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Acoustic emission from the crystallization of potassium bromide doped with lead Cook, Adrian P.

Abstract

Simultaneous optical and acoustic information was used to elucidate the mechanism that causes acoustic emission (AE) during growth of potassium bromide (KBr) crystals. The generation mechanism postulated here is one of inter-crystal interaction. AE was used to monitor the growth of different crystal structures of KBr grown with varying degrees of lead (Pb) dopant (0-32000 ppm). Digital images were collected simultaneously at magnifications up to x75. Integrated AE profiles and image whitening curves were calculated. Best agreement between these was found for the crystal growth of the dendritic form of KBr (4080-8120 ppm Pb dopant), and the doped forms of 12210 ppm Pb and above. The dendritic form of KBr observed between 4080 and 8120 ppm Pb consisted of finger-like needles that grew outward from the crystallizing solution. Scanning electron microscopy showed these fingers to consist of contiguous chains of tiny (< 25 μm dia.) octahedra. This microscopic structure provided many potential sources of emission. The total AE observed per gram of crystals produced during the growth of the different KBr morphologies was found to vary in a reproducible manner over 60 experiments. The dendritic form of KBr exhibited 10 times the acoustic emission per gram of crystals produced compared with the undoped (cubic) form. The mass of crystals grown for individual experiments was 0.004-0.212 g. AE was able to reliably follow the growth of these very small masses of crystals whose morphologies consisted of clusters of crystals in intimate contact. A confirmatory mechanistic study was carried out using ammonium chloride (NH₄Cl) which is known to produce classic dendritic growth. In contrast to the clusters of tiny microcrystals observed for the dendritic form of KBr, one would not expect the growth of the single crystal branches of NH₄Cl to exhibit acoustic emission, as there is no means to generate AE by the inter-crystal interaction mechanism. Single branch fracture is possible but rare. Experimental results showed very limited emission from NH₄Cl, and thus supported the hypothesis of inter-crystal interaction. Primary nucleation could not be detected acoustically. The end of crystal growth was acoustically determinable. The AE associated with the growth of the dendritic form of KBr finished abruptly when the maximum image whiteness was reached. The acoustic waveforms detected during the growth of the KBr morphologies were compared with artificially produced signals of bulk fracture and crystal impact. Acoustic waveform analysis showed these classes of signals to be distinguishable.

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