Si Diffusion Research Articles

Hybrid structures of Si Nanocrystals-Carbon Nanotubes with bead architecture

In present work, hybrid structures of Si Nanocrystals (Si-ncs) and Carbon Nanotubes (CNTs) with bead-like architecture have been synthesized by exposing pristine CNTs to silane (SiH4) at 200 ºC. The exposed CNTs were annealed in Ar ambient at 500ºC in a catalytic chemical vapor deposition chamber. High-resolution transmission electron microscopy (HRTEM) elucidated that the bead-like architecture has well-defined crystalline nature. X-ray photoelectron spectroscopy (XPS) was used to study the nature of chemical bonding and structural functionalization/defects caused by silane exposure and annealing on the nanotube surface. XPS results indicated in-diffusion of Si in the CNT cavity as well reduction in oxygen content on the top surface of the CNTs. X-ray diffractometry was used to further confirm the formation of the crystalline hybrid structures. From the present work it is inferred that a controlled synthesis of hybrid structures (CNT-Si ncs) with bead-like architecture can be achieved by a simple CVD method for various applications

Activation energy of silicon diffusion in gallium oxide: Roles of the mediating defects charge states and phase modification

Silicon (Si) is an efficient n-type dopant in gallium oxide (Ga2O3)—an ultra-wide bandgap semiconductor promising in a number of applications. However, in spite of the technological importance for device fabrication, the activation energy for Si diffusion in Ga2O3 is missing in the literature. In the present work, we do such measurements in ion implanted monoclinic β-Ga2O3 samples employing anneals in air ambient, also admitting the influence of potential ion beam induced phase modifications on diffusion. Importantly, we show that Si diffusion in β-Ga2O3 fits with the concentration dependent diffusion model, involving neutral and single negatively charged point defects to mediate the process; so that we assumed gallium vacancies in the corresponding charge states to assist Si diffusion in β-Ga2O3 with activation energies of 3.2 ± 0.3 and 5.4 ± 0.4 eV, respectively. Moreover, we also found that a preexisting phase modified surface layer efficiently suppressed Si diffusion in β-Ga2O3 for temperatures up to 1000 °C.

Fabrication and Evaluation of Low-Cost CrSi2 Thermoelectric Legs

CrSi2 is a promising thermoelectric material constituted of non-toxic and earth abundant elements that offer good perspectives for the mass production of inexpensive and reliable thermoelectric modules for waste heat recovery. Realization of robust metallic contacts with low electrical and thermal resistances on thermoelectric materials is crucial to maximize the conversion efficiency of such a device. In this article, the metallization of an undoped CrSi2 with Ti and Nb using a conventional Spark Plasma Sintering process is explored and discussed. These contact metals were selected because they have compatible thermal expansion coefficients with those of CrSi2, which were determined in this study by X-ray Diffraction in the temperature range 299–899 K. Ti was found to be a promising contact metal offering both strong adhesion on CrSi2 and negligible electrical contact resistance (<1 μΩ cm2). However, metallization with Nb resulted in the formation of cracks caused by large internal stress inside the sample during the fabrication process and the diffusion of Si in the metallic layer. A maximum conversion efficiency of 0.3% was measured for a sandwiched Ti/CrSi2/Ti thermoelectric leg placed inside a thermal gradient of 427 K. The preliminary results obtained and discussed in this article on a relatively simple case study aim to initiate the development of more reliable and efficient CrSi2 thermoelectric legs with an optimized design.

Self-diffusion mechanisms based defect complexes in MoSi2

MoSi2 is widely concerned due to excellent electrical conductivity, oxidation resistance as a typical transition metal silicide. The high-temperature diffusion behavior is one of the important factors for the degradation of MoSi2 coatings. However, the diffusion mechanism in MoSi2 is still unclear. Prior theoretical work mostly focused on defect formation energy, but these are not consistent with the self-diffusion experiments because the migration behaviors were not considered. Therefore, the purpose of this work was to investigate the microscopic diffusion mechanisms of Mo and Si atoms in MoSi2 using density functional theory and the CI-NEB method. We confirmed that the temperature-dependent vibrational contribution has a significant impact on the defect formation free energy. The isolated point defects in MoSi2 will tend to aggregate to form defect complexes, which participate in the atomic diffusion as mediators. The defect migration behaviors of atoms for vacancy mediated, vacancy complex mediated, and antisite assisted jumps were obtained based on electronic structures analysis. The results show that Si diffusion is mediated by intrasublattice jumps of the nearest neighbor Si vacancies. Moreover, the destroyed covalent Mo–Si bonds by Si vacancies and the non-directional weak metal bonds formed by the Mo antisites and Mo atoms could improve the mobility of the Mo atom which results in the low migration barrier. The agreement between our calculations and the reported experimental results indicates that the dominant diffusion mechanism for Mo atoms is mediated by vacancy complex mediated jumps and antisite assisted jumps. It is concluded that the Si vacancy-based defect complexes are likely the diffusion mediators for Mo atom self-diffusion in MoSi2. This work provides a deeper insight into the connection between the atomic mechanism and the macroscopic behavior for the diffusion in the MoSi2, and establishes the basis for further optimizing high-temperature coating materials.

