Multilayer porous refractory metal ionizer design with thick, porous, large-grain substrates and thin, porous micron-grain substrate

Husmann, O. K.

English

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NATIONAL AERONAUTICS AND SPACE ADMINISTRATION 
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SOTE - Xf t h i s  patent covers an invention made by a 
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Enclosuzee 
Copy of Patent csited above 
https://ntrs.nasa.gov/search.jsp?R=19710013570 2020-03-17T03:01:04+00:00Z
Jan. 14, 1969 
Filed June 7, 1965 
0. I(. HUSMANN 3,42 1,864 
MULTILAYER POROUS IONIZER 
W 
- _  . +  
(THRU) 
d7J 4 6  
(CODE) 
Fi led  June 7, 1965 
0. K. HUSMANN 
MULTILAYER POROUS IONIZER 
Sheet 2 of 3 
7 
Jan. 14, 1969 0. K. HUSMANN 3,42 1,864 
MULTILAYER POROUS IONIZER 
Filed June 7, 1965 Sheet 3 of 3 
3,42 1,864 
Patented Jan. 14, 1969 Unite 
1 
3,421,864 
~ U L T ~ ~ A ~ ~  POROUS IO1\;IIIzEHz 
Otto K. Husmam, Santa Monica, Calif., assignor, by 
mesne assignments, to The United States of America as 
represented by the Administrator of the National Aero- 
nautics and Space Administration 
Filed June 7,1965, Ser. No. 461,765 
US. 'CL 29-182.2 8 Claims 
fnt. CI. B22f 3/00; 122f 3/00; 
ABSTRACT OF THE DISCLOSURE 
An ion source is provided comprising at least two cen- 
ter layers. One of these layers, the substrate, is thick and 
porous, and is composed of large-sized grains. The layer 
on the substrate is rel'atively thin, is porous, and has 
grains of micron size. Both materials are relractory metais 
such as tungsten. 
7his invention relates to a refractory porous structure 
or article for use as a vapor phase ionizer and particularly 
a multibayer porous refractory metal ionizer for vapor 
phase contact ionization purposes. The invention described 
herein was made in the performance of work under a 
NASA contract and is subject to the provisions of Sec. 
305 of the National Aeronautics & Space Act of 1958, 
Public Law 85-568 (72 Stat. 435; 42 U.S.C. 2457). 
A two-layer porous ion source instead of a one-layer 
source, provides an improved solution for minimizing the 
values of neutral efflux and critical temperature for surface 
ionization. Such a two-layer ionizer comprises a thin 
layer of micron pore size material formed over a thicker 
porous base layer composed of larger size grains. The 
base layer need not be of the same material as the thin 
over-la yer. 
Accordingly, it is a primary object of this inventiun to 
provide a porous ionizer body composed of a plurality of 
porous layers wherein each layer contains pores or inter- 
stices of different size ranges to optimize or permit varia- 
tions of the ion emission characteristics of the various 
ionization surfaces. 
Another object of this invention is t o  provide a multi- 
layer porous ionizer having ionizing surfaces of optimum 
uniformity for evaporation of ions from the hot ionizing 
surfaces of the ionizer with a miniinum of atom evapora- 
tion. 
Additional objects of the invention will become ap- 
parent from the following description, whioh is given 
primarily for purposes of illustration, and not limitation. 
Stated in general terms, the objects of the invention are 
attained by applying a thin layer of porous refractory 
material, such as a thin layer of a porous metal or metal 
alloy, upon a thicker, higher pore density layer or sub- 
strate material, such as high temperature stable porous 
structures, including refractory metals, refractory metal 
alloys and ceramics. 
