CNT-Based Solar Thermal Coatings: Absorptance Vs. Emittance

coatings

Article CNT-Based Solar Thermal Coatings: Absorptance vs. Emittance

Yelena Vinetsky, Jyothi Jambu, Daniel Mandler * and Shlomo Magdassi * Institute of Chemistry, The Hebrew University of Jerusalem, Jerusalem 9190401, Israel; [email protected] (Y.V.); [email protected] (J.J.) * Correspondence: [email protected] (D.M.); [email protected] (S.M.)

Received: 15 October 2020; Accepted: 13 November 2020; Published: 17 November 2020

Abstract: A novel approach for fabricating selective absorbing coatings based on carbon nanotubes (CNTs) for mid-temperature solar–thermal application is presented. The developed formulations are dispersions of CNTs in water or solvents. Being coated on stainless steel (SS) by spraying, these formulations provide good characteristics of solar absorptance. The eﬀect of CNT concentration and the type of the binder and its ratios to the CNT were investigated. Coatings based on water dispersions give higher adsorption, but solvent-based coatings enable achieving lower emittance. Interestingly, the binder was found to be responsible for the high emittance, yet, it is essential for obtaining good adhesion to the SS substrate. The best performance of the coatings requires adjusting the concentration of the CNTs and their ratio to the binder to obtain the highest absorptance with excellent adhesion; high absorptance is obtained at high CNT concentration, while good adhesion requires a minimum ratio between the binder/CNT; however, increasing the binder concentration increases the emissivity. The best coatings have an absorptance of ca. 90% with an emittance of ca. 0.3 and excellent adhesion to stainless steel.

Keywords: carbon nanotubes (CNTs); binder; dispersion; solar thermal coating; absorptance; emittance; adhesion; selectivity

1. Introduction Solar energy conversion by concentrated solar power (CSP) systems is a leading technology in renewable energies. According to the Strategic Energy Technology (SET) Plan, the goal is that by 2030, up to 27% of EU electricity will be generated by solar energy. In that report, the global electricity share of CSP is envisioned to reach 11% by 2050 [1]. The spectrally selective solar coatings (SSCs) of the collector are an essential element in increasing the efficiency of mid-range temperature CSP systems for non-evacuated solar-trough or linear Fresnel collectors. An appealing material for solar absorbers, which is still not fully explored, is carbon nanotubes (CNTs) [2]. CNTs are known for their unique optical, electrical, and mechanical properties, and are considered as one of the best light-absorbing materials [3,4]. Moreover, CNTs exhibit high physical and chemical stability, a large surface area, and high thermal conductivity [5]. However, to obtain very high absorptance, >95%, which is a prerequisite for efficient solar thermal coatings, the CNTs must be grown from the vapor phase, which is suitable only for small-scale devices, by using costly equipment. Wang et al. reported the visible and the near-infrared radiative properties of vertically aligned, multiwalled carbon nanotubes [6]. The results showed that CNTs do not only reflect light weakly but also strongly absorb it. These combined features make CNTs an ideal candidate for realizing a “super black coating.” A CNT forest (composed of densely aligned nanotubes grown perpendicularly to the surface) has been used as an optical coating [7,8]. This type of coating shows

Coatings 2020, 10, 1101; doi:10.3390/coatings10111101 www.mdpi.com/journal/coatings Coatings 2020, 10, 1101 2 of 12 good light absorption and anti-reflective performance, but its production requires unique equipment and, specific conditions are required for chemical vapor deposition (CVD), such as high temperature and pressure. This process is expensive and has several drawbacks, including limited type substrates, small collector area, and poor adhesion to the substrate. Mizuno et al. fabricated vertically aligned CNTs that showed an extremely high absorptance of 0.98–0.99 [9]. Cao et al. constructed a tandem structure of aligned CNTs on Au, which exhibited an absorptance of 0.95 with no spectral selectivity [10]. Selvakumar et al. reported an absorptance of 0.95 and remarkably low emittance of 0.2 using a CNT-based tandem absorber fabricated by CVD [11,12]. The application of carbon nanotubes in solar collectors was reported by several researchers, as will be briefly described here. Abdelkader et al. reported the fabrication of solar selective coating for Flat plate solar air heaters (SAHs) by embedding CNTs and cupric oxide nanoparticle (CuO) into the black paint [13]. The coating showed high solar selective properties with solar absorptance and thermal emittance reaching 0.964 and 0.124, respectively. Li et al. demonstrated a high performance solar thermoelectric generator (STEG) system combined with solar concentrators and a carbon nanotubes absorber, which can greatly improve the solar–thermal conversion process [14]. The enhanced efficiency is ensured by the optimized system thermodynamics as a result of the combination of solar concentration devices and the CNT-based solar absorber. Sobhansarbandi et al. confirmed that CNT sheet coatings improved the solar energy phase change and also increased solar energy absorption [15]. In a different study, it was found that CNTs can increase the evaporation efficiency of a steam generation [16]. Kasaeian et al. evaluated the effect of carbon nanotube/ethylene glycol on a direct absorber solar collector attached to a parabolic trough [17]. The optical efficiency was 71.4%, and the thermal efficiency of the collector was 17% higher than that of the base fluid. Multiwalled carbon nanotubes (MWCNTs) were found to absorb almost 100% of solar energy as compared to other nanoparticles, making them ideal for application in a direct absorber solar collector (DASC) due to their smaller agglomeration and high stability [18]. Shende et al. investigated the DASC using MWCNTs as a working fluid and compared the behavior in water and ethylene glycol [19]. Wet deposition processes with CNTs as a functional material were also reported. For example, Rincon et al. fabricated spectrally selective coatings using a CNT–TiO2 composite by a sol-gel process combined with dip-coating, but these coatings exhibited low absorptance [20]. Roro et al. fabricated multiwalled CNT–nickel oxide nanocomposite coatings also by dip-coating, yielding an absorptance of 0.84 and emittance of 0.2 [2]. From the literature survey, it is evident that most of the CNT-based coatings suffer from low spectral selectivity. Making coatings composed of CNTs brings a major challenge in dispersing them in liquids. Bibi et al. reported the treatment of multiwalled carbon nanotubes with gamma irradiation in the air or by using dilute acids (H2SO4/HNO3) combined with 20 kHz ultrasound to compare their dispersion [21,22]. The CNT microstructure was investigated using transmission electron microscopy, which revealed that both methods effectively modified the CNTs to overcome aggregation of the nanotubes, resulting in efficient dispersion in ethanol. Raiah et al. reported that imidazolium-based ionic liquids with a long hydrocarbon chain enable high concentration dispersion of double-walled carbon nanotubes (DWCNTs) in water, having a low viscosity [23]. Dispersibility of DWCNTs increased as the length of the hydrocarbon chain increased. The suspensions of partially de-bundled DWCNTs proved to be stable for over a month. Ferreira et al. studied ionic liquid suspensions of CNTs functionalized with carboxylic and alkane groups in various solvents [24]. CNTs were functionalized using H2SO4/HNO3 and subsequently functionalized by dodecylamine (DDA). It was found that the dispersion stability was strongly dependent on the solvent and carbon nanotube surface interactions, which can vary with the chemical nature of the solvent. The work of Lee et al. [25] provided a comparative investigation of the thermal efficiency of a volumetric receiver using MWCNT–water nanofluid and a surface receiver with conventional fluid. The study demonstrated that the volumetric solar receivers (VRs) with water-based MWCNTs can achieve higher efficiency, compared to the conventional surface-based solar receivers over 10%. Coatings 2020, 10, 1101 3 of 12

