This study was made to reveal some causes of break-down of the folded heater and thus to establish more effective way of preventing the break-down of heater wire.
The broken parts of heater wire were brittle, and our microscopic observations found that the brittle parts showed large spheroidal crystal grains, whereas the unbroken part showed grains similar to fiber structure.
Treatments of heater wire by three different methods, (e.g., wet hydrogen annealing, dry hydrogen annealing and oxygenate annealing treatments), before coating of the insulating pastes over its surface, showed that the oxygenate annealing treatment was most effective to prevent the break-down of heater wire under our experimental conditions.
The reason seems of be that the oxygenate annealing treatments etch off the metallographically instable surface of heater wire caused by the graphite lubricants and other impurities enfolded during the drawing process of wire, and serve to prevent coarsing of the crystal grains.
The heater insulating pastes containing some nitrocelluloses which are added as the binder are inferior to the pastes containing 9 aqueous aluminium nitrates for the purpose to prevent the break-down of heater wire.
Considering the facts that the above mentioned instable surfaces which cause the break-down of heater wire certainly are not removed sufficiently in the case of nitrocellulose pastes, we can understand the difference between the nitrocellulose pastes and the aluminium nitrate pastes for the prevention of the break-down of heater wire. The break-dwon of heater wire was prevented very effectively using the tungsten wire doped with 0.2% of thoria. The cause of break-dwon of bent part of heater results from the fact that the insulating coating of the bent part is scraped off by the sleeve during long operation and as a result the short circuit accident is induced.
The short circuit accidents may be prevented by lessening the size of fault spot which is subjected to scrapping during the bending process.
It seems that the size of fault spot has some relations to binder contained in insulating pastes and the shape of forming brade of the heater folding machine.