A more detailed description of specific embodiments of 
the invention is given below with reference to the ac- 
companying drawings, wherein: 
FIG. 1 is a partial schematic sectional view drawn to 
a greatly enlarged scale showing the relationship between 
5 
10 
15 
20 
25 
30 
35 
40 
45 
50 
55 
60 
2 
a thin layer of micron grain size material over a thicker 
porous layer of larger grains in a two-Iayer ionizer 
structure; 
F1G. 2 is a graph showing the relationship of the per- 
cent neutral efflux to pore density in pores per square 
centimeters for porous tungsten and porous rhenium; 
FIG. 3 is a graph showing the relationship between the 
efRux in percent and the cesium ion current density h 
milliamperes per square centimeter actually observed; 
FIG. 4 is a graph showing the dependence of the critical 
temperature on porous tungsten on tmhe number of pores 
per square centimeter; and 
FIG. 5 is a view similar to  that of FIG. 1, but showing 
a three-layer embodiment of the ionizer of the invention. 
In general, application of a thin layer of porous re- 
fractory metal o r  metal alloy 10, as shown in FIG. 1, on 
the high pore density substrate 11, increases tile pore 
density of the ionizing surface, if the thin layer density 
does not exceed about 70% of its theoretical density while 
the substrate density ranges between about 72 and about 
82 percent of its theoretical density. The surface layer 10 
is not required to provide mechanical strength. Therefore, 
high porosity, connected with high pore density can be 
achieved. 
The layer density of the thin layer 10 can range be- 
tween 60 and about 70 percent of the theoretical maxi- 
mum density. The grain size of the coating material 10 
should not be below about 1 micron, A preferred lower 
limit in grain size is between about 2.U and about 3.0 
microns. In the case of porous tungsten, the grain size of 
the particles in thin layer 10 should be in the range from 
about 2.0 to 3.0 microns to about 5.5 to 6.0 microns, 
preferably about 4.0 microns. 
This grain size limit is chosen to prevent sintering of 
the coating 10 during operation in the temperature range 
between about 1,300" and about 1,500" K. In this man- 
ner pore densities by the line intercept technique exceed- 
ing the low lo6 pores per cm.2 range of the prior ark are 
possible, The pore density of thin layer 10 preferably 
should range from about 5x 106 to about 5x 1W pores 
per square centimeter and the density preferably should 
be about 75 percent of the theoretical maximum density. 
The pore density of the substrate layer 11 preferably 
is in the range from about 5 x 10s to about 5 x 10s pores 
per square centimeter and the grain size may cover any 
range necessary to achieve this requirement. Narrow grain 
size particle distributions of the original refractory metal 
powder, preferably spherical particles, have given the best 
results. Sintering stability tests indicate satisfactory re- 
sults have been obtained for tungsten grain sizes between 
about 4.0 to about 7.0 imicrons, preferably about 6.0 
microns grain size and density of about 80 to about 82 
percent of the theoretical maximum density. 
Neutral efilux, pore density and iun current density €or 
the use of a porous tungsten ionizer with cesium vapor, 
are connected by the equation: 
Log F=0.8 (log j+8.75-1og N )  where F is the neu- 
tral efflux in percent, j is the ion current density in amps/ 
cm.2, and N is the pore density in pores per cm.2. v i s  re- 
lationship is plotted in FIG. 2 for a cesium ion cwrent 
density of 10 ma./cm.2, where the ordinate i s  the neutral 
efflux in percent and the abscissa is the pore density in 
pores per square centimeter. Thus, to improve the surface 
3,421,864 
3 4 
ionization characteristics, a high pore density substrate 11, layer ionizer can be made the same way as described for 
as shown in FIG. 1, of refractory metal or metal alloy is layer 10 in the two layer structure of FIG. 1. 
coated with a thin layer 10 of low compaction density. Obviously, many other modifications and variations of 
Ionization takes place on the surface layer 10 upon dif- the present invention are possible in the light of the above 
fusion through the uorous structure formed by the surface 5 teachings. It is therefore to be understood that. within 
layer and theporous substrate 11. 
FIG. 3 indicates the experimentally investigated im- 
provement of the surface ionization of cesium on coated 
refractory metal pellets. For comparison neutral efflux 
data is added in FIG. 3 for the uncoated material to  show 
the relationship between Mod. E and Mod. B coated versus 
Mod. E and Mod. B uncoated. The higher pore density 
substrate Mod. E yields the lower neutral efflux with the 
same coat, as the low pore density Mod. B material. 
