Development and application of high-performance boron nitride materials
As a new ceramic material with excellent performance and great development potential, boron nitride includes five isomers, namely hexagonal boron nitride (h-BN), cubic boron nitride (c-BN), fiber Zinc mineral boron nitride (w-BN), rhombohedral boron nitride (r-BN) and rhombic boron nitride (o-BN).
Applications of Boron Nitride
Current research on BN mainly focuses on its hexagonal phase (h-BN) and cubic phase (c-BN). The former has lubricity, thermal conductivity and good high-temperature performance; the latter is also in a thermodynamic equilibrium and stable state at normal temperature and pressure. The main application area of h-BN is as a raw material for the synthesis of cubic boron nitride.
Hexagonal boron nitride
Hexagonal boron nitride is a material with high temperature resistance, corrosion resistance, high thermal conductivity, high insulation and excellent lubrication properties. According to the current situation, simplifying the process, reducing production costs and increasing the service life of components are the current comparisons of this type of material. Active research directions. Main applications: crucibles, boats, liquid metal delivery pipes, rocket nozzles, high-power device bases, etc. for smelting evaporated metals. It can also be used as various material additives.
cubic boron nitride
Used as abrasive material. Small particles of cBN single crystal can be used as abrasive material. CBN abrasive tools use the action of a bonding agent to bond cBN abrasive grains into products with a certain geometric shape as a superhard material abrasive tool.
Used as tool material. PcBN overcomes the shortcomings of cBN single crystal, such as easy cleavage and anisotropy, and is mainly used to make tool materials. PcBN cutting tools are particularly suitable for high-speed cutting and can also be used for high-precision cutting. They have been widely used in CNC machine tools and are suitable for cutting high-hardness materials.
With the continuous advancement of science and technology and the increasing demand for applications, boron nitride has broad prospects for future development. Here are some possible trends:
Improve preparation efficiency: Improving preparation efficiency is one of the ways to achieve large-scale production of boron nitride, and developing more efficient and economical preparation methods is its development goal.
At present, the preparation efficiency of boron nitride is low, requires higher temperature and pressure conditions, and the preparation cycle is long. One of the future research directions is to develop more efficient and economical preparation methods to improve the preparation efficiency of boron nitride.
Develop new materials: In addition to conventional boron nitride materials, new materials such as two-dimensional boron nitride and porous boron nitride will receive attention. These new materials have unique structures and properties and are expected to be used in a wider range of fields.
Expand application fields: Boron nitride has been widely used in electronics, optoelectronics, materials science and other fields. Its excellent performance can expand more application fields in the future, such as biomedicine, environmental protection and other fields.
Improve performance and stability: The mechanical and chemical properties of boron nitride can be improved by controlling the crystal structure and purity to meet higher application requirements in the future.
Effect of ultrafine fly ash powder on cement properties
Fly ash is a small particle produced during the combustion process of coal-fired power plants. It is mainly composed of glass, minerals and carbon. Ultrafine powder refers to powder particles with a particle size less than 0.1 mm. In cement production, ultrafine fly ash powder can be used as an auxiliary cementing material to improve the performance of cement.
Effect of ultrafine fly ash powder on cement properties
1. Improve cement strength
Ultrafine fly ash powder can significantly improve the strength of cement. This is because the ultrafine fly ash powder has high activity and can react with the hydration products in the cement to form a denser structure, thus improving the strength of the cement. In addition, fly ash ultrafine powder can also fill the pores of cement, reduce the occurrence of cracks, and further enhance the strength of cement.
2. Improve cement fluidity
Fly ash ultrafine powder has good flow properties and can improve the fluidity of cement. Adding an appropriate amount of ultrafine fly ash powder to cement can reduce the viscosity of the mixture and improve its fluidity, making construction more convenient and faster.
3. Reduce cement hydration heat
Ultrafine fly ash powder can reduce the heat of hydration of cement. This is because the ultrafine fly ash powder can react with the minerals in the cement to form low-calorie compounds, thereby reducing the hydration heat of the cement. This is of great significance for the construction of large-volume concrete and can reduce the occurrence of temperature cracks.
