Si Matrix Research Articles

Microstructure and oxidation resistance of a Ti–Mo–Nb–Al–Si titanium matrix composite reinforced with in situ TiC prepared by powder metallurgy

The mixture of the Ti–Mo–Nb–Al–Si matrix powder and Carbon nanotubes (CNTs) particles was used as the starting powder to produce the in-situ TiC reinforced TMCs using reaction synthesis method. The effect of the adding various CNTs content on the microstructure, phase composition, hardness, and oxidation behavior of the composites was investigated. The results show that the phases of the composites were composed of mainly α-Ti, β-Ti and TiC. With the increase of adding CNTs content, the porosity increased gradually, and the hardness initially increased and then decreased. The hardness of the composites with adding 1.0 wt% CNTs content was 496.1 HV, which was about 13.89% higher than that of the matrix alloy. With the increase of adding CNTs content, the oxidation mass gain first decreased and then increased. The oxidation mass gain of composites with adding 1.0 wt% CNTs content was 2.212 mg cm−2, which was about 63.33% less than that of the matrix alloy, and the parabolic rate constant (kp) was the minimum, which was 1.67 × 10−3 mg2 cm−4 h−1. The oxidation resistance of the composites was better than the matrix alloy.

Formation of Crystallographically Oriented Metastable Mg1.8Si in Mg Ion-Implanted Si

Metastable hexagonal Mg–Si was synthesized by implanting Mg ion into Si. Single-crystalline Si(111) was irradiated with Mg ions at elevated temperature followed by thermal annealing under vacuum. Microstructural analysis with transmission electron microscopy revealed the formation of precipitates with sizes of several 10 nm in the damaged crystalline Si matrix. Using electron diffraction, the precipitates were identified as Mg1.8Si, and the crystallographic orientation relationship between Mg1.8Si precipitates and Si was determined. The phase stability of Mg1.8Si in the Si matrix is discussed in terms of the internal stress generated during the precipitation process.

Experimental analysis of hybrid metal matrix composite reinforced with Al2O3 and graphite

Recently the development and use of metal matrix composites. instead of conventional metals, has led to manufacturing cost optimisation to meet various light weight to strength. The MMC has several applications in the field of automotive and aerospace sectors. The paper presents the results of tribological research on Al–Si, Al2O3 and graphite-aluminium metal matrix composite material. It analyses the mechanical properties and compares among themselves with different weight fraction samples. This composite is used as high-tech construction material in many applications, particularly for power to weight applications like pistons, cylinder liners, and bearings. The tribological properties of these materials can be significantly changed by the introduction of graphite, silicon carbide particles. The wear tests were carried out using the pin-on-disc tribometer and analysed by Ducom software. The sliding and wear resistance of the matrix and composite alloy specimens have been subjected to comparative analysis. It was demonstrated that the composite reinforced with graphite is characterised by a lower friction coefficient and lower wear rate was achieved by adding silicon carbide to the matrix composite.

Effect of interaction between impurity elements on silicon purification during Al–Si solvent refining

Al–Si solvent refining is a promising method to purify silicon. Boron and phosphorus are key impurity elements during the purifying process. The common raw materials for Al–Si solvent refining are metallurgical grade silicon and industrial aluminum. There are many impurity elements except boron and phosphorus in the metallurgical grade silicon and the industrial aluminum. The effect of these impurity elements on boron and phosphorus removal during Al–Si solvent refining is studied in this work. The hypereutectic Al–Si melts with these impurity elements were solidified, and the hypereutectic Al–Si melts without these impurity elements were solidified at the same cooling rate. Boron and phosphorus contents in purified silicon are determined by inductively coupled plasma optical emission spectrometer (ICP-OES). It is found that when silicon ratio increases from 20% to 70% in samples without other impurity elements, boron removal rates decrease from 0.4 to 0.3, and phosphorus removal rates decrease from 0.96 to 0.92. When silicon ratio increases from 20% to 40% in samples with other element impurities, boron removal rates decrease from 0.87 to 0.57, and phosphorus removal rates increase from 0.63 to 0.9. The different removal rates between samples with and without other impurity elements are attributed to the interaction between impurity elements. In addition, boron-containing intermediate compound forms and locates in the Al–Si matrix, while phosphorus-containing intermediate compound forms and locates in the primary silicon phase.

