Estimation of heat distribution in the acoustic lens of an ultrasonic microscope with the carrier frequency of 1 GHz

The propagation of heat was analyzed in the lens of an
acoustic microscope used for testing of living cells at the frequency of 1~GHz.
Information concerning the propagation of heat is necessary for determination of
thermal boundary conditions which influence the temperature increase in the
tested samples representing acoustical properties of water. The time of
temperature propagation from water, heated due to high absorption, to the
sapphire body of the lens was estimated to be 0.77 ms. To carry out these
calculations the derivations of Carslow and Jaeger [2] and of Tautz [6] were
adjusted. On the other hand the propagation time of the acoustic wave in the
sapphire body equalled 0.0093 [ampersand]mu;s only. The time of image formation in the
microscope is rather long being equal from one to several seconds due to
mechanical inertia of the support vibrating together with the tested sample. The
heat capacities of the water volume and the sapphire body were found to be
comparable. However, if the heat capacity of the water volume would be many time
smaller then the time of the finally attained temperature would be elongated.
This effect can be neglected since the time of image formation is 3 orders of
magnitude longer than the time of penetration of the sapphire body by the heat
supplied by water. As the result a temperature equilibrium will be obtained with
the average boundary temperature of water. In such a case no heat flux will
penetrate the boundary water - sapphire and the condition of the thermal
insulation at the boundary will be fulfilled. This thermal boundary condition
makes it possible to determine the real temperature increase in biological
specimens.