Which processes can be used to make nano powders?

Nanopowder, also known as ultrafine powder or ultrafine powder, generally refers to powder or particles with a particle size below 100nm. It is a solid particle material in an intermediate state between atoms, molecules and macroscopic objects. It has a specific surface Effect, small size effect, volume effect, quantum size effect and macroscopic quantum tunneling effect are widely used. 

Nano powder preparation method classification

According to whether a chemical reaction occurs, it is divided into physical method, chemical method and physical chemical method.

Physical methods involve physical change processes such as evaporation, melting, solidification, deformation, and particle size changes.Chemical methods include vapor deposition, precipitation, hydrothermal synthesis, benzene thermal synthesis, sol-gel method, microemulsion method, vacuum condensation method, etc.

The comprehensive method means that the preparation process must be accompanied by some chemical reactions, and at the same time it involves the change process of the particle's physical state, and even certain physical means must be applied during the preparation process to ensure the smooth progress of the chemical reaction.

According to the state of the material in the reaction process, it can be classified into three categories: solid-phase method, liquid-phase method and gas-phase method.

The solid-phase method is to manufacture nano-powders by changing from solid phase to solid phase, without accompanying state (phase) changes from gas phase to solid phase, liquid phase to solid phase. The micronization mechanism of substances can be roughly divided into two categories: one is the method of very finely dividing (size reduction) bulk substances without changing the substance, there are mechanical pulverization methods, explosive sintering methods, dissolution methods (chemical treatment), etc.; One is the method of combining the smallest units (molecules or atoms) (construction process), changes in substances, thermal decomposition methods, solid-state reaction methods, etc.

The liquid phase method is a method widely used in laboratories and industries to prepare nanopowders. According to chemical means, nanopowders can be prepared through a simple solution process without complex instruments. Including precipitation method, hydrothermal method, solution evaporation method, solution gel method, radiation chemical synthesis method, etc.

The gas phase method directly uses gas or transforms substances into gas by various means, so that physical and chemical reactions occur in the gas state, and finally condenses and grows to form nanoparticles during the cooling process. Gas phase methods are roughly divided into gas evaporation and condensation methods, chemical vapor phase reaction methods, chemical vapor phase condensation methods, and sputtering methods. The electric explosion method is a special resistance heating method, which is a kind of gas phase method. 

1. Solid phase process 

1.1 Mechanical alloying process 

The mechanical alloying method is a solid-state reaction method for preparing powders. It realizes alloying in the solid state and is not restricted by physical properties such as vapor pressure and melting point of the substance. Alloying, and the synthesis of some quasi-steady states, non-equilibrium states and new substances far from thermodynamic equilibrium are possible. Main features: The advantage of the mechanical alloying method is that the process is simple, and it can prepare high-melting point metals, solid solutions of immiscible systems, nano-intermetallic compounds and nano-metal-ceramic composite materials that are difficult to prepare by conventional methods; the particle size can be adjusted according to needs. Process parameters are controlled; the yield is high and can be used for industrialized mass production. The disadvantages are high energy consumption, wide particle size distribution and easy introduction of impurities.

1.2 Electric explosive wire process 

The electric explosion method is to charge the capacitor bank through the high-frequency voltage of the pulse circuit, and then discharge it suddenly, and the high-density stored electricity generated is used as the energy source. Main features: The electric explosion method uses the resistance heat energy storage of the metal wire, and the energy conversion efficiency is high. The electric explosion can vaporize the entire metal wire almost at the same time, and the steam generated is more uniform than the vapor obtained from the surface gasification of the pulse laser and the particle beam, so , the obtained nano-powder has a high degree of uniformity. By changing the size of the discharge electricity, it is possible to produce nano-metal powder with a suitable particle size and diameter, and the produced powder has high purity and no pollution, which is a very environmentally friendly method.

1.3 Amorphous crystallization process 

A rapid solidification method is used to prepare amorphous strips from liquid metal, and then heat-treat the amorphous strips to crystallize them to obtain nanocrystalline strips. The process is relatively simple and the chemical composition is accurate.

2. Gas phase process 

2.1 Evaporation-condensation process 

Basic principle: Introduce low-pressure inert gas (He or Ar) into the vacuum evaporation chamber, heat and evaporate the evaporation material, and the atoms of the evaporated material collide with the atoms of the inert gas to lose energy and condense to form nanoparticles. Alloy nanoparticles can be obtained by simultaneously evaporating two or several metals.

Main features: Since the formation of nanoparticles is completed under a very high temperature gradient, the particle size range of the obtained nanoparticles is narrow, and the morphological characteristics such as agglomeration and aggregation of the particles can be controlled. The nano-particles prepared by the inert gas evaporation-coagulation method have good crystallinity, clean surface and a protective dense oxide film on the surface, which is convenient for handling and safe storage. However, this method has certain limitations, and it is more suitable for the preparation of metal nanoparticles with low melting point. The heating methods used by IGC to prepare nano-powders can be mainly divided into six types: resistance heating, plasma jet heating, induction heating, electron beam heating, laser heating and glow plasma sputtering.

