Nanomaterials have unique properties such as mechanics, light, heat, electricity, magnetism, adsorption, and gas sensitivity. Adding nanopowders to traditional materials will greatly improve their performance or bring unexpected properties. However, in the actual application process, due to the small size of the nanoparticles and the high surface activity, they are prone to agglomerate to form larger-sized agglomerates, which severely hinders the application of nano-powders and the preparation of corresponding nano-materials.

Nano powder agglomeration phenomenon

At present, many nano-products on the market have been characterized and found to contain both nano-scale particles and micro-scale particles, which cannot achieve the true nanometer standard (1-100nm). The main reason is that the powder agglomeration phenomenon is serious, and the nano-scale particles agglomerate into large particles. .

1. Why does the nano powder reunite?

The so-called agglomeration of nano-powder refers to the phenomenon that the original nano-powder particles are connected to each other during the preparation, separation, processing and storage process, and a large particle cluster is formed by multiple particles. It is generally divided into two types: soft agglomeration and hard agglomeration. kind.

The agglomeration and dispersibility of nanopowder depends on its morphology and surface structure. The morphology and surface structure of nano-powders are related to its internal structure, impurities, surface adsorption and chemical reaction, preparation technology, environmental conditions and other factors, which leads to the complexity and diversity of the agglomeration and dispersion mechanism of nano-powders.

2. How to solve the agglomeration problem of nano powders?

To solve the agglomeration problem of nano powders, it is necessary to use certain means to uniformly disperse the nano powders. Nano powder dispersion methods mainly include ultrasonic dispersion, mechanical dispersion and chemical dispersion. At present, the most widely used is chemical dispersion, that is, surface modification.

Surface modification refers to the use of surface additives to cause chemical reactions and physical effects on the surface of particles to change the surface state of particles, such as surface atomic layer structure and functional groups, surface hydrophobicity, electrical properties, chemical adsorption and reaction characteristics. Surface modification can improve the dispersibility, durability, weather resistance, and surface activity of the powder, so that new physical, chemical, and optical properties can be produced on the surface of the particles, suitable for different application requirements, broadening its application fields, and significantly Improve the added value of materials.

There are many methods for surface modification of nanopowders, mainly including coating modification, precipitation reaction modification, surface chemical modification, mechanochemical modification, high-energy treatment modification, encapsulation modification, microemulsion modification, etc.

(1) Modification of coating treatment

Package treatment modification is also called coating and coating. It is a method of coating the surface of powders such as inorganic or organic substances, main surfactants, water-soluble or oil-soluble polymer compounds and fatty acid soaps to achieve modification. Such as including the use of adsorption, adhesion and simple chemical reactions or precipitation phenomena for coating.

(2) Modification of precipitation reaction

The method of using chemical reaction and depositing its product on the surface of the modified powder to form one or more "modified layers" to change the surface characteristics of the nanopowder material to achieve the desired use Requirements, this is the main method of wet modification.

(3) Surface chemical modification

This is the most important and commonly used method of surface modification. Surface chemical modification uses the surface modifier to chemically react with the particle surface, so that the surface modifier covers the surface of the particle, changes the surface structure and state of the nano powder particles, and achieves the purpose of surface modification. In addition to surface functional groups, surface chemical modification methods also use free radical reactions, chelating reactions, sol adsorption and coupling agents.

Nanoparticles have a large specific surface area, the surface bond state and electronic state are different from the inside of the particle, and the incomplete coordination leads to a large number of dangling bonds, which provides favorable conditions for the use of chemical reactions to modify the surface of nanoparticles.

The surface chemical modification of nano-powder materials is affected by many factors. Mainly include: the influence of the properties of the surface of the nano powder particles; the influence of the type, dosage and use method of the surface modifier; the influence of the process equipment and operating conditions, etc.

(4) Mechanochemical modification

Mechanochemical modification is the purposeful activation of the powder surface through the process of ultrafine pulverization and other strong mechanical forces, and to a certain extent, it changes the crystal structure (surface amorphization), solubility, chemical adsorption and Reactivity (increase the active point of the surface or the active point from the point) and so on.

(5) High-energy treatment modification

Through the action of high-energy particles such as corona, ultraviolet light, plasma radiation, microwave, etc., active points are generated on the surface of nanoparticles, increasing surface activity, and easily chemically reacting or adsorbing with other substances, and modifying the surface of nanoparticles to achieve easy dispersion Purpose.

(6) Encapsulation modification

Encapsulation modification is a surface modification method in which a homogeneous and thick film is covered on the surface of powder particles.

(7) Microemulsion modification

In the preparation of nanoparticles, a thermodynamically stable, isotropic, transparent or translucent dispersion system of two or more immiscible liquids is formed, and one or more microscopically stabilized by the surfactant interface film A microemulsion composed of droplets of two liquids. The surface of the particles is coated with a layer of surfactant molecules to make it difficult to coalesce between the particles. By selecting different surfactants and co-surfactants, the particles can be modified and the size of the particles can be controlled.

From the current development of nanomaterials, the agglomeration of nanopowders has severely restricted the application of nanomaterials. Nanopowder dispersion and surface modification technologies are the "most important" technologies in the development of nanomaterials. Only by dealing with these problems, nano-powder materials can play a huge function. The uniform dispersion of nano-powder particles is the key to improving the performance of various materials after modification. The use of various nano-powder surface modification technologies can make the surface of the nano-powder compatible with the matrix.