Porous Stack Research Articles

Use of Selective Laser Melting for Manufacturing the Porous Stack of a Thermoacoustic Engine

In this study the authors investigate the possibility of using Selective Laser Melting (SLM) to manufacture the porous element (stack) of a thermoacoustic engine. Two stacks, provided with different internal geometries but with the same hydraulic radii, have been produced: one with parallel plates and one with a new, oblique pin array configuration. The pin array stacks provide smaller amount of viscous losses during the thermal to acoustic power conversion when working fluids with Pr < 1, e.g. air. Furthermore, the oblique configuration is designed to reduce the heat transmission from the hot to the cold surface of the stack, thus to achieve more easily the temperature gradient for the onset of the thermoacoustic phenomenon. In general, the pin array configurations are difficult to manufacture with conventional technologies. However, the SLM provides precise control of the features of the printed object, thus allowing to objectively compare the behavior of the two stacks. To this aim, the temperature gradient between the hot and cold faces of the stacks and the corresponding sound pressure levels measured at 15 cm from the open end of the engine are measured and compared.

Experimental investigations of the performance of a thermoacoustic refrigerator based on the Taguchi method

Conventional refrigeration system poses a major threat to the environment due to emission of harmful gases (CFC, HCFC). Hence, there is a need for an alternative. Thermoacoustic refrigeration, an alternative to conventional refrigeration, offers a wide scope for further research. It functions by passing high intensity sound waves through a porous stack or regenerator placed inside a resonator tube. Due to the pressure pulsations and the oscillatory motion of the gas, a temperature gradient is created on either side of the stack. Heat exchangers utilize this cooling. This paper deals with the fabrication of the model of the system and analyzes the performance in terms of temperature difference, by varying the stack material, its position inside the resonator, type of input wave and the frequency of the wave. Optimization by design of experiments is also done. A maximum temperature difference of 5.42 oC was obtained at the best combination of its parameters. Results obtained from the experiment are in agreement with the results obtained from Taguchi analysis.

Permeability, strength and electrochemical studies on ceramic multilayers for solid-state electrochemical cells.

An electrochemical reactor can be used to purify flue gasses. Such a reactor can be a multilayer structure consisting of alternating layers of porous electrodes and electrolytes (a porous cell stack). In this work optimization of such a unit has been done by changing the pore former composition and the electrode powder pre-treatment. The effect on permeability, mechanical strength and electrochemical behavior was studied in this work. The effects were evaluated by measuring the pressure difference over the samples in relation to the flow through the sample, by the ball on ring method and by electrochemical impedance spectroscopy in air at temperatures between 300 and 450 °C. The resulting structures were also evaluated with scanning electron microscopy.The work showed a dependence on the pore former composition and electrode powder pre-treatment resulting in variations in porosity, strength and flow resistance. A higher porosity gives a lower backpressure. The electrochemical performance shows that both thickness and amount of pore former in the electrolyte is important, but almost no dependence of electrode composition on the polarization resistances within the tested compositions.

Self-Assembled Zinc/Cystine-Based Chloroplast Mimics Capable of Photoenzymatic Reactions for Sustainable Fuel Synthesis.

Prototypes of biosystems provide good blueprints for the design and creation of biomimetic systems. However, mimicking both the sophisticated natural structures and their complex biological functions still remains a great challenge. Herein, chloroplast mimics have been fabricated by one-step bioinspired amino acid mineralization and simultaneous integration of catalytically active units. Hierarchically structured crystals were obtained by the metal-ion-directed self-assembly of cystine (the oxidized dimer of the amino acid cysteine), with a porous structure and stacks of nanorods, which show similar architectural principles to chloroplasts. Porphyrins and enzymes can both be encapsulated inside the crystal during mineralization, rendering the crystal photocatalytically and enzymatically active for an efficient and sustainable synthesis of hydrogen and acetaldehyde in a coupled photoenzymatic reaction.

Self‐Assembled Zinc/Cystine‐Based Chloroplast Mimics Capable of Photoenzymatic Reactions for Sustainable Fuel Synthesis

AbstractPrototypes of biosystems provide good blueprints for the design and creation of biomimetic systems. However, mimicking both the sophisticated natural structures and their complex biological functions still remains a great challenge. Herein, chloroplast mimics have been fabricated by one‐step bioinspired amino acid mineralization and simultaneous integration of catalytically active units. Hierarchically structured crystals were obtained by the metal‐ion‐directed self‐assembly of cystine (the oxidized dimer of the amino acid cysteine), with a porous structure and stacks of nanorods, which show similar architectural principles to chloroplasts. Porphyrins and enzymes can both be encapsulated inside the crystal during mineralization, rendering the crystal photocatalytically and enzymatically active for an efficient and sustainable synthesis of hydrogen and acetaldehyde in a coupled photoenzymatic reaction.

