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1.
《Journal of the European Ceramic Society》2021,41(13):6778-6783
The sintering behaviour of freeze-granulated UO2-PuO2 powders containing 33 and 15 mol% Pu/(U + Pu) was investigated under reducing conditions up to 1700 °C. For both compositions, the “grain size versus relative density” trajectory was constructed. All the experimental points form a single trajectory meaning that a relative density/grain size pair obtained after sintering seems independent of the thermal path (heating rate, soak time, soak temperature) and of the Pu content. Exploiting the “grain size versus relative density trajectory” enabled also to propose that densification was controlled by grain boundary diffusion and grain growth by the grain boundaries whatever the Pu content. An activation energy around 510 kJ/mol was obtained for densification, which was close to the value reported for the grain boundary diffusion of plutonium cations in U1-xPuxO2 polycrystals. Whatever the Pu/(U + Pu) content, the sintered microstructure of 98 % dense samples possesses a homogeneous distribution of plutonium and uranium cations. 相似文献
2.
Dense nanocrystalline UO2+x fuel pellets synthesized by high pressure spark plasma sintering
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Tiankai Yao Spencer M. Scott Guoqing Xin Bowen Gong Jie Lian 《Journal of the American Ceramic Society》2018,101(3):1105-1115
Nanocrystalline UO2+x powders are prepared by high‐energy ball milling and subsequently consolidated into dense fuel pellets (>95% of theoretical density) under high pressure (750 MPa) by spark plasma sintering at low sintering temperatures (600°C‐700°C). The grain size achieved in the dense nano‐ceramic pellets varies within 60‐160 nm as controlled by sintering temperature and duration. The sintered fuel pellets are single phase UO2+x with hyper‐stoichiometric compositions as derived by X‐ray diffraction, and micro‐Raman measurements indicate that random oxygen interstitials and Willis clusters dominate the single phase nano‐sized oxide pellets of UO2.03 and UO2.11, respectively. The thermal conductivities of the densified nano‐sized oxide fuel pellets are measured by laser flash, and the fuel stoichiometry displays a dominant effect in controlling thermal transport properties. A reduction in thermal conductivity is also observed for the dense nano‐sized pellets as compared with micron‐sized counterparts reported in the literature. The correlation among the SPS sintering parameters—microstructure control—properties is established, and the nano‐sized UO2+x pellets with controlled microstructure can serve as the model systems for fundamental understandings of fuel behaviors and obtaining critical experimental data for multi‐physics MARMOT model validation. 相似文献
3.
《Journal of the European Ceramic Society》2005,25(7):1057-1065
The high sintering temperature required for aluminum nitride (AlN) at typically 1800 °C, is an impediment to its development as an engineering material. Spark plasma sintering (SPS) of AlN is carried out with samarium oxide (Sm2O3) as sintering additive at a sintering temperature as low as 1500–1600 °C. The effect of sintering temperature and SPS cycle on the microstructure and performance of AlN is studied. There appears to be a direct correlation between SPS temperature and number of repeated SPS sintering cycle per sample with the density of the final sintered sample. The addition of Sm2O3 as a sintering aid (1 and 3 wt.%) improves the properties and density of AlN noticeably. Thermal conductivity of AlN samples improves with increase in number of SPS cycle (maximum of 2) and sintering temperature (up to 1600 °C). Thermal conductivity is found to be greatly improved with the presence of Sm2O3 as sintering additive, with a thermal conductivity value about 118 W m−1 K−1) for the 3 wt.% Sm2O3-doped AlN sample SPS at 1500 °C for 3 min. Dielectric constant of the sintered AlN samples is dependent on the relative density of the samples. The number of repeated SPS cycle and sintering aid do not, however, cause significant elevation of the dielectric constant of the final sintered samples. Microstructures of the AlN samples show that, densification of AlN sample is effectively enhanced through increase in the operating SPS temperature and the employment of multiple SPS cycles. Addition of Sm2O3 greatly improves the densification of AlN sample while maintaining a fine grain structure. The Sm2O3 dopant modifies the microstructures to decidedly faceted AlN grains, resulting in the flattening of AlN–AlN grain contacts. 相似文献
4.
