Formation of TiO2(B) Nanocrystallites in Sol-Gel-Derived SiO2-TiO2 Film

TiO₂(B), one of the polymorphs of TiO₂, has been formed by annealing a sol-gel-derived SiO₂-TiO₂ amorphous film on a silicon wafer at 900°C in air. Transmission electron microscopy (TEM) revealed that nanocrystallites with a size of 5-10 nm were dispersed in the amorphous SiO₂ matrix in the film. The X-ray diffraction pattern and lattice fringe spacing in high-resolution TEM images corresponded to those of TiO₂(B). These TiO₂(B) nanocrystallites are probably stable with the presence of surrounding SiO₂ in the film at 900°C, because previous works reported that this phase should be converted to anatase at temperatures higher than 550-700°C.     

Interpretation of the Kaolinite-Mullite Reaction Sequence from Infrared Absorption Spectra

The phases in the kaolinite-mullite reaction sequence were reexamined by ir absorption spectrophotometry. Particular attention was paid to the controversial intermediate Al-containing phases. Amorphous materials were leached from fired kaolinite samples with NaOH to help identify crystalline phases. Metakaolinite partially decomposes, releasing amorphous γ-Al₂O₃ and SiO₂, before the "950°C" exothermic reaction in which metakaolinite is completely decomposed. The resulting spinel-type phase, which is associated with amorphous SiO₂ and some poorly crystalline "primary" mullite, is γ-Al₂0₃ (crystalline) rather than an Al-Si spinel. There is some evidence, however, that a fraction of the γ-Al₂O₃, may be an Al-Si spinel. At ≥1100°C secondary mullite therefore forms primarily from the γ-Al₂O₃/amorphous SiO₂ reaction and the recrystallization of primary mullite, whereas excess amorphous SiO₂ eventually crystallizes as cristobalite.     

Reaction and Formation of Crystalline Silicon Oxynitride in Si–O–N Systems under Solid High Pressure

Oxidized amorphous Si₃N₄ and SiO₂ powders were pressed alone or as a mixture under high pressure (1.0–5.0 GPa) at high temperatures (800–1700°C). Formation of crystalline silicon oxynitride (Si₂ON₂) was observed from amorphous silicon nitride (Si₃N₄) powders containing 5.8 wt% oxygen at 1.0 GPa and 1400°C. The Si₂ON₂ coexisted with β-Si₃N₄ with a weight fraction of 40 wt%, suggesting that all oxygen in the powders participated in the reaction to form Si₂ON₂. Pressing a mixture of amorphous Si₃N₄ of lower oxygen (1.5 wt%) and SiO₂ under 1.0–5.0 GPa between 1000° and 1350°C did not give Si₂ON₂ phase, but yielded a mixture of α,β-Si₃N₄, quartz, and coesite (a high-pressure form of SiO₂). The formation of Si₂ON₂ from oxidized amorphous Si₃N₄ seemed to be assisted by formation of a Si–O–N melt in the system that was enhanced under the high pressure.     

Crystallization Behavior of Al2O3 in the Presence or SiO2

The independent crystallization sequence of an Al₂O₃ component is modified in the presence of SiO₂ and vice versa. Mixed SiO₂-Al₂O₃, gel (28 wt% SiO₂ and 72 wt% Al₂O₃) forms neither cristobalite nor γ-Al₂O₃ and corundum at 1000°C but forms Si-Al spinel; an amorphous aluminosilicate phase invariably also forms after the gel is heated. However, the composition of this amorphous aluminosilicate phase is not as yet known.     

