A.P. Gonçalves

Purification of metallurgical grade silicon by acid leaching

Abstract The purification of metallurgical grade silicon (≈98% Si) from Companhia Portuguesa de Fornos Electricos by acid leaching is studied as a function of the particle size, time, temperature and concentration of leaching agents (HNO3, H2SO4, HCl and HF). It was found that using only hydrochloric acid (16%, 5 h, 80°C) and a relatively coarse fraction (116 μm) it is possible to remove ∼85% of the impurities and to obtain 99.9% pure Si after further leaching with hydrofluoric acid (2.5%, 2 h, 80°C). All the other acids and their mixtures, including aqua regia, gave results poorer than those for hydrochloric acid. The finer fractions were more difficult to purify. These results differ from other previously reported on purification studies of silicon by acid leaching and are ascribed to the unusual composition of the MG-Si used in this study.

Conducting glasses as new potential thermoelectric materials: the Cu–Ge–Te case

Recent approaches to improve performance of bulk thermoelectric (TE) materials show that they should have complex structures, include inclusions and impurities, possess mass fluctuations, disorder and be based on heavy elements. Glasses can possess these properties. In order to identify glasses with interesting TE potential, attention should be focused on small gap semiconducting or semimetallic glasses. Chalcogenide glasses with Cux+yGe20−xTe80−y (0 ≤ x ≤ 20; 0 ≤ y ≤ 10) compositions were prepared by melt spinning. Their powder X-ray diffraction analyses point to a short-range order analogous to Ge20Te80, with Cu atoms most likely replacing Ge atoms in the GeTe4 structural unit. It also indicates, together with the differential scanning calorimetry results, a reduction in the glass stability with the increase in Cu concentration. The enhancement of Cu content dramatically reduces (five orders of magnitude) the electrical resistivity, while keeping the Seebeck coefficients at large values (∼400 μV K−1). As a consequence, a huge increase in the power factor is observed, up to a maximum value of 60 μW K−2 m−1 for the Cu27.5Ge2.5Te70 glass at T = 300 K. Ge20Te80 has extremely low lattice thermal conductivity values (∼0.1 W K−1 m−1 at 300 K), which points to relatively high values for the figure of merit ZT for this family of glasses, and indicates Cux+yGe20−xTe80−y based glasses as good candidates for obtaining high performance thermoelectric materials.

Nanostructured silicon and its application to solar cells, position sensors and thin film transistors

This paper reports the performance of small area solar cells, 128 linear integrated position sensitive detector arrays and thin film transistors based on nanostructured silicon thin films produced by plasma-enhanced chemical vapour deposition technique, close to the onset of dusty plasma conditions, within the transition region from amorphous to microcrystalline. The small area solar cells, produced in a modified single chamber reactor, exhibited very good electrical characteristics with a conversion efficiency exceeding 9%. The 128 integrated position sensitive detector arrays, based on a similar pin structure, allow real-time 3D object imaging with a resolution higher than 90 l p/mm. The thin film transistors produced exhibited field effect mobility of 2.47 cm2/V/s, threshold voltage of 2 V, on/off ratio larger than 107 and sub-threshold slopes of 0.32 V/decade, which are amongst the best results reported for this type of device.

Infrared Spectra and Quantum Chemical Calculations of the Uranium Carbide Molecules UC and CUC with Triple Bonds.

Laser evaporation of carbon-rich uranium/carbon alloys followed by atom reactions in a solid argon matrix and trapping at 8 K gives weak infrared absorptions for CUO at 852 and 804 cm(-1). A new band at 827 cm(-1) becomes a doublet with mixed carbon 12 and 13 isotopes and exhibits the 1.0381 isotopic frequency ratio, which is appropriate for the UC diatomic molecule, and another new band at 891 cm(-1) gives a three-band mixed isotopic spectrum with the 1.0366 isotopic frequency ratio, which is characteristic of the linear CUC molecule. CASPT2 calculations with dynamical correlation find the C[triple bond]U[triple bond]C ground state as linear 3Sigma(u)+ with 1.840 A bond length and molecular orbital occupancies for an effective bond order of 2.83. Similar calculations with spin-orbit coupling show that the U[triple bond]C diatomic molecule has a quintet (Lambda = 5, Omega = 3) ground state, a similar 1.855 A bond length, and a fully developed triple bond of 2.82 effective bond order.

Extended Miedema model: Predicting the formation enthalpies of intermetallic phases with more than two elements

Abstract A generalisation of the Miedema model for the calculation of formation enthalpies of ternary or higher-order intermetallics is presented. The comparison of model calculations with experimental data in MgCu2, MgZn2 and CaCu5-type structure compounds supports the validity of the proposed model.

