SiC Compounds Research Articles

Switching it up: New mechanisms revealed in wurtzite-type ferroelectrics.

Wurtzite-type ferroelectrics have drawn increasing attention due to the promise of better performance and integration than traditional oxide ferroelectrics with semiconductors such as Si, SiC, and III-V compounds. However, wurtzite-type ferroelectrics generally require enormous electric fields, approaching breakdown, to reverse their polarization. The underlying switching mechanism(s), especially for multinary compounds and alloys, remains elusive. Here, we examine the switching behaviors in Al1-xScxN alloys and wurtzite-type multinary candidate compounds we recently computationally identified. We find that switching in these tetrahedrally coordinated materials proceeds via a variety of nonpolar intermediate structures and that switching barriers are dominated by the more-electronegative cations. For Al1-xScxN alloys, we find that the switching pathway changes from a collective mechanism to a lower-barrier mechanism enabled by inversion of individual tetrahedra with increased Sc composition. Our findings provide insights for future engineering and realization of wurtzite-type materials and open a door to understanding domain motion.

Functionally graded structure of a nitride-strengthened Mg2Si-based hybrid composite

The ex-situ incorporation of the secondary SiC reinforcement, along with the in-situ incorporation of the tertiary and quaternary Mg3N2 and Si3N4 phases, in the primary matrix of Mg2Si is employed in order to provide ultimate wear resistance based on the laser-irradiation-induced inclusion of N2 gas during laser powder bed fusion. This is substantialized based on both the thermal diffusion- and chemical reaction-based metallurgy of the Mg2Si–SiC/nitride hybrid composite. This study also proposes a functional platform for systematically modulating a functionally graded structure and modeling build-direction-dependent architectonics during additive manufacturing. This strategy enables the development of a compositional gradient from the center to the edge of each melt pool of the Mg2Si–SiC/nitride hybrid composite. Consequently, the coefficient of friction of the hybrid composite exhibits a 309.3% decrease to –1.67 compared to –0.54 for the conventional nonreinforced Mg2Si structure, while the tensile strength exhibits a 171.3% increase to 831.5 MPa compared to 485.3 MPa for the conventional structure. This outstanding mechanical behavior is due to the (1) the complementary and synergistic reinforcement effects of the SiC and nitride compounds, each of which possesses an intrinsically high hardness, and (2) the strong adhesion of these compounds to the Mg2Si matrix despite their small sizes and low concentrations.

An investigation into transition states of cyclic tetra-atomic silicon and germanium interstellar dust compounds: SixC4-x, GexC4-x, and GexSi4-x (x ∈ {1,2,3}).

Presented in this work is a thorough determination of the transition states between the different isomers of cyclic tetra-atomic silicon carbide, germanium carbide, and germanium silicide clusters. Through use of density functional theory (B3LYP-D3BJ, M06-2X, ωB97X-D4, and B2GP-PLYP) in conjunction with the aug-cc-pVTZ basis set, transition state structures and their barrier heights are determined for the interconversions between the various isomers for the family of tetra-atomic SiC, GeC, and GeSi compounds. SiC dust grains are known to be prevalent in interstellar dust, and among this group, so far only diamond-shaped (d-)SiC3 has been detected in the interstellar medium (ISM). Determining which other structures might be detectable not only depends on their intrinsic spectroscopic features, but whether or not they are likely to exist as isomers in interstellar environments. By examining the energy barrier heights for transitions between isomers, we determined that many of these structures are unlikely to exhibit interconversion in the ISM, outside of hotter circumstellar environments. Although Boltzmann population ratios at approximate circumstellar temperatures suggest the presence of higher energy minima, it is likely that once interconversion happens, as molecules travel away from a star and cool, they will get kinetically trapped in the potential energy well they inhabit, making how the ratios freeze out dependent on the time and pathways the molecules take to cool down. As such, many of these higher energy minima may still be good candidates for detection including (rhomboidal) r-SiC3, r-GeC3, r-GeSi3, (trapezoidal) t-Si2C2, r-Ge2C2, and d-Si3C.

