Tuquabo Tesfamichael

Oxidation Kinetics of Nickel Particles: Comparison Between Free Particles and Particles in an Oxide Matrix

Abstract The degradation of selective solar absorbers through oxidation has been studied. We compare the oxidation kinetics of nickel particles of various sizes. Both free particles and particles embedded in an oxide matrix were studied. The oxidation kinetics of polycrystalline nickel nanorods was determined by IR spectroscopy in the temperature range 300–500°C. The particles were oxidized when situated in the porous alumina matrix of an electrochemically deposited solar absorber coating. The oxidation kinetics was compared to that of free nanometer-particles at the same temperature and to micron-sized polycrystalline nickel particles, which were studied by thermogravimetry in a wider temperature range. It was found that the rate constant was markedly lower for the particles in the matrix. Implications for the durability of selectively solar absorbing coatings are discussed.

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.

A theoretical feasibility study of pigments for thickness-sensitive spectrally selective paints

We present a model for thickness-sensitive spectrally selective paints and use it to optimize their optical properties with respect to the particle size of the pigment. Pigments were chosen from different classes of materials such as metals, low band gap insulators and semiconductors and carbon. Silicone was chosen as the binder and the paint thickness was varied from 1 to 4 µm. Scattering and absorption cross sections were derived from Mie theory for spherical particles, and the particle radii ranged between 10 and 500 nm. The reflectance was derived from a radiative transfer formulation of a four-flux model, assuming a mono-disperse particle ensemble. The integrated values for near-normal solar absorptance and thermal emittance at 100°C were calculated from the total near-normal spectral reflectance in the wavelength range 0.3–30 µm. It was found that all the pigments investigated have an optimal particle radius of about 100 nm in the case of a 1.0 µm thick paint layer and a particle volume fraction of 0.20. The optimal particle size increases slightly for thicker films. It was also found that direct, low band gap semiconductors give the best spectral selectivity. A solar absorptance of 0.91 and a thermal emittance of 0.13 were computed for PbS particles of volume fraction 0.20 in a 2.0 µm thick paint layer on aluminium.

Bactericidal Effects of Natural Nanotopography of Dragonfly Wing on Escherichia coli

Nanotextured surfaces (NTSs) are critical to organisms as self-adaptation and survival tools. These NTSs have been actively mimicked in the process of developing bactericidal surfaces for diverse biomedical and hygiene applications. To design and fabricate bactericidal topographies effectively for various applications, understanding the bactericidal mechanism of NTS in nature is essential. The current mechanistic explanations on natural bactericidal activity of nanopillars have not utilized recent advances in microscopy to study the natural interaction. This research reveals the natural bactericidal interaction between E. coli and a dragonfly wings (Orthetrum villosovittatum) NTS using advanced microscopy techniques and proposes a model. Contrary to the existing mechanistic models, this experimental approach demonstrated that the NTS of Orthetrum villosovittatum dragonfly wings has two prominent nanopillar populations and the resolved interface shows membrane damage occurred without direct contact of the bacterial cell membrane with the nanopillars. We propose that the bacterial membrane damage is initiated by a combination of strong adhesion between nanopillars and bacterium EPS layer as well as shear force when immobilized bacterium attempts to move on the NTS. These findings could help guide the design of novel biomimetic nanomaterials by maximizing the synergies between biochemical and mechanical bactericidal effects.

Treatment of antireflection on tin oxide coated anodized aluminum selective absorber surface

Nickel-pigmented anodic aluminum oxide, Ni–Al2O3 was pyrolytically coated with tin oxide (SnO2). The undesirable increase of reflectance in the solar spectrum due to the high refractive index of the SnO2 film was compensated by an antireflection layer. The layer was applied by a simple dipping technique in a bath containing a commercial colloidal silica sol which forms a silica (SiO2) layer. The infrared reflectance is nearly unaffected by the silica sol treatment process. Preliminary test results indicate that treated samples are resistant to temperatures as high as 300°C as well as to corrosion in an 8% sulfuric acid solution. In addition, the optical properties were unaffected by outdoor exposure for two months.

