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.     

Effect of β to α phase transformation on microstructure and thermal conductivity of SiC ceramic densified with Y2O3-MgO additives in argon

SiC ceramic was fabricated by spark plasma sintering of β-SiC powder and Y₂O₃-MgO additives in argon. The effects of β→α phase transformation of SiC on microstructure and thermal conductivity of densified bulks were systematically investigated, in comparison to the counterparts using α-SiC as starting powder. The β→α phase transformation led to a “unimodal to bimodal” transition in grain size distribution. After sintering at 1850 ^oC, the incomplete β→α phase transformation induced the appearance of β/α heterophase boundary with strong effect of phonon-scattering. After sintering at 2050 ^oC, the completion of β→α phase transformation resulted in enlarged grains and disappearance of β/α heterophase boundary in SiC ceramic. The lattice oxygen content was decreased primarily by enhanced grain growth and oxygen picking-up of sintering additives, and possibly some contribution from β→α phase transformation. The optimized microstructure enabled SiC ceramic to obtain a remarkable increase in thermal conductivity from 126 to 204 W/mK after the replacement of α-SiC by β-SiC as starting powder and the accomplishment of β→α phase transformation.     

Complex impedance spectroscopy study of a liquid-phase-sintered α-SiC ceramic

The electrical response of a liquid-phase-sintered (LPS) α-SiC with 10 wt.% Y₃Al₅O₁₂ (YAG) additives was studied from near-ambient temperature up to 800 °C by complex impedance spectroscopy. The electrical conductivity of this LPS SiC ceramic was found to increase with increasing temperature, which was attributed to the semiconductor nature of the SiC grains. It was concluded that the contribution of the SiC grains to the electrical conductivity of the LPS SiC ceramic at moderate temperatures (<450 °C) is a somewhat greater than that of the YAG phase. In contrast, at higher temperatures the SiC grains control the electrical conductivity of the LPS SiC ceramic. It was also found that there are two activation energies for the electrical conduction process of the α-SiC grains. These are 0.19 eV at temperatures lower than ∼400 °C and 2.96 eV at temperatures higher than ∼500 °C. The existence of two temperature-dependence conduction regimes reflects the core–shell substructure that develops within the SiC grains during the liquid-phase sintering, where the core is pure SiC (intrinsic semiconductor) and the shell is mainly Al-doped SiC (extrinsic semiconductor).     

Rare-earth nitrate additives for the sintering of silicon carbide

Fourteen rare earth elements in their nitrate form were evaluated as sintering additives for β-SiC. All rare earth nitrates transformed to oxides by a reaction with the surface-adsorbed thin SiO₂ during heat treatment, which enhanced the density of the SiC monolith without decomposing SiC. In particular, Sc, Yb, Tm, Er and Ho were quite effective sintering additives; a > 99% relative density was observed by the addition of 5 wt.% rare earth oxide, whereas the other rare earth additives (Lu, Dy, Tb, Gd, Eu, Sm, Nd, Ce and La) revealed 77–92% density. Moreover, a fine 156 nm-sized SiC grain could be acquired by Sc addition, whereas the other additives showed a SiC grain size of approximately 1 μm. The mean hardness and K_Ic of the dense SiC containing rare earth elements were 24–27 GPa and 3.3–5.0 MPa m^1/2, respectively.     

Electromagnetic wave absorption properties of hot-pressed Si3N4-SiC ceramic nanocomposites

High-density Si₃N₄-SiC ceramic nanocomposites have exceptional mechanical properties, but little is known about their electromagnetic wave absorption (EMA) capabilities. In this paper, the effects of sintering temperature and starting material compositions on the dielectric and EMA properties of hot-pressed Si₃N₄-SiC ceramic nanocomposites were investigated. The real and imaginary permittivities of Si₃N₄-SiC ceramic nanocomposites increase with increasing sintering temperature or SiC content, particularly at the sintering temperature of 1850°C and SiC content of 50 wt.%. This is primarily due to the improvement of interfacial and defect polarizations, which is caused by the doping of nitrogen into the SiC nanocrystals during the solution-precipitation process. The real and imaginary permittivities of Si₃N₄-SiC ceramic nanocomposites show decreasing trends as sintering aid content increases. Si₃N₄-SiC ceramic composites have both good EMA and mechanical properties when they are sintered at 1850°C with 30 wt.% SiC and 5–8 wt.% sintering aids. The minimum reflection loss and maximum flexural strength reach -58 dB and 586 MPa, respectively. Materials with multilayered structural designs have both strong and broad EMA properties.     

