L’amélioration des performances des dispositifs pyrotechniques requiert une description fine de la transition choc/détonation (TCD) dans les explosifs. Les modèles de TCD existant comportent une part d’empirisme qu’il est souhaitable de réduire à l’aide de données expérimentales à l’échelle microscopique. Pour cela, nous avons mis au point un dispositif permettant de générer des chocs de 2 à 30 GPa et d’analyser en temps réel l’évolution du matériau par spectroscopie. Le système de mesure comporte un laser sonde et un ensemble de spectroscopie Raman rapide. Ce dispositif a été utilisé pour l’étude des mécanismes d’initiation d’un explosif appelé TATB. L’évolution des spectres Raman en fonction de la pression révèle un couplage entre les modes de vibration des groupements NO2 et NH2 provenant de la présence d’un réseau de liaisons hydrogène au sein du TATB. Ce réseau est responsable de la grande stabilité de la molécule. Quelques différences entre les régimes statique et dynamique, imputables à l’effet du chauffage par le choc, sont mises en évidence ; elles semblent indiquer un affaiblissement du réseau de liaisons hydrogène. Les résultats font également apparaître une atténuation progressive du signal Raman sous choc avec la pression. A partir de 9 GPa, le signal n’est plus détectable. Des visualisations par caméras rapides montrent que cette atténuation du signal Raman est accompagnée d’un assombrissement progressif du TATB qui devient totalement opaque à 9 GPa. Des expériences de réflectivité sous choc ont montré que ces deux phénomènes sont dus à un élargissement de la bande d’absorption du TATB. / Improving performances and safety of pyrotechnic devices requires a sharp knowledge of the shock to detonation transition phenomenon in high explosives. Current models to describe this phenomenon largely involve empiric parameters based on macro scale experiments. To improve predictive capability of these models, it is necessary to get experimental data at a microscopic scale. To provide such data, we developed an experimental setup to shock a high explosive up to 30 GPa and perform in-situ measurement of its Raman spectra under this loading. The device includes a shock generator based on explosive driven plate impact triggered by a laser pulse and a diagnostic involving an excitation laser and a spectrometer coupled with an intensified CCD. This experiment has been applied to an insensitive high explosive named TATB. Pressure driven evolution of the Raman spectra reveals an important coupling between NO2 and NH2 vibration modes that is due to a strong H bonding within TATB crystal. This bonding is clearly linked to TATB high stability. Differences observed between dynamic and static loading are attributed to shock heating resulting in H bonding weakening. Moreover, results show a progressive decrease in Raman spectra intensity with increasing shock pressure down to a complete signal loss at about 9 GPa. High speed visualisations reveal a progressive darkening of the sample leading to complete opacity at 9 GPa. Reflectivity measurements under shock loading show that these two phenomena are due to a shock-induced enlargement of the TATB absorption band.

Semiconductor nanostructures such as CdSe quantum dots and colloidal nanoplatelets exhibit remarkable optical properties, making them interesting for applications in optoelectronics and photocatalysis. For both areas of application a detailed understanding of the electronic structure is essential to achieve highly efficient devices. The electronic structure can be probed using the fact that optical properties of semiconductor nanoparticles are found to be extremely sensitive to the presence of excess charges that can for instance be generated by means of an electrochemical charge transfer via an electrode. Here we present the use of EMAS as a versatile spectroelectrochemical method to obtain absolute band edge positions of CdSe nanostructures versus a well-defined reference electrode under ambient conditions. In this, the spectral properties of the nanoparticles are monitored with respect to an applied electrochemical potential. We developed a bleaching model that yields the lowest electronic state in the conduction band of the nanostructures. A change in the band edge positions caused by quantum confinement is shown both for CdSe quantum dots and for colloidal nanoplatelets. In the case of CdSe quantum dots these findings are in good agreement with tight binding calculations. The method presented is not limited to CdSe nanostructures but can be used as a universal tool. Hence, this technique allows the determination of absolute band edge positions of a large variety of materials used in various applications

No description available.