Amorphous Si Research Articles

Deciphering the Impact of Current, Composition, and Potential on the Lithiation Behavior of Si-Rich Silicon-Graphite Anodes.

Adding silicon (Si) to graphite (Gr) anodes is an effective approach for boosting the energy density of lithium-ion batteries, but it also triggers mechanical instability due to Si volume changes upon (de)lithiation reactions. In this work, component-specific (de)lithiation dynamics on Si-rich (30 and 70wt.% Si) SiGr anodes at various charge/discharge C-rates are unveiled and compared to a graphite-only electrode (100Gr) via operando synchrotron X-ray diffraction coupled with differential capacity plots analysis. Results show preferential lithiation of amorphous Si above ≈200mV and competing lithiation of Gr, amorphous Si, and crystalline Si below ≈200mV. Discharge proceeds via sequential delithiation of Gr and amorphous lithium silicide. Si shifts the interconversion potentials of graphite intercalation compounds, lowering the Gr state of charge compared to 100Gr. In the 30%Si electrode, crystalline Si amorphization at potentials <110mV is found to be kinetically hindered at C-rates higher than C/5, which can be key for enhancing the cycling stability of SiGr anodes. The 70%Si electrode exhibits restricted lithium diffusion in Gr, full Si amorphization, and Li15Si4 formation. These findings related to the potential- and current-dependent dynamic changes on SiGr blends are crucial for designing stable high energy density SiGr anodes.

Ammonium nutrition enhances rhizosphere mobilization and uptake of silicon in white lupin grown in low phosphorus soil

Abstract Background and Aims While nitrogen (N) supply can enhance plant silicon (Si) accumulation, the mechanisms by which different forms of N affect Si mobilization in the rhizosphere are not well understood. This study aims to elucidate how pH changes induced by ammonium (NH4+) and nitrate (NO3−) affect Si availability in the rhizosphere, especially under low phosphorus (P) conditions. Methods White lupin (Lupinus albus) plants were grown in non-fertilized low-P soil, supplied with a low dose of N, either as NH4+ or NO3−, with or without supply of monosilicic acid. We measured Si levels in various rhizosphere soil pools, along with different plant and rhizosphere soil parameters. Results The addition of NH4+ significantly lowered rhizosphere pH and decreased both Si adsorbed to pedogenic Fe/Mn oxides and amorphous phytogenic Si, resulting in higher concentrations of plant available Si in the white lupin rhizosphere. This led to greater Si uptake and improved plant growth compared to both the –N and + NO3− treatments. The supply of Si further enhanced these effects, with NH4+ showing a consistently different pattern of influence compared to NO3−. Additionally, –N white lupin plants accumulated more P than those treated with N, while Si supply significantly improved P acquisition across all treatments. Conclusions Our study demonstrates that rhizosphere acidification induced by NH4+ nutrition can significantly enhance Si mobilization from the rhizosphere soil in the absence of Si supply and reduce Si adsorption when Si is applied. These findings may have practical implications for improving both Si mobilization in the rhizosphere and the effectiveness of Si fertilizers.

Incubation Experiments Characterize Turbid Glacier Plumes as a Major Source of Mn and Co, and a Minor Source of Fe and Si, to Seawater

AbstractGlaciers are a source of fine‐ground rock flour to proglacial and coastal marine environments. In these environments, suspended rock flour may affect light and (micro)nutrient availability to primary producers. Due to high loads of glacier rock flour, the particulate metal load of glacier runoff typically exceeds the dissolved metal load. As glacier rock flour is deposited in downstream environments, short‐term exchange between particulate and dissolved metal phases may have a moderating influence on dissolved metal concentrations. Here we compare the behavior of iron (Fe), manganese (Mn), cobalt (Co) and silica (Si) following the addition of different glacier‐derived sediments into seawater under conditions of varying sediment load (20–500 mg L−1), time (0.5 hr–21 days), temperature (4–11°C) and light exposure (dark/2,500 Lux). Despite a moderately high labile Fe content across all particle types (0.28–3.50 mg Fe g−1 of dry sediment), only 0.27–7.13 μg Fe g−1 was released into seawater, with less efficient release as sediment load increased. Conversely, Si, Mn, and Co exhibited a more constant rate of release, which was less sensitive to sediment load. Dissolved Si release was equivalent to 17% ± 22% of particulate amorphous Si after 1–2 weeks. Dissolved Mn concentrations in most incubations exceeded dissolved Fe concentrations within 1 hr despite labile Mn content being 12‐fold lower than labile Fe content. Our results show the potential for glacier‐derived particles to be a large source of Mn and Co to marine waters and add to the growing evidence that Mn may be the bio‐essential metal most affected by glacier‐associated sources.

