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Synthesis of porous magnetite [Fe.sub.3][O.sub.4] and its application in thermal control coatings as new black pigment.

Abstract In the present work, firstly, porous magnetite [Fe.sub.3][O.sub.4] was synthesized using mesoporous silica molecular sieve (SBA-15) as a template. Finally, the infiltrated ferrous precursor into silica mesopores was subjected to an infiltration process, in order to obtain the porous [Fe.sub.3][O.sub.4]. The structural, morphological, and porosity characteristics and optical property of assynthesized material were investigated by X-ray diffraction, field emission scanning electron microscopy, [N.sub.2] adsorption, and diffuse reflectance spectroscopy. Evaluation of the obtained results indicated that the porous [Fe.sub.3][O.sub.4] has well-defined morphology and higher light absorption in the UV/Vis region than not only the common [Fe.sub.3][O.sub.4] and the other inorganic black pigments, but also common carbon black. Investigation of the prepared coating with this pigment indicated the higher absorption (0.98) and lower reflectance (~2%) than commercial black thermal control coatings (TCC) with inorganic pigments. This pigment presents the best selection for black TCC with inorganic pigment and stable in the space environment. The as-prepared porous magnetite [Fe.sub.3][O.sub.4] also has high surface area (~24.78 [m.sup.2]/g) and excellent magnetic properties ([M.sub.s] = 70 emu/g) at room temperature.

Keywords Porous [Fe.sub.3][O.sub.4], Optical properties, Light absorption. Black pigment, thermal control coatings

Introduction

Porous materials and molecular sieves attract much attention due to their remarkable characteristics. They possess high surface and high sorption capacity compared to nonporous objects, as well as the possibility to perform selective sorption of various substrates by regulation of pore diameter or easy chemical modification. (1) In recent years, much attention has focused on porous structures of transition metal oxides and the most studied porous materials are those built from diamagnetic units, such as carbon, (2,3) oxides of p elements, (4,5) etc. Among these structures, iron oxides, especially [Fe.sub.3][O.sub.4], have aroused great interest in various fields--for example, catalysis, (6) lithium-ion batteries, (7) and for absorption of electromagnetic waves. (8)

Satellite temperatures in a space environment are often passively controlled by thermo-optical properties of suitable surfaces. (9) Thermal control coatings (TCCs) control the spacecraft temperatures by their thermooptical properties including spectral absorptance (ocs) and thermal emittance (st). (10) Black coatings with low reflectivity in visible and infrared spectral ranges, such as the prepared coatings by anodizing of aluminum, (9) plasma electrolytic oxidation, (11) or electroless nickel coatings, (12) are now being actively developed and studied. Such materials can be used as anti-reflection coatings in optical devices, in solar energy, and other alternative energy applications. Depending on the application purpose of such materials (coatings and absorbers), they should possess a certain series of important properties. (13) For example, solar thermal plants, the same as TCC, should effectively absorb radiation in a wide wavelength range regardless of the angle of incidence or the polarization of incident radiation. In other words, optical characteristics of such coatings should be close to those of a black body. (13) One of the most important fields that requires the whole absorption of wavelength regions (UV-Vis-NIR) is in passive thermal control systems such as black thermal control coatings. Black TCCs play an important role in thermal management of spacecrafts and satellites by their optical property including high solar absorptance. One example of these applications includes earth observations by satellite imaging systems where high brightness contrast exists between desired field of view and other background sources which can illuminate and scatter from elements of the instrument. This bright light diffracts from instrument structures, rattles around, and invariably contaminates measurements. Astrophysical observations are also influenced by stray light that obscures very dim objects and degrades signal to noise in spectroscopic measurements. Stray light is controlled by utilizing low reflectance structural surface treatments and by using baffles and stops to limit this background noise. Development of this technology will provide numerous benefits including the increase in observational fidelity by reducing scattered light in high contrast regions. (14) To achieve the above purposes and produce an excellent black body, we suggested utilizing the porous magnetite [Fe.sub.3][O.sub.4] as pigment in black coatings for the first time. The high light absorption of this material makes it exceptionally good as absorber, with the potential to provide order-of-magnitude improvement in stray-light suppression when used in an optical system or in TCC as black pigments.

