US3450558A - Vapor plating beryllium - Google Patents

Description

United States Patent 3,450,558 VAPOR PLATING BERYLLIUM Thomas P. Whaley, Glenview, Ill., and James M. Wood, Jr., Baton Rouge, La., assignors, by mesne assignments, to the United States of America as represented by the United States Atomic Energy Commission No Drawing. Filed Feb. 1, 1965, Ser. No. 429,637 Int. Cl. C23c 12/02 US. Cl. 117-107.2 11 Claims ABSTRACT OF THE DISCLOSURE Process for coating a substrate with beryllium which comprises heating the substrate, generating a plating gas comprising di-tert-butylberyllium and contacting the heated substrate and generated plating gas. Substrate should be heated to a temperature from about 245 C. to about 370 C. and it is preferred that nonoxidizing conditions be present for all process steps.

BACKGROUND OF THE INVENTION Noentirely satisfactory method has been developed for the deposition of pure, dense, continuous, and adherent coatings of beryllium. Electroplating from aqueous solutions is not known to be feasible, nor has a satisfactory electrolyte for electroplating from nonaqueous media been developed. Furthermore, as brought out in the publication entitled Vapor-Plating by Powell, Campbell, and Gonser (John Wiley and Sons, Inc., 1955), beryllium metal cannot be vapor-deposited rapidly or efficiently by any known method. The authors disclose various techniques directed at producing beryllium coatings utilizing beryllium halide compounds, both by hydrogen reduction and thermal decomposition. Summarizing the authors comments, it can be seen that high quality beryllium coatings are extremely ditficult to realize and only then at high temperatures, viz. above 700 C.

More recent attempts to produce beryllium coatings have been based on the pyrolysis of organoberyllium compounds. For example, Coates and Glockling, J. Chem. Soc. 1954, 2227,in their article entitled Diisopropylberyllium and Some Beryllium Hydrides disclose that isopropylberyllium hydride decomposes to the metal at 220-250 C. Goubeau and Walter in Zeit. fur anorg. und Allgeim. Chem. 322, 58 (1963) reported that dimethylberyllium decomposes to beryllium carbide at a temperature of 220-230" C. In these and other related publications, none of the authors have reported success at effecting an integral beryllium coating. The only exception is the earlier work conducted by Paneth and Loleit, J. Chem. Soc., 1935, 369, who disclosed the preparation of a beryllium mirror on gold foil by the decomposition of diethylberyllium. However the authors did not disclose the process and conditions under which the coating was realized nor the composition of the coating. In view of results achieved pursuant to the practice of the present invention, it is unlikely that a coating of satisfactory purity was obtained. Therefore, a comprehensive vapor plating process for effecting a pure, dense, continuous, and adherent beryllium coating would be a novel and welcome contribution to the art.

An object of this invention is to provide a vapor plating process whereby a pure, dense, continuous, and adherent beryllium coating can be effected. A more specific object of this invention is to provide a vapor plating process utilizing a heat-decomposable organoberyllium compound whereby an integral beryllium coating of high purity and having distinct physical characteristics is efiicaciously produced. These and further objects will come to light as the discussion proceeds.

The above objects are accomplished pursuant to the present process which comprises heating a substrate in a non-oxidizing atmosphere to a temperature within the range of from about 245 C. to about 370 C.; generating vapors of a plating gas comprising di-t-butylberylliurnunder conditions to prevent decomposition of the compound; contacting the heated substrate under non-oxidizing conditions with the plating gas; maintaining said temperature and non-oxidizing conditions until the desired thickness of coating is achieved; and thereafter discontinuing contact between the coated substrate and vaporous plating gas and allowing the coated substrate to cool under non-oxidizing conditions.

For reasons unknown, it has been found that beryllium coatings having optimum property characteristics are realized pursuant to the present invention when the plating vapors are not allowed to remain in close proximity or contact with the heated substrate for more than about 20 seconds, especially for less than about 10 seconds, and even more preferably for less than about one second. A preferred embodiment of the present process thus comprises heating a substrate in a plating system under nonoxidizing conditions to a temperature within the range of from about 245 C. to about 370 C.; generating vapors of a plating gas comprising di-t-butylberyllium under conditions to prevent decomposition of the compound; injecting the vapors in the plating system which is continually maintained under non-oxidizing conditions whereby vapors contacting the heated substrate are decomposed and a beryllium coating is effected upon the substrate; continually removing by-products of decomposition and any undecomposed vapors at a rate such that the plating vapors are not allowed to remain in close proximity to the heated substrate for more than 20 seconds; maintaining said temperature and vapor contact conditions until the desired thickness of coating is achieved, and thereafter discontinuing contact between the coated substrate and the vaporous plating gas and allowing the coated substrate to cool under non-oxidizing conditions. It can be seen that control of the vapor flow rate to realize the desired residence time of the vapors in the system will be mainly dictated by the size and geometric configuration of the substrate.

