Nickel has properties such as magnetism, conductivity, and high temperature stability. The preparation of ultrafine nickel powder has great surface effect and volume effect, and shows a series of special properties in terms of electrical properties, wave absorption, thermal resistance, light absorption, chemical activity, etc., so it is used in electronic pastes and metal ceramics , Shielding absorbing materials, catalysts, battery materials and many other fields have broad application prospects.
 
A series of physical and chemical properties of ultrafine nickel powder change with the morphology, particle size and distribution of the powder. How to prepare particles with a certain shape, size and distribution that meet the needs by controlling the reaction conditions is the study of ultrafine nickel powder an important aspect.
 
Preparation method of superfine nickel powder
 
1. Gas phase method
 
1.1 Evaporation-condensation method
The process of producing ultrafine nickel powder by evaporation-condensation method is: heating metallic nickel to 1425℃ to vaporize, and the steam is rapidly condensed to obtain nickel powder. Evaporation in a vacuum environment can reduce the evaporation temperature, such as heating to 700°C under a pressure of 1.33 Pa to obtain nickel vapor. The evaporation-condensation method can theoretically prepare any material. Its characteristics are that the prepared ultrafine powder has a clean surface, adjustable particle size, and generally spherical crystal shape, which is especially suitable for the preparation of metal ultrafine powder.
 
1.2 Thermal decomposition method of nickel carbonyl
The thermal decomposition method of nickel carbonyl was proposed by Mond et al., England in 1889. It is mainly carried out in two steps: the first step is to prepare nickel carbonyl, and the second step is to decompose nickel carbonyl to obtain nickel powder. This method is more practical, the purity of the produced nickel powder is very high, and the application is relatively wide.
 
1.3 Chemical vapor precipitation method
The chemical vapor deposition method is also known as the vapor phase hydrogen reduction method. The method is to volatilize nickel chloride at a high temperature, and then reduce to metallic nickel atoms in a hydrogen atmosphere, and obtain spherical ultrafine nickel powder through processes such as nucleation, growth, and collision. Due to its high crystallization temperature, the chemical vapor deposition method produces nickel powder with good crystallinity, high purity and controllable particle size. This method can produce spherical ultrafine nickel powder with uniform particle size at a low production cost. It is suitable for replacing metal palladium electrode materials in MLCC. Its price can compete with traditional capacitor electrode materials, but the required equipment is relatively expensive. And the equipment is corroded seriously.
 
1.4 Electric explosive wire method
The electric explosive wire method is a relatively new method for preparing nickel powder. It applies a direct current high voltage to the nickel wire in a reaction chamber filled with inert gas to form a high current density inside the nickel wire, making the nickel wire Explosion to obtain ultra-fine nickel powder. The nickel wire can automatically enter the reaction chamber through a wire feeding system, so that the above process can be repeated.
 
2. Liquid phase method
 
2.1 High-pressure hydrogen reduction method
In the autoclave, in the presence of a catalyst, hydrogen can be used to reduce the ammonia aqueous solution of nickel or water-insoluble slurries such as basic nickel carbonate and nickel hydroxide 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. The reaction mechanism is an oxidation-reduction reaction. The reducing agents used are generally hydrazine hydrate, NaBH4, KBH4 and polyols. The advantages of the liquid-phase reduction method are a wide range of sources of raw materials, simple equipment, easy operation, high product purity, small particle size, and uniform distribution. But its disadvantage is 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. The system contains one or two liquids stabilized by the interfacial film formed by surfactants. Droplets. Microemulsion disperses the continuous medium into tiny spaces, and the microemulsion method has been widely used in the preparation of ultrafine nickel powder. Gao Baojiao et al. studied the reduction of nickel sulfate in a reversed microemulsion system of water (solution)/xylene/sodium lauryl sulfate/n-pentanol with hydrazine hydrate in a strong alkaline environment under constant temperature water bath conditions. The composition of the microemulsion system can adjust the particle size of the product 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 to produce particles with unique properties. This method uses the high-energy dispersion mechanism of ultrasonic waves. The mother liquid of the target precursor passes through an ultrasonic atomizer to produce micron-level droplets, which are carried by carrier gas. It is brought into a high-temperature reactor and thermally decomposed, thereby obtaining an ultrafine powder material with uniform particle size. The ultrasonic atomization-thermal decomposition method is easy to control the target components, the precursors are widely sourced, the product particle size distribution is narrow and the particle size is controllable.
 
2.5 Electrolysis
Add a solution containing Ni2 to the electrolytic cell, use a nickel plate as an anode, and graphite or precious metal as a cathode. Turn on the power and periodically change the direction of the current. The nickel powder generated is deposited on 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. The process needs to be improved.
 
