Ag Grains Research Articles

Structural Evolution and Phase Stability of Hume-Rothery Phase in a Mechanically Driven Nanostructured Ag-15 at. pct Sn Alloy

The paper reports phase evolution in mechanically driven Ag-15 at. pct Sn alloy powder starting with elemental powders in order to establish the feasibility of designing nanocomposites of a Ag-Sn solid solution. This alloy lies in the phase field of the hexagonal ζ-phase which is a well-known Hume-Rothery electron compound with an electron-to-atom ratio of about 1.45 and hexagonal crystal structure (a = 0.2966 nm, c = 0.4782 nm). Through a systematic use of X-ray diffraction and transmission electron microscopy, the results establish the formation of the ζ-phase which co-exists with the Ag solid solution during the initial phase of milling. Mechanical milling for long duration (55 hours) destabilizes the ζ-phase. A complete solid solution of Ag with a grain size of ~8 nm could be achieved after 60 hours of milling. Additional milling can induce decomposition of the solid solution that results in a reappearance of ζ-phase. We present a detailed thermodynamic calculation which indicates that complete Ag solid solution of the present alloy composition would be possible if the crystallites size can be reduced below a certain critical size. In particular, we show that both Ag and ζ-phase grain sizes need to be taken into account for determining the metastable equilibrium and the phase change that has been experimentally observed. Finally, we argue that recrystallization processes set a limit to the achievable size of the nanoparticles with metastable Ag solid solution.

Structural and Morphological Changes in Ag:TiN Nanocomposite Films Promoted by In-Vacuum Annealing

Thin films composed of titanium nitride doped with silver were deposited by DC reactive sputtering, with Ag contents varying between 0 and 50 at.%. The as-deposited samples were subjected to vacuum annealing treatments, with a range of temperatures varying from 200 to 500 °C, in order to study the morphological and structural changes that may occur. The as-deposited samples showed three main zones of basic characteristics, which differ both in terms of morphology and structural features. By increasing the annealing temperature, the thermodynamic stability is accelerated, giving rise to (i) a uniform dispersion of silver particles at 200 °C; (ii) the start of segregation at 300 °C; (iii) at 400 °C the coalescence of the segregated Ag particles takes place and finally (iv) at 500 °C the formation of large Ag clusters is evident, particularly within the zone that comprises the samples with higher Ag content. In addition to corroborate the presence of free Ag in Ag:TiN thin films, the increasing annealing temperature promotes the improvement of the coatings crystallinity, as well as Ag grain growth.

Influence of Ag content on mechanical and tribological behavior of DLC coatings

Ag–DLC coatings with Ag contents ranging from 1.3at.% to 13.1at.% were deposited by DC magnetron sputtering. The coatings were characterized with respect to their structure (by means of XRD and Raman spectroscopy), mechanical and tribological properties (by scratch test, nanoindentation, residual stress measurements and pin-on-disk test). The incorporation of 13.1at.% Ag resulted in the formation of Ag grains of 2–3nm which promoted the increase of graphite like bonds organized in rings. Regarding the mechanical properties, no variations were found for films with Ag contents lower than 13at.%; a reduction of both hardness and compressive residual stress was then observed for higher values. Pin-on-disk tests were performed at two different contact pressures (690MPa and 1180MPa) in dry sliding conditions against a zirconia counterpart. For the lower contact pressure the variations in the wear rate are well correlated with the coating structure and mechanical properties, while for the higher contact pressure the presence of Ag is relevant and Ag–DLC coatings are showing higher wear rate than DLC one. SEM analysis revealed the formation of Ag aggregates on the wear track and adhesion of silver to the counterpart.

Effect of wettability on surface morphologies and optical properties of Ag thin films grown on glass and polymer substrates by thermal evaporation

A series of Ag films with different thicknesses were deposited on BK-7 glass, PET and PC substrates under identical conditions by thermal evaporation. The effect of the wettability on the morphology and optical properties of Ag/glass and Ag/polymer films was studied by atomic force microscopy and spectrophotometry. The experimental results show that the wettability of Ag grains with polymer is stronger than with glass, which results in the aggregation of bigger grains in initial layer. During deposition the interaction of interlayer plays an important role for the formation of the surface morphology. The strong wettability activates the nonlinear optical properties of Ag grains grown on polymer substrates, which result in the strong absorbance in short wavelength. The effect of the bare substrate on the transmittance of Ag films is more obvious than the reflectance. With the increasing of the thickness, the effect of the wettability on the morphology and optical properties of Ag films decline. In this experiment when the thickness is above 50nm, the effect almost vanished.

