CoSn3 Phase Research Articles

Highly Reversible Conversion-Type CoSn2 Cathode for Fluoride-Ion Batteries.

An all-solid-state fluoride-ion battery (FIB) is one of the promising candidates for the next-generation battery owing to its high energy density and high safety. For the practical application of FIBs, it is an urgent task to operate FIBs at lower temperatures. However, there are still two major difficulties in conventional conversion-type pure metal cathodes: low F- ion conductivities and poor cycle stabilities. Here, the conversion-type Sn-based intermetallic alloy is proposed as a new cathode that can overcome the above issues. The present CoSn2 cathode retains the discharge capacity of 229 mAh g-1 after 250 cycles, even at 60°C. CoSn2 is decomposed into CoF2 and SnF2 nanocrystals in the charging process, and the nanoscale network structure of SnF2 provides the fast F- ion conduction path throughout the cathode, facilitating the battery operation at lower temperatures. Moreover, the formed CoF2 and SnF2 phases are merged into the original CoSn2 phase in the discharging process, leading to a highly reversible redox reaction and the high cycle stability of CoSn2. These findings should pave the way to enhance the performance of all-solid-state FIBs at lower temperatures.

Effect of Electrolysis Conditions on Electrodeposition of Cobalt-Tin Alloys, Their Structure, and Wettability by Liquids.

The aim of this study was a systematic analysis of the influence of anions (chloride and sulfate) on the electrochemical behavior of the Co-Sn system during codeposition from gluconate baths. The pH-dependent multiple equilibria in cobalt-tin baths were calculated using stability constants. The codeposition of the metals was characterized thermodynamically considering the formation of various CoxSny intermetallic phases. The alloys obtained at different potentials were characterized in terms of their elemental (EDS and anodic stripping) and phase compositions (XRD), the development of preferred orientation planes (texture coefficients), surface morphology (SEM), and wettability (water; diiodomethane; surface energy). The mass of the deposits and cathodic current efficiencies were strongly dependent on both the deposition potential and the bath composition. The morphology and composition of the alloys were mainly dependent on the deposition potential, while the effect of the anions was less emphasized. Two-phase alloys were produced at potentials -0.9 V (Ag/AgCl) and lower, and they consisted of a mixture of tetragonal tin and an uncommon tetragonal CoSn phase. The preferential orientation planes of tin grains were dependent on the cobalt incorporation into the deposits and anion type in the bath, while the latter did not affect the preferential orientation plane of the CoSn phase. The surface wettability of the alloys displayed hydrophobicity and oleophilicity originating from the hierarchical porous surface topography rather than the elemental or phase composition. The codeposition of the metals occurs within the progressive nucleation model, but at more electronegative potentials and in the presence of sulfate ions, a transition from progressive to instantaneous nucleation can be possible. This correlated well with the partial polarization curves of the alloy deposition and the texture of the tin phase.

Investigating the impact of cobalt incorporation on the transformation of Cu6Sn5 layer to (Cu, Co)6Sn5: Microstructural and mechanical insights

In this study, cobalt (Co) particles were incorporated as reinforcing agents into the Sn58Bi solder. The solderability of the solder and the strength of the solder joints were investigated. The results indicated that adding Co particles did not alter the low melting point characteristics of the Sn58Bi solder, yet its undercooling was reduced, and the wettability was enhanced. In the Sn58Bi/Cu solder joint, adding Co led to forming free (Cu, Co)6Sn5 intermetallic compound (IMC) and generating the CoSn2 phase through microalloying. The interfacial IMC was gradually transformed from a fan-shaped Cu6Sn5 to a flatter layer-like (Cu, Co)6Sn5 IMC, eventually exhibiting a coral-like structure with increased Co content. Adding Co particles increased the thickness of the IMC layer, but at low levels, the Co could refine the IMC grains. Simultaneously, implementing grain refinement and Orowan strengthening resulted in heightened microhardness and shear strength. In the shear test, the fracture of solder joints predominantly transpired within the solder matrix. Excessive Co particles were prone to aggregating within the solder, forming irregular block-shaped (Cu, Co)6Sn5. Concurrently, the coral-like IMC at the interface led to a decline in the mechanical performance of the solder joints, with some interfacial IMC being exposed in the fracture region.