Rapid growth of ultra-long Al whiskers from TiN/Al/Si island structures

This paper aims to fabricate longer Al whiskers with shorter heating time by using stress-induced migration. The island structure sample, which consists of a rectangular Al thin film on a Si wafer covered with a TiN layer, is heated for rapid growth of Al whiskers. Unique, compressive stress distribution in the Al thin film from the center to the edge of islands during the heating contributes to whisker growth. Si diffusion into the Al thin film during heating promotes atomic diffusion. In order to obtain longer whiskers, a lower heating temperature of 300 °C was found to be better than 500 °C. As a result, a longer Al whisker of approximately 0.5 mm was obtained for 3 h heating time.

Effects of A-site atoms in Ti2AlC and Ti3SiC2 MAX phases reinforced Mg composites: Interfacial structure and mechanical properties

AZ91D magnesium composites reinforced by Ti3SiC2 MAX phase were fabricated by the stir casting technique. Tensile tests reveal that the reinforced effects of Ti3SiC2 and Ti2AlC on Mg matrix are different. Unlike Ti2AlC-AZ91D composite for which Ti2AlC delamination happens during the fracture, Ti3SiC2-AZ91D composite fractures by interfacial debonding mechanisms. Transmission electron microscopy characterization shows that few nanometric Mg grains form in the vicinity of Ti3SiC2 particles, while huge amount of nanometric Mg grains form along with Ti2AlC particles. Elements mapping confirms that Al atoms of Ti2AlC diffuse into Mg matrix. In contrast, no outward diffusion of Si of Ti3SiC2 is observed. This outcome points out that it is possible to regulate the mechanical properties of their relevant Mg composite through adjusting the content of Al and Si in MAX phases, such as Ti3SiAlC2 solid solution.

Highly surface-active Si-doped TiO2/Ti3C2Tx heterostructure for gas sensing and photodegradation of toxic matters

Si@TiO2/Ti3C2Tx heterostructures were synthesized in situ from Ti3AlC2 MAX phase by a combination of high-temperature Si diffusion and HF etching. The optimal processing conditions and optimal structure were set up. For the optimal heterostructure, small TiO2 particles were distributed over Ti3C2Tx MXene with significantly increased surface area. Unlike Ti3C2Tx MXene, the Si@TiO2/Ti3C2Tx heterostructure behaved like a semiconductor. The heterostructure demonstrated superb 2,4-Dinitrophenol (DNP) photodegradation capability and good room-temperature NO2-sensing performance. In particular, the remarkable photocatalytic DNP degradation was explained on the basis of the broad-range light absorption and effective carrier separation by the Schottky contact. Furthermore, density-functional theory (DFT) calculations indicated that Si atoms might facilitate the NO2 adsorption process. The output of this work may highlight the potential of Si@TiO2/Ti3C2Tx heterostructure as a multi-functioning nanomaterial.

In Situ Synthesis of Core-Shell-Structured SiCp Reinforcements in Aluminium Matrix Composites by Powder Metallurgy

SiCp reinforced aluminium matrix composites (AMCs), which are widely used in the aerospace, automotive, and electronic packaging fields along with others, are usually prepared by ex situ techniques. However, interfacial contamination and poor wettability of the ex situ techniques make further improvement in their comprehensive performance difficult. In this paper, SiCp reinforced AMCs with theoretical volume fractions of 15%, 20%, and 30% are prepared by powder metallurgy and in situ reaction via an Al-Si-C system. Moreover, a combined method of external addition and an in situ method is used to investigate the synergistic effect of ex situ and in situ SiCp on AMCs. SiC particles can be formed by an indirect reaction: 4Al + 3C → Al4C3 and Al4C3 + 3Si → 3SiC + 4Al. This reaction is mainly through the diffusion of Si, in which Si diffuses around Al4C3 and then reacts with Al4C3 to form SiCp. The in situ SiC particles have a smooth boundary, and the particle size is approximately 1–3 μm. A core-shell structure having good bonding with an aluminium matrix was generated, which consists of an ex situ SiC core and an in situ SiC shell with a thickness of 1–5 μm. The yield strength and ultimate tensile strength of in situ SiCp reinforced AMCs can be significantly increased with a constant ductility by adding 5% ex situ SiCp for Al-28Si-7C. The graphite particle size has a significant effect on the properties of the alloy. A criterion to determine whether Al4C3 is a complete reaction is achieved, and the forming mechanism of the core-shell structure is analysed.

