WO1994001379A1

WO1994001379A1 - Welding procedure for a closed tube assembly

Info

Publication number: WO1994001379A1
Application number: PCT/AU1993/000349
Authority: WO
Inventors: Michael John Bannister
Original assignee: Commonwealth Scientific And Industrial Research Organisation
Priority date: 1992-07-14
Filing date: 1993-07-14
Publication date: 1994-01-20

Abstract

A method for forming an assembly which comprises a hollow body of a ceramic material having an aperture which is closed by a closure consisting of another body of the same or another ceramic material, said method being characterised in that it comprises the steps of: positioning the closure over or within the aperture of the hollow body; partially evacuating the hollow body; and heating the assembly of the bodies to thereby weld the bodies together.

Description

"WELDING PROCEDURE FOR A CLOSED TUBE ASSEMBLY"

This invention relates to procedures for the manufacture of closed tube assemblies, particularly oxygen sensors and like devices. More specifically, the invention is concerned with welding procedures for use in fabrication of such devices.

Oxygen sensors of the type in question are disclosed, for example, in United States Patents Nos.4,046,661; 4,193,857 and 4,240,891. In their simplest forms, such sensors comprise a disc of a solid electrolyte material, such as yttria-doped zirconia, which is sealed to one end of a tube of a ceramic material, such as alumina, aluminous porcelain or magnesium aluminate spinel.

Variations of this basic structure are described in the prior art, including the patents mentioned above, and include, for example, pellets or plugs of the solid electrolyte material which are sealed wholly or partly within the ceramic tube; and domed, cup- shaped or other hollow-form bodies of electrolyte material sealed to the end of the tube.

Whatever the configuration employed, it is essential that the electrolyte body and the tube be joined in such a manner that a seal is formed which is both fluid tight and mechanically strong.

The general approach to fabricating such sensors, as illustrated by reference to the disc/tube configuration, involves fusion butt welding a thin disc of the solid electrolyte material across the end of the ceramic tube. Typically this is done by placing the disc on top of the vertically-held tube, the end of which is first coated with a small amount of a paste of ground-up electrolyte material. The paste helps to hold the disc in position during subsequent heating.

The tube and disc assembly is then heated to the final welding temperature in a gas- heated refractory enclosure, during which time the tube is rotated around its vertical axis to ensure uniform heating. While simple in concept, this technique has its disadvantages.

Firstly, there is a tendency for the disc to move or jump off the end of the tube during heating. This probably occurs as a result of the paste drying out and losing its adherence, combined with pressure build-up within the sensor tube due to rapid local heating. Once the disc has moved the whole operation has to be aborted, the enclosure allowed to cool, and the procedure recommenced with a fresh assembly.

Secondly, the disc occasionally separates from the tube when in use. Examination of sensors that had failed, and others sectioned before use, revealed that the bond between the disc and tube generally involved only a thin zone at the outer perimeter of the tube. Most of the wall thickness of the tube was not bonded to the sensor.

Thirdly, the back surface of the disc, i.e. that within the tube, retains adhering particles of the paste material after welding, resulting in a surface which is not flat.

In use, the front and back surfaces of the disc are commonly coated with an electrode material such as platinum paste, to which electrical contacts are often made by means of spring-loaded conductors. After welding and prior to applying the electrodes the outer face is generally machined flat, but it is difficult to grind the inner face after welding and normally no attempt is made to do so. If the back surface is irregular because of the presence of paste material difficulty is often encountered in achieving the required contact to the inner electrode. In particular, it is not possible to achieve a reliable low resistance contact using a flat internal contact plate or disc as shown at the outer electrode in Figure 1 of U.S. Patent 4,193,857.

We have now found that by maintaining a vacuum inside the sensor during the welding operation all of these problems are overcome. The disc is then held firmly against the machined end of the tube by atmospheric pressure during heating, so that a paste-free weld can be made and the back surface of the disc remains flat and clean. The vacuum inside the tube prevents internal pressure build-up during heating, so that the disc does not jump off the tube prior to welding. Also as melting occurs at the disc /tube interface, the external air pressure forces the disc more firmly against the tube and squeezes molten material across the interface towards the inner wall of the tube. This results in a more extensive, and therefore stronger, bond.

The method of the invention is also suitable for other closed tube assemblies comprised of ceramic materials, other than those used in oxygen sensors. It will be appreciated by those skilled in the art that the materials to be joined will need to have closely matched thermal expansion coefficients.

Accordingly, one aspect of the present invention provides a method for forming an assembly which comprises a hollow body of a ceramic material having an aperture which is closed by a closure consisting of another body of the same or another ceramic material, said method being characterised in that it comprises the steps of: positioning the closure over or within the aperture of the hollow body; partially evacuating the hollow body, and heating the assembly of the bodies to thereby weld the bodies together.

In a further aspect, the present invention provides a method for forming a closed tube assembly which comprises a tube or other hollow body of a ceramic material one end of which is closed by a disc or other body of the same or another ceramic material, said method being characterised in that it comprises the steps of: positioning the disc or other body on or within the end of the tube or other hollow body; partially evacuating the tube or other hollow body; and heating the assembly of the bodies to thereby weld the bodies together.

