CN1564722A

CN1564722A - Method for the production of protective coatings on the surface of chemically active materials

Info

Publication number: CN1564722A
Application number: CNA028199219A
Authority: CN
Inventors: K·春托诺夫
Original assignee: KONSTANTIN TECHNOLOGIES GmbH
Current assignee: KONSTANTIN TECHNOLOGIES GmbH
Priority date: 2001-10-08
Filing date: 2002-07-05
Publication date: 2005-01-12
Also published as: WO2003031100A1; RU2004113946A; CA2462214A1; ATA15852001A; AR036758A1; MY144170A; KR20050005398A; EP1434664A1; IL161237A0; AT410947B; TW593748B; JP2005504886A

Abstract

The invention relates to a method for the production of protective coatings on the surface of chemically active materials, comprising a mixture consisting of a chemically active metal and a meltable stable element.The inventive method is characterized in that it comprises the following steps: provision of at least one chemically active metal A; provision of at least one meltable stable element B; mixing metal A and element B in order to form a mixture; treating said mixture on the surface thereof with a liquid medium L which can dissolve metal A but which cannot dissolve element B at a temperature which is higher than the melting point of element B, whereby a coating is created on the surface of the mixture, substantially consisting of element B; the treatment is terminated when the desired thickness of coating is obtained; the liquid medium is removed and the mixture is purified and dried.

Description

Method for producing protective coatings on the surface of chemically active materials

The invention relates to a method for producing a protective coating on the surface of a chemically active material, which comprises a mixture of a chemically active metal and a stable meltable element.

Most metallic materials are susceptible to corrosion and must be protected from the environment. In particular, the protection of reactive metals which are rapidly damaged even in short-term contact with the atmosphere in general is of great importance. The problem of inhibiting the high chemical activity of Li, Na, K, Rb, Cs, Mg, Ca, Sr, Ba, Ra, La, Pr, Er, Eu, Yb, Cl, Pu and some other metals such as Tl is encountered in almost all of its processing and application stages.

As a means of protecting the metal from the harmful effects of the atmosphere and moisture, either the material is completely isolated from the environment by a coating or sealed enclosure, or the metal-reactivity is partially reduced by mixing the metal with another specifically selected component. The choice of protection method depends on the requirements of the material at the specific stage of its application.

Some established methods for depositing protective coatings from the vapour or liquid phase, such as physical or chemical deposition, spraying, sputtering, electrochemical plating, enamelling, oxidation, nitriding, etc., are unsuitable in the case of reactive metals, precisely because of their high chemical reactivity. Thus, there aretwo methods of protecting the material from the environment when using reactive metals: using a sealed container such as a metal container or glass ampoule or using a mixture of the active metal and other materials.

The first method is reliable during the storage and transport phases of the active material, but requires special aids to open the shell so as not to expose the metal provided to minute doses. The second method is well suited for treating the material at the application stage, but has serious drawbacks when the process controllability requirements for releasing these metals are taken as the main requirements for the active metal source.

In alkali metal complex reagents (spender) [ Della Porta p., Rabusin e., US-patent No.3579459, 1971]or in barium-evaporable getters (Gettern) [ Ferrario b. vacuum 1996, 47, 363]the vapour of the active metal a is generated as a result of the reaction taking place in the powder:

inorganic compounds (e.g. chromates or dichromates) or intermetallic compounds (e.g. Al) in which AX is a metal A₄Ba), Me is the reducing agent, and A ≈ is the desired vapor. Disadvantages of these steam sources are the hygroscopic nature of AX, the gas injection during evaporation, the exothermic nature of the reaction, the nature of the chromium compoundCancerous, etc.

For the preparation of the metal vapors of A, it has also been proposed to use thermal decomposition reactions of intermetallic compounds:

wherein AB is an intermetallic powder or a melt of the AB composition and B is a second component. An advantage of this type of source is that it allows for precise control of the evaporation stream by varying the time and temperature of thermal decomposition [ VanVuchT j.h.n., Fransen j.j.b.us-Patent No.3945949, 1976; hellier S.J.US-Patent No.4195891, 1980]. However, the use of such sources is not widespread, since the chemical activity of this intermetallic component AB, in particular in the form of a powder, is still very high.

However, another class of processes, which also take place in the presence of reactive metals, occurs in many reactions of organic synthesis, which are generally expressed as:

or

Wherein AY is a compound of active metals A and Y, R is an organic substance, and Solv is a specific solution of liquid ammonia or tetrahydrofuran. Typical problems are the necessity of separating the reaction products and removing the solvent, in which case the high explosion risk and flammability of the reagents must be taken into account.

It is an object of the present invention to provide a process for producing a protective coating on the surface of a chemically active material comprising a mixture of a chemically active metal and a stable meltable element, which process provides a usable source of metal vapour in a more efficient manner than hitherto suggested.

It is another object of the present invention to provide a chemically active material which is particularly suitable as a source of metal vapor and which overcomes the aforementioned disadvantages of the prior art.