Metal-Organic Frameworks-Derived Mesoporous Si/SiOx @NC Nanospheres as a Long-Lifespan Anode Material for Lithium-Ion Batteries.

Silicon (Si)-based anode materials with suitable engineered nanostructures generally have improved lithium storage capabilities, which provide great promise for the electrochemical performance in lithium-ion batteries (LIBs). Herein, a metal-organic framework (MOF)-derived unique core-shell Si/SiOx @NC structure has been synthesized by a facile magnesio-thermic reduction, in which the Si and SiOx matrix were encapsulated by nitrogen (N)-doped carbon. Importantly, the well-designed nanostructure has enough space to accommodate the volume change during the lithiation/delithiation process. The conductive porous N-doped carbon was optimized through direct carbonization and reduction of SiO2 into Si/SiOx simultaneously. Benefiting from the core-shell structure, the synthesized product exhibited enhanced electrochemical performance as an anode material in LIBs. Particularly, the Si/SiOx @NC-650 anode showed the best reversible capacities up to 724 and 702 mAh g-1 even after 100 cycles. The excellent cycling stability of Si/SiOx @NC-650 may be attributed to the core-shell structure as well as the synergistic effect between the Si/SiOx and MOF-derived N-doped carbon.

Characterization of mechanically alloyed and pressureless sintered Al-7 wt% Si-2 wt% LaB6-2 wt% (MoSi2, WSi2) hybrid composites

The purpose of this research is to investigate the dual effect of silicide (MoSi2 or WSi2) and LaB6 reinforcing particles on the microstructural and mechanical properties of Al-7 wt% Si (Al7Si) matrix. Hypoeutectic Al7Si blends prepared from elemental Al and Si powders were mechanically alloyed (MA’d) for 12 h in a planetary ball mill (at 300 rpm). Afterwards, 2 wt% silicide reinforcements (MoSi2 or WSi2) with various particle size distributions (micron, bimodal, submicron) were separately added into these MA’d Al7Si powders together with 2 wt% of LaB6 particles. Powders having compositions of Al7Si, Al7Si-2 wt% LaB6, Al7Si-2 wt% LaB6-2 wt% MoSi2 and Al7Si-2 wt% LaB6-2 wt% WSi2 were milled for 30 min using a high-energy ball mill (at 1200 rpm) in order to obtain homogeneous distribution throughout the microstructure. Compositional, microstructural and mechanical characterization studies were performed on the sintered samples. The results showed that high-energy ball milling ensured the homogeneous distribution of micron-sized MoSi2 and WSi2 particles within the matrix rather than those of bimodal and submicron-sized ones. Micron-sized MoSi2 and WSi2 reinforced hybrid composites displayed dramatically higher hardness and wear resistance than those of composites reinforced with different size of MoSi2 and WSi2 particles. The striking point of the study was the remarkably higher hardness and wear resistance properties of the hybrid composites compared to those of un-reinforced and only LaB6-reinforced ones. As a conclusion, hybrid composites extremely displayed promising mechanical properties.

A new approach for improving the elevated-temperature strength and ductility of Al–Cu–Mg–Si alloys with minor amounts of dual-phased submicron/nanosized TiB2–TiC particles

