xSix Alloys Research Articles

Real-time study of the interplay between phase formation and stress evolution at the W1−xSix/a−Si interface

Nanoscale W1−xSix layers with different Si content were deposited by magnetron sputtering on amorphous silicon layers. The structure and stress evolution during deposition were monitored and in real time. Thus, it was possible to disentangle different origins of stress built-up (interface formation, phase formation, grain growth, and texture) on the nanoscale. It was found that the bcc phase with a composition-dependent texture forms at low Si content (less than 10% Si), but only above a critical thickness. At intermediate Si content (13.9% and 16.5% Si), or low Si content and low thicknesses (≲4.5 nm), the β phase with A15 structure and coexisting phases (bcc or amorphous) form, while a single, amorphous phase is observed at high Si content (≳22% Si). An A15 formation driven by impurities (O,C,N) was excluded by combined x-ray photoelectron spectroscopy and x-ray diffraction measurements on a second sample series. calculations of substitutional bcc and A15 W1−xSix alloys support the formation of A15 at higher Si content. The A15-containing interlayer should be taken into account when discussing the superconductivity of W/Si multilayers. The stress evolution during deposition of W1−xSix was correlated with the microstructure evolution, and compared to similar observations for Mo1−xSix. Due to the crystalline phase (bcc/A15) and bcc texture ([111] and [110]) competition, the structure formation of W1−xSix shows a higher complexity. This explains the wide range of stress states (from tensile to compressive) observed after deposition of W1−xSix. Nanoscale W-Si based stress-compensation layers could be employed for tailoring the stress state of nonepitaxial semiconductor devices. Published by the American Physical Society 2024

Effects of metalloid Si on the microstructure and mechanical properties of Fe36Ni36Cr10Mo1Al17−XSiX alloys

Multi-component alloys with good mechanical properties and casting liquidity have become a hotspot in the materials field. Typically, the addition of minor metal elements plays a vital role in strengthening the mechanical properties. However, doping metalloid elements such as Si into multi-component alloys is also an effective method to enhance the mechanical properties. In the present study, the effect of Si (0 ∼ 4 at%) on the microstructure and mechanical properties was investigated within both as-cast and thermal mechanical treatment states. In as-cast state, all 4 alloys exhibited dendrites and interdendrites structure. The fine structure enhanced tensile strength for Si1 and improved ductility for Si2. Once the Si content reached 4 at%, both the strength and ductility dropped dramatically. Phase selective recrystallization and aging greatly increased the tensile strength and ductility simultaneously. The alloys with addition of Si had similar mechanical properties after thermal mechanical treatment process. Si2 after recrystallization and aging was considered a promising candidate for further application with a yield strength of 960 MPa, tensile strength of 1463 MPa and fracture strain of 20.5%. This work provided a new prospect to design multi-component alloys with nonmetallic elements, which may be applied to wear, corrosion and oxidation resistance fields.

Exploring the heat capacity and magnetocaloric behaviors of rare-earth based multicomponent (Ce0.71Pr0.07Nd0.22)2Fe17‐xSix alloys

The rare–earth based multicomponent (Ce0.71Pr0.07Nd0.22)2Fe17−xSix (x = 0–0.6) alloys were fabricated by conventional arc melting following the standard annealing and quenching processes. The prepared alloys were characterized by X-ray diffraction (XRD), backscattered scanning electron (BSE) microscope, energy dispersive X-ray (EDX) spectroscopy, magnetization, and heat capacity measurements. Combination of Rietveld refinement of XRD patterns, EDX/BSE analyses confirm the formation of stable rhombohedral Th2Zn17 type crystal structure with R-3 m space group in all studied samples. The investigated samples experience second-order phase transition as confirmed through phenomenological universal scaling analysis. The magnetic entropy change (−ΔSM) and associated relative cooling power (RCP) values for all compositions were found in the range of 1.1–1.4 Jkg−1K−1 and 55–66 Jkg−1 under magnetic field change (ΔH) of 0–1 T. The ΔSM curves as a function of temperature exhibit a wide working temperature range (δTFWHM) over 40 K for all studied compositions. The performance-cost ratio − ΔSM/cost and RCP/cost values are estimated to be 0.1 JK−1$−1 and 5.0 J$−1, respectively. In particular, a large adiabatic temperature change of 7.36 K and 8.09 K was achieved for x = 0.2 and x = 0.4, respectively, at low ΔH= 0–1 T. The better performance-cost ratio and good magnetocaloric properties with near-zero magnetic hysteresis loss may make these alloys promising candidates for magnetic refrigeration.

