Properties of spray-deposited cobalt oxide selective coatings on aluminium and galvanised iron substrates

Highly stable selective coatings of cobalt oxide have been prepared on commercial aluminium and galvanised iron substrates by the method of spray pyrolysis. The optothermal, structural and optical properties of these films have been investigated to determine the optimum growth parameters. Best selectivity values are obtained for films with thicknesses 0·30 μm on aluminium and 0·32 μm on galvanised iron. The optimised films on aluminium give = 0·88 and _100°C = 0·15 whereas those on galvanised iron give = 0·88 and _100°C = 0·13. The films are extremely adherent and are stable up to 500°C in the case of aluminium and 350°C in the case of galvanised iron substrates.     

Blackening of aluminium-zinc (galvalume) coatings for solar energy utilization

The authors have reported a method for immersion blackening of Galvalume coatings for use as a selective surface for solar collectors. Such coatings have a solar absorptance () of 0.90–0.92 and thermal emittance (ε) of 0.25–0.40. The coating has moderate corrosion resistance. In order to improve this, a post-treatment is necessary. The post-treated coatings in dichromate solution offer good corrosion resistance. Thermal cycling tests show that the coatings are stable to 220°C. Tape tests show that the coating is strongly adherent.     

Synthesis of LiCoO2 from cobalt—organic acid complexes and its electrode behaviour in a lithium secondary battery

Well-crystallized, layered LiCoO₂ has been prepared by heating cobalt—organic acid complexes (such as malic acid and succinic acid) at 900 °C in air after preheating at 400 °C (2 h) and at 650 °C (6 h). LiCoO₂ obtained by this method shows a high (003) peak intensity and low (104) or (101) intensities in X-ray diffraction (XRD). The first discharge capacity of LiCoO₂ obtained from this method in ester-based electrolyte is 132 mA h g⁻¹ on cycling between 4.3 and 3.7 V. The value is larger than that obtained by the conventional method. X-ray diffraction studies and open-circuit voltage curves show the presence of at least two types of reaction. A two-phase reaction occurs in the region of 0.71<χ <1.0 in Li_xCoO₂. The lithiation proceeds as a homogeneous reaction together with expansion of the c-axis in the region of 0.47<χ<0.71. The expansion of the c-axis againstΔχ at x=0.56 corresponds well with the voltage jump observed in the charge/discharge curves.     

Chemical vapour deposited SnO2:Sb heat mirror coatings for cylindrical solar collectors

A chemical vapour deposition technique for growth of SnO₂:Sb heat mirror coatings on the inner walls of long cover glass tubes for cylidrical solar collectors is reported. The best performance of the tin-oxide film is obtained for those films grown from a source of SnCl₂ + 1 mol% Sb on Corning glass tubes at 520°C. These films, supported on 2 mm glass substrates, have a solar transmittance of 0.85 and an infrared reflectance of 0.8. The heat mirror coatings are observed to increase the stagnation temperature of the absorber in anevacuated tubular collector from 142 to 161°C under incident optical flux of 1150 W/m²     

Membranes based on phosphotungstic acid and polybenzimidazole for fuel cell application

Composite membranes based on phosphotungstic acid (PWA) adsorbed on silica (SiO₂) and polybenzimidazole (PBI) have been prepared and their physico-chemical properties have been studied. The membranes with high tensile strength and thickness of less than 30 μm can be cast. They are chemically stable in boiling water and thermally stable in air up to 400°C. Proton conductivity is influenced by the temperature (range: 30–100°C), relative humidity and PWA loading in the membrane. Maximum conductivity of 3.0×10⁻³ S/cm is obtained at 100% relative humidity and 100°C with membrane containing 60 wt.% PWA/SiO₂ in PBI. Conductivity measurements performed at higher temperatures, in the range from 90°C to 150°C, give almost stable values of 1.4–1.5×10⁻³ S/cm at 100% relative humidity.     

Nickel-black solar absorber coatings

A new electrolyte has been proposed for the deposition of nickel-black selective coatings for use in flat-plate solar collectors. The authors have studied the influence of various ingredients and operating parameters on the appearance of the black coatings so obtained with special reference to their optical values (, ε). The coating exhibits better corrsion resistance than the well known black-nickel coatings (which contains nickel, zinc and sulphur) apart from having thermal stability upto 380°C. With no zinc in the deposit, this can be used in place of black nickel coatings with less corrosion problems.     

