[신간의 별자리x] 우리/미술, 그리고 ‘슬픔의 박물관’

: The unpolarized absorption and circular dichroism spectra of the fundamental vibrational transitions of the chiral molecule, 4-methyl-2-oxetanone, are calculated ab initio. Harmonic force fields are obtained using Density Functional Theory (DFT), MP2, and SCF methodologies and a 5S4P2D/3S2P (TZ2P) basis set. DFT calculations use the Local Spin Density Approximation (LSDA), BLYP, and Becke3LYP (B3LYP) density functionals. Mid-IR spectra predicted using LSDA, BLYP, and B3LYP force fields are of significantly different quality, the B3LYP force field yielding spectra in clearly superior, and overall excellent, agreement with experiment. The MP2 force field yields spectra in slightly worse agreement with experiment than the B3LYP force field. The SCF force field yields spectra in poor agreement with experiment.The basis set dependence of B3LYP force fields is also explored: the 6-31G* and TZ2P basis sets give very similar results while the 3-21G basis set yields spectra in substantially worse agreements with experiment. jg

Ab initio calculation of vibrational absorption and circular dichroism spectra using density functional force fields

Potassium-sodium niobate lead-free piezoelectric materials: past, present, and future of phase boundaries.

Feature size is a natural determinant of material properties. Its design offers the technological perspectives for material improvement. Grain size, crystallite size, domain width, and structural defects of different nature constitute the classical design elements. Common ferroelectric ceramics contain micrometer grain sizes and submicrometer domain widths. Domain wall mobility is a major contribution to their macroscopic material properties providing approximately half of the macroscopic output in optimized materials. The extension into the dynamic nanoworld is provided by relaxor ferroelectrics. Ionic and nanoscale field disorders form the base to a state with natural nanometer-size polar structures even in bulk materials. These polar structures are highly mobile and can dynamically change over several orders of magnitude in time and space being extremely sensitive to external stimuli. This yields among the largest dielectric and piezoelectric constants known. In this feature article, we want to outline how lead-free relaxors will offer a route to an environmentally safer option in this outstanding material class. Properties of uniaxial, planar, and volumetric relaxor compositions will be discussed. They provide a broader and more interesting scope of physical properties and features than the classical lead-containing relaxor compositions.

Lead-Free Relaxor Ferroelectrics

We present a critical review that encompasses the fundamentals and state-of-the-art knowledge of barium titanate-based piezoelectrics. First, the essential crystallography, thermodynamic relations, and concepts necessary to understand piezoelectricity and ferroelectricity in barium titanate are discussed. Strategies to optimize piezoelectric properties through microstructure control and chemical modification are also introduced. Thereafter, we systematically review the synthesis, microstructure, and phase diagrams of barium titanate-based piezoelectrics and provide a detailed compilation of their functional and mechanical properties. The most salient materials treated include the (Ba,Ca)(Zr,Ti)O3, (Ba,Ca)(Sn,Ti)O3, and (Ba,Ca)(Hf,Ti)O3 solid solution systems. The technological relevance of barium titanate-based piezoelectrics is also discussed and some potential market indicators are outlined. Finally, perspectives on productive lines of future research and promising areas for the applications of these ma...

BaTiO3-based piezoelectrics: Fundamentals, current status, and perspectives

XRD, Raman and electrical studies on the (1−x)(Na0.5Bi0.5)TiO3−xBaTiO3 lead free ceramics

Ceramics with composition Ba1-xNaxTi1-xNbxO3 are of either classical ferroelectric (for 0 ≤ x < 0.075) and ferro- or antiferroelectric (for 0.55 < x ≤ 1) or relaxor ferroelectric type (for 0.075 ≤ x ≤ 0.55), the transition at Tc being only diffuse without any frequency dispersion for this last region. All the corresponding dielectric characteristics, i.e. diffusivity of the ferroelectric-paraelectric transition, frequency dispersion of e'r, shift of Tm with frequency deviation from the Curie-Weiss law, are determined. The relaxor behaviour is more relaxor the more the composition deviates from BaTiO3 and NaNbO3. This study is in the field of preparation of relaxor ceramics free from lead in the interest of the environment, which present a transition temperature close to room temperature.

Relaxor or classical ferroelectric behaviour in ceramics with composition Ba1-xNaxTi1-xNbxO3

A polycrystalline sample of BaTi0.85(Fe1∕2Nb1∕2)0.15O3 is synthesized by high temperature solid state reaction technique. This sample crystallizes in cubic pervoskite structure at room temperature. The dielectric properties have been investigated at temperature range from 80to450K at various frequencies (100–107Hz). Frequency dielectric dispersion phenomena in a BaTi0.85(Fe1∕2Nb1∕2)0.15O3 ceramic has been analyzed by impedance spectroscopy in the temperature range from 250to450K. The Cole-Cole relaxation equation modified by introducing the conductivity was used to describe the experimental dielectric spectra of a high permittivity. Excellent agreement has been obtained in wide frequency domain (100–107Hz) between the measured and calculated permittivities in the 250–450K temperature range. The dielectric relaxation frequency obeys the Arrhenius behavior with activation energy of 0.181eV. A separation of the grain and grain boundary properties has been achieved using an equivalent circuit model. The diffe...

Hamadi Khemakhem

Papers

XRD, Raman and electrical studies on the (1−x)(Na0.5Bi0.5)TiO3−xBaTiO3 lead free ceramics

Relaxor or classical ferroelectric behaviour in ceramics with composition Ba1-xNaxTi1-xNbxO3

Dielectric relaxation in BaTi0.85(Fe1∕2Nb1∕2)0.15O3 perovskite ceramic

Structural, optical, and electrical properties of Nd-doped Na0.5Bi0.5TiO3

Dielectric properties and ferroelectric phase transitions in BaxSr1−xTiO3 solid solution