Effect of CaO doping on densification and microstructure control of highly transparent La0.4Gd1.6Zr2O7 ceramics

Calcium oxide (CaO) as sintering additive was first used to fabricate La_0.4Gd_1.6Zr₂O₇ transparent ceramics by a simple solid-state reaction and one-step vacuum sintering method. The effects of CaO dopant amount on the densification, as well as sintering behaviors and microstructure evolution of the as-fabricated La_0.4Gd_1.6Zr₂O₇ ceramics, were systematically investigated. Under the different sintering temperatures, the relationships during the sintering process between grain growth and zpore elimination were analyzed as well. It was found that 0.1 wt% CaO doping can effectively control the rate of grain growth and promote densification dominated by surface diffusion. Furthermore, Ca²⁺ entered the lattice of La_0.4Gd_1.6Zr₂O₇ ceramics to accelerate ion diffusion and suppress grain boundary migration. With the introduction of 0.1 wt% CaO doping, the highly transparent La_0.4Gd_1.6Zr₂O₇ ceramics (T = 80.4% at 1100 nm) were successfully fabricated at the traditional sintering temperature (1850°C).     

Effects of sintering aids on microstructure,mechanical, and optical properties of AlON ceramics synthesized by SPS

Aluminum oxynitride (AlON) ceramics doped with different sintering aids were synthesized by spark plasma sintering process. The microstructures, mechanical, and optical properties of the ceramics were investigated. The results indicate that the optimal amount of sintering aids is 0.06 wt% La₂O₃ + 0.16 wt% Y₂O₃ + 0.30 wt% MgO. The addition of La³⁺ and Mg²⁺ decreases the rate of grain boundary migration in ceramics, promotes pore elimination, and inhibits grain growth. The addition of Y³⁺ facilitates liquid-phase sintering of AlON ceramics. Moreover, the addition of Mg²⁺ effectively promotes twin formation in the ceramics, which hinders crack propagation and dislocation motion when the ceramics are loaded. Hence, the AlON ceramic doped with 0.06 wt% La₂O₃ + 0.16 wt% Y₂O₃ + 0.30 wt% MgO exhibits a relative density of 99.95%, an average grain size of 9.42 μm, and a twin boundary content of 10.3%, which contributes to its excellent mechanical and optical properties.     

Effects of dopants on the microstructure and phase-purity control in PMN-PT ceramics

The effects of Pb²⁺, Ba²⁺, La³⁺, Y³⁺ and Gd³⁺ dopants on the densification and microstructure evolution of PMN-PT ceramics are investigated. The ceramics were reactively-sintered between 850 and 1250?°C in oxygen atmosphere. After sintering, all ceramics reached a relative density greater than 98%. XRD and SEM results revealed that lanthanum doping produces pure perovskite phase with unimodal rounded grains whereas other dopants yield microstructures with faceted grains due to the liquid phase formed in the intermediate stage. The mechanism for the observed differences in phase and morphology is also explored. The slower densification rate of La-doped samples helps maintain an open porosity network and favors the outgassing of the initial excess of PbO. The delayed densification contributes to the formation of pure perovskite at a temperature of 850?°C. Preferential A-site occupancy of La³⁺ lowers the oxygen vacancy concentration which suppresses grain growth.     

Doping effect of divalent cations on sintering of polycrystalline yttria

The sintering behavior of Y₂O₃ doped with 1 mol% of Ca²⁺, Mg²⁺, Mn²⁺, Ni²⁺, Sr²⁺ or Zn²⁺ was investigated by pressureless sintering in air at a sintering temperature in the range 900–1600 °C. The sintering temperature required for full densification in Y₂O₃ was reduced by 100–400 °C by the cation doping, while undoped Y₂O₃ was densified at 1600 °C. The most effective dopant among the examined cations was Zn²⁺. The grain growth kinetics of undoped and cation-doped Y₂O₃ was described by the parabolic law. The grain boundary mobility of Y₂O₃ was accelerated by doping of the divalent cations. High-resolution transmission electron microscopy (HRTEM) observations and nano-probe X-ray energy dispersive spectroscopy (EDS) analyses confirmed that the dopant cations tended to segregate along the grain boundaries without forming amorphous layers. The improved sinterability of Y₂O₃ is probably related to the accelerated grain boundary diffusion owing to the grain boundary segregation of the dopant cations.     

