Si Clusters Research Articles

Charge Transport Mechanism and Trap Origin in Methyl‐Terminated Organosilicate Glass Low‐κ Dielectrics

The charge transport and trap nature responsible for the leakage current through thermally cured methyl‐terminated organosilicate low‐κ dielectric films are studied. It is found that the Frenkel emission does not describe correctly the charge transport in the studied films. The charge transport occurs via the phonon‐assisted electron tunneling between neutral traps as described in the Nasyrov–Gritsenko model. The obtained thermal trap energy value 1.2 eV is close to that for the oxygen divacancy (SiSiSi cluster) in SiO2. The electron energy loss spectra, photoluminescence excitation of 2.7 eV blue band spectra, and data from the simulation within the density functional theory for the model SiCOH low‐κ structure confirm the presence of oxygen vacancy and divacancy in the studied films. The thermal trap energy value estimated as half the Stokes shift of the blue luminescence also gives a value close to 1.2 eV. It proves the correctness of the Nasyrov–Gritsenko model for describing the charge transport mechanism and the conclusion that oxygen divacancies are traps responsible for the leakage current in the studied low‐κ films.

Design of solute clustering during thermomechanical processing of AA6016 Al–Mg–Si alloy

Solute clustering is a technologically important microstructural process in Al alloys. Exerting control over this process to enhance the alloy properties and reduce the energy costs of production is a major scientific and technological focus, with automotive sheet applications serving as a key driver. In this work, we detail changes in the state of clustering arising from the insertion of a thermomechanical pre-ageing process, via a coil-cooling step in a commercial production line, immediately after solution treatment. This pre-ageing step effectively mitigates the negative effects of the natural ageing on the mechanical properties that would otherwise occur after solution treatment and ahead of the final paint-bake step. Our work was focussed on a short duration/low temperature single-step pre-ageing process on a Cu-lean grade of AA6016. Using a combination of atom probe tomography and first principles density functional theory simulations, we are able to reveal and explain the atomic-scale microstructure arising from this thermomechanical process. Mg–Si co-clusters ≥ 20 solute atoms were found responsible for the excellent properties in the material exposed to the pre-ageing plus paint-baking process. Our simulations revealed that vacancies can effectively stabilise single-species Si clusters enabling them to attract further solute and serving as a pathway to formation of the larger Mg–Si co-clusters that are so beneficial to alloy properties. Our simulations also revealed that the nearest neighbour configurations are a critical aspect to the stability of the clusters.

Microstructural characterization of cold-worked 316 stainless steel flux thimble tubes irradiated up to 100 dpa in a commercial Pressurized Water Reactor

Two flux thimble tubes (FTTs) made of 15% cold-worked 316 stainless steel (SS) were harvested from Ringhals Pressurized Water Reactor (PWR) Unit 2, with peak damages of 76 and 100 displacements per atom (dpa) after 29 and 34 years’ service, respectively. Specimens sectioned from parent tubes were comprehensively characterized with nominal damage levels of ∼0, ∼41, ∼74, 76, and 100 dpa at a nominal temperature range of 285–323 °C. Both FTTs contained helium and hydrogen gases as transmutation products. The helium follows a production rate of ∼9.8 appm/dpa, while environmental factors complicate hydrogen production obscuring an exact H/dpa ratio. Irradiation-induced dislocation loops, nano-cavities, solute clusters, and microsegregation were all observed. The dislocation loops and nano-cavities indicated saturation at 41 dpa. The solute clusters continued to evolve with Ni–Si clusters formed at 41 dpa, and Ni–Si–Mn–P clusters formed at 74 and 100 dpa, but neither clusters exhibited distinct diffraction patterns at any damage levels. Solute clusters were observed to frequently be co-located with dislocation loops, but fully decorated loops were rarely detected. Significant radiation-induced segregation (RIS) was observed around grain boundaries at all damage levels. The modified inverse Kirkendall (MIK) model captured the RIS behavior of major elements. Large cavities within or around an Mn–S rich region were observed for the first time. Through all the damage levels, void swelling is always below 0.05%, making significant dimensional change unlikely in core internals when used at similar conditions. Meanwhile, the role of overwhelming nanocavities, presumably helium bubbles, should be considered in other potential degradation mechanisms, including irradiation-assisted stress corrosion cracking, embrittlement, and loss of fracture toughness, which remain the concerns for extended operation of nuclear power plants.

