Residual Si Research Articles

Experimental and theoretical analyses of the thermal conductivity of diamond /SiC composites with SiC matrix

Diamond/SiC composites with SiC matrix were prepared by using a liquid silicon infiltration method. The interface morphology and thermal conductivity of the composites have been analyzed by scanning electron microscopy (SEM), and a laser flash thermal conductivity test, respectively. The thermal resistance of the interface structures has been calculated by using the acoustic and diffusion mismatch models. After the addition of mono-sized SiC particles, an increased diamond content resulted in an increased thermal conductivity of the composites. After the addition of SiC with a graded mix-sized particle under fixed diamond content condition, although the residual Si content decreased, the thermal conductivity of the composites showed little changes. Furthermore, the thermal conductivity of the composites have been calculated by applying Cubic Structure (C-S) and Hasselman-Johnson (H-J) models. The result shows that the calculations based on the C-S model showed the best agreement with the experimental thermal conductivity of the experiments with the addition of mono-sized SiC particles compared to the results analyzed by the other calculation models. The calculations based on the H-J model showed the best agreement with the experimental thermal conductivity of the graded mix-sized SiC particles composites compared to results analyzed by the other calculation models.

Effect of gradation on thermal and mechanical properties of reaction‐bonded silicon carbide ceramics

AbstractParticle grading is a reliable method to improve mechanical properties of reaction‐bonded silicon carbide (RBSC) ceramics. This work aims to explore the effects of particle grading on thermal and mechanical characteristics of RBSC ceramics. Results showed that the powder grading enabled to increase sintering density of silicon carbide (SiC) ceramics. The use of large particle SiC powder in grading led to a 28% increase in thermal conductivity but a 22% decrease in bending strength. Meanwhile, the addition of finer SiC powder exerted minimal effect on thermal conductivity but noticeably increased bending strength by 10%. Therefore, properly adjusting particle size in grading allows for the control of residual Si content and microstructure, thereby simultaneously increasing thermal conductivity and bending strength of the material.

Microstructure and properties of different carbon fiber reinforced ceramic composites brazed joint by Si–Ti alloy filler

The brazing of C/C and Cf/SiC composites was achieved using Si–14Ti eutectic filler. The wetting behavior of the Si–Ti filler on the composites, as well as the microstructure and performance of the joints at different brazing temperatures, were characterized. The results showed that the contact angle of Si–Ti eutectic filler on precursor infiltration and pyrolysis (PIP)-Cf/SiC composite surface is only 60°, while it exhibits good spreading effect on liquid silicon infiltration (LSI)-Cf/SiC and C/C composite surfaces. Although Si–Ti filler and carbon fiber reinforced composites (C/C, PIP-Cf/SiC and LSI-Cf/SiC) react to form SiC, the joint shows different mechanical performance. The liquid filler infiltrates into the gaps between fiber bundles and reacts with C/C to form a SiC reaction layer at the interface. However, the presence of numerous defects at the brazed joint between Si–Ti filler and PIP-Cf/SiC results in the shear strength of only 28 MPa for the C/C-PIP-Cf/SiC joint. While for the C/C-LSI-Cf/SiC joint, due to the residual Si in the LSI-Cf/SiC composites, the liquid filler infiltrates intensely into LSI-Cf/SiC to form a unique Cf/SiC–Si–TiSi2 compound infiltration layer. The shear strength of C/C-LSI-Cf/SiC joints reaches 47 MPa, with fracture occurring in both substrates and brazed seam. The present study offers theoretical support for the investigation of Si-based fillers in brazing carbon fiber reinforced ceramic matrix composites.

Significant impact of carbon source on microstructure and water–oxygen corrosion behavior of Si–Y alloy modified SiC/SiC composite

This study investigated the influence of carbon source on the microstructure and water–oxygen corrosion behavior of Si–Y alloy modified SiC/SiC composites. The results indicated that incorporating a small-sized and diffusely distributed carbon source within the matrix not only facilitates the preparation of a highly dense matrix, but also enables a uniform dispersion of the reaction-generated SiC. Furthermore, with the assistance of residual Si–Y alloy, the dispersed SiC enhanced the matrix strength, leading to a stepwise fracture behavior of the composites. Yttrium silicate with a gradient structure formed around the dispersed SiC could effectively exert its water–oxygen corrosion resistance. After 100 h of water–oxygen corrosion test at 1300 °C, the flexural strength of the composites with this structure reached 416 MPa, with a retention rate of 85 %. Both bending strength and retention were found to be at a high level in the current research on matrix modification.

