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High-temperature tolerance in WTi-Al2O3 cermet-based solar selective absorbing coatings with low thermal emissivity
Nano Energy 37 (2017) 232–241
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High-temperature tolerance in WTi-Al2O3 cermet-based solar selective absorbing coatings with low thermal emissivity
MARK
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Xiaoyu Wanga,b, Junhua Gaoa, , Haibo Hua,b, Hongliang Zhangc, Lingyan Lianga, Kashif Javaida, ⁎ Fei Zhugec, Hongtao Caoa, , Le Wangd a Key Laboratory of Additive Manufacturing Materials of Zhejiang Province, Ningbo Institute of Materials Technology & Engineering, Chinese Academy of Sciences, Ningbo 315201, China b University of Chinese Academy of Sciences, 19 Yuquan Road, Beijing 100049, China c Key Laboratory of Graphene Technologies and Applications of Zhejiang Province, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China d College of Optical and Electronic Science and Technology, China Jiliang University, Hangzhou 310018, China
A R T I C L E I N F O
A BS T RAC T
Keywords: Solar thermal conversion Tungsten alloy nanoparticles Segregation Thermal stability Emissivity
Cermet-based solar selective absorbing coatings are widely used, however, the long-term thermal instability and pretty high infrared emissivity at high temperatures ( > 550 °C) are still challenging issues to be addressed, which essentially lies in suppressing the growing up and agglomeration behaviors of metal nanoparticles (NPs) and maintaining the interface integrity in the multi-layer stacked structure. Herein, we develop and explore WTi-Al2O3 cermet-based absorbing coatings, demonstrating a solar absorptance of ~93% and a very low thermal emissivity of 10.3% @500 °C even after annealing at 600 °C for 840 h in vacuum. It is revealed that the surface segregation of solute Ti atoms from the parent alloyed NPs and their partial oxidation to form protective layer restrain outward diffusion of W element, agglomeration of NPs, and interface structure degradation, in favor of enhancing the thermal tolerance of the coatings. These results suggest that the WTi-Al2O3 based absorbing coating is a good candidate for high-temperature solar thermal conversion.
1. Introduction Among solar energy utilizations, solar thermal techniques play a more and more important role in solar-energy harvesting, due to high energy-conversion efficiency and appealing energy storage functionality [1]. Currently, parabolic trough collector (PTC) has been commercially developed [2], in which solar selective absorbing coatings are one of the critical components to govern the overall performance of PTC system. Ideally speaking, this coating should possess complete absorptance in the solar spectral range with low heat loss to the ambient via infrared radiation. Certainly, good chemical inertness and thermal stability of the coating at elevated temperatures are also indispensable for long-term power generation. In addition, the higher temperature of the heat-transfer fluid (HTF) in PTC system, the higher Carnot efficiency of power plants [3]. In the conventional PTC system, mineral or synthetic oil is utilized as HTF, limiting the operating temperature around 400 °C. One of promising candidates of HTF is molten salt (such as 60% KNO3+40% NaNO3), as it can be operated at much higher temperatures (more than 550 °C) [2]. Along with the advance on molten salt-based PTC technologies, new spectrally selective coatings ⁎
Corresponding authors. E-mail addresses: [email protected] (J. Gao), [email protected] (H. Cao).
http://dx.doi.org/10.1016/j.nanoen.2017.05.036 Received 25 April 2017; Received in revised form 16 May 2017; Accepted 16 May 2017 Available online 17 May 2017 2211-2855/ © 2017 Elsevier Ltd. All rights reserved.
with good optical performance and excellent thermal stability are highly desirable for parabolic trough solar thermal power generation. In recent years, numerous efforts have been made for developing high-temperature solar selective absorbing coatings. Most of the studies focus on cermet-based coatings [4–11], where metal NPs are embedded in dielectric matrices to construct cermet absorbing layers. For example, Pt-Al2O3 coatings have been developed with high absorptance and low emissivity [11–14]. However, this coating is too expensive to be commercialized. Mo-SiO2 cermet is also intensively investigated to build up absorbing coatings based upon a double cermet layer configuration [4,5,15]. Nevertheless, the tolerable temperature for such coating is not high enough. Note that W-Al2O3 absorbing coatings have exhibited reasonably high thermal robustness [16]. After annealing at 580 °C for 2 days in vacuum, there was no obvious optical and structural degradation in the coating. Rebouta et al. [17] also found that the W-Al2O3-based coatings were quite stable even after heat-treatment at 580 °C for 850 h in vacuum. For further improving its operating temperature, Cao et al. [18] had constructed WNi-Al2O3 cermet-based coatings, demonstrating an almost constant solar absorptance of ~90% during annealing treatment at 600 °C for 7 days.
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Analogously, by using YSZ (yttria-stabilized zirconia) to replace Al2O3, the fabricated WNi-YSZ coating exhibited a higher solar absorptance of 91% with a lower thermal emissivity of 13% @500 °C after annealing at 600 °C [19]. Based on the above surveys, we believe that there is still room to improve the optical performance of cermet-based absorbing coatings, especially for Al2O3-matrix cermet cases. Generally speaking, the thermal-induced optical degradation of cermet-based coatings is mainly ascribed to the element migration, coalescence and/or oxidation of metal NPs, inter-diffusion between adjacent layers, and diffusion of substrate atoms into the coatings [20– 22]. In a sense, the thermal tolerance of metal NPs is the most importance concern for cermet-based absorbing coatings. Up to now, numerous efforts have been implemented to boost the thermal stability of metal NPs, such as covering oxide passivation layers [23–25], alloying [26–29], etc. For example, the Pt@SiO2 NPs could maintain their core-shell configuration even up to 750 °C [23]. PtRh alloy NPs still exhibited admirable catalytical capability at 650 °C [26]. In our previous study [30,31], the AgAl-Al2O3 cermet films could exhibit excellent thermal stability at 500 °C for ~1000 h. Likewise, the utilization of WNi alloy in ceramic matrix was beneficial for stabilizing WNPs-based cermets at elevated temperatures [18,19]. Thus, alloying method is believed to be an effective way to improve the thermal stability of metal NPs. Besides the thermal robustness, low thermal emissivity is another criterion for high temperature solar selective absorbing coatings. According to the Stefan-Boltzmann law [32], the emissivity is proportional to T4 (T is the working temperature). Thus, for designing hightemperature absorbing coatings, the issue of thermal emissivity should be put in more priority than the solar absorptance to be concerned, in order to minimize the thermal radiation loss as low as possible. Previous studies suggested that tungsten is one of the good candidates used as the infrared reflector layer [17,18]. In this work, we attempted to further strength the thermal tolerance of W-Al2O3 cermet by introducing Ti to form WTi alloy NPs, and the validity of this method was revealed by that the absorptance of singlelayer cermet underwent negligible changes after annealing at 600 °C for 312 h in argon ambient. A computer simulation was carried out to design W-Al2O3 absorbing coating with double cermet layer-stacked configuration. Based on the simulations, the target absorbing coatings with an Al2O3 anti-reflection layer/W-Al2O3 LMVF (low metal volume fraction) layer/WTi-Al2O3 HMVF (high metal volume fraction) layer/ Al2O3 barrier layer/W infrared reflective layer structure were successfully prepared. After annealing at 600 °C for 840 h in vacuum, the coating still exhibited a low thermal emissivity of 10.3% @500 °C and a desirable solar absorptance of ~93%. Finally, the thermal strengthening mechanism of the proposed coatings was systematically investigated. The excellent thermal stability with low infrared emissivity provides a great opportunity for the designed coatings to find applications in high-temperature solar-thermal conversion.
sputtering pressure of 0.21 Pa. The detailed preparation parameters are summarized in Supporting Information. For single-layer cermet's phase structure and compositional analysis, the fabricated W- and WTi-Al2O3 films were deposited onto quartz for absorptance and XRD testing or onto Si (100) for XPS analysis. For the Al2O3/W-Al2O3/Al2O3 or Al2O3/WTi-Al2O3/Al2O3 sandwich structure, the sputtering time of the cermet layer was 55 min, and the deposition time was 30 and 40 min for the bottom and top Al2O3 layer with a thickness of ~18 and 24 nm, respectively. With regard to solar selective absorbing coatings, the tandem structure was Al2O3(1)/double-cermet layer/Al2O3(2)/W from top to the substrate, where the Al2O3(1) and Al2O3(2) layer was served as the antireflection and diffusion barrier layer, respectively. The substrate was stainless steel or Si (100). All the annealing treatments were carried out in a tube furnace. The tube furnace was filled with argon gas for the annealing of single-layer cermet films, while the solar selective absorbing coating samples were firstly loaded into sealed quartz tubes exhausted to a pressure less than 0.1 Pa and then were packed into the tube furnace for heat treatment. For the annealing procedure, the temperature was rapidly increased to 600 °C firstly, then kept at 600 °C for 10 h, and subsequently rapidly dropped to room temperature. After the optical measurement, the samples were loaded into sealed quartz tubes again for long-term annealing. During the long-term heat treatment, the samples were heated up to 600 °C and then cooled down to room temperature after an interval of ~50 h (one cycle in our case), followed by the repeating heat treatment cycle.
