The peculiarities of transformation of organosilicon polymer into ceramic products under mechanochemical treatment

The effect of mechanical treatment on polygermasilethyne for different periods of time was studied. The structural and chemical transformations of polygermasilethyne into ceramic products induced during a high-energy ball milling were investigated by infrared and Raman spectroscopy, thermal analysis, X-ray analysis, as well as scanning electron microscopy analysis with using EDS spectrometer. It was shown that high-energy mechanical treatment leads to the cross-linking process including the formation of new covalent bonds between molecules and rigid three-dimensional network. Simultaneously an amorphous solid network is converted to nano- and microsized inorganic phases, namely, silicon carbide, carbon and germanium-containing phase.     

Direct Combustion Synthesis of Silicon Carbide Nanopowder from the Elements

Direct synthesis of silicon carbide (SiC) nanopowders (size 50–200 nm, BET ~20 m²/g) in Si–C system is conducted in an inert atmosphere (argon) using a self‐propagating high‐temperature synthesis (SHS) approach. A preliminary short‐term (e.g., minutes) high‐energy ball milling (HEBM) of the initial mixture, which involves pure Si and C powders, is used to enhance system reactivity. Two conditions of HEBM with different force fields (17G and 90G) are applied and the results are compared. The influence of HEBM's conditions on the microstructure of mechanically treated mixtures and combustion products is also investigated and discussed. Obtained results suggest that by changing the intensity of mechanical treatment one may prepare a completely amorphous reactive mixture containing carbon and silicon, or gradually change the ratio of (Si/C)–SiC phases and finally produce pure silicon carbide powder during the milling process. The influence of HEBM on the combustibility of the Si/C mixture possesses a critical character: the self‐sustained reaction becomes feasible only after a critical time of ball milling (i.e., 10 min for 90G; 30 min for 17G). Comparison of the microstructures for as‐milled and as‐synthesized powders reveals that for all investigated conditions the morphologies of the as‐milled reactive Si/C media are essentially the same as that for SiC combustion products. The mechanism for direct synthesis of SiC by combustion reaction is also proposed.     

Mechanism of Hardening of a Pressure-Sintered Material Based on a SiC–C Solid Solution

The kinetics of sintering of a powdered solid solution of carbon in silicon carbide sintered under a high pressure is considered. The role of diamond-like carbon clusters in the structure of silicon carbide in the process of structure formation is determined. It is shown that the parameters of the microstructure of sintered ceramics based on a SiC–C solid solution are correlated with its hardness.     

SHS of Ti3SiC2: ignition temperature depression by mechanical activation

The influence of high-energy mechanical alloying on the self-propagating high-temperature synthesis (SHS) of titanium silicon carbide (Ti₃SiC₂) was investigated. A depression of the SHS ignition temperature as a function of milling time was observed. After 107 min of milling, a spontaneous combustion reaction (MASHS) occurred within the milling vial at 67 °C, which corresponded with an 25 °C rise in the vial temperature. Observed changes in the microstructure of the milled powders gave considerable insight into the process, which provides a means of controlling a previously difficult synthesis procedure.     

Pressureless Sintering of ZrB2–SiC Ceramics

A pressureless sintering process was developed for the densification of zirconium diboride ceramics containing 10–30 vol% silicon carbide particles. Initially, boron carbide was evaluated as a sintering aid. However, the formation of a borosilicate glass led to significant coarsening, which inhibited densification. Based on thermodynamic calculations, a combination of carbon and boron carbide was added, which enabled densification (relative density >98%) by solid-state sintering at temperatures as low as 1950°C. Varying the size of the starting silicon carbide particles allowed the final silicon carbide particle morphology to be controlled from equiaxed to whisker-like. The mechanical properties of sintered ceramics were comparable with hot-pressed materials with Vickers hardness of 22 GPa, elastic modulus of 460 GPa, and fracture toughness of ∼4 MPa·m^1/2. Flexure strength was ∼460 MPa, which is at the low end of the range reported for similar materials, due to the relatively large size (∼13 μm long) of the silicon carbide inclusions.     