Densification and high-temperature oxidation behavior of SiC sintered with multicomponent rare-earth additives

This study examined the effects of rare-earth (RE) elements such as Sc, Y, Ce, and Yb on the densification and oxidation of SiC. After adding binary or ternary RE nitrates in liquid form to β-SiC, hot pressing was performed at 1750 °C for 2 h under 20 MPa. RE nitrate was transformed into RE oxide and formed a liquid phase during sintering by a reaction with SiO₂ present on the SiC surface, where the total amount of RE oxide was fixed at 5 wt%. RE-based silicate melts acted as sintering additives without decomposing SiC at high sintering temperatures. SiC containing Sc–Y as an additive showed a much higher density (≥ 99%) than SiC containing the conventional Al–Y additive (∼95%). The multicomponent RE additive with a melting point (T_m) < 1550 °C had a relatively lower density than that with a higher T_m, owing to the evaporation of the additive at 1750 °C. The density of SiC also depended on the additive composition. The oxidation test, conducted at 1300 °C for up to 168 h in air, exhibited a parabolic weight gain. The SiC sample sintered with the Sc–Yb additive achieved the highest resistance of 3.23 × 10⁻⁵ mg/cm⁴·s.     

Microstructure evolutions of SiC whiskers at high temperature and its effects on silicon carbide ceramics

SiC whisker with a single-crystal structure is promising in enhancing the strength and toughness of advanced structural ceramics, owing to its excellent properties. However, studies on its microstructure evolution at high temperature (>2000 °C) are scarce. Herein, SiC whiskers were calcined at 2100 °C, and XRD, SEM, and TEM were employed to analyze microstructure evolutions. Compared with raw whiskers, XRD results indicated serious annihilation of stacking faults after calcination. The annihilation led to the fracture of whiskers and the formation of β-SiC grains, and then partial grains underwent the phase transformation to form hexagonal prism and triangular prism α-SiC grains with diameters of about 10 μm, according to SEM and TEM results. Furthermore, SiC ceramics containing different whisker contents were innovatively fabricated by pressureless solid-state sintering. The flexural strength and fracture toughness of SiC ceramic containing 10 vol% whiskers were 540 MPa and 5.1 MPa m^0.5, resulting in 38% and 11% higher values than those without whiskers, respectively.     

Improvement in fracture strength of porous reaction-bonded silicon carbide with unique surface layer by incorporating α-silicon carbide powder as a secondary phase

This study investigated the effect of incorporating α-phase silicon carbide (α-SiC) powder as a secondary phase on the fracture strength of porous reaction-bonded silicon carbide with a surface layer (porous RB-SiC) mainly based on β-phase SiC (β-SiC). The porous RB-SiC was composed of a highly porous body and surface layer with lower porosity than the porous body. α-SiC powder was incorporated into the porous RB-SiC through powder-based direct foaming followed by reaction sintering. The fracture strength and porous structure of the porous RB-SiC were investigated by varying the amount and particle size of the α-SiC powder. The porous structure composed of β-SiC grain in the porous RB-SiC was strongly influenced by the incorporation of α-SiC powder. Fracture strength of porous RB-SiC increased from 27.1 ± 1.4 MPa (without α-SiC powder) to 39.9 ± 2.0 MPa as maximum by incorporation of α-SiC, resulting in fracture strength improvement by up to 1.5 times.     

