The temporal evolution of the negative ion density in a low duty cycle pulsed reactive magnetron discharge

Time-resolved O^? density measurements have been made in the bulk plasma of a unipolar pulsed-DC magnetron using laser photodetachment. The magnetron was operated with a titanium target at a total pressure of 1.3 Pa and a fixed oxygen-to-argon partial pressure ratio of 10%. The duty cycle was maintained at 5% but the peak on-time discharge power was varied from 120 to 720 W.For all discharge powers, both the electron (n_e) and negative ion (n_?) densities increase during the plasma on-time, with the electron density reaching a maximum at the end of this phase. In the off-time, the electron density initially decreases at a rapid rate (characteristic decay time ~ 25 μs) for the first 50 μs, followed by a slower rate (~ 150 μs) for the remainder of the off-time, however, the negative ion density continues to increase in this phase, reaching a maximum at about ~ 150 μs after the termination of the discharge power. Both the electron and negative ion densities increase with discharge power. The maximum negative ion density was 2.5 × 10¹⁶ m^?3 for a peak power of 720 W, corresponding to a negative ion-to-electron density ratio n_?/n_e (α) of about 3. In the long afterglow, this ratio reaches a maximum value of 12 as the electron density decreases faster than the negative ion density. This shows that in the afterglow the plasma is highly electronegative.     

Plasma electrolytic oxidation of hafnium

This paper presents the results of the investigation of plasma electrolytic oxidation (PEO) of hafnium. Atoms ionized during the PEO micro-discharging were identified using optical emission spectroscopy. The spectral line shape analysis of the hydrogen Balmer line H_β indicated the presence of two types of micro-discharges characterized by electron number densities of around 2.5·10²¹ m^− 3 and 1.3·10²² m^− 3. Scanning electron microscopy and X-ray diffraction were employed to investigate surface morphology and phase composition of the PEO coatings obtained. The coatings were crystalline and composed of monoclinic HfO₂. Diffuse reflectance spectroscopy has shown that HfO₂ coatings have a broad absorption band in the range from 200 nm to 400 nm. Optical band gap of HfO₂ coatings was around 5.4 eV, as estimated from absorption spectra. Photoluminescence measurements show that HfO₂ coatings have broad emission band in the visible region, with a maximum at around 480 nm. The highest photoluminescence was obtained for the excitation wavelength of 270 nm. Intensity of photoluminescence increased with PEO time and is related to an increase of oxygen vacancy defects in HfO₂ coatings formed during the process.     

The evolution of the IEDFs in a low-pressure HiPIMS discharge

Using an energy-resolved mass spectrometer the evolution of the ion energy distribution function (IEDF) in a high-power impulse magnetron sputtering (HiPIMS) plasma has been measured at low operating pressures. The plasma was maintained using a low-power DC power supply as a pre-ionizer in conjunction with a conventional HiPIMS power supply, allowing reduced times between the initiation of the HiPIMS pulse and plasma ignition. As a result, a stable HiPIMS discharge can be operated at the working pressure down to 86 mPa with a typical pulse width, repetition rate and peak power density of 100 μs, 100 Hz and 450 W cm^? 2, respectively. Time-averaged IEDF measurements at 10 cm from a carbon target surface showed two distinct energy peaks one at low energy (~ 0 eV) and the other at high energy (~ 10 eV) for the detected species Ar⁺ and C⁺. The origin of these two peaks can be revealed using a time-resolved acquisition technique. It was found that the high energy peak (corresponding with an effective stopping voltage of 15 V) was created during the on-time phase of HiPIMS pulse while the low energy peak dominated the IEDF during the off-time. Increase of the operating pressure to 750 mPa results in the depletion of intensity and energy of the energetic peak. At the low pressure condition, with a mean-free-path comparable with the mass spectrometer orifice-target distance, energetic ions are transported ballistically to the instrument orifice. This suggests that forming well-structured films may be achieved in the condition of the collision-free operation.     

