Phase Equilibria in the System SrO-PbO-SiO2

Phase relations in the ternary system were determined by solid-state reaction, quenching, and electron microscopy of the immiscible liquid region. The limits of the stable 2-liquid region and the approximate limits of the metastable 2-liquid region were determined. Relations in the region of homogeneous glass formation in the system, which included the joins SrSiO₃-PbSiO₃, SrSiO₃-Pb₂SiO₄, and SrSiO₃-Pb₄SiO₆, were determined by quenching. The solid-solution limits of SrSiO₃ in Pb₂SiO₄ and Pb₄SiO₆ and of PbSiO₃ in SrSiO₃ were established. Two new compounds were discovered, 3SrO.2PbO.SiO₂ and 2SrO.3PbO.SiO₂, and extensive solid-state trials permitted the compatibility triangles to be established. The accumulated data were assembled in the form of an equilibrium diagram for the system, including joins, boundary lines, and isotherms.     

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.     

The System KAlSiO4–Mg2SiO4–KAlSi2O6

Schairer's study (1954) on phase relations in the system KalSi₂O₆–Mg₂SiO₄–SiO₂ was extended to include the system KalSiO₄–Mg₂SiO₄–KalSi₂O₆. It is shown that this join is ternary; however, the relatively high vapor pressure of the condensed phases prohibits study by the usual quenching techniques. The apparent intersection of the (KalSiO₄–Mg₂SiO₄–SiO₃) join with the primary phase volume of spinel is attributed to loss of the alkali-silicate constituents by vapor transport. This results in the effective bulk composition being moved away from this join toward the primary phase volume of spinel in the system K₂O–MgO–Al₂O₃–SiO₂.     

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.     

Phase Relations in the System Iron Oxide–NiO–SiO2 under Strongly Reducing Conditions

Liquidus and solidus phase relations have been determined for the system iron oxide–NiO–SiO₂ under strongly reducing conditions obtained by using CO₂–CO gas mixtures in controlled proportions. The phase relations were determined with the well-known quenching method: oxide mixtures were equilibrated in vertical tube resistance furnaces, followed by quenching to room temperature and identification of phases with transmitted- and reflected-light microscopy and X-ray diffraction. Three crystalline phases are present on the liquidus surface: olivine (Fe₂SiO₄–Ni₂SiO₄ solid solutions), oxide ("FeO"–NiO solid solutions), and silica (tridymite or cristobalite, depending on temperature). The "ternary peritectic" point where these three phases coexist with liquid is at 1571°C, with a liquid composition of approximately 19 wt%"FeO", 47 wt% NiO, 34 wt% SiO₂.     

Phase Equilibria in High-Alumina Part of the System CaO–MgO–Al2O3–SiO2

Results are presented of a study of phase equilibria among crystalline and liquid phases in the quaternary system CaO–MgO-Al₂O₃–SiO₂ at Al₂O₃ contents greater than 35%. Equilibrium diagrams shown are for the five triangular joins CaAl₂Si₂O₃-Ca₂Al₂SiO₇-MgAl₂O₄, Ca₂Al₂SiO₇-MgAl₂O₄-Al₂O₃, CaAl₂Si₂O₈-MgO-Al₂O₃, CaAl₂Si₂O₈-Mg₂SiO₄-MgAl₂O₄, and CaAl₂Si₂O₈-MgO-Mg₂SiO₄. The composition and nature of the four quaternary peritectic points and the relationships of univariant lines and primary phase volumes are discussed.     

Phase Equilibria in the System Li2O-CaO-SiO2

Phase relations in the system Li₂O-CaO-SiO₂ were studied by the quenching method. Four stable ternary compounds were found (Li₂Ca₃Si₆O_l6, Li₂Ca₄Si₄O₁₃, Li₂Ca₂Si₂O₇, and Li₂CaSiO₄) as well as phase Y , which is probably a metastable orthosilicate fairly close to Ca₂SiO₄ in composition. X-ray powder data are given for the new phases. Eleven subsolidus compatibility triangles and thirteen liquidus invariant points were located. Melting relations were determined for that part of the system bounded by Li₂SiO₃, Li₂CaSiO₄, Ca₂SiO₄, and SiO₂. The join Li₂SiO₃-CaSiO₃ is binary.     

