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i54 _' ICROELECTRONIC CIRCUITS for com munications. Controlled permeability _11 films for drug delivery systems. Pro- tein-specific sensors for the monitoring of bio- chemical processes. Catalysts for the produc- tion of fuels and chemicals. Optical coatings for window glass. Electrodes for batteries and fuel cells. Corrosion-resistant coatings for the pro- tection of metals and ceramics. Surface active agents, or surfactants, for use in tertiary oil recovery and the production of polymers, paper, textiles, agricultural chemicals, and cement. What do these products have in common? They all are based on materials that have pre- cisely defined microstructures and/or surface and near-surface properties. In fact, surfaces, interfaces, and microscale structures are im- portant in virtually every aspect of modern technology; they influence the quality and value of a broad range of products. Modern high- technology materials and products can be thought of as populations of molecules that are distin- guished by the ways in which they are spatially organized to provide useful, often unique prop- erties and performance. Organizational forms include microstructures such as domains, mi FRONTIE]RS IN CHEAUIą~AL ENGI1NTEE`Z2IAAYG crocrystallites, thin films, micelles, and micro- composites that are assembled into more com- plex structures on scales from microscopic to larger. The ultimate products formed from mi- crostructures may include tailored forms of particles, fibers, sheets, porous and sponge-like structures, and a vast range of composites and assemblages. So it is that microstructures, in- terfaces, and surfaces represent an expanding and exciting frontier of untold potential. The frontier extends from humanly designed mate- rials and products all across energy and natural resources processing, environmental engineer- ing, and biochemical and biomedical technolo- g~es. The Nature of Structure Figure 9.1 illustrates a variety of different structures. This selection is by no means all- inclusive; a host of related structures such as colloids, microstrands, thin films, microporous solids, microemulsions, and gels could also have been shown. The parts of each of these struc- tures are distinguished by the zones inter- faces between them, which often seem to be (I) ,, ~/~/~5. ~META'I h - 3) METAL l: I O ~ _ Jne~n^~ ' n~JD4~_ no ---a--- n tPt ~' 1 \ ";'-' / ~ DRAIN\ SOURCE ~_, avers\ _--~11~-I've a''' ~'P'- ~ ~ BURIED \ X - ~ITA~T \ (2) MA I C V^IVe GHAN-STOP \ AVIS I A_ I \ a, (3) \ EMDn \ DMD \ P - SUBSTRATE ~. ENHANCEMENT DEPLETION CHANNEL CHANNEL IMPLANT FIGURE 9.1 Examples of different types of surfaces, interfaces, and micro- structures. (1) A biological membrane composed of phospholipid molecules in which protein molecules are embedded. (2) An NMOS logic circuit. (3) A section of ZSM-5 zeolite.
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SURFACES, I1YTERFACES, AND MICROSTRUCTURES so sharply defined that they are called surfaces. Well-defined surfaces occur between solids and either gases or liquids and thus are commonly found in catalytic and electrode reactions. More diffuse interfaces may occur between solids, as in microelectronic devices, and between fluids or semifluids, as in many polymeric and colloidal systems. Whether they are called surfaces or inter- faces, when the zones between parts of a struc- ture are "thin" from a fraction of a micrometer (the limit of the ordinary microscope) down to molecular dimensions-the matter in them as- sumes a character that is somewhat different from that seen when the same matter is in the form of a bulk solid. This special character of a molecular population arranged as an interfacial zone is manifested in such phenomena as surface tension, surface electronic states, surface reac- tivity, and the ubiquitous phenomena of surface adsorption and segregation. And the structuring of multiple interfaces may be so fine that no part of the resulting material has properties characteristic of any bulk material; the whole is exclusively transition zones of one kind or another. The finer the scale of structuring in a material, the more the material is taken up by interfacial zones. As the proportion of material in inter- facial zones increases, the special character of those zones begins to dominate the properties of the total structure. This is why the perform- ance of highly microstructured materials is often marked by superior physical properties (e.g., mechanical strength, toughness, and elasticity) or chemical properties (e.g., resistance to oxi- dation or corrosion, selective permeability, and catalytic activity). Further, since "the perfor- mance is in the interface," there are strong economic incentives to develop high-perform- ance products from inexpensive bulk materials, by modifying their surface and interfacial prop- erties, to compete with existing products com- posed of expensive, homogeneous materials. Relationship to Applications of Chemical Engineering Surface and interfacial properties and pro- cesses affect virtually every aspect of modern 155 chemical engineering. The ability to produce microstructures with the desired properties is leading to a wide variety of new materials and products that promise to improve our quality of life and to provide new business opportunities for U.S. industry. Five specific examples of the societal and technological impacts of micro- structural engineering, corresponding to Chap- ters 3 through 7 in this report, are given in the following subsections. Biochemical and Biomedical Engineering Many biological processes depend on cellular microstructure. These include selective and re- action-coupled transport, antibody-antigen in- teractions, enzyme catalysis, and the synthesis of proteins and membranes. Surface and inter- facial phenomena affect cell growth through their influence on cell immobilization, cell-cell interactions, and cell disruption and separation of constituents. A wide variety of therapeutic products, then, exert their influence on living systems by influencing molecular events occur- ring at biological interfaces. In addition, the practical implementation of cell culture for the commercial production of biochemicals (bio- technology) is heavily dependent on advances in understanding how cell-surface interactions mediate important cellular events. Finally, sur- face and interfacial phenomena are important to the function of a variety of biomedical de- vices, including artificial tissues and organs, sustained-release drug delivery systems, and future generations of biosensors. Electronic, Photonic, and Recording Materials and Devices Verv-lar~e-scale integrated (VLSI) micro electronic circuits epitomize intricately de- signed solid microstructures that are painstak- ingly built under meticulously controlled conditions. Structure scales in microelectronic devices are now at the 1 M-level and are expected to reach the 1-nary level in the next decade. The production of such devices depends on the carefully controlled deposition, pattern- ing, and etching of thin layers of metals, semi- conductors, polymers, and ceramics. The grow
