Frontiers in Chemical Engineering Research Needs and Opportunities (1988) / Chapter Skim
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9 Surfaces, Interfaces, and Microstructures
9 Surfaces, Interfaces, and Microstructures
Pages 153-174
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From page 154... ...
Organizational forms include microstructures such as domains, mi FRONTIE] RS IN CHEAUI¢~AL ENGI1NTEE`Z2IAAYG crocrystallites, thin films, micelles, and microcomposites that are assembled into more complex structures on scales from microscopic to larger.
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From page 155... ...
As the proportion of material in interfacial zones increases, the special character of those zones begins to dominate the properties of the total structure. This is why the performance of highly microstructured materials is often marked by superior physical properties (e.g., mechanical strength, toughness, and elasticity)
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From page 156... ...
Thus, magnetic memories require ultrafine particulate coatings, whereas the new field of optical storage requires polymeric and inorganic thin films that undergo finely tuned structural changes upon illumination. The production of these and many other materials will present a growing set of challenges to chemical engineers in the future.
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From page 157... ...
Moreover, the activity and specificity of the catalyst itself are often influenced by the way the catalyst is deployed on the supporting surface, by the nature of that surface, and by the catalyst's interactions with the surface. The development of new and improved catalysts requires advances in our understanding of how to make catalysts with specified properties; the relationships between surface structure, composition, and catalytic performance; the dynamics of chemical reactions occurring at a catalyst surface; the deployment of catalytic surface within supporting microstructure; and the dynamics of transport to and from that surface.
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From page 158... ...
Such models could be used not only to guide the preparation of existing materials, but also to explore possibilities for making novel catalysts. Characterization of Catalyst Structure Characterization of catalyst structure and composition is essential to achieving a fundamental understanding of the factors controlling catalyst activity, selectivity, and stability.
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From page 159... ...
Unfortunately, many of the currently known techniques must be used ex situ, making it difficult to observe catalyst structure and composition during use or to examine the dynamics of the changes in these properties. Therefore, it is essential that greater attention be given to developing in-situ characterization techniques based on infrared spectroscopy, Raman spectroscopy, EXAFS, NMR, and neutron diffraction.
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From page 160... ...
Knowledge gained from such studies combined with information on the structure of the catalyst surface will lead to an improved understanding of what types of centers are critical for achieving high activity and selectivity and the role of poisons and other substances in causing catalyst deactivation. The information gained from such studies will provide vital input to large-scale scientific computations of molecular dynamics aimed at predicting the influence of surface composition and structure on catalyst performance.
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From page 161... ...
Since electrochemical processes involve coupled complex phenomena, their behavior is complex. Mathematical modeling of such processes improves our scientific understanding of them and provides a basis for design scale-up and optimization.
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From page 162... ...
The challenge to chemical engineers is to understand the fundamental elements of each processing step at a level where this knowledge can be used to guide the design and fabrication of high-density, superfast circuits and storage devices. The scientific problems that must be addressed to meet the challenges posed by the decreasing feature and domain size include the following: · characterization of microstructures; · identification of the factors affecting the controlled application and development of photoresists; · determination of the elementary processes involved in chemical vapor deposition, plasma deposition, and etching of thin films; and · mathematical modeling of all aspects of microstructures formation (e.g., in photoresist spincoating, resist patterning, and thin film deposition and etching)
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From page 163... ...
The conventional macroscopic models will have to be augmented with microscopic treatments of interface formation so that process conditions and interface properties can eventually be related. A close collaboration between experimentalists and theoreticians will lead to detailed models for simulating such processes as chemical vapor deposition, plasma etching, photoresist spinning, and photoresist development.
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From page 164... ...
