Traditional preparation method of microcrystals
There are many methods for preparing microcrystals (ultrafine particles), which can be divided into two categories: physical methods and chemical methods according to their nature.
Vacuum condensation method
Vacuum evaporation, heating, high-frequency induction and other methods are used to vaporize the metal raw material or form a plasma, and then quickly cool, and finally form ultra-fine particles with a diameter of about 10 mm on the condenser. By adjusting the parameters such as evaporation temperature and gas pressure, the size of ultrafine particles (microcrystals) can be controlled.
The minimum particle size of microcrystals prepared by this method can reach 2nmThe advantage of this method is that the ultrafine particles (micronization) prepared have high purity, good crystal structure, controllable particle size and uniform distribution, and are suitable for any evaporable elements and compounds;
The disadvantage is that the technology and equipment requirements are higher
In the grinding process, the purpose of compounding is achieved by the effects of grinding media and particles, extrusion, shearing, impact between particles.
In the study of the composite mechanism, it is believed that the grinding process is a process of cyclic deformation of the mother particles under the action of various forces, which can lead to the recombination of the large-angle grain boundaries of the particles, which gradually refines the powder structure and finally reaches different atoms.
Mutual penetration and diffusion can obtain ultra-fine composite materials that are difficult to prepare by other traditional micronization methods.
High energy ball milling
High energy ball milling is a method that uses the rotation or vibration of a ball mill to make the hard balls strongly impact, grind and stir the raw materials to crush the powder into ultrafine particles (micronization).
If two or more metal powders are put into the ball mill’s ball mill tank for high-energy ball milling at the same time, the powder particles are subjected to the repeated processes of calendering, pressing, crushing, and re-pressing, and finally the structure and composition can be evenly distributed. Superfine composite particles.
Because this method uses mechanical energy to achieve alloying, so that some systems that cannot react under normal conditions can directly perform chemical reactions at lower temperatures, the method of preparing alloy powders by high-energy ball milling also belongs to mechanochemical methods.
The blending method is one of the most original compound methods.
It first premixes ultrafine particles (micronization) at room temperature, and then blends and mixes under heating. Under the action of extrusion and shear forces, larger composite particles can also be split into smaller microcrystals. Composite particles. This compounding method is similar to the mechanical grinding method in some respects.
The difference is that the blending method is usually an organic/inorganic particle compound, and the stirring speed is lower than the former, so there will be no organic particles when mixing. Soften the high temperature, so it is usually heated or cooled by a jacket or other devices during the mixing process.
This composite method is simple and can produce high-performance ultra-fine particle materials.
Evaporation in gas
Evaporation in gas, also known as evaporative condensation, is essentially a classic method of preparing ultrafine particles (microcrystals) using physical methods.
Preparation method of microcrystals
This method is usually filled with low-pressure inert gas (N2, He, Ne, Ar, etc.) in the vacuum evaporation chamber, and heated by resistance, plasma, electron beam, laser, high-frequency induction, etc. to vaporize the raw material or form a plasma. It collides with inert gas atoms to lose energy, and then quenches to condense it into ultrafine particles (microcrystals) .
Particle size control of microcrystals
The particle size of ultrafine particles (microcrystals) can be controlled by changing the gas pressure, heating temperature and type of inert gas.
- Under the condition that the heater temperature and the type of inert gas are fixed, the average particle size increases with the increase of gas pressure;
- Under the condition of a certain type of inert gas and pressure, the particle size of the particles increases as the heating temperature increases;
- When the pressure and temperature of the inert gas are the same, different inert gases are used, and the particle size of the generated particles is also different.
The evaporated metal atoms diffuse in the inert gas and continuously collide with the gas atoms to lose energy, so the metal vapor is cooled in the inert gas. When the metal vapor cools below the saturation critical temperature, nucleation occurs until it is deposited on the wall or metal plate.
The in-gas evaporation method is particularly suitable for the preparation of non-oxide ultrafine particles (microcrystals) that are difficult to synthesize directly by the liquid-phase method and the solid-phase method.
The particle size is usually below 100 mm and the dispersion is very good. In addition, the resulting particles have high purity, good crystalline structure, and controllable particle size.
The chemical preparation method of ultrafine particles (microcrystals) generally adopts the “top-down” method, that is, through appropriate chemical reactions, ultrafine particles (microcrystals) are prepared from molecules and atoms
Colloidal chemical method is also called surface modification method.
At present, most of ultrafine particles (microcrystals) are prepared in colloidal solution.
Therefore, the method of preparing microcrystals with colloidal solution is relatively mature and the process is simple. In this method, a certain stabilizer is generally added to bond the atoms on the surface of the microcrystals to prevent the agglomeration between the particles.
The particle size of microcrystals synthesized by this method is controllable, and the surface properties of ultrafine particles are improved, but flocculation is prone to occur. Some people abroad used sulfur powder as a stabilizer in methanol systems to generate CdS emulsions.
After drying with nitrogen, Cds ultrafine particles were obtained, and the size of ultrafine particles was controlled by adjusting the ratio of sulfur powder and sodium sulfide. Chemseddinet et al. proposed the size-selective precipitation method to precipitate particles of different sizes in a uniform and transparent colloidal solution. Centrifugal separation to obtain microcrystals with uniform particle size.
The template method has been widely paid attention to the preparation of microcrystal materials.
In the template method, the template can be selected according to the size and morphology of the synthetic material, and the size, morphology, structure, arrangement, etc. of the synthetic material can be controlled according to the space-limiting effect of the template and the regulation effect of the template agent.
The template method can be divided into hard template and soft template according to its own characteristics and limited domain capabilities.