Endogenous silicon-activated rice husk biochar prepared for the remediation of cadmium-contaminated soils: Performance and mechanism

Biochar is widely used for the remediation of heavy metal-contaminated soils. However, pristine biochar generally has limited active functional groups and adsorption sites, thereby exhibiting low immobilization performance for heavy metals. In addition to carbon (C), silicon (Si) is another common macro-element present in rice husk biochar, but it often exists in the form of amorphous oxide and therefore contributes little to the adsorption performance for heavy metals. The transformation of amorphous Si oxide to dissolved silicate through a precipitation effect can significantly improve its heavy metal immobilization capability. Herein, the amorphous Si oxide in rice husk biochar was activated by sodium hydroxide and then the dissolved silicate was immobilized by calcium salt. The as-synthetized Si-activated biochar was used to remediate cadmium (Cd)-contaminated soils. The results indicated that Si-activated rice husk biochar could reduce Cd migration and environmental risks by the transformation from exchangeable Cd into carbonate-bound and residual Cd. With increasing Ca: Si molar ratio, the content of CaCl2 and H2O-extractable Cd exhibited a decreasing trend. Moreover, a higher addition amount of Si-activated biochar improved the Cd immobilization efficiency. The application of 1.0% Ca/Si molar ratio of 2: 2 Si-activated rice husk biochar decreased the CaCl2-Cd and H2O-Cd concentration by a maximum of 83.7% and 90.5% compared with pristine rice husk biochar, respectively. The present work proposes an approach for highly efficient remediation of Cd-polluted soils by biochar.

Metastable phases of Ag–Si: amorphous Si and Ag-nodule mediated bonding

Metastable phases such as supersaturated solid solutions, supercooling, and amorphous phases are well-known in metallurgy. They are often composed in non-equilibrium states and can be transformed into a stable phase by overcoming an energy barrier with driving forces. Particularly, it has been widely used for material strengthening and heterogeneous nucleation of precipitates in solids is mainly induced by heat treatments for supersaturated solid solutions. However, little is known about the metastable phases of the Ag–Si alloy, although it is a well-known simple binary eutectic alloy. Here, we show that the metastable phases composed of amorphous Si and supersaturated Ag solid solution are induced by the eutectic reaction under rapid cooling of Ag–Si. Furthermore, the solute Si in the Ag matrix reacts with oxygen to precipitate Ag by-products, which grow as nodules. The Ag nodules have high crystallinity and robust interfacial structures, and the nodule growth leads to the formation of cross-links between the Ag–Si particles. We also demonstrate the Ag nodule-mediated bonding where the rapidly cooled Ag–Si ribbon is directly used as a bonding medium, indicating the possibility of using it as a high-temperature bonding material with low-temperature processes.

Investigation on the machining mechanism of silicon modified by Au ion irradiation in elliptical vibration diamond cutting