Electronic and magnetic behaviors of Cr and Mn (co)-doped zinc-blende SiC compound

Electronic and magnetic behaviors of Cr and Mn (co)-doped zinc-blende SiC compound

Copper-containing tetra-phenyl polyhedral oligomeric silsesquioxane endows epoxy resin with reinforcements in the thermal stability, flame retardancy and smoke suppression

Massive release of black smoke during combustion severely impede epoxy resin (EP) applications in the fields with high fire safety requirements. Herein, Cu was grafted into Si-O-Si cage to obtain copper-containing tetra-phenyl polyhedral oligomeric silsesquioxane (Cu-Ph-POSS). The chemical structure of Cu-Ph-POSS was fully characterized. Cu-Ph-POSS was adopted to solve the flame retardant and smoke suppression problems of EP. The incorporation of 2 wt% Cu-Ph-POSS into EP distinctly improved the thermal stability, meanwhile, decreasing the peak of heat release rate (p-HRR), peak of smoke production rate (p-SPR) and peak of CO production rate (p-COP) by 56.8%, 44.4% and 59.4%, respectively. The interior char layers richened in polycyclic aromatic and Si-C compounds, which effectively prohibited the release of heat, smoke and toxic volatiles. Thus, imparting the fabricated EP composites to superior flame retardant and smoke suppression performances.

Microstructural and nanoindentation study of TaN incorporated ZrB2 and ZrB2–SiC ceramics

This study assessed the sinterability and microstructure of ZrB2-SiC-TaN and ZrB2-TaN ceramics. Spark plasma sintering at 2000 °C and 30 MPa for 5 min produced both ceramics. The relative density of ZrB2 ceramic containing TaN was 95.3%; the addition of SiC increased this value to 98.1%. SiC’s contribution to the elimination of ZrB2 surface oxides was the primary factor in the advancement of densification. The in situ formation of hexagonal boron nitride at the interface of TaN and ZrB2 was confirmed by high-resolution transmission electron microscopy, field emission-electron probe microanalyzer, X-ray diffractometry, and field emission scanning electron microscopy. Moreover, the in situ graphite might be produced as a byproduct of the SiC-SiO2 process, hence boosting the reduction of oxide compounds in the ternary system. The SiC compound had the highest hardness (29 ± 3 GPa), while the ZrB2/TaN interface exhibited the greatest values of elastic modulus (473 ± 26 GPa) and stiffness (0.76 ± 0.13 mN/nm).

Synthesis and thermomechanical behavior of SiC/Si compounds derived from wood waste

The traditional method of manufacturing SiC compounds is associated with a serious environmental problem, mainly due to the need for large amounts of energy (generally derived from oil) to reach processing temperatures (typically above 2500 ºC). In addition, the chemical reaction that gives rise to the formation of SiC has CO and CO2 as by-products. Therefore, in this work an alternative method to manufacture SiC/Si composites using waste from the wood industry as the main raw material was developed. SiC/Si composites were fabricated by infiltration of molten silicon into carbon preforms at 1500 °C. The carbon preforms were obtained by pyrolysis (in an inert Ar atmosphere) of four types of resin-carbon mixtures. The carbon used in the mixtures was obtained by pyrolysis of sawdust powder.The mechanical and thermomechanical behavior in uniaxial compression was studied at a constant compression rate of 0.05 mm/min at different temperatures (ambient, 1100 °C and 1400 °C). The maximum resistance values found were in the range of 58 and 384 MPa, while the Young's modulus values were between 40 and 120 GPa. The porosity found in the materials was between 1 and 4%. Finally, the fabricated compounds presented a homogeneous microstructure of interconnected silicon carbide in gray contrast and dispersed and unconnected whitish phases of uniformly distributed silicon.

