In high temperature vacuum equipment, the low vapor pressure and sealing balance of phenyl silicone oil need to be considered comprehensively from many aspects such as the characteristics of phenyl silicone oil, equipment working conditions and sealing design. The following is a specific analysis:

Characteristics and low vapor pressure of phenyl silicone oil
The composition and characteristics of phenyl silicone oil: Phenyl silicone oil is a product formed by phenyl replacing part of the methyl group in methyl silicone oil. Its flash point is higher than 300℃, the freezing point is as low as -70℃, the surface tension is about 2.1×10⁻⁴～2.85×10⁻⁴ N/cm, the relative density is between 1.00～1.11, and the refractive index is 1.425～1.533. These characteristics make it an ideal lubricant, heat exchange fluid, insulating oil and carrier for gas-liquid chromatography.

Low vapor pressure and high vacuum performance: Phenyl silicone oil has low saturated vapor pressure and can remain stable in a high vacuum environment. The phenyl silicone oil used in high vacuum diffusion pumps needs to have high vacuum performance and good thermal stability. Its saturated vapor pressure is extremely low, which helps to achieve a higher vacuum degree.

Viscosity and temperature resistance: The viscosity of phenyl silicone oil changes significantly with temperature, but silicone oil with a high phenyl content will only oxidize when heated above 250°C, and the viscosity will increase. In particular, silicone oil with a phenyl mole fraction of 50% can be used for hundreds of hours at 300-350°C after adding stabilizers.
Key factors for sealing balance

Viscosity and fluidity: The viscosity of phenyl silicone oil affects its fluidity in the sealing gap. Low-viscosity oils have strong fluidity but are prone to leakage; high-viscosity oils have good sealing properties, but may cause local overheating due to insufficient fluidity. Therefore, the appropriate viscosity needs to be selected according to the equipment working conditions.
Thermal expansion coefficient matching: The thermal expansion coefficients of phenyl silicone oil and sealing materials need to match to avoid changes in the sealing gap caused by mismatched thermal expansion coefficients at high temperatures.
Chemical compatibility: Phenyl silicone oil needs to be compatible with sealing materials to avoid sealing failure caused by chemical reactions. For example, phenyl silicone oil may swell with certain rubber materials, so it is necessary to select sealing materials that are compatible with them.

Balance strategy and engineering practice
Graded seal design: The main seal and auxiliary seal are combined. The main seal bears the main pressure and the auxiliary seal fills the small gap to improve the sealing effect.
Dynamic compensation mechanism: The gap change caused by thermal expansion is compensated by elastic elements (such as bellows) to ensure the constant sealing gap.
System-level optimization: including temperature gradient control, pressure monitoring and feedback, material surface treatment, etc. For example, the equipment is divided into high-temperature zone and low-temperature zone, and silicone oil circulates in the low-temperature zone to reduce the impact of vapor pressure on the seal; piezoelectric sensors are installed at the seal to monitor the leakage rate in real time; laser cladding treatment is performed on the sealing surface to improve the sealing performance.