3.1. Static loading
Figure 2 shows how the voltage (V) varied during static bending loading at − 190 °C and 20 °C. The ceramics were loaded until a sample deflection of about 1.8 mm was achieved. As seen in the early loading stage at 20 °C, the electric voltage increased to about 3.5 V, at which time the ceramic was deflected by 0.2 mm. With further loading, the voltage decreased to 0 and then with further loading still, generated a negative voltage. Similar variation of the voltage was seen at − 190 °C; however, the maximum voltage (about 2.7 V) was lower than that at room temperature, and a negative voltage was not obtained. The different positive and negative voltages were generated due to different amounts of stress. Differences in the voltages at − 190 °C and 20 °C were related to the material properties, which will be discussed in a later section of this paper.
Figure 3 shows the profile of the ceramic before and after bending loading. The convex shape of the ceramic is formed at 20 °C before the loading. The deflection of the convex shape is about 0.2 mm, as shown in Fig. 1, but the profile is altered after the loading, which causes the ceramic to be deformed permanently to a concave shape, shown by the yellow lines. Compressive stress occurs early in the loading process due to the convex shape of the ceramic, while tensile stress is generated later in the process, because of the permanent deforamtion. This causes the voltage to change from positive to negative (Fig. 2).
Figure 3 also shows the profile of the ceramic plate at − 190 °C before mechanical loading (see the blue dotted line). It can be seen that the ceramic is significantly bent with a convex shape, due to the high stress caused by the different thermal expansion coefficients of the brass plate and PZT ceramic. As severely bent PZT ceramic results in high compressive stress during the loading process, negative voltages were not obtained for the ceramic at − 190 °C, as shown in Fig. 2.
Figure 4 shows how the maximum voltage varies as a function of the sample temperature. Figure 4(a) shows that the voltage increases to about 40 V immediately after the sample is put in liquid nitrogen and is cooled from 20 °C to 0 °C. Upon further cooling to about − 100 °C, the voltage is relatively stable. This may be an effect of heat insulation, due to the air gap created around the sample surface. The voltage increases to 100 V when the sample temperature is decreased from − 120 °C to − 190 °C, and the cooling rate increases to about − 70 °C/s. However, the voltage decreases to 0 when the sample temperature becomes stable at − 190 °C for a certain period of time, i.e. a spontaneous electric polarization cannot be reversed in the presence of an electric field.
When the ceramic, cooled to − 190 °C, is put into water Fig. 4(b), a negative voltage (–80 V) is generated rapidly in the temperature range of − 190 °C to − 150 °C. This is followed by a decrease of the voltage during warming to 20 °C, while the rate of increase of the sample temperature decreases. This could be due to the weak electric field of the PZT ceramic, i.e., the lower the heating rate, the lower the electric generation. Note that the reason for the negative electric voltage is considered to be the thermal tensile stress applied to the PZT ceramic.
The ceramic was further warmed from 20 °C to 180 °C using the heater, and the voltage increased slowly without saturation, Fig. 4(c). This is unlike the result of the heating process in water (–190 °C to 20 °C). This reason is the low heating rate of about 12 °C/s when using the heater (20 °C to 180 °C), compared to 60 °C/s when using the cold water (–190 °C to 20 °C). It is of interest to note that the rate of increase of the voltage changes at around 70 °C, as indicated by the dashed circle. This may be due to a change in the lattice or material characteristics [9]. However, there is no clear evidence of this, so further discussion will be required in the future.
3.2. Cyclic loading
The voltage was further investigated under cyclic loading at low temperature (–190 °C) and room temperature (before and after cooling to − 190 °C). Figure 5 shows the voltage obtained during the cyclic loading. Note that the cyclic loading at − 190 °C was performed in liquid nitrogen. Figure 5 shows that a lower voltage was detected at the lower temperature (Vmax = 15 V). This is about 70% lower than the voltage obtained at 20 °C (Vmax = 45 V). The low voltage at − 190 °C is due to the thermal stress at low temperatures, as shown in Fig. 3. In previous work, piezoelectric properties were measured in a temperature range between − 288.8 °C and the Curie point on undoped and Fe-doped PZT samples, and a pronounced relaxation below 127 °C occurred due to domain wall vibrations [10]. Zhang et al. have also examined dielectric and piezoelectric properties of modified lead titanate zirconate ceramics at temperatures between − 288.8 °C and 27 °C, and piezoelectric properties, including the piezoelectric constant, worsened with decreasing sample temperature [5, 11].
The voltage of the ceramic increases at 20 °C after warming from − 190 °C (Fig. 5(b)); however, the maximum voltage is slightly lower than that obtained before cooling to − 190 °C (see the red line vs. the blue dashed line). This may be attributed to damage of the ceramic (90° domain switching), arising from the high thermal stress. To verify this, the thermal stress of the ceramic was calculated. The different strain values (ε) caused by the different thermal shrinkages of the brass and the PZT ceramic result in high stress on the PZT ceramic. The thermal strain can be estimated using the formula ε = α × ΔT. It is clarified using this formula that εPZT = 0.0016 and εbrass = 0.0037. Knowing the different strain values, the thermal stress of the PZT ceramic can be calculated using the formula σ = EPZT × {(εPZT – εbrass)/2} = 82 GPa × 109 × (– 0.00105) = − 86.1 MPa (in compressive stress).
To examine whether or not 90° domain switching occurs in the PZT ceramic, an attempt was made to analyze the crystal orientation characteristics using the bulk PZT ceramic after compressive stress (86.1 MPa) was mechanically applied. The crystal orientations obtained before and after application of compressive stress are shown in Fig. 6. The observations were made in the same area of the sample before and after loading. Figure 6 shows that the crystal orientations were changed after the application of compressive stress, and 90° domain switching occurred (see the pole figures with the lattice formations). From this result it can be inferred that the decrease of the electric voltage after the cooling process is influenced by material failure, i.e., domain switching. One of the authors has examined the domain switching characteristics using a similar PZT ceramic [12]. In that study, domain switching was detected when the bending loading was more than 0.8 Py (bending yield strength).