Adhesion and thermal stability of thickness insensitive spectrally selective (TISS) polyurethane-based paint coatings on copper substrates

Thickness insensitive spectrally selective (TISS) paint coatings based on a polyurethane polymeric binder deposited on copper substrates were investigated to obtain information about their service lifetime. The degradation of TISS paint coatings was performed according to the methodology worked out within Task 10 of the IEA's Solar heating and the cooling programme. The activation energy (E_a) for the degradation process was derived from vibrational band changes of the polyurethane binder recorded in the infrared hemispherical reflectance spectra of TISS paint coatings exposed to different thermal loads. The results of the vibrational band analysis were correlated with cross-cut tests, showing that the coatings started to lose integrity at 190 °C but protected the copper substrate against oxidation perfectly even at 200 °C (15 days). An accelerated test procedure confirmed that TISS coatings could be safely used in solar collectors for at least 45 years.     

Polyhedral oligomeric silsesquioxane trisilanols as pigment surface modifiers for fluoropolymer based Thickness Sensitive Spectrally Selective (TSSS) paint coatings

Thickness Insensitive Spectrally Selective (TISS) paint coatings based on black pigment (PK 3060, Ferro Company) dispersed in a fluoropolymeric resin binder (Lumiflon, Asahi Company, Japan) have recently been made without added aluminium flakes and their properties have been reported for the first time. In this study we investigated in more detail the effect of trisilanol isobutyl (IB₇ T₇(OH)₃) polyhedral oligomeric silsesquioxane (trisilanol POSS) on the surface modification of PK 3060 pigment. Infrared spectral analysis of the surface modified pigment particles provided firm evidence for the formation of a POSS layer on the surface of the pigment particles, substantiated by the corresponding TEM and Energy Dispersive X-ray Spectroscopy (EDXS) measurements of functionalized and as-received pigments. SEM micrographs of the diluted dispersions in fluoropolymeric resin binder revealed uniform distribution of pigment particles with an average size of ∼300 nm and the beneficial effect of the pigment functionalization was assessed from the measured spectral selectivity of coatings of various thicknesses.     

Thermal stability and cross-linking studies of diisocyanate cured solar absorptance low-emittance paint coatings prepared via coil-coating process

Thickness-sensitive spectrally-selective (TSSS) paint coatings were prepared by using FeMnCuO_x pigment in combinations with phenoxy and silicone resins. The spectral selectivity expressed as a ratio between solar absorptance (a_s) and thermal emittance (e_T) was 0.92/0.14 for the first and 0.87/0.18 for the second type of the paint. The surface of phenoxy resin based coatings is characterized by pigment particles protruding from the surface, while silicone based paints exhibit a much smoother surface. Abrasion resistance and adhesion of both types of coatings were enhanced by cross linking the resin binder with diisocyanate hardener. Cured coatings withstand temperatures up to 135°C. Cross-linking and degradation mechanisms of both types of coatings were studied by using FT-IR spectroscopy and thermogravimetric measurements, cycling and ageing tests.     

Preparation of a TiMEMO nanocomposite by the sol–gel method and its application in coloured thickness insensitive spectrally selective (TISS) coatings

