Silicon Carbide Cylinder Manufacturer: An Introduction to Silicon Carbide Technology

　　Silicon Carbide Cylinder Manufacturer: An Introduction to Silicon Carbide Technology

　　 Silicon carbide cylinder Manufacturer: Silicon carbide abrasives typically use quartz and petroleum coke as the primary raw materials. During the material preparation process, these raw materials are ground to appropriate particle sizes and then mixed according to chemical calculations to form the furnace charge. To adjust the permeability of the abrasive furnace charge, an appropriate amount of sawdust is added during batching. When manufacturing green silicon carbide, an appropriate amount of salt should be added to the furnace charge.

　　Silicon Carbide Cylinder Manufacturer: The charge material is placed into an intermittent resistance furnace. The resistance furnace is equipped with end walls at both ends, and graphite electrodes are positioned near the center. The furnace core is connected between these two electrodes. Surrounding the furnace core is the reactive charge material, while the outer layer consists of insulating materials. During smelting, the electric furnace is energized, causing the temperature of the furnace core to rise to between 2600 and 2700 degrees Celsius. Heat is transferred from the surface of the electrically heated furnace core to the charge material, gradually raising its temperature. Once the temperature exceeds 1450 degrees Celsius, a chemical reaction occurs, producing silicon carbide and releasing carbon monoxide. As time goes on, the high-temperature zone within the furnace expands continuously, leading to the formation of increasing amounts of silicon carbide. This silicon carbide continually forms, evaporates, moves around, crystallizes, and grows into cylindrical crystal cylinders. Due to the high temperatures on the inner walls of the crystal cylinders—exceeding 2600 degrees Celsius—the silicon at those areas begins to decompose. The decomposed silicon then combines with carbon from the charge material, forming new silicon carbide. In the initial stage of power transmission, most of the electrical energy is used to heat the charge material, with only a small portion devoted to generating the heat needed for silicon carbide formation. In the middle stage of power transmission, the heat used for silicon carbide formation accounts for a significant proportion. In the later stages of power transmission, heat loss becomes the dominant factor. By adjusting the relationship between power supply and time and selecting favorable power-off periods, it is possible to achieve higher efficiency in the utilization of electrical energy. For high-power resistance furnaces, it is common to schedule power supply for approximately 24 hours, which facilitates operational planning. On this basis, the furnace power should be adjusted in accordance with the furnace specifications.

　　Silicon carbide cylinder manufacturer: During the resistive heating process, in addition to the fundamental reactions that produce silicon carbide, various impurities present in the charge material also undergo a series of chemical and physical changes and displacements. Salt is no exception. As refining progresses, the charge material continuously decreases, and its surface deforms and sinks. The carbon monoxide generated during the reaction diffuses into the atmosphere, becoming a harmful component that pollutes the surrounding air.

　　After the power outage, the reaction process has essentially come to an end. However, due to the large size of the furnace and its substantial thermal inertia, it takes some time for the furnace to cool down. The temperature inside the furnace remains high enough to sustain chemical reactions, so a small amount of carbon monoxide continues to escape from the furnace surface. In high-power electric furnaces, this residual reaction can persist for 3 to 4 hours continuously. Compared with the reactions occurring during power transmission, the reactions at this stage are negligible. Nevertheless, the surface temperature of the furnace has already begun to drop, resulting in incomplete combustion of the carbon monoxide. From the perspective of occupational safety, we should pay sufficient attention to this situation.

　　After the power outage, once the furnace has cooled down for a period of time, the furnace walls can be removed, and then the various materials inside the furnace can be gradually taken out.

