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Expanding the Vision of Sensor Materials
FIGURE C-2 Example of thermocouple transducer.
based on the magnitude of the energy change to be detected. For example, the detection of chemical signals is generally less than the measurement of chemical bonding or binding energies, which are likely to be in the range of 0.025 volt. In contrast, the energies associated with thermal signals are significantly greater, corresponding to wavelengths and intensities in the thermal energy spectrum. Similarly, sensing of radiant, electrical, and magnetic signals requires the detection of energies in the relevant parts of the electromagnetic spectrum. For example, the long-wavelength infrared detectors discussed in Chapter 5 depend upon the use of semiconductor materials that can efficiently absorb radiation in the wavelength band of 8 to 14 microns; this requirement necessitates the use of sensor materials with band gaps less than about 130 MeV.
Figure C-5 shows that sensitivity requirements for sensors used in materials processing depend not only upon the signal energy magnitude and form, as discussed above, but also on the scale of the material property to be measured. In the case of metal processing, the properties of interest can range from nanostructural features, such as point defects and dislocation densities, through microstructural features (phase transformations, grain size) to millistructural and macrostructural properties such as tensile strength. The selection and design of an appropriate sensor depends upon an understanding of the scale of measurement and sensitivity required for the proposed application. The communication tool presented in Chapter 2 offers an effective means for matching the measurement scale of the application with the capabilities of candidate sensor technologies. In the polymer matrix composite processing example discussed in Chapter 3, sensing requirements to determine part quality are based on the determination of bulk properties. In contrast, the in situ diagnostic techniques under development to monitor the fabrication of band-structure-engineered semiconductor materials depend on the determination of materials properties at nanostructural and microstructural scales.
Figure C-6 illustrates some of the practical constraints associated with sensor applications for materials processing. These constraints apply regardless of the signal form or transduction type. For example, there is frequently a requirement for sensing