Effects of sintering conditions on the microstructure and mechanical properties of SiC prepared using powders recovered from kerf loss sludge

The effects of sintering conditions on the microstructure and mechanical properties of the sintered SiC prepared using the SiC powder recovered from the kerf loss sludge were investigated. The recovered SiC powders were consolidated by spark plasma sintering (SPS) and conventional sintering methods. The effects of sintering temperature, time and methods (SPS and conventional sintering) on the phase, grain size and density of SiC were systematically studied. The Vickers hardness of spark plasma-sintered (SPSed) samples was higher than that of conventional sintered samples due to small grain size. When holding time was increased from 10 to 30 min, the grain size and relative density of SPSed samples were also increased, which lead to the almost constant Vickers hardness by competing effects of grain size and relative density. When holding time was over 30 min, no appreciable change of the relative density and grain size were observed, which can lead to similar values of Vickers hardness. SPS process can be used to make SiC with high density and hardness at relatively low temperature compared with the conventional sintering process.     

Synthesis of High-Density Pellets of Uranium Dioxide by Spark Plasma Sintering in Dies of Different Types

A procedure for electric pulse consolidation of commercial UO₂ powders of various ceramic grades into high-density fuel pellets using dies of various types was studied. The influence of the parameters of spark plasma sintering (SPS) on the chemical composition of the initial powders and quality of ready UO₂ pellets was examined. The main advantages and drawbacks of the SPS process for consolidation of UO₂ powder in standard graphite dies were revealed. A novel alternative procedure for SPS of UO₂-based fuel pellets in Mo-based dies was suggested. High quality of the pellets (density 97.5–98.4% of theoretical, no C impurity, mean grain size no more than 3 μm) and mild sintering conditions (1100°С, 141.5 MPa, 25 min) make the process highly promising. The results obtained are of interest for the development and possible implementation of an industrial SPS process for production of ceramic nuclear fuel.     

放电等离子快速烧结纳米3Y-TZP材

本文采用放电等离子烧结技术（ＳＰＳ）快速烧结结纳米３Ｙ－ＴＺＰ材料,利用ＳＰＳ技术快速烧结,可制备出完整、致密的３Ｙ－ＴＺＰ材料,在烧结温度为１３００℃,保温３ｍｉｎ条件下,相对密度达９８．２％,晶粒仅１００￣１３０ｎｍ,研究发现,材料的密度随烧结温度的变化趋势与一般快速烧结有明显区别;材料的晶粒随烧结温度的提高而长大,但长大幅度小于其他一些烧结方法所得的３Ｙ－ＴＺＰ材料,本研究对这些现象进行了理     

Spark-plasma-sintering of fine BaTiO3 powder prepared by a sol-crystal method

Spark-Plasma-Sintering (SPS) has been applied to fine BaTiO₃ powder (several nanometers) prepared by a sol-crystal method. The starting powder was densified to more than 95% of the theoretical X-ray density, and the obtained SPS pellets had an average grain size of less than 1 m, owning to the relatively low sintering temperature (900–1000°C) and short sintering period of ca. 3 min. The room temperature permittivity at 1 kHz for SPS samples sintered at 1000°C is ca. 10,000. This high room temperature permittivity value is attributed to electrical inhomogeneities within pellets; a resistive surface layer covers the inner pellet core that consists of oxygen-deficient BaTiO₃. The reduced pellet core is characteristic of SPS pellets formed from powders that contain small amounts of residual organic matter.     

Spark plasma sintering on nanometer scale WC–Co powder

Nanometer scale WC–11Co powder was sintered by spark plasma sintering (SPS) process in order to improve the properties of the cemented carbides. Properties such as density and hardness were measured. The microstructures of sintered WC–11Co cemented carbides were observed. The grain size of WC in alloys was also obtained. The results showed that spark plasma sintering could lower the sintering temperature, increased the density and circumscribed the growth of grain size of WC. Besides, the hardness of the sintered cemented alloys that was dependent on the grain size and densification could also be improved by SPS. SPS was an effective method to get WC–11Co cemented carbides with fine grain size and good properties.     

Consolidation of SiC/BN composite through MA-SPS method

Hexagonal-BN has been selected as a second phase for SiC/BN composite to improve SiC’s machinability and thermal shock resistance. In this research, nano-metric SiC/BN was prepared through mechanical alloying (MA) from Si + C + BN powder and then consolidated by SPS without any sintering aids. XRD results after MA revealed the absence of sharp peaks corresponding to SiC and BN. The density and the intensity of the SiC and BN peaks on XRD increased with temperature during SPS. The final density of the composite reached approximately 90–99% with 50/50 of SiC/BN to 100/0. During the consolidation process, crystallization, phase separation, and ordering were observed simultaneously. This phenomenon could accelerate the mass transfer for the consolidation and the preparation of bulk SiC/BN composite without any sintering aids. In a 50/50 SiC/BN ratio, the Vickers hardness of the nano-structured reference sample prepared by the conventional method with sintering aids could not be measured due to high porosity. However, the well-consolidated sample prepared in our research showed a hardness of approximately 3 GPa.     