Investigation of Nanoscale Phase Formation in Rapidly Solidified Fe<sub>20</sub>Co<sub>20</sub>Ni<sub>20</sub>Cr<sub>20</sub>B<sub>20−</sub><i><sub>x</sub></i>Si<i><sub>x</sub></i> Alloys

In this study, nanoscale phase formation in rapidly solidified Fe20Co20Ni20Cr20B20−xSix alloys was investigated using atom probe tomography (APT) and transmission electron microscopy (TEM). According to previous X-ray diffraction measurements and scanning electron microscopy with energy-dispersive X-ray spectroscopy results, it was thought that this alloy system was composed of almost a single face-centered cubic (FCC) solid solution phase when the Si content was 5 at.%. However, APT and TEM results at the nanoscale showed that both an FCC solid solution and compound phases formed for all alloy compositions. The rapidly solidified alloy ribbon with a Si content of 5 at.% showed the presence of nanoscale FCC and Cr2B phases. Furthermore, in addition to the nanoscale FCC and Cr2B phases, a nanoscale Cr3Ni5Si2 phase was observed when the Si content exceeded 7.5 at.%.

Tuning the optical properties through bandgap engineering in Si-doped YAuPb: ab initio study

In order to probe the bandgap engineering to tune optical properties in YAuPb1−xSix (x = 0, 0.25, 0.50, 0.75 and 1) alloys, we used all-electron full-potential linearized augmented plane wave (FP-LAPW+lo) method within the framework of the density functional theory. The optimized structural parameters were in good agreement with other theoretical and experimental results. The calculated results of elastic constant satisfy the condition for mechanical stability at each composition for cubic symmetry. In addition, the study of elastic parameters is summarized for the calculation bulk modulus, Young’s modulus, shear modulus, Kleinman parameters, Poisson’s ratio and Lame’s co-efficient. To predict the brittle (ductile) nature of this composition, the Cauchy pressure, Poisson’s ratio, and B/G ratio were also calculated. Using modified Becke and Johnson GGA, the bandgap values of each composition were computed precisely. Further, it was observed that for 0.25 < x < 0.75, the bandgap structure revealed a direct bandgap configuration. In order to analyze the electronic structure of each composition, the total and partial densities of states have been investigated in detail. Furthermore, the investigation of optical parameters in terms of dielectric functions revealed the potential of these alloys for optoelectronic devices.

Electronic structures and related properties of Ce2Fe17-based magnetocaloric material upon Nd/Si substitution

The electronic structures and magnetocaloric properties of Ce2Fe17-based materials doped with Nd/Si have been investigated by means of experiments and first-principles calculations based on density functional theory (DFT). Ce1.6Nd0.4Fe17−xSix alloys crystallize in a rhombohedral Th2Zn17-type 2:17R main structure with trace amounts of the α-Fe secondary phase, and Si substitution for Fe contributes to the 2:17R phase. Compared with the parent Ce2Fe17 compound, Nd/Si substitution increases the Curie temperature (TC) to 273 K for Ce1.6Nd0.4Fe17 and to 297 K for Ce1.6Nd0.4Fe16.67Si0.33. DFT calculations reveal that Nd/Si addition increases the c/a values and Fe-Fe interatomic distances at the 6c site, thus leading to an enhanced TC. In addition, Si atoms in Ce2Fe17-based alloys energetically prefer to occupy the 18h site. The local density of states and charge density difference map for a single Si and its first-nearest-neighbor Fe atoms confirm a strong hybridization between the electronic orbitals of Si and Fe atoms, leading to slight lattice contraction and a decrease in the formation energy of the 2:17R phase. The Ce1.6Nd0.4Fe17−xSix alloys exhibit a maximum isothermal magnetic entropy change (ΔSM) of 3.7–4.1 J/kg K, working temperature range (∆TFWHM) of 90.5–95.5 K, and a relative cooling power (RCP) of 350–371 J/kg at μ0∆H = 5 T, suggesting that Nd/Si substitution contributes to the optimization of Ce2Fe17-based alloys as potential room-temperature magnetic refrigerants.

