Low-Temperature Synthesis of Hexagonal Anorthite via Hydrothermal Processing

Hexagonal anorthite (CaAl₂Si₂O₈) has been prepared by hydrothermal processing of monocalcium aluminate and quartz at temperatures as low as 200°C. The successful development of this phase is dependent upon several processing parameters, including the hydration of the calcium aluminate precursor material to the hydrogarnet phase (Ca₃Al₂O₆·6H₂O) prior to hydrothermal treatment and the use of quartz as opposed to amorphous sources of SiO₂. Quartz has partial solubility in the hydrogarnet lattice for additions up to 40 wt%. Increased SiO₂ substitution has been shown to reduce the conversion of hydrogarnet to Ca₄Al₆O₁₃·3H₂O, thereby increasing its thermal stability and improving its strength characteristics at temperatures greater than 200°C. Quartz additions greater than 43 wt% lead to the formation of CaAl₂Si₂O₈ as the sole reaction product. The moderate temperatures involved in forming this anhydrous material are an order of magnitude lower than those necessary to form this phase by melt crystallization, making it a true chemically bonded ceramic. The reaction can form a bonded matrix with strengths up to 40000 psi (280 MPa). Strengths are limited due to density changes during anorthite formation, but the matrix is thermally stable up to 1000°C.     

Melting Phenomena in the System CaO–SiO2– H2O: I, The Join Ca2SiO,-Ca(OH)2

The liquidus-solidus relations along the join Ca₂SiO₄-Ca(OH), in the system CaO-SiO₂-H₂O have been determined at 1000 atm up to 1110°C. This join is binary and contains the calcium silicate hydrate, calciochondrodite, Ca5-(SiO₄(OH)₂. Calciochondrodite melts incongruently to Ca₂SiO₂+ liquid (composition 23 wt% Ca₂Si0₄) at 955°C. The eutectic between calcium hydroxide and calciochondrodite lies at 13% Ca₂Si0₄ and 822°C. Preliminary experiments, also at 1000 atm, in the ternary system CaO-Ca₂Si0₄-Ca(OH), indicate that the eutectic at which the fields of primary Ca(OH)₂, CaO, and Ca₂(Si0₄)₂(OH)₂ meet is close to the CaO-Ca. (OH), side of the triangle at approximately 805° C. The ternary reaction point Ca₂SiO_l+ liquid ⇌Ca₅(SiO₄)₂(OH)₂+ CaO + liquid is believed to lie in the low-CaO (<5%) high-Ca(OH)₂ (>70%) part of the system.     

Chemistry and Morphology of Hydrogarnets Formed in Cement-Based CASH Hydroceramics Cured at 200° to 350°C

We have studied the chemistry and the morphology of hydrogarnet crystals produced in cement-based hydroceramic materials at elevated temperatures (200°–350°C) with silica and alumina additions. Such materials lie within the hydrothermal CaO–Al₂O₃–SiO₂–H₂O (CASH) system. Hydrogarnet Ca₃Al₂(SiO₄)_{3− y} (OH)_{4 y} is the dominant aluminum bearing phase formed and its composition is influenced mainly by the curing temperature and to a lesser degree by the addition of silica. The composition parameter y was estimated by Rietveld refinement of X-ray diffraction (XRD) data. Electron probe microanalysis (EPMA) shows that the hydrogarnets incorporate minor elements such as Fe, Mg, and S. EPMA data confirmed the hydrogarnet composition estimated from XRD. Both octahedral and icositetrahedral forms are observed. The icositetrahedral form is associated with higher minor element content.     

Synthesis and Thermal Stability of Aluminum Titanate Solid Solutions

Aluminum titanate solid solutions with empirical formulas of Al₂Ti_1-xZr_xO₅, Al_6(2-x)(6+x)Si_6x/(6+x)□_6x/(6+x)TiO₅, and Al_2(1-x)Mg_xTi_1+xO₅ were synthesized by reaction sintering and annealed at 900° to 1300°C in air to evaluate the thermal stability. Substitution of Al in Al₂TiO₅ by Si and 2Al by Mg and Ti ions to form solid solutions such as AI_{6(2-x)/(6+x)l-Si}6x/(6+x)□_6x/(6+x)TiO₅, and Al_2(1-x)Mg_xTi_1+xO₅ was effective in controlling the thermal decomposition, but substitution of Ti by Zr had little effect.     