2. Experimental section
In this paper, considering the fact that the content of metallic Ti in WTi-Al2O3 films is less than 10 at%, the computational design was mainly focused on W-Al2O3 coating for guiding the design of WTiAl2O3 coating. The detailed simulation processes are described in Supporting Information.
2.2. Characterization The optical spectra (including reflectance (R) and transmittance (T)) were obtained by a Lambda 950 spectrophotometer equipped with an integrating sphere (angle of incidence 0° and 8° for transmittance and reflectance, respectively) in the wavelength region ranging from 0.3 to 2.5 µm. The absorptance (A) was calculated by A=1-R-T. As for the mid-infrared region (2.5–25 µm), a Fourier transform infrared (FTIR) spectrometer (Thermo Nicolet Model-6700, angle of incidence 12°), with a mid-infrared integrating sphere, was employed to record the reflectance spectra at room temperature, which was calibrated with a standard Au reference. The XRD patterns were acquired using Cu Kα radiation (λ=1.54 Å) at a glazing incidence angle of 1°. XPS analysis was implemented to characterize the chemical compositions and the valence states. Planar and cross-sectional TEM images were acquired to identify the microstructural features of the samples. Focused ion beam (FIB) milling method was utilized for preparing cross-sectional TEM specimens. STEM analysis in conjunction with X-ray energydispersive spectroscopy (EDS) was carried out to detect the elemental distribution in WTi-Al2O3 coatings. 2.3. Computational design of solar selective absorbing coating
2.1. Samples preparation and annealing treatment Cermet films and solar selective absorbing coatings were deposited onto various substrates at room temperature by a multi target sputtering system (Jsputter8000, manufactured by ULVAC Co., Ltd.). The commercially available high-purity Al2O3 (99.99%, 2 in. diameter), W (99.99%, 2 in. diameter), and Ti (99.99%, 2 in. diameter) targets were used as the source materials. Before deposition, all the substrates were ultrasonically cleaned in acetone and alcohol baths for 20 min, then rinsed with deionized water, and finally dried under nitrogen flow. Two radio-frequency (RF, 13.56 MHz) power supplies were used to operate the Al2O3 target and the W target, while the Ti target was sputtered using a DC power source. The base pressure was less than 1×10−4 Pa, and all the depositions were implemented at a constant
3. Results and discussion 3.1. Thermal stability, phase structure and chemical composition of WTi-Al2O3 cermets We deposited W- and WTi-Al2O3 cermet layers on quartz substrates with a sandwich structure (Al2O3/cermet layer/Al2O3). Then the samples were successively annealed at 600 °C in argon ambient for 12 h, 112 h and 312 h. The preparation details are summarized in Table S1 (see Supporting Information). The top and bottom Al2O3 layer 233
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Fig. 1. Absorptance spectra of W-Al2O3 (a) and WTi-Al2O3 (~8 at% Ti) (b) samples under different annealing conditions. (c) XRD patterns of W- and WTi-Al2O3 (~8 at% Ti) cermet layers (including pristine and annealed in Ar ambient). The 47–1319 PDF card of β-W is also shown at the bottom of panel c. (d) XPS spectra of Ti 2p core levels for the WTi-Al2O3 (~8 at % Ti) samples. All the samples have the same sandwich structure of Al2O3/cermet layer/Al2O3.
nanocrystals [22,36]. After annealing, the intensity of the (210) peak almost kept constant. For the W-Al2O3 case, however, the diffraction intensity of each peak encountered a detectable reduction after annealing, probably due to the oxidation of metallic W during the annealing process. This speculation is clearly confirmed by the Raman results shown in Fig. S1 (see Supporting Information). The XPS spectrum of Ti 2p core levels in the WTi-Al2O3 sample was acquired after Ar plasma etching for 1920s to remove the top alumina layer completely. As shown in Fig. 1d, two Ti 2p bumps (shown by black line) are observed, which can be further deconvolved. Specifically, the binding energy located at 460.3 eV and 454.2 eV can be assigned to Ti 2p1/2 and Ti 2p3/2 of metallic Ti, respectively, in line with the previous study [37–39]. Furthermore, the oxidation state of Ti, with relatively high binding energies, can not be ruled out [40,41]. But nevertheless, the additives of Ti in the cermet mainly exist in the form of metallic state. After carefully calculating, the atomic ratio of Ti/ W was estimated to be about 1/11, corresponding to the atomic percent (Ti/(W+Ti)) of 8 at%. According to the previous studies on WTi alloys [42], the atomic ratio of Ti/W of 1/11 is within the solid solubility range between them. The inclusion of Ti would induce the heterogeneous nucleation and growth of metallic W (see Fig. S2 in Supporting Information), double confirming the successful alloying process. Meanwhile, no phase related to pure Ti was found based on TEM observations shown in Fig. S2. Taken the aforementioned results together, it is believed that the WTi-Al2O3 cermet is consisted of unalloyed W and alloyed WTi NPs, in which the oxidized state of titanium is also present in the alumina matrix.
in the sandwich structure is served as the barrier to isolate cermet from the surrounding environment and substrate, respectively. The optical absorption spectra ranging from 300 to 2500 nm are displayed in Figs. 1a and b. For these two samples annealed at 600 °C for 12 h, the absorptance intensity did not show any significant changes in the entire measurement range with respect to the pristine cases. After 312 h heat treatment, however, the absorptance of the W-Al2O3 sample decreased distinctly. In contrast, the absorption spectra of the WTi-Al2O3 specimen were almost overlapped with each other, implying that the inclusion of Ti into the W-Al2O3 system can improve its thermal tolerance significantly. In general, the reliable physical properties of the films are coupling well with their phase structure stability [29,33]. Fig. 1c depicts the Xray diffraction (XRD) patterns for the W- and WTi-Al2O3 samples under different annealing times in argon ambient. For the WTi-Al2O3 sample, three peaks centered at 35.5°, 39.8°, 43.8° are always clearly discernible during annealing, which are respectively assigned to the (200), (210), (211) lattice plane of β-W (A15 crystal structure) [34,35]. It should be noted that no diffraction peaks from pure Ti or WTirelated alloys are observed within the detection limit of the XRD instrument. One of the possible reasons lies in the relative low atomic concentration of Ti in the WTi-Al2O3 layer (less than 10 at% for Ti/(W +Ti) atomic ratio by X-ray photoelectron spectrometry (XPS) analysis), and the other is due to the difficulties in distinguishing diffraction peaks among W, Ti and WTi nanocrystals. For example, the typical (100), (002) and (101) peaks for hexagonal close packed (hcp) Ti are partially overlapped with the broad (200) and (210) peaks of W 234
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Fig. 2. (a) Structural diagram of the simulated absorbing coating. (b) The comparison between the measured and simulation results. Effect of metal volume fraction (c) and absorption layer thickness (d) on ηFOM of the W-Al2O3 absorbing coatings.
ings, it is necessary to balance the mutually exclusive optical performances between the high solar absorptance and low infrared emissivity. Thus, we introduce photo-thermal conversion efficiency η as the figure of merit (FOM) [44]:
3.2. Theoretical simulation Before preparing solar selective absorbing coatings, optical simulation is carried out to guide the coating design. With the aid of related professional softwares, it is convenient to assign the refractive index, extinction coefficient and the thickness of each layer, and then to extract the reflectance spectra. Then the predicted performance of the coatings would enable us to fabricate the target coatings. In this study, the absorbing coating structure is: an Al2O3 anti-reflection layer, double cermet layers, an Al2O3 barrier layer, and a W metal infrared reflective layer. Note that the insertion of an Al2O3 barrier layer is aimed to inhibit thermal inter-diffusion, as reported elsewhere [30,31]. The double cermet layers are LMVF layer of W-Al2O3 and HMVF layer of W- or WTi-Al2O3. In order to simplify numerical simulation, we adopted the following laminated structure: Al2O3/W-Al2O3 LMVF layer (cermet layer 1)/W-Al2O3 HMVF layer (cermet layer 2)/Al2O3 (barrier layer)/W, as shown in Fig. 2a. Based on the effective medium approximation (EMA), we employed a characteristic matrix to calculate the reflectance, as reported previously [43]. The details are described in Supporting Information. In order to confirm the validity of our simulations, the simulated and measured results were compared and shown in Fig. 2b, demonstrating that the reflectance spectra are in a good agreement with each other. So our simulations are competent in guiding the design of absorbing coatings. The detailed information is summarized in the Table S2 (see Supporting Information). According to the simulated reflection spectra, it is convenient to acquire the solar absorptance α and thermal emissivity ε using spectral weighted integration as given in Eqs. (S7) and (S8) (see Supporting Information). For the optimization of solar selective absorbing coat-
ηFOM = Bα −
εσT 4 , CI
(1)
where C, I, σ and T are the solar concentration ratio, the solar flux intensity, the Stefan-Boltzmann constant, and the operating temperature, respectively. The constant of B, relating to the transmittance of glass envelope, is typically selected to be 0.91. The other parameters were set as follows: solar concentration of 100 suns, solar flux intensity of 863 W/m2, and operating temperature of 773 K. Firstly, the effect of Al2O3 barrier layer thickness on the reflectance of W-Al2O3 absorbing coating was studied. The simulation conducted in Fig. S3 (see Supporting Information) suggests that the barrier layer used is slightly detrimental to the infrared reflectance. Therefore, the Al2O3 layer thickness ranging from 5 to 10 nm was selected in this study. Then, as shown in Fig. S4 (see Supporting Information), the contour plots of the spectrally average α and ε at 773 K for W-Al2O3 absorbing coatings are depicted as a function of metal volume fraction and cermet layer thickness. Based on the previous simulation, we could obtain the corresponding contour plot of the FOM based on the Eq. (1). Fig. 2c shows a contour plot of the FOM for the W-Al2O3 absorbing coatings as a function of metal volume fraction. The metal volume fraction, determined by desirable high FOM ( > 0.84), can be found in the range of 0.18–0.27 for cermet layer 1 and 0.35–0.49 for cermet layer 2, respectively. Similarly, according to Fig. 2d, the ideal film thickness is obtained in the range of 25–45 nm and 30–50 nm for cermet layer 1 and 235
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Fig. 3. Reflectance spectra of the W-Al2O3 (a) and WTi-Al2O3 (b) absorbing coatings under different annealing conditions. The inset in panel (a, b) shows the structural diagram of the absorbing coatings. The solar absorptance (c) and the thermal emissivity @500 °C (d) evolution for the W- and WTi-Al2O3 absorbing coatings in panel (a, b).