One aspect of the present invention provides a method of producing a protective coating on the surface of a chemically active material comprising a mixture of a chemically active metal and a stable meltable element, characterised in that the method comprises the steps of:

providing at least one chemically active metal A

Providing at least one stable meltable element B

Mixing metal A and element B to form a mixture

Treating the mixture on its surface with a liquid medium L at a temperature above the melting point of the element B, the medium L being capable of dissolving the metal A but not the element B, thereby forming a coating consisting essentially of the element B on the surface of the mixture

Stopping the treatment when the desired coating thickness is reached

-removing the liquid medium

-cleaning and drying the mixture.

The essence of the process of the invention consists in treating the surface of a mixture consisting of a chemically active metal A and a stable meltable element B with a liquid medium L which dissolves the chemically active metal A but does not react with the other components of the material.

The result is that the metal a is extracted from the mixture, resulting in a stable excess of element B on the surface of the material. During the treatment, a continuous liquid film is thus formed in excess (see fig. 3). The membrane is such that the material is no longer in direct contact with the liquid medium L. After the treatment is stopped, a stable film or stable coating is formed on the surface of the material, which coating consists mainly of element B and protects the material, in particular the active metal a, from the atmosphere or other influences.

In the prior art processes for producing protective coatings, the coating material is always from the outside, whereas in the process of the invention the formation of the protective coating takes place as a result of an internal material source of the object to be treated.

For the process of the invention, it is important that the treatment temperature is above the melting point (T) of the element B_f). If the method is carried out at a temperature below the melting point of the element B, the intermediate layer formed by the treatment and consisting of B atoms is still penetrable by the molecules of the liquid L, whereby the etching of the mixture AB continues until it is completely destroyed within the material.

If the process temperature T>T_f(B) The intermediate layer B formed by the structural transformation occurring inside is dense. At the beginning, nuclei of the B melt appear in the mass of the intermediate layer, from which islands are then formed, which islands formed from the B melt grow to form new islands, etc. (see fig. 4), until finally all of them are combined into a continuous liquid film of component B. From this point on, the extraction process of metal a moves to the diffusion-controlled range and the growth of film B becomes controllable.

The process of the invention makes it possible for the first time to produce protective coatings on intermetallic compounds containing the most electropositive metals.

The metal a may thus be chosen from alkali metals, alkaline earth metals, rare earth metals and/or actinides. In particular, the metal A may be selected from the group consisting of lithium, sodium, potassium, rubidium, cesium, magnesium, calcium, strontium, barium, radium, lanthanum, praseodymium, erbium, europium, ytterbium, uranium, plutonium and thallium.

Element B may be selected from elements of groups III, IV, V and/or VI of the periodic table and binary and ternary combinations thereof with each other. In particular, the element B may be selected from gallium, indium and/or tin and binary and ternarycombinations thereof with one another.

The composition of the protective layer will be selected according to the use of the material. Since the activation temperature of the material is determined by the melting temperature of the layer of element B, different elements B or combinations thereof with each other can be used in order to obtain an activation temperature which is suitable for a wide range of applications. Ga coatings with a melting point temperature of about 30 ℃ are suitable for organic synthesis applications, Sn coatings are proposed for vacuum-applications, for example in the preparation of alkali metal sources, Pb coatings are most suitable for applications in acidic media, etc. Some examples of metallic materials having a fixed temperature transition from solid to liquid are shown in the following table.

TABLE 1 possible protective coatings

Cladding material Atom%	Ga- 14％In	Ga- 8,4％Sn	Ga	In- 21,5％Bi	In- 50％Sn	Sn- 43％Bi	In	Sn- 26％Pb	Se	Sn	Bi	Pb
Cladding material Atom%	Ga- 14％In	Ga- 8,4％Sn	Ga	In- 21,5％Bi	In- 50％Sn	Sn- 43％Bi	In	Sn- 26％Pb	Se	Sn	Bi	Pb	The temperature of the melt-down phase is, ℃	15.3	20.5	29.8	72.7	120	139	156	183	221	232	271.3	327.5

when e.g. having a eutectic composition c_e(see FIG. 7) and is the most suitable material for forming the protective layer, in order to produce pellets, a binary alloy having a composition A-B should be prepared₁-B₂Has a ternary mixture according to the line A-c_eI.e. it has a concentration of e.g. c_eSame as B in (1)₁And B₂The ratio of (a) to (b).

The liquid medium L may be selected from (a) substances having a boiling point above and a melting point below that of the element B, (B) mixtures of substances according to (a) and (c) solutions of substances according to (a) or mixtures thereof (B) in solvents which are neutral to both the metal a and the element B, i.e. the liquid medium L should satisfy the condition:

m.p.(L)＜T_f(B)＜b.p.(L)

wherein m.p. (L) is the melting point of substance L and b.p. (L) is its boiling point.

The liquid medium L may preferably be selected from CH-acids, aliphatic alcohols, polyols, higher carboxylic acids, condensed aromatic hydrocarbons and/or macrocyclic polyethers and mixtures and/or solutions thereof.

These species can be divided into two groups depending on whether gaseous hydrogen is produced during film growth.

The first group consists of inorganic compounds which react with a according to the following substitution reaction:

wherein l, s, g each represent a liquid, solid and gas state.

