Fabrication of thin silicon flakes through dealloying from laser directed energy deposition Ca-Si alloys

In this study, thin silicon flakes were fabricated through dealloying from laser directed energy deposition (LDED) Ca-Si alloys, and the solidification mechanism of the lamellar eutectic microstructure was studied. With 1200 W laser power and 450 mm/min scanning speed, the LDED Ca-Si alloys have a fine lamellar eutectic microstructure and are easily separated from copper substrates. After dealloying in the diluted HCl solution for 6 h, the thin silicon flakes ranging in thickness from 100 nm to 0.5 μm were obtained. The formation of the thin flakes was attributed to fine lamellar Si phases in precursor alloys. The rapid solidification of laser processing could refine the microstructure, especially Si phases, and improve the chemical homogeneity of the material. Combining laser processing and dealloying, a cost-effective and scalable strategy was devised to fabricate fine powder from a laser processing optimized microstructure.


Introduction
Silicon (Si) has widespread applications in the metallurgical [1], semiconductor [2], and photovoltaic [3] industries due to its extensive supply sources, unique photoelectric properties, and stability.Additionally, Si possesses a high theoretical specific capacity and is considered the next-generation anode material.There are two commercial Si-based anode technical routes: combining with carbon as Si/C composite and disproportionation reaction as SiO x [4].In the Si/C composite route, the whole composite materials only contain <10% Si, and the size of Si is required to be fine to maintain the conductivity and stability of the anode.However, the cost of commercial Si/C composites is still high because of the long process and complex equipment for preparing nanoscale Si.There are three main approaches to synthesizing nanoscale silicon in the industry: ball milling [5,6], physical ways based on evaporation and solidification [7,8], and chemical ways, including decomposing of silane [9,10].Ball milling is a simple and scalable method that is extensively used in powder industrial production.Nevertheless, the milling time will surge for nanoscale materials, and roller balls may introduce impurities.Physical ways require a high vacuum system, while chemical ways produce hydrogen with poor security stability.Therefore, finding a simple, scalable, cost-effective, and safe method of preparing small-size silicon is necessary for broadening the application of Si-based materials.Dealloying of silicon-based alloys is potential.
The main idea of dealloying silicon-based alloys contains two steps: dissolution of silicon in the metal as precursor materials and selective leaching of active metal elements.Such a method has already been studied in obtaining bulk Si crystals [11] or porous Si (P-Si) [12].However, the millimeter scale bulk Si crystals and the micron scale P-Si need following grinding to gain the finer powders.The structure of dealloying products is determined by the etching processes and the microstructure of precursor materials.Hence, optimizing the microstructure of precursor materials, especially Si phases, is key to getting fine Si powders with the dealloying method.
LDED is one effective way to refine crystal grain and promote chemical homogeneity through rapid solidification.It can extend the equilibrium solubility limit and produce metastable or amorphous phases.After introducing laser processing in silicon-based alloys, the microstructure of precursor materials will be optimized and controllable.As a result, finer Si powders will come from the finer Si phase in precursor materials.Cao et al [13] have fabricated a planar island Si with a 50-300 nm thickness on the porous Cu matrix through a laserfabricated Al-Si-Cu layer, followed by dealloying with HCl.Lien et al [14] have obtained ultrafine Si in hypereutectic Al-Si alloy by laser surface remelting.Huang et al [15] have fabricated porous Si with interconnected micro-sized dendrites and tunable morphology through dealloying a laser remelted Al-Si alloy.Most laser treatment researchers focus on the Al-Si system and remelting or deposition on the substrate.As a result, the morphology of products is limited, and the laser treatment layer needs to be cut from the substrate as an extra process.Thus, the raw materials and process apart from the laser treated layer need to be improved.
In this work, the Ca-Si alloy and pure Cu are chosen as raw materials and substrates.Ca-Si alloys, used as oxygen absorbers in the steel industry, have broad resources, high Si content, and removable intermediate phases.Pure Cu substrate could be a cooling medium because of its significant heat dissipation capabilities and notable reflectivity.The microstructure of raw Ca-Si alloys, single layer deposition, and thin silicon flakes are characterized by SEM.Crystallographic orientation relationships (OR) of thin lamellar eutectic are also studied by EBSD.The results show that rapid solidification and special eutectic growth conduct thin lamellar Si phases.As a result, thin Si flakes are obtained from dealloying.LDED introduces a highly flexible and continual manufacturing process.Thus, the proposed method has great potential in the industrial fabrication of nanoscale materials with the designed composition and structure.