Grain Growth of Silica-Added Zirconia Annealed in the Cubic/Tetragonal Two-Phase Region

The grain growth in silica-doped 3-mol%-yttria-stabilized tetragonal zirconia polycrystals (SiO₂-doped 3Y-TZP) and undoped 3Y-TZP has been examined in the temperature range of 1400°-1800°C. The presence of a SiO₂ phase inhibits rather than promotes the grain growth in 3Y-TZP, particularly at high temperatures. During the grain growth in 3Y-TZP, yttrium ions are partitioned between grains, and the grain growth mechanism can be understood from Ostwald ripening dominated by lattice diffusion of cations. In SiO₂-doped 3Y-TZP, an amorphous SiO₂-rich phase exists only in the grain-boundary corners or junctions, not in the grain-boundary faces. The grain growth in SiO₂-doped 3Y-TZP is controlled by using different mechanisms below and above the eutectic temperature of the zirconia-silica (ZrO₂-SiO₂) system. The glass phase does not have a major role in grain growth below the eutectic temperature, and the grain growth is dominated by a similar mechanism in undoped 3Y-TZP. The grain growth is more effectively retarded by the presence of a SiO₂ phase above the eutectic temperature and is likely to be controlled by a solution-reprecipitation process in the amorphous phase at the grain-boundary corners or junctions.     

Phase Transformation of Diphasic Aluminosilicate Gels

Aluminosilicate gels with compositions Al₂O₂/SiO₂ and 2 were prepared by gelling a mixture of colloidal pseudo-boehmite and a silica sol prepared from acid-hydrolyzed Si(OC₂H₅)₄. Upon heating the pseudo-boehmite transforms to γ-Al₂O₃ around 400°C, then to δ-Al₂O₃ at 1050°C, and at 1200°C reacts with amorphous SiO₂ to form mullite. Some twinned θ-Al₂O₃ forms before mullite. Nonstoichiometric specimens have a similar transformation sequence, but form mullite grains with inclusions of either Al₂O₃ or cristobalite, often associated with dislocation networks or micropores. Mullite grains are formed by nucleation and growth and have equiaxed shape.     

Novel Low-Temperature Processing Route of Dense Mullite Ceramics by Reaction Sintering of Amorphous SiO2-Coated γ-Al2O3 Particle Nanocomposites

Dense mullite ceramics were successfully produced at temperatures below 1300°C from amorphous SiO₂-coated gamma-Al₂O₃ particle nanocomposites (AS-gammaA). This method reduces processing temperatures by similar/congruent300°C or more with respect to amorphous SiO₂-coated alpha-Al₂O₃ particle microcomposites (AS-alphaA) and to other Al₂O₃-SiO₂ reaction couples. The good densification behavior and the relatively low mullite formation temperature make AS-gammaA nanocomposites an excellent matrix raw material for polycrystalline aluminosilicate fiber-reinforced mullite composites.     

Study of Ordering in High-Quartz Solid Solutions by Substitutions Affecting Superlattice Reflections

Ordering of the interstitial and network atoms in stuffed high-quartz solid-solution ( ss ) phases was studied by correlating suitable substitutions in the structure with the resultant changes in the superlattice reflections arising from these phases. In an MgO·Al₂O₃·3SiO₂ high-quartz ss , the special ordering of the interstitial Mg atoms, which produces satellite reflections around superlattice positions, was disturbed by replacement of ∼20 mol% of the MgO by Li₂O or ZnO. The interstitial atoms were further disordered by larger substitutions. The streaked superlattice reflections arising from an Li₂O·Al₂O₃·2SiO₂ high-quartz ss disappeared on replacement of ∼40 mol% of the Li₂O by MgO. This result implies that the interstitial Li atoms are ordered and were disordered by the substitution. The superlattice reflections were much intensified when ∼25 mol% of the SiO₂ was replaced by GeO₂. However, these reflections were intensified to a lesser degree when presumably both SiO₂ and Al₂O₃ were partially replaced by GeO₂ and Ga₂O₃, respectively. These observations strongly suggest an ordering of the network Si and Al atoms, which, together with the ordering of the interstitial Li atoms, contributed to the original streaked superlattice reflections.     

