Optical properties of sol–gel deposited Al2O3 films

Highly transparent, uniform and corrosion resistant Al₂O₃ films were prepared on stainless-steel and quartz substrates by the sol–gel process from stable coating solutions using aluminum-sec-butoxide, Al(OBu^s)₃ as precursor, acetylacetone, AcAcH as chelating agent and nitric acid, HNO₃, as catalyzer. Films up to 1000 nm thick were prepared by multiple spin coating deposition, and were characterized by X-ray diffractometry (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR), optical spectroscopy and micro Vickers hardness test. XRD of the film heat treated at 400°C showed that they had an amorphous structure. XPS confirmed that they were stoichiometric Al₂O₃. The refractive index (n) and extinction coefficient (k) were found to be n=1.56±0.01 and k=0.003±0.0002 at 600 nm, respectively. The surface microhardness and corrosion resistance investigations showed that Al₂O₃ films improved the surface properties of stainless-steel substrates.     

Preparation of Cu(In,Ga)Se2 thin films from Cu–Se/In–Ga–Se precursors for high-efficiency solar cells

Improved preparation process of a device quality Cu(In,Ga)Se₂ (CIGS) thin film was proposed for production of CIGS solar cells. In–Ga–Se layer were deposited on Mo-coated soda-lime glass, and then the layer was exposed to Cu and Se fluxes to form Cu–Se/In–Ga–Se precursor film at substrate temperature of over 200°C. The precursor film was annealed in Se flux at substrate temperature of over 500°C to obtain high-quality CIGS film. The solar cell with a MgF₂/ITO/ZnO/CdS/CIGS/Mo/glass structure showed an efficiency of 17.5% (V_oc=0.634 V, J_sc=36.4 mA/cm², FF=0.756).     

CIS film growth by metallic ink coating and selenization

A novel technique was demonstrated for the growth of CuInSe₂ (CIS) thin films. The technique used an ink formulation containing sub-micron size particles of Cu–In alloys. A metallic precursor layer was first formed by coating this ink onto the substrate by spraying. The precursor film was then made to react with Se to form the CIS compound. The morphology of the CIS layers depended on the initial composition of the Cu–In particles as well as the post-deposition treatments. Solar cells were fabricated on CIS absorber layers prepared by this low-cost ink-coating approach and devices with a conversion efficiency of over 10.5% were demonstrated.     

Corrosion behavior of TiN-coated stainless steel as bipolar plate for proton exchange membrane fuel cell

Stainless steel is a potential material to be used as the bipolar plate for proton exchange membrane fuel cell (PEFC) because of its suitable physical and mechanical properties. Several coating techniques have been applied to improve its corrosion resistance. But seldom study is focused on the microstructure evolution with corrosion. In the present study, the use of TiN-coated stainless steel as the bipolar plate is evaluated. Two surface coating techniques, pulsed bias arc ion plating (PBAIP) and magnetron sputtering (MS), are adoped to prepare the TiN-coated stainless steel. Their corrosion resistances and electrical conductivities of the coated substrates are evaluated. The performance shows strong dependance on microstructural characteristics. The corrosion of SS304/Ti₂N/TiN prepared by MS mainly occurs on the grain boundary. The corrosion of SS304/TiN prepared by PBAIP mainly takes place from the large particles on the coating. The Ti₂N/TiN multilayer coating provides superb corrosion protective layer for stainless steel. Both the TiN and Ti₂N/TiN coatings provide low contact resistance.     

Lanthanum oxide-coated stainless steel for bipolar plates in solid oxide fuel cells (SOFCs)

