What is the effect of high temperature after cooling after baking and feeding of prebaked anode green body?

Solid-phase method means that nano-powder is made of solid-phase raw materials, which can be divided into mechanical crushing method and solid-phase reaction method according to its processing characteristics. 3. 1. 1 mechanical pulverization The main process of mechanical pulverization is to mix matrix powder and nano powder, ball milling and then sintering. It is difficult to prepare nano-powder by ordinary grinding method, but high-energy ball milling can provide great driving force for solid-state reaction. Combining high-energy ball milling with solid-state reaction, nano-composite powder can be directly synthesized through the reaction between particles. Such as synthetic metal carbide, fluoride, nitride, metal oxide composite nano-powder, etc. Matteazzi P of Italy and Calka of Australia have done a lot of research work on the preparation of the above nano-ceramic powders by high-energy ball milling. Nano-AlN powder [1] can be obtained by high-energy ball milling aluminum powder at room temperature and N2 atmosphere. There are some problems in mechanical pulverization, such as the difficulty in controlling the particle size of powder, which brings difficulties to industrial production. Ball milling itself can not completely destroy the agglomeration between nanoparticles, ensure the uniform dispersion of the two-phase composition, and make the dispersed particles agglomerate and settle after ball milling, resulting in further unevenness. In addition, the pollution caused by ball milling and oxidation will also reduce the purity of nano-ceramic powder. If, on the basis of mechanical mixing and dispersion, the agglomeration is destroyed by high-power ultrasound and the pH value of the system is adjusted, the electric double layer structure of the suspended particles of the two powders has electrostatic stability, and the final dispersion can be improved. 3.2.2 Solid-state reaction method Solid-state reaction method is divided into combustion method and thermal decomposition method. Combustion method refers to fully mixing metal salts or metal oxides according to the formula, grinding and calcining, and after solid-state reaction, directly obtaining nano-ceramic powder or grinding to obtain nano-ceramic powder. For example, a common preparation method of BaTiO _ 2 _ 3 is to mix TiO _ 2 and BAC _ O _ 3 in equimolar, then calcine to produce solid-state reaction, and then synthesize BaTiO2 _ 3 and then crush it to obtain nano-ceramic powder. The law of thermal decomposition is to prepare nano-ceramic materials through the thermal decomposition of metal compounds. Such as oxalate and carbonate, can be thermally decomposed to prepare nano-oxides. It can also decompose chelates formed by metals and some chelating agents (such as citric acid and lactic acid). ) preparing high-performance nano-ceramic powder by heating. 3.2 liquid phase method liquid phase method is a widely used method to prepare nano-ceramic powder at present. Its basic technological principle is: select one or more suitable soluble metal salts, prepare a solution according to the composition of the prepared materials, then select a suitable precipitant or use evaporation, sublimation, hydrolysis and other operations to uniformly precipitate or crystallize metal ions, and finally dehydrate or decompose the precipitation or crystallization to obtain nano-ceramic powder. 3.2. 1 precipitation precipitation method can be divided into direct precipitation method, * * * precipitation method and homogeneous precipitation method, all of which are prepared by precipitation generated by liquid phase reaction. * * * precipitation method can complete the reaction and doping process in the preparation process, so it is widely used in the preparation of electronic ceramics. Barium titanate is an important electronic ceramic material with high dielectric constant and excellent ferroelectric and piezoelectric properties. TiCl4 _ 4, H _ H2O2 _ 2 and BaCl2 _ 2 were used as raw materials to prepare titanium peroxide precursor by * * precipitation method, and BaTi03 _ 3 nanocrystals with particle size less than 30 nm were prepared by dispersion dehydration and thermal decomposition of anhydrous ethanol. 3.2.2 Hydrothermal hydrothermal method is to synthesize substances in aqueous solution or steam at high temperature and high pressure, and then obtain nanoparticles through separation and heat treatment. Hydrothermal conditions can accelerate and promote ion reaction and hydrolysis reaction, so that some thermodynamic reactions with slow reaction rate at normal temperature and pressure can be carried out quickly under hydrothermal conditions. According to different reaction types, it can be divided into: hydrothermal oxidation, reduction, precipitation, synthesis, hydrolysis, crystallization and so on. Fe203，Ti TiO2，ZrO2，BaO？ A series of nano-oxide powders such as 6Fe2O3 and Ce02 [4-5]. Hydrothermal method is more suitable for the synthesis of oxide materials and the preparation of a few sulfides that are insensitive to water. 3.2.3 Sol-gel method Sol-gel method is to prepare uniform sol of metal oxide or metal hydroxide through hydrolysis and polymerization of metal alkoxide, and then concentrate the sol into transparent gel through solvent, catalyst and complexing agent. The gel can be dried and heat-treated to obtain the desired nanoparticles. Among them, the main parameters to control the gelation of sol are pH value of solution, solution concentration, reaction temperature and time. By adjusting the process conditions, nano-powders with small particle size and narrow particle size distribution can be prepared. The sol-gel process is simple and the particle size is controllable. The prepared nano-powder has high purity, but the cost is high. 3.2.4 Hydrolysis Many compounds can be hydrolyzed to produce precipitates, some of which are also widely used to synthesize nano-ceramic powders. The product of hydrolysis reaction is usually hydroxide or hydrate. After filtration, drying, roasting and other processes, oxide nano-ceramic powder can be obtained. In the process of preparing nano-ceramic powder, metal alkoxide hydrolysis is usually used. In this method, alkoxide is dissolved in an organic solvent, and alkoxide is hydrolyzed and polymerized by adding distilled water to form sol. After the sol is formed, water is added to convert it into gel, and the gel is dried at low temperature in vacuum to obtain loose xerogel, and then the xerogel is calcined at high temperature to obtain oxide nano-ceramic powder. For example, Mazdiyashi et al. used this method to synthesize fine BaTiO3 nano-ceramic powder with a particle size of 5- 15nm [6]. 3.3 Gas Phase Method The gas phase method is a method that directly uses gas or converts substances into gas by various means, so that they undergo physical changes or chemical reactions in the gas state, and finally condense and grow into nanoparticles in the cooling process. This method can prepare nano-ceramic powder with high purity, good particle dispersibility, narrow particle size distribution and small particle size. Gas phase method can be divided into evaporation in gas, chemical gas phase reaction, sputtering source method, vacuum deposition on flowing oil surface and metal gas phase synthesis method. 3.3. 1 Vaporization in gas is to vaporize a metal, alloy or compound in an inert gas (such as He, Ar, Xe, etc.). ) or active gases (such as O2, CH4, NH3, etc. ) heating in vacuum, and then condensing in gas medium to form nano-ceramic powder. The particle size is controlled by evaporation temperature, gas type and pressure. The general particle size is about 10nm. Among them, the evaporation source can be heated by resistance and high frequency induction, and the high melting point substance can be heated by plasma, laser and electron beam. 1987 Sicgel of Argonne laboratory in the United States prepared Ti02 ceramic powder with an average particle size of 12 nm by this method, and then nano-ceramic powders such as ZrO2 with a particle size of 4-8nm and Y203 with a median particle size of 4 nm were also prepared by this method in the laboratory. The method is suitable for preparing low melting point powder; For carbides and nitrides with high melting point, the energy consumption is too large, and the equipment is huge, complex and expensive. 3.3.2 Chemical Vapor Reaction Method Chemical vapor reaction method is to use the vapor of volatile metal compounds to generate the required compounds through chemical reaction, and quickly condense them in a protective gas environment to prepare nanoparticles of various substances. This method is also called chemical vapor deposition (CVD). Since 1980s, CVD technology has been gradually applied to the synthesis of powdery and fast materials and fibers, and a variety of ultrafine particles such as SiC, Si304 and AlN have been successfully prepared [8]. The original CVD reactor was heated by an electric furnace. Although this thermal CVD technology can synthesize ultrafine particles of some materials, the synthesized particles are not only large in size, but also easy to agglomerate and sinter due to the small temperature gradient in the reactor, which is also the biggest limitation of thermal CVD synthesis of nanoparticles. On this basis, people have developed a variety of