Three Planes in the Quaternary System CaO-ZnO-Al2O3-SiO2

Equilibrium relations in part of the quaternary system CaO-ZnO-Al₂O₃-SiO₂ have been investigated by the quenching method. Liquidus surfaces and primary-phase fields have been determined in planes containing respectively 8, 10, and 15% A1₂O₃, as well as six quaternary invariant points. The minimum melting point in the system is 1030°± 5°C at a composition of 18.5% CaO, 22.5% ZnO, 8.2% A1₂O₃, and 50.8% SiO₂,. At this point the condensed phases in equilibrium are tridymite, willemite, melilite, anorthite, and liquid. The results have a bearing on reactions occurring in lead blast-furnace slags, in some ceramic glazes, and in certain zinc-bearing rocks and ores.     

Mullite Crystallization from SiO2-Al2O3 Melts

SiO₂-Al₂O₃ melts containing 42 and 60 wt% A1₂O₃ were homogenized at 2090°C (∼10°) and crystallized by various heat treatment schedules in sealed molybdenum crucibles. Mullite containing ∼78 wt% A1₂O₃ precipitated from the 60 wt% A1₂O₃ melts at ∼1325°± 20°C, which is the boundary of a previously calculated liquid miscibility gap. When the homogenized melts were heat-treated within this gap, the A1₂O₃ in the mullite decreased with a corresponding increase in the Al₂O₃ content of the glass. A similar decrease of Al₂O₃ in mullite was observed when crystallized melts were reheated at 1725°± 10°C; the lowest A1₂O₃ content (∼73.5 wt%) was in melts that were reheated for 110 h. All melts indicated that the composition of the precipitating mullite was sensitive to the heat treatment of the melts.     

Glass-Ceramic Formation in the ZnO-P2O5 System and the Effect of Silica as a Nucleating Agent

Crystallization of a series of ZnO-P₂O₅ based glasses was investigated. ZnO-P₂O₅-CaO glasses could be converted most readily to glass-ceramics and crystallization of these led to formation of alpha-Zn₂P₂O₇, alpha-CaZn₂(PO₄)₂, and ß-CaZn₂(PO₄)₂ phases. A further phase has been tentatively identified as monoclinic (Zn,Ca)₂P₂O₇. The most promising glass-ceramic composition Z15 (59.4ZnO·33P₂O₅·6.6CaO·1SiO₂) crystallized to alpha-Zn₂P₂O₇ and ß-CaZn₂(PO₄)₂, the latter phase being stabilized by the presence of SiO₂ which also encouraged volume nucleation giving a fine-scale (submicrometer) microstructure.     

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%).     

The System Al2O3-Cr2O3-Sio2

Liquidus phase equilibrium data are presented for the system Al₂O₃-Cr₂O₃-SiO₂. The liquidus diagram is dominated by a large, high-temperature, two-liquid region overlying the primary phase field of corundum solid solution. Other important features are a narrow field for mullite solid solution, a very small cristobalite field, and a ternary eutectic at 1580°C. The eutectic liquid (6Al₂O₃-ICr₂O₃-93SiO₂) coexists with a mullite solid solution (61Al₂O₃-10Cr₂O₃-29SiO₂), a corundum solid solution (19Al₂O₃-81Cr₂O₃), and cristobalite (SO₂). Diagrams are presented to show courses of fractional crystallization, courses of equilibrium crystallization, and phase relations on isothermal planes at 1800°, 1700°, and 1575°C. Tie lines were sketched to indicate the composition of coexisting mullite and corundum solid solution phases.     

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.     

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.     

Fabrication of Transparent, Sintered Sc2O3 Ceramics

We report here the fabrication of transparent Sc₂O₃ ceramics via vacuum sintering. The starting Sc₂O₃ powders are pyrolyzed from a basic sulfate precursor (Sc(OH)_2.6(SO₄)_0.2·H₂O) precipitated from scandium sulfate solution with hexamethylenetetramine as the precipitant. Thermal decomposition behavior of the precursor is studied via differential thermal analysis/thermogravimetry, Fourier transform infrared spectroscopy, X-ray diffractometry, and elemental analysis. Sinterability of the Sc₂O₃ powders is studied via dilatometry. Microstructure evolution of the ceramic during sintering is investigated via field emission scanning electron microscopy. The best calcination temperature for the precursor is 1100°C, at which the resultant Sc₂O₃ powder is ultrafine (∼85 nm), well dispersed, and almost free from residual sulfur contamination. With this reactive powder, transparent Sc₂O₃ ceramics having an average grain size of ∼9 μm and showing a visible wavelength transmittance of ∼60–62% (∼76% of that of Sc₂O₃ single crystal) have been fabricated via vacuum sintering at a relatively low temperature of 1700°C for 4 h.     