Sintering of Al–Zr based oxide ceramics using thermal plasma

A novel and low cost extended arc thermal plasma heating (EATPH) reactor [Roul et al., JMSP 6 (1) (1998)] has been used to sinter Al–Zr oxide ceramics within a few minutes in Ar atmosphere instead of a few tens of hours by conventional furnace heating. Critical experimental parameters such as plasma power (kW), plasmagen gas flow rate, sintering time and electrode spacing distance are optimized to achieve high density sintered materials. It is noted that sintering time is approximately reciprocal to plasma power. Higher plasma power with less sintering time can generate high-density homogeneous material without significant grain growth. XRD and SEM studies were carried out to characterize and evaluate the sintered materials. Surface morphology revealed uniform particle size distribution in long-range order, with no runway grain growth during this EATPH sintering process. This simple EATPH method provides an alternative and quick technique for tailoring high temperature ceramic materials over conventional sintering method due to fast heat and mass transfer kinetics inside thermal plasma reactor.     

Structure and strength of aluminum with sub-micrometer/micrometer grain size prepared by spark plasma sintering

A spark plasma sintering (SPS) technique has been applied to prepare fully dense Al samples from Al powder. By applying a sintering temperature of 600 °C and a loading pressure of 50 MPa, fully recrystallized samples of nearly 100% density with average grain sizes of 5.2 μm, 1.3 μm and 0.8 μm have been successfully prepared using a sintering time of less than 30 min and without the need for a nitrogen atmosphere. A similarity between the grain size and powder particle size is found, which suggests a potential application of the SPS technique to prepare samples with a variety of grain sizes by tailoring the initial powder particle size. The SPS samples show higher strength than Al samples with an identical grain size prepared using thermo-mechanical processing, and a better strength–ductility combination, with the 1.3 μm grain size sample showing a yield strength (σ_0.2%) of 140 MPa and a uniform elongation of more than 10%. This higher strength is related to the presence of oxide particles in the grain boundaries of the samples. It is concluded that SPS is an excellent technique for the production of very fine grained Al materials with high strength, by combining both grain boundary and oxide dispersion strengthening.     

制备工艺对n型Bi_2Te_3基材料热电性能和抗压强度的影响

以商用区熔(ZM)n型Bi2Te3基材料为原料,采用简单研磨结合放电等离子烧结技术(ZM+SPS)和熔体旋甩(MS)结合放电等离子烧结技术(MS+SPS)制备了n型Bi2Te3基块体热电材料.对三种不同工艺制备出样品的微结构、热电性能和力学性能进行了研究.FESEM微结构表征结果表明:区熔样品的晶粒粗大,有较强的取向性;经SPS烧结后,晶粒细化,取向性大为降低;而区熔样品经MS+SPS后,晶粒得到进一步细化,且没有明显的取向性.对三组样品进行的热电性能和抗压强度测试,结果表明:区熔原料最大ZT值为0.72(430K),抗压强度仅为40MPa;经SPS后,样品的最大ZT值为0.68(440K),抗压强度为110MPa,相比区熔样品提高了175%;MS+SPS样品的最大ZT值为0.96(320K),其室温ZT值相比区熔样品提高了64%,抗压强度相比区熔样品提高了400%,达到200MPa.     

Effect of submicron grains on ionic conductivity of nanocrystalline doped ceria

Doped ceria has been considered for high oxygen ion conductivity for solid oxide fuel cells. In the present study, 20 mole% samarium doped nano ceria powder was prepared by wet chemical synthesis and sintered at different temperatures to retain submicron grains (> 92-96% density). ionic conductivity of the sintered pellets was measured using impedance spectroscopy as a function of temperature (200-800 degrees C). The total maximum conductivity was 1.0 x 10(-2)S.cm(-1) (at 600 degrees C) for samples sintered at 1200 degrees C. The activation energy at higher test temperature decreases with the decrease in the sintering temperature (by 25%). The grain boundary, grain interior conductivity and activation energy of the electrolyte were correlated to the resulting microstructure. It has been demonstrated that use of doped nano ceria powder as precursor not only reduced the sintering temperature but also provided segregation free grain boundary for engineering higher conductivity dense electrolytes.     