In addition to toxic substances, these are, for example, some high-boiling alkenes (triphenylmethane, etc.), high-boiling ethers R-O-R 'and esters (diethylene glycol, malonic esters, etc.), higher aldehydes R-CHO and ketones C-CO-R' (tolualdehyde, 2-heptanone, etc.), higher aliphatic and polyhydric alcohols R-OH (cetyl alcohol, propylene glycol, ethylene glycol, glycerol, sorbitol, xylitol, etc.), higher carboxylic acids R-COOH (stearic acid, palmitic acid, oleic acid, etc.) and other substances, mixtures thereof and solutions thereof in liquid diluents D which are inert toward AB.

Among such diluents are liquid ammonia, tetrahydrofuran and other ethers (anisole, diphenyl ether, etc.), aromatic hydrocarbons (benzene, xylene, etc.), alkanes (heptane, hexane, paraffin, etc.).

A second group of extractants L which react with the metal A by addition reactions of the formula comprising condensed aromatics (naphthalene, anthracene, etc.), macrocyclic polyethers ([18]-crown ethers [6], etc.)and others, mixtures thereof and solutions in the above-mentioned diluent D,

preferably, the treatment with the liquid medium L is ended by lowering the temperature below the melting point of the element B.

The coating thickness may be 1 μm or more, preferably 10 μm or more.

The thickness of the coating can be controlled by adjusting the duration and/or temperature of the treatment with the liquid medium L.

There are 4 approaches available to affect the dissolution rate of metal a and thus the thickness of coating B: changing the process temperature; changing the duration of the treatment; changing the liquid L and changing the hydrodynamic state close to the surface of the mixture consisting of a and B.

The temperature and the extraction time can accurately control the thickness of the growing coating: the higher the temperature and the longer the treatment time, the thicker the coating consisting of B.

Changing the liquid L to another or diluting the liquid L with a neutral diluent D that does not react with the AB material components will also affect the formation and growth kinetics of the film consisting of B.

To improve the uniformity of the coating, convection is used in the liquid L. In the case of a large area of the AB mixture, a flow L of liquid flowing around the surface is generated (see fig. 5). In the case of small particles of AB, the particles move in the unmoving medium L, for example under the influence of gravity (see fig. 6).

When the desired coating thickness is reached, the process is stopped. For this purpose, the process is carried out using an AB-L systemThe temperature is reduced to T<T_f(B) It is sufficient. The liquid film B then solidifies and the diffusive transport of the a atoms through the solid continuous layer B becomes impossible.

Preferably, a mixture consisting of the metal a and the element B is immersed in the liquid medium L.

The mixture consisting of metal a and element B can be shaped as desired before treatment with the liquid medium L. It will be appreciated that the shape of the mixture of metal a and element B may also be susceptible to change when treated with the liquid L.

Preferably the mixture of metal a and element B is substantially spherical before treatment with the liquid medium L. In this case, the substantially spherical mixture will preferably fall into a trough of liquid medium L.

Furthermore, the mixture of metal A and element B can also be cylindrical or flat before treatment with the liquid medium L.

The process for encapsulating small particles consisting of AB having an average linear dimension of from-1.0 to 5.0mm can preferably be carried out in the following two variants:

dropping a solution consisting of AB into the liquid L or

-throwing solid particles AB, in any desired form or in the form of cast granules, into the liquid L, while heating the upper layer of the liquid L to a temperature T>T_f(B)。

In the first case, the melt consisting of AB is extruded via a capillary opening into a flight tube with a particle collector, which is filled with liquid L (see fig. 8). The droplet solidifies in the liquid L and is covered by the layer B in the downward movement. The large thermal energy storage in the AB melt droplet allows the upper part of the bath to be heated to a temperature T>T_f(B) Is not necessary. But the lower layer of liquid L should be kept at T<T_f(B) The temperature range of (a).

In the second case, solid particles consisting of AB are thrown downwardly into a vertically extending bath containing a liquid L (see fig. 6). The upper region of the tank is heated to a temperature T>T_f(B) The temperature of the lower zone is T<T_f(B) In that respect As the particles sink into the hot zone, they are covered by liquid layer B. This layer is then solidified in the lower cold zone of L. The thickness of the coating is controlled by two parameters, namely the length of the hot zone Δ h ═ h₁-h₀And the temperature T of the zone₁. The moment of formation of the liquid film B can be visually observed during the adjustment of the treatment conditions if a shiny metallic surface is shown on the particles.

The first test for the quality of the coating was at a temperature T<T_f(B) Water is allowed to act on the encapsulated particles, and no gas escapes during the action indicating the continuity of the coating.

After surface passivation, the product is thoroughly washed in a suitable solvent to remove medium L, dried and then used for the corresponding purpose.

In addition, the method of the invention has other main characteristics and advantages that:

high productivity: the duration required for the formation of the protective coating is a few seconds

Versatility: the method of forming the layer is independent of the surface size or surface shape. The process can be used for a wide range of products which differ in the kind of active ingredient, the number of such ingredients and their concentrations. In particular, the process makes it possible for the first time to produce protective coatings on materials containing alkali metals or alkaline earth metals.

Ease of process: the process does not require complex equipment, is carried out at low temperatures, and can be carried out using inexpensive and readily available reagents.

Controllability of the process: the process of film growth can be completely controlled using only two parameters, namely the treatment time and temperature of the liquid medium L.

