Inorganic glasses normally exhibit a network of interconnected, covalent-bonded, structural elements that has no long-range order. In silicate glasses, the network formers are based on SiO4 tetrahedra interconnected through oxygen atoms at the corners. Conventional wisdom implies that alkaline and alkaline-earth orthosilicate materials cannot be vitrified, because they do not contain sufficient network-forming SiO2 to establish the needed interconnectivity. We studied a bulk magnesium orthosilicate glass obtained by containerless melting and cooling. We found that the role of network former was largely taken on by corner and edge sharing of highly distorted, ionic Mg-O species that adopt 4-, 5-, and 6-coordination with oxygen. The results suggest that similar glassy phases may be found in the containerless environment of interstellar space.

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Abstract. Here we report the first synthesis of a forsterite (Mg2SiO4) composition glass as an essentially phase-pure bulk material. Under containerless conditions, with heterogeneous nucleation sites minimized, glass forms by cooling ca. 1 mm liquid Mg2SiO 4 droplets in oxygen at 700 K/s. 29Si NMR spectroscopic data indicate that the SiO n tetrahedra and MgO6 octahedra exist in a comer sharing arrangement in the glass, but upon crystallization the polyhedral units reorganize to form edge-sharing linkages. Transposed temperature drop calorimetry shows that the glass is 61.4 + 1.3 kJ/mol higher in enthalpy than the crystal.

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Glass Formation at the Limit of Insufficient Network Formers. -As revealed by XRD, neutron diffraction, and Raman spectroscopy on a bulk Mg 2 SiO 4 glass obtained by containerless melting and cooling, the role of network former is largely taken on by corner-and edge-sharing of highly distorted, ionic Mg-O species that adopt 4-, 5-, and 6-coordination with oxygen. The results suggest that similar glassy phases may be found in the containerless environment of interstellar space. -(KOHARA, S.; SUZUYA, K.; TAKEUCHI, K.; LOONG*, C.-K.; GRIMSDITCH, M.; WEBER, J. K. R.; TANGEMAN, J. A.; KEY, T. S.; Sci.

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The rheological properties of commercially available polycarbosilane, SMP-10, were analyzed as a function of temperature, to guide development of thermal treatment processes for the improved yield and functionality of polymer ceramic precursors. The curing onset temperature for SMP-10 was determined to be as low as 100°C for a heating rate of 1°C/min enabling a heat treatment process at 90°C, where low molecular weight oligomers volatilize from the liquid precursor prior to curing. By driving off the low molecular weight oligomers before fabrication of a composite, the mass yield of SMP-10, from a room temperature liquid state was increased from 77% to 83%. The development of B-staging processes, or a semicure of SMP-10, were also demonstrated. B-staging processes were then applied to polymer infiltration and pyrolysis processing and compared with traditional wet layup CMC processing. It was determined that B-staging processes did not adversely affect ceramic matrix composite fabrication. B-staged processing methods were determined to result in less waste, allow ply-by-ply control of matrix compositions, and enable time independent processing when compared to traditional wet layup processing methods. K E Y W O R D Sceramic matrix composites, microstructure, pre-ceramic polymers, silicon carbide

The volatilization of polycarbosilanes is important to the processing and performance of polymer infiltration and pyrolysis‐based ceramic matrix composites. Low molecular weight (MW) polycarbosilane is often present in preceramic polymers and enhances viscosity for the purpose of composite infiltration. Due to the volatility of low MW chains, a model was developed to semi‐empirically determine the MW distribution and then predict the mass yield and evolution of the MW distribution as a function of temperature and time for StarPCS™ SMP‐10. The enthalpy of vaporization, the temperature dependence of the enthalpy of vaporization, the temperature dependence of the normal boiling point and a representation of the molecular weight distribution were fit using a series of thermogravimetric measurements, involving isothermal holds on a particular batch of SMP‐10. Once calibrated for SMP‐10 in this fashion, the molecular weight distribution of different batches of SMP‐10 could be fit using a thermogravimetric measurement involving a reduced temperature‐time series. The model was then predictive of mass loss over time for temperatures below the onset of curing (>90°C). Understanding this volatilization enables improved SiC yield, reduced processing time and minimizing void/bubble formation.