US3096155A - Process for the continuous production of thin, elongated, single crystals of zinc oxide and antimony tri-oxide - Google Patents

Description

.1. E. GORDON ETAL 3,096,155 PROCESS FOR THE CONTINUOUS PRODUCTION OF THIN, ELONGATED, SINGLE CRYSTALS OF ZINC July 2, 1963 OXIDE AND ANTIMONY TRI-OXIDE Filed Nov. 25, 1959 FIG.|.

FIG.2.

PROCESS FOR THE CONTINUOUS PRODUCTION OF THIN, ELONGATED, SINGLE CRYSTALS F ZINC OXIDE AND ANTIMONY TRI-OXIDE James Edward Gordon, Christopher Charles Evans, and

Noel James Parratt, all of Safiron Walden, England, asslgnors to T. I. (Group Services) Limited, Aston, England, a company of Great Britain Filed Nov. 25, 1959, Ser. No. 855,431 Claims priority, application Great Britain Nov. 26, 1958 1 Claim. (Cl. 23144) This invention is concerned with the production of whiskers for use as reinforcement. It is known that various materials, when in the form of extremely thin elongated single crystals known as whiskers," have strengths many time that of the bulk material, and approaching the theoretical maximum strength deduced from considerations of inter-atomic forces. Accordingly, it has been proposed to use such whiskers as reinforcement for a matrix in which they are embedded.

Various methods are known for the production of whiskers, but hitherto these have only been on a laboratory scale and not suited to quantity production. Such methods include the subjection of a metal to extreme mechanical pressure, the chemical reduction of metal compounds, deposition from vapour or firom solutions, and electrolysis.

:It is an object of the invention to enable whiskers to be produced in an economical manner on a commercial scale such as to make their use as reinforcement in commercially produced materials a practical possibility.

It is now proposed, according to the invention, to produce whiskers on a commercial scale by carrying out one of the above processes, such as deposition from solution or from vapour, or chemical reduction, in a continuous manner in a chamber from which the grown crystals are deposited continuously by the action of gravity or are otherwise removed when they have reached the desired size. For example, a vapour may be circulated through a chamber similar to a vertical shaft furnace, the chamber being subjected to a light upward current of The conditions in the chamber are arranged to be such as to promote crystal growth, which may be initiated by artificial nucleation at the input end. The vapour is continuously recirculated and only those crystals exceeding a certain size will have a sufficient weight in relation to their air resistances to overcome the upward current and be deposited. They may be harvested continuously from the bottom of the chamber by means of a conveyor, on which they are oriented before incorporation in a matrix to form a structural material.

The chamber may be arranged horizontally, with the incoming vapour or solution entering on one side and leaving from the opposite side, and any crystals which reach the desired minimum size fall out of the horizontal current into the stagnant region below to be collected.

Where the whiskers are produced by a chemical reaction, it is necessary to control the speed at which the reaction takes place, since if the reaction occurs too rapidly, a powder is formed, the whiskers having insufiicient time to grow. The speed is controlled by limiting the supply of one of the reactants, for example, by diluting the reactants in a stream of inert gas.

The invention will now be described by way of example with reference to the accompanying drawings, in which:

FIGURE 1 shows one form of apparatus suitable for producing whiskers by a straightforward physical evaporation process; and

FIGURE 2 shows apparatus suitable for producing whiskers by a continuous chemical reaction.

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Three examples of processes according to the invention will be described.

Example I This is an example of the production of whiskers of copper phthalocyanine by physical evaporation using the apparatus of FIGURE 1. Dry carbon dioxide is blown at a rate of 6 litres per minute into a heating zone A which is maintained, by thermostatic control means not shown, at a temperature of 760 C. With the carbon dioxide there is borne along one gram per minute of commercial copper phthalocyanine powder and this sublimes to a vapour in the zone A. The zone A leads to a chamber E, of which, in the zone B, the walls are maintained at a temperature of C. The majority of the phthalocyanine vapour is carried by the gas stream into this zone B, and the vapour condenses out into whiskers. The chamber E has an outlet C at its upper end, and so there is a general upward current of gas to this outlet, and the dimensions of the chamber E are such that this upward current has a mean velocity of approximately 1 cm. per second in the zone B. Thus the light whiskers are held suspended in this zone, but as they grow and become heavier the upward current is unable to support them and they fall down to the base D of the chamber E, from which they can be withdrawn from time to time. It will be understood that in this way only whiskers or bunches of whiskers of a certain predetermined minimum size reach the base D for collection.

