You are looking for thermal conductive fillers:
Under the name of Güpo®tim you find fillers with:
By Güpo®tim you can achieve filling grades in e.g. PA 6 until 65 Vol.-% and thermal conductivities beyond values of 8 W/(m*K) integral.
Güpo®tim is a brand of Güpo GmbH. Güpo provides customers with synthetic and natural minerals as for grinding, polishing and cleaning as for use in paints, ceramics and cosmetics for more than 20 years. An own laboratory together with co-operations with national as well as international research institutions enable customer-adapted developments on the latest state of the art.