Formula and process have been developed for coating refractory metal surfaces with high voltage electrical insulation for use at temperatures to 600 C. Coatings were specifically developed as an insulation for the surface of a perforated, molybdenum, ion-accelerator grid, but are not limited to this application

Lent, W. E.

English

NASA Technical Reports Server

June 1972 
•	 NASA TECH BRIEF 
Lewis Research Center
B72-10290 
NASA Tech Briefs announcenew technology derived from the U.S. space program. They are issued to encourage 
commercial application. Tech Briefs are available on a subscription basis from the National Technical Information 
Service, Springfield, Virginia 22151. Requests for individual copies or questions relating to the Tech Brief program may 
be directed to the Technology Utilization Office, NASA, Code KT, Washington, D.C. 20546. 
High Voltage Electrical Insulation 
Coating for Refractory Materials 
A formula and process have been developed for coat- filler, and (3) high resistivity glass sealer coat. 
ing refractory metal surfaces with a high voltage electri- A	 generalized	 formula	 for	 a refractory	 dielectric 
cal insulation for use at temperatures to 600°C. Coatings subcoat is as follows: 
previously available are of two types: glass, and flame-
sprayed polycrystalline ceramic. Glass coatings are better Percent for use at low temperatures, since they are limited by 
softening point and viscosity. Flame-sprayed ceramic 100-200 mesh pre-fired refractory
	 70 to 95 
coatings withstand higher temperatures but have low dielectric powder, thermal expansion 
dielectric strength, poor adherence
	 and porosity. The coefficient, 2 to 4.5 x 1fr6/°C 
new	 coatings	 give excellent high voltage
	 protection, 
because impervious coatings can be applied up to 0.102 High resistivity glass (fusion temper-
	 30 to 5 
.	 cm in thickness, which is not generally attainable with ature 850 to 1050°C, thermal ex-
other processes. Adherence to the metals is comparable pansion similar to the substrate metal) 
to that for glass coatings. The temperatures at which new 
coatings may be used are equivalent to those of the An example of a specific formula for basic high re-
flame-sprayed type. The new coatings will not break sistivity glass frit is as follows: 
down in vacuum at less than 3000 V compared to less 
than 1000 V for the conventional thinner glass coat- Percent ings. Magnesium	 9.7 
The new coatings were specifically developed as a Calcite	 4.8 
high voltage insulation for the surface of a perforated, Alumina	 10.6 
molybdenum, ion-accelerator grid, but are not limited to Boric Acid	 41.0 
this application. Advantages of the new coatings are: Silica	 33.9 (1) it is possible to apply thick, adherent and impervious 
coatings,	 (2)	 electrical insulation in vacuum can be All materials are prepared to less than200-mesh, dry 
achieved up to 3 kV or higher, (3) the coatings will blended and melted at 1400°C for 4 hours, then poured 
protect the surface and remain intact at temperatures up into water to quench and shatter. The shattered glass is 
to 600°C in air, inert atmosphere or vacuum, (4) good crushed to pass an 8-mesh screen. The crushed glass is 
coverage of a perforated surface is achieved, especially used in the following formula. 
around sharp corners and down into holes, (5) a voltage 
puncture is repairable with filler and sealer glasses, (6) Percent 
distortion of a thin molybdenum sheet by the thick 
coating can be held to a minimum with proper firing 200-mesh mullite powder	 80 
jigs, and (7) other low expansion refractory metals such 8-mesh basic glass frit	 16 
as tungsten and tantalum may also be protected by Enamel clay	 4 
these coatings. 100 
The coating system consists of three components: Water	 95 (1) refractory dielectric subcoat, (2) low viscosity glass Sodium nitrite	 0.05 
(continued overleaf) 
This document was prepared under the sponsorship of the National
	 Government assumes any liability resulting from the use of the 
Aeronautics and Space Administration. Neither the United States
	 information contained in this document or warrants that such use Government nor any person acting on behalf of the United States will be free from privately owned rights.
https://ntrs.nasa.gov/search.jsp?R=19720000289 2020-03-17T03:28:28+00:00Z
This mixture is ball milled for four hours in a porce-
lain mill with Alundum stones and the milled slip passed 
through a 100-mesh screen. 
The metal surface is prepared by any of the following 
methods: degreasing, sandblasting, liquid honing, electro-
polishing or oxidation.. The wet coating is applied with 
a spray gun, 0.102 to 0.203 mm thick per coat. A 
thickness of at least 0.38 mm is built up by drying 
and fusing between coats. If thicker subcoats are re-
quired, each application is thoroughly dried and fused. 
Fusion is accomplished at 1100°C for 30 minutes in 
argon at a pressure of at least one inch of water above 
atmospheric pressure. Argon flow should be sufficient to 
carry away volatile products during heating. Heat re-
quired for bonding coating to metal is supplied by an 
inductively heated graphite susceptor. 
Next, the low viscosity glass filler coat is applied. Re-
quired properties of filler glasses are low viscosity at melt 
temperature, thermal expansion close to mullite and 
high electrical resistivity. The formula for this coat con-
sists of the following: 
silica (20 g) 
magnesia (13 g) 
alumina (10 g) 
boric acid (80 g) 
calcite (15 g) 
The formula is dry blended, melted at 1350°C 
quenched in water and dried. The resulting frit is 
ground with 3 to 6 percent enamel clay and sufficient 
water to produce a sprayable slip. A 0.051 to 0.152 mm 
thick coating of this slip is applied over the previously 
fired refractory subcoat. The refractory subcoat is satu-
rated with the filler coat(s) by heating to 1000 to
1050°C in argon atmosphere. 
Next, the glass sealer coat is applied. The glass sealer 
coat is composed of either of the following: (1) the 
previously described filler coat fired at least 100°C lower 
than when used for a filler, or (2) basic high resistivity 
glass frit (96 percent), enamel clay (4 percent) and water 
to produce a sprayable slip. This mixture is ball milled 
four hours and sprayed over the subcoat and filler. This 
coating is fired to 900 to 9500C for 30 minutes in 
argon. 
Notes: 
1. The following documentation may be obtained from: 
National Technical Information Service 
Springfield, Virginia 22151 
Single document price $3.00 
(or microfiche $0.95) 
Reference: NASA CR.72677 (N70-28290), Low 
Specific Impulse Ion Engine 
2. Technical questions may be directed to: 
Technology Utilization Officer 
Lewis Research Center 
21000 Brookpark Road 
Cleveland, Ohio 44135 
Reference: B72-10290 
Patent status: 
No patent action is contemplated by NASA. 
Source: W. E. Lent of 
Hughes Aircraft Co.

under contract to

Lewis Research Center

(LEW-1 1479) 
B72-10290	 Category 04


High voltage electrical insulation coating for refractory materials

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server