Phase Stability and Ionic Conductivity of Solid Solutions with NASICON Structure

Formation of heterovalent Zr-substituted solid solutions (up to 7 mol%) for Yb³⁺ in Na₆Yb₃(PO₄)₅ and LiNa₅Yb₃(PO₄)₅ complex phosphates was studied by ceramic technique at 950°C. Obtained samples were investigated with X-ray powder diffraction, infrared, and impedance spectroscopy. Zr-substituted (7 mol%) Na₆Yb₃(PO₄)₅ has ionic conductivity of 1.6·10⁻² S/cm at 300°C and 4.8·10⁻⁵ S/cm at room temperature. An updated version of phase diagram for ScPO₄–Na₃PO₄–Li₃PO₄ quasi-ternary system was provided.     

Ionic Conductivity and Sinterability of NASICON-Type Ceramics: The Systems NaM2(PO4)3+yNa2o (M = Ge, Ti, Hf, and Zr)

Sodium-rich NASOCON-type ceramics, the NaM₂(PO₄)₃+_yNa₂O (M = Ge, Ti, Hf, Zr) system, were investigated in order to obtain a material having a high Na⁺ conductivity and high density. The ionic conductivity and the sinterability were greately improved by an increase in the valve of y for all of the system examined. Added Na₂O was not souble in teh NASICON-type skeletton, sice the lattice constants and teh X-ray diffraction patterns were not changed by the Na₂O addintion in all of the samples. Na₂O acts as a flux for obtaining highly dense ceramics and highly conductive grain boundaries. Partial A₂ site insertion by Na⁺ ions is effective for the enhancement of conductivity, because the conductivity for Na_1.5M(III)_0.5Zr_1.5(PO₄)₃ (M = In or Y) is about 1 order of magnitude higher than the maximum conductivity of the NaZr₂(PO₄)₃+_yNa₂O system.     

Porous Glass-Ceramic in the CαO–TiO2–P2O5 System

A porous glass-ceramic in the CaO–TiO₂—P₂O₅ system has been prepared by crystallization and subsequent chemical leaching of the corresponding glass. By applying a two-step heat treatment to 45CaO · 25TiO₂· 30P₂O₅ glasses containing a few mol% of Na₂O, volume crystallization results in the formation of dense glass-ceramics composed of CaTi₄(PO₄)₆ and β-Ca₃(PO₄)₂ phases. By leaching the resultant glass ceramics with HCI, β-Ca₃(PO₄)₂ is selectively dissolved out, leaving a crystalline CaTi₄(PO₄)₆ skeleton. The surface area and mean pore radius of the porous glass-ceramics were approximately 40 m²/g and 13 nm, respectively.     

Preparation of a Calcium Titanium Phosphate Glass–Ceramic with Improved Chemical Durability

A calcium titanium phosphate glass–ceramic for use as a dental material with excellent chemical durability was derived from a mother glass with a small amount of fluorine. The laser Raman spectroscopic analysis showed that 35CaO–10CaF₂–30P₂O₅–25TiO₂ glass, as the nominal composition, consists of ortho-, pyro-, and meta-phosphate groups. On heating the glass at 865°C, orthophosphate crystals, such as fluorine-containing oxyapatite (Ca₁₀(PO₄)₆(O,F₂)) and the Nasicon-type phase (CaTi₄(PO₄)₆), were preferentially precipitated; the apatite particles of several tens of nanometers in size were embedded in the CaTi₄(PO₄)₆ phase. The pale bluish color of the glass–ceramic indicated that titanium ions were included in the residual glassy phase. When the glass–ceramic was treated with dilute hydrochloric acid, only the apatite particles at the surface were leached out, while no CaTi₄(PO₄)₆ phase was etched; the dissolution of the glass–ceramic was effectively controlled. Almost no dissolution of ions from the glass–ceramic occurred in water. It was suggested that the behavior is a result of the microstructure of the glass–ceramic, which consists of crystalline and glassy phases with excellent chemical durability.     

Hydration Products and Reactivity of Blast-Furnace Slag Activated by Various Alkalis

Pastes of blast-furnace slag were cured for up to 90 d using sodium silicate (waterglass), NaOH, and three different mixtures of Na₂CO₃–Na₂SO₄–Ca(OH)₂ to activate reactions. The highest slag reactivity was observed for NaOH activation and the least for waterglass, although nonevaporable water indicated similar amounts of hydration products formed. The main hydration products found using X-ray diffractometry in all systems were calcium silicate hydrate (C-S-H) and a hydrotalcite-type phase. Microanalysis was performed on pastes activated using 50% Na₂CO₃·25% Na₂SO₄·25% Ca(OH)₂, NaOH, and waterglass; the chemical composition of the C-S-H in the waterglass case was different relative to the other two alkalis. For all alkaline agents used, the C-S-H seemed finely intermixed with a hydrotalcite-type phase of Mg/Al = 1.82, on average.     

