EP0568863A1 - Fine metal particles - Google Patents

Abstract

The present invention relates to fine powders consisting of the metals B, Al, Si, Ti, Zr, Hf, V, Nb, Ta and/or Cr with a defined particle size between 1.0 nm and less than 3 mu m. <IMAGE>

Description

The present invention relates to finely divided powders of the metals B, Al, Si, Ti, Zr, Hf, V, Nb, Ta and / or Cr with a defined particle size between 1.0 nm and less than 3 μm.

The properties of the starting powder are of decisive importance for the mechanical properties of powder-metallurgically manufactured components. In particular, a narrow particle size distribution, high powder purity and a lack of coarse particles or agglomerates have a positive effect on the properties of corresponding components.

Numerous processes have become known for the technical production of fine metal powders.

In addition to the purely mechanical comminution and classification processes, which have the disadvantage that only powders of a certain fineness and with a relatively wide particle size distribution can be produced, a large number of processes for separation from the gas phase have also been proposed.

Through sometimes very small energy sources, e.g. thermal plasma or laser beam, or with turbulent flames, e.g. a chlorine oxyhydrogen burner, the particle size and particle size of the powders produced cannot be exactly controlled, and the reaction conditions usually lead to a wide particle size distribution and to the occurrence of individual particles whose diameter is a multiple of the average particle size.

According to the currently known large-scale industrial powder production process, it is hardly or very difficult to produce powders with average grain sizes of <0.5 µm, measured according to FSSS (and not individual particle size). With these conventionally produced fine powders, it cannot be practically ruled out that a certain percentage of coarse grain is contained in the material, which has a detrimental effect on the mechanical properties of components made therefrom. With conventional grinding processes, a very broad particle size distribution is also obtained, which cannot be significantly restricted by these steps with these powders.

Other gas phase processes do not work with a flow-optimized hot wall reactor, but use a plasma flame or other energy sources such as laser beams for the implementation. Disadvantages of these processes are essentially the reaction conditions which cannot be controlled in practice in various areas of the reaction zone with very high temperature gradients and / or turbulent currents. This creates powders with a broad particle size distribution.

Numerous proposals have been made for processes for producing the finest metal powders, but all of them have disadvantages.

EP-A 0 290 177 describes the decomposition of transition metal carbonyls for the production of fine metallic powders. Here, up to 200 nm fine powders can be obtained

In the search for metals with new mechanical, electrical and magnetic properties, ever finer metal powders are required.

The noble gas condensation process enables the production of the finest metal powders in the lower nanometer range. However, only quantities on a milligram scale can be obtained here. In addition, the powders produced in this way do not have a narrow particle size distribution.

The object of this invention is therefore to provide finely divided metal powders which do not have the disadvantages described for the powders of the prior art

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

The invention relates to finely divided powders of the metals B, Al, Si, Ti, Zr, Hf, V, Nb, Ta and / or Cr with a defined particle size between 1.0 nm and less than 3 μm, less than 1% of the individual particles a deviation of more than 40% and no individual particles have a deviation of more than 60% from the average grain size.

Preferably, less than 1% of the individual particles have a deviation of more than 20% and no individual particles have a deviation of more than 50% from the mean grain size. Particularly preferably less than 1% of the individual particles have a deviation of more than 10% and no individual particles Deviation of more than 40% from the average grain size. The powders according to the invention preferably have particle sizes from 1 to less than 500 nm, particularly preferably from 1 to less than 100 nm and very particularly preferably from 1 to less than 50 nm.

The metal powders according to the invention are notable for their high purity. They preferably have an oxygen content of less than 5,000 ppm and particularly 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, preferably less than 50 ppm.

The non-oxide impurities are also very low. Thus, the sum of their impurities, with the exception of the oxidic impurities, is preferably less than 5,000 ppm, particularly preferably less than 1,000 ppm.

In a very particularly preferred embodiment, the sum of their impurities, with the exception of the oxide impurities, is less than 200 ppm.

The powders according to the invention are available on an industrial scale. They are preferably present in amounts of more than 1 kg.

The powders according to the invention are obtainable in a process for the production of finely divided metal powders by reacting corresponding metal compounds and corresponding reactants in the gas phase -CVR-, the metal compound (s) and the other reactants being reacted in a gaseous state, directly from the gas phase condensed homogeneously with the exclusion of any wall reaction and then separated from the reaction medium, which is characterized in that the metal compounds and the reactants are introduced into the reactor separately from one another at at least the reaction temperature. In the event that several metal compounds and / or reactants are to be introduced, the respective gas mixtures should be selected so that no reaction occurs during the heating process. which leads to solid reaction products. This process can be carried out particularly advantageously in a tubular reactor. It is particularly favorable if the metal compounds, the reactants and the product particles flow through the reactor in a laminar manner.

