High resolution x ray diffraction study of phase and domain structures and thermally induced phase transformations in PZN (4 5 9)%PT 10

transformation occurs over a range of temperature involving concurrent formation of R* and T* domains The effect of heating to higher temperature on the dielectric behaviour and the room temperature piezoelectric properties after each heating-cooling cycle were then investigated The results showed that, for PZN-4.5%PT, perceptible sign of dielectric hysteresis loop could be noted (Figure 9.9) and that the resultant room temperature piezoelectric properties (i.e., dielectric constant (KT) and electromechanical constant (k31)) started to degrade after the sample was heated above 105°C (Figure 9.10) From this figure, the TDP of PZN-4.5%PT is estimated at about 105°C above which perceptible property degradation begins The TDP of PZN-7%PT was determined similarly, giving TDP ≅ 95 °C The TDP values for PZN-4.5%PT and PZN-7%PT determined as described above agree with the TR-T(L) obtained from the first anomaly in ε’ given in Figure 9.6 It also corroborates with our HD-XRD results to within 10 °C As shown in Figure 9.7(b), emergence of (100)T domains were noted at about 105 °C The emergence of (100)T domains not only indicates R-T transformation but also concurrent thermal depolarization of the crystal Should R-T transformation occur without thermal depoling, as stated in case (b) above, then only (001)T domains would be detected in the (002) mappings instead This is because in the poled condition, c-axis hence (001)T 168 a) J (mA/m ) 100 110 120 -1 -2 b) J (mA/m ) 80 90 100 110 120 -1 40 60 80 100 120 140 160 180 200 220 o Temperature ( C) Figure 9.5 PP-ZFH J of the [001]-annealed-and-poled (a) PZN-4.5%PT and (b) PZN-7%PT single crystals 169 ε' (x10 ) P Z N -4 % P T P Z N -7 % P T T R -T (L ) T R -T (L ) 50 100 150 200 T em p eratu re ( o C ) Figure 9.6 PP-ZFH ε’ curves of the [001]-annealed-and-poled PZN-4.5%PT and PZN-7%PT single crystals from which the “TR-T” was determined from the first anomaly 170 (a) 100ºC: R*+T# Figure 9.7 (b) 125ºC: R*+T+T* (c) 135ºC: T+T* Temperature dependent (002) RSMs taken from fractured surfaces of the [001]-annealed-and-poled PZN-4.5%PT single crystal: (a) 100 ºC, (b) 125 ºC, and (c) 135 ºC The intensity contours are in log scale T# indicates the vague T diffractions TNT denotes the T at ∆ω ≠ 0º 171 (a) 105ºC: R+T# Figure 9.8 (b) 110ºC: R+T# (c) 115ºC: T+T* Temperature dependent (002) RSMs taken from fractured surfaces of the [001]-annealed-and-poled PZN-7%PT single crystal: (a) 105 ºC, (b) 110 ºC, and (c) 115 ºC The intensity contours are in log scale T# indicates the vague T diffractions TNT denotes the T at ∆ω ≠ 0º 172 o ε' (x10 ) (a) 100 C ε' (x104) o (b) 105 C ε' (x104) (c) 115oC ε' (x104) o (d) 125 C 20 40 60 80 100 120 140 Temperature (oC) Figure 9.9 Dielectric hysteresis behaviors of [001]-poled PZN-4.5%PT recorded during the heating-cooling cycles to (a) 100 ºC, (b) 105 ºC, (c) 115 ºC, and (d) 125 ºC, respectively After heating to 105 ºC, PZN-4.5%PT showed clear signs of hysteresis on cooling to room temperature Above this temperature, the area of the dielectric hysteresis increases with increasing heating temperature The heating and cooling rate is 1.5 ºC/min 173 (a) T K (x10 ) k31 0.6 0.4 0.2 Figure 9.10 (b) 20 40 60 80 100 120 o Temperature ( C) 140 (a) KT and (b) k31 of four plate samples of [001]-poled PZN4.5%PT taken at room temperature after cooling from the temperatures indicated on the x-axis Both the KT and k31 of PZN-4.5%PT started to degrade after it was heated to 105 ºC 174 domains are parallel to the E-field direction which is normal to the fractured surface being x-rayed Our results thus indicate that thermal depolarization occurs with the emergence of R-T phase transformation in PZN-PT single crystals, giving TR-T(L) ≅ TDP ≅105-110 °C for PZN-4.5%PT and ≅90-100 °C for PZN-7%PT These results, including the upper bound values of TR-T, i.e., TR-T(U) or the temperature at which