2.6 Ray radiation synthesis method
The basic principle of γ-ray radiation metal nickel salt solution to prepare ultra-fine nickel powder is: water can produce a large number of particles under γ-ray radiation. The hydrated electrons and hydrogen atoms in these particles have strong reducing power, which can remove the metal nickel ions. Stage reduction, the newly generated nickel atoms gather into nuclei, and finally generate ultra-fine particles. By controlling the solution concentration, pH value, and irradiation dose, the size and shape of the particles can be controlled.
 
3. Solid phase method
 
3.1 Mechanical crushing method
Mechanical crushing is a method that uses mechanical force to break large pieces of material into required particles. According to the difference of mechanical force, it can be divided into air impact method, mechanical ball milling method and ultrasonic crushing method. Mechanical ball milling is currently a relatively economical method for preparing ultrafine nickel powder. The mechanical ball milling method has the advantages of simple operation process, low cost, high preparation efficiency, and can 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
V. Rosenbanddeng uses graphite as a reactor to heat and decompose solid nickel formate in an argon atmosphere to prepare nickel powder. The average particle size of the obtained nickel powder is 0.4-0.6μm, and the shape of the powder is nearly spherical. The nickel powder prepared by this method has a high purity. By adjusting the process parameters, nickel powder meeting the conditions for the internal electrode of the MLCC can be prepared, but the cost of preparing the nickel powder by this method is relatively high.
 
Application of ultrafine nickel powder
 
1. Battery Materials
Ni-MH batteries and lithium-ion batteries, as new secondary batteries, are playing an increasingly important role in new energy materials. Lithium-ion batteries are widely used in military and civilian electrical appliances because of their high specific energy, high battery voltage, wide operating temperature range, and long storage life. The development of cathode materials for lithium-ion batteries has also experienced a tortuous course. Currently, LiCoO2 is widely used, and cheap LiNiO2 and LiMnO2 are being extensively 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 issue that must be broken through in the current large-scale development of LiNiO2 cathode materials.
 
2. Magnetic materials
Ultrafine nickel powder is an excellent magnetic material, which can be dispersed in a carrier liquid to form a magnetic fluid; nano-nickel powder with standardized rod or linear shape can be used to make a high-density "quantum disk". Some people use Ni-Fe and Ni -Co and other magnetic resistance effect trial-produced magnetic heads. This kind of magnetic head has a high read voltage and does not require a coil, which avoids the shortcomings of slow response of the inductive magnetic head when the recording density is high.
 
3. Cemented carbide
Since the advent of cemented carbide in 1923, metallic cobalt has been regarded as the best bonding metal. However, due to its high price and unstable supply, nickel, which has the same bonding properties but is relatively cheap, has gradually been favored.
 
4. Catalytic materials
Ultrafine nickel powder is an excellent chemical catalyst material. The particle size is less than 5nm, and the Ni nanoparticle catalyst with Si as the carrier not only has good surface activity, but also makes the selectivity in the hydrogenation reaction of propionaldehyde sharply increased; when Ni/SiO2 is used as the catalyst for the hydrogenolysis of ethane, when the particle size is 22nm When it is reduced to 2.5nm, the catalytic reaction rate increases by 10 times; nano-nickel powder catalyzes the hydrogenation of cyclooctadiene to cyclooctene, the activity is 2-7 times that of the traditional framework Ni, and the selectivity is increased by more than 5 times.
 
5. Absorbing materials
Utilizing its excellent electrical and magnetic properties, the electromagnetic wave shielding material can be prepared by combining the ultrafine nickel powder with the polymer matrix material. The conductive coating based on ultrafine nickel powder has strong electronic vector ability to absorb and scatter electromagnetic rays, and large magnetic vector attenuation. After special treatment, it has excellent anti-oxidation, anti-corrosion and anti-moisture capabilities. Therefore, it is used in electromagnetic wave shielding materials. The proportion is getting bigger and bigger.
 
6. Military special materials
Ultrafine nickel powder is mainly used in nano-catalytic composite materials for solid rocket propellants and explosives in the military field. The use of nickel powder can increase the burning rate of the same body propellant and explosive and reduce the critical partial pressure. According to reports, adding about 1% of nano-nickel powder to the solid rocket propellant increases the combustion efficiency by 100 times.
 
7. Multilayer ceramic capacitor MLCC
With the adjustment of the electronic product market structure, the rapid development of mobile communication equipment and portable computers has brought huge market space for the development of MLCC. The traditional MLCC electrode material is Pd/Ag alloy or pure Pd. At present, the price of imported Pd30/A970 internal electrode slurry is higher than 25,000 yuan/kg, so the use of base metal materials to replace Pd/Ag electrodes is the development of MLCC Important trends. In order to meet the requirements of large capacity and low cost, the 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 improving the friction and wear properties of lubricants.