Silicon Carbide Foams Research Articles

Processing of polymer-derived silicon carbide foams and their adsorption capacity for non-steroidal anti-inflammatory drugs

A process is described for preparing light weight and low cost polymer-derived silicon carbide foams by impregnation with polymeric precursor and polyurethane foam, followed by pyrolysis in inert atmosphere. Scanning electron microscopy showed that the resultant silicon carbide foams had a fully open and interconnected porous structure with dense struts. The suitability of these silicon carbide foams for the adsorption of non-steroidal anti-inflammatory drugs and their metabolites was studied. The results showed that silicon carbide foams adsorbed the target compounds and adsorption capacity increases with increasing the contact time between the foam and the analytes investigated.

Low‐Density Open Cellular Silicon Carbide Foams from Sucrose and Silicon Powder

Open cellular SiC foams with low densities were prepared by thermo‐foaming and setting (130°C–150°C) of silicon powder dispersions in molten sucrose followed by pyrolysis and reaction sintering at 1500°C. The bubbles generated in the dispersion by water vapor produced by the –OH condensation was stabilized by the adsorption of silicon particles on the air‐molten sucrose interface. The composition of a sucrose‐silicon powder mixture for producing SiC foam without considerable unreacted carbon was optimized. The sucrose in the thermo‐foamed silicon powder dispersion leaves 24 wt% carbon during the pyrolysis. The sintering additives such as alumina and yttria promoted the silicon‐carbon reaction. SiC nanowires with diameters in the range of 35–55 nm and length >10 μm observed on the cell walls as well as in the fractured strut region were grown by both vapor–liquid–solid and vapor–solid mechanisms. Large SiC foam bodies without crack could be prepared as the total shrinkage during pyrolysis and reaction sintering was only ~30 vol%. The relatively low compressive strength (0.06–0.41 MPa) and Young's modulus (14.9–24.2 MPa) observed was due to the large cell size (1.1–1.6 mm) and high porosity (93%–96%).

Pre-ceramic polymer-derived open/closed cell silicon carbide foam: microstructure, phase evaluation, and thermal properties

The presented article describes a simple technique to fabricate open/closed cell silicon carbide (SiC) foams. The SiC foams, especially pyrolyzed either at 1200 °C (PSiC_12) or 1500 °C (PSiC_15), offered the best thermophysical properties suitable for advance thermal management systems. Both PSiC_12 and PSiC_15 possessed almost equal concentrations of open and closed pores (~22–24 vol%). On the contrary, the 1800 °C-pyrolyzed specimen (PSiC_18) had 40 vol% open pores and only ~9 vol% closed pores. X-ray Micro-CT observation also revealed that the PSiC_15 had significant concentration of closed pores having pore volume in the range of (2–20) × 10−4 mm3. While PSiC_15 contained small β-SiC crystallites of ~13-nm size, PSiC_18 possessed ~55 nm crystallites of α-SiC. From room temperature to 1000 °C, thermal conductivity values of PSiC_12 and PSiC_15 varied from 0.5 to 1.6 W/m K. Both these specimens also offered outstanding oxidation resistance at 1000 and 1500 °C for up to 100 h. Extremely low thermal conductivity, the presence of closed pores that restricted inward diffusion of oxygen even at the elevated temperatures, and the formation of impervious oxide scale at the initial oxidation stage were the key factors behind obtaining such excellent oxidation resistance. On the contrary, due to the presence of much higher channel pores and relatively higher thermal conductivity (~3 W/m K), oxidation of PSiC_18 was found to be much exaggerated resulting in a few orders of magnitude higher oxidation weight gain compared to those obtained for PSiC_12 and PSiC_15.

