e investigated the effects of post-annealing in vacuum, nitrogen, and hydrogen atmospheres on the structural, electrical, and optical properties of 600 nm thick Al-doped ZnO (ZnO:Al) thin films deposited by RF magnetron sputtering at room temperature. Post-annealing in hydrogen atmosphere at 400℃ for 1 hour showed the most significant improvement in electrical properties. Resistivity decreased from 9.11×10⁻³ to 1.4×10⁻³ Ω·cm, electron mobility increased from 4.11 to 18.23 cm²/V·s, and electron carrier concentration increased from 1.63×10²⁰ to 4.85×10²⁰ cm⁻³. In contrast, post-annealing in vacuum and nitrogen atmospheres resulted in degraded electrical properties due to oxygen and nitrogen chemisorption at grain boundaries. The enhancement in hydrogen-annealed films was attributed to the formation of additional oxygen vacancies and desorption of adsorbed oxygen species from grain boundaries. All films maintained excellent optical transparency of 80-90% in the visible range. The optical bandgap exhibited a blue-shift from 3.365 eV to 3.624 eV due to the Burstein-Moss effect induced by the increased electron carrier concentration. These results confirmed that hydrogen atmosphere post-annealing is the most effective method for enhancing the electrical conductivity of ZnO:Al thin films while maintaining high optical transparency.

In this study, the electrical properties of zinc oxide (ZnO) thin-film transistors (TFTs) based on oxide semiconductors were analyzed. As interest in next-generation transparent and flexible displays grows, ZnO, which offers high field-effect mobility and transparency, has emerged as a promising material to overcome the limitations of amorphous silicon (a-Si)-based TFTs. ZnO has a wide bandgap and optical transparency and can be deposited on various substrates at low temperatures, making it a suitable channel material for future display devices. In this study, ZnO TFTs were fabricated with an inverted staggered structure using a p⁺⁺ Si wafer coated with SiO₂ as the substrate. The ZnO channel layer was deposited by RF magnetron sputtering, and the ITO source/drain electrodes were formed using an e-beam evaporator. The electrical characteristics was evaluated using Keithley 4200A-SCS parameter analyzer. Mobility, On/Off ratio, and subthreshold swing (SS) were calculated from the measurements.

Mechanoluminescence (ML) is a phenomenon where the application of mechanical force to ML materials generates an electric field and produces light, holding significant promise as an eco-friendly technology. However, challenges in commercializing ML technology has arisen due to its low brightness and short luminous lifetime. To address this, in this work, we enhance ML efficiency by mixing carbon nanotubes (CNTs) into a ZnS: Cu embedded in a polydimethylsiloxane composite ML device. The inclusion of CNTs boosts ML intensity by 98% compared to devices without CNTs, as the increasing CNT fraction elevates conductivity, thereby amplifying ML intensity. However, this increase in CNT fraction also leads to enhanced light absorption within the device. Consequently, we observe a trend where ML intensity rises initially but declines beyond a CNT fraction of 0.0015 wt%. Based on these findings, we anticipate that our research will make valuable contributions to the advancement of electrical powerless mechanoluminescent technology.

Recently, as power and electronic devices have increased in frequency and capacity, it has become a major concern to protect electronic circuits and electronic components used in these devices from abnormal voltages such as various surges and pulse noise. To respond to variously rated voltages applied to power electronic devices, the rated voltages of various varistors can be obtained by controlling the size of internal particles of the varistor or controlling the number of layers of the varistor. During bonding, the problem of unbalanced thermal runaway occurring between the electrode and the varistor interface causes degradation of the varistor and shortens its life of the varistor. In this study, to solve the problem of unbalanced heat distribution of stacked varistors to adjust the operating voltage, the contents of the ZnO-based varistor composition were 96 wt% ZnO, 1 mol% Sb₂O₃, 1 mol% Bi₂O₃, 0.5 mol% CoO, 0.5 mol% MnO, and 1 mol% TiO₂. A multi-layered ZnO varistor was modeled by bonding a single varistor with a composition in three layers according to the operating voltage. The thermal distribution of the triple-layered ZnO varistor was analyzed for the thermal runaway phenomenon that occurred during varistor operation using the finite element method according to Comsol 5.2.

