V. Corregidor

Development of GaSb Photoreceiver Arrays for Solar Thermophotovoltaic Systems

In comparison to conventional solar photovoltaics, where sun radiation is converted into electricity directly by solar cells, solar thermophotovoltaic (STPV) conversion has some specific advantages. These advantages come from the fact that in thermophotovoltaics the photon radiator is always inside the conversion system and near the photovoltaic cells. For these reasons we are developing small prototypes with sun heated emitters and photoreceiver arrays to be installed inside complete STPV systems. In order to achieve these complete STPV systems, the first step is to determine the optimum way of packaging the TPV cells into STPV arrays, choosing the best series/parallel configurations depending on the TPV cell band gap, the size of arrays, and the materials. This is the goal of this paper. To carry out the calculations, 18 and 24 cell arrays have been connected following different series and parallel configurations, using the PSPICE commercial circuit-simulation software. Each TPV cell is simulated as a block consisting of the well-known photogenerated current source, two dark diodes of ideality factors equal to one and two, and two resistances, one in parallel and the other in series. As a result, recommendations about the size and front grid design of the GaSb cells are obtained. When the optimally designed cells are connected to be included in two specific systems, recommendations about the best parallel/series connection are achieved. Evaluation on the performance of the arrays at nonuniform illumination is also carried out. The first photoreceiver arrays are being constructed and implemented in real STPV systems following these recommendations.

Combination of IBA Techniques for Composition Analysis of GaInAsSb Films

Quaternary GaInAsSb films alloys were grown by MOVPE technique on GaSb substrates with different growth conditions such as substrate orientation and thickness. The composition of the films determines their bandgap, and also how well they are lattice matched to the substrate. It is thus essential to determine it accurately, which is not a trivial task in this system. The composition of the samples was studied with a combination of Particle Induced Xray Emission (PIXE) and Rutherford Backscattering Spectrometry (RBS) experiments. The RBS experiments were done with a 2 MeV 4He+ or H+ ion beam, according to the thickness of the films, and were used to determine the thickness of the samples. The PIXE experiments were performed at grazing angle conditions and provided accurate elemental composition information. It was found that for thin layers (300 nm) there is a dependence of In incorporation into the matrix according to the substrate orientation, although this tendency was not found for thicker films (24m).

Compositional characterization of iron gall inks in manuscripts

This work aims to characterize the elemental and molecular composition of iron gall inks and papers by means of several non-destructive techniques as: Particle Induced X-ray Emission (PIXE), Rutherford Backscattering spectrometry (RBS), X-ray diffraction technique (XRD), and Fourier Transformed Infrared spectroscopy (FTIR). Ultraviolet (UV) and infrared (IR) photography were used for ink differentiation and detection of paper stains. The basic ingredients of iron gall inks, besides water, are: iron sulphate (vitriol), Arabic gum and tannins. By means of PIXE analysis , among other elements, sulphur, calcium and iron were identified. By XRD gallic acid and glucose were identified in the tannin sources and different iron sulphate hydrate compounds in the vitriol. In papers written with the prepared inks, by RBS technique it was possible to identify light elements as oxygen and carbon. An estimation of ink depth profiling revealed ink penetration in the paper of the nanometer order. By XRD it was possible to identify calcite and cellulose. After the determination of the right experimental conditions which assure the pristine conditions of the documents, different documents dated from the early XX century and belonging to a private collection were studied. Besides the elements listed above, traces of strontium and arsenic were identified in the documents’ papers. Papers are mainly composed of cellulose and calcite. The FTIR analysis could however discriminate between ink and paper compounds.