Previous liquid formulations developed in our group for solar selective coatings [26–28] were directly applied on Inconel or aluminum substrates by spraying. The coatings had a multilayer structure composed of different materials with specific optical properties. Additionally, a self-cleaning CNT-based near-perfect solar absorber coating was fabricated for non-evacuated CSP applications [29]. The coating shows high durability and is super hydrophilic (0◦ contact angle) after plasma treatment, without affecting the solar absorptance and excellent coating adhesion. In the previous publication, a multilayer coating was presented, in which the first absorbing layer was made of CNTs, which gave high absorptance but also high emittance. The second layer was an infrared (IR)-reflecting layer composed of indium tin oxide (ITO). The ITO layer enabled the reduction of the emittance; however, it simultaneously reduced the absorptance. The third layer, which functioned as an anti-reflecting layer, was made of boehmite (AlOOH), whose presence made it possible to improve the absorptance without a significant change in the emittance. The outcome was a multilayer CNT/ITO/AlOOH coating with very good spectral characteristics. However, the fabrication of the multilayer coating is complex, and a simpler solution is required. Here we present novel formulations for a one-step fabrication of a single-layer CNT coating on stainless steel (SS), which is commonly used for the manufacturing of solar collector pipes. The coatings are made by water- or solvent-based liquid formulations composed of dispersed CNTs and a ceramic binder. We studied the effect of CNTs and the binder type and their concentrations on the absorptance, emittance, and adhesion of the thin coatings.

2. Materials and Methods

2.1. Materials Nanocyl (NC-7000TM) MWCNTs were purchased from Nanocyl SA, Sambreville, Belgium; Short Cheap Tube MWCNTs (outer diameter 10–20 nm) from Cheap Tubes, Cambridgeport, MA, USA; aluminum oxyhydroxide (AlOOH, boehmite) powder (Disperal P3) from SASOL, Germany; SILRES REN 100 (silicone resin) from Wacker Chemie AG, Munchen, Germany; BYK 333 and BYK 9077 from Byk-Chemie GmbH, Wesel, Germany; Solsperse 46,000 from The Lubrizol Corporation; methyltrimehtoxysilane (MTMS) from Alfa Aesar, Heyshem, UK; and N,N-dimethylformamide (DMF) from Fischer Scientiﬁc, Loughborough, UK.

2.2. Coating with Water-Based CNTs Dispersion The first 0.24 g of Cheap Tubes CNTs was placed in a 28 mL vial and 0.12 g of Solsperse 46,000 was added, followed by the addition of 4 g of propylene glycol. Then 15.64 g of distilled water was added, and the mixture was sonicated for 7.5 min at 750 W in pulses for 1 s on and 1 s off with an amplitude of 85% (Sonics Vibra Cell Sonicator, Sonics & Materials, Inc., Newtown, CO, USA). For binder preparation, 10% of AlOOH was dispersed in distilled water by stirring for 2 h and then 1 g of MTMS was added, followed by stirring for 2.5 h. For the preparation of the final composition, 5 g of CNT dispersion, 3.65 g of distilled water, 0.1 g of wetting agent (Byk333), and 1.25 g of the binder were mixed and stirred for 24 h. The obtained formulation (2.0 mL) was sprayed onto a heated substrate (60–80 C) with an area of 5 5 cm2. ◦ × The sample was initially heated to 100 ◦C with a heating rate of 5 ◦C/min for 30 min. Then the sample was heated to 350 ◦C with a heating rate of 10 ◦C/min for 30 min. Finally, it was heated to 400 ◦C with a heating rate of 10 ◦C/min for 60 min. Flat metal substrate was thoroughly washed with aqueous soap and then rinsed with ethanol and acetone. Coatings 2020, 10, 1101 4 of 12

2.3. Coating with Solvent-Based CNT Dispersion An amount of 0.1 g of Nanocyl CNTs was placed in a 28 mL vial and 2 g of 10% Byk-9077 and 17.9 g of DMF were added. The mixture was sonicated for 20 min at 750 W in pulses of 2 s on and 1 s oﬀ with an amplitude of 85% (Sonics Vibra Cell Sonicator, Sonics & Materials, Inc., USA). An amount of 6 g of Ren 100 (10% in DMF) was added to the CNT dispersion, the obtained mixture was shaken well and then sonicated in a bath for 5 min. The coating was prepared by spraying 5 mL of the mixture onto a heated (100–120 C) substrate, with an area of 5 5 cm2. The samples were ◦ × cured in an oven at 350 ◦C for 2 h with a heating rate of 15 ◦C/min.

2.4. Absorptance and Emittance Measurements The reflectance spectra of the coatings were recorded in the region of 280–2500 nm. With a CARY 5000 UV–vis–NIR spectrophotometer (Varian Analytical Instruments, Palo Alto, CA, USA) using an integrated sphere. The absorptances of the coatings were calculated from the UV–vis–NIR (NIR: near-infrared) reflectance spectra using the equation α + R + T = 1, in which α is the absorptance, R is the reflectance, and T is the transmittance. As the coatings were fabricated on a stainless steel substrate, the transmittance is zero and therefore, the absorptance α = 1 R. The contribution of nanotubes was − determined by comparing the complete sample reflectivity with the reflectivity of the stainless steel substrate alone, as presented in the Supplementary Information and Figure S1. Emittance measurements were carried out by the Emissometer detector connected to the RD1 voltmeter reading display (Devices & Services Company, Dallas, TX, USA).

2.5. Thickness Measurement The thickness of the coatings was measured by Focused Ion Beam Scanning Electron Microscopy (FIB-SEM, Model Helios Nanolab 460F1 Lite, Thermo Fisher Scientiﬁc, Hillsboro, OR, USA).

2.6. SEM Measurements SEM images were obtained using a UHR-SEM (Ultra High-Resolution Scanning Electron Microscope) Magellan 400 L produced by FEI (Field Emission Instruments, Noord-Brabant, The Netherlands).

2.7. Adhesion Tests Adhesion tests were performed by using the Cross-Cut Tester 1 mm according to standards ASTM D3359 [30] and ISO 2409. [31] In this test, a lattice pattern was cut into the coating, penetrating through the substrate. A tape was then placed on the cut pattern and peeled. The coating area was observed and the adhesion was rated, following a standard scale.

3. Results and Discussion In our recent studies, CNT coatings were deposited on aluminum and Inconel substrates from solvent-based dispersions [27,28]. Since pipes for solar selective absorbers are usually manufactured from SS, we studied the eﬃciency of CNT coatings on this substrate as solar selective absorbers.

3.1. Coatings with Various Types of CNTs Scheme1 illustrates the fabrication of the CNT coatings on SS substrate. The liquid coating formulation was sprayed onto the substrate, followed by a heat treatment, to convert the pre-ceramic polymer into a ceramic matrix. Preliminary tests were performed with several types of multiwalled CNTs (MWCNTs). The most stable and low viscosity formulations were obtained with a 0.6% water-based dispersion of Cheap Tubes CNTs (α = 97.3%, ε = 0.79) and with a 0.38% solvent-based Coatings 2020, 10, 1101 5 of 12 dispersion of Nanocyl CNTs (α = 94.8%, ε = 0.80). These coatings have high adhesion to stainless steel CoatingssubstratesCoatings 20 202020, (95%),10 10, ,x x FOR FOR due PEER PEER to theREVIEW REVIEW presence of the binder. 55 of of 12 12

SchemeScheme 1. 1. Fabrication Fabrication of of carbon carbon nanotube nanotube ( CNT( (CNT)CNT) )coating coating on on a a stainless stainless steel steel substrate. substrate.

