Physical and Optical Properties of Ultra-black Nickel–Phosphorus for a Total Solar Irradiance Measurement

The high solar absorber was obtained by the etching process of Ni–P electroless plating. The Ni–P deposition process is described as follows. The aluminum alloy (Al-98.2 Wt%, Si-1.2 Wt%, and Mg-0.6 Wt%) substrates with a 30 mm diameter were mechanically polished with SiC paper. Each sample was degreased in an acetone ultrasonic bath, Ridoline® solution (alkaline degreasing, Henkel products), and was subsequently activated by etching in 15 vol% nitric acid (HNO₃) and 6 vol% sulfuric acid (H₂SO₄), at 70°C. A double zincate treatment was done using Alumon EN® (zincating solution).

The Ni–P electroless bath included nickel sulfate (NiSO₄.7H₂O, 30 g L⁻¹) as nickel source, sodium hypophosphite (NaH₂PO₂, 10 g L⁻¹) as a reducing agent, sodium citrate (Na₃C₆H₅O₇ · 2H₂O, 12 g L⁻¹), sodium acetate (CH₃COONa, 5 g L⁻¹), and thiourea (NH₂CSNH₂, 1 mg L⁻¹). The bath pH was maintained in three different steps: 4.90 ± 0.05 for 160 minutes, 4.30 ± 0.05 for 100 minutes, and 4.00 ± 0.05 for 180 minutes. The three steps were used to obtain multilayers of Ni–P with different phosphorus content using the same bath. The pH was adjusted using a sulfuric acid and ammonia solution. After the deposition, the surface was etched with an oxidizing acid (HNO₃, 9M) at 40°C during 40–60 s to produce ultra-black Ni–P.

The morphology images were taken using the scanning electron microscopy–field emission gun (SEM–FEG) by TESCAN MIRA 3 equipped with an energy dispersive X-ray spectroscopy (EDS) for an elemental analysis. The surface roughness of the samples was characterized using a surface profilometer, Wyko NT1100. The coatings were characterized by X-ray diffraction (XRD; Panalytical X'Pert PRO) to investigate the crystal structure and Scherrer's equation (Langford & Wilson 1978) was used to estimate the average of the particle size:

$$\begin{eqnarray}&&L=\displaystyle \frac{k.\lambda }{\left(\beta .\cos \theta \right)},\end{eqnarray} \tag{ 1 }$$

where λ is the wavelength of the X-rays; β is the full-width at half maximum of the X-ray peak; k is the shape factor, dimensionless, usually taken close to unity; and θ is the diffraction angle.

The BRDF device was developed at the National Institute for Space Research and a comprehensive description and system characterization are given by Berni et al. (2015). The system is composed of a rotating arm to collect the light in the zenith direction from −70 to +70 degrees and in the azimuth angle over 360 degrees (Figure 1(A)). The halogen light source (50 W) is mounted on a turntable arm that can rotate from 0 to 60° in the zenith direction. The scattered collected light wavelength can be selected by a set of interference filters with an average bandwidth of 8.0 nm. The detector used is a Si photodiode (Oriel 70336). The BRDF measurement was calculated according to Bartell et al. (1981). The measurements were made in pairs, a sample and a reference with Lambertian reflectance National Institute of Standards and Technology (NIST)-traceable Spectralon® (Labsphere Inc., SRS-99). They were quantified by their voltages, V_ref and V_s, in the following way:

$$\begin{eqnarray}&&\displaystyle \frac{{f}_{s}\left({\theta }_{I},{\varphi }_{I},{\theta }_{R},{\varphi }_{R},\lambda \right)}{{f}_{\mathrm{ref}}\left({\theta }_{I},{\varphi }_{I},{\theta }_{R},{\varphi }_{R},\lambda \right)}={\rho }_{\mathrm{ref}}\displaystyle \frac{{V}_{s}}{{V}_{\mathrm{ref}}},\end{eqnarray} \tag{ 2 }$$

where θ_i is the incident zenith angle, θ_r is the reflected zenith angle, φ_i and φ_r are the incident and reflected azimuth angles. The method requires reference data for each measured position (ρ_ref).