Characterization by Multinuclear High-Resolution NMR of Hydration Products in Activated Blast-Furnace Slag Pastes

The effect of activators on the hydration of granulated blast-furnace slag (gbfs) was studied through compressive strength measurements, ²⁹Si, ²⁷Al, and ²³Na high-resolution nuclear magnetic resonance, and X-ray diffraction. Four different activations containing sodium hydroxide, sodium silicate, and/or calcium hydroxide (CH) were considered, at fixed amounts of alkali: 5% Na₂O, 5% Na₂O-2.5% CH, 5% Na₂O-7.5% SiO₂, and 5% Na₂O-2.5% CH-7.5% SiO₂. Silicate-activated gbfs cements have greater compressive strength than Portland cements over the whole period of study (1 yr). Also, silicate-free activated gbfs cements have poorer mechanical strength than silicate-activated cements. In fact, substantial structural differences were observed between hydration products in both kinds of activations. In silicate-activated pastes there exists an intimate mixture of C-S-H layers and AFm-like arrangements containing Al in octahedral sites bonded to the silicate layers, originated either from phase intergrowths or from a high density of Ca-Al incorporation in the interlayer spaces of C-S-H. In pastes obtained from silicate-free activation of gbfs there is a better chemical and structural definition among C-S-H and calcium aluminate hydrate domains (AFm and hydrogarnet).     

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.     

An Alternative Chemical Route for the Synthesis and Thermal Stability of Chemically Enriched Hydroxyapatite

Hydroxyapatite (HAp: Ca₁₀(PO₄)₆(OH)₂) was synthesized by aqueous precipitation using CaCl₂ and Na₃PO₄ with NaOH added to ensure completion of the reaction at room temperature. The HAp powder prepared using stoichiometric amounts of NaOH was stable even at 1200°C, but the HAp prepared with sub-stoichiometric amounts of NaOH resulted in its transformation into β-tricalcium phosphate at 600°C. The reaction pH, X-ray diffraction, thermal analysis, scanning electron microscopy, Fourier transform infrared analyses and inductively coupled plasma-optical emission spectroscopy were used to characterize the phase purity, thermal stability, morphology, and chemical composition of the synthesized HAp powder.     

Early Hydration of Tricalcium Silicate

The hydration of tricalcium silicate (C₃S) in the preacceleration stages was studied. The C₃S particles carry a positive charge during the early stages of hydration. Following a rapid hydrolysis of C₃S, calcium ions adsorbed on the Si-rich surface of C₃S particles, greatly reducing their further dissolution, thus initiating the induction period. The [Ca²⁺] and [OH^-] continue to increase at lower rates and, because Ca(OH)₂ crystal growth is inhibited by silicate ions, become supersaturated with respect to Ca(OH)₂. When the supersaturation reaches a value of ∼1.5 to 2.0 times the saturation concentration, nuclei are formed, and rapid growth of Ca(OH)₂ and C-S-H is initiated. These products act as sinks for the ions in solution, thus enhancing the further dissolution of C₃S.     

Preparation of Porous Glass-Ceramics with a Skeleton of NASICON-Type Crystal CuTi2(PO4)3

A novel porous glass-ceramic with a skeleton of CuTi₂(PO₄)₃ was prepared by controlled crystallization of a glass and subsequent chemical leaching of the resulting dense glass-ceramic. A volume-crystallized dense glass-ceramic composed of CuTi₂(PO₄)₃ and Cu₃(PO₄)₂ whose surface was covered by a CuO thin layer was prepared by reheating a glass with a nominal composition of 50CuO·20TiO₂30P₂O₅ (in mol%) glass in air. When the resultant glass-ceramic was leached with dilute H₂SO₄, Cu₃(PO₄)₂ and CuO phases were dissolved out selectively, leaving a crystalline CuTi₂(PO₄)₃ skeleton. The specific surface area and the average pore radius of the porous glass-ceramic obtained were approximately 45 m₂g_-1 and 9 nm, respectively. The porous glass-ceramic showed catalytic activity in the conversion reaction of propene into acrolein.     