Novel self-illuminated smart windows were fabricated consisting of Cu-doped ZnS (ZnS:Cu) powder and polymer-dispersed liquid crystal (PDLC). This smart window shows not only switchable transparency but also self-illumination without any attachable luminous body. Its electro-optical characteristics, transmittance, and luminance were investigated in relation to various applied voltages and composition ratios. The optical transmittance and luminous intensity increased with increasing applied voltages. However, the optical transmittance decreased with increasing ZnS:Cu powder content. One of the self-illuminated smart windows, which was fabricated with 9 wt% of ZnS:Cu, achieved the optical transmittance of 60.5% (at 550 nm) and the luminance of 11.0 cd/m² at 100 V. This smart window could be used as a normal switchable smart window in daytime and light-emitting signage at night.

Ultrasonic sensor is suitable as a next-generation autonomous driving assist device because its lower price compared to that of other sensors and its sensing stability in the external environment. Although Pb(Zr, Ti)O₃ (PZT)-relaxor ferroelectric system has excellent piezoelectric properties, the change in capacitance is large in the daily operating temperature range due to the low phase transition temperature. Recently, many studies have been conducted to improve the temperature stability of ferroelectric ceramics by controlling the grain size and crystal structure, so it is necessary to study the effect of the grain size on the piezoelectric properties and the temperature stability of PZT-relaxor ferroelectric system. In this study, the piezoelectric properties, phase transition temperature, and temperature coefficient of capacitance (TCC) of 0.9 Pb(Zr_1-xTi_x)O₃-0.1 Pb(Zn_1/3Nb_2/3)O₃ (PZT_x-PZN) ceramics with various grain sizes were investigated. PZT_x-PZN ceramics with larger grain size showed higher piezoelectric properties and temperature stability, and are expected to be suitable for ultrasonic devices in the future.

Since the ZnO varistor is a semiconductor device, the internal thermal distribution during the varistor operation is recognized as an important factor in the performance and deterioration of the varistor. For an optimal varistor structure design, the thermal runaway phenomenon during the varistor operation was interpreted using the Comsol 5.2 analysis program by a finite element analysis. The maximum temperature of the center measured in the cross section of the ZnO varistor was confirmed to increase as the temperature moved from the lower electrode to the center towards the upper electrode up to 572.6 K. The electrodes are thinned so that the influence of the Schottky barrier is not great. The heat gradient balance is determined to be improved when the electrode of the hybrid form is introduced. The thickness, density, pore distribution, impurity uniformity, and particle size of the ZnO varistor are required, and it is determined that the pyrolysis gradient will be improved regardless of the electrode thickness. When these results are applied to design the ZnO varistor, the optimal structure of the ZnO varistor can be obtained.

In this work, we synthesized alloy-core InZnP quantum dots, which are more efficient than single-core InP quantum dots, using a solution process method. The effect of synthesis conditions of alloy core on optical properties was investigated. We also investigated the conditions that make up the gradient shell to minimize defects caused by lattice mismatch between the InZnP core and ZnS is 7.7%. The stable synthesis temperature of the InZnP alloy core was 200℃. Quantum dots consisting of three layered ZnSe gradient shell and single layered ZnS exhibited the best optical property. The properties of quantum dots synthesized in 100 ml and in 2,000 ml flasks were almost equal.

Indium-doped Mg_0.05Zn_0.95O thin films were deposited on glass substrates by a sol-gel method. Three types of indium precursors such as indium chloride, indium acetate, and indium nitrate were used as doping sources. Physical properties of fabricated thin films were analyzed through XRD (x-ray diffraction), UV-vis spectrophotometer, Hall effect measurement, and EDS (energy dispersive x-ray spectroscopy). All In-doped thin films grown in this study exhibited a preferred orientation of (002) with over 80% transmittance. The results showed that the Mg_0.05Zn_0.95O thin film from indium chloride as the indium precursor has higher crystallinity and transmittance with lower resistivity when compared with those from other indium precursors.

In the case of ZnO:Al thin films, it is the best material that can replace ITO that is mainly used as a transparent electrode in electronic devices such as solar cells and flat-panel displays. In this study, ZnO:Al films were fabricated by using the RF dual magnetron sputtering method at various substrate temperatures. As the substrate temperature increased, the crystallinity of the ZnO:Al thin films was improved, and the electrical conductivity and electrical properties of the thin film improved owing to the increase in grain size. In addition, the surface roughness of the ZnO:Al thin films increased due to changes in the surface and density of the thin films. Moreover, the substrate temperature increased the density of thin films and improved their transmittance. To be applied to solar cells and other several electronic devices in the future, the hardness and adhesion properties of the thin film improve as the substrate temperature increases.

