Optimization of sputtered zirconium thin films as an infrared reflector for use in spectrally-selective solar absorbers

Thin Solid Films 627 (2017) 17–25

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Optimization of sputtered zirconium thin films as an infrared reflector for use in spectrally-selective solar absorbers B. Usmani a, V. Vijay b, R. Chhibber c, Ambesh Dixit a,⁎ a b c

Department of Physics & Center for Solar Energy, Indian Institute of Technology Jodhpur, Rajasthan 342011, India Department of Mathematics, Indian Institute of Technology Jodhpur, Rajasthan 342011, India Department of Mechanical Engineering, Indian Institute of Technology Jodhpur, Rajasthan 342011, India

a r t i c l e

i n f o

Article history: Received 2 October 2016 Received in revised form 3 February 2017 Accepted 25 February 2017 Available online 27 February 2017 Keywords: Thermal emittance DC sputter deposition Zirconium thin film Spectrally solar selective coatings

a b s t r a c t Thermal emittance is an important parameter for the solar thermal collectors as thermal radiative losses from the solar thermal collector increase to the fourth power of temperature. This should be minimized using infrared reflectors in designing spectrally selective absorber coatings for solar thermal applications. The thermal emittance of zirconium (Zr) film as an infrared reflector has been investigated for the use in the spectrally selective absorber. The Zr metallic films are deposited using DC magnetron sputtering process on stainless steel and glass substrates and the deposition process has been optimized to achieve the minimum thermal emittance. The effect of structural, microstructural and surface morphological properties of Zr films is investigated on the emittance of fabricated structures. The X-ray diffraction analysis revealed that the Zr film coatings consist of both cubic and hexagonal Zr crystallographic phase. The optimized deposition time and temperature showed 0.12 and 0.14 emittance values for Zr film coatings on stainless steel and glass substrates respectively. © 2017 Elsevier B.V. All rights reserved.

1. Introduction Solar energy is one of the most abundant sources of renewable energy and has attracted attention because of its easy availability, naturefriendliness, and CO2 emission free source etc. [1–4]. There are efforts to harness the incident solar energy using solar thermal energy technology [5–9]. The collector/receiver is an important sub-system of the solar thermal system and is used to collect and convert the concentrated solar energy into thermal energy followed by its transfer to the fluid medium. The efficient collection of solar energy relies on the absorber coatings, which may be single or multilayer thin absorbers/surfaces. The main feature of these absorber coatings is to have high absorptance (α) in the solar spectrum range (0.3–2.5 μm) and low thermal emittance (ε) in the infrared spectrum range (2.5–25 μm) to minimize the thermal radiative losses [9–11]. Intrinsic absorbers, such as few transition metal oxides and semiconductors, show spectrally selective properties. However, the selectivity of these intrinsic absorbers is not up to the desired level and need to modify the intrinsic absorbers. Several approaches/designs such as a multilayer, tandem absorber – reflector, absorber – reflector tandem etc. are explored to obtain the low emittance and high absorptance in spectrally selective absorbers [10,12–24]. Normally, the absorber – reflector tandem structures are used in conjunction with a thin metal layer, as an infrared reflector, between the substrates and ⁎ Corresponding author. E-mail address: [email protected] (A. Dixit).

http://dx.doi.org/10.1016/j.tsf.2017.02.055 0040-6090/© 2017 Elsevier B.V. All rights reserved.