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~ Ad; r6; ing field of optoelectronics depends on the formation of carefully tailored glass films and fibers to serve as light guides. Recording ma- terials also depend on the careful generation and control of microstructure. 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 illumina- tion. The production of these and many other materials will present a growing set of challenges to chemical engineers in the future. Polymers, Ceramics, and Composites Microstructures, surfaces, and interfaces play a central role in the production of ceramics, glasses, and organic and inorganic polymers. The mechanical and chemical properties of cement and concrete are also highly dependent on the formation of the proper microstructure. Interfacial chemistry is critical in determining the strength and durability of fiber- and fabric- reinforced composites and laminated high- performance polymer composites. Exciting opportunities are now emerging for molecular composites of rod- and coil-type polymers. Processing of Energy and Natural Resources The recovery of petroleum from sandstone and the release of kerogen from oil shale and tar sands both depend strongly on the micro- structure and surface properties of these porous media. The interracial properties of complex liquid agents mixtures of polymers and sur- factants are critical to viscosity control in tertiary oil recovery and to the comminution of minerals and coal. The corrosion and wear of mechanical parts are influenced by the com- position and structure of metal surfaces, as well as by the interaction of lubricants with these surfaces. Microstructure and surface properties are vitally important to both the performance of electrodes in electrochemical processes and the effectiveness of catalysts. Advances in syn- thetic chemistry are opening the door to the design of zeolites and layered compounds with tightly specified properties to provide the de ['8~-I\Ji i~'pS ]~1\' WEAL E\~RIATC sired catalytic activity and separation selectiv- ity. Environmental Protection, Process Safety, and Hazardous Waste Management The production of aerosols, soot, ash, and fines during combustion, calcining, and incin- eration depends on a large number of physical and chemical processes at gas-solid and/or gas- liquid interfaces. Similar phenomena are im- portant in the formation of suspensions, slimes, sludges, slurries, and other waterborne partic- ulates from natural resource processing and all sorts of chemical manufacture. The separation of particulates from air or water requires highly microstructured separators (e.g., membranes, colloidal absorbents, porous absorbents, and micellar and reverse-micelle scavengers). Like- wise, the understanding of soil contamination and decontamination by hazardous wastes de- pends on knowing soil and mineral microstruc- ture as well as the interactions of waste materials with these porous matrices. INTELLECTUAL FRONTIERS Crucial to the future of chemical engineering are two profound questions that define the field's frontiers: · How do the properties of a system or product-and thus its processing characteris- tics depend on the local structure of the system (i.e., the size and shape of its parts, their contacts and connectivity, and their composi- tion)? · How does local structure depend on the starting materials and processing conditions by which the system or product was created? How should the process be designed and controlled to achieve reliably the desired structuring? The answers to both questions can come only from interdisciplinary research. The first ques- tion impels physicists, chemists, materials sci- entists, geologists, biologists, and engineers alike. So does the second, but it serves as a more potent driving force to materials engineers and process engineers. If chemical engineers do
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I, I.\rimERFACES, A^N7~.~OSTRU<~S FIGURE 9.2 This high-resolution electron micrograph shows the unique pore structure of the ZSM-5 zeolite catalyst. Molecules such as methanol and hydrocarbons can be catalytically converted within the pores to valuable fuels and lubricant products. Courtesy, Mobil Research and Development Corpo- ration. not take up the research opportunities that come to them in the area of structure, researchers in other disciplines will. But chemical engineering has much to contribute to interdisciplinary at- tacks on structure-function relationships and process-structure connections. These potential contributions, and the research opportunities associated with them, are discussed in the remainder of this section. Catalysis Catalysts are most often used to promote reactions of fluid reactants. They are, with some exceptions, colloidal, amorphous, or micro- crystalline states that, to be accessible by the reactants, are deployed on supporting matrices with a large ratio of surface area to volume. The largest possible ratios are achieved, though, by suspending fine particles containing the ca- talyst in the fluid of reactants, thus creating the problem of removing the fine catalytic particles from the fluid after the reaction is complete. Alternatively, the fine particles may be packed together in one of several ways (e.g., as a porous bed; as the internal surface of a fine, consolidated porous medium; as an intersper- sion of connected solid; or as a semisolid and :57 connected porespace filled with fluid). But as the dimensions of the supporting matrix become smaller, access to the catalyst on the matrix surface becomes more strongly controlled by diffusion, a process quite slow compared to convection, which is favored by larger interstices and pores. Moreover, the activity and spec- ificity of the catalyst itself are often influenced by the way the catalyst is deployed on the sup- porting surface, by the nature of that surface, and by the cata- lyst's interactions with the sur- face. The development of new and improved catalysts requires ad- vances in our understanding of how to make catalysts with spec- ified 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 support- ing microstructure; and the dynamics of trans- port to and from that surface. Research oppor- tunities for chemical engineers are evident in four areas: catalyst synthesis, characterization of surface structure, surface chemistry, and design. Catalyst Synthesis The introduction of new types of catalytic materials has often led to the development of new or improved chemical processes. Examples are zeolite catalysts for petroleum cracking and organic synthesis (Figure 9.2), platinum-based reforming catalysts, Ziegler-Natta catalysts for Glenn polymerization, and catalysts for control of automobile exhaust emissions. Synthetic in- organic chemistry is currently opening up ways of preparing new multicomponent compounds, many of which have compositional and geo- metrical characteristics that suggest they might have potential as catalysts. Such materials in- clude the molecular sieves based on silicon aluminum phosphates, pillared clays and other
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158 layered materials, supported transition metal clusters, and metal carbides and nitrides. There is a growing interest in studying the influence of promoters on the catalytic properties of nonnoble metals, which could lead to a reduction in the demand for cat- alysts based on metals such as platinum, palladium, and rho- dium metals for which the United States is almost totally dependent on foreign sources. Catalysts that do not contain these metals but possess many of their catalytic properties have re- cently been developed. A challenge particularly suited to chemical engineers is the de- velopment of process models for predicting the microstructure and surface structure of catalysts as a function of the conditions of their preparation. Such models could be used not only to guide the preparation of existing ma- terials, but also to explore pos- sibilities for making novel cata- lysts. Characterization of Catalyst Structure Characterization of catalyst structure and composition is es- sential to achieving a fundamen- tal understanding of the factors controlling catalyst activity, se- lectivity, and stability. During the past 15 years, the application of surface science techniques (e.g., low-energy electron dif- fraction ELEED], Auger electron spectroscopy LAESl, x-ray pho- toelectron spectroscopy EXPS], and ultraviolet photoelectron spectroscopy PUPS]) has led to a very rapid advance in under- standing how metallic catalysts function. Knowledge of the FRONTIERS IN CHEMICAL ENGINEERING
