John F. MacDowell

Bio: John F. MacDowell is an academic researcher from Corning Inc.. The author has contributed to research in topics: Glass-ceramic & Ceramic. The author has an hindex of 17, co-authored 31 publications receiving 902 citations.

Immiscibility and Crystallization in A12O3‐SiO2 Glasses

Abstract: Metastable glass-in-glass separation was observed on rapid quenching of A12O3-SiO2 melts containing from 10 to 50 mol% A12O3. Nucleation and subsequent crystallization of mullite within the high-alumina-dispersed glass phase may occur either during cooling from the melt or on reheating. The metastable binary two-liquid region is compositionally defined, and a structural interpretation of the phase separation and the effects of small oxide additions on it is offered. Evidence for both classical and possible spinodal nucleation mechanisms during liquid segregation in this system is presented.

Glass laminated bodies comprising a tensilely stressed core and a compressively stressed surface layer fused thereto

Abstract: This invention relates to sound, high strength, laminated articles of glass, glass-ceramic, glass and glass-ceramic materials. Such articles are made by means of a continuous hot-forming process wherein glasses are melted for the individual layers and these layers are then simultaneously fused together and shaped into a laminated structure of a desired configuration. Where a glass-ceramic article is desired, the laminated glass structure is subsequently heat treated in a particular manner to cause the glass to crystallize in situ.

Crystallization and Chemical Strengthening of Nepheline Glass-Ceramics

Abstract: Electron micrographic and X-ray diffraction techniques were used to study the nucleation and growth of titania-nucleated nepheline in glass. On heating, the glasses phase-separated as a prelude to the crystallization sequence. The first crystalline phase identified was the metastable phase, carnegieite. With time, the equilibrium phase, nepheline, crystallized and the titania crystallized to anatase. The resulting materials were nonporous and largely crystalline. These nepheline glass-ceramics were chemically strengthened by treatments in molten potassium salts. A K+→ Na+ exchange took place and effected transformation of nepheline to kalsilite. This transformation was a function not only of the exchange treatment, but of the composition of the initial nepheline crystals. Confining the kalsilite surface against a volume increase during the phase transformation created a surface compressive stress. Glass-ceramics containing nepheline crystals with an appropriate structure were chemically strengthened in this manner to yield bulk, abraded modulus of rupture values above 200,000 psi.

Aluminoborate Glass‐Ceramics with Low Thermal Expansivity

Abstract: The synthesis and properties of calcium, strontium, and barium aluminoborate glass-ceramics are described. Glass powder compacts near the RO-Al 2 O 3 -B 2 O 3 stoichiometry were sintered and crystallized below 1000°C to form dense glass-ceramics containing RAl 2 B 2 O 7 as the principal phase. Thermal expansion coefficients as low as 6.10 -7 /°C, 15.10 -7 /°C, and 23.10 -7 /°C were measured on SrAl 2 B 2 O 7 , CaAl 2 B 2 O 7 , and BaAl 2 B 2 O 7 glass-ceramics, respectively. Two forms of the ultra-low-expansion SrAl 2 B 2 O 7 were not previously reported in the literature and were indexed as cubic and hexagonal crystals. The unique radial spherulitic interbladed microstructures and other important properties of these glass-generated ceramics are also revealed

Coated refractory article and method

Abstract: of the Disclosure For protection against oxidation or other adverse chemical deterioration, substrates such as carbon, inter-metallic aluminides, and refractory metal alloys are provided with barrier coatings of an aluminoborate glass-ceramic coating having a composition, in weight percent, of about 10-55% Al2O3, 4-40% B2O3, up to 70% total of RO, wherein RO consists of one or more alkaline earth metal oxides selected in amounts not exceeding the indicated proportions from the group consisting of up to 70% BaO, up to 60% SrO, up to 40% CaO, and up to 25% MgO, up to 35%total of R2O, wherein R2O consists of one or more alkali metal oxides selected in amounts not exceeding the indicated proportion from the group consisting of up to 35% Na2O, up to 30% K2O, and up to 15% Li2O, up to 25% total of metal oxides selected from the group consisting of ZnO and MnO, up to 30% of SiO2, and up to 10% F.

