Ali Ata

Capillary forces between surfaces with nanoscale roughness

The flow and adhesion behavior of fine powders (approx. less than 10 microm) is significantly affected by the magnitude of attractive interparticle forces. Hence, the relative humidity and magnitude of capillary forces are critical parameters in the processing of these materials. In this investigation, approximate theoretical formulae are developed to predict the magnitude and onset of capillary adhesion between a smooth adhering particle and a surface with roughness on the nanometer scale. Experimental adhesion values between a variety of surfaces are measured via atomic force microscopy and are found to validate theoretical predictions.

Role of surface roughness in capillary adhesion

The adhesion forces between a smooth spherical particle and flat surfaces of alumina, silver, and titanium-coated Si wafers were measured with an atomic force microscope (AFM) under various humidity conditions. The results showed that there is a discrepancy between the experimentally observed and the theoretically predicted values of capillary adhesion forces. The reason for the discrepancy is explained considering the relative humidity of the surrounding atmosphere and the surface roughness profiles of the contacting surfaces. Two geometrical configurations to define the contact region profile are suggested. The equations to calculate the capillary adhesion forces were modified using the new geometries. The discrepancy was largely eliminated using the modified equations.

Rotating Disk Electrode Study of Supported and Unsupported Catalysts for PEMFC Application

The electrochemical study of different supported and unsupported Pt-based catalysts used in polymer electrolyte fuel cells (PEMFCs) has been made by means of thin film rotating disk electrode (RDE) method. The comparison of electrochemical surface area (ESA) was made using cyclic voltammetry measurements in H2SO4 or HClO4 at room temperature and scanning rates of 20 and 100 mV/ s. Oxygen reduction activity for aerogel-supported and carbon-supported Pt catalysts was evaluated at various rotation speeds in the range of 0 – 2500 rpm and compared to the catalytic activity of unsupported Pt-black catalyst. The calculated values of Levich constant for the oxygen reduction reaction (ORR) indicate dependence on the applied voltage and either 4 or 2‐4 electron transfer mechanism. The hydrogen peroxide by-product formation was determined with a rotating ring disk electrode (RRDE) and was observed to take place mostly at voltages below 0.6 V due to the limitations from mass transport effects. The results obtained at different Pt loadings in the range of 10– 46.5 wt % Pt demonstrated that XC-72 and aerogel-based catalysts are foreseen to have higher ESA in comparison to other tested supported and unsupported commercially available catalysts. DOI: 10.1115/1.2349524

First-Principles Study of Thermoelectric Properties of Covalent Organic Frameworks

Covalent organic frameworks (COFs) are new emerging functional porous materials. Strong covalent bonds result in molecular building blocks that can be arranged in layered two-dimensional (2D) or three-dimensional (3D) periodic networks. However, to the best of our knowledge, there have been no reports on experimental and theoretical studies of thermoelectrical properties of COFs to date. Therefore, density functional theory (DFT) and the Boltzmann transport equation have been applied in this work to calculate the semiclassical transport coefficients for phthalocyanine (Pc)-based NiPc, NiPc-benzothiadiazole (BTDA), and Pc COFs. Owing to the well-ordered stacking of the phthalocyanine units and linkers in these compounds, charge-carrier transport is facilitated in the stacking direction. In all studied compounds, the highly directional character of π-orbitals provides band-structure engineering and produces a type of low-dimensional hole transport along the stacking direction. All studied compounds are indirect semiconductors. The low-dimensional transport of holes and the localized states in both valence and conduction bands prevent the electron–hole compensation effect in the Seebeck coefficients, correlating with the large Seebeck coefficients of the studied compounds. Insertion of the electron-deficient building block benzothiadiazole in the NiPc-BTDA COF leads to positive Seebeck coefficients along the a-, b-, and c-directions. The relaxation time was estimated in our investigations from DFT band-structure calculations and the experimentally defined mobility, leading to determination of the electrical conductivity and electronic contribution to the thermal conductivity, as well as figure of merit (ZT) estimation. Ni atom provided greater electrical conductivity along the c-direction in comparison with metal-free Pc COF, and NiPc COF showed the highest thermoelectric performance among the studied COFs.

Magnetically assisted impaction coating process to synthesize engineered particulates with controlled surface characteristics

A novel dry coating technique called magnetically assisted impaction coating (MAIC) to synthesize engineered particulates with controlled surface coatings has been developed. In this method, the impaction of magnetic particles, core particles and secondary particles results in dry coating on the surface of the core particle. The attachment of particles to surfaces and factors affecting the efficiency of the process have been discussed.

The Effect of Humidity on the Adhesion Behavior of AC Anodised Aluminium

The effect of humidity on the surface forces of hot AC anodised AA6060 alloy was studied. The surface adhesion forces were measured in various humidity environments using an atomic force microscope (AFM). The environmental durability of the joints bonded at different humidity conditions was investigated using wedge test experiments. The results from AFM and durability tests indicate that there is a strong capillary effect at around 70–80% relative humidity. This effect was attributed to the change in the adsorption behavior of water on the anodised oxide surface forming bulk liquid and hence dissociating the bonds across the interface. At the lower humidity levels below 60% RH, no capillary effect was observed and the total adhesion forces followed the dry case values.

Hydrogen Generation from Alkaline Solutions of Methanol and Ethanol by Electrolysis

In this study, electrolysis of alkaline methanol and ethanol solutions was carried out on platinized electrodes in actual cells with different cell preparation and catalyst coating methods. The electrolysis reaction products were analyzed by X-Ray and titrimetric methods. The gas phase products were analyzed by Gas Chromatography technique. Thermodynamic calculations and analysis results suggest that the carbonate formation is highly favorable in alkaline KOH solutions. However, methane formation reaction along with carbonate and oxygen formations may be possible during ethanol electrolysis. Alkaline methanol solutions were electrolyzed easily using platinum coated carbon paper electrodes. It was found that Nafion based MEAs are not essential for electrolysis of alkaline solutions of methanol and ethanol. One of the best performances was obtained using micro porous polyethersulfone membranes in a special cell configuration.