CVD of TiO2 and TiO2/Ag antimicrobial layers: Deposition from the hexanuclear μ-oxo Ti(IV) complex as a precursor, and the characterization

Hexanuclear μ-oxo titanium(IV) complex, of the general formula [Ti6O6(H2O)2(OiBu)6(O2CtBu)6], has been used as a new type of TiO2 chemical vapor deposition precursor. Thin titanium dioxide layers of a different polymorphic structure were grown on the surface of titanium foil or titanium surgical implants, as substrates, in a wide range of substrate temperatures 693–853K, under Ar atmosphere. The best antimicrobial activity revealed TiO2 covers deposited between 713 and 733K. Loose packing layers produced at 733K were doped by silver grains by CVD method using silver(I) 3,3-dimethyl tert-butyrate complex with triethylphosphane as a precursor. The increase of the antimicrobial activity of TiO2–Ag system with the increase of Ag grains content was noticed. Crystallinity, composition, and morphology of TiO2 and TiO2/Ag layers were analyzed by X-ray diffraction method, high resolution transmission electron microscopy, and scanning electron microscopy.

Microstructure and Properties of Ag-1.33%Mg-0.54%Ni Alloy after Severe Plastic Deformation and Internal Oxidation

Abstract Ag-1.33Mg-0.54Ni (at%) alloy with high strength and high electrical conductivity, was subjected to accumulative rolling with 98% thickness reduction and internal oxidation, and studied by scan electron microscope (SEM) and X-ray diffraction (XRD). Conventional mechanical properties before and after internal oxidation were evaluated. Results show that the hardness, tensile strength, tensile elongation and electrical resistivity of the alloy after internal oxidation were 1350 MPa, 450 MPa, 6% and 1.73 μΩ·cm, respectively. Further, texture studies suggest that the alloy during internal oxidation had the {110} Goss and {110} Brass textures. A few of MgO and NiO precipitates in a silver matrix hindered the growth of Ag grains and kept the orientation of Ag grains quite stable and enhanced the mechanical properties of the alloy.

Fabrication of superoleophobic surfaces on Al substrates

An easy method of fabricating superoleophobic surfaces on Al substrates by constructing reentrant structures is reported. The reentrant micro/nanometer-scale structures comprise micrometer-scale, rectangular-shaped, and step-like Al structures obtained by electrochemical etching and nanometer-scale Ag grains resulting from immersion in [Ag(NH3)2]+ solution. Surface energy is reduced by perfluorooctanoic acid (PFOA) containing –CF3 and –CF2– groups. The PFOA-modified micro/nanometer-scale rough structures enable the formation of a composite solid–liquid–air interface with peanut oil. These structures show good superoleophobicity with a peanut oil contact angle of 160.0 ± 2° and a sliding angle of 8°. Nanometer-scale structures can effectively transform the micrometer-scale non-reentrant structures into reentrant structures. With the aid of suitable low surface energy materials such as PFOA, fabricating superoleophobic surfaces on Al substrates can be easier.

Evaluation of Pressure Free Nanoparticle Sintered Silver Die Attach on Silver and Gold Surfaces

A commercially available silver nanoparticle based die attach material was used in a pressure free process to bond 2.5 mm square Ag plated Si die to Ag and Au plated substrates. The two substrate types were 5mm square Ni/Ag plated silicon substrate and a W/Ni/Au metallised cofired alumina package. The assemblies were stored at 300 °C for up to 500 h and the morphology of the sintered Ag and the shear strength was monitored as a function of time. Bondline thickness measurements were carried out after following the paste manufacturer's drying and sintering temperature profile. On Ag substrate it was found that die shear strength increased with storage time. The fracture surfaces of the sheared die and substrate as well as cross sections of untested die were examined using electron microscopy. It was found that the Ag grains grew in size and porosity decreased over time. There was also a clear difference in morphology between sintered Ag at the die edge and centre. During shearing the Ni layer was found to separate from the chip at the edges of the die after ageing. On Au substrate, it was found that die shear strength decreased with storage time. It was found that the Au diffused into the Ag, creating a low porosity Au-Ag layer. Ag also migrated towards the Au surface, leaving behind a layer of voids which contributed to weakening of the joint. Rapid Au diffusion was associated with the high density of grain boundaries arising from the sintering process.

