Thermal Stability Of Si Research Articles

Improving Thermal Stability of Si-Based Anodes for Lithium-Ion Batteries by Controlling Bulk and Surface Layer Compositions

Si-based anodes are promising candidates for next-generation lithium-ion batteries (LIBs). Apart from electrochemical performances, their thermal stability needs immediate attention as it relates to battery safety and commercialization. In this work, we use differential scanning calorimetry (DSC) to systematically compare the thermal behaviors of silicon (Si) and silicon monoxide (SiO) electrodes with different state of charge and electrolytes. It is possible to reduce heat generation not only by reducing the amount of lithiation but also with the incorporation of oxygen matrix in the material. Moreover, the existence of robust F-containing species on the electrode surface with the addition of fluoroethylene carbonate (FEC) into the electrolyte reduces reaction between electrolyte and active material, leading to lower heat generation. Though, larger amount of FEC results in more heat as FEC is more exothermic than ethylene carbonate. Better thermal stability of Si and SiO is achievable by optimizing the FEC content in the electrolyte.

Local Arrangement of Substitutional C Atoms and the Thermal Stability of Epitaxial Si:C(P) Grown by CVD

Si:C alloys are potential candidates for source-drain stressor applications meant for n-type FinFETs. To function as an effective stressor, the substitutional C concentration (Csub) must be ≥ 1.5%. Increasing the C content to achieve high strain results in local arrangement of a fraction of Csub atoms in the form of third nearest neighbors (3 nn). We investigate if this 3 nn arrangement can be controlled by choosing optimal growth conditions and we also explore if this arrangement has an impact on the thermal stability of the Si:C layers, if they are subjected to high temperature processing. For a given Csub concentration, the use of a cyclic deposition and etching scheme reduces the 3 nn distribution in comparison to a single step deposition process. The 3 nn concentration also varied with respect to the choice of the Si precursor. Faster growth rates and adding in-situ doping did not significantly influence the 3 nn concentration for disilane, whereas the 3 nn concentration was considerably lower if trisilane was used as Si precursor. For the Si:C layers with comparable crystalline quality, the Csub loss during thermal annealing did not show a regular trend with respect to 3 nn distribution.

Kinetic effect of boron on the thermal stability of Si–(B–)C–N polymer-derived ceramics

The isothermal mass loss of two polymer-derived ceramics with compositions SiC 1.4N 0.9 and SiC 1.5N 1.0B 0.05 were measured as a function of time using thermal gravimetric analysis at various temperatures ranging between 1580 and 1720 °C. The process of mass loss, attributed to the reaction Si 3N 4 + 3C → 3SiC + 2N 2 ↑, takes substantially more time for the boron-containing ceramic compared with the boron-free one. The continuous formation of SiC crystallites as the product of the reaction between Si 3N 4 and C was revealed through X-ray diffraction (XRD) measurements during the course of the reaction. The kinetics of this reaction was studied using a generalized model for the analysis of chemical reaction kinetics. Consequently, the effective activation energies for the Si 3N 4 degradation were estimated to be 11.6 ± 0.5 eV and 17.1 ± 0.7 eV for the Si–C–N and Si–B–C–N ceramics, respectively. Moreover, the results obtained indicate that the dominant mechanisms of the Si 3N 4 degradation are strongly influenced by the presence of boron. For the Si–C–N ceramic, the chemical reaction at interfaces of the reactants and the crystallization of SiC as the reaction product are proposed to be the main probable stages controlling the progress of the investigated reaction. However, the local diffusion of C out of BNC x turbostratic layers surrounding the Si 3N 4 nanocrystals and the gas (N 2) release from the reaction zone are suggested to be the most plausible processes limiting the progress of Si 3N 4 degradation for the Si–B–C–N ceramic.

Kinetics of Ge diffusion, desorption and pit formation dynamics during annealing of Si0.8Ge0.2/Si(001) virtual substrates

Thermal stability of Si(0.8)Ge(0.2)/Si(001) virtual substrates (VS) is studied as a function of annealing temperature and time by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). Two regimes describing different Ge behavior are observed when the Si(0.8)Ge(0.2) VS is annealed. Heating the substrate from room temperature to 500 degrees C results in some degree of Ge segregation. The surface morphology however remains relatively smooth and there is no formation of 3D islands on the surface. Above 500 degrees C, Ge is preferentially lost from the surface and microscopic pits with edges aligned along 110 azimuth are formed. As temperature increases, Ge% decreases and the size of pits also increases. The decrease in Ge% and the formation of holes at the surface are attributed to Ge desorption from the surface. A kinetic model involving diffusion and desorption processes is proposed to describe the Ge behavior and pits formation in this regime.

Effects of Si 4+ and B 3+ doping on the photoluminescence of BaMgAl 10O 17:Eu 2+ phosphor under UV and VUV excitation

Si 4+ and B 3+ were doped into the spinel block of blue-emitting BaMgAl 10O 17:Eu 2+ phosphor. Their photoluminescence, such as emission characteristic and thermal stability, were investigated under ultraviolet (UV) and vacuum ultraviolet (VUV) excitation. The results showed that proper quantities of Si 4+ or B 3+ doping could obviously improve the emission intensity. However, the thermal stability of Si 4+ or B 3+ doped phosphors was worse than that of undoped samples. Based on lattice defect theory, the mechanisms of these interesting phenomena were discussed and explained, which revealed that the thermal degradation of BaMgAl 10O 17:Eu 2+ had great correlation with the lattice environment.

