Tricalcium Silicate T1 and T2 Polymorphic Investigations: Rietveld Refinement at Various Temperatures Using Synchrotron Powder Diffraction

The lattice parameters, cell volume, and structure of a sample of phase pure triclinic tricalcium silicate were determined using in situ, high-temperature synchrotron powder diffraction and full-profile Rietveld refinement. The temperature range covered was from ambient to 740°C. Evidence of superstructure was found. The T₂ type structure with disordered SiO₄ tetrahedra was observed, and an average structure for the subcell ( P     , a = 11.7416(2) Å, b = 14.2785(2) Å, c = 13.7732(2) Å, α= 105.129(1)°, β= 94.415(1)°, and γ= 89.889(1)°) is presented. Differential thermal analysis and X-ray fluorescence was also performed.     

Tricalcium Silicate and Its Stability Within the System CaO-SiO2

A portion of the system CaO-SiO₂ has been reinvestigated by high-temperature microscopy. Direct observations at the liquidus establish a primary-phase region for tricalcium silicate, which is shown in a revised phase diagram to melt incongruently. The tricalcium silicate primary-phase field extends from the eutectic with dicalcium silicate at 2050 °C., 69.5% CaO, to the incongruent boundary with lime at 2070°C., 71.5% CaO, with a possible error in composition of ± 0.25%. The estimated accuracy of these temperatures is ±3° relative to one another, and ± 10° relative to the melting points of alumina and dicalcium silicate, for which the values 2040°C. and 2130°C. taken from the literature have been assumed.     

Effect of P2O5 on the Crystallization of Lead Silicate Glasses

The effect of P₂O_S on the devitrification of binary lead silicate glasses containing 64 and 59 mol% PbO was studied. Glasses underwent isothermal crystallization treatments at 400°, 450°, 500°, and 550°C. A polymorph of 3PbO₂SiO₂ was the major product of crystallization for all compositions of glasses. Secondary products of crystallization were found to be a polymorph of PbOSiO₂ in the 59 mol% and a low-temperature polymorph of 2PbOSiO₂ in the 64 mol% PbO glasses. Dominant mode of crystallization in both binary glasses was surface devitrification at all temperatures studied. Addition of P₂O₅ promoted internal crystallization in the form of spherulites. 400°C was found to be the most effective temperature for nucleating spherulitic growth. Crystallization at 400°C led to high concentrations of spherulites in all glasses containing at least 0.5 mol% P₂O₅. Concentrations of 0.5 mol% P₂O₅ were needed to produce detectable levels of spherulitic nucleation at r<400°C.     

Hydration of Tricalcium Silicate Followed by 29Si NMR with Cross-Polarization

The use of cross-polarization (CP) NMR in conjunction with magic angle sample spinning (MASS) to examine the hydration reaction of tricalcium silicate (C₃S) is described. In particular the very early stages of the reaction both with and without admixtures has been studied as well as the hydration in a ball mill. The combination of CP and non-CP ²⁹Si NMR permits the distinction between silicate units associated with protons, i.e., in hydrated material, and those in anhydrous material. It has been found that in paste hydration there is steady formation of a small amount of hydrated monomeric silicate units during the induction period. In ball mill hydration the formation of the crystalline calcium silicate hydrate, afwillite, which contains only hydrated monomeric silicate species, can be monitored. These results are interpreted in terms of possible mechanisms for C₃S hydration.     

Synthesis and Structural Characterization of Trimetallic Perovskite-Type Rare-Earth Orthoferrites, LaxSm1–xFeO3

Ultrafine powders of trimetallic orthoferrites containing lanthanum and samarium in various ratios were synthesized by thermal decomposition at low temperatures of the corresponding hexacyano complexes to modulate the functional properties of these perovskite-type oxides. The precursors and their decomposition products were analyzed by simultaneous thermogravimetric and differential thermal analyses, X-ray diffractometry, and Raman spectroscopy. Single-phase trimetallic precursors and oxides were obtained. The crystal structure of the perovskite-type oxides was orthorhombic, and the lattice parameters were affected by the ionic size of the rare-earth elements present in the oxides. Raman spectra showed a broadening of the vibrational bands with increased lanthanum content. This was ascribed to some disorder in the oxygen sublattice, related to distortions of the cation–oxygen coordination, and to a reduction of the orthorhombic distortion in the unit-cell basal plane. Most of the Raman modes above 200 cm⁻¹, associated with the vibration of oxygen ions, showed a frequency increase with effective cation mass, defined as m _eff= xm _La+ (1 – x ) m _Sm, i.e., with samarium content. This was explained by assuming that the force constants increased with decreased Ln–O and Fe–O interatomic distances observed for high samarium content.     

