Preparation Process and Application of Ultrafine Nickel Powder

Nickel has properties such as magnetism, conductivity, and high temperature stability. Ultrafine nickel powder has great surface effect and volume effect, and shows a series of special properties in electrical properties, wave absorption, thermal resistance, light absorption, chemical activity, etc., so it has broad application prospects in many fields such as electronic paste, metal ceramics, shielding and wave absorbing materials, catalysts, and battery materials. 

A series of physical and chemical properties of ultrafine nickel powder change with the powder morphology, particle size and distribution. How to prepare particles with a certain shape, size and distribution that meet the needs by controlling the reaction conditions is an important aspect of ultrafine nickel powder research.

Preparation method of ultrafine nickel powder

01 Gas phase method

1.1 Evaporation-condensation method

The process of producing ultrafine nickel powder by evaporation-condensation method is: heating metal nickel to 1425℃ to vaporize, and the vapor is rapidly condensed to obtain nickel powder. Using vacuum environment evaporation can reduce the evaporation temperature, such as heating to 700℃ under a pressure of 1.33Pa to obtain nickel vapor. The evaporation-condensation method can theoretically prepare any material. Its characteristics are that the surface of the ultrafine powder produced is clean, the particle size is adjustable, and the crystal shape is generally spherical, which is particularly suitable for the preparation of ultrafine metal powders.

1.2 Carbonyl nickel thermal decomposition method

The carbonyl nickel thermal decomposition method was proposed by Mond et al. in the UK in 1889. It is mainly divided into two steps: the first step is to prepare carbonyl nickel, and the second step is to decompose carbonyl nickel to obtain nickel powder. This method is more practical, the purity of the nickel powder produced is very high, and it has a wide range of uses.

1.3 Chemical vapor deposition method

The chemical vapor deposition method is also called gas phase hydrogen reduction method. This method is to volatilize nickel chloride at high temperature, and then reduce it to metallic nickel atoms in a hydrogen atmosphere, and obtain spherical ultrafine nickel powder through nucleation, growth, collision and other processes. Due to its high crystallization temperature, the nickel powder produced by the chemical vapor deposition method has good crystallinity, high purity and controllable particle size. This method can produce spherical ultrafine nickel powder with uniform particle size at a lower production cost. It is suitable for electrode materials replacing metal palladium in MLCC. Its price can compete with traditional capacitor electrode materials, but the required equipment is relatively expensive and the equipment is severely corroded.

1.4 Electric Explosion Wire Method

The electric explosion wire method is a relatively new method for preparing nickel powder. It applies DC high voltage to nickel wire in a reaction chamber filled with inert gas to form a very high current density inside the nickel wire, so that the nickel wire explodes to obtain ultrafine nickel powder. The nickel wire can automatically enter the reaction chamber through a wire supply system, so that the above process can be repeated.

02 Liquid Phase Method

2.1 High-pressure Hydrogen Reduction Method

In an autoclave, under the condition of the presence of a catalyst, hydrogen can be used to reduce the ammoniacal aqueous solution of nickel or the water-insoluble basic nickel carbonate, nickel hydroxide and other aqueous slurries to obtain ultrafine nickel powder.

2.2 Liquid Phase Reduction Method

The liquid phase reduction method is to prepare the reactants into a solution of a certain concentration and use a reducing agent to reduce the nickel in the liquid phase. Its reaction mechanism is redox reaction. The reducing agents used are generally hydrazine hydrate, NaBH4, KBH4 and polyols. The advantages of the liquid phase reduction method are wide sources of raw materials, simple equipment, easy operation, high product purity, small particle size and uniform distribution. However, its disadvantages are that the reducing agent sodium borohydride is expensive and hydrazine hydrate is toxic.

2.3 Microemulsion method

"Microemulsion" is defined as a thermodynamically stable, isotropic, transparent or translucent dispersion system formed by two immiscible liquids, and the system contains droplets of one or two liquids stabilized by an interfacial film formed by a surfactant. Microemulsion disperses the continuous medium into tiny spaces. The microemulsion method has been widely used in the preparation of ultrafine nickel powder. Gao Baojiao et al. studied the reduction of nickel sulfate with hydrazine hydrate in a water (solution)/xylene/sodium dodecyl sulfate/n-pentanol reverse microemulsion system under constant temperature water bath conditions in a strong alkaline environment. By controlling the composition of the microemulsion system, the particle size of the product can be adjusted to obtain spherical ultrafine metal nickel powder with uniform particle size distribution.

2.4 Ultrasonic atomization-thermal decomposition method

Ultrasonic atomization-thermal decomposition method is an important method for producing microparticles with unique properties. This method utilizes the high-energy dispersion mechanism of ultrasound. The target precursor mother liquor passes through the ultrasonic atomizer to produce micron-sized droplets, which are carried into the high-temperature reactor by the carrier gas for thermal decomposition, thereby obtaining ultrafine powder materials with uniform particle size. Ultrasonic atomization-thermal decomposition method has the advantages of easy control of target components, wide source of precursors, narrow product particle size distribution and controllable particle size.

2.5 Electrolysis method

Add Ni2+ solution to the electrolytic cell, use nickel plate as anode, graphite or precious metal as cathode, turn on the power and change the current direction periodically, and the generated nickel powder is deposited at the bottom of the electrolytic cell, and then collected with magnetic materials. This method is currently a method that is widely used in industrial production, but it has the problems of strong corrosiveness, poor working conditions, high energy consumption and easy to cause a certain degree of environmental pollution, and the process needs to be improved.