Kinetic studies on production of silicon carbide from rice husks

Abstract

A kinetic study on a non-conventional route for the production of silicon carbide was carried out on a laboratory scale. Silicon carbide was produced by vacuum pyrolysis of rice husks in the temperature range 1200–1600°C. The resulting products were characterised by XRD and chemical analysis. Attempts were made to develop the parametric relationships correlating the yield of silicon carbide to the process variables and starting material characteristics. Empirical relationships defining the rate of silicon carbide formation as a function of temperature, compaction pressure, CO gas diffusion, and compact porosity are proposed.     

The growth of 3D carbon fiber lattices based on silicon oxide micro-wires

A highly cross-linked network of carbon fibers was formed during the dilute oxypyrolysis of methane over a silicon carbide surface. Electron microscopy and focused ion beam milling studies demonstrate that the fibers are comprised of a carbon sheath and a silicon oxide core. Examination of surfaces exposed for different periods of time revealed that the growth of the network was a two-step process involving the formation of a silicon oxide micro-wire network, followed by carbon deposition and thickening of the fibers. Secondary reactions within the fibers are proposed which disrupt the internal structure and introduce internal pores.     

Optimal process conditions of shrinkage and warpage of thin‐wall parts

Optimal process conditions of thin‐wall injection molding of a cellular phone cover were investigated with the consideration of interaction effects between process parameters. L₂₇ experimental tests based on Taguchi's method were performed, and then Cyclone Scanner, PolyCAD and PolyWorks were used to measure the shrinkage and warpage of the thin‐wall injected parts to determine the optimal process conditions. Based on the results of the analysis of variables and the F‐test, interaction effects for each observed factor were determined. The results indicated that the packing pressure was the most important process parameter affecting the shrinkage and warpage of the thin‐wall part. The optimal process conditions were different for the shrinkage and the warpage. This was because during the injection process, the mechanisms affecting shrinkage or warpage were different. Compared with the results obtained with simplified thin‐wall parts in the literature, it was found that the geometry of a real commercial part did affect the optimal process conditions and the order of influence of process parameters. The optimal process conditions determined by Taguchi's method for reducing the shrinkage and warpage were verified experimentally in this work. Polym. Eng. Sci. 44:917–928, 2004. © 2004 Society of Plastics Engineers.     

Liquid-Phase Reaction-Bonding of Silicon Carbide Using Alloyed Silicon-Molybdenum Melts

We have investigated reaction-forming of silicon carbide by the infiltration of carbonaceous preforms using alloyed silicon melts, in order to synthesize composite materials free of the residual silicon phase that has previously limited mechanical properties and upper use temperatures. In this approach, rejection of the alloying component(s) from the primary silicon carbide phase into the remaining melt results in the formation of a secondary refractory phase, such as a silicide, in place of residual free silicon. Experiments conducted in the Si-Mo melt system show that relatively dense (>90%) silicon carbide-molybdenum silicide materials free of residual silicon and residual carbon can be obtained. A model for reactive infiltration based on time-dependent permeabilities is proposed. Processing variables important for control of the reaction rate relative to the infiltration rate, and associated processing flaws, are discussed.     

Fabrication of wood-like porous silicon carbide ceramics without templates

The porous silicon carbide ceramics with wood-like structure have been fabricated via high temperature recrystallization process by mimicking the formation mechanism of the cellular structure of woods. Silicon carbide decomposes to produce the gas mixture of Si, Si₂C and SiC₂ at high temperature, and silicon gas plays a role of a transport medium for carbon and silicon carbide. The directional flow of gas mixture in the porous green body induces the surface ablation, rearrangement and recrystallization of silicon carbide grains, which leads to the formation of the aligned columnar fibrous silicon carbide crystals and tubular pores in the axial direction. The orientation degree of silicon carbide crystals and pores in the axial direction strongly depends on the temperature and furnace pressure such as it increases with increasing temperature while it decreases with increasing furnace pressure.     

