C. K. Jeong, G.-T. Hwang, D. Y. Park, J. H. Park, Dr. M. Byun, Prof. K. J. Lee Department of Materials Science and Engineering Korea Advanced Institute of Science and Technology (KAIST) 291 Daehak-ro , Yuseong-gu , Daejeon 305-701 , South Korea E-mail: keonlee@kaist.ac.kr Dr. J. Lee, Dr. S. Han, Prof. S. H. Ko Department of Mechanical Engineering Seoul National University 1 Gwanak-ro , Gwanak-gu , Seoul 151-742 , South Korea E-mail: maxko@snu.ac.kr Dr. J. Lee, Prof. S. S. Lee Department of Mechanical Engineering Korea Advanced Institute of Science and Technology (KAIST) 291 Daehak-ro , Yuseong-gu , Daejeon 305-701 , South Korea Dr. J. Ryu Functional Ceramic Group Korea Institute of Materials Science (KIMS) 797 Changwon-daero Seongsan-gu Changwon , Gyeongsangnam-do 642–831 , South Korea

A Hyper‐Stretchable Elastic‐Composite Energy Harvester

In addition to x-ray diffraction, Raman spectrum measurements provide direct evidence of the tetragonal and orthorhombic phases coexistence in lead-free ceramics (1−x)K0.5Na0.5NbO3 (KNN)–xLiTaO3 when x equals 5mol%. This is caused by the phase transition temperature between tetragonal and orthorhombic decreasing to around room temperature due to the Li and Ta doping in KNN, and not by constituting a region of morphotropic phase boundary as presented by most published papers. The results indicate that although this kind of ceramic displays good properties, it needs further study to verify if it is suitable to be used in a varying temperature environment.

Phase transitional behavior in K0.5Na0.5NbO3–LiTaO3 ceramics

This paper provides a review of a systematic micro-Raman scattering study on various lead- and barium-based ABO3 perovskites (A = Pb, La, Ba, Sr, Ce, Gd, Nd and B = Ti, Zr), Aurivillius-type Bi-layered compounds (SrBi2Ta2O9, Bi4Ti3O12) and their solid solutions useful in a broad range of device applications. Various factors that influence the material properties such as particle size, stresses, stoichiometry, compositional homogeneity and their effects on phase transition were investigated. The processing conditions, A- and B-site substitution, size-dependent Raman spectra and the structure–property correlations are discussed in the bulk, thin film and nano-crystalline forms of these materials. A film thickness dependence stress study on lead titanate (PT) and lead zirconate titanate (PZT) films indicated that the nature of stress depends strongly on the lattice parameters of the film and substrate. The size effect was found to decrease the ferroelectric transition temperature in lead-based perovskite materials. Both ionic charge and radii induced changes in the Raman spectra of A- and B-site substituted perovskites and layered compounds were studied. A-site substitution in strontium bismuth tantalite (SBT) was found to induce a relatively linear variation of transition temperature compared with the B-site substituted SBT. Raman spectra of layered compounds and their solid solutions exhibited a strong dependence on dopants resulting in structural modifications. Copyright © 2002 John Wiley & Sons, Ltd.

Studies on ferroelectric perovskites and Bi‐layered compounds using micro‐Raman spectroscopy

Photochromic materials have been extensively studied because they are quite attractive and promising for many applications. Tungsten oxide (WO3), a typical photochromic material, has attracted considerable attention. So far, many advances have been achieved in different subfields of WO3 photochromism, mainly focused on the enhancement of WO3 photochromic properties, which depends not only on the chemical features but also on the microstructures, interface and hybrid components. This review aims to summarize recent progress in different advanced strategies to improve WO3 photochromic properties, such as a fast photoresponse (within 1 min), strong and stable photochromism, visible-light coloration and reversible photochromism. However, it is still challenging to achieve these properties for large-scale commercial applications. In addition, many previous studies have been limited to lab research purposes, although some photochromic WO3-based devices have been fabricated. To address the above-mentioned issues clearly, this review will provide a discussion of the following aspects. First, the fundamental photochromism of WO3 is described. Secondly, various advances towards improving its photochromic properties will be summarized. Thirdly, the optical applications of WO3-based photochromic materials will be reviewed and discussed. And finally, a perspective on the development trends in WO3-based photochromic materials and devices is proposed.

