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.     

Machinable Ceramics Containing Rare-Earth Phosphates

Two-phase composites consisting of LaPO₄ or CePO₄ and alumina, mullite, or zirconia were found to be machinable; i.e., they can be cut and drilled using conventional tungsten carbide metal-working tools. Single-phase LaPO₄ was also machinable. Measurements of drilling rates, grinding rates, and normal forces are used to compare the ease of machining in these materials and in a conventional machinable glass-ceramic material, and to provide preliminary information on the relation between microstructure and machining properties. In Hertzian contact experiments these materials showed extensive nonlinear behavior associated with a damage zone beneath the contact site, similar to other machinable ceramics. Mechanisms of material removal are discussed.     

Dehydration Behavior of Aluminum Sulfate Hydrates

An exothermic transition is observed near 400°CC on thermal dehydration of highly crystalline AI₂(SO₄)3.16H₂O, Al₂(S0₄)₃ 14H₂O, and Al₂(S0₄)₃ 9H₂O when the early stages of heating are carried out in vacuum. Amorphous or partially crystalline hydrates do not show the exotherm. No systematic relation is apparent between the decomposition behavior and the pore volume distribution of the various anhydrous A1₂(SO₄)₃ products.     

Crystallization of a Tetrasilicic Fluormica Glass

The crystallization of a tetrasilicic synthetic mica glass of nominal composition K₂Mg₅Si₈O₂₀F₄ was investigated at selected temperatures from 560° to 1150°C using DTA, density measurements, electron microscopy, and X-ray diffraction techniques. Crystallization is shown to occur in 2 stages. (1) Initial structural ordering occurs at ∼600°C, as indicated by a strongly exothermic reaction, an increase in density, and the appearance of X-ray diffraction lines. (This transformation temperature is related to fluorine content.) (2) The crystal morphology of the glass-ceramic changes at 900° to 1150°C. A mechanism is proposed for the transformation from amorphous to crystalline structure in this synthetic mica system.     

Characterization of glass-ceramic corrosion and durability

The chemical durability of some selected glass-ceramic materials based on the Li₂O- (MnO, CaO)MgO---Al₂O₃---SiO₂ glass composition, has been determined by HCl, using the powder test.

The leachability of the glass-ceramic was gradually increased by replacing MgO with MnO, while it was decreased by addition of MnO instead of CaO. Calcium oxide had a retarding effect on the durability of the material when it was added instead of MgO and/or MnO. However, on the addition of Al₂O₃ in replacing Li₂O, the durability of the material was markedly improved. The leaching data were found to be dependent mainly on the proportion of the glass oxide constituents i.e. MgO/MnO, CaO/MnO and Li₂O/Al₂O₃ ratios present in the glasses.

The results were correlated to different views concerning the effect of various ions on the rate of interdiffusion between the crystalline-glass materials and leaching solution, the type, and proportions of the crystalline phases developed in glass-ceramics and residual glass phase.     

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.     

Microstructure of Mica-Based Nanocomposite Glass-Ceramics

A high-strength machinable glass-ceramic was prepared by controlling the crystallization of ZrO₂-containing Ca_0.43K_0.14Mg₃ (Si₃AlO₁₀)F₂ glass. A unique microstructure of the glass-ceramic was observed, using transmission electron microscopy (TEM): zirconia particles of nanometer size, 20–50 nm, were embedded in calcium mica crystal. After zirconia particles grew in nanometer size, the mica crystals were precipitated to embed the particles. The particles were identified as transformable tetragonal zirconia crystals. This texture leads to high strength (500 MPa in bending) and good machinability.     

Internal Friction in Lithium Aluminosilicate Glass-Ceramics

This paper describes the results of an investigation of the anelastic behavior of two glass-ceramic materials with similar compositions, one nucleated with TiO₂ and the other with ZrO₂. The influence of the grain size, residual glass fraction, and composition was examined. A low-frequency torsion pendulum was constructed for this purpose. The as-received TiO₂-nucleated. Corning Code 9608 ceramic exhibits an attenuation peak at about 750°C; when heat-treated at 1200°C for 167 h, the attenuation peaks at approximately 850°C. The ZrO nucleated glass-ceramic showed no such peak. Our results indicate that this anelastic behavior is due to grain boundary sliding, but the rates and magnitude of sliding are strongly dependent on boundary chemistry.     