Synthesis and Characterization of Nanocrystalline SrTiO3

The synthesis and densification of fairly dense nanocrystalline SrTiO₃ and its characterization are described in this paper in detail. Significant grain growth was avoided by the application of a two-stage sintering process using hot pressing. High-resolution transmission electron microscopy and electron energy loss spectroscopy characterizations indicate pure material with no detectable chemical inhomogeneities. The electrical measurements indicate the disappearance of bulk contribution to the electrical conduction due to the overlap of depleted space charge layers if the grain size is below 100 nm. Owing to the overlap, the capacitance appears as bulk-like rather than due to space charge polarization. Non-contact sub-millimeter optical spectroscopy measurements reveal strong suppression of the dielectric constant values at low temperature.     

Synthesis and Characterization of a Remarkable Ceramic: Ti3SiC2

Polycrystalline bulk samples of Ti₃SiC₂ were fabricated by reactively hot-pressing Ti, graphite, and SiC powders at 40 MPa and 1600°C for 4 h. This compound has remarkable properties. Its compressive strength, measured at room temperature, was 600 MPa, and dropped to 260 MPa at 1300°C in air. Although the room-temperature failure was brittle, the high-temperature load-displacement curve shows significant plastic behavior. The oxidation is parabolic and at 1000° and 1400°C the parabolic rate constants were, respectively, 2 × 10⁻⁸ and 2 × 10⁻⁵ kg²-m⁻⁴.s⁻¹. The activation energy for oxidation is thus =300 kJ/mol. The room-temperature electrical conductivity is 4.5 × 10⁶Ω⁻¹.m⁻¹, roughly twice that of pure Ti. The thermal expansion coefficient in the temperature range 25° to 1000°C, the room-temperature thermal conductivity, and the heat capacity are respectively, 10 × 10⁻⁶°C⁻¹, 43 W/(m.K), and 588 J/(kgK). With a hardness of 4 GPa and a Young's modulus of 320 GPa, it is relatively soft, but reasonably stiff. Furthermore, Ti₃SiC₂ does not appear to be susceptible to thermal shock; quenching from 1400°C into water does not affect the postquench bend strength. As significantly, this compound is as readily machinable as graphite. Scanning electron microscopy of polished and fractured surfaces leaves little doubt as to its layered nature.     

Formation and Structural Characterization of 1:1 Ordered Perovskites in the Ba(Zn1/3Ta2/3)O3–BaZrO3 System

The phase stabilities in the(1−x)Ba(Zn_1/3Ta_2/3)O₃ (BZT)-xBaZrO₃(BZ)system have been investigated using samples prepared by the mixed oxide method. The substitution of Zr⁴⁺destabilizes the 1:2 cation ordering in BZT and pro-motes the formation of a cubic, 1:1 ordered structure with a doubled perovskite repeat. The homogeneity range of the 1:1 phase extends from x = 0.04 to approximately x = 0.25; substitutions beyond this range stabilize a disordered perovskite. The limits of stability of the 1:1 ordering coin-cide with compositions previously found to exhibit anoma-lies in their dielectric loss. The range of homogeneity is consistent with a "random layer" model for the 1:1 ordered "Ba{β';_1/2β^1/2}O₃" structure. In this model the B× positions are assumed to be occupied exclusively by Ta⁵⁺, and the b× sites by a random distribution of Zn₂₊, Zr₄₊, and the remaining Ta 5+ cations. The validity of the model, where the ordered solid solutions can be represented by Ba{[Zn_{2− y /3}Ta_{(1−2 y )/3}Zr y ]_1/2[Ta]_1/2}O₃(y =2x)was con-firmed by Rietveld refinements conducted using data col-lected with a synchrotron X-ray source.     

Structural and Thermophysical Properties of (PbO)0.5(SiO2)0.5 Glasses: Effect of SnO2 Incorporation

¹¹⁹Sn and ²⁹Si solid-state nuclear magnetic resonance studies on lead silicate glasses containing different amounts of SnO₂ confirmed that tin exists in the glass as distorted SnO₆ polyhedra and there is no direct interaction between tin and silicon structural units. Transmission electron microscopic studies have established that tin structural units are uniformly distributed in the glass. Significant changes in the values of glass transition temperature, microhardness, and thermal expansion coefficient with SnO₂ incorporation into the glass have been attributed to the increased rigidity of the glass network brought about by the replacement of weaker Pb–O linkages with stronger Sn–O linkages.     

