A specially textured surface of pyrolytic graphite exhibits extremely low yields of secondary electrons and reduced numbers of reflected primary electrons after impingement of high energy primary electrons. An ion flux having an energy between 500 eV and 1000 eV and a current density between 1.0 mA/sq cm and 6.0 mA/sq cm produces surface roughening or texturing which is in the form of needles or spines. Such textured surfaces are especially useful as anode collector plates in high efficiency electron tube devices

Curren, A. N.

Forman, R.

Sovey, J. S.

Wintucky, E. G.

English

NASA Technical Reports Server

https://ntrs.nasa.gov/search.jsp?R=19830001847 2020-03-21T23:22:28+00:00Z
u. Eltent Sep. 14, 1982 
1.2 
40 
c 
Sheet 1 of 2 4,349,424 
I I I 1 
60 400 600 800 1000 1206 1400 1600 I800 2000 
PRIMARY ELECTRON ENERGY, Ep, eV 
FIG. I 
4V’ 
I I 
10 400 600 800 1000 1200 1400 1600 1800 2000 
PRIMARY ELECTRON ENERGY, Ep, eV 
FIG. 2 
U.S. Patent Sep. 14,1982 Sheet 2 of 2 4,349,424 
FIG. 3 
4,349,424 
L 
as grit blasting and preparing metal blacks by evaporat- 
ing gold, silver, or copper in relatively high residual 
pressure of various gases. Certain disadvantages were 
observed with these prior art methods. For example, the 
' 
1 
ION SPUTTER TEXTURED GRAPHITE 
Origin of the Invention 
The invention described herein was made by employ- 5 secondary electron emiSSiOn yields Were reht 
being greater than 0.5- Also. relatively high ees of the United States Government and d a y  be manu- 
mental purposes without the payment of any royalties 
thereon or therefor. 
Description 
Technical Field 
This invention relates to ion sputter texturing. The 
invention is particularly directed to texturing pyrolytic 
graphite to reduce both the secondary electron emission 
yield and the number of reflected primary electrons. 
factured and used by or for the Government for govern- 
The textured graphite is used in high efficiency electron 
tube devices. 
Broadband, high power traveling wave tube micro- Gutnajer Pat. No. 3,619,286 discloses a process 
wave amplifiers are used in military electronic counter- '' wherein a graphite electrode is coated with pyrolytic 
measure and communications satellite applications 
where high electrical power efficiency is an important 
requirement. Electron beams with energies on the order 
of 10 keV are used to amplify the microwave signals. 
The electrical efficiency of a traveling wave tube is 
substantially improved by the use of a multistage de- 
pressed collector of the type shown in U.S. Pat. No. 
3,702,951 to H. Kosmahl. 
Electrons entering the multistage depressed collector 
are slowed by a retarding electric field and strike the 30 electrons. 
collector plates at reduced energies which are experi- 
mentally observed to be generally in the range of 500 
e v  to ev .  The overall traveling wave tube effi- surfaces are eswmy useful on the 
ciency depends strongly on the collector efficiency. An static co~~ector for charged 
important factor influencing collector efficiency is the 35 in U.S. pat. N ~ .  3,702,951. 
effectiveness of the electrode surfaces in absorbing elec- 
trons. Significant energy losses can occur through emis- 
sion of electrons from the surface as a result of electron 
bombardment. The electron emission process, called 4o yields of sputter-textured 
secondary electron emission, includes both electrons plotted against primary ele reflected elastically without loss of energy (reflected is a graph of the primaries) and electrons which undergo inelastic colli- 
sions with the lattice and are emitted at lower energies 
(true secondaries). Reflected primary electrons gener- 
ally constitute 5 to 10 percent of the secondary electron 45 
emission. A measure of secondary electron emission is 
the secondary electron emission yield which is defined 
as the ratio of the total number of emitted electrons to 
the number of incident primary electrons. For large 
electron absorption and consequently optimum collec- 50 
tor efficiency, one obviously needs collector electrode 
surfaces with low secondary electron emission yields. 
Previous techniques used to limit secondary electron 
emission yield include the proper choice of appropriate 
materials, such as tantalum carbide, titanium carbide, 
and soot for the collector. Carbon black, or soot, is one 
