Band-gap engineering of Cu2ZnSn1-xGe xS4 single crystals and influence of the surface properties

Thin film solar cells based on Cu2ZnSn(S,Se)4 are very promising, because they contain earth-abundant elements and show high absorptivity. However, the performance of these solar cells needs to be improved in order to reach efficiencies as high as that reported for Cu(In,Ga)Se 2-based devices. This study investigates the potential of band-gap engineering of Cu2ZnSn1-xGexS 4 single crystals grown by chemical vapour transport as a function of the [Ge]/([Sn] + [Ge]) atomic ratio. The fundamental band gap E0 is found to change from 1.59 to 1.94 eV when the Ge content is increased from x = 0.1 to x = 0.5, as determined from spectroscopic ellipsometry measurements. This knowledge opens a route to enhancing the performance of kesterite-based photovoltaic devices by a Ge-graded absorber layer. Furthermore, the formation of GeO2 on the surface of the as-grown samples was detected by X-ray photoelectron spectroscopy, having an important impact on the effective optical response of the material. This should be also taken into account when designing photovoltaic solar cellsRC acknowledges financial support from Spanish MINECO within the program Ramón y Cajal (RYC-2011-08521). This work was supported by the Marie Curie-IRSES project (PVICOKEST, GA: 269167), MINECO projects (KEST-PV, ENE2010-21541-C03-01/-02/-03) and Marie Curie-ITN project (KESTCELL, GA: 316488

Atmospheric corrosion of magnesium alloys AZ31 and AZ61 under continuous condensation conditions

This paper studies the corrosion rate of magnesium alloys AZ31 and AZ61 exposed in humid air under continuous condensation conditions. The shape of the gravimetric curves for corrosion progress suggests that the process is controlled by factors related with the corrosion product layer growing on the metallic surface according to gravimetric results there is an initial period in which only a small part of the corroded metal is incorporated in the corrosion product layer, but after longer testing times the proportion of metal that comes to form part of this layer tends to increase very significantlyfinancial support for this work from the Ministry of Science and Innovation of Spain (MAT 2009-13530)Peer reviewe

Effect of naturally formed oxide films and other variables in the early stages of Mg-alloy corrosion in NaCl solution

The influence of initial surface conditions on the subsequent corrosion behaviour of AZ31 and AZ61 magnesium alloys in 0.6 M NaCl solution has been studied using electrochemical impedance spectroscopy. For obtaining the different surface conditions, some of the specimens were immersion tested with the surface in the as-received condition, while others were tested immediately after mechanical polishing, and part of the polished specimens after six months of exposure to the laboratory atmosphere. Considering the evolution of the high-frequency capacitive arc of the Nyquist diagram, whose diameter is related to the corrosion process, a clear effect of the initial surface conditions is observed only in the early stages of testing. This effect is especially significant for the freshly polished specimensfinancial support for this work from the Ministry of Science and Innovation of Spain (MAT 2009-13530)Peer reviewe

Effect of the chemistry and structure of the native oxide surface film on the corrosion properties of commercial AZ31 and AZ61 alloys

The purpose of this study has been to advance in knowledge of the chemical composition, structure and thickness of the thin native oxide film formed spontaneously in contact with the laboratory atmosphere on the surface of freshly polished commercial AZ31 and AZ61 alloys with a view to furthering the understanding of protection mechanisms. For comparative purposes, and to more fully describe the behaviour of the native oxide film, the external oxide films formed as a result of the manufacturing process (as-received condition) have been characterised. The technique applied in this research to study the thin oxide films (thickness of just a few nanometres) present on the surface of the alloys has basically been XPS (X-ray photoelectron spectroscopy) in combination with ion sputtering. Corrosion properties of the alloys were studied in 0.6 M NaCl by measuring charge transfer resistance values, which are deduced from EIS (electrochemical impedance spectroscopy) measurements after 1 h of exposure. Alloy AZ61 generally showed better corrosion resistance than AZ31, and the freshly polished alloys showed better corrosion resistance than the alloys in as-received condition. This is attributed to a combination of (1) higher thickness of the native oxide film on the AZ61 alloy and (2) greater uniformity of the oxide film in the polished condition. The formation of an additional oxide layer composed by a mixture of spinel (MgAl2O4) and MgO seems to diminish the protective properties of the passive layer on the surface of the alloys in as-received conditionfinancial support for this work from the Ministry of Science and Innovation of Spain (MAT 2009-13530Peer reviewe

Microstructure study of nicraly and fecraly exposed to superheated steam at 800 C

