Composite Glass-Ceramics in the Systems MgO-SiO2, MgO-Al2O3-SiO2 and Fluorapatite Obtained by Sol-Gel Technology

The crystallization processes in the synthesis of composite glass-ceramics in the systems fluorapatite (FA)-MgO·SiO₂ gel-glass (GG) and FA-MgO·Al₂O₃·SiO₂ (GG) have been studied. The composites were prepared by mixing of MgO·SiO₂ and MgO·Al₂O₃·2SiO₂ gel-glasses with FA (from 20 mol% to 80 mol%). The obtained samples were thermally treated at 950°C, 1050°C, 1150°C and 1250°C for 2 hours and the structural changes were investigated, applying XRD-analysis, IR-spectroscopy, Raman spectroscopy and SEM. It has been proved that depending on the chemical composition of the gel-glass and the temperature of treatment, composite glass-ceramics may be prepared consisting only FA, FA and a silicate phase or FA, other phosphate phases and a silicate phase. Because of the absence of calcium or aluminum ions in the composition of the GG, FA is easily decomposed and phases as 7CaO·2MgO·3P₂O₅ and MgO·2P₂O₅ are formed.     

Crystallization of Gels in the Apatite-Mullite System

Crystallization processes in gels of the apatite-mullite system were studied to obtain information for the synthesis of bioglass-ceramics and composite materials. SiO2-sol, Al(NO3)3·9H2O, Ca(NO3)2·4H2O, (NH4)3PO4·3H2O and CaF2 were used as precursors. CaF2 was added before and after gelation. Mixtures of mullite gel-glass and fluorapatite in the range 10 to 90 mol% were investigated for synthesis of composites. All the samples were heat treated at different temperatures in the range 950–1250°C and the structural changes were established using X-ray diffraction and IR-spectroscopy. When the gels were treated at 1050°C and at 1150°C, the main crystalline phases found were fluorapatite and mullite independent of the CaF2 content and the manner of its addition. At 1250°C the relative amounts of fluorapatite and mullite decrease and gehlenite appears. Composite materials containing fluorapatite and mullite as main crystalline phases can be obtained only when the content of mullite gel-glass in the initial mixture is more than 60 mol%.     

Rh Inclusion in Sol-Gel SiO2. Effects of Rh Precursors on Metal Dispersion and SiO2-Rh Thermal Behavior

RhCl₃·nH₂O and [RhCl(C₂H₄)₂]₂ are used as precursors for the preparation of 1%Rh in sol-gel derived SiO₂. The gelling process of Si(OEt)₄ is carried out in the absence of solvent and under strong acid catalysis. The thermal behavior of Rh precursors, of SiO₂ gel and Rh-SiO₂ composites is independently studied by analysing organic species released at definite temperature intervals and concomitantly collecting infrared, XPS, TEM, XRD and porosity data. Results indicate that nanometric Rh particles may be obtained from [RhCl(C₂H₄)₂]₂, their dispersion being homogeneous, dense and stable up to 250°C, whereas RhCl₃·nH₂O affords less metallic dispersion with other crystalline Rh-species; in both cases, well-shaped Rh metal crystallites are obtained at 650°C. The different synthetic approaches used for the preparation of RhCl₃- and [RhCl(C₂H₄)₂]₂-derived samples, are invoked to account for the features of Rh dispersion obtained by mild temperature treatment. Moreover, the particular procedures for sol-gel SiO₂ synthesis are related to the high-temperature maintenance of great porosity and elevated specific surface area.     

