DIELECTRIC AND PIEZOELECTRIC PROPERTIES OF(K,Na)NbO_(3)-BASED PIEZOELECTRIC CERAMICS

Solid solutions based on Li,Ta and Sb-doped(K_(0.5)Na_(0.5))NbO_(3)(KNN)lead-free perovskite systems were created using standard solid-state methods.X-ray diffraction was used to conrm that all compositions were single phase and to verify the phase transition from tetragonal to cubic at T_(C)=302℃.The three compositions examined,originally developed by Saito and Li,were shown to be strongly ferroelectric with sharp peaks in permittivity present at the Curie temperature.The optimum composition had loss tangent values below 5%up to 100 kHz at room temperature.Bipolar hysteresis measurements showed high values for both maximum polarization(25 and 21μC/cm^(2))and remenant polarizations(20 and 16
μC/cm^(2))for undoped and 0.2 wt%CuO-doped samples.Maximum strain values of greater than 0.23%were observed.

Synthesis and electrical properties of BaBiO_(3)and high resistivity BaTiO_(3)-BaBiO_(3)ceramics

Ceramics of the composition BaBiO_(3)(BB)were sintered in oxygen to obtain a single phase with monoclinic I2/m symmetry as suggested by high-resolution X-ray diffraction.X-ray photoelectron spectroscopy confirmed the presence of bismuth in two valence states—3þand 5þ.Optical spectroscopy showed presence of a direct bandgap at-2.2 eV and a possible indirect bandgap at-0.9 eV.This combined with determination of the activation energy for conduction of 0.25 eV,as obtained from ac impedance spectroscopy,suggested that a polaron-mediated conduction mechanism was prevalent in BB.The BB ceramics were crushed,mixed with BaTiO_(3)(BT),and sintered to obtain BT–BB solid solutions.All the ceramics had tetragonal symmetry and exhibited a normal ferroelectric-like dielectric response.Using ac impedance and optical spectroscopy,it was shown that resistivity values of BT–BB were orders of magnitude higher than BT or BB alone,indicating a change in the fundamental defect equilibrium conditions.A shift in the site occupancy of Bi to the A-site is proposed to be the mechanism for the increased electrical resistivity.

Relationship between tolerance factor and temperature coefficient of permittivity of temperature-stable high permittivity BaTiO_(3)–Bi(Me)O_(3) compounds

The temperature coefficient of permittivity(TCε)of BaTiO_(3)–Bi(Me)O_(3) solid solutions were investigated.It was determined that as the tolerance factor was decreased with the addition of Bi(Me)O_(3),the TCε shifted from large negative values to TCε values approaching zero.It is proposed that the different bonding nature of the dopant cation affects the magnitude and temperature stability of the permittivity.This study suggests that the relationship between tolerance factor and TCε can be used as a guide to design new dielectric compounds exhibiting temperature-stable high permittivity characteristics,which is similar to past research on perovskite and pyrochlore-based microwave dielectrics.

Modeling Evaporating Droplets in Complex Unsteady Flows

In many applications, a moving fluid carries a suspension of droplets of a second phase which may change in size due to evaporation or condensation. Examples include liquid fuel drops in engines and raindrops or ice-crystals in a thunderstorm. If the number of such particles is very large, and, if further, the flow is inhomogeneous, unsteady or turbulent, it may be practically impossible to explicitly compute all of the fluid and particle degrees of freedom in a numerical simulation of the system. Under such circumstances Lagrangian Particle Tracking (LPT) of a small subset of the particles is used to reduce the computational effort. The purpose of this paper is to compare the LPT with an alternate method that is based on an approximate solution of the conservation equation of particle density in phase space by the method of moments (MOM). Closure is achieved by invoking the assumption that the droplet size distribution is locally lognormal. The resulting coupled transport equations for the local mean and variance of the particle size distribution are then solved in conjunction with the usual equations for the fluid and associated scalar fields. The formalism is applied to the test case of a uniform distribution of droplets placed in a non homogeneous temperature field and stirred with a decaying Taylor vortex. As a benchmark, we perform a direct numerical simulation (DNS) of high resolution that keeps track of all the particles together with the fluid flow.

Effects of A-site nonstoichiometry on oxide ion conduction in 0.94Bi_(0.5)Na_(0.5)TiO_(3)-0.06BaTiO_(3) ceramics

Lead free 0.94(Bi_(0.5)Na_(0.5))TiO_(3)-0.06BaTiO_(3)ceramics were prepared by conventional solid-state mixed oxide route with the A-site stoichiometry modified to incorporate donor-doping(through Bi-excess)and acceptor-doping(through Na-excess).Both stoichiometric and nonstoichiometric ceramics exhibited a single perovskite phase with pseudo-cubic symmetry.A significant improvement in the dielectric properties was observed in Bi-excess compositions and a deterioration in the dielectric properties was observed in Na-excess compositions.Impedance spectroscopy was utilized to analyze the effects of A-site nonstoichiometry on conduction mechanisms.Compositions with Bi-excess resulted in an electrically homogeneous microstructure with an increase in resistivity by
3-4 orders of magnitude and an associated activation energy of 1.57 eV which was close to half of the optical bandgap.In contrast,an electrically heterogeneous microstructure was observed in both the stoichiometric and Na-excess compositions.In addition,the Na-excess compositions exhibited low resistivities(ρ-10^(3)Ω-cm)with characteristic peaks in the impedance data comparable to the recent observations of oxide ion conduction in(Bi_(0.5)Na_(0.5))TiO_(3).Long term annealing studies were also conducted at 800℃to identify changes in crystal structure and electrical properties.The results of this study demonstrates that the dielectric and electrical properties of 0.94(Bi_(0.5)Na_(0.5))TiO_(3)-0.06BaTiO_(3)ceramics are very sensitive to Bi/Na stoichiometry.

Thermal stability of additively manufactured austenitic 304L ODS alloy

Thermal stability and high-temperature mechanical properties of a 304 L austenitic oxide dispersion strengthened(ODS)alloy manufactured via laser powder bed fusion(LPBF)are examined in this work.Additively manufactured 304 L ODS alloy samples were aged at temperatures of 1000,1100,and 1200℃for 100 h in an argon atmosphere.Microstructure characterization of LPBF 304 L ODS alloy before and after the thermal stability experiments revealed that despite the annihilation of dislocations,induced cellular substructure by the LPBF process was partially retained in the ODS alloy even after aging at1200℃.The size of Y-Si-O nanoparticles after aging at 1200℃increased from 25 to 50 nm.EBSD analysis revealed that nanoparticles retained the microstructure of LPBF 304 L ODS and hindered recrystallization and further grain growth.At 600℃and 800℃,the yield stress of the 290 and 145 MPa were measured,respectively,which are substantially higher than 113 MPa,and 68 MPa for 304 L at the same temperatures.Furthermore,the creep properties of LPBF 304 L ODS alloy were evaluated at a temperature of 700℃under three applied stresses of 70,85,and 100 MPa yielding a stress exponent(n)of 7.7;the minimum creep rate at 100 MPa was found to be about two orders of magnitude lower than found in the literature for wrought 304 L stainless steel.