SbSI is a ferroelectric material with the c-axis as ferroelectric axis and the most strongly piezoelectric material known. The permittivity of SbSI single crystal of the c-direction is about 25000 whereas that of the a-direction is 25. Till now, due to the difficulties in obtaining large crystal, it is very hard to use this material for practical applications. Specimens used in piezoelectric measurement reported by RCA group consisted of thin discs cut from bundles of SbSI needles embedded in Araldite.
SbSI single crystal tends to grow in form of thin needles along the c-axis. In this report, the hot-pressing technique was applied to the powder of SbSI crystals in order to obtain the hard polycrystalline ingot having the oriented c-axis and systematic experiments were carried out on the effects of the hot-pressing temperature (T_m), the pressure (P_h) and the time (t_p) influencing upon the orientation of the c-axis in the hot-pressed specimens.
The results obtained are summarized as follows.
(1) When P_h was applied to the z-direction of the press mould, the c-axis of SbSI microcrystals were oriented in the x-y plane of the press mould. In this case, the permittivity of the x-direction (ε_x) is 1400 at the Curie point whereas that of the z-direction (ε_z) is 160.
(2) When the pre-forming pressure to the y-direction and the hot-pressing pressure to the z-direction were applied, the c-axis were oriented to the x-direction of the pressed specimens. In this case, the permittivity of x, y, and z-directions at the Curie point are 3000, 1000, and 240 respectively.
(3) The orientation of the c-axis in the hot-pressed specimens was also ascertained by the observation of the etched surface by mean of a microscope.
(4) The piezoelectric constants such as k₃₃, s₃₃^E, s₃₃^D, d₃₃, f_R⋅t were measured for the hot-pressed specimens prepared under a condition of T_m=370°C, P_h=250kg/cm², and t_p=1hr. This specimens showed an electromechanical coupling coefficient k₃₃ of 35%.
(5) The Curie temperature of the hot-pressed SbSI of 30°C is higher by 10°C than that of the SbSI single crystal If the phase transition of SbSI is order-disorder type, it may be considered that the rise of the Curie point by the hot-pressing is the effect of the residual strain generated while hot-pressing.