mentioned that the emissivity at 500 °C is calculated ranging from 1 to 25 µm (see the equation (S8)). The absorptance α was achieved to be 93.1% and 92.2% for the pristine W-Al2O3 and WTi-Al2O3 coating, respectively. After annealing at 600 °C for 630 h, the absorptance and emissivity@500 °C for the W-Al2O3 coating was changed dramatically to be 88.6% and 16.2% (ε@500 °C for the pristine W-Al2O3 coating is 12.1%), respectively. Whereas for the WTi-Al2O3 coating after annealing at 600 °C for 840 h, the absorptance even experienced an increase from 92.2% to 92.8% and the emissivity @500 °C slightly increased to 10.3%. According to the literatures, the emissivity is generally larger than 13% @500 °C [16–19], highlighting that our proposed coating has sound solar thermal properties. As we know, small emissivity can effectively reduce the heat losses. And meanwhile, good thermal stability can ensure the long-term reliable optical performance at elevated temperatures. According to the previous studies on thermal stability of cermet-based absorbing coatings [16,31], one can figure out the thermal stability trend based on the current experimental data with finite time scale. The WTi-Al2O3 coatings in our study can be estimated to possess long-term thermal robustness at 600 °C in vacuum. For further probing the structural variations of the W- and WTiAl2O3 coatings shown in Fig. 3, the cross-sectional transmission electron microscopy (TEM) images of the annealed coatings are exhibited in Fig. 4. Considering the poor conductivity of the samples, a gold overlay layer was first deposited on the top of Al2O3 antireflection layer. Subsequently, a Pt layer as a protective cover was employed after the gold layer deposition to avoid the damages during FIB milling. For the W-Al2O3 coating, Figs. 4a and c show the high-resolution TEM (HRTEM) image of the LMVF and HMVF cermet layer, respectively, indicating that the W nanocrystals in the double cermet layer still possess β-W phase structure, in line with the single cermet layer
layer 2, respectively. The simulations offer us an opportunity to predict the optical performance and construct the target absorbing coatings with a tandem structure (Al2O3/W-Al2O3/WTi-Al2O3/Al2O3/W). 3.3. Optical, microstructural and compositional evolution of solar selective absorbing coatings under annealing With the aid of the previous simulations, the solar selective absorbing coatings were fabricated with the following structure: SS (stainless steel) substrate/W infrared reflective layer/ultra-thin Al2O3 barrier layer/double-cermet layers/Al2O3 anti-reflection layer. It should be emphasized that in actual operation, the precise tuning on thickness and composition of each layer was implemented based upon the simulations. The detailed information of the prepared coatings was provided in Supporting Information (see Fig. S5, 6 and Table S4, 5). As shown from the sketches in Figs. 3a and b, the difference between these two structures lies in the HMVF cermet layer. Fig. 3a shows the reflectance spectra of the W-Al2O3 absorbing coating before and after annealing in vacuum. The pristine coating possesses satisfying absorption in the solar visible region and high reflectance in the mid-infrared range ( > 74% at 2500 nm). After annealing at 600 °C for 630 h, the overall reflectance presented an obvious increase at short wavelengths ( < 1900 nm) and a significant drop at the wavelength region ranging from 1900 to 8000 nm. Instead, the WTi-Al2O3 counterpart exhibited desirable thermal robustness, as seen that the reflectance spectrum almost remains unchanged after annealing (Fig. 3b). More importantly, comparing with the previous reports [17,18], our WTi-Al2O3 coating has superior thermal stability at 600 °C. The solar absorptance α and thermal emissivity ε are calculated from the reflectance spectra, as reported elsewhere [45]. It should be 236
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Fig. 4. Bright-field cross-sectional TEM images of the (a-c) W-Al2O3 and (d-f) WTi-Al2O3 absorbing coatings after annealing. The inset in panel (a, c, d and f) shows the magnified HRTEM image corresponding to the yellow dashed rectangular frame. (a-c) annealed at 600 °C for 630 h in vacuum, (d-f) annealed at 600 °C for 840 h in vacuum.
W and Ti element across the WTi-Al2O3 cermet layer (indicated by the fuchsia arrow in Fig. 5a), demonstrating a spatially inhomogeneous distribution. This is further confirmed by the elemental mappings (corresponding to the yellow rectangular frame in Fig. 5a) depicted in Fig. 5e, i.e., Ti atoms are spatially segregated with respect to W element. On the other hand, as demonstrated in Figs. 5f and g, some Ti nanocrystals were found to graft onto W grain surface, further suggesting the surface segregation of Ti atoms from the parent nanoparticles. It should be emphasized that the solute segregation of alloy NPs generally leads to improved solvent stability based on both theoretical [46,47] and experimental [48,49] investigations. Thus, in our study, the thermal-induced segregation of Ti might greatly contribute to inhibiting the coalescence of the W nanocrystals so as to maintain the microstructural stability at elevated temperatures. For comparison, the cross-sectional STEM and the pertaining chemical component mapping via EDS analysis were also performed on the pristine WTi-Al2O3 coating, as shown in Fig. S7 (see Supporting Information). There is no evident spatial segregation of Ti with respect to W. And no phase related to pure Ti (namely, the isolated Ti nanocrystals) was found in the pristine WTi-Al2O3 cermet layer (see the HRTEM observations in Fig. S7). Figs. 6a and b depict XPS depth profile of the W-Al2O3 cermetbased absorbing coating before and after annealing at 600 °C for 630 h in vacuum, respectively. Compared with the pristine case, the average W atomic concentration in the HMVF W-Al2O3 layer of the annealed sample is decreased from 47.5 at% to 38.5 at%. Based on the cross-
presented in Fig. 1c. However, annealing at 600 °C for 630 h in vacuum resulted in the distinct agglomeration of the W nanocrystals (see Figs. 4b and c). Meanwhile, it is clear that the interface between the two cermet layers has become blurred. In particular, severe microstructural collapse of the double cermet stack appeared in some areas, as indicated by the fuchsia dashed circle in Fig. 4b. Evidently, the destruction on the structural integrity is responsible for optical performance degradation of the W-Al2O3 coating displayed in Fig. 3. Fig. 4e shows the cross-sectional TEM image of the annealed WTiAl2O3 coating, demonstrating desirable structural integrity with sharp interfaces among different layers. More importantly, no severe agglomeration and structural destruction were observed in the double cermet stacked layer. The phase structure of W nanoparticles in the cermet layers is also in the form of β-W, as verified in Figs. 4d and f. Reminiscent of the images exhibited in Figs. 4a and c, β-W structure was maintained independent of the addition of Ti or not, in a good agreement with the XRD result shown in Fig. 1c. Taken together, it can be confirmed that the introduction of Ti is capable of suppressing the agglomeration of W nanocrystals to maintain structural integrity of the multilayer coating, yielding improved optical stability. To gain deep insight into the thermal stability improvement mechanism, the cross-sectional scanning TEM (STEM) and the pertaining chemical component mapping via EDS analysis were performed. Fig. 5a illustrates the cross-sectional HAADF (high angle annular dark-field) STEM image of the annealed WTi-Al2O3 coating. The relevant line-scanning profile in Fig. 5b reveals the EDS counts of 237
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Fig. 5. (a) HAADF-STEM image of the sample shown in Fig. 4e, in which the fuchsia arrow denotes the location and direction of the line scan data shown in panel (b), and the yellow box represents the elemental mapping region; (c) spatial distribution of W, (d) spatial distribution of Ti, and (e) overlapped distribution of W (red) and Ti (green). The yellow dashed lines in (c-e) indicate the interfaces in the vicinity of the HMVF layer. (f) and (g) show the bright-field cross-sectional TEM images of the double cermet layer.
