Preparation of KHSO4/SiO2 Nanocomposites and Their Physical Properties

A series of SiO₂/KHSO₄ nanocomposites with various SiO₂/salt ratios was prepared where the active compound was added before gelation. The sol was prepared by mixing of these hydrogen salts, TEOS (tetraethyl orthosilicate) and water. After gelation and heat treatment (heating slowly to 200–220°C under vacuum), the samples were characterized by X-ray diffraction, Differential Scanning Calorimetry (DSC), IR spectroscopy, Scanning Electron Microscopy (SEM), and High Resolution Transition Electron Microscopy (HR TEM). DSC measurements showed phase transition temperature shifts that depended on the SiO₂/salt ratio. The properties of the nanocomposite samples were compared with the bulk materials. The shift in the phase transitions to lower temperatures was attributed to the particle size effect.     

Local structure and chemical durability of FeOOH-fixed sodium silicate glass prepared from water glass

A 12M HCl solution of iron oxyhydroxide (a-FeOOH: goethite) was mixed with water glass (18Na₂O^.36SiO₂ ^.46H₂O) at room temperature. The mixture (sol) changed into a dry gel when dried at 25 °C for 120 hours in air. Glass-ceramic and glass samples were prepared when the dry gel was heated for 1-3 hours in an electric furnace at 800 and 900 °C, respectively. The ⁵⁷Fe Mössbauer spectrum of the dry gel is composed of a magnetic hyperfine structure owing to the formation of g-FeOOH (lepidocrocite). By contrast, the ⁵⁷Fe Mössbauer spectrum of glass-ceramic and glasses is composed of paramagnetic Fe(III) with distorted tetrahedral symmetry. This proves that Fe(III) atoms occupy network-forming Si(IV) sites in the FeOOH-fixed sodium silicate glass. A leaching test of the silicate glass in the acid rain simulant composed of HNO₃ (pH 3.5) and H₂SO₄ (pH 3.5) revealed high chemical durability, indicating that Fe(III) is firmly fixed in the glass matrix.     

Mössbauer study of the thermal decomposition of co-precipitated Fe/NH4/2/SO4/2.6H2O and Ni/NH4/2/SO4/2.6H2O

Using Mössbauer spectroscopy, the thermal decomposition products of co-precipitated Fe/NH_4/2/SO_4/2.6H₂O and Ni/NH_4/2/SO_4/2.6H₂O for two hours at various temperatures in open air have been studied and identified. It has been found that NiFe₂O₄, formed at 900 °C and 1100 °C, has been the final product.     

Mössbauer study of iron reduction in hematite containing mineral originated from Aswan area in Egypt

Mössbauer spectroscopy was used to study iron reduction in mineral originated from Aswan area in Egypt. The Mössbauer spectra of samples heat treated at 1000 °C in hydrogen atmosphere show a gradually reduction process of hematite. The main phase of sample aged for 1 h is magnetite, while alpha-iron as well as some silicates and wustite type oxide can be detected in sample aged for 3 h. Further aging of sample results in dissolution of alloying elements /Si, Al/, being present in the samples. This process starts already in samples aged at 1000 °C for 4 h, but it is more expressive in minerals heat treated at 1200 °C or 1300 °C, when all paramagnetic phases disappeared.     

M?ssbauer study of transformation mechanism of Fe cations in olivine after thermal treatments in air

The transformation mechanism of Fe cations in natural olivine after thermal treatments in air has been studied using mainly⁵⁷Fe Mössbauer spectroscopy. -Fe₂O₃ nanoparticles appear as the primary Fe³⁺ phase in Mössbauer spectra of olivine samples heated at 600-900 °C. These nanoparticles are thermally unstable and they are transformed to -Fe₂O₃ with the increase of heating time. Another transformation mechanism of iron related with the complete decomposition of olivine structure has been observed at temperatures of 1000 °C and higher. The mixed oxide MgFe₂O₄ with the spinel structure and enstatite MgSiO₃ were identified as iron-bearing decomposition products.     

