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Summary BiVO4 is a promising photoanode material for solar-assisted water splitting in a photoelectrochemical cell but has a propensity to degrade. Investigations carried out here in 0.1 M Na2SO4 electrolyte showed that degradation is by dissolution of V in the electrolyte while Bi is retained on the anode probably in the form of solid Bi oxide (Bi2O3, Bi4O7). Accumulation of Bi oxide on the anode surface leads to passivation from further degradation. Thermodynamic modeling of possible degradation reactions has provided theoretical support to this mechanism. This self-passivation is accompanied by a decrease in photocurrent density, but it protects the anode against extensive photocorrosion and contributes to long-term stability. This is a more definitive understanding of degradation of BiVO4 during water splitting in a photoelectrochemical cell. This understanding is imperative for both fundamental and applied research., Graphical Abstract, Highlights • A mechanism of degradation is developed for BiVO4 photoanode during photolysis • Degradation occurs by V dissolution and Bi accumulation on the anode as oxide • Accumulating Bi oxide passivates the anode • Thermodynamic modeling supports this mechanism, Electrochemical Energy Storage; Electrochemical Energy Conversion; Chemical Reactions in Materials Science; Materials Characterization

The corresponding authors are professors in Materials Science and Engineering at the Nanyang Technological University, Singapore. Thirumany Sritharan hails from Sri Lanka and received his PhD from the University of Sheffield, UK. He worked in the University of Melbourne and Comalco Research Centre, Australia, before joining NTU Singapore in 1992 when the Materials Engineering discipline was inaugurated. Zhong Chen hails from China and received his PhD from the University of Reading, UK. He worked in ASTAR Singapore before joining NTU in 2000. Both have worked in different areas of materials science with extensive industrial research experience following their PhD degrees.

The monoclinic scheelite‐type BiVO4 is recognized as one of the promising candidate materials for a photoanode because of its 9.1 % theoretical efficiency for half‐cell solar‐to‐hydrogen conversion. Although significant research efforts have been devoted to improving the performance of the photoelectrochemical cell (PEC) of this material, they have mainly been in small anode areas with only a handful of studies on scaled‐up sizes. Herein, a facile metal–organic decomposition synthesis method was used to produce scaled‐up Mo‐doped BiVO4 photoanodes. Multiple modifications were explored and incorporated to enhance the performance of the photoanode. A large‐area (5 cm×5 cm) photoanode was successfully prepared with all modifications. The resulting photoanode gave rise to an initial photocurrent density of 2.2 mA cm−2 at 1.23 V versus reversible hydrogen electrode, under AM 1.5G illumination in a PEC, which remained at 79 % of this value after 1 h of operation. A deleterious effect of the increased anode surface area on the photocurrent density was observed, which we termed the “areal effect”. Understanding the reasons for the areal effect is indispensable for the development of large‐scale PEC devices for water splitting. NRF (Natl Research Foundation, S’pore) Accepted version

In this report, a three-dimensional (3-D) network of core–shell TiO2 (P25)–mesoporous SiO2 (P25@mSiO2) nanocomposites was prepared via a controllable surfactant-assisted sol–gel method. The nanocomposites were investigated for photocatalytic reactions of organic dye degradation, water splitting, and CO2 reduction to understand the roles of the mSiO2 shell in these photocatalytic reactions. It was found that the mSiO2 shell accelerates the photodegradation of the organic dye, but dramatically reduces the photocatalytic activity of P25 in water splitting and CO2 reduction. The roles played by the mSiO2 shell in the photocatalytic reactions are summarized as: (1) effective prevention of agglomeration of P25 nanoparticles, (2) facilitating the transfer of uncharged photo-generated •OH radicals via the abundant –OH groups on the mesoporous surface, (3) provision of increased reaction sites between •OH radicals and dye molecules by its mesoporous nanostructure and large surface area, and (4) prevention of diffusion of the photo-generated charge carriers (photoelectrons and photoholes) because of its insulating nature. NRF (Natl Research Foundation, S’pore) MOE (Min. of Education, S’pore) Accepted version

Fabrication of catalytically active electrodes by electrodeposition is attractive due to its in situ nature, easy controllability, and large-scale operation capability. Most recently, modifying the electrodes with phosphate ligands through electrodepositing electrode materials has shown promising results in improving the kinetics of some reactions. However, it is unclear how the presence of phosphate anions affects the electrodeposition process and functions in catalyzing reactions. Here, we report a systematic study on electrodeposition of Cu in the presence of phosphate anions. The phosphate anions form a complex with free Cu(II) cations, competing with the electrodeposition process. The competition between the two processes results in an insufficient supply of free Cu(II) for electrodeposition, especially at the diffusion layer. This is evidenced by the calculation of free Cu(II) concentration and the electrodeposition current at identical applied potentials. We also found that the electrodeposition of Cu in the presence of phosphate generates Cu-oxyo/hydroxyo-phosphate species on the deposited copper surface. The modified electrodes with phosphate species exhibit higher selectivity for HCOOH formation (faradaic efficiency ∼80%) from the electrochemical reduction of CO2 as compared with Cu foil (faradaic efficiency ∼33%). The effect of phosphate ligands promoting HCOOH selectivity is further verified by stripping off the ligands and regenerating the ligands.