Facile synthesis of Co3O4@MnO2 core–shell nanocomposites for high-performance supercapacitor

Co3O4@MnO2 core–shell arrays hybrid electrode directly grown on Ni foam for supercapacitor was obtained by two-step hydrothermal processes. Co3O4@MnO2 core–shell arrays hybrid electrode exhibited a high specific capacitance of 1920Fg−1 at current density of 1Ag−1. This was attributed to relative high conductivity of the Co3O4 nanowire arrays and the large surface area provided by the ultrathin MnO2 nanosheets. The porous MnO2 nanosheets stack provided numerous channels for rapid diffusion of electrolyte ions and fast electron transport, which enhanced the electrochemical reactions. The synergetic effect between Co3O4 nanowire and MnO2 nanosheets also improved the supercapacitor performance. In addition, the Co3O4@MnO2 core–shell arrangement also exhibited excellent cyclic stability.

A 3D dual pore-system leaching model. Part 1: Study on fluid flow

Two scale pore networks are retrieved from computerised tomographic (CT) images of a leaching column based on the detected dual pore-size distribution. The ‘macro-pore’ network is comprised of the void space between ore particles, where the liquid is allowed to flow as in free channels, while the ‘micro-pore’ network resides in the ore particles, through which the flow path can only be achieved by percolation. The dual pore system refers to the porous ore stack, where both macro- and micro-pore networks exist and two different flow patterns prevail. The characterisation of pore networks is carried out firstly by generating finite element geometries and meshes and simultaneously estimating the porosity; the anisotropic intrinsic permeability of both pore networks is calculated by free flow simulation (using Navier–Stokes equations) within the pore networks based on Darcy's definition of permeability. The dual pore-system is then derived by integrating the micro-pore characteristics into the macro-pore ore stack. Subsequently, with Navier–Stokes equations governing the free flow in the macro-pore network and Brinkman's equations governing the seepage in the micro-pore network, the flow in the dual pore-system model is established and solved. Ultimately, a comparison of the fluid velocity among the uniform flow model, the single pore scale model, and the dual pore-system model is made, and the preferential flow pathways in the heterogeneous models are analysed, suggesting that it is necessary to adopt a dual pore-system model to capture the flow behaviour within a leaching system.

Characterization of Etch Residues Generated on Damascene Structures

For patterned TiN/silicon oxide/low-k dielectric stack, fluorinated etch residues were detected on the TiN surface, the dielectric sidewall and bottom, regardless of the low-k material used in the stack. XPS results showed that they consisted of polymer-based (CFx) residues deposited on trench sidewall and bottom, and metal-based (TiFx) residues mainly deposited on top surface. In terms of post-etch residue removal, the efficiency of various wet clean solutions can be clearly distinguished for CFx, and TiFx using the same patterned porous low-k stack. These results also demonstrate that the removal of both TiFx and CFx residues generated during the plasma is possible in one step with optimized chemical and process.

Characterization of Patterned Porous Low-k Dielectrics: Surface Sealing and Residue Removal by Wet Processing/Cleaning

The results described in this paper first demonstrate key differences between a plasma-exposed blanket porous dielectric sample and a patterned structure in terms of surface sealing induced by the patterning plasma using ellipsometric porosimetry characterization. While the blanket CVD organosilicate glass (nominal k-value = 2.4, ∼20% of porosity) surface was sealed by the C4F8/CF4-based plasma, the surface of the analogous patterned structure of 45 nm 1/2 pitch was found to be completely open after the same patterning plasma process. Second, the surface composition of the patterned feature, type of the residues generated during the plasma etch, and the effect of a subsequent wet clean step are presented. The experimental results demonstrate that the use of a relevant patterned test structure and its characterization represent an appropriate approach for the optimization of the etch and cleaning processes. The removal efficiency of various wet clean solutions, including dilute HF, citric acid, tetramethylammonium hydroxide:H2O2:H2O mixture, and a slightly alkaline formulated mixture, with regard to polymer residues, CFx, and metal-containing residues, TiFx, can be clearly distinguished using the patterned porous low-k stack.

