Design, fabrication and characterization of TiO2-SiO2 multilayer with tailored color glazing for thermal solar collectors

Materials & Design 130 (2017) 275–284

Contents lists available at ScienceDirect

Materials & Design journal homepage: www.elsevier.com/locate/matdes

Design, fabrication and characterization of TiO2-SiO2 multilayer with tailored color glazing for thermal solar collectors

MARK

Iulian Panaa,b,⁎, Catalin Vitelarua, Adrian Kissa, Nicolae Catalin Zoitaa, Mihaela Dinua, Mariana Braica a b

National Institute for Optoelectronics, 409 Atomistilor St., 077125 Magurele, Romania Faculty of Physics, University of Bucharest, 405 Atomistilor St., 077125 Magurele, Romania

A R T I C L E I N F O

A B S T R A C T

Keywords: Multilayer design Optical coatings Colored glazing Magnetron sputtering

The paper reports on the design, fabrication, and characterization of a 7-layer SiO2-TiO2 colored coating for solar thermal façades. A combined theoretical and experimental approach was employed for designing the colored coating by using optical modeling and individual layers characterization, targeting for high solar transmittance and pronounced yellow-green reflected color. SiO2-TiO2 multilayers were deposited on glass and silicon substrates by using the reactive RF magnetron sputtering of stoichiometric oxide targets. The coatings were investigated by UV–Vis spectrophotometric methods for optical properties, surface profilometry, X ray reflectivity and SEM for the thickness of individual layers composing the multilayer and AFM for surface roughness. Hardness H, reduced modulus Er, and adhesion were determined by nanoindentation and scratch tests. The deposited colored glazing exhibited a prominent reflectivity peak of ~54% centered at 574 nm, specific visible reflectance of ~45.39% and an integrated solar transmittance of about 80.3%. Its optical properties were preserved in the ambient for temperatures up to 200 °C and the color was not modified for incidence angles up to 45°. H values of 7.18 GPa, Er of 88.31 GPa, H/Er ratio of about 0.081 and good scratch resistance up to 13 mN were measured.

1. Introduction A large number of studies indicated the importance of solar thermal technologies development, in terms of consumption of fossil fuels and greenhouse gas emissions reduction [1]. The thermal solar collectors with high energy conversion efficiency are absorber coatings with high solar absorption and low thermal emission. As reviewed by Schüler and coworkers, [2] the deposition of the thermal collector coatings by a variety of processes such as electrodeposition, magnetron sputtering, vacuum evaporation, electrochemical or sol-gel processes, or even selective paints, resulted in dark-bluish or black colors. Up to now, many research efforts were conducted for solving the architectural integration issue of thermal solar collectors into modern buildings [3,4], one of the main issues being the demand for adapted colored façades [5,6]. It should be underlined that color modification of solar collectors according to the needs of the architectural design, aiming for other colors than blue and gray, should maintain the overall performance of the thermal collectors. Schüler and coworkers proposed the use of glazing colored collectors in front of the thermal absorber [2]. A simple solution for colored glazing is to use colored glass, but this

⁎

implies light absorption and corresponding diminishing of energy transmitted to the thermal collector and overall efficiency. Another approach, based on colored interference coatings [2], is taking advantage of the structural colored films known in nature [7], and of the dielectric multilayers used in optics and laser applications [8]. Generally, the interest for improving the optical properties of different multilayer structures is continuously growing, as these may be used in a variety of applications. Besides the colored solar thermal collectors [9] other well-known applications are: Bragg mirrors [10], transparent heat reflectors [11], energy-saving spectrally-selective coatings [12], antiglare side-view car mirrors [13], or UV reflective and near-infrared (NIR) anti-reflective filters [14,15], to name only a few. As the production of the colored interference coatings is currently run by the industry, the production process of such coatings is supposed to be quickly accepted. However, unlike in the applications in optics, the potential use of the spectrally-selective multilayer as color glazing for thermal solar collectors should consider the need for large coated surfaces, imposing economical restriction such as the use of abundant materials and an economical design of the multilayer. This means that the multilayer should have some key features to be competitive,

Corresponding author at: National Institute for Optoelectronics, 409 Atomistilor St., 077125 Magurele, Romania. E-mail address: [email protected] (I. Pana).

http://dx.doi.org/10.1016/j.matdes.2017.05.063 Received 16 January 2017; Received in revised form 19 May 2017; Accepted 22 May 2017 Available online 24 May 2017 0264-1275/ © 2017 Elsevier Ltd. All rights reserved.

Materials & Design 130 (2017) 275–284

I. Pana et al.

alternating layers of TiO2 and SiO2, by sputtering of stoichiometric oxide targets in an Ar/O2 gas mixture with low O2 partial pressure, to compensate for oxygen loss during the sputtering process.

namely: low number of individual layers, thermal stability of the optical characteristics at temperatures reachable by surfaces when exposed to direct sunlight in open atmosphere, superior mechanical properties such as a good mechanical resistance and adhesion to the glass substrate. The optical interference filters, comprising successive high- and low- refractive index layers, represent a valuable choice for the color glazing, as they might exhibit high solar transmission and high reflectivity only in a narrow visible spectral range. This paper will focus on the development of such a colored interference filter. For materials' selection, we considered the “multi-material by design” approach for the multilayer development, to congregate the specific requirements [16]. The selection of the materials to be used in multilayer took into consideration the environmental impact, in order to include the concept of sustainability into the multilayer design and production process [17]. Moreover, we applied the computational tool presented in ref. [17] related to the materials selection and a clean environment, by including energy consumption for their production, materials' performance, abundance, recyclability and reusability, manufacturing process acceptance by industry, manufacturing defects and health risks for humans. In order to achieve the desired optical characteristics for the colored multilayer, the current used materials are TiO2, ZnS, Al2O3 (high index) and SiO2 or MgF2 (low index). The individual layers to be used in the multilayer structure should be very smooth, with small roughness at the interfaces, and should be also nonabsorbing in the visible and near infrared region [6]. As previously reported in the literature, the values of the optical constants for SiO2 and TiO2 thin films obtained by magnetron sputtering are ranging between 1.46–1.475 [18,19] and 2.1–2.45 [20–22] respectively. The extinction of SiO2 thin films is situated in the interval from 0 to 1 × 10− 7, while for TiO2 thin films is ranging from 5 × 10− 2 to 1 × 10− 3. The higher scores were obtained for TiO2 and SiO2 films deposited by magnetron sputtering technique, because they are abundant, are recyclable and pose no threats for human health. So far, SiO2TiO2 multilayer stacks have been obtained by several techniques, including magnetron sputtering [19,23], sol-gel process [24], spray pyrolysis [25], or oblique angle deposition by electron-beam [26]. The magnetron sputtering deposition technique is suitable to cover large area surfaces, being already used in industry for the deposition of black solar absorbers [27]. It is therefore interesting to obtain color glazing for thermal solar collectors by using the same deposition technology. Moreover, the magnetron sputtering deposition technique permits a wide choice of deposition parameters, so the optical properties can be tuned within a large range. Therefore, based on the above considerations we chose as materials for the colored glazing SiO2 and TiO2 films deposited by magnetron sputtering. Besides the optical properties defining the functionality of the multilayer structure, the coating should also exhibit reliable mechanical properties, ensuring for the colored façades deposited on glass a prolonged lifetime in open atmosphere. Due to the fragile nature of the glass substrates, we used the nanoindentation technique [28,29] for the mechanical tests. The very low indentation loads, ranging from 1 to 10 mN are well adapted for investigating the multilayer structures with typical individual layer thicknesses as low as 10 nm and total thicknesses of only few hundred of nanometers. As previously reported, higher loads lead to considerably higher penetration depths, masking the relevant data related to film properties and possibly damaging the substrate [29–32]. The work presented in this paper is focusing on a combined theoretical and experimental approach, aiming to obtain an all-oxide multilayer filter, with only few individual layers, exhibiting high solar transmittance, combined with a green-yellow reflected color, stability at temperatures as high as 200 °C, and valuable mechanical properties related to wear resistance. Reactive radio-frequency (RF) magnetron sputtering technique was used to obtain a color glazing for thermal solar collectors consisting of an optical interference filter comprising