Separation of Impurity Iron from Polysilicon by Pulsed Electric Current

The effect of using a pulsed electric current to remove impurity iron from molten polysilicon was investigated. Using optical microscopy observation and area statistics of iron-rich content, it was found that iron tends to accumulate at the bottom of an ingot under the action of a pulsed electric current. A new separation mechanism is proposed, based on the decreased solubility of iron in polysilicon under conditions including a pulsed electric current. Thermodynamic calculations indicate the theoretical possibility of the formation of iron-rich Si clusters. These clusters sink to the bottom of an ingot under the effect of gravity and form iron-rich precipitates with silicon, thereby achieving the iron removal. This technique provides a new method for purification of polysilicon.

The role of Ti and TiC nanoprecipitates in radiation resistant austenitic steel: A nanoscale study

This work encompasses an in-depth transmission electron microscopy and atom probe tomography study of Ti-stabilized austenitic steel irradiated with Fe-ions. The focus is on radiation induced segregation and precipitation, and in particular on how Ti and TiC affect these processes. A 15-15Ti steel (grade: DIN 1.4970) in two thermo-mechanical states (cold-worked and aged) was irradiated at different temperatures up to a dose of 40 dpa. At low irradiation temperatures, the cold-worked and aged materials evolved to a similar microstructure dominated by small Si and Ni clusters, corresponding to segregation to small point defect clusters. TiC precipitates, initially present in the aged material, were found to be unstable under these irradiation conditions. Elevated irradiation temperatures resulted in the nucleation of nanometer sized Cr enriched TiC precipitates surrounded by Si and Ni enriched shells. In addition, nanometer sized Ti- and Mn-enriched G-phase (M6Ni16Si7) precipitates formed, often attached to TiC precipitates. Post irradiation, larger number densities of TiC were observed in the cold-worked material compared to the aged material. This was correlated with a lower volume fraction of G-phase. The findings suggest that at elevated irradiation temperatures, the precipitate-matrix interface is an important point defect sink and contributes to the improved radiation resistance of this material. The study is a first of its kind on stabilized steel and demonstrates the significance of the small Ti addition to the evolution of the microstructure under irradiation.

Theoretical study of the structures and electronic properties for NiX (X = Na–Cl) clusters

Using the first-principles calculations, we study the structural, electronic and magnetic properties of the neutral and ionic Nin-1X (n = 19–23; X = Na–Cl) clusters. The calculations are performed using the density functional theory with the PBE exchange–correlation energy functional. The results reveal that the most stable structures of Nin-1X (X = Na, Mg, Al, Si) clusters are all similar to those of corresponding Nin clusters, while there are substantial structural deformations of Nin-1X (X = P, S, Cl) clusters. From the optimized results, a systematic analysis is carried out to obtain the relative stability, charge transfer, magnetic moments, density of states, electron affinity and ionization potential of Nin-1X clusters. Our findings also suggest how to change the stability, electronic and magnetic properties by doping different atoms in Ni clusters.

Design of Solute Clustering During Thermomechanical Processing of AA6016 Al-Mg-Si Alloy

Solute clustering is a technologically important microstructural process in Al alloys. Exerting control over this process in order to enhance the alloy properties and reduce the energy costs of production is a major scientific and technological focus, with automotive sheet applications serving as a key driver. In this work, we detail changes in the state of clustering arising from the insertion of a thermomechanical pre-ageing process, via a coiling step, immediately after solution treatment. This pre-ageing step effectively mitigates the negative effects of the natural ageing on the mechanical properties that would otherwise occur after solution treatment and ahead of the final paint-bake step. Our work sought to raise the strength of AA6016, which nominally contains little or no Cu, to levels equivalent to the higher Cu-bearing 6xxx grade alloys, whilst preserving excellent ductility. This novel combination of process design and alloy selection has been studied in detail using a combination of atom probe tomography (APT) and first principles density functional theory (DFT) simulations. The thermomechanical process described here invokes a strong cluster strengthening phenomenon. We report on the details of the single-species Si-Si and Mg-Mg clusters and those of the Mg-Si co-clusters and relate the APT observations to our DFT calculations. Mg-Si co-clusters ≥ 20 solute atoms are found to be responsible for the excellent properties in the material exposed to the pre-ageing and paint-baking process. Our results revealed that vacancies can effectively stabilise single-species Si clusters enabling them to attract further solute and serving as a pathway to formation of the larger Mg-Si co-clusters that are so beneficial to alloy properties.

