Helium outgassing process for fused glass coating on ion accelerator gri

Banks, B. A.

English

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NATIONAL AERONAUTICS AND SPACE ADMINIST T I 0  
I7CPLY TO 
ATTN OF: Gp 
FROM: 
~ s ~ / S c i e n t i f i c  61 Technical Information Division 
Attention: Mise Winnie M. Morgan 
~ ~ / ~ f f i c e  of Assistant General Counsel fo r  
Patent Matters 
. , 
SUBJECT: Announcement of NASA-Owned U. S . Patents i n  STAR 
In accordance with the procedures agreed upon by Code GP 
and Code U S I ,  the attached NASA-owned U. S. Patent is being 
forwarded fo r  abstracting and announcement i n  NASA STAR. 
The following information is prwidedt 
U. S. Patent No. 
Government or  
Corporate Employee 
Supplementary Corporate 
Source ( i f  applicable) t 
NASA Patent Case No. 
NOTE - If t h i s  patent  covers an invention made by a corporate 
employee of a NASA Contractor, the following is applicable: 
yes NOH 
Pursuant t o  Section 305(a) of the National Aeronautics and 
Space Act, the name of the Administrator of NASA appears on 
the first page of the patent; however, the name of the actual  
inventor (author) appears a t  the  heading of Column No. 1 of 
the Specification, following the words . . with respect t o  
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Enclosure 
Copy of Patent c i t ed  abwc 3 
(CODE> 
/A 
$ (NASA CR OR TMX OR AD NUMBER) (CATEGORY) 
https://ntrs.nasa.gov/search.jsp?R=19710019106 2020-03-17T03:14:12+00:00Z
April 6, 1971 6. A. BANKS 
L1 4 
FIG. 2 
INVENTOR 
BRUCE A. BANKS 
BY 
ATTORNEYS 
3,573,973 United States Patent Ofice . a,n,, A pl.*, ,,,, 
3,573,977 drawings wherein like numerals are used throughout to 
PROCESS FOR GLASS COA'FI[NG AN ION identify like parts. 
ACCELERATOR GRID DESCRIPTION OF THE DRAWLNG 
Bmce A. Banks, North Olimsted, Ohio, assignor to the In the drawing: United States 06 America as represented by the Ad- 5 
minisffator of the National Aeronautics and Space FIG. 1 is an isometric view 'with parts broken away of 
Admidstration an ion thrustor having a single grid accelerator system 
Filed Sqpt. 19, 1968, Ser. No. 760,928 constructed in accordance with the present invention, and 
Int. CI. C23d 5/04,7/00 FIG. 2 is an enlarged sectional view taken along the line 
U.S. CI. 117-224 5 Claims 2-2 in FIG. 1. 
~n 
ABSTRACT OF THE DISCLOSURE 
Minimizing bubbles in a fused glass coating on a per- 
forated plate forming the single grid of an ion thrustor ac- 
celerator system. A slurry coated grid is first heated in a 
helium atmosphere which is subsequently changed to argon 
so that entrapped helium diffuses out of the glass. 
STATEMENT O F  GOVERNMENT OWPJERSMIP 
The invention described herein was made by an em- 
ployee of the United States Government and may be manu- 
factured and used by or for the Government for govern- 
mental purposes without the payment of any royaities 
thereon or therefor. 
BACKGROUND OF THE INVENTION 
This invention is concerned with improving the insulat- 
ing coating on a single grid accelerator system in an ion 
thrustor. The invention is particularly directed to increas- 
ing the effective electrical breakdown strength of such a 
coating. 
Single grid accelerator systems have been utilized with 
ion thrustors to solve certain problems encountered in 
conventional systems having two spaced grids. One such 
system has a dielectric material interposed between the 
accelerator grid and the thrustor ionization chamber for 
protecting the grid against direct ion impingement as de- 
scribed in U.S. patent application Ser. No. 758,540 and 
now Pat. No. 3,443,376. The ion extraction capability of 
such a single grid system is improved by utilizing a glass 
coated grid as described in U.S. patent application Ser No. 
758,390 and now abandoned. Certain problems have been 
encountered while such a grid is being coated with glass 
in that environment gas is encapsulated by the glass par- 
ticles resulting in smail gas filled bubbles trapped in the 
fused glass coating. When a high electrical potential is im- 
posed across the coating, the bubbles become potential 
electrical breakdown sites. If an electrical discharge occurs 
