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Chemical and Molecular Engineering in the 21st Century
Pages 1-28

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From page 3...
... Unlike conventional methods of producing electricity, such as steam turbines, the efficiency of a fuel cell is not limited by the Carnot cycle; therefore, fuel cells can have energy-conversion efficiencies of 60 to 80 percent. This makes fuel cells environmentally friendly energy-conversion devices, and they have been proposed for use in applications ranging from large-scale power production to transportation to battery replacement.
From page 4...
... , and a porous anode and cathode. To obtain appreciable oxygen ion conductivity in the electrolyte, the system must operate at high temperatures.
From page 5...
... Thus, one would prefer to have an anode half-cell reaction that involves the direct oxidation of a hydrocarbon fuel, such as the following:
From page 6...
... In a fuel cell, this produces rapid deactivation. This is demonstrated in Figure 2, which shows the current output of an SOFC with a Ni/YSZ cermet anode as a function of time.
From page 7...
... Coke formation from hydrocarbons is thermodynamically favorable at high temperatures; however, this reaction proceeds very slowly in the absence of a catalyst. Thus, the key to producing an SOFC anode that is active for the direct oxidation of hydrocarbons is to use materials that do not catalyze carbon-deposition reactions.
From page 8...
... Direct oxidation of hydrocarbons also greatly simplifies fuel cell systems by eliminating the need to cofeed water when using methane, eliminating the need for external hydrocarbon reformers, and eliminating the need to store and transport H2. Direct oxidation has the added advantage that the current infrastructure for the production and distribution of hydrocarbon-based fuels could be used.
From page 9...
... 9 o 0.6 Decane - 0.6 ,= ~ .
From page 10...
... 2001. Tape cast solid oxide fuel cells for the direct oxidation of hydrocarbons.
From page 11...
... Evidence for dimension-dependent properties of amorphous polymers has been observed in measurements of the glass transition temperature, Tg. Experiments by several research groups, including ours, report that the Tg of polymer films with nanoscopic dimensions can be significantly different from the corresponding bulk value (Forrest and Dalnoki-Veress, 2001~.
From page 12...
... by advanced lithography. To reach critical patterning dimensions of less than 100 nm, for example, the industry may be forced to use ultrathin films of polymer photoresist in conjunction with 157 nm and extreme ultraviolet (13.4 nm)
From page 13...
... These observations are consistent with models of capillary forces that predict that the force acting on the resist structures is inversely proportional to the distance between structures. They are also consistent with beam-bending models that predict that the maximum deformation of the resist structure in response to the imposed force is proportional to the aspect ratio cubed and is inversely proportional to the stiffness (Young's modulus)
From page 14...
... Scanning probe techniques, such as measuring the torsion of a cantilever required to push over a photoresist structure with dimensions of 100 to 200 nm are not convenient because of the gross mismatch in stiffness between standard cantilever beams and the polymer. To circumvent these problems, we have developed a formalism in which well defined forces are imposed on three-dimensional polymer nanostructures using simple and elegant principles from the classical thermodynamics of surface tens~on.
From page 15...
... Figure 3 shows some of our results for nanoscopic structures having widths as small as 100 nm; the agreement between continuumlevel predictions using bulk material properties and experimental data is quantitative. Note, however, that below 100 nm, preliminary experimental results suggest that mechanical properties start to deviate from the bulk.
From page 16...
... The material properties, surface tension, and contact angle were all determined from independent experiments on bulk samples.
From page 17...
... On the right, we show the results of molecular simulations for a material with exactly the same bulk mechanical constants. Clearly, the stress distribution from continuum mechanics is unable to capture the phenomena at play in nanostructures; for the models considered in our work, the assumption of a mechanical continuum starts to deviate from the continuum observed in molecular models of nanoscopic polymeric structures between 50 and 70 nm.
From page 18...
... Near the polymer-substrate interface the material undergoes a pronounced layering, which gives rise to large local elastic moduli. In the core of the structure, however, the local elastic constants
From page 19...
... CONCLUSIONS Our experiments and simulations to date have revealed that, above approximately 100 nm, the mechanical behavior of nanoscopic polymeric structures can be described using continuum formalisms and bulk material properties. At smaller length scales, our results suggest that amorphous polymeric structures exhibit dimension-dependent, anisotropic, elastic constants.
From page 20...
... 2002. Monte Carlo simulation of freestanding polymer films near the glass transition temperature.
From page 21...
... The mapping of the laws of mass, momentum, and energy conservation to the continuum results in field equations that describe the dynamics of the continuum. These field equations, variously known as the equations of motion, the equations of change, or simply the conservation equations, are nonlinear, partial differential equations that can be solved, in principle, when combined with the appropriate constitutive information]
From page 22...
... Solid mechanics is significantly simpler than fluid mechanics because of the absence of the nonlinear convection term, and the finite element method has become the standard method. In fluid mechanics, however, the finite element method is primarily used for laminar flows, and other methods, such as the finite difference and finite volume methods, are used for both laminar and turbulent flows.
From page 23...
... 3The development of graphical user interfaces for commercial CFD codes in the last 10 to 15 years has significantly increased their accessibility. 4The algebraic equations are linear if the associated partial differential equations are linear, for example, when the constitutive behavior is linear and diffusion dominates.
From page 24...
... It follows that the economic benefit of using CFD has been substantial, although detailed economic analyses are rarely reported. A case study of the economic benefit of the application of CFD in one chemical and engineeredmaterial company over a six-year period conservatively estimated that the application of CFD generated approximately a six-fold return on the total investment in CFD (Davidson, 2001a)
From page 25...
... ENTERPRISE-WIDE ACCESS Although the commercial CFD industry has greatly simplified the use of CFD codes by providing CAD-like preprocessors, automatic mesh generation, graphical user interfaces for all aspects of model definition, and on-line documentation, the industrial practice of CFD is still primarily in the hands of specialists. Regular use of a general-purpose code requires significant expertise in transport phenomena, an understanding of the capabilities and limitations of the modeling approaches used to handle turbulence and dispersed multiphase flows,
From page 26...
... Digital experts would automate geometry construction, mesh generation, solver selection, and other processes behind the scenes. In addition, they would contain all of the algorithms necessary to nurse the CFD codes to solution automatically, without having to ask the user to define satisfactory convergence, for example.
From page 27...
... In spite of the limitations of CFD, the economic value of industrial applications has been demonstrated in a variety of industnes, and its value as a research tool has been accepted in many areas, such as meteorology, medicine, and astrophysics. In industry, CFD is presently primarily in the hands of specialists, but the development of digital experts and tools to facilitate the development of digital experts may revolutionize the way industry uses CFD by providing ready access throughout the enterpnse.
From page 28...
... 1966. Elementary Partial Differential Equations.


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