Independence from the original surface state: since the upper layer of material is partly removed and partly re-structured when treated with the liquid medium L, there is no need to pre-treat the surface. This is very important in view of the chemical activity of the material.

Another aspect of the present invention is to provide a chemically active material having a protective coating on its surface, which is obtained by the method of the above invention.

The new material of the invention is an intermetallic compound, the free surface of which is covered by a thin film consisting of a stabilizing element B. Its main advantage is that it can be handled, stored and transported like various other usual substances. For example, the material of the present invention need not be stored in a vacuum or in a protective gas.

When the material of the present invention is heated, the vapors of the chemically active material a can escape in a controlled manner. The evaporation temperature is related to the composition of the material. In the case of materials containing Na, K and/or Cs as metal A, the evaporation temperature is, for example, at 400-700 ℃.

Encapsulated intermetallic precursors and their advantages are described in: vacuum 47, 79-82,1996；Vacuum，47，463-466，1996；Vacuum 55，101-107，1996(ii) a Or k.a.chuntonov and t.b.stenitzer at the second international conference on inorganic materials, santa berbara, USA, show report "international recursors" in 9 months 2000; none of these publications disclose a method for preparing such intermetallic precursors.

The material of the present invention combines the two aspects of the prior art to solve the protection problem of active materials, namely the application of a mixture of cladding and components. This eliminates both of these disadvantages and takes advantage of their great advantages.

The structure of the sealed small particles of the present invention is shown in fig. 1: core A between chemically active metals_nB_n(hereinafter AB) is enclosed in a stable cladding of a meltable element B, in whichA is an active metal. Under normal conditions, the solid coating B is not penetrated by water, air and other substances which do not chemically react with the element B. Such small particles can be handled by the method of treating element B without any safety measures.

When the temperature rises to T>T_f(B) I.e. greater than the melting point T of the element B_f(B) When activated, the small particles are activated. When the composition of the core of the small particles is C ≤ C₁(see FIG. 2) the cladding layer becomes of composition c_lAnd allows the a atoms to reach the surface of the small particles. In this case, the small particles are a controllable source of the metal a (fig. 1b) and can be used as an alkali metal-complexing agent or evaporable getter for the various reagents R or as a pure source of the atoms a.

When the core body is formed into C₁＜c≤C₄(see FIG. 2), when T>T_f(B) In the case of cladding B reacting with the core as follows:

wherein n is 2, 3 or 4. As a result the protective film disappears, showing a particularly active discontinuous layer on the surface of the small particles that can open up pathways for reaching the substantially active core Cn. This type of material is an excellent chemical getter and can be used as a non-evaporable getter in instruments closed in a vacuum or in filters for gas purification.

Another aspect of the invention is to provide for the use of the active material of the invention, such as a source of steam; as a chemical absorbent; formed as an active metal source in the form of a catalyst or as a component of the product produced for use in chemical synthesis and/or for use in the production of specific alloys, sublimation pumps and/or particle accelerators.

The chemically active material is preferably used as a vapor source in the production of photoemissive devices (e.g., photovoltaic cells, photomultipliers, photoconductive cameras, image converters) and in the production of organic light-emitting diodes.

The chemically active material can also be used as a chemical getter (e.g., evaporable and non-evaporable getters) in the preparation of vacuum-tight instruments such as solar cells, electron tubes such as CRT, X-ray tubes, lamps, dewar flasks, vacuum barrier panels and tubes, field emission displays, and the like.

The chemically active material can also be used as a chemical getter for gas purification such as evaporable and non-evaporable getters in plasma displays, gas filters, etc.

The chemically active material may also have other applications, such as special alloys (in case the elements Eu, Yb, Na, Li, etc. are applied), in sublimation pumps, in particle accelerators and in many other possible applications.

The present invention will now be described with reference to the accompanying drawings and preferred embodiments, which are not intended to limit the scope of the invention.

FIG. 1:

shows the structure of the present invention encapsulating small particles, wherein

(a) The method comprises the following steps Expressed in T<T_f(B) Wherein 1a is a solid cladding B and 2a is an intermetallic core;

(b) the method comprises the following steps Expressed in T>T_f(B) An embodiment of the small particles, wherein 1b is of the composition C₁2b is of composition C ═ C (see fig. 2)₁The core of (1).

(c) The method comprises the following steps Expressed in T>T_f(B) Another embodiment of the small particles, where 1C is of the composition C₁+C₂+ … a discontinuous layer, 2C being of composition C₁＜c≤C₄The core of (1).

Two behaviors of small particles with respect to the composition of the core under heating are shown. When the core body has the composition C less than or equal to C₁(FIG. 2) when T>T_f(B) When forming a composition of c_lIn the case of a liquid cladding (fig. 2) of (a) the core is partially molten and acts as a semi-permeable film for metal a, allowing atoms a to penetrate from the core AB to the cladding c_lOf the outer surface of (a). Atom A can be removed from cladding c_lSuch as in a vapour source of an alkali metal or in an evaporable getter, or into a corresponding liquid medium consuming the active metal a, such as into an organic synthesis reaction. When the core body has the composition C₁＜c≤C₄(FIG. 2) when the small particles are heated to T>T_f(B) The cladding B reacts with the core, thereby forming a sponge-like discontinuous layer C on the surface₁+C₂+ …, which (as empirically shown) is chemically more reactive than pure metal a. The material is used in airtight instrumentEmpty excellent chemical absorbents and their use for producing pure gases, for example in corresponding filters. It has a low activation temperature Ta ≡ T_f(B) And operated at room temperature, in this respect is superior to a standard non-evaporable getter (NEG).