Controllable dielectric properties and strong electromagnetic wave absorption properties of SiC/spherical graphite-AlN microwave-attenuating composite ceramics

Fully dense SiC/spherical graphite-AlN microwave-attenuating composite ceramics were manufactured via hot-pressing sintering, in which, apart from the primary SG (spherical graphite) attenuating agent, 5–30 wt% semiconductive α-SiC was employed as the second attenuating agent. The incorporation of SiC contributed to a slightly decreasing electrical conductivity and enhanced polarization relaxation. Controllable complex permittivities were obtained, namely, both the real and imaginary permittivities exhibit first a decrease and then an increase with the SiC addition, and which delivers an optimized impedance matching of the composites. RL_min values below ?10 dB (more than 90% absorption) were achieved by all the composites containing 5–20 wt% SiC with the sample thickness of 1–1.4 mm, and the absorption performance characteristics were significantly tunable by controlling the of SiC content at 8.2–12.4 GHz. Impressively, a superior reflection loss of ?46 dB (1.1 mm) and wide effective absorption bandwidth of 2.1 GHz were achieved at a 5 wt% SiC content, respectively, rendering SiC/SG–AlN composites a potential ultra-thin and highly efficient microwave-attenuating ceramic candidate.     

Microstructure and densification of ZrB2–SiC composites prepared by spark plasma sintering

ZrB₂–SiC composites were prepared by spark plasma sintering (SPS) at temperatures of 1800–2100 °C for 180–300 s under a pressure of 20 MPa and at higher temperatures of above 2100 °C without a holding time under 10 MPa. Densification, microstructure and mechanical properties of ZrB₂–SiC composites were investigated. Fully dense ZrB₂–SiC composites containing 20–60 mass% SiC with a relative density of more than 99% were obtained at 2000 and 2100 °C for 180 s. Below 2120 °C, microstructures consisted of equiaxed ZrB₂ grains with a size of 2–5 μm and α-SiC grains with a size of 2–4 μm. Morphological change from equiaxed to elongated α-SiC grains was observed at higher temperatures. Vickers hardness of ZrB₂–SiC composites increased with increasing sintering temperature and SiC content up to 60 mass%, and ZrB₂–SiC composite containing 60 mass% SiC sintered at 2100 °C for 180 s had the highest value of 26.8 GPa. The highest fracture toughness was observed for ZrB₂–SiC composites containing 50 mass% SiC independent of sintering temperatures.     

Pressureless sintering of beta silicon carbide nanoparticles

This study reports the pressureless sintering of cubic phase silicon carbide nanoparticles (β-SiC). Green blended compounds made of SiC nano-sized powder, a fugitive binder and a sintering agent (boron carbide, B₄C), have been prepared. The binder is removed at low temperature (e.g. 800 °C) and the pressureless sintering studied between 1900 and 2100 °C. The nearly theoretical density (98% relative density) was obtained after 30 min at 2100 °C.The structural and microstructural evolutions during the heat treatment were characterised. The high temperatures needed for the sintering result in the β-SiC to α-SiC transformation which is revealed by the change of the composite microstructure. From 1900 °C, dense samples are composed of β-SiC grains surrounding α-SiC platelets in a well-defined orientation.TEM investigations and calculation of the activation energy of the sintering provided insight to the densification mechanism.     

Effects of SiC on phase transformation and microstructure of spark plasma sintered α/β-SiAlON composite ceramics

α/β-SiAlON/SiC composite ceramic tool materials were prepared via spark plasma sintering. The effects of content and size of SiC particles and sintering temperature on phase composition, mechanical properties, and microstructure were investigated. The results indicated that SiC restrained the transformation of β-SiAlON to α-SiAlON, but higher SiC content (≥10 wt.%) resulted in a higher Vickers hardness of the composite. The large size of SiC particles raised the densification temperature of α/β-SiAlON composites, and small SiC particles benefited to improve microstructure. There were more equiaxed α-SiAlON grains and β-SiAlON with a larger aspect ratio (${\overline{\alpha }}_{95}$ = 5.1) in the α/β-SiAlON composite containing 100 nm SiC. The sample containing 10 wt.% 100 nm SiC particles sintered at 1700°C had the optimal properties with a Vickers hardness and fracture toughness of 18.5 ± .2 GPa, 6.4 ± .2 MPa m^1/2, respectively.     