absorber layer followed by the antireflecting layer [25]. The substrates, known as the base materials, are stainless steel, copper, and aluminum, commonly used for solar thermal applications. The substrate itself may play a prominent role in the infrared reflection. The thermal conductivity of these substrates, namely copper and aluminum, is also very high, which helps in efficient heat transfer between absorber and heat transfer medium [26]. The cleaned substrates usually show low emittance ~0.12–0.13, depending on surface conditions [27]. Yet, these substrates usually degrade the solar thermal performance at higher temperatures because of iron, chromium, copper and aluminum diffusion from substrates into the absorber coating structures [27]. In addition, these substrates, used for solar collectors, are not resistant to the microclimatic conditions [26]. Thus, it becomes important to introduce an additional layer between substrate and absorber, which may provide low emittance and simultaneously avoid the diffusion of substrate elements into the absorber and thus, protect the solar thermal performance at elevated temperatures. Simultaneously, the reflector should also withstand high temperature and exhibit high thermal tolerance, as the temperature of these tandem spectrally selective coating structures may increase more than 400 °C [10] during the operational period (mostly Sun hours) and reduce up to ambient temperatures (mostly off Sun hours). This infrared reflecting layer also plays an important role of a barrier layer between the substrate and absorbing layer to protect the corrosion and thus providing the long-term stability to these absorber structures [28]. Normally, silver (Ag), and gold (Au) are the ideal reflector in the infrared region.

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However, these reflectors suffer from agglomeration at ~350 °C, even in a vacuum, which finally, degrades the infrared reflection properties, causing undesired enhanced thermal emittance [29]. Additionally, the cost is also one of the major issues of these IR reflectors. Nickel has also been used as an infrared reflector and as a corrosion barrier on copper and aluminum substrates. The electroplated nickel film is normally used as an infrared reflector in electroplated black chrome and black nickel solar absorbers [30–32]. Moreover, sputtered deposited nickel has been integrated as an infrared reflector in nickel-nickel oxide and nickel-silicon oxide solar selective absorbers [33,34]. This has been observed that sputtered deposited absorber surfaces with nickel as infrared reflector show low infrared reflectance as compared to that of only absorber layers deposited directly on bare copper and aluminum substrates. Thermal emittance of these absorber structures with nickel barrier layer is 0.15 [35] and without nickel layer is 0.10 [34]. The electrodeposited absorber coatings with and without nickel infrared reflector on copper substrate showed the same emittance values within experimental error bar [29]. In addition, there is a problem in depositing pure nickel because of its ferromagnetic nature, specific magnetron needs to be designed and optimized, which is again a difficult task for large area deposition. Thus, keeping in mind the above constraints, such as hightemperature stability, cost, the resistance to the microclimatic conditions and ease of fabrication, there is a need to use a suitable infrared reflector, which may withstand high temperature without losing solar thermal properties, especially the emissivity property. Transition metals are suitable candidates for IR reflector applications. Moreover, Ag, Ni, Ti, Mo, Cr, Ta and W metals are already studied as IR reflector in solar absorber applications [12,27,36,37]. Zirconium (Zr) metal is also a promising candidate for IR reflector in solar absorbers because of its refractory nature with high melting point (1855 °C). The optimized zirconium IR reflector has been used in zirconium carbonitride based absorbermetal tandem structure as IR metal reflector, these tandem structures were developed for basically parabolic trough applications [38]. The Sputtering parameters are very important for depositing a highquality thin film maintaining the low emittance [39]. IR reflectance of the metal film is influenced by the film structure and properties, such