Microporous Glass-Ceramics in NASICON-Type Copper(II) Titanium Phosphate

A novel porous glass-ceramic with a skeleton of a NASICON-type copper(II) titanium phosphate was prepared via the controlled crystallization of a glass and the subsequent chemical leaching of the resulting dense glass-ceramic. A volume-crystallized dense glass-ceramic comprised of CuTi₂(PO₄)₃ and Cu₃(PO₄)₂, whose surface was covered by a thin layer of CuO, was prepared by reheating a glass with a nominal composition of 50CuO20TiO₂30P₂O₅ (in mol%) in air. When the resulting glass-ceramic was leached with dilute HCl, the Cu₃(PO₄)₂ and CuO phases were dissolved out selectively, and a cuprous NASICON crystal of CuTi₂(PO₄)₃ was converted to its cupric type, CuTi₄(PO₄)₆, which was left as a skeleton of the porous materials. The specific surface area and the average pore radius of the porous glass-ceramic obtained were ∼70 m²/g and ∼7 nm, respectively. The porous glass-ceramic showed high catalytic activities for the dehydration of 2-propanol.     

New Approach to the β→α Polymorphic Transformation in Magnesium-Substituted Tricalcium Phosphate and its Practical Implications

The transformation β→α in Mg-substituted Ca₃(PO₄)₂ was studied. The results obtained showed that, contrary to common belief, there is, in the system Mg₃(PO₄)₂–Ca₃(PO₄)₂, a binary phase field where β+α-Ca₃(PO₄)₂ solid solutions coexist. This binary field lies between the single-phase fields of β- and α-Ca₃(PO₄)₂ solid solution in the Ca₃(PO₄)₂-rich zone of the mentioned system. In the light of the results and the Palatnik–Landau's Contact Rule of Phase Regions, a corrected phase equilibrium diagram has been proposed. The practical implications of these findings with regard to the synthesis of pure α- and β- Mg-substituted Ca₃(PO₄)₂ powders and to the sintering of related bioceramics with improved mechanical properties are pointed out.     

Thermoelastic Behavior in Ca2SiO4 Solid Solutions

Dicalcium silicate solid solutions (C₂S(ss)) doped with Na₂O, A1₂O₃, and Fe₂O₃ were examined by high-temperature optical microscopy. Surface deformation caused by a possible martensitic transformation between a'_L and β phases was observed in situ under the microscope during temperature changes, indicating that the transformation was thermoelastic. Both the start and finish temperatures of the a'_L-to-β and β-to-a'_L transformations decreased with increased Na:(Na + Ca) ratio. Because of the athermal nature of the a'_L-to-β transformation, the a'_L phase, when cooled below the finish temperature, should have been completely converted to the β phase.     

Spectroscopic Properties of Er3+-Doped Na2O–La2O3–Al2O3–SiO2 Glasses

Er³⁺-doped sodium lanthanum aluminosilicate glasses with compositions of (90− x )(0.7SiO₂·0.3Al₂O₃)· x Na₂O·8.2La₂O₃· 0.6Er₂O₃·0.2Yb₂O₃·1Sb₂O₃ (in mol%) ( x = 12, 20, 24, 40, 60 mol%) were prepared and their spectroscopic properties were investigated. Judd–Ofelt analysis was used to calculate spectroscopic properties of all glasses. The Judd–Ofelt intensity parameter Ω _t ( t = 2, 4, 6) decreases with increasing Na₂O. Ω₂ decreases rapidly with increasing Na₂O while Ω₄ and Ω₆ decrease slowly. Both the fluorescent lifetime and the radiative transition rate increase with increasing Na₂O. Fluorescence spectra of the ⁴ I _13/2 to ⁴ I _15/2 transition have been measured and the change with Na₂O content is discussed. It is found that the full width at half-maximum decreases with increasing Na₂O.     

Hydration of Beta Dicalcium Silicate Alone and in the Presence of CaCl2 or C2H5OH

Beta C₂S was hydrated at room temperature with and without added CaCl₂ or C₂H₅OH by methods previously studied for the hydration of C₃S, i.e. paste, bottle, and ball-mill hydration. The amount of reacted β-C₂S, the Ca(OH)₂ concentration in the liquid phase, the CaO/SiO₂ molar ratio, and the specific surface area of the hydrate were investigated. A topochemical reaction occurs between water and β-C₂S, resulting in the appearance of solid Ca(OH)₂ and a hydrated silicate with a CaO/SiO₂ molar ratio of ≃1. As the liquid phase becomes richer in Ca(OH)₂, the first hydrate transforms to one with a higher CaO/SiO₂ ratio. Addition of CaCl₂ increases the reaction rate and the surface area of the hydrate but to a much lesser extent than in the hydration of C₃S, whereas C₂H₆OH strongly depresses the hydration rate of β-C₂S, as observed for C₃S hydration.     

