In this paper the results of numerical simulations and experimental studies are presented which describe potential and limitation of applications of single-layer (SLG) and multi-layer (MLG) graphene ...for thermal conductivity enhancement (TCE) of copper. A series of composite structures were studied which are representative of most widely used systems. The influence of structural parameters on the macroscopic thermal conductivity was analyzed, both experimentally and by numerical simulations. Analytical and Finite Element Method modeling were carried out to investigate a wide range of phenomena, including the effect of copper-MLG interface, copper grain size, volume fraction, thickness and orientation of MLG platelets as well as spatial distribution of MLG defined by percolation factor. Both modeling and the experimental results show that the volume fraction of MLG regions, their size, orientation and spatial distribution may significantly affect the thermal conductivity of metal matrix composites. TCE can be obtained for the laminate-like structure or particulate composites with highly aligned MLG regions. The thermal conductivity of such composites is strongly anisotropic and enhanced in the direction perpendicular to the layers. The results obtained in this study predict that SLG will have a negative effect on the thermal conductivity of copper matrix composites.
Display omitted
•Thermal conductivity of copper-graphene composites were studied by modeling and experimental techniques•Spatial distribution and orientation of graphene platelets is crucial for macroscopic thermal conductivity of composites•The enhancement of thermal conductivity is observed for the structures with high anisotropy•Thermal conductivity increases with the number of graphene layers and for single layered graphene always decreases.
In the present paper, we demonstrate how modifications of the microstructure and the chemical composition can influence the electrochemical behavior of cathodes for molten carbonate fuel cells ...(MCFCs). Based on our experience, we designed new MCFC cathode microstructures combining layers made of porous silver, nickel oxide or nickel foam to overcome common issues with the internal resistance of the cell. The microstructures of the standard NiO cathode and manufactured cathodes were extensively investigated using scanning electron microscopy (SEM) and porosity measurements. The electrochemical behavior and overall cell performance were examined by means of electrochemical impedance spectroscopy and single-cell tests in operation conditions. The results show that a porous silver layer tape cast onto standard NiO cathode and nickel foam used as a support layer for tape cast NiO porous layer substantially decrease resistance components representing charge transfer and mass transport phenomena, respectively. Therefore, it is beneficial to combine them into a three-layer cathode since it facilitates separation of predominant physio-chemical processes of gas and ions transport in respective layers ensuring high efficiency. The superiority of the three-layer cathode has been proven by low impedance and high power density as compared to standard NiO cathode.
•New layered MCFC cathodes were designed, manufactured and characterized.•Presence of porous silver layer decreases charge transfer resistance of MCFC cathode.•Presence of nickel foam decreases mass transfer resistance of MCFC cathode.•The combined effects of silver layer and nickel foam is demonstrated.
•SiC crystals were grown by the PVT method with the presence of Ce impurity.•Cerium is the impurity for promotion of the 4H polytype nucleation.•The crystals showed a high, near 100% uniformity of ...the 4H polytype.•High cerium and nitrogen concentrations influenced the crystal’s surface morphology.•The reduced carbonization of the crystals’ backsides was observed.
Effect of cerium impurity in the SiC source material on the 4H-SiC growth was investigated. 4H-SiC crystals were grown on 6H-SiC crystal seeds by physical vapor transport and using the open seed backside method. Cerium is the impurity used for promotion of the 4H polytype nucleation. The optimal amount of CeO2 for the growth of 4H-SiC with a good crystalline quality and without undesired 15R- and 6H-SiC inclusions was identified. The open seed backside method combined with the presence of cerium impurity in the growth chamber were more effective for the crystal growth process due to a lower degradation of the backside surface of the grown crystal.
► Pore size dispersion (not only average) influences other structural parameters of foams. ► Pore size dispersion has an effect on foam properties. ► New model is proposed especially for porosity ...lower than 70%. ► The model proposed here gives results closer to those obtained experimentally.
Voronoi tessellations are widely used to represent various cellular structures found in the nature. In this study we propose using a model based on Laguerre–Voronoi tessellations (LVT) to simulate the geometry of engineering foams. It is demonstrated that geometrical features of the modeled foam structures, such as the number of faces per pore, are close to the ones observed experimentally. The LVT approach, used here, allows for the investigation of the influence of variations in the pore size on the specific surface area and porosity of the foams. Based on the results obtained and an analysis of the models available in literature, corrections to the volume fraction and model of specific surface area of pores is proposed, which are especially important when the foam porosity is smaller than 70%.
The relationships between different structural parameters of the models of foam structures are provided. These parameters are compared with those proposed by other authors and their applicability is verified using commercially available alumina foams.
In the present paper, advanced characterization and modeling techniques are combined to analyze the microstructure of open-porous tape-cast materials and its influence on the permeability. Four ...samples with different pore former content were fabricated via the tape casting method. Subsequent firing of the manufactured green tapes allowed to obtain open-porous structures with porosity in range 45–50%. Quantitative image analysis of 3D micro-computed tomography data was performed and detailed porous microstructure characteristics were obtained. Based on the acquired data, representative model of the open-porous microstructure has been developed. Finite volume method was utilized to calculate the permeability of various microstructural scenarios. Results obtained indicate that porosity, mean pore size and constrictivity directly influence the fluid flow properties. Notable variations in the constrictivity and resulting permeability may be caused by a significant addition of porogen, for which the connectivity of the pores change and creation (or its lack) of free-paths affects the fluid flow.
