Several boron-modified polysilazanes of general type {B[C2H4Si(R)NH]3}
n
(C2H4 = CHCH3
or CH2CH2) were synthesized and their thermal behavior studied. In contrast to the known
derivatives with R = alkyl or aryl, we describe ceramic precursors in which the bulky moieties
R are substituted with lower weight groups and/or reactive entities. Reactive units enable
further cross-linking of the polymeric framework and therefore minimize depolymerization
during ceramization. The polymer-to-ceramic conversion of all synthesized polymers was
monitored by thermogravimetric analysis. Both low molecular weight substituents and/or
cross-linking units increase the ceramic yield from 50% (R = CH3) to 83−88%. High-temperature thermogravimetric analysis in an inert gas atmosphere indicates the ceramics
obtained are stable up to ∼2000 °C. XRD studies of the fully amorphous materials point out
that, with increasing temperature, formation of α-SiC or α-SiC/β-Si3N4 crystalline phases
occurs at 1550−1750 °C, depending on the material's composition. The resistance of these
novel materials toward oxidative attack was investigated by TGA in air up to 1700 °C and
SEM/EDX, indicating that the materials efficiently self-protect toward oxidation.
Thermodynamic parameters for solid and liquid phases in the Al2O3‐Y2O3 system are assessed using new calorimetric measurement for the YAG (Y3Al5O12), YAP (YAlO3) and YAM (Y4Al2O9) phases. The calculated phase diagram of the Al2O3‐Y2O3 system is in reasonable agreement with experimental data. According to the calculations, the YAP phase melts congruently and is stable down to the low temperatures, while the YAM phase disproportionates to a mixture of YAP and Y2O3 phases at temperatures below 1385 K. The calculated entropy of the YAG phase 300.1 J/(mol · K) is between 2 experimentally determined values 284.8 and 349.1 J/(mol · K). However, the difference between calculated and experimental values exceeds uncertainty limits of adiabatic calorimetry data. The enthalpies of melting for the YAG and YAP phases calculated in this study are in reasonable agreement with DTA measurements. The calculated enthalpy of melting for the YAG phase is not consistent with estimates based on solution calorimetric data. New independent measurement of the standard entropy and enthalpy of melting are desirable for the YAG, YAM and YAP phases. The liquidus surface and isothermal section at 2000 K for ternary Al‐Y‐O system are calculated in this study.
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