Sarwan S. Sandhu

A kinetic model of the pyrolysis of phenolic resin in a carbon/phenolic composite

The potential for damage during the pyrolysis of phenolic/carbon composite material in the processing of carbon/carbon components is high. Accurately modeling the kinetics of the pyrolysis reaction could lead to improvements in processing. A mechanism for the pyrolysis of phenolic/carbon composite describing the reaction as occurring in three major regions is the foundation of a kinetic model. The peak separation of thermogravimetric weight loss data has been used to separately evaluate the three major regions of the reaction. Kinetic parameters for each region have been determined using a thermogravimetric data analysis method developed to account for the effect of temperature variation on the thermodynamic state of the pyrolyzing composite. The resulting model is shown to accurately predict the extent of reaction of the pyrolyzing phenolic/carbon composite as a function of temperature.

Testing of Lithium-Ion 18650 Cells and Characterizing/Predicting Cell Performance

The performance of lithium-ion cells, as determined from in-house testing, is primarily a function of cell design/materials, charge/discharge rate, ambient temperature, and the number of charge/discharge cycles. Testing of lithium-ion 18650 cells was performed in order to characterize their behavior and to eventually predict the performance of lithium-ion cells of various sizes. AC impedance spectroscopy was used to determine the interfacial resistance of the lithium-ion cells as a function of temperature, state-of-charge, and cycle number. From these results, a nonisothermal mathematical model was developed and preliminary results are presented.

Direct methanol polymer electrolyte fuel cell modeling: reversible open-circuit voltage and species flux equations

By the application of thermodynamic fundamentals of phase and electrochemical reaction equilibria, a mathematical equation has been developed to predict the reversible open-circuit voltage of a direct methanol fuel cell (DMFC), in the absence of electrode poisoning and methanol crossover. The equation accounts for the effect of the nonideal behavior of the fluid phases on the reversible open-circuit voltage. Sample computed results are presented. The species (CH 3 OH, H 2 O, H + ) flux equations to compute molar fluxes through the polymer electrolyte membrane of a DMFC are also presented.

An empirical intrinsic chemical kinetic model for the carbon-carbon composite pyrolysis process

Lightweight, high mechanical strength matrix-carbon fibre reinforced composites are manufactured from phenol-formaldehyde matrix/carbon fibre composites by the pyrolysis process. Phenol-formaldehyde matrix/carbon fibre composites are heated in an inert gas environment to high temperatures, for example, up to 800°C in the so-called first carbonization process. Evolution of gaseous species in a relatively thick composite, during the transient heating period, leads to internal pressure build-up, resulting in the composite delamination. The composite delamination depends on the rate of thermal energy flux into the composite material, production of gaseous species via intrinsic chemical kinetics, and transport of the produced gases to the composite environment through its porous structure. A refined transient process model, incorporating the effects of intrinsic chemical kinetics and transport phenomena of heat and mass, still needs to be developed to predict/control the carbonization process to avoid delamination of a composite of thickness greater than 0.001 m. To this end, the overall intrinsic chemical kinetic model being presented here was developed using the experimental data, free of mass and heat transfer effects, on the pyrolysis of a phenol-formaldehyde resin [1]. Insight gained from the thermogravimetric data analyses [2-5] on thermal degradation of polymeric materials was helpful in the chemical kinetic model development. The molecular structure of the matrix changes, with simultaneous evolution of water vapour, methane, hydrogen, carbon monoxide, carbon dioxide, etc. via intrinsic chemical reactions, during the pyrolysis process employed to manufacture a carbon-carbon composite from a phenol-formaldehyde resin [1, 6-9]. Based on this information, it is reasonable to consider that the activation energy of the intrinsic overall pyrolysis reaction is a function of the composite material temperature. The overall pyrolysis reaction rate with the assumed first-order is given in terms of the rate of change of the reaction extent, ~, as:

Thermodynamic equations for a model lithium-ion cell

Fundamentals of classical thermodynamics of electrochemical systems have been employed to formulate mathematical equations for a model lithium-ion and lithium electrode concentration cell. Equations have been formulated to determine the energetic lithium interaction coefficients in the solid lithium-ion electrode phases and the salt mean activity coefficient in the electrolytic solution used in the model cell from the measured experimental data. In addition, a mathematical equation to predict the open-circuit voltage of the model cell for the case of differing electrolyte compositions in its porous electrodes has been developed.

Diffusion-Limited Self-Discharge Reaction in the Hubble Space Telescope Battery

Abstract The self-discharge rate of aerospace flight-quality nickel-hydrogen batteries is limited by hydrogen diffusion within the nickel electrode active material. A diffusion-limited reaction model is developed which accounts for the observed self-discharge behavior. Effective hydrogen diffusion coefficients were calculated from the open-circuit self-discharge data on Hubble space telescope flight-quantified nickel-hydrogen cells and ranged from 1.12×10 −10 cm 2 /sec at 25°C to 7.45×10 −12 cm 2 /sec at 0°C.

Characterization of Iron Phthalocyanine as the Cathode Active Material for Lithium-Ion Batteries

The developed thermodynamic functions for the determination of Gibbs free energy, enthalpy, and entropy of formation of solid lithium-iron phthalocyanine (LixFePc) from solid lithium and iron phthalocyanine as a function of x, defined as g-moles of the intercalated lithium per g-mole of iron phthalocyanine, at a fixed set of temperature and pressure conditions are presented. In addition, a proposed expression for the evaluation of lithium diffusion coefficient in solid iron phthalocyanine as a function of both x and temperature, and the experimental results from the ongoing research/development work on the lithium/iron phthalocyanine cells are included

Theoretical and experimental analysis of a 3-volt lithium/polymer rechargeable battery

The cell voltage and discharge capacity as a function of discharge rate for a room temperature rechargeable lithium/polymer electrolyte/lithium manganese oxide cell can be accurately simulated by a simple diffusion-limited reaction model and by a transient, two dimensional mass transfer and generation model. The importance of using the intrinsic lithium chemical diffusion coefficient in modeling lithium insertion cathode materials is addressed.