《Journal of the European Ceramic Society》2014,34(7):1791-1801
High density uranium dioxide (UO2) pellets with grain sizes between 0.9 μm and 9 μm were produced by spark plasma sintering (SPS). A systematic study was performed by varying the sintering temperature between 750 °C and 1450 °C and hold time between 0.5 min and 20 min to obtain UO2 pellets with a range of theoretical densities (TD) and grain sizes. The microstructure development in terms of grain size, density and porosity distribution was investigated. The oxygen/uranium (O/U) ratio of the resulting pellets was found to decrease after SPS. The thermal conductivity of UO2 pellets increased with the theoretical density but the grain size in the investigated range had no significant influence. The measured thermal conductivity values up to 900 °C were consistent with the reported literature for conventionally sintered UO2 pellets. The benefits of using SPS over the conventional sintering of UO2 are summarized. 相似文献
5.
E. De Bona L. Balice L. Cognini M. Holzhäuser K. Popa O. Walter M. Cologna D. Prieur T. Wiss G. Baldinozzi 《Journal of the European Ceramic Society》2021,41(6):3655-3663
Three different spark plasma sintering (SPS) treatments were applied to highly sinteractive, near-stoichiometric UO2.04 nanocrystalline (5 nm) powders produced by U(IV) oxalate hydrothermal decomposition at 170 °C. The sintering conditions for reaching 95 % theoretical density (TD) in regular SPS, high pressure SPS (HP-SPS), and, for the first time, two-step SPS (2S-SPS), were determined. Densification to 95 % TD was achieved at 1000 °C in regular SPS (70 MPa applied pressure), 660 °C in HP-SPS (500 MPa), and 650?550 °C in 2S-SPS (70 MPa). With the goal of minimising the grain growth during densification, the sintering treatments were optimised to favour densification over coarsening, and the final microstructures thus obtained are compared. Equally dense UO2 samples of different grain sizes, ranging from 3.08 μm to 163 nm, were produced. Room-temperature oxidation of the powders could not be avoided due to their nanometric dimensions, and a final annealing treatment was designed to reduce hyperstoichiometric samples to UO2.00. 相似文献
6.
Cheng-Fan Lee Cheng-Feng Wu Jau-Ho Jean 《Journal of the American Ceramic Society》2019,102(1):158-166
Effects of CuO on constrained sintering of a polycrystalline TiO2 ceramics have been investigated. The densification temperature of TiO2 is reduced from 1100-1200°C for pure TiO2 to 900°C with the presence of 0.5-3 mol% CuO under free sintering. For the samples with 1 mol% CuO, the constrained densification is slowed down, but a high sintered density of >95% at 950°C, which is close to that sintered freely, is still obtained. The above results are caused by the formation of CuO-rich film at the grain boundaries, which reduces grain-boundary energy and enhances grain-boundary migration kinetics of TiO2. To confirm the above findings, molecular dynamics simulation, at which the ratio of grain boundary energy of TiO2 between with and without CuO agrees well with that obtained experimentally, is conducted. 相似文献
7.
《Ceramics International》2022,48(15):21756-21762
Understanding the densification and grain growth processes is essential for preparing dense alumina fibers with nanograins. In this study, the alumina fibers were prepared via isothermal sintering at 1200, 1300, 1400, and 1500 °C for 1–30 min. The phase, microstructure, and density of the sintered fibers were investigated using XRD, SEM, and Archimedes methods. It was found that the phase transformation during the isothermal sintering enhances the densification of Al2O3 fibers in the initial stage, while the pores generated during the phase transformation retard the densification in the later period. The kinetics and mechanisms for the densification and grain growth of the fibers were discussed based on the sintering and grain growth models. It was revealed that the densification process of the fibers sintered at 1500 °C is dominated by the lattice diffusion mechanism, while the samples sintered at 1200–1400 °C are dominated by the grain boundary diffusion mechanism. The grain growth of the Al2O3 fibers sintered at 1200–1300 °C is governed by surface-diffusion-controlled pore drag, and that sintered at 1400 °C is dominated by lattice-diffusion-controlled pore drag. 相似文献
8.