Preparation of high-strength silicon carbide by SLS-RS

One major limitation of selective laser sintering (SLS) plus reaction sintering (RS) SiC ceramics was the poor mechanical properties related to those of the traditional reaction sintered SiC due to the excess residual Si and C. TiC as a new carbon source in SLS was explored in this work to improve the performance of sintered SiC. It's found that TiC could react with liquid Si and form liquid TiSi2 during the processing, and then reduce the residual Si from 26.41 vol% to 0.53 vol% in the final products. As the content of TiC increased, the flexural strength of the sintered sample increased from 274.3 MPa to 434.6 MPa. The maximum value was reached when the TiC content was 25 wt%. Continuing to add TiC would generate more TiSi2, and the flexural strength of the sintered sample would decrease.

Mechanical and microstructure characterisation of 2.5D C/C-SiC composites applied for the brake disc of high-speed train

Carbon fibre reinforced silicon carbide (C/C-SiC) composite materials have attracted increasing attention in brake system of high-speed trains, due to their low density, high specific strength, thermal stability, and frictional wear performance. This study aims to explore the mechanical properties of the 2.5D C/C-SiC composites, in order to characterise its potential application to the friction braking system of high-speed trains. To comprehensively evaluate the mechanical performance of the 2.5D C/C-SiC composites, the chemical vapor infiltration (CVI) method was adopted to prepare novel samples with high densification and low content of residual Si. The mechanical properties and failure mechanisms of the composites were investigated under various loading conditions, including tension, compression, bending, and shear tests. The experimental results indicated that the 2.5D C/C-SiC composites exhibited superior mechanical properties, with average in-plane tensile strength, compressive strength, bending strength, and interlaminar shear strength reaching 127.67 MPa, 326.92 MPa, 355.74 MPa, and 9.77 MPa, respectively, which are 2–8 times higher than the mechanical properties of C/C-SiC composites in existing publications. Under different loading conditions, the composites demonstrated characteristics of ductile fracture, pseudo-plastic compression fracture, pseudo-plastic bending fracture, and brittle shear fracture. Both high fibre content and the formation of SiC structure were advantageous for enhancing the load-bearing performance of C/C-SiC composites. The outstanding mechanical properties of the 2.5D C/C-SiC composite render the brake disc highly resistant to deformation and cracking under high stress, thereby enhancing its durability and establishing it as the ideal material for the next generation of high-speed train brake discs.

Enhanced mechanical/electrical properties of in situ synthesized SiC-TiB2 ceramic composites by reactive hot-pressing sintering

In this work, in situ formed TiB2-reinforced SiC ceramic composites were prepared by reactive hot pressing using SiC, TiC, B4C, and Si as raw materials. Phase composition, microstructure, and mechanical as well as electrical properties were investigated. The formation of the TiB2 phase could be realized from the reaction among TiC, B4C, and Si. In the sintered SiC–TiB2 ceramic composites, the content of α-SiC decreased while that of the in situ formed TiB2 increased. In addition, residual Si only existed in the SiC–TiB2 ceramic composites with in situ TiB2 below 20 mol%. Notably, the comprehensive performance of the SiC–TiB2 ceramic composites was enhanced with the increase of TiB2 content due to the reduction of pores and residual Si. Finally, nearly fully dense SiC–TiB2 ceramic composites were obtained when in situ formed TiB2 was above 20 mol%. The SiC–TiB2 ceramic composites, with in situ TiB2 of 30 mol%, exhibited hardness of 26.9 ± 2.3 GPa, fracture toughness of 6.83 ± 0.6 MPa m1/2, and electrical resistivity of 0.3 mΩ cm, respectively.