In this study, ZnO:Ga thin films were fabricated on a glass substrate using various Ar flows by an RF magnetron sputter system at room temperature. The dependencies of Ar flow on different properties were investigated. An appropriate control over the Ar flow led to the formation of a high-quality thin film. The ZnO:Ga films were formed as a hexagonal wurtzite structure with high (002) preferential orientation. The films exhibited a typical columnar microstructure and a smooth top face. The average transmittance was 85~89% within the visible area. By decreasing the Ar flow, the sheet resistance was decreased due to an increase in the grain size and a decrease in the root mean square roughness. The lowest sheet resistance of 86 Ω/□ was obtained at room temperature for the 40 sccm Ar flow.

In this study, Mg_xZn_1-xO thin films, which can be applied not only to active layers of light-emitting devices (LEDs), such as UV-LEDs, but also to solar cells, high mobility field-effect transistors, and power semiconductor devices, are fabricated using the sol-gel method. ZnO and Mg_0.3Zn_0.7O solution synthesized by the sol-gel method and the thin film were grown by spin coating on a Si (100) substrate and sapphire substrate. The solutions are synthesized by dissolving precursor materials in 2-methoxyethanol (2-ME) solvent, and then monoethanolamine (MEA) was added to the mixed solution as a sol stabilizer. Zinc acetate dihydrate is used as a ZnO precursor, while Mg nitrate hexahydrate and Mg acetate tetrahydrate are used as an MgO precursor. Then, the optical and structural characteristics of the fabricated thin films are compared. The molar concentration of the Zn precursor in the solvent is fixed at 0.3 M, and the amount of the Mg precursor is 30% of Mg²⁺/Zn²⁺. The optical characteristics are measured using an UV-vis spectrophotometer, and the transmittance of each wavelength is measured. Structural characteristics are measured using X-ray diffraction (XRD) and transmission electron microscopy (TEM). Composition analyses are performed using energy dispersive X-ray spectroscopy (EDS). The Mg_0.3Zn_0.7O thin film was well formed at the ratio of the Mg precursor added regardless of the type of Mg precursor, and the c-axis of the thin film was decreased, while the band gap was increased to 3.56 eV.

In this study, we investigated the crystal defects and grain boundary properties in a ZZCCC (ZnO-Zn₂BiVO₆-Co₃O₄-Cr₂O₃-CaCO₃) varistor, with the liquid-phase sintering aid Zn₂BiVO₆ developed by our laboratory. The ZZCCC varistor sintered at 1,200℃ exhibited excellent nonlinear current-voltage characteristics (α=63), with oxygen vacancy (V₀^·; 0.35 eV) as a main defect, and an apparent activation energy of 1.1 eV with an electrically single grain boundary. Therefore, among the various additives to improve the electrical properties of ZnO varistors, if Zn₂BiVO₆ is used as a liquid phase sintering aid, it will be ideal to use Co for the oxygen vacancy and Ca for the electrically single grain boundary. This will allow the good properties of ZnO varistors to be maintained up to high sintering temperatures.

This work presents the response characteristics of a ZnO varistor to conductive EMP. An E1 pulse, standardized to MIL-STD-188-125-1, was applied to the varistors wherein the residual current and response times were measured with the applied E1 pulse current. Additionally, the response time was measured according to the length of the connection path. Consequently, the amplitude of the residual voltage through the ZnO varistors was increased with increasing amplitude of the applied E1 pulse current. As the length of the connection path increased, the operating response time and residual peak voltage also increased. These results indicate that the response characteristics of ZnO varistors can be applied to basic data to support the use of varistors as a protective measure against conductive EMP.

In this study, photoluminescence (PL) analysis was performed to evaluate the optical properties of commercial ZnO substrates. Particular attention was paid to the bound exciton (BX) luminescence, which is usually the strongest emission intensity of commercial substrates. At 15 K, PL analysis revealed that the BX peak due to donor-type impurities (donor-bound-exciton; DX) dominated, while two-electron satellite (TES) emission, donor-accepter pair (DAP) emission, and LO-phonon replica emission were also observed. The impurity concentration of the ZnO substrate was determined to be 10¹⁵ to 10¹⁶/cm³ by examination of the temperature variation of DAP, while the half width and intensity change of the luminescence revealed that the temperature change of BX can be interpreted almost the same as the analysis of free-exciton emission.