Optical characterization and modeling of black pigments used in thickness-sensitive solar-selective absorbing paints

The performance of black pigments used in thickness-sensitive solar-selective absorbing paints for solar thermal collectors depends on the optical properties of the pigments. Knowledge of the intrinsic optical properties of most paint pigments is very limited. Pellets made from either FeMnCuOx or black carbon dispersed in KBr matrix have been used to determine the effective optical coefficients of the pigments. Scattering and absorption cross-sections were derived from reflectance and transmittance measurements at near normal angle of incidence in the wavelength range 0.3 to 2.5 μm. Volume concentrations of the pigments that gave linear dependencies for the coefficients were found to be between 0.053 and 0.53% (FeMnCuOx) and 0.076 and 0.31% (black carbon). Subsequently, a four-flux model has been used for calculation of reflectance for thickness-sensitive spectrally-selective paints. The paints were obtained from the FeMnCuOx and black carbon pigments embedded in silicone and phenoxy resin, respectively. We have used the experimentally determined scattering and absorption cross-sections of the pigments as input to the four-flux theory and the calculations have been performed for different thicknesses and/or pigment volume fraction of the paints. The calculated reflectances were compared with experiments and the results for thick films fit well.

Angular solar absorptance of absorbers used in solar thermal collectors

The optical characterization of solar absorbers for thermal solar collectors is usually performed by measurement of the spectral reflectance at near-normal angle of incidence and calculation of the solar absorptance from the measured reflectance. The solar absorptance is, however, a function of the angle of incidence of the light impinging on the absorber. The total reflectance of two types of commercial solar-selective absorbers, nickel-pigmented anodized aluminum, and sputtered nickel nickel oxide coated aluminum are measured at angles of incidence from 5 to 80 in the wavelength range 300-2500 nm by use of an integrating sphere. From these measurements the angular integrated solar absorptance is determined. Experimental data are compared with theoretical calculations, and it is found that optical thin-film interference effects can explain the significant difference in solar absorptance at higher angles for the two types of absorbers.

Characterization of a commercial dye-sensitised titania solar cell electrode

We have characterised a dye-sensitised nanoporous nanocrystalline titania film used in prototype photoelectrochemical solar cell production. From transmission electron microscopy the particles were seen as mixtures of tetrahedral and rhombohedral geometries with size distribution in the range between 10 and 25 nm. These particles were identified by X-ray diffraction as nanocrystals of anatase and brookite phases. The film was sensitised with a ruthenium (II) based chromophore for different times (between 0.5 and 24 h) and the penetration and coverage of the dye was studied using secondary ion mass spectroscopy. The dye was found to percolate through the whole of the titania film and was distributed uniformly. Using Rutherford backscattering, the composition of the film was determined and found to be 1 wt% Ru on maximum sensitisation. The optical properties of the dye-sensitised films were also measured which resulted in an increase of absorbance and a decrease of transmittance for dyeing times up to 8 h. Beyond this time the values remained unchanged and thus a semi-transparent film with luminous transmittance between 0.12 and 0.60 were obtained.

Optical properties of silicon pigmented alumina films

Plates of Al–Si alloy were anodized in a sulfuric acid solution. This treatment provides a Si–Al2O3 coating growing at a rate of 0.14 μm/min. The Si particles had sizes between 1 and 10 μm, as seen by scanning electron microscopy. Optical measurements showed a continuous decrease of reflectance with increasing film thickness. The reflectance of the Si–Al2O3 coated aluminum could be understood from a four flux radiative transfer theory. In order to explain our measurements, it was found necessary to include a free-carrier term in the dielectric permittivity of Si. The free carriers are probably due to doping with Al. Hence, the relaxation time of the free carriers is determined by scattering from the charged Al impurities.

Optical characterization method for black pigments applied to solar-selective absorbing paints

We propose a novel, to our knowledge, method for characterizing the optical properties of pigment particles or powders. Measurements of the diffuse and the total transmittance as well as the diffuse and the total reflectance are used to obtain effective scattering and absorption coefficients per unit length for the particles that are dispersed in a continuous matrix. For dilute dispersions in the single-scattering regime scattering and absorption cross sections of the particles were obtained. The method was applied to two pigments, namely, FeMnCuO(x) and black carbon. The data were obtained by use of pellets consisting of low concentrations of FeMnCuO(x) or black-carbon pigments dispersed in a KBr matrix. The pigment volume concentrations used to evaluate the scattering and the absorption coefficients ranged from 0.053% to 0.530% for FeMnCuO(x) and 0.076% to 0.310% for the black carbon. These ranges were found to exhibit the linear dependence of the coefficients as a function of volume fraction, as given by single-scattering theory.