Dielectric response of spherical particles of graded materials
  We have studied the effective response of composites of spherical particles with a dielectric profile which varies along the radius of the particles. We developed a differential effective dipole theory to compute the dipole moment of individual spherical particle and hence the effective dielectric response of a dilute suspension. The theory has been applied to two model dielectric profiles. Moreover, we compare the approximate results with the exact results of the power-law profile and the agreement is excellent. 
K. W. Yu (Chinese University of Hong Kong), G. Q. Gu, J.P. Huang