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SURFACES, INTERFACES, AND MICROSTRUCTURES structure of supported metal ca- talysts has been advanced through the use of high-resolution trans- mission electron microscopy (TEM), extended x-ray absorp- tion fine structure spectroscopy (EXAFS), and, more recently, solid-state nuclear magnetic res- onance spectrometry (NMR). Major advances in understanding zeolite structure have resulted from combining information ob- tained from x-ray diffraction, sil- icon-29 and aluminum-27 NMR, infrared spectroscopy, and neu- tron diffraction. Unfortunately, many of the currently known techniques must be used ex situ, making it difficult to observe cat- alyst structure and composition during use or to examine the dynamics of the changes in these properties. Therefore, it is es- sential that greater attention be given to developing in-situ char- acterization techniques based on infrared spectroscopy, Raman spectroscopy, EXAFS, NMR, and neutron diffraction. Surface Chemistry Knowledge of the structure and bonding of molecules to sur- faces has been obtained from such techniques as LEED, elec- tron energy-loss spectroscopy (EELS), secondary-ion mass spectrometry (SIMS), infrared spectroscopy (IRS), Raman spectroscopy, and NMR spec- trometry. The scope of such studies needs to be greatly ex- panded to include the effects of coadsorbates, promoters, and poisons. Greater emphasis should be given to developing new pho- ton spectroscopies that would permit observation of adsorbed species in the presence of a gas
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FRONTIERS IN CHEMICAL E1~INEERI.~G or liquid phase. Attention also needs to be given to studies of surface reaction dynamics to obtain a fundamental understanding of the elementary reaction steps involved in the decomposition or rearrangement of an adsorbed species and of its reaction with coadsorbed species to form either desirable or undesirable products. 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 perform- ance. Catalyst Design Recent theoretical studies have demonstrated that it is possible to calculate accurately adsor- bate structure and energy levels, to explain trends with variations in metal composition, and to interpret and predict the influence of pro- moters and poisons on the adsorption of reac- tants. Additional efforts along these lines will contribute greatly to understanding how catalyst structure and composition influence catalyst- adsorbate interactions and the reactions of ad- sorbed species on a catalyst surface. With sufficient development of theoretical methods, it should be possible to predict the desired catalyst composition and structure to catalyze specific reactions prior to formulation and test- ing of new catalysts. Electrochemistry and Corrosion Electrochemical processes (e.g., electrolysis, electroplating, electromachining, current gen- eration, and corrosion [Plate 81) are distin- guished by their occurrence in a boundary region between an electrolyte (liquid or solid) and an electrode. The course of these processes is strongly dependent on the potential at the elec- trode surface, the composition and structure of the electrode, the composition of the electrolyte, and the microstructure of the electrolyte in the boundary layer near the electrode surface. In certain applications, the pore size and connec- tivity of the electrode can also be important. Charge Transfer There are two issues of fundamental impor- tance to the kinetics of electrochemical pro- cesses. The first is the dependence on distance of electron transfer between sites that are not in contact. An understanding of this is critical for creating three-dimensional catalytic struc- tures through which charge percolates to fixed sites. The second is the kinetics of electron transfer at well-defined sites such as individual defects on single crystals or on selected planes of carbon. An improved understanding of the physical processes governing charge percolation and conduction through an electrode and the factors influencing electron transfer at the elec- trolyte-electrode interface is needed. Such knowledge would make it possible to choose electrode materials and tailor their microstruc- ture to suit particular applications. The identi- fication of cheap electrode materials to replace platinum would be a very significant accom- plishment. Molecular Dynamics Mechanistic studies are needed on a select number of electrochemical reactions, particu- larly those involving oxygen. These studies are far from routine and require advances in knowl- edge of molecular interactions at electrode sur- faces in the presence of an electrolyte. Recent achievements in surface science under ultrahigh vacuum conditions suggest that a comparable effort in electrochemical systems would be equally fruitful. There is no fundamental theory for electro- crystallization, owing in part to the complexity of the process of lattice formation in the pres- ence of solvent, surfactants, and ionic solutes. Investigations at the atomic level in parallel with studies on nonelectrochemical crystalli- zation would be rewarding and may lead to a theory for predicting the evolution of metal morphologies, which range from dense deposits to crystalline particles and powders. Many electrochemical reactions consist of
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SUREACES, ~'NTERFACES, BAND M' CROSTRUCTURES complex sequences of steps, such as in elec- troorganic synthesis. In these, the key to high yield is knowledge of the sequence so that adroit choices of electrode and solution materials can be made. More thorough documentation of rate and equilibrium constants is mandatory to trans- fer such scientific understanding into engineer- ing practice. The influence of added agents and inhibitors is important in processes that involve cor rosion, electrodeposition, or etching. Mechanistic details re- main essentially unknown. Im- proved insight would benefit technologies that depend on the formation and stabilization of controlled surfaces. Supramolecular Microstructures As noted earlier, the kinetics of electrochemical processes are influenced by the microstructure of the electrolyte in the electrode boundary layer. This zone is pop- ulated by a large number of spe- cies, including the solvent, re- actants, intermediates, ions, in- hibitors, promoters, and impur- ities. The way in which these species interact with each other is poorly understood. Major im- provements in the performance of batteries, electrodeposition systems, and electroorganic syn- thesis cells, as well as other elec- trochemical processes, could be achieved through a detailed un- derstanding of boundary layer structure. Since electrochemical pro- cesses involve coupled complex phenomena, their behavior is complex. Mathematical model- ing of such processes improves our scientific understanding of them and provides a basis for design scale-up and optimiza- tion. The validity and utility of such large-scale models is ex- pected to improve as physically correct descrip- tions of elementary processes are used. Electronic, Photonic, and Recording Materials and Devices The creation of microstructure with well- defined electrical or optical properties is critical to the production of integrated circuits and recording materials. The processes used to de
