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.     

The Join Ca2SiO4-CaMgSiO4

New data obtained on the join Ca₂SiO₄-CaMgSiO₄ established a limit of crystalline solubility of Mg in α-Ca₂SiO₄ corresponding to the composition Ca_1.90Mg_0.10SiO₄ at 1575°C. The α-α'Ca₂SiO₄ inversion temperature is lowered from 1447° to 1400°C by Mg substitution in the lattice. α'-Ca₂SiO₄ takes Mg into its lattice up to the composition Ca_1.94Mg_0.06SiO₄ at 1400°C and to Ca_1.96Mg_0.04SiO₄ at 900°C. A new phase (T) reported previously by Gutt, with the approximate composition Ca_1.70Mg_0.30SiO₄, was stable between 979° and 1381°C, and should be stable at liquidus temperatures in multicomponent systems involving CaO–MgO–SiO₂.     

The Ternary System CaO—MnO—SiO2

Phase equilibrium data resulting from quenching experiments are presented for the ternary system CaO-MnO-SiO₂. An atmosphere of controlled oxygen pressure having P_o2, ≅ 10⁻⁶ atm at 1555°C was used to maintain the manganese in the divalent state. The ternary liquidus surface is largely one of low-lying liquidus temperatures. Three ternary liquidus minima dominate this surface. These have the following compositions (in weight percent CaO, MnO, and SiO₂): (a) 5.0, 48.4, and 46.6%, (b) 17.5, 45.0, and 37.5%, and (c) 15.0, 53.0, and 32.0%. Temperatures measured at these points are (a) 1265° C, (b) 1195°C, and (c) 1204°C. Isofracts of the quenched glasses are presented. Crystallization paths of ternary mixtures are represented by a series of fractionation curves and selected isothermal planes. Partition of manganese between coexisting pairs of crystalline phases (e.g., meta-silicate, olivine, and (Ca,Mn)O solid solutions) favors concentration of manganese in the more basic phase. Subsolidus equilibria involving these phases and also Ca₃Si₂O₇ and Ca₃SiO₅ are discussed. Ca₃Si₂O₇ and Ca₃SiO₆ do not admit any appreciable amounts of Mn⁺⁺ into their lattices.     

Phase Equilibrium Studies in the System CaO-Cr2O3-SiO2

Results are presented of a study in air of mixtures in the system CaO-Cr₂O₃-SiO₂. The phase equilibrium diagram shows relations at liquidus temperatures for all but the high-lime part of the system. In this omitted part chromium in the mixtures oxidizes in air to higher valence forms. The compound Ca₃Cr₂Si₃O₁₂ (uvarovite) occurs at subsolidus temperatures, decomposing at 1370°C. to α-CaSiO₃ and Cr₂O₃. The inhibiting action of chromium oxide on the inversion of high-temperature forms of Ca₂SiO₄ to the low-temperature γ-Ca₂SiO₄ is discussed in the light of new data. Evidence is presented for the existence of a pentavalent chromium compound, Ca₃(CrO₄)₂, having solid-solution relations with Ca₃SiO₄.     

Stability of CaNiSi2O6 ("Niopside") and Activity–Composition Relations of CaMgSi2O6– CaNiSi2O6 Solid Solutions at 1350°C

A complete solid-solution series exists between diopside (CaMgSi₂O₆) and its nickel analogue, "niopside"(CaNiSi₂O₆). Activity–composition relations within this solid solution, and the stability of the end member CaNiSi₂O₆, have been determined by equilibrating CaNiSi₂O₆ with SiO₂, CaSiO₃, and metallic Ni in atmospheres of known oxygen pressures. Within limits of accuracy of the experiments, the solution is ideal at 1350°C. From the experimental data obtained in the present investigation, the standard free energy (Δ G °) of formation of CaNiSi₂O₆ according to the equation CaO + NiO + 2SiO₂= CaNiSi₂O₆ is calculated to be Δ G °=−165862 + 42.40 T J. Experiments in the system CaO–NiO–SiO₂ have shown that the nickel analogue of the phase pseudo-enstatite (MgSiO₃) is unstable with respect to SiO₂ and nickel olivine (Ni₂SiO₄), and the nickel analogues of the phases akermanite (Ca₂MgSi₂O₇) and monticellite (CaMgSiO₄) are unstable relative to the phase assemblage pseudo-wollastonite (CaSiO₃) plus NiO. In the system CaO–MgO–NiO–SiO₂, however, substitution of Ni for Mg in these phases was observed. The percentage substitution of Ni for Mg in the phases is given in parentheses: diopside (100%), olivine (100%), enstatite (18%), akermanite (20%), and monticellite (57%).     

Effect of Temperature and Water-Solid Ratio on Growth of Ca(OH)2 Crystals Formed During Hydration of Ca3SiO5

The growth behavior, time of nucleation, and morphology of Ca(OH)₂ crystals formed during the hydration of Ca₃SiO₅, at 15°, 25°, and 35°C at water-solid ratios ( w/s ) from 0.3 to 5.0 were studied by optical microscopy. In samples with w/s >0.5 growth of Ca(OH)₂ in the c -axis direction is initially dominant. Growth in this direction ends after a few hours, but growth perpendicular to the c axis continues for several days and produces a dendritic morphology. Growth behavior is not so well defined for w/s <0.5, in part because of the large number of unhydrated particles engulfed. Increasing temperature resulted in an increase in the number of Ca(OH)₂ nuclei and a decrease in nucleation time and crystal size. Increasing the w/s ratio improved the euhedral character of the Ca(OH)₂ crystals, decreased the number of engulfed Ca₃SiO₅ particles, and increased the nucleation time. Dendritic morphology was most pronounced in the samples for which w/s = 1. Growth rates and the ultimate size of the Ca(OH)₂ crystals varied within a given sample. The effects of temperature and the w/s ratio on the heat evolved during the hydration were studied by isothermal calorimetry. The times of nucleation of crystalline Ca(OH)₂ estimated from calorimetry were similar to those derived from growth curves determined by optical microscopy.     

Low-Temperature Synthesis of β-Ca2SiO4 from Hillebrandite

Experiments on hydrothermal synthesis were conducted using quartz or silicic acid and lime as starting materials at Ca/Si = 2.0. It is possible to synthesize pure hillebrandite (Ca₂(SiO₃)(OH)₂) having the theoretical composition by heating at 200°C for 10 h or at 250°C for 5 h. The synthesized product is fibrous, open at each end, and has a length of 20 to 30 μm. Calcium silicate hydrate gels are produced at the initial stage of the reaction. These react further with the unreacted lime to give hillebrandite. However, when silicic acid is used as silica, hillebrandite with tricalcium silicate hydrate is observed at 250°C because of the high reaction rate of silica. On heating, hillebrandite starts to decompose at about 500°C and produces low-crystalline β-Ca₂SiO₄, which is stable at room temperature and has a remarkably large specific surface area of about 7 m²/g. The decomposition reaction rate in a single crystal is rapid, and the reaction is considered to proceed topotactically.     

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.     

Role of Cuspidine (3CaO.2SiO2.CaF2) in the System CaO–SiO2–CaF2

Cuspidine is a well-defined ternary compound with a stability field in the subsystem CaF₂–CaSiO₃–Ca₂SiO₄. Cuspidine is easily formed by solid-state reactions in the subsystem mentioned and is stable above its apparently congruent fusion point if heated in welded platinum containers. Above 1450° decomposition and the formation of a mixture of CaF₂ and Ca₂SiO₄ is observed. Cuspidine also is easily formed by secondary reactions in solid mixtures of the subsystem CaF₂–CaSiO₃ and in ternary mixtures of these with free SiO₂ if heated in open crucibles. The existence of double compounds of CaF₂ and CaSiO₃ is not confirmed.     

Studies in the System CaO-Al2O3-SiO2H2O: III, New Data on the Polymorphism of Ca2SiO2 and Its Stability in the System CaO-SiO2- H2O

The nature of the low-temperature inversions γ-α' and α'-β was investigated by various techniques: hydrothermal and "dry" quenching runs, differential thermal analysis at atmospheric and elevated nitrogen pressures, X-ray diffractometer patterns obtained at elevated temperatures, "static" pressure techniques, and infrared absorption spectrometry. A revised energy-temperature diagram is presented for Ca₂SiO₄, with the transition γ' to α' taking place at about 725°C. and the α'-β transition, although not reversible at an exact temperature, taking place at about 670° C. At low water pressures (2000 lb. per sq. in.) the inversion γ-α' was placed at 675°C. Attempts to extrapolate the value obtained at 2000 lb. per sq. in. to obtain a more accurate reversible inversion temperature at atmospheric pressure, although limited in accuracy by the reliability of heat-of-transition data, would indicate a temperature of about 725° C. at atmospheric pressure. Three new compounds, 8CaO.3SiO₂ -3H₂O (X), 6CaO 3SiO₂.H₂O (Y), and 9CaO-6SiO₂ H₂O (Z), were found to be stable above 700°C. at H₂O pressures greater than 7500 lb. per sq. in.     

