The common feature of the preparation of powder particles by liquid phase chemical method is that it starts from a homogeneous solution, separates the solute from the solvent through various means, and forms particles of a certain shape and size to obtain the precursor of the desired powder, which is then pyrolyzed to obtain the powder particles. Currently, common preparation methods include liquid phase chemical precipitation, hydrolysis, hydrothermal, spray, sol-gel, etc.
1. Liquid phase chemical precipitation method
Liquid phase chemical precipitation method refers to a chemical preparation method in which a soluble salt solution produces a precipitate after encountering a precipitant in a liquid phase system, and the precipitate is thermally decomposed or separated from the aqueous phase to prepare the desired powder particles. The liquid phase chemical precipitation method is a method that can obtain powder particles with excellent composition uniformity. Liquid phase chemical precipitation method can also be divided into single phase precipitation method and multiphase precipitation method according to the number of phases
(1) Single phase precipitation method
In a solution system containing one or more cations, after adding a precipitant, the precipitate prepared is a single compound, which is called single phase precipitation method.
For example, the preparation of BaTiO3 powder using oxalic acid as a precipitant is a single-phase precipitation method. After adding oxalic acid as a precipitant to the nitrate solution of Ba and Ti, a single-phase compound precipitate of BaTiO(C2O4)2·4H2O is formed; the precipitate is calcined and decomposed to obtain BaTiO3 powder. The experimental device for preparing BaTiO3 powder using oxalic acid as a precipitant is shown in the figure below.
(2) Multiphase precipitation method
In a solution system containing multiple cations, after adding a precipitant, the precipitate prepared is a mixture, which is called a multiphase precipitation method. The method of preparing zirconium oxide-yttrium oxide powder by chemical precipitation is a multiphase precipitation method.
The obtained hydroxide coprecipitate can be washed, dehydrated, and calcined to obtain ZrO2-Y2O microparticles with good sintering activity. The multiphase precipitation process is very complicated. Different types of cations in the solution cannot be precipitated at the same time. The order of precipitation of various ions is also closely related to the pH of the solution.
2. Hydrolysis method
The hydrolysis method is a type of liquid phase chemical preparation method. It refers to a method in which a reaction compound in an aqueous solution can be hydrolyzed to form a precipitate and prepared into powder particles. Usually, the raw materials are metal salts and water, and the hydrolysis reaction product is hydroxide or hydrate. High-purity powder particles can be easily obtained from high-purity metal salts. Metal salts that can be used to prepare powder particles by hydrolysis include chlorides, sulfates, nitrates, ammonium salts, etc. In addition, metal alcohol salts can also be used as raw materials for preparing powder particles. The hydrolysis method is divided into two types: the inorganic salt hydrolysis method and the metal alcohol salt hydrolysis method.
(1) Inorganic salt hydrolysis method
The inorganic salt hydrolysis method refers to a method of preparing powder particles by hydrolysis reaction using solutions of metal chlorides, sulfates, nitrates, etc. The most common method is to prepare metal oxides or hydrated metal oxides. For example, the preparation of zirconium oxide nanopowder is to cyclically hydrolyze zirconium tetrachloride and zirconium oxychloride in hot water. The resulting precipitate is hydrous zirconium oxide, whose particle size, shape, and crystal form vary with the initial concentration and pH of the solution, and uniform powder particles can be obtained.
(2) Metal alkoxide hydrolysis method
Metal alkoxide is a general term for compounds that replace the hydrogen in the hydroxyl group of the alcohol with a metal element.
The general formula of metal alkoxide is M(OR), where M represents the metal element and R is an alkyl group. Metal alkoxide can also be called a metal compound. Metal alkoxide is a different concept from commonly used organic metal compounds. Alkoxide is a compound that combines metal and oxygen to form an M-O-C bond, which is called a metal organic compound. Organic metal compounds refer to compounds that directly combine alkyl groups with metals to form a -C-M bond.
Metal alkoxide hydrolysis refers to a method of preparing powder particles by hydrolyzing metal organic alkoxides in organic solvents to form hydroxide or oxide precipitates. This preparation method has the following characteristics:
●Organic reagents are generally of high purity, so using organic reagents as solvents for metal alkoxides can produce oxide powders of high purity.
● Analytically pure composite metal oxide powder particles can be prepared. One of the most important indicators of composite metal oxide powder is the uniformity of the composition between oxide powder particles. The alcohol salt hydrolysis method can obtain particles with the same composition.
The hydrolysis process does not require the addition of alkali, so there are no harmful anions and alkali metal ions. The reaction conditions are mild and the operation is simple, but the cost is high.
3. Hydrothermal method
The hydrothermal method refers to a method for preparing powder particles by conducting relevant chemical reactions in a solvent (such as water, ethanol benzene, etc.) under high temperature and high pressure. The minimum particle size of the powder particles prepared by the hydrothermal method can reach the nanometer level. According to the different reaction types, the hydrothermal method can also be subdivided into hydrothermal oxidation method, hydrothermal precipitation method, hydrothermal synthesis method, hydrothermal reduction method, etc.
4. Solvent evaporation method
The solvent evaporation method refers to a method for preparing mixed oxide powder products by using soluble salt as raw material, dissolving it in a solvent, evaporating the solvent by various means, and then obtaining a mixed oxide powder product by a thermal decomposition reaction. Compared with the precipitation method and the hydrolysis method, the solvent evaporation method does not have the problems of difficult precipitation of colloids, easy mixing of impurities in washing and filtering powders, etc.
According to the different solvent evaporation methods, the solvent evaporation method can be divided into spray drying, supercritical fluid drying, freeze drying, etc.
Freeze drying refers to the process of atomizing a solution containing metal ions into tiny droplets and rapidly cooling it to solidify it. The frozen droplets obtained in this way evaporate all the water through sublimation to form anhydrous salts of solutes, and this salt is calcined at low temperature to prepare powder particles. Freeze drying is a widely used method for preparing highly active powder particles.
Its characteristics are:
① It can prepare powder raw materials with complex compositions from soluble salt solutions.
② By rapid freezing, the uniform mixing state of metal ions in the solution can be maintained.
③ Anhydrous salts can be easily prepared by freeze drying. The hydration melting of anhydrous salts generally occurs under conditions much lower than the melting temperature of anhydrous salts, thus avoiding the composition separation of mixed salts during melting.
④ A porous dry body is generated by freeze drying, so the obtained powder has good gas permeability. The gas generated during calcination is easy to release, and its crushing property is also good, so it is easy to be fined.
5. Sol-gel method
The sol-gel method refers to a method in which a precursor is dissolved in a solvent to form a uniform solution, a sol is prepared by hydrolysis or alcoholysis reaction between the solute and the solvent, and then the pre-treated coated powder suspension is mixed with it. Under the action of a gelling agent, the sol is transformed into a gel after aging, and then a coated composite powder is obtained by high-temperature calcination.