Effects of diamond as a main carbon source on the fabrication and mechanical properties of reaction-bonded SiC

RBSC with a low residual Si less than 5 vol% was fabricated using SiC–C preform with diamond as a main carbon source by a conventional reaction sintering process at the temperature range between 1500 °C and 1800 °C. The diamond content in the SiC–C preform varied from 10 wt% to 17 wt%. The diamond as the main carbon source used in SiC–C preform effectively minimized the residual Si in RBSC by increasing the reaction of diamond due to the enhanced Si melt infiltration even in SiC–C preform with high carbon content. The Si melt infiltration should be performed above 1600 °C for the complete reaction of diamond particles with size of 0.5 μm because of the relatively low dissolution rate of diamond in Si melt. The flexural strength of RBSC with residual Si content less than 7 vol% exceeded 400 MPa. The flexural strength of RBSC was increased with decreasing residual Si content, but sharply decreased by the residual carbon accompanying pores in RBSC. RBSC with low residual Si content displayed moderate flexural strengths around 200 MPa even above Si melt temperature because of the formation of continuous SiC structure in RBSC by the enhanced grain growth of SiC. Vickers hardness of RBSC was steadily increased from 17.7 GPa to 24.6 GPa with decreasing residual Si.

Scaling behavior of Si in capacitive deionization (CDI) systems for brackish groundwater desalination

Capacitive deionization (CDI) is a potentially cost-effective method of desalinating brackish groundwaters though the technology has not yet been widely utilised for this particular application. Silicon (Si) is a common element in groundwaters which may cause problems due to its tendency to induce severe fouling of equipment used for water treatment. In this study we focus on the impacts of the presence of Si on the performance of CDI. A systematic evaluation procedure is proposed, which is used to distinguish the “dynamic” and residual Si scaling and the impacts of these types of fouling on desalination performance. Results show that Si has minimal influence on CDI performance within low concentration ranges (<0.5 mM). Sorption of deprotonated silicic acid species is largely avoided in CDI, which contributes to sustaining the removal of major ions and stable operation of the system. However, at high concentrations of Si (>2.0 mM), scaling was observed to form on the electrode surface, leading to noticeable deterioration in CDI electrode performance. The presence of ion exchange membrane in CDI (i.e., membrane CDI or MCDI) can significantly alleviate Si scaling of the desalination system. Results of this study clearly demonstrate the scaling behavior of CDI and its derivatives when dealing with Si-laden water matrices, providing insights into the design and maintenance of CDI systems in practical applications.

Substrate dependence of the self-heating in lead zirconate titanate (PZT) MEMS actuators

Lead zirconate titanate (PZT) thin films offer advantages in microelectromechanical systems (MEMSs) including large motion, lower drive voltage, and high energy densities. Depending on the application, different substrates are sometimes required. Self-heating occurs in the PZT MEMS due to the energy loss from domain wall motion, which can degrade the device performance and reliability. In this work, the self-heating of PZT thin films on Si and glass and a film released from a substrate were investigated to understand the effect of substrates on the device temperature rise. Nano-particle assisted Raman thermometry was employed to quantify the operational temperature rise of these PZT actuators. The results were validated using a finite element thermal model, where the volumetric heat generation was experimentally determined from the hysteresis loss. While the volumetric heat generation of the PZT films on different substrates was similar, the PZT films on the Si substrate showed a minimal temperature rise due to the effective heat dissipation through the high thermal conductivity substrate. The temperature rise on the released structure is 6.8× higher than that on the glass substrates due to the absence of vertical heat dissipation. The experimental and modeling results show that the thin layer of residual Si remaining after etching plays a crucial role in mitigating the effect of device self-heating. The outcomes of this study suggest that high thermal conductivity passive elastic layers can be used as an effective thermal management solution for PZT-based MEMS actuators.

