Primary Silicon Particles Research Articles

A New Approach to Synthesis Micrometer-Sized Porous Silicon Anode for Lithium Ion Batteries

A New Approach to Synthesis Micrometer-sized Porous Silicon Anode for Lithium Ion batteries Haiping Jia, Xiaolin Li, and Ji-Guang Zhang Pacific Northwest National Laboratory, Richland, WA, United States. With the increasing demand on high energy Li-ion batteries (LIBs) for electric vehicles and other large scale energy storage applications, silicon (Si) based composite material has been regarded as one of the most promising candidate to replace the state-of-the-art graphite anodes and can further boost the specific energy of LIBs to be more than 300 Wh/kg. Silicon has the highest practical capacity of 3,579 mAh g-1 and a relatively low lithiation potential of 0.2 V vs. Li/Li+. However, fast capacity fade still largely limits the practical application of Si based anode. A large volume change during lithiation and delithiation processes lead to pulverization and subsequent loss of electrical contact, continuous consumption of electrolyte, repeated breaking/formation of the solid electrolyte interphase (SEI) and an increase in the overall resistance. There has been significant effort to understand and mitigate the capacity fade in Si-based anodes exploiting nanostructured electrodes, surface coatings, additives and novel binders. These advances have paved the way for practical application of Si-based anodes for Li-ion battery applications. Although significant progress has been made, most nanostructured Si materials have to be prepared by high-cost processes (such as chemical vapor deposition) that are difficult to scale up. Therefore, there is an increasing need to prepare Si microparticles (SiMP) with high conductivity and stable structures. In this work, we developed a low-cost and scalable synthesis processes to make desired micronmeter scale SiO2/MWNT spheres. The highly spherical SiO2/MWNT macrobeads ranging from 500 nm to 10 µm in diameter were employed as template and silicon precursor (Figure 1a, b). The as-prepared material basically retains the original morphology of template and demonstrates unique hierarchical structure (Figure 1c, d). The fine primary silicon particles can shorten the Li+ transport route, and avoid the pulverization of SiMP. The obtained silicon composite delivers a reversible capacity of 1290 mAh g-1 at 1C and 87% capacity can be retained after 300 cycles. We attribute this promising cycling stability to the well-designed hierarchical structure, which provides: 1) enough void space to accommodate the volume change so that the swelling of the composite can be minimized; 2) good mechanical structure and electronic conductivity from the MWNT matrix. More details on the synthesis process and electrochemical performance of SiMP will be reported in this report. Figure 1. (a, b) SEM images of microsphere SiO2/MWNT; (c, d) SEM images of the obtained Si/MWNT. Figure 1

On the formation mechanism of the ring-like microstructure of high-pressure die-cast A390 alloy

The microstructure of the A390 alloy during high pressure die casting (HPDC), including pores, primary silicon particles (PSPs) and Cu-rich phases was characterized using synchrotron X-ray tomography. On the cross section of the sample exhibited ring-like or band microstructure, which comprised multiple PSPs-free and PSPs-rich zones. The PSPs distributed evenly in both size and morphology regardless of the ring-like microstructure. The size of aluminum dendrites changed according to the location of the ring-like microstructure, and was highly dependent on the local latent heat released from the PSPs as well as the distribution of the ‘net’ Cu-rich phases. Along the radial direction of the sample exhibited an abrupt increase of pores, indicating a tendency for forming a defect band in but such tendency was inhibited because of the huge amount of latent heat released by the PSPs during solidification, which was quite different from that of either magnesium alloys or hypoeutectic aluminum alloys.

Zeolite-Templated Mesoporous Silicon Particles for Advanced Lithium-Ion Battery Anodes.

For the practical use of high-capacity silicon anodes in high-energy lithium-based batteries, key issues arising from the large volume change of silicon during cycling must be addressed by the facile structural design of silicon. Herein, we discuss the zeolite-templated magnesiothermic reduction synthesis of mesoporous silicon (mpSi) (mpSi-Y, -B, and -Z derived from commercial zeolite Y, Beta, and ZSM-5, respectively) microparticles having large pore volume (0.4-0.5 cm3/g), wide open pore size (19-31 nm), and small primary silicon particles (20-35 nm). With these appealing mpSi particle structural features, a series of mpSi/C composites exhibit outstanding performance including excellent cycling stabilities for 500 cycles, high specific and volumetric capacities (1100-1700 mAh g-1 and 640-1000 mAh cm-3 at 100 mA g-1), high Coulombic efficiencies (approximately 100%), and remarkable rate capabilities, whereas conventional silicon nanoparticles (SiNP)/C demonstrate limited cycle life. These enhanced electrochemical responses of mpSi/C composites are further manifested by low impedance build-up, high Li ion diffusion rate, and small electrode thickness changes after cycling compared with those of SiNP/C composite. In addition to the outstanding electrochemical properties, the low-cost materials and high-yield processing make the mpSi/C composites attractive candidates for high-performance and high-energy Li-ion battery anodes.