Synthesis of ZrO2 and Y2O3-Doped ZrO2 Thin Films Using Self-Assembled Monolayers

Undoped or Y₂O₃-doped ZrO₂ thin films were deposited on self-assembled monolayers (SAMs) with either sulfonate or methyl terminal functionalities on single-crystal silicon substrates. The undoped films were formed by enhanced hydrolysis of zirconium sulfate (Zr(SO₄)·4H₄O) solutions in the presence of HCl at 70°C. Typically, these films were a mixture of two phases: nanocrystalline tetragonal- ( t -) ZrO₂ and an amorphous basic zirconium sulfate. However, films with little or no amorphous material could be produced. The mechanism of film formation and the growth kinetics have been explained through a coagulation model involving homogeneous nucleation, particle adhesion, and aggregation onto the substrate. Annealing of these films at 500°C led to complete crystallization to t -ZrO₂. Amorphous Y₂O₃-containing ZrO₂ films were prepared from a precursor solution containing zirconium sulfate, yttrium sulfate (Y₂(SO₄)₃8·H₂O), and urea (NH₂CONH₂) at pH 2.2–3.0 at 80°C. These films also were fully crystalline after annealing at 500°C.     

Proton Conductivity in Zr4+-Ion-Doped P2O5–SiO2 Porous Glasses

The effect of zirconium ions on glass structure and proton conductivity was investigated for sol-gel-derived P₂O₅–SiO₂ glasses. Porous glasses were prepared through hydrolysis of PO(OCH₃)₃, Zr(OC₄H₉)₄, and Si(OC₂H₅)₄. Chemical bonding of the P⁵⁺ ions was characterized using ³¹P-NMR spectra. The phosphorous ions, occurring as PO(OH)₃ in the ZrO₂-free glass, were polymerized with one or two bridging oxygen ions per PO₄ unit with increased ZrO₂ content. The chemical stability of these glasses was increased significantly on the addition of ZrO₂, but the conductivity gradually decreased from 26 to 12 mS/cm at room temperature for 10P₂O₅·7ZrO₂·83SiO₂ glass. A fuel cell was constructed using 10P₂O₅·5ZrO₂·85SiO₂ glass as the electrolyte; a power of ∼4.5 mW/cm² was attained.     

Sc2O3 Nanopowders via Hydroxyl Precipitation: Effects of Sulfate Ions on Powder Properties

Hydroxyl-type Sc₂O₃ precursors have been synthesized via precipitation at 80°C with hexamethylenetetramine as the precipitant. The effects of starting salts (scandium nitrate and sulfate) on powder properties are investigated. Characterizations of the powders are achieved by elemental analysis, X-ray diffractometry (XRD), differential thermal analysis/thermogravimetry (DTA/TG), high-resolution scanning electron microscopy (HRSEM), and Brunauer-Emmett-Teller (BET) analysis. Hard-aggregated precursors (γ-ScOOH·0.6H₂O) are formed with scandium nitrate, which convert to Sc₂O₃ at temperatures ≥400°C, yielding nanocrystalline oxides of low surface area. The use of sulfate leads to a loosely agglomerated basic sulfate powder having an approximate composition of Sc(OH)_2.6(SO₄)_0.2·H₂O. The powder transforms to Sc₂O₃ via dehydroxylization and desulfurization at temperatures up to 1000°C. Well-dispersed Sc₂O₃ nanopowders (∼64.3 nm) of high purity have been obtained by calcining the basic sulfate at 1000°C for 4 h. The effects of SO₄²⁻ on powder properties are discussed.     

Properties of Sm2O3─Al2O3─SiO2 Glasses for In Vivo Applications

The compositional range for glass formation below 1600°C in the Sm₂O₃─Al₂O₃─SiO₂ system is (9–25)Sm₂O₃─(10–35)Al₂O₃─(40–75)SiO₂ (mol%). Selected properties of the Sm₂O₃─Al₂O₃─SiO₂ (SmAS) glasses were evaluated as a function of composition. The density, refractive index, microhardness, and thermal expansion coefficient increased as the Sm₂O₃ content increased from 9 to 25 mol%, the values exceeding those for fused silica. The dissolution rate in 1 N HCl and in deionized water increased with increasing Sm₂O₃ content and with increasing temperature to 70°C. The transformation temperature ( T _g ) and dilatometric softening temperature ( T _d ) of the SmAS glasses exceeded 800° and 850°C, respectively.     

Influence of SO3 on the Phase Relationship in the System CaO–SiO2–Al2O3–Fe2O3

The influence of the additive SO₃ on the phase relationships in the quaternary system CaO-SiO₂-Al₂O₃-Fe₂O₃ was investigated by observing the change of volume ratio of 3CaOSiO₂ (C₃S) to 2CaOSiO₂ (C₂S) + CaO (C) in the sintered material with the increase of SO₃ content. The primary phase volume of C₃S in the quaternary phase diagram shrank with the increase of SO₃ and disappeared when the SO₃ content exceeded 2.6 wt% in the sintered material. Changes in the peritectic reaction relationship between CaO (C), 2CaOSiO₂ (C₂S), 3CaOSiO₂ (C₃S), 3CaOAl₂O₃ (C₃A), 4CaOAl₂O₃Fe₂O₃ (C₄AF), and liquid were also observed and discussed.     

