Zero-expansion silica bricks are also called fused silica bricks or thermal shock-resistant silica bricks. It has the characteristics of extremely low thermal expansion coefficient, excellent resistance to rapid cooling and heat, good high temperature structural strength, and high SiO2 content. This product can be widely used in rapid thermal repair projects of various coke ovens. The product does not crack and peel off during use, and its low thermal expansion performance is close to zero, so that the long-term use of the coke oven is safe and reliable after repair.
The following mainly analyzes its low thermal expansion and high thermal stability from the crystal structure of silica:
(1) Its main mineral phase is amorphous silica, accounting for more than 90%
It uses high-grade quartz ore to prepare fused silica material through electric arc furnace melting, cooling, crushing, screening, pickling and other processes. The particles are mixed with a small amount of mineralizer and sintering agent to form semi-dry mud. It is made by pressing, drying and firing processes. The main mineral component is amorphous amorphous fused silica. The coefficient of thermal expansion of the zero-expansion silica brick mainly composed of amorphous fused silica is only 0.55×10′, and the coefficient of thermal expansion of the product is almost zero, so it is called zero-expansion silica brick.
(2) Atomic structure of amorphous amorphous fused silica
The same is silicon dioxide, but the structure of amorphous SiO2 is different from that of crystalline SiO2. The above diagram shows a silicon-oxygen tetrahedron [SiO4] composed of each silicon atom and oxygen atom of crystalline silicon oxide, and each tetrahedron passes through the corner apex. The connection becomes a three-dimensional development network structure; the figure below shows the structure of amorphous silicon oxide. It can be seen that there are vacancies in the combination of silicon and oxygen in the disordered amorphous SiO2, and its atomic structure is long-range disordered. The bond angles are distorted irregularly, forming a three-dimensional structure cross-linked.
The thermodynamic properties of the material are determined by the atomic crystal structure. SiO2 has a variety of homogeneous crystal structures. The crystal phases formed under different conditions have different structures, so the thermodynamic properties are different.
Crystalline SiO2—Once the bond angle changes when heated, it will begin to undergo homogeneous transformation at 180°C, accompanied by huge volume expansion, so its thermal stability is extremely poor, and it will crack after being water-cooled once at 1100°C. Amorphous SiO2 has the characteristics of small thermal expansion coefficient, excellent thermal stability, high viscosity at high temperature, and small thermal conductivity. However, amorphous SiO2 is an unstable state at high temperature, and there will be no volume change when heated within 1100 ° C. However, as the temperature rises, amorphous SiO2 begins to crystallize. Because the crystallization process is slow, it is still a problem during hot repair operations. Can well meet the construction requirements.
It takes effective measures to control the crystallization speed and crystallization ratio of amorphous amorphous fused silica at high temperature during the production process, so it can better meet the thermal requirements,
(3) thermal stability test
After heating the zero-expansion silica brick to a high temperature of 1100°C, it was immediately put into normal temperature water, and there was no change after 30 times of water cooling cycle test. After being placed directly from room temperature to a high temperature of 1500°C for 12 hours, there is no change after taking it out. The actual test results also prove its superior thermal stability.
