Helium Mass Spectrometry Principles for Battery Seal Hermeticity Testing
Helium mass spectrometry isolates sub-micron battery seal defects down to 10-12 mbar·L/s, preventing field electrolyte drying and moisture ingress.
Knudsen Effusion
Meaning
Thermodynamic process describing the escape of gas molecules from a container through an opening so small that collisions between the molecules are negligible. Measuring the rate of knudsen effusion allows for the determination of vapor pressures of solid or liquid materials at high temperatures. The hole size is the defining constraint of the system.
Mathematical Basis
Mass loss from the cell is governed by the molecular weight of the gas and the temperature of the sample. In knudsen effusion experiments, the rate of flow is inversely proportional to the square root of the molar mass.
Vapor Pressure
Calculations based on the mass loss and the orifice area yield the equilibrium vapor pressure of the substance inside the cell. This knudsen effusion method is accurate for pressures between ten to the minus eight and ten to the minus two bar.
Cell Geometry
Cell construction must use materials that do not react with the sample at high temperatures. Quartz or refractory metals are common choices for knudsen effusion studies in metallurgy and battery chemistry research. The ratio of the orifice area to the sample surface area is a critical design parameter.
If the hole is too large, the equilibrium state inside the cell is disturbed and the measurement becomes invalid. Accurate data requires a stable temperature gradient across the entire enclosure to prevent condensation of the vapor on the internal walls.