Production of silicon carbide reinforced molybdenum disilicide composites using high-pressure sintering

Studying the mechanical and thermal properties as well as the relationship between microstructure evolution and strengthening mechanisms is crucial for obtaining superior high-temperature refractory molybdenum disilicide-silicon carbide composites. In this study, molybdenum disilicide and silicon carbide powders with different volume ratios were used to fabricate molybdenum disilicide-silicon carbide composites via high pressure and high temperature at 5.5 GPa/1773 K. The introduction of the second phase improved the materials' mechanical properties. Transmission electron microscopy demonstrated that severe plastic deformation generated numerous micro-defects and grain refinement during the high pressure sintering process. The second phase enhancement also improved the samples’ performances. Under high pressure and high temperature sintering process, the thermal and mechanical properties of the materials increased dramatically, including a well Vickers hardness (~20.6 GPa), fracture toughness (~10.3 MPa·m^1/2) and thermal stability (>1673 K).     

Production of Fine, High-Purity Beta Silicon Carbide Powders

Submicrometer SiC ( β -form) powders were synthesized by reacting silica and carbon black at temperatures between 1450° and 1800°C. Simultaneous application of vacuum and mixing provides the condition for full conversion of silica to SiC. It was shown that two different reaction mechanisms are possible, depending on the reaction temperature and the partial pressure of CO. At lower temperatures (below approximately 1400°C), the dominant mechanism for silicon carbide formation involves the solid-state reaction of silica and carbon. At higher temperatures (above approximately 1400°C), the dominant mechanism is the reaction between gaseous SiO and C. Above 1400°C, the rate of SiC formation is controlled by the rate of SiO formation. In as-synthesized form, the SiC powders typically contain < 0.2 wt% of unreacted silica and free carbon in the range between 6 and 15 wt%. Precise control of partial pressure of CO in the reaction chamber and continuous mixing of the reactants provide the conditions under which the rate of silicon carbide formation may be increased by one order of magnitude. The process is suitable for large-scale commercial production of SiC, requiring no postfabrication acid leaching or major milling.     

Synthesis and Reaction Mechanism of Ti3SiC2 by Mechanical Alloying of Elemental Ti, Si, and C Powders

Formation of titanium silicon carbide (Ti₃SiC₂) by mechanical alloying (MA) of Ti, Si, and C powders at room temperature was experimentally investigated. A large amount of granules less than 5 mm in size, consisting of Ti₃SiC₂, smaller TiC particles, and other silicides, have been obtained after ball milling for only 1.5 h. The effect of excess Si in the starting powders on the formation of Ti₃SiC₂ was studied. The formation mechanism of Ti₃SiC₂ was analyzed. It is believed that a mechanically induced self-propagating reaction is ignited during the MA process. A possible reaction mechanism was proposed to explain the formation of the final products.     

Selective laser sintering of clay‐reinforced polyamide

An experimental investigation has been carried out to study the feasibility of processing blended powder of polyamide (PA) and organically modified nanoclay using selective laser sintering. The effect of nanoclay on the sintering parameters and mechanical properties of the sintered specimen have been studied. This article presents the details of preprocessing studies required and conducted to find suitability of the blended powder material to be processed on SLS machine. A suitable part bed temperature has also been found to avoid curling as well as to ensure the powder reusability. In this work, laser power, beam speed, scan spacing have been considered as influential operating process parameters and are explored using Taguchi's L₉ orthogonal array. Tensile specimens of PA and PA/clay composite have been fabricated as per ASTM D638 standard and tested for ultimate tensile strength, elongation at break, and Young's modulus using universal testing machine. Ultimate tensile strength and elongation at break are found to be decreased in case of PA/clay composite when compared with virgin polyamide. To understand the sintering insight and to explain the behavior of obtained mechanical properties, further investigations have been carried out using material characterization techniques like X‐ray diffraction and scanning electron microscopy. POLYM. COMPOS., 2010. © 2009 Society of Plastics Engineers     

Mechanically Enhanced Reactivity of Silicon for the Formation of Silicon Nitride Composites

The chemical reactivity of silicon can be enhanced by mechanical activation. Ball milling elemental silicon powders with a small amount of carbon addition at ambient temperature has resulted in an enhanced nitridation of silicon at high temperatures. In comparison with powder mixtures without milling, the nitridation process at 1250°C has been accelerated by a factor of 9 and 23 by milling powder mixtures in N₂ and NH₃, respectively, before nitridation. The enhanced nitridation process for powders milled in N₂ is primarily attributed to the mechanical activation, whereas for powders milled in NH₃, the trapped nitrogen within the powder mixtures during milling also contributes to the enhanced nitridation process, in addition to the effect of the mechanical activation.     