For the WTi-Al2O3 sample, there is a slight decrease of W content in the double-cermet layer before and after annealing (see Fig. 6c, d). However, the average atomic concentration of Ti in the WTi-Al2O3 HMVF cermet layer experienced a decrease from 4.6 to 2.9 at% after annealing. In particular, the Ti element in the pristine sample just existed in the HMVF and the Al2O3 barrier layer. A certain amount of element Ti present in the Al2O3 barrier layer is due to the presputtering of a metallic Ti target before depositing the HMVF layer. After annealing, the Ti element in the LMVF cermet layer is also detectable, implying the outward diffusion of Ti atoms from the HMVF layer. Owing to the penetration of external oxygen into the WTi-Al2O3 coating during annealing process, there is an increase of the oxygen concentration in the double cermet layer, similar to the W-Al2O3 case.
sectional TEM images of W-Al2O3 coating (Fig. S10 in Supporting Information) before and after annealing, it was demonstrated that the thickness of the double cermet layer was increased from 65 nm (27 nm and 38 nm for the LMVF and HMVF layer) to 73 nm (30 nm and 43 nm for the LMVF and HMVF layer). And the thicknesses of the alumina antireflection and barrier diffusion layer were almost kept constant before and after annealing. Thus, the ‘expansion’ of the double cermet layer can account for the decrease of W content in the cermet layers, due to that the fat double cermet layer makes the W concentration diluted. Correspondingly, the average oxygen concentration in the double cermet layer and Al2O3 barrier layer of the annealed W-Al2O3 coating has a certain increase, probably due to inward diffusion of environmental oxygen into the coating. 238
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Fig. 6. XPS depth profiles for the W-Al2O3 absorbing coating before (a) and (b) after annealing at 600 °C for 630 h in vacuum, respectively. (c) and (d) depict the XPS depth profiles for the WTi-Al2O3 absorbing coating before and after annealing at 600 °C for 840 h in vacuum, respectively.
one hand, the atomic radius (R) of Ti is larger than that of W (RTi=147 pm, RW=139 pm [56]). It was demonstrated that the Ti atoms in the mechanical mixture of W and Ti (20 at%) tended to segregate onto the surface of W NPs after thermal treatment [48], in order to lower system free energy [48,57]. With the aid of first-principle calculation, Wu et al. also predicted that the solute Ti atoms prefer migrating outward to keep the system free energy as low as possible [46]. Therefore, the thermodynamic-favored surface segregation of Ti from WTi NPs does happen under heating, which is a self-stabilization behavior. In our study, the cross-sectional STEM analysis in Fig. 5 experimentally confirmed the surface segregation of Ti, in line with the previous study [48]. Similarly, the solute segregation in nanocrystalline alloy Fe-Mg (10 at%) was also reported [58], leading to improved thermal stability. On the other, compared with W, the element Ti is easier to be oxidized in an aerobic environment, because the Gibbs energy of formation of TiO2 and WO3 at 600 °C is −799 kJ/mol and −618 kJ/mol [59], respectively. Hence, it is expected that the ease oxidation of Ti can be regarded as additional driving force for the preferential outward diffusion of Ti from WTi NPs when surrounding by an aerobic environment (alumina, in this case), forming metallic Ti and Ti-related oxide-phase segregation near WTi/ alumina interfaces. Further, the generated titanium oxide local layer, acting as the protective layer partially covering the surface of the alloy NPs, is capable of prohibiting the thermal diffusion of W, similar to the Al2O3 armor layer in our previous work [30,31]. It should be noted that in some previous studies [60–62], formation behaviors of oxide layers partially covering surfaces of bimetallic NPs were systematically investigated in aerobic environments by utilizing in-situ TEM visulization. For the pristine WTi-Al2O3 absorbing coating, pure W and WTi mixed NPs are included in its HMVF layer (i.e. WTi-Al2O3 cermet), as revealed in Fig. 1, which can be further revealed by the cross-sectional
The detailed XPS analysis for the valence states of Ti is shown in Fig. S9 (see Supporting Information). Although a considerable number of the metallic Ti still existed in the cermet layer, the titanium-related oxide phase was increased after annealing. Namely, partial metallic Ti in the HMVF or/and out-diffusion Ti from HMVF layer was oxidized during the annealing process. It should be noted that the oxidation state of W is not detectable by the comparison studies on the XPS spectra of W 4f core levels in the WTi-Al2O3 coating before and after annealing (Fig. S11, see Supporting Information). In addition, the out-diffusion of W atoms from the coating can also be ruled out, due to the absence of W in the Al2O3 anti-reflection layer or on the outermost surface of coating after annealing (see Figs. 4 and 6 and S11). Based upon the above XPS analysis, it can be concluded that the addition of Ti into W-Al2O3 cermet layer can effectively suppress the long-range diffusion of W (W element missing) in the corresponding absorbing layer, coupling well with the microstructural observations illustrated in Fig. 4. 3.4. Discussion on thermal strengthening mechanism For bimetallic NPs under heating, the atoms with low surface energy (often with large atomic radius) generally prefer to diffuse towards their surface regions to form surface segregation [50–54]. For example, in the PdPt bimetallic NPs, heat treatment can induce more Pd atoms to migrate to the particle surface, since the surface free energy (γ) of Pd is smaller than that of Pt (γPd=1.98 J/m2, γPt=2.43 J/m2 at 425 °C) [54]. Furthermore, the oxidizing environment could promote the surface segregation of Pd [54]. In like manner, the bimetallic AuPt NPs surrounded by the oxidizing matrix also undergo surface segregation during annealing [55]. As for WTi bimetallic NPs with Ti as the solute, on 239
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Scheme 1. Schematic illustrations of the microstructure evolution of the WTi-Al2O3 absorbing coating that was successively annealed at 600 °C for 840 h in vacuum.
is HMVF/barrier layer/W infrared reflective layer) are maintained (see Fig. 4e), thus ensuring excellent thermal tolerance for the whole stacked structure.
HRTEM images of W- and WTi-Al2O3 cermet layer in Fig. S12 (see Supporting Information). It is well known that there are generally some structural defects (voids, grain boundaries, etc.) within the films deposited by sputtering at low temperatures. In kinetics, these defects would provide rapid paths for atom diffusion. For the annealed WTiAl2O3 absorbing coating, the preferential diffusion-induced segregation of solute Ti atoms and their partial oxidation play a critical role in maintaining the microstructure robustness (see Fig. 4e) and further improving the optical stability at elevated temperatures (see Fig. 3b). Meanwhile, as revealed by the XPS results presented in Fig. 6, S8 and S9 (see Supporting Information), partial metallic Ti coming from WTiAl2O3 HMVF layer had diffused (gradient diffusion) into the W-Al2O3 LMVF layer. Thus, the long-range migration of W element was suppressed at the expense of Ti, ensuring the structural stability whether for the cermet component or for the interface between the double cermet layer. Based on the aforementioned detailed analysis, the thermal stability strengthening mechanism is proposed, as depicted in Scheme 1. At the beginning of the annealing process, Ti atoms in the WTi NPs would preferentially migrate out to gather on the NPs surface, resulting in that the diffusion paths were locally occupied by Ti atoms and correspondingly W atoms would become inert to migrate out. Some of metallic Ti atoms, segregated to the WTi NPs’ surface, are easily oxidized under an aerobic environment to generate titanium oxide layer. Thus, the WTi NPs’ surface is partially covered by out-diffused metallic Ti as well as generated oxide protective layer, further suppressing the nanoparticle agglomeration via Ostwald ripening. Of course, there are still some un-alloyed W NPs, whose agglomeration would be ignited under heating, just to lower the surface free energy. However, pure W NPs and WTi NPs with self-passivation layer are intertwined together, reducing the probability to form larger agglomerations. Along with annealing treatment proceeding, metallic Ti would reach to LMVF layer via long-distance diffusion driven by the concentration gradient, which can contributes to suppress the W-atom transfer near the LMVF/HMVF interface. Further, owing to the absent of severe agglomerations and the utilization of alumina barrier layer, welldefined interfaces (one is in between LMVF and HMVF, and the other
4. Conclusion In this article, we proposed the alloying of W NPs with Ti to inhibit the thermal diffusion of W atoms and the agglomeration of NPs in the cermet films, resulting in that the thermal stability of WTi-Al2O3 cermet film was improved significantly. Based on the well-performed WTi-Al2O3 cermet and the guidance of optical simulation, we developed solar selective absorbing coatings with a structure of SS substrate/W infrared reflective layer/ultra-thin Al2O3 barrier layer/WTiAl2O3 HMVF layer/W-Al2O3 LMVF layer/Al2O3 anti-reflection layer, demonstrating a high solar absorptance ~93% and a low thermal emissivity ~10.3% @500 °C after annealing at 600 °C for 840 h in vacuum. The segregation of solute Ti atoms and their partial oxidation to form local protective layer could suppress the outward diffusion of W and wild agglomeration of NPs, thereby ensuring the microstructure, interface structure and optical stability at elevated temperatures. This concept is quite different from the conventional method by using refractory alloy or compound with high melting point dispersed into ceramic matrixes (WNi-Al2O3 [18], AlNi-Al2O3 [8], MoSi2-Al2O3 [63], and so on). It is expected that the WTi-Al2O3 cermet-based solar selective absorbing coatings, with low thermal emissivity and excellent thermal stability, have great potential for high-temperature solar thermal applications. Acknowledgements We thank Dr. Hubin Luo for useful discussions. The authors acknowledge support from the National Natural Science Foundation of China (Grant No. 51302277), the Natural Science Foundation of Zhejiang Province (Grant No. LY17E020012), the Key Research and Development Plan of Jiangsu Province (Grant No. BE2015049), and the program for Ningbo Municipal Science and Technology Innovative Research Team (Grant No. 2016B10005). 240
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High-temperature tolerance in WTi-Al2O3 cermet-based solar selective absorbing coatings with low thermal emissivity
MARK
⁎
Xiaoyu Wanga,b, Junhua Gaoa, , Haibo Hua,b, Hongliang Zhangc, Lingyan Lianga, Kashif Javaida, ⁎ Fei Zhugec, Hongtao Caoa, , Le Wangd a Key Laboratory of Additive Manufacturing Materials of Zhejiang Province, Ningbo Institute of Materials Technology & Engineering, Chinese Academy of Sciences, Ningbo 315201, China b University of Chinese Academy of Sciences, 19 Yuquan Road, Beijing 100049, China c Key Laboratory of Graphene Technologies and Applications of Zhejiang Province, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China d College of Optical and Electronic Science and Technology, China Jiliang University, Hangzhou 310018, China
A R T I C L E I N F O
A BS T RAC T
Keywords: Solar thermal conversion Tungsten alloy nanoparticles Segregation Thermal stability Emissivity
Cermet-based solar selective absorbing coatings are widely used, however, the long-term thermal instability and pretty high infrared emissivity at high temperatures ( > 550 °C) are still challenging issues to be addressed, which essentially lies in suppressing the growing up and agglomeration behaviors of metal nanoparticles (NPs) and maintaining the interface integrity in the multi-layer stacked structure. Herein, we develop and explore WTi-Al2O3 cermet-based absorbing coatings, demonstrating a solar absorptance of ~93% and a very low thermal emissivity of 10.3% @500 °C even after annealing at 600 °C for 840 h in vacuum. It is revealed that the surface segregation of solute Ti atoms from the parent alloyed NPs and their partial oxidation to form protective layer restrain outward diffusion of W element, agglomeration of NPs, and interface structure degradation, in favor of enhancing the thermal tolerance of the coatings. These results suggest that the WTi-Al2O3 based absorbing coating is a good candidate for high-temperature solar thermal conversion.