Influence of excitation frequency on structural and electrical properties of spray pyrolyzed CuInS2 thin films

This paper reports the cost effective deposition of the copper indium sulfide (CuInS₂) thin films under atmospheric conditions via ultrasonic spray pyrolysis. Structural and electrical properties of these films have been tailored by controlling the nozzle excitation frequency and the solution loading. Smoother films have been obtained via 120 kHz excitation frequency compare to the 48 kHz. Band gap energy of the films has also been tailored via excitation frequency. UV–vis–NIR analysis revealed that films deposited at 48 kHz excitation frequency had lower band gap energies. Although, both excitation frequencies resulted chalcopyrite structure, crystallinity of the CuInS₂ films was better for 120 kHz. On the other hand, better optical absorption in visible and near infrared region was observed at 48 kHz. Moreover, room temperature electrical conductivity of the samples deposited at 48 kHz excitation frequency was higher than that of samples deposited at 120 kHz. Temperature dependent electrical conductivity data showed that variable range hopping mechanism can be used to explain the conduction of spray pyrolyzed CuInS₂ thin films. Electrical mobility as high as 48 cm²/Vs has been observed for the sample deposited from 0.51 ml/cm² loading at 48 kHz excitation frequency. This value is very close to the mobility of vacuum deposited thin films like amorphous silicon, which is one of the most commonly used semiconductor in electronic and energy applications.     

Effect of boron doped fullerene C60 film coating on the electrochemical characteristics of silicon thin film anodes for lithium secondary batteries

In this work, boron doped fullerene (B:C₆₀) films were prepared by the radio frequency plasma assisted thermal evaporation technique for use as a coating material for the silicon thin film anode in lithium secondary batteries. Raman and XPS analyses revealed that the boron atoms were well inserted into the fullerene film lattices. The effect of the B:C₆₀ film on the electrochemical characteristics of the silicon thin film was studied by charge–discharge tests, electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV). The B:C₆₀ coated silicon film exhibited a high reversible capacity of more than 1200 mAh g^?1 when cycled 50 times between 0 and 2 V at a current density of 1200 μA cm^?2 (1.5 C). The film also showed good rate capacity at different current densities and a more improved coulombic efficiency of 87.7% in the first cycle in comparison with that of the C₆₀ coated film electrode.     

Time-resolved plasma parameters in the HiPIMS discharge with Ti target in Ar/O2 atmosphere

A High Power Impulse Magnetron Sputtering System (HiPIMS) equipped with a titanium target 5 cm in diameter has been investigated by means of a time-resolved Langmuir probe and time-resolved ion flux probe. The plasma parameters have been measured in a distance of 70 mm from the target face and below the racetrack. The ion flux at the substrate (placed at the same position as the Langmuir probe) has been determined from the voltage drop across a resistor connected with the planar probe. The planar probe was pulsed at a repetition rate of 50 kHz, duty cycle 50%. The effects of working gas pressure and mean discharge current have been investigated.Fine temporal resolution of the measured probe characteristics revealed a decrease in the effective electron temperature T_eff during pulse-ON time followed by a steady value at 0.6 eV for all the pressures during the rest of the period. For the pressure of 2 Pa, we observed a local maximum in T_eff (0.9 eV) at the end of cathode voltage pulse. Furthermore, during the pulse-OFF time we observed an exponential-like decay of the electron temperature for all the pressures. The plasma density demonstrated a steep increase during pulse-ON time followed by an exponential-like decrease during plasma OFF phase. The ion flux measurements have revealed approximately 3 times higher magnitude in the ion flux on the substrate at low pressure than at higher-pressures. The influence of these phenomena on the total energy flux at substrate and on thin film properties is discussed.     

