Pulsed DC Plasma Research Articles

DC-Plasma over Aqueous Solution for the Synthesis of Titanium Dioxide Nanoparticles under Pressurized Argon.

Nanomaterials that comprise titanium dioxide (TiO2) nanoparticles have received much attention owing to their wide applications; presently, the green synthesis of TiO2 nanoparticles is a developing research area. In this study, the TiO2 nanoparticles were synthesized through a DC-pulsed discharge plasma over an aqueous solution surface under a high-pressure argon environment. The titanium-rod electrode was utilized as the material source for the TiO2 nanoparticle generation. Experiments were performed at room temperature with pressurized argon at 1–4 MPa. To generate a pulse electrical discharge plasma, a DC power supply of 18.6 kV was applied. The Raman spectroscopy showed that the TiO2 nanoparticle with a brookite structure was formed dominantly. The scanning transmission electron microscopy equipped with energy dispersion spectroscopy (STEM coupled with EDS) indicated that TiO2 coated with carbon and that without carbon coating were successfully produced at the nanoscale. The process presented here is an innovative process and can update the existing information regarding the synthesis of metal-based nanoparticles using pulsed discharge plasma under an argon environment.

Influence of hydrogen supply on Mo(C,N) films synthesized by plasma-enhanced chemical vapor deposition using bis(tert‑butylimido) bis(dimethylamido) molybdenum

Complex composites of Mo(C,N) are deposited at temperatures of 150 °C and a working pressure of 1.3 Pa by plasma-enhanced chemical vapor deposition using bis(tert‑butylimido) bis(dimethylamido) molybdenum as precursor, argon as carrier gas to transport the vapors of the liquid metal–organic precursor at 80 °C, and hydrogen gas as reactant. The effect of hydrogen as reactive gas in the deposition process is investigated. For plasma generation in the plasma cleaning and deposition process, pulsed DC plasma is utilized at a negative voltage of 600 V with a frequency of 150 kHz and a pulse reversal time of 1 μs. As a result, hydrogen ambient decreases the depositio rate and surface roughness and increases the hardness of the films. Under the flow rate condition of hydrogen gas over 300 sccm, the hardness reaches high values over 26 GPa, and the adhesion is over 30 N. X-ray diffraction spectroscopy measurements reveal hexagonal phased molybdenum carbide as the preferred crystallization of the Mo(C,N) films, and nanocrystalline molybdenum nitride is mixed with the molybdenum carbide. Analysis of the chemical binding of the Mo(C,N) films by X-ray photoelectron spectroscopy and Raman spectroscopy shows that the dominant binding state is the molybdenum carbide, amorphous graphitic carbon and carbon nitride composites are identified from the evaluation of the binding energy of carbon atoms.

Characteristics of DLC/oxynitriding duplex-treated V8 tool steel by DC-pulsed PECVD

ABSTRACTIn this paper, DLC films are produced by DC-pulsed plasma enhanced CVD after oxynitride-treating of Vanadis 8 tool steel. The experimental parameters include various Ar gas flows (1, 1.5, 2 and 2.5 sccm) with asymmetric bipolar pulsed voltages, the gas flow of CH4 and a deposition time of 90 min, respectively. The experimental results showed that optimal properties occurred at an appropriate argon gas flow of 1.5 sccm. The wear tests showed the lowest wear loss volume when loads of 2 and 5 N were 3.81 × 10−3 and 4.01 × 10−3 mm3, respectively. Meanwhile, the hardness of the DLC film was 17.78 GPa and the elastic modulus was 191.63 GPa, respectively. Moreover, the water contact angle was 78.11°, which indicated that the DLC films could make Vanadis 8 tool steel more hydrophobic.