Study of Manganese Recovery in the CaO-SiO<sub>2</sub>-MgO-Al<sub>2</sub>O<sub>3</sub>-MnO-Fe<sub>2</sub>O<sub>3</sub> System by Thermodynamic Simulation

The results of thermodynamic simulation of the manganese recovery in the CaO-SiO2-MgO-Al2O3-MnO-Fe2O3 system by carbon are presented. Parameters of the initial system are temperature range 1400-at a step of , a total pressure of 0.1 MPa, and N2. The composition of the oxide system is corresponded by the manganese ore (wt %) 1.1 MnO2, 44.3 MnO, 28.4 CaO, 9.3 SiO2, 5.4 MgO, 0.3 Al2O3, 11.2 Fe2O3 and silicomanganese slag. It contains (wt %) 16.3 MnO, 18.4 CaO, 52 SiO2, 7.8 MgO, 5.26 Al2O3, and 0.24 FeO. The amount of silicomanganese slag in the system was 0, 5, 12, and 25%. Carbon is used as a reducing agent. Its consumption is increased by 5% from the stoichiometry for the recovery of Fe and Mn and by 8% of the metal mass for the formation of iron, manganese, and silicon carbides. The simulation is carried out using HSC Chemistry 6.12 () software package. The thermodynamic characteristics of the Fe3C, Fe2O3, FeO, MnO2, Mn, Mn3C, Mn5C2, Mn7C3, Mn23C6, and SiC compounds existing in the database are refined. It was determined that an increase in the melt temperature from 1400 to increases the degree of manganese recovery (ηMn) for all compositions of the systems. An increase of the silicomanganese slag content in the mixture from 0 to 25% decreases ηMn from 89.3 to 85% at and from 95.8 to 91.5% at . The chemical composition of the high-carbon ferromanganese alloy is (wt %): 71.9-72.8 Mn, 16.6-17.9 Fe, 0.015-1.64 Si, and 9-. The simulation results can be used to develop a technology for producing a high-carbon ferromanganese when silicomanganese slag is involved in metallurgical processing.

Effect of Ti, Al, Si on the Structure and Mechanical Properties of Boron-Rich Fe–B–C Alloys

The effects of substitution of Fe in the boron-rich Fe–B–C alloys, containing 10.0–14.0 % B; 0.1–1.2 % C; Fe – the remainder, 5.0 % Ti, Al, or Si (in wt. %) have been studied with optical microscopy, X-ray diffractometry, scanning electron microscopy, energy dispersive spectroscopy. Mechanical properties, such as microhardness and fracture toughness, have been measured by Vickers indenter. The microstructure of the master Fe–B–C alloys cooled at 10 and 103 K/s consists of primary dendrites of Fe(B,C) solid solution and Fe2(B,C) crystals. It has been found that titanium has the lowest solubility in the constituent phases of the Fe–B–C alloys, with preferential solubility observed in the Fe(B,C) dendrites, where Ti occupies Fe positions. This element has been shown to be mainly present in secondary phases identified as TiC precipitates at the Fe2(B,C) boundaries. Titanium slightly enhances microhardness and lowers fracture toughness of the boron-rich Fe–B–C alloys due to substitutional strengthening of Fe(B,C) dendrites and precipitation of the secondary phases. The level of the content of Al or Si in the Fe(B,C) and Fe2(B,C) solid solutions and quantity of the secondary phases observed in the structure suggest that more Al or Si are left in the constituent phases as compared with Ti. These elements mainly enter the crystal lattice of Fe2(B,C) phase replacing iron atoms and form at their boundaries AlB12C and SiC compounds respectively. The additions of Al and Si to the boron-rich Fe–B–C alloys help to modify their fragility: while they slightly decrease microhardness values, addition of these elements improves the fracture toughness of the constituent phases. Increase in a cooling rate from 10 to 103 K/s does not bring about any noticeable changes in the solubility behavior of the investigated alloying elements. The rapid cooling gives rise to microhardness and fracture toughness of the phase constituents which average sizes significantly decrease. The effects of the alloying elements on the structure and mechanical properties of the investigated boron-rich Fe–B–C alloys have been explained considering differences in the atomic radii and electronic structure of the solute Ti, Al, or Si atoms.

