Substituted Tobermorites: 27Al and 29Si MASNMR, Cation Exchange, and Water Sorption Studies

²⁷Al and ²⁹Si magic angle spinning nuclear magnetic resonance spectroscopy revealed that the maximum amount of Al that can be substituted for Si in the tobermorite structure is 15 to 20 mol%. Powder X-ray diffraction and cation exchange studies corroborate the above finding. Anomalous tobermorite structures resulted in all cases, and hydrogarnet appeared beyond the 15 to 20 mol% Al substitution limit for Si. The sorption of water molecules by synthetic [Al³⁺+ Na⁺]-substituted tobermorites and their cation-exchanged forms heated at 200°C under vacuum (≥10⁻³ torr) was measured at 25°C in order to probe the nature of the ion-exchange cavity. Samples with ≤30 mol% of Al substitution for Si showed isotherms of Brunauer, Demming, Demming, Teller (BDDT) type I and gave better linearity with the Langmuir plot than with the BET plot. Samples with ≥35 mol% of Al substitution for Si showed BDDT type II isotherms and gave better linearity with the BET than with the Langmuir equation. The Langmuir monolayer capacity was found to depend on the Al content and on the cationic form. The Li⁺- and Na⁺-exchanged tobermorites with about 20 mol% of Al substitution for Si showed the highest Langmuir monolayer water sorption capacity. The Cs⁺-exchanged tobermorites showed a smaller capacity than the Li⁺- or Na⁺-exchanged samples, which can be ascribed to clogging of the ion-exchange cavity by the large Cs⁺ ions.     

Influence of Several Metal lons on the Gelation Activation Energy of Silicon Tetraethoxide

The effects of nine metal cations (Li⁺, Na⁺, Mg²⁺, Ca²⁺, Sr²⁺, Cu²⁺, Al³⁺, La³⁺, and Y³⁺) on silica gel formation have been investigated by studying the hydrolysis and polycondensation of silicon tetraethoxide (TEOS) in the presence of metal nitrates. The influence of the water: TEOS mole ratio, metal ion concentration, and the reaction temperature has been investigated. The overall activation energy for gel formation has been determined from the temperature dependence of the time of gelation for each system. The activation energy for network formation is 54.5 kJ/mol. The gel formation time as well as the activation energy sharply increases in the presence of Cu²⁺, Al³⁺, La³⁺, and Y³⁺. In contrast, the presence of Li⁺, Na⁺, Mg²⁺, Ca²⁺, or Sr²⁺ lowers the gelation time but has no appreciable effect on the activation energy. This difference may be attributed to the participation or nonparticipation of the metal ions in the formation of the three-dimensional polymeric network during polycondensation. The concentration of metal ion (Mg²⁺, Ca²⁺, Y³⁺) or the water: TEOS mole ratio had no appreciable effect on the gelation activation energy. A simple test has been proposed to determine whether a metal ion would act as a network intermediate or modifier in silica and other glassy networks.     

Effect of Divalent Cation Additives on the γ-Al2O3-to-α-Al2O3 Phase Transition

The effect on the γ-Al₂O₃-to-α-Al₂O₃ phase transition of adding divalent cations was investigated by differential thermal analysis, X-ray diffractometry, and surface-area measurements. The cations, Cu²⁺, Mn²⁺, Co²⁺, Ni²⁺, Mg²⁺, Ca²⁺, Sr²⁺, and Ba²⁺, were added by impregnation, using the appropriate nitrate solution. These additives were classified into three groups, according to their effect: (1) those with an accelerating effect (Cu²⁺ and Mn²⁺), (2) those with little or no effect (Co²⁺, Ni²⁺, and Mg²⁺), and (3) those with a retarding effect (Ca²⁺, Sr²⁺, and Ba²⁺). The crystalline phase formed by reaction of the additive with γ-Al₂O₃ at high temperature was a spinel-type structure in groups (1) and (2) and a magnetoplumbite-type structure in group (3). In groups (2) and (3), a clear relationship was found between the transition temperature and the difference in ionic radius of Al³⁺ and the additive (Δ r ): The transition temperature increased as Δ r increased. This result indicates that additives with larger ionic radii are more effective in suppressing the diffusion of Al³⁺ and O²⁻ in γ-Al₂O₃, suppressing the grain growth of γ-Al₂O₃, and retarding the transformation into α-Al₂O₃.     