NOx Conversion of Porous LSF15-CGO10 Cell Stacks

A porous electrochemical reactor, made of La0.85Sr0.15FeO3 as electrode and Ce0.9Gd0.1O1.95 as electrolyte, was studied for the electrochemical reduction of NO with Propene. In order to enhance the effect of polarization, the reactor was impregnated with Ce0.9Gd0.1O1.95, CeO2 or Ce0.8Pr0.2O2-d nanoparticles. The HC-SCR on the cells was increased on the impregnated cells, but no electrochemical enhancement of this was observed. The applied overpotential on the impregnated cells changed the oxidation reaction of NO into NO2 which is considered an intermediate in the NO reduction to nitrogen.

Monolithic High Performance Surface Anchored Metal−Organic Framework Bragg Reflector for Optical Sensing

We report the fabrication of monolithic dielectric mirrors by stacking layers of metal–organic frameworks (MOFs) and indium tin oxide (ITO). Such Hybrid Photonic Band-Gap (PBG) Materials exhibit high optical quality (reflectivities of 80%) and are color tunable over the whole visible range. While the ITO deposition is accomplished by using a conventional sputter process, the highly porous MOF layers are deposited using liquid-phase epitaxy (LPE), therefore yielding crystalline, continuous, and monolithic HKUST-1 SURMOF thin films with high optical performance. We demonstrate the optical sensing capabilities of these monolithic and porous Bragg stacks by investigating the chemo-responsive optical properties (PBG shift and modulation of the intensity of the PBG maximum) upon the exposure to different organic solvents.

Electrochemical reduction of NO with propene in the presence of oxygen on LSCoM/CGO porous cell stacks impregnated with BaO

The electrochemical reduction of NO with propene in the presence of 10 % O2 was studied on a La0.85Sr0.15Co0.03Mn0.97O3-Ce0.9Gd0.1O1.95 11-layer electrochemical reactor. BaO was impregnated into the porous reactor, and electrochemical impedance spectroscopy was used for characterisation in the temperature range of 300–400 °C. They were subjected to different magnitudes of polarisations, and the BaO impregnation was shown to have increased the NO reduction with increasing polarisation in the presence of excess oxygen. At 350 °C, more than 60 % was removed and a current efficiency for the cell stack of up to 30 % was achieved. In the presence of propene, the electrochemical promotion on the NOx conversion decreased and only 13 % was removed. A propene oxidation of 35 % was achieved as the highest at 400 °C. The propene acted as a reducing agent on the barium nitrates and caused a greater reduction of the nitrates and hereby releasing more NO than in the gas without propene. This caused an increase in NO in the outlet gas since not all of it was reduced further to nitrogen. The results indicate that the electrochemical reduction of the nitrates when no propene is present is a surface reaction, whereas the propene enables further reduction of the bulk nitrate.

Effect of Co3O4 and Co3O4/CeO2 Infiltration on the Catalytic and Electro-catalytic Activity of LSM15/CGO10 Porous Cells Stacks for Oxidation of Propene

The objective of this work was to study the effect of Co3O4 and Co3O4/CeO2 infiltration on the propene oxidation catalytic activity of a La0.85Sr0.15MnO3/Ce0.9Gd0.1O1.95 electrochemical porous cell stack (11 layers, 5 single cells in series). The effect of the infiltration of Co3O4 and Co3O4/CeO2 on the electrochemical properties of the porous cell stack was also investigated by electrochemical impedance spectroscopy (EIS).Co3O4 and Co3O4/CeO2 exhibited high catalytic activity for propene oxidation. The increase of propene oxidation rate with +4V (0.8V/cell) polarization reached 10% for the Co3O4 infiltrated reactor and 48% of efficiency at 300°C.The Co3O4/CeO2 co-infiltration decreased the reactor polarization resistance, while Co3O4 infiltration had negligible effect on reactor electrochemical performance. The beneficial effect of CeO2 on the electrode activity was attributed to the increased concentration of stable oxygen species on the electrode surface.

