Theoretical analysis of experimental densification kinetics in final sintering stage of nano-sized zirconia

The experimental densification kinetics of 7.8 mol% Y₂O₃-stabilized zirconia was analyzed theoretically during isothermal sintering in the final stage. By taking concurrent grain growth into account, a possible value of the grain-size exponent n was examined. The Coble’s corner-pore model recognized widely was found not to be applicable for explaining the densification kinetics. The corner-pore model of n = 4 shows a significant divergence in the kinetics at different temperatures. Microstructural observation shows that most pores are not located at grain corners and have a size comparable to the surrounding grains. The observed pore structure is similar to the diffusive model where single pore is surrounded by dense body. The diffusive model combined with theoretical sintering stress predicts n = 1 or n = 2, which shows a good consistence to the measured densification kinetics. During sintering of nano-sized powder, it is found that the densification kinetics can be explained distinctively by the diffusive single-pore model.     

Predicting powder densification during sintering

The milestone of sintering studies is to predict the densification resulting from the coalescence of powder particles under different thermal histories. Although sintering is a common process to produce devices with dense and/or porous materials, predicting the outcome from a given heating procedure is still a challenge. Hence, manufacturing sintered products is currently guided by previous work at very specific experimental settings, full material characterization or trial and error approaches. Here we develop and validate a simple model to overcome those issues towards efficient thermal cycles design by predicting densification during sintering. Additionally, this model may be helpful in diverse areas beyond the scope of sintering, such as in reaction systems, and for modeling other Arrhenius-like phenomena.     

Influence of TiO2 on the densification behaviour of Yb2O3

The effect of temperature and heating rate on the densification of ytterbia (Yb₂O₃), with and without titania (TiO₂) doping was investigated. It is shown that up to a certain doping level, titania doping enhances the densification behaviour of ytterbia. The effect of titania doping on crystal structure confirms that titania is substitutionally incorporated in ytterbia up to the solubility limit, which corresponds well with the densification results. The increased densification rate of titania-doped ytterbia is attributed to the formation of cation vacancy and lattice distortion. Using constant heating rate experiments, the activation energy for densification has been calculated and it is shown that in the intermediate density range (60 % to 85 %), the activation energy is independent of the density. Titania doping increases the activation energy for densification.     

Microstructural evolution and densification behavior of porous kaolin-based mullite ceramic added with MoO3

Microstructural evolution and densification behavior of porous kaolin-based mullite ceramic added with MoO₃ were investigated. The results indicated that MoO₃ addition not only lowered the secondary mullitization temperature to below 950?°C, but also facilitated effectively the anisotropic growth of mullite grains. Fine mullite whiskers grew and interlocked with one another in the pre-existing pore regions, in-situ forming a stiff 3D skeleton structure of mullite whiskers, which arrested further densification of the sample. On the other hand, due to the great capillary attraction of small pores, the liquid phase tended to spread over small grains, which favored the growth from small mullite grains into whiskers at the expense of the liquid phase. Consequently, competitive mechanisms of sintering and crystal growth of mullite functioned, which further limited the sample densification. As a result, the total linear shrinkage of the sample added with MoO₃ after firing at 1400?°C was only ??2.75%, and its porosity was retained at as high as 67%.     

Densification and grain growth kinetics of Ce0.9Gd0.1O1.95 in tape cast layers: The influence of porosity

The sintering behavior of Ce_0.9Gd_0.1O_1.95 (CGO) tape cast layers with different porosity was investigated by an extensive characterization of densification, microstructural evolution, and applying the constitutive laws of sintering. The densification of CGO tapes associates with grain coarsening process at the initial sintering stage at T < 1150 °C, which is mainly influenced by small pores and intrinsic characteristics of the starting powders. At the intermediate sintering stage, densification is remarkably influenced by large porosity. Moreover, the sintering constitutive laws indicate that increasing the initial porosity from 0.38 to 0.60, the densification at the late stage is thermally activated with typical activation energy values increasing from 367 to 578 kJ mol⁻¹. Similar effect of the porosity is observed for the thermally activated phenomena leading to grain growth in the CGO tapes. The analysis of sintering mechanisms reveals that the grain growth behavior at different porosity can be described using an identical master curve.     

Effects of sintering atmosphere on the densification and microstructure of yttrium aluminum garnet fibers prepared by sol-gel process

Yttrium aluminum garnet (YAG) fibers were prepared by a sol-gel method, and then sintered in air or nitrogen atmosphere, respectively. The effects of sintering atmosphere on the densification process and microstructure of YAG fibers were investigated. No obvious difference can be found in the fibers sintered below 800 °C. At 1100 °C, the grain size of YAG fibers sintered in nitrogen is much smaller than in air. This difference is much clearer at the higher temperature of 1200 °C. The fine grains are explained by the existence of residual carbon in YAG fibers, which can be trapped at the grain boundaries to hinder the movement of grain boundary. Meanwhile, the densification degree of fibers sintered in nitrogen is higher than in air at 1200 °C, due to the smaller grain size and higher oxygen vacancy concentration generated in the nitrogen atmosphere, which leads to a higher fiber densification rate.     

