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Fig. 4 Microstructural features, morphology and point analysis of Na2SiF, a-b scanning electron micrographs, e EDS spectrum and d-e high resolution transmission electron images

Figure 4 Microstructural features, morphology and point analysis of Na2SiF, a-b scanning electron micrographs, e EDS spectrum and d-e high resolution transmission electron images

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Figure | also shows the cumulative distribution and histo- gram curves of particles. Static laser diffraction analysis is a fast, feasible, convenient, and non-destructive rout to deter- mine the particle size distribution of inorganic powders by measuring the scattering angle and intensity of laser after pass- ing through a diluted particle dispersions suspended in a prop- er liquid [39]. Due to the low solubility of Na,SiF.¢, water was selected as dispersion media and the combination of stirring
Figure | also shows the cumulative distribution and histo- gram curves of particles. Static laser diffraction analysis is a fast, feasible, convenient, and non-destructive rout to deter- mine the particle size distribution of inorganic powders by measuring the scattering angle and intensity of laser after pass- ing through a diluted particle dispersions suspended in a prop- er liquid [39]. Due to the low solubility of Na,SiF.¢, water was selected as dispersion media and the combination of stirring
and ultra-sonication would allow the sufficient time for dis- agglomeration of Na,SiF¢ sample is particles. It reveals that the particle size of in the range of 38-121 um with the average size of 77 um. Type of the used raw materials and the applied synthesis process size distribution o have strong effects on particle growth and f the final NazSiF¢ powder. The nucleation and growth of Na»SiF¢ particle immediately happen in the solution during precipitation synthesis. After calcination pro- cess, particles cou d join to form larger angular shaped aggre- gates. The particle morphology will be discussed later in the microstructural analysis section. a huge amount of mass loss (54.02 wt%). The temperature of the maximum dissociation phenomena was determined as 556.2 °C and 556.5 °C corresponding to the sharp endother- mic peaks in the representative DSC and DTG curves, respec- tively. Sodium hexa-fluorosilicate is decomposed to sodium fluoride (NaF) and silicon tetrafluoride (SiF,) by heating over 500 °C according to the Eq. 4 [40, 41].
and ultra-sonication would allow the sufficient time for dis- agglomeration of Na,SiF¢ sample is particles. It reveals that the particle size of in the range of 38-121 um with the average size of 77 um. Type of the used raw materials and the applied synthesis process size distribution o have strong effects on particle growth and f the final NazSiF¢ powder. The nucleation and growth of Na»SiF¢ particle immediately happen in the solution during precipitation synthesis. After calcination pro- cess, particles cou d join to form larger angular shaped aggre- gates. The particle morphology will be discussed later in the microstructural analysis section. a huge amount of mass loss (54.02 wt%). The temperature of the maximum dissociation phenomena was determined as 556.2 °C and 556.5 °C corresponding to the sharp endother- mic peaks in the representative DSC and DTG curves, respec- tively. Sodium hexa-fluorosilicate is decomposed to sodium fluoride (NaF) and silicon tetrafluoride (SiF,) by heating over 500 °C according to the Eq. 4 [40, 41].
inexpensive green way for the production of NaF without using HF. Likewise; this phenomenon was applied in the syn- thesis of mesoporous silica and silicon nitride materials under controlled atmosphere [25, 26]. The kinetic mechanism of Na»SiF¢ dissociation was studied in some researches to reveal the effects of heating condition on reaction rate. Kashiwaya et al. determined a formula for the rate constant of dissociation as a function of temperature and the activation energy was calculated as 106.6+0.7 kJ/mol over a temperature range of 427 to 1347 °C in gas phase [40]. In another work, the optimal processing condition of complete dissociation of NazSiF¢ was proposed as heating at 650 °C for 40 min under pure nitrogen atmosphere [42]. be said that the NajSiF¢ material shows a good crystallinity with a very few miscellaneous peaks in the X-ray diffraction pattern. Sodium hexa-fluorosilicate crystallize in a non- centro-symmetric (NCS) space group of P321 hexagonal crys- tal structure with the lattice parameters of a= b = 8.859 A and c= 5.038 A [43]. This crystal system is highly symmetric and composed of two independent regular [SiF¢]” octahedrons with six F' in contact to a single Si** ion. Two crysta graphically independent sodium cation (Na'*) are placed both corners of the octahedral. On the other hand, this octal lo- on he- dral structure is identified by six Si-F bonds [28]. In general, Na,SiF, has a similar crystal structure to other ternary A, ByF¢ hexa-fluorides compounds such as NajPdF.6, NazTi Fé, Na MnF6, NaplrF¢, Li,SiF6, Na»GeF¢, Na» PtF 6, and Na2RhF¢ [43]. According to the crystal structure and cell pa- rameters of Na>SiF¢, the theoretical density is close to 2.74 g/ cm? which is comparable to the measured value of the 2.708 g/ cm? obtained from helium gas pycnometry.