Lattice Boltzmann study on thermoacoustic onset in a Rijke tube

Nonlinear thermoacoustic self-excited onset was numerically studied in this work. A lattice Boltzmann model for viscous compressible flow and the implicit-explicit finite difference method were used to develop a solver. Nonlinear onset in an open-open Rijke tube with a constant-temperature stack was simulated with the solver. Based on the numerical results, overall onset process and self-excited standing wave in the Rijke tube are observed. The length of the Rijke tube along the x-direction covers a 1/4 wavelength of the standing wave and the main component of this standing wave is 171.2 Hz. These results agree well with the theoretical prediction. Instantaneous velocity and temperature fields at several phases under the limit cycle are presented and discussed. The maximal Mach number is about 0.035, indicating that the flow in the Rijke tube is a low Mach number compressible flow. This solver can also be applied for simulations of some other complex flows, such as the flow in porous media stack in thermoacoustic engine.

Kerfless layer-transfer of thin epitaxial silicon foils using novel multiple layer porous silicon stacks with near 100% detachment yield and large minority carrier diffusion lengths

Two important aspects for the success of the porous silicon-based layer transfer method in producing kerfless thin (<50µm) silicon foils for future silicon solar modules are addressed in this work: achieving high detachment yield and high minority carrier diffusion lengths. The detachment characteristics of the porous silicon-based lift-off process is studied using finite element modeling as well as experiments. It is shown that for easy detachment and high detachment yield, a low density of thin silicon pillars must be attained in the high porosity detachment layer (HP-DL) after high temperature sintering. This is elegantly achieved by increasing the thickness of the low porosity template layer (LP-TL) which acts as the vacancy supply to increase the post-anneal porosity of the HP-DL. In this way, near 100% detachment yield has been achieved. However, a thicker LP-TL results in a poorer quality epitaxial growth surface. To circumvent this trade-off, novel triple and quadruple layer porous silicon stacks are introduced which decouple the function of the LP-TL that acts as both the template for epitaxy and as the vacancy supply for the HP-DL. In these new stacks, a surface zone of very low void size and density (nearly void-free) is created which allows high quality epitaxy on easily-detachable porous silicon stacks. Minority carrier lifetime measurements on epitaxial foils grown on such a triple layer stack has resulted in an effective lifetime of ~350µs at the injection level of 1015cm−3 which corresponds to a minimum minority carrier diffusion length of ~670µm (> 16 times the silicon thickness). With such high quality epitaxial foils combined with high detachment yield, very high efficiency solar devices on thin silicon substrates would be a reality in the near future.

Modulated porous wick evaporator for loop heat pipes: Experiment

The available studies indicate the significantly enhanced pool boiling heat transfer by the modulated porous wick sintered on the heater wall. This paper sinters the modulated porous wick as the primary wick on the evaporator wall for loop heat pipes (LHPs). Three types of evaporators were fabricated to integrate with other LHP components: MWE (microchannel/wick evaporator), MME (modulated monoporous wick evaporator) and MBE (modulated biporous wick evaporator). Experiments were performed with water as the working fluid at various tilt angles. The major findings are (1) MBE LHP significantly shortens start-up time and stabilizes all temperatures during the steady operation; (2) MBE LHP decreased the evaporator wall temperatures by 20∼50°C at moderate or high heat loads compared with MWE LHP at similar conditions; (3) MBE LHP achieved the evaporator wall temperature of 63°C at the heat load of 200W for the anti-gravity operation, under which the heat flux attained 40W/cm2, which is 1.7∼6.7 times of those reported in references; (4) MBE behaves the nucleate boiling heat transfer at small head loads, and film evaporation heat transfer at moderate or large heat loads; (5) A properly designed MBE LHP achieved better performance when the evaporator is above the condenser; (6) Effect of liquid charge ratios was studied with the best liquid charge ratio of 51.3%. The best geometric parameters for porous stacks and vapor channels, as well as the best particle size were obtained.The MBE multiscale behavior majorly accounts for the performance improvement: small pores (∼μm scale) creating great capillary force for liquid suction, large pores (∼10μm scale) between clusters increasing surface area for liquid film evaporation, and vapor channels (∼mm scale) for vapor venting. Besides, small contact thermal resistance and reduced heat flow path in the evaporator also improve the LHP performance.

