The preparation of HfC/C ceramics via molecular design

Abstract

Polymer derived ceramics have received lots of attention throughout the last few decades. Unfortunately, only a few precursor systems have been developed, focusing on silicon based polymers and ceramics, respectively. Herein, the synthesis of novel hafnium containing organometallic polymers by two different approaches is reported. Dialkenyl substituted hafnocene monomers were synthesized and subsequently polymerized via a free radical mechanism. Salt metathesis reactions of hafnocene dichloride with bifunctional linkers led to the formation of polymeric materials. NMR spectroscopic methods--in solution as well as in the solid state--were used to characterize the organometallic polymers. Ceramics were finally obtained after cross-linking and thermal treatment under argon (T(max) = 1800 °C). SEM investigations, elemental analyses, Raman spectroscopy and XRD investigations identified the pyrolyzed products as partially crystalline HfC/C mixed phases.

Polymer‐Derived Ultra‐High Temperature Ceramics (UHTCs) and Related Materials

“…A second preparative tool to produce UHTC phases from polymeric precursors is based on organometallic precursors. [124][125][126] Already in the 80s, titanocene derivatives CpTiCl 3 , Cp 2 TiCl 2 , Cp 2 Ti(R)Cl, and Cp 2 TiR 2 (Cp cyclo-pentadienyl; R alkyl, aryl) were used to prepare titanium carbide by gas-phase approaches (MO CVD) or thermal decomposition (see ref. [124] and references therein).…”

“…[124] Recently, the synthesis of Hf-containing polymeric precursors based on organometallic compounds and their hightemperature conversion into HfC/C composites was reported. [125] Typically, Cp 2 HfCl 2 was reacted with bifunctional dilithio diynes (e.g., 1,4-dilithio-1,3-butadiyne, or 1,4-bis-(lithioethynyl)-benzene) to generate the corresponding polymeric precursors. [125] Additionally, a dialkenyl-substituted hafnocene was polymerized upon thermal treatment in the presence of dicumyl peroxide as radical polymerization catalysts (Figure 11).…”

Synthesis, Characterization, and Microstructure of Hafnium Boride‐Based Composite Ceramics Via Preceramic Method

“…However, the usage of this kind of ceramic materials in extreme conditions is limited due to their poor oxidation resistance. Metal boride or carbide can form a protective metal‐oxide layer at high temperature in oxidation atmospheres, but the generation of volatile by‐products will result in the increase in porosity in surface and the decrease in oxidation resistance . Silicon carbide has been proved to be effective against oxidation below 1600°C .…”

“…Sol–gel method is an effective approach to prepare nano‐sized powders at low temperature due to the intimate contact of different ingredients . However, relatively long aging time and complicated process procedures are key points which limit its further applications . Preceramic polymer method is an alternative method for preparing ceramics, especially, appropriate for preparing ceramic matrix composites (CMCs) by polymer impregnation and pyrolysis (PIP).…”

Synthesis of soluble and meltable pre‐ceramic polymers for Zr‐containing ceramic nanocomposites

“…Similar work was performed by Cai, in which Si was introduced to synthesize ZrC/SiC ceramic precursors; the existence of a Si–C bond in the polymer was found to improve its solubility. Inzenhofer et al synthesized HfC/C ceramic precursors by two different synthetic routes: (1) the radical polymerization of alkenyl‐substituted hafnocene derivatives and (2) salt‐metathesis reactions of hafnocene dichloride with dilithio‐alkynyl compounds . However, the resulting HfC/C ceramics were not ceramic nanocomposites because the crystallite size was on the micrometre scale.…”