Increase In Breakdown Voltage Research Articles

(Invited, Digital Presentation) The Challenges of Overcoming Defects in GaN Hemts for Operation in Extreme Environments

GaN high electron mobility transistors (HEMTs) are known to contain a high density of crystal defects. The material properties of GaN such as high breakdown voltage, carrier mobility, critical electric field strength, and higher thermal conductivity than Si, make it an ideal candidate material for use in power switches in extreme environments. In high power environments (>1.5kV), vertical GaN device geometry is emerging as an effective technique to improve performance, but the more mature lateral GaN HEMT technology is being consistently improved upon to increase breakdown voltage and overall performance in the high-power regime. In high temperature environments, GaNs wide bandgap helps in preventing unwanted thermal generation of carriers, but devices also experience increased gate-leakage current and ohmic contact resistance degradation at high temperatures. The role of defects in these limiting factors of GaN HEMT performance in extreme high power and high temperature environments is reviewed.

Improved Breakdown Voltage and Low Damage E-Mode Operation of AlON/AlN/GaN HEMTs Using Plasma Oxidation Treatment

Low-damage plasma oxidation treatment for AlN/GaN high-electron-mobility transistors (HEMTs) was developed in this letter, with which high-performance enhancement-mode (E-mode) AlON/AlN/GaN HEMTs were demonstrated. After removal of in situ SiN cap layer within gate area, remote plasma oxidation (RPO) treatment of 4.9nm AlN barrier layer at 300 °C leads to formation of AlON insulator layer and effective depletion of 2-D electron gas channel, without etch-induced damage or serious interfacial issues compared with the commonly used gate- recess metal-insulator-semiconductor HEMTs. Thanks to the low-damage RPO treatment, the achieved E-mode HEMTs with threshold voltage of 0.4 V exhibit quite a small loss of output current (7.8%) compared with the depletion-mode devices. In addition, the AlON gate insulator layer results in a remarkable increase in breakdown voltage of devices from 20 V to 74 V, which benefits the high-voltage operation of AlN/GaN technology. Further investigation shows that the oxidation process of AlN barrier tends to be saturated with treatment time increasing, providing a promising solution to the high-performance E-mode operation of scaled GaN-based HEMTs in RF applications.

Sensitivity of electrical behaviours of ZnO nanoparticle–Bi2O3–Mn2O3 varistor system to various La2O3 doping compositions

The effects of La2O3 on the behaviours of ZnO nanoparticle–Bi2O3–Mn2O3 varistors were determined. The discs with various La2O3 compositions (0.0–3.0 mol%) were prepared via traditional ceramic techniques and their characteristics were investigated. The results demonstrate that La2O3 remarkably affects the varistor samples, in which the size of grain diminished from 29.3 to 8.5 µm according to increasing La2O3 content from 0.0 to 3.0 mol%. The significant response within the varistor sample fabricated from ZnO nanoparticles through sintering process at 1200oC perhaps caused by the surface properties of the powdered ZnO nanoparticles. X-ray diffraction results show that the addition of La2O3 to the varistor resulted in additional peaks, which can be attributed to the construction of La-based phases through the sintering process at high temperature. The spinal phases of the (Bi48ZnO73) and (Mn3O4) diminish with the 3.0 mol% content of La2O3. The varistor samples with 2.0 mol% La2O3 show a superior spinal phase construction of (Bi48ZnO73) and (Mn3O4) and La-based phases, resulting in homogeneous grain size. La2O3 doping substantially enhanced the electrical behaviours of the disc sample with a notable increase in breakdown voltage from 2614.8 to 5730.7 V/mm caused by the increase in La2O3 doping concentration from 0.0% to 3.0 mol%. The improvement was associated with the potential lanthanide ion segregation caused by its greater ionic radius to the boundaries between ZnO grains, thus enhancing its electrical properties. Moderate La2O3 doping concentration at 1.0 mol% improved the non-linear characteristics of the sample (α = 66) and 4400 V/mm breakdown voltage was introduced, while a significant amount of La2O3 doping concentration (more than 1.0 mol%) caused its deterioration (α = 35). The increase in donor concentration Nd to 2.1 × 1020 cm−3 for the varistor doped with 1.0 mol% La2O3 is accompanied by a decrease of interfacial states density NIS to 2.2 × 1012 cm−2 eV−1, in addition to decrease in depletion layer ω to 3.9 nm. The leakage currents of the sample decreased from 0.473 to 0.310 mA/cm2 with the increase of La2O3 doping contents from 0.0% to 2.0 mol% and then increased to 0.596 mA/cm2 at 3.0 mol% La2O3 doping concentration. Therefore, La2O3 may be added to dominate the microstructural characteristics and electrical behaviours of ZnO–Bi2O3–Mn2O3 varistor manufactured using powdered ZnO nanoparticles.