TheThe final final ﬁnal absorptance absorptance absorptance of of ofthe the thecoating coating coating depends depends depends on on the the on CNT CNT the concentration CNTconcentration concentration in in the the dispersion, dispersion, in the dispersion, as as well well asas on well on the the as ability on ability the ability of of the the of CNTs CNTs the CNTs to to form form to form a a high high a high-quality-quality-quality dispersion dispersion dispersion without without without agglomeration agglomeration agglomeration or or or rapid rapid sedimentation.sedimentation. It It It was was was found found found that that thethat water-based the the water water-based- formulationbased formulation formulation must contain mus must a t higher contain contain CNT aa higher concentrationhigher CNT CNT concentrationtoconcentration achieve the to to same achieve achieve absorptance the the same same that absorptance absorptance is obtained that that for is is a obtained obtained solvent-based for for a a solvent CNTsolvent dispersion.-based-based CNT CNT Nevertheless, dispersion. dispersion. Nevertheless,theNevertheless, Cheap Tubes thethe type CheapCheap of CNTs TubesTubes was typetype better of of dispersed CNTs CNTs waswas in water,betterbetter and dispersed dispersed the obtained in in water, water, formulations and and the the were obtained obtained more formulationsstableformulations as compared were were more more with stable solvent-basedstable as as compared compared ones. w withith solvent solvent--basedbased ones. ones. AA coatingcoating coating withwith with goodgood good spectrallyspectrally spectrally selectiveselective selective propertiesproperties properties is is characterizedcharacterized is characterized byby highhigh by high absorptance,absorptance, absorptance, lowlow emittance,lowemittance, emittance, good good good adhesion adhesion adhesion to to the the to underlying underlyingthe underlying metal metal metal substrate, substrate, substrate, and and andhigh high high thermal thermal thermal stability stability stability for for an for an extendedanextended extended duration. duration. duration. In In general, general, In general, all all formulati formulati all formulationsonsons and and types andtypes typesof of CNTs CNTs of CNTs resulted resulted resulted in in high high in absorptance, highabsorptance, absorptance, very very goodverygood good adhesion, adhesion, adhesion, and and highand high high thermal thermal thermal stability stability stability of of the of the the coatings. coatings. coatings. However, However, However, the the the disadvantage disadvantage disadvantage of of of these these coatingscoatings was was their their high high emittance emittance (0.80). (0.80).(0.80). FigureFigure 1 1 shows shows SEMSEM imagesimages ofof the thethe coatings coatingscoatings prepared preparedprepared with withwith various variousvarious CNTs CNTsCNTs disperseddispersed inin waterwater andand solvents.solvents. The The diameter diameter of of the the CNTs CNTs was was in in the the range range of of 15 15 15–100––100100 nm nm and and they they were were densely densely packed packed and and wellwell embedded embeddedembedded within withinwithin the thethe polymeric polymericpolymeric matrix. matrix.matrix.

aa bb

Figure 1. SEM images of coatings fabricated with an aqueous dispersion of Cheap Tubes CNTs (a), FigureFigure 1. 1. SEMSEM images images of of coatings coatings fabricated fabricated with with an an aqueous aqueous dispersion dispersion of of Cheap Cheap Tube Tubess CNTs CNTs (a (a),), and a solvent-based dispersion of Nanocyl CNTs (b). andand a a solvent solvent-based-based dispersion dispersion of of Nanocyl Nanocyl CNTs CNTs ( (bb).).

3.2.3.2. Coatings Coatings with with a a Water Water Water-Based-Based-Based Dispersion Dispersion of of CNTs CNTs HighHigh thermal thermal stability stability and and good good adhesion adhesion of of the the coatings coatings require require a a binder binder in in the the formulation formulation suchsuch as as silicon silicon resin resin Silres Silres REN REN REN-100-100-100 for for solvent solvent solvent-based-based-based dispersion dispersion and and a amixture mixture mixture of of of boehmite boehmite boehmite (AlOOH): (AlOOH): (AlOOH): polysiloxanepolysiloxane (MTMS) (MTMS) for forfor water-based waterwater--basedbased dispersion. dispersion. dispersion. However, However, However, it was it it was was found found found that that the that presence the the presence presence of a binder of of a a bindergreatlybinder greatly increases greatly increases increases the emittance the the emittance emittance of the coating. of of the the This coating. coating. eﬀect was This This studied effect effect was for was various studied studied CNT for for concentrations, various various CNT CNT concentrationsasconcentrations shown in Figure, ,as as shown shown2 for coatings in in Figure Figure with 2 2 for for and coatings coatings without with with binders. and and without without binders. binders.

Coatings 2020, 10, x FOR PEER REVIEW 6 of 13

Coatings 2020, 10, x FOR PEER REVIEW100 1.0 6 of 12 0.9 95 0.8

100 90 1.0 0.7 CoatingsCoatings2020 2020, 10, 10, 1101, x FOR PEER REVIEW 0.9 66 of of 12 12 0.6 95 85 0.8 0.5 0.4

0.7 Emittance 90 80

Adsorptance,% 0.3 100 1.0 0.6 0.2 85 75 0.9 0.5 0.1 95 0.4

0.8 Emittance 80 70 0.0

Adsorptance,% 0.00 0.02 0.04 0.06 0.08 0.100.3 0.12 0.14 0.02 0.04 0.06 0.08 0.10 0.12 0.14 90 0.7 CNTs,% 0.2 75 0.6 CNTs,% 85 0.5 0.1 70 0.4 Figure 2. The absorptance and emittance0.0 of coatings made of various CNT concentrations, prepared

0.00 0.02Emittance 0.04 0.06 0.08 0.10 0.12 0.14 80 with and without binder composed of0.02 boehmite0.04 (AlOOH0.06 ) 0.08and polysiloxane0.10 0.12 (MTMS0.14 ) with a ratio of Adsorptance, % 0.3 CNTs,% ◆ CNTs,% 0.2 2:1. The binder/CNT weight ratio was 8.4:1 ( —without binders, █—with binders). 75 Figure 2. The absorptance0.1 and emittance of coatings made of various CNT concentrations, prepared 70 0.0 0.00 0.02 0.04 0.06 0.08 0.10 with0.12 and0.14 without0.02 binder0.04 composed0.06 of0.08 boehmite0.10 (AlOOH0.12 ) 0.14and polysiloxane (MTMS) with a ratio of CNTs,% 2:1. The binder/CNT weight◆ ratio wasCNTs,% 8.4:1 (◆—without binders, █—with binders).

FigureFigure 2. 2.The The absorptance absorptance and and emittance emittance of of coatings coatings made made of of various various CNT CNT concentrations, concentrations, prepared prepared withwith and and without without binder binder composed composed of of boehmite boehmite (AlOOH) (AlOOH and) and polysiloxane polysiloxane (MTMS) (MTMS with) with a ratio a ratio of 2:1. of The2:1. binderThe binder/CNT/CNT weight weight ratio wasratio 8.4:1was (8.4:1◆—without (◆—without binders, binders,█—with ■— binders).with binders).