Accelerated aging tests of chromium containing amorphous hydrogenated carbon coatings for solar collectors

Chromium containing amorphous hydrogenated carbon films (a-C : H/Cr) have been prepared by simultaneous rf plasma activated chemical vapour deposition of methane and magnetron sputtering of a chromium target. During deposition the substrates were heated (up to 300°C) and DC biased (−200 and −600 V) in order to obtain films with high chemical stability. Constant temperature tests were performed at 250°C in air with coatings deposited on silicon substrates. The degradation of the coatings was monitored by Raman spectroscopy and reflectance and transmission measurements. The main degradation mechanisms are discussed and the relevant parameters which improve the durability of the coatings are presented. Furthermore, the durability of solar selective, multilayered coatings which were deposited on copper sheets was investigated. Based on accelerated aging tests at different temperature loads in air (at 220°C, 250°C and 300°C) and in a humid environment (80°C sample temperature in humid air with 85°C and 95% relative humidity) the service lifetime in a flat plate collector is predicted to amount to more than 25 years.     

Novel device structure for Cu(In,Ga)Se2 thin film solar cells using transparent conducting oxide back and front contacts

Cu(In_1−xGa_x)Se₂ (CIGS)-based thin film solar cells fabricated using transparent conducting oxide (TCO) front and back contacts were investigated. The cell performance of substrate-type CIGS devices using TCO back contacts was almost the same as that of conventional CIGS solar cells with metallic Mo back contacts when the CIGS deposition temperatures were below 500 °C for SnO₂:F and 520 °C for ITO. CIGS thin film solar cells fabricated with ITO back contacts had an efficiency of 15.2% without anti-reflection coatings. However, the cell performance deteriorated at deposition temperatures above 520 °C. This is attributed to the increased resistivity of the TCO’s due to the removal of fluorine from SnO₂ or undesirable formation of a Ga₂O₃ thin layer at the CIGS/ITO interface. The formation of Ga₂O₃ was eliminated by inserting an intermediate layer such as Mo between ITO and CIGS. Furthermore, bifacial CIGS thin film solar cells were demonstrated as being one of the applications of semi-transparent CIGS devices. The cell performance of bifacial devices was improved by controlling the thickness of the CIGS absorber layer. Superstrate-type CIGS thin film solar cells with an efficiency of 12.8% were fabricated using a ZnO:Al front contact. Key techniques include the use of a graded band gap Cu(In,Ga)₃Se₅ phase absorber layer and a ZnO buffer layer along with the inclusion of Na₂S during CIGS deposition.     

Low temperature μc-Si film growth using a CaF2 seed layer

This paper describes low temperature thin film Si growth by remote plasma chemical vapor deposition system for photovoltaic device applications. Using CaF₂/glass substrate, we were able to achieve an improved μc-Si film at a low process temperature of 300°C. The μc-Si film on CaF₂/glass substrate shows that a crystalline volume fraction of 65% and dark conductivity of 1.65×10⁻⁸ S/cm with the growth conditions of 50 W, 300°C, 88 mTorr, and SiH₄/H₂=1.2%. XRD analysis on μc-Si/CaF₂/glass showed crystalline film growth in (1 1 1) and (2 2 0) planes. Grain size was enlarged as large as 700 Å for a μc-Si/CaF₂/glass structure. Activation energy of μc-Si film was given as 0.49 eV. The μc-Si films exhibited dark- and photo-conductivity ratio of 124.     

Cd-free buffer layers for CIGS solar cells prepared by a dry process

ZnSe buffer layers for Cu(ln,Ga)Se₂/buffer/ZnO solar cells have been prepared by metal organic chemical vapor deposition (MOCVD). Using photoassisted MOCVD, deposition temperatures down to 265°C are possible. It is shown that deposition temperatures well below 300°C are essential as well as deposition times not much longer than 3 min. Higher temperatures and longer deposition times lead to absorber degradation. With optimized buffer deposition efficiencies of 11% have been obtained on CIGS absorbers from the Siemens pilot production line.     

Solar preparation of SiOx (x ≈ 1) nanopowders from silicon vaporisation on a ZrO2 pellet. XPS and photoluminescence characterisation

SiO_x nanoparticles were prepared by vaporisation and condensation of melted silicon droplets put on zirconia pellets in a solar reactor at the focus of a 2 kW solar furnace. The size of the grains were nanometric, generally included in the range 20–40 nm, and the O/Si atomic ratio values were close to stoichiometry (O/Si ≈ 1 ± 0.2). XPS, DRX and TEM analyses show that these nanoparticles are amorphous with various silicon chemical environments which can be described as constituted with polysubstituted Si-(O_4−nSi_n) tetrahedral configurations. The estimated oxygen atomic concentrations for these nanoparticles was in good agreement with thermodynamic equilibrium calculations for the system ZrO₂–Si at high temperature. The predominant gaseous species is the SiO molecule. The SiO_x nanoparticles present photoluminescence property similar to those currently reported for electrolytic porous silicon.     

