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Natural wood-based materials are directly utilized to construct ultra-thick all-wood-structured supercapacitors with ultra-high capacitance and energy density.
Herein we show that hydrazine intercalation into 2D titanium carbide (Ti 3 C 2 -based MXene) results in changes in its surface chemistry by decreasing the amounts of fluorine, OH surface groups and intercalated water. It also creates a pillaring effect between Ti 3 C 2 T x layers pre-opening the structure and improving the accessability to active sites. Furthermore, the hydrazine treated material has demonstrated a greatly improved capacitance of 250 F g –1 in acidic electrolytes with an excellent cycling ability for electrodes as thick as 75 μm.
Experimentally and theoretically demonstrated water quality or level monitoring by inkjet printed coplanar capacitive sensors with high sensitivity.
The Anderson localization effect has been exploited in the design of high-temperature dielectric polymers, resulting in reduced conduction loss and outstanding capacitive energy storage performance over a wide temperature range up to 250 °C.
Reliable energy storage at subzero temperatures is crucial for polar and space applications. Nitrogen-doped multiscale porous carbon aerogels deliver 96% capacitance retention at −40 °C and 1.43 Wh L −1 energy density. In conclusion, enhanced nitrogen content and enlarged ion-accessible surface synergistically enable exceptional low-temperature supercapacitor performance.
Process uniformity of macro-scale parameters such as electron and ion densities is critical during any plasma process. Pulsed operation of multiple frequency capacitively coupled plasmas (CCPs) has been shown to improve profile characteristics of features during plasma etching. In this work, we consider pulsing of both power sources in a dual frequency CCP. The impact of phase lag between the high frequency and low frequency power pulses on plasma uniformity is examined using a two-dimensional computational plasma model. Results for Ar/CF{sub 4} gas mixture indicate that phase lag allows one to control plasma uniformity by modulating the time for which the high or low frequency source is on.
We theoretically investigated the effect of phase difference between powered electrodes in capacitively coupled plasma (CCP) discharges. Previous experimental result has shown that the plasma potential could be controlled by using a phase-shift controller in CCP discharges. In this work, based on the previously developed radio frequency sheath models, we developed a circuit model to self-consistently determine the bias voltage from the plasma parameters. Results show that the present theoretical model explains the experimental results quite well and there is an optimum value of the phase difference for which the V{sub dc}/V{sub pp} ratio becomes a minimum.
A two-dimensional (2D) self-consistent fluid model is developed to describe the formation, subsequent growth, transport, and charging mechanisms of nanoparticles in a capacitively coupled silane discharge applied by two very high frequency (VHF) sources with phase shift. In this discharge process, large anions are produced by a series of chemical reactions of anions with silane molecules, while the lower limit of the initial nanoparticles are taken as large anions (Si{sub 12}H{sub 25}{sup -} and Si{sub 12}H{sub 24}{sup -}) to directly link the coagulation module with the nucleation module. And then, by using the coagulation module, the particle number density quickly decreases over several orders of magnitude, whereas the particle size strongly increases. We investigate in particular the growth of the nanoparticles ranging in size from {approx}1 to 50 nm in coagulation processes. The influences of controlled phase shifts between VHF (50 MHz) voltages on the electron density, electron temperature, nanoparticle uniformity, and deposition rate, are carefully studied. It is found from our simulation that the plasma density and nanoparticle density become center high and more uniform as the phase shift increases from 0 to 180 deg. Moreover, the role of phase-shift control in the silane discharge diluted with hydrogen gas is also discussed.
An atmospheric-pressure capacitive discharge source driven by radio-frequency power supply at 13.56 MHz has been developed experimentally that is capable of producing a homogeneous and cold glow discharge in O{sub 2}/Ar. With respect to the influence of oxygen component when diluted into argon plasma discharge on the discharge characteristics, the measurements of the electrical parameters (impedance, phase angle, resistance, and reactance) are made systematically and the densities of the metastable and resonant state of argon are determined by means of optical emission spectroscopy (OES). It is shown that the admixture of oxygen into argon plasma not only changes the electric characteristics but also alters the optical emission spectra greatly due to strong interaction between the oxygen content and the argon in the plasma environment.
