Preparation And Properties Of Ions Doped Lead Zirconate Titanate Pyroelectric Ceramics

Pyroelectric materials are the key to the development of modern uncooled infrared focal plane array (UFPA) whose properties directly determine the performance of pyroelectric infrared detectors. The Zr-rich lead zirconate titanate with a Zr/Ti ratio of 95/5 (denoted as PZT95/5), is one of the most important ferroelectric materials for infrared thermal imaging and pyroelectric infrared detectors due to its high pyroelectric coefficient, low dielectric constant (εr) and dielectric loss (tan δ). It has a phase transition from the low-temperature ferroelectric rhombohedral (FR(LT)) to high-temperature ferroelectric rhombohedral (FR(HT)) with the phase transition temperature much lower than its Curie temperature (Tc). During the phase transition, the change of dielectric constant and dielectric loss of the materials is very small, and the phase transition can recur by the change of temperature. However, the FR(LT)-FR(HT) transition in nature is first order along with significant thermal hysteresis. Moreover, it is difficult for the densification of pure PZT95/5 ceramics by sintering and high annealing temperature is needed, which causes the volatilization of some components of the ceramics and gives rise to the compositional variation and finally even the formation of the PbZrO3 (PZ) antiferroelectric ceramics. Meanwhile, during the practical application of PZT95/5 ceramics, there are also some problems needed to be addressed such as larger dielectric loss, the difficulity of impedance matching and low pyroelectric coefficient. Therefore, it is of great significance to modify the existed materials and develop novel materials with high-performance.In this thesis, the phase composition, microstructure, electrical performance and the infrared application of pure PZT95/5 ceramics and doped PZT95/5 ceramics were investigated systematically to address the problems occureed during the fabrication and application of PZT95/5 ceramics. The main research contents are:(1) The effects of Nb2O5 doping on the microstructure, phase structure and electrical properties of Pb0.99Bi0.01(Zr0.95Ti0.05)O3(PBNZT) system were investigated. All the prepared ceramics show that rombohedral perovskite phase and PbZrO3 phase coexist. It is found that even a small doping amount of Nb ions could improve the ferroelectric and pyroelectric properties of the ceramics. With increasing Nb contents, dielectric constant shows an increasing trend and dielectric loss keep rather stable, the polarization electric field (P-E) hysteresis loops of ceramics become well-saturated loops, while the pyroelectric coefficient and figure of merit firstly increase and then decrease. However, when excess of Nb is added, Nb cations accumulate at the grain boundaries, which worse the ferroelectric properties and pyroelectric properties. The ceramics with Nb doping amount of 0.4 wt% show the optimal electrical properties at FR(LT)-FR(HT), with pyroelectric coefficient and detectivity figure-of-merit (FD) of 15.6×10-8C·cm-2·K-1 and 12.7×10-5Pa-1/2, respectively.(2) In order to further improve the pyroelectric performance of the PbNb0.02(Zr0.95Ti0.05)0.98O3 (PNZT95/5) ceramics, the phase structure, microstructure, and electrical properties of Mn-doped PNZT95/5 pyroelectric ceramic were fabricated. The investigations demonstrate that Mn addition does not cause a remarkable change in rhombohedral perovskite structure, but significantly alter ferroelectric and pyroelectric properties of the PNZT95/5 ceramics. PNZT95/5 ceramics usually possess a normal single hysteresis loop character when no aging process was employed, whereas Mn doped PNZT95/5 (PNMZT95/5) ceramics exhibit a double-hysteresis-like loop character because of the defect dipoles (Mn"zr/Ti-Vo") formed by Mn2+ and O2- vacancies. Meanwhile, ferroelectric features such as the polarization and coercive field show strong frequency and temperature dependences. There is an alleviate of the pinched loop to some degree with increasing temperature or decreasing frequency. Furthermore, Mn substitution results in the weakened dielectric property and enhanced pyroelectric property of the ceramics. The relevant pyroelectric coefficient (p) and detectivity figure of merit (FD) increase from 7.90×10-8 to 9.15×10-8 C·cm-1·K-1 and 8.69×10-5 to 16.74×10-5 Pa-1/2, respectively, which make the PNMZT95/5 ceramics potential in commercial infrared detectors.