2. Experimental The magnetron sputtering deposition system used for multilayer deposition is equipped with three cathodes (Φ = 2.55 cm) in a confocal geometry. In our experiment we used two cathodes with SiO2 and TiO2 targets respectively (99.99% purity, Kurt Lesker) fed by RF generators (13.56 MHz). The distance between the magnetron targets and the substrates holder was kept constant (12 cm). Films with high uniformity ( ± 2%) on a 10 cm diameter circle area were deposited by continuously rotating the substrates holder (15 rpm). The argon and oxygen gas flow rates were controlled by using mass flow controllers (MFC), and the pressure inside the deposition chamber was kept constant by using an automatic pressure controller. Prior to all deposition runs, the base pressure in the system was < 1.3 × 10− 4 Pa. The gas flow ratio Ar/O2 = 21/0.21 and the working pressure (0.67 Pa) were maintained constant during the deposition process. All films were deposited at room temperature on 1 mm thick (10 × 15 mm2) glass (Heinz-Herenz, Hamburg, Germany) and 0.525 mm thick Si (100) substrates. Prior to each deposition process the substrates were ultrasonically cleaned in isopropyl alcohol and sputter cleaned in the deposition chamber for 15 min, in pure argon atmosphere at 0.67 Pa, applying 50 W RF power on the substrate holder. For all deposition experiments, the power fed into both SiO2 and TiO2 targets was kept constant at 50 W, corresponding to a cathode voltage of 340 V and 350 V, respectively. During the deposition processes the substrates were RF biased at −60 V. The deposition rate of TiO2 and SiO2 monolayers were calculated from deposition time and the thickness (δ) measured by surface profilometry (Dektak 150 Bruker). The surface morphology of the coated glass was assessed by using a Veeco-INNOVA atomic force microscope (AFM) operated in tapping mode with a RTESPA tip made from antimony-doped Si. The characteristics of the tip are as follows: curvature radius of about 8 nm, resonance frequency ~300 kHz and a spring constant of 40 N/m. The measurement was performed in ambient air with a scan rate of 1 μm/s and 512 points/line. The RMS roughness was calculated on 1 μm2 image by using SPMLab Analysis Software (Veeco, USA). The refractive index (n) and the extinction coefficient (k) wavelength dependencies were derived from the transmission (T) and reflection (R) spectra. All the T-R spectra were measured on glass substrates and recorded by using a Jasco V670 spectrophotometer equipped with an integrating sphere (60 mm) for specular reflectance measurements in ambient air. The transmission measurements were performed at normal incidence, whereas the specular reflectance measurements were carried out at 5° incidence angle. The T-R measurements were performed in the same position on the samples surface, in the wavelength range of 350–1500 nm, with a scan speed of 400 nm/min and a wavelength step of 1 nm. The refractive index and the extinction coefficient of the monolayers were calculated from the experimental T-R curves, by using the OptiLayer software [33]. The reverse engineering OptiLayer procedure, as used in the following, implies the post-deposition characterization of multilayers, considering as input data the design and the measured T-R values, for estimation of the optical constants and total and individual layers' thickness. The dispersion behavior of refractive index was assessed by using a standard multi-parametric model (Cauchy), while the extinction coefficient was estimated by using a nonparametric model, as discussed in the next section. Out-of-plane X-ray reflectometry (XRR) measurements on coated glass were used to evaluate the surface and interfaces roughness, layers thickness and their mass density. The XRR data were recorded on a 276

Materials & Design 130 (2017) 275–284

I. Pana et al.

Rigaku SmartLab X-ray diffractometer equipped with a 9 kW Copper rotating anode, operating in parallel beam mode, with high-resolution optics for Cu Kα1 radiation placed on the incident beam (2-bounces Ge (220) monochromator and incident slit of 0.05 mm), and narrow aperture slits of 0.15 mm on the receiving beam. XRR scans were taken on a 2θ range of 0.2–2° with a speed of 0.16°/min and a step size of 0.004°. Experimental data were fit with the Rigaku's GlobalFit software package. The XRD patterns of the multilayer were also investigated at room temperature in Bragg-Brentano geometry, by using the same equipment. We investigated the thermal stability in air of the samples for temperatures up to 200 °C, using a computer controlled furnace delivering a positive temperature gradient of 8 °C/min. The coated glass samples were maintained for 60 min at 200 °C, and then freely cooled down to room temperature. T-R spectra of the thermally treated samples were recorded. Nanoindentation and nanoscratch resistance tests were performed by using a Hysitron TI Premier nanoindenter equipped with a 100 nm radius Berkovich diamond tip, enabling a maximum 13 mN normal force to be used. Prior to indentation experiments the contact area was calibrated using a standard fused quartz sample, to account for the imperfections of the indenter. During the indentation experiments on the TiO2 - SiO2 multilayered structure with thickness of 500 nm, individual indents on the sample's surface were made by using different forces in order to obtain the hardness (H) and the reduced modulus (Er) values for penetration depths in the 40–310 nm range. The indents (3 to 5 indentations at the same maximum force) were located at least 7.5 μm apart to avoid possible interference effects between the indentation points. The values of H and Er were extracted from load-displacement curves according to Oliver-Pharr formalism [34]. Cross-sectional Scanning Electron Microscopy (SEM) imaging of SiO2-TiO2 multilayer deposited on Si was performed using a NanoSAM Lab system (ScientaOmicron GmbH) equipped with a Gemini column. The images were obtained at 15 kV accelerating voltage and 200 pA current.