Clustering of Si–Ni in cementite in 18MND5 steel weldment

ABSTRACT In this work, a new phenomenon—clustering of Si–Ni in cementite was observed in a thermal aged 18MND5 weldment and the formation mechanism was briefly discussed. 18MND5 weldment mock-up was thermally aged at 400°C for 3000 h and its microstructure and composition were investigated by SEM, TEM and 3DAP. Silicon–nickel clusters in cementite were observed in the heat-affected zone of 18MND5 base metal by atom probe tomography. The formation of Si–Ni cluster in cementite is closely linked to the welding, tempering and thermal aging processes. This clustering phenomenon might be explained by that in the process of cementite formation, an alloying element ‘spike’ generated ahead of the interface of cementite and α-Fe matrix based on the partition local equilibrium model and silicon atoms were trapped for Mn, Cr and Mo ‘spikes’ act as a barrier to Si diffusion. As a result of thermal aging, those trapped Si atoms were compelled to generate metastable clusters in cementite for lowering the free energy.

Preparation, properties and high-temperature oxidation resistance of MoSi2-HfO2 composite coating to protect niobium using spent MoSi2-based materials

Industrial spent MoSi2-based materials and HfO2 were recycled as raw materials to fabricate MoSi2-HfO2 composite coating by spark plasma sintering (SPS). The microstructural evolution of the coatings was characterized and the 1500 °C oxidation behavior was explored. Cracks penetrated through the MoSi2 coating while no cracks can be found in the HfO2-containing composite coating owing to the reduction of the mismatch of thermal expansion coefficient (CTE). Good metallurgical bonding was exhibited since (Mo,Nb)5Si3 diffusion layer was found in the HfO2-containing coating by the diffusion of Nb and Si across the interface without gaps. After 1500 °C oxidation of 20 h, cracks appeared in the surface of SiO2 layer on MoSi2 coating while the HfO2-containing composite coating possessed crack-free oxide scale. HfSiO4 with high temperature (>2900 °C) is formed during oxidation and it inlays in the silica oxide scale to improve the stability. Compared to MoSi2 coating, Nb coated MoSi2-HfO2 has thinner oxide scale with lower mass gain during oxidation, thus presenting better high-temperature anti-oxidation properties.

Atomic and electronic structures of Si/Ge(100) interfaces studied by high-resolution photoelectron spectroscopy and scanning tunneling microscopy

The close similarity of silicon and germanium, isoelectronic group-IV elements, makes the integration of Ge layers on Si substrates suitable for technology development, but the atomic and electronic structures of ${\mathrm{Si}}_{1\ensuremath{-}x}{\mathrm{Ge}}_{x}$ surfaces are still an open issue, in particular, for the alloy systems where Si is deposited on the Ge substrate. In this study, utilizing low-energy electron diffraction, scanning tunneling microscopy, and photoelectron spectroscopy using synchrotron radiation, we demonstrate that the formation mechanisms of the Si-on-Ge structures are controlled by two interface phenomena, namely Si indiffusion and Ge segregation on top of this surface. Employing these phenomena and controlling the Si quantity, one can synthesize the well-defined crystalline Ge-(2 \ifmmode\times\else\texttimes\fi{} 1)/${\mathrm{Si}}_{1\ensuremath{-}x}{\mathrm{Ge}}_{x}$/Ge(100) stacks where the number of Si atoms at the host Ge lattice sites can be tuned. Using the obtained data on the atomic and electronic structures of such systems, we also propose a method for interface engineering of Ge/Si/Ge stacks with tailored properties as promising templates for growing the device junctions.