Phase Relations in the System Si3N4-SiO2-MgO and Their Interrelation with Strength and Oxidation

Phase relation studies of Si₃N₁, SiO₂, and MgO have established three important subsolidus tie lines, viz. Si₃N₄-MgO, Si₃N₄-Mg₂SiO₄, and Si₂N₂O-Mg₂SiO₄ for nonoxidizing fabrication conditions. Strength measurements at 1400°C show that optimum strengths are obtained for compositions approaching the Si₃N₄-MgO and Si₃N₄-Si₂N₂O tie lines and that inferior strengths are obtained for compositions approaching the Si₃N₄-Mg₂SiO₄ tie line. Oxidation measurements at 1375°C show that the oxidation kinetics depend on the content of MgO and Mg₂SiO₄ phases. Optimum oxidation resistance is observed for compositions approaching the Si₃N₄-Si₂N₂O tie line. Strength and oxidation results are discussed with regard to phase equilibrium considerations.     

Raman and Infrared Spectroscopic Studies of the Crystalline Phases in the System Pb2SiO4-PbSiO3

The stable and metastable phase relations in the system Ph₂SiO₄-PhSiO₃, were reexamined using Raman and ir spectroscopy to characterize the phases. Three stable phases, H-Ph₂SiO₄, M-Ph₂SiO₄, and M-Ph₂SiO₄ (alamosite) and four metastable phases, L-Ph₂SiO₃, phase ^X, L-PhSiO₃ and hexagonal PhSiO₃, occur in the portion of the system investigated. The Raman and ir spectra allow some conclusions to be drawn about the structures of silicate groups in the lead silicate crystalline phases.     

Analytical Electron Microscopic Studies of Doped Dicalcium Silicates

Dicalcium silicates having CaO/SiO₂ molar ratios of 1.8 to 2.2 were sintered at 1450°C for 90 min with or without small quantities of dopants (K₂O or Al₂O₃) and were air quenched. The microstructures of the fired samples were characterized using electron microscopy (SEM and TEM) and associated microanalytical techniques. There was no evidence for the existence of Ca_1.8SiO_3.8 or Ca_2.2SiO_4.2. Amorphous grain-boundary phases were observed between grains and as inclusions within the grains; the amounts decreased as CaO/SiO₂ ratios increased. The compositions of the amorphous phases were always rich in dopants and had a CaO/SiO₂ ratio close to that of wollastonite. High levels of Al₂O₃ were observed to enter the β-Ca₂SiO₄ grains under lime-rich conditions (CaO/SiO₂= 2.2) up to a saturation level of about 3.0 wt%. Some additional crystalline phases were observed to form depending on stoichiometry and dopant level.     

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%).     

Phase Equilibria and Thermodynamics in the Al2O3–SiO2 System—Modeling of Mullite and Liquid

The Al₂O₃–SiO₂ system has been reassessed using a solution model for mullite extending from sillimanite to a hypothetical state of alumina. The property of sillimanite, to be used to describe one of the end-members, was extracted from an analysis of the T – P phase diagram for Al₂SiO₅ polymorphs. It was possible to represent the information on the range of stability of mullite, including some showing that mullite extends to higher SiO₂ contents than represented by the composition of 3:2 mullite. An attempt was made to model the liquid with the ionic two-sublattice model using a new species AlO₂⁻¹. The pressure dependence of Al₂SiO₅ polymorphs was optimized by a new model recently implemented in Thermo-Calc.     

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.     

Chromium-Doped Forsterite Nanoparticle Synthesis by Flame Spray Pyrolysis

Synthesis of chromium-doped forsterite (Mg₂SiO₄:Cr) nanoparticles by flame spray pyrolysis (FSP) was investigated. The morphology, crystalline phase, and photoluminescence of the products were evaluated. Crystalline Mg₂SiO₄:Cr nanoparticles of several 10 nm in diameter were obtained, although a small amount of the submicrometer-sized particles and the unreacted MgO phase existed. The product powder showed electron-spin resonance signals from Cr⁴⁺ and photoluminescence typical for Cr⁴⁺ in Mg₂SiO₄, suggesting that a part of the Cr⁴⁺ ions were incorporated into Si⁴⁺ sites by FSP. On the other hand, the effects of excess SiO₂ addition on the structural and optical characteristics of Mg₂SiO₄:Cr were examined. Addition of excess SiO₂ up to 20 mol% did not influence these characteristics of the products. Further addition of excess SiO₂ (60–100 mol%) enhanced the formation of the amorphous phase and resulted in the emission from Cr³⁺ in the amorphous phase in addition to an emission from Cr⁴⁺ in Mg₂SiO₄.     