Fig. 1. Measured transmission and reflection spectra of a) TiO2 (δ = 174 nm) and b) SiO2 (δ = 237 nm) monolayers compared with those of glass substrate.

coworkers [35]:

⎛1 ⎜ ⎝L

⎞2

L

∑ j =1 S (nλj , kλj , δ, λj ) ) − Sλj⎟⎠

L

+ a∑

[k ′′λj ]2

3. Results and discussion

DF2 =

The presented results are grouped in 5 sections. In the first part (Section 3.1) the optical properties of both monolayer oxides are extracted, by combining the spectrophotometric measurements and optical modeling. The so determined complex refractive indices are used in the second part (Section 3.2) as input parameters for the design of a colored multilayer. The concordance between designed and deposited multilayer, concerning the layer thicknesses, is also assessed in the same section. The overall optical properties of the multilayer were evaluated in the third part (Section 3.3), taking into account the color characteristics, the reflectivity angular dependency and the performance of the multilayer as colored glazing in conjunction with solar thermal collectors. The mechanical characteristics of the multilayer are examined in section (Section 3.4), in terms of hardness, reduced elastic modulus and scratch resistance. In the last section (Section 3.5) the thermal stability of the multilayer is evaluated.

where: λj – wavelength grid (1 nm step), n(λj) and k(λj) – refractive index and extinction coefficient, L – number of spectral points defined in the visible spectral range, S(n, k, δ, λj) – theoretical T and R spectral characteristics obtained from the dispersion model, while Ŝ(λj) are the measured T and R spectral characteristics of investigated layer. The second term of the DF is referring to a non-parametric optical model, where the second-order derivative of extinction coefficient, k″(λj)and a weight parameter a used to calculate the wavelength dependency of extinction coefficient k(λj). The refractive index of the glass substrate, nglass(λ) and of TiO2 monolayer, nTiO2(λ), were described by a Sellmeier [36] (Eq. (2)) and Cauchy [6] (Eq. (3)) dispersion relationship, respectively:

n 2 (λ ) = A 0 +

(1)

A1 (λ )2 A (λ ) 2 + 23 λ2 − A2 λ − A4

n (λ ) = B0 +

3.1. Optical characterization of glass substrate and oxide monolayers

j =1

(2)

B1 B + 42 λ2 λ

(3) −2

and resulting A0 = 2.17, A1 = −0.25, A2 = − 4.28, A3 = 7 × 10 A4 = 2.8 × 10− 2, and B0 = 2.2, B1 = 3.9 × 10− 2 and B2 = 5 × 10− 3 with λ expressed in microns. The Cauchy model used for nTiO2(λ) modeling assumed the presence of a homogeneous overlayer, with higher porosity (~ 50%) than the bottom one. As this overlayer is thin, its refractive index was calculated by using the Maxwell-Garnett effective medium theory, not considering its thickness [37]. The value of the total film thickness δtotal obtained from model (174 nm) is very close to the one obtained by surface profilometry measurements (180 ± 5 nm), confirming the validity of

The measured T-R curves of the glass substrate and of TiO2/glass and SiO2/glass monolayers are represented in Fig. 1. Both TiO2 and SiO2 layers are transparent in the Vis-NIR domain (350 to 1500 nm), presenting an insignificant absorption on the whole spectral range. Few interference fringes are observed on the T and R spectra of the TiO2 sample only. The wavelength dependency of their refractive indices “n” was obtained by a fitting procedure based on a discrepancy function DF (Eq. (1).) which describes the closeness between experimental and model derived T and R spectral characteristics, as defined by Amotchkina and 277

Materials & Design 130 (2017) 275–284

I. Pana et al.

Fig. 2. Refractive index n and extinction coefficient k vs. wavelength for TiO2 monolayer.

the model results. A DF(TiO2) value of 0.97 was obtained, which is smaller than the accepted maximum value of 2 [38]. Fig. 2 depicts the nTiO2(λ) and kTiO2(λ) dispersion curves, which are close to the typical values reported in the literature [39]. The extinction coefficient has values smaller than 10− 3 on the whole spectral range indicating an almost non-absorbing material. The lack of interference fringes on the transmission curve of SiO2 monolayer and the very small difference between its optical properties and those of glass substrate (Fig. 1b), makes it difficult to extract relevant nSiO2(λ) data from T-R measurements of SiO2/glass sample. To overcome this inconvenient, we created a simplified multilayer design, called MD1 hereafter, consisting of a 50 nm SiO2 layer surrounded by two adjacent TiO2 layers of 59 nm each. The optical parameters of the SiO2 layer were deduced from the measured T and R curves of MD1 multilayer by a reverse engineering procedure [33], using as initial input data the optical constants of TiO2 previously determined (Fig. 2) and of SiO2 from literature [40]. The DF(SiO2) value for the optimized nSiO2(λ) optical constant was < 1.23. The thicknesses derived from the model were 55.5 nm and 61.9 nm for bottom and top TiO2 layers respectively, and 52.9 nm for SiO2 layer, at < 10% difference from those derived experimentally, for each individual layer. The obtained nSiO2(λ) values of the deposited SiO2 are presented in Fig. 3, along with reference data for SiO2 refractive index dispersion curve of bulk fused silica [40], used as input data in optimization process, and of a 580 nm a-SiO2 monolayer deposited by magnetron sputtering [41] (blue curve). In our particular case, the nSiO2(λ) seems to have values close to bulk fused silica, which are lower than those of the a-SiO2 monolayer reference for λ > 600 nm, and higher values than both references at lower wavelengths. It has been shown that smaller values of refractive index may be resulting from the method we used to derive them, i.e. n(λ) of individual layers deduced from a multilayer system by reversed engineering [42]. That could explain the difference between our nSiO2(λ) and the one from ref. [41] for λ > 600 nm. Larger values of nSiO2 are reported also in the literature, being ascribed to several causes such as refractive indices inhomogeneity, interface roughness's or thickness wedges [43]. As presented in the following, the deposited SiO2 films have a low surface and interface roughness, such as the slightly larger values of our data for λ < 600 nm can be accounted for by the waviness of the interfaces observed on SEM image if the multilayer (Fig. 6), which might mimic a faulty interface roughness or even inhomogeneity of the refractive index. Also, the possible intermixing of SiO2/TiO2 layers at their interfaces, as resulted from XRR measurements discussed in the following, may also account for the observed increase of the refractive index at shorter wavelengths. As for the extinction coefficient k of SiO2 monolayer the estimated values are as low as 10− 3. Therefore the obtained SiO2 was considered

