Joint QUEST research by Exeter and Oxford has been published by the open-access journal APL Materials.
In this paper, the development of a cold-pressing technique was reported for fabricating composites composed of a polytetrafluoroethylene-polymer (PTFE) matrix and a wide range of volume-fractions of MnZn-ferrite fillers (0%–80%).
The production of "low-volume-fraction" ferrite composites (i.e., below 50% volume) is relatively straightforward, such as by mixing ferrite powders with elastomers before curing. However, high volume fractions (above 70% volume) is more challenging, due to the increased viscosity of the mixture.
A simple and reliable method is described in this work, allowing for the fabrication of materials with user-specified values of permittivity and permeability. The natural logarithm of both the relative complex permittivity and permeability shows an approximately linear dependence with the volume fraction of ferrite. Thus, this simple method allows for the manufacture of bespoke materials required in the design and construction of devices based on the principles of transformation optics.
The images show ferrite particles as light regions in the darker PTFE matrix. Although there is evidence of agglomeration of ferrite particles, as well as a slightly uneven distribution, the PTFE has spread throughout the ferrite sufficiently such that there
were no large clusters of MnZn ferrite particles. (Image taken from the online version of the paper.)
The paper was published online on 4 October 2013.