Preparation of Ultra-High Temperature Ceramics–Based Materials by Sol-Gel Routes

Abstract

Ultra-high temperature ceramics (UHTCs) are a class of inorganic materials that have melting point over 3000°C and are typically borides, carbides, and nitrides of early transition metals. UHTCs are considered as the promising candidate used in the extreme environment involved with the hypersonic aviation thermal protective system. Synthesis of UHTCbased materials can be divided into solid-based and solution-based protocols according to the state of the raw materials. A sol-gel technique is one of the solution-based protocols for the preparation of UHTC-based materials, which involves the hydrolysis, condensation of the metal organic and/or metal inorganic compounds, gelation, and the posthigh temperature treatment of the dried gels. The sol-gel technique enables the synthesis of UHTC-based materials at 1300-1600°C. UHTC-based materials with desired shapes, such as nanopowders, fibers, and porous monoliths, can also be prepared via sol-gel routes.Recent interest in UHTCs has been motivated by the search for materials that can withstand extreme environments, such as extreme temperature, chemically aggressive environments, and rapid heating/cooling, etc. [5,6]. Due to the combination of series of excellent physical and chemical properties, such as high hardness, good high temperature stability, and excellent solid-phase stability, UHTCs are considered as promising candidates for a variety of high-temperature structural applications, including engines, hypersonic vehicles, plasma arc electrode, cutting tools, furnace elements, and high temperature shielding. However, owing to the strong covalent bond and relatively low volume diffusion coefficient, UHTCs are difficult to get successfully sintered. Densification of these materials is typically performed by hot pressing and spark plasma sintering with or without sintering aids.UHTCs can be synthesized by a variety of routes, which can be divided into solid phase protocols and solution based protocols based on the state of the starting materials [7,8]. Among the solid-phase protocols, transition metal oxides, carbon black, boron oxide, and boron carbide are commonly used as the starting materials. After ball-milling and high temperature carbothermal reduction, the mixtures are converted into UHTC powders. Examples of the reactions used to synthesize borides and carbides are listed in Table 1 [7]. These reactions are written for ZrB 2 and ZrC, but analogous processes produce other borides and carbides. These solid phase protocols are very straightforward and simple and are used to produce UHTC powders commercially. However, due to the heterogeneity of the reactants, the reactions need to be performed at high temperatures for several hours to complete. Also, those synthesized UHTC powders are micron-sized particles. Figure 1. A comparison of the melting temperatures of the most refractory members of several classes of materials.

Sol-gel derived porous ultra-high temperature ceramics

“…Similar to many other porous oxide ceramics, the highly porous ZrC foams with spherical cell structures demonstrate huge strain before crushing, as depicted in Fig. 13(a) [23,25,37]. The stress-strain curve shows a linear elastic region followed by a plateau region, and ended with a densification region at large strain.…”

“…13 Compressive stress-strain curves of the sol-gel derived porous UHTCs. (a) ZrC foams prepared by foaming method [37]. Reproduced with permission from Ref.…”

“…Reproduced with permission from Ref. [37], © IntechOpen 2019. (b) Porous ZrB 2 /ZrC/SiC ceramics prepared by solvent evaporation method [22].…”

“…After sol-gel reactions, shaping with specific techniques, drying at moderate conditions, and pyrolysis at high temperatures, the porous UHTCs with desired compositions are obtained. Numbers of papers focus on the design and pyrolysis of UHTCs precursors to fine UHTCs powders through sol-gel methods [37]. However, rare works have been published on sol-gel derived porous monolithic UHTCs.…”

“…The synthesis of UHTCs powders from sol-gel or liquid precursor routes will not be discussed here. Sacks et al [40], Dollé et al [41], Yan et al [42], Ang et al [43], Cai et al [44], and www.springer.com/journal/40145 Li et al [37] are suggested for those interested in these research topics.…”