Pratik Joshi

Plasma assisted hydrophobic coatings on porous materials: influence of plasma parameters

Cellulose-based filter paper was plasma treated for hydrophobicity and the permeation of the coating investigated. A five-layer stack of filter paper was the model porous medium and perfluoromethylcyclohexane (PFMCH) was the model monomer used in this study. Water-drop-absorption time (time needed for certain size droplet of water to be completely absorbed) was recorded as a measure of coating effectiveness. Video contact angle measurements were used to evaluate surface hydrophobicity and x-ray photoelectron spectroscopy was used to analyse the surface chemistry of each surface in the stack. It is clear that the outer surface of normally hydrophilic filter paper can easily be made hydrophobic by treatment with PFMCH plasma. By adjusting plasma power, pressure and time, a filter paper can be treated to have one side hydrophobic (water repellent) and the other side hydrophilic (water absorbent). The control of coating penetration into inner layers needs more careful study: it is seen that plasma permeation depends to a small extent on plasma power, negligible extent on background pressure, but to a very large extent on treatment time. During initial deposition, both, the water-drop-absorption rate and the contact angle, have exponential relationships with plasma time. It is shown that CF2 and CF3 functional groups are related to hydrophobic behaviour and that the contact angle and water-absorption time can be correlated to the total fluorine concentration on the surface. A very small amount of surface F is needed for hydrophobic behaviour.

Structural investigation of graphitic foam

Structural and chemical characteristics of pitch-based graphitic foam have been studied using scanning electron microscopy, transmission electron microscopy (TEM) and x-ray photoelectron spectroscopy. Chemically, the carbon atoms in these materials are found to have identical bonding states as those in pure graphite single crystals. Microstructural studies indicate that they have a cellular morphology with the cell walls made up of graphitic layers. The walls can be smooth or stepped depending upon the orientation of graphitic layers with respect to the cells. Ligaments between neighboring cells and junctions of ligaments (corners of three or more cells) distinctly show layers of graphitic planes, irregular flakes, and beam-like protruding structures made up of folded layers of graphite. The network of interconnecting pores have openings on the cell walls that have ruptured edges and sharp corners. This indicates that they were formed after hardening of the precursor, resulting in brittle fracture of the ...

Thin Films for Coating Nanomaterials

Abstract For nano-structured solids (those with one or more dimensions in the 1-100 nm range), attempts of surface modification can pose significant and new challenges. In traditional materials, the surface coating could be several hundreds nanometers in thickness, or even microns and millimeters. In a nano-structured material, such as particle or nanofibers, the coating thickness has to be substantially smaller than the bulk dimensions (100 nm or less), yet be durable and effective. In this paper, some aspects of effective nanometer scale coatings have been discussed. These films have been deposited by a non-line of sight (plasma) techniques; and therefore, they are capable of modifying nanofibers, near net shape cellular foams, and other high porosity materials. Two types of coatings will be focused upon: (a) those that make the surface inert and (b) those designed to enhance surface reactivity and bonding. The former has been achieved by forming 1-2 nm layer of —C—CF 2 — (and/or CF 3 ) groups on the surface, and the latter by creating a nano-layer of SiO 2 -type compound. Nucleation and growth studies of the plasma-generated film indicate that they start forming as 2-3 nm high islands that grow laterally, and eventually completely cover the surface with 2-3 nm film. Contact angle measurements indicate that these nano-coatings are fully functional even before they have achieved complete coverage of 2-3 nm. They should therefore be applicable to nano-structural solids. This is corroborated by application of these films on vapor grown nanofibers of carbon, and on graphitic foams. Coated and uncoated materials are infiltrated with epoxy matrix to form composites and their microstructure, as well as mechanical behaviors are compared. The results show that the nano-oxide coating can significantly enhance bond formation between carbon and organic phases, thereby enhancing wettability, dispersion, and composite behavior. The fluorocarbon coating, as expected, reduces bond formation, and therefore, effective as an inert layer to passivate nanomaterials.

Profiling of the SiO2-SiC interface using x-ray photoelectron spectroscopy

The implementation of SiC based sensors and electronics for operation in chemically harsh, high temperature environments depends on understanding the SiO 2/SiC interface in field effect devices. We have developed a technique to fabricate wedge polished samples (angle ~ 1x10 -4 rad) that provides access to the SiO 2/SiC interface via a surface sensitive probe such as xray photoelectron spectroscopy (XPS). Lateral scanning along the wedge is equivalent to depth profiling. Spatially resolved XPS images of the O 1s and Si 2p core levels were obtained of the interfacial region. Samples consist of device-quality thermally grown oxides on 4H-SiC single crystal substrates. The C 1s spectrum suggests the presence of a graphitic layer on the nominally bare SiC surface following thermal oxidation.