within a bubble, the conductivity of the plasma in the 
bubble reduces the effective breakdown strength of the 
coating in a line through the bubble. Thus, either relative- 
ly large bubbles or a multiplicity of smaller bubbles can 
significantly reduce the effective electrical breakdown 
strength of the coating. 
These problems have been solved by coating the metal 
grid in accordance with the present invention. A powdered 
glass and water slurry is sprayed onto the metal grid. The 
sprayed-on slurry coating is fused first in helium and then 
in an environment having a very low helium partial pres- 
sure. 
It is, therefore, an object of the present invention to 
provide an accelerator grid having a bubble free glass coat- 
ing with an improved electrical breakdown strength. 
Another object of the invention is to provide a process 
for fusing glass onto the surface of an ion thrustor acceIera- 
tor grid to provide a glass coating having improved sealing 
and bonding properties at high temperatures. 
These and other objects of the invention will be ap- 
parent from the specification which follows and from the 
- - 
DESCRI'PTION OF THE PREFERRED 
EMBODIMENT 
In an electron bombardment idn thrustor of the type de- 
scribed in U.S. Pat. No. 3,156,090, mercury vapor is fed 
l5 into an ionization chamber through a distributor plate. 
Mercury atoms are bombarded by electrol~s emitted from 
a cathode in the chamber to ionoize this propellant. High 
electric fields in an accelerator system at the opposite end 
of the thrustor accelerate ions in the near vicinity through 
20 openings in an accelerator grid. Thrust is produced as 
these ions accelerate through this grid. 
Referring now to the drawing an ion tbrrrstor is shown 
in FIG. 1 which has an ionization chamber 10 for contain- 
ing an ionized propellant. This propellant may 6- mercury 
'' that is ionized by electron bombardment as described In 
the aforementioned U.S. Pat. No. 3,156,090. 
The ion thrustor further includes an accelerator system 
12 constructed in accordance with the present invention for 
30 accelerating propellant ions in the direction of the arrows 
in FIGS. 1 and 2. The accelerator system 12 utilizes a sin- 
gle grid 14 of an electrically conducting material. An eiec- 
trical power source impresses a potential on lhc grid 14 
that is highly negative relative to the ionization chamber 
10. The grid 14 is electrically conductive to establish the 
35 proper electrostatic field for accelerating the ions in the 
chamber 10. 
An insulator 16 serves to electricaliy isolate the electrl- 
cally conductive grid 14 from the metal houslng formmg 
40 the ionization chamber 10. The insulator 16 has m annular 
configuration and extends about the perrphery of the 
ionization chamber 10. 
The accelerator system in a conventional ion thrustor 
of the type described in U.S. Pat. No. 3,156,090 has both 
43 a screen grid and an accelerator grid. The screen grid 
serves to contain the discharge plasma in the ionization 
chamber while forming the necessary ion optics to pA revent 
direct impingement of accelerated ions cm the accelera- 
tor grid. A plasma sheath is formed near the upstream sur- 
30 face of the screen grid. 
According to the present invention a glass coating 18 is 
bonded to the electrically conducting grid 14 as shown in 
FIGS. 1 and 2. The plasma sheath is believed formed near- 
the upstream surface of the glass coating 318. Because of 
55 the high electrical resistance of the glass, charges can 
build up on its surface to form a virtual screen grid. In 
ihis manner the plasma sheath is moved closcr to the 
negative accelerator grid 14. This increa%s the field 
strength for a given voltage which, in turn. increases the 
60 ion beam current density. 
The fused glass coating 18 is obtained by spraying a 
slurry onto the metai grid 14. This slurry is prepared by 
suspending finely milled particles of glass in a solutron of 
water or some other organic or inorganic solvent. This 
65 slurry covers one face and the walls of the holes ic s 
perforated plate forming the grid 14. The surfaces of the 