4. Improve cement impermeability
Fly ash ultrafine powder can improve the impermeability of cement. This is because the ultrafine fly ash powder can react with the minerals in the cement to form a denser structure, reduce the generation of pores, and thus improve the impermeability of the cement. This is of great significance for projects such as basements that require waterproofing requirements.
Fly ash ultrafine powder is an industrial waste with high utilization value and can play an important role in cement production. By adding an appropriate amount of ultrafine fly ash powder, the properties of cement can be improved, increasing its strength, fluidity, impermeability and durability. At the same time, the application of ultrafine fly ash powder can also reduce cement production costs and environmental pollution, meeting the requirements of sustainable development.
Characteristics of conventional powders in the chemical industry
Characteristics of talc powder
Talcum powder, whose main component is hydrated magnesium silicate, is a white or off-white fine sand-free powder. It has excellent physical and chemical properties such as lubricity, fire resistance, acid resistance, insulation, high melting point, and chemical inertness.
Characteristics of kaolin clay
Kaolin, also known as dolomite, is a non-metallic mineral mainly composed of clay minerals of the kaolinite family, forming clay and clay rock.
In terms of chemical properties, kaolin has excellent electrical insulation properties, good acid solubility resistance, very low cation exchange capacity, high refractoriness and other physical and chemical properties.
Characteristics of mica powder
Mica powder is a non-metallic mineral whose main components are silica and aluminum oxide.
In terms of chemical properties, mica powder shows good acid and alkali corrosion resistance, high temperature resistance and other properties. In addition, plastic mica powder processed through special processes has the characteristics of high diameter-to-thickness ratio, high temperature resistance, acid and alkali resistance, and wear resistance. It is a natural functional powder filling material.
Characteristics of silica powder
Microsilica powder is a fine granular solid material with a particle size generally less than 1 micron. It is a new functional mineral raw material composed of natural microcrystalline quartz (a-quartz). It is mainly white or off-white.
Microsilica powder has a series of excellent properties: low thermal expansion coefficient, excellent dielectric properties, high thermal conductivity and good suspension performance.
Characteristics of aluminum hydroxide
In the chemical industry, aluminum hydroxide is mainly used as a flame retardant. It is not only flame retardant, but also prevents smoke, dripping, and toxic gases. Therefore, it has been widely used in electronics, chemicals, cables, plastics, rubber and other industries. In particular, ultrafine aluminum hydroxide has become the most widely used and widely used low-smoke, halogen-free material due to its multiple functions such as flame retardancy, smoke suppression, filling, and environmental protection.
Characteristics of alumina
Aluminum oxide, with the chemical formula Al2O3, is an inorganic substance. It is a compound with high hardness and a melting point as high as 2054°C. It is a typical ionic crystal and can be ionized at high temperatures.
Chemically, alumina is a highly hard material with a Mohs hardness of up to 9, which makes it widely used as a wear-resistant and corrosion-resistant material in many applications. Alumina has good thermal conductivity, and Al2O3 with high purity requirements is generally prepared by chemical methods.
In terms of industrial applications, aluminum oxide is widely used in the materials industry due to its high hardness, wear resistance and corrosion resistance.
Characteristics of barium sulfate
Barium sulfate is a colorless orthorhombic crystal or white amorphous powder with stable chemical properties and insoluble in water, acid, alkali or organic solvents. Barium sulfate is made from barite as the main raw material, and is processed through a series of processes such as mineral processing, mineral washing, and crushing.
Characteristics of diatomite
Diatomaceous earth is a naturally occurring inorganic mineral with colors such as white, off-white, gray and light gray brown, and has the characteristics of fine, loose, light and porous. It has very strong water absorption and permeability, so it is often used as a filler or anti-settling agent in paint, coating, rubber, plastic and other industries.
Diatomite also has good stability and is an important industrial material for heat insulation, grinding, filtration, adsorption, anticoagulation, demoulding, filling, carrier, etc.