Modeling of a Solar Receiver for Superheating Sulfuric Acid

A volumetric solar receiver for superheating evaporated sulfuric acid is developed as part of a 100 kW pilot plant for the hybrid sulfur (HyS) cycle. The receiver, which uses silicon carbide foam as a heat transfer medium, heats evaporated sulfuric acid using concentrated solar energy to temperatures of 1000 °C or greater, which are required for the downstream catalytic reaction to split sulfur trioxide into oxygen and sulfur dioxide. Multiple parallel approaches for modeling and analysis of the receiver are used to design the prototype. Focused numerical modeling and thermodynamic analysis are applied to answer individual design and performance questions. Numerical simulations focused on fluid flow are used to determine the best arrangement of inlets, while thermodynamic analysis is used to evaluate the optimal dimensions and operating parameters. Finally, a numerical fluid mechanics and heat transfer model is used to predict the temperature field within the receiver. Important lessons from the modeling efforts are given, and their impacts on the design of a prototype are discussed.

ChemInform Abstract: Silicon Carbide Foam as a Porous Support Platform for Catalytic Applications

AbstractReview: 121 refs.

Selective hydrogenation of cinnamaldehyde over a rotating stirrer reactor made of SiC foam supported Al2O3 and Pt catalysts

A rotating stirrer reactor using aluminum oxide (Al2O3) and platinum (Pt) loaded silicon carbide (SiC) foam as both the stirring blade and active catalyst was applied for selective hydrogenation of cinnamaldehyde (CAL). The variations of the catalyst activity and selectivity against the CAL concentration and rotating speed were studied. The reaction rate increased with the increasing of stirring speed and showed a maximum value at 300rpm, while the highest selectivity of 75% could be achieved at conversion as high as 90% at 100rpm.

An experimental study of [formula omitted] dilution effects on [formula omitted]–[formula omitted] flame stabilization characteristics in a two-section porous medium

N2 dilution effects on CH4–O2 flame stabilization in a two-section porous medium burner were experimentally investigated for its potential application to the destruction of highly N2-diluted CF4. The burner was made up of two axially-stacked silicon carbide foams having different pore sizes. Various flame behaviors, such as stable flame, flashback, blowout, extinction and transient flames, were observed and, for the stable flame, temperature distributions and exhaust gas emissions were measured. Results showed that the flame was only stable in a belt-like region on the domain of fuel flow rate and oxygen concentration. The region boundaries, termed as O2-enriched and O2-deficient limits, were determined for various equivalence ratios. It was found that our burner configuration was able to sustain the submerged combustion even under the N2 dilution amounting to at least 10.4 times of CH4 flow rate. In addition, temperature measurements showed that the stable flames could have their peak temperatures higher than 1600°C, which is known to be a thermal destruction temperature of CF4. These results may indicate that porous media burners can practically applied in destructing highly N2-diluted CF4 with a reduced fuel consumption.

Development of a novel cooking stove based on catalytic hydrogen combustion

A hydrogen based cooking stove was developed and tested in respect to efficiency, functionality and hydrogen safety. Therefore the developed novel combustion unit technology is based on the catalytic oxidation of hydrogen on a platinum coated highly porous reticulated silicon carbide foam ceramics. Compared to other works [1], premixing of hydrogen and air is avoided, thus providing the basis for safe and reliable operation. With this catalytic combustion technology, no initial hydrogen ignition is required, implying a high application security. Separate air and hydrogen supplies make regulation of temperature and power efficient, simple and save.Catalytic hydrogen oxidation is performed already at room temperature; whereby at high hydrogen load, a small hydrogen slip is detected under cold start conditions, nevertheless in operation this becomes negligible. In order to overcome this issue, a hydrogen flow regulation, based on the hydrogen combustion temperature, was introduced.A heat exchanger is integrated into the exhaust line, in order to recover heat for the incoming air, required for the catalytic combustion. This leads to a stove efficiency of up to 80%, measured according to the DIN EN 30-2-1: 2005–08 test standard and greatly exceeds the postulated standard minimum efficiency of 35%. For comfortable and save use, the combustion unit is mounted under a glass ceramic coverage, as common for conventional electrical stoves.

Silicon carbide foam as a porous support platform for catalytic applications

This review provides an overview of the use of foam-structured SiC as a porous support platform in some typical catalytic processes both for gas-phase and liquid-phase reactions, such as H2S selective oxidation, Friedel–Crafts benzoylation and Fischer–Tropsch synthesis.