as crystallinity, grain size, porosity, surface roughness, strain, dislocation density, surface morphology [40]. These properties can be controlled by sputtering process conditions, such as substrate temperature and argon working pressures, to achieve the enhanced infrared reflectance [26, 41–44]. The main motive/objective of the present study is to optimize the thermal emittance of sputter deposited zirconium film by varying sputtering process parameters and to understand the structureproperly-process parameter correlation affecting the thermal emittance of metallic zirconium thin films. 2. Experimental details Zirconium thin films were deposited in an RF/DC magnetron sputtering system using only DC sputtering conditions. A Zirconium (Zr) target of 101.6 mm diameter and 6.35 mm thickness was used to deposit zirconium thin film on stainless steel (SS) and glass substrates. One mm thick 304 SS substrates were cut into a square shape (35 mm × 35 mm) pieces and were mechanically polished using 2000 grade SiC abrasive paper. Further, these substrates were cleaned in 2propanal and acetone, respectively in an ultrasonic agitator for 10 min and rinsed with DI water, followed by nitrogen drying. Normal microscopic slides (76 mm × 26 mm) were used as the glass substrates. The glass substrates were also cleaned using acetone in an ultrasonic agitator for 3 to 5 min, and rinsed in DI water, followed by nitrogen drying. These cleaned and dried SS and glass substrates were mounted on the sample holder inside the deposition chamber. The chamber was pumped down to the base pressure of 2.0 × 10−6 mbar. Argon was introduced into the chamber at a flow rate of 50 sccm (standard cubic centimeter per minute at standard temperature and pressure (STP)), to maintain the working pressure of ~2.5 × 10−2 mbar for Zr thin film deposition. The target was pre-sputtered using argon (Ar) plasma for ~10 to 15 min to ensure the removal of any residual surface impurity. All Zr thin films were deposited at 110 W DC target power. In this work, the optimization of sputtering conditions is aimed to achieve the maximum IR reflectance for minimum emittance values for their possible use in spectrally selective coating structures. The parameters, such as substrate temperature, and deposition time, are methodically investigated

Fig. 1. X-ray diffraction (XRD) patterns of the Zr thin films sputtered at different deposition time (a) and temperature (b) on stainless steel substrates.

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Fig. 2. XRD patterns of the Zr thin films sputtered at different deposition times (a) and temperature (b) on glass substrates.

to achieve the minimum emittance, keeping other parameters constant during deposition. For example, the substrate temperature was kept constant at 350 °C with other parameters, while deposition time was used as a variable and the deposition time was kept constant for 2 h, while deposition temperature was varied for Zr thin film deposition. Structural characterization of the films was carried out using X-ray diffraction (XRD-Bruker, D8) in thin film geometry configuration at 2–3° of the grazing incident angle, to investigate the development of respective crystallographic phases and microstructure. XRD graphs were recorded in the range of 20° to 80° at 0.02° step size using copper Kα radiation (λ = 1.5406 Å). The grain size (t) was calculated using the Scherrer formula [45],

t¼

0:94λ B cosθ

ð1Þ

where B is the half-width at half maximum of the diffraction peak measured in radians, the strain (ε) was calculated from the slope of B cosθ versus sinθ plot using Eq. (2) [46],

B¼

λ −ε tanθ t cosθ

ð2Þ

formula [47],

δ¼

ð3Þ

The microstructural and surface properties were investigated using Carl Zeiss EVO 18 especial edition scanning electron microscope (SEM) and Park System XE-70 atomic force microscope (AFM) systems. All AFM micrographs were acquired on 2 × 2 μm2 scanning areas, at 256 × 256 pixel resolution. The elemental compositions were measured using energy dispersive X-ray (EDX) system, equipped as an accessory with SEM equipment. The thickness of these coatings was calculated using AFM system and cross-sectional scanning electron microscope (SEM) measurements. In AFM measurements, a scanning was carried out on 45 × 45 μm2 area in contact mode at the step near film surfaces and substrate, to get the morphology, and XEI software was used to calculate the thickness of the film in line profile. A Bruker vertex 70v FTIR spectrophotometer with a gold mirror as a reference for spectral reflectance has been used to measure the reflectance in 2.5–25 μm wavelength range. These FTIR reflectance measurements were used to calculate the room temperature thermal emittance, ε, using Eq. (4);

The dislocation density (δ), defined as the number of dislocation lines per unit volume of the crystal, was evaluated using the following

Table 1 Microstructural properties of Zr thin films as a function of deposition time on stainless steel and glass substrates for 350 °C deposition temperature.

