Qijie Zhai

Dealloying behavior of rapidly solidified Al–Ag alloys to prepare nanoporous Ag in inorganic and organic acidic media

The dealloying processes of Al-Ag alloy ribbons consisting of two distinct phases of α-Al (Ag) and Ag2Al in the 5 wt.% H2SO4, 10 wt.% H3PO4, 10 wt.% C2H2O4 and 5 wt.% HCl solutions were investigated. The as-dealloyed samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive X-ray (EDX) analysis coupled with SEM. It has been found that the resultant microstructure of the as-dealloyed samples is significantly influenced by the initial alloy composition and the acid kind. The four acid solutions are all able to leach out the Al from both the α-Al (Ag) and Ag2Al phases and the as-dealloyed samples exhibit the typical three dimensional (3D) bi-continuous nanoporous structure. The dealloying duration in four acid solutions ranges from 1.1 to 24 h, and the dealloying rates are influenced by acidity, acid concentration, and the diffusion coefficient (Ds) of Ag atoms which is affected by anions and interactions between anions and Al atoms. The specific surface area of the nanoporous silver (NPS) from the Al-15 at.% Ag alloy dealloyed in HCl was determined as 3.47 ± 0.08 m2 g−1 through N2 adsorption experiments and Brunauer–Emmett–Teller (BET) analysis. The Uv-vis absorption spectrum indicates that surface plasmon resonance (SPR) of silver nanostructure exists on the NPS samples, implying the potential application of NPS in surfaced enhanced Raman scattering (SERS)-active substrate, surface plasmon-based analytical devices, etc.

Size-dependent melting properties of Sn nanoparticles by chemical reduction synthesis

Tin nanoparticles with different size distribution were synthesized using chemical reduction method by applying NaBH4 as reduction agent. The Sn nanoparticles smaller than 100 nm were less agglomerated and no obviously oxidized. The melting properties of these synthesized nanoparticles were studied by differential scanning calorimetry. The melting temperatures of Sn nanoparticles in diameter of 81, 40, 36 and 34 nm were 226.1, 221.8, 221.1 and 219.5 ℃, respectively. The size-dependent melting temperature and size-dependent latent heat of fusion have been observed. The size-dependent melting properties of tin nanoparticles in this study were also comparatively analyzed by employing different size-dependent theoretical melting models and the differences between these models were discussed. The results show that the experimental data are in accordance with the LSM model and SPI model, and the LSM model gives the better understanding for the melting property of the Sn nanoparticles.

Effect of electrodes and thermal insulators on grain refinement by electric current pulse

Abstract The application of electric current pulse (ECP) to a solidification process refers to the immersion of electrodes into the liquid metal and the employment of thermal insulators on the upper surface of metal. In order to ascertain the effects of these two factors on the structure refinement by the ECP technique, three groups of experiments were performed with different types of electrodes or various thermal insulators. By the comparison between solidification structures under different conditions, it is followed that the electrode and the thermal insulator have an obvious influence on the grain refinement under an applied ECP, and further analysis demonstrates that the thermal conditions of the liquid surface play a vital role in the modification of solidification structure. Also, the results support the viewpoint that most of the equiaxed grains originate from the liquid surface subjected to an ECP.

Effect of special combined electromagnetic stirring mode on macrosegregation of high strength spring steel blooms

Abstract The effect of various electromagnetic stirring (EMS) modes on reducing macrosegregation has been investigated by conducting plant trials on a continuous bloom caster. A new combined mould and final EMS mode combination was investigated using 60Si2CrVAT high strength spring steel. The centre carbon segregation was significantly reduced.

Recent Development of Nano-solder Paste for Electronics Interconnect Applications

Conventional lead-free solders, with a solder alloy particle size in the micrometer range, present some major disadvantages, such as relatively high melting temperatures, which can result in defects and build up stresses during reflow processing, and limited application for high density, ultra- small pitch electronic applications. By decreasing the size of the solder alloy particles to the nanometer range, one can both decrease the melting temperature of the solder alloy and use such solders in very fine pitch applications. Besides lower melting temperature, particles in the nanometer size range present many other extraordinary properties, such as, large surface area per unit volume, large surface energy, supermagnetism, extraordinary optical properties, self- purification properties and quantum size effects. It is all these extraordinary properties that have attracted the attention of both scientific and technological communities all over the world. The main focus of this paper is the recent development of both composite solders and pure nano-solder pastes and their application as electronic interconnect materials. The paper starts by giving and introduction to the subject of nanoparticles, including definitions, advantages and general applications. This is followed by a section dealing with the main manufacturing processes presently being used to manufacture solder alloy nanoparticles. The two main sections of this article deal with composite solders and pure nanosolder pastes. The first part, regarding composite solders, deals with the issues related to adding nanoreinforcements into conventional micrometer-sized solders and the effect of such reinforcements on both the mechanical and physical properties of solder alloys. The second part deals with pure nano-solder pastes and their application in electronic interconnect applications.

Formation of amorphous structure in Sn3.5Ag droplet by in situ fast scanning calorimetry controllable quenching

Attributing to sensitive fast scanning calorimetry, combined with focused ion beam and high resolution transmission electron microscopy, we observed the solidification structure of single Sn3.5Ag droplet quenched at controllable rate. Amorphous layers in nanometer adjacent to some Ag3Sn crystals were directly detected. Based on solid state amorphization, a nano diffusion couple between primary formed β-Sn matrix and Ag3Sn intermetallic was put forward. Quenched at 15 000 K/s, the concentration gradient in this diffusion area was up to 109 m−1, which could seriously suppress the growth and further homogeneous nucleation of Ag3Sn, leading to the formation of amorphous structure.

Influence of magnetic field on dealloying of Al-25Ag alloy and formation of nanoporous Ag

The dealloying behavior of bi-phase Al–Ag alloys in 5 wt.% HCl in non-treated and magnetic treated conditions was comparatively investigated. The as-dealloyed samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive X-ray (EDX) analysis coupled with SEM. The results reveal that an external magnetic field would accelerate the dealloying process and the as-dealloyed samples from magnetic treatment are characterized by a finer and more homogeneous three dimensional (3D) bi-continuous nanoporous structure compared with those from non-treatment dealloying. The smaller diffusion coefficient (Ds) of silver and the larger diffusion activation energy (Eα) under a magnetic field can be responsible for the finer nanoporous structure. It is deemed that the formation of diamagnetic reaction products (during the etching of Al) and the crystal nucleation of silver (during the dealloying of Ag2Al) are accelerated by the external magnetic field. In addition, not only the pre-exponential factor (D0), obtained from the Arrhenius equation for diffusion of Ag, but also the activation energy (Eα) were influenced by the magnetic field, implying that different diffusion mechanisms and also the complicated underlying dealloying mechanism were involved.

Structure of slowly solidified 30Cr2Ni4MoV casting with surface pulsed magneto-oscillation

Abstract Large scale 30Cr2Ni4MoV ingots play a crucial role in nuclear power plants. Shrinkage and carbon segregation are the most common defects in the manufacture of these large scale ingots. Large scale ingots have very low cooling rates. In the present work, an experimental method was employed to achieve similarly low cooling rates with the aim of simulating the solidification process of large ingots in smaller 30Cr2Ni4MoV ingots.Thus, we examined the effect of surface pulsed magneto-oscillation (SPMO)on solidification structure in a laboratory setting. Our experimental results showed an SPMO treated ingot with less carbon segregation and a smaller shrinkage cavity than in an untreated one. Finally, the action mechanism was analysed by numerical simulation.

Characterization of Nanoparticles of Lead Free Solder Alloys

Nanoparticles of Sn-4.0Ag-0.5Cu and Sn-0.4Co-0.7Cu (wt% composition) lead free solder alloys were manufactured and characterized for potential applications in microelectronics packaging. Scanning electron microscopy (SEM) analysis was carried out in order to study the morphology and size distribution of the nanoparticles. It was observed that nanoparticles of lead free alloys were almost spherical in shape. The observed size distribution of both lead free solder alloy nanoparticles is between 20 ~ 80 nm. High resolution transmission electron microscopy (HRTEM) was used to analyse the detailed nanostructure and oxide layer present on the nanoparticles surface. The observed oxide layer thickness is about 25Aring (2.5 nm). The depression in melting point due to nanosize effect was calculated theoretically as a function of particle size for the above mentioned lead free solder alloys and the theoretical results were compared with differential scanning calorimeter (DSC). Onset calculation was used to determine the depression in melting point due to nanosize effect in respect to bulk material. The melting point difference obtained by DSC for Sn-4.0Ag-0.5Cu and Sn-0.4Co-0.7Cu lead free solders are between 1.1 to 7.8degC and 0.24 to 2.4degC respectively depending on the definition of the melting point determination by DSC

Columnar to equiaxed transition during solidification of small ingot by using electric current pulse

A new approach to applying the electric current pulse (ECP) with parallel electrodes to the promotion of the transition from columnar crystal to equiaxed crystal and the improvement of macrosegregation was introduced. The ECP was applied to different stages of the solidification. The results showed that the application of the ECP in both the initial stage (the thickness of solidified shell reached 2 mm approximately) and the late stage (the thickness of solidified shell reached 14 mm approximately) of solidification can promote the columnar to equiaxed transition (CET). The analysis showed that during solidification, a large number of nuclei around the upper surface fell off due to ECP, which subsequently showered on the melt and impinged the growth front of the columnar crystal. Therefore, the CET occurred. In addition, this method was also employed to influence the solidification process of bearing steel, and the results showed that the structure was changed from columnar crystal to equiaxed crystal, indicating that ECP can enhance the homogeneity of structure and composition of bearing steel.