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162 fine microstructure in this context are described in Chapter 4. Common to the fabrication of all electronic and photonic materials are the dep- osition, patterning, and etching of thin films. The preparation of magnetic recording materials also involves the creation of very small magnetic particles and their distribution in a thin layer of binder on a substrate. Virtually all aspects of these processes involve surface and interracial phenomena. The challenge to chemical engi- neers 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 ad- dressed 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 pho- toresists; · 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 dep- osition and etching). Characterization of Microstructure Advances in integrated circuit technology and in the production of high-density storage devices depend on making ever-smaller microstruc- tures. An essential aspect of this problem is the ability to characterize the physical and chemical properties of domains having dimensions be- tween 0.1 and 1 Em. Visualization and elemental mapping of microstructural elements at this scale have been accomplished by use of scan- ning electron microscopes and, more recently, high-resolution scanning Auger and x-ray pho- toelectron spectrometers. If chemical engineers are to play an effective part in the future of the electronics and photonics industries, they must be familiar with such modern analytical devices. ~3~ERS IN CuEMiC~L ENGI.VEE.~.~G Photoresist Processing Polymer films that are sensitive to light, x- rays, or electrons known es photoresists are used extensively to transfer the pattern of an electronic circuit onto a semiconductor surface. Such films must adhere to the semiconductor surface, cross-link or decompose on exposure to radiation, and undergo development in a solvent to achieve pattern definition. Virtually all aspects of photoresist processing involve surface and interracial phenomena, and there are many outstanding problems where these phenomena must be controlled. For example, the fabrication of multilayer circuits requires that photoresist films of about 1-,um thickness be laid down over a semiconductor surface that has already been patterned in preceding steps. A planar resist surface is essential to the successful execution of subsequent steps, but it is as yet difficult to attain. A knowledge of the ways in which polymer viscoelasticity, sur- face tension, and surface adhesion affect the rheology of resist How is needed. Another area requiring research is the solvent development of resist films after exposure of the films to radiation through a mask. In this step, it is essential to remove only those parts of the polymer that have been degraded by the radia- ~tion. Research is needed to understand how solvent composition, residual polymer stress, polymer adhesion, and the swelling of an unir- radiated polymer affect the geometric definition of the developed well. Such problems become increasingly important as resolution is pushed to 1 am and then to 0.1 ,um. Chemical Vapor Deposition and Plasma Deposition/Etching of Thin Films Both thermal and plasma-assisted chemical vapor deposition techniques are used routinely to deposit thin films, and plasma etching is used to define fine features in the films. Understand- ing the fundamental reactions involved in these processes is essential to developing an under- standing of how best to control the deposition or etching of thin films and the design of equip- ment to carry out such steps. To make progress in this area, chemical engineers need to identify the chemical species present in the gas phase
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SURFACES, INTERFACES, AND MICROSTRUCTURES of both thermal and plasma reactors through the use of such techniques as emission spec- troscopy, laser-induced fluorescence spectros- copy, electron spin resonance spectroscopy, and mass spectrometry. The dynamics of gas- phase chemical reactions need to be understood for processes involving not only neutral species but also electrons and ions. Another task of equal importance is understanding how reactive gas-phase species interact with solid surfaces to achieve film growth or etching. While some of the elementary processes are similar to those occurring at the surface of catalysts, others, such as ion bombardment and photon-assisted etching, are specific to systems found in the electronics and photonics industries. Because of their knowledge of transport phenomena, chemical engineers are expected to contribute significantly to an understanding of how local electrical fields and concentration gradients in- teract to influence such processes as the ani- sotropic etching of semiconductors. Mathematical Modeling The systems involved in microelectronic processing are usually so complex that they can rarely be described by simple conceptual models. It is therefore necessary to develop mathemat- ical models that incorporate fundamental infor- mation in order to understand such processes adequately. The advantage of mathematical modeling has been demonstrated for simple systems, and more detailed models will continue to appear with the growing access to large-scale computing. The conventional macroscopic models will have to be augmented with micro- scopic 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. Colloids, Surfactants, and Fluid Interfaces The area of colloids, surfactants, and fluid interfaces is large in scope. It encompasses all fluid-fluid and fluid-solid systems in which in i63 terfacial properties play a dominant role in determining the behavior of the overall system. Such systems are often characterized by large surface-to-volume ratios (e.g., thin films, sots, and foams) and by the formation of macroscopic assemblies of molecules (e.g., colloids, micelles, vesicles, and Langmuir-Blodgett films). The peculiar properties of the interfaces in such media give rise to these otherwise unlikely (and often inherently unstable) structures. The formation of ordered two- and three- dimensional microstructures in dispersions and in liquid systems has an influence on a broad range of products and processes. For example, microcapsules, vesicles, and liposomes can be used for controlled drug delivery, for the con- tainment of inks and adhesives, and for the isolation of toxic wastes. In addition, surfactants continue to be important for enhanced oil re- covery, ore beneficiation, and lubrication. Ce- ramic processing and sol-gel techniques for the fabrication of amorphous or ordered materials with special properties involve a rich variety of colloidal phenomena, ranging from the produc- tion of monodispersed particles with controlled surface chemistry to the thermodynamics and dynamics of formation of aggregates and micro- crystallites. The current status and the emerging oppor- tunities in the science of colloids, surfactants, and fluid interfaces can be addressed conveni- ently by considering a threefold hierarchy of systems as follows: individual molecules (e.g., surfactants), ~ self-assembling (associated) microstruc- tures of surfactants and other molecules in the colloidal size range; and · macroscopic (often structured) systems made up of associated microstructures and bulk phases. This last category may also include fluid interface systems with unstructured bulk phases and/or moderate surface-to-volume ratios. Significant breakthroughs have been made in recent years in the identification, preparation, characterization, and understanding of entities at all these levels, creating new opportunities for the successful use of colloidal and interracial phenomena in chemical engineering and pre- senting new challenges as described below. New surfactant molecules are being designed
