Solid-phase synthesis of advanced ceramic powders

The solid phase method is a method of preparing ceramic powders by taking solid materials as raw materials and avoiding certain physical and Chemical process: unlike the gas phase method and the liquid phase method, which are accompanied by the state (phase) changes of gas solid phase and liquid solid phase, the prepared solid phase powders and the original solid phase raw materials can be the same or not the same substance.

1. Preparation of raw materials

Solid phase raw materials can be natural minerals, chemical raw materials (oxides), chemical reagents, etc. The starting materials of the earliest solid phase method are mostly natural minerals, such as clay, feldspar, quartz, talc, Calcite, bentonite, Magnesite, Wollastonite, etc., which are commonly referred to as the traditional solid phase method. Oxide ceramics, on the other hand, use industrial oxide raw materials as solid phase materials, which are commonly used in structural ceramics, device ceramics, and insulating ceramic substrates.

2. Ingredients and Mixing

After weighing the solid phase raw materials of the required components for ceramics according to the required proportion of the product, they must be thoroughly mixed (which also includes a certain refining effect) to ensure uniform distribution and sufficient contact between the various raw materials, in order to facilitate the synthesis of the mixture in the next stage. Mixing is a physical process that can generally be divided into two types: dry mixing and wet mixing. The main equipment for mixing includes vibration mills and ball mills.

3. Synthesis of raw materials

At a certain temperature, the chemical reactions between each component of the mixture are carried out more fully and completely in order to obtain the required phase. Because this chemical reaction occurs between solid phases, it is also called Dry media reaction (also called synthesis). As a Dry media reaction, it actually contains many contents, such as Combination reaction, Chemical decomposition, solid solution reaction, Redox, exsolution reaction and phase change. In practical work, several reactions often occur simultaneously, and the products of the reactions need to be crushed. Here we focus on introducing the following reactions.

(1) Combination reaction method

The Combination reaction method is two or more kinds of powders, which are mixed and react under certain thermodynamic conditions and atmosphere to form composite powders, sometimes with some gases escaping.

(2) Thermal Chemical decomposition method

Thermal Chemical decomposition is not only limited to solid phase, but also can occur in gas and liquid.

(3) Oxide reduction method

In modern industrial production, the raw material powder of non oxide ceramics is generally prepared by oxide reduction method, either through reduction carbonization or reduction nitridation

(4) Self propagating high-temperature synthesis method

The self-propagating high-temperature synthesis technology was first successfully studied by the Soviet Institute of Macrodynamics of Structural Sciences in 1967, and is now capable of producing nearly a thousand compound powders using this technology. The production of powder using this technology can be divided into two main directions.

① Elemental synthesis

If there are no gas-phase reactants or products in the reaction, it is called gas-free combustion. If the reaction is carried out in a mixed system of solid and gas phases, it is called gas phase permeation combustion.

② Compound synthesis

An example is the reaction where metal or non-metallic oxides are used as oxidants and active metals are used as reducing agents (such as Al, Mg, etc.). This is actually the chemical method mentioned earlier, also known as Al (or Mg) thermal method.

(5) Explosive method

Explosion method is a method of preparing micro powder using instantaneous high-temperature and high-pressure reactions, which is a continuous powder preparation process. The prepared powder is spherical in shape and generally has a size range of 20-30nm.

4. Refinement process

After synthesis, the material with the required phase can be obtained, and then refined to become the powder (or ceramic material) required for preparing ceramic products. In the solid phase method, the refinement adopts mechanical method, which is also called comminution. For advanced ceramics, the most commonly used comminution equipment includes vibration mill, ball mill, colloid mill, air flow mill, etc. Later, eccentric ball mill and Sander were developed.

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