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Expanding the Vision of Sensor Materials EXECUTIVE SUMMARY DEFINITIONS AND ISSUES Sensor technology is a rapidly growing field that has significant potential to improve the operation, reliability, serviceability, and utility of many engineering systems. Advances in materials science and engineering have paved the way for the development of new and more capable sensors. U.S. industry is experiencing shorter innovation cycles, growing technical complexity of its products, and increased costs in conducting and commercializing research and development (R&D). The nature and scope of commercial R&D programs in sensor technologies, including sensor materials, are largely determined by these market drivers plus the potential size of the available market. The trend is toward the development of materials tailored to specific, or targeted, applications rather than toward fundamental R&D without a specific application-focus (i.e., toward materials development driven by ''market pull" as opposed to "technology push"). However, technology-driven, leading edge research is vitally important, since the results from these efforts have the potential to create entirely new products and markets. Two categories of sensors and sensor materials development can be distinguished: high-volume, low-cost sensors and low-volume, high-cost sensors. In the case of sensors for mass market applications, clearly defined R&D strategies have frequently been identified and implemented. Often, the selection and development of materials for niche market applications do not originate from a logical, top-down strategy but rather from a serendipitous combination of technical and commercial considerations. Such innovative developments have led to major breakthroughs in "pathfinding" applications and have served as a means to gain much needed operational experience with a new technology. Exceptions to this generalization of development approaches include niche markets, such as well-defined defense applications. Advancing the sensor technology state of the art has been limited by the lack of a widely accepted language for describing sensor needs and performance. Potential users of sensor technology often speak a different technical language from the sensor technologists involved in developing sensors. In response, the Committee on New Sensor Technologies: Materials and Applications prepared a communication tool that employs a common set of descriptors to map sensor application requirements to sensor technology attributes, and vice versa. This tool can provide guidance in identifying opportunities for sensor materials development and application areas in which materials can be further exploited. MATERIALS FOR SENSORS: IDENTIFYING NEEDS A considerable body of published work relates to the vast topic of sensor technologies. The committee compiled an extensive bibliography of the recent
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Expanding the Vision of Sensor Materials literature to document the state of the art. A review of the literature indicates that there is at present no common language concerning the definition and classification of sensors. The domain of endeavor is very broad and encompasses all technical disciplines. Sensors can be categorized in a number of different ways, most notably by either their chemical composition or their principle of operation. To address this lack of standardization and to establish a solid framework for its own analysis of the field, the committee defined the basic terms associated with sensor technology. A sensing element is the fundamental transduction mechanism (e.g., a material) that converts one form of energy into another. A sensor supplies a usable output in response to a specific measurand (input) in a predictable way; a sensor's physical configuration includes the sensing element together with its physical packaging and external connections (e.g., electrical or optical). A sensor system is a sensor and its assorted signal processing hardware (analog or digital) with the processing either in or on the same package or discrete from the sensor itself. The diversity of sensor technologies and applications and the resulting diversity of materials needs led the committee to conclude that the concept of an "ideal" sensor material is inadvisable. It is frequently possible to fulfill a given sensing need by more than one type of sensor. Thus, identification of the "best" sensor material should only be done within the context of a specific application. SUMMARY OF CASE STUDIES Since the present report could not realistically provide a comprehensive list of research opportunities for the entire field of sensor materials, a "case study" method was used to illustrate the different driving forces and considerations that affect the development and incorporation of new sensor technologies. Examples of sensor materials technologies were synthesized from two applications areas (manufacturing, and structural monitoring and control) and two sensor technology categories (long-wavelength infrared sensors and chemical sensors). Manufacturing Intelligent processing can reduce product variability, optimize use of processing facilities, and decrease costs. As defined by the committee, intelligent processing involves event-based control of process variables. Sensors are currently the weak link in intelligent processing. They must perform reliably in hostile manufacturing environments and provide data that permit accurate representation in time and space of the changes occurring as the material is processed. Intelligent (self-directed) processing of aerospace structural polymeric composites (e.g., thermosetting resin systems) is an illustrative example. In this case, the state of cure of the polymer is determined, and then the process variables are adjusted to achieve the most efficient cure cycle. There is a significant need to improve sensor materials and technologies for this application, notably by directly measuring the molecular structure of the polymer to more precisely determine the degree of cure. Laser-fiberoptic sensor technology has considerable potential in this regard. The fabrication and processing of complex semiconductor materials is an important and expanding industrial endeavor. There is a significant need for in situ diagnostics to permit precise on-line process control during epitaxial growth of electro-optical thin films. The manufacture of low-cost, reproducible, uniform, and tailorable structures needs noninvasive sensors to measure film thickness, composition, interlayer sharpness, and other properties. Notable opportunities exist for optical sensing technologies (e.g., ellipsometry, laser-induced fluorescence, and fiberoptic probes) that can significantly improve the processing of complex semiconductor devices. Structural Monitoring and Control "Smart" structures that incorporate active materials are emerging as an important broad-based discipline. A step toward the development of such "smart" structures is the incorporation of sensor systems that provide accurate information describing the state of a structure at any given time throughout its life cycle. This life-cycle management
