La_0.5Sr_0.5CrO₃ 세라믹스의 전기전도특성

정우환,Jung, Woo-Hwan 한국재료학회 2016 한국재료학회지 Vol.26 No.1

The electrical transport properties of $La_{0.5}Sr_{0.5}CrO_3$ below room temperatures were investigated by dielectric, dc resistivity, magnetic properties and thermoelectric power. Below $T_c$, $La_{0.5}Sr_{0.5}CrO_3$ contains a dielectric relaxation process in the tangent loss and electric modulus. The $La_{0.5}Sr_{0.5}CrO_3$ involves the transition from high temperature thermal activated conduction process to low temperature one. The transition temperature corresponds well to the Curie point. The relaxation mechanism has been discussed in the frame of electric modulus spectra. The scaling behavior of the modulus suggests that the relaxation mechanism describes the same mechanism at various temperatures. The low temperature conduction and relaxation takes place in the ferromagnetic phase. The ferromagnetic state in $La_{0.5}Sr_{0.5}CrO_3$ indicates that the electron - magnon interaction occurs, and drives the carriers towards localization in tandem with the electron - lattice interaction even at temperature above the Curie temperature.

DyCoO₃ 세라믹스의 교류전도특성

정우환,Jung, Woo-Hwan 한국재료학회 2010 한국재료학회지 Vol.20 No.3

The ac, dc conductivity and dielectric properties of $DyCoO_3$ were reported in the temperature range of 77 - 300K and in the frequency range of 20 Hz - 100 kHz. It was observed that at low temperature, ac conductivity is much higher than dc conductivity and the hopping carrier between localized states near the Fermi level was the dominant loss mechanism. A comparison of the measured ac conductivity $\sigma(\omega)$ was made with some of the models of hopping conductivity of the proposed earlier in the literature. It was observed that in $DyCoO_3$ the measured ac conductivity, over the entire frequency and temperature region, can be explained reasonably well by assuming two contributions $\sigma_1(\omega)$ and $\sigma_2(\omega)$ to the measured $\sigma(\omega)$. The first, $\sigma_1(\omega)$, which dominates at low temperature, may be due to impurity conduction in a small polaron; the second, $\sigma_2(\omega)$, which dominates at higher temperatures, depending on the frequency of measurements, may be due to the hopping of a small polaron and is reasonable for the dielectric relaxation peak.

LaNi_1-xTi_xO₃(x∼0.5) 세라믹스의 전기전도 특성

정우환,Jung, Woo-Hwan 한국재료학회 2009 한국재료학회지 Vol.19 No.4

Thermoelectric power and resistivity are measured for the perovskite $LaNi_{1-x}Ti_xO_3$ ($x{\leq}0.5$) in the temperature range 77 K - 300 K. The measured thermoelectric power of $LaNi_{1-x}Ti_xO_3$ ($x{\leq}0.5$) increases linearly with temperature and is represented by A + BT. The x = 0.1 sample showed metallic behavior, the x = 0.3 showed metal and insulating transition around 150 K, and x = 0.5 showed insulating behavior the over the whole temperature range. The electrical resistivity of x = 0.1 shows linear temperature dependence over the whole temperature range and $T^2$ dependence. On the other hand, the electrical resistivity of x = 0.3 shows a linear relation between $ln{\rho}$ and $T^{-1/4}$ (variable range hopping mechanism) in the range of 77 K to 150 K. For x = 0.5, the temperature dependence of resistivity is characteristic of insulating materials; the resistivity data was fitted to an exponential law, such as ln(${\rho}/T$) and $T^{-1}$, which is usually attributed to a small polaron hopping mechanism. These experimental results are interpreted in terms of the spin polaron (x = 0.1) and variable range hopping (x = 0.3) or small polaron hopping (x = 0.5) of an almost localized $Ni^{3+}$ 3d polaron.

La_0.7Sr_0.3FeO₃ 세라믹스의 교류 전도특성

정우환,Jung, Woo-Hwan 한국세라믹학회 2007 한국세라믹학회지 Vol.44 No.11

We have studied the ac conductivity of insulating $La_{0.7}Sr_{0.3}FeO_3$ in the frequency range 20 Hz-l MHz and in the temperature range 80-300 K. We have analyzed experimental results in the frame works of the quantum-mechanical tunneling mechanism (QMT) and the hopping of barrier mechanism (HOB). We observed that small polaron QMT model is the most suitable mechanism for the low temperature ac conductivity of $La_{0.7}Sr_{0.3}FeO_3$.

Gd_0.33Sr_0.67FeO₃ 세라믹스의 전기전도 특성

정우환,Jung, Woo-Hwan 한국세라믹학회 2006 한국세라믹학회지 Vol.43 No.2

In this study, the dielectric, magnetic and transport properties of $Gd_{0.33}Sr_{0.67}FeO_3$ have been analyzed. The dielectric loss anomaly was found to be around 170 K. The activation energy corresponding to relaxation process of this dielectric anomaly was 0.17 eV. From the temperature dependence of the characteristic frequency, we concluded that the elementary process of the dielectric relaxation peak observed is correlated with polaron hopping between $Fe^{3+}\;and\;Fe^{4+}$ ions. The electrical resistivity displayed thermally activated temperature dependence above 200 K with an activation energy of 0.16 eV. In addition, the temperature dependence of thermoelectric power and resistivity suggests that the charge carrier responsible for conduction is strongly localized.

