Preparation Methods of Amorphous Boron Powder
Amorphous boron powder is mainly prepared by six mainstream methods: metal thermal reduction, boron halide hydrogen reduction, plasma synthesis, borane pyrolysis, electrolysis, self-propagating high-temperature synthesis & silicon thermal reduction. Among them, magnesium thermal reduction is the most widely used in industry, while plasma synthesis and boron trichloride hydrogen reduction are preferred for high-purity and nano-grade products.
1. Magnesium Thermal Reduction (Mainstream Industrial Method, Low Cost)
Principle
Dehydrate boric acid to prepare boron trioxide, then reduce it with magnesium at high temperature.
Process
Boric acid → dehydration → boron anhydride → mixing with magnesium powder → high-temperature reduction at 850–950℃ → crude boron product → pickling with hydrochloric acid → water washing → secondary purification → drying → sieving.
Advantages & Disadvantages
- Advantages: Low cost, stable mass production, particle size 0.5–2 μm, purity 92%–98%.
- Disadvantages: Contains magnesium oxide and boron-magnesium impurities requiring deep purification; difficult to reach electronic grade purity.
2. Boron Halide Hydrogen Reduction (First Choice for High-Purity & Electronic Grade)
Principle
High-purity boron trichloride reacts with hydrogen under high temperature gas phase condition to form amorphous boron.
Reaction temperature: 1200–1500℃
Advantages & Disadvantages
- Advantages: High purity up to 99.9%–99.999%, ultra-low impurity content, controllable particle size 0.1–1 μm, ideal for semiconductor doping.
- Disadvantages: Expensive equipment, boron trichloride is highly toxic and corrosive, high production cost.
3. Plasma Synthesis Method (Nano High-Purity Grade)
Principle
Boron trichloride and hydrogen react instantly under ultra-high temperature plasma arc, rapid quenching inhibits crystallization to directly synthesize nano amorphous boron powder.
Advantages & Disadvantages
- Advantages: Nano particle size, high chemical activity, high purity, stable amorphous structure.
- Disadvantages: Complex equipment, high energy consumption, limited large-scale production capacity.
4. Borane Pyrolysis Method (Laboratory & Small-Batch High-Purity Production)
Principle
Diborane is pyrolyzed at 400–800℃ to produce amorphous boron; crystal boron will form when temperature exceeds 1000℃.
Features
Available with purity up to 99.99% and ultra-fine particle size; diborane is toxic, spontaneous combustible and explosive, only applicable to laboratory research and small batch production.
5. Molten Salt Electrolysis Method (Special & Nuclear Grade)
Principle
Take fluoroborate as molten electrolyte, amorphous boron precipitates on cathode through electrolysis at 700–800℃.
Features
Purity reaches 95%–98%, suitable for boron-10 enriched nuclear shielding materials; high-temperature corrosion resistance required for equipment, high energy consumption, narrow application range.
6. Self-Propagating High-Temperature Synthesis & Silicon Thermal Reduction
- Self-propagating synthesis: Trigger rapid reaction by local ignition, low purity 92%–94%, fine uniform particles.
- Silicon thermal reduction: Prepare spherical amorphous boron powder, by-products are water-soluble and easy to remove via washing.
Comparison of Various Preparation Methods
| Preparation Method | Purity Range | Particle Size | Production Cost | Typical Application |
|---|---|---|---|---|
| Magnesium Thermal Reduction | 92%–98% | 0.5–2 μm | Low | Solid propellant, ceramic sintering additive |
| Boron Halide Hydrogen Reduction | 99.9%–99.999% | 0.1–1 μm | High | Semiconductor doping, electronic industry |
| Plasma Synthesis | 99.9%–99.97% | 30–100 nm | Medium-High | Nano polishing materials, high-energy materials |
| Borane Pyrolysis | Up to 99.99% | 50–200 nm | Extremely High | Scientific research, special advanced materials |
| Molten Salt Electrolysis | 95%–98% | 1–5 μm | Medium | Nuclear radiation shielding, boron isotope e |