Correlation of microstructure with thermal and tensile creep properties of novel Sn-5Sb-0.5Cu alloy reinforced with Co particles for soldering applications

The present research examined how cobalt microalloying additions of 0.25, 0.5, 0.75, and 1 weight percent affected the microstructural properties, thermal features, and tensile creep characteristics of eutectic Sn-5 wt% Sb- 0.5 wt% Cu (SSC) lead-free solder alloy. According to the results, cobalt additions of 0.25, 0.5, and 0.75 wt% did not affect SbSn phase but significantly refined β-Sn grains, facilitating the formation of fine fibers (Cu,Co)6Sn5 together with plate-like CoSn3 phases, and preventing the formation of Cu6Sn5 phases. Furthermore, a large amount of cobalt (1 wt%) addition accumulated in the coarsening of the CoSn3 phase. Additions of 0.25 wt% Co, 0.75 wt% Co, and 1 wt% Co did not affect the melting temperatures, but pasty ranges had been slightly lowered, which may enhance the thermal characteristics. Addition of 0.5 wt% Co had unfavorable effects on both melting point and pasty range. This has significant effects on solder reliability and electronic service performance. In terms of creep behavior, the SSC-0.75 wt% Co specimens displayed the highest creep resistance because of the fine dispersion of intermetallic compounds (IMCs) and extended the creep-rupture life to a level that is 3.0 times greater than the SSC baseline. Lower creep resistance was observed in SSC-0.25 wt% Co specimens, which was mostly due to the smaller volume fraction of the precipitate phases and the absence of the CoSn3 phase. Depending on the determined stress exponents and activation energies, it is suggested that the dominant deformation mechanism in SSC-xCo solders is the dislocation climb controlled by short-circuit pipe diffusion across the whole temperature range that was examined.

Investigation of the microstructural evolution and detachment of Co in contact with Cu–Sn electroplated silicon chips during solid-liquid interdiffusion bonding

Solid-liquid interdiffusion (SLID) bonding is one of the most promising novel methods for micro-(opto)-electromechanical system (MEMS/MOEMS) wafer-level packaging. However, the current SLID bonding solutions require the use of an electrochemical deposition method for MEMS/MOEMS wafers as well, which significantly complicates the process integration options. Hence, this work proposes Co as a potential option for compatible contact metallization on MEMS/MOEMS wafers to utilize mature Cu–Sn SLID bonding. The focus of this study is on gaining a fundamental understanding of the microstructural formation and evolution of Co substrates in contact with Cu–Sn electroplated silicon wafers and identifying possible failures of joints during bonding, which are prerequisites for guaranteeing devices’ manufacturability, functionality, and long-term reliability. The effect of bonding time and temperature on the microstructural evolution and phase formation of Co substrates in contact with Cu–Sn electroplated silicon chips was investigated. Moreover, a phase diagram of the Co–Cu–Sn ternary system was thermodynamically evaluated based on experimental data. Samples were successfully bonded at 250 °C for 1500 and 2000 s and at 280 °C for 1000 s. The main interfacial intermetallic compounds were identified as (Cu,Co)6Sn5, Cu3Sn, and (Co,Cu)Sn3. Co stabilized the high-temperature hexagonal (η) Cu6Sn5 phase down to room temperature. Bond detachment was observed when applying either a higher bonding temperature or a longer bonding time. Two critical factors that cause detachment during bonding were recognized: first, a change in thermodynamic equilibrium when exceeding the maximum allowed Co content in Cu6Sn5 formed adjacent to the CoSn3 phase and a discontinuous change in the Co content in the Cu6Sn5 grown on the Cu and Co sides; second, stress exerted due to the rapid growth of (Co,Cu)Sn3 between the Co substrate and (Cu,Co)6Sn5. Therefore, achieving successful bonding in the Co–Sn–Cu SLID system requires governing the amount of dissolved Co atoms in liquid Sn and the CoSn3 formation, both of which can be achieved by manipulating the relative thickness of the Co, Cu, and Sn layers. The observations and calculations in this work show that a prerequisite for obtaining successful bonding in the Co–Sn–Cu SLID system at 250 °C is a Co–to–Sn thickness ratio of 0.04 or less.

