An article takes you to interpret the "magic" nano powder

What is nano-powder?

Nano powder is also called nano particles, generally refers to the size of 1-100nm ultrafine particles, some people call it ultrafine particles. Its scale is larger than the atomic cluster and smaller than the average particle. According to its size, assuming that each atom is 1 angstrom in size, it contains between 1000 and 1 billion atoms. It is smaller than the average biological cell, and the size of the virus. The form of the nanoparticles includes spherical, plate-like, rod-like, angular, sponge-like, etc. The component of the nanoparticles can be metal, oxide, or other various compounds.

Nanomaterials are divided into three categories: nano-powder materials, nano-solid materials, and nano-assembly systems. Nano-powder material is the most basic kind of nano-materials. Nanosolids are assembled and combined by separate materials. The nano assembly system is the deformation of nano powder materials.

metal nano powder is what?

Some common metal materials are made into nano-scale metal nano-powders, which have different abilities from ordinary materials. Now the common metal nano powder are: nano zinc powder, nano copper powder, nano silver powder, nano cobalt powder and so on, with the development of science and technology, these metal nano powder is gradually applied to many fields, such as ultra-fine metal powder to add lubricating oil in an appropriate way, can greatly improve the maximum non-seizure load of lubricating oil.

Metal nanopowders are widely used in the field of catalysts. Due to the small size and surface properties of nano-metal powders, nano-metal powders are used as catalysts, making them more widely used in the metallurgical and chemical industries. The general catalyst must be small in size and large in effect, and the metal nano-powder meets this requirement well. The volume is small: nanoscale; the effect is large: the specific surface area is larger than the general catalyst, and the reactant is more fully contacted to make the reaction faster.

Three effects of nano powder

(1). small size effect

As the amount of particles changes, when the size of the nanoparticles is equivalent to or smaller than the physical size characteristics such as light waves, conduction electron de Broglie wavelength, and the coherent length or transmission depth of the superconducting state, the periodic boundary conditions will be destroyed, and the acoustic, optical, electrical, magnetic, thermal, force and other properties will be qualitatively changed. Changes in macroscopic physical properties due to the reduction in particle size become small size effects.

(2). surface and interface effects

Nanoparticles are small in size, large in surface, and a large proportion of atoms located on the surface. Due to the decrease of nano-particle size, the activity of surface atoms will eventually increase, which will not only cause the change of the transport and configuration of the surface atoms of the nanoparticles, but also cause the change of the surface electron spin conformation and electron energy spectrum. These properties are referred to as "surface and interface effects".

(3). quantum size effect

When the particle size decreases to a certain value, the phenomenon that the electron energy level near the Fermi level of the metal changes from quasi-continuous to discrete energy levels becomes a quantum size effect.

Morphological Analysis of Nanometer Powder

In the world of powder materials, morphological diversity brings us a wealth of choices and unlimited possibilities. Different forms of powder materials, such as spherical, tubular, flake and whisker/wire, show their own unique performance advantages. By selecting the appropriate powder material form, we can better meet product design and performance requirements, and promote technological progress and industrial development.

1. Spherical powder materials-nano-spherical powder, such as nano-metal powder and some metal oxides, have high specific surface area and uniform particle size distribution. This makes them outstanding in the fields of catalysis, energy storage and sensors. The good fluidity and uniformity of the spherical powder make its handling and processing more convenient, which helps to improve the production efficiency of the product.

2. Tubular powder materials-nanotubes, such as carbon nanotubes and titanium dioxide nanotubes, have a unique hollow structure and high specific surface area. This makes them have important applications in energy storage, transmission and optoelectronic devices. The tubular structure provides a stronger carrier transport channel and a larger reactive surface area, thereby improving the electrochemical performance and light absorption capacity of the material.

3. Flaky powder materials-nanosheets, such as flake silver powder, flake copper powder, graphene, hexagonal boron nitride and molybdenum disulfide, show good electrical and thermal conductivity. This makes them widely used in electronic devices, conductive coatings and thermal interface materials. The highly two-dimensional characteristics of the sheet structure have excellent electronic and thermal conductivity, which provides a strong support for the preparation of high-performance materials and devices.

4. Whisker-like/wire-like powder materials-nanowires, such as silicon carbide whiskers, nano-silver wires, nano-copper wires, nano-silicon wires and zinc oxide nanowires, have high tensile strength and good electrical conductivity. This gives them great potential in areas such as flexible electronics, sensors and reinforcement materials. The high interfacial activity and plasticity of the whisker/thread-like structure enables the material to be adapted to a variety of complex application requirements.

