IL111279A - Method for the preparation of zinc sulfide or selenide and reactor therefor - Google Patents

Description

METHOD FOR THE PREPARATION OF ZINC SULFIDE OR SELENIDE AND REACTOR THEREOF THE APPLICANT: STATE OF ISRAEL, MINISTRY OF DEFENSE, ARMAMENT DEVELOPMENT AUTHORITY, P.O.Box 2250 , Haifa 31021. 31021 Π£3 ■> Π ,2250 . T . Π THE INVENTORS: Meir HEFETZ.

Moshe LEVITZKY.

Dr. Igal E. KLEIN.

The present invention relates to a roethod and reactor for the manufacture of a good optical quality of poly-crystalline infra-red transmitting materials. More particularly the invention relates to an improved roethod and reactor for the manufacture of Chemical Vapour Deposition (CVD) zinc sulfide possessing improved optical and mechanical properties .

BACKGROUND OF THE INVENTION.

The development of the infra-red technology for various military applications, such as in FLIR (Forward Looking Infra-Red) sensors for use on high performance imaging systems, homing devices and various aerospaces windows, required new materials which possess improved properties in order to answer to the demands. The traditionally windows blanks have been produced by a number of technologies, such as glass castings, hot pressing and crystal growth. None of these methods was found to be suitable, the main disadvantages being the non-uniformity of trans-missivity and refractivity , presence of impurities, size limitations and high costs .

The CVD-zinc sulfide or zinc selenide were selected for such purposes, in view of their optical properties, which to a certain extent are quite useful . The known method is based on a low pressure CVD method, wherein the CVD chamber walls were made of carbon or fused silica, which act as substrates for large area deposition of polycrys-talline zinc sulfide or zinc selenide. The volatile compounds of the elements comprising the material to be deposited, were continuously fed into a chamber under controlled conditions and reacted on a surface whose temperature allowed the compounds to decompose or react to form a solid adherent layer. The volatile by-products of the chemical reaction are pumped away, in order to allow a thermodynamically steady state growth conditions. The main mandatory requirement in order to operate the method to a full completion, is a constant supply of reactants. Hydrogen sulfide and hydrogen selenide are gases and their flow can be easily controlled. A problem encountered with the zinc feeding, is its solid form at room temperature. The zinc sulfide is a brittle solid optical ceramic material whose mechanical properties such as hardness, fracture toughness and rupture modulus, are largely dependent upon the microstructure of the material which in turn is set by the growth conditions during deposition. As will be realized, the deposition of a zinc powder is completely undesired and a thickness uniformity in the lateral &3 well as vertical directions are most desired. In a paper by J.A.Savage et.al. (SPIE, Vol. 505, 1984, p.47- 51), it is suggested to feed the solid zinc in the center of the bottom part of a heated vertical chamber, while the gaseous reactants are fed along the perimeter .

It was already recognized, that the main factors which govern the manufacture of ZnS and ZnSe, are the reactants flow rates and the design of the nozzles through which the reactants flow into the reaction chamber. According to the U.K Patent Application No. 2,173,512 the apparatus suggested for this purpose , operates at reduced pressure . The gaseous reactants are introduced through one nozzle to a first passageway in a turbulent flow and the vaporised reactant from the molten material through another nozzle to a second passageway.

According to the DDR Patent Number 288,844, the CVD reactor for depositing zinc sulfide or zinc selenide was made from graphite segments . These segments are separated by thin carbon plates, having sharp edge protrusions inside the reactor, which determine that the growth of the zinc sulfide will occur under a turbulent flow. - 4 - 111,279/2 Among the main disadvantages of the CVD zinc sulfide prepared by the known methods, it should be mentioned the low deposition of the product and its non-uniform thickness. Therefore, it is a long felt need to produce a higher quality of the CVD zinc sulfide or selenide, to be useful for various aerospace systems. It is an object of the present invention to provide a method for the manufacture of an improved quality of CVD zinc sulfide or selenide. It is another object of the present invention to provide a method for the manufacture of CVD zinc sulfide or selenide at a relatively high yield. It is yet another object of the present invention to provide a novel reactor useful for the production of an improved CVD zinc sulfide or selenide.

