What gases can be passed through an experimental rotary tube furnace?

The experimental rotary tube furnace can be filled with various gases according to experimental requirements, and the gas selection mainly depends on material properties, reaction types, and process requirements. The following is a detailed explanation of common gas types and their application scenarios:

1. Inert gas
Nitrogen (N ₂)
Application scenarios:
Protective atmosphere: prevent metal, ceramic and other materials from oxidizing at high temperatures.
Carrier gas: used for transporting powders or gaseous reactants.
Example: The sintering process of lithium battery cathode materials (such as LiCoO ₂) needs to be completed in nitrogen gas.

Argon gas (Ar)
Application scenarios:
High purity protection: suitable for materials sensitive to oxidation, such as titanium alloys and semiconductor materials.
Plasma treatment: used as a substrate gas in plasma assisted deposition or etching processes.
Example: In metal organic chemical vapor deposition (MOCVD), argon gas is used to stabilize the reaction chamber.

Helium (He)
Application scenarios:
Thermal conductivity testing: used as a carrier gas in gas chromatography analysis.
Rapid cooling: Utilizing its high thermal conductivity to accelerate sample cooling.
Example: After heat treatment of high-temperature alloys, helium can shorten the cooling time to several minutes.

2. Reducing gas
Hydrogen (H ₂)
Application scenarios:
Metal reduction: The reduction of metal oxides to pure metals (such as tungsten and molybdenum).
Carburizing treatment: forming a carbide layer on the surface of steel to increase hardness.
Example: The hydrogen reduction process of tungsten powder requires the introduction of hydrogen gas above 1000 ℃.

Carbon monoxide (CO)
Application scenarios:
Metal reduction: replacing hydrogen for high-temperature reduction (such as direct reduction of iron ore).
Carbonization reaction: used as a carbon source in the synthesis of silicon carbide (SiC).
Example: CO reacts with silicon powder at 1400 ℃ to form SiC.

Ammonia gas (NH3)
Application scenarios:
Nitriding treatment: forming a nitride layer (such as TiN coating) on the metal surface.
Doping gas: Introducing nitrogen elements into semiconductor processes.
Example: Nitrogen atoms produced by ammonia cracking can infiltrate the surface of steel to form a hardened layer.

3. Oxidizing gas
Oxygen (O ₂)
Application scenarios:
Oxidation sintering: promotes the densification of ceramic materials (such as Al ₂ O3 ceramics).
Catalyst activation: adjust the surface oxygen species in the preparation of noble metal catalysts (such as Pt/Al ₂ O ∨).
Example: Alumina ceramics sintered at 1600 ℃ in oxygen can achieve high density.

air
Application scenarios:
Economic oxidation: Materials with high oxidation resistance, such as glass and some metal oxides.
Example: The melting of ordinary glass can be carried out in air.

4. Reactive gas
Methane (CH ₄)
Application scenarios:
Chemical Vapor Deposition (CVD): Decomposes to form graphene on a nickel substrate.
Carburizing treatment: forming a high carbon layer on the surface of steel.
Example: Methane can be decomposed at 1000 ℃ to deposit graphene with controllable thickness.

Hydrogen chloride (HCl)
Application scenarios:
Surface etching: Removing metal impurities in semiconductor processes.
Chlorination reaction: Reacts with metals to generate volatile chlorides (such as SiCl ₄).
Example: HCl reacts with polycrystalline silicon at 900 ℃ to purify silicon.

Water vapor (H ₂ O)
Application scenarios:
Hydrothermal reaction: synthesis of nanomaterials in an autoclave.
Oxidation treatment: Forming an oxide layer on the surface of a metal (such as anodizing aluminum).
Example: Water vapor reacts with titanium at 600 ℃ to form TiO ₂ nanotubes.

5. Mixed gas
Ar + H₂（90:10）
Application scenarios:
Reductive protection: prevents oxidation during tungsten wire annealing while providing a reducing environment.
Example: Introducing this mixture of gas into tungsten wire at 1800 ℃ can eliminate processing stress.

N₂ + H₂（95:5）
Application scenarios:
Nitriding treatment: forming a nitride layer on the surface of steel to improve wear resistance.
Example: The mixture can be treated at 550 ℃ to obtain a 0.3mm thick nitriding layer.

Ar + 5% CH₄
Application scenarios:
CVD precursor: provides a carbon source in diamond film deposition.
Example: The mixed gas can achieve diamond nucleation at 900 ℃.

6. Key factors in gas selection
Material compatibility:
Avoid chemical reactions between gases and materials or furnace tubes (such as quartz, corundum).

Security risks:
Flammable gases such as hydrogen and methane require leak detection and explosion-proof devices.

Process control:
High precision mass flow controller (MFC) can achieve precise adjustment of gas ratio (error ≤± 0.5%).

7. Application Cases
Positive electrode material for lithium batteries:
Process: Li ₂ CO ∝+Co ∝ O ₄ → LiCoO ₂
Conditions: 850 ℃, oxygen atmosphere, rotation speed of 5 rpm

Preparation of Graphene:
Process: CH ₄ → C (graphene)+2H ₂
Conditions: 1000 ℃, copper substrate, methane flow rate of 50 sccm

Metal carburizing:
Process: Fe+CH ₄ → Fe ∝ C+H ₂
Conditions: 950 ℃, Ar+5% CH ₄ mixture, processing time 2 hours

Summarize
The gas selection for experimental rotary tube furnaces should be based on material characteristics, reaction mechanisms, and process objectives. Inert gases (such as N ₂, Ar) are suitable for protective sintering, reducing gases (such as H ₂, CO) are used for metal reduction, oxidizing gases (such as O ₂, air) are used for oxidation treatment, active gases (such as CH ₄, HCl) are used for chemical deposition or etching, and mixed gases can achieve multifunctional synergistic effects. In practical applications, it is necessary to ensure the reliability and safety of the experiment through gas flow control, atmosphere purity monitoring, and safety protection measures.