Preparation of Portland Cement Components by Poly(vinyl alcohol) Solution Polymerization

The four components portland cement-dicalcium silicate, C₂S (Ca₂SiO₄); tricalcium silicate, C₃S (Ca₃SiO₅); tricalcium aluminate, C₃A (Ca₃Al₂O₆); and tetracalcium aluminate iron oxide, C₄AF (Ca₄Al₂Fe₃O₁₀)-were formed using a solution-polymerization route based on poly(vinyl alcohol) (PVA) as the polymer carrier. The powders were characterized using X-ray diffraction techniques, BET specific surface area measurements, and scanning electron microscopy. This method produced relatively pure, synthetic cement components of submicrometer or nanometer crystallite dimensions, high specific surface areas, as well as extremely high reactivity at relatively low calcining temperatures. The PVA content and its degree of polymerization had a significant influence on the homogeneity of the final powders. Two types of degree of polymerization (DP) PVA were used. Lower crystallization temperatures and smaller particle size powders were obtained from the low-DP-type PVA at optimum content.     

Study of Crystallization Kinetics in Glasses along the Diopside–Ca-Tschermak Join

We report on the thermal stability and crystallization kinetics of the glasses in the diopside (CaMgSi₂O₆)–Ca-Tschermak (CaAl₂SiO₆) system. Four glasses with compositions corresponding to different diopside/Ca-Tschermak ratio were studied. Structural investigations on the glasses have been made by employing Infrared spectroscopy (FTIR). Activation energies for structural relaxation and viscous flow have been calculated using the data obtained from differential thermal analysis. The existence of glass-in-glass phase separation was observed in all the glasses. Kinetic fragility of the glasses along with other thermal parameters have also been calculated. Nonisothermal crystallization kinetic studies have been employed to study the mechanism of crystallization in all the four glasses. The Avrami parameter for the glass powders is ∼2, indicating the existence of intermediate mechanism of crystallization. Crystallization sequence in the glasses has been followed by X-ray diffraction analysis, scanning electron microscopy, and FTIR.     

Preparation and Characterization of Nanocrystalline Misch-Metal-Substituted Yttrium Iron Garnet Powder by the Sol–Gel Combustion Process

Nanocrystalline Y_{3− x} MM _x Fe₅O₁₂ powders (MM denotes Misch-metal, x =0.0, 0.25, 0.5, 0.75, and 1.0) were synthesized by a sol–gel combustion method. Magnetic properties and crystalline structures were investigated using X-ray diffraction (XRD), a vibrating sample magnetometer (VSM), and a scanning electron microscope. The XRD patterns showed that the single-phase garnet of Y_{3− x} MM _x Fe₅O₁₂ was formed at x values ≤1.0. The saturation magnetization of powders increased with decreasing MM content and reached the maximum value at Y₃Fe₅O₁₂. The crystallite size of powders calcined at 800°C for 3 h was in the range of 38–53 nm.     

Reaction-Formed Porous Yb4Si2N2O7 Materials with Uniform Open-Cell Network Structure

A novel porous Yb₄Si₂O₇N₂ material with uniform open-cell network structure was fabricated from the reaction between Si₃N₄, Yb₂O₃, and SiO₂. The formation of Yb₄Si₂O₇N₂ during heating was studied using X-ray diffractometry. The porous structure was characterized using scanning electron microscopy and mercury porosimeter. It is shown that the formation of Yb₄Si₂O₇N₂ phase starts at ∼1150°C and completes at 1350°C for 4 h, accompanied by the development of open-cell network structure. The necks between Yb₄Si₂O₇N₂ particles become much thicker with increasing temperature because of the coarsening of Yb₄Si₂O₇N₂ particles, thus leading to a uniform three-dimensional network structure. Furthermore, the pore size can be well controlled by adjusting reacting temperature and altering atmosphere.     

Yttrium Silicate Powders Produced by the Sol–Gel Method, Structural and Thermal Characterization

Yttrium silicate (Y₂SiO₅) powders of high purity have been synthesized using the sol–gel method. Alkoxide precursors were used with commercial tetraethyl orthosilicate as the silica source and yttrium propoxide synthesized from YCl₃. Powders calcined from the xerogel showed submicrometer crystal sizes. These powders were sintered at temperatures <1300°C and are suitable for coating applications such as a thermal barrier system for SiC/SiC composites.     

