Gel-glass transformation in the SiO2Fe2O3 system

Gel-glass transformation has been studied by Mössbauer spectroscopy, DTA-TG analyses and X-ray diffractometry for four compositions in the SiO₂Fe₂O₃ system (A: 5 wt% Fe₂O₃, B: 10 wt% Fe₂O₃, C: 20 wt% Fe₂O₃, D: 40 wt% Fe₂O₃).The gels were prepared by the hydrolysis of silicon tetraethoxide and iron triethoxide and successively dried and heated in oxygen in the temperature range 40–1000°C.Samples A and B gave typical amorphous X-ray patterns up to 700°C; heating at higher temperature yielded the precipitation of quartz, cristobalite and hematite in sample A, cristobalite and hematite in sample B. Crystallization was also detected by DTA in sample A for which X-ray diffraction exhibited a larger effect.In samples C and D crystallization took place starting from 300°C with the precipitation of hematite, which remained the only crystalline phase up to 1000°C.The presence of hematite was confirmed by the obtained Mössbauer spectra which showed the characteristic sextet. The apportion of iron ions in the Fe³⁺ and Fe²⁺ oxidation states was also determined, together with the attribution of the probable coordination states for Fe³⁺ ions.Complex magnetic structure appeared in samples treated above 800°C.     

Crystallization kinetics of amorphous Se60S20Te20

The temperature and the time dependence of the electrical conductivity (6) of a-Se₆₀S₂₀Te₂₀ are investigated. Conductivity anomalies are observed during the crystallization growth from both the amorphous solid and supercooled liquid phases. The changes of 6 during different isothermal annealing of a-Se₆₀S₂₀Te₂₀ samples with time are used to determine the ratio transformed from amorphous to crystalline. A value of 0.97±0.16 eV has been obtained for the activation energy of the crystal growth in the temperature range of 120–170°C.     

Preparation and properties of ferroelastic/ferroelectric polytypes of antimony(III) oxide iodine,Sb5O7I

Antimony oxide iodide, Sb₅O₇I (?7Sb₂O₃.SbI₃), is monoclinic and exists in at least 8 polytypic modifications. Crystals of these (transparent, pseudohexagonal prisms up to 15 × 10 × 5 mm³) were grown by vacuum sublimation and identified by optical and X-ray methods. Depending on the stacking sequence of characteristic slabs of antimony and oxygen in the (monoclinic) b-direction, centric and acentric polytypes result. The former show pure ferroelastic, the latter combined ferroelastic + ferroelectric behaviour. All polytypes transform reversibly into hexagonal, nonferroic phases at characteristic Curie temperatures between 165 and 208°C. The crystals can assume one of the three stable orientation states which may be mutually switched into each other by successive application of comprehensive stress (of the order of 10⁶ N/m²) on the three pairs of prism faces. In the ferroelectric polytypes the spontaneous polarization (about 5 × 10^-3 C/m²) is not reversible but only reorientable by ± 120° by coercive fields in the order of 2–5 kV/cm. Ferroelectricity and ferroelasticity are fully coupled.     

Electron microscopic study of the crystallization kinetics of the amorphous alloy Pd80Si20

Amorphous Pd₈₀Si₂₀ foils, prepared by the rapidly rotating mill device method, were quenched from the liquid state, temperatures of the melt being 1100, 1500 and 1600°C. The foils were linearly heated at a rate of 1.5 °C/min over a temperature range of 20–600°C. The structural changes were observed by methods of optical microscopy, transmission electron microscopy and electron diffraction. The beginning of crystallization of the amorphous phase is affected by the temperature of the melt. The crystallization of the amorphous phase was terminated at a temperature of 390–400°C. Silicides Pd₉Si₂ and Pd₃Si, the morphology of which varied with temperature, were formed during crystallization.     