Novel Si-C compounds with semiconducting and metallic properties: A DFT study

A set of novel silicon-carbon alloys with chemical formula hSi6-xCx (x=1,2,⋯,6) is proposed. The structural stability of the optimized geometries was assessed using different methods, and the mechanical (elastic constants), electronic (Kohn-Sham band structures, partial densities of states, carrier effective masses) and optical properties of the stable crystals were obtained using the GGA-PBE and the hybrid HSE06 exchange–correlation functionals. In particular, the hC6 crystal exhibits a wide direct band gap of 3.29 eV (near ultraviolet), smaller than the band gap of diamond (5.5 eV) with comparable elastic constants and, together with other direct/nearly direct gap hSi6-xCx alloys covering the energy range between 0.43 and 2.42 eV (infrared to green light), are potentially useful for optoelectronic applications. Surprisingly, the theoretical calculations predict a metallic character for one of the hSi3C3 compounds. One can speculate that a combination of these materials could be used to produce light-emitting devices covering the full range between the near infrared and the near ultraviolet wavelengths, as well as for charge transport applications.

Effect of high-temperature oxidation on Si3N4 containing Ti3AlC2

In this work, a dense Si3N4-based composite was fabricated by spark plasma sintering the mixture of Si3N4 and Ti3AlC2. The nitrogen atoms of Si3N4 diffused into the TiCx lattice and leaded to the in-situ formation of TiC0.3N0.7 and SiC compounds. After annealing in air at 900 °C, TiC0.3N0.7 was oxidized to form rutile-TiO2 particles (about 3–4 μm). Meanwhile, it was found that the oxidized composite displayed superior mechanical and tribological properties, compared to Si3N4 and Si3N4-based composite before oxidation. Typically, the fracture toughness reached 9.97 MPa m1/2, which was 59.5% higher than that of Si3N4. In addition to cracks branch, bridge and deflection, the surface barrier of oxides was another major toughening mechanism. Besides, due to the pinning effect of TiC0.3N0.7, sliding-induced microcracks were completely suppressed, and the composite exhibited excellent friction and wear performances. In this case, the composite is expected to be fabricated into ceramic parts for service in high temperature environment.

Molecular dynamics study of pristine and defective hexagonal BN, SiC and SiGe monolayers

The structural and thermodynamical properties of 2D honeycomb hybrids are investigated over a wide temperatures range by carrying out atomistic simulations using Tersoff-type interatomic empirical potential. The variance of the radial distribution as well as the height correlation functions in terms of temperature and buckling parameter are discussed for the SiC, SiGe and BN compounds. A non-monotonic growth of mean square displacement is observed when increasing the defect concentration. The energetic of Stone–Wales defects and vacancies as well as the magnitude of thermally excited ripples for both pristine and defective monolayers increase significantly at higher temperatures, which is caused by the increase in bond lengths and atom’s velocities, resulting in higher kinetic energy. It follows that thermal rippling is intricately related to thermal expansion.

Unexpected SiC nanowires growth during spark plasma sintering of WC-10Si: A comparative study on phase formation and microstructure properties against WC-10Co cermet

In the present work, SiC nanowires were formed unexpectedly during spark plasma sintering of WC-10 wt%Si cermet. The microstructure, hardness, bending strength and fracture toughness of WC-10 wt%Si were compared with conventional WC-10 wt%Co. The sintering temperatures were chosen according to time-temperature-displacement curves to enhance maximum shrinkage of composites containing Si and Co at temperatures of 1850° and 1350 °C, respectively. Thermodynamic investigations showed negative Gibbs free energy of the formation of WSi2 and SiC compounds during sintering process while the positive amounts of ΔG were calculated for the possible formation of intermetallic compounds in WC-10 wt%Co composite. XRD investigations approved the formation of WSi2 and SiC compounds while the reaction between WC and Co led to the formation of η phase as expected. The microstructure observations revealed nanowire shapes of SiC particles distributed uniformly. The bending strength of 583 ± 31 MPa and fracture toughness of 6.2 MPa m1/2 were measured for WC-10 wt%Si composite, these amounts were lower compared to bending strength and fracture toughness of WC-10 wt%Co composite (1032 ± 26 MPa and 16.4 MPa m1/2, respectively). Besides, the hardness of 2147 ± 31 and 1622 ± 28 Vickers was obtained for WC-10 wt%Si and WC-10 wt%Co composites, respectively. The weak mechanical properties of WC-10 wt%Si composite were related to the presence of pores between SiC nanowires which acted as crack propagation centers.

