Guisheng Zou

Joining of Silver Nanomaterials at Low Temperatures: Processes, Properties, and Applications.

A review is provided, which first considers low-temperature diffusion bonding with silver nanomaterials as filler materials via thermal sintering for microelectronic applications, and then other recent innovations in low-temperature joining are discussed. The theoretical background and transition of applications from micro to nanoparticle (NP) pastes based on joining using silver filler materials and nanojoining mechanisms are elucidated. The mechanical and electrical properties of sintered silver nanomaterial joints at low temperatures are discussed in terms of the key influencing factors, such as porosity and coverage of substrates, parameters for the sintering processes, and the size and shape of nanomaterials. Further, the use of sintered silver nanomaterials for printable electronics and as robust surface-enhanced Raman spectroscopy substrates by exploiting their optical properties is also considered. Other low-temperature nanojoining strategies such as optical welding of silver nanowires (NWs) through a plasmonic heating effect by visible light irradiation, ultrafast laser nanojoining, and ion-activated joining of silver NPs using ionic solvents are also summarized. In addition, pressure-driven joining of silver NWs with large plastic deformation and self-joining of gold or silver NWs via oriented attachment of clean and activated surfaces are summarized. Finally, at the end of this review, the future outlook for joining applications with silver nanomaterials is explored.

Highly localized heat generation by femtosecond laser induced plasmon excitation in Ag nanowires

Photo-excitation of plasmons in nano-systems initially results in highly localized heating, but the final temperature distribution in irradiated nanostructures is almost uniform because heat diffusion equilibrates the overall temperature within ∼10−12 s. Here, we show that irradiation with femtosecond pulses enables visualization of the location of plasmonic heating because thermal effects such as plasmon-induced melting are frozen in at the initial location of energy deposition. Simulations show plasmonic heating is related to the orientation of the laser polarization and to the geometry of partially melted nanowires. This may provide a useful tool in joining, cutting, and reshaping nano-objects.

Microstructures and mechanical properties of Ti–24Al–17Nb (at.%) laser beam welding joints

Abstract Laser beam welding of Ti–24Al–17Nb (at.%) alloy was conducted to investigate the microstructures and the mechanical properties of its joints. The results indicated that the weld microstructure consisted primarily of retained ordered β phase (namely B2 phase) and was independent of the laser welding parameters, while the size and the orientation of the weld solidification structures and then the bend ductility of the joints were related to the welding conditions. The microstructures became coarser and the strains of inducing crack and fracturing decreased as the heat input increased. The fracture occurred in the base metal when the transverse tensile test of the joints was conducted. The tensile strength of the joints was equal to that of the base material and the tensile ductility could reach 12∼17%, which was near to that of the base material.

Low Temperature Bonding of Cu Metal through Sintering of Ag Nanoparticles for High Temperature Electronic Application

Lead-based solders bring pollution to the environment and result in health threat to humans. The preparation and application of metallic nanoparticles provide a potential method to develop Pb-free bonding materials. In this article, bonding of Ag-coated Cu bulks was realized through low temperature sintering by directly using the chemically-reduced Ag nanoparticle paste and baked nanoparticle powders at 60 o C, respectively. The results indicate that the capillary flow of paste caused a ring-like deposit on substrate coating, while this phenomenon disappeared when using the powders. Increasing bonding temperature facilitated the sintering, and shear strengths of 20 MPa and 84 MPa was obtained at bonding temperature of 250 o C for 30 min under 20 MPa when using the Ag paste and powders, respectively. High joint strength value of bonded region using paste is due to the high effective bonding pressure and small effective bonded area. Finally, for the extensive application in packaging industry especially for high temperature electronics, challenges such as the improvement of screen and stencil printing ability of paste, avoiding bonding pressure and lowering the cost were pointed out based on our and other researchers achievements. Cu nanoparticle or Cu-containing nanoparticle mixture pastes are promising but the problems of oxidation and bonding stability must be resolved urgently.

Polymer-Protected Cu-Ag Mixed NPs for Low-Temperature Bonding Application

A simple method has been proposed to prepare polymer-protected Cu-Ag mixed nanoparticles (NPs), which are suitable for use as low-temperature bonding materials. The polymer coated on the Cu-Ag mixed NPs can protect them from oxidation effectively when heated in air at temperature lower than 280°C. The low-temperature bonding process utilizing Cu-Ag mixed NPs as the bonding material is investigated. The bonding experiments show that robust joints are formed using Cu-Ag mixed NPs at 160°C in air. The shear test shows that addition of copper to silver is helpful for improving joint strength. This novel sintering-bonding technology using Cu-Ag mixed NPs as an interconnection material has potential for application in the electronics packaging industry.