LDED process
Commercial Ca-Si alloy powders (Ca31Si60, 100 μm) were procured from the Sichuan Green Forest Technology Co., Ltd.A single mode fiber laser (MFSC-2000W, Maxphotonics) was employed.The laser is a continuous fiber type with an adjustable power range of 200-2000 W. Its efficiency is greater than 30% nominal, and the laser wavelength of 1060-1100 nm belongs to the infrared range, where the thermal effect is noticeable.Industrial pure Copper substrates were chosen in this study because of their high reflection and high thermal conductivity.In order to ensure the stability and separability of the deposition, it is necessary to control the relative flatness of the processing surface.The top surface of the substrate is ground by surface grinders before ethanol cleaning.After drying, the substrate is ready as a suitable heat dissipation medium.Low direct laser energy absorption and rapid heat dissipation process promise no melting of copper subtracts and a high gradient between the LDED layer and subtract.Thus, shrinkage will happen on the interfaces, and separating the LDED layer from the subtract becomes easy.Laser processing system parameters were selected to ensure the separability and continuity of deposition layers, which is important for large-scale continuous production.It means that deposition layers need to be separated from the substrate easily while they are thin for adequate cooling and wide for productivity.In this study, the diameter of the focused laser beam is 3 mm.Protect gas flow of 5 l min −1 , laser power of 1200 W, and scanning speed of 450 mm min −1 are used.The scanning trajectory is unidirectional mode and single layer (figure1(b)).A part feeder driven by the electromagnetic vibrator is chosen as a delivery system.The vibration frequency controlling delivery rate is 135 HZ.With such parameters, the size of the deposition layer is 2 mm in width and 0.5 mm in height.After every scanning, residual powders were removed from the substrate to protect other tracks from oxidation.Deposition layers were separated from the copper substrate and ground for dealloying.

Preparation of Si flakes
A regent bottle with a magnetic stirring was charged with 0.1 g deposition layer powders and 80 ml HCl solution (3 mol L −1 ).Dealloying processes at room temperature continued for 6 h to form black precipitates.The precipitates were washed with deionized water and then dispersed in pure ethanol with an ultrasonic cell crusher (DS-1000Y) for 10 min.After sediment within 6 h, supernatant liquid ethanol is collected and dried in a vacuum at 60 °C.Figure1 (a) illustrates the whole process of laser treatment and dealloying.

Characterization of Si flakes
X-ray diffraction (XRD, DX-2700) with Cu Kα radiation at 40 kV and 30 mA was used to determine the phase compositions of samples.The morphology and elemental analyses of the raw powders, deposition layer powders, and dealloying products were characterized by scanning electron microscopy (SEM, JOEL JSM-7900F) with energy-dispersive x-ray scope (EDS).The microstructural features of the lamellar eutectic structure were characterized by electron backscattering diffraction (EBSD) with SEM.The size of Si flakes, lamellar eutectic thickness, and lamellar spacing were measured through Back-scattered Electron (BSE) images.

