Investigation of different liquid media and ablation times on pulsed laser ablation synthesis of aluminum nanoparticles

Aluminum nanoparticles were synthesized by pulsed laser ablation of Al targets in ethanol, acetone, and ethylene glycol. Transmission Electron Microscope (TEM) and Scanning Electron Microscope (SEM) images, Particle size distribution diagram from Laser Particle Size Analyzer (LPSA), UV-visible absorption spectra, and weight changes of targets were used for the characterization and comparison of products. The experiments demonstrated that ablation efficiency in ethylene glycol is too low, in ethanol is higher, and in acetone is highest. Comparison between ethanol and acetone clarified that acetone medium leads to finer nanoparticles (mean diameter of 30 nm) with narrower size distribution (from 10 to 100 nm). However, thin carbon layer coats some of them, which was not observed in ethanol medium. It was also revealed that higher ablation time resulted in higher ablated mass, but lower ablation rate. Finer nanoparticles, moreover, were synthesized in higher ablation times.     

Fuel quality and operational issues for polymer electrolyte membrane (PEM) fuel cells

Polymer electrolyte membrane (PEM) fuel cells are susceptible to degradation due to the catalyst poisoning caused by CO present in the fuel above certain limits. Although the amount of CO in the fuel may be within the permissible limit, the fuel composition (% CO₂, CH₄, CO and H₂O) and the operating conditions of the cell (level of gas humidification, cell temperature and pressure) can be such that the equilibrium CO content inside the cell may exceed the permissible limit leading to a degradation of the fuel cell performance. In this study, 50 cm² active area PEM fuel cells were operated at 55–60 °C for periods up to 250 hours to study the effect of methane, carbon dioxide and water in the hydrogen fuel mix on the cell performance (stability of voltage and power output). Furthermore, the stability of fuel cells was also studied during operation of cells in a cyclic dead end / flow through configuration, both with and without the presence of carbon dioxide in the hydrogen stream. The presence of methane up to 10% in the hydrogen stream showed a negligible degradation in the cell performance. The presence of carbon dioxide in the hydrogen stream even at 1–2% level was found to degrade the cell performance. However, this degradation was found to disappear by bleeding only about 0.2% oxygen into the fuel stream.     

Target-plane deposition of diamond-like carbon in pulsed laser ablation of graphite

In pulsed Nd:YAG laser ablation of highly oriented pyrolytic graphite (HOPG) at 10⁻⁶ Torr, diamond-like carbon (DLC) are deposited at laser wavelengths of 1064, 532, and 355 nm on substrates placed in the target-plane. These target-plane samples are found to contain varying sp³ content and composed of nanostructures of 40-200 nm in size depending on the laser wavelength and laser fluence. The material and origin of sp³ in the target-plane samples is closely correlated to that in the laser-modified HOPG surface layer, and hardly from the backward deposition of ablated carbon plume. The surface morphology of the target-plane samples shows the columnar growth and with a tendency for agglomeration between nanograins, in particular for long laser wavelength at 1064 nm. It is also proposed that DLC formation mechanism at the laser-ablated HOPG is possibly via the laser-induced subsurface melting and resolidification.     

Femtosecond laser ablation of polypropylene for breathable film

A polypropylene (PP) film was ablated using a femtosecond laser with a center wavelength of 785 nm, a pulse width of 184 fs and a repetition rate of 1 kHz. Increments of both the pulse energy and the shot number of pulses lead to co-occurrence of photochemical and thermal effect, demonstrated by the spatial expansion of rim on the surface of PP. The shapes of the laser-ablated PP films were imaged by a scanning electron microscope (SEM) and measured by a 3D optical measurement system (NanoFocus). And, the gas and water vapor transmission rate, mechanical properties of PP film micropatterned by fs laser pulses was characterized. Our results demonstrate that a femtosecond pulsed laser is an efficient tool for breathable packaging films in modifying the flow of air and gas, where the micropatterns are specifically tailored in size, location and number of which is easily controlled by laser processing conditions.     

