Khaled Youssef

Ultrahigh strength and high ductility of bulk nanocrystalline copper

We have synthesized artifact-free bulk nanocrystalline copper samples with a narrow grain size distribution (mean grain size of 23nm) that exhibited tensile yield strength about 11 times higher than that of conventional coarse-grained copper, while retaining a 14% uniform tensile elongation. In situ dynamic straining transmission electron microscope observations of the nanocrystalline copper are also reported, which showed individual dislocation motion and dislocation pile-ups. This suggests a dislocation-controlled deformation mechanism that allows for the high strain hardening observed. Trapped dislocations are observed in the individual nanograins.

Ultratough nanocrystalline copper with a narrow grain size distribution

We report a unique way of using mechanical milling/in situ consolidation at both liquid-nitrogen and room temperature to produce artifact-free nanocrystalline Cu(23nm) with a narrow grain size distribution. This nanocrystalline Cu exhibits an extraordinarily high yield strength (770MPa), as predicted from a Hall–Petch extrapolation, along with good ductility (comparable with ∼30% uniform tensile elongation). Possible factors leading to this excellent optimization of strength and ductility are discussed.

Bulk Nanostructured Metals and Alloys: Processing, Structure, and Thermal Stability

1Department of Chemical Engineering and Materials Science, University of California, Davis, CA 95616, USA 2Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC 27695-7907, USA 3Department of Materials Science and Engineering, University of North Texas, Denton, TX 76203, USA 4Department of Metals and Materials Engineering, University of British Columbia, Vancouver, BC, Canada V6T 1Z4

Mechanical Properties of Nanocrystalline Materials Produced by In Situ Consolidation Ball Milling

This paper reviews a method, “in situ consolidation ball milling” that provides artifactfree bulk nanocrystalline samples for several ductile metals such as Zn, Al and Al alloys, and Cu and Cu alloys. The preparation method is described in this paper and examples of the mechanical behavior of nanocrystalline materials made by this technique are given. It is found that in such artifact-free metals, combinations of both high strength and good ductility are possible.

Effect of oxygen and associated residual stresses on the mechanical properties of high growth rate Czochralski silicon

The mechanical properties of Czochralski silicon (Cz-Si) crystals grown in vacancy rich regimes with elevated axial oxygen concentrations ranging from ∼6 × 1017 to ∼12 × 1017 atoms/cm3 have been investigated using nano- and micro-indentation techniques. Both hardness and fracture toughness were found to decrease with increasing oxygen concentration, while major differences in mechanical properties were found between the central core and the edge of the high oxygen concentration wafers. Photoluminescence imaging and Nomarski optical microscopy of high-oxygen wafers revealed the presence of a ring and swirl-like distributions of micro defects, including oxidation induced stacking faults. Micro-Raman analysis was used to measure local residual stress profiles associated with these characteristic defects. These results provide a quantitative understanding of the influence of the oxygen content and the associated defects resulting from the sub-optimal growth regimes within the Cz-Si process.

Proximity gettering of Cu at a (110)/(001) grain boundary interface formed by direct silicon bonding

We have demonstrated that a direct silicon bonded (110)/(001) interface, fabricated using hybrid orientation technology (HOT), acts as a proximity gettering center for Cu during quench annealing. The Cu gettering efficiency, which increases with annealing temperature, can reach more than 99%. Gettered Cu impurities tend to form colonylike precipitates at the (110)/(001) grain boundary (GB) interface when quenched from an elevated temperature. It is believed that interfacial GB imperfections initiate the Cu precipitation, and then, due to the release of a large number of self-interstitials, dislocation loops form and become new gettering centers which enhance the Cu gettering efficiency. These results are of interest for the defect engineering of the HOT wafer in advanced microelectronics and for enhancing our understanding of GBs in multicrystalline photovoltaic silicon.

Recombination via nano-precipitates … a new mechanism for efficiency loss in solar silicon?

Fast diffusing metals in silicon can form large precipitates. Although such precipitates can be damaging because of junction leakage they are not powerful recombination centers. In contrast we have investigated the precipitation of slow diffusing metals and found that they can produce nano-precipitates with dimensions of 2 to 10 nm. These have strong recombination activity. In this paper we focus on molybdenum nano-precipitates and present an analysis of recombination behavior derived from deep level transient spectroscopy. The chemical species of the nano-precipitates has been determined using transmission electron microscopy.

Silicon PV Wafers: Mechanical Strength and Correlations with Defects and Stress

Nanoindentation was used to measure the mechanical properties of 200mm diameter (100) CZ Si wafers subjected to the initiation and propagation of micro-crack defects. Silicon amorphization and phase changes were observed and accompanied by a monotonic decrease in hardness and elastic modulus, as the nanoindent tip approached the micro-crack shank or point. Identification and profiling of these localized phase transitions was obtained in the vicinity of the micro-cracks using electron back-scattered diffraction (EBSD) and Raman spectroscopy. It was found that the amorphous Si regions extend for about 10 µm at the edges and ahead of a moving crack tip. Wafers from ingots grown at faster growth rates with enhanced thermal gradients and associated point defect/impurity produce large localized stresses in the wafer core, which are capable of changing the path of propagating cracks. FTIR and Raman spectroscopy analysis were used to quantify local stresses due to radial oxygen precipitate variations. The resulting stress modified crack deviates considerably from energetically favorable [110]/(111) directions, following a radial path suggesting a ductile fracture failure mode.