Densification and grain growth of Gd2Zr2O7 nanoceramics during pressureless sintering

Gd₂Zr₂O₇ nanoceramics were fabricated using pressureless sintering method, in which the nanopowders were synthesized via solvothermal approach. The effects of starting powders on grain growth and densification during sintering of ceramics were revealed. Two distinct pressureless sintering methods were investigated, including conventional and two-step sintering. The sample grain size increases abruptly as sintering temperature increases during conventional sintering. In contrast, in two-step sintering, abnormal or discontinuous grain growth was suppressed in the second step, leading to Gd₂Zr₂O₇ nanoceramics formation (average grain size 83 nm, relative density ∼93%). Such distinct behaviors may originate from the interplay between kinetic factors such as grain boundary migration and diffusion. Moreover, suppression of grain growth and promotion of densification in the two-step sintering are mainly due to dominant role of grain boundary diffusion during the second-step sintering process.     

Reactive flash sintering of high-entropy oxide (La0.2Nd0.2Sm0.2Eu0.2Gd0.2)2Zr2O7: Microstructural evolution and aqueous durability

Herein, the phase evolution, densification and grain growth process of the high entropy ceramics during flash sintering were systematically characterized and quantified to understand the microstructural evolution for the first time. It was demonstrated that the densification rate of (La_0.2Nd_0.2Sm_0.2Eu_0.2Gd_0.2)₂Zr₂O₇ by flash sintering in this work was generally around 60 times that of conventional sintering at 1600 °C, while the grain growth rate by flash sintering was only around 1.5–6 times that of conventional sintering, indicating that grain growth was suppressed during flash sintering. The grain growth mechanisms by flash sintering and conventional sintering could be both attributed to surface diffusion and volume diffusion. In addition, the flash sintered high-entropy ceramics as promising immobilization materials for high-level radioactive waste (HLW) exhibited excellent aqueous durability with normalized leaching rates of Nd, Gd and Zr approximately 10^?6～10^?7 g m^?2 d^?1 after 42 days, which were much lower than most reported pyrochlore materials.     

Thermodynamics and kinetics of sintering of Y2O3

Surface energy (γ_S) and grain boundary energy (γ_GB) of yttrium oxide (Y₂O₃) were determined by analyzing the heat of sintering (ΔH_sintering) using differential scanning calorimetry (DSC). The data allowed quantification of sintering driving forces, which when combined with a thorough kinetic analysis of the process, provide better understanding of Y₂O₃ densification as well as insights into effective strategies to improve its sinterability. The quantitative thermodynamic study revealed moderate thermodynamic driving force for densification in Y₂O₃ (as compared to other oxides) represented by a dihedral angle of 152.7° calculated from its surface and grain boundary energies. The activation energy was determined as 307 ± 61 kJ/mol, consistent with activation energies previously reported for processes relevant to sintering of Y₂O_3, such as Y³⁺ diffusion and grain boundary mobility. Finally, we propose that a refined deconvolution study on the DSC curve for Y₂O₃ sintering, combined with the associated material's microstructure evolution, may help identify shifts in sintering mechanisms, and therefore, specific activation energies at increasing temperatures.     

Effects of Gd Substitution on Sintering and Optical Properties of Highly Transparent (Y0.95−xGdxEu0.05)2O3 Ceramics

Highly transparent (Y_0.95?xGd_xEu_0.05)₂O₃ (x = 0.15–0.55) ceramics have been fabricated by vacuum sintering at the relatively low temperature of 1700°C for 4 h with the in‐line transmittances of 73.6%–79.5% at the Eu³⁺ emission wavelength of 613 nm (~91.9%–99.3% of the theoretical transmittance of Y_1.34Gd_0.6Eu_0.06O₃ single crystal), whereas the x = 0.65 ceramic undergoes a phase transformation at 1650°C and has a transparency of 53.4% at the lower sintering temperature of 1625°C. The effects of Gd³⁺ substitution for Y³⁺ on the particle characteristics, sintering kinetics, and optical performances of the materials were systematically studied. The results show that (1) calcining the layered rare‐earth hydroxide precursors of the ternary Y–Gd–Eu system yielded rounded oxide particles with greatly reduced hard agglomeration and the particle/crystallite size slightly decreases along with increasing Gd³⁺ incorporation; (2) in the temperature range 1100°C–1480°C, the sintering kinetics of (Y_0.95?xGd_xEu_0.05)₂O₃ is mainly controlled by grain‐boundary diffusion with similar activation energies of ~230 kJ/mol; (3) Gd³⁺ addition promotes grain growth and densification in the temperature range 1100°C–1400°C; (4) the bandgap energies of the (Y_0.95?xGd_xEu_0.05)₂O₃ ceramics generally decrease with increasing x; however, they are much lower than those of the oxide powders; (5) both the oxide powders and the transparent ceramics exhibit the typical red emission of Eu³⁺ at ~613 nm (the ⁵D₀→⁷F₂ transition) under charge transfer (CT) excitation. Gd³⁺ incorporation enhances the photoluminescence and shortens the fluorescence lifetime of Eu³⁺.     