of the lowest known secondary electron emitting mate- 
rials. Soot has poor adhesion and lacks the mechanical 
integrity necessary for space applications. However, 
carbon in the form of pyrolytic graphite is very attrac- 60 
tive as a collector electrode material for space applica- 
tions. This material is relatively light in weight and 
mechanically strong with suitable electrical properties. 
It also has both high planar thermal conductivity and 
high thermal emittance for the enhancement of collec- 
tor cooling. 
Other methods for limiting secondary electron emis- 
sion yields include surface roughening techniques such 
gas ionizes and a carbon film forms on the elctrode. 
Surfaces of pyrolytic 
like are roughened by 
tured surfaces exhibit ex 
of 
BRIEF DESCRIPTION 0 
FIG. 
BEST MODE FOR 
4,349,424 
3 
ion current density, and sputter duration. Their values 
depend on which argon ion bombardment technique IS 
employed. 
One sample was sputter-textured for 13.7 hours in a 
radio frequency generated plasma at an average argon 
ion energy of 1 SO0 eV and a current density of approxi- 
mately 0.7 mA/cm2. The resulting surface microstruc- 
ture consisted typically of scattered clumps of micron- 
size toothlike protuberances. This sample formed the 
basis for curves 12 in FIG. 1 and 32 in FIG. 2. 
In a second technique the surfaces were sputter-tex- 
tured by a normally incident 750 eV ion beam from a 30 
cm-diameter argon ion source. Such an ion source is 
described in “Advances in Electronics and Electron 
Physics” by H. R. Kaufman, vol. 36, pages 365-373. 
Beam extraction was accomplished by a dished, two- 
grid ion optics system. Such a system is described in 
AIAA paper No. 76-1017 entitled “A 30 cm Diameter 
Argon Ion Source”. 
The electron bombardment ion source is located in a 
vacuum facility which is sufficiently large to minimize 
back sputtered facility material from contaminating the 
pyrolytic graphite. The vacuum facility is maintained at 
a pressure of about 4x torr during the operation of 
the ion source. Neutralization of the ion beam is 
achieved by secondary electrons released by ion bom- 
bardment of the vacuum facility walls. 
The surface of the pyrolytic graphite sample to be 
textured is positioned normal to the ion beam at a loca- 
tion of about 20 centimeters from the ion source. Based 
on current ion source technology, it is contemplated 
that ion beam textured graphite surfaces may be easily 
obtained with dimensions up to 0.5 X0.5 meters. 
A short exposure to the ion beam of 0.5 hours at an 
ion current density of 1.1 mA/crnZ produced a surface 
microstructure consisting of a dense array of uniformly- 
sized small cones. This sample supplied the information 
for curve 14 in FIG. 1 and 34 in FIG. 2. A longer expo- 
sure of 4.6 hours also at 1.1 mA/cm2 produced a notice- 
ably different cone structure in which the cones are of 
much larger size, randomly distributed and of varying 
peak heights on the order of microns. This sample sup- 
plied the data for curves 16 in FIG. 1 and 36 in FIG. 2. 
Ion beam sputter-texturing for 4.7 hours at a lower ion 
current density of 0.5 mA/cn2 resulted in a similar cone- 
like microstructure, but the cones were both smaller 
and less clustered. 
Both ion beam and R F  plasma techniques necessi- 
tated that sputter-texturing be performed at low ion 
current densities of 1.1 mA/cm2 or less. Because pyro- 
lytic graphite is an inherently low sputter yield material, 
the corresponding sputter-etch rates were low. 
To enhance the sputter-etch rate, the 30-cm diameter 
argon ion source was converted to a DC triode sputter- 
ing configuration which provided for much higher ion 
current densities. In this approach, the ion beam focus 
grids were replaced by an electrically isolated plate 
with an orifice having nearly the same geometry as the 
sample holder. The pyrolytic graphite target was then 
biased negative relative to the discharge plasma poten- 
tial. Ion current densities of 5 mA/cm2 were readily 
obtained in this manner with current densities of 10 
mA/cm2 o r  higher possible. 
The samples were sputter-textured at ion energies of 
4 
duced the data for curve< 18 in FIG. 1 and 38 in F[G. 2 
consisted of tall slender cones or spircs randomly dis- 
:ributecl in a dense background of smaller cones. Vary- 
ing the etch time from 4 to 19 hours did not significantly 