There is a need for the development of transparent conductive materials that are economical and environmentally friendly with exhibit low resistivity and high transmittance in the visible spectrum. In this study, the deposition rate and uniformity of Al-doped ZnO-thin films were improved by changing the Z-motion of the sputtering system. The deposition rate and the uniformity were determined to be 3.44 nm/min and 1.23%, respectively, under the 10 mm Z-motion condition. During O2 plasma treatment, the intrusion-type metal elements in the thin film were reduced, which contributed to an oxygen vacancy reduction in addition to structural stabilization. Moreover, the sheet resistance was more easily saturated.

Epitaxial ZnO nanowires (NWs) were synthesized on sapphire (001) substrates using a hydrothermal process. The effects of the pH value of the precursor solution on the structural and optical properties of the resulting NWs was studied. The epitaxial relationship and the domain matching configuration between the sapphire (001) substrate and the as-grown ZnO NWs were determined using synchrotron X-ray diffraction measurements. The (002) plane of wurtzite ZnO NW grows in the surface normal direction parallel to the sapphire (001) direction. However, three types of in-plane domain matching configurations were observed, such as the on-position, 30°-rotated position, and ±8.5°-rotated position relative to the on-position, which might be attributed to inheriting the in-plane domain configuration of the ZnO seed layer.

In particular, gas sensors require characteristics such as high speed, sensitivity, and selectivity. In this study, we fabricated a NO_X gas sensor by using a multi-walled carbon nanotube (MWCNT)/zinc oxide (ZnO) composite film. The fabricated MWCNT/ZnO gas sensor was then treated by a 450℃ temperature process to increase its detection sensitivity for NOx gas. We compared the detection characteristics of a ZnO film gas sensor, MWCNT film gas sensor, and the MWCNT/ZnO composited film gas sensor with and without the heat-treatment process. The fabricated gas sensors were used to detect NO_X gas at different concentrations. The gas sensor absorbed NO_X gas molecules, exhibiting increased sensitivity. The sensitivity of the gas sensor was increased by increasing the gas concentration. Additionally, while changing the temperature inside the chamber for the MWCNT/ZnO composite film gas sensor, we obtained its sensitivity for detecting NO_X gas. Compared with ZnO, the MWCNT film gas sensor is excellent for detecting NO_X gas. From the experimental results, we confirmed the enhanced gas sensor sensing mechanism. The increased effect by electronic interaction between the MWCNT and ZnO films contributes to the improved sensor performance.

In this paper, the ZnS nanoparticles were synthesized according to the process conditions of hydrothermal synthesis. When the molar ratio of Zn to S was 1:1.2, it was confirmed that it had a cubic single phase and a high crystal phase. After the molar ratio is fixed, hydrothermal synthesis was conducted at 180℃ for 24, 36, 72 and 96 h in order to confirm the structural change with the change of hydrothermal synthesis times. As the hydrothermal synthesis times increased, the particle size increased. The hydrothermal synthesized particle size for 72 h was considered to be suitable for sintering. The ZnS ceramic had a density of 99.7% and an excellent transmittance of ~70% in the long-wavelength region.

The rod-shaped Ni_0.5Zn_0.5Fe₂O₄ particles were synthesized via a topotactic reaction, in which goethite (α-FeOOH) particles are the main constituents. The phases, microstructures and magnetic properties of these particles were studied using XRD, FE-SEM and VSM. The precursor solution consisted of NiSO₄·xH₂O, ZnSO₄·xH₂O, goethite and D.I. water werereacted at four different temperatures (50, 70, 90, 100℃) to generate four differently precipitated particles respectively. During the co-precipitation reaction, the pH of the solution was maintained at 8.0 using NaOH. The particles coprecipitated and calcined at a temperature of 700℃, exhibited a rod-shape similar to its original goethite, which means that the shape of Ni-Zn ferrite particles can be topotactically controlled by the goethite. The particles synthesized at 70 and 90℃ have a saturation magnetization of 29 and 35 emu/g respectively; representing better values than the ones synthesized at the 50 and 100℃, in which some second phases such as Fe₂O₃ were observed.

