Flexible and thermal-stable SiZrOC nanofiber membranes with low thermal conductivity at high-temperature

Abstract

Ceramic nanofibers with excellent thermal stability and low thermal conductivity are highly desired for high-temperature thermal insulation applications. However, the incompatibility of thermal stability and low thermal conductivity at high-temperatures largely limit the practical use of conventional single-phase ceramic nanofibers. Here, we report the preparation of multi-phase SiZrOC nanofiber membranes (NFMs) composed of amorphous SiOC and ZrO₂ nanocrystals via electrospinning technique. The fabricated SiZrOC NFMs exhibited excellent high-temperature stability (∼1200 °C in Ar) and low thermal conductivity (∼0.1392 W m⁻¹ K⁻¹ at 1000 °C in N₂). The decreased thermal conductivity is achieved through a synergistic mechanism, that the multi-phase interfaces and the ZrO₂ nanocrystals create thermal transfer barriers to reduce the heat transfer, whilst the SiOC phase effectively suppresses radiative heat transfer. This unique combination of amorphous SiOC and ZrO₂ nanocrystals provides a novel strategy to prepare high-performance thermal insulation materials, and the obtained SiZrOC NFMs are promising high-temperature thermal insulation materials.

Introduction

For the past decades, high-temperature (∼1200 °C) thermal insulation materials have attracted tremendous attention due to the growing demand for thermal protection in aerospace crafts, industrial furnaces and high-temperature fuel cells [[1], [2], [3], [4]]. Ceramic fibers (SiO₂, ZrO₂, Al₂O₃, SiC and SiOC etc.) with low density, high strength, excellent fire and corrosion resistance, have been recognized as ideal candidates for high-temperature thermal insulation [[4], [5], [6], [7], [8], [9]].

Currently, some oxides ceramic fibers (SiO₂, ZrO₂ and Al₂O₃ etc.) have served as thermal insulation materials due to their low thermal conductivity at ambient temperature (such as thermal conductivity of fiber mats: 0.038 W·m⁻¹ K⁻¹ for SiO₂, 0.028 W·m⁻¹ K⁻¹ for ZrO₂, 0.039 W·m⁻¹ K⁻¹ for Al₂O₃) [[10], [11], [12]]. Nevertheless, the radiative infrared heat transfer rises rapidly with increasing temperature because these oxide ceramic fibers are infrared transparent [13,14]. As a result, the total thermal conductivities of these oxide ceramic fibers are high at high-temperatures (such as thermal conductivity of fiber mats: 0.08 W·m⁻¹ K⁻¹ for SiO₂ at 700 °C, 0.230 W·m⁻¹ K⁻¹ for ZrO₂ at 900 °C, 0.295 W·m⁻¹ K⁻¹ for Al₂O₃ at 1200 °C) [[10], [11], [12]]. Moreover, owing to the crystallization-induced embrittlement behaviour of oxide ceramic fibers, they often suffer from serious strength degradation and pulverization under long-term high-temperature exposure [1,15,16]. For practical application, thermal insulation materials are expected to be robust and flexible to fit the spaces of various shapes and durable enough to bear loads when exposed in extreme environments [1,4]. Therefore, the oxide ceramic fibers have limited application in the field of high-temperature thermal insulation.

Although some non-oxide ceramic fibers (such as SiC and C etc.) show higher thermal stability and infrared extinction coefficient than oxide ceramic fibers, their high thermal conductivity (the thermal conductivity of SiC is 490 W·m⁻¹ K⁻¹ and C is 800 W·m⁻¹ K⁻¹) restrict their usage as thermal insulation materials [17]. In short, traditional single-phase ceramic fibers can’t meet the requirements of high-temperature insulation materials for high thermal stability and low thermal conductivity. Therefore, the research and development of a low thermal conductivity and high-temperature stability materials with a relatively good flexibility and high strength simultaneously is the top priority.

Recently, multi-phase ceramics (SiOC [18], SiCN [19] and SiBNC [20] etc.) have gained significant attention because they integrate excellent performance of each composed phase and demonstrate fascinating properties. In particular, the multi-phase SiOC (composed of SiO₂, SiC and some free carbon phases) fibers have shown promising applications as high-temperature thermal insulation materials due to the amorphous microstructures of the SiOC fibers heat-treated up to 1000 °C. Amorphous structure indicates that the microstructure of SiOC fibers is long-range disordered and there are a large number of boundaries between the nanocrystals. These boundaries significantly scatter the phonons and lead to the heat transfer decreased. As a result, the thermal conductivity of the fiber is low. [18]. Besides, the SiC and free carbon phases within SiOC fibers have high infrared extinction coefficient, which contributes to decrease the radiation heat transfer at high-temperatures [21,22]. Furthermore, the SiOC fibers with high strength and good chemical stability [23].

However, the most reported SiOC fibers could only worked at below 1200 °C [24,25]. The main reason is that the decomposition of SiOC phase degradation into SiO₂ and free carbon at the temperature above 1000 °C. At higher temperatures (T>1200 °C), the carbothermal reduction occurs between the SiO₂ and free carbon result in great weight loss and numerous defects, which is detrimental to the mechanical properties of the SiOC fibers [24].

In order to improve the thermal stability of SiOC ceramics, great efforts have been focused on inhibiting SiOC phase decomposition by incorporating hetero-elements (such as B [26,27], Ti [28,29], Hf [25,30] and Zr [24,28,31,32] etc.). Specially, Zr element has attracted more attention in developing high-performance SiZrOC fibers because Zr atoms can strongly capture oxygen atoms to form thermally stable Zr-O structure [28]. In addition, the formed Zr-O structure has low thermal conductivity since zirconia has lower phonon energy [33]. Inspired by this, incorporating Zr element into the SiOC fibers to fabricate multi-phase SiZrOC fibers could simultaneously enhance high-temperature stability and thermal insulation of the SiOC fibers. Recently, the SiZrOC fibers have been developed by mechanical spinning from a viscous melt or a sol-gel solution. Yamaoka et al. [28] firstly used the polycarbosilane and zirconium acetylacetonate as the precursor and successfully fabricated the SiZrOC fiber by the melt-spun process. Then, our group fabricated ZrO₂/SiC nanofibers through electrospinning a polymer blend of polycarbosilane and zirconium acetylacetonate. The obtained fibers show excellent high-temperature stability and erosion resistance [32]. Su et al. [31] reported the fabrication of SiZrOC fibers via polyvinyl pyrrolidone assisted sol-gel process. Nevertheless, to the best of our knowledge, currently studies on SiZrOC fibers only pays attention to their high-temperature resistance, and their thermal insulation performance haven’t been studied. Moreover, the Zr content of the reported SiZrOC fibers is relatively low (< 3 wt%) because high amount of Zr content may lead to poor spinnability [31,32]. It is believed that the higher Zr content is, the more thermal stable Zr-O structure will be, which is helpful to improve both high-temperature stability and thermal insulation performance.

To this end, this work focuses on filling this research gap to fabricate multi-phase SiZrOC nanofiber membranes (NFMs) by electrospinning technique combined with sol-gel process. The prepared SiZrOC NFMs possess high Zr content (∼51.7 wt %), exhibit excellent flexibility, oxidation resistance (850 °C in air), mechanical properties, high-temperature stability (1200 °C in Ar) and low thermal conductivity (∼0.0511 W·m⁻¹ K⁻¹ at 25 °C in Air, ∼0.1392 W·m⁻¹ K⁻¹ at 1000 °C in N₂), which have broad prospects in high-temperature insulation.