BiVO4 is a promising photoanode material for solar assisted water splitting in a photoelectrochemical cell but has a propensity to degrade. Investigations carried out here in 0.1 M Na2SO4 electrolyte showed that degradation is by dissolution of V in the electrolyte while Bi is retained on the anode probably in the form of solid bismuth oxide (Bi2O3, Bi4O7). Accumulation of Bi oxide on the anode surface leads to passivation of the photoanode from further degradation. Thermodynamic modelling of possible degradation reactions has provided theoretical support to this mechanism. This self-passivation is accompanied by a decrease in photocurrent density but, it protects the anode against extensive photocorrosion and contributes to long-term stability. This disclosure is a more definitive understanding of degradation of BiVO4 during water splitting in a photoelectrochemical cell. This understanding is imperative for both fundamental and applied research.

Author(s): Shukla, S; Ager, JW; Xiong, Q; Sritharan, T | Abstract: Iron pyrite (FeS2) holds an enormous potential as a low cost and non-toxic photoelectrochemical and energy-harvesting material owing to its interesting optical, electronic, and chemical properties along with elemental abundance. In this Review, low cost and scalable processing techniques to synthesize phase-pure pyrite thin films and nanocubes are described, and the application of this material in various energy-harvesting devices such as dye-sensitized solar cells, photodiodes, and heterojunction solar cells is discussed. A detailed analysis of the electron transport in single-crystal iron pyrite is presented to shed light on its bulk- and surface-conduction properties, which could be useful in designing better pyrite solar cells and could be exploited for novel device architectures. Finally, future prospects and directions are discussed.

Mn-Co containing spinel oxides are promising, low-cost electrocatalysts for the oxygen reduction reaction (ORR). Most studies are devoted to the design of porous Mn-Co spinels or to strongly coupled hybrids (e.g., MnCo

The effects of ultrasonic irradiation treatment at room temperature on the nonisothermal reaction kinetics for the formation of 0.94(K0.5Na0.5)NbO3-0.06LiNbO3 (0.94KNN–0.06LN) lead-free piezoelectric ceramics were studied. It was observed that sonication dramatically decreased the sizes of the starting material aggregates and enhanced the interaction between the individual reactants even at room temperature. Through quantitative analyses on the activation energies of the decomposition of the starting alkali carbonates and formation of the resulting perovskite phase, it was found that the activation energies decreased with increasing ultrasonic power. More interestingly, the activation barrier for the decomposition of the carbonates was reduced to below the critical value for the crystallization of the perovskite phase with the high irradiation power, so that the reaction of forming the perovskite phase was not rate-limited by the decomposition of the carbonates. This finding highlights the significance of ultrasound utility in processing KNN-based ceramics with reduced synthesis temperature and thus improved control of chemical stoichiometry.

The coexistence of low-index facets with a highly photoactive {001} facet in anatase TiO2 nanocrystals has been recently found beneficial to enhance the photocatalytic performance of TiO2 via a synergistic effect. In this paper, this synergistic effect has been further extended from a single crystal to interconnected nanocrystals with dominating {001} or {010} facet in intact hierarchical TiO2 nanostrucutre. The particles synthesized at the optimal condition showed outstanding photocatalytic hydrogen production of 364.2 μmol·g–1·h–1, which is about four times as much as that of commercial P25 (96.5 μmol·g–1·h–1). Femtosecond transient spectroscopy and density functional theory (DFT) study indicates that effective electron–hole separation takes place within these nanostructures. This new prototype of synergy, which we denote as quasi-heterojunctions, shows that enhanced photocatalytic performance could be derived in the same anatase phase by synthesizing appropriate faceted nanostructures. This work provid...

Growing oxide shells on seed nanoparticles requires the control of several processes: (a) the nucleation and growth of the shell material; (b) the "wetting" of the shell material on the seeds; and (c) the aggregation of the nanoparticles. These processes are influenced by a number of factors, many of which are related. Without understanding the interdependence of these contributing factors, it is difficult to circumvent problems and achieve rational synthesis. We first did a case study on encapsulating Au nanoparticles with ZnO to understand the multiple roles of polyvinylpyrrolidone (PVP) and their dependence on other factors. We developed a general method for coating ZnO on a variety of seeds, including metals, oxides, polymer nanoparticles, graphene oxide, and carbon nanotube. This method can be further extended to include Fe3O4, MnO, Co2O3, TiO2, Eu2O3, Tb2O3, Gd2O3, β-Ni(OH)2, ZnS, and CdS as the shell materials. The understanding obtained in this systematic study will aid rational design and synthesis of other core-shell nanostructures.