Fabrication of transparent double-walled carbon nanotubes flexible matrix touch panel by laser ablation technique

This study reports on the fabrication of transparent double-walled carbon nanotubes (DWNTs) flexible matrix touch panel using the method of laser ablation. We employed an Nd:YAG laser (1064 nm) to pattern transparent DWNT thin film pre-coated on a PET substrate and successfully fabricated a flexible matrix touch panel. By increasing the laser energy, the ablation depth of transparent DWNT flexible thin film is increased but the sheet resistance (Ω/sq) is decreased. When the laser energy intensity reaches 117 mJ/cm², the DWNTs can be completely ablated from transparent DWNT flexible thin film. This method is rapid, simple, applicable to large-area processing and thus is potential for mass production.     

Catalytic activity of carbon-supported iridium oxide for oxygen reduction reaction as a Pt-free catalyst in polymer electrolyte fuel cell

Iridium oxide supported on Vulcan XC-72 carbon black (IrO₂/C) as a cathode catalyst for polymer electrolyte fuel cell (PEFC) has been characterized by transmission electron microscopy (TEM) and X-ray diffraction (XRD) measurement. The IrO₂ particles were 8-160 nm in diameter. The oxygen electroreduction activity was studied by cyclic voltammetry (CV). It was found that IrO₂/C had high oxygen reduction reaction (ORR) activity. The performance of the membrane electrode assemble (MEA) was also tested in a single PEFC and showed that IrO₂/C catalyst would be potential candidates for use as cathode catalyst in PEFC.     

Polymer ablation: From fundamentals of polymer design to laser plasma thruster

UV-Laser ablation of polymers is a well-established method to structure and deposit polymers, but the mechanisms of ablation are still controversial, i.e. photothermal or photochemical processes. An approach to probe the ablation mechanisms and to improve ablation is to incorporate photoactive groups into the polymer structure.The investigation of the ablation behavior of designed triazene polymers showed that the ablation mechanism is always a combination of both photothermal and photochemical processes, but the ratio can be changed by using different polymers and irradiation wavelengths. Also the quality of structures in the triazene polymers is superior at an irradiation wavelength of 308 nm compared to commercially available polymers.Polymers can be designed not only for UV irradiation, but also for applications in the IR range, but with different requirements. One application for designed polymers in the near-IR range is as fuel for the laser plasma thruster, which is used as propulsion system for small satellites. With commercially available polymers the necessary thrust could not be achieved. A specially designed polymer-absorber system for this application produce more energy in the form of thrust, than the laser delivered.     

In situ observation of water distribution and behaviour in a polymer electrolyte fuel cell by synchrotron X‐ray imaging

In situ visualization of the distribution and behaviour of water in a polymer electrolyte fuel cell during power generation has been demonstrated using a synchrotron X‐ray imaging technique. Images were recorded using a CCD detector combined with a scintillator (Gd₂O₂S:Tb) and relay lens system, which were placed at 2.0 m or 2.5 m from the fuel cell. The images were measured continuously before and during power generation, and data on cell performance was recorded. The change of water distribution during power generation was obtained from X‐ray images normalized with the initial state of the fuel cell. Compared with other techniques for visualizing the water in fuel cells, this technique enables the water distribution and behaviour in the fuel cell to be visualized during power generation with high spatial resolution. In particular, the effects of the specifications of the gas diffusion layer on the cathode side of the fuel cell on the distribution of water were efficiently identified. This is a very powerful technique for investigating the mechanism of water flow within the fuel cell and the relationship between water behaviour and cell performance.     

Determination of the optical bandgap for diamond-like carbon films obtained by laser plasma deposition

We have obtained and analyzed the optical transmission spectra of diamond-like carbon films deposited on quartz substrates by pulsed laser deposition (λ = 1064 nm, τ = 20 nsec, q = 4.9·10⁸ W/cm²) under vacuum (p = 2.6·10⁻³ Pa). Based on the spectra obtained, we have estimated the size of the bandgap by the Tauc method, and also have studied the growth dynamics of the coatings formed. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 5, pp. 637–641, September–October, 2007.     

