Gradient Estimates for the Perfect Conductivity Problem
  This paper concerns optimal gradient estimates of solutions for the perfect conductivity problem with closely spaced interfacial boundaries. The problem arises from composite material. Our estimates exhibit different blow up rates of the gradients, as the distance between the inclusions goes to zero, in dimensions n=2, n=3 and n\ge 4, which become higher as dimensions increase. 
Ellen Shiting Bao, YanYan Li and Biao Yin