Proximity Effect Study of Macroscopic-Scale Carbon Nanotube Fiber Yarn in MHz Region

The AC resistance of carbon nanotube fiber (CNTF) yarn wires has rarely been studied at megahertz (MHz) frequencies. However, there is a growing demand for magnetic devices, including transformers and inductors, implemented in high-frequency power-converter circuits. Recently, a study on the skin effect of CNTF yarn in this MHz-range was published. This paper reports a study on the proximity effect. The pure proximity-effect component of the AC resistance was measured by minimizing the skin-effect losses. For this, a constant uniform magnetic field was applied to the device under test through an air gap. The measuring instrument was a transformer with a higher bandwidth than the range of frequencies tested and had a low power loss. An impedance gain analyzer was used to measure the AC resistance. The proximity losses of the CNTF yarn wires as well as those of some commonly used solid and Litz copper conductors were measured. For further investigations, lead wires with a conductivity close to that of the CNTF yarns were tested. The measured data show that the CNTF yarn wire had less proximity losses than copper and lead in this frequency range. For a theoretical investigation of the test results, COMSOL (finite element method simulation program) models for the test setup were developed with the same physical dimensions. The model results for the CNT yarn, solid copper, Litz, and lead wires were compared with the measured data. Both the simulation and measurement results are discussed in the paper.