Effects of Mn addition on microstructure and hardness of Al-12.6Si alloy

In this work, eutectic Al-12.6Si alloy with and without manganese (Mn) have been developed through gravity casting route. The effect of Mn concentration (0.0 wt.%, 1 wt%, 2 wt% and 3 wt%) on microstructural morphology and hardness property of the alloy has been investigated. The eutectic Al-12.6 Si alloy exhibits the presence of combine plate, needle and rod-like eutectic silicon phase with very sharp corners and coarser primary silicon particles within the α-Al phase. In addition of 1wt.% of Mn in the eutectic Al-12.6Si alloy, sharp corners of the primary Si and needle-like eutectic Si are became blunt and particles size is reduced. Further, increase in Mn concentration (2.0 wt.%) in the Al-12.6Si alloy, irregular plate shape Al6(Mn,Fe) intermetallics are formed inside the α-Al phase, but the primary and eutectic phase morphology is similar to the eutectic Al-12.6Si alloy. The volume fraction of Al6(Mn,Fe) increases and Al6(Mn,Fe) particles appear as like chain structure in the alloy with 3 wt.% Mn. An increase in Mn concentration in the Al-12.6Si alloys result in the increase in bulk hardness of the alloy as an effects of microstructure modification as well as the presence of harder Al6(Mn,Fe) phase in the developed alloy.

Selective laser melting of hypereutectic Al-Si40-powder using ultra-short laser pulses

We investigate the use of ultra-short laser pulses for the selective melting of Al-Si40-powder to fabricate complex light-weight structures with wall sizes below 100,upmu text {m} combined with higher tensile strength and lower thermal expansion coefficient in comparison to standard Al–Si alloys. During the cooling process using conventional techniques, large primary silicon particles are formed which impairs the mechanical and thermal properties. We demonstrate that these limitations can be overcome using ultra-short laser pulses enabling the rapid heating and cooling in a non-thermal equilibrium process. We analyze the morphology characteristics and micro-structures of single tracks and thin-walled structures depending on pulse energy, repetition rate and scanning velocity utilizing pulses with a duration of 500,text {fs} at a wavelength of 1030,text {nm}. The possibility to specifically change and optimize the microstructure is shown.

Hybrid multi-objective optimization of microstructural and mechanical properties of B4C/A356 composites fabricated by FSP using TOPSIS and modified NSGA-II

Abstract A356 alloy was used as the base metal to produce boron carbide (B4C)/A356 composites using friction stir processing (FSP). The microstructural and mechanical properties of B4C/A356 composites were optimized using artificial neural network (ANN) and non-dominated sorting genetic algorithm-II (NSGA-II). Firstly, microstructural properties of the composites fabricated in different processing conditions were investigated. Results show that FSP parameters such as rotational speed, traverse speed and tool pin profile significantly affect the size of the primary silicon (Si) particles of the base metal, as well as the dispersion quality and volume fraction of reinforcing B4C particles in the composite layer. Higher rotational to traverse speeds ratio accompanied by threaded pin profile leads to better particles distribution, finer Si particles and smaller B4C agglomerations. Secondly, hardness and tensile tests were performed to study mechanical properties of the composites. FSP changes the fracture mechanism from brittle form in the as-received metal to very ductile form in the FSPed specimens. Then, a relation between the FSP parameters and microstructural and mechanical properties of the composites was established using ANN. A modified NSGA-II by incorporating diversity preserving mechanism called the ɛ elimination algorithm was employed to obtain the Pareto-optimal set of FSP parameters.