Vat photopolymerization of large-aperture high performance SiC mirror through multiphase carbon infiltration modification

Vat photopolymerization technique (VPP) shows advantages in rapidly preparation of complex structure silicon carbide matrix composites (Si/SiC). However, compared with traditional preparation technology, the properties of Si/SiC prepared via VPP are insufficient due to the presence of residual Si and carbon. Herein, the reaction mechanism of Si and carbon were systematically studied. Multiphase carbon was introduced into porous SiC preform via carbon precursor infiltration pyrolysis (CPIP) and graphitization methods. The content of residual Si was significantly decreased after reactive melt infiltration (RMI), and the residual carbon was eliminated through partial graphitization of the carbon. The influence mechanism of the carbon content and crystallinity on the reaction process of Si and carbon was studied. It suggested that the diameter of capillary channel and crystallinity of carbon are the main factors affecting residual carbon content in the final body after RMI. The sample with multiphase carbon achieved the best performance after RMI. The residual Si content, bulk density, flexural strength and elastic modulus are 11.58%, 3.04 g/cm3, 280.09 MPa and 355.48 GPa. In addition, the lowest coefficient of linear expansion was 2.39×10−6 K−1. Finally, a large-aperture SiC mirror with a diameter of 500 mm was prepared. Therefore, this research lays a foundation for promoting the application of large-aperture Si/SiC prepared via 3D printing.

Impact of trace silicon on irradiation hardening and embrittlement of RAFM steel subjected to neutron irradiation

The content of silicon (Si) element in reduced activation ferritic/martensitic (RAFM) steels varies significantly among different candidates of fusion reactor structural materials. Si is not intentionally added to the RAFM steel but rather remains as a residue during the deoxidation process in steel fabrication. To control and reduce the Si content, removing processes and special treatments are required. The additional refining processes significantly increase the cost of steel fabrication. Currently, the role of residual or trace Si in neutron irradiation of steel remains unclear. In the presented study, two 9Cr-2 W RAFM steels with Si contents of 0.1 wt.% and 0.01 wt.% were fabricated and subjected to a neutron irradiation at 563 K with a neutron fluence of 4.8 × 1023m−2. The main results show that 0.1 wt.% Si can contribute to the suppression of irradiation hardening and embrittlement, as evidenced by the reduction in irradiation-increased hardness and yield strength, while maintaining preferable elongation. Microscopic analysis suggests that an appropriate content of Si helps refine the grains in RAFM steel, increases the ductile fracture region of the fracture surface, and effectively reduces the density of dislocation loops induced by neutron irradiation. The findings evidence that retaining a certain amount of trace Si should be an informed and rational choice in the design of RAFM steel.

Forms of Silicon in Rice Soils of Kerala, India

Aim: The plant-available silicon (PAS) content is highly influenced by different forms/fractions of silicon (Si) in which it exists in the soil. The solubility of these forms varies and affects the Si concentration in soil solution. Quantification of Si pools in soils is needed for a better understanding of biogeochemical processes that govern Si dynamics in soils, the magnitude of Si release, and Si-cycling between soil and plant systems. In this context, an attempt was made to elucidate the status and distribution of Si pools in major rice-growing soils of Kerala.&#x0D; Methodology: The soil samples were collected from five different locations representing the major rice growing tracts (Kuttand, Kole, Pokkali, sandy and lateritic soils) of Kerala, India. The processed soil samples were analyzed to estimate various physicochemical properties. Then, using standard procedure, different fractions of Si in these soils were estimated and their correlation with physico-chemical properties of soils were worked out.&#x0D; Results: The forms of Si estimated in these soils were mobile Si, adsorbed Si, organic Si, amorphous Si, and residual Si. The percentage distributions of Si pools in these soils were mainly in the order; of residual Si &gt; amorphous Si &gt; occluded Si &gt; organic Si &gt; mobile Si &gt; adsorbed Si. Mobile and adsorbed Si were the smallest fractions and amorphous Si was the largest fraction in these soils.&#x0D; Conclusion: Mobile and occluded Si were found to be the major contributors of PAS in major rice soils of Kerala.

Investigation of Impact of C/Si Ratio on the Friction and Wear Behavior of Si/SiC Coatings Prepared on C/C-SiC Composites by Slurry Reaction Sintering and Chemical Vapor Infiltration

Carbon/carbon (C/C)-SiC composite materials have a series of outstanding advantages, such as a light weight, resistance to thermal degradation, excellent friction performance, and good stability in complex environments. In order to improve the wear resistance of the C/C-SiC composite matrix, Si/SiC coatings were prepared by a combination of chemical vapor infiltration and reactive sintering. The wear performance of Si/SiC coatings with different amounts of silicon carbide was investigated. When the carbon silicon ratio in the slurry was 1:3, the SiC particle content in the coating was 93.0 wt.%; the prepared Si/SiC coating exhibited the lowest wear rate of 3.2 × 10−3 mg·N−1·m−1 among the four coatings; and its frictional coefficient was 0.95, which was higher than that of the substrate. As the residual Si content in the coating decreased, the continuity between SiC particles in the coating was improved. Both the high hardness of SiC and the dense coating contributed significantly to enhancing the coating’s wear resistance.

