Thirumany Sritharan

Interface reaction between copper and molten tin-lead solders

The formation and growth of Cu–Sn intermetallic film at the interface between molten Sn–Pb solders and Cu were studied at different temperature and exposure times. The η-phase (Cu6Sn5) was observed to form at all conditions except at the lowest Sn level of 27 wt% and at the two highest temperatures of 290 and 310°C. The var epsilon-phase (Cu3Sn) was then obtained. At high Sn contents and short times, a cellular film with a rugged interface was obtained which evolved into a compact film with a scalloped interface as the Sn content decreased and exposure time increased. The var epsilon-phase film always formed with a relatively planar interface. The intermetallic grains showed preferred crystallographic orientations. Thickness measurements showed that the net growth rate depends not only on diffusion through the film but also on the film dissolution. Precipitation of the η-phase whiskers was detected during cooling, particularly when the temperature was significantly high.

Large strain and high energy storage density in orthorhombic perovskite (Pb0.97La0.02)(Zr1−x−ySnxTiy)O3 antiferroelectric thin films

Antiferroelectric (Pb0.97La0.02)(Zr1−x−ySnxTiy)O3 (PLZST) thin films with orthorhombic perovskite structure were prepared on Si substrates by a chemical solution deposition process. A secondary pyrochlore phase, which was not detectable with x-ray diffraction, was revealed with transmission electron microscopy. The pyrochlore phase was effectively suppressed by the introduction of polyethylene glycol (PEG) in the precursor solution and applying PbO capping layer on the surface of the films. With the persistent and detrimental pyrochlore phase removed completely, our PLZST antiferroelectric thin films exhibited excellent electrical and electromechanical properties. A large energy storage density up to 13.7 J/cm3 was exhibited from the polarization measurement, and a strain of 0.49% under the clamping of the substrate was also achieved in the thin film with high Zr content.

Nonlinear dielectric thin films for high-power electric storage with energy density comparable with electrochemical supercapacitors

Although batteries possess high energy storage density, their output power is limited by the slow movement of charge carriers, and thus capacitors are often required to deliver high power output. Dielectric capacitors have high power density with fast discharge rate, but their energy density is typically much lower than electrochemical supercapacitors. Increasing the energy density of dielectric materials is highly desired to extend their applications in many emerging power system applications. In this paper, we review the mechanisms and major characteristics of electric energy storage with electrochemical supercapacitors and dielectric capacitors. Three types of in-house-produced ferroic nonlinear dielectric thin film materials with high energy density are described, including (Pb_0.97La_0.02)(Zr_0.90Sn_0.05Ti_0.05)O₃ (PLZST) antiferroelectric ceramic thin films, Pb(Zn_1/3Nb_2/3)O_3-Pb(Mg_1/3Nb_2/3) O_3-PbTiO₃ (PZN-PMN-PT) relaxor ferroelectric ceramic thin films, and poly(vinylidene fluoride) (PVDF)-based polymer blend thin films. The results showed that these thin film materials are promising for electric storage with outstandingly high power density and fairly high energy density, comparable with electrochemical supercapacitors.

Iron Pyrite Thin Film Counter Electrodes for Dye-Sensitized Solar Cells: High Efficiency for Iodine and Cobalt Redox Electrolyte Cells

Iron pyrite has been the material of interest in the solar community due to its optical properties and abundance. However, the progress is marred due to the lack of control on the surface and intrinsic chemistry of pyrite. In this report, we show iron pyrite as an efficient counter electrode (CE) material alternative to the conventional Pt and poly(3,4-ethylenedioxythiophene (PEDOT) CEs in dye-sensitized solar cells (DSSCs). Pyrite film CEs prepared by spray pyrolysis are utilized in I3(-)/I(-) and Co(III)/Co(II) electrolyte-mediated DSSCs. From cyclic voltammetry and impedance spectroscopy studies, the catalytic activity is found to be comparable with that of Pt and PEDOT in I3(-)/I(-) and Co(III)/Co(II) electrolyte, respectively. With the I3(-)/I(-) electrolyte, photoconversion efficiency is found to be 8.0% for the pyrite CE and 7.5% for Pt, whereas with Co(III)/Co(II) redox DSSCs, efficiency is found to be the same for both pyrite and PEDOT (6.3%). The excellent performance of the pyrite CE in both the systems makes it a distinctive choice among the various CE materials studied.