Carbon fiber reinforced composites have stimulated increasing attention due to their excellent mechanical properties. However, the interface tends to be weak because carbon fiber does not naturally have a good wettability and adsorption with most polymers matrix, which could not effectively transfer the load from the matrix to the fibers. Here we proposed to chemically functionalize carbon fiber (CF) by poly (amido amine) (PAMAM), which has more concentration of amino groups than traditional coupling agents. The chemical bond formation between acid-treated CF and PAMAM was confirmed. The PAMAM functionalization did not change the surface morphology while significantly increased the wettability of CF surface. The interfacial shear strength has an increase of 85% compared to that without functionalization. The interfacial enhancement mechanism was also explored in details.

Iron Pyrite has received a lot attention of the researchers to develop low cost and non-toxic solar energy conversion system. Despite of having a band gap of 0.95 eV and extraordinarily high optical absorption, the efficiency is hampered by photovoltage loss. Maximum photovoltage achieved so far could not exceed more than 200 mV. We addressed this problem by identifying photocarrier loss pathways within the high quality, faceted iron pyrite nanocubes prepared by hot-injection method. In this work, we probed charge carrier dynamics by ultrafast laser spectroscopy and analyze the carrier relaxation/recombination lifetimes. We have also analyzed the impact of defects on transport properties of pyrite. To further examine the nature intrinsic defects in iron pyrite, we did low temperature magnetic measurements and found the evidence of phase impurities that relate to sulfur vacancies. On the basis of comprehensive characterization and analysis of the system, we presented a model that explains the origin of low photovoltages in iron pyrite. Fast carrier localization to shallow states followed by deep level relaxation is found to be the main cause. The elimination of the problem lies in preparing samples with clean band gap and passivating intrinsic defects.

Electromechanical cantilevers comprising antiferroelectric (Pb, La) (Zr, Sn, Ti)O3 (PLZST) thin films were fabricated through bulk micro-machining process on silicon wafers, and their electromechanical properties including strain-fatigue behaviors of the antiferroelectric thin film cantilevers were investigated. The antiferroelectric cantilevers showed the distinct digital actuation characteristics with the strain generated due to the antiferroelectric–ferroelectric transformation. The maximum displacement per unit voltage around the phase switching field reached 16.7 μm/V, significantly larger than the typical piezoelectric cantilevers. Moreover, the antiferroelectric PLZST cantilevers exhibited superior strain-fatigue resistance compared to the similar piezoelectric microstructures. These results show the promising future of antiferroelectric materials in micro electromechanical systems. Accepted version

Coupling effects among mechanical, electrical and magnetic parameters in thin film structures including ferroic thin films provide exciting opportunity for creating device functionalities. For thin films deposited on a substrate, their mechanical stress and microstructure are usually determined by the composition and processing of the films and the lattice and thermal mismatch with the substrate. Here it is found that the stress and structure of an antiferroelectric (Pb0.97,La0.02)(Zr0.90,Sn0.05,Ti0.05)O3 (PLZST) thin film are changed completely by a ferroelastic strain in a magnetic shape memory (MSM) alloy Ni-Mn-Ga (NMG) thin film on the top of the PLZST, despite the existence of the substrate constraint. The ferroelastic strain in the NMG film results in antiferroelectric (AFE) to ferroelectric (FE) phase transformation in the PLZST layer underneath. This finding indicates a different strategy to modulate the structure and function for multilayer thin films and to create unprecedented devices with ferroic thin films.

The Poisson's ratio of perovskites with a high degree of anisotropy can change from negative to positive and become much larger than the normally observed range of 0.2–0.4 and even greater than the isotropic upper limit 0.5 along certain crystallographic orientations. These results pave the way for a better understanding of elastic-mediated physical properties of perovskites by engineering the orientation-dependent Poisson's ratios.

In the present study, amorphous Fe–18Cr–4Mn– x N alloys were successfully synthesized by mechanical alloying (MA) of pure elemental powders under a nitrogen gas atmosphere. The microstructure of the milled products at different stages of milling was characterized by X-ray diffraction (XRD) and high-resolution transmission electron microscopy (TEM). Furthermore, differential scanning calorimetry (DSC) was conducted to investigate the thermal stability and to determine the glass formation ability and the crystallization behavior of the amorphous as-milled powders. The results showed that by increasing the milling time, the amorphous phase content increases and after 126 h the amorphization process becomes complete. The alloys exhibited a distinct glass transition and a wide supercooled liquid region that increased by increasing the milling time. It was found out that dissolved nitrogen atoms play a crucial role in the amorphization and the exhibition of the large supercooled liquid region.