Although the method of the invention is especially directed to the production of closed tube assemblies which comprise a tube or other hollow body closed by a disc, the method is equally applicable to other types of assembly. For example, the disc may be replaced by a pellet or plug, or a domed, cup-shaped or other hollow body which is closed at one end. In one particularly preferred embodiment (as already indicated) the closed tube assembly is an oxygen sensor or like device which comprises a tube or other hollow body of ceramic material the end of which is closed by a disc or other body of solid electrolyte.

The invention is further described and illustrated by the following examples. These examples are not to be construed as limiting the invention in any way. Reference is also made to the accompanying drawing which shows in diagrammatic form a simple apparatus for producing an oxygen sensor by the method of the invention.

As shown in the drawing, the apparatus comprises a furnace consisting of a refractory enclosure 1 provided with apertures in the side wall and base for the entry of flames from gas burners 2 . An aperture 4 is provided in the base of the enclosure for entry of the sensor assembly to be welded.

As previously described, the sensor assembly consists of disc 5 of solid electrolyte material and ceramic tube 6. The sensor assembly is supported on the end of a hollow rotatable shaft 7 by a flexible coupling 8, for example a short length of thick- walled rubber tubing. The bottom of shaft 7 is supported by and journalled for rotation in the collar of an O-ring rotary seal 9, which is attached to the end of a vacuum line 10. The latter is connected to a vacuum pump and relief valve (not shown). Means (not shown) are provided for rotating the shaft 7.

In use the sensor assembly is raised into the refractory enclosure 1, connected to flexible coupling 8, and the vacuum is applied (generally only a modest vacuum is necessary, e.g. an absolute pressure of 0.1 atm. (lO^a) or less). Rotation and heating of the sensor assembly are then commenced and continued until the weld is properly formed. Burner 3 is used to preheat the assembly to a temperature preferably in the range 1200 ° to 1500 °C, then burner 2 is introduced to raise the temperature at the disc/tube interface to above the eutectic temperature (1860 °C, in the case of a zirconia-based disc and an alumina tube). Care must be taken to avoid completely melting the disc or the end of the tube. Formation of the weld liquid may be monitored visually, through welding goggles When the weld is judged to be complete, both burners are turned off and the senso is allowed to cool. When the sensor is cool enough to handle, the rotation is stoppe and the vacuum is turned off. Air is admitted to the inside of the sensor, and th sensor is uncoupled from tubing 8 and removed from enclosure 1. When the senso reaches room temperature the quality of the weld can be assessed by internal ai pressurisation at an absolute pressure of about 300 kPa and submersion under water Leaks in the weld are indicated by air bubbles emerging from the sensor.

Example 1

Using the above-described apparatus, 40 sensors were made by the method of th invention. For some the vacuum source was a rotary backing pump capable o achieving an absolute pressure of lPa. For others the vacuum source was the laboratory vacuum line with an absolute pressure of approximately lOkPa. The sensors comprised discs of solid electrolyte 8mm in diameter and 1 to 2mm thic joined to alumina tubes 500mm long of outer diameter 8mm and inner diameter 5mm. The discs, of composition 50 weight percent alumina and 50 weight percent of a zirconia-scandia solid solution containing 95.3 mole percent zirconia and 4.7 mole percent scandia, were made by blending alumina, zirconia and scandia powders in the required proportions, pressing to shape and sintering to high density at 1700 °C in air.

The welding operation was found to be much simpler and more reliable than before. No weld had to be aborted because of disc movement during heating. Productivit was thus improved, not only because of a lower incidence of failure, but also because a new sensor could be inserted in the apparatus and the welding procedure begun while the refractory enclosure was still hot. Previously, the chamber had to be completely cooled between welds in order to protect the paste from rapid drying. Example2

Three of the sensors produced as described in Example 1 were sectioned and examined under the optical microscope. No gaps were observed at the disc/tube interface, suggesting that complete bonding had been achieved. Furthermore, the back face of each disc was clean and flat, with no evidence that the weld melt had spread beyond the disc /tube interface. Sectioning and examination of prior art welded sensors of similar design showed extensive gaps at the disc/tube interface, with bonding only at the outer circumference of the disc. The prior art sensors showed accumulations of partly sintered paste material covering more than half of the back face of the disc, extending inwards from the inner wall of the tube.

Example 3

Measurements were made of the load needed to break the bond between the disc and the tube for prior art sensors and sensors made according to the present invention. In each case the sensor comprised an alumina/zirconia/scandia disc, as described in Example 1, joined by welding to an alumina tube. Six prior art sensors and six of the sensors produced as described in Example 1 were used, the sensors being randomly selected. For measurements, each sensor were cemented to a steel collar and supported on a cylindrical steel die with the welded end of the sensor located within an axial hole in the die. Force was applied to the back face of the disc using a 4mm diameter hardened steel pin which was flat at the end in contact with the disc. Loading was performed on an Instron Universal Testing Machine at a crosshead speed of 0.1mm per minute. The loads needed to force the discs from the ends of the sensors are given in Table 1. Table 1

These results yield a mean breaking strength of 270 Newtons for prior art sensors and 683 Newtons for sensors made according to the present invention, Le. the present invention provides a 2.5 fold improvement in average strength. Of particular significance is the fact that the lowest recorded strength for the new sensors was above the best strength given by the prior art sensors, and was five times the lowest strength of prior art sensors.

