Definitions

• the present invention relates to the technology of the creation of anticorrosive coatings on extremely sensitive metal particles, specifically introducing an efficient industrial method of encapsulation of intermetallic particles A n Ga m , where A is an alkali or alkaline earth metal and n and m are indices.
• the chemically active material according to the present invention can be used as vapor source of alkali and alkaline earth metals in the production of photoemission devices, in the production of organic light emitting diodes, in the production of film chemisorbents.
• Particles of intermetallic compounds of the general formula A n Ga m can be used as convenient sources of the metal A component in a variety of vacuum technologies for producing thin films of metal A (see Chuntonov K.A., Postovalov V.G., Kesarev A.G. Vacuum 55 (1999) pages 101-107).
• A is an alkaline or an alkaline earth metal
• m and n are stoichometric indices
• the Me film solidifies reliably insulating the sensitive intermetallic core material of the particle against the environment.
• the thickness of the coating Me is determined by the leaching time, and the choice of the extractant L is determined by the melting point of the metal Me and the requirement of efficient wetting of the crystal A n Me m with its cover of melted metal Me in the presence of L.
• water can serve as an extractant (RU 2056661 CI) owing to its suitable physical constants.
• organic extractants with higher temperature boundaries for the liquid state must be used, e.g. polyatomic alcohols, carboxylic acids, etc. (WO 03/031100).
• the main detrimental aspect is the high tendency of as-encapsulated particles to conglutination.
• the mass which is collected in the receiver of the extraction column is far from uniform: only a small part of the product consists of separately encapsulated particles, whereas the major part must be described as agglomerates of conglutinated particles containing residues of the extractant L in their voids. Removal of these residues of the extractant L and its reaction byproducts (metal hydroxides, alkoxides, carboxylates etc.) from such a material is not an easy task as any attempts to separate the conglutinated particles either mechanically or chemically lead to damage of the protective shell of the particles.
• the unacceptable quality of the product obtained following the technology of prior methods (RU 2056661 CI) and (WO 03/031100) is also caused by variations in the structure of the initial materials.
• a polycrystalline structure of the particles is unfavourable for efficient encapsulation: during the leaching process any granules with needle or laminar structures melt into a spherical shape; this process proceeds very fast and leads to an unacceptable loss of the active component A.
• the extractant penetrates along the grain boundaries of the polycrystallites into the particle and is immured there after the formation of the outside shell, contaminating the particle.
• the present inventor conducted experiments and showed that the disadvantages of the extraction method in its prior variants can be overcome with the help of new solutions based on a) using a pre-floatation state of the intermetallic particles while it reacts with water, b) the phenomenon of self-passivation of the coating metal when it is subjected to a controlled exposure to water and air, and c) observing strict requirements regarding the microstructure of the initial particles and considering their structure on the molecular level.
• a new encapsulation technology which can be carried out in two variants, a conveyor and a cassette methodology, has been developed as applied specifically to A n Ga m particles.
• the gallides provide a particularly high grade of purity.
• An unprecedented quality of the particles prepared by the new method is guaranteed by carefully selecting the starting material, by keeping the particles isolated from each other prior to passivation, and by a sequence of process steps which e.g. make any cleaning of the metallic particles from residues of organic extractants, such as from stearic acid or glycerol (WO 03/031 100) unnecessary.
• the process is also not only yielding a better product and is more economical, but also environmentally benign because it uses no solvents other than water.
• Another object of the present invention is to provide chemically active materials, which are especially qualified as perfect vapor sources of alkali and alkaline earth metals.
• the above-stated objects of the invention can be attained by a process for the production of monocrystalline binary intermetallic compounds defined by the general formula A n Ga m with 0 ⁇ n ⁇ 22 and 0 ⁇ m ⁇ 39, wherein n and m are indices and wherein compound A is a metal and is selected from the group consisting of lithium, sodium, potassium, rubidium, cesium, calcium, strontium, barium and radium, comprising the steps of a) leaching of single crystals of the compound of said formula A n Ga m in water, which dissolves metal A, but which cannot dissolve gallium, at a temperature which is higher than the melting point of gallium, to produce a melted cover layer consisting essentially of gallium metal b) terminating the treatment as soon as the desired thickness of the coating is reached, c) solidification of the cover layer made of gallium metal, and d) passivation of the gallium cover layer of the particles.