FIG. 4 shows the decrease of the critical temperature 
To with increasing pore density for surface ionization. 
Above pore density N greater than 106, the dependence 
of the critical temperature on the pore density is quite 
small. In comparison to  solid tungsten at 10 milliamperes 
per square centimeter ion current density, on porous 20 
tungsten with 1@ pores per square centimeter, the in- 
crease in critical temperature is 31" C. The points at 108 
pores per square centimeter are for solid tungsten. 
A procedure for conducting a thin film, fine grain re- 
fractory metal powder deposition to produce a multi-layer 25 
ionizer of the invention has been developed. The fine 
grain coating material employed is mixed with sufficient 
deionized water to make a pasty slurry. Without adding 
any binder, this slurry is dispersed with a fine brush over 
the ion emission side of the porous substrate material. 30 
The surface layer thickness should be in the range of 
about 10 mils. As long as the paste has a thick consistency, 
its surface should be flattened and somewhat compressed. 
The air dryed coating and substrate materials are 
covered with a weight to apply about 1 kg./cm.2 pressure 35 
on the fine grain coating while being presintered at  an 
appropriate sintering temperature. The sintering tempera- 
ture employed depends on the grain size of the coating 
material. As an example, a sintering temperature of 
2,000" K. for 15 minutes produces a densification of 76 40 
percent of the theoretical maximum density for a 3.9 
micron spherical tungsten powder. Thus the presintering 
temperature should not exceed about 1,600" K. for more 
than about 15 minutes for the tungsten powder to avoid 
After presintering the pellet in a hydrocarbon-free oven 
for 15 minutes, the coat and substrate combination struc- 
ture is sintered in a vacuum furnace free of hydrocarbons 
is the 10-7 Torr pressure range at temperatures exceed- 
ing 1,700" C. A sintering pressure of 0.5 to 1.0 kg. per 60 
cm.2 may improve the adherence of the coat t o  the sub- 
strate. Organic binders should be avoided. 
Excessive sintering temperatures and sintering times 
yield surface layers having unduly low pore densities. 
TO insure long term stability of the porous ionizer, a 55 
fairly coarse substrate material is of advantage because 
sintering of the porous structure depends on the original 
powder grain size. The original powder grain size pro- 
vides both the substrate mechanical strength and sintering 
stability. However, the most important requirement is 60 
low neutral efflux as connected with high pore density. 
To combine low neutral efflux with a high mechanical 
strength substrate we have to coat the substrate with fine 
grain refractory material of low density. The best dis- 
tribution, connected with minimum sintering during ion- 65 
izer operation, is achieved if several layers of porous ma- 
terial are employed on the substrate in such way that the 
grain size decreases with distance from the substrate. 
For tungsten, as a specific example, the substrate or bot- 
tom layer 14 can be made from powder with a grain size 70 
between about 6 and about 10 microns (FIG. 5 ) + a n d  
sintered to a relatively high density, such as about SO to 
82 percent. The next or intermediate layer 13 can be 
made from powder in the grain size range between about 
5 and 7 microns and the upper or top layer 12 in a three 75 
overdensification of the thin film top layer. 45 
the scope of the appended claims, the invention can be 
practiced otherwise than as specifically described. 
What is claimed is: 
1. A sintered, multilayer porous ionizer which consists 
0 of a relatively thin porous outer layer of refractory ma- 
terial of high pore density ranging from about 5x l@ to 
about 5x 107 pores per square centimeter, the thin layer 
being on a relatively thick porous substrate layer of re- 
fractory material of high pore density in the range from 
5 about 5x 105 to about 5 X 106 pores per square centimeter 
adjacent the relatively thin layer of porous material. 