Influence of the Precursor Cross-Linking Route on the Thermal Stability of Si−B−C−O Ceramics

The specifics of the cross-linking route of tris(1-(dichloro(methyl)silyl)ethyl)borane to form a preceramic polymer suitable for the subsequent synthesis of Si−B−C−O ceramics has significant influence on the local phase composition and the microstructural features of the resulting samples after their ceramization and subsequent annealing. Cross-linking of tris(1-(dichloro(methyl)silyl)ethyl)borane via hydrolysis used for the preceramic polymer synthesis increases the amount of nanocrystalline SiC in the resulting Si−B−C−O ceramics up to 1400 °C, while cross-linking with polyvalent alcohols allows the homogeneous component distribution to be preserved and the formation of crystalline phases to be retarded. The different contents of bound oxygen in the preceramic polymers, depending on the cross-linking route, may explain the observed different stabilities of amorphous Si−B−C−O against phase separation and crystallization.

Epitaxial Growth of Si:C by means of Gas Source MBE

Si:C films have been successfully grown by the gas-source molecular beam epitaxy. The thermal stability of Si:C was comprehensively studied, and the formation of SiC and the diffusion of carbon atoms from the substitutional sites were observed when the film was annealed higher than 800 C. It was shown that the SOI substrate was very promising substrate in order to realize the strained Si/strain-relaxed Si:C structure, which is applicable to the vertical MOSFETs. Finally, the relaxation mechanism of Si:C was studied. It was shown that the Si:C was hard to form the misfit dislocations at the interface and that the stress was released by the cusp formation. The phenomenon is suitable for the application of Si:C to the source/drain stressor.

Effect of CO2partial pressure on oxidation of low-oxygen SiC fibers (Hi-Nicalon) in Ar-CO2gas mixtures

The oxidation behavior and thermal stability of Si—C fibers (Hi-Nicalon) in Ar-CO2 gas mixtures were investigated at 1773 K, through mass change determination, XRD analysis, resistivity measurement, SEM observation and tensile tests. Mass gain and cristobalite formation were observed at p CO 2 ≧ 103 Pa, showing the occurrence of passive-oxidation of the fibers. On the other hand, the active-oxidation was characterized by the mass loss, no formation of SiO2 film and a marked increase in resistivity at p CO 2 ≦ 5 × 102 Pa. The oxygen potential for the active-to-passive oxidation transition in Ar-CO2 gas mixtures was nearly identical to that in Ar-O2 gas mixtures. About 50% of the strength in the as-received state was retained after the active-oxidation in Ar-CO2 gas mixtures.

Molecular beam epitaxy growth and thermal stability of Si 1− xGe x layers on extremely thin silicon-on-insulator substrates

Pseudomorphic Si and SiGe layers have been grown by molecular beam epitaxy (MBE) on extremely thin silicon-on-insulator (SOI) substrates with top Si layer widths down to about 10 nm. A low-temperature substrate cleaning procedure ( T S<800°C) is required in order to avoid Si cluster formation induced by holes and defects of the SOI substrates. The relaxation behavior of very thin SiGe on SOI layers is studied which is expected to take place by elastic strain relaxation of the structure concomitant with a gliding at the Si/SiO 2 interface. Novel concepts described here are Ge diffusion into SOI layers during deposition at high temperatures and relaxation of SOI/SiGe structures with very thin layers. They promise fabrication of relaxed virtual SOI/SiGe substrates without any dislocations. Partial strain relaxation takes place by annealing SOI/Si 0.7Ge 0.3 structures at T A=750°C. Si layers with tensile strain up to about 0.3% have been realized on top of such relaxed buffer structures. Optical and atomic force microscopy (AFM) reveal smooth surfaces without cross-hatch. Transmission electron microscopy (TEM) indicates a few stacking faults which seem to be related to defects of the underlying SIMOX substrate.

Dopants on Si(100) surfaces: useful probes of silicon atomic layer epitaxy?

Submonolayer coverages of N, B, Ge and Sn were studied on Si(100) surfaces using time-of-flight scattering and recoiling spectroscopy. Thin Si layers were grown on top of the dopants, and these layers are designated Si ∗. The thermal stability of Si ∗/N/Si(100), Si ∗/B/Si(100), Si ∗/Sn/Si(100) and Si ∗/Ge/Si(100) structures is compared here. Attenuation of the elemental dopant signals by Si ∗ is used to study Si atomic layer epitaxy. The dopant is called an elemental “marker” in this context. Plots of direct recoiling intensity vs. incident angle are used to distinguish surface marker layers from marker layers buried under Si ∗.

Strain Modification and Thermal Stability of SixGe1−x Films Grown by Ion-Assisted Molecular Beam Epitaxy

AbstractSignificant changes in strain are produced in SixGe1−x epitaxial films grown on Si and Ge (001) substrates as a result of low energy ion beam assisted molecular beam epitaxy (IAMBE). Films grown with concurrent Ar+ or Xe+ ion bombardment are coherent and uniformly strained in the growth direction by up to 1.5% in Ge films and 0.5% in Si films and contain no dislocations. The dependence of the films strain perpendicular to the growth surface on ion-atom flux ratio, and ion energy can be explained by the injection of uniformly distributed point defects. Post-growth isochronal annealing of SixGe1−x films suggests that the existing defects in the IAMBE films are defect complexes and that the strain relaxation path is determined by the overall thermodynamic driving force toward the strain-relieved state.