Ar-H2 Plasmas Research Articles

Interfacial modification of titanium dioxide to enhance photocatalytic efficiency towards H2 production

Strong demand for affordable clean energy to support applications ranging from conventional energy supply to space propulsion places spotlight on advanced energy generation using photovoltaic and wind power. Yet, the intermittent nature of solar and wind sources drives the search for energy storage solutions that would permit the needed level of resilience and support further growth in the use of renewable sources of power. Hydrogen generation using sunlight is a promising pathway to decouple demand from supply. Herein, we show how exposure to reactive Ar-H2, Ar-H2-N2, and Ar-O2 plasma environments can notably enhance surface properties of photocatalytic TiO2 nanosheets used in advanced energy generation systems. Treatment using Ar-H2 plasmas produced highly hydrogenated, surface-disordered TiO2 nanosheets with oxygen vacancies, whereas exposure to Ar-H2-N2 plasmas resulted in N doping. Surprisingly, Ar-O2 plasma treatment did not change surface properties of TiO2. Optical emission spectroscopy was used to monitor transient species to further understand surface modification in plasma. Direct measurements demonstrated that among thus-produced samples, hydrogenated TiO2 nanosheets exhibit the highest photocatalytic H2-generation activity under visible-light irradiation, which is also greater than the activity of pure, untreated nanosheets. The mechanism of enhancing the visible-light photocatalytic H2-generation activity on hydrogenated TiO2 nanosheets is also proposed. The level of surface disorder and oxygen vacancies plays an important role in enhancing visible-light absorption and reducing the recombination of photogenerated electrons and holes.

Lead-free solder wettability of oxidized-aluminum enhanced by Ar-H2 plasmas for flip-chip bumping

PurposeAn investigation has been performed on the improved solder wettability of oxidized aluminum (Al) with lead-free solder (96.5Sn-3.5Ag) using Ar-H2 plasmas. The lead-free solder wettability was raised from 62.2 per cent wetting for Al oxidized in air at 250 C for 4 h to 98.4 per cent wetting of oxidized Al modified by Ar-H2 plasmas at a certain H2 flow rate. This study aims to gain insight on the surface characteristics of Al affecting the solder wettability with a liquid lead-free solder.Design/methodology/approachAr-H2 plasmas at certain H2 flow rates are intended to reduce Al oxides on the surfaces of oxidized Al substrates both by physical bombardments via Ar plasmas and chemical reductions with H2 plasmas, while Al substrates are exposed in Ar-H2 plasmas to improve the solder wettability with a liquid lead-free solder.FindingsSurface characteristics of oxidized Al substrates have been identified to play key roles for enhanced lead-free solder wettability using Ar-H2 plasmas. A decrease in polar surface free energy and an increase in dispersive surface free energy on the surfaces of oxidized Al substrates are exploited to advance the lead-free solder wettability. Decreased composition ratios of O to Al, detected by X-ray photoelectron spectroscopy (XPS) for oxidized Al substrates, are crucial for improved lead-free solder wettability.Originality/valueXPS is typically used to analyze the surface compositions of Al oxides. To provide a rapid and non-expansive method to identify the surfaces of Al substrates prior to soldering to assure lead-free solder wettability, this study proposes a measurable skill, a so-called sessile drop test method, to investigate surface free energies such as total, polar and dispersive surface free energy on the surfaces of Al substrates, to illuminate how the lead-free solder wettability of oxidized Al is improved by Ar-H2 plasmas.

Influence of post‐deposition annealing on electrical and optical properties of ZnO‐based TCOs deposited at room temperature

The post‐deposition modification of ZnO‐based transparent conductive oxides (TCOs) can be the key to produce thin films with optoelectronic properties similar to indium tin oxide (ITO), but at a much lower cost. Here, we present electro‐optical results achieved for post‐deposition annealing of Al–Zn–O (AZO), AZO:H, Ga–Zn–O:H (GZO:H), and Zn–O:H (ZNO:H) thin films deposited by RF sputtering at room temperature. These studies comprise results of thermal annealing at atmospheric pressure, vacuum, forming gas, H2 and Ar atmospheres, and H2 and Ar plasmas, which lead to significant enhancement of their electro‐optical properties, which are correlated to morphological and structural improvements. The post‐deposition annealing leads to an enhancement in resistivity above 40% for AZO, AZO:H, and GZO:H, reaching ρ ≈ 2.6–3.5 × 10−4 Ωcm, while ZnO:H showed a lower improvement of 13%. The averaged optical transmittance in the visible region is about 89% for the investigated TCOs. Such results match the properties of state‐of‐art ITO (ρ ≈ 10−4 Ωcm and transmittance in VIS range of 90%) employing much more earth‐abundant materials.