Solid oxide fuel cells typically operate at temperatures of about 1000 °C. At these temperatures only ceramic interconnects such as LaCrO₃ can be employed. The development of intermediate-temperature solid oxide fuel cells (IT-SOFCs) can potentially bring about reduced manufacturing costs as it makes possible the use of an inexpensive ferritic stainless steel (STS) interconnector. However, the STS suffers from Cr₂O₃ scale formation and a peeling-off phenomenon at the IT-SOFC operating temperature in an oxidizing atmosphere. Application of an oxidation protective coating is an effective means of providing oxidation resistance. In this study, we coated an oxidation protective layer on ferritic stainless steel using a precursor solution prepared from lanthanum nitrate, ethylene glycol, and nitric acid. Heating the precursor solution at 80 °C yielded a spinable solution for coating. A gel film was coated on a STS substrate by a dip coating technique. At the early stage of the heat-treatment, lanthanum-containing oxides such as La₂O₃ and La₂CrO₆ formed, and as the heat-treatment temperature was increased, an oxidation protective perovskite-type LaCrO₃ layer was produced by the reaction between the lanthanum-containing oxide and the Cr₂O₃ scale on the SUS substrate. As the concentration of La-containing precursor solution was increased, the amount of La₂O₃ and La₂CrO₆ phases was gradually increased. The coating layer, which was prepared from a precursor solution of 0.8 M, was composed of LaCrO₃ and small amounts of (Mn,Cr)O₄ spinel. A relatively dense coating layer without pin-holes was obtained by heating the gel coating layer at 1073 K for 2 h. Microstructures and oxidation behavior of the La₂O₃-coated STS444 were investigated.     

Time-dependent corrosion behavior of electroless Ni–P coating in H2S/Cl− environment

As a mature technology, electroless Ni–P alloy coating is widely applied in the protection of chemical equipment and pipelines owing to its excellent corrosion resistance, but its application and long-term service evaluation in the field of high-sulfur oil and gas are rare. Therefore, the time-dependent corrosion behavior of Ni–P coating, which was plated on the L360 steel surface, was investigated in a saturated H₂S medium by the method of surface analysis. The results indicate that Ni–P coating with a thickness of about 52.6 μm could significantly reduce the corrosion rate compared with uncoated pipeline steel. This is related to the structure of the dense, protective film on the surface. The uncoated pipeline steel suffered local corrosion during the immersion process, and then it developed into uniform corrosion with the formation of a large number of corrosion products. In comparison, Ni–P coatings corroded relatively mildly with only a thin corroded layer. However, during prolonged corrosion testing, the corrosive medium penetrated the coating/substrate interface at inherent defects, leading to severe local corrosion of the substrate.     

An investigation into effect of cationic precursor solutions on formation of CuInSe2 thin films by SILAR method

SILAR deposition of CuInSe₂ films was performed by using Cu²⁺–TEAH₃ (cupric chloride and triethanolamine) and In³⁺–CitNa (indium chloride and sodium citrate) chelating solutions with weak basic pH as well as Na₂SeSO₃ solution at 70 °C. A separate mode and a mixed one of cationic precursor solutions were adopted to investigate effects of the immersion programs on crystallization, composition and morphology of the deposited CuInSe₂ films. Chelating chemistry in two solution modes was deducted based on IR measurement. The XRD, XPS and SEM results showed that well-crystallized, smoothly and distinctly particular CuInSe₂ films could be obtained after annealing in Ar at 400 °C for 1 h by using the mixed cationic solution mode.     

Self-assembled graphene film to enable highly conductive and corrosion resistant aluminum bipolar plates in fuel cells

We report in this paper a novel method to form protective graphene film on aluminum substrate, which is particularly applicable to bipolar plates in proton exchange membrane (PEM) fuel cells. By simply immersing an aluminum sheet in an aqueous solution of graphene oxide (GO), a layer of cross-linked GO gel forms on the aluminum sheet, taking advantage of dissociated aluminum ions as a cross-linker. Then the cross-linked GO is converted to graphene at 400 °C in hydrogen atmosphere. The chemistry of the self-assembled GO layer and its conversion to graphene film is revealed by FTIR and XPS. Under simulated fuel cell environment the graphene coated aluminum sheet shows a corrosion current density of <1 × 10^?6 A/cm², which is around four orders of magnitude lower than a bare aluminum sheet. Meanwhile, the graphene film on aluminum results in a much lower and more stable interfacial contact resistance (ICR) of <5 mΩ cm². These enable the graphene coated aluminum sheet to meet the U.S. DOE targets of 2020 for bipolar plates in terms of both the corrosion and electrical resistance. Thus the proposed method is very promising for protecting aluminum bipolar plates in PEM fuel cells.     