In this research, the effect of nitrogen on the thermal behavior and micro-hardness of Fe–Cr–Mn–N amorphous alloys synthesized by mechanical alloying under a nitrogen atmosphere has been considered. The characterization of the as-milled powders by X-ray diffraction, scanning and transmission electron microscopy showed that a fully amorphous structure has been developed by the mechanical alloying process. Differential scanning calorimetry results revealed that the glass transition temperatures and onset crystallization temperatures are in the ranges of 764–766 K and 855–861 K, respectively, for the alloys containing 3.45–3.95 wt.% nitrogen, giving considerable supercooled liquid regions of 91–95 K. The amorphous alloys exhibited an increase in the glass-forming ability by increasing the nitrogen amount. Furthermore, the as-milled amorphous powders showed high micro-hardness values of nearly 1015–1070 HV with an elastic–plastic deformation feature during the indentations. A decrease in the micro-hardness values was found by increasing the nitrogen content.

In this paper, we developed nc-Al/a-Al2O3 nanocomposite thin films using magnetron sputtering. The nc-Al/a-Al2O3 films were sputtered on p-type Si substrates from pure Al target in gas mixture of Ar and O2. X-ray photoelectron spectroscopy and high resolution transmission electron microscope studies confirm that the nanocrystalline Al are embedded in amorphous Al2O3 matrix thus nc-Al/ a-Al2O3 nanocomposite forms. This nanocomposite thin film exhibits memory effect as a result of charge trapping.

Amorphous FeCrMnN alloys were synthesized by mechanical alloying (MA) of the elemental powder mixtures under a nitrogen gas atmosphere. The phase identification and structural properties, morphological evolution, thermal behavior and magnetic properties of the mechanically alloyed powders were evaluated by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), differential scanning calorimetry (DSC) and vibrating sample magnetometer (VSM), respectively. According to the results, at the low milling times the structure consists of the nanocrystalline ferrite and austenite phases. By progression of the MA process, the quantity and homogeneity of the amorphous phase increase. At sufficiently high milling times (>120 h), the XRD pattern becomes halo, indicating complete amorphization. The results also show that the amorphous powders exhibit a wide supercooled liquid region. The crystallization of the amorphous phase occurs during the heating cycle in the DSC equipment and the amorphous phase is transformed into the crystalline compounds containing ferrite, CrN and Cr 2 N. The magnetic studies reveal that the magnetic coercivity increases and then decreases. Also, the saturation magnetization decreases with the milling time and after the completion of the amorphization process (>120 h), the material shows a paramagnetic behavior. Although the magnetic behavior does not considerably change by heating the amorphous powders up to the crystallization temperature via DSC equipment, the material depicts a considerable saturation magnetization after the transformation of the amorphous phase to the nanocrystalline compounds.

Nd-doped SrBi 2 Ta 2 O 9 films were sputtered on Pt/Ta/SiO 2 /Si substrates under various substrate biases. The radio frequency bias results in the reduction of the Aurivillius phase crystallization temperature. At 48 W, the crystallization temperature of film is lowered at a magnitude of about 80 °C. When the bias further increases, Aurivillius phase formation is suppressed due to too deficient Bi in film. The film deposited at 48 W after annealing at 670 °C shows ferroelectric characteristics. The remnant polarization of the films increases as the annealing temperature is increased.

In this study, the formation of Fe18Cr8MnxN alloys by mechanical alloying (MA) of the elemental powder mixtures was investigated by running the milling process under nitrogen and argon gas atmospheres. The effect of the milling atmosphere on the microstructure and phase contents of the as-milled powders was evaluated by X-ray diffraction and transmission electron microscopy. The thermal behavior of the alloyed powders was also studied by differential scanning calorimetry. The results revealed that in the samples milled under nitrogen, three different phases, namely ferrite (α), austenite (γ), and a considerable amount of amorphous phase are present in the microstructure. In contrast, in the samples milled under argon, the structure contains the dominant crystalline ferrite phase. By progression of MA under the nitrogen atmosphere, the ferrite-to-austenite phase transformation occurs; meanwhile, the quantity and stability of the amorphous phase increase, becoming the dominant phase after 72 h and approaching 83.7 wt% within 144 h. The quantitative results also showed that by increasing the milling time, grain refinement occurs more significantly under the nitrogen atmosphere. It was realized that the infused nitrogen atoms enhance the grain refinement phenomenon and act as the main cause of the amorphization and α-to-γ phase transformation during MA. It was also found out that the dissolved nitrogen atoms suppress the crystallization of the amorphous phase during the heating cycle, thereby improving the thermal stability of the amorphous phase.

