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PENCIL LEAD FIELD EMITTER
R. Khairnar, D. Joag
To cite this version:
R. Khairnar, D. Joag. PENCIL LEAD FIELD EMITTER. Journal de Physique Colloques, 1989, 50
(C8), pp.C8-85-C8-90. �10.1051/jphyscol:1989815�. �jpa-00229913�
COLLOQUE DE PHYSIQUE
C o l l o q u e C8, s u p p l 6 m e n t a u n o 11, Tome 50, novembre 1989
PENCIL LEAD FIELD EMITTER
R.S. KHAIRNAR a n d D.S. JOAG
D e p a r t m e n t of P h y s i c s , U n i v e r s i t y o f Poona, Pune-411 007, I n d i a
Abstract
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Field electron emission from W and HB grades of pencil lead has been studied. The p e n c i l l e a d f i e l d emitter is found t o obey t h e Fowler-Nordheim characteristics. The emission cut-rent fluctuations a r e found to increase with the residual gas pressure and the emission current. The current density of the order ofl o 6
amp/cm2 c o u l d be drawn from t h e s e emitters. The emission s t a b i l i t y over t h e o p e r a t i o n of s i x hours h a s been found t o be reasonably good. The f i e l d i o n microscopy of t h e 2H and HB grade p e n c i l l e a d has been c a r r i e d o u t d e p i c t i n g t h e lesser degree of graphitization i n the 2H grade i n comparison with the HB grade. A comparison of the two grades of emitters has been made i n view of the respective degrees of graphitization.Various fonns of ca&m such a s carbon f i b r e /1,2/, glassy carbon /3/ a s w e l l a s carbon based composites /4/ have been studied i n the past a s promising f i e l d e m i t t e r s because of t h e i r chemical i n e r t n e s s , s t r u c t u r a l s t a b i l i t y , r e s i s t a n c e t o i o n bombardment e t c . For example, carbon f i b r e s have been shown tm exhibit long emission l i f e p a r t i c u l a r l y i n the residual gas pressure of the order of 10'~ mbar /5/. These a r e a l s o employed a s emitters i n t h e cathode r a y tube /6,7/. P e n c i l l e a d is a w e l l known g r a p h i t e composite c o n s i s t i n g of graphite flakes bonded together by means of a clay. Recently, t i p s of HB grade pencil lead have been s u b j e c t e d t o a f i e l d i o n and e l e c t r o n e m i s s i o n microscopic i n v e s t i g a t i o n i n an e f f o r t to utxkrstand their morphology and emission c h a r a c t e r i s t i c s /8/. It has been observed t h a t t h e f i e l d e l e c t r o n emission p a t t e r n shows emission p a t c h e s d i s p l a y i n g i n t e n s i t y fluctuations consisting of a combination of emission spots turning on and off andomly and a localised f l i c k e r of individual spots. Generally speaking, t h i s type of noise is typical of the carbon f i b r e and the glassy carbon f i e l d emitters.
Pencil lead o f f e r s i t s e l f a s a readily available, inexpensive composite of graphite.
The p e n c i l l e a d emitter t i p s have t h e advantage of e a s e of f a b r i c a t i o n , viz. mechanical p o l i s h i n g using emery paper o r conventional e l e c t r o c h e m i c a l etching. Various g r a d e s of p e n c i l l e a d can be used t h u s g i v i n g a n o p p o r t u n i t y t o s t u d y t h e e f f e c t of t h e degree of g r a p h i t i z a t i o n . As a r e s u l t of t h e e x t r u s i o n p r o c e s s by means of which t h e p e n c i l l e a d is manufactu&, the graphite flakes a r e oriented with t h e i r a-axes approximately p a r a l l e l t o the pencil lead axis. Ihe e l e c t r i c a l conductivity of graphite along the a-axis is two orders
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1989815
of magnitude higher than along the c-axis /9/. This s i t u a t i o n is, therefore, favourable f o r the pencil lead to be a promising f i e l d emitter. The aim of the present work is t o find out a s to' w h e t h e r the pencil lead f i e l d emitter is a b e t t e r a l t e r n a t i v e to the above mentioned carbon based emitters which u s u a l l y need e l a b o r a t e processing. Some miscellaneous observations on the 2H and'the HB grades of pencil lead emitters, from the point of view of their degrees of graphitization, a r e a l s o reported.