ItIt can can be be seen seen that that decreasing decreasing the CNT the concentration CNT concentration led to a decrease led to a of decrease emittance of and emittance absorptance. and Sinceabsorptance. the volumes Since of the the volumes sprayed formulationof the sprayed were formulation constant, the were above constant, trend in the absorptance above trend and in It can be seen that decreasing the CNT concentration led to a decrease of emittance and emittanceabsorptance was and probably emittance due was to reducingprobablyIt the due can film to be reducing thickness. seen that the Interestingly,decreasingfilm thickness. the there I CNTnterestingly, was concentration no significant there was led to a decrease of emittance and absorptance. Since the volumes of the sprayed formulation were constant, the above trend in changeno significant in the emittancechange in atthe various emittanceabsorptance. concentrations at various Since concentrations of CNTs the volumes with of binders. CNTs of the with This sprayed binders. means formul thatThisation atmeans low were constant, the above trend in absorptance and emittance was probably due to reducing the film thickness. Interestingly, there was CNTthat at concentrations low CNT concentrations the binder isthe responsibleabsorptance binder is responsible for and most emittance of for the most emittance.was of probably the emittance. Yet, due while to Yet,reducing allthe while coatings the all filmthe thickness. Interestingly, there was no significant change in the emittance at various concentrations of CNTs with binders. This means containing binders had good adhesion,no significant formulations change without in the binders emittance had poorat various adhesion. concentrations Therefore, of CNTs with binders. This means coatings containing binders had good adhesion, formulationsthat at low without CNT co bindersncentrations had thepoor binder adhesion. is responsible for most of the emittance. Yet, while all the the presence of a binder is, on one hand,that essential at low CNT for good concentrations adhesion and, the on binder the other is responsible hand, increases for most the of the emittance. Yet, while all the Therefore, the presence of a binder is, on one hand, essentialcoatings containingfor good adhesion binders had and, good on theadhesion, other formulations without binders had poor adhesion. emittance significantly. All our attemptscoatings to findcontaining a ceramic binders binder had with good significantly adhesion, formulations low emittance without binders had poor adhesion. hand, increases the emittance significantly. All ourTherefore, attempts the to presence find a of ceramic a binder binder is, on one with hand, essential for good adhesion and, on the other were unsuccessful. Therefore, the presence of a binder is, on one hand, essential for good adhesion and, on the other significantly low emittance were unsuccessful. hand, increases the emittance significantly. All our attempts to find a ceramic binder with hand, increases the emittance significantly. All our attempts to find a ceramic binder with TableTable1 1shows shows the the absorptance absorptance and and emittance emittance of of a significantlya few few water-based water -lowbased emittance formulations formulations were withunsuccessful. with various various binders.binders. A constant constant concentration concentration ofsignificantly of Cheap Cheap Tube Tubes lows CNTs CNTsemittance of ofTable0.032% 0.032% were 1 showswas unsuccessful. was used the used inabsorptance all in formulations. all formulations. and emittance This of a few water-based formulations with various Thisconcentration concentration gave gave the thehighest highest difference differenceTable in in 1emissivity emissivityshows binthe betweenders. betweenabsorptance A constant formulations formulations and concentration emittance with and of Cheapa withoutwithout few waterTube aa s- CNTsbased offormulations 0.032% was usedwith in various all formulations. This binderbinder (Figure (Figure2 ).2). binders. A constantconcentration concentration gave of Cheap the highest Tubes differenceCNTs of 0.032% in emissivity was used between in all formulations.formulations withThis and without a concentration gave binderthe highest (Figure difference 2). in emissivity between formulations with and without a TableTable 1. 1.E Effectffect of of binder binder type type for for water-based waterbinder-based (Figure formulations formulations 2). containing containing 0.032% 0.032% CNTs CNTs on on the the selective selective propertiesproperties of of the the coatings. coatings. Binder Binder/CNT/CNT ratio ratio was was 8.4:1. 8.4:1. Table 1. Effect of binder type for water-based formulations containing 0.032% CNTs on the selective Table 1. Effect of binderproperties type for of waterthe coatings.-based Binderformulations/CNT ratio containing was 8.4 :1.0.032% CNTs on the selective Binder Absorptance (α) Emittance (ε) Binder propertiesAbsorptance of the coatings. (α) Binder Emittance/CNT ratio (ε) was 8.4:1. WithoutWithout Binders Binders 80.7 80.70.1 ± 0.1 0.270.27 ±0.02 0.02 Binder Absorptance (α) Emittance (ε) ± ± AlOOH 86.4 0.3Binder 0.64 0.03 WithoutAbsorptance Binders (α) Emittance80.7 ± 0.1 (ε) 0.27 ± 0.02 AlOOH 86.4± ± 0.3 0.64± ± 0.03 MTMS 81.6 0.5 0.66 0.03 AlOOH 86.4 ± 0.3 0.64 ± 0.03 MTMS 81.6± ± 0.5Without Binders0.66± ± 0.03 80.7 ± 0.1 0.27 ± 0.02 AlOOH and MTMS (2:1) 87.0 0.1 0.78 0.04 MTMS 81.6 ± 0.5 0.66 ± 0.03 AlOOH and MTMS (2:1) 87.0± ± 0.1 AlOOH0.78± ± 0.04 86.4 ± 0.3 0.64 ± 0.03 MTMS AlOOH and MTMS81.6 ± 0.5(2:1) 87.00.66 ±± 0.10.03 0.78 ± 0.04 TheThe resultsresults showshow that while the the coating coating without without theAlOOH the binder binder and had hadMTMS an an emittance (2:1) emittance of 87.0 ofonly only ± 0.27,0.10.27, the 0.78 ± 0.04 The results show that while the coating without the binder had an emittance of only 0.27, the the addition of any of the binder components, i.e., AlOOH or MTMS, increased the emittance to addition of any of the binder components, i.e., AlOOHaddition or MTMS, of any increased of the binder the emittance components, to 0.64 i.e.,– AlOOH or MTMS, increased the emittance to 0.64– 0.64–0.78. Interestingly, the presence of AlOOHThe results [27], show enhanced that while the absorptance. the coating Sincewithout the the presence binder had an emittance of only 0.27, the 0.78. Interestingly, the presence of AlOOH [27], enhanced0.78. Interestingly,the absorptance. the presenceSince the of presence AlOOH [27],of a enhanced the absorptance. Since the presence of a of a binder is necessary for good adhesionaddition to of the any substrate, of the binder we evaluated components, the ei.e.,ffect AlOOH of binder or/ CNTMTMS, increased the emittance to 0.64– binder is necessary for good adhesion to the substrate, binderwe evaluated is necessary the effect for good of binder/CNT adhesion to theweight substrate, we evaluated the effect of binder/CNT weight weight ratio on the properties of the0.78. coatings. Interestingly, These experiments the presence were of carriedAlOOH out [27], with enhanced formulations the absorptance. Since the presence of a ratio on the properties of the coatings. These experiments were carried out with formulations containingcontaining 0.064% 0.064% of of CNTs CNTs (Table (Table2). 2).binder is necessary for good adhesion to the substrate, we evaluated the effect of binder/CNT weight ratio on the properties of the coatings. These experiments were carried out with formulations containing 0.064% of CNTs (Table 2).

Coatings 2020, 10, 1101 7 of 12

Coatings 2020, 10, x FOR PEER REVIEW 7 of 12 Table 2. Eﬀect of binder/CNT weight ratio on the properties of the coatings with 0.064% CNTs. TheTable properties 2. Effect of of the binder/CNT coating with weight 0.128% ratio CNTs on arethe presentedproperties for of comparison.the coatings with 0.064% CNTs. The propertiesCNTs, of the % coating Binder with 0.128%/CNT RatioCNTs are presentedα,% for comparison.ε Adhesion CNTs, % Binder/CNT8.4:1 Ratio 94.9 α0.05, % ε 0.60 Adhesion 65%–85% ± 4:18.4:1 93.794.90.02 ± 0.05 0.60 0.43 65%–85% ± 0.064 2:14:1 92.893.70.05 ± 0.02 0.43 0.38 0.064 2:1 92.8± ± 0.05 0.38 <35% 1:1 92.1 0.07 0.39 <35% 1:1 92.1± ± 0.07 0.39 0:1 92.3 0.06 0.35 0:1 92.3± ± 0.06 0.35 0.128 8.4:1 96.0 0.03 0.80 >95% 0.128 8.4:1 96.0± ± 0.03 0.80 >95%