1. Introduction Among solar energy utilizations, solar thermal techniques play a more and more important role in solar-energy harvesting, due to high energy-conversion efficiency and appealing energy storage functionality [1]. Currently, parabolic trough collector (PTC) has been commercially developed [2], in which solar selective absorbing coatings are one of the critical components to govern the overall performance of PTC system. Ideally speaking, this coating should possess complete absorptance in the solar spectral range with low heat loss to the ambient via infrared radiation. Certainly, good chemical inertness and thermal stability of the coating at elevated temperatures are also indispensable for long-term power generation. In addition, the higher temperature of the heat-transfer fluid (HTF) in PTC system, the higher Carnot efficiency of power plants [3]. In the conventional PTC system, mineral or synthetic oil is utilized as HTF, limiting the operating temperature around 400 °C. One of promising candidates of HTF is molten salt (such as 60% KNO3+40% NaNO3), as it can be operated at much higher temperatures (more than 550 °C) [2]. Along with the advance on molten salt-based PTC technologies, new spectrally selective coatings ⁎
Corresponding authors. E-mail addresses: [email protected] (J. Gao), [email protected] (H. Cao).
http://dx.doi.org/10.1016/j.nanoen.2017.05.036 Received 25 April 2017; Received in revised form 16 May 2017; Accepted 16 May 2017 Available online 17 May 2017 2211-2855/ © 2017 Elsevier Ltd. All rights reserved.
with good optical performance and excellent thermal stability are highly desirable for parabolic trough solar thermal power generation. In recent years, numerous efforts have been made for developing high-temperature solar selective absorbing coatings. Most of the studies focus on cermet-based coatings [4–11], where metal NPs are embedded in dielectric matrices to construct cermet absorbing layers. For example, Pt-Al2O3 coatings have been developed with high absorptance and low emissivity [11–14]. However, this coating is too expensive to be commercialized. Mo-SiO2 cermet is also intensively investigated to build up absorbing coatings based upon a double cermet layer configuration [4,5,15]. Nevertheless, the tolerable temperature for such coating is not high enough. Note that W-Al2O3 absorbing coatings have exhibited reasonably high thermal robustness [16]. After annealing at 580 °C for 2 days in vacuum, there was no obvious optical and structural degradation in the coating. Rebouta et al. [17] also found that the W-Al2O3-based coatings were quite stable even after heat-treatment at 580 °C for 850 h in vacuum. For further improving its operating temperature, Cao et al. [18] had constructed WNi-Al2O3 cermet-based coatings, demonstrating an almost constant solar absorptance of ~90% during annealing treatment at 600 °C for 7 days.
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Analogously, by using YSZ (yttria-stabilized zirconia) to replace Al2O3, the fabricated WNi-YSZ coating exhibited a higher solar absorptance of 91% with a lower thermal emissivity of 13% @500 °C after annealing at 600 °C [19]. Based on the above surveys, we believe that there is still room to improve the optical performance of cermet-based absorbing coatings, especially for Al2O3-matrix cermet cases. Generally speaking, the thermal-induced optical degradation of cermet-based coatings is mainly ascribed to the element migration, coalescence and/or oxidation of metal NPs, inter-diffusion between adjacent layers, and diffusion of substrate atoms into the coatings [20– 22]. In a sense, the thermal tolerance of metal NPs is the most importance concern for cermet-based absorbing coatings. Up to now, numerous efforts have been implemented to boost the thermal stability of metal NPs, such as covering oxide passivation layers [23–25], alloying [26–29], etc. For example, the Pt@SiO2 NPs could maintain their core-shell configuration even up to 750 °C [23]. PtRh alloy NPs still exhibited admirable catalytical capability at 650 °C [26]. In our previous study [30,31], the AgAl-Al2O3 cermet films could exhibit excellent thermal stability at 500 °C for ~1000 h. Likewise, the utilization of WNi alloy in ceramic matrix was beneficial for stabilizing WNPs-based cermets at elevated temperatures [18,19]. Thus, alloying method is believed to be an effective way to improve the thermal stability of metal NPs. Besides the thermal robustness, low thermal emissivity is another criterion for high temperature solar selective absorbing coatings. According to the Stefan-Boltzmann law [32], the emissivity is proportional to T4 (T is the working temperature). Thus, for designing hightemperature absorbing coatings, the issue of thermal emissivity should be put in more priority than the solar absorptance to be concerned, in order to minimize the thermal radiation loss as low as possible. Previous studies suggested that tungsten is one of the good candidates used as the infrared reflector layer [17,18]. In this work, we attempted to further strength the thermal tolerance of W-Al2O3 cermet by introducing Ti to form WTi alloy NPs, and the validity of this method was revealed by that the absorptance of singlelayer cermet underwent negligible changes after annealing at 600 °C for 312 h in argon ambient. A computer simulation was carried out to design W-Al2O3 absorbing coating with double cermet layer-stacked configuration. Based on the simulations, the target absorbing coatings with an Al2O3 anti-reflection layer/W-Al2O3 LMVF (low metal volume fraction) layer/WTi-Al2O3 HMVF (high metal volume fraction) layer/ Al2O3 barrier layer/W infrared reflective layer structure were successfully prepared. After annealing at 600 °C for 840 h in vacuum, the coating still exhibited a low thermal emissivity of 10.3% @500 °C and a desirable solar absorptance of ~93%. Finally, the thermal strengthening mechanism of the proposed coatings was systematically investigated. The excellent thermal stability with low infrared emissivity provides a great opportunity for the designed coatings to find applications in high-temperature solar-thermal conversion.
sputtering pressure of 0.21 Pa. The detailed preparation parameters are summarized in Supporting Information. For single-layer cermet's phase structure and compositional analysis, the fabricated W- and WTi-Al2O3 films were deposited onto quartz for absorptance and XRD testing or onto Si (100) for XPS analysis. For the Al2O3/W-Al2O3/Al2O3 or Al2O3/WTi-Al2O3/Al2O3 sandwich structure, the sputtering time of the cermet layer was 55 min, and the deposition time was 30 and 40 min for the bottom and top Al2O3 layer with a thickness of ~18 and 24 nm, respectively. With regard to solar selective absorbing coatings, the tandem structure was Al2O3(1)/double-cermet layer/Al2O3(2)/W from top to the substrate, where the Al2O3(1) and Al2O3(2) layer was served as the antireflection and diffusion barrier layer, respectively. The substrate was stainless steel or Si (100). All the annealing treatments were carried out in a tube furnace. The tube furnace was filled with argon gas for the annealing of single-layer cermet films, while the solar selective absorbing coating samples were firstly loaded into sealed quartz tubes exhausted to a pressure less than 0.1 Pa and then were packed into the tube furnace for heat treatment. For the annealing procedure, the temperature was rapidly increased to 600 °C firstly, then kept at 600 °C for 10 h, and subsequently rapidly dropped to room temperature. After the optical measurement, the samples were loaded into sealed quartz tubes again for long-term annealing. During the long-term heat treatment, the samples were heated up to 600 °C and then cooled down to room temperature after an interval of ~50 h (one cycle in our case), followed by the repeating heat treatment cycle.
2. Experimental section
In this paper, considering the fact that the content of metallic Ti in WTi-Al2O3 films is less than 10 at%, the computational design was mainly focused on W-Al2O3 coating for guiding the design of WTiAl2O3 coating. The detailed simulation processes are described in Supporting Information.