Enhanced lead-free solder wettability of oxidized-nickel by Ar–H2 plasmas for flip chip bumping

Solder wettability (SW) of oxidized-nickel (ON) with liquid lead-free solder (LFS) (96.5Sn–3.5Ag) is notably improved by Ar–H2 plasmas from 0 % wetting of nickel substrate oxidized in air gases at 200 °C for 2 h to 100 % wetting of ON substrate treated by Ar–H2 plasmas at certain plasma settings. The SW of ON substrates with LFS is dependent on the surface characteristics of ON substrates. The enhanced SW of ON substrates with LFS by Ar–H2 plasmas is mostly due to a decrease in polar surface free energy and an increase in dispersive surface free energy on the surfaces of ON substrates. The ratios of the compositions of O to Ni are critical to the SW of ON substrate with LFS. The atomic compositions of Ni and O on ON substrates are analyzed by an energy-dispersive X-ray spectroscope. The nickel oxides NiO and Ni2O3 on the surfaces of ON substrate are identified by an X-ray photoelectron spectroscope.

Vacuum ultraviolet emission from microwave Ar-H2 plasmas

Vacuum ultraviolet emission from Ar-H2 wave driven microwave (2.45 GHz) plasmas operating at low pressures (0.1–1 mbar) has been investigated. The emitted spectra show the presence of the Ar resonance lines at 104.8 and 106.7 nm and of the Lyman-α,β atomic lines at 121.6 nm and 102.6 nm, respectively. The increase of the hydrogen amount in the mixture results in an abrupt increase of the Werner and Lyman molecular bands intensity. The Lyman-β intensity shows little changes in the range of 5%–30% of hydrogen in the mixture while the Lyman-α intensity tends to decrease as the percentage of hydrogen increases.

Enhanced anisotropic conductive film (ACF) void-free bonding for Chip-On-Glass (COG) packages by means of low temperature plasmas

The voids at the interface of anisotropic conductive film (ACF) to glass are typically formed during bonding process for Chip-On-Glass (COG) packages. The low temperature (∼23°C) ArH2 plasma pretreatment on glasses are investigated to enhance the void-free bonding of ACF to glass for COG packages. The void fraction of glass with ACF is significantly reduced from 52% of untreated-glass to 0% of ArH2 plasma-pretreated glass. The improved void-free ACF bonding is found to be highly dependent on the surface characteristics of glass. The surface energies, surface morphologies and surface compositions of glasses are discussed how to affect the enhanced void-free bonding of ACF to glass by low temperature ArH2 plasmas. The surface energies, such as total surface energies, polar surface energies and dispersive surface energies of glasses are obtained by calculating the basis of the measured contact angles of three various surface tensions of liquids. The surface morphologies of glasses are observed by field emitted scanning electron microscopy (FESEM). The surface compositions, such as atomic compositions and chemical bond compositions of glasses are analyzed using X-ray photoelectron spectroscopy (XPS).

RF sputtering of ZnO films in Ar and Ar–H2 gas mixtures: Role of H incorporation in developing transparent conductive coatings

Fundamental and applied investigation of ZnO has been recently experiencing a renaissance due to its prospective use in various technological domains and, in particular, as transparent conductive oxide (TCO). In this respect, the present work aims to study the structural and physical properties of ZnO thin films deposited by RF sputtering in pure Ar and Ar–H2 plasmas, at various concentrations (0–50%). The plasma chemical species were followed in function of the different gas mixture settings by optical emission spectroscopy (OES). X-ray photoelectron spectroscopy (XPS) and ATR-FTIR (Attenuated Total Reflection Fourier-Transformed Infrared) spectroscopy were used to study the bulk and surface chemical composition of the films, X-ray Diffraction (XRD) analysis allowed lattice structure and grain size determination while samples morphology was checked with a scanning electron microscope (SEM). The films were also characterized for their electrical and optical properties.The introduction of hydrogen in the plasma phase strongly affected the structural, chemical and physical properties of the films. In particular a pronounced change in the film electrical behavior was observed which become conductive when H is added in the gas mixture ([H2]>6%). The film transparency was on the other hand maintained. By combining XPS, ATR-FTIR and OES data we could correlate the established conductivity and its variations with intentional hydrogen incorporation in the crystal structure in the form of hydroxide species.

Yttria-Stabilized Zirconia Thermal Barriers Sprayed Using N2-H2 and Ar-H2 Plasmas: Influence of Processing and Heat Treatment on Coating Properties

Thermal barrier coatings were produced using both Ar and N2 as the primary plasma gas. Various aspects of the process and the coatings were investigated. It was found that higher in-flight particle temperatures could be produced using N2, but particle velocities were lower. Deposition efficiencies could be increased by a factor of two by using N2 as compared to Ar. Coatings having similar values of porosity, hardness, Young’s modulus, and thermal diffusivity could be produced using the two primary gases. The coatings exhibited similar changes (increased hardness, stiffness, and thermal diffusivity) when heat-treated at 1400 °C. However, the N2-processed coatings tended to have lower values of Young’s modulus and thermal diffusivity following such treatment. The results point to the potential advantage, in terms of reduced powder consumption and increased production rate, of using N2 as compared to Ar as the primary plasma gas for TBC deposition.