1 t2

Z25 ð1−RðλÞÞEb ðλÞdλ ε¼

2:5

ð4Þ

Z25 Eb ðλÞdλ 2:5

Time (min)

Stainless steel substrate B (rad)

t (nm)

δ × 1016 ε (line/m2)

B (rad)

Glass substrate t (nm)

δ × 1016 ε (line/m2)

90 120 150 180

0.711 0.586 0.516 0.692

11.7 14.2 16.12 12.01

0.7305 0.4959 0.3848 0.6932

0.708 0.529 0.487 0.701

11.74 15.74 17.1 11.87

0.7255 0.4036 0.3419 0.7097

2.2527 1.856 1.6348 2.1924

2.2431 1.676 1.543 2.221

Eb(λ) is the spectral radiance of a black body at temperature, T, and is given by Plank's law Eq. (5) [40]. All emittance values are calculated at room temperature (T = 300 K). Here Eq. (4) describes the portion of the black-body radiation absorbed by the sample with respect to the total black-body radiation and is called as emissivity at given

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Table 2 Microstructural properties of Zr thin films as a function of deposition temperature on stainless steel and glass substrates for 2 h deposition time. T (°C)

Stainless steel substrate B (rad)

t (nm)

δ × 1016 (line/m2)

ε

Glass substrate

100 200 300 350

0.966 0.785 0.583 0.638

8.61 10.6 14.27 13.04

1.3489 0.8899 0.4959 0.588

3.06 2.4871 1.8471 2.021

B (rad)

t (nm)

δ × 1016 (line/m2)

ε

0.837 0.652 0.532

9.94 12.76 15.62

1.012 0.6141 0.4098

2.6519 2.065 1.6855

temperature T (in present case T = 300 K). Eb ¼

C1 λ5 ½eC 2 =λT −1

ð5Þ

where C1 = 3.743× 10−16 W m2 and C2 =1.4387× 10−2 mK. 3. Results and discussion 3.1. Structural analysis X-ray diffraction (XRD) studies are carried to analyze the evolution of Zr thin film structure and its correlation with their thermal emittance. The XRD patterns are shown in Fig. 1 for Zr thin films on SS substrates as a function of deposition time, Fig. 1(a) and as a function of substrate temperature Fig. 1(b). The XRD spectrum suggests that Zr thin films are polycrystalline and polyphase with mixed cubic and hexagonal crystallographic Zr phases. The Zr thin films with smaller thickness (30 min deposited) are nearly amorphous and no significant diffraction of Zr diffraction pattern has been observed, Fig. 1(a). In the case of SS substrates, only chromium diffraction peaks at 2θ = 43.54°, 44.44° and 50.55°, have been observed [48], and a less intensity XRD peak at 2θ = 29.89° corresponds to (111) zirconium oxide phase has also been noticed [49]. This zirconium oxide phase is forming due to the presence of residual oxygen either in the deposition chamber or due to impurity in sputtering gas. However, as the deposition time was increased to the 60 min, Zr thin

films started showing crystallographic appearance, as can be observed from the onset of (100) diffraction peak at 2θ = 31.97°, which corresponds to the hexagonal zirconium phase [50], in conjunction with substrate diffraction peaks. The Zr metal film showed the onset of 2θ = 34.38°, which corresponds to the (111) preferred orientation of cubic phase with increasing the film thickness Fig. 1(a) [51]. The crystallinity of the film has increased with increasing the thickness, due to the initial stress release for the deposited zirconium thin films on SS substrates [52]. Moreover, the development of Zr phase on glass substrates also follows the similar pattern with increasing the deposition time, as shown in Fig. 2(a). The structure evolution is an atomistic process during thin film deposition and affected by process parameters such as target power, substrate temperature, deposition time, and reactive/sputtering gas pressure [52,53]. Moreover, these process parameters may lead to the variation in grain size, texture orientation and film density of deposited thin film structures [54]. Here, the substrate temperature has been used as another controlling parameter for thin films structural and physical properties [55]. To understand such physical parameters and their impact on solar thermal performance, Zr metallic thin films have been deposited at different substrate temperatures, keeping the deposition time constant for 2 h. These thin films exhibited the polycrystalline nature having mixed cubic and hexagonal crystallographic phase, similar to the constant temperature deposited Zr thin films for different deposition intervals. At low substrate temperature, the deposited Zr films showed cubic crystallographic phase in majority as confirmed with the observed (111) preferred orientation at 2θ = 34.38° diffraction peaks as shown in Fig. 1(b) and 2(b) [51]. The hexagonal crystallographic phase in Zr thin films starts increasing as the substrate temperature increased from 350 °C and 500 °C. The onset of (100) hexagonal orientation and its' enhanced relative intensity with the temperature substantiate that the enhancement in the Zr hexagonal phase in Zr thin films with temperature, as shown in Fig. 1(b) and 2(b). The Zr thin films, deposited for lower deposition time and temperature, exhibit the weak zirconium oxide peaks, suggesting the presence of oxygen. However, with increasing substrate temperature, the presence of residual oxygen minimized as zirconium oxide crystallographic diffraction peaks reduced for both SS and glass substrate cases. At low temperatures, the diffraction of (111) cubic crystallographic planes is insignificant, Fig. 1(b) and

