Chul Hyun Yo

Electrical and magnetic properties of NiCrxFe2−xO4 spinel (0 ≤ x ≤ 0.6)

The spinel ferrite NiCrxFe2−xO4 system was synthesized under atmospheric pressure, and showed single spinel phase in the XRD pattern. The lattice parameter decreased with increasing amount of Cr. For the study of cation distribution in this system, magnetic measurements and Mossbauer spectroscopy were carried out. It was found that ions at the B site moved to the A site, and that this system varied from an inverse to a normal spinel structure. It was observed that the magnetic moment of the systems and Curie Temperature decreased by substitution effect. Using a thermopower measurement, this system was found to exhibit n-type semiconductivity. In the electrical conductivity measurements, an abrupt change exists near the Tc, owing to the magnetic spin arrangement relaxation. The electrical conduction mechanism of spinel is an electron transfer between the +2 and +3 cations at the B site, so called polaron hopping mechanism. From the conductivity result, this system is closely related to the electron-hole-compensation between electrons and holes.

Characterization of the magnetic properties and transport mechanisms of CoxFe3−xO4 spinel

CoxFe3−xO4 spinels were synthesized at 1100°C in air and their phases were investigated by XRD. In the IR spectra of these samples, the stretching vibration of tetrahedral group was observed at 583cm−1, which means that the tetrahedral structure was not influenced by the concentration of Co ion. The cation distribution determined by Mossbauer spectroscopy is closely related to the lattice parameter as well as magnetic properties. From the measurements of the magnetic moment, it has been known that the arrangement of magnetic moment between tetrahedral(A) and octahedral(B) site is canted in this spinel. The n-type and p-type conductions were observed in the samples with x<1.0 and x=1.2, respectively, showing that p-type to n-type transition occurred at 1140K in the sample of x=1.0. The n-type and p-type conductions are due to the formation of Fe2+ ion and Co3+ ion forming Co2++Co4+ exciton in B site.

Study of the nonstoichiometric composition op zinc oxide

The x values in the nonstoichiometrie chemical formula Zn1+xO have been measured in the temperature range from 400 to 1200°C under oxygen pressures from 1 to 10−6 atm, and also 1 atm of air. The x values varied between 0.00824 and 0.06370 under the various oxygen pressures. The enthalpies of formation of excess zinc in zinc oxide were in general less than 7.40 kcal/mole under the above conditions, all positive values representing an endothennic process. The plots of log x vs log Po2 (or log x = 1/n log Po2) are linear, and the 1/n values from the slopes of the plots are −114.0to−111.1 in the temperature range 400–900°C. Many physical properties of zinc oxide such as electrical conductivity, catalytic effects and defects can be explained on the basis of the x values and the mechanism of formation of the nonstoichiometric compositions of the oxide.

Influence of the technological conditions on the luminescence of Eu3+ ions in Sr2SnO4

Different synthesis conditions have been used to prepare the stoichiometric compounds of the Sr2−xEuxSnO4 with x=0.00, 0.01, 0.02 and 0.03. As the 5D0→7F0 transition is forbidden by a selection rule of the Judd–Ofelt theory, the J=0→J=0 level gains some intensity from the mixing of J states in the Sr2−xEuxKxSnO4 and Sr1−xEuxSnO4 samples. For Sr2−xEuxSnO4 samples prepared under a slightly reducing atmosphere, the J=0→J=0 transition is split into two peaks, which confirms the substitution of Eu3+ ions for different crystallographic sites.

Study of the nonstoichiometry and physical properties of the SrxDy1−xFeO3−y ferrite system

A series of samples in the Sr/sub x/Dy/sub 1-x/FeO/sub 3-y/ system (x = 0.00, 0.25, 0.50, 0.75, and 1.00) have been produced by heating the reactants to 1200/sup 0/C in air under atmospheric pressure. Mohr salt analysis shows that DyFeO/sub 3/ is a stoichiometric compound and that Fe/sup 4 +/ ions and oxygen deficiency compensate for the cation charge deficiency which is caused by substitution of Sr/sup 2 +/ at Dy/sup 3/..mu.. sites. X-ray powder diffraction assigns the compositions x = 0.00 and 0.25 to the orthorhombic system and the composition x = 0.50, 0.75, and 1.00 to the cubic system. The electrical conductivity sharply increases and the activation energy decreases with the presence of mixed iron valency. The Moessbauer spectra of the samples with x = 0.25 and 0.50 are composed of two sets of lines and a single broad peak for x = 0.75.