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~- with novel and special proper- ties, particularly with the inclu- sion of fluorine atoms and sili- con-containing substituents (both yielding surface activity in or- ganic media). Multifunctional surfactants are being designed as coupling agents, release agents, rewetting agents, and steric sta- bilizers for wet colloids. Other recent developments include the synthesis of all-tail (and even tri- tail) surfactants for use in the preparation of thin films and membranes. Despite these ad- vances, a host of possibilities for structural modifications of hy- drophilic and hydrophobic groups in surfactants remains unex- plored. Self-assembling structures in- clude monolayers and micelles, both of which have received much study. However, new ap- proaches and possibilities for these structures are emerging. For instance, chromophores are being incorporated in monolayer assemblies to produce Lang- muir-Blodgett films with a vari- ety of unique optical properties. There has been great interest in incorporating chemical function- ality into monolayer-forming sur- factants to permit lateral poly- merization, either in monolayers on liquid substrates or in Lang- muir-Blodgett films on solids, thus yielding exceedingly thin films or membranes with structural integ- rity. For micelles, greater refine- ment in the determination of mi- cellar shapes, structures, and properties, as well as the inves- tigation 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. Notable advances have also been made in the study of FRO^N'ti^~RS Slur CH~E~76~.~t ~x,3~.,'`i'2E~,~c other microstructural assemblies, and new en- tities have been discovered and identified. These include inverse micelles, vesicles, liposomes, bilayers, microemulsions, liquid crystallites, and a variety of as yet unnamed entities formed by the interaction of dissolved polymers (often
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SUREAŁES, ., AND .~OSTR&C ';~'^2L'~§ proteins) or other macromolecules with the above structures. Many of these exist in the cellular makeup of living tissues (their study is called membrane mimetic chemistry), and host- guest systems or artificial enzymes may also be produced. Methods to determine and control the properties of individ- ual surfactant molecules and to determine the conditions needed to produce well-defined molec- ular assemblies are just begin- ning to emerge. We are at the threshold of being able to pro- duce deliberately structured su- pramolecular entities with prop- erties tailored to meet special applications. Some additional examples of problems that will have significant impacts over the next one or two decades follow: · New methods to produce large quantities of mono-sized particles of nearly any inorganic material desired (e.g., metals, oxides, silicates, sulfides) are needed for the processing of ce- ramics, electronic materials, and other engineered materials. · New methods of emulsion polymerization, particularly the use of swelling agents, to pro- duce monodisperse latexes of any desired size and surface chem- istry are also needed. Perfect spheres as large as 100 Am can now be produced in the zero- gravity environment provided by the space shuttle. These spheres and other mono-sized particles of various shapes can be used as model colloids to study two- and three-dimensional many-body systems of very high complexity. · Refinements in the theory of interparticle long-range van der Waals forces (the Landau- Lifshitz theory) are within reach. New techniques are now available for measuring the complex dielectric constants of various media required for the implementation of that theory. · Recognition and description of new inter- particle interaction forces such as those owing to magnetic dipoles, steric and electrosteric repul
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166 i FRONTIERS IN CHE.lIICAL ENGINEERING sign. and long-range solvent ordering offeroppor tunities to study interracial molecular pheno- mena that were previously difficult to describe. · New experimental techniques for the direct measurement of interparticle forces are now available and can be used to understand the physicochemical factors that control adhesion, coating phenomena, tribology, and others. · New optical (static as well as dynamic) techniques for the study of long-range order in structured continua are beginning to appear and can be used to understand the constitutive properties and relations in complex (polymeric, nematic, and other structured) fluids. · New application of modern statistical me- chanical methods to the description of structured continua and supramolecular fluids have made it possible to treat many-body problems and cooperative phenomena in such systems. The increasing availability of high-speed computa- tion and the development of vector and parallel processing techniques for its implementation are making it possible to develop more refined de- scriptions of the complex many-body systems. · Because of the increasing level of control that is now possible in the preparation of model colloids and surfactants, model many-body systems can be created in the laboratory and studied by non- intrusive instrumental techniques in parallel with computational and theoretical sophistication. In view of the above developments, it is now possible to formulate theories ofthe complex phase behavior and critical phenomena that one observes in structured continua. Furthermore, there is cur- rently little data on the transport properties, rheo- logical characteristics, and thermomechanical properties of such materials, but the thermody- namics and dynamics of these materials subject to long-range interparticle interactions (e.g., dis- joining pressure effects, phase separation, and viscoelastic behavior) can now be approached systematically. Such studies will lead to significant intellectual and practical advances. Ceramics, Cements, and Structural Composites The development and control of microstruc- ture are critical in the processing of ceramics and cements. The chemical engineer's knowl- edge of reaction kinetics, surface phenomena, and transport phenomena could contribute ef- fectively to the development of new materials. Bulk ceramics are produced conventionally by the sintering of powders. The strength, toughness, thermal stability, and dielectric properties of the fired ceramic depend strongly on the size and uniformity of the precursor powder and on the chemical properties of the powder surface. Examples of the need for improved ceramics technology, either to produce ceramics more economically or to produce ceramics with im- proved performance, abound in both structural and electronic applications. They include au- tomobile engine components, armor, welding nozzles, artificial hip joints, wear elements of valves and pumps, cutting tools, electronic packages, and a host of other current or future applications of this exciting new area of mate- rials science. Among new developments in ce- ramic materials are ceramic glasses, micropo- rous ceramic filtering media, ceramic-ceramic composites and microcracked composite ce- ramics for catalyst supports, ceramic fibers, ceramic thin films and coatings, permselective membranes for application in separations and sensors, and a variety of high-performance ce- ments. Essential to these improved ceramics is the control of particle size and uniformity through well-characterized chemical reactions. Chemi- cal engineers have rich opportunities for con- tribution through surface and interracial engi- neering of preceramic particles and powders. 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 re- spect to the properties of finished ceramic bod- ies, such as surface energy and susceptibility to crack propagation. Sintering mechanisms and kinetics represent a very important area for scientific investigation. Progress in addressing