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Expanding the Vision of Sensor Materials approach has the goal of combining the traditional fabrication and customer-use periods of a product. The availability of advanced sensors and actuators, together with developments in signal processing, communication, and control technologies, has led to a surge of interest in smart structural materials systems that can adapt to an ill-defined, changing environment. Important issues include evaluating the long-term performance of sensors used for in-service monitoring, understanding the sensor/host interactions for embedded-sensor applications, and focusing on improved long-term reliability of sensors for in-service environments. Long-Wavelength Infrared Sensors The sensing of electromagnetic radiation is essential for a wide variety of activities. Sensing radiation in the long-wavelength infrared (LWIR) window (spectral region with a wavelength of 8 mm to 14 mm) allows detection of unilluminated objects that are approximately at room temperature. Infrared sensors are attractive because they are non-contracting and can quickly sense a temperature change over an area. This capability is important for applications such as measuring part temperature, detecting process defects, enabling night vision, and identifying chemical species. There are three materials strategies—at different levels of maturity—for obtaining high-efficiency photodetector LWIR sensors. Mercury cadmium telluride (MCT) compounds. The quality of LWIR MCT has improved over the last decade, and continued incremental improvements may eventually yield temporally stable, uniform detector arrays for LWIR applications. However, materials instabilities still result in major challenges. Artificially structured materials with tailored band gaps based on multiple-quantum-well devices (e.g., GaAs/AlGaAs system). The theoretical sensitivity of these sensors is lower than that of MCT; however, because of superior response uniformity, arrays of these structures already exhibit performance exceeding that of MCT detector arrays for selected applications. These materials can be produced using real-time sensor-based process controls developed for GaAs. Artificially structured materials with tailored band gaps based on strained-layer superlattice structures (e.g., those that exploit In [As, Sb] alloys). The fundamental detectivity limit of these materials is higher than that of MCT. It is the least mature of the technologies, and substantial improvements in performance appear to be possible, since no fundamental limitations have emerged as obstacles. Chemical Sensors The committee defines chemical sensors as devices or instruments that determine the detectable presence, concentration, or quantity of specific chemical substances (analytes). Arguably the most severe limitation on current chemical sensor technology is the inability to obtain a selective response to a target analyte, given the millions of known molecular species, the variations in environmental conditions (presence of water, etc.), and the variations in analyte amount or concentration by factors of 1023 or greater. Applications of chemical sensors include monitoring manufacturing processes, environmental sensing, and health monitoring. Direct-reading selective sensors, such as electrochemical sensors, detect species in the gas or liquid phase. They achieve molecular selectivity through interaction at the sensor-sample interface. This selectivity depends on recognition of the size, shape, or dipolar properties of the analyte by molecular films, phases, or "receptor sites," with resultant selective binding, absorption, or permeation of the analyte. Selectivity of direct-reading chemical sensors can be enhanced through the development of analyte-specific films, membranes, and coatings. Miniaturization of these sensor systems could lead to compact, lightweight, portable monitoring systems. As an alternative, analyte selectivity can be addressed by using sensors that incorporate preliminary sample separation steps, such as chromatography and electrophoresis. Conventional analytical chemistry methods, such as mass spectroscopy, may then be used if the analysis can be performed fast enough and the equipment is significantly compact
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Expanding the Vision of Sensor Materials and inexpensive. Miniaturized and integrated platforms that incorporate both separation and detection systems would be preferable. GENERAL CONCLUSIONS Trends in Sensor Technology Current sensor development is trending toward increased complexity in sensor systems. The greater flexibility and lower cost of advanced, integrated electronic technology allows signal processors to be reduced to a microelectronic chip; however, from the perspective of the end user, the sensor system appears simpler. The principal technical drivers for sensor development are becoming enabling/supporting technologies other than materials technology. Most recent advances in sensors have not originated from the synthesis of new transduction materials; they have been due to sensor technologies not heavily dependent on transduction materials and to innovations that have significantly decreased the cost of a sensor system. Networking of large sensor systems can provide improved spatial and temporal sampling in low-cost, low-maintenance systems. A network of sensors can provide data to a central processor that monitors performance or helps characterize defects. Also, the individual outputs from an array of sensitive but moderately selective transducers can provide a composite indication that is both sensitive and selective. Sensor R&D lends itself to dual use and commercialization efforts. Sensors are an enabling technology with a wide spectrum of applications. R&D Strategy Few programs have existed to develop sensors solely for the sake of advancing sensor technology. Historically, sensor research and development efforts have been funded as an adjunct to large application programs that required sensors. A new approach