Fine-Particle Metal Powders

This invention relates to fine-particle powders of the metals B, Al, Si, Ti, Zr, Hf, V, Nb, Ta and/or Cr which have a defined particle size of 1.0 nm to less than 3 .mu.m.

The mechanical properties of components produced by powder metallurgical techniques are critically determined by the properties of the starting powders. More particularly, a narrow particle size distribution, high powder purity and the absence of oversize particles or agglomerates have a positive effect on the properties of corresponding components.

There are many known processes for the industrial production of fine metal powders.

In addition to purely mechanical size-reducing and grading processes, which have the disadvantage that only powders up to a certain fineness and with a relatively broad particle size distribution can be produced, a large number of processes for deposition from the gas phase have also been proposed.

Due in part to very small energy sources, such as for example, thermal plasmas or laser beams, or where turbulent flames, such as for example a chlorine detonating gas burner, are used, the particle size distribution and particle size of the powders produced cannot be exactly controlled. The reaction conditions normally lead to a broad particle size distribution and to the occurrence of individual particles several times larger in diameter than the average particle size.

It is very difficult, if not impossible, to produce powders having average particle sizes of <0.5 .mu.m, as measured by FSSS (and not individual particle sizes), by known industrial powder production processes. In the case of these conventionally produced fine powders, it is not possible in practice to prevent a certain percentage of oversize particles being present in the material to the detriment of the mechanical properties of components produced therefrom. Conventional grinding processes also give a very broad particle size distribution which, in the case of these powders, cannot be significantly narrowed even by sizing steps.

Instead of a flow-optimized hot wall reactor, other gas-phase processes use a plasma flame or other energy sources, such as laser beams, for the reaction. Disadvantages of these processes are essentially the uncontrollable reaction conditions prevailing in various parts of the reaction zone with very steep temperature gradients and/or turbulent flow conditions. As a result, the powders formed have broad particle size distributions.

Numerous proposals for processes for the production of ultrafine metal powders have been put forward, but are all attended by disadvantages.

EP-A 0 290 177 describes the decomposition of transition metal carbonyls for the production of fine metallic powders. Powders having a particle fineness of up to 200 nm can be obtained by this process.

In the search for metals having improved mechanical, electrical and magnetic properties, there is a demand for increasingly finer metal powders.

Ultrafine metal powders in the lower nanometer range can be produced by the noble gas condensation process. However, it is only possible by this process to produce quantities on the milligram scale. In addition, the powders obtained by this process do not have a narrow particle size distribution.

Accordingly, the problem addressed by the present invention was to provide fine-particle metal powders which would not have any of the described disadvantages of known powders.

Metal powders which satisfy these requirements have now been found. These powders are the subject of the present invention.

Accordingly, the present invention relates to fine-particle powders of the metals B, Al, Si, Ti, Zr, Hf, V, Nb, Ta and/or Cr which have a defined particle size of 1.0 nm to less than 3 .mu.m, less than 1% of the individual, particles deviating by more than 40% from the average particle size and no individual particles deviating by more than 60% from the average particle size.

In a preferred embodiment, less than 1% of the individual particles deviate by more than 20% from the average particle size and no individual particles deviate by more than 50% from the average particle size. In a particularly preferred embodiment, less than 1% of the individual particles deviate by more than 10% from the average particle size and no particles deviate by more than 40% from the average particle size. The powders according to the invention preferably have particle sizes in the range from 1 to less than 500 nm, more preferably in the range from 1 to less than 100 nm and most preferably in the range from 1 to less than 50 nm.

The metal powders according to the invention are highly pure. Thus, they preferably have an oxygen content of less than 5,000 ppm and, more preferably, less than 1,000 ppm. Particularly pure metal powders according to the invention are characterized in that they have an oxygen content of less than 100 ppm and preferably less than 50 ppm.

The non-oxidic impurities are also minimal. In a preferred embodiment, the sum total of their impurities, except for the oxidic impurities, is less than 5,000 ppm and, more preferably, less than 1,000 ppm.

In a particularly preferred embodiment, the sum total of their impurities, except for the oxidic impurities, is less than 200 ppm.

The powders according to the invention can be obtained on an industrial scale and, accordingly, are preferably present (i.e., produced) in quantities of more than 1 kg.