The location of the nucleation can be limited by separately preheating the process gases to at least the reaction temperature. The laminar flow in the reactor ensures a narrow residence time distribution of the germs or the particles. A very narrow grain size distribution can be achieved in this way.

The metal compounds and the reactants should therefore preferably be introduced into the reactor as coaxial laminar partial streams.

However, in order to ensure that the two coaxial partial flows are mixed, a Karman vortex street, defined in terms of intensity and expansion, is generated by installing a disturbing body in the otherwise strictly laminar flow.

A preferred embodiment of this method therefore consists in that the coaxial, laminar partial streams of the metal compound (s) and the reactants are mixed in a defined manner by means of a Karman vortex street.

In order to prevent the energetically highly preferred separation of the reactants on the reactor wall, the reaction medium is preferably shielded from the reaction wall by an inert gas layer. This can be done by introducing an inert gas stream into the reactor wall through specially shaped annular gaps, which is applied to the reactor wall via the Coanda effect. The metal powder particles created in the reactor by a homogeneous separation from the gas phase with typical residence times between 10 and 300 msec leave this together with the gaseous reaction products (e.g. HCl), the unreacted reactants and the inert gases, which act as carrier gas, purge gas and for the purpose of reduction the HCl adsorption are blown in. Yields, based on the metal component, of up to 100% can be achieved by the process according to the invention.

The metal powders are then preferably separated off at temperatures above the boiling or sublimation temperatures of the metal compounds used, reactants and / or forced products formed during the reaction. The separation can advantageously be carried out in a blow-back filter. If this at high temperatures of e.g. 600 ° C is operated, the adsorption of the gases, especially the non-inert gases such as HCl on the very large surface of the powder can be kept low.

The remaining interfering substances adsorbed on the powder surfaces can be added in a downstream Vacuum containers are further removed, preferably again at temperatures of about 600 ° C. The finished powders should then be discharged from the system in the absence of air.

Preferred metal compounds for the purposes of this invention are one or more from the group consisting of metal halides, partially hydrogenated metal halides, metal hydrides, metal alcoholates, metal alkyls, metal amides, metal azides and metal carbonyls.

Hydrogen is used as a further reaction partner. Other characteristics of the powders are their high purity, high surface purity and good reproducibility.

Depending on the grain size and substance, the powders according to the invention can be very sensitive to air or pyrophoric. In order to eliminate this property, these powders can be surface-modified in a defined manner by exposure to gas / steam mixtures.

Fig. 1 is a schematic representation of a device with which the powders according to the invention can be produced. The implementation of this method is explained below with reference to FIG. 1. The process, material and / or device parameters explicitly mentioned here represent only selected options among many and thus do not limit the invention.

The solid, liquid or gaseous metal compounds are metered into an outside evaporator (1) or inside the high temperature furnace (1a), evaporated there at temperatures from 200 ° C to 2000 ° C and with an inert carrier gas (N₂, Ar or He) transported into the gas preheater (2a). The further reactant (3) H₂ is also heated in a gas preheater (2). Before entering the tubular reactor (4), the turbulent individual flow threads emerging from the gas preheaters (2) are formed into two coaxial, laminar and rotationally symmetrical flow threads in a nozzle (5). In the tubular reactor (4), the middle flow thread (6), which contains the metal component, and the enveloping flow thread (7), which contains the hydrogen, mix under defined conditions. The reaction occurs at temperatures between 500 ° C and 2000 ° C, for example according to the following case examples;



TaCl₅ + 2 1/2 H₂ → Ta + 5 HCl




BCl₃ + 1 1/2 H₂ → B + 3 HCl



In order to ensure the mixing of the two coaxial current threads, a Karman vortex street can be created by installing a disturbing body (17) in the otherwise strictly laminar flow. The two coaxial flow threads are separated by a weak inert gas flow (16) at the nozzle outlet in order to prevent growth at the nozzle (5).

In order to prevent the energetically highly preferred heterogeneous deposition of these substances on the hot reactor wall, this is flushed through annular gaps (8) with an inert gas stream (9) (N₂, Ar or He) which is applied to the reactor wall via the Coanda effect. The metal powder particles created in the reactor by a homogeneous separation from the gas phase leave it together with the gaseous reaction products (e.g. HCl), the inert gases and the unreacted reactants and go directly to a blow-back filter (10) in which they are separated. The blow-back filter (10) is operated at temperatures between 300 ° C and 1000 ° C, whereby the adsorption of the gases, especially the non-inert gases such as HCl on the very large surface of these powders is kept at a low level. In a subsequent container (11), the residues of the adsorbed gases on the powders are further reduced by preferably alternately applying a vacuum and flooding with various gases at 300 ° C. to 1000 ° C. Good effects are achieved when gases such as N₂, Ar or Kr are used. SF₆ is particularly preferably used.

