Woong-Kil Choo

Modified ferroelectric high density Strontium Barium Niobate ceramics for pyroelectric applications

Abstract Modified Strontium Barium Niobate (SBLLN) ceramics were prepared by the conventional oxide technique. The permittivities of thermally depoled SBLLN ceramics measured at 100 kHz are much smaller than that of intrinsic Sr0.75Ba0.25Nb2O6 crystal. The EP hysteresis loops of the materials are of the slim-loop type. Ceramic SBLLN wafers show a good transparency from the absorption edge of 0.4 μm to 6 μM. Such physical characteristics of ceramic SBLLN look suitable for pyroelectric as well as electro-optical application.

Diffuse phase transition of barium-substituted lead-ytterbium-niobate ceramics

The behaviour of the diffuse phase transition of (Pb1-xBax)(Yb12/Nb12/)O3 (0 Tt, where in m is the peak value of in at Tt and n is an exponent. The value of n is dramatically increased for 0.1<or=x<or=0.14, and then increased linearly with increasing Ba-content (dn/dx=1.78*10-2 %-1) for 0.14<or=x<or=0.3. TN decreases markedly for 0<or=x<0.12, whereas the value of Tt is reduced linearly with increasing x-value (dTt/dx=-6.61 degrees C %-1) for 0.14<or=x<or=0.3. Another remarkable feature is an anomalous enhancement of in m between x=0.1 and x=0.14. Heat capacity measurements have revealed a well-defined anomaly associated with the AFE-PE transition, increasingly broadening with Ba-content for 0<or=x<0.1. For 0.1<or=x<or=0.14, the heat capacity anomaly is extremely broadened and a small deviation from the extrapolated baseline can be seen. In particular, the authors cannot detect any heat capacity anomaly associated with Tt above x=0.14, which is suggested as additional evidence for DPT. These salient features are discussed in the light of the formation of locally ordered polar microdomains within a non-polar matrix and the pseudo-cubic structure above x=0.12.

The dielectric properties and phase transitions of (1−x)Pb(Yb1/2Ta1/2)O3−xPbZrO3

Abstract The dielectric properties and phase transition behaviors of antiferroelectric-antiferroelectric (1− x )Pb(Yb 1/2 Ta 1/2 )O 3 − x PbZrO 3 [(1− x )PYT− x PZ] solid solution system (0.0⩽ x ⩽0.10) have been studied by X-ray diffraction, permittivity and E–P hysteresis measurement. With increasing the PbZrO 3 amount, the initially split Bragg peaks (400) and (440) begin to merge into single lines. The temperature of the secondary phase transition of (1− x )PYT− x PZ whose character is relaxor-like decreases rapidly rather than that of the first order transition whose character is frequency independent with increasing the PbZrO 3 addition. From the E–P hysteresis loop measurement, the remanent polarizations and the coercive field of the system go to zero as the temperature and Zr 4+ concentration increase.

A crystallographic and dielectric studies in Pb(Yb1/2Nb1/2)O3 - Pb(Fe1/2Nb1/2)O3

Abstract The phase change pattern of antiferroelectric and B-site ordered Pb(Yb1/2Nb1/2)O3 on doping ferroelectric and disordered Pb(Fe1/2Nb1/2) O3 has been examined. Initial x-ray diffraction and dielectric constant results show the reduction of the transition temperature with increasing x in (1-x)Pb(Yb1/2Nb1/2)O3 - (x)Pb(Fe1/2Nb1/2)O3. For 0.0 ≤ x ≤ 0.13, the antiferroelectric-paraelectric phase transition is relatively sharp, the low temperature crystal structure of the solid soluion may be analyzed based on an orthorhombic crystal structure. For 0.15≤ x ≤0.30, the low temperature crystal structure is pseudocubic, and the phase transition changes from a sharp to a diffuse one.