Result and discussion
Figure 1(c) shows the XRD pattern of raw powders, the LDED layers, and the dealloying products.The compositions of raw powders include main phase CaSi 2 , Si, and impurity FeSi 2 .No oxide phases such as CaO or SiO 2 were detected in the XRD pattern, which might be attributed to the low content of such phases.Ca31Si60 alloys consist of 66.7 wt.% Si when focusing on the Ca-Si binary system.According to the Ca-Si binary phase diagram (figure 2(d) [16]), such alloys contain CaSi 2 and Si phases with hypereutectic composition at equilibrium conditions.The impurity of FeSi 2 is often unavoidable in the fabrication processes of commercial Ca-Si alloys.The XRD pattern of the LDED layer discloses no new phase appearing during the laser processing.However, no CaSi 2 peaks are observed in the XRD pattern of the dealloying product after the treatment with dilute HCl, which means CaSi 2 almost disappeared.According to the research about leaching impurities in metallurgical grade silicon [17], the CaSi 2 phases are dissolvable in HCl.Drawing on the reaction between Mg 2 Si and HCl, the CaCl 2 and SiH 4 may be reaction products.Indeed, bubbles are generated during the reaction.However, these products are not detected with XRD because CaCl 2 is soluble in water and SiH 4 is gas.Nevertheless, FeSi 2 still survives because of its high resistance to acid.Based on the research about Fe-Si alloys [18], Fe-Si alloys contain FeSi 2 phases and show lower capacity than micro-sized Si.Although FeSi 2 phases may influence capacity performance, they could improve capacity retention and reduce severe aggregation of Si as a conductive matrix to buffer expansion of Si [19].
BSE images of the cross-section of raw powders are shown in figure 2(a).The raw powders have irregular shapes like mechanically crushed scraps.Notably, raw powders contain two individual phases rather than a clear eutectic structure.It may be caused by the uneven solute distribution and slow cooling solidification during the industrial production of Ca-Si alloys [20].Figure 2(b) illustrates that the main phase is grey in contrast with the black and white second phases.According to the results of XRD and EDS-point scans (table 1), there are white FeSi 2 phases, black Si phases, and grey CaSi 2 main phases.The Si phases in the CaSi 2 matrix possess various shapes with micron scale; some individual Si phases even have a size of ∼100 μm.Most of the FeSi 2 phases are thin strip-like embedded in the CaSi 2 matrix.Such microstructure is not beneficial for the fine powder obtained from the dealloying process, and another study chose NaOH treatment to purify bulk Si [21].
After laser treatment (figure S1 shows a macroscopic morphology image of single layer deposition samples), the microstructure of materials became a typical eutectic structure.As shown in figure 2(c), a small amount of polygonal primary Si distributes on the CaSi 2 matrix, while the eutectic Si exhibits a lamellar structure (figure S2 shows the compositional analysis of the single layer deposition sample).Si and FeSi 2 become tiny and distribute at 1024 °C, and the melting temperature of Si is 1414 °C.The current parameter of laser processing has enough energy to melt individual Si, and the alloys' composition meets the eutectic reaction requirement.This lamellar structure is thought to be caused by the alternation growth of layers of two phases [22].Due to the high melting point of silicon phases, they could be leading crystalline phases of eutectic crystallization and substrates for crystallization of CaSi 2 phases.After crystallization of the silicon phases, the Ca atoms will be enriched in the liquid ahead of the eutectic-liquid interface.It allows CaSi 2 phases to crystallize and grow on the silicon substrates, conducting enrichment of Si atoms in the liquid ahead of the eutectic-liquid interface.As for redistribution of Si and Ca, Si atoms diffuse toward the Si phase layers, and diffusion of Ca is in the direction of the CaSi 2 phase layers.As this process repeats, alternation layers with a lamellar configuration are formed because of a short atomic diffusion distance.With repeated nucleation, CaSi 2 and Si phases may grow continuously in a 'bridging' way.As a result, the same phase layers grow from the same crystal, and a specific orientation relationship between two phases may occur in one eutectic cell.Notably, the lamellar eutectic spacing (∼0.5 μm from figure 2(c)) becomes relatively thinner compared with the traditional casting method (∼10 μm) in the research [23].During the laser processing, alloys will suffer rapid remelting and solidification.Such a high cooling rate allows little time for diffusion to form equilibrium phases and reduce its nucleation growth.Thus, finer lamellar eutectic structure appears because of shorter diffusion distance.In addition, the nucleation rate in solidification is negatively correlated with Gibbs free energy required for critical nucleation (∆G*) by classical nucleation theory [24].∆G* is positively correlated with degree of supercooling.A high nucleation rate brings fine microstructure by more nucleation in the same areas.Besides, the laser processing owning convection stirring effect improves the redistribution of Si atoms, and more eutectic microstructures will be obtained by extending the equilibrium solubility limit effect.Figure 3 shows SEM-EBSD on a lamellar eutectic structure in the LDED layer.Based on the successful identification of red Si phases and green CaSi 2 phases in figure 3(a), the lamellar eutectic structure consists of these two phases.However, few FeSi 2 phases are discriminated because of their size below the instrument's resolution.Figure 3(b) illustrates a crystal orientation map, and all Si phases show the same crystal orientation   ( ̅ ̅ ) CaSi 2 1 1 0 2 are still the same, which means they also comply with above special orientation relationship.To further explain the reason for this crystal orientation relationship, the crystal structure of the two phases needs to be studied.At ambient pressure, CaSi 2 processes a trigonal structure (space group R-3m), consisting of double Si layers alternating with Ca's trigonal layers.The double-layer Si atoms connecting as puckered hexagons look like a honeycomb along to ( ) 0001 direction.Common Si has a diamond cubic crystal structure (space group Fd-3m), containing two interpenetrating face-centered cubic.The ( ) 111 planes also consist of double-layer Si atoms.It is notable that Si double-layers on ( ) CaSi 0001 2 planes are similar to puckered ( ) Si 111 planes, which have the lowest lattice mismatch of 0.4% [26].Such a specific crystal growth relationship can minimize the interfacial energy, keeping the whole system stable.
After dealloying in diluted HCl solution, deposition layer powders were broken into black fragments.Figure 4(a) reveals the morphology of the dealloying product.Compared with raw materials, the particle size of silicon phases decreased because they were extracted from finer eutectic structures after laser treatment.Some possess the expected flake shape, but others show chunk type.The chunks come from two possible origins: the primary silicon or incompletely dealloying products from the CaSi 2 matrix.The morphology of dealloying product flakes is illustrated in figure 4(b).The thickness of one flake in the center of the figure is about 0.36 μm, a sub-micro scale.
Based on the BSE image of the LDED sample and the secondary electronic (SE) image of dealloying products (figure 2(c), figure 4(a)), the thickness of lamellar Si phases and Si flakes is measured (figures 4(c), (d)).The mean thickness of the Si phase is 0.18 μm with uniform distribution, whereas the flakes range in thickness from 100 nm to 0.5 μm with a mean value of 0.21 μm.Similar mean thicknesses disclose the inheritance of microstructures from laser deposition samples to dealloying products.The proposed method could obtain a promising shape and size of the dealloying product by controlling the microstructure of the precursor.The length of the Si flake basically becomes the level of micro 1 μm to 6.5 μm, which matches the lamellar structure of eutectic Si phases.As for the wide distribution of the thickness, it might come from the uneven microstructure of the LDED sample.The solidification process will differ due to different heat transfer conditions in different melting deposition parts.Thus, dealloying products need centrifugal separation to ensure the uniformity and controllability of Si flakes.