FIG. 2:

FIG. 2 shows an extensive phase diagram of system A-B, where A is a reactive metal and B is a stable meltable component; c_p、C₁、C₂… is an intermetallic compound; c. C_lIs a liquid phase composition; at a temperature T_dIt is mixed with crystal C₁The balance is present; in the concentration range C₂In A:

phase boundary in the case where A is an alkali metal

… … phase boundary in the case where A is an alkaline earth metal.

Fig. 2 illustrates in an exemplary binary system how small particles of material can be selected for various applications. Suitable for use as a controllable generator for the vapor A also comprises an evaporable getter or as a controllable source of the metal A in the chemical reaction, the composition of the small active particles being such that C ═ C₁. In this case, at a temperature T_d＜T_f(C1) In which T is_f(C1) Is a compound C₁Melting temperature ofPassing the metal A through the liquid cladding c_lMigration occurs with quasi-steady state conditions, i.e. with constant velocity migration. For use as a non-evaporable getter, use should be made of materials having a relatively high active metal concentration, such as C₂、C₃…. This compound forms a thermodynamically unstable pair with cladding B and is heated to T>T_f(B) A reaction occurs and then decomposition of the protective layer occurs.

FIG. 3:

fig. 3 shows the starting state of the treatment with the liquid medium L, where AB is an intermetallic mixture consisting of a metal a and an element B, L is the liquid medium, O is an atom a, is an atom B, 31 is an interlayer consisting of B.

Fig. 3 shows the mechanism of inducing an interlayer B31 at the interface of a solid AB and a liquid L. The sponge-like structure of the interlayer B is at a temperature T<T_f(B) Cannot prevent the material from corroding, but when the temperature is increasedT＞T_f(B) When this occurs, a condition of structural change is created, resulting in the appearance of a continuous liquid cladding B.

FIG. 4:

fig. 4 shows the main steps of the film forming process:

(a) indicating an initial state (corresponding to the state of fig. 3) when the atoms 41 of the metal a are dissolved in the liquid medium L;

(b) islands 42 of B melt occur in the spongy interlayer;

(c) a phase 42 extending from the island structure;

(d) the continuous film 43, constituted by the solution B, is changed to the diffusion condition of extraction;

(e) the temperature drops and a solid protective coating 43 consisting of B is formed.

FIG. 4 shows the steps in the formation of the protective coating, i.e. the melt B forms crystalline nuclei islands 42(B), which islands 42 grow (c) and combine into a continuous liquid film 43(d) of component B until the temperature is reduced to T<T_f(B) Curing (e) takes place, the most important point in the overall process corresponding to step (d), after which the temperature of the system AB-L may be lowered to end the extraction of metal a, or the temperature may be maintained to allow further growth of the coating thickness. Earlier cooling of the system, for example from step (c) is premature, which can lead to surface defects, i.e. the presence of open (unprotected) regions in the cladding.

FIG. 5:

fig. 5 shows an example of passivation of the bar, where reference numeral 51 is the container containing the bar of the AB mixture, 52 is the gas pilot tube, 53 is the liquid L stream, and 54 is the coating made of element B formed by this method.

The figure shows how the flat surface of the bar AB is treated with a liquid L to obtain a protective coating B.A cylindrical container 51 containing a slug of the AB mixture is placed in a gas pilot tube 52, which has an opening in its lower part for the outflow of liquid L. Heating to a temperature T>T_f(B) Is introduced from above into the open surface of the bar, washed and flows down the outer vessel wall with the dissolved product therein.

When displayed on the surface of the barIn the case of a glossy, pure film 54, the temperature of the extraction medium 53 is reduced to a value T<T_f(B) And the feed was stopped. The surface of the bar and the entire vessel were thoroughly washed with a neutral solvent to remove residual liquid L. The vessel was then placed in a vessel with distilled water to judge the continuity of the coating, and finally the water was washed off with acetone or ethanol. The product is dried using air blowing or vacuum.

FIG. 6:

FIG. 6 illustrates a technique for encapsulating small particles, wherein

(a) A metering device with a tube 61, a hopper 62 and a shifter (Schaltwerk) 63;

(b) passivation device with a trough 64 containing medium L, a bridge 65, a cylinder 66 and a scoop67;

(c) temperature distribution in the cell

(d) View of the hopper 62 and shifter 63 along line N-N in (a).

One of the methods for implementing the process for encapsulating solid particles (chunks or granules) is shown in fig. 6. The main parts of the device are: a metering device (a) whose task is to roll the AB particles down one by one in order to avoid the particles sticking to each other in the groove 64 after the liquid coating B has appeared; a tank 64 containing a medium L in which a liquid envelope is formed and solidification takes place; the barrel 66 is removed as soon as the final product has accumulated without interruption of the process.