Interface engineering of β-Ga2O3 MOS-type Schottky barrier diode using an ultrathin HfO2 interlayer

Low leakage current and high breakdown voltage are critically important to design high performance Schottky barrier diodes. In this work, a low leakage current was reported by studying the effect of inserting an ultrathin HfO2 layer using atomic-layer deposition with different cycle number at the interface between Ni and β-Ga2O3 for different annealing temperatures. A remarkable decrease in leakage current from 5.68 × 10−3 to 2.97 × 10−6 A/cm² at -300 V and a breakdown voltage increase from -467 to -556 V, were achieved when two cycles of HfO2 are inserted and annealed at 400°C. The leakage current decreases from 9.87 × 10−5 to 2.14 × 10−5 A/cm² for 200°C annealing temperature and from 1.035 × 10−4 to 2.56 × 10−6 A/cm² for as-deposited Metal-oxide semiconductor based Schottky barrier diode (MOSSBD) only after 2-cycles (∼2 Å) HfO2 is inserted. As the number of HfO2 cycles was increased (2–8 cycles) and the annealing temperature is varied, leakage current and breakdown voltage are affected. To investigate these variations in leakage current and breakdown voltage, Silvaco TCAD simulator is used. Good agreement is obtained between experiment and simulation by considering the fixed charge density in HfO2 (positive fixed charge density is related to oxygen vacancy density in HfO2 and the interfacial trap (reduced by annealing), and, where necessary, also considering the inhomogeneity of the Schottky barrier height to understand the main reason for the abrupt increase in leakage current at a specific reverse voltage.

Charge collection efficiency study on neutron-irradiated planar silicon carbide diodes via UV-TCT

The material properties of silicon-carbide (SiC) make it a promising candidate for application as a particle detector at high beam rates. In comparison to silicon (Si), the increase in charge carrier saturation velocity and breakdown voltage allows for high intrinsic time resolution while mitigating pile-ups. A larger bandgap and higher atomic displacement threshold energy suppresses dark current and potentially improves radiation hardness, respectively. In addition to the lower susceptibility to temperature variations, it allows the operation of irradiated devices at room temperature and daylight illumination. Although already known for several decades, recent developments in industrial power electronics made SiC more accessible as a potential particle detector. Repeatability, large amplitudes and timing possibilities of signal pulses produced by the ultraviolet transient current technique (UV-TCT) allow for precise performance comparison of differing samples, while efficiently monitoring the influence of alternating external conditions. We present measurement results on the performance of neutron irradiated 4HSiC p-on-n planar diodes using such a setup. Dark current levels remain in the nA range for all fluences (5×1014neq/cm2−1×1016neq/cm2), while charge collection efficiency decreases with irradiation fluence, partially compensated when operating samples at reverse voltage conditions far above full depletion.