Preparation of NiAl2O4/SiO2 and Co2+-Doped NiAl2O4/SiO2 Nanocomposites by the Sol–Gel Route

NiAl₂O₄/SiO₂ and Co²⁺-doped NiAl₂O₄/SiO₂ nanocomposite materials of compositions 5% NiO – 6% Al₂O₃– 89% SiO₂ and 0.2% CoO – 4.8% NiO – 6% Al₂O₃– 89% SiO₂, respectively, were prepared by a sol–gel process. NiAl₂O₄ and cobalt-doped NiAl₂O₄ nanocrystals were grown in a SiO₂ amorphous matrix at around 1073 K by heating the dried gels from 333 to 1173 K at the rate of 1 K/min. The formations of NiAl₂O₄ and cobalt-doped NiAl₂O₄ nanocrystals in SiO₂ amorphous matrix were confirmed through X-ray powder diffraction, Fourier transform infrared spectroscopy, differential scanning calorimeter, transmission electron microscopy (TEM), and optical absorption spectroscopy techniques. The TEM images revealed the uniform distribution of NiAl₂O₄ and cobalt-doped NiAl₂O₄ nanocrystals in the amorphous SiO₂ matrix and the size was found to be ∼5–8 nm.     

Stability of CaNiSi2O6 ("Niopside") and Activity–Composition Relations of CaMgSi2O6– CaNiSi2O6 Solid Solutions at 1350°C

A complete solid-solution series exists between diopside (CaMgSi₂O₆) and its nickel analogue, "niopside"(CaNiSi₂O₆). Activity–composition relations within this solid solution, and the stability of the end member CaNiSi₂O₆, have been determined by equilibrating CaNiSi₂O₆ with SiO₂, CaSiO₃, and metallic Ni in atmospheres of known oxygen pressures. Within limits of accuracy of the experiments, the solution is ideal at 1350°C. From the experimental data obtained in the present investigation, the standard free energy (Δ G °) of formation of CaNiSi₂O₆ according to the equation CaO + NiO + 2SiO₂= CaNiSi₂O₆ is calculated to be Δ G °=−165862 + 42.40 T J. Experiments in the system CaO–NiO–SiO₂ have shown that the nickel analogue of the phase pseudo-enstatite (MgSiO₃) is unstable with respect to SiO₂ and nickel olivine (Ni₂SiO₄), and the nickel analogues of the phases akermanite (Ca₂MgSi₂O₇) and monticellite (CaMgSiO₄) are unstable relative to the phase assemblage pseudo-wollastonite (CaSiO₃) plus NiO. In the system CaO–MgO–NiO–SiO₂, however, substitution of Ni for Mg in these phases was observed. The percentage substitution of Ni for Mg in the phases is given in parentheses: diopside (100%), olivine (100%), enstatite (18%), akermanite (20%), and monticellite (57%).     

Formation and Sintering of TiO2 (Anatase) Solid Solution in the System TiO2-SiO2

In the TiO₂-SiO₂ system, anatase solid solutions (ss) containing up to similar/congruent ∼15 mol% SiO₂ are formed in the as-prepared state by the hydrazine method. The lattice parameters a and c decrease linearly from 0.3785 to 0.3776 nm and from 0.9514 to 0.9494 nm, respectively, with increased SiO₂ content. At high temperatures, the solid solutions by transformation decompose into rutile and amorphous SiO₂. The anatase(ss) powders have been characterized for particle size and surface area. They consist of very fine particles (7-25 nm). Surface areas at low temperatures are very high and do not drop below 60 m²/g at 1000°C. Nanostructured anatase(ss) ceramics, with greaterthan/equal to 99.5% of theoretical density and an average grain size of 72 nm, have been fabricated by hot isostatic pressing for 1 h at 850°C and 196 MPa. Their mechanical and electrical properties have been examined.     

Formation of Silicon-Aluminum Spinel

Characterization of the intermediate cubic phase formed during the transformation of coprecipitated SiO₂-Al₂O₃ gel on heating was studied and X-ray diffraction methods are reviewed and criticized. Coprecipitated gels of different SiO₂/Al₂O₃ ratios were prepared; all showed a 980°C exotherm followed by crystallization of the cubic phase and liberation of SiO₂. Alkali extraction of SiO₂ showed two types present in the 980°-heated product. One variety is free amorphous SiO₂ and the other, chemically bonded to alumina in the crystalline cubic phase, was isolated and characterized as Si-Al spinel with the same composition as mullite. Thus, its formation from the gel of mullite composition shows the highest exotherm and the measured density agrees approximately with the theoretically calculated value.     