Magnetism and Thermomechanical Properties in Si Substituted MnCoGe Compounds

MnCoGe-based compounds have been increasingly studied due to their possible large magnetocaloric effect correlated to the magnetostructural coupling. In this research, a comprehensive study of structure, magnetic phase transition, magnetocaloric effect and thermomechanical properties for MnCoGe1−xSix is reported. Room temperature X-ray diffraction indicates that the MnCoGe1−xSix (x = 0, 0.05, 0.1, 0.15 and 0.2) alloys have a major phase consisting of an orthorhombic TiNiSi-type structure with increasing lattice parameter b and decreasing others (a and c) with increasing Si concentration. Along with M-T and DSC measurements, it is indicated that the Tc value increased with higher Si concentration and decreased for structural transition temperature Tstr. The temperature dependence of the magnetization curves overlaps completely, indicating that there is no thermal hysteresis, and it is shown that the transition is the second-order type. It is also shown that the decreased magnetization on the replacement of Si for Ge decreases the value of −ΔSM from −ΔSM~8.36 J kg−1 K−1 at x = 0 to −ΔSM~5.49 J kg−1 K−1 at x = 0.2 with 5 T applied field. The performed Landau theory has confirmed the second-order transition in this study, which is consistent with the Banerjee criterion. The magnetic measurement and thermomechanical properties revealed the structural transition that takes place with Si substitution of Ge.

Enhancement of the glass forming ability of superelastic Ni-Ti-Cu-Zr-Si alloy

In this study, the use of (Ni0.35Ti0.30Cu0.15Zr0.20)100−xSix (x = 0, 0.5, 1.0, 2.0 at%) alloy as a metallic glass precursor for a superelastic alloy, following thermoplastic forming, is investigated. The addition of Si improved the glass forming ability such that a fully amorphous sample with a thickness of ~450 µm was obtained with the x = 2.0 at% alloy. The addition of Si effectively stabilises the supercooled liquid by significantly increasing the activation energy of glass transition. The incubation time, τ, increased from 89 s for the x = 0 alloy to 346 s for the x = 2.0 at% alloy. The parameters relevant to superelastic effects, such as the maximum indentation depth (Dmax) and remnant indentation depth (Drem), remained almost constant. However, the remnant depth ratio increased slightly with the addition of Si. The fabrication of millimetre-scale Ni-Ti-Cu-Zr-Si metallic glass was achieved via the hot pressing of overlapped as-cast plates.

Reduced Annealing Time and Enhanced Magnetocaloric Effect of La(Fe, Al)13 Alloy by La-nonstoichiometry and Si-doping

LaFe13−xMx (M = Si, Al) alloys are promising for use in magnetic refrigeration. However, they require long annealing time (30 days) in order to optimize the magnetocaloric properties. Research has shown that the addition of extra La in off-stoichiometric alloys can greatly shorten the annealing time. Therefore, the purpose of this study is to investigate the influence of the extra addition of La on the annealing properties of a new off-stoichiometric La1.7Fe11.6Al1.4−xSix (x = 0, 0.1, 0.4) alloys. It was demonstrated that after a 36h annealing time, a large volume fraction of 1:13 magnetocaloric phase was obtained for all alloys. Further microstructural analysis of the off-stoichiometric La1.7Fe11.6Al1.4−xSix alloys revealed a facet-like grain morphology. The La1.7Fe11.6Al1.4 and La1.7Fe11.6Al1Si0.4 alloys were shown to contain large 1:13 phase precipitates separated in a La-rich matrix, while the La1.7Fe11.6Al1.3Si0.1 alloy had a continuous 1:13 phase matrix with a fine dispersion of La-rich precipitates throughout. When the magnetic field varied between 0 and 2 T, the corresponding magnetic entropy change and relative cooling capacity for the La1.7Fe11.6Al1.3Si0.1 specimen were determined as 4.58 J/kg K and 173.6 J/kg, respectively. More importantly, the La1.7Fe11.6Al1.3Si0.1 alloy displayed only a slight volume change when the meta-magnetic phase transition occurred, which is promising for cyclic use.