Effect of Substituent Ions on Martensitic Transformation Temperatures in Dicalcium Silicate Solid Solutions

Five types of Ca₂SiO₄ solid solutions, doped with either P⁵⁺, Ge⁴⁺, Fe³⁺, Mg²⁺, or Ba²⁺, were prepared and examined by high-temperature X-ray diffractometry up to 800°C. The starting and finishing temperatures were determined for the α'_L-to-β martensitic transformation during cooling and the reverse (β-to-α'_L) transformation during heating. These four types of transformation temperatures for the preparations doped with either P⁵⁺ or Ge⁴⁺ steadily decreased with increasing substituted fraction. The effect of the substitution on the decrease for each transformation temperature was quantitatively evaluated by Δ T / x , where Δ T is the difference in the transformation temperatures between the solid solutions and pure Ca₂SiO₄, and x represents the fraction substituted for Si⁴⁺ or Ca²⁺ in the α'_L-phase structure. The evaluated value for the substitution of P⁵⁺ was more than 3 times that of Ge⁴⁺. The effect of the substituent ions mentioned above, together with Na⁺ and Sr²⁺, on the lowering of the starting temperature of the α'_L-to-β transformation was principally determined by the differences in the ionic radius between the interchanging cations.     

Effect of Ba2+ Substitution on Dielectric and Electric-Field-Induced Strain Properties of PMN–PZ–PT Ceramics

Effect of Ba²⁺ substitution for Pb²⁺ on the dielectric and electric-field-induced strain characteristics of the PMN–PZ–PT ceramics has been investigated in the compositions of the tetragonal-rich 0.2PMN–0.36PZ–0.44PT and rhombohedral-rich 0.2PMN–0.4PZ–0.4PT ceramics. The phase approached cubic structure from the tetragonal and rhombohedral, and grain size was reduced when the Ba²⁺ cation was substituted. As Ba²⁺ content increased, frequency-dependent relaxor-like behavior of the dielectric constant was observed at temperatures below the dielectric maximum ( T _max) for compositions with 20 and 25 mol% Ba²⁺. Electric-field-induced strain was maximized in the 12 mol% Ba²⁺-substituted 0.2PMN–0.4PZ–0.4PT specimen ( S _max= 0.15%), and maximum piezoelectric, d ₃₁, was 300 in the 14 mol% Ba²⁺-substituted 0.2PMN–0.4PZ–0.4PT specimen.     

Immiscibility and Surface Tension of Some Simple Borates

This paper reports the minimum concentrations of second cations that must be added to phase-separated binary borates to produce single-phase glasses when batches of approximately 100 gm. are melted at 1300°C. and poured into an 8- by 10.5-cm. steel mold at room temperature. Surface tensions are also reported for the melts at 900°, 1000°, 1100°, 1200°, and 1300°C. Alkali and alkaline-earth cations and Pb⁺⁺, Cd⁺⁺, and Al⁺⁺⁺ were used as auxiliary cations to make single phased the phase-separated binary borates of the principal cations Ba⁺⁺, Sr⁺⁺, Ca⁺⁺, Cd⁺⁺, Zn⁺⁺, and Mg⁺⁺. Within any group of the periodic table the minimum homogenizing concentration increases with decreasing ionic radius of either the auxiliary or the principal cation. The surface tensions of layered binary melts are independent of cation identity and content, indicating an upper layer of nearly pure B₂O₃. For layered or unlayered ternary melts containing alkali auxiliary cations, surface tension increases with increasing cation content at rates that decrease with the ionic radius of either cation, indicating that in these layered ternary melts the cations enter the upper (B₂O₃-rich) layer more freely than in the corresponding binary melts.     

Aluminum-Substituted Gyrolite as Cation Exchanger

A successful Al-substituted gyrolite has been hydrothermally synthesized at 220°C under saturated steam pressure, and its cation exchange characteristics for K⁺ and Cs⁺ ions have been investigated. Al-substituted gyrolite took up K⁺ and Cs⁺ ions of 14 and 20 mequiv/100 g, respectively, and these removal amounts are smaller than those by Al-substituted tobermorites (ca. 80 mequiv/100 g). It was found that Al-substituted gyrolite exhibited selectivity for univalent cations in the following order: Na⁺ < K⁺ < Cs⁺ in the low-concentration regions of K⁺ and Cs⁺ ions. Al-substituted gyrolite did not lose its crystallinity and morphology, and its basal spacing did not change in the cation exchange. These results, therefore, suggest that Al-substituted gyrolite may be useful for separation and waste disposal.     

