The influence of CaO on alumina grain boundary mobility

The influence of CaO on the evolving microstructure of alumina has been studied in a range of concentrations below the solubility limit. The amount of Ca in the alumina was determined by conducting fully standardized wavelength dispersive spectroscopy, and the change in grain boundary mobility as a function of the amount of dopant was characterized using scanning electron microscopy. Unlike segregating dopants which reduce grain boundary mobility by solute-drag, CaO increases the rate of grain growth, and a trend of increased mobility with increasing dopant level was shown. The increased mobility with Ca segregation is believed to be due to an increase in vacancy concentration in the vicinity of the grain boundaries, thus facilitating faster grain boundary motion.     

Toward Knowledge‐Based Grain‐Boundary Engineering of Transparent Alumina Combining Advanced TEM and Atomistic Modeling

Transition element dopants (e.g., Y, La) are commonly used as sintering aids in polycrystalline alumina ceramics, which segregate to the grain boundaries and control the grain‐boundary mobility. However, due to the extremely thin (<2 nm) layer of segregated dopants, the experimental characterization of the segregated alumina grain boundaries is a complex task. Computational studies have focused only on tilt grain boundaries, which are only a small fraction in a sintered alumina sample. In this study, a quantitative characterization of the segregation of Y and La at general high angle grain boundaries in transparent alumina is carried out using a unique combination of advanced TEM and near coincidence grain‐boundary atomistic simulations. The result show that high angle grain boundaries may lead to enhanced grain growth in comparison to symmetric tilt twin grain boundaries due to the reduced configuration entropy for dopant segregation and higher order grain‐boundary complexions. On the other hand, multidoping with different dopants was shown to be more beneficial than single doping due to its contribution in increasing the configurational entropy for segregation. The advanced TEM analysis showed Y and La distributions and concentrations on a series of general grain boundaries in very good agreement with the atomistic simulations. This validation of atomistic modeling technique used in this study means, as it a generic method, can be used as a predictive tool to design ceramic microstructure and properties.     

Influence of micro- and nanoparticles of zirconium oxides on the dielectric properties of CaCu3Ti4O12

This work presents the results of Zr oxide doping of a CaCu₃Ti₄O₁₂ (CCTO) ceramic prepared by a solid-state reaction. Different stoichiometries (ZrO and ZrO₂) and grain sizes (micro- and nanoparticles) were added as dopants at concentrations of 0.5 and 1.0 wt%. Zr-doping controls the grain size growth, leading to a reduction of the grain size as observed by scanning electron microscopy. For both dopant concentrations, all of the samples exhibited lower dielectric loss and a smaller dielectric constant than those of undoped CCTO. The sample doped with 0.5% of the non-stoichiometric ZrO exhibits a dielectric constant over 3200 and a dissipation factor of 0.02 at 1 kHz. The impedance spectroscopy analysis confirms that the decrease of dielectric loss is mainly due to an increase in resistivity at grain boundaries, which is attributed to the suppression of oxygen-loss promoted by dopants.     

On the phase and grain boundaries in dual phase carbide/boride ceramics from micro to atomic level

The phase and grain boundary characteristics of recently developed fine-grained dual-phase high-entropy (Ti-Zr-Nb-Hf-Ta)C/(Ti-Zr-Nb-Hf-Ta)B₂ was investigated throughout all the accessible length scales using scanning electron microscopy (SEM), aberration-corrected scanning transmission electron microscopy (STEM), energy dispersive X-ray spectroscopy (EDS) and electron energy loss spectroscopy (EELS). The system exhibits relatively homogeneous grain size distribution where the average size is approximately 0.97 µm, with chemical composition (Ti_0.14 Zr_0.2 Nb_0.2 Hf_0.2 Ta_0.26)C + (Ti_0.38 Zr_0.18 Nb_0.22 Hf₀.₁₁₅ Ta_0.105)B₂. SEM analyses revealed no micro-crack formation and second – phase segregation at the boundaries or micro-pores at the triple – points. Investigation down to the sub-nanometer scale revealed that the phase and grain boundaries were typically clean and sharp with an indistinct 1 – 1.5 nm thin gradient of metallic elements at boride/boride and carbide/carbide interfaces. The sharp phase and grain boundaries do exhibit elemental enrichment from a trace amount of Fe being incorporated in interstitial positions of carbide and boride grains locally at boride/carbide boundaries or are present in boride and carbide grains in the form of continuous thin layer at boride/boride and carbide/carbide interfaces with the probably origin from starting powders.     