The effect of manganese additions on the structure changes proceeding in rapidly solidified Al-Cu alloys

The effect of Mn additions (0… 2 wt%) on the decomposition of rapidly solidified Al-4.0 wt% Cu alloys (cooling rate 10³ to 10⁴ K/s; LQ treatment) were studied during ageing between RT and 450 °C by hardness, X-ray methods and electron microscopy. The results were compared with alloys homogenized in the region of the solid solution (SQ treatment). (i) The LQ treatment results in a quite better homogeneous distribution of the alloyed elements than the SQ one, that is less particles of intermetallic phases are present in the ascast state. (ii) At T < 250 °C Mn additions affect the decomposition kinetics by trapping of vacancies (retardation) and the diminution of the solubility of Cu atoms (acceleration). The first effect dominates in the stage of G.P. zone formation, the second one during precipitation of intermediate phases. (iii) At T ≧ 300 °C the intermetallic compound Cu₂Mn₃Al₂₀ forms associated with a significant increase of the hardness.     

The crystallization of vitreous and metastable Pb5Ge3O11

Vitreous Pb₅Ge₃O₁₁ was prepared by water quenching and by roller quenching. The crystallization was studied by DTA, X-ray diffraction, and TEM. After a glass transition at 325°C, meta-stable Pb₅Ge₃O₁₁ crystallized exothermically at 370°C. This was subsequently followed by the formation of the stable phase of the same composition, again exothermically at 490°C.     

Ion exchange between glass melts of the system Na2ORb2OSiO2

Ion exchange between glass melts of the quasi-binary system Na₂O · 3SiO₂Rb₂O₂ was investigated at 700—1300°C by means of a special capillary method. Concentration profiles were obtained by electron microprobe analysis and were evaluated for a concentration-dependent quasi-binary interdiffusion coefficient $\text{tilde}\text{D}$ using a modified Boltzmann-Matano method. At 700–1000°C interdiffusion could be obtained in pure form with $\text{tilde}\text{D}$ values ranging from about 10^?7 ?5 × 10^?6 cm² s^?1. Above 1000°C convection processes superimposed interdiffusion, making a further evaluation impossible. The data are compared with those from a 0g rocket experiment and are discussed with respect to a mixed-alkali effect and in terms of the Nernst-Planck diffusion model.     

Glass forming ability and crystallization in the PbO-Al2O3 system

The formation of glass by the splat-quenching of PbO-Al₂O₃ melts was confirmed in the range 67-95 mol.% PbO. The crystallization of the glasses obtained in this way was examined by thermal analysis, dark field microscopy, X-ray diffraction and IR spectroscopy. This crystallization is generally a two-step process: orthorhombic PbO crystallizes first around 280-36-°C depending on the composition, then the aluminate PbO · Al₂O₃ crystallizes around the rather constant temperature of 600°C.     

Study of the region of glass formation and electrical properties of the AsSeSb system

Glassy samples containing As, Se and Sb in different ratios are synthesized under a vacuum of the order of 10^?5 Torr and at a temperature of (950 ± 20)°C. The region of glass formation of this system is determined. It is found that antimony can be added in amounts of up to approximately 19 at.% in As₂Se₃ while the glassy state is still preserved. Thermal activation energy for the samples is found to decrease with the increase of antimony content. The conductivity at 20°C and the logarithmic value of the constant σ₀ are determined for each composition investigated. Annealing of the samples (induces crystallization) reduces both thermal activation energy and room temperature resistivity. Preliminary devitrification characteristics of some samples in this system using X-ray and differential thermal analysis are given.     

Thermal-induced phase transition and assembly of hexagonal metastable In2O3 nanocrystals: A new approach to In2O3 functional materials

This paper reports on the thermal-induced performance of hexagonal metastable In₂O₃ nanocrystals involving in phase transition and assembly, with particular emphasis on the assembly for the preparation of functional materials. For In₂O₃ nanocrystals, the metastable phase was found to be thermally unstable and transform to cubic phase when temperature was higher than 600 °C, accompanied by assembly as well as evolution of optical properties, but the two polymorphs coexisted at the temperature ranging from 600 to 900 °C, during which the content of product phase and crystal size gradually increased upon increasing temperature. The assembly of In₂O₃ nanocrystals can be developed to fabricate In₂O₃ functional materials, such as various ceramic materials, or even desired nano- or micro-structures, by using metastable In₂O₃ nanocrystals as precursors or building blocks. The electrical resistivity of In₂O₃ conductive film fabricated by a hot-pressing route was as low as 3.72×10⁻³ Ω cm, close to that of In₂O₃ single crystal, which is important for In₂O₃ that is always used as conductive materials. The findings should be of importance for both the wide applications of In₂O₃ in optical and electronic devices and theoretical investigations on crystal structures.     

