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Thermochromic VO2-based smart radiator devices with ultralow refractive index cavities for increased performance
Solar Energy Materials & Solar Cells 205 (2020) 110260
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Thermochromic VO2-based smart radiator devices with ultralow refractive index cavities for increased performance R. Beaini *, B. Baloukas, S. Loquai, J.E. Klemberg-Sapieha, L. Martinu Department of Engineering Physics, Polytechnique Montr�eal, Montreal, QC, H3C 3A7, Canada
A R T I C L E I N F O
A B S T R A C T
Keywords: Thermochromic VO2 Thermal regulation Smart radiator devices Variable emissivity Ultralow refractive index
VO2 thermochromic coatings show great potential as the main component in smart radiator devices (SRD) for spacecraft, most notably for micro- and nano-satellites. Indeed, the inherent metal to insulator transition (MIT) of VO2 allows such a coating to act as a lightweight thermal regulator, eliminating the need for heavy and failureprone mechanical louvers. However, spacecraft industry standards require an emissivity variation Δε of at least 60%, a value which, to our knowledge, has yet to be demonstrated. To reach and surpass this value, we first apply a modeling approach to optimize the optical properties and thicknesses of the individual constituent films of a typical SRD with the following architecture: mirror | dielectric resonant cavity | VO2. This study then highlights various possible avenues to enhance the performance of the devices, one of these being the use of an infrared transparent ultralow refractive index dielectric material for the resonant cavity. This theoretical pre diction is then confirmed by the deposition of various prototype devices implementing a CaF2 cavity layer with n @ 10 μm ¼ 1.17 and with measured values of Δε in excess of 60% for the full 3 to 25μm wavelength range. In fact, we demonstrate a prototype device with a maximum Δε value of 66%, thus bringing this technology one step closer to implementation.
1. Introduction As the number of space missions keeps on growing, micro- and nanosatellites are becoming an appealing option for spacecraft companies around the globe [1]. Defined by a mass between 1 kg and 100 kg, such satellites present an important engineering challenge due to their smaller size. For instance, the operating temperature of a satellite must be kept between 0 � C and 40 � C [2]. Traditionally, the necessary thermal load management system was based on the use of mechanical louvers [3]. However, with the constant miniaturization of satellites, these louvers become a bigger part of the satellite, leading to an increase in launching costs as well as being prone to wear and mechanical failure [4]. Alternative approaches towards thermal regulation are therefore required, and as such, smart radiator devices (SRD) are amongst the most promising candidates. One of the simplest ways to design a SRD is through the use of a thermochromic material. Such materials exhibit a change in their optical properties when reaching their transition temperature TC . For example, VO2, with a TC close to room temperature (TC ¼ 68 � C), is an ideal candidate as it transitions from semi-conducting properties to metallic-
like properties when below and above its TC , respectively. It is also possible to lower its TC by doping it with a transition metal; e.g., 2% of W for a TC of 25 � C [5]. The drastic change in electrical properties results in an important variation of the reflectance in the infrared (IR) and thus, a variable and tunable emissivity, Δε. To meet the spacecraft industry standards for thermal regulation, this value should ideally surpass 60% while keeping the solar absorption below 20% [6]. At the moment, the most promising SRD configuration consists of a bottom mirror, followed by a dielectric resonant cavity, an active VO2 film, and a top protective/antireflective film. When properly adjusted, such a device presents a low absorption/high reflectance when below Tc, and a high absorption/low reflectance when above Tc. By appropriately adjusting the optical thickness of the resonant cavity, one can choose the wavelength at which the absorption reaches a maximum, and thus optimize the emissivity variation Δε [7]. As VO2 is thermally driven, this allows a SRD to self-regulate its temperature: at low temperatures, heat is trapped inside the spacecraft keeping it warm, and as the temperature rises, the SRD transitions into its absorbing state, allowing heat to be re-emitted into space, thus cooling down the spacecraft. Previous works on SRDs show limited studies on the impact of the
* Corresponding author. E-mail address: [email protected] (R. Beaini). https://doi.org/10.1016/j.solmat.2019.110260 Received 7 August 2019; Received in revised form 23 October 2019; Accepted 25 October 2019 Available online 31 October 2019 0927-0248/© 2019 Published by Elsevier B.V.
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layer thicknesses [8–10], and rarely follow an optimization approach. Table 1 shows examples of recently proposed SRD architectures and their corresponding performance. The architecture proposed by Hendaoui et al. [8] gives a good overall performance at high temperature as the selected SiO2 thickness acts as a resonant cavity generating a high absorption at about 7 μm. However, by choosing SiO2 as the cavity material, the performance at low tem perature is hindered, as silica has an absorption peak at around 9.25 μm (Si-O-Si stretching) [11], which is relatively close to the peak emissivity of a blackbody at 25 � C (~9.7 μm). Wang et al. proposed a very similar architecture in which the cavity layer was replaced by HfO2 [9], a ma terial which is transparent between 5 μm and 12 μm [12]. The properties at low temperature of their SRD are thus highly improved, and to our knowledge, this is currently the design showing the highest performance in the literature with a Δε value of 55%. However, their optimization approach was mainly empirical, and very little is said on the impact of each individual films’ optical properties and thicknesses. Finally, Sun et al. proposed an interesting metamaterial architecture [10], in which the VO2 and the top Al2O3 layers are patterned. This design allows one to increase the absorption at high temperature, due to the presence of a plasmonic resonance generated by the VO2 at high temperature, while also reducing the low temperature absorption due to the decrease in coverage of the VO2. In their most promising design, each feature measured 2.7 μm � 2.7 μm with a 0.5 μm gap between each square. Although they showed an increase in the absorption in the high tem perature state, the absorption at low temperature is above 30%, greatly reducing the overall performance. This is, once again, mainly due to the presence of SiO2 as the cavity material. In the present work, a model-based approach is first implemented to clearly assess the impact of each individual film with respect to the overall performance; the main goal being to maximize the emissivity variation between the low and high temperature states. Following this systematic approach allows for a better understanding of the SRD’s behaviour and thus serves as a guide for properly choosing the materials,
their thickness and their incorporation into the most appropriate ar chitecture. The implemented optical properties are obtained in house by spectroscopic ellipsometry ranging from the UV to the IR. The models clearly indicate that using an ultralow refractive index infrared trans parent material with an appropriate thickness as the resonant cavity can lead to higher than 60% Δε values. Prototype samples are then deposited using a Au | CaF2 | Si3N4 | VO2 architecture in which an ultralow refractive index CaF2 film is used as the resonant cavity. The resulting optical performance confirms the developed modeling approach, and devices with emissivity variations as high as 66% are demonstrated. Finally, further potential avenues of optimization are discussed. 2. Methodology 2.1. Optical modeling Optical modeling and design is an important part of the present work as it allows one to predict the optical response of a multilayer coating. It also allows for a better understanding of each layers’ role in the overall SRD performance, as well as hinting towards ideal material combina tions that should be implemented for optimal performance. Modeling was carried out using the open source software OpenFilters [13] as well as Matlab, both applying the well known matrix approach [14]. Calculations were performed for the 300 nm to 25 μm range, and the resulting reflection (R) and transmission (T) spectra were used to calculate the SRD’s absorption assuming AðλÞ ¼ 1 RðλÞ TðλÞ. Using Kirchoff’s law of thermal equilibrium, the absorption coeffi cient at a specific wavelength αðλÞ is considered as equal to the emis sivity at that same wavelength (αðλÞ ¼ εðλÞ) [15]. The emittance at low temperature (εL ) and at high temperature (εH ) is then found to be the ratio between the sample’s emission and a blackbody’s emission at the same temperature. The emission is evaluated in the wavelength interval [λ1 ; λ2 ], typically from 3 μm to 25 μm: R λ2 αðλ; TÞEb ðλ; TÞdλ ε ¼ λ1 R λ2 ; (1) Ebλ ðλ; TÞdλ λ1
Table 1 Recently published smart radiator architectures, their corresponding perfor mance and observations. Architecture
Performance
εL
εH
Δε
Au | SiO2 (850 nm) | VO2 (30 nm)
0.22
0.71
0.49
Ag | HfO2 (800 nm) | VO2 (50 nm)
0.13
0.68
0.55
(25 � C)
Si | Al2O3 (50 nm)| Al (80 nm) | SiO2 (1200 nm)| patterned VO2 (50 nm)| patterned Al2O3 (6 nm)
0.31
(100 � C)
0.79
0.48
Observations
Reference
The overall performance is hindered by the high value of εL due to the absorption band of SiO2 near 9 μm.