The powders according to the invention are obtainable by a process for the production of fine-particle metal powders by reaction of corresponding metal compounds and corresponding reactants in the gas phase -CVR-, the metal compound(s) and the other reactants being reacted in the gas phase in a reactor, homogeneously condensed directly from the gas phase in the absence of any wall reactions and subsequently removed from the reaction medium, characterized in that the metal compounds and the reactants are introduced separately from one another into the reactor at at least the reaction temperature. In cases where several metal compounds and/or reactants are to be introduced, the particular gas mixtures should be selected so that no reaction leading to solid reaction products takes place during the heating phase. In a particularly advantageous embodiment, the process is carried out in a tube reactor. It is particularly favorable for the metal compounds, the reactants and the product particles to pass through the reactor under laminar flow conditions.

By separately preheating the process gases to at least the reaction temperature, the nucleation site can be confined. The laminar flow conditions prevailing in the reactor provide for a narrow residence time distribution of the nuclei or particles. A very narrow particle size distribution can be obtained in this way. Accordingly, the metal compounds and the reactants should preferably be introduced into the reactor in the form of coaxial laminar streams.

However, to ensure that the two coaxial streams are intermixed, a Karman vortex path of defined intensity and extent is produced by the incorporation of an obstacle in the otherwise strictly laminar flow.

In a preferred embodiment of this process, therefore, the coaxial laminar streams of the metal compound(s) and the reactants are mixed under defined conditions by means of a Karman vortex path.

In order to prevent deposition of the reactants on the walls of the reactor, for which there is considerable preference in energy terms, the reaction medium is preferably screened off from the reactor wall by a layer of inert gas. This may be done, for example, by introducing an inert gas stream through specially shaped annular gaps in the reactor wall, this inert gas stream keeping to the reactor wall under the Coanda effect. The metal powder particles formed in the reactor by homogeneous condensation from the gas phase for typical residence times of 10 to 300 msec leave the reactor together with the gaseous reaction products (for example HCl), the unreacted reactants and the inert gases which are introduced as carrier gas, purging gas and for the purpose of reducing the adsorption of HCl. Yields of up to 100%, based on the metal component, can be obtained by the process according to the invention.

The metal powders are then preferably removed at temperatures above the boiling or sublimation temperatures of the metal compounds used, the reactants and/or any by-products inevitably formed during the reaction. The metal powders are advantageously removed in a blowback filter. If this filter is operated at high temperatures, for example 600.degree. C., the adsorption of the gases, particularly the non-inert gases, such as HCl, to the very large surface of the metal powders can be minimized.

The remaining troublesome substances adsorbed onto the powder surfaces can be removed in a following vacuum vessel, again preferably at temperatures of the order of 600.degree. C. The final powders should then be discharged from the plant in the absence of air.

According to the invention, preferred metal compounds are one or more metal compounds from the group consisting of metal halides, partly hydrogenated metal halides, metal hydrides, metal alcoholates, metal alkyls, metal amides, metal azides and metal carbonyls.

Hydrogen is used as another reactant. Further characteristics of the powders include their high purity, their high surface purity and their good reproducibility.

Depending on the particle size and the constituent material, the powders according to the invention can be highly sensitive to air or pyrophoric. To eliminate this property, the powders may be subjected to a defined surface modification by treatment with gas/vapor mixtures.

FIG. 1 diagrammatically illustrates an apparatus with which the powders according to the invention can be produced. The working of the process is described in the following with reference to FIG. 1. The process, material and/or apparatus parameters specifically mentioned are selected from many possibilities and, accordingly, do not limit the invention in any way.

The apparatus shown in FIG. 1 generally comprises a gas preheater (23), a gas-introduction part (24), a flow shaping part (25), a reaction tube (4) and a product discharge device (26).

The solid, liquid or gaseous metal compounds are introduced into an externally arranged evaporator (1) or into an evaporator (1a) arranged inside the high-temperature furnace, vaporized therein at temperatures of 200.degree. to 2000.degree. C. and transported into the gas preheater (2a) with an inert carrier gas (N.sub.2, Ar or He). The other reactant (3) H.sub.2 is also heated in at least one gas preheater (2). Before entering the tube reactor (4), the turbulent individual streams issuing from the gas preheaters (2) are combined in a nozzle (5) into two coaxial, laminar and rotationally symmetrical streams. The middle stream (6) containing the metal component and the surrounding stream (7) containing the hydrogen are mixed under defined conditions in the tube reactor (4). The reaction takes place at temperatures of 500.degree. C. to 2000.degree. C., for example in accordance with the following case examples:

To ensure that the two coaxial streams are intermixed, a Karman vortex path can be produced by incorporation of an obstacle (17) in the otherwise strictly laminar flow. In a preferred embodiment of the present invention, the obstacle (17) is disposed in the flow-shaping part (25), preferably along the longitudinal axis of the central coaxial nozzle (i.e., the nozzle which produces the middle stream (6)). The two coaxial streams are separated at the nozzle outlet by a weak inert gas stream (16) to prevent growths around the nozzle (5).

