How does barium sulfate contribute to the creation of high-quality coating materials?
Barium sulfate is highly favored primarily due to its exceptional filling capacity. This means that while maintaining paint film performance, it can effectively optimize formulation costs and is widely used in various fields, from industrial coatings to decorative paints.
More importantly, thanks to its small particle size, uniform distribution, large specific surface area, and excellent fluidity, barium sulfate exhibits very low abrasiveness during processing. This characteristic directly translates into production efficiency: it significantly reduces wear and tear on mixing, pumping, and spraying equipment, extending equipment lifespan and making the production process smoother and more economical.
This advantage is fully demonstrated in the application of automatic primer surface coatings. Even under high filling rate production requirements, barium sulfate ensures excellent stability and leveling properties of the paint slurry, resulting in exceptional uniformity and smoothness. This provides a flawless "canvas" for subsequent topcoat application, which is crucial for achieving efficient, automated, and high-quality coating.
Barium sulfate is far more than just a simple filler. It is a multifunctional additive that combines high filling capacity, low abrasion, and excellent leveling properties. Choosing it means selecting a reliable "foundation of quality" for your coatings, enhancing product performance while also ensuring efficient production.
Applications of advanced ceramic materials
Applications in High-Speed Aircraft
High-speed aircraft are strategic equipment that major military powers are vying to develop. Their supersonic flight and sharp structures lead to serious aerodynamic heating problems. The typical thermal environment for high-speed aircraft involves high temperatures and complex, harsh thermo-mechanical loads. Existing high-temperature alloys can no longer meet the requirements, leading to the emergence of ceramic matrix composites. In particular, SiCf/SiC composite ceramic materials have been widely used in hot structural components such as turbine blades, nozzle guide vanes, and turbine outer rings of aero-engines. Their composite material density is approximately 1/4 that of high-temperature alloys, resulting in significant weight reduction. Furthermore, they can operate at temperatures up to 1400°C, greatly simplifying cooling system design and enhancing thrust.
Applications in Lightweight Armor
Lightweight composite armor is crucial for maintaining the survivability of modern equipment. The development of ceramic fibers and fiber-reinforced ceramic matrix composites is fundamental to the application of lightweight composite armor. Currently, the main protective ceramic materials used include B4C, Al2O3, SiC, and Si3N4. Silicon carbide ceramics, with their excellent mechanical properties and cost-effectiveness, have become one of the most promising bulletproof ceramic materials. Their diverse applications in various armor protection fields, including individual soldier equipment, army armored weapons, armed helicopters, police and civilian special vehicles, give them broad application prospects. Compared to Al2O3 ceramics, SiC ceramics have a lower density, which is beneficial for improving equipment mobility.
Applications in Small Arms
Small arms, as an important component of weaponry, generally include pistols, rifles, machine guns, grenade launchers, and special individual equipment (individual rocket launchers, individual missiles, etc.). Their main function is to launch projectiles to the target area to kill or destroy enemy targets. The operating conditions of small arms include high temperature, low temperature, high altitude, humid heat, dust, rain, dust-rain, salt spray, and immersion in river water. Corrosion resistance is crucial. Currently, the main anti-corrosion processes for small arms include bluing, hard anodizing, ion-controlled penetration technology, diamond-like carbon coatings, and plasma nitriding. Especially for weapons and equipment used in marine environments, the requirement for corrosion resistance in salt spray environments for more than 500 hours poses a significant challenge to traditional coating treatments.
Applications in Gun Barrels
The gun barrel is a core component of projectile weapons. The internal structure of the gun barrel includes the chamber, the forcing cone, and the rifling, with the chamber and rifling connected by the forcing cone. Traditional gun barrels are generally made of high-strength alloy steel. During firing, the inside of the gun barrel is subjected to the combined effects of propellant gases and projectiles, leading to cracks and coating detachment on the inner wall of the barrel. The damage to the gun barrel's bore is a result of the repeated action of high-temperature, high-pressure, and high-velocity propellant gases and projectiles on the barrel wall. The forcing cone and the muzzle are usually the first parts to fail.