Structure, stability and bonding features of AlnSim clusters

Al–Si alloys have excellent properties for building engine blocks, and their microstructures have important effect on the performance. Investigations on the electronic structures of Al–Si clusters help to understand the microstructural evolution and improve the properties of the alloys. This paper studies the geometric structures, stabilities and bonding features of AlnSim (n,m = 2,4,6 and n + m = 8,10,12) clusters by using genetic algorithm combined with density functional theory and QCISD models. The results show there are a lot of isomers with close energies, and the geometric structures suggest no obvious Al–Si segregation. The electronic structures of the Al–Si clusters are similar to the metal clusters; the density of states of the aluminum-rich clusters agrees with the jellium shells. Doping silicon enhances the stabilities of the aluminum clusters significantly; the binding energies increase considerably with the Si/Al ratios. The bonding features and energetics demonstrate the Si–Si and Si–Al interactions are much stronger than the Al–Al interaction. As the dispersion of Si atoms will form more Si–Al bonds, it suggests the silicon atoms can form highly dispersed states in the hypoeutectic and eutectic alloys (≤ 12.6% Si).

The influence of Sc–Si clusters on aging hardening behavior of dilute Al-Sc-(Zr)-(Si) alloy

The aging hardening behaviors of Al-0.12Sc and Al-0.12Sc-0.2Zr (wt.%) alloys with the addition of Si range from 0.05 wt% to 0.3 wt% were investigated by microhardness and transmission electron microscopy. The results indicate that content of ∼0.15% Si is optimal and Sc–Si clusters forming at the early ageing stage hinder the evolution process of Al3Sc. Upon aging isochronally, Al-Sc-(Zr)–Si alloy shows two stages hardening. The first strengthen stage occurs because of the formation of Sc–Si clusters, and peak hardness of the second stage owes to (Al,Si)3Sc evolving from the clusters. During aging isothermally at 275 °C, Al-Sc-Si and Al-Sc-Zr-Si show similar hardening characteristics. But ageing at 225 °C, the Sc–Si clusters are relatively stable, the second stage increase in the Al-Sc-(0.15, 0.3%)Si alloys have no appearance. Clustering behavior of Sc and Si does not be changed by the addition of Zr. The Al-Sc-Zr-Si alloy exhibits better thermal stability. Because partition of Zr to the preceding formed (Al,Si)3Sc reduces the coarsening rate of the precipitates.

Molecular dynamic investigation of the structure and stress in crystalline and amorphous silicon during lithiation

Silicon (Si) is a promising anode material for lithium-ion batteries with a high specific capacity, but the severe volume expansion and mechanical stress generated during electrochemical cycling hinders its practical application. Considering the coupled effect between electrochemical reaction and mechanical stress, the structure and stress evolution in lithiated crystalline and amorphous Si was investigated using molecular dynamics simulation. The results show that the lithiation in both crystalline and amorphous Si is featured by the shift of the phase boundary. Lithiation is favoured along 〈1 1 0〉 crystallographic orientation in crystalline Si due to the atomic channels along this direction. The lithiation induced compressive stress is accumulated at the Si-Li phase boundary of crystalline Si and reaches −6 GPa, which slows down the lithiation process. On the other hand, the stress in amorphous Si can be mitigated through deformation at the phase boundary, and the lithiation speed is higher than that in crystalline Si. The lithiation in crystalline Si is related to a peeling-off mechanism, and Si clusters form and disintegrate during lithiation. More Si clusters are generated during the lithiation in crystalline Si than in amorphous Si.

Microstructural and elemental evolution of polycrystalline α-SiC irradiated with ultra-high-fluence helium ions before and after annealing

Helium atoms are the main reaction products that affect the properties of SiC, which is an important structural material used in the nuclear industry. Understanding the behaviour of helium in SiC is important for predicting the lifetimes of SiC in nuclear power systems. Unfortunately, the behaviour of SiC toward ultra-high concentrations (near 50 at.%) of helium have not been investigated extensively. In this study, polycrystalline α-SiC was irradiated with 300 keV He2+ ions at a fluence of 7.3 × 1017 He2+/cm2 at room temperature, followed by annealing at 900 ℃ and 1200 ℃, with microstructural and elemental evolution investigated using transmission electron microscopy, energy-dispersive X-ray and Raman spectroscopies. The evolution of helium bubbles, as well as C and Si clusters were investigated by molecular dynamics simulations. Significant volume swelling occurred during helium irradiation, and the material recrystallised after annealing at 900 ℃. Stacking faults and dislocation loops were observed in the recrystallised and crystalline layers, respectively. This work is expected to provide insight into the damage mechanism of SiC exposed to ultra-high-fluence He ions and will provide references for the application of SiC in future advanced nuclear energy systems.