• steps a), b), c) and d) are carried out with individual particles isolated from each other to prevent any aggregation.
• Leaching according to step a) of the compound of the formula A n Ga m is carried out on a support, especially a mesh made of metal.
• the termination of the treatment in step b) is reached by lowering the temperature below the melting point of gallium metal.
• the preferred thickness of the gallium coating is 10 ⁇ m, the average diameter of the A n Ga m particles is in the range from 0.2 mm to 3.5 mm.
• the monocrystalline particles of the formula A n Ga m grown from a stoichometric melt or from a melt with a small excess of gallium, are used as the initial material, the monocrystals of the compound A n Ga m being chosen from the group consisting of LiGa, NaGa*, Na 22 Ga 39 , KGa , RbGa 3 , CsGa 3 , CsgGa ⁇ , CaGa 4 , Ca Ga 8 , SrGa 4 , SrGa 2 and BaGa 4 ,.
• a particle in the beginning a particle is exposed to hot water on a slowly rotating horizontal mesh, whereafter the particle is thrown down into a small extraction column where in the lower cold part of the column the melted gallium-shell solidifies (step c) and the particles get on to a conveyor belt moving the particles to air followed by a rinsing of the product and rapid passivation of the gallium surface layer of the particles according to step d) is carried out in streams of water and/or air including drying of the particles with an air-flow.
• a metallic sieve divided into a large number of cells is employed to carry the A n Ga m particles, which after loading with particles is placed into an extraction tank and exposed to water for the set time, and wherein hot water is subsequently displaced by cold water from below by feeding it upwards through a damping mesh and creating a hydrodynamic backing, which induces crystallisation of a gallium shell under non- contact conditions when a particle is in a suspension state, and wherein passivation of the created gallium shell by intensive rinsing of particles with distilled water and blowing with dustfree atmospheric air is conducted.
• the chemically active material according to the present invention can be used as vapor source of alkali and alkaline earth metals in the production of photoemission devices and in the production of organic light emitting diodes, as chemisorbent including getters for vaporable and non- vaporable substrates, in the production of gas filters and sealed vacuum apparatus, for use as a source for active metals in chemical synthesis in the form of a catalyst or as a reaction component to produce chemicals and alloys, as well as in supplementation pumps and/or particle accelerators.
• the inventor of the present invention found that the extraction method according to the present invention offers high reproducibility in the properties of encapsulated particles and the required purity of the product then and only then, when monocrystalline A n Ga m particles (single crystals), grown from a melt of a strictly stoichiometric composition or from a melt with a small excess of gallium, are used as the initial material.
• the material used for the monocrystalline A n Ga m particles can contain impurities of up to 3 atomic percent of base metals including iron, nickel, molybdenum, tungsten, chromium and titanium.
• X-ray diffraction analysis of particles can serve as a convenient means of initial material quality control.
• An example of such control is given in Fig. 7 and 8 and also in Fig. 9a, 9b, 10a, and 10b, showing morphology of crystal LiGa and Cs 8 Ga ⁇ i correspondingly.
• the second important step in the technological improvement of the process consists in the replacement of bulky extraction columns by small baths containing metallic mesh devices serving as a support for the particles.
• the effect of the bubble jacket depends on the particle size: for particles smaller than 0.2 mm the tendency to floating-up is strong, while for particles larger than about 3.5 mm partial extrusion of gallium melt from under a particle and the appearance of coating defects is observed.
• the optimum size for encapsulation of the particles was found to be in the range from about 0.2 mm to about 3.5 mm. This range determines properly the borders of mesh applicability in the extraction method.
• the method according to the present invention eliminates the mentioned drawback due to the usage of monocrystalline particles and due to the passivation of the gallium coating, which eliminates the ability of the particles to conglutinate. Also the isolated treatment of the particles under conditions of the absence of contact between them provides very high yield of the end product.
• the method according to the present invention provides a practical solution for the problem of production of chemically active materials with guaranteed high purity and this method for the first time allows manufacturing this kind of materials on an industrial scale.
• the conveyer-variant The process of leaching and formation of a protective coating is performed in the following way. In the beginning, a particle is exposed to hot water on a slowly rotating horizontal mesh (Fig. 2). Subsequently, the particle is thrown into a small extraction column, in the lower, cold part of which the melted shell of gallium metal solidifies, and the particle is then placed onto a moving conveyer belt, which exposes it to the air and subjects it to a series of operations of rinsing and air passivation of the coating, including finally drying of the particle in a stream of air.