The System CaO–"CaCr2O4"–CaAl2O4 in Air and under Mildly Reducing Conditions

The system CaO–chromium oxide in air is reinvestigated and the existence of intermediate phases with chromium in oxidation states >3+ (Ca₅Cr₃O₁₂, Ca₃(CrO₄)₂, and Ca₅(CrO₄)₃) confirmed. Under reducing conditions these phases are unstable. A metastable, polymorphic form of calcium chromite, δ -CaCr₂O₄, is observed. In the CaO-rich section of the CaO–Al₂O₃–Cr₂O₃ system a ternary intermediate phase, chrome-haüyne, Ca₄[(Al,Cr³⁺)₆O₁₂](Cr⁶⁺O₄), coexists with calcium chromate and calcium aluminate phases. In air, low melting temperatures are preserved in all assemblages containing calcium chromate phases. Under reducing conditions a new ternary phase, Ca₆Al₄Cr₂O₁₅, coexists with CaO, CaCr₂O₄, chrome-haüyne, and calcium aluminate phases. The influence of chromium oxide additions on the solidus temperatures of the CaO–Al₂O₃ system is insignificant.     

Phase Equilibrium in the Fluorapatite–Anorthite–Diopside System

The binary systems Ca₅[PO₄]₃F–CaAl₂Si₂O₈ and Ca₅[PO₄]₃F–CaMgSi₂O₆ have been investigated, via annealing and quenching in the experimental method, direct observation of the melting behavior of the samples, X-ray diffraction analysis, petrography, and transmission electron microscopy. Phase equilibrium in the ternary system Ca₅[PO₄]₃F–CaAl₂Si₂O₈–CaMgSi₂O₆ was determined by combining information from the structure of the binary boundary systems and additional experimental data that were obtained from ternary compositions. The glass-formation region of the fluorapatite–anorthite–diopside system was studied, and the glass compositions for the development of glass-ceramics for technical and medical applications were identified.     

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.     

Parallel Solution-Based Synthesis Approach for Search of Lanthanoid-Activated Ca2SnO4 Phosphor Materials

In the search for new fluorescent materials among lanthanoid-activated Ca₂SnO₄ compounds, a parallel solution-based synthesis approach was applied and a new blue-emission Ca₂SnO₄:Ce phosphor was discovered; its emission intensity reaches 80% of the intensity of one of the best commercial Y₃Al₅O₁₂:Ce³⁺ phosphors. Among 14 lanthanoid-activated materials, three categories could be distinguished with respect to the effect of the dopant ion on the excitation and emission properties of the phosphor. In addition, potential phosphors in the A–Sn–O (A=Mg, Ca, Sr, and Ba) systems were investigated and Ca₂SnO₄ was found to be the most suitable host compound for the lanthanoid-activated phosphors.     

Formation of Infinite-Layered (Ca1-xSrx) CuO2 and NaCuO2-type (Ca1-yNay)0.85CuO2 in Tartrate Route

Both NaCuO₂-type Ca_0.85CuO₂ and infinite-layered (Ca_{1- x} Sr _x )CuO₂ could be prepared much more easily by firing the dried solids from mixed acetate aqueous solutions titrated with tartaric acid than by normal calcination. The presence of a narrow solid-solution composition range of 0.10 < x > 0.16 was confirmed in infinite-layered (Ca_{1- x} Sr _x )CuO₂ in the preparation using the tartrate route. The calcium could also be substituted by sodium in a range of y > 0.15 in NaCuO₂-type (Ca_{1- y} Na _y )_0.85CuO₂ using the same route. Further substitution of Ca²⁺ with Y³⁺ might also be possible in infinite-layered (Ca_{1- x} Sr _x )CuO₂, but resulted in the NaCuO2-type compound in the substitution with Na⁺.     

Compositional Model for Calcium Silicate Hydrate (C-S-H) Gels, Their Solubilities, and Free Energies of Formation

A compositional model based on available structural evidence is proposed for amorphous calcium silicate hydrogel. It is applicable to gels in the (Ca/Si),_solid range 1.0 to 1.4 and is formulated to take account of dimeric silicate species in the solid. Its composition is represented by Ca _x H_{6−2 x} Si₂O₇· z Ca(OH)₂· n H₂O where x and z are not independently obtainable; x + z , however, is evaluated from the Ca/Si ratio. The model is applied to representative solubility data; K_sp and free energies of formation for C-S-H's in this composition range are evaluated.     

Trivalent-Cation-Substituted Calcium Oxyhydroxyapatite

Synthesis of stable oxy- and oxyhydroxyapatkes [(Ca,M)-Ap] was attempted by substitution of Ca²⁺ with M³⁺ (M = Al, Y) assuming the composition Ca_10-xM_x(PO₄)₆(OH)_2-xO_x. Yttrium-calcium oxyhydroxyapatite was successfully obtained. Its structure was analyzed and confirmed by X-ray diffraction and infrared spectroscopy. Dielectric dispersion and absorption were not so distinct in (Ca,Y)-Ap as in hydroxyapatite (Ca-Ap). The ac electrical conductivity of (Ca_10-x,Y_x)-Ap was found to decrease with increasing x. These results are discussed in terms of a proposed charge transport mechanism.     