Sol–gel deposited nickel oxide films for electrochromic applications

The electrochromic (EC) behavior, the microstructure, and the morphology of sol–gel deposited nickel oxide (NiO_x) coatings were investigated. The films were produced by spin and dip-coating techniques on indium tin oxide (ITO)/glass and Corning glass (2947) substrates.The coating solutions were prepared by reacting nickel(II) 2-ethylhexanoate as the precursor, and isopropanol as the solvent. NiO_x was heat treated at 350 °C for 1 h. The surface morphology, crystal structure, and EC characteristics of the coatings were investigated by scanning electron microscopy (SEM), electron dispersive spectroscopy (EDS), atomic force spectroscopy (AFM), X-ray diffractometry (XRD), and cyclic voltammetry (CV).SEM and AFM images revealed that the surface morphology and surface characteristics of the spin- and dip-coated films on both types of substrate were different. XRD spectra revealed that both films were amorphous, either on ITO or Corning glass substrates. CV showed a reversible electrochemical insertion or extraction of the K⁺ ions, cycled in 1 M KOH electrolyte, in both type of film. The crystal structure of the cycled films was found to be XRD amorphous. Spectroelectrochemistry demonstrated that dip-coated films were more stable up to 1000 coloration–bleaching cycles, whereas spin-coated films gradually degraded after 500 cycles.     

Deposition and characterization of Al–Si metallic TBC precursor on Mo–Si–B turbine materials

The interfacial reactions of Mo–Si–B with Al–Si metallic coatings during processing to achieve high-temperature corrosion protective mullite coating for advanced turbine materials are discussed. Al–Si is being deposited on the Mo–Si–B substrate material by immersion in liquid Al–Si alloy, molten salt cathodic deposition, and organic electrolysis, to achieve adhesion and compositional gradients across the interfacial region. The interfacial region during Al–Si deposition is the precursor to the formation of compositionally graded mullite by annealing and subsequent oxidation. The optimum Al–Si content of this metallic layer, which will be attained by deposition combined with silicon diffusion from Mo–Si–B, is described. Characterization of the constitution and gradients across the interfacial region is reported. Transport modeling in this precursor layer and the substrate is discussed.     

The effect of annealing on the properties of diamond-like carbon protective antireflection coatings

In addition to its similarity to genuine diamond film, diamond-like carbon (DLC) film has many advantages, including its wide band gap and variable refractive index. Therefore, as one of the diverse applications, DLC film can be utilized as a protective coating for IR windows and an anti-reflective coating for solar cells. For this study, DLC films were prepared by the radio frequency-plasma enhanced chemical vapor deposition (RF-PECVD) method on silicon substrates using methane (CH₄) and hydrogen (H₂) gas. We examined the effects of the post-annealing temperature and the annealing ambient on structural, electrical and optical properties of DLC films. The films were annealed at temperatures ranging from 300 to 900 °C in steps of 200 °C using rapid thermal annealing equipment in nitrogen ambients. The thickness of the film was observed by scanning electron microscopy (SEM) and surface profile analysis. The variation of structure according to the annealing treatment was examined using Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and high-resolution transmission electron microscopy (HRTEM). The reflectance of DLC thin film was investigated by UV–vis spectrometry and its electrical properties were investigated using a four point probe and I–V meter. The carrier lifetime of the film was also checked.     

Optical and electrochemical characteristics of sol–gel deposited iron oxide films

Homogenous, crack free iron oxide films are prepared by the sol–gel spin coating technique from a solution of iron iso-propoxide and isopropanol. The films were characterized by X-ray diffractometry (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), UV-visible (UV–Vis) spectroscopy and cyclic voltammetry (CV). XRD of the films showed that they had an amorphous structure. The optical constants refractive index (n) and extinction coefficient (k) were measured by scanning spectrometer in the wavelength range of 390–990 nm. The n and k values were found n =2.3±0.01 and k =0.2±0.002 at 650 nm. The electrochemical behavior investigated in 0.5 M LiClO₄ propylene carbonate (PC) electrolyte-CV examinations showed good rechargeability of the Li⁺/e⁻ insertion extraction process beyond 300 cycles. Spectroelectrochemistry showed that these films exhibit weak cathodic coloration in the spectral range of 350–800 nm.     