An organic–inorganic nanocomposite was prepared via sol–gel processing from 3-(trimethoxysilyl)propyl methacrylate (MAPTMS) and titanium(IV) isopropoxide (TIP) precursors (TiMEMO) in the form of a viscous resin, and used as a binder for the preparation of coloured thickness insensitive spectrally selective (TISS) paints and corresponding solar absorber coatings. The spectral selectivity of TiMEMO-based TISS paints was optimized by varying the concentrations of binder and different pigments: black, coloured (red, green and blue) and aluminium flakes, the latter imparting low thermal emittance, which was correlated to the presence of titanium in the TiMEMO sol–gel host. The formation and the ensuing structure of the sol–gel TiMEMO hybrid was studied in detail and the nanocomposite structure of the TiMEMO binder formed was assessed from infrared and ²⁹Si NMR measurements, which confirmed the formation of Ti–O–Si linkages established after the hydrolysed precursors condensed into a compliant resinous material. XRD measurements provided additional information about the existence of small coherent domains of silsesquioxane units in the sol–gel host. The abrasion resistance of the non-pigmented TiMEMO binder deposited in thin film form on a PMMA substrate was assessed by the Taber test, and its hardness compared with other resin binders which have been used for making TISS paint coatings. The surface properties of the non-pigmented TiMEMO binder and the ensuing TISS paint coatings were determined from contact angle measurements. The results showed that the water contact angles of non-pigmented TiMEMO binder increased from 70° to 125–135° for the corresponding pigmented TISS paint coatings, inferring the influence of surface roughness on surface energy in the presence of pigments. SEM measurements revealed a striking similarity in the surface morphology of the TISS paint coatings with some other surfaces exhibiting the Lotus effect.     

Antireflection treatment of Thickness Sensitive Spectrally Selective (TSSS) paints for thermal solar absorbers

There are several methods to produce solar absorbers, and one cheap alternative is painted absorbers, preferably painted with a spectrally selective paint. The optical properties of Thickness Sensitive Spectrally Selective (TSSS) paints are, however, limited by the thickness of the paint layer. In this study it is shown that the solar absorptance of two commercial TSSS paints can be increased between 0.01 and 0.02 units with an antireflection treatment using a silicon dioxide layer deposited from silica-gel. It was found that the thermal emittance (100 °C) did not change significantly after the treatment.     

Silicone-based thickness insensitive spectrally selective (TISS) paints as selective paint coatings for coloured solar absorbers (Part I)

The objective of this study was to make paints having a variety of colours and whose spectral selectivity would be independent of the thickness of the deposited layer of paint (thickness insensitive spectrally selective (TISS) paint coatings). TISS paint coatings combine the advantages of paints (longevity and chemical resistance achieved by a high thickness of the applied layer, variety of colours and simple application) with spectral selectivity. Low emittance is attained by the addition of bare aluminium, coloured aluminium flake pigments or copper flake pigments, while other inorganic pigments impart various colours to the paints. Pigments were dispersed in silicone resin binder imparting the TISS paint coatings high-temperature tolerance, excellent adhesion, uv resistance, flexibility and weather-durability, which make them suitable coatings for coloured glazed or unglazed solar absorbers.     

Spectral selectivity of black and green painted surfaces

Black, green and mixed paints were prepared from organically modified siloxane resin. The solar absorptance (a_s) of prepared black paint/metal coatings was 0.90 with corresponding thermal emittance (e_T) 0.20, whereas green paint coatings did not reach satisfactory solar absorptance (a_s<0.8O). To improve the absorptance of the green coating, the black paint was admixed into the green paint. Optical properties of the prepared coatings were determined by the help of Kubelka–Munk formalism.     

Characterisation of high-temperature-resistant spectrally selective paints for solar absorbers

The spectrally selective paint coatings were prepared from organically modified siloxane resin and inorganic pigment (FeMnCuO_x-P320). To optimise the low-emittance properties, different thicknesses of paints were applied on high-reflecting Al foil by a draw bar coater. For all paints, optical and thermal properties were determined as well as their adhesion resistance. Pigment to volume concentration ratio was 20% and for thicknesses of about 1.7–2.0 g/m², the solar absorptance for these samples were a_s=0.90–0.92 with corresponding thermal emittance of e_T=0.20–0.25. Temperature stability of these samples was followed by FT-IR spectroscopy at 300°C. The obtained results indicated good temperature stability of prepared paint coatings.     