《Ceramics International》2016,42(6):6619-6623
UO2 beads from the sol supported precipitation method were calcined at a low temperature in order to obtain porous micro-beads, composed of nanometric particles. The sintering behaviour of the beads in spark plasma sintering was investigated. The powder had a good sinterability and the final grain size of the pellets could be tailored by varying the processing conditions, in order to resemble the microstructure of the traditionally fabricated UO2 pellets (i.e. grains of several µm size), or to achieve sub-micrometre size as observed in the high burnup structure. Dense UO2 pellets with a grain size as small as 300 nm were obtained by sintering at 835 °C without dwell time, whereas 3 µm grained pellets were obtained at 1000 °C and a 5 min dwell time. 相似文献
9.
《Ceramics International》2020,46(8):11577-11583
In this research, for the first time, the stability of SnO2 based varistor ceramics sintered in the range of 1250–1350 °C against DC-accelerated aging and impulse surge current tests, was systematically studied. Microstructural study of the sintered samples by XRD and FESEM indicated that the sintering temperature only affects densification and grain size, while phase composition remains intact. With the increase of sintering temperature from 1250 °C to 1350 °C, the mean grain size increased from 1.6 to 8 μm. The maximum nonlinear coefficient of 50 and the minimum leakage current density of 1.5 μA/cm2 were obtained in the sample sintered at 1300 °C. The breakdown electric field decreased from 800 V/mm to 270 V/mm, when sintering temperature increased from 1250 °C to 1350 °C. The samples sintered at 1250 °C did not show stability against neither of DC-accelerated aging and impulse current tests. The varistors sintered at 1300 °C exhibited the excellent resistance to DC-accelerated aging degradation, while ceramics sintered at 1350 °C showed the best resistance to impulse current degradation. 相似文献
10.
Multi-phase ceramics in the TaC–TaB2–C system were prepared from TaC and B4C mixtures by reactive pressureless sintering at 1700–1900 °C. The pressureless densification was promoted by the use of nano-TaC and by the presence of active carbon in the reaction products. The presence of TaB2 inhibited grain growth of TaC and increased the hardness compared to pure TaC. If a coarse TaC powder was used, the compositions did not densify. In contrast, pure nano-TaC was pressureless sintered at 1800 °C by the addition of 2 wt.% carbon introduced as carbon black or graphite. The introduction of carbon black resulted in fully dense TaC ceramics at temperatures as low as 1500 °C. The grain size of nominally pure TaC ceramics was a strong function of carbon stoichiometry. Enhanced grain size in sub-stoichiometric TaC, compared to stoichiometric TaC, was observed. Additional work is necessary to optimize processing parameters and evaluate the properties of ceramics in the TaC–TaB2–C system. 相似文献
11.
《Journal of the European Ceramic Society》2002,22(11):1847-1855
The objective of this work was to lower the sintering temperature of lead zirconate titanate (PZT) without reducing the piezoelectric performance. PZT was sintered using PbO–WO3 additive of eutectic composition, which assists the densification process by liquid-phase formation. Sintering was carried out from 1075 to 1125 °C between 1 and 4 h. Density, dielectric properties and piezoelectric properties were measured. Microstructure and fracture mechanism have been studied by SEM. At the mildest sintering conditions, the additive has a positive effect on dielectric and piezoelectric properties. The liquid-phase sintering leads to a denser material without additional grain growth. PZT with PbO–WO3 additive is mechanically weaker than pure PZT. The liquid phase leads to weaker grain boundaries and the material cracks in intergranular fracture, whereas pure PZT has a mixture of intergranular and transgranular fracture, and PZT sintered conventionally at 1260 °C has transgranular fracture. 相似文献
12.
《Journal of the European Ceramic Society》2007,27(2-3):801-805
The effect of small amounts (0.2–2.0 wt.%) of bismuth oxide on the sintering behavior and microstructural development of Ce0.9Gd0.1O1.95 (GDC) submicronized powders has been studied using XRD for the lattice parameter measurements, the constant heating rate (CHR) method in air to monitor the shrinkage kinetics of powder compacts, and scanning electron microscopy (SEM) to study the microstructure of the sintered samples. Sintering of GDC compacts was significantly improved by adding small amounts of Bi2O3 (≤2.0 wt.%), and samples of doped-GDC sintered at 1200–1400 °C for 2–4 h were dense bodies (98–99.5% of theoretical density). Measurements showed that the addition of Bi2O3 could reduce the sintering temperature by about 250–300 °C lower than that for undoped-GDC samples. A liquid phase-assisting mechanism was assumed as the main cause for the enhancement of the densification process. The average grain size of doped-GDC sintered samples grew with the increasing of Bi2O3 addition up to 1.0 wt.%, and then decreased indicating a poor wetting properties of the formed liquid phase. 相似文献
13.