Defect Chemistry of BaTiO3 with Additions of CaTiO3

The substitution of up to 5% Ca²⁺ for Ba²⁺ in BaTiO₃ results in a shift in the oxygen pressure dependence of the equilibrium electrical conductivity that is in the same direction as that caused by addition of acceptor impurities such as Al³⁺ or Ca²⁺ substituted for Ti⁴⁺. In contrast to the latter effect, however, the shape of the conductivity plot is not changed, the conductivity value at the conductivity minimum is not affected, and the amount of the shift increases with decreasing temperature of measurement. It is shown that the shift is primarily due to an increase in the enthalpy of reduction and a decrease in the enthalpy of oxidation as increasing amounts of Ba²⁺ are replaced by Ca²⁺.     

Segregation of Isovalent Impurity Cations at the Surfaces of MgO and CaO

Enthalpies of segregation for isovalent impurities in magnesium and calcium oxide as a function of surface concentration were calculated by using an atomistic computer simulation method. We have considered Be²⁺, Mg²⁺, Ca²⁺, Ba²⁺, and Ni²⁺, segregating to both (001) and (110) faces. The results obtained can be extrapolated to predict the behavior of other impurities including Mn²⁺, Fe²⁺, and Co²⁺, We find, for example, that Fe²⁺, Mn²⁺, Ca²⁺, Sr²⁺, and Ba²⁺ will concentrate at the (001) surface of MgO, while Ni²⁺ will be depleted. The enthalpy of segregation is found to vary substantially with coverage particularly for the larger impurities. The enthalpy becomes less negative with increasing impurity concentration due to the increasing lattice strain until the surface is nearly saturated. Then additional stabilization is obtained by restructuring of the surface layer. We predict reconstructed surfaces for both the (001) and (110) faces, which contain a high concentration of a larger impurity ion. The enthalpy of segregation shows a maximum at around 50% surface coverage implying a bimodal surface distribution of segregant. The influence of segregation on surface energy suggests two unusual effects. The (001) surface energy of the impure crystal becomes negative for surface concentrations of impurity greater than 10% Ba²⁺ or 75% Sr²⁺ in MgO. This implies a thermodynamic barrier to sintering. At high coverages of Ba²⁺ in MgO the (110) surface becomes more stable than the (001) face suggesting that facetting may occur.     

The Effect of Sr Substitution on Phase Formation and Magnetic Properties of Y-type Hexagonal Ferrite

The effect of Sr substitution in Y-type hexagonal ferrite of composition Ba_{2– x} Sr _x Zn_1.2Co_0.4Cu_0.4Fe₁₂O₂₂ was investigated. x varied from 0 to 2.0. The samples were prepared by the solid-state reaction method. Phase formation was characterized by powder X-ray diffraction. The microstructure was observed via scanning electron microscopy and magnetic properties were measured using an impedance analyzer and vibrating sample magnetometer (VSM). Results reveal that Sr²⁺ cannot completely substitute for Ba²⁺ ions in Y-type hexagonal ferrites. The maximum solubility of Sr in the Y-type ferrite is about 1.8. With Sr above 1.8, the lattice mismatch induced by the difference in ionic radii of Ba²⁺ and Sr²⁺ ions prevents the formation of Y-type phase. While Sr content is equal or less than 1.0, the magnetic properties may be enhanced. As x >1.0, because of the internal stress resulting from the lattice mismatch, demagnetizing field significantly degrades initial permeability.     

Effect of Acceptor and Donor Dopants on the Dielectric and Tunable Properties of Barium Strontium Titanate

The effects of different concentrations of Mn²⁺, Mg²⁺, Al³⁺, Fe³⁺, La³⁺, and Nb⁵⁺ on the dielectric and tunable properties of Ba_0.6Sr_0.4TiO₃ ceramics were investigated. It was found that doping in small amounts with acceptor ions such as Mg²⁺, Fe³⁺, and Al³⁺ could meliorate the dielectric properties clearly. Decrease of dielectric loss was attributed to the formation of compensating defects originating from acceptor substitution. It was concluded that the tunability was linked to both the dielectric constant and the grain size. A higher figure of merit was obtained by doping the ceramics with smaller ions of Al and Fe, compared to Ti.     