Bentonite characteristics
Bentonite, also known as bentonite, bentonite or bentonite, is a non-metallic mineral whose main mineral component is montmorillonite.
The color of bentonite is usually white or light yellow, but due to changes in iron content, it may also appear light gray or light green.
Characteristics of transparent powder
Transparent powder is a new type of functional filler. It is a composite silicate. Its main component is a composite silicate containing magnesium, aluminum and calcium, which is an inorganic salt. Its characteristics are as follows:
1. High transparency
2. Good hardness and gloss
3. Low oil absorption
4. Good collapse resistance and less dust during use.
5. Ultra-transparent ultra-fine powder material is developed through the process of raw material selection-mixing-melting-rough grinding-fine grinding-grading.
Dry fine grinding for agrochemical applications
The reason why pesticide manufacturers develop specific components and dosage forms is to use the active ingredients at the right time and in the right amount when crops need protection, to effectively reduce factors that are detrimental to crop growth. Therefore, a plant protectant is essentially a mixture of different ingredients. These ingredients can basically be summarized into three major categories: active ingredients in the formula; fillers used to dilute the active substances, such as clay, talc, kaolin or silica; auxiliaries and additives used to improve the quality of the formula (such as stabilizers, Wetting agents, protective agents, defoaming agents, etc.).
In the pesticide production process, the first step is feeding and mixing; the second step is grinding. Through different types of grinding equipment shown below, the mixed material particles are ground and dispersed to the target fineness to meet the application requirements. After grinding, it goes through a screening process to prevent the possible presence of oversized particles. Finally, add additives or fillers that do not require grinding and perform dispersion and mixing again.
The reason why pesticide particles are required to be ultra-fine particles and have a narrow particle size distribution: the finer the active ingredient particles, the more effective they are, which means that a smaller amount can be used to achieve the same effect. This is beneficial for safety, environmental and economic reasons: reducing toxic effects on people in the spray area; reducing environmental pollution; reducing the use of the most expensive active ingredients in the formulation, thereby reducing pesticide production costs and increasing profits .
The narrow particle size distribution facilitates a simplified pesticide application procedure: the powder is dispersed in water before application on crops. The finer the particles, the more stable the suspension will be and no sedimentation will occur during operation. During the pesticide spraying process, the problem of large particles clogging the nozzles of the spraying system is effectively reduced.
Choosing the right mill is crucial, and ALPA offers different dry grinding technologies depending on the fineness and specifications required by the pesticide manufacturer.
Impact grinding machine CSM with classifying function
This type of classifying mill offers the possibility of achieving both grinding and classifying functions in one system. The CSM classifier is a combination of a fine impact classifier and a guide wheel classifier. Using two independent motor drives, one for the grinding disc and the other for the grading wheel, the CSM can precisely adjust the grading wheel speed to obtain a wide range of final product fineness from d97=9μm to 200μm. By utilizing the geometry of the classifier impeller and the air seal between the classifier wheel and the machine top cover, precise control of the upper limit of the particle size of the grinding material is ensured, thereby achieving fine classification.
Fluidized bed jet mill
This jet mill is suitable for ultra-fine grinding of materials of various hardnesses (soft to extremely hard). In the grinding area, the particles are driven by high-speed airflow to collide and grind with each other. There are no additional grinding parts. The dynamic classifier controls the maximum particle size. The air flow velocity at the nozzle outlet in the grinding chamber can reach 500 to 600 m/s. Because high grinding energy and impact speed can be generated in the fluidized bed, it is possible to achieve D50 fineness of 1 to 5 μm.
Due to such structural characteristics, the airflow mill has a very attractive feature: during the grinding process, there will be no temperature increase in the grinding chamber. The reason is that the heat generated when particles collide with each other is offset by the cooling phenomenon produced by the expanding compressed gas, so that the temperature in the grinding chamber remains constant and the active material molecules will not be destroyed.
Currently, pesticide production is of increasing strategic importance. There must be a re-evaluation to place greater emphasis on environmental constraints, both during the production of products and their use on agricultural crops. However, meeting the needs of the world's population remains a huge challenge. The role of chemical engineering is to produce pesticides in the best possible way, which requires selecting the most suitable grinding technology to achieve this.