On the growth mechanism of the primary silicon particle in a hypereutectic Al-20 wt%Si alloy using synchrotron X-ray tomography

The 3-D morphology of the primary silicon particle (PSP) in a hypereutectic Al-20wt%Si alloy was characterized using synchrotron X-ray tomography. It was found that the most probable particle morphology always had a shape factor of 0.85, indicating either a regular octahedron or a twinned shape with a/h=2.5 (a was the edge length and h was the distance between the outer surface and the {111} plane). A growth transition model was then developed to describe the growth mechanism for the PSPs. It is believed that both octahedral and twinned shapes are a transition from a so-called facetted growth unit, which has a morphology of just half octahedron and evolves from a spherical nucleation. Further growth of the silicon particles can lead to the formation of hollow hoppers due to the mismatch of the growth velocity along different directions.

The Effect of Water Mist Cooling of Casting Die on the Solidification, Microstructure and Properties of AlSi20 Alloy

Abstract Unmodified AlSi20 alloy were casted at the research station, allowing for sequential multipoint cooling using a dedicated computer- controlled program. This method allows for the formation of the microstructure of hypereutectic AlSi20 alloy and also increases hardness. Primary silicon dendrites were found in the microstructure of cooled samples. Based on these dendrites, the formation of primary silicon particles is explained. Cooling of casting die with a water mist stream causes changes in solidification, which leads to expansion of the boundary layer with columnar crystals and shrinkage of the core zone with equiaxed crystals. It also causes more regular hardness distribution around pre-eutectic Si crystals, which can lead to tensile strength and machinability improvement.

Characterization of the morphology of primary silicon particles using synchrotron X-ray tomography

The 3D morphology of primary silicon particles in a hypereutectic Al-20wt.% Si alloy was characterized using synchrotron X-ray tomography. According to the statistical analysis of the primary silicon particles, it was found that both octahedral shape and twinned spinel shape were identified as the typical morphologies. For the octahedral shape, the measured growth speed ratio between <100> and <111> directions was close to 3, i.e. approaching the theoretical value for a regular octahedron. The measured growth speed ratio between <111> and <112¯> for most twinned particles was about 0.93, indicating a perfect twinned spinel shape. However, influenced by surrounding solidification conditions, such perfect twinned growth of the primary silicon particles was hard to maintain, i.e. with medium or low cooling rate, the particle morphology tended to be regular while with high cooling rate, a plate-like morphology would become favorable.

Formation of equiaxed crystal structures in directionally solidified Al-Si alloys using Nb-based heterogeneous nuclei.

The design of chemical compositions containing potent nuclei for the enhancement of heterogeneous nucleation in aluminium, especially cast alloys such as Al-Si alloys, is a matter of importance in order to achieve homogeneous properties in castings with complex geometries. We identified that Al3Nb/NbB2 compounds are effective heterogeneous nuclei and are successfully produced in the form of Al-2Nb-xB (x = 0.5, 1 and 2) master alloys. Our study shows that the inoculation of Al-10Si braze alloy with these compounds effectively promotes the heterogeneous nucleation of primary α-Al crystals and reduces the undercooling needed for solidification to take place. Moreover, we present evidences that these Nb-based compounds prevent the growth of columnar crystals and permit to obtain, for the first time, fine and equiaxed crystals in directionally solidified Al-10Si braze alloy. As a consequence of the potent heterogeneous particles, the size of the α-Al crystals was found to be less dependent on the processing conditions, especially the thermal gradient. Finally, we also demonstrate that the enhanced nucleation leads to the refinement of secondary phases such as eutectic silicon and primary silicon particles.

Needle Like Fe-Containing Intermetallic Compounds of High Silicon Aluminum Alloy with Fe Modified by Mn and Ultrasonic Vibration

In this study, the modification effects and mechanism of manganese (Mn) and ultrasonic vibration (USV) on the needle-like Fe-containing intermetallic compounds of Al-20Si-xFe-2.0Cu-0.4Mg-1.0Ni (x=1, 2 wt.%) alloy have been studied respectively. The effect of Fe-containing phases on volume fraction of hard phases is also investigated. The results show that the mechanism and effect of Fe-containing intermetallic compounds improved by Mn are in close relationship with Fe content. Mn can promote to form less harmful α-Al15(Fe,Mn)3Si2 phase, or replace some Fe atoms of β-Al5FeSi and δ-Al4FeSi2 according to different Fe content. When USV was applied to this alloy containing 2%Fe near liquidus temperature, most of the acicular β phases formed in traditional process are substituted by fine plate δ phases. With the combined effects of 0.5%Mn and USV, the acicular β phases are almost repressed and the Fe-containing phases exist in form of fine Al4(Fe,Mn)Si2 and Al5(Fe,Mn)Si particles about 20~30μm. Consequently, the total volume fraction of hard phases which are composed of primary silicon particles and Fe-containing phases increases significantly.