During magnetron sputtering deposition of SrBi 2 Ta 2 O 9 thin films, oxygen gas is introduced in the deposition chamber to compensate the oxygen loss during sputtering. In this paper, the influence of oxygen partial pressure is systematically investigated in sputtering of Sr 0.8 Nd 0.3 Bi 2.5 Ta 2 O 9+ x thin films, using X-ray diffraction, X-ray photoelectron spectroscopy and field emission scanning electron microscopy. The studies confirm that at various oxygen partial pressure Nd 3+ ions substitute into the bismuth layered perovskite structure, preferentially at the Sr 2+ site. The deposition rate, composition, microstructure and polarization properties of the films highly depend on the oxygen partial pressure.

Three common Al–Au intermetallics, Al2Au, AlAu2, and AlAu4, were oxidized in the air and characterized using x-ray photoelectron spectroscopy in terms of the elemental chemical state. It was found that there is an increasing trend of oxidation in these intermetallics as the Au content increases. AlAu4 shows the greatest tendency to oxidize with two extra peaks appearing on the Au 4f spectra after long exposure time in air. The surface of AlAu2, although fully oxidized, reveals only one Au 4f peak shift as depth increases. Al2Au was the least oxidizing compound, and the oxide is thin. The binding energies of Al 2p and Au 4f peaks were measured and reported. The Au atoms trapped in the oxide layers exhibit higher binding energy emissions compared to those of elemental Au.

Conventional depth-sending indentation and novel cyclic indentation were performed to study the mechanical properties and strengthening mechanism of pure gold and Ca-doped gold of up to 90 ppm. The system was chosen because it is popular materials for microelectronics interconnection, yet the strengthening mechanism is not well-understood. Conventional depthsensing indentation showed an increase in hardness and modulus of gold with increasing Ca content of up to 90 ppm, which remained significantly higher after annealing. Cyclic depth sensing indentation, where the specimen is loaded to a specific value, unloaded and immediately reloaded, produced a cyclic loop between unloading and reloading curves. The path of the unloading and reloading curves tell the story of the different response of the dislocations generated under the indentation, in different materials, pure gold and Ca-doped gold at different level. Thus their dislocation movement and deformation behavior could be studied by cyclic depth-sensing indentation method. This paper reports cyclic indentation study performed in calcium doped gold widely used in wire bonding and shows that a few ppm level of calcium could result in dislocationssolute interactions, leading to significant strengthening of the materials. TEM micrographs of the material shows that it consists of elongated grains parallel to the drawing direction about 200 nanometers in diameter and a few micrometers in length. It is concluded that cyclic nanoindentation could be used as an extended technique to extract sensitive material information that are not reflected in the conventional test.

Iron pyrite has received significant attention due to its high optical absorption. However, the loss of open circuit voltage (Voc) prevents its further application in photovoltaics. Herein, we have studied the photophysics of pyrite by ultrafast laser spectroscopy to understand fundamental limitation of low Voc by quantifying photocarrier losses in high quality, stoichiometric, and phase pure {100} faceted pyrite nanocubes. We found that fast carrier localization of photoexcited carriers to indirect band edge and shallow trap states is responsible for major carrier loss. Slow relaxation component reflects high density of defects within the band gap which is consistent with the observed Mott-variable range hopping (VRH) conduction from transport measurements. Magnetic measurements strikingly show the magnetic ordering associated with phase inhomogeneity, such as FeS2−δ (0 ≤ δ ≤ 1). This implies that improvement of iron pyrite solar cell performance lies in mitigating the intrinsic defects (such as sulfur vacancies) by blocking the fast carrier localization process. Photocarrier generation and relaxation model is presented by comprehensive analysis. Our results provide insight into possible defects that induce midgap states and facilitate rapid carrier relaxation before collection. NRF (Natl Research Foundation, S’pore) MOE (Min. of Education, S’pore)

Porous tungsten oxide/copper tungstate (WO3/CuWO4) composite thin films were fabricated via a facile in situ conversion method, with a polymer templating strategy. Copper nitrate (Cu(NO3)2) solution with the copolymer surfactant Pluronic®F-127 (Sigma-Aldrich, St. Louis, MO, USA, generic name, poloxamer 407) was loaded onto WO3 substrates by programmed dip coating, followed by heat treatment in air at 550 °C. The Cu2+ reacted with the WO3 substrate to form the CuWO4 compound. The composite WO3/CuWO4 thin films demonstrated improved photoelectrochemical (PEC) performance over WO3 and CuWO4 single phase photoanodes. The factors of light absorption and charge separation efficiency of the composite and two single phase films were investigated to understand the reasons for the PEC enhancement of WO3/CuWO4 composite thin films. The photocurrent was generated from water splitting as confirmed by hydrogen and oxygen gas evolution, and Faradic efficiency was calculated based on the amount of H2 produced. This work provides a low-cost and controllable method to prepare WO3-metal tungstate composite thin films, and also helps to deepen the understanding of charge transfer in WO3/CuWO4 heterojunction.