Inhibition of grain growth in liquid-phase sintered SiC ceramics by AlN additive and spark plasma sintering

SiC ceramics were prepared from nanosized β-SiC powder with different compositions of AlN and Y₂O₃ sintering additives by spark plasma sintering (SPS) at 1900 °C for 600 s in N₂. The relative density of the sintered SiC specimens increased with increasing amount of AlN, reaching a relative density higher than 99%, while at the same time grain size decreased significantly. The smallest average grain size of 150 nm was observed for SiC sample sintered with 10 vol% of additives consisting of 90 mol% AlN and 10 mol% Y₂O₃. Fully dense nanostructured SiC ceramics with inhibited grain growth were obtained by the AlN additive and SPS technique. The flexural strength of the SiC body containing 70 mol% AlN and 30 mol% Y₂O₃ additives reached the maximum value of 1000 MPa. The SiC bodies prepared with AlN and Y₂O₃ additives had the fracture toughness of around 2.5 MPam^1/2.     

Effects of oxidation cross-linking and sintering additives (TiN,B) on the formation and heat-resistant performance of polymer-derived SiC(Ti,B) films

Effects of oxidation cross-linking and sintering additives (TiN, B) on the microstructure formation and heat-resistant performance of freestanding SiC(Ti, B) films synthesized from Ti, B-containing polycarbosilane (TiB-PCS) precursor were investigated. TiB-PCS green films were first cross-linked for 1 h, 2 h, 3 h and 4 h, respectively, and then pre-sintered at 950 °C. Finally, they were sintered at 1800 °C to complete the conversion from organic films to inorganic SiC(Ti, B) films. The results reveal that curing time has a great impact on the uniformity and density of SiC(Ti, B) films. TiB-PCS films cured for 3 h yield the best quality SiC(Ti, B) films, which are composed of β-SiC crystals, C clusters, α-SiC nano-crystals, a small amount of TiB₂ and B₄C. TiB₂ and B₄C are both steady phases which can inhibit abnormal growth of β-SiC, effectively reduce sintering temperature and help consume excess C from decomposition of amorphous SiOxCy. After high temperature annealing at 1500 °C, 1600 °C and 1700 °C in argon, SiC(Ti, B) films still keep excellent mechanical properties, which makes them attractive candidate materials for microelectromechanical systems (MEMS) used at ultra-high temperatures (exceeding 1500 °C).     

The densification behavior,microstructure, mechanical,and thermionic emission properties of LaB6-SiC composite

The LaB₆-SiC composite with the different SiC content (0, 15, 30, 36, 50, 90, and 100 wt.%,) was densified by spark plasma sintering. The effects of SiC content on the densification behavior, microstructure, mechanical, and thermionic emission properties of LaB₆-SiC composite were systemically investigated. The results show that all the rapid shrinkage occurred at the heating stage during densification, and LaB₆-36 wt.%SiC composite owned the maximum shrinkage rate of 1.5 mm/min at T = 1798°C. The highest relative density of the composite decreased from 98.18% to 95.01% as the SiC content increased from 15 wt.% to 90 wt.%, under which the morphology of LaB₆ grain evaluated from the equiaxed to elongated structure, and LaB₆ grain size varied in the range of 5.05–11.42 μm. The similar eutectic structures were observed in the LaB₆-36 wt.% SiC composite because of some LaB₆ grains melting. Both the highest fracture toughness of 5.15 ± 0.56 MPa.m^1/2 and the highest bending strength of 313 ± 4.7 MPa belonged to the LaB₆-36 wt.% SiC composite, which also exhibited thermionic emission current density of 10.74 A/cm² and work function of 2.99 eV at T = 1873 K.     

Highly resistive SiC ceramics sintered with Al2O3-AlN-Y2O3 additions

A polycrystalline SiC ceramic prepared by pressureless sintering of α-SiC powders with 3 vol% Al₂O₃-AlN-Y₂O₃ additives in an argon atmosphere exhibited a high electrical resistivity of ~10¹³ Ω cm at room temperature. X-ray diffraction revealed that the SiC ceramics consisted mainly of 6H- and 4H-SiC polytypes. Scanning electron microscopy and high resolution transmission electron microscopy investigations showed that the SiC specimen contained micron-sized grains surrounded by an amorphous Al-Y-Si-O-C-N film with a thickness of ~4.85 nm. The thick boundary film between the grains contributed to the high resistivity of the SiC ceramic.     