Subsolidus Studies in the System PbO-SiO2

The subsolidus region of the PbO-SiO₂ system was studied by DTA and X-ray diffraction. X-ray diffraction analysis showed the presence of five compounds: 4PbO.SiO₂, 3PbO·SiO₂, 2PbO·SiO₂, 3PbO·2SiO₂, and PbO·SiO₂. The compound 4PbO·SiO₂ has previously been reported to have three polymorphic forms; there are two polymorphs of 2PbO·SiO₂ with the inversion at 460°±15°C. The compounds 3PbO·SiO₂ and 3PbO·2SiO₂ were unstable above 430°±10° and 585°±15°C, respectively; PbO·SiO₂ was unstable below 525°±15°C. DTA patterns were determined for glasses of the composition of each of these compounds.     

Stability of Molybdenum Disilicide in Combustion Gak Environments

The stability of MoSi₂ in combustion gas at 1370° and 1600°C was evaluated using SOLGASMIX-PV thermodynamic modeling, periodic weight measurements, and characterization via XRD, SEM, EDS, and image analysis. Passive oxidation occurred at both temperatures. During an initial stage of exposure, specimen surfaces oxidized to form MoO_3(g) and amorphous SiO₂ via reduction of CO₂ and H₂O. After a short time (<6.5 min at 1370°C, <1 min at 1600°C), the oxidation mechanism switched; Mo₅Si₃ and amorphous SiO₂ formed as oxidation products. The first mechanism esulted in the formation of 46.1 vol% at 1370°C and 42.6 vol% at 1600°C of the amorphous silica surface coating. The attainment of a near-terminal weight gain implied silica formation was limited by H₂O and CO₂ diffusion through the silica coating.     

Incorporation of Aluminum into C–S–H Structures: From Synthesis to Nanostructural Characterization

C–A–S–H (C=CaO, A=Al₂O₃, S=SiO₂, H=H₂O in cement nomenclature) phases have been synthesized from CaO, SiO₂, and AlNaO₂. The initial CaO/SiO₂ (C/S_initial) ratios varied from 0.8 to 1.5 and the initial Al₂O₃/SiO₂ (A/S_initial) ratio was set to 0.1. Samples were characterized by X-ray diffraction and chemical analyses of their equilibrium solutions. This paper describes experiments using a low-voltage scanning transmission electron microscopy (STEM-in-SEM) imaging system that allows transmission observations in an environmental scanning electron microscope. Observations of the nanostructure were also performed by transmission electron microscopy (TEM). Two types of morphologies were clearly observed: fine fibrillar aggregates and small plates, the latter being particularly sensitive to beam damage. Despite their different appearance, both of these phases were amorphous, and the small plates were richer in aluminum. The fraction of the small plate phase increased with the C/S_initial ratio. TEM interpretations showed that C–A–S–H phases were not stable under the electron beam and high-magnification observations could significantly modify their structure. Images and chemical analyses acquired with STEM-in-SEM appeared as valuable sources of information because they offered a large observation field comparable to a transmission electron microscope and better magnification resolution than a classical scanning electron microscope.     

Solubility of Si3N4 in Liquid SiO2

The nitrogen solubility in the SiO₂-rich liquid in the metastable binary SiO₂-Si₃N₄ system has been determined by analytical TEM to be 1%–4% of N/(O + N) at 1973–2223 K. Analysis of the near edge structure of the electron energy loss peak indicates that nitrogen is incorporated into the silicate network rather than being present as molecular N₂. A regular solution model with a positive enthalpy of mixing for the liquid was used to match the data for the metastable solubility of N in the presence of crystalline Si₃N₄ and to adjust the computed phase diagram. The solubility of Si₃N₄ in fused SiO₂ is far less than reported in liquid silicates also containing Al, Mg, and/or Y. Apparently, these cations act as modifiers that break anion bridges in the silicate network and, thereby, allow further incorporation of Si₃N₄ without prohibitive amounts of network cross-linking. Finally, indications emerged regarding the diffuse nature of the Si₃N₄-SiO₂ interface that leads to amorphous regions of higher N content.     

