Gas Phase Diffusion Research Articles

Surface passivation of black silicon phosphorus emitters with atomic layer deposited SiO2/Al2O3 stacks

Black silicon (b-Si) is a promising surface structure for solar cells due to its low reflectance and excellent light trapping properties. While atomic layer deposited (ALD) Al2O3 has been shown to passivate efficiently lightly-doped b-Si surfaces and boron emitters, the negative fixed charge characteristic of Al2O3 thin films makes it unfavorable for the passivation of more commonly used n+ emitters. This work studies the potential of ALD SiO2/Al2O3 stacks for the passivation of b-Si phosphorus emitters fabricated by an industrially viable POCl3 gas phase diffusion process. The stacks have positive charge density (Qtot = 5.5·1011 cm-2) combined with high quality interface (Dit = 2.0·1011 cm-2eV-1) which is favorable for such heavily-doped n-type surfaces. Indeed, a clear improvement in emitter saturation current density, J0e, is achieved with the stacks compared to bare Al2O3 in both b-Si and planar emitters. However, although the positive charge density in the case of black silicon is even higher (Qtot = 2.0·1012 cm-2), the measured J0e is limited by the recombination in the emitter due to heavy doping of the nanostructures. The results thus imply that in order to obtain lower saturation current density on b-Si, careful optimization of the black silicon emitter profile is needed.

A-site Ba-deficiency layered perovskite EuBa1−xCo2O6−δ cathodes for intermediate-temperature solid oxide fuel cells: Electrochemical properties and oxygen reduction reaction kinetics

A-site Ba-deficiency layered perovskite oxides, EuBa1−xCo2O6−δ (EB1−xCO, x = 0.02 and 0.04), have been synthesized by a citric acid-ethylene diamine tetraacetic acid complexation sol-gel method, and evaluated as potential cathode materials for intermediate-temperature solid oxide fuel cells (IT-SOFCs). Room temperature powder X-ray diffraction patterns indicate that the EB1−xCO oxides crystallize in an orthorhombic symmetry with space group Pmmm. Among all of components, EB0.98CO exhibits a good chemical compatibility with Ce0.9Gd0.1O1.95 (CGO) electrolyte, as evidenced by phase analysis of mixed EB0.98CO-CGO after calcining at 950 °C for 12 h in air. Thermal expansion analysis gives an average thermal expansion coefficient of 16.7 × 10−6 K−1 for EB0.98CO. Thermogravimetric measurement confirms the high oxygen nonstoichiometric characteristic of EB0.98CO at elevated temperatures. The electrical conductivity values of EB0.98CO exceed 300 S cm−1 in the temperature range of 100–750 °C. When tested as cathode in IT-SOFCs, the polarization resistance of 0.107 Ω cm2 and the overpotential of 10 mV at current density of 77 mA cm−2 are achieved in the EB0.98CO cathode at 700 °C in air. The EB0.98CO cathode-based anode-supported single cell delivers the maximum power density of 505 mW cm−2 at 700 °C. Finally the rate-limiting steps for oxygen reduction reaction at the EB0.98CO cathode interface are determined to be the charge transfer reaction and gas-phase diffusion process.

Pore network modelling of molecular diffusion in a single‐block model during lean gas injection, investigating the effect of throat sorting

Based on analogy between isothermal drying of porous media and molecular diffusion in a single‐block model containing a volatile liquid during lean gas injection, a detailed procedure is presented for two‐dimensional quasi‐static pore network modelling of diffusion. A modified invasion percolation algorithm is used to account for evaporation of liquid clusters formed during gas injection process. Diffusion in gas phase, capillary‐induced flow in liquid phase, and evaporation in gas‐liquid interface were modelled to trace the movement of gas‐liquid interface and desaturation profile. Using a set of throat sizes with normal distribution, 15 cases of a regular pore network model were proposed by rearranging throat positions in the model. Results of this study indicate that diffusion time, breakthrough time, vapour partial pressure gradient, liquid saturation gradient, liquid cluster frequency, gas‐liquid interface frequency, and evaporation potential are strong functions of throat sorting. Correlation coefficient of throat size with position was used to capture the disorder of porous medium. It was shown that such a correlation coefficient is not capable of describing diffusion characteristics.

