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.     

DTA and X-Ray Analysis Study of Nucleation and Crystallization of MgO-Al2O3-SiO2 Glasses Containing ZrO2, TiO2, and CeO2

Crystallization sequences of glasses with compositions in the tridymite primary phase field of the MgO-Al₂O₃-SiO₂ system were studied by DTA, X-ray diffraction, and other techniques. Crystallization was catalyzed by the addition of 7 wt% of either ZrO₂ or TiO₂. Up to 10 wt% CeO₂ was also added to some glasses. Metastable solid solutions with the high-quartz structure exhibiting varying lattice parameters commonly occurred at low temperatures, transforming into a high cordierite at higher temperatures. Depending on the composition and heat treatment, other phases also appeared, e.g. Ce₂Ti₂O₄ (Si₂O₇). The rate of crystallization was markedly dependent on the catalyst. Colloidal precipitation of the catalyst accompanied by bulk crystallization of the glass was observed with ZrO₂, but no crystalline TiO₂ was detected. In the presence of CeO₂, TiO₂ was a more effective catalyst than ZrO₂. Although CeO₂ lowered the melting temperatures of the glass-ceramics, it increased the stability of the glasses and inhibited volume nucleation, causing coarse structures to form on crystallization.     

Crystallization of MgO-Al2O3-SiO2-ZrO2 Glasses

The crystallization of MgO-Al₂O₃-SiO₂-ZrO₂ glasses at 1000°C was studied. Isothermal heat treatments of a cordierite-based glass (2MgO.2Al₂O₃.5SiO₂= Mg₂Al₄Si₅O₁₈) with 7 wt% ZrO₂ produced surface crystallization of α-cordierite and tetragonal ZrO₂ ( t -ZrO₂). These phases advanced into the glass by cocrystallization of t -ZrO₂ rods in an α-cordierite matrix with a well-defined orientation relation. The t -ZrO₂ rods were unstable with respect to diffusional breakup (a Rayleigh instability) and decomposed into rows of aligned ellipsoidal and spheroidal particles. The t -ZrO₂ was very resistant to transformation to monoclinic symmetry. With a similar glass containing 15 wt% ZrO₂, surface crystallization of α-cordierite and t -ZrO₂ was accompanied by internal crystallization of t -ZrO₂ dendrites. Transformation of the dendrites to mono-clinic symmetry was observed under some conditions.     

Seeding Effects on Crystallization of KAlSi3O8, RbAlSi3O8, and CsAlSi3O8 Gels and Glasses

Crystallization of stoichiometric KAlSi₃O₈, RbAlSi₃O₈, and CsAlSi₃O₈ gels (single-phase and multiphasic gels) and glasses with and without seeding has been investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectrometry (EDS). The pure gels of K-, Rb-, and Cs-feldspar compositions crystallized mainly to leucite (KAlSi₂O₆), a Rb analogue of leucite (RbAlSi₂O₃), and pollucite (CsAlSi₂O₆), respectively. The KAlSi₃O₈ glass also yielded leucite. No feldspar phases crystallized in the temperature region studied. The leucite crystallization temperature was lowered by using multiphasic gels. The crystallization sequence of the KAlSi₃O₈ glasses and single-phase gels did not change upon seeding with isostructural feldspar crystals. No significant crystallization effects were detected with the use of RbAlSi₃O₈ and CsAlSi₃O₈ multiphasic gels. However, K-feldspar (sanidine) was crystallized by using both compositionally and structurally different gels of KAlSi₃O₈, i.e., multiphasic gels seeded With feldspar crystals. The microstructure of oriented sanidine crystals around the seeds indicated an epitaxial crystallization mechanism.     

Low-Fire Processing of ZrO2–SnO2–TiO2 Ceramics

Effects of a liquid-phase-sintering aid, BaCuO₂+ CuO (BCC), on densification and microwave dielectric properties of (Zr_0.8Sn_0.2)TiO₄ (ZST) ceramics have been investigated. The densification kinetics of ZST are greatly enhanced with the presence of 2.5–5 wt% BCC, but become retarded when the amount of BCC increases further. At a given BCC content, moreover, slower densification kinetics are observed with a larger particle size of ZST. The above results are attributed to a chemical reaction taking place at the interface of BCC/ZST during firing. The ZST dissolves into BCC, forming crystalline phases of ZrO₂, SnO₂, CuO, and BaTi₈O₁₆ which reduce the amount of BCC flux available for liquid-phase sintering. The crystallization kinetics become more significant, compared with densification kinetics, with increasing the amount of BCC and the particle sizes of ZST. For samples with 2.5–5 wt% BCC, a high relative sintered density is obtained at 1000°C and the resulting microwave ceramics have a dielectric constant and a value of Q at 7 GHz in the ranges of 35–38 and 2800–5000, respectively.     