　　Silicon carbide cylinder manufacturer: After refining, the materials inside the furnace are structured from outside to inside as follows:

　　(1) Unreacted substances

　　This portion of the charge material does not reach the reaction temperature during smelting, so no chemical reactions occur; instead, it serves solely to maintain thermal insulation. The location of this material within the furnace is referred to as the insulation zone. The preparation methods for the insulation-zone charge material and the reaction-zone charge material differ, as do the methods for utilizing this portion of the charge material after refining. One process involves placing new materials into a specific area of the insulation zone within the furnace and, after refining, blending them into the reactive charge material. This method is known as "calcined charge." If the unreacted materials from the insulation zone are regenerated—by adding a small amount of coke and an appropriate quantity of sawdust to produce reusable insulation material—such materials are referred to as "waste materials." (2) Oxygen-carbon silicon layer

　　In fact, the material in this layer is semi-reactive, consisting primarily of unreacted carbon and silica, with a smaller proportion—approximately 20% to 50%—of already reacted silicon carbide. The unreacted silica and carbon have undergone significant morphological changes, making them distinct from their original forms. In the oxygen-silicon carbide layer, the silica and carbon no longer retain their original structures; instead, they have combined to form an amorphous substance that appears gray or yellow. However, these compounds are loosely bonded, and upon cooling, they can easily be reduced to powder with gentle pressure. On the other hand, the particle size and shape of the silica in the waste material remain nearly identical to those before melting, though its transparency has changed. As for the carbon powder, it has simply sintered into larger particles, with volatile substances having been removed. Consequently, the oxygen-silicon carbide layer is readily identifiable.

　　 Silicon carbide cylinder Manufacturer: (3) Adhesive Layer

　　The bonding layer is a material that adheres tightly between the silicon oxycarbide and the amorphous layer; it contains a high concentration of impurities such as iron, aluminum, calcium, magnesium, and others. When expressed in terms of metal oxides, the impurity content can reach as high as 5–10%. The phase composition of the adhesive mainly consists of unreacted silica, carbon, and formed silicon carbide (SiC, accounting for approximately 40–60%), as well as silicates of iron, aluminum, calcium, and magnesium. This layer can sometimes be difficult to distinguish from the silicon oxycarbide and the amorphous material itself. However, when there are many impurities present in the furnace, this layer becomes quite distinct—especially in black silicon carbide furnaces.

　　(4) Amorphous layer

　　The amorphous layer is primarily composed of cubic silicon carbide (-SiC), with silicon carbide accounting for about 70% to 90% of its composition. It also contains significant amounts of incompletely reacted carbon and silica, while the total content of metal oxides such as iron, aluminum, calcium, and magnesium can reach 2% to 5%. This material is relatively loose, making it easy to crush into powder. In black silicon carbide furnaces, the amorphous material is typically black, whereas in green silicon carbide furnaces, it is often yellowish-green; some samples exhibit a color gradient from black on the outside to gradually grayish-yellow or yellowish-green toward the interior. The amorphous layer is usually uniform. However, when the silica particles used as raw materials are too coarse or the fixed carbon content in the coke is too low, the amorphous layer may sometimes contain more small gas bubbles.

　　(5) Secondary silicon carbide

　　The layer is SiC, but its crystals are small and brittle, with a high impurity content. The silicon carbide content is only 90–95%, making it unsuitable for use as an abrasive material. The secondary product can be easily distinguished from the amorphous material in appearance. The amorphous SiC layer appears as a powdery, matte substance, whereas the secondary product consists of hexagonal crystals with well-defined crystal faces that exhibit a glossy, mirror-like reflection. There is no fundamental difference between the secondary-grade and primary-grade materials; they are simply categorized into secondary-grade and primary-grade layers on the crystallization cylinder based entirely on their intended use and quality requirements. Some of the other material layers mentioned above differ significantly from one another in terms of properties and appearance. Certain layers naturally form in layered structures. Like the amorphous material, the second layer sometimes contains small, bubble-like cavities.

　　(6) First-grade silicon carbide crystal ingot

　　This is the main product of the resistance furnace—a coarse silicon carbide crystal with a silicon carbide content of over 96%. Its thickness ranges from 50 mm to 450 mm, depending on the furnace’s power and location. The crystals are thick and dense, and they appear black or green. (7) Graphite in the furnace core.

　　 Silicon carbide cylinder Manufacturer’s note: The graphite located close to the inner wall of the crystallization cylinder is derived from the decomposition of silicon carbide; its shape still retains the original crystal appearance of the silicon carbide. On the inner side of the decomposed graphite layer lies the graphite core that was originally placed in the furnace. After multiple high-temperature treatments, this core has achieved an even more complete degree of graphitization. However, its shape differs from the original configuration. These two types of graphite are mixed and recycled as core material for the next furnace batch.