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Acoustic emission from the synthesis of dichloro(pyrazine)zinc(II)

University of British Columbia

The reaction of pyrazine with ZnCl₂ m aqueous solution results in copious acoustic emission in the ultrasonic region. When first reported, no satisfactory explanation was given for this observation. The aims of this thesis are to determine the source of the acoustic emission and, where possible, to establish experimental relationships between the system's chemical and physical properties and its acoustic emission. The detection system used consisted of a piezoelectric transducer mounted beneath the reaction vessel. Its conditioned output was connected to a peak acoustic emission intensity integrator, and to a transient digitizer, which allowed individual signals to be captured. The acoustic signals were broad-band between 300-800 kHz; this work indicates that they originate from crystal fracture. The reagent concentrations, temperature, pH, and type of solvent were found to affect the acoustic emission intensity vs. time profiles and the crystal habit formed. In most instances, the integrated acoustic emission intensity value was proportional to the concentrations of the aqueous reagents used. High concentrations effected dendrite crystal habits, whereas intermediate ones produced granular crystal habits. At low reactant concentrations, polymerization did not commonly occur and very low acoustic emission counts were observed. The integrated acoustic emission traces were strongly dependent on the crystal habit formed: emission from dendrites consisted of two distinct exponential curves (a sharp rise of approximately 20 min duration, followed by a slower increase lasting about 2 h), whereas granular growths gave only the latter exponential rise. X-ray diffraction studies indicated no difference in crystallinity between the dendritic and granular forms. Temperature studies (30-60 °C) were performed for 0.30, 0.40, and 0.50 M solutions of pyrazine and ZnCl₂- At higher temperatures, the granular habit was favoured with the trend towards larger integrated acoustic emission values. Rate constants for the acoustic emission obtained from 0.40 M pyrazine and 0.40 M ZnCl₂ at each temperature were estimated and fitted to the Arrhenius equation. The values determined for the pre-exponential factor and the activation energy were 3.1 ± 0.3 x 10⁵ s⁻1 and 51.7 ± 0.6 kJ/mole respectively. The predominant crystal habit changed from dendrite to granular when the pH value was lowered from 3.0 to 1.0. A gravimetric study of dichloro(pyrazine)zinc(II),s mass as a function of time revealed that most of the acoustic emission was detected after the polymer was formed. When ethanol was used as the solvent, acoustic emission bursts were two orders of magnitude greater in intensity than those generated in water. However by 5 min into the reaction, all acoustic emission activity had stopped. The resultant polymer's habit was a finely divided powder. Analogous systems which substituted ZnBr₂ or ZnI₂ for ZnCl₂ and pyrimidine for pyrazine were also acoustically active. However, those which replaced ZnCl₂ with CdCl₂, CoCl₂, Co(NO₃3)₂, or Cu(NO₃)₂ were not, despite the formation of similar polymeric structures. No polymer was formed when ZnCl₂ was substituted with ZnCO₃ or ZnSO₄, and no emission was detected.

Text

2010-11-25

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http://hdl.handle.net/2429/30138

Chemistry

University of British Columbia

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