Impact of Anode Doping on Avalanche in Vertical GaN Pin Diodes with Hybrid Edge Termination Design

Introduction: Large area GaN power devices are seldom reported to avalanche and the theoretical studies of their edge termination still struggle to match experimental results especially for planar structures. We report for the first time a punch through avalanche on 1.2 kV vertical GaN diode with optimized hybrid edge termination design to fit the device structure. Furthermore, we provide a systematic study that pairs the Sentaurus TCAD simulation to the reverse characteristics of three different anode designs with three different remaining dose in the anode extension region. Device Fabrication: PiN diodes with three different anode doping were fabricated on a non-homogenous GaN substrate. 5×1017, 1×1018, and 2×1019 cm-3 Mg-doped layer were grown on a MOCVD lightly doped layer of 8um with doping concentration of the order of 1-2×1016 cm-3. The Edge termination process starts with etching a trench 1µm depth and about 140µm from anode edge, followed by isolation nitrogen implant using a box profile with three different energies and total depth of ~650 nm. Finally, the junction termination extension and guard rings (JTE/GR) hybrid is implemented by nitrogen implant to achieve a full planar device structure with final depth of 300 nm. Figure 1 depicts the device structure. The same JTE/GR hybrid implant depth into three different anode doping which results in different residual charge in the extension region thickness that is expected to impact the field management. Experimental Result: Three devices with the different anode doping were tested under identical forward and reverse biases conditions. The three devices display similar forward behavior with ideality factor ranging 2<n<2.2 as shown in Figure 2. Their reverse characteristics however, are varying proportional to the anode doping. The highest breakdown voltage and the sharpest breakdown characteristics was displayed by anode doping of 1×1018 cm-3. The other two anode doping devices fail short to achieve the expected breakdown voltage and both devices also exhibit higher leakage than the 1×1018 cm-3.. All devices are designed to withhold 1.2 kV, thus 2×1019 and 5×1017 cm-3 anode are ~ 500 V away from the targeted value. Considering the fact that these three wafers have the same drift region thickness and doping, and were processed together through the edge termination therefore, the drastic difference in the breakdown voltage can be only explained by the remaining dose in the extension region. To further understand the edge termination efficacy in these devices an avalanche test was conducted, reverse sweep was applied at 25,100,150, &200 °C for all three devices. Figure 3 depicts the avalanche results. Both devices with 1×1019 and 5×1017 are struggling to avalanche due to the high leakage and inconsistent trend with temperature. The PiN diode with 1×1018 cm-3 avalanched with an increase of breakdown voltage 10 V for every 50 °C. This trend of the breakdown with temperature is possible due to the increase in leakage current, consequently a temperature coefficient α= 3.1×10-4 K-1 was calculated using equation: . To better understand this difference a simulation was conducted using Sentaurus TCAD. The model is designed to observe the behavioral trends thus the absolute numbers in the model do not capture the devices non-idealities. The 2D distribution of the electric field is displayed in Figure 4 for all 3 doing levels. Conclusion: The reverse characteristic and TCAD simulation indicate an improvement in the field management with the decrease in the anode doping to 1×1018 cm-3 which subsequently means less charges in the edge termination region than the standard 2×1019 doping level . These results show that the anode dose is a function of anode doping level, implant depth and the edge termination region thickness. To optimize edge termination design one needs to manage the dose in the termination region which is a function of anode doping level, implant depth and the edge termination region thickness. Figure 1

Projected Cost of Gallium Oxide Wafers from Edge-Defined Film-Fed Crystal Growth

Gallium oxide (Ga2O3) is an emerging ultra-wide bandgap semiconductor that has unique properties ideal for high-power, high-temperature, optoelectronic, and sensing applications and has piqued interest over the last decade. It has the potential to be technologically and economically superior to commercially available wide bandgap semiconductor materials, such as silicon carbide and gallium nitride, because its wider bandgap enables increased breakdown voltages and lower on-state resistances, and its ability to be grown from melt enable cost-competitive economics. In this study, we present a techno-economic analysis that projects the cost of 6″ β-Ga2O3 wafers fabricated from crystals grown via edge-defined film-fed growth (EFG). At a manufacturing volume of 5000 wafers per month, we predict a unit cost of $320 for a 6″ EFG grown β-Ga2O3 epi-wafer. We determine that, when calculated using 2021 iridium crucible costs, EFG has a 2× cost advantage compared to previously reported epi-wafers grown via the Czochralski (CZ) method. We further identify key cost parameters for 6″ β-Ga2O3 epi-wafers and present cost-sensitivity analysis of their impact on the final cost.