Preparation and characterization of nanosize nickel-substituted cobalt ferrites (Co1−xNixFe2O4)

Nanosize nickel-substituted cobalt ferrites were prepared using aerosol route and characterized by TEM, XRD, magnetic and Mössbauer spectroscopy. The particle size of as obtained samples was found to be ∼10 nm which increases upto ∼80 nm on annealing at 1200 °C. The unit cell parameter ‘a’ decreases linearly with the nickel concentration due to smaller ionic radius of nickel. The saturation magnetization for all the samples after annealing at 1200 °C lies in the range 47.6-84.5 emu/g. Room temperature Mössbauer spectra of as obtained samples exhibit a broad doublet, suggesting super paramagnetic nature of the sample. The broad doublet is further resolved into two doublets corresponding to the iron atoms residing at the surface and internal regions of the particle. The samples annealed at 1200 °C showed broad sextet, which is resolved into two sextets, corresponding to tetrahedrally and octahedrally coordinated Fe cations. Cation distribution calculated using XRD and Mössbauer data indicates a decrease in Fe³⁺(oct.)/Fe³⁺(tet.) ratio with increasing nickel concentration.     

Chemical reduction of hematite with starch

Chemical reduction of hematite with starch in air at elevated temperature was investigated using X-ray powder diffraction, FT-IR and ⁵⁷Fe Mössbauer spectroscopies. On heating the starting mixture for 0.5 and 2 hours at 300 °C, magnetite and a small fraction of hematite were identified by XRD. With the heating time prolonged up to 24 hours, magnetite reoxidized and hematite was obtained again. The formation of magnetite was observed even at 580 °C. However, the magnetite formed at this temperature was substoichiometric, as shown by XRD and Mössbauer spectroscopy. Characteristic IR bands of oxide phases were monitored by FT-IR spectroscopy. Chemical reduction of hematite with starch into a Fe⁰ state was not observed in any sample.     

Thermal decomposition and structural reconstruction effect on Mg-Fe-based hydrotalcite compounds

The thermal decomposition and structural reconstruction of Mg-Fe-based hydrotalcites (HT) have been studied through thermogravimetric analyses, X-ray powder diffraction (XRD), Fourier transform infrared spectroscopy and Mössbauer spectroscopy. The destruction of the layered structure took place at about 300°C. The broad peaks observed in the X-ray diffractograms suggest that the resultant oxides constitute a solid solution. For samples treated at temperatures higher than 500°C, the formation of the MgO and MgFe₂O₄ spinel phases is observed. ⁵⁷Fe Mössbauer spectroscopy was employed to monitor the Fe chemical environment for the samples annealed at different temperatures (100-900°C). In situ XRD experiments revealed that the HTs start an interlayer contraction at about 180°C. This phenomenon is identified as being due to a grafting process for which the interlamellar anions attach to the layers through a covalent bond. The reconstruction of the HTs was also investigated and its efficiency depends on the thermal annealing temperature and the Mg/Fe ratio. The structure of the reconstructed samples was found to be exactly the same as the parent structure.     

Physico-Chemical and Catalytic Study of the Co/SiO2 Catalysts

This paper is focused on the physico-chemical and catalytic properties of Co/SiO₂ catalysts. Silica-supported cobalt catalysts were prepared by sol-gel and impregnation methods and characterized by BET measurements, temperature programmed reduction (TPR_H2), X-ray diffraction (XRD), and thermogravimetry-mass spectroscopy (TG-DTA-MS). The sol-gel method of preparation leads to metal/support catalyst precursor with a homogenous distribution of metal ions into bulk silica network or on its surface. After drying the catalysts were calcined at 500, 700, and 900°C. The reducibility of the supported metal oxide phases in hydrogen was determined by TPR measurements. The influence of high temperature—atmosphere treatment on the phase composition of Co/SiO₂ catalysts was investigated by XRD and TG-DTA-MS methods. At least five crystallographic cobalt phases may exist on silica: metallic Co, CoO, Co₃O₄, and two different forms of Co₂SiO₄ cobalt silicate. Those catalysts in which cobalt was chemically bonded with silica show worse reducibility as a result of strongly bonded Co-O-Si species formed during high-temperature oxidation. The TPR measurements show that a gradual increase in the oxidation temperature (500–900°C) leads to a decrease in low-temperature hydrogen reduction effects (<600°C). The decrease of cobalt oxide reduction degree is caused by cobalt silicate formation during the oxidation at high temperature (T 1000°C). The catalysts were tested by the reforming of methane by carbon dioxide and methanation of CO₂ reactions.     