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SURFACES, INTERFACES, Aged MICR0STRDCTURES these issues may permit the application of ce- ramics of known high-performance character- istics to areas in which their use is now unec- onomical. Finally, there is a need for chemical engineers to bring their expertise in surface and interracial engineering to the problems of developing better varieties of Portland cement and concrete. Both these commodities are produced in vast quan- tities each year, and major improvements in their properties (e.g., freeze-thaw durability, corrosion resistance, and compressive strength) would tremendously benefit society. The prop- erties of Portland cement and concrete are controlled by the microstructure of the mate- rials. The microstructure of concrete is devel- oped through a remarkably complex series of steps and can be influenced by a host of low-concentration addi- tives. Examples include the su- perplasticizers, which not only reduce the viscosity of freshly mixed concrete but also affect its final microstructure. Collab- orative research among chemical engineers, civil engineers, and colloid and surface chemists can accelerate progress toward achieving superior formulations for cement and concrete. Membranes Membranes are thin two-di- mensional structures designed to pass preferentially certain com- ponents. Highly efficient sepa- rations of gaseous or liquid com- ponents can be achieved with such technologies as reverse os- mosis, ultrafiltration, gas sepa- ration, microfiltration, dialysis, and electrodialysis. In these sys- tems, separations are driven either by pressure or by a gradient in chemical or electrochemical po- tential. Membranes are also find- ing increasing use in controlled drug release devices and bio- sensors. Traditional applications of membrane technology have barely scratched the surface of an exciting and rapidly developing area. There are two major frontiers in membrane research, one technological and the other sci- entific. At the technological frontier, chemical engineers can make important contributions to the development of new materials, the engi- neering of structure or morphology into mem- branes, and the identification of new ways of using permselective membranes. On the materials side, there is considerable interest in developing novel membrane mate- rials that are functionalized to selectively adsorb a specific component from a fluid phase. Mem- brane materials that are environmentally stable and resistant to fouling are also needed. Since
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higher fluxes of permeates can be achieved by decreasing mem- brane thickness, there is increas- ing emphasis on building struc- tural integrity into the membrane. Possibilities include the use of laminated polymer membranes and porous ceramic substrates for ultrathin polymer layers. On the applications side, in- tegrated membrane processes represent an attractive area to be developed, in which a membrane separation is combined with a conventional separation to ac- complish a job neither process by itself could do. An example is the distillation of azeotropic mixtures, where a pervaporation module can be used to get around the azeotropic composition. An- other example is the use of hol- low fiber membranes in bioreac- tors. Here the membrane acts as a support for an immobilized cell or enzyme and at the same time facilitates the supply of oxygen and/or nutrients. In a particularly elegant extension, a permselec- tive membrane may be combined with a catalytic membrane to selectively remove a dilute reac- tant from a stream containing inerts and to generate a product stream in which the product concentration is many-fold higher than that of the reactant going in. Finally, there are some very exciting opportunities for the development of "smart" membranes that respond to the types or concentrations of species present in the fluids contacting them. One example is a membrane that regulates the delivery of insulin from a reservoir into a patient's bloodstream in re- sponse to blood sugar level. The scientific base for rationally designing membrane polymers for specific applications is very limited, and hence there is an immense frontier to be conquered. Work is also needed on transport fundamentals, structure-permea- bility relationships, and elucidation of how to control membrane morphology. While phenom FRO\TIERS IN CHEWS ENGI.~1~G enological transport models already exist, mo- lecular-scale models for describing the transport of organic permeants and the transport of con- densible vapors through glassy or nonequilib- rium matrices have yet to be developed. The application of structural probes, such as carbon- 13 NMR spectroscopy and XPS, could contrib- ute 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. RESEARCH NEEDS To understand how the properties and per- formance of a material are tied to its microstruc- ture and how microstructure depends on pro
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SURFACES, llNfERF~CES, A~iD`~iCROST~ UG fU~iE:S ceasing, researchers must be able to detect microstructure, characterize it, resolve its shape and connectivity, and measure its size and composition. They need to visualize the micro- structure, whether directly through some sort of microscopy or indirectly by means of theory based on model-dependent synthesis from mea- surements. The challenges are enormous be- cause of the small size and complexity of mi- crostructures, the fluidity and thermal fluctuations of liquid and semiliquid systems, and the rap- idity of many physical transformations and chemical reactions. Instrumentation Instrumentation for experimental observation and measurement is paramount in microstruc ture-related research. One rea- son that surfaces, interfaces, and more complicated microstruc- tures are a frontier of chemical engineering and processing re- search is that modern science has recently spawned a number of microstructural probes of un- precedented resolution and util- ity. For the first time, we have the proper tools to attack the molecular and chemical basis of m~crostructures. Of course, our understanding of microstructures will be ad- vanced through an interplay of observation, conceptualization, experiment, and theory. But in this area of engineering science, advances will come most often when already developed instru- mental probes are adapted to new systems or new probes are perfected to answer questions arising from practical problems. The adaptation and development of instrumental probes for sys- tems of interest to chemical en- gineers demand cooperative ef- forts with the originating scientific disciplines and with instrument manufacturers. In such efforts, chemical engineers can bring important refine- ments or innovations to instrumental practice. This has already occurred, for example, in the development of video-enhanced optical micros- copy, rapid-freezing cryo-electron microscopy, the analysis of solid catalytic surfaces, and the probing of solid-liquid interfaces important in electrochemical catalysis. Microscopy and Microtomography The direct visualization of microstructure may be accomplished by various forms of mi- croscopy. Recent refinements in microscopy techniques are epitomized by video-enhanced interference phase-contrast microscopy, which is emerging as a workhorse probe for colloidal suspensions and other microstructured liquids.
