PREPARATION OF NANOP ARTICLES

Background of the invention

The invention relates to a method and equipment for preparing metal nanoparticles based on the aerosol technology.

Metallic powders are employed in many applications, among others, in powder metallurgy, as catalysts, and in electronics. At present, the requirements of metal powders concerning the particle size, for example, are ever stricter and the need for nanoparticles has increased. Let us mention the need for cobalt nanopowders in the hard metal industry in manufacturing micro drills, and the exploitation of nickel nanopowders in electronics in manufacturing conductive materials; for example, when manufacturing multilayer capacitors. There are corresponding needs for many other metal nanopowders. However, it is considerably more difficult to produce metal nanoparticles than metal oxide nanoparticles and, in addition, not many preparation methods of metal nanoparticles that are based on the aerosol technology or some other method can be scaled up to an industrial scale.

Metal nanoparticles have been prepared by the hydrogen reduction method as described in publications JP 2004300480 (Fujihura Ltd), JP 11-314917 (Sumitomo Metal Mining Co Ltd) and KR 2002029888 (Korea Institute of Geoscience and Mineral Resources). Corresponding methods are also described in publications Yong Jae Suh et al., Kinetic of gas phase reduction of nickel chloride in preparation for nickel nanoparticles, Materials Research Bulletin 40 (2005), 2100-2109, Jang et al., Preparation of cobalt nanoparticles in the gas phase: kinetics of cobalt dichloride reduction, J. Ind. Eng. Chem. vol 9, no 4 (2003), 407-411 and Jang et al., Preparation of cobalt nanoparticles by hydrogen reduction of cobalt chloride in the gas phase, Materials Research Bulletin 39 (2004), 63-70.

Summary of the invention According to the independent claims, a method and equipment have now been invented for preparing metal nanoparticles by reducing the salt of a metal in a gas phase. The other claims present some preferred embodiments of the invention.

According to the invention, the salt of the metal is evaporated into the flow of a carrier gas in an evaporation layer that consists of porous material and permeates the salt. In this way, a continuous, steady and controllable feed is obtained. The continuous, steady feed is a prerequisite for a larger-scale continuous production in preparing crystalline particles with a primary size of less than 100 nm.

The invention can be used in the preparation of cobalt and nickel nanoparticles, in particular.

Detailed description of the invention

In this invention, the metal nanoparticles are produced by a reduction reaction of the evaporated salt of the metal and a reducing gas. The salt of the metal, such as cobalt or nickel, is evaporated in a porous evaporation layer that permeates the evaporated salt. The reducing gas is preferably hydrogen, whereby the metallic salt reacts with the hydrogen, producing metal and the acid of the salt. Suitable salts include espe- cially those with an evaporation temperature lower than the decomposition temperature. Suitable salts mostly include chlorides, sulphates, formates, nitrates, and acetates. For example, in the case of metal chloride, the metal chloride reacts with hydrogen, forming metal and hydrogen chloride, as in the reaction below

MCL +-H, → M + nHCl

wherein M denotes the metal and n a stoichiometric amount of material.

According to the invention, a continuous, steady and controllable feed can be provided.

The evaporation temperature is preferably 600 °C - 1000 ⁰ C, typically 700 ⁰ C - 900 ⁰ C. The reaction temperature of the reduction is preferably 700 ⁰ C - 1000 ⁰ C, typi- cally 800 ⁰ C - 950 ⁰ C.

When the evaporation temperature is lower than the reaction temperature that is used, the evaporated mixture of salt and carrier gas must be heated to the reaction temperature. The mixture is preferably heated separately from the reducing gas. By heating the carrier gas, the condensation of the salt vapour and the formation of

drops are also prevented. The reducing gas is also preferably heated to the reaction temperature before bringing it into contact with the salt.

The metallic salt can be fed as a powder or a liquid in a carrier gas flow into the evaporation layer, which the metallic salt does not permeate until evaporated.

The porous material is preferably a ceramic, especially aluminium oxide. Aluminium oxide pellets are preferred, in particular.

The carrier gas is preferably nitrogen or argon.

The volumetric ratio of the reducing gas of the total flow is preferably 10 % - 90 %, typically 20 % - 50 %.