To improve gun barrel life, chromium plating of the bore is the most common method, but the oxidation resistance temperature of the chromium plating layer does not exceed 500°C. With the continuous increase in chamber pressure during firing and the exponential increase in gun barrel life requirements, the pressure and temperature borne by the gun barrel are also increasing. Utilizing the high hardness, high strength, and high-temperature chemical inertness of ceramics can effectively reduce gun barrel erosion and extend its service life.
Applications in Ammunition
The main components of ammunition are the warhead and the fuze. As the most direct component for causing damage, the warhead mainly consists of the casing, fragmentation elements, explosive charge, and fuze. Continuously improving the lethality of the warhead has always been a goal pursued in weapon development. Especially for area-effect grenades, the fragments produced by the warhead explosion are the terminal killing elements, and efficient fragmentation technology has always been a research challenge in this field.
Four major application areas of silicon nitride ceramics
Mechanical Field
Silicon nitride ceramics are mainly used as valves, pipes, classification wheels, and ceramic cutting tools in the mechanical industry. The most widespread application is silicon nitride ceramic bearing balls. Silicon nitride ceramics are widely recognized as the best bearing material, and the most critical "core player" in bearings—silicon nitride ceramic bearing balls—are the true "unsung heroes" that support equipment performance. These small ceramic balls, ranging in diameter from a few millimeters to tens of millimeters, may seem insignificant, but with their "lightweight, hard, stable, and insulating" properties, they play a "key role" in new energy vehicles, roller skates, dental drills, and even high-end bicycles.
Aerospace Field
Silicon nitride ceramic materials have advantages such as high strength, high temperature resistance, and good chemical stability, which can meet the stringent requirements for materials in the aerospace field. Silicon nitride ceramics have two classic applications in the aerospace field: firstly, silicon nitride is considered one of the few monolithic ceramic materials capable of withstanding the severe thermal shock and thermal gradients generated by hydrogen/oxygen rocket engines, and is used in rocket engine nozzles; secondly, the excellent properties of silicon nitride ceramics and their composites, such as heat resistance, wave transmission, and load-bearing capacity, make them one of the new generation of high-performance wave-transmitting materials under research.
Semiconductor Field
As electronic devices develop towards miniaturization and high performance, semiconductor packaging requires higher demands on heat dissipation materials. Silicon nitride ceramics have a thermal conductivity of up to 90-120 W/(m·K), and a high degree of matching with the thermal expansion coefficient of third-generation semiconductor substrate SiC crystals, making them the preferred material for SiC power device packaging substrates. Internationally, Japanese companies such as Toshiba and Kyocera dominate the market, while domestic companies such as Sinoma Advanced Materials have achieved technological breakthroughs.
In addition to being a key packaging material, silicon nitride ceramics show broad application prospects in semiconductor manufacturing equipment. In the wafer processing process, silicon nitride ceramics can be used to manufacture high-temperature resistant and thermal shock resistant heating elements, meeting the stringent operating conditions of equipment such as CVD (chemical vapor deposition) and diffusion furnaces.
Biomedical Field
As an emerging bioceramic material, silicon nitride shows great application potential in medical implants due to its excellent mechanical properties and biocompatibility. Specifically, silicon nitride has been used as an orthopedic biomaterial and successfully applied in bearing components of prosthetic hip and knee joints to improve wear resistance and extend the lifespan of the prostheses. Furthermore, silicon nitride materials have been used to promote bone fusion in spinal surgery. Silicon nitride ceramic materials demonstrate excellent stability and reliability in the medical field. Silicon nitride also exhibits strong cell adhesion and osteoconductivity, providing an important biological basis for its application in bone repair. However, the inherent brittleness of silicon nitride ceramics remains a major challenge for its application in bone repair engineering. In addition, silicon nitride materials are difficult to degrade in vivo, which hinders the growth of new bone tissue into the repair site and its complete replacement of the original repair material, thus limiting the breadth of its clinical applications.
Why is aluminum hydroxide so effective in treating stomach problems?
Aluminum oxide, also known as alumina, with the chemical formula Al2O3, is the second most abundant oxide in the Earth's crust after silicon dioxide, and is widely found in minerals such as feldspar and mica. Industrially, it is often refined from natural mineral raw materials—bauxite—to obtain alumina.
In a broader sense, aluminum oxide is a general term for aluminum oxides and aluminum hydroxides, a class of compounds composed of aluminum, oxygen, and hydrogen. Due to its multiple forms and properties, aluminum oxide can be divided into hydrated and anhydrous aluminum oxide.
