Three preparation methods of nano powder-solid phase method

Nano-powder, also known as ultra-fine powder or ultra-fine 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 specific surface effect, small size effect, volume effect, quantum size effect and macroscopic quantum tunneling effect, and is widely used.

Preparation of nano materials

From small to large: atoms → clusters → nanoparticles

Preparation Requirements of Nanomaterials

- Size, size controllable (generally less than 100 nm)

- Controllable composition (elemental composition)

- Shape controllable (shape)

- Crystal type controllable (crystal structure, superlattice)

- Controllable surface physical and chemical properties (surface state)

- (Surface modification and surface coating)

The preparation method of nano powder is often classified as follows

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

Physical methods involve physical changes such as evaporation, melting, solidification, deformation, and particle size changes.

Chemical methods include vapor deposition method, precipitation method, hydrothermal synthesis method, benzene thermal synthesis method, sol-gel method, microemulsion method, vacuum condensation method, etc.

The comprehensive method is that the preparation process should be accompanied by some chemical reactions, and at the same time it involves the change of the state of the particles, and even certain physical means should be applied to ensure the smooth progress of the chemical reaction in the preparation process.

The material state in the reaction process can be divided into three categories: solid phase method, liquid phase method and gas phase method.It is also a common method for the preparation of nanoparticles.

The solid-phase method is to produce a nano-powder by changing from a solid phase to a solid phase, and is not accompanied by a state (phase) change of gas phase →solid phase and liquid phase →solid phase. The micronizing mechanism of substances can be roughly divided into two categories: one is the method of extremely fine division (size reduction) of large substances, and the substances have no change, including mechanical crushing method, explosive sintering method, dissolution method (chemical treatment), etc. The other is the method of combining the smallest units (molecules or atoms) (construction process), the substance changes, thermal decomposition method, solid phase reaction method, etc.

Liquid phase method is a widely used method to prepare nano-powder in laboratory and industry. According to chemical means, nano-powders can be prepared by a simple solution process without the need for 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 through various means to turn substances into gas, so that physical and chemical reactions occur in the gas state, and finally in the cooling process to form nanoparticles. The gas phase method is roughly classified into a gas evaporation condensation method, a chemical vapor reaction method, a chemical vapor condensation method, a sputtering method, and the like. The electric explosion method is a special resistance heating method and is a kind of gas phase method.

solid phase method

mechanical alloying method

Mechanical alloying (Mechanical Alloying, referred to as MA) refers to the use of mechanical energy to make the composition of the material in the solid state to achieve alloying material preparation technology. In the process of MA, due to the effect of high energy mechanical energy, the material undergoes a series of microstructure changes and non-equilibrium phase transitions, resulting in the formation of various non-equilibrium structures, such as nanocrystalline, amorphous, metastable phase, supersaturated solid solution, etc. These metastable structural materials often exhibit excellent physical, chemical and mechanical properties.

Process of Material Preparation by High Energy Ball Milling

High energy ball milling is the main method to realize MA. The diagram schematically shows the process of alloying in ball milling of two metal powders. The powder is severely deformed by the impact of the steel ball and cold welded to form a sheet structure. With the continuous progress of ball milling collision, the layer structure is more and more refined. Due to the large number of crystal defects introduced by deformation and the existence of a large number of interfaces introduced by cold welding, as well as the temperature rise caused by ball milling collision, the diffusion capacity of the component is greatly enhanced. Interdiffusion occurs through the interlaminar interface to cause a phase transition, thereby forming an alloy of an amorphous phase, a quasicrystal phase, and a nanocrystalline phase, an intermetallic compound, a metastable phase, a supersaturated solid solution, and the like.

As a traditional milling technology, mechanical ball milling is widely used, but it has low efficiency and serious medium pollution. At the same time, a single mechanical energy is difficult to induce material activation, material phase change and chemical reaction, and it is not easy to achieve precise control of the material structure, and some compounds are difficult to synthesize. This makes the traditional ball mill in the preparation of high-performance functional powder facing a great development bottleneck.

Main features: mechanical alloying method has the advantages of simple process, can be prepared by regular methods difficult to prepare high melting point metal, immiscible system of solid solution, nano intermetallic compounds and nano metal ceramic composite materials. The disadvantages are high energy consumption, wide particle size distribution and easy introduction of impurities.

electric explosion method

Electric explosion refers to a physical phenomenon in which the physical state of the conductor material itself changes sharply when a strong pulse current passes through the conductor wire in a certain medium (such as inert gas, water, etc.), and the electrical energy is rapidly converted into other forms of energy (such as heat energy, plasma radiation energy, shock wave energy, etc.). The preparation of nano-powder materials by electrical explosion can be used as a purely physical method (such as the preparation of nano-powder under inert gas environment), and can also be developed as a comprehensive preparation method (by controlling the chemical reaction between high temperature and high pressure metal steam and environmental medium after explosion).

