Sintering behavior of calcium lanthanum sulfide ceramics in field‐assisted consolidation

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.     

Flash Spark Plasma Sintering of 3YSZ: Modified sintering pathway and impact on grain boundary formation

Yttria stabilized zirconia (3 mol% YSZ) ceramics were prepared by Flash-SPS, while allowing high heating rates up to 200 °C/s, which led to the extremely fast densification within a few seconds. The high heating rates had strong impact on sintering mechanisms, in terms of densification and grain growth. While the specimens ended with 5–15 vol% porosity and limited grain growth (< 350 nm), their hardness is higher than fully dense counterpart SPSed ceramics. Using the sintering trajectories, microstructural observations, and impedance spectroscopy, we highlight altered sintering mechanism which resulted in very thin grain boundaries compared to SPS. It appears that densification is largely advanced at grain boundary interfaces, with no residual nano-pores at the grain junctions, where some pores with size comparable to grain size were present. This opens up opportunities for the fabrication of porous lightweight ceramics with good mechanical properties.     

两步烧结法制备纳米氧化钇稳定的四方氧化锆陶瓷

采用共沉淀法制备纳米氧化钇稳定的四方氧化锆（yttria stabilized tetragonal zirconia,3Y-TZP）粉体。利用X射线衍射、N2吸附–脱附等温线,透射电子显微镜对3Y-TZP粉体的物理性能和化学性能进行表征。研究了纳米3Y-TZP粉体的烧结曲线,分析了3Y-TZP素坯在烧结过程中的致密化行为和显微结构,探讨了两步烧结工艺对3Y-TZP纳米陶瓷微观结构的影响。结果表明：采用共沉淀法,在600℃煅烧2h后,可获得晶粒尺寸为13nm、晶型发育良好、团聚较少的纳米3Y-TZP粉体;采用两步烧结法,将素坯升温至1200℃保温1min后,再降温到1050℃保温35h,可获得相对密度大于98%,晶粒尺寸约为100nm的3Y-TZP陶瓷。两步烧结法通过控制煅烧温度和保温时间,利用晶界扩散及其迁移动力学之间的差异,使晶粒生长受到抑制,样品烧结致密化得以维持,实现在晶粒无显著生长前提下完成致密化。     

Two step sintering of cubic yttria stabilized zirconia using Field Assisted Sintering Technique/Spark Plasma Sintering

Starting from homogenous highly packed green bodies, two sintering methods have been combined in order to limit grain growth during the densification of fully stabilized zirconia. Densification behavior and microstructures in relation with green body shaping have been investigated. It could be shown that two step sintering schedules carried out by FAST/SPS are very effective in reducing both processing time and grain growth.     

Influence of grain boundary sliding on diffusion in yttria doped zirconia

Grain boundary sliding during high temperature deformation can lead to stress concentrations and an enhancement of diffusion in mobile boundaries. Experiments were conducted on a fine grained 3 mol% yttria stabilized tetragonal zirconia, under conditions associated with superplastic flow involving grain boundary sliding. Tracer diffusion studies under creep conditions and without load indicate that there is no enhancement in either the lattice or grain boundary diffusivities. The experimental creep data are consistent with an interface controlled diffusion creep mechanism.     

Sintering Behavior of Nanocrystalline Zirconia Doped with Alumina Prepared by Chemical Vapor Synthesis

Powders of nanocrystalline zirconia doped with 3–30 mol% alumina have been synthesized using chemical vapor synthesis (CVS). Dense or mesoporous ceramics of small and narrowly distributed grain and pore sizes in the nanometer range are obtained via pressureless vacuum sintering. The microstructural development of the doped samples is strongly dependent on the alumina content. Sintering of zirconia samples with 3 and 5 mol% alumina at temperatures of 1000°C for 1 h results in fully dense, transparent ceramics with grain sizes of 40–45 nm and homogeneous microstructures.     