At high temperatures, the weakened strength and decreased ductility of Al alloys limit their industrial applications. (TiB2–TiC)/Al–Cu–Mg–Si composites were synthesized by adding in situ dual-phased and bimodal-sized (TiB2–TiC)/Al master alloys to molten Al–Cu–Mg–Si alloys. The addition of minor amounts of TiB2 and TiC particles (0.05 and 0.1 wt%, respectively) effectively refined the α-Al grains and θ′ and Q′ precipitates and increased the elevated-temperature strength and uniform elongation of the Al–Cu–Mg–Si alloys. The 0.1 wt% (TiB2–TiC)/Al–Cu–Mg–Si composite showed the best yield strength (279 MPa), ultimate tensile strength (366 MPa), and uniform elongation (10.6%) at 493 K, with enhancements of 9.4%, 15.1%, and 24.7% compared to the Al–Cu–Mg–Si matrix alloy (255 MPa, 318 MPa, and 8.5%). The simultaneous increases in the elevated temperature strength and ductility of the composites were attributed to the strengthening effects of the bimodal-sized TiB2 and TiC particles, θ′ and Q′ precipitation strengthening, and the refined partial recrystallization microstructure.

Surface Effects and Optical Properties of Self-Assembled Nanostructured a-Si:Al.

We present a study of the surface effects and optical properties of the self-assembled nanostructures comprised of vertically aligned 5 nm-diameter Al nanowires embedded in an amorphous Si matrix (a-Si:Al). The controlled (partial) removal of Al nanowires in a selective etching process yielded nanoporous a-Si media with a variable effective surface area. Different spectroscopy techniques, such as X-ray photoelectron spectroscopy (XPS), UV-Vis spectrophotometry and photoluminescence (PL), have been combined to investigate the impact of such nanostructuring on optical absorption and emission properties. We also examine long-term exposure to air ambient and show that increasing level of surface oxidation determines the oxide defect-related nature of the dominant PL emission from the nanoporous structures. The role of bulk, nanosize and surface effects in optical properties has been separated and quantified, providing a better understanding of the potential of such nanoporous a-Si:Al structures for future device developments.

Characterization Investigations of 2 wt% LaB6 Particulate-Reinforced Al-7 wt% Si Matrix Composites Fabricated by Powder Metallurgy Methods

In this study, first of all, sub-micron LaB6 reinforcing particles were synthesized in the size range of 100–500 nm via mechanochemical synthesis and leaching steps. Afterwards, Al-7 wt% Si matrix alloy and 2 wt% laboratory synthesized LaB6 powders were mechanically alloyed (MA’d) at different durations to constitute Al-7 wt% Si/2 wt% LaB6. Followed by compaction under a pressure of 450 MPa in a uniaxial cold press, samples were sintered at 570 °C for 2 h in flowing Ar gas atmosphere. Microstructural characterizations of the powders and sintered samples were performed using X-Ray Diffractometry (XRD) and Scanning Electron Microscopy (SEM) techniques. Effects of both MA (0, 1, 4 and 8 h) and LaB6 reinforcement (0 and 2 wt%) on the mechanical properties of the composites were determined in terms of microhardness and dry sliding tests. Both MA process and LaB6 reinforcement were found to make positive contribution to the hardness and wear resistance values. As-blended and sintered Al-7 wt% Si matrix alloy had a hardness value of 0.43 GPa. On the other hand, 8 h MA’d and sintered Al-7 wt% Si/2 wt% LaB6 composite had the highest hardness (0.97 GPa) and the 4 h MA’d one had the lowest wear rate (3.25 × 10−6 mm3/N mm) among all the sintered samples. The hardness and wear resistance values were increased by approximately 2 times in the 4 h MA’d and sintered Al-7 wt% Si-2 wt% LaB6 composite compared to its 1 h MA’d counterpart.

Si-Based Materials for Thermoelectric Applications.

Si-based thermoelectric materials have attracted attention in recent decades with their advantages of low toxicity, low production costs, and high stability. Here, we report recent achievements on the synthesis and characterization of Si-based thermoelectric materials. In the first part, we show that bulk Si synthesized through a natural nanostructuring method exhibits an exceptionally high thermoelectric figure of merit zT value of 0.6 at 1050 K. In the second part, we show the synthesis and characterization of nanocomposites of Si and metal silicides including CrSi2, CoSi2, TiSi2, and VSi2. These are synthesized by the rapid-solidification melt-spinning (MS) technique. Through MS, we confirm that silicide precipitates are dispersed homogenously in the Si matrix with desired nanoscale sizes. In the final part, we show a promising new metal silicide of YbSi2 for thermoelectrics, which exhibits an exceptionally high power factor at room temperature.