The emitter t i p s were made from 0.3 mm diameter 2H and HB grade p e n c i l l e a d by electropolishing i n 2N potassium hydroxide solution. The t i p was crimped i n a nickel tube, spot welded to the tungsten loop and mounted i n a g l a s s f i e l d emission microscope (FEM). The F E M tube was evacuated by means of a cold trapped diffusion pump and a s p u t t e r ion pump to a p r e s s u r e of t h e o r d e r of lxlo-' mbar. The c u r r e n t (I)- v o l t a g e (V) c h a r a c t e r i s t i c s were observed by means of a highly regulated dc power supply (Fluke 410B). The emission current s t a b i l i t y was s t u d i e d by means' of a n x-t c h a r t recorder. The dependence of c u r r e n t f l u c t u a t i o n s on t h e r e s i d u a l g a s p r e s s u r e and t h e c u r r e n t drawn from t h e emitter was studied.
The t i p morphology was observed i n a f i e l d i o n microscope (FIM). For t h i s purpose, a pencil lead emitter t i p was mounted i n an a l l s t a i n l e s s steel FIM and imaged i n helium g a s f i l l e d a t a p r e s s u r e of 6 x l 0 - ~ mbar a f t e r evacuating t h e chamber t o t h e p r e s s u r e below 1x10-* mbar. The t i p was cooled with l i q u i d nitrogen.
The t i p s were observed i n a scanning electron microscope (SEN) f o r understanding the gross features of these emitters. The various grades of pencil lead were a l s o subjected to x-ray powder d i f f r a c t i o n (XRD) s t u d y and energy d i s p e r s i v e x-ray a n a l y s i s (EDAX). T h i s analysis shows t h a t pencil lead consists of hexagonal graphite c r y s t a l s and a small quantity of s i l i c o n oxide possibly contributed by the clay used a s a binder.
3-RESULTS AND DISCUsSJ.CN
Figure l ( a ) shows the Fowler-Nordheim (F-N) p l o t a f t e r annealing the t i p a t about 1200 L Some outgassing was a l s o observed to take place a t t h i s temperature. The overall nature of the F-N p l o t shows the pencil lead e m i t t e r to be metallic i n Mture. The f i e l d e m i s s i o h p a t t e r n is shown i n fig.l(b). The FEM p a t t e r n c o n s i s t s of b r i g h t patches a s w e l l a s emission spots turning on and off randomly. Some of the emission spots were found to f l i c k e r with a c e r t a i n frequency f o r a few seconds. Figure 2 depicts the s h o r t term emission current fluctuations a t the residual gas pressure of mbar a t three d i f f e r e n t values of the i n i t i a l l y set current, f o r the 2H grade emitter. Similar study f o r the HB grade pencil lead has been done earlier and has been presented e l s e w h e r e /8/. The fluctuations are found to be s i m i l a r t o those of t h e HB grade emitter. The f l u c t u a t i o n s i n t h e form of r a p i d rise t i m e pulses were found to increase a s the residual gas pressure increases. They w e r e a l s o found t o i n c r e a s e w i t h t h e i n c r e a s i n g c u r r e n t . Such a p r e s s u r e and c u r r e n t dependant n o i s e has a l s o been observed i n carbon f i b r e /1/ and g l a s s y carbon /3/ f i e l d emitters. S i m i l a r t o these e m i t t e r s this noise is suppressed by the inclusion of a high resistance (Mega ohm) i n series w i t h t h e emitter by way of e x t e r n a l s t a b i l i z a t i o n . Figure 3 shows t h e long term current s t a b i l i t y of the 2H pencil lead e m i t t e r when operated a t the pressure of 1 x 1 0 ~ mbar i n a s e a l e d tube w i t h t h e i n c l u s i o n of a 30 Mega ohm r e s i s t a n c e i n series. The c u r r e n t is o f t e n found to make excursion to higher current due t o switching on of an emission s i t e . The c u r e n t s t a b i l i t y is found t o improve w i t h reducing t h e p r e s s u r e and i n c r e a s i n g t h e s t a b i l i z i n g resistance. Similar behaviour is observed f o r the HB grade emitter.- emission is supposedly t a k i n g p l a c e from t h e g r a p h i t e f l a k e s p r o t r u d i n g o u t of t h e t i p surface. I t is, therefore, d i f f i c u l t to estimate the current density a s the f i e l d enhancement f a c t o r f o r each emitting site is unknown. The l i n e a r i t y of the F-N p l o t allows to estimate the CUrrent
Fig.l(a)- F-N p l o t of the 2H grade Fig.l(b)- A t y p i c a l FEM pattern o f the 2H emitter t i p upon annealing. grade pencil lead t i p (3.6 kV).
Fig.2- Short term emission current f l u c t a a t i o n s f o r the 2H e m i t t e r a t the residual gas pressure o f 4x10-~ mbar a t the current l e v e l s (a) 1 PA, (b) 0.5 PA and (c) 0.1 PA.
T I M E ( h o u r s )
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Fig.3- The long term current s t a b i l i t y o f the 2H grade p e n c i l lead emitter with a 30 Mega ohm resistance i n series ( p r e s s u r e r l x l ~ - ~ mbar).
Fig.4- F i e l d i o n micrographs of ( a ) 2H (9.0 kV) and (b) HB (7.9 kV) grade p e n c i l lead emitter t i p s imaged a t liquid nitrogen temperature i n helium gas.
d e n s i t y by t h e method o u t l i n e d by Charbonnier and Martin
/lo/.
Accordingly, t h e c u r r e n t density of the order oflo6
amp/cm2 is obtained f o r t h i s multitip-like f i e l d emitter.It is f& t h a t the HB grade pencil lead emitter a c t s a s a getter while operating as a f i e l d emitter. Typically, the pressure i n the system dropped from 7 x 1 0 ~ mbar to 3 x 1 0 ~ m b a r during operation. This pumping e f f e c t is a t t r i b u t a b l e to the t u r i a l of the residual gas ions i n between t h e open s t r u c t u r e of t h e g r a p h i t i c l a y e r s . Such a pumping e f f e c t could n o t be observed f o r the 2H grade pencil lead emitter operated under similar conditions.
Figure 4 shows t h e f i e l d i o n micrographs of t h e 2H and HB grade p e n c i l l e a d e m i t t e r t i p s imaged a t l i q u i d n i t r o g e n temperature. These images have been o b t a i n e d a f t e r f i e l d evaporation. It has been noticed while f i e l d evaporating t h a t the e n t i r e flake chips off the t i p and t h e v o l t a g e r e q u i r e d f o r t h i s removal is much higher t h a n t h e best image voltage.
