Intermediate Phases in Mullite Synthesis Via Aluminum‐ and Alumina‐Filled Polymethylsiloxane

Abstract

Active filler pyrolysis of polymethylsiloxane/aluminum (PMS/Al) mixtures leads to the formation of mullite at 1400°C, and by 1700°C mullite is the only crystalline phase in an amorphous matrix. These mixtures show a complex series of oxidation and reduction reactions prior to the formation of mullite. Initial oxidation of PMS at low temperatures results in the formation of an amorphous product before any significant oxidation of the Al. After melting of the Al phase, redox reactions lead to the formation of elemental Si and SiC as intermediate crystalline phases along with Al2O3 both as the crystalline α‐phase and dissolved in the amorphous matrix. Mullite formation occurs by precipitation from the matrix at temperatures above 1400°C. In contrast, PMS/Al2O3 mixtures did not show mullite formation until heat treatment at 1700°C. In this case no significant intermediate crystalline phases occur, except for some limited cristobalite crystallization from the matrix.

Influence of nano-aluminum filler on the microstructure of SiOC ceramics

“…The goal of such studies was to produce dense mullite ceramics at relatively low temperatures. [17][18][19][20] Detailed studies of the evolution of phases during the pyrolysis are also presented in the above mentioned literature and show that the thermal decomposition of the polymers in the presence of aluminum in air proceeds by a complex series of reactions which shows elemental Si, SiC and ␣-Al 2 O 3 as intermediates. Moreover, the size and morphology of the starting Al powder controls the pyrolysis reaction pathway and kinetics.…”

Mullite formation from the pyrolysis of aluminium-loaded polymethylsiloxanes: The influence of aluminium powder characteristics

“…This has been discussed in detail in earlier work. 7 There is a weak exotherm (peak 1) and associated weight loss at 250 • C associated with a cross-linking reaction and a more intense exotherm (peak 2) and weight loss around 400-500 • C that characterises the pyrolysis of the cross-linked polymer, loss of organic side chains and conversion to an amorphous SiO 2 material. Fig.…”

“…The sharply defined endotherm at 1416 • C (peak 7) occurs at the melting point of pure Si, the presence of Si was reported by Michalet et al 6 and was also seen in our earlier work. 7 The exotherms that occur in the temperature range of 1450-1550 • C are thought to be associated with mullite formation. Most of the differences between the thermal analysis of the PMS/Al mixtures and of the PMS and Al in isolation can be explained by the increased availability of oxygen from the amorphous SiO 2 .…”

“…Fig. 1 presents scanning electron microscope (SEM) micrographs of the 7 Samples were prepared by dissolving the PMS resin in ethanol:acetone, ratio of 95:5 vol.%, to which the Al powder was then added and stirred to obtain a homogeneous solution. Aluminium acetylacetonate (Al(acac) 2 ) (1 wt.%) was added as a catalyst (Sigma-Aldrich, Dorset, UK).…”

“…Michalet et al used a fine sub-micron sized spherical Al powder whereas in our earlier work a flake morphology Al was used of sub micron thickness but with a flake diameter of approximately 70 m. 7 The current study has focused on the effect of the difference in Al powder characteristics (in terms of morphology and particle size) used as filler particles loaded in the PMS on the mullitisation temperature and mechanism of mullite formation.…”