Fine-grained SiC Research Articles

Investigation of the effect of pressure, sintering temperature and time on silicon carbide microstructure

In this research, SiC ceramics were densified via spark plasma sintering (SPS) with 0.5wt.% B4C and 1.5wt.% C additions at temperatures ranging from 1900 to 2000?C for 5-65min under 10-50MPa applied pressure with an intermediate dwell at 1400?C, and the effects of applied pressure, sintering temperature, and dwelling time were examined. The samples sintered under 50MPa applied pressure had high density (>99%), and showed high elastic modulus (~420MPa). However, lower applied pressure caused a decrease in density and elastic properties. The increase of sintering temperature from 1900 to 2000?C, while sintering time and pressure remained the same, caused grains coarsening. Increasing the dwelling time for the samples sintered at 1900 and 2000?C showed that sintering at a relatively lower temperature for a longer period of time did not increase grain size significantly. On the other hand, increasing the dwelling time at 2000?C caused excessive grain growth. The results show that fine-grained highly dense SiC can be produced by spark plasma sintering at 1900?C for 5min under 50MPa.

Microstructure and mechanical properties evolution of thermal-treated SiC layer with fine grain size in TRISO particles

The previous studies showed that the SiC layer in TRISO particle with smaller grain sizes can provide better mechanical property, higher oxidation resistance, and containment of fission products more effectively. Hence, the behavior of fine-grained SiC layer at high temperatures is of great significance. However, there are little reports on the effect of high temperature properties of SiC layers with fine-grained size. This study investigated the phase change, evaporation process, microstructure and hardness and Young’s modulus change of SiC layers with fine-grained size after high temperatures. The experiment result indicated that the amorphous SiC have a better crystallinity after high-temperature treatment. Especially, it could be observed that there is an evaporation-precipitation process of SiC layer at 1800 °C. So the intact OPyC layer can act as an effective obstacle to this process. When the fine-grained SiC was annealed at 1900°C, the grain size of the fine SiC layer increased to about 600 nm from 100 nm. Moreover, a novel formation mechanism of the pores in SiC layers is proposed. The results showed that the fine-grained SiC layers have better resistance to the formation of the pores at high temperatures than the SiC layers with large size. Besides, the fine-grained SiC layers show higher hardness and Young’s modulus than larger SiC layers before and after 1900°Cannealing. This research will further our understanding of the high temperature resistance of the fine-grained SiC layers and its crucial role in TRISO particles.

Structure and phase constitution of graphite-loaded reaction-bonded sic

The influence of porous SiC preforms densities for the siliconizing process on the structure and phase constitution of graphite-loaded reaction-bonded SiC (G-SiSiC) was studied. It was found that varying the densities of porous SiC preforms containing artificial graphite of similar grain size with the dimensions less than 25 mm (in height or diameter) can lead to the G-SiSiC samples with low free Si content (less 4 wt.%.). It was also shown that reaction sintering of G-SiSiC samples with the optimized densities during the siliconizing process results in the formation of a dense fine-grained SiC layer. Moreover, during the siliconizing process, a dense SiC gradient matrix is formed in which graphite and Si inclusions are uniformly dispersed in bulk.

TEM study on the morphology and interface microstructure of C/C-SiC composites fabricated by liquid infiltration

Carbon fiber reinforced carbon and silicon carbide dual matrix composites (C/C-SiC) have been widely used in aerospace field thanks to their superior high temperature properties. However, the interfacial microstructure of the composites is still not thoroughly understood in details. In this study, we focus on the morphology of the reaction-formed SiC and the interfacial microstructure between carbon and SiC in the C/C-SiC composites fabricated by liquid silicon infiltration performed at 1700 °C. The fine microstructure of the composites was investigated systematically by using transmission electron microscopy. The results show that two main types of interface are formed in the composites: (i) the fine-grained SiC is formed adjacent to the pyrolytic carbon, followed by a layer of coarse-grained SiC; and (ii) some extremely large SiC grains are formed directly on the pyrolytic carbon, and different pyrolytic carbon coatings can be connected by these large SiC grains. Additionally, some subgrains with grain size of a few tens of nanometers or even smaller can be found near the pyrolytic carbon or carbon fibers. The formation mechanism of the SiC/carbon interface is also discussed based on the fine microstructure observed.