Beryllium coatings effected by way of the above process exhibit unique properties and fulfill a much felt need in the art inasmuch as they are pure, dense, continuous, and adherent. They are further characterized as being highly oriented in that the z axis of the hexagonal beryllium crystallites is perpendicular to the substrate surface. The latter structure is realized to a high order of precision and constitutes a distinct and unique feature of beryllium coatings prepared pursuant to the present invention. The latter feature is borne out through analysis of the present coatings by X-ray diffraction spectra. The diffraction spectra of coatings prepared pursuant to the present process consists almost exclusively of one line, namely at d=l.79 A. (Angstrom Unit).

The present process is preferably conducted at a temperature within the range of from about 280 C. to about 305 C. As brought out above for the reasons discussed hereinafter, the present process is conducted under nonoxidizing conditions which are preferably realized by conof coating thickness per hour. Growth rates higher than this makes it difficult to maintain coating uniformity and quality without taking extra precautionary measures, e.g. specially designed plating chambers, and the like. The latter parameter is mainly a function of the substrate size and particular plating temperature employed. A particularly preferred embodiment of the present invention is thus a process comprising heating a substrate in a plating system under non-oxidizing conditions to a temperature within the range of from about 280 C. to about 305 C.; generating vapors of a plating gas comprising di-t-butylberyllium under conditions to prevent decomposition of said compound; injecting the vapors in the plating system which is continually maintained at a pressure less than about millimeters of mercury whereby vapors contacting the heated substrate are decomposed and a beryllium coating is effected upon the substrate; continually removing by-products of decomposition and any undecomposed vapors at a rate such that the plating vapors are not allowed to remain in close proximity to the heated substrate for more than about seconds; maintaining said temperature, pressure, and vapor contact conditions until the desired thickness of coating is achieved at a growth rate of less than about 2 mils of coating thickness per hour; and thereafter discontinuing contact between the coated substrate and the vaporous plating gas and allowing the beryllium coated substrate to cool under nonoxidizing conditions.

A non-oxidizing condition or an inert atmosphere is maintained during plating to avoid oxidation of the coating, substrate, or di-t-butylberyllium plating compound because of its deleterious effect on the quality of the finished product. There are several techniques whereby nonoxidizing conditions can be provided. One is to continually purge the plating system with an inert gas which is compatible with the substrate, plating compound, and effected coating. Examples of suitable inert gases are nitrogen, hydrogen, helium, neon, argon, krypton, xenon, gaseous aliphatic hydrocarbons, and the like. The more preferred method to provide non-oxidizin g conditions as brought out above is to conduct this process at subatmospheric pressure. By evacuating the plating system during operation undesirable contaminants will be readily removed. Additionally, the use of reduced pressure during the plating operation will assure expeditious removal of the by-prodnets of decomposition. Low pressure operation also makes it easier to maintain a fairly constant vapor flow rate. Moreover, this method of providing an inert atmosphere offers an economical advantage in that the expense of an inert medium is avoided.

For certain applications it may be desirable to utilize a carrier gas in conjunction with vaporization and transportation of the plating compound while simultaneously and continually inducing a vacuum on the plating system. In which case, the carrier gas is preferably introduced from aseparate source and used to sweep the plating vapors into the plating chamber. Also, the di-t-butylberyllium plating compound can be dissolved in a solvent in which it is compatable and the resultant solution heated to generate the plating gas. The solvent is generally a high boiling solvent to the extent that it will not immediately flash when subjected to process conditions. Aromatic hydrocarbons are preferred solvent, especially toluene, however, other solvents such as the alkanes, e.g. 2,2,3-trimethylpentane; cycloalkanes; and the like can be employed as long as di-t-butylberyllium is soluble therein. This procedure minimizes the pyrophoricity of the plating compound as well as providing a method whereby a more uniform rate of vaporization of the plating compound is assured. However, the use of a carrier gas or solvent is neither mandatory nor preferred.