Schematic diagram of device for preparing nano-powder by electric explosion of metal wire

The basic process of electric explosion can usually be divided into five stages: solid-state heating stage of metal conductor-melting of metal conductor, vaporization stage-explosion stage-arc breakdown stage-condensation stage.

On the whole, the preparation of nano-powder materials by the electrical explosion method is affected by the precursor material, atmosphere, energy storage and discharge parameters such as pulse discharge voltage, current density, pulse width and other factors, at the same time, the formation of high temperature and high pressure in the electrical explosion stage and the subsequent cooling rate on the impact of powder particle size also need to be quantified.

The advantages of preparation of nano powder by electric explosion method:

1. High energy conversion efficiency. Depending on the resistance of the metal wire in the discharge circuit, it is easy to convert electrical energy into heat energy;
2. The prepared material has more uniform particle size distribution and high purity. Pulse discharge can gasify the entire metal wire at the same time, and the gasification effect is more uniform than that obtained by pulse laser and particle beam gasification from the metal surface, and the surrounding environment medium is well controlled, which can ensure the high purity of nano-powder (the purity of metal nano-materials can reach 99%);
3. Convenient adjustment of process parameters. By adjusting the capacitance, charging voltage and the size of the explosive wire, the particle size can be effectively controlled;
4. The generality of the method is strong. Various types of nano-powder materials can be prepared by the electrical explosion method;
5. It does not produce harmful substances and does not damage the environment. It is a "green" method for preparing nano-powders.

Research on the preparation of nano powder by electric explosion method According to the principle of electric explosion powder technology, the material used for electric explosion must be a good conductive material, so the nano powder of general metal, alloy, intermetallic compound and metal oxide can be prepared by the method of electric explosion. When the cavity is filled with inert gas or gas that does not react with the explosive wire, metal nano-powder is generated; metal thin film can also be deposited on the substrate under quasi-vacuum (10-6Torr); in an environment containing oxygen, metal oxide nano-powder and composite metal oxide are generated; in a nitrogen or ammonia environment, metal nitride nano-powder is prepared; metal carbide with high melting point can be prepared in a mixture containing organic matter. Such as metal powder Al, Cu, Fe, Ni, Mg, Ag, Ti, metal compounds Al2O3, Fe2O3, AlN, TiN, AgF, PtF2, nano alloy materials Cu-Zn, Fe-Ni, Ni-Al, Al-Cu, etc. have made some progress. On the other hand, the electrical explosion method can prepare non-metallic nanomaterials. For example, carbon nano-powder, carbon nanotubes and fullerenes are prepared by using very fine carbon fibers in nitrogen or argon, and GaAs clusters of 5 to 10nm can be prepared in inert gas or hydrogen. At present, the non-metallic materials prepared by the electrical explosion method are mainly carbon nanomaterials. The use of powder as an electric explosive material, and the preparation of carbon nanomaterials can be organically combined with the development of natural graphite resources, reflecting the "green, low carbon".

amorphous crystallization method

The method of preparing amorphous strips from liquid metal by rapid solidification method, and then crystallizing the amorphous strips by heat treatment to obtain nanocrystalline strips. The process is simple and the chemical composition is accurate.

The preparation methods of amorphous silicon crystallization mainly include thermal annealing method, laser annealing method, microwave annealing method and so on. Among them, the thermal annealing method is the most commonly used. The method transforms the amorphous silicon into an ordered crystal by heating. Although the thermal annealing method is simple and easy, it is difficult to prepare and process large-area films.

References:

Peng Chucai, etc. Research Progress of Nano-powder Materials Prepared by Electrical Explosion Method

Peng Chu-cai. Study on the preparation and mechanism of nano powder by electrical explosion method

Li Zhiyong. Study on the preparation of carbon nano materials by powder electric explosion

Mao Zhiguo. Study on Preparation of nano powder by electric explosion metal wire

Chen Bin. Study on Preparation Technology and Equipment of Metal Nanopowder

baidu encyclopedia. preparation of nano powder

Mao Zhiguo. Study on Preparation of nano powder by electric explosion metal wire

Liu Jun. Study on Preparation of inorganic functional nano powder by vacuum freeze drying

Sun Weimin. Study on the stabilization treatment and application of nano powder prepared by arc method

Bao Jiusheng, etc. Research Progress in Preparation of Nano-powders by Evaporation-Condensation

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