TheThe results results show show that that decreasing decreasing the binder the binder/CNT/CNT ratio decreased ratio decreased significantly significantly the emittance the emittance while affwhileecting affecting only slightly only theslightly absorptance. the absorptance. This decrease This decrease is presumably is presumably attributed attributed to the thinning to the thinning of the coatings.of the coatings. At the same At time,the same the adhesion time, the was adhesion negatively was anegativelyffected by the affected decrease by in the the decrease binder/ CNT in the ratio.binder/CNT Specifically, ratio. the adhesionSpecifically, with the the adhesion highest binder with concentration the highest (i.e.,binder 8.4:1 concentration binder/CNT ratio(i.e., and 8.4:1 0.128%binder/CNT CNTs) ratio gave and an adhesion 0.128% CNTs) of 95%, gave while an theadhesion adhesion of 95%, decreased while the to ca. adhesion 65%–85% decreased at the CNT to ca. concentration65%–85% at 0.064% the CNT and the concentration same binder 0.064%/CNT ratio and of the 8.4:1. same At lower binder binder/CNT/CNT ratio ratios, of 8.4:1. the adhesion At lower wasbinder/CNT poor, i.e., onlyratios, 35%. the adhesion was poor, i.e., only 35%. ThicknessThickness measurements measurements carried carried out out by by FIB-SEM FIB-SEM showed showed that that a binder a binder/CNT/CNT ratio ratio of 8.4:1of 8.4:1 gave gave a coatinga coating thickness thickness of 1.1–1.2 of 1.1–µ1.2m, μ twicem, twice than than that that obtained obtained without without the bindersthe binders (0.46–0.52 (0.46–0.52µm). μm). ItIt should should be b notede noted that that a twofolda twofold increase increase in in the the CNT CNT concentration concentration (from (from 0.064 0.064 to to 0.128) 0.128) at at the the samesame binder binder/CNT/CNT ratio ratio increased increase slightlyd slightly the absorptance the absorptance (due to (due thickening to thickening of the coating); of the however, coating); thehowever, emittance the increased emittance substantially increased substantially from 0.6 to 0.8.from The 0.6 selectivity to 0.8. The of selectivi the coatingsty of the (α/ εcoatings) is presented (α/ε) is inpresented Figure3. Asin seen,Figure the 3. selectivityAs seen, the decreased selectivity with decreased increasing with the increasing binder /CNT the ratio,binder even/CNT though ratio, theeven totalthough absorptance the total slightlyabsorptance increased. slightly increased.

3.0

2.5

2.0

a/e 1.5

1.0

0.5

0.0 0 1 2 3 4 5 6 7 8 9 10 Binder/CNTs ratio

FigureFigure 3. 3.The The selectivity selectivity (α (/αε)/ε as) as a functiona function of of binder binder/CNT/CNT ratio. ratio.

TheThe UV–vis–NIR UV–vis–NIR absorptance absorptance spectra spectra of of CNTs CNT coatingss coatings with with and and without without a bindera binder are are shown shown in in FigureFigure4. The 4. The broad broad absorptance absorptance band band in the in solar the spectrum solar spectrum with a minimum with a minimum at 1100 nm at was 1100 observed nm was forobserved the CNTs for coating the CNT withouts coating a binder,without whereasa binder, introducing whereas introducing the binder the caused binder an caused increase an increase of the absorptanceof the absorptance at wavelengths at wavelengths range between range 550between and 1550 550 nmand This 1550 explained nm This theexplained increase the in emittanceincrease in withemittance the addition with the of theaddition binder. of the binder.

Coatings 2020, 10, 1101 8 of 12 Coatings 2020, 10, x FOR PEER REVIEW 8 of 12 Coatings 2020, 10, x FOR PEER REVIEW 8 of 12 100 100

95 95

90 90

Absorptance, % 85 Absorptance, % 85

80 80 0 500 0 1000 5001500 10002000 15002500 2000 2500 Coatings 2020, 10, x FOR PEER REVIEW 9 of 12 Wavelength, Wavelength,nm nm

Figure3.3. 4. CoatingsAbsorptance with spectra Solvent of-Based CNT Dispersion coatings without of CNT binders (red line) and with a binder/CNT ratio Figure 4. AbsorptanceFigure 4. spectraAbsorptance of CNT spectra coatings of withoutCNT coatings binders without (red line) binders and with (red a binderline) and/CNT with a binder/CNT of 8.4:1 (blue line). ratio of 8.4:1 (blueratio line). of 8.4:1 Since (blue the line). use of formulations based on an aqueous dispersion of CNTs did not result in good selectivity, coatings based on dispersions of CNTs in an organic solvent were evaluated. As shown in Table1, the absorptance increased due to the presence of AlOOH in the binder’s As shown inAs Table shown 1, theFirs in absorptancet,Table the effect 1, the of increased absorptance CNT concentration due increasedto the presenceon thedue absorptance to of theAlOOH presence and in the emittance of binder’ AlOOH ofs coatingsin the binder’ obtaineds by mixture. Additionally, we prepared a two-layer coating, where the ﬁrst layer was made of 0.064% CNTs mixture. Additionally,mixture. Additionally,a we solvent preparedFigure 5 -based wea dispersiontwo prepared-layer coatof a Nanocyl twoing,- layerwhere CNTs coat the without ing,first wherelayer a binder was the madefirstwas studied.layer of 0.064% was As madepresented of 0.064% in Figure 6, dispersion without the binder. The second layer was deposited by spraying on top of the ﬁrst layer, CNTs dispersionCNT swithout dispersionthe theabsorptance binder. without The increasedthe second binder. withlayer The CNT was second depositedconcentration layer wasby spraying and deposited leveled on top byoff spraying ofat 0.25%the first CNTson top, where of the itfirst reached a mixture of binders (AlOOH:MTMS with ratio 2:1) at various binder/CNT ratios, which was thermally layer, a mixturelayer, of abinders mixture96.6%. (AlOOH:MTMS Theof binders emittance (AlOOH:MTMS alsowith increased ratio 2:1) and withat various leveled ratio binder/CNT2:1) off at at 0.25% various andratios, binder/CNT reached which 0.74. was ratios, These coatingswhich was adhered treated as before (Figure5). thermally treatthermallyed as before verytreat (Figurepoorlyed as before to5). the (Figuresubstrate 5). and the addition of a binder was crucial. 100 1.0 100 100 100 1.0 1.0 1.0 0.9 95 95 0.9 0.9 95 95 0.9 0.8 0.8 0.8 90 0.8 90 90 90 0.7 0.7 0.7 0.7 85 85 85 0.6 0.6 0.6 85 0.6 80 80 0.5 0.5 0.5 80 80 0.5 0.4 0.4 0.4 75 Emittance

