Optical constants of sputtered Ni/NiO solar absorber film—depth-profiled characterization

An optical analysis of a solar absorber with a graded index coating of nickel–nickel oxide deposited in a continuous process for reactive DC magnetron sputtering is reported. The optical constants were determined by a set of 14 samples deposited under the same sputtering conditions as in the continuous process representing the composition at 14 different depths in the continuously graded index coating solar absorber. The optical constants of the 14 layers were derived from optical measurements of reflectance and transmittance in the wavelength range 300–2500 nm. The optical constants were used to construct a model for the graded refractive index in the solar-absorbing coating. It was found that the graded index was not linear in depth profile and hence could be modelled with four layers: two metallic layers in the base closest to the substrate, a dielectric layer at the top of the coating and one intermediate layer representing the change from metallic to dielectric properties in the middle part of the coating.     

Optical Characterization Of Industrially Sputtered Nickel–Nickel Oxide Solar Selective Surface

Tandem absorbers are often used in the design of solar absorbers for photo thermal conversion. They consist of a thin coating, selectively absorbing in the wavelength range of the solar spectrum, on a metal substrate. The optical performance of a tandem absorber depends on the optical constants and thickness of the absorbing coating and also on the reflectivity of the underlying metal. A very high solar absorptance is achieved when the coating has a non-uniform composition in the sense that the refractive index is highest closest to the metal substrate and then gradually decreases towards the front surface. This type of composition suppresses coating interference and gives a low front surface reflection if the refractive index at the front surface is low. We report on optical analysis of a solar absorber with a graded index coating of sputtered nickel–nickel oxide deposited on aluminium. The optical constants have been determined from reflectance, transmittance and ellipsometry data by fitting the data to a two-layer model of the coating. The optical constants of the two layers can be regarded as effective optical constants for the lower and upper part of the graded index coating respectively. It is found that the effective refractive index of the top layer is somewhat lower than for the base layer. The extinction coefficient is higher in the lower part of the coating. Both effective refractive index and extinction coefficient of the base layer increase monotonically with increasing wavelength as for metallic materials.     

New method to optimize a solar absorber graded film profile

Normalized dc conductivity was used to analyze the inhomogeneously graded depth profile structure of deposits from dc magnetron reactive sputtering in a roll coater. The graded coating was obtained by moving the substrate through the deposition zone with a non-uniform oxidation along the zone mainly due to the asymmetrical position of the oxygen inlet to the right of the target. With this arrangement, the composition of the deposited film was gradually changed from metal to metal oxide as the substrate moved from left to right through the zone. The profile control therefore relied on the non-uniform oxidation along the sputtering zone. The study shows that the normalized dc conductivity of stationary samples in this roll coater offers a simple and effective method to optimize the graded composition in spectrally selective solar absorber coatings. A solar absorptance of 0.91 with thermal emittance of 0.05 at 100 °C was achieved for a single graded film without antireflection treatment.     

Angular solar absorptance and incident angle modifier of selective absorbers for solar thermal collectors

The solar absorptance of absorbers for thermal solar collectors is usually characterized at near normal angle of incidence. The solar absorptance is however a function of the angle of the incident light on the absorbers. In this paper the angular solar absorptance of commercial nickel pigmented aluminum oxide and sputtered nickel/nickel oxide solar selective absorbers are reported. The solar absorptance was calculated from experimental total reflectance spectra in the wavelength range 300–2500 nm for angles of incidence between 5 and 80°. It was found that the solar absorptance at higher angles of incidence is lower for the sputtered nickel/nickel oxide than for the nickel pigmented aluminum oxide coating. This could be understood from theoretical calculations based on microstructure models of the two types of coatings. The nickel pigmented aluminum oxide with a double-layer structure of its coating has an enhanced higher angle solar absorptance due to thin film interference effects which can not be achieved from a graded-index thin film coatings as is the case for the sputtered nickel/nickel oxide absorber. When the absorbers were covered by glass, as is common for most solar collectors, a negligible difference in optical performance at the higher angles of incidence has been obtained. These results were consistent with a theoretical calculation by use of an incident angle modifier model.     

