Surface Of Si Particles Research Articles

Building an Elastic Mechanical Network for Highly Efficient Silicon‐Based Anodes

AbstractSilicon (Si), known for its high theoretical capacity, holds promise for developing high‐energy‐density lithium‐ion batteries (LIBs). However, the intrinsic low conductivity and substantial volume expansion limit its commercial application. Developing a binder that can suppress the massive volume expansion while improving Si conductivity is particularly important for LIBs based on Si anode. Here, grafting natural adhesive cationic guar gum (CGG) with small molecular acrylamide (AM) is proposed and applied it to the Si‐based anode. The rigid guar gum main chain and the elastic polymerized acrylamide (pAM) side chain forms a 3D network that completely encapsulated Si nanoparticles and cross‐linked with the natural hydroxyl groups on the surface of Si particles through abundant functional groups. Numerous link anchors and robust mechanical networks facilitate lithium‐ion transport and effectively mitigated the significant stress generated by silicon during charge and discharge cycles. Based on this innovative design, the Si@CGG@pAM@SP (Super P) electrode demonstrates excellent long‐term cycle stability and rate performance. The Si@CGG@pAM@SP electrode preserves 2086 mAh g−1 discharge capacity after 100 cycles (corresponding to 91.5% capacity retention). Physical characterization and electrochemical tests confirm the rationality of the polymer binder design, serving as a valuable reference for the material design of lithium‐ion batteries.

Engineering a Low-Strain Si@TiSi2@NC Composite for High-Performance Lithium-Ion Batteries.

The huge volume expansion/contraction of silicon (Si) during the lithium (Li) insertion/extraction process, which can lead to cracking and pulverization, poses a substantial impediment to its practical implementation in lithium-ion batteries (LIBs). The development of low-strain Si-based composite materials is imperative to address the challenges associated with Si anodes. In this study, we have engineered a TiSi2 interface on the surface of Si particles via a high-temperature calcination process, followed by the introduction of an outermost carbon (C) shell, leading to the construction of a low-strain and highly stable Si@TiSi2@NC composite. The robust TiSi2 interface not only enhances electrical and ionic transport but also, more critically, significantly mitigates particle cracking by restraining the stress/strain induced by volumetric variations, thus alleviating pulverization during the lithiation/delithiation process. As a result, the as-fabricated Si@TiSi2@NC electrode exhibits a high initial reversible capacity (2172.7 mAh g-1 at 0.2 A g-1), superior rate performance (1198.4 mAh g-1 at 2.0 A g-1), and excellent long-term cycling stability (847.0 mAh g-1 after 1000 cycles at 2.0 A g-1). Upon pairing with LiNi0.6Co0.2Mn0.2O2 (NCM622), the assembled Si@TiSi2@NC||NCM622 pouch-type full cell exhibits exceptional cycling stability, retaining 90.1% of its capacity after 160 cycles at 0.5 C.

Investigation of the oxidation mechanism of Si-Al2O3-fused mullite containing Ti2O3 composite

In this work, Si-Al2O3-fused mullite containing Ti2O3 composite was prepared and heat-treated at 1100 °C, 1300 °C and 1500 °C for 3 h respectively in an air atmosphere to explore its oxidation mechanism. The results show that the Si-Al2O3-fused mullite containing Ti2O3 composite has excellent oxidation resistance at evaluated temperatures. The reason for this is the formation of a complete oxide layer and a transition layer in the exterior to absorb and filter O2, allowing the interior without oxidation. The oxidation of Si and Ti2O3 in the exterior is accompanied by volume expansion. SiC and mullite are formed in the transition layer at 1300 °C. At 1500 °C, SiO2, mullite and Si2N2O are formed in the transition layer. O′-Sialon and Ti(C, N)ss are formed in the interior of the samples at both 1300 °C and 1500 °C. At 1100 °C, Si oxidizes to form SiO2 coating on the surface of Si particles. The industrial CT characterization of the pore evolution of the samples after oxidation is consistent with the oxidation mechanism. A reaction model is developed.

A new strategy for de-oxidation of diamond-wire sawing silicon waste via the synergistic effect of magnesium thermal reduction and hydrochloric acid leaching.