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.     

Mechanical Properties and Microstructure of Ca2SiO4–CaZrO3 Composites

Three types of dicalcium silicate (Ca₂SiO₄–calcium zirconate (CaZrO₃) composites were fabricated and their microstructures correlated with their mechanical properties. In the first type, Ca₂SiO₄ was added as a minor phase. The second type consisted of a 50 vol% Ca₂SiO₄-50 vol% CaZrO₃ mixture, while in the third type, CaZrO₃ constituted the minor phase. Pure CaZrO₃ was also studied as a control and found to have a toughness which depended on its grain size. In composites with Ca₂SiO₄ as the minor phase, a toughness increase was observed and found to be a function of matrix grain size. The composite with the second type of microstructure had the highest toughness of about 4.0 Mpa. m^1/2, which was about double that of the monolithic CaZrO₃. No evidence was found for transformation toughening by the orthorhombic (β) to monoclinic (γ) transformation in Ca₂SiO₄. The main toughening mechanisms identified were crack deflection and crack branching. Microstructural observations indicated the existence of weak grain boundaries in CaZrO₃ agglomerates as well as weak interfaces between the two phases.     

Activities in Borosilicate Melts: II, Some Mixtures of Calcium Borates-Calcium Silicates

Simple expressions of activities, as functions of composition, were developed from structural models for some borosilicate melts. Their applicability was tested with data from the liquidus curves of phase diagrams. If the heat of fusion is known, a value of the activity-composition function can be calculated for each temperature of the liquidus curve. These values are compared with activities computed from the structural model. Calorimetric heats of fusion were available for the calcium borates studied. Experimental data from published phase diagrams for the systems Ca₃(BO₃)₂–Ca₂SiO₄, Ca₂B₂O₆–Ca₂SiO₄, Ca₃(BO₃)₂–CaSiO₃, Ca(BO₂)–CaSiO₃, Ca₂B₂O₅–CaSiO₃, CaSiO₃–BaSiO₃, and CaSiO₃–K₂SiO₃ were used. Simple ionic models seemed to be applicable in most cases, except for melts of highly polymerized mixed ions (metaborate-metasilicate), for which the so-called "group" model appeared to be useful. A value for the heat of fusion of CaSiO₃ was calculated from this work (13,200 cal. per mole).     

Phase Equilibria in the Systems CaO–CuO and CaO-Bi2O3

New data are presented on the phase equilibria of the binary systems CaO-CuO and CaO-Bi₂O₃. Corrected compositions are reported for Ca.Bi₆O₁₃ and Ca₂Bi₂O₅ and a new metastable high-temperature phase is reported for a composition near Ca₆Bi₇O_16.5. The composition and decomposition temperatures for Ca_1–x.CuO₂ are given for both air and 1 atm of oxygen at 755 ± 5° and 835 ± 5°C, respectively.     

A Novel Temperature-Compensated Microwave Dielectric (1−x)(Mg0.95Ni0.05)TiO3−xCa0.6La0.8/3TiO3 Ceramics System

The microstructure and microwave dielectric properties of a (1− x )(Mg_0.95Ni_0.05)TiO₃− x Ca_0.6La_0.8/3TiO₃ ceramics system have been investigated. The system was prepared using a conventional solid-state ceramic route. In order to produce a temperature-stable material, Ca_0.6La_0.8/3TiO₃ was added for a near-zero temperature coefficient (τ_f). With partial replacement of Mg²⁺ by Ni²⁺, the dielectric properties of the (1− x )(Mg_0.95Ni_0.05)TiO₃− x Ca_0.6La_0.8/3TiO₃ ceramics can be promoted. The microwave dielectric properties are strongly correlated with the sintering temperature and the composition. An excellent Q × f value of 118,000 GHz can be obtained for the system with x =0.9 at 1325°C. For practical application, a dielectric constant (ɛ_r) of 24.61, a Q × f value of 102,000 GHz, and a temperature coefficient of resonant frequency (τ_f) of −3.6 ppm/°C for 0.85(Mg_0.95Ni_0.05)TiO₃−0.15Ca_0.6La_0.8/3TiO₃ at 1325°C are proposed. A parallel-coupled line band-pass filter is designed and simulated using the proposed dielectric to study its performance.     