75 Emittance Emittance 75 75 0.4 0.3 Emittance Absorpsion, % 0.3 0.3

Absorpsion, % 70 Absorptance, % 70 0.3 Absorpsion, % 70 70 0.2 0.2 0.2 65 65 0.2 0.1 0.1 0.1 65 65 0.1 60 60 0.0 0.0 0.0 0 1 260 01234567893 40.0 5 60.1 7 0.28 9 0.3 0.4 0.0 0.5 Binder/CNTs0 1 2 ratio3 4 5 6 7 8 9 Binder/CNTsCNTs concentration, ratio % Binder/CNTs ratio Figure 5. AbsorptanceFigure 5. andAbsorptanceFigure emittance 6. Absorptance as and a function emittance and of emittance asbinder a function/CNT of coatings ratio of binder (calculated as/ CNTa function ratio from (calculatedof the Nanocyl content fromCNT theconcentration content in in an

in each layer)Figureeach for layer) a 5. two AbsorptanceN,N’- forlayer a- two-layerdimethylformamide coating. and coating. CNT emittance concentration CNT (DMF) as concentration a function-based in the formulation of formulation inbinder the formulation/CNT without was ratio 0.064%. was (calculated a binder. 0.064%. (◆— (from◆—Absorptance,—Absorptance, the content ■— Absorptance, in■—Emittance). — eachEmittance) layer)Emittance).Figure 6 . for a two-layer coating. CNT concentration in the formulation was 0.064%. (◆— Absorptance, ■—Emittance). It can be seenIt that can as be theFigure seen binder/CNT 7 that shows as ratio the effectdecreased binder of/ CNTthe, the binder/CNT ratioemittance decreased, ratiodecreased, on thethe as absorptance emittance well, without decreased, and emittance as well,for 0.05% affecting thewithout absorptance. aﬀandecting Yet,0.1% thethe CNTs. absorptance.binder/CNT The binder ratio Yet, used played the was binder alsoa pre/ CNTa -majorceramic ratio role polymer, played in the alsoadhesion i.e., a Silres major to Ren the role 100. in theInterestingly, adhesion this It can be seen that as100 the binder/CNT ratio decreased, the emittance decreased,1.0 as well, without substrate. Onlyto thecoatings substrate.binder with adid Only high not coatingsbinder/CNT affect noticeably with ratio a high (8.4:1) the binder absorptancehad /goodCNT adhesion ratio and (8.4:1) emittance (more had than, goodas 95%) compared adhesion to to (more the binder than-free affecting the absorptance. Yet, the binder/CNT ratio played also a major role in the adhesion to the the SS substrate.95%) Hence, to theformulations, SSthe substrate. binder/CNT for Hence, all ratio binder/CNTthe was binder a compromise ratios./CNT H ratioowever, between was the acompromise good binder adhesion concentration between0.9 and low goodsignificantly adhesion affected and the substrate. Only coatings with95 a high binder/CNT ratio (8.4:1) had good adhesion (more than 95%) to emittance. Itlow can emittance.be coadhesionncluded It can thatof the be the concludedcoating use of to water the that SS-based thesubstrate. use dispersions of water-basedFor 0.05% with CNTs a dispersionslow at concentration binder/CNT0.8 with aratios of low 1.5:1 concentration and lower, the the SS substrate. Hence, the binder/CNT ratio was a compromise between good adhesion and low CNTs (0.064%)of CNTsmakes (0.064%)itadhesion possible makesdrastically to obtain90 it possiblecoatings decreased with to and obtain a relativelydropped coatings tohigh value with absorptance <35% a relatively at binder/CNT of 92.8. high Under absorptance ratios 0.75:1 (Table of 92.8. 3). At emittance. It can be concluded that the use of water-based dispersions with0.7 a low concentration of these conditions,Under it was thesethe impossible conditions, same time, to achieveit for was 0.1% impossibleboth CNTs, good adhesion adhesion to achieve began and both emittance to good decrease adhesion lower already than and 0.59. at emittance a binder/CNT lower ratio than of 2:1. CNTs (0.064%) makes it possible85 to obtain coatings with a relatively high absorptance0.6 of 92.8. Under 0.59. Therefore, for good adhesion of this coating, the binder/CNT ratio should not be less than 3:1. these conditions, it was impossible to achieve both good adhesion and emittance0.5 lower than 0.59. 80 0.4 75 Emittance 0.3

% Absorptance, 70 0.2 0.1 65 0.0 0.0 0.1 0.2 0.3 0.4 0.5 CNTs concentration, %

Coatings 2020, 10, x FOR PEER REVIEWFigure 5 8 of 12

100

95 100 1.0 95 0.9 0.8 90 90 0.7 85 0.6

Absorptance, % 85 80 0.5

75 0.4 Emittance Coatings 2020, 10, 1101 0.3 9 of 12 80 Absorptance, % 70 0 500 1000 1500 2000 2500 0.2 Coatings 2020, 10, x FOR PEER REVIEW 9 of 12 65 Wavelength, nm 0.1 3.3. Coatings with Solvent-Based Dispersion of CNT 3.3. Coatings with60 Solvent-Based Dispersion of CNT 0.0 Figure 4. AbsorptanceSince the spectra use of of formulationsCNT coatings0123456789 without based onbinders an aqueous (red line) dispersionand with a binder of CNTs/CNT did not result in good ratio of 8.4:1 (blue line). Since the use of formulations based on an aqueous dispersion of CNTs did not result in good selectivity, coatings based on dispersionsBinder/CNTs of CNTs in an ratio organic solvent were evaluated. First,selectivity the effect, coatings of CNT concentrationbased on dispersions on the absorptanceof CNTs in an and organic emittance solvent of were coatings evaluated. obtained by a As shown in Table 1, Firsthe t,absorptance the effect of increased CNT concentration due to the onpresence the absorptance of AlOOH and in theemittance binder’ ofs coatings obtained by solvent-based dispersion of Nanocyl CNTs without a binder was studied. As presented in Figure6, mixture. Additionally,a wesolvent prepared -based a dispersion two-layer ofcoat Nanocyling, where CNTs the without first layer a binder was wasmade studied. of 0.064% As presented in Figure 6, the absorptance increased with CNT concentration and leveled off at 0.25% CNTs, where it reached CNTs dispersion withoutthe absorptancethe binder. The increased second with layer CNT was concentrationdeposited by sprayingand leveled on topoff atof 0.25%the first CNTs , where it reached 96.6%. The emittanceFigure 6 also increased and leveled off at 0.25% and reached 0.74. These coatings adhered layer, a mixture of binders96.6%. (AlOOH:MTMS The emittance also with increased ratio 2:1) and at variousleveled offbinder/CNT at 0.25% and ratios, reached which 0.74. was These coatings adhered very poorly to the substrate and the addition of a binder was crucial. thermally treated as beforevery poorly(Figure to 5). the substrate and the addition of a binder was crucial.

100 1.0 100 100 1.0 1.0 0.9 0.9 0.9 95 95 95 0.8 0.8 0.8 90 90 90 0.7 0.7 0.7 85 85 85 0.6 0.6 0.6 80 0.5 0.5 0.5 80 80 0.4 0.4 0.4 75 Emittance Emittance

Emittance

75 75 0.3 Absorpsion, % 0.3 0.3

Absorpsion, % 70 % Absorptance, 70 70 0.2 0.2 0.2 65 0.1 0.1 0.1 65 65 60 0.0 0.0 0.0 0 1 2 0.03 40.0 0.15 0.16 0.27 0.28 0.39 0.3 0.40.4 0.5 0.5 Binder/CNTsCNTs ratio CNTsconcentration, concentration, % %

Figure 5. AbsorptanceFigure 6. andFigureAbsorptance emittance 6. Absorptance as anda function emittance and ofemittance binder of coatings/CNT of coatings ratio as (calculated aas function a function from of of Nanocylthe Nanocyl content CNTCNT concentration in an in each layer)in for an a N,N’-dimethylformamide twoN,N’-layer -dimethylformamide coating. CNT concentration (DMF)-based (DMF)-based in the formulation formulation formulation without without was 0.064%. a a binder. binder. (◆— (◆—Absorptance,—Absorptance, ■— Absorptance, ■—Emittance).—Emittance)Emittance)..