Microstructural tailoring and its influence on oxidation resistance of carbon fiber-reinforced C-SiC matrix composites

In order to satisfy the requirements of structural and functional applications, tailoring microstructures and improving oxidation resistance of C/C-SiC is essential. In this study, microstructure tailoring is achieved by pre-treatment of fiber at 1500 °C in a nitrogen atmosphere. It is found that the pre-treatment of carbon fibers increases the mass fraction of the SiC matrix in composites, the distribution of the SiC matrix becomes homogenous, and the fine-grained SiC layer enwraps the fibers forming the dense shell. Thermo-gravimetric measurements indicate that microstructural tailoring by fiber treatment can significantly improve the oxidation resistance of C/C-SiC composites, attributed to the high mass fraction and the homogenous distribution of the SiC matrix.

SiC nanowire-induced fabrication of fine-grained and highly-density SiC coating by pressure-less reactive sintering

Fine-grained and high-density SiC coating was fabricated on carbon materials by incorporating SiC nanowires during the pressure-less reactive sintering for the first time. The grain size, relative density, and fracture toughness of the as-fabricated coating are 1.5 ± 0.6 μm, 95%, and 5.03 ± 0.73 MPa m1/2, respectively, resulting in a good oxidation protective ability, which can efficiently protect carbon materials from oxidation at 900 °C for more than 70 h or at 1500 °C for more than 90 h. The formation of such microstructure may be associated to the increasing nucleation rate resulting from the microstructure evolution and phase transformation of SiC nanowires during the sintering process.

High-temperature thermal conductivity of biomorphic SiC/Si ceramics

Thermal conductivity of biomorphic SiC/Si, a silicon carbide + silicon containing two phase material, was evaluated using the laser steady-state heat flux method. These materials were processed via silicon melt infiltration of wood-derived carbon scaffolds. In this approach, heat flux was measured through the thickness when one side of the specimen was heated with a 10.6-µm CO2 laser. A thin mullite layer was applied to the heated surface to ensure absorption and minimize reflection losses, as well as to ensure a consistent emissivity to facilitate radiative loss corrections. The influence of the mullite layer was accounted for in the thermal conductivity calculations. The effect of microstructure and composition (inherited from the wood carbonaceous performs) on measured conductivity was evaluated. To establish a baseline for comparison, a dense, commercially available sintered SiC ceramic was also evaluated. It was observed that at a given temperature, thermal conductivity falls between that of single-crystal silicon and fine-grained polycrystalline SiC and can be rationalized in terms of the SiC volume fraction in biomorphic SiC/Si material.

Highly dense and fine-grained SiC with a Sc-nitrate sintering additive

Abstract To explain the sintering behavior of 5 wt.% Sc-nitrate-added SiC, which showed a 99.3% density with the fine 156 nm-sized grains, a microstructural study using high resolution electron microscopy (HREM) was performed and the results were compared with those of Tm-added SiC, which had a mean grain size of 753 nm. Contact flattening was proposed as a governing sintering mechanism for Sc-added SiC based on the experimental observations of an inter-granular phase thinner than 1 nm along with the straight grain boundaries. On the other hand, the Tm-added SiC showed a several nm-thick inter-granular phase with a curved grain boundary morphology, which is the typical microstructure that can be obtained by a solution–reprecipitation mechanism combined with Ostwald ripening.