Single-crystal silicon (Si) has important applications in semiconductor, infrared optics, and photovoltaic industries. However, Si is difficult to be machined precisely due to its hard and brittle characteristics. Ion irradiation is proposed as an advanced technology to reduce the hardness and brittleness for a covalent crystal, which is beneficial for the ductile machining process. In the present research, the simulation and experimental investigation of Si with Au ion irradiation were carried out firstly. As following, the grooving experiment is carried out by elliptical vibration cutting (EVC). the machining performance is compared with ordinary cutting (OC) and the material removal mechanism is elaborated. The critical depth of cut for brittle-to-ductile transition is nearly 7 times higher by EVC compared to OC. Initial verification of material modification was conducted by Raman spectra and cutting microgrooves. Finally, the ion irradiation damage mechanism and amorphous Si (a-Si)/crystalline Si (c-Si) machining deformation mechanism were analyzed in detail by transmission electron microscopy. The irradiated sample contains an amorphous layer of 1050 nm, a transition layer containing dislocations and nanocrystals, and a fully crystalline layer. During machining a-Si/c-Si interface, the machining defects in the amorphous layer will first absorb the energy and ensure that the crystalline layer does not produce subsurface damage. In summary, ion-irradiated Si can be achieved in the amorphous region at any depth position and the substrate ductile machining without subsurface damage generation by EVC.

Optimization of microstructure and mechanical performance of clay-rich sand-washing slurry-based geopolymers

As a byproduct of sand-washing process, sand-washing slurry (SWS) showed a potential geopolymer precursor due to the abundant clay minerals. This study investigated the impact of calcination temperature of SWS and SiO2/Na2O ratio of alkaline solution on the microstructure and mechanical properties of SWS-based geopolymers. The analysis results indicated that at a calcination temperature of 550 °C, some clay minerals in SWS underwent a transition from crystalline to amorphous phases, resulting in increased amounts of amorphous Si and Al, thereby enhancing their alkaline reactivity. However, due to the low content of amorphous phases, the formed geopolymer structure was loose, leading to lower compressive strength. Compared to geopolymer treated at 550 °C, the geopolymer produced under temperatures ranging from 650 °C to 750 °C exhibited denser microstructures and higher compressive strength. The improvement in performance was attributed to more clay minerals undergoing dehydroxylation reactions, resulting in more amorphous Si and Al participating in the polymerization reaction. Nevertheless, upon reaching a calcination temperature of 850 °C, the microstructure of the formed geopolymers became loose with diminished compressive strength due to reduced activity of Si and Al in the amorphous phase.Furthermore, under a calcination temperature of 750 °C, the amorphous content in geopolymers with SiO2/Na2O ratios in the alkaline solution of 0.8 and 1.4 was 62.4 % and 64.5 %, respectively, while decreasing to 54.5 % in geopolymers with a ratio of 2. Geopolymers with a SiO2/Na2O ratio of 1.4 exhibited denser microstructures and higher compressive strength, indicating that alkaline solutions with this ratio promoted the formation of more amorphous materials and enhanced geopolymer strength. These findings suggested that calcination around 750 °C improved the microstructure and mechanical properties of SWS-based geopolymers, with the appropriate SiO2/Na2O ratio in alkaline solutions facilitating geopolymerization.

Self‐Encapsulated N‐Type Semiconducting Photoresist Toward Complementary Organic Electronics

AbstractSemiconducting photoresists hold great promise for scale‐up manufacturing of organic field‐effect transistors (OFETs) for integrated organic electronics. While photolithographic p‐type OFETs have achieved a considerable balance among patterning precision, electrical properties and process stability, it remains challenging for n‐type OFETs due to the inherent limited mobility and ambient instability. Herein, a n‐type semiconducting photoresist (SPr) is developed that is compatible with photolithography procedures. By utilizing the solvent‐driven force, a self‐encapsulated blend film with gradient semiconductor phase is prepared, where the underneath transistor active layer is protected by the upper cross‐linked network, avoiding solvent erosion and air doping. As such, a mobility up to 1.1 cm2 V−1 s−1 that is comparable with amorphous Si is achieved, with remained mobility by ≈90% after long‐term exposure to developer and stripper or atmospheric conditions. The sub‐micrometer patterning accuracy of SPr enables the fabrication of organic transistor arrays with a density of 9 × 105 units cm−1, which is comparable to other state‐of‐the‐art devices fabricated by the printing or photolithography, demonstrating immense potential in integrated organic electronics.