《Ceramics International》2020,46(4):4730-4736
UO2/Mo composite pellets with enhanced thermal conductivity have been considered for the novel construction of high-safety fuel systems. UO2/Mo core-shell composite pellets with a reasonable porosity of 4–5% were fabricated by spark plasma sintering (SPS). In the SPS UO2/Mo composite, the majority of pores remained homogeneous in the UO2 matrix, while it had a dense morphology in the continuous Mo channel for the heat conduction. The sintering behaviour of the UO2/Mo composite indicated that the incorporating Mo impeded densification of UO2, and the primary densification temperature ranged from 903 K to 1270 K. To introduce 2 vol% Mo to UO2, the thermal conductivity (TC) was enhanced to 4.02 W/m·K at 1073 K. The above result represented a 23.31% improvement over the value of 3.26 W/m·K (Fink) for a pure UO2 pellet, which was approximated by the revised Hasselman-Johnson model. In particular, the coefficient of thermal expansion (CTE) was reduced to 10.0 × 10−6/K from 298 K to 1673 K, representing an 8.83% reduction from the value of 10.98 × 10−6/K (Martin) for a pure UO2 pellet. The reduction effect on the CTE was superior to that of other UO2 matrix composites fuel systems and, hence, offers an external safety aspect for reactors at the elevated temperatures close to accident conditions. These results provide a feasible method for the fabrication of a UO2/Mo core-shell pellet as an accident tolerant fuel. 相似文献
14.
《Ceramics International》2016,42(3):4221-4227
Spark plasma sintering (SPS) is a powerful technique to produce fine grain dense ferrite at low temperature. This work was undertaken to study the effect of sintering temperature on the densification, microstructures and magnetic properties of magnesium ferrite (MgFe2O4). MgFe2O4 nanoparticles were synthesized via sol–gel self-combustion method. The powders were pressed into pellets which were sintered by spark plasma sintering at 700–900 °C for 5 min under 40 MPa. A densification of 95% of the theoretical density of Mg ferrite was achieved in the spark plasma sintered (SPSed) ceramics. The density, grain size and saturation magnetization of SPSed ceramics were found to increase with an increase in sintering temperature. Infrared (IR) spectra exhibit two important vibration bands of tetrahedral and octahedral metal-oxygen sites. The investigations of microstructures and magnetic properties reveal that the unique sintering mechanism in the SPS process is responsible for the enhancement of magnetic properties of SPSed compacts. 相似文献
15.
《Ceramics International》2016,42(16):18333-18337
The effect of CuO/MnO additives on phase composition, microstructures, sintering behavior, and microwave dielectric properties of 3ZrO2-3TiO2-ZnNb2O6 (3Z-3T-ZN) ceramics prepared by conventional solid-state route were systematically investigated. CuO/MnO doped ceramics exhibited a main phase of α-PbO2-structured ZrTi2O6 and a secondary phase of rutile TiO2. SEM results showed that the grain size of MnO doped ceramics became larger with increasing amount of dopants. The presence of CuO/MnO additives effectively reduced the sintering temperature of 3Z-3T-ZN ceramics to 1220 °C. MnO doped into ceramics could enhance the Q×f values significantly. The 0.5 wt% CuO doped 3Z-3T-ZN ceramics with 0.5 wt% of MnO, sintered at 1220 °C for 4 h, was measured to show superior microwave dielectric properties, with an εr of 41.02, a Q×f value of 44,230 GHz (at 5.2 GHz), and τf value of +2.32 ppm/°C. 相似文献
16.
《Ceramics International》2015,41(8):9692-9700
Spark plasma sintering followed by hot isostatic pressing was applied for preparation of polycrystalline alumina with submicron grain size. The effect of additives known to influence both densification and grain growth of alumina, such as MgO, ZrO2 and Y2O3 on microstructure development was studied. In the reference undoped alumina the SPS resulted in some microstructure refinement in comparison to conventionally sintered materials. Relative density >99% was achieved at temperatures >1200 °C, but high temperatures led to rapid grain growth. Addition of 500 ppm of MgO, ZrO2 and Y2O3 led, under the same sintering conditions, to microstructure refinement, but inhibited densification. Doped materials with mean grain size <400 nm were prepared, but the relative density did not exceed 97.9%. Subsequent hot isostatic pressing (HIP) at 1200 and 1250 °C led to quick attainment of full density followed by rapid grain growth. The temperature of 1250 °C was required for complete densification of Y2O3 and ZrO2-doped polycrystalline alumina by HIP (relative density >99.8%), and resulted in fully dense opaque materials with mean grain size<500 nm. 相似文献
17.
