**3.5 Catalyst supports and catalysts**

Ceramic nanofibers with their unique properties have been extensively explored as either catalyst supports or catalysts for various types of heterogeneous reactions, such as sinter-resistant catalysts [68], photocatalytic reaction [71], electrocatalytic reaction [69], hydrogenation reaction [70], oxidation reaction [66], and Suzuki coupling reactions [67].

Fu et al. [68] prepared γ-Al2O3 nanofibers with a loofah-like surface using single-needle electrospinning for use as Pt supports. After sintering at 500°C, the Al2O3 nanofiber supported Pt catalysts were employed in catalytic reduction of p-nitrophenol and 4-times higher reaction rate constant (6.8 s−1 mg−1) was observed compared to Pt nanocrystals. The high performance of the Al2O3 nanofiber supported Pt catalysts was attributed to the special surface structure and the strong metal–support interactions between Pt and γ-Al2O3 [68]. There are many studies on the use of ceramic nanofibers as photocatalysts in water remediation studies, as already explained previously. Other than water remediation studies, ceramic nanofibers are also employed in the photocatalytic H2 evolution from water splitting, which is a promising renewable energy generation process. Using electrospinning method, Wang et al. [71] fabricated MgTiO3 nanofibers and compared their photocatalytic H2 generation ability with the MgTiO3 nanoparticles and P25. The MgTiO3 nanofibers showed high efficiency and stability in photocatalytic H2 generation under ultraviolet light. Attributed mainly to their large specific surface area, special 1D structure, unique mesh morphology, and pure phase, photoelectrochemical measurements showed that the MgTiO3 nanofibers facilitated the transport and separation of the photoinduced charge carriers [71]. Ceramic nanofibers are also utilized in electrocatalysis to speed up the charge transfer reaction between electrodes and electrolytes. Hosseini et al. [69] fabricated CuO/NiO composite nanofibers and investigated their photocatalytic performance as anode catalyst for hydrazine oxidation in alkaline media. The best catalytic performance was observed when the proportion of Cu(OAc)2:Ni(OAc)2 was 25:75 in polymeric solution [69]. Liu et al. [70] electrospun mesoporous CeO2-based ultrathin nanofibers in fibril-intube configuration. The fibril-in-tube configuration was achieved by choosing two metal precursors with different decomposition rates. Al(acac)3, which rapidly led to the growth kinetics varied along the radial direction of nanofibers by releasing

gaseous pieces, was selected as Al2O3 precursor, and made critical contribution to the formation of fibril-in-tube structure. The novel fibril-in-tube CeO2 nanofibers with different amount of homogenous Al2O3 elemental distribution were investigated as Pt supports. The developed catalytic system exhibited sinter-resistant catalytic activity in the hydrogenation of p-nitrophenol, which was 13-times higher than that of Pt@Al2O3 catalyst [70]. Electrospun ceramic nanofibers are also employed as heterogeneous catalysts for oxidation reactions. Formo et al. [67] reported the use of TiO2 and ZrO2 nanofibers decorated with Pt, Pd, and Rh nanoparticles as a catalytic system for Suzuki cross-coupling reactions. The new catalytic system offered an efficient process, which was cost-effective as it could be fully regenerated and repeatedly used [67]. Formo et al. deposited Pt nanoparticles on the TiO2 porous nonwoven mats, which acted as superior catalyst toward methanol oxidation reaction [66].

### **3.6 EMI shielding applications**

Intensive effort is being spent to develop high-performance EMI shielding materials to protect people from the potential damages of high frequency electromagnetic waves generated by the electronic devices. Among many different electromagnetic wave absorbers such as carbon nanotubes, hollow carbon microspheres, graphene, Fe3O4 microspheres, composite spheres, needle-like metal oxides, and carbon nanofibers, specifically SiC nanofibers are considered as an important candidate because of their high temperature stability, mechanical strength, and resistance to oxidation and corrosion [75].