Solid state synthesis of three complex ceramics, nickel ferrite, manganese ferrite and strontium bismuth tantalate, by mechanochemical activation in a high energy ball mill was investigated. It was shown that mechanochemical activation prior to reaction synthesis at elevated temperatures is generally beneficial to reduce the necessary synthesis temperature. Synthesis of manganese ferrite occurred even during the ball milling but not to completion. Subsequent heating to complete the synthesis resulted in deterioration of the manganese ferrite back into the constituent oxides. In strontium bismuth tantalate, the other oxides dissolved initially in bismuth oxide during the activation process.

Nd-substituted SrBi 2 Ta 2 O 9 (SNBT) thin films are sputtered on Pt/Ta/SiO 2 /Si substrates. X-ray diffraction and x-ray photoelectron spectroscopy studies indicate that Nd 3+ is substituted into the bismuth layered perovskite structure, preferentially at the Sr 2+ site. The annealed thin film is polycrystalline with plate/needle-like grain microstructure. Secondary ion mass spectrometry results show that elements in SNBT thin film homogeneously distribute along film depth and interfacial diffusion takes place during post annealing. The Nd substitution leads to enhanced remnant polarization (2 P r = 18 μC/cm 2 ) and reduced coercivity (2 E c = 64 kV/cm) at 180 kV/cm measured at 25 °C. After 10 10 switching cycles, around 9% remnant polarization is decreased.

The reaction mechanisms and related microstructures in the Cu/Si, Ru/Si, and Cu/Ru/Si metallization system were studied experimentally. With the help of sheet resistance measurements, x-ray diffraction, field-emission scanning electron microscopy, secondary ion mass spectroscopy, and transmission electron microscopy, the metallization structure with Ru barrier layer was observed to fail completely at temperatures around 700 °C, regardless of the Ru thickness because of the formation of polycrystalline Ru2Si3 followed by Cu3Si protrusions.

Interface phase transformations in 1 μm aluminum + 1 μm gold thin film couples upon 24-h isothermal annealing at 125, 150 and 175 °C were studied. It is shown that temperature change does not affect the type of phases formed but only alters their growth rates. The layered structures consisting of multiple intermetallics were identified by X-ray diffraction and electron microscopy. Interface tracking using secondary ion mass spectroscopy depth profiling showed that Au is the predominant diffusant in this system. Intermetallic thickness growth measurements were made and the activation energy calculated to be 33.602 kJ/mol.

Samples of Al 2 Au and AlAu 4 were examined using XPS after controlled oxidation in air. AlAu 4 showed a strong tendency to oxidize compared to Al 2 Au. The binding energies (b.e.) of Au 4f and Al 2p XPS emissions were determined for both intermetallics. Heavy oxidation of AlAu 4 resulted in a unique Au 4f emission near the surface which was attributed to Au dissolved in aluminum oxide.

Thin film diffusion couples of 1 μm Au with 0.3 μm and 1 μm Al were annealed at different temperatures and for different times and the resulting interface reactions were tracked by X-ray diffraction and electron microscopy. The phase formation sequence was identified and the interface microstructure was shown to consist of layers of the different phases. Interface tracking by secondary ion mass spectroscopy depth profiling showed that Au is the predominant diffusant in this system. Intermetallic thickness measurements were made and the activation energy calculated.

The grain growth dependence of microstructure and its effects on magnetic and transport properties are studied in the polycrystalline La 0.67 Ba 0.33 MnO 3 oxides. It is found that a lateral growth manner along a certain direction and a concentric terrace pattern along three orthogonal axes occur in the samples sintered at 1573 and 1673 K, respectively. Lamella-like twin microstructure forms in the concentric terrace growth pattern and the magnetoresistance properties can be enhanced by the twin microstructure. It suggests that the twin-boundaries in twin-grains may possibly induce spin-dependent scattering of electrons that is field reduced, or spin-polarized tunneling of electrons that is field enhanced, thus strengthening the effect of grain boundaries.

A four-point bend technique (4PBT) was employed to characterize adhesion strength of nanoporous silica (SiCOH-based low- κ ) to a hard mask layer (TEOS). Adhesion energy was analyzed as a function of low- κ porogen loading (3, 6, 9, and 11%). Effect of an adhesion promoter (AP) layer would also be discussed. Low fracture energies ( 2 ) confirm the weak mechanical properties of such highly porous materials. Fracture energy decreases as the porosity increases for samples without AP. However it becomes slightly higher and independent on the porosity when AP is applied. Low- κ /TEOS interface is the main delamination path for samples without AP. On the other hand, AP/TEOS interface becomes the interface that controls the adhesion for samples with AP hence it becomes independent on the low- κ porosity.