Common hydrated aluminum oxides include industrial aluminum hydroxide, gibbsite, boehmite, pseudoboehmite, diaspore, corundum, and tohdite. Among these, industrial aluminum hydroxide, gibbsite, and boehmite are trihydrated aluminum oxides, diaspore and corundum are monohydrated aluminum oxides, and pseudoboehmite and tohdite are polyhydrated aluminum oxides.
In a broader sense, aluminum hydroxide is a general term for monohydrated aluminum oxide (meta-aluminum hydroxide) and trihydrated aluminum oxide (ortho-aluminum hydroxide). Hydrated aluminum oxide is not a true hydrate of aluminum oxide, but rather emphasizes a crystalline structure of aluminum hydroxide, where aluminum and hydroxide ions are connected by ionic bonds, and all hydroxide ions are equivalent. Aluminum hydroxide is usually a white powder, odorless, non-toxic, inexpensive, and widely used. Aluminum hydroxide is best known for its use as a flame retardant added to polymer matrix materials, where it exhibits excellent flame retardant properties.
Have you noticed that in daily life, aluminum hydroxide is often used to make stomach medicine? It has antacid, adsorbent, local hemostatic, and ulcer-protective effects. Aluminum hydroxide gel can be used to neutralize stomach acid and has a therapeutic effect on some common stomach diseases.
The principle is simple: aluminum hydroxide is a typical amphoteric oxide; it can react with both acids and bases. Therefore, aluminum hydroxide can neutralize or buffer stomach acid. When aluminum hydroxide reacts with stomach acid, the resulting aluminum chloride has an astringent effect, which can provide local hemostasis, but may also cause constipation as a side effect. Aluminum hydroxide, when mixed with gastric juice, forms a gel that covers the surface of ulcers, creating a protective film. This film isolates the gastric mucosa from irritation and damage caused by gastric acid, pepsin, and other harmful substances, promoting the repair and healing of the gastric mucosa and aiding in the treatment of gastritis, gastric ulcers, and other related diseases.
Secondly, aluminum ions bind with phosphates in the intestines to form insoluble aluminum phosphate, which is then excreted in the feces. Therefore, in patients with uremia, taking large amounts of aluminum hydroxide can reduce the absorption of intestinal phosphates, thereby alleviating acidosis.
Furthermore, nanoscale aluminum hydroxide can be used as a drug carrier to encapsulate drugs or antigens, improving drug stability and targeting. In addition, aluminum hydroxide is often used as a pharmaceutical excipient in the preparation of oral medications and vaccines, ensuring drug stability and safety.
Aluminum hydroxide: Why can't it be used directly?
Inorganic amphoteric hydroxides—aluminum hydroxide (Al(OH)3, ATH)—possess highly efficient flame retardant, smoke suppressant, and filling properties. Upon thermal decomposition, it does not produce toxic or corrosive gases and can be used as a flame retardant filler in polymeric organic materials. Currently, the use of ATH as a flame retardant is increasing year by year, and ATH has become the most important inorganic flame retardant globally.
Modification First, Then Flame Retardancy
Generally, manufacturers typically fill flammable materials with powdered aluminum hydroxide (ATH) or coat the surface of flammable materials with a flame-retardant coating containing ATH to improve the flame-retardant properties of polymeric organic materials.
Furthermore, because ATH contains three hydroxyl groups (-OH), its surface is asymmetrical and highly polar. The surface hydroxyl groups exhibit hydrophilic and oleophobic properties, making it prone to agglomeration when added to polymeric organic materials, directly affecting the material's mechanical properties.
Therefore, aluminum hydroxide needs to be surface modified before use.
Surface Modification of Aluminum Hydroxide
Surface modification is one of the key technologies for optimizing the properties of inorganic powder materials, playing a crucial role in improving the application performance and value of inorganic powders. Surface modification of inorganic particles refers to the adsorption or encapsulation of one or more substances on the surface of inorganic particles, forming a core-shell composite structure. This process is essentially a composite process of different substances.
Types and Characteristics of Modifiers
There are many types of powder surface modifiers, but there is no standard classification method. Modifiers for inorganic powder modification are mainly divided into two categories: surfactants and coupling agents.
