Due to its unique optical, electrical, magnetic, thermal and chemical properties, nano-metal copper powder is widely used in high-efficiency catalysts, conductive plasmas, ceramic materials, high electrical conductivity, high specific strength alloys and solid lubricants. At present, the preparation methods of nano metal copper mainly include: chemical reduction method, microemulsion method, polyol method, organic precursor thermal decomposition method, electrochemical method and the like.
1. Chemical reduction method
The chemical reduction method is currently the most commonly used preparation method for preparing nanometers in the laboratory and industry.
The method comprises the steps of: selecting a suitable soluble copper salt precursor and reacting with a suitable reducing agent such as N2H4H2O, NaBH4 ascorbic acid in a liquid phase, Cu2+ reduction, and nucleation growth to a nano copper powder. In the process of preparing metal nanoparticles by chemical reduction, nano-copper is easy to oxidize or agglomerate, which limits its practical application. Surface modification technology provides a practical way for surface modification of nanoparticles. Through the modification of the surface of the nanoparticles, the dispersion stability of the nanoparticles can be improved, and at the same time, new physical and chemical properties can be generated on the surface of the particles, and the compatibility between the nanoparticles and other substances can be improved, thereby effectively solving the agglomeration of the nanoparticles. Oxidation inactivation and other issues. Common molecular ligands for the preparation of copper nanomaterials by chemical reduction include surfactants, various polymers and dendrimers, thiols and their derivatives.
The advantages of the chemical reduction method are: easy operation and easy control. For example, the particle size and morphology can be controlled by changing the reaction parameters such as the type of reducing agent, the concentration of the precursor, the reaction temperature and time, especially the amount and type of surfactant, and controlling the nucleation and growth process. In addition, this method has low requirements on equipment, the raw materials used are inexpensive inorganic salts, the reaction can be carried out under mild conditions, the process flow is simple, and it is easy to expand to industrial production.
2, microemulsion method
Microemulsions are generally transparent, isotropic thermodynamically stable systems composed of surfactants, co-surfactants, and oils. In the microemulsion, the tiny “pool” is a microemulsion surrounded by a monolayer composed of a surfactant and a co-surfactant, that is, a “microreactor” having a size ranging from several to several tens of nanometers. . It has a large interface, which is beneficial to the chemical reaction and is an effective medium for preparing nanomaterials. Compared with other chemical methods, this method can effectively control the size and particle size distribution of nanoparticles.
At present, the preparation of nano-powder particles by microemulsion method has gradually deepened, and some new and improved methods have appeared, such as microemulsion method based on supercritical fluid technology. A supercritical fluid is a fluid that can control its physical and chemical properties such as density and dielectric constant by adjusting its temperature and pressure. Compared with ordinary fluids, it usually has a higher density and diffusion coefficient and a lower viscosity. Therefore, by using it in a microemulsion, the exchange transfer process of the nanoparticles between the micelles can be controlled by adjusting the properties of the solvent, thereby providing a new reaction medium for preparing the size-controlled nanoparticles.
Advantages of microemulsion method: It can control the particle size of the product and prevent particle agglomeration; the disadvantage is that it is limited to the laboratory and is not used on a large scale.
At present, the application of microemulsion to the preparation of nano-metal copper powder, the nature, formation and mechanism of microemulsion, and how to choose a suitable microemulsion system is still a need for further exploration and research.
Fig.1 SEM picture of nano-metal copper powder particles with different morphologies prepared by microemulsion method
(A cubic square B tetrahedral type C rod shape)
3. Polyol method
The polyol method is a method for preparing nanoparticles by heating a precursor of a metal salt to a certain temperature by using a polyol as a solvent and a reducing agent. Although the polyol is a type of reducing agent having a relatively low reducing ability, it is effective to reduce the metal salt to zero valence under heating conditions (the reaction temperature is usually the boiling point of the alcohol). The biggest advantage of this method is that the nucleation and growth process of the nanoparticles are separate, and it is easy to control the reaction process by changing experimental conditions such as the amount of reducing agent, reaction temperature, and precursor addition rate, and effectively control the size and distribution of the nanoparticles. . At the same time, the non-aqueous solvent is used as the reaction medium, and the oxidation and agglomeration of the nanoparticles can be effectively avoided.