GeO2/SiO2-glasses from gels to increase the oxidation resistance of porous silicon containing ceramics

GeO₂/SiO₂-glasses with relatively high expansion coefficients (between 5 and 7.3 × 10^?6 °C^?1) were prepared by hydrolysis of metal alkoxides (tetramethoxy silane+tetraethoxy germane as well as mixtures of the silane with GeO₂-powder) with subsequent heat treatment up to 1000°C. It is shown that the preparation of thin films on glasses and ceramics is possible but also that this technique can be used for liquid infiltration of porous SiC- and Si₃N₄-materials to increase the oxidation behaviour in the temperature range 1000 to 1400°C. In the case of molybdenumdisilicide as a ceramic material it is shown that the catastrophic inner oxidation in the low temperature range (600 to 800°C) of porous specimens (tested with plasma-sprayed MoSi₂-layers on refractory metals) can be hindered if these materials are infiltrated and heat treated.     

Single crystals of the fluorite nonstoichiometric phase Eu 0.9162+ Eu 0.0843+ F2.084 (conductivity, transmission, and hardness)

The nonstoichiometric phase EuF_2+x has been obtained via the partial reduction of EuF₃ by elementary Si at 900–1100°C. Eu_0.916²⁺Eu_0.084³⁺F_2.084 (EuF_2.084) single crystals have been grown from melt by the Bridgman method in a fluorinating atmosphere. These crystals belong to the CaF₂ structure type (sp. gr. Fm $ \bar 3 $ \bar 3 m) with the cubic lattice parameter a = 5.8287(2) ?, are transparent in the spectral range of 0.5–11.3 μm, and have microhardness H _μ = 3.12 ± 0.13 GPa and ionic conductivity σ = 1.4 × 10⁻⁵ S/cm at 400°C with the ion transport activation energy E _a = 1.10 ± 0.05 eV. The physicochemical characteristics of the fluorite phases in the EuF₂ − EuF₃ systems are similar to those of the phases in the SrF₂ − EuF₃ and SrF₂ − GdF₃ systems due to the similar lattice parameters of the EuF₂ and SrF₂ components. Europium difluoride supplements the list of fluorite components MF₂ (M = Ca, Sr, Ba, Cd, Pb), which are crystal matrices for nonstoichiometric (nanostructured) fluoride materials M _{1 − x} R _x F_{2 + x} (R are rare earth elements).     

Effect of heat-treatment condition on crystallization behavior and thermal expansion coefficient of Li2O-ZnO-Al2O3-SiO2-P2O5 glass-ceramics

Utilizing P₂O₅ as nucleation agent, a Li₂O-ZnO-Al₂O₃-SiO₂ glass was prepared by conventional melt quenching technique and subsequently converted to glass-ceramics with different crystal phases. During the processing, two-step heat-treatments including nucleation and crystallization were adopted. The effects of heat-treatment on the crystal type, the microstructure and the thermal expansion behavior of the glass-ceramics were studied by means of differential scanning calorimetry, X-ray powder diffraction analysis, scanning electron microscopy and thermal expansion coefficient tests. It was shown that the crystallization of occurred after the glass was treated at 580 °C. As the temperature increased from 580 °C to 630 °C, cristobalite and were identified as main and second crystal phases, respectively, in the glass-ceramic. An increase in the temperature to 700 °C, the β-quartz solid solution in the glass-ceramic accompanied by a decrease in cristobalite content. The transformation from to γ₀-Li₂ZnSiO₄ took place from 700 °C to 750 °C. The resulting crystallization phases in the glass-ceramics obtained at the temperature higher than 750 °C were β-quartz solid solution and γ₀-Li₂ZnSiO₄. The glass-ceramics containing or β-quartz solid solution crystal phase possessed a microstructure formed by the development of dendritic crystals. The thermal expansion coefficient of the glass-ceramics varied from 36.7 to 123.8 × 10⁻⁷ °C⁻¹ in the temperature range of 20-400 °C, this precise value is dependent on the type and the proportion of the crystalline phases presented.     

