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Preparation method and analysis of advantages and disadvantages of nano cobalt powder

2019-05-23
In recent years, with the rapid development of industry and science and technology, the demand for nano cobalt powder is increasing day by day, and the requirement for its quality is more strict.Nano-cobalt powder, in particular, has the characteristics of high recording density, high coercivity, high signal-to-noise ratio and good oxidation resistance.It is widely used in sealing shock absorption, medical equipment, sound regulation, light display and so on.Nano-cobalt powder also has a special absorption effect on electromagnetic waves and is used as invisible material for military equipment against millimeter wave, visible light and infrared ray.The sintering temperature of powder metallurgy products and high temperature ceramic products can be greatly reduced by using nano cobalt powder as sintering additive.Metal ceramics containing 80% nano cobalt powder are excellent high temperature resistant materials for rocket jet ports.Currently, the electronic industry is developing towards miniaturization, which requires the use of finer cemented carbide drill bits to process circuit boards. However, the commonly used micro tungsten carbide powder and nano cobalt powder cannot produce such small diameter drill bits, and the carbide powder and nano cobalt powder below 0.5 m are required.In addition, the use of nano-cobalt powder cemented carbide not only reduces the porosity, to avoid the emergence of cobalt pool, but also because the appearance of tungsten carbide adhere to a layer of cobalt, is conducive to the separation of tungsten carbide grain, which is conducive to improve the comprehensive performance of cemented carbide.There are also many preparation methods for two nanometer cobalt powder, which are detailed as follows:

1.High pressure water mist method
Water mist method is widely used. This method is a physical powder making method which USES high pressure water flow with pressure of 5~50 MP to directly crush liquid metal or alloy.Its technological process is: according to the requirements of the weighing cobalt, activates the induction furnace and high pressure water atomized powder and cobalt piece of melting into it, will get the molten metal liquid leaking into the package sold out through the bottom and furnace into the atomization device, the annular nozzle fluid of high pressure water will break into thousands of tiny beads, small droplet through condensation, formation water, powder mixtures and the atomizing cylinder at the bottom of the current stabilizer, wet powder, was finally get after dehydration, drying treatment of nanometer cobalt powder.

2 Electrolytic process
The principle of metal nano-cobalt powder prepared by electrolysis method is to use electrolytic cobalt plate as anode and highly polished stainless steel water jacket as cathode. Generally, the electrolyte is cobalt chloride or cobalt sulfate aqueous solution.Direct current is introduced into the electrolytic cell, and the electrolyte ionizes. At this time, cobalt ions gather at the cathode and discharge and deposit, forming nano-cobalt powder. Usually, in order to prevent the agglomeration of nano-cobalt powder, an appropriate amount of surfactant is added to the electrolyte.The performance of metal nano-cobalt powder is related to the temperature, electrode distance, electrolyte composition, electrolyte concentration, surfactant and other factors during the experiment. Meanwhile, in order to obtain high-purity nano-cobalt powder, electrolytic nano-cobalt powder needs to be cleaned.

3.High temperature pyrolysis
At present, high temperature pyrolysis is a popular and relatively mature method in the field of nano-cobalt powder production for cemented carbide.The basic principle is that cobalt oxalate can be cleaved into nano cobalt powder and carbon dioxide to prepare metal nano cobalt powder under the condition of isolation from air and high temperature.The process steps are mainly: first, using pure hydrochloric acid solution will be of high purity electrolytic cobalt or cobalt dissolved into a solution containing the cobalt ions, adding to the solution with oxalic acid or oxalic acid, ammonia oxalate ions in the reaction of cobalt ions generated white precipitate cobalt oxalate, then dry cobalt oxalate dehydrate, finally puts cobalt oxalate isolate air airtight container fragment into nanometer cobalt powder and carbon dioxide.
The cobalt oxide hydrogen reduction method is also widely used in the preparation of nano-cobalt powder for cemented carbide.This method is mainly based on cobalt oxide (Co2O3, Co3O4 and Co O) as raw materials, and directly reduced by hydrogen at 400~600 C to obtain nano cobalt powder.

 
4.Cobalt oxalate reduction
Cobalt oxalate hydrogen reduction method is usually the main method to prepare nano-cobalt powder for cemented carbide.The technological process of this method is simple, direct under 600 ~ 700 ℃ by hydrogen reduction preparation nanometer cobalt oxalate cobalt powder.
 
5. Polyol reduction method
Polyol reduction method based on Co (OH) 2 as precursor, use of polyols (such as ethylene glycol, propylene glycol 1.2) as the reducing agent, the specific technological process is: first, the precursor and polyol mixed in a beaker, using mixer to mix solution, and then put a beaker containing mixed solution in water bath heating container, start heating device make precursor heating and reducing, and get the reduction product metal nanometer cobalt powder.In the experiment, polyols play an important role: first, they are mixed with Co (OH) 2 in the form of liquid phase to make it in suspension state, and at the same time, as a reductant, the precursor is reduced to nano cobalt powder.

6.Irradiation preparation method
The basic principle of ray irradiation method is to irradiate aqueous solution with ray to produce a kind of active electron (eaq-) with strong reducing ability, which can reduce cobalt ions to cobalt elemental substance at room temperature and obtain nano cobalt powder after drying treatment.Its technological process is: make a certain concentration of cobalt solution, adding suitable amount of isopropyl ketone as the first free radical cleaning agent, and adding suitable amount of pva as dispersant, through a drip into the right amount of acetic acid (d) or Na OH solution to adjust the PH value of solution, then make a good solution for ultrasonic degassing treatment, ventilation with nitrogen reduce the oxygen content in the solution at the same time, the solution is to make use of gamma irradiation, ray irradiation dose rate for 70 Gy/min of 60 co, finally to collect the product.Ray irradiation has been widely used in the preparation of metal nano-cobalt powder.

7.Hydrazine liquid phase reduction method
The principle of hydrazine liquid phase reduction method is as follows: using hydrazine with strong reducing capacity as reducing agent, Co2+ is reduced to cobalt in alkaline solution, and the reaction equation is as follows:
 
 
8. The microemulsion method
Microemulsion is made up of water, oil, surfactant and surfactant, nucleation, hitches in micro bubble coalescence, clear transparent, reunion, heat treatment of isotropic thermodynamic system, through the collision of micelle particles, restructuring and integration, separation process, promote water within the nuclear material exchange or transfer, the nuclear reaction within the product, then the product within the nuclear nucleation and growing up.The specific process flow is as follows: first, the anionic surfactant is dissolved in the iso-octane solution (wherein the surfactant is 2-ethylhexylsulfonate sodium, the concentration is about 0.27 mol/L), and the Co Cl2 and NABH4 with a certain concentration are added to the solution to obtain two kinds of microemulsions.Then the two solutions were mixed and stirred evenly to cause chemical reactions in the nucleus. The color of the solution changed from light color to black, and acetone and water were used as flocculants to make the colloid agglomerate. After full washing, the colloid was filtered.Due to the high cost of required raw materials, the reaction time, environmental temperature and aging time have complex effects on the preparation process, making it difficult to achieve large-scale production.