2.2. Characterization The optical spectra (including reflectance (R) and transmittance (T)) were obtained by a Lambda 950 spectrophotometer equipped with an integrating sphere (angle of incidence 0° and 8° for transmittance and reflectance, respectively) in the wavelength region ranging from 0.3 to 2.5 µm. The absorptance (A) was calculated by A=1-R-T. As for the mid-infrared region (2.5–25 µm), a Fourier transform infrared (FTIR) spectrometer (Thermo Nicolet Model-6700, angle of incidence 12°), with a mid-infrared integrating sphere, was employed to record the reflectance spectra at room temperature, which was calibrated with a standard Au reference. The XRD patterns were acquired using Cu Kα radiation (λ=1.54 Å) at a glazing incidence angle of 1°. XPS analysis was implemented to characterize the chemical compositions and the valence states. Planar and cross-sectional TEM images were acquired to identify the microstructural features of the samples. Focused ion beam (FIB) milling method was utilized for preparing cross-sectional TEM specimens. STEM analysis in conjunction with X-ray energydispersive spectroscopy (EDS) was carried out to detect the elemental distribution in WTi-Al2O3 coatings. 2.3. Computational design of solar selective absorbing coating
2.1. Samples preparation and annealing treatment Cermet films and solar selective absorbing coatings were deposited onto various substrates at room temperature by a multi target sputtering system (Jsputter8000, manufactured by ULVAC Co., Ltd.). The commercially available high-purity Al2O3 (99.99%, 2 in. diameter), W (99.99%, 2 in. diameter), and Ti (99.99%, 2 in. diameter) targets were used as the source materials. Before deposition, all the substrates were ultrasonically cleaned in acetone and alcohol baths for 20 min, then rinsed with deionized water, and finally dried under nitrogen flow. Two radio-frequency (RF, 13.56 MHz) power supplies were used to operate the Al2O3 target and the W target, while the Ti target was sputtered using a DC power source. The base pressure was less than 1×10−4 Pa, and all the depositions were implemented at a constant
3. Results and discussion 3.1. Thermal stability, phase structure and chemical composition of WTi-Al2O3 cermets We deposited W- and WTi-Al2O3 cermet layers on quartz substrates with a sandwich structure (Al2O3/cermet layer/Al2O3). Then the samples were successively annealed at 600 °C in argon ambient for 12 h, 112 h and 312 h. The preparation details are summarized in Table S1 (see Supporting Information). The top and bottom Al2O3 layer 233
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Fig. 1. Absorptance spectra of W-Al2O3 (a) and WTi-Al2O3 (~8 at% Ti) (b) samples under different annealing conditions. (c) XRD patterns of W- and WTi-Al2O3 (~8 at% Ti) cermet layers (including pristine and annealed in Ar ambient). The 47–1319 PDF card of β-W is also shown at the bottom of panel c. (d) XPS spectra of Ti 2p core levels for the WTi-Al2O3 (~8 at % Ti) samples. All the samples have the same sandwich structure of Al2O3/cermet layer/Al2O3.
nanocrystals [22,36]. After annealing, the intensity of the (210) peak almost kept constant. For the W-Al2O3 case, however, the diffraction intensity of each peak encountered a detectable reduction after annealing, probably due to the oxidation of metallic W during the annealing process. This speculation is clearly confirmed by the Raman results shown in Fig. S1 (see Supporting Information). The XPS spectrum of Ti 2p core levels in the WTi-Al2O3 sample was acquired after Ar plasma etching for 1920s to remove the top alumina layer completely. As shown in Fig. 1d, two Ti 2p bumps (shown by black line) are observed, which can be further deconvolved. Specifically, the binding energy located at 460.3 eV and 454.2 eV can be assigned to Ti 2p1/2 and Ti 2p3/2 of metallic Ti, respectively, in line with the previous study [37–39]. Furthermore, the oxidation state of Ti, with relatively high binding energies, can not be ruled out [40,41]. But nevertheless, the additives of Ti in the cermet mainly exist in the form of metallic state. After carefully calculating, the atomic ratio of Ti/ W was estimated to be about 1/11, corresponding to the atomic percent (Ti/(W+Ti)) of 8 at%. According to the previous studies on WTi alloys [42], the atomic ratio of Ti/W of 1/11 is within the solid solubility range between them. The inclusion of Ti would induce the heterogeneous nucleation and growth of metallic W (see Fig. S2 in Supporting Information), double confirming the successful alloying process. Meanwhile, no phase related to pure Ti was found based on TEM observations shown in Fig. S2. Taken the aforementioned results together, it is believed that the WTi-Al2O3 cermet is consisted of unalloyed W and alloyed WTi NPs, in which the oxidized state of titanium is also present in the alumina matrix.
in the sandwich structure is served as the barrier to isolate cermet from the surrounding environment and substrate, respectively. The optical absorption spectra ranging from 300 to 2500 nm are displayed in Figs. 1a and b. For these two samples annealed at 600 °C for 12 h, the absorptance intensity did not show any significant changes in the entire measurement range with respect to the pristine cases. After 312 h heat treatment, however, the absorptance of the W-Al2O3 sample decreased distinctly. In contrast, the absorption spectra of the WTi-Al2O3 specimen were almost overlapped with each other, implying that the inclusion of Ti into the W-Al2O3 system can improve its thermal tolerance significantly. In general, the reliable physical properties of the films are coupling well with their phase structure stability [29,33]. Fig. 1c depicts the Xray diffraction (XRD) patterns for the W- and WTi-Al2O3 samples under different annealing times in argon ambient. For the WTi-Al2O3 sample, three peaks centered at 35.5°, 39.8°, 43.8° are always clearly discernible during annealing, which are respectively assigned to the (200), (210), (211) lattice plane of β-W (A15 crystal structure) [34,35]. It should be noted that no diffraction peaks from pure Ti or WTirelated alloys are observed within the detection limit of the XRD instrument. One of the possible reasons lies in the relative low atomic concentration of Ti in the WTi-Al2O3 layer (less than 10 at% for Ti/(W +Ti) atomic ratio by X-ray photoelectron spectrometry (XPS) analysis), and the other is due to the difficulties in distinguishing diffraction peaks among W, Ti and WTi nanocrystals. For example, the typical (100), (002) and (101) peaks for hexagonal close packed (hcp) Ti are partially overlapped with the broad (200) and (210) peaks of W 234
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Fig. 2. (a) Structural diagram of the simulated absorbing coating. (b) The comparison between the measured and simulation results. Effect of metal volume fraction (c) and absorption layer thickness (d) on ηFOM of the W-Al2O3 absorbing coatings.
ings, it is necessary to balance the mutually exclusive optical performances between the high solar absorptance and low infrared emissivity. Thus, we introduce photo-thermal conversion efficiency η as the figure of merit (FOM) [44]:
3.2. Theoretical simulation Before preparing solar selective absorbing coatings, optical simulation is carried out to guide the coating design. With the aid of related professional softwares, it is convenient to assign the refractive index, extinction coefficient and the thickness of each layer, and then to extract the reflectance spectra. Then the predicted performance of the coatings would enable us to fabricate the target coatings. In this study, the absorbing coating structure is: an Al2O3 anti-reflection layer, double cermet layers, an Al2O3 barrier layer, and a W metal infrared reflective layer. Note that the insertion of an Al2O3 barrier layer is aimed to inhibit thermal inter-diffusion, as reported elsewhere [30,31]. The double cermet layers are LMVF layer of W-Al2O3 and HMVF layer of W- or WTi-Al2O3. In order to simplify numerical simulation, we adopted the following laminated structure: Al2O3/W-Al2O3 LMVF layer (cermet layer 1)/W-Al2O3 HMVF layer (cermet layer 2)/Al2O3 (barrier layer)/W, as shown in Fig. 2a. Based on the effective medium approximation (EMA), we employed a characteristic matrix to calculate the reflectance, as reported previously [43]. The details are described in Supporting Information. In order to confirm the validity of our simulations, the simulated and measured results were compared and shown in Fig. 2b, demonstrating that the reflectance spectra are in a good agreement with each other. So our simulations are competent in guiding the design of absorbing coatings. The detailed information is summarized in the Table S2 (see Supporting Information). According to the simulated reflection spectra, it is convenient to acquire the solar absorptance α and thermal emissivity ε using spectral weighted integration as given in Eqs. (S7) and (S8) (see Supporting Information). For the optimization of solar selective absorbing coat-
ηFOM = Bα −
εσT 4 , CI
(1)
where C, I, σ and T are the solar concentration ratio, the solar flux intensity, the Stefan-Boltzmann constant, and the operating temperature, respectively. The constant of B, relating to the transmittance of glass envelope, is typically selected to be 0.91. The other parameters were set as follows: solar concentration of 100 suns, solar flux intensity of 863 W/m2, and operating temperature of 773 K. Firstly, the effect of Al2O3 barrier layer thickness on the reflectance of W-Al2O3 absorbing coating was studied. The simulation conducted in Fig. S3 (see Supporting Information) suggests that the barrier layer used is slightly detrimental to the infrared reflectance. Therefore, the Al2O3 layer thickness ranging from 5 to 10 nm was selected in this study. Then, as shown in Fig. S4 (see Supporting Information), the contour plots of the spectrally average α and ε at 773 K for W-Al2O3 absorbing coatings are depicted as a function of metal volume fraction and cermet layer thickness. Based on the previous simulation, we could obtain the corresponding contour plot of the FOM based on the Eq. (1). Fig. 2c shows a contour plot of the FOM for the W-Al2O3 absorbing coatings as a function of metal volume fraction. The metal volume fraction, determined by desirable high FOM ( > 0.84), can be found in the range of 0.18–0.27 for cermet layer 1 and 0.35–0.49 for cermet layer 2, respectively. Similarly, according to Fig. 2d, the ideal film thickness is obtained in the range of 25–45 nm and 30–50 nm for cermet layer 1 and 235
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Fig. 3. Reflectance spectra of the W-Al2O3 (a) and WTi-Al2O3 (b) absorbing coatings under different annealing conditions. The inset in panel (a, b) shows the structural diagram of the absorbing coatings. The solar absorptance (c) and the thermal emissivity @500 °C (d) evolution for the W- and WTi-Al2O3 absorbing coatings in panel (a, b).