Experimental Investigation and Optimization of Cutting Force and Tool Wear in Milling Al7075 and Open-Cell SiC Foam Composite

In the present study, aluminium alloy-based metal matrix composites were fabricated by infiltrating Al7075 into a three-dimensional open-cell silicon carbide (SiC) foam using the liquid metallurgy method. The effects of machining variables on the milling force and tool wear during milling of both Al7075 and the open-cell SiC foam metal matrix composite (MMC) using an uncoated carbide cutting tool were studied. The milling experiments were performed based on the Taguchi $${{\rm L}_{27}}$$ full-factorial orthogonal array, and the milling variables were optimized for cutting force and tool wear. The test results showed that the cutting depth was the most significant cutting parameter affecting milling force in the milling of both workpiece materials. Cutting tool wear was directly affected by the cutting depth in the milling of MMC, and the feed rate was the most influential factor on the tool wear in the milling of Al7075. Uncoated carbide tool showed an excellent machining performance below a machining speed of 220 m/min in finish milling Al7075 workpiece material, but excessive edge chipping was observed on the cutting tool surface in the milling of MMCs. Second-order mathematical models with respect to milling parameters were created for prediction of cutting force and tool wear.

Ultralight, Strong, Three-Dimensional SiC Structures.

Ultralight and strong three-dimensional (3D) silicon carbide (SiC) structures have been generated by the carbothermal reduction of SiO with a graphene foam (GF). The resulting SiC foams have an average height of 2 mm and density ranging between 9 and 17 mg cm(-3). They are the lightest reported SiC structures. They consist of hollow struts made from ultrathin SiC flakes and long 1D SiC nanowires growing from the trusses, edges, and defect sites between layers. AFM results revealed an average flake thickness of 2-3 nm and lateral size of 2 μm. In-situ compression tests in the scanning electron microscope (SEM) show that, compared with most of the existing lightweight foams, the present 3D SiC exhibited superior compression strengths and significant recovery after compression strains of about 70%.

Representative elementary volumes required to characterize the normal spectral emittance of silicon carbide foams used as volumetric solar absorbers

The objective of this work is to determine the representative elementary volumes (REVs) required to accurately characterize the normal spectral emittances of SiC open-cell foams and their dependency on the textural features. Because the REVs determined for porosity are often used as radiative REVs, these REVs are also investigated through a parallel study. For this purpose, several numerical tools have been combined. First, a numerical foam generator that provides realistic textures and that enables control over textural parameters such as the porosity and the cell size distribution is presented. Second, a Monte Carlo ray-tracing (MCRT) code that considers the 3D geometry of a foam and its intrinsic optical properties facilitates the computation of their normal spectral emittances. Both numerical tools are validated through comparison with the textural and radiative properties experimentally measured on a real reference α-SiC foam sample using X-ray μ-tomography and infrared spectroscopic measurements, respectively. Then, a set of 3D virtual α-SiC open-cell foams with prescribed textural features (porosity in the range of 0.4–0.9 and cell sizes in the range of 0.5–1.7mm) were investigated, and a large number of MCRT simulations were performed to determine their respective REVs for both porosity and emittance. The findings indicate that the minimal REV for porosity cannot be directly used for determining the normal spectral emittance. Some practical considerations are then proposed to characterize the normal spectral emittance of α-SiC open-cell foams. Finally, a linear relationship that links the normal spectral emittance to the porosity and the intrinsic optical properties of an α-SiC open-cell foam is presented. The results indicate that this relationship is independent of the cell size.