[8]
Implementing an IR-transparent material as the cavity decreases the absorption in the low temperature state. This metamaterial design allows to reach new heights for εH . The overall performance is, however, hindered by the high value of εL due to the absorption band of SiO2 near 9 μm.
where Eb is the emissivity of a blackbody at the desired temperature given by Planck’ law. By calculating the performance at both tempera tures, the total emissivity variation of the multilayer Δε can be obtained (Δε ¼ εH εL ). The solar absorption αs can be calculated using the following equa tion: R 2500 αðλÞ φs ðλÞdλ αs ¼ 300R 2500 (2) φs ðλÞdλ 300 where φs denotes the solar irradiance with an air mass of 1.5 (ASTMG173-03 - Global tilt).
[9]
2.2. Optical characterization The refractive index and extinction coefficient of each layer were obtained by variable-angle spectroscopic ellipsometry in the 0.3 to 25 μm range using a combination of two ellipsometers from J.A. Woollam Co.: the RC2-XI (300–2500 nm) and the IR-VASE (2–25 μm). Ψ and Δ ellipsometric parameters were obtained for four angles of incidence (45� , 55� , 65� , and 75� ) on backside roughened c-Si (100) substrates for visible characterization, and 2-side-polished undoped-Si for IR charac terization. The VO2 films’ optical properties were measured at 25 � C (low temperature state) and 100 � C (high temperature state) using heat cells installed on both ellipsometers. The results were analyzed and modeled in the CompleteEase and WVASE software also from J.A. Woollam Co. Fig. 1 shows the optical properties of VO2 in its low and high temperature states, which are in accordance with the work of Wan et al. [16].
[10]
2
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glass substrate. The VO2 was then RF-sputtered in a reactive argon and oxygen plasma with the addition of substrate biasing. Table 2 shows a detailed list of the deposition parameters. 3. Results 3.1. Optical modeling and SRD architecture To optimize the emissivity tunability of a SRD, it is essential to un derstand each layer’s role. Let us first consider the initial metallic mirror film typically based on either gold, silver or aluminum. This layer is present for the SRD to possess a low emissivity in its low temperature state. Indeed, VO2, at low temperatures, behaves like a semiconductor and displays a high NIR and IR transmission, while at high temperatures, it displays metallic-like properties and thus a higher reflection/absorp tion. However, adding VO2 alone on top of a metallic mirror does not offer a sufficiently high performance. As an example, a Au mirror | Si3N4 | VO2 test sample was deposited and is shown in Fig. 2; the obtained variation in Δε is only about 1.9%. By optimizing the VO2 layer thick ness, higher Δε values could potentially be reached, up to 22% with a thickness of 250 nm; yet, they still remain insufficient for SRD applications. The optical performance is greatly improved by designing a FabryPerot resonant cavity consisting of a bottom IR mirror, a dielectric cavity and a top VO2 absorber. As will be shown, the dielectric spacer must be transparent in the NIR and IR regions and compatible with VO2. By carefully adjusting the dielectric spacer thickness, one can choose the desired absorption wavelength of the coating; this thickness is typically close to a quarter-wave (λabs � 4nt, where n is the refractive index and t the thickness). At such a thickness, the electric field reaches a maximum at the cavity-VO2 interface, thus increasing the absorption in the high temperature state [18]; it is interesting to note that this same effect has also been implemented to generate highly colored anticounterfeiting optical security devices [19]. In the context of a SRD, a judicious choice of wavelength for maximum absorption is λabs ¼ 8μm, which corresponds to the peak in emissivity of a black body at 100 � C. This design allows for a clear distinction between both temperature states (see Fig. 3 for an example): at low temperature, the VO2 is highly transparent, so most of the light is reflected by the bottom Au mirror film which results in a low εL , while at high temperature, the VO2 becomes reflective/absorbing thus completing the resonant cavity requirements and increasing the ab sorption near λabs , thus significantly increasing εH . Now the example shown in Fig. 3 is based on a SiO2 cavity material which presents a peak in absorption in the IR. The precise modeled ar chitecture is inspired by the work of Hendaoui et al. [8] and consists in a Au | SiO2 [850 nm] | VO2 [30 nm] layer configuration. The modeling results show an emissivity variation of 52.9%, with the emittance at low and high temperatures being, respectively, 17.7% and 70.6%, values
Fig. 1. Refractive index (continuous line) and extinction coefficient (dotted line) of a 70-nm-thick VO2 film deposited on Si obtained by ellipsometry. At low temperature (~25 � C), VO2 is transparent in the IR spectrum while at high temperature (100 � C), VO2 displays metallic-like properties. A minor and manageable offset is observed in the transition between the RC2 and IR-VASE modeled data at 2 μm.
As for the emissivity, directional infrared reflectance measurements were performed on a SOC-100 reflectometer from Surface Optics Corps, from 2.5 to 25 μm by measuring R(λ) at a 10� angle of incidence for unpolarized light. Measurements were also done at low (25 � C) and high temperature (100 � C). Assuming Kirchoff’s law for thermal equilibrium and a transmission TðλÞ ¼ 0 (the gold mirror film is highly reflective), we find that:
εðλÞ ¼ αðλÞ ¼ 1
RðλÞ
TðλÞ ¼ 1
RðλÞ:
(3)
By using equation (1), it is then possible to evaluate εL and εH . 2.3. Sample deposition Samples were deposited onto B270 glass substrates which were cleaned using soapy water and isopropanol, rinsed with deionized water and dried with nitrogen. The deposition of the gold mirror films was performed in a magnetron sputtering system equipped with a 2-inch diameter Au target using a RF power supply set at 150 W in a 5 mTorr argon atmosphere. Note that the substrates were first pretreated with an oxygen and argon plasma (100 V, 20 mTorr, 2:1 Ar/O2 ratio, 5 min) to improve adhesion, and rotated during deposition for increased uniformity. The CaF2 films were then prepared by electron beam evaporation using a Thermoionics E-Beam evaporator where the base pressure was below 9 � 10 6 Torr. Pure CaF2 pellets (99.9%) were placed inside a crucible and targeted by the electron beam. The current was then increased up to approximately 0.75 A until the microbalance showed a 2 nm/s deposition rate. This high deposition rate increased the film’s porosity and consequently diminished its refractive index. As VO2 requires specific growth conditions, a thin Si3N4 film, which acted both as a passivating layer and a barrier coating, was added below the VO2 [17]. The Si3N4 and VO2 depositions were carried out in a CMS-18 sputtering system from Kurt J. Lesker using three-inch Si and V targets, respectively, and a 150 mm diameter substrate holder. The base pressure for depositions was below 2 � 10 7 Torr. The Si3N4 was RF-sputtered in an Ar and N2 reactive plasma (2:1 ratio) at room tem perature. Afterwards, the temperature setpoint was set to 650 � C with a 10 � C/min ramp rate, followed by a 1-h pause once the setpoint was reached. The surface temperature was approximately 400 � C as measured using a thermocouple in direct contact with the surface of the
Table 2 Deposition parameters for each layer of the prototype SRDs.