Influence of a Ceramic Substrate on Aqueous Precipitation and Structural Evolution of Alumina Nano-Crystalline Coatings

Either boehmite (γ-AlOOH) or gibbsite (γ-Al(OH)₃) nanocrystalline thin films (h≈100 nm) can be precipitated from AlCl₃ solution at fixed pH and temperature onto different substrates. It depends on the nature of the substrate (mica flakes, SiO₂ flakes, or α-Al₂O₃ flakes), on their crystallographic properties (crystalline or amorphous), and on some experimental parameters (agitation rate, addition rate). According to the surface charge of the substrates, different alumina species are involved in the precipitation process. When negative charges are present on the substrate, the [Al₃O(OH)₃(OH₂)₉]⁴⁺ polycation is promoted, leading to the formation of the (Al₄) tetramer ([Al₄O(OH)₁₀(OH₂)₅]^o) and then to the precipitation of bohemite. When positive charges are present, a ligand bridge containing complex ([Al₃O(OH)₃(O₂H₃)₃(OH₂)₉]⁺) is likely favored, giving rise to hexagonal ring structures or amorphous solids that lead to the formation of gibbsite. Besides the surface effects, crystalline substrates can act as a template during precipitation of aluminum species as shown for the formation of gibbsite on muscovite. Finally, calcination at 850°C of boehmite samples leads to porous γ-Al₂O₃ layers, while calcination of gibbsite leads to δ-Al₂O₃ layers.     

Monopyroxenic Basalt-Based Glass-Ceramics

Uniform, ultrafine, microcrystalline, hard, pyroxenic glass-ceramic materials have been obtained successfully from basalt rock; instead of adding nucleation catalysts, the FeO:Fe₂O₃, CaO:Na₂O, and CaO:MgO ratios have been rectified. This process has been accomplished by deliberately adding the smallest permissible amounts of oxidizers, limestone, dolomite, and soda ash (as additives) that are necessary to fulfill the monominerality requirements; these requirements affect the melting, workability, crystallization, and microstructure of the glass-ceramics. The melting temperature decreases as the ratios decrease (beyond certain limits); in addition, the workability, crystallization, and microstructure also improve as the ratios decrease. An almost-stable solid solution of augite or aegirine-augite composition is the only crystalline phase that is formed. The minimal FeO:Fe₂O₃ ratio and the likelihood of a greater affinity of the Na⁺ cation for the Fe³⁺ cation, rather than the Al³⁺ cation, may be responsible for increasing the stability and widening of the crystallization field of the complex aluminum-bearing pyroxene solid solution.     

High-Strength Mica-Containing Glass-Ceramics

Glass-ceramics containing barium–mica in the system Ba_0.5 Mg₃ (Si₃AIO₁₀)F₂–2MgO · 2Al₂O₃· 5SiO₂–Ca₃ (PO₄)₂ are two to three times stronger than conventional mica-containing glass-ceramics. Moreover, the barium-mica glass-ceramics are easier to machine, as confirmed by a drilling test using conventional steel tools. Such mechanical properties are attributable to the microstructure of the barium–mica glass-ceramics. Very fine, interlocking mica crystals are precipitated in the glass, and a crack-deflection mechanism is observed by scanning electron microscopy.     

Compatibility of Hydrated Sodium MetasiIicate Glasses with Inorganic and Organic Compounds

By dehydrating crystalline Na₂SiO₃·9H₂O, a very low melting glass of the approximate composition Na₂SiO₃·3H₂O was obtained. The compatibility of selected inorganic compounds with this molten glass was found to be related to the nature of the cation or anion present in each compound. The behavior of organic indicator dyes in the glass melt provided further information on the nature of the molten glass, whereas the infrared spectrum of the glass gave an inconclusive result.     

Reaction of Vaporized Sodium Sulfate with Aluminous Refractories

A method was devised and tested whereby the action of an alkali salt, in the vapor or in the liquid state, on refractory materials may be studied under controlled conditions of temperature and pressure. Using Na₂SO₄, the progress of reaction was followed by (1) change of weight, (2) determination of SO₃ by precipitation, and (3) determination of Na₂O by flame photometer. The materials studied were a commercial-grade high-alumina brick (88% Al₂, 11 % SiO₂) * and fused crystalline alumina (99.5% Al₂O₃).
The dissociation of sodium sulfate, reacting in contact with high-alumina refractory materials, is speeded more by the presence of SiO₂, probably as the compound mullite, than by crystalline alpha-alumina. The ultimate product in the commercial brick is probably nephelite, NaAl-SiO₄; in the alumina, sodium alumina, NaAlO₄.
Exposure at 1200°C. in a vacuum of 150 μ of mercury approximately doubles the rate of deposition of sodium sulfate on the brick studied and adds one-third to the rate on fused alumina, as compared with the rate in air at atmospheric pressure.     

Crystallization Behavior of a High-Zinc-Content Li2O-ZnO0-SiO2 Glass-Ceramic and the Effect of K2O Additions

The crystallization behavior of an Li₂O-Zn0-SiO₂ glass with a ZnO content of −28.5 wt% and nucleated with P₂O₅ was investigated by infrared spectroscopy, X-ray diffraction, and scanning electron microscopy. A three-stage process, consisting of the nucleation, crystallization, and transformation of Li₃Zn_0.5Si0₄ and the emergence of a silica phase, occurs during heat treatment of this system. A small addition of K₂O varies the sequence of crystallization and the phase composition of the resulting glass-ceramic.     