Characterization and Microwave Dielectric Properties of M2+Nb2O6 Ceramics

This paper details the investigation of the quality factor ( Q ), dielectric permittivity (ɛ_r) and temperature coefficient of resonant frequency (τ_f) of the TE_01δ mode of the columbite binary niobate ceramics, with the formula MNb₂O₆ where M=²⁺ cation, in relation to their degree of sintering, microstructure and phase composition. The ceramics were made from a mixed oxide preparative route and fired over a range of temperatures from 800° to 1400°C, and most formed the columbite structure. A comprehensive study was made of the niobates containing the transition metal cations M=Mn²⁺, Co²⁺, Ni²⁺, Cu²⁺, and Zn²⁺, and the group II metal cations M=Mg²⁺, Ca²⁺, Sr²⁺, and Ba²⁺. All columbite niobates were found to have ɛ_r between 17 and 22 and negative τ_f values between –45 and –76 ppm/°C, and ZnNb₂O₆, MgNb₂O₆, CaNb₂O₆, and CoNb₂O₆ had high Q f values of 84 500, 79 600, 49 600, and 41 700 GHz, respectively. The Q f of MgNb₂O₆ was found to rise to over 95 000 GHz when heated at 1300°C for 50 h.     

Preparation and Optical Properties of Transparent Eu3+:Y3Al5(1−x)Sc5xO12 Ceramics

Using a novel combustion method, Eu-doped Eu:yttrium aluminum garnet (YAG) and Eu:YSAG powders, and transparent Eu:YSAG ceramics were fabricated. The optical properties of these transparent ceramics have been measured, and a reduced peak splitting of Eu³⁺ for ⁵D₀→⁷F₁ and ⁵D₀→⁷F₂ was observed when 10 at.% Al³⁺ was substituted by Sc³⁺. The enhanced symmetry of the Eu sites in YAG lattice, which resulted from the expanded YSAG lattice by Sc³⁺ doping, is the main reason for the reduced peak splitting.     

Effect of Composition on the Mechanical Properties of Aluminosilicate and Borosilicate Glasses

Elastic moduli and fracture toughness were determined for several glasses in the systems soda-alumina-silica, calcia-alumina-silica, and soda-boric oxide-silica. Results for the aluminosilicates are analyzed in terms of Al³⁺:Na⁺ ratios. The mechanical properties do not show maxima or minima at the Al³⁺:Na⁺ ratio of 1, in contrast to conductivity, helium permeability, and refractive index. The moduli and toughness increase with Al³⁺:Na⁺ ratio, which is consistent with increased coherency of the glass network. Glasses which contain B₂O₃ instead of Al₂O₃ have slightly higher moduli but are considerably tougher. The moduli of calcium aluminosilicate glasses are ∼25% greater than sodium aluminosilicates, whereas the fracture toughnesses are similar.     

Coordination of Aluminum Ions in Tricalcium Aluminate

The coordination of aluminum ions in tricalcium aluminate (3CaO·Al₂O₃) was examined using infrared absorption, X-ray emission spectroscopy, and molar refraction techniques. Each of these methods indicated the presence of aluminum in fourfold coordination. No evidence was found for aluminum in sixfold coordination. The isomorphous substitution of Sr²⁺ for Ca²⁺ and Ga³⁺ for Al³⁺ and a comparison between the absorption frequencies for an SiO₄ group and an AlO₄ group were used to identify the infrared absorption bands in tricalcium aluminate.     

Scandium-Containing Oxynitride Glasses

Oxynitride glasses were made by atom-for-atom substitution of Sc³⁺ for Mg²⁺ in the Mg-Si-O-N and Mg-Si-Al-O-N systems. Good-quality glasses containing up to 7 mol% Sc₂O₃ were obtained. For a constant cation ratio, an increased N/O ratio gives glasses with increased elastic modulus, Vickers microhardness, density, and glass transition temperature.     