Microstructural Analysis of Liquid-Phase-Sintered β-Silicon Carbide

The microstructures of fine-grained β-SiC materials with α-SiC seeds annealed either with or without uniaxial pressure at 1900°C for 4 h in an argon atmosphere were investigated using analytical electron microscopy and high-resolution electron microscopy (HREM). An applied annealing pressure can greatly retard phase transformation and grain growth. The material annealed with pressure consisted of fine grains with β-SiC as a major phase. In contrast, the microstructure in the material annealed without pressure consisted of elongated grains with half α-SiC. Energy-dispersive X-ray analysis showed no differences in the amount of segregation of aluminum and oxygen atoms at grain boundaries, but did show a significant difference in the segregation of yttrium atoms at grain boundaries along SiC grains for the two materials. The increased segregation of yttrium ions at grain boundaries caused by the applied pressure might be the reason for the retarded phase transformation and grain growth. HREM showed a thin secondary phase of 1 nm at the grain boundary interface for both materials. The development of a composite grain consisting of a mixture of β/α polytypes during annealing was a feature common to both materials. The possible mechanisms for grain growth and phase transformation are discussed.     

Segregation-controlled densification and grain growth in rare earth-doped Y2O3

Cation doping of Y₂O₃ is an established approach for tailoring densification and grain growth during sintering. However, the segregation of doped cations to the grain boundary and their impact on processing are still not completely understood. Segregation can be driven by electrostatic effects due to charge mismatch with the host lattice or elastic effects induced by ion size mismatch. While segregation is caused by thermodynamics, it impacts diffusion and the kinetics of grain boundaries during densification and microstructure evolution. In this study, we utilize two isovalent dopants (La³⁺ and Gd³⁺), that is we focus on the elastic component of segregation. We investigate the densification as well as the grain growth kinetics of both doped and undoped Y₂O₃ during field-assisted sintering/spark plasma sintering (FAST/SPS). While Gd³⁺ is showing no significant effect on densification, La³⁺ resulted in a strongly reduced sintering activity. Furthermore, the analysis of the grain growth behavior during sintering and on predensified samples revealed a decrease in the grain growth coefficient, with La³⁺ having the strongest impact. The structure and chemistry at the grain boundary were observed by aberration-corrected TEM. While no structural change was caused by doping, the chemical analysis showed a strong segregation of La³⁺ to the grain boundary, which could not be observed for Gd³⁺. The results indicate that segregated La³⁺ causes a drastic decrease in grain boundary migration rates through solute drag as well as much slower sintering kinetics, likely caused by a decrease in the grain boundary self-diffusion due to segregation. This study further underlines the importance of the elastic contribution to cation segregation and establishes a clear relationship to grain growth and sintering kinetics, which are both decreased by segregation.     

Atomic structures of Ti-doped α-Al2O3 Σ13 grain boundary with a small amount of Si impurity

Doping is a fundamental technique to control sintering of α-Al₂O₃, and many types of elements have been used to control grain growth and material properties. Such dopants tend to be concentrated at the grain boundaries and modify grain boundary properties. It is therefore important to investigate the variation of grain boundary atomic structures induced by additives. However, in the sintering process it is difficult to prevent small amounts of unintentional impurity (such as Si), and the potential influence of additives and impurities on the grain boundary structure should be taken into account. Here we show that two types of grain boundary atomic structures are formed in a Ti-doped Σ13 []/() α-Al₂O₃, which has been determined by atomic-resolution scanning transmission electron microscopy. Combining with atomic-scale spectroscopic analyses, we elucidate that the local concentration of Si impurities significantly alters the grain boundary atomic structures and the valence state of Ti additives. The present results suggest that the subtle change in concentrations of both additives and impurity should have a strong impact on sintering processes and resultant properties in α-Al₂O₃.     

Grain boundary segregation of iron,chromium an scandium in polycrystalline magnesium oxide

Utilizing x-ray microanalysis in samples studied with scanning transmission electron microscopy, segregation of Fe, Cr and Sc has been found at grain boundaries of polycrystalline MgO. Samples studied contain between 500 and 1200 cation ppm of each solute, or of all three. The level of grain boundary segregation of Fe and Sc was approximately proportional to the bulk concentration while the boundary concentration of Cr was less than the other solutes, more so at higher concentrations. This results is attributed to the higher association energy of Cr_MgV_Mg and Cr′_Mg-V″_Mg-Cr′_Mg complexes which have a negative or neutral charge in the MgO matrix, thus not contributing to the space charge layer.     