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170 This technique is capable of re- solving structures at distances approaching the wavelength of visible light (350 to 800 nary). Another useful tool, and ar- guably the most powerful probe of surface topography on scales from those of the light micro- scope down to 5 nm, is scanning electron microscopy with x-ray microanalysis. This technique , · . ,~ . combines magnifying power, depth of field, and ability to ana- lyze local composition. It may also be used to study the internal microstructure of specimens by fracturing them (sometimes after freezing). Scanning electron mi- croscopy is certain to become a very useful tool in the hands of chemical engineers, particularly as they apply the principles of . · . . . . cnemlca1 engineering science (e.g., a sophisticated under- standing of heat and mass transfer, phase change, and chemical reactions) to interpreting images and developing ancillary techniques. Even greater magnifying power is provided by transmission electron microscopy, and in some instances this technique can be comple- mented with energy loss spectroscopy. Trans- mission electron microscopy can resolve micro- structure down to atomic scales (0.1 to 0.5 nm) and requires the skillful application of special- ized techniques to extremely thin, solid, or solidified specimens (or replicas of specimen surfaces such as internal surfaces of fractured samples). Correct interpretation of images re- quires not only considerable experience, but also a fundamental understanding of sample behavior during preparation and under the elec- tron beam and of the contrast mechanisms underlying an image. 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 sur- faces immersed in gas or liquid, whereas elec- tron 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. The microscope collects digital information that is plotted by a computer. Bands on the hills are contours assigned by the computer to help researchers see how crystalline structures are formed. Copyright AT&T, Microscapes. "environmental stages" that function only with severely reduced effectiveness). However, the intense electrical fields of the scanning tip can strongly affect the specimen locally. Equipment and techniques are rapidly being refined, and it appears that scanning tunneling microscopy will be playing an important role as a probe of active solid-gas and solid-liquid interfaces. X-ray microtomography is a new develop- ment of great promise for reconstructing, dis- playing, and analyzing three-dimensional mi- crostructures. Resolution of around 1 Am has been demonstrated with currently available synchrotron sources of x-rays, x-ray de- tectors, algorithms, and large-scale computers. The potential for microstructural research in composites, porous materials, and suspensions at this and finer scales appears to be tremendous. Magnetic resonance imaging, or microtomog- raphy by multinuclear magnetic resonance, is another new development that is even more exciting because it provides three-dimensional mapping of the abundance of a variety of atoms. Compositional aspects of microstructure can thereby be resolved. However, the resolution
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SURFACES, INTERFACES, AND MICROSTRUCTURES of currently available instruments does not yet approach 1 lam. Scattering Methods Beams of electromagnetic radiation of appro- priate wavelength are scattered when they in- teract with the gradients inherent in structured materials. By measuring the ways in which the intensity of scattered radiation varies as a func- tion of the angle at which the radiation initially strikes the sample, the wavelength of the radia- tion, and the time, many aspects of the structure of materials can be inferred. Bulk heterogeneities and surface topography are both marked by electron distributions that vary in their polarizability. These variations are capable of scattering photons. In liquid and semiliquid materials, where the variations them- selves fluctuate over time and space, static light scattering and its dynamic complement photon correlation spectroscopy are important probes of larger colloidal-scale microstructures and their thermal motions, which are often finger- prints of structure. For solids, the scattering of x-ray radiation can be used to characterize the structure of both crystalline and amorphous materials. Of particular interest in terms of amorphous materials is the technique of ex- tended x-ray absorption fine structure, which provides information on atomic coordination number and local bond distances. Generally, the more intense the available beam source, the shorter the time scales, the weaker the heterogeneities, and the longer the distances that can be probed by a scattering method. Hence, there is a strong drive to utilize high-powered lasers, synchrotrons, and intense neutron sources in research on surfaces, inter- faces, and microstructures. This is particularly true in the study of liquid materials and of systems that undergo rapid physical transfor- mations or chemical reactions. Resonance Spectroscopies The interaction of radiation with a material can lead to an absorption of energy when the radiation frequency matches one of the resonant frequencies of the material. The exact frequency 171 at which the absorption occurs and the shape of the absorption feature can provide detailed information about electronic structure, molec- ular bonding, and the association of molecules into microstructural units. Nuclear magnetic resonance (NMR) spec- troscopy is an enormously powerful tool that, in chemistry, has become a mainstay for ana- lyzing molecular structure and environment. In recent years, NMR spectroscopy has proved useful for studying catalysts, amorphous semi- conductors, and colloidal-scale microstructure and molecular aggregates. Examples of the application of NMR spectroscopy to problems of interest in chemical engineering include iden- tification of the secondary building units in- volved in zeolite synthesis, analysis of the development of bicontinuous "liquid micro- sponge" in surfactant-oil-water systems, the clustering of hydrogen in amorphous silicon photovoltaic devices, and the structural char- acterization 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. Thus, for example, time-re- solved NMR techniques have made it possible to characterize the dynamics of forming the precursors to zeolite synthesis. The vibrations of molecular bonds provide insight into bonding and structure. This infor- mation can be obtained by infrared spectroscopy (IRS), laser Raman spectroscopy, or electron energy loss spectroscopy (EELS). IRS and EELS have provided a wealth of data about the structure of catalysts and the bonding of adsor- bates. IRS has also been used under reaction conditions to follow the dynamics of adsorbed reactants, intermediates, and products. Raman spectroscopy has provided exciting information about the precursors involved in the synthesis of catalysts and the structure of adsorbates present on catalyst and electrode surfaces. Molecular-level characterization of surface composition and structure can be obtained through a variety of electron and ion spectros- copies. The two-dimensional structure of sur- faces and ordered arrays formed by adsorbates is revealed by low-energy electron diffraction
