Hydrocarbon Thin Films Research Articles

High protonic resistance of hydrocarbon-based cathodes in PEM fuel cells under low humidity conditions: Origin, implication, and mitigation

Hydrocarbon-based electrodes for proton-exchange membrane fuel cells face challenges in closing the performance gap with electrodes based on perfluorosulfonic acid ionomers, particularly under low humidity conditions. Alongside increased oxygen transport resistance and higher kinetic-induced overpotentials, the protonic resistance of these fluorine-free electrodes is the primary hurdle to improved performance. This study systematically investigates the origin and impact of the cathode protonic resistance on fuel cell performance, utilizing sulfonated phenylated polyphenylenes as hydrocarbon ionomers. Electrochemical characterization at low relative humidity (≤50 %) reveal a high protonic resistance arising from both lower conductivity of the hydrocarbon thin film compared to the bulk membrane and increased cathode tortuosity at a gas transport-optimized ionomer to carbon (I/C) ratio of 0.2. The poor protonic resistance at low relative humidities leads to a non-homogeneous current distribution across the thickness of the cathode electrode, resulting in lower catalyst utilization. To address this issue, reducing the thickness of the cathode CL while maintaining a constant Pt loading (i.e., increasing the Pt on carbon ratio) significantly reduces protonic resistance. This improvement compensates for the kinetic disadvantages of highly loaded carbon particles and results in a considerable performance increase by 40 % at 0.75 V under low relative humidities.

Cross-section data for the reactions 12C(3He,p0)14N to 12C(3He,p6)14N at energies up to 6 MeV

The cross-sections of the 12C(3He,px)14N reactions were studied for x = 0...6 at energies from 1.5 to 6 MeV and reaction angles of 135∘ and 175∘ using thin amorphous hydrocarbon (a–C:H) film targets on silicon and chromium substrates. The cross-sections were determined relative to the Rutherford scattering cross-section of 3He from gold, the accuracy of the measured cross-sections ranges from 2 to 4% at 135∘ and 3 to 10% at 175∘. The determined cross-sections were validated using benchmark measurements from bulk carbon targets. The results are presented in graphical and tabular form.

Polyaromatic hydrocarbon thin film layers on glass, dust, and polyurethane foam surfaces

An investigation was conducted into the dynamic behavior of two polyaromatic hydrocarbon (PAH) semi-volatile organic compound (SVOC) naphthalene (NAP) and benzo [ghi]perylene (BghiP) in air and on various surfaces including glass, dust, and polyurethane foam (PUF) to understand their interaction with different media. A confocal fluorescence microscope and an infrared microscope were employed to detect and monitor the concentration-, time-, and temperature-dependent changes of the aromatic NAP and BghiP species on the surfaces. Infrared two-dimensional mapping of the vibrational characteristic peaks was used to track the two PAHs on the surfaces. Gas chromatography-mass spectrometry (GC-MS) was employed to measure the gaseous concentrations. The sorption of NAP and BghiP on the surfaces was estimated using Arizona desert sand fine (ISO 12103-1 A2) dust and organic contaminant household (SRM 2585) dust. The surface-to-air partition coefficients of NAP and BghiP were estimated on the different surfaces of glass, dust, and PUF. Molecular dynamic simulations were performed on dust surfaces based on the Hatcher model to understand the behavior of NAP and BghiP on dust surfaces. The Weschler-Nazaroff model was introduced to predictPAH film accumulation on the surfaces, providing a better understanding of PAH interaction with different environmental media. These findings could contribute to developing effective strategies to mitigate the adverse impact of PAHs on the environment and human health.

Gamma-ray generation optimized by long and short pulses jointly driving double-layer target