Insight into impurity scavenging effect of gadolinium-doped ceria electrolytes

In this study, Ce_0.89+xGd_0.1−2xSr_xEu_0.01O_2−δ (x = 0–0.04) ceramic electrolytes sintered at different temperatures were prepared and the impurity scavenging effect of Sr²⁺ dopants on their grain boundary (GB) electrical properties were investigated. The phase compositions, microstructures, and ionic conductivities of the samples were investigated using X-ray diffraction, scanning electron microscopy, and AC impedance spectroscopy, respectively. The incorporation of Sr²⁺ improved the overall GB conduction of the electrolytes by decreasing their GB area and space-charge potential. It hindered conduction to some extent by weakening the impurity dilution effect. The scavenging effect could be observed only when the sintering temperature was higher than the eutectic temperature of Si GB impurities. At low sintering temperatures, the Sr²⁺ dopants could not scavenge the GB impurities and increased the GB impurity coverage proportion. The findings of this study provide insights into the optimisation of the GB electrical properties of oxygen-ion conductors by selecting appropriate dopants and sintering temperatures to induce the GB impurity scavenging effect.     

Microstructure and Electrical Properties of Sodium-Diffused and Potassium-Diffused SrTiO3 Barrier-Layer Capacitors Exhibiting Varistor Behavior

In sodium-diffused strontium titanate internal-boundary-layer capacitors exhibiting good varistor properties, high-resolution transmission electron microscopy and scanning transmission electron microscopy revealed sodium segregation at grain boundaries in the absence of intergranular phases. The segregation layer is narrow (≅10 nm), unlike much broader diffused boundary layers which have also been observed. The nonohmic behavior in these and in potassium-diffused specimens suggests that segregated acceptor alkali ions act as grain-boundary electron traps leading to varistor electrical barriers.     

Engineering grain boundary anisotropy to elucidate grain growth behavior in alumina

Current grain growth models have evolved to account for the relationship between grain boundary energy/mobility anisotropy and the five degrees of grain boundary character. However, the role of grain boundary networks on overall growth kinetics remains poorly understood. To experimentally investigate this problem, a highly textured Al₂O₃ was fabricated by colloidal casting in a strong magnetic field to engineer a unique spatial distribution of grain boundary character. Microstructural evolution was quantified and compared to an untextured sample. From this comparison, a prevalence of (0001)/(0001) terminated grain boundaries with anisotropic networks were identified in the textured sample. These boundaries and their networks were found to be driving grain growth at a faster rate than predicted by models. These findings will allow better modelling of grain growth in real systems by experimentally exploring the impact thereon of grain boundary plane anisotropy and relative energy/mobility differences between neighboring boundaries.     

Effects of Space Charge, Grain-Boundary Segregation, and Mobility Differences Between Grain Boundary and Bulk on the Conductivity of Polycrystalline Al2O3

The potential difference between grain boundary and bulk and the concentrations of native and foreign point defects in the bulk, the grain boundary ( gb ), and the subgrain boundary space-charge region ( sg ) of polycrystalline Al₂O₃ doped with acceptors are computed for the case that the dopants segregate at the grain boundaries, with either the ionized or the nonionized acceptor as the dominant species. Expressions are derived for the effective dc conductivity of a polycrystalline material on the basis of a model in which the grain has one or two shells with conductivities different from that of the bulk. Combination of the two results yields expressions for the effective ionic and electronic conductivity of doped A1₂O₃ as a function of grain size with distribution coefficients gb/sg , mobility ratios in the various regions, and equilibrium constants as parameters. At impurity concentrations normally found in ceramics, the contribution by subgrain boundaries to conductivity may be neglected. The theoretical results are compared to published data for Al₂O₃:Mg, Fe, and Ti.     

Development of microstructure during creep of polycrystalline mullite and a nanocomposite mullite/5 vol.% SiC

The microstructures of as-sintered and creep tested polycrystalline mullite and mullite reinforced with 5 vol.% nano-sized SiC particles have been characterized by scanning and transmission electron microscopy. The dislocation densities after tensile creep testing at 1300 and 1400 °C were virtually unchanged as compared to the as-sintered materials which indicates diffusion-controlled deformation. Mullite matrix grain boundaries bending around intergranular SiC particles suggest that grain boundary pinning, in addition to a reduced mullite grain size, contributed to the increased creep resistance of the mullite/5 vol.% SiC nanocomposite. Both materials showed pronounced cavitation at multi-grain junctions after creep testing at 1400 °C which suggests that unaccommodated grain boundary sliding, facilitated by softening of the intergranular glass, occurred at this temperature. This is consistent with the higher stress exponents at 1400 °C.     

