide body of high mechanical strength to form a body from asilicon carbide and carbon, and to sinter the body under the effect of silicon or a silicon compound. The reaction between carbon and silicon carbide leads to the formation of additional silicon carbide which to a large extent fills the pores present in the body. In this method known as "reaction sintering" a certain proportion of free silicon remains in the body depending upon the quantity of carbon used, the quantity of silicon used and the size of the pores. Another method of forming dense silicon carbide articles is disclosed in U.S. Patent Specification No. 3205043. In this case, the carbon is produced by a conversion process, for example, by thermal decomposition of a phenol resin or the like.

Silicon compound or compounds can be provided in vapour form (British Patent Specification No. 886813). Further, liquid silicon can be introduced into a porous body containing silicon and carbon, the liquid silicon entering the body by capillary action. It is also known in the case of bodies produced by drawing to treat these with SiO in the reaction sintering process, so that by removing part of the C in the surface zone, the surface porosity, which has been reduced by the drawing operation, is improved (see British Patent Specification No. 1180918).

Advantageously, the binder should be a eutectic mixture of silicon and metal, preferably the same metal as the metal part to be fastened. This ensures a thermally stable mechanical and/or electrical connection between the silicon carbide part and the metal (iron) part.

The binder may be mixture of metal powder and silicon powder (mixed in the proportion, for example, of 1:1).

Use of electrodes for flame monitoring in oil and gas burners is already known. Basically, the presence of the flame causes ionisation of the air between the flame electrode and the burner jet which permits a small electric current to flow between the jet and the electrode.

The current operates a control relay. In the event of flame failure no current flows, the relay is not operated and, for example, fuel flow to the burner jet is cut off. In blue-flame burners from which no visible light is emitted, an ultraviolet-detector has been used. Often however, such a detector cannot be used, e.g.

when the burner is surrounded by a recirculation chamber. In such burners a flame electrode is, therefore, preferred, but such an electrode has the drawback that its material disintegrates at the very high operating flame temperature (which in blue-flame burners is normally about 1,400 C-1,500 C), so that the electrode has a relatively short life.

The present invention also provides, therefore, a more robust and flame resistant flame electrode, the electrode comprising a high density (e.g. reaction sintered) silicon carbide part and a metal part fastened together by the method according to the invention, the silicon carbide part being in the form of a rod.

Preferably, the length of that part of the rod consisting of the silicon carbide material is such that its region of connection to the metallic part of the rod lies, in use, outside the flame area. That length depends on two conflicting factors. The fragility of the silicon carbide part means that its length should be kept as short as possible. On the other hand, if it is too short its connection with the metal part may be exposed, in use, to high and damaging temperatures.Even immediately outside the flame, the connection will be exposed to a high temperature of about 600 C-700 C. By forming the electrode in accordance with the invention, however, the conflict between these factors is not so acute since the silicon carbide part itself is more robust (it being high density, e.g. reaction sintered) and since the connection between it and the metal part is more durable and more heat resistant (especially when the binder is a eutectic mixture since it diffuses into the silicon carbide and metal (iron) parts to form a stronger bond).

Each of the two parts of the electrode may be in the form of a rod, the rods being joined together end-to-end. One of the rods (preferably the silicon carbide rod) may be placed in an oversized hole in the end of the other rod and the binder introduced into that hole. The binder may then be subjected to a temperature of at least 1,200 C.

The present invention also provides a method comprising surrounding a body (for example, a rod) of high density (for example, reaction sintered) silicon carbide with a metal powder and sintering and/or pressing (for example, by applying isostatic pressure; with or without the presence of the aforesaid binder-the powder and/or body to form a shell of metal round, and fastened to, the body. A mechanically stable electrode can be formed in this way, which is easy to handle, the sintered and/or pressure-formed metal shell protecting (e.g. during transport or installation) the relatively fragile silicon carbide body (rod). Of course, that part of the sintered metal shell which is actually exposed to the flame, in use will corrode and/or disintegrate.

Further, a metal part of the electrode not sintered to the silicon carbide part may be weakened to facilitate bending of the electrode to desired position without damage to the relatively fragile silicon carbide part.

