• 한국재료학회지
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• 제14권6호
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• pp.420-424
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• 2004

The microstructure and electrical properties of $ZnO-Pr_{6}$$O_{11}$ $-CoO-Y_2$$O_3$-based varistors were investigated with and without various metal oxide additives (NiO, MgO, and $Cr_2$$O_3$). The addition of NiO promoted the grain growth while that of Cr$_2$O$_3$ decreased average grain size. Thereby, the varistor voltage was higher in $Cr_2$$O_3$-added composition. Among $ZnO-Pr_{6}$ $O_{11}$ /$-CoO-Y_2$$O_3$-based varistors, the$ Cr_2$$O_3$-added varistor exhibited the highest nonlinear exponent (51.2), the lowest leakage current (1.3 $\mu$A), and the lowest dielectric dissipation factor (0.0433).

• 한국세라믹학회지
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• 제43권6호
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• pp.383-388
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• 2006

The varistor properties and DC accelerated aging characteristics of $ZnO-Pr_{6}O_{11}-CoO-Cr_{2}O_{3}$-based varistors were investigated at different $La_{2}O_3$ contents in the range of $0{\sim}2.0mol%$. The varistors doped with 0.5 mol% $La_{2}O_3$ exhibited good nonlinearity, with 81.6 in nonlinear coefficient. Increasing the $La_{2}O_3$ content further to 2.0 mol% caused the sintered density to increase, and the breakdown voltage and nonlinearity to decrease abruptly. The varistors with 0.5 mol% $La_{2}O_3$ exhibited the high electrical stability, with -1.14% in variation rate of breakdown voltage, -3.7% in variation rate of nonlinear coefficient, and +100% in variation rate of leakage current for specified DC accelerated aging stress condition (95% of breakdown voltage/$150^{\circ}C$/24 h).

• 대한전기학회:학술대회논문집
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• 대한전기학회 2004년도 하계학술대회 논문집 C
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• pp.1631-1633
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• 2004

The surge current characteristics of ZnO-$Pr_6O_{11}$-CoO-$Cr_2O_3-Dy_2O_3$-based varistors were investigated with various $Dy_2O_3$ contents. The sintered density decreased in the range of $5.2{\sim}4.6g/cm^3$ with increasing $Dy_2O_3$ content. The incorporation of $Dy_2O_3$ markedly enhanced the nonlinear properties of varistors above 10 times in nonlinear exponent, compared with the varistor without $Dy_2O_3$. The varistor ceramics doped with 0.5 mol% $Dy_2O_3$ exhibited the highest electrical stability. However, the remainder varistors resulted in thermal runaway due to low density of varistor ceramics. The clamping voltage ratio exhibited a minimum value of 2.03 in 1.0 mol% $Dy_2O_3$ at surge current of 100 A(8/20 ${\mu}s$).

• Transactions on Electrical and Electronic Materials
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• 제13권5호
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• pp.262-265
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• 2012

The Microstructure and nonlinear electrical properties of the ZPCCYT (ZnO-$Pr_6O_{11}$-CoO-$Cr_2O_3$-$Y_2O_3$-$Tb_4O_7$) varistors were investigated for different amounts of $Tb_4O_7$. The addition of $Tb_4O_7$ has a significant effect on microstructure and electrical properties. Analysis of the microstructure indicated that the ceramics consisted of ZnO grain as a main phase and a few secondary phases as a mix of $Pr_6O_{11}$, $Y_2O_3$, and $Tb_4O_7$. As the amount of $Tb_4O_7$ increased, the sintered densities of pellets increased from $\rho$ = 5.70 to $5.78g/cm^3$ and the average grain size decreased from d = 4.8 to $3.6{\mu}m$. The increase in the amount of $Tb_4O_7$ increased from $E_B$ = 7,473 to 10,035 V/cm and from ${\alpha}$ = 39.7 to 52.2. In particular, it was found that the ceramics modified with 1.0 mol% in the amount of $Tb_4O_7$ are suited for the varistors for high voltage in the light of a high sintered density and a high voltage gradient.

• 한국전기전자재료학회논문지
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• 제20권6호
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• pp.508-513
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• 2007

The electrical properties and stability of $ZnO-Pr_6O_{11}-CoO-Cr_2O_3-based$ varistors were investigated for different $Tb_4O_{11}$ amounts in the range of $0{\sim}1.0\;mol%$. As the $Tb_4O_{11}$ amount increased, the sintered density increased in the range of $99.1{\sim}101.1%$ of theoretical density and the average grain size decreased in the range of $7.7{\sim}4.8{\mu}m$. The varistor voltage increased in the range of $280.9{\sim}715.8V/mm$ and the nonlinear coefficient increased in the range of $26.4{\sim}44.4$ with the increased of $Tb_4O_{11}$ amount. The 0.25 mol% $Tb_4O_{11}$-doped varistors exhibited the high electrical stability, with -0.1% in variation rate of varistor voltage, -0.7% in variation rate of nonlinear coefficient, and +17.4% in variation rate of leakage current for specified dc accelerated aging stress of $0.95V_{1mA}/150^{\circ}C/24h$.