mentioned that the emissivity at 500 °C is calculated ranging from 1 to 25 µm (see the equation (S8)). The absorptance α was achieved to be 93.1% and 92.2% for the pristine W-Al2O3 and WTi-Al2O3 coating, respectively. After annealing at 600 °C for 630 h, the absorptance and emissivity@500 °C for the W-Al2O3 coating was changed dramatically to be 88.6% and 16.2% (ε@500 °C for the pristine W-Al2O3 coating is 12.1%), respectively. Whereas for the WTi-Al2O3 coating after annealing at 600 °C for 840 h, the absorptance even experienced an increase from 92.2% to 92.8% and the emissivity @500 °C slightly increased to 10.3%. According to the literatures, the emissivity is generally larger than 13% @500 °C [16–19], highlighting that our proposed coating has sound solar thermal properties. As we know, small emissivity can effectively reduce the heat losses. And meanwhile, good thermal stability can ensure the long-term reliable optical performance at elevated temperatures. According to the previous studies on thermal stability of cermet-based absorbing coatings [16,31], one can figure out the thermal stability trend based on the current experimental data with finite time scale. The WTi-Al2O3 coatings in our study can be estimated to possess long-term thermal robustness at 600 °C in vacuum. For further probing the structural variations of the W- and WTiAl2O3 coatings shown in Fig. 3, the cross-sectional transmission electron microscopy (TEM) images of the annealed coatings are exhibited in Fig. 4. Considering the poor conductivity of the samples, a gold overlay layer was first deposited on the top of Al2O3 antireflection layer. Subsequently, a Pt layer as a protective cover was employed after the gold layer deposition to avoid the damages during FIB milling. For the W-Al2O3 coating, Figs. 4a and c show the high-resolution TEM (HRTEM) image of the LMVF and HMVF cermet layer, respectively, indicating that the W nanocrystals in the double cermet layer still possess β-W phase structure, in line with the single cermet layer
layer 2, respectively. The simulations offer us an opportunity to predict the optical performance and construct the target absorbing coatings with a tandem structure (Al2O3/W-Al2O3/WTi-Al2O3/Al2O3/W). 3.3. Optical, microstructural and compositional evolution of solar selective absorbing coatings under annealing With the aid of the previous simulations, the solar selective absorbing coatings were fabricated with the following structure: SS (stainless steel) substrate/W infrared reflective layer/ultra-thin Al2O3 barrier layer/double-cermet layers/Al2O3 anti-reflection layer. It should be emphasized that in actual operation, the precise tuning on thickness and composition of each layer was implemented based upon the simulations. The detailed information of the prepared coatings was provided in Supporting Information (see Fig. S5, 6 and Table S4, 5). As shown from the sketches in Figs. 3a and b, the difference between these two structures lies in the HMVF cermet layer. Fig. 3a shows the reflectance spectra of the W-Al2O3 absorbing coating before and after annealing in vacuum. The pristine coating possesses satisfying absorption in the solar visible region and high reflectance in the mid-infrared range ( > 74% at 2500 nm). After annealing at 600 °C for 630 h, the overall reflectance presented an obvious increase at short wavelengths ( < 1900 nm) and a significant drop at the wavelength region ranging from 1900 to 8000 nm. Instead, the WTi-Al2O3 counterpart exhibited desirable thermal robustness, as seen that the reflectance spectrum almost remains unchanged after annealing (Fig. 3b). More importantly, comparing with the previous reports [17,18], our WTi-Al2O3 coating has superior thermal stability at 600 °C. The solar absorptance α and thermal emissivity ε are calculated from the reflectance spectra, as reported elsewhere [45]. It should be 236
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Fig. 4. Bright-field cross-sectional TEM images of the (a-c) W-Al2O3 and (d-f) WTi-Al2O3 absorbing coatings after annealing. The inset in panel (a, c, d and f) shows the magnified HRTEM image corresponding to the yellow dashed rectangular frame. (a-c) annealed at 600 °C for 630 h in vacuum, (d-f) annealed at 600 °C for 840 h in vacuum.
W and Ti element across the WTi-Al2O3 cermet layer (indicated by the fuchsia arrow in Fig. 5a), demonstrating a spatially inhomogeneous distribution. This is further confirmed by the elemental mappings (corresponding to the yellow rectangular frame in Fig. 5a) depicted in Fig. 5e, i.e., Ti atoms are spatially segregated with respect to W element. On the other hand, as demonstrated in Figs. 5f and g, some Ti nanocrystals were found to graft onto W grain surface, further suggesting the surface segregation of Ti atoms from the parent nanoparticles. It should be emphasized that the solute segregation of alloy NPs generally leads to improved solvent stability based on both theoretical [46,47] and experimental [48,49] investigations. Thus, in our study, the thermal-induced segregation of Ti might greatly contribute to inhibiting the coalescence of the W nanocrystals so as to maintain the microstructural stability at elevated temperatures. For comparison, the cross-sectional STEM and the pertaining chemical component mapping via EDS analysis were also performed on the pristine WTi-Al2O3 coating, as shown in Fig. S7 (see Supporting Information). There is no evident spatial segregation of Ti with respect to W. And no phase related to pure Ti (namely, the isolated Ti nanocrystals) was found in the pristine WTi-Al2O3 cermet layer (see the HRTEM observations in Fig. S7). Figs. 6a and b depict XPS depth profile of the W-Al2O3 cermetbased absorbing coating before and after annealing at 600 °C for 630 h in vacuum, respectively. Compared with the pristine case, the average W atomic concentration in the HMVF W-Al2O3 layer of the annealed sample is decreased from 47.5 at% to 38.5 at%. Based on the cross-
presented in Fig. 1c. However, annealing at 600 °C for 630 h in vacuum resulted in the distinct agglomeration of the W nanocrystals (see Figs. 4b and c). Meanwhile, it is clear that the interface between the two cermet layers has become blurred. In particular, severe microstructural collapse of the double cermet stack appeared in some areas, as indicated by the fuchsia dashed circle in Fig. 4b. Evidently, the destruction on the structural integrity is responsible for optical performance degradation of the W-Al2O3 coating displayed in Fig. 3. Fig. 4e shows the cross-sectional TEM image of the annealed WTiAl2O3 coating, demonstrating desirable structural integrity with sharp interfaces among different layers. More importantly, no severe agglomeration and structural destruction were observed in the double cermet stacked layer. The phase structure of W nanoparticles in the cermet layers is also in the form of β-W, as verified in Figs. 4d and f. Reminiscent of the images exhibited in Figs. 4a and c, β-W structure was maintained independent of the addition of Ti or not, in a good agreement with the XRD result shown in Fig. 1c. Taken together, it can be confirmed that the introduction of Ti is capable of suppressing the agglomeration of W nanocrystals to maintain structural integrity of the multilayer coating, yielding improved optical stability. To gain deep insight into the thermal stability improvement mechanism, the cross-sectional scanning TEM (STEM) and the pertaining chemical component mapping via EDS analysis were performed. Fig. 5a illustrates the cross-sectional HAADF (high angle annular dark-field) STEM image of the annealed WTi-Al2O3 coating. The relevant line-scanning profile in Fig. 5b reveals the EDS counts of 237
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Fig. 5. (a) HAADF-STEM image of the sample shown in Fig. 4e, in which the fuchsia arrow denotes the location and direction of the line scan data shown in panel (b), and the yellow box represents the elemental mapping region; (c) spatial distribution of W, (d) spatial distribution of Ti, and (e) overlapped distribution of W (red) and Ti (green). The yellow dashed lines in (c-e) indicate the interfaces in the vicinity of the HMVF layer. (f) and (g) show the bright-field cross-sectional TEM images of the double cermet layer.