The present invention further provides a method for connecting a high density (for example, a reaction sintered) silicon carbide part to a metal part, the method comprising push-fitting a metal part over at least the end of the silicon carbide part-with or without the presence of the aforesaid binder-and then heating (for example, to a temperature of at least 1,100"C.) the two parts so that they are sintered together. The sintering results in a kind of shrinking together of the parts and at the same time an intimate mechanical and electrical connection is obtained at the junction, which is constant also at high temperatures. If the temperature is raised to more than 1 ,200C., this method will also give a eutectic mixture which ensures an even more stable junction and good electric connection.

If the silicon carbide part is in the form of a rod this can result in a suitable flame electrode construction, the electrode being completed by soldering, welding or otherwise fastening the metal part to other metal parts of the electrode which, in use, lie outside the flame zone.

Preferably, the metallic part is pressuresintered, possibly through isostatic pressure, onto the silicon carbide part. This provides a very close and thorough connection of the sintered-on metal part.

The present invention further provides a method for connecting a high density (for example, a reaction sintered) silicon carbide part to a metal part, the method comprising pressure-sintering (preferably through isostatic pressure)-with or without the presence of the aforesaid binder-the metal part onto the silicon carbide part.

The present invention relates to specification of a method for reliable and heat-resistant connection of metallic parts with silicon carbide parts.

According to the present invention a reaction-sintered carbide part is fastened to a metal- or iron part by means of a binder of silicon and a metal, with subsequent heating of the connection.

Preferably, the binder should be a eutectic mixture of silicon and the metal used, e.g.

iron. This ensures a thermically stable mechanical and electrical connection between the silicon carbide part and the iron part.

Use of an ionisation electrode for flame monitoring in oil- and gas-burners is already known. Such electrodes are based on the already known principle that the electrode, e.g. an iron electrode, is inserted in the flame area, causing the flame to ionise the gap between the fuel nozzle of the burner and the electrode, permitting conduction of an electric current. In blue-flame burners from which no visible light is emitted, an ultraviolet-detector has been used. Often, however, such a detector cannot be used, e.g. when the burner is surrounded by a recirculation chamber.In such burners an ionisation electrode is therefore preferred, but such electrode has the drawback that the material is disintegrated so that the electrode has a relatively short life at the very high flame temperature which is in blue-flame burners normally around 1,400 1 ,500'C.

Therefore, the invention also relates to specification of a more resistant electrode. According to the invention this is obtained when the electrode is rod-shaped and when at least the part to the rod protruding into the flame consists of a reaction-sintered silicon carbide semiconductor material.

Preferably, the semiconductor material should be of such an extent that the connection to the metallic part is mostly outside the flame area. The fragility of the end of the electrode points towards the electrode being kept as short as possible. On the other hand, the high temperature causes problems with the connection to the metal part of the electrode. Even immediately outside the flame the connection will be exposed to a high temperature of about 600-700 C. At such temperatures the method according to the invention is particularly suitable for connecting the metal part of the ionisation electrode with the silicon carbide rod. By fastening, according to Claim 1 of the invention, the end of the electrode to the metallic part by means of a binder of silicon and iron, preferably a eutectic mixture of silicon and iron, we shall get a durable electrode. The eutectic mixture will enter into connection with both the silicon carbide rod and the iron part.

The connection proper can be made by placing the silicon carbide rod in a hole in an iron rod and by putting a mixture of silicon powder and iron powder into the space between the silicon carbide rod and the iron rod, and then heating the unit to more than 1,200 C. The mixture may e.g. be in the proportion of 1:1. Through this, the siliconand the iron powder will form a eutectic mixture which will enter into connection with the silicon carbide rod and the iron part.

The sintered-on metal part may possibly completely enclose the silicon carbide rod.

This gives a mechanically stable unit which is easy to handle, the sintered shell protecting the rather fragile ceramic silicon carbide rod.

Of course, the sintered metal part which is exposed to the flame will disintegrate but will leave a residue enclosing the stable silicon carbide rod in the area a short distance from the flame, in which area the temperature is not higher than 700-800 C. Therefore, the only part corroding will be the one which is directly in the high-temperature part of the flame.

Further, the metal electrode outside the sintered part may be weakened so that the fragile tip of the electrode can be bent to a desired position without damage to the fragile part.

Another method is to push in a metal part covering at least the end of the silicon carbide rod and to concrete the two parts through heating to a temperature of 1,1 00 C. The sintering results in kind of shrinking, and at the same time an intimate mechanical and electrical connection is obtained in the junction, constant also at high temperatures. If the temperature is raised to more than 1,2qO"C., this method will also give a eutectic mixture which ensures an even stabler junction and A good electric connection.