will require the implementation of a research planning process that addresses the needs of a broad set of users and applications. A generally accepted framework to describe both sensor application requirements and sensor performance capabilities is needed. A common set of descriptors for use by sensor users, suppliers, and researchers was identified by the committee as the most important step in facilitating the identification of sensor materials R&D opportunities and in accelerating the development and use of advanced sensor technologies. Experience in establishing centers of excellence for sensor development provides useful guidelines for improving sensor R&D strategy. Essential characteristics include: a multidisciplinary approach with emphasis on teamwork; capabilities ranging from an initial proof of concept through engineering prototypes; focus on selected sensor technologies for a broadly defined range of applications; and strong linkage to industry to guide the general relevance of the research. Opportunities in Materials R&D Sensor materials include all materials required by the sensing system. These materials encompass those required by the transducer, package, and leads. Sensor materials R&D can be divided into two main categories: the development of new materials and materials engineering for particular applications. These two categories frequently require very different approaches to materials R&D. High payoff opportunities for new sensor materials in the near term will come primarily from R&D on existing materials rather than synthesis of new compositions of matter. The committee identified fundamental research on new compositions of matter as the highest risk element of sensor materials R&D programs, although this approach also offers the highest potential payoff. Exploiting materials developed for purposes other than sensing can lead to rapid sensor technology advancements at relatively low cost and risk. Materials processing science will be the foundation for developing affordable sensor materials. Materials synthesis and processing will facilitate the transfer of innovations in materials science to commercially viable products. Universities and federal research laboratories play a critical role in conducting frontier research. Universities are well positioned to conduct frontier research and to use such programs as vehicles to educate students. Federal research laboratories generally
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Expanding the Vision of Sensor Materials sponsor frontier research and conduct a portion of the research in-house to keep abreast of the leading-edge technologies. Long-term commitment to such research is essential to remain technically competitive internationally. GENERAL RECOMMENDATIONS R&D Strategy In the view of the committee, focused programs in which sensors are treated as a separate field of endeavor, as opposed to an adjunct to larger programs, will significantly accelerate the development and use of advanced sensors. The R&D approach aimed at satisfying high payoff opportunities in sensor technology should emphasize the multidisciplinary integration of existing technologies for specific or generic applications. The committee recommends use of a communication tool to facilitate interdisciplinary communication in the identification of research opportunities and needs for sensor systems and technologies. A standard terminology is needed to describe sensing requirements and technology attributes. The communication tool and descriptors should form the basis for an evolutionary methodology that can be augmented and refined for use by specific research groups and applications. Organizations undertaking sensor R&D programs should maintain a broad research base with critical core competencies. These organizations should possess, or have access to, expertise from all technical disciplines involved in the sensor technology under investigation. To give a sense of relevance and urgency to any applied R&D program, a customer or end user with a specific implementation need should be identified and charged with demonstrating the potential payoff in a joint effort with the developer. R&D programs that develop sensor materials should focus on selected classes of materials. In view of the diverse range of sensor materials and the high costs of fabrication facilities for many advanced sensor materials, the committee recommends that specific R&D programs set priorities for selected classes of materials rather than attempting to encompass a very broad range of endeavor. Priorities in Materials R&D The committee recommends that sensor materials R&D be pursued in three main areas. In order of decreasing priority these are development of processing techniques for existing sensor materials; assessment and development of sensing capabilities in existing materials that have properties not yet exploited for sensor applications; and fundamental investigation of novel sensing approaches, such as using multiple physical responses to a sensing phenomenon and new compositions of matter. ORGANIZATION OF THE REPORT The report is divided into three parts. Part I, "Definitions and Issues," has two chapters that provide the basic definitions used throughout the report and suggests approaches to planning and conducting research and development in sensor materials. The basic definitions and discussion of the state of art are contained in Chapter 1. The generic framework and suggested approaches to sensors R&D is the subject of Chapter 2. Part II, "Materials for Sensors: Identifying Needs," has four chapters that contain the illustrative examples of opportunity areas for sensor development. Topics discussed are manufacturing (Chapter 3), structural monitoring and control (Chapter 4), LWIR sensors (Chapter 5), and chemical sensors (Chapter 6). R&D opportunities in sensor materials that arose from these case studies are highlighted at the end of each chapter. Part III, "Opportunities, Conclusions and Recommendations," contains two chapters that summarize the materials development opportunities that originated from the illustrative case studies described in Part II, as well as generalized conclusions and recommendations that resulted from the committee's synthesis of the material in parts I and II. Chapter 7 is a synopsis of the material development opportunities from Part II. Chapter 8 discusses the committee's general conclusions and recommendations. In addition, there are seven appendices, which contain more detailed information on the sensor terminology and technologies that are discussed in the report.
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