preparation technologies, such as plasma CVD and laser CVD. 3.3.3 Sputtering source method Sputtering source method adopts two metal plates as anode and cathode, the cathode is made of evaporated material, inert gas Ar(40-250 Pa) is filled between the two electrodes, and the voltage applied between the two electrodes is (0-3 1.5V). Due to the glow discharge between the two electrodes, Ar ions are formed. Under the action of electric field, Ar ions impact the surface of the cathode target, so that the target atoms evaporate from the surface to form ultrafine particles, which are deposited on the attached surface. The size and size distribution of particles mainly depend on the voltage, current and air pressure between the two electrodes. The larger the target area, the higher the evaporation rate of atoms and the more nano-ceramic powder can be obtained [9]. Commercial magnetron sputtering equipment can be used to prepare nano-ceramic molecular clusters with a diameter of 7-50 nanometers. The formation of ceramic nano-products such as TiO2 _ 2, Zr02 and Zr02 was studied by magnetron sputtering. 3.3.4 Vacuum deposition on flowing oil surface (VEROS method) The principle of vacuum deposition on flowing oil surface is to heat and evaporate raw materials with electron beam under high vacuum, so that the evaporated substances are deposited on the flowing oil surface on the lower surface of the rotating disk, and the evaporated atoms are combined in the oil to form nano-ceramic powder [10]. Its advantage is that the average particle size is very small, about 3nm, and the particle size is very neat. In addition, nano-ceramic powder is dispersed in oil as soon as it is formed, and it is in an isolated state. Its disadvantage is that the generated nano-ceramic powder is difficult to separate from oil and the yield is low. Generally speaking, the nano-ceramic powder obtained by gas phase method has high purity, less agglomeration and good sintering performance, but the equipment is expensive and the output is low, so it is not easy to popularize. The equipment used in solid-phase method is simple and easy to operate, but the powder obtained is often not pure enough and the particle size distribution is large, so it is suitable for occasions with low requirements. Liquid phase method is between gas phase method and solid phase method. Compared with the gas phase method, the liquid phase method has the advantages of simple equipment, no need of harsh physical conditions such as vacuum and easy amplification. At the same time, compared with the powder prepared by solid-state method, it has higher purity and less agglomeration, which is easy to realize industrial production and has broad development prospects. 4. Thermodynamic characteristics of nano-ceramics 4. 1 sintering of nano-ceramics 4. 1. 1 change of sintering temperature The sintering temperature of nano-ceramic powder is low. The results show that the agglomeration-free nano-powder containing zirconia (particle size: 10-20nm) can be sintered to 95% of the theoretical density at 1200℃, and the heating rate can reach 500℃/min. The holding time is only 2min, while the micron sintering temperature is about 1650℃. Literature [l4] through the study of the initial dynamic process of γ-TZP nano-powder, it is proposed that grain boundary diffusion is the dominant factor leading to the initial shrinkage of sintering, and the following sintering dynamic equations are derived: where is the grain boundary diffusion coefficient; ω is the vacancy volume; R is the particle radius; K is often caused by Boltzmann; T is the sintering temperature; Sintering time. The experimental results show that for ultrafine powder without agglomeration, the shrinkage of green body at the initial stage of sintering is linear with sintering time. 4. 1.2 sintering kinetics The huge specific surface area of ultrafine powder means that the surface energy as the driving force of powder sintering increases sharply, resulting in an increase in diffusion rate and a decrease in diffusion path. In the sintering process with chemical reaction, the contact surface of particles increases, which increases the probability of reaction and speeds up the reaction. All these lead to the decrease of sintering activation energy, accelerate the whole sintering process, reduce the sintering temperature and shorten the sintering time. The grain growth in the whole sintering process, that is, recrystallization process, will also accelerate, and the reduction of sintering temperature and shortening of sintering time will slow down the recrystallization process. It is necessary to re-understand and study the role of these mutually promoting and restricting factors. So as to establish the kinetics suitable for ultrafine particle sintering. 