Morphological and chemical modifications and plume ejection following UV laser ablation of doped polymers: Dependence on polymer molecular weight

This work investigates the effect of polymer molecular weight (M_W) on the morphological and chemical modifications and on the plume ejection of doped polymethyl methacrylate (PMMA), and polystyrene (PS) films following irradiation at 248 nm. Micro-bubbles develop in the irradiated films of the low absorbing PMMA-based substrates. The extent and kinetics of the observed morphological changes are respectively larger and last longer in the low M_W polymer, as evidenced by optical microscopy and real-time monitoring of transmission of a CW laser. The changes observed in the Raman spectra upon irradiation indicate that degradation occurs to a higher extent in larger M_W polymers. Laser induced fluorescence (LIF) probing of the plume reveals the presence of NapH and PhenH products from, respectively, NapI- and PhenI-doped films and a slower ejection process in the plume of low M_W polymer. For highly absorbing PS, a less dramatic dependence on M_W is observed. Results are discussed in the framework of the bulk photothermal model, according to which ejection requires a critical number of bonds to be broken.     

Quantitative visualization of a gas diffusion layer in a polymer electrolyte fuel cell using synchrotron X‐ray imaging techniques

A gas diffusion layer (GDL) in a polymer electrolyte fuel cell (PEFC) is quantitatively visualized using synchrotron X‐ray micro‐computed tomography. For three‐dimensional reconstruction, an adaptive threshold method is used. This method is compared with the conventional method, i.e. Otsu's method. Additionally, the spatial and temporal variations of the porosity distribution of the GDL under freeze‐and‐thaw cycles are investigated experimentally. The freeze‐and‐thaw cycles are established simply using a CRYO system and light source illumination, respectively. Structural defects are found to largely affect the porosity of the GDL. In addition, a cyclic porosity variation is observed in the GDL under freeze‐and‐thaw cycles. The heterogeneous porosity is irreversibly decreased with the progress of repetitive cycles.     

A domain decomposition method for two-phase transport model in the cathode of a polymer electrolyte fuel cell

Using Kirchhoff transformation, we develop a Dirichlet–Neumann alternating iterative domain decomposition method for a 2D steady-state two-phase model for the cathode of a polymer electrolyte fuel cell (PEFC) which contains a channel and a gas diffusion layer (GDL). This two-phase PEFC model is represented by a nonlinear coupled system which typically includes a modified Navier–Stokes equation with Darcy’s drag as an additional source term of the momentum equation, and a convection–diffusion equation for the water concentration with discontinuous and degenerate diffusivity. For both cases of dry and wet gas channel, we employ Kirchhoff transformation and Dirichlet–Neumann alternating iteration with appropriate interfacial conditions on the GDL/channel interface to treat the jump nonlinearities in the water equation. Numerical experiments demonstrate that fast convergence as well as accurate numerical solutions are obtained simultaneously owing to the implementation of the above-described numerical techniques along with a combined finite element-upwind finite volume discretization to automatically control the dominant convection terms arising in the gas channel.     

Calibrating the X‐ray attenuation of liquid water and correcting sample movement artefacts during in operando synchrotron X‐ray radiographic imaging of polymer electrolyte membrane fuel cells

Synchrotron X‐ray radiography, due to its high temporal and spatial resolutions, provides a valuable means for understanding the in operando water transport behaviour in polymer electrolyte membrane fuel cells. The purpose of this study is to address the specific artefact of imaging sample movement, which poses a significant challenge to synchrotron‐based imaging for fuel cell diagnostics. Specifically, the impact of the micrometer‐scale movement of the sample was determined, and a correction methodology was developed. At a photon energy level of 20 keV, a maximum movement of 7.5 µm resulted in a false water thickness of 0.93 cm (9% higher than the maximum amount of water that the experimental apparatus could physically contain). This artefact was corrected by image translations based on the relationship between the false water thickness value and the distance moved by the sample. The implementation of this correction method led to a significant reduction in false water thickness (to ～0.04 cm). Furthermore, to account for inaccuracies in pixel intensities due to the scattering effect and higher harmonics, a calibration technique was introduced for the liquid water X‐ray attenuation coefficient, which was found to be 0.657 ± 0.023 cm^?1 at 20 keV. The work presented in this paper provides valuable tools for artefact compensation and accuracy improvements for dynamic synchrotron X‐ray imaging of fuel cells.     