perforated plate to be coated are initially oxidized prior 
to being covered with the sprayed slurry. 
The glass used in the bonded coating 18 is of a type 
70 that forms a good seal with the material of the gric! 14. 
Corning glass 7052 has been found to be satisfactory for 
coating a molybdenum grid. Corning glass 7052 is a boro- 
3 
silicate glass having a nominal composition of about 80% 
silica, 14% boric oxide, 4% soda, and 2% alumina. 
According to the present invention the sprayed metal 
grid is dried in air to remove the water. The powdered 
glass and grid are then heated in an inert atmosphere of 
heliil,.n gas to fuse the glass. During this time helium 
bubbles are formed in the glass. 
The helium atmosphere is then changed to an argon 
atmosphere to provide an environment with zero helium 
partial pressure. The entrapped helium diffuses out of the 
glass and the bubbles collapse. This results in a nearly 
bubble-free coating with a substantially enhanced elec- 
trical breakdown strength. 
By way of example, a 0.3 mm. thick Corning glass 7052 
coating was fused to one side of a 0.5 mm. thick perforated 
molybdenum sheet. This was accomplished by first spray- 
Ing a slurry prepared in the aforementioned manner onto 
tho molybdenum sheet. The environment was purged 
zfter drying, and both the glass and grid were heated in 
a helium gas atmosphere at  1170' C., for approximately 
5 minutes. The helium environment was then removed, 
and argon gas was substituted. The grid was maintained 
at the elevated temperature for an additional 15 minutes 
to enable trapped helium to diffuse out of the bubbles 
which collapse. The result after cooling was a nearly 
bubble-free coating. 
It was found that the effective breakdown strength 
of a coating prepared in accordance with the invention 
was increased over eight times by eliminating the bubbles. 
By way of example the effective breakdown strength of 
coatings made by the helium diffusion process was 8 . 7 5 ~  
lo7 volts per meter for coatings approximately 0.035 
cm. thick, 
While the preferred embodiment of the invention has 
been shown slid described it will be appreciated that 
various modifications may be made to the disclosed proc- 
ess without departing from the spirit of the invention or  
4 
same from direct ion impingement comprising the steps 
of 
suspending fine particles of a borosilicate glass in a 
liquid to form a sluiry, 
spraying said slurry onto said plate to form a coating 
thereon, 
heating said coating in a helium atmosphere to fuse 
said glass particle onto the surface of said plate 
thereby entrapping helium in bubbles in said glass, 
displacing said helium atmosphere with an inert at- 
mosphere having a low helium partial pressure 
whereby said entrapped helium in said bubbles dif- 
fuses out of said glass and said bubbles collapse 
thereby enhancing the electrical breakdown strength 
15 of said coating, and 
cooling said plate with said glass coating thereon to 
ambient temperature. 
2. A method of coating a plate as claimed in claim 1 
wherein said plate is perforated, and 
20 the surfaces of said perforated plate to be coated are 
initially oxidized prior to being sprayed with said 
slurry. 
3. A method of coating a plate as claimed in claim 2 
wherein the slurry is sprayed onto those surfaces of the 
25 perforated plate to be protected from direct ion impinge- 
ment. 
4. A method of coating a plate as claimed in claim 3 
wherein the sprayed slurry is dried onto said surfaces of 
said perforated plate prior to heating said coating to fuse 
30 said glass. 
5. A method of coating a plate as claimed in claim 4 
wherein the helium atmosphere is displaced with an 
argon atmosphere. 
References Cited 
35 UNITED STATES PATENTS 
3,015,032 12/1961 Noyer et al. -------- 250-84.5 
the scope of the subjoined claims: WILLIAM L. JARVIS, Primary Examiner 
What is claimed is: 
I. A method of coating a plate for a single grid ac- 40 U.S. C1. X.R. 
cekrator system in an ion thrustor for protecting the 117-125, 98; 60-202; 65-60 


Process for glass coating an ion accelerator grid Patent

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