The following runs were conducted employing a cylindrical plating chamber which further comprised substrate mounting means. The plating chamber was equipped with inlet and outlet connections which established a flow path in which the substrate was positioned. Means for preheating the vaporous plating gas was provided in combination with the plating chamber. The plating compound reservoir was connected to the inlet of the plating chamber, the plating compound reservoir further comprising heating means for generating the requisite di-t-butylberyllium vapors. Also connected to the inlet of the plating chamber was means containing a source of hydrogen. The outlet or discharge from the plating chamber was connected to a dry ice trap which discharged to a liquid nitrogen trap which in turn was connected to vacuum inducing means. Suitable valves were arranged in the conduit connecting these components such that they could be selectively isolated for reasons as brought out hereinafter. Flow meters were provided in the lines connecting the source of hydrogen and plating compound reservoir with the plating chamber. An induction heater was provided for heating the substrate, the temperature of which was indicated by a thermocouple provided in combination with the substrate supporting means.

,After positioning the substrate in the plating chamber, the system was sealed off from the atmosphere and evacuated throughout the operation -by the vacuum inducing means. The substrate was gradually heated and hydrogen was allowed to flow through the system to remove residual oxygen. Upon attainment of the desired temperatures of the substrate and plating compound, the hydrogen flow was terminated and the plating compound reservoir was opened to the system whereby vaporous di-t-butylberyllium flowed into the system. The vaporous di-t-butylberyllium passed first through the preheater and then into the plating zone where it contacted the heated substrate.

EXAMPLE I Substrate-Copper cylinder, diameter x 3" long Substrate temperature-248 C. Compound temperature (preheater).177 C. Pressure.0.3 mm. Hg Time.- minutes Coating thickness.0.l4 mil Plating rate.0.08 mil./ hr. Results-Smooth, light gray, metallic, adherent, and coherent coating.

EXAMPLE II Substrate.Mild steel cylinder, /2" diameter x 1 /2 long Substrate temperature.289 C.

Compound temperature (preheater).l83 C.

Pressure.0.70 mm. Hg

Time-105 minutes Coating thickness.--0.8 mil Plating rate.-0.46 mil/ hr.

Results.-Smooth, light gray, metallic, adherent, and coherent coating.

EXAMPLE III Substrate.-Mild steel cylinder, /2" diameter x 1 /2" long Substrate temperature.300 C.

Compound temperature (preheater).196 C.

Pressure.0.8 mm. Hg

Time.--105 minutes Coating thickness-averaged 1.6 mil Plating rate.-1.1 mil/hr.

Results.-Srnooth, light gray, metallic, adherent, and coherent coating which assayed at 94.8 weight percent beryllium.

EXAMPLE IV Substrate.Mild steel cylinder, 1" diameter x long Substrate temperature.287 C.

Compound temperature (preheater).l75 C. Pressure-0.3 mm. Hg

Time.450 minutes Coating thickness.--Averaged 1.7 mil Plating rate-0.23 mil/ hr.

Results-Light gray, metallic, adherent, and coherent containing a few tiny nodules. A portion was removed and assayed at 9.8 weight percent beryllium.

EXAMPLE V In the following runs a similar apparatus arrangement was employed except that the substrate was positioned on a rotating hotplate and heated by conduction. The temperature of the substrate was indicated by a thermocouple welded to the surface of the hotplate. Argon was utilized in lieu of hydrogen for purging the plating chamber of residual oxygen.

EXAMPLE VI Substrate-Aluminum disc, 1%" diameter x 0.04" thick Substrate temperature.370 C. Compound temperature (preheater).25 C. Pressure.1.25 mm. Hg Time.3.3 minutes Coating thickness-03 mil Plating rate.5 .4 mil/hr. Results-Light gray, metallic, adherent, and coherent coating.

EXAMPLE VII Substrate.8 sapphire discs, each 1 cm./ diameter x 1 mm. thick Substrate temperature-256 C.

Compound temperature (preheater).25 C.

Pressure.0.42 mm.

Time-45 minutes Coating thickness-26 mil Plating rate..34 mil/hr.

Results.-Light gray, metallic, adherent, and coherent coatings containing a few tiny nodules.

EXAMPLE VIII Substrate-8 glass squares, 0.7 cm. length x 1 mm. thick Substrate temperature-290 C.

Compound temperature (preheater).-25 C.

Pressure-0.90 mm. Hg

Timeminutes Coating thickness-58 mil Plating rate.1.7 mil/hr.

Results-Light gray, metallic, adherent, and coherent coating on each member, with some having a few small nodules.

EXAMPLE IX Substrate-4 fused quartz discs, 1 cm, diameter x 1 mm.

thick.

Substrate temperature.--2S4 C.

Compound temperature (Preheater). C.

Pressure-0.70 mm.

Time.--18 minutes Coating thickness-.14 mil Plating rate.-.47 mil/hr.

Results-Smooth, light gray, adherent, and coherent metallic coating on each disc.

