Residual Si Research Articles

Evaluation of Microstructure Formation and Phase Equilibria for Thermoelectric β-FeSi2 Composite Alloys

ABSTRACTThermoelectric composite alloys consisting of the β-FeSi2 matrix and SiO2 particles dispersion were fabricated by a so-called combined reactions sintering process using reduction and oxidation reactions between eutectoid Si decomposed from α-Fe2Si5 and added Fe-oxide powder. Typical microstructure may include some of residual eutectoid Si particles, intermediate product Fe2SiO4 particles, and/or remaining reduced Fe particles depending on the composite alloy compositions and the process conditions. Partitioning of doping element, n-type Co or p-type Mn, during the process plays an important role to control the optimum carrier concentration of the composite alloys. Thermal conductivity can be reduced, as expected, by the dispersion of SiO2 particles. The solubility of doping elements, Co, Mn, Al, and Ru was evaluated in α-Fe2Si5 at 1373 K and in β-FeSi2 at 1073 K being based on the isotherm determination. It is suggested that suitable dopants for the present process are n-type Co and p-type Mn, since they have sufficiently large solubility around 10 at% in both α-Fe2Si5 and β-FeSi2 phases.

Microstructure and mechanical properties of reaction bonded B4C-SiC composites: The effect of polycarbosilane addition

Reaction bonded B4C-SiC composites were prepared by infiltrating silicon melt into porous B4C-SiC green preforms at 1500°C in vacuum. The porous green preform was obtained from a mixture of polycarbosilane (PCS) and particle size graded B4C after pre-sintering at 1600°C. For the first time, PCS was used to adjust the phase composition and microstructure of the reaction bonded boron carbide composites. It is indicated that the addition of PCS and its content has a significant influence on the microstructure as well as the mechanical properties of the subsequent reaction bonded B4C-SiC composites. For the B4C-SiC composite with 5wt% PCS added, a flexural strength of 319±12MPa, and an elastic modulus of 402±18GPa can be achieved, which is 23% and 15% higher than those of the composite without PCS addition, respectively. While, with the higher content of PCS addition, the mechanical properties of the composites are decreased drastically due to the large amount of residual Si agglomeration in the composites. The reaction mechanisms as well as their microstructure evolution processes correlated with the mechanical properties of the reaction bonded B4C-SiC composites are further discussed in our work.

Pilot Optimization and Power Allocation for OFDM-Based Full-Duplex Relay Networks With IQ-Imbalances

In OFDM relay networks with IQ imbalances and full-duplex relay station (RS), how to optimize pilot pattern and power allocation using the criterion of minimizing the sum of mean square errors (Sum-MSE) for the frequency-domain least-squares channel estimator has a heavy impact on self-interference cancellation. Firstly, the design problem of pilot pattern is casted as a convex optimization. From the KKT conditions, the optimal analytical expression is derived given the fixed source power and RS power. Subsequently, an optimal power allocation (OPA) strategy is proposed and presented to further alleviate the effect of Sum-MSE under the total transmit power sum constraint of source node and RS. Simulation results show that the proposed OPA performs better than equal power allocation (EPA) in terms of Sum-MSE, and the Sum-MSE performance gain grows with deviating $\rho$ from the value of $\rho^o$ minimizing the Sum-MSE, where $\rho$ is defined as the average ratio of the residual SI channel at RS to the intended channel from source to RS. For example, the OPA achieves about 5dB SNR gain over EPA by shrinking or stretching $\rho$ with a factor $4$. More importantly, as $\rho$ decreases or increases more, the performance gain becomes more significant.

Defining the tensile properties of Al-12Si parts produced by selective laser melting

Al-12Si samples were fabricated by selective laser melting. Different processing parameters, including four different hatch styles, as well as contour and base plate heating, were employed to evaluate their impact on the room temperature tensile properties. The samples with different hatch styles show similar crystallite sizes and lattice parameters of Al and comparable amount of free residual Si, but different levels of texture. The yield strength (YS) varies between 235 MPa and 290 MPa, the ultimate tensile strength (UTS) between 385 MPa and 460 MPa and the tensile ductility ranges between 2.8% and 4.5%. Similarly, the samples produced with and without contour show differences in their tensile properties. Base plate heating was employed with three different temperatures and the samples display improved plasticity with increasing base plate temperature (BP 473 K, 573 K and 673 K samples show a ductility of 3.5%, ∼3% and 9.5%, respectively). The results indicate that the room temperature tensile properties can be tuned (between YS: 115 MPa–290 MPa, UTS: 220 MPa–460 MPa and ductility: 2.8%–9.5%) in-situ during the selective laser melting process giving an opportunity to define the mechanical properties of the SLM parts to suit their service requirements.

