Characterization of ThO2 and (Th,U)O2 pellets consolidated from NSD-sol gel derived nanoparticles

Sinterable ThO₂ and mixed 70%ThO₂-30%UO₂ nanopowders were synthesized and calcined at a low temperature by surfactant assisted sol-gel process using three different nonionic structure directing (NSD) agents: TritonX100, Polysorbate 80 and Polysorbate 20. The produced powder samples, with high surface area and micro and meso pores, were pressed and sintered to form high density ThO₂ and (Th,U)O₂ pellets. The calculated green and sintered densities of the fabricated pellets, and the results of their microstructural characteristics studies, utilizing SEM images, show that developed synthesizing method yields a satisfactory sinterability of the synthesized ThO₂ and ThO₂-UO₂ nanopowders at low temperature. Also, the final grain size of the pellets reached less than a micrometer in size. The ThO₂ and (Th,U)O₂ pellets fabricated by synthesized powder using Triton X100 had a higher density and better grain growth. The uranium presence in the mixed oxide pellets led to better sinterability and more normal grain growth.     

Transmission Electron Microscope Study of Antimony-Doped Zinc Oxide Ceramics

A transmission electron microscope investigation of a ZnO powder and low-temperature air-sintered pellets with small additions of Sb₂O₃ was conducted in order to study the thin Sb-rich film on the surfaces of ZnO particles. This film was found to be noncrystalline at temperatures below 700°C, and partially crystalline or completely crystalline above this temperature. An oriented overgrowth of the spinel Zn₇Sb₂O₁₂ was observed on the prismatic planes as well as on the basal planes of ZnO crystallites. It is shown that these coherently overgrown spinel films are responsible for the inhibition of grain growth at high temperatures.     

Formation of translucent nanostructured zirconia ceramics

In this work the mechanisms that affect the optical transparency of nanostructured translucent ZrO₂ ceramics are studied. The translucent ceramic samples were obtained from a low agglomeration nanosized powder at low pressure and low temperature sintering. Even low pressures cause structural changes and defect creation in the nanocrystals. Annealing was used to study the grain formation, structure and impact of defects. Significant changes in translucency were observed with increase in pore size. In order to further understand the defect creation, the obtained ceramics were doped with Er³⁺ ions and studied optically. Photoluminescence studies revealed a change in the ratio of green to red parts of the spectrum as well as luminescence quenching when samples were pressed into pellets. Additionally, grain and pore size dependence on annealing temperatures was studied using X-ray diffraction, scanning electron microscopy and transmission electron microscopy.     

Hybrid Microwave Sintering of Infrared Transparent Nano‐Y3Al5O12 Synthesized by a Modified Combustion Technique

Infrared transparent ceramics found to have numerous civilian and defense applications. In the present work, Y₃Al₅O₁₂ nanoparticles were synthesized by an auto‐igniting modified single‐step combustion method. The structure and morphology of the as‐prepared powder revealed the phase purity and ultrafine nature of the powder having an average crystallite size of 16 nm and well‐defined lattice planes. Coupling of the resistive and microwave heating at precise proportion leads to a sintered density of the powder with 99.3% of the theoretical density at a temperature as low as 1470°C for a soaking duration of just 20 min. Marked reduction in grain size and the porosity was also observed for the hybrid sintered pellets. An average grain size of 167 nm was measured for the sintered pellets, which also showed a high transmittance of 80% in the UV–vis region and 82.5% in the mid‐IR region.     

Formