Synthesis of nano-crystalline Gd0.1Ce0.9O2−x for IT-SOFC by aerosol flame deposition

Nano-sized gadolinia-doped ceria (GDC) can be used as an IT-SOFC electrolyte, oxygen gas sensor or abrasives. In this study, nano-sized GDC powders with bimodal particle distribution of about 10 nm and 200 nm particle size were successfully synthesized by aerosol flame deposition (AFD). The resulting effects of sintering temperature on microstructure and electrical properties were investigated in the sintering temperature range 1100–1400 °C. The pellet had a completely dense microstructure after sintering at 1400 °C for 10 h. Raman measurement showed an increase of oxygen vacancy due to shift between reduced and oxidized states (Ce³⁺ ↔ Ce⁴⁺) with increasing sintering temperature. The formation of oxygen vacancies noticeably increased the ionic conductivity above 1300 °C.     

Microstructure development of the single-step, low-temperature sintered SrTiO3, GBBL capacitor materials

The microstructure development of the single-step, low-temperature sintered SrTiO₃, GBBL capacitor materials is studied according to the microstructure analysis, mass transfer and defect chemical reactions during sintering process. The sintering mechanism is the reactive liquid phase sintering with the participation of solid state diffusion caused by the volatilization of oxygen. Insulating grain boundaries are formed by the grain boundary segregation of Li₂O, which is caused by the donor enhanced volatilization of oxygen. Grains are semiconductorized during grain growth process which incorporates the donor dopant into SrTiO₃ lattice.     

Processing Effects on Microstructure and Superconducting Properties of Sintered YBa2Cu3O6+x

The relationships between the microstructure of sintered YBa₂Cu₃O_{6+ x} superconductors and processing variables (sintering time, sintering temperature, and oxygen partial pressure) were examined. Large-grained microstructures were obtained in 100 kPa oxygen sintering atmospheres, while fine-grained microstructures were obtained in 2 kPa oxygen. The formation of liquid phases below the peritectic decomposition temperature of YBa₂Cu₃O_{6+ x} was found to have an effect on both the microstructure (as observed by optical and transmission electron microscopy) and the transport critical current density ( J_c ). The critical current density was found to be highest for sintering below the lowest invariant point, which is a function of the oxygen partial pressure. However, over the range of conditions examined here, there does not appear to be any correlation between microstructural features, such as average grain size and aspect ratio, and the transport J_c .     

Effect of sintering atmospheres on the densification behavior of CuO ceramics

In this study, the effect of oxygen partial pressure on the densification and the resultant microstructure change of CuO were examined. When CuO green bodies were heated up to 1100 °C for sintering in air, denser CuO samples were obtained than the samples sintered in oxygen. When the samples were kept at 1100 °C for a prolonged period of time, however, the densification of the sample in oxygen was accelerated whereas that of the sample in air was hindered. This is attributed to the phase transformation from CuO to Cu₂O which accompanies oxygen release during sintering in air. On the basis of the results, dense CuO samples could be obtained by switching the sintering atmosphere from air to oxygen at the suitable stage so that oxygen release by reduction could be suppressed.     

放电等离子烧结La0.67Sr0.33MnO3-δ粉体的输运性质

用自蔓延高温合成技术合成La0.67Sr0.33MnO3-δ粉体.探讨了放电等离子烧结的烧结温度对合成粉体的显微结构和输运性质的影响.扫描电镜和密度测试结果表明:放电等离子烧结后的晶粒大小均匀,烧结体致密度高.随着烧结温度的升高,样品的电阻率明显下降.由于氧缺位的存在,金属-绝缘相转变温度随着烧结温度的升高而降低.相对于传统的固相烧结,放电等离子工艺制得的样品的致密度较高导致颗粒间的巨磁电阻有所提高.     

Effect of Oxygen Partial Pressure during Sintering on the Microstructure of Mg-PSZ with Strontia and Silica Additions

The microstructure, average grain size, and density of Mg-PSZ sintered with SrO and SiO₂ additions are found to depend on the partial pressure of oxygen in the sintering atmosphere. Over the range of 10⁻¹ to 10° atm, both the average grain size and the density increase with oxygen partial pressure, for a constant SrO/SiO₂ ratio. The partial pressure of oxygen also affects the microstructural distribution of the remnant liquid phase. At high partial pressures the liquid phase is uniformly distributed, whereas at lower oxygen pressures it is preferentially located near the surface. It is proposed that the microstructure produced by sintering is the result of a competition between liquid-phase-enhanced densification and the migration of the liquid phase to the free surface. The migration is attributed to vaporization of MgO from the liquid phase which increases with decreasing partial pressure of oxygen.     

Grain growth kinetics of 3 mol. % yttria-stabilized zirconia during flash sintering

Grain growth kinetics of dense 3 mol. % yttria-stabilized zirconia (3YSZ) ceramics during both DC flash sintering and conventional annealing were investigated using the grain size as a marker of microstructure evolution. The results indicated faster grain growth under greater current density. In contrast to conventionally annealed specimen, the grain boundary mobility was enhanced by almost two orders of magnitude with the applied electric current, revealing that joule heating alone was not sufficient to account for the experimental results. Instead, activation energy for grain growth decreased significantly due to electro-sintering. Systematic characterization of graded microstructure further indicated that local oxygen vacancies and specimen temperature were responsible for a grain size transition. Based on electrochemical reaction involved in flash sintering, grain size reduction at the cathode was proposed to be attributed to the local rearrangement of lattice cations and generated oxygen ions.     

Microstructure and phase development in NiMn2O4 spinel ceramics during isothermal sintering

Studies of the sintering behaviour of NiMn₂O₄ semiconducting ceramics at 1100°C under oxygen atmosphere show the advantage of a powder prepared by thermal decomposition of oxalate mixed crystals NiMn₂ (C₂O₄)₃. 6H₂O at 450°C for the formation of dense ceramics with homogeneous microstructure. The microstructure of the semiconductor ceramics was established by image analysis. SEM and EDX indicate a phase separation related to partial oxygen loss as observed by thermogravimetry and redox analytic measurements. Reoxidation to a single-phase homogeneous microstructure by annealing at 800°C is possible only in porous samples. NiMn₂O₄ is not stable in oxygen at 1100°C. The spinel decomposes into NiO and a Mn-rich spinel matrix Ni_x^IIMn_1−x^IIMn₂^IIIO₄. For producing a reproducible semiconducting ceramics it is necessary to stimulate sintering by separation of NiO. A one-phase spinel is obtained by reoxidation of long duration.     

先驱体制备SiC纤维的发展历程与研究进展

碳化硅(SiC)纤维具有高强度、高模量、耐高温、抗蠕变、抗氧化等优异性能,是增强耐高温陶瓷基复合材料的关键材料。介绍了先驱体法制备3代SiC纤维的发展历程:从第1代高氧碳含量SiC纤维发展到第2代低氧高碳含量SiC纤维,再到第3代近化学计量比SiC纤维,SiC纤维的微结构从非晶到微晶显著变化,纤维的耐热性能也显著提高。重点比较了第3代近化学计量比SiC纤维(Hi-Nicalon Type S纤维、Tyranno SA和Sylramic纤维等)的性质,结果表明:SiC纤维的热稳定性由近化学计量比SiC微晶的致密度和微结构决定,Sylramic和Tyranno SA纤维的组成和微结构可通过控制Si-C-O纤维的碳热还原反应来实现,烧结助剂的采用及陶瓷烧结工艺的有效应用可提高纤维的致密度。Hi-Nicalon Type S纤维的组成和微结构取决于聚碳硅烷分解过程中特定的气氛和温度。简介了SiC纤维的研究进展并讨论了其发展趋势。     

The origin of microstructural inhomogeneity in vacuum-sintered ZrO2-doped Lu2O3 transparent ceramics