Liquid phases in ZnO varistors cause more complex phase development and microstructure, which makes the control of electrical properties and reliability more difficult. Therefore, we have investigated 2 mol% CaCO₃ doped ZnO-Co₃O₄-Cr₂O₃-La₂O₃ (ZCCLCa) bulk ceramics as one of the compositions without liquid phase sintering additive. The results were as follows: when CaCO₃ is added to ZCCLCa (644 Ωcm) acting as a simple ohmic resistor, CaO does not form a secondary phase with ZnO but is mostly distributed in the grain boundary and has excellent varistor characteristics (high nonlinear coefficient α＝78, low leakage current of 0.06 μA/㎠, and high insulation resistance of 1×10¹¹ Ωcm). The main defects Zn_i^·· (AS: 0.16 eV, IS & MS: 0.20 eV) and V˙_o (AS: 0.29 eV, IS & MS: 0.37 eV) were found, and the grain boundaries had 1.1 eV with electrically single grain boundary. The resistance of each defect and grain boundary decreases exponentially with increasing the measurement temperature. However, the capacitance (0.2 nF) of the grain boundary was ~1/10 lower than that of the two defects (~3.8 nF, ~2.2 nF) and showed a tendency to decrease as the measurement temperature increased. Therefore, ZCCLCa varistors have high sintering temperature of 1,200℃ due to lack of liquid phase additives, but excellent varistor characteristics are exhibited, which means ZCCLCa is a good candidate for realizing chip type or disc type commercial varistor products with excellent performance.

This study introduces a new investigation report on the microstructural and electrical property changes of ZnO-Zn₂BiVO₆-Mn₃O₄ (ZZMn), where 0.33 mol% of Mn₃O₄ and 0.5 mol% of Zn₂BiVO₆ were added to ZnO (99.17 mol%) as liquid phase sintering aids. Zn₂BiVO₆ contributes to the decrease of sintering temperatures by up to 800℃, and segregates its particles at the grain boundary, while Mn₃O₄ enhances α, the nonlinear coefficient, of varistor properties up to α＝62. In comparison, when the sintering temperature is increased from 800℃ to 1,000℃, the resistivity of ZnO grains decreases from 0.34 Ωcm to 0.16 Ωcm, and the varistor property degrades. Oxygen vacancy (V_o^·) (P1, 0.33~0.36 eV) is formed as a dominant defect. Two different kinds of grain boundary activation energies of P2 (0.51~0.70 eV) and P3 (0.70~0.93 eV) are formed according to different sintering temperatures, which are tentatively attributed to be ZnO/Zn₂BiVO₆-rich interface and ZnO/ZnO interface, respectively. Accordingly, this study introduces a progressive method of manufacturing ZnO chip varistors by way of sintering ZZMn-based varistor under 900℃. However, to procure a higher reliability, an in-depth study on the multi-component varistors with double-layer grain boundaries should be executed.

The piezoelectric properties of 0.65Pb(Zr_1-xTi_x)O₃-0.35Pb(Zn_1/6Ni_1/6Nb_2/3)O₃ (PZT_x-PZNN) ceramics with 0.530≤ x≤0.555 were investigated for application to piezoelectric energy harvesters. Although a morphotropic phase boundary (MPB) was found at approximately x＝0.545, the ceramic with the highest figure of merit (FOM) (d₃₃×g₃₃) was observed at a composition of x＝0.540. Values of this figure of merit, d₃₃×g₃₃, of 19.6 pm²/N and 20.2 pm²/N were obtained from PZT_0.540-PZNN ceramics sintered at 920℃ and 950℃, respectively. A high output power of 937 μW and a high power density of 3.3 mW/cm³ were obtained from unimorph-type piezoelectric energy harvesters fabricated using PZT_0.540-PZNN ceramic sintered at 920℃ for 4h.

In this work, in order to effectively improve the electrical conductivity and visible light transmittance of ZnO thin films, ZnO single layer and ZnO/Ag bi-layer films were deposited on glass substrates by radio frequency and direct current magnetron sputtering, and then, the effects of an Ag buffer layer and electron beam irradiation on the electrical and optical properties of the films were investigated. The observed results indicate that ZnO 100 nm / Ag 7 nm films show higher opto-electrical performance than the ZnO single layer film. In addition, electron beam irradiation also effectively enhanced the visible transmittance and electrical conductivity of the ZnO/Ag bi-layer films.

We prepared ZnS thin films via chemical bath deposition (CBD) in an aqueous solution of ammonia (NH₃) and hydrazine (N₂H₄). The composition ratio of hydrazine used was 0%, 17%, 22%, 29%, or 50%. We investigated the effects of hydrazine and ammonia on the growth, and the structural and optical properties of ZnS in terms of surface uniformity, voids, and grain size. We found that during the growth of ZnS films, hydrazine was very effective for improving the surface morphology and layer uniformity with fast layer formation, while it had no effect on the bandgap energy, Eg.