Field effects in electron-irradiated GaP LEDs

The paper presents the results of the study of field effects in non-irradiated and irradiated by electrons (E = 2 MeV, F = 8.2∙1016 cm–2) gallium phosphide (GaP) light emitting diodes (LEDs) under reverse bias. The avalanche multiplication of charge carriers and tunneling breakdown in the space charge region has been considered. An increase of breakdown voltage after electron irradiation has been revealed. The effects of the annealing of non-irradiated and irradiated diodes in the temperature range of 20 to 500 °C have been analyzed.

Research of Electrophysical Properties of Thin Gate Dielectrics Obtained by Rapid Thermal Processing Method

Researches of the electrophysical characteristics of gate dielectrics obtained by the rapid thermal processing (RTP) method by two-stage and three-stage processes have been carried out. Each photonic processing (stage) was carried out for 12 s at a constant power of halogen lamps and heating the wafers to a maximum temperature of 1250 °C. The first two stages of the process were carried out in an oxygen atmosphere, the third - in nitrogen or a forming gas. It was found that for dielectrics obtained by the process with final processing in a nitrogen atmosphere, the absolute value of the voltage of flat zones is 0.42 V less, than for insulators, formed by a two-stage process. This is the consequence of the elimination of a significant part of the defects, responsible for the presence of Coulomb centers in the dielectric layer. Carrying out photonic processing in anitrogen atmosphere at high temperatures of procedures for proceeding of the restructuring of the structure of the dielectric layer. For insulators obtained by a three-stage process with final processing in N2, an increase in dielectric strength and breakdown voltage by 1 V and 3.3 MV/cm, respectively, is observed in comparison with dielectrics, obtained by a two-stage process. An increase in dielectric strength indicates relaxation of elastic stresses of deformed bonds and compensation for dangling bonds both in the dielectric and at its interface with Si during high-temperature photonic treatment. Passivation by nitrogen atoms of deformations at the dielectric/semiconductor interface will also have a positive effect on the strength of the insulator.

β-(AlxGa1−x)2O3/Ga2O3 heterostructure Schottky diodes for improved VBR2/RON

We propose and demonstrate the use of a β-(AlxGa1–x)2O3 capping layer to achieve increased breakdown voltage, VBR, and VBR2/RON figure of merit in β-Ga2O3 Schottky diodes, where RON is the on-state resistance. We demonstrate that the addition of a 30 nm-thick β-(Al0.22Ga0.78)2O3 cap to an n-type β-Ga2O3 layer grown by metal organic chemical vapor deposition increases the breakdown voltage from 246 to 387 V in diodes with Pt contacts and 3 × 1016 cm−3 n-type doping. The cap increases the surface Schottky barrier with Pt, resulting in reduced carrier injection under reverse bias. The results are in good agreement with simulations which show that the addition of the capping layer enables the peak electric field at breakdown to increase from 2.5 to 3.6 MV/cm. Simulations further show that RON penalty associated with the (AlxGa1–x)2O3 cap can be almost completely eliminated by grading the capping layer. Thus, by raising the barrier height beyond the limit imposed by the metal work function and Fermi level pinning, the proposed heterojunction helps to improve VBR by reducing the reverse leakage current in ultrawide bandgap semiconductor diodes where bipolar doping remains a challenge.