3
Parameters
Au
CaF2
Si3N4
VO2
Gases O2 (N2): Ar Pressure [Torr]
Ar 0 5 � 10
– – < 9� 10 6
Ar þ N2 1:2 5 � 10 3
Ar þ O2 3.4:30 5 � 10 3
Power supply Average power [W] Cathode voltage [V] Substrate bias voltage [V] Substrate temperature [� C] Deposition rate [nm/ min]
RF 150 280 0
DC 300 – –
RF 450 220 90
RF 450 158 180–190
No heating 16
No heating
No heating 2
~400
3
120
0.75
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Fig. 4. Study of the impact of the refractive index of a transparent dielectric cavity in a Au mirror | cavity [optical thickness of ~1700–1850 nm] |VO2 [20 nm] system on the overall reflectance. Each curve from green to blue rep resents a material with a refractive index varying from 1.17 to 2.17 @ 10 μm by 0.2 increments and an identical extinction coefficient, in this case that of the deposited CaF2 film. The shaded areas represent the normalized emission of a black body at low (blue) and high temperatures (pink). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
Fig. 2. Modeled (dotted lines) and experimental (continuous lines) results of a Au | Si3N4 (34 nm) | VO2 (36 nm) architecture at low (25 � C) and high tem perature (100 � C). The resulting Δεis about 1.9%.
index values were not chosen by chance, as the lowest refractive index we could experimentally obtain with CaF2 was ne1:17 at 10 μm (typical refractive index values for CaF2 are ~1.32 at 10 μm [13]), and the highest value was close to previously-deposited ZnS films (n@10μme2:10Þ. It is clear from Fig. 4 that decreasing the refractive index leads to a broadening of the absorption region at higher temper atures. As a lower refractive index has little impact on the low temper ature state and broadens the absorption at high temperature, the overall performance in Δε of the coating is significantly improved going from 60.7% for an index of 2.17 up to 70.8% for an index at 1.17. Performing the same exercise but with an absorbing SiO2 film (not shown here) also shows a broadening of the absorption at high tem perature. However, once again, the performance is not improved as much due to the presence of SiO2’s absorption peak in the 8–10 μm range. Fig. 3. Modeled results based on a Au | SiO2 [850 nm] | VO2 [30 nm] archi tecture. The resulting Δε is 52.9%.
3.3. Thermochromic layer thickness
which are in close agreement with the experimental results shown in Ref. [8] (see Table 1). Also observed in Fig. 3, the absorption peak around 9–10 μm (Si-O-Si stretching bond) in the low temperature state has a significant impact on εL , while rendering the SRD slightly less effective at high temperature. From these results, it is clear that imple menting an IR-transparent material would be largely beneficial.
Another important parameter to consider is the impact of the thickness of the thermochromic VO2 film as too thin a layer may not present enough absorption in the resonant cavity mode, and too thick a layer may inhibit light from getting into the cavity itself. In Fig. 5, the thickness of the CaF2 cavity remains unchanged at 1400 nm, while the thickness of the VO2 is varied between 15 nm and 40 nm by 5 nm in crements. One can observe that the optimal thickness is between 15 and 25 nm, as a high absorption is observed near 8 μm without a significant increase in reflection at higher wavelengths. Table 3 lists the resulting Δε values. A final optimization was performed for a situation where both the cavity and the VO2 thicknesses were varied in order to obtain the ab solute maximum in emissivity variation using the previously established thickness values as starting points (see Fig. 6). The highest variation in emissivity is reached for a VO2 thickness in the 20 nm range and for a CaF2 thickness of ~1250 nm. It is important to note that an additional antireflective (AR) coating could also be added on top of the VO2 layer to further lower the reflection at high temperatures. However, in the
3.2. Ultra-low index material as the resonant cavity The use of an IR-transparent material is consequently the next logical step. In addition, it is worthwhile to consider what would be the ideal refractive index of such a material. Fig. 4 shows the results of various Au mirror | transparent cavity|VO2 [20 nm] architectures in which the cavity material’s refractive index was numerically varied between 1.17 and 2.17 @ 10 μm by 0.2 increments. The thickness of the layer was also adjusted to maintain the position of maximum absorption at λabs and thus optimize Δε. The resulting optical thickness (nt) of the cavity therefore slightly varied between 1700 nm and 1850 nm. The refractive 4
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lead to an increase in the absorption in the low temperature state; as a result, it has not been added to our prototype samples. 4. Experimental results 4.1. Individual layers Many challenges can occur during deposition, especially in the context of VO2 coatings as they require a high deposition temperature, an appropriate seed/barrier layer (Si3N4 in our case) and, most impor tantly, a precise control of the oxygen content to obtain the correct stoichiometry [20]. When depositing at high temperatures, interdiffu sion between the different layers of the stack and crystallization of these same layers can also take place. To potentially observe such changes, each layer was individually studied as well as sequentially characterized during each step of the deposition process. As was shown in Fig. 2, the experimental results of the Au mirror | Si3N4 [34 nm]|VO2 [36 nm] sample corresponded very well to the op tical model, indicating that the optical properties of the individual ma terials have been correctly assessed. Following this confirmation, we then proceeded with the deposition of a prototype Fabry-Perot-like SRD containing a CaF2 cavity layer. Fig. 7 shows the resulting reflection spectrum of the Au mirror |CaF2 [1240 nm] sample in comparison with the model. While there are some slight discrepancies, the reflectivity is on average predicted within 2% in the 3–15 μm range. Specifically, the intensity of the water absorption peaks at ~6.1 μm [21] are over estimated, most probably due to a change in the water content in the film, while the onset of absorption for the CaF2 above 17 μm is under estimated; nevertheless, these differences remain below 5%.
Fig. 5. Variation of the VO2 thickness from 15 to 40 nm in a Au mirror |CaF2 [1400 nm] |VO2 architecture. The shaded areas represent the normalized emission of a black body at low (blue) and high temperatures (pink). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.) Table 3 Emissivity variation for each VO2 thickness configuration as shown in Fig. 5. VO2 thickness
Δε
[nm]
[%]
15 20 25 30 35 40
75.3 76.0 74.0 70.8 67.0 63.0
4.2. SRD deposition The first prototype sample consisted in a CaF2 [960 nm] |Si3N4 [31 nm]| VO2 [46 nm] system, giving rise to Δε ¼ 56%. This result provided a proof of concept for the use of an ultra-low refractive index cavity and demonstrated that VO2 could grow on a porous calcium fluoride surface. Note that a Si3N4 film was nevertheless added to the stack in order to limit modifying the growth conditions of the VO2. The layers were then modeled and reoptimized, and the desired thickness for CaF2 was found to be between 1200 and 1400 nm as predicted in Fig. 6. Indeed, in the second set of depositions with a 1240 nm thickness of CaF2, all samples exhibited a large absorption band in the high tem perature state with Δε values greater than 60%, with the highest values
Fig. 6. Two-dimensional mapping of a Au mirror | CaF2 (800–1800 nm) | Si3N4 (30 nm) | VO2 (10–40 nm) architecture where the thicknesses of the CaF2 and VO2 are varied in the indicated intervals. The scale bar on the right side in dicates the emissivity variation.