(1) Coupling Agents
Coupling agents are suitable for various composite material systems of organic polymers and inorganic fillers. After surface modification with coupling agents, the inorganic material's compatibility and dispersibility with the polymer are increased. The surface of the inorganic material changes from hydrophilic and oleophobic to oleophilic and hydrophobic, increasing its affinity with the organic polymer.
Coupling agents are diverse and can be classified into four main categories based on their chemical structure and composition: organic complexes, silanes, titanates, and aluminates.
(2) Surfactants
Surfactants are substances that can significantly alter the surface or interfacial properties of a material when used in very small amounts. They include anionic, cationic, and nonionic surfactants, such as higher fatty acids and their salts, alcohols, amines, and esters. Their molecular structure is characterized by a long-chain alkyl group at one end, similar to polymer molecules, and polar groups such as carboxyl, ether, and amino groups at the other end.
How can the modification effect be determined?
Is modified aluminum hydroxide reliable? How reliable is it? This requires evaluating and characterizing the modification effect.
Currently, the flame-retardant effect of aluminum hydroxide flame retardants can be evaluated through direct methods such as testing the material's oxygen index, vertical and horizontal flammability index, smoke production, thermogravimetric analysis, and mechanical properties during combustion; or indirectly by measuring powder absorbance, activation index, and oil absorption value to indirectly test its modification effect.
(1) Absorbance
Unmodified ATH has hydrophilic and oleophobic hydroxyl groups on its surface, allowing it to dissolve in water or settle freely to the bottom. After modification, the surface of ATH becomes hydrophilic and oleophobic, with surface properties completely opposite to the unmodified form. It cannot dissolve or settle to the bottom and can only float on the surface. However, modified ATH can dissolve or precipitate well in oils (such as liquid paraffin).
(2) Activation Index
Unmodified ATH has very strong polarity due to the nature of its surface hydroxyl groups (-OH), allowing it to dissolve or settle freely in water with similar properties. After modification, ATH has a layer of lipophilic groups attached to its surface, with surface hydroxyl groups (-OH) encapsulated within. The better the modification effect, the higher the lipophilic group coverage rate of the ATH surface, and the more modified ATH floats on the water surface.
(3) Oil Absorption Value
Measuring the oil absorption value requires adding castor oil to ATH and stirring. Before modification, ATH, due to its hydrophilic and oleophobic properties, requires more castor oil to form spheres. After surface modification, it becomes hydrophilic and oleophobic, improving the dispersibility of ATH in the polymer and reducing voids formed by powder agglomeration.
Understanding Super Strong Materials—NdFeB
Sintered NdFeB, as the earliest preparation process and the most universally applicable, has driven the rapid development of rare earth permanent magnet materials. Sintered NdFeB, with its strong magnetic anisotropy and low-cost raw material input, has become a research target for many countries. Sintered NdFeB permanent magnet materials utilize powder metallurgy. The smelted alloy is made into powder and pressed into a compact in a magnetic field. The compact is then sintered in an inert gas or vacuum to achieve densification. Furthermore, to improve the coercivity of the magnet, aging heat treatment is usually required. The process flow is as follows: raw material preparation → smelting → powder preparation → pressing → sintering and tempering → magnetic testing → grinding → machining → electroplating → finished product.
Unlike sintered NdFeB, the individual powder particles of bonded magnets need to have sufficiently high coercivity. Once the multiphase structure and microstructure required for high coercivity are severely damaged during the powder preparation process, it will be impossible to produce good bonded magnets. Therefore, by using the method of melt-spinning rapid quenching magnetic powder, the hot molten alloy is first poured or sprayed onto a high-speed rotating water-cooled copper wheel to form a thin strip with a thickness of 100 μm.
The manufacture of hot-pressed/hot-deformed magnets requires starting with rapidly quenched Nd-Fe-B magnetic powder, rather than directly using cast alloys. By employing over-quenching (rapid cooling) conditions, finer grains, or even amorphous magnetic powder, are prepared. During hot pressing and hot deformation, the grains are heated and grown to near single-domain size, thus achieving high coercivity in the final magnet. The hot pressing process involves placing the magnetic powder in a mold and applying pressure at high temperature to force it into an isotropic, solid-density magnet.
