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Initiation of hot-cathode discharges
VACUUM Classified Abstracts
IV
Special Subsidiary Subjects --
IV
Abstract No. and References
Contd.
frequencies, higher electrostatic fields operate against these cathodes which do not possess sufficient mechanical s t r e n g t h and surface regularity. Since the basic materials of the oxide cathode (the alkaline earths) a p p e a r to be the m o s t satisfactory emitters, it has been a t t e m p t e d recently to devise cathodes using these materials b u t eliminating the m a j o r disadvantages associated with the conventional designs. One such development is the 'L' cathode which is described b y H. J. L e m m e n s , M. J. J e n s o n and R. Loosjes, (Philips Tech. Rev., xI, 341). The L-cathodes have low electrical resistance, great mechanical s t r e n g t h and a s m o o t h surface which m a y be machined. T h e y also w i t h s t a n d electron b o m b a r d m e n t and recover quickly from the effects of poisoning and ion b o m b a r d m e n t . V e n e m a (Le Vide, 9, 267) describes some applications of L-cathodes including a disc triode w i t h cathode-grid spacing of 0.04 mm. I t has been stated t h a t L-cathodes h a v e lives greater t h a n 8,000 h o u r s w h e n operated at 950°C (brightness) and with an emission density of 1.3 A./cmL I n a n o t h e r cathode development an i m p r e g n a t e d W disc is used instead of the oxide pellet and sintered disc of the L-cathode. R. Levi (Le Vide, 9, 284) uses a sintered m i x t u r e of W and b a r i u m aluminates giving cathodes which can be machined to close tolerances. D. MacNair, R. T. L y n c h and N. B. H a n n a y (J. Appl. Phys,, 24, 1335) use moulded cathodes made b y compressing m i x t u r e s of nickel powder and alkaline earth carbonates in steel dies. After sintering the cathodes can be machined as desired. These cathodes are highly resistant to poisoning, stable at high voltages and have emission characteristics between sprayed oxide and L-cathodes. A f u r t h e r form of cathode has been described b y W. Balas, J. Dempsey, and E. F. Rexer (J. Appl. Phys. 26, 1163, Sept. 1955) in which sintered nickel is impregnated with a strong solution of b a r i u m and s t r o n t i u m acetates in water, followed b y a solution of a m m o n i u m carbonate to precipitate the carbonates in the pores. These cathodes give emission densities of a b o u t 10 A./cm. ~ and have a work function of a b o u t 1.0-1.25 eV.
Article by F. A. Vick
Sci. Progr.
No. 173, Oct. 1955 65-71
45/iv
Initiation of Hot-Cathode Discharges
United Kingdom. An experimental s t u d y has been made of the effect of a voltage fall along a h o t cathode on the initiation of a gas discharge in a diode at low pressure. Plasma forms first near the negative end of the cathode and spreads along the cathode with increase in t u b e current. Positive ions diffuse in the inter-electrode space t o w a r d s the positive end of the cathode, where the potential difference between cathode and anode w h e n p l a s m a is p r e s e n t m a y be well below the ionisation potential of the gas. Observations have been made on the appearance of the discharge before and after p l a s m a forms, and on oscillations generated b y the discharge.
(Authors) Improvements i.o.r.t. Coating Metals with Composite Metallic Aluminium Compounds
Article by N. A. Daly & K. G. Emeleus
Brit. J. Appl. Phys. 6, Oct. 1955 370-372 46/IV
Germany. I t is k n o w n practice to use Al-coated Ni, Fe, etc., to make anodes of radio valves; the metal is coated with A1 b y rolling after which on heating to a high t e m p e r a t u r e an intermetallic c o m p o u n d is formed which has good heat-radiating properties, A difficulty is t h a t spots of impurities form on the outer surface, causing h o t spots and spoiling the v a c u u m in the finished valve. The invention describes a modification in which the heating is adjusted so t h a t the intermetallic c o m p o u n d is formed thinly at the interface and the remaining A1 is t h e n dissolved a w a y to expose the compound. I t is claimed t h a t the surface has a darker colour when formed in this w a y and t h a t because of the reduced heating the base metal is less affected. An example is described of the t r e a t m e n t of Ni sheet: The sheet is coated with a layer of A1 10 micron thick a n d t h e n heated for 10 min. at 600°C to form the c o m p o u n d NiA1 v The unconverted A1 can t h e n be removed b y a weak solution of caustic soda, sodium carbonate or hydrochloric acid, or alternatively by a m a l g a m a t i o n with m e r c u r y or a m e r c u r y salt.