Measurements with a rf compensated Langmuir probe and optical emission spectroscopy are carried out in capacitively coupled rf (13.56 MHz) pure nitrogen N{sub 2} discharges at fixed rf voltage over a wide range of pressure, 30 to 400 mTorr. The electron energy probability function (EEPF) measured below 100 mTorr resembles a bi-Maxwellian-type distribution. At pressure range of 100-200 mTorr, the EEPF has non-Maxwellian distribution with a ''dip'' near 4.5 eV. At the highest pressure of 400 mTorr, the EEPF evolves into a Druyvestein-like distribution and the ''dip'' disappears. The electron density significantly decreases with increase in the N{sub 2} pressure. On the other hand, the electron temperatures gradually decrease with an increase in N{sub 2} pressure, reaching minimum at 150 mTorr, beyond which it abruptly increases. Such evolution of the EEPFs shape with gas pressure has been discussed in terms of non-local electron kinetics and heating mode transition. The emission intensities of nitrogen (0-0) band of second positive system at 337.1 nm and (0-0) band of first negative systems at 391.4 nm are used to determine the dependence of their radiative states N{sub 2}(C{sup 3}{Pi}{sub u}) and N{sub 2}{sup +}(B{sup 2}{Sigma}{sub u}{sup +}) with nitrogen pressure. It is observed that the pressure influences the radiative states differently owing to their different populating mechanisms. The vibrational temperature T{sub {nu}ib} and rotational temperature T{sub rot} are measured for the sequence ({Delta}{nu}=-2) of N{sub 2} second positive system (C{sup 3}{Pi}{yields}B{sup 3}{Pi}{sub g}) using the method of comparing the measured and calculated spectra with a chi-squared minimization procedure. It was found that both T{sub {nu}ib} and T{sub rot} have similar dependences with N{sub 2} pressure; peaked at 100 mTorr beyond which it monotonically decreases with increase in the N{sub 2} pressure. The correlation between the observed maximum value of T{sub {nu}ib} around 100 mTorr and the detected ''dip'' in the EEPF in the same pressure range has been discussed.
The effect of the transition from capacitive (E) to inductive (H) mode on propanethiol plasma polymer films properties was investigated by optical emission as well as by x-ray photoelectron spectroscopy. The E mode is characterized by low deposition rate and by high sulfur content in the films ({approx}40% vs {approx}20% in H mode). After aging, a strong decrease of sulfur to carbon content (from {approx}0.75 to 0.13), attributed to desorption of unbounded sulfur-based molecules (e.g., H{sub 2}S), is detected at low power in E mode. The importance of the E-H transition for altering the film properties is highlighted.
Through the use of particle-in-cell simulations, we demonstrate that the power deposition in capacitively coupled discharges (in argon) can be increased by replacing sinusoidal waveforms with Gaussian-shaped voltage pulses (with a repetition frequency of 13.56 MHz). By changing the Gaussian pulse width, electron heating can be directly controlled, allowing for an increased plasma density and ion flux for the same gas pressure and geometrical operating conditions. Analysis of the power deposition profiles and electron distribution functions shows that enhanced electron-sheath heating is responsible for the increased power absorption.
The discharge mode transition from uniform plasma across the gas gap to the {alpha} mode happens at the rising phase of the pulsed radio frequency capacitively coupled plasma (PRF CCP). This transition is attributed to the fast increasing stochastic heating at the edge of sheath. In the second stage with the stable current and voltage amplitude, the consistency between experimental and numerical spatial-temporal 777 nm emission profile suggests that He* and He{sub 2}* dominate the production of O({sup 5}p{sub 1}) through dissociation and excitation of O{sub 2}. Finally, the sterilization efficiency of PRF CCP is found to be higher than that of plasma jet.
Charge buildup during plasma etching of dielectric features can lead to undesirable effects, such as notching, bowing, and twisting. Pulsed plasmas have been suggested as a method to achieve charge-free plasma etching. In particular, electronegative plasmas are attractive as the collapse of the plasma potential during the after-glow period of pulsed capacitively coupled plasmas (CCPs) can allow for extraction of negative ions into the feature. The extraction of negative ions in the after-glow of pulsed CCPs sustained in CF{sub 4} containing gas mixtures is computationally investigated. In this paper, the consequences of pulse frequency and gas chemistry on negative ion flux to the wafer are discussed. A low negative ion flux to the wafer was observed only in the late after-glow period of low pulse frequencies. The negative ion flux was found to significantly increase with the addition of highly electronegative gases (such as thermally attaching Cl{sub 2}) even at a high pulse frequency of 10 kHz. As the production of negative ions during the after-glow diminishes, alternative strategies to enhance the flux were also pursued. The flux of negative ions was found to increase by the addition of a pulsed dc voltage on the top electrode that is 180 Degree-Sign out-of-phase with the rf pulse.