Corrosion resistance and electrical conductivity of plasma nitrided titanium

The continuous and dense Ti–N compound layers with a thickness ranging from 0.7 to 2.1 μm were formed on the titanium by plasma nitriding at 700 °C for different times with hollow cathode discharge assistance. Scanning electron microscope (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) were used to characterize the nitrided layer. XRD and XPS results showed that the compound layer was mainly composed of Ti₂N phase. The corrosion current density of 4 h nitrided titanium was 0.016 μA/cm² (cathode) and 0.03 μA/cm² (anode), respectively. The electrical conductivity of samples was evaluated by means of the interfacial contact resistance (ICR). The value of 4 h nitrided titanium was 4.94 mΩ-cm² which was much lower than that of original titanium 26.25 mΩ-cm² under applied force of 150 Ncm^?2 after corrosion test. The results showed that the electrical conductivity and corrosion resistance of the titanium bipolar plates (BPs) were apparently improved with the formation of Ti₂N compound layer.     

Improvement of corrosion and electrical conductivity of 316L stainless steel as bipolar plate by TiN nanoparticle implantation using plasma focus

The present work reports the results of TiN-ions implantation into the SS316L samples as bipolar plates by a 4 kJ Mather type Plasma Focus (PF) device operated with nitrogen gas for 10, 20, and 30 shots in order to improve the corrosion resistance and electrical conductivity of samples. The PF can generate short lived (10–100 ns) but high temperature (0.1–2.0 keV) and high density (10¹⁸–10²⁰ cm⁻³) plasma, and the whole process of PF lasts just a few microseconds. X-ray diffraction (XRD) results reveal the formation of a nanocrystalline titanium nitride coating on the surface of substrate. The interfacial contact resistance (ICR) of samples is measured, and the results show that the conductivity of samples increase after coating because of high electrical conductivity of TiN coating. The electrochemical results show that the corrosion resistances are significantly improved when TiN films are deposited into SS316L substrate. The corrosion potential of the TiN coated samples increases compared with that of the bare SSI316L and corrosion currents decrease in TiN implanted samples. Scanning Electron Microscopy (SEM) indicates changes in surface morphology before and after potentiostatic test. The thickness of coated layer which is obtained by cross sectional SEM is about 19 μm.     

CdTe/CdS Solar cells on flexible molybdenum substrates

Development of CdTe/CdS solar cells on flexible metallic substrates is highly interesting due to the light weight and flexible nature of the solar modules. We have deposited CdTe films onto flexible molybdenum substrates using close-spaced sublimation technique and the CdTe/CdS junction was developed by depositing a thin layer of CdS onto the CdTe substrate from a chemical bath. The devices were characterized by Current–voltage (I–V) and photocurrent spectroscopy techniques. Prior to the deposition of the transparent conducting layer, the devices were annealed in air at different temperatures and found that the devices annealed at 400°C have better photovoltaic parameters. The efficiency of a typical device under 60 mW cm⁻² illumination was estimated as 3.5%.     

Spectrally selective laminated glazing consisting of solar control and heat mirror coated glass: preparation, characterization and modelling of heat transfer

In this study, solar control coatings were prepared by sequential depositions of thin films of ZnS (40 nm)–CuS (150 nm) and ZnS (40 nm)–Bi₂S₃ (75 nm)–CuS (150 nm) from chemical baths on 3 mm thick commercial sheet glass. These were laminated to 3 mm thick clear glass or commercially available SnO₂ based heat mirror coating of sheet resistance 15 Ω on float glass of 3 mm thickness using a poly(ethylene vinyl acetate), EVA, sheet of 0.36 mm thickness in a vacuum process at 120 °C for 30 min. In total, the thickness of the glazing was 6.35 mm. The glazings possess visible transmittance, weighted for D65 solar spectra and sensitivity of the human eye for daylight vision, of 36% or 14% with solar absorptance of 71% or 78% depending on the coating type, i.e ZnS–CuS or ZnS–Bi₂S₃–CuS-heat mirror respectively. The solar heat gain coefficient (SHGC) was evaluated for these glazings at exterior temperatures of 15 and 32 °C for an exterior convective heat transfer coefficient (h_ex) of 6–100 Wm⁻² K⁻¹ using a mathematical model. The model predicts the extent of reduction in SHGC through the presence of the heat mirror coating as a function of h_ex and hence helps to decide on the relative benefit, which may be derived through their use in different locations. Though the deposition technique mentioned here involves longer duration compared with vacuum techniques, it may be developed into a low throughput, low-capital alternate technology for small-scale production.     