BRIEF DESCRIPTION OF THE FIGURES Figure 1 , illustrates the Infra-red spectrum of the zinc domes.

Figure 2, illustrates schematically the reactor for obtaining the zinc domes.

BRIEF DESCRIPTION OF THE INVENTION The invention relates to a method for the manufacture of CVD zinc sulfide or selenide, wherein the zinc is fed into a reactor from a multitude of plates at a controlled vapour pressure and the gaseous reactants are fed into the reactor through at least one annular nozzle under laminar flow conditions.

A reactor to manufacture the respective CVD zinc sulfide or selenide, as well as a special design for the nozzles present in said reactor are also described. The method minimizes the co-production of the scum zinc oxide which normally floats on the top of the melt material, which avoids a constant supply of the zinc vapours. The yield production of acceptable domes manufactured according to the invention was above 72%, which is considered a significant improvement over the yields obtained in the existent methods .

DETAILED DESCRIPTION OF THE INVENTION.

According to the invention, the solid zinc in the form of powder, granules or coarse pellets is placed in a number of shallow plates, made of an adequate material, such as graphite or carbon. The height of zinc in each plate, is not critical and depends on the height of the reactor, generally being in the range of between 5 to 75 mm. In order to feed the zinc vapour reactant into the reactor, a series of plates, one on the top of the other is provided. In order to get an efficient feeding of the zinc, a minimum of four plates is required .The plates are maintained at a predetermined temperature profile, in order to maintain a controlled vapour pressure of the zinc. An inert gas, such as argon, is introduced into the center of the first plate, flowing along the surface of the zinc material, collecting some vapours, and is leaving the plate through its perimeter. In the eecond plate, the gaseous stream is flowing from the perimeter to the center, in the proximity of the zinc material. The flowing in the third plate iB similar to the first plate and in the fourth plate is similar to the second one . Of course, a reverse order of feeding the plates is also possible. In this manner, the inert gas environment is gradually enriched in zinc vapours until reaching the saturation point. This arrangement will avoid splashing of zinc droplets. When no more zinc is available on a plate, or the scum produced forms a barrier to its vaporization, the inert gas will carry zinc from another plate. Thus, the amount of zinc fed into the reactor can be easily controlled, either by regulating the flow rate of the inert gas or by controlling the temperature ramp of the plates zone. The number and diameter of the platee are determined by the reactor size, a preferred minimal limit of the diameter being about 30 mm.

One of the advantages of the method according to the present invention, ie the fact that the zinc to be fed, is not absolutely required to be in a melt form. Temperatures below the melting point of zinc, at atmospheric pressure, are feasible provided that its vapour pressure is adequately regulated in order to maintain the required flow-rate. This is particular important, when the product quality and its properties require very low rates of growth. The reactor is made from graphite or carbon and according to a preferred embodiment, the wall sections are made of two types of graphite having different densities or crystal orientations. In this manner, during separating the deposits from the graphite walls, the grown zinc on each wall cracked along the line between the wall sections. This embodiment, also imparts an increased yield of zinc sulfide production.

In order to produce a homogeneous CVD zinc sulfide growth with improved thickness in the lateral and vertical directions, it is suggested a particular design for the nozzles which allows high flow velocities of the fresh reactants into the reactor. The nozzle which feeds the reactant with a heavier molecular weight, should be located close to the bottom of the reactor's walls, while the nozzle feeding a lower molecular weight reactant will be located at the center of the bottom section, having an annular construction.

It was found that improved CVD zinc sulfide or selenide products are obtained when laminar flow conditions of the reactants in the reactor prevail . This may be achieved by providing a narrow mixing chamber at the bottom of the reactor. Under turbulent flow conditions in the reactor as described in the prior art, the quality of the CVD zinc sulfide or selenide ie significantly lower than that obtained according to the present inventio .