Fig. 3. Refractive index of SiO2 monolayer resulted after optical characterization of the multilayer MD1 (red curve), compared with the ones of fused silica (black curve with data from [40]) and a-SiO2 580 nm SiO2 monolayer (blue curve with data from [41]). Thickness values of MD1 multilayer as derived from: i) design; ii) measured by profilometry; iii) reverse engineering fitting by using the measured optical characteristics. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

to be a non-absorbing material [28,44,45] and its extinction coefficient was further neglected in the optical design of multilayers and. 3.2. Design and post-production characterization of SiO2/TiO2 multilayer The basic requirements for colored solar multi-layered coating, as defined by Boudaden and co-workers in [46] are: near-zero absorption for the individual monolayers, significant colored reflection and an acceptable high solar transmittance (Tsol > 85%) on the spectral interval of interest. As presented above, a low absorption was already attained for both SiO2 and TiO2 monolayers Figs. 1 and 2 [28,45]. The design of the colored multilayer structure MD2 was performed using as input parameters the previously obtained nSiO2(λ) and nTiO2(λ) data, targeting simultaneous attainment of a high transmission in the Vis-NIR range and a pronounced green-yellow color corresponding to a reflection peak centered at 575 nm, while aiming for an economical design, with a low number N of monolayers. It is common knowledge that for each monolayer a certain thickness threshold is required, in order to obtain a continuous film [47]. Consequently, apart from the input parameters and the described target, two supplementary criteria were imposed: N ≤ 10, for an economical design, and a minimum thickness for individual layers δmonolayer > 10 nm. These two “physical” criteria were considered in the design in order to obtain a multilayer with a manageable architecture from the experimental point of view. After the optimization procedures, a design with N = 7 and δtotal = 500 nm was selected, among other designs with similar performance. Note that more complex designs were also evaluated, but they presented only small improvements of reflectance in the 560–590 nm spectral range. Fig. 4 is showing the successive arrangement in the multilayer MD2, the layer (1) being on the substrate, along with the thickness of each monolayer. For each monolayer, the four bars represent the thickness values, as follows: as designed, as measured by SEM, estimated from XRR measurements, and estimated by reverse engineering procedure, using as input the measured T and R curves of MD2. All three thickness values present a close match, not exceeding variations larger than 5% of total thickness. A likely source of differences may be related to production errors, mainly due to small fluctuations of deposition rates. The measured and fitted XRR curves as well as the values of the surface and interfaces roughness and of the mass density of the 278

Materials & Design 130 (2017) 275–284

I. Pana et al.

Fig. 6. Cross-sectional SEM image of the multilayer deposited on Si substrate, indicating the layers' and multilayer thickness values.

Fig. 4. Design of MD2 multilayer - monolayers arrangement in the multilayer and monolayers' thicknesses.

static charging of the dielectric multilayer. The cross sectional SEM image of the multilayer in Fig. 6 shows the successive layers, with smooth interfaces, the substrate being on the bottom of the image. The markers indicate the thickness values of the layers, as well as the total thickness of the multilayer. The image evidences also the interface between the 6-th and the 5-th monolayer, as well as the thin SiO2 layers. The measured thicknesses of the individual layers are well correlated with the designed values, and with those resulted from XRR fitting and reverse engineering, as shown in Fig. 4.

3.3. Optical properties of “physical” multilayer The typical T-R curves measured for the deposited MD2 multilayer (black line), in comparison with the ones resulted from modeling (red line) and from the reverse engineering process (blue dotted line) are presented in Fig. 7. There is a quite good agreement between the designed and Fig. 5. The measured (black line) and fitted (red line) XRR curves for MD2 multilayer. The inset shows both interface roughness and mass density values of each monolayer resulted after the XRR fitting procedure (full symbols - SiO2; hollow symbols - TiO2). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

individual layers resulted from XRR analysis are shown in Fig. 5. The XRR fitting delivered a goodness of fit value of χ2 = 0.00217. There were imposed constraints forcing the same mass density values for all TiO2 and for all SiO2 monolayers, resulting density values of about 4.14 g/cm3 and 2.20 g/cm3, respectively. These values are consistent with other reported data for TiO2 and SiO2 obtained by sputter deposition [48,49]. The surface roughness was estimated to about 0.8 nm, which is consistent with the one deduced from AFM measurements of about 0.2 nm (Fig. 9). Usually higher surface roughness values result from XRR fitting in comparison to those measured by AFM [50]. The interface roughness values are slightly higher, in the nanometer range, suggesting the presence of inter-mixing effects at the SiO2/TiO2 interfaces [51]. The concordance between the designed structure and the multilayer deposited on Si substrate was investigated by SEM imaging. As the multilayer was deposited in the same run on Si and glass substrates, we considered that the two layers are identical. The (100) cut Si wafer was used as substrate for the cross sectional images as it produces better fracture patterns than the amorphous thicker glass substrate. The fractured sample was coated with a 2 nm Pt thin film by magnetron sputtering prior to SEM image acquisition, in order to avoid electro-

Fig. 7. Comparison between the modeled, experimental, and reverse engineered R and T curves for MD2 multilayer. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)

279

Materials & Design 130 (2017) 275–284

I. Pana et al.

Fig. 8. CIE coordinates of the reflected color for the designed and fabricated MD2 multilayer, at near normal incidence [53]. The inset shows the angular dependency (0° to 60°) of color coordinates for the designed multilayer. The meaning of the uppercase letters: Red, Blue, Green, Yellow, Orange, Purple, Pink; the lowercase letters label the colors depicted by using the “-ish” suffix.

Fig. 9. Theoretical and experimental transmission spectra of MD2 multilayer obtained at different angles of incidence.

values for 60° angle of incidence were 73.3% for the deposited multilayer and 80.61% for the designed one. Even if Tsol value obtained at normal incidence is slightly lower than the 85% value reported by Boudaden [9], it is worthy for the aimed applications, due to its color characteristics. An alternative approach for energy-effectiveness evaluation is the use of the figure of merit, M [6]:

measured spectral characteristics in the domain of interest, the overall differences between the curves being around 10% or even smaller (< 4% in the visible range). The agreement between reverse engineering T-R curves and the experimental ones is even better, the curves being almost identical in the visible range. Note that the reverse engineering curves represent a fit of the experimental data, fit that generated the T-R curves represented in Fig. 7 and the individual thicknesses presented in Fig. 4. The transmission in NIR spectral interval of MD2 design is reaching 90%, and exceeding 87% for the deposited multilayer. A maximum reflectivity of 58% at 574 nm was designed, and about 54% was obtained, with a wavelength shift lower than 10 nm. Using the CIE color coordinates, for angles close to normal incidence the reflected color is green-yellow, as shown in Fig. 8 [52,53]. For the aimed architectural purposes, the angular dependency of reflected color is important, because the general appearance should remain almost unchanged. Consequently, we carried out a theoretical analysis to evaluate the color changes in a quite wide angle, from 0° (normal incidence) to 60°. The corresponding color coordinates are presented in the inset of Fig. 8, indicating that the reflected color remains in the yellow-green interval for observation angles up to 45°. At 60° angle the color shifts into the yellowish-green interval, keeping the same general appearance, except for an increased yellowish-green component. The angular dependency evaluation of the transmission curves was also performed, both experimentally and theoretically. The curves, shown in Fig. 9, exhibit the shift of the minimum towards lower wavelengths, indicating a small loss in amplitude of only few percent in the 0–45° range. The highest lost in transmission was obtained for 60°, but the transmission still remains around 80% outside the interval defining the reflected color. A quantitative way to describe the efficiency of the MD2 multilayer as a colored glazing for solar thermal collectors is the evaluation of the integrated solar transmittance in the range of 350–1500 nm, by using the following formula [6]:

M = Rvis Rsol

where Rvis and Rsol are the specific visible and solar reflectance of the multilayer. M quantifies the energy efficiency of the coloration, introducing the concept of “brightness per energy cost”. In fact, when dealing with designs that should have to carry out a double function, a certain compromise should be considered. For Vis reflectance around 10–20%, the solar transmittance is typically higher than 90%, however no distinct/prominent reflected color is observed [2]. Our design was conceived to ensure a much higher visible reflectance, the resulting Rvis being 45.39%, corresponding to a merit factor of 2.23, respectively. This value is comparable to other theoretical designs reported in the literature (Rvis ~40% and M = 2.6) [2], identifying this design as strongly colored. It qualifies mainly for use on large area collecting surfaces integrated into façades or roofs, where the color component is important, and the large area can compensate well for the actual reflection losses [6]. 3.4. Mechanical characterization of MD2 multilayer structure Besides the optical properties, that define the functionality of the multilayer structure, the coating should also exhibit reliable mechanical properties, ensuring a prolonged lifetime. Nanoindentation tests are usually conducted on films [28,29] deposited on mechanically fragile substrates, such as glass. Due to the small indentation depths encountered when performing nanoindentation, typically tens to hundreds of nanometers, it is necessary to take into account the surface roughness of the sample [54]. Hence, the surface morphology of the multilayer was evaluated by atomic force microscopy (AFM), a (1 μm × 1 μm) image being presented in Fig. 10. The RMS roughness is of about 0.2 nm, indicating the fine smoothness of the film surface. Moreover, the highest peak on the investigated area was smaller than 0.7 nm, as indicated in the height scale in Fig. 10. It was then concluded that the surface roughness is small enough to be neglected when evaluating the nanoindentation results, knowing that the typical indentation depths will exceed 50 nm. For MD2 multilayer, typical plots of displacement curves vs. the

1500

Tsol =

∫350 T (λ) Isol (λ) dλ 1500

∫350 Isol (λ) dλ

(5)

(4)

where Isol is the intensity of the solar spectrum and T(λ) is the measured transmission spectral curve of the multilayer. The resulted Tsol values for normal incidence were 80.3% and 81.7%, for the fabricated and designed multilayer, respectively. The 280

Materials & Design 130 (2017) 275–284

I. Pana et al.

where Hs and Es are representing the hardness and the reduced modulus of the substrate, hmax is the maximum penetration depth obtained after nanoindentation and δ is total thickness of the film. H and Er are the hardness and reduced modulus evaluated form the registered forcedisplacement curves using the Oliver-Pharr formalism, without taking into account the substrate effects. The validity of the model requires the use of suitable indentation forces making possible to evaluate H and Er values correctly by using the Oliver-Pharr formalism [34]. In fact, the penetration depths should be situated between the minimum requirements for the Berkovich tip (40 nm in this case) and up to 90% of film thickness. A large interval of penetration depths is necessary to obtain an accurate fit of the input data in the model. However, the borders of the interval should remain in the validity limits of the measuring technique and the nanoindentation system used. Consequently, we used in the model the data obtained from indentations between 1 and 11 mN load force. This corresponds to maximum penetration depth of 310 nm, which represent 60% of total film thickness. Tests were also performed for higher loads, up to 13 mN, but the acquired curves were not corresponding to the selection criteria's for valid indentations. Only the indentation curves exhibiting the same typical shape as those shown in Fig. 11 were selected for fitting. Additionally, there were selected only the curves which exhibited identical loading parts on the shared common intervals. The resulting H and E values for indentation are represented in Fig. 12, along with the fitted profile resulted from the model. As shown in Fig. 11, the hardness (Hf) and the reduced Young modulus (Ef) of MD2 multilayer were found to be 7.18 GPa and 88.3 GPa, respectively. The observed decrease of the H and Er as the penetration depth increase is to be expected since the values corresponding to the multilayer are higher than the ones of the glass substrate (Fig. 12). This corresponds to a mean H/Er value of 0.081, well in the range of the value 0.077 reported in Ref. [28] for multilayer structure composed of same materials, but well superior to the values reported in Ref. [55]. As currently acknowledged, hardness may stand as a rough measure of strength, whereas the material's tendency to elastically deform is described by the elastic modulus. Therefore, the H/Er ratio might be considered as an indicator of the amount of strain a coating can experience before a permanent deformation arises, a higher H/Er ratio indicating a better wear resistance. Scratch resistance tests were also performed using the same Berkovich tip, by increasing the indentation force from 0 to 13 mN (maximum value reachable with the instrument). The surface was scanned by using the same Berkovich tip in order to obtain twodimensional images of the imprinted scratch trace, and the depth

Fig. 10. AFM image of MD2 multilayer deposited on glass (RMS roughness = 0.2 nm).

Fig. 11. Typical force – displacement curves of MD2 multilayer. The inset shows a scan of an imprint obtained on MD2 surface for 1.5 mN indentation force.

applied force are shown in Fig. 11, corresponding to values ranging from 0.5 mN up to 3 mN. A surface Scanning Probe Microscopy image obtained using the indentation tip after a typical indentation measurement at 1.5 mN is also represented as inset in Fig. 11, the size of the image being identical with the AFM scan size previously presented, 1 μm2 respectively. As for the shape of the loading-unloading curves, the observed hysteresis effect was ascribed to a typical plastic-elastic behavior of the material, as the deformed region undergoes only partial elastic recovery. Therefore, the final penetration depths, at the end of the indentation process, will be smaller than the maximum depths, achieved at maximum load, as will be further discussed in this section. For 0.5 mN indentation force the contact depth is smaller than 40 nm, the results on H and Er being potentially affected by the poor definition of the tip area function. For 1 mN force on the other hand, the penetration depth is about 10% from total film thickness (i.e. 50 nm), and the measurement can be affected by the substrate mechanical properties [30]. Moreover, in a multilayer structure, the arrangement of the individual layers and their thicknesses affect the overall performance in terms of hardness – H and reduced Young modulus – Er [28]. The evaluation of H and Er for thin films can be challenging, especially if the thickness of the film is only a few hundred nanometers. In order to eliminate the substrate effect we used the model proposed by Martyniuk et al. [30] and used by Mazur [29] for similar multilayer structure (i.e. TiO2/SiO2). The model allows for the estimation of the Hf and Ef using dependencies of the type:

Hf = H (Hs , h max , δ )

(6)

Ef = E (Es , h max , δ )

(7)

Fig. 12. Plots of hardness (H) and reduced modulus (E) versus maximum penetration depth normalized to the MD2 multilayer thickness (hmax/δ).