Towards a Generalized Electric Breakdown Mechanism of Insulating Nanofluids

Recently, addition of nanofillers to insulating fluids to increase its breakdown voltage finds a great interest from researchers. Understanding the reasons of the increased breakdown voltage with addition of nanofillers at certain loadings to insulating fluids is of great importance. Hence, understanding how electric breakdown occurs in nanofluids can help researchers to select the more suitable nanofiller material, nanofiller particle size ....... etc. to be added to insulating fluids. Therefore, in this paper, a generalized electric breakdown mechanism of insulating nanofluids is presented. The generalized mechanism takes different parameters into consideration. These parameters are nanoparticle permittivity, nanoparticle size and insulating fluid temperature. To demonstrate the validity of the generalized breakdown mechanism, theoretical computations are carried out using finite element analysis. Also, breakdown experiments considering transformer oil are carried out considering different nanofiller materials, nanofiller sizes at different temperatures.

Investigation of the switching characteristics of high-pressure subnanosecond gas dischargers with the purpose of a sharp increasing of the breakdown voltages and the switching speed

The paper analyzes the data obtained in the subnanosecond time range on the times (t br) and speeds (V br) of switching of hydrogen diode dischargers. These data were obtained in a wide range of hydrogen pressures (p) and the degree of the discharge gap overvoltage (the length of the cathode–anode gap d) in a uniform electric field. It is shown that the reduced strength of the average electric field E br/p in the discharge gap at the moment of the beginning of the breakdown significantly decreases when the gas pressure increases from 5 atm to 50 atm. An increase in pressure from 50 atm to 60 atm leads to a sharp (by 40% ÷ 135%, depending on the d) increase in the pulse breakdown voltage (U br) and an increase in E br/p. In proportion to the growth of E br/p the switching speed V br of the discharge gas gap increases. The observed effect is explained by the change in the discharge initiation mechanisms. The limitation of U br and V br in the hydrogen pressure range from 5 atm to 50 atm occurs as a result of gas ionization by runaway electrons and the subsequent development of a multi-avalanche discharge in the volume of the discharge gap. With a further increase in pressure, the discharge develops according to the streamer type. To design ultrafast gas dischargers of the subnanosecond range intended for switching high voltages, it is necessary to select an appropriate range of working gas pressures in order to ensure the development of a streamer-type discharge.

Effectiveness Of Rice Husk Charcoal to Improve Breakdown Voltage of Used Transformer Oil

Environmental pollution due to the burning of rice husk biomass and discarded transformer oil has a negative impact on the environment and livings. Utilization of rice husk biomass apart from being used as animal feed, fertilizer, can also be used as a purification of used transformer oil so that used transformers that were originally going to become waste can be reused in a long period of time. This study aims to analyze the effectiveness of rice husk activated charcoal in improving the breakdown voltage of used nynas libra transformer oil. The activator material used in rice husk charcoal is NAOH 30% as much as 100 ml with a chemical method for the activation process. To obtain the effectiveness value in terms of the percentage increase in breakdown voltage before and after purification, then the value of the voltage of the two test treatments is also reviewed against the standard breakdown voltage of transformer oil SPLN 49-1. The results showed that the average breakdown voltage increase of 5 times in each process was 166.99%. It can be concluded that the use of rice husk activated charcoal as a purification agent is effective in improving the breakdown voltage of used transformer oil

Improved f T/f max in wide bias range by steam-annealed ultrathin-Al2O3 gate dielectrics for InP-based high-electron-mobility transistors

In this study, we successfully achieved flat cutoff frequency (f T) and maximum oscillation frequency (f max) across a wide bias range on InP-based high-electron-mobility transistors (HEMTs) using a steam-annealed ultrathin-Al2O3 gate dielectric due to the reduction in gate leakage current and increase in forward breakdown voltage. Fourier transform infrared spectroscopy analysis demonstrated that steam annealing reduced defects in atomic layer deposited-Al2O3 because of the hydrolysis of carbon impurities and that reducing electron traps suppressed the increase in sheet resistance due to dielectric passivation. Consequently, the insulated-gate HEMTs developed show a high f max of > 700 GHz across a wide bias range.