After the reaction, the mixture of particles and carrier gas is cooled, preferably as quickly as possible to minimize sintering. This is preferably carried out by conducting colder dilution gas into the mixture. The temperature of the dilution gas is preferably -100 ⁰ C - +200 ⁰ C, typically 0 ⁰ C - 50 ⁰ C. The ratio between the particle and carrier gas mixture and the dilution gas is preferably 1 :1 - 1 :50, typically 1 :5 - 1 :20. For example, the temperature of the diluted mixture is 0 ⁰ C - 200 ⁰ C. The dilution gas is preferably combined with the flow of particles and carrier gas in a co- axially surrounding relationship. The dilution gas is also preferably nitrogen or argon.

By using the feeding method of the invention, the carrier gas flow can be saturated with metallic salt vapour, maximizing the yield of the nanoparticles. Metallic nanoparticles of less than 100 nm can be produced from the entire saturated metallic salt vapour. The reduced metal is nuclided into nanoparticles, when the temperature in the reaction part is high but, however, clearly below the melting point of the metal. By means of the aerosol technology, the method enables the production of large amounts of metal nanoparticles, because the flow that was saturated with the metallic salt vapour at the evaporation temperature can be used to produce metal particles with a primary diameter of less than 100 nm, corresponding to the entire amount of evaporated vapour.

The equipment according to the invention comprises:

- a flow channel

- a means for conveying the salt, which is to be reduced, in the carrier gas flow into the flow channel,

- an evaporation layer in the flow channel, wherein the salt is evaporated,

- a reaction part after the evaporation layer, and

- a means for conducting the reducing gas into the reaction part and into the flow of carrier gas and salt vapour.

In the reaction part, the flow direction is preferably upwards. When the reducing gas is extremely light-weight, such as hydrogen, the buoyant force has a strong impact on the flow direction. In this way, it is ensured that no reducing gas ends up in the other parts of the equipment and, thus, mixing of the reducing gas with the mixture of metallic salt and carrier gas can be controlled.

In the flow channel before the reaction part, there is preferably a horizontal portion. Before the horizontal portion, the flow channel most preferably also comprises a vertical portion, wherein the flow direction is downwards, and the evaporation layer is located in this portion.

After the reaction part, there is preferably provided a dilution part, wherein the growth and sintering of the particles are prevented by a quick and lossless dilution by means of the dilution gas. The equipment preferably comprises a means for conducting the dilution gas to the mixture of particles and carrier gas coaxially.

For conducting the reducing gas to the reaction part, there is preferably a tube leading to the flow of salt vapour and carrier gas inside the same!.

The equipment preferably also contains a means for heating the salt vapour and carrier gas mixture and the reducing gas before the reaction part.

Equipment for implementing the invention is presented schematically in Fig. 1. The equipment comprises a U-shaped flow channel. At the end of the first branch, the feeding branch of the channel, there is a feeding device 1, by which the solid or liquid salt to be reduced is fed into the channel in a carrier gas (e.g., nitrogen or argon). Below the feeding device and spaced therefrom, there is an evaporation layer 2 consisting of porous material (such as aluminium oxide pellets), which layer closes the channel and is heated. The solid or liquid salt cannot permeate the evaporation layer. However, the temperature of the evaporation layer is kept so high (e.g.,

600 ⁰ C - 1000 ⁰ C) that the salt evaporates, whereby it is allowed to travel in a gaseous form along with the carrier gas flow and through the layer. The rate of evaporation can be adjusted by changing the temperature and the gas flow. The temperature depends on the salt and the desired yield.

The part after the evaporation layer of the flow channel constitutes a reactor, which is heated in two parts 3 and 4 (e.g., 700 ⁰ C - 1000 ⁰ C). A feed pipe 5 for heated hydrogen, which is used for mixing the hydrogen with the carrier gas flow directly upwards and sideways, leads directly from below to about the middle of the second branch of the channel. Hydrogen reduces the salt into metal. The temperature in the reactor is kept below the melting point of the metal, whereby the metal is nuclided into particles. At the tail of the flow channel, there is a diluter 6. It comprises a dilution channel that is located coaxially around the end of the channel, cold dilution gas being conducted from the channel (e.g., -100 ⁰ C - +200 ⁰ C). The dilution gas can be the same as or a different gas from the carrier gas. The volumetric ratio of the hydrogen of the total flow before the dilution is 10 % - 90 %. The degree of dilution is 1 :1 - 1 :50.