Experimental

Reagent and materials

Tetraethyl orthosilicate (TEOS, Merck) as silica source, poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (P123, Aldrich) as a structure-directing agent, HC1, ethanol, and Fe [(N[O.sub.3]).sub.3]x9[H.sub.2]O (Merck) were used as received. The [Fe.sub.3][O.sub.4] powder, carbon black spherical powder with particle size distribution of 0.4-12 [micro]m, and potassium silicate solution were purchased from the local market and used as received.

Preparation of SBA-15 template

The SBA-15 sample was prepared according to the synthesis procedure reported by Zhao et al. (15) Typically, 4 g of P123 was dissolved in the solution composed of 130 g deionized water and 20 g of 37% HC1 under stirring at 40[degrees]C. Then, 8.5 g of TEOS was added dropwise and the resultant mixture was stirred at 40[degrees]C for 24 h, followed by aging statically in a Teflon-lined autoclave at 100[degrees]C for 48 h. The solid product was filtered, washed with water, dried at room temperature, and calcined in air at 497[degrees]C for 5 h.

Preparation of porous [Fe.sub.3][O.sub.4]

Synthesis of porous [Fe.sub.3][O.sub.4] was done according to the reported method by Jiao et al.lfi but with SBA-15 as hard and silica template. Typically, 1 g of SBA-15 was dispersed in a solution of Fe[(N[O.sub.3]).sub.3]x9[H.sub.2]O with a ratio of 1:1 in ethanol and stirred at room temperature for 1 h. Ethanol was removed by heating at 80[degrees]C. The resulting powder with a red-brown color was calcinated at 600[degrees]C for 6 h. The impregnation steps were repeated twice. The resulting sample was treated four times with a hot (2 M) NaOEl solution to remove the silica template. Finally, the obtained [Fe.sub.2][O.sub.3] at the end of this step was reduced to [Fe.sub.3][O.sub.4] by heating at 400[degrees]C for 1 h under 5% [H.sub.2]-95% Ar atmosphere.

Paint preparation

To examine the effectiveness of the synthesized porous [Fe.sub.3][O.sub.4] as a new pigment for TCC formulation, it was incorporated into potassium silicate binder at a pigment to binder ratio of 1:1 and then this coating was compared with the prepared coatings with commercial [Fe.sub.3][O.sub.4] and carbon black. For preparation of the so-called coatings, the specific amounts of pigment, binder, and distilled water (as solvent) were mixed together, and the pigment particles were dispersed in binder with a pearl mill apparatus for 5 h. Then the prepared coatings were sprayed on the aluminum (Al 6061) plates to achieve a dry film thickness of 100-120 [micro]m. After the coatings were fully dried, their reflectance was measured.

Materials characterization

The crystal structure of as-prepared products was characterized by powder X-ray diffraction (XRD) on a Bruker D8 Advance X-ray diffractometer using CuK[alpha] radiation (40 kV, 40 mA, and [lambda] = 0.1541 nm). XRD patterns were recorded from 0[degrees] to 10[degrees] with a scanning step of 0.02[degrees]/s. Morphology and size of the sample were analyzed by a Hitachi S-4160 Field Emission Scanning Electron Microscope (FE-SEM) at an accelerating voltage of 15 kV. The diffuse reflectance spectra of prepared powders were measured by a JASCO V-670 UV-Vis Spectrophotometer (in the range of 220-2200 nm). The specific surface areas were measured by the Brunauer-Emmet-Teller (BET) method employing [N.sub.2] adsorption at 77 K after treating the sample at 393 K and 10 4 Pa for 15 h, using a TriStar-3000 apparatus. Magnetization measurement was carried out on a superconducting quantum interference device at room temperature. The thermal emittance of the films was measured by a Temp 2000 A infrared reflectometer (AZ Technology, USA) in the range of 3-30 [micro]m.

Results and discussion

The corresponding XRD is illustrated in Figs, la and lb for SBA-15 template and porous [Fe.sub.3][O.sub.4], respec tively. The diffraction peaks match well with the standard values of [Fe.sub.3][O.sub.4] (JCPDS NO. 19-0629), indicating that the as-prepared product is completely [Fe.sub.3][O.sub.4] phase. (6,7,17-19) The XRD patterns indicate that silica structure also was in good agreement with the reported data and the SBA-15 template was produced successfully. (20,21)

Field emission scanning electron microscopic observations with low and high magnification have shown that the obtained [Fe.sub.3][O.sub.4] has collective-like morphology, as illustrated in Fig. 2. These nanopowders are 20-30 nm in mean diameter. Further, the nitrogen sorption measurement was conducted to evaluate the porous structure and specific surface area for such magnetite nanoparticles. Figure 3 shows the nitrogen adsorption-desorption isotherm. Based on the desorption curve, the BET surface area of the porous magnetite was calculated to be 25 [m.sup.2]/g. In addition, the sorption exhibits type IV isotherm, and the pore analysis has revealed that the pore diameters in the porous powder mainly fall into 36 nm, as seen in the inset of Fig. 3.