281

Materials & Design 130 (2017) 275–284

I. Pana et al.

Fig. 14. Mechanical behavior curves corresponding to scratch test presented in Fig. 13.

applied force, asymptotically reaching an equilibrium value of approx. 0.3. 3.5. Thermal stability of TiO2/SiO2 colored multilayer Because the main application envisaged for the MD2 multilayer is related to its use for architectural solar panels, one must consider the temperature increase due to the incident solar radiation. For this reason, one of the important features of such a multilayer is the preservation of its optical characteristics at temperatures higher than the environmental ones. The monolayers and MD2 transmission and reflection spectra were measured as follows: a) as-deposited, b) after 35 days of air exposure at room temperature, c) after the thermal treatment. The purpose of these experiments was to evaluate: (i) the stability in time of the optical properties (by comparing (a) and (b) results), and (ii) the thermal stability of the layers (by comparing (b) and (c) results). The results are presented in Fig. 15. A good stability in time of the multilayer sample can be observed after its storage in open atmosphere at room temperature. As for the thermal stability, TiO2 and SiO2 monolayer samples proved to be resistant to the heat treatment, only small changes, < 1% on T and R data, were evidenced (not shown here). The thermal treatment effects on the transmission and reflection spectra of the colored multilayer sample are in the same range of ~1%, while only a very small shift (< 1 nm) towards smaller wavelengths is visible

Fig. 13. Typical nanoscratch trace obtained by increasing force from 0 to 10 mN (a), and the corresponding sections into the trace (b).

profile. In all the experiments no peeling of the film was observed, showing its good adhesion to the substrate. Nevertheless, significant pile-up at the end of the scratch test was observed for large force values (10 to 13 mN), making it difficult to obtain good quality images by scanning probe microscopy imaging. A typical scratch trace obtained by using an increasing normal force from 0 to 10 mN is represented in Fig. 13a, along with a few sections of the trace corresponding to different forces, shown in Fig. 13b. From the shape of the trace, with uniform shape borders, it can be seen that no peeling effect appeared. The lateral pile-up appears to be asymmetric, mainly due to the shape of the indentation tip and its orientation with respect to scratch direction. Indeed, when looking at the shape of the indentation (Figs. 11 and 13a) one can see that the scratch direction (vertical downwards) is situated between face forward and edge forward orientations, such as a higher pile-up is formed on the side corresponding to the edge. For low indentation forces as 1.5 mN, the lateral pile up is very low, being barely visible on the traces (Fig. 13b). On the contrary, for high indentation forces as 10 mN, an important pile-up is formed at the end of the scratch line, so that the section is not relevant and was not shown in Fig. 13b. The pile up of the coating material becomes important for forces higher than 6 mN, being visible both as lateral pile-up and as added material to the bottom of the trace, determining the decrease of the measured values of the final depths. The penetration depths illustrated in Fig. 13b represent the final values, registered after the loading force was released. In order to have a complete picture of the thin film behavior under typical scratch conditions one needs to analyze also the behavior during the test. In Fig. 13 the evolutions of the normal displacement and the friction coefficient during the scratch test are plotted vs. the applied force. On the same figure the distribution of layer thicknesses is also presented, to illustrate the indenter penetration in different layers of the structure. As seen in Fig. 14, the maximum penetration is around 325 nm, going as deep as the fifth layer from surface, as resulting from Fig. 4. No sudden variations are observed when passing from one layer to another, showing a good interface quality and lack of any failure mechanism. This behavior is related to the fact that the mechanical characteristics of both materials are quite close. The friction coefficient increases with the

Fig. 15. Visible spectral reflection and transmission spectra of MD2 multilayer: a) as deposited (black curves), b) 35 days post deposition (red curves), c) 35 days post deposition and thermal treatment (green curves). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

282

Materials & Design 130 (2017) 275–284

I. Pana et al.

facades, Sol. Energy 135 (2016) 408–422 doi.org/10.1016/j.solener.2016.06.006. [4] G. Leone, M. Beccali, Use of finite element models for estimating thermal performance of facade-integrated solar thermal collectors, Appl. Energy 171 (2016) 392–404 doi.org/10.1016/j.apenergy.2016.03.039. [5] D.C. Zhu, S.X. Zhao, Chromaticity and optical properties of colored and black solarthermal absorbing coatings, Sol. Energy Mater. Sol. Cells 94 (10) (2010) 1630–1635 doi.org/10.1016/j.solmat.2010.05.019. [6] J. Boudaden, R.S.C. Ho, P. Oelhafen, A. Schuler, C. Roecker, J.L. Scartezzini, Towards coloured glazed thermal solar collectors, Sol. Energy Mater. Sol. Cells 84 (1–4) (2004) 225–239 doi.org/10.1016/j.solmat.2004.02.042. [7] S. Kinoshita, Structural Colors in the Realm of Nature, World Scientific, 2008. [8] Z. Knittl, Optics of Thin Films: An Optical Multilayer Theory, Wiley, 1976. [9] J. Boudaden, P. Oelhafen, A. Schuler, C. Roecker, J.L. Scartezzini, Multilayered Al2O3/SiO2 and TiO2/SiO2 coatings for glazed colored solar thermal collectors, Sol. Energy Mater. Sol. Cells 89 (2–3) (2005) 209–218 doi.org/10.1016/j.solmat.2005. 01.015. [10] C.G. Alvarado-Beltran, J.L. Almaral-Sanchez, R. Ramirez-Bon, Low temperature processing of multilayer dielectrics mirrors by sol-gel method, Mater. Lett. 161 (2015) 523–526 doi.org/10.1016/j.matlet.2015.09.020. [11] G.K. Dalapati, S. Masudy-Panah, S.T. Chua, M. Sharma, T.I. Wong, H.R. Tan, D. Chi, Color tunable low cost transparent heat reflector using copper and titanium oxide for energy saving application, Sci. Rep. 6 (2016) 1–14, http://dx.doi.org/10.1038/ srep20182. [12] M.F. Al-Kuhaili, S.H.A. Ahmad, S.M.A. Durrani, M.M. Faiz, A. Ul-Hamid, Energysaving spectrally-selective coatings based on MoO3/Ag thin films, Mater. Des. 73 (2015) 15–19 doi.org/10.1016/j.matdes.2015.02.025. [13] P. Eiamchai, M. Horprathum, V. Patthanasettakul, P. Limnonthakul, N. Nuntawong, P. Limsuwan, P. Chindaudom, Designs and investigations of anti-glare blue-tint side-view car mirrors, Mater. Des. 31 (7) (2010) 3151–3158 doi.org/10.1016/j. matdes.2010.02.033. [14] D.S. Hinczewski, M. Hinczewski, F.Z. Tepehan, G.G. Tepehan, Optical filters from SiO(2) and TiO(2) multi-layers using sol-gel spin coating method, Sol. Energy Mater. Sol. Cells 87 (1–4) (2005) 181–196 doi.org/10.1016/j.solmat.2004.07.022. [15] J.P. Dai, W. Gao, B. Liu, X.L. Cao, T. Tao, Z.L. Xie, H. Zhao, D.J. Chen, H. Ping, R. Zhang, Design and fabrication of UV band-pass filters based on SiO2/Si3N4 dielectric distributed bragg reflectors, Appl. Surf. Sci. 364 (2016) 886–891 doi.org/ 10.1016/j.apsusc.2015.12.222. [16] H. Wargnier, F.X. Kromm, M. Danis, Y. Brechet, Proposal for a multi-material design procedure, Mater. Des. 56 (2014) 44–49 doi.org/10.1016/j.matdes.2013.11.004. [17] R. Zhao, G. Neighbour, P. Deutz, M. McGuire, Materials selection for cleaner production: An environmental evaluation approach, Mater. Des. 37 (2012) 429–434 doi.org/10.1016/j.matdes.2012.01.014. [18] G. Brauer, Large area glass coating, Surf. Coat. Technol. 112 (1–3) (1999) 358–365 doi.org/10.1016/S0257-8972(98)00737-3. [19] P.R. Sagdeo, D.D. Shinde, J.S. Misal, N.M. Kamble, R.B. Tokas, A. Biswas, A.K. Poswal, S. Thakur, D. Bhattacharyya, N.K. Sahoo, S.C. Sabharwal, Deposition and characterization of titania-silica optical multilayers by asymmetric bipolar pulsed dc sputtering of oxide targets, J. Phys. D. Appl. Phys. 43 (4) (2010) 1–11, http://dx.doi.org/10.1088/0022-3727/43/4/045302. [20] M.M. Hasan, A. Haseeb, R. Saidur, H.H. Masjuki, M. Hamdi, Influence of substrate and annealing temperatures on optical properties of RF-sputtered TiO2 thin films, Opt. Mater. 32 (6) (2010) 690–695 https://doi.org/10.1016/j.optmat.2009.07.011. [21] N. Martin, C. Rousselot, D. Rondot, F. Palmino, R. Mercier, Microstructure modification of amorphous titanium oxide thin films during annealing treatment, Thin Solid Films 300 (1–2) (1997) 113–121 doi.org/10.1016/S0040-6090(96) 09510-7. [22] M. Turowski, T. Amotchkina, H. Ehlers, M. Jupe, D. Ristau, Calculation of optical and electronic properties of modeled titanium dioxide films of different densities, Appl. Opt. 53 (4) (2014) A159–A168 doi.org/10.1364/AO.53.00A159. [23] S. Mertin, V. Hody-Le Caer, M. Joly, I. Mack, P. Oelhafen, J.L. Scartezzini, A. Schur, Reactively sputtered coatings on architectural glazing for coloured active solar thermal facades, Energy Build. 68 (2014) 764–770. [24] M. Nocun, S. Kwasny, J. Zontek, Optical properties of SiO2/TiO2 thin layers prepared by sol-gel method, Opt. Appl. 41 (4) (2011) 979–987 doi.org/10.1016/j. enbuild.2012.12.030. [25] M. Kitui, M.M. Mwamburi, F. Gaitho, C.M. Maghanga, Optical properties of TiO2 based multilayer thin films: application to optical filters, Int. J. Thin Films Sci. Technol. (2015) 17–21. [26] D.J. Poxson, F.W. Mont, M.F. Schubert, J.K. Kim, J. Cho, E.F. Schubert, Demonstration of optical interference filters utilizing tunable refractive index layers, Opt. Express 18 (23) (2010) A594–A599 doi.org/10.1364/OE.18.00A594. [27] W. Graf, F. Brucker, M. Kohl, T. Troscher, V. Wittwer, L. Herlitze, Development of large area sputtered solar absorber coatings, J. Non-Cryst. Solids 218 (1997) 380–387 https://doi.org/10.1016/S0022-3093(97)00283-4. [28] M. Mazur, D. Wojcieszak, J. Domaradzki, D. Kaczmarek, S. Song, F. Placido, TiO2/ SiO2 multilayer as an antireflective and protective coating deposited by microwave assisted magnetron sputtering, Opto-Electron. Rev. 21 (2) (2013) 233–238, http:// dx.doi.org/10.2478/s11772-013-0085-7. [29] M. Mazur, D. Wojcieszak, D. Kaczmarek, J. Domaradzki, S. Song, D. Gibson, F. Placido, P. Mazur, M. Kalisz, A. Poniedzialek, Functional photocatalytically active and scratch resistant antireflective coating based on TiO2 and SiO2, Appl. Surf. Sci. 380 (2016) 165–171 doi.org/10.1016/j.apsusc.2016.01.226. [30] M. Martyniuk, J. Antoszewski, B.A. Walmsley, C.A. Musca, J.M. Dell, Y.G. Jung, B.R. Lawn, H. Huang, L. Faraone, Determination of mechanical properties of silicon nitride thin films using nanoindentation, Conference on Spaceborne Sensors II, Orlando, FL, 2005, pp. 216–225.

