Si Diffusion Research Articles

Influence of annealing treatment on Si morphology and strength of rapid solidified Al-12 wt% Si powders

An experimental study is reported on heat treating rapid solidified Al-12 wt% Si alloy with 355–425 μm powder size. Fast heat treatments at 813 K revealed morphological shifts of eutectic Si from the as-atomized condition to those after annealing. The Si network of the as-atomized samples showed a fine dispersed coral fibrous architecture. This feature via rapid cooling brings the prospect of quickly disintegrating and spheroidizing Si in a few minutes without chemical modification. By adopting different exposure times, it was possible to investigate fragmentation and coarsening of the eutectic Si. The integrity of the Si network is very sensitive to the Si spheroidization treatment transforming into a network of round Si particles. It is also seen that the material softens in the earlier stages of heat treatment, especially after 2 min of heat treatment. The breakdown in Si rods in a rapidly solidified alloy is shown to take place with a mixed mechanism of interface or internal rod diffusion and diffusion of supersaturated Si through the α primary phase to the Si rods in the eutectic. The quantitative analysis of the microstructures coupled with an analytical strengthening model resulted in good agreement between experimental and calculated data.

Microstructural investigation of the coatings prepared by simultaneous aluminizing and siliconizing process on γ-TiAl

In this research, formation of aluminide/silicide diffusion coatings on ?-TiAl[Ti-48Al-2Nb?2Cr (at.%)] alloy using gasphase diffusion pack cementation process has been investigated. The application of powder mixtures with various chemical compositions in the pack cementation process performed at 1000oC for 6 hours in order to achieve simultaneous diffusion of Al and Si, showed that the composition of the powder mixture could have a significant effect on the structure and thickness of the aluminide/silicide coatings. The identification and analysis of aluminide/silicide microstructures formed as a result of simultaneous diffusion of Al and Si, which was comprehensively and qualitatively done for the first time in this study, showed that the sequential mechanism is dominant in the formation of the above-mentioned coatings. Furthermore, Kirkendall phenomenon and volumetric changes caused by the formation of Ti5Si3 and Ti5Si4, were considered as the two dominant mechanisms in the formation of porous segregated structure in these coatings. In this study, the effect of decreasing the activity of Si, through two approaches of reducing the amount of Si in the powder mixture and using Al- 20wt.%Si alloyed powder instead of pure Al and Si depositing elements, on the microstructural modification coatings was investigated. The results showed that reducing the Si activity at the surface of the coating and, consequently, reducing the flux of active silicon atoms (JSi), has a significant effect on the formation of coating with an ideal structure.

Damage Recovery and Dopant Diffusion in Si and Sn Ion Implanted β-Ga2O3

The commonly used n-type dopants, Si and Sn, were implanted into bulk (−201) β-Ga2O3 over a 2 order of magnitude dose range and annealed at temperatures from 1000–1150°C. The original lattice parameters were restored by annealing at 1150°C for the highest dose Si implants, while only partial recovery was observed in Sn implanted samples. The Sn implanted samples had overall lower lattice parameters compared to the Si implanted samples, indicating that Sn generates tensile strain in the Ga2O3 lattice. The rocking curve FWHM was observed to increase with the annealing process, indicating that the annealing process does not improve the crystal quality. Transmission electron microscopy showed removal of the end-of-range lattice damage after 1150°C anneals of the heaviest implanted species, Sn. Cathodoluminescence at 5K showed recovery of intensity of the common UV band around 3.2 eV after annealing. Secondary Ion Mass Spectrometry profiling showed the presence of concentration-dependent diffusion of both Si and Sn, with values for diffusivity at 1150°C of 9.5 × 10−13 cm.s−1 for Si and 1.7 × 10−13 cm.s−1 for Sn obtained by fitting through the FLOOPS simulation package.

New insights into Li diffusion in Li-Si alloys for Si anode materials: role of Si microstructures.

Li ion transport is very important to the rate capability of electrode materials in Li ion batteries. For Si anodes, due to huge structural changes of Si structures during the process of charging and discharging, Li ion transport is essentially affected by the Si internal microstructures. Herein, we studied the effect of Si microstructures on Li ion diffusion in Li-Si alloys using first-principles molecular dynamics calculations. Our results demonstrate that the Li diffusion coefficients are closely related to the aggregation degree of Si atoms, regardless of whether it is the low Li concentration phase LiSi or the high Li concentration phase Li2Si under consideration. Furthermore, through counting the number of Si microstructures, such as rings, chains and small clusters, the relationship between the aggregation degree of Si atoms and the number of Si microstructures is established. A large number of Si microstructures corresponds to the low aggregation degree of Si atoms, thus resulting in small Li diffusion coefficients due to the strong interaction between Li and Si atoms. Conversely, a small number of Si microstructures originates from the high aggregation degree of Si atoms, consequently leading to large Li diffusion coefficients. Our study provides a deep insight into the relationship between the Li ion diffusion and the Si distribution, which facilitates the performance improvement of future Si anode materials.