presence of an ultra-low refractive index material such as CaF2, RðλÞ is already very low (< 10%) between the 8 and 10 μm, which reduces the need and the impact of the AR coating. Adding an extra layer could also
Fig. 7. Modeled (discontinuous line) and experimental (continuous line) reflection curves for a Au | CaF2 [1240 nm] sample. 5
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being 64.9% and 66.0% (see Fig. 8 and Table 4 for the results of a series of samples consisting of Au | CaF2 (1240 nm) | Si3N4 (31 nm) | VO2 (40 nm) - some slight modifications to the heating ramp rates and waiting times once the deposition temperature was reached were also tested). Although note shown here, optimized SRDs with ZnS cavities were also deposited (n ¼ 2.2 @ 10 μm) and, as predicted by the modeling approach, they exhibited a narrower absorption band in the high tem perature state thus limiting the obtained Δε values. One may note that the VO2 thickness was chosen as 40 nm despite the previous models indicating that a 15–25 nm thickness would be better suited. In fact, we did deposit thinner VO2 films but were unable to observe any beneficial impact on performance, most probably due to a change in their optical properties; the latter being intrinsically related to the growth and crystallinity of the films. Note that the optical properties were originally measured on a 70-nm-thick film (see Fig. 1) deposited directly on Si and as such, one may expect the seed layer to affect the growth conditions of the VO2. In Fig. 9, we compare Sample IV with the predicted model curves. We can observe that the trends are similar despite some differences which play in our favor. As the model fitted well before the deposition of the Si3N4/VO2 films, we surmise that these differences are most probably due to a combination of several effects: a) changes in the optical prop erties of the VO2 film as it is deposited on a rougher surface due to the presence of a thick and porous CaF2, b) potential oxidation of the Si3N4 layer due to the presence of an O2 plasma during the VO2 deposition, c) slight changes in the materials due to the high temperature of the deposition process and, finally, d) the possible presence of interdiffusion between adjacent layers. These effects can be studied in more details in future experiments.
Table 4 Measured emissivity variations for various prototype samples with a Au | CaF2 (1240 nm) | Si3N4 (31 nm) | VO2 (40 nm) architecture.
4.3. Solar absorption
Fig. 9. Comparison between the model (dotted lines) and the experimental results at low and high temperatures (continuous lines) for sample IV. The shaded areas represent the normalized emission of a black body at low (blue) and high temperatures (pink). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
This work’s focus is to optimize the emissivity variation (Δε) of the SRD. However, the solar absorption is also an important parameter, especially for low-orbit satellites. Using equation (2), sample V’s ab sorption was calculated to be 37.8%, a value which is, unfortunately, above the 20% ideal one [6]. Reducing the thickness of the VO2 layer should potentially lower αs as it absorbs between 300 nm and 2500 nm (see Fig. 1 for the optical properties of VO2). A 2D map of the absorption as a function of the CaF2 and VO2 thickness is shown in Fig. 10. While going thinner in VO2 does decrease αs , there are clearly limitations when using the present architecture.
Sample
εL (%)
εH (%)
Δε (%)
I II III IV V
14.1 16.8 17.5 14.8 13.9
80.1 79.4 78.9 79.6 78.2
66.0 62.5 61.4 64.9 64.4
Fig. 10. Two-dimensional mapping of a Au mirror | CaF2 (800–1800 nm) | VO2 (10–40 nm) architecture where the thicknesses of the CaF2 and VO2 are varied in the indicated intervals. The scale bar on the right side indicates the solar absorption at low temperature.
5. Conclusions In the present work, we have shown that a modeling-based approach can help further understand the impact of each individual layer on the
Fig. 8. Measured reflectivity at low temperature (25 � C) and high temperature (100 � C) on sample IV. The architecture of this sample was Au | CaF2 (1240 nm) | Si3N4 (31 nm) | VO2 (40 nm). 6
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overall performance of the SRD. For instance, an ultra-low refractive index cavity substantially enhances Δε as it broadens the absorption peak in the high temperature state, as also confirmed by the experi mental results with a CaF2 film with n ¼ 1.17 @ 10 μm. The modeling approach also allows one to choose the appropriate thicknesses for a given layer. Following a detailed optimization of the SRD design, an emissivity variation of up to 66% was obtained for a system consisting of a Au mirror | CaF2 (1240 nm) | Si3N4 (31 nm) | VO2 (40 nm) architec ture. Such values are fully compatible with spacecraft requirements such as in the case of nanosatellites. As previously discussed, future experi ments should focus on the impact of heating on the layers’ optical properties and on the interfaces between adjacent layers, as well as a more thorough study of VO2 growth on different seed layers. Finally, a multilayer design specifically aimed at reducing αS could be designed to lower the solar absorption to less than 20%. The metamaterial approach proposed by Sun et al. could potentially yield lower αS values due to a smaller coverage area of VO2 [10], and by combining it with our ultra-low refractive index material, further enhance the performance of SRDs for space applications.
[2] L. Vincent, Pisacane, Fundamentals of Space Systems, second ed., Oxford University Press, Oxford, 2015. [3] D. Gilmore, Spacecraft thermal control handbook, in: Fundamental Technologies vol. I, American Institute of Aeronautics and Astronautics, Inc., Washington, DC, 2002, https://doi.org/10.2514/4.989117. [4] M. Soltani, M. Chaker, E. Haddad, R. Kruzelecky, Thermochromic Vanadium Dioxide (VO2) Smart Coatings for Switching Applications, (n.d.) 23. [5] T.D. Manning, I.P. Parkin, M.E. Pemble, D. Sheel, D. Vernardou, Intelligent window coatings: atmospheric pressure chemical vapor deposition of tungsten-doped vanadium dioxide, Chem. Mater. 16 (2004) 744–749, https://doi.org/10.1021/ cm034905y. [6] D. Nikampour, Personal Communications, Canadian Space Agency. [7] M.A. Kats, D. Sharma, J. Lin, P. Genevet, R. Blanchard, Z. Yang, M.M. Qazilbash, D. N. Basov, S. Ramanathan, F. Capasso, Ultra-thin perfect absorber employing a tunable phase change material, Appl. Phys. Lett. 101 (2012), https://doi.org/ 10.1063/1.4767646, 221101. � � Haddad, Highly tunable-emittance radiator [8] A. Hendaoui, N. Emond, M. Chaker, E. based on semiconductor-metal transition of VO2 thin films, Appl. Phys. Lett. 102 (2013), 061107, https://doi.org/10.1063/1.4792277. [9] X. Wang, Y. Cao, Y. Zhang, L. Yan, Y. Li, Fabrication of VO2-based multilayer structure with variable emittance, Appl. Surf. Sci. 344 (2015) 230–235, https:// doi.org/10.1016/j.apsusc.2015.03.116. [10] K. Sun, C.A. Riedel, A. Urbani, M. Simeoni, S. Mengali, M. Zalkovskij, B. Bilenberg, C.H. de Groot, O.L. Muskens, VO2 thermochromic metamaterial-based smart optical solar reflector, ACS Photonics 5 (2018) 2280–2286, https://doi.org/ 10.1021/acsphotonics.8b00119. [11] V.A. Zeitler, C.A. Brown, The infrared spectra of some Ti-O-Si, Ti-O-Ti and Si-O-Si compounds, J. Phys. Chem. 61 (1957) 1174–1177, https://doi.org/10.1021/ j150555a010. [12] J.D. Traylor Kruschwitz, W.T. Pawlewicz, Optical and durability properties of infrared transmitting thin films, Appl. Opt. 36 (1997) 2157, https://doi.org/ 10.1364/AO.36.002157. [13] S. Larouche, L. Martinu, OpenFilters: open-source software for the design, optimization, and synthesis of optical filters, Appl. Opt. 47 (2008) C219, https:// doi.org/10.1364/AO.47.00C219. [14] C.C. Katsidis, D.I. Siapkas, General transfer-matrix method for optical multilayer systems with coherent, partially coherent, and incoherent interference, Appl. Opt. 41 (2002) 3978, https://doi.org/10.1364/AO.41.003978. [15] D.R.L. Borissova, F. Smarandache, L. Borissova, Progress in physics, J. Adv. Stud. Theor. Exp. Phys. Incl. Relat. Themes from Math. 4 (2009) (Infinite Study). [16] Steffen Richter, Tifei Sun, M. Mumtaz Qazilbash, Rudiger Schmidt-Grund, Carsten Ronning, Shriram Ramanathan, A. Kats Mikhail, Optical Properties of Thin-Film Vanadium Dioxide from the Visible to the Far Infrared, ArXiv E-Prints, 2019. [17] S. Loquai, B. Baloukas, J.E. Klemberg-Sapieha, L. Martinu, HiPIMS-deposited thermochromic VO 2 films with high environmental stability, Sol. Energy Mater. Sol. Cells 160 (2017) 217–224, https://doi.org/10.1016/j.solmat.2016.10.038. [18] J. Callaway, Optical absorption in an electric field, Phys. Rev. 130 (1963) 549–553, https://doi.org/10.1103/PhysRev.130.549. [19] B. Baloukas, W. Trottier-Lapointe, L. Martinu, Fabry-Perot-like interference security image structures: from passive to active, Thin Solid Films 559 (2014) 9–13, https://doi.org/10.1016/j.tsf.2013.10.030. [20] S. Loquai, B. Baloukas, O. Zabeida, J.E. Klemberg-Sapieha, L. Martinu, HiPIMSdeposited thermochromic VO2 films on polymeric substrates, Sol. Energy Mater. Sol. Cells 155 (2016) 60–69, https://doi.org/10.1016/j.solmat.2016.04.048. [21] Coblentz Society, NIST Chemistry WebBook, NIST Standard Reference Database Number 69, Eds. P.J. Linstrom and W.G. Mallard.