Several impact ultra-fine grinding in industry process
The impact ultra-fine grinding process generally refers to the grinding and classification process to prepare particle size distribution d9, ≤10 micron. It can be divided into two types: dry method and wet method. The ultra-fine crushing unit operations (i.e. one-stage ultra-fine crushing) currently used in industry include the following types.
(l) Open circuit process. Generally, flat or disc type, circulating tube type and other airflow mills have self-grading function, so this open circuit process is often used. In addition, this process is often used for intermittent ultrafine grinding. The advantage of this process flow is that the process is simple. However, for ultra-fine grinders that do not have the function of self-classification, since there is no classifier in this process, qualified ultra-fine powder products cannot be separated in time, so the particle size distribution range of general products is wide.
(2) Closed-circuit process, which is characterized by a classifier and an ultra-fine grinder forming a closed-circuit system of ultra-fine crushing and fine classification. This process is often used in continuous powder operations of ball mills, stirring mills, high-speed mechanical impact mills, vibration mills, etc. Its advantage is that it can separate qualified ultrafine powder products in time, so it can reduce the agglomeration of fine particles and improve the efficiency of ultrafine crushing operations.
(3) The open circuit process with pre-classification is characterized by the fact that the materials are classified before entering the ultra-fine pulverizer. The fine-grained materials are directly used as ultra-fine powder products, and the coarse-grained materials then enter the ultra-fine pulverizer for crushing. When the feed contains a large amount of qualified ultrafine powder, using this process can reduce the load of the crusher, reduce the energy consumption per unit of ultrafine powder product, and improve the efficiency of the operation.
(4) Closed-circuit process with pre-grading. This process is essentially a combination of two processes. This combined operation not only helps improve crushing efficiency and reduce energy consumption per unit product, but also controls the particle size distribution of the product. This process can also be simplified to only one grader, that is, pre-grading and inspection grading are combined into the same grader.
(5) Open circuit process with final classification. The characteristic of this crushing process is that one or more classifiers can be installed after the crusher to obtain two or more products with different fineness and particle size distribution.
(6) With pre-grading and final grading open circuit process, this process is essentially a combination of two processes. This combined operation can not only pre-separate some qualified fine-grained products, but also reduce the load on the crusher, and the final classification equipment can obtain two or more products with different fineness and particle size distribution.
The number of crushing stages mainly depends on the particle size of the raw materials and the required product fineness. For raw materials with relatively coarse particle sizes, a process of fine crushing or fine grinding and then ultra-fine crushing can be used. Generally, the raw materials can be crushed to 200 mesh or 325 mesh and then an ultra-fine crushing process can be used; for product particle size requirements For materials that are very fine and easy to agglomerate, a multi-stage ultra-fine crushing process in series can be used to improve operating efficiency. However, generally speaking, the more crushing stages, the more complex the process flow and the greater the engineering investment.
In terms of grinding methods, ultra-fine grinding processes can be divided into three types: dry (one or more stages) grinding, wet (one or more stages) grinding, and dry-wet combined grinding. The following introduces several typical ultra-fine grinding process flows.
Application of ultra-fine grinding technology in cosmetics
Ultrafine grinding refers to the unit operation of crushing coarse-grained materials to a particle size of less than 10~25 μm. When the material is crushed to a particle size of less than 10 μm, the ultra-fine particles have high surface activity, void ratio and surface energy, thus giving the material Excellent solubility, adsorption, fluidity and unique optical, electrical, magnetic and other properties. Ultra-fine grinding technology is widely used in food, medicine, information materials, microelectronics, thermal insulation materials, advanced refractory materials, high-tech ceramics, Coatings, fillers and new material industries.、
As one of the most effective equipment for ultra-fine pulverization of powders, jet pulverizer uses supersonic airflow to impact materials to cause the materials to collide with each other to achieve the purpose of ultra-fine pulverization. Therefore, the jet pulverizer equipment is simple to operate, pollution-free, and has high product purity. High, good activity maintenance, good powder dispersion, small particle size and narrow distribution, smooth particle surface, especially suitable for ultra-fine crushing of heat-sensitive and moisture-sensitive drugs.