Meandering growth of in-plane silicon nanowire springs

Despite the fundamental difference in material systems and temporal evolution, self-oscillating growth of silicon nanowires (SiNWs), led by metal droplets, resembles very much natural river meanders in terms of their sinuosity, fractal dimensions, and scaling law. Both of them are driven by the release of higher potential energy stored in the disorder hydrogenated amorphous Si (a-Si:H) matrix or at highlands, tailored by a streamwise flow mechanism and subject to an erodible boundary constraint imposed by the a-Si:H thin film or the soil banks, respectively. Under specific conditions, the cross-droplet/stream velocity difference can be magnified, during the in-plane growth of SiNWs, to stimulate regular swaggering dynamics that produce continuous and smooth SiNW meanders. This interesting phenomenon indicates a rather simple and highly efficient strategy to shape complex elastic channels with only a few control parameters. A kinetic model has been established to explain the underlying mechanism of the self-oscillating meandering growth, which has unique potential to transform rigid SiNW channels into elastic forms for flexible or stretchable electronic applications.

Low‐frequency dielectric performance of polyimide composites containing amorphous polymer‐derived SiCN ceramics

AbstractHigh‐dielectric‐constant polymer‐matrix composites have potential applications as embedded capacitors. In this paper, an amorphous polymer‐derived SiCN ceramic was incorporated into a polyimide (PI) matrix by in situ polymerization for the first time. The resulting SiCN/PI composite showed good thermal stability below 500°C, with the SiCN filler being homogeneously dispersed in the PI matrix. The dielectric properties of the SiCN/PI composites were investigated over a broad frequency range (0.1‐106 Hz). The dielectric properties of the SiCN/PI composites at low frequencies revealed a weak percolation phenomenon similar to that shown by conductive/polymer composites and the calculated percolation threshold was 22.8 vol%. At SiCN loadings of 25 vol%, the SiCN/PI composites showed a high dielectric constant of 190 at 0.1 Hz, which is 57 times higher than that of pure PI (3.3). The conductivity of the 25 vol% SiCN/PI composite was as low as 8.5 × 10−10 S·cm−1. These results could be explained by the unique microstructure of the polymer‐derived SiCN ceramic, containing an amorphous Si matrix and a free carbon phase. This study revealed SiCN/PI composites as promising dielectric materials with promising applications in energy storage fields.

Effects of Cu-Coated SiC Content on Microstructure and Properties of Laser Cladding SiCp/Al⁻Si Composite Coatings.

SiC particles (SiCp)-reinforced Al–Si matrix composite coatings were synthesized on 4032 aluminum alloy by laser cladding using powder mixtures of Al-20 wt.% Si alloy and electroless copper-plated SiC particles (SiCp-Cu). The effects of SiCp-Cu content on microstructure, phase composition, and microhardness of the SiCp/Al–Si laser cladding layer (LCL) were investigated systematically. The results showed that the microstructure of SiCp-Cu/Al–Si LCL was mainly composed of undissolved SiCp, lump-like primary Si, lump-like Al2Cu, plate-like Al4SiC4, and Al–Si–Cu ternary eutectic. In addition, the eutectic microstructure became finer with the increasing of SiCp-Cu content. The average microhardness of the LCL increased with the increasing of SiCp-Cu content. When SiCp-Cu content was 50 wt.%, the average microhardness of the LCL reached 508 HV0.05, which was about 3.5 times larger than that of the substrate. The LCL reinforced with a SiCp-Cu content of 30 wt.% exhibits the best wear resistance.

Development of the ReaxFF Reactive Force Field for Inherent Point Defects in the Si/Silica System.