With the rapid development of photovoltaic industry, the amount of diamond-wire sawing silicon waste (DWSSW) increased dramatically. Recently, how to achieve cost-effective deoxygenation of DWSSW should be the most critical issue in silicon waste recycling. Herein, a new strategy for de-oxidation from DWSSW, employing Mg vapor as de-oxidation agent and hydrochloric acid (HCl) as leaching agent, under low temperature was suggested. The de-oxidation mechanism and the mineralogical evolution of different phases during the magnesium thermal reduction and acid leaching were clarified. Results showed that the impurity oxygen in DWSSW exists as amorphous SiO2 on the surface of Si particles, which can be eliminated by transforming it into crystalline MgO using magnesium thermal reduction and followed by acid leaching. As the reduction condition was controlled at 1333 K for 2 h, the residual oxygen content in DWSSW was 0.9643% and the de-oxidation fraction attained 93%. Additionally, the removal rate of impurities Al and Fe were 99.06% and 92.20%, respectively, and the Si recovery rate achieved 88%. Taking into consideration the recoverability of Mg, the cost of the process will be reduced significantly, making this process more viable in industry.

A Stable Core-Shell Si@SiOx/C Anode Produced via the Spray and Pyrolysis Method for Lithium-Ion Batteries.

In the critical situation of energy shortage and environmental problems, Si has been regarded as one of the most potential anode materials for next-generation lithium-ion batteries as a result of the relatively low delithiation potential and the eminent specific capacity. However, a Si anode is subjected to the huge volume expansion–contraction in the charging–discharging process, which can touch off pulverization of the bulk particles and worsens the cycle life. Herein, to reduce the volume change and improve the electrochemical performance, a novel Si@SiOx/C anode with a core–shell structure is designed by spray and pyrolysis methods. The SiOx/C shell not only ensures the structure stability and proves the high electrical conductivity but also prevents the penetration of electrolytes, so as to avoid the repetitive decomposition of electrolytes on the surface of Si particle. As expected, Si@SiOx/C anode maintains the excellent discharge capacity of 1,333 mAh g−1 after 100 cycles at a current density of 100 mA g−1. Even if the current density reaches up to 2,000 mA g−1, the capacity can still be maintained at 1,173 mAh g−1. This work paves an effective way to develop Si-based anodes for high-energy density lithium-ion batteries.

In-situ X-ray investigation of growth behavior of primary silicon particles and aluminum dendrites during solidification of a hypereutectic Al-Si-Cu alloy

The solidification behavior of hypereutectic Al-18 wt%Si-10 wt%Cu alloy was investigated using in-situ X-ray radiography. Sheet-like samples were solidified both under isothermal cooling conditions and in a moderate temperature gradient with different process parameters. The real-time observation method allows for the investigation of the growth of faceted primary Si particles and non-faceted Al dendrites. Cooling down below the liquidus temperature, firstly primary silicon particles nucleate and grow. At low cooling rates just a few Si particles nucleate and grow continuously to large sizes, whereas at higher cooling rates a larger number of smaller Si particles appear. As cooling continues below the nucleation temperature of aluminium, simultaneous growth of both columnar and equiaxed Al dendrites in the melt surrounding the primary Si particles was observed. In-situ observation as well as post-mortem analysis of the solidification morphology demonstrated that nucleation and growth of Al dendrites in the melt was nearly independent of existing Si particles and no indication was found that the Al phase starts growing from the surface of Si particles. The growth of the equiaxed Al dendrites can be explained by the thermal undercooling controlled by the application of a constant and continuous cooling, but the reason for the fast growing columnar Al dendrites in an isothermal melt is still not clear.