Effect of niobium doping on the densification and grain growth in alumina

The effect of niobium doping on the densification and grain growth of nano-sized α-Al₂O₃ powders during sintering has been investigated. The dopant concentration added ranged from 0.1 to 0.5 mol%. It was observed that addition of niobium oxide could improve the densification of the pure alumina with a lower sintering temperature, a shorter sintering time. The effect is strengthened by increasing the amount of dopant. It also demonstrated that niobium dopant significantly promotes the grain growth of alumina during sintering and the grain size of alumina increases with increasing the amount of dopant in the added range.     

Study on AlON phase evolution and densification behavior as a function of particle size and doping amount

Three groups of AlON powders with D₅₀ = 0.5, 1.1 and 2.7 μm (referred as P0.5, P1.1 and P2.7) were prepared to study the pressureless sintering behavior of AlON. These powders were doped with 0−0.75 wt.% Y₂O₃ to investigate the combined effect of powder size and doping amount on the phase transformation, microstructure evolution and densification process during heating. The addition of high additive amount to P0.5 and P1.1 powders resulted in massive transformation into α-Al₂O₃ and AlN, which resulted in agglomeration of α-Al₂O₃ and isolation of AlN. Although low amount of additive to all three powders can effectively inhibit agglomeration of α-Al₂O₃ and homogenization of particle size before sintering, its sintering ability is insufficient. 0.05, 0.10 and 0.50 wt.% Y₂O₃ is the optimal addition to P0.5, P1.1 and P2.7 powders, respectively, to obtain high density, i.e. the larger of AlON powder size, the more Y₂O₃ is needed.     

炭质材料对连铸保护渣烧结性能的影响

借鉴耐火材料烧结性能的研究方法 ,通过研究连铸保护渣的炭质材料种类 (中超炭黑、半补强炭黑、鳞片石墨、土状石墨和焦炭 )及含量对烧结性能的影响 ,提出了评价保护渣烧结特性的方法和影响渣圈形成的原因。结果发现 :致密化起始温度Tq和致密化速率U可作为衡量渣圈生成的主要指标 ,U高或Tq低的保护渣的烧结倾向大 ,在使用中易结渣圈 ;在保护渣中加入炭质材料 ,可以降低保护渣的U ,从而抑制保护渣的烧结 ;在试验所用的炭质材料中 ,炭黑降U的效果优于石墨 ,其中中超炭黑的效果最好 ,其次为半补强炭黑和鳞片石墨。     

炭质吸附剂孔径分布与焦化废水有机组分分离的相关性

选取4种孔隙结构不同的炭质吸附剂木质（A₁）、椰壳（A₂）、煤质（A₃）和焦炭（H）吸附焦化废水中的总有机碳（TOC）成分,考察吸附性能、分子量大小等因素对吸附效果的影响,同时利用傅里叶红外光谱、比表面积及介孔/微孔分析仪对吸附剂进行表征,探究吸附剂表面化学性质和孔径分布对焦化废水吸附差异相关性。结果表明：4种吸附剂表面性质相近,孔隙结构不同是其吸附性能差异的主要因素。比表面积：A₁（1723.59m²/g） > H（1716.19m²/g） > A₂（911.55m²/g） > A₃（505.23m²/g）,平均孔径：A₁（5.14nm） > H（5.02nm） > A₃（3.81nm） > A₂（3.45nm）。Redlich-Peterson吸附等温线方程能更好地拟合吸附数据。分子量分布、UV₂₅₄、SUVA和EEMs说明微孔面积较大的A₁和A₂优先吸附低分子量（< 1000）有机物,A₃和H能够回收高分子量（1000～0.45μm）有机物,降低废水芳香构造化程度。焦化废水TOC中94.29%的有机物分子量小于10000,微孔（< 2nm）和较小中孔（2～10nm）更适合用焦化废水吸附处理。上述研究指出,吸附材料、孔结构与孔径分布、焦化废水性质、有机物分子结构之间存在相关性,通过性质的匹配来实现废水预处理优化的吸附分离工艺。     