Plasmonic engineering of metal-oxide nanowire heterojunctions in integrated nanowire rectification units

We show that irradiation with femtosecond laser pulses can produce robust nanowire heterojunctions in coupled non-wetting metal-oxide Ag-TiO2 structures. Simulations indicate that joining arises from the effect of strong plasmonic localization in the region of the junction. Strong electric field effects occur in both Ag and TiO2 resulting in the modification of both surfaces and an increase in wettability of TiO2, facilitating the interconnection of Ag and TiO2 nanowires. Irradiation leads to the creation of a thin layer of highly defected TiO2 in the contact region between the Ag and TiO2 nanowires. The presence of this layer allows the formation of a heterojunction and offers the possibility of engineering the electronic characteristics of interfacial structures. Rectifying junctions with single and bipolar properties have been generated in Ag-TiO2 nanowire circuits incorporating asymmetrical and symmetrical interfacial structures, respectively. This fabrication technique should be applicable for the in...

Effect of PVP on the low temperature bonding process using polyol prepared Ag nanoparticle paste for electronic packaging application

There has been an increasing interest in developing low temperature interconnection process using metal nanoparticles. In this study the Ag nanoparticles (NPs) for this low temperature bonding process applications were prepared based on the polyol method using polyvinylpyrrolidone (PVP) as the protecting agent. The effect of PVP on the Ag nanoparticle size, nanoparticle solution viscosity and the bondability of the Ag nanoparticle paste were studied. The silver nanoparticle with diameter of 90 nm, 40 nm and 20 nm were synthesized by adjusting PVP concentration. The bonding processes using different kind of Ag nanoparticle paste were conducted. The joint with shear strength of 50 MPa were formed using Ag nanoparticle (NP) paste prepared with appropriate PVP concentration.

effect of post-weld heat treatment on microstructure and properties of ti-23al-17nb alloy laser beam welding joints

Abstract The mechanical properties of Ti-23Al-17Nb (mole fraction, %) laser beam welding alloy joint at room temperature are comparable to that of the base materials. However, the strength and ductility of the as-welded joint deteriorate seriously after high temperature circulation. The effect of post-welded heat treatment on the microstructure and mechanical properties of the joint was investigated. The heat treatment was taken at 980 °C for 1.5 h, then furnace cooling and air cooling were performed separately. The results indicate that proper post-welded heat treatment improves the ductility of the joint at high temperature.

In situ nanojoining of Y- and T-shaped silver nanowires structures using femtosecond laser radiation.

We report the in situ joining of spatially separated silver nanowires without additional filler material by controlled irradiation with femtosecond laser pulses. Nanojoining under these conditions arises from highly localized heat generation in the vicinity of the gap between adjacent silver nanowires. Melting, followed by the flow of silver into the gap, is optimized by adjusting the direction of laser polarization relative to gap geometry. Our results show that melting of silver occurs on both nanowires in the vicinity of the gap between the two components. Successful formation of a joint is found to be a function of the angle between the long axis of the nanowires and the gap distance. Finite element simulations show that the strong localized electric field generated by optical excitation determines the location and the morphology of the resulting bond. Light coupling and the resulting emission properties of these Y-shaped nanowire structures have been simulated and are compared to similar structures where the gap remains open. It is suggested that joined Y-shaped couplers will have a higher switching ratio between emitted nanowire ends than those occurring in open-gap structures. Nanojoining induced by localized heating under strong field excitation may enable the production of robust branched metal nanowire structures for optical applications.

Femtosecond laser-induced phase transformations in amorphous Cu77Ni6Sn10P7 alloy

In this study, the femtosecond laser-induced crystallization of CuNiSnP amorphous ribbons was investigated by utilizing an amplified Ti:sapphire laser system. X-ray diffraction and scanning electronic microscope were applied to examine the phase and morphology changes of the amorphous ribbons. Micromachining without crystallization, surface patterning, and selective crystallization were successfully achieved by changing laser parameters. Obvious crystallization occurred under the condition that the laser fluence was smaller than the ablation threshold, indicating that the structural evolution of the material depends strongly on the laser parameters. Back cooling method was used to inhibit heat accumulation; a reversible transformation between the disordered amorphous and crystalline phases can be achieved by using this method.