• Korean Journal of Materials Research
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• v.29 no.10
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• pp.586-591
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• 2019

This study describes the doping effect of $Yb_2O_3$ on microstructure, electrical and dielectric properties of $ZnO-V_2O_5-MnO_2-Nb_2O_5$ (ZVMN) ceramic semiconductors sintered at a temperature as low as $900^{\circ}C$. As the doping content of $Yb_2O_3$ increases, the ceramic density slightly increases from 5.50 to $5.54g/cm^3$; also, the average ZnO grain size is in the range of $5.3-5.6{\mu}m$. The switching voltage increases from 4,874 to 5,494 V/cm when the doping content of $Yb_2O_3$ is less than 0.1 mol%, whereas further doping decreases this value. The ZVMN ceramic semiconductors doped with 0.1 mol% $Yb_2O_3$ reveal an excellent nonohmic coefficient as high as 70. The donor density of ZnO gain increases in the range of $2.46-7.41{\times}10^{17}cm^{-3}$ with increasing doping content of $Yb_2O_3$ and the potential barrier height and surface state density at the grain boundaries exhibits a maximum value (1.25 eV) at 0.1 mol%. The dielectric constant (at 1 kHz) decreases from 592.7 to 501.4 until the doping content of $Yb_2O_3$ reaches 0.1 mol%, whereas further doping increases it. The value of $tan{\delta}$ increases from 0.209 to 0.268 with the doping content of $Yb_2O_3$.

• Journal of Ceramic Processing Research
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• v.19 no.5
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• pp.372-377
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• 2018

MgO or gadolinium-doped ceria (GDC, $Ce_{0.9}Gd_{0.1}O_{2-{\delta}}$) was added as a promoter to improve the oxygen transfer kinetics of $MgMnO_3$ oxygen carrier material for chemical looping combustion. Neither MgO nor GDC reacted with $MgMnO_3$, even at the high temperature of $1100^{\circ}C$. The average oxygen transfer capacities of $MgMnO_3$, 5 wt% $MgO-MgMnO_3$, and 5 wt% $GDC-MgMnO_3$ were 8.74, 8.35, and 8.13 wt%, respectively. Although the addition of MgO or GDC decreased the oxygen transfer capacity, no further degradation was observed during their use in 5 redox cycles. The addition of GDC significantly improved the conversion rate for the reduction reaction of $MgMnO_3$ compared to the use of MgO due to an increase in the surface adsorption process of $CH_4$ via oxygen vacancies formed on the surface of GDC. On the other hand, the conversion rates for the oxidation reaction followed the order 5 wt% $GDC-MgMnO_3$ > 5 wt% $MgO-MgMnO_3$ >> $MgMnO_3$ due to morphological change. MgO or GDC particles suppressed the grain growth of the reduced $MgMnO_3$ (i.e., (Mg,Mn)O) and increased the specific surface area, thereby increasing the number of active reaction sites.

• Journal of the Korean Magnetics Society
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• v.14 no.1
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• pp.38-44
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• 2004

The iron doped colossal magnetoresistance materials with La-Ba-Mn-O perovskites structure have been synthesized by chemical reaction of sol-gel methods. Their crystallographic and magnetic properties have been studied with x-ray diffraction, VSM, RBS, Mossbauer spectroscopy, and magnetoresistance measurements. The crystal structure of the La$\_$0.67/Ba$\_$0.33/Mn$\_$0.99/Fe$\_$0.01/ $O_3$ at room temperature was determined to be orthorhombic of Pnma. The lattice parameters a$\_$0/ and c$\_$0/ increased gradually, but b$\_$0/ deceased with increase of iron substitution. The magnetization and coercivity deceased, also the Curie temperature decreased from 360 K as x increased from 0.00 to 0.05. Magnetoresistence measurements were carried out, and the maximum MR ($\Delta$$\rho$/$\rho$(0)) was observed at 281 K, about 9.5 % in 10 kOe. The temperature of maximum resistance (R$\_$MAX/) decreased with increasing substitution of Fe ions and a semiconductor-metal transition temperature (T$\_$SC-M/) decreased too. This phenomena show that ferromagnetic transition temperature decreased by substituting Fe for Mn ions, it decreases double exchange interaction. This result accords with magnetic structure of neutron diffraction. Mossbauer spectra of La$\_$0.67/Ba$\_$0.33/Mn$\_$0.99/Fe$\_$0.01/ $O_3$were taken at various temperatures ranging from 15 to 350 K. With lowering temperature of the sample, two magnetic phases were increased and finally it showed the two sharp sextets of spectra at 15 K. The isomer shift at all temperature range is about 0.3 mm/s relative to Fe metal, which means that both Fe ions are Fe$\^$3+/ states.Fe$\^$3+/ states.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.22 no.2
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• pp.78-83
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• 2012

The structural change and the electrical conductivity with Sr content in $La_{1-x}Sr_xMn_{0.8}Cu_{0.2}O_3$ (LSMCu) were studied. $La_{0.8}Sr_{0.2}MnO_3$ (LSM) and $La_{1-x}Sr_xMn_{0.8}Cu_{0.2}O_3$ ($0.1{\leq}x{\leq}0.4$) were synthesized by EDTA citric complexing process (ECCP). A decrease in the lattice parameters and lattice volumes was observed with increase of Sr content, and these results were attributed to the increasing $Mn^{4+}$ ions and $Cu^{3+}$ ions in B-site. The electrical conductivity measured from $500^{\circ}C$ to $1000^{\circ}C$ was increased with increase of Sr content in the $0.1{\leq}x{\leq}0.3$ composition range, and it was 172.6 S/cm (at $750^{\circ}C$) and 177.7 S/cm (at $950^{\circ}C$, the maximum value) in x = 0.3. The electrical conductivity was decreased in x = 0.4 because of the presence of the second phase in the grain boundaries. The lattice volume was contracted by increase of $Mn^{4+}$ ions and $Cu^{3+}$ ions in B-site according to increase of Sr content and the electrical conductivity was increased with increase of charge carriers which were involved in the hopping mechanism.