The polyol used in such a reaction is generally a lower polyol represented by ethylene glycol. This type of alcohol is more polar, and the alkali and metal salt precursors are directly soluble in such alcohols, generally heated to reflux. The metal salt precursor can be reduced at a temperature, but the particle size of the resulting particles is generally large, usually several tens of nanometers.
The polyol method for preparing nano-metal copper powder has many advantages:
1 production process and required equipment is simple;
2 Polyol can be used as solvent, reducing agent, surfactant, generally not protected by inert gas, which is beneficial to the industrialization of the prepared materials.
The disadvantage is that in the polyol process, there is still a problem that the solution is viscous after the reaction, difficult to separate, and requires an organic dispersion medium. Therefore, further improvement of the polyol process has important practical significance.
4. Organic precursor thermal decomposition method
The organic precursor thermal decomposition method is a method of preparing a nanoparticle by heating and decomposing an organic precursor in a high temperature organic solvent. The precursor used in the preparation of the nano-metal copper powder is generally copper acetylacetonate and copper oil which have a low thermal decomposition temperature and are relatively inexpensive, and the organic solvent used is generally oleylamine.
The advantage of the organic precursor thermal decomposition method is that the reaction rate can be better controlled, the nucleation rate is faster and the growth rate is slow, so that the monodispersity of the nanocrystal size can be greatly improved.
Disadvantages are: the reaction conditions are too harsh, the reaction temperature is high, the raw materials are not easy to obtain or expensive, and have certain toxicity, which is an environmentally unfriendly synthetic route, which limits its large-scale production.
5, electrochemical method
Electrochemical method refers to a process in which metal ions are reduced to atoms at the cathode and then nucleated to form nanostructures under an applied voltage. Electrochemical methods (especially electrochemical deposition methods) can control and control the potential or current, implement potential or current steps, and add AC perturbation signals due to their own characteristics. The nanoparticles provide a convenient and feasible experimental method.
The electrochemical method is simple in equipment, convenient in operation, easy to control, mild in reaction conditions, high purity of the obtained nanoparticles, and less environmental pollution.
At present, due to the late start of the research on electrochemical synthesis of nanomaterials, this method has not been applied in the mass synthesis of nanomaterials. Nevertheless, electrochemical methods are still a very promising method for preparing nanoparticles and assembling ordered arrays of nanoparticles.
Second, the application of nano metal copper powder
Nano-copper has attracted more and more attention due to its unique physical and chemical properties and its wide application in the fields of optics, electronics, catalysis, antibacterial, lubrication, polymer filling and modification.
(1) Metal nano-lubricating additive: Adding 0.1~0.6% to the lubricating oil and grease, the self-lubricating and self-repairing film is formed on the surface of the friction surface during the friction process, which significantly improves the anti-wear and anti-friction performance of the friction pair. .
(2) Metallic and non-metallic surface conductive coating treatment: Nano-aluminum, copper, and nickel powders have a highly activated surface, and the coating can be applied at a temperature lower than the melting point of the powder under anaerobic conditions. This technology can be applied to the production of microelectronic devices.
(3) High-efficiency catalyst: Copper and its alloy nano-powder are used as catalysts, with high efficiency and strong selectivity, and can be used as a catalyst in the reaction process of carbon dioxide and hydrogen synthesis of methanol.
(4) Conductive paste: used for terminal and internal electrodes of MLCC to miniaturize microelectronic devices. It can replace the precious metal powder to prepare the electronic slurry with superior performance, which can greatly reduce the cost and optimize the microelectronic process.
(5) Raw materials for bulk metal nanomaterials: bulk copper metal nanocomposite structural materials are prepared by inert gas protection powder metallurgy sintering.
(6) Drug Additives: It is used to treat new specific drugs such as osteoporosis and bone hyperplasia.
(7) Nano-metal self-repairing agent: It is added to the metal friction pair lubricating oil of various mechanical equipments to realize self-repair of metal friction and wear parts, save energy and reduce consumption, improve equipment service life and maintenance period.