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Midsize Facilities: The Infrastructure for Materials Research H Personal Perspectives from Howard K. Birnbaum CENTERS FOR MICROCHARACTERIZATION: ESSENTIAL AND ENABLING TOOLS FOR MATERIALS RESEARCH H.K. Birnbaum Materials Research Laboratory University of Illinois In this age of introspective research funding, it is appropriate to question the funding of the microcharacterization centers in the same manner that we question other activities. While the sciences that comprise the field of microcharacterization are widely used by chemists, physicists, materials scientists and engineers, and by researchers in a number of other disciplines, microcharacterization plays an important educational role only in a limited number of fields other than materials science. This somewhat anomalous situation exists, despite the fact that about 31 percent of recent experimental articles published in the materials physics literature and about 45 percent of recent experimental articles published in the materials chemistry literature utilized one or more methods of microcharacterization. For the purpose of this statement, microcharacterization is defined as the sciences utilized to specify the structure and/or chemistry of solids on a scale appropriate to the measurements or synthesis being carried out. In almost all cases, this characterization is carried out at a size scale that is small (of the order of nanometers) in at least one dimension. Even in the case of surface analysis, while
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Midsize Facilities: The Infrastructure for Materials Research the measured area may be relatively large, the third dimension, depth into the surface, is microscopic in dimension. Included within the scope of microcharacterization are the sciences and methods of electron microscopy (transmission electron microscopy [TEM], scanning transmission electron microscopy [STEM], low-energy electron microscopy [LEEM], spin polarized low-energy electron microscopy [SPLEEM], scanning electron microscopy [SEM], reflection high-energy electron diffraction [RHEED], etc.), surface analysis (Auger electron spectroscopy [AES], scanning Auger electron spectroscopy [SAES], ultraviolet photoelectron spectroscopy [UPS], x-ray photoelectron spectroscopy [XPS], secondary ion mass spectrometry [SIMS], scanning tunneling microscopy [STM], atomic force microscopy [AFM], other scanning probe microscopies, etc.), ion beam analyses (Rutherford backscattering [RBS], particle induced x-ray emission [PIXE], channeling, etc.), x-ray and neutron scattering (wide angle x-ray scattering [WAXS], small angle x-ray scattering [SAXS], fluorescence analysis, etc.), and the group of techniques classified as field emission, field ion, and atom probe microscopies. A somewhat broader view of microcharacterization would include techniques such as nanoindentation. The incomplete listing above demonstrates that the field of microcharacterization is a highly dynamic one and one that makes use of a wide range of physical phenomena. These techniques meet the needs of materials scientists to understand the structure and chemistry of complex systems in order to understand the properties that interest them. In view of the above, it is fair to ask why the field of microcharacterization remains the “Rodney Dangerfield” of the physical sciences. The lack of incorporation of these sciences into the departments of physics and chemistry in the United States stems from a number of factors and differs for the different techniques. X-ray scattering, in its various manifestations, is accepted in these science departments, perhaps due to its quantitative nature, although academic departments rarely hire faculty whose main research interest is in this field. Surface analysis techniques are widely accepted in departments of chemistry as tools to be used but are rarely a part of the teaching curricula. Electron microscopy methods, although they are widely used by the physics and chemistry research communities, are not considered as subjects for inclusion in the curricula. Rarely are faculty whose area of interest lies in electron microscopy included in the faculty of these departments. While the science underlying electron microscopy methods is considerable and is sophisticated, it is only recently that the field has progressed to the stage where quantitative results can be obtained. It is this lack of quantification that probably lies at the core of
