The Densification Of NaNbO₃-KNbO₃-Based Lead-free Piezoelectirc Ceramics By Air Pressure Sintering

Today, the researches of the lead-free piezoelectric ceramics and theirapplications are extremely important as a result of the implementing of thestrategy in which the society of the world can sustainably develop, and thestrengthening of the consciousness of environmental protection.(Na_0.5K_0.5)NbO₃（NKN） system ceramics exhibit typical ABO₃-typeperovskite structure. The Nb, K and Na metals are environment-friendly andhuman-friendly materials. Its performance is much better than other lead-freepiezoelectric ceramic system, such as high piezoelectricity and high Curietemperature. Therefore, NKN ceramics has been paid to a lot attention andbecome the main promising candidate for the lead-containing piezoelectricmaterials.In this thesis, lead free piezoelectric ceramic of （Na1/2K1/2）NbO₃（NKN）and0.95NKN-0.05AB （AB=CaTiO₃（CT）, LiSbO₃（LS）, LiTaO₃（LT）,（Ba1/2Sr1/2）TiO₃（BST）） system were prepared by the conventional solid-statereaction method sintered at air pressure. The influences of the secondcomponent and the sintering conditions on the phase structure, the density andelectric properties of the NKN-based ceramics were studied systematically.X-ray diffraction （XRD） measurement confirms that phase-pure perovskiteNKN ceramics of all the composition can be prepared by the conventionalsolid-state reaction method via atmosphere sintering at certain sinteringtemperature range. Based on the integral analysis of the microstructure,relative density and electrical properties of the sintered ceramics, theappropriate sintering temperatures for the NKN-based ceramics is determinedas being1020-1080℃. The results indicate that the proper amount of dopingpromotes densification and electric properties of the NKN-based ceramics.The0.94NKN-0.03LS-0.03LT ceramics sintered at1040℃exhibit themaximum relative density and the maximum value of d33, which are94.72%and228pC/N, respectively. The0.95NKN-0.05LT ceramics sintered at1040℃exhibit the maximum value of ε_max（1kHz）, being7256, and exhibitthe maximum value of Pr, being24.33C/cm2. The0.95NKN-0.025LS-0.025LT ceramics sintered at1040℃exhibit themaximum value of Q_m, which is62.3. Addition of appropriate sintering additives can produce liquid-phasesintering in the NKN-based lead-free piezoelectric ceramics, inhibit graingrowth, eliminate gas pores fully, promote the development of grains andimprove density of the ceramics. In this thesis, the NKN-based ceramics withthe appropriate composition were chosen, and the influences of the sinteringadditives on the NKN-based ceramics were studied. The phase structure,microstructure morphology, and electrical properties of the WO₃-, Fe₂O₃-,ZnO-, and CuO-doped NKN based ceramics were studied in detail. Smallamount of WO₃-and CuO doping can improve density and electricalproperties of the0.95NKN-0.05CT ceramics, in which the doping effect ofCuO is better than that of WO₃. For the Fe₂O₃-and CuO-doped0.95NKN-0.05LS ceramics, pure perovskite structure can be obtained sinteredat1020-1040℃, and the sintered ceramics exhibit slightly lower relativedensity. Within the measuring frequency, the variation of the value of thedielectric constant with frequency is much small between the TCand TO-Tphase transition temperatures range, indicating that the sintered ceramicsexhibit better temperature stability. The1mol%Fe₂O₃-doped0.95NKN-0.05LS ceramics sintered at1060℃exhibit the maximum value ofd33, which is190pC/N. The1mol%CuO-doped0.95NKN-0.05LS ceramicssintered at1040℃exhibit the maximum value of Q_m, which is218. For theFe₂O₃-, ZnO-and CuO-doped0.94NKN-0.03LS-0.03LT ceramics, pureperovskite structure can be obtained sintered at relatively low sinteringtemperatures. With the increase of sintering temperature, impurity appears inthe sintered ceramics. The relative density and the value of d33of the doped0.94NKN-0.03LS-0.03LT ceramics decrease slightly, whereas the value ofε_maxdecreases apparently. The Q_mvalue of the Fe₂O₃-doped0.94NKN-0.03LS-0.03LT ceramics decreases slightly, whereas the Q_mvalueof the ZnO-and CuO-doped ceramics increases, in which the Q_mvalue of theCuO-doped ceramics increase greatly. The Q_mvalue of the0.5mol%CuO-doped0.94NKN-0.03LS-0.03LT ceramics tends to increase with theincrease of sintering temperature, where the ceramics sintered at1080℃exhibit the maximum value of Q_m, which is152.