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors wish to thank Mr. F. Turcot and Mr. C. Cl� ement for their technical assistance. This research was supported in part by the Natural Sciences and Engineering Research Council of Canada (NSERC) through the NSERC Multisectorial Industrial Research Chair in Coatings and Surface Engineering (grant IRCPJ 433808-11), and it has benefited from the partial support from the Fonds de recherche du Qu� ebec – Nature et technologies (FRQNT) through its grant to the Quebec Advanced Ma terials Strategic Research Cluster (RQMP). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.solmat.2019.110260. References [1] S. DelPozzo, Caleb Williams, Bill Doncaster, Nano/microsatellite Market Forecast, ninth ed., SpaceWorks, 2019, pp. 1–32, 2019.
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Thermochromic VO2-based smart radiator devices with ultralow refractive index cavities for increased performance R. Beaini *, B. Baloukas, S. Loquai, J.E. Klemberg-Sapieha, L. Martinu Department of Engineering Physics, Polytechnique Montr�eal, Montreal, QC, H3C 3A7, Canada
A R T I C L E I N F O
A B S T R A C T
Keywords: Thermochromic VO2 Thermal regulation Smart radiator devices Variable emissivity Ultralow refractive index
VO2 thermochromic coatings show great potential as the main component in smart radiator devices (SRD) for spacecraft, most notably for micro- and nano-satellites. Indeed, the inherent metal to insulator transition (MIT) of VO2 allows such a coating to act as a lightweight thermal regulator, eliminating the need for heavy and failureprone mechanical louvers. However, spacecraft industry standards require an emissivity variation Δε of at least 60%, a value which, to our knowledge, has yet to be demonstrated. To reach and surpass this value, we first apply a modeling approach to optimize the optical properties and thicknesses of the individual constituent films of a typical SRD with the following architecture: mirror | dielectric resonant cavity | VO2. This study then highlights various possible avenues to enhance the performance of the devices, one of these being the use of an infrared transparent ultralow refractive index dielectric material for the resonant cavity. This theoretical pre diction is then confirmed by the deposition of various prototype devices implementing a CaF2 cavity layer with n @ 10 μm ¼ 1.17 and with measured values of Δε in excess of 60% for the full 3 to 25μm wavelength range. In fact, we demonstrate a prototype device with a maximum Δε value of 66%, thus bringing this technology one step closer to implementation.
1. Introduction As the number of space missions keeps on growing, micro- and nanosatellites are becoming an appealing option for spacecraft companies around the globe [1]. Defined by a mass between 1 kg and 100 kg, such satellites present an important engineering challenge due to their smaller size. For instance, the operating temperature of a satellite must be kept between 0 � C and 40 � C [2]. Traditionally, the necessary thermal load management system was based on the use of mechanical louvers [3]. However, with the constant miniaturization of satellites, these louvers become a bigger part of the satellite, leading to an increase in launching costs as well as being prone to wear and mechanical failure [4]. Alternative approaches towards thermal regulation are therefore required, and as such, smart radiator devices (SRD) are amongst the most promising candidates. One of the simplest ways to design a SRD is through the use of a thermochromic material. Such materials exhibit a change in their optical properties when reaching their transition temperature TC . For example, VO2, with a TC close to room temperature (TC ¼ 68 � C), is an ideal candidate as it transitions from semi-conducting properties to metallic-
like properties when below and above its TC , respectively. It is also possible to lower its TC by doping it with a transition metal; e.g., 2% of W for a TC of 25 � C [5]. The drastic change in electrical properties results in an important variation of the reflectance in the infrared (IR) and thus, a variable and tunable emissivity, Δε. To meet the spacecraft industry standards for thermal regulation, this value should ideally surpass 60% while keeping the solar absorption below 20% [6]. At the moment, the most promising SRD configuration consists of a bottom mirror, followed by a dielectric resonant cavity, an active VO2 film, and a top protective/antireflective film. When properly adjusted, such a device presents a low absorption/high reflectance when below Tc, and a high absorption/low reflectance when above Tc. By appropriately adjusting the optical thickness of the resonant cavity, one can choose the wavelength at which the absorption reaches a maximum, and thus optimize the emissivity variation Δε [7]. As VO2 is thermally driven, this allows a SRD to self-regulate its temperature: at low temperatures, heat is trapped inside the spacecraft keeping it warm, and as the temperature rises, the SRD transitions into its absorbing state, allowing heat to be re-emitted into space, thus cooling down the spacecraft. Previous works on SRDs show limited studies on the impact of the
* Corresponding author. E-mail address: [email protected] (R. Beaini). https://doi.org/10.1016/j.solmat.2019.110260 Received 7 August 2019; Received in revised form 23 October 2019; Accepted 25 October 2019 Available online 31 October 2019 0927-0248/© 2019 Published by Elsevier B.V.
