M. Khairul Alam

Processing and characterization of aligned vapor grown carbon fiber reinforced polypropylene

This investigation describes a method for aligning vapor grown carbon nano-fibers suspended in a polymer flow during extrusion to produce an improved thermoplastic composite. A twin-screw extruder was used to shear mix and extrude fiber/polypropylene mixtures through a converging-annular die that generates fiber alignment along the flow direction. The degree of fiber alignment was quantified by using X-ray diffraction. It was shown that fiber alignment could be improved by increasing the residence time in the die channel. Tensile specimens were fabricated by molding the extruded strands and the strength properties of the aligned samples increased with fiber content. The tensile strength improved with greater fiber orientation, however, more fiber alignment had little affect on the modulus. The addition of vapor grown carbon nano-fiber also increased the thermal conductivity and decreased the electrical resistivity.

The Effect of van der Waals and Viscous Forces on Aerosol Coagulation

An analysis is carried out to determine the combined effect of van der Waals and viscous fluid forces on coagulation of spherical aerosol particles in the free molecular, transition, and continuum regimes. The effect of viscous forces is taken into account by modifying the particle diffusion coefficient. An asymptotic solution is substituted for the classical formulation of viscous forces. The results of free molecular and continuum regimes are then extended to the transition size range by an interpolation formula.

Strength prediction of partially aligned discontinuous fiber-reinforced composites

An experimental and theoretical approach has been described for the determination of the strength of partially aligned discontinuous fiber-reinforced composites. The fiber alignment information was obtained as a Gaussian or normal distribution function by using an x-ray-diffraction technique. The distribution function was then used in the composite strength equation to calculate the theoretical strength. This approach was applied to a composite of vapor grown carbon fiber (VGCF) in a polypropylene matrix, and the experimental and theoretical results were compared. As expected, the composite strength increased with increase in fiber volume fraction and the degree of fiber alignment. It was also observed that the composite strength was sensitive to variation in fiber length when the average fiber length was less than the critical fiber length. At higher fiber volume fractions the composite strength was much lower than predicted by theory. This is most likely due to incomplete wetting and infiltration of the VGCF.

Thermal conductivity of thermoplastic composites with submicrometer carbon fibers

This is an experimental study of the thermal properties of polypropylene composites reinforced with aligned Pyrograf III™ vapor-grown carbon fiber. The fiber reinforcement was uniaxially aligned in the composite samples by an extrusion process, thereby producing improved properties along the preferred direction. The density, specific heat, and thermal diffusivity were determined experimentally and used to calculate the thermal conductivity in both the longitudinal and transverse directions of the test specimens. The experimental thermal conductivity results were then compared with those predicted by the Lewis and Nielsen semitheoretical model to evaluate the axial thermal conductivity of the Pyrograf III fiber.

THERMAL CONDUCTIVITY OF GRAPHITIC CARBON FOAMS

Carbon foams are being developed as a new class of thermal management materials. These foams are produced with a wide variety of thermo-mechanical properties; however, very few studies of the properties of carbon foams have been reported in literature. This article reports on an experimental study that was conducted to determine the thermal conductivity of various forms of graphitic carbon foam by using the flash diffusivity and guarded hot plate method. To reduce errors introduced by porous specimen, the test samples were vacuum infiltrated with epoxy. The thermal diffusivity results from the flash diffusivity instrument were used to determine the thermal conductivity of the samples. Some foam samples were determined to have large variations in thermal properties within the sample block. A theoretical and numerical model has been used to examine the effect of the filler epoxy on the experimental results and the influence of pore characteristics on the thermal conductivity of these foams. It was determined that accurate measurement of thermal properties of graphitic foam samples requires careful selection of sample size and measurement technique.

Use of membrane collectors in electrostatic precipitators.