Surface‐Oxide‐Controlled InAlGaN/GaN High‐Electron‐Mobility Transistors Using Al2O3‐Based Insulated‐Gate Structures with H2O Vapor Pretreatment

Herein, Al2O3/InAlGaN/GaN metal–insulator–semiconductor high‐electron‐mobility transistors (MIS‐HEMTs) using H2O vapor pretreatment to decrease the amount of deficient indium oxide (InOx) is successfully developed. InOx acts as an electron trap in the oxide on the surface of the InAlGaN barrier layer. It is clarified that when O2 plasma is used as an oxidant source for atomic‐layer‐deposited (ALD) Al2O3 (O2 plasma‐Al2O3), the forward breakdown voltage increases using the MIS‐HEMT, due to a high density and a low leakage current of O2 plasma‐Al2O3. Furthermore, when using O2 plasma‐Al2O3 for InAlGaN/GaN MIS‐HEMTs, H2O vapor pretreatment is effective in decreasing the amount of InOx and electron traps in the oxide on the surface, thus suppressing current collapse. These results demonstrate the improved output power characteristics of the fabricated Al2O3/InAlGaN/GaN MIS‐HEMT and show the potential of surface‐oxide‐controlled MIS structures as promising process technologies for next‐generation high‐power radio frequency devices.

Effect of Environmental Parameters on Streamer Discharge in Short Air Gap between Rod and Plate

Environmental meteorological parameters (temperature, humidity, air pressure, etc.) have an important impact on short gap streamer discharge. Based on the streamer theory and particle continuity equation of gas discharge, a short gap streamer discharge simulation model is established, and the influence of environmental parameters on the dynamic development process of streamer discharge is simulated and analyzed. The short gap air gap discharge characteristics were tested in a self-made gas discharge chamber. The results show that at low humidity (relative humidity &lt;80%), with the increase of humidity, the electron density of the streamer channel increases, the field strength of the streamer head increases, the streamer development speed speeds up, and the gap breakdown voltage decrease; At higher humidity (relative humidity &gt;80%), with the increase of humidity, the electron density of the streamer channel decreases, the field strength of the streamer head decreases, the development speed of the streamer slows down, and the gap breakdown voltage increase. With a decrease of gas pressure, the electron density of the streamer channel decreases, the field strength of the streamer head decreases, the development speed of the streamer increases, and the gap breakdown voltage decreases; The temperature change has little effect on the streamer development process.

Probing the Use of Silane-Grafted Fumed Silica Nanoparticles to Produce Stable Transformer Oil-Based Nanofluids.

In this experimental investigation, hydrophobic silane-grafted fumed nano-silica was employed in transformer oil to formulate nanofluids (NFs). A cold-air atmosphere-pressure plasma reactor working on the principle of dielectric barrier discharge was designed and utilized to functionalize the surface of these nanoparticles. A field emission scanning electron microscope (FE-SEM) coupled with energy-dispersive X-ray (EDX) module and Fourier transform infrared (FTIR) spectroscopy were used to scan surface features of new and plasma-treated nanoparticles. The study revealed considerable changes in the surface chemistry of nanoparticles, which led to good dispersibility and stability of nanofluids. The measurements of AC breakdown voltages (AC-BDV) of nanofluids so prepared were conducted according to IEC-Std 60156, and a significant improvement in the dielectric strength was achieved. A statistical analysis of these results was performed using Weibull probabilistic law. At a 5% probability of failure, modified nanofluid remarkably exhibited a 60% increase in breakdown voltage. The dielectric properties such as variation of and tan δ in temperature of up to 70 °C were measured and compared with untreated fluid. Results exhibit an increase in tan δ and a slight decrease in permittivity of nanofluids. The analysis also revealed that while unpolar silane coating of NPs increased the breakdown strength, the polar-amino-silane-coated NPs in oil resulted in a drastic reduction. Details of this antagonistic trend are elaborated in this paper.