Characterization and strengthening effects of different precipitates in Al–7Si–Mg alloy

The microstructure of the nanoscale precipitates (especially the β phase) formed in the primary a-Al grains of a cast Al–7Si–Mg alloy during ageing at different temperatures, and the strengthening effects of them on the mechanical properties have been systematically investigated by transmission electron microscopy (TEM) and tensile test. The interactions of dislocations with different types of precipitates were analyzed in depth. It was found that the GP zones and β″ phase can be easily sheared by dislocations along the close-packed {111}Al planes, leading to good deformation compatibility and elongation. The step-like planar defects were found in the deformed β″ phase, indicating that this needle-shaped precipitate can be sheared not only along the short axis but also along the long axis. The β phase and nanoscale Si particles are usually bypassed by dislocations, showing a typical Orowan strengthening mechanism, leading to the highest strength. This may be related to their large size (about 100 nm in diameter), high shear modulus and modest number density. In addition, the moiré fringes and FFT pattern of β phase were discussed by calculation of diffraction vectors. The moiré fringes with about 2.389 nm and 2.664 nm spacing displayed in HRTEM image results from the interactions between the 004Al/006β and 2¯22Al/3¯33β diffraction respectively. The corresponding complex FFT pattern is caused by strong double diffraction of [110]β and [110]Al reflections. Furthermore, an interesting phenomenon that the β phase tends to nucleate and grow on the surface of Si particle was firstly observed and discussed in depth.

Multivalent Amide-Hydrogen-Bond Supramolecular Binder Enhances the Cyclic Stability of Silicon-Based Anodes for Lithium-Ion Batteries.

A supramolecular polymer, poly(N-acryloyl glycinamide) (PNAGA), with a bisamide group on each side of the chain forming multiple amide-hydrogen bonds was synthesized in this work as a binder for silicon (Si)-based anodes. This supramolecular polymer binder with improved mechanical properties presents good interfacial adhesion with Si particles forming hydrogen bonds and enhances the adhesive strength between the electrode material film and the copper current collector. Benefiting from the highly stable inter- and intramolecular multiple amide-hydrogen bonds of the PNAGA binder, the electrode structure maintains integrity and a stable solid electrolyte interphase (SEI) layer is formed on the surface of Si particles. The effect of different binders on the composition of the SEI film was also investigated by X-photoelectron spectroscopy (XPS) characterization. In comparison with polyacrylamide (PAM), which has a similar structure to PNAGA, and the traditional sodium alginate (SA) binder, the Si electrode containing the PNAGA binder shows improved electrochemical performance. The capacity retention is 84% after 100 cycles at 420 mA g-1, and the capacity remains at 1942.6 mAh g-1 after 400 cycles at 1260 mA g-1. Even with a mass loading of 1.2 mg cm-2 Si, the electrode with the PNAGA binder exhibits high initial areal capacity (2.64 mAh cm-2) and good cycling performance (81% capacity retention after 50 cycles). Moreover, the application of the PNAGA binder also brings a stable cycle performance to the commercial Si-graphite (SiC) anode material.

Recycling silicon from silicon cutting waste by Al–Si alloying

Abstract Large amounts of silicon cutting waste (SCW) will be generated as the photovoltaic (PV) industry grows, which not only wastes valuable resources, but also causes environmental issues if SCW is not addressed. In this study, a novel and effective method to recycle Si from SCW by Al–Si alloying with a cryolite additive is proposed. The influence of the smelting parameters including smelting temperature, holding time and cryolite/SCW ratio, on the Si content of alloys and the utilization ratio of SCW was investigated and optimized. The optimal conditions were a smelting temperature of 1150 °C, holding time of 30 min and cryolite/SCW ratio of 6.3. Under these optimized conditions, the Si content of alloys and the utilization ratio of SCW were 12.02 wt% and 64.25%, respectively. The characterization analysis results of the products showed that Al–Si alloys with a uniform distribution of Si were successfully prepared. Furthermore, the mechanism of the alloying process was studied. Cryolite additives can efficiently dissolve the SiO2 film on the surface of Si particles and hence contribute to the subsequent transfer of the exposed Si to molten Al. The study of the slag-metal interface demonstrates that Si is recycled with a highly efficiency from SCW. Despite the relatively large amount of cryolite, the generated slag can be recycled and used as an additive for the next batch of experiments. This method not only recycles silicon scrap for use as valuable raw materials, but also reduces the production cost of Al–Si alloys, and thus, this method is a feasible way to commercially recycle SCW in an economically and environmentally friendly manner.