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layer thicknesses [8–10], and rarely follow an optimization approach. Table 1 shows examples of recently proposed SRD architectures and their corresponding performance. The architecture proposed by Hendaoui et al. [8] gives a good overall performance at high temperature as the selected SiO2 thickness acts as a resonant cavity generating a high absorption at about 7 μm. However, by choosing SiO2 as the cavity material, the performance at low tem perature is hindered, as silica has an absorption peak at around 9.25 μm (Si-O-Si stretching) [11], which is relatively close to the peak emissivity of a blackbody at 25 � C (~9.7 μm). Wang et al. proposed a very similar architecture in which the cavity layer was replaced by HfO2 [9], a ma terial which is transparent between 5 μm and 12 μm [12]. The properties at low temperature of their SRD are thus highly improved, and to our knowledge, this is currently the design showing the highest performance in the literature with a Δε value of 55%. However, their optimization approach was mainly empirical, and very little is said on the impact of each individual films’ optical properties and thicknesses. Finally, Sun et al. proposed an interesting metamaterial architecture [10], in which the VO2 and the top Al2O3 layers are patterned. This design allows one to increase the absorption at high temperature, due to the presence of a plasmonic resonance generated by the VO2 at high temperature, while also reducing the low temperature absorption due to the decrease in coverage of the VO2. In their most promising design, each feature measured 2.7 μm � 2.7 μm with a 0.5 μm gap between each square. Although they showed an increase in the absorption in the high tem perature state, the absorption at low temperature is above 30%, greatly reducing the overall performance. This is, once again, mainly due to the presence of SiO2 as the cavity material. In the present work, a model-based approach is first implemented to clearly assess the impact of each individual film with respect to the overall performance; the main goal being to maximize the emissivity variation between the low and high temperature states. Following this systematic approach allows for a better understanding of the SRD’s behaviour and thus serves as a guide for properly choosing the materials,
their thickness and their incorporation into the most appropriate ar chitecture. The implemented optical properties are obtained in house by spectroscopic ellipsometry ranging from the UV to the IR. The models clearly indicate that using an ultralow refractive index infrared trans parent material with an appropriate thickness as the resonant cavity can lead to higher than 60% Δε values. Prototype samples are then deposited using a Au | CaF2 | Si3N4 | VO2 architecture in which an ultralow refractive index CaF2 film is used as the resonant cavity. The resulting optical performance confirms the developed modeling approach, and devices with emissivity variations as high as 66% are demonstrated. Finally, further potential avenues of optimization are discussed. 2. Methodology 2.1. Optical modeling Optical modeling and design is an important part of the present work as it allows one to predict the optical response of a multilayer coating. It also allows for a better understanding of each layers’ role in the overall SRD performance, as well as hinting towards ideal material combina tions that should be implemented for optimal performance. Modeling was carried out using the open source software OpenFilters [13] as well as Matlab, both applying the well known matrix approach [14]. Calculations were performed for the 300 nm to 25 μm range, and the resulting reflection (R) and transmission (T) spectra were used to calculate the SRD’s absorption assuming AðλÞ ¼ 1 RðλÞ TðλÞ. Using Kirchoff’s law of thermal equilibrium, the absorption coeffi cient at a specific wavelength αðλÞ is considered as equal to the emis sivity at that same wavelength (αðλÞ ¼ εðλÞ) [15]. The emittance at low temperature (εL ) and at high temperature (εH ) is then found to be the ratio between the sample’s emission and a blackbody’s emission at the same temperature. The emission is evaluated in the wavelength interval [λ1 ; λ2 ], typically from 3 μm to 25 μm: R λ2 αðλ; TÞEb ðλ; TÞdλ ε ¼ λ1 R λ2 ; (1) Ebλ ðλ; TÞdλ λ1
Table 1 Recently published smart radiator architectures, their corresponding perfor mance and observations. Architecture
Performance
εL
εH
Δε
Au | SiO2 (850 nm) | VO2 (30 nm)
0.22
0.71
0.49
Ag | HfO2 (800 nm) | VO2 (50 nm)
0.13
0.68
0.55
(25 � C)
Si | Al2O3 (50 nm)| Al (80 nm) | SiO2 (1200 nm)| patterned VO2 (50 nm)| patterned Al2O3 (6 nm)
0.31
(100 � C)
0.79
0.48
Observations
Reference
The overall performance is hindered by the high value of εL due to the absorption band of SiO2 near 9 μm.
[8]
Implementing an IR-transparent material as the cavity decreases the absorption in the low temperature state. This metamaterial design allows to reach new heights for εH . The overall performance is, however, hindered by the high value of εL due to the absorption band of SiO2 near 9 μm.
where Eb is the emissivity of a blackbody at the desired temperature given by Planck’ law. By calculating the performance at both tempera tures, the total emissivity variation of the multilayer Δε can be obtained (Δε ¼ εH εL ). The solar absorption αs can be calculated using the following equa tion: R 2500 αðλÞ φs ðλÞdλ αs ¼ 300R 2500 (2) φs ðλÞdλ 300 where φs denotes the solar irradiance with an air mass of 1.5 (ASTMG173-03 - Global tilt).
[9]
2.2. Optical characterization The refractive index and extinction coefficient of each layer were obtained by variable-angle spectroscopic ellipsometry in the 0.3 to 25 μm range using a combination of two ellipsometers from J.A. Woollam Co.: the RC2-XI (300–2500 nm) and the IR-VASE (2–25 μm). Ψ and Δ ellipsometric parameters were obtained for four angles of incidence (45� , 55� , 65� , and 75� ) on backside roughened c-Si (100) substrates for visible characterization, and 2-side-polished undoped-Si for IR charac terization. The VO2 films’ optical properties were measured at 25 � C (low temperature state) and 100 � C (high temperature state) using heat cells installed on both ellipsometers. The results were analyzed and modeled in the CompleteEase and WVASE software also from J.A. Woollam Co. Fig. 1 shows the optical properties of VO2 in its low and high temperature states, which are in accordance with the work of Wan et al. [16].
[10]
2
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glass substrate. The VO2 was then RF-sputtered in a reactive argon and oxygen plasma with the addition of substrate biasing. Table 2 shows a detailed list of the deposition parameters. 3. Results 3.1. Optical modeling and SRD architecture To optimize the emissivity tunability of a SRD, it is essential to un derstand each layer’s role. Let us first consider the initial metallic mirror film typically based on either gold, silver or aluminum. This layer is present for the SRD to possess a low emissivity in its low temperature state. Indeed, VO2, at low temperatures, behaves like a semiconductor and displays a high NIR and IR transmission, while at high temperatures, it displays metallic-like properties and thus a higher reflection/absorp tion. However, adding VO2 alone on top of a metallic mirror does not offer a sufficiently high performance. As an example, a Au mirror | Si3N4 | VO2 test sample was deposited and is shown in Fig. 2; the obtained variation in Δε is only about 1.9%. By optimizing the VO2 layer thick ness, higher Δε values could potentially be reached, up to 22% with a thickness of 250 nm; yet, they still remain insufficient for SRD applications. The optical performance is greatly improved by designing a FabryPerot resonant cavity consisting of a bottom IR mirror, a dielectric cavity and a top VO2 absorber. As will be shown, the dielectric spacer must be transparent in the NIR and IR regions and compatible with VO2. By carefully adjusting the dielectric spacer thickness, one can choose the desired absorption wavelength of the coating; this thickness is typically close to a quarter-wave (λabs � 4nt, where n is the refractive index and t the thickness). At such a thickness, the electric field reaches a maximum at the cavity-VO2 interface, thus increasing the absorption in the high temperature state [18]; it is interesting to note that this same effect has also been implemented to generate highly colored anticounterfeiting optical security devices [19]. In the context of a SRD, a judicious choice of wavelength for maximum absorption is λabs ¼ 8μm, which corresponds to the peak in emissivity of a black body at 100 � C. This design allows for a clear distinction between both temperature states (see Fig. 3 for an example): at low temperature, the VO2 is highly transparent, so most of the light is reflected by the bottom Au mirror film which results in a low εL , while at high temperature, the VO2 becomes reflective/absorbing thus completing the resonant cavity requirements and increasing the ab sorption near λabs , thus significantly increasing εH . Now the example shown in Fig. 3 is based on a SiO2 cavity material which presents a peak in absorption in the IR. The precise modeled ar chitecture is inspired by the work of Hendaoui et al. [8] and consists in a Au | SiO2 [850 nm] | VO2 [30 nm] layer configuration. The modeling results show an emissivity variation of 52.9%, with the emittance at low and high temperatures being, respectively, 17.7% and 70.6%, values
Fig. 1. Refractive index (continuous line) and extinction coefficient (dotted line) of a 70-nm-thick VO2 film deposited on Si obtained by ellipsometry. At low temperature (~25 � C), VO2 is transparent in the IR spectrum while at high temperature (100 � C), VO2 displays metallic-like properties. A minor and manageable offset is observed in the transition between the RC2 and IR-VASE modeled data at 2 μm.