• Korean Journal of Materials Research
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• v.22 no.12
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• pp.675-681
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• 2012

We aimed to examine the co-doping effects of 1/6 mol% $Mn_3O_4$ and 1/4 mol% $Cr_2O_3$ (Mn:Cr = 1:1) on the reaction, microstructure, and electrical properties, such as the bulk defects and grain boundary properties, of ZnO-$Bi_2O_3-Sb_2O_3$ (ZBS; Sb/Bi = 0.5, 1.0, and 2.0) varistors. The sintering and electrical properties of Mn,Cr-doped ZBS, ZBS(MnCr) varistors were controlled using the Sb/Bi ratio. Pyrochlore ($Zn_2Bi_3Sb_3O_{14}$), ${\alpha}$-spinel ($Zn_7Sb_2O_{12}$), and ${\delta}-Bi_2O_3$ (also ${\beta}-Bi_2O_3$ at Sb/Bi ${\leq}$ 1.0) were detected for all of the systems. Mn and Cr are involved in the development of each phase. Pyrochlore was decomposed and promoted densification at lower temperature on heating in Sb/Bi = 1.0 system by Mn rather than Cr doping. A more homogeneous microstructure was obtained in all systems affected by ${\alpha}$-spinel. In ZBS(MnCr), the varistor characteristics were improved dramatically (non-linear coefficient, ${\alpha}$ = 40~78), and seemed to form ${V_o}^{\cdot}$(0.33 eV) as a dominant defect. From impedance and modulus spectroscopy, the grain boundaries can be seen to have divided into two types, i.e. one is tentatively assigned to ZnO/$Bi_2O_3$ (Mn,Cr)/ZnO (0.64~1.1 eV) and the other is assigned to the ZnO/ZnO (1.0~1.3 eV) homojunction.

• Korean Journal of Materials Research
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• v.22 no.12
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• pp.689-695
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• 2012

We have examined the co-doping effects of 1/2 mol% NiO and 1/4 mol% $Cr_2O_3$ (Ni:Cr = 1:1) on the reaction, microstructure, and electrical properties, such as the bulk defects and the grain boundary properties, of ZnO-$Bi_2O_3-Sb_2O_3$ (ZBS; Sb/Bi = 0.5, 1.0, and 2.0) varistors. The sintering and electrical properties of Ni,Cr-doped ZBS, ZBS(NiCr) varistors were controlled using the Sb/Bi ratio. Pyrochlore ($Zn_2Bi_3Sb_3O_{14}$), ${\alpha}$-spinel ($Zn_7Sb_2O_{12}$), and ${\delta}-Bi_2O_3$ were detected for all of compositions. For the sample with Sb/Bi = 1.0, the Pyrochlore was decomposed and promoted densification at lower temperature by Ni rather than by Cr. A homogeneous microstructure was obtained for all of the samples affected by ${\alpha}$-spinel. The varistor characteristics were not dramatically improved (non-linear coefficient, ${\alpha}$ = 5~24), and seemed to form ${Zn_i}^{{\cdot}{\cdot}}$(0.17 eV) and ${V_o}^{\cdot}$(0.33 eV) as dominant defects. From impedance and modulus spectroscopy, the grain boundaries were found to have been divided into two types, i.e., one is tentatively assigned to ZnO/$Bi_2O_3$ (Ni,Cr)/ZnO (0.98 eV) and the other is assigned to a ZnO/ZnO (~1.5 eV) homojunction.

• Korean Journal of Materials Research
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• v.22 no.12
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• pp.682-688
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• 2012

In this study we aimed to examine the co-doping effects of 1/6 mol% $Co_3O_4$ and 1/4 mol% $Cr_2O_3$ (Co:Cr = 1:1) on the reaction, microstructure, and electrical properties, such as the bulk defects and the grain boundary properties, of ZnO-$Bi_2O_3-Sb_2O_3$ (ZBS; Sb/Bi = 0.5, 1.0, and 2.0) varistors. The sintering and electrical properties of Co,Cr-doped ZBS, ZBS(CoCr) varistors were controlled using the Sb/Bi ratio. Pyrochlore ($Zn_2Bi_3Sb_3O_{14}$), ${\alpha}$-spinel ($Zn_7Sb_2O_{12}$), and ${\delta}-Bi_2O_3$ were formed in all systems. Pyrochlore was decomposed and promoted densification at lower temperature on heating in Sb/Bi = 1.0 by Cr rather than Co. A more homogeneous microstructure was obtained in all systems affected by ${\alpha}$-spinel. In ZBS(CoCr), the varistor characteristics were improved (non-linear coefficient, ${\alpha}$ = 20~63), and seemed to form ${Zn_i}^{{\cdot}{\cdot}}$(0.20 eV) and ${V_o}^{\cdot}$(0.33 eV) as dominant defects. From impedance and modulus spectroscopy, the grain boundaries were found to be composed of an electrically single barrier (0.94~1.1 eV) that is, however, somewhat sensitive to ambient oxygen with temperature. The phase development, densification, and microstructure were controlled by Cr rather than by Co but the electrical and grain boundary properties were controlled by Co rather than by Cr.