Influence of titanium to boron ratio on the ability to grain refine aluminium-silicon alloys

Abstract Grain refinement is achieved in aluminium alloys by inoculating the molten metal with small amounts of titanium and boron. Grain refiners containing high concentrations of titanium and boron are available commercially for this purpose. However, these grain refiners come in different chemical compositions giving rise to a variety of Ti/B ratios. Although it is accepted that increasing the amount of boron in the grain refiner gives beneficial effects on its performance, evidence for this is available principally in wrought aluminium alloys or in pure aluminium. Foundry alloys which contain high concentrations of silicon did not appear to respond in a manner similar to the other aluminium alloys. Hence, this work was undertaken to investigate the effects of various grain refiners on Al Si a alloys. One Al Si alloy of a hypo-eutectic composition and another of eutectic composition were chosen for the study. The performances of three Tibor alloys Al-6%T, Al-5%Ti-1%B and Al-2.5%Ti-2.5%B were assessed on the two Al Si. The results show that the grain refiner Al-2.5%Ti-2.5%B is the most potent in both Al Si Alloys while the Al-6%Ti gave the worst performance. The refined grain size depends on the Ti content, Ti/B ratio and the matrix alloy composition. A logarithmic dependence of grain size on Ti level was noticed in some cases while a power law dependence was appropriate for some others.

Coexistence of ferroelectric triclinic phases in highly strained BiFeO3 films

The structural evolution of the strain-driven morphotropic phase boundary (MPB) in BiFeO3 films has been investigated using synchrotron x-ray diffractometry in conjunction with scanning probe microscopy. Our results demonstrate the existence of mixed-phase regions that are mainly made up of two heavily tilted ferroelectric triclinic phases. Analysis of first-principles computations suggests that these two triclinic phases originate from a phase separation of a single monoclinic state accompanied by elastic matching between the phase-separated states. These first-principle calculations further reveal that the intrinsic piezoelectric response of these two low-symmetry triclinic phases is not significantly large, which thus implies that the ease of phase transition between these two energetically close triclinic phases is likely responsible for the large piezoelectric response found in the BiFeO3 films near its MPB. These findings not only enrich the understandings of the lattice and domain structure of epitaxial BiFeO3 films but may also shed some light on the origin of enhanced piezoelectric response near MPB.

Ruthenium Barrier/Seed Layer for Cu/Low- κ Metallization Crystallographic Texture, Roughness, Diffusion, and Adhesion

Ru is examined as a barrier/seed layer in different layer stacks, with and without Ta, for a CU/IOW-K system. Ru was found to promote a more pronounced Cu(lll) texture than Ta, and the texturing appears to increase with the increase of the Ru-layer thickness. Physical-vapor-deposited Cu films on both Ru and Ru/Ta layers are adequately smooth with root-mean-square roughness of ∼ 0.8-1.4 nm. Ru crystals on a low-κ layer have a columnar microstructure. Cu and Ru diffusion into the low-κ occurred after thermal annealing at 300°C for 1 h. A four-point bend adhesion study demonstrates a sufficiently high RU/IOW-K adhesion strength (∼6 J/m 2 ) in comparison to Ta/low-κ (∼6.5 J/m 2 ).

Multiferroic properties of sputtered BiFeO3 thin films

A cosputtering method was used to deposit BiFeO3 thin films on Pt∕Ti∕SiO2∕Si substrates. It was confirmed as a polycrystalline film with a tetragonal crystal structure in the annealed state. Both Fe2+ and Fe3+ ions were found to coexist in the film. The leakage current density is as low as 10−3A∕cm2 at 120kV∕cm. This sputtered film shows multiferroic properties exhibiting a saturated ferroelectric loop with a large remnant polarization of 37μC∕cm2 and a saturated ferromagnetic loop with saturation magnetization of 21emu∕cm3 at room temperature.

Nanoscale domains in strained epitaxial BiFeO3 thin Films on LaSrAlO4 substrate

BiFeO3 thin films with various thicknesses were grown epitaxially on (001) LaSrAlO4 single crystal substrates using pulsed laser deposition. High resolution x-ray diffraction measurements revealed that a tetragonal-like phase with c-lattice constant ∼4.65 A is stabilized by a large misfit strain. Besides, a rhombohedral-like phase with c-lattice constant ∼3.99 A was also detected at film thickness of ∼50 nm and above to relieve large misfit strains. In-plane piezoelectric force microscopy studies showed clear signals and self-assembled nanoscale stripe domain structure for the tetragonal-like regions. These findings suggest a complex picture of nanoscale domain patterns in BiFeO3 thin films subjected to large compressive strains.

Pulsed laser ablation of preferentially orientated ZnO:Co diluted magnetic semiconducting thin films on Si substrates

ZnO:Co thin films with room-temperature ferromagnetism have been synthesized on (001) Si substrates by pulsed laser deposition using a Zn0.95Co0.05O ceramic target. Single-phase wurtzite thin films with (002) preferential orientation were grown at 450°C in vacuum. There is no indication of Co nanocluster formation. However, copious edge dislocations appear to have formed during the film growth. A saturation magnetization of 1.04μB∕Co and a coercivity of 25Oe were obtained at room temperature. In addition to O vacancies, the Zn interstitial induced by edge dislocations may also contribute to the ferromagnetic properties in this oxide-diluted magnetic semiconductor.