for the main reflectance peak (Fig. 15). Also, the width of the main reflection feature remains constant, as well as the color characteristic of the multilayer. The multilayer structure is therefore proven to be stable at temperatures as high as 200 °C, maintaining all its major optical properties. To quantify the minor temperature effects, the area of the main reflection and transmission peak centered at 574 nm was calculated, resulting almost identical values for the as deposited film and the one stored for 35 days. After the thermal treatment, the reflection peak area changed by < 1% as compared to the one obtained before thermal treatment. The very small changes of the optical properties of the multilayer after the thermal treatment are ascribed to the fact that at temperatures below 300 °C no crystallization is expected for the oxide layers. In fact, the XRD patterns obtained in Bragg-Brentano geometry (not shown here) indicated no structural changes after the thermal treatment, the samples maintaining the amorphous character. 4. Conclusions A combined theoretical and experimental approach was employed to produce a yellow-green colored glazing TiO2/SiO2 multilayer for thermal solar collectors. An optimized design consisting of 7 layers with a total thickness of 500 nm was designed by OptiLayer software and subsequently fabricated by RF magnetron sputtering method. The concordance between the deposited multilayer structure and the design was independently assessed by SEM, XRR, surface profilometry and optical reverse engineering. The experimentally measured reflectivity curve of the multilayer coating exhibited a prominent peak (~ 54%) at about 574 nm (corresponding to a green-yellow color), a visible reflectance of ~45.39% and a solar transmittance of about 80%. Angle dependency analyses proved that the reflected color remains within the desired interval (green-yellow), whilst the total transmittance is stable, except for a ~10% decrease for high angles around 60o. The multilayer has smooth surface and interface roughness values, as evidenced by AFM, SEM and XRR measurements. The mechanical testing by nanoindentation and nanoscratch showed moderate hardness and elastic modulus values (7.18 and 88.31 GPa respectively) and good scratch resistance, for forces up to 13 mN. A mean value of the hardness on reduced modulus ratio of 0.081 indicates a good wear resistance of the colored glazing. Moreover, the thermal stability of the multilayer was investigated, proving its utility for solar thermal façades. The spectral transmission and reflection measurements, performed before and after the thermal treatment, indicated no significant modifications of the optical properties. The yellow-green color, the good mechanical properties and the thermal stability of the multilayer recommend it as a potential solution for color glazing in front of thermal solar collectors, enabling their architectural integration on building façades. Acknowledgements This work was funded by the Romanian Ministry of Research and Innovation, through projects PN 16.40.01.01 and PN 16.40.01.02. The nanoindentation and nanoscratch experiments were carried out by using the equipment acquired by the infrastructure project INOVAOPTIMA SMIS code 49164, contract no. 658/2014. References [1] M.M. Probst, C. Roecker, Towards an improved architectural quality of building integrated solar thermal systems (BIST), Sol. Energy 81 (9) (2007) 1104–1116 doi. org/10.1016/j.solener.2007.02.009. [2] A. Schuler, C. Roecker, J. Boudaden, P. Oelhafen, J.L. Scartezzini, Potential of quarterwave interference stacks for colored thermal solar collectors, Sol. Energy 79 (2) (2005) 122–130 doi.org/10.1016/j.solener.2004.12.008. [3] R. O'Hegarty, O. Kinnane, S.J. McCormack, Review and analysis of solar thermal