Wang et al. [75] fabricated flexible, hydrophobic, corrosion resistant, and thermally stable SiC ceramic nanofibers, which showed excellent EMI shielding properties with an effective absorption bandwidth of 4–18 GHz. 90% of electromagnetic waves below −10 dB were absorbed. The maximum reflection loss (RL) reached −19.4 dB at 5.84 GHz. Besides, they were found to be environmentally stable in 2 mol L−1 NaOH solution for 2 h and at high temperatures of 500°C in air atmosphere. The developed SiC nanofibers were suggested as candidates for EMI shielding materials in harsh environments [75]. In another study [76], they incorporated graphite into SiC/Si3N4 composite nanofibers and investigated the relationship between processing, fiber microstructure, and their electromagnetic wave absorption performance. The annealing atmosphere and temperature were observed to affect the electromagnetic wave absorption capability and effective absorption bandwidth. The nanofibers after annealing at 1300°C in Ar showed a minimum RL of −57.8 dB at 14.6 GHz with EAB of 5.5 GHz. The nanofibers annealed at 1300°C in N2 exhibited a minimum RL value of −32.3 dB at a thickness of 2.5 mm, and the EAB reached 6.4 GHz over the range of 11.3–17.7 GHz. The superior EMI shielding properties (high reflection loss together with wider EAB) imparted the composite nanofibers the potential to be used as reinforcements in polymer and ceramic matrix composites with EMI shielding properties [76]. Huo et al. [72] prepared heterogeneous SiC/ZrC/SiZrOC hybrid nanofibers with different ZrC contents and analyzed them in terms of electrical conductivity, average diameter, and microwave-absorbing capability. When the ZrC concentration increased from 0 to 10 wt.%, decrease in average nanofiber diameter from 800 nm to 200 nm and increase in electrical conductivity from 0.3448 to 2.5676 S cm−1 were observed. The minimum reflection loss was measured as −40.38 dB at 14.1 GHz for the SiC/ZrC/SiZrOC hybrid nanofibers and they were suggested as promising high temperature microwave-absorbing materials [72]. Using a combined process of electrospinning and calcination, Huo et al. [73] produced silicon carbide/carbon (SiC/C) hybrid nanofibers, which possessed a high aspect ratio and a scaly surface.

**69**

*Recent Advances in Applications of Ceramic Nanofibers DOI: http://dx.doi.org/10.5772/intechopen.97118*

(SBET) increased from 51.5 to 131.1 m<sup>2</sup>

**3.7 Thermal insulation materials**

which could be further improved to 20.1 dB at 600°C [74].

improve the insulation properties of ceramic nanofibers [131].

The EMI shielding properties of the SiC/C composite nanofibers were studied in the range from 2 GHz to 18 GHz. A paraffin matrix, which was reinforced with 30 wt. % composite nanofibers and had a thickness of 3 mm showed satisfactory dielectric loss value and a minimal reflection loss of approximately −36 dB at 6.8 GHz. Moreover, the maximum effective absorption (<−10 dB) bandwidth (EAB) was approximately 4.1 GHz (12.6–16.7 GHz) with a 1.5 mm thickness, which could cover most of the Ku-band [73]. Hou et al. [74] also fabricated flexible and lightweight ZrC/SiC hybrid nanofiber mats. When ZrC was added into SiC electrospinning solution, the viscosity decreased and the conductivity increased as a result of which the average diameter reduced from 2.6 μm to 330 nm, the specific BET surface area

from 1.5 × 10−6 to 1.3 × 10−1 S cm−1. It is found that 3-layer ZrC/SiC nanofiber mats with a thickness of 1.8 mm showed EMI shielding effectiveness (SET) of 18.9 dB,

Ceramic nanofibers exhibit excellent thermal stability and low thermal conductivity, which make them highly desirable for high-temperature thermal insulation applications [26]. Additionally, polymer phase used in electrospinning of ceramic nanofibers results in the formation of nanosized pores after calcination and helps to