The Breakdown of Gases Subject to Crossed Electric Fields
Telefunken G.m.b.H.
BriL Pat. 750,637
47/IV
United Kingdom. The hypothesis is made t h a t in ultra-high frequency b r e a k d o w n where the drift m o v e m e n t imposed on an electron b y the field is very restricted, the localisation of the electron contributes to breakdown. An experimental s y s t e m is described in which an auxiliary unidirectional or relatively low-frequency field is imposed at right angles to a microwave field to destroy the localisation. W h e n the auxiliary field is unidirectional, or of relatively low frequency (0.86 Me/s) its application raises the b r e a k d o w n stress, b u t w h e n the frequency is raised to 9.7 Mc/s the t w o fields a p p e a r to act quite independently in producing breakdown. A partial explanation is advanced. The gases studied include air, oxygen, nitrogen, h y d r o g e n and neon.
(Authors) Some Factors Influencing Field Emission and the Fowler-Nordheim Law
Article by W. A. Prowse & P. E. Lane
Prec. Phys. See. 69 B, Jan. 1956 33-46 48/IV
United Kingdom. Investigations of field emission up to fields of a p p r o x i m a t e l y 7 × 10 7 V cm. -1 show t h a t the experimentally observed c u r r e n t s tend to be progressively less t h a n t h a t predicted b y the Fowler-Nordheim law in the region between 10 ~ V cm. -1 and 7 × 10 7 V cm. -1. The a u t h o r does not disagree with the view t h a t these departures from t h e o r y are due to space charge distortion of the electric field b u t draws attention to other second-order effects which m a y produce departures from the law at higher fields. The Fowler-Nordheim law was established on the a s s u m p t i o n t h a t it was legitimate to use the image law and t h a t the field is uniform over distances of the order of 1.0 ~ from the surface. The a u t h o r points o u t t h a t it has been indicated t h a t it is unlikely t h a t the image law holds within 3 ~ of the surface and t h a t since the surface barrier is reduced as the field is increased it is unlikely, in the range up to 7 × l0 T V cm. -I, t h a t the surface barrier would be like t h a t deduced using the image law. As a result the usual Fowler-Nordheim linear plot would be modified to lower currents in the high field range. This has been ascertained using clean t u n g s t e n emitters and also using t u n g s t e n
October, 19.56
Vacuum Vol. VI
305
IV
Special Subsidiary Subjects --
IV
Abstract No. and References
Contd.
frequencies, higher electrostatic fields operate against these cathodes which do not possess sufficient mechanical s t r e n g t h and surface regularity. Since the basic materials of the oxide cathode (the alkaline earths) a p p e a r to be the m o s t satisfactory emitters, it has been a t t e m p t e d recently to devise cathodes using these materials b u t eliminating the m a j o r disadvantages associated with the conventional designs. One such development is the 'L' cathode which is described b y H. J. L e m m e n s , M. J. J e n s o n and R. Loosjes, (Philips Tech. Rev., xI, 341). The L-cathodes have low electrical resistance, great mechanical s t r e n g t h and a s m o o t h surface which m a y be machined. T h e y also w i t h s t a n d electron b o m b a r d m e n t and recover quickly from the effects of poisoning and ion b o m b a r d m e n t . V e n e m a (Le Vide, 9, 267) describes some applications of L-cathodes including a disc triode w i t h cathode-grid spacing of 0.04 mm. I t has been stated t h a t L-cathodes h a v e lives greater t h a n 8,000 h o u r s w h e n operated at 950°C (brightness) and with an emission density of 1.3 A./cmL I n a n o t h e r cathode development an i m p r e g n a t e d W disc is used instead of the oxide pellet and sintered disc of the L-cathode. R. Levi (Le Vide, 9, 284) uses a sintered m i x t u r e of W and b a r i u m aluminates giving cathodes which can be machined to close tolerances. D. MacNair, R. T. L y n c h and N. B. H a n n a y (J. Appl. Phys,, 24, 1335) use moulded cathodes made b y compressing m i x t u r e s of nickel powder and alkaline earth carbonates in steel dies. After sintering the cathodes can be machined as desired. These cathodes are highly resistant to poisoning, stable at high voltages and have emission characteristics between sprayed oxide and L-cathodes. A f u r t h e r form of cathode has been described b y W. Balas, J. Dempsey, and E. F. Rexer (J. Appl. Phys. 26, 1163, Sept. 1955) in which sintered nickel is impregnated with a strong solution of b a r i u m and s t r o n t i u m acetates in water, followed b y a solution of a m m o n i u m carbonate to precipitate the carbonates in the pores. These cathodes give emission densities of a b o u t 10 A./cm. ~ and have a work function of a b o u t 1.0-1.25 eV.