• the conveyer variant is characterized by a continuous motion of the treated particles one by one in certain intervals, and their sequential transfer from one technological zone to the other. In each of the zones a set of conditional parameters is maintained.
• the cassette- variant Unlike in the previous case, in this variant a large number of particles, isolated from each other, simultaneously and at once undergo a sequence of states, which constitutes the essence of the encapsulation process. The change of the states occurs by changing the parameters of the media into which the treated particles are placed.
• a metallic sieve divided into a large number of cells, one particle in each, is put into a tank with hot water for a set time. Then, the hot water is displaced by cold water from below (Fig. 5). The cold water is fed upwards through a damping mesh and is creating a hydrodynamic backing allowing crystallization of the gallium shell of the particles under non-contact conditions with the particle in a suspension state.
• the remains of the alkaline solution are removed and the passivation process of the cover layer of the particles is started.
• the particles are rinsed and dried with a fan, the procedure being repeated several times.
• both variants are based on the same set and sequence of technological stages: leaching of an active component A from the surface of a monocrystalline particle and formation of a molten gallium coating; crystallisation of this coating either at immersing the particle into cold water under the influence of gravitation forces, or while the particle is in a suspension state in upward water flows; passivation of the as-created shell of gallium metal by extensive rinsing of the particles with distilled water and blowing with dust-free atmospheric air.
• the new technology and the new products have the following advantages: the new material in a form of monocrystalline particles of A n Ga m with a gallium coating is characterized by higher chemical purity and stability of physical parameters than previous products; consequently, the new material is a vapor source for metal A of higher, unprecedented quality; the new technology is more perfect and simple, it allows increasing the yield of the product per time unit by a large factor and to increase the capacity of the extraction method to an industrial level.
• the new material according to the present invention can be used in the production of thin films by vacuum deposition. Especially it can be used in the production of photoemission devices, in the production of organic light emitting diodes and in the production of film chemisorbents.
• Fig. 1 shows a particle of A n Ga m in hot water on a metal mesh support. a) 1 - intermetallic core, 2 - an island of molten Ga on a particle surface, 3 - stainless steel mesh, 4 - hydrogen bubbles; b) free access for water to the entire particle surface; c) free outlet of the products of dissolution of metal A in H 2 O from the particle surface.
• Fig. 2 is a general plan view of the conveyer line.
• a doser 5 can be moved along the arch NM and than be fixed.
• a device for pushing a particle into a column (see 3 in Fig. 6) is not shown here.
• the packaging line can direct the particles to any of the two positions, A or B.
• Fig. 3 is a longitudinal section of the apparatus. 1 - a heater, 2 - an extraction column, 3 - a stepper motor shaft (see 7 in Fig. 2.); 4 - a plate with a mesh; 5 - a discharge pipe; 6 - a refrigerator; 7 - a funnel; 8 - a transporter; 9 - a conduit; 10 - a water inlet; 1 1 - a particle; 12 - a shower; 13 - a receiving funnel of a rinsing basin; 14 - a silicone serpentine tube; 15 - a cuvette; 16 - a transporter; 17 - a guiding collar; 18 - a discharge pipe.
• a particle is placed on the mesh and pushed with a jet into the column 2, where it falls down with a rate V.
• a gas jacket serves as an excellent lubrication and the falling rate is sufficiently fast, about 0.1 m/s.
• the Ga-shell solidifies. Through the funnel 7 the particles get into the moving conveyer 8, and then with its help reach the rinsing basins 12 - 13 - 14 - 15.
• the collar 17 retains the particles in the track. Subsequently, the transporter 16 brings the particles to the second rinsing basin, and so on.
• Fig. 4 is a scheme of rinsing and drying. a) 1 - a sieve in a rectangular frame; 2 - a metal carcass; 3 - a stainless steel mesh; 4 - hooks or protrusions for hanging the sieve in a tank or a bath; 5 - honey-combs made of metal foil; b) 1 - a bath; 2 - a carcass with a sieve; 3 - a shower; 4 - clamps; 5 - a detachable bottom of a bath; 6 - a valve; 7 - a mesh with particles.
• the honey-combs 5 and the sieve 1 are shown separately for better understanding.