Nanoporous Ag–CeO2 ribbons prepared by chemical dealloying and their electrocatalytic properties

A novel and facile strategy for the preparation of a monolithic Ag–CeO2 composite is reported. Here, nanoporous Ag–CeO2 ribbons are successfully prepared through dealloying melt-spun Al79.5Ag20Ce0.5 alloy in a 5 wt% NaOH aqueous solution, followed by calcining in air. The Ag–CeO2 ribbons with a homogeneous pore/grain structure are thermally stable up to 973 K because of the formation of Ag cores and CeO2 shells. Uniform CeO2 particles with lengths of about 5 nm are dispersedly loaded on the fine Ag grains of the porous structure. The Raman spectra show that a larger number of oxygen vacancies are formed in the CeO2 nanoparticles with an increase of the calcination temperature. The electrochemical tests reveal that the nanoporous Ag–CeO2 catalyst exhibits an enhanced activity toward the direct oxidation of sodium borohydride. The superior catalytic activity is attributed to the enhancement of the interfacial interaction between Ag and CeO2. The present work opens up a novel method to prepare metal–CeO2 nanocomposites with high catalytic performance and low cost.

Enhanced electrical performance of Ag–Cu thin films after low temperature microwave processing

In the present study, thin films of binary Ag–Cu alloys with different Cu content were prepared by a cosputtering technique and then annealed by microwave heating. The metallographic and electrical properties of the thin films were observed experimentally. It was found that the electrical performance of Ag–Cu thin films enhanced after microwave processing when compared to vacuum annealing. Based on the fact that Cu has a low solubility in Ag, it was chosen as the alloying element. The low solubility favored segregation of Cu at the surface and grain boundaries. This prevented Ag grain boundary diffusion and agglomeration. The as-deposited thin films were more resistive when compared to the microwave processed films. Comparison of the two postdeposition annealing techniques, microwave and vacuum, showed that microwave annealing is a clean, faster, and efficient process that can improve the electrical performance of Ag–Cu thin films. Results from this investigation demonstrated that microwave annealing is a suitable replacement of more expensive manufacturing techniques currently used in the field of interconnects technology.

Oxygen, boron, chromium and niobium enrichment in native Au and Ag grains: A case study from the Linglong gold deposit, Jiaodong, eastern China

Quartz vein-type gold mineralization in the Linglong deposit constitutes one of the important sources of gold in China. Here we report the first finding of anomalous enrichment of boron (B), oxygen (O), chromium (Cr) and niobium (Nb) in native gold and Ag–Te grains from the Linglong deposit. The oxygen content of gold and silver grains ranges up to 75.83at.%, boron up to 97.85at.%, chromium up to 26.23at.%, and tellurium up to 33.4at.%. The Au precipitation is mainly linked to pyrite precipitation under reducing conditions. The HFSE enrichment including chromium (Cr) and niobium (Nb) in the gold and silver grains indicate that the ore-forming fluids were derived from high temperature magmas sourced from depth. The enrichment of low temperature Te in the native gold and silver grains, combined with the typical features of fluid ‘boiling’ as inferred from fluid inclusion studies indicate that the ore-fluids derived from depth upwelled rapidly, and metal precipitation occurred through decompression at shallower levels. Our results suggest good prospect for gold and silver at depth in Linglong.

Phase constitution and interface structure of nano-sized Ag-Cu/AlN multilayers: Experiment and ab initio modeling

Nano-sized Ag-Cu8nm/AlN10nm multilayers were deposited by reactive DC sputtering on α-Al2O3(0001) substrates. Investigation of the phase constitution and interface structure of the multilayers evidences a phase separation of the alloy sublayers into nanosized grains of Ag and Cu. The interfaces between the Ag grains and the quasi-single-crystalline AlN sublayers are semi-coherent, whereas the corresponding Cu/AlN interfaces are incoherent. The orientation relationship between Ag and AlN is constant throughout the entire multilayer stack. These observations are consistent with atomistic models of the interfaces as obtained by ab initio calculations.