In order to verify that the large-scale low-density plasma has a significant gain effect on the quality of the ultra-hot electron beam in ultra-strong ultra-short pulse laser interactions with matter, on the Shenguang-II upgraded laser facility, we carry out an experimental study on the optimization of relativistic electron beams that combine long and short pulse lasers to generate large-scale low-density pre-plasma. A nanosecond laser is used to ablate the thin hydrocarbon film. After a period of time, a large-scale low-density plasma is formed. The second picosecond short pulse laser then interacts with the formed low-density plasma, and accelerates the electrons to the relativity magnitude. Through the comparative analysis between different experimental conditions in the experiment, it is found that the existence of large-scale low-density plasma significantly increases the intensity of the relativistic electron beam along the laser propagation direction. In the comparative test of three shooting methods (long and short pulse combined driving double-layer target, short pulse driving double-layer target, short pulse driving single-layer target), we find that the energy bands above 1 MeV can be produced by the long pulse and the short pulse jointly driving double-layer target. The gamma-ray intensity is nearly twice that of the other two schemes. In addition, we also find in the experiment that when the time interval between nanosecond-picosecond pulses changes, the pre-plasma electron density and density scale length of the CH film irradiated by the nanosecond laser will be different. When the time interval is 0 ns, the expansion of the plasma is still insufficient, and the density distribution of the pre-plasma is steep. Part of the picosecond laser energy will be reflected at the critical density surface. Insufficient absorption of picosecond laser energy reduces the generation of relativistic electrons.When the time interval is 1 ns, the plasma state is more appropriate. The picosecond laser will form a self-focusing plasma channel in the plasma, thereby efficiently depositing energy, breaking through the energy limit of ponderomotive force acceleration, and obtaining more higher energy relativistic electrons. When the time interval is longer, such as 1.5 ns or more, the plasma can be fully expanded, the electron density becoming too low. Most of the picosecond laser will pass through the pre-plasma, affecting energy deposition and failing to generate more relativistic electrons.

Formation of Complex Organic Molecules (COMs) from Polycyclic Aromatic Hydrocarbons (PAHs): Implications for ISM IR Emission Plateaus and Solar System Organics

The presence of complex organic molecules (COMs) in a variety of solar system objects has prompted various theories regarding their origins. This paper reports on a series of proton, electron, and UV radiation experiments, conducted singly and in combination, on polycyclic aromatic hydrocarbon (PAH) thin films to determine if PAH-related materials could be related to COMs. Fourier transform infrared (FTIR) and mass spectra reveal that the PAHs are fragmented and altered, producing new infrared features and a mass spectral pattern similar to those from small aliphatic and larger saturated hydrocarbon chain and ring systems. Mass spectra after proton and electron irradiation exhibit significant differences. The proton irradiation products appear to contain more oxygen-related species, possibly the result of higher H2O concentrations in the vacuum chamber, whereas electron irradiation generates a more abundant, larger mass organic species mass spectral pattern. Combined irradiation produces a superposition of the proton and electron results, with some subtle differences. Mass spectral patterns resulting from electron and combined irradiation compare favorably to Cassini ion neutral mass spectrometer (INMS) measurements of particles falling from Saturn’s inner rings into its upper atmosphere. FTIR results are compared to PAH emission from interstellar sources and absorption components in IR spectra observed toward low-mass young stellar objects (YSOs). Notably, some newly reported broad IR features appear very similar in structure to the plateaus underlying the astronomical 3.2−3.6, 6−9, and 10–15 μm emission plateaus and several of the absorption components found in spectra observed toward YSOs. The studies also indicate that the presence of other species, such as H2O, may significantly impact the radiation products. These results point to the fact that a top-down synthesis of solar system COMs is possible via irradiative processing of PAHs.

Review Article: Unraveling synergistic effects in plasma-surface processes by means of beam experiments.

The interaction of plasmas with surfaces is dominated by synergistic effects between incident ions and radicals. Film growth is accelerated by the ions, providing adsorption sites for incoming radicals. Chemical etching is accelerated by incident ions when chemical etching products are removed from the surface by ion sputtering. The latter is the essence of anisotropic etching in microelectronics, as elucidated by the seminal paper of Coburn and Winters [J. Appl. Phys. 50, 3189 (1979)]. However, ion-radical-synergisms play also an important role in a multitude of other systems, which are described in this article: (1) hydrocarbon thin film growth from methyl radicals and hydrogen atoms; (2) hydrocarbon thin film etching by ions and reactive neutrals; (3) plasma inactivation of bacteria; (4) plasma treatment of polymers; and (5) oxidation mechanisms during reactive magnetron sputtering of metal targets. All these mechanisms are unraveled by using a particle beam experiment to mimic the plasma–surface interface with the advantage of being able to control the species fluxes independently. It clearly shows that the mechanisms in action that had been described by Coburn and Winters [J. Appl. Phys. 50, 3189 (1979)] are ubiquitous.

