Measurement Of Intrinsic Thermal Conductivity Of Carbon Fiber Using Direct Electrical Heating Method

It is usually very difficult to measure the intrinsic thermal conductivity of fibers using direct electrical heating method,due to the presence of lateral heat loss.This study demonstrates that the intrinsic thermal conductivity and lateral heat transfer coefficient of fibers can be extracted simultaneously via multiple measurements on the same fiber.In this experiments,three samples of various lengths were prepared from an individual polyacrylonitrile-based carbon fiber of 5.6 ?m in diameter and measured with the direct electrical heating method.Each sample can get a curve of thermal conductivity vs lateral heat transfer coefficient.This study showed that the intrinsic thermal conductivity and lateral heat transfer coefficient can be extracted from the intersection of these curves.The results also showed that ignoring the lateral heat loss can result in an overestimation in thermal conductivity of carbon fibers by more than3 times.If lateral heat loss is large in the heat transfer,the resulting temperature distribution curve will be very flat,and the direct electrical heating method is not applicable in this case.In the experiment using direct electrical heating method,the carbon fiber can be heated through applying the rated voltage or rated current to the experimental circuit.Different heating methods bring different uncertainties of the carbon fiber's intrinsic thermal conductivity.In this study,the carbon fiber was heated with rated voltage and rated current respectively,and the uncertainty of each parameter measured directly was analyzed in the experiment.The lateral heat transfer coefficient and the uncertainty of intrinsic thermal conductivity obtained by these two methods were calculated by Monte Carlo simulation method.The results show that the uncertainty of the lateral heat transfer coefficient and thermal conductivity obtained by applying rated voltage to the circuit is close to the uncertainty of the lateral heat transfer coefficient and thermal conductivity obtained by applying rated current.The results are more accurate when measured at lower temperatures.In this paper,the direct electrical heating method is used to measure the intrinsic thermal conductivity of carbon fiber on the premise that the real lateral heat transfer coefficient and intrinsic thermal conductivity of samples of different lengths are consistent.In this study,we discussed how much error the thermal conductivity of carbon fiber would be if the real lateral heat transfer coefficient or intrinsic thermal conductivity of two different lengths of carbon fiber were different.The influence of the ratio of real intrinsic thermal conductivity or lateral heat transfer coefficient,environment temperature and the ratio of two different lengths of carbon fiber on the error of lateral heat transfer coefficient and intrinsic thermal conductivity were compared respectively.