(Invited) A Study of the SEI Layer Cross-Talk in Si/C Composite Li-Ion Anodes

Composite electrodes, especially silicon/carbon (Si/C) anodes, present significant opportunities for advancing the energy density of lithium-ion batteries (LIBs). However, challenges emerge, particularly in performance metrics when a high weight percentage of Si beyond 8-10 wt.% is employed 1. A nuanced understanding of the solid electrolyte interphase (SEI) in Si/C composites is imperative to overcome these limitations and enhance energy density. Conventional techniques such as ex-situ X-ray photoelectron spectroscopy and cryogenic electron microscopy face inherent drawbacks, including exposure to ultrahigh vacuum and the need for solvent washing, potentially altering the SEI layer 2. In contrast, nano-FTIR, utilizing Fourier transform infrared near-field spectroscopy, overcomes these challenges, enabling non-destructive probing of the SEI layer's chemistry and structure at room temperature within an inert atmosphere 3–5.In this study, we engineer custom-patterned electrodes featuring amorphous Si on atomically flat highly ordered pyrolytic graphite (HOPG) to emulate model composite electrodes. Leveraging nano-FTIR spectroscopy, near-field nanoscale white-light imaging, and atomic force microscopy measurements, our investigation reveals that the SEI on HOPG is enriched in organic lithium ethylene decarbonate (LiEDC), while the SEI on lithiated silicon unveils the presence of inorganic Li2CO3. Notably, we unveil a unique "mixed" SEI layer zone at the Si/HOPG interface, comprising a mixture of dominant SEI species from the surfaces of both the lithiated Si and the HOPG. The coexistence of these species suggests the formation of an SEI layer with presumably unique physicochemical properties, forming along Si-C interfaces in Si/C composite electrodes. These findings provide a comprehensive approach to studying model composite electrodes and offer valuable insights into optimizing the surface passivation of Si/C composite electrodes for high-performance LIBs. Acknowledgments This research was supported by the US Department of Energy (DOE)’s Vehicle Technologies Office under the Silicon Consortium Project directed by Brian Cunningham and managed by Anthony Burrell. References V. G. Khomenko and V. Z. Barsukov, Electrochim Acta, 52, 2829–2840 (2007).J. Wu, M. Ihsan-Ul-Haq, Y. Chen, and J. K. Kim, Nano Energy, 89, 106489 (2021).X. He, J. M. Larson, H. A. Bechtel, and R. Kostecki, Nat Commun, 13, 1398 (2022).I. Yoon, J. M. Larson, and R. Kostecki, ACS Nano, 17, 6943–6954 (2023).A. Dopilka, Y. Gu, J. M. Larson, V. Zorba, and R. Kostecki, ACS Appl Mater Interfaces, 15, 6755–6767 (2023).

Benchmarking the Match of Porous Carbon Substrate Pore Volume on Silicon Anode Materials for Lithium-Ion Batteries.

Silicon (Si) is one of the most promising anode materials for high-energy-density lithium-ion batteries. However, the huge volume expansion hinders its commercial application. Embedding amorphous Si nanoparticles in a porous carbon framework is an effective way to alleviate Si volume expansion, with the pore volume of the carbon substrates playing a pivotal role. This work demonstrates the impact of pore volume on the electrochemical performance of the silicon/carbon porous composites from two perspectives: 1) pore volume affects the loadings of Si particles; 2) pore volume affects the structural stability and mechanical properties. The smaller pore volume of the carbon substrate cannot support the high Si loadings, which results in forming a thick Si shell on the surface, thereby being detrimental to cycling stability and the diffusion of electrons and ions. On top of that, the carbon substrate with a larger pore volume has poor structural stability due to its fragility, which is also not conducive to realizing long cycle life and high rate performance. Achieving excellent electrochemical performances should match the proper pore volume with Si content. This study will provide important insights into the rational design of the silicon/carbon porous composites based on the pore volume of the carbon substrates.