Structural analysis of mixed tin oxides produced by the sol-gel method

Mixed oxides SiO₂/SnO₂ with 80/20 nominal weight composition have been obtained by the sol-gel method with different precursors. X-ray diffraction and low temperature transmission Mössbauer spectroscopy have been used to follow the structural evolution of the samples after treatments in the temperature range RT-1050 °C. The main results are that changes in the precursor nature and gel preparation affect the Sn(IV) homogeneity in the SiO₂ matrix and that the use of Sn-alkoxides increases the content of residual carbon which promotes reduction of Sn(IV) at intermediate temperatures.     

Mössbauer spectroscopic and X-ray diffraction study of the thermal decomposition of Fe(CH3COO)2 and FeOH(CH3COO)2

Thermal decomposition of iron(II) acetate, Fe(CH₃COO)₂, and iron(III) acetate hydroxide, FeOH(CH₃COO)₂, has been studied using⁵⁷Fe Mössbauer spectroscopy and X-ray diffraction. Samples were thermally treated in air atmosphere between 150°C and 1000°C. The formation of maghemite '-Fe₂O₃, and hematite, -Fe₂O₃, is discussed. Hematite appears as the final decomposition product.     

Cation Distribution of the Transparent Conductor and Spinel Oxide Solution Cd1+xIn2−xSnxO4

The cation distribution in the transparent conducting oxide Cd_1+xIn_2−xSn_xO₄ was investigated to determine if there is a correlation between structure and electronic properties. Combined Rietveld refinements of neutron and X-ray diffraction data and ¹¹⁹Sn Mössbauer spectroscopic analysis were used to show that the cation distribution changed with x(0≤x≤0.7) from a primarily normal spinel (x=0) to an increasingly random spinel. CdIn₂O₄ quenched from 1175°C has an inversion parameter of 0.31 (i.e., (Cd_0.69In_0.31)^tet(In_1.69Cd_0.31)^octO₄). The inversion parameter decreases to 0.27 as the quench temperature is lowered from 1175°C to 1000°C. The decrease in inversion parameter with temperature correlates with an increase in optical gap from 3.0 eV to 3.3 eV for specimens prepared at 1175°C and 800°C, respectively. We show that this is a consequence of an increase in the fundamental band gap.     

Thermischer Zerfall von AlPO4−SiO2-Mischkristallen

Zusammenfassung AlPO₄-10 Mol% SiO₂-Mischkristalle mit stapelfehlgeordneter Cristobalit-Tridymit-Struktur zeigen gegenüber reinem AlPO₄ mit Cristobalit-Struktur eine auffällige Hemmung der thermischen Zersetzung. Dies wird auf die Bildung thermisch stabiler diffusionshemmender SiO₂–P₂O₅-Schmelzhäute zurückgeführt. Zusätze von GeO₂, SnO₂ oder TiO₂ zu AlPO₄ bewirken diesen Hemmungseffekt nicht. Schmelzen von GeP₂O₇, SnP₂O₇ und TiP₂O₇ zerfallen bei 1400°C schnell. Mechanische Gemenge von AlPO₄ und SiO₂ zeigen den Hemmungseffekt nur schwach. AlAsO₄ zerfällt mit und ohne SiO₂-Zusatz rasch oberhalb 1000°C.