It is particularly preferred to incorporate the evaporator within the high temperature furnace, for example, within the gas preheater (2a). This avoids the need for feed pipes outside the reactor, thus avoiding corrosion and the resulting impurities. By locating the evaporator within the preheater it is also possible to use non-metal materials for the construction of the evaporator, so that evaporation temperatures can be employed which are higher than the temperatures for which metal materials are designed.

In order to prevent the heterogeneous deposition of these substances on the hot walls of the reactor, for which there is a considerable preference in energy terms, the hot reactor wall is purged through annular gaps (8) with an inert gas stream (9) (N.sub.2, Ar or He) which keeps to the reactor wall under the Coanda effect. The metal powder particles formed in the reactor by homogeneous condensation from the gas phase leave the reactor together with the gaseous reaction products (for example HCl), the inert gases and the unreacted reactants and pass directly into a blowback filter (10) in which they are deposited. The blowback filter (10) is operated at temperatures of 300.degree. C. to 1000.degree. C., so that adsorption of the gases, more particularly the non-inert gases, such as HCl, to the very large surface of these powders is kept at a low level. In a following vessel (11), residues of the adsorbed gases on the powders are further reduced by preferably alternate application of a vacuum and flooding with various gases at 300.degree. C. to 1000.degree. C. Good results are obtained when such gases as N.sub.2, Ar or Kr are used. It is particularly preferred to use SF.sub.6.

Metastable systems and core/shell particles can also be produced by this process. Metastable systems are obtained by establishing very high cooling rates in the lower part of the reactor.

Core/shell particles are obtained by introducing additional reaction gases in the lower part of the reactor.

From the vacuum vessel (11), the powders enter the cooling vessel (12) before passing through the lock (13) into the collecting and transport vessel (14). In the cooling vessel (12), the particle surfaces can be subjected to defined surface modification by exposure to various gas/vapor mixtures.

Coated graphite, more particularly fine-particle graphite, is preferably used as the constituent material of those components which are exposed to temperatures of up to 2000.degree. C. and higher, such as the heat exchangers (2) and (2a), the nozzle (5), the reactor (4) and the tube (15) surrounding the reactor. Coating may be necessary, for example, if the necessary chemical stability of the graphite to the gases used, such as metal chlorides, HCl, H.sub.2 and N.sub.2, at the temperatures prevailing is inadequate or if erosion at relatively high flow rates (0.5 to 50 m/sec.) is very high or if the impermeability of graphite to gases can thus be increased or if the surface roughness of the reactor components can thus be reduced.

For example SiC, B.sub.4 C, TiN, TiC and Ni (only up to 1200.degree. C.) may be used for the layers. Combinations of various layers, for example with a "characteristic" outer layer, are also possible. These layers may advantageously be applied by CVD, plasma spraying and electrolysis (Ni).

In cases where only low temperatures are required, metallic materials may also be used.

To adjust the particle sizes of the metal powders, three measures may simultaneously be applied:

establishing a certain ratio between the reaction gases and inert gases.

establishing a certain pressure.

establishing a certain temperature/residence time profile along the reactor axis.

The temperature/residence time profile is established as follows:

by two or more heating zones from the beginning of the gas preheater (2) to the

end of the tube reactor (4).

by varying the cross-section of the reactor along its longitudinal axis.

by varying the gas throughputs and hence--for a predetermined reactor crosssection--the flow rates.

A significant advantage of the variability of the temperature/residence time profile is the possibility of separating the nucleation zone from the nucleus growth zone. Accordingly, it is possible--for the production of "relatively coarse" powders over short residence times at very low temperatures (i.e. small reactor cross-section for a certain length)--to allow the formation of only a few nuclei which can then grow into "coarse" particles over long residence times at high temperatures (large reactor cross-section). "Fine" powders can also be produced: numerous nuclei are formed in a zone of high temperature and relatively long residence time and, further along the reactor, grow only slightly over short residence times at low temperatures (small reactor cross-section). Any transitions between the extreme cases qualitatively illustrated here may also be adjusted.

The powders, of which some are highly sensitive to air or pyrophoric, can be desensitized in the cooling vessel (12) by injection of a suitable gas/vapor mixture. The particle surfaces of these metal powders may be coated both with an oxide layer of defined thickness and with suitable organic compounds, such as higher alcohols, amines or even sintering aids, such as paraffins, in an inert carrier gas stream. The powders may also be coated to facilitate their further processing.