The Influence of Composition on the Clustering and Precipitation Behavior of Al-Mg-Si-Cu Alloys

The natural aging (NA) and artificial aging (AA) behavior of Al-Mg-Si-Cu alloys with different Mg/Si ratios and Cu additions were systematically investigated by means of hardness test, atom probe tomography, transmission electron microscopy, and Monte Carlo simulation. The Si-rich low-Cu alloys displayed higher hardness compared to the Mg-rich equivalents because Si atoms play a dominant role in clustering of solute atoms during both natural and artificial aging. In the high-Cu alloys, Cu did not obviously change the cluster distribution during NA, but significantly refines the clusters and precipitates due to the strong interaction of Cu atoms with Mg atoms during AA. In contrast to the low-Cu alloys, the Mg-rich high-Cu alloys exhibit higher hardness in the early and over-aged stages of artificial aging, with similar or slightly higher hardness in the peak aging condition compared to their Si-rich counterparts. Three types of precipitates (β″, Q′, and L) are favored in the high-Cu alloys. The Mg-rich high-Cu alloy has more L phase, while the Si-rich variant is abundant in Q′ phase. The negative effect of NA on subsequent AA behavior is less dependent on Mg/Si ratio in the high-Cu alloys due to a synergistic action of the residual Si and Cu atoms, but is closely related to Mg/Si ratio in low-Cu alloys.

High‐temperature Na2SO4 deposit‐assisted corrosion of silicon carbide–II: Effects of B, C, and Si

AbstractThe effects of boron, carbon, and silicon on the induced hot corrosion of sintered‐α (Hexoloy) and CVD‐SiC coupons were studied to elucidate the hot corrosion of SiC‐based ceramic matrix composites. The extent of corrosion was quantified after 24 hour exposures at 1000°C using mass change measurements, inductively coupled plasma optical emission spectrometry analysis of corrosion products, and optical profilometry of pitting on the substrate surface. In addition, scanning electron microscopy and X‐ray diffraction were used to better understand the morphology, distribution, and phase composition of corrosion products. It was found that Si was more resistant to hot corrosion than SiC, indicating that residual Si in a ceramic matrix composites matrix should not negatively impact hot corrosion resistance of the composite in highly oxidizing conditions. Carbon did not have a large impact on hot corrosion of SiC, whereas the presence of boron made the hot corrosion attack more severe.

Investigation of the residual stress in reaction-bonded SiC under irradiation

The development of reaction-bonded SiC (RB-SiC) for an application in nuclear reactor environments has been suspended for many years because of the possible swelling mismatch between SiC and Si under irradiation. Therefore, we studied the residual stresses in RB-SiC due to the irradiation-induced swelling mismatch and the mismatch between the coefficients of thermal expansion. The maximum irradiation-induced swelling stress for the amorphous state was obtained by irradiation with 2MeV Au2+ ions (1×1016ions/cm2) at room temperature. Utilizing Raman piezo-spectroscopy, nanoindentation tests and thermal expansion coefficient measurements, we demonstrate that the irradiation-induced swelling stress is remarkably high at low temperatures, whereas the thermal stress becomes the dominant residual stress at medium (415°C<T<700°C) or higher temperatures. By optimizing the residual Si content in the RB-SiC, the thermal stress and the irradiation-induced swelling stress can be adjusted and the initiation of cracks can be prevented.