Increased Breakdown Voltage in Vertical Heterostructure III-V Nanowire MOSFETs With a Field Plate

Vertical III-V heterostructure MOSFETs exhibit outstanding performance at reduced supply voltages. In this letter, we report on a novel process of extending high-speed device operation towards higher voltages. The device vertical geometry allows for engineering a field plate by covering the nanowire drain area with a 10-nm-thick SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> film. The film acts as a field moderator in the device drain region. Reference devices without a field plate exhibit a transconductance of 2.5 mS/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> , while devices with a 120-nm-long field plate show 1.5 mS/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> but a three times increase in breakdown voltage. Measurements show that the field-screening effect attributes to reduced band-to-band tunneling and impact ionization, thereby reducing the parasitic bipolar effect in the MOSFET channel as well. The devices show promise in applications in circuits and systems requiring large power-handling.

Effect of Concentration of Al2O3 Nanoparticles on Electrical Properties of Mineral Oil

This study aimed to compare the electrical characteristics of mineral oil-based nanoparticles to those of pure mineral oil. Mineral oil used in distribution transformers was mixed with aluminum oxide (Al2O3) nanoparticles, which serve as an insulator, with 0.01, 0.03, and 0.05 % mineral oil volume concentrations. The moisture content in mineral oil was measured beforehand, and the electrical tests were subsequently conducted in accordance with international standards. The moisture content test revealed that the combination of nanoparticles and mineral oil resulted in an increase in moisture content. As regards the test of AC withstanding voltage and negative polarity lightning impulse voltage, it was found that the mineral oil mixed with the nanoparticles had higher dielectric strength than the original mineral oil. In addition, the test results of positive polarity lightning impulse voltage demonstrated that with a mixture of mineral oil and the nanoparticles at a 0.01 % concentration, the positive polarity lightning impulse dielectric strength of the mineral oil increased. However, at concentrations of 0.03 and 0.05 %, it did not rise, regardless of the increasing volume of nanoparticles. The results, thus, showed promising directions for applications of nanoparticles in the development of electrical properties of mineral oil. HIGHLIGHTS The addition of Al2O3 nanoparticles to the mineral oil resulted in an increase in the oil moisture content. However, it was found that the moisture content in the mineral oil tended to decrease with the increase in the amount of nanoparticles The optimum nanofluid combination in terms of increased AC breakdown strength was created by adding Al2O3 nanoparticles to mineral oil at a rate of 0.05 vol% The Al2O3 nanoparticles being added to the mineral oil can give an increase in the impulse breakdown voltage with the negative polarity For the positive polarity of the impulse breakdown voltage, the best nanofluid mixture in terms of increased breakdown strength was produced by the addition of Al2O3 nanoparticles at a rate of 0.01 vol% to the mineral oil GRAPHICAL ABSTRACT

Research on the aging of micro-supercapacitor under the condition of longtime storage

Given the phenomenon that the capacity retention rate, anti-surge voltage capability, and frequency characteristic of the micro-supercapacitors (MSCs) would become poor under the condition of long time storage, a study was carried on a long time natural aging of MSCs with (PPy/GO-RuO2) membrane electrode and (PVA-KOH-KSCN-EG) gel electrolyte, and it fell into two parts: one was to measure the self-discharge, breakdown voltage, and capacitance value diffusion of samples with various natural storage time, and the second to carry on a scanning electron microscopy (SEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) test of the membrane electrode. The possible causes of a decrease of the capacity retention rate and the capacitance value diffusion threshold, an increase of the leakage current was found, based on the two test results. SEM analysis showed that the three-dimensional structure collapsed in the surface area of the membrane electrode with an increase of storage time, which made the pore and specific surface area of the active substance decreasing, resulting in capacity retention rate decrease, and the leakage current increases. XRD and XPS analysis showed that the amorphous RuO2•nH2O lost water gradually, which made RuO2 and Ru constantly appear, resulting in the polarization voltage of the membrane electrode and the breakdown voltage increase, and the specific capacity of MSCs reduce. Meanwhile, the amino group (- NH) protonation in PPy weakened the electron-donating ability, which increased faraday resistance RF, leading to the continuous decrease of capacitance diffusion threshold. The above research results were beneficial for improving the manufacturing process and service conditions of MSCs.