2.1.1 Resistance heating process 

The evaporated raw material is heated on a resistance heater. The amount produced by one evaporation is small, and the laboratory scale is less than 100g at one time.

2.1.2 High frequency induction process 

Using a high-frequency induction coil as a heat source, the conductive material in the crucible is heated under the action of eddy currents, evaporated in a low-pressure inert gas, and the evaporated atoms collide with the inert gas atoms to cool and condense into nanoparticles. Features: Using a crucible, generally only low melting point substances are prepared. The particle size is easy to control, and it can run at high power for a long time.

2.1.3 Plasma beam heating

A plasma beam is used to heat metallic material in a water-cooled copper crucible. The laboratory-scale output is 20-30g per batch, which is suitable for almost all metals.

2.1.4 Electron beam heating

The pressure difference is maintained between the high vacuum electron beam generating chamber and the evaporation chamber with a pressure of 133Pa, and the raw material is powder. It can produce Ta, W and other high melting point metals and TiN, AlN and other high melting point compounds. 

2.1.5 Laser beam heating

A continuous, high-energy laser beam is focused on the raw material through a lens. It can evaporate minerals, compounds, etc., and is effective for metal compounds such as SiC.

2.1.6 Sputtering process 

Principle: Under the action of an electric field, Ar ions impact the surface of the cathode target, causing the target atoms to evaporate from the surface to form ultrafine particles, which are deposited on the attachment surface.

Features: It can prepare metals with high melting point and low melting point; it can prepare multi-component compound nanoparticles; the amount of nanoparticles obtained can be increased by enlarging the sputtered cathode surface.

2.1.7 Flowing liquid surface vacuum evaporation process

Metal atoms evaporated in high vacuum form extremely ultrafine particles in the flowing oil surface, and the product is paste oil containing a large number of ultrafine particles. 

Features: ①It can prepare small particles with an average particle size of about 3nm; ②The particle size is uniform and the distribution is narrow; ③The nanoparticles are evenly distributed in the oil; ④The size of the particle size is controllable.

2.1.8 Electric heating evaporation method

The carbon rod is in contact with the metal, and the metal is melted by electric heating. The metal reacts with the high-temperature carbon rod and evaporates to form carbide ultrafine particles. 

2.2 Chemical vapor deposition method

The chemical vapor deposition method is to use one or several reaction gases under the action of heat, microwave, laser, plasma, etc. to initiate a chemical reaction between the reaction gases and generate the desired compound, which is rapidly condensed in the gas phase environment, thereby Methods of preparing various nanoparticles.

Main features: When preparing nanoparticles by CVD method, there are many controllable process parameters, such as concentration, flow rate, temperature, composition and ratio, etc. Therefore, active control of nanoparticle composition, morphology, size, and crystal phase can be achieved by controlling process parameters. In the gas phase state, there is a large space for particle nucleation and growth, so that the prepared nanoparticles have a narrow particle size distribution, good monodispersity, and uniform morphology.

2.3 Laser-induced vapor deposition method

LICVD uses the absorption of specific wavelength laser beams by reactive gas molecules to cause pyrolysis or chemical reactions of reactive gas molecules, and the formation and growth of nuclei to produce nanoparticles.

Main features: Due to the fast heating rate of LICVD method, short high temperature residence time (about 10-3s), and fast cooling rate, the prepared nano-powder has small particle size and uniform distribution. At the same time, because the reaction center area and the reactor are separated by raw materials, the pollution is small, and the purity of the prepared nanopowder is high. The disadvantage is the high preparation cost.

2.4 Chemical evaporation condensation method (CVC)

Under the high temperature and high pressure environment, the organic raw materials are pyrolyzed to form clusters and further condense into nano-scale particles. Features: large output, small size, narrow distribution. 

2.5 DC arc plasma method

Plasma vapor phase synthesis is one of the main methods for preparing nanopowders. The formation of particles in the low-temperature plasma method is the result of chemical reaction and nucleation growth, and its principle is similar to the thermochemical reaction process of high-temperature pyrolysis reaction and laser-induced reaction. The formation of particles in the high-temperature plasma method is the result of cooling and condensation of the reactive gas after plasmaization. The plasma gas phase synthesis method is divided into direct current arc plasma method (DC method), high frequency plasma method (RF method) and composite plasma method.

The DC arc plasma method can not only synthesize metal nanopowders, but also synthesize metal-ceramic nanopowders, ceramic nanopowders and carbon nanotubes. The advantage is that the equipment is simple and easy to operate, and the powder synthesis speed is fast, the purity is high, there are many types, and the activity is strong, which is suitable for industrial batch production.