Change in microstructures and physical properties of ZrB2–SiC ceramics hot-pressed with a variety of SiC sources

ZrB2–SiC ceramics were fabricated by hot pressing with a variety of SiC sources in order to examine the effect of the SiC size on the microstructures and physical properties, such as hardness and thermal conductivities, of ZrB2–SiC composite ceramics. Three different ZrB2–SiC ceramics, ZPS (ZrB2–20vol% polycarbosilane), ZFS (ZrB2–20vol% fine-grained SiC), and ZNS (ZrB2–20vol% nano-sized SiC), were prepared for this study. PCS is effectively transformed into β-SiC after hot pressing. By using PCS as a precursor for SiC, ZrB2 particles are surrounded by fine particles of SiC, which results in the grain-growth inhibition of ZrB2. The effects of the SiC size on the microstructures and the physical properties of ZrB2–SiC ceramics were also investigated. ZrB2–SiC ceramics were produced by using various SiC sources in order to investigate the grain-growth inhibition and the mechanical/thermal properties of ZrB2–SiC. The sizes of ZrB2 or SiC particles in the sintered bodies highly depend on the initial size of SiC. ZrB2–SiC ceramics with smaller SiC show enhanced mechanical properties, consistently with the Hall–Petch relation. The thermal conductivities of ZrB2–SiC ceramics with nano-SiC or PCS-derived SiC are higher than that of ceramics with conventional SiC, which can be explained by the percolation theory.

Role of recombination kinetics and grain size in radiation-induced amorphization

Using a rate theory model for a generic one-component material, we investigated interactions between grain size and recombination kinetics of radiation-induced defects. Specifically, by varying parametrically nondimensional kinetic barriers for defect diffusion and recombination, we determined the effect of these parameters on the shape of the dose to amorphization versus temperature curves. We found that whether grain refinement to the nanometer regime improves or deteriorates radiation resistance of a material depends on the barriers to defect migration and recombination, as well as on the temperature for the intended use of the material. We show that the effects of recombination barriers and of grain refinement can be coupled to each other to produce a phenomenon of interstitial starvation. In interstitial starvation, a significant number of interstitials annihilate at the grain boundary, leaving behind unrecombined vacancies, which in turn amorphize the material. The same rate theory model with material-specific parameters was used to predict the grain-size dependence of the critical amorphization temperature in SiC. Parameters for the SiC model were taken from ab initio calculations. We find that the fine-grained SiC has a lower radiation resistance when compared to the polycrystalline SiC due to the presence of high-energy barrier for recombination of carbon Frenkel pairs and due to the interstitial starvation phenomenon.

Enhancement of high-temperature deformation in fine-grained silicon carbide with Al doping

Fine-grained SiC was hot-pressed with Al, B, and C additives under 150 MPa at 1850 °C. The grains had an equiaxed shape and the average grain size was 360 nm in as-sintered SiC. Al was detected at grain boundaries of Al, B, C-doped SiC by using energy-dispersive X-ray spectroscopy. The uni-axial compression tests were performed at constant crosshead speed at 1772 °C in He. The strain rates of Al, B, C-doped SiC in the low-stress region were ∼1 order of magnitude faster than those of B, C-doped SiC. The stress exponent of Al, B, C-doped SiC was 1.4 in the higher stress region, and increased to 2.6 with decreasing stress. The transition of the stress exponent, which is often observed in the superplasticity of metals and oxides, e.g., ZrO 2, appeared in fine-grained SiC also.

Swelling and time-dependent crack growth in SiC/SiC composites

SiC continuous-fiber composites are considered for nuclear applications but concern has centered on the differential materials response of the fiber, fiber/matrix interphase (fiber coating), and matrix. In our study, a continuous-fiber composite is simulated by four concentric cylinders to explore the magnitude of radial stresses when irradiation swelling of the various components is incorporated. The outputs of this model were input into a time-dependent crack-bridging model to predict crack growth rates in an environment where thermal and irradiation creep of SiC-based fibers is considered. Under assumed Coulomb friction the fiber–matrix sliding stress decreases with increasing dose and then increases once the pyrocarbon swelling reaches ‘turn around’. This causes an initial increase in crack growth rate and higher stresses in crack bridging fibers at higher doses. An assumed irradiation creep law for fine-grained SiC fibers is shown to dominate the radiation response.