• the detachable bottom of a bath 5 (Fig. 4b) is connected to the lower flange of bath 1 through a sealing gasket with the help of clamps 4.
• Fig. 5 is an apparatus for the cassette method. a) 1 - a tank; 2 - a carcass with a sieve; 3 -hooks, 4 - a discharge pipe; 5 - a vessel with pure water; 6 - a thermocouple; 7 - a valve; 8 - a refrigerator; 9 - a damping mesh; 10 - a resistive heater; 11 - a valve; b) 1 - a resistive heater; 2 - a flange; 3 - a thermocouple.
• the sieve 2 with the particles is inserted into the tank 1 with heated water and from the moment, when gas bubbles appear, the exposure time starts to be counted off. After the leaching process is over, the valve 7 is opened and ice-cold water replaces the hot water. The hot water is let out through the pipe 4.
• Fig. 6 shows the upper part of the extraction apparatus (see also 1 - 9 in Fig. 2) 1 - an extraction column; 2 - a doorway in a bath wall; 3 - an injector for wash-off of a particle; 4 - a plate; 5 - a stepper motor shaft; 6 - hot air flows; 7 - a doser spout; 8 - a bath body; 9 - a radial partition; 10 -a discharge pipe valve; 11 - a mesh; 12 - a flat clamping disk; 13 - hot water; 14 - heaters; 15 - a falling particle.
• a doser spout 7 is a nozzle with two channels, through which hot air is constantly fed for preventing appearance of condensate on the wall of the inside channel. At the moment when rotation of the plate 4 is stopped the dozer throws the next particle into the bath. During the same stop the injector 3, the lower end of which is always down in hot water, injects the set portion of water, which pushes the particle exposed in the bath into the extraction column.
• Fig. 7 shows the single crystal X-ray diffraction pattern of LiGa (cubic).
• Fig. 8 shows single crystal X-ray diffraction pattern of Cs 8 Ga ⁇ (rhombohedral).
• Fig. 9a, 9b show single crystals of LiGa.
• the agglomerate (before crushing) clearly shows the cubic symmetry of the individual crystal.
• Fig. 10a, 10b show single crystals of Cs 8 Ga ⁇ j.
• the extraction column 1 and a cylindrical bath 2, in which a mesh disk 4 is placed, represent the complete reservoir (Fig. 2).
• the mesh disk is tightly pressed from below to a plate 4 (Fig. 6), which is fixed to a shaft 5 and connected with a stepper motor 7 as also shown in Fig. 2.
• Partitions 6 are inserted into the radial clearances of the plate, forming the side walls of the cells 8.
• the outside cylindrical surface of the plate and the inside cylindrical surface of the bath serve as the other borders of the cells.
• the process time is divided into small alternating intervals ⁇ r m and ⁇ s : the plate rotates for ⁇ m seconds, then for ⁇ s seconds it is motionless, and at this stage, during the time ⁇ r m , the disk turns to an angle corresponding to an arc ⁇ / (Fig. 2).
• the operation of the apparatus is tuned in such a way that by the moment, when a particle is known to acquires a uniform gallium coating, its cell coincides with a doorway 9 in the cylindrical wall of the bath (Fig. 2), then during the stop time ⁇ r s this particle is carried to the extraction column (Fig.6) by a submerged jet, created by an injector 3 (Fig. 6).
• a new particle is thrown into a cell, which is k units away from the cell coinciding with the doorway 9,counting against the direction of the disk movement (Fig. 2).
• T is the total extraction time
• t is the time of a particle's passing through the hot zone of the extraction column.
• the height of the extraction column is 10 to 15 times shorter than in the method proposed previously (RU 2056661 CI) and does not exceed one meter.
• a particle is moving through the upper zone, heated by the heater 1 (Fig. 3), it is still reacting with water and has a molten shell.
• the trajectory of the falling particles is characterized by a certain scattering cone.
• the diameter of the column should be bigger than the base of a scattering cone.
• the gallium shell of the particles solidifies in ice-cold water, cooled with the help of an outside cooler 6, and then the particles are focused through a funnel 7 to a median of a conveyor belt 8 (Fig.
• This setup with a cascade of four basins guarantees a cleaning of the product from metal hydroxides (OH " anions) to reach lower a pH.