Density functional theory calculations of self- and Xe diffusion in U3Si2

Uranium silicide, U3Si2, has been proposed as an advanced nuclear fuel to be used in light water reactors (LWRs). Development of this alternative to the predominant current fuel, UO2, is motivated by enhanced accident tolerance as a result of higher thermal conductivity as well as improved fuel cycle economics through increased uranium density. In order to accurately model the fuel performance of U3Si2, the diffusion rate of point defects, which is related to self-diffusion, and of fission gas atoms must be determined. DFT calculations are used to predict the U and Si point defect concentrations, the corresponding self-diffusivities, the preferred Xe trap site and the Xe diffusivity. Effects of irradiation are not considered. A low defect formation energy and a high entropy for Si interstitials give rise to Si-rich non-stoichiometry at elevated temperatures. Both U and Si self-diffusion and Xe diffusion are anisotropic as a consequence of the tetragonal crystal structure of U3Si2. Si diffusion occurs by interstitial mechanisms in both the a-b plane and along the c axis, while the U c axis diffusion rate is controlled by a vacancy mechanism. Interstitial diffusion of U is very fast in the a-b plane of the U3Si2 crystal structure. Xe atoms prefer to occupy U vacancy trap sites. The highest Xe diffusion rate occurs by a vacancy mechanism in both the a-b plane and along the c axis. The diffusion rate is similar in the a-b plane and along the c axis. U and Si self-diffusion and Xe diffusion are all faster in U3Si2 than intrinsic U and Xe diffusion in conventional UO2 nuclear fuel.

Dissimilar joining of TC4 alloy to ST16 steel by GTAW

The feasibility of direct fusion welding of TC4 alloy to ST16 steel by GTAW using CuSi3 filler wire was investigated. Defect-free fusion welded joints were obtained. Various intermetallic compounds, such as Ti2Cu, TiCu, AlCu2Ti, TiCu2 and Ti5Si3 phases, orderly formed in Ti/weld transition zone. Ti and Si diffused into ST16 steel, and an interface formed between copper weld and ST16 steel substrate. The resultant joints fractured at the ST16 steel substrate, and the tensile strength reached 282 MPa. The formation of various TixCuy IMCs and the solution strengthening induced by diffusion of Ti and Si led to the increase of microhardness in Ti/weld transition zone and copper/steel interface, respectively.

Coarsening behavior of (Ni, Co)2Si particles in Cu–Ni–Co–Si alloy during aging treatment

The coarsening behavior of (Ni, Co)2Si particles in Cu–Ni–Co–Si alloy was investigated by experimental observations and coarsening kinetics calculations when aged at 450, 500, 550 and 600 °C for different durations. The results show that the critical particle radius for coherence mismatch is found to be 10.3 nm, and particles larger than 25 nm are generally semi-coherent. The relationship of (Ni, Co)2Si particles size and aging time follows Lifshitz, Slyosov and Wagner (LSW) theory. The particle size distributions fit well to the LSW theoretical distribution. The activation energy for (Ni, Co)2Si coarsening is accurately determined to be (216.21 ± 5.18) kJ·mol−1 when considering the effect of temperature on the solution concentrations in matrix. The coarsening of (Ni, Co)2Si particles in Cu–Ni–Co–Si alloy is controlled by diffusion of Ni, Co and Si in Cu matrix. The growth of particles for long durations suggests that vacancies can be trapped within the structure for long time despite their mobility.

Direct wafer bonding of Ga2O3–SiC at room temperature

Integration of Ga2O3 on SiC substrate with a high thermal conductivity is one of the promising solutions to reduce the self-heating of Ga2O3 devices. Direct wafer bonding of Ga2O3–SiC at room temperature was achieved by surface activated bonding (SAB) using a Si-containing Ar ion beam. An average bonding energy of ~2.31 J/m2 was achieved. Both the structure and the composition of the interface were investigated to understand the bonding mechanism. According to the interface analysis, a ~2.2 nm amorphous SiC layer and a ~1.8 nm amorphous β-Ga2O3 layer originating from the ion beam bombardment for surface activation were found at the interface. A slight diffusion at the interface might already happen at room temperature, which should contribute to the strong bonding. To confirm the diffusion at a low temperature and investigate the possible interfacial variation during device operation, an annealing process was carried out at 473 K. The same analysis was applied on the annealed bonding interface. The interfacial layer shrank by ~0.5 nm after annealing. The further diffusion of Ga and Si at the interface caused by the annealing was confirmed. Besides, the position of the Ar count peak inside the amorphous Ga2O3 layer shifted by ~0.5 nm toward SiC.