Zoom In Zoom Out Reset image size

The spatial reflectance device mapped the sample over an area of ∼50 mm² using the experimental setup shown in Figure 1(B). Three laser wavelengths at 375, at 532, and at 633 nm were used as the light source. The light goes through a collecting lens and focuses on the center of the entrance and exit of the integrating sphere. The integrating sphere was mounted on a platform to move on two axes. The reflected light was measured using a Si photodiode detector. More details of the experimental setup can be found in Suter (2014).

The results (V_s) for each position were measured relative to an NIST-traceable standard (Labsphere Inc., SRS-02) as reference (V_ref):

$$\begin{eqnarray}&&\rho ={\rho }_{\mathrm{ref}}\displaystyle \frac{{V}_{s}-{V}_{\mathrm{dark}}}{\left\langle {V}_{\mathrm{ref}}-{V}_{\mathrm{dark}}\right\rangle },\end{eqnarray} \tag{ 3 }$$

where the dark current signal (V_dark) was acquired to provide the system with stray light and then it is subtracted from the V_s and V_ref.

Ultra-black Ni–P coatings (Figure 2) were obtained by etching Ni–P as deposited. Figure 3 presents the XRD of Ni–P as deposited and after etching. The x-ray diffractogram shows two broad coalescing peaks around 44° and 52°. It corresponds to the pattern reference reflections of Ni (111) and Ni (200) planes. As phosphorus concentration increases, the peaks become broad, indicating the continuous Ni grain size decrease (Mahalingam et al. 2007; Pillai et al. 2012). The codeposition of phosphorus in octahedral interstitial on the Ni faced centered cubic (FCC) lattice leads to expansion and structure distortion (Daly & Barry 2003). Alloys with lower P content are a crystalline and microcrystalline solid solution of phosphorus in nickel. As the P content increases, the deposit structure undergoes transition from crystalline to nanocrystalline and becomes amorphous above 9 wt% of P (Agarwala & Agarwala 2003; Pillai et al. 2012). The Scherrer analysis indicates the structure crystallite size to be 1.66 nm and 2.48 nm for Ni–P and ultra-black Ni–P, respectively. For crystallites with sizes less than about 200 nm, the distinction between alloys being finely crystalline and amorphous could be inaccurate (Brown et al. 2002).

Zoom In Zoom Out Reset image size

Zoom In Zoom Out Reset image size

It is well known the Ni–P alloy phosphorus content and morphological structure influence the chemical (Riedel 1991), electrochemical (Gu et al. 2005; Elias et al. 2016), and mechanical properties. The electroless deposits content are strongly dependent on the bath composition, pH, temperature, and loading factor (the ratio between the substrate area and solution volume). Different electroless Ni–P coating characteristics can be obtained by adjusting the plating bath variables. The concentration of nickel metal and the phosphorus source can affect the bath equilibrium and the optimum quantity ratio can improve the efficiency of deposition (Osifuye et al. 2014; Sun et al. 2015).

Electroless deposition processes are a heterogeneous reaction. The electrochemical mechanism (Malecki & Micek-Ilnicka 2000) establishes that for each mole of metal deposition, three moles of H⁺ are produced during the reaction progress. Several authors have demonstrated the effect of pH in the Ni–P composition (Sudagar et al. 2013; Sun et al. 2015). The decreasing of pH results in nickel deposits with higher P content and a lower deposition rate. The main reason is due to pH's influence on the Ni and P speed reaction. Generally, when the pH of the bath reduces, there is a higher rate of P formation and lower Ni formation (Anijdan et al. 2018). Ni–P presented a typical cauliflower morphology (Figure 4) consisting of globular grains due the tendency of Ni–P precipitates (with higher phosphorus contents) for spheroidization to reduce surface energy. The spherical shape and size depends on the deposition rate and the nucleation sites on the surface (Anijdan et al. 2018).