5. Powder materials with irregular morphology, such as silicon powder, boron powder, etc.

agglomeration phenomenon of nano-powder

At present, many nano-products on the market are characterized to contain both nano-scale particles and micron-scale particles, and cannot realize the real nano-standard (1-100nm). The main reason is that the phenomenon of powder agglomeration is serious, and the nano-scale particles are agglomerated into large particles. The so-called agglomeration of nano-powder refers to the phenomenon that the primary nano-powder particles are connected to each other in the process of preparation, separation, treatment and storage, and a large particle cluster is formed by multiple particles. Generally, it is divided into soft agglomeration and hard agglomeration.

For nano-powder to take appropriate means to prevent the primary particles from reuniting to make the agglomerates completely dispersed, in order to obtain a good dispersion effect, therefore, in the dispersion process must make each newly formed particle surface quickly wetted by the medium, that is, isolated by the dispersion medium, in order to prevent re-aggregation, and have high enough energy to prevent the particles from mutual expansion contact, re-agglomeration.

The agglomeration and dispersion of nanopowders depend on their morphology and surface structure. The morphology and surface structure of nano-powders are related to their internal structure, impurities, surface adsorption and chemical reactions, preparation process, environmental conditions and other factors, which leads to the complexity and diversity of the agglomeration and dispersion mechanism of nano-powders.

How to solve the problem of agglomeration of nano powder?

To solve the problem of agglomeration of nano-powder, it is necessary to use certain means to uniformly disperse the nano-powder. The main dispersion methods of nano-powder are 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, thereby changing the surface state of particles, such as surface atomic layer structure and functional groups, surface hydrophobicity, electrical properties, chemical adsorption and reaction characteristics. Through surface modification, the dispersion, durability, weather resistance and surface activity of the powder can be improved, so that the particle surface has new physical, chemical and optical properties, which are suitable for different application requirements, broaden its application fields, and significantly improve the added value of the material.

Theory of Nano-powder Dispersion Stability

The nano-powder is dispersed in the medium, and the suspension formed belongs to the category of colloid because the size of the particles is at the nanometer level. In general, colloidal suspensions are thermodynamically unstable due to the very large surface energy of nanopowders, but colloids can be kinetically stable if an energy barrier exists. This stabilizing effect is derived from the repulsion of the electric double layer and the static interaction force. If the interaction force is large enough to provide an energy barrier, agglomeration of the nanoparticles can be prevented or destroyed. All the dispersion problems of nanopowders in media are related to this.

The stability of the dispersion system refers to a certain degree of invariance of certain properties (such as the concentration of the dispersed phase, particle size, system viscosity and density, etc.). Since the particle size of nanoparticles is similar to that of colloidal particles, the dispersion of nanoparticles in liquid media can be discussed with the help of colloidal stability theory.

In summary, the stability or coagulation of colloids depends on the repulsive and attractive forces between colloidal particles. The repulsive force disperses the particles and increases the stability of the particles in the medium, while the attractive force causes the particles to agglomerate. Therefore, according to the causes of these two forces and their interaction, the dispersion of nano-powder in the medium can be discussed by colloid theory. There are three stabilization theories for colloidal dispersion: DLV0 theory, steric hindrance stabilization theory, and vacancy stabilization theory, which are introduced separately below.

1. DLV0 Theory

DLV0 theory is a theory to study the stability of charged colloidal particles. It was independently proposed by Delgakun and Landon (Dariaguin and Landon) of the former Soviet Union in l941 and by Verwey and Overbeek (Verwey and Verwey) of the Netherlands in l948, taking the first letter of the four names. Therefore, it is called DLV0 theory. DLV0 theory mainly explains the mechanism of the stability of the dispersion system and the factors affecting the stability through the electric double layer theory of particles. According to the electric double layer model, the surface of the particle is charged, and the particle is surrounded by an ionic atmosphere (as shown in Figure 1).

Figure 2

Figure 2

Figure 3

In Figure 1, the colloidal particles are positively charged, and the coil represents the range of positive charge. Due to the shielding effect of counter ions in the ion atmosphere, the coil is not affected by the charge of colloidal particles. Therefore, when the two particles approach and the ion atmosphere has not yet contacted, there is no repulsion between the particles. When the particles approach each other to overlap the ion atmosphere (see fig. 2), the ion concentration in the overlapping region is obviously large, which destroys the symmetry of the original charge distribution and causes the redistribution of charge in the ion atmosphere, that is, the ions diffuse from the high concentration interval to the non-overlapping interval, so that the positively charged particles are separated from each other by a repulsive force, which is expressed by the inter-particle distance. A peak U appears on the potential energy curve, called the potential barrier. As long as the potential barrier is high enough that the movement of the particles cannot overcome it, the colloid remains stable.