Improved Localized Laser Doping of Boron‐Doped Si Paste for High‐Efficiency Solar Cells

Laser doping (LD) of born‐doped Si paste (Si paste) is a potential boron‐doping approach for advanced solar cells. It is aimed to develop a top‐hat LD instead of a Gaussian LD and a rear‐side polishing process to fabricate P++ local contacts for improved passivated emitter and rear contact (i‐PERC) cells. With the help of vision alignment, LD is conducted on a dashed line pattern of printed Si paste, which decreases the consumption of paste. The variations of boron content in Si paste and the laser pulse energy density can help customize the doping profile. The predicted result of the boron LD profile is a peak dopant density of 2 × 1019 atoms cm−3 and a dopant depth of over 4.0 μm. When higher firing temperature, it can obtain thicker localized back surface field of over 7 μm, since the gradient of the residual Si concentration at the Al/Si interface can suppress the formation of voids. The overall increment in fill factor, short‐circuit current, and open‐circuit voltage enhances the average efficiency by 0.11% for i‐PERC cells and 0.09% for i‐bifacial PERC cells, respectively, due to the improved local contacts.

Effect of CNF in-situ growth on microstructure and mechanical properties evolution of RMI-deriver C/C–SiC composites

The present paper investigates the effect of carbon nanofibers (CNFs) on the microstructure and mechanical properties of Carbon fiber reinforced carbon and silicon carbide binary matrix (C/C–SiC) composite prepared by using the reactive melt infiltration technique. The CNFs used in this paper were generated through in-situ reaction in the pores of C/C preforms using the chemical vapor deposition. Results indicate introducing additional carbon sources in the form of CNFs in the C/C preform can remarkably improve the permeability of molten Si, resulting in higher density and more uniform microstructure of the RMI-deriver C/C–SiC composites. Consequently, the in-situ formation of CNFs reduces the residual Si content by 64.71% and increases the mechanical strength and modulus at room temperature by [22.31% and 17.89%] and after heat treatment at 1500 °C by [35.6% and 47.48%] with respect to no-CNF containing composites due to a lower amount of residual Si content and additional reinforcement effect of the unreacted CNFs.

Pressureless joining of SiC by molten Si infiltration to the SiC/C filler tape and residual stress analysis

This study examined the joining of monolithic SiC by Si-C reaction bonding using a 100 µm-thick filler tape prepared using different SiC/C compositions (30/70, 50/50, and 70/30 wt%). Joining was performed at 1450, 1500, and 1550 °C for 1 h in a vacuum without pressure after inserting the filler tape followed by molten-Si infiltration. The sample joined at 1500 °C using a SiC/C = 70/30 filler tape revealed a dense joint morphology without pores or cracks, showing the highest joint strength of 254 MPa. A larger amount of SiC in the filler tape resulted in less residual Si at the joining interface and higher joint strength. Moreover, the distribution of residual stresses at the joint was simulated using finite element analysis, which was well-suited to experimental observations based on micro-Raman spectroscopy.