Structural, magnetic, and electrical properties of nonstoichiometric perovskite Ho1−xCaxCoO3−y system

Abstract A series of samples of the Ho 1− x Ca x CoO 3− y ( x = 0.00, 0.15,0.25, 0.75 and 1.00) system have been prepared by the dripping pyrolysis method at 1000 °C under an atmospheric pressure. The samples for the compositions of x = 0.00 to x = 0.25 are identified as GdFeO 3 -type structure with a space group Pbnm . When Ca 2+ (r o = 1.34 A ) ions are substituted with Ho 3+ ( r 0 = 1.12 A ) ions, structural distortions decrease. All samples exhibit paramagnetic behaviour in the temperature range from 300 to 900 K. There is not any spin state change of Co 3+ ions in that temperature range. Paramagnetic Curie temperatures have negative values which represent an antiferromagnetic interaction in the Co 3+  O 2− Co 3+ bond. The samples of x = 0.00,0.15 and 0.25 contain high spins Co 3+ with increasing order. All the compositions of Ho 1− x ,Ca x CoO 3− y are included in the semi-conducting range. The composition of x = 0.15 with the highest amount of Co 4+ ions shows the highest conductivity. The collective Co 4+ ions favor low spin states which act as positive holes. The linearity in the CoOCo bond enhances covalency that can decrease the band gap between α ∗ and π ∗ bands. Activation energies for the electrical conduction of the samples are relatively higher than those of other cobaltates due to small size of A-site ions. The small size ions of A-site broaden the band gap as a result of the competing σ bond of CoA and π bond of CoO 2− . The transition of semi-conductor to metal above 1000 K represents the high order transition due to the collective σ ∗ and π ∗ bands overlapping.

Electrical and magnetic properties of the Sn4+ substituted spinel ferrite, ZnSnxFe2−4x/3O4 (0.00 ≤ x ≤ 0.30)

Abstract ZnSnxFe2−4x/3O4 spinels (0.0 ≤ x ≤ 0.3) which have cubic symmetry were synthesized using a ceramic technique at 1150°C for 24 h in air and rapidly quenched to room temperature. From the XRD. TGA and DTA results, it was found that all samples had single phase and thermal stability in the range of room temperature up to 1000 K. As Sn4− concentration was increased from x = 0 to 0.3, the lattice parameter and electrical conductivity increased. The results of Mossbauer spectroscopy and magnetic moment measurements showed that the tetrahedral (A) site is occupied by a Zn+ ion and magnetic moments depend only upon Fe3+ ion in an octahedral (B) site. The magnetic moments decreased with an increasing amount of non-magnetic Sn (d10) ion. n-type semiconductivity was observed in all samples. However, the conductivities did not depend on Sn substitution. It was assumed that the conduction mechanism would be derived from the competitive relation between the formation of Fe2+ and elongation of distance between neighboring Fe cations.

Infrared spectra and electrical conductivity of the solid solutions xSrO + (1 − x)Eu2O3, 0.01 ≤ x ≤ 0.08

Abstract The solid solutions xSrO + (1−x)Eu2O3 were synthesized at 1050 °C in air. Their phases were investigated by XRD and IR spectroscopy. Through XRD, we observed that the lattice parameters increased with increasing amount of SrO. In particular, effective replacement of Eu by Sr for all solid solutions was identified by the peak shifts of the IR spectra. With the addition of SrO, the vibration of pure Eu2O3, which is normally observed at 528 cm−1, is shifted to 532 cm−1. The solid solutions show electronic n-type conductivity at oxygen partial pressures lower than 2 × 10−3 atm above 720 °C owing to the singly and doubly effectively ionized oxygen vacancies. At oxygen partial pressures higher than 2 × 10−3 atm above 720 °C, the electrical conduction is p-type, owing to the triply effectively ionized europium vacancies.

Synthesis and characteristics of soluble polypyrroles with mixed dopants

Abstract Soluble conducting polypyrroles(Ppy) were prepared by the oxidative polymerization of pyrrole using mixed dopants. Compared to the dodecylbenzenesulfonic acid doped polypyrrole [Ppy-DBSA (σRT ∼ 10−1 S/cm)], the Ppy with mixed dopants (Ppy-mixed dopants) showed higher absorption intensity in near IR region in UV-Vis./NIR solution spectra and partly crystalline X-ray diffraction (XRD) patterns and higher conductivity of ∼20 S/cm for free standing films. The Ppy-DBSA film showed three dimensional variable range hopping (3D-VRH) conducton while Ppy-mixed dopant systems 1D-VRH conduction. These results are explained by the packing effect of planar dopant between the polymer chains.

Electrical conductivity of the solid solutions xCeO2 + (1-x)Dy2O3; 0.01 ⩽ x ⩽ 0.10

Abstract CeO 2 -doped Dy 2 O 3 systems containing 1, 4, 7 and 10 mol.% CeO 2 were found to be solid solutions by X-ray diffraction techniques. The lattice parameter a was obtained by the Nelson-Riley method, and its value increased with increasing dopant content. The average residual factors ( R ) obtained from X-ray intensity analysis based on oxygen interstitial and vacancy models were found to be 0.0631 and 0.1085, respectively, suggesting the oxygen interstitial model. The ratios of measured to calculated densities were found to be greater than 0.96, justifying the oxygen interstitial model. The results of thermal analysis showed that no phase transition occurred in the temperature range 500–1100 °C. The electrical conductivity was measured as a function of temperature from 500 to 1100 °C and of oxygen partial pressure from 5 × 10 −5 to 2 × 10 −1 atm. The exponential dependence of the electrical conductivity (σ) on the oxygen partial pressure ( P O 2 ) gives σαP O 2 1/ n with n = 6, the main defect and charge carriers being the oxygen interstitial and the electron hole, respectively.