Rare-earth Silicides Research Articles

High-temperature oxidation of yttrium silicides

Current silicon melt-infiltrated (SMI) ceramic matrix composites (CMCs) are limited by the melting temperature of silicon (1414 °C) and the volatility of the thermally grown SiO2 scale in high-temperature water vapor environments. Replacement of the melt-infiltrated (MI) silicon with a rare-earth silicide offers the potential to address both limitations of SMI CMCs. This study focuses on the ability of yttrium silicides to form yttrium silicates (phases with greater stability in high-temperature water vapor than SiO2) in high-temperature oxidizing environments. Yttrium silicides with compositions of 41, 67 and 95 at.% Si–Y were fabricated using arc melting and oxidized in air at 1000 and 1200 °C for up to 24 h. Oxidation resulted in the rapid formation of a non-protective Y2O3 scale and rejected Si. Additional minor oxide phases of Y–Si–O, Y2SiO5, Y2Si2O7 and SiO2 were observed to form on and beneath the specimen surface. Characterization of the microstructural evolution with time and temperature helped elucidate the diffusion mechanisms that control oxide growth rates. Replacement of MI silicon with a MI yttrium silicide would significantly compromise the high-temperature performance of a CMC due to Y2O3 CTE mismatch with SiC, high oxygen permeability and the large volume change associated with its rapid subsurface formation. Results are utilized to examine the viability of other rare-earth silicides as MI materials for CMCs.

Experimental formation enthalpies for intermetallic phases and other inorganic compounds

The standard enthalpy of formation of a compound is the energy associated with the reaction to form the compound from its component elements. The standard enthalpy of formation is a fundamental thermodynamic property that determines its phase stability, which can be coupled with other thermodynamic data to calculate phase diagrams. Calorimetry provides the only direct method by which the standard enthalpy of formation is experimentally measured. However, the measurement is often a time and energy intensive process. We present a dataset of enthalpies of formation measured by high-temperature calorimetry. The phases measured in this dataset include intermetallic compounds with transition metal and rare-earth elements, metal borides, metal carbides, and metallic silicides. These measurements were collected from over 50 years of calorimetric experiments. The dataset contains 1,276 entries on experimental enthalpy of formation values and structural information. Most of the entries are for binary compounds but ternary and quaternary compounds are being added as they become available. The dataset also contains predictions of enthalpy of formation from first-principles calculations for comparison.

On the diffraction pattern of bundled rare-earth silicide nanowires on Si(0 0 1)

Motivated by the complex diffraction pattern observed for bundled rare-earth silicide nanowires on the Si(0 0 1) surface, we investigate the influence of the width and the spacing distribution of the nanowires on the diffraction pattern. The diffraction pattern of the bundled rare-earth silicide nanowires is analyzed by the binary surface technique applying a kinematic approach to diffraction. Assuming a categorical distribution for the (individual) nanowire size and a Poisson distribution for the size of the spacing between adjacent nanowire-bundles, we are able to determine the parameters of these distributions and derive an expression for the distribution of the nanowire-bundle size. Additionally, the comparison of our simulations to the experimental diffraction pattern reveal that a (1 × 1)-periodicity on top of the nanowires has to be assumed for a good match.

Effects of Domain Boundaries on the Diffraction Patterns of One-Dimensional Structures

Motivated by diffraction experiments on the (2√3 x √3) R30◦ reconstructed Si(111) surface due to deposition of rare earth elements (Dy, Tb) and silicide formation, we analyse the splitting and non-splitting of superstructure diffraction spots. For this purpose, we model diffraction patterns for one-dimensional structures generated by the binary surface technique and use supercell models to keep the analysis simple. Diffraction patterns are calculated in the framework of the kinematical diffraction theory, and they are analyzed as a function of the domains and domain boundaries. Basic properties of the diffraction pattern are analyzed for model systems of a two-fold and a three-fold periodicity. The rules derived from these calculations are applied to the “real-world” system of Si(111)-(2√3 × √3) R30◦-RESix (RE = Dy or Tb). Depending on the combination of domains and domain boundaries of different types, a plethora of different features are observed in the diffraction patterns. These are analyzed to determine the sizes of both domain boundaries and domains from experimentally observed splitting of specific superstructure spots.