Effects of Mixing Ratio of Silicon Carbide Particles on the Etch Characteristics of Reaction-Bonded Silicon Carbide

We prepared a number of reaction-bonded silicon carbides (RBSCs) made from various mixing ratios of raw SiC particles, and investigated their microstructure and etch characteristics by Reactive Ion Etch (RIE). Increasing the amount of 9.5 μm-SiC particles results in a microstructure with relatively coarser Si regions. On the other hand, increasing that of 2.6 μm-SiC particles produces much finer Si regions. The addition of more than 50 wt% of 2.6 μm-SiC particles, however, causes the microstructure to become partially coarse. We also evaluated their etching behaviors in terms of surface roughness (Ra), density and weight changes, and microstructure development by employing Confocal Laser Scanning Microscope (CLSM) and Scanning Electron Microscope (SEM) techniques. During the etching process of the prepared samples, we confirmed that the residual Si region was rapidly removed and formed pits isolating SiC particles as islands. This leads to more intensified ion field on the SiC islands, and causes physical corrosion on them. Increased addition of 2.6 μm-SiC particles produces finer residual Si region, and thus decreases the surface roughness (Ra.) as well as causing weight loss after etching process by following the above etching mechanism.

A 60 GHz CMOS Full-Duplex Transceiver and Link with Polarization-Based Antenna and RF Cancellation

This paper presents a fully integrated 60 GHz direct-conversion transceiver in 45 nm SOI CMOS for same-channel full-duplex (FD) wireless communication. FD operation is enabled by a novel polarization-based wideband reconfigurable self-interference cancellation (SIC) technique in the antenna domain. The antenna cancellation can be reconfigured from the IC to combat the variable SI scattering from the environment during in-field operation. A second RF cancellation path with $ > 30\;\text{dB}$ gain control and $ > 360^{\circ}$ phase control from the transmitter (TX) output to the LNA output further suppresses the residual SI to achieve the high levels of required SIC. With antenna and RF cancellation together, a total SI suppression of $ > 70\;\text{dB}$ is achieved over a cancellation bandwidth of 1 GHz and can be maintained in the presence of nearby reflectors. In conjunction with digital SIC (DSIC) implemented in MATLAB, a FD link is demonstrated over 0.7 m with a signal-to-interference-noise-and-distortion ratio (SINDR) of 7.2 dB. To the best of our knowledge, this work achieves the highest integration level among FD transceivers irrespective of the operation frequency and demonstrates the first fully integrated mm-wave FD transceiver front-end and link.

Characterization of the effects of nitrogen and hydrogen passivation on SiO2/4H-SiC interface by low temperature conductance measurements**Project supported by the National Natural Science Foundation of China (Nos. 61106080, 61275042) and the National Science and Technology Major Project of the Ministry of Science and Technology of China (No. 2013ZX02305).

We investigate the effects of NO annealing and forming gas (FG) annealing on the electrical properties of a SiO2/SiC interface by low-temperature conductance measurements. With nitrogen passivation, the density of interface states (DIT) is significantly reduced in the entire energy range, and the shift of flatband voltage, ΔVFB, is effectively suppressed to less than 0.4 V. However, very fast states are observed after NO annealing and the response frequencies are higher than 1 MHz at room temperature. After additional FG annealing, the DIT and ΔVFB are further reduced. The values of the DIT decrease to less than 1011 cm−2 eV−1 for the energy range of EC − ET > 0.4 eV. It is suggested that the fast states in shallow energy levels originated from the N atoms accumulating at the interface by NO annealing. Though FG annealing has a limited effect on these shallow traps, hydrogen can terminate the residual Si and C dangling bonds corresponding to traps at deep energy levels and improve the interface quality further. It is indicated that NO annealing in conjunction with FG annealing will be a better post-oxidation process method for high performance SiC MOSFETs.

Effect of TiSi2/Ti3SiC2 matrix phases in a reaction-bonded SiC on mechanical and high-temperature oxidation properties

The reaction-bonding method is used to fabricate a dense SiC sintered body by filling a SiC preformed structure with a molten Si. Reaction-bonded SiC has both excellent mechanical strength and high-temperature corrosion resistance; however, a low fracture toughness and low melting point of the residual Si constitute major drawbacks of this material. Therefore, in this study, in order to improve the mechanical properties of the reaction-bonded SiC, the composition of the matrix phase was modified by using a preform that is composed of TiC powder. Two types of matrix phases (TiSi2 and Ti3SiC2) were developed, depending on the amounts of reacted TiC and Si. Ti3SiC2 was formed at a TiC to Si molar ratio of 3:2, and TiSi2 was obtained with increasing amount of Si. The fracture toughness (7.4MPa√m) of reaction-bonded SiC consisting of the Ti3SiC2 matrix phase was higher than that (3.0MPa√m) of the the conventional reaction-bonded SiC. However, the resistance to high-temperature steam oxidation at 1200°C decreased when TiSi2 and Ti3SiC2 were substituted for Si as the matrix phase.