Example II This is an example of a combined reaction and condensation process for producing whiskers of antimony trioxide, using apparatus which is simple enough not to need illustration. A stream of air is passed over a bath of molten antimony at a temperature of 800 C. and the oxygen in the air reacts with the antimony to form antimony trioxide vapour, with which the air stream becomes saturated. This saturated stream is then passed into a large condensing chamber held at 550 C. and the antimony trioxide condenses out in the form of regularly shaped needles or whiskers. These needles or whiskers are carried out of the condensing chamber by the velocity of the gas stream and the stream is then passed to a filter in which they are collected, or into a further chamber in which the gas velocity is reduced to a point at which the Whiskers fall out under gravity and are collected.

Example III This is an example of the production of whiskers by continuous chemical reaction, using the apparatus of FIG- URE 2. Metallic zinc -F is boiled in a side chamber G and is carried, by means of a gentle flow or argon gas passing through that chamber, into a main reaction chamber H in which the atmosphere is largely of argon. The chamber H is maintained at a temperature of 1000 C. Around its walls are arranged nozzles K through which pure oxygen is fed in stoichiometric quantities in relation to the zinc, that is to say, in the right proportion to the quantity of zinc entering the chamber to convert all the zinc to zinc oxide without excess oxygen being left. The rate of delivery of zinc and oxygen is kept down to a level such that the zinc oxide has time to grow in whiskers and is not merely formed at a large number of nuclei simultaneously, which would result in the formation of zinc oxide powder. The whiskers in fact grow out in buches from common nuclei, and these bunches or groups fall to the bottom of the chamber H, from which the whiskers can be withdrawn from time to time in .a tangled mass.

It will be appreciated that all the processes described in'the examples are operated continuously, and this way, in spite vof the comparatively slow growth rate of individual whiskers, it becomes commerically practical to produce Whiskers in sufiicient quantities for use as reinforcing fibers in otherimaterials and -to'=achieve thereby a final product at a competitive price. The invention' is not limited to any particular materials for the Whiskers, as the process may be carried out with any materials that undergo whisker formation under the right conditions, and the parameters such astte'mperatur'aipressure, and rate of delivery of the reactants mustlobviously he selected to suit' the reactants involved. "These parameters also exercise some control over the size and length/diameter ratio of the resulting whiskers. l

' We claim: I

A process for the continuous production of thin elongated, single crystals of the classknown as whiskers comprising the steps of subjecting a metallic compound selected from the group consisting of oxide, and antimony tri-oxide to heat in a heating zone which is maintained at'a temperature above'the vaporizing temperature of said compound, continuouslyv passing vaporized compound formed in said heating zone from said zone into a region of a condensing vessel, maintaining said region at a temperature below the vaporizing temperature of said compound, continuously passing an upward current of inert gas through said region, controlling the mean velocity of said upward current so that it initially maintains in suspension condensed crystals of said compound formed in said gas in said region as a result of the lowered temperature thereof until said crystals have grown to a size wherein they fall by gravity against said upward current of gas, and collecting from the bottom of the vessel the crystals of metallic compound that are formed and fall under gravity. 7

References Cited in the file of this patent UNITED STATES PATENTS Trump et a1 May 12,1912 Kaufiman et a1. June 1-8, |1957 OTHER REFERENCES Brenner articles in Acta Metallurgica, vol. 4, January ,1956, pages 62-64, 74, and vol. 4, May 1956, pages 268- 270. 1

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