Fig. 3. Schematic representation of Zr thin film growth on SS substrate with varying deposition time and temperature, showing the impact of lattice mismatching between substrate and Zr.

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temperature, leading to the better growth conditions [56]. Thus, the relative intensity has increased with increasing the substrate temperature, which is attributed to the enhanced sticking force between target particles and substrate, in conjunction with low surface energy for (111) crystallographic planes. The (111) diffraction peak has reduced in intensity and (100) hexagonal zirconium diffraction peak started for high temperature ~ 400 °C and 500 °C deposited thin film structures. This has led to enhanced grain/particulate size with substrate temperature, as observed in grain size calculation, summarized in Table 2. The full width at half maxima (FWHM) (B), grain size (t), dislocation density (δ), and strain (ε) of these zirconium films are calculated and summarized in Tables 1 and 2 for both SS and glass substrates, deposited for different deposition time and substrate temperature, respectively. It has been observed that the crystallite size increases and the dislocation density decrease with increasing deposition time and substrate temperature and approaches towards the constant values. A slight reduction in grain size and increase in dislocation density have been observed with further increase in deposition time and temperature. The atomic force microscopic results also substantiate these observations, where average grain size has shown enhancement initially with time and temperature. The grain size showed a decrease with further increase in deposition time and temperature, which is attributed to the formation of smaller grains on the previously deposited larger grains and consistent with reported literature [46]. The dislocation density and strain are the demonstration of dislocation network in the films, and a decrease in the strain and dislocation density shows the formation of high-quality films at relatively higher substrate temperatures. Moreover, smaller B (HWFM) and larger thickness t values suggest the enhanced crystallization of the zirconium metallic films. 3.2. Microstructural analysis

Fig. 4. 2D AFM images and SEM micrographs of sputtered deposited Zr thin films on stainless steel substrate at different temperatures: (a) RT, (b) 100 °C, (c) 200 °C, (d) 300 °C, (e) 350 °C, (f) 400 °C, (g) 500 °C.

Fig. 2(b), however, the contribution of (111) peak increases with increasing substrate temperature up to 350 °C. The surface mobility of sputtered particles has increased with increasing substrate

