Boron: Properties, Production and Uses

Boron is the second hardest element, surpassed only by the diamond allotrope of carbon. α-rhombohedral boron ranges from red to brown, β-rhombohedral boron is lustrous gray to black, and amorphous boron is brown to gray.

The electrical resistivity of boron exhibits a drastic change with temperature, varying from 10¹¹ W.cm⁻¹ at 160 °C to 10⁶ W.cm⁻¹ at 20 °C and to 0.1 W.cm⁻¹ at 700 °C for polycrystalline β-rhombohedral boron, demonstrating semiconductor behavior.

2. Chemical Properties of Boron

Boron’s small atomic radius (0.25 nm) and high ionization potentials significantly influence its chemical properties. The following key properties and reactions are summarized:

Ionization energy:

• B → B⁺: 798 kJ/mol (8.27 eV)
• B⁺ → B²⁺: 2426 kJ/mol (25.15 eV)
• B²⁺ → B³⁺: 3658 kJ/mol (37.92 eV)

Standard electrode potential:

B + 3 H₂ + OH → H₃BO₃ + 3 H⁺ + 3 e^– (0.73 V)

Electron affinity: 32 kJ/mol (0.332 eV)

Electronegativity:

• Pauling: 2.04
• Mulliken: 2.01

Ionic radius: 0.25 nm

Atomic radius: 0.80 – 0.95 nm (depending on bonding type)

Standard enthalpy of formation:

• BF₃: 1136 kJ/mol
• BCl₃: 402 kJ/mol
• BBr₃: 239 kJ/mol
• B₂O₃: 1269 kJ/mol
• BN: 256 kJ/mol

Boron’s electronic configuration, 2s²2p¹, dictates predominant trivalency, though simple B³⁺ ions do not exist. Boron has more orbitals (4) for bonding than electrons (3), making it an electron-pair acceptor, a Lewis acid, and prone to forming multi-center bonds. It exhibits a high affinity for oxygen and tends to combine with most metals, forming refractory alloy-like metal borides.

Elemental boron’s chemical behavior depends on its morphology and particle size. Crystalline boron is generally unreactive, while amorphous boron reacts more readily. All boron modifications show relative resistance to chemical attack at room temperature.

Reactions include:

• Indirect synthesis of boron hydrides with hydrogen

• Trihalide formation via reaction with halogens: 2 B + 3 X₂ → 2 BX₃

• Spontaneous reaction with fluorine and reactions with chlorine and bromine at elevated temperatures

• Reaction with oxygen, sulfur, selenium, and nitrogen under specific conditions

• Boron-carbon reaction to produce B₁₂C₃ above 2000 °C, and reactions with silicon to form B₄Si and B₆Si

• Reactivity with various heterocyclic compounds containing C, N, S, O, and boron

Elemental boron serves as an effective reducing agent in reactions with water vapor, carbon monoxide, nitrogen oxides, sulfur dioxide, and transition metal oxides.

Boron is unresponsive to aqueous NaOH but reacts completely with molten Na₂CO₃ or molten mixtures of sodium carbonate and sodium nitrate. Boiling nonoxidizing acids, such as HF, HCl, HBr, or dilute sulfuric or phosphoric acid, do not attack boron, but concentrated HNO₃ or HNO₃–H₂O₂ mixtures lead to vigorous reactions.

3. Production of Boron

Various preparative methods for obtaining boron exist, with the following considered the most significant:

1.Reduction of Boric Oxide with Magnesium:

B₂O₃ + 3 Mg → 2 B + 3 MgO

MgO simultaneously reacts with excess B₂O₃:

MgO + B₂O₃ → Mg(BO₂)₂

MgO + 2 B₂O₃ → MgB₄O₇

This reaction is rapid, highly exothermic, and finely divided material may react explosively.

A smoother reaction occurs with an excess of B₂O₃:

2 B₂O₃ + 3 Mg → 2 B + Mg₃(BO₃)₂

4 B₂O₃ + 3 Mg → 2 B + 3 Mg(BO₂)₂

The optimum B₂O₃ : Mg ratio is about 1.8 : 3.