Flow Regime Transition in Open-Cell Solid Foam Packed Reactors: Adaption of the Relative Permeability Concept and Experimental Validation

The trickle-to-pulse flow regime transition in silicon-infiltrated silicon carbide (SiSiC) foam packed fixed bed reactors has been investigated. Based on the film stability concepts of Grosser et al. [AIChE J. 1988, 34, 1850. DOI: 10.1002/aic.690341111] as well as Attou and Ferschneider [Chem. Eng. Sci. 2000, 55, 491. DOI: 10.1016/S0009-2509(99)00344-9], two predictive models have been adapted to foams’ specific geometric parameters. To account for the different nature of solid foams and their interactions with various fluids, the fixed bed characteristics (specific surface area and bed porosity) and fluid specific parameters (gas and liquid density, liquid viscosity, surface tension) have been incorporated in the model. Ergun parameters and static liquid holdup which are required for the modeling of the prevailing tractive forces were determined experimentally. The modeling results were compared to regime transition measurements performed for SiSiC solid foams with different linear pore densities (20, 30...

Rapid Sintering of Ceramics with Gradient Porous Structure by Asymmetric Thermal Radiation

In this study, thermal radiation was employed for sintering silicon carbide foams that achieved a gradient porous structure. The simultaneous use of graphite and carbon fiber reinforced carbon composite (Cf/C) radiators resulted in an axial temperature gradient of ~600°C along the cylindrical sample, as confirmed by both numerical simulation and experimental measurement. By sintering the cylinder top at 1600°C for 5 min, the porous SiC body achieved an axial pore size gradient from ~106 ± 36 μm to ~250 ± 84 μm and an open porosity from 41.4 to 79.8 vol%. This work indicates the potential of sintering by intense thermal radiation technique for rapid manufacturing functionally graded materials through asymmetric assembly of thermal radiators.

Investigation of surface roughness in the milling of Al7075 and open-cell SiC foam composite and optimization of machining parameters

In the present study, aluminum alloy 7075 (Al7075)-based open-cell silicon carbide (SiC) foam composite was fabricated and the machinability of both Al7075 and the open-cell SiC foam Al metal matrix composite was investigated during milling using an uncoated carbide tool. The machining trials were conducted using the Taguchi L27 full-factorial orthogonal array, and the milling parameters were optimized for surface roughness. Analysis of variance was employed to determine the effect of the cutting variables on surface roughness. The experimental results were evaluated by signal-to-noise ratio, 3D surface graphs, artificial neural networks (ANNs) and main effect graphs. The analysis results show that the feed rate was the most significant milling parameter affecting surface roughness of both Al7075 and the open-cell SiC foam composite. Prediction models have been developed for the surface roughness through regression analysis and ANNs. Confirmation experiments were performed to identify the performance of mathematical models, and the surface roughness was predicted with a mean squared error equal to 1.6 and 0.24 % in the milling of Al7075 and open-cell SiC foam composite, respectively. The test result showed that the three-dimensional open-pore SiC foam network reinforcement was restricted the movement of the soft matrix and provided an acceptable surface quality in the milling of MMCs.

Application of La-ZSM-5 Coated Silicon Carbide Foam Catalyst for Toluene Methylation with Methanol

The performance of toluene methylation reaction was studied on H-ZSM-5 catalyst modified with La, Ce and Nb at different percentage loading. It was found that 10% metal loading produced the best performance in the reaction in terms of toluene conversion. The catalyst was coated on silicon carbide foam support which showed better conversion than the pelleted catalyst. Again, among the treated and untreated H-ZSM-5, the La-ZSM-5 catalyst is chosen for the reaction for its highest selectivity towards xylene, the main product. All catalysts were characterized in terms of surface properties, SEM, XRD and NH3-TPD. Kinetic study was done on La-ZSM-5 catalyst with 10% loading. In this kineticstudy, Langmuir Hinshelwood kinetic model with surface reaction as rate controlling step was selected as the rate equation. The activation energy was found to be 47 kJ/mol. © 2015 by Authors, Published by BCREC Group. This is an open access article under the CC BY-SA License (https://creativecommons.org/licenses/by-sa/4.0)

High-efficiency aerosol filters based on silicon carbide foams coated with ceramic nanowires