Studies on ionic conductivity of stabilized zirconia ceramics (8YSZ) densified through conventional and non-conventional sintering methodologies

Densification studies of 8 mol% yttria stabilized zirconia ceramics were carried out by employing the sintering techniques of conventional ramp and hold (CRH), spark plasma sintering (SPS), microwave sintering (MWS) and two-stage sintering (TSS). Sintering parameters were optimized for the above techniques to achieve a sintered density of >99% TD. Microstructure evaluation and grain size analysis indicated substantial variation in grain sizes, ranging from 4.67 μm to 1.16 μm, based on the sintering methodologies employed. Further, sample was also sintered by SPS technique at 1425 °C and grains were intentionally grown to 8.8 μm in order to elucidate the effect of grain size on the ionic conductivity. Impedance spectroscopy was used to determine the grain and grain boundary conductivities of the above specimens in the temperature range of RT to 800 °C. Highest conductivity of 0.134 S/cm was exhibited by SPS sample having an average grain size of 1.16 μm and a decrease in conductivity to 0.104 S/cm was observed for SPS sample with a grain size of 8.8 μm. Ionic conductivity of all other samples sintered vide the techniques of TSS, CRH and MWS samples was found to be ∼0.09 S/cm. Highest conductivity irrespective of the grain size of SPS sintered samples, can be attributed to the low densification temperature of 1325 °C as compared to other sintering techniques which necessitated high temperatures of ∼1500 °C. The exposure to high temperatures while sintering with TSS, CRH and MWS resulted into yttria segregation leading to the depletion of yttria content in fully stabilized zirconia stoichiometry as evidenced by Energy Dispersive Spectroscopy (EDS) studies.     

Densification and grain growth of 8YSZ containing NiO

The effects of NiO addition on sintering yttria-stabilized zirconia were systematically studied to understand the role of the additive in the sintering process of the solid electrolyte. Specimens of 8 mol% yttria-stabilized zirconia with NiO contents up to 5.0 mol% were prepared using different Ni precursors and sintered at several dwell temperatures and holding times. Densification and microstructural features were studied by apparent density measurements and scanning electron microscopy observations, respectively. The sintering dynamic study was carried out by following the linear shrinkage of powder compacts containing 0-0.75 mol% NiO. Small (up to 1.0 mol%) NiO addition was found to improve the sinterability of yttria-stabilized zirconia. The activation energy for volume diffusion decreases with increasing NiO content, whereas the grain boundary diffusion seems to be independent on this additive. The grain growth of yttria-stabilized zirconia is found to be enhanced even for small NiO contents.     

Diffusion of calcium in yttria stabilized zirconia ceramics

Bulk and grain boundary diffusion of calcium in yttria fully stabilized zirconia was studied in air in the temperature range from 1100 to 1400°C. Secondary ion mass spectrometry (SIMS) was used to determine the diffusion profiles as average concentration vs. depth in the B-type kinetic region. The obtained results allowed the calculation of the temperature dependence of the bulk diffusion coefficient D and the grain boundary diffusion parameter D′δs. Activation energies of these processes amount to 333 and 367 kJ mol⁻¹, respectively. Diffusion data of calcium were compared to those of titanium obtained previously using the same zirconia material.     

Size-induced grain boundary energy increase may cause softening of nanocrystalline yttria-stabilized zirconia

An increase in hardness with reducing grain sizes is commonly observed in oxide ceramics in particular for grain sizes below 100 nm. The inverse behavior, meaning a decrease in hardness below a critically small grain size, may also exist consistently with observations in metal alloys, but the causing mechanisms in ceramics are still under debate. Here we report direct thermodynamic data on grain boundary energies as a function of grain size that suggest that the inverse relation is intimately related to a size-induced increase in the excess energies. Microcalorimetry combined with nano and microstructural analyses reveal an increase in grain boundary excess energy in yttria-stabilized zirconia (10YSZ) when grain sizes are below 36 nm. The onset of the energy increase coincides with the observed decrease in Vickers indentation hardness. Since grain boundary energy is an excess energy related to boundary strength/stability, the results suggest that softening is driven by the activation of grain boundary mediated processes facilitated by the relatively weakened boundaries at the ultra-fine nanoscale which ultimately induce the formation of an energy dissipating subsurface crack network during indentation.     