The Effect of Chilling and Ce Addition on the Microstructure and Mechanical Properties of Al-23Si Alloy

The present work involves the study of the effect of varying concentration of Ce addition on microstructure and mechanical properties of Al-23%Si alloys. Melt-treated alloys were solidified in copper, brass, stainless steel molds to assess the effect of cooling rate. The effect on microstructure was assessed by measuring the fineness of primary silicon and eutectic silicon particle characteristics. The Ce melt treatment transformed the coarse and irregular primary silicon into refined polyhedral silicon crystals, and the effect was more significant at higher cooling rates. Although the melt treatment had refined the eutectic silicon at lower cooling rates, it did not show any considerable effect on the eutectic silicon at higher cooling rates. The mechanical properties of the alloy increased significantly with increase in cooling rates and cerium concentration. Analysis of the results and literature reveals that the refined primary silicon was formed as a result of an invariant reaction between Ce compounds and primary silicon at higher temperatures.

Evaluation of microstructure of A356 aluminum alloy casting prepared under vibratory conditions during the solidification

The objective of this investigation was to evaluate the effect of vibrations (during solidification) on the metallurgical properties of A356 aluminum casting. Mechanical vibrations were applied to A356 aluminum alloy through set up. A356 melt has been subjected to mechanical vibration with the frequency range from 0 to 400 Hz with constant amplitude 5 µm. Grain refinement was obtained through mold vibration. Metallurgical properties were examined through optical microstructure, tensile fracture scanning electron microscope (SEM) and SEM image of test specimens prepared under different conditions of solidification. Results indicate that mold vibration effectively modified the microstructure of A356 casting and it has uniform and smaller grain size with fibrous silicon particle than nonvibrated casting. Grain refinement results increase in mechanical properties with increase in frequency of vibration of mold during the solidification. SEM micrograph of tensile fracture surface was carried out to study the influence of microstructure on fracture mode. SEM image of tensile fractured surface shows transgranular cleavage facets due to fracture of primary silicon particles. Fractures are brittle in nature so observation indicates low ductility and brittle fracture.

A method for improving the mechanical properties of a hypereutectic Al–Si alloy by introducing the α-Al phase

Abstract The low ductility of hypereutectic Al–Si alloys, which is attributed to the coarse primary silicon particles in the microstructure, limits their application potential. The mechanical properties of hypereutectic Al–Si alloys strongly depend on the size and morphology of the primary Si. This study focused not only on the effect of the primary Si content, but also on the ductile α-Al phase, which does not occur in the microstructure of hypereutectic Al–Si alloys when using traditional alloying methods. We propose a novel method which allows introducing the α-Al phase in order to improve the mechanical properties of hypereutectic Al–Si alloys, e.g., the hardness, tensile strength, and ductility. The formation of the α-Al phase and a refinement of the primary Si were simultaneously achieved via an unconventional method, which is based on mixing a high-temperature molten Al–Si alloy with low Si content into a low-temperature semi-solid Al–Si alloy with high Si content. The resulting alloy showed an improved ductility, hardness and tensile strength compared to a traditionally-cast Al–Si alloy with the same nominal Si content.

Effects of Silicon on Microstructures and Properties of Al-40Zn-xSi Filler Metal

Abstract The effect of Si on the brazability and the microstructures of Al-40Zn-xSi filler metals were studied, and the microstructures and the mechanical properties of the joints brazed with Al-40Zn-xSi filler metals were investigated. The results indicate that the Al-40Zn-xSi filler metal presents the best wettability on 6061 aluminum alloy when Si content is and 4.0 wt%. The microstructure of the filler metal indicates that the primary silicon particles could be found when the silicon content exceeds 4.0 wt%. Al-Si eutectic mixed with Zn-Al eutectoid is located at α-Al interdendritic regions in the brazed seam after cooling with water. Moreover, the Al-40Zn-4Si joint possesses the optimum shear strength of 142.28 MPa. However, the excess of Si would increase the amount of brittle eutectic structure and primary silicon particles in the brazed joints, and thereby the mechanical properties will be deteriorated.

