General knowledge of coating: gas phase preparation of nano materials

Core tip: nano material: refers to the material with at least one dimension in the nano scale range (1-100 nm) or composed of them as basic units in three-dimensional space Compared with conventional materials, nano materials show some physical effects and unique physical properties Preparation technology is the key of nanotechnology It affects the microstructure and macroscopic properties of nano materials Through different preparation technologies, we can get nano particle materials, nano film materials, nano solid materials and so on The preparation of nanomaterials From small to large: atomic cluster nanoparticles From childhood Paint agent, top 10 Chinese wall paint brands Recommended introduction: interior wall paint and exterior wall paint can achieve beautiful, clean and pleasant effects, but the effects in different environments are different, so the color of the paint will be different There are ten brands of Chinese paint, ten brands of global paint, ten brands of Chinese wall paint, and ten brands of Chinese paint, Famous trademark of Chinese paint and coating In private rooms, most of us choose according to our own personality, personal hobbies and living habits The younger generation will choose the interior wall paint with rich colors and bright colors, while the older ones will have some working experience Coating home news: nano material: refers to the material with at least one dimension in the nano scale range (1-100 nm) in three-dimensional space or composed of them as basic units Compared with conventional materials, nano materials show some physical effects and unique physical properties Preparation technology is the key of nanotechnology It affects the microstructure and macroscopic properties of nano materials Through different preparation technologies, we can get nano particle materials, nano film materials, nano solid materials and so on The preparation methods of nanomaterials are from small to large: atomic cluster nanoparticles from small to large: the preparation requirements of atomic cluster nanoparticles - controllable size and size (generally less than 100 nm) - controllable composition (element composition) - controllable morphology (external form) - controllable crystal type (crystal structure, Superlattice) - controllable surface physical and chemical properties (surface state) - (surface modification and surface coating) common preparation methods of nanoparticles: • gas phase method 1 Gas condensation method, 2 Reactive hydrogen molten metal reaction method, 3 Sputtering method, 4 Vacuum evaporation method on flowing liquid surface, 5 Electric heating evaporation method, 6 Mixed plasma method, 7 laser induced chemical vapor deposition (LICVD), 8 exploding wire method, 9 chemical vapor condensation method (CVC) and combustion flame chemical vapor condensation (CFCVC) liquid phase 1 precipitation method, 2 spray method, 3 hydrothermal method (high temperature hydrolysis method), 4 solvent volatilization method, 5 sol gel method (Colloid Chemistry method), 6 radiation chemical synthesis method, 7 microemulsion method Solid phase method 1 Thermal decomposition of salts, 2 Gas condensation method for the preparation of nano materials by ball milling mechanical alloy method This method is to heat metals in inert gases such as AR and he at low pressure, evaporate them, and then condense them in gas medium to form nano particles Through the evaporation and condensation process in the pure inert gas, the cleaner nano powder is obtained The heating sources are as follows: (I) resistance heating method; (II) plasma spraying method; (III) high frequency induction method; (I V) electron beam method; (V) laser method NOK = "F" O: CO necttype = "rect" gradientshapeok = "t" > 1 Preparation of nanoparticles by gas phase method 1 Evaporation condensation method This method is to heat metals in inert gases such as AR and he at low pressure, evaporate them, and then condense them in gas medium to form 5-100 nm nanoparticles Through the evaporation and condensation process in the pure inert gas, the cleaner nano powder is obtained The following figure is a typical device of this method 1) resistance heating method: Place the substances to be evaporated (such as metal, CaF2, NaCl, fef2 plasma compound, transition metal nitride and oxide, etc.) in the crucible Gradually heat and evaporate them through the heating device such as tungsten resistance heater or graphite heater to generate the source material smoke The smoke moves upward from the opposite flow of inert gas and approaches the cooling rod filled with liquid nitrogen (cold well, 77K) In the evaporation process, the atoms emitted by the source material collide with the inert gas atoms to cool down due to the rapid loss of energy This effective cooling process causes a high local supersaturation in the source material vapor, which will lead to a uniform nucleation process Therefore, in the process of approaching the cooling rod, the vapor of the source material first forms an atomic cluster, and then forms a single nanoparticle Finally, it accumulates on the surface of the cooling rod, scrapes off with a Teflon scraper and collects the nano powder Features: the heating method is simple, the working temperature is limited by the crucible material, and it may react with the crucible Therefore, it is generally used to prepare low melting point metal nanoparticles such as Al, Cu, Au Schematic diagram of inert gas condensation and in-situ pressurization device 2) the high frequency induction method uses the high frequency induction coil as the heat source to heat the conductive substances in the crucible under the action of eddy current, evaporate in the low-pressure inert gas, and the evaporated atoms collide with the inert gas atoms to cool and condense to form nano particles Features: crucible is generally used to prepare low melting point materials like low melting point metals 3) The reactive hydrogen molten metal reaction method produces an arc between the plasma containing hydrogen and the metal, which melts the metal, ionizes gases such as N2, AR and H2 into the molten metal, and then releases them to form metal nanoparticles or hydrides in the gas 4) The principle of sputtering method: two metal plates are used as anode and cathode respectively, and the cathode is the source material for evaporation Ar