Preparation and properties of MgAl2O4 spinel ceramics by double-doped CeO2 and La2O3

Magnesium aluminate spinel (MgAl₂O₄) ceramics are high-performance and carbon-free materials widely used in both military and civilian fields. However, it is usually challenging to densify during the solid-state sintering process. The excellent properties of some rare earth oxides have been proved to promote the densification of MgAl₂O₄ spinel ceramics. But the mechanism of promoting sintering is not clear. In the present work, MgAl₂O₄ spinel ceramics have been successfully fabricated by co-doping CeO₂ and La₂O₃ via a single-stage solid-state reaction sintering. The effects of addition amounts of CeO₂ and La₂O₃ on phase compositions, microstructures, sintering characteristics, cold compressive strength, and thermal shock resistance of as-prepared MgAl₂O₄ spinel ceramics were systematically investigated. The results show that by co-doping CeO₂ and La₂O₃ can increase the defect concentration due to the lattice distortion. This could promote the movement of Al³⁺ and Mg²⁺ at high temperature, which is beneficial to the formation of more secondary MgAl₂O₄ spinel. t-ZrO₂ with more Ce⁴⁺ filling between spinel grains could prevent the growth of grains and promote the densification, besides the new-formed LaAlO₃ that was mainly distributed along the grain boundary of the MgAl₂O₄ phase, both of which were favorable for the formation of dense microstructure of MgAl₂O₄ spinel materials. At the same time, the formation of more secondary MgAl₂O₄ spinel and sintering densification also improve the mechanical properties of spinel ceramics. La³⁺ will segregate to the spinel grain boundary, preventing grain boundary movement and absorbing the main crack's fracture energy. With 3 wt% CeO₂ and 3 wt% La₂O₃ co-doping, the bulk density of the sample increased from 3.02 g∙cm⁻³ to 3.55 g∙cm⁻³; the apparent porosity decreased from 12.21% to 9.97%; the cold compressive strength increased from 172.88 MPa to 189.54 MPa; and the residual strength retention ratio after thermal shock increased from 84.92% to 89.15%.     

Low-temperature sintering of Al2O3 ceramics doped with 4CuO-TiO2-2Nb2O5 composite oxide sintering aid

Dense alumina ceramics doped with 5 wt% 4CuO-TiO₂-2Nb₂O₅ composite sintering aids were obtained at low sintering temperatures of 950∼975 °C. The ceramic sintered at optimal condition shows good microwave dielectric properties (ε_r = 12.7, Q × f = 7400 GHz), high thermal conductivity (18.4 W/m K) and high bending strength (320 MPa). TEM and EDS analysis revealed that amorphous Cu-Ti-Nb-O interfacial films with nanometer thickness formed at the grain boundaries, which could provide paths of mass transportation for densification. Al³⁺ ions may be involved in mass transportation through substitution by Ti³⁺ and Ti⁴⁺ ions near the grain boundary during the sintering process. The accumulation of copper ions at the trigeminal grain boundary was observed. The migration and reaction of copper ions in grain boundaries may also play an important role in promoting mass transportation and low-temperature densification of alumina ceramics.     

Doping effect on sinterability of polycrystalline yttria: From the viewpoint of cation diffusivity

The sintering behavior of Y₂O₃ doped with 1 mol% of a trivalent or tetravalent cation was investigated by pressureless sintering in air. Ga³⁺ or Ge⁴⁺-doped Y₂O₃ bodies exhibited higher relative densities than the undoped Y₂O₃, while the La³⁺ or Zr⁴⁺-doping suppressed the densification of Y₂O₃. An interdiffusion experiment was performed on the diffusion couples of polycrystalline Er₂O₃ and Y₂O₃ doped with Ni²⁺ or Zr⁴⁺, which are some of the most effective and least effective dopants for the improvement of the sinterability, respectively. The lattice and grain boundary diffusion coefficients of the Er³⁺ cation in Y₂O₃ were increased by the Ni²⁺-doping, but were decreased by the Zr⁴⁺-doping. High-resolution transmission electron microscopy observations and nano-probe X-ray energy dispersive spectroscopy analyses confirmed that the dopant cations segregate along the grain boundaries without forming an amorphous phase. The doping effect on the sinterability of Y₂O₃ must result from the change in the diffusivity in Y₂O₃.     