Highly transparent Lu₂O₃ ceramics were prepared by using co-precipitation combined with vacuum sintering method with ZrO₂ as sintering aid. The effect of ZrO₂ on densification, transmittance, and microstructure evolution of the Lu₂O₃ ceramics was carefully studied. It was found that the addition of ZrO₂ was very effective in improving densification of Lu₂O₃. The highest transmittance (at 600 nm) of the 3 at.% ZrO₂ doped Lu₂O₃ ceramic (1.6-mm thickness) sintered at 1800°C reached 80.1%. Microstructural inhomogeneity was found after vacuum sintering with larger grain sizes at the central. The microstructural inhomogeneity mainly occurred in the final stage of sintering. Doping ZrO₂ mitigated microstructural inhomogeneity and decreased markedly the grain size of Lu₂O₃ ceramics. Raman measurements indicated that the disordered structure was formed due to oxygen vacancy, and the oxygen vacancy concentration at the central was higher than at the peripheral. The oxygen vacancy concentration gradient is the dominant factor governing nonuniform microstructure evolution during vacuum sintering. Furthermore, a uniform microstructure was obtained by the inhibition of oxygen vacancy formation by the Lu₂O₃/ZrO₂ double powder bed.     

Role of oxygen on the phase stability and microstructure evolution of CaCu3Ti4O12 ceramics

Phase stability and microstructure evolution of polycrystalline CaCu₃Ti₄O₁₂ (CCTO) ceramics were studied by controlling the partial pressure of oxygen (from a poor to an oxygen rich atmosphere) during the sintering process at high temperatures. The samples were analyzed by X-ray powder diffraction, scanning electron microscopy and X-ray energy dispersive spectroscopy. Our results show that the oxygen partial pressure during the sintering process is an important parameter that controls the phase stability, non-stoichiometry, and decomposition process of the CCTO phase as well as the densification and grain growth mechanisms on these polycrystalline ceramics. These results provided us further insight into the important role of copper reduction and copper/oxygen diffusion on the crystalline structure and morphological characteristics of polycrystalline CCTO ceramics.     

Sintering of Nanosized MnZn Ferrite Powders

The sintering and microstructural evolution of nanosized MnZn ferrite powders prepared by a hydrothermal method were investigated. The microstructure of sintered ferrite compacts depends strongly on the strength of the agglomerates formed during the compacting of nanosized ferrite powders. It was found that at 700°C the theoretical density of sintered compacts can almost be reached, while above 900°C an increase of porosity was identified. The formation of extra porosity at higher sintering temperatures is caused mainly by the oxygen release which accompanies the dissolution of relatively large grains of residual alpha-Fe₂O₃ in the spinel lattice.     

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.     

烧结参数对镍粉毛细芯性能的影响

毛细芯（多孔介质）是环路热管的核心部件,金属粉末烧结是目前制备毛细芯的常用方法,研究烧结参数对毛细芯性能的影响有利于提高环路热管的性能。以镍粉为原料,利用烧结的方法制备了具有不同烧结参数的毛细芯,实验研究了烧结参数对毛细芯微观结构和性能参数的影响。实验表明,烧结参数对毛细芯的微观结构和性能参数具有明显的影响。随着烧结温度的升高和保温时间的延长,毛细芯的孔隙率、平均孔径和渗透率呈下降趋势;毛细芯的抽吸质量和抽吸速率与毛细芯的渗透率呈正相关;在750~800℃范围内改变烧结温度和在40~50 min范围内改变保温时间,毛细芯结构和性能参数变化明显。     

三氧化钨陶瓷的压敏特性和机理

采用传统电子陶瓷烧结工艺,制备了无掺杂物的三氧化钨(WO3)陶瓷。分析了陶瓷样品经淬火和不同气氛下处理后的微结构和压敏电学特性。研究表明:陶瓷冷却过程中在氧吸附的作用下,WO3陶瓷晶粒表面呈现氧元素富集。分析认为,晶粒表面吸附的氧与晶粒内的电子作用,在晶粒表面形成界面态,并进一步在晶界形成Schottky势垒,这可能是WO3陶瓷压敏行为的起源。根据实验结果,提出了一种修正的晶界Schottky势垒模型,解释了WO3陶瓷的压敏行为。     

Tailoring the microstructure by a proper electric current control in flash sintering: The case of barium titanate