저온에서 La_2/3+xTiO_3-_δ (x = 0, 0.13)세라믹스의 전자전도특성

정우환,Jung, Woo-Hwan 한국재료학회 2014 한국재료학회지 Vol.24 No.11

The thermoelectric power and dc conductivity of $La_{2/3+x}TiO_{3-{\delta}}$ (x = 0, 0.13) were investigated. The thermoelectric power was negative between 80K and 300K. The measured thermoelectric power of x = 0.13 increased linearly with increased temperatures and was represented by $S_0+BT$. The x = 0 sample exhibited insulating behavior, while the x = 0.13 sample showed metallic behavior. The electric resistivity of x = 0.13 had a linear temperature dependence at high temperatures and a T3/2 dependence below about 100K. On the other hand, the electric resistivity of x = 0 has a linear relation between $ln{\rho}/T$ and 1/T in the range of 200 to 300K, and the activation energy for small polaron hopping was 0.23 eV. The temperature dependence of thermoelectric power and the resistivity of x = 0 suggests that the charge carriers responsible for conduction are strongly localized. This temperature dependence indicates that the charge carrier (x = 0) is an adiabatic small polaron. These experimental results are interpreted in terms of spin (x = 0.13) and small polaron (x = 0) hopping of almost localized Ti 3d electrons.

이중 층 La_1.4(Sr_0.2Ca_1.4)Mn₂O₇ 세라믹스의 저온에서의 Small Polaron Hopping 전도

정우환,Jung, Woo-Hwan 한국재료학회 2008 한국재료학회지 Vol.18 No.1

The dc resistivity and thermoelectric power of bilayered perovskite $La_{1.4}(Sr_{0.2}Ca_{1.4})Mn_2O_7$ were measured as a function of the temperature. In the ferromagnetic phase, ${\rho}(T)$ was accurately predicted by $a_0+a_2T^2+a_{4.5}T^{4.5}$ with and without an applied field. At high temperatures, a significant difference between the activation energy deduced from the electrical resistivity and thermoelectric power, a characteristic of small polarons, was observed. All of the experimental data can be feasibly explained on the basis of the small polaron.

La₂NiO₄₊_δ세라믹스의 유전이완 및 전기전도특성

정우환,Jung, Woo-Hwan 한국재료학회 2011 한국재료학회지 Vol.21 No.7

Thermoelectric power, dc conductivity, and the dielectric relaxation properties of $La_2NiO_{4.03}$ are reported in the temperature range of 77 K - 300 K and in a frequency range of 20 Hz - 1 MHz. Thermoelectric power was positive below 300K. The measured thermoelectric power of $La_2NiO_{4.03}$ decreased linearly with temperature. The dc conductivity showed a temperature variation consistent with the variable range hopping mechanism at low temperatures and the adiabatic polaron hopping mechanism at high temperatures. The low temperature dc conductivity mechanism in $La_2NiO_{4.03}$ was analyzed using Mott's approach. The temperature dependence of thermoelectric power and dc conductivity suggests that the charge carriers responsible for conduction are strongly localized. The relaxation mechanism has been discussed in the frame of the electric modulus and loss spectra. The scaling behavior of the modulus and loss tangent suggests that the relaxation describes the same mechanism at various temperatures. The logarithmic angular frequency dependence of the loss peak is found to obey the Arrhenius law with activation energy of ~ 0.106eV. At low temperature, variable range hopping and large dielectric relaxation behavior for $La_2NiO_{4.03}$ are consistent with the polaronic nature of the charge carriers.

Ca₃Mn₂O₇ 세라믹스의 전기전도 및 자기적 특성

정우환,Jung, Woo-Hwan 한국세라믹학회 2005 한국세라믹학회지 Vol.42 No.9

We present the results of a combined magnetization, dc and magneto-transport study of the n = 2 Ruddlesden-Popper compound $Ca_3Mn_2O_7$ The negative thermoelectric powder is observed. The magnetic measurement data show that there is sharp magnetic transition at 134 K. However, the de and magnetoresistance of $Ca_3Mn_2O_7$ show no particular transport. Transport properties of the compound $Ca_3Mn_2O_7$ are interpreted in terms of activated hopping of small polarons in non-adiabatic regime. Polarons are most probably formed around $Mn^{3+}$ sites created by oxygen sub-stoichiometry.

n=3인 Ruddlesden-Popper형 La_2.1Sr_1.9Mn₃O₁₀ 세라믹스의 Small polaron Hopping 전도 (II)

정우환,이준형,손정호,Jung, Woo-Hwan,Lee, Joon-Hyung,Sohn, Jeong-Ho 한국세라믹학회 2002 한국세라믹학회지 Vol.39 No.9

Mn계 층상 perovskite 세라믹스 $La_{2.1}Sr_{1.9}Mn_3O_{10}$의 전기저항 및 열기전력의 온도의존성을 측정하였다. 실험결과 $La_{2.1}Sr_{1.9}Mn_3O_{10}$의 전기전도는 Emin-Holstein의 단열 small polaron model에 의하여 이루어지고 있었다. Curie 온도이상의 small polaron hopping 영역에서의 열기전력 측정결과는 이론적인 $Mn^{4+}$ 가전자 관점에서 예측되었던 열기전력 측정 결과와 거의 동일하였다. 이 실험결과 역시 $La_{2.1}Sr_{1.9}Mn_3O_{10}$ 세라믹스의 전기전도가 small polaron에 의하여 이루어지고 있음을 의미한다. Electrical resistivity and thermoelectric power measurements on Mn-based $La_{2.1}Sr_{1.9}Mn_3O_{10}$ with layered perovskite structure as functions of temperature are presented. The experimental results demonstrate that the electronic transport in $La_{2.1}Sr_{1.9}Mn_3O_{10}$ is well described by the Emin-Holstein adiabatic small polaron model. The thermoelectric power data in the small polaron regime above Curie temperature is nearly equal to that predicted by nominal $Mn^{4+}$ valence arguments. This indicates that transport involves small polaron hopping.