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Midsize Facilities: The Infrastructure for Materials Research the lack of acceptance as a field of physics. In the case of chemistry departments, this lack of acceptance probably stems from the very recent use of the methods by chemists. Only within the relatively small fields of metallurgy and ceramics have these methods won acceptance and respectability. The microcharacterization centers are faced with the somewhat daunting prospect of providing access to a wide range of techniques based on sophisticated instrumentation and sciences to a user base that would be greatly hampered by the absence of such access. Yet the fields of science to which these capabilities are provided, and in which they play a crucial role, have only a limited acceptance of microcharacterization science as a valid field of research. In a very real sense, the capabilities provided by the sciences of microcharacterization and by the centers would be appreciated only if they become unavailable. Microcharacterization centers play two crucial roles in support of Department of Energy (DOE) science. The first is that they provide a primary tool for study of important problems—studies that could not be carried out without access to these techniques. Examples of this role abound and a few are cited below. Investigations of grain boundary and interface structures would remain empty exercises in geometry and simulation without the direct observation provided by atomic resolution TEM. Multilayer structures and the defect structure of thin films and multilayers (interface dislocations, threading dislocations, microstructure evolution in polycrystalline layers, etc.) would remain in the realm of speculation without the methods of cross-sectional TEM. Surface reconstructions and their role in controlling surface chemistry and film growth would not have been discovered or understood without the use of high-resolution TEM, LEEM, and STM techniques. The electronic structures of surfaces and of molecules adsorbed onto the surfaces would not have been understood without the use of XPS and UPS methods. Atomic site locations of solutes were established using ion channeling. The structure of sonochemically synthesized catalysts and solids was established using SEM and TEM methods. Determination of the crystallographic dependence of catalytic activity required the use of TEM and STEM based techniques. Fracture processes at crack tips would have remained speculation without the use of x-ray topography and in situ TEM methods. Elucidation of hydrogen effects on deformation, fracture, and hydride precipitation would not have been possible without in situ environmental cell TEM and SEM methods.
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Midsize Facilities: The Infrastructure for Materials Research The chemistry of grain boundaries and interfaces can only be understood with Auger, XPS, UPS, STEM/EDS, and STEM/EELS methods. Many aspects of dislocation behavior would remain exercises in the application of elasticity theory without the direct observations of TEM and in situ TEM experiments. Medium-range atomic correlations and fluctuations, of great interest in amorphous materials and glasses, can only be studied with TEM and coherent x-ray methods. Nanotubes were discovered using TEM and are still best studied with TEM techniques. Much of what is known of epitaxial growth results from the use of TEM, LEEM, STM, and x-ray methods. Quasicrystals and their unusual symmetries were discovered using TEM methods. The structure and chemistry of amorphous grain boundary films in ceramics have been determined using TEM and microchemical TEM based methods. Mass transport during oxidation would not be directly measurable without the isotopic sensitivity of the SIMS method. Development of self-assembling nanostructure structures and thin films is dependent upon the observations using TEM, SEM, AFM, and STM and on the surface chemical knowledge developed using XPS, UPS, and SAES. The atomic structure of domains in ferroelectric relaxor materials has been established using STEM imaging methods. Microcharacterization science also provides an extremely important tool for the support of other kinds of measurements. In many cases, this secondary role is so important that it can be considered to be enabling in allowing the science to be carried forth. In these cases, the microcharacterization is often carried out as a cooperative effort between colleagues, one of whom is expert in the microcharacterization methods and interpretations. Examples of this type are: Correlations between physical properties (magnetic, mechanical, electrical, etc.) and structure (grain shape, preferred orientation, and distribution, particle size, shape and distribution, defect structure, interface structure, etc.) are often critical in understanding the behavior of materials. Development of thin film and multilayer growth methods requires
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Midsize Facilities: The Infrastructure for Materials Research close interactions between the processing scientist and the structure analysis that is generally based on determination of the chemistry and defect structure of interfaces and of the crystal layers. Growth and properties of quantum confinement structures, such as quantum dots and quantum lines, were greatly enabled by the ability to visualize these structures using TEM and STM methods. SIMS is essential for the understanding of doping of semiconductors. The first observations of dislocations and other lattice defects were carried out with TEM. Development of our understanding of these defects was enabled by TEM methods. Our understanding of micromechanics is based on the understanding of dislocations and defect interactions based on TEM observations. Flux lattice behavior in superconductors was established by direct microscopic observations. Imaging of discommensurations in charge density waves has