Browsing by Author "Pannhorst, Wolfgang"

Now showing 1 - 9 of 9
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    Crystallization sequence of barium osumilite and MAS osumilite glass-ceramics with low thermal expansion
    (Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1993) Winter, Wolfgang; Bogdanow, Christina; Müller, Gerd; Pannhorst, Wolfgang
    Glass powders with compositions of the series BaMg2Al6Si9O30 (barium osumilite)-Mg2Al4Si11O30 were crystallized isothermally and with a constant heating rate schedule. The phase diagram of the series was constructed from equilibrium solid state reaction studies in order to distinguish stable from metastable crystallization products. Depending on the glass composition and the conditions of heat treatment different phases can crystallize. Metastable crystallization products are a high-quartz solid solution (MgAl2SixO4+2x). hexacelsian (BaAl2Si2O8) and MAS osumilite (Mg2Al4Si11O30). As accessories which can also be metastable with respect to the chosen conditions, cordierite, celsian and a spinel-Al2O3 solid solution appear in the products. BaO-rich glasses of the series crystallize easily to give a monophase barium osumilite glass-ceramic. Glass-ceramics which mainly consist of MAS osumilite can be prepared by crystallizing glasses of the series with low BaO content. Barium osumilite and especially MAS osumilite have very low thermal expansion coefficients: 2.6 ∙ 10^-6 and 1.7 ∙ 10^-6 K^-1 respectively, in the temperature range 25 to 1000 °C. The expansion behaviour of MAS osumilite is similar to that of the low-expansion phase cordierite.
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    Microstructural development of a P2O5-modified cordierite glass ceramic during sintering Part 1. Microscopic characterization
    (Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1991) Rudolph, Thomas; Szabó, Dorothée V.; Pannhorst, Wolfgang; Weisskopf, Karl-L.; Petzow, Günter
    The crystallization sequence of a P2O5-modified cordierite glass was investigated using conventional and analytical transmission electron microscopy, as well as transmission optical microscopy. The experiments show surface nucleation for both bulk and powdered glass specimens. The crystallization process leads to a preferred orientation of high-cordierite crystals via a metastable high-quartz solid solution. The hexagonal crystal lamellaes grow with their c axis perpendicular to the surface. A distinct dependence of the cordierite lamellae size on the crystallization temperature is observed. The residual glassy phase between the cordierite lamellae develops via a phase-separation process into a SiO2-rich amorphous phase and a polycrystalline matrix with magnesium orthophosphate as the major phase.
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    Microstructural development of a P2O5-modified cordierite glass ceramic during sintering Part 2. Densification experiments
    (Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1991) Rudolph, Thomas; Weisskopf, Karl-L.; Pannhorst, Wolfgang; Petzow, Günter
    The non-isothermal sintering of a P2O5-modified cordierite glass powder was investigated using sintering dilatometry, DTA and x-ray diffractometry. A typical densification curve as obtained by dilatometry with a constant heating rate is discussed. To clarify the dependence of the final degree of porosity on the heating rate additional porosity measurements were carried out on selected specimens using a stereological image processing system. The porosity exhibits a distinct minimum for a heating rate of 15 K/min. This effect is discussed based on a variation of the effective sintering interval which ranges from Tg of the glass to the onset temperature of crystallization. Together with the crystallization behaviour, as given in part 1 of this paper, a schematic description of the microstructural development is presented.