Phase Equilibria in the System Lithium Metasilicate-Forsterite–Silica

The primary fields of cristobalite, tridymite, Li₂O·2SiO₂, Li₂O·SiO₂, MgSiO₃, Mg₂SiO₄, and a new compound, Li₂O·MgO·SiO₂, have been located for the system lithium metasilicate-forsterite–silica. Most of the primary field for the compound Li₂O·MgO·SiO₂ lies below the join Li₂O·SiO2-Mg₂SiO₄, but a small portion of it lies in the composition triangle Li₂SiO₃-Mg₂SiO₄-MgSiO₃. The positions of four invariant points and three composition triangles have been established.     

Phase Equilibria in the System Na2SiO3-Li2SiO3

Phase equilibria were studied for the system Na₂SiO_:rLi₂SiO₃. The 2 end-member metasilicates show limited mutual solid solubility with the (Na_2-χLi_χ)SiO₃ solid solutions being particularly extensive at solidus temperatures (0 x 1.06). Ordering of the latter solid solution occurs at the NaLiSiO_;) composition with a_supercellx= 6a_subcell During cooling of the solid solutions, metastable phase transformations occur; a twinned monoclinic metastable phase, low (Na, Li)₂SiO₃, has been characterized.     

Activities and Structure of Some Melts in the System Na2SiO3-Na2Si2O5

Different structural models for melts between Na₂SiO₃ and Na₂Si₂O₅ were tested by comparison of activities computed from the models with activities determined from the calorimetric heat of fusion. Data were used from the liquidus curve of the phase diagram for the system Na₂SiO₃-Na₂Si₂O₅ (Kracek and Morey and Bowen). A group model involving (SiO₃)_x^2x- rings or chains, with a random distribution of (SiO_2.5)₂ pairs which bridge between rings or chains, gives activities in good agreement with those determined from the calorimetric heat of fusion.     

Structural Study on PbO–SiO2 Glasses by X-Ray and Neutron Diffraction and 29Si MAS NMR Measurements

Structure of x PbO–(100− x )SiO₂ ( x =25–89) glasses has been investigated by means of the X-ray and neutron diffraction and ²⁹Si MAS NMR measurements. In the radial distribution functions of all the glasses, the Pb–O correlation was observed at 0.23 nm, indicating that the PbO₃ trigonal pyramids units do exist in the whole glass forming composition range. Furthermore the existence of the first Pb–Pb correlation at ∼0.385 nm in the whole composition range suggests that the basic structural unit is considered to be a Pb₂O₄ unit, which consists of the edge-shared PbO₃ trigonal pyramids. These results strongly imply that the Pb₂O₄ units participate in the glass network constructed by SiO₄ tetrahedra even at low PbO content. Differing from other lead-containing glass systems, these structural characteristics of Pb ions in the PbO–SiO₂ glass system are responsible for the extremely wide glass-forming region.     

Crystallization of BeO, Be2SiO4, and SiO2 from Li2MoO4-MoO3

Single crystals of phenacite (Be₂SiO₄), bromellite (BeO), and tridymite (SiO₂) were grown from an Li₂MoO₄-MoO₃ flux. Phenacite, with rhombohedral symmetry, grew in three distinct shapes with aspect ratios (length/width) as follows: needles (>3), rods (>1.1 to 1.5), and rhombohedral-faced crystals (=1). The latter grew as single crystals; the others were twinned on the     . For most experiments the temperature was held constant at 1165°C and the Li₂MoO₄/MoO₃ ratio at 1/16. The growth mechanism for crystallization was the evaporation of MoO₃. The system produced one to three phases, depending on the BeO/SiO₂ ratio. Bromellite grew until a BeO/SiO₂ ratio of 0.8 was attained. It grew as a hemipyramidal crystal having a short prism with a curved     top or as a hexagonal plate. The pyramid- and prism-shaped crystals were twinned, although a few hexagonal plates were single. Tridymite grew in small hexagonal plates when the BeO/SiO₂ ratio was less than 1.5. The effect of temperature, nucleation, and flux composition on crystal shape, twinning, and occurrence is discussed.     

Dilatometric and X-Ray Data for Lead Compounds, II

Linear thermal-expansion data are presented for PbAl₂O₄, Pb₂GeO₄, PbMoO₄, PbWO₄, the perovskite-type structures Pb₂MgWO₆, Pb₂FeTaO₆, and Pb₃Fe₂WO₉, and the apatite-type structures Pb₅(SiO₄)(VO₄)₂, Pb₅(GeO₄)(VO₄)₂, and Pb₅(GeO₄)(PO₄)₂. X-ray powder data are given, and the behavior of the compounds is correlated with previous information from the literature.