M. Di Sabatino

Light-induced boron-oxygen defect generation in compensated p-type Czochralski silicon

D.M. is supported by an Australian Research Council QEII Fellowship, L.J.G. acknowledges SenterNovem for support, and B.L. and J.S. acknowledge the support of the German Academic Exchange Service.

Distribution of iron in multicrystalline silicon ingots

The distribution of iron in multicrystalline silicon ingots for solar cells has been studied. A p- and a n-type multicrystalline ingot were intentionally contaminated by adding 53ppmwt (μg∕g) of iron to the silicon feedstock and compared to a reference p-type ingot produced from ultrapure silicon feedstock. The vertical total iron distribution was determined by neutron activation analysis and glow discharge mass spectrometry. For the intentionally Fe-contaminated ingots, the distribution can be described by Scheil’s equation with an effective distribution coefficient of 2×10−5. The interstitial iron concentration was measured in the p-type ingots. In the Fe-contaminated ingot, it is almost constant throughout the ingot and constitutes about 50% of the total concentration, which is in conflict with the previous studies. Gettering had a large impact on the interstitial iron levels by reducing the concentration by two orders of magnitude. Considerable trapping was observed at crystal defects on as-cut wafers...

In situ investigation of spinodal decomposition in hypermonotectic Al–Bi and Al–Bi–Zn alloys

Spinodal decomposition of hypermonotectic Al-6 wt.%Bi, Al-8 wt.%Bi and Al-6 wt.%Bi-8 wt.%Zn alloys has been investigated using synchrotron radiography. In the case of the 6 and 8 wt.%Bi binary alloys undercoolings of 70 and 110 K, respectively, were required to initiate the L ! L1 + L2 reaction, which appeared to occur very close to the monotectic reaction temperature. The nucleated L2 droplets were set in collective size- dependent motion by forces coupled to external fields (gravity and imposed temperature gradient) as well as forces arising due to internal fluctuations of the system. With experimental conditions similar to those realized during strip casting of the same materials, it was found that the size-dependant droplet velocity field combined with Stokes drag at the L1-L2 interfaces as well as attractive and repulsive diffusion-coupling between adjacent L2 droplets, yield complex meso- to microscale hydrodynamics. The hydrodynamics are the dominating mechanisms for L2 droplet coagulation, and are accordingly decisive for the final size distribution and geometrical dispersion of the soft Bi-rich component in the cast material. A different decomposition mode was observed in the Al-6 wt.%Bi-8 wt.%Zn ternary alloy, with the L2 droplets undergoing an immiscible-miscible-immiscible transition. In contrast to what

An improved method for fluidity measurement by gravity casting of spirals in sand moulds

Abstract This study describes a new equipment for gravity casting of fluidity spirals in a sand mould. The fundamental characteristic of the equipment is a constant pouring temperature, which gives a constant melt superheat and since the molten metal is poured into the spiral sand mould from the same height, the equipment gives a constant initial pressure head and pouring velocity. By comparing data from an earlier version of the equipment, an improvement in reproducibility has been shown. The effect of melt superheat on fluidity has been measured by the new improved equipment and has been confirmed to increase linearly with increasing melt superheat.

Effect of hydrogen content, melt cleanliness and solidification conditions on tensile properties of A356 alloy

Abstract In this study, the influence of hydrogen content, melt cleanliness and solidification conditions on the porosity distribution and mechanical properties have been investigated in a laboratory scale step mould casting with an A356 alloy. Three hydrogen levels, namely 0·10, 0·20 and 0·40 mL/100 g, were reached by treatment of the melt by Ar degassing, Ar–10H2 and Ar water vapour mixture respectively. The hydrogen content of the melt was continuously measured. For each hydrogen level, two castings were produced, namely with and without filter. Porosity was increased with increasing hydrogen content but there was no significant effect of filtration. Moreover, similar tensile properties were achieved with different hydrogen levels and different porosity levels.