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    New Developments in the Field of Glass Ceramics
    (Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 2003) Pannhorst, Wolfgang
    [no abstract available]
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    Nucleation at cordierite glass surfaces: Kinetic aspects
    (Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1996) Müller, Ralf; Reinsch, Stefan; Pannhorst, Wolfgang
    The surface nucleation of high-quartz sohd Solution crystals at fractured surfaces of glasses of the stoichiometric cordierite composition (2MgO · 2AI₂O₃ · 5SiO₂) was studied by optical microscopy. Particular attention was focused on the nucleation kinetics. A constant nucleation density, Ν ≈ 10⁻⁴ μm⁻², was found not to be significantly influenced by the time and the temperature of nucleation treatment. Even a very fast heating of samples employing heating rates u p to 1200 K/min does not lower Ν substantially. However, for small average crystal diameters ( < 2 0 μm) a distribution of crystal size in the same order of magnitude is detectable indicating a simultaneous appearance of b o t h measurable nucleation rates and growth velocities. It can be concluded that the surface nucleation of µ-cordierite occurs during the thermal treatment from a limited number of preferred nucleation sites; these sites are "used Up" rapidly enough to cause a strong Saturation effect of nucleation, but slow enough to cause a crystal size distribution at the same time. The surface nucleation rate, Is , was calculated from the observed distribution of crystal sizes. Is progressively increases with rising temperature similar to the crystal growth velocity indicating a broad temperature ränge of essential nucleation activity. The latter must be regarded as t he main obstacle to measure or to control surface nucleation density by means of two-step nucleation and growth treatments and must therefore be claimed to be mainly responsible for the observed constancy of N.
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    Nucleation in a precursor glass for a Li2O-Al2O3-SiO2 glass ceramic Part 1. Nucleation kinetics
    (Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1987) Schiffner, Ulrich; Pannhorst, Wolfgang
    The nucleation of the parent glass of a Li2O-Al2O3-SiO2 glass ceramic is determined. The two-step development method is applied utilizing a specially developed device which allows quantitative determination of nucleation rates despite of the strong tendency for nucleation. The influence of nucleation on the transmission of the glass ceramic is investigated to obtain criteria for optimal nucleation conditions for ceramization of the glass. Nucleation is markedly characterized by non-stationary behaviour. The existence of delay times which depend as well on development temperature as on thermal history of the glass is of great importance for the processing of the glass. The impact of nucleation kinetics on the production of the glass ceramic is explained by means of TTT diagrams.
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    Nucleation in a precursor glass for a Li2O-Al2O3-SiO2 glass ceramic Part 2. Variation of the nucleating agent concentrations
    (Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1987) Schiffner, Ulrich; Pannhorst, Wolfgang
    The influence of nucleating oxide concentration on nucleation in a parent glass of a Li2O-Al2O3-SiO2 glass ceramic is investigated by means of a method described in part 1 of this paper. Furthermore it is investigated to which extent glasses with different concentrations of nucleating agents can be transformed into useable glass ceramics. It is shown that with decreasing concentration of the nucleating agents the nucleation process is slowed down. The data present a basis for improved processing. The study of properties of ceramized glasses with varied concentration of nucleating agents clearly indicates that a reduction in the nucleation tendency can be counteracted by changing the nucleation conditions during ceramizing.
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    Sintering and crystallization of glasses with roedderite-type composition (Na, K)2Mg5Si12O30
    (Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1992) Winter, Wolfgang; Müller, Gerd; Pannhorst, Wolfgang
    Two glass powders of roedderite-type composition (Na2Mg5Si12O30 and NaKMg5Si12O30) have been prepared for sintering and crystallization experiments. Glass powder of sodium roedderite composition sinters to a dense body after 15 to 60 min at 800 to 850 °C before crystallization. The sintered body crystallizes easily to give roedderite as the main phase. Sodium-potassium-roedderite glass powder can be sintered to a dense body, too, though the sintering rate is lower. This is due to the higher viscosity of the sodium-potassium-roedderite glass. The amount of crystallized roedderite is smaller in the sodium-potassium composition whereas the amount of residual glass is higher. This influences the thermal expansion of the compact, which is higher than in the sodium roedderite glass-ceramic. The thermal expansion coefficient of the latter is close to that of pure crystaline sodium roedderite.
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    Sintering of a crystalline cordierite/ZrO2 composite
    (Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1989) Semar, Wolfgang; Pannhorst, Wolfgang; Hare, Thomas M.; Palmour III, Hayne
    Dilatometry was used to investigate the sintering of a composite material with 70 wt% crystalline cordierite and 30 wt% 3Y-TZP. Using a precision digital dilatometer, results for constant rate of heating sintering and rate-controlled sintering are compared. In both cases dense bodies with homogeneous distributions of both components are obtained. Reactions between the two components are observed only in the case of sintering with constant rate of heating and low heating rates.