We redeveloped the ReaxFF force field parameters for Si/O/H interactions that enable molecular dynamics (MD) simulations of Si/SiO2 interfaces and O diffusion in bulk Si at high temperatures, in particular with respect to point defect stability and migration. Our calculations show that the new force field framework (ReaxFFpresent), which was guided by the extensive quantum mechanical-based training set, describes correctly the underlying mechanism of the O-migration in Si network, namely, the diffusion of O in bulk Si occurs by jumping between the neighboring bond-centered sites along a path in the (110) plane, and during the jumping, O goes through the asymmetric transition state at a saddle point. Additionally, the ReaxFFpresent predicts the diffusion barrier of O-interstitial in the bulk Si of 64.8 kcal/mol, showing a good agreement with the experimental and density functional theory values in the literature. The new force field description was further applied to MD simulations addressing O diffusion in bulk Si at different target temperatures ranging between 800 and 2400 K. According to our results, O diffusion initiates at the temperatures over 1400 K, and the atom diffuses only between the bond-centered sites even at high temperatures. In addition, the diffusion coefficient of O in Si matrix as a function of temperature is in overall good agreement with experimental results. As a further step of the force field validation, we also prepared amorphous SiO2 (a-SiO2) with a mass density of 2.21 gr/cm3, which excellently agrees with the experimental value of 2.20 gr/cm3, to model a-SiO2/Si system. After annealing the a-SiO2/Si system at high temperatures until below the computed melting point of bulk Si, the results show that ReaxFFpresent successfully reproduces the experimentally and theoretically defined diffusion mechanism in the system and succeeded in overcoming the diffusion problem observed with ReaxFFSiOH(2010), which results in O diffusion in the Si substrate even at the low temperature such as 300 K.

Investigation of wear and corrosion behavior of graphene nanoplatelet-coated B4C reinforced Al–Si matrix semi-ceramic hybrid composites

The present study aims to produce graphene nanoplatelet-coated B4C ceramic particle using semi-powder method and to investigate the effect of graphene nanoplatelets on wear and corrosion performance of Al–Si-based metal matrix hybrid composites. For this purpose, first graphene nanoplatelets at different ratios (0.25, 0.5, and 1 vol.%) were coated to the surfaces of B4C particles and then the Al–Si alloy was infiltrated into the reinforcements by gas pressure infiltration method. The characterization of graphene nanoplatelet-coated B4C powders and its composites was carried out by X-ray diffraction, differential scanning calorimetry, scanning electron microscope, and transmission electron microscope analysis. Tribological properties were investigated by reciprocating ball-on-flat method under three different loads (10–20–40 N) in a dry environment. The corrosion resistance was carried out with Tafel polarization method in 3.5% NaCl solution. Characterization results show that graphene coated on the B4C surface was successfully achieved by semi-powder method. After infiltration process, a new phase formation was not observed, but porosity increased with the increase of graphene content. When the boron carbide surface was coated with 0.5vol.% graphene, it was determined that the specific wear resistance increased by 55% and the corrosion resistance decreased by 12%.

Mechanical properties and microstructures of Al-10Mg-4.5Si matrix composites reinforced by carbon nanotubes

Carbon nanotubes (CNTs) reinforced Al-Mg-Si matrix composites were produced by powder metallurgy technique. The mechanical properties and microstructures of the composites were highly dependent on the alloy elements and the content of CNTs. Two types of precipitates were detected in the fabricated composites. It was found that the CNTs content has a series of effects on the microstructures of the fabricated composites, including densification, the dispersion condition of CNTs and interfacial bonding. Concurrently, the CNTs content also significantly affects the distribution of CNTs and oxygen content, which in turn affects the size, distribution and chemical composition of the precipitates during the composite fabrication process. Owing to the closely bonded interface, and uniform dispersion of CNTs and precipitates, the composite reinforced by 0.5 vol% CNTs achieved well balanced strength and ductility.