Selective paint coatings for coloured solar absorbers: Polyurethane thickness insensitive spectrally selective (TISS) paints (Part II)

Red, green and blue paints were prepared for use as thickness insensitive spectrally selective (TISS) paint coatings for solar façade absorbers. The paints were composed of a polyurethane resin binder in which various pigments were incorporated in such a way that they formed stable paint dispersions, satisfying stability criteria for façade coatings. A low emittance of the paints was achieved by using low-emittance aluminium flake pigments combined with iron oxide (red coloured paints). Black pigment was added to adjust solar absorptance. Blue and green paints were made by the addition of coloured aluminium flake pigment and the solar absorptance was also adjusted by the addition of black pigment. Efficiency for photo-thermal conversion of solar radiation was assessed by evaluation of the corresponding performance criteria, which enabled the selection of paints whose performance criteria values were higher than 0 (spectrally non-selective black coating). The results confirmed that blue and green paints and to minor extent red ones, combined selectivity with colour. The morphology of the paints was assessed, revealing that the colours originated from the deposition of finely dispersed colour and/or black pigment on the surface of the aluminium flakes during paint preparation.     

Sol–gel derived CuCoMnOx spinel coatings for solar absorbers: Structural and optical properties

A novel black coloured coating with the composition CuCoMnO_x was prepared using sol–gel synthesis. The coatings were deposited using the dip-coating technique from alcoholic sols based on Mn-acetate and Co- and Cu-chloride precursors. Thermogravimetric analysis showed that xerogels become crystalline at 316°C while X-ray diffraction analysis revealed that the coatings and powders correspond predominantly to CuCoMnO_x spinels. Rutherford back scattering (RBS) and transmission electron microscopic (TEM) studies combined with energy dispersive X-ray spectroscopy (EDXS) measurements confirmed that Cu, Mn and Co are present in the films in stoichiometric ratios close to that in the initial sols. IR spectroscopy has been employed to study the formation of sols by following the changes in the vibrational bands of the acetate groups during both thermal hydrolysis and the ageing of sols to xerogels. It was found that ageing of xerogels was accompanied by the formation of −COO⁻ bridging units, which at 250°C are no longer visible in the IR spectra but substituted by the vibrational modes characteristic for CuCoMnO_x. The solar absorptance (a_s) and thermal emittance (e_T) of the coatings when deposited on an Al-substrate are a_s=0.9 and e_T=0.05, which rank deposited black sol–gel CuCoMnO_x spinels among the promising candidates for spectrally selective absorber coatings for solar collectors and solar facades.     

Influence of the management strategy and operating conditions on the performance of an adsorption chiller

The aim of this experimental work was to study the influence of the operation mode (i.e. cycle time and relative duration of ads-/desorption phases, R) as well as of the operating conditions on the performance of an adsorption chiller. The testing campaign demonstrated that the management optimization strongly improves the performance of such kind of machines. The Coefficient of Performance (COP) and the Specific/Volumetric Cooling Power (SCP, VCP) vary, respectively, in a range of ±133% and ±43% when the cycle time (τ_cycle) increases from 5 to 20 min at fixed boundary conditions (T_e = 15 °C, T_c = 35 °C, T_h = 90 °C) while a further increasing in performance (up to 15%) is reached, at fixed cycle time, by protracting the duration of the adsorption phase at the expense of the desorption one. The complete set of results allowed to draw a map of performance suitable for the optimization of the management mode taking into account the specific application. At T_e = 15 °C, T_c = 35 °C, T_h = 90 °C, if high SCP is required (e.g. automotive air conditioning), the optimal choice is τ_cycle = 7 min and R = 2.5 (SCP = 394 W/kg, COP = 0.60, VCP = 223 W/m³) while to assure a good efficiency (e.g. solar cooling) the proper management is τ_cycle = 20 min and R = 1 (SCP = 204 W/kg, COP = 0.69, VCP = 116 W/m³).     

