| Piezoelectric ceramics are widely used in modern industry to achieve the transformation between the electrical signal and the mechanical strain. Particularly, the lead-based ones represented by Pb(Zr, Ti)O3 (abbreviated as PZT) ceramics are popularly utilized in a variety of electronic components and devices because of their excellent piezoelectric and dielectric properties. However, strict regulations that limit the use of Pb-containing materials have been enacted in many countries from viewpoints of human health and environment protection. Accordingly, (K, Na)NbO3-based ceramics have attracted considerable attention in recent years as a type of promising lead-free piezoelectric materials to replace the traditional lead-based ones.It has been found that improving of the relative density is quite effective to enhance piezoelectric of KNN-based ceramics. Our group has fabricated the dense (K0.50Na0.50)NbO3 and (K0.45Na0.55)0.98Li0.02Nb0.77Ta0.18Sb0.05O3 by two-step sintering technique in recent years. Their relative density values were increased from 94.9% and 95.8%(which is the value of ceramics prepared by ordinary sintering) to 97.1% and 98.2%, and their d33 value was raised from 125 pC/N and 413 pC/N to 143 pC/N and 436 pC/N, respectively.Previously, the author’s group has made a systematical study of piezoelectric grain-size effect in BaTiO3 ceramics. For BaTiO3 ceramics prepared from micro-sized BaTiO3 powder by conventional sintering, d33 show a maximum value of 413 pC/N around the average grain size of 1.2 μm. It is expected that some similar grain-size effects might also exist in the (K, Na)NbO3-based ceramics. However, it is usually very difficult to fabricate dense and different grain-sized (K, Na)NbO3-based ceramics by conventional sintering.Under the above background, study of fabrication, microstructure and piezoelectric properties of (K, Na)NbO3-based ceramics was carried out, and their grain-size effects were discussed in this thesis. The main contents and results are as follow:1. Among those (K0.50Na0.50)NbO3 ceramics prepared by hot-press sintering, the one densified under the condition of 930℃/2h exhibits the best piezoelectric properties. Its d33 value is 155 pC/N at room temperature. The piezoelectric grain-size effect of (K0.50Na0.50)NbO3 ceramics was studied. It is found that d33 increases with decreasing the average grain size, reaches the maxmium value around 0.7 μm, then decreases with decreasing the average grain size. The relationship between TO-T and average grain size was examined. In general, the finer the grain size, the higher the TO-T value.2. The hot-press sintering technique was attempted to fabricate the dense (K0.50Na0.50)0.94Li0.06NbO3 ceramics. They show a d33 value of 241 pC/N, while the ceramic conventionally-sintered just has a d33 value of 223 pC/N. The piezoelectric grain-size effect was investigated for the (K0.50Na0.50)0.94Li0.06NbO3 ceramics, and an interesting physical phenomenon that d33 increases with increasing the average grain size and reaches the maxmium value around 7.6 μm was observed.TO-T (orthorhombic-tetragonal phase transition temperature) shows surprisingly large changes with sintering and annealing treatment conditions. In general, fine-grained ceramic exhibits a higher TO-T value.3. Investigations on the preparation technology of KNN-based ceramics were made. The (K0.50Na0.50)o.96Li0.04Nb0.85Ta0.15O3 ceramics were fabricated by two-step sintering, and d33 was improved from 258 pC/N to 285 pC/N. Remanent polarization Pr tends to decrease with increasing the grain size. However, coercive field Ec increases from 9.4 kV/cm to 11.9 kV/cm with by increasing the grain size from 2.5 μm to 3 μm and decreases to 10.7 kV/cm with a further increase in grain size to 4.8 μm. |
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