The di-t-butylberyllium plating compound employed in the instant invention is well known and readily available. For the purposes of the present invention it is preferably prepared by adding beryllium chloride to a pure etherate of (t-buty1) Be. Pure di-t-butylberyllium can also be prepared by treating an etherate thereof with an alkali metal salt whereby a di-t-butylberyllium alkali metal salt complex is formed. The ether is then removed at reduced pressure and pure, ether free di-t-butylbery-llium realized by thermal dissociation of the complex salt.

Instead of ether free di-t-butylberyllium, its diethyl etherate can be used as the platig compound. This material is more readily available and does not require an ether removal step. Coatings produced from the etherate generally contain about 2 Weight percent oxygen. Otherwise, such coatings are generally the same with regard to orientation, hydride content, carbide content, adherence, coherence, etc. When employing a di-t-butylberyllium etherate as the plating agent along with a carrier gas, it is convenient to employ diethyl ether vapor as a carrier gas. The etherate is preferably heated to a temperature within the range of from about 50 C. to about C. to generate the requisite plating vapors which are similarly decomposed as pure di-t-butylberyllium pursuant to the present process.

While not mandatory, it is preferred to preheat the plating vapors since this minimizes cooling of the substrate by the vapors and thereby reduces temperature gradients in the substrate. It is preferred to preheat the plating vapors to a temperature within the range of from about C. to about 200 C. Contact time of the plating vapors with the preheater should be brief to prevent undesirable decomposition reactions.

The substrate, i.e., the object or material upon which the beryllium derived by decomposition of the di-t-butylberyllium compound is to be plated, or coated, can be any substrate which is stable at the conditions employed to effect decomposition of the compound and to the plating vapors. By stable is meant that the substrate does not decompose under the decomposition conditions utilized to deposit the beryllium plate or coating. Thus, for example, if thermal decomposition of the di-t-butylberyllium compound is the decomposition technique being employed, and the temperature necessary to effect such decomposition is high enough to cause the substrate upon which the beryllium is to be deposited to begin to soften, but, however, such substrate still retains enough rigidity to effectively support the beryllium deposit being laid down, then such a substrate is stable for the purposes of this invention.

The substrates employed in this invention range all the ways from metals and metalloids, especially those of Groups III through VIII of the Periodic Chart of the Elements (Fisher Scientific Company, 1955), to fibers, ceramics, and refractory materials. When the term fiber is employed herein the fiber can be in the form of filaments, fabrics, strands, cloth and the like.

The metals which are employed as substrates in the process of this invention are those metals which are solid at the conditions employed to plate the beryllium metal upon the metal substrate. Thus it can be seen that such metals as mercury, which is a liquid under normal conditions, is not contemplated as a substrate within the scope of this invention. Additionally, metals which would react with the plating gas to form undesirable coatings and prevent deposition of the beryllium metal are likewise not attractive. Thus, the metal substrates of this invention include, for example: scandium, yttrium, lanthanum, titanium, zirconium, hafnium, vanadium, niobium. tantalum, chromium, molybdenum, tungsten, manganese, technetium, rhenium, boron, indium, tellurium, germanium, silicon, antimony, bismuth, iron, cobalt, nickel, ruthenium, rhodium, osmium, iridium, magnesium, copper, zinc, and the like as well as alloys thereof.

Exemplary of other suitable substrates for use in the instant invention are: fiber glass, ceramics, cermets, graphits, carbides, nitrides, oxides, and the like.

For certain applications the substrate to be plated preferably subjected to an initial preparation step. This is especially desirable in the case of metal substrates which are seldom commercially supplied free from contaminants. In other words, the degree of adherence achieved through the present plating process can in certain instances be further improved by appropriate metal surface pretreatment. A suitable metal surface preparation is achieved through degreasing with a solvent such as 1,1,2-trichloroethylene or the like followed by light sandblasting. It is well known that vapor blasted coatings exhibit better adherence when they are effected on slightly uneven surfaces, such as those created by sandblasting as opposed to a highly polished surface. However, other desirable methods can be employed, such as acid pickling. On substrates such as graphiie and ceramics where the surface is already nonuniform, it is feasible only to degrease the surface preparatory to plating. Other well known methods of substrate pretreatment can be employed in lieu of the above.

