Non-Stoichiometric Compounds

Sodium hexa-fluorosilicate (Na 2 SiF 6) is a synthetic inorganic material with distinguished chemical, thermal and optical properties. In this research, a pure sodium hexa-fluorosilicate sample was systematically investigated via various characterization techniques. The amounts of constitutional elements and impurities were investigated using X-ray fluorescence and the chemical purity was determined as 99.89 wt%. Physical properties were calculated as 2.7085 g/cm 3 for density, 0.98 m 2 /g for specific surface area and a mean particle size (D 50) of 77.3 μm. Thermal analysis was applied to understand the degradation of Na 2 SiF 6 till 900°C. The mineral composition and morphology was studied using X-ray diffraction, scanning and transmission electron microscopy and revealed the existence of P321 type hexagonal structure associated by elongated prism shaped precipitated crystals. Spectroscopy studies were performed via different techniques at different ranges of measurement. Thermal stability was investigated by post-mortem phase analysis of heat treated Na 2 SiF 6 samples and showed a well stability till 400°C. It is proposed that the thermal dissociation of Na 2 SiF 6 can be utilized as a facile, inexpensive and green way for the synthesis of sodium fluoride instead of routine hydrofluoric acid-based methods. The obtained results suggested that the examined sodium hexa-fluorosilicate material could potentially be used as a reliable precursor in the synthesis of fluoride and silicon based materials due to its acceptable physical properties, chemical purity and thermal behavior.

Öz Sodyum silikoflorür, Na2SiF6'nın kimyasal formülü ile son zamanlarda seramik, metalurji ve kimya mühendisliği gibi çeşitli endüstrilerde birçok uygulama alanı bulan inorganik sentetik bir malzemedir. Bu tozun en önemli kullanımlarından bazıları opal cam-seramik, sırlar ve fritlerdir. Uygulama alanlarının gün geçtikçe artmasına karşı, literatürde sodium silikoflorür (SSF) ile ilgili sınırlı sayıda kaynak mevcuttur. Bu araştırmada, ticari bir sodyum silikoflorür tozunun ısıl davranış ve mikroyapı açısından karakterize edilmiştir. Termal analiz, hava atmosferinde 900 °C'de diferansiyel termal ve termo gravimetri eğrilerine göre araştırılmıştır. Mikroyapısal analizler, taneciklerin tane büyüklüğü ve morfolojisini incelemek için taramalı ve geçirimli elektron mikroskopları kullanılarak yapılmıştır. Sonuçlar, ticari sodyum silikoflorür malzemesinin, eşlik eden bir endotermik reaksiyonla havada 430-600 °C sıcaklık aralığında çözündüğünü göstermiştir. Mikroyapısal karakterizasyonlarla, incelenen SSF tozunun granüler morfolojisi, altıgen şekilli prizmaların ve ilgili gözeneklerin varlığına da açıklık getirmiştir.

Öz Sodyum silikoflorür, Na2SiF6'nın kimyasal formülü ile son zamanlarda seramik, metalurji ve kimya mühendisliği gibi çeşitli endüstrilerde birçok uygulama alanı bulan inorganik sentetik bir malzemedir. Bu tozun en önemli kullanımlarından bazıları opal cam-seramik, sırlar ve fritlerdir. Uygulama alanlarının gün geçtikçe artmasına karşı, literatürde sodium silikoflorür (SSF) ile ilgili sınırlı sayıda kaynak mevcuttur. Bu araştırmada, ticari bir sodyum silikoflorür tozunun ısıl davranış ve mikroyapı açısından karakterize edilmiştir. Termal analiz, hava atmosferinde 900 °C'de diferansiyel termal ve termo gravimetri eğrilerine göre araştırılmıştır. Mikroyapısal analizler, taneciklerin tane büyüklüğü ve morfolojisini incelemek için taramalı ve geçirimli elektron mikroskopları kullanılarak yapılmıştır. Sonuçlar, ticari sodyum silikoflorür malzemesinin, eşlik eden bir endotermik reaksiyonla havada 430-600 °C sıcaklık aralığında çözündüğünü göstermiştir. Mikroyapısal karakterizasyonlarla, incelenen SSF tozunun granüler morfolojisi, altıgen şekilli prizmaların ve ilgili gözeneklerin varlığına da açıklık getirmiştir.

Pure nanocrystalline forsterite could be fabricated by mechanochemical synthesis in the presence of fluorine ion. Mechanochemical process and post-heat treatment were performed using MgCO 3 , SiO 2 and (NH 4) 2 SiF 6 powders as starting materials. Thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques were utilized to characterize the synthesized powders. Pure forsterite could be obtained after 5 h of mechanical activation and post-heat treatment for 1 h at 900 • C. Nanocrystalline forsterite was fabricated with the grain size of about 30 nm. The synthesized forsterite nanopowder had particle size smaller than 100 nm. The results showed that in the presence of fluorine ion, the rate of forsterite formation was increased by combustion reaction.