Diffusion-Related SOFC Stack Degradation

Solid Oxide Fuel Cell systems for stationary applications should exhibit low degradation (<< 1%/1,000h voltage loss) to ensure long-term operation (> 40,000h). The rather high operation temperature of SOFCs (≥ 700°C) is on the one hand beneficial for oxygen conduction through the ceramic-based electrolyte, but on the other hand all diffusion-related, energetically-driven mechanisms are also increased. This counts less for gas diffusion processes but is more pronounced for solid-state diffusion (surface, bulk, grain boundary). For gas diffusion processes other parameters like gas stream, individual partial pressures and partial pressure gradients are dominating. Within two different stack tests with four-plane short stacks and their intensive post-test characterization two varying diffusion-related degradation mechanisms have been investigated. One is a short-term (~1250h) test with two different chromium evaporation protection layers on the air side metallic interconnect and frame and the other one is a long-term endurance test (~ 35,000h). For the first stack two planes were coated with a manganese oxide layer applied by wet powder spraying (WPS) while the other two planes were coated by atmospheric plasma spraying (APS) with a manganese-cobalt-iron-spinel layer. Thus the protection layers do not only differ by coating technology but also by material. The voltage loss in the planes with WPS-coated interconnect was markedly higher than in those coated by APS. Finally it turned out that the layers microstructure plays the key role in Cr evaporation minimization. In this stack, gas-phase diffusion is dominating degradation. In the long-term operated stack, the influence of a diffusing element from the cathodic contact layer towards and through the electrolyte has been characterized. Manganese, presumably originating from the contact layer (but it’s additionally also part of the protection layer), diffuses via solid-state diffusion and reacts while reaching reducing conditions in the fuel compartment with the zirconia-based electrolyte. This interaction forms a secondary sponge-like porous phase at the boundary electrolyte-anode. This secondary phase causes stresses and leads, after growing to big islands of some ten micrometers in diameter, to electrolyte-anode delamination and finally to complete cell failure. For the Mn-related degradation solid state diffusion has been supposed, as the electrolyte is coated by a dense thin-film diffusion barrier layer which suppresses gas diffusion. Both results indicate that volatile elements (which could either evaporate via gas phase or diffuse via solid state diffusion) should be avoided as they can lead to severe cell, stack and system failure even after thousands of hours of high-temperature operation.

Electrochemical Behavior of Nanostructured La0.8Sr0.2MnO3 as Cathodes for Solid Oxide Fuel Cells

In this work we outline the mechanisms contributing to the oxygen reduction reaction in nanostructured cathodes of La0.8Sr0.2MnO3 (LSM) for Solid Oxide Fuel Cells (SOFC). These cathodes, developed from LSM nanostructured tubes, can be used at lower temperatures compared to microstructured ones, and this is a crucial fact to avoid the degradation of the fuel cell components. This reduction of the operating temperatures stems mainly from two factors: i) the appearance of significant oxide ion diffusion through the cathode material in which the nanostructure plays a key role and ii) an optimized gas phase diffusion of oxygen through the porous structure of the cathode, which becomes negligible. A detailed analysis of our Electrochemical Impedance Spectroscopy supported by first principles calculations point towards an improved overall cathodic performance driven by a fast transport of oxide ions through the cathode surface. Figure 1

Diffusion-Related SOFC Stack Degradation

As part of two different stack tests with four-plane short stacks and their intensive post-test characterization, two varying diffusion-related degradation mechanisms were investigated. The first was a short-term test (~1250h) with two different chromium evaporation protection layers on the air-side metallic interconnect and frame and the second was a long-term endurance test (~ 35,000h). For the first stack, two planes were coated with a manganese oxide layer applied by wet powder spraying (WPS), while the other two planes were coated with a manganese–cobalt–iron spinel layer by atmospheric plasma spraying (APS). The voltage loss in the planes with a WPS-coated interconnect was markedly higher than in those coated by means of APS. Finally, it was shown that the microstructure of the layers plays a key role in minimizing Cr evaporation. In this stack, gas-phase diffusion prevails over degradation. In the long-term stack, severe degradation due to solid-state manganese diffusion was observed. This paper draws an interaction hypothesis.

