AU427790B2 - Seals between sintered ceramic parts - Google Patents

Description

13 3 91 no

The present invention relates to the formation of

seals between sintered ceramic parts.

Such seals can be used in the formation of envelopes

of translucent polycrystalline alumina ceramic for high

pressure sodium discharge lamps. It is very difficult to

secure and maintain a hermetic seal in such envelopes because

of the high operating temperature and the corrosive action

of the sodium vapour. The ends of a tubular envelope can

be closed by shells of niobium metal sealed to the envelope

by means of a reactive metal brazing alloy containing

titanium and zirconium but it is found that the majority of

lamp failures are the result of the breakdown of this seal.

An alternative method would be to use plugs of the same

chemical composition as the tubular envelope but in order

to achieve a satisfactory seal an intermediate sealing material must be used and this gives rise to similar

deficiencies.

In accordance with the present invention there is

provided a method of forming sintered ceramic articles from

a number of parts with homogenous seals between the parts formed by differential shrinkage of the parts during

sintering wherein at least one part is made from powder

which has been spray-dried giving it a higher bulk density

resulting in smaller shrinkage during sintering than that

of another part made directly from powder.

427,790

13.3 91 /0

Thus by making the tubular envelope in the normal

way from amorphous ball-milled powder and the end plugs from

spray-dried particulate material the envelope can be caused

to shrink on to the plugs and form a homogeneous seal. The usual two-stage process of formation may be used for both the tube and the plugs in which the starting material is

compressed and fired at a lower temperature, typically 1100°C.,

to produce a part in the "green" state and then, after

machining to the required size, is sintered at a higher

temperature typically 18000°C. The plugs in the green state are made to be a tight fit in the tube and on subsequent

sintering the tube shrinks more than the plugs because of its

lower initial density. By way of example one process in accordance with the invention for the formation of envelopes for high pressure sodium discharge lamps will now be described in more detail. In this process the envelope for a lamp is composed of a cylindrical tube and two end plugs. The ceramic tube is prepared by isostatically pressing high purity, sub-micron

particle size, alumina powder, containing a small amount of magnesia, to a pressure of five tons per square inch and then firing the compact, thus formed, at 11000°C for 4 hours, the density of the compact at this stage being 1.84 gm.cm-3.

The plug is prepared from the same. powder as the tube but. in this case the powder is spray dried prior to compaction.

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13.3 91 no

Spray drying is a method whereby an aqueous suspension

of the powder, which may contain a small proportion of

organic binding agent, is introduced, in the form of a

fine spray, into a chamber counter to a current of hot

air. Under the action of surface tension forces, the

spray forms itself into regular droplets which are rapidly

dehydrated at the temperature of the chamber. The

resulting stable spheroids may readily be separated from any fine material produced, by passing the product through

an air cyclone. The particles thus produced have diameters

of the order of a few hundred microns, depending upon the

drying conditions. Particles produced by spray drying are

free flowing, dust free and display a higher bulk density

than the starting amorphous powder.

The plugs produced by compaction of this powder at

five tons per square inch and subsequent firing are a tight 3 fit inside the tube and have a density of 2.15 gm.cm The

tube, fitted with a plug in each end, is then fired in

hydrogen for several hours to allow the complete assembly

to attain almost complete densification (i.e.73.95 gm.cm-3).

As the initial density of the tube is less than that of the

plugs its shrinkage during sintering is correspondingly greater and this results in intimate contact being produced

between the tube and plug surfaces. This, in turn, allows

grain growth, across the interface, to occur during sintering

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13 3 91 no

thus producing an effective hermetic seal. Examination

of the completed assembly shows that the diameters of the

portions of the tube which are sealed to the plugs are

0.004" 0.006" greater than the rest of the tube. hicro-

structural examination of the seal interface reveals a homogeneous structure with only small isolated porosity. Preferably, identical starting material is used for

both the tube and the plugs, but the same process could be

used with materials of differing composition, as a

supplementary means of controlling differential shrinkage. Further control of shrinkage can be achieved by pressing

the plugs at a higher pressure than thetube or bypartially

sintering the compacted plugs to increase their density before they are machined to fit the tube.

The plugs are formed with a hole or holes through which an electrode and, if necessary, an exhaust tube may

be-sealed. The magnesia in the mixture may be replaced by other

additives for controlling grain growth.

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427.790

13.3 9 1/70

TIM CLAIMiS DEFINING THIS INV!ENTIONI ARE AS FOLLOWS:

1. A method of forming sintered ceramic articles from a number of parts with homogenous seals between the parts formed by differential shrinkage of the parts during sintering wherein at least one part is made from powder which has been spray-dried giving it a higher bulk density resulting in smaller shrinkage during sintering than that of another part made directly from powder.

2. A method as claimed in claim I in which the said one part is a plug for closing the end of a cylindrical tube forming the other part.

3. A method as claimed in claim I or 2 in which the two parts are composed of the same material.

4. A method as claimed in claim 3 in which the material* is alumina with a small amount of magnesia.

A method as claimed in any of the preceding claims in which the parts 'are initially formed by compression and firing and are then machined to fit before sintering.

6. A method as claimed in claim 5 in which the parts are compressed at different pressures.

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13 .391no

Dated this 1st day of April-, 1970

THORN LIGHTING LIMITED By its Patent Attorney

A. WALLACE SMITH

.l0 7 427.7