Abstract This work describes the evaluation of the performance of ceramic foams coated with nanowires as high-efficiency aerosol filters. A process of catalyst-assisted pyrolysis was employed to grow Si 2 N 2 O nanowires in situ on the cell walls of commercial α-SiC foams. The resulting bodies presented a hierarchical reticulated structure comprised of large cells (0.5–5 mm) with struts coated with a layer (8–207 μm thick) of nanowires of 200–400 nm in diameter. Permeability to air flow and particle collection efficiency of coated foams were experimentally assessed and compared with those of filter groups defined by the European standards EN 779:2002 and EN 1822:2009. Coated foams were more permeable than filters from the Coarse and Fine groups, but possessed a collection efficiency for NaCl nanoparticles typical of filter groups EPA (E12) and HEPA (H13). The progressive stacking of samples to increase filter thickness promoted a linear increase in pressure drop, but with an exponential improvement in collection efficiency by the diffusion and interception mechanisms. The integrity of nanowires was maintained after exposure to air flow at 679 °C. These features indicate the applicability of ceramic foams coated with nanowires as aerosol filters with enhanced performance to operate under aggressive airflow conditions, in which conventional glass fiber media are not recommended.

Mechanism and analytical models for the gas distribution on the SiC foam monolithic tray

Silicon carbide (SiC) foam material has been applied as monolithic tray for distillation column in our previous study. A systematic understanding of the gas distribution process on the foam tray should help to the design of commercial application. In this article, local gas holdup distribution and bubble size distribution are used to measure the gas distribution. The local gas holdup is tested by the conductive probe and the number of test point is counted in different local gas holdup. The bubbles are captured by the high‐speed camera to measure the bubble size. Bubble size is calculated as ellipsoidal bubble and counted with different pore sizes. Furthermore, a three‐stage process model is put forward to explain the uneven distribution of gas phase, and verified by the experimental values. The results show that the structure and the thickness of SiC foam is the decisive factor for the gas distribution performance. © 2015 American Institute of Chemical Engineers AIChE J, 61: 4509–4516, 2015

Improvement in underwater acoustic absorption performance of open-celled SiC foam

Abstract Open-celled silicon carbide (SiC) foam has been proven of a promising underwater acoustic absorption material. However, further improvement of its sound absorption capacity especially at low frequency is restricted by the acoustic impedance mismatch between the foam material and the water surrounding it. How to tailor the existing SiC foam in order to achieve an optimized impedance match is a key issue. In this work, the underwater acoustic absorption performance of open-celled SiC foam is studied by both analytical and experimental methods. First, based on Zwikker–Kosten’s theory, the modified Johnson–Allard (J–A) model is proposed to analytically predict the absorption performance of liquid-saturated SiC foam. Then three approaches including (i) backing the test samples by a 30-mm-thick air cavity, (ii) enclosing a certain amount of water and (iii) filling the samples with silicon oil are proposed to improve the underwater sound absorption performance of SiC foam. The underwater acoustic absorbency of the silicon oil filled SiC foam is significantly improved, especially in the low frequency band. A sound absorption coefficient near unity is achieved by this manner over a significant portion of the investigated frequency range, eg. 750–4000 Hz. The foam partially filled with water also shows enhanced sound absorbency at low frequency, but the foams backed by air gap and saturated fully by water display degraded sound absorption capacity.

Performance study of an induced air porous radiant burner for household applications at high altitude

Porous radiant burners are presented as an alternative technology for improving the thermal efficiency of conventional burners. A performance study of an induced air porous radiant burner (IAPRB) with submerged combustion using natural gas was performed at high altitude to assess the feasibility of employing a porous burner operated with induced air in household applications. The experiments were performed in two-layer porous media. The preheating and combustion zones consisted of 400 ppi alumina honeycomb and 90% porosity silicon carbide foam, respectively. Three power per unit area levels, 370 kW/m2, 480 kW/m2 and 670 kW/m2, were evaluated. Pollutant emissions (CO and NOx), the radiation efficiency, the temperature profile along the bed, the primary air rate and the pressure drop across the porous materials were measured. A maximum burner thermal efficiency near 50% was obtained for 370 kW/m2, with a radiation efficiency of 27%. The preheating of the premix caused an increased bed pressure drop, which resulted in a reduction in ambient air entrainment and an air deficiency in the reaction zone. The CO emissions exceeded the standard allowable emissions.