Phase transition and nano-mechanical properties of directionally solidified Sn–Co peritectic alloy

In this work, directional solidification method was used to investigate the growth characteristics of intermetallic compounds whose solubilities are narrow during solidification of Sn-9at.%Co peritectic alloy (L+CoSn→CoSn2). In this work, the transition of the leading phase during solidification is observed in all samples. By the application of the Interface Response Function (IRF), the calculated velocity range for the transition of the primary phase by the peritectic phase ranges from 0.501 μm/s to 398 μm/s. Moreover, the phase transition of CoSn and CoSn2 intermetallic compounds does not initiate from the initial growth interface, and the distance required for the occurrence of the phase transition is found to increase with increasing growth velocity. The measurement of the hardness and Young's modulus was accomplished by nanoindentation with the continuous stiffness measurement (CSM) technique. The elastic modulus is nearly constant while the hardness increases with increasing growth velocity. In addition, the hardness of CoSn2 phase is lower than that of CoSn phase.

Non-isothermal crystallization kinetics of an intermetallic CoSn phase

Herein, the crystallization kinetics of the intermetallic CoSn phase, nucleated and grown in the microstructure of a binary CoSn alloy, have been reported. Differential scanning calorimetry traces under non-isothermal conditions were obtained and used to extract the activation energy () for the crystallization of the CoSn phase at various heating rates. The obtained values of were validated by the application of several isoconversion methods. The minimum value of was 1000 kJ mol−1, which occurred at 853 K. The observed behavior of showed a strong temperature dependence, wherein it decreased with increasing temperature. This suggests that the rate constant for the crystallization of the CoSn phase is determined by the rates of two processes—nucleation and diffusion. The precise description of the crystallization process for the CoSn phase reveal that the reaction mechanism of this phase is following the first-order reaction.

Effect of Co content on the microstructure, spreadability, conductivity and corrosion resistance of Sn-0.7Cu alloy

The effect of Co content on the microstructure, spreadability, conductivity and corrosion resistance of Sn-0.7Cu (mass%, mass fraction except specified) based lead-free solder was studied in this paper. With the addition of Co, the microstructure of Sn-0.7Cu-xCo (x = 0.5, 1.0, 1.5, 2.0) alloy consists of Cu6Sn5, CoSn2 intermetallics, and β-Sn matrix. The morphology of CoSn2 phase changes from small blocks to flake like, short rod-like, and then to massive bulk shape with the increase of Co content. A small amount of CoSn phase formed in Sn-0.7Cu-2.0Co solder alloy. Adding Co reduce the spreading area of Sn-0.7Cu solder alloy slightly. The spread area decreases with the increase of Co content from 0.5% to 1.5%, and then increases again with the addition of 2.0% Co. Sn-0.7Cu-0.5Co solder obtains the largest spreading area among the Sn-0.7Cu-xCo solders. The thickness of reaction layer between Sn-0.7Cu-xCo solder alloys and copper plate is increased with the increase of Co content. The conductivity of the Sn-0.7Cu alloy decreases with the increase of Co content, but the leave of decrease is acceptable. Meanwhile, corrosion resistance of Sn-0.7Cu solder alloy is improved with the addition of Co under salt spray atmosphere. The mass loss rate of Sn-0.7Cu-xCo solder alloy decreases with the increase of Co content.

Growth and mechanical properties of intermetallic compound between solid cobalt and molten tin

Transient liquid phase (TLP) bonding has gained attention due to the advantage of producing bond that has a higher melting point than the bonding temperature. Cobalt (Co) is a potential candidate for base metal in TLP bond. This work studied the interfacial reaction in binary cobalt–tin (Co–Sn) system, growth kinetics and mechanical properties of Co–Sn intermetallic compound (IMC) for transient liquid phase (TLP) joints. Dipping method was employed for the investigation of Co–Sn IMC growth. Solid Co was immersed into molten Sn statically at varying temperatures (250–300 °C) and durations (15–60 min). The IMC formed was characterized by field emission scanning electron microscope (FESEM) coupled with energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD) tests and nanoindentation. The results showed the formation of a CoSn3 layer with plate-like morphology at temperature range of 250–300 °C. The thickness of the CoSn3 layer increases with dipping temperature and duration. The growth of CoSn3 is suggested to be controlled by only diffusion reaction at 250 °C. As temperature increases to 300 °C, the growth of CoSn3 is controlled by chemical reaction. The average nanohardness and Young’s modulus of CoSn3 phase reported in this work are 4.25 ± 0.6 GPa and 98.30 ± 9.0 GPa, respectively.