Preparation of porous nanorod polyaniline film and its high electrochemical capacitance performance

Nano-sized polyaniline (PANI) films were electrochemically deposited onto an ITO substrate by a pulse galvanostatic method (PGM) in an aqueous solution. The morphology of the as-prepared PANI film was characterized using a field emission scanning electron microscope (FESEM). It was observed that the as-prepared PANI films were highly porous, and showed a nano-sized rod-like or coralline-like morphology depending on the charge loading performed in the electropolymerization process. Furthermore, the PANI films were electrochemically measured by the galvanostatic charge–discharge (GCD), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) tests in 1 mol L^?1 HClO₄ solution. The results showed that such PANI films had a favorable electrochemical activity and an excellent capacitance. The rod-like PANI film prepared with the charge loading of 1000 mC showed the highest discharge capacitance of 569.1 F g^?1 at a low current density of 1 A g^?1. The discharge capacitance retained 97.7% after 1000 cycles at a large current density of 10 A g^?1.     

Plasma electrolytic oxidation of titanium in heteropolytungstate acids

This article is a review of our recent research of plasma electrolytic oxidation (PEO) process of titanium in heteropolytungstate acids (12-tungstosilicic acid and 12-tungstophosphoric acid). It has been detected that spatial density of microdischarges is the highest in the early stage of the PEO process, while the percentage of oxide coating area covered by active discharge sites decreases with PEO time. The elements and their ionization stages present in PEO microdischarges are identified using optical emission spectroscopy technique. The spectral line shape analysis of hydrogen Balmer line H_β (486.13 nm) indicates presence of two types of microdischarges during PEO. The discharges are characterized by relatively low electron number densities of N_e ≈ 0.8 × 10¹⁵ cm^? 3 and N_e ≈ 2.1 × 10¹⁶ cm^? 3. Oxide coatings formed by PEO process were characterized by AFM, SEM–EDX and XRD. The elemental components of PEO coatings are Ti, W and O. The oxide coatings are partly crystallized and mainly composed of WO₃ and anatase.     

Synthesis of polyindole and its evaluation for Li-ion battery applications

This study is intended to develop a polyindole-based Li-polymer secondary battery system, which has a high electromotive force together with excellent cycle property and is capable of fast charging and discharging. The batteries include polyindole as the cathode and Li as the anode. LiBF₄ was used as the electrolytic solution with about 3.0 V electromotive force. The battery achieves about 80–70 mAh/g at discharge current densities of 10–10³ A/m². As the theoretical capacity of polyindole is 84 mAh/g, its capacity occurrence rate is 95% at the discharge current density of 10 A/m² with a very high reaction rate. In addition, a discharge capacity at discharge current density of 10³ A/m² maintains 87% of capacity relative to that at 10 A/m². This indicates that this battery is excellent in fast charge and discharge properties. The cyclic life of the battery, which is measured at the current density of 10 A/m² with the discharge depth 60% at 25, is about 30,000 times. This shows the battery system has very excellent cycle property. Summarily, this Li–polyindole battery system would be promising in future applications such as hybrid electric vehicle with the development of the battery system.     

Formation and characterization of graphite-encapsulated cobalt nanoparticles synthesized by electric discharge in an ultrasonic cavitation field of liquid ethanol

Cobalt nanoparticles encapsulated in graphitic shells were successfully synthesized using an advanced and cost-effective synthesis method. The graphite-encapsulated nanoparticles were synthesized using an electric plasma discharge generated in an ultrasonic cavitation field of liquid ethanol, followed by the separation, drying and annealing of particles. Prior to annealing, the core-shell structured nanoparticles were characterized by high-resolution transmission electron microscopy and X-ray diffraction, which revealed the presence of a Co₃C and carbon supersaturation in the face-centered cubic β-Co phase in the core of carbon nanocapsules; some cores were discovered in an amorphous state. The majority of carbon nanocapsules were found to be spherical particles, less than 10 nm in diameter. The annealed powder samples, in which cobalt carbides and amorphous inner cores transformed to crystallized α-Co and β-Co cores, showed coercivity values of 15.5 kA m⁻¹and saturation magnetization values of about 50 A m² kg⁻¹, which is approximately 32% of the value of bulk cobalt particles.     