Flash sintering is arousing growing interest because high-density ceramics can be obtained at lower temperatures and shorter dwell times than conventional sintering. However, not only temperature and dwell times should be controlled during flash sintering but also parameters such as the electric field and electric current should be considered. Controlling all the parameters during the processing allows comprehensive control of the microstructure and, consequently, functional properties can be improved. In this work, it is evidenced that an exhaustive control of the flash electric current is a crucial factor for tailoring the microstructure of BaTiO₃ ceramics. The results reveal that the most suitable way to control the sintering process is by using non-linear current profiles because better densification and improved grain growth is achieved. Although the results focus on BaTiO₃, this work offers a new pathway to tailor the microstructure of flash sintered ceramics, which may be extended to other materials.     

Dependence of Electrical Conductivity of ZnO on Degree of Sintering

The electrical resistivity of ZnO doped with Al₂O₃ was measured in air and under reduced pressure (∼0.5 mm Hg) in the range from 30° to 680°C as a function of the degree of sintering. The data obtained were explained in terms of the effects of the microstructure of the sintered body and the chemisorbed oxygen. There are two mechanisms of electrical conduction involved, only one of which is affected by the microstructure.     

Microstructural Evolution of Transparent PLZT Ceramics Sintered in Air and Oxygen Atmospheres

Transparent lanthanum-doped lead zirconate titanate (PLZT) ceramics were fabricated by air-pressure sintering. When the PLZT (9/65/35) specimens were sintered in air, the microstructure was not uniform throughout the body; the outer region near the surface was completely dense, while the inner region of the body was porous. The thickness of the outer dense layer increased parabolically with sintering time. When the specimen was sintered in air at 1200°C for 8 h, the thickness of the dense layer was ∼0.25 mm. Therefore, when the specimen had a thickness of <0.5 mm, it was dense and transparent. This difference in microstructure was attributed to the formation of lattice vacancies as a result of PbO evaporation from the surface. The sintering atmosphere also was important in determining the thickness of the dense layer. The thickness was strongly dependent on the oxygen partial pressure of the atmosphere. The oxygen-gas trapped in pores was deemed to migrate easily through the lattice vacancies. By sintering in an oxygen-gas atmosphere at 1200°C for 8 h, a transparent PLZT with thickness up to 2 mm was fabricated.     

Role of Zinc Volatilization on the Microstructure Development of Manganese Zinc Ferrites

The role of Zn atmosphere on the microstructure development of Mn-Zn ferrite prepared by alcholic dehydration was investigated. Discontinuous grain growth and intragranular porosity were observed throughout the specimen when a low oxygen partial pressure was employed during sintering. The microstructure of Mn-Zn ferrite sintered in air resembled a mixture between Mn ferrite (uniform grain size) and Zn ferrite (exaggerated grain growth). Exaggerated grain growth was also observed on the surface of Mn ferrite sintered in the presence of a Zn-rich atmosphere. The presence of Zn vapor phase enhanced the grain growth kinetics. The overall results indicate that a strict control of atmosphere is required in order to achieve a uniform pore-free microstructure.     

Varistor Performance of ZnO-Based Ceramics Related to Their Densification and Structural Development

Electrical potential barriers are often observed in ZnO-based ceramics. Earlier studies on ZnO photoconduction have shown that the narrow regions, where the sintered grains have grown together, control the resistance of the entire sample. In those regions, the surface/volume ratio is sufficiently high for the acceptor concentration (which occurs because of adsorbed oxygen) to exceed the donor concentration inside the ZnO grains. More recent works have shown that Schottky barriers result from interface states because of the chemisorbed oxygen ion at the ZnO-ceramic grain boundaries. The work reported in this paper involves the relationship between the densification of the microstructure and the varistor performance of ZnO ceramics. The emphasis of densification percentage as an indicator of the degree of sintering shows the desirability of continuity across ZnO grain boundaries, without the presence of voids or films of second phases, in optimizing varistor behavior. The effect of oxygen partial pressure on the development of varistor microstructure and electrical properties, as well the kinetics of grain growth, during the sintering process have been determined and are discussed.