Covalent anchoring of Mo(VI) Schiff base complex into SBA-15 as a novel heterogeneous catalyst for enhanced alkene epoxidation

Molybdenum(VI) Schiff base complexes modified mesoporous SBA-15 hybrid heterogeneous catalysts were synthesized by the reaction of MoO2(acac)2 with mesoporous SBA-15 functionalized by grafting procedures of 3-aminopropyl-triethoxysilane and salicylaldehyde, respectively. The physico-chemical properties of the as-synthesized catalysts were analyzed by ICP-AES, XRD, N2 adsorption–desorption, FT-IR, SEM, TEM and EDX. The as-synthesized catalysts were effective in the catalytic epoxidation of cyclohexene. The catalytic activity can be further enhanced by silylation of the residual Si–OH groups using Me3SiCl, which was largely due to the higher content of Mo active sites. The conversion and selectivity reached to 97.78 and 93.99 % using tert-butyl hydroperoxide as oxidant for Mo–CH3–SA–NH2–SBA-15, while 81.97 and 89.41 % in conversion and selectivity for Mo–SA–NH2–SBA-15. At the same time, the catalytic performances of the hybrid materials were further systematically investigated under various reaction conditions (solvent, oxidants and alkenes, etc.). Mo–CH3–SA–NH2–SBA-15 catalyst can be recycled effectively and reused four cycles with little loss in activity. In addition, the results from hot filtration demonstrated that the catalytic activity mostly resulted from the heterogeneous catalytic process.

Dependence of overcurrent failure modes of IGBT modules on interconnect technologies

Insulated gate bipolar transistor (IGBT) modules which can fail to short circuit mode have great of applications in electricity network related fields. Single IGBT samples have been constructed with the standard Al wire bonding, flexible printed circuit board (PCB) interconnect and sandwich structure technologies. The overcurrent failure modes of the constructed IGBT samples have been tested under a range of energy levels, and the structures of the tested samples have been characterized with scanning electronic microscopy and three-dimensional X-ray computed tomography imaging. The results obtained indicate that the IGBT samples constructed with the three interconnect technologies can fail to both open circuit mode and short circuit mode. The sandwich structure IGBT sample can fail to short circuit mode under an energy level of 750J which can meet realistic industrial applications. The networked conductive phases within the solidification structure and the Sn-3.5Ag filled in the cracks within the residual Si IGBT are responsible for forming the conducting paths in the tested samples. Both liquid phase and gas phase can be formed and the highest temperature can reach the boiling point of Si even if the sandwich structure IGBT sample is tested with short circuit failure mode.

Integrated synthesis of zeolites 4A and Na–P1 using coal fly ash for application in the formulation of detergents and swine wastewater treatment

Several researchers have reported zeolite synthesis using coal ash for a wide range of applications. However, little attention has been given to green processes, including moderate synthesis conditions, using waste as raw material and effluent reuse or reduction. In this study, Brazilian coal fly ashes were used for integrated synthesis of zeolites 4A and Na–P1 by two different routes and under moderate operating conditions (temperature and pressure). Both procedures produced zeolites with similar conversions (zeolite 4A at 82% purity and zeolite Na–P1 at 57–61%) and high CEC values (zeolites 4A: 4.5meqCa2+g−1 and zeolites Na–P1: 2.6–2.8meqNH4+g−1). However, process 1 generated less effluent for the zeolite mass produced (7mLg−1), with low residual Si and Al levels and 74% of the Si available in the coal fly ash incorporated into the zeolite, while only 55% is used in process 2. For use as a builder in detergents, synthetic zeolite 4A exhibited conformity parameters equal to or greater than those of the commercial zeolite adopted as reference. Treatment of swine wastewater with zeolite Na–P1 resulted in a high removal capacity for total ammoniacal nitrogen (31mgg−1).