The hopper 62 serves the same purpose.

The charging of the hopper 62 and the downward movement of the granules AB from the tube 61 are carried out under an atmosphere of flowing Ar the granules are caused to enter the tank one by adjusting the suitable inclination angle α and the rotational frequency of the shifter 63₁-h₂) Coated, in the cold zone (h)₀-h₂) The envelope is cured and the encapsulated small particles then pass along the bridges 65 into the cylinder 66 where they are periodically removed with a scoop 67.

The minimum length of the hot zone, which corresponds to the transition of the process to the diffusion zone, i.e. to the controlled zone, under given conditions, can be determined by visual inspection of the state of the surface of the small particles as they move through the hot zone and inspection of the continuity of the resulting product coating.

The process given requires less stringent conditions in terms of volatility of the liquid L than processes for particle encapsulation starting from a solution.

FIG.7:

FIG. 7 shows A-B₁-B₂Concentration triangle of system, wherein A is active metal, B₁And B₂Is a meltable element, c_eIs a binary system B₁-B₂Eutectic composition of (1); along line A-c_eThe area drawn with dashed lines is the preferred compositional range for the small particle active core of the present invention.

If a binary or ternary eutectic mixture consisting of the stabilizing element B is applied, the possible composition range of the protective cladding can be greatly broadened. This broadening is related not only to the melting temperature of the protective cladding, but also to its chemical properties. Ternary system A-B in FIG. 7₁-B₂It is given how the choice of cladding material is linked to the composition of the active core of the small particles. Component B of the latter₁And B₂Should preferably be in the ratio to the eutectic c used for the cladding_eThe same is true. This means that the composition of the small particle core should be on line A-c_eThe above.

FIG. 8:

FIG. 8 shows an apparatus for preparing encapsulated small particles, wherein numeral 81 is a bar of the mixture AB, 82 is a melting chamber, 83 is a wire, 84 is a glass film having a capillary, 85 is a flight tube, 86 is a suction tube, 87 is a tapered connecting base, 88 is a vapor of a volatile metal A, 89 is a melt of the mixture AB, 810 is a droplet of the melt of the mixture AB, 811 is a tank containing a liquid L and 812 is a heating furnace.

As shown in fig. 8, the apparatus for preparing encapsulated small particles directly from the melt is composed of three parts: a melting chamber 82, a flight tube 85 with a suction tube 86, and a glass channel 811 connected to the flight tube with a conical connecting base 87.The tank 811 is filled with a liquid medium L having a very low vapor pressure at room temperature. The melting chamber 82 is divided into two parts by a glass membrane 84, which is centered by a capillary tube having a wire 83 therein.

The device comprises the following process steps: the slug 81 is introduced into the melting chamber 82 counter-currently to the argon from the suction tube 86. The upper charge portion of the melting chamber 82 is then closed as shown in fig. 9. The furnace 812 is moved from above onto the melting chamber 82 and the charge is melted by continuous pumping. The slug melts and the melt is slowly forced out through the capillary opening due to its own hydrostatic pressure and the vapor pressure of metal a.

The temperature of the furnace was slowly increased as long as the melt was flowing to maintain a constant frequency of dropping.

The droplet runs down the wire 83 extending from the capillary 84 drop by drop and is released from the wire and reaches the well 811 containing the liquid L where it is encapsulated. After the drop had passed, the apparatus was filled with argon gas, and the tank 811 was separated from the flight tube 85, and washing of the product was started.

The melting chamber is cut off slightly above the flight tube and a new melting chamber is welded in place to allow the apparatus to be used in the next cycle.

FIG. 9:

FIG. 9 shows the operation of adding the bar, where reference numeral 913 is a cone, 914 is a test tube, 915 is a plug, 916 is a hook, 917 is a cap, and 918 is a neck. 81 and 82 are bars or melting chambers as shown in fig. 8.

The addition of the bar to the encapsulation device was performed as follows: the slug is fed into the test tube 914 in an argon containing box, the test tube is closed with a plug 915 and removed from the box in the same manner. A large cone 913 is fixed to the upper portion of the melting chamber 82 (see also fig. 8) and feeds argon from the lower edge tube 82. Test tube 914 with slug 81 is fed into cone 913, the plug is opened, and slug 81 is discharged.

The slug 81 is lowered into the depth of the melting chamber 82 with a hook 916 and the test tube 914, plug 915 and cone 913 are removed. The upper part of the tube is hermetically closed by a cap 917 by forming a neck 918 on the tube by heating without disconnecting the argon flow, and the upper part of the tube is sealed around the neck after the argon is sucked. The device is thus ready for dripping and particle encapsulation.

Fig. 10-19 are discussed in detail in example 7 below.

Examples

Example 1:

will contain Na₈In₁₁The thin-walled Ni cylinder of the bar was mounted under the heptane layer in the Pyrex test tube as shown in figure 5. Diethylene glycol was directed from above onto the surface of the bar and the temperature of the diethylene glycol was heated to 180 ℃. Once a shiny layer of In appeared on the surface of the bar, the temperature of the diethylene glycol was lowered to 100 ℃, after confirming that the In-coating had cured, feeding of the diethylene glycol was stopped, and washing of the Ni cartridge with hot water and acetone was started.