Optical and structural properties of the sol-gel-prepared ZnO thin films and their effect on the photocatalytic activity

ZnO thin films were obtained by the sol-gel method, using the dip-coating procedure. Glass slides were used as substrates. The sintering temperature (T_s) was varied in the range of 200-600 °C in intervals of 50 °C, in an open atmosphere. Films with 1 and 5 coatings were prepared for each T_s. An increase of the grain size from 10 to 34 nm as the T_s increased was observed from X-ray diffraction measurements. The thickness of the films prepared starting from five coatings, decreased by 36% when T_s increased, and denser films were obtained. This result was corroborated with the refractive index values, calculated from the UV-Vis transmission spectra. The films were tested as a photocatalyst by the photobleaching of methylene blue in an aqueous solution under UV light exposure during 5 h. The photocatalytic activity (PA) increased with T_s, around 72% for the films with one coating and 66% for those with five coatings. The samples with one coating and a T_s=500 °C showed the best PA. However, the glass substrate had a negative effect on the PA for T_s>500 °C, even when the surface morphology of the samples showed an increase in roughness when T_s increased. The observed negative effect can be due to the presence of an amorphous compound formed by Si, Zn and O at the glass-ZnO interface.     

The potential of polyurethane bio-based solid polymer electrolyte for photoelectrochemical cell application

A photoelectrochemical cell was developed from bio-based polyurethane (PU), solid polymer electrolyte with lithium iodide as conducting material. At the initial stage, PU prepolymer was prepared via prepolymerization technique by reacting palm kernel oil-based monoester-OH (PKO-p) and 2,4′-methylene diphenyl diisocyanate (2,4′-MDI). The polyurethane electrolyte film was then prepared by inclusion of varying amount of lithium iodide (LiI) via solution casting technique. The formation of urethane linkages (NHCO backbone) and the chemical interaction between segmented polyurethane and lithium ion from LiI salts were confirmed by ATR-FTIR technique. Thermal studies carried out by TGA have proven the occurrence of polymer-salt complexation. Structural analysis by XRD has revealed that polyurethane electrolytes with 25 wt.% LiI reduced the semi-crystalline characteristics of plasticized polyurethane. The SEM morphological observation on the fractured film indicated the absence of phase separation. The ionic conductivity increased with the addition of 25 wt.% LiI resulted in the highest conductivity of 7.6 × 10⁻⁴ S cm⁻¹. The temperature dependence conductivity of the electrolytes obeyed the Arrhenius law with the pre-exponential factor, σ_o of 2.4 × 10⁻³ S cm⁻¹ and activation energy, E_a of 0.11 eV. A dye-sensitized solar cell of FTO/TiO₂-dye/PU-LiI-I₂/Pt give a response under light intensity of 100 mW cm⁻² indicated the photovoltaic effect with the J_sc of 0.06 mA cm⁻² and V_oc of 0.14 V respectively. These properties exhibited promising potentials for photoelectrochemical cell giving the focus on bio-based polymer electrolyte.     

HCFC22-based absorption cooling systems,part III: Effects of different absorbers and condenser temperatures

Generally in a vapour absorption refrigeration system (VARS) heat rejection temperatures at absorber (T_a) and condenser (T_c) are taken to be equal. However, different temperatures can exist when the cooling water flows in series through the two components. Under such situations, it is essential to know which of T_a and T_c has greater influence on the performance of the VARS. Here the influence of different T_a and T_c on the performance of a single-stage VARS working with HCFC22 as a refrigerant and three organic solvents, namely DMA, DMF and DEMTEG, as absorbents is studied. Results are obtained over a wide range of operating temperatures. To improve the performance of HCFC22-based VARS, results reveal that (i) the cooling water in parallel pipe connections should be used at low values of temperatures at evaporator, cooling water and heat source, and (ii) cooling water should first flow through condenser and then through the absorber when evaporator and heat source temperatures are high over the complete range of cooling water temperatures. COP_th is more sensitive to T_c than to T_a.     