Interconnected structure Si@TiO2-B/CNTs composite anode applied for high-energy lithium-ion batteries

As an anode with ultrahigh capacity (4200 mAh g−1), silicon (Si) has inherent defects, such as drastic volume change (~300%) and poor conductivity. To cope with these problems, constructing a rigid shell (e.g. TiO2) on the surface of Si particles is an effective method. Typically, monoclinic titanium dioxide (TiO2-B) has the more excellent electrochemical performance among the various crystalline of TiO2 (anatase, rutile, etc.) for its open channel and characteristic pseudocapacitive effect. In addition, carbon nanotubes (CNTs) are good conductor of electricity. It is hopeful to increase the conductivity of the Si anode by adding CNTs. In this work, therefore, a Si@TiO2-B/CNTs composite is prepared by hydrothermal method. The composite exhibits superior electrochemical properties during the half-cell test (1184 mAh g−1 under 0.1 A g−1 at 200th cycles). Furthermore, the electrochemical kinetics of the materials were studied by cyclic voltammetry, which demonstrated that coating TiO2-B could increase the capacitive effects of the composite (68.75%).

A Modified Natural Polysaccharide as a High-Performance Binder for Silicon Anodes in Lithium-Ion Batteries.

A modified natural polysaccharide (carboxymethylated gellan gum) is investigated as a water-soluble high-performance binder for silicon anodes in lithium-ion batteries to improve poor cycle life and fast capacity fade of silicon anodes due to dramatic volume expansion during lithiation/delithiation process. The numberof carboxyl and acetyl groups distributed homogeneously in the modified polysaccharide polymer chain can form strong hydrogen bonds with the surface of Si particle and copper current collector, thus effectively restricting the volume change of silicon and maintaining electronic integrity of Si electrodes during repeated charge/discharge cycles. As a result, Si anodes with carboxymethylated natural polysaccharide polymer present high capacity performance, excellent rate capability, and stable cycling.

Healing of Micro-Cracks in Silicon-Aluminum Anodes for Lithium-Ion Cells: Role of Silicon Particle Size and Electrolyte Composition

Secondary lithium-ion batteries are attractive energy storage devices with high gravimetric and volumetric capacity and the ability to deliver high rates of power. But the constantly increasing power requirements for pure and hybrid electric vehicles has challenged the existing lithium-ion battery technology, and that prompted intense interest in developing battery electrodes with still higher energy densities. As a high capacity anode and also capable of sustaining high power rates, Si has attracted attention due to its highest known theoretical charge capacity (4,200 mAhg-1 for Li4.2Si). Although the theoretical specific capacity of Si is more than four times higher than the Sn (991 mAhg-1 for Li4.4Sn) and ten times higher than the graphite (372 mAhg-1 for LiC6), Si has limited applications. During electrochemical cycling in Li-ion batteries, Si anodes undergo volume changes that results in fracture and fragmentation of Si. In this work, we have considered Si electrodes as components in a Si-Al “composite” where micron-sized Si particles are embedded in a ductile Al matrix. Formation and closure of micro-cracks that caused their healing in Si particles was observed in anodes fabricated from Si-Al alloys when electrochemically cycled in ethylene carbonate (EC)- and propylene carbonate (PC)-based electrolytes (vs. Li/Li+). In-situ optical microscopy performed during electrochemical experiments indicated the development of a network of cracks in Si particles during lithiation as the voltage decreased from 1.50 V to 0.02 V (Fig.1). Cycling in EC-based electrolyte promoted formation of a constantly growing and non-uniformly distributed electrolyte reduction products on Si (Fig.1), whereas in presence of PC, more damage of Si occurred by loss of material. Cross-sectional high-resolution transmission electron microscopy (HR-TEM) indicated that the SEI formed on Si consisted of LiCl, Li2O2 and Si nano-fragments that were possibly comminuted from the Si surfaces. Optical profilometry and SEM observations were used to discuss the changes in electrode surface morphology, and FIB results revealed the evolution of Si subsurface microstructure due to cycling. The Al matrix stopped propagation of the cracks in Si. Al, having a high fracture toughness of 21 MPa.m0.5, exerted a larger resistance to crack advance than Si that has a fracture toughness of 1 MPa.m0.5 (Fig.1). In-situ micro-Raman spectroscopy and HR-TEM revealed that an amorphous layer of a depth of ~100 nm formed at the surface of Si particles. Crevices were formed in this amorphous layer due to propagation of compressive shear cracks. The crevices also acted as stress concentrators to initiate the cracks in the crystalline interior. The crack surfaces also provided easy paths for Li-ion diffusion and the intercalated Li-ions formed amorphous regions ~ 5-6 nm wide on each side of the cracks. The amorphous zones having an increased volume (2.4%) applied compressive stresses causing closure of cracks as the voltage increased from 0.02 V to 4.00 V during de-lithiation. The role of the crack formation and healing on Si particle size during lithiation and de-lithiation will be discussed. It was observed that the self-healing occurred in two steps; arresting of micro-cracks at the Si/Al interfaces, and closure of the cracks at the end of the electrochemical cycle. The observation of the self-healing process should have technological implications in development of this type of composite Si particulate electrodes, as one of the major problems in monolithic Si electrodes, pulverization, could be alleviated in this way. Therefore, the results are significant for battery technology. Composite materials containing high capacity Si particles and a ductile phase around them could, therefore, provide durable electrodes for Li-ion batteries and enhance the capacity retention capability leading to prolonged battery life. Figure 1