Chrysotile fibers were synthesized from glass in hydrothermal conditions. The starting materials were first held at 1650°C and then rapidly quenched down to room temperature. The resulting glass, after the addition of mineralizing agents, was hydrothermically altered in the following conditions: temperature 300°C-400°C; pressure 100-200 MPa; time 48-480 h. X-ray powder diffraction, scanning and transmission electron microscopy were used to examine/study the starting materials and products. Cylindrical fiber morphology was prevalent, but proto-chrysotile was also detected, not entirely showing welldefined crystallinity, as revealed by electron diffraction patterns of selected areas. The mineralizing agent and chemical composition of the glass play an important role in the yield of chrysotile fibers. The effect of growth parameters on the size and abundance of chrysotile fibers is also discussed, in the light of possible recrystallization of glass obtained by thermal treatment of chrysotile-asbestos-containing materials. I.

Chrysotile fibers were synthesized from glass in hydrothermal conditions. The starting materials were first held at 1650°C and then rapidly quenched down to room temperature. The resulting glass, after the addition of mineralizing agents, was hydrothermically altered in the following conditions: temperature 300°C-400°C; pressure 100-200 MPa; time 48-480 h. X-ray powder diffraction, scanning and transmission electron microscopy were used to examine/study the starting materials and products. Cylindrical fiber morphology was prevalent, but proto-chrysotile was also detected, not entirely showing welldefined crystallinity, as revealed by electron diffraction patterns of selected areas. The mineralizing agent and chemical composition of the glass play an important role in the yield of chrysotile fibers. The effect of growth parameters on the size and abundance of chrysotile fibers is also discussed, in the light of possible recrystallization of glass obtained by thermal treatment of chrysotile-asbestos-containing materials. I.

In this paper, a study was carried out on the evolution of the effective mass of the electron and the effective mass of the crystal parameters in solar cells based on non-stoechiometric absorbers Cu (In 1-x Ga x) Se 2. Usually the stoechiometry is defined for this type of cells in relation to the ratio In Cu for x values between 0.1 and 0.9. We have noted that the ratio     In Cu increases with x and the energy gap Eg of the material, and that m e also increases with this ratio. The lattice parameters increase with the ratio     In Cu and with the gap energy Eg. In the end, two relations linking the crystal parameters and this ratio were determined.

Pure nanocrystalline forsterite could be fabricated by mechanochemical synthesis in the presence of fluorine ion. Mechanochemical process and post-heat treatment were performed using MgCO 3 , SiO 2 and (NH 4 ) 2 SiF 6 powders as starting materials. Thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques were utilized to characterize the synthesized powders. Pure forsterite could be obtained after 5 h of mechanical activation and post-heat treatment for 1 h at 900 • C. Nanocrystalline forsterite was fabricated with the grain size of about 30 nm. The synthesized forsterite nanopowder had particle size smaller than 100 nm. The results showed that in the presence of fluorine ion, the rate of forsterite formation was increased by combustion reaction.

The influence of mechanical activation of the mixture basic MgCO -amorphous SiO on reaction of forsterite formation was studied 3 2 with and without the presence of fluorine ion. Beneficial influence of mechanical activation on the reaction in the mixture without fluorine ion is a consequence of the change of the granulometry during the intense milling. Mechanical activation in the presence of fluorine ion affects the mechanism of forsterite formation via different compounds from the humite group. With non-activated mixture dominant transition phase is hondrodite, while with activated mixture, clinohumite is the transition compound in the process of formation of end reaction products. Fluorine ion is released from the system by hydrolysis. q 2001 Elsevier Science S.A. All rights reserved.

The thermal behaviour of two crocidolite asbestos samples was characterized. Infrared spectroscopy (FT-IR), powder X-ray diffraction, scanning electron microscopy and thermal analysis (DTA and TG) with evolved gas analysis were carried out on samples of crocidolite asbestos as received and after heating in order to observe their structural changes and dehydroxylation process. The results show that the dehydroxylation process of crocidolite asbestos occurs at different temperatures and is in the range 400-650°C. This temperature is dependent on the origin of asbestos samples and may be associated with an increased amount of magnesium in the structure of asbestos. For the sample which comes from the Republic of South Africa, the dehydroxylation process occurs at lower temperature in contrast to Russian crocidolite asbestos sample from cement-asbestos material. However, this temperature range is lower than those reported by some authors, who state that the decomposition temperature of crocidolite should be at least 900°C. This finding may have a significant influence on thermal utilization of asbestos materials, e.g. cement-asbestos, because the temperature of the heat treatment in the utilization process should be as low as possible.