Interface engineering in ferromagnetic high-thermal conductivity iron-diamond/metal composites for electric conversion applications

The objective of this work is to investigate whether any combination of metal and magnetic particles may fit the specifications of electric conversion applications, which require, among other properties, sufficiently high magnetic permeability and thermal conductivity and a low (adjustable) thermal expansion coefficient. After having explored a wide variety of combinations, guided by both chemical and physical considerations, it was decided to investigate composites fabricated by gas pressure infiltration of Ag or Ag3wt%Si alloys into compacts of bimodal mixtures of diamond (high thermal conductivity) and iron particles (high magnetic permeability). Three average particle sizes of each component were used to fabricate the composites, namely, diamond particles of 230, 285 and 295 μm and iron particles of 30, 42 and 398 μm. In addition the volume fraction varied in the ranges 0.1-0.59 (diamond) and 0.12-0.43 (iron). In order to avoid alloying with the infiltrating metal and iron-diamond reaction, iron particles were coated with amorphous carbon. The results indicate that only composites containing a volume fraction of carbon-coated iron particles higher than 0.4 showed properties (a thermal conductivity higher than 200 W/mK and a relative magnetic permeability above 0.3) within the range valid for electric conversion applications. Composites containing non-coated iron particles reached in almost all cases very low values of both properties.