Surface morphology, roughness, and microstructures are also important parameters to optimize the thermal emittance of the metal films, as emittance also relies on surface properties of the materials [40]. Twodimensional (2D) surface AFM images of Zr film coatings are shown in Fig. 4 for stainless steel substrates deposited at the different substrate temperatures. The granular morphology has been observed for all these films, which showed an increase in granules size with increasing the substrate temperature, consistent with XRD measurements. The root mean square (rms) surface roughness (Rq), average surface roughness (Ra) and grain size are calculated from the scanned surface morphology, as shown in Fig. 4. These measurements are summarized in Fig. 5 for Zr thin films on SS substrate as a function of deposition time and substrate temperature. The rms surface roughness has increased with increasing the deposition time (Fig. 5a), initially, followed by a decrease for larger deposition time and temperatures, suggesting that surface defects have decreased, contributing towards reduced surface roughness. Simultaneously, the grain sizes have increased with increasing the deposition time and temperature. The similar observations have been observed in XRD measurements for these structures and discussed previously. The Zr films on glass substrates also showed the similar microstructural properties with deposition time and substrate temperature. SEM images of Zr films on SS substrate are shown in Fig. 4, for different substrate temperatures, in conjunction with AFM microstructures. The surface micrographs suggest that Zr structures are dense and showing granular morphology. The surface imprints present on the substrates can also be seen even after film deposition. The geometrical size of these granules on the Zr surfaces has increased with increasing temperature and became denser at higher temperatures. As the emittance is also a surface property of the materials [40], the low surface roughness of the film coatings is important to achieve the low thermal emittance of the film, deposited on the substrates. The thickness of these Zr coatings has been measured using AFM system and also with SEM cross sectional measurements. The

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Fig. 5. Root mean square (rms) surface roughness and grain size of Zr thin films deposited on SS substrate with different deposition time (left panel) and substrate temperature (right panel).

morphology scan at a step of Zr film and substrate has been collected and a line profile has been used to calculate the thickness of Zr thin films deposited for different deposition time and substrate temperature for both SS and glass substrate. The measured step and film structure are shown in Fig. 6(a) for a Zr film on a glass substrate. The line profiles are also shown in the inset, used for thickness measurements. The measured film thicknesses are summarized in Fig. 7(a) and 7(b) for Zr/ Glass structures against the deposition time and temperature, respectively. The thicknesses of the films are also calculated using cross sectional SEM measurements of the samples. A representative cross sectional SEM image of zirconium (Zr) film on SS substrate for lowest emittance value is shown in Fig. 6(c) and measured thicknesses are in agreement with that of the step method using AFM measurements. The thickness of Zr film has increased with increasing deposition time,

Fig. 7(a). In contrast, with temperature, the film thickness has increased initially, up to 200 °C and became insensitive to the increase in temperature anymore Fig. 7(b). This has been attributed to enhancement in compactness of Zr film, as compared to the low-temperature Zr thin film structures. The elemental composition has been evaluated using energy dispersive X-ray analysis and a schematic EDX spectra collected on Zr/SS (deposited at 350 °C substrate temperature for 2 h) is shown in Fig. 6(b), and elemental contribution has been summarized in the respective tables. In conjunction with Zr elemental contribution (61.09 wt%, 40.27 atm. %), chromium (Cr), nickel (Ni), titanium (Ti), iron (Fe) and strontium (Sr) elemental contributions have also been observed from SS substrate. Moreover, EDX spectrum also exhibits a small peak for oxygen, which is consistent with XRD observation, where small zirconium oxide phase in Zr thin films was observed. The schematic

Fig. 6. (a) AFM image of Zr thin film with a step (light yellow color), used for measurement of thickness with line profile, shown in inset for Zr thin film on glass substrate. (c) EDX spectra of Zr film deposited on SS substrate for minimum emittance values. (c) SEM cross sectional image of Zr film deposited on SS substrate for minimum emittance values. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Fig. 7. Variation of Zr film thickness as a function of deposition time (a) and substrate temperature (b) on glass substrate.