Unraveling the abnormal dependence of phase stability on valence electron concentration in Ni–Mn-based metamagnetic shape memory alloys

Valence electron concentration (e/a) dependence of phase transition temperature TM, i.e., a higher e/a leading to an elevated TM, is a well-accepted criterion for the Ni–Mn-based alloys. However, this tendency is not always obeyed by certain alloy systems, such as the Ni2Mn(Ga, Z) alloys (Z = Si, Ge, and Sn). The origin of this abnormal behavior remains uncovered. In this work, by first-principles calculations, the origin of the abnormal e/a dependence of phase stability in the Ni2MnGa1−xSix (x = 0–1) alloys is elucidated through examining the electronic structure, phonon, and magnetism. We find that the abnormal e/a dependence of phase stability intrinsically originated from the chemical composition change. The composition variation brings about a reduction of the minority-spin electronic states near the Fermi energy and the weakness of the Fermi surface nesting. Moreover, the substitution of Si for Ga leads to a decreased magnetization of austenite and an increased magnetization of martensite, which also makes a non-negligible contribution to the abnormal phase stability. The conclusions drawn for the Ni2MnGa1−xSix alloys can be well extended to understand the structural transition in other abnormal alloying systems, such as the Ni2MnGa1−xZx alloys (Z = Ge and Sn). This work clarifies the origin of the abnormal dependence of phase stability on e/a in the Ni–Mn-based alloys and provides solid knowledge for the design of advanced magnetic shape memory alloys.

Effect of Si Addition on Martensitic Transformation and Microstructure of NiTiSi Shape Memory Alloys

The effect of Si addition on microstructure and martensitic transformation of the Ni-Ti-Si SMA was investigated in this work. The Ni49−0.5xTi51−0.5xSix alloys with various amounts of silicon (2.1 ≤ × ≤ 21.1 at. pct) were prepared by the self-propagating high-temperature synthesis (SHS). Temperatures of the phase transformation increase with the increasing amount of silicon in the samples, which also causes the change of the Ti/Ni ratio in the matrix. The temperatures of austenite transformations are around the temperature of 373 K.

Ab Initio Studies of Phase Transformations in Fe100 – xSix

A concentration phase diagram for Fe100 – xSix (9.375 ≤ x ≤ 25.0 at %) alloys has been built on the base of the structural and magnetic phase transition temperatures estimated theoretically from the first principles. The structural phase transition temperatures for the experimentally observed crystal structures are obtained from the geometric optimization of the crystal structure. The Curie temperatures are estimated in a molecular field approximation using the constants of magnetic exchange interaction calculated ab initio. As temperature increases, the structural transitions from the ordered cubic phase to a completely disordered structure occur via a partially disordered structure over the entire concentration range. As for the magnetic transformations, the ferromagnet–paramagnet transition is observed for all the compositions, but in various crystal phases.

Mechanical properties, microstructural and thermal evolution of Mg65Ni20Y15−xSix (X = 1, 2, 3) alloys by mechanical alloying

We report on a work of the influence of the mechanical alloying on the microstructure, thermal and mechanical features of Mg65Ni20Y15−xSix (X = 1, 2, 3) alloys. The Mg-based alloys were produced by mechanical alloying technique from mixtures of pure crystalline Mg, Ni, Y and Si powders. These alloys were investigated using a variety of analytical techniques including x-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectrometry (EDX) and differential scanning calorimetry (DSC). The mechanical properties of the alloys were investigated by Vickers microhardness (HV) tester. After 75 h of milling time, three different intermetallic phases were obtained. These phases were defined as Mg24Y5, Mg2Ni3Si and Mg2Ni by XRD data. The particle and crystallite sizes of the Mg-based alloys were decreased by increasing milling time and they were calculated 2 μm and ∼9 nm, respectively. From the EDX analysis, it was determined that compositional homogeneity of the Mg-based alloys was fairly high. The microhardness values of the Mg65Ni20Y15−xSix (X = 1, 2, 3) alloys increased by increasing Si into the alloys and were determined 101, 131 and 158 HV, respectively.