In order to produce thick deposits of above 1 ram, it is suggested that each wall should comprise at least two vertical sections of graphite . The number of sections , should be so selected that on each section only one product unit - dome or window - was grown. In this manner, the dismantling of the chamber walls and separation of product walls may be carried out quite easy and without any substantial cracking of the products. As a result, the yield of the products will be increased.

The invention will be hereafter illustrated by the folow-ing Examples, being understood that these are presented only for a better understanding of the invention, without imposing any limitation on the method and apparatus, as covered by the appended Claims .

EXAMPLE 1.

Zinc powder was fed into a reactor through a multitude of plates. The temperature profile was regulated In a manner wherein the temperature at the lowest plate was 470oC and at the highest plate was 550oC . The zinc and hydrogen sulfide were introduced into the mixing chamber through annular nozzles, which provide a laminar flow. The walls of the mandrel were made from graphite, and preforms of a diameter of 140 mm domes were milled into the walls. Each wall with a total length of 900 mm was made out of two vertical sections .

At the end of the growth, the walls were dismantled, the yield production of suitable domes, was above 75%. The Infra-red spectrum of these domes are given in the attached Figure 1.

EXAMPLE 2.

The experiment as in Example 1 was repeated in the same reactor under the same conditions, but preforms with a diameter of 220 ram domes were milled into the graphite walls .

The yield production of acceptable domes was above 72%. The Infra-red spectrum was the same as that in the previous Example, as illustrated in Figure 1.

Claims (13)

- 1 i - 111 ,279/2 C L A I M S

1. A method for the manufacture of Chemical Vapour Deposition (CVD) zinc sulfide or zinc selenide from solid zinc and gaseous hydrogen sulfide or hydrogen selenide, the solid zinc being in the form of powder, granules or coarse pellets, located in a number of shallow plates, which are fed into a reactor at a controlled vapour pressure, the gaseous reactants being fed through at least one annular nozzle under laminar flow conditions.

2. The method for the manufacture of CVD zinc sulfide or zinc selenide according to Claim 1 , wherein the number of the shallow plates in the reactor are at least four.

3. The method for the manufacture of CVD zinc sulfide or selenide according to Claims 1 or 2, wherein said solid zinc is passed from one plate to another by an inert gas.

4. The method for the manufacture of CVD zinc sulfide or selenide according to Claim 3, wherein the inert gas is entering through the center of the first plate, collecting vapours of zinc and is leaving the plate through its perimeter.

5. The method for the manufacture of CVD zinc sulfide or selenide according to Claims 1 to 4, wherein said plates are maintained at a predetermined temperature profile, in order to maintain a controlled vapour pressure.

6. A reactor useful for the manufacture of CVD zinc sulfide or zinc selenide made from graphite or carbon, the wall sections being made of two types of graphite having different densities or crystal orientations.

7. The reactor for the manufacture of CVD zinc sulfide or zinc selenide according to Claim 6, containing at least four plates having a diameter of at least 30 mm. 111 279/2

8. The reactor according to Claims 6, or 7, containing particular designed nozzles, which allow high flow velocities of the fresh reactants into the reactor.

9. The reactor according to Claim 8, wherein the nozzles feeding the reactants with a heavier molecular weight, are located close to the bottom of the reactor walls.

10. The reactor according to Claims 6 to 9, wherein laminar flow conditions of the reactants, are maintained by providing a narrow mixing chamber at the bottom of the reactor.

11. The reactor according to Claims 6 to 10, wherein each wall comprises at least two vertical sections of graphite.

12. A method for the manufacture of CVD zinc sulfide or zinc selenide, substantially as described in the specification and in any one of Claims 1 to 5.

13. A reactor for the manufacture of CVD zinc sulfide or zinc selenide, substantially as described in the specification and in any one of Claims 6 to 11. For the Applicants Patent Attorney