Nanophase glass-ceramics

Abstract: Future applications for glass-ceramics are likely to capitalize on designed-in, highly specialized properties for the transmission, display, and storage of information. Glass-ceramics with microstructures comprised of uniformly dispersed crystals <100 nm in size offer promise for many potential new applications as well as provide unique attributes for many current products. This paper focuses on two types of nanocrystalline glass-ceramics: transparent glass-ceramics and tough, high-modulus glass-ceramics with precisely engineered surfaces. Transparent glass-ceramics are formed from certain aluminosilicate glasses capable of efficient crystal nucleation and slow growth. The key crystalline phases include β-quartz solid solutions, characterized by low-thermal-expansion behavior; spinel, with high hardness and elastic modulus; and mullite, which shows unique chromium-luminescence behavior.

Stable and Metastable Equilibria in the System SiO2-Al2O3

Abstract: Concentration profiles of Al/sub 2/O/sub 3/ in diffusion couples made from sapphire and fused silica were used to determine the stable equilibrium phase diagram of the system SiO/sub 2/--Al/sub 2/O/sub 3/. The intermediate compound mullite, 3Al/sub 2/O/sub 3/.2SiO/sub 2/, melts incongruently at 1828 +- 10/sup 0/C; its stable solid-solution region ranges from 70.5 to 74.0 wt percent Al/sub 2/O/sub 3/ below 1753/sup 0/C and from 71.6 to 74.0 wt percent at 1813/sup 0/C. The microstructures of diffusion zones and heat-treated specimens also indicate the incongruency of mullite. Additional information is given for 3 metastable systems: SiO/sub 2/-Al/sub 2/O/sub 3/ in the absence of mullite, SiO/sub 2/-ordered-mullite in the absence of alumina, and SiO/sub 2/-disordered mullite in the absence of alumina. Under metastable conditions, ordered mullite melts congruently at approximately 1880/sup 0/C and its solid-solution range extends up to approximately 77 wt percent Al/sub 2/O/sub 3/. The solid-solution range of disordered mullite extends to approximately equal to 83 wt percent Al/sub 2/O/sub 3/ with an estimated congruent melting temperature of approximately 1900/sup 0/C. The existence of metastable systems is associated with superheating of mullite above the incongruent melting temperature and with nucleation of alumina and mullite in supercooled aluminum-silicate liquids. (auth)

Oxidation Kinetics of Silicon Carbide Crystals and Ceramics: I, In Dry Oxygen

Abstract: The oxidation kinetics of several single-crystal and polvcrystalline silicon carbide materials and single-crystal silicon in dry oxygen over the temperature range 1200° to 1500°C were fitted to the linear-parabolic model of Deal and Grove. The lower oxidation rates of silicon carbide compared to silicon can be rationalized by additional consumption of oxidant in oxidizing carbon to carbon dioxide. The (000J) Si face of the silicon carbide platelets exhibited lower parabolic oxidation rates than the (0001) C face, by a factor of 10 at 1200°C. Apparent activation energies increased from a value of ∼120 kJ/mol below 1400°C to a value of ∼300 kJ/mol above this temperature. The (0001) Si face exhibited this high activation energy over the entire temperature range. The controlled nucleation thermally deposited material exhibited the highest oxidation rates of the polycrystalline materials followed by the hot-pressed and sintered α-silicon carbides. In general, the oxidation rates of the polycrystalline materials were bracketed by the oxidation rates of the basal planes of the single-crystal materials. Higher impurity concentrations and higher density of nucleation sites led to a greater susceptibility to crystallization of the scale which significantly complicated the oxidation behaviors observed. When crystallization of the oxide scale occurred in the form of a layer of spherulitic cristobalite crystals, a retardation of the oxidation rates was observed. An accelerated oxidation behavior was found when this coherent layer was superseded by the formation of fine mullite crystals.

Critical evaluation and optimization of the thermodynamic properties and phase diagrams of the CaO-Al 2 O 3 , Al 2 O 3 -SiO 2 , and CaO-Al 2 O 3 -SiO 2 systems

Abstract: All available thermodynamic and phase diagram data have been critically assessed for all phases in the CaO-Al2O3, Al2O3-SiO2, and CaO-Al2O3-SiO2 systems at 1 bar pressure from 298 K to above the liquidus temperatures. All reliable data for the binary systems have been simultaneously optimized to obtain, for each system, one set of model equations for the Gibbs energy of the liquid slag and all solid phases as functions of composition and temperature. The modified quasichemical model was used for the slag. With these binary parameters and those from the optimization of the CaO-SiO2 system reported previously, the quasichemical model was used to predict the thermodynamic properties of the ternary slag. Two additional small ternary parameters were required to reproduce the ternary phase diagram and ternary activity data to within experimental error limits. The calculated optimized phase diagram and thermodynamic properties are self-consistent and are the most reliable currently available estimates of the true values.