The entire process of passivating the surface lasts for several minutes. The resulting In-coated bar can be used as a source of Na-vapor In the MBE-chamber or In place of Ti In the sublimation-getter pump.

Example 2

Cylindrical rods of In-20 atomic% K (as depicted In fig. 8 and 9) 14mm In diameter and 40mm high were introduced into a Pyrex apparatus to drip the melt. The capillary tube in the center of the membrane 84 (FIG. 8) has an inner diameter of about 1mm and a length of about 10 mm. A nichrome wire having a diameter of 0.8mm was placed in the capillary tube, which passed through the entire capillary tube and extended from the lower end of the capillary tube by about 3.0 mm. After the device was evacuated, the charge was melted and the melt started to flow down at a frequency of 1.0 to 0.25 sec/drop at a slowly increasing temperature.

The droplet reached a glycerol-containing tank, the upper layer of which was about 10cm high and was temporarily heated to about 80-100 ℃ and the lower layer of which was about 15cm high and had room temperature. Hydrogen evolved during the encapsulation process is continuously pumped out.

When the process is complete, the small particles of glycerol are washed with hot water and ethanol and dried. Thickness measurements of the In-cladding layer showed that the particles with an average size of about 3.0mm had a coating thickness of 80-100 μm.

The In-20 atomic% K particles with the In-coating can be used as a controllable and safe In-operation source of potassium In organic synthesis reactions or as a controllable potassium vapor generator In the preparation of photoemissive devices.

Example 3:

mixing Na with diameter of 1.2-1.5mm₂₂Ga₃₉The granules of (2) are introduced into a tank of diethanol (FIG. 6) by means of a metering device. Hot zone T of the cell₁120 deg.C, length delta h 250mm, and cold zone T ₂10 ℃. Then Ga-containing encapsulated particles Na₂₂Ga₃₉(prepared as described in FIG. 6) was washed with distilled water and acetone at a temperature not exceeding 20 ℃ followed by drying in vacuo.

Another portion of the same composition and size of the pellets was charged to a heptanone-containing tank, the hot zone T of the tank₁120 deg.C, length delta h 250mm, and cold zone T ₂10 ℃. The product was washed of liquid reactants with acetone at 20 ℃ and dried under vacuum.

In order to avoid the particles sticking together due to the meltable Ga-coating, the product should be stored at temperatures not exceeding 20-22 ℃.

Na with Ga-cladding₂₂Ga₃₉The granules are a good source of Na-vapor in vacuum applications: it can withstand outgassing in vacuum up to 400 ℃ and, upon heating to 450-600 ℃, it can generate Na-vapor with the required strength for the preparation of photocathodes. Other fields of application for the particles are chemical reactions in organic synthesis requiring the presence of Na.

Example 4:

spherical particles of composition InLi and average diameter 2.8mm are charged T from a metering device (FIG. 6) heated to about 180 ℃ by flowing argon₁180 ℃ and T ₂25 ℃ diethylene glycol.

The hot zone length was 220mm for the first 20 particles. The encapsulated particles are removed with a spoon 67 and washed with water and ethanol. Chemical and metallographic analysis showed that the In-cladding thickness was about 0.2 mm. The encapsulation process was repeated at the same temperature for a second portion of InLi-particles consisting of 20 particles, but with a hot zone length of 340 mm. Thickness measurements of the In-cladding layer showed that the thickness In this case was 0.35 mm. For a third portion of the InLi-particles, consisting of 20 particles, the hot zone length was increased to 700mm, which resulted In growth of the In-cladding thickness to about 0.45 mm.

InLi-particles coated with In-are excellent working materials for Li-evaporable getters, and alsoLi-sources for reactions In organic synthesis.

Example 5:

a piece of alloy with a composition of Sn-40 atomic% Ba, with an average linear size of 1.8-2.5mm, is fed from a metering device (FIG. 6) into a glycerol tank, the T of which is₁＝245℃，T₂The hot zone length was 300mm at 25 ℃. The product was washed with hot water and acetone. Chemical analysis showed the product to have a composition of Sn-33 atomic% Ba. This corresponds to a Sn-coating thickness of about 120 mm.

Such particles are a controllable source of Ba and are used as Ba-evaporable getters in instruments sealed in vacuum or as ultra-pure reagents for the precise incorporation of Ba-cations into organic compounds.

Example 6:

mixing an intermetallic compound Cs with an average line size of 1.8-2.0mm₂In₃The blocks were treated as in example 5, using a cell containing a 15% malonic acid ester in diphenyl ether solution, T₁＝170℃，T₂The hot zone length was 400mm at 30 ℃. The resulting product was washed first with isopropanol and then with distilled water to remove liquid reactants and dried under vacuum.

In-coated Cs₂In₃The particles can also be used as reliable reagents in the preparation of photocathodes, controllable sources of Cs-vapor in organic Li-emitting-diodes or in organic synthesis of Cs-containing substances.