Role of the slow diffusion species in the dewetting of compounds: The case of NiSi on a Si isotope multilayer studied by atom probe tomography

Dewetting or agglomeration is a crucial process in material science since it controls the stability of thin films or can be used for film nanostructuration by formation of islands. The models developed for dewetting usually assume diffusion at the interface and/or at the surface but no direct evidence of such diffusion was demonstrated. Moreover, these models are usually dealing with elemental materials and not with compounds in which several elements can diffuse. The mechanisms behind agglomeration of polycrystalline compound thin films are still not fully understood. In this work, Si isotope multilayers coupled with atom probe tomography (APT) are used to reveal the agglomeration mechanism of NiSi, a binary compound. The diffusion of Si, the less mobile species in NiSi, at the NiSi/Si interface is demonstrated through comparison between the three dimension redistribution of the Si isotopes determined by APT and models taking into account grooving and agglomeration. The implication for the understanding and control of agglomeration in polycrystalline compound thin films are highlighted.

Reaction synthesis of spark plasma sintered MoSi2-B4C coatings for oxidation protection of Nb alloy

MoSi2-B4C coatings with different B4C contents were prepared on Nb alloy by spark plasma sintering (SPS) process. Powder mixtures of Mo, Si and B4C were used as the coating starting materials. Besides MoSi2 and B4C phases, small amounts of SiC and MoB are also found in the coatings because of the reactions of Mo, Si and B4C powders during sintering. Compared with single MoSi2 coating, the MoSi2-B4C coatings show better oxidation resistance at 1450 ℃, and dense B2O3-SiO2 oxide scales form after 100 h oxidation. The B4C or MoB in the MoSi2-B4C coatings can serve as the B donor for the formation of B2O3. A slight degradation in the microstructure of the MoSi2-B4C coatings after oxidation is observed, which can be attributed to the presence of an NbB layer in the inter-diffusion zone of the coatings that retards the inward diffusion of Si from the coating into the substrate alloy. The microstructure development and oxidation behavior of the MoSi2-B4C coatings have been discussed.

Thermal expansion coefficient of Diamond/SiC composites prepared by silicon vapor infiltration in vacuum

Diamond/SiC composites were fabricated by Si-vapor reactive infiltration where graphite was used to generate SiC via an SiC reaction. The composites exhibited an inward growth of SiC and cracks within the diamond. In the strong interfacial bonding, Si diffusion occurred on the surface of the diamond. The thermal expansion coefficient (CTE) and thermal residual stress at different infiltration temperatures were simultaneously investigated. It was found that the CTE values increased from 1.2 to 4.5 × 10−6 K−1 in the measured temperature range, while the presence of large-sized crushed diamond, high SiC content and high infiltration temperature enhanced the CTE values. In the low-temperature range, the measured CTEs agreed with those predicted by the Turner model; while in the high-temperature range they agreed with those predicted by the lower-bound Schapery model. Additionally, based on the Raman spectra, the thermal residual stresses were found to be 0.85, 1.76, 2.15 and 2.67 GPa at the infiltration temperatures of 1550, 1600, 1650, and 1700 °C, respectively.

Self-diffusion and chemical diffusion in peridotite melt at high pressure and implications for magma ocean viscosities