Donor Stabilized Diatomic Gr.14 E2 (E = C-Pb) Molecule D-E2-D (D = NHC, aNHC, NNHC, NHSi, NHGe, cAAC, cAASi, cAAGe): A Theoretical Insight.

Quantum chemical calculations have been carried out to explore the detailed electronic structure and bonding scenario in various bis-donor stabilized E2 compounds (E = C-Pb). Our computational findings reveal that the thermodynamic stabilities of the E2 core gradually decrease as we move down the group. A linear D-E-E'-D framework is observed for C2 systems, while the heavier group 14 analogues possess trans-bent geometries. Consideration of few compounds as viable targets for synthesis is suggested by their corresponding calculated formation energies. In addition, the thermodynamic stabilities of C2 systems notably increase with the saturation of the donor ring framework and are even more pronounced for boron-substituted saturated NHD ligand. QTAIM calculations affirmed that the covalent nature of E-E' bonds shifts toward the donor-acceptor region as one traverses from top to bottom along group 14. The E-D and E'-D bonds in the C2 systems have covalent nature, whereas those in Si2-Pb2 systems are characterized by donor-acceptor bonds. In addition, we have computed proton affinities and vertical ionization potentials (VIPs) of these compounds. An excellent correlation was obtained between calculated VIPs and orbital energies of HOMOs. Furthermore, in the present study, we also explored the effect of bis-donors in the stabilization of heterodiatomic SiC compounds. Our calculations indicate that a typical bonding description of the SiC(D)2 compounds should be represented by a combination of a classical double bond between C-D with significant donor-acceptor interaction in Si-D, i.e., D → Si═C═D. The SiC(D)2 systems are found to be less stable than the corresponding dicarbon compounds C2(D)2, but they show significant stabilization compared to the corresponding disilicon systems Si2(D)2.

Strain-assisted band gap modulation in intrinsic and aluminum doped p-type SiC

The uniaxial strain-induced electronic structure variations in intrinsic and Al doped (3C-, 4H-) SiC are studied by using first-principle calculations. The main findings are summarized as follows: (I) the tensile strain leads to a structural transformation in Al doped 3C-SiC, which is signed by the total energy and lattice characteristics; (II) the band gap Eg modulation with large reductions is achieved by applying strains up to 2% in all compounds; (III) The high-symmetry points of valence band maximum and conduction band minimum in intrinsic and Al doped 4H-SiC show the abnormal characteristic under strain, corresponding to the maximum Eg as the strain is ∼-1%. These results suggest that the strain approach could be used to tune the electronic structures of SiC compounds.

A study of the wear resistance of a TiC–SiC composite ceramic material prepared by spark plasma sintering

The effect of the silicon carbide content and the spark plasma sintering temperature on the porosity and wear resistance of a composite material is determined. An increase in the silicon carbide content leads to an increase in the porosity from 4 to 12%. The temperature effect on the porosity becomes apparent above 1200°C. The material consists of a matrix represented by titanium carbide grains with a size of 1–3 μm and silicon carbide grains with a size of 10–30 μm. It is found that the TiC–20%SiC compound exhibits the highest wear resistance, which is 12 times higher than that of hot-pressed silicon carbide.