Highly transparent Yb:Y2O3 ceramics obtained by solid-state reaction and combined sintering procedures

(Y_0.87-xLa_0.1Zr_0.03Yb_x)₂O₃ (x?=?0.02, 0.04, 0.05) transparent ceramics were obtained by solid-state reaction and combined sintering procedures with La₂O₃ and ZrO₂ as sintering additives. A method based on two-step intermediate sintering in air followed by vacuum sintering was applied in order to control the densification and grain growth of the samples during the final sintering process. The results indicate that La₂O₃ and ZrO₂ co-additives can improve the microstructure and optical properties of Yb:Y₂O₃ ceramics at relatively low sintering temperature. On the other hand, the addition of Zr⁴⁺ ions leads to the formation of dispersed scattering volumes in the ceramic bodies. Transmittance of 78.8% was measured for the 2.0?at% Yb:Y₂O₃ ceramic sample at the wavelength of 1100?nm. The spectroscopic properties of Yb:Y₂O₃ ceramics were investigated at room temperature. The obtained results show that the absorption cross-section at 978?nm is in the range of 2.08?×?10^–20 to 2.36?×?10^–20 cm², whereas the emission cross-section at 1032?nm is ~1.0?×?10^–20 cm².     

The Effects of Na2O and SiO2 on Liquid Phase Sintering of Bayer Al2O3

To determine how grain‐boundary composition affects the liquid phase sintering of MgO‐free Bayer process aluminas, samples were singly or co‐doped with up to 1029 ppm Na₂O and 603 ppm SiO₂ and heated at 1525°C up to 8 h. Na₂O retards densification of samples from the onset of sintering and up to hold times of 30 min at 1525°C compared to the undoped samples, but similar to the as‐received, MgO‐free Al₂O₃, Na₂O‐doped samples sinter to 98% density with average grain sizes of ~3 μm after 8 h. Increasing SiO₂ concentration significantly retards densification at all hold times up to 8 h. The estimated viscosities (20?400 Pa·s) of the 0.3 to 1.8 nm thick siliceous grain‐boundary films in this study indicate that diffusion greatly depends on the composition of the liquid grain‐boundary phase. For low Na₂O/SiO₂ ratios, densification of Bayer Al₂O₃ at 1525°C is controlled by diffusion of Al³⁺ through the grain‐boundary liquid, whereas for high Na₂O/SiO₂ ratios, densification can be governed by either the interface reaction (i.e., dissolution) of Al₂O₃ or diffusion of Al³⁺. Increasing Na₂O in SiO₂‐doped samples increases diffusion of Al³⁺ and Al₂O₃ solubility in the liquid, and thus densification increases by 1%. Based on these findings, we conclude that Bayer Al₂O₃ densification can be manipulated by adjusting the Na₂O to SiO₂ ratio.     

Influence of Yb and Si on the fabrication of Yb:YAG transparent ceramics using spherical Y2O3 powders

Yb:YAG (yttrium aluminum garnet) transparent ceramics were fabricated by the solid-state method using monodispersed spherical Y₂O₃ powders as well as commercial Al₂O₃ and Yb₂O₃ powders. Pure YAG phase was obtained at low temperature due to homogeneous mixing of powders. Under the same sintering conditions, the Yb:YAG ceramics with different doping contents of Yb³⁺ had similar morphologies and densification rates. After being sintered at 1700 °C in vacuum, the ceramic samples had high transparencies. The Yb:YAG ceramics doped with 0.5 wt% SiO₂ formed Y–Si–O liquid phase and nonstoichiometric point defects that enhanced sintering. Compared with Nd doping, Yb doping hardly affected the YAG grain growth, sintering densification or optical transmittance, probably because Yb³⁺ easily entered the YAG lattice and had a high segregation coefficient.     

Densification of nanocrystalline Y2O3 ceramic powder by spark plasma sintering

Nanocrystalline Y₂O₃ powders with 18 nm crystallite size were sintered using spark plasma sintering (SPS) at different conditions between 1100 and 1600 °C. Dense specimens were fabricated at 100 MPa and 1400 °C for 5 min duration. A maximum in density was observed at 1400 °C. The grain size continuously increased with the SPS temperature into the micrometer size range. The maximum in density arises from competition between densification and grain growth. Retarded densification above 1400 °C is associated with enhanced grain growth that resulted in residual pores within the grains. Analysis of the grain growth kinetics resulted in activation energy of 150 kJ mol^?1 and associated diffusion coefficients higher by 10³ than expected for Y³⁺ grain boundary diffusion. The enhanced diffusion may be explained by combined surface diffusion and particle coarsening during the heating up with grain boundary diffusion at the SPS temperature.     