Alignment of swift cluster ions in high-energy-density plasma

Unusual position exchange among ions constituting a swift cluster was found in a high-energy-density plasma. In this paper, the peculiar ion behavior appears in a Si cluster, moving with a speed of ~0.2c (c: speed of light) in a solid Al plasma in the context of the cluster-ion beam driven inertial confinement fusion (ICF). The ion position exchange behavior of cluster ions may influence the cluster-ion beam ICF (CIF) target structure design. In this paper we discuss a possibility of cluster-ion beams as the energy driver in CIF. In conclusions, the present work suggests that the following two points in CIF: (1) The unusual ion position exchange of the cluster ions would not induce a remarkable change in the ion stopping range in CIF targets, and (2) the swift cluster ions may not provide a significant vicinage effect on the stopping range.

Geometry control of size selected Pt clusters bound to Si substrate surface by cluster impact deposition.

Geometry of platinum clusters, PtN (N = 30-71), supported on a silicon substrate was investigated, aiming to control the geometry. The supported clusters were prepared by the impact of size-selected PtN + onto the substrate at a given collision energy (cluster-impact deposition), and their geometry was observed by means of a scanning-tunneling microscope. Even at the collision energy of 1 eV per Pt atom, sufficiently strong Pt-Si interaction between PtN (N = 30 and 45) and the Si substrate allows them to be supported as close-packed monatomic-layered Pt disks, while at N = 60, multilayered shapes exist besides the monatomic-layered shape, the fraction of which increases at N = 71. When the collision energy is increased, Si atoms located at the interface between the cluster and Si substrate dissolve into the cluster, and with further increase in the collision energy, the Pt-Si cluster is partially implanted into the substrate. The transition in the shape of the supported clusters with the collision energy and the cluster size was explained according to the deformation of the clusters and the substrate surface by the cluster impact. It is proposed that the momentum of PtN + per its cross section is a good index to control the geometry in the case of strong cluster-support interaction such as Pt and Si.

Structure-rate performance relationship in Si nanoparticles-carbon nanofiber composite as flexible anode for lithium-ion batteries

A flexible silicon-carbon nanofibre composite is reported as an anode material for lithium-ion batteries. Self-standing, binder-free and flexible anodes composed of Si nanoparticles embedded inside carbon nanofibers of different fibre diameter are fabricated via electrospinning. The silicon nanoparticles are effectively protected from direct exposure to the electrolyte by the carbon fibre encapsulation, leading to vastly improved capacity retention during galvanostatic half-cell cycling. Cycling results also showed that an electrode with 230 nm fibre diameter has enhanced cyclability and rate capability when compared to one with 620 nm diameter. SEM (scanning electron microscopy) and EIS (electrochemical impedance spectroscopy) post-cycling investigations of the electrodes reveals an appropriate structural stability and lower impedance during cycling for the electrode with thinner carbon fibres. This behaviour is associated with the low linear density of the Si nanoparticles along the thin carbon nanofibers, which prevents the fracture of the carbon fibres at the sites of Si clusters.

Effect of hydrogen dilution on the silicon cluster volume fraction of a hydrogenated amorphous silicon film prepared using plasma-enhanced chemical vapor deposition

We investigated the effect of hydrogen dilution on the Si cluster volume fraction of hydrogenated amorphous films by varying the hydrogen dilution ratio at 0.5 Torr and compared it to that obtained at pure silane discharge at 0.3, 0.4, and 0.5 Torr. The correlation between the plasma emission characteristic, deposition rate, and cluster volume fraction in the hydrogen dilution plasma was described. The cluster volume fractions of films under hydrogen dilution conditions were similar to those of the pure silane but showed a higher deposition rate. The results suggest that under hydrogen dilution conditions, it is possible to maintain a higher deposition rate with a lower cluster incorporation rate.

Quantification of Solute Topology in Atom Probe Tomography Data: Application to the Microstructure of a Proton-Irradiated Alloy 625

The analysis of solute clustering in atom probe tomography (APT) has almost exclusively relied on a simple algorithm based on the simple friend-of-friend analysis where a threshold distance or maximum separation defines whether atoms are part of a cluster or part of the matrix. This method is however limited to very specific microstructures and is very sensitive to parameter selection. To expand the range and applicability of current APT analysis tools, we introduce new quantitative data analysis methods based on density-based hierarchical clustering algorithms and relevant to solute clustering and segregation. We demonstrate the methods’ performance on the complex microstructure developing in a proton-irradiated Alloy 625, specifically focusing on the analyses of nanoscale Al clusters, Si clusters, and Si-decorated dislocation loops.