Gasochromic windows

Gasochromic windows can change their transmittance over a wide range. This change is caused by a thin layer of tungsten oxide (WO₃), covered by a very thin layer of platinum. Exposing this coating to diluted hydrogen gas leads to reduction of the WO₃, resulting in colouring. This process can be reversed by introducing diluted oxygen.The hydrogen and oxygen are produced by an electrolyser. Only small amounts of gas are needed for the switching process. The coatings are produced by sputtering. Water is needed in the WO₃ films to allow rapid transport of the hydrogen. However, this water should not escape when the system is operated at higher temperatures, which can exceed 60°C. By adjusting the conditions of the sputtering process appropriately, a large amount of water can be incorporated in the films, which remains even up to temperatures above 100°C.The best transmittance values obtained for a coated double-glazed unit with a moderate film thickness (560 nm) and hydrogen concentrations below the combustion limit are 76% and 77% for solar and visual transmittance, respectively, in the bleached state and 5% and 6% for solar and visual transmittance, respectively, in the coloured state. Darker states can be obtained by applying thicker films of tungsten oxide without reducing the transmittance in the bleached state.Gasochromic coatings can also be deposited easily on plastic substrates and—because their coating structure is so simple—combined with prismatic microstructures, which allow light to be redirected. In addition, the gasochromic systems technology can be used with metal hydride systems.     

A comparative study of the optical properties of nickel pigmented alumina films of different thicknesses exposed to elevated temperature and humidity

Commercial solar absorbers of nickel pigmented anodized aluminium are composed of an inner nickel pigmented sublayer of about 0.3 μm thickness and a 0.4–0.5 μm thick top layer of plain alumina. Thermal emittance can be reduced from 0.17 to 0.12 if the top layer is made thinner, to be about 0.1 μm. The solar absorptance is 0.96 as for the thicker coating. In this study degradation is analysed for samples with thin or thick alumina top layer after exposure to elevated temperature, 300–500°C, or humidity. The results from these tests show that a thinner aluminium oxide top layer has the same durability as a thicker top layer. The implication of making commercial nickel pigmented anodized aluminium with an oxide half as thick as today is a reduction of the anodization time to about half the time and lower manufacturing costs.     

A novel design in composities of various materials for solar selective coatings

This study describes the development of multilayer metal-dielectric graded index solar selective coatings in which the metallic volume fraction increases with depth, from top (air–film interface) to bottom (film–substrate interface). The work is based on computer simulation followed by validation through fabrication of the coatings and optical measurements. The influence of the choice of the number of layers present in a graded index composite selective absorber and results obtained for a new destructive interference bilayer (four-layer system) coating, designed using the computer model, were studied. The design and optimization of the composite coating was undertaken using a computer tool developed within this program of research employing Bruggeman and Maxwell–Garnett effective medium formalisms. The design tool enabled all key design parameters, with the exception of particle size and orientation, to be varied systematically to permit the sensitivity of the optical properties of the selective absorber coating to be studied.The model was validated with a supporting program of experimental research in which many different selective absorbers were prepared by co-sputtering of metal and dielectric materials.Although the best compositional gradation can be achieved by increasing the number of layers, the variation in optical performance beyond a certain number of layers is minimal. The destructive interference produced between adjacent layers contributes to the absorptance. The effect of the number of layers (single, four and 10) has been calculated for various materials such as nickel, vanadium, tungsten, cobalt and chromium based coatings. Solar absorptance of 0.98 and 0.96 was achieved by simulation and experimental findings with less than 0.07 thermal emittance at 300 K for 200 nm thick, 4-PGSAC (four-layer system) of V : Al₂O₃ composites. Other designs showed lower optical performance for all the material combinations regardless of their individual optical properties. Use of such thin film coating on the absorbers of solar thermal appliances can reduce thermal losses significantly, which could be of importance to the relevant industry.     

Influence of oxygen on the sputtering of aluminum oxide for the surface passivation of crystalline silicon

While sputtering has been shown to be capable of depositing aluminum oxide suitable for surface passivation, the mechanisms for this are yet to be firmly established and its potential realized. In this paper, we investigate the relationships between the oxygen in the sputtering process to the resulting composition of the deposited film and the surface passivation obtained. We find that surface passivation is not strongly dependent on the bulk composition of the film. Instead the results indicate that the interfacial silicon oxide layer that forms after annealing between the aluminum oxide film and the silicon is a much more important factor; it is this combined structure of aluminum oxide, silicon oxide and silicon that is crucial for obtaining negative charges and excellent surface passivation.     