Alumina as a Devitrification Inhibitor during Sintering of Borosilicate Glass Powders

Cristobalite is formed when borosilicate glass (Corning Code 7740) is sintered at temperatures ranging from 700^o to 1000^o C. The precipitation kinetics, determined by XRD analysis, exhibit a characteristic incubation period which decreases with increasing sintering temperature, from 60-120 min at 700^oC to 3-5 min at 1000^o C. Activation analysis of precipitation shows an activation energy of 75 kJ/mol, which is close to that for the diffusion of Na+ in borosilicate glass, suggesting mass-transport controlled kinetecs. ^1.,5With added alumina content greater than a critical value, however, the cristobalite formation in the borosilicate glass is completely prevented at the sintering temperatures investigated. The critical alumina content is found to decrease with decreasing alumina particle size but with increasing sintering temperature. The above result, similar to observations previously made in a binary glass mixture containing a low-softening borosilicate glass (BSG) and a high-softening high silica glass (HSG) ³ is attributed to a strong coupling between Al³⁺ from alumina and Na⁺ from borosilicate glass. The coupling reaction causes segregation of Na⁺ in boraosilicate glass to alumina, thus forming a Na⁺-and Al³⁺-rich reaction layer around alumina particles far too rapid for cristobalite formation.     

Gamma Irradiation Studies of Some Borate Glasses

The gamma-ray-induced optical absorption in a series of cabal (calcium-boron-aluminum) glasses was studied and is interpreted, wherever possible, in terms of structural concepts. A resolution of the observed absorption spectra showed that three Gaussian-shaped bands were induced with their maxima at about 2.3, 3.5, and 5.0 e.v. (550, 350, and 250 m_μ). The 2.3-e.v. band decreased in intensity with increasing CaO content, reaching a minimum intensity at a composition corresponding to the four-coordination of about 20% of the boron. Further increase in CaO content was associated with an increase in the intensity of this band. The intensity of the 3.5-e.v. band decreased gradually with increased mole per cent of CaO and increased with increased Al₂O₃. The 5.0-e.v. band showed an abrupt increase in intensity which corresponded to the appearance of non-bridging oxygens in the network. Replacing Ca²⁺ by Mg²⁺, Sr²⁺, or Ba²⁺ or replacing Li⁺ by Na⁺ or K⁺ showed that glasses containing large ions of low field strength give less induced absorption than glasses containing small ions of high field strength. A potassium alumina borate glass melted under reducing conditions gave a considerably higher ultraviolet transmission, before irradiation, as compared with the same glass melted under normal conditions. The gamma-induced absorption of these two glasses showed that reducing conditions resulted in a decrease in the intensity of the 2.3- and 3.5-e.v. bands, whereas it caused an increase in the far-ultraviolet-induced absorption. The effect of additions of arsenic, thallium, titanium, germanium, and some rare-earth oxides is discussed.     

Phase Equilibria and Structural Species in MgF2-MgO, MgF2-CaO, and MgF2-Al2O3 Systems

Partial equilibrium phase diagrams for the systems MgF₂-MgO, MgF₂-CaO, and MgF₂-Al₂O₃ were determined by differential thermal analysis. Simple eutectics were observed at 8.5 mol% MgO and 1228°± 3°C in the MgF₂-MgO system, at 7.5 mol% CaO and 1208°± 3°C in the MgF₂-CaO system, and at 2.5 mol% Al₂O₃ and 1250°± 3°C in the MgF₂-Al₂O₃ system. On the basis of agreements between the activities calculated by the Clausius-Clapeyron equation and Temkin's model using the present data, the eutectic melt consists of Mg²⁺, F^-, and O^2- ions in the MgF₂-MgO system; Mg²⁺, Ca²⁺, F^-, and O^2- ions in the MgF₂-CaO system; and Mg²⁺, Al³⁺, F^-, and AlO_2¯ ions in the MgF₂-Al₂O₃ system. Well-defined long needles of MgO in the MgF₂-MgO system, less defined needles of CaO in the MgF₂-CaO system, and Al₂O₃ grains in the MgF₂-Al₂O₃ system were observed by optical microscopy.     

Room-Temperature Preparation of Crystallized Ba1-χSrχWO4 Solid-Solution Films by an Electrochemical Method

Films of a complete series of solid-solution oxides, Ba^1-χSr^χWO⁴ (O ≥ X ≥ 1), have been prepared on tungsten substrates in an electrolytic solution containing Ba²⁺ and Sr²⁺ ions by an electrochemical method at room temperature (25°C). The composition X could easily be controlled by the concentrations of Ba and Sr species in the starting solutions. X-ray diffraction and X-ray photoelectron spectroscopy analyses showed that a complete series of well-crystallized solid-solution oxides was formed even at room temperature.