Indirect Dissolution of (Al, Cr)2O3 in CaO—MgO—Al2O3—SiO2 (CMAS) Melts

The dissolution of (Al, Cr)₂O₃ into CaO—MgO—Al₂O₃—SiO₂ melts, under static and forced-convective conditions was investigated at 1550°C in air. With sufficient MgO in the melt, or sufficient Cr₂O₃ in (Al, Cr)₂O₃, a layer consisting of a spinel solid solution, Mg(Al, Cr)₂O₄, formed at the (Al, Cr)₂O₃/melt interface. The dissolution kinetics of 1.5 and 10 wt% Cr₂O₃ specimens were determined as a function of immersion time, specimen rotation rate, and magnesia content of the melt. Electron microprobe analysis was used to characterize concentration gradients in the (Al, Cr)₂O₃ sample, the Mg(Al, Cr)₂O₄ spinel, or in the melt after immersion of specimens containing 1.5 to 78 mol% Cr₂O₃. The dissolution kinetics and microprobe analyses indicated that a steady-state condition was reached during forced-convective, indirect (Al, Cr)₂O₃ dissolution such that spinel layer formation was rate limited by solid-state diffusion through the spinel layer and/or through the specimen, and spinel layer dissolution was rate limited by liquid-phase diffusion through a boundary layer in the melt. This is consistent with a model previously developed for the indirect dissolution of sapphire in CMAS melts.     

The System SiO2–P2O2

Phase relations in the binary system between SiO₂-P₂O₅ and SiO₂ were investigated by the quenching method using sealed platinum tubes to prevent the loss of P₂O₅. The compound Si0₂-P₂O₅ exists in two forms, the low-temperature β form inverting sluggishly but reversibly to the high-temperature β form at 1030°C. The β form melts congruently at 1290°C. The compound 2SiO₂-P₂O₅ melts incongruently at 1120°C to a silica-rich liquid and SiO_a-P₂O₅. In the region between 5 and 25 mole % PO₂, reactions were so sluggish that no data could be obtained by quenching.     

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.     

Microstructure and Properties of Co2+: ZnAl2O4/SiO2 Nanocomposite Glasses Prepared by Sol–Gel Method

Transparent bulk Co²⁺: ZnAl₂O₄/SiO₂ nanocomposites containing nanocrystalline Co²⁺: ZnAl₂O₄ dispersed in silica glass matrix were obtained by the sol–gel method. The gels of composition 89SiO₂–6Al₂O₃–5ZnO− x CoO ( x =0.2, 0.4, 0.6, 0.8, 1.0) (mol%) were prepared at room temperature by using two different aluminum salts, aluminum nitrate and aluminum alkoxide (aluminum-iso-propoxide, Al(OPrⁱ)₃), as starting materials. The transparent gels were converted to the crystalline phase of gahnite by heating above 900°C. The microstructural evolution of gels was characterized. The effect of Co²⁺ concentration on spectroscopic properties was also discussed. Co²⁺: ZnAl₂O₄ nanocrystals dispersed in the SiO₂-based glass are formed at lower heat-treatment temperature and shorter heating time by using Al(OPrⁱ)₃ as raw material.     

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.     

Preparation of NiAl2O4/SiO2 and Co2+-Doped NiAl2O4/SiO2 Nanocomposites by the Sol–Gel Route

NiAl₂O₄/SiO₂ and Co²⁺-doped NiAl₂O₄/SiO₂ nanocomposite materials of compositions 5% NiO – 6% Al₂O₃– 89% SiO₂ and 0.2% CoO – 4.8% NiO – 6% Al₂O₃– 89% SiO₂, respectively, were prepared by a sol–gel process. NiAl₂O₄ and cobalt-doped NiAl₂O₄ nanocrystals were grown in a SiO₂ amorphous matrix at around 1073 K by heating the dried gels from 333 to 1173 K at the rate of 1 K/min. The formations of NiAl₂O₄ and cobalt-doped NiAl₂O₄ nanocrystals in SiO₂ amorphous matrix were confirmed through X-ray powder diffraction, Fourier transform infrared spectroscopy, differential scanning calorimeter, transmission electron microscopy (TEM), and optical absorption spectroscopy techniques. The TEM images revealed the uniform distribution of NiAl₂O₄ and cobalt-doped NiAl₂O₄ nanocrystals in the amorphous SiO₂ matrix and the size was found to be ∼5–8 nm.