Sintering and densification behavior of pure and alkaline earth (Ba2+, Sr2+ and Ca2+) substituted La2Mo2O9

The present work studies densification and grain growth of fast oxide ion conductor, lanthanum molybdenum oxide (La₂Mo₂O₉). Sintering variables are experimentally analyzed using sintering laws. This paper investigates sintering of pure as well as Ca²⁺, Sr²⁺ and Ba²⁺ doped compositions by linear shrinkage measurement using dilatometer, relative theoretical density calculation and microstructural analysis using scanning electron microscope. In this work fine powder prepared by citrate–nitrate auto-combustion synthesis is used as source of powders for different compositions. The investigations indicate that sintering is enhanced in compare to the powders prepared by the other synthesis routes reported earlier. The evaluation of activation enthalpy of pure La₂Mo₂O₉ and its dependence with dopant addition are also studied with the help of sintering models and equations. The sintering mechanism is found to be dominated by grain boundary diffusion. The sintering inhibition is observed for the substituted compositions.     

Densification of Y2O3 by flash sintering under an AC electric field

Frequency dependence of the densification behavior of undoped Y₂O₃ sintered by the AC-flash sintering was systematically investigated at 500 V·cm^?1 over a frequency range from 0.05 Hz to 1 kHz. The Y₂O₃ bodies sintered under an AC field showed a uniform microstructure, without an asymmetric grain size distribution between the electrodes. Almost fully-densified Y₂O₃ body was consolidated at 1 kHz exhibited a relative density greater than 99 % and an average grain size of 1.6 μm. The almost full densification probably resulted from the high input power at the relatively high onset temperature of 1300 °C at this frequency. The temperature dependence of the power dissipation during the AC-flash sintering experiments can be ascribed to the periodic fluctuations of the specimen temperature at low frequencies and to the phase shift between the applied field and the specimen current at high frequencies.     

Sintering and grain growth kinetics in La0.85Sr0.15MnO3–Ce0.9Gd0.1O1.95 (LSM–CGO) porous composite

The sintering kinetics in La_0.85Sr_0.15MnO₃–Ce_0.9Gd_0.1O_1.95 (LSM–CGO) porous composite was studied by applying a two-stage master sintering curve (MSC) approach and comparing with LSM and CGO single-phase materials. In the two-stage MSC, sintering mechanisms occurring at different stages were separated with respect of density, giving a typical apparent activation energy values for each sintering stage of the LSM–CGO system. Compared with the single-phase materials, retardant effect of the different phases on mass diffusion leads to much higher apparent activation energy for densification of the composite. Similarly, constrain effect was also observed in grain growth in the composite. Particularly, in the investigated temperature range (1100–1250 °C), the determined grain boundary mobility of CGO in the LSM–CGO composite (10⁻¹⁸–10⁻¹⁶ m³ N⁻¹ s⁻¹) is comparable with the single-phase CGO, while the grain boundary mobility of LSM in the composite (10⁻¹⁷–10⁻¹⁶ m³ N⁻¹ s⁻¹) is around 1 order of magnitude smaller than the single-phase LSM.     

Influence of Gd2O3 on the microstructure and properties of transparent MgAl2O4: Cr3+ ceramic

In this work, MgAl₂O₄: Cr³⁺ transparent ceramics have been synthesized by the hot press sintering techniques, and the effect of the sintering aid Gd₂O₃ and its content on the densification, microstructure, and optical, photoluminescence was studied and discussed. The relative density reached 99.29% with 0.8 wt% Gd₂O₃ as a sintering aid, and the optical transmittance at 686 nm and 1446 nm were approximately 76%. As Gd₂O₃ content continued to increase, the grain size of the ceramics became smaller and uniform, accompanied by some pores with the size of ～1 μm. The ceramics with 4.0 wt% Gd₂O₃ showed a higher transmittance, of 82% at 1446 nm. Additionally, Gd₂O₃ was helpful for Cr³⁺ in the sites of octahedral symmetry, which increased the quantum yield. The quantum yield of MgAl₂O₄: Cr³⁺ with 0.8 wt% Gd₂O₃ was about 0.175, which was 36% higher than that of ceramic without Gd₂O₃. In short, the sintering aid Gd₂O₃ not only contributed to improving the densification, homogenizing the grain size, and heightening the optical transmittance but also enhanced the quantum yield of Cr³⁺.     

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.