《Journal of the European Ceramic Society》2005,25(7):1067-1072
Sintering behavior of three different compositions in the AlN–Al2O3 system using Y2O3 as a sintering aid was investigated. Samples with various ratios of AlN/Al2O3 were sintered in nitrogen atmosphere using a gas pressure furnace in the temperature range 1750–1950 °C. The densification of the samples was studied by shrinkage and relative density measurements. Results showed that samples containing 1 and 70 wt.% alumina were sintered to near theoretical density at 1800 °C; whereas the sample with 20 wt.% alumina never reached densities higher than 93% in the temperature range considered. It was found that the AlN/Al2O3 ratio and the sintering temperature had a great influence on the microstructure and crystalline phases present in the samples, namely, AlN, γ-AlON, 27R, and YAG. In the sample with 20 wt.% alumina, porosity formation prevented further densification. These porosities were probably due to the release of oxygen during sintering. 相似文献
18.
In this study, calcium lanthanum sulfide (CaLa2S4, CLS) ceramics with the cubic thorium phosphate structure were sintered at different temperatures by field‐assisted sintering technique (FAST). Densification behavior and grain growth kinetics were studied through densification curves and microstructural characterizations. It was determined that the densification in the 850°C‐950°C temperature range was controlled by a mixture of lattice or grain‐boundary diffusion, and grain‐boundary sliding. It was revealed that grain‐boundary diffusion was the main mechanism controlling the grain growth between 950°C and 1100°C. The infrared (IR) transmittance of the FAST‐sintered CLS ceramics was measured and observed to reach a maximum of 48.1% at 9.2 μm in ceramic sintered at 1000°C. In addition, it was observed that the hardness of the CLS ceramics first increased with increasing temperature due to densification, and then decreased due to a decrease in dislocations associated with grain growth. 相似文献
19.
Jiwei Fan Huijun Zhao Yanjun Xi Yunchao Mu Feng Tang Robert Freer 《Journal of the European Ceramic Society》2010,30(2):545-548
Varistors based on SnO2 have attracted increasing interest in recent years. However, the combined effect of CoO–MnO on SnO2 ceramics is still unclear. In this study, the non-Ohmic behaviour of the 98.95 mol%SnO2–0.5 mol%CoO–0.5 mol%MnO–0.05 mol%Nb2O5 system, the microstructures and the influence of sintering temperature were investigated. The samples were prepared by the mixed oxide route, and were sintered at temperatures in the range 1250–1450 °C. SEM observation and EDS analysis revealed that the ceramics have a two-phase microstructure comprising SnO2 primary grains and a Mn, Co rich secondary phase of small particles. The sintered density of the samples increased with the increase in sintering temperature. The maximum non-linear coefficient (α = 10) was obtained at a sintering temperature of 1350 °C. 相似文献
20.
《Ceramics International》2021,47(22):31160-31167
In this study, a novel UO2/SiC nanocomposite pellet was constructed via in-situ hydrothermal synthesis and SPS. Such method could avoid the problem of traditional mechanical mixing that could obtained the molecular level mixing during a chemical process. Using such method, SiC was dispersed uniformly in the UO2 matrix. Its thermal conductivity is significantly higher than those of UO2 pellet fabricated using hydro-thermally prepared powder and traditional UO2 pellets at both working temperature (400 °C) and near-accident temperature (1000 °C). The thermal conductivity of UO2/SiC nanocomposite pellet increased 23.7 % over traditional UO2 and 48.9 % over UO2 pellet fabricated using hydro-thermally prepared powder at 400 °C. It also increased 33.6 % over traditional UO2 and 74.8 % over UO2 pellet fabricated using hydro-thermally prepared powder at 1000 °C. These advantages are expected to maintain high thermal conductivity of fuels, enhance heat transferring efficiency of reactors, and minimize risks of pellet failure in the entire fuel life cycle. 相似文献