NKN-BaTiO3 based lead free piezoelectric ceramics
Piezoelectric ceramics have been widely used as actuator, transducer, and sensor materials. It is well known that a series of lead-based perovskite ceramics, such as Pb(Zr,Ti)O3, Pb(Mg1/3Nb2/3)O3-PbTiO3, Pb(Zn1/3Nb2/3)O3-PbTiO3, show excellent piezoelectric and electromechanical properties. However, the toxicity of lead-based ceramics has drawn our attention for the sake of environmental protection, and there is an ever-increasing demand for lead-free piezoelectric materials. Nowadays, lead free ceramics have been intensively studied due to the environmental concern.
Among the various lead-free candidates, sodium potassium niobate (Na0.5K0.5NbO3 or NKN) solid solution has been considered one of the most promising alternatives to replace highly efficient, lead-based piezoelectrics because of its high Curie temperature and good ferroelectric and piezoelectric properties. NKN is a combination between anti-ferroelectrics NaNbO3 (NN) and ferroelectrics KNbO3 (KN) which is reported to have high piezoelectric properties near the morphology phase boundary or MPB when x = 0.5.
Generally, ceramics with high density often present good properties. In order to obtain the optimum density, ceramics for each composition are usually sintered at different sintering temperatures (from 1000 to 1600ºC). The sintering process is interplay between densification and grain growth, and is divided into three overlapping sintering stages, depending on the changes in the grain size and shape, the pore size and shape, and the related densification kinetics.
Barium titanate is a dielectric ceramic used in capacitors applications. BaTiO3 ceramics with a perovskite structure are capable of dielectric constant values as high as 7000; other ceramics, such as titanium dioxide (TiO2), have values between 20 and 70. Over a narrow temperature range, values as high as 15000 are possible; the dielectric constants of most common ceramic and polymer materials are less than 10. It is a piezoelectric material for microphones and other transducers.
Barium titanate is the most widely studied ceramic material, due to its excellent dielectric, ferroelectric and piezoelectric properties. The high dielectric constant of BaTiO3 ceramics results from its crystal structure. Barium titanate has three crystalline forms: cubic, tetragonal, and hexagonal. The tetragonal polymorph is the most widely used because of its excellent ferroelectric, piezoelectric, and thermoelectric properties. Temperature has a strong effect on the crystal structure and polarization characteristics of BaTiO3. Above 120°C (and up to 1400°C), BaTiO3 is cubic and the BaTiO3 has a spontaneous random polarization.
Therefore, NKN-based ceramics (e.g., solid solution of NKN–BaTiO3, NKN–SrTiO3, and NKN–CaTiO3) have received considerable attention mainly for the following two reasons: (i) Piezoelectric properties exist over a wide range of temperature and (ii) Several possibilities for substitution and additions.
Hence, the proposed research work is concerned with the synthesis and characterization of sodium potassium niobate (NaKNbO3) and barium titanate based lead free piezoelectric ceramics.
Some of the specimens (samples) that will be studied are as follows:
- (1-x)Na0.5K0.5NbO3-xBaTiO3 with x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0
- (1-x)Na0.5K0.5NbO3-xSrTiO3 with x = 0.00, 0.02, 0.04, 0.06, 0.08, 0.10
- (1-x)Na0.5K0.5NbO3-x(Ba0.95Sr0.05TiO3) with x = 0.00, 0.02, 0.04, 0.06, 0.08, 0.10
- (1-x)Na0.5K0.5NbO3-xBa(Zr0.52Ti0.48)O3 with x = 0.00, 0.02, 0.03, 0.04, 0.05, 0.07
- (1-x)Na0.5K0.5NbO3-xBa(SnxTi1-x)O3 with x = 0.00, 0.02, 0.04, 0.06, 0.08, 0.10