gas (40-250pa) is filled between the two electrodes, and the applied voltage range between the two electrodes is 0.3-1.5kv Due to the glow discharge between the two poles, Ar ions form Under the action of electric field, Ar ions impact the surface of cathode target, making atoms evaporate from the surface, condense and form nanoparticles, and deposit on the attachment surface The size and size distribution of particles mainly depend on the voltage, current and gas pressure between the two electrodes The larger the surface area of the target, the higher the evaporation rate of atoms and the more nanoparticles are obtained 5) The basic principle of the floating surface vacuum evaporation method is: the metal atoms evaporated in the high real air form the extremely nano particles in the flowing oil surface, and the product is the paste oil containing a large number of ultrafine particles When the raw materials in the water-cooled copper crucible are heated and evaporated, the shutter is opened to make the evaporated materials plated on the surface of the rotating disk to form nanoparticles The oil containing nanoparticles is thrown into the container along the wall of the vacuum chamber, and then the oil with very low content of ultrafine particles is distilled under vacuum to make it a concentrated paste containing nanoparticles The advantages of the floating surface vacuum evaporation method: ① AG, Au, PD, Cu, Fe, Ni, Al, in and other nanoparticles can be prepared, with an average particle size of about 3nm, but it is difficult to obtain such small particles by the inert gas evaporation method; ② the particle size is uniform and the distribution is narrow, as shown in the right figure ③ Nanoparticles are dispersed in oil ④ The particle size can be controlled by changing the evaporation conditions, such as evaporation speed, oil viscosity, disk speed, etc High disk speed, high evaporation speed and high oil viscosity all increase the particle size up to 8 nm 6) Electric heating evaporation method this method is through the carbon rod and metal contact, electric heating to melt the metal Metal and high temperature carbon reaction and evaporation to form carbide nanoparticles The following figure shows the device for preparing SiC ultrafine particles The carbon rod is in contact with the Si plate (evaporation material), and is filled with AR or he gas in the evaporation chamber, with a pressure of 1-10kp An alternating current (several hundred a) is applied between the carbon rod and the Si plate The Si plate is heated by the heater under it, and rises with the temperature of the Si plate, When the temperature is higher than 2473k, the "smoke" of small SiC particles is formed around it, and then they are collected to get SiC nanoparticles With this method, Cr, Ti, V, Zr, HF, Mo, Nb, Ta and W carbide nanoparticles can also be prepared 7) The preparation method is to use RF plasma and DC plasma to obtain nanoparticles As shown in the figure, the high frequency magnetic field (several MHz) generated by the induction coil outside the central British tube ionizes the gas to generate RF plasma The raw materials carried by the internal carrier gas are heated by the plasma, reacted to generate nanoparticles and attached to the cooling wall DC (direct current) plasma arc is used to prevent RF plasma flame from being disturbed, so it is called "mixed plasma" method Characteristics of mixed plasma method: ① when RF plasma is produced, no electrode is used, and no electrode material (melting or evaporation) is mixed into the plasma, resulting in impurities in the plasma, so the purity of nano powder is high; ② The plasma is located in a large space, and the gas velocity is slower than that of DC plasma, which results in a long residence time of reactants in the plasma space, and the reactants can be fully heated and reacted; ③ The non inert gas (reactive gas) can be used Therefore, the compound nanoparticles can be prepared That is to say, the mixed isoseparation method can not only prepare the metal nanoparticles, but also the compound nanoparticles, so as to diversify the products 8) In recent years, laser-induced chemical vapor deposition (LICVD) has been widely used to prepare nano powders The laser beam forms a reaction flame on the reaction gas, and the particles are formed in the flame after the reaction The particles are carried by argon into the upper particle trap This method uses the absorption of the reaction gas molecule (or photosensitizer molecule) to the laser beam of a specific wavelength to cause the reaction gas molecule laser photolysis (UV photolysis or infrared multi light photolysis), laser pyrolysis, laser photosensitization and laser-induced chemical synthesis reaction Under certain technological conditions (laser power density, reaction pool pressure, ratio and flow rate of reaction gas, reaction temperature, etc.), the Space nucleation and growth of nanoparticles 9) Chemical vapor condensation (CVC) is a method to obtain nano ceramic powders by pyrolysis of organic or organometallic molecules The principle is to use high-purity inert gas as carrier gas to carry organic molecular raw materials, such as hexamethyldisilane, into the molybdenum wire furnace, the temperature is 1100-1400 ℃, the pressure of atmosphere is kept at 1-10 mbar, in this environment, the raw materials are pyrolyzed to form clusters to further agglomerate into nano SiC particles, and finally attached to a rotating substrate filled with liquid nitrogen, The nano powder is collected by scraping with a scraper, as shown in the figure below Advantages: large output, small particle size, narrow distribution 10) This method is suitable for preparing nano metal and alloy powders The basic principle is to fix the wire in a reaction chamber filled with inert gas (50bar) The two ends of the wire are clamped with two electrodes, which are connected with a large capacitance to form a circuit, and heated under 15kV high voltage and 500-800ka wire At the moment when the current stops, the high voltage on the clamp is discharged at the melting point, so that the molten metal enters into a circuit during the discharge process Step heating turns into steam, which collides in inert gas to form nano particles and settles at the bottom of the container The wire can automatically enter between the two clamps through a wire feeding system, so that the above process can be repeated Coatings