Research Briefs

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research briefs bulletin f i r s t This article first appears exclusively in the Bulletin, and can later be found online on Ceramic Tech Today. Acoustic detection method may identify early cracks in ceramic capacitors and beyond A Food and Drug Administration report from 2006 found that battery or capacitor abnormalities accounted for 23.6% of failed pacemakers and implanted defibrillators. Although only 0.65% of the almost three million such devices examined in the study failed, 61 patients died because of device failures. Although that is a small percentage of the overall study population, the data mean that 61 lives could potentially have been saved by better materials or better ways to monitor those materials. Researchers at the National Institute of Standards and Technology may be on to something that can help with better materials monitoring— they have devised a new nondestructive method that may be able to detect cracks in materials before they lead to device failure. The NIST scientists, in collaboration with scientists at the University of Maryland, NASA Goddard Space Flight Center, and Colorado State University, have developed a testing prototype that they show can detect cracks in barium titanate-based multilayer ceramic capacitors. The method uses a brief electric field to make the capacitor vibrate at a particular frequency and then measures how the signal decays over time. The scientists measure how the frequency shifts in relation to the vibration. This provides important clues into the integrity of the capacitor, because cracks in the material create greater frequency shifts. Absolute shifts in frequency are important—but this approach also has a significant and integral advantage in that it can assess various capacitors, according to a NIST press release about the work. “This nonlinear approach—focusing on frequency shifts relative to signal strength rather than the frequency shifts alone—is especially useful because it is not affected by slight variations in size of the capacitors.” To test how well it could work, the scientists applied the technique to 41 barium titanate capacitors (2 mm × 3 mm) before and after a round of thermal abuse designed to generate cracks in the ceramic. Using a box furnace, the scientists heated the capacitors to 189°C and then plunged them into ice water, generating surface-breaking cracks in some of the specimens (27 of the 41), the authors write in the report’s abstract. They then compared the before and after acoustic measurements to determine if the data revealed noticeable differences between measurements for visibly-cracked and nonvisibly cracked capacitors. “The nonlinear acoustic results were strongly correlated with the presence of visible cracks: Measurements on 25 of the 27 visibly cracked capacitors yielded results that were outside the range of those for capacitors without cracks,” according to the NIST release. The authors say these results, although not perfect, show the potential of the method to reliably detect cracks. They call it a “promising approach,” meaning that the results could pan out with further tweaking and optimization of the method. In addition to ceramic capacitors, the researchers also say that the method may help detect structural anomalies in other materials as well. The open-access paper, published in conference proceedings from the 2015 Annual Review of Progress in Quantitative Nondestructive Evaluation, is “Time–domain analysis of resonant acoustic nonlinearity arising from cracks in multilayer ceramic capacitors” (DOI: dx.doi. org/10.1063/1.4940511). n Credit: NIST/NASA NIST studied 3-mm-long capacitors (top), looking for cracks similar to the one shown in the NASA photo (bottom). 12 www.ceramics.org | American Ceramic Society Bulletin, Vol. 95, No. 3


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