After the diluter, there is a particle collector 7. The particles can be collected, for example, by a bag filter, a flat folded filter, electrostatically or by means of thermo- phoresis.

Hydrogen gas is very light; therefore, the buoyant force has a strong impact on the flow direction of hydrogen. In the reaction part 4 of the equipment, the flows are directed upwards, ensuring that no hydrogen is allowed to enter the other parts of the equipment and, thus, mixing the hydrogen with the metallic salt and carrier gas mixture can be controlled. The growth and sintering of the particles are prevented by a quick and lossless dilution of the gas in the diluter 6. By using heat at the forward end 3 of the reaction part, the condensation of salt is also prevented.

Examples

In the following, examples about the preparation of metal nanoparticles by means of the equipment according to Fig. 1 are described.

Example 1

The preparation of cobalt nanoparticles of less than 100 nm

Cobalt chloride was used as a source material and nitrogen as the carrier gas. The cobalt chloride powder was evaporated from porous aluminium oxide pellets at a temperature of 650 ⁰ C. The cobalt chloride vapour and nitrogen mixture and the hydrogen, both of which had been heated to a reaction temperature of 900 ⁰ C, were mixed together from the upper end of the pipe, which was in the middle of the reactor, straight up and to the sides so that the total flow was upwards. The volume fraction of the hydrogen from the total flow before dilution was 30 %. After the reaction, the particle and gas mixture was diluted coaxially by nitrogen in a ratio of 1 : 10. After the dilution, the particles were collected onto a flat folded filter.

As a result, metallic cobalt nanoparticles were obtained, their primary particle size being less than 100 nm. The grid structure of the particles is a face-centred cubic. The TEM-image of the particles is shown in Fig. 2.

Example 2

The preparation of cobalt nanoparticles of less than 100 nm.

The source material was cobalt chloride and the carrier gas was nitrogen. The cobalt chloride powder was evaporated from porous aluminium oxide pellets at a temperature of 800 ⁰ C. The cobalt chloride vapour and nitrogen mixture and the hydrogen, which had been heated to a reaction temperature of 950 ⁰ C, were mixed together. After the reaction, the particle and gas mixture was diluted coaxially by nitrogen in a ratio of 1 : 10. After the dilution, the particles were collected by a bag filter.

As a result, metallic cobalt nanoparticles with a primary particle size of less than 100 nm were obtained. The grid structure of the particles is a face-centred cubic. The TEM-image of the particles is shown in Fig. 3. A diffraction pattern that shows the face-centred cubic crystal structure of the particles with the TEM image of the diffraction spot is in Fig. 4.

Example 3

The preparation of nickel nanoparticles of less than 80 nm

The source material was nickel chloride and the carrier gas was nitrogen. The nickel chloride powder was evaporated from porous aluminium oxide pellets at a tempera- ture of 650 ⁰ C. The nickel chloride vapour and nitrogen mixture and the hydrogen, which had been heated to a reaction temperature of 900 ⁰ C, were thus mixed together. After the reaction, the particle and gas mixture was diluted coaxially by ni-

trogen in a ratio of 1 :5. After the dilution, the particles were collected by a flat folded filter.

As a result, metallic nickel nanoparticles with a primary particle size of less than 80 nm were obtained. The grid structure of the particles is a face-centred cubic. The TEM-image of the particles is in Fig. 5.

Example 4

The preparation of nickel nanoparticles of less than 100 nm

The source material was nickel chloride and the carrier gas was nitrogen. The nickel chloride powder was evaporated from porous aluminium oxide pellets at a tempera- ture of 800 ⁰ C. The nickel chloride vapour and nitrogen mixture and the hydrogen, which had been heated to a reaction temperature of 900 °C, were mixed together. After the reaction, the particle and gas mixture was diluted coaxially by nitrogen in a ratio of 1 : 10. After the dilution, the particles were collected by a bag filter.

As a result, metallic nickel nanoparticles with a primary particle size of less than 80 nm were obtained. The grid structure of the particles is a face-centred cubic. The TEM-image of the particles is in Fig. 6. A diffraction pattern that shows the face- centred cubic crystal structure of the particles is in Fig. 7 with the TEM image of the diffraction sample.