4.2 Mechanical properties of nano-ceramics. 4.2. The improvement of mechanical properties of1shows that the mechanical properties of the material can be greatly improved by introducing nano-dispersed phase into the material matrix for compounding. The main performance is that the fracture strength and fracture toughness are greatly improved, and the high temperature resistance of the material is obviously improved. Fig. 1 shows the influence of SiC content in A 1203/SiC nanocomposites on the strength and toughness of composite ceramics [1 1]. Fig. 2 shows the variation of strength and fracture toughness of Si3N4/SiC composite ceramics with nano-SiC content [12]. Fig./influence of kloc-0/SiC content on strength and toughness (A 1203/SiC system) sic (volume fraction, the same below)% < 25% can improve the mechanical properties, and at the same time improve the hardness, elastic modulus, thermal shock resistance and high temperature resistance of the material. Adding 25% SiC nanoparticles to Si3N4 nanoparticles can reduce the fracture toughness of Si3N4 nanoceramics from 4.5MPa. M 1/2 to 6.5 MPa? M 1/2, the strength increased from 850 MPa to 1550MPa[ 16]. 4.2.2 Superplasticity Superplasticity refers to the ability to produce unusually large tensile deformation under stress without damage. Ceramic materials are transition bonds with directional ionic bonds and valence bonds. The dislocation density is low, and the grain boundary is difficult to slip, which makes the ceramics hard and brittle, and ordinary ceramic materials hardly produce plastic deformation at room temperature. Only when the temperature is above 1000℃, the thermal movement between the crystal and the grain boundary will accelerate, and the ceramics will have certain plasticity. Recent studies have found that with the decrease of particle size, the deformation rate sensitivity of nano-Ti02 and Zn0 ceramics is obviously improved, which is mainly due to the decrease of pores in the samples. It can be considered that this trend is inherent in fine-grained ceramics. The sensitivity of the deformation rate of the finest grain is about 0. This shows that these ceramics have ductility. Although they do not show room temperature superplasticity, this possibility exists with the further reduction of grain size. Atomic force microscope (AFM) shows that after cyclic tensile test at room temperature, the fracture area of nano 3Y -T7P ceramics (about 100nm) undergoes local superplastic deformation, and a large number of slip lines usually appear on the metal fracture surface are observed from the fracture side. 4.2.3 Toughening and toughening mechanism It is generally believed that ceramics superplasticity should meet two conditions: (1) smaller particle size; (2) Rapid diffusion path (the diffusion ability of lattice and grain boundary is enhanced). At present, the known toughening mechanisms can be roughly divided into five types: dispersion toughening, crack toughening, ductile phase toughening, ceramic microscopic (whisker) toughening and phase transformation toughening. According to the research of Shinhara Hiroichi [14], it is considered that the strengthening and toughening of nano-composite ceramics are mainly realized by the following functions: 1) Dispersion phase can effectively inhibit the growth and abnormal growth of matrix grains; 2) The local stress around the dispersed phase or dispersed phase is caused by the expansion mismatch between the matrix and dispersed phase, resulting in dislocation in the cooling stage. When nanoparticles are pinned or enter the dislocation zone, potential grain boundaries are formed in the matrix grains, and the grains are refined, thus weakening the role of main grain boundaries; 3) Transgranular fracture is induced by local tensile stress around nanoparticles, and toughening is caused by the reflection of A 1203 hard particles at the crack tip; 4) Nanoparticles inhibit dislocation movement at high temperature, and improve high-temperature mechanical properties such as hardness, strength and creep resistance. Study [15] Through the hot-pressing synthesis experiment of A 1203/SiC nanocomposites, it is considered that the segregation of cracks by intragranular particles, microcracks and surface compressive stress caused by processing are not the main mechanisms of strengthening and toughening; The change of fracture mode, that is, from intergranular fracture of pure matrix to transgranular fracture of composite materials, may be the main reason for enhancing the toughness of materials, and the occurrence of transgranular fracture is related to the nano-scale effect existing in the structure.