• OH " anions metal hydroxides
• the rinsing result is even better, because the concentration of the alkaline solution in the extraction column never reaches 1% due to constant renewal of the water mass in the column (see 5 and 10 in Fig. 3).
• the final stage of the process is the drying of the particles.
• a water jet with particles flows from the last rinsing basin with a rate, the horizontal constituent of which is close to the speed of the mesh belt of the long conveyor 17 (Fig. 2).
• a row of fans 16 (see also a-a) is installed above the belt, which creates laminar filtered airflows with the temperature kept between 12 and 15°C.
• the dry particles arrive at a distributor 18, which either routes the product for charging into the corresponding containers, e.g. boats, or into dust free boxes where they are stored in unpacked form at a temperature not higher than 18-20°C.
• a sieve 1 (Fig. 4), consisting of a light carcass 2, mesh 3 and thin-walled honey-combs 5, is charged - in a fume box under n-heptane - with particles of A n Ga m . Then this sieve is placed into an extraction tank 1 (Fig. 5a), hanging it on hooks 3 (see also 4 in Fig. 4a). In the part of the tank where the particles are located, water is heated with the help of a low-inertia insulated resistive element 10 (see also 1 in Fig. 5 b) up to close to the boiling point. In the lower part of the tank, under the metallic mesh 9, the water has a temperature close to the temperature of melting ice.
• the leaching process lasts not longer than 2 minutes, after which a valve 7 is opened and cold water, moving upwards, replaces hot water, which flows out through discharge pipes 4. With the temperature decrease the jacket of gas bubbles, surrounding each particle, starts decreasing and then disappears. Owing to this effect, for maintaining particle floatability the rate of the cold water feed is gradually increased up to the value of ⁇ V s (see text to Fig. 3), such that crystallization of the shell of gallium metal takes place without any contact with the mesh.
• the time required for the solidification of the shell of gallium metal does not exceed 2 seconds. Therefore rinsing of the tank with cold water is stopped as soon as the temperature of the water flowing through the pipes 4 reaches about 5°C.
• the sieve is taken out from the tank and transferred to the washing bath 1 (Fig. 4b). From above a shower 3 is moved to the bath, water is switched on, and the bath is filled to a level that the particles are covered with water.
• the water is drained through a valve 6, the rinsing procedure is repeated 4 - 5 times, the shower device 3 is replaced by an air fan for dust free laminar air flow, the bottom 5 of the bath is taken away and drying of the particles is started.
• the particles feature the required degree of purity with respect to residual AOH contaminants, and a passivated gallium coating.
• encapsulated Cs 8 Ga ⁇ particles For manufacturing encapsulated Cs 8 Ga ⁇ particles according to the conveyer variant 1, monocrystalline particles of the composition CsgGa ⁇ with an average linear size of 2 mm are charged to a doser 5 (Fig. 2), where they are kept under n-heptane.
• the water temperature in an extraction bath 2 and in the upper zone of column 1 is set to the range of 96 - 98°C, the water temperature in the lower zone of the column to about 0°C.
• the height of the hot zone of the column is 0.75 m, the height of the hot zone is 0.25 m.
• the temperature of the running water in the four rinsing basins is about 10°C.
• the capacity of the conveyer line in a steady state regime is about 50 g of encapsulated particles per hour.
• the final product are particles of CsgGan in a shell of gallium metal with the maximum baking temperature of 220° C and an evaporation temperature for cesium metal of 275° C and above.
• the baking temperature of such a product is not higher than 280 °C and evaporation becomes noticeable starting from 320 °C.
• monocrystalline Na 22 Ga 9 particles are charged under n-heptane into a sieve with hexangular cells with an edge length of 4 mm.
• the total number of particles in one charge is about 8.000 items.
• the sieve is moved down into a tank with water kept at a temperature of 94 - 96°C and the particles are thus exposed to the bath for 75 seconds before they are rinsed with ice-cold water for 2 minutes. Subsequently the particles are rinsed with a shower four times and dried with air as described above for the cassette variant. The whole procedure takes approximately 15 minutes for one load of particles. The capacity of the given method is about 70 g of encapsulated particles per hour.
• the final product are particles of Na 22 Ga 9 in a shell of gallium metal with a maximum baking temperature of 350°C and an evaporation temperature for sodium metal of 475°C and above.

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• 2005
  • 2005-08-22 EP EP05773154A patent/EP1782448A1/de not_active Withdrawn

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