Microstructure and thermal conductivity of fully ceramic microencapsulated fuel fabricated by spark plasma sintering

Fully ceramic microencapsulated pellet (FCM), consisting of tristructural isotropic (TRISO) particles embedded in silicon carbide (SiC) matrix, was fabricated using spark plasma sintering. The parameters affecting the densification of SiC matrix were first investigated, and then FCM pellets were prepared using TRISO particles with/without outer pyrolytic carbon (OPyC) layer. Effects of thermal exposure on the TRISO particles during SPS were evaluated. In addition, the thermal condcutvitities of FCM pellet, as well as the SiC matrix, were measured using laser flash. It was revealed that the TRISO particles with OPyC layers significantly lower the thermal conductivity of FCM pellet. Based on Maxwell‐Eucken model, the predicted effective thermal conductivities of TRISO particles with/without OPyC layers were 14.4 W/m K and 25.2 W/m K, respectively. Finite elements simulation indicated that the SiC layer in TRISO particle plays a dominant role on the thermal conductivity of FCM. The presence of OPyC layers would generate gaps/porous SiC near the interface and resist the heat flows, leading to a lower thermal conductivity of FCM.     

New quaternary additive for processing fully ceramic microencapsulated fuels without applied pressure

To apply SiC ceramics as a matrix for fully ceramic microencapsulated (FCM) fuels, the equivalent boron content (EBC) factors of elements in the sintering additives should be considered as an important criterion. A previously developed quaternary additive composition based on AlN–Y₂O₃–Sc₂O₃–MgO contained Sc, which has a relatively high EBC factor (8.56 × 10^?3). This study proposes a novel quaternary additive composition (AlN–Y₂O₃–CeO₂–MgO), in which Sc is replaced by Ce (EBC factor = 6.36 × 10^-5). The new additive composition achieved successful densification of the SiC matrix at 1850 °C without applied pressure. FCM pellets containing 36 vol% tristructural isotropic (TRISO) particles were successfully sintered at 1850 °C using the above matrix without applied pressure. The thermal conductivities of the FCM pellets prepared via pressureless sintering with 36 vol% TRISO particles were 43.9 W·m^-1·K^-1 and 25.8 W·m^-1·K^-1 at 25 °C and 500 °C, respectively.     

Pressureless sintered silicon carbide matrix with a new quaternary additive for fully ceramic microencapsulated fuels

This study suggests a new additive composition based on AlN–Y₂O₃–Sc₂O₃–MgO to achieve successful densification of SiC without applied pressure at a temperature as low as 1850 °C. The typical sintered density, flexural strength, fracture toughness, and hardness of the SiC ceramics sintered at 1850 °C without applied pressure were determined as 98.3%, 510 MPa, 6.9 MPa·m^1/2, and 24.7 GPa, respectively.Fully ceramic microencapsulated (FCM) fuels containing 37 vol% tristructural isotropic (TRISO) particles could be successfully sintered at 1850 °C using the above matrix without applied pressure. The residual porosity of the SiC matrix in the FCM fuels was only 1.6%. TRISO particles were not damaged during processing, which included cold isostatic pressing under 204 MPa and sintering at 1850 °C for 2 h in an argon atmosphere. The thermal conductivity of the pressureless sintered FCM pellet with 37 vol% TRISO particles was 44.4 Wm^?1 K^?1 at room temperature.     

Effects of starting powder on microstructure and thermal conductivity of pressureless-sintered fully ceramic microencapsulated fuels

The effects of the starting SiC powder (α or β) with the addition of 5.67 wt% AlN–Y₂O₃–CeO₂–MgO additives on the residual porosity and thermal conductivity of fully ceramic microencapsulated (FCM) fuels were investigated. FCM fuels containing ～41 vol% and ～37 vol% tristructural isotropic (TRISO) particles could be sintered at 1870 °C using α-SiC and β-SiC powders, respectively, via a pressureless sintering route. The residual porosities of the SiC matrices in the FCM fuels prepared using the α-SiC and β-SiC powders were 1.1% and 2.3%, respectively. The thermal conductivities of FCM pellets with ～41 vol% and ～37 vol% TRISO particles (prepared using the α-SiC and β-SiC powders, respectively) were 59 and 41 Wm^?1K^?1, respectively. The lower porosity and higher thermal conductivity of FCM fuels prepared using the α-SiC powder were attributed to the higher sinterability of the α-SiC powder than that of the β-SiC powder.     

