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Supercritical fluids (SCFs) have been recently considered to be used as insulating media due to properties that show exceptional dielectric strength, high heat transfer capability, and low viscosity. This paper reports the result of breakdown experiments on carbon dioxide (CO2) from gaseous to supercritical state. Experiments are performed under the isothermal condition of 310 K (37°C). The dielectric strength test is conducted in a 0.1 mm gap under uniform dc electric field. To interpret the result, a theoretical model that combines the thermodynamic calculation and existing data from the structure analysis by small angle x-ray scattering is developed. Our experiments suggest that the dielectric behavior of supercritical CO2 shows a discontinuity of the dielectric strength near the critical point. This phenomenon can be well explained by the theoretical model, which calculates the molecular cluster size and considers the local fluid structure. 

Supercritical fluids (SCFs) are being investigated as a dielectric medium for their low viscosity, high dielectric strength, high heat transfer capability, low cost, and environmental friendliness. This paper introduces a straightforward, cost-effective, and commercially available sensor to measure SCF viscosity for the characterization of the dielectric medium. Quadratic and cubic fitting between the sensor current output and viscosity of He, H2, CO2, SF6, and N2 were made in the ambient lab environment. Experimental temperatures range from 19.3°C to 22.0°C and fluid pressure from 0.1 MPa-1.5 MPa. This manuscript introduces preliminary data for a methodology to correlate SCF viscosity to the output signal of a commercially available sensor. This will enable viscosity measurement of mixtures of dielectric fluids.