Robust silicon/carbon composite anode materials with high tap density and excellent cycling performance for lithium-ion batteries

Achieving high density while ensuring structural stability and low volume expansion during cycling remains challenging for Si-based anode materials in lithium-ion batteries (LIBs). Herein, we introduce a novel approach to address this issue by developing high tap-density carbon-coated sub-nano-Si-embedded activated carbon (ACSC) anode materials. The resulting ACSC exhibits an ultra-high true density exceeding 0.99 g cm−3 and a Si content of 48 % (abbreviated as ACS0.48C), leading to an impressive volumetric capacity of 2182 mA h cm−3. Despite the high tap density and Si content of ACS0.48C, the ACS0.48C/artificial graphite (ACS0.48C/AG) mixture demonstrates a remarkable capacity retention rate of 97.9 % after 200 cycles at 0.5 C, with a modest volume expansion rate of 7.3 % after 50 cycles. The outstanding electrochemical performance can be attributed to the structural stability of ACS0.48C throughout cycling. The AC scaffold provides a robust mechanical framework to prevent volume expansion and agglomeration of sub-nano-sized Si particles. Furthermore, the homogeneous mixing of amorphous Si and carbon at the atomic level ensures isotropic expansion, thereby enhancing structural stability. The unique structure of ACS0.48C, combining high tap density and superior cycling performance, offers a solution to the low tap density issue in nanostructures and introduces innovative concepts for the morphology and structural design of Si/C secondary particles.

Silicon heterojunction solar cells: Excellent candidate for low light illuminations

The current solar cells and modules are marketed according to the behaviour at standard test conditions (STC), however, these devices are more often operated at lower irradiance levels. The dependence on illumination stems mainly from the voltage at the open circuit and at the maximum power point. The latter might also be strongly influenced by serial resistance, but that is more an engineering problem not addressed here. More fundamentally, the voltage is determined by quasi-Fermi levels at the contacts. In the case of unconstrained conductivity between the absorber and electrode, the quasi-Fermi levels are flat, and determined by their splitting in the absorber. Our analysis shows that the modulation doping mechanism working in Si heterojunction solar cell between the doped amorphous Si layer with a higher bandgap and crystalline absorber with a lower bandgap can, for certain parameter settings, lead to strongly depleted contact layer that limits conduction. This is a prerequisite for decoupling the quasi-Fermi level between contact and absorber and offers the possibility for additional voltage increase. Based on this understanding, we simulate a heterojunction solar cell with a varying thickness and doping of amorphous silicon p-type contact. We demonstrate that for a certain combination of thinner or lower-doped contact, higher efficiency at low illumination can be achieved compared to the technological baseline. This is fully in line with the experimental findings. This analysis is crucial not only for using solar cells for indoor applications but also for designing photovoltaic modules optimized for low irradiance, potentially increasing the level of self-sufficiency of buildings.

Comprehensive effect on the development of multifunctional low emissive and transparent conductive amorphous-SIZO/Ag/amorphous-SIZO multilayer structure

Wide band gap oxide semiconductors based on transparent conducting electrodes (TCEs) are promising for the fabrication of a wide range of electronic and optoelectronic devices, which include flat panel displays, photovoltaics, transparent heaters, and smart windows. High optical transmittance in the visible spectral range and low electrical resistivity as obtained by intrinsic and/or extrinsic doping, lead to a high figure of merit (FoM) for oxide based TCEs. Recently, we have used wide band gap amorphous Si–In–Zn–O (SIZO) to realize a relatively new class of TCEs using SIZO/Ag/SIZO multilayer structures, which exhibit high FoM in addition to infrared reflection capability due to the embedded ultrathin Ag metal layer. These multifunctional TCEs are found to be highly useful for low emissive smart window applications, which minimize the outside infrared to coming inside and efficiently reflect inside heat to emit outside. In addition, amorphous OMO based TCEs show excellent mechanical endurance, unlike crystalline ITO based coatings. The optical and low emission characteristics of amorphous SIZO OMO were optimized to transmittance of 93.4 % at 550 nm and 58.6 % at 950 nm.

Effect of surface treated amorphous Si–Zn–Sn–O on the electrical properties of thin film transistors by Ar plasma treatment

Effect of surface treated amorphous Si–Zn–Sn–O on the electrical properties of thin film transistors by Ar plasma treatment

Characteristics of Si (C,N) Silicon Carbonitride Layers on the Surface of Ni-Cr Alloys Used in Dental Prosthetics.

