182 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1957 



and distinct chemical actions occurring in the oxide cathode itself. Both 

 actions are side issues in no way essential to the basic functioning of the 

 cathode and it seems probable that both can be eliminated if sufficient 

 understanding of their nature is available. The first action is the growth 

 of a resistive interface layer between the oxide matrix and its supporting 

 nickel core, discussed briefly in an earlier section. This effect is assumed 

 to be due to silicon contamination of the nickel core metal. 



4 BaO + Si = Ba2Si04 + 2 Ba. 



The resistance of the laj^er of barium orthosilicate rises as it loses its 

 barium activator and approaches the intrinsic state. The effect of the in- 

 terface resistance is to bring negative feedback to bear on the tube with 

 resulting loss of transconductance. The second deleterious action is loss 

 of electron emission from the oxide cathode by direct destruction of its 

 essential excess barium metal by oxidizing action of residual gases. Such 

 gases result from an imperfect processing technology. 



These two problems have been approached in the 6P12 tube in a some- 

 what novel manner. The conventional nickel core is replaced by platinum 

 of such high purity (99.999 per cent) that the possibility of appreciable 

 interface growth from impurities can be disregarded. The only factor to 

 be considered is the appearance of high resistive products of a possible 

 interaction between platinum and the alkaline earth oxides. Batch tests 

 over a period of 30,000 hours have failed to show any sign whatever of 

 such an action occurring and workers at Dollis Hill now regard the pure 

 platinum-cored tube as free from the interface resistance phenomenon. 



The problem of avoiding gas deactivation of the cathode is a more 

 difficult one and so far has been reduced in magnitude rather than elim- 

 inated. It is now appreciated that the dangerous condition arises from 

 "gas generators" left in the tube and not from a true form of residual 

 gas pressure left after seal-off from the pump. These gas generators are 

 solid components of the tube which give off a continual stream of gas 

 over a prolonged period of time. The gas evolution rates are usually so 

 small that they cannot be detected by reverse grid current measurement 

 but they tend to integrate gas by absorption on the cathode and to de- 

 stroy its activity. The gas generators are usually of finite magnitude and, 

 depending mainly on diffusion phenomena, evolve gas at a rate which 

 falls in roughly exponential fashion with time. The probability of trans" 

 conductance failure is therefore highest in early life and tends to lessen 

 with time as the generators move to exhaustion. 



One particularly useful feature of the platinum-cored cathode is its 

 freedom from core oxidation during gas attack and this leaves the tube 