The electrical asymmetry effect (EAE) allows an almost ideal separate control of the mean ion energy, , and flux, {Gamma}{sub i}, at the electrodes in capacitive radio frequency discharges with identical electrode areas driven at two consecutive harmonics with adjustable phase shift, {theta}. In such geometrically symmetric discharges, a DC self bias is generated as a function of {theta}. Consequently, can be controlled separately from {Gamma}{sub i} by adjusting the phase shift. Here, we systematically study the EAE in low pressure dual-frequency discharges with different electrode areas operated in argon at 13.56 MHz and 27.12 MHz by experiments, kinetic simulations, and analytical modeling. We find that the functional dependence of the DC self bias on {theta} is similar, but its absolute value is strongly affected by the electrode area ratio. Consequently, the ion energy distributions change and can be controlled by adjusting {theta}, but its control range is different at both electrodes and determined by the area ratio. Under distinct conditions, the geometric asymmetry can be compensated electrically. In contrast to geometrically symmetric discharges, we find the ratio of the maximum sheath voltages to remain constant as a function of {theta} at low pressures and {Gamma}{sub i} to depend on {theta} at the smaller electrode. These observations are understood by the model. Finally, we study the self-excitation of non-linear plasma series resonance oscillations and its effect on the electron heating.
The multi-holed electrode that has been reported to enhance the electron density of the capacitively coupled plasma is now being adopted to speed up the processes. However, the discharge condition when the multi-holed electrode enhances the electron density of the discharge at fixed power is not studied. At low pressure, the multi-holed electrode increased the electron density of the plasma at fixed power. However, the multi-holed electrode is experimentally revealed to lower the electron density at high pressure. In this paper, the different roles of the multi-holed electrode are experimentally studied.
The electron bounce resonance heating (BRH) in dual-frequency capacitively coupled plasmas operated in oxygen and argon has been studied by different experimental methods. In comparison with the electropositive argon discharge, the BRH in an electronegative discharge occurs at larger electrode gaps. Kinetic particle simulations reveal that in the oxygen discharge, the bulk electric field becomes quite strong and is out of phase with the sheath field. Therefore, it retards the resonant electrons when traversing the bulk, resulting in a suppressed BRH. This effect becomes more pronounced at lower high-frequency power, when the discharge mode changes from electropositive to electronegative.
By using a rf compensated Langmuir probe and optical emission spectroscopy, the effects of driving frequency (13.56-50 MHz) on the electron energy probability function (EEPF), electron density, electron temperature, and the vibrational and rotational temperatures in capacitively coupled nitrogen discharge were investigated. Measurements were performed in the pressure range 60-200 mTorr, and at a fixed voltage of 140 V (peak-to-peak). With increasing the driving frequency, the dissipated power and electron density markedly increased along with the intensity of the optical emission lines belonging to the 2nd positive (337.1 nm) and 1st negative systems (391.4 nm) of N{sub 2}. The EEPF at low pressure 60 mTorr is two-temperature (bi-Maxwellian) distribution, irrespective of the driving frequency, in contrast with argon and helium discharges in the similar conditions. The mechanism forming such bi-Maxwellian shape was explained by two combined effects: one is the collisionless sheath-heating effect enhancing the tail electron population, and the other is the collision-induced reduction of electrons at the energy 2-4 eV where the collision cross-section for the vibrational excitation has a resonantly large peak. The two-temperature EEPF structure was basically retained at moderate pressure 120 mTorr and high pressure 200 mTorr. The vibrational temperature T{sub vib} and rotational temperature T{sub rot} are measured for the sequence ({Delta}{nu}=-2) of N{sub 2} second positive system (C{sup 3}{Pi}{sub u}{yields}B{sup 3}{Pi}{sub g}) using the method of comparing the measured and calculated spectra with a chi-squared minimization procedure. It was found that, both of T{sub vib} and T{sub rot} are a weakly dependent on driving frequency at low pressure 60 mTorr. At higher pressure (120 and 200 mTorr), T{sub vib} rises monotonically with the driving frequency, whereas the T{sub rot} slightly decreases with frequency below 37 MHz, beyond which it relatively increases or saturated.