Dog-bone copper specimens prepared by one-step spark plasma sintering

Copper dog-bone specimens are prepared by one-step spark plasma sintering (SPS). For the same SPS cycle, the influence of the nature of the die (graphite or WC–Co) on the microstructure, microhardness, and tensile strength is investigated. All samples exhibit a high Vickers microhardness and high ultimate tensile strength. A numerical electro-thermal model is developed, based on experimental data inputs such as simultaneous temperature and electrical measurements at several key locations in the SPS stack, to evaluate the temperature and current distributions for both dies. Microstructural characterizations show that samples prepared using the WC–Co die exhibit a larger grain size, pointing out that it reached a higher temperature during the SPS cycle. This is confirmed by numerical simulations demonstrating that with the WC–Co die, the experimental sample temperature at the beginning of the dwell is higher than the experimental control temperature measured at the outer surface of the die. This difference is mostly ascribed to a high vertical thermal contact resistance and a higher current density flowing through the WC–Co punch/die interface. Indeed, simulations show that current density is maximal just outside the copper sample when using the WC–Co die, whereas by contrast, with the graphite die, current density tends to flow through the copper sample. These results are guidelines for the direct, one-step, preparation of complex-shaped samples by SPS which avoids waste and minimizes machining.     

Fabrication of silicon nitride nanoceramics—Powder preparation and sintering: A review

Fine-grained silicon nitride ceramics were investigated mainly for their high-strain-rate plasticity. The preparation and densification of fine silicon nitride powder were reviewed. Commercial sub-micrometer powder was used as raw powder in the “as-received” state and then used after being ground and undergoing classification operation. Chemical vapor deposition and plasma processes were used for fabricating nanopowder because a further reduction in grain size caused by grinding had limitations. More recently, nanopowder has also been obtained by high-energy milling. This process in principle is the same as conventional planetary milling. For densification, primarily hot pressing was performed, although a similar process known as spark plasma sintering (SPS) has also recently been used. One of the advantages of SPS is its high heating rate. The high heating rate is advantageous because it reduces sintering time, achieving densification without grain growth. We prepared silicon nitride nanopowder by high-energy milling and then obtained nanoceramics by densifying the nanopowder by SPS.     

Fabrication of silicon nitride nanoceramics—Powder preparation and sintering: A review

Fine-grained silicon nitride ceramics were investigated mainly for their high-strain-rate plasticity. The preparation and densification of fine silicon nitride powder were reviewed. Commercial sub-micrometer powder was used as raw powder in the “as-received” state and then used after being ground and undergoing classification operation. Chemical vapor deposition and plasma processes were used for fabricating nanopowder because a further reduction in grain size caused by grinding had limitations. More recently, nanopowder has also been obtained by high-energy milling. This process in principle is the same as conventional planetary milling. For densification, primarily hot pressing was performed, although a similar process known as spark plasma sintering (SPS) has also recently been used. One of the advantages of SPS is its high heating rate. The high heating rate is advantageous because it reduces sintering time, achieving densification without grain growth. We prepared silicon nitride nanopowder by high-energy milling and then obtained nanoceramics by densifying the nanopowder by SPS.     

Recent progress on RE2O3-Mo/W emission materials

RE2O3-Mo/W cathodes were prepared by powder metallurgy method. La2O3-Y2O3-Mo cermet cathodes prepared by traditional sintering method and spark plasma sintering (SPS) exhibit different secondary emission properties. The La2O3-Y2O3-Mo cermet cathode prepared by SPS method has smaller grain size and exhibits better secondary emission performance. Monte carlo calculation results indicate that the secondary electron emission way of the cathode correlates with the grain size. Decreasing the grain size can decrease the positive charging effect of RE2O3 and thus is favorable for the escaping of secondary electrons to vacuum. The Scandia doped tungsten matrix dispenser cathode with a sub-micrometer microstructure of matrix with uniformly distributed nanometer-particles of Scandia has good thermionic emission property. Over 100 A/cm2 full space charge limited current density can be obtained at 950Cb. The cathode surface is covered by a Ba-Sc-O active surface layer with nano-particles distributing mainly on growth steps of W grains, leads to the conspicuous emission property of the cathode.     

A mechanistic approach of the sintering of nuclear fuel ceramics

The CEA and the COGEMA, as part of their effort to model the different stages of the MOX fuel fabrication process, have specifically worked, on the sintering stage. A physical mechanistic model of MOX fuel sintering is proposed, as well as the numerical schemes that will lead to the achievement of the corresponding software tool. The model takes into account surface diffusion, grain boundary diffusion, volume diffusion, exchanges between solid and gas, as well as the mechanical strains of grains. The scale at which concentrations and strains are taken into account is smaller than grain size (0.1 μm for green pellets and 10 μm for sintered ones). The numerical resolution schemes of this problem have been conceived, and are also presented. They mainly consist of relaxation procedures to uncouple partial derivative mass conservation equations, flux balances along interfaces, non-linear potential equality conditions at interfaces, and Navier–Lamé equations over each grain. A code (SALAMMBO) is under construction, based on the latter; once validated by experimental and parametric tests, this code will describe Pu distribution, grain and pore size distribution, and local density of the pellets as functions of the sintering conditions, thus enabling further developments of the fabrication process such as the obtention of advanced microstructures of improved MOX fuels.     