Influence of carbon content and tempering temperature on fatigue threshold characteristics of a low alloy steel

This paper attempts to describe the effect of carbon content on the fatigue threshold characteristics ΔK_th in various heat treated conditions. Essentially it has been shown that a tempering treatment increased ΔK_th while increasing the carbon content of steels from 0·13% to 0·8% significantly decreased the ΔK_th value by over 100%. At intermediate fatigue crack growth rates all the data show a linear relationship with ΔK level.

In terms of yield strength σ_y, the threshold stress intensity level could be given by the expression: ΔK_th = 8·74 − 3·42 × 10⁻³(σ_y).

At near threshold fatigue crack growth levels significant amounts of isolated intergranular failure were observed in the 400°C tempered condition. In the other heat treated microstructures only a flat trans-granular ductile striated failure mode was evident. A maxima in the amount of intergranular facets occurred at ΔK values approaching 15 to 20 MPa√m. It has been shown the existence of intergranular failure resulted from environmentally induced fracture (through the diffusion of hydrogen) that occurred within the crack tip enclave.     

Instrumented drop-weight test results for normalised and tempered 9Cr1Mo steel

From instrumented drop-weight tests, the nil ductility transition temperature (T_NDT), and a conservative estimate of dynamic fracture toughness (K_Id), at T_NDT for normalised and tempered 9Cr---1Mo steel, are determined to be −25°C and 70 MPa√m, respectively. The latter value agrees well with that determined from pre-cracked Charpy tests. The K_Id/σ_Yd (σ_Yd is the dynamic yield stress) ratio at T_NDT is estimated to be 0·076 √m, in agreement with previous estimates. The uncertainties in crack profile measurement and effect of microstructural variation in the heat affected zone on fracture loads are also discussed.     

Forced steady-state convections from pin-fin arrays

The steady-state thermal and air-flow resistance performances of horizontally-based pin-fin assemblies have been investigated experimentally. The effects of varying the geometrical configurations of the pin-fins and the air-flow rates have been studied. The optimal pin-fin separation S_xopt in the span-wise direction, to achieve a maximum rate of heat transfer from the assembly, is 1·0 ± 0·2 mm ≤ S_x ≤ 3·0 ± 0·2 mm for the pin-fins arranged either in the in-line or the staggered array. The optimal pin-fin separations in the stream-wise direction for these two arrays are 7·6 ± 0·2 mm and 7·8 ± 0·2 mm respectively. The general correlation of the data is:

     

Optical properties of black nickel selective surface by conversion coating process

Optical properties of nickel black selective surfaces, prepared by a conversion coating process, on zincated aluminium alloy surfaces are studied in relation to the processing parameters. It is possible to obtain nickel black surfaces with solar absorptances ranging from 0.95 to 0.97 and thermal emittances ranging from 0.2 to 0.4. The coatings are stable up to 200°C but show degradation under humid conditions. Surface studies indicate that the optical properties may be explicable by the structure of these nickel films.     

Investigation of the impact of atmospheric particles on solar radiation at Nsukka, Nigeria

Using a well-calibrated integrating nephelometer of sensitivity 10⁻⁷/m, we have successfully measured the scattering extinction coefficient of atmospheric fine particles, b_sp at Nsukka, Nigeria, at latitude 6.8°N and longitude 7.35°E and an elevation of 488 m above sea level.

The values of b_sp recorded are 10.0 × 10⁻⁴/m (maximum) and 0.17 × 10⁻⁴/m (minimum). We have also investigated the impact of the scattering extinction coefficient on solar radiation. Results of our investigation reveal that, for high b_sp, mean insolation is generally low, and the reverse is the case for low b_sp. It appears that there is no correlation between instantaneous (hourly) extinction coefficient and insolation for the period covered.     