With the rapid development of the cosmetics industry in the past 20 years, a large number of bioactive substances and Chinese herbal medicine powders have been widely used in various cosmetics. However, the raw materials have large particles and are difficult to dissolve in water at low temperatures or are difficult to be absorbed by the skin when applied directly. By ultrafinely crushing the active ingredients, the dissolution temperature of the active ingredients can be greatly reduced, which is beneficial to the maintenance of activity and transdermal absorption. In addition, airflow crushing technology is used in the manufacturing of high-end pressed powder cosmetics to improve the powder structure and greatly improve the pressed powder performance and product quality. Airflow crushing technology has broad application prospects in the cosmetics industry.
1) Micronization technology is a complete set of processes and technologies, and it is a systematic process that must meet the requirements of cosmetics hygiene standards during the manufacturing process of cosmetics. To apply it to the industrialization of cosmetics, we should also combine the characteristics of the cosmetics industry to design ultra-fine grinding equipment that is easy to clean and disinfect, does not pollute products during the manufacturing process, does not produce dust, and has low energy consumption.
2) Strengthen the basic theoretical research on ultra-fine grinding, combine the properties of various powders, conduct module design on the basis of experiments, establish data models, develop multi-functional, integrated airflow grinding equipment, and improve comprehensive supporting performance and automatic control capabilities With the processing capacity of a single machine, it can obtain ultrafine powder with narrow particle size distribution, and can be adapted to the processing of materials with different characteristics and various hardnesses.
3) Find effective ways to reduce the wear and tear of airflow grinding equipment during crushing, extend the service life of the equipment, and reduce product pollution. Focus on solving the material problems of the airflow grinding chamber and nozzle ring, and develop alloy materials with high wear resistance. In addition, appropriate process flow is also an effective measure to reduce airflow abrasion.
4) Find effective ways to reduce energy consumption and improve energy utilization, and overcome the biggest shortcoming of low energy utilization of jet mills.
5) The development of airflow grinding technology will provide technical support for the development of high-quality, high-tech, and excellent cosmetics and enhance the market competitiveness of products. Airflow grinding technology can not only be widely used in pressed powder cosmetics and facial mask products, but also has broad application prospects in the pretreatment of active raw materials and Chinese herbal medicines.
Grinding of API in Oral Solid Dosage Process
In the production process of oral solid dosage forms, bulk drug crushing is often an extremely critical unit operation. On the one hand, the particle size of the API may affect drug absorption. For poorly soluble oral solid preparations, the smaller the particle size of the raw material, the faster the dissolution, and the bioavailability of the drug may also be improved. In addition, the particle size of the API has an important impact on the fluidity of the powder, the mixing process and the stratification of the powder, and these factors have an important impact on the stability of the production process.
In the synthesis process, raw materials for oral solid dosage forms are often obtained by crystallization. By controlling the crystallization process, the particle size of the raw material drug can be controlled to a certain extent. However, in many cases, the particle size and particle size distribution of the API obtained by crystallization often cannot meet the needs of the preparation. Therefore, it is necessary to further process the API during preparation production, that is, crush the API to control the particle size within the target range.
Generally speaking, grinding methods can be divided into dry and wet methods according to the different media dispersed during grinding. The wet method is to disperse the API in a liquid medium for pulverization, while the dry method is to pulverize the API in a gas (air, nitrogen, etc.). The dry method is mostly used for crushing raw materials of solid preparations.