Zoom In Zoom Out Reset image size

Figure 5 shows SEM images of the cross-sections of Ni–P (A) and ultra-black Ni–P (B). An EDS cross sectional elemental line scan consists of the analysis of the major alloying elements of the coating (Ni and P) and substrate (Al) across the layer. According to Figure 5(C), the phosphorus content progressively decreased due to the pH maintained during electroless deposition. The P content varies from 10.3 wt.% P at the top to 4.8 wt.% P near to the Ni–P/Al interface, indicating the multilayer coating formation. The P content along the layer can be classified as medium (5 to 9 wt.% P) and high (more than 10 wt.% P) according to ASTM B733-15.⁴ The phosphorus content has a substantial influence on Ni–P alloy properties. The low P (1 to 2% P) is rapidly attacked by acid, and the higher P content was more resistant to acid attack than pure metal (Brenner et al. 1950). When Ni–P coating is etched by oxidizing acid, the coating turns black. Brown et al. (2002) had found the influence of phosphorus content before etching in the morphologies and optical properties of black nickel surfaces. They also observed the ultra-black Ni–P coating's lowest reflectance with P content is between 5 and 9%. In preliminary results, we tested the influence of phosphorus content through different pH maintenance approaches. Initially, the pH was adjusted only at the beginning of operation or it was kept constant during the deposition time. However, our experimental results show the lowest reflectance and deeper black surface etching with a multilayer coating. The blue dotted line (Figure 5) corresponds to the approximate thickness of the Ni–P film removed in the acid etching. The region with a continuous decrease in P (∼13 μm) was removed in the acid etching. In this configuration, the phosphorus content is higher at the top and decreases along the layer. We propose that the outermost layer is attacked for a long time due to the higher P content, and as the oxidizing solution penetrates the lower P content, it is more easily attacked, forming deeper pores. Surfaces with higher phosphorus contents at the top were not attacked with the method employed.

Zoom In Zoom Out Reset image size

The pores have a crucial role in obtaining low reflectance (Johnson 1980; Brown et al. 2002; Saxena et al. 2006). They confine the radiation leading to multiple internal reflections until the absorption. This unique morphology (Figure 6) is a result of the etching process. It is strongly influenced by the Ni–P for deposited coating properties and the blackening process conditions. The oxidant acid attacks the nickel atoms preferentially (Cui et al. 2006) and it is responsible for oxide film (NiO and NiO₂) formation (Stupnišek-Lisac & Karšulin 1984). The different pore diameters and depths can be attributed to the coating composition variability and structure defects. The pore depths reach up to 25 μm and have different shapes. Figure 7 shows ultra-black Ni–P roughness profiles in the sample center and near the edge (∼0.6 mm from the edge). The roughness parameters (Ra) were computed by averaging the distance from the measured point to the mean plane. The Ra values obtained were 1.7 to 4.2 μm. The average roughness parameter confirms the non-uniformity along the surface. This variation can be attributed to the variability of acid etching along the surface. The scattering properties are directly related to surface roughness and wavelength. There is a relationship between the diffuseness or glossiness and the parameter Ra. As Ra gets smaller, there is a tendency to increase the glossiness (Yonehara et al. 2004).

Zoom In Zoom Out Reset image size

Zoom In Zoom Out Reset image size

To investigate the black Ni–P absorption (α) features, the reflectance (1−α) was measured. An 8 mm diameter circle in a flat sample was scanned line by line with a line/point step of 0.3 mm. The laser was focused on multiple locations normal to the surface, and the reflected light was measured. Figure 8 shows the 2D (X–Z) reflectance maps and the reflectance average results are listed in Table 1. The absorptance average over the illuminated area is 99.71%, 99.77%, and 99.81% at 375 nm, 532 nm, and 633 nm, respectively. The results show a wavelength-dependent absorptivity influence of about 0.1%. The reflectance is distributed in the same sample within the range of 1554–6288 ppm at 633 nm. The reflectance maps provided an understanding of the impact of the size and depth of the pores formed in the etching process (see also Section 3.2). The maps showed that the reflectance is non-uniform over the surface and demonstrated the existence of different intensity with some points of higher reflection. The non-uniformity suggests that the etching process removes more intense material in regions rich in nickel. Consequently, regions rich in nickel present a different morphology and thickness of the black layer than the overall surface, which determines the electromagnetic radiation absorption characteristics (Brown et al. 2002). As expected, the reflectance varies in different sample regions due to the inherent electroless characteristics variability and the blackening process.