Si et al. [77] electrospun ultra-soft and strong silica nanofibers (SNF) from a sol–gel solution containing NaCl, the incorporation of which significantly enhanced the tensile strength of the SNF membranes from 3.2 to 5.5 MPa. The calcination temperature and the NaCl content in the precursor solution were found to affect the morphology and mechanical properties of the membranes. The membranes exhibited a ultra-softness of 40 mN, relative high tensile strength of 5.5 MPa and an ultra-low thermal conductivity of 0.0058 W m−1 K−1, which made them promising candidates for bunker clothing [77]. Dong et al. [79] fabricated fine-grained mullite nanofibers derived from the diphasic mullite sol (polymethyl siloxane and aluminum tri-sec-butoxide) by electrospinning and subsequent pyrolysis at 1500°C. Mullite fibers with 216 nm average diameter and ~ 100 nm grain size were obtained after sintering at 1500°C were suggested as high-temperature thermal insulation materials [79]. Si et al. [78] reported a scalable strategy to create superelastic lamellar-structured ceramic nanofibrous aerogels by combining SiO2 nanofibers with aluminoborosilicate matrices. The developed nanofibrous aerogels exhibited the integrated properties of flyweight densities of >0.15 mg cm−3, zero Poisson's ratio, rapid recovery from 80% strain, and temperature-invariant superelasticity up to 1100°C, robust fire resistance and thermal insulation performance [78]. Dou et al. [81] designed a hierarchical cellular structured silica nanofibrous aerogel by using electrospun SiO2 nanofibers (SNFs) and SiO2 nanoparticle aerogels (SNAs) as the matrix and SiO2 sol as the high-temperature analogue. They obtained randomly deposited SNFs with the SNAs evenly distributed on the fibrous cell wall. The unique hierarchical cellular structure of the ceramic nanofibrous aerogels exhibited ultralow density of ~0.2 mg cm−3, ultralow thermal conductivity (23.27 mW m−1 K−1), negative Poisson's ratio, temperature-invariant superelasticity from −196 to 1100°C, and editable shapes on a large scale. The novel nanofibrous aerogels with their favorable properties were suggested as thermal insulation materials for aerospace, industrial, and even extreme environmental conditions [81]. Using electrospinning technique, Zhang et al. [26] prepared multi-phase SiZrOC nanofiber membranes composed of amorphous SiOC and ZrO2 nanocrystals to solve the incompatibility between thermal stability and low thermal conductivity

g−1 and the electrical conductivity increased

*Recent Advances in Applications of Ceramic Nanofibers DOI: http://dx.doi.org/10.5772/intechopen.97118*

*Nanofibers - Synthesis, Properties and Applications*

nol oxidation reaction [66].

**3.6 EMI shielding applications**

resistance to oxidation and corrosion [75].

gaseous pieces, was selected as Al2O3 precursor, and made critical contribution to the formation of fibril-in-tube structure. The novel fibril-in-tube CeO2 nanofibers with different amount of homogenous Al2O3 elemental distribution were investigated as Pt supports. The developed catalytic system exhibited sinter-resistant catalytic activity in the hydrogenation of p-nitrophenol, which was 13-times higher than that of Pt@Al2O3 catalyst [70]. Electrospun ceramic nanofibers are also employed as heterogeneous catalysts for oxidation reactions. Formo et al. [67] reported the use of TiO2 and ZrO2 nanofibers decorated with Pt, Pd, and Rh nanoparticles as a catalytic system for Suzuki cross-coupling reactions. The new catalytic system offered an efficient process, which was cost-effective as it could be fully regenerated and repeatedly used [67]. Formo et al. deposited Pt nanoparticles on the TiO2 porous nonwoven mats, which acted as superior catalyst toward metha-

Intensive effort is being spent to develop high-performance EMI shielding materials to protect people from the potential damages of high frequency electromagnetic waves generated by the electronic devices. Among many different electromagnetic wave absorbers such as carbon nanotubes, hollow carbon microspheres, graphene, Fe3O4 microspheres, composite spheres, needle-like metal oxides, and carbon nanofibers, specifically SiC nanofibers are considered as an important candidate because of their high temperature stability, mechanical strength, and

