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ZnO-Zn<sub>2</sub>BiVO<sub>6</sub>-Co<sub>3</sub>O<sub>4</sub> 바리스터의 전류-전압 및 임피던스의 온도 특성
홍연우 ( Youn Woo Hong ),김유비 ( You Bi Kim ),백종후 ( Jong Hoo Paik ),조정호 ( Jeong Ho Cho ),정영훈 ( Young Hun Jeong ),윤지선 ( Ji Sun Yun ),박운익 ( Woon Ik Park ) 한국센서학회 2016 센서학회지 Vol.25 No.6
This study introduces the characteristics of current-voltage (I-V) and impedance variance for ZnO-Zn<sub>2</sub>BiVO<sub>6</sub>-Co<sub>3</sub>O<sub>4</sub> (ZZCo), which is sintered at 900℃, according to temperature changes. ZZCo varistor demonstrates dramatic improvement of non-linear coefficient, α = 66, with lower leakage current and higher insulating resistivity than those of ZZ (ZnO-Zn<sub>2</sub>BiVO<sub>6</sub>) from the aspect of I-V curves. While both systems are thermally stable up to 125℃, ZZCo represents a higher grain boundary activation energy with 1.05 eV and 0.94 eV of J-E-T and from IS & MS, respectively, than that of ZZ with 0.73 eV and 0.82 eV of J-E-T and from IS & MS, respectively, in the region above 180℃. It could be attributed to the formation of V<sub>O</sub> (0.41~0.47 eV) as dominant defect in two systems, as well as the defect-induced capacitance increase from 781 pF to 1 nF in accordance with increasing temperature. On the other hand, both the grain boundary capacitances of ZZ and ZZCo are shown to decrease to 357 pF and 349 pF, respectively, while the resistances systems decreased exponentially, in accordance with increasing temperature. So, this paper suggests that the application of newly formed liquid phases as sintering additives in both Zn<sub>2</sub>BiVO<sub>6</sub> and the ZZCo-based varistors would be helpful in developing commercialized devices such as chips, disk-type ZnO varistors in the future.
HP 및 SPS법에 의한 ZnS의 소결과 소결체의 광학적 특성
홍연우 ( Youn Woo Hong ),김유비 ( You Bi Kim ),이영진 ( Young Jin Lee ),김세기 ( Sei Ki Kim ),백종후 ( Jong Hoo Paik ),김진호 ( Jin Ho Kim ) 대한금속재료학회(구 대한금속학회) 2015 대한금속·재료학회지 Vol.53 No.12
The sintering behaviors of commercial ZnS nanopowders (purity 99.999%) processed with the two different sintering techniques of hot pressing (HP) and spark plasma sintering (SPS) and optical properties of sintered material are investigated here. The results are compared between the two different sintering techniques in terms of the sintering parameters (density, microstructure, X-ray diffraction pattern) and the optical properties (infrared transmission in the range of 2~20 μm) of the sintered ZnS ceramics. The maximum density was obtained at 950℃ irrespective of the sintering method. The wurtzite (hexagonal) phase causes birefringence due to optical anisotropy which begins at a temperature of more than 920℃. The optimized optical properties were found to be ZnS ceramics sintered with HP at 950℃ and with SPS at 90 0℃. However, contamination from the graphite mold and its influence on the optical properties were confirmed to be very serious, especially in HP and SPS. In the SPS method, the optical properties by light scattering were found in this study to worsen considerably due to the refractive index differences among the ZnS, carbon impurities, and pores.
ZnO-Bi2O3-Sb2O3-Mn3O4-NiO 바리스터의 소결과 입계 특성
홍연우 ( Youn Woo Hong ),이영진 ( Young Jin Lee ),김세기 ( Sei Ki Kim ),김진호 ( Jin Ho Kim ) 대한금속재료학회(구 대한금속학회) 2013 대한금속·재료학회지 Vol.51 No.5
We prepared ZnO-Bi2O3-Sb2O3-Mn3O4-NiO(Sb/Bi=0.5, 1.0, and 2.0) varistors to examine the co-doping effect of 1/6 mol% Mn3O4 and 0.5 mol% NiO (Mn:Ni = 1:1) on the reaction, microstructure, and electrical properties, such as J-E characteristics, bulk defects and grain boundary properties. The sintering and electrical properties of the ZnO-Bi2O3-Sb2O3-Mn3O4-NiO varistor were controlled by Sb/Bi ratio. Pyrochlore (Zn2Bi3Sb3O14) was decomposed and promoted densification at lower temperature in Sb/Bi = 1.0. Pyrochlore reproduced on cooling, α-spinel (Zn7Sb2O12), and β-Bi2O3 were formed in all systems above 1000℃. More homogeneous microstructure was obtained in all systems by Mn doping due to the α-spinel formation. The varistor performance was drastically improved as non-linear coefficient of α = 22~84. It was found to form Zni (0.20 eV) and V0 (0.33 eV) as dominant defects. The grain boundaries consisted of an electrically single type (0.90~0.93 eV) by the co-doping effect of Mn and Ni.