Hydration Study of the System Ca3Al2O6–CaSO4· 2H2O–Ca(OH)2–H2O with and without Citric Acid

Hydration occurring in the system Ca₃Al₂O₆–CaSO₄· 2H₂O–Ca(OH)₂–H₂O has been studied at different temperatures and it was found that the reactions are diffusion controlled. The kinetic data obeyed Jander's equation and the rate of reaction increased with increasing temperature. X-ray diffraction studies and calorimetric measurements show that when gypsum is consumed, ettringite is converted into monosulfate. The rate of this conversion also increased with the increasing temperature and decreased in the presence of citric acid. Spectroscopic studies showed that there was some interaction between citric acid and the cement and that the product of hydration is of colloidal nature. Zeta potential measurements show that retardation of Ca₃Al₂O₆ hydration in the presence of gypsum and Ca(OH)₂ is not due to SO²⁻₄ adsorption. Electrical conductivity and thermoelectric potential measurements of solid Ca₃Al₂O₆ show that Ca₃Al₂O₆ is an n -type semiconductor and contains defects. The retardation of Ca₃Al₂O₆ may be due to poisoning of reaction sites by gypsum and Ca(OH)_2.     

Thermodynamic Analysis of the Chemical Vapor Deposition of Composite 〈Si3N4〉-〈BN〉 Coatings

A thermodynamic analysis of the chemical vapor deposition of the composite ceramic (Si₃N₄(BN) ⋆ was performed using a general computer program for the calculation of heterogeneous equilibria. Reactants assumed were (H₂SiCl₂), (BCI₃), and (NH₃). The equilibrium conditions for the deposition of condensed phases in the B-Si-N system wire determined as a function of the variables temperature, total system pressure, and gram-atomic reactant gas fractions B/(Si + B) and N/(CI + N). The phase assemblage (Si₃N₄)-〈BN〉 was found to be stable over a large region. Predominant gaseous species at equilibrium were (HCI), (N₂), (H₂), and the silicon chlorides. Deposition efficiencies at equilibrium for (Si₃N₄) and (BN) were high, particularly in the presence of excess (NH₃) and at temperatures below 1600 K.     

Calcium Phosphate Cements Prepared by Acid–Base Reaction

We investigated the characteristics of calcium phosphate cements (CPC) prepared by an exothermic acid–base reaction between NH₄H₂PO₄-based fertilizer (Poly-N) and calcium aluminate compounds (CAC), such as 3CaO · Al₂O₃ (C₃A), CaO · Al₂O₃ (CA), and CaO · 2Al₂O₃ (CA₂), in a series of integrated studies of reaction kinetics, interfacial reactions, in-situ phase transformations, and microstructure development. Two groups were compared: untreated and hydrothermally treated CPC specimens. The extent of reactivity of CAC with Poly-N at 25°C was in the following order: CA > C₃A ≫ CA₂. The formation of a NH₄CaPO₄· x H₂O salt during this reaction was responsible for the development of strength in the CPC specimens. The in-situ phase transformation of amorphous NH₄CaPO₄· x H₂O into crystalline Ca₅(PO₄)₃(OH) and the conversion of hydrous Al₂O₃ gel →γ-AIOOH occur in cement bodies during exposure in an autoclave to temperatures up to 300°C. This phase transformation significantly improved mechanical strength.     

Melt Differentiation Induced by Zonal Structure Formation of Calcium Aluminoferrite in a CaO–SiO2–Al2O3–Fe2O3 Pseudoquaternary System

The phase stability in part of the P₂O₅-bearing pseudoquaternary system CaO–SiO₂–Al₂O₃–Fe₂O₃ has been studied by electron probe microanalysis, optical microscopy, and powder X-ray diffractometry. At 1973–1653 K, the α-Ca₂SiO₄ solid solution [α-C₂S(ss)] and melt coexisted in equilibrium, both chemical variations of which were determined as a function of temperature. The three phases of melt, calcium aluminoferrite solid solution (ferrite), and C₂S(ss) coexisted at 1673–1598 K. On the basis of the chemical compositions of these phases, a melt-differentiation mechanism has been, for the first time, suggested to account for the crystallization behavior of Ca₃Al₂O₆ solid solution [C₃A(ss)]. When the α-C₂S(ss) and melt were cooled from high temperatures, the melt would be induced to differentiate by the crystallization of ferrite. Because the local equilibrium would be continually attained between the rims of the precipitating ferrite and coexisting melt during further cooling, the melt would progressively become enriched in Al₂O₃ with respect to Fe₂O₃. The resulting ferrite crystals would show the zonal structure, with the Al/(Al+Fe) value steadily increasing up to 0.7 from the cores toward the rims. The C₃A(ss) would eventually crystallize out of the differentiated melt between the zoned ferrite crystals in contact with their rims.