Helium reduces bubbles in glass coatings of accelerator grids for ion thrustors. Fusing the coating in a helium atmosphere creates helium bubbles in the glass. In an argon atmosphere, entrapped helium diffuses out of the glass and the bubbles collapse. The resultant coating has a substantially enhanced electrical breakdown strength

Banks, B.

English

NASA Technical Reports Server

June 1968	 Brief 68-10214 
S	 NASA TECH BRIEF	 1 
NASA Tech Briefs are issued to summarize specific innovations derived from the U.S. space program, 
to encourage their commercial application. Copies are available to the public at 15 cents each 
from the Clearinghouse for Federal Scientific and Technical Information, Springfield, Virginia 22151. 
Reducing Bubbles in Glass Coatings Improves Electrical 
Breakdown Strength 
.
Recent investigation of fused glass coated accelera-
tor grids for ion thrusters, has shown that elimination 
of the naturally occurring bubbles in the fused glass 
coating can increase its effective electrical breakdown 
strength by over 800 percent. 
The fused glass coatings are obtained by spraying a 
powdered glass and water slurry onto the metal grid. 
The coating is dried in air and then heated in an oven 
to fuse the glass particles to each other and bond the 
coating to the grid. During this process, air or inert 
gas is entrained in the slurry, resulting in small gas-
filled bubbles trapped in the fused glass coating. When 
a high electrical potential is imposed across the coat-
ing, the bubbles become potential electrical break-
down sites. If an electrical discharge occurs within a 
bubble, the conductivity of the plasma in the bubble 
reduces the breakdown strength of the coating in a 
line through the bubble. Thus, either relatively large 
bubbles or a multiplicity of smaller bubbles can sig-
nificantly reduce the effective electrical breakdown 
strength of the coating. 
A new technique for eliminating the bubbles is 
based on the very high diffusion rate of helium 
through the glass at elevated temperatures. The 
sprayed-on slurry coating is first fused in a helium 
atmosphere during which time helium bubbles are 
formed in the glass. The helium atmosphere is then 
changed to an argon atmosphere to provide an en-
vironment with zero helium partial pressure. The 
entrapped helium diffuses out of the glass and the 
bubbles collapse. The result is a nearly bubble-free
coating with a substantially enhanced electrical break-
down strength. 
These glass coatings have good sealing and bonding 
properties, can withstand high temperatures, and have 
high electrical breakdown strength for small thick-
nesses. This invention should be useful for many 
electrical and electronic applications in addition to 
accelerator grids. 
Notes: 
I. The effective breakdown strength has been in-
creased over eight times by reduction of the bub-
bles. The effective breakdown strength of the coat-
ings made by this helium diffusion process is 8.75 
X 107 volts/meter for coatings of approximately 
0.035 centimeter thickness. 
2. The technique of applying the fused glass coatings 
to charged particle accelerator grids which resulted 
in a successful single grid accelerator system is 
described in Tech Brief 68-10215. 
3. Inquiries concerning this invention may be di-
rected to:
Technology Utilization Officer 
Lewis Research Center 
21000 Brookpark Road 
Cleveland, Ohio 44135 
Reference: B68-10214 
Patent status: 
Inquiries about obtaining rights for the commercial 
use of this invention may be made to NASA, Code 
GP, Washington, D.C. 20546.
Source: Bruce Banks 
(LEW- 10278) 
Category 03 
This document was prepared under the sponsorship of the National 
Aeronautics and Space Administration. Neither the United States 
Government nor any person acting on behalf of the United States
Government assumes any liability resulting from the use of the 
information contained in this document, or warrants that such use 
will be free from privately owned rights.
https://ntrs.nasa.gov/search.jsp?R=19680000214 2020-03-12T05:35:04+00:00Z


Reducing bubbles in glass coatings improves electrical breakdown strength

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