Elsevier

Renewable Energy

Volume 33, Issue 2, February 2008, Pages 348-353
Renewable Energy

Preparation of selective absorbers based on CuMn spinels by dip-coating method

https://doi.org/10.1016/j.renene.2007.05.017Get rights and content

Abstract

CuMn-spinel thin films were prepared on aluminum substrates by the so-called dip-coating method. The layers were deposited from alcoholic solutions based on nitrate precursors and subsequently sintered in air at 500 °C. Reflectance spectra in the NIR–vis–UV interval were measured for samples with different composition and thickness. The absorber quality of the films was checked by calculating the solar absorptance. The films displaying the best reflectance spectra and the highest solar absorptances (αs>0.87) were deposited from solutions containing molar ratio Cu/Mn=1. The analysis of composition showed that Cu/Mn ratio in the film was very close to the ratio in the dip-in solution and supported the formation of a spinel-like material of stoichiometry Cu1.5Mn1.5O4. Solar absorptance was dramatically improved when a SiO2 antireflective layer was deposited onto the spinel. By optimizing film thickness of both CuMn-spinel and SiO2 layers optical parameter values as good as αs=0.94 and εT(1 0 0)=0.06 were achieved.

Introduction

The efficiency in solar–thermal energy conversion systems strongly depends on the materials used as absorber coatings, especially on their optical properties and thermal stability. At this moment, the conventional coatings used in the solar energy industry include: electroplated black chrome, which employs Cr–Cr2O3 cermet material and is the most widely used solar absorber [1], [2]; nickel-pigmented anodic Al2O3, which is produced by electrochemical treatment of an aluminum sheet [3], [4], the absorber from Sunselect-Alanod, which is also a cermet prepared by sputtering and finally the so-called TINOX, which is an evaporated titanium nitride film. The preparation and investigation of new selective absorbers for solar collectors has attracted considerable attention, especially during the last decades. Among the possible materials, black-colored transition-metal oxides with spinel-like structure could be promising candidates since they can attain high absorptance and low emittance values and display also a very high thermal stability. Therefore, the aim of this work has been the preparation of CuMn-spinel thin films to be used as selective absorbers in low-temperature solar–thermal applications. As deposition method, a dip-coating process developed in our group for preparing metal oxides coatings has been chosen [5]. This method presents several advantages like being simple, cost-effective, easy to scale up and versatile in operation. The optical properties of the absorber have been improved by subsequently depositing an antireflective layer of SiO2 by sol–gel. Since our goal has been to obtain an absorber for low-temperature applications, aluminum has been chosen as substrate for preparing the samples as it also has a very high IR reflectivity.

Section snippets

Experimental

CuMn-spinel layers were deposited on commercially available highly polished Al foils. Metallic precursors were copper and manganese nitrates dissolved in absolute ethanol at various concentrations. A complexing agent and a wetting additive were also added to stabilize the solution and improve the film adherence. Such solutions showed long-term stability, which made the absorber deposition both reproducible and reliable provided the viscosity of the solution was kept constant. Depositions were

Results and discussion

In order to check the quality of CuMn-spinels as selective absorbers, reflectance spectra in the range 300–2500 nm were measured and solar absorptance was calculated for each case. It is well known that the thickness of the absorber layer also determines the final absorptance of the system. In our case, we did not measure the thickness of the CuMn-spinel layers but it could be estimated to be about 100–200 nm, which is inferred from the position of the minima appearing in the reflectance spectra.

Conclusions

CuMn-spinel layers were successfully deposited on Al substrates by the dip-coating method. Reflectance spectra in the NIR–vis–UV interval were measured for samples with different composition and thickness and solar absorptance was calculated in order to check their quality as solar absorbers. The films displaying the best reflectance spectra and the highest solar absorptances (αs>0.87) were deposited from a solution containing [Cu2+]=0.15 M and [Mn2+]=0.15 M (i.e. molar ratio Cu/Mn=1) at

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