Characteristics of TiAlCN ceramic coatings prepared via pulsed-DC PACVD, part I: Influence of precursors’ ratio

In this study, titanium aluminium carbonitride (TiAlCN) ceramic coatings with different precursors' ratio of AlCl3/TiCl4 were deposited on H13 hot work tool steel substrates, using pulsed-DC plasma assisted chemical vapor deposition method. The coatings showed a nanocomposite microstructure consisted of fcc-TiAlN and hcp-AlN nanocrystalline grains and an amorphous carbon phase. Increasing the precursors' ratio from 0.5 to 3 led to an increase in Al content from ∼10 to ∼42 at.%. Moreover, with increasing the precursors' ratio, the surface roughness of the coating reduced initially from 21.39 to 14.44 nm, due to the creation of more nuclei of AlN and then it rose up to 18.26 nm. Coating with the precursors' ratio of 0.5 presented the highest microhardness of 3840 HV0.01,due to the less amount of chloride impurity and hcp-AlN phase. The precursors’ ratio of 0.5 resulted in the highest wear resistance and lowest coefficient of friction of around 0.17.

Photocatalytic antibacterial study of N-doped TiO2 thin films synthesized by ICP assisted plasma sputtering method

Specific plasma engineering based synthesis can induce interesting photocatalytic antibacterial activities in TiO2 films. This work presents the combination of pulsed DC (PDC) plasma with inductively coupled plasma (ICP) to synthesize undoped and nitrogen doped (N-doped) TiO2 thin films using magnetron sputtering. The surface properties of resulting films were systematically analyzed in detail and correlated to the photocatalytic antibacterial activity against Gram negative Escherichia coli (E.coli). The addition of ICP source to PDC discharge generated TiO2 films with enriched surface roughness due to the bombardment of highly activated species on the substrate. The ICP assisted films exhibited the morphology of nanoclusters with clear defined boundaries, higher O content and higher surface wettability. It was found that the films deposited with N-doping and addition of ICP source significantly increased the eradication of bacteria both in dark as well as in white light (artificial indoor light). Higher antibacterial activity of N-doped TiO2 films in white light has been explained by photocatalytic degradation mechanism.

Plasma Enhanced CVD Growth of Graphene on Cu and Ge

Thermal chemical vapor deposition (T-CVD) is currently the process of choice to grow large area graphene with good quality on metal surfaces like copper (Cu) or nickel (Ni) due to their catalytic properties. However the typical growth temperatures are still around 1000°C, and the process cannot be easily adapted to alternative, i.e. non-metallic, substrates. Regarding these issues we used a commercially available 4” Aixtron Black Magic system for establishing a plasma enhanced CVD process (PE-CVD) for graphene growth. A pulsed DC plasma was applied to dissociate the precursor material –methane- already in the gas phase and hence reduce the growth temperature. This process facilitates graphene fabrication with good quality down to a growth temperature of 600°C on Cu substrates. By studying the growth rate for different temperatures we have been able to extract a characteristic activation energy of 1.8 eV in PE-CVD, which is reduced by 2.2 eV as compared to T-CVD owing to the dissociation of methane already in the gas phase. In order to understand the plasma enhanced growth process, we analyzed the graphene formed for different growth times at a growth temperature of 700°C. After 45 min (Fig. 1a, top) we observe amorphous carbon around high quality graphene flakes. This is supported by Raman measurements (Fig. 1b), where Raman signatures of both, amorphous carbon and crystalline graphene are found. By increasing the growth time to 3 h, a continuous graphene film is obtained (Fig. 1a, bottom). In Raman spectroscopy, the amorphous signal disappears and Raman signatures of monolayer graphene with strongly reduced defect peak are detected. This indicates that through a recrystallization process the amorphous carbon turns into a graphene film around the crystalline graphene nuclei. The fabricated graphene film has a sheet resistance down to < 0.5 kΩ/sq. when transferred onto a Si/SiO2 substrate. Establishing a PE-CVD process at growth temperatures down to 600°C indicates that the need for catalytical substrates become superfluous. We thus adapted our approach to non-metallic substrates. As an example, we chose crystalline Ge(100), a CMOS compatible material that is used for example in broadband photodetectors in combination with graphene [1]. Graphene growth on Ge is typically performed by T-CVD at temperatures of 900°C or above. Such high temperatures might be critical for practical applications because of the temperature dependent dopant diffusion. [2] We demonstrate that our PE-CVD process can be applied to reduce the growth temperature of graphene on Ge(100) down to 800°C. We observed that by increasing the growth time from minutes to hours a 2D peak appears and the D and G bands isolate indicating growth is initiated from amorphous carbon and proceed to nanocrystalline graphene. In contrast to PE-CVD growth on Cu, a significant defect peak is observed for graphene grown on Ge(100), which we attribute to the numerous nucleation sites arising from the 100 nm large terraces as reported by Luskosius et al. [3] Fig. 1: Graphene growth in PE-CVD at 700°C on Cu foil at different growth times. a) Exemplary SEM images of the graphene flakes at growth times of 45 min (top) and 3 h (bottom). b) Average Raman spectra of the samples presented in a), extracted from the corresponding Raman maps.