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Thermal decomposition of AlPO₄–SiO₂ mixed crystals

AlPO₄-10 mole% SiO₂ solid solutions, representing the stacking-disordered cristobalite-tridymite structure, in contrast to pure AlPO₄ show a remarkable reduction of rate of thermal decomposition. This is brought into connection with the formation of molten surface layers of SiO₂–P₂O₅. Simple mixtures of AlPO₄ with SiO₂ in the form of quartz powder or amorphous silica gel show this effect only scarcely. Additions of GeO₂, SnO₂ or TiO₂ do not show this effect at all. In contrast to melts of the composition SiP₂O₇, melts of GeP₂O₇, SnP₂O₇ and TiP₂O₇ decompose quickly at 1400°C. AlAsO₄ even with the addition of SiO₂ decomposes very rapidly above 1000°C.

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Mit 5 Abbildungen

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Herrn Prof. Dr.H. Nowotny gewidmet.     

Mössbauer studies in thermally treated iron-samarium mixed oxide system

Mössbauer spectroscopy and X-ray diffractometry have been used to study Sm–Fe mixed oxides (with different SmFe atomic ratios) annealed at 550, 850, 1000 and 1250 °C. The room temperature Mössbauer spectra can be interpreted in terms of one, two or three sextets and in some cases by an additional doublet depending on the composition and the heat treatment. The sextets have been associated with SmFeO₃ perovskite, Sm₃Fe₅O₁₂ garnet and -Fe₂O₃ hematite. These results are in agreement with those of X-ray diffractometric measurements, which give a clear, evidence of the presence of these phases.     

New Synthesis and Magnetic Properties of NiFe2O4/SiO2 and Co0.5Zn0.5Fe2O4/SiO2 Nanocomposites Using Tetraglycolatosilane as Precursor

In this work the new synthesis and magnetic properties of NiFe₂O₄/SiO₂ and Co_0.5Zn_0.5Fe₂O₄/SiO₂ nanocomposites using a water‐soluble silica precursor, tetraglycolatosilane (THEOS), by the sol‐gel method were reported. Nanocomposite were obtained by the thermal decomposition of the organic part at different annealing temperatures varying from 400 to 900 °C. Studies carried out using XRD, FT‐IR, TEM, STA (TG‐DTG‐DTA) and VSM techniques. XRD patterns show that NiFe₂O₄ and Co_0.5Zn_0.5Fe₂O₄ have been formed in an amorphous silica matrix at annealing temperatures above 600 and 400 °C, respectively. It is found that when the annealing temperature is up to 900 °C NiFe₂O₄/SiO₂ and Co_0.5Zn_0.5Fe₂O₄/SiO₂ samples show almost superparamagnetic behavior with a magnetization 4.66 emu/g and ferromagnetic behavior with a magnetization 10.11 emu/g, respectively. The magnetization and coercivity values of nanocomposites using THEOS were considerably less than previous reports using TEOS. THEOS as a silica matrix network provides an ideal nucleation environment to disperse ferrite nanoparticles and thus to confine them to aggregate and coarsen. By using THEOS over the currently used TEOS and TMOS, organic solvents are not needed due to the entire solubility of THEOS in water. Synthesized nanocomposites with adjustable particle sizes and controllable magnetic properties make the applicability of ferrites even more versatile.     

A Mössbauer study of oxygen vacancy and cation distribution in 6H-BaTi1−xFexO3−x/2

We have synthesized samples in the system BaTi_1−xFe_xO_3−x/2 with x=0.1−0.6 at temperatures of 1200-1300°C under reducing conditions of oxygen fugacity. After drop quenching, samples were characterized using the electron microprobe, X-ray diffraction and Mössbauer spectroscopy. All samples were hexagonal with a 6H-BaTiO₃ type structure. Mössbauer spectroscopy showed all iron to be present as Fe³⁺, occurring in octahedral and pentahedral sites. Analysis of area ratios indicates that oxygen vacancies are distributed randomly over O1 sites, and that a random distribution of Fe and Ti cations over M1 and M2 sites is consistent with the data. No evidence for ordering of oxygen vacancies was found. Results are consistent with conductivity results, which show generally increasing ionic conductivity with increasing oxygen vacancy concentration.     