Phase Evolution in Heat-Treated Si3N4 with Additions of Yb2O3

The heat treatment of silicon nitride (Si₃N₄) ceramics with additions of 8, 12, and 16 wt% Yb₂O₃ was carried out at different temperatures and the evolution of grain boundary (GB) phase was investigated systematically by X-ray diffraction (XRD) as well as scanning electron and transmission electron microscopic analyses. XRD results reveal that the extent and the ease of GB crystallization increase with increasing the Yb₂O₃ content, and that high heat-treatment temperatures in general favor crystallization of the quaternary compounds such as the Yb₄Si₂O₇N₂ phase. These results provide an insight into the GB phase evolution in the Yb-system Si₃N₄ ceramics subjected to a postsintering heat treatment.     

Densification, Structure, and Thermophysical Properties of Ytterbium–Gadolinium Zirconate Ceramics

(Yb _x Gd_{1− x} )₂Zr₂O₇ (0≤ x ≤1.0) ceramic powders synthesized with the chemical-coprecipitation and calcination method were pressureless-sintered at 1550–1700°C to develop new thermal barrier oxides with a lower thermal conductivity than yttria-stabilized zirconia ceramics. (Yb _x Gd_{1− x} )₂Zr₂O₇ ceramics exhibit a defective fluorite-type structure. The linear thermal expansion coefficients of (Yb _x Gd_{1− x} )₂Zr₂O₇ ceramics increase with increasing temperature from room temperature to 1400°C. The measured thermal conductivity of (Yb _x Gd_{1− x} )₂Zr₂O₇ ceramics first gradually decrease with increasing temperature and then slightly increase above 800°C because of the increased radiation contribution. YbGdZr₂O₇ ceramics have the lowest thermal conductivity among all the composition combinations studied.     

Synthesis of Yttrium–Aluminum–Garnet Hollow Microspheres by Reverse-Emulsion Technique

Yttrium–aluminum–garnet (YAG, Y₃Al₅O₁₂) hollow microspheres were synthesized by reverse-emulsion (w/o) technique starting with aqua-based precursors of oxides. The non-ionic surfactant was used as the emulsifying agent. The gel powders were calcined at 700°–1200°C. The synthesized powders were characterized by differential thermal analysis (DTA), thermogravimetry, Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and scanning electron microscopy (SEM). The appearance of an exothermic peak at 932°C in the DTA curve revealed the crystallization of YAG, which was further confirmed by XRD and FTIR studies. SEM confirmed the formation of hollow microspheres.     

Sol–Gel Route to Nanocrystalline Lithium Metasilicate Particles

Crystalline lithium metasilicate (Li₂SiO₃) nanoparticles have been synthesized using a sol–gel process with tetraethylorthosilicate and lithium ethoxide as precursors. The particle size examined by using transmission electron microscopy and BET-specific surface area techniques is in the range 5–50 nm, depending on the temperature at which the material is calcined. The crystalline Li₂SiO₃ forms at ambient temperature (∼40°C), and it remains in this phase after calcination at temperatures up to 850°C. The BET-specific surface area is ∼110 m²/g for material calcined at temperatures below 500°C, decreasing to ∼29 and ∼0.7 m²/g following calcination at 700° and 850°C, respectively. Solid-state ²⁹Si NMR spectroscopy shows the presence of only Q ² structural units in the material. The lithium metasilicate is further characterized using differential scanning calorimetry and thermogravimetric analysis, and Fourier transform infrared spectroscopy.     

Rapid Formation of Active Mesoporous TiO2 Photocatalysts via Micelle in a Microwave Hydrothermal Process

Rapid formation of active, mesoporous, and crystalline TiO₂ photocatalysts via a novel microwave hydrothermal process is presented. Crystalline anatase mesoporous nanopowders 100–300 nm in size with worm hole-like pore sizes of 3–5 nm were prepared by a modified sol–gel of titanium tetra-isopropoxide, accelerated by a microwave hydrothermal process. The organic surfactant, tetradecylamine, which is used as a self-assembly micelle in the sol–gel and microwave hydrothermal process, enables to harvest crystallized mesoporous anatase nanoparticles with a high-surface area. Mesoporous worm hole-like and crystalline powders with surface areas of 243–622 m²/g are obtained. X-ray diffraction, N₂-adsorption isotherms (Barrett–Joyner–Halenda and Brunauer–Emmet–Teller method), scanning electron microscope, and transmission electron microscope are used to identify the characteristics and morphologies of the powders. It is shown that crystallization by calcination at 400°C/3 h inevitably reduced the surface area, while the microwave hydrothermal process demonstrated a rapid formation of crystalline mesoporous TiO₂ nanopowders with a high-surface area and excellent photocatalytic effects.     