Fabrication and microstructure development of Yb:YAG transparent ceramics from co-precipitated powders without additives

Sintering additives are generally considered to be important for improving densification in fabrication of transparent ceramics. However, the sintering aids as impurities doped in the laser materials would decrease the laser output power and produce additional heat during laser operation. In this work, Yb:YAG ceramics were vacuum-sintered without additives at different temperatures for various soaking time through using ball-milled powders synthesized by co-precipitation route. The densification behavior and grain growth kinetics of Yb:YAG ceramics were systematically investigated through densification curves and microstructural characterizations. It was determined that the densification in the 1500°C-1600°C temperature range was controlled by a grain-boundary diffusion. It is revealed that the volume diffusion is the main mechanism controlling the grain growth between 1600°C and 1750°C. Although SiO₂ additives can promote densification during low-temperature sintering, the optical transmittance of Yb:YAG ceramic with no additives, sintered at 1800°C for 15 hours, reaches a maximum of 83.4% at 1064 nm, very close to the measured transmittance value of Yb:YAG single crystal. The optical attenuation loss was measured at 1064 nm in Yb:YAG transparent ceramic, to be 0.0035 cm⁻¹, a value close to that observed for single crystals.     

Grain Boundary Diffusion of Magnesium in Zirconia

This paper reports the transport kinetics of Mg in cubic yttria-stabilized zirconia (containing 10% mol of Y₂O₃ (10YSZ)) involving the bulk and the grain boundary diffusion coefficients. The diffusion-controlled concentration profiles of Mg were determined using secondary ion mass spectrometry (SIMS) in the range 1073–1273 K. The determined bulk diffusion coefficient and the grain boundary diffusion product may be expressed as the following functions of temperature, respectively: D = 5.7 exp[(−390 kJ/mol)/ RT ] cm²·s⁻¹ and D 'αδ= 3.2 × 10⁻¹⁵ exp[(−121 kJ/mol)/ RT ] cm³·s⁻¹, where α is the segregation enrichment factor and δ is the boundary layer thickness. The grain boundary enhancement factor decreases with temperature from 10⁵ at 1073 K to 10³ at 1273 K.     

Coarse-grain CeO2 doped ZrO2 ceramic prepared by spark plasma sintering

In the present work, coarse grain cerium stabilized zirconia bulk ceramic was prepared by spark plasma sintering technique. The relatively high temperature of 2000 °C used for sintering led to enormous grain growth up to approximately 100 μm. Sintering at high temperatures and in the vacuum caused oxygen depletion and thus transformation from tetragonal to cubic phase during the sintering process. The tetragonal phase was recovered by annealing at 1400 °C in air. This led to a change in fracture behavior. Mostly transgranular fracture of the cubic phase was changed to intergranular fracture after recovering the tetragonal phase. On the intergranular fracture surface, twinning-like structure and structures similar to antiphase domain were observed.Mechanical properties represented by indentation hardness of prepared samples were evaluated.     

Densification mechanism and microstructure characteristics of nano- and micro- crystalline alumina by high-pressure and low temperature sintering

Fully dense ceramics with retarded grain growth can be attained effectively at relatively low temperatures using a high-pressure sintering method. However, there is a paucity of in-depth research on the densification mechanism, grain growth process, grain boundary characterization, and residual stress. Using a strong, reliable die made from a carbon-fiber-reinforced carbon (C_f/C) composite for spark plasma sintering, two kinds of commercially pure α-Al₂O₃ powders, with average particle sizes of 220 nm and 3 μm, were sintered at relatively low temperatures and under high pressures of up to 200 MPa. The sintering densification temperature and the starting threshold temperature of grain growth (T_sg) were determined by the applied pressure and the surface energy relative to grain size, as they were both observed to increase with grain size and to decrease with applied pressure. Densification with limited grain coarsening occurred under an applied pressure of 200 MPa at 1050 °C for the 220 nm Al₂O₃ powder and 1400 °C for the 3 μm Al₂O₃ powder. The grain boundary energy, residual stress, and dislocation density of the ceramics sintered under high pressure and low temperature were higher than those of the samples sintered without additional pressure. Plastic deformation occurring at the contact area of the adjacent particles was proved to be the dominant mechanism for sintering under high pressure, and a mathematical model based on the plasticity mechanics and close packing of equal spheres was established. Based on the mathematical model, the predicted relative density of an Al₂O₃ compact can reach ～80 % via the plastic deformation mechanism, which fits well with experimental observations. The densification kinetics were investigated from the sintering parameters, i.e., the holding temperature, dwell time, and applied pressure. Diffusion, grain boundary sliding, and dislocation motion were assistant mechanisms in the final stage of sintering, as indicated by the stress exponent and the microstructural evolution. During the sintering of the 220 nm alumina at 1125 °C and 100 MPa, the deformation tends to increase defects and vacancies generation, both of which accelerate lattice diffusion and thus enhance grain growth.     