Review and comparison of equations relating bulk lifetime and surface recombination velocity to effective lifetime measured under flash lamp illumination

Photoconductance measurements are frequently used to determine the minority-carrier effective lifetime (τ_eff), from which the bulk lifetime (τ_b) and surface recombination velocity (S) must be extracted. The exact solution to the continuity equation is used to determine the conditions of validity for three approximate equations relating τ_eff to τ_b and S: the steady-state approximation, the transient approximation, and the widely used simple equation (1/τ_eff=1/τ_b+2S/W). We show that only the steady-state approximation matches the exact solution over the entire range of τ_eff, when the lamp time constant is 2.3 ms. When τ_eff>20 μs, all the equations give approximately the same result. However, when τ_eff<10 μs, only the steady-state equation is valid, while the error in the transient and simple equations may exceed 100%.     

The Second Law of Thermodynamics for a Two-Temperature Model of Heat Transport in Metal Films

ABSTRACT

The second law of thermodynamics asserts that heat will always flow “downhill”, i.e., from an object having a higher temperature to one having a lower temperature. For a parabolic rigid heat conductor with a single temperature T and a single heat-flux q this amounts to the statement that the inner product of q and ?T must be non-positive for every point x of the conductor and for every non-negative time t. For a homogeneous and isotropic body in which classical Fourier law with a heat conductivity coefficient k is postulated, the second law is satisfied if k is a positive parameter. For ultra-fast pulse-laser heating on metal films, a parabolic two-temperature model coupling an electron temperature T_e with a metal lattice temperature T_l has been proposed by several authors. For such a model, at a given point of space x and a given time t there are two different temperatures T_e and T_l as well as two different heat-fluxes q _e and q _l related to the gradients of T_e and T_l, respectively, through classical Fourier law. As a result, for a homogeneous and isotropic model the positive definiteness of the heat conductivity coefficients k_e and k_l corresponding to T_e and T_l, respectively, implies that the second law of thermodynamics is satisfied for each of the pairs (T_e, q _e) and (T_l, q _l), separately. Also, the positive definiteness of k_e and k_l, and of the corresponding heat capacities c_e and c_l as well as of a coupling factor G imply that a temperature initial-boundary value problem for the two-temperature model has unique solution. In the present paper, an alternative form of the second law of thermodynamics for the two-temperature model with k_l = 0 and q _l =  0 is obtained from which it follows that in a one-dimensional case the electron heat-flux q_e(x, t) has direction that is opposite not only to that of ?T_e(x, t)/?x but also to that of ?T_l(x, t + τ_T)/?x, where τ_T is an intrinsic small time of the model. Also, for a general two-temperature rigid heat conductor in which k_e, k_l, c_e, c_l, and G are positive, an inequality of the second law of thermodynamics type involving a pair (T_e ? T_l, q _e ?  q _l) is postulated to prove that a two-heat-flux initial-boundary value problem of the two-temperature model has a unique solution. For a one-dimensional case, the semi-infinite sectors of the plane ( q _l, q _e) over which uniqueness does not hold true are also revealed.     

Electrical and photoluminescence properties of evaporated CuIn1−xGaxTe2 thin films

Polycrystalline thin films of CuIn_1−xGa_xTe₂ have been deposited by flash evaporation on Corning glass 7059 substrates at T_s=200°C. Hall and resistivity measurements have been carried out down to 77 K. These films are p-type and the variation of the resistivity may be linked to defects, disorder of the material or grain boundaries. The PL spectra of these films after annealing in argon atmosphere at T_a=450°C have showed a broad band emission between 0.98 and 1.12 eV in which the main peak appears at 1.05 eV (at 4.2 K).     