Ag droplets nano-shape design on SiC: Study on wetting and energetics

This work describes the effect of the annealing temperature on the size and wetting of nanostructures obtained on 4H–SiC substrate by the dewetting process of a deposited nanoscale-thick Ag film. The work focuses, in particular, on the difference between solid-state dewetting (below Ag melting temperature) and liquid-state dewetting (above Ag melting temperature). Following the deposition of a 58 nm-thick Ag film on the SiC substrate, annealings were perfomed at 800 °C, 850 °C and 885 °C so to induce the Ag film solid-state dewetting. In addition, annealing at 1040 °C was performed so to induce the Ag film liquid-state dewetting. The dewetting processes led to the formation of nanostructures on the SiC surface. Plan-view and cross-view scanning electron microscopy analyses were carried out to quantify the evolution of the average planar size and of the average contact angle (to the SiC surface) of the Ag nanostructures versus the annealing temperature. These analyses allowed us to extract information on the wetting behavior of the nanostructures on the SiC surface by calculating the temperature-dependent adhesion work. In fact, the nanostructures obtained by the 800 °C and 850 °C processes show mean contact angle of 118,9° and 117,4°, respectively, and mean work of adhesion of 0,45 and 0,48 J/m2, respectively. On the other hand, the nanostructures obtained by the 885 °C and 1040 °C show mean contact angle of 48,3° and 52,2° respectively, and work of adhesion of 1,45 and 1,40 J/m2, respectively. The variation of the wetting properties is identified in Si diffusion from the SiC substrate to the Ag nanostructures at the higher temperatures, as confirmed by energy dispersive x-ray analyses. Finally, the optical properties of the Ag nanostructures/SiC systems were probed by absorbance and transmittance measurements allowing to recognize the occurrence of a peak around 460 nm which is attributed to localized surface plasmon resonances due to the Ag nanostructures.

Oxidation and degradation of amorphous SiAlN coating via forming Si-Si bond

The oxidation and degradation of amorphous SiAlN coatings consisting of Si3N4 and AlN phases, with 750 nm or 300 nm Mo interlayers on Zr alloy in steam environment at 1000 °C were investigated. After 1 h exposure, no detectable oxide scale formed on SiAlN/Mo (750 nm) coatings whereas SiAlN/Mo (300 nm) formed oxide scale. The downward diffusion of Si followed by relatively faster downward diffusion of N generated excessive Si and lean N, forming Si-Si bond in outermost surface of SiAlN, thereby resulting in oxidation. The degradation mechanism of amorphous nitrides was determined by elemental composition instead of reported amorphous or crystalline status.

Complex diffusion saturation of carbon steel 1045 with boron, chromium, titanium and silicon

The article presents the results of studies on the formation of a diffusion layer upon saturation of steel 1045 with boron together with chromium, titanium and silicon. The results of tests of diffusion coatings on steel 1045 obtained by simultaneous diffusion saturation with boron, chromium, titanium and silicon under the action of a sulfuric acid solution under conditions of abrasive wear are presented. A comparison is made of the service life of knives for shredding lead-acid batteries made of steel HARDOX 600 and steel 1045 after diffusion saturation with boron, chromium, titanium and silicon. It was found that the simultaneous strengthening with boron, chromium, titanium and silicon made it possible to increase the operational durability of knives for shredding lead-acid batteries by 2.4 times. The possibility of highly efficient use of products made of simple carbon steel with a multicomponent (B + Cr + Ti + Si) diffusion coating instead of products made of hardened alloy steel is shown.

Role of helium ion irradiation in the corrosion of CVD−SiC in molten LiF−NaF−KF salt

The role of helium ion irradiation in the corrosion of CVD−SiC in molten LiF−NaF−KF salt was studied. Results indicate that the corrosion of SiC relates to the diffusion of element Si. Impurities in molten LiF−NaF−KF salt react with SiC to drive the outward diffusion of element Si and graphitic carbon is further formed. Energetic helium ions break down the SiC bonds to form defects. Slight disorder has little effect on the corrosion of SiC, while the amorphous induced by helium ion irradiation significantly enhances the diffusion of element Si and thus increases the corrosion of SiC.

Si diffusion across the liquid/solid interface of capillary driven (Al–Si)-KxAlyFz micro-layers

Si diffusion across the liquid/solid interface of capillary driven (Al–Si)-KxAlyFz micro-layers

Формирование мелкодисперсного термоэлектрика Si-=SUB=-1-x-=/SUB=-Ge-=SUB=-x-=/SUB=- при электроимпульсном плазменном спекании

The kinetics of diffusion processes occurring d0uring the formation of polycrystalline Si1-xGex nanostructures (x=0.20, 0.35) by electro-pulse plasma sintering in the temperature range 20-1200°C was studied for the first time. A mechanism for the formation of a solid solution of SiGe is proposed as a result of a comprehensive study of the microstructure and phase composition of samples with particle sizes from 150 nm to 100 μm, together with the analysis of experimental sintering maps. It is based on the phenomenon of mutual diffusion of Si and Ge atoms that occurs during the entire sintering process. For the selected sintering modes, the grain size of the formed SiGe corresponds to the size of the initial powder particles.