Layered Si-C Nanocomposites Synthesized from Layered Silicate As High Performance Anodes for Lithium-Ion Batteries

1. Introduction Lithium Ion Batteries (LIB) are already widely used for small potable electronics (cellular phone, lab top computer, MP3 etc.) as a main energy part due to light weight, good energy storage rate, and long term stability (cyclaility and durability). Existing LIBs use graphite as an anode material. This carbon material has high cycle stability and electrical conductivity. However, the specific lithiation capacity (370 mA h g-1) of the graphite can’t satisfy the demands for higher energy density batteries. Silicon (Si) has emerged as a promising material to produce high capacity anodes for LIBs, due to its high theoretical capacity (~4200 mA h g-1) which is 10 times higher than that of graphite. However, Si anodes for LIBs have a disadvantage from the large volume change. The expanding to 3-4 times its original size often break the electrical contacts with current collector. Therefore, we focus on making ideal particle size and effective structure of Si. Layered Si particles are prepared by kanemite as a raw material, followed by magnesiothermic reduction. And then, the phenol-formaldehyde liquid resin pyrolyzed carbon coated on the surface of Si particles and inserted into layered Si inner space. In this research, This novel anode active materials exhibit a high reversible specific capacity and good capacity retention. 2. Materials and Method The natural clays with layered Na+ silicates are used in constructing the structure of the silicon layers. A layered silicate kanemite (NaHSi2O5ㆍ3H2O) was prepared by dispersing δ-Na2SiO5 (1.0g) in deionized water (100mL) with stirring for 1h. After that, the kanemite was filtrated, washed, and then dried. The kanemite powder (200mg) was mixed with Mg powder (200mg), and calcined for magnesiothermic reduction at 650°C under Ar atmosphere for 5h. 0.1 M of HCl solution remove redundant Mg, MgO and Mg2Si. We used HF and ethanol solution for remove remained SiO2 in the layered silicon particles. 200 mg of synthesized layered silicon particles, 100ml of Phenol-Formaldehyde (PF) liquid and 1ml of HCl were dispersed in the mixture by ultrasonication for 2h. The mixed solution was filtrated with 0.2mm size of membrane filter. During the filtrate process, dispersed silicon thin plates are stacked like bricks. The filter and mixture become phenolic resin structure after heated at 128°C for 12h. Finally, the silicon-phenolic resin solid compound was calcined at 700°C for 5h in Ar atmosphere in order to be layered Si-C nanocomposites. For the characteristic comparison, we prepare commercial original Si nanoparticles (40~60nm). 3. Results and discussion As shown in Fig. 1a,b, the PF resin pyrolized carbon and carbon black showed constant reversible capacity of 170mA h g-1 and 150mA h g-1 respectively. The layered Si and commercial Si particle showed a first reversible capacity of almost 1100mAhg-1 and 1200mAhg-1 in Fig. 1c,d. however, the reversible capacity in both samples were less that 100mAhg-1 after 25 cycles. On the other hand, the reversible capacity on the layered Si-C nanocomposite is still as high as 792mA h g-1 after 25 cycles at a rate of 100mAg-1 as shown in fig. 1e. This means capacity retention is 90% because the first reversible capacity was 875 mA h g-1. Fig. 1f result indicating that the layered Si-C nanocomposite has an excellent cycling performance. One possibility of the improved performance maybe accounted to the fact that the thin layer structures of nano silicon may have less volumetric changes and restrain the pulverization of the electrode materials during lithium ion insertion and extraction. Another reason for the enhanced behavior could be assigned to the carbon, which can improve the electrical conductivity of the composite and suppress the polarization. 4. Conclusions In summary, layered Si plates were prepared by the means of magnesiothermic reduction of Kanemite. And then, layered Si-C nanocomposites were successfully synthesized by dispersing layered silicon particles in a liquid phenol-formaldehyde followed by a subsequent pyrolysis process. This process enables a strong contact of the silicon layers and carbon sources. The layered Si-C nanocomposites exhibit an initial reversible capacity of 875mAhg-1 and a high retention of 90% after 25cycles at a current density of 100mAg-1. The highly enhanced electrochemical performance of layered Si-C nanocomposite maybe attributed to the intercalated and coated carbon on the composites which suppress the volume change of Si upon cycling, prevent the pulverization of the electrode, and increase the electrical conductivity of nanocomposite. The extremely thin Si layer (<1nm) will also provide better strength than general Si particles from the volume change, as same function as the Si thin film. Figure 1

