Nano powder, also known as (powder) is the basis of nano-science and technology, is causing a wide range of world view of concern. The nanomaterials referred to by modern materials and physicists refer to ultrafine particles (also known as nanopowders) with solid particles as small as nanoscale and solid and thin films with small grain size to nanometer poles.
Nano - powder morphology and particle size control
1. Examples of nano-powder morphology requirements
The requirements for particle size and morphology of micro-nano powders vary depending on the application. Iron oxide α, β, γ three crystal. Wherein the water gas conversion reaction, the butane dehydrogenation reaction catalyst is required for the α crystal form of the iron oxide catalyst, and the ultrafine iron oxide magnetic powder for the magnetic recording medium is required to have a γ crystal type, the particle size is less than 0.3 pm, the shape is an aspect ratio greater than 8 Of the needle. In addition, the α-Fe2O3 of the pigment is preferably in the form of a rod, a disc, and a sheet. A12O3 has α, γ, θ, η and other eight crystal form, the catalyst and the carrier of alumina should be η-A12O3 or γ-A12O3, and α-A12O3 is an important ceramic material. Alumina hydrate mainly has three kinds of trihydrate and two kinds of monohydrate, flame retardant material with the requirements of trihydrate, and fine particle size, a reasonable level, good transparency, particle shape for the sheet, fine prism The
The spherical nickel hydroxide powder used as the nickel-hydrogen battery material requires a certain distribution width of the particle size so that the small particles can be filled between the voids of the large particles to increase the energy density of the electrodes; and as the preparation of the oxidation for the electronics industry Nickel powder calcined precursors require particle size in submicron and distributed as narrow as possible. Table 2.1 also lists some industrial products on the particle shape requirements, nano-powder in the application of this special requirements.
2. The complexity of morphology and particle size control
In the preparation of ultrafine powder, it is quite difficult to control the particle size and morphology, which is mainly due to the complexity of the preparation process itself. Liquid phase precipitation is one of the most commonly used wet milling methods. It has been widely used for its excellent quality, simple method, low cost and easy expansion of production. The precipitation reaction of this method is one of the most important steps in wet milling, which has a decisive influence on the particle size and morphology of the final powder particles.
The physical model of particle size and morphology control of precipitated particles is also very complex. There is a coupling interaction between the product and the process. In the practical application process, it is necessary to make full use of the boundary condition, limit condition or some special condition of the system to simplify some of these items in order to calculate and solve , And the solution itself is very cumbersome.
Therefore, the morphology of powder particles and particle size control is a complex process.
3. Morphology and the significance of particle size control
The particle size and distribution of the powder are the most basic morphological features, which basically determine the overall and surface properties of the powder. In addition, the structural features of the powder include the shape of the powder, chemical composition, internal and external surface area, volume and surface defects, etc., together they determine the overall performance of the powder. Therefore, in recent years, the morphology and particle size control of powder has gradually become an important part of powder research.
In the preparation of most of the powder material in the process of particle size and morphology and other aspects of the special requirements. The application of different materials in the field of functional powder particle size and particle size requirements for the development of powder material preparation technology put forward a new topic, that is, in its preparation and processing of particle morphology and particle size control. Therefore, in the process of micro-nano powder preparation, according to its application needs to be powder structure, morphology control is very important.
In the process of preparation and processing of functional powder materials, particle morphology and particle size control are often dependent on the analysis of particle formation mechanism. The mechanism of particle formation is the basis of powder morphology and particle size control. The mechanism of particle formation is shown in Figure 3.1.
During the wet chemical precipitation process, the formation of powder particles undergoes nucleation, growth and agglomeration.
The so-called nano-powder agglomeration refers to the original nano-powder in the preparation, separation, processing and storage process interconnected by a number of particles to form a larger particle clusters phenomenon. As the agglomeration particle size is small, the surface atomic ratio is large, the specific surface area is large, the surface energy is large, is in the energy unstable state, so the fine particles tend to gather together, it is easy to agglomerate to form the secondary particles, Big.
In the wet chemical preparation of powder materials, agglomeration is the main way of particle growth, agglomeration mechanism can not only prepare monodisperse spherical particles, can also be prepared ellipsoid, rod and cube and other non-spherical powder. Wet chemical process In the process of milling, agglomeration is generally carried out at high concentrations, supersaturated and presence of surface active substances, and precursors are generally at nanoscale.
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