Microstructure stability of fine-grained silicon carbide ceramics during annealing

Fine-grained silicon carbide ceramics with an average grain size of ∼140 nm or smaller were prepared by low-temperature hot-pressing of very fine β-SiC powders using Al2O3-Y2O3-CaO (AYC) or Y-Mg-Si-Al-O-N glass (ON) as sintering additives. The microstructure stability of the resulting fine-grained SiC ceramics was investigated by annealing at 1850°C and by evaluating quantitatively the grain growth behavior using image analysis. The β → α phase transformation of SiC in AYC-SiC was responsible for the accelerated abnormal grain growth of platelet-shaped grains. In contrast, the β → α phase transformation in ON-SiC was suppressed, which resulted in a very stable microstructure.

Presolar nanodiamonds: faster, cleaner, and limits on platinum-HL

We have developed a procedure that allows extraction of clean nanodiamond samples from primitive meteorites for isotopic analyses of trace elements on a timescale of just a week. This procedure includes microwave digestion and optimization of existing isolation techniques for further purification. Abundances of trace elements that are difficult to dissolve using standard procedures (e.g., Ir) are lower in the diamond residues prepared using the new technique. Accelerator mass spectrometry (AMS) was explored as a means for isotopic measurements. Results obtained on diamond fractions from Allende and Murchison show the need for suitable matrix-adjusted standards to correct for fractionation effects; nevertheless they allow putting an upper limit on the abundance of 198Pt-H in nanodiamonds of ∼1 × 10 14 atoms/g. This limit is on the order of what can be expected from predictions of competing nucleosynthesis models and extrapolation of the apparently mass dependent abundance trend of the associated noble gases. Unfortunately, and unexpectedly, presolar silicon carbide is almost quantitatively dissolved during microwave digestion with HCl/HF/HNO 3. Re-evaluation of the standard extraction technique, however, shows that it also may lead to severe loss of fine-grained SiC, a fact not commonly appreciated. A lower limit to SiC abundance in Murchison is 20 ppm, and previous conclusions that Murchison SiC is unusually coarse-grained compared to SiC in other primitive meteorites seem not to be warranted. Graphite and silicon nitride may survive and possibly can be separated after this step as suggested by a simulation experiment using terrestrial analog material, but the detailed behavior of meteoritic graphite requires further study.

Oxidation behavior of fine-grained SiC–B 4C/C composites up to 1400 °C

Fine-grained B 4C–SiC/C composites were fabricated using a ball-milling dispersion process. The oxidation behaviors of both fine-grained B 4C–SiC/C composites and coarse-grained B 4C–SiC/C composites at temperatures of up to 1400 °C were analyzed by the differential thermal analysis technique, and the surface morphology of the composites after isothermal oxidation at 800, 1200 and 1400 °C was examined by scanning electron microscopy (SEM). The results indicated that fine-grained B 4C–SiC/C composites had excellent oxidation resistance with self-healing properties at 1400 °C. A general model and mechanism for self-protection against oxidation of carbon materials were proposed.

Effects of heat treatment and sintering additives on thermal conductivity and electrical resistivity in fine-grained SiC ceramics

