High quality material - magnesium oxide powder

What is the role of magnesium oxide powder in the electric heating tube?

Since my country had the first electric heating tube factory to produce electric heating elements in 1958, today, electric heating tube products have become more and more widely used. With the development of society and national economy, electric heating tubes have become more and more widely used in the market. Popularity has entered thousands of households, and as one of the most critical materials for electric heating tubes, magnesium oxide powder is in increasing demand. Because it has a great impact on the performance of electric heating tubes, it is necessary for us to have a good understanding of it. With a full understanding, the following is a summary of my personal work experience in this area in order to share with you.

Magnesium Oxide Introduction and Material Properties

1. At present, the main filling materials for electric heating tubes are: quartz sand, alumina, and electrical grade magnesium oxide. And why choose magnesium oxide as the filling material? The main reason is that it is an electric insulator, a good conductor of heat, high stability, cheap price, easy to buy, and large storage capacity, so it has been widely used in electric heating tubes for a long time.

2. Manufacture method of magnesia After refining, magnesia needs to undergo electric melting at 2800 ℃ to make its crystallization completely stable and then cooled into a block, the middle part has a higher purity (specific gravity 3.58-3.6) due to the influence of cohesion. The block is crushed by a hammer, and then the iron filings are removed, and then the mesh is mixed to form the magnesium oxide powder as the filling material currently used. The mesh specification currently used is 40-325 mesh.

3. Magnesium oxide related data Molecular formula: MgO Molecular weight: 40.3 Specific gravity: 3.58 Melting point: 2800°C Hardness: 5. Specific heat: 0.25Kcal/kg°C Seawater magnesium oxide crystals are cube-shaped, and magnesite magnesium oxide crystals are spherical, so The filling density is better with magnesite magnesia. In terms of electrical properties, seawater magnesia is better because it contains more impurities, but the performance after processing is better with magnesite magnesia, because the content of B2O3 in magnesite magnesia is less.

Magnesium oxide contains other impurities that increase thermal conductivity but reduce insulation.

4. Heat treatment of magnesia After magnesia is heat-treated, the color becomes thicker, indicating that more impurities are precipitated by crystallization. When the particle diameter is up to 1mm, Fe is not easy to precipitate after heat treatment. If the particle diameter is up to 0.5mm, Fe will be precipitated and combined with oxygen to form Fe2O3 after heat treatment.

5. The mesh size of magnesium oxide has too much influence on the electric heating tube. The mesh size is too large: its particle size is small, because the surface is scale-like and the area per unit weight is large, it is easy to absorb moisture, which affects the life of the electric heating tube; to serious pollution. The mesh size is too small: the larger the particle size, the surface of the heating wire will be crushed after shrinking, which will affect the life of the heating tube. When filling, it is easy to cause bad phenomena such as filament eccentricity, messy wire, and hollow core of the heating wire ring.

6. Hygroscopic properties of magnesium oxide Magnesium oxide will produce chemical moisture absorption after being stored for more than one year. Before use, it must be dried at 500°C for 4 hours. For short-term storage, it is best to dry at 150°C for 1 hour before use.

7. The relationship between magnesium oxide heat conduction and temperature. When the filling density of the electric heating tube is 2.9/cm3, the heat conduction coefficient of the linear electric heating tube. When the bending radius of the electric heating tube is 15mm, the heat conduction of the curved part is 1/2 times that of the straight line. It will cause broken wire, and the remedy is to hydraulically process the bent part.

8. Magnesium oxide sintering phenomenon After the electric heating tube is heat-treated at 1000-1050°C, the MgO particles are bonded with cement. This phenomenon is called sintering. The reason for this phenomenon is mainly caused by B2O3 or Fe2O3 contained in magnesium oxide.

9. Magnesium oxide impurity capacity such as Fe2O3 contained in MgO will gradually reduce the insulation resistance of the electric heating tube when the electric heating tube is used continuously. This is caused by the partial pressure of O2 in Fe2O3.

Definition of electrical grade magnesium oxide

The fused crystalline magnesium oxide blocks are crushed and mixed with different particle sizes (mesh numbers) in a certain proportion, and used directly or after modification in tubular electric heating elements as an insulating medium for heat conduction at high temperatures.

Classification of electrical grade magnesium oxide

Magnesium oxide is divided into the following four categories according to the production method and main application:

1. Common magnesium oxide classification code P

2. Classification code of low-temperature moisture-proof magnesium oxide D

3. Classification code Z for medium temperature and moisture-proof magnesium oxide

4. Classification code of high temperature magnesium oxide G

Main advantages and disadvantages of electrical grade magnesium oxide

1. Advantages

a. Good insulation performance and electric strength

b. It has good thermal conductivity and can conduct the heat from the heating wire to the metal tube

c. Good heat resistance and vibration resistance

2. Disadvantages

It is easy to absorb carbon dioxide and water in the air, and is slightly soluble in pure water, but the solubility increases rapidly after absorbing carbon dioxide, and reacts with water to form magnesium hydroxide, which makes the insulation resistance drop rapidly. Therefore, the electrical insulation performance of the electric heating tube filled with magnesium oxide raw materials decreases during storage and use, causing electrical breakdown and damaging the electric heating tube. This is the root cause of the short service life of electric heating tubes filled with magnesium oxide raw materials.

In order to overcome the water absorption of magnesia raw materials, magnesium powder manufacturers have tried many methods, one of which is to modify fused magnesia with organic silicon.

Matters needing attention and problems in the production and use of magnesium oxide powder

1. Since magnesium oxide is one of the most critical materials in electric heating tubes, it is necessary to perform related performance tests on magnesium powder before adding magnesium powder, so as to prevent problems before they occur. Especially leakage current test is more important.

2. At present, in order to save costs, many enterprises often lower the grade and require low-priced magnesium powder instead of high-priced magnesium powder or use medium-temperature powder instead of high-temperature powder, resulting in frequent tube explosions, magnesium powder carbonization and blackening of electric heating tubes, decreased insulation resistance, and withstand voltage. Poor or poor thermal performance and so on. Therefore, you should be cautious when choosing magnesium powder, and you can't just ignore the quality for the sake of cost, which will cause serious consequences.

3. The storage and placement of magnesium powder should be ventilated and dry, without sunlight and also to prevent dust and other pollution.

The reason for the blackening of magnesium oxide powder

1. The precipitation of ions in stainless steel causes the magnesium powder to turn black.

2. Due to the low oxygen pressure in the electric heating tube, the magnesium powder turns black.