Zoom In Zoom Out Reset image size

Figure 9 shows the reflectance profile measured in three incident zenith angles (15°, 30°, and 60°) at 500 nm. The distributions are bell-shaped, which resulted from diffuse reflectance. The profiles show the distribution variation in a function of incident zenith angles. Although predominating diffuse reflectance for θ_I = 15° and θ_I = 30°, a component that coincides with specular reflection (Figure 10) is observed. Larger zenith angles cause an increase in the BRDF. Figure 11 shows the reflectance obtained by total integrated scattering (TIS). The increment in the incident zenith angles increases the reflectance. Ultra-black Ni–P has low wavelength dependence on the reflectance at θ_I = 15° and θ_I = 30° between 500 and 800 nm. However, there is a reflectance increase in the function of wavelength at θ_I = 60°.

Zoom In Zoom Out Reset image size

Zoom In Zoom Out Reset image size

Zoom In Zoom Out Reset image size

The ultra-black Ni–P surface morphology has a significant influence on the reflectance. The incident beam at lower angles results in low reflectance due to the ability of the electromagnetic radiation to enter and interact with the pores. These interactions repeatedly occur until the electromagnetic radiation is almost completely absorbed. The pores perpendicular to the surface confine the incident light (Amemiya et al. 2012; Kodama et al. 1990). In this way, the highest incident angles may have part colliding with the lateral wall of the pores, resulting in a reflectance increase. The ultra-black Ni–P optical properties with a thicker absorption layer yield a lower spectral reflectance, especially at longer wavelengths. Ultra-black Ni–P with low reflectance depends on several parameters such as pore depth and size, the thickness of the absorption layer, and the geometric factor of incident light and porous position.

The BRDF analysis indicated the scattered light depends on the incidence angle, with the absorptance of the Ni–P film decreasing from 99.77% at θ_I = 0° to 98.4% at θ_I = 60°. Although it is high absorption, it is desirable to design absolute radiometers with even a higher absorption. We can increase the overall absorptance by employing cavities instead of flat surfaces. Most of the absolute radiometers designed for TSI measurements are based on this approach to improve the absorption of solar radiation. To estimate the absorbtance of cavities, we model a cone-shaped cavity by employing an optical engineering software (Zemax® OpticStudio). Figure 12(A) shows the model configuration. An 8 mm diameter beam in front of the instrument illuminates the cavity, which has a diameter of 17 mm and a length of 71 mm and its internal coating is based on the experimental BRDF data. We used the mode non-sequential (NSC) ray-tracing, where which rays hit the objects depends upon the object geometry and the angle and position of the input ray (ZEMAX 2016).

Zoom In Zoom Out Reset image size

Figure 12(B) presents the results of the irradiance distribution of the reflected beam in a virtual detector rectangle (size: 20 × 20 mm and pixels: 100 × 100) positioned in front of the cavity. The color code indicates the flux density (W m⁻²) in the pixel area. It is noticeable that the reflected flux out of the cavity is non-uniform, with a higher flux density at the center region of the cavity. This pattern occurs due to the result of the geometric relationship among the incident beam, cavity geometry, and the pores perpendicular to the cavity surface. The salt-and-pepper flux distribution seems to be due to the incident angle of the beam in the pores.

The estimated absorption for the modeled cavity is approximately 99.9998%. However, we expect lower absorption in real cavities due to the inhomogeneous distribution of the coating Ni–P that arises during the deposition process.