Strain-induced quasi-one-dimensional rare-earth silicide structures on Si(111)

After deposition of rare-earth elements (Dy, Tb) on Si(111) at elevated temperatures, a formerly unknown $\left(2\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}\right)R{30}^{\ensuremath{\circ}}$ reconstruction is observed by low-energy electron diffraction, while scanning tunneling microscopy measurements exhibit a $\left(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}\right)R{30}^{\ensuremath{\circ}}$ reconstruction. On the basis of density-functional theory calculations, the structure of the larger unit cell is explained by periodically arranged subsurface Si vacancies. The vacancy network in the first subsurface layer has a $\left(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}\right)R{30}^{\ensuremath{\circ}}$ periodicity, while strain is released by a $\left(2\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}\right)R{30}^{\ensuremath{\circ}}$ Si vacancy network in the second subsurface layer. In addition, this vacancy network forms quasi-one-dimensional structures (striped domains) separated by periodically arranged antiphase domain boundaries. The diffraction spot profiles are explained in detail by kinematic diffraction theory calculations, and average domain widths are deduced.

The Ce-Ni-Si system as a representative of the rare earth-Ni-Si family: Isothermal section and new rare-earth nickel silicides

The Ce-Ni-Si system has been investigated at 870/1070K by X-ray and microprobe analyses. The existence of the known compounds, i.e.: Ce2Ni15.8Si1.2 (Th2Ni17-type), Ce2Ni15-14Si2-3 (Th2Zn17-type), CeNi8.6Si2.4 (BaCd11-type), CeNi8.8Si4.2 (LaCo9Si4-type), CeNi6Si6 (CeNi6Si6-type), CeNi5Si1-0.3 (TbCu7-type), CeNi4Si (YNi4Si-type), CeNi2Si2 (CeGa2Al2-type), Ce2Ni3Si5 (U2Co3Si5-type), Ce3Ni6Si2 (Ce3Ni6Si2-type), Ce3Ni4Si4 (U3Ni4Si4-type), CeNiSi2 (CeNiSi2-type), ~CeNi1.3Si0.7 (unknown type structure), Ce6Ni7Si4 (Pr6Ni7Si4-type), CeNiSi (LaPtSi-type), CeNi0.8-0.3Si1.2-1.7 (AlB2-type), ~Ce2Ni2Si (unknown type structure), ~Ce4.5Ni3.5Si2 (unknown type structure), Ce15Ni7Si10 (Pr15Ni7Si10-type), Ce5Ni1.85Si3 (Ce5Ni1.85Si3-type), Ce6Ni1.4Si3.4 (Ce6Ni1.67Si3-type), Ce7Ni2Si5 (Ce7Ni2Si5-type) and Ce3NiSi3 (Y3NiSi3-type) has been confirmed in this section.Moreover, the type structure has been determined for ~Ce2Ni2Si (Mo2NiB2-type Ce2Ni2.5Si0.5) and ~Ce4.5Ni3.5Si2 (W3CoB3-type Ce3Ni3-2.7Si1-1.3) and new ternary phases Ce2Ni6.25Si0.75 (Gd2Co7-type), CeNi7-7.6Si6-5.4 (GdNi7Si6-type) and ~Ce27Ni42Si31 (unknown type structure) have been identified in this system.Quasi-binary phases, solid solutions, were detected at 870/1070K for CeNi5, CeNi3 and CeSi2; while no appreciable solubility was observed for the other binary compounds of the Ce-Ni-Si system.As a prolongation of Rare Earth-Ni-Si system’s isostructural rows, LaNi7Si6 and YNi6.6Si6.1 (GdNi7Si6-type), ScNi6Si6 (YCo6Ge6-type), NdNi6Si6 (YNi6Si6-type), {Tb, Ho}2Ni15Si2 (Th2Zn17-type), Nd2Ni2.3Si0.7 and Sm2Ni2.2Si0.8 (Mo2NiB2-type), Nd3Ni2.55Si1.45 (W3CoB3-type) and {Tb, Dy}7Ni50Si19 (Y7Ni49Si20-type) compounds were synthesized and investigated.Magnetic properties of the CeNi6Si6, CeNi7Si6, CeNi8.8Si4.2, Ce6Ni7Si4, CeNi5Si, Ce2Ni2.5Si0.5, Nd2Ni2.3Si0.7 and Dy7Ni50Si19 compounds have also been investigated and are presented here.