Synthesis and Characterization of Bulk Zr2Al3C4 Ceramic

Polycrystalline Zr₂Al₃C₄ was fabricated by an in situ reactive hot-pressing process using zirconium (zirconium hydrides), aluminum, and graphite as starting materials. The investigation on reaction path revealed that the liquid Al played an important role in triggering the formation of ternary zirconium aluminum carbides. The mechanical properties of Zr₂Al₃C₄ at room temperature were measured (Vickers hardness of 10.1 GPa, Young's modulus of 362 GPa, flexural strength of 405 MPa, and fracture toughness of 4.2 MPa·m^1/2). The electrical resistivity and thermal expansion coefficient were determined as 1.10 μΩ·m and 8.1 × 10⁻⁶ K⁻¹, respectively.     

Effect of Bi2O3 on Impurity Ion Distribution and Electrical Resistivity of Li-Zn Ferrites

Preventing the incorporation of impurities in Li-Zn ferrite grains during sintering is essential for production of ceramics with reproducible magnetic and electrical properties. Li-Zn ferrites of composition Li_0.3Zn_0.4Mn_0.05Fe_2.25O₄ were prepared with Bi₂O₃ and borosilicate sintering additives. The distribution of impurity ions in the sintered ferrites was investigated using transmission electron microscopy (TEM) coupled with energy dispersive spectroscopy (EDS). Ceramics prepared with Bi₂O₃ contained Si, Ca, and S impurities, located at grain boundaries and triple point regions. The low viscosity and good wetting properties of the Bi₂O₃ and to a lesser extent the borosilicate liquid phase allowed impurities to be selectively removed from the growing ferrite phase during sintering, thus improving sample resistivities.     

Synthesis and Characterization of a New Sillenite Compound—Bi12(B0.5P0.5)O20

A new sillenite compound, Bi₁₂(B_0.5P_0.5)O₂₀, was synthesized using a solid-state reaction method. The stoichiometry was confirmed by XRD analyses, microstructural investigations, and quantitative elemental analysis. An investigation of the dielectric properties at frequencies from 100 Hz to 1 MHz revealed a broad, highly frequency-dispersive, relaxor-like dielectric anomaly, which appeared in the temperature range of −80°–100°C. The permittivity, Q × f value, and temperature coefficient of the resonant frequency, measured at ∼5.5 GHz, were determined to be 37.4, 850 GHz, and −19 ppm/K, respectively.     

Chemical Durability of Sodium Silicate Glasses Containing AI2O3 and ZrO2

A sodium silicate glass and ternary glasses derived from it by substituting AI₂O₃ and ZrO₂ for SiO₂ were exposed to water and aqueous solutions of pH 1.4 to 12.7; the kinetics of the reactions were studied. Diffusion of alkali ions and leaching of alkali and SiO₂ from the glasses were influenced by the occupancy of surface sites by alkali ions above a critical pH; however, the activation energies of the processes varied linearly with the logarithm of mole fraction of surface sites occupied by H^plus;. Identical slopes were obtained for all glasses for a given process. The results are explained on the basis that transport of alkali ions is retarded as a result of increased boundary concentration and that suitable sites for reaction are lacking.     

Synthesis and Characterization of Sr2Ce2Ti5O16 in the System SrO-CeO2-TiO2

The ternary system SrO-CeO₂-TiO₂ was investigated using X-ray diffractometry. The formation of a new compound, Sr₂Ce₂Ti₅O₁₆, was established, and its compatibilities with SrO, SrCeO₃, and SrTiO₃ were studied. The results revealed the existence of a series of compounds Sr_6–12xCe_6xTi₅O₁₆ and solid solutions Sr_2+nCe₂Ti_5+nO_16+3n ( n ≤ 6).     

Hydrothermal Synthesis and Characterization of CdWO4 Nanorods

Cadmium tungstate (CdWO₄) nanorods were successfully synthesized by a hydrothermal process at as low a temperature as 70°C. The products were characterized by X-ray powder diffraction, transmission electron microscopy, and photoluminescent spectra techniques. The results showed that the morphology of nanocrystallites significantly varied with the reaction temperature, and CdWO₄ nanorods exhibited a better luminescent property than nanofibers.     