The crushing principle of the hammer mill is mainly to continuously beat the raw drug particles through high-speed rotating hammers/hammers, and the particles further collide with the crushing cavity or between particles. These processes can effectively reduce the particle size. When the particle size is small enough to pass through the selected sieve holes, it will be discharged from the crushing chamber. The hammer mill has a large production capacity and low energy consumption, and is more suitable for crushing brittle drugs. Some viscous materials are not prone to particle breakage through mechanical beating and are not suitable for hammer crushing. However, the materials can be cooled to increase the brittleness of the materials and increase the ease of crushing. In addition, hammer crushing generates serious heat, so attention must be paid to the stability of the material. Compounds with a melting point lower than 100°C are not suitable for mechanical crushing methods such as hammer crushing. Hammer mills are generally suitable for crushing particle sizes above 10 μm. Factors related to the crushing effect of the hammer mill generally include the shape and installation method of the hammer blade, rotation speed and feed speed, etc.
Spiral jet pulverizer is a relatively common airflow pulverizer with relatively simple mechanical structure and crushing operation. The pressurized air flow brings the materials into the crushing chamber at a certain speed through the feeding nozzle. There are several nozzles on the same plane around the annular crushing chamber, which spray airflow with a speed of up to 300~500 meters/second into the crushing chamber, forming a vortex airflow, causing the particles entering the crushing chamber to move at high speed with the airflow, and the particles and other particles or the crushing chamber The body was shattered by violent collision and friction. The crushing process mainly involves the collision between particles, followed by the collision between particles and the crushing cavity. The circular motion of particles in the airflow will generate a certain centrifugal force. As the crushing progresses, the particle size and mass decrease, and the centrifugal force received becomes smaller and smaller. When the centrifugal force is small enough, the airflow discharged from the crushing chamber will bring the particles to the center of the vortex airflow, and then be discharged from the crushing chamber with the airflow to complete the crushing process. This vortex airflow allows the crushing and classification processes to be carried out simultaneously, which is beneficial to obtaining a final product with a narrower particle size distribution.
Application and research of modified wollastonite
Wollastonite is an extremely important non-metallic mineral. Its main chemical composition is calcium metasilicate (CaSiO3). It belongs to the trigonal crystal system and is gray-white. Wollastonite has a large aspect ratio, a natural needle-like structure, and stable performance, making it an excellent reinforcing material. In addition to its natural fibrous structure, wollastonite also has extremely low oil absorption, electrical conductivity and dielectric loss. It is widely used in plastics, rubber, paints, coatings and other fields, and can significantly improve the mechanical and tribological properties of the matrix. Improve the thermal stability and dimensional stability of products.
However, natural wollastonite is hydrophilic, and when blended with organic polymers, the dispersion is uneven due to different polarities, thus reducing the mechanical properties of its filled products. In order to improve its dispersion and compatibility in organic matrices, as well as the mechanical properties of products, wollastonite often needs to be surface modified.
Wollastonite modification technology
The surface modification technology of wollastonite can be divided into: organic surface modification and inorganic surface modification.
For organic surface modification, commonly used surface modifiers include silane coupling agents, titanate and aluminate coupling agents, surfactants, and methyl methacrylate. Among them, silane coupling agent modification is one of the commonly used surface modification methods for wollastonite powder, and dry modification process is generally used. The dosage of coupling agent is related to the required coverage and the specific surface area of the powder. The dosage is generally 0.5% to 1.5% of the wollastonite mass.
The technical background of inorganic surface modification is that wollastonite as a polymer filler often causes the filler material to become darker in color, has a larger wear value, and easily wears processing equipment; inorganic surface coating modification can improve the silicone Gray stone fiber fills the color of polymer materials and reduces their wear value. At present, the inorganic surface modification of wollastonite mineral fibers mainly uses chemical precipitation method to coat the surface with nanometer calcium silicate, silica and nanometer calcium carbonate.
Application and research of modified wollastonite
(1) Plastic
Polypropylene (PP), as one of the five general-purpose plastics, has better comprehensive properties than other general-purpose plastics. It is increasingly widely developed and used in the fields of automobiles, aerospace, construction and medicine.
(2) Papermaking
The application of wollastonite in the paper industry is quite different from other fillers. It is not a simple filling like traditional fillers. It mainly relies on a higher aspect ratio to realize the interweaving of wollastonite and plant fibers to form plant fibers. The network structure of fiber-mineral fiber can replace some short fibers of plants, which can effectively improve the opacity and printing adaptability of produced paper, improve uniformity and reduce manufacturing costs.