反应烧结碳化硅高温性能的研究进展

反应烧结碳化硅具有优良的力学性能、抗侵蚀性能和抗氧化性能等优点,是一种高致密度、低成本和净尺寸成型的材料.但由于反应烧结法的特殊工艺,反应烧结碳化硅中常含有较多游离硅,严重损害了材料的高温性能.主要阐述了反应烧结碳化硅高温力学性能、抗氧化性能、导热性能和抗热震性能的研究现状,并总结了近年来降低游离硅含量、提高反应烧结碳...     

Process parameters analysis of direct laser sintering and post treatment of porcelain components using Taguchi's method

The effects of laser sintering parameters (laser power, scan speed and hatching space) and post sintering process (heating rate, sintering temperature and holding time) on the physical and mechanical properties of porcelain components have been investigated. The study has been carried out using the Taguchi's method for the experimental design. In the laser sintering process, lower laser energy density and higher hatching space will increase the final mechanical properties of the porcelain components. A stress relief principle has been put forward to explain the different influence of the factors. The appropriate laser sintering parameters are attained in this paper: laser power 50 W; scan speed 85 mm/s; and hatching space 0.6 mm. Sintering temperature has been determined to be the most important factor in the post sintering process. Appropriate sintering temperature for the laser sintered porcelain bodies is in the range of 1425–1475 °C regarding the mechanical properties of the porcelain components. The maximum bending strength, 34.0 ± 4.9 MPa, is reached.     

电石渣首次在新型干磨干烧生产线的成功应用

安徽皖维高新材料股份有限公司为处理排放的18万t／a工业废渣(其中湿排电石渣为7．5万t/a)，决定新建1条1000t/d水泥熟料生产线。在充分分析利用湿排电石渣生产水泥熟料的现状和对电石渣脱水性能的试验研究基础上，采用了温排电石渣的机械脱水处理→配料后用立磨烘干粉磨→采用窑外分解技术进行熟料煅烧的新型干磨干烧工艺，生产出优质高标号水泥。该工程实践的成功，填补了国内在这一领域内的一项空白。     

Processing of Al2O3/SiC nanocomposites—part 1: aqueous colloidal processing

An aqueous sol preparation route was investigated for the production of Al₂O₃/SiC nanocomposites. Different alumina and silicon carbide powders were used as well as two polyelectrolyte dispersing agents. The alumina powders were dispersed first, via wet ball milling, with silicon carbide being added during a second milling stage. Dispersed sols were prepared with silicon carbide powders having an average particle size ranging from 0.5 μm down to 50 nm. The rheological characteristics of the sols were determined and used together with the results from TEM studies of the sols to ascertain the spatial arrangement of sol particles or ‘sol architecture’. It was found that whilst a polydispersion approach is suitable for the coarser SiC powders, this cannot be applied to very fine particles. Rather, the pH of the sol has to be lowered to encourage heterocoagulation.     

High productive machining of C/SiC preceramics

High productive machining of C/SiC preceramics is investigated in relation to the fabrication of complex-shaped reaction-bonded silicon carbide ceramics. Machinability is analyzed at different manufacturing steps of ceramics preparation. Machining after the carbonization step is shown to be the most efficient. Great value of material removal rate of 360 mm³/s is achieved by high productive CNC-milling of carbonized preceramics at a feed rate of 50 mm/s without any defects upon the processed surfaces, edges, and corners. Diamond tool wears approximately .01% (weight loss per mass of material removed) in the process of green CNC-milling is two orders lower compared with the milling of sintered ceramics (2.8%). Specifics of surface processing are investigated depending on carbon content in preceramics. The increase of bonding carbon from 8 to 16 vol.% decelerates loose abrasive grinding three times, improves the accuracy of surface leveling, and leads to a change of fracture mechanism. The obtained results can be helpful for the advantageous manufacture of complex-shaped silicon carbide ceramics.