Wang et al. [75] fabricated flexible, hydrophobic, corrosion resistant, and thermally stable SiC ceramic nanofibers, which showed excellent EMI shielding properties with an effective absorption bandwidth of 4–18 GHz. 90% of electromagnetic waves below −10 dB were absorbed. The maximum reflection loss (RL) reached −19.4 dB at 5.84 GHz. Besides, they were found to be environmentally stable in 2 mol L−1 NaOH solution for 2 h and at high temperatures of 500°C in air atmosphere. The developed SiC nanofibers were suggested as candidates for EMI shielding materials in harsh environments [75]. In another study [76], they incorporated graphite into SiC/Si3N4 composite nanofibers and investigated the relationship between processing, fiber microstructure, and their electromagnetic wave absorption performance. The annealing atmosphere and temperature were observed to affect the electromagnetic wave absorption capability and effective absorption bandwidth. The nanofibers after annealing at 1300°C in Ar showed a minimum RL of −57.8 dB at 14.6 GHz with EAB of 5.5 GHz. The nanofibers annealed at 1300°C in N2 exhibited a minimum RL value of −32.3 dB at a thickness of 2.5 mm, and the EAB reached 6.4 GHz over the range of 11.3–17.7 GHz. The superior EMI shielding properties (high reflection loss together with wider EAB) imparted the composite nanofibers the potential to be used as reinforcements in polymer and ceramic matrix composites with EMI shielding properties [76]. Huo et al. [72] prepared heterogeneous SiC/ZrC/SiZrOC hybrid nanofibers with different ZrC contents and analyzed them in terms of electrical conductivity, average diameter, and microwave-absorbing capability. When the ZrC concentration increased from 0 to 10 wt.%, decrease in average nanofiber diameter from 800 nm to 200 nm and increase in electrical conductivity from 0.3448 to 2.5676 S cm−1 were observed. The minimum reflection loss was measured as −40.38 dB at 14.1 GHz for the SiC/ZrC/SiZrOC hybrid nanofibers and they were suggested as promising high temperature microwave-absorbing materials [72]. Using a combined process of electrospinning and calcination, Huo et al. [73] produced silicon carbide/carbon (SiC/C) hybrid nanofibers, which possessed a high aspect ratio and a scaly surface.

**68**

The EMI shielding properties of the SiC/C composite nanofibers were studied in the range from 2 GHz to 18 GHz. A paraffin matrix, which was reinforced with 30 wt. % composite nanofibers and had a thickness of 3 mm showed satisfactory dielectric loss value and a minimal reflection loss of approximately −36 dB at 6.8 GHz. Moreover, the maximum effective absorption (<−10 dB) bandwidth (EAB) was approximately 4.1 GHz (12.6–16.7 GHz) with a 1.5 mm thickness, which could cover most of the Ku-band [73]. Hou et al. [74] also fabricated flexible and lightweight ZrC/SiC hybrid nanofiber mats. When ZrC was added into SiC electrospinning solution, the viscosity decreased and the conductivity increased as a result of which the average diameter reduced from 2.6 μm to 330 nm, the specific BET surface area (SBET) increased from 51.5 to 131.1 m<sup>2</sup> g−1 and the electrical conductivity increased from 1.5 × 10−6 to 1.3 × 10−1 S cm−1. It is found that 3-layer ZrC/SiC nanofiber mats with a thickness of 1.8 mm showed EMI shielding effectiveness (SET) of 18.9 dB, which could be further improved to 20.1 dB at 600°C [74].

## **3.7 Thermal insulation materials**

Ceramic nanofibers exhibit excellent thermal stability and low thermal conductivity, which make them highly desirable for high-temperature thermal insulation applications [26]. Additionally, polymer phase used in electrospinning of ceramic nanofibers results in the formation of nanosized pores after calcination and helps to improve the insulation properties of ceramic nanofibers [131].

