Thermal oxidation of stainless steel substrate with tunable spectral selectivity: Transition from a reflecting to a highly absorbing Cr–Fe spinel surface

Highlights

• Thermal treatment of SS 304 in air at 900 °C carried out to make it an absorbing surface.

• High absorptance (0.920) and relatively low thermal emittance (0.37) achieved, suitable for solar collector applications.

• Formation of Cr₂O_3, Fe₃O₄ and Fe_3-xCr_xO₄ spinel phases confirmed on the surface of SS 304 substrate.

• Light absorption is due to spinel layer and multiple internal reflections along Cr₂O₃ pyramids.

Abstract

The conventional methods of obtaining solar selective surfaces for high temperature solar thermal applications involve coating of the substrate by various methods such as physical vapor deposition, plasma spraying, anodization, etc. The present work is an attempt to enhance the optical properties of metals by heat treatment. The oxide layers formed by annealing of stainless steel 304 (SS 304) enhance the absorptance in the solar spectrum region. Influences of oxidation temperature and oxidation time span on the values of solar absorptance and thermal emittance have been studied. The annealing of SS 304 substrate was carried out in air at 600–900 °C. The time period of annealing plays a crucial role in the amount of oxides formed and thus is a variable parameter. The absorptance values obtained with isothermal oxidation at shorter and longer durations have been compared at a temperature of 900 °C. A cyclic loading approach is also employed to arrive at the optimal absorptance of the samples. It is further used to study the dependency of solar absorptance on the reaction kinetics with respect to varying oxidation time. Under the optimized annealing conditions, heat-treated SS 304 sample exhibited an absorptance of 0.920 and an emittance of 0.37. A plausible model for the high optical absorption in these oxidized surfaces with relatively low thermal emittance is rationalized. High temperature materials such as Inconel and Nimonic have also been subjected to isothermal annealing and the absorptance values were found to be 0.887 and 0.880, respectively.

Introduction

Solar radiation is the most abundant and clean source of energy. Unlike fossil fuel, no greenhouse gases are emitted from the solar energy. The solar energy is harnessed by (i) photovoltaic and (ii) solar thermal technologies. In the case of solar thermal technologies, the solar radiation is first converted into thermal energy and then to electrical energy. The economic feasibility of solar components for concentrated solar power (CSP) technologies is a key point for their large scale deployment. In the CSP technologies, a high temperature heat source is created by concentrating a large amount of solar radiation, reflected from reflectors onto a central receiver (or receiver tubes). The efficient utilization of the available solar radiation depends primarily on the absorber property of the receiver surface [1]. The approach employed in creating a selective surface also plays a crucial role in the efficiency of the absorber. Present day CSP methodologies that are pervasive on the industrial scale are exacting in terms of time consumed, expenditure and environmental factors [2]. As the concentrating solar power utilizes focused sunlight onto a smaller area, it is evident that the operational temperature of the absorbers exceeds 500 °C [3]. This necessitates the thumb of rule that the absorber surface must exhibit high solar absorptance (α > 0.95) in the wavelength range of 250–2500 nm. It should simultaneously have a least possible value of thermal emittance (ε < 0.10) in the infrared (IR) range of wavelength (λ > 2500 nm) [4]. A lower emittance value ensures reduced radiation losses as it varies as the fourth power of temperature [5]. The spectral selectivity (α/ε) of a given surface, that is, very high absorptance in solar spectrum region and very low thermal emittance in IR region plays very important role in capturing useful solar radiation.

Stainless steel 304 (SS 304) is widely used material for various collectors in the solar thermal technologies because of its high temperature stability and high corrosion resistance [6]. Therefore, thermal oxidation of SS 304 in air is employed in the present work to generate a solar selective surface, which is a facile and cost-effective technique. This method involves the formation of intrinsic oxide layers over bare metal substrates by air annealing, leading to the formation of an absorber/reflector system. One of the pioneering works carried out by Valkonen et al. [7] on the oxidation of alloys, was to create spectrally selective surfaces. The solar absorptance values ranging between 0.850 and 0.900 were achieved on stainless substrates heat treated for a duration ranging from 0.5 to 120 min at 900 °C. Douglass and Pettit [8] investigated the spectral selectivity of various pure metals like Ni, Cu, etc. Reacted with air, oxygen or water to form their corresponding oxide films. It could, however, not rationalize the enhancement of absorptance in all the cases of experiments. The two schools of thought for the attenuation of reflection in an absorber-reflector system exists; (i) one of them is based on the intrinsic absorption by the oxides. This phenomenon is not found to be very effective owing to the wide energy-gap of oxides [9] (ii) the second notion is the interference controlled absorption, wherein, the absorptivity is sensitive to the position and magnitude of the interference minimum and maximum.

The optical constants of the oxide films such as Fe₃O₄ and Cr₂O₃ have been examined on a separate basis on their corresponding metals [10]. The choice of metallic substrate, which acts as an IR reflector majorly decides the emissivity of the tandem absorber. SS 304, with its ease of availability, lower cost in comparison with the other high temperature materials, high temperature oxidation resistance, high temperature creep strength and excellent durability makes it an attractive material for the purpose. The maximum service temperature of SS 304 is 925 °C under continuous operations. Further, stainless steels are heat-treated routinely in the industry for stress relieving, hardening and annealing, strengthening the ductility and to improve corrosion resistance under controlled environments. Several other works on thermal treatment of metallic substrates have generated renewed interest in the literature [6,11]. But, still there is a tremendous scope to develop spectrally selective surfaces, widely used as collectors in solar thermal technologies, with facile processes such as simple thermal treatment of SS 304 substrates. This work explores the potential use of oxidized SS 304 as a solar selective surface and an optimum schematic of isothermal annealing in air is arrived at for the same. The variation in the values of solar absorptance with heat treatment duration is also presented. Finally, a comparative study of Inconel, Nimonic and stainless steel substrates is presented for the optimized time of annealing and temperature.