The reaction is conducted in vertical steel retorts shielded from oxygen by an argon flow, initiated by an electric spark, igniter mixture, or external heating. After cooling, the reaction mass is crushed and leached with hydrochloric acid to obtain crude amorphous boron of 86 – 88% purity.

Upgrading methods include treatment with B₂O₃ or KHF₂ and KBF₄, subsequent leaching with acid, and final heating in a vacuum to remove boron suboxide and metals.

2. Reduction of KBF₄ by Sodium:

KBF₄ + 3 Na → 3 NaF + KF + B

This method was used for commercial boron production in Germany until the end of the 1950s. Challenges include incomplete reaction and the formation of nonremovable metal borides.

3. Reduction of Boron Halides with Hydrogen:

Very pure boron (> 99% B) samples can be obtained by reducing boron halides with hydrogen, especially BBr₃ and BCl₃. This method is also preferred for laboratory synthesis.

The halides can be purified by distillation before reduction. Efficiency of the reaction is relatively low, with yields of 5 – 25%, and unreacted boron halide must be recycled or removed, making the process complex and expensive.

4. Thermal Decomposition of Boron Compounds:

Very pure boron can be obtained by the thermal decomposition of BI₃ or boron hydrides on tungsten wires or other incandescent filaments. Boron of 99.9999% purity (six nines) was obtained by decomposing diborane and subsequent zone melting.

5. Electrolysis of Molten Borates or KBF₄ is not a prominent method for commercial or laboratory preparation, despite being mentioned in numerous articles.

Refinement of boron can be achieved by zone melting or the volatilization of impurities in high vacuum or hydrogen at 2000 °C.

4. Uses of Boron

Amorphous boron, particularly in the range of 90% to 95% purity, finds diverse applications in various industries:

1. Boron is used as additives in pyrotechnic mixtures, including flares, igniters, delay compositions, solid rocket propellant fuels, and explosives.
2. Bron is required for the preparation of refractory metal boride additives utilized in cemented carbides.
3. Boron serves as a useful reducing additive in fluxes for soldering stainless steel.
4. High-purity boron (> 99.99%) is employed in electronics. It acts as a ppm additive for germanium and silicon to produce p-type semiconductors.
5. Crystalline high-purity boron is used in thermistors and delay lines.
6. Boron filaments were developed as reinforcing materials for lightweight, stiff composites used in commercial and military aircraft. However, graphite filaments have largely replaced boron filaments in many applications.
7. Thin films of boron are used in neutron counters and boron powder dispersed in polyethylene castings serves for shielding against thermal neutrons in nuclear centers.
8. Boron composites with Al or Fe, especially when enriched in 10B, are utilized as neutron shields and absorbers in nuclear reactors.
9. Boron functions as an effective deoxidizing agent, particularly in the production of pure copper.
10. It plays a role in the preparation of amorphous magnetic metals and alloys.
11. In the form of ferroboron boron is used in microalloyed steel (0.001% B) to impart excellent resistance to stress cracks and enhance tensile strength and hardness.

5. Toxicology of Boron

Elemental boron is considered to be nontoxic because of its chemical inertness and insolubility.

Boron is a naturally occurring element that is found in a variety of minerals and compounds. It is an essential trace nutrient for humans, but exposure to high levels of boron can be toxic.

Acute Toxicity

The acute toxicity of boron depends on the route of exposure. Ingestion of large amounts of boron can cause nausea, vomiting, diarrhea, and abdominal pain. In severe cases, it can lead to death. Inhalation of boron dust can cause irritation of the lungs and respiratory problems. Skin contact with boron compounds can cause irritation, redness, and burning.

Chronic Toxicity

Long-term exposure to high levels of boron can cause a number of health problems, including:

• Reproductive problems
• Developmental problems in children
• Kidney damage
• Liver damage
• Immune system problems

Carcinogenicity

Boron is not considered to be a human carcinogen.

Reference

• Boron and Boron Alloys; Ullmann’s Encyclopedia of Industrial Chemistry. – https://onlinelibrary.wiley.com/doi/10.1002/14356007.a04_281