inexpensive green way for the production of NaF without using HF. Likewise; this phenomenon was applied in the syn- thesis of mesoporous silica and silicon nitride materials under controlled atmosphere [25, 26]. The kinetic mechanism of Na»SiF¢ dissociation was studied in some researches to reveal the effects of heating condition on reaction rate. Kashiwaya et al. determined a formula for the rate constant of dissociation as a function of temperature and the activation energy was calculated as 106.6+0.7 kJ/mol over a temperature range of 427 to 1347 °C in gas phase [40]. In another work, the optimal processing condition of complete dissociation of NazSiF¢ was proposed as heating at 650 °C for 40 min under pure nitrogen atmosphere [42]. be said that the NajSiF¢ material shows a good crystallinity with a very few miscellaneous peaks in the X-ray diffraction pattern. Sodium hexa-fluorosilicate crystallize in a non- centro-symmetric (NCS) space group of P321 hexagonal crys- tal structure with the lattice parameters of a= b = 8.859 A and c= 5.038 A [43]. This crystal system is highly symmetric and composed of two independent regular [SiF¢]” octahedrons with six F' in contact to a single Si** ion. Two crysta graphically independent sodium cation (Na'*) are placed both corners of the octahedral. On the other hand, this octal lo- on he- dral structure is identified by six Si-F bonds [28]. In general, Na,SiF, has a similar crystal structure to other ternary A, ByF¢ hexa-fluorides compounds such as NajPdF.6, NazTi Fé, Na MnF6, NaplrF¢, Li,SiF6, Na»GeF¢, Na» PtF 6, and Na2RhF¢ [43]. According to the crystal structure and cell pa- rameters of Na>SiF¢, the theoretical density is close to 2.74 g/ cm? which is comparable to the measured value of the 2.708 g/ cm? obtained from helium gas pycnometry.
Table 2 X-ray diffraction data of as-received Na2SiF¢ sample
Table 2 X-ray diffraction data of as-received Na2SiF¢ sample
Fig. 3 Representative X-ray diffraction patterns of as-received and heated Na2SiF, samples
Fig. 3 Representative X-ray diffraction patterns of as-received and heated Na2SiF, samples
Fig. 5 Spectral curves of Na)SiF¢ a FT-IR, b UV-Vis-NIR and ¢ Raman analysis results. Thermal dissociation of NazSiF¢ is shown to be a facile, eco-friendly and inexpensive method for the production of NaF instead of the conventional hydrofluoric acid-based methods. presence of major vibrating peaks at 629, 476 and 414 cm’ which are the main characteristic Raman band of Na,SiF, material [32]. As there is no evidence of other non-related peaks, it seems that the studied NajSiF¢ material is a nearly pure hexagonal phase of Malladrite which is in agreement with Zhao et al. study [32]. In the presence of major impurity ions, the inhibition of the characteristic peaks or the signs of new local vibration modes may be appeared in the Raman spectra of Na2SiF¢. It was reported that the characteristic peak of 476 cm ' could be slightly suppressed in the presence of impurity metal cations such as Ce**, Tb** and Eu** for doped synthesized NazSiF.¢ samples [32].
Fig. 5 Spectral curves of Na)SiF¢ a FT-IR, b UV-Vis-NIR and ¢ Raman analysis results. Thermal dissociation of NazSiF¢ is shown to be a facile, eco-friendly and inexpensive method for the production of NaF instead of the conventional hydrofluoric acid-based methods. presence of major vibrating peaks at 629, 476 and 414 cm’ which are the main characteristic Raman band of Na,SiF, material [32]. As there is no evidence of other non-related peaks, it seems that the studied NajSiF¢ material is a nearly pure hexagonal phase of Malladrite which is in agreement with Zhao et al. study [32]. In the presence of major impurity ions, the inhibition of the characteristic peaks or the signs of new local vibration modes may be appeared in the Raman spectra of Na2SiF¢. It was reported that the characteristic peak of 476 cm ' could be slightly suppressed in the presence of impurity metal cations such as Ce**, Tb** and Eu** for doped synthesized NazSiF.¢ samples [32].
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