Improvement on the corrosion protection of conductive polymers in pemfc environmets by adhesives

The corrosion protection of polypirrol (PPY) and polyaniline (PANI) coatings electrochemically deposited with and without polyvinil alcohol (PVA) as adhesive onto 304 type stainless steel has been evaluated using electrochemical techniques. Environment included 0.5 M H₂SO₄ at 60 °C whereas employed techniques included pothentiodynamic polarization curves (PC), linear polarization resistance (LPR) and electrochemical impedance spectroscopy (EIS) measurements. Results showed that the free corrosion potential, of the substrate, E_corr, was made more noble up to 500 mV with the polymeric coatings. The corrosion rate was lowered by using the polymers, but with the addition of PVA, it was decreased further, one order of magnitude for PPY and up to three orders of magnitude for PANI. Impedance spectra showed that the corrosion mechanism is under a Warburgh-type diffussional process of the electrolyte throughout the coating, and that the uptake of the environment causes the eventual failure of the coating corroding the substrate.     

Substrate to substrate aluminized bonding of 10 cm round silicon substrates for application in solar photovoltaics

Large area silicon solar cells with screen printed contacts have been realized for the first time on 10 cm diameter, p-type, Cz silicon wafers which were bonded to silicon substrates by alloying of a suitably thick screen printed layer of Al on them. In cells made on 300 μm thick wafers without texturization, antireflection coating and passivation of the front surface, the values of the open-circuit voltage (V_oc), the short-circuit current density (J_sc), curve factor (CF) and the efficiency (η) were found to be in the range 572–579 mV, 16–19.2 mA cm⁻², 0.53–0.61 and 5.5–5.89%, respectively, under simulated tungsten halogen light of 100 mW cm⁻² intensity. Using thinner wafers and having optical confinement, surface passivation and effective back surface field, the cell performance would be substantially improved. In fact, an efficiency close to 18% (AM1.5) would be realizable with this approach. Another attractive feature of this approach is that a low-cost silicon substrate could be used at the bottom that would act as support for the thin top surface without disadvantage to the cell performance. In this paper only the principle has been demonstrated experimentally. Possible improvements have been shown by computer simulation.     

Effect of copper diffusion on photovoltaic characteristics of CuGaSe2–GaAs cells

CuGaSe₂–GaAs heterojunctions were fabricated by fast evaporation of polycrystalline CuGaSe₂ from a single source on n-type GaAs substrates. The best CuGaSe₂–GaAs photocell (without an antireflective coating) exhibited an efficiency of 11.5%, J_sc=32 mA/cm², V_oc=610 mV and FF=0.60. The spectral distribution of photosensitivity of CuGaSe₂–GaAs junctions extends from 400 to 900 nm. The CuGaSe₂ films were characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM) techniques. XRD analysis indicated that the thin films were strongly oriented along the (1 1 2) plane. SEM studies of CuGaSe₂ films showed nearly stoichiometric composition with grain size about 1–2 μm. The energy dispersive X-ray spectroscopy (EDX) analysis of Cu concentration distribution in n-type GaAs showed that Cu diffused from the film into n-type GaAs during the growth process resulting in formation of the latent p–n homojunction in substrate. The diffusion coefficient of Cu in GaAs at growth temperature (520°C) estimated from EDX measurements was 6×10⁻⁸ cm²/s.     

Formation and characterization of porous silicon layers for application in multicrystalline silicon solar cells

The electrochemical formation of porous silicon (PS) layers in the n⁺ emitter of silicon p–n⁺ homojunctions for solar energy conversion has been investigated. During the electrochemical process under constant polarization, a variation of the current density occurs. This effect is explained by considering the doping impurity gradient in the emitter and by TEM characterization of the PS layer structure. Optical transmission measurements indicate that modifications of the refractive index and absorption coefficient of PS are mainly related to the porosity value. Reflectivity measurements, spectral response and I–V characteristics show that PS acts as an efficient antireflection coating layer. However, beyond a critical layer thickness, i.e. when PS reaches the p–n⁺ interface, the junction properties are degraded.