Characterization and Sintering Behavior of Alkoxide-Derived Aluminosilicate Powders

Submicrometer SiO₂-Al₂O₃ powders with compositions of 46.5 to 76.6 wt% Al₂O₃ were prepared by hydrolysis of mixed alkoxides. Phase change, mullite composition, and particle size of powders with heating were analyzed by DTA, XRD, IR, BET, and TEM. As-produced amorphous powders partially transformed to mullite and Al-Si spinel at around 980°C. The compositions of mullite produced at 1400° and 1550°C were richer in Al₂O₃ than the compositions of stable mullite solid solutions predicted from the phase diagram of the SiO₂-Al₂O₃ system. Particle size decreased with increasing Al₂O₃ content. The sintered densities depended upon the amount of SiO₂-rich glassy phase formed during sintering and the green density expressed as a function of particle size.     

Liquid–Solid Equilibria in the System NiO–TiO2–SiO2 in the Temperature Range 1430° to 1660°C

Equilibrium relations in the system NiO–TiO₂–SiO₂ in air have been investigated in the temperature range 1430° to 1660°C. The most conspicuous feature of the phase relations is the existence of a cation-excess spinel-type phase, in addition to NiO and NiTiO₃, on the liquidus surface and at subsolidus temperatures down to 1430°C. Three invariant points have been located on the liquidus. There is a peritectic at 1540°C characterized by coexisting NiO ( ss ), spinel( ss ), cristobalite, and liquid of composition 47 wt% NiO, 29 wt% TiO₂, and 24 wt% SiO₂. Two eutectics are present, one at 1480°C, with spinel( ss ), NiTiO₃, cristobalite, and liquid (42 wt% NiO, 43 wt% TiO₂, and 15 wt% SiO₂), as the coexisting phases. The other is at 1490°C with NiTiO₃, rutile, cristobalite, and liquid (32 wt% NiO, 56 wt% TiO₂, and 12 wt% SiO₂). A liquid miscibility gap extends across the diagram from the two bounding binary systems NiO–SiO₂ and TiO₂–SiO₂.     

Reexamination of the Kaolinite-to-Mullite Reaction Series

The kaolinite-to-mullite reaction series was reexamined with special attention to the nature of the metaphase, the eontroversial spinel phase, and the cause of the exothermic peak at 980°C. Amorphous SiO₂ forms during the exothermic reaction; it can be leached by alkali extraction. When the residual cubic phase is heated further, it forms mullite only. This result indicates that the cubic phase is an Al-Si spinel and that metakaolinite is an AI₂O₃-SiO₂ compound. It was established that the exotherm exhibited by kaolinite at 980°C represents the sudden transformation of metakaolinite to Al-Si spinel, the crystallization of mullite, and the liberation of amorphous SiO₂. The AI-Si spinel has the same composition as mullite, containing both AI(IV) and AI(VI). This spinel transforms into mullite at the second exothermic peak; no amorphous SiO₂ is liberated.     

Effects of Amorphous and Crystalline SiO2 Additives on γ-Al2O3-to-alpha-Al2O3 Phase Transitions

This paper focused on the effects of various phases of SiO₂ additives on the γ-Al₂O₃-to-α-Al₂O₃ phase transition. In the differential thermal analysis, the exothermic peak temperature that corresponded to the theta-to-α phase transition was elevated by adding amorphous SiO₂, such as fumed silica and silica gel obtained from the hydrolysis of tetraethyl orthosilicate. In contrast, the peak temperature was reduced by adding crystalline SiO₂, such as quartz and cristobalite. Amorphous SiO₂ was considered to retard the γ-to-α phase transition by preventing γ-Al₂O₃ particles from coming into contact and suppressing heterogeneous nucleation on the γ-Al₂O₃ surface. On the other hand, crystalline SiO₂ accelerated the α-Al₂O₃ transition; thus, this SiO₂ may be considered to act as heterogeneous nucleation sites. The structural difference among the various SiO₂ additives, especially amorphous and crystalline phases, largely influenced the temperature of γ-Al₂O₃-to-α-Al₂O₃ phase transition.