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Advances on tungsten oxide based photochromic materials: strategies to improve their photochromic properties

Perovskite solar cells with record power conversion efficiency are fabricated by alloying both hybrid and fully inorganic compounds. While the basic electronic properties of the hybrid perovskites are now well understood, key electronic parameters for solar cell performance, such as the exciton binding energy of fully inorganic perovskites, are still unknown. By performing magneto-transmission measurements, we determine with high accuracy the exciton binding energy and reduced mass of fully inorganic CsPbX3 perovskites (X = I, Br, and an alloy of these). The well-behaved (continuous) evolution of the band gap with temperature in the range of 4–270 K suggests that fully inorganic perovskites do not undergo structural phase transitions like their hybrid counterparts. The experimentally determined dielectric constants indicate that at low temperature, when the motion of the organic cation is frozen, the dielectric screening mechanism is essentially the same for both hybrid and inorganic perovskites and is do...

Impact of the Halide Cage on the Electronic Properties of Fully Inorganic Cesium Lead Halide Perovskites

Solid State Communications 112 (1999) 383–386

The vibrational properties of WO3 ceramics in the 30–300 K temperature range were probed by Raman spectroscopy. Starting with WO3 ceramics at room temperature presenting monoclinic (V) and triclinic (VI) mixed phases, structural transitions related to decreasing temperature were observed. At low temperatures a monoclinic (VII) and a new mixed phase were observed, the former characteristic of WO3 bulk and the latter associated to microcrystals. The analysis of the spectra indicated that the new phase develops gradually and it comes mainly from the monoclinic (V) phase. Copyright © 2001 John Wiley & Sons, Ltd.

Phase transition in WO3 microcrystals obtained by sintering process

Single-crystal samples of taurine, NH3C2H4SO3, were studied by Raman spectroscopy in a diamond-anvil cell up to pressures of 5.6 GPa. From the analysis of the results we observed that the crystal undergoes a phase transition at about 0.7 GPa. The transition is accompanied by the appearance of a phonon in the external mode region of the spectrum and by changes of both the wavenumber of the CCN bending and of the CSH torsional vibrations. Additionally evidence for a second phase transition occurring at 5.2 GPa was observed. Pressure coefficients for all modes observed in this work are also given. Copyright © 2001 John Wiley & Sons, Ltd.

High-pressure Raman study of taurine crystal

We have performed x-ray, electrical resistivity, and Raman scattering measurements in ${\mathrm{La}}_{070}{\mathrm{Sr}}_{030}{\mathrm{Mn}}_{1\ensuremath{-}x}{\mathrm{Fe}}_{x}{\mathrm{O}}_{3}$ manganites, with x varying from 0 to 040 The appearance of new peaks in the Raman spectra as the Fe content increases is attributed to either otherwise forbidden Raman modes that become active due to symmetry breaking effect or to the phonon density of states features, which arise due to a disorder-induced phenomenon The softening of some phonon modes suggests the evidence of magnetic polaronic states in the insulating paramagnetic phase of these manganites Our observations are discussed in connection with the x-ray diffraction and electrical resistivity data

Evidence of magnetic polaronic states in La 0.70 Sr 0.30 Mn 1 − x Fe x O 3 manganites

We have demonstrated in this work that single phase of α-Fe2O3 nanoparticle can be prepared using gelatin. It was characterized by X-ray powder diffraction (XRPD) technique, gas sorption technique (BET), thermogravimetric analysis (TGA) and transmission electron microscopy (TEM). The nanoparticle was obtained from an aqueous solution of gelatin and FeCl3 · 6H2O, as Fe source, annealed in the temperature range of 200–600 °C for 12 h. The mean particle size of this powder measured with XRPD is about 100 nm and this size agrees well with TEM experiments. Also, the TEM result shows agglomeration that was confirmed by BET technique. These results show that gelatin could be an alternative organic precursor to produce metal oxide powders with nanometer dimensions.

José Marcos Sasaki

Papers

Solid State Communications 112 (1999) 383–386

Phase transition in WO3 microcrystals obtained by sintering process

High-pressure Raman study of taurine crystal

Evidence of magnetic polaronic states in La 0.70 Sr 0.30 Mn 1 − x Fe x O 3 manganites

Sintering of nanoparticles of α-Fe2O3 using gelatin