The effects of heat treatment and sintering additives on the thermal conductivity and electrical resistivity of fine-grained SiC materials were investigated. The thermal conductivity and the electrical resistivity of dense SiC materials were measured at room temperature by a laser flash technique and a current-voltage method, respectively. The results indicated that the thermal conductivity and electrical resistivity of the SiC materials were dependent on the sintering additives and the resultant microstructure. Annealed materials with oxide additives developed microstructures consisting of elongated grains of various α/β–SiC polytypes. In contrast, annealed materials with oxynitride additives had microstructures consisting of fine equiaxed grains entirely of β–SiC phase. For the annealed materials with oxide additives the observed thermal conductivity was over 110 W/mK. For the annealed materials with oxynitride additives the observed value was 47 W/mK. The electrical resistivity of a hot-pressed material with oxide sintering additives decreased after annealing. For annealed materials with oxynitride additives, the electrical resistivity was even lower. High-resolution electron microscopy revealed a thin amorphous phase along the grain boundaries. Energy dispersive x-ray spectroscopy results showed that there was segregation of both Al and O atoms and a very small amount of Y atoms at grain boundaries. The results indicated that the chemistry and structure of the grain boundary has significant influence on thermal and electrical properties in SiC.

Densification and Thermal, Mechanical and Electrical Properties of SiC Ceramics Sintered with Addition of MgO.

The suitability of MgO as a densification additive for SiC was investigated. Dense and fine-grained SiC ceramics could be obtained by hot-pressing at 1900°C with addition >1mass% (and up to 29mass%) of MgO. X-ray diffraction (XRD) analysis of the SiC ceramics showed little reaction between SiC and MgO except for formation of small amount of Mg2SiO4, which indicated the presence of a liquid phase at high temperature. XRD analysis also showed the transformation from 3C-SiC to 4H-SiC during hot-pressing. The grain sizes of the SiC sintered with 2-29mass% MgO were almost the same and in the range of 100-300nm. Mechanical properties, thermal conductivity and electrical properties of these ceramics were measured and discussed in relation to their microstructure.

A Method of Predicting the Elastic Characteristics of Diamond Composites

A model is considered for a representative cell, which is based on structural research on a particular composite, which consists of relatively large diamond single crystals in a relatively fine-grained SiC matrix; the elastic constants are calculated from an asymptotic averaging method.

Growth of SiC particles in reaction sintered SiC

Abstract For reaction sintered SiC (RSSC) prepared at 1600°C by conventional melt infiltration technique, experimentation with two different particle sizes of initial SiC, viz., 0.2 and 23.65 μm, showed that the large SiC particles remained unaltered and the sizes of the fine-grained SiC increased several times yielding well-developed faceted crystals in the final material. To study the process further, compacts of SiC powder of particle sizes varying between 0.20 and 8.99 μm were reacted with pure Si at 1600°C and the resulting SiC–Si boundaries were studied by optical microscopy. A distinct boundary layer with no penetration of Si in the compact of SiC of 0.2 μm was observed and the width of the SiC–Si boundary was found to be increasing linearly with time. Detailed SEM examination establishes the growth of the SiC upto around 4 μm from 0.2 μm starting powder. No such growth was observed in the case of starting SiC powder coarser than 0.2 μm. The growth of SiC is explained in terms of solution-reprecipitation mechanism.

Properties of silicon-based ceramics produced by hot isostatic pressing ultrafine powders

Abstract The densification of ultrafine laser-synthesized SiC (below 20 nm) and of mixtures of ultrafine SiSiC with conventional carbon powders has been performed by encapsulated hot isostatic pressing (HIP). Density and grain size as functions of temperature, HIP pressure, additive content and porosity distribution of the green body are discussed for the case of poor SiC samples. The results of measurements of thermomechanical properties and the influence of the grain size are presented. Hardness shows a significant increase with decreasing grain size, especially in specimens with a high density. Hot hardness measurements up to 1100°C on a specimen with a grain size of about 300nm revealed a relatively high hardness level compared with conventional SiC ceramics in the intermediate temperature range. Indentation fracture toughness slightly decreased with decreasing grain size. Thermal conductivity of the fine-grained SiC-based samples are lower than the values of conventional SiC ceramics, indicating the influence of the fine grain size on the phonon scattering. Measurements at temperatures up to 1500°C reveal that the reduction in the thermal conductivity in fine-grained SiC is significantly reduced at higher temperatures.