Study On Ba-x(Bi-(1/2)Na-(1/2))-(1-x)TiO-3 Systems For Lead-Free Piezoelectric Ceramics

Since environmental issues are world-wide and concern every human activity fundamental improvements in the materials being used are needed to safeguard the natural environment. Lead-free piezoelectric ceramics thus attract more and more interest. There has been evidence showing Bax(Bi1/2Na1/2)1-xTiO3 (abbr. BNBT) ceramics could be an excellent candidate as lead-free piezoelectric ceramics.In the present study, BNBT ceramics were prepared by conventional ceramic method, the crystallization, the chemical states and constitutions, the microstructures, and the electrical properties including dielectric, piezoelectric and ferroelectric properties were characterized. Some relationships among composition, process, microstructure and properties were studied and understood.In the thesis, BNBT ceramics were prepared by solid-state method. The processes of ceramic preparation were studied and the key parameters such as calcining and sintering temperature and maintaining time were established. The results show that calcining temperatures for BNBT are in the range of 800C~900C depending on the Ba concentration and two hours is suitable as the calcining time. The sintering parameter is determined by combing the sintering temperature and the maintaining time. A high sintering temperature is needed by a short maintaining time and vice visa. The sintering temperature can be ranged from 1050C to 1200C.The main results are obtained from the characterizations of BNBT samples:1) XRD analyses show the BNBT ceramic samples have good crystallization. Meantime, they have mono-perovskite structure expect for some of those sintered for a long maintaining time.2) XPS analyses indicate Ba, Bi, Na, Ti and O mostly exist in the form of Bax(Bi1/2Na1/2)1-xTiO3, but the constitutions are not in good agreement with the stoichiometry by quantitative analysis.3) The fact that most of samples have high densification is illustrated by SEM analyses.4) The sintering parameters and the Ba concentration have great effects on the crystallization, the chemical states, and the microstructure of BNBT ceramics. High sintering temperature and long maintaining time lead to the sufficient crystallization and the full densification, but result in the worsening of the deviation from the stoichiometry. Ba doping induces the distortion of crystal lattice and the changes in lattice size. At room temperature, the rhombohedral-tetragonal morphotropic phase boundary exists approximately at the Ba composition of x=0.06.5) The dielectric measurement show a relative dielectric constant of BNBT ceramics not higher than 800 at 10 kHz. From the relationship between relative dielectric constant and frequency, before 10 kHz, the decrease of the relative dielectric constant is dramatic. After 10 kHz, the trend of decrease is rather tempered. Generally, for the same sample, the relative dielectric constant before poled is more than that of poled one. From the relationship of relative dielectric constant versus chemical constitution, the dielectric peak exists near 6% of Ba concentration at10kHz.6) Piezoelectric and ferroelectric tests imply big piezoelectricity and ferroelectricity. The value of d33 of BNBT ceramic more than 136X10-12C/N and a Ps more than 5uC/cm2 can be realized, which implies a good piezoelectric application perspective.