Supercritical water-cooled reactor (SCWR) was chosen as Generation IV reactor concept in Canada to utilize Canada’s expertise and technical capabilities from past research and designs. The conceptual design of Canadian SCWR has a core outlet temperature of 650 C at 25 MPa and a peak cladding temperature as high as 800 C. Corrosion/oxidation resistance is an important factor in material selections and also coating considerations. Most of the reported supercritical water (SCW) test data have been obtained at temperatures up to 700 C as no autoclave exists that can operate above 700 C at supercritical pressures and temperatures. Superheated steam (SHS) is used as a surrogate fluid to SCW in this study to evaluate two coating materials, FeCrAlY and NiCrAl, at 800 C. The results showed that the FeCrAlY became discolored rapidly while NiCrAl still maintained some metallic sheen after 600 h. The weight change results suggest that more oxide formation took place on FeCrAlY than NiCrAl. In particular, grain boundary oxide (Al2O3) formed on FeCrAlY surface upon exposure to steam after 300 h. Further exposure caused more intragranular Al2O3 to form, in addition to magnetite formation on the grain boundary regions. For NiCrAl samples, NiO formed after steam exposure for 300 h. Spinel and (Cr,Al)2O3 were also found after 300 h along with very limited amount of Al2O3. After 600 h, Al2O3 became well developed on NiCrAl and the coverage of spinel and Cr2O3 on the surface reduced

The Application of X-Ray Photoelectron Spectroscopy in Understanding Corrosion Mechanisms of Magnesium and Mg-Al Alloys

The authors present a review of their research work on the application of X-ray photoelectron spectroscopy (XPS) to characterise thin (~ 3 nm) natural oxide films that spontaneously form on the surface of magnesium and its alloys in contact with the oxygen in air, and thick (> 1 μm) corrosion films rich in carbonates and other species that form during exposure to a humid atmosphere. The results obtained can provide much useful information about the protection mechanisms of these materials. Emphasis is placed on the relationship between the alloy's Al content and the characteristics of the natural oxide film and the relationship between segregation of calcium impurities from the bulk to the outer surface of the metal and repercussions on electrochemical behaviour. The natural oxide film may alone be sufficiently protective in less aggressive environments, making it of interest to know its characteristics. A detailed study will also be made of the changes in chemical composition that may occur in this passivating film with increasing exposure time to a humid atmosphere, deteriorating or reinforcing its anticorrosive properties. In particular, an attempt will be made to research the effect on corrosion behaviour of: (a) the time the different specimens are exposed to the aggressive agent (humid atmosphere); (b) the possible formation during corrosive attack of layers that are rich in carbonates and other species; and (c) the presence of aluminium (alloying element) on the protective properties of the corrosion layers.Peer Reviewe

Study of the interfacial chemistry of poly(vinyl chloride) paint on steel exposed to the ultraviolet-water condensation test

The effect of exposure to the UVCON (humidity and UV radiation) accelerated corrosion test of a poly(vinyl chloride) paint applied over cleaned, abraded, and pre-rusted mild steel substrates has been investigated from the interfacial chemistry and adhesion standpoints. The X-ray photoelectron spectroscopy (XPS) data reveal significant differences in the locus and mode of adhesion loss, which depend on the time of exposure and the initial presence of a rust layer on the mild steel. The adhesion values obtained with the pre-rusted mild steel were greater than those obtained with the cleaned and abraded mild steel before exposure. After 15 days of exposure, it was observed that changes in the interfacial surface composition of the PVC/cleaned and abraded mild steel systems might have important positive effects on the adhesion strength values. The XPS data suggest the oxidation of both interfacial surfaces incorporating polar C-O and C=O species, which would be expected to enhance the adhesion of paint to metal. In contrast, the PVC paint/pre-rusted mild steel system showed no change in adhesion values or interfacial surface composition over the entire period of exposure.Peer Reviewe

Suppression of hydrogenated carbon film deposition by scavenger techniques and their application to the tritium inventory control of fusion devices

The well-known radical and ion scavenger techniques of application in amorphous hydrogenated carbon film deposition studies are investigated in relation to the mechanism of tritium and deuterium co-deposition in carbon-dominated fusion devices. A particularly successful scheme results from the injection of nitrogen into methane/hydrogen plasmas for conditions close to those prevailing in the divertor region of present fusion devices. A complete suppression of the a-C: H film deposition has been achieved for N2/CH4 ratios close to one in methane (5%)/hydrogen DC plasma. The implications of these findings in the tritium retention control in future fusion reactors are addressed.© 2002 IOP Publishing LtdSupported by Dirección General de Investigación Científica y Técnica (DGICYT) under Project FTN2000-0915-C03. MECD of Spain, under Project SB2000-0051.Peer Reviewe