How to control temperature uniformity in a bell jar furnace?

Controlling temperature uniformity in a bell jar furnace is a complex but crucial process that directly affects the sintering quality and yield of the product. Here are some key control measures and methods:
1. Furnace design and material selection
Furnace structure design:
Maintain consistent thermal resistance around the furnace to ensure even distribution of heat within the furnace.
The shape of the furnace is generally circular or square to reduce the influence of corner effects on temperature uniformity.
The heat dissipation at the top of the furnace is relatively large, so the insulation layer at the top should be appropriately thickened to reduce heat loss.
Material selection:
Choose materials with high insulation performance, such as lightweight furnace structures that can be anchored and composite with alumina fibers, polycrystalline fibers, high alumina fibers, etc; The heavy furnace structure can be constructed using materials such as corundum mullite bricks and alumina hollow sphere bricks.
These materials have good insulation properties, which help reduce heat loss in the furnace and improve temperature uniformity.
2. Distribution and Control of Heating Elements
Heating element distribution:
Heating elements (such as silicon molybdenum rods) should be evenly arranged on the side of the furnace, taking into account the differences in heat release coefficients among the six sides. Based on thermal resistance insulation design, theoretically achieving equal heat release per unit area on all six sides.
For high-temperature bell jar furnaces, special silicon molybdenum rod heating elements can be used, with a lower compensation independent temperature zone set up to reduce the temperature difference between the upper and lower sections of the furnace.
Temperature control:
Adopting advanced temperature control systems, such as cascade temperature control systems, the furnace temperature is controlled by two sets of thermocouples connected in series near the heater and substrate, ensuring that the temperature at the location of the substrate to be burned can be accurately reflected.
Multiple temperature sensors can be installed inside the furnace to monitor the temperature at various points in real-time, and adjust the power output of the heating element based on feedback signals to maintain temperature uniformity inside the furnace.
3. Convective heat transfer and gas flow
Forced convection heat transfer:
By introducing compressed air and other media, they are regulated by an electric control valve and sent into the furnace to form a uniform airflow. Gas is taken in from multiple small holes in the distribution box on one side of the furnace and discharged from the exhaust box on the opposite side, forming a uniform airflow field in the furnace.
This forced convection heat transfer method helps to improve the problem of uneven heating of products in the effective space of the furnace due to uneven radiation distance.
Gas flow control:
For bell jar furnaces that require atmosphere protection, it should be ensured that the atmosphere flows uniformly within the furnace. The flow condition of gas in the furnace can be controlled by adjusting parameters such as injection gas flow rate and pressure, in order to achieve the goal of controlling the uniformity of furnace temperature.
4. Operation and maintenance
Standardized operation:
Operators should be familiar with the operating procedures and precautions of the bell jar furnace, and strictly follow the process requirements for loading, heating, insulation, and discharging operations.
During the operation, attention should be paid to observing the temperature and atmosphere changes inside the furnace, and adjusting the power output and atmosphere flow rate of the heating element in a timely manner.
Regular maintenance:
Regularly maintain and inspect the bell jar furnace, including cleaning and replacing heating elements, temperature sensors, atmosphere control systems, and other components.
Check if the sealing performance of the furnace is good to prevent gas leakage from affecting temperature uniformity.
In summary, through reasonable furnace design and material selection, scientific distribution and control of heating elements, effective convective heat transfer and gas flow control, as well as standardized operation and maintenance measures, good temperature uniformity control can be achieved in bell jar furnaces.