Pentacoordinate Silicon Complexes as Precursors to Silicate Glasses and Ceramics

The recent discovery that pentacoordinate alkali glycolato silicates such as monomeric MSi(OCH₂CH₂O)₂OCH₂-CH₂OH or dimeric M₂Si₂(OCH₂CH₂O)s (where M = Li, Na, K, or Cs) can be synthesized directly from SiO₂, ethylene glycol, and MOH suggests that these compounds may serve as useful, inexpensive precursors to alkali silicate glasses or ceramics, either by sol–gel processing or by simple pyrolysis. We report here studies on the chemical changes and phase transformations that occur during the pyrolytic transformation of these compounds to ceramic and glassy materials. The evolutionary processes encountered as the materials are heated to selected temperatures were followed by X-ray powder diffraction, diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and thermal analysis techniques. In general, the crystalline compounds oxidatively decompose at temperatures above 300°C to form amorphous materials. At higher temperatures, the line compounds M₂Si₂O₅ begin to crystallize. During the transformation process, ligand oxidation forms CO₂ and H₂O, which react with the alkali metals to form small amounts of carbonates as seen by DRIFTS. At higher temperatures, the carbonates decompose with coincident formation of the primary crystalline phase, except in the case of the potassium compound, which exhibits some phase segregation.     

Devitrification Kinetics and Mechanism of K2O–CaO–SrO–BaO–B2O3–SiO2 Glass-Ceramic

The devitrification kinetics and mechanism of a low-dielectric, low-temperature, cofirable K₂O–CaO–SrO–BaO–B₂O₃–SiO₂ glass-ceramic have been investigated. Crystalline phases including cristobalite (SiO₂) and pseudowollastonite ((Ca,Ba,Sr) SiO₃) are formed during firing. Activation energy analysis shows that the nucleation of the crystalline phases is controlled by phase separation of the glass. The crystallization kinetics of both cristobalite and pseudowollastonite obey Avrami-like behavior, and the results show an apparent activation energy close to that of the diffusion of alkaline and alkali ions in the glass, suggesting that diffusion is rate limiting. The above conclusion is further supported by analysis of measured growth rates.     

Synthesis of Er3+ Doped Y2O3 Nanophosphors

The synthesis and characterization of yttrium hydroxyl carbonate (Y(OH)CO₃²⁻) and yttrium nitrate hydroxide hydrate (Y(OH)NO₃H₂O) precursor materials as well as Y₂O₃ nanoparticles are reported. The resultant precursor particle size is about 10–12 nm with a narrow size distribution by the enzymatic decomposition method, whereas the particle size was smaller than those acquired by the homogeneous and alkali precipitation methods. The formation of Y(OH)CO₃²⁻ and Y(OH)NO₃H₂O species was also evident from the fourier-transform infrared spectrometry (FT-IR) analysis. Precipitated Y(OH)CO₃²⁻ precursors have an amorphous nature whereas Y(OH)NO₃H₂O precursors have a crystalline nature, which was manifested from the XRD analysis. Moreover, precipitated Y(OH)NO₃H₂O precursors were found in the agglomerated form and Y(OH)CO₃²⁻ was established in the monodispersed form, as determined from the FE-SEM, TEM and DLS measurements. It was demonstrated that calcination of precursor materials at 900°C eventually removed the inorganic anions from the precursors and consequently produced crystalline Y₂O₃ nanoparticles, which was evident from the XRD and FT-IR analysis. The EDS analysis confirms Er³⁺ doping in the Y₂O₃ nanoparticles. The morphology and the size of the Y₂O₃ nanoparticles are almost unchanged before and after the calcination.     

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.     

Environment-Sensitive Hardness and Machinability of Al2O3

The influences of water, toluene, and n -alcohols on the pendulum hardness of Al₂O₃ monocrystals and on the ease of drilling of mono- and polycrystalline Al₂O₃ were studied. Adsorption-induced increases in pendulum hardness were produced by certain alcohol environments; these increases resulted in rates of drilling with diamond-studded core bits of up to 10 times those measured under water. A correlation between pendulum hardness, machinability, and surface charge (as indicated by ζ-potential measurements) was established and revealed that Al₂O₃ is hardest and most effectively drilled with a diamond core bit at its zero point of charge. The possible use of this correlation in developing more cost-effective and ecologically viable cutting fluids for the machining of Al₂O₃ is discussed.     

Reaction Sintering and Properties of Silicon Oxynitride Densified by Hot Isostatic Pressing

Silicon oxynitride ceramics were reaction sintered and fully densified by hot isostatic pressing in the temperature range 1700°C to 1950°C from an equimolar mixture of silicon nitride and silica powders without additives. Conversion to Si₂N₂O increases steeply from a level around 5% of the crystalline phases at 1700°C to 80% at 1800°C, and increases a few percent further at higher temperatures. α -Si₃N₄ is the major residual crystalline phase below 1900°C. The hardness level for materials containing 85% Si₂N₂O is approximately 19 GPa, comparable with the hardness of Si₃N₄ hot isostatically pressed with 2.5 wt% Y₂O₃, while the fracture toughness level is around 3.1 MPa. m^1/2, being approximately 0.8 MPa.m^1/2 lower. The three-point bending strength increased with HIP temperature from approximately 300 to 500 MPa.