Conclusion
In summary, thin Si flakes were prepared from the Ca-Si alloys by hybrid laser processing in combination with dealloying.The cheap and industrial Ca-Si alloys and pure Cu substrate are chosen to extend the product's shape and improve the deposition's separability from the substrate.The microstructure of Ca-Si alloys was optimized, conducting a fine lamellar eutectic microstructure, which is beneficial for dealloying.This is mainly due to less time for diffusion during rapid solidification and special crystal orientation between CaSi 2 and Si.The mean thickness of Si flakes after dealloying is 0.21 μm.This work provides a scalable way of fabricating fine powders from industrial-grade raw materials, which could be extended to other materials based on their microstructure.

Figure 1 .
Figure 1.(a) Strategy for preparation of thin flake Si by combining laser processing and dealloying.(b) Laser scanning trajectory (c) The XRD patterns of raw powders, LDED layers, and dealloying products.

Figure 2 .
Figure 2. (a) Cross-section BSE image of raw powders.(b) EDS point scan area image of raw powders.(c) BSE image of a part of laser melting single layer deposition sample.(d) Ca-Si phase diagram.Reprinted from [16], Copyright (2003), with permission from Elsevier.

in figure 3
could identified crystal orientation relationship between CaSi 2 and Si in the eutectic structure.It was shown that ( with the research of fabricating CaSi 2 on Si Substrates[25].As for two types of crystal orientation of CaSi 2 , the pole figures of (

Figure 4 .
Figure 4. (a) SE image of dealloying products after HCl wash.(b) High-magnification SE image of silicon flakes.Thickness distribution histogram of (c) eutectic Si phases in the single layer deposition sample and (d) Si flakes after dealloying.

Table 1 .
Element contents of spots in figure 2(b), (at.%).This is one piece of evidence that proves 'bridging' growth in the lamellar eutectic structure.Nevertheless, areas in yellow and purple color indicate two types of crystal orientation of CaSi 2 .Otherwise, the pole figures of ( )