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~2 (LEED). This technique can also be used to follow phase changes and surface reconstruction in real time. The atomic composition of surfaces can be determined by Auger electron spectroscopy (AES), x-ray photoelectron spectroscopy (XPS), and sec- ondary ion mass spectrometry (SIMS). While SIMS provides the highest elemental sensitivity, AES and XPS can resolve spatial variations in composition down to 0.1 ~m. XPS, in addition, gives information on the valence state of individual atoms, from which details of interatomic bonding can often be inferred. The density of electrons in bond- ing orbitals can be obtained from ultraviolet photoelectron spec- troscopy (UPS). When carried out with monochromatic beams of synchrotron radiation, this technique can also identify the orientation of individual atomic and molecular orbitals at a solid surface. With the exception of LEED, each of these techniques can be used to characterize poly- crystalline films, amorphous materials, and powders, as well as single crystals. Other Important Methods The statics and dynamics of microstructures are governed by the forces that create or main- tain them. Rarely can the forces be measured directly. But forces between special surfaces immersed in fluid can now be accurately gauged at separations down to 0.1 nm with the direct force measurement apparatus, an ingenious combination of a differential spring, a piezo- electric crystal, an interferometer, and crossed cylindrical surfaces covered by atomically smooth layers of cleaved mica (Figure 9.41. This recent development is finding more and more appli- cations 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 force- measuring 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. Mica surfaces, either raw or coated, are the primary surfaces used in this apparatus. The separation between the surfaces is measured by optical techniques to better than 10 nm. The distance between the two surfaces is controlled by a three-stage mechanism that includes a voltage-driven piezoelectric crystal tube supporting the upper mica surface; this crystal tube can be displaced less than 10 nm in a controlled fashion. A force-measuring spring is attached to the lower mica surface and its stiffness can be varied by a factor of 1,000 by shifting the position of a movable clamp. Reprinted with permission from Proc. Natl. Acad. Sci. USA, 84, July 1987, 4722. Its use will continue to expand as its cost falls, its complexity decreases, and its capabilities multiply. The electron tunneling microscope tip is cur- rently gaining recognition as the most exquisite micromanipulator for measuring local deform- ability of solid surfaces, down to nanometers and smaller. For microstructures on scales of micrometers and larger, micromanipulation ap- paratus from biology and biophysics is turning up in probes of deformability and force; mi- croelectronic devices are in the offing. Micro- electrode probes continue to evolve. Laser- doppler motion probes capable of micrometer resolution and birefringence and dichroism mea- surements are becoming important in the char- acterization of many surfaces.
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174 emphasized. In the past, many of the major problems in the processing of structured ma- terials have yielded to analysis once sufficient computational power was provided to permit the utilization of very detailed physical models. Supercomputers have made possible significant advances in the modeling of plasma reactors, complex electrochemical systems, coating Hows, and stress fracturing of polymers and ceramics. Advanced computational tools will become even more important as chemical engineers attack the important and highly complex problems now on the cutting edge of research on surfaces, interfaces, and microstructures. IMPLICATIONS OF RESEARCH FRONTIERS There is an increasing societal need for ma- terials with surface and interracial properties tailored to meet specific application. This spec- trum 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. The devel- opment and production of such advanced ma- terials, and of surface active agents, will be rich in technical challenges for chemical engineers. To address these challenges, chemical engi- neers will need state-of-the-art analytical in- struments, particularly those that can provide information about microstructures for sizes down to atomic dimensions, surface properties in the presence of bulk fluids, and dynamic processes FRONTIERS IN CHEMICAL ENGINEERING with time constants of less than a nanosecond. It will also be essential that chemical engineers become familiar with modern theoretical con- cepts of surface physics and chemistry, colloid physical chemistry, and rheology, particularly as it applies to free surface flow and flow near solid boundaries. The application of theoretical concepts to understanding the factors control- ling surface properties and the evaluation of complex process models will require access to supercomputers. Funding must be provided to support research at academic and industrial institutions. Re- searchers in universities will require funds for research assistants, instrumentation, computer time, and travel to use special facilities such as synchrotron radiation sources, neutron sources, and atomic resolution microscopes. The primary support for these efforts should come from federal agencies, with additional support pro- vided by industry. Industry will also need to finance its own research and development ef- forts. One should anticipate that generic long- range work will be carried out at universities, whereas research leading to specific products and processes will be conducted primarily in industrial laboratories. Collaborative investi- gations between university and industry scien- tists should be strongly encouraged, since such efforts will help define the goals and objectives of intermediate- and long-range research and facilitate the transfer of new ideas and tech- niques into practice.
Representative terms from entire chapter: