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Hydrothermally grown uniform TiO2 coatings on ZrO2 fibers and their infrared reflective and thermal conductive properties
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Hydrothermally grown uniform TiO2 coatings on ZrO2 fibers and their infrared reflective and thermal conductive properties Dehua Maa, Luyi Zhua,∗, Benxue Liub,∗∗ a b
State Key Laboratory of Crystal Materials and Institute of Crystal Materials, Shandong University, Jinan, 250100, PR China Qilu University of Technology (Shandong Academy of Science), Advanced Materials Institute, Jinan, 250014, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Infrared reflection Thermal properties TiO2 ZrO2 fibers Hydrothermal growth
Improving the infrared reflectivity of ZrO2 polycrystalline fibers is of great benefit to its thermal applications. In the present research, we cast a highly uniform TiO2 coating with a thickness ranging from dozens to hundreds of nanometers on ZrO2 fibers by utilizing hydrothermal growth. The coating bonds tightly to the ZrO2 fibers via Zr–O–Ti chemical linkages, and the thickness of the coating can be tailored by varying the hydrothermal growth time. The TiO2 coating, acting as a sheath towards electromagnetic radiation, not only reflected light with wavelengths ranging from the visible region to the infrared region and up to 8 μm but also shielded the Raman signals of the ZrO2 fibers. The present research provides an efficient way to cast controllable and uniform coatings on flexible fiber materials. The obtained ZrO2 fibers coated with TiO2 may have applications such as reinforcement for bulk ceramics, thermal barrier coatings, aerogels, etc., thus performing the dual functions of mechanical strengthening and thermal insulation.
1. Introduction Manipulation of infrared radiation plays a significant role in thermal management because the infrared radiation uptakes a considerable amount of thermal energy, especially when exposed to a high temperature [1]. An object interacts with infrared radiation by means of transmission, adsorption and reflection. Tailoring the infrared reflection of an object has turned into a hot topic [2] recently because, by efficiently reflecting infrared radiation, applications such as thermal insulation and infrared stealth could be achieved [3,4]. Zirconia (ZrO2) fibers [5–8] are leading-edge polycrystalline fibers due to their large aspect ratios, high mechanical strength, and flexibilities [9,10], as well as some intrinsic physicochemical properties of ZrO2, for instance, high melting point, phase transformation induced toughness, corrosion and oxidation resistance, low phonon conduction, etc. [11–15]. These fibers are promising thermal insulating materials [16] in various thermal protection systems utilized in both civilian and military areas. Thus, the improvement of the infrared reflection of ZrO2 fibers may result in the enhanced thermal insulation and infrared stealth performances of a thermal protection system using ZrO2 fibers. However, to our knowledge, there have only been a few reports concerning this topic. Reflective coating is an efficient way to modify infrared reflective properties without altering the intrinsic properties of substrates. ∗
According to the Fresnel equation, a refractive index higher than that of the substrate promotes enhanced infrared radiation [17]. Hence, casting reflective coatings with a high refractive index on ZrO2 fibers may be a promising way to improve infrared radiation reflection and thermal properties [18]. Regardless of the substrates, coating methods involve dip [19], spin [20], deposition by physical processes (e.g., sputtering [21]), chemical processes (e.g., chemical vapor deposition [22]), etc. However, applying these coating processes for flexible materials produces drawbacks, such as the need for flat or near-flat substrates for spin and deposition methods and a higher cost. Dip coating seems to be efficient for casting infrared reflective coatings for ZrO2 fibers. However, it is less controllable, thus causing the thickness and uniformity of the coating to be difficult to manipulate. During the dip coating of flexible fiber materials, when two or more fibers are in close contact, the coating easily bridges the two individual fibers instead of uniformly coating them [23,24]. Recently, hydrothermal growth to cast coatings on flexible fiber materials attracted research interests because the resulting coatings are found to be very uniform [25]. Meanwhile, it is easy to scale up cost-effectively to meet the large area requirements of commercial applications. Based on the above statements, in the present research, we try to hydrothermally grow infrared reflective coatings on ZrO2 fibers in order to enhance their infrared radiation reflection. Titanium dioxide (TiO2)
Corresponding author. Corresponding author. E-mail addresses: [email protected] (L. Zhu), [email protected] (B. Liu).
∗∗
https://doi.org/10.1016/j.ceramint.2019.10.050 Received 15 August 2019; Received in revised form 11 September 2019; Accepted 5 October 2019 0272-8842/ © 2019 Published by Elsevier Ltd.
Please cite this article as: Dehua Ma, Luyi Zhu and Benxue Liu, Ceramics International, https://doi.org/10.1016/j.ceramint.2019.10.050
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has a high refractive index (2.52–2.72) that is greater than that of ZrO2 (ca. 2.17), which is an ideal infrared reflective material [26]. In particular, TiO2 has a high melting point, better chemical stability, and oxidation resistance [27–30], making the TiO2 coated ZrO2 fibers appropriate for high-temperature thermal applications. Herein, we prepared TiO2 infrared reflective coatings on ZrO2 fibers by the hydrothermal growth process. The microstructures of TiO2 coating, the growth-time dependent thickness, the bonding between coating and substrate, the infrared radiation reflective properties, and the thermal conductivities of the coated ZrO2 fibers were fully studied. Specifically, the commonly used titanium precursors, e.g., tetrabutyl titanate and titanium tetrachloride, usually result in large thicknesses, reaching thicknesses up to a micrometer size, which is comparable with the diameters of the ZrO2 fibers. In the present research, we report the use of an acetylacetonate modified titanium precursor, polyacetylacetonatotitanium [31], to achieve more controllable TiO2 coatings with the thicknesses of dozens to hundreds of nanometers. 2. Experimental 2.1. Starting materials A modified titanium precursor, polyacetylacetonatotitanium (PAT), was synthesized by the coordination reaction between partially hydrolyzed titanium tetrachloride and acetylacetonate in a molar ratio of 1:1. The chloride ions were fully removed by a precipitation reaction after adding stoichiometric amount of triethylamine. PAT was obtained by distillation, yielding yellowish powders. The detailed synthesis of PAT could be referred in previous literature [31]. The remaining chemical reagents were purchased from chemical agents without further purification, such as, isopropanol (≥99.9%, Aladdin), hydrofluoric acid (≥40%, Aladdin), zirconia fibers (Shandong Dipole New Energy Saving Materials Co., Ltd).
Fig. 1. SEM images of (a) pristine ZrO2 fibers and (b) the fibers coated with TiO2, diameter distributions of (c) pristine ZrO2 fibers and (d) the fibers coated with TiO2. (e) XRD patterns and (f) Raman spectra of pristine ZrO2 fibers and the fibers coated with TiO2.
Thermal conductivity was measured by using a NETZSCH LFA457 instrument (LFA457, NETZSCH, Germany), and the value at each point was the average of triplicate analysis with a standard derivation of less than 0.01 W/(m·K).
2.2. Hydrothermal growth of the TiO2 coating on the ZrO2 fibers TiO2 coatings were cast on ZrO2 fibers via a hydrothermal growth process in stainless-steel high-pressure reactors equipped with Teflon linings at 180 °C. The filled degree of the Teflon lining reached 80%, thus achieving a satisfactory high-pressure environment. A typical growth solution was acquired by mixing 1 g PAT, 0.5 ml hydrofluoric acid, and 40 ml isopropanol under vigorous stirring. In each bath, 0.1 g of fluffy ZrO2 fibers were added into the dissolved growth solution. The hydrothermal time variable indicated that the growth at 180 °C lasted for 6 h, 12 h, or 24 h. Then, the reactors were taken out of the furnace and quenched with cold water, and the TiO2 -coated ZrO2 fibers were obtained.
3. Results and discussion Before coating, the ZrO2 fibers presented continuous morphologies with uniform diameters as shown in Fig. 1a. In previous studies, we found that these fibers show considerable mechanical strengths, i.e., above 1 GPa [10], guaranteeing the integrity of the fibers under hydrothermal growth process. After coating at 180 °C with a duration of 12 h, the fibers were obviously thicker than that of the pristine one (Fig. 1b). The coated fibers maintained continuous morphologies, indicating the hydrothermal growth did not break the fibers. However, they felt harder and more fragile than the pristine fibers due to the increased diameters and emerged defects caused by the hydrothermal growth. We analyzed the diameters of the fibers before and after the hydrothermal growth (Fig. 1c and d). Most of the pristine ZrO2 fibers have diameters lower than 3.5 μm, showing a peak value in the range of 1.5–2 μm and a statistical average diameter of 2.0 μm ± 0.4 μm. After the hydrothermal coating procedure, the peak value shifts to the range of 2–2.5 μm, and the statistical average diameter of the coated fibers increased to 2.4 μm ± 0.4 μm, indicating the coatings have uniform thicknesses. This result demonstrates that the hydrothermal growth is preferred to give a uniform coating. By contrast, dip coating on fibers results in nonuniform coating because of numerous variables [32]. Fig. 1e presents the XRD patterns, in which the pristine ZrO2 fibers were in crystallized tetragonal phases [33]; after the hydrothermal growth, (101) facets assigned to anatase TiO2 appeared at 25° [34], indicating the presence of TiO2. There is a peak envelop located below 20° ascribed to fractural characters of amorphous TiO2, which is probably because of the low crystallinity of TiO2 [35]. As seen in Fig. 1f, the Raman spectra further demonstrate that TiO2 coating on ZrO2 fibers has the spectra of anatase TiO2 [36]; in addition, a TiO2 sheath@ZrO2 core
2.3. Characterizations A Hitachi S-4800 scanning electron microscope (SEM) was used to observe the morphologies and microstructures of the coatings grown on the fibers. The X-ray diffraction (XRD) patterns for the samples were collected using a Bruker AXSD8 Advance X-ray diffractometer with Cu Kα radiation (λ = 1.540598 Å) with a graphite monochromator in the 2θ range of 10–80° and a step size of 0.02°. Transmission electron microscopy (TEM) images were recorded using a JEM-200CX electron microscope operating at 20 kV. Diffuse reflectance (DR) spectra were measured using a Hitachi U-4150 UV–Vis spectrometer with an integrating sphere accessory. The chemical environment of the fibers was characterized by an X-ray photoelectron spectrophotometer of VG Scientific, with an X-ray source of Al Kα radiation at 1486.6 eV. IR spectra were recorded on a Nicolet 5DX-FTIR spectrometer using the KBr pellet method in the range of 4000–375 cm−1. Raman spectra were measured on a LabRAM HR800 spectrometer equipped with a CCD detector at room temperature and an excitation laser at 632.8 nm. 2
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Fig. 2. (a) XPS survey scan of pristine ZrO2 fibers and the fibers coated with TiO2, and the corresponding Ti 2p spectra (b), O 1s spectra (c), Zr 3d spectra.
coated with TiO2 shown in Fig. 4a clearly demonstrates that the TiO2 coating is uniform along with thickness. Seen from the SEM image, the coating bond the surfaces of the fibers firmly to form a compacted sheath that we have demonstrated in TEM and Raman spectra. The sheath is expected to be efficient for reflecting radiation for the central ZrO2 fibers which is characterized in the next section. The thicknesses of TiO2 coatings were measured and are shown in Fig. 4b–d. The thickness of the coating showed a growth-time dependence, producing thicknesses of 89 nm, 156 nm, and 236 nm for the TiO2 coatings grown for 6 h, 12 h, and 24 h, respectively. In Fig. 1, we have concluded that the statistical average diameter of the coated ZrO2 fibers hydrothermally grown for 12 h is 2.4 μm ± 0.4 μm in comparison with a 2.0 μm ± 0.4 μm diameter obtained for pristine ZrO2 fibers, indicating an increase of ca. 400 nm in diameter. The thickness is estimated qualitatively to be ca. 200 nm, which is close to the measured value in Fig. 4c, i.e., 156 nm. This result demonstrates that the coating is uniform in the thickness. After hydrothermal growth for 6 h, the prominent XRD peak, anatase (101) facets at 25°, was absent in the patterns of the coated fibers (Fig. 4e). With the growth-time increase, this peak appeared in the diffraction patterns of the coated fibers hydrothermally grown for 12 h and 24 h. Due to the high diffraction intensities of ZrO2 fiber, other related diffraction peaks ascribed to TiO2 were not observed in the XRD patterns. Whereas the Raman spectra (Fig. 4f) showed almost exclusively signals for TiO2 coatings because of the TiO2 sheath@ ZrO2 core structure, even in the fibers hydrothermally grown for 6 h, such signals were rarely observed by XRD diffraction. It is known that TiO2 has a high refractive index (2.52–2.72) compared with that of ZrO2 (ca. 2.17). Therefore, the TiO2 coatings on ZrO2 fibers are expected to reflect a considerable amount of electromagnetic radiation from ZrO2 fibers, improving physical properties that are radiation dependent. Herein, we first characterized the reflectivity of radiation with wavelengths in the range of 0.3–3 μm (Fig. 5a). The TiO2 coated fibers have low reflectivity below 0.4 μm. The interactions between electromagnetic radiation and an object involve reflection, adsorption, and transmission. Anatase TiO2 has a band gap of ca. 3 eV, resulting in high adsorption towards radiation with wavelengths below
structure was suggested because tetragonal ZrO2 [37] could not be detected in the coated fibers due to the shielding effects of TiO2 sheath. XPS spectra were used to collect further information about the TiO2 coating. In the survey scan (Fig. 2a), the spectra demonstrate the incorporation of Zr, C, O, Y (being as phase stabilizer) in pristine ZrO2 fibers. After the hydrothermal growth by 12 h, Ti 2p peak was observed in the fibers coated with TiO2 at ca. 460 eV. A high-resolution scan of Ti 2p (Fig. 2b) showed bimodal peaks with a differential value of ca. 6 eV, corresponding to quadrivalent Ti(Ⅳ) [34]. After deconvolution of O 1s spectra, the new component located at ca. 530 eV may attribute to TiO2, further confirming the incorporation of TiO2 coating on ZrO2 fibers. The coated TiO2 on ZrO2 fibers caused an obvious enlargement in the binding energy of Zr 3d, as shown in Fig. 2d. This result indicates that the TiO2 coating and the ZrO2 fibers are connected by chemical linkages of Ti–O–Zr which substitute the intrinsic Zr–OH on fiber surfaces and cause the decrease of charge density around Zr and an increase of binding energy [38]. Fig. 3a shows a typical TEM image of an individual ZrO2 fiber coated with TiO2 grown for 12 h. Due to electronic contrast, TiO2 coating was distinguished on the central ZrO2 fibers unambiguously, showing the thickness of ca. 150 nm. Preferentially grown (101) facets of anatase TiO2 have an interplanar spacing of 0.35 nm (Fig. 3b) [39], which was further confirmed by the FFT image (Fig. 3d). The ZrO2 fibers were thick, as shown in the TEM image, and they were very difficult to penetrate with an electron beam; therefore, the interface between TiO2 and ZrO2 was unclear when evaluated by electron images (Fig. 3b). Because the XPS results illustrated chemical bonding between TiO2 and ZrO2, the crystallographic textures of TiO2 on ZrO2 may have a preferred orientation that is yet to be determined. Nevertheless, the cross-section (zone B in Fig. 3a) of the ZrO2 fibers gave thin ZrO2 crystal domains in which the crystal texture of ZrO2 could be characterized. Fig. 3c shows HRTEM image of ZrO2 fibers and the (101) facets of tetragonal ZrO2 with an interplanar spacing of 0.29 nm [40]. Two primary crystal facets, (101) and (102), are observed in the FFT images, as shown in Fig. 3e. A typical SEM image of a cross-section of an individual ZrO2 fiber 3
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Fig. 3. (a) TEM image of a typical TiO2 coating on ZrO2 fibers, HRTEM image of (a) zone A in TEM image showing crystallographic TiO2 and (d) corresponding FFT image, HRTEM image of (c) zone B in TEM image showing crystallographic ZrO2 and (e) corresponding FFT image.
Fig. 4. SEM image of (a) a typical cross-section of individual ZrO2 fibers coated with TiO2, showing the homogeneous thickness. SEM images of (b) thickness measurements of the fibers with TiO2 coatings by hydrothermal growth 6 h, (c) 12 h, (d) 24 h. (e) XRD patterns and (f) Raman spectra of the fibers with and without TiO2 coatings by hydrothermal growth at different times.
0.4 μm. In contrast, ZrO2 has low ultraviolet adsorption so that the pristine ZrO2 fibers exhibited high reflectivity for the radiation below 0.4 μm. Because of the high refractive index, the TiO2 coatings reflected the majority of radiation above 0.4 μm, endowing the fibers a high reflectivity for both the visible region and the near-infrared region. The TiO2 coating grown for 6 h has a higher reflectivity than that of the pristine ZrO2 fibers. As the growth-time increased to 12 h, the
Fig. 5. (a) Visible and infrared reflective abilities of the fibers with and without a TiO2 coating. (b) Thermal conductivities of pristine ZrO2 fibers and the fibers coated with TiO2 grown for 12 h.
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reflectivity of the fibers amplified, indicating the TiO2 coatings improve the reflective abilities of the ZrO2 fibers toward electromagnetic radiation unambiguously. However, as the growth-time continued to increase, the reflectivity of the fibers with TiO2 coating reduced, as the reflectivity obtained after 24 h is lower than that obtained after 12 h. The interaction between radiation and an object not only depends on the intrinsic properties, i.e., the refractive index, but is also affected by the surface morphologies of the interacted plane [41]. In our situation, we suggest that the decrease of reflectivity for the fibers coated with TiO2 grown for 24 h may result from the coarsening of TiO2 grains, improving diffuse reflection on fiber surfaces. The radiation with wavelengths above 3 μm seem to be sensitive to the roughness of the interacted plane, as shown in the inset of Fig. 5a. The transmittance of the fibers with and without a TiO2 coating exhibited a tendency in which the increase of thickness of the TiO2 coating reduced the transmittances of infrared radiation. This reduction may be due to either being reflected by the TiO2 coating or absorbed by the incorporation of TiO2. Radiation is a major pathway of heat transfer, especially at high temperature. Thus, the reflection of radiation is expected to prevent heat transfer. Following this instruction, we tested the thermal conductivities of ZrO2 fibers with and without a TiO2 coating in a temperature range from 600 to 900 °C by a laser flash diffusivity apparatus in a vacuum. The vacuum eliminates convection heat transfer, permitting only conduction and radiation heat transfer. According to the Wien law (i.e., λ·T = b, b = 0.002897 m K), radiation with wavelengths of 3.5–2.5 μm corresponds to the above temperature range of 600–900 °C. Herein, we choose the ZrO2 fibers with TiO2 coatings grown for 12 h as the study object, which show higher radiation reflection in the range of 2.5–3.5 μm compared to those of the other samples. However, as shown in Fig. 5b, the fibers with TiO2 coatings exhibited larger thermal conductivities compared to that of the pristine ZrO2 fibers overall. Both the pristine ZrO2 fibers and the fibers coated with TiO2 presented increased thermal conductivities as the temperature increases. This increase occurs because both the conduction and radiation increase as the temperature increases. It should be noted that the fibers coated with TiO2 showed a lower increased rate of thermal conductivities versus temperature compared to that of the pristine ZrO2 fibers (see the slopes of fitting curves of thermal conductivities). Concerning these results of the thermal conductivities, we provide the following discussion: first, the fibers coated with TiO2 exhibited higher thermal conductivities compared to that of the pristine ZrO2 fibers because the TiO2 coating remained in contact within the test samples, forming thermal bridges through which the heat transfer could occur faster than that observed in ZrO2 fibers (schematically shown in the inset of Fig. 5b). Although the radiation heat transfer was suppressed to some extent by the TiO2 coating, the enhanced conduction heat transfer dominates in the final thermal conductivities. Nevertheless, the thermal conductivities of the fibers coated with TiO2 exhibited weak temperature dependence, which was likely because of the suppressed radiation heat transfer by the coating. Based on these results, we suggested that the ZrO2 fibers with radiation reflective TiO2 coatings may find appropriate applications as reinforcement for bulk ceramics, thermal barrier coatings, aerogels, etc., in which the fibers are dispersive, thus avoiding the thermal bridges produced by surface coated TiO2. Related studies are ongoing in our laboratories, and further results will be reported later.