Heating of the substrate can be accomplished by many well known means. Generally, resistance heating, infrared heating, or induction heating means are utilized since they are particularly suitable for heating materials without contaminating them. The latter type of heating is especially attractive where the temperature of the substrate is to be varied during the plating operation. Quite often, the nature of the substrate will influence the selection of a particular type of heating means. For instance, induction heating is preferred when working with small intricate substrates which do not possess a high thermal conductivity. Flat substrates, such as metal plates, are generally heated by conduction from resistance heating apparatus such as a hotplate. The determination of a suitable heating means is additionally influenced by the plating environment in relation to such factors as to whether the plating operation is a continuous or batch process, the number of substrates to be plated in unit time, and additionally to the physical and structural arrangement of the plating equipment. These factors are a matter of design within the abilities of one skilled in the art.

Beryllium coated materials as well as pure beryllium structural components produced by the process of this invention find utility in a wide range of applications. Advantage can be taken of berylliums high heat transfer rate, its light weight for aeronautical applications, its low mass absorption coefficient for X-ray equipment, and the like. For example, beryllium sheeting for the preparation of windows for X-ray tubes is obtained by plating beryllium onto a mild steel sheet to the desired thickness pursuant to the present invention. Thereafter, the steel substrate is stripped with nitric acid leaving intact a coherent baryllium sheet which is employed for fabrication of such windows. As a further illustration, beryllium tubing is produced by coating beryllium onto a mild steel mandrel to the desired thickness. Thereafter, the mandrel is dissolved away with nitric acid leaving the tubular beryllium coating or structure intact.

We claim:

1. A vapor plating process for depositing a beryllium coating upon a substrate which process comprises:

(a) heating a substrate under non-oxidizing conditions in a plating system to a temperature within the range of from about 280 to about 305 C.,

'(b) generating vapors of a plating gas comprising dit-butylberyllium under non-oxidizing conditions, said vapors also being generated under conditions to prevent decomposition of said compound,

(c) contacting the heated substrate under nonoxidizing conditions in the plating system with a plating gas, the pressure in the plating system being less than about 10 millimeters of mercury absolute,

(d) controlling contact between the heated substrate and the plating vapors so that the plating vapors are not in contact with the heated substrate for more than about seconds,

(e) maintaining said temperature and pressure conditions while continually contacting the substrate with the vaporous plating gas as above such that the beryllium coating is effected at a growth rate less than about 2 mils of coating thickness per hour, and thereafter (t) discontinuing contact between the coated substrate and the vaporous plating gas when the desired thickness of coating is achieved and allowing the coated substrate to cool under non-oxidizing conditions whereby a beryllium coating having a diffraction spectra consisting almost exclusively of 1.79 A. is realized.

2. A plating process for depositing a beryllium coating upon a substrate comprising, in combination,

(a)heating the substrate in a non-oxidizing atmosphere to a temperature of from about 245 C. to about 370 C.

(b) generating a plating gas comprising di-t-butylberyllium in a non-oxidizing atmosphere under conditions to prevent the decomposition of said plating gas,

(c) contacting the heated substrate and the generated plating gas in a non-oxidizing atmosphere at a subatmospheric pressure of less than about I0 mm. of mercury absolute,

(d) maintaining said substrate at said temperature during said contacting until the desired thickness of coating is achieved,

(e) discontinuing contact between the coated substrate and said plating gas, and

(f) cooling the coated substrate in a non-oxidizing atmosphere.

3. The process of claim 2 further characterized by said non-oxidizing atmosphere for the contacting step being provided by conducting the contacting at a subatmospheric pressure of less than about 1 mm. of mercury absolute and said temperature being from about 280 C. to about 305 C.

4. The process of claim 2 further characterized by said subatmospheric pressure being less than about 1 mm. of mercury absolute.

5. The process of claim 2 further characterized by said temperature being from about 280 C. to about 305 C.

6. The process of claim 2 further characterized by said non-oxidizing atmosphere for the contacting step being provided by an inert fluid.

7. The process of claim 2 further characterized by said contacting the heated substrate and the generated plating gas being effected in the presence of a carrier gas for said generated plating gas.

8. The process of claim 2 further characterized by said generating a plating gas being substantially continuous and said contacting being controlled to prevent the generated plating gas and heated substrate from being in contact for a period of more than about 20 seconds.

9. The process of claim 8 further characterized by said period being of more than about 10 seconds.

10. The process of claim 2 further characterized by said contacting being controlled to provide a coating thickness growth rate of less than about 2 mils per hour.

11. The process of claim 10 further characterized by said coating thickness growth rate being less than about 1 mil per hour.

References Cited Coates et al., Journal of the Chemical Society (Brit.), July 1954, pp. 2526 to 2529 relied upon.

Head et al., Journal of the American Chemical Society, vol. 79, 1957, pp. 3687 to 3689 relied upon.

ALFRED L. LEAVITI, Primary Examiner.

A. GOLEAN, Assistant Examiner.