Successive phase transitions in Pb2CoWO6

Abstract The structural phase transitions in Pb2CoWO6 have been investigated by means of x-ray powder diffraction, dielectric constant and DSC measurements. The basic structure of the incommensurate phase (II) has been identified as monoclinic from the x-ray diffraction profile. The cubic-incommensurate phase transition at Tc1 ≅27°C on cooling is strongly first order, which is confirmed by dielectric thermal hysteresis. The incommensurate-orthorhombic phase transition at Tc2≅-35°C on cooling has been characterized by very weak dielectric constant anomaly and distinct heat capacity anomaly. DSC curves have revealed that the enthalpy change associated with Tc1 is twice as much as that associated with Tc2

X-ray and electrical study of the phase transitions in (Pb1-xBax)(Yb1/2Nb1/2)O3

Abstract In order to study the effect caused by atomic substitution on the phase transition in Pb(Yb1/2Nb1/2)O3, crystallographic and dielectric properties of (Pb1-xBax)(Yb1/2Nb1/2)O3 ceramics system have been investigated for 0≤x≤0.5, as a function of temperature. A continuous series of solid solutions with ordered perovskite structure has been found. While the antiferroelectric transition temperature markedly decreases with increasing Ba concentration, the value of dielectric constant in the immediate vicinity of the peak is maximum at x=0.14. It has been revealed that the low symmetry phase changes from the antiferroelectric state to ferroelectric one near x=0.12. The antiferroelectric-paraelectric transition is relatively sharp for 0≤x≤0.1, but the ferroelectric-paraelectric transition is diffuse. A brief discussion of the effect caused by Ba substitution is given.

Electron microscopy study of incommensurate modulation in barium sodium niobate

Abstract Electron diffraction and high resolution electron microscopy (HREM) have been employed to investigate the incommensurate modulation in barium sodium niobate (BSN). Especially, we have characterized the texture of the phase front at the lock-in transition through the proper heat treatment of speciments. We have observed that the stable phase near Tc is the singly-modulated orthorhombic phase. The topological features of the discommensurations and the characteristics of the corresponding electron diffration pattern have been presented.

Effects of DC bias fields on dielectric properties and phase transition behaviour of the (1-x)((Pb0.7Ba0.3)(Mg13/Nb23/)O3)-xPbTiO3 system of morphotropic phase boundary compositions

The effect of DC bias on the dielectric properties and the phase transition behaviour of the (1-x)((Pb0.7Ba0.3)(Mg13/Nb23/)O3)-xPbTiO3 ceramic system near the morphotropic phase boundary (0.35<or=x<or=0.5) have been investigated. The phase transition goes through a series of character changes from being a diffuse phase transition at x=0.35 to being a more or less normal phase transition at x=0.5. Between the extreme compositions, the system has shown a rather anomalous phase transition behaviour due to the existence of the morphotropic phase boundary centred around x=0.44. On subjection to a series of external biases, the averaged dielectric constant maximum temperatures of the specimens of intermediate compositions tend to oscillate with respect to the bias field. Such anomalous dielectric behaviour is ascribed to the coexistence of multiple phases in a narrow composition band whose centre of gravity may be assigned as the morphotropic phase boundary.

A HREM study of La and Li-modified strontium barium niobate ceramics

Abstract The incommensurate (INC) modulation and diffuse phase transition (DPT) in La and Li-substituted SBN ceramics have been studied by the high resolution electron microscopy (HREM), TEM and X-ray diffractions, and dielectric constant measurements. INC satellite diffraction patterns in the tetragonal phase along with its images were obtained. Among the interesting features is the appearance of diffraction streaks along . In accordance with the diffraction pattern change with the solute concentration, the HREM image shows the presence of zones (or {110} platelets of an unit cell spacing). These extended defects seem to induce the stabilization of INC phase and at the same time enhance the nature of DPT.

Carrier-enhanced Ferromagnetism in Cr-doped ZnO

We have investigated the effects of Al codoping on the structural, electrical transport, and magnetic properties of oxide diluted magnetic semiconductor thin films prepared by reactive sputtering. Nondoped thin films show semiconducting transport behavior and weak ferromagnetic characteristic. The Al doping increases the carrier concentration and results in an decrease of resistivity and metal-insulator transition behavior. With increasing carrier concentration, the magnetic properties drastically change, exhibiting a remarkable increase of the saturation magnetization. These results show carrier-enhanced ferromagnetic order in Cr-doped ZnO.