Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter.
Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 153
OCR for page 154
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.
OCR for page 155
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
OCR for page 156
~ 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
OCR for page 157
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
OCR for page 158
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
OCR for page 159
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
OCR for page 160
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
OCR for page 161
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
OCR for page 162
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
OCR for page 163
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
OCR for page 164
~-
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
OCR for page 165
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
OCR for page 166
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
OCR for page 167
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
OCR for page 168
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
OCR for page 169
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.
OCR for page 170
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
OCR for page 171
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
OCR for page 172
~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.
OCR for page 173
SURFACES, US, AND ^~ICROSTRD CTURE;
Particulate microstructures, as well as the
fragments obtained by disrupting more exten-
sive structures, are separated by equipment of
varying cost and sophistication: ultrafilters, ul-
tracentrifuges, gel permeation and size exclu-
sion chromatography, and electrophoretic sep-
arators. The ultimate goal is rapid, automatic
sorting of individuals from a population of
particles upon sensing one or more properties
of each. In the laser spectroscopy cell sorter,
this capability has reached down to the scale of
living cells through application of several tech-
nologies, including inkjet printing. The cell
sorter has already opened up research into the
population structure of cell cultures, a basic
research problem important in biotechnology.
Cost and Availability
Instruments for probing microstructures and
their changes typically follow a rule that the
costs of purchase, installation, operator exper-
tise, and equipment maintenance become higher
as the dimensions of the structure to be meas-
ured become smaller, as the connectivity and
shape to be examined become more complex,
and as the time between events to be resolved
becomes shorter. Management of such research
gets complicated as its scale moves into the
gray area between small science and big science.
The need arises to share instruments within a
department, an institution, or a regional center,
or, in some cases, a national or international
facility, an activity that can become cumber-
some when the instruments being shared are
central to an investigation. Cutting-edge re-
search often calls for improvement, adaptation,
and augmentation of equipment. Research can
be stultified when scheduling a shared instru-
ment inhibits hot pursuit of a finding or an idea,
when the needs of others prevent modification
of an instrument, or when a key experiment
faces the risk of a temporary shutdown of an
instrument.
Over the long run, the costs of sophisticated
equipment fall to the level where the equipment
can be acquired for use and adaptation by
individual research groups. But in the interim,
while costs are sufficiently high that the level
of usage of advanced equipment does not justify
173
widespread acquisition, or while the requisite
financial resources cannot be marshaled, an
effective strategy may be for funding agencies
or institutions to provide reasonably complete
subsets of the sophisticated instruments needed
for surface and microstructural engineering to
selected groups that would focus on a coherent
theme. The result would be to create a small
community of users (e.g., three to four faculty
members) with similar interests in terms of use
of the equipment. While some needs of the
group may go unmet, requiring the use of
equipment in other locations, the investigators
should generally be able to carry out the major
portion of their preparative and analytical work
in close proximity to their laboratories.
Theory
Significant advances are needed in our current
understanding of how molecules interact with
surfaces and with each other to form micro-
structural units. Theoretical efforts along these
lines should be carried out starting at the mo-
lecular level and extending to the level of bulk
materials. The development of a hierarchy of
theoretical methods for predicting the behavior
of increasingly complex ensembles of molecules
will be invaluable in understanding how best to
process materials. Examples of specific areas
requiring development were discussed earlier
in this chapter.
Development of the necessary theoretical
models will involve a careful integration of
insights from different disciplines. Concepts
new to chemical engineers (e.g., fractals, Monte
Carlo methods, and percolation theory) will
have to be introduced to provide more accurate
and/or computationally efficient means for for-
mulating process descriptions. Chemical engi-
neers 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. In par-
ticular, collaborative efforts between theoreti-
cians and experimentalists should be encour-
aged as a means to new theoretical approaches
and insights.
The need for access to supercomputers, dis-
cussed in detail in Chapter 8, cannot be over
OCR for page 174
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:
thin films