Enhancing the adhesion of diamond-like carbon films to steel substrates using silicon-containing interlayers

Diamond-like carbon (DLC) films were deposited on AISI 316L stainless steel by using a unipolar pulsed-DC plasma assisted chemical vapor deposition (PACVD) system. In order to improve the adhesion of the DLC films to the substrate, three different silicon-containing interlayers (a-Si:H, a-SiCx:H, and a-SiNx:H) were deposited at two temperatures of 150 and 350 °C using silicon tetrachloride, methane, nitrogen, and hydrogen precursors. The DLC films with silicon-containing interlayers, deposited at 150 °C, did not show appropriate adhesion to the substrate due to the presence of a high amount of chlorine and hydrogen in the structure of the interlayers. By increasing the temperature of the interlayer deposition, the adhesion of DLC films improved significantly. The results of Raman spectroscopy, Nano indentation and adhesion tests showed that the DLC film with a-SiNx:H interlayer exhibited the lowest ID/IG ratio, highest hardness, and highest adhesion to the substrate.

Improving the bio-corrosion behavior of AISI316L stainless steel through deposition of Ta-based thin films using PACVD

In this study, the Ta and Ta/TaN films were deposited on AISI 316L stainless steel (SS) using pulsed-DC plasma assisted chemical vapor deposition (PACVD) method to simultaneously improve its corrosion resistance and biocompatibility. The results showed that the deposited Ta/TaN films represents a uniform and flawless microstructure with better adhesion to substrate compared to single layer Ta due to the enhancement of Ta nucleation suppressed by TaN interlayer and also lower lattice mismatch between Ta and TaN. The Ta and Ta/TaN films showed the better surface wettability compared to the bare SS owing to the different hydrophilic nature of their spontaneously formed oxide layers. The Ta/TaN film exhibited the excellent corrosion resistance which is mainly attributed to its dense and flaw-free microstructure. The cell viability observations revealed that due to lower corrosion rate, the adhesion and also proliferation of cells on Ta/TaN film are meaningfully higher than that of bare SS and single layer Ta. After biofilm removal, the Ta/TaN film exhibited a higher integrity and adhesion to substrate compared to the single layer Ta.

Non-Conventional Atmospheric Pressure Plasma Sources for Production of Hydrogen

ABSTRACTThe atmospheric pressure plasma sources with a coaxial geometry were used for generation of the radio frequency, microwave and pulsed dc plasmas inside water and aqueous solutions. Pulsed dc plasma generated in ethanol-water mixtures leads to production of the hydrogen-rich synthesis gas with hydrogen content up to 65 %. The effect of various plasma generation regimes on the performance of plasma, on the hydrogen production efficiency and on the hydrogen-rich synthesis gas production was examined. A role of the composition of the ethanol-water mixture was investigated.

DC-pulse atmospheric-pressure plasma jet and dielectric barrier discharge surface treatments on fluorine-doped tin oxide for perovskite solar cell application

Nitrogen DC-pulse atmospheric-pressure plasma jet (APPJ) and nitrogen dielectric barrier discharge (DBD) were applied to pre-treat fluorine-doped tin oxide (FTO) glass substrates for perovskite solar cells (PSCs). Nitrogen DC-pulse APPJ treatment (substrate temperature: ~400 °C) for 10 s can effectively increase the wettability, whereas nitrogen DBD treatment (maximum substrate temperature: ~140 °C) achieved limited improvement in wettability even with increased treatment time of 60 s. XPS results indicate that 10 s APPJ, 60 s DBD, and 15 min UV-ozone treatment of FTO glass substrates can decontaminate the surface. A PSC fabricated on APPJ-treated FTO showed the highest power conversion efficiency (PCE) of 14.90%; by contrast, a PSC with nitrogen DBD-treated FTO shows slightly lower PCE of 12.57% which was comparable to that of a PSC on FTO treated by a 15 min UV-ozone process. Both nitrogen DC-pulse APPJ and nitrogen DBD can decontaminate FTO substrates and can be applied for the substrate cleaning step of PSC.

Pulsed DC plasma CVD system for the deposition of DLC films

Pulsed DC plasma chemical vapour deposition (CVD) system with a compact vacuum chamber has been developed using a simple configured pulsed DC power supply. Pulsed DC discharge was examined with variation of operation parameters as well as source gas materials. Diamond like carbon (DLC) films were deposited on silicon (Si) substrates from acetylene (C2H2) or carbon-monoxide (CO) at two distinct temperatures. The fabricated films were characterized by Raman spectroscopy, FTIR spectroscopy, and scanning electron microscopy (SEM). The results reveal that the films prepared from CO gas exhibited soft conductive DLCs with a large number of graphite like structures whereas mostly insulating hydrogenated amorphous carbon DLCs were obtained from C2H2 gas. The deposition rate from CO was lower than from C2H2 due to etching by oxygen produced from the source gas. As the substrate temperature increased, progressive graphitization was incorporated to the films deposited from both gases. By using the pulsed DC plasma CVD system, the energy efficiency for CVD as the ratio of growth volume to energy consumption was dramatically improved with some distinctive differences in film properties as compared to our previous RF plasma CVD. The results showed many advantageous features of pulsed DC plasma CVD for replacing conventional RF plasma CVD.

Surface modification of Ti 13Nb 13Zr by plasma electrolytic oxidation

Titania coatings were produced on Ti 13Zr 13Nb by a unipolar pulsed DC plasma electrolytic oxidation (PEO) process in an electrolyte containing 1M H2SO4+0.1M H3PO4. The samples were galvanostatically anodized under a constant charge-carrier density of 63Ccm−2 and current-densities reaching from 14mAcm−2 up to 700mAcm−2. Both a constant current mode and a unipolar pulsed mode with a duty-cycle of 50% and frequencies of 1Hz to 50Hz were performed. The surface layers were characterized by scanning electron microscopy (SEM/EDX), X-ray diffraction (XRD) and nanoindentation. Titanium oxide coatings with a thickness of several micrometers and a porous structure could be generated. The coatings morphology was evaluated. It can be shown that the porosity can be reduced with increasing frequency. The coatings consist of anatase and rutile, the phase composition of the coatings can be adjusted by the PEO current density. The corrosion resistance of the layers was tested in a simulated body fluid (SBF) with the addition of 0.1M H2O2 by open circuit potential measurements, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). EIS data fitting indicates a composed layer structure of an outer porous layer and an inner barrier layer. Higher current densities during the PEO process formed thicker coatings, which consequently resulted in lower corrosion currents. The addition of zirconia nanoparticles into the electrolyte increased the surface hardness to an average value of 12.8GPa, compared to that of an untreated sample (4.1GPa) and PEO samples without particle addition (8.5GPa).

