The Cracking of Glass Envelope of Miniature Electron Tube by Thermal Shock

Abstract

We discussed mainly the various methods of thermal shock testing of glass envelope of electron tubes in our previous report.
In this paper, it is pointed out that the stiffness of the stem pin, the firing condition of stem molding and the seal-in process have a great deal to do with the thermal shock cracking.
It is intended in the present paper to clear up the causes of thermal shock failure based on these results.
(1) If the glass base of the electron tube is strongly tempered in the seal-in process, strong radial compression stress is set up and crack occurs radially across the base through the pin on the downward thermal shock test. On the other hand, if the base is lightly tempered, strong radial tension stress is set up and crack occurs radially across the base through the pin or the crack tangentially on the circle of the pin through the pin in the upward thermal shock test.
(2) The relation between the test of stiffness of the Ni outer pin of the stem (annealing treatments of the Ni outer pin) and the thermal shock test with deflexion cone has been examined, and if the pin is too stiff, crack occurs tangentially on the circle of the pin through the pin in the thermal shock testing.
(3) Crack occurs radially across the base between the pins in the thermal shock test when the temperature of stem molding is too low. It appears that laps are formed between the stem beads, and they act as the cause of the fracture.
(4) If the final shape of the sealed electron tube is deformed, crack occurs tangentially on the seal in the thermal shock test.
(5) The cracks of the glass base are classified into four types, that is,
Type A: Crack radially across the base through the pin
Type B: Crack tangentially on the circle of the pin through the pin
Type C: Crack radially across the base between the pins
Type D: Crack tangentially on the seal.