Effect of mechanically alloyed sintering aid on sinterability of TiB2

High-melting-point borides are attractive for structural and functional materials used in extreme environments. However, their poor sinterability due to their high melting point and strong covalent bonding have prevented their further applications. In this study, a new sintering process, called transient liquid-phase diffusion sintering (TLPDS), has been developed for high-melting-point borides. TiB₂ compacts were fabricated by spark plasma sintering at 50 MPa for 15 min at temperatures of 1300 and 1600 °C via TLPDS of TiB₂ powder with a eutectic TiB–Ti powder as the sintering aid, prepared via mechanical alloying (MA). Differential scanning calorimetry results indicated that the melting temperature of the obtained sintering aid was lower than its equilibrium melting point. MA of the sintering aid suppressed the open pore fraction to one-third of that in the compact sintered with the aid prepared without MA when sintering at 1300 °C. We also propose a possible mechanism for TLPDS of TiB₂.     

Dimensional change behavior of porous MgTi2O5 in reactive sintering

Volume-shrinkage of a sample in reactive sintering generally tends to be larger than that in conventional sintering. New techniques to suppress the volume shrinkage are eagerly needed for actual manufacturing. Recently, we have reported that reactively sintered porous MgTi₂O₅ from hydromagnesite and TiO₂ rutile showed less volume shrinkage than that from hydromagnesite and TiO₂ anatase. The result demonstrated that the compositional control of starting polymorphs can be a potential technique to optimize the volume shrinkage. In this paper, in order to evolve the reactive sintering technique, volume-changes during reactive sintering were dynamically monitored by thermomechanical analysis (TMA). The dimensional change behavior measured by TMA was linked up with the reaction behavior clarified by high-temperature X-ray diffraction (HT-XRD). In dilatometry curves, transient volume expansions were observed and they were well-explained by the formation and crystal growth of intermediate MgTiO₃ and objective MgTi₂O₅ particles.     

Kinetic and thermodynamic effects of manganese as a densification aid in yttria-stabilized zirconia

This work demonstrates the role of Mn as an effective sintering aid in Yttria-Stabilized-Zirconia (YSZ) is a result of the concomitant reduction of activation energies and change of interfacial energies caused by Mn segregation. Kissinger analyses of the heat of sintering showed a decrease in activation energy from 219.9 kJ/mol for YSZ to 103.4 kJ/mol for YSZ containing 3-mol% Mn. Direct microcalorimetry analyses showed that the average surface and grain boundary energies of YSZ decreased from 0.94 and 0.71 J/m², respectively, to 0.70 and 0.17 J/m² for 3-mol% Mn doped YSZ. The decrease in the ratio between surface and grain boundary energies indicates an increase in dihedral angle from 137.5° to 166.6°, meaning an increase in sintering stress. Segregation of manganese to grain boundaries was experimentally observed and is discussed to be responsible for both kinetic and thermodynamic changes in the system while suggesting interconnection by the thermodynamic extremal principle.     

Densification behavior of mullite-Al2TiO5 composites by reaction sintering of natural andalusite and TiO2

In this work, mullite-Al₂TiO₅ composites were fabricated by natural andalusite with TiO₂ as an additive. The densification characteristic, phase composition and mullitization process of andalusite with TiO₂ addition was investigated by the Archimedes’ principle, dilatometry, X-ray diffraction and scanning electron microscopy (SEM-EDS) techniques. The results showed that the incorporation of TiO₂ not only enhanced the thermal stability of in-situ Al₂TiO₅ in the silica liquid yielded from the mullitization of andalusite, but also accelerated andalusite decomposition and retarded mullite formation, which facilitated the sintering and densification of mullite-Al₂TiO₅ composites.     

Initial stage sintering mechanism of NaNbO3 and implications regarding the densification of alkaline niobates

The behavior of submicron- and nano-sized NaNbO₃ powder compacts during conventional sintering was studied using optical dilatometry and microstructure analysis. Microstructure-development trajectories revealed the dominance of grain growth during the initial sintering stage, while densification occurred only during later stages. Surface diffusion with low activation energy in the range of 50–60 kJ/mol was found to be the dominant material-transport mechanism during the initial sintering stage. The early activation of surface diffusion reduced the sintering driving force, decreased the rate of the densifying mechanisms and was thus identified as the main cause for poor densification of NaNbO₃. Same explanation could be valid also for other alkaline niobate based lead-free piezoelectric ceramics. Finally, alternative sintering methods are discussed and the efficiency of the pressure-assisted sintering was demonstrated in successful production of highly-dense fine-grained NaNbO₃ ceramics, with relative density and grain size of 98% and 700 nm, respectively.     