Preparation and characterization of silicate gel glasses containing Cr2O3

The effect of thermal treatment on the structural incorporation of Cr₂O₃ in xCr₂O₃·(100 – x)SiO₂ and 5R _n O·xCr₂O₃·(95 – x)SiO₂ (where x=0.01–1 mol.% and R=Li, Na and Ca) gel glasses was studied by optical absorption spectrophotometry, DTA-TG, XRD and electron microscopy.Samples heat treated at 60°C have green color due to the presence of Cr³⁺ in octahedral coordination. The optical transmission, and color (yellow, orange or ambar), of the samples heat treated between 200 and 700°C prove that Cr³⁺ (octahedrally coordinated) and Cr⁶⁺ (tetrahedrally coordinated) are both present. Segregation of Cr₂O₃ take place at temperatures above 800°C.In reducing conditions the gel glasses were green due to the presence of a high content of Cr³⁺ ions. Samples containing Li or Na show crystalline phases at temperatures below 850°C.     

Structural and thermal interpretation of the synergy and interactions between the fire retardants magnesium hydroxide and zinc borate

The mechanism of cooperative action of commercial fire retardants is interpreted as resulting from specific chemical reaction and phase changes. This investigation focuses on the thermally initiated interactions between two forms of commercially available fire retardant compounds. The fire performance of a polyolefin with a metal hydroxide fire retardant, magnesium hydroxide, can significantly reduce the heat release rate through absorption of heat during conversion to its metal oxide. Formation of water, followed by vaporisation, decreases heat and dilutes volatiles from polymer degradation. The second form of fire retardant compounds are zinc borates (2ZnO·3B₂O₃·3H₂O and 4ZnO·B₂O₃·H₂O), that undergo dehydration with increasing temperature. Differential thermal analysis and wide-angle X-ray spectroscopy indicated that various structural changes occurred during heating. Endothermic transitions were observed for all components, while zinc borate (2ZnO·3B₂O₃·3H₂O) showed an exothermic crystallisation transition at relatively high temperature. The exotherm was modified by the development of a new crystalline phase, magnesium orthoborate (3MgO·B₂O₃) that formed on reaction with magnesium oxide (MgO) at temperatures greater than 500 °C. Formation of crystalline zinc oxide (ZnO) was also detected. From zinc borate (4ZnO·B₂O₃·H₂O), ZnO was primarily formed. No new crystalline phases were observed in the presence of MgO over the temperature range investigated.     

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.     

Phase Formation in Gels of the Apatite-Anorthite System

Phase formation in heat-treated composites containing anorthite gel and fluorapatite has been investigated. For the sol-gel synthesis TEOS, Al(NO3)3·9H2 O, Ca(NO3)2·4H2O, (NH4)3PO4·3H2O, NH4F and CaF2 were used as precursors. The gel mixtures were treated from 200 to 1250°C. It was found that the gels remain in an amorphous state up to 900°C. From X-ray diffraction structural analysis of anorthite gel-glasses heat treated at 600°C and 800°C it was found that a shift of the first maximum occurred from 1.85 to 1.77 Å, which could be interpreted as a transformation of the gel-glass structure with increasing temperature. During the heat treatment of the mixed gels from the apatite-anorthite system it was found that fluorapatite is present as a major crystal phase at 950°C. At 1250°C anorthite and gehlenite are the major crystalline phases.     

The synthesis and properties of zeolite CF-3

The novel zeolite CF-3, with a high ratio of SiO₂/Al₂O₃ and a characteristic X-ray powder diffraction pattern, has been synthesized hydrothermally from a TMEDA-Na₂O–SiO₂–Al₂O₃–H₂O system at 200°C. The molar composition of CF-3 is (0.4–0.6)Na₂O·(1.5–6.3)TMEDA·Al₂O₃·(80–400)SiO₂·(0–17)H₂O·CF-3 is similar to ZSM-39 and melanophlogite, which have a clathrate-type structure.     