This process also enables the production of metastable material systems and particles with core / shell structures. Metastable material systems are obtained by setting very high cooling rates in the lower part of the reactor.

The particles with a core / shell structure are obtained by introducing additional reaction gases into the lower part of the reactor.

The powders reach the cooling container (12) from the evacuation container (11) before they pass through the lock (13) into the collection and shipping container (14). The particle surfaces can be surface-modified in a defined manner in the cooling container (12) by blowing in various gas / steam mixtures

Coated graphite, in particular fine-grain graphite, can preferably be used as the material for those components which are exposed to temperatures of up to 2000 ° C. and more, such as heat exchangers (2) and (3), nozzle (5), reactor (4) and reactor jacket tube (15). be used. A coating can e.g. be necessary if the necessary chemical resistance of the graphite against the gases used such as metal chlorides, HCl, H₂ and N₂, is not sufficient at the given temperatures or if the erosion is very considerable at higher flow rates (0.5 to 50 m / sec) or if the gas tightness of the graphite can be increased as a result or if the surface roughness of the reactor components can thus be reduced.

As layers e.g. SiC, B₄C, TiN, TiC and Ni (only up to 1200 ° C) can be used. Combinations of different layers, e.g. with an "own" top layer are possible. These layers can advantageously be applied by means of CVD, plasma spraying and electrolysis (Ni).

If only low temperatures are necessary, the use of metallic materials is also possible.

• Einstellen eines bestimmten Verhältnisses der Reaktions- und Inertgase.
• Einstellen eines bestimmten Druckes.
• Einstellen eines bestimmten Temperatur-Verweilzeit-Profils längs der Reaktorachse.

There are three ways to adjust the particle sizes of the metal powders:

• Setting a certain ratio of the reaction and inert gases.
• Setting a certain pressure.
• Setting a specific temperature-residence time profile along the reactor axis.

• Durch zwei oder mehrere Heizzonen vom Beginn der Gasvorwärmer (2) bis zum Ende des Rohrreaktors (4).
• Durch Variation des Reaktorquerschnitts entlang seiner Längsachse.
• Durch Variation der Gasdurchsätze und damit bei vorgegebenem Reaktorquerschnitt der Strömungsgeschwindigkeiten.

The temperature residence time profile is set as follows:

• Through two or more heating zones from the start of the gas preheater (2) to the end of the tubular reactor (4).
• By varying the reactor cross section along its longitudinal axis.
• By varying the gas throughputs and thus with a given reactor cross-section of the flow velocities.

A major advantage of the variability of the temperature-residence time profile is the possibility of decoupling the nucleation zone from the nucleation zone. This makes it possible to produce "coarser" powders at a very low temperature and with a short residence time (ie small reactor cross section for one certain length) to allow the formation of only a few nuclei, which can then grow into "coarse" particles at high temperature and a long residence time (large reactor cross section). It is also possible to produce very "fine" powders: in a region of high temperature and a relatively long residence time, a large number of nuclei are formed, which only grow slightly in the further reactor at low temperatures and a short residence time (small reactor cross section). It is possible to set all transitions between the borderline cases shown here qualitatively.

In the cooling container (12), a passivation of the partly. very air sensitive to pyrophoric powder possible. The particle surfaces of these metal powders can be coated with an oxide layer of defined thickness as well as with suitable organic compounds such as higher alcohols, amines or sintering aids such as paraffins in an inert carrier gas stream. The coating can also be carried out with regard to the further processing possibilities of the powders.

Because of their mechanical, electrical and magnetic properties, the nanoscale powders according to the invention are suitable for the production of novel sensors, actuators, structural metals and cermets.

The invention is explained below by way of example, without any limitation being seen therein.

example 1

Ta became according to the reaction equation



TaCl₅ + 2 1/2 H₂ → Ta + 5HCl



1 in an apparatus, with an excess of H₂ was maintained.

