Hydrothermal method: The most ideal preparation method for multilayer ceramic capacitor powder raw materials?

I. Multilayer Ceramic Capacitors and Hydrothermal Method
Multilayer ceramic capacitors (MLCC) are the most consumed and fastest growing chip components in the world. Due to its compact structure, low dielectric loss, high specific volume, small size and low price, MLCCs are widely used in bypass, tuning, filtering, coupling and oscillation circuits of civil electronic equipment such as mobile communication, measuring instruments, home computers and medical equipment, which greatly improve the filtering performance, high frequency switching performance and anti-interference performance and reduce the weight and volume of equipment. In aerospace, military signal control, weapons warhead control and other military electronic equipment.
Barium titanate powder is the main raw material for MLCC. the development of MLCC requires higher and higher quality of barium titanate powder, and the demand is increasing. At present, the preparation methods of barium titanate powder are solid phase method, liquid phase method and gas phase method, among which solid phase method and hydrothermal method have been industrialized. Compared with other methods, the solid phase method has mature technology, cheap and easy to obtain raw materials and high yield. However, this method has high reaction temperature, high energy consumption, large product size, cannot produce powder below 100 nm, serious agglomeration, and non-uniform composition, which cannot meet the development needs of MLCC.
The complete development, fine and uniform barium titanate grains can be synthesized in one step by hydrothermal, and the powder has less agglomeration, high purity, uniform chemical components, high sintering activity, cheap and easy to obtain raw materials, no sintering ball mill, low energy consumption, low pollution and low input, so the hydrothermal method is more advantageous to prepare barium titanate powder required for MLCC. Hydrothermal method is therefore also considered to be the most ideal method to promote the development of MCLL in the direction of miniaturization and high performance, and is the current aspect of high barium titanate powder generation enterprises focus on the development.
Second, the research status of hydrothermal method
Hydrothermal method refers to the effective method of inorganic synthesis and material treatment by creating a relatively high temperature and high pressure reaction environment by heating and pressurizing the reaction system (or autogenous vapor pressure) in a special closed reactor (autoclave) with an aqueous solution as the reaction system to dissolve and recrystallize the normally insoluble or insoluble substances.
The presence of water in a hydrothermal reaction has many roles. Not only does water act as a solvent and participate in the reaction as a chemical component, but it also acts as a medium for transferring pressure. By controlling the physical and chemical factors and accelerating the reaction penetration, crystals can be formed and grown rapidly.
According to the research objects and purposes, hydrothermal methods can be divided into hydrothermal crystal growth, hydrothermal synthesis, hydrothermal reaction and hydrothermal treatment, which are used to grow various single products, prepare functional ceramic powders, complete certain organic reactions or treat some organic wastes that are harmful to the human survival environment, and complete the sintering of certain ceramic materials at relatively low temperatures, respectively.
Depending on the equipment, the hydrothermal method can be divided into "general hydrothermal method" and "special hydrothermal method". The so-called "special hydrothermal method" refers to the addition of other force fields in the hydrothermal reaction system, such as direct current electric field, magnetic field, microwave field, etc.
According to the reaction temperature can be divided into low temperature hydrothermal method and supercritical hydrothermal synthesis method. The temperature range of low-temperature hydrothermal method is generally between 100℃-250℃. Supercritical hydrothermal synthesis refers to the synthesis reaction in a supercritical state (i.e., the critical temperature of water is 374 ℃, the critical pressure is above 22.1 MPa) using the nature of water as a reaction medium, as well as the special properties of the reactants under high temperature and high pressure hydrothermal conditions.
The main advantages of the hydrothermal method are as follows:
(1) hydrothermal method mainly uses medium and low temperature liquid phase control, the process is simple, no need for high temperature treatment to obtain a complete crystalline shape, uniform particle size distribution, good dispersion of the product, relatively lower energy consumption.
(2) Wide applicability, can prepare ultra-fine particles, larger particle size single crystal and inorganic ceramic film.
(3) Relatively cheap and easy availability of raw materials, the resulting product with uniform phase, high purity, good crystallization, high yield, and controllable product morphology and size.
(4) The reaction and crystal growth can be effectively controlled by changing the reaction temperature, pressure and reaction time.
(5) The closed conditions of hydrothermal synthesis are favorable to toxic reaction systems that are harmful to human health and minimize environmental pollution.
Regarding the hydrothermal reaction kinetics and crystallization mechanism, it is believed that the crystal growth under hydrothermal conditions mainly includes the following steps:
(1) Dissolving the raw material in the hydrothermal medium and entering the solution after dissolution in the form of ions, molecules or ionic groups.
(2) Using the strong convection of the solution in the kettle due to the temperature difference, the ions, molecules or ionic groups generated after dissolution of the raw material are transported to the nucleation growth zone (low temperature zone) to form a supersaturated solution.
(3) Adsorption, decomposition, and desorption of ions, molecules, or ionic groups at the growth interface.
(4) Movement of adsorbed substances at the interface. 
(5) Crystallization of dissolved substances.
C. Hydrothermal synthesis of barium titanate powder
The synthesis of high-purity nanoscale barium titanate powder has been a hot spot for research. Compared with the solid-phase method, the technology of hydrothermal synthesis method is still immature, but its advantages are self-evident. The hydrothermal process mainly deals with the mixture of ceramic precursors, which is generally reacted at a temperature of 25°C~250°C and under atmospheric or pressurized conditions. The hydrothermal synthesis of barium titanate powders was first reported by Christensen et al. because of the low activity of the Ti precursors used, and the reaction was carried out at high temperature and pressure (380°C-450°C, 30-50 MPa).
The reactivity of the precursors plays a particularly important role in the reaction conditions of hydrothermal synthesis of barium titanate. Solid powders, colloids, and mixtures of powders and colloids can be precursors, Ba(OH)2-8H2O and Ba(CH3COO)2, solid TiO2, and amorphous TiO2 gels, all of which are commonly used as raw materials for the preparation of barium titanate powders.
In hydrothermal synthesis, from the thermodynamic aspect, barium titanate grains can be synthesized only in the presence of OH ions; from the kinetic point of view, OH contributes to the crystalline transformation. Barium titanate can be synthesized only in the hydrothermal reaction system with PH "13, so Ba(OH)2 as the source of barium, Ba needs to be in excess, or adding mineralizing agent, such as NaOH, KOH: the study shows that only when Ba/Ti is greater than 1, the pure phase of barium titanate can be synthesized, and with the increase of Ba/Ti, barium titanate gradually changes from cubic phase to four directions.
Summary
The barium titanate powder used in barium titanate electronic ceramics is generally in tetragonal phase. In order to synthesize tetragonal phase barium titanate directly, it is necessary to use highly active precursors or to promote the formation of tetragonal phase barium titanate by increasing the barium/Ti ratio and the alkalinity of the reaction system, adding additives such as surfactants, and combining other techniques such as microwave hydrothermal method, sol-gel hydrothermal method, hydrothermal-precipitation method and hydrothermal-thermoelectrochemical method.
Although the preparation technology of barium titanate nanopowders is developing rapidly, there are still many problems that need to be solved, such as the formation mechanism of barium titanate nanoparticles, the reasons for the stable existence of sub-stable cubic phase, the size of critical size, etc.; the industrialization of synthesis devices, the limitation of powder characterization means, the accurate measurement of tetragonal phase content, etc.