Interaction of Embedded Tri-Isotropic Fuel Particles with Melt of Alkaline Borosilicate Glass

A silicon carbide (SiC) layer is an outer-coated layer of spent tri-isotropic (TRISO) fuel particles and it is known to be a pressure vessel for retaining fission products, and preventing contamination in the primary circuit of a nuclear reactor. The goal of this article is to elucidate the chemical bonding and an interface formation of an alkaline borosilicate glass (ABG) with the coating layer of TRISO fuel particles. Particular emphasis is placed on the analysis of the intermediate chemical phase at the interface SiC/glass as a function of the material impurity and thickness of the SiC layer. The findings provide valuable information regarding the restriction parameters of immobilisation TRISO particles in glass. The interaction between the glass and SiC caused a total destruction of a thin SiC layer (10 μm), a random partial interaction to a thick SiC layer (40 μm) and formation of bubbles (CO₂, CO) to an inner pyrolitic carbon (IPYC). The Raman spectroscopy analysis revealed that the interaction of ABG with the SiC layer occurred at a point, where a low excess of carbon was co-deposited during chemical vapour deposition process. The interaction resulted in a formation of a mono-crystal SiC, dispersed in vitreous silica as a crystalline inclusion.     

Densification of Alumina-Silicon Carbide Powder Composites: I, Effects of a Polymer Coating on Silicon Carbide Particles

One possible approach to improving the densification of powder composites containing a major crystalline phase which densifies (e.g., Al₂O₃) and a difficult-to-sinter phase (e.g., SiC) is to accommodate the matrix volume shrinkage with a "disappearing" polymer coating. A polymer coating prevents contact between the nonsinterable particles and the surrounding matrix. The coating can be burned off before sintering, allowing the matrix phase to "shrink-fit" around the nonsinterable particles during sintering. The effects of a polymer coating on the densification of a two-phase particle system were tested using SiC powder dispersed in an Al₂O₃ matrix. The composites processed with a polymer coating showed more densification during equivalent firing cycles than did those processed without a polymer coating. Densification during sintering was approximately proportional to the amount of polymer adsorbed on SiC, suggesting that the Al₂O₃ matrix did shrink-fit into the gaps between the SiC particles and the surrounding Al₂O₃ matrix. Differences in the pore-size distributions of polymercoated green compacts and uncoated compacts indicated a perturbation of the green microstructure by the gaps. The estimated average thickness of the gap is approximately 20 nm, ∼8% of the average radius of the SiC powder used in this study.     

An Original Way to Investigate Silver Migration Through Silicon Carbide Coating in TRISO Particles

Extensive release of the metastable silver nuclide Ag^110m from fully intact tristructural‐isotropic (TRISO) particles raises concerns over safety of advanced nuclear reactors. In this study, we propose a new model to interpret the silver migration mechanism in SiC based on experimental observations from both Ag/SiC composite pellets and TRISO particles with an entrapped silver layer. For the Ag/SiC composite pellets heat treated at 1450°C, silicon was detected in the silver phase, amorphous carbon was found, and new β‐SiC had formed at the Ag/SiC interface. The results indicate that Ag in its liquid state reacts with SiC by forming a Ag–Si alloy. Carbon precipitates as a second phase or reacts with the Ag–Si alloy to form new SiC. Results from the heat‐treated TRISO particles trapping Ag show that Ag penetrates through the SiC layer and is present in either “finger‐shape” or “wedge shape” at the SiC grain boundaries. Ag was also found inside abnormally large SiC grains at the trailing edge between Ag and SiC, indicating the recrystallization of SiC. A dissolution‐reaction model was proposed to explain Ag migration through SiC, and this model is supported by thermodynamic calculations.     