소결온도가 ZnO-Co3O4-Cr2O3의 결함과 전기적 특성에 미치는 영향
홍연우 ( Youn Woo Hong ),김유비 ( You Bi Kim ),이영진 ( Young Jin Lee ),김세기 ( Sei Ki Kim ),백종우 ( Jong Hoo Paik ),조만호 ( Man Ho Jo ) 대한금속재료학회(구 대한금속학회) 2014 대한금속·재료학회지 Vol.52 No.12
The defects and origin of the good varistor properties in the ZnO-Co3O4-Cr2O3 system were investigated by admittance spectroscopy, I-V characteristics, and impedance and modulus spectroscopy. Two kinds of defects were detected, but (0.27 eV) was identified as a major donor level by admittance spectroscopy. The ZnO grain resistivity of ~0.4 Ωcm was calculated but somewhat increased with sintering temperature. J-E characteristics with varistor behavior was seen in this system while the nonlinear coefficient α changed from 9 to 92 with sintering temperature. The single potential barrier of 0.64-1.01 eV at the grain boundary region was confirmed by impedance and modulus spectroscopy. The origin of a good varistor behavior in ZnO-Co3O4-Cr2O3 would be due to the formation and stabilization of a double Schottky barrier by the redox reaction of Co ions and the existence of small Cr ions in the grain boundaries.
홍연우,이영진,김세기,김진호,Hong, Youn-Woo,Lee, Young-Jin,Kim, Sei-Ki,Kim, Jin-Ho 한국전기전자재료학회 2012 전기전자재료학회논문지 Vol.25 No.12
In this study, we have investigated the effects of Mn and Co co-doping on defects, J-E curves and grain boundary characteristics of ZnO-$Bi_2O_3$ (ZB) varistor. Admittance spectra and dielectric functions show two bulk defects of $Zn_i^{{\cdot}{\cdot}}$ (0.17~0.18 eV) and $V_o^{\cdot}$ (0.30~0.33 eV). From J-E characteristics the nonlinear coefficient (${\alpha}$) and resistivity (${\rho}_{gb}$) of pre-breakdown region decreased as 30 to 24 and 5.1 to 0.08 $G{\Omega}cm$ with sintering temperature, respectively. The double Schottky barrier of grain boundaries in ZB(MCo) ($ZnO-Bi_2O_3-Mn_3O_4-Co_3O_4$) could be electrochemically single type. However, its thermal stability was slightly disturbed by ambient oxygen because the apparent activation energy of grain boundaries was changed from 0.64 eV at lower temperature to 1.06 eV at higher temperature. It was revealed that a co-doping of Mn and Co in ZB reduced the heterogeneity of the barrier in grain boundaries and stabilized the barrier against an ambient temperature (${\alpha}$-factor= 0.136).
Ni를 첨가한 ZnO-Bi<sub>2</sub>O<sub>3</sub>-Sb<sub>2</sub>O<sub>3</sub>계의 소결과 전기적 특성
홍연우,신효순,여동훈,김종희,김진호,Hong, Youn-Woo,Shin, Hyo-Soon,Yeo, Dong-Hun,Kim, Jong-Hee,Kim, Jin-Ho 한국전기전자재료학회 2009 전기전자재료학회논문지 Vol.22 No.11
The present study aims at the examination of the effects of 1 mol% NiO addition on the reaction, microstructure development, resultant electrical properties, and especially the bulk trap and interface state levels of $ZnO-Bi_2O_3-Sb_2O_3$ (Sb/Bi=0.5, 1.0, and 2.0) systems (ZBS). The samples were prepared by conventional ceramic process, and characterized by density, XRD, SEM, I-V, impedance and modulus spectroscopy (IS & MS) measurement. The sintering and electrical properties of Ni-doped ZBS (ZBSN) systems were controlled by Sb/Bi ratio. Pyrochlore ($Zn_2Bi_3Sb_3O_{14}$) was decomposed more than $100^{\circ}C$ lowered in ZBS (Sb/Bi=1.0) by Ni doping. The reproduction of pyrochlore was suppressed by the addition of Ni in ZBS. Between two polymorphs of $Zn_7Sb_2O_{12}$ spinel ($\alpha$ and $\beta$), microstructure of ZBSN (Sb/Bi=0.5) composed of a-spinel was more homogeneous than $Sb/Bi{\geq}1.0$ composed of $\beta$-spinel phase. In ZBSN, the varistor characteristics were not improved drastically (non-linear coefficient $\alpha\;=\;6{\sim}11$) and independent on microstructure according to Sb/Bi ratio. Doping of Ni to ZBS seemed to form ${V_0}^{\cdot}$ (0.33 eV) as dominant bulk defect. From IS & MS, especially the grain boundaries of Sb/Bi=0.5 systems were divided into two types, i.e. sensitive to oxygen and thus electrically active one and electrically inactive intergranular one with temperature.