Effect of temperature on the morphological and photovoltaic stability of bulk heterojunction polymer:fullerene solar cells

In high performance polymer:fullerene bulk heterojunction solar cells the nanoscale morphology of interpenetrating acceptor:donor materials is optimized through appropriate preparation conditions such as annealing and choice of solvent, but this initial state-of-the-art morphology will not remain stable during long-term operation. We report the effects of prolonged storage at elevated temperatures on both the morphology and the photovoltaic performance for the model systems MDMO-PPV:PCBM and P3HT:PCBM as compared to ‘High T_g PPV’:PCBM based solar cells, where the ‘High T_g PPV’ is characterized by its high glass transition temperature (138 °C). In situ monitoring of the photocurrent–voltage characteristics at elevated temperatures, in combination with a systematic transmission electron microscopy (TEM) study and complementary optical spectroscopy, reveals distinct degradation kinetics and morphological changes that indicate the occurrence of different underlying physico-chemical mechanisms.     

Optical and electrochemical characteristics of niobium oxide films prepared by sol-gel process and magnetron sputtering A comparison

Electrochromic niobia (Nb₂0₅) coatings were prepared by the sot-gel spin-coating and d.c. magnetron sputtering techniques. Parameters were investigated for the process fabrication of sol-gel spin coated Nb₂0₅ films exhibiting high coloration efficiency comparable with that d.c. magnetron sputtered niobia films. X-ray diffraction studies (XRD) showed that the sot-gel deposited and magnetron sputtered films heat treated at temperatures below 450°C, were amorphous, whereas those heat treated at higher temperatures were slightly crystalline. X-ray photoelectron spectroscopy (XPS) studies showed that the stoichiometry of the films was Nb₂0₅. The refractive index and electrochromic coloration were found to depend on the preparation technique. Both films showed low absorption and high transparency in the visible range. We found that the n, k values of the sot-gel deposited films to be lower than for the sputtered films. The n and k values were n = 1.82 and k = 3 × 10⁻³, and n = 2.28 and K = 4 × 10⁻³ at 530 urn for sot-gel deposited and sputtered films, respectively. The electrochemical behavior and structural changes were investigated in 1 M LiC10₄/propylene carbonate solution. Using the electrochemical measurements and X-ray photoelectron spectroscopy, the probable electrode reaction with the lithiation and delithiation is Nb₂O₅ + x Li⁺ + x e⁻ ↔ Li_xNb₂0₅. Cyclic voltametric (CV) measurements showed that both Nb₂0₅ films exhibits electrochemical reversibility beyond 1200 cycles without change in performance. “In situ” optical measurement revealed that those films exhibit an electrochromic effect in the spectral range 300 < λ < 2100 nm but remain unchanged in the infrared spectral range. The change in visible transmittance was 40% for 250 nm thick electrodes. Spectroelectrochemical measurements showed that spin coated films were essentially electrochemically equivalent to those prepared by d.c. magnetron sputter deposition.     

Schottky solar cells with amorphous carbon nitride thin films prepared by ion beam sputtering technique

This paper reports on the successful deposition of amorphous carbon nitride thin films (a-CN_x) and fabrication of ITO/a-CN_x/Al Schottky thin-film solar cells by using the technique of ion beam sputtering. XPS and Raman spectra are used to characterize the deposited thin films. Nitrogen atoms are incorporated into the films in the form of carbon–nitrogen multiple bands. Their optical properties are also investigated using a spectroscopic ellipsometer and UV/VIS/NIR spectrophotometer. The refraction of the carbon nitride thin films deposited lies in the range of 1.7–2.1. The Tauc optical band gap is about 0.6 eV. The photovoltaic values of the device, short-circuit current and open-circuit voltage are 1.56 μA/cm² and 250 mV, respectively, when exposed to AM1.5 illumination (100 mW/cm², 25°C).     

Upper bounds for the yearly energy delivery of stationary solar concentrators and the implications for concentrator optical design

Compound parabolic concentrator (CPC) type collectors have been viewed as the optimal design for totally stationary concentrators. However the CPC is ideal only for uniform incident solar flux averaged over the energy collection period. The actual yearly-averaged incident flux map turns out to be highly non-uniform, as a function of projected incidence angle, which implies that concentration can be increased markedly if optical collection efficiency is compromised. The question then becomes: what concentrator angular acceptance function is best matched to nature's radiation flux input, and how much energy can such a concentrator deliver? The recently-invented tailored edge-ray concentrator (TERC) approach could be used to determine optimal reflector contours, given the optimal acceptance angle function. We demonstrate that totally stationary TERCs can have around three times the geometric concentration of corresponding optimized stationary CPCs, with greater energy delivery per absorber area, in particular for applications that are currently being considered for stationary evacuated concentrators with the latest low-emissivity selective coatings, e.g. solar-driven double-stage absorption chillers (at around 170°C) and solar thermal power generation (at around 250°C).