Self-diffusion coefficients of Si, O, Mg, and Ca and chemical diffusion coefficients of Ni and Co have been determined experimentally up to 24 GPa and 2623 K in anhydrous peridotite liquid using a multi-anvil press. Each diffusion couple consisted of a cylinder of isotopically-natural peridotite enriched in 1 wt% CoO that was mated to a cylinder of chemically similar peridotite enriched in 18O, 30Si, 25Mg, 44Ca, and 1 wt% NiO. Isotope abundance (18O, 30Si, 25Mg, 44Ca) profiles along each sample length and perpendicular to the diffusion interface were analyzed by secondary ion mass spectrometry, and Ni and Co concentration profiles were measured by electron microprobe analysis. The rates of self-diffusion of Si, O, Mg, Ca and chemical diffusion of Ni and Co decrease with increasing pressure from 4 to 8 GPa, where diffusivity reaches a minimum, and then increase with a further pressure increase to reach a maximum at ~12 GPa. The observed change in the pressure dependence at ~8–10 GPa is likely caused by pressure-induced structural changes and the formation of high-coordinated intermediate species. Above ~12 GPa, diffusivities again decrease weakly with increasing pressure most likely as a result of further melt compaction when the coordination changes are complete. Predicted viscosities from self-diffusion coefficients for oxygen, obtained using the Eyring equation, are similar to viscosities measured by falling sphere viscometry, including the observed changes of the pressure dependence. The effect of pressure on diffusion and viscosity in peridotite melt is small compared with the effect of temperature, indicating that the viscosity along the melting curve decreases with depth. Diffusion parameters determined in this study are used to estimate the viscosity of a magma ocean with a depth of ~700 km. The results indicate that the viscosity increases from ~10 mPa s near the surface to a reach a maximum of ~50 mPa s at a depth of ~250 km. At higher pressures the viscosity decreases to ~10 mPa s at a depth of ~300 km, below which it remains almost constant.

Influence of annealing conditions on threading dislocation density in Ge deposited on Si by reduced pressure chemical vapor deposition

The influence of annealing conditions on the crystallinity of Ge deposited on Si(001) is investigated. Ge deposited with postannealing at 800 °C and 850 °C, five cycles of postannealing at 750 °C and 850 °C (temperature-swing postannealing) and several cycles of annealing at 800 °C or 850 °C during the Ge growth by interrupting the deposition step (cyclic annealing) are compared. To check the threading dislocation density (TDD) of the deeper part of the Ge, thinning by HCl vapor phase etching (VPE) followed by Secco defect etching is performed for 5 μm thick Ge of all three annealing variants. By comparing the TDD of the same Ge thickness with and without HCl VPE, TDD reduction by VPE is observed for the sample using the cyclic annealing process only. Lower TDD is observed at higher postannealing temperature. By the five cycles of temperature swinging, TDD becomes around a half compared to conventional postannealing at 850 °C. In the case of the cyclic annealing process significant improvement of TDD is observed with increasing Ge thickness. Even at a maximum temperature of 800 °C, the same or lower TDD levels were observed for higher than 2 μm thick Ge compared to that with five cycles of temperature-swing postannealing. For the sample with cyclic annealing at 800 °C, a lower Si diffusion length into Ge is also observed for the cyclic annealing process indicating a lower thermal budget. A lower amount of tilted Ge planes at the interface is confirmed showing higher crystal quality also in the deeper part of the Ge layer.

Interfacial ferrite band formation to suppress intergranular liquid copper penetration of solid steel

Intergranular liquid metal penetration, which is detrimental to the integrity of welded structures, has been widely investigated. Its mechanisms are now more clearly understood, but the suppression method is limited to optimizing processing parameters. In this paper, a metallurgical suppression technique by building an interfacial ferrite band to prevent intergranular liquid Cu penetration of solid steel was proposed. A ternary chemical potential prediction model was established and predicted that Si in liquid copper can accumulate at the interface and diffuse into steel during the interaction between molten Si-containing Cu and solid steel. This interaction can induce the formation of a ferrite band with a high resistance to intergranular liquid copper penetration of solid steel. A model for interfacial ferrite band formation based on Si diffusion during the interaction between solid steel and Si-containing molten copper was proposed and experimentally verified by welding-brazing copper to steel with Si-containing copper wire. The results indicated that long-range intergranular penetration was effectively suppressed, and harmless micropenetration appeared between the ferrite band and weld. The intergranular micropenetration mechanism is considered a grain boundary liquidation phenomenon induced by Si diffusion, differing from classical wetting theory.

Effect of Shot Peening on Oxidation Behavior of SS304H Upon Long Term Steam Exposure.

Specimens of austenitic stainless steel 304H (SS304H) were shot peened and exposed to steam at 600, 650, and 700 °C for 10,000 h, 15,000, and 20,000 h. After steam exposure, un-peened SS304H has three oxide layers, Fe-O, Fe-Cr-O and Cr2O3, while shot peened specimens have an Fe-Cr-O layer, a Cr2O3 and an amorphous Fe-Si-O layer. The evolution of the oxide layers as a function of duration of steam exposure and temperature reveals that the growth of the Fe-O and the Fe-Cr-O layers is inhibited by shot peening. The oxide layers in shot peened specimens are much thinner, indicating that shot peening enhances the oxidation resistance of SS304H. Grain refinement during shot peening enhances the diffusion of Cr and Si to help form a protective, oxidation resistant surface.