Oxidation resistant zirconium diboride–silicon carbide coatings for silicon carbide coated graphite materials

Four ZrB2–SiC/SiC dual-layer coatings were prepared on the surface of graphite matrix by pack cementation to improve the oxidation and thermal shock resistance of graphite. The crystalline structure and morphology as well as the resistance to oxidation and thermal shock of these coatings were investigated. The results indicated that the weight loss of the ZS50 coating sample, whose pack powders contained 52.4wt.% ZrB2, 39.2wt.% Si and 8.4wt.% graphite, was only 2.9% after oxidation in air at 1500°C for 19h and 6.5% after thermal shocks between 1500°C and room temperature for 15 cycles. With the increasing silicon in pack powders, some residual silicon appeared in ZS50 coating, which was considered to be beneficial to oxidation resistance and thermal shock resistance because it can improve the density of coating and the SiO2 formed by oxidation of residual Si can heal the microcracks at high temperature.

Microporous inorganic/organic hybrids via oxysilylation of a cubic symmetry nanobuilding block [(HMe&lt;sub&gt;2&lt;/sub&gt;SiOSiO&lt;sub&gt;1.5&lt;/sub&gt;)&lt;sub&gt;8&lt;/sub&gt;] with R&lt;i&gt;&lt;sub&gt;x&lt;/sub&gt;&lt;/i&gt;Si(OEt)&lt;sub&gt;4&amp;minus;&lt;/sub&gt;&lt;i&gt;&lt;sub&gt;x&lt;/sub&gt;&lt;/i&gt;

The B(C6F5)3 catalyzed Piers-Rubinsztajn (oxysilylation) reaction of the cubic symmetry Q-cage [(HMe2SiOSiO1.5)8] with ethoxysilanes in hexane forms microporous 3-D networks coincident with ethane evolution. Slow drying provides monoliths whereas fast drying provides powders. The reaction is most efficient if initiated at 60°C for 5 min and then allowed to progress at ambient. The products offer high specific surface areas [SSAs > 700 m2/g, e.g. with Si(OEt)4], with micropores of 0.6–2.0 nm, mesopores of 2–40 nm, total pore volumes ≈ 0.5 cc/g, and thermal stabilities to 320°C. Changes in reaction conditions (times, solvent volumes, catalyst concentrations) do not significantly change product properties pointing to rapid and complete reaction as evidenced by the near absence of residual Si–H under all conditions for 1:1: Si–H:Si–OEt ratios. RSi(OEt)3 gives similar 3-D microporous gels. Smaller R groups give higher SSAs than those with large R groups; however, R = n-octyl is not porous. Rigid, bridged compounds [(EtO)3Si–R–Si(OEt)3] (R = phenyl, biphenyl) offer high SSAs whereas flexible bridges [R = (CH2)2 or 8] give reduced SSAs. All materials were characterized by FTIR, TGA, XRD and BET. XRD studies show periodicities suggesting some long range ordering as might be expected for reactions with cubic symmetry Q8 cages. However, the fast reaction rates likely generate kinetic rather than thermodynamic products that cannot be expected to exhibit high degrees of ordering. Gel affinities for specific organics were studied showing some preferential ability to absorb specific solvents for example the 1:1 OHS:vinylSi(OEt)3 gels were more selective for toluene than the 1:1 OHS:TEOS gels.

Molybdenum(VI) complex with a tridentate Schiff base ligand immobilized on SBA-15 as effective catalysts in epoxidation of alkenes

Two novel molybdenum(VI) tridentate Schiff base complexes supported onto modified mesoporous SBA-15 were prepared by a covalent grafting method, one of which involved the postsynthesis trimethylsilylation of Mo-FSAP-Cl-SBA-15 to remove the residual surface silanol groups. The prepared complex materials were characterized by FT-IR, XRD, SEM, TEM, 29Si CP–MAS-NMR, TGA, ICP-AES and nitrogen adsorption–desorption. Both two were active in the catalytic epoxidation of cyclohexene at 80°C using tert-butylhydroperoxide (TBHP) as oxidant and 1,2-dichloroethane as solvent. The catalytic activity of the heterogenized organo catalyst can be further enhanced by silylation of the residual Si–OH groups using Me3SiCl, due to the higher content of Mo active sites and better surface hydrophobicity. As a result, using the more efficient catalyst Mo-FSAP-CH3-Cl-SBA-15 for catalytic reaction, the conversion and selectivity were 94.38% and 91.63%, respectively, under the optimized condition. After being used five times, the conversion and selectivity of Mo-FSAP-CH3-Cl-SBA-15 were still above 80.00% and 90.00% indicating the good reusability of Mo-FSAP-CH3-Cl-SBA-15.