Optimization of CdO layer in a Se---CdO photovoltaic cell

An experimental study was made on a Se---CdO photovoltaic cell of conventional structure to optimize the conditions during the dc reactive sputtering of the CdO layer. The parameters varied were: sputtering current, pressure, gas flow rate, deposition time and ratio of oxygen to argon in the sputtering gas. These parameters were found to be very critical in determining photovoltaic cell performance at higher light levels. The electrical resistance and optical transparency in the CdO, are determined by its stoichiometry and this in turn was found to be controlled by the film deposition rate, through sputtering current and pressure and by oxygen content in the sputtering gas. A laboratory-fabricated Se---CdO photovoltaic cell with an optimized CdO window layer (but without an optimized collecting grid or antireflecting coating) was found to yield a conversion efficiency of about 2.5% under 100 mW/cm² of solar irradiance and about 5% under fluorescent room light of irradiance 0.13 mW/cm². For a standard commercial selenium photometry cell the corresponding efficiency values were 0.3% and 3%, respectively.     

Replacement of the CBD–CdS buffer and the sputtered i-ZnO layer by an ILGAR-ZnO WEL: optimization of the WEL deposition

As shown earlier the window extension layer (WEL) concept for thin film solar cells based on chalcopyrites results in device performances exceeding those of corresponding chemical bath deposited cadmium sulfide (CBD–CdS) buffered reference cells. The WEL concept is extended and it will be demonstrated, that now a single WEL successfully replaces both, the conventional buffer and the intrinsic part of the window bi-layer usually deposited by sputtering. Thus, one part of the window is deposited directly onto the absorber by a soft process called ion layer gas reaction (ILGAR). The optimization of ILGAR-ZnO WELs on Cu(In,Ga)(S,Se)₂ absorbers with respect to the efficiencies of the completed solar cells is presented. This effort results in ‘total area’ efficiencies of 14.5% (best cell) which are comparable to those of devices with CBD–CdS buffer (14.7%—best cell) without any antireflecting coating.     

Effect of magnetron sputtered anode functional layer on the anode-supported solid oxide fuel cell performance

Nickel oxide-yttria stabilized zirconia (NiO-YSZ) thin films were reactively sputter-deposited by pulsed direct current magnetron sputtering from the Ni and ZrY targets onto heated commercial NiO-YSZ substrates. The microstructure and composition of the deposited films were investigated with regard to application as thin anode functional layers (AFLs) for solid oxide fuel cells (SOFCs). The pore size, microstructure and phase composition of both as-deposited and annealed at 1200 °C for 2 h AFLs were studied by scanning electron microscopy and X-ray diffractometry and controlled by changing the deposition process parameters. The results show that annealing in air at 1200 °C is required to improve structural homogeneity of the films. NiO-YSZ films have pores and grains of several hundred nanometers in size after reduction in hydrogen. Adhesion of deposited films was evaluated by scratch test. Anode-supported solid oxide fuel cells with the magnetron sputtered anode functional layer, YSZ electrolyte and La_0.6Sr_0.4Co_0.2Fe_0.8O₃/Ce_0.9Gd_0.1O₂ (LSCF/CGO) cathode were fabricated and tested. Influence of thin anode functional layer on performance of anode-supported SOFCs was studied. It was shown that electrochemical properties of the single fuel cells depend on the NiO volume content in the NiO-YSZ anode functional layer. Microstructural changes of NiO-YSZ layers after nickel reduction-oxidation (redox) cycling were studied. After nine redox cycles at 750 °C in partial oxidation conditions, the cell with the anode NiO-YSZ layer showed stable open circuit voltage values with the power density decrease by 11% only.     

Computations of the optical properties of metal/insulator-composites for solar selective absorbers

Solar selective absorbers are very useful for photo thermal energy conversion. The absorbers normally consist of thin films (mostly composite), sandwiched between the antireflection layer and (base layer on) a metallic substrate, selectively absorbing in the solar spectrum and reflecting in the thermal spectrum. The optical performance of the absorbers depends on the thin film design, thickness, surface roughness and optical constants of the constituents. The reflectivity of the underlying metal and porosity of the antireflection coating plays important roles in the selectivity behavior of the coatings. Computer simulations, applying effective medium theories, have been used to investigate the simplest possible design for composite solar selective coatings. A very high solar absorption is achieved when the coating has a non-uniform composition in the sense that the refractive index is highest closest to the metal substrate and then gradually decreases towards the air interface. The destructive interference created in the visible spectrum has increased the solar absorption to 98%. This paper also addresses the optical performance of several metals/dielectric composites like Sm, Ru, Tm, Ti, Re, W, V, Tb, Er in alumina or quartz on the basis of their refractive indices. The antireflection coating porosity and surface roughness has been analyzed to achieve maximum solar absorption without increasing the thermal emittance. Antireflection layer porosity is a function of dielectric refractive index and has nominal effect on the performance of the coating. While, up to the roughness of 1×10⁻⁷ m RMS, the solar absorption increases and for higher roughness, the thermal emittance increases only.     