Rare-earth silicide thin films on the Si(111) surface

Rare-earth induced layered structures on the Si(111) surface are investigated by a combined approach consisting of ab initio thermodynamics, electron and x-ray diffraction experiments, angle-resolved photoelectron spectroscopy, and scanning tunneling microscopy. Our density functional theory calculations predict the occurrence of structures with different periodicity, depending on the rare-earth availability. Microscopic structural models are assigned to the different silicide phases on the basis of stability criteria. The thermodynamically stable theoretical models are then employed to interpret the experimental results. The agreement between the simulated and measured scanning tunneling microscopy images validates the proposed structural models. The electronic properties of the surfaces are discussed on the basis of the calculated electronic band structure and photoelectron spectroscopy data.

Self-organized growth and magnetic properties of epitaxial silicide nanoislands

Self-organized transition-metal (Ni and Fe) and rare-earth (Er) silicide nanostructures were grown on Si(111) and Si(001) surfaces under low coverage conditions, in a “solid phase” and “reactive deposition” epitaxial regimes. Island evolution was continuously monitored in-situ, using real-time scanning tunneling microscopy and surface electron diffraction. After anneal of a Ni/Si(111) surface at 700°C, we observed small hemispherical Ni-silicide nanoislands ∼10nm in diameter decorating surface steps in a self-ordered fashion and pinning them. Fe-silicide nanoislands formed after a 550°C anneal of a Fe-covered surface, were also self-ordered along the surface step-bunches, however were significantly larger (∼70×10nm) and exhibited well-developed three-dimensional polyhedral shapes. Ni-silicide islands were sparsely distributed, separated by about ∼100nm from one another, on average, whereas Fe-silicide islands were more densely packed, with only ∼50nm mean separation distance. In spite of the above differences between both types of island in size, shape, and number density, the self-ordering in both cases was close to ideal, with practically no islands nucleated on terraces. Superconducting quantum interference device magnetometry showed considerable superparamagnetism, in particular in Fe-silicide islands with ∼1.9μB/Fe atom, indicating stronger ferromagnetic coupling of individual magnetic moments, contrary to Ni-silicide islands with the calculated moments of only∼0.5μB/Ni atom. To elucidate the effects of the island size, shape, and lateral ordering on the measured magnetic response, we have controllably changed the island morphology by varying deposition methods and conditions and even using differently oriented Si substrates. We have also begun experimenting with rare-earth silicide islands. In the forthcoming experiments we intend to compare the magnetic response of these variously built and composed islands and correlate between their composition, crystal and morphological characteristics, and the resultant magnetic properties.

Ternary rare earth silicides RE2M3Si4 (RE = Sc, Y, Lu; M = Mo, W): crystal structure, coloring and electronic properties.

The ternary compounds Sc2Mo3Si4, Y2Mo3Si4, Lu2Mo3Si4 and Sc2W3Si4 have been synthesized using arc melting and structurally characterized using single crystal X-ray diffraction. The compounds are isostructural with Gd5Si4 but with coloring (order of the rare earth and transition metals) on the Gd site. In contrast to group 4 and 5 ternaries of the same type, we observe no site mixing between the rare earth and transition metals. The Y compound displays a different, less common coloring from the others and through DFT calculations and investigation of the solid solution between Sc2Mo3Si4 and Y2Mo3Si4 it is shown that the different coloring of the latter is only marginally more stable. The electronic structures of the ternary compounds have been investigated using DFT calculations, yielding densities of states very similar to Gd5Si4. These predict metallic behavior and no magnetism, which is confirmed through resistivity and magnetization measurements.