Example 7:

to demonstrate the film-forming mechanism when treating solid particles AB with liquid L, particle Na is shown in FIGS. 10-19₈In₁₁The result of the electron microscope analysis of (1). The granules, 2.5-3.0mm in diameter, were treated at 165 ℃ for various lengths of time in a tank with a paraffin solution containing 30% stearic acid. After a.tau.sec treatment, the particles were removed from the tank, washed several times with hot heptane and analyzed. The results are set forth in Table 2 below:

TABLE 2 morphological changes during particle encapsulation

Test No	In the extracting agent Treatment of Time tau(s)	Surface state	Reference to
Test No	In the extracting agent Treatment of Time tau(s)	Surface state	Reference to	1	0	Compound surface uniformity	FIG. 10: global picture of particles before treatment FIG. 11: elemental analysis table for mark area The surface composition is similar to the integral composition
2	1	The presence of a single In island	FIG. 12: bright spots on the surface	1	0	Compound surface uniformity
2	1	The presence of a single In island	FIG. 12: bright spots on the surface	3	3	Development of In-islands Is a system	FIG. 13: number of bright spots on the surface Visible increase in eyes and size
4	5	In is substantially continuous Surface of	FIG. 14: almost the entire surface is bright FIG. 15: when enlarged, the original can be seen Triangular depressions in the surface (coating defects) Trap) FIG. 16: the analysis of the bright-area elements shows that, it is composed of metal In	3	3	Development of In-islands Is a system
4	5	In is substantially continuous Surface of		5	6	Continuous uniform In surface	FIG. 17: encapsulationAfter particle morphology FIG. 18: when enlarged, FIG. 15 can be seen Is out of the depression

For comparison, Na In the In-cladding with a diameter of 1.7mm is shown In FIG. 19₈In₁₁-the morphology of the particles. The particles are obtained by treating solid spherical particles by feeding them into a glycerol tank having a T₁The treatment time in this medium is about 1 second at 175 deg.c and Δ h at 200mm (fig. 6).

Conclusion from this example:

the formation phase of the continuous protective coating is technologically important, after which time the thickness growth of the coating is controllable (example 4);

the duration until a continuous coating is shown is determined by the temperature of the liquid L and its chemical composition.

Claims

1. A process for producing a protective coating on the surface of a chemically active material comprising a mixture of a chemically active metal and a stable meltable element, characterized in that the process comprises the steps of:

providing at least one chemically active metal A

Providing at least one stable meltable element B

Mixing metal A and element B to form a mixture

Stopping the treatment when the desired coating thickness is reached

-removing the liquid medium and

-cleaning and drying the mixture.

2. The process as claimed in claim 1, characterized in that the metal A is selected from the group consisting of alkali metals, alkaline earth metals, rare earth metals and/or actinides.

3. The method of claim 2, wherein metal a is selected from the group consisting of lithium, sodium, potassium, rubidium, cesium, magnesium, calcium, strontium, barium, radium, lanthanum, praseodymium, erbium, europium, ytterbium, uranium, plutonium, and thallium.

4. A method according to any one of the preceding claims, characterized in that element B is selected from the elements of groups III, IV, V and/or VI of the periodic Table and their binary and ternary combinations with each other.

5. The method as claimed in claim 4, characterized in that the element B is selected from the group consisting of gallium, indium and/or tin and binary and ternary combinations thereof with one another.

6. The process as claimed in any of the preceding claims, characterized in that the liquid medium L is selected from the group consisting of (a) substances whose boiling point is above and whose melting point is below that of the element B, (B) mixtures of substances according to (a) and (c) solutions of substances according to (a) or mixtures thereof (B) in solvents which are neutral to both the metal A and the element B.

7. The process as claimed in claim 6, characterized in that the liquid medium L is selected from CH-acids, aliphatic alcohols, polyols, higher carboxylic acids, condensed aromatic hydrocarbons and/or macrocyclic polyethers and mixtures and/or solutions thereof.

8. Method according to one of the preceding claims, characterized in that the end treatment is reached by lowering the temperature below the melting point of element B.

9. A method according to any of the preceding claims, wherein the thickness of the coating is 1 μm or more, preferably 10 μm or more.

10. Method according to one of the preceding claims, characterized in that the coating thickness is controlled by adjusting the duration and temperature of the treatment with the liquid medium L.

11. Method according to one of the preceding claims, characterized in that a mixture of metal A and element B is immersed in the liquid medium L.

12. Method according to one of the preceding claims, characterized in that the mixture of metal A and element B is brought into the desired shape before the treatment with the liquid medium L.

13. The method of claim 12, wherein the mixture of metal a and element B is shaped substantially spherical prior to treatment with the liquid medium L.

14. The method according to claim 12, characterized in that the mixture of metal a and element B is shaped into a cylinder or plate before the treatment with the liquid medium L.

15. The method as claimed in claim 13, characterized in that the mixture in the form of spheres is dropped into a tank for the liquid medium L.

16. A chemically active material having a protective coating on its surface, preparable by the method of any one of claims 1 to 15.

17. Use of the chemically active material according to claim 16 as a vapour source in the manufacture of light emitting instruments and organic light-emitting-diodes, as a chemical getter, including evaporable and non-evaporable getters, in the manufacture of gas filters and vacuum tight instruments, as a catalyst in chemical synthesis or as a component of the products produced and/or as an active metal source in the manufacture of special alloys, sublimation pumps and/or particle accelerators.