Views: 31
Nanomaterials have unique properties of mechanics, light, heat, electricity, magnetism, adsorption and gas sensitivity. Adding nanomaterials to traditional materials will greatly improve their properties or bring unexpected properties.
However, in the practical application process, due to the small particle size and high surface activity of nanoparticles, they tend to agglomerate and form large agglomerates, which seriously hinders the application of nano-powders and the preparation of corresponding nano-materials
At present, many nano-calcium carbonate products on the market are characterized and found to contain both nano-sized particles and micron-sized particles, which cannot achieve the true nano-standard (1-100nm). The main reason is that the powder agglomeration phenomenon is serious, and nano-sized particles are aggregated into large particles.
1. Why are nano powders agglomerated?
The so-called agglomeration of nano powder refers to the phenomenon that the original nano powder particles are connected with each other in the process of preparation, separation, treatment and storage, and a large particle cluster is formed by multiple particles, which is generally divided into soft agglomeration and hard agglomeration.
The agglomeration and dispersion of nanometer powders depend on their morphology and surface structure.
The morphology and surface structure of nanometer powders are related to the internal structure, impurities, surface adsorption and chemical reaction, preparation process and environmental state, which leads to the complexity and diversity of the agglomeration and dispersion mechanisms of nanometer powders.
2. How to solve the agglomeration problem of nano powders?
In order to solve the agglomeration problem of nanometer powders, it is necessary to use some means to evenly disperse the nanometer powders.
The dispersion methods of nano powder mainly include ultrasonic dispersion, mechanical force dispersion and chemical dispersion.
The most widely used chemical dispersion is surface modification.
Surface modification refers to the use of surface additives to make the particle surface chemical reaction and physical action, so as to change the particle surface state, such as surface atomic layer structure and functional groups, surface hydrophobicity, electricity, chemisorption and reaction characteristics.
Through surface modification, the dispersibility, durability and weatherability of powder can be improved, and the surface activity can be improved, so that the particle surface can produce new physical, chemical and optical characteristics, which can be applied to different application requirements, expand its application field, and significantly increase the added value of materials.
There are many methods for surface modification of nano powders, such as coating modification, precipitation reaction modification, surface chemical modification, mechanical chemical modification, high-energy treatment modification, encapsulation modification and microemulsification modification.
(1) coating treatment modification
Inclusion treatment modification also known as coating and coating, is the use of inorganic or organic matter, the main surfactant, water soluble or oil soluble polymer compounds and fatty acid soap powder surface coating to achieve modification.
Such as including the use of adsorption, adhesion and simple chemical reactions or precipitation phenomenon for the envelop.
(2) precipitation reaction modification
The main method of wet modification is to change the surface characteristics of nanometer powder materials and make them meet the required requirements by using the method of chemical reaction and precipitation of the products on the surface of the modified powder to form one or more "modified layers".
(3) surface chemical modification
This is the most important and common method of surface modification.
Surface chemical modification through the chemical reaction between the surface modifier and the particle surface, the surface modifier can cover the particle surface, change the surface structure and state of nano powder particles, and achieve the purpose of surface modification.
In addition to surface functional groups, the surface chemical modification method also USES free radical reaction, cogging reaction, sol adsorption and coupling agent.
The specific surface area of the nanoparticles is large, the surface bond states and the electronic states are different from the interior of the particles, and the incomplete coordination leads to a large number of suspended bonds, which provides a favorable condition for the surface modification of the nanoparticles by chemical reaction.
The surface chemical modification of nanometer powder materials is affected by many factors.
The main effects are: the surface properties of nano powder particles;
The influence of the type, dosage and application method of surface modifier;
The influence of process equipment and operating conditions.
(4) mechanical and chemical modification
The mechanochemical modification is to activate the powder surface purposedly through the process of ultramicro pulverization and other strong mechanical force action, and to a certain extent change the crystal structure (surface amorphous), solubility, chemisorption and reactivity of the particle surface (increase the active point or passive point of the surface).
(5) high energy treatment modification
Through the action of corona, ultraviolet light, plasma radiation, microwave and other high-energy particles, the active spots are generated on the surface of nanoparticles to increase the surface activity, which is easy to have chemical reactions or adsorption with other substances, and the surface modification of nanoparticles to achieve the purpose of easy dispersion
(6) capsule modification
Encapsulation modification is a surface modification method that covers the surface of powder particles with homogeneous film with a certain thickness.
(7) microemulsion modification
In the preparation of nanoparticles, two or more kinds of insoluble liquids are formed as thermodynamically stable, isotropic, transparent or translucent dispersion systems, and microemulsions are composed of droplets of one or two kinds of liquids stabilized by surfactant interface membrane.
The particle surface is coated with a layer of surfactant molecules, which makes it difficult for particles to coalesce. By choosing different surfactants and cosurfactants, the particles can be modified and the size of particles can be controlled.
From the current development of nanomaterials, the agglomeration of nanomaterials severely limits the application of nanomaterials, and the dispersion and surface modification of nanomaterials are the "most important" technologies in the development of nanomaterials.
Only by dealing with these problems can nanometer powder materials play a great role.
The uniform dispersion of nano-powder particles is the key to improve the performance of modified materials. The surface and substrate of nano-powder can be compatible with each other by adopting various surface modification technologies of nano-powder.