Enhanced strength-conductivity trade-off in Al-Mg-Si alloys with optimized Mg/Si ratio

This study evaluated the potential for an enhanced strength–conductivity trade-off in Al-Mg-Si alloys with optimized Mg/Si ratios. In cases of the consistent total amounts of Mg and Si atoms, evident differences in hardness and conductivity manifested during artificial aging, contingent on the Mg/Si ratio investigated. The alloy with an Mg/Si ratio of 1.18 displayed a superior trade-off between strength and conductivity, yielding a yield strength of 305 MPa coupled with a remarkable conductivity of 51.57%IACS and 199.7 W/m/K. Alloys characterized by low Mg/Si ratios exhibited a significantly higher precipitates fraction than those with high Mg/Si ratios because of the accelerated age-precipitation kinetics. This resulted in the improved strength owing to the enhanced precipitation strengthening contribution. A constitutive model considering both fraction and structural composition of the precipitates was established to elucidate the mechanism of varying Mg/Si ratio on conductivity. Variations in the residual Mg and Si within the matrix have been demonstrated to predominantly account for the disparities in conductivity across alloys with distinct Mg/Si ratios. The substantial precipitation in the alloy with an Mg/Si ratio of 1.18 induced heightened depletion of both Mg and Si, consequently achieving a comparatively elevated conductivity concomitant with its relatively superior strength.

A novel porous carbon synthesized to serve in the preparation of highly dense and high-strength SiC/SiC by reactive melt infiltration

SiC/SiC composites, prepared by reactive melt infiltration (RMI), exhibit exceptional properties. However, the presence of residual Si in the composite matrix can significantly compromise their high-temperature mechanical properties. Addressing the issue of high-content residual Si aggregation remains a crucial focus of research in order to fully unlock the application potential of SiC/SiC composites prepared by RMI. In this work, a special porous carbon (Cg) is successfully synthesized and introduced into the porous 2D SiC/SiC composites prepared by CVI. The unique pore structure of the SiC/SiC-Cg is thoroughly analyzed, along with its liquid Si infiltration process. The difference in coefficient of thermal expansion (CTE) between SiC and Cg can cause thermal stress, which can destory the Cg structure. Consequently, the C particles detach from the Cg skeleton and react with liquid Si to create SiC. This process effectively separates residual Si and helps alleviate the negative effects of residual Si aggregation. The microstructure and phase distribution of SiC/SiC composites obtained by liquid Si infiltrating different C structures are investigated and compared, and it demonstrates the positive effect of Cg on the uniform phase distribution of matrix in the composites. The as-received SiC/SiC composites possess a density of 2.94 g/cm3 with open porosity of 1.23 %, and a flexural strength of 808.7 ± 10.2 MPa, a fracture toughness of 25.5 ± 1.8 MPa·m1/2, a tensile strength of 317.4 ± 12.4 MPa and a proportional ultimate stress of 157.33 ± 4.1 MPa. Acoustic emission (AE) and digital image correlation (DIC) technology are used to study the special mechanical behavior of composites.

Microstructure evolution of Ni-Cr-Mo superalloy fabricated by microwave-assisted combustion synthesis

In this study, the microwave-assisted self-propagation high-temperature synthesis (SHS) technique was utilized to successfully synthesize the Nistelle super C commercial alloy with the chemical composition of Ni59Cr23Mo18. The precursors used for this purpose were NiO, Cr2O3, and MoO3 oxide compounds. The thermodynamicall calculations were carried out by the Factsage software. The synthesized alloys' chemical composition, phase, and microstructure were evaluated by induction-coupled plasma, X-ray diffraction, optical microscopy, and scanning electron microscopy. The findings revealed considerable residual silicon in the final product due to using Si as the reducing agent. The presence of residual Si in the chemical composition led to forming a eutectic Niγ-FCC+ Mo2Ni3Si microstructure in the alloys produced by the alumino/silicothermic method. The high silicon content in the excess Al-containing sample led to forming of proeutectic Mo2Ni3Si phases with a petaloid morphology. Besides, this alloy displayed small islands of the interdendritic nickel silicide phase. Conversely, the aluminothermic sample showed a nearly single-phase structure with low amounts of the precipitates of chromium and molybdenum-rich phases. The phase diagram plotted for this sample indicated that the precipitates were probably TCP phases like σ and P. In all the samples, the segregation of Mo occurred significantly, while Cr was distributed almost uniformly in all regions. The formation of the Mo2Ni3Si intermetallic phase resulted in the significant segregation of Mo and depletion of the Niγ-FCC matrix from this element. However, the segregation of Mo was also observed near the grain boundaries in the single-phase sample. The results showed that the dissolution of the alloying elements, especially molybdenum, caused severe offset in some peaks of the Niγ-FCC XRD pattern.