Depth profile analysis of solar cell silicon by GD-MS

In this work we have assessed the capability of depth profile analysis by glow discharge mass spectrometry (GD-MS) for several impurities relevant for solar cell silicon. Fast-flow direct-current high resolution GD-MS has been used. Six multicrystalline p-type silicon samples with contamination of B, P, Ti, Fe and Cu have been investigated. Ion implantation has been used for impurity contamination with a target depth of 3 μm. The acquisition time was approximately 30 seconds, giving a depth resolution of approximately 0.5 μm. The GD-MS concentration profiles of the samples contaminated with B, P and Ti agreed well with the levels implanted. Because Fe and Cu are fast diffusers, their distribution deviates from the target implantation. This indicates that for fast diffusing transition metallic impurities, such as Fe and Cu, different impurity distribution mechanisms occur and should be taken into account when analysing their depth profiles.

Influence of Oxide Additions on the Porosity Development and Mechanical Properties of A356 Aluminium Alloy Castings

Three A356 aluminium alloy melts with different oxide contents were produced by mixing alloy turnings with ingot material. The melts had a constant hydrogen level and were cast in a step mould steel die. The resulting casting porosity was characterized using the Archimedes’ method and image analysis. Melt quality was accessed using a Porous Disc Filtration Apparatus (PoDFA) and the Reduced Pressure Test (RPT). It was shown that oxide additions lead to higher amounts of fine structured oxide films resulting in an increase of the fraction of small pores. Shrinkage porosity with its inherent partially elongated and interdendritic pore morphology was predominant. Ultimate tensile strength (UTS) and elongation from thin and thick sections in the step castings deteriorated with increased oxide content; UTS decreased from 3–18 % and elongation from 18–38 % with oxide additions as the section thickness increased. The melt cleanliness analyses from PoDFA were consistent with the oxide additions.

Fluidity evaluation methods for Al–Mg–Si alloys

Abstract Much work has been carried out to assess the fluidity of casting alloys by various fluidity test methods. This study compares two tests which have been used to evaluate the fluidity of three Al– Mg–Si alloys for High Pressure Die Casting (HPDC) processes: Al–5Mg–2.5Si, Al–5Mg–1.5Si, and Al–3Mg–0.5Si (compositions in wt-%). The fluidity of the alloys has been measured using the commercially available strip fluidity test method and an experimental test method using a spiral sand mould designed by SINTEF (Norway). Reproducibility and fluidity measurements obtained using the two methods are reviewed and discussed. The experimental results show that both fluidity test methods give the same trends and internally consistent results.

EBIC, EBSD and TEM study of grain boundaries in multicrystalline silicon cast from metallurgical feedstock

Grain boundaries in multicrystalline silicon material grown from metallurgical feedstock, were investigated in detail using Electron Beam Induced Current (EBIC), Electron Back-Scattered Diffraction (EBSD) and Transmission Electron Microscopy (TEM) techniques. The EBSD analysis showed that small angle grain boundaries, with misorientation angles lower than 2°, gave high EBIC contrast, i.e., high recombination activity. EBIC combined with TEM showed that at low temperatures, silicon oxide was found to be recombination centers both at grain boundaries and on decorated dislocations in the bulk. The grain boundaries containing multi-metallic silicides were found to have random misorientations and showed strong contrast in the EBIC image. Clean twins showed less or no contrast in the EBIC image. The metallic precipitates observed in the sample contain mainly nickel silicide with an iron rich core.

Effect of grain refinement and dissolved hydrogen on the fluidity of A356 alloy

Abstract The influence of grain refinement and dissolved hydrogen on the fluidity of A356 alloy has been investigated. A spiral casting test method, recently developed, has been used to measure fluidity in a reproducible way. The grain refinement reduces the grain size of the spirals, particularly at the tip, but no significant influence on the fluidity has been revealed. The hydrogen additions in the melt have not affected the fluidity but have, of course, significantly increased the porosity.