(3) The Pb(Zn1/3Nb2/3)O3 (PZN) is a typical relaxor ferroelectric which has the relative high Curie tempareture, excellent dielectric properties and low synthesis temperature. The composite ceramics composed of PZN and PZT (xPZN-(1-x)PZT) are expected to exhibit better electrical properties than the corresponding single ceramics. In order to further improve the pyroelectric properties for the practical application of different infrared detectors, the effects of the PZN content on the phase structure, microstructureand electrical property of the multi-compositionalxPb(Zn1/3Nb2/3)O3-(1-x)Pb(Zr0.95Ti0.05)O3(PZN-PZT) ceramics were investigated. The results reveal that perovskite phase and PbZrO3 (PZ) phase coexist in the samples when PZN contents are lower. The proper amount of PZN doping (x=0.75) favors the formation of rhombohedral phase and improves the the ferroelectric properties of the materials due the formation of liquid phase and the inhibition of the formation of PZ phase. However, with the further increase of PZN content, excess liquid phase accumulates at the grain boundaries and the microstructure is worsen, resulting in the worsening of electrical property of the ceramics. Tc decreases monotonously with the increase of PZN content, and the variation in Tc is a nearly linear function of PZN content following the equation of Tc=-240x+244. The diffusivity parameter (y) increases from 1.0266 to 1.3899 when PZN content increases. Meanwhile, the pyroelectric coefficient, remnant polarization and coercive field increase firstly and then decrease. The ceramics prepared with x=0.075 have the optimum performances with p=14.2×10-8 C·cm-2·K-1,Fv=0.22 m2/C, FD=7.35×10-5 Pa-1/2, εr=300, tanδ=0.023 and Pr=32.45 μC/cm2.(4) Fe-doped 0.075 Pb(Zn1/3Nb2/3)03-0.925Pb(Zr0.95Ti0.05)O3 (PZN-PZT-Fe) pyroelectric ceramics were prepared by conventional oxide-mixed method (one-step) and precursor method (two-step), respectively. X-ray diffraction results indicate that the ceramics fabricated by the one-step method possess a mixture of perovskite phase and pyrochlore phase, while the two-step method could suppress the appearance of pyrochlore phase during the synthesis procedure by avoiding the direct reaction between Pb3O4 and Nb2O5. Scanning electron microscopy observation suggests that the ceramics prepared by the two-step method have a dense microstructure with uniform grains. The dielectric, ferroelectric and pyroelectric properties were measured, which reveal that the ceramics with single phase and dense microstructure have enhanced ferroelectric and pyroelectric properties.(5) xPb(Fe1/2Nb1/2)O3-(1-x)Pb(ZryTi1-y)O3 (PFN-PZT) ceramics were synthesized by a two-step solid state reaction method. Three Zr/Ti ratios were selected and the effects of PFN content on the phase structure and electrical property of the ceramics were investigated. The results reveal that the as-prepared ceramics have a single perovskite phase structure, and the X-ray diffraction peaks shift to low angles with increasing PFN contents and Zr/Ti ratio. The pyroelectric properties as well as ferroelectric properties are also influenced by the composition variation.(6) 0.3Pb(Fe1/2Nb1/2)O3-0.7Pb(ZryTi1-y)O3 (0.3PFN-0.7PZT) pyroelectric ceramics with various Zr/Ti ratio were prepared by the wolframite method. The microstructure and electrical performance of the ceramics were investigated systematically. X-ray diffraction results indicated that the as-prepared ceramics have a single perovskite phase structure. The sample fabricated with y= 0.50 shows a morphotropic phase boundary (MPB) with coexisted tetragonal and rhombohedral phase. With increasing Zr/Ti ratio (y increased from 0.50 to 0.95), the lattice structure changes from the coexistence of tetragonal and rhombohedral phase to single rhombohedral phase. Meanwhile, the samples show an increasing lattice volume and grain size. Furthermore, the dielectric constant of samples shows a decreasing trend while the dielectric loss keep rather stable, and the pyroelectric coefficient and figure of merit show an increasing trend. The enhancement of the pyroelectric performance with increasing Zr/Ti ratio is attributed to the improved spontaneous polarization with changing temperature. When the Zr/Ti ratio is in the range of 80/20-85/15, the system shows the optimum pyroelectric performance, with a room-temperature pyroelectric coefficient higher than 11×10-8C·cm-2·K-1 and a detectivity figure of merit as high as 16×10-5 Pa-1/2.