Novel Synthesis of SrBi2Nb2O9 Powders From Hydroxide Precursors

Simple hydroxide precursors were used for the first time for the synthesis of a typical Aurivillius compound (SrBi₂Nb₂O₉ (SBN)) at a low temperature. This method is very advantageous because it circumvents the use of SrCO₃ in the case of conventional ceramics as well in the coprecipitation methods, thereby lowering the formation of the product phase. Commercially purchased strontium hydroxide is mixed thoroughly with freshly precipitated bismuth and niobium hydroxides in a stoichiometric ratio and heated at different temperatures ranging from 100°C to 750°C for 12 h. The sequence of the reaction and evolution of the product phase was monitored by X-ray diffraction (XRD) studies by recording the XRD for samples calcined at different temperatures. The incipient SBN phase begins to form at temperatures as low as 400°C, and phase formation was complete only at 650°C as revealed by the XRD observations. The differential thermal/thermogravimetric analyses) also corroborate this result. The morphology and average particle size of these powders were investigated by transmission electron microscopy studies.     

Hydrothermal Synthesis and Characterization of KxNa(1−x)NbO3 Powders

NaNbO₃, KNbO₃, and K _x Na_{(1− x )}NbO₃ powders were successfully prepared by the hydrothermal method. The phase of the products was identified to be orthorhombic structure by X-ray diffraction (XRD) technique, and the XRD results revealed that the x value of the K _x Na_{(1− x )}NbO₃ gradually increased with the increase in the ratio of K⁺ to Na⁺ in alkaline solution. The morphology and the microstructure were investigated by scanning electron microscopy, energy-dispersive spectroscopy, and transmission electron microscopy, and the results indicated that the ratio of K⁺ to Na⁺ in the solution had a great effect on the morphology and the size of products. Na_0.5K_0.5NbO₃ with morphotropic phase boundary composition could be synthesized when the molar ratio of K⁺ to Na⁺ was between 4:1 and 6:1 in the solution. A possible formation mechanism of the K _x Na_{(1− x )}NbO₃ crystal was also proposed based on the experimental results.     

Single-Source Sol-Gel Synthesis of Nanocrystalline ZnAl2O4: Structural and Optical Properties

Nanometer-sized zinc aluminate (ZnAl₂O₄) particles were synthesized from heterometal alkoxides, [ZnAl₂(OR)₈], possessing an ideal cation stoichiometry for the ZnAl₂O₄ spinel. ZnAl₂O₄ is formed at 400°C, which is the lowest temperature reported for the formation of monophasic ZnAl₂O₄. ²⁷Al magic-angle spinning nuclear magnetic resonance spectroscopy revealed that ZnAl₂O₄ possesses an inverse structure at <900°C, while the normal spinel phase is observed at higher temperatures. The homogeneity of the in-depth composition and Zn:Al stoichiometry (1:2) was confirmed by electron spectroscopy for chemical analysis. Evaluation of the valence-band spectra of ZnAl₂O₄ and ZnS suggested that the hybridization of O 2 p and Zn 3 d orbitals is responsible for lowering the bandgap in the latter. The average crystallite size showed an exponential relationship to the calcination temperature (X-ray diffractometry and transmission electron microscopy data). The optical spectra of different spinel powders (average particle sizes, 20–250 nm) showed that the absorption edge exhibits a blue shift as particle size decreases.     

Effects of Calcium Substitution on the Structural and Microwave Dielectric Characteristics of [(Pb1−xCax)1/2La1/2](Mg1/2Nb1/2)O3 Ceramics

The effects of calcium substitution on the structural and microwave dielectric characteristics of [(Pb_{1− x} Ca _x )_1/2La_1/2](Mg_1/2Nb_1/2)O₃ ceramics (with x = 0.01–0.5) were investigated. All the materials were observed to have an ordered A(B_1/2^'B_1/2^")O₃-type perovskite structure; however, the space group of the structure changed from Fm 3 m to Pa 3 as the calcium content increased to x = 0.1, and then from Pa 3 to R 3¯ at the x = 0.5 composition. During the structural evolution, the lattice parameter of the perovskite cell decreased linearly, and the dielectric constant ( k ) also decreased, from k = 80 to k = 38. However, the product of the quality factor and the resonant frequency ( Q × f ) increased from 50 000 GHz to 90 000 GHz as the calcium content increased. Also, the temperature coefficient of resonant frequency (τ_ƒ) gradually changed from 120 ppm/°C to −40 ppm/°C as the calcium content increased. At the x = 0.3 composition, a combination of properties— k ∼ 50, Q × f ∼ 86 000 GHz, and τ_ƒ∼ 0 ppm/°C—can be obtained.