The resulting unit is hard-soldered (brazed), welded or otherwise fastened to the parts of the electrode leading out of the flame zone.

Preferably, the metallic part should be pressure-sintered on, possibly through isostatic pressure. This will give a very close and thorough connection of the sintered-on metal part.

Various flame electrodes constructed in accordance with the invention and methods of making them according to the invention will now be described, by way of example, with reference to the accompanying drawings in which: Figure 1 illustrates a method of forming flame-monitoring electode with a silicon carbide rod inserted in an iron-holder; Figure 2 illustrates the finished electrode according to the method illustrated in Fig. 1; Figure 3 illustrates flame-monitoring electrode in which a silicon carbide rod is completely enclosed in sintered-on metal material; Figures 3, 4 and 5 illustrate the sensor shown in Fig. 3 after it has been used for some time, and Figures 6(a) and (b) illustrate two variations of a silicon carbide rod with a sintered and shrunk-on metal part.

Referring to the accompanying drawings, a high density ceramic silicon carbide rod is made in a known way, preferably by reactionsintering a mixture of silicon carbide (SiC) and graphite with liquid silicon (Si) at a high temperature, through which free graphite reacts with silicon to form silicon carbide. Fig. 1 illustrates how such a silicon carbide part 1 can be connected to a metallic electrode part 2 of iron. The silicon carbide rod is placed in a bore 7 in the iron rod. The space between the bore and the rod is filled with a mixture 3 of silicon and iron powder, e.g. in the proportion of 1:1. Alternatively, the space may be filled with a pre-prepared powder consisting of a mixture of silicon and iron, e.g. a eutectic mixture.The whole assembly is now heated to more than 1,200 C., at which temperature the powder mixture will form a eutectic mixture and enter into connection with both the silicon carbide rod and the iron. Also other metal compounds may be used as the binder, e.g. a mixture of silicon and nickel.

Fig. 2 shows the finished electrode tip 1, 2, made by the method illustrated in Fig. 1. The iron part 2 of the silicon carbide rod forms an intermediate part which can now be fastened as desired to the rest 4 of the electrode rod 5.

Fig. 3 illustrates a method by which iron powder is pressed around a silicon carbide rod, e.g. by isostatic pressure, i.e. applying a uniform pressure from all directions. The whole assembly is then heated to about 1,200 C., preferably 1200 C to 1,220 C., through which iron powder will concrete, and at the same time silicon from silicon carbide rod will diffuse into iron powder, forming a emetic mixture which forms a thermally stable, rhhanical and electrical connection between the silicon carbide rod and the iron powder. The latter electrode is sturdier and withstands transporttand handling better than the rod illustrated in Figs. 1 and 2.

As illustrated in Figs. 4 and 5, the part of the metal shell protruding into the flame will slowly be corroded away and only the stable silicon carbide rod will remain.

Fig. 4 further illustrates how the rod 4 behind the intermediate part 2 of the ceramic silicon carbide rod 1 can be made with weakenings 6 which may either, as desired be made all around the rod or be made only in part of the circumference. This facilitates tilting of the silicon carbide part of the rod relative to the rest of the electrode by simple manual bending of the electrode at the weakening and without damage to the fragile ceramic outer part. The other forms of electrode described above can also be made with such weakening(s).

Figs. 6(a) and (b) illustrate two ways in which a metal part 8, here an annular part, can be shrunk and sintered on to a silicon carbide rod.

The high density silicon carbide parts referred to throughout this specification and claims can be made by a number of different processes, for example, by the processes described in British Patent Specification Nos.

886813, 1180918, 1459252 or 1548748 or U.S. Patent Specification No. 3205043, and reference is directed to those specifications for full details.

Claims

1. A method for connecting a high density silicon carbide part with a metal part, the method comprising fastening the two parts together with a binder of silicon and metal, the binder being heated to effect the fastening.

2. A method as claimed in claim 1, in which the binder is a eutectic mixture of silicon and metal.

3. A method as claimed in claim 1 or claim 2, in which the metal of the binder is the same as the metal of the metal part.

4. A method as claimed in any one of claims 1 to 3, in which the metal part is made of iron.

5. A method as claimed in any one of claims 1 to 4, in which each of two parts is in the form of a rod, the rods being fastened together end-to-end.