Conventional depth sensing nanoindentation test which produces a load–depth curve for a monotonically increasing load can be used to determine many mechanical properties. Alternatively, if the specimen is loaded to a specific value, unloaded and immediately reloaded, a cyclic nanoindentation curve is produced. The unloading–reloading paths do not necessarily overlap in all materials. The reason for this is thought to be the different response of the dislocations generated under the indentation, in different materials. The dislocation behaviour will be controlled by the chemistry and microstructure of the material. Therefore, sensitive dislocation–microstructure interactions may be detectable in the unloading–reloading curves during cyclic indentation. This paper reports such cyclic nanoindentation investigation done in calcium-doped gold used in wire bonding. It is shown that calcium, even in ppm levels, can influence the cyclic load–depth curves to measurable extents. The loading and unloading profiles obtained are reported and the effect of increasing dopant level is evident in them. This is attributed to dislocation–solute interactions. The microstructure of the material consists of elongated (drawn) grains of diameter of about 200 nm and length of a few micrometers. It is concluded that cyclic nanoindentation could be used as an extended technique to extract sensitive material information that are not reflected in the conventional test.

SrBi2Nb2O9 (SBN) is an attractive ferroelectric material that is being considered in non-volatile random access memory cells. This study reports the effects of doping SBN at the Bi sites by the rare earth element Sm in the concentration range corresponding to x = 0–1 in the formula of SrBi2−xSmxNb2O9. A solid-state double sintering method is adopted to synthesize and sinter the compounds starting from raw materials. It is shown that the crystal structure of SBN is retained but with slight changes to the lattice parameter. The sintered density, electric polarization and magnetization are enhanced by Sm addition. This system could be considered ferroelectromagnetic in nature. © 2005 Elsevier B.V. All rights reserved.

Chemical vapour deposited (CVD) low- k films using tri-methyl-silane (3MS) and tetra-methyl cyclo-tetra-siloxanes (TMCTS) precursors were studied. A 4-point bend test (4PBT) was performed to assess the adhesion property of the low- k films to Si substrates and the results were compared with that of simpler method, nanoscratch test (NST), as a quality control tool despite its drawbacks. Adhesion energy, G c , of the low- k /Si interface as measured by 4PBT and critical scratch load, P c , as obtained by NST display a linear relationship with hardness and modulus of the low- k film. The lowering of G c as the hardness of the film decreases can be explained by the effects of the C introduction into the Si O networks found in these films. Lower carbon content for higher hardness films is thought to cause them to be more “silica-like”, and thus, exhibit better adhesion with the Si substrate. Two failure modes were observed for specimens under 4PBT. On one hand, films with low hardness ( G c ( 2 ) with an adhesive separation of low- k from the Si substrate. On the other hand, films of high hardness (>5 GPa) display interfacial energies in excess of 10 J/m 2 with delamination of epoxy from the Si substrate, thus, indicating excellent adhesion between the low- k films and Si substrate. For the low hardness films, good correlation exists between P c and G c . However, the two data points of the high hardness films that gave the two highest P c and G c values do not lie on the correlation line drawn for the low hardness film data points due to different factors governing the failure in both tests and a change in the 4PBT failure mechanism.

Chemical vapor deposited (CVD) low-k films using tri methyl silane (3MS) precursors and tetra methyl cyclo tetra siloxanes (TMCTS) precursors were studied. Films were deposited by means of four processes, namely, O"2, O"2+He process and CO"2, CO"2+O"2 process for 3MS and TMCTS precursors, respectively. Interfacial adhesion energy (G"c), of low-k/Si samples, as measured by a 4-point bending test displayed a linear relationship with film hardness and modulus. Fractography studies indicated two possible failure modes with the primary interface of delamination being either at low-k/Si or Si/epoxy interface. In the former, once delamination initiated at the low-k/Si interface, secondary delamination at the Si/epoxy and epoxy/low-k interfaces was also observed. Films with low hardness ( 5 GPa) displayed interfacial energies in excess of 10 J/m^2 with separation of Si/epoxy and epoxy/low-k interfaces, thus indicating excellent adhesion between the Si and low-k films. Films with high hardness have less carbon in the system causing it to be more ''silicon dioxide'' like and exhibiting better adhesion with the Si substrate.

This paper reports the effects of ternary alloying additions Al, Cu and Nb to Fe 75 Si 25 in high energy ball milling to produce nano-crystalline alloy powder, and its microstructural stability during subsequent high temperature annealing. It is shown that all additions generally retard grain growth up to some temperature. Nb appears to amorphise the alloy. The binary base alloy and Al containing alloy forms the DO 3 ordered structure at high temperatures accompanied by rapid grain growth. The Cu and Nb containing alloys precipitate Cu and Nb 2 Fe at high temperatures but do not become ordered.