By virtue of their mechanical, electrical and magnetic properties, the nano-scale powders according to the invention are suitable for the production of new sensors, actors, cutting ceramics and cermets.

The following Examples are intended to illustrate the invention without limiting it in any way.

TaCl.sub.5 was produced in accordance with the following reaction equation:

in an apparatus of the type shown in FIG. 1, an excess of H.sub.2 being used.

To this end, 100 g/min. TaCl.sub.5 (solid, boiling point 242.degree. C.) were introduced into the evaporator (1a), vaporized and heated to 1300.degree. C. together with 50 Nl/min. Ar in the gas preheater (2a). The reactant H.sub.2 was introduced into the gas preheater (2) at 200 Nl/min. The reactants were separately preheated to a temperature of approximately 1300.degree. C. Temperature was measured with a W5Re-W26Re thermocouple (18) at the place marked in FIG. 1 (1450.degree. C.). Before entering the reaction tube (4), the turbulent individual streams issuing from the gas preheaters (2) were combined in the outer part of the nozzle (5) into a homogeneous, rotationally symmetrical and laminar annular stream. The gas stream issuing from the gas preheater (2a) was also laminarized in the nozzle (5) and introduced into the annular flow. The nozzle (5) consisted of three component nozzles arranged coaxially of one another. An inert gas stream (16) issued from the middle nozzle and shifted the point where the reaction begins, i.e. where the two streams (6) and (7) are combined, away from the nozzle into the reaction tube. A Karman vortex path was produced in the inner stream by the obstacle (17) with a characteristic size of 3.0 mm (arranged in the longitudinal axis of the nozzle). For an overall length of 1100 mm, the reaction tube had an internal diameter of 40 mm at the nozzle outlet, an internal diameter of 30 mm 200 mm below the nozzle and an internal diameter of 50 mm at the outlet. The internal cross-section was steadily varied taking the laws of flow into account. The reaction tube (4) was made up of 18 segments joined by spacer and centering rings. Annular gaps (8) were formed at these places. The reaction tube (4) was adjusted to a temperature of 1230.degree. C. as measured on the outside wall of the reactor 400 mm below the nozzle with the W5Re-W26Re thermocouple (19). The pressure in the reaction tube (4) was virtually identical with the pressure in the blowback filter (10) which was 250 mbar excess pressure. The reactor wall was purged with 200 Nl/min. Ar through 18 annular gaps (8). If the reactor wall is not purged with an inert gas, growths can be formed and, in part, can lead very quickly to blockage of the reactor and hence to termination of the process. In any event, a varying product is obtained on account of the varying geometry of the reactor. To reduce the HCl partial pressure, 200 Nl/min. Ar was introduced into the reaction tube (4) through the 6th annular gap from the bottom by means of an additional gas injector. The product (Ta with a uniform particle size of .about.25 nm) was separated from the gases (H.sub.2, HCl, Ar) in the blowback filter (10) at a temperature of 600.degree. C.

This temperature was chosen to keep the primary coating of the very large particle surfaces (18 m.sup.2 /g) with HCl at a low level (.about.0.8% Cl).

The Ta thus produced was collected for 40 mins. (i.e. 2000 g) in the blowback filter and was then transferred to the vacuum vessel (11). In this vessel, 8 pumping/flooding cycles with final vacuums of 0.1 mbar absolute were carried out over a period of 35 minutes. The vessel was flooded with Ar to a pressure of 1100 mbar abs. After 35 minutes, the Ta powder thus treated was transferred to the cooling vessel (12). In this vessel, the powder can also be "surface-tailored" by exposure to various gas/vapor mixtures. After cooling to <50.degree. C., the powder was transferred to the collecting and transport vessel through the lock (13) so that it did not come into contact with the outside air.

For a specific BET surface of 17 m.sup.2 /g (as measured by the N.sub.2 -1-point method according to DIN 66 131), corresponding to 25 nm, the pyrophoric Ta powder showed an extremely narrow particle size distribution.

An SEM micrograph of this Ta powder with its specific surface of 25 m.sup.2 /g showed the very narrow distribution of the particle sizes and the absence of oversize particles. According to the micrograph, less than 1% of the individual particles deviate by more than 10% from the average particle size and no individual particles deviate by more than 40% from the average particle size. According to the present state of the art in the field of measurement, reliable information on the particle size distribution of such extremely fine powders can only be obtained by imaging methods (for example SEM, TEM).

Analysis of this Ta powder revealed an oxygen content of 70 ppm and a sum total of non-oxidic impurities of 430 ppm.