Phase evolution of SiOC‐based ceramic nanocomposites derived from a polymethylsiloxane modified by Hf‐ and Ti‐alkoxides

Abstract

SiOC/HfO2‐based ceramic nanocomposites with in situ formed HfO2 nanoparticles were prepared via a single‐source precursor (SSP) approach starting from a polymethylsilsesquioxane (PMS) modified by Hf‐ and Ti‐alkoxides. By varying the alkyl‐group of the employed Hf‐alkoxides, SiOC/HfO2‐based ceramic nanocomposites with different HfO2 polymorphs formed via thermal decomposition of the SSP under the same heat‐treatment conditions. Using PMS chemically modified by Hf(OnBu)4, tetragonal HfO2 phase was formed after the synthesis at 1100°C in Ar, whereas both, tetragonal and monoclinic HfO2 nanocrystals, were analyzed when replacing Hf(OnBu)4 by Hf(OiPr)4. After oxidation of the synthesized nanocomposites in air at 1500°C, a facile formation of oxidation‐resistant HfSiO4 (hafnon) phase occurred by the reaction of HfO2 nanocrystals with silica present in the SiOC nanocomposite matrix derived from Hf(OiPr)4‐modified SSPs. Moreover the amount of hafnon is dramatically increased by the additional modification of the polysiloxane with Ti‐alkoxides. In contrast, ceramic nanocomposites derived from Hf(OnBu)4‐modified SSPs, almost no HfSiO4 is detected after oxidation at 1500°C even though in the case of Ti‐alkoxide‐modified single‐source precursor.

Microstructure and phase evolution of polymer‐derived SiHfOC ceramic microspheres

“…One reason why the initial temperature declines is that the introduction of Hf atoms reduces the thermal stability of Si-C bonds. 34,35 In the third stage, above 849.4 • C, the weight of modified microspheres remains the same until 1500 • C, and the DSC curve shows a more obvious endothermic peak at 1118 • C. In summary, the Hf-PSN microspheres have a significantly higher ceramic yield than unmodified microspheres. It is suggested that the introduction of Hf may improve the ceramic yield by suppressing the decomposition of the Si-O-C bonds and enhancing the stability of the amorphous SiOC matrix in the pyrolysis process.…”

“…It has been widely reported that the multicomponent SiMOC ceramics synthesized via the polymer precursors route can uniformly precipitate nanocrystalline MO x from the amorphous SiOC matrix at elevated temperatures, and the nanocrystalline MO x can enhance the structural stability of the PDC matrix as a reinforcement. [34][35][36] Ionescu et al 37 synthesized polymer-derived SiOC/HfO 2 ceramic nanocomposites using polysilsesquioxane and hafnium tetra (n-butoxide) and investigated the structural changes and phase evolution at different pyrolysis temperatures. The results indicated that the phase separation occurred in the SiOC matrix, and hafnium nanoprecipitate crystallized at temperatures ranging from 1100 to 1300 • C. Sujith et al 38 studied the crystallization and microstructural evolution of HfO 2 from an amorphous matrix of the SiOC system by adding hafnium alkoxide.…”

Polymer-Derived Si-Based Ceramics: Recent Developments and Perspectives

“…The first is to directly mix the metal (oxide) powders in the ceramic precursors, yet it is difficult to form monodispersed solution; another one is to directly use the metal element-containing polymers, however, the process of constructing polymers containing metal elements is complicated with high cost. In addition to mixing the dopants with the precursor polymers directly, modification of the precursor polymers [7,8,14,42,56] is also an effective way. Xu et al [56] used polyhydromethylsiloxane (PHMS) and tetramethyl-tetravinyl-cyclete-trasiloxane (D4Vi) as precursors, and polydimethylsiloxane (PDMS) was added to the precursors, then the three were mixed and crosslinked.…”

“…The recent research progress and trends of PDCs are to functionalize them through various methods (such as additive doping [30,31,[41][42][43][44][45]) and to combine PDCs with 3D printing [4,10,[13][14][15]29,46] to explore new techniques to fabricate PDCs. By functionalizing PDCs, they have excellent mechanical, thermal, electrical, and dielectric properties.…”

High-Temperature Properties and Applications of Si-Based Polymer-Derived Ceramics: A Review

“…Initially, at the crosslinking stage, B atoms may improve the formation of the crosslinking network and increase the yield and structural density of SiBOC [ 11 , 12 ]. Upon polymer-to-ceramic conversion, doped elements may form additional phases (e.g., TiO 2 , HfO 2 , ZrO 2 ) and lead to nanocomposite-like structures of metal-oxide/amorphous ceramics [ 25 ] or dissolve into and alter the amorphous phase (e.g., SiBOC, SiAlOC) which eventually segregate into additional phases (BN, Al 2 O 3 ) at higher temperatures. At the amorphous stage, the superior stability of the B- or Al-doped amorphous phase significantly enhances the oxidation resistance of SiBOC or SiAlOC ceramics [ 11 , 13 ].…”