(3) Friction materials
Wollastonite products for friction materials are wollastonite needle-like powders. Compared with traditional application scenarios, they are mostly used as fillers in brake pads, clutches, etc. The acicular powder of wollastonite is an ideal substitute for short fiber asbestos. It can improve the stability of friction materials, reduce cracking, improve wear resistance and recovery properties and other mechanical properties to a certain extent.
(4) Coating
Wollastonite can be used as an extender pigment and a partial substitute for white pigments in paints. In addition, according to the characteristics of wollastonite itself, it can also be used as a coating modification additive to expand the functionality of the material. For example, wollastonite has good corrosion resistance and can be widely used in the field of anti-corrosion coatings.
(5) Rubber
In the rubber industry, wollastonite powder can replace part of titanium dioxide, white carbon black, clay, light calcium, lithopone and other materials, play a certain reinforcing effect, and can improve the hiding power of some colorants.
(6) Cement/fiber reinforced concrete
Fibrous wollastonite replaces short asbestos fibers and glass fibers and is added to cement, concrete and other building materials, which can improve the impact resistance, bending strength, wear resistance and dimensional stability of the materials.
The importance of silicon carbide powder modification process
Silicon carbide (SiC) is an inorganic non-metallic material with a wide range of uses and good development prospects. After being made into ceramics, it is an excellent structural material. It has high elastic modulus and specific stiffness, is not easy to deform, and has High thermal conductivity and low thermal expansion coefficient have now become one of the main considerations for high-temperature heat engine materials, and can be used in high-temperature nozzles, turbine blades, turbocharger rotors, etc.
Therefore, the industry has put forward higher requirements for SiC ceramics in terms of geometric accuracy, strength, toughness and reliability, and the molding process is a crucial part. Different molding processes have a greater impact on the performance of ceramic products, such as difficulty in demoulding, difficulty in preparing products with complex shapes, insufficient density of ceramics, etc. The existence of these defects will restrict their application in high-end fields. Therefore, it is necessary to To prepare ceramic products with excellent performance and high reliability, it is necessary to explore the factors that affect the effectiveness of the molding process.
The silicon dioxide layer on the surface of silicon carbide will affect the dispersion of the powder in the aqueous phase. The silicon dioxide will form silicon hydroxyl groups "Si-OH" in the water phase. The silicon hydroxyl groups are acidic in the water phase, so the dispersion of silicon carbide is The isoelectric point is acidic. The more silicon dioxide there is, the closer the isoelectric point of silicon carbide is to the acidic end. When the pH value is lower than the isoelectric point of the powder, silanol will attract hydrogen ions, making the particle surface positively charged and thus the Zeta potential. becomes a positive value. Under alkaline conditions, silanol will react with the high concentration of OH- in the solution to form [Si-O]- on the surface of the powder, making the surface of the particles negatively charged, so the Zeta potential is also negative.
The dispersion of powder in the water phase is closely related to the absolute value of Zeta potential, so the silica layer formed on the surface of the powder plays a great role in the dispersion of the powder.
Chemical modification method refers to the chemical reaction that occurs during the surface coating process. This is the most common method in powder modification. Surface coating is divided into two types: inorganic coating and organic coating. It mainly deposits a layer of oxide, hydroxide or organic matter on the surface of inorganic powder. When the coating is an oxide or hydroxide, it is called inorganic coating. When the coating is organic, it is called organic coating.
Inorganic coating methods mainly include alkoxide hydrolysis method, uniform precipitation method, non-uniform nucleation method, and sol. Gel method, etc. Among them, the best method is the non-uniform nucleation method. Organic coating modification improves the electrostatic and steric hindrance of inorganic powder, thereby improving its dispersion. Organic coating methods mainly include organic surface grafting, surface adsorption coating and encapsulation modification. It is mainly used in the dispersion of inorganic composite materials or fillers to improve the wettability and compatibility of inorganic powders and organic matrices. It is also used to improve the dispersion of inorganic powder in water.