Self-healing of cracks formed in Silicon-Aluminum anodes electrochemically cycled at high lithiation rates

Abstract Lithiation-induced volume changes in Si result in fracture and fragmentation of Si anodes in Li-ion batteries. This paper reports the self-healing behaviour of cracks observed in micron-sized Si particles dispersed in a ductile Al matrix of a Si-Al electrode electrochemically cycled vs. Li/Li + using a high lithiation rate of 15.6 C. Cross-sectional high-resolution transmission electron microscopy and Raman spectroscopy revealed that an amorphous layer with a depth up to ∼100 nm was formed at the surface of Si particles. In-situ optical microscopy performed during electrochemical experiments revealed development of cracks in Si particles as the voltage decreased to 0.02 V during lithiation. Self-healing of cracks in Si particles occurred in two steps: i) arresting of the crack growth at the Si/Al interface as the surrounding Al matrix had a higher fracture toughness and thus acted as a barrier to crack propagation, and ii) closure of cracks due to compressive stresses applied to the crack faces by the amorphous zones formed on each side of the crack paths.

Invited: Critical Role of Si Nanoparticles Surface on Lithium Cell Electrochemical Performance

The surface of Si particles is strongly involved in different phenomena of crucial importance for the electrochemical performance of lithium batteries with Si-based composite electrodes. Indeed, Si electrodes suffer from poor cyclability due to the large volumetric expansion of Si particles upon cycling. The successive swelling and shrinkage upon alloying/dealloying of the stack of Si particles within the confined space of the electrochemical cell leads to major geometrical change of granular texture with a vast redistribution of inter-particle contacts, the formation of cracks within the composite electrode and loss of electrical contacts at the current collector interface. The repetition of all these processes with frictional sliding of the particles leads to a damaging of the SEI layer that exposes again the Si particles surface to the liquid electrolyte. It results in an irreversible capacity loss at each cycle by the reduction at low potential of some liquid electrolyte on the exposed surface [1]. Silicon is always recovered by a native silicon oxide layer. An electrochemical reduction of the SiO2 surface layer occurs at the first cycle with the formation of lithium silicate, Li4SiO4, and Li2O as degradation products as proven by XPS [2]. Moreover, the silicon oxide layer is composed of hydroxylated silanol (SiOH) groups which likely dramatically increases the reactivity compared to the dehydroxylated siloxane phase (Si-O-Si) [3]. As a consequence various efforts have been made to reduce the thickness of SiO2 layer on silicon nanoparticles to decrease the first irreversible capacity and to enhance electrochemical performance. Although the surface of Si particles is playing a major role in the electrochemical performance, it has rarely been characterized in depth. A combination of techniques is here used to finely describe the surface of Si nanoparticles: Diffuse Reflectance Infrared Fourier Transform (DRIFT) spectroscopy combined with TGA, Raman spectroscopy, solid-state high-resolution Nuclear Magnetic Resonance (MAS NMR), TEM coupled with Electron Energy Loss Spectroscopy (EELS), X-Rays Photoelectron Spectroscopy (XPS), and finally Broad Band Dielectric Spectroscopy (BDS). With respect to previous works and common belief, we demonstrate, on the electrochemical performance, a favorable effect of a particular thin layer silicon oxide with a well-defined SiO2 composition at its extreme surface.