Application
Permanent Magnet Motors
In permanent magnet motors, the use of permanent magnets for excitation not only reduces power consumption and saves energy, but also improves motor performance.
Magnetic Machinery
Magnetic machinery operates using the repulsive force of like poles or the attractive force of unlike poles in magnets. This requires permanent magnets with high remanence and high intrinsic coercivity. Furthermore, due to the principle of attraction between unlike poles, magnetic drives can be constructed using non-contact transmission, offering advantages such as no friction and noise. Therefore, high-performance Nd-Fe-B magnets are widely used in drive components of mining machinery, magnetic bearings in gyroscopes and turbines in satellites and spacecraft, and rotor bearings in centrifugal pumps for assisting cardiac function in medical equipment.
Aerospace
Rare-earth permanent magnet materials are indispensable for rocket launches, satellite positioning, and communication technologies. High-performance sintered Nd-Fe-B is particularly useful in microwave transmitting/receiving systems for radar. Utilizing the combined effect of a constant magnetic field and an alternating microwave magnetic field, ferromagnetic resonance occurs, allowing for the fabrication of microwave circulators, isolators, etc. Consumer Electronics
3C consumer electronics has always been an important downstream industry for sintered NdFeB. Sintered NdFeB possesses characteristics such as high magnetic energy product, which aligns with the miniaturization, lightweighting, and thinning trends in 3C consumer electronics products. It is widely used in electronic components such as VCMs, mobile phone linear motors, cameras, headphones, speakers, and spindle drive motors.
Neodymium iron boron waste recycling: an unmissable treasure trove
Neodymium iron boron (NdFeB) permanent magnets are widely used in wind power generation, new energy vehicles, and electronic products due to their excellent magnetic properties, earning them the title of "King of Magnets." However, the scrap rate in the NdFeB magnet production process is as high as 30%, and coupled with their limited lifespan, this results in a large amount of NdFeB waste.
These wastes contain up to 30% rare earth elements, far exceeding the content of primary rare earth ores, making them a highly valuable secondary resource. Efficiently recovering rare earth elements from NdFeB waste is crucial for ensuring rare earth resource security, reducing environmental pollution, and promoting sustainable development.
Characteristics and Sources of NdFeB Waste
NdFeB waste mainly originates from scraps, defective products, and retired electronic products containing magnets during the magnet manufacturing process. Its chemical composition is complex; in addition to the main rare earth elements Nd and Pr, elements such as Dy and Tb are often added to improve coercivity, and elements such as Co, Al, and Cu are added to improve overall performance. Based on rare earth element (REE) content, NdFeB waste can be classified into three categories: low rare earth (REEs < 20%), medium rare earth (20%–30%), and high rare earth (> 30%).
Currently, the recycling processes for NdFeB waste are mainly divided into pyrometallurgical, hydrometallurgical, and novel recycling technologies.
(I) Pyrometallurgical Recycling Processes
Pyrometallurgical recycling separates rare earth elements from iron through high-temperature reactions. The main methods include selective oxidation, chlorination separation, liquid alloying, and slag-metal fusion separation.
Selective oxidation is based on the fact that rare earth elements have a much higher affinity for oxygen than iron. At high temperatures, rare earth elements are selectively oxidized to form oxides, which are then separated from metallic iron. Nakamoto et al. successfully prepared mixed rare earth oxides with a purity exceeding 95% and a recovery rate exceeding 99% by precisely controlling the oxygen partial pressure.
Chlorination separation utilizes the strong affinity between rare earth elements and chlorine. Chlorinating agents such as NH4Cl, FeCl2, or MgCl2 are used to convert rare earth elements into chlorides before separation. Uda used FeCl2 as a chlorinating agent, reacting at 800℃, achieving a rare earth recovery rate of 95.9% and a product purity exceeding 99%.
The liquid alloying method utilizes the difference in affinity between rare earth elements and iron for other metals to achieve effective enrichment and separation of rare earth elements and iron. Rare earth element Nd can form various low-melting-point alloys with Ag, Mg, etc.
The slag-metal separation method is based on the characteristic that rare earth elements in NdFeB waste more readily combine with oxygen. All the metals in the NdFeB waste are converted into metal oxides. Simultaneously, under the high temperature of a slagging agent, the iron oxides are converted into metallic Fe by controlling the reducing conditions.