Crystal growth kinetics in undercooled melts of pure Ge, Si and Ge-Si alloys.

We report on measurements of crystal growth dynamics in semiconducting pure Ge and pure Si melts and in Ge100-x Si x (x = 25, 50, 75) alloy melts as a function of undercooling. Electromagnetic levitation techniques are applied to undercool the samples in a containerless way. The growth velocity is measured by the utilization of a high-speed camera technique over an extended range of undercooling. Solidified samples are examined with respect to their microstructure by scanning electron microscopic investigations. We analyse the experimental results of crystal growth kinetics as a function of undercooling within the sharp interface theory developed by Peter Galenko. Transitions of the atomic attachment kinetics are found at large undercoolings, from faceted growth to dendrite growth.This article is part of the theme issue 'From atomistic interfaces to dendritic patterns'.

Magnetostructural transformation and large magnetocaloric effect in Mn0.9Cu0.1CoGe1−xSix alloys

The structural transition temperature Tt increases largely with the introduction of Si in Mn0.9Cu0.1CoGe1-xSix alloys due to the enhancement of degree of hexagonal distortion. Thus, the first-order magnetostructural transformation from ferromagnetic orthorhombic to paramagnetic hexagonal phase can be obtained in the range of 0.15 < x < 0.21. Large magnetocaloric effect (MCE) under low magnetic field change of 2 T is obtained due to the magnetostructural transformation, e.g., the maximum magnetic entropy change (-ΔSM) value for x = 0.16 is 10.3 J/kg K at 297 K, which is comparable to or even larger than those of some typical room-temperature magnetocaloric materials. The nature of magnetostructural transition has been studied by different methods, and it is found that the Arrott plots fail to determine the order of phase transition. In contrast, the universal curve of ΔSM is proved to be a more effective criterion to distinguish the order of phase transition. Besides, a linear relationship between -ΔSM and Δμ0H is found, and so the -ΔSM values for higher field changes can be estimated by linear fitting. Consequently, large MCE induced by magnetostructural transformation suggests that Mn0.9Cu0.1CoGe1-xSix alloys could be promising candidates for room-temperature magnetocaloric materials.

Interaction influence of S/D GeSi lattice mismatch and stress gradient of CESL on nano-scaled strained nMOSFETs

The intrinsic stress of thin films has become an important resource as advanced strain engineering is introduced into nanoscaled semiconductor devices. The performance of preferred device infrastructure can be studied by estimating stress-induced mobility gain. However, traditional simulation approaches for device stress typically do not consider the eventual consequence of the stress gradient of strained thin films on nanoscaled transistor stress prediction. Thus, the actual operating characteristic of concerned devices can be easily misunderstood. To resolve this issue, this research presents a fabrication-oriented simulation methodology for device stress and combines it with a multi-layered deposition model for thin film. This study aims to explore stress-induced effects on the performance of a Ge-based testing vehicle for 20nmn-type metal-oxide-semiconductor field-effect transistors (nMOSFETs) with Ge1−xSix alloys embedded into the source/drain regions of the device. Intrinsic stress is introduced via a 1.0GPa tensile contact etch stop layer (CESL). Analysis results indicate that stress contours adjacent to the top of the device gate are different from those obtained using the conventional simulation method. Moreover, greater CESL stress passes through the spacer and reaches the device channel. Furthermore, a relationship between the stress components and piezoresistivity coefficients of the Ge material is adopted to estimate the width dependence of the Ge-based nMOSFET on its mobility gain. A maximum mobility gain of up to 60.87% is acquired from the estimated results, and a channel width of 200nm is preserved.