The corresponding magnetization curve of porous [Fe.sub.3][O.sub.4] powder as a function of magnetic field, or the M vs H, is shown in Fig. 4. The saturation magnetization ([M.sub.s]) of the magnetite [Fe.sub.3][O.sub.4] is about 70 emu/g, higher than the other porous [Fe.sub.3][O.sub.4] samples. (22-24) The saturation magnetization of the various [Fe.sub.3][O.sub.4] samples, which were prepared with different methods, is presented in Table 1.

The diffuse reflectance spectra of as-synthesized porous material, its prepared coating, and the prepared coatings with commercial [Fe.sub.3][O.sub.4] and carbon black are indicated in Figs. 5(a), 5(b), and 5(c), for more comparison. As seen in this figure, the diffuse reflectance amount of porous magnetite [Fe.sub.3][O.sub.4] and its prepared coating were very low and their absorption was very high. The reflectance of porous [Fe.sub.3][O.sub.4] pigment is almost 2%. The reflectance of porous [Fe.sub.3][O.sub.4] coating, equal to ~3%, is lower than common [Fe.sub.3][O.sub.4] coating, equal to ~5%. However, when pigments are mixed with the binder, the optical absorption of the coating deteriorates. If reflectance of the pigment is 1-2%, reflectance of thermal control coating based on the pigment is 3-5%. (29) The porous [Fe.sub.3][O.sub.4] showed lower reflectance and consequently higher light absorption than the other used inorganic black pigments in spacecraft TCC (30-32) and the other thermal control surfaces. (33-37) The high light absorption of porous [Fe.sub.3][O.sub.4] arises from its porous structure. According to the Garnett theory, (38) the porous structure of the pigment leads to reduction of light scattering and increasing of absorption. Thermal emittance (et) of the prepared coating with porous [Fe.sub.3][O.sub.4] is equal to 0.96, which is higher than common TCC with thermal emittance equal to 0.93.

Conclusion

In the present work, porous magnetite [Fe.sub.3][O.sub.4] was synthesized and its reflectance investigated. The obtained results indicated that UV/Vis absorbance of this structure was very high and it almost absorbs the whole wavelength range. Because of the multiple light reflections through the porous skeleton, UV/Vis absorption was increased and reflection was very low. This feature is very important and considerable for black TCC and solar thermal collectors to absorb all of the wavelengths. Therefore, porous [Fe.sub.3][O.sub.4] is an excellent selection for new black TCC with inorganic pigments that are more stable in a destructive space environment.

DOI 10.1007/s11998-015-9700-5

M. Mashayekhi, K. Ghani ([mail]) Department of Chemistry, Malek Ashtar University of Technology, Shahin Shahr, Isfahan, Iran e-mail: kghani@mut-es.ac.ir

R. Shoja Razavi, N. Kiomarsipour Department of Materials Engineering, Malek Ashtar University of Technology, Shahin Shahr, Isfahan, Iran

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Table 1: Saturation magnetization of the various
[Fe.sub.3][O.sub.4] samples that were prepared with
different methods

Particle shape             Saturation     Particle    Reference
                          magnetization   Size (nm)
                           ([M.sub.s])
                              emu/g

Present work                   70           25-50        --
[Fe.sub.3][O.sub.4]           87.15        50-100        25
  porous microspheres
[Fe.sub.3][O.sub.4]            60           25-50        26
  nanoparticles
[Fe.sub.3][O.sub.4]           28.4          11-13        27
  nanoparticles
Magnetite nanoparticles       58.72         15-30        28


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Author:Mashayekhi, Moslem; Ghani, Kamal; Razavi, Reza Shoja; Kiomarsipour, Narges
Publication:Journal of Coatings Technology and Research
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Date:Nov 1, 2015
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