The Iron Behaviour in Aluminosilicate Gel-Derived Materials

Electron spin resonance (ESR) measurements have been carried out on a 10Fe₂O₃ · 10Al₂O₃ · 80SiO₂ gel heat-treated at different temperatures in air and under reducing conditions. ESR spectra were obtained at 300, 50 and 5 K. The effective g value (g = h/H), linewidth (H_pp) and ESR amplitude (A) depend on heat-treatment temperature of the gel-derived samples. ESR spectra exhibit different magnetic characteristics as a function of heat-treatment temperature and atmosphere. X-ray diffraction (XRD), scanning electron microscopy (SEM) and a.c. susceptibility (_a.c.) analyses were used to better understand the ESR results. The results show that in the samples heat-treated in air, up to 700°C, Fe³⁺ ions are incorporated in the glass network in tetrahedral and/or octahedral co-ordinations. In the samples heat-treated between 250 and 700°C was not detected, by ESR, the presence of iron oxide aggregates. However, the formation of hematite particles was observed by XRD and SEM. The presence of iron oxide aggregates was detected (by ESR) in the samples heat-treated at temperatures higher than 700°C. These aggregates are formed, at 1200 and 1300°C, by hematite and magnetite particles as proved by XRD. The ESR spectra and a.c. susceptibility, of the samples heat-treated at 250°C (under reducing conditions), show a behaviour characteristic of small magnetite particles presence. The sample heat-treated at 500°C (under reducing conditios) contains magnetite particles (XRD). In the ESR spectra of the sample heat-treated at 1000°C, under reducing conditions, was not detected any resonance signal.     

Nanocomposites NiFe2O4/SiO2 and CoFe2O4/SiO2-Preparation by Sol-Gel Method and Physical Properties

This paper aims to characterise the systems NiFe₂O₄/SiO₂ and CoFe₂O₄/SiO₂ prepared by the sol-gel method. After heat treatment, the various samples have been studied by means of X-ray diffraction, Mössbauer spectroscopy, magnetic measurements and transmission electron microscopy (HR TEM).X-ray diffraction and Mössbauer spectra confirmed the presence of the spinel phase. HR TEM observations revealed the nanocrystals with the size in the range of 2–25 nm. Magnetic measurements showed a superparamagnetic behaviour of the samples heated at lower temperature (800°C) and ferrimagnetic character for the samples heated at higher temperature (900, 1000°C).The final phase composition of the heated samples depends on the preparation conditions. The samples, treated up to 300°C in vacuum and then subsequently heated at 800°C or 900°C, do not contain hematite (the most stable phase at higher temperatures). On the contrary, the samples heated at 1000°C or 1250°C display certain content of hematite.     

Crystallization and second harmonic generation in potassium-sodium niobiosilicate glasses

Transparent glasses having molar composition (23−x)K₂O·xNa₂O·27Nb₂O₅·50SiO₂ (x=0, 5, 10, 15 and 23) have been synthesized by the melt-quenching technique and their devitrification behaviour has been investigated by DTA and XRD. Depending on the composition, the glasses showed a glass transition temperature in the range 660-680 °C and devitrified in several steps. XRD measurements showed that the replacement of K₂O by Na₂O strongly affects the crystallization behaviour. Particularly, in the glasses with only potassium or low sodium content the first devitrification step is related to the crystallization of an unidentified phase, while in the glass containing only sodium, NaNbO₃ crystallizes. For an intermediate sodium content (x=10 and 15) a potassium sodium niobate crystalline phase, belonging to the tungsten-bronze family, is formed by bulk nucleation. This system looks promising to produce active nanostructured glasses as the tungsten-bronze type crystals have ferroelectric, electro-optical and non-linear optical properties. Preliminary measurements evidenced SHG activity in the crystallized glasses containing this phase.     

Preparation and Optical Properties of Submicron SiO2 Spheres Doped with YAG:Nd3+ Nanocrystallites

The sol-gel technology has been applied to obtain SiO₂ spheres of submicron size. The spheres have been doped with YAG:Nd³⁺ nanocrystallites. The nanocrystallites have been obtained from aqueous solutions of citric acid, yttrium, aluminum and neodymium chlorides. The obtained gels have been heated up to 800°C. Emission spectra as well as the excited state lifetimes have been measured at room and liquid nitrogen temperatures. The structural characterization has been performed by means of transmission and scanning electron microscopies (TEM, SEM) and powder diffraction (XRD) measurements. We have observed that the YAG:Nd³⁺ nanocrystallites demonstrate pronounced dependence of the emission intensities on the excitation power as compared to the YAG:Nd³⁺ crystallites embedded into the submicron SiO₂ spheres. The results suggest that silica spheres/YAG:Nd³⁺ composites are expected to be good hosts for microlaser systems.     