Molten-Salt Synthesis and Characterization of Nickel-Doped Forsterite Nanocrystals

Nickel-doped forsterite (Ni²⁺:Mg₂SiO₄) nanocrystals have been synthesized by a facile molten-salt approach in the presence of NaCl and a surfactant (NP-7.5). The products were characterized by X-ray diffraction, transmission electron microscopy (TEM), high-resolution TEM, selected area electron diffraction (SAED), and luminescence spectra measurements. The crystal size could be controlled by tailoring the synthesis parameters. TEM, high-resolution TEM, and SAED results revealed the single crystalline character of Mg₂SiO₄ nanoparticles. A possible model for the growth of Ni²⁺:Mg₂SiO₄ nanocrystals was postulated. The obtained Ni²⁺:Mg₂SiO₄ nanocrystals show strong, super broad, near-infrared luminescence at room temperature. These doped Mg₂SiO₄ nanocrystals are promising gain mediums for super broadband optical amplification.     

Hydration in the System Ca2SiO4–Ca3(PO4)2 at 90°C

Compositions along the Ca₂SiO₄–Ca₃(PO₄)₂ join were hydrated at 90°C. Mixtures containing 15, 38, 50, 80, and 100 mol% Ca₃(PO₄)₂ were fired at 1500°C, forming nagelschmidtite + a ¹-CaSiO₄, A -phase and silicocarnotite and a -Ca₃(PO₄)₂, respectively. Hydration of these produces hydroxylapatite regardless of composition. Calcium silicate hydrate gel is produced when Ca₂SiO₄≠ 0 and portlandite when Ca₂SiO₄ is >50%. Relative hydration reactivities are a -Ca₃(PO₄)₂ > nagelschmidtite > α ¹-Ca₂SiO₄ > A -phase > silicocarnotite. Hydration in the presence of silica or lime influences the amount of portlandite produced. Hydration in NaOH solution produces 14-A tobermorite rather than calcium silicate hydrate gel.     

Composition Microheterogeneity of Uvarovite–Grossularite Garnets

A solid-solution series with up to 50 mol% uvarovite (Ca₃Cr₂Si₃O₁₂) and grossularite (Ca₃Al₂Si₃O₁₂) was synthesized by the sol–gel method at 1543 K. Colloidal silica was used as the starting reagent, together with chloride salts of the remaining components. X-ray diffraction analysis of the evolution of the systems showed the formation of uvarovite in a nominal composition of 100 mol% uvarovite; a garnet-phase solid solution; and gehlenite (Ca₂Al₂SiO₇), pseudowollastonite (α-CaSiO³), and α-cristobalite in a nominal composition of 100 mol% grossularite. In some samples, small amounts of impurity phases appeared along with the major phase. The general microstructure was observed by scanning electron and transmission electron microscopy, and an agglomeration of crystals with 2-μm average size was revealed. The crystals were similar and rounded in shape for all compositions. Additional observations carried out by transmission electron microscopy (TEM) (using the powder replica method) showed that the end series consists of cubic crystals displaying some deformation. Quantitative and semiquantitative scanning electron microscopy/energy-dispersive X-ray (SEM/EDX) and transmission electron microscopy/scanning electron microscopy/energy-dispersive X-ray (TEM/STEM/EDX) microanalysis (by the Cliff-Lorimer method, working at 200 kV) were also conducted in all compositional ranges of the solid solutions synthesized in the present study.     

FTIR Spectra, Lattice Shrinkage, and Magnetic Properties of CoTi-Substituted M-Type Barium Hexaferrite Nanoparticles

Single-phase BaCoTiFe₁₀O₁₉ (BaCoTi-M) nanoparticles were prepared by a modified sol–gel process, using metallic chlorides as starting materials. The physical chemistry process of BaCoTi-M formation, the interdependences between composition, technological conditions, microstructure, and magnetic properties were studied by X-ray diffraction (XRD), Fourier transform-infrared (FTIR), scanning electron microscope (SEM), transmission electron microscope (TEM), and vibrating sample magnetometer (VSM). XRD and FTIR results show that BaCoTi-M nanoparticles formed directly from γ-Fe₂O₃, spinel ferrite, and barium salts without the formation of α-Fe₂O₃ and BaFe₂O₄. The lattice shrinkage of BaCoTi-M nanoparticles that occurred on increasing the calcining temperature from 973 to 1173 K under holding for 2 h or on increasing the holding time in the range 0–2 h at 1173 K was discovered by analyzing the dependences of lattice parameters on the heat-treatment conditions. The shrinkage led to a relatively higher concentration of magnetic Fe³⁺ cations in the unit cell, and resulted in an increase of specific saturation magnetization under the corresponding conditions. Microstructural characterization shows that the evolutions of coercivity, remnant magnetization, and squareness ratio depended on the crystal growth and the reduction of structural defect as well as a decrease of grain boundary.     