development of thin film structure has been summarized in Fig. 3, as a function of deposition time and thickness. The lower thickness, up to ~200 nm or less, of the Zr film, has led to highly disordered phase, due to large mismatch between the lattice parameters between Zr and SS/ glass substrate. As thickness increases with deposition time, the initial Zr layer may act as a buffer layer for upper Zr thin film structures, thus, increasing the crystallinity and reduced defect density, as observed in XRD measurements and summarized in Tables 1 & 2. Finally, the phase pure crystallographic phase has been deposited due to the pseudo-lattice matching of Zr metallic thin film structures, which has shown the lowest emittance values, as explained later. (See Fig. 9.) 3.3. Optical analysis The reflectance spectra were recorded for these Zr thin film structures using a 30° reflectance specular accessories attached with an FTIR spectrophotometer in the 2.5–25 μm wavelength range, at room temperature. The measured reflectance spectra are plotted in Fig. 8(a, b) and Fig. 9(a, b) for SS and glass substrates respectively and are used for calculating the thermal emittance using Eqs. (4) and (5). The calculated emittance values are plotted as an inset as a function of deposition time and temperature in respective figures. The emittance values strongly depends on the thickness of deposited thin film structures and showed a sharp decrease with increasing thickness and becomes nearly constant at higher thickness for both SS and glass substrates. The minimum emittance value ~ 0.12 has been observed for the 120 min deposited Zr thin films. In addition, emittance values also showed dependence on deposition temperature and the lowest value has been observed for Zr thin film structures deposited at 350 °C. The decrease in emittance values with temperature is mainly due to the enhanced crystallinity and reduced defects, as observed from XRD, SEM and AFM measurements. Furthermore, the low emittance values for Zr films with larger thicknesses are attributed to reduced defects such as grain boundaries, pin hole, dislocation, vacancies and interferential

defects [57]. The free electrons in the metal thin films also play important role to minimize the thermal emittance, since the optical response in infrared region is determined by contribution of free electrons in films [58]. Thus, crystalline quality and low defect densities may contribute to the lower emittance value because of larger free electron contribution in Zr thin films. The depth of light penetration, dp, and λ , where λ is the waveextinction coefficient (k) are related as dp ¼ 4πk length of the light [59]. This indicates that the depth of light penetration is inversely proportional to the extinction coefficient. The values of extinction coefficient should be lower for avoiding possible absorption. These two contradictory requirements suggest the probable tradeoff between thickness and emittance values of these thin film structures and thus, the thickness of thin films should be greater than the depth of penetration of light for lower emittance values. However, the emittance values of Zr thin films have shown increase with increasing thickness and became almost constant after certain thickness, critical thickness. Above the critical thickness, emittance values are nearly insensitive to the thickness. The experimental observations suggest that the minimum emittance values are achieved for Zr thin films, deposited at 350 °C for 2 h. These optimized conditions can be used for depositing the Zr metallic infrared reflector thin films in conjunction with an absorber and antireflecting structures for the development of spectrally selective coating structures.

4. Conclusion DC-magnetron sputtered Zr films have been studied and sputtered conditions have been optimized in order to achieve the minimum thermal emittance/maximum infrared reflectance in 2.5–25 μm wavelength range. The structure–property-process correlations suggest that thermal emittance values of Zr infrared reflectors strongly depends on the sputtering conditions. The minimum thermal emittance has been observed for Zr thin film structures with minimum dislocation densities

Fig. 8. Reflectance spectra of Zr thin films deposited on SS substrate for different deposition time ranging from 30 min. to 180 min., with inset showing the respective emittance variation with the deposition time (a) and for different substrate temperatures ranging from room temperature to 500 °C, with inset showing the measured emittance values as a function of temperature.

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Fig. 9. Reflectance spectra of Zr film deposited on glass substrate with changes of time from 30 min to 180 min (a): in inset show the calculated emittance vs deposition time. (b) With changes of substrate temperature from room temperature to 500 °C: in inset, showing the calculated emittance values vs substrate temperature.