Enhanced Wettability of Oxidized Copper with Lead-Free Solder by Ar-H2 Plasmas for Flip-Chip Bumping

Enhanced solder wettability (SW) of oxidized-Cu (OC) with 96.5Sn-3Ag-0.5Cu lead-free solder (LFS) by Ar-H2 plasmas was investigated. The SW of OC was significantly improved from 0% wetting of Cu oxidized in air at 260°C for 1 h to 100% wetting of OC modified by Ar-H2 plasmas for 10 min. The SW of Cu was found to be highly dependent on the surface characteristics of Cu. By decreasing the total surface energy (TSE), decreasing the polar surface energy (PSE), and increasing the dispersive surface energy (DSE) on the surfaces of OC modified by Ar-H2 plasmas, the SW with LFS improves. X-ray photoelectron spectroscopy (XPS) indicates that Ar-H2 plasma treatment is used to remove the copper oxides CuO and Cu2O from the OC surfaces. The ratio of the total amount of Cu2O to CuO was found to be a good indication of how the copper oxides CuO and Cu2O affect the PSE, DSE, and SW of Cu.

Enhancement of paint adhesion to cold rolled steel in salt bath by low temperature Ar–H2 plasmas

This study investigates the enhanced adhesion of paint to cold rolled steel (CRS) in salt bath by Ar–H2 plasmas. It is found that the surface treatment on CRS, prior to painting, by low temperature plasma cleaning with argon–hydrogen plasmas at room temperature (23°C), can be used for improving the adhesion of paint to CRS in salt bath. The tape test (ASTM 3359 Method) demonstrated this improvement, with a rating of “0” for untreated CRS for aging at a 3.5wt% NaCl solution for 6 days and “5” for argon–hydrogen plasma-cleaned CRS at certain plasma conditions even for aging at a 3.5wt% NaCl solution for 6 days. The adhesion performance of paint to CRS in salt bath is highly dependent on the surface characteristics of the CRS and the work of adhesion of paint to CRS.

Role of atomic hydrogen density and energy in low power chemical vapor deposition synthesis of diamond films

Polycrystalline diamond films were synthesized on silicon substrates without diamond seeding by a very low power (∼40–80 W) microwave plasma continuous vapor deposition reaction of a mixture of helium–hydrogen–methane (48.2/48.2/3.6%) or argon–hydrogen–methane (17.5/80/2.5%). However, predominantly graphitic carbon films or no films formed when neon, krypton, or xenon was substituted for helium or argon. The films were characterized by time of flight secondary ion mass spectroscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, scanning electron microscopy, and X-ray diffraction. It is proposed that each of He+ and Ar+ served as a catalyst with atomic hydrogen to form an energetic plasma since only plasmas having these ions in the presence of atomic hydrogen showed significantly broadened H α lines corresponding to an average hydrogen atom temperature of >100 eV as reported previously. It was found that not only the energy, but also the H density uniquely increases in He–H2 and Ar–H2 plasmas. Bombardment of the carbon surface by highly energetic hydrogen formed by the catalysis reaction may play a role in the formation of diamond. Then, by this novel pathway, the relevance of the CO tie line is eliminated along with other stringent conditions and complicated and inefficient techniques which limit broad application of the versatility and superiority of diamond thin film technology.

Three-Dimensional Simulation of Thermal Plasma Spraying of Partially Molten Ceramic Agglomerates

Thermal plasma spraying of agglomerated nanostructured ceramic particles has been studied using computational fluid dynamics. The plasma jet is modeled as a mixture of Ar-H2 plasmas issuing into a quiescent atmosphere. The particles, modeled as micron-sized spheres, are introduced into the jet outside the plasma gun exit with radial injection. The existence of a simple target in front of the plasma gun is taken into account. The trajectories and state histories of particles of various sizes during their flight through the jet are presented. Moreover, the solid-liquid interface within the particles is tracked in an attempt to predict the amount of unmelted material retained in these particles at various axial distances from the gun exit. The effects of turbulence in the jet on these particle histories are accounted for. It is shown that, for the range of particle sizes and the plasma gun operating conditions studied, both the deposition location and the retained unmolten fraction are strongly affected by the size of the particles. The predictions are significant in terms of showing general trends, which will be useful in identifying processing windows for producing optimally nanostructured coatings.