Reverse reaction

This entry is made by Compilation and application of scientific encyclopedia entries of "Science Popularization China" to examine.

Reverse reaction refers to the reaction from product to reactant (or from right to left). Under the same conditions, the reaction in both positive and negative directions is called reversible reaction. In reversible reaction, the reaction from reactant to product (or from left to right) is a positive reaction.

Chinese name: Reverse reaction
Foreign name: reversible reaction
Exists in: Reversible reaction

Definition: Reaction from product to reactant
Example: N2+3H2==(reversible number) 2NH3
Conditions: Under the same conditions as the positive reaction

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brief introduction

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Reverse reaction refers to the reaction from product to reactant (or from right to left) Under the same conditions The reaction that goes in both positive and negative directions is called reversible reaction. In reversible reaction, the reaction from reactant to product (or from left to right) is a positive reaction, The reaction from product to reactant (or from right to left) is a reverse reaction.

For example: N two +3H two ==(Reversible number) 2NH three , where NH three Generate N two And H two The reaction of is reverse reaction^[1] 。

The difference between reciprocal reaction and reciprocal reaction

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(1) Reciprocal reaction means that under certain conditions, the product of a reaction can react to form the original reactant.

(2) They are mutually reversible reactions, so the reverse reaction must be Under the same conditions Can occur.

(3) For example, if hydrogen and oxygen are ignited to generate water, the reverse reaction is water electrolysis to generate hydrogen and oxygen, but the conditions are different.

Nitrogen and hydrogen generate ammonia under high temperature and pressure and with catalyst, and the ammonia generated under the same conditions can be decomposed into nitrogen and hydrogen, which is a reversible reaction.

Mechanism of reverse reaction sintering

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Reverse reaction sinter The process is to burn non oxide composite materials in a conventional sintering furnace under oxidizing atmosphere. The basic requirements of the process are: first, control the formation of enough new oxides or nitrogen oxide And then form a firm surface film to seal it for high-temperature sintering.

Silicon nitride - silicon carbide During the preparation of composites, Si three N four And SiC are covalently bonded compounds, which are generally difficult to sinter. Reverse reaction sintering is a process in which Si three N four Oxidation to SiO two Or Si two N two O fired. In this case, the oxides or suboxides formed by oxidation are new, very active, and dispersed in Si three N four It is easy to sinter refractory particles together because of their low melting point at the boundary between SiC particles and SiC particles. When a small amount of metal Si is added, it can not only act as a sintering aid, but also make the oxidation product become Si two N two O， Make the material have Si two N two The properties of O/SiC composites further improve their corrosion resistance. The key to this process is to control the composition, sintering system and the preparation of brick blanks. Because it is fired in oxidizing atmosphere, it can be fired in ordinary refractory industrial furnace, so it is easy to promote and reduce production costs^[2] 。

Reverse reaction sintering process

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In the process of preparing silicon carbide/silicon nitride composites by reverse reaction sintering, Si three N four /When SiC composites are oxidized, Si three N four It will be oxidized before SiC, and the oxidation product can be SiO two , or Si two N two O， They can form active sintering. Surface oxide film, no matter SiO two Or Si two N two Neither O nor Si three N four Or SiC, which can protect the matrix from oxidation. The results of sintering process show that the best sintering process is the heating rate of 50 ℃/h, holding at 800 ℃ for more than 4 hours, and then sintering at 1450 ℃ for 2 hours. This process can obtain sintered samples with high specific strength, density increase rate and moderate residual nitrogen rate^[3] 。

Relevant conclusions:

(1) Si with physical properties and nitridation reaction sintering of metal Si can be prepared by reverse reaction sintering process three N four /SiC composite equivalent products.

(2) Reverse reaction sintered Si three N four /SiC composites have higher resistance to cryolite melt than reaction sintered products. The reason is that the matrix structure is compact, the pores are evenly distributed, and the nitrides nitrogen oxide It is non wetting with silicon carbide to the melt, so it can prevent penetration. For the sample without iS, fine crystals can be seen from the melt. It indicates that there are some materials melted or involved in erosion reaction.

(3) Reverse reaction sintered Si three N four /The corrosion resistance and permeability of SiC composites are better if there is metal silicon in them. The reason is that the oxidation product is two N two Mainly O, Si two N two O is more than SiO two With stronger corrosion resistance, no fine crystals can be seen on the corrosion boundary after melting into the melt.

(4) Preparation of Si by reverse reaction sintering three N four /The mechanism of SiC composites is Si three N four Active Si generated after oxidation two N two O or SiO two Results. These active fine particles are dispersed in Si three N four And SiC boundary, so as to form active sintering, so that products can be fired at 1450 ℃.

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