2. A sintered, multilayer porous ionizer which con- 
sists of a relatively thin outer porous layer of refractory 
material of particles of grain size in the range from about 
2 to about 6 microns and of high pore density ranging 
from about 5 x  106 to about 5X 107 pores per square 
centimeter, the thin layer being on a relatively thick 
porous substrate layer of high pore density refractory 
material in the range from about 5X lo5 to about 5x lo6 
pores per square centimeter adjacent the relatively thin 
layer of porous material. 
3. A sintered, multilayer porous ionizer which con- 
sists of a relatively thin porous layer of refractory ma- 
terial having a density of about 60 to about 70 percent 
of the theoretical maximum density, particles of grain 
size on the order of 4 microns, and a pore density in the 
range from about 5 X 106 to about 5 X 107 pores per square 
centimeter on a relatively thick porous substrate layer of 
refractory material of particles of grain size on the order 
of 6 microns, and a high pore density in the range from 
about 5 x 105 to about 5 x  l@ pores per square centimeter. 
4. A sintered multilayer porous tungsten ionizer which 
consists of a relatively thin porous tungsten layer having 
a high pore density ranging from about 5X 106 to  5x IO7 
pores per square centimeter, the thin tungsten layer being 
on a relatively thick porous tungsten substrate layer of 
hight pore density on the range from about 5x105 to 
about 5x 106 pores per square centimeter. 
5. A sintered, multilayer porous tungsten ionizer which 
consists of a relatively thin porous tungsten layer of par- 
ticles of grain size in the range from about 2 to about 6 
microns and of high pore density ranging from about 
5X 106 to about 5 x 107 pores per square centimeter, the 
thin tungsten layer being on a relatively thick porous 
tungsten substrate layer of high pore density in the range 
from about 5x lo5 to about 5x 106 pores per squrae 
centimeter. 
6. A sintered, multilayer porous tungsten ionizer which 
consists of a relatively thin porous tungsten layer of par- 
ticles of grain size in the range from about 3.0 to about 
5.5 microns, and a density in the range from about 60 to  
about 70 percent of the theoretical maximum density on 
a relatively thick porous tungsten substrate layer of par- 
ticles of grain size in the range from about 5.6 to about 
7.0 microns, and a density in the range from about 72 
to about 82 percent of the theoretical maximum density. 
7. A sintered, multilayer porous tungsten ionizer which 
consists of a relatively thin porous tungsten layer of 
spherical particles of grain size of about 4.0 microns and 
a density of about 75 percent of the theoretical m i m u m  
density on a relatively thick porous tungsten substrate 
layer of spherical particles of grain size of about 6.0 mi- 
crons and a density of about SO percent of the theoretical 
maximum density. 
8. A sintered, multilayer porous ionizer which con- 
sists of a relatively thin porous top layer of porous 
tungsten particles having a grain size in the range from 
about 2 to about 6 microns and a density ranging from 
about 60-70%, an intermediate layer of porous tungsten 
of particles having a grain size in the range from about 5 
3,421,864 
5 
to about 7 microns and a density ranging from about 
72-82%, and a bottom layer of porous tungsten of par- 
ticles having a grain size in the range from about 6 to 
about 10 microns and a density ranging from about 
80-82%. 5 
References Cited 
UNITED STATES PATENTS 
2,464,517 3/1949 Kurtz _ _ _ _ _ _ _ _ _ _ _  29-182.2 X 
2,722,626 11/1955 Coppola et al. _ _ _ _ _ _  313-346 lo 
2,754,442 711956 Boutry et al. 
3,076,915 2/1963 Gal et al. ______-___ 313-346 
3,183,396 5/1965 
3,243,637 3/1966 
3,285,714 1111966 
3,354,644 11/1967 
E 
Becker et al. _ _ _ _ _  29-182.3 X 
Affleck _I___________ 313-346 
Davies et al. _ _ _ _ _ _  29-182.2 X 
Moore _ _ _ _ _ _ _ _ _ _ _ _ _ _  60-202 
CARL D. QUARFORTH, Primary Examiner. 
A. J. STEINER, Assistant Examiner. 


Multilayer porous ionizer Patent

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