Method of making a ceramic evacuatable enclosure

Details Method of making a ceramic evacuatable enclosure Method of making a ceramic evacuatable enclosure

1984-11-05

1987-05-05

Weston, Caleb

Adhesive bonding and miscellaneous chemical manufacture

Methods

Surface bonding and/or assembly therefor

1563042, 228193, 228195, 228196, 264 56, 264 66, B32B 1800, B32B 3126

Patent

active

046629580

DESCRIPTION:

BRIEF SUMMARY
This invention relates to a ceramic evacuatable enclosure.
Ceramic evacuatable enclosures are required for example for laser waveguides and other optical and electronic devices such as vacuum tubes. Ceramic laser waveguides are known, and comprise a ceramic enclosure in the form of a waveguide cavity in a ceramic material such as alumina (Al.sub.2 O.sub.3). For acceptable performance efficiency, the waveguide cavity requires an accurately linear longitudinal axis, a constant cross-section and a high quality internal surface finish. Furthermore, to achieve and maintain laser action in the waveguide cavity, it must contain a mixture of gases in specific relative proportions at low pressure. The proportions and pressure must be maintained over the working life of the device. These criteria apply particularly to CO.sub.2 laser waveguides formed in alumina.
A number of techniques have been investigated and applied to the production of CO.sub.2 laser waveguides. One such technique is to employ an ultrasonic drill to machine a laser bore in an alumina block. Laser waveguide cavities have been fabricated with bores of 1 to 2 mm in diameter and 250 mm in length. Straightness has been achieved to camber of less than 10.sup.-3 (ratio of mid-bore centre deviation from colinearity with the line of bore end centres to bore length). Good bore surface finish and acceptable constancy of cross-section have also been achieved, with good leak-tightness for retention of the laser gas medium. However, laser output power varies approximately in proportion to cavity length, and cavity machining accuracy worsens rapidly with increasing drilling length. The limit of bore length which retains acceptable cavity accuracy is probably not greater than 350 mm, which sets an upper limit to the maximum laser power obtainable. In fact, all ultrasonic drilling of the standard required for laser cavities is time consuming and expensive. For some applications requiring high power compact lasers, a folded-path waveguide structure would be desirable. This is not technically practicable by ultrasonic drilling, since it imposes very stringent bore alignment criteria.
It is also known to produce a ceramic waveguide in the form of a covered channel. A waveguide channel is formed in a ceramic substrate, and a cover is placed over the channel. The substrate and cover are then clamped together. This arrangement is not leak-tight, and a surrounding vacuum vessel is required to contain the laser gas medium communicating with the waveguide cavity. Excitation electrode connections and the laser light path must pass through the outer vessel, resulting in an undesirably complex and bulky structure.
It is well known to join together ceramic components to form composite structures. Earthenware articles such as crockery may be provided with handles etc by treating surfaces to be joined with a slip or aqueous suspension of their constituent clay. This is not appropriate for insoluble ceramics or where precision geometries are required. Glazes or ceramic to metal seals may also be employed. In particular, Allen and Borbidge describe alumina to platinum bonding in J. Mat. Sci. 18 (1983) pp 2835-2843. However, both glazes and ceramic to metal seals have the disadvantage that their joints produce an inhomogeneity or discontinuity of composition between the parts being joined, leading to difficulties and weaknesses due to differential thermal expansion. Moreover, particularly in the case of the production of cavities by glazing, it is difficult to avoid the glaze material flowing into the cavity region from the joint. A further drawback in the case of a ceramic to metal seal is the problem that an electrically conducting region is produced which affects the electrical insulation of the joint. These disadvantages are particularly serious in the case of an alumina laser waveguide, which requires an accurate, vacuum tight, optically homogeneous bore clear of extraneous sealing materials and providing electrical insulation.
It is an object of the present invention to pro

REFERENCES:
patent: 2756892 (1956-07-01), Bleuze et al.
patent: 3239323 (1966-03-01), Folweiler
patent: 3789499 (1974-02-01), Pequignot
patent: 3982204 (1976-09-01), Andringa
patent: 4241319 (1980-12-01), Papayoanou
"Engineering Ceramics--When to Use Them", Design Engineering, Nov. 1977, pp. 79-82.
R. P. Harrison, "Report on the Ultrasonic Inspection of Two Bonded Ceramic Components", Fullmer Technical Services.

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