Dielectric properties of spark-plasma-sintered BaTiO3

Spark-plasma-sintering (SPS) has been applied to BaTiO₃ to prepare dense ceramics consisting of submicrometre-sized powder. Relatively dense (typically 97% of the theoretical X-ray density) pellets with an average grain size remaining similar to that of the starting powder, approximately 0.6 m, were obtained by the SPS process. Fixed frequency (1 kHz) measurements show the room temperature permittivity of SPS ceramics to be relatively high, approximately 3500, and at least double the value of conventionally sintered ceramics, approximately 1500. Alternating current (a.c.) impedance spectroscopy measurements show that SPS is an effective process to reduce the influence of intergranular (grain boundary) effects on the permittivity and direct current (d.c.) resistance characteristics of BaTiO₃ ceramics substantially.     

放电等离子烧结制备高导热SiC_P/Al电子封装材料

为了满足电子封装材料越来越高的性能要求,采用放电等离子烧结(SPS)工艺制备了SiCP/Al复合材料。研究了烧结温度和保温时间等工艺条件对SiCP/Al复合材料组织形貌和性能的影响。结果表明:采用SPS烧结,温度为700℃、保温时间为5 min时,所制备的70 vol%SiCP/Al复合材料热导率达到195.5 W(m.K)-1,与传统15%W-Cu合金相当,是Kovar合金的10倍,但密度小,仅为3.0 g.cm-3;其热膨胀系数为6.8×10-6K-1,与基板材料热膨胀系数接近;抗弯强度为410 MPa,抗拉强度为190 MPa,达到了电子封装材料对热学性能和力学性能的要求。     

Dielectric Ba(Ti1?xSnx)O3 (x?=?0.13) ceramics, sintered by spark plasma and conventional methods

A two-step sintering approach composed of spark-plasma-sintering (SPS) technique at 1000 °C for 1 min and under a uniaxial pressure of 63 MPa followed by conventional sintering at 1400 °C for 3 h is proposed for synthesis of dense Ba(Ti_0.87Sn_0.13)O₃ ceramics. Starting powders had grain size of about 90 nm and were obtained by co-precipitation. The SPS pellets consist of submicron (300–500 nm) grains. X-ray diffraction analysis of as-prepared Ba(Ti_0.87Sn_0.13)O₃ ceramic shows the occurrence of cubic and tetragonal phase coexistence for the pellets obtained after SPS processing and the presence of only tetragonal phase in the samples after the second (conventional) sintering. Grain uniformity in the final product is high, with average size of ~2 μm. The apparent densities of the sintered pellets at temperature of 1400 °C were ~92% of the theoretical value of Ba(Ti_0.87Sn_0.13)O₃. The ceramics exhibit a high relative dielectric constant of 6,550 and a dielectric loss (tan δ) = 0.078 at Curie temperature of 63 °C and 10 Hz.     

二氧化铀陶瓷燃料块工艺研究进展

简述了铀及铀氧化物、二氧化铀陶瓷燃料块制备工艺的发展和应用.重点阐述了超化学计量二氧化铀芯块在微氧化气氛下低温活化烧结的最新研究进展.从理论上,提出了扩散烧结和强化烧结是二氧化铀陶瓷燃料块低温烧结的过程机理;从工艺上,提出了工艺稳定性和提高晶粒尺寸是二氧化铀陶瓷燃料块低温烧结的热点课题.展望了二氧化铀陶瓷燃料块微氧化低温烧结的工业应用前景.     

Comparisons of grain size-density trajectory during spark plasma sintering and hot-pressing of zirconia

Spark plasma sintering (SPS) and hot-pressing (HP) of a granulated stabilized zirconia powder have been investigated for a fixed macroscopic compaction pressure of 100 MPa and a fixed heating rate (25 °C/min for HP, 50 °C/min for SPS). The “relative density/grain size” trajectories have been established for both sintering methods.HP is shown to be similar to SPS for the manufacturing of polycrystalline TZ3Y materials with a final grain size well below the micrometer. Independently of the sintering technology employed, it is interesting to note that three kinds of microstructures are obtained depending on the experimental parameters: porous materials (opened porosity, relative density between 61 and 90%) with a nanometer grain size (around 75-80 nm), dense materials (closed porosity, relative density between 90 and 98%) with a nanometer grain size (around 75-80 nm), fully dense material with a submicron grain size (around 160 nm using SPS and around 105 nm using HP).