Improved performance of polyaniline/reduced-graphene-oxide supercapacitor using atmospheric-pressure-plasma-jet surface treatment of carbon cloth

This study evaluates carbon cloth treated using dc-pulse nitrogen atmospheric pressure plasma jet (APPJ) for a polyvinyl alcohol (PVA)/sulfuric acid (H2SO4) gel-electrolyte supercapacitor with a polyaniline (PANI)/reduced graphene oxide (rGO) nanocomposite electrode materials. The water contact angle of the carbon cloth decreases from 144.71° to 0° after the scanning APPJ surface treatment. The improved wettability can facilitate the penetration of the electrolyte into the porous electrodes, thereby improving the capacitance. X-ray photoelectron spectroscopy (XPS) indicates the introduction of nitrogen doping into to the carbon fibers of carbon cloth through the nitrogen APPJ treatment. Without APPJ treatment of carbon cloth, the specific (areal) capacitance of the fabricated supercapacitor is 315.0 F g−1 (55.67 mF/cm2); it increases to 580.2 F g−1 (106.89 mF/cm2) with APPJ surface treatment of carbon cloth before screen-printing the PANI/rGO nanocomposite. Electrochemical impedance spectroscopy (EIS) indicates a decreasing charge-transfer impedance at the electrode/electrolyte interface for supercapacitors with APPJ treatment of carbon cloth. This also improves the supercapacitive performance. After 1000-cycle cyclic voltammetry stability test, the capacitance retention rate is ∼85%. Our experimental results suggest that nitrogen dc-pulse APPJ in scanning mode is an efficient tool for enhancing the supercapacitance performance of a PANI/rGO supercapacitor.

Atmospheric-pressure plasma jet processed Pt/ZnO composites and its application as counter-electrodes for dye-sensitized solar cells

Nitrogen dc-pulse atmospheric pressure plasma jet (APPJ) is used to fabricate Pt/ZnO composites as the counter electrodes (CEs) of dye-sensitized solar cells (DSSCs). Due to the synergetic effect of the reactive plasma species and heat in nitrogen APPJ, the spin-coated precursors including chloroplatinic acid and zinc acetate can be reduced on fluorine-doped tin oxide (FTO) glass substrates in a few seconds. X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) analyses indicate that the precursors are reduced to Pt/ZnO under APPJ calcination. Electrochemical impedance spectroscopy (EIS) and Tafel measurement show the promising catalytic activities of Pt/ZnO CEs with low charge transfer resistance and high exchange current density. The efficiency of a DSSC with a 30-s APPJ-calcined Pt/ZnO CE is similar to that with a conventional furnace-annealed Pt CE for 15 min. The results indicate that nitrogen dc-pulse APPJ treatment is an efficient tool for rapidly fabricating Pt/ZnO composite CEs of DSSCs.

Structure and tribological behavior of diamond-like carbon coatings deposited on the martensitic stainless steel: The influence of gas composition and temperature

The relationship between the tribological behavior and inherent features of the DLC coatings (i.e. structural and mechanical properties) as a consequence of deposition conditions was investigated. Hence, a plasma nitrocarburising process as pretreatment followed by Diamond-Like Carbon (DLC) deposition by pulsed DC Plasma Assisted Chemical Vapor Deposition (PACVD) method, was utilized. Although a remarkable reduction in the friction coefficient of coated samples was observed, almost the same friction coefficient was measured for the DLC coatings (i.e. 0.2), regardless of their diverse intrinsic properties. As the H/C ratio in feeding gas and temperature increased up to certain values, the hardness and elastic modulus increased while the wear rate decreased. Therefore, the DLC coating deposited with the highest Hydrogen fraction at 160°C, exhibited the highest hardness and elastic modulus (i.e. 25.4GPa and 177.4GPa, respectively), as well as the lowest wear rate of 5.9×10−06mm3/N·m.

Generation of energetic electrons in pulsed microwave plasmas

Pulsed microwave driven microplasma at atmospheric pressure is proposed as an efficient source of energetic electrons for low‐temperature biomedical applications. Energetic electrons are required to break the covalent bonds of nitrogen, oxygen, and water molecules in humid air and produce reactive species. Particle‐in‐cell (PIC) simulations demonstrate that the energetic electrons with energy greater than 10 eV (capable of breaking the bond of nitrogen molecules) can be produced much more efficiently in pulsed microwave helium plasmas than in pulsed DC or continuous microwave plasmas. In particular, fast heating and cooling of electrons are observed at the rising edge and the falling edge of the pulse, respectively. This phenomenon is consistent with the fluid analysis showing the electrons’ energy gain via collective motion and collisional loss to the neutral particles.