Chromium- and cobalt-based alloys, as well as chrome-nickel steels, are most used in dental prosthetics. Unfortunately, these alloys, especially nickel-based alloys, can cause allergic reactions. A disadvantage of these alloys is also insufficient corrosion resistance. To improve the properties of these alloys, amorphous Si (C,N) coatings were deposited on the surfaces of metal specimens. This paper characterizes coatings of silicon carbide nitrides, deposited by the magnetron sputtering method on the surface of nickel-chromium alloys used in dental prosthetics. Depending on the deposition parameters, coatings with varying carbon to nitrogen ratios were obtained. The study analyzed their structure and chemical and phase composition. In addition, a study of surface wettability and surface roughness was performed. Based on the results obtained, it was found that amorphous coatings of Si (C,N) type with thicknesses of 2 to 4.5 µm were obtained. All obtained coatings increase the value of surface free energy. The study showed that Si (C,N)-type films can be used in dental prosthetics as protective coatings.

Oxidation of SiC fibers in water vapor

AbstractThe current study focuses on the role of composition in the oxidation behavior of three commercial SiC fibers (Hi‐Nicalon Type S (HNS), Tyranno ZMI, and Tyranno SA) in a steam‐containing environment at 1000°C. Oxide scales that form on each of the three fiber types differ in morphology, microstructure, and composition. Differences are attributed to O and C contents, crystallinity, and minor alloying elements. Oxidation kinetics of the HNS fibers follow linear/parabolic (Deal–Grove) predictions over the entire range of exposure times (to 96 h). In the other fibers, deviations from the initial linear/parabolic trend occur when either the scale crystallizes in situ during oxidation (as on the ZMI fibers) or when structural changes internal to the fiber alter the chemical environment at the scale/fiber interface (on the SA fibers). Adaptations of the linear/parabolic oxidation model are used to rationalize the growth kinetics in the latter two cases. Collectively, the data and analyses indicate a two‐fold difference in permeabilities of crystalline and amorphous silica scales. Inferred interface reaction rates further suggest faster oxidation for fibers with greater amounts of amorphous Si–O–C. Moreover, crystalline scales are more prone to cracking, because of their higher viscosity at elevated temperatures and a deleterious phase transformation upon cooling.

C54-TiSi2 formation using nanosecond laser annealing of A-Si/Ti/A-Si stacks

Low resistive Ti-based contacts are required to reach the targeted performances of 3D imaging devices. To fulfill this request, the C54-TiSi2 phase appears to be the most efficient. However, in view of monolithic integration, the temperature needed to form C54-TiSi2 must be drastically reduced by at least 100 °C in order to avoid damage in other parts of the device. Indeed, as these 3D imaging devices require the co-integration of Ti and Ni based silicides, it is crucial to develop a thermal treatment which allows to form the C54-TiSi2 phase without agglomerating the NiSi layer (i.e. at temperatures lower than 600 °C). In this work, a three-layer stack made of a Ti layer in between two amorphous Si thin films deposited on (100) Si substrate has been studied. Nanosecond laser annealing (NLA) followed by rapid thermal annealing (RTA) from 500 to 800 °C were performed on those stacks. Sheet resistance and X-Ray Diffraction measurements showed that the C54-TiSi2 phase could be formed with a RTA treatment at a temperature as low as 600 °C in this case. In line with some previous work, this was due to another phase sequence involving the C40-TiSi2 during the laser annealing. The most favorable microstructure to form C54-TiSi2 by a subsequent RTA treatment seemed to be a matrix made of amorphous titanium silicide containing grains of the C40-TiSi2 template phase. This hypothetical microstructure was formed by laser annealing at a lower energy density when layers of amorphous Si were used.