As for the emissivity, directional infrared reflectance measurements were performed on a SOC-100 reflectometer from Surface Optics Corps, from 2.5 to 25 μm by measuring R(λ) at a 10� angle of incidence for unpolarized light. Measurements were also done at low (25 � C) and high temperature (100 � C). Assuming Kirchoff’s law for thermal equilibrium and a transmission TðλÞ ¼ 0 (the gold mirror film is highly reflective), we find that:
εðλÞ ¼ αðλÞ ¼ 1
RðλÞ
TðλÞ ¼ 1
RðλÞ:
(3)
By using equation (1), it is then possible to evaluate εL and εH . 2.3. Sample deposition Samples were deposited onto B270 glass substrates which were cleaned using soapy water and isopropanol, rinsed with deionized water and dried with nitrogen. The deposition of the gold mirror films was performed in a magnetron sputtering system equipped with a 2-inch diameter Au target using a RF power supply set at 150 W in a 5 mTorr argon atmosphere. Note that the substrates were first pretreated with an oxygen and argon plasma (100 V, 20 mTorr, 2:1 Ar/O2 ratio, 5 min) to improve adhesion, and rotated during deposition for increased uniformity. The CaF2 films were then prepared by electron beam evaporation using a Thermoionics E-Beam evaporator where the base pressure was below 9 � 10 6 Torr. Pure CaF2 pellets (99.9%) were placed inside a crucible and targeted by the electron beam. The current was then increased up to approximately 0.75 A until the microbalance showed a 2 nm/s deposition rate. This high deposition rate increased the film’s porosity and consequently diminished its refractive index. As VO2 requires specific growth conditions, a thin Si3N4 film, which acted both as a passivating layer and a barrier coating, was added below the VO2 [17]. The Si3N4 and VO2 depositions were carried out in a CMS-18 sputtering system from Kurt J. Lesker using three-inch Si and V targets, respectively, and a 150 mm diameter substrate holder. The base pressure for depositions was below 2 � 10 7 Torr. The Si3N4 was RF-sputtered in an Ar and N2 reactive plasma (2:1 ratio) at room tem perature. Afterwards, the temperature setpoint was set to 650 � C with a 10 � C/min ramp rate, followed by a 1-h pause once the setpoint was reached. The surface temperature was approximately 400 � C as measured using a thermocouple in direct contact with the surface of the
Table 2 Deposition parameters for each layer of the prototype SRDs.
3
Parameters
Au
CaF2
Si3N4
VO2
Gases O2 (N2): Ar Pressure [Torr]
Ar 0 5 � 10
– – < 9� 10 6
Ar þ N2 1:2 5 � 10 3
Ar þ O2 3.4:30 5 � 10 3
Power supply Average power [W] Cathode voltage [V] Substrate bias voltage [V] Substrate temperature [� C] Deposition rate [nm/ min]
RF 150 280 0
DC 300 – –
RF 450 220 90
RF 450 158 180–190
No heating 16
No heating
No heating 2
~400
3
120
0.75
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Fig. 4. Study of the impact of the refractive index of a transparent dielectric cavity in a Au mirror | cavity [optical thickness of ~1700–1850 nm] |VO2 [20 nm] system on the overall reflectance. Each curve from green to blue rep resents a material with a refractive index varying from 1.17 to 2.17 @ 10 μm by 0.2 increments and an identical extinction coefficient, in this case that of the deposited CaF2 film. The shaded areas represent the normalized emission of a black body at low (blue) and high temperatures (pink). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
Fig. 2. Modeled (dotted lines) and experimental (continuous lines) results of a Au | Si3N4 (34 nm) | VO2 (36 nm) architecture at low (25 � C) and high tem perature (100 � C). The resulting Δεis about 1.9%.
index values were not chosen by chance, as the lowest refractive index we could experimentally obtain with CaF2 was ne1:17 at 10 μm (typical refractive index values for CaF2 are ~1.32 at 10 μm [13]), and the highest value was close to previously-deposited ZnS films (n@10μme2:10Þ. It is clear from Fig. 4 that decreasing the refractive index leads to a broadening of the absorption region at higher temper atures. As a lower refractive index has little impact on the low temper ature state and broadens the absorption at high temperature, the overall performance in Δε of the coating is significantly improved going from 60.7% for an index of 2.17 up to 70.8% for an index at 1.17. Performing the same exercise but with an absorbing SiO2 film (not shown here) also shows a broadening of the absorption at high tem perature. However, once again, the performance is not improved as much due to the presence of SiO2’s absorption peak in the 8–10 μm range. Fig. 3. Modeled results based on a Au | SiO2 [850 nm] | VO2 [30 nm] archi tecture. The resulting Δε is 52.9%.
3.3. Thermochromic layer thickness
which are in close agreement with the experimental results shown in Ref. [8] (see Table 1). Also observed in Fig. 3, the absorption peak around 9–10 μm (Si-O-Si stretching bond) in the low temperature state has a significant impact on εL , while rendering the SRD slightly less effective at high temperature. From these results, it is clear that imple menting an IR-transparent material would be largely beneficial.
Another important parameter to consider is the impact of the thickness of the thermochromic VO2 film as too thin a layer may not present enough absorption in the resonant cavity mode, and too thick a layer may inhibit light from getting into the cavity itself. In Fig. 5, the thickness of the CaF2 cavity remains unchanged at 1400 nm, while the thickness of the VO2 is varied between 15 nm and 40 nm by 5 nm in crements. One can observe that the optimal thickness is between 15 and 25 nm, as a high absorption is observed near 8 μm without a significant increase in reflection at higher wavelengths. Table 3 lists the resulting Δε values. A final optimization was performed for a situation where both the cavity and the VO2 thicknesses were varied in order to obtain the ab solute maximum in emissivity variation using the previously established thickness values as starting points (see Fig. 6). The highest variation in emissivity is reached for a VO2 thickness in the 20 nm range and for a CaF2 thickness of ~1250 nm. It is important to note that an additional antireflective (AR) coating could also be added on top of the VO2 layer to further lower the reflection at high temperatures. However, in the
3.2. Ultra-low index material as the resonant cavity The use of an IR-transparent material is consequently the next logical step. In addition, it is worthwhile to consider what would be the ideal refractive index of such a material. Fig. 4 shows the results of various Au mirror | transparent cavity|VO2 [20 nm] architectures in which the cavity material’s refractive index was numerically varied between 1.17 and 2.17 @ 10 μm by 0.2 increments. The thickness of the layer was also adjusted to maintain the position of maximum absorption at λabs and thus optimize Δε. The resulting optical thickness (nt) of the cavity therefore slightly varied between 1700 nm and 1850 nm. The refractive 4
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lead to an increase in the absorption in the low temperature state; as a result, it has not been added to our prototype samples. 4. Experimental results 4.1. Individual layers Many challenges can occur during deposition, especially in the context of VO2 coatings as they require a high deposition temperature, an appropriate seed/barrier layer (Si3N4 in our case) and, most impor tantly, a precise control of the oxygen content to obtain the correct stoichiometry [20]. When depositing at high temperatures, interdiffu sion between the different layers of the stack and crystallization of these same layers can also take place. To potentially observe such changes, each layer was individually studied as well as sequentially characterized during each step of the deposition process. As was shown in Fig. 2, the experimental results of the Au mirror | Si3N4 [34 nm]|VO2 [36 nm] sample corresponded very well to the op tical model, indicating that the optical properties of the individual ma terials have been correctly assessed. Following this confirmation, we then proceeded with the deposition of a prototype Fabry-Perot-like SRD containing a CaF2 cavity layer. Fig. 7 shows the resulting reflection spectrum of the Au mirror |CaF2 [1240 nm] sample in comparison with the model. While there are some slight discrepancies, the reflectivity is on average predicted within 2% in the 3–15 μm range. Specifically, the intensity of the water absorption peaks at ~6.1 μm [21] are over estimated, most probably due to a change in the water content in the film, while the onset of absorption for the CaF2 above 17 μm is under estimated; nevertheless, these differences remain below 5%.