For the WTi-Al2O3 sample, there is a slight decrease of W content in the double-cermet layer before and after annealing (see Fig. 6c, d). However, the average atomic concentration of Ti in the WTi-Al2O3 HMVF cermet layer experienced a decrease from 4.6 to 2.9 at% after annealing. In particular, the Ti element in the pristine sample just existed in the HMVF and the Al2O3 barrier layer. A certain amount of element Ti present in the Al2O3 barrier layer is due to the presputtering of a metallic Ti target before depositing the HMVF layer. After annealing, the Ti element in the LMVF cermet layer is also detectable, implying the outward diffusion of Ti atoms from the HMVF layer. Owing to the penetration of external oxygen into the WTi-Al2O3 coating during annealing process, there is an increase of the oxygen concentration in the double cermet layer, similar to the W-Al2O3 case.
sectional TEM images of W-Al2O3 coating (Fig. S10 in Supporting Information) before and after annealing, it was demonstrated that the thickness of the double cermet layer was increased from 65 nm (27 nm and 38 nm for the LMVF and HMVF layer) to 73 nm (30 nm and 43 nm for the LMVF and HMVF layer). And the thicknesses of the alumina antireflection and barrier diffusion layer were almost kept constant before and after annealing. Thus, the ‘expansion’ of the double cermet layer can account for the decrease of W content in the cermet layers, due to that the fat double cermet layer makes the W concentration diluted. Correspondingly, the average oxygen concentration in the double cermet layer and Al2O3 barrier layer of the annealed W-Al2O3 coating has a certain increase, probably due to inward diffusion of environmental oxygen into the coating. 238
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Fig. 6. XPS depth profiles for the W-Al2O3 absorbing coating before (a) and (b) after annealing at 600 °C for 630 h in vacuum, respectively. (c) and (d) depict the XPS depth profiles for the WTi-Al2O3 absorbing coating before and after annealing at 600 °C for 840 h in vacuum, respectively.
one hand, the atomic radius (R) of Ti is larger than that of W (RTi=147 pm, RW=139 pm [56]). It was demonstrated that the Ti atoms in the mechanical mixture of W and Ti (20 at%) tended to segregate onto the surface of W NPs after thermal treatment [48], in order to lower system free energy [48,57]. With the aid of first-principle calculation, Wu et al. also predicted that the solute Ti atoms prefer migrating outward to keep the system free energy as low as possible [46]. Therefore, the thermodynamic-favored surface segregation of Ti from WTi NPs does happen under heating, which is a self-stabilization behavior. In our study, the cross-sectional STEM analysis in Fig. 5 experimentally confirmed the surface segregation of Ti, in line with the previous study [48]. Similarly, the solute segregation in nanocrystalline alloy Fe-Mg (10 at%) was also reported [58], leading to improved thermal stability. On the other, compared with W, the element Ti is easier to be oxidized in an aerobic environment, because the Gibbs energy of formation of TiO2 and WO3 at 600 °C is −799 kJ/mol and −618 kJ/mol [59], respectively. Hence, it is expected that the ease oxidation of Ti can be regarded as additional driving force for the preferential outward diffusion of Ti from WTi NPs when surrounding by an aerobic environment (alumina, in this case), forming metallic Ti and Ti-related oxide-phase segregation near WTi/ alumina interfaces. Further, the generated titanium oxide local layer, acting as the protective layer partially covering the surface of the alloy NPs, is capable of prohibiting the thermal diffusion of W, similar to the Al2O3 armor layer in our previous work [30,31]. It should be noted that in some previous studies [60–62], formation behaviors of oxide layers partially covering surfaces of bimetallic NPs were systematically investigated in aerobic environments by utilizing in-situ TEM visulization. For the pristine WTi-Al2O3 absorbing coating, pure W and WTi mixed NPs are included in its HMVF layer (i.e. WTi-Al2O3 cermet), as revealed in Fig. 1, which can be further revealed by the cross-sectional
The detailed XPS analysis for the valence states of Ti is shown in Fig. S9 (see Supporting Information). Although a considerable number of the metallic Ti still existed in the cermet layer, the titanium-related oxide phase was increased after annealing. Namely, partial metallic Ti in the HMVF or/and out-diffusion Ti from HMVF layer was oxidized during the annealing process. It should be noted that the oxidation state of W is not detectable by the comparison studies on the XPS spectra of W 4f core levels in the WTi-Al2O3 coating before and after annealing (Fig. S11, see Supporting Information). In addition, the out-diffusion of W atoms from the coating can also be ruled out, due to the absence of W in the Al2O3 anti-reflection layer or on the outermost surface of coating after annealing (see Figs. 4 and 6 and S11). Based upon the above XPS analysis, it can be concluded that the addition of Ti into W-Al2O3 cermet layer can effectively suppress the long-range diffusion of W (W element missing) in the corresponding absorbing layer, coupling well with the microstructural observations illustrated in Fig. 4. 3.4. Discussion on thermal strengthening mechanism For bimetallic NPs under heating, the atoms with low surface energy (often with large atomic radius) generally prefer to diffuse towards their surface regions to form surface segregation [50–54]. For example, in the PdPt bimetallic NPs, heat treatment can induce more Pd atoms to migrate to the particle surface, since the surface free energy (γ) of Pd is smaller than that of Pt (γPd=1.98 J/m2, γPt=2.43 J/m2 at 425 °C) [54]. Furthermore, the oxidizing environment could promote the surface segregation of Pd [54]. In like manner, the bimetallic AuPt NPs surrounded by the oxidizing matrix also undergo surface segregation during annealing [55]. As for WTi bimetallic NPs with Ti as the solute, on 239
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Scheme 1. Schematic illustrations of the microstructure evolution of the WTi-Al2O3 absorbing coating that was successively annealed at 600 °C for 840 h in vacuum.
is HMVF/barrier layer/W infrared reflective layer) are maintained (see Fig. 4e), thus ensuring excellent thermal tolerance for the whole stacked structure.
HRTEM images of W- and WTi-Al2O3 cermet layer in Fig. S12 (see Supporting Information). It is well known that there are generally some structural defects (voids, grain boundaries, etc.) within the films deposited by sputtering at low temperatures. In kinetics, these defects would provide rapid paths for atom diffusion. For the annealed WTiAl2O3 absorbing coating, the preferential diffusion-induced segregation of solute Ti atoms and their partial oxidation play a critical role in maintaining the microstructure robustness (see Fig. 4e) and further improving the optical stability at elevated temperatures (see Fig. 3b). Meanwhile, as revealed by the XPS results presented in Fig. 6, S8 and S9 (see Supporting Information), partial metallic Ti coming from WTiAl2O3 HMVF layer had diffused (gradient diffusion) into the W-Al2O3 LMVF layer. Thus, the long-range migration of W element was suppressed at the expense of Ti, ensuring the structural stability whether for the cermet component or for the interface between the double cermet layer. Based on the aforementioned detailed analysis, the thermal stability strengthening mechanism is proposed, as depicted in Scheme 1. At the beginning of the annealing process, Ti atoms in the WTi NPs would preferentially migrate out to gather on the NPs surface, resulting in that the diffusion paths were locally occupied by Ti atoms and correspondingly W atoms would become inert to migrate out. Some of metallic Ti atoms, segregated to the WTi NPs’ surface, are easily oxidized under an aerobic environment to generate titanium oxide layer. Thus, the WTi NPs’ surface is partially covered by out-diffused metallic Ti as well as generated oxide protective layer, further suppressing the nanoparticle agglomeration via Ostwald ripening. Of course, there are still some un-alloyed W NPs, whose agglomeration would be ignited under heating, just to lower the surface free energy. However, pure W NPs and WTi NPs with self-passivation layer are intertwined together, reducing the probability to form larger agglomerations. Along with annealing treatment proceeding, metallic Ti would reach to LMVF layer via long-distance diffusion driven by the concentration gradient, which can contributes to suppress the W-atom transfer near the LMVF/HMVF interface. Further, owing to the absent of severe agglomerations and the utilization of alumina barrier layer, welldefined interfaces (one is in between LMVF and HMVF, and the other
4. Conclusion In this article, we proposed the alloying of W NPs with Ti to inhibit the thermal diffusion of W atoms and the agglomeration of NPs in the cermet films, resulting in that the thermal stability of WTi-Al2O3 cermet film was improved significantly. Based on the well-performed WTi-Al2O3 cermet and the guidance of optical simulation, we developed solar selective absorbing coatings with a structure of SS substrate/W infrared reflective layer/ultra-thin Al2O3 barrier layer/WTiAl2O3 HMVF layer/W-Al2O3 LMVF layer/Al2O3 anti-reflection layer, demonstrating a high solar absorptance ~93% and a low thermal emissivity ~10.3% @500 °C after annealing at 600 °C for 840 h in vacuum. The segregation of solute Ti atoms and their partial oxidation to form local protective layer could suppress the outward diffusion of W and wild agglomeration of NPs, thereby ensuring the microstructure, interface structure and optical stability at elevated temperatures. This concept is quite different from the conventional method by using refractory alloy or compound with high melting point dispersed into ceramic matrixes (WNi-Al2O3 [18], AlNi-Al2O3 [8], MoSi2-Al2O3 [63], and so on). It is expected that the WTi-Al2O3 cermet-based solar selective absorbing coatings, with low thermal emissivity and excellent thermal stability, have great potential for high-temperature solar thermal applications. Acknowledgements We thank Dr. Hubin Luo for useful discussions. The authors acknowledge support from the National Natural Science Foundation of China (Grant No. 51302277), the Natural Science Foundation of Zhejiang Province (Grant No. LY17E020012), the Key Research and Development Plan of Jiangsu Province (Grant No. BE2015049), and the program for Ningbo Municipal Science and Technology Innovative Research Team (Grant No. 2016B10005). 240
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