where 0 is the relative density; t is the time (sec); P is the applied pressure (dyncm-2); q is the viscosity (poise); ~ is the strain rate (see-l); A, B are the constants; DE is the lattice-diffusion coefficient (cm 2 s e e l ) ; D b is the grain-boundary diffusion coefficient (cm z see -1); Q is the volume of vacancy (cm3); a~ is the effective stress (dyncm-2); k is Boltzmann's constant (1.38 x 10-16ergK-1); G is the average grain diameter (cm); T is the absolute temperature (K); and W is the grain-boundary width (era). For undirectional hot-pressing, El = constant, where l is the height of specimens in the die (cm). Then the strain rate ~ = l-1 dl/dt = 0-1 do/dt obtains and Equation 2 becomes