Display omitted
•Tape-cast open-porous materials with various amounts of porogen were analyzed by micro-CT coupled with image analysis•Models of the tape-cast microstructures were developed. Material scenarios with different porous characteristics were studied•Fluid flow phenomena in several microstructure scenarios were studied based on numerical simulations•Influence of constrictivity on permeability in open-porous materials has been indicated
This paper demonstrates the benefits of using a metallic foam support within molten carbonate fuel cell (MCFC) cathodes. A state-of-the-art fabrication process based on tape casting has been ...developed to produce microporous electrodes with a nickel foam scaffold. Surfactant was added to control the depth to which the slurry infiltrated the foam. New cathodes were used as an alternative to the traditional cathode in the single cell assembly and were tested for power density. Mechanical properties were compared with the current state-of-the-art. The results show that the use of metallic foams for high temperature fuel cell electrodes is beneficial from the technological point of view, especially in larger scale production. It was also found that the resultant continuous metallic structure of the microporous electrodes delivered a slight enhancement to fuel cell power density.
Display omitted
•Metallic foam-supported electrodes for molten carbonate fuel cells were fabricated and characterized.•The new electrodes exhibit superior mechanical strength and flexibility with increased power density.•The proposed innovation also facilitates the manufacturing and assembly of molten carbonate fuel cells.
The effect of annealing on the mechanical and corrosion response of 316L stainless steel produced using laser powder bed fusion (LPBF) was investigated. The as-printed materials were subjected to ...annealing at a temperature of 1000 °C for 2 h. Microstructural observations revealed that after annealing the cellular network inside the grains disappeared, and a slight increase in grain size and the proportion of low-angle grain boundaries (LAGBs) occurred. The most significant microstructural changes occurred with the nano-inclusions, as the annealing resulted in change in their chemical composition, a decrease in their density (number per unit volume), while simultaneously their size increased. All these changes caused a decline in the mechanical properties of the steel and an increase in its susceptibility to localized corrosion.
In this study, the effect of microstructure of porous nickel electrode on the performance of high temperature fuel cell is investigated and presented based on a molten carbonate fuel cell (MCFC) ...cathode. The cathode materials are fabricated from slurry consisting of nickel powder and polymeric binder/solvent mixture, using the tape casting method. The final pore structure is shaped through modifying the slurry composition - with or without the addition of porogen(s). The manufactured materials are extensively characterized by various techniques involving: micro-computed tomography (micro-XCT), scanning electron microscopy (SEM), mercury porosimetry, BET and Archimedes method. Tomographic images are also analyzed and quantified to reveal the evolution of pore space due to nickel in situ oxidation to NiO, and infiltration by the electrolyte. Single-cell performance tests are carried out under MCFC operation conditions to estimate the performance of the manufactured materials. It is found that the multi-modal microstructure of MCFC cathode results in a significant enhancement of the power density generated by the reference cell. To give greater insight into the understanding of the effect of microstructure on the properties of the cathode, a model based on 3D tomography image transformation is proposed.
Display omitted
•Molten carbonate fuel cell cathodes with various microstructures are fabricated.•Materials are extensively characterized before and after operation.•The evolution of pore space due to oxidation and infiltration is described.•Multi-modal microstructure results in doubling the power density of the fuel cell.
The size, shape and surface topology have a strong influence on powders properties, such as: mechanical, optical, catalytic, etc. In addition, when particles have a nanometer size, the dispersion of ...these features plays an important role.
There are a number of techniques, which could be used in order to characterize powders in terms of particle size and shape. However, due to the scale of analysis, well beyond the wavelength of visible light, most of them cannot be applied for investigations of nanopowders.
In this paper, transmission electron microscopy (TEM) image analysis and X-ray methods are presented as promising and complementary techniques. An example of their application to ZrO
2 nanopowder is shown. The advantages and limitations of each method are described.
The unique properties of ceramic foams make them well suited to a range of applications in science and engineering such as heat transfer, reaction catalysis, flow stabilization, and filtration. ...Consequently, a detailed understanding of the transport properties (i.e. permeability, pressure drop) of these foams is essential. This paper presents the results of both numerical and experimental investigations of the morphology and pressure drop in 10ppi (pores per inch), 20ppi and 30ppi ceramic foam specimens with porosity in the range of 75–79%. The numerical simulations were carried out using a GPU implementation of the three-dimensional, multiple-relaxation-time lattice Boltzmann method (MRT-LBM) on geometries of up to 360 million nodes in size. The experiments were undertaken using a water channel. Foam morphology (porosity and specific surface area) was studied on post-processed, computed tomography (CT) images, and the sensitivity of these results to CT image thresholding was also investigated. Comparison of the numerical and experimental data for pressure drop exhibited very good agreement. Additionally, the results of this study were verified against other researchers׳ data and correlations, with varying outcomes.
•We investigate pressure drop of foam filters experimentally and numerically.•10ppi (pore-per-inch), 20ppi and 30ppi foams of porosity 75–79% are used.•Experiments are done in water channel, simulations use D3Q19 MRT-LBM.•Excellent agreement between simulations and experiment is reported.•Comparison to external data and correlations yields varying outcomes.
PDF