Si et al. [77] electrospun ultra-soft and strong silica nanofibers (SNF) from a sol–gel solution containing NaCl, the incorporation of which significantly enhanced the tensile strength of the SNF membranes from 3.2 to 5.5 MPa. The calcination temperature and the NaCl content in the precursor solution were found to affect the morphology and mechanical properties of the membranes. The membranes exhibited a ultra-softness of 40 mN, relative high tensile strength of 5.5 MPa and an ultra-low thermal conductivity of 0.0058 W m−1 K−1, which made them promising candidates for bunker clothing [77]. Dong et al. [79] fabricated fine-grained mullite nanofibers derived from the diphasic mullite sol (polymethyl siloxane and aluminum tri-sec-butoxide) by electrospinning and subsequent pyrolysis at 1500°C. Mullite fibers with 216 nm average diameter and ~ 100 nm grain size were obtained after sintering at 1500°C were suggested as high-temperature thermal insulation materials [79]. Si et al. [78] reported a scalable strategy to create superelastic lamellar-structured ceramic nanofibrous aerogels by combining SiO2 nanofibers with aluminoborosilicate matrices. The developed nanofibrous aerogels exhibited the integrated properties of flyweight densities of >0.15 mg cm−3, zero Poisson's ratio, rapid recovery from 80% strain, and temperature-invariant superelasticity up to 1100°C, robust fire resistance and thermal insulation performance [78]. Dou et al. [81] designed a hierarchical cellular structured silica nanofibrous aerogel by using electrospun SiO2 nanofibers (SNFs) and SiO2 nanoparticle aerogels (SNAs) as the matrix and SiO2 sol as the high-temperature analogue. They obtained randomly deposited SNFs with the SNAs evenly distributed on the fibrous cell wall. The unique hierarchical cellular structure of the ceramic nanofibrous aerogels exhibited ultralow density of ~0.2 mg cm−3, ultralow thermal conductivity (23.27 mW m−1 K−1), negative Poisson's ratio, temperature-invariant superelasticity from −196 to 1100°C, and editable shapes on a large scale. The novel nanofibrous aerogels with their favorable properties were suggested as thermal insulation materials for aerospace, industrial, and even extreme environmental conditions [81]. Using electrospinning technique, Zhang et al. [26] prepared multi-phase SiZrOC nanofiber membranes composed of amorphous SiOC and ZrO2 nanocrystals to solve the incompatibility between thermal stability and low thermal conductivity

of single-phase ceramic nanofibers at high temperatures, which limit their practical use. The fabricated SiZrOC nanofibers 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 unique combination of amorphous SiOC and ZrO2 nanocrystals offered a novel strategy to produce high-performance thermal insulation materials. While the multi-phase interfaces and the ZrO2 nanocrystals created thermal transfer barriers to reduce the heat transfer, the SiOC phase effectively suppressed radiative heat transfer [26]. Zhang et al. [82] produced ultra-strong, superelastic, and high temperature resistant, lamellar multiarc structured ceramic nanofibrous aerogels by combining ZrO2 − Al2O3 nanofibers with Al(H2PO4)3 matrices. The resulting ZrO2 − Al2O3 nanofibrous aerogels displayed a recovery of 90%, compression strength of more than 1100 kPa, temperature-invariant superelasticity, and high fatigue resistance, thermal insulation performance with low thermal conductivity (0.0322 W m−1 K−1), and temperature resistance up to 1300°C [82]. Peng et al. [80] fabricated yttria-stabilized zirconia mixed silica (YSZ/SiO2) nanofibers by the electrospinning method. The use temperature of the nanofibers increased by nearly 300°C and their maximum strength reached 5.9 ± 0.8 MPa. The YSZ/SiO2 nanofibers, showing good thermal insulation performance and hydrophobic properties, were suggested for use as high-temperature insulation materials [80].

#### **3.8 Other applications**

CNFs have been intensely investigated and applied in many different applications since their first production in 2002. There is still an ever-growing interest in the field with emerging applications that influence different aspects of life.