(II) Wet Recovery Process
Wet recovery is currently the most widely used method, mainly including the total dissolution method, hydrochloric acid preferential dissolution method, double salt precipitation method, and solvent extraction method.
(III) New Recycling Processes
New recycling technologies aim to solve the problems of high energy consumption and high pollution associated with traditional methods, including hydrogen explosion, bioleaching, and electrochemical methods.
Comparison of Different Recycling Processes and Environmental Impact
Pyrometallurgical processes have short flow rates and large processing capacities, but high energy consumption and difficulty in separating single rare earth elements; hydrometallurgical processes have high recovery rates and high product purity, but high acid consumption and high wastewater treatment costs; newer processes such as bioleaching and electrochemical methods are environmentally friendly, but are mostly in the laboratory stage and have not yet been applied on a large scale.
In terms of environmental impact, traditional recycling processes often use strong acids, strong alkalis, and high temperatures, generating large amounts of waste liquid and waste gas, increasing the environmental burden. Therefore, developing green and low-consumption recycling processes is crucial.
NdFeB waste recycling is a key way to alleviate rare earth resource shortages and reduce environmental pollution. Through technological innovation and policy guidance, the NdFeB recycling industry will develop towards greening, low cost, short processes, and high recovery rates, injecting new impetus into sustainable development.
Application and development of inorganic powder materials in the rubber industry
Rubber is widely used in transportation, machinery, electronics, defense, and other sectors of the national economy. However, rubber also has its own significant drawbacks, such as weak intermolecular forces, large free volume, and poor self-crystallization ability, resulting in low strength and modulus, and poor wear resistance in rubber materials. Therefore, it is necessary to add inorganic non-metallic fillers to meet the requirements of these applications.
Generally speaking, inorganic non-metallic fillers in rubber mainly serve the following functions: reinforcement, filling (increasing volume) and cost reduction, improving processing performance, regulating vulcanization characteristics, and imparting special functions.
Commonly Used Inorganic Non-metallic Mineral Fillers in Rubber
(1) Silica
Silica is currently the second most widely used reinforcing agent in the rubber industry after carbon black. The chemical formula of silica is SiO2·nH2O. Its particle structure contains many voids. When these voids are in the range of 2nm-60nm, they easily combine with other polymers, which is the main reason why silica is used as a reinforcing agent. As a reinforcing agent, silica can greatly improve the wear resistance and tear resistance of materials. It can also significantly improve the mechanical properties of tires and is widely used in vehicles, instruments, aerospace, and other fields.
(2) Light Calcium Carbonate
Light calcium carbonate is one of the earliest and most widely used fillers in the rubber industry. Large amounts of light calcium carbonate added to rubber can increase the volume of the product, thereby saving expensive natural rubber and reducing costs. Light calcium carbonate filling rubber can achieve higher tensile strength, wear resistance, and tear strength than pure rubber vulcanizates. It has a significant reinforcing effect in both natural and synthetic rubber, and can also adjust consistency. In the cable industry, it can provide a certain degree of insulation. (3) Kaolin
Kaolinite is a hydrous aluminosilicate, a common clay mineral. Its practical application in rubber enhances the rubber's elasticity, barrier properties, elongation, and flexural strength. Adding modified kaolinite to styrene-butadiene rubber (SBR) significantly improves the rubber's elongation, tear strength, and Shore hardness, while also extending its service life.
(4) Clay
Clay can be added during tire manufacturing, depending on the production process requirements. Clay is used as a filler to reduce costs. However, it must be activated clay to facilitate bonding with rubber. Activated or modified clay can partially replace carbon black in the formulation.
Studies show that as the amount of clay increases, the hardness, 300% tensile stress, and tensile strength of the rubber compound decrease slightly, but this can be compensated for by adjusting the vulcanization system. When used in tread formulations, after system optimization, it can also reduce rolling resistance.
(5) Barium Sulfate
It can effectively enhance the anti-aging and weather resistance of rubber products such as tire rubber and belts. In addition, it can improve the surface smoothness of rubber products. As a powdered rubber filler, it can not only improve the powder application rate, but also has obvious advantages in terms of economic cost.
