Concentration dependence of diffusion coefficient in separating dilute solid mixtures of 4He in 3He

The precise pressure measurements of the phase separation kinetics of dilute solid mixtures of 4He in 3He are made in the temperature range from 100 to 200 mK at 4He concentrations x = 2,2-3,3% and pressures between 32 and 35 bar. It is found that the characteristic time constant of separation, τ, is very sensitive to the degree of supercooling. When the mixture is supercooled by more than 40-50 mK relative to the separation temperature of the initial mixture, the value of t becomes lower than 103 s and remains almost unchanged with further decrease in temperature. For low supercooling levels the values of t may be as much as 4 centre dot 104 s and decrease appreciably with temperature. A relation between the measured values of t and the effective mass diffusion coefficient is obtained by solving the diffusion problem with involving a surface resistance appearing under transition of 4He impurity atoms from the mixture to a new phase inclusion. It is shown that an adequate treatment of the experimental data in terms of the kinetic theory of tunnel transport of impurities under their strong interaction with one another may be obtained assuming that the diffusion coefficient of 4He in 3He is proportional to x-'4/3. The nuclei concentration and the average area size per a nucleus are calculated. The analysis of the experimental data on nucleation kinetics at phase separation indicates that they can be treated in terms of the homogeneous nucleation theory. The values of interphase surface tension coefficient obtained appear to be in agreement with the available data. The relaxation time accounted for by the finite resistance on impurity penetration through the matrix - new phase inclusion boundary was estimated for the first time