Structure and property modulation of primary CoSn and peritectic CoSn2 intermetallic compounds through ultrasonicating liquid Sn–10%Co alloy

The effects of power ultrasound on the peritectic solidification of binary Sn–10%Co peritectic type alloy were investigated. If introducing ultrasound in the whole solidification process, primary CoSn intermetallic compound which grew as coarse strips with a preferred grain orientation under static condition was considerably refined and exhibited a random orientation distribution. Moreover, the significant increase in thickness ratio of peritectic CoSn2 to primary CoSn compounds indicated that the peritectic transformation was remarkably enhanced. To further reveal the underlying mechanism, power ultrasound was respectively applied to primary phase growth and peritectic transformation stages. As compared with the latter one, the former procedure led to a finer primary CoSn phase and a higher peritectic transformation extent. The results showed that ultrasound induced refinement of primary CoSn phase played the dominant role, while ultrasound accelerated atomic diffusion between primary and peritectic solid phases may also contributed to the enhancement of peritectic transformation. Both the yield strength and ultimate compressive strength of Sn–10%Co alloy were improved after ultrasonic solidification.

Microstructure Evolution, Thermal and Mechanical Property of Co Alloyed Sn-0.7Cu Lead-Free Solder

The effect of Co addition (0.5-2.0 wt.%) on the microstructure evolution, thermal and mechanical property of Sn-0.7Cu (wt.%) solder was investigated. The microstructure of Sn-Cu alloy was refined with the addition of Co, and CoSn2 phase formed during solidification. The volume fraction of CoSn2 phase increases with the increase of Co content. The melting temperature of Sn-0.7Cu-xCo alloys increases with increasing Co content. The melting range of these alloys is small (< 4°C), and decreased with the increase of Co content. The ultimate tensile strength and microhardness of Sn-Cu-xCo (x = 0.5, 1.0, 1.5, 2.0) alloys increase with the increase of the Co content. The relationships between tensile strength/microhardness and Co content of Sn-Cu-xCo alloy are established. Meanwhile, Sn-0.7Cu-1.5Co (wt.%) and Sn-0.7Cu-2.0Co (wt.%) alloys revealed an indentation size effect. Different ISE behaviors of Sn-0.7Cu-xCo alloys are mainly related to the presence of CoSn2 phase and the volume fraction of it. The indentation creep property of Sn-Cu-xCo alloys is modified with the addition of Co. Enhanced creep property of the alloys is depended on the solid solution of Co and precipitate out of CoSn2 phase.

Study of the effect of Sn grain boundaries on IMC morphology in solid state inter-diffusion soldering

This paper reports on 3D phase field simulations of IMC growth in Co/Sn and Cu/Sn solder systems. In agreement with experimental micrographs, we obtain uniform growth of the CoSn3 phase in Co/Sn solder joints and a non-uniform wavy morphology for the Cu6Sn5 phase in Cu/Sn solder joints. Furthermore, simulations were performed to obtain an insight in the impact of Sn grain size, grain boundary versus bulk diffusion, IMC/Sn interface mobility and Sn grain boundary mobility on IMC morphology and growth kinetics. It is found that grain boundary diffusion in the IMC or Sn phase have a limited impact on the IMC evolution. A wavy IMC morphology is obtained in the simulations when the grain boundary mobility in the Sn phase is relatively large compared to the interface mobility for the IMC/Sn interface, while a uniform IMC morphology is obtained when the Sn grain boundary and IMC/Sn interface mobilities are comparable. For the wavy IMC morphology, a clear effect of the Sn grain size is observed, while for uniform IMC growth, the effect of the Sn grain size is negligible.