CrN–Ag nanocomposite coatings: Control of lubricant transport by diffusion barriers

1-μm-thick self-lubricating CrN–Ag composite coatings containing 16at.% Ag were deposited on Si substrates by reactive co-sputtering at Ts=400°C, and were covered with CrN cap layers with a columnar microstructure and a thickness d=0–1000nm. Vacuum annealing at Ta=500 and 600°C for 1h causes Ag transport to the sample surface and the formation of Ag surface grains. Quantitative scanning electron microscopy and energy dispersive spectroscopy analyses show that increasing d from 0 to 10 to 100nm for Ta=500°C leads to a decrease in the areal density of Ag surface grains from 0.86 to 0.45 to 0.04μm−2, while their lateral size remains constant at 360±60nm. However, increasing Ta to 600°C causes a doubling of the Ag grain size, and a 4–30 times larger overall Ag transport. These results are explained by kinetic barriers for Ag diffusion through the porous cap layer with a porosity that decreases with increasing d, resulting in an effective activation barrier for Ag transport that increases from 0.78eV in the absence of a cap layer to 0.89eV for d=10nm and 1.07eV for d=30nm. Auger electron spectroscopy depth profile analyses of annealed layers reveal no detectable Ag within the CrN cap layer and a uniform depletion of the Ag reservoir throughout the composite coating thickness, indicating unhindered Ag transport within the composite. The overall results show that a CrN diffusion barrier cap layer is an effective approach to control Ag lubricant transport to the surface of CrN–Ag composite coatings.

A study of chemical reactions of silver and indium at 180 °C

The silver-indium system has become important in electronic packaging applications. The In-3.5 % Ag eutectic alloy has been used for laser diode attachment since 1980. The Ag-rich alloys were less studied but have received significant attention recently for producing high temperature joints at low bonding temperature. In multilayer Ag-rich bonding structure design, typical bonding temperature is 180 °C and the process can be made entirely fluxless. The melting temperature of joints fabricated is higher than 695 °C. Despite success in several bonding processes, the chemical reactions of Ag and In at 180 °C were little known. In this study, we performed systematic experiments to investigate these reactions. In experimental design, copper (Cu) substrates were electroplated with 40 μm Ag layer, followed by indium layers of 1, 3, 5, 10, and 15 μm, respectively. Thick Ag layer was chosen to prevent In from reacting with the Cu substrate. The samples were annealed at 180 °C in 0.1 torr vacuum for 5 min to emulate the bonding conditions. The microstructure and composition on the surface of the samples were evaluated using scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDX) as well as X-ray diffraction. They were then cut in cross section, polished, and examined by SEM/EDX. The results show that, for samples with 1 μm thick In, the In layer converts to Ag2In entirely after annealing. For samples with In thickness of 3 and 5 μm, AgIn2, Ag2In, and solid solution (Ag) all form after annealing. No indium was identified. For samples having 10 and 15 μm thick In, In covers almost over the entire sample surface after annealing. The effect of annealing Ag layer was investigated. After Ag plating, samples were annealed at 450 °C for 3 h to grow Ag grains. This was followed by plating 10 μm In and annealing at 180 °C. The result shows that, by annealing Ag before plating In, more In is kept in the structure during annealing at 180 °C. In theory, this effect is caused by larger Ag grains and thus fewer grain boundaries resulting from annealing the Ag layer. Other than scientific values, the results of this investigation are useful in designing better bonding structures.

Three-dimensional macroporous Sn–Ag thin film anode prepared by electro-less reduction method: effect of micro-structure

A three-dimensional Sn–Ag thin film with open interconnected walls consisting of active small grains of Sn and Ag was fabricated by electro-less reduction method as anode for lithium-ion batteries application. The morphologies and electrochemical performance of the macroporous Sn–Ag thin films were investigated by using scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction, cyclic voltammetry, and galvanostatical charge–discharge measurement. The results demonstrated that through controlling the plating time, the electrode with optimized microstructure exhibited the largest reversible capacities, maintaining a capacity of 583 mAh g−1 after 100 cycles, due to the well-preserved micro-porous structure after extended cycling.