High-Temperature Mechanical Properties of Ceramic Materials: V, (Zn0.65, Mg0.35)2SiO4

The composition (0.65Zn,0.35Mg)₂ SiO₁ was investigated. Its thermal expansion was 32 × 10^-7/°C from room temperature to 1000°C. Modulus of rupture was approximately 7000 psi between room temperature and 800°C, whereas Young's modulus held at approximately 11 × 10° psi over the same range. The substitution of 0.35 m oles Mg⁺⁺ for Zn⁺⁺ in Zn₂Si0₄ causes little change in many of the physical properties, but the solid solution sinters much more readily than pure Zn₂Sio₄. The willemite solid solution studied has very good thermal shock resistance between room temperature and 1000°C.     

Studies on 4CaOAl2.O3.13H2O and the Related Natural Mineral Hydrocalumite

Single-crystal X-ray and electron-diffraction studies show the existence in one polymorph of 4CaO.Al₂O₃. 13H₂O of a hexagonal structural element with α= 5.74 a.u., c = 7.92 a. u. and atomic contents Ca₂(OH)₇- 3H₂O. These structural elements are stacked in a complex way and there are probably two or more poly-types as in SiC or ZnS. Hydrocalumite is closely related to 4CaO.A1₂O₃.13H₂O, from which it is derived by substitution of CO₃^2-for 20H^-+ 3H₂O once in every eight structural elements; similar substitutions explain the existence of compounds of the types 3CaO Al₂O₃.Ca Y ₂- xH₂O and 3CaO Al₂O₃ Ca Y xH₂O. On dehydration, 4CaO.Al₂O₃.13H₂O first loses molecular water and undergoes stacking changes and shrinkage along c. At 150° to 250°C., Ca(OH)₂ and 4CaO.3Al₂O₃.3H₂O are formed and, by 1000°C., CaO and 12CaO.7Al₂O₈. The dehydration of hydrocalumite follows a similar course, but no 4CaO.3Al₂O₃.3H₂O is formed.     

Equilibria of SiF4 with SiO2, Be2SiO4, and BeO in Molten Li2BeF4

Equilibrium partial pressures of SiF₄ were measured for the reactions 2SiO₂( c )+2BeF₂( d )⇋SiF₄( g )+Be₂SiO₄( c ) (log P _siF4(mm) = [8.790 - 7620/ T ] ±0.06(500°–640°C)) and Be₂SiO₄( c ) +2BeF₂( d )⇋SiF₄( g ) +4BeO( c )(log P _siF4(mm) = [9.530–9400/T] ±0.04 (700°–780°C)), wherein BeF₂ was present in solution with LiF as molten Li₂BeF₄. The solubility of SiF₄ was low (∼0.04 mol kg^-1 atm^-1) in the melt. The results for the first equilibrium were combined with available thermochemical data to calculate improved Δ H_f and Δ G_f values for phenacite (–497.57 ±2.2 and –470.22±2.2 kcal, respectively, at 298°K). The few measurements above 700°C for the second equilibrium are consistent with the temperature of the subsolidus decomposition of phenacite to BeO and SiO₂ and with the heat of this decomposition as determined by Holm and Kleppa. Below 700°C, the pressures of SiF₄ generated showed an increasing positive deviation from the expression given for the equilibrium involving Be₂SiO₄ and BeO. This deviation might have been caused by the formation of an unidentified phase below 700°C which replaced the BeO; it more likely resulted from a metastable equilibrium involving BeO and SiO₂.     

The Ternary System MgO-MnO-SiO2

A ternary phase equilibrium diagram is presented for the system Mg0-Mn0-Si0₂. Two peritectic points exist at 1536° and 1538°C. at liquid compositions of (a) 32 MgO, 5 MnO, 63 SiO₂ and (b) 36 MgO, 5.5 MnO, 58.5 SO,. Each of these liquids is in equilibrium with a hodonite (Mn-SiO,) and a high enstatite (MgSiO,) solid solution, the third crystalline phase being cristobalite (SiO₂) with (a) and olivine ((Mg,Mn]₂,Si0₄) with (b). Metasilicate solid solutions form a congruently melting rhodonite series in the central part of the MgSi0₃-MnSi0₃ join, but melt incongruently to olivine plus liquid at the MgSiO₃ end and to tridymite plus liquid at the MnSi0₃ end. X-ray diffraction data are given for two new high enstatite solid solution structures. Rhodonite solid solutions form a continuous structural series extending from MnSi0, to 94.5 weight % MgSi03. Continuous series of solid solutions exist between Mg,Si0₄ and Mn₂SiO₄ and between MgO and MnO. Compositional data are given for coexisting condensed phases, and courses of crystallization are described for certain ternary mixtures. A plot is presented of variation of refractive index of glasses which form an almost colorless series in the region of the rhodonite and high enstatite fields. The quenching technique was used in the investigation, with oxygen partial pressure controlled to maintain manganese as Mn²⁺.     