6. A method as claimed in claim 5, in which one of the rods is placed in an oversized hole at the end of the other rod and the binder is introduced into that hole.

7. A method as claimed in claim 5, in which the metal rod has the hole.

8. A method as claimed in any one of claims 1 to 7, in which a heating temperature of at least 1200 C is used.

9. A method of connecting a high density silicon carbide part to a metal part substantially as hereinbefore described with reference to, and as illustrated by, Figs. 1 and 2 of the accompanying drawings.

10. A method comprising surrounding a body of high density silicon carbide with a metal powder and sintering and/or pressing the powder and/or body to form a shell of metal round, and fastened to, the body.

11. A method as claimed in claim 10, in which a a sintering temperature of about 12004C. is used.

12. A method as claimed in claim 10 or 11, in which a sintering temperture of 1200 C. to 1220 C. is used.

13. A method as claimed in any one of claims 10 to 12. in which the body is rodshaped.

14. A method substantially as hereinbefore described with reference to, and as illustraterd by, Figs. 3 to 5 of the accompanying drawings.

15. A method for connecting a high density silicon carbide part to a metal part, the method comprising push-fitting a metal part over the silicon carbide part and then heating the two parts so that they are sintered together.

16. A method as claimed in claim 15, in which a heating temperature of at least 1 100'C. is used.

17. A method as claimed in Claim 15 or claim 16, in which the silicon carbide part is rod shaped.

18. A method as claimed in claim 17, in which the metal part is in the form of a washer or a cap.

19. A method substantially as hererinbefore described with reference to, and as illustrated by Fig. 6a or Fig. 6b of the accompanying drawing.

20. A flame-monitoring electrode comprising a high density silicon carbide part and a metal part fastened together as claimed in any one of claims 1 to 9.

21. A flame-monitoring electrode made by the method as claimed in any one of claims 10 to 14.

22. A flame-monitoring electrode made by the method as claimed in any one of claims 15 to 19.

23. A flame-monitoring electrode as claimed in any one of claims 20 to 22, in which a metal part of the electrode remote from the silicon carbide part is provided with a weakening to facilitate bending.

24. A flame-monitoring electrode as claimed in claim 23, in which the metal part having the weakening has been joined to the metal part which has been fastened to the silicon carbide part.

25. A flame-monitoring electrode substantially as hereinbefore described with reference to, and as shown in Fig. 1 or Fig. 2 of the accompanying drawings.

26. A flame-monitoring electrode substantially as hereinbefore described with reference to, and as shown in Figs 3, 4 or 5 of the accompanying drawings.

27. A flame monitoring electrode substantially as hereinbefore described with reference to, and as shown in Figs. 6aor 6bof the accompanying drawings.

28. A method for connecting reaction-sintered carbide parts with iron- or metal parts, characterised by the silicon carbide part being fastened to the metal- or iron part by means of a binder of silicon and a metal, with subsequent heating of the connection.

29. A method according to claim 28, characterised by the binder being a eutectic mixture of silicon and iron.

30. An electrode of mainly metallic material for ionisation flame monitoring in oil- and gas burners, made by the method according to claims 28 or 29 characterised by the electrode being rod-shaped and by at least the part of the rod protruding into the flame consisting of a reaction-sintered silicon carbide semiconductor material.

31. An electrode according to claim 30, characterised by the semiconductor material being of such an extent that the connection to the metallic part is mostly outside the flame area.

32. A method for embodiment of an electrode according to claims 30 and 31, characterised by the connection being made by placing the silicon carbide rod in a hole in an iron rod, and putting a mixture of silicon powder and iron powder into the space between the silicon carbide rod and the iron rod, and the unit then being heated to more than 1 200C.

33. A method for embodiment of an electrode according to either of claims 30 and 31, characterised by the sintered-on metal part completely enclosing the silicon carbide rod.

34. An electrode according to one of claims 30 to 32, characterised by the metal electrode outside the sintered part having a weakening.

35. Method for embodiment of an electrode according to claims 30 and 31, characterised by a metal part being pushed in and covering at least the end of the silicon carbide rod and by the two parts being concreted through heating to a temperature of more than 1100 C.

36. Method according to claims 28, 29, 33, 34 and 35, characterised by the metallic part being pressure-sintered on, possibly through isostatic pressure.