Effects of A-elements (A[dbnd]Si, Ge or Sn) on the structure and electrical contact properties of Ti–A–C–Ag nanocomposites

Ti–A–C–Ag (A is Si, Ge or Sn) nanocomposite coatings have been deposited by dc magnetron sputtering in an ultra high vacuum chamber. Electron microscopy, energy-dispersive x-ray spectroscopy, x-ray photoelectron spectroscopy, and x-ray diffraction show that all coatings contain nanocrystalline TiC and Ag grains in a matrix of mainly amorphous C. A C/Ti ratio above unity yields a homogenous distribution of Ag with a reduced grain size. From a chemical point of view, the addition of Ge and Sn to the Ti–C–Ag system should increase the conductivity of the coatings since the formation of more metallic phases than Si. We demonstrate that Si can be replaced with Ge and Sn and still yield a homogeneous distribution of Ag. The incorporation of Ge and Sn to the Ti–C–Ag system results in elemental precipitation and intermetallic phases, respectively. This gives improved electrical properties compared to Ti–Si–C–Ag coatings, and a contact resistance at loads of ~1N against an Au probe (radius of 0.7mm) that is comparable to that of Ag.

Improvement of the Hydrogen Microprint Technique on AHSS Steels

Preferential hydrogen diffusion paths in low-carbon ferritic steel and transformation-induced plasticity (TRIP) steel characteristic of new advanced high strength sheet steels (AHSS) were evaluated via the hydrogen microprint technique (HMT). As a consequence of hydrogen diffusion and evolution from the sample after charging, the technique involves reduction of AgBr emulsion applied to the surface of a charged sample to produce discrete Ag grains which identify, via scanning electron microscope (SEM) imaging, locations where hydrogen evolved. Enhancements to the results are revealed through a newly developed image-before-etch HMT method, consisting of SEM imaging before and then again after etching. The modified technique was implemented to facilitate direct comparison of silver grains with the features of an etched multi-phase TRIP steel microstructure. Results from SEM imaging demonstrated the effectiveness of the image-before-etch method to avoid artifacts and to positively identify hydrogen evolution sites in AHSS TRIP steel microstructures.

Research on Natural Aging Behavior of High Purity Ag Sheets After Severe Plastic Deformation

High purity silver (Ag) powder was melted in vacuum at ~10−3 Pa and in an atmosphere with charcoal dust as a protective agent. Both samples were subjected to accumulative rolling with 95% thickness reduction and naturally aged for 1 year. The mechanical properties and microstructural studies of as melted and processed samples were evaluated. Results showed that after natural ageing for a year the sample melted in vacuum was responsible for total recrystallization and the sample melted in atmospheric conditions showed slight recovery. It is suggested that grain growth and rotation of Ag is hampered by some oxygen interstitial solute and a few of Ag2O particles which can keep the orientation of Ag grains stable and obviously resist natural aging and softening.

Fractal pattern formation in thermal grooving at grain boundaries in Ag films on Si(111) surfaces

Growth of Ag films on Br- and H-passivated Si(111) surfaces and the annealing behaviour have been investigated by Rutherford backscattering spectrometry, scanning electron microscopy and photoemission electron microscopy techniques. Upon annealing the phenomenon of thermal grooving was observed in the Ag films. Depending on the annealing temperature, at an intermediate annealing time Ag depletion (evaporation) from the grain boundaries produces fractal patterns of Ag-depleted regions. Continued annealing eventually produces a percolated network of Ag-depleted regions (thermal grooves) along the grain boundaries and isolated Ag grains appear as the depth of the grooves reaches the substrate. For the fractal structures produced by thermal grooving, the fractal dimension has been estimated to be 1.60±0.04. Observation of a fractal pattern in thermal grooving was not hitherto reported. A thorough analysis of the experimental results has been carried out in the context of current theories. These theories are inadequate to describe the experimental results.

Interplay of structural and temperature effects on plasmonic excitations at noble-metal interfaces

The interplay of structural and electronic properties on plasmon modes was investigated for a thin Ag film grown at room temperature on Cu(111). Surface plasmons are confined within Ag grains, as indicated by the analysis of their dispersion relationship, which is dispersionless up to a critical wave-vector. Surface plasmon confinement is removed upon annealing at 400 K. The thermal treatment induces a flattening of the Ag adlayer with a merging of Ag islands, and, moreover, a strong enhancement of surface conductivity.