• Transactions on Electrical and Electronic Materials
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• 제7권3호
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• pp.123-128
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• 2006

The microstructure and electrical properties of the varistors, which are composed of $ZnO-Pr_6O_{11}-CoO-Cr_2O_3-La_2O_3-based$ ceramics, were investigated for different $La_2O_3$ contents. The increase of $La_2O_3$ content led to more densified ceramics, whereas abruptly decreased the nonlinear properties by incorporating beyond 1.0 mol%. The highest nonlinearity was obtained from 0.5 mol% $La_2O_3$, with the nonlinear coefficient of 81.6 and the leakage current of $0.2{\mu}A$. The increase of sintering time and temperature greatly decreased the nonlinear properties. As the $La_2O_3$ content increased, the donor density increased from $0.64\times10^{18}/cm^3\;to\;16.89\times10^{18}/cm^3$ and the barrier height greatly decreased with increasing $La_2O_3$ content, reaching a maximum (1.47 eV) in 0.5 mol% $La_2O_3$.

• 대한전기학회:학술대회논문집
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• 대한전기학회 1999년도 추계학술대회 논문집 학회본부 C
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• pp.1003-1007
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• 1999

This paper is reported for the stability of ZnO-$Pr_{6}O_{11}-CoO-Er_{2}O_{3}$ based ceramic varistors with $Er_{2}O_{3}$ added in the addition range 0.0 to 2.0 mol%. The varistors sintered at $130^{\circ}C$ exhibited abrupt positive current creep phenomena, which accompany thermal run away within short times, even under weak d.c. stress. As a result, these varistors were completely degraded. On the contrary, the stability of varistors sintered at $1350^{\circ}C$ was far better than that of $1300^{\circ}C$. In particular, the varistor containing 0.5 mol% $Er_{2}O_{3}$ showed a excellent stability, which the variation rate of the varistor voltage, the nonlinear coefficient, and leakage current is below 1%, 2%, and 3.5%, respectively, even under more severe d.c. stress, such as ($0.8V_{1mA}/90^{\circ}C/12h$) + ($0.85V_{1mA}/115^{\circ}C/12h$) + ($0.9V_{1mA}/120^{\circ}C/12h$) + ($0.9V_{1mA}/150^{\circ}C/12h$). Consequently, it is estimated that the basic composition of ZnO-$Pr_{6}O_{11}-CoO-Er_{2}O_{3}$ based varistor contain 0.5 mol% $Er_{2}O_{3}$ will be used to the fabrication of the varistors for high performance and stability in a forthcoming.

• 대한전기학회:학술대회논문집
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• 대한전기학회 2000년도 하계학술대회 논문집 C
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• pp.1673-1675
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• 2000

The nonlinearity and degradation characteristics of $Pr_{6}O_{11}$-based ZnO varistors doped with, $Er_{2}O_3$ were investigated. The varistors were sintered at 1335$^{\circ}C$ in the addition range of 0.0 to 2.0 mol% $Er_{2}O_3$, respectively The varistor doped with $Er_{2}O_3$ exhibited more higher nonlinearity than that without $Er_{2}O_3$. Most of the varistors containing 0.5 mol% $Er_{2}O_3$ showed nonlinear exponent more than 70 and a excellent stability, which the variation rate of the varistor voltage and the nonlinear exponent is -0.85% and -1.43%. respectively, even under 3rd d.c stress, such as (0.80 $V_{1mA}/90^{\circ}C$/12h)+(0.85$V_{1mA}/115^{\circ}C$/12h)+(0.90$V_{1mA}/120^{\circ}C$/12h). Consequently, since $Pr_{6}O_{11}$-based 2nO varistors doped with 0.5 mol% $Er_{2}O_3$ have an excellent stability as well as good nonlinearity, it is expected to be usefully used to develop the advanced varistors in future.

• 한국전기전자재료학회논문지
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• 제18권6호
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• pp.543-549
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• 2005

The electrical and dielectric properties of $ZnO-Pr_6O_{11}-CoO-Cr_2O_3-based$ varistors were investigated in the sintering time range of $1\~3$ h. Increasing sintering time improved the densification of ceramics, whereas deteriorated the nonlinearity, in which the nonlinear exponent greatly decreased from 55.3 to 38.2 and the leakage current increased from 0.1 to $1.1\;{\mu}A$. The donor concentration and density of interface states increased in the range of $(1.06\~1.60)\times10^8/cm^3\;and\;(3.15\~3.67)\times10^{12}/cm^2$ with increasing sintering time, respectively. while, in dielectric properties, the increase of sintering time increased in the range of $0.0228\~0.0505$ in tans and $1.32\~1.36\;{\mu}s$ in relaxation time.

• 한국전기전자재료학회논문지
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• 제11권11호
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• pp.969-974
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• 1998

The effects of $La_2O_3$on the microstructure and electrical characteristics of Pr-based ZnO varistors were investigated. The average grain size increased in the range of 21.9~56.3$\mu$ m with increasing $La_2O_3$additive content(0.0~2.0 mol%). La was, of course grain boundary, largely segregated at the nodal point. As $La_2O_3$additive content increases, threshold voltage and nonlinear coefficient decreased and leakage current increased. In particular, 2.0 mol% $La_2O_3$-added varistor exhibited low threshold voltage 17.0V/mm and nonlinear coefficient of about 6. Based on these results, this varistor can be said to be used as low-voltage varistor, if nonlinear coefficient is somewhat improved forward.