Article by F. A. Vick
Sci. Progr.
No. 173, Oct. 1955 65-71
45/iv
Initiation of Hot-Cathode Discharges
United Kingdom. An experimental s t u d y has been made of the effect of a voltage fall along a h o t cathode on the initiation of a gas discharge in a diode at low pressure. Plasma forms first near the negative end of the cathode and spreads along the cathode with increase in t u b e current. Positive ions diffuse in the inter-electrode space t o w a r d s the positive end of the cathode, where the potential difference between cathode and anode w h e n p l a s m a is p r e s e n t m a y be well below the ionisation potential of the gas. Observations have been made on the appearance of the discharge before and after p l a s m a forms, and on oscillations generated b y the discharge.
(Authors) Improvements i.o.r.t. Coating Metals with Composite Metallic Aluminium Compounds
Article by N. A. Daly & K. G. Emeleus
Brit. J. Appl. Phys. 6, Oct. 1955 370-372 46/IV
Germany. I t is k n o w n practice to use Al-coated Ni, Fe, etc., to make anodes of radio valves; the metal is coated with A1 b y rolling after which on heating to a high t e m p e r a t u r e an intermetallic c o m p o u n d is formed which has good heat-radiating properties, A difficulty is t h a t spots of impurities form on the outer surface, causing h o t spots and spoiling the v a c u u m in the finished valve. The invention describes a modification in which the heating is adjusted so t h a t the intermetallic c o m p o u n d is formed thinly at the interface and the remaining A1 is t h e n dissolved a w a y to expose the compound. I t is claimed t h a t the surface has a darker colour when formed in this w a y and t h a t because of the reduced heating the base metal is less affected. An example is described of the t r e a t m e n t of Ni sheet: The sheet is coated with a layer of A1 10 micron thick a n d t h e n heated for 10 min. at 600°C to form the c o m p o u n d NiA1 v The unconverted A1 can t h e n be removed b y a weak solution of caustic soda, sodium carbonate or hydrochloric acid, or alternatively by a m a l g a m a t i o n with m e r c u r y or a m e r c u r y salt.
The Breakdown of Gases Subject to Crossed Electric Fields
Telefunken G.m.b.H.
BriL Pat. 750,637
47/IV
United Kingdom. The hypothesis is made t h a t in ultra-high frequency b r e a k d o w n where the drift m o v e m e n t imposed on an electron b y the field is very restricted, the localisation of the electron contributes to breakdown. An experimental s y s t e m is described in which an auxiliary unidirectional or relatively low-frequency field is imposed at right angles to a microwave field to destroy the localisation. W h e n the auxiliary field is unidirectional, or of relatively low frequency (0.86 Me/s) its application raises the b r e a k d o w n stress, b u t w h e n the frequency is raised to 9.7 Mc/s the t w o fields a p p e a r to act quite independently in producing breakdown. A partial explanation is advanced. The gases studied include air, oxygen, nitrogen, h y d r o g e n and neon.
(Authors) Some Factors Influencing Field Emission and the Fowler-Nordheim Law
Article by W. A. Prowse & P. E. Lane
Prec. Phys. See. 69 B, Jan. 1956 33-46 48/IV
United Kingdom. Investigations of field emission up to fields of a p p r o x i m a t e l y 7 × 10 7 V cm. -1 show t h a t the experimentally observed c u r r e n t s tend to be progressively less t h a n t h a t predicted b y the Fowler-Nordheim law in the region between 10 ~ V cm. -1 and 7 × 10 7 V cm. -1. The a u t h o r does not disagree with the view t h a t these departures from t h e o r y are due to space charge distortion of the electric field b u t draws attention to other second-order effects which m a y produce departures from the law at higher fields. The Fowler-Nordheim law was established on the a s s u m p t i o n t h a t it was legitimate to use the image law and t h a t the field is uniform over distances of the order of 1.0 ~ from the surface. The a u t h o r points o u t t h a t it has been indicated t h a t it is unlikely t h a t the image law holds within 3 ~ of the surface and t h a t since the surface barrier is reduced as the field is increased it is unlikely, in the range up to 7 × l0 T V cm. -I, t h a t the surface barrier would be like t h a t deduced using the image law. As a result the usual Fowler-Nordheim linear plot would be modified to lower currents in the high field range. This has been ascertained using clean t u n g s t e n emitters and also using t u n g s t e n
October, 19.56
Vacuum Vol. VI
305