complete R-T transformation was detected from our HR-XRD studies, are summarized in Table 9.2 The slight difference in the various temperature values can be attributed to the different temperature measurement instruments used in the tests In summary, the R-T transformation in relaxor single crystals occurs over a temperature range, manifested by the coexistence of R* and T* domains, a string of thermal current signals, and continued degradation of KT and k31 The TR-T determined from the conventional ε’-temperature curve corresponds to TR-T(L), which marks the beginning of R-T phase transformation with concurrent thermal depolarization The TRT(L) can thus be taken as the TDP of the crystal above which perceptible property degradation begins Thermal current and HR-XRD results suggest TR-T ≅ 100-135 °C for [001]-annealed-and-poled PZN-4.5%PT and 95-115 °C for [001]-annealed-andpoled PZN-7%PT single crystals 175 Table 9.2 TDP, TR-T(L), and TR-T(U) of [001]-poled PZN-PT single crystals PT (%) Tmax (oC) TDP (oC) TRT(L) (oC) [a] [c] [d] TRT(U)(oC) [a] [b] 4.5 ≅155 ≅105 ≅105 ≅107 ≅100 ≅120 ≅135 ≅165 ≅95 ≅90 ≅92 ≅115 ≅115 ≅105 [c] [d] [a] Determined from the ε’-temperature plot of [001]-poled crystals TR-T(L) is the temperature corresponding to the first dielectric anomaly of the plot; Tmax is the temperature at which the ε’ is at its maximum Determined by means of the temperature cycling tests of Honoso et al [77, 78] and [b] Rajan et al [104] TDP is defined as the temperature above which perceptible degradation of dielectric and electromechanical properties begins [c] [d] Determined from the thermal current density data of [001]-poled crystals Determined by means of HR- XRD results 176 9.5 Summary of main observations (a) A revised phase diagram for the PZN-PT system has been constructed Two new evident features of this revised phase diagram are: (a) the expanded (R+T) twophase MPB region, and (b) a (T+C) two-phase region at high temperature before the crystal transforms completely into the single C phase (c) The expanded (R+T) MPB region can be further divided into two regions In the lower PT region, 0.06 ≤ x ≤ 0.08, the T phase is metastable stabilized by the residual stress in the crystal In the high PT region, 0.09 ≤ x ≤ 0.10, both the (R+T) phase are thermodynamically stable phases at room temperature (d) The FWHM suggests that with lower PT contents (i.e., 0.045 ≤ x ≤ 0.07), the R phase has a mixed micro- and nanotwin structure of both {100}-type and {110}type At high PT content (i.e., x ≈ 0.08), the R phase is made up predominantly of {110}-type micro/nanotwin, as manifested by the single R peak of much smaller FWHM (e) In lower PT contents (i.e., 0.045 ≤ x ≤ 0.07), the room temperature R phase of poled crystals is under strained in which the structure appeared to elongate along a certain crystal direction on comparing to the unpoled samples of broad R phase The strained state of the poled crystals is the result of poling induced stresses in the crystals 177 (f) In the event of PP-ZFH in PZN-(4.5-7)%PT, the TR-T occurs in a temperature range of 10-35 ºC The beginning of R-T transformation with concurrent thermal depolarization shows perceptible property degradation This suggests that the TRT(L) corresponds to the TDP Thermal current and HR-XRD results suggest TRT ≅ 100–135 °C for [001]-poled PZN-4.5%PT and ≅90–115 °C for [001]-poled PZN7%PT 178 ... emergence of R-T phase transformation in PZN- PT single crystals, giving TR-T(L) ≅ TDP ≅1 05- 110 °C for PZN- 4 .5% PT and ≅90 -100 °C for PZN- 7%PT These results, including the upper bound values of TR-T,... (b) 110 ºC, and (c) 1 15 ºC The intensity contours are in log scale T# indicates the vague T diffractions TNT denotes the T at ∆ω ≠ 0º 172 o ε'' (x1 0 ) (a) 100 C ε'' (x1 04) o (b) 1 05 C ε'' (x1 04)... 4 .5 ≅ 155 ≅1 05 ≅1 05 ? ?107 ? ?100 ≅120 ≅1 35 ≅1 65 ≅ 95 ≅90 ≅92 ≅1 15 ≅1 15 ≅1 05 [c] [d] [a] Determined from the ε’-temperature plot of [001]-poled crystals TR-T(L) is the temperature corresponding to the