Low Temperature Sintering of Zn3Nb2O8 Ceramics from Fine Powders

A possibility to produce microwave (MW) dielectric materials by liquid-phase sintering of fine particles was investigated. Zn₃Nb₂O₈ powders with a grain size 50–300 nm were obtained by the thermal decomposition of freeze-dried Zn–Nb hydroxides or frozen oxalate solutions. The crystallization of Zn₃Nb₂O₈ from amorphous decomposition products was often accompanied by the simultaneous formation of ZnNb₂O₆. Maximum sintering activity was observed for single-phase crystalline Zn₃Nb₂O₈ powders obtained at the lowest temperature. The sintering of as-obtained powders with CuO–V₂O₅ sintering aids results in producing MW dielectric ceramics with a density 93%–97% of the theoretical, and a Q × f product up to 36 000 GHz at sintering temperature ( T _s)≥680°C. The high level of MW dielectric properties of ceramics was ensured by intensive grain growth during the densification and the thermal processing of ceramics.     

Crystalline Phases and YAG Laser-Induced Crystallization in Sm2O3–Bi2O3–B2O3 Glasses

The glass formation region, crystalline phases, second harmonic (SH) generation, and Nd:yttrium aluminum garnet (YAG) laser-induced crystallization in the Sm₂O₃–Bi₂O₃–B₂O₃ system were clarified. The crystalline phases of Bi₄B₂O₉, Bi₃B₅O₁₂, BiBO₃, Sm _x Bi_{1− x} BO₃, and SmB₃O₆ were formed through the usual crystallization in an electric furnace. The crystallized glasses consisting of BiBO₃ and Sm _x Bi_{1− x} BO₃ showed SH generations. The formation of the nonlinear optical BiB₃O₆ phase was not confirmed. The formation (writing) region of crystal lines consisting of Sm _x Bi_{1− x} BO₃ by YAG laser irradiation was determined, in which Sm₂O₃ contents were∼10 mol%. The present study demonstrates that Sm₂O₃–Bi₂O₃–B₂O₃ glasses are promising materials for optical functional applications.     

Crystallization Kinetics and Mechanism of Low-Dielectric, Low-Temperature, Cofirable CaO-B2O3-SiO2 Glass-Ceramics

The crystallization kinetics and mechanism of low-dielectric, low-temperature, cofirable CaO-B₂O₃-SiO₂ glass-ceramics were investigated. Crystalline phases formed during firing included calcium silicates (CaSiO₃, Ca₃Si₂O₇, Ca₂SiO₄) and calcium borate (CaB₂O₄), with crystalline wollastonite (CaSiO₃) the major phase. The crystallization kinetics of wollastonite followed an Avrami equation. The results of the present study showed an apparent activation energy of 200-260 kJ/mol. Combined with the results of reduced growth rate (growth rate × viscosity) and thermal analysis, the rate-controlling mechanism of crystallization appeared to be a two-dimensional surface nucleation growth. As the amount of crystalline wollastonite increased, the dielectric constant decreased, but the thermal expansion coefficient remained relatively unchanged.     

Microstructure and Kinetics of Crystallization of MgO-Al2O3−Si02 Glass-Ceramics

Quantitative X-ray diffraction and microscopy were used to study the morphology development and overall crystallization rate between 900° and 990°C of MgO-Al₂O₃−SiO₂ glasses with added ZrO, TiO₂, CaF₂, or CeO₂. Three basic stages of micro-structural development were distinguishable: I, an induction period, II, a spherulitic crystallization stage, and III, a final crystallization stage. The duration of the induction period, the crystallization rate of the high-quartz solid solution, and the microstructures varied markedly with prior nucleation treatment and the type of modifier present in a glass of nearly equal silica content. The roles of major (high-quartz ss , high cordierite) and of minor crystalline and liquid phases in textural development are discussed, and it is postulated that nucleants (ZrO₂, TiO₂) act also as growth-modifying "impurities" in crystal growth.     

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.     

Microwave Dielectric Properties of Low-Fired Ba5Nb4O15

The effect of B₂O₃ on the sintering temperature and microwave dielectric properties of Ba₅Nb₄O₁₅ has been investigated using X-ray powder diffraction, scanning electron microscopy, and a network analyzer. Interactions between Ba₅Nb₄O₁₅ and B₂O₃ led to formation of second phases, BaNb₂O₆ and BaB₂O₄. The addition of B₂O₃ to Ba₅Nb₄O₁₅ resulted in lowering the sintering temperature from 1400° to 925°C. Low-fired Ba₅Nb₄O₁₅ could be interpreted by measuring changes in the quality factor ( Q × f ), the relative dielectric constant (ɛ_r), and the temperature coefficient of resonant frequency (τ_f) as a function of B₂O₃ additions. More importantly, the formation of BaNb₂O₆ provided temperature compensation. The microwave dielectric properties of low-fired Ba₅Nb₄O₁₅ had good dielectric properties: Q × f = 18700 GHz, ɛ_r= 39, and τ_f= 0 ppm/°C.     

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.     

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.     