Experimental Grain Refiners for Carbon Steels

A series of master alloys designed to make rare earth oxide, rare earth aluminum oxide, or rare earth sulfide inclusions were created to determine which inclusions acted as heterogeneous nuclei. Plate test castings having additions of the various master alloys were produced to understand the refinement produced and the mechanical properties. The plates underwent tensile testing and metallographic analysis. For comparison, no addition and rare earth silicide addition heats were also poured. The rare earth oxide and sulfide master alloys formed the intended inclusions. The rare earth aluminum oxide master alloy had very poor homogeneity and contained inclusions with far lower aluminum contents than intended. Only the rare earth silicide reduced the grain size, resulting in improved mechanical properties. This would indicate that simple oxides or sulfides do not play a significant role in the nucleation in steel, but the more complex oxides and probably oxysulfides play the dominate roles.

Studies of valence of selected rare earth silicides determined using Si K and Pd/Rh L2,3 XANES and LAPW numerical studies

We report on the investigation of Si and Pd/Rh chemical environments using X-ray Absorption Near Edge Spectroscopy in two different families of rare earth silicides R2PdSi3 (R=Ce, Nd, Tb, Dy, Ho, Er) and HoRh2−xPdxSi2 (x=0, 0.5, 0.75, 1.0, 1.5, 1.8, 2.0). The Si K, Pd L3 and Rh L3 absorption edges were recorded in order to follow their changes upon the variation of 4f and 4d5s electron numbers. In both cases it was found that the Si K edge was shifted ≈0.5eV toward lower energies, relative to pure silicon. In the first family, the shift decreases with increasing number of f-electrons, while the Si K edge remains constant upon rhodium–palladium substitution.In all cases the Pd L3 edge was shifted to higher energies relative to metallic Pd. No visible change in the Pd L3 position was observed either with a varying 4f electron count or upon Pd/Rh substitution. Also, the Rh L3 edge did not change.For two selected members, Ho2PdSi3 and HoPd2Si2, the Wien2K’09 (LDA+U) package was used to calculate the electronic structure and the absorption edges. Si K edges were reproduced well for both compounds, while Pd L3 only exhibited a fair agreement for the second compound. This discrepancy between the Pd L3 theory and experiment for the Ho2PdSi3 sample can be attributed to the specific ordered superstructure used in the numerical calculations.The observed changes indicate that despite possessing a formal inter-metallic character, the chemical bond between the R–Si and R–Pd interactions are different. The variation and the direction of the chemical shift of the Si K edge suggests a weak ionic character of the R–Si bonds, in agreement with the localized character of the 4f electrons. In turn, the changes of the Pd/Rh edge are consistent with a metallic band that is affected by its long range chemical environment.

Characterization of Rare Earth Inclusions from Cast 1010 Steel

An electron microscopy investigation was conducted on two cast 1010 steel plate samples that had rare earth silicide additions. A scanning electron microscope (SEM) and transmission electron microscope (TEM) with energy dispersive spectrometer (EDS) characterized the inclusions found. The SEM primarily observed rare earth oxide particles. No particles less than 1 μm in diameter were found in the TEM which indicates that any possible heterogeneous nuclei are only about 1 μm in size. Electron diffraction and EDS spectra from the rare earth inclusions examined indicated that they were comprised of multiple rare earth phases. The matrix and one rare earth inclusion with a face centered cubic (FCC) like crystal structure had an eight degree alignment between crystallographic axes. This suggested that the body centered cubic (BCC) structure of ferrite and some of the FCC like rare earth compounds could align with each other. Such alignment would be necessary for any particle to act as a heterogeneous nucleus of δ-ferrite.