2.6 Radiation synthesis method

The basic principle of preparing ultrafine nickel powder by γ-ray irradiation of metal nickel salt solution is that water can produce a large number of particles under γ-ray radiation. The hydrated electrons and hydrogen atoms in these particles have strong reducing ability, which can reduce metal nickel ions step by step. The newly generated nickel atoms gather into nuclei and finally form ultrafine particles. The size and shape of the particles can be controlled by controlling the solution concentration, pH value and irradiation dose.

03 Solid phase method

3.1 Mechanical crushing method

The mechanical crushing method is a method of using mechanical force to crush large blocks into required particles. According to the different mechanical forces, it can be divided into air flow impact method, mechanical ball milling method and ultrasonic crushing method. Mechanical ball milling method is currently a relatively economical method for preparing ultrafine nickel powder. The advantages of mechanical ball milling method are simple operation process, low cost, high preparation efficiency, and the ability to prepare high melting point metal ultrafine particles that are difficult to obtain by conventional methods. Its disadvantages are uneven particle size distribution and low purity.

3.2 Solid phase decomposition method

1. Rosenbanddeng used graphite as a reactor and heated and decomposed solid nickel formate in an argon environment to prepare nickel powder. The average particle size of the obtained nickel powder was 0.4-0.6μm, and the powder shape was nearly spherical. The nickel powder obtained by this method has a high purity. By adjusting the process parameters, nickel powder that meets the conditions for MLCC internal electrodes can be obtained, but the cost of preparing nickel powder by this method is relatively high.

Application of ultrafine nickel powder

1. Battery materials

Nickel-hydrogen batteries and lithium-ion batteries, as new secondary batteries, play an increasingly important role in new energy materials. Lithium-ion batteries are widely used in military and civilian electrical appliances due to their high specific energy, high battery voltage, wide operating temperature range, and long storage life. The development of lithium-ion battery positive electrode materials has also experienced a tortuous process. Currently, LiCoO2 is widely used, while cheap LiNiO2 and LiMnO2 are being widely studied and tried. The crystal structure of LiNiO2 is similar to that of LiCoO2, but its price is quite low and its specific capacity is large. The synthesis conditions of LiNiO2 are relatively harsh, which is also a key problem that must be overcome in the large-scale development of LiNiO2 positive electrode materials.

2. Magnetic materials

Ultrafine nickel powder is an excellent magnetic material. It can be dispersed in a carrier liquid to form a magnetic fluid; nano nickel powder with a standard rod or linear shape can be used to make a "quantum disk" with high storage density. Some people use the magnetoresistance effect of Ni-Fe and Ni-Co to test the magnetic head. This head has a high readout voltage and does not require a coil, avoiding the disadvantage of slow response of the inductive head when the recording density is very high.

3. Cemented carbide

Since the advent of cemented carbide in 1923, metal cobalt has been considered the best bonding metal, but because of its high price and unstable supply, nickel, which has the same bonding properties but is relatively cheap, has gradually become popular.

4. Catalytic materials

Ultrafine nickel powder is an excellent chemical catalyst material. The Ni nanoparticle catalyst with a particle size of less than 5nm and Si as the carrier not only has good surface activity, but also makes the selectivity in the propionaldehyde hydrogenation reaction increase sharply; when Ni/SiO2 is used as a catalyst for ethane hydrogenolysis, when the particle size is reduced from 22nm to 2.5nm, the catalytic reaction rate increases by 10 times; the activity of nano-nickel powder in catalyzing the hydrogenation of cyclooctadiene to produce cyclooctene is 2-7 times that of traditional skeleton Ni, and the selectivity is increased by more than 5 times.

5. Absorbing materials

Using its excellent electrical and magnetic properties, ultrafine nickel powder can be composited with polymer matrix materials to prepare electromagnetic wave shielding materials. The conductive coating based on ultrafine nickel powder has strong electronic vector ability to absorb and scatter electromagnetic rays, and large magnetic vector attenuation amplitude. After special treatment, it has excellent anti-oxidation, anti-corrosion and anti-moisture capabilities, so it accounts for an increasing proportion in electromagnetic wave shielding materials.

6. Military special materials

Ultrafine nickel powder is mainly used in the military field for nano-catalytic composite materials of solid rocket propellants and explosives. The use of nickel powder can increase the burning rate of solid propellants and explosives and reduce the critical partial pressure. It is reported that adding about 1% nano nickel powder to solid rocket propellant increases its combustion efficiency by 100 times.

7. Multilayer ceramic capacitor MLCC

With the adjustment of the market structure of electronic complete machine products, mobile communication equipment and portable computers have developed rapidly, bringing huge market space for the development of MLCC. The traditional MLCC electrode material is Pd/Ag alloy or pure Pd. The price of the imported slurry of Pd30/A970 inner electrode with the largest consumption is higher than 25,000 yuan/kg, so the use of base metal materials to replace Pd/Ag electrodes is an important trend in the development of MLCC. In order to take into account the requirements of large capacity and low cost, base metal Ni electrode is the best choice.

In addition, ultrafine nickel powder is also widely used in porous materials, chiral materials, spraying materials, nano-composite electroplating materials, and in improving the friction and wear properties of lubricating oils.