Microstructure and mechanical properties of Al-nano/micro SiC composites produced by stir casting technique

In this research, composites of A356- nano SiC (0.5 and 1.5wt%) and A356-5wt% of micro SiC were produced by stir casting technique. After applying T6 heat treatment on a part of cast ingots, all samples were studied by optical and SEM microscopy and mechanical testing techniques. It was found that adding SiC particles decreases dendrite lengths because the particles act as the preferential nucleation sites for the solidification of matrix. The best modifying effect was observed in the composite containing micro SiC particles. The T6 heat treatment on A356 resulted in the dissociation and globularization of Si platelikes and formation of complex AlMg SiFe intermetallics and nano-sized Mg2Si particles. The mechanical strength of A356 increased considerably by combining T6 heat treatment and adding micro/nano SiC particles. The highest strength was obtained in the composite containing 1.5wt% nano SiC. The SEM observations showed that fracture in cast A356 occurs along the dendrite boundaries. However, the T6 treated alloy showed a combined interdendritic and cleavage fracture. The same combined mode of fracture was observed for the alloy with 0.5wt% nano SiC. The brittle fracture in the composites was attributed to the existence of SiC and/or intermetallic compounds in the matrix. The fracture mode for other composites was fully cleavage.

Structural and Microstructural Properties of Si-C and Si-C-B Powders Obtained by High-Energy Ball Milling

This work evaluated on the structural modification during high-energy ball milling of the Si-50C, Si-42.9C-19.1B e Si-33.3C-44.4B (at.-%) powder mixtures from elemental powders. Electron images revealed on occurrence of continuous fracture mechanisms in brittle particles during their processing, which presented rounded particles lower than 10 μm. X-ray diffraction results of Si-50C powders indicated that the intensity of Si peaks was slightly reduced after milling for 17 h, which were moved to the direction of larger diffraction angles after 7 h of milling, suggesting that carbon atoms were dissolved into the Si lattice in order to form an extended solid solution. Following, these values were increased due to the discrete exothermic formation of the SiC compound. In Si-C-B powder mixtures, the SiC and B4C compounds were formed after milling for 7 h.

Nanoscale SiC production by ballistic ion beam mixing of C/Si multilayer structures

The ion beam-induced mixing process using Ar+, Ga+, and Xe+ ion irradiation has been used to form SiC rich layers on the nanometer scale at the interfaces of C/Si/C/Si/C multilayer structures. The SiC depth distributions were determined by Auger electron spectroscopy (AES) depth profiling and were compared to the results of analytical models developed for ballistic ion mixing and local thermal spike induced mixing. In addition, the measured SiC depth distributions were correlated to the Si and C mixing profiles simulated by the TRIDYN code which can follow the ballistic ion mixing process as a function of ion fluence. Good agreement has been found between the distributions provided by AES depth profiling and TRIDYN on the assumption that the majority of the Si (C) atoms transported to the neighboring C (Si) layer form the SiC compound. The ion beam mixing process can be successfully described by ballistic atomic transport processes. The results show that SiC production as a function of depth can be predicted, and tailored compound formation on the nanoscale becomes feasible, thus leading to controlled synthesis of protective SiC coatings at room temperature.

Microwave Assisted Self-Propagating High-Temperature Synthesis for Joining SiC Ceramics and SiC/SiC Composites by Ni-Al System

Microwave has been applied to ignite the Self-propagating High-temperature Synthesis (SHS) of compacted Ni-Al mixtures, having 1:1 atomic ratio, in order to join Chemical Vapor Deposition (CVD) SiC ceramics and SiC/SiC composites. The average joint thickness of CVD SiC joint is about 200 μm and the Coefficient of Thermal Expansion (CTE) mismatch between CVD SiC and Ni-Al intermetallic compounds results in a interface bond strength inferior to that of the substrate and joining material; on the other hand, for the SiC/SiC composite joints, as a result of the porosity of SiC/SiC composites, the SHS products readily infiltrated into the pore spaces of the composite, leading to an increased porosity of the joint area and a better lower interface than the upper one. The mechanical strength of the joints has been evaluated by Single-Lap (SL) shear test at room temperature; neither of the ceramic joints nor the composites joint gave satisfactory results, but the ceramic joints reaching a maximum shear strength value of 56MPa exhibited a positive aspect for further experiments.