Changes in ferroelectric domain structures of triglycine sulfate after heat treatment have been examined in detail by the nematic liquid crystal method. The observed time evolution of domain structures can be explained in terms of an accumulation of local polarization reversal occurring spontaneously by the surface tension at the domain walls. Under ordinary circum-stances, such a polarization reversal is accompanied by subsequent reversal of the neighboring polarization in the opposite direction, conserving the total polarization during the evolution. Since the evolution does not progress unless compensating charges are supplied to the reversed polarization, the surface conductivity, therefore, influences the evolution of domain structures.
A Fe3O4 film prepared by ferrite plating exhibited, in an aqueous solution, a potential depending linearly on pH (=5_??_8) at a rate of -55 mV/pH with a short response time (several seconds), advantageous for practical applications to an electrode-type sensor. Fixing urease on the Fe3O4 film intermediated by dextran ((C6H12O5)n, n=1200_??_1800), we made a urea-sensor. The output potential in-creases monotonically as the urea concentration changes in the range 10-5_??_10-1 g/ml, with a sensitivity of 30 mV per factor of 10 change in urea concentration. The sensor has the remakable feature of requiring no impedance transducer or FET, since the Fe3O4 ion-sentitive film is conductive.