Tetrahedral amorphous carbon deposited with the pulsed plasma arc-discharge method as a protective coating against solid impingement erosion

The solid impingement erosion resistance of a tetrahedral amorphous carbon (ta-C) coating (sp³ bonding fraction ∼80%, thickness 20 μm) was compared to the erosion resistance of stainless steels, WC–Co hard metal, sintered SiC, sintered Al₂O₃, synthetic ruby (Al₂O₃, grain size of the order of mm) and a commercial TiN coating. The ta-C coating was deposited by the filtered pulsed plasma arc-discharge method on an AISI316L stainless steel sample. All other materials, except the ta-C and the TiN, were in a bulk form. The experiments revealed that the volume removal rate of bulk materials was 1.5–540 times higher than that of ta-C, depending on the material. The extensive chipping of TiN hindered a meaningful comparison of the measured results to those received from bulk materials. The erosion experiments were performed with a test apparatus, which used pressurized air to accelerate angular Al₂O₃ particles (60–77 μm in diameter). The erosion damage was analyzed with a surface profilometer and an optical microscope. The critical thickness for the coating that was able to resist catastrophic delamination under particle exposure, was found to be approximately 1 μm. The extremely low erosion rate of ta-C, when eroded with low values of angle of attack (∼20°), implies that ta-C erodes in a brittle manner.     

Size effect on the irradiation performance of coated fuel particles

Outer coatings that were as near alike as possible were applied to two different sizes of inert TRISO particles that were larger than those commonly used to fuel HTGR reactors and these particles were then irradiated in a test reactor to observe the influence of particle size on outer coating failures that resulted from irradiation-induced shrinkage of coatings onto the more stable SiC substrates over which they were applied. Outer coatings of plain pyrocarbon and of Si-alloyed pyrocarbon were used to make up two test pairs of particles with diameters of about 1050 and 1300 μm. For a fast-neutron fluence of 5.5 × 10²⁵ n/m² (E > 29 fJ) at an irradiation temperature of 1125K, failure was about twice as high in the larger 1300 μm particle of each test pair as in the smaller 1050 μm particle (16 vs 8%), with each of the coating types having roughly the same behavior. This observed size effect is somewhat greater than predicted by volume-dependent Weibull theory, which estimates failure of the larger size particles at 13% when the smaller size particles fail at a rate of 8%. However, experimental uncertainties are sufficient to account for the difference between observed size effects and those predicted by Weibull theory. The conclusion that the irradiation performance of coated particles is size dependent is reinforced by the fact that failure for regular 850 μm fueled particles in the same irradiation test was essentially zero.     

Effect of hydrothermal corrosion on the fracture strength of SiC layer in tristructural-isotropic fuel particles

The hydrothermal corrosion behavior of SiC layer in tristructural-isotropic (TRISO) fuel particles and its effect on the fracture strength were investigated. The corrosion test was performed using the static autoclave at 400°C/10.3 MPa. The SiC layer exhibited a thickness loss and the corrosion rate followed a linear law. During corrosion, carbon was formed on the SiC surface due to the loss of Si. The corrosion was found preferentially occurred at the grain boundary of SiC, leading to the grain detachment and pit formation. The rate determining step of the corrosion was SiO₂ formation rather than SiO₂ dissolution in the hydrothermal environment. The fracture strength of SiC shell after corrosion was evaluated using the crush test. It showed a slight decrease with an increase in corrosion time, due to the thickness reduction in SiC layer. The results of this study demonstrated that the SiC in TRISO particles has good corrosion resistance in the hydrothermal environment.     

Preparation of SiC layer with sub-micro grain structure in TRISO particles by spouted bed CVD

The previous studies showed that the SiC layer with sub-micro grain structure can prevent fission products release more effectively. Usually, the silicon carbide (SiC) layer in TRISO (Tristructural isotropic) particles was deposited at 1500–1600 °C by spouted bed chemical vapor deposition (CVD) from methyltrichlorosilane in H₂ environment. It has grain size in micro scale (1–10 μm). In this work, we proposed a novel method for preparation of sub-micro grained SiC layer by adjusting the deposition parameters. The as-prepared SiC layer had grain size about 200 nm and pure phase (β-SiC). The fined grain SiC layer also exhibited higher mechanical property than SiC layer with larger grain size.     