Formation of a p-type emitter with the involvement of surfactants in GaAs photoelectric converters

The comparative characteristics of photovoltaic converters (of laser radiation) based on gallium arsenide with a p-type emitter formed by gas-phase diffusion in the presence of surfactants (isovalent impurities) and without them are reported. It is shown that the use of indium and phosphorus in the process of the formation of a p–n junction significantly affects the characteristics of the obtained devices.

Microstructure and growth kinetics of silicide coatings for TiAl alloy

In order to improve the oxidation resistance of TiAl alloy, silicide coatings were prepared by pack cementation method at 1273, 1323, and 1373K for 1-3 hours. Scanning electron microscopy (SEM), energy dispersive spectrometry (EDS) and X-ray diffraction (XRD) were employed to investigate the microstructures and phase constitutions of the coatings. The experimental results show that all silicon deposition coatings have multi-layer structure. The microstructure and composition of silicide coatings strongly depend on siliconizing temperatures. In order to investigate the rate controlling step of pack siliconizing on TiAl alloy, coating growth kinetics was analyzed by measuring the mass gains per unit area of silicided samples as a function of time and temperature. The results showed that the rate controlling step was gas-phase diffusion step and the growth rate constant (k) ranged from 1.53 mg2/(cm4·h2) to 2.3 mg2/(cm4·h2). Activation energy (Q) for the process was calculated as 109 kJ/mol, determined by Arrhenius' equation: k = k 0 exp[–Q/(RT)].

Oxygen Reduction Mechanisms in Nanostructured La0.8Sr0.2MnO3 Cathodes for Solid Oxide Fuel Cells

In this work we outline the mechanisms contributing to the oxygen reduction reaction in nanostructured cathodes of La0.8Sr0.2MnO3 (LSM) for Solid Oxide Fuel Cells (SOFC). These cathodes, developed from LSM nanostructured tubes, can be used at lower temperatures compared to microstructured ones, and this is a crucial fact to avoid the degradation of the fuel cell components. This reduction of the operating temperatures stems mainly from two factors: i) the appearance of significant oxide ion diffusion through the cathode material in which the nanostructure plays a key role and ii) an optimized gas phase diffusion of oxygen through the porous structure of the cathode, which becomes negligible. A detailed analysis of our Electrochemical Impedance Spectroscopy supported by first principles calculations point towards an improved overall cathodic performance driven by a fast transport of oxide ions through the cathode surface.

The influence of MOVPE growth conditions on the shell of core-shell GaN microrod structures

A core-shell geometry is employed for most next-generation, three-dimensional opto-electric devices based on III–V semiconductors and grown by metal organic vapor phase epitaxy (MOVPE). Controlling the shape of the shell layers is fundamental for device optimization, however no detailed analysis of the influence of growth conditions has been published to date.We study homogeneous arrays of gallium nitride core-shell microrods with height and diameter in the micrometer range and grown in a two-step selective area MOVPE process. Changes in shell shape and homogeneity effected by deliberately altered shell growth conditions were accurately assessed by digital analysis of high-resolution scanning electron microscope images.Most notably, two temperature regimes could be established, which show a significantly different behavior with regard to material distribution. Above 900°C of wafer carrier temperature, the shell thickness along the growth axis of the rods was very homogeneous, however variations between vicinal rods increase. In contrast, below 830°C the shell thickness is higher close to the microrod tip than at the base of the rods, while the lateral homogeneity between neighboring microrods is very uniform. This temperature effect could be either amplified or attenuated by changing the remaining growth parameters such as reactor pressure, structure distance, gallium precursor, carrier gas composition and dopant materials. Possible reasons for these findings are discussed with respect to GaN decomposition as well as the surface and gas phase diffusion of growth species, leading to an improved control of the functional layers in next-generation 3D V–III devices.