Miniature fuel cell with monolithically fabricated Si electrodes – Uniformity of Catalyst Layer Thickness -

Uniformity of catalyst layer thickness was improved to obtained higher output with our miniature fuel cells. Though the miniature fuel cells demonstrated high power density about 500mW/cm2 with 1 mm2 reaction area, it was difficult to maintain the power density with larger reaction area and careful observation of the prototypes revealed that the porous Si layer, which should be etched away completely, remained on the channel bottoms. Nonuniform porous Pt catalyst layer thickness was suspicious for the residual porous Si, and rotation of the plating vessel in the Pt deposition process was performed to mitigate variation of the catalyst layer thickness. Though the power density was still low with larger reaction area, the larger output was successfully obtained with the vessel rotation.

Steric spacing of molecular linkers on passivated Si(111) photoelectrodes.

Surfaces with high photoelectrochemical and electronic quality can be prepared by tethering small molecules to single-crystalline Si(111) surfaces using a two-step halogenation/alkylation method (by Lewis and co-workers).1-7 We report here that the surface coverage of custom-synthesized, phenyl-based molecular linkers can be controlled by varying the steric size of R-groups (R=CH3, C6H11, 2-ethylhexyl) at the periphery of the linker. Additionally, the linkers possess a para triflate group (-O2SCF3) that serves as a convenient analytical marker and as a point of covalent attachment for a redox active label. Quantitative X-ray photoelectron spectroscopy (XPS) measurements revealed that the surface coverage systematically varies according to the steric size of the linker: CH3 (6.7±0.8%), CyHex (2.9±1.2%), EtHex (2.1±0.9%). The stability of the photoelectrochemical cyclic voltammetry (PEC-CV) behavior was dependent on an additional methylation step (with CH3MgCl) to passivate residual Si(111)-Cl bonds. Subsequently, the triflate functional group was utilized to perform Pd-catalyzed Heck coupling of vinylferrocene to the surface-attached linkers. Ferrocene surface coverages measured from cyclic voltammetry on the ferrocene-functionalized surfaces Si(111)-8a/CH3-Fc (R=CH3) and Si(111)-8c/CH3-Fc (R=2-EtHex) are consistent with the corresponding Fe 2p XPS coverages and suggest a ∼1:1 conversion of surface triflate groups to vinyl-Fc sites. The surface defect densities of the linker/CH3 modified surfaces are dependent on the coverage and composition of the organic layer. Surface recombination velocity (SRV) measurements indicated that n-Si(111)-8a/CH3 and the ferrocene coupled n-Si(111)-8a/CH3-Fc exhibited relatively high surface carrier lifetimes (4.51 and 3.88 μs, respectively) and correspondingly low S values (3880 and 4510 cm s(-1)). Thus, the multistep, linker/Fc functionalized surfaces exhibit analogously low trap state densities as compared to the fully passivated n-Si(111)-CH3 surface.

Preparation and characterization of diamond-silicon carbide-silicon composites by gaseous silicon vacuum infiltration process

Diamond-SiC-Si composites have been prepared using gaseous silicon vacuum infiltration. The evolution of the phases and microstructures of the composites have been analyzed using X-ray diffraction technique and scanning electron microscopy. It has been found that the diamond-SiC-Si composite is composed of β-SiC, diamond, and residual Si. The diamond particles were distributed homogeneously in the dense matrix of the composites. Besides, the effects of particle size and content of diamond on the properties of diamond-SiC-Si composites have been analyzed. The thermal conductivity of the composites increases with particle size and content of diamond. When the particle size and content of diamond are 300 μm and 80 wt %, respectively, the thermal conductivity of the composites approaches the value of 280 W·m−1·K−1.