The effect of surface nano-corrugation on the squeeze-out of molecular thin hydrocarbon films between curved surfaces with long range elasticity

The properties of linear alkane lubricants confined between two approaching solids are investigated by a model that accounts for the roughness, curvature and elastic properties of the solid surfaces. We consider linear alkanes of different chain lengths from to , confined between corrugated solid walls. The pressure necessary to squeeze out the lubricant increases rapidly with the alkane chain length, but is always much lower than in the case of smooth surfaces. The longest alkanes form domains of ordered chains and the squeeze-out appears to nucleate in the more disordered regions between these domains. The short alkanes stay fluid-like during the entire squeeze out process which result in a very small squeeze-out pressure which is almost constant during the squeeze-out of the last monolayer of the fluid. In all cases we observe lubricant trapped in the valley of the surface roughness, which cannot be removed independent of the magnitude of the squeezing pressures.

Bioactive surface modifications on inner walls of poly-tetra-fluoro-ethylene tubes using dielectric barrier discharge

Bioactive surface modification can be used in a variety of medical polymeric materials in the fields of biochips and biosensors, artificial membranes, and vascular grafts. In this study, the surface modification of the inner walls of poly-tetra-fluoro-ethylene (PTFE) tubing was carried out to improve vascular grafts, which are made of biocompatible material for the human body in the medical field. Focus was centered on the cell attachment of the inner wall of the PTFE by sequential processes of hydrogen plasma treatment, hydrocarbon deposition, and reactive plasma treatment on the PFTE surface using micro plasma discharge. Micro plasma was generated by a medium-frequency alternating current high-voltage generator. The preliminary modification of PTFE was conducted by a plasma of hydrogen and argon gases. The hydrocarbon thin film was deposited on modified PTFE with a mixture of acetylene and argon gases. The reactive plasma treatment using oxygen plasma was done to give biocompatible functionality to the inner wall surface. The hydrophobic surface of bare PTFE is made hydrophilic by the reactive plasma treatment due to the formation of carbonyl groups on the surface. The reactive treatment could lead to improved attachment of smooth muscle cells (SMCs) on the modified PTFE tubing. Fourier transform infrared absorption spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, and water contact angle measurement were used for the analysis of the surface modification. The SMC-attached PTFE tube developed will be applicable to in vitro human vasculature-mimetic model systems, and to medical vascular grafts.

Tuning the hydrophobicity of mica surfaces by hyperthermal Ar ion irradiation

The hydrophobicity of surfaces has a strong influence on their interactions with biomolecules such as proteins. Therefore, for in vitro studies of bio-surface interactions model surfaces with tailored hydrophobicity are of utmost importance. Here, we present a method for tuning the hydrophobicity of atomically flat mica surfaces by hyperthermal Ar ion irradiation. Due to the sub-100 eV energies, only negligible roughening of the surface is observed at low ion fluences and also the chemical composition of the mica crystal remains almost undisturbed. However, the ion irradiation induces the preferential removal of the outermost layer of K(+) ions from the surface, leading to the exposure of the underlying aluminosilicate sheets which feature a large number of centers for C adsorption. The irradiated surface thus exhibits an enhanced chemical reactivity toward hydrocarbons, resulting in the adsorption of a thin hydrocarbon film from the environment. Aging these surfaces under ambient conditions leads to a continuous increase of their contact angle until a fully hydrophobic surface with a contact angle >80° is obtained after a period of about 3 months. This method thus enables the fabrication of ultrasmooth biological model surfaces with precisely tailored hydrophobicity.

Adhesion of tungsten carbide reinforced amorphous hydrocarbon thin films (WC/a-C:H) to steel substrates for tribological applications

We have explored the adhesive interlayer structure for a tungsten carbide reinforced amorphous hydrocarbon thin film coating (WC/a-C:H) that demonstrated excellent coating adhesion under highly stressed tribological contact. Transmission electron microscopy (TEM) analysis including cross-sectional imaging, electron diffraction, and energy dispersive spectroscopy was performed on abrupt and gradient interfaces within the multilayer film architecture. Interpretation of these results is aided by quantum mechanical calculations that were performed to investigate bonding interactions of the Cr adhesive interlayer to the Fe substrate surface within a ∼3 nm thick interfacial region. Low levels of oxygen present in the coating deposition chamber during deposition were found at the Fe–Cr interface using high-resolution TEM. Molecular orbital calculations for a linear three-atom molecular model Fe–O–Cr demonstrate the role of O in strengthening Fe to Cr bonding within that interfacial region.