Effect of annealing on grain growth and Y segregation behavior in tetragonal ZrO2 thin film

In order to fabricate tetragonal yttria stabilized zirconia samples with large grain size, 3 mol% Y₂O₃ doped zirconia thin films were grown on (0001) α-Al₂O₃ substrate by pulsed laser deposition (PLD) followed by subsequent high temperature annealing. The thin film samples were annealed at 1200°C, 1250°C, 1300°C, and 1350°C in order to obtain larger grain size without Y segregation. The microstructure and chemical composition of these annealed films were analyzed using atomic force microscopy, scanning transmission electron microscopy, and energy-dispersive X-ray spectroscopy. The as-grown thin film was found to be composed of [111]-oriented grains of ∼100 nm connected with small-angle tilt boundaries. Based on analysis of annealed thin films, it was revealed that grain growth of tetragonal zirconia occurred anisotropically. Cross section scanning transmission electron microscopy observations revealed that such grain growth behavior is affected by the step-terrace structures of the sapphire substrate. Energy-dispersive X-ray spectroscopy showed that Y was found to distribute almost uniformly below 1300°C but to segregate at the grain boundaries at 1350°C. As a conclusion, the 1300°C-annealed sample shows the largest grain size with homogeneous Y distributions.     

Effects of liquid phases on densification of TiO2-doped Al2O3–ZrO2 composite ceramics

This study investigated the densification behaviors and microstructural evolution of Al₂O₃–ZrO₂ (3Y) composite ceramics doped with four different amounts of TiO₂ (0, 1, 4, and 8 wt%; denoted as 0T, 1T, 4T, and 8T, respectively) to clarify the effect of TiO₂ dopants on densification. The shrinkage rate during densification increased with the increase in the amount of TiO₂. The development of grain boundary feature was also examined. The undoped ceramic showed clean grain boundaries. Thin liquid grain boundary phases were observed in 1T, whereas large liquid phases were found on the grain boundary and at the junction pockets in 4T and 8T. The results were discussed in terms of the relationship between densification and grain boundary feature.     

Influence of SiAlON addition on the microstructure development of hot-pressed ZrB2–SiC composites

The impact of SiAlON on densification behavior and microstructure of the ZrB₂-SiC composite was investigated. ZrB₂, SiC, and SiAlON were used as the initial materials to produce ZrB₂-SiC composite by hot pressing at 1900 °C. A fully dense composite was obtained having ~99.9% relative density. High-resolution X-ray diffraction (HRXRD) assessment verified the in-situ formation of ZrC, and the presence of residual carbon, SiAlON, and ZrB₂ and SiC phases in the as-sintered ceramic. Furthermore, the thermodynamic calculations confirmed the results attained by HRXRD. In addition, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were utilized for the microstructural investigation. SEM fractographs indicated the impact of SiAlON on the hindering of grain growth and the formation of flaky phases (graphitized carbon or solidified liquid phase) at the grain boundaries. TEM studies revealed the presence of a transparent glassy phase at the particle interfaces. A significant impact of liquid phase sintering was also affirmed in the clean interfaces.     

Characterization of Grain Boundaries in Superplastically Deformed Y-TZP Ceramics

The effects of compressive deformation on the grain boundary characteristics of fine-grained Y-TZP have been investigated using surface spectroscopy, impedance analysis, and transmission electron microscopy. After sintering at low temperature (1150°C), the grain boundaries are covered by an ultrathin (1nm) yttrium-rich amorphous film. After deformation at 1200°–1300°C under low stress, some grain boundaries are no longer covered by the amorphous film. Yttrium segregation seems to occur only at wetted grain boundaries. Evidence has been found that the extent of dewetting increases with increasing applied stress.     

PTCR Characteristics and Fabrication of Porous, Sb-Doped BaTiO3 Ceramics

Sb-doped BaTiO₃ ceramics containing corn-starch were prepared by sintering at 1350°C for 1 h in air. In this study, the effect of corn-starch on positive temperature coefficient of resistivity (PTCR) characteristics and microstructures of Sb-doped BaTiO₃ ceramics was investigated. It was found that the porosity and pore size increased and the grain size slightly decreased with increasing corn-starch content. XRD results showed the presence of BaTiO₃ peaks only in the Sb-doped BaTiO₃ ceramics with and without corn-starch. The PTCR jump of the Sb-doped BaTiO₃ ceramics with corn-starch was over 10⁶ and 1–2 orders higher than that of samples without corn-starch. The increase in the room-temperature resistivity with increasing corn-starch content was attributed mainly to the increase in the electrical barrier height of grain boundaries and the porosity as well as the partial decrease in the donor concentration of grains and the grain size. It was also noticed that the grain boundary resistivity contributed largely to the total resistivity of the Sb-doped BaTiO₃—corn-starch ceramics.     

Local field electron emission from grain boundaries of CVD diamond film

In this article, we have investigated local field electron emission from grain boundaries of diamond films with (100) preferential orientation by double-probe scanning electron microscopy (SEM). Compared with the field emission from the plane area of diamond film, local field emission from grain boundary area is greatly enhanced at the same applied field, and further increased with the increasing of grain boundary number density. This result provides a direct evidence that grain boundary plays an important role in field emission from diamond film because a great deal of sp² graphitic carbon phases exists in grain boundary areas as electron transport channels for the surface field emission process.