Apart from the above discussed applications, ceramic nanofibers are also utilized in separation of nuclear waste and recycling of nuclear fuels. Liu et al. [84] electrospun TiO2 nanofibers with high porosity and functionalized the membranes with silver nanoparticles and nanocrystal metal organic frameworks to capture gases of interest in order to recycle nuclear and industrial waste. These ceramic nanofiber materials showed high porosity, loading capacity, permeability, stability in extreme conditions and were effective in recycling of nuclear waste back into the fuel cycle [84].

Ramaseshan and Ramakrishna [85] produced zinc titanate ceramic nanofibers by electrospinning technology using polyvinylpyrrolidone as a binder for use as catalysts for the detoxification of chemical warfare agents, which are known to react with metal oxides (Mg, Al, Fe, Ti, Zn, Cr, Cu, Mn, etc.) and detoxify them into nontoxic harmless by-products. The zinc titanate nanofibers were found to be effective in decontamination of agents, such as dimethyl methyl phosphonate and chloroethyl ethyl sulfide. The extent of detoxification was measured and the products of reaction of zinc titanate against the simulants were found to be relatively harmless. The nanofibers obtained were suggested to replace conventional-activated carbon by electrospun ceramic nanofibers for face masks and protective clothing [85].

Abdo et al. [86] fabricated titanium dioxide (TiO2) and carbon nanofibers by electrospinning technique followed by calcination process and utilized these nanofibers to reinforce magnesium matrix composites. When 5 wt.% TiO2 nanofibers were added the ultimate compressive strength increased to 281 MPa, which was about 12.4% higher than the pure Mg. As a result of the addition of carbon nanofibers into magnesium matrix composites, hardness increased to 64.4% with slight sacrifice in the mechanical properties [86].

CNFs were also utilized in development of highly sensitive and reliable capacitive pressure sensors. Using flexible TiO2 ceramic nanofibrous networks, Fu et al. [87] developed a capacitive pressure sensor and studied the capacitance-to-pressure

**71**

**Author details**

**4. Conclusions**

many different fields.

publication of this article.

**Conflict of interest**

**Funding**

Nuray Kizildag

Sabanci University, Istanbul, Turkey

provided the original work is properly cited.

Integrated Manufacturing Technologies Research and Application Center,

© 2021 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

\*Address all correspondence to: nuray.kizildag@sabanciuniv.edu

research, authorship, and/or publication of this article.

*Recent Advances in Applications of Ceramic Nanofibers DOI: http://dx.doi.org/10.5772/intechopen.97118*

sensitivity. The ceramic pressure sensors, which exhibited high sensitivity

(≈4.4 k Pa−1), fast response speed (<16 ms), ultralow limit of detection (<0.8 Pa), low fatigue over 50000 loading/unloading cycles, high temperature stability, were suggested for use as real-time health monitoring and motion detection [87].

The development of ceramic nanofibers has attracted a significant interest over the recent years. Among different production techniques such as magnetron sputtering, solution blowing, laser spinning, chemical vapor deposition, template synthesis, phase separation, hydrothermal treatment; electrospinning enabled the production of uniform CNFs with high surface areas, very small diameters, extremely long length, and small pore size. Ceramic nanofibers became indispensable materials in many applications. In this chapter, electrospun ceramic nanofibers are reviewed with an emphasis on their applications in tissue engineering, gas sensors, water remediation, batteries, catalyst supports/catalysts, electromagnetic interference shielding and thermal insulation materials. Although there has been a lot of progress since their first production, there is still a lot to be explored regarding their production, and properties, and there is a great potential for their uses in

The author(s) received no financial support for the research, authorship, and/or

The author(s) declared no potential conflicts of interest with respect to the

sensitivity. The ceramic pressure sensors, which exhibited high sensitivity (≈4.4 k Pa−1), fast response speed (<16 ms), ultralow limit of detection (<0.8 Pa), low fatigue over 50000 loading/unloading cycles, high temperature stability, were suggested for use as real-time health monitoring and motion detection [87].