Low-Temperature Synthesis of 0.65 PbMg1/3Nb2/3O3–0.35PbTiO3 Ceramics

Nanocrystalline 0.65 PbMg_1/3Nb_2/3O₃–0.35PbTiO₃ powders were synthesized by citrate gel method. The gel was prepared using citrate (titanium and niobium) and nitrate (lead and magnesium) salts. The hard gel obtained after completion of the reaction was treated to get the desired phase. Thermal analysis of the gel was done to optimize the calcination temperature. The calcined powders were characterized by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The crystallite size and the effective strain were found to be 50 nm and 0.03584 N, respectively.     

Phase Equilibria in the System CaO-Yb2O3

Phase equilibrium relations in the system CaO-Yb₂O₃ were studied. Results of this work demonstrated the existence of four crystalline phases: Yb₂O₃.3CaO, Yb₂O₃.2CaO, Yb₂O₃°CaO, and 2Yb₂O₃°CaO. The 2Yb₂O₃°CaO phase is metastable at all temperatures and was obtained only by rapid quenching from the melt. The crystalline solubility limit of Yb_aO₃ in CaO at 1850°C is slightly greater than 8 mole %, whereas no solubility of CaO in Yb₂O₃ was detected. All four compounds have subsolidus minimums of stability and dissociate into the component oxides below 1800°C. Data are also presented for the systems CaO-Gd₂O₃ and CaO-La₂O₃.     

MgSiN2 Addition as a Means of Increasing the Thermal Conductivity of β-Silicon Nitride

Si₃N₄ powders with the concurrent addition of Yb₂O₃ and MgSiN₂ were sintered at 1900°C for 2–48 h under 0.9 MPa nitrogen pressure. Microstructure, lattice oxygen content, and thermal conductivity of the sintered specimens were evaluated and compared with Si₃N₄, Yb₂O₃, and MgO addition. MgSiN₂ addition was effective for improving the thermal conductivity of Si₃N₄ ceramics, and a material with high thermal conductivity over 140 W·(m·K)⁻¹ could be obtained. For both specimens, lattice oxygen content was decreased with sintering time. However, the thermal conductivity of the MgSiN₂-doped specimen was slightly higher than the MgO-doped specimen with the same oxygen content.     

Synthesis of Li4SiO4 by a Modified Combustion Method

We report for the first time the synthesis of Li₄SiO₄ by the modified combustion method, a rapid chemical process that takes 5 min for completion. This method uses nonoxidizer compounds instead of nitrate mixtures, which are not always commercially available.
The effects of the following parameters on the production of Li₄SiO₄ were studied: (1) different lithium hydroxide:silicic acid:urea (LiOH:H₂SiO₃:CH₄N₂O) molar ratios; (2) the presence of air flow in the furnace chamber; and (3) the furnace heating temperature. It was found that LiOH:H₂SiO₃:CH₄N₂O molar ratios 6:1:3 heated at 1100°C in the presence of additional air in the muffle chamber formed the best precursors to produce Li₄SiO₄.     

Mechanism of LaFeO3 Perovskite-Type Oxide Formation from the Thermal Decomposition of d-f Heteronuclear Complex La[Fe(CN)6]-5H2O

Nanosized, single-phase perovskite-type LaFeO₃ powders are synthesized by the thermal decomposition at 600°C of d-f heteronuclear complex La[Fe(CN)₆]-5H₂O. The mechanism of the formation of LaFeO₃ has been studied by simultaneous thermogravimetric and differential thermal analysis (TG/DTA), Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), and scanning electron microscopy (SEM). The formation of LaFeO₃ from the complex involves the formation of a carbonated, orthorhombic transition phase, which facilitates the formation of the LaFeO₃ orthorhombic structure at low temperatures with a peculiar morphology.     

Phases in the System Ba2SiO4-Ca2SiO4

The system Ba₂SiO₄-Ca₂SiO₄ was studied by heating mixtures of Ba₂SiO₄ and Ca₂SiO₄ at 1723 K. Six distinct phases resulted; they were examined by both X-ray diffraction and differential thermal analysis. The phases β -(Ba_0.05Ca_1.95)SiO₄ and α-(Ba_0.15Ca_1.85)SiO₄ are isostructural with corresponding high-temperature modifications of Ca₂SiO₄. The X phase (Ba_0.48Ca_1.52SiO₄) is orthorhombic, is a pure phase rather than a solid solution, and is defined for the first time in the present work. The T phase (Ba_1.31Ca_0.69SiO₄) is hexagonal and interpreted in terms of a unit cell with a doubled c parameter, in contrast with literature data.