입계형 바리스터의 유전특성과 등가회로

Abstract

본 연구에서는 ZnO계 바리스터에 대하여 미세구조(SEM 촬영)와 X선 회절실험 결과로부터 입계세라믹스형임을 확인하였다. 연구의 주요 내용으로서 SrTiO3계 및 ZnO계와 같은 입계형 바리스터에 대하여 이중쇼트키장벽 모델을 기초로 한 거시적 등가회로를 제안하고 그 특성을 분석하였다. 이 등가회로는 입계형 바리스터에 있어서 유전층, 반도전층 및 공핍층을 고려한 것이며 핀형 리드부 바리스터인 경우는 핀리드의 인덕턴스 및 표유 정전용량도 고려한 것이다. 그리고 시판중의 SrTiO3계 및 ZnO계 바리스터에 대해 정전용량 및 유전손실계수()를 측정하고 제안된 등가회로를 적용하여 바리스터 물질의 특성을 나타내는 회로정수를 커브피팅법으로 구하였다. 또한 전압-전류(V-I) 특성을 측정하여 그 특성분석에 누설전류를 고려한 식을 사용함으로써 실제의 데이터와 잘 일치함을 보였고 관련 특성 파라미터를 분석하였다.

In this study the SEM photographs and the X-ray diffraction data for ZnO-based varistors were analyzed. As the important theme in this study, a macroscopic equivalent circuit based on Double Schottky Barrier(DSB) model is proposed for SrTiO3- or ZnO-based varistor and the characteristics of the varistor are analyzed. In the equivalent circuit dielectric layer, semiconducting layer and depletion layer of the grain-boundary type varistor are considered as elements of the circuit and also in case of the varistor with pin-leads the inductance of pin-leads and the stray capacitance are considered together. This equivalent circuit were adapted to the experimental data of capacitance and ？炚뽄힝？ for SrTiO3- and ZnO-based commercial varistors and the circuit-constants were determined by the curve-fitting method. In V-I characteristics of varistors the empirical equation considering ohmic leakage current and non-ohmic current together was well consistent with experimental V-I data and the related parameters were analyzed. Results of above studies are follows : [1] Dielectric properties (1) SrTiO_(3)-based varistors ① Among 4 types of equivalent circuits for simulation the circuit consisting of series elements considered dielectric layer(parallel connection of capacitance and conductance of dielectric loss), semiconducting layer(parallel connection of capacitance and low resistance) and depletion layer(parallel connection of capacitance and high resistance) was well consistent with experimental data. ② According to parameters obtained by above simulation, the higher the varistor voltage is, capacitances of the dielectric layer, the semiconducting layer and the depletion layer are smaller, while parallel resistances of the semiconducting layer and the depletion layer are larger. This means that thickness of the dielectric layer, the semiconduction layer and the depletion layer are larger as the varistor voltage is higher. ③ Using parameters obtained as curve fitting results, dielectric properties of the varistor could be reproduced in the more broad frequency region. As the results two relaxations attributed to deep donors and shallow donors could be found. The relaxation by deep donors was shown at low frequency region(0.2 ∼ 14.4 [Hz]) and the relaxation by shallow donors was shown at high frequency(1×10^(6) ∼ 8.3×10^(6) [Hz]). These facts confirm that the equivalent circuit proposed in this study is well adapted to the Double Schottky Barrier model. ④ In tanζ-frequency spectra the relaxation peak by shallow donors is larger than the relaxation peak by deep donors. This means that since the varistor voltage of SrTiO_(3)-based varistors is relatively low properties of the varistor are dominated by the effect of shallow donors. ⑤ Relaxation phenomena by shallow and deep donors could be found in resistance vs frequency plots too. (2) ZnO-based varistor ① From the SEM photographs of ZnO-based varistors ZnO grains, pyrochlore phase and spinel phase located in the grain boundaries could be found and also these phases could be confirmed by the X-ray diffraction data. The grain size of each varistor sample was determined by the linear-intercept method on the photographs and as the result it could be found that the higher the varistor voltage is, the grain size is smaller. ② It could be found that the equivalent circuit considered the inductance of pin-leads to above mentioned equivalent circuit is well adapted to the experimental data for ZnO-based varistor with pin leads and various parameters of the circuit were determined by the curve fitting method. ③ Using parameters except the inductance dielectric spectra of ZnO-based varistors were reproduced in broad frequency region and the relaxations by deep donors and shallow donors could be found. ④ In tanζ-frequency spectra the relaxation peak of low frequency region(3.6×10^(-4) ∼ 2.1×10^(-3) [Hz]) by deep donors is larger than the relaxation peak of high frequency region(1×10^(5) ∼ 7×10^(5) [Hz]) by shallow donors. These facts means that properties of varistor are dominated by the effect of deep donors and this is consistent with the report of other reference[64]. ⑤ The relaxation frequency by deep donors is inclined to decrease as the varistor voltage increase. This means that response of deep donors to alternating signals is more difficult as to as the higher varistor voltage. ⑥ In resistance-frequency plots relaxation phenomena by deep and shallow donors could be found. [2] Voltage-Current Characteristics ① The empirical equation added the leakage current of prebreakdown region to the traditional V-I equation were well fitted with experimental V-I data of varistors. ② In the empirical equation the reciprocal of K_(1) meaning leakage resistance is very high value and is nearly consistent with the resistance of extremely low frequency in the resistance-frequency plots. K_(2) meaning the conductance for nonlinear voltage(V^(α) ) shows smaller value as to as the higher varistor voltage. This means that the insulation for nonlinear voltage in prebreakdown region is better as to as the higher varistor voltage. ③ The nonlinear coefficient α of ZnO-based varistors showed much more large value than that of SrTiO3-based varistors.