Effects of Annealing and Deformation on Sagging Resistance of a Hot‐Rolled, Four‐Layered Al Alloy Clad Sheet

Multilayer brazeable aluminum alloy sheet is prone to collapse during high‐temperature brazing process. The sagging resistance of the aluminum composite sheet needs to be further improved for quality control. Effects of annealing and rate of reduction on sagging resistance, microstructure, and Si diffusion of a hot‐rolled, four‐layered Al clad sheet (4343/3003/6111/3003) were investigated by means of a sagging device, OM, SEM, and TEM. Results showed that once annealed at 360°C, the sagging distance was increased from 3 to 15.7 mm as the reduction rate changed from 10% to 40%. By increasing annealing temperature to 410°C, those were changed from 3.1 to 20.8 mm accordingly. At 360°C/40% and 410°C/40%, specimens exhibited weak sagging resistance, whereas fine recrystallized grains were formed in the core promoting Si penetration along grain boundaries. While the specimens were treated at 360°C/10% and 410°C/10%, better sagging resistance was observed due to the formation of coarse recrystallized grains that can suppress erosion of Si. At the same reduction rate, the sagging resistance was higher for the sample annealed at a lower temperature as more precipitates appeared in the core (at 360°C), thus leading to an increase in strength.

Microstructure and oxidation behaviour of MoSi2 coating combined MoB diffusion barrier layer on Mo substrate at 1300 °C

To improve the oxidation resistance of a Mo substrate at high temperatures, MoSi2–MoB double layers were prepared on the surface of the substrate using a two-step pack cementation method. The isothermal oxidation tests indicate that the weight loss of MoSi2–MoB double layers was only 6.3824 mg/cm2 after oxidation at 1300 °C for 15 h, whereas the untreated Mo experienced serious weight loss and the weight loss was 606.775 mg/cm2 after only 90 min. Following oxidation exposure, the oxidized coating became six complex layers due to correlation diffusion among Mo, Si and B. The double layers showed excellent high-temperature oxidation resistance due to the formation of a dense and continuous SiO2 layer with low O2 permeability. Meanwhile, in situ fabrication of a novel Mo5SiB2 layer could prevent inward diffusion of Si.

Effect of Calcination Time on the Catalytic Activity of Ni/γ-Al2O3 Cordierite Monolith for Dry Reforming of Biogas

Ni/γ-Al2O3 wash coated cordierite monolith catalysts are calcined in air at 800 °C for 4, 10, and 20 h in order to study the effect of calcination time on the activity of the catalysts for dry reforming of model biogas. Catalytic activity studies are performed at 800 °C with three different CH4/CO2 ratios of 1.0, 1.5, and 2.0. The catalyst calcined for the longest time (C-20) displays higher stability and activity in terms of CH4 and CO2 conversion compared to those calcined for 4 h (C-4) and 10 h (C-10). XRD data and TPR analysis detect the maximum amount of NiAl2O4/MgAl2O4 phases and strongest metal-support interaction, respectively, for the C-20 sample. FESEM reveals the particle size of the calcined and reduced C-20 sample to be smaller than that of the C-4 and C-10 samples. Whereas, H2 pulse-chemisorption characterization demonstrates the highest metal surface area, metal dispersion, and smallest Ni particle size for the C-20 catalyst. While, no carbon deposition on any catalyst occurs for the CH4/CO2 ratio of one, lowest amount of carbon nanotubes is formed on the C-20 sample for the CH4/CO2 ratio of 1.5 and 2.0, as observe by DTA-TGA. EDX reveals concentration variation of Mg and Si from the cordierite monolith wall along the thickness of the coating for all the samples. In addition, the maximum amount of these elements is observed for the calcined C-20 catalyst coating. These implies that the diffusion of Mg and Si from the cordierite monolith to the catalyst coating during calcination contribute significantly in controlling the physicochemical properties of the catalysts. As a result, the higher stability and activity of the C-20 could be attributed to the formation of higher amount of the Ni– Mg- alumina spinel complex in the catalyst coating during longer calcination time, which leads to the improved metal-support interaction and higher nickel dispersion over monolith.