Growth behaviors of CoSn2/CoSn4 composite particles in the solidification of Sn–Co alloy under a high magnetic field

Binary Sn–Co alloy was solidified under various high magnetic fields (HMFs). The results show that metastable peritectic CoSn4 phase instead of equilibrium CoSn3 phase is formed, irrespective of the HMFs. Also, fine and coarse CoSn2/CoSn4 composite particles, fine single-phase CoSn4 particles and βSn matrix are distributed in the specimens from the bottom up. In three dimensions, the starting central CoSn2 crystals in the composite particles prefer growing along [001] and have a tetragonal prism-like shape. The outer CoSn4 crystals grow in a spiral manner and exhibit a multi-layered platelike shape. For a single plate, the faceted large-area surface and side surfaces correspond to the (001) and {111} planes, respectively. Moreover, an orientation relationship (OR) (100)CoSn2//(100)CoSn4, [001]CoSn2//[001]CoSn4 exists, which is attributed to the similar atomic arrangements of the two phases. Additionally, the HMFs tend to “levitate” the intermetallic particles and align the fine composite particles regularly. Also, both the CoSn2 and CoSn4 crystals in the fine composite particles tend to orient with the [001] axes parallel to the HMF direction. The “levitation” of the intermetallic particles arises from the HMF-induced magnetic viscosity resistance force. The preferential orientation and regular alignment are associated with the magnetocrystalline anisotropy of the CoSn2 crystals.

Growth and evolution kinetics of intermetallic compounds in Sn-0.7Cu-10Bi-0.15Co/Cu interface

The morphology and composition of intermetallic compounds (IMCs) formed between substrate and solder play a vital role in the service life and failure of the solder joint, which is the most important part in the research of electronic packaging. Therefore, it is very important to investigate the growth and evolution of IMCs layer in the aging process. In this paper, there are particular phenomena in the interfacial reaction between the Sn-0.7Cu-10Bi-0.15Co solder and Cu substrate. The 0.15 wt% Co added to the matrix solder causes the abnormal transformation in the thickness and morphology of the IMCs. Because the CoSn3 phase is rapidly growing in a porous structure, so the intermetallic compounds are planar in morphology with an increasing thickness. In addition, the thickness of IMCs layer decreases because the CoSn3 with porous structure promotes the dissolution of the IMCs during aging for 200 h at 70 °C. Analytical results indicate that the Co element changes the diffusion channel and mechanism of the IMCs growth at the interface, and affects the growth kinetics of the CoSn3 phase in products; it in turn impacts the morphology and thickness of IMCs and reliability of solder joint.

Thermal stability of intermetallic compounds at Sn-0.7Cu-10Bi-xNi/Co interface during reflows

In this paper, thermal stability of Sn-0.7Cu-10B-xNi/Co (SCB-xNi/Co, x = 0.05, 0.10 and 0.15, in wt.%) interface during multiple reflows was investigated. Both (Ni,Co)Sn3 and (Cu,Ni)6Sn5 phases were formed at the SCB-xNi/Co interface after the initial reflow. With the increase of reflow number, the thermal stability of intermetallic compounds (IMCs) layer at high temperature was significantly different. The content of Ni atoms determined the thermal stability of IMCs layer in the interface reaction process. At the IMCs layer at the SCB-0.15Ni/Co interface, Ni atoms were enriched excessively, but they were present in a dispersive distribution at SCB-0.05Ni/Co and SCB-0.10Ni/Co interfaces. Ni atoms occupied sublattice sites of Co atoms in CoSn3 phase, which could improve the high-temperature stability of IMCs layer, so the IMCs layer had the strongest thermal stability at the SCB-0.15Ni/Co interface.

Theoretical and experimental investigations on mechanical properties of Co1−xNixSn2 intermetallic compounds

The effects of Ni element on mechanical properties of CoSn2 phase were investigated by means of first-principles calculations and nano-indentation measurements in this work. Theoretical models of Co1−xNixSn2 (x = 0, 0.25, 0.5, 0.75 and 1) were built to research the phase stability, elastic constants and electronic structures of Ni-free and Ni-doped CoSn2 structures. Analyses about phase stability indicated that the Ni solubility in (Co,Ni)Sn2 phase could reach to 33.33 at.%. The calculated results showed that the additional Ni atoms weakened the mechanical properties of CoSn2 and the Young’s modulus decreased from 149.70 to 107.55 GPa. Besides, the ductility of CoSn2-based structures was improved after the doping of Ni. All studied crystal structures were anisotropic. The results of electronic structures demonstrated that the bonding energy of (Co,Ni)Sn2 phase decreased with the adding of Ni, which was the determinant of the decrement of mechanical properties. The experimental results obtained from nano-indentation showed that the Young’s modulus and hardness were 159.9 ± 11.3 GPa and 6.91 ± 0.65 GPa for CoSn2 while 140.5 ± 9.3 GPa and 6.23 ± 0.82 GPa for (Co,Ni)Sn2. By combining theoretical and experimental methods, it was certainly concluded that the introduced Ni element weakened the mechanical properties of CoSn2.