Using liquid crystal and anodized aluminum oxide film (AAO) formed by polyoxyethylene surfactants as soft templates, semiconductor sulfide ultrafine wires and mesoporous ultrafine materials were prepared in the solution system. Carbide ultrafine tubes are prepared by the reaction of oxides and carbon ultrafine tubes.
Further research shows that stable carbon ultrafine tubes may function as a template to control the reaction in the ultrafine tubes and then synthesize ultrafine rods . In recent years, the use of biomolecules as templates to synthesize ultrafine particles has also received more and more attention.
The sol-gel method is a wet chemical method for preparing microcrystal powders.
Its basic principle is to prepare a metal inorganic salt or a metal alkoxide precursor with a liquid chemical reagent. The precursor is dissolved in a solvent to form a uniform solution.
A hydrolysis or alcoholysis reaction occurs between the solute and the solvent.After the reaction product is aggregated , Generally generate particles with a particle size of about 1 nm and form a sol. Generally, the reactants are required to be uniformly mixed and reacted in the liquid phase.
The reaction product is a stable sol system, and no precipitation should occur during the reaction. After a long period of storage or drying, the sol is converted into a gel.
The gel usually contains a large amount of liquid phase, and the liquid medium needs to be removed by extraction or evaporation, and heat treatment is performed at a temperature far lower than the traditional sintering temperature, and the corresponding compound powder is finally formed.
Inorganic salt sol-gel method and Organic salt sol-gel method
According to the different starting solutions used, the sol-gel method can be divided into inorganic salt sol-gel method and Organic salt sol-gel method. The starting material of the former is an inorganic salt solution, which is formed by adding a precipitant or adjusting the pH; the starting material of the latter is an organic salt solution, which uses the gel chain polymerization of organic molecules to form a sol.
Inorganic salt sol-gel method
Due to the relatively high price of organic substances, the production cost is high. Moreover, some organic substances cause serious environmental pollution and affect human health. The use of inorganic salts instead of organic salts can overcome these deficiencies.
Therefore, the inorganic salt sol-gel method has received widespread attention.For example, Wang Xiaohui and other companies have prepared CoO cubes with an average particle size of 6 nm using CC solutions and NaCO solutions as raw materials, and Sveg and others have synthesized Co(OH ) Then the pH was adjusted with CH1COOH, concentrated to colloid, and then calcined at 300 ℃ and 500 ℃ to get Co3O4 and lithium-doped Co3O4.
Advantages of sol-gel method
The sol-gel method is used to prepare microcrystals.
The outstanding advantages are high uniformity, good purity, few side reactions, easy control of the reaction process, and easy industrialization.
Sol-gel-freeze drying method
In the preparation process of the sol-gel method, after the sol is converted into a gel, it usually contains a large amount of liquid phase.
The specific surface area and pore volume of the powder obtained by the ordinary drying method are very small, and the particle agglomeration is quite serious.
In the freeze-drying method, when the colloidal particles are precipitated to form a network-structured gel, a large amount of dispersion medium is adsorbed therein, and a large number of capillaries are formed accordingly. During the drying process, the surface tension and surface energy cause the gel to shrink and aggregate. Knot causing agglomeration between particles.
Freeze-drying sublimates the solidified medium under low temperature and negative pressure, so as to eliminate the dispersion medium.
Because the colloidal particles are frozen in the original liquid medium, and there is no gas-liquid interface with huge surface tension in the capillary between the particles, the serious agglomeration caused by “liquid bridge” is avoided.
Sol-gel-freeze-drying method is a method of combining freeze-drying technology with sol-gel method to prepare ultrafine particles, which not only solves the problem of powder particle agglomeration in the process of gel drying, but also avoids the preparation process Impurities such as anionic surfactants and organic solvents that are not easily removed are added to cause defects that affect product quality.
Pyrolysis of metal organic compounds
The pyrolysis method of metal organic compounds is also called metal organic compound precursor method.It uses the coordination between the coordination compound and different metal ions to obtain a highly dispersed composite precursor.Finally, the organic ligand is removed by thermal decomposition superfine particles.
The advantages of the preparation of microcrystals by pyrolysis of metal organic compounds are:
- Since metal organic compounds can achieve high purity through rectification or recrystallization, the purity of ultrafine particles is guaranteed;
- There are many types of metal organic compounds, with a wide range of selectivity;
- Metal Organic compounds can be dissolved in many solvents, so ultrafine particles can be prepared in many media.
However, metal organic compounds are inherently toxic, which limits their scope of application
The microemulsion method generally dissolves the reactant of the synthetic particles in the microemulsion solution.
Under vigorous stirring, the reactant undergoes a chemical reaction (including precipitation reaction, redox reaction, hydrolysis reaction, etc.) in the water core, and the product Nucleate and grow in the water core. When the particles in the water core grow to a certain size, the surfactant will attach to the surface of the particles to stabilize the particles and prevent their further growth.After the reaction is complete, centrifuge or add solvents such as water and acetone.
The ultrafine particles can be obtained by removing the oil phase and surfactant attached to the surface of the particles, and then drying at a certain temperature.
The components of microemulsion are generally oil phase, water phase, surfactant and co-surfactant.
There are three types of microemulsion, namely W/O type, O/W type and oil-water bi-continuous type.
Two kinds of reaction pathways
Generally, W/O microemulsion is used to prepare ultrafine powder, and there are two kinds of reaction pathways:
- The first isThe reactant is added to the microemulsion containing another reactant to perform the osmotic reaction;
- The second type is the microemulsion containing the reactant added to another emulsion containing the reactant to perform the fusion reaction