5 alter the surface microsirccture. 
At 1 0 0  eV, the structure was a more uniform. dense 
array of tall, thin spires less than 1 pm diam.. 5-10 pm 
high and appeared similar for etch times of 6 and 18.5 
hours. This sample formed the basis for curves 20 in 
10 FIG. 1 and 40 in FIG. 2. The photomicrograph shown 
in FIG. 3 shows this surface at an etch time of 6 hours. 
Measurements of secondary electron emission and 
reflected primary electron characteristics were made 
{cith an Auger cylindrical mirror analyzer in a high- 
15 vacuum chamber at pressures below 1 . 3 3 ~  lo--’ N/m* 
torr). The primary electrons were normally inci- 
dent o n  the sample surface at energies ranging from 200 
to 2000 eV. 
Surface contaminants, such as absorbed gaseous im- 
20 purities, may alter the secondary electron emission lev- 
els. To ensure clean surfaces, it is necessary to first bake 
out the samples. ’The samples were heated to tempera- 
tures above 850“ C. both by resistance heaters mounted 
on the sample holder and by electron bombardment. 
25 Temperatures were monitored by a platinum vs plati- 
num - 13% rhodium thermocouple embedded inside the 
disk. A residual gas analyzer indicated when the sur- 
faces were free of gaseous impurities. 
Soot (carbon black) has exceptionally reproducible 
30 secondary electron emission characteristics and is useful 
as a standard, control surface. One half of each disk was 
coated with soot after careful removal of the sputter 
texture. Measurements of electron emission characteris- 
tics were made successively on both soot and the test 
35 surface during the same run. 
The secondary electron emission yield, 8, defined as 
the ratio of secondary electrons emitted to primary 
electrons incident per unit time, is experimentally deter- 
mined for a given primary energy, Ep, from measure- 
40 ments of the primary electron current, Ip, and the cur- 
rent passing through the sample to ground, IT, accord- 
ing to 
( Ep) = I p  - IT/ Ip 
45 
The quantity I,--IT is the total emitted secondary 
electron current, which is collected by the surrounding 
grounded vacuum chamber. 
The reflected primary electrons are detected by an 
50 energy analyzer technique in which only those elec- 
trons emitted with energies equal to the primary elec- 
tron energy are selectively focused and collected. The 
resulting analyzer output signal, although representa- 
tive of the magnitude of the reflected primary electron 
55 current, is not by itself meaningful unless it is indexed 
relative to similar measurements with a standard sur- 
face. A useful parameter. which enables meaningful 
comparison between the reflected primary electron 
characteristics of various surfaces, is the “reflected 
60 primary electron index,” defined here as the ratio of the 
analyzer output signal amplitude for the test surface at a 
given primary energy to that for soot at a primary en- 
ergy of 1000 eV. 
FIG. 1 is B dot  of secondarv electron emission vield. 
500 and 1600 eV. A’ high ion current density of 5 65 versus primarv electron energy, E,, for seven surfaces. 
mA/cm2 bombardment produced a much more dense Five of !he surfaces are pyrolytic graphite. One surface 
cone-like microstructure than the low current density is soot. and one surface IS copper for comparison. The 
techniques. The structure formed at 500 eV which pro- first curve 10 is for untextured pyrolytic graphite while 
4,349,424 
5 6 
the second curve 12 is for the surface textured by the emission from such a surface is observed to be lower 
RF  plasma. than that of soot, whose secondary emission is among 
Curves 14 and 16 are for the ion beam textured sur- the lowest of any material. At a primary electron en- 
faces for 0.5 hours and 4.6 hours as previously de- ergy of 1000 eV, the secondary electron emission yield 
scribed. Curves 18 and 20 are for DC triode sputtered 5 of smooth Cu is about 350 percent greater than the 
samples as previously described. For comparison, a lowest value obtained for sputter-textured pyrolytic 
typical S vs. Ep curves 22 for soot and 24 for smooth graphite. The reflected primary electron index of 
copper are also presented. smooth Cu is a factor of 80 greater. If the secondary 
FIG. 2 is a plot of the reflected primary electron electron emission yield is reduced to 0.3, which appears 
