Kun-Hong Lee

Electrochemical impedance spectroscopy of porous electrodes : the effect of pore size distribution

A new model for the impedance of porous materials was developed. The transmission line model with pore size distribution (TLM-PSD) is based on the transmission line equivalent circuit, considering the effect of pore size distribution (PSD). The PSD was represented by an analytical distribution function, for example, the log normal distribution. With this model, impedance was a sole function of the representative penetrability αμ when the standard deviation σ is constant. Phase angle at low frequency or high penetrability changes from −90 degrees at σ=0 to −45 degrees at σ=infinity. This indicates that a wider distribution leads to more porous characteristics of impedance. Experimental impedance data of a gold powder electrode were successfully fitted with the TLM-PSD.

The effect of pore size distribution on the frequency dispersion of porous electrodes

The transmission line model with pore size distribution (TLM-PSD) developed for blocking electrodes without faradaic reactions in our previous work was reformulated on the basis of the distribution of the penetrability coefficient α0, which is a more generalized concept than the distribution of pore size. Impedances were simulated using the TLM-PSD model for four different pore size distributions (PSD). Although different shapes of the Nyquist curve was obtained depending upon the different PSD, all simulated impedances share a common point that an increase in the width of a distribution (Pσ) leads to a more inclined Nyquist curve at high penetrability (αμ). This is because the effect of small pores on the total impedance increases with Pσ. We fitted the experimental impedance data of a carbon membrane with the TLM-PSDs and the de Levie’s model (TLM-δ). The fitting by TLM-PSD was successful while TLM-δ could not describe the impedance of the porous electrode at low frequency range. In addition, the geometric parameters estimated by TLM-PSD agreed with physically available values.

The generalized van der Waals partition function. II: Application to the square-well fluid

Abstract A formulation of the generalized van der Waals partition function derived earlier is used here, together with insights obtained from computer simulation, to develop two new equations of state for the square-well fluid. One of these equations is remarkably simple in form, and leads to compressibility factor, vapor pressure and orthobaric density predictions which are as good as, or better than, those obtained with the much more complicated equations of state which have been proposed in the past for the square-well fluid. Further, using this equation, with square-well parameters obtained from second virial coefficient data, and only one adjustable parameter, remarkably good predictions are obtained for the P-V-T and saturation properties of argon and methane.

Metal hydride compacts of improved thermal conductivity

Abstract Metal hydrides begin as hydride-forming metal alloys with good thermal conductivity. However, undergoing hydriding/dehydriding reactions and subsequently causing large strain changes, they decrepitate and finally form a powder bed. Such a powder bed exhibits poor thermal conductivity (k eff ∼0.1 W / m K ) and reduces the heat transfer process to and from the bed that occurs with hydrogen absorption and desorption. A newly developed technique reported here (recompressed expanded graphite technique), that allows one to significantly improve the thermal conductivity of a metal hydride, LaNi5, is presented. The compacts with LaNi5 and recompressed expanded graphite were made and their thermal conductivity measurements were taken. Recompressed expanded graphite is used to allow good heat transfer while providing efficient mass transfer. This study reports that the manufactured metal hydride compact has the thermal conductivity in the range of keff∼3– 6 W / m K that shows a greater potential in developing high-power metal hydride devices. It should be pointed out that a minute amount of expanded graphite increases the thermal conductivity of metal hydride significantly.

The generalized van der Waals partition function. III. Local composition models for a mixture of equal size square-well molecules

Abstract New local composition models for mixtures of equal size molecules with differing attractive forces are presented, and compared with our results of Monte Carlo computer simulations for mixtures of square-well molecules which are also reported here. Unlike most previous local composition models, these new models predict random mixing in the high density limit and Boltzmann factor nonrandomness at low density; both limiting behaviors are in agreement with statistical mechanical theory. The predictions of these new models as a function of composition, density and temperature are in good agreement with the Monte Carlo computer simulation results.