Porous Glass-Ceramics with a Skeleton of the Fast-Lithium-Conducting Crystal Li1+xTi2−xAlx(PO4)3

Porous glass-ceramics with a skeleton of the fast-lithium-conducting crystal Li_{1+ x} Ti_{2− x} Al _x (PO₄)₃ (where x = 0.3–0.5) were prepared by crystallization of glasses in the Li₂O─CaO─TiO₂─Al₂O₃–P₂O₅ system and subsequent acid leaching of the resulting dense glass-ceramics composed of the interlocking of Li_{1+ x} Ti_{2− x} Al _x (PO₄)₃ and β-Ca₃(PO₄)₂ phases. The median pore diameter and surface area of the resulting porous Li_{1+ x} Ti_{2− x} Al _x (PO₄)₃ glass-ceramics were approximately 0.2 μm and 50 m²/g, respectively. The electrical conductivity of the porous glass-ceramics after heating in LiNO₃ aqueous solution was 8 × 10⁻⁵ S/cm at 300 K or 2 × 10⁻² S/cm at 600 K.     

Strengthening Glass-Ceramics by Application of Compressive Glazes

Glasses in the Na₂O–Ba0–A1₂O₃-Si0₂ system, nucleated with TiO₂, were heat-treated to effect controlled crystallization. Resulting materials consisted of a dense, micro-crystalline mixture of nepheline (Na₂0–A1₂O₃-2SiO₂) and barium feldspar (BaO-A1₂O₃-2Si0₂) in a glassy matrix. Thermal expansion coefficients (O° to 300° C) of these bodies ranged from 75 to 125 × 10 –7/°C. Glazes in the Na₂O-CaO-PbO-B₂O₂-A1₂0₃-SiO₂ system having expansion coefficients of about 40 to 80 × 10 ^-7/0°C were applied to the glass-ceramics. On firing, the glazes matured well and reacted with the bodies to form interlocking crystals at the interface. This interfacial region was investigated using several instrumental techniques, and the crystals were identified as plagioclase feldspar. Applying these compressive glazes resulted in modular of rupture up to five times that of the initial glass-ceramic. Calculated strengths correlated well with experimental values.     

Fundamentals of Glass-to-Metal Bonding: II, Reactions of Tantalum and Sodium Silicate Glass

A compound, identified by X-ray diffraction analysis as sodium metatantalate (Na₂Ta₂O₆), was found at the interface between a sodium silicate glass and tantalum metal which had been heated in vacuum to 1000°C. Thermodynamic calculations, followed by further experimental evidence, were used to determine the chemical reaction between tantalum and sodium silicate which resulted in the formation of Na₂Ta₂O₆. The compound was synthesized by other methods and its structure was investigated.     

Hot Corrosion of Sintered α-Sic at 1000°C

The hot corrosion of sintered α-Sic by thin films of Na₂SO₄ and Na₂CO₃ was studied at 1000°C in controlled gas atmospheres. Under all conditions corrosion led to 10 to 20 times the amount of SiO₂ formed in pure oxidation after a 48-h exposure. In addition, small amounts of sodium silicate formed. Melts of Na₂SO₄/SO₃ caused uniform pitting of the Sic substrate; Na₂CO₃/CO₂ melts caused localized pitting and grain-boundary attack. In all cases the protective SiO₂ layer dissolved to form silicate, leading to corrosion. In the sulfate case, free carbon in the Sic promotes this process. In all cases the presence of liquid films is responsible for rapid transport rates and the subsequent rapid reaction.     

Novel Preparation Method of Hydroxyapatite Fibers

A novel method for preparing calcium hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂: HAp) fibers has been developed. HAp fibers can be prepared successfully by heating a compact consisting of calcium metaphosphate (ß-Ca(PO₃)₂) fibers with Ca(OH)₂ particles in air at 1000°C and subsequently treating the resultant compact with dilute aqueous HCl solution. The ß-Ca(PO₃)₂ fibers and the Ca(OH)₂ in the compact were converted into fibrous HAp and CaO phases by the heating, and the CaO phase was removed by acid-leaching. HAp fibers obtained in the present work were 40-150 µm in length and 2-10 µm in diameter. The fibers had almost the same dimensions as those of the ß-Ca(PO₃)₂ fibers.