index, P, versus primary electron energy, Ep, for the 10 possible with sputter-textured pyrolytic graphite, the 
same surfaces as in FIG. 1. Curves 30 to 44 in FIG. 2 collector efficiency could be 
correspond to curves 10 to 24, respectively, in FIG. 1. percent over that of smooth copper. 
The emission characteristics, 6 and T, in FIGS. 1 and Similar surface features ar 
2 correlate well with the height and density of the sur- lytic and isotropic graphite. This yields flexibility in 
face microstructures. In progressing from the relatively 15 choice of the graphite material for various applications. 
smooth surface of untextured pyrolytic graphite to the It is further contemplated that the texture will also exist 
dense, tall spire-like structure, 6 and P are in general on graphite fiber, cloth, or mesh. 
correspondingly reduced. The higher and denser micro- 
structures appear to provide more effective electron U.S. Pat. No. 3,702,951, the textured graphite is useful 
traps. 20 in plasma devices or plasma probes 
For Ep< 1000 eV, all of the sputter-textured surfaces electron emission is undesirable. 
reduce S and P below that of untextured pyrolytic 
graphite. For Ep> lo00 eV, both 6 and P for the sparse been described, it is evident that various modifications 
structure are about equal to or slightly greater than for may be made to the invention without departing from 
untextured pyrolytic graphite. The 6 for the surface 25 the spirit thereof or the scope of the subjoined claims. 
with the small cone structure is about the same as for We claim: 
untextured surface for Ep > lo00 eV, but P is somewhat 1. A method of treating the surface of a pyrolytic 
reduced. graphite electrode plate for use in a multistage de- 
For Ep< 500 eV, the measured 6 is approximately the pressed collector to attain low secondary emission 
same as for soot, whose electron emission characteris- 30 yields and reduced numbers of reflected primary elec- 
tics are among the lowest observed for any material. trons from the collector comprising the steps of 
The P for surface 16 is slightly less than that of soot nvironment of about 
whereas P for surface 18 is slightly more. Both surfaces 4~ 10-5 torr, and 
16 and 18 have some tall cone-like microstructure. For f said plate to ions 
Epin the range of 500 to lo00 eV, the measured 6 and P 35 having an energy between about 500 eV and lo00 
for both surfaces are approximately equal to or lower eV and an ion beam current density between about 
than for soot. For Ep> IO00 the same two surfaces, 16 1.0 mA/cm2 and 6.0 mA/cm2 for about 4 to 6 
and 18 have 6 lower than soot, whereas P is nearly hours. 
equal to or slightly greater than soot. 2. A method of treating the surface of a pyrolytic 
Both S and P for the surface with the dense, tall 40 graphite electrode plate as claimed in claim 1 wherein 
spire-like microstructure are well below those of soot an ion flux is produced by an argon ion beam. 
over the entire range of Ep. In the primary electron 3. A method of treating the surface of a pyrolytic 
energy range of 500 to 1000 eV, which is of consider- graphite electrode as claimed in claim 2 wherein the ion 
able interest for collector applications, sample 20, 40 flux is produced by a D.C. plasma triode system. 
shows reductions in 6 and T of at least 45 and 83 per- 45 4. A method of treating the surface of a pyrolytic 
cent, respectively, when compared to untextured pyro- graphite electrode as claimed in claim 3 wherein the 
lytic graphite. argon ions have a current density of about 5 mA/cm2 
FIGS. 1 and 2 demonstrate that sputter texturing and is substantially uniform over the surface of the 
effectively suppresses secondary electron emission from plate. 
pyrolytic graphite surfaces. A dense, tall, thin spire-like 50 5. A method of treating the surface of a pyrolytic 
microstructure shown in FIG. 3, readily obtained at ion graphite electrode plate as claimed in claim 4 wherein 
energies of 1000 eV and ion current densities of 5 the energy of said beam is about 1000 eV. 
mA/crnz, is the most effective. The secondary electron 
In addition to electron tubes of the type sh 
While the preferred embodiment of 
placing said plate in a vacuu 
exposing at least one surfa 
* * * * *  


Ion sputter textured graphite

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