High and Low Frequency Scanning Acoustic Microscopes

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This is a proposal for the purchase of high (burst model) and low (pulse mode) frequency scanning acoustic microscopes (SAM). Micro- mechanical measurement of materials is now possible at resolutions down to the order of 0.1 mum using scanning acoustic microscopy at frequencies up to 2 GHz. Ultrasonic scanning at the energy involved is notably a non-destructive technique; materials are left intact and unaffected by this analytical technique. In the micro-mechanical measurement of materials there is a trade-off, however, between resolution and depth of penetration. The higher the frequency the greater the lateral resolution but, the depth of penetration is limited, especially for those materials with high acoustic impedance values. The low frequency SAM is particularly useful for imaging of internal defects and interface discontinuities. In summary, a distinct advantage of SAM is the ability to investigate the properties of internal and subsurface structures in addition to the surface properties of most materials, including those that are optically opaque. Another advantage of this technique, especially for those studies involving biological tissues is that a liquid couplant (usually water) must be used to transmit the acoustic waves, from the acoustic lens to the specimen being investigated. The "wet" environment reduces the potential for introducing artifacts in those biological specimens that are sensitive to desiccation. The high and low frequency SAM would be a shared resource and a cadre of 5 NIH-funded scientists have been identified as primary users. The application of the proposed instrumentation to these projects runs the gamut from micro- mechanical measurement at the cellular/tissue level to characterization of subsurface flaws and interfacial coupling defects in experimental oxirane/polyol composites. One of the scientists is interested in determining whether the lack of mechanical strain permits the osteocyte to send signals initiating bone resorption (P01AR46798). This investigator would use SAM to determine the micro-mechanical properties of bone after the applications of various magnitudes of mechanical strain; this instrumentation would provide the investigator the unique opportunity to image the micro-mechanical properties of the bone at the level of individual osteonic lamellae. In a separate study (P01 DE09696) SAM would be used to study the fracture mechanics of newly synthesized low-shrinking and low-stressing producing resin composite restorative materials. SAM would be used in combination with micro- Raman spectroscopy to determine the molecular structure and elastic properties of the d/a interface in situ (R01 DE12487). These measurements will e completed not only on the same specimen but the same small region of the interface.