Structural stability and evolution of terbium‐doped silicon clusters and influence of 4f → 5d electronic transition mechanism on magnetism and appearance of photoelectron spectroscopy for TbSin0/− (n = 6‐18) clusters

AbstractThe global minimal structures of terbium‐doped Si clusters and their anions TbSi n0/− (n = 6‐18) are confirmed by employing the ABCluster unbiased global search technique combined with a B2PLYP double‐hybrid density functional and comparing consistency of simulated and experimental photoelectron spectroscopy (PES). The results demonstrated that structural evolution patterns for neutral clusters prefer Tb‐substitutional to Tb‐encapsulated configuration starting from n = 16. While for the corresponding anionic clusters, the growth pattern adopts Tb‐linked structures to encapsulated motif. The Natural Population Analysis revealed that the 4f electrons of Tb atom in TbSi n0/− (n = 6‐18) clusters participate in bonding. The way to participate in bonding is one 4f electron transition to 5d orbital ([Xe]6s24f9 → [Xe]6s24f85d1), which significantly affects the cluster's magnetism and appearance of PES. The total magnetic moments of neutral TbSi n and the corresponding anions maintain at 7 μB and 6 μB, respectively, which are larger than that of an isolated Tb atom. The HOMO‐LUMO energy gap, relative stability, and chemical bonding analysis demonstrated that superatomic TbSi16− cluster is a magic cluster with fine thermodynamic and moderate chemical stability.

Understanding the importance of the energetics of Mn, Ni, Cu, Si and vacancy triplet clusters in bcc Fe

Numerous experimental studies have found the presence of (Cu)-Ni-Mn-Si clusters in neutron irradiated reactor pressure vessel steels, prompting concerns that these clusters could lead to larger than expected increases in hardening, especially at high fluences late in life. The mechanics governing clustering for the Fe-Mn-Ni-Si system are not well-known; state-of-the-art methods use kinetic Monte Carlo (KMC) parameterized by density functional theory (DFT) and thermodynamic data to model the time evolution of clusters. However, DFT-based KMC studies have so far been limited to only pairwise interactions due to lack of DFT data. Here, we explicitly calculate the binding energy of triplet clusters of Mn, Ni, Cu, Si, and vacancies in bcc Fe using DFT to show that the presence of vacancies, Si, or Cu stabilizes cluster formation, as clusters containing exclusively Mn and/or Ni are not energetically stable in the absence of interstitials. We further identify which clusters may be reasonably approximated as a sum of pairwise interactions and which instead require an explicit treatment of the three-body interaction, showing that the three-body term can account for as much as 0.3 eV, especially for clusters containing vacancies.

Effect of Low-Temperature Ion Irradiation on the Nanostructure of 12% Chromium ChS-139 Steel

Specimens of ChS-139 steel (0.2 С–12 Cr–Ni–Mo–W–Nb–V–N–B) were irradiated up to 6 displacements per atom (dpa) with Fe ions at 250, 300, and 400°C. The radiation-induced clusters and Cottrell atmospheres enriched in Ni, Si, and Mn were revealed in irradiated samples by atom probe tomography. The typical sizes of the radiation-induced clusters were about 2–7 nm, and their number density was about (2–20) × 1023 m–3. It was shown that the highest irradiation temperature (400°C) is compliant with the lowest enrichment of clusters in Ni, Si, and Mn as well as with the largest average size of clusters (~5 nm) and the least number density (~2 × 1023 m–3). A noticeable decrease in the number density of radiation-induced clusters with an increase in the damage dose to 4 dpa at 300°C was found. At the same time, it was shown that the Cr–V–Nb–N clusters found in the initial state dissolve with the increase in the irradiation dose at all considered temperatures. The value of detected radiation-induced effects is indicative of their significant role in the low temperature radiation embrittlement of ChS-139 steel.

Adsorption and dissociation of H2 on Al4Sim (m = 2, 3, and 4) clusters

AbstractHydrogen in alanates forms unit and the bonding strength depends on the electron donor property of the metal atoms. The electronic structures of Al─Si clusters are similar to metal clusters but the Al atom is a weaker electron donor than that in light metal alanates, so hydrogen may form weaker Al─H bonds in Al─Si systems. This article studies the adsorption and dissociation of H2 on Al4Si m (m = 2, 3, and 4) clusters by using density functional theory. The physisorption of H2 molecules on Al4Si m is very weak, the average binding energies of two H atoms on Al4Si m fall in the range –0.61 to –1.50 eV. It suggests that Al─Si hydrides can be candidates for hydrogen storage at ambient conditions. The investigation on the H2 dissociation processes shows that the lowest energy barrier is 0.76 eV, which implies that H2 dissociation can occur at moderate temperatures.