US2484519A

US2484519A - Method of coating surfaces with boron

Info

Publication number: US2484519A
Application number: US686453A
Authority: US
Inventors: Martin Graham Robert
Original assignee: Individual
Current assignee: Individual
Priority date: 1946-01-15
Filing date: 1946-07-26
Publication date: 1949-10-11
Anticipated expiration: 1966-10-11

Description

Patented Oct. 11, 1949 y f2,4s4;519,; METHOD OF COATING SURFACES WITH ORON Graham Robert Martin, Montreal, Quebec, Canada No Drawing. Application July 26,

1946, Serial N0.

686,453. In Canada January 15, 1946 4 Claims. 1

This invention relates to the coating of various surfaces with boron and more particularly to a method of forming a thin film containing boron on metallic, glass or other surfaces.

Because boron under the influence of fluxes of thermal neutrons emits charged particles the ionization due to which can be measured, it has been used in ionization chambers employed in detecting neutrons and in measuring their intensities. Previous methods have employed an adhesive vehicle containing a boron compound, such as boron carbide, in finely divided condition. Boron obtained by the thermal decomposition of diborane has been used but diborane is difficult to obtain. Other uses of surfaces coated with a thin film of boron will suggest themselves to those skilled in the art.

The object of the present invention is to provide a method whereby readily available boron compounds may be employed to deposit from the vapour phase a continuous film containin boron uniformly distributed on a surface to be coated.

In carrying out the invention boron alkyl compounds are used to provide the boron. Boron trimethyl is preferred. The triethyl and the tripropyl compound may be used but at somewhat lower temperatures. Boron trimethyl is conveniently prepared in accordance with the following equations:

An ethereal medium is used, and, for convenience in handling the boron trifiuoride is added in the form of its ether adduct, (C2H5)2OZBF3. Since the product is spontaneously inflammable, the reaction is carried out in an atmosphere of an inert gas, such as nitrogen, and the boron trimethyl is separated by condensation in a trap cooled by liquid nitrogen. It is purified by fractional distillation at 95 C. and is stored as a gas. It boils at 20.2 C.

In forming the coating the article to be coated is placed in an oven in which it may be heated to the desired temperature of about 550 C. and the gaseous boron alkyl compound is passed into the oven under a pressure of about 1 cm. of mercury to contact the article. Provision is necessarily made to evacuate the oven before admitting the gaseous compound. The temperature of the article may be maintained by any suitable furnace electrically or otherwise heated. The boron alkyl compound is thus thermally decomposed at and on the heated surface of the article yielding free boron which deposits adherently on the 2 surface. A portion of carbon usually deposits along with the boron, but for use in ionization chambers this is of little consequence.

For coating a steel surface of 1000 sq. cm. about 8 liters of gaseous boron trimethyl passed over the surface, maintained at a temperature of about 550 C., during a period of about two to three hours gave a good coating.

When the surface to be coated is relatively small in relation to the volume of the furnace, it has been found advantageous to add the boron alkyl compoimd to the oven intermittently. Thus the compound may be released under reduced pressure into the oven where the pressure may reach about 2 cm. of mercury and allowed to react for a short period when the residue of gases is pumped off to a pressure of say 0.1 mm. and then more of the boron alkyl compound is admitted. This is repeated until the desired thickness of coating is obtained. With this procedure the amount of the boron compound required for a given coating is reduced.

These ionization chambers are useful for measuring fluxes of slow neutrons. For ionization chambers for the monitoring and control of piles a coating of about 1 mg. of boron per square centimeter is sufficient. The nuclear reaction involved is:

The range of alpha particles from the reaction corresponds to a slightly smaller thickness than this, and the slow neutron flux is reduced to the extent of about 4% per mg./sq. cm. of coating, with a consequent reduction in the ionization current produced.

It will be apparent that various forms of apparatus may be employed for coating surfaces by the method described.

I claim:

1. A method of coating surfaces which comprises maintaining the article to be coated at a temperature of about 550 C. in an evacuated chamber and passing one of a group consisting of trimethyl boron, triethyl boron and tripropyl boron in the vapour phase'and under reduced pressure into contact with the surface of the heated article to cause boron to be deposited thereon in a thin film.

2. A method as defined in claim 1 wherein said compound is admitted to the chamber intermittently.

3. A method as defined in claim 1 wherein said compound is continuously admitted to the chamber.

4. A method of coating surfaces of articles with REFERENCES CITED boron in a chamber which comprises evacuating the chamber, heating the article in the chamber The following references are of record in the to a temperature of about 550 C. and admitting file of this patent: to the chamber one of a group consisting of tri- 5 methyl boron, triethyl boron and tripropyl boron UNITED STATES PATENTS to a pressure of about one centimeter of mercury Number Name Date to contact the surfaces of the heated article and 597,172 WilmOWSkY 11, 1893 deposit boron thereon. 1,774,410 Van Arkel Aug. 26, 1930 GRAHAM ROBERT MARTIN. H 10 '2.303,658 Porter Dec. 1, 1942