RESUMO
Fused silica crucibles are commonly used in the fabrication process of solar grade silicon ingots. These crucibles are manufactured from high purity natural quartz sand and as a consequence, their properties are influenced by the presence of water and hydroxyls in the raw quartz. In this work, diffuse reflectance IR, 1H magic angle spinning NMR, and Raman spectroscopy were used to investigate the influence of thermal treatment on water and hydroxyl groups in high purity natural quartz sand. Most of the water in dry sand is present in the form of closed inclusions within the quartz grains which were detected in Raman imaging studies, even after thermally treating the samples at 600 °C. Only after heating to 900 °C did this water completely vanish, most likely as a result of rupturing of the inclusions. However, newly formed OH groups, identified as isolated and hydrogen bound OH were observed as products of the reaction between water and quartz. Similarly, liquid water was observed in NMR spectra even after treatment at 600 °C while at temperatures >900 °C, only non-interacting silanol groups were present. The comparison of the temperature dependence of the IR and NMR spectra also yields insight into the assignment of the OH stretching mode region of the IR spectrum in this system. The intensity of water related bands decreases while the intensity of OH bands first increases and then decreases with increasing temperature. The band intensity of Al-rich defects as well as the characteristic feature at 3200 cm-1 does not follow the temperature dependence of typical water peaks. It is also shown that leaching the quartz sand in HF solution helps to remove water from inclusions, likely by forming pathways for fluid flow inside the quartz grains. Milling of the samples caused formation of an additional type of hydroxyl group, possibly due to partial amorphisation of the surfaces of the quartz grains surface during the process. The results improve the basis for a knowledge-based processes development for the processing of high purity natural quartz.
RESUMO
A general synthetic approach to variously polarised merocyanines and a cyanine with enhanced thermal and (photo)chemical stability by a locked all-trans conformation (derived from a rigidified hexatriene unit and a variety of common donor and acceptor groups) is presented as well as a systematic study of their (non)linear optical properties. Apart from the UV/Vis absorption and fluorescence behaviour, the ground- and excited-state dipoles, the first-, second- and third-order molecular polarisabilities were determined by electro-optical absorption measurements (EOAM) and degenerate four-wave mixing (DFWM) techniques in solution. Large values for the second- and third-order polarisability up to beta(0)=461x10(-50) C V(-2) m(3) (1242x10(-30) esu) and [gamma(LL)]=183x10(-60) C V(-3) m(4) (15x10(-34) esu) were found. The linear and nonlinear optical properties were related to the ground-state polarisation and the resonance structure of the chromophores. In order to reveal the influence of the length of the polymethinic chain (number of pi electrons within the chromophore), some lower homologues shortened by one C=C (double) bond were also taken into account. The unexpectedly high gamma values of some of the merocyanines cannot be explained by a two-level model. Molecular vibrational third- order polarisabilities (calculated from absolute Raman intensities in solution) were qualitatively correlated to the DFWM results. Furthermore, the dependence of the (13)C NMR chemical shifts of the polymethinic carbons within the merocyanines upon ground-state polarisation was investigated and compared to those within a corresponding cyanine.