Silicon extraction from x-ray amorphous soil constituents: a method comparison of alkaline extracting agents

The growing interest in amorphous silica (ASi) within the fields of soil science and ecology underscores the necessity for a reliable protocol to estimate ASi contents in soil. Alkaline wet chemical extraction methods are commonly employed for silicon (Si) extraction from operationally defined (x-ray) amorphous Si phases or short-range ordered mineral phases in soils and marine sediments. In our study we conducted a comparative analysis of four alkaline extraction methods (1% sodium carbonate, 0.5 M sodium carbonate, 0.2 M sodium hydroxide, and 0.1 M Tiron), assessing their extraction selectivity as well as effectiveness using soils artificially enriched with varying, defined amounts of ASi. While extraction effectiveness was evaluated by determining the recovery rate of initially added ASi, extraction selectivity was determined by measuring aluminum (Al) and iron (Fe) concentrations as indicators of the dissolution of non-target mineral phases. Microwave plasma atom emission spectrometry was used to analyze Al, Fe, and Si concentrations in the extracts. Our results indicate that extraction with 0.2 M sodium hydroxide yields the best outcomes in terms of both extraction effectiveness and selectivity. This more recent extraction technique is conducted at the most alkaline pH (13.3) of all four methods tested, but at ambient temperature (21°C) decreasing the dissolution of non-target mineral phases. Though, no wet-chemical extraction used on heterogeneous samples like soil is precisely selective, and thus able to quantify the target analyte only. Hence, data obtained by such procedures still need to be interpreted with caution considering all their limitations.

The ablation behavior and modification mechanism of SiC under different laser energy

The laser modification pretreatment method is an effective way to address the machining difficulties of silicon carbide (SiC), a typical hard-to-machine material. Therefore, the ablation behavior and modification mechanism of SiC under different laser energy were explored in this paper. A new multi-scale model of laser irradiation SiC that couples heat transfer and fluid motion is first established. Then, a series of experiments is carried out to evaluate the model's accuracy, and the interaction between laser and SiC is discussed in detail. The results show that the dominant modification mechanism changes from coulomb explosion to multiphoton absorption, incubation effect, and heat accumulation with the laser energy increase. This leads the surface topography of SiC to transition from nanoparticle formation to disorder to a melting state. In ablative state, micro/nano porous and humps are formed at the edge of ablation groove due to surface tension, generation and rupture of bubbles, respectively. Furthermore, the surface roughness is not proportional to the laser energy due to the plasma shielding effect, and the surface roughness can be reduced by enhancing the flow of the molten material. Amorphous Si–O–C, Si and spherical SiO2 exist in deposition area, leading to SiC elastic modulus decreases from 347 GPa to 103.82 GPa, and the shear strength decreases from 20.9 GPa to 17.25 GPa. The results of this study can provide references for parameters selection and theoretical support for improving the machinability of SiC.

Feasibility analysis of the preparation of geopolymers from different types of coal based ash: Reaction, synthesis, and properties

This study compared the feasibility of circulating fluidized bed boiler fly ash (CFBFA), pulverized coal furnace fly ash (PCFA), and coal gasification slag (CGS) for the preparation of geopolymers by investigating the Si and Al dissolution reaction of the raw materials, product structure, and the development of the mechanical properties of the geopolymers. The results indicated that the reaction mechanism for preparing geopolymers were the same for the three kinds of coal based ash, i.e., the depolymerization and repolymerization of Si-O and Al-O bonds in raw materials. The loose and porous microstructure of CFBFA resulted in a high dissolution ratio of amorphous Si and Al, and its self-hardening properties generated ettringite crystals. This resulted in a high initial strength of the CFBFA based geopolymer (CFBFAG). The active Si and Al in PCFA and CGS were released slowly during the curing process, which was conducive to an enhanced compressive strength of the geopolymer in the later stage. The compressive strengths of the CGS based geopolymer (CGSG) after curing for 90 days reached 52.1 MPa, which was 139 % and 11.4 % higher than that of CFBFAG and PCFAG for the same curing time respectively. A microstructure analysis showed that the CGSG had a denser microstructure and more ordered geopolymer gel network than the other geoploymers. Compared with the CFBFA and PCFA, CGS was found to have a greater application potential in the preparation of geopolymers.