Revised Phase Diagram for the System Al2O3—SiO2

On the basis of 190 runs made up to 1860°C in sealed noble-metal containers the following revisions have been made in the equilibrium diagram for the system A1₂O₃–SiO₂. Mullite melts congruently at 1850°C. The extent of equilibrium solid solution in mullite at solidus temperature is from approximately 60 mole % Al₂O₃ (3/2 ratio) to 63 mole % A1₂O₃. Metastable solid solutions can be prepared up to about 67 mole % Al₂O₃. There is no evidence for stable solubility of excess SiO₂ beyond the 3/2 composition at pressures below 3 kbars. Refractive indices are presented for glasses containing up to 60 mole % Al₂O₃ and from them the composition of the eutectic is confirmed at 5 mole % SiO₂. The variation in lattice constants of the mullite solid solution is not an unequivocal guide to composition since mullites at one composition produced at different temperatures show differences in spacing, no doubt reflecting Al-Si ordering phenomena. The possibility of quartz and corundum being the stable assemblage at some low temperatures and pressures cannot be ruled out. A new anhydrous phase in the system is described, which was previously thought to be synthetic andalusite; it is probably a new polymorph of the Al₂SiO₅ composition with ortho-rhombic unit-cell dimensions a =7.55 A, b =8.27 A, and c = 5.66 A.     

The System Si3N4-SiO2-Y2O3

Subsolidus phase relations were established in the system Si₃N₄-SiO₂-Y₂O₃. Four ternary compounds were confirmed, with compositions of Y₄Si₂O₇N₂, Y₂Si₃O₃N₄, YSiO₂N, and Y₁₀(SiO₄)₆N₂. The eutectic in the triangle Si₃N₄-Y₂Si₂O₇-Y₁₀(SiO₄)₆N₂ melts at 1500°C and that in the triangle Si₂N₂O-SiO₂-Y₂Si₂O₇ at 1550°C. The eutectic temperature of the Si₃N₄-Y₂Si₂O₇ join was ∼ 1520°C.     

Oxidation State of Chromium in CaO–Al2O3–CrOx–SiO2 Melts under Strongly Reducing Conditions at 1500°C

Equilibrium ratios Cr²⁺/Cr³⁺ of chromium oxide dissolved in CaO–chromium oxide–Al₂O₃–SiO₂ melts have been determined by analysis of samples equilibrated at 1500°C under strongly reducing conditions ( p _o2= 10^−9.56 to 10^−12.50 atm). The majority of the chromium is divalent (Cr²⁺) under these conditions and Cr²⁺/Cr³⁺ ratios at given constant oxygen pressures decrease with increasing basicity of the melts, expressed as CaO/SiO₂ ratios. In addition, Cr²⁺/Cr³⁺ ratios, at a given CaO/SiO₂ ratio, are relatively unaffected by the amount of Al₂O₃ present.     

Formation Mechanism and Synthesis of Apatite-Type Structure Ba2+xLa8−x(SiO4)6O2−δ

The formation process of Ba₂La₈(SiO₄)₆O₂ was clarified using thermogravimetry–differential thermal analysis (TG-DTA) and a high-temperature powder X-ray diffraction (HT-XRD) method. Phase changes identified from the HT-XRD data surprisingly corresponded to the weight loss and/or endothermic peaks observed in the TG-DTA curves. Raw material with the composition Ba₂La₈(SiO₄)₆O₂ was completely reacted at 1400°C and produced only an apatite-type compound without a secondary phase. Moreover, the synthesis of Ba_{2+ x} La_{8− x} (SiO₄)₆O_2−δ crystals with x = 0–2 was attempted using a solid-state reaction.     

Effect of Bi2O3 Addition on the Sintering Temperature and Microwave Dielectric Properties of Zn2SiO4 Ceramics

Bi₂O₃ was added to a nominal composition of Zn_1.8SiO_3.8 (ZS) ceramics to decrease their sintering temperature. When the Bi₂O₃ content was <8.0 mol%, a porous microstructure with Bi₄(SiO₄)₃ and SiO₂ second phases was developed in the specimen sintered at 885°C. However, when the Bi₂O₃ content exceeded 8.0 mol%, a liquid phase, which formed during sintering at temperatures below 900°C, assisted the densification of the ZS ceramics. Good microwave dielectric properties of Q × f =12,600 GHz, ɛ_r=7.6, and τ_f=−22 ppm/°C were obtained from the specimen with 8.0 mol% Bi₂O₃ sintered at 885°C for 2 h.