The discs and tube ends were examined under the optical microscope after fracture. Extensive areas of partly sintered paste material were observed on the discs and inner walls of the tubes of prior art sensors, whereas for sensors made according to this invention each tube wall and disc face was clean and the disc faces remained flat. In every case the fracture occurred within or close to the weld zone. In the prior art sensors less than 30 percent of the possible disc/tube interface contributed to the bond, whereas for sensors made according to this invention between 50 and 100 percent of the possible disc/tube interface had been involved in the bond. In general, the bond strength correlated with the extent of bonding at the disc/tube interface.

Example 4

Flat discs of chromium /alumina cermet were used to make spring-loaded electrode contacts to the back faces of prior art and present invention sensors, which were then tested for their performance in air and oxygen at 300° to 600 °C. Prior art sensors showed very high resistances, sensitivity to electrical interference and emf errors, even at 600 °C. These problems were even worse at lower temperatures. Sensors made according to the present invention showed resistances which, at equivalent temperatures, were lower by at least a factor of ten. The lower resistances are consistent with better face-to-face contact between the cermet and solid electrolyte discs resulting from the cleaner and flatter sohd electrolyte back face provided by the present invention. As consequences of the lower resistance, electrical interference was less and emf errors were greatly reduced.

Example 5

Using the above described apparatus, six closed-end alumina tubes were made by the method of the invention, the vacuum source being the laboratory vacuum line with an absolute pressure of approximately 10 kPa. The alumina tubes were 8mm outer diameter and 5mm inner diameter; the discs, made by pressing and sintering alumina powder at 1700 °C for 15 hours, were 1 to 2mm thick. Compared with the welding of sensors, in which liquid is formed at the tube /disc interface by the zirconia/ alumina eutectic reaction, more careful positioning of burner 2 was required in order to prevent premature melting of the tube.

All six closed-end tubes were tested for leaks by internal air pressurisation at 300 kPa and immersion in water. No leaks were observed. All six tubes were strength tested using the procedure described in Example 3. The loads needed to force the discs from the ends of the tubes are listed in Table 2.

Table 2

These results give a mean breaking strength of 444 ± 144 Newtons, not quite as high as the strength achieved using solid electrolyte discs (Example 3) but adequate for many applications of closed end tubes.

In the above description, attention has been directed principally towards the disc/ tube low-temperature sensor. It will be obvious that the method of the invention can also be equally well applied to welding other types of sensor, such as the high-temperature, pellet-in-tube sensors described in U.S. Patent No. 4,046,661. When welding this latter type of construction, the pellet and/or tube bore may need to be tapered or stepped, to avoid sucking the pellet completely inside the tube.

Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described. It is to be understood that the invention includes all such variations and modifications which fall within its spirit and scope.

Claims

CLAIMS:

1. A method for forming an assembly which comprises a hollow body of a ceramic material having an aperture which is closed by a closure consisting of another body of the same or another ceramic material, said method being characterised in that it comprises the steps of: positioning the closure over or within the aperture of the hollow body; partially evacuating the hollow body, and heating the assembly of the bodies to thereby weld the bodies together.

2. A method for forming a closed tube assembly which comprises a tube or other hollow body of a ceramic material one end of which is closed by a disc or other body of the same or another ceramic material, said method being characterised in that it comprises the steps of: positioning the disc or other body on or within the end of the tube or other hollow body partially evacuating the tube or other hollow body, and heating the assembly of the bodies to thereby weld the bodies together.

3. A method as claimed in claim 2, characterised in that the closed tube assembly comprises a tube or other hollow body closed by a disc.

**.

4. A method as claimed in claim 2, characterised in that the closed tube assembly is an oxygen sensor or like device which comprises a tube or other hollow body of ceramic material the end of which is closed by a disc or other body of solid electrolyte.

5. A method as claimed in any one of claims 2 to 4, characterised in that prior to the welding step, the assembly is preheated to a temperature below the welding temperature and thereafter additional heating is applied to the tube or other hollow body and to the disc or other body in the vicinity of their interface to bring about the welding of the bodies.

6. An oxygen sensor or other closed tube assembly whenever made by the method described in any preceding claim.

7. Apparatus for forming a closed tube assembly by welding, said assembly comprising a tube or other hollow body of a ceramic material one end of which is closed by a disc or other body of the same or another ceramic material, said apparatus being characterised in that it comprises a refractory enclosure provided with an aperture for entry of the sensor assembly to be welded; means for heating the interior of the enclosure and the assembly when located therein; means for supplying additional heat to the vicinity of the interface between the tube or other hollow body and the disc or other body, sufficient to bring about the welding of the bodies; means for at least partially evacuating the assembly while within the enclosure; and means for rotating the partially evacuated assembly while within the enclosure.