A study of the gasification of carbon black with molten salt of Li2CO3 and K2CO3 for application in the external anode media of a direct carbon fuel cell

The characteristics of gasification reactions for carbon–carbonate mixtures were experimentally investigated at high temperatures up to 900 °C, considering the application of the mixtures to the external anode media of a direct carbon fuel cell. A thermo-gravimetric analysis (TGA) was conducted in either a nitrogen or carbon dioxide ambient environment for Li₂CO₃, K₂CO₃ and a mixture of these two substances with carbon black. Changes in the exit gas composition were also monitored during the heating process. It was shown that gasification in the mixture media occurs much more rapidly than carbonate decomposition at elevated temperatures, even for low concentrations of CO₂. It was also shown that the loading of carbonates to carbon significantly affects the global gasification reaction; it increased the reaction rate by an order of magnitude and decreased its activation energy. Based on the experimental observations, a simplified reaction model of gasification was suggested for the anode media of a DCFC, regarding carbonate-catalysed and metal-catalysed pathways of Boudouard reactions.     

Structural and electrical studies of sodium iodide doped poly(vinyl alcohol) polymer electrolyte films for their application in electrochemical cells

Solid polymer electrolyte films based on poly(vinyl alcohol) (PVA) complexed with sodium iodide (NaI) were prepared using solution cast technique. The structural properties of pure and complexed PVA polymer electrolyte films were examined by X-ray diffraction (XRD) studies. The XRD results revealed that the amorphous domains of PVA polymer matrix was increased with the increase in NaI salt concentration. The variation of film morphology was examined by scanning electron microscopy (SEM) studies. Fourier transform infrared spectral studies for pure and complexed PVA films revealed the vibrational changes that occurred due to the effect of dopant salt in the polymer. Direct current conductivity was measured in the temperature range of 303–373 K, and the conductivity was found to increase with the increase in dopant concentration as well as temperature. Measurement of transference number was carried out to investigate the nature of charge transport in these polymer electrolyte films using Wagner’s polarization technique. Transport number data showed that the charge transport in these polymer electrolyte systems was predominantly due to ions. Using these polymer electrolytes, solid-state electrochemical cells were fabricated. Various cell parameters like open-circuit voltage, short circuit current, power density, and energy density were determined.     

Limit analysis for laser removal of micron contaminant colloidal silicon dioxide particles from the super-smooth optical glass substrate by pulse Nd:YAG laser

Multimode Nd:YAG pulse laser was applied to remove micron and submicron particles by vaporizing a thin paint film pre-coated on super-smooth optical substrate surface. By analyzing the poor absorption of the optical glass substrate to the irradiative Nd:YAG pulse laser, the removal mechanism of contaminated colloidal particles from the super-smooth surface through vaporization of a volatile solid film is described. A limit analysis was proposed to determine the lower and the upper threshold of laser fluence for cleaning the SiO₂ contaminants from super-smooth K8 optical substrate. Relevant experiments on laser cleaning of micron-polishing particles from super-smooth K8 optical substrate confirmed the usefulness of this method in assisting the selection of effective cleaning fluence for accomplishing high cleanliness, which was in a range of 80–90% of the predicted upper threshold.     

微晶硅锗薄膜作为近红外光吸收层在硅基薄膜太阳电池中的应用

采用射频等离子体增强化学气相沉积技术,制备了具有一定晶化率不同Ge含量的氢化微晶硅锗(μcSi1-xGex:H)薄膜.通过Ⅹ射线荧光谱、拉曼光谱、X射线衍射谱、傅里叶红外谱、吸收系数谱和电导率的测试,表征了μc-Si_(1-x)Ge_x:H的材料微结构随Ge含量的演变.研究表明:提高Ge含量可以增强μc-Si_(1-x)Ge_x:H薄膜的吸收系数.将其应用到硅基薄膜太阳电池的本征层中可以有效提高电池的短路电流密度(J_(sc)).特别是在电池厚度较薄或陷光不充分的情况下,长波响应的提高会更为显著.应用ZnO衬底后,在Ge含量分别为9%和27%时,μc-Si_(1-x)Ge_x:H太阳电池的转换效率均超过了7%.最后,将μc-Si_(1-x)Ge_x:H太阳电池应用在双结叠层太阳电池的底电池中,发现μc-Si_(0.73)Ge_(0.27):H底电池在厚度为800 nm时即可得到比1700 nm厚微晶硅(μc-Si:H)底电池更高的长波响应.以上结果体现μc-Si_(1-x)Ge_x:H太阳电池作为高效近红外光吸收层,在硅基薄膜太阳电池中应用的前景.