Color-tunable luminescence of Eu3+ in LaF3 embedded nanocomposite for light emitting diode

Eu³⁺-doped transparent nanocomposite of SiO₂–Al₂O₃–NaF–LaF₃ was fabricated by melt-quenching and subsequent heating. X-ray diffraction and transmission electron microscopy analyses evidenced that hexagonal LaF₃ nanocrystals were homogeneously precipitated among the aluminosilicate glass matrix. The distribution of Eu³⁺ ions in the nanocomposite was investigated by energy dispersive X-ray spectroscopy, photoluminescence and time-resolved luminescence spectra. The nanocomposite exhibited intense blue and green emissions originating from the ⁵D_1,2,3 levels of Eu³⁺ incorporated in the low-phonon-energy LaF₃ nanocrystals, and red emission corresponding to the Eu³⁺:⁵D₀ → ⁷F_J (J = 0, 1, 2, 3, 4) transitions, under the excitation of single wavelength light at 394 nm. By adjusting the Eu³⁺ doping content, various luminescent colors, including perfect white light, were easily tuned through cross relaxation processes between Eu³⁺ ions. The results suggest that the Eu³⁺-doped transparent nanocomposite could be potentially applicable as a white-light-emitting material under UV chip excitation.     

Investigation of polyaniline co-doped with Zn2+ and H+ as the electrode material for electrochemical supercapacitors

Polyaniline co-doped with Zn²⁺ and H⁺ was synthesized in aqueous HCl solution containing ZnCl₂, and tested for its supercapacitive behavior in three-electrode or two-electrode system with 1.0 M H₂SO₄ as electrolyte by cyclic voltammetry, charge–discharge and electrochemical impedance spectroscopy (EIS). Scanning electron microscopy (SEM) and Fourier transform infrared (FTIR) techniques were employed for characterization of polyaniline co-doped with Zn²⁺ and H⁺. Compared with polyaniline doped with H⁺, it shows a larger specific capacitance of 369 F g^?1. Moreover, the specific capacitance remains 90% after 1000 cycles at a current density of 0.5 A g^?1, indicating good cycleability.     

Fabrication of LiMn2O4 thin film cathode from chitosan-containing solution

Thin film of spinel LiMn₂O₄ was obtained by spin coating the chitosan-containing precursor solution on a platinumized Si substrate, followed by a two-step annealing procedure at 300 and 700 °C, respectively. It was demonstrated that the addition of the appropriate amount of chitosan to the precursor solution enhanced the deposition of LiMn₂O₄ films. The thickness of the deposited film from chitosan-containing precursor solution is about 5.2 μm after five-time spin coating under a spinning speed of 2500 rpm. Without the addition of chitosan in precursor solution, the deposited film was as thin as 0.16 μm under the same processing parameters. Furthermore, the electrochemical behavior for the deposited LiMn₂O₄ film calcined at 700 °C for 1 h was characterized by the charge–discharge test. The result shows that the 1st discharge capacity is 56.31 μAh cm⁻² μm⁻¹ at a discharge rate of C/2 and the fading rate of the discharge capacity is only 0.19% cycle⁻¹ after 50 cycles.     

Effect of metal coating on machinability of high purity germanium using wire electrical discharge machining