Low-density, hydrophobic aerogels

Abstract Low-density, hydrophobic aerogels synthesis consisting of a two-step synthesis, supercritical drying with liquid carbon dioxide and surface modification by vapor-phase methoxylation is described. No alcohol was added in the first stage of oligomer synthesis. Wet gels were produced from tetramethoxysilane or tetraethoxysilane under basic conditions by adding water and NH 4 OH. Non-alcoholic diluents were mixed to control the density of aerogels. The gelation time decreased with decreasing amount of diluents. Wet gels were extracted with liquid CO 2 followed by supercritical drying at 40°C and 100 atm. Aerogel densities of 0.034–0.38 g/cm 3 were produced without cracks. Hydrophilic surfaces of aerogels were modified with methanol vapor. Methoxylation was carried out at 240°C for 10 h. The density of the hydrophobic aerogel was unchanged for 40 days under a humid, open atmosphere. The Brunauer, Emmett and Teller surface area was in the range of 750–870 m 2 /g for aerogel densities of 0.05–0.2 g/cm 3 .

The generalized van der Waals partition function: IV: Local composition models for mixtures of unequal-size molecules

Abstract In this study, we present the results of Monte Carlo computer simulations for binary mixtures of square-well molecules of equal energy but differing size ratios at a variety of temperatures and densities. These data are used to determine the extent of the local composition effects in these mixtures, and to test various local composition models which had been proposed. We then show how, by use of the most successful of the local composition models and the generalized van der Waals partition function, a new two-parameter activity coefficient model and a density-dependent mixing rule can be derived. The new activity coefficient model is found to be quite successful in correlating data for the aqueous nonelectrolyte solutions on which it was tested, with the important advantages of having two parameters which are not intercorrelated and only slightly temperature-dependent. The density-dependent equation of state mixing rule we derived was found, when compared to simulation data, to be of some improvement over the van der Waals one-fluid mixing rule for mixtures of molecules of different square-well depths, but of no advantage for molecules of similar energies but different size.

Enhancement of heat and mass transfer in silica-expanded graphite composite blocks for adsorption heat pumps. Part II. Cooling system using the composite blocks

Abstract Cooling systems using graphite/silica-gel composite blocks were built and their performances were compared with that of the silica gel packed bed. Spatial and temporal profiles of temperature in the composite blocks were obtained in axial and radial directions. The amount of adsorption and specific cooling powers for each composite block were also measured. It was shown that the axial profile of temperature was affected mainly by mass transfer while the radial distribution of temperature was largely due to poor heat transfer. When compared with a pure silica gel packed bed, both heat transfer and mass transfer were enhanced by using the composite blocks. As a result, the good mass and heat transfer of the composite blocks led to enhancement of performances of the cooling systems.

Manufacture of biodegradable packaging foams from agar by freeze-drying

Cellular foams were made from the aqueous solution of agar by freeze-drying. A narrow range (5–20°C min-1) of freezing rate was required to avoid damage to the microstructure of the agar foams. The size of cells in the foam decreased with increasing freezing rate. Agar foams of more than 4 wt% agar content absorbed more energy than a polystyrene foam in compression tests. Foams with a higher agar content absorbed more energy. The behaviour of agar foams in compression tests could be explained by the modified beam theory for cellular foams. Agar foams were thermally stable up to 200°C, and were also stable in a humid environment.

Gas-phase synthesis of AlN powders from AlCl3-NH3-N2

Aluminium nitride powders were synthesized by the gas-phase reaction of AlCl3-NH3-N2 system. The yield of AlN powders from AlCl3 and their crystallinity increased as the reaction temperature and the mole ratio of NH3/AlCl3 were increased. AlN powders were also formed outside the reactor and their crystallinity was amorphous. The yield of AlN powders was as high as 80% when the reactor temperature was 1000 °C and the mole ratio of NH3/AlCl3 was 8. Most of the HCl by-product was recovered as NH4Cl outside the reactor. Powder characteristics, such as shape, size distribution and crystallinity, were also studied.