Effects of surface roughness and energy on ice adhesion strength

The aim of this study is to investigate the effects of surface roughness and surface energy on ice adhesion strength. Sandblasting technique was used to prepare samples with high roughness. Silicon-doped hydrocarbon and fluorinated-carbon thin films were employed to alter the surface energy of the samples. Silicon-doped hydrocarbon films were deposited by plasma-enhanced chemical vapor deposition, while fluorinated-carbon films were produced using deep reactive ion etching equipment by only activating the passivation step. Surface topographies were characterized using scanning electron microscopy and a stylus profilometer. The surface wetting properties were characterized by a video-based contact angle measurement system. The adhesion strength of ice formed from a water droplet on these surfaces was studied using a custom-built shear force test apparatus. It was found that the ice adhesion strength is correlated to the water contact angles of the samples only for surfaces with similar roughness: the ice adhesion strength decreases with the increase in water contact angle. The study also shows that smoother as-received sample surfaces have lower ice adhesion strength than the much rougher sandblasted surfaces.

Plasma deposited Pt-containing hydrocarbon thin films as electrocatalysts for PEM fuel cell

High performance electrocatalytic hydrocarbon thin films containing Pt nano-clusters are deposited by simultaneous plasma polymerization of ethylene and sputtering of a Pt target. The proposed synthetic approach largely simplifies the PEMFC electrocatalyst fabrication process with respect to conventional methods. The deposition of the hydrocarbon matrix provides the mechanical support and electronic continuity, ensuring a uniform metal dispersion, avoiding Pt nanoparticle agglomeration. The Pt catalyst is dispersed as small vertically stacked clusters with size less than 10 nm in columnar thin (250–500 nm) films. PEMFC testing with plasma deposited 500 nm thick film and 0.513 mg cm−2 Pt load as anodic electrocatalyst led to a maximum reproducible power density as high as 300 mW cm−2.

NEXAFS microscopy studies of the association of hydrocarbon thin films with fine clay particles

The nature of organic species associated with clay minerals plays a significant role in several processes, from hydrocarbon recovery in oil sands to contaminated soil remediation and water treatment. In this work, we address the use of scanning transmission X-ray microscopy (STXM) in conjunction with near edge X-ray absorption fine structure (NEXAFS) spectroscopy to study the microstructure and chemistry of organic–clay associations in situ. A model system based on methylene blue and illite is used to explore the sensitivity of NEXAFS microscopy to these interactions, and to identify and resolve experimental challenges in these measurements. We find that sample contamination from X-ray induced photodeposition is a significant problem in STXM microscopy, but also that this problem can be substantially reduced with a liquid nitrogen cooled anticontaminator. With appropriate sample preparation and experimental procedures, we find that STXM microscopy is sensitive to thin carbon adsorbates on clay surfaces.

Engineering and validation of a novel lipid thin film for biomembrane modeling in lipophilicity determination of drugs and xenobiotics

BackgroundDetermination of lipophilicity as a tool for predicting pharmacokinetic molecular behavior is limited by the predictive power of available experimental models of the biomembrane. There is current interest, therefore, in models that accurately simulate the biomembrane structure and function. A novel bio-device; a lipid thin film, was engineered as an alternative approach to the previous use of hydrocarbon thin films in biomembrane modeling.ResultsRetention behavior of four structurally diverse model compounds; 4-amino-3,5-dinitrobenzoic acid (ADBA), naproxen (NPX), nabumetone (NBT) and halofantrine (HF), representing 4 broad classes of varying molecular polarities and aqueous solubility behavior, was investigated on the lipid film, liquid paraffin, and octadecylsilane layers. Computational, thermodynamic and image analysis confirms the peculiar amphiphilic configuration of the lipid film. Effect of solute-type, layer-type and variables interactions on retention behavior was delineated by 2-way analysis of variance (ANOVA) and quantitative structure property relationships (QSPR). Validation of the lipid film was implemented by statistical correlation of a unique chromatographic metric with Log P (octanol/water) and several calculated molecular descriptors of bulk and solubility properties.ConclusionThe lipid film signifies a biomimetic artificial biological interface capable of both hydrophobic and specific electrostatic interactions. It captures the hydrophilic-lipophilic balance (HLB) in the determination of lipophilicity of molecules unlike the pure hydrocarbon film of the prior art. The potentials and performance of the bio-device gives the promise of its utility as a predictive analytic tool for early-stage drug discovery science.