This paper reports on the research of wire electrical discharge machining (WEDM) as a cutting process for n-type high purity germanium (HP Ge). WEDM requires sufficient electrical conductivity of the work piece for discharges to occur. Owing to the very high material resistivity of HP Ge (32.8 Ω cm), the electrical conduction is too low for WEDM to be efficient. To temporarily enhance the conduction, metals (aluminum and nickel) were deposited on the HP Ge on 1 or 2 sides with various thicknesses (1.0, 2.0, and 3.0 μm) using sputter deposition. This shortens the path of conduction between the HP Ge and the WEDM ground and also serves to trigger the discharges. Machining experiments were performed to determine the correlation between the slicing rate and locally enhanced HP Ge through various discharge energies (potential voltage: 150, 200, 250 and 300 V and capacitance: 1, 3.3, 5.5, 9.9 and 21.4 nF). From the results, the obtained maximum slicing rate is 7.7 mm²/min for Al coating (2 sides, 1.0 μm thickness) at high energy (300 V, 21.4 nF), which is improved as much as 27 times over uncoated HP Ge. The fastest cutting without creating subsurface microcracks was measured as 1.12 mm²/min performed at 150 V and 9.9 nF. Additional slicing experiments at reverse polarity (positive wire and negative work piece, uncommon polarity for WEDM) were performed at 150 V and various capacitances. The experiment proved that there were rectifying contacts at the metal coating surface. It was found that under identical EDM settings, a faster slicing rate also showed a reduction in kerf size due to less lateral discharge energy. Scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS) were used to investigate microcracks and to analyze surface impurities.     

The role of anthraquinone sulfonate dopants in promoting performance of polypyrrole composites as pseudo-capacitive electrode materials

We report the role of anthraquinone sulfonate dopants in promoting performance of electro-synthesized polypyrrole (PPy) composites for use in electrochemical supercapacitors. The incorporation of anthraquinone sulfonate species into the polymer matrix can significantly improve the surface area of PPy composites that are composed of submicron-/nano-sized particles, as evidenced from scanning electron microscopy (SEM) results. Cyclic voltammetry and galvanostatic charge–discharge measurements in 1 M KCl solution reveal that these dopants result in an improved specific capacitance, a wide working potential range and enhanced long-cycle stability as compared to ClO₄^? dopant. Among the samples investigated, the resulting PPy/AQS (9,10-anthraquinone-2-sulfonic acid sodium salt) composite exhibits the highest specific capacitance of 608 F g^?1 at a scan rate of 5 mV s^?1 within a potential range between ?0.9 and 0.5 V (vs. saturated calomel electrode, SCE).     

Synthesis and properties of nanostructured dense LaB6 cathodes by arc plasma and reactive spark plasma sintering

Nanostructured polycrystalline LaB₆ ceramics were prepared by the reactive spark plasma sintering method, using boron nanopowders and LaH₂ powders with a particle size of about 30 nm synthesized by hydrogen dc arc plasma. The reaction mechanism of sintering, crystal structure, microstructure, grain orientations and properties of the materials were investigated using differential scanning calorimetry, X-ray diffraction, Neutron powder diffraction, Raman spectroscopy, transmission electron microscopy and electron backscattered diffraction. It is shown that nanostructured dense LaB₆ with a fibrous texture can be fabricated by SPS at a pressure of 80 MPa and temperature of 1300 °C for 5 min. Compared with the coarse polycrystalline LaB₆ prepared by traditional methods, the nanostructured LaB₆ bulk possesses both higher mechanical and higher thermionic emission properties. The Vickers hardness was 22.3 GPa, the flexural strength was 271.2 MPa and the maximum emission current density was 56.81 A cm^?2 at a cathode temperature of 1600 °C.     

