Since the first production of electric heating elements by a Chinese factory in 1958, the application of electric heating tubes has become increasingly widespread. With the development of the national economy, these components have entered nearly every household. As one of the most critical materials in a heating tube, the demand for Magnesium Oxide (MgO) powder is growing. Because it significantly impacts the performance of the heating tube, it is essential to understand it thoroughly.
Introduction to Material Characteristics of Magnesium Oxide
1. Why Choose Magnesium Oxide?
Currently, the primary filling materials for heating tubes include quartz sand, alumina, and electrical-grade magnesium oxide. MgO is chosen because it is both an electrical insulator and a thermal conductor. It offers high stability, is inexpensive, and is easy to source, leading to its long-term widespread use in heating tubes.
2. Manufacturing Method
MgO is refined and then fused in an electric furnace at 2800°C to achieve complete crystallization and stability. Once cooled into blocks, the inner portion reaches high purity (specific gravity of 3.58–3.6) due to cohesive forces. These blocks are crushed, iron impurities are removed, and the material is screened into the current filling powder, typically ranging from 40 to 325 mesh.
3. Technical Data
- Molecular Formula: MgO
- Molecular Weight: 40.3
- Specific Gravity: 3.58
- Melting Point: 2800°C
- Hardness: 5.5 (Mohs)
- Specific Heat: 0.25 Kcal/kg°C
- Crystal Shape: Seawater-derived MgO forms square crystals, while magnesite-derived MgO forms spherical crystals. Consequently, magnesite MgO offers better filling density. In terms of electrical performance, seawater MgO is often superior due to higher purity after processing, while magnesite MgO typically has lower
B2O3B_2O_3B2O3content. - Note: Other impurities in MgO can improve thermal conductivity but will decrease insulation performance.
4. Heat Treatment
After heat treatment, the darker the MgO color, the more impurities have precipitated from the crystals. In particles with a 1mm diameter, Iron (Fe) does not precipitate easily. However, in particles of 0.5mm, Fe precipitates and combines with oxygen to form Fe2O3.
5. Impact of Mesh Size
Mesh size has a massive impact on the heating tube:
- Fine Mesh (Small particles): These have a “fish scale” surface and a high surface area per unit weight. They absorb moisture easily, shortening the tube’s lifespan and potentially polluting the filling environment.
- Coarse Mesh (Large particles): If particles are too large, they can damage the surface of the heating wire, causing eccentricity (off-center wires), “disordered” wires, or hollow cores within the heating coil.
6. Hygroscopicity (Moisture Absorption)
MgO stored for over a year undergoes chemical moisture absorption. Before use, it must be dried at 500°C for 4 hours. For short-term storage, it should be dried at 150°C for 1 hour before use.
7. Thermal Conductivity and Temperature
At a filling density of 2.9g/cm³, straight heating tubes have a specific thermal conductivity coefficient. However, if a tube is bent (e.g., radius = 15mm), the thermal conductivity at the bend drops to roughly half that of the straight section. This high heat accumulation can cause the wire to break. The remedy is to apply hydraulic pressure to the bent section during processing.
8. Sintering
At temperatures above 1000°C, MgO particles may bond together like cement; this is called sintering. This phenomenon is primarily caused by impurities like B2O3 and Fe2O3.
9. Impurity Capacity
The Fe2O3 contained in MgO will cause the insulation resistance of the heating tube to gradually decrease during continuous use. This is due to the partial pressure of O2 within the Fe2O3.
Definition of Electrical Grade Magnesium Oxide
Electrical grade MgO is produced by crushing fused crystalline magnesium oxide blocks and blending different particle sizes (mesh) in specific proportions. It is used directly or after modification as a high-temperature, thermally conductive, and insulating medium.
Classification of Electrical Grade MgO
MgO is classified into four categories based on production method and application:
- Common MgO (Code: P)
- Low-Temperature Moisture-Proof MgO (Code: D)
- Medium-Temperature Moisture-Proof MgO (Code: Z)
- High-Temperature MgO (Code: G)
Main Advantages and Disadvantages
Advantages:
- Excellent insulation properties and high dielectric strength.
- High thermal conductivity, effectively transferring heat from the wire to the metal sheath.
- Good heat resistance and vibration resistance.
Disadvantages:
MgO easily absorbs Carbon Dioxide (CO2) and water from the air. While it is only slightly soluble in pure water, its solubility increases rapidly after absorbing CO2. It reacts with water to produce Magnesium Hydroxide [Mg(OH)₂], which causes insulation resistance to drop sharply. This moisture absorption is the fundamental reason for short lifespans or electrical failure in heating tubes. To overcome this, manufacturers often use silicon-modified fused MgO.
Precautions for Production and Use
- Testing: Since MgO is critical, it must undergo performance testing (especially leakage current tests) before being added to the production line.
- Quality vs. Cost: Many companies attempt to save costs by using lower-grade powder (e.g., using medium-temp powder for high-temp applications). This leads to frequent tube bursting, carbonization (blackening), decreased insulation, and poor pressure resistance. Do not sacrifice quality for cost.
- Storage: MgO should be stored in a ventilated, dry environment, away from direct sunlight and dust.
Causes of MgO Powder Blackening
- Ion Precipitation: Precipitation of metal ions from the stainless steel sheath can discolor the powder.
- Low Oxygen Pressure: Low oxygen levels within the sealed heating tube can cause the MgO powder to turn black.