Synthesis and characterization of multi functional NaNbO3-KNbO3 mixed ceramics

Abstract

Abstract The Ferroelectric perovskite oxides in the form of single crystals, polycrystalline ceramics and thin or thick films constitute an important class of materials widely used in capacitors, electro mechanical systems, ferroelectric memories, etc. Ferroelectric ceramic materials NaxK1-x NbO3(NKN) are synthesized by solid state double sintering method. The synthesized materials are characterized using X-ray diffraction, SEM, EDS and DSC. Dielectric properties and impedance are studied.NKN is a highly researched material as a lead-free piezoelectric ceramic. It is a mixture between ferroelectric KNbO3 and anti-ferroelectric NaNbO3 where there is a morphotropic phase boundary (MPB) for the (Na0.52K0.48)NbO3 formulation; most research, however, is conducted at an equal ratio of sodium to potassium. Like BNT, NKN is known for its sintering difficulties, and suffers from deliquescence (degradation in contact with moisture). Typical properties of NKN include Pr and Ec of 15 µC/cm2 and 13 kV/cm. Doping NKN significantly improve the density, microstructure and piezoelectric properties. In the present study NKN samples with x= 0.0, 0.4, 0.5, 0.6 and 1 compositions are synthesized and characterized. The SEM micrographs indicate a gradual change in the morphology of the samples. For NaNbO3 samples spherical grains are observed. Growth of microfibers is observed in x= 0.5 samples. For higher compositions the microfiber morphology is changed to elongated grains. DSC of the samples showed phase transitions at around 320°C temperature. The x-ray diffraction studies indicate orthorhombic structure except x = 0.4. Dielectric constant versus temperature plots indicated ferroelectric to paraelectric phase transitions. These transitions are of diffuse type. Ferroelectric hysteresis loops are observed to be lossy type without saturation. Impedance spectroscopy studies show relaxation in the samples. The relaxations are observed in the temperature region of 400-600°C. The complex impedance plots in this temperature range indicate single semi-circle. The relaxations are observed to be of non-Debye type. The equivalent circuits contain parallel combinations of resistance and capacitance elements. The mechanism of relaxation is discussed in the paper. An attempt is made to understand the results obtained in terms of possible charge accumulation mechanism.