Effect of surface self-nanocrystallization and Si infiltration on Si diffusion behavior, hardness and magnetic properties of pure Fe

Surface nanocrystallization of pure Fe was performed using an improved surface treatment process. The phase transformation and Si infiltration depth of the pure Fe before and after surface mechanical attrition treatment (SMAT) were compared by X-ray diffraction, scanning electron microscopy and energy-dispersive X-ray spectroscopy. The results indicated that nanocrystallization of Fe surface was achieved using SMAT, which resulted in deeper penetration of Si. Prolonging time of SMAT and Si infiltration also resulted in increasing microhardness, with the hardness first increasing with increasing distance from the surface and then decreasing. Furthermore, longer Si infiltration time, nanocrystallization of Si and longer SMAT time resulted in higher saturation magnetization (MS). The greatest Si penetration depth (150 μm), maximum hardness (280 HV), and maximum MS (1.849 × 106 A/m) were achieved after SMAT for 45 min and Si infiltration for 9 h. The interaction between adjacent grains after surface nanocrystallization leads to a region of the magnetic domain wall structure located at the grain boundary, which causes the remanence enhancement effect.

Ablation Behaviors of ZrC-20vol.%MoSi<sub>2</sub> Composite Coating under Different Heat Fluxes

ZrC-20vol.%MoSi2 (ZM) composite coating was fabricated by vacuum plasma spray and the ablation resistance was assessed using plasma flame under low (1.94 MW/m2) and high (3.01 MW/m2) heat fluxes, respectively. Results showed that the ultimate surface temperatures of ZM coating were about 2100 °C and 2400 °C, respectively. ZM coating exhibited good ablation resistance at low heat flux, which benefited from the low evaporation of SiO2 and the diffusion of Si derived from MoSi2 decomposition. However, bubble-burst event took place under high heat flux. The different ablation behaviors of ZrC-MoSi2 coating were analyzed, which might contribute to the application of ultra-high temperature ceramic coatings.

(Invited) Determining Si Composition in SiGe Alloys with < 1% Si Concentrations Using Raman Spectroscopy

This paper proposes a methodology based on Raman spectroscopy to determine the Si concentration in SiGe films with less than 1.2% Si. The methodology is validated on SiGe films with Si concentrations varying between 0.10% and 1.83% using SIMS results. It is next applied in a study of diffusion of Si in annealed GeSi films.

Thermodynamics and kinetics of core-shell versus appendage co-precipitation morphologies: An example in the Fe-Cu-Mn-Ni-Si system

What determines precipitate morphologies in co-precipitating alloy systems? We focus on alloys of two precipitating phases, with precipitates of the fast-precipitating phase acting as heterogeneous nucleation sites for a second phase manifesting slower kinetics. We study a FeCuMnNiSi alloy using the combination of atom probe tomography and kinetic Monte Carlo simulations. It is shown that the interplay between interfacial and ordering energies, plus active diffusion paths, strongly affect the selection of core-shell verses appendage morphologies. Specifically, the ordering energy reduction of the MnNiSi phase heterogeneously nucleated on a pre-existing copper-rich precipitate exceeds the energy penalty of a predominantly Fe/Cu interface, leading to initial appendage, rather than core-shell, formation. Diffusion of Mn, Ni and Si around and through the Cu core towards the ordered phase results in subsequent appendage growth. We further show that in cases with higher primary precipitate interface energies and/or suppressed ordering, the core-shell morphology is favored.

Influence of C addition on solid-state reaction of Ti-Si-C system

The effect of C addition on synthesis behavior, microstructure, reaction mechanism and high-temperature oxidation resistance of Ti-Si system was investigated. The Ti5Si3Cx-TiC composite was synthesized by introducing the C into Ti-Si system and using mechanically activated self-propagating high-temperature synthesis (MASHS). The addition of 1 mol C into 5Ti+3Si reduced activation energy of reaction from 207.5 kJ/mol to 189 kJ/mol. The reaction in the Ti-Si-C system was initiated by the formation of TiC and followed by synthesis of an interstitial solid solution of Ti5Si3Cx, while reaction in Ti-Si was activated by solid-state diffusion of Si into Ti. The ignition temperature of the reaction was reduced from 1000 °C to 900 °C when the combustion temperature slightly increased as a result of the C addition into the system. The wave velocity of the reaction was also declined from 55 mm/s to 48 mm/s. The microstructural observations show that dissolution of the C in Ti5Si3 crystal structure restricted microcracks formation through coarse grains. Moreover, the oxidation resistance of titanium silicide was improved by the formation of the interstitial solid solution.