As can be seen from fig. 3, when the two particles are far apart, the ion atmosphere has not yet overlapped, and the attraction force of "long distance" between the particles is at work, I .e. gravity is dominant, the curve is below the horizontal axis, and the total potential energy is negative. With the shortening of the distance, the ion atmosphere overlaps. At this time, the repulsion begins to appear, the total potential energy gradually increases to a positive value, and an energy peak UR appears at a certain distance. The potential energy rises to the maximum point, which means that the two particles cannot get closer, or they will separate after collision. If the potential energy peak is crossed, the potential energy drops rapidly, indicating that when the distance between particles is very close, the repulsive force generated by the ion atmosphere is an important factor for the particles to avoid agglomeration, and the repulsive force generated by the ion atmosphere depends on the thickness of the electric double layer. Therefore, the potential can be lowered by adding an electrolyzable substance such as sodium hexametaphosphate, sodium chloride, sodium nitrate to the suspension, and an ionic surfactant having the same charge as the surface of the particles can also be added, since its adsorption leads to an increase in the surface dynamic potential, thereby improving the stability of the system.

2. steric stability theory

The application of DLV0 theory to explain the stability of some high polymer or non-ionic surfactant colloidal system is often difficult, the important reason is to ignore the role of the adsorption polymer layer. After the colloid adsorbs the polymer, a new kind of repulsive potential energy-space repulsion potential energy is produced, so when there is a polymer adsorption layer, the total potential energy between particles:

E=EATH EX (5-1)

where EA-attraction energy between particles;

ER-Repulsive energy between particles;

Es -- space repulsion potential energy between particles.

It can be seen from the above formula that ES plays an important role in the stability of colloids, so the stability theory is called the steric hindrance stability theory.

3. Vacancy Stability Theory

Due to the negative adsorption of the particles to the polymer, the polymer concentration in the surface layer of the particles is lower than the bulk concentration of the solution. This negative adsorption phenomenon leads to the formation of a "vacancy layer" on the surface of the particles, and when the vacancy layer overlaps, it will produce repulsive energy or attractive energy, which will change the potential energy curve of the system. In low concentration solution, the attraction energy dominates and the colloidal stability decreases. In high concentration solutions, the repulsive energy dominates, stabilizing the colloid. Since this stability depends on the formation of the vacancy layer, it is called the vacancy stability theory.

In the study of colloidal stability, dispersants have become the focus of research because they can significantly change the surface state and interaction of suspended particles. The dispersant can be adsorbed on the surface of the particles in the suspension, increasing the repulsive potential of the particles and preventing the agglomeration of the particles. However, the adsorption of the dispersant on the surface of the powder has an optimal value. Only when the dispersant reaches the saturated adsorption capacity, the viscosity of the suspension is the smallest and the system is stable. At the same time, the study also found that the acid and alkali of the solution significantly affected the adsorption of dispersant on the surface of the powder.

Surface Modification of Nanometer Powder

There are many methods for surface modification of nano-powder, mainly including coating treatment modification, precipitation reaction modification, surface chemical modification, mechanical chemical modification, high-energy treatment modification, encapsulation modification, micro-emulsion modification, etc.

(1) Coating treatment modification

Coating 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 and other powder surface coating to achieve the modified method, such as including the use of adsorption, adhesion and simple chemical reaction or precipitation phenomenon for coating.

(2) Precipitation reaction modification

The use of chemical reaction and its products precipitated on the surface of the modified powder, so that the formation of one or more layers of "modified layer" method, in order to change the surface characteristics of nano powder materials, so that it can meet the required requirements, which 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 through the surface modifier and the particle surface chemical reaction, so that the surface modifier to cover the surface of the particles, change the surface structure and state of nano-powder particles, to achieve 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, surface bond states, and electronic states are different from those inside the particles, and incomplete coordination leads to a large number of dangling bonds, which provides favorable conditions for the surface modification of nanoparticles by chemical reactions.

The surface chemical modification of nano-powder materials is affected by many factors. The main effects are: the nature of the surface of nano-powder particles; the impact of the type, dosage and use of surface modifiers; the impact of process equipment and operating conditions.

(4) mechanical chemical modification

Mechanical chemical modification is through the process of ultrafine grinding and other strong mechanical force on the powder surface purposefully activated, to a certain extent, change the crystal structure of the particle surface (surface amorphous), solubility, chemical adsorption and reaction activity (increase the surface of the active point or activity from the point).

(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 to increase surface activity, which is easy to chemically react or adsorb with other substances, and modify the surface of nanoparticles to achieve the purpose of easy dispersion.

(6) Capsule modification

Encapsulation modification is a surface modification method in which the surface of powder particles is covered with a homogeneous film with a certain thickness.

(7) micro emulsion modification

In the preparation of nanoparticles, a thermodynamically stable, isotropic, transparent or translucent dispersion of two or more immiscible liquids is formed, and a microemulsion is composed of droplets of one or two liquids stabilized by a surfactant interfacial film. The surface of the particles is coated with a layer of surfactant molecules, so that the particles are not easy to coalesce. 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 nano-powders severely limits the application of nanomaterials, and the dispersion and surface modification of nano-powders 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 improve the performance of various materials after modification, and the use of various nano-powder surface modification technologies can make the surface of nano-powder compatible with the matrix.