It can be seenFigure that as7 showstheFigure binder/CNT the 7 shows e ffect ratiothe of theeffect decreased binder of the/CNT, thebinder/CNT emittance ratio on theratio decreased, absorptance on the asabsorptance well and, without emittance and emittance for 0.05% for and 0.05% affecting the0.1% absorptance. CNTs.and The 0.1%Yet, binder the CNTs. binder/CNT used The wasbinder aratio pre-ceramic used played was alsoa polymer,pre a- ceramicmajor i.e., role polymer, Silres in the Ren adhesioni.e., 100. Silres Interestingly, to Ren the 100. Interestingly, this binder this substrate. Onlydid coatings not affbinderect with noticeably dida high not binder/CNT theaffect absorptance noticeably ratio (8.4:1) and the emittance, absorptance had good asadhesion and compared emittance (more to than the, as binder-free compared95%) to to formulations, the binder-free the SS substrate.for all Hence, binderformulations, the/CNT binder/CNT ratios. for all However, ratio binder/CNT was thea compromise ratios. binder H concentrationowever, between the bindergood significantly adhesion concentration and affected low significantly the adhesion affected of the emittance. Itthe can coating be coadhesionncluded to the of SSthat the substrate. the coating use of to Forwater the 0.05% SS-based substrate. CNTs dispersions at For binder 0.05% with/CNT CNTs a low ratios at concentrationbinder/CNT 1.5:1 and ratios lower, of 1.5:1 the and adhesion lower, the CNTs (0.064%)drastically makes adhesionit decreasedpossible drastically to andobtain dropped coatingsdecreased to with value and a droppedrelatively<35% at to binderhigh value absorptance/ CNT<35% ratiosat binder/CNT of 0.75:192.8. Under (Table ratios 3). 0.75:1 At the (Table same 3). At these conditions,time, it for wasthe 0.1% impossible same CNTs, time, to adhesion achieve for 0.1% both began CNTs, good to adhesion decreaseadhesion began alreadyand emittanceto decrease at a binder lower already/ CNTthan at0.59. ratio a binder/CNT of 2:1. Therefore, ratio of 2:1. for goodTherefore, adhesion of for this good coating, adhesion the of binder this coating,/CNT ratio the binder/CNT should not beratio less should than 3:1.not be less than 3:1.

Coatings 2020, 10, x FOR PEER REVIEW 9 of 12

3.3. Coatings with Solvent-Based Dispersion of CNT Since the use of formulations based on an aqueous dispersion of CNTs did not result in good selectivity, coatings based on dispersions of CNTs in an organic solvent were evaluated. First, the effect of CNT concentration on the absorptance and emittance of coatings obtained by a solvent-based dispersion of Nanocyl CNTs without a binder was studied. As presented in Figure 6, the absorptance increased with CNT concentration and leveled off at 0.25% CNTs, whereCoatings it reached 2020, 10, x FOR PEER REVIEW 6 of 12 Figure 7 96.6%. The emittance also increased and leveled off at 0.25% and reachedCoatings 0.74. 2020These, 10 coatings, 1101 adhered 10 of 12 very poorly to the substrate and the addition of a binder was crucial. 100 1.0 100 0.9 1.0 100 1.0 95 0.8 0.9 0.9 95 90 0.7 0.8 0.6 0.8 90 90 85 0.5 0.7 0.7 0.4 85 0.6 80 Emittance 0.6

Adsorptance,% 0.3 0.5 80 0.2 0.5 80 75 0.1 0.4 0.4

Emittance Emittance 75 70 0.0 0.3 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.02 0.04 0.06 0.08 0.10 0.12 0.14

Absorpsion, % 0.3 Absorptance, % Absorptance, 70 CNTs,% CNTs,% 70 0.2 0.2 0.1 Figure 2. The absorptance and emittance of coatings made of various CNT concentrations, prepared 65 0.1 0.0 with60 and without binder composed of boehmite (AlOOH) and0.0 polysiloxane (MTMS) with a ratio of 0.0 0.1 0.2 0.3 0.4 0.5 2:1. The binder0123456/CNT weight ratio was 8.4:1 (◆—without binders, █—with binders). CNTs concentration, % Binder/CNTs ratio

Figure 6. Absorptance and emittance of coatings as a function of Nanocyl CNTFigure concentration 7. Eﬀect in of an Silres Ren 100/CNT ratio on selective properties of Nanocyl’s CNT coatings. N,N’-dimethylformamide (DMF)-based formulation without a binder. (◆—absorptance—Absorptance, and■—◆—emittance for 0.05% CNTs; —absorptance and —emittance for 0.1% CNTs. • • Emittance). The optimal coatings (with good adhesion) are marked for both 0.05% (green) and 0.1% (yellow) CNT concentrations. Figure 7 shows the effect of the binder/CNT ratio on the absorptance and emittance for 0.05% and 0.1% CNTs. The binder used was a pre-ceramic polymer, i.e., Silres Ren 100. Interestingly,Table 3. ItE ﬀthis canect of be binder seen/ CNT that ratio decreasing on adhesion the CNTproperties concentration of CNT coatings. led to a decrease of emittance and absorptance. Since the volumes of the sprayed formulation were constant, the above trend in binder did not affect noticeably the absorptance and emittance, as compared to the binder-free Adhesion formulations, for all binder/CNT ratios. However, the binder concentration significantly affectedabsorptance the and emittance was probably dueBinder to reducing/CNT Ratio the film thickness. Interestingly, there was 0.1% CNTs 0.05% CNTs adhesion of the coating to the SS substrate. For 0.05% CNTs at binder/CNT ratios 1.5:1 andno lower, significant the change in the emittance at various concentrations of CNTs with binders. This means adhesion drastically decreased and dropped to value <35% at binder/CNT ratios 0.75:1 (Tablethat at3) low. At> 95%CNT concentrations>95% the binder is responsible 6:01 for most of the emittance. Yet, while all the the same time, for 0.1% CNTs, adhesion began to decrease already at a binder/CNT ratiocoatings of 2:1. containing> 95% binders> had95% good adhesion, 3:01formulations without binders had poor adhesion. 65% >95% 2:01 Therefore, for good adhesion of this coating, the binder/CNT ratio should not be less thanTherefore, 3:1. the presence of a binder is, on one hand, essential for good adhesion and, on the other - 65–85% 1.5:1 hand, increases the emittance significantly. All our attempts to find a ceramic binder with 35–45% - 1:01 significantly<35% low emittance

Acknowledgments: The Hebrew University center for Nanocharacterization is warmly acknowleged. Conﬂicts of Interest: The authors declare no conﬂict of interest. Coatings 2020, 10, 1101 11 of 12