and strain, minimum surface roughness and maximum grain sizes. The optimized deposition time ~120 min–150 min and substrate deposition temperature 300 °C–350 °C at 110 W sputtering power showed the minimum thermal emittance of 0.12 and 0.14 on SS and glass substrates. The small variation in emittance values is attributed to the substrate surface conditions. These results suggest that the sputtered Zr metallic thin films with optimal surface properties can be used as an effective infrared reflector for the development of spectrally selective coating structures and the emittance values may further be reduced by controlling the residual oxygen during deposition. Acknowledgment Authors acknowledge the financial assistance from the Ministry of New & Renewable Energy (MNRE), India through grant 15/40/201011/ST to carry out this work. References [1] S.-Y. Liu, Y.-H. Perng, Y.-F. Ho, The effect of renewable energy application on Taiwan buildings: what are the challenges and strategies for solar energy exploitation? Renew. Sustain. Energy Rev. 28 (2013) 92–106, http://dx.doi.org/10.1016/j.rser. 2013.07.018. [2] V. Bosetti, M. Catenacci, G. Fiorese, E. Verdolini, The future prospect of PV and CSP solar technologies: an expert elicitation survey, Energy Policy 49 (2012) 308–317, http://dx.doi.org/10.1016/j.enpol.2012.06.024. [3] M.M. Aman, K.H. Solangi, M.S. Hossain, A. Badarudin, G.B. Jasmon, H. Mokhlis, et al., A review of Safety, Health and Environmental (SHE) issues of solar energy system, Renew. Sustain. Energy Rev. 41 (2015) 1190–1204, http://dx.doi.org/10.1016/j. rser.2014.08.086. [4] S. Mekhilef, R. Saidur, A. Safari, A review on solar energy use in industries, Renew. Sustain. Energy Rev. 15 (2011) 1777–1790, http://dx.doi.org/10.1016/j.rser.2010. 12.018. [5] V. Siva Reddy, S.C. Kaushik, K.R. Ranjan, S.K. Tyagi, State-of-the-art of solar thermal power plants - a review, Renew. Sustain. Energy Rev. 27 (2013) 258–273, http://dx. doi.org/10.1016/j.rser.2013.06.037. [6] X. Zhang, X. Zhao, S. Smith, J. Xu, X. Yu, Review of R&D progress and practical application of the solar photovoltaic/thermal (PV/T) technologies, Renew. Sustain. Energy Rev. 16 (2012) 599–617, http://dx.doi.org/10.1016/j.rser.2011.08.026. [7] A. Fernández-García, E. Zarza, L. Valenzuela, M. Pérez, Parabolic-trough solar collectors and their applications, Renew. Sustain. Energy Rev. 14 (2010) 1695–1721, http://dx.doi.org/10.1016/j.rser.2010.03.012. [8] A. Kumar, P. Baredar, U. Qureshi, Historical and recent development of photovoltaic thermal (PVT) technologies, Renew. Sustain. Energy Rev. 42 (2015) 1428–1436, http://dx.doi.org/10.1016/j.rser.2014.11.044. [9] N. Selvakumar, H.C. Barshilia, Review of physical vapor deposited (PVD) spectrally selective coatings for mid- and high-temperature solar thermal applications, Sol. Energy Mater. Sol. Cells 98 (2012) 1–23, http://dx.doi.org/10.1016/j.solmat.2011.10. 028. [10] C.E. Kennedy, Review of mid-to high-temperature solar absorber materials, NREL/ TP-520-31267, National Renewable Energy Laboratory, Colorado, July 2002, 2002. [11] F. Cao, K. McEnaney, G. Chen, Z. Ren, A review of cermet-based spectrally selective solar absorbers, Energ. Environ. Sci. 7 (2014) 1615, http://dx.doi.org/10.1039/ c3ee43825b. [12] Z.Y. Nuru, L. Kotsedi, C.J. Arendse, D. Motaung, B. Mwakikunga, K. Roro, et al., Thermal stability of multilayered Pt-Al2O3 nanocoatings for high temperature CSP systems, Vacuum 120 (2015) 115–120, http://dx.doi.org/10.1016/j.vacuum.2015.02. 001. [13] X.-H. Gao, Z.-M. Guo, Q.-F. Geng, P.-J. Ma, A.-Q. Wang, G. Liu, Structure, optical properties and thermal stability of SS/TiC–ZrC/Al2O3 spectrally selective solar absorber,

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