The dielectric constant of a material in powder form is generally lower than in its solid form, mainly due to porosity, air inclusions, and interface effects. If you need accurate dielectric measurements for a material, using a fully dense solid sample is preferable.

The solid structure determines the intrinsic dielectric constant of a material while powder structure affects the effective dielectric constant of the powder sample due to the presence of air gaps and other factors related to the particulate nature of the material. Though, if you have powder sample and wishes to determine its dielectric constant, you may make a suspension of it by adding into a liquid of known dielectric constant. Or have it pressed to remove the air gaps in between particulates. And you may also take into consideration the effects of the power particles sizes because sizes and surfaces of the power has also effects on the dielectric constant value.