The grain growth and ordering behaviors in nanocrystalline Fe 75 Si 25 powders produced by mechanical alloying was studied. In the temperature range 573–703 K, the disordered α-Fe(Si) showed only limited growth with a small activation energy of 32.3 kJ mol −1 . However, in the range 773–853 K ordering to DO 3 structure occurred accompanied by a sharp increase in average grain size to a maximum value that depended on the temperature. A grain growth model available in the literature for nanocrystalline materials yielded a good fit to the experimental data. Activation energy of 278.9 kJ mol −1 determined using this model for grain growth is similar to that for lattice diffusion of Si in DO 3 Fe 3 Si. The reason for the concurrence of ordering and grain growth is attributed to the Si desegregation at grain boundaries upon ordering transformation. The levels of segregation are estimated from lattice parameter measurements.

There is currently little information regarding the physiochemical effects of common bonding wire dopants such as Ca and Pd despite the use of gold bonding wire as an electrical interconnection in over 80% of semiconductor devices. This paper presents experimental results on the residual microstrains in 4 N gold bonding wires with different concentrations of Ca–Pd, measured using peak broadening in X-ray diffraction. The effect of Ca was found to be more prominent. In order to understand more about the physiochemical action of Ca and its effects on the microstructure, TEM and depth sensing nano-indentation (DSI) were used to study 2 mm Ca-doped hard-drawn rods.

The 4 N (99.99% Au) gold ball bonding on Al alloy pad metallisation is widely used in microelectronic packaging assembly. The formation of intermetallics is an essential part of the wirebonding process that gives mechanical strength to the ball–pad interconnection. Bake testing in air, during which intermetallics thicken and grow, is commonly employed in the electronics packaging industry to test the mechanical integrity of ballbonds. While ballbonds are observed to be robust at extended aging times of up to 1000 h, brittle fractures occur within the intermetallic. The mechanisms of such failures are poorly understood but may be related to corrosion (oxidation), possibly induced by contamination. While oxidation studies of structural aluminides are common, few studies have been conducted on gold aluminides and the relevance of such reactions to ballbond reliability has not yet been established. A prerequisite to analysing suspected ballbond oxidation is to understand the oxidation behaviour of bulk gold aluminides. This paper examines the oxidation behaviour of two types of bulk gold aluminides, AuAl 2 and Au 4 Al, using thermogravimetry. Initial results on bulk materials show that appreciable amounts of oxidation can occur in these intermetallics.

Tensile testing was carried out at constant cross-head speed of 0.5 mm/min on specimens of two Cu blocks of length 45 mm with rectangular area of cross-section (10 mm×3 mm) soldered using Sn–Pb solder alloys with 200–300 μm thick. Four compositions of Sn–Pb alloys were used. The testing specimens were aged at 175 °C in an oven with a temperature measuring accuracy of ±1 °C for different times of up to 16 days to study the effect of isothermal ageing on the strength of the joints. The fracture surface was analysed both in the plan and cross-sectional views to identify the fracture location in the solder joint. It was identified that the morphology of the intermetallic compound (IMC) layer plays a role on the strength of the joint, especially when it fails through the solder/IMC interface. Also, the Pb-content in the solder alloy has a profound effect on the fracture path.

Nanocrystalline Fe90Si10, Fe75Si25, and Fe75Si15Al10 alloy powders were produced by high-energy planetary ball milling, starting from elemental powders. The resulting powders were annealed at various temperatures for 30 min and their grain sizes were determined by X-ray diffraction peak broadening using the Williamson–Hall approximation and confirmed by transmission electron microscope (TEM) observation. A minimum crystallite size of 13–17 nm could be achieved in Fe90Si10, Fe75Si25, and Fe75Si15Al10 alloys. The lattice parameters of as-milled Fe90Si10, Fe75Si25, and Fe75Si15Al10 attained minimum values of about 0.2860, 0.2842, and 0.2862 nm, respectively after 60 h, and thereafter remained constant for prolonged milling times. Annealing at 873 K increased the grain size of Fe90Si10 from approximately 17 nm to about 35 nm. Fe75Si25 exhibited some stability against grain growth at low temperatures but not above 673 K. Fe75Si15Al10 grains were more stable with no significant grain growth until 873 K. For Fe75Si25 and Fe75Si15Al10 alloys annealing at 773, 873, and 973 K resulted in the formation of ordered DO3 structure with long-range order (LRO) parameters ranging from 0.64 to 0.71. The internal strains of all three compositions decreased steadily with annealing from the range 0.63–0.32% to the range 0.23–0.07%.

The interfacial intermetallics between Cu and solder were studied for four Sn-Pb compositions at the annealing temperatures of 125°C, 150°C, and 175°C for up to 30 days. The η-phase (Cu6Sn5) layer formed during reflow continues to grow during annealing. An additional layer of ɛ-phase (Cu3Sn) forms at the η/Cu interface after an incubation annealing time. The thickness results fit a power-law relationship against time with average exponents 0.69 and 0.44 for the η phase and the ɛ phase, respectively. On prolonged annealing, the proportions of the individual phases in the total layer reach a steady state.