Inclusion of a dithiadiazolyl radical in a seemingly non-porous solid.

Inclusion of the dithiadiazolyl radical PhCNSSN˙ into the dynamically porous metallocycle [Cu2(L1)2Cl4], where L1 is the bidentate ligand 1,3-bis(imidazol-1-ylmethyl)-2,4,6-trimethylbenzene, has been achieved by gas phase diffusion. Single crystal X-ray diffraction, powder X-ray diffraction, UV-visible spectroscopy, EPR and SQUID magnetometry studies confirm inclusion of the radical into this seemingly non-porous material, and illustrate the antiferromagnetic coupling between the paramagnetic host and guest species. The radical guest is readily released by heating or by the addition of solvent (CH2Cl2).

Diffusion in sphere and spherical-cavity arrays with interacting gas and surface phases

Three-dimensional computations of coupled gas and surface diffusion in periodic arrays of solid spheres and spherical cavities are undertaken to assess the role of pore size, void fraction, and pore geometry on permeability. With nano-dimensioned pores, gas-phase diffusion is hindered by the small mean-free path, so the overall permeability hinges on surface diffusion. We prescribe the intrinsic gas-phase diffusivity to be self-consistent with the Knudsen diffusivity obtained from molecular-kinetic simulations reported in the literature. The continuum model of Albaalbaki and Hill then couples the gas and surface phases via an equilibrium adsorption isotherm and kinetic-exchange parameters. Solving these equations in complex pore-scale geometries provides new insights into the coupling. To interpret the results, we decompose the net flux into (i) gas and surface contributions that can be calculated without knowledge of the concentration perturbations to equilibrium, but which depend on the pore geometry, and (ii) counterparts that must be evaluated from integrals of the concentration perturbations to equilibrium. These measures are directly related to the familiar, but less precise, gas and surface tortuosity parameters, which are often used to interpret experiments. The results highlight (i) discontinuous variations in the fluxes when passing through void and solid percolation thresholds, (ii) how the surface flux depends on the specific surface area, and (iii) the pore sizes at which surface transport compares with void transport.

Case Studies in Physiology: The Case of the Giant Giraffe.

N/A.

A Semitransparent and Flexible Single Crystal Si Thin Film: Silicon on Nothing (SON) Revisited.

Ultrathin single crystal Si films offer a versatile vehicle for high performance flexible and semitransparent electric devices due to their outstanding optoelectric and mechanical properties. Here, we demonstrate the formation of an ultrathin (100) single crystal Si film based on morphological evolution of nanoporous Si during high temperature annealing. Square arrays of cylindrical Si pores are formed by nanoimprint lithography and deep reactive etching and then subjected to annealing in hydrogen ambient. By controlling the aspect ratio of nanoporous Si, defect-free single crystal Si membranes with controlled thicknesses from 330 to 470 nm are formed on a platelike void after the annealing. In addition, we investigate the role of oxygen impurities in a hydrogen atmosphere on defect formation on a Si surface and eliminate the oxygen-related defects on Si by controlling gas phase diffusion of oxygen impurities during annealing in a conventional tube furnace. Finally, we demonstrate the transfer of a defect-free, flexible, and wafer scale Si membrane with thickness of 470 nm onto a PDMS substrate, utilizing the platelike void under the membrane as a releaser. The ultrathin flexible Si film on PDMS shows optical transmittance of about 30-70% in visible and near-infrared light.