The Dy–Ni–Si system as a representative of the rare earth–Ni–Si family: Its isothermal section and new rare-earth nickel silicides

The Dy–Ni–Si system has been investigated at 1070K by X-ray and microprobe analysis. The system contains the 12 known compounds DyNi10Si2, DyNi5Si3, DyNi6Si6, DyNi4Si, DyNi2Si2, Dy2Ni3Si5, DyNiSi3, Dy3Ni6Si2, DyNiSi2, DyNiSi, Dy3NiSi3, Dy3NiSi2, and the new compounds Dy34Ni16−27Si50−39 (AlB2-type), Dy2Ni15.2−14.1Si1.8−2.9 (Th2Zn17-type), ~Dy11Ni65Si24, ~Dy16Ni62Si22 (unknown structures), DyNi7Si6 (GdNi7Si6-type), Dy3Ni8Si (Ce3Co8Si-type), DyNi2Si (YPd2Si-type), ~Dy40Ni47Si13 and ~Dy5Ni2Si3 (unknown structures). Quasi–binary solid solutions were detected at 1070 (870K) for Dy2Ni17, DyNi5, DyNi7, DyNi3, DyNi2, DyNi, DySi2 and DySi1.67. No detectable solubility is observed for the other binary compounds of the Dy–Ni–Si system.The crystal structures of new phases RNi7Si6 (GdNi7Si6-type), R3Ni8Si (Ce3Co8Si-type), RNi2Si (YPd2Si-type) and R3Ni12Si4 (Gd3Ru4Al12-type), with R=Y, Gd–Tm, has been studied. Magnetic properties of few representative compounds are also reported.

Mechanochemical synthesis and spark plasma sintering of the cerium silicides

The cerium silicides, Ce5Si3, Ce3Si2, CeSi, CeSi2−y, and CeSi2−x, have been prepared from the elements by mechanochemical processing in a planetary ball mill. Preparation of the cerium silicide Ce5Si4 was unsuccessfully attempted and potential reasons for this are discussed. Temperature and pressure of the milling vial were monitored in situ to gain insight into the mechanochemical reaction kinetics, which include a mechanically-induced self-propagating reaction (MSR). Some prepared powders were consolidated by spark plasma sintering to high density. Starting materials, as-milled powders, and consolidated samples were characterized by X-ray diffraction, scanning electron microscopy, and energy dispersive spectroscopy. The results obtained help elucidate key questions in mechanochemical processing of intermetallics, showing first phase formation similar to thin films, MSR ignition times that are composition- and milling speed-dependent, and sensitivity of stable compound formation on the impact pressure. The results demonstrate mechanochemical synthesis as a viable technique for rare earth silicides.

Concomitant charge-density-wave and unit-cell-doubling structural transitions inDy5Ir4Si10

The tetragonal rare-earth transition-metal silicide system with three-dimensional crystallographic structure ${R}_{5}{T}_{4}{\mathrm{Si}}_{10}$, where $R$ is Dy, Ho, Er, Tm, and Lu, and $T\phantom{\rule{0.16em}{0ex}}=\phantom{\rule{0.16em}{0ex}}\mathrm{Ir}$ and Rh, has been shown to exhibit fascinating charge-density-wave (CDW) phase transitions, a phenomenon largely found in otherwise low-dimensional systems. In this study, we report the investigations of CDW in Dy${}_{5}$Ir${}_{4}$Si${}_{10}$ at different temperatures using transmission electron microscopy techniques including electron diffraction and dark-field imaging. Incommensurate superlattice spots along the $c$ axis were observed in the electron-diffraction patterns when the sample was cooled below the CDW transition temperature at $\ensuremath{\sim}$208 K. CDW becomes commensurate with further cooling and configurations of CDW dislocations convincingly show that the CDW phase transition is accompanied by a concomitant cell-doubling crystallographic structural phase transition. Intriguingly, the cell-doubling transition is featured by a broken inversion symmetry along the $c$ axis and a disparity in the CDW-modulation vectors with opposite signs, which gives rise to two sets of CDW domains with reversed contrasts. The profound physics underlining this notable domain-contrast behavior is discussed.