Microstructural analysis of sintering behavior of intra-grain pores

A latticebased Monte Carlo simulation approach has been developed for studying the behavior of intragrain pores during the intermediate and final stages of sintering. The changes of the microstructures and the resulting properties of intragrain pores during sintering are easily examined. The sintering behavior such as pore size distribution, average pore size, etc. is in very good agreement with the experimental observations. In addition, the relationships between the number of pores and the average pore volume agree well with theory.     

Effect of Eu2O3 on sintering densification and corrosion resistance of magnesium aluminate spinel

The effect of the doping amount of Eu₂O₃ on the densification behaviour of magnesium aluminate spinel (MAS) and its corrosion resistance to aluminium electrolyte were studied. The relative density, phase composition, micro morphology and hardness of the sintered samples were characterised by Archimedes’ drainage method, X-ray diffractometer, scanning electron microscope and automatic micro Vickers hardness tester. Results showed that the doping of Eu₂O₃ was conducive to the densification of MgAl₂O₄. When the content of Eu₂O₃ was 3 wt.%, the relative density of MAS was the largest (99.32%), the microstructure was more compact and the hardness was the largest (2293.4 kgf/mm²). The MAS sample with 3 wt.% Eu₂O₃ had the best corrosion resistance to aluminium electrolyte, and the corrosion depth was 80.99 μm. It was speculated that the electrolyte may penetrate into the sample through the micropores, and the fluoride salt chemically reacted with MgAl₂O₄ to form Al₂O₃, NaF and MgF₂.     

Effect of Al2O3 addition on texturing in a rotating strong magnetic field and densification of B4C

The properties of ceramics can be improved by controlling the microstructure through texturing ceramics in a strong magnetic field. Fabricating dense boron carbide (B₄C) requires high temperature sintering, therefore sintering additives are often used in order to densify B₄C ceramics at lower temperatures. However, combined effect of texturing and sintering additives on densification of B₄C has not been made clear yet. Here we report the effect of alumina (Al₂O₃) sintering additive on texturing in a strong magnetic field and densification of B₄C. Texturing was performed by rotating superconducting magnet at 12 T during slip casting process. Electron backscatter diffraction (EBSD) was used to observed the texturing projection. {0001} plane is clearly oriented in the plane parallel to rotating magnetic field. In addition, Lotgering factor was also calculated as quantitatively evaluation of texturing degree. Results on densification showed that addition of Al₂O₃ successfully increased density of B₄C sintered by spark plasma sintering (SPS) at 1800^oC to 97.8%. Formation of aluminum borate (Al₅BO₉) as secondary phase was detected by X-Ray diffraction (XRD). It is considered that the generation of Al₅BO₉ assisted finer densification of B₄C ceramic. Textured B₄C sintered at 1700^oC by SPS without alumina addition exhibited the highest orientation of c-axis. Addition of alumina caused decrease in degree of orientation of c-axis.     

Effect of temperature and holding time on the densification of alumina obtained by two-step sintering

The two-step sintering technique is a process of controlling the sintering curve, which provides materials with higher density and smaller grain size when compared to conventional sintering. This technique was evaluated by optical dilatometry with three commercial alumina powders of different purity (92, 96 and 99 wt% of Al₂O₃) and particle size (between 0.73 and 2.16 µm). Different sintering conditions in the first (temperature, T1) and second (temperature, T2, and holding time, t2) steps were studied in order to evaluate the effect of these variables on densification and grain growth. Considering T1 as the temperature at which a relative density (D_rel) of 83% was achieved, and for the range of conditions tested, it was found that higher D_rel values and lower grain size of alumina were obtained with higher T2 and lower t2. Alumina with 99 wt% purity sintered at T1 of 1550 °C for 5 min and T2 of 1500 °C for 4 h showed the best relationship between higher densification (~96% relative density) and reduced grain size (0.94±0.15 µm). Thus, this work demonstrated that suppression of grain growth can also be obtained for commercial alumina.     

Effect of chromium additive on sintering and oxidation behavior of HfB2-SiC ceramics

The effect of chromium admixture on the processes in the HfB₂-SiC ceramic powder system during its pressureless sintering at 1600?°C was studied. It was shown that an increase in chromium content from 0% to 15.5% in the HfB₂-SiC ceramic powder mixture leads to a continuous increase in its relative density up to 90%. A transient liquid phase Cr-Si-C-B is formed at 1600?°C, and it promotes intense sintering of HfB₂ and SiC powders. The oxidation resistance of HfB₂-SiC-Cr ceramics was studied in static air at 1000–1500?°C. It was shown that the oxidation resistance is greatly improved due to a decrease in the porosity of the sintered ceramic system because of chromium additive. The presence of chromium oxide in the formed surface glassy layer can also lead to the increase in the oxidation resistance. These results suggest that chromium can be considered as a promising sintering additive for HfB₂-SiC and similar systems.