Investigation on the decrease in the reduction rate of oxygen carriers for chemical looping combustion

Copper oxide on alumina is often used as oxygen carrier for chemical looping combustion owing to its very high reduction rates at lower temperatures and its very good mechanical and chemical stability at not too high temperatures. In this work, the redox kinetics of CuO/Al2O3 have been measured and analysed with many consecutive reduction (50% H2, 50% N2) and oxidation (air) cycles at different temperatures using thermogravimetric analysis. In all the redox cycles, a cyclic steady state in the redox kinetics is observed after several redox cycles, where a sudden decrease in the reaction rate is observed after a certain solids conversion, typically around 75%, depending in particular on the operating temperature. Models developed in the literature fail at predicting these redox kinetics and they are mostly shrinking core/grain type models with an ad-hoc combination and modification of chemical reaction and gas phase diffusion. The oxygen carrier was characterized in detail with different techniques including N2 physisorption, XRD and SEM-EDX to investigate morphological changes in the particle over the redox cycles in order to better understand why all the models developed in literature fail. After the study we can conclude that i) gas-phase diffusion is not playing a limiting role on the redox kinetics despite significant morphological changes in the particle; ii) migration of copper from the inside of the particle to the surface of the particle does not occur or does not influence the kinetics; iii) oxygen uncoupling is not influencing the kinetics being equally fast for Cu2O and CuO, although it can be significant for the final maximum particle conversion; iv) solid phase oxygen diffusion limitations may be responsible for the observed drop in reaction rate.

Analysis of the site-specific carbon isotope composition of propane by gas source isotope ratio mass spectrometer

Site-specific isotope ratio measurements potentially provide valuable information about the formation and degradation of complex molecules—information that is lost in conventional bulk isotopic measurements. Here we discuss the background and possible applications of such measurements, and present a technique for studying the site-specific carbon isotope composition of propane at natural abundance based on mass spectrometric analysis of the intact propane molecule and its fragment ions. We demonstrate the feasibility of this approach through measurements of mixtures of natural propane and propane synthesized with site-specific 13C enrichment, and we document the limits of precision of our technique. We show that mass balance calculations of the bulk δ13C of propane based on our site-specific measurements is generally consistent with independent constraints on bulk δ13C. We further demonstrate the accuracy of the technique, and illustrate one of its simpler applications by documenting the site-specific carbon isotope signature associated with gas phase diffusion of propane, confirming that our measurements conform to the predictions of the kinetic theory of gases. This method can be applied to propane samples of moderate size (tens of micromoles) isolated from natural gases. Thus, it provides a means of studying the site-specific stable isotope systematics of propane at natural isotope abundances on sample sizes that are readily recovered from many natural environments. This method may also serve as a model for future techniques that apply high-resolution mass spectrometry to study the site-specific isotopic distributions of larger organic molecules, with potential applications to biosynthesis, forensics and other geochemical subjects.

Gas and Liquid Phase Diffusivities of Isomeric Metal Complexes Derived from Multifold Ring-Closing Metatheses: Ion Mobility Mass Spectrometry Trumps DOSY NMR

Isomeric octahedral osmium and rhenium complexes of formulas M(CO)y(X)z(P((CH2)n)3P) and M(CO)y(X)z(P(CH2)n−1CH2)((CH2)n)(P(CH2)n−1CH2) (n/M/y/z/X = 14/Os/2/2/Cl, 14/Os/2/2/Br, 14/Re/3/1/Cl, 18/Os/2/2/Br), in which the phosphorus donor atoms occupy trans positions, are examined by the title techniques. The former gyroscope-like complexes are spherical or ovoid in shape, whereas the latter have phosphacycle “flaps” that increase the volume. Both types diffuse at identical rates in C6D6 as assayed by DOSY NMR, but ions derived from the latter exhibit up to 10% slower drift times in the gas phase by ion mobility mass spectrometry (IMMS). Thus, IMMS, which is seldom applied to isomeric organometallic or coordination compounds, provides superior differentiation.