In this study, we fabricated an NOX gas sensor using a composite film of multi-walled carbon nanotubes (MWCNT)/zinc oxide (ZnO). Carbon nanotubes (CNTs) show good electronic conductivity and chemical-stability, and zinc oxide (ZnO) is a wide band gap semiconductor with a large exciton binding energy. Gas sensors require characteristics such as high speed, sensitivity, and selectivity. The fabricated gas sensor was used to detect NOX gas at different NOX concentrations. The sensitivity of the gas sensor increased with increasing gas concentrations. Additionally, while changing the temperature inside the chamber containing the MWCNT/ZnO gas sensor, we obtained the sensitivity and normalized responses for detecting NOX gas in comparison to ZnO and MWCNT film gas sensors. From the experimental results, we confirmed that the gas sensor sensing mechanism was enhanced in the composite-film gas-sensor and that the electronic interaction between MWCNT and ZnO contributed to the improved sensor performance.

In a Pb-included piezoelectric composition, Sr_yPb_1-y[(Zn_1/3Nb_2/3)_x-(Ni_1/3Nb_2/3)_0.2-(Zr_0.46Ti_0.54)_0.8-x]O₃ was selected in order to attain high piezoelectric properties. According to the PZN ratio (x) and the amount of Sr doping (y), the crystal structure, microstructure and piezoelectric properties were measured and evaluated. In the case of Sr 4 mol% doping, the piezoelectric properties were the highest for a PZN ratio of 0.1. In this condition, the grain size was larger and the intensity was higher. With the PZN ratio fixed and varying the Sr doping, the piezoelectric properties increased until 10 mol% doping and then decreased for over 12 mol% doping. In the case of x＝0.1 and y＝10 mol%, the best piezoelectric properties were obtained, i.e., d₃₃＝660 pC/N and k_p＝68.5%, and these values seem to be related to the grain size and crystal structure.

Transparent conductive thin films (TCFs) are essential materials for solar cells, organic light-emitting diodes, and display panels. Indium tin oxide (ITO) is one of the most widely used commercial materials to create TCFs’; however, new materials that can possibly replace ITO at a lower cost and/or those possessing mechanical flexibility are urgently needed. Silver nanowire (AgNW) is one of those promising materials, as it is less expensive and possesses superior mechanical flexibility as compared to ITO. We used AgNW and sol-gel ZnO to fabricate composite thin films by spray coating. We propose two spray-coating methods: the ‘metal－organic chemical vapor deposition (MOCVD)/AgNW’ method and the Mixture method. These two methods are expected to be commercialized for high-quality and low-cost products, respectively.

Ga-doped ZnO (GZO) films were deposited by an RF magnetron sputtering method on glass substrates using ZnO as a target containing 5 wt% Ga₂O₃ powder (for Ga doping). The structural, electrical, and optical properties of the GZO thin films were investigated as a function of the substrate temperatures. The deposition rate decreased with increasing substrate temperatures from room temperature to 350℃. The films showed typical orientation with the c-axis vertical to the glass substrates and the grain size increased up to a substrate temperature of 300℃ but decreased beyond 350℃. The resistivity of GZO thin films deposited at the substrate temperature of 300℃ was 7×10^-4 Ωcm, and it showed a dependence on the carrier concentration and mobility. The optical transmittances of the films with thickness of 3,000 Å were above 80% in the visible region, regardless of the substrate temperatures.

As a ZnO varistor is subjected to electrical and environmental stresses, it degrades gradually, which may result in power interruption by short circuit. This study investigates changes in the electrical characteristics of ZnO varistors due to deterioration owing to energy absorption, and determines the optimal parameters for on-line diagnosis of the varistor. Two types of varistors were used for an accelerated aging experiment involving the application of the 8/20 μs standard lightning impulse current. The electrical characteristics in terms of the reference voltage, total leakage current, resistive leakage current, and third-harmonic component of the total leakage current were measured, and their change rates were analyzed. The results revealed that the total leakage current increased slightly with an increase in the varistor absorbed energy, while the resistive leakage current and the third-harmonic component increased apparently. Therefore, the third-harmonic component of the total leakage current was proposed as the optimal parameter for on-line monitoring of ZnO varistor conditions.