Microprobe study of Portuguese ancient silver coins of uncertain provenance

Ion Beam Analytical (IBA) techniques such as Particle Induced X-ray Emission (PIXE) and Elastic Backscattering Spectrometry (EBS) are finding increasing applications in the study of ancient coins [1] because they are non-destructive and can determine the coin chemical fingerprint down to the ppm range, which in some cases can be related with the ore provenance or the metal purification process, thus giving information about the fabrication period. In ancient coins, surface inhomogeneities created by centuries of corrosion growth, require a micro beam to characterize them. The analysis of -PIXE and -EBS spectra taken simultaneously allows to differentiate the superficial corrosion layer from the bulk, giving elemental surface composition distribution and elemental depth profiling, respectively. Using 1.0 and 2.0 MeV proton beams from the nuclear microprobe (resolution 3 4 m) located at the Ion Beam Laboratory at CTN (Sacavém Portugal), this approach was used to try to clarify about the authenticity of two XVI century Portuguese 91.6%wt silver (stipulated by decree of law) coins (Fig. 1) as the stylistic analysis by numismatic experts was not conclusive. Three coeval genuine Portuguese coins were also studied and served as a comparison. 2D-PIXE maps (as shown in Fig. 2) were acquired for all coins, followed by point analysis (3 4 m) in areas with thicker (higher Fe and Br content) and thinner (higher Ag and Cu content) corrosion layer. Figure 3 shows fitted -PIXE and -EBS spectra taken simultaneously using a 1.0 MeV proton beam. These fits performed on all five coins showed that: a) major elements Ag and Cu: genuine coins have 95 wt% in Ag and 5 wt% in Cu homogenously distributed, while the two coins of uncertain provenance show 70-80 wt% in Ag and 20-30 wt% in Cu; b) trace elements Au and Bi (which are related with ore provenance): not detected on all coins, but when detected the concentration is around 0.05 wt%; c) trace element Pb (which is related with Ag refining process): detected on all coins, with 0.2 wt% quantified. From these results it was not possible to give a definite answer about the authenticity of the two coins of uncertain provenance. The low content in Ag is very uncommon in ancient coins, even more knowing that Ag-Cu alloys get surface enriched in silver, as observed for the genuine coins. However, in terms of trace elements, data are consistent with genuine coins.

Electroless plating of Au, Pt, or Ru thin film layer on CdZnTe

The electrical and structural properties of thin metallic films (Au, Pt, or Ru) on CdZnTe (CZT) deposited by electroless deposition method have been investigated by means of Atomic Force Microscopy (AFM), Scanning Electron Microscope (SEM), Energy Dispersion X-ray (EDX), and Rutherford Backscattering Spectroscopy (RBS) measurements. SEM and AFM techniques put in evidence the modification of morphology and the change of roughness versus deposition time on the surface of contacts. The surface of Pt or Ru layer on CZT material present micro-cracks at a critical thickness, whereas it does not occur with Au layer. The thickness of Au layer with different deposition time was obtained by RBS depth profiles, which indicates a fast increase of thickness at a short deposition time and a saturation to an asymptotic value of 120 nm after 1 hour deposition.

Phase Separation on GaInAsSb Films for Thermophotovoltaic Devices

Ga0.81In0.19As0.14Sb0.86 layers were grown on (100)-Te doped GaSb substrates 2º missoriented towards (110), (111)A and (111)B directions by metalorganic vapour deposition (MOVPE) at 540 °C. X-ray reciprocal space maps done in symmetric (224) and asymmetric (115) directions show a super-lattice structure due to the phase separation with a 5 nm period and independent of substrate orientation. The x-ray maps show different stage of relaxation of the films and in same cases an interdiffusion region near the substrate. Despite of the phase separation, channelling experiments with H ions as projectiles showed a good quality of the films. Channelling experiments show that the crystalline quality gets worse with increasing the In and As concentration.

Bridgman Solidification of Concentrated GaInSb Alloys with Variable Growth Rate

Experimental works and numerical simulations on concentrated Ga1−xInxSb alloys directional solidification at high crucible pulling rates, show a damping solutal effect on the thermally driven convection which leads to a significant increase of the chemical heterogeneity of the sample and of the solid‐liquid interface curvature. Analytical calculations which give a quantitative description of the solutal effect, show that the melt convection damping can be avoided if low pulling rates of the crucible are used for the growth process. A Bridgman growth method, which uses a variable pulling rate in order to reduce the damping solutal effect and to improve the axial chemical homogeneity of the sample, is numerically investigated. The growth process can be started at high pulling rates which are reduced progressively during the solidification. From the numerical modeling, it is found that the axial and radial variations of solute concentration as well as the interface curvature are maintained at lower values wh...

Thermophotovoltaic Systems based on Gallium Antimonide Infrared Cells

This paper overviews the fundamentals of the thermophotovoltaic systems, which convert infrared radiation into electricity by means of photovoltaic cells. First, the basic configuration and the main system components are discussed, and a short description of their most important applications is carried out. Besides, the manufacture of gallium antimonide infrared cells is presented, and their typical optical and electrical characteristics are analyzed.