A Si–SiC Oxidation‐resistant Coating for Carbon/Carbon Composites by Hot‐pressing Reaction Technique

A Si–SiC coating was prepared by hot‐pressing reactive sintering (HPRS) technique for protecting carbon/carbon (C/C) composites against oxidation. The Si–SiC coating has a dense and crack‐free structure with a thickness of 70–90 μm. The Si–SiC coating by HPRS has a higher SiC content and lower Si content than the coating by pressure‐less reactive sintering (PRS). It also exhibits better oxidation‐protective ability than that prepared by PRS. With hot‐pressing, the flexural strength of the Si–SiC coated C/C composites decreases from 121 MPa to 99 MPa, and the interface bonding strength increases from 6 MPa to 10 MPa.     

脉冲电沉积Ni-SiC复合镀工艺的研究

采用脉冲电镀在45#钢表面制备含有SiC微粒的镍基复合镀层,研究了镀液中SiC的质量浓度、脉冲平均电流密度、pH值对Ni-SiC复合镀层的影响规律.结果表明:电镀工艺条件的改变影响复合镀层中SiC的共沉积量和镀层的硬度,当镀液中SiC质量浓度为20 g/L,平均电流密度为40 A/dm2时,镀层中SiC体积分数为14.3%,硬度约为镍镀层的1.7倍.     

裂解碳包覆对SiC纳米纤维增强SiC陶瓷基复合材料性能的影响

以SiC纳米纤维(SiC_nf)为增强体,通过化学气相沉积在SiC纳米纤维表面沉积裂解碳(PyC)包覆层,并与SiC粉体、Al₂O₃-Y₂O₃烧结助剂共混制备陶瓷素坯,采用热压烧结工艺制备质量分数为10%的SiC纳米纤维增强SiC陶瓷基(SiC_nf/SiC)复合材料。研究了PyC包覆层沉积时间对SiC_nf/SiC陶瓷基复合材料的致密度、断裂面微观形貌和力学性能的影响。结果表明:在1 100 ℃下沉积60 min制备的PyC包覆层厚度为10 nm,且为结晶度较好的层状石墨结构;相比于纤维表面无包覆层的复合材料,复合材料的断裂韧性提高了35%,达到最大值(19.35±1.17) MPa·m^1/2,抗弯强度为(375.5±8.5) MPa,致密度为96.68%。复合材料的断裂截面可见部分纳米纤维拔出现象,但SiC_nf/SiC陶瓷基复合材料界面结合仍较强,纳米纤维拔出短,表现为脆性断裂。     

Coating of carbon-fiber surface by silicon carbide via sol-gel mixtures of tetraethyl orthosilicate and phenolic resin

The preparation of a SiC coating on a carbon fiber surface using a sol-gel mixture of tetraethyl orthosilicat (TEOS) and phenolic resin was studied. FTIR and SEM investigations indicated that the SiC coating can be formed by carbothermal reduction of the sol-gel mixture at 1420°C for 15–20 min in an argon atmosphere. TGA of the coated fiber was also performed, showing that the SiC coating improves the thermooxidative stability of the carbon fiber. With the thickness of the obtained coating of 0.47 μm using a C/Si ratio of 4, this treatment does not affect the carbon fiber strength.     

Oxidation treatments for SiC particles and its compatibility with glass

Different thermal treatments were performed to produce a protective coating on the surface of SiC particles in order to allow their incorporation in a glass matrix. These oxidation treatments were carried out in air at different temperatures ranging from 800 °C to 1500 °C and different times at 1200 °C (10 min–48 h). The oxidation kinetics followed the Deal–Grove model and the thickness of the protective coating increased with temperature and SiC particle size. Protected SiC particles with different particle sizes were incorporated in a borosilicate glass. With small particles sizes foam glasses were obtained, whereas particles with higher grain size, i.e., higher coating thickness, were stable in the glass matrix and a smooth glass was obtained.     