Physico-chemical properties of metallosilicate supports and cobalt catalysts based on them

The specific surface and the porosity of silicate supports (SiO₂, ZrO₂ · SiO₂, CoO · SiO₂) were determined. The adsorption properties and the reducing ability of the catalysts containing 10 % Co were studied. The spectra of the thermo-programmed desorption of CO below 250°C possess two signals typical of the adsorption of the catalyst on the oxide and metal phases. The formation of liquid hydrocarbons from CO and H₂ is assumed to proceed at surface bifunctional centers.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 668–672, April, 1993.     

Microcalorimetric Studies of Zn—Al Mixed Oxides Obtained From Hydrotalcite-type Precursors

A series of Zn–Al hydrotalcites with Zn/Al molar ratios of 1, 2, 3 and 6 were prepared by co-precipitation method. TG-DTG results showed that the hydrotalcites decompose in two stages, corresponding to the two endothermic peaks around 180 and 220°C. After calcination at 400°C, the samples were converted into Zn–Al mixed oxides with the only XRD pattern of ZnO, except for the sample with the ratio of 6. The Zn–Al mixed oxides possess similar surface acidity revealed by microcalorimetric adsorption of NH₃. The basicity of the samples increases with the order: ZnO>6Zn/Al>1Zn/Al>Al₂O₃.This revised version was published online in November 2005 with corrections to the Cover Date.     

Contributions to the mechanism of water glass hardening by using thermal analysis methods and X-ray diffractometry

Thermal analysis methods and X-ray diffractometry provided data on and permitted practical use of the eutectic mixture between Na₂O·2SiO₂ and SiO₂, which melts at 790°C. Based on this, water glass was used as a binder to obtain artificial cluster granules, ceramically hardened by heating at 800°C. The process of water glass hardening in the presence of hardening reagents such as Na₂SiF₆, NH₄Cl, silica gel and ultra-fine silica was studied by thermal analysis. In the first stage, gelification of the SiO₂ sol takes place by neutralization of the NaOH deflocculant, while the second stage involves tridimensional cross-linking by polycondensation, promoted by powders rich in SiO₂.

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Zusammenfassung Die durch Thermoanalyse und Röntgendiffraktion gewonnenen Daten ermöglichten die Nutzung eines eutektischen Gemisches aus Na₂O·2SiO₂ und SiO₂ mit einem Schmelzpunkt von 790°C. Auf dieser Basis wurde Wasserglas als Bindemittel zur Herstellung von künstlichen Cluster-Granulaten genutzt, die bei 800°C keramisch gehärtet wurden. Dieser Härtungsprozess von Wasserglas in Gegenwart von Härtemitteln wie z.B. Na₂SiF₆, NH₄Cl, Silikagel und ultrafeiner Kieselerde wurde mittels Thermoanalyse untersucht. Im ersten Schritt der Gelbildung aus dem SiO₂-Sol erfolgt die Neutralisation des NaOH-Deflokulanten, während der zweite Schritt durch Polykondensation eine dreidimensionale Vernetzung umfaßt, die durch SiO₂-reiche Pulver unterstützt wird.

