This invention relates to a cathode ray tube having an envelope comprising a metal rear housing having a wall portion defining an opening and a substantially flat glass faceplate which extends over the opening and is sealed in a vacuum-tight manner to the surface of a flange on the rear housing extending around the opening by means of a compliant pressure bonded seal between the faceplate and the flange and comprising a pressure deformable material.
The rear housing, which in the finished cathode-ray tube contains the electron gun and other components of the tube, is usually referred to as the cone portion of the envelope, although it may not be strictly, or even remotely, conical in the geometric sense. For example, a new type of rear housing which is very relevant to the present invention and which may be referred to as a "flat-can" rear housing has the form of a shallow rectangular, metal can with a generally flat, slightly dished bottom and an open top surrounded by a flange. An envelope having this type of rear housing is used in so-called flat cathode-ray tubes. To form the envelope a rectangular flat glass faceplate is sealed to the flange of the can.
In order to reduce costs, it is advantageous to make the envelope using cheap components which can be sealed using simple apparatus and at relatively low temperatures. Thus the metal rear housing may consist of mild steel for cheapness and ease of forming. The faceplate may consist of flat toughened float glass which is significantly cheaper than conventional moulded glass faceplates and need not be so thick so that it is lighter. Such materials are not ideally matched as regards their coefficients of thermal expansion but by using a compliant pressure bonded seal to bond the faceplate and metal rear housing together and provide a seal therebetween, the effects of the difference in the coefficients of thermal expansion can be accommodated. Examples of compliant, pressure bonded seals are given in Applicant's British Patent Specification No. 1598888, corresponding to U.S. Pat. No. 4,268,712 whose disclosure in this respect is incorporated herein by reference. Whilst the bonding achieved by such seals might not be as stiff as that provided by a glass frit seal, the latter form of seal is comparatively brittle and not suited to bonding together materials having significantly differigg coefficients of thermal expansion. Moreover, glass frit seals require the application of considerably higher temperatures when forming the seal which, besides being more expensive to implement, can be undesirable as regards the risk of possible damage to components within the envelope.
British Patent Specification No. 2123210 describes a cathode ray tube envelope having a construction as defined in the opening paragraph. In the described construction, the peripheral portion of the flat glass faceplate overlies the flange of the rear housing and this flange projects outwardly from the opening defined by the rear housing.
In the manufacture of a cathode-ray tube having an envelope of this construction the faceplate deflects inwards slightly upon evacuation of the envelope and there is a consequent tendency for the faceplate to peel away from the seal between the faceplate and the flange of the rear housing and/or for the seal to peel away from the flange.
To counteract this tendency, it is proposed in the aforementioned British Patent Application No. 2123210 that spring clamps be provided on the edges of the faceplate and flange, each such clamp consisting of, for example, a spring stee channel member with wall portions which converge towards one another and which engage with the front and rear surfaces of the faceplate and flange respectively to exert a clamping force perpendicular to these surfaces to hold the faceplate and flange together. While being effective, the provision of these clamps complicates the construction of the cathode ray tube envelope and adds to the overall cost of the cathode ray tube.