Synthesis and Structural Characterization of Core-Shell Si-SiC Composite Particles by Thermal Plasma In-Flight Carburization of Silicon Powder

Unique core-shell-structured micron spheres of Si-SiC consisting of a layer of silicon carbide nanocrystallites and a silicon single-crystal core were synthesized by thermal plasma in-flight carburization of silicon micron powders in Ar-H2-CH4 plasma. Raman spectroscopy and energy dispersive X-ray spectroscopy (EDX) revealed the formation of silicon carbide particles on the surface of Si particles, and transmission electronic microscopy (TEM) showed a uniform distribution of SiC nano-sized grains (~80 nm) at a depth of 500 nm beneath the surface of the composite spheres. Micron voids and nano pores were observed at the SiC-Si interface and in the Si matrix. The formation of long and thick SiC nanotubes was observed in the Si matrix near the SiC-Si interface. The mechanism of carburization reactions and the growth of silicon carbide layers and SiC nanotubes are discussed.

Preparation of Si-carbon nanotube composite by decomposition of tetramethylsilane (TMS) and its hydrogen storage property

Hydrogen storage property of Si–carbon nanotube composite was studied. It was found that the carbon nanotube coated Si particle was prepared by the decomposition of tetramethylsilane by using Ni catalyst. The obtained composite was consisted of the spherical Si particle with 300 nm diameter and the surface of Si particle was coated with carbon nanotube with 10 nm diameter. The obtained Si–carbon nanotube composite shows a fairly large hydrogen storage capacity of 2.5 wt%. The amount of storage H2 increased with increasing the preparation temperature and the largest hydrogen storage amount could be achieved at 1273 K for 6 h. Fairly large hydrogen storage capacity could be sustained after 3 cycles of adsorption and desorption.

In Situ HRTEM Observation of Solid-Liquid Interfaces

The processes of melting and freezing of alumina, Al and Al-Si alloy have been observed directly in a conventional transmission electron microscope (CTEM). The solid-liquid and solid-solid interfaces in these materials have been observed at an atomic level. The liquid phase nucleated preferentially at the surfaces of the Al particles at the melting. The liquid phase then propagated into the interior of the Al particles at the expense of the solid. The solid-liquid interface of alumina showed the atomically flat interface. The layer-by-layer growth of alumina on the solid-liquid interface was observed. The solid-liquid interface in an Al-Si alloy was atomically flat, while the solid-solid interface was atomically rough. When Al melted, the surface of Si particle showed characteristic reconstructed structures.

Sintering Behavior of Green Compacts of Si and B&lt;sub&gt;4&lt;/sub&gt;C Mixed Powder in Nitrogen

The Sintering Behavior of powder mixture compacts of Si and B4C in nitrogen atmosphere at 1350°C was investigated. Mainly, Si was changed into α-Si3N4 and β-Si3N4 by a reaction with nitrogen, and B4C was changed into BN and C, and C was converted into β-SiC by a reaction with Si. BN seemed to have an effect on reducing the number of pores in the nitrided body. B4C retarded nitridation of Si. An Si-B-C-N-O layer and BN were formed on the surface of unnitrided Si particle in the nitrided body. An SiO2-B2O3 layer formed on the surface of the Si particle in the nitriding stage seemed to retard nitridation of Si.