     

Photocatalytic degradation of Orange II by titania addition to sol–gel glasses

Glasses on SiO₂–CaO–ZnO–B₂O₃–K₂O–Al₂O₃ oxide system modified by addition of titania (0, 3, 5, 12, and 20% w) have been prepared by sol–gel method. The obtained gels were aged, dried and fired at 600 °C/1 h in order to stabilise the glass. The resulting fired powders were characterised by UV–Vis–NIR spectroscopy, scanning electron microscopy, transmission electron microscopy (TEM) and X-ray diffraction (XRD). Their photocatalytic capacity on the degradation of Orange II dye has been studied. The XRD and TEM studies indicate that system becomes amorphous with a nanostructured microstructure. From UV–Vis–NIR results the band gap calculated is around 3.5 eV for all modified glasses. Photoactivity of powders depends on amount of titania in glass composition and the specific surface area of prepared samples. The sample with highest surface area and lowest addition of titania (3% w sample) shows similar activity than commercial anatase used as reference.     

Monitoring of the evolved gases in apatite-ammonium sulfate thermal reactions

Summary Thermal reactions in natural fluorapatite or fluorcarbonate apatite and ammonium sulfate mixtures with mole ratio 1:4 at calcination up to 500°C were studied by simultaneous thermogravimetry and FTIR analysis of the evolved gases. The composition of natural apatite has little impact on the release of NH₃. Upon the evolution of NH₃ nitrous oxides were found in minor amounts. The release of SO₂ at temperatures above 400°C is more intensive and occurs at lower temperatures in the case of fluorapatite than of carbonate containing apatites. Evolution of CO₂ starts at 250°C with maximum at 350-360°C.     

Mechanism and kinetics of glass-ceramics formation in the LiO2-SiO2-CaO-P2O5-CaF2 system

Two glasses based on lithium disilicate (LS₂), with and without fluorapatite (FA), were synthesised in the Li₂O-SiO₂-CaO-P₂O₅-CaF₂ system with P₂O₅: CaO: CaF₂ ratios corresponding to fluorapatite. Glass-ceramics have then been prepared by thermal treatment. The mechanism and kinetics of crystallization as functions of grain size and rate of heating were investigated using thermal analysis methods. The smaller particles crystallize preferentially by surface crystallization, which is replaced by volume crystallization at larger particle sizes. Inclusion of FA in the LS₂ favours crystallization through the surface mechanism. The onset limit for volume crystallization replacing the surface mechanism is at about 0.3 mm for pure LS₂ glass and 0.9 mm for glass containing FA. The calculated activation energies of the glasses (₂99 ± 1 kJ mol-1 for pure LS₂ glass and ₂88 ± 7 kJ mol⁻¹ for glass containing FA according to Kissinger, or 313 ± 1 kJ mol-1 for pure LS₂ glass and 303 ± 8 kJ mol-1 for glass containing FA according to Ozawa) indicate that the tendency of the glasses to crystallize is supported by the FA presence. Bioactivity of all samples has been proved in vitro by the formation of new layers of apatite-like phases after soaking in SBF.      

The hydrates of sodium metasilicate peroxyhydrates

Summary A study of solubility in the system Na₂SiO₃-H₂O₂-H₂O at 0 and 15° has shown that sodium metasilicate forms a number of compounds with the general formula Na₂SiO₃· ·xH₂O₂·yH₂O, where x=2.5–3 and y=0–1.     

Deformation investigation on iPP/SiO2 composites: Influence of stretching temperature and particle size on morphology evolution and crystalline structure of thin films

 In the present work, structure changes during stretching of isotactic polypropylene (emPP) and emPP/silicon dioxide (SiO₂) composites have been investigated systematically. The α-form crystal structure of both iPP and emPP/SiO₂ composites is destroyed and transforms into the mesophase as the samples are stretched at a low temperature (35℃), while stretching at high temperatures (90℃ and 120℃) can restrain the appearance of defects and keep the perfection of crystal structure. FTIR results reveal that the stretching temperatures show no obvious difference of the effect on the orientation of pure iPP, however, the orientation of emPP/SiO₂ composites is greatly changed by the tensile temperature. In the case of micron-sized SiO₂ particles (average particle diameter d>1 μm), the orientation of the composites is lower than that of pure iPP at all stretching temperatures. The above results suggest that the stretching temperature and the SiO₂ particle size have great influence on the structure variation and orientation behavior of emPP/SiO₂ composites.