3. Liquid phase process

3.1 Sol-gel process

Basic principle: react an easily hydrolyzed metal compound (inorganic salt or metal alkoxide) with water or other substances in a certain solvent, gradually gel it through the process of hydrolysis and polycondensation, and then undergo heat treatment such as drying/calcination and reduction Finally, the desired nanoparticles are produced. Main features: The operating temperature is low, the preparation process is easy to control, and can be used to prepare materials in various shapes such as powder, film, fiber, tube, and rod.

3.2 Microemulsion polymerization process

Basic principle: The microemulsion polymerization process is divided into three stages: nucleation, growth and completion.

Main features: The size distribution of nanoparticles prepared by microemulsion method is narrow and easy to control; by modifying the surface of particles with different surfactant molecules, nanoparticles with special physical and chemical properties can be obtained; Coating one or several layers of surfactant molecules, the aggregation of nanoparticles is not easy to occur, so it has good stability and can be placed for a long time; the active agent layer on the surface of the nanoparticles is similar to an "active film". It can be replaced by corresponding organic groups to prepare special nano-functional materials; the coating of the surface of nanoparticles improves the interface properties of nano-materials, and at the same time significantly improves its optical, catalytic and electrorheological properties.

3.3 Precipitation process 

Precipitation method is the most commonly used method for preparing nano-powder by liquid-phase chemical reaction. Mix the soluble salt solution, control the reaction to form nano-precipitation of insoluble salt, and if necessary, calcinate the precipitate to become nano-powder. The precipitation method can be further divided into direct precipitation method, co-precipitation method, uniform precipitation method and precipitation conversion method.

3.3.1 Direct precipitation process 

A certain metal cation in the solution reacts chemically to form a precipitate. The main disadvantage is the non-uniformity of supersaturation in the reactor, resulting in non-uniform particle size of precipitated particles.

3.3.2 Uniform precipitation process 

No precipitant is added, but the precipitant is slowly generated in the solution, the local inhomogeneity of the precipitant is eliminated, and the purity of the precipitate is very high. Since the generated precipitant is consumed immediately, its concentration is kept at a very low state, so the precipitate has high purity and is easy to perform filtration and cleaning operations. Nanoparticles such as NiO, MgO, Er2O3, and ZnO can be prepared by using this method. 

3.3.3 Precipitation conversion process 

First a precipitate is formed, and then another solution is added to convert the precipitate into a precipitate of another substance. This method also eliminates local supersaturation of the solution caused by direct precipitation.

Precipitation conversion method is conducive to the formation of monodisperse nanopowders. However, there are some problems: ①Because the generated precipitate is gel-like, it is difficult to wash and filter; ②The precipitant is easily mixed into the powder as an impurity; ③The components may be separated during the precipitation process; ④A part of the precipitation occurs during washing. substance redissolved.

3.4 Hydrolysis process 

The hydrolysis method has the advantages of simple process, easy control, precise composition, uniform dispersion, high purity, fine particle size and large scale, and is a very promising preparation method of oxide nanopowder. Mainly divided into inorganic salt hydrolysis and metal alkoxide hydrolysis.The inorganic salt hydrolysis method is to hydrolyze some metal salt solutions such as alum salt solution, sulfate solution, and halide solution at high temperature to form hydroxide or hydrated oxide precipitation, and obtain nano-oxide powder after heating and decomposition.The metal alkoxide hydrolysis method is to react the metal alkoxide with water, filter, and dry to obtain oxide nanopowders with a particle size ranging from tens to hundreds of nanometers. 

3.5 spray drying process 

The solvent evaporation method is to heat the metal salt solution to evaporate the solvent. According to the characteristics of materials and different processes, it can be divided into freeze drying method, spray drying method and spray pyrolysis method.The spray drying method is a method in which the solution is dispersed into small droplets and sprayed into hot air to make it dry quickly. It is also possible to use such a method, that is, spray the solution into a high-temperature immiscible liquid (such as kerosene), so that the solvent evaporates rapidly.

3.6 Spray pyrolysis process 

The spray pyrolysis method is a method in which the precursor solution is sprayed into a high-temperature atmosphere, and at the same time, the evaporation of the solvent and the thermal decomposition of the metal salt are caused instantaneously, thereby directly synthesizing the oxide powder. The advantage of this method is that the liquid-phase substance precursor is used to obtain the final product through the aerosol process, so there is no need for processes such as filtration, washing, drying, sintering and re-grinding. The product has high purity, good dispersibility, uniform and controllable particle size, and can Prepare multi-component composite nanopowder, especially suitable for continuous preparation, with high production efficiency. The main disadvantage is that there are many hollow particles in the generated nanoparticles, and the particle size distribution is not uniform. 

3.7 Freeze-drying process

The freeze-drying method belongs to the solvent evaporation method in the liquid phase method of preparing nano powder. Generally, the preparation of nano-powder by freeze-drying method will go through four steps, that is, preparation of precursor solution or sol, freezing of precursor solution or sol, freeze-drying of frozen material and heat treatment of dried material.