(6) Talc
Talc powder is usually divided into general industrial talc powder and ultrafine talc powder. The former, as a rubber filler, does not play a reinforcing role and has a negligible effect on improving the physical properties of rubber. Therefore, general industrial talc powder is often used as a separating agent. Ultrafine talc powder, on the other hand, has a good reinforcing effect. If it is used as a rubber filler, the tensile strength of the rubber itself is equal to the effect produced by silica.
(7) Graphite
Graphite belongs to the lamellar silicate non-metallic minerals and has good thermal conductivity, electrical conductivity, and lubricity. Using graphite as a rubber filler involves a similar process to that used for montmorillonite, where graphite is broken down into nano-sized particles using a special technique. When these nanoparticles combine with the rubber matrix, various functional properties of the rubber are improved. For example, electrical conductivity, thermal conductivity, airtightness, and mechanical properties are all significantly enhanced.
Types and Applications of Powder Spheroidization Technology
Powder spheroidization technology, an indispensable component of modern industry and science, can improve the surface characteristics and physical properties of powders, optimize material performance, and meet multifunctional requirements. Currently, powder spheroidization technology has penetrated numerous fields, including pharmaceuticals, food, chemicals, environmental protection, materials, metallurgy, and 3D printing.
Spherical powder preparation technology involves multiple disciplines, including expertise in chemistry, materials science, and engineering. Below, we will explore the various technologies involved in powder spheroidization.
Mechanical Shaping Method
Mechanical shaping methods primarily utilize a series of mechanical forces, such as collision, friction, and shear, to plastically deform and adsorb particles. Continuous processing results in denser particles, and sharp edges are gradually smoothed and rounded by the impact force. Mechanical shaping methods utilize high-speed impact mills, media stirred mills, and other pulverizing equipment to produce fine powder materials. Combined with dry and wet grinding, these methods yield powder materials with finer particle size, narrower particle size distribution, and a certain spheroidization rate.
Mechanical shaping is widely used in the spheroidization and shaping of natural graphite, artificial graphite, and cement particles. It is also suitable for crushing and pulverizing brittle metal or alloy powders. Mechanical shaping utilizes a wide range of low-cost raw materials, fully utilizing existing resources. It offers advantages such as simplicity, environmental friendliness, and industrial scalability. However, this method is not very selective in terms of materials, and cannot guarantee the sphericity, tap density, and yield of the processed particles. Therefore, it is only suitable for producing spherical powders with lower quality requirements.
Spray Drying
Spray drying involves atomizing a liquid substance into droplets, which are then rapidly evaporated in a hot air stream, solidifying into solid particles. The advantages of spray drying are its simplicity and ease of controlling product properties. It is primarily used in the fields of military explosives and batteries.
Gas-Phase Chemical Reaction
Gas-phase chemical reaction uses gaseous raw materials (or evaporates solid raw materials into a gaseous state) to produce the desired compound through a chemical reaction. This compound is then rapidly condensed to produce ultrafine spherical powders of various substances.
Hydrothermal Method
The hydrothermal method utilizes a reactor under high temperature and pressure conditions, using water or an organic solvent as the reaction medium for a chemical reaction. Particle size can be effectively controlled by adjusting parameters such as the hydrothermal temperature, hydrothermal time, pH, and solution concentration.
Precipitation Method
The precipitation method combines metal ions with a specific precipitant through a chemical reaction in a solution, generating tiny, semi-solid colloidal particles and forming a stable suspension. Subsequently, by further adjusting precipitation reaction conditions, such as static aging, slow stirring, or changing the solution environment, these colloidal particles gradually aggregate and grow toward spherical shape, forming a primary spherical precipitate. The resulting precipitate is then dried or calcined to ultimately produce a spherical powder material.
Sol-Gel Method
The sol-gel method typically involves three stages: sol preparation, gel formation, and spherical powder formation. Heat treatment can further improve the structure and properties of the spherical powder, enabling precise control of the particle size and morphology.
Microemulsion Method
The microemulsion method is a liquid-liquid two-phase system preparation method. This method involves adding an organic solvent containing a dissolved precursor to an aqueous phase to form an emulsion containing tiny droplets. Spherical particles are then formed through nucleation, coalescence, agglomeration, and heat treatment. Microemulsion methods are widely used in the preparation of nanoparticles and organic-inorganic composite materials.