Fig. 5. Variation of the VO2 thickness from 15 to 40 nm in a Au mirror |CaF2 [1400 nm] |VO2 architecture. The shaded areas represent the normalized emission of a black body at low (blue) and high temperatures (pink). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.) Table 3 Emissivity variation for each VO2 thickness configuration as shown in Fig. 5. VO2 thickness
Δε
[nm]
[%]
15 20 25 30 35 40
75.3 76.0 74.0 70.8 67.0 63.0
4.2. SRD deposition The first prototype sample consisted in a CaF2 [960 nm] |Si3N4 [31 nm]| VO2 [46 nm] system, giving rise to Δε ¼ 56%. This result provided a proof of concept for the use of an ultra-low refractive index cavity and demonstrated that VO2 could grow on a porous calcium fluoride surface. Note that a Si3N4 film was nevertheless added to the stack in order to limit modifying the growth conditions of the VO2. The layers were then modeled and reoptimized, and the desired thickness for CaF2 was found to be between 1200 and 1400 nm as predicted in Fig. 6. Indeed, in the second set of depositions with a 1240 nm thickness of CaF2, all samples exhibited a large absorption band in the high tem perature state with Δε values greater than 60%, with the highest values
Fig. 6. Two-dimensional mapping of a Au mirror | CaF2 (800–1800 nm) | Si3N4 (30 nm) | VO2 (10–40 nm) architecture where the thicknesses of the CaF2 and VO2 are varied in the indicated intervals. The scale bar on the right side in dicates the emissivity variation.
presence of an ultra-low refractive index material such as CaF2, RðλÞ is already very low (< 10%) between the 8 and 10 μm, which reduces the need and the impact of the AR coating. Adding an extra layer could also
Fig. 7. Modeled (discontinuous line) and experimental (continuous line) reflection curves for a Au | CaF2 [1240 nm] sample. 5
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being 64.9% and 66.0% (see Fig. 8 and Table 4 for the results of a series of samples consisting of Au | CaF2 (1240 nm) | Si3N4 (31 nm) | VO2 (40 nm) - some slight modifications to the heating ramp rates and waiting times once the deposition temperature was reached were also tested). Although note shown here, optimized SRDs with ZnS cavities were also deposited (n ¼ 2.2 @ 10 μm) and, as predicted by the modeling approach, they exhibited a narrower absorption band in the high tem perature state thus limiting the obtained Δε values. One may note that the VO2 thickness was chosen as 40 nm despite the previous models indicating that a 15–25 nm thickness would be better suited. In fact, we did deposit thinner VO2 films but were unable to observe any beneficial impact on performance, most probably due to a change in their optical properties; the latter being intrinsically related to the growth and crystallinity of the films. Note that the optical properties were originally measured on a 70-nm-thick film (see Fig. 1) deposited directly on Si and as such, one may expect the seed layer to affect the growth conditions of the VO2. In Fig. 9, we compare Sample IV with the predicted model curves. We can observe that the trends are similar despite some differences which play in our favor. As the model fitted well before the deposition of the Si3N4/VO2 films, we surmise that these differences are most probably due to a combination of several effects: a) changes in the optical prop erties of the VO2 film as it is deposited on a rougher surface due to the presence of a thick and porous CaF2, b) potential oxidation of the Si3N4 layer due to the presence of an O2 plasma during the VO2 deposition, c) slight changes in the materials due to the high temperature of the deposition process and, finally, d) the possible presence of interdiffusion between adjacent layers. These effects can be studied in more details in future experiments.
Table 4 Measured emissivity variations for various prototype samples with a Au | CaF2 (1240 nm) | Si3N4 (31 nm) | VO2 (40 nm) architecture.
4.3. Solar absorption
Fig. 9. Comparison between the model (dotted lines) and the experimental results at low and high temperatures (continuous lines) for sample IV. The shaded areas represent the normalized emission of a black body at low (blue) and high temperatures (pink). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
This work’s focus is to optimize the emissivity variation (Δε) of the SRD. However, the solar absorption is also an important parameter, especially for low-orbit satellites. Using equation (2), sample V’s ab sorption was calculated to be 37.8%, a value which is, unfortunately, above the 20% ideal one [6]. Reducing the thickness of the VO2 layer should potentially lower αs as it absorbs between 300 nm and 2500 nm (see Fig. 1 for the optical properties of VO2). A 2D map of the absorption as a function of the CaF2 and VO2 thickness is shown in Fig. 10. While going thinner in VO2 does decrease αs , there are clearly limitations when using the present architecture.
Sample
εL (%)
εH (%)
Δε (%)
I II III IV V
14.1 16.8 17.5 14.8 13.9
80.1 79.4 78.9 79.6 78.2
66.0 62.5 61.4 64.9 64.4
Fig. 10. Two-dimensional mapping of a Au mirror | CaF2 (800–1800 nm) | VO2 (10–40 nm) architecture where the thicknesses of the CaF2 and VO2 are varied in the indicated intervals. The scale bar on the right side indicates the solar absorption at low temperature.
5. Conclusions In the present work, we have shown that a modeling-based approach can help further understand the impact of each individual layer on the
Fig. 8. Measured reflectivity at low temperature (25 � C) and high temperature (100 � C) on sample IV. The architecture of this sample was Au | CaF2 (1240 nm) | Si3N4 (31 nm) | VO2 (40 nm). 6
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overall performance of the SRD. For instance, an ultra-low refractive index cavity substantially enhances Δε as it broadens the absorption peak in the high temperature state, as also confirmed by the experi mental results with a CaF2 film with n ¼ 1.17 @ 10 μm. The modeling approach also allows one to choose the appropriate thicknesses for a given layer. Following a detailed optimization of the SRD design, an emissivity variation of up to 66% was obtained for a system consisting of a Au mirror | CaF2 (1240 nm) | Si3N4 (31 nm) | VO2 (40 nm) architec ture. Such values are fully compatible with spacecraft requirements such as in the case of nanosatellites. As previously discussed, future experi ments should focus on the impact of heating on the layers’ optical properties and on the interfaces between adjacent layers, as well as a more thorough study of VO2 growth on different seed layers. Finally, a multilayer design specifically aimed at reducing αS could be designed to lower the solar absorption to less than 20%. The metamaterial approach proposed by Sun et al. could potentially yield lower αS values due to a smaller coverage area of VO2 [10], and by combining it with our ultra-low refractive index material, further enhance the performance of SRDs for space applications.
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Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors wish to thank Mr. F. Turcot and Mr. C. Cl� ement for their technical assistance. This research was supported in part by the Natural Sciences and Engineering Research Council of Canada (NSERC) through the NSERC Multisectorial Industrial Research Chair in Coatings and Surface Engineering (grant IRCPJ 433808-11), and it has benefited from the partial support from the Fonds de recherche du Qu� ebec – Nature et technologies (FRQNT) through its grant to the Quebec Advanced Ma terials Strategic Research Cluster (RQMP). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.solmat.2019.110260. References [1] S. DelPozzo, Caleb Williams, Bill Doncaster, Nano/microsatellite Market Forecast, ninth ed., SpaceWorks, 2019, pp. 1–32, 2019.
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