been carried out by TEM methods. Theories of kinks on dislocations and dislocation kinetics were confirmed by direct observations in the TEM. Catalysis cannot be understood at the level of microscopic rate-structure correlations without the use of XPS and UPS techniques. Understanding of the dependence of catalytic activity on crystal orientation was made possible by the use of TEM and STEM based microchemical measurements. Understanding of the rate-limiting reaction paths in thin film interfacial reactions necessary for developing improved diffusion barriers in fields as diverse as microelectronics and wear protection was made possible with the use of TEM and STEM based microchemical techniques. Development of combustion exhaust remediation depended critically on the knowledge of catalytic and support structures developed by TEM methods and on detailed chemical analysis of the catalyst and substrates during poisoning or deactivation. The new methods of combinatorial synthesis are dependent upon the ability to characterize the microdots structurally and chemically with TEM, x-ray, XPS, and UPS methods. The above examples, by no means a complete list, illustrate the important role played by microcharacterization science in materials science, condensed matter physics, and materials chemistry. Considering the portfolio of research at the DOE laboratories, the role of the characterization centers is perhaps greater than in the average research organization. Microcharacterization certainly plays an extremely
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Midsize Facilities: The Infrastructure for Materials Research important role in industrial laboratories as evidenced by their large investments in and support of microcharacterization efforts. As the above clearly establishes, there is very significant science carried out using microcharacterization methods and even more extensively enabled by microcharacterization science. The DOE microcharacterization centers play an essential role in the DOE science and technology portfolio. In many ways they fulfill complementary roles to those played by the large DOE facilities such as the synchrotron and neutron facilities. While both are important to the DOE effort, there are distinct differences in their modes of operation (in addition to the very large differences in their operating budgets). The large facilities provide intensive user time at limited periods during the year. They generally require a proposal approval for access requiring advance planning. In contrast, the microcharacterization centers generally provide frequent access to their instrumentation and generally (but not always) do not require a proposal process for access. Thus, synchrotron users may have access to synchrotron beams 3 to 4 times per year or less while microcharacterization center users often use the instruments on a weekly or more frequent basis. At many of the microcharacterization centers, the researchers make use of multiple techniques for their studies—indeed this is essential in addressing complex materials problems. Their very nature requires that synchrotrons and neutron scattering facilities serve large regional or national clienteles. In contrast, the microcharacterization facilities serve more local communities, although in many cases, they have unique instruments and capabilities that attract users nationally or internationally. Since time at the large synchrotron and neutron facilities is available only on a very competitive basis, the user community generally consists of scientists who are experts at the scattering methods. In contrast, there is a greater spectrum of users at the microcharacterization facilities. The users range from experts to novices who are being educated in the techniques. In addition, there is a large cadre of users requiring the information from microcharacterization methods and who work in cooperation with the expert users. It is often argued that scientists active in the fields of microcharacterization are primarily interested in the instruments and their development. This attitude mirrors the lack of appreciation by the traditional sciences of the importance of the sciences of microcharacterization. As in many fields, instruments have become so complex that it is folly to try to construct them in house when they are available commercially. Hence, as the methods become more comprehensive and capable, there is a desire to obtain that
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Midsize Facilities: The Infrastructure for Materials Research greater capability to enable the sciences researchers want to carry out. In recent years, the rate of significant advances is such that instruments should be replaced with a frequency of about 10 years. Is this too large an investment? I think not, when the increased capability to answer important questions in the very wide range of science enabled by microcharacterization is taking into account. Thus in many ways, the large facilities and the microcharacterization centers play complementary roles in the range of DOE facilities. While they serve complementary purposes, it would be difficult to imagine a DOE research environment with either of the two types of facilities missing.