Plasma Spheroidization
With the rapid development of high-tech and the urgent need for new nanomaterials and novel preparation processes, the research and application of plasma chemistry are gaining increasing attention. Plasma spheroidization, characterized by high temperature, high enthalpy, high chemical reactivity, and controllable reaction atmosphere and temperature, is ideal for producing high-purity, small-particle spherical powders.
Other methods include deflagration, Gas Combustion Flame Pelletization, Ultrasonic Atomization, Centrifugal Atomization, wire cutting, punching, and remelting, and pulsed micropore spraying.
How to modify the surface of silicon nitride powder?
Surface modification of silicon nitride powder primarily involves treating the surface of the powder through various physical and chemical methods to improve the physical and chemical properties of the particles.
Surface modification can reduce the mutual attraction between powder particles, allowing for better dispersion of the powder in the medium and improving the dispersibility of the powder slurry. It can also enhance the surface activity of the silicon nitride powder, increasing its compatibility with other substances and thus developing new properties.
The main principle of powder surface modification is that the interaction between the powder and the surface modifier enhances the wettability of the powder surface and improves its dispersion in aqueous or organic media.
1. Surface Coating Modification
Surface coating modification technology utilizes physical or chemical adsorption to uniformly attach the coating material to the surface of the coated object, forming a uniform and complete coating layer. The coating layer formed during the coating process is typically a monolayer.
Coating modification is generally categorized as inorganic and organic. Inorganic coating primarily involves depositing appropriate oxides or hydroxides on the surface of ceramic particles to modify the powder, but this modification only affects physical properties. Organic coating, on the other hand, involves selecting organic substances as coating materials. These organic substances bond with groups on the surface of the powder particles and selectively adsorb onto the surface, imparting the properties of the coating layer to the powder.
This modification technology offers low cost, simple steps, and easy control, but the resulting results are often limited.
2. Surface Acid and Alkali Treatment
Ceramic molding processes generally require ceramic slurries with high solids content and low viscosity. The charge density on the powder surface significantly influences the rheological and dispersibility of the slurry. Washing the ceramic powder surface (acid and alkaline treatments) can alter the surface charge properties of the powder. As the name suggests, this modification method involves thoroughly mixing and washing the silicon nitride powder with acid or alkaline solutions of varying concentrations.
At the same time, alkaline treatment at a certain concentration can also react with the surface of ceramic powders. Research by Wang Yongming et al. has shown that alkaline washing can reduce the silanol content on the surface of silicon carbide powder, lowering its degree of oxidation, altering the electrostatic repulsion between particles, and improving the rheological properties of the slurry.
3. Dispersant Modification
Based on the differences between different types of ceramic powders, selecting an appropriate dispersant or designing a new one plays a key role in increasing the solid content of the ceramic slurry. The type and amount of dispersant added can significantly alter the effect on ceramic properties.
Dispersants generally have both hydrophilic and hydrophobic structures, and it is through the interaction between these hydrophilic and hydrophobic groups that they adjust the dispersion properties of the ceramic slurry. Dispersants include surfactants or polymer electrolytes, with surfactants including cationic and anionic surfactants.
Polymer electrolytes include polyvinyl sulfonic acid, polyacrylic acid, polyvinyl pyridine, and polyethyleneimine. Dispersants can undergo adsorption reactions with the powder surface, including chemical and physical adsorption, leveraging interparticle forces (van der Waals forces and electrostatic repulsion) and the potential for steric effects.
4. Surface Hydrophobicity Modification
Surface hydrophobicity modification involves converting the hydroxyl groups in ceramic powder into hydrophobic groups, such as hydrocarbon groups, long-chain alkyl groups, and cycloalkyl groups. These organic groups bind to the ceramic powder surface, exerting a strong hydrophobic effect, enabling better dispersion in the dispersion medium and preventing agglomeration.
When polymers are grafted onto the surface of silicon nitride powder, the long polymer chains attach to the powder surface, while the hydrophilic chains at the other ends extend into the aqueous medium. Throughout the dispersion process, the powder particles experience both interparticle repulsion and steric hindrance created by the long polymer chains, resulting in better slurry dispersion.