New Wear Resistant Hypereutectic AlSi4Cu4FeCrMn Alloys for High Pressure Die Casting

In order to adapt the AlSi14Cu4FeCrMn alloy to the production of monolithic engine blocks in HPDC as well as to improve their tribological properties, the following study was performed. The silicon content in the hypereutectic Al-Si alloy was reduced to allow for a lower casting temperature. The addition of iron to the alloy compensated for the reduced volume fraction of hard primary silicon particles and improved the tribological properties.

Effect of Electromagnetic Stirring on the Reciprocating Wear Characteristics of A390 Alloy

Present study reports the effect of electromagnetic stirring (EMS) on reciprocating wear characteristics of A390 alloy. The microstructure features showed the refinement of eutectic silicon and reduction in size of primary silicon particles due to the effect of EMS. Hardness of the A390 alloy (100.28 BHN) prepared by EMS was higher than conventional A390 alloy (66.86 BHN). Reciprocating wear tests were carried out at different contact loads (15N to 90N) and speeds (0.2m/s to 1m/s) using pin-on-reciprocating plate tribometer for a constant sliding distance of 500m. As the reciprocating velocity increases from 0.2m/s to 1m/s, the wear loss of both EMS stirred and without stirred A390 alloys were found to decrease gradually up to the critical velocity 0.6m/s. It was noticed that above critical reciprocating velocity 0.6m/s, the wear loss was found to increase abruptly. Also, as the load increases from 15N to 90N, the wear loss of both the alloys were found to increase gradually up to 45N and thereafter abrupt increase in wear loss was noticed up to 90N. This clearly indicates a transition in the mode of wear from mild oxidative to severe metallic wear regime. Wear rate of without stirred A390 alloy in the region 45N to 90N showed predominant increase compared to EMS stirred A390 alloy. This trend suggests that EMS stirred A390 alloy is more coherent and stable under dry sliding wear conditions than without stirred A390 alloy.

The effect of Nb–B inoculation on binary hypereutectic and near-eutectic LM13 Al–Si cast alloys

Hyper-eutectic Al–Si alloys are used for wear-resistant components, such as pistons, because their microstructure is composed by ductile aluminium dendrites and hard primary silicon particles. In this study the effect of Nb–B inoculation on the microstructural features of binary hyper-eutectic and near-eutectic LM13 Al–Si alloys is assessed. It is found that the inoculation with Nb-based compounds (i.e. NbB2 and Al3Nb) leads to the refinement of the microstructural features (i.e. finer and more homogeneous distribution of the eutectic phase as well as smaller primary Si particles as secondary effects). The study also demonstrates that the addition of Nb–B inoculants do not interfere with additions of strontium (used to modify the morphology of the eutectic phase) or phosphorous (added to nucleate primary Si particles).

Effect of Alternating Traveling Magnetic Field on the Removal of Inclusions from Aluminum Melt

Alternating traveling magnetic field (TMF) was introduced to agglomerate the inclusions with a density smaller than surrounding melt. Primary silicon particles precipitating from the solidification process of hypereutectic Al-Si alloy was regarded as inclusions need removing. Results indicated that alternating TMF was more effective to promote the inclusions to agglomerate into clusters than downward TMF. The effect of alternating TMF to agglomerate the inclusions increases with the increase of current and frequency. There exists the best alternating time to get the best agglomeration effect. In this study, 10s is the best alternating time.

Preparation and wear properties of TiB2/Al–30Si composites via in-situ melt reactions under high-energy ultrasonic field

TiB2/Al–30Si composites were fabricated via in-situ melt reaction under high-energy ultrasonic field. The microstructure and wear properties of the composite were investigated by XRD, SEM and dry sliding testing. The results indicate that TiB2 reinforcement particles are uniformly distributed in the aluminum matrix under high-energy ultrasonic field. The morphology of the TiB2 particles is in circle-shape or quadrangle-shape, and the size of the particles is 0.1–1.5 μm. The primary silicon particles are in quadrangle-shape and the average size of them is about 10 μm. Hardness values of the Al–30Si matrix alloy and the TiB2/Al–30Si composites considerably increase as the high energy ultrasonic power increases. In particular, the maximum hardness value of the in-situ composites is about 1.3 times as high as that of the matrix alloy when the ultrasonic power is 1.2 kW, reaching 412 MPa. Meanwhile, the wear resistance of the in-situ TiB2/Al–30Si composites prepared under high-energy ultrasonic field is obviously improved and is insensitive to the applied loads of the dry sliding testing.