The image spots appear to be elongated due t o uneven magnification i n the f i e l d ion imaging of t h e g r a p h i t e flakes. The edge of each g r a p h i t e f l a k e is thought t o g i v e rise t o a s t a c k of image spots a s seen i n the micrographs. Similar stacks of image points have been observed i n the FIM images of graphite /11/. The interpretation of these images has been discussed i n the e a r l i e r communication /8/. The graphite flakes a r e c l e a r l y imaged i n the HB grade while many additional spots a r e seen i n the micrograph of 2H grade emitter. The difference i n the two micrographs may be a t t r i b u t e d t o t h e d i f f e r e n t d e g r e e s of g r a p h i t i z a t i o n i n t h e two grades. In t h e process of g r a p h i t i z a t i o n /9,12/, t h e p a r a l l e l s t a c k s of g r a p h i t i c p l a n e s grow i n a l l d i r e c t i o n s w i t h a minimal c r i s - c r o s s i n g between t h e two a d j a c e n t p a r a l l e l planes. A perfect graphite c r y s t a l is o m i n which there a r e rn cris-crossing b x i s between the carbon atoms i n the two adjacent planes. 9% m n graphitizing carbon consists of stacks of planes randomly oriented. Because of the small p a r t i c l e s i z e and t h e i r random orientation with respect to each other, the c r y s t a l l i t e s a r e arrested i n their developement even a t high temperature. The less g r a p h i t i z e d one h a s g l a s s y f e a t u r e s and hence is hard t o w r i t e on a paper. From t h i s p o i n t of view, t h e 2H grade is less g r a p h i t i z e d and hence t h e f i e l d i o n micrograph does not show a c l e a r arrangement of p a r a l l e l stacks of planes.
The scanning electron microscopy of these emitters shows graphite flakes with t h e i r a- a x e s approximately p a r a l l e l t o t h e p e n c i l l e a d axis. F i e l d evaporated t i p s show thinned graphite flakes protruding o u t of the surface. Figure 5 shows the o v e r a l l appearance of the f i e l d evaporated t i p surface. A rough estimate of the dimensions of the graphite flake can be made. With a magnification of
lo5
along the longer dimension, the length of the flake a s estimated from the length of the stack i n the FIM image w i l l be a few hurdred angstroms.This agrees with the dimensions observed i n the SEM /8/.
The g e t t e r i n g i n e f f i c i e n c y of t h e 2H grade p e n c i l l e a d e m i t t e r may be due t o t h i s lesser degree of g r a p h i t i z a t i o n . There is l i k e l y t o be more c r i s - c r o s s i n g between t h e p a r a l l e l stacks of planes thereby obstructing the burial of residual gas ions.
The c u r r e n t d e n s i t y o b t a i n a b l e from t h e p e n c i l l e a d emitters seems t o be q u i t e a t t r a c t i v e f o r t h e i r use a s c o l d cathodes i n poor vacuum devices. The e m i t t e r may g i v e b e t t e r s t a b i l i z a t i o n upon pulsed f i e l d o p e r a t i o n and by using an e x t e r n a l c o n t r o l l i n g feedback c i r c u i t f o r its use a s a p r a c t i c a l emitter as has been achieved f o r carbon f i b r e emitter /7/. A study of various grades of emitter tips may be done i n order t o further judge their s u i t a b i l i t y a s cathodes.
a ) 9% pencil lead composite behaves l i k e a multitip emitter. Current density of the order of
lo6
amp/cm2 can be obtained.b) The emission current is noisy and the step-like fluctuations exhibited by these e m i t t e r s a r e s i m i l a r to those of other forms of carbon based emitters.
c) The pencil lead emitter is more o r less s i m i l a r t o the carbon based f i e l d emitters a s f a r a s the emission c h a r a c t e r i s t i c s and the l i f e a r e concerned. The advantages seem to be the ease of t i p preparation and t h e lower c o s t with respect to the c a r h based emitters.
d) The differences between the 2H and the HE grades of pencil lead can be explained
on
the b a s i s of their d i f f e r e n t degrees of graphitization.Fig.5
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SEM photograph of the f i e l d evaporated W grade pencil lead tip.The authors a r e thankful to Ms. Eeena Kuruvilla f o r experimental assistance. Thanks a r e due t o Dr.C.VJ>harmadhikari f o r v a l u a b l e discussions. The a u t h o r s wish t o thank Prof.
A.S.Nigavekar, Head of t h e Department o f Physics f o r providing t h e f a c i l i t i e s and Prof.
Kanitkar f o r encouragement. F i n a l l y , f i n a n c i a l support from DAE and DRDO, Government of M i a , is gratefully acknowledged.
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