SiO2 films as heat resistant layers for protection of expandable polystyrene foam from flame torch–induced heat

Currently, polymeric insulation materials are widely used for energy saving in buildings. Despite of all benefits, these materials are generally sensitive to heat and highly flammable. This work discusses possibility to improve heat resistance of expanded polystyrene (EPS) foam using thin silicon dioxide (SiO2) films deposited by magnetron sputtering technique. In order to increase surface energy and adherence of SiO2 thin films to substrate EPS was plasma pretreated before films’ depositions using pulsed DC plasma generator for 40 s in argon gas. SiO2 formation was done in reactive argon and oxygen gas atmosphere. Laboratory made equipment was used for flame torch–induced heat resistance experiments. Results showed that silicon oxide films remains stable during heat resistance experiments up to 5 s and fully protects polystyrene (PS) substrate. Films are relatively stable for 30 s and 60 s and partially protect PS from melting and ignition. Scanning electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy analysis confirmed that SiO2 layer, which is distributed uniformly on the EPS surface, could work as a good heat resistant material.

Pulsed DC plasma as a tool for the generation of nanomaterials that mitigate the alkali-aggregate reaction in Portland cement concretes

The efficacy of the pulsed DC plasma method in the mitigation of the aggregate alkali reaction, by the deposition of thin films of Al, Ti and Fe nanomaterials on potentially reactive aggregates was investigated from analytical techniques. These included field emission scanning electron microscope, X-ray dispersive energy microprobe, electron back-scatter diffraction, Raman and the resulting expansion in the modified accelerated mortar-bar method. In the 30days of the test, the values of expansion were obtained in samples with in nature aggregates of (0.77±0.03)% and in those treated of (0.16±0.01)%, (0.16±0.01)% and (0.16±0.00)%, respectively.

Application of atmospheric-pressure plasma jet processed carbon nanotubes to liquid and quasi-solid-state gel electrolyte supercapacitors

We use a dc-pulse nitrogen atmospheric-pressure plasma jet (APPJ) to calcine carbon nanotubes (CNTs) pastes that are screen-printed on carbon cloth. 30-s APPJ treatment can efficiently oxidize and vaporize the organic binders, thereby forming porous structures. As indicated by X-ray photoelectron spectroscopy (XPS) and electron probe microanalysis (EPMA), the oxygen content decreases after APPJ treatment owing to the oxidation and vaporization of ethyl cellulose, terpineol, and ethanol. Nitrogen doping was introduced to the materials by the nitrogen APPJ. APPJ-calcination improves the wettability of the CNTs printed on carbon cloth, as evidenced by water contact angle measurement. Raman spectroscopy indicates that reactive species of nitrogen APPJ react violently with CNTs in only 30-s APPJ processing time and introduce defects and/or surface functional groups on CNTs. Carbon cloths with calcined CNT layers are used as electrodes for liquid and quasi-solid-state electrolyte supercapacitors. Under a cyclic voltammetry test with a 2mV/s potential scan rate, the specific capacitance is 73.84F/g (areal capacitance=5.89 mF/cm2) with a 2M KCl electrolyte and 66.47F/g (areal capacitance=6.10 mF/cm2) with a H2SO4/polyvinyl alcohol (PVA) gel electrolyte.

Active screen cage pulsed dc discharge for implanting copper in polytetrafluoroethylene (PTFE)

Polymers such as polytetrafluoroethylene (PTFE) are widely used in artificial organs where long-term anti-bacterial properties are required to avoid bacterial proliferation. Copper or silver ion implantation on the polymer surface is known as a viable method to generate long-term anti-bacterial properties. Here, we have tested pulsed DC plasma with copper cathodic cage for the PTFE surface treatment. The surface analysis of the treated specimens suggests that the surface, structural properties, crystallinity and chemical structure of the PTFE have been changed, after the plasma treatment. The copper release tests show that copper ions are released from the polymer at a slow rate and quantity of the released copper increases with the plasma treatment time.