Highly dispersible micron-sized SiC powder is a necessary condition for obtaining ceramic products with high accuracy, strength, toughness and reliability. Therefore, it is of great significance to explore related technologies to prepare silicon carbide ceramics that can be used in high-end fields.
Important steps in the production of diamond powder - grinding and shaping
At present, the most common diamond powder is produced by grinding, purifying, classifying and other processes of artificial diamond.
Among them, the diamond crushing and shaping process plays an important role in the production of micro powder, and directly affects the shape of the micro powder particles and the content of the target particle size. Different crushing methods will produce different crushing effects. The scientific and reasonable crushing and shaping process can not only quickly crush coarse-grained diamond raw materials (conventional particle size 100-500 microns) into diamond powder particles with a particle size range of approximately (0-80 microns), but also optimize the particle shape. , making the particles of micro powder products more round and regular, reducing or even completely eliminating long strips, flakes, pins and rods and other particles that affect the final quality of micro powder. Maximize the proportion of marketable target particle size output.
In the production of micro powder, the crushing method can be divided into dry method and wet method. Different crushing and shaping methods are used, and their working principles and process parameters are also different.
Process control points of dry grinding method of ball mill
Taking the horizontal ball mill dry grinding method as an example, the main process control points are ball mill speed, ball-to-material ratio, filling coefficient, steel ball ratio, etc. In actual production, they can be flexibly controlled according to different raw materials and the purpose of crushing and shaping.
1. Ball mill speed
The reasonable rotation speed of the ball mill is an important condition for exerting its production capacity. When the diameter of the ball mill barrel is the same. The higher the rotational speed, the greater the centrifugal force generated, and the higher the distance the steel ball is driven to rise along the cylinder wall.
It is generally believed that the suitable working speed of the ball mill is 75%-88% of the theoretical critical speed.
2. Filling coefficient, ball-to-material ratio
In the crushing and shaping process, the appropriate ball-to-material ratio and filling coefficient are crucial. If the ball-to-material ratio and filling coefficient are too high or too low, they will affect the production efficiency and product quality of the ball mill. If the ball-to-material ratio is too high or the filling coefficient is too low, the feeding capacity of a single machine will be restricted.
Practice has proved that for the crushing of diamond raw materials, the loading coefficient is generally 0.45. The ratio of ball to material is 4:1.
3. Steel ball diameter and ratio
In order to crush diamond more effectively, when the ball mill filling coefficient and ball loading amount are determined, steel balls of different diameters should be selected and assembled in proportion to obtain better particle shape and faster crushing and shaping efficiency.
Segmented grinding
In the production process of micro powder, wet crushing is more effective than dry crushing. Because when dry crushing reaches a certain fineness, wall sticking is easy to occur, reducing the crushing effect; with wet crushing, the raw materials always exist in the form of slurry, and it is easy to increase the proportion of fine particle size.
In order to control the particle size ratio, when more fine-grained micro powder needs to be produced, segmented crushing should be used, especially wet segmented crushing is better. This can not only avoid excessive crushing of materials, but also achieve segmentation according to strength during the crushing process.
Jet milling
Another crushing method is the airflow pulverizer crushing method. The airflow pulverizer uses compressed air as the working medium. The compressed air is sprayed into the crushing chamber at high speed through a special supersonic nozzle. The airflow carries the material in high-speed movement, causing the material to move between them. Produce strong collision, friction and shear to achieve the purpose of crushing. Fragmentation occurs when the force acting on the particle is greater than its failure stress. High-speed impact collision causes volumetric fragmentation of particles, while shearing and grinding effects cause surface fragmentation of particles. This crushing method is very beneficial to the production of diamond powder because it can produce ideal particle shapes. The biggest advantage of the airflow pulverizer is that it is not limited by the mechanical linear speed and can produce very high airflow speeds. In particular, the supersonic airflow pulverizer can produce a flow rate several times the speed of sound, so it can generate huge kinetic energy and it is easier to obtain micron-level particles. and submicron ultrafine powders.