Understanding ion irradiation resistance of a silicon diffused nanostructured ferritic alloy-chromium carbide–carbon composite

Cladding materials play the essential role in the safe, efficient, and economic operation of a nuclear reactor. In this work, a silicon diffused nanostructured ferritic alloy-chromium carbide–carbon (NFA|Si-Cr3C2-C) composite is studied as a nuclear fuel cladding material and ion irradiation with highly energetic Fe++ ions has been carried out to induce a peak damage level of ∼250 dpa. Elemental distribution, phase composition, and defect structure are investigated in order to understand the ion irradiation response of the NFA|Si-Cr3C2-C composite. SRIM simulation reveals displacement damage profiles for different phase compositions. The NFA|Si phase has good ion irradiation resistance with newly induced dislocation loops. The C phase in the NFA|Si matrix is prone to being partially amorphized whereas the Cr3C2 phase in the NFA|Si matrix is completely amorphized. This work offers important guidance for nuclear cladding material development.

Structural characterization of tin nanocrystals embedded in silicon by atomic probe tomography

Tin nanocrystals embedded in silicon are studied by atom probe tomography and by photoluminescence spectroscopy in the 0.76–1.07 eV region of emission energies. The nanocrystals have been fabricated by molecular beam epitaxy followed by a post-growth annealing step at various temperatures. One particular sample, annealed at a temperature of 725 °C, shows a distinctly higher optical activity. It is found, however, that the distinct behavior cannot be explained by variations in the nanocrystal composition or in the properties of Sn atoms dissolved in the surrounding Si matrix, which can be investigated by atom probe tomography.

Vortex-free stir casting of Al-1.5 wt% Si-SiC composite

This paper investigates the avoiding of air entrapment and the quick realizing of uniform distribution of SiC particles in stir-casting Al-1.5 wt% SiSiC composite by vortex-free stir casting (VFSC). The air entrapment can be completely avoided by vortex-free high-speed electromagnetic-mechanical stirring and the porosity of composite can be reduced from about 3 vol% to less than 0.04 vol%. Meanwhile, the uniform distribution of SiC particles in the Al-1.5 wt% Si slurry can be quick realized in the crucible with tilt blades by adjusting the horizontally and radially tilt angles of blade. In addition, the primary Al particles and the high viscosity of slurry can restrain the settling of SiC particles and maintain the uniform distribution of SiC particles during casting in metal mold. The microstructure and composition of composite are determined by SEM, XRD, TEM and HRTEM. The (020) plane of Al is parallel to the (1¯103) plane of 6H α-SiC and their 0.053 nm lattice misfit results in a semi-coherent and clean interface. The compact, homogeneous and well-combined VFSC Al-1.5 wt% SiSiC composite possesses outstanding performance. The tensile strength of Al-1.5 wt% Si matrix can be increased 64.9%, the hardness can be enhanced from 60 to 78 BHN and the wear rate can be reduced 92.4% by 8 vol% SiC particles. This investigation provides a quick stir-casting mode for advanced AlSiC composite.

Precipitation of metal nitride nanoparticles from amorphous (M,Si)-(N,O) thin films (M =Nb, Zr)

Abstract Crystallization of metal nitride nanoparticles from amorphous matrix upon thermal annealing was investigated by studying the plasmonic absorption behavior of the nanoparticles. Amorphous silicon oxynitride thin films containing various amounts of M (M = Zr, Nb) were deposited by RF magnetron sputtering from a M/Si composite target using nitrogen as a reactive sputtering gas. The optical transmittance at around 700 nm of the annealed thin film decreased due to plasmonic absorption by the metallic NbN nanoparticles. The nanoparticles precipitated homogeneously in the Si(N,O) matrix upon annealing the Nb1-xSix(N,O) thin film with x = 0.7 at 700°C. In contrast, for Zr1-ySiy(N,O) thin films with y = 0.4 and 0.5, ZrN nanoparticles did not precipitate even after thermal annealing due to the stability of zirconium oxide in the Si(N,O) matrix, resulting in no significant change in the interference color of the thin films.