Formation of Nepheline Glass-Ceramics Using Nb2O5 as a Nucleation Catalyst

The crystallization of several Nb₂O₅-catalyzed glasses in the Na₂O.Al₂O.SiO₂ system was studied using DTA, X-ray diffraction, and electron microscopy. The Nb₂O₅ was an effective nucleation catalyst; fine-grained body-nucleated glass-ceramic materials containing hexagonal nepheline and NaNbO₃ were obtained. The crystallization sequence and final crystalline phases in these compositions were quite different from those found in the equivalent TiO₂ analogs. The addition of small amounts of carbides and nitrides at the expense of oxides in the initial glass batch markedly affected the final crystalline phases.     

Phase Relations and Ordering in the Systems MgO-Y2O3-ZrO2 and CaO-MgO-ZrO2

The phase relations in the systems MgO-Y₂O₃-ZrO₂ and CaO-MgO-ZrO₂ were established at 1220° and 1420°C. The system MgO-Y₂O₃-ZrO₂ possesses a much-larger cubic ZrO₂ solid solution phase field than the system CaO-MgO-ZrO₂ at both temperatures. The ordered δ phase (Zr₃Y₄O₁₂) was found to be stable in the system ZrO₂-Y₂O₃ at 1220°C. Two ordered phases φ₁ (CaZr₄O₉) and φ₂ (Ca₆Zr₁₉O₄₄) were stable at 1220°C in the system ZrO₂-CaO. At 1420°C no ordered phase appears in either system, in agreement with the previously determined temperature limits of the stability for the δ, φ₁, and φ₂ phases. The existence of the compound Mg₃Y_zO₆ could not be confirmed.     

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.     

Microstructure and Dielectric Properties of a LaAlO3–TiO2 Diffusion Couple

Near-field scanning microwave microscopy was applied to investigate the dielectric properties and microstructure in a polycrystalline LaAlO₃–TiO₂ diffusion couple, which included three regions containing different phases and microstructures. Relatively low (La₂Ti₄Al₁₈O₃₈), high (α-La_2/3TiO₃), and intermediate (La₄Ti₉O₂₄) dielectric constant phases were distinguished at the inter-diffusion interface in optical, backscattered electron scanning electron microscopy, and scanning microwave microscopy (SMM) images. The relative ranking of dielectric constants based on SMM examination was as follows: TiO₂>α-La_2/3TiO₃>La₄Ti₉O₂₄>LaAlO₃>La₂Ti₄Al₁₈O₃₈. La_2/3TiO₃ and LaAlO₃ will form solid solutions in the LaAlO₃-rich region. The reaction paths leading to phase development are discussed.     

Rapid Crystallization of SiO2-Al2O3 Glasses

The crystallization of Al₂O₃-rich glasses in the system SiO₂-Al₂O₃ which were prepared by flame-spraying and/or splat-cooling was studied by DTA, electron microscopy, and X-ray diffraction. Over a wide range of compositions, the crystallization temperature ( T_x ) remained near 1000°C, changing smoothly with composition. In all cases crystallization of mullite was detected by X-ray diffraction. In the low-Al₂O₃ region, coarsening of the microstructure during crystallization was observed by electron microscopy. In the high-Al₂O₃ region mullite and γ-Al₂O₃ cocrystallized; this behavior may be interpreted as evidence of a cooperative process of crystallization at the respective T_x 's. The crystallite size of the mullite immediately after rapid crystallization increased continuously with increasing Al₂O₃ content. In light of the T_x data, the adequacy of the evidence for the proposed metastable miscibility gap in the SiO₂-Al₂O₃ system is questioned.     

Preparation of Ba6−3xNd8+2xTi18O54 via Ethylenediaminetetraacetic Acid Precursor

Ba_{6−3 x} Nd_{8+2 x} Ti₁₈O₅₄ ceramic powders were synthesized by the modified Pechini method using ethylenediaminetetraacetic acid (EDTA) as a chelating agent. A purplish red, molecular-level, homogeneously mixed gel was prepared, and transferred into a porous resin intermediate through charring. Single-phase and well-crystallized Ba_{6−3 x} Nd_{8+2 x} Ti₁₈O₅₄ powders were obtained from pulverized resin at a temperature of 900°C for 3 h, without formation of any intermediate phases. Meanwhile, the molar ratio of EDTA to total metal cation concentration had a significant influence on the crystallization behavior of Ba_{6−3 x} Nd_{8+2 x} Ti₁₈O₅₄. The Ba_{6−3 x} Nd_{8+2 x} Ti₁₈O₅₄ ( x = 2/3) ceramics prepared via EDTA precursor have excellent microwave dielectric characteristics: ɛ= 87, Qf = 8710 GHz.     

Crystallization of MgO-CaO-SiO2-P2O5 Glass

The crystallization behavior of a glass with a composition of 40 wt% 3CaO · P₂O₅−60 wt% CaO · MgO · 2SiO₂ was investigated. The primary crystalline phase was apatite with a dendritic form and ellipsoidal shape. β-(3CaO · P₂O₅) and CaO · MgO · 2SiO₂ were crystallized as samples heated to 990°C, and a three-layer structure was obtained. The development and morphology of this construction were explained by both the surface crystallization of the apatite and CaO · MgO · 2SiO₂ and the bulk crystallization of apatite and the CaO · MgO · 2SiO₂-β-(3CaO · P₂O₅) composite.