Removal of NOx with Porous Cell Stacks with La0.85Sr0.15CoxMn1-xO3+δ-Ce0.9Gd0.1O1.95 Electrodes Infiltrated with BaO

Porous cell stacks with composite electrodes of La0.85Sr0.15CoxMn1-xO3-Ce0.9Gd0.1O1.95 were tested for activity toward selective electrochemical reduction of NOx to N2 in the presence of 10% O2. The cell stacks were produced by tape casting, laminating and sintering the backbone structure followed by infiltration with BaO or La0.85Sr0.15MnO3+δ. The cell stacks were tested in an atmosphere of 1000 ppm NO or NO2 + 10% O2 in Ar with 10% O2 in Ar as reference, and in the temperature range of 250 to 500°C. The cell stacks were investigated electrochemically with cyclic voltammetry and polarization, and the outlet gas composition was monitored. Doping with Co increases the electrodes activity for reducing NOx, and up to 93% conversion in the presence of 10% O2 at 400°C was measured. Infiltration with BaO was necessary to achieve any formation of N2. Square wave polarization increased the activity of the electrodes achieving a current efficiency as high as 20% at 350°C, and this indicates a problem with mass transfer. An atmosphere containing NO2 also increased the activity of the electrodes, which points to a mechanism with a NO2 related intermediate.

Pool boiling heat transfer on uniform and non-uniform porous coating surfaces

Pool boiling heat transfer experiments were performed using acetone as the working fluid at atmospheric pressure on seven heater surfaces, which can be divided into three types. The first, second and third types refer to the plain surface with poor heat transfer, the open channel surface and uniform porous coating surface with moderate heat transfer enhancement, and the 2-D/3-D porous coating surfaces with significant heat transfer enhancement. The open channel surface and uniform porous coating surface had intercrossed boiling curves, with better performance for the uniform porous coating surface due to the enhanced nucleation heat transfer at small or moderate wall superheats, and better performance for the open channel surface at large wall superheats due to the vapor channel provided. The 2-D porous coating surface with the pitch width of 1.6mm yields the best heat transfer performance and the largest critical heat fluxes among the three 2-D surfaces. The 3-D porous coating surface (TS#7) significantly increases the critical heat flux, which can be 3.7 times of that for the plain surface. Boiling curves were shifted to smaller wall superheat region and critical heat fluxes were increased with increases in liquid subcoolings. Two distinct bubble patterns existed for 2-D/3-D porous coating surfaces: continuously-isolated-bubble-escape pattern at small or moderate heat fluxes, and periodically-large-bubble-escape pattern at large heat fluxes. The hollow well at the heater center and/or the porous stacks exposed in liquid are acted as the liquid supply duct at high heat fluxes. This explains the reason that the porous coating surfaces with periodically varied particle layer thicknesses had larger critical heat fluxes than other heater surfaces.

Porous silicon Bragg mirrors on single- and multi-crystalline silicon for solar cells

Porous silicon Bragg mirrors at back-side of single crystalline and multicrystalline silicon solar cell were numerically simulated by transfer matrix method. It allows to choose the optimal parameters of porous stack of bi-layers (indexes of refraction, number of bi-layers) when the increase of photon absorption in 900–1050 nm spectral region is achieved. Application of Bragg mirrors at back-side of single crystalline solar cell can improve the efficiency on more than 0.8% in absolute for 200 μm both-side textured thickness wafer. The simulated results were compared with characteristics of Bragg mirrors fabricated by electrochemical etching of single- and multi-crystalline silicon. It is shown that despite the natural crystallites disorientation the efficient Bragg mirrors can be fabricated on multicrystalline silicon wafers in such way. Maximum measured reflectivity for Bragg mirrors on multicrystalline substrate achieves approximately 62%, whereas for single crystalline silicon the reflectivity in maximum is more than 90%.

STANDING WAVE THERMOACOUSTIC PIEZOELECTRIC SYSTEM AND APPARATUS FOR GENERATING ELECTRICAL ENERGY FROM HEAT ENERGY

A standing wave thermoacoustic piezoelectric apparatus capable of generating electrical energy from heat energy is provided. The standing wave thermoacoustic piezoelectric apparatus includes a housing, a porous stack and a piezoelectric bimorph. The housing comprises a compressible fluid and has a first portion and a second portion. The second portion receives the heat energy from a heat source for creating a temperature gradient between the first portion and the second portion. A cold heat exchanger within the first portion is positioned at one end of the porous stack. The compressible fluid traverses between the first portion and the second portion through the porous stack to generate standing acoustic waves for generating acoustic energy. The piezoelectric bimorph positioned at an end of the first portion opposite to an end of the first portion having the cold heat exchanger, oscillates based on the acoustic energy for generating the electrical energy.