| The high temperature piezoelectric ceramics have no phase structure transition which affects the piezoelectric properties in high temperature. Every parameter maintains stability when they were used in a wide temperature range, which is able to keep them working for a period in high temperature. The PbNb2O6-based high temperature piezoelectric ceramics were studied as a main research object in this thesis. The ceramics were prepared by the conventional solid-state reaction method. The influences of preparation process on the electrical properties have been studied. Meanwhile, the effects of A, B site co-substitutions and oxide doping on the phase structure, microstructure, and electrical properties have also been studied. The obtained results are as following:(1) The influences of preparation process on the electrical properties of Pb0.925Ba0.075Nb2O6—x wt%TiO2(x=0.4,0.5) have been studied. Firstly, the effects of density, diameter and thickness of samples on radial resonant frequency have been studied. The results show that the radial resonant frequency of sample is inversely proportional to its density, diameter and thickness. Secondly, the effects of pressure on the diameter and density of ceramics have also been discussed. It shows that the changes of diameter and density were identical with that of pressure. The radial resonant frequency is changed by adjusting the pressure. But the density of ceramics decreases with the decrease of the pressure. The low density of ceramics will be disadvantageous to the later process of artificially polarization. The results show that the optimum pressure is a range of 8 to lOMpa. Finally, the effect of temperature of polarization on the piezoelectric property has been studied. It shows that electric domain tends to directionally aligned with the increase of the temperature. But a heighten temperature of polarization may cause the decrease of resistivity and increase of leakage current, easily for breakdown of pellet. The optimum temperature of polarization varies from 175 to 185℃.(2) A series of (Pb0.55La0.01)1+yNb1-yTiy)2O6 (0≤y≤0.1) (PLNT) ceramics were prepared by a solid-state reaction method. The crystal structure and microstructures of specimens were investigated by x-ray diffraction (XRD) and scanning electron microscope (SEM). All PLNT ceramics sintered at temperatures from 1250℃to 1290℃are shown to be a single orthorhombic phase. The formation of orthorhombic phase is promoted by excess PbO existed in the Ti-bearing samples during calcination processing. The increasing of the tolerance factor due to a substitution of Ti4+ for Nb5+ is also responsible for the stability of orthorhombic phase. With an increase of Ti content, grain shape varies gradually from columnar to equiaxed shape. The high density of ceramics with a minor porosity has been obtained. The measurement of dielectric and piezoelectric properties of PLNT ceramics reveals that a substitution of Ti4+ for Nb5+ reduces the dielectric loss (tanδ) and dielectric constant (εr), and enhances the piezoelectric constant (d33) and Curie temperature (Tc). The optimum component with y=0.075 possesses the excellent electrical properties:εr=182, tanδ= 0.18%,d33=84pC/N and Tc=564℃. Ti and La co-doped lead metaniobate ceramics can be used as piezoelectric transducers operating at high temperatures and high frequencies.(3) A series of Pb0.97La0.02(Nb0.95Ti0.0625)2O6+x wt% TiO2 (0≤x≤2) (PLNT-x T) ceramics were prepared by a solid-state reaction method. The results show that TiO2 doping increases the density of PLNT-x T ceramics. XRD results show that all PLNT ceramics sintered at temperatures from 1230℃to 1290℃are shown to be a single orthorhombic phase. Ti4+ enters into crystal structure. SEM photographs show that TiO2 doping refines the grain and forms a closed packed structure. As a small amount of TiO2 is added, Ti4+ enters into Nb5+ sites strengthening the ferroelectric property, thus the Curie temperature and piezoelectric property are enhanced. With an increase of TiO2 content, Ti4+ enters into A and C-sits resulting appearance of Pb vacancy that maintains the valence balance and inhibits the long-range coupling effect, as a result, the Curie temperature is decreased. The optimum component with x=0.4 possesses the excellent electrical properties:εr=40, tanδ=0.006, Tc=498℃,d33=82pC/N. It can be used at high temperatures and high frequencies. |
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