Microstructure porosity

Simulating the appropriate size, geometry and architecture of natural extracellular matrix is of critical importance for tissue engineering and three-dimensional (3D) tissue culture. Essential parameters for tissue scaffolds are microstructures, porosity, pore size, surface area / surface chemistry and mechanical properties (35). With these properties in mind, several iterations of scaffolds have been produced and evaluated. 

Concrete Microstructure Porosity and Permeability. A model has been developed that lays the foundation for relating porosity to permeability. This is based on knowledge gained from previous work as well as experimental and theoretical input from the present program. A linear combination of log-normal distribution may be used to define the pore structure. 86 pages. SHRP-C-628... 

Although the nucleation of cavities does not seem to be well understood at present it is clear that cavitation depends on microstructure. Porosity and second-phase particles, which are sources of stress concentration (see Chapter 18), can act as nucleation sites for cavitation and subsequent crack growth. Remember pores can be found in most ceramics even pore-free materials such as hot-pressed alumina may contain small pores. Cavitation also occurs in ceramics with IGFs. Nucleation of the cavities will usually occur at regions where the IGF is not homogeneous, e.g., nonwetted regions, gas bubbles, and impurity particles. 

Overpotential of Ni-YSZ cermet anode is significantly less compared to other ceramic fuel cell components. Normally, planar SOFCs having standard cell components is capable of producing more than 500 mW/cm of power. However, a careful control of the anode substrate fabrication could even deliver very high power densities, even up to 1.8 W/cm ( 3.5 A/cm at 0.5 V) at 800°C (Kim et al. 1999). The detailed information and fundamental studies on this aspect is available in numerous articles—calculations on the optimal morphology, microstructure, porosity and thickness of such cermet anodes have been reported by Minh et al. (1995), Costamagna et. al. (1998),... 

To further characterize the event it is first necessary to identify critical features of the initial configuration that will strongly influence the process. For powder compacts, the most obvious features are the morphological characteristics of the powders, their microstructures, and the porosity of the compact. For solid density samples, the grain structure, grain boundaries, defect level, impurities, and inclusions are critical features. 

Porous ultrafine tin oxide ethanol gas sensors92 in the form of a thin film have been prepared from tin alkoxide by the sol-gel process. The microstructural evolution of the tin oxide films, which affected the ethanol gas-sensing properties of the films, was investigated as a function of firing temperature and solution concentration. Theoretically, it was expected that ethanol gas sensitivity would increase monotonically with decreasing film thickness, but experimental results showed a maximum sensitivity at about 70 nm. The sudden decrease of the sensitivity below the thickness of 70 nm seemed to be due to the sudden decrease of film porosity, i.e., the sudden decrease of the number of the available sites for the oxidation reaction of ethanol molecules. Thus, it seemed that below the thickness of 70 nm, the sensitivity was governed by microstructure rather than by film thickness. 

The fabrication procedure affects the product s microstructure including grain size, grain-boundary width, and porosity. In addition, different procedures introduce various amounts of impurities to the product. Therefore, the electrical conductivity and activation energy are affected by the fabrication procedure since, as mentioned above,... 

The general requirements for an SOFC anode material include [1-3] good chemical and thermal stability during fuel cell fabrication and operation, high electronic conductivity under fuel cell operating conditions, excellent catalytic activity toward the oxidation of fuels, manageable mismatch in coefficient of thermal expansion (CTE) with adjacent cell components, sufficient mechanical strength and flexibility, ease of fabrication into desired microstructures (e.g., sufficient porosity and surface area), and low cost. Further, ionic conductivity would be beneficial to the extension of... 

The microstructure, properties, and performance of Ni-YSZ anodes depend sensitively on the microscopic characteristics of the raw materials (e.g., particles size and morphology of NiO and YSZ powders). The particle sizes of the starting YSZ powders vary usually from 0.2 to 0.3 pm, whereas those for the NiO powders are 1 pm. The Ni to YSZ volume ratio usually varies from 35 65 to 55 45. For example, the reported Ni to YSZ volume ratios include 34 66 [20, 21], 40 60 [24], 43 57 [22], and 55 45 [23], For a bilayer anode, the functional anode layer in contact with the electrolyte contains 45 to 50 vol% Ni, whereas the anode support layer has 35 to 40 vol% Ni [25, 26], A pore former is usually added to tailor the shrinkage (for the cofiring) and to achieve sufficient porosity (>30 vol%) in the anode or the anode support layer. 

In addition to bilayered electrodes with a functional layer and a support layer, electrodes have also been produced with multilayered or graded structures in which the composition, microstructure, or both are varied either continuously or in a series of steps across the electrode thickness to improve the cell performance compared to that of a single- or bilayered electrode. For example, triple-layer electrodes commonly utilize a functional layer with high surface area and small particle size, a second functional layer (e.g., reference [26]) or diffusion layer with high porosity and coarse structure, and a current collector layer with coarse porosity and only the electronically conductive phase (e.g., reference [27]) to improve the contact with the interconnect. 

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