Aqueous phase hydrogenation of levulinic acid using a porous catalytic membrane reactor

Membrane reactors offer an alternative approach for conducting three-phase heterogeneous chemical reactions. The membrane acts as a liquid/gas phase contactor, while also serving as the support for a solid catalyst. A significant benefit from this approach is circumvention of gas phase dissolution and diffusion in the liquid phase to reach catalytic sites. This method of gas phase mass transfer allows a significant reduction in operating pressure compared to traditional three-phase reactors that often require higher gas pressures due to low gas solubility and diffusivity in the liquid phase. The membrane reactor in this work consists of a porous expanded polytetrafluoroethylene (ePTFE) membrane with deposited Ru catalyst particles. The reaction studied is the aqueous phase hydrogenation of levulinic acid to produce γ-valerolactone. The highly hydrophobic PTFE material provides an almost impermeable barrier to the liquid phase while allowing hydrogen gas to freely transport through the pores to reach catalytic sites located at the liquid/membrane interface. The reaction kinetics displayed by the membrane reactor favorably compare to those of a packed bed reactor (PBR). In terms of hydrogen pressure the maximum catalytic benefit in comparison to the PBR is obtained at pressures greater than 0.7bar, and a more pronounced and continuously increasing catalytic benefit is obtained with increasing temperature.

Analytical expression for gas-particle equilibration time scale and its numerical evaluation

We have derived a time scale τeq that describes the characteristic time for a single compound i with a saturation vapour concentration Ceff,i to reach thermodynamic equilibrium between the gas and particle phases. The equilibration process was assumed to take place via gas-phase diffusion and absorption into a liquid-like phase present in the particles. It was further shown that τeq combines two previously derived and often applied time scales τa and τs that account for the changes in the gas and particle phase concentrations of i resulting from the equilibration, respectively. The validity of τeq was tested by comparing its predictions against results from a numerical model that explicitly simulates the transfer of i between the gas and particle phases. By conducting a large number of simulations where the values of the key input parameters were varied randomly, it was found out that τeq yields highly accurate results when i is a semi-volatile compound in the sense that the ratio of total (gas and particle phases) concentration of i to the saturation vapour concentration of i, μ, is below unity. On the other hand, the comparison of analytical and numerical time scales revealed that using τa or τs alone to calculate the equilibration time scale may lead to considerable errors. It was further shown that τeq tends to overpredict the equilibration time when i behaves as a non-volatile compound in a sense that μ > 1. Despite its simplicity, the time scale derived here has useful applications. First, it can be used to assess if semi-volatile compounds reach thermodynamic equilibrium during dynamic experiments that involve changes in the compound volatility. Second, the time scale can be used in modeling of secondary organic aerosol (SOA) to check whether SOA forming compounds equilibrate over a certain time interval.

Electrochemical response of several cathode configurations prepared with Ba0.5Sr0.5Co0.8Fe0.2O3-δ and Ce0.9Gd0.1O1.95 for IT-SOFC

The electrochemical response of Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) electrodes prepared by an acetic acid-based gel route has been investigated by impedance spectroscopy (IS) as a function of temperature (400 ≤ T ≤ 900 °C) and oxygen partial pressure (1 × 10−3 ≤ pO2 ≤ 1 atm). Several electrode configurations were studied using Ce0.9Gd0.1O1.95 (GDC) as the electrolyte. These consisted of one BSCF layer (cell A), a BSCF layer with an intermediate porous GDC layer (cell B), and graded electrodes using a composite BSCF+GDC, with variations in the surface area of GDC (cells C and D). The optimum heat treatment for the electrode assemblages was determined to be around 850–900 °C. Analysis of the impedance spectra shows that at T ≥ 600 °C a low frequency (LF) contribution, associated to the gas phase diffusion is systematically the rate-limiting step. All the electrodes show an intermediate frequency (IF) arc related to mixed processes. For cells A and B, the IF response is related to the oxide ion transfer at the electrode/electrolyte surface and the charge transfer at the electrode surface, while for cells C and D the mixed process involves the charge transfer and the molecular oxygen dissociation at the electrode surface.