What gases can be passed through a high-temperature vacuum atmosphere tube furnace?

The high-temperature vacuum atmosphere tube furnace can be filled with various gases according to different process requirements, including inert gases, reducing gases, oxidizing gases, carbon/nitrogen source gases, composite gases, and special gases. These gases play a crucial role in material preparation, heat treatment, surface modification, and other processes. The following are specific gas types and their application instructions:

1. Inert gas
Function: Protect materials from oxidation and maintain stability in a vacuum environment.
Common gases:
Argon (Ar):
Applications: metal sintering (such as titanium alloys, high-temperature alloys), ceramic preparation (such as silicon nitride, silicon carbide), semiconductor processes (such as wafer annealing).
Advantages: Stable chemical properties, no reaction with materials, suitable for high temperature environments (up to 2000 ℃ or above).
Example: When sintering alumina ceramics at 1600 ℃, introducing argon gas can prevent the body from oxidizing and increase the density.
Helium (He):
Application: Processes that require rapid heating/cooling (such as nanomaterial synthesis, rapid heat treatment).
Advantages: High thermal conductivity, which can shorten the heating time, but the cost is relatively high.

2. Reducing gas
Function: Reduce metal oxides or remove surface oxide layers of materials.
Common gases:
Hydrogen (H ₂):
Applications: metal reduction (such as tungsten powder reduction), catalyst preparation (such as Pt/C catalyst reduction), semiconductor doping (such as silicon wafer phosphorus diffusion).
Advantage: Strong restoration ability, but strict concentration control is required to avoid explosion risk.
Example: In the preparation of fuel cell catalysts, hydrogen gas (vacuum degree 10 ⁻ ² Pa) is introduced to reduce Pt precursor into nanoparticles.
Carbon monoxide (CO):
Application: Reduction of specific metal oxides (such as iron oxide reduction to iron powder).
Attention: Toxic, exhaust gas treatment device is required.
Ammonia gas (NH3):
Application: Nitriding treatment (such as metal surface nitriding), preparation of nitride ceramics (such as Si ∝ N ₄).
Advantage: Simultaneously providing nitrogen source and reducing environment.

3. Oxidizing gas
Function: Control the thickness of the surface oxide layer of the material or achieve specific oxidation reactions.
Common gases:
Oxygen (O ₂):
Application: Metal oxidation (such as aluminum oxidation to form Al ₂ O ∝ coating), ceramic sintering (such as zirconia stabilization).
Control: The flow rate needs to be precisely adjusted to avoid excessive oxidation.
Example: In the heat treatment of aircraft engine turbine blades, the introduction of trace amounts of oxygen can form a dense oxide layer, improving the high-temperature corrosion resistance.
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Application: Low cost oxidation process (such as steel bluing treatment).
Restrictions: Contains moisture and impurities, needs to be dried before use.

4. Carbon/nitrogen source gas
Function: Participate in chemical reactions to generate carbides or nitrides.
Common gases:
Methane (CH ₄):
Application: Chemical vapor deposition (CVD) is used to prepare diamond films and silicon carbide coatings.
Reaction: Decompose into carbon and hydrogen at high temperature, and carbon deposits on the surface of the substrate.
Condition: It needs to be used in conjunction with hydrogen gas (such as CH ₄: H ₂=1:4), and the temperature is usually between 800-1200 ℃.
Acetylene (C ₂ H ₂):
Application: Rapid deposition of carbon layers (such as hard coatings on tool steel surfaces).
Advantages: Low decomposition temperature and fast sedimentation rate.
Nitrogen (N ₂):
Application: Nitriding treatment (such as metal surface nitriding), preparation of nitride ceramics (such as AlN).
Attention: Catalysts (such as iron) are required at high temperatures to accelerate the nitriding reaction.

5. Composite gas
Function: To achieve multifunctional processes through mixed gases.
Common combinations:
Ar+H₂：
Application: Combining metal reduction with protective atmosphere (such as avoiding re oxidation after tungsten powder reduction).
Advantages: Balancing reducibility and safety.
N₂+CH₄：
Application: Simultaneously achieving nitriding and carbonization (such as preparing TiCN composite coatings).
Condition: The temperature must be above 800 ℃ to promote the reaction.
Ar+O₂：
Application: Control the thickness of the oxide layer (such as surface passivation treatment of stainless steel).
Control: Accurately adjust the oxygen ratio through a flow meter.

6. Special gases
Function: To meet specific process requirements (such as doping, luminescence, catalysis).
Common gases:
Silane (SiH ₄):
Application: Deposition of semiconductor silicon-based materials (such as polycrystalline silicon thin films).
Attention: Flammable and explosive, strict safety measures are required.
Borane (B ₂ H ₆):
Application: Boron doping (such as doping silicon chips to form P-N junctions).
Control: The concentration should be below 100ppm to avoid lattice damage.
Hydrogen chloride (HCl):
Application: Corrosion cleaning (such as removing metal impurities from the surface of silicon wafers).
Subsequent processing: Rinse with deionized water and dry.

7. Gas selection principle
Process objective:
Protect materials → select inert gas (such as Ar);
Reduce metals → choose H ₂ or CO;
Generate carbides → choose CH ₄ or C ₂ H ₂.
Material compatibility:
Avoid gas and material reactions (such as Al reacting with O ₂ at high temperatures to form Al ₂ O ∝, if protection is required, use Ar).
Temperature range:
Ar or He is preferred for high temperatures (>1500 ℃);
For medium and low temperatures (<1000 ℃), N ₂ or air can be selected.
Security:
Flammable gases such as H ₂ and CH ₄ must be controlled in concentration ( Toxic gases such as CO and HCl require tail gas treatment.
Cost:
He has high cost and is only used for special processes;
Ar and N ₂ have lower costs and are commonly used choices.