Structure and magnetism of the Sm7.5Y2.5Fe90−xSix (x=0.0, 2.5, 5.0, 7.5 and 10) alloys

Sm7.5Y2.5Fe90−xSix (x=0.0, 2.5, 5.0, 7.5 and 10) alloys have been prepared by arc melting method and equilibrium disordered Th2Zn17-type phases, (Sm,Y)2−y(Fe,Si)17+2y, with relative lower rare-earth content than the ordered Th2Zn17-type phase, have been obtained. Compared to the ordered Th2Zn17-type structure, the X-ray diffraction (XRD) intensity of the superstructure lines of the (Sm,Y)2−y(Fe,Si)17+2y decreases with the increase of the Si content and becomes zero for x=10. According to the refinement with the disordered Th2Zn17-type structure, the occupation rates of the R atoms at (3a) and (6c) sites tend to reach the same value with the increase of the Si content, and the lattice parameter a decreases while the lattice parameter c increases, leading to an increase of c/a. It was found that the atomic ratio of Fe(Si)/Sm(Y) in the disordered Th2Zn17-type structure increases with the increase of Si content and reaches a maximum value of 9.07 with x=10. The XRD diagrams of the magnetic aligned samples indicate that the easy magnetization direction (EMD) of the (Sm,Y)2−y(Fe,Si)17+2y is in the a-b plane, and the change of the EMD in a-b plane has also been observed due to the Si preferred site occupation. The remanence ratios along the easy direction are higher than that along hard direction; however, all the remanence ratios are less than 0.5. The magnetocrystalline anisotropy constant K increases first and then decreases with increasing the Si content. The Curie temperature of Sm7.5Y2.5Fe90−xSix alloys increases by about 65K per Si. The saturation magnetization increases first and then decreases with a maximum of 135.5emu/g observed for x=2.5 at room temperature.

Electronic and Magnetic Properties of Co2CrGa1−x Si x Heusler Alloys

First-principles calculations within full-potential linear-augmented plane-wave method using the generalized gradient approximation were carried out for the electronic and magnetic properties of the Co2CrGa1−xSix Heusler alloys. The electronic structure calculations show a conservation of the minority spin gap supporting the half-metallic character of the Co2CrGa1−xSix alloys. The substitution of Ga by Si results in linear increasing of total magnetic moment as silicon concentration increases following the Slater–Pauling rule that is in agreement with experimental data.

Effects of in-situ formed Mg2Si phase on the hydrogen storage properties of Mg[sbnd]Li solid solution alloys

MgLi solid solution alloys, Mg90Li10−xSix (x=0,2,4,6), were synthesized via sintering and ball-milling. Analysis of the samples, via X-ray diffraction (XRD), revealed that the Mg2Si phase formed with the addition of Si. Moreover, measurements of the pressure composition isotherms (PCT) revealed reversible hydrogen storage capacities of 6.47wt.%, 5.69wt.%, 5.56wt.%, and 5.28wt.%, respectively, at 623K, for the Mg90Li10−xSix (x=0,2,4,6) alloys. The Mg2Si acted as a catalyst that reduced the apparent activation energy (Ea), enhanced the reversible hydrogenation reaction, and improved the kinetic and thermodynamic performance of the hydrogenation/dehydrogenation process of the MgLi solid solution.

Formation of donors in germanium–silicon alloys implanted with hydrogen ions with different energies

The distributions of hydrogen-containing donors in Ge1–xSix (0 ≤ x ≤ 0.06) alloys implanted with hydrogen ions with an energy of 200 and 300 keV and a dose of 1 × 1015 cm–2 are studied. It is established that, at the higher ion energy, the limiting donor concentration after postimplantation heat treatment (275°C) is attained within ~30 min and, at the lower energy, within ~320 min. In contrast to donors formed near the surface, a portion of hydrogen-containing donors formed upon the implantation of ions with the higher energy possess the property of bistability. The limiting donor concentration is independent of the ion energy, but decreases from 1.3 × 1016 to 1.5 × 1015 cm–3, as the Si impurity content in the alloy is increased from x = 0.008 to x = 0.062. It is inferred that the observed differences arise from the participation of the surface in the donor formation process, since the surface significantly influences defect-formation processes involving radiation-induced defects, whose generation accompanies implantation.