Development of n-μc-SiOx:H as cost effective back reflector and its application to thin film amorphous silicon solar cells

Development of doped silicon oxide based microcrystalline material as a potential candidate for cost-effective and reliable back reflector layer (BRL) for single junction solar cells is discussed in this article. Phosphorus doped μc-SiO_x:H layers with a refractive index ∼2 and with suitable electrical properties were fabricated by radio frequency plasma enhanced chemical vapor deposition (RF-PECVD) technique, using the conventional capacitively coupled reactors. Optoelectronic properties of these layers were controlled by varying the oxygen content within the film. The performance of these layers as BRL have been investigated by incorporating them in a single junction amorphous silicon solar cell and compared with the conventional ZnO:Al based reflector layer. Single junction thin film a-Si solar cells with efficiency ∼9.12% have been successfully demonstrated by using doped SiO:H based material as a back reflector. It is found that the oxide based back reflector shows analogous performance to that of conventional ZnO:Al BRL layer. The main advantage with this technology is that, it can avoid the ex-situ deposition of ZnO:Al, by using doped μc-SiO:H based material grown in the same reactor and with the same process gases as used for thin-film silicon solar cells.     

Characterization of black nickel solar absorber coatings electroplated in a nickel chlorine aqueous solution

Black nickel coatings electroplated in a nickel- and sodium chlorine aqueous solution have been prepared and parameters in the process have been optimized to achieve optimal solar selectivity. The best result is a solar absorptance of 0.96 and a thermal emittance of 0.10, as also has been obtained previously for the same type of coating ¹H(¹⁵N, αγ)¹²C nuclear resonance reaction (NRR) have contributed with new information about surface chemical composition, morphology and atomic composition in depth profile. These investigations reveal that the coating is porous and contains mainly metallic nickel at the substrate–coating interface and mainly nickel hydroxide at the front surface.The stability of the coating has been tested regarding high-temperature and condensation at high humidity. It has been found that the solar absorptance changes initially, during the first hour of exposure at high temperature, but will then stabilize. Condensation causes a more severe attack on the coating by cracking it. This is contradictory to the results from previous tests at high temperature and humidity in which the black nickel coating was found to be resistant     

Fabrication and properties of anode-supported tubular solid oxide fuel cells

Tubular solid oxide fuel cell (SOFC) systems have many desirable characteristics over their planar counter-parts. Anode-supported tubes provide an excellent platform for individual cells. They allow for a thin electrolyte layer to be applied to the outside of the tube, which helps to minimize polarization losses. This paper describes the fabrication of nickel–zirconia (Ni–YSZ)-based anode tubes via extrusion of a plastic mass through a die of the required dimensions. The anode tubes were then dried and fired. Tests were performed on the tubes to determine the effects of firing temperature on porosity to allow for a pinhole-free electrolyte coating to be applied. Thin layer coating techniques, including vacuum-assisted dip coating and painting, were compared. Ni–YSZ anode-supported tubular SOFCs with a gas-tight thin YSZ electrolyte layer were then realized. Microstructure of the anode support, electrolyte and cathode thin films was also examined.     

High performance sputtered Ni : SiO2 composite solar absorber surfaces

A solar selective absorber can be prepared by dispersing suitably sized metallic particles in an insulating host matrix. Absorption is assisted by controlling optical interference within the composite film. Graded index thin films of metallic nickel in quartz (Ni : SiO₂) were made by co-sputtering with metal volume fractions ranging from 10–90% from top (anti reflecting coating) to bottom (base layer) of the structure, to minimise optical interference peaks. The films are 100–170 nm thick with an additional 70 nm anti-reflection (AR) coating. Coatings of different thickness, metal volume fraction and compositional gradient were investigated. Substrates were Al and Cu and films were deposited on either the bare substrate or substrates coated with evaporated nickel. The influence of substrate choice on the optical properties was studied. Films with solar absorptance, α, in the range 0.90–0.96 and thermal emittance, ,=0.03–0.14 were achieved. The dependence of these properties on thickness, film composition and gradient and substrate were determined. A computer programme which calculates the solar absorptance and thermal emittance based on the assumptions of both Maxwell Garnett and Bruggeman theories for metal fill factors below and above 0.3, respectively, was used to design the structure of the composite films. The theoretical results are not presented here.     