Phase evolutions and growth kinetics in the Co–Sn system

Co/Sn diffusion couples are annealed at 175–220 °C. The reaction phase CoSn3 is found to have a narrow homogeneity range of 2 at% with composition 74–76 at% Sn. The growth of this phase is found to be reaction–controlled initially followed by diffusion–controlled later. The marker experiment indicates that Sn is the faster diffusing element in this phase. The CoSn2 phase is found to grow in the Co/CoSn3 incremental couples at 200–220 °C.

First-principles Study of Mechanical and Electronic Properties of Co-Sn Intermetallics for Lithium Ion Battery Anode

The equilibrium structures, formation energy, mechanical properties and electronic properties of Co-Sn intermetallics have been systemically studied by first-principles study. The results show that the CoSn phase is more thermodynamically stable than any other stoichiometry of Co-Sn intermetallics. With the increasing of Co content in Co-Sn intermetallics, the mechanical properties change into brittle behavior from ductility character. Adding proper amount of Co to Co-Sn intermetallics can improve the cycle performance for lithium ion battery anode. However, high Co content will lead to a poor cycle performance for Co-Sn intermetallics.

Formation and characterization of intermetallic compounds in electroplated cobalt–tin multilayers

Intermetallic compounds (IMC) form during the metallurgical bonding processes when the interfaces involve solid metal and molten solder. Cobalt (Co) is a promising candidate to be used as under bump metallization (UBM) material, tin (Sn)-based solder alloying element, and transient liquid phase bonding base metal. This work aims at studying the intermixing reaction in Co–Sn system from electroplated Co and Sn multilayers. Co–Sn couples were sequentially electroplated and reflowed at 400 °C for 1 and 4 h. The microstructure and composition of the phases formed at different reflow duration were characterized by field emission scanning electron microscopy (FESEM) coupled with energy dispersive X-ray spectroscopy (EDX), and X-ray diffraction (XRD). Mechanical properties of the IMC layers formed in the samples were characterized using the nanoindentation technique under quasi-static and continuous measurement modes. FESEM/EDX and XRD analysis showed a mixture of CoSn+CoSn2 phases was formed in the 1 h reflow sample. As reflow time was increased to 4 h, only CoSn phase was found. Pure CoSn phase exhibits high nanohardness of 9.51 GPa while the region with CoSn+CoSn2 phases gives nanohardness value of 6.83 GPa.

Metastable CoSn4 formation induced by minor Ga addition and effective suppression effect on the IMC growth in solid-state Sn–Ga/Co reactions

Solid-state interfacial reactions between Co and Sn-based solders doped with different levels of Ga (0.02 to 0.5 wt%) were investigated. With minor Ga addition of less than 0.05 wt%, the CoSn3 layer was uniformly formed at the interface. Compared to the pure Sn/Co reaction, the CoSn3 growth was greatly suppressed by 95 % with only 0.02 wt%Ga addition at 160 °C. The growth kinetics of CoSn3 were systematically studied at temperatures of 160 to 200 °C. The CoSn3 phase clearly exhibited a linear growth with aging time and the suppression effect was more significant with decreasing the aging temperature. It could be attributed to the fact that Ga atoms doped in the CoSn3 phase retarded the nucleation and growth of CoSn3. In particular, with 0.1 wt%Ga addition, the reaction layer changed to the metastable CoSn4 phase, rather than CoSn3. Most importantly, the CoSn4 growth was also strongly suppressed. It was only ~3-μm thick at 180 °C even after aging for 480 h. Similarly, a thin layer of CoSn4 was formed in the reactions with 0.2 to 0.4 wt%Ga. When the Ga content was increased to 0.5 wt%, the dominant reaction phase changed to the CoGa, which was very thin and stably present at the interface. The CoSn4 and α-CoSn3 phases with similar orthorhombic crystal structures and the reasons for the phase change were further discussed.