홍연우,신효순,여동훈,김종희,김진호,Hong, Youn-Woo,Shin, Hyo-Soon,Yeo, Dong-Hun,Kim, Jong-Hee,Kim, Jin-Ho 한국전기전자재료학회 2009 전기전자재료학회논문지 Vol.22 No.11
In this study, we investigated the effects of Cr dopant (1.0 at% $Cr_2O_3$ sintered at $1000^{\circ}C$ for 1 h in air) on the bulk trap (i.e. defect) and interface state levels of ZnO using dielectric functions ($Z^*$, $M^*$, $Y^*$, $\varepsilon^*$, and $tan{\delta}$), admittance spectroscopy (AS), and impedance-modulus spectroscopy (IS & MS). For the identification of the bulk trap levels, we examine the zero-biased admittance spectroscopy and dielectric functions as a function of frequency and temperature. Impedance and electric modulus spectroscopy is a powerful technique to characterize grain boundaries of electronic ceramic materials as well. As a result, three kinds of bulk defect trap levels were found below the conduction band edge of ZnO in 1.0 at% Cr-doped ZnO (Cr-ZnO) as 0.11 eV, 0.21 eV, and 0.31 eV. The overlapped defect levels ($Zn^{..}_i$ and $V^{\cdot}_0$) in admittance spectra were successfully separated by the combination of dielectric function such as $M^*$, $\varepsilon^*$, and $tan{\delta}$. In Cr-ZnO, the interfacial state level was about 1.17 eV by IS and MS. Also we measured the resistance ($R_{gb}$) and capacitance ($C_{gb}$) of grain boundaries with temperature using impedance-modulus spectroscopy. It have discussed about the stability and homogeneity of grain boundaries using distribution parameter ($\alpha$) simulated with the Z"-logf plots with temperature.
Cr을 첨가한 ZnO-Sb<sub>2</sub>O<sub>3</sub> 세라믹스의 바리스터 응용
홍연우,신효순,여동훈,김진호,Hong, Youn-Woo,Shin, Hyo-Soon,Yeo, Dong-Hun,Kim, Jin-Ho 한국전기전자재료학회 2010 전기전자재료학회논문지 Vol.23 No.11
In this study, we have investigated the effects of Cr dopant on the sintering and electrical properties of ZnO-$Sb_2O_3$ (ZS) ceramics for varistor application. Spinel phases including $\alpha-$ and $\beta$-type was formed at ZS system and $\alpha$-spinel was stabilized by Cr doping in ZS system. Densification of ZS and ZSCr system was retarded to $1000^{\circ}C$ by the formation of spinel at $800^{\circ}C$. The morphology and its distribution of spinel phases in ZS system was homogeneous but disturbed by Cr doping. In ZSCr the densification of ZnO compared with ZS system was more retarded by low concentration of Zn interstitial defects induced by Cr doping in addition to the effect of spinel phase formation. The defects in each system were identified as attractive coulombic center (ZS: 0.13 eV, ZSCr: 0.12 eV) and singly charged oxygen vacancy $V_0^{\cdot}$ (ZSCr: 0.33 eV). In all ZS and ZSCr system have week varistor behavior by the formation of double Schottky barrier at grain boundary but its stability of barrier was very sensitive to sintering temperature.
홍연우,이영진,김세기,김진호,Hong, Youn-Woo,Lee, Young-Jin,Kim, Sei-Ki,Kim, Jin-Ho 한국전기전자재료학회 2012 전기전자재료학회논문지 Vol.25 No.11
In this study we aims to examine the co-doping effects of 1/3 mol% $Mn_3O_4+Co_3O_4$ (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,Co-doped ZBS, ZBS(MCo) varistors were controlled by Sb/Bi ratio. Pyrochlore ($Zn_2Bi_3Sb_3O_{14}$) was decomposed and promoted densification at lower temperature on heating in Sb/Bi=1.0 by Mn rather than Co. Pyrochlore on cooling was reproduced in all systems however, spinel (${\alpha}$- or ${\beta}$-polymorph) did not formed in Sb/Bi=0.5. More homogeneous microstructure was obtained in $Sb/Bi{\geq}1.0$ In ZBS(MCo), the varistor characteristics were improved drastically (non-linear coefficient, ${\alpha}$=30~49), and seemed to form $Zn_i^{..}$(0.17 eV) and $V_o^{\bullet}$(0.33 eV) as dominant defects. From impedance and modulus spectroscopy (IS & MS), the grain boundaries have divided into two types, i.e. the one is tentatively assign to $ZnO/Bi_2O_3(Mn,Co)/ZnO$ (0.47 eV) and the other ZnO/ZnO (0.80~0.89 eV) homojunctions.
홍연우,이영진,김세기,김진호,Hong, Youn-Woo,Lee, Young-Jin,Kim, Sei-Ki,Kim, Jin-Ho 한국재료학회 2012 한국재료학회지 Vol.22 No.12
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.