纳米复相陶瓷材料的烧结技术

纳米复相陶瓷的烧结与普通陶瓷的烧结不同，在纳米复相陶瓷的烧结过程中需要采取相应的措施尽可能地控制晶粒的长大。目前，国内外研究者主要是通过改进传统烧结技术或采用新型烧结技术烧结制备纳米复相陶瓷。文章综述了目前国内外用的比较多的几种制备纳米复相陶瓷的烧结技术，并对其特点和应用情况进行了总结。     

Size-induced room temperature softening of nanocrystalline yttria stabilized zirconia

Nanocrystalline oxides exhibit exceptional resistance to plastic deformation, manifesting increased strength and hardness with reduced grain size that qualitatively follows the so-called Hall-Petch relationships. However, below a critical grain size, softening has been observed to occur, in the so-called inverse Hall-Petch regime. The mechanisms underlying these phenomena are still not well understood in oxides. Here we observe, using nanopillar compression, that the yield strength initially increases with decreasing grain size for yttria-stabilized zirconia ceramics produced by high-pressure spark plasma sintering. A hardening-to-softening transition occurs at grain sizes below ≈21 nm. The experiments indicate that this transition depends on strain rate, and the onset of the decrease in yield strength occurs before any shear fracture begins. Nanopillar compression combined with in situ electron diffraction demonstrates the onset of softening coincides with an increase in the amount of crystallographic rotation per unit strain, suggesting a change in deformation mechanism.     

Analysis of nonisothermal sintering of nanocrystalline and microscaled ceramic materials

Abstract

Abstract

Sintering involves consolidation of powders under the application of heat to form solids of higher density and is often the final step in the processing of ceramic materials. The time–temperature cycles used in sintering affect the kinetics and, in turn, influence the quality of the sintered product. Considering the densification mechanisms controlled by grain boundary diffusion along with interface reaction and the grain growth mechanism, this paper presents a systematic numerical study on the sintering of nanocrystalline yttria tetragonal stabilised zirconia and microscaled α-alumina, to bring out the effects of the time–temperature cycles on their sintering behaviour. Effects of initial grain size are also examined. Based on the studies, empirical correlations are developed that relate the final grain size and the sintering time to the temperature cycle. The results serve as guidelines in the design of time–temperature cycles for the sintering of the two material systems considered.     

Electrical and hydrogen reduction enhances kinetics in doped zirconia and ceria: II. Mapping electrode polarization and vacancy condensation in YSZ

Knowing the correlation between grain boundary mobility and oxygen potential in yttria‐stabilized zirconia (YSZ), we have utilized the grain size as a microstructural marker to map local oxygen potential. Abrupt oxygen potential transition is established under a large current density and in thicker samples. Cathodically depressed oxygen potential can be easily triggered by poor electrode kinetics or in an oxygen‐lean environment. Widespread cavitation in the presence of highly reducing oxygen potential suggests oxygen vacancy condensation instead of oxygen bubble formation as commonly assumed for solid oxide fuel/electrolysis cells. These results also suggest electrode kinetics has a direct influence on the microstructure and properties of ceramics sintered under a large electric current.     

Grain coalescence in (Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)C during spark plasma sintering

High-entropy (Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)C ceramics (HEC) are fabricated via spark plasma sintering using different die configurations, including the conductive and insulating dies. Compared to the conductive die, the grain sizes of samples sintered in the insulating die are significantly larger, which is attributed to the higher local temperature as a result of the higher current density in the sample. Furthermore, the microstructure evolution and grain growth mechanism of HEC are investigated for the first time. We find that at moderate temperatures (∼1600°C), the grain growth of HEC can occur by a grain coalescence mechanism, forming numerous irregular grains in the porous sample. Three factors are crucial to induce grain coalescence, including the formation of partial melting layers on particle surfaces, nanograin rearrangement via rotation and sliding, and the formation of low-angle grain boundaries. During the final sintering stage, the irregular grains will change into polyhedral shapes by grain boundary migration. These findings are of assistance to better understand and control the microstructure evolution of HEC and other ultrahigh-temperature carbide ceramics.