First, the mechanism of particle growth during reduction

The particle size of the tungsten powder produced during the reduction process varies with the reducing conditions, that is, it will grow under certain conditions, such as high temperature and high humidity. There are various opinions on the growth mechanism. The following are two The main point of view.

(1) Mechanism of chemical vapor migration growth

Hydrated tungsten oxides have a much higher volatility than pure tungsten oxide. During the reduction process, water vapor first reacts with tungsten oxide or fine-grained tungsten powder to form hydrated tungsten oxide, which migrates to other particles through the gas phase and is then reduced, thereby causing the particles to grow. High temperature and wet hydrogen reduction have the most favorable chemical vapor migration conditions.

(2) Oxidation-reduction mechanism

The finer the powder particles, the larger the specific surface area and the surface activity. Therefore, the fine particle powder may be oxidized by water vapor or oxygen in the gas phase to form volatile hydrated tungsten oxide, which is then subjected to chemical vapor migration and reduced on coarser particles. Make the particles grow.

Second, the main factors affecting powder particle size and grain shape change

(a) temperature

Increasing the temperature accelerates the reduction reaction, correspondingly increases the rate of formation of water vapor, and promotes the chemical vapor phase shift reaction. Promote particle growth and agglomeration.

(2) Water vapor partial pressure

Water vapor is a basic condition for a chemical vapor phase shift reaction, and the amount thereof includes water vapor contained in the hydrogen gas and a reduction reaction. It is not a constant value during the restore process. All the factors that contribute to the reaction rate and all the factors affecting the diffusion process (such as temperature, grain thickness, hydrogen flow direction and flow rate, particle size, boat geometry, etc.) and push speed affect the actual water vapor. The partial pressure affects the particle size and morphology of the powder. The effects of temperature and humidity (dew point of hydrogen) on the relative growth rate of WO ₂ are shown in Table 1.

Table 1 Relative growth rate of WO ₂ particle size at different temperatures and hydrogen temperatures

(3) Properties of raw material powder

Studies have shown that the reduction activity of tungsten oxide has a significant effect on the particle size of tungsten powder. It is easy to obtain fine-grained tungsten powder by reducing the active material.

(4) Impurities and additives

The influence of impurity elements on the variation of tungsten powder particles can be divided into three categories:

The first category, represented by alkali metal, they can play the role of oxygen carrier, oxygen extended residence time in the powder layer, to promote the chemical vapor transport reactor, the tungsten powder particle growth enhancement.

The second category is represented by calcium, magnesium and silicon, and their effect on the growth of tungsten powder particles is not obvious.

The third type is represented by aluminum , which can form a thin oxide layer with high stability on the surface of tungsten crystals, and inhibit the growth of tungsten powder particles.

(5) Operating system

Since the particle growth process mainly occurs during the reduction of WO ₃ to WO ₂ , in order to obtain fine particles, it is necessary to ensure a low temperature, low water vapor partial pressure state at the initial stage of reduction. Therefore, the speed of pushing the boat is too fast, on the one hand, the material quickly enters the high temperature zone, which is beneficial to the growth of particles such as WO _2.9 , and at the same time, the reduction speed is accelerated, and the H ₂ O vapor concentration is increased, which is beneficial to the growth of the particles, so Fine particles generally require slow pushing speed. At the same time, the furnace temperature is lower and the temperature gradient is smaller.

If the loading amount is too much and the material layer is too thick, the internal water vapor will be difficult to discharge, and the internal particles will grow and the upper and lower layers will be uneven in particle size.

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