Small angle X-ray scattering study of spinodal decomposition in B2O3---PbO---Al2O3 glass

SAXS measurement for 80B₂O₃·15PbO·5Al₂O₃ (wt%) glass can be performed with high accuracy. SAXS spectra were obtained for glasses which had been heat-treated for different lengths of time at 380°C and 400°C. A converging point k′_c and a common curve for k >k′_c were observed in the SAXS spectra of glasses subjected to appropriate aging. This suggests the validity of Cook's modification of Cahn's theory. At the very early stage of the decomposition a relaxation takes place of $\text{?″}\text{and}\text{D}\text{?}$ and the spectra of Cahn-Cook's amplification factor show a typical aspect of the early stage of decomposition after the relaxation. Plots of k³I(k) versus k³ suggest that the interface between the two phases is diffuse and the demixing process is in the early stage of the spinodal decomposition.     

Crystallization of amorphous Ni35Zr65 and Fe40Ni40P14B6 under proton irradiation

Two amorphous alloys, Ni₃₅Zr₆₅ and Fe₄₀Ni₄₀P₁₄B₆, were irradiated using 400 keV protons at several temperatures below the crystallization temperature, T_x, to peak doses in the neighborhood of 3.5 to 4.5 dpa. Irradiation at 250°C resulted in the crystallization of both alloys, which were examined by transmission electron microscopy of samples electrolytically polished to various distances from the irradiated surface to study the effect of dose. Samples masked from the proton beam remained amorphous during irradiation. In the Ni₃₅Zr₆₅ alloy crystallization of the equilibrium phases propagated throughout the entire sample, while the in the Fe₄₀Ni₄₀P₁₄B₆ alloy crystallization was observed only in those parts of the samples lying within the proton range. Neither alloy crystallized during irradiation at 100°C. In both these alloys the amorphous phase is therefore evidently stable at irradiation temperatures below approximately 0.6 T_x. An examination of the literature on irradiation damage of binary alloys and intermetallic compounds suggests that there is a tendency for initially amorphous alloys to remain amorphous at irradiation temperatures, T_irr < 0.3 T_L, where T_L (≈T_x) is the “melting” temperature (either a eutectic, peritectic or congruent melting temperature). Also, these same alloys, even when they are initially crystalline, transform to the amorphous state during irradiation at T < 0.3 T_L. Some other crystalline alloys have also been shown to transform to the amorphous state at T_irr < 0.3 T_L even though they have never been prepared in this condition by rapid quenching techniques. The temperature 0.3 T_L appears to be a lower limit, however, since the crystalline to amorphous transformation occurs in many of these alloys at temperatures greater than 0.3 T_L. It is suggested, by analogy with results on void formation in irradiated metals, that this low temperature limit is related to the low mobility of vacancies in these materials, although the mechanism of crystallization, or conversely amorphization, is not fully understood.     

Crystallization and local order of bulk AsxTe1−x glasses

The crystallization and the local order of the bulk As_xTe_1?x (0.2 ? × ? 0.8) glasses have been investigated by DSC, X-ray and Mössbauer methods. During the crystallization a metastable phase has been observed between 0.35 ? × ? 0.5 and it was identified as fcc AsTe. The local surrounding of these As_xTe_1?x glasses characterized by the ¹²⁵Te Mössbauer measurements were compared with those of the stable monoclinic As₂Te₃, hexagonal Te and metastable fcc AsTe.     