There has been great interest in incorporating chemical functionality into monolayer-forming surfactants to permit lateral polymerization, either in monolayers on liquid substrates or in Langmuir-Blodgett films on solids, thus yielding exceedingly thin films or membranes with structural integrity. For micelles, greater refinement in the determination of micellar shapes, structures, and properties, as well as the investigation of the kinetics of micelle formation and disintegration have become possible thanks to recent advances in the use of photon correlation spectroscopy, small-angle neutron scattering, and neutron spin-echo spectroscopy.
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From page 165... ...
Many of these exist in the cellular makeup of living tissues (their study is called membrane mimetic chemistry) , and hostguest systems or artificial enzymes may also be produced.
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From page 166... ...
Specific research areas include the study of chemical reactions affecting powder particle nucleation, precipitation, surface structure and composition, size distribution, shape, shape distribution, surface charges, agglomeration, deagglomeration, tribological characteristics, and rheology. Important research opportunities in surface and interracial engineering also exist with respect to the properties of finished ceramic bodies, such as surface energy and susceptibility to crack propagation.
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From page 167... ...
At the technological frontier, chemical engineers can make important contributions to the development of new materials, the engineering of structure or morphology into membranes, and the identification of new ways of using permselective membranes. On the materials side, there is considerable interest in developing novel membrane materials that are functionalized to selectively adsorb a specific component from a fluid phase.
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From page 168... ...
The application of structural probes, such as carbon13 NMR spectroscopy and XPS, could contribute to the development of structure-permeability probes. Likewise, elucidation of the physical and chemical processes involved in membrane synthesis could aid in producing membranes with the desired microstructures.
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From page 169... ...
The challenges are enormous because of the small size and complexity of microstructures, the fluidity and thermal fluctuations of liquid and semiliquid systems, and the rapidity of many physical transformations and chemical reactions. Instrumentation Instrumentation for experimental observation and measurement is paramount in microstruc ture-related research.
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From page 170... ...
Scanning tunneling microscopy is a recent invention of great potential (Figure 9.31. Capable of resolving surface topography down to atomic dimensions, it operates perfectly well on surfaces immersed in gas or liquid, whereas electron microscopy requires that the specimen be studied under a vacuum (except for special FRONTIERS IN CHEMICAL EA'CI`VEERING FIGURE 9.3 Measuring less than 1/lOO,OOO,OOOth of an inch, the hills in this micrograph are individual atoms on a silicon crystal that have been enlarged more than 1 billion times using a scanning tunneling microscope.
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From page 171... ...
Examples of the application of NMR spectroscopy to problems of interest in chemical engineering include identification of the secondary building units involved in zeolite synthesis, analysis of the development of bicontinuous "liquid microsponge" in surfactant-oil-water systems, the clustering of hydrogen in amorphous silicon photovoltaic devices, and the structural characterization of carbonaceous deposits that lead to formation of coke on catalysts. In addition to providing time-averaged information, NMR spectroscopy can be used to probe the dynamics of molecular motion on time scales ranging from 106 to 1 second.
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From page 172... ...
This recent development is finding more and more applications in research on liquid and semiliquid microstructures, thin films, and adsorbed layers. FRO.\'T~S IN (~E.~ICAL El\`GI.~EER~.~G ,~ geometry ~ J light to spectrometer microscope objective, ~Upper rod disks variable stiffness forcemeasuring spring O cm ~white light movable clamp main support stiff double~cantilever spring helical spring FIGURE 9.4 The direct force measurement apparatus shown here can measure the forces between two curved molecularly smooth surfaces in liquids.
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From page 173... ...
will have to be introduced to provide more accurate and/or computationally efficient means for formulating process descriptions. Chemical engineers will need to become more familiar with recent advances in applied mathematics and computer science in order to work productively with researchers from these disciplines.
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From page 174... ...
IMPLICATIONS OF RESEARCH FRONTIERS There is an increasing societal need for materials with surface and interracial properties tailored to meet specific application. This spectrum of materials is extremely broad; it ranges from thin films for microelectronic circuits, to high-strength concrete for roads and buildings, to membranes for food protection.
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