Gadolinium silicide/silicon composite with excellent high-rate performance as lithium-ion battery anode

Novel composite materials consisted of elemental Si and rare-earth metal silicides, Sm–Si, Gd–Si, and Dy–Si, were synthesized and were evaluated for the first time as a high-performance anode material of Li-ion battery. Thick-film electrodes of the silicide/Si composites were prepared by an arc melting method and a successive gas-deposition method. Among them, the Gd–Si/Si composite electrode exhibited reversible Li-insertion/extraction reactions and the best cycling performance: the initial Coulombic efficiency was 80% and the discharge capacity at the 1000th cycle was 840mAhg−1. In addition to this, the composite electrode delivered a superb high-rate performance with the capacity of 2100mAhg−1 even at the high current rate of 12.0Ag−1 (4.8C). The remarkable performances demonstrated that the gadolinium silicide is favorable to significantly enhance anode properties of Si-based composite electrodes.

Experimental parameters during optical floating zone crystal growth of rare earth silicides

Optical floating zone (FZ) crystal growth involving growth stability and as-grown crystal perfection is affected by experimental conditions and the specific material. Referring to rare earth silicides, high purity of raw rare earth elements and ambient argon atmosphere are crucial to grow high-quality crystals; the maximum zone height is determined by equating the capillary forces of the surface tension; and asymmetric counter rotation of crystal and feed rod with convex (toward the melt) interfaces are favored to reach single crystals. Influences of several other growth parameters were also discussed in detail in this paper.

The electronic structure of rare-earth iron silicide R2Fe3Si5 superconductors

The electronic structures of R2Fe3Si5 (where R = Lu, Tm, Er, Tb, Yb) intermetallics have been calculated from first principles in local-spin density (LSDA) and LSDA + U approaches. The majority of rare-earth iron silicides, except for the heavy-fermion Yb-based compound, exhibit almost equal values of density of states at the Fermi level (EF) as well as very similar Fermi surface topology. The electronic structure around EF in the 235-type Fe-based compounds is completely dominated by the Fe 3d states. Thus the different superconducting properties of some members of the R2Fe3Si5 family are rather related to a presence of local magnetic moments of R-atoms than to electronic-structure features at EF.

Understanding the Mechanism of Grain Refinement in Plain Carbon Steels

This paper documents a series of plate casting experiments in 1010 steel. A set of castings with various levels of rare earth (RE) silicide and engineered grain refiner were produced via induction melting and green sand molds. One set of castings also contained a high phosphorous content to enable etching for the solidification structure. Metallographic and tensile specimens were examined to determine the resulting microstructure and property changes. Yield strength, ultimate tensile strength and percent elongation significantly increased with the RE additions. This increase appeared to be primarily a function of the cerium content of the final castings. Examination of the phosphorous containing castings revealed the solidification morphology changed from a columnar to equiaxed type. Electron microscopy revealed RE oxides formed in both sets of castings. The strength and solidification morphology changes were likely caused by the nucleation of δ-ferrite on these RE oxides.

Dy–Mn–Si as a representative of family of ‘Dy–Transition Metal–Si’ systems: Its isothermal sections, empirical rProd. Type: FTPules and new rare-earth manganese silicides

Abstract X-ray and microprobe analyses were employed for the investigation of Dy–Mn–Si system at 870/1070/1170 K. The Dy–Mn–Si system, contains the known DyMn 2 Si 2 , DyMnSi and Dy 2 Mn 3 Si 5 compounds and DyMn 4 Si 2 , Dy 2 MnSi 2 and Dy 3 Mn 2 Si 3 were new compounds identified first time and their structure are of the type TmCu 4 Sb 2 , Sc 2 CoSi 2 and Hf 3 Ni 2 Si 3 respectively. The quasi-binary solid solutions were detected at 870/1070/1170 K: the ThMn 12 -type Dy 8 Mn 87 Si 5 , Th 6 Mn 23 -type Dy 23 Mn 72 Si 5 , MgCu 2 -type Dy 33 Mn 58 Si 9 and AlB 2 -type Dy 38 Mn 2 Si 58 . The other binary compounds of the Dy–Mn–Si system do not show any visible solubility. New phases R 2 MnSi 2 and R 3 Mn 2 Si 3 ( R =Gd, Tb, Ho–Tm) were found out and their structure of the type Sc 2 CoSi 2 and Hf 3 Ni 2 Si 3 respectively. The specific features of ‘Dy–Transition Metal–Si’ systems were discussed.