For this purpose, 100 g / min TaCl₅ (solid, boiling point 242 ° C) were metered into the evaporator (1a), evaporated and heated together with 50 Nl / min Ar in the gas preheater (2a) to 1300 ° C. The reaction partner H₂ (200 Nl / min) was introduced into the gas preheater (2). The reactants were preheated separately from one another to a temperature of approximately 1300 ° C. The temperature was measured using a W5Re-W26Re thermocouple (18) at the location shown in FIG. 1 (1450 ° C). Before entering the reaction tube (4), the turbulent individual flow threads emerging from the gas preheaters (2) were formed in the outer part of the nozzle (5) to form a homogeneous, rotationally symmetrical and laminar ring flow. The gas stream emerging from the gas preheater (2a) was also laminarized in the nozzle (5) and introduced into the ring flow. The nozzle (5) consisted of three sub-nozzles arranged coaxially to one another. An inert gas stream (16) emerged from the middle part nozzle, which moved the location of the start of the reaction, ie the meeting of the two partial flows (6) and (7) away from the nozzle into the reaction tube. In the inner filament was with the disturbing body (17), with a characteristic dimension of 3.0 mm (arranged in the longitudinal axis of the nozzle) a Karman vortex street. The tube reactor had a total length of 1100 mm at the nozzle outlet with an inner diameter of 40 mm, 200 mm below the nozzle with an inner diameter of 30 mm and 50 mm at the outlet. The reaction tube (4) was composed of 18 segments, the segments each being connected by a spacer and centering ring. An annular gap (8) was implemented at each of these locations. The temperature of the reaction tube (4) was set at 1230 ° C., measured on the outer wall of the reactor, 400 mm below the nozzle, using the W5Re-W26Re thermocouple (19). The pressure in the reaction tube (4) was practically identical to the pressure in the blow-back filter (10). This was 250 mbar overpressure. The reactor wall was flushed through 18 ring gaps (8) with 200 Nl / min Ar. If the reactor wall is not flushed with an inert gas, accretions can occur which can sometimes lead to the reactor closure and thus to the process being terminated very quickly; in any case, because of the changing reactor geometry, a likewise changing product is produced. To reduce the HCl partial pressure, 200 Nl / min Ar was blown into the reaction tube (4) through the 6th annular gap from below with an additional gas inlet device. The product (Ta with a uniform particle size of ∼25 nm) was separated from the gases (H₂, HCl, Ar) in the blow-back filter (10) at a temperature of 600 ° C.

This temperature was selected in order to keep the primary coverage of the very large particle surfaces (18 m² / g) with HCl at a low level (∼0.8% Cl).

The Ta produced in this way was collected in the blow-back filter for 40 min (ie 2000 g), in order to then be transferred to the evacuation container (11). 8 pump-flood cycles with a final vacuum of 0.1 mbar abs were carried out in this container over a period of 35 min . run through. The container was filled with Ar up to a pressure of 1100 mbar abs. flooded, after 35 min. the Ta powder thus treated was transferred to the cooling container (12). Targeted surface tailoring is also possible in this container by blowing in various gas / steam mixtures. After the powder had cooled to <50 ° C., it was transferred through the lock (13) into the collection and shipping container without contact with the outside air.

The pyrophoric Ta powder showed an extremely narrow particle size distribution at a specific surface area of 17 m² / g, according to BET, measured according to the N₂-1-point method (DIN 66 131), corresponding to 25 nm.

A SEM image of this Ta powder with a specific surface area of 25 m² / g showed the very narrow distribution of the particle dimensions and the absence of oversize particles. According to this, less than 1% of the individual particles have a deviation of more than 10% and no individual particles have a deviation of more than 40% from the mean grain size. According to the current state of measurement technology, reliable statements about a Particle size distribution of such extremely fine powders can only be obtained using imaging methods (e.g. B, SEM, TEM).

The analysis of this Ta powder showed an oxygen content of 70 ppm and the sum of the non-oxide impurities was 430 ppm.

Claims (12)

Fine-particle powders of the metals B, Al, Si, Ti, Zr, Hf, V, Nb, Ta and / or Cr with a defined particle size between 1.0 nm and 3 µm, characterized in that less than 1% of the individual particles have a deviation of more than 40% and no individual particles have a deviation of more than 60% from the average grain size. Metal powder according to claim 1, characterized in that less than 1% of the individual particles have a deviation of more than 20% and no individual particles have a deviation of more than 50% from the mean grain size. Metal powder according to one of claims 1 or 2, characterized in that less than 1% of the individual particles have a deviation of more than 10% and no individual particles have a deviation of more than 40% from the mean grain size. Metal powder according to one or more of claims 1 to 3, characterized in that the particle size is from 1 to less than 500 nm. Metal powder according to one or more of claims 1 to 4, characterized in that the particle size is from 1 to less than 100 nm, preferably 1 to less than 50 nm. Metal powder according to one or more of claims 1 to 5, characterized in that it has an oxygen content of less than 5,000 ppm. Metal powder according to one or more of claims 1 to 6, characterized in that it has an oxygen content of less than 1,000 ppm. Metal powder according to one or more of claims 1 to 7, characterized in that it has an oxygen content of less than 100 ppm, preferably less than 50 ppm. Metal powder according to one or more of claims 1 to 8, characterized in that the sum of its impurities, with the exception of the oxidic impurities, is less than 5,000 ppm. Metal powder according to one or more of claims 1 to 9, characterized in that the sum of their impurities, with the exception of the oxidic impurities, is less than 1,000 ppm. Metal powder according to one or more of claims 1 to 10, characterized in that the sum of their impurities, with the exception of the oxidic impurities, is less than 200 ppm. Metal powder according to one or more of claims 1 to 11, characterized in that it is present in quantities of greater than 1 kg.

Images