Cr-N-O型选择性吸收涂层制备及热稳定性研究

采用反应磁控溅射方法制备SiO_x/Cr-N-O/Al选择性吸收涂层,该涂层太阳吸收比为95.9%、发射比3.8%、吸收发射比25.2。结合光学显微镜微观形貌分析、X射线衍射结构分析(XRD)、X射线光电子能谱成分分析(XPS)探讨涂层在250和400℃大气环境下热稳定性机理:250℃大气热处理后,涂层保持较高光谱选择性,表面形貌与物相结构未出现明显变化,SiO_x层氧化程度增大导致太阳吸收比升高,金属Cr和金属Al相互扩散导致发射比升高;400℃大气热处理后,涂层光谱选择性降低,表面出现微米级孔洞,XRD及XPS结果表明Cr-N-O吸收层被氧化,导致太阳吸收比降低,金属Cr和金属Al相互扩散导致发射比明显升高。     

Performance analysis of new cathode materials for molten carbonate fuel cells

The slow dissolution of the lithiated nickel oxide cathode represents one of the main causes of performance degradation in molten carbonate fuel cells (MCFC). Two main approaches were studied in ENEA laboratories to overcome this problem: protecting the nickel cathode covering it by a thin layer of a material with a low solubility in molten carbonate and stabilizing the nickel cathode doping it with iron and magnesium.Among several materials, due to its low solubility and good conductivity, lithium cobaltite was chosen to cover the nickel cathode and slow down its dissolution. A nickel electrode covered with a thin layer of lithium cobaltite doped with magnesium, was fabricated by complex sol-gel process. To simplify electrode preparation, no thermal treatments were made after covering to produce lithium cobaltite, and during the cell start-up LiMg_0.05Co_0.95O₂ was obtained in situ.To stabilize the nickel cathode, metal oxides Fe₂O₃ and MgO were chosen as dopant additives to be mixed with NiO powder in a tape-casting process (Mg_0.05Fe_0.01Ni_0.94O).On the prepared materials TGA analysis, morphological analysis by scanning electron microscopy (SEM-EDS) and electrical conductivity measurements were carried out.A conventional nickel cathode, the nickel cathode covered by lithium cobaltite precursors and the nickel cathode stabilized by iron and magnesium oxides were each tested in a 100 cm² fuel cell.Polarization curves and internal resistance (iR) measurements were acquired during the cell lifetime (1000 h) and the effect of gas composition variation on the cell performance was studied.From a comparison with the conventional nickel cathode it can be observed that the new materials have similar performance and show a good potential stability during the cell operating time. From the post-test analysis both the nickel cathode covered by lithium cobaltite and the nickel cathode doped with iron and magnesium seem to succeed in reducing nickel dissolution.     

Electrochemically induced electrochromic properties in nickel thin films deposited by DC magnetron sputtering

Nickel oxide thin films and other electrochromic materials receive particular attention due to the great variety of practical applications in energy conservation and in semitransparent optical devices. In this work, nickel thin films were produced by DC magnetron sputtering on ITO substrates. The nickel–ITO thin films were studied by electrochemical techniques, and electrochromic properties were induced in the films after several different cyclic voltammetry runs. The cyclic potential range was set from −400 to 600 mV and the scan rates were varied from 6.6 to 10 mV/s. The electrochromic phenomena was observed just after 80 cycles as derived from voltammograms and color changes in the nickel oxide films were observed close to 100 cycles. The optical properties of as-deposited films and of the ones tested in the electrochemical cell were determined by optical spectrophotometry in the visible range. The structural properties of the films were studied by X-ray diffractometry, scanning and transmission electron microscopy in conventional and high-resolution modes. The electrochemical properties were studied principally by the cyclic voltammetry technique. Noticeable differences in induced electrochromic behavior were observed between the nickel films deposited on two sets of ITO substrates, prepared by DC magnetron sputtering and spray pyrolysis.     

Thermal emittance of sputter deposited infrared reflectors in spectrally selective tandem solar absorbers

The thermal emittance of infrared reflectors, deposited by DC-magnetron sputtering on glass substrates of spectrally selective solar absorbers was studied. The deposition process was optimized in order to decrease the thermal emittance of the absorber. The sputter deposition process was optimized with regard to applied power and argon pressure for nickel–vanadium, copper–nickel and copper. The results show that the thermal emittance of the infrared reflector in a tandem solar absorber can be reduced from 0.12 to 0.06 by replacing nickel–chromium by copper–nickel. The copper–nickel alloy has a higher deposition rate and is less sensitive to the sputtering conditions, which is also favorable in large-scale industrial production.