Magnetization of the Fe sublattices in UFexAl12−x (4≤x≤5.8) studied by Mössbauer spectroscopy

Abstract An 57 Fe Mossbauer study of UFe x Al 12− x (4≤ x ≤5.8) intermetallics with ThMn 12 -type structure was performed. In order to study samples with the highest degree of purity, homogeneity and atomic ordering, the measurements were performed on crushed single crystals. A consistent description of the Mossbauer spectra could be obtained by using more than one subspectrum for Fe on each crystallographic site when x ≥4.2. As a result, the Fe atoms on the same crystallographic site were found to have different magnetic hyperfine fields, whose relative values are discussed on the basis of the Fe–Fe intersite exchange interactions.

Phase relations and single crystal growth of U-Fe-M (M = Al, Si) compounds with ThMn12-type structure

Abstract The U-Fe-M ( M = Al , Si ) phase diagrams were investigated around UFe 12− x M x compositions by SEM/EDS in order to detect intermetallic congruent melting compounds with ThMn 12 -type structure. Single crystals with UFe 12− x Al x (6.2 ⩽ x ⩽ 8.2) and UFe 9.2 Si 1.8 compositions, both with ThMn 12 -type structure, were grown from the melt using the Czochralski method.

Infrared spectra and quantum chemical calculations of the uranium-carbon molecules UC, CUC, UCH, and U(CC)2.

Laser evaporation of carbon rich uranium/carbon alloy targets into condensing argon or neon matrix samples gives weak infrared absorptions that increase on annealing, which can be assigned to new uranium carbon bearing species. New bands at 827.6 cm(-1) in solid argon or 871.7 cm(-1) in neon become doublets with mixed carbon 12 and 13 isotopes and exhibit the 1.0381 carbon isotopic frequency ratio for the UC diatomic molecule. Another new band at 891.4 cm(-1) in argon gives a three-band mixed isotopic spectrum with the 1.0366 carbon isotopic frequency ratio, which is characteristic of the anti-symmetric stretching vibration of a linear CUC molecule. No evidence was found for the lower energy cyclic U(CC) isomer. Other bands at 798.6 and 544.0 cm(-1) are identified as UCH, which has a uranium-carbon triple bond similar to that in UC. Evidence is found for bicyclic U(CC)(2) and tricyclic U(CC)(3). This work shows that U and C atoms react spontaneously to form the uranium carbide U≡C and C≡U≡C molecules with uranium-carbon triple bonds.

Oxide semiconductors: Order within the disorder

The effect is considered of order and disorder on the electrical and optical performance of ionic oxide semiconductors used to produce optoelectronic devices such as p–n heterojunction solar cells and thin-film transistors (TFTs). The results obtained show that p-type c-Si/a-IZO/poly-ZGO solar cells exhibit efficiencies above 14% in device areas of about 2.34 cm2, whereas amorphous oxide TFTs based on the Ga–Zn–Sn–O system demonstrate superior performance to the polycrystalline ZnO TFTs, with I ON/I OFF ratio exceeding 107, turn-on voltage below 1–2 V and saturation mobility above 25 cm2 V−1 s−1. In addition, preliminary data on a p-type oxide TFT based on the Zn–Cu–O system are presented.

Isothermal section at 750°C of the U–Fe–Sn ternary system

A systematic investigation of the isothermal section at 850 8C of the U–Fe–Al ternary system was done by means of scanning electron microprobe analysis and X-ray powder diffraction. At this temperature the phase diagram is characterized by the formation of seven ternary phases and two extended ranges of solubility. Three compounds form with non-existing, or negligible, homogeneity domains: UFe2Al10 (YbFe2Al10-type, aZ8.9146(3) A u , bZ10.1986(3) A u , cZ9.0114(3) A u ); U2Fe3.6Al13.4 (Th2Ni17-type, aZ8.8589(2) A u , cZ8.9824(2) A u ); and U2Fe12Al5 (Th2Ni17-type, aZ8.5631(7) A u , cZ8.438(1) A u ). Four other phases exhibit more or less extended homogeneity ranges: UFe1CxAl1Kx (MgZn2-type); U3Fe4CxAl12Kx (Gd3Ru4Al12-type); U2Fe17KxAlx (Th2Zn17-type); and UFexAl12Kx (ThMn12-type). The two extended solid solutions, UAl2KxFex and UFe2KxAlx, are formed from the UAl2 and UFe2 binary compounds, respectively, both crystallizing in the cubic MgCu2-type structure. q 2004 Elsevier Ltd. All rights reserved.