signals of the ZrO2 fibers. In a temperature range of 600–900 °C, the ZrO2 fibers with the TiO2 coating exhibited considerable reflective abilities for the corresponding wavelengths of 2.5–3.5 μm, which are expected to reduce thermal conductivities. However, the fibers coated with TiO2 had higher thermal conductivities compared to that of the pristine ZrO2 fibers because the TiO2 coating caused the fibers to contact each other, forming thermal bridges in the test sample and endowing high thermal conductivities. The ZrO2 fibers coated with TiO2 may be applied in a dispersive state, for instance, as reinforcement for bulk ceramics, thermal barrier coatings, aerogels, etc. Further studies are in progress in our laboratory and will be reported later. Acknowledgements China Postdoctoral Science Foundation (2019M652452), Shandong Provincial Natural Science Foundation of China (ZR2017BEM009), Youth Science Funds of Shandong Academy of Sciences (2018QN0031), and the Shandong University Young Scholars Program (2016WLJH27) are acknowledged for their financial support. References [1] X. Yue, T. Zhang, D. Yang, F. Qiu, G. Wei, Y. Lv, A robust Janus fibrous membrane with switchable infrared radiation properties for potential building thermal management applications, J. Mater. Chem. A. 7 (2019) 8344–8352. [2] C. Xu, G.T. Stiubianu, A.A. Gorodetsky, Adaptive infrared-reflecting systems inspired by cephalopods, Science 359 (2018) 1495–1500. [3] X. Yue, T. Zhang, D. Yang, F. Qiu, Z. Li, G. Wei, Y. Qiao, Ag nanoparticles coated cellulose membrane with high infrared reflection, breathability and antibacterial property for human thermal insulation, J. Colloid Interface Sci. 535 (2019) 363–370. [4] M.J. Moghimi, G. Lin, H. Jiang, Broadband and ultrathin infrared stealth sheets, Adv. Eng. Mater. 20 (2018) 1800038. [5] D.B. Marshall, F.F. Lange, P.D. Morgan, High-strength zirconia fibers, J. Am. Ceram. Soc. 70 (1987) C187–C188. [6] G. De, A. Chatterjee, D. Ganguli, Zirconia fibers from the zirconiumn-propoxideacetylacetone-water-isopropanol system, J. Mater. Sci. Lett. 9 (1990) 845–846. [7] Y. Abe, H. Tomioka, T. Gunji, Y. Nagao, T. Misono, A one-pot synthesis of polyzirconoxane as a precursor for continuous zirconia fibers, J. Mater. Sci. Lett. 13 (1994) 960–962. [8] B. Clauss, A. Grub, W. Oppermann, Continuous yttria-stabilized zirconia fibers, Adv. Mater. 8 (1996) 142–146. [9] X. Mao, H. Shan, J. Song, Y. Bai, J. Yu, B. Ding, Brittle-flexible-brittle transition in nanocrystalline zirconia nanofibrous membranes, CrystEngComm 18 (2016) 1139–1146. [10] L. Wang, B. Liu, Y. Xie, D. Ma, L. Zhu, X. Wang, Effect of high-pressure vapor pretreatment on the microstructure evolution and tensile strength of zirconia fibers, J. Am. Ceram. Soc. 102 (2019) 4450–4458. [11] H.G. Scott, Phase relationships in the zirconia-yttria system, J. Mater. Sci. 10 (1975) 1527–1535. [12] K.W. Schlichting, N.P. Padture, P.G. Klemens, Thermal conductivity of dense and porous yttria-stabilized zirconia, J. Mater. Sci. 36 (2001) 3003–3010. [13] M. Hoch, M. Nakata, H.L. Johnston, Vapor pressures of inorganic substances. XII. zirconium dioxide, J. Am. Chem. Soc. 79 (1954) 2651–2652. [14] H.A. Battez, R. Gonzalez, J.L. Viesca, J.E. Fernandez, J.M. Diaz Fernandez, A. Machado, R. Chou, J. Riba, CuO, ZrO2 and ZnO nanoparticles as anti-wear additive in oil lubricants, Wear 265 (2008) 422–428. [15] I. Espitia-Cabrera, H. Orozco-Hernández, R. Torres-Sánchez, M.E. Contreras-Garcıa, P. Bartolo-Pérez, L. Martinez, Synthesis of nanostructured zirconia electrodeposited films on AISI 316L stainless steel and its behavior in corrosion resistance assessment, Mater. Lett. 58 (2004) 191–195. [16] T. Wang, Q. Yu, J. Kong, Preparation and heat-insulating properties of biomorphic ZrO2 hollow fibers derived from a cotton template, Int. J. Appl. Ceram. Technol. 15 (2018) 472–478. [17] P. Jeevanandam, R.S. Mulukutla, M. Phillips, S. Chaudhuri, L.E. Erickson, K.J. Klabunde, Near infrared reflectance properties of metal oxide nanoparticles, J. Phys. Chem. C 111 (2007) 1912–1918. [18] X. Gan, Z. Yu, K. Yuan, C. Xu, X. Wang, L. Zhu, G. Zhang, D. Xu, Preparation of CeO2-nanoparticle thermal radiation shield coating on ZrO2 fibers via a hydrothermal method, Ceram. Int. 43 (2017) 14183–14191. [19] S.D. Wang, J.Y. Wang, Anti-reflective and superhydrophobic films prepared from a sol at different withdrawal speeds, Appl. Surf. Sci. 476 (2019) 1035–1048. [20] E. Mudra, I. Shepa, O. Milkovic, Z. Dankova, A. Kovalcikova, A. Annusova, E. Majkova, J. Dusza, Effect of iron doping on the properties of SnO2 nano/microfibers, Appl. Surf. Sci. 480 (2019) 876–881. [21] Z. Duan, X. Zhao, J. Xu, P. Wang, W. Liu, Influence of Ni13+ ions irradiation on the microstructure, mechanical and tribological properties of Mo-S-Ti composite films, Appl. Surf. Sci. 480 (2019) 438–447. [22] L. Chen, E. Liu, F. Teng, T. Zhang, J. Feng, Y. Kou, Q. Sun, J. Fan, X. Hu, H. Miao,
4. Conclusions In summary, we hydrothermally grew uniform TiO2 coatings with thicknesses ranging from dozens to hundreds of nanometers on ZrO2 fibers. The coating tightly bonds with the ZrO2 fibers via Zr–O–Ti chemical linkages, and the thicknesses could be tailored by varying the growth time. TiO2 coating, acting as a sheath towards electromagnetic radiation, not only reflected light with wavelengths ranging from the visible region to infrared region up to 8 μm but also shielded the Raman 5
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Hydrothermally grown uniform TiO2 coatings on ZrO2 fibers and their infrared reflective and thermal conductive properties Dehua Maa, Luyi Zhua,∗, Benxue Liub,∗∗ a b
State Key Laboratory of Crystal Materials and Institute of Crystal Materials, Shandong University, Jinan, 250100, PR China Qilu University of Technology (Shandong Academy of Science), Advanced Materials Institute, Jinan, 250014, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Infrared reflection Thermal properties TiO2 ZrO2 fibers Hydrothermal growth
Improving the infrared reflectivity of ZrO2 polycrystalline fibers is of great benefit to its thermal applications. In the present research, we cast a highly uniform TiO2 coating with a thickness ranging from dozens to hundreds of nanometers on ZrO2 fibers by utilizing hydrothermal growth. The coating bonds tightly to the ZrO2 fibers via Zr–O–Ti chemical linkages, and the thickness of the coating can be tailored by varying the hydrothermal growth time. The TiO2 coating, acting as a sheath towards electromagnetic radiation, not only reflected light with wavelengths ranging from the visible region to the infrared region and up to 8 μm but also shielded the Raman signals of the ZrO2 fibers. The present research provides an efficient way to cast controllable and uniform coatings on flexible fiber materials. The obtained ZrO2 fibers coated with TiO2 may have applications such as reinforcement for bulk ceramics, thermal barrier coatings, aerogels, etc., thus performing the dual functions of mechanical strengthening and thermal insulation.
1. Introduction Manipulation of infrared radiation plays a significant role in thermal management because the infrared radiation uptakes a considerable amount of thermal energy, especially when exposed to a high temperature [1]. An object interacts with infrared radiation by means of transmission, adsorption and reflection. Tailoring the infrared reflection of an object has turned into a hot topic [2] recently because, by efficiently reflecting infrared radiation, applications such as thermal insulation and infrared stealth could be achieved [3,4]. Zirconia (ZrO2) fibers [5–8] are leading-edge polycrystalline fibers due to their large aspect ratios, high mechanical strength, and flexibilities [9,10], as well as some intrinsic physicochemical properties of ZrO2, for instance, high melting point, phase transformation induced toughness, corrosion and oxidation resistance, low phonon conduction, etc. [11–15]. These fibers are promising thermal insulating materials [16] in various thermal protection systems utilized in both civilian and military areas. Thus, the improvement of the infrared reflection of ZrO2 fibers may result in the enhanced thermal insulation and infrared stealth performances of a thermal protection system using ZrO2 fibers. However, to our knowledge, there have only been a few reports concerning this topic. Reflective coating is an efficient way to modify infrared reflective properties without altering the intrinsic properties of substrates. ∗
According to the Fresnel equation, a refractive index higher than that of the substrate promotes enhanced infrared radiation [17]. Hence, casting reflective coatings with a high refractive index on ZrO2 fibers may be a promising way to improve infrared radiation reflection and thermal properties [18]. Regardless of the substrates, coating methods involve dip [19], spin [20], deposition by physical processes (e.g., sputtering [21]), chemical processes (e.g., chemical vapor deposition [22]), etc. However, applying these coating processes for flexible materials produces drawbacks, such as the need for flat or near-flat substrates for spin and deposition methods and a higher cost. Dip coating seems to be efficient for casting infrared reflective coatings for ZrO2 fibers. However, it is less controllable, thus causing the thickness and uniformity of the coating to be difficult to manipulate. During the dip coating of flexible fiber materials, when two or more fibers are in close contact, the coating easily bridges the two individual fibers instead of uniformly coating them [23,24]. Recently, hydrothermal growth to cast coatings on flexible fiber materials attracted research interests because the resulting coatings are found to be very uniform [25]. Meanwhile, it is easy to scale up cost-effectively to meet the large area requirements of commercial applications. Based on the above statements, in the present research, we try to hydrothermally grow infrared reflective coatings on ZrO2 fibers in order to enhance their infrared radiation reflection. Titanium dioxide (TiO2)
Corresponding author. Corresponding author. E-mail addresses: [email protected] (L. Zhu), [email protected] (B. Liu).
∗∗
https://doi.org/10.1016/j.ceramint.2019.10.050 Received 15 August 2019; Received in revised form 11 September 2019; Accepted 5 October 2019 0272-8842/ © 2019 Published by Elsevier Ltd.
Please cite this article as: Dehua Ma, Luyi Zhu and Benxue Liu, Ceramics International, https://doi.org/10.1016/j.ceramint.2019.10.050
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has a high refractive index (2.52–2.72) that is greater than that of ZrO2 (ca. 2.17), which is an ideal infrared reflective material [26]. In particular, TiO2 has a high melting point, better chemical stability, and oxidation resistance [27–30], making the TiO2 coated ZrO2 fibers appropriate for high-temperature thermal applications. Herein, we prepared TiO2 infrared reflective coatings on ZrO2 fibers by the hydrothermal growth process. The microstructures of TiO2 coating, the growth-time dependent thickness, the bonding between coating and substrate, the infrared radiation reflective properties, and the thermal conductivities of the coated ZrO2 fibers were fully studied. Specifically, the commonly used titanium precursors, e.g., tetrabutyl titanate and titanium tetrachloride, usually result in large thicknesses, reaching thicknesses up to a micrometer size, which is comparable with the diameters of the ZrO2 fibers. In the present research, we report the use of an acetylacetonate modified titanium precursor, polyacetylacetonatotitanium [31], to achieve more controllable TiO2 coatings with the thicknesses of dozens to hundreds of nanometers. 2. Experimental 2.1. Starting materials A modified titanium precursor, polyacetylacetonatotitanium (PAT), was synthesized by the coordination reaction between partially hydrolyzed titanium tetrachloride and acetylacetonate in a molar ratio of 1:1. The chloride ions were fully removed by a precipitation reaction after adding stoichiometric amount of triethylamine. PAT was obtained by distillation, yielding yellowish powders. The detailed synthesis of PAT could be referred in previous literature [31]. The remaining chemical reagents were purchased from chemical agents without further purification, such as, isopropanol (≥99.9%, Aladdin), hydrofluoric acid (≥40%, Aladdin), zirconia fibers (Shandong Dipole New Energy Saving Materials Co., Ltd).
Fig. 1. SEM images of (a) pristine ZrO2 fibers and (b) the fibers coated with TiO2, diameter distributions of (c) pristine ZrO2 fibers and (d) the fibers coated with TiO2. (e) XRD patterns and (f) Raman spectra of pristine ZrO2 fibers and the fibers coated with TiO2.
Thermal conductivity was measured by using a NETZSCH LFA457 instrument (LFA457, NETZSCH, Germany), and the value at each point was the average of triplicate analysis with a standard derivation of less than 0.01 W/(m·K).
2.2. Hydrothermal growth of the TiO2 coating on the ZrO2 fibers TiO2 coatings were cast on ZrO2 fibers via a hydrothermal growth process in stainless-steel high-pressure reactors equipped with Teflon linings at 180 °C. The filled degree of the Teflon lining reached 80%, thus achieving a satisfactory high-pressure environment. A typical growth solution was acquired by mixing 1 g PAT, 0.5 ml hydrofluoric acid, and 40 ml isopropanol under vigorous stirring. In each bath, 0.1 g of fluffy ZrO2 fibers were added into the dissolved growth solution. The hydrothermal time variable indicated that the growth at 180 °C lasted for 6 h, 12 h, or 24 h. Then, the reactors were taken out of the furnace and quenched with cold water, and the TiO2 -coated ZrO2 fibers were obtained.
3. Results and discussion Before coating, the ZrO2 fibers presented continuous morphologies with uniform diameters as shown in Fig. 1a. In previous studies, we found that these fibers show considerable mechanical strengths, i.e., above 1 GPa [10], guaranteeing the integrity of the fibers under hydrothermal growth process. After coating at 180 °C with a duration of 12 h, the fibers were obviously thicker than that of the pristine one (Fig. 1b). The coated fibers maintained continuous morphologies, indicating the hydrothermal growth did not break the fibers. However, they felt harder and more fragile than the pristine fibers due to the increased diameters and emerged defects caused by the hydrothermal growth. We analyzed the diameters of the fibers before and after the hydrothermal growth (Fig. 1c and d). Most of the pristine ZrO2 fibers have diameters lower than 3.5 μm, showing a peak value in the range of 1.5–2 μm and a statistical average diameter of 2.0 μm ± 0.4 μm. After the hydrothermal coating procedure, the peak value shifts to the range of 2–2.5 μm, and the statistical average diameter of the coated fibers increased to 2.4 μm ± 0.4 μm, indicating the coatings have uniform thicknesses. This result demonstrates that the hydrothermal growth is preferred to give a uniform coating. By contrast, dip coating on fibers results in nonuniform coating because of numerous variables [32]. Fig. 1e presents the XRD patterns, in which the pristine ZrO2 fibers were in crystallized tetragonal phases [33]; after the hydrothermal growth, (101) facets assigned to anatase TiO2 appeared at 25° [34], indicating the presence of TiO2. There is a peak envelop located below 20° ascribed to fractural characters of amorphous TiO2, which is probably because of the low crystallinity of TiO2 [35]. As seen in Fig. 1f, the Raman spectra further demonstrate that TiO2 coating on ZrO2 fibers has the spectra of anatase TiO2 [36]; in addition, a TiO2 sheath@ZrO2 core
2.3. Characterizations A Hitachi S-4800 scanning electron microscope (SEM) was used to observe the morphologies and microstructures of the coatings grown on the fibers. The X-ray diffraction (XRD) patterns for the samples were collected using a Bruker AXSD8 Advance X-ray diffractometer with Cu Kα radiation (λ = 1.540598 Å) with a graphite monochromator in the 2θ range of 10–80° and a step size of 0.02°. Transmission electron microscopy (TEM) images were recorded using a JEM-200CX electron microscope operating at 20 kV. Diffuse reflectance (DR) spectra were measured using a Hitachi U-4150 UV–Vis spectrometer with an integrating sphere accessory. The chemical environment of the fibers was characterized by an X-ray photoelectron spectrophotometer of VG Scientific, with an X-ray source of Al Kα radiation at 1486.6 eV. IR spectra were recorded on a Nicolet 5DX-FTIR spectrometer using the KBr pellet method in the range of 4000–375 cm−1. Raman spectra were measured on a LabRAM HR800 spectrometer equipped with a CCD detector at room temperature and an excitation laser at 632.8 nm. 2
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Fig. 2. (a) XPS survey scan of pristine ZrO2 fibers and the fibers coated with TiO2, and the corresponding Ti 2p spectra (b), O 1s spectra (c), Zr 3d spectra.
coated with TiO2 shown in Fig. 4a clearly demonstrates that the TiO2 coating is uniform along with thickness. Seen from the SEM image, the coating bond the surfaces of the fibers firmly to form a compacted sheath that we have demonstrated in TEM and Raman spectra. The sheath is expected to be efficient for reflecting radiation for the central ZrO2 fibers which is characterized in the next section. The thicknesses of TiO2 coatings were measured and are shown in Fig. 4b–d. The thickness of the coating showed a growth-time dependence, producing thicknesses of 89 nm, 156 nm, and 236 nm for the TiO2 coatings grown for 6 h, 12 h, and 24 h, respectively. In Fig. 1, we have concluded that the statistical average diameter of the coated ZrO2 fibers hydrothermally grown for 12 h is 2.4 μm ± 0.4 μm in comparison with a 2.0 μm ± 0.4 μm diameter obtained for pristine ZrO2 fibers, indicating an increase of ca. 400 nm in diameter. The thickness is estimated qualitatively to be ca. 200 nm, which is close to the measured value in Fig. 4c, i.e., 156 nm. This result demonstrates that the coating is uniform in the thickness. After hydrothermal growth for 6 h, the prominent XRD peak, anatase (101) facets at 25°, was absent in the patterns of the coated fibers (Fig. 4e). With the growth-time increase, this peak appeared in the diffraction patterns of the coated fibers hydrothermally grown for 12 h and 24 h. Due to the high diffraction intensities of ZrO2 fiber, other related diffraction peaks ascribed to TiO2 were not observed in the XRD patterns. Whereas the Raman spectra (Fig. 4f) showed almost exclusively signals for TiO2 coatings because of the TiO2 sheath@ ZrO2 core structure, even in the fibers hydrothermally grown for 6 h, such signals were rarely observed by XRD diffraction. It is known that TiO2 has a high refractive index (2.52–2.72) compared with that of ZrO2 (ca. 2.17). Therefore, the TiO2 coatings on ZrO2 fibers are expected to reflect a considerable amount of electromagnetic radiation from ZrO2 fibers, improving physical properties that are radiation dependent. Herein, we first characterized the reflectivity of radiation with wavelengths in the range of 0.3–3 μm (Fig. 5a). The TiO2 coated fibers have low reflectivity below 0.4 μm. The interactions between electromagnetic radiation and an object involve reflection, adsorption, and transmission. Anatase TiO2 has a band gap of ca. 3 eV, resulting in high adsorption towards radiation with wavelengths below
structure was suggested because tetragonal ZrO2 [37] could not be detected in the coated fibers due to the shielding effects of TiO2 sheath. XPS spectra were used to collect further information about the TiO2 coating. In the survey scan (Fig. 2a), the spectra demonstrate the incorporation of Zr, C, O, Y (being as phase stabilizer) in pristine ZrO2 fibers. After the hydrothermal growth by 12 h, Ti 2p peak was observed in the fibers coated with TiO2 at ca. 460 eV. A high-resolution scan of Ti 2p (Fig. 2b) showed bimodal peaks with a differential value of ca. 6 eV, corresponding to quadrivalent Ti(Ⅳ) [34]. After deconvolution of O 1s spectra, the new component located at ca. 530 eV may attribute to TiO2, further confirming the incorporation of TiO2 coating on ZrO2 fibers. The coated TiO2 on ZrO2 fibers caused an obvious enlargement in the binding energy of Zr 3d, as shown in Fig. 2d. This result indicates that the TiO2 coating and the ZrO2 fibers are connected by chemical linkages of Ti–O–Zr which substitute the intrinsic Zr–OH on fiber surfaces and cause the decrease of charge density around Zr and an increase of binding energy [38]. Fig. 3a shows a typical TEM image of an individual ZrO2 fiber coated with TiO2 grown for 12 h. Due to electronic contrast, TiO2 coating was distinguished on the central ZrO2 fibers unambiguously, showing the thickness of ca. 150 nm. Preferentially grown (101) facets of anatase TiO2 have an interplanar spacing of 0.35 nm (Fig. 3b) [39], which was further confirmed by the FFT image (Fig. 3d). The ZrO2 fibers were thick, as shown in the TEM image, and they were very difficult to penetrate with an electron beam; therefore, the interface between TiO2 and ZrO2 was unclear when evaluated by electron images (Fig. 3b). Because the XPS results illustrated chemical bonding between TiO2 and ZrO2, the crystallographic textures of TiO2 on ZrO2 may have a preferred orientation that is yet to be determined. Nevertheless, the cross-section (zone B in Fig. 3a) of the ZrO2 fibers gave thin ZrO2 crystal domains in which the crystal texture of ZrO2 could be characterized. Fig. 3c shows HRTEM image of ZrO2 fibers and the (101) facets of tetragonal ZrO2 with an interplanar spacing of 0.29 nm [40]. Two primary crystal facets, (101) and (102), are observed in the FFT images, as shown in Fig. 3e. A typical SEM image of a cross-section of an individual ZrO2 fiber 3
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Fig. 3. (a) TEM image of a typical TiO2 coating on ZrO2 fibers, HRTEM image of (a) zone A in TEM image showing crystallographic TiO2 and (d) corresponding FFT image, HRTEM image of (c) zone B in TEM image showing crystallographic ZrO2 and (e) corresponding FFT image.
Fig. 4. SEM image of (a) a typical cross-section of individual ZrO2 fibers coated with TiO2, showing the homogeneous thickness. SEM images of (b) thickness measurements of the fibers with TiO2 coatings by hydrothermal growth 6 h, (c) 12 h, (d) 24 h. (e) XRD patterns and (f) Raman spectra of the fibers with and without TiO2 coatings by hydrothermal growth at different times.
0.4 μm. In contrast, ZrO2 has low ultraviolet adsorption so that the pristine ZrO2 fibers exhibited high reflectivity for the radiation below 0.4 μm. Because of the high refractive index, the TiO2 coatings reflected the majority of radiation above 0.4 μm, endowing the fibers a high reflectivity for both the visible region and the near-infrared region. The TiO2 coating grown for 6 h has a higher reflectivity than that of the pristine ZrO2 fibers. As the growth-time increased to 12 h, the
Fig. 5. (a) Visible and infrared reflective abilities of the fibers with and without a TiO2 coating. (b) Thermal conductivities of pristine ZrO2 fibers and the fibers coated with TiO2 grown for 12 h.
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reflectivity of the fibers amplified, indicating the TiO2 coatings improve the reflective abilities of the ZrO2 fibers toward electromagnetic radiation unambiguously. However, as the growth-time continued to increase, the reflectivity of the fibers with TiO2 coating reduced, as the reflectivity obtained after 24 h is lower than that obtained after 12 h. The interaction between radiation and an object not only depends on the intrinsic properties, i.e., the refractive index, but is also affected by the surface morphologies of the interacted plane [41]. In our situation, we suggest that the decrease of reflectivity for the fibers coated with TiO2 grown for 24 h may result from the coarsening of TiO2 grains, improving diffuse reflection on fiber surfaces. The radiation with wavelengths above 3 μm seem to be sensitive to the roughness of the interacted plane, as shown in the inset of Fig. 5a. The transmittance of the fibers with and without a TiO2 coating exhibited a tendency in which the increase of thickness of the TiO2 coating reduced the transmittances of infrared radiation. This reduction may be due to either being reflected by the TiO2 coating or absorbed by the incorporation of TiO2. Radiation is a major pathway of heat transfer, especially at high temperature. Thus, the reflection of radiation is expected to prevent heat transfer. Following this instruction, we tested the thermal conductivities of ZrO2 fibers with and without a TiO2 coating in a temperature range from 600 to 900 °C by a laser flash diffusivity apparatus in a vacuum. The vacuum eliminates convection heat transfer, permitting only conduction and radiation heat transfer. According to the Wien law (i.e., λ·T = b, b = 0.002897 m K), radiation with wavelengths of 3.5–2.5 μm corresponds to the above temperature range of 600–900 °C. Herein, we choose the ZrO2 fibers with TiO2 coatings grown for 12 h as the study object, which show higher radiation reflection in the range of 2.5–3.5 μm compared to those of the other samples. However, as shown in Fig. 5b, the fibers with TiO2 coatings exhibited larger thermal conductivities compared to that of the pristine ZrO2 fibers overall. Both the pristine ZrO2 fibers and the fibers coated with TiO2 presented increased thermal conductivities as the temperature increases. This increase occurs because both the conduction and radiation increase as the temperature increases. It should be noted that the fibers coated with TiO2 showed a lower increased rate of thermal conductivities versus temperature compared to that of the pristine ZrO2 fibers (see the slopes of fitting curves of thermal conductivities). Concerning these results of the thermal conductivities, we provide the following discussion: first, the fibers coated with TiO2 exhibited higher thermal conductivities compared to that of the pristine ZrO2 fibers because the TiO2 coating remained in contact within the test samples, forming thermal bridges through which the heat transfer could occur faster than that observed in ZrO2 fibers (schematically shown in the inset of Fig. 5b). Although the radiation heat transfer was suppressed to some extent by the TiO2 coating, the enhanced conduction heat transfer dominates in the final thermal conductivities. Nevertheless, the thermal conductivities of the fibers coated with TiO2 exhibited weak temperature dependence, which was likely because of the suppressed radiation heat transfer by the coating. Based on these results, we suggested that the ZrO2 fibers with radiation reflective TiO2 coatings may find appropriate applications as reinforcement for bulk ceramics, thermal barrier coatings, aerogels, etc., in which the fibers are dispersive, thus avoiding the thermal bridges produced by surface coated TiO2. Related studies are ongoing in our laboratories, and further results will be reported later.
signals of the ZrO2 fibers. In a temperature range of 600–900 °C, the ZrO2 fibers with the TiO2 coating exhibited considerable reflective abilities for the corresponding wavelengths of 2.5–3.5 μm, which are expected to reduce thermal conductivities. However, the fibers coated with TiO2 had higher thermal conductivities compared to that of the pristine ZrO2 fibers because the TiO2 coating caused the fibers to contact each other, forming thermal bridges in the test sample and endowing high thermal conductivities. The ZrO2 fibers coated with TiO2 may be applied in a dispersive state, for instance, as reinforcement for bulk ceramics, thermal barrier coatings, aerogels, etc. Further studies are in progress in our laboratory and will be reported later. Acknowledgements China Postdoctoral Science Foundation (2019M652452), Shandong Provincial Natural Science Foundation of China (ZR2017BEM009), Youth Science Funds of Shandong Academy of Sciences (2018QN0031), and the Shandong University Young Scholars Program (2016WLJH27) are acknowledged for their financial support. References [1] X. Yue, T. Zhang, D. Yang, F. Qiu, G. Wei, Y. Lv, A robust Janus fibrous membrane with switchable infrared radiation properties for potential building thermal management applications, J. Mater. Chem. A. 7 (2019) 8344–8352. [2] C. Xu, G.T. Stiubianu, A.A. Gorodetsky, Adaptive infrared-reflecting systems inspired by cephalopods, Science 359 (2018) 1495–1500. [3] X. Yue, T. Zhang, D. Yang, F. Qiu, Z. Li, G. Wei, Y. Qiao, Ag nanoparticles coated cellulose membrane with high infrared reflection, breathability and antibacterial property for human thermal insulation, J. Colloid Interface Sci. 535 (2019) 363–370. [4] M.J. Moghimi, G. Lin, H. Jiang, Broadband and ultrathin infrared stealth sheets, Adv. Eng. Mater. 20 (2018) 1800038. [5] D.B. Marshall, F.F. Lange, P.D. Morgan, High-strength zirconia fibers, J. Am. Ceram. Soc. 70 (1987) C187–C188. [6] G. De, A. Chatterjee, D. Ganguli, Zirconia fibers from the zirconiumn-propoxideacetylacetone-water-isopropanol system, J. Mater. Sci. Lett. 9 (1990) 845–846. [7] Y. Abe, H. Tomioka, T. Gunji, Y. Nagao, T. Misono, A one-pot synthesis of polyzirconoxane as a precursor for continuous zirconia fibers, J. Mater. Sci. Lett. 13 (1994) 960–962. [8] B. Clauss, A. Grub, W. Oppermann, Continuous yttria-stabilized zirconia fibers, Adv. Mater. 8 (1996) 142–146. [9] X. Mao, H. Shan, J. Song, Y. Bai, J. Yu, B. Ding, Brittle-flexible-brittle transition in nanocrystalline zirconia nanofibrous membranes, CrystEngComm 18 (2016) 1139–1146. [10] L. Wang, B. Liu, Y. Xie, D. Ma, L. Zhu, X. Wang, Effect of high-pressure vapor pretreatment on the microstructure evolution and tensile strength of zirconia fibers, J. Am. Ceram. Soc. 102 (2019) 4450–4458. [11] H.G. Scott, Phase relationships in the zirconia-yttria system, J. Mater. Sci. 10 (1975) 1527–1535. [12] K.W. Schlichting, N.P. Padture, P.G. Klemens, Thermal conductivity of dense and porous yttria-stabilized zirconia, J. Mater. Sci. 36 (2001) 3003–3010. [13] M. Hoch, M. Nakata, H.L. Johnston, Vapor pressures of inorganic substances. XII. zirconium dioxide, J. Am. Chem. Soc. 79 (1954) 2651–2652. [14] H.A. Battez, R. Gonzalez, J.L. Viesca, J.E. Fernandez, J.M. Diaz Fernandez, A. Machado, R. Chou, J. Riba, CuO, ZrO2 and ZnO nanoparticles as anti-wear additive in oil lubricants, Wear 265 (2008) 422–428. [15] I. Espitia-Cabrera, H. Orozco-Hernández, R. Torres-Sánchez, M.E. Contreras-Garcıa, P. Bartolo-Pérez, L. Martinez, Synthesis of nanostructured zirconia electrodeposited films on AISI 316L stainless steel and its behavior in corrosion resistance assessment, Mater. Lett. 58 (2004) 191–195. [16] T. Wang, Q. Yu, J. Kong, Preparation and heat-insulating properties of biomorphic ZrO2 hollow fibers derived from a cotton template, Int. J. Appl. Ceram. Technol. 15 (2018) 472–478. [17] P. Jeevanandam, R.S. Mulukutla, M. Phillips, S. Chaudhuri, L.E. Erickson, K.J. Klabunde, Near infrared reflectance properties of metal oxide nanoparticles, J. Phys. Chem. C 111 (2007) 1912–1918. [18] X. Gan, Z. Yu, K. Yuan, C. Xu, X. Wang, L. Zhu, G. Zhang, D. Xu, Preparation of CeO2-nanoparticle thermal radiation shield coating on ZrO2 fibers via a hydrothermal method, Ceram. Int. 43 (2017) 14183–14191. [19] S.D. Wang, J.Y. Wang, Anti-reflective and superhydrophobic films prepared from a sol at different withdrawal speeds, Appl. Surf. Sci. 476 (2019) 1035–1048. [20] E. Mudra, I. Shepa, O. Milkovic, Z. Dankova, A. Kovalcikova, A. Annusova, E. Majkova, J. Dusza, Effect of iron doping on the properties of SnO2 nano/microfibers, Appl. Surf. Sci. 480 (2019) 876–881. [21] Z. Duan, X. Zhao, J. Xu, P. Wang, W. Liu, Influence of Ni13+ ions irradiation on the microstructure, mechanical and tribological properties of Mo-S-Ti composite films, Appl. Surf. Sci. 480 (2019) 438–447. [22] L. Chen, E. Liu, F. Teng, T. Zhang, J. Feng, Y. Kou, Q. Sun, J. Fan, X. Hu, H. Miao,
4. Conclusions In summary, we hydrothermally grew uniform TiO2 coatings with thicknesses ranging from dozens to hundreds of nanometers on ZrO2 fibers. The coating tightly bonds with the ZrO2 fibers via Zr–O–Ti chemical linkages, and the thicknesses could be tailored by varying the growth time. TiO2 coating, acting as a sheath towards electromagnetic radiation, not only reflected light with wavelengths ranging from the visible region to infrared region up to 8 μm but also shielded the Raman 5
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