Etching of Bacillus atrophaeus by oxygen atoms, molecules and argon ions

The etching of spores of Bacillus atrophaeus by oxygen atoms, molecules and argon ions is investigated in a particle beam experiment. Thereby, the conditions occurring in an argon oxygen plasma are mimicked and fundamental inactivation mechanisms are revealed. It is shown that only the combined impact of argon ions and of O atoms or O2 molecules causes significant etching of the spores. This is explained by the process of chemical sputtering, where an ion induced defect at the surface of the spore reacts with either the incident bi-radical O2 or with an incident O atom. This leads to the formation of CO, CO2 and H2O and thus to erosion. This process is compared to the plasma etching of hydrocarbon thin films as an atomistic model system for the spore coat. It is shown that the etch rate in an inductively coupled argon oxygen plasma is only maximal if both, the electron density and thus the ion flux towards the surface and the O atom flux are optimized simultaneously.

Production of high-density capacitive plasma by the effects of multihollow cathode discharge and high-secondary-electron emission

High-density capacitively coupled plasma with electron density of 1011cm−3 was produced with the effects of the multihollow cathode discharge and the high-secondary-electron emission from radio frequency (rf)-biased electrode using Ar gas. It was found that the optimum pressure was around 3–15Pa. In the case of only multihollow cathode discharge, the plasma density increased from 1.2×1010to8×1010cm−3 with the increasing distance z from the cathode electrode for 5mm<z<15mm. Moreover, plasma density increased with increasing voltage of rf-biased electrode. The rate of deposited amorphous hydrocarbon thin films of about 200nm∕min was attained with the high-density rf plasma enhanced chemical vapor deposition using CH4 gas.

Investigation of hydrocarbon coated porous silicon using PECVD technique to detect CO2 gas

In the present work, we investigate the influence of hydrocarbon (CHx) thin film coating on porous silicon (PS) by plasma enhanced chemical vapor deposition (PECVD) technique to detect CO2 gas. The fabricated CHx/PS heterojunction device shows up to one and two orders of magnitude enhancement in current under CO2 gas exposure. FTIR spectroscopy measurements reveal a remarkable structural modification of the CHx/PS device during CO2 gas exposure. Further, the enhancement of CHx related absorbance bands by a factor 6.2 for the CHx/PS specimen in comparison with PS confirm the good quality of the deposited CHx thin films.

Hydrocarbon film growth by energetic CH 3 molecule impact on SiC (0 0 1) surface

Classic molecular dynamics (MD) calculations were performed to investigate the deposition of thin hydrocarbon film. SiC (1 0 0) surfaces were bombarded with energetic CH 3 molecules at impact energies ranging from 50 to 150 eV. The simulated results show that the deposition yield of H atoms decreases with increasing incident energy, which is in good agreement with experiments. During the initial stages, with breaking Si–C bonds in SiC by CH 3 impacting, H atoms preferentially reacts with resulting Si to form Si–H bond. The C/H ratio in the grown films increases with increasing incident energy. In the grown films, CH species are dominant. For 50 eV, H–C sp3 bond is dominant. With increasing energy to 200 eV, the atomic density of H–C sp2 bond increases.

Plasma-chemical modification of the structure and properties of poly(ethylene terephthalate) track membranes

A process of extraction of the low-molecular products of the synthesis from the poly(ethylene terephthalate) track membranes modified by plasma has been investigated. It is shown that the deposition of a thin polymeric hydrocarbon film by cyclohexane plasma on the membrane surface with preliminary treatment in a plasma of non-polymerizing gases, for example oxygen, allows one to produce membranes possessing a high productivity. Their advantages are much better hydrodynamic properties and a small amount of the low-molecular products of the synthesis extracted by organic solvents.

Effects of applied substrate bias during reactive sputter deposition of nanocomposite tantalum carbide/amorphous hydrocarbon thin films

Nanocomposite tantalum carbide/amorphous hydrocarbon (TaC/a-C:H) thin film composition, structure, and mechanical properties depend on the direct current bias voltage ( V b) level applied to the substrate during reactive sputter deposition. A set of TaC/a-C:H films was deposited across the range V b = 0 to − 300 V with all other deposition parameters held constant except substrate temperature, which was allowed to reach its steady state during the depositions. Effects of V b on film composition and structure were explored, including TaC crystallite size and dispersion using X-ray diffraction and high resolution transmission electron microscopy. In addition, the dependency of stress and hardness on V b was studied with an emphasis on relationships to a-C:H phase structure.