ABSTRACT Membrane collection surfaces, developed and patented by researchers at Ohio University, were used to replace steel plates in a dry electrostatic precipitator (ESP). Such replacement facilitates tension-based rapping, which shears the adhered particle layer from the collector surface more effectively than hammer-based rapping. Tests were performed to measure the collection efficiency of the membranes and to quantify the potential improvements of this novel cleaning technique with respect to re-entrainment. Results indicate that even semiconductor materials (e.g., carbon fibers) collect ash nearly as efficiently as steel plates, potentially indicating that collection surface resistivity is primarily dictated by the accumulated ash layer and not by the underlying plate conductivity. In addition, virtually all sheared particles separated from the collecting membranes fell within the boundary layer of the membrane, indicating extremely low potential for re-entrainment.

Finite Element Analysis of TV Panel Glass during Cooling

Abstract A finite element software has been used to determine the temperature history in forming a TV glass panel during hot pressing in a metal mold and the controlled cooling cycle. The temperature history is then used to calculate the stress and strain history in the glass panel. The goal of the simulation model is to predict the resulting stresses and strains due to forming and cooling that takes place during the manufacturing of the TV panel. DEFORM 3D software has been used to carry out the numeric simulation.

Analysis of Deformation and Residual Stresses in Composites Processed on a Carbon Foam Tooling

Carbon foam is a low cost, durable, light-weight material with a low coefficient of thermal expansion (CTE) and it has cure process compatibility with aerospace composites. Therefore, carbon foam is being investigated as an alternative to traditional composite tooling material for aerospace applications. However, the dramatic differences in properties between traditional tooling materials and carbon foam makes it difficult to fully predict the performance of this novel material as a tooling alternative. This article is a comparative analytical study of the effects of processing of composite geometries on a carbon foam tooling vs. conventional tooling materials. Simulation is carried out on graphite-epoxy composite system using a carbon foam tooling and the stresses and the resulting deformations are predicted for three different geometric features of composites. The results show that both the lower CTE and modulus of the carbon foam tooling can contribute to a reduction of residual stresses in the part and the tooling; however, the mechanical strength of the foam needs to be considered in the tooling design.

Thermal Management Using Composite Structures

The continuing increase in integrated circuits (IC) power levels and microelectronics packaging densities has resulted in a need of materials with higher thermal conductivities and new thermal management solution designs. A base plate design that uses integrated metalized dielectric including Aluminum nitride or polycrystalline diamond (PCD) joined to a diamond/Al composite heat spreader with fins or foam attached by a single step casting is proposed. A numerical study of the design parameters is presented and analyzed. The new material and integrated bonding design can be further improved by changing the materials to a diamond/Copper base plate with polycrystalline diamond dielectric and graphitic foams for convective transport.Copyright

DETERMINATION OF HEAT TRANSFER THROUGH METAL FOILS AND CERAMIC FIBER MATS DURING COMPOSITE FABRICATION

Heat transfer behavior during consolidation of metal matrix composites (MMCs) was investigated. Such composites are often fabricated by putting foils and fiber mats in a layup under pressure and heating to the consolidation temperature; the applied pressure is then increased to the final value. In the present work, experiments were carried out in which plies made of foils and fiber mats are placed together in a layup and pressed between a hot top die and a cool lower die. Various combinations of foils and fiber mats were compressed under different loads while the temperature transients in the lower die were recorded. From the resulting plots, the thermal conductance for different combinations of foils and fiber mats and for a single ply are determined. The deformation history of the foils and fiber are analyzed on the basis of micrographs taken after the compression tests.Heat transfer behavior during consolidation of metal matrix composites (MMCs) was investigated. Such composites are often fabricated by putting foils and fiber mats in a layup under pressure and heating to the consolidation temperature; the applied pressure is then increased to the final value. In the present work, experiments were carried out in which plies made of foils and fiber mats are placed together in a layup and pressed between a hot top die and a cool lower die. Various combinations of foils and fiber mats were compressed under different loads while the temperature transients in the lower die were recorded. From the resulting plots, the thermal conductance for different combinations of foils and fiber mats and for a single ply are determined. The deformation history of the foils and fiber are analyzed on the basis of micrographs taken after the compression tests.