References

1. EC. Towards an Integrated Roadmap: Research Innovation Challenges and Needs of the EU Energy System. In Towards an Integrated Roadmap: Research Innovation Challenges and Needs of the EU Energy System; 2014. Available online: https://setis.ec.europa.eu/system/files/Towards%20an%20Integrated%20Roadmap_0.pdf (accessed on 15 October 2020). 2. Roro, K.; Tile, N.; Mwakikunga, B.W.; Yalisi, B.; Forbes, A. Solar absorption and thermal emission properties of multiwall carbon nanotube/nickel oxide nanocomposite thin films synthesized by sol–gel process. Mater. Sci. Eng. B 2012, 177, 581–587. [CrossRef] 3. Chen, Z.; Boström, T. Electrophoretically deposited carbon nanotube spectrally selective solar absorbers. Sol. Energy Mater. Sol. Cells 2016, 144, 678–683. [CrossRef] 4. Abendroth, T.; Althues, H.; Mäder, G.; Härtel, P.; Kaskel, S.; Beyer, E. Selective absorption of Carbon Nanotube thin films for solar energy applications. Sol. Energy Mater. Sol. Cells 2015, 143, 553–556. [CrossRef] 5. Bindushree, N.; Dhabale, A.; Dhanush, M.S.; Honakeri, A.; Ankit, A.; Anusha, M.K.; Kumar, R.; Choudhary, H.K.; Khopkar, V.; Sekhar, K.C.; et al. Role of Composition in Enhancing Heat Transfer Behavior of Carbon Nanotube-Ethylene Glycol Based Nanofluids. Electron. Mater. Lett. 2020, 16, 595–603. [CrossRef] 6. Wang, X.J.; Flicker, J.D.; Lee, B.J.; Ready, W.J.; Zhang, Z.M. Visible and near-infrared radiative properties of vertically aligned multi-walled carbon nanotubes. Nanotechnology 2009, 20, 215704. [CrossRef][PubMed] 7. Shi, H.; Ok, J.G.; Baac, H.W.; Guo, L.J. Low density carbon nanotube forest as an index-matched and near perfect absorption coating. Appl. Phys. Lett. 2011, 99, 211103. [CrossRef] 8. Gheitaghy, A.M.; Ghaderi, A.; Vollebregt, S.; Ahmadi, M.; Wolffenbuttel, R.; Zhang, G.Q. Infrared absorbance of vertically-aligned multi-walled CNT forest as a function of synthesis temperature and time. Mater. Res. Bull. 2020, 126, 110821. [CrossRef] 9. Mizuno, K.; Ishii, J.; Kishida, H.; Hayamizu, Y.; Yasuda, S.; Futaba, D.N.; Yumura, M.; Hata, K. A black body absorber from vertically aligned single-walled carbon nanotubes. Proc. Natl. Acad. Sci. USA 2009, 106, 6044–6047. [CrossRef] 10. Cao, A.; Zhang, X.; Xu, C.; Wei, B.; Wu, D. Tandem structure of aligned carbon nanotubes on Au and its solar thermal absorption. Sol. Energy Mater. Sol. Cells 2002, 70, 481–486. [CrossRef] 11. Selvakumar, N.; Krupanidhi, S.B.; Barshilia, H.C. Carbon Nanotube-Based Tandem Absorber with Tunable Spectral Selectivity: Transition from Near-Perfect Blackbody Absorber to Solar Selective Absorber. Adv. Mater. 2014, 26, 2552–2557. [CrossRef] 12. Selvakumar, N.; Biswas, A.; Rao, D.S.; Krupanidhi, S.B.; Barshilia, H.C. Role of component layers in designing carbon nanotubes-based tandem absorber on metal substrates for solar thermal applications. Sol. Energy Mater. Sol. Cells 2016, 155, 397–404. [CrossRef] 13. Abdelkader, T.K.; Zhang, Y.; Gaballah, E.S.; Wang, S.; Wan, Q.; Fan, Q. Energy and exergy analysis of a flat-plate solar air heater coated with carbon nanotubes and cupric oxide nanoparticles embedded in black paint. J. Clean. Prod. 2020, 250, 119501. [CrossRef] 14. Li, L.; Gao, X.; Zhang, G.; Xie, W.; Wang, F.; Wang, Q. Combined solar concentration and carbon nanotube absorber for high performance solar thermoelectric generators. Energy Convers. Manag. 2019, 183, 109–115. [CrossRef] 15. Sobhansarbandi, S.; Martinez, P.M.; Papadimitratos, A.; Zakhidov, A.; Hassanipour, F. Evacuated tube solar collector with multifunctional absorber layers. Sol. Energy 2017, 146, 342–350. [CrossRef] 16. Wang, X.; He, Y.; Liu, X.; Zhu, J. Enhanced direct steam generation via a bio-inspired solar heating method using carbon nanotube films. Powder Technol. 2017, 321, 276–285. [CrossRef] 17. Kasaeian, A.; Daneshazarian, R.; Rezaei, R.; Pourfayaz, F.; Kasaeian, G. Experimental investigation on the thermal behavior of nanofluid direct absorption in a trough collector. J. Clean. Prod. 2017, 158, 276–284. [CrossRef] 18. Hordy, N.; Rabilloud, D.; Meunier, J.-L.; Coulombe, S. High temperature and long-term stability of carbon nanotube nanofluids for direct absorption solar thermal collectors. Sol. Energy 2014, 105, 82–90. [CrossRef] 19. Shende, R.C.; Ramaprabhu, S. Thermo-optical properties of partially unzipped multiwalled carbon nanotubes dispersed nanofluids for direct absorption solar thermal energy systems. Sol. Energy Mater. Sol. Cells 2016, 157, 117–125. [CrossRef] Coatings 2020, 10, 1101 12 of 12

20. Rincón, M.E.; Molina, J.; Sánchez, M.; Arancibia, C.; Garcia, E. Optical characterization of tandem absorber/reflector systems based on titanium oxide–carbon coatings. Sol. Energy Mater. Sol. Cells 2007, 91, 1421–1425. [CrossRef] 21. Bibi, S.; Yasin, T.; Nawaz, M.; Price, G.J. Comparative study of the modification of multi-wall carbon nanotubes by gamma irradiation and sonochemically assisted acid etching. Mater. Chem. Phys. 2018, 207, 23–29. [CrossRef] 22. Price, G.J.; Nawaz, M.; Yasin, T.; Bibi, S. Sonochemical modification of carbon nanotubes for enhanced nanocomposite performance. Ultrason. Sonochem. 2018, 40, 123–130. [CrossRef][PubMed] 23. Raiah, K.; Djalab, A.; Hadj-Ziane-Zafour, A.; Soula, B.; Galibert, A.-M.; Flahaut, E. Influence of the hydrocarbon chain length of imidazolium-based ionic liquid on the dispersion and stabilization of double-walled carbon nanotubes in water. Colloids Surfaces A Physicochem. Eng. Asp. 2015, 469, 107–116. [CrossRef] 24. Ferreira, F.; Francisco, W.; De Menezes, B.R.C.; Cividanes, L.D.S.; Coutinho, A.D.R.; Thim, G.P. Carbon nanotube functionalized with dodecylamine for the effective dispersion in solvents. Appl. Surf. Sci. 2015, 357, 2154–2159. [CrossRef] 25. Lee, S.H.; Choi, T.J.; Jang, S.P. Thermal efficiency comparison: Surface-based solar receivers with conventional fluids and volumetric solar receivers with nanofluids. Energy 2016, 115, 404–417. [CrossRef] 26. Bera, R.K.; Azoubel, S.; Mhaisalkar, S.; Magdassi, S.; Mandler, D. Fabrication of Carbon Nanotube/Indium Tin Oxide “Inverse Tandem” Absorbing Coatings with Tunable Spectral Selectivity for Solar-Thermal Applications. Energy Technol. 2015, 3, 1045–1050. [CrossRef] 27. Bera, R.K.; Mhaisalkar, S.; Mandler, D.; Magdassi, S. Formation and performance of highly absorbing solar thermal coating based on carbon nanotubes and boehmite. Energy Convers. Manag. 2016, 120, 287–293. [CrossRef] 28. Bera, R.K.; Binyamin, Y.; Mhaisalkar, S.; Magdassi, S.; Mandler, D. Highly Selective Solar Thermal Sprayable Coating Based on Carbon Nanotubes. Sol. RRL 2017, 1, 1700080. [CrossRef] 29. Bera, R.K.; Binyamin, Y.; Frantz, C.; Uhlig, R.; Magdassi, S.; Mandler, D. Fabrication of Self-Cleaning CNT-Based Near-Perfect Solar Absorber Coating for Non-Evacuated Concentrated Solar Power Applications. Energy Technol. 2020.[CrossRef] 30. ASTM D3359 Standard Test Methods for Rating Adhesion by Tape Test; ASTM: West Conshohocken, PA, USA, 2009. 31. Paints and Varnishes–Cross–Cut Test. Available online: https://www.iso.org/obp/ui/#iso:std:iso:2409:ed-5:v1: en (accessed on 15 October 2020).

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