Effect of AlN-content on the microstructure and fracture toughness of hot-pressed and heat-treated LPS–SiC ceramics

The influence of additive content on the microstructural development of hot-pressed and heat-treated LPS–SiC has been investigated using AlN–Y₂O₃ mixtures at a molar ratio of 80:20, varying the total amount from 5, 10, 15 to 20 wt.%. Specimen were hot-pressed at 1900 °C for 1 h in nitrogen atmosphere under an applied pressure of 25 MPa and subsequently heat-treated at 2000 °C for 1, 2, 4 and 8 h.It has been found that the transformation rate of β- into α-SiC is retarded by higher AlN-contents and the formation of the 6H α-SiC polytype is favored. Furthermore, grain growth during annealing is also effectively inhibited. While hardness remained almost unchanged, fracture toughness varied with additive content and/or duration of the heat-treatment. Fracture toughness increased during the first 1 or 2 h of annealing,depending on the AlN-content, and diminishing for more prolonged treatments. The maximum fracture toughness has been determined for samples containing 10 wt.% of additives, hot-pressed and annealed during 1 h.     

Microstructure and mechanical properties of carbon-precursor-added B4C and B4C−SiC ceramics subjected to pressureless sintering

Organic-carbon-precursor-added B₄C and B₄C–SiC ceramics were subjected to pressureless sintering at various temperatures. The carbon precursor increased the densification of the B₄C and B₄C–SiC ceramics sintered at 2200 °C to 95.6 % and 99.1 % theoretical density (T.D.), respectively. The pyrolytic carbon content of the B₄C–SiC composite decreased with increasing SiC content. The graphitization degree of pyrolytic carbon decreased slightly with the amount of carbon precursor and content of SiC. The 95 wt. % B₄C–5 wt. % SiC composite added with 7.5 wt. % carbon precursor and sintered at 2200 °C outperformed the other B₄C–SiC composites, and its sintered density, flexural strength, Young’s modulus, and microhardness were 98.6 % T.D., 879 MPa, 415 GPa, and 28.5 GPa, respectively. These values were higher than those of composites prepared via pressureless sintering and comparable to those of composites fabricated via hot pressing and/or using metal or oxide additives.     

Pressureless sintered silicon carbide tailored with aluminium nitride sintering agent

This study reports the influence of aluminium nitride on the pressureless sintering of cubic phase silicon carbide nanoparticles (β-SiC). Pressureless sintering was achieved at 2000 °C for 5 min with the additions of boron carbide together with carbon of 1 wt% and 6 wt%, respectively, and a content of aluminium nitride between 0 and 10 wt%. Sintered samples present relative densities higher than 92%. The sintered microstructure was found to be greatly modified by the introduction of aluminium nitride, which reflects the influence of nitrogen on the β-SiC to α-SiC transformation. The toughness of sintered sample was not modified by AlN incorporation and is relatively low (around 2.5 MPa m^1/2). Materials exhibited transgranular fracture mode, indicating a strong bonding between SiC grains.     

Effects of the initial α-SiC content on the microstructure, mechanical properties, and permeability of macroporous silicon carbide ceramics

By using α- and/or β-SiC staring powders, the effects of the initial α-phase content on the microstructure, mechanical properties, and permeability of macroporous SiC ceramics were investigated. When β-SiC powder or a mixture of α/β powders containing a small amount (≤10%) of α-SiC powder was used, the microstructure consisted of large platelet grains. In contrast, when using α-SiC powder or α/β powders containing a large amount (>10%) of α powders, the microstructure consisted of small equiaxed grains. The development of large α-SiC platelet grains in the microstructure did not result in any improvement of the flexural strength of the macroporous SiC ceramics because of the accompanying pore growth and grain growth. The growth of the platelet-SiC grains was beneficial in increasing the gas permeability of the macroporous SiC ceramics from 4.12 × 10⁻¹³ m² for macroporous SiC with an equiaxed microstructure to 1.89 × 10⁻¹² m² for macroporous SiC ceramics with large platelet grains.