Structure and phase composition of nanocrystalline Ce1−xLuxO2−y

The microstructure and phase stability of nanocrystalline mixed oxide Lu_xCe_1−xO_2−y (x=0-1) are described. Nano-sized (3-4 nm) oxide particles were prepared by the reverse microemulsion method. Morphological and structural changes upon heat treatment in an oxidizing atmosphere were studied by transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman and Yb³⁺ emission spectroscopy, the latter ion being present as an impurity in the Lu₂O₃ starting material. Up to 950 °C, the samples were single phase, with structure changing smoothly with Lu content from fluorite type (F) to bixbyite type (C). For the samples heated at 1100 °C phase separation into coexisting F- and C-type structures was observed for 0.35<x<0.7. It was also found that addition of Lu strongly hinders the crystallite growth of ceria during heat treatment at 800 and 950 °C.     

Aqueous Syntheses of Forsterite (Mg2SiO4) and Enstatite (MgSiO3)

Forsterite MgSiO₄ and enstatite MgSiO₃ were synthesized by two different aqueous processes. TEOS was directly hydrolyzed in aqueous solutions of magnesium nitrate, giving solutions of magnesium nitrate and silicic acid. For the first process these solutions were spray-dried and the powders heat treated to decompose the nitrate; and for the second one they were precipitated in a solution of ethylenediamine as a base, the resulting precipitate was filtered, washed and dried. Spray-dried or precipitated, no specific thermal event was detected by thermal analysis for the crystallization of forsterite (500–1000°C) while a strong and sharp exothermic peak traduced the crystallization of enstatite at 800°C. Very minor secondary phases could be detected by X-ray diffraction up to 1200°C for the spray-dried powders, while the precipitated powders presented a higher chemical homogeneity, but much care had to be taken for a quantitative precipitation. As some minor secondary phases like SiO₂ or some polymorphs of MgSiO₃ could be not detected by XRD up to 1300°C, higher thermal treatments were necessary to control the purity or the desired phase.     

Sol-Gel Synthesis of Humidity-Sensitive P2O5-SiO2 Amorphous Films

The sol-gel synthesis of silicophosphate gels using phosphoryl chloride and tetraethoxysilane as molecular precursors is reported and discussed. Gel-derived glasses and films having the molar compositions 10P₂O₅ · 90SiO₂ and 30P₂O₅ · 70SiO₂ have been obtained. The structure of the dried gels as well as the structural modifications that occurs during the transformations in gel-derived glasses are analyzed by Fourier transform infrared spectroscopy (FTIR). It has been found that the evidence of the P—O—Si linkages begins to appear only on the FTIR spectra of the bulk gels heat treated up to 400°C while they are well resolved on the FTIR spectra of the bulk gel samples heated up to 1000°C indicating that at this temperature the transformation in the corresponding gel-derived glasses occurs. The humidity sensitive properties of the gel-films have been evaluated by electrochemical impedance spectroscopy (EIS). The phosphorous content as well as the temperature of the heat treatments strongly affect the sensitivity to RH of the gel-derived films.     

The Synthesis of SnS2 Nanoflakes from Tetrabutyltin Precursor

Hexagonal phase SnS₂ nanoflakes have been synthesized by reactions between an organotin precursor tetrabutyltin [TBT, (CH₂CH₂CH₂CH₃)₄Sn] and carbon disulfide in hexanes at 180-200°C for 10-40 h. The structure, morphologies, composition, and properties have been characterized by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), selected area electron diffraction (SAED), X-ray photoelectron spectroscopy (XPS), ICP-AES, and Raman and Mössbauer spectroscopies. XRD patterns determined the hexagonal SnS₂ with lattice parameters a=3.6384 Å, c=5.9201 Å obtained in n-hexane, and a=3.6389 Å, c=5.9288 Å in cyclohexane. The flakelike morphologies were mainly caused by the anisotropic growth of SnS₂. A possible mechanism is given in the paper.