Electrical and impedance spectral characterisation of ITO/DAG/In device

A Schottky device, with configuration ITO/DAG/In is fabricated using diazopheny diamino glyoxime (DAG) as an n-type organic material. Current–voltage characteristics and impedance spectroscopy measurements were carried out, which reveals that the injection and transport properties are dominated by negative charge carriers. Space charge limited current theory with an exponential distribution of traps is very well followed by observations resulted through current–voltage characteristics at high voltage region. This gives a traps density N_t of about 4.5×10²¹ m⁻³ and mobility of electron is about 3.9×10⁻¹⁰ m² V⁻¹ s⁻¹. It is found that DAG behaves as an n-type materials as it forms Schottky barrier with ITO (high work function electrode) and conduction is governed by majority carriers, i.e. electrons. Using temperature and bias dependence of impedance spectral characteristics in a broad frequency range, i.e. 40 Hz to 100 kHz, it is found that the ac behaviour of In/DAG/ITO device shows several features, described by the simple double RC circuit representing a depleted junction region and an undepleted bulk region. From the large frequency range of impedance spectroscopy two distinct processes were identified, corresponding to bulk DAG layer and junction region. The activation energy of the relaxation times coincides well with the results obtained from the temperature dependent dc conductivity. The temperature dependent capacitance–voltage measurements were analysed at a frequency of 40 Hz. The obtained inherent donor concentration from 1/C²–V plots varies from 1.8×10²³ m⁻³ at room temperature to 5.9×10²³ m⁻³ at 360 K.     

High charge/discharge rate polypyrrole films prepared by pulse current polymerization

Polypyrrole (PPy) films doped by p-toluenesulfonic (PTS) ion were prepared by pulse current polymerization (PCP PPy) and direct current polymerization (DCP PPy) in aqueous solution. During polymerization of DCP PPy films, the sustained high anodic potentials can result in overoxidation and –CH₂– formation at pyrrole rings, which was confirmed by the FTIR spectroscopy. PCP PPy films exhibited more homogeneous and smoother appearance than DCP PPy films. The apparent diffusion coefficient of PCP PPy films was one order of magnitude bigger than that of DCP PPy films, which was closely correlated with the more ordered structure of PCP PPy films confirmed by XRD. When the scanning rate reached up to 500 mV s^?1, the CV curves of PCP PPy films showed rectangular shape within voltage range of ?0.8 V to +0.8 V. High charge/discharge rate of PCP PPy films can be attributed to well wettability, very small charge transfer resistance, high electronic conductivity and ions apparent diffusion coefficient. After 50,000 charge/discharge cycles, the specific capacitance of PCP PPy films only decreased by 14% at a charge/discharge current of 20 A g^?1 within voltage range of 0–0.4 V.     

Effect of Al addition on SiC–B4C cermet prepared by pressureless sintering and spark plasma sintering methods

SiC–B₄C–Al cermets containing 5, 10 and 20 wt.% of Al were fabricated by high-energy planetary milling followed by conventional sintering and spark plasma sintering (SPS) techniques separately. The average particle size reduced to ~ 3 μm from an initial size of 45 μm after 10 h of milling. The as-milled powders were conventionally sintered at 1950 °C for 30 min under argon atmosphere and SPS was carried out at 1300 °C for 5 min under 50 MPa applied pressure. The formation of Al₈B₄C₇ and AlB₁₂ phases during conventional sintering and SPS were confirmed by X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses. The formation of Al₈B₄C₇ at 700 °C and AlB₁₂ at 1000 °C was well supported by XRD and differential scanning calorimetry (DSC). The maximum relative density, microhardness and indentation fracture resistance of SiC–B₄C–10Al consolidated by SPS are 97%, 23.80 GPa and 3.28 MPa·m^1/2, respectively.     

Properties of single-walled carbon nanotube-based aerogels as a function of nanotube loading

Here, we present the synthesis and characterization of low-density single-walled carbon nanotube-based aerogels (SWNT-CA). Aerogels with varying nanotube loading (0–55 wt.%) and density (20–350 mg cm^?3) were fabricated and characterized by four-probe method, electron microscopy, Raman spectroscopy and nitrogen porosimetry. Several properties of the SWNT-CAs were highly dependent upon nanotube loading. At nanotube loadings of 55 wt.%, shrinkage of the aerogel monoliths during carbonization and drying was almost completely eliminated. Electrical conductivities are improved by an order of magnitude for the SWNT-CA (55 wt.% nanotubes) compared to those of foams without nanotubes. Surface areas as high as 184 m² g^?1 were achieved for SWNT-CAs with greater than 20 wt.% nanotube loading.