Quenched glasses in the systems of Li2O with Al2O3, Ga2O3 and Bi2O3

By rapid quenching in a twin roller apparatus, glass was found to occur widely in the systems of Li₂O with Al₂O₃, Ga₂O₃, Bi₂O₃ and in mixed systems. Examination of the resulting flakes by X-ray powder diffraction, differential thermal analysis, and capacitance data revealed the occurrence of glass, glass transitions, crystallization exotherms and the nature of some of the crystallization paths.The log ionic conductivity of the glasses was found to follow a linear relationship with the Li concentration. Evidence was observed for three new metastable crystalline phases, one in the Li₂OAl₂O₃ system and two in the Li₂OBi₂O₃ system. The latter system also showed evidence for the occurrence of two glasses at almost all compositions.     

Growth of single crystals of NaH2PO4 (NaDP), NaH2PO4 · H2O (NaDP · H2O), and NaH2PO4 · 2H2O (NaDP · 2H2O) sodium dihydrogenphosphate based on the analysis of the Na2O-P2O5-H2O phase diagram

The crystallization conditions for the NaH₂PO₄, NaH₂PO₄ · H₂O, and NaH₂PO₄ · 2H₂O solid phases have been established from the analysis of the phase diagram of solubility of the ternary Na₂O-P₂O₅-H₂O system in the temperature range from 0 to 100°C. Based on these data, the methods for growing sodium dihydrogenphosphate single crystals of the above compositions are developed. The initial components for preparing mother solutions were H₃PO₄ and NaOH solutions taken in certain weight ratios. For the first time, NaDP, NaDP · H₂O, and NaDP · 2H₂O single crystals were grown on a seed by the method of temperature decrease. The habits of the NaDP and NaDP · H₂O single crystals are determined. __________ Translated from Kristallografiya, Vol. 47, No. 5, 2002, pp. 937–944. Original Russian Text Copyright ? 2002 by Soboleva, Voloshin.     

Anomalous amorphous SiO2 films

Anomalous SiO₂ films have been prepared by sputtering Si in a mixture of Ar-10% O₂ at 77 K. The same sputtering conditions at room temperature yield normal SiO₂ which means that the anomaly is produced by the low temperature deposition. The anomaly reveals itself in several physical properties. The density of the anomalous SiO₂ is 1.72 as compared with 2.20 for bulk and the dielectric constant is about 50% larger than bulk and with a much stronger temperature dependence. The infrared (ir) spectrum of the anomalous SiO₂ is only slightly different from bulk SiO₂ but esr experiments reveal about 3 × 10¹⁸ spins cm which do not exist in bulk SiO₂. These anomalous films are extremely stable: upon heating only a small amount of oxygen (1 part in 10⁵) evolves at 440°C but the density and IR spectrum remain unchanged up to 1300°C. Annealing at 1500°C completely removes the ESR signal and returns the ir spectrum and the density to that of cristobalite. An electron diffraction and transmission electron microscopy study reveals that the anomalous SiO₂ films consist of essentially bulk like SiO₂ clusters about 250 Å in diameter separated by a low density network. The low density network undoubtedly contains unbound O atoms and the SiSi bonds which give rise to the esr signal. The structural model can account for all the anomalous properties.     

Structural analysis and devitrification of glasses based on the CaO-MgO-SiO2 system with B2O3, Na2O, CaF2 and P2O5 additives

Glasses, whose basic composition was based on the CaO-MgO-SiO₂ system and doped with B₂O₃, P₂O₅, Na₂O, and CaF₂, were prepared by melting at 1400 °C for 1 h. Raman and infrared (IR) spectroscopy revealed that the main structural units in the glass network were predominantly Q¹ and Q² silicate species. The presence of phosphate and borate units in the structure of the glasses was also evident in these spectra. X-ray analysis showed that the investigated glasses devitrified at 750 °C and higher temperatures. The crystalline phases of diopside and wollastonite dominated, but weak peaks, assigned to akermanite and fluorapatite, were also registered in the diffractograms. The presence of B₂O₃, Na₂O, and CaF₂ had a negligible influence on the assemblage of the crystallized phases, but it caused a reduction of crystallization temperature, comparing to similar glasses of the CaO-MgO-SiO₂ system.