Controlling micro-porous size in TiO2 pellets processed by sol-gel and rapid liquid phase sintering

A fast and lower electric energy consumption process to synthesize TiO₂ pellets with interconnected micropores, is proposed. Pellets were prepared by rapid liquid-phase sintering (RLPS) at different temperatures (900, 1000 and 1100 °C) and times (2, 5, 7 and 10 min). The density of these samples increases when temperature rises and decreases for longer sintering times; the highest density, of 2.78 g/cm³ was obtained when sintering at 1100 °C/2min. The addition of PEG and the annealing at 450 °C/2 min produced pores of 38.51 ± 27.51 μm and 48.98 ± 32.34 μm when PEG3350 and PEG8000 respectively, were used. An additional RLPS at 1100 °C/2 min gives rise to TiO₂ pellets in a rutile phase, with pores of 76.82 ± 34.23 μm and 173.04 ± 68.03 μm for PEG3350 and PEG8000, respectively. Interconnectivity of pores is obtained in all samples. The elastic module of these pellets was 39.22 ± 0.16 GPa, for the sample prepared with PEG3350; and 121.30 ± 0.04 GPa for the one made with PEG8000. The achieved pore size and interconnectivity at 1100 °C/2 min are a result of the optimized sintering conditions and the better control of PEG vapor pressure released when the intermediate annealing at 450 °C/2 min is introduced.     

Electrophoretic deposition of ceramic coatings on ceramic composite substrates

Abstract

The applicability of electrophoretic deposition (EPD) for the fabrication of single layer and multilayer ceramic coatings on dense ceramic composite materials has been examined. Al₂O₃/Y-tetragonal zirconia polycrystal (TZP) functionally graded composites of tubular shape were successfully coated with a two layer coating comprising porous alumina and dense reaction bonded mullite layers. The dual layer coating structure was designed to eliminate the numerous cracks caused by volume shrinkage during sintering of the individual EPD formed layers. In another example, mullite fibre reinforced mullite matrix composites were coated with a thin layer of nanosized silica particles using EPD. The aim was to achieve a compressive residual stress field in the silica layer on cooling from sintering temperature, in order to increase composite fracture strength and toughness. The EPD technique proved to be a reliable method for rapid preparation of single layer and multilayer ceramic coatings with reproducible thickness and microstructure on ceramic composite substrates.     

Fine grained Al2O3/SiC composite ceramic tool material prepared by two-step microwave sintering

Fine-grained Al₂O₃/SiC composite ceramic tool materials were synthesized by two-step microwave sintering. The effects of first-step sintering temperature (T1), content and particle size of SiC on the microstructure and mechanical properties were studied. It was found that the sample with higher content of SiC was achieved with finer grains, and the incorporation of SiC particles could bridge, branch and deflect the cracks, thus improving the fracture toughness. Higher T1 was required for the densification of the samples with higher content of SiC (>5?wt%). The sample containing 3?wt% SiC particles with the mean particle size of 100?nm, which was sintered at 1600?°C (T1) and 1100?°C (T2) for 5?min had the fine microstructure and optimal properties. Its relative density, grain size, Vickers hardness and fracture toughness obtained were 98.37%, 0.78?±?0.31?μm, 18.40?±?0.24?GPa and 4.97?±?0.30?MPa?m^1/2, respectively. Compared to the sample prepared by single-step microwave sintering, although near full densification can be achieved in both two methods, the grain size was reduced by 36% and the fracture toughness was improved by 28% in two-step microwave sintering.     

Room temperature impact-induced deposition of pure SiC coating layer by vacuum kinetic spraying

This study successfully manufactured a thick, pure SiC coating layer with a thickness of 83.3 μm using vacuum kinetic spray process and investigated the unique impact-induced deposition behavior at room temperature. The simulated result of SiC particle collides with the metallic matrix, or predeposited SiC layer confirmed that particle shock pressure could increase up to the maximum pressure of 27.2 GPa. Moreover, the particles were predicted to fracture to submicrometer size after plastic deformation and those characteristics also matched microstructural observations made with a transmission electron microscope. The SiC coating layer formed an unexpected microstructure composed of bent lattices, fractured submicrometer-sized particles, and amorphous layers. Correlating the microstructure and simulation results suggested that a pure SiC coating layer could be formed using mechanical anchoring and amorphous bonding at room temperature.