For a piece of material, the steady-state flow across its thickness is 0.297 *10^-3 kg/m^2 *h. If the concentration on the high pressure surface is 2.43 kg/m^3 and the low pressure side concentration is 0.102 kg/m *h. Calculate the thickness of the material (in mm). D = 0.75 * 10^-10 m^2/s.