Silicon carbide rods for high-temperature refractory materials

I. Definition:

Silicon carbide (SiC) rods are rod-shaped or tubular non-metallic high-temperature electric heating elements made primarily from high-purity green hexagonal silicon carbide, processed into blanks according to a specific material ratio, and then sintered at 2200℃ through siliconization and recrystallization. Normal operating temperature in an oxidizing atmosphere can reach 1450℃, with continuous use up to 2000 hours. It is a rod-shaped non-metallic high-temperature electric heating element. Domestic SiC rod manufacturers use largely similar processes and manufacturing techniques, and the operating temperature has consistently remained below 1520℃.

II. Chemical Properties:

Silicon carbide rods have good chemical stability and strong acid resistance. Under high-temperature conditions, alkaline substances have a corrosive effect on them.

Prolonged use of silicon carbide rod elements above 1000℃ can lead to the following reactions with oxygen and water vapor: ①SiC + 2O2 → SiO2 + CO2 ②SiC + 4H2O = SiO2 + 4H2 + CO2. This causes a gradual increase in the SiO2 content within the element, resulting in a slow increase in resistance, signifying aging. Excessive water vapor will promote SiC oxidation. The H2 produced by reaction ② combines with O2 in the air to form H2O, creating a vicious cycle and reducing component lifespan. Hydrogen (H2) can reduce the mechanical strength of components. Nitrogen (N2) can prevent SiC oxidation below 1200℃, but reacts with SiC above 1350℃, causing SiC decomposition. Chlorine (Cl2) can completely decompose SiC.

III. Precautions:

1. Silicon carbide rods are hard and brittle, easily broken by severe vibration and impact. Therefore, extra care must be taken during transportation and handling.

2. The length of the heating element of the silicon carbide rod should be equal to the width of the furnace chamber. If the heating element extends into the furnace wall, it can easily burn the furnace wall.

3. The length of the cold end of the silicon carbide rod should be equal to the furnace wall thickness plus the length of the cold end extending out of the furnace wall. Generally, the cold end extension length is 50~150mm to allow for cooling of the cold end and connection clamps.

4. The inner diameter of the furnace through which the silicon carbide rods are installed should be 1.4 to 1.6 times the outer diameter of the cold end. If the furnace opening is too small or the filling material inside is too tight, it will hinder the free expansion and contraction of the silicon carbide rods at high temperatures, leading to rod breakage. During installation, the silicon carbide rods should be able to rotate freely 360 degrees.

5. The distance between the silicon carbide rods and the heated object and furnace wall should be greater than or equal to three times the diameter of the heating element. The center-to-center distance between silicon carbide rods should be no less than four times the diameter of their heating elements.

6. The cold end of the silicon carbide rod is connected to the main circuit using aluminum braids or aluminum foil. The clamps on the cold end must be securely fastened.

7. Newly built furnaces or electric furnaces that have not been used for a long time should be preheated before use, using old rods or other heat sources.

8. Silicon carbide rods should be protected from moisture during storage. Moisture can easily cause the aluminum layer on the cold end to decompose and peel off, increasing the contact resistance between the cold end and the clamps, and making the silicon carbide rods prone to cracking when energized.

9. Before use, silicon carbide rods must be matched for resistance. Connect silicon carbide rods with the same or similar resistance values ​​together first.

10. Equip silicon carbide rods with a voltage regulator. Initially, the voltage should be half of the normal operating voltage. After stabilizing for a period, gradually increase the voltage. This prevents the silicon carbide rods from breaking due to rapid temperature rise.

11. Silicon carbide rods have a long continuous service life but a short intermittent service life.

12. When using silicon carbide rods, select appropriate surface load density and operating temperature. The operating temperature should not exceed 1650℃; in environments with harmful gases, prevent the silicon carbide rods from reacting chemically with the harmful gases.

13. When replacing silicon carbide rods, select those with resistance close to that of the silicon carbide rods currently operating in the furnace. If necessary, replace the entire furnace of silicon carbide rods. This helps extend the service life of the silicon carbide rods. Silicon carbide rods that cannot be removed, if with suitable resistance values, can be replaced later in the furnace's operation.

14. Prevent molten metal from splashing onto the silicon carbide rods, as this can easily cause them to break.

15. Prevent corrosion of the silicon carbide rod by alkalis, alkaline earth metals, and alkaline oxides.

16. Regularly observe whether the readings of the ammeter, voltmeter, and thermometer are normal; whether the cold end clamps are loose, oxidized and blackened, or sparking; whether the silicon carbide rod is broken; and whether the heating element of the silicon carbide rod is uniformly heated.