Optical properties and thermal stability of pulsed-sputter-deposited AlxOy/Al/AlxOy multilayer absorber coatings

Spectrally selective Al_xO_y/Al/Al_xO_y multilayer absorber coatings were deposited on copper (Cu) and molybdenum (Mo) substrates using a pulsed sputtering system. The Al targets were sputtered using asymmetric bipolar-pulsed DC generators in Ar+O₂ and Ar plasmas to deposit an Al_xO_y/Al/Al_xO_y coating. The compositions and thicknesses of the individual component layers were optimized to achieve high solar absorptance (α=0.950-0.970) and low thermal emittance (ε=0.05-0.08). The X-ray diffraction data in thin film mode showed an amorphous structure of the Al_xO_y/Al/Al_xO_y coating. The X-ray photoelectron spectroscopy data of the Al_xO_y/Al/Al_xO_y multilayer absorber indicated that the Al_xO_y layers present in the coating were non-stoichiometric. The optical constants (n and k) of the multilayer absorber were determined from the spectroscopic ellipsometric data. Drude's free-electron model was used for generating the theoretical dispersion of optical constants for Al films, while the Tauc-Lorentz model was used for modeling optical properties of the dielectric Al_xO_y layers. In order to study the thermal stability of the Al_xO_y/Al/Al_xO_y coatings, they were subjected to heat treatment (in air and vacuum) at different temperatures and durations. The multilayer absorber deposited on Cu substrates exhibited high solar selectivity (α/ε) of 0.901/0.06 even after heat-treatment in air up to 400 °C for 2 h. At 450 °C, the solar selectivity decreased significantly on Cu substrates (e.g., α/ε=0.790/0.07). The coatings deposited on Mo substrates were thermally stable up to 800 °C in vacuum with a solar selectivity of 0.934/0.05. The structural stability of the absorber coatings heat treated in air (up to 400 °C) and vacuum (up to 800 °C) was confirmed by micro-Raman spectroscopy measurements. Studies on the accelerated aging tests suggested that the absorber coatings on Cu were stable in air up to 75 h at 300 °C and the service lifetime of the multilayer absorber was predicted to be more than 25 years. Further, the activation energy for the degradation of the multilayer absorber heat treated for longer durations in air is of the order of 64 kJ/mol.     

Silver-based low-emissivity coatings for architectural windows: Optical and structural properties

Spectrally selective coatings are, nowadays, deposited onto architectural windows to be employed in commercial and residential buildings for the purpose of saving energy. In this work, the fundamental optical and structural properties of two types of low-emissivity silver-based coatings deposited onto glass, Ag and SnO₂/Ni-Cr/Ag/Ni-Cr/SnO₂ (Vitrage à couches avec une forte reflexion des rayonnements thermiques, European Patent EP 0 506 507 B1) are analyzed. It has been demonstrated that the silver-based multilayer coatings give an efficient heat isolation due to their low emissivity values, thus not deteriorating the optical properties in the visible range given by the glass substrate. From Atomic Force Microscopy measurements it has been determined that the Ag layer has a more homogeneous grain size when is deposited onto the Ni-Cr layer than when is deposited onto glass.     

Equations for estimating global solar radiation in data sparse regions

The knowledge of the amount of solar radiation in an area/region is very essential in the field of Solar Energy Physics. In this work two equations are put forward for estimating global solar radiation from common climate variables in data sparse regions. The first is the Hargreaves equation, R_s=0.16R_aT_d^0.5 where R_a is the extraterrestrial solar radiation and T_d is the temperature difference (maximum minus minimum), while the second is the Angstrom equation, R_s=R_a(0.28+0.39n/N) where n and N are the measured sunshine hours and the maximum daylight duration respectively. The global solar radiation estimated by the two equations for three sites, Owerri (5°28′N, 7°2′E), Umudike (5°29′N, 7°33′E) and Ilorin (8°32′N, 4°46′E), located in different climate zones of in Nigeria, West Africa, are in agreement with those of earlier workers and that from Photovoltaic Geographic Information System (PVGIS) project. The implication of this in solar photovoltaic applications has been stressed.