283

Materials & Design 130 (2017) 275–284

I. Pana et al.

[44] T.V. Amotchkina, M.K. Trubetskov, V. Pervak, B. Romanov, A.V. Tikhonravov, On the reliability of reverse engineering results, Appl. Opt. 51 (22) (2012) 5543–5551 doi.org/10.1364/AO.51.005543. [45] C. Battaglin, F. Caccavale, A. Menelle, M. Montecchi, E. Nichelatti, F. Nicoletti, P. Polato, Characterisation of antireflective TiO2//SiO2 coatings by complementary techniques, Thin Solid Films 351 (1–2) (1999) 176–179 doi.org/10.1016/S00406090(99)00212-6. [46] A. Schuler, J. Boudaden, P. Oelhafen, E. De Chambrier, C. Roecker, J.L. Scartezzini, Thin film multilayer design types for colored glazed thermal solar collectors, Sol. Energy Mater. Sol. Cells 89 (2–3) (2005) 219–231, http://dx.doi.org/10.1016/j. solmat.2004.11.015. [47] I. Pana, C. Vitelaru, N.C. Zoita, M. Braic, Tunable optical properties of SiNx thin films by OES monitoring in a reactive RF magnetron plasma, Plasma Process. Polym. 13 (2) (2016) 208–216, http://dx.doi.org/10.1002/ppap.201400202. [48] B.Q. Li, T. Fujimoto, I. Kojima, Structural characterization of radiofrequency magnetron sputter deposited SiO2 thin films, J. Phys. D. App. Phys. 32 (12) (1999) 1287–1292 https://doi.org/10.1088/0022-3727/32/12/302. [49] V. Sittinger, A. Pflug, W. Werner, C. Rickers, M. Vergohl, A. Kaiser, B. Szyszka, Production of MF and DC-pulse sputtered anti-reflective/anti-static optical interference coatings using a large area in-line coater, Thin Solid Films 502 (1–2) (2006) 175–180 doi.org/10.1016/j.tsf.2005.07.270. [50] Y. Fujii, Improvement of X-ray reflectivity analysis on surface and interface roughness estimation, Am. J. Phys. Appl. 3 (2) (2015) 21–24, http://dx.doi.org/10. 11648/j.ajpa.20150302.12. [51] E. Sutter, P. Sutter, J.J. Moore, Microstructure of sputter deposited TiO2/SiO2 multilayer optical coatings, MRS Proc. 654 (2000), doi.org/10.1557/PROC-654AA3.38.1. [52] W.D. Wright, A re-determination of the trichromatic coefficients of the spectral colours, Trans. Opt. Soc. 30 (4) (1929) 141. [53] S. Gage, Optoelectronics: Fiber-Optics Applications Manual, second ed., McGrawHill Book Company, New York, 1981. [54] G. Cheng, X. Sun, Y.X. Wang, S.L. Tay, W. Gao, Nanoindentation study of electrodeposited Ag thin coating: An inverse calculation of anisotropic elasticplastic properties, Surf. Coat. Technol. 310 (2017) 43–50 doi.org/10.1016/j. surfcoat.2016.12.056. [55] J.J. Roa, V. Rico, M. Oliva-Ramirez, A.R. Gonzalez-Elipe, E. Jimenez-Pique, Nanoindentation and scratch resistance of multilayered TiO2-SiO2 coatings with different nanocolumnar structures deposited by PV-OAD, J. Phys. D. App. Phys. 49 (13) (2016) 7, http://dx.doi.org/10.1088/0022-3727/49/13/135104.

[31] H.N. Shah, R. Jayaganthan, A.C. Pandey, Nanoindentation study of magnetronsputtered CrN and CrSiN coatings, Mater. Des. 32 (5) (2011) 2628–2634 doi.org/ 10.1016/j.matdes.2011.01.031. [32] J.N. Liu, B.S. Xu, H.D. Wang, X.F. Cui, L.N. Zhu, G. Jin, Measurement for mechanical behavior and fatigue property of Cu films by nanoscale dynamic load method, Mater. Des. 65 (2015) 1136–1142 doi.org/10.1016/j.matdes.2014.08.043. [33] A.V. Tikhonravov, M.K. Trubetskov, OptiLayer Software v. 11.65e. http://www. optilayer.com, (2016). [34] W.C. Oliver, G.M. Pharr, An improved technique for determining hardness and elastic-modulus using load and displacement sensing indentation experiments, J. Mater. Res. 7 (6) (1992) 1564–1583 doi.org/10.1557/JMR.1992.1564. [35] T.V. Amotchkina, V. Janicki, J. Sancho-Parramon, A.V. Tikhonravov, M.K. Trubetskov, H. Zorc, General approach to reliable characterization of thin metal films, Appl. Opt. 50 (10) (2011) 1453–1464 doi.org/10.1364/AO.50.001453. [36] W. Sellmeier, Zur Erklärung der abnormen Farbfolge im Spektrum einiger Substanzen, Annalen der Physik und Chemie 219 (1871) 272–282, http://dx.doi. org/10.1002/andp.18712190612. [37] G.A. Niklasson, C.G. Granqvist, O. Hunderi, Effective medium models for the optical properties of inhomogeneous materials, Appl. Opt. 20 (1) (1981) 26–30 doi.org/10. 1364/AO.20.000026. [38] A. Macleod, Experimental Determination of Thin Film Optical Constants, Optical Coatings: Material Aspects in Theory and Practice, Springer-Verlag, Berlin, Berlin, 2014, pp. 117–158. [39] S. Ben Amor, G. Baud, J.P. Besse, M. Jacquet, Elaboration and characterization of titania coatings, Thin Solid Films 293 (1) (1997) 163–169 doi.org/10.1016/S00406090(96)09106-7. [40] I.H. Malitson, Interspecimen Comparison of the Refractive Index of Fused Silica, J. Opt. Soc. Am. 55 (10) (1965) 1205–1209 doi.org/10.1364/JOSA.55.001205. [41] L.H. Gao, F. Lemarchand, M. Lequime, Exploitation of multiple incidences spectrometric measurements for thin film reverse engineering, Opt. Express 20 (14) (2012) 15734–15751, http://dx.doi.org/10.1364/OE.20.015734. [42] A.V. Tikhonravov, T.V. Amotchkina, M.K. Trubetskov, R.J. Francis, V. Janicki, J. Sancho-Parramon, H. Zorc, V. Pervak, Optical characterization and reverse engineering based on multiangle spectroscopy, Appl. Opt. 51 (2) (2012) 245–254 doi.org/10.1364/AO.51.000245. [43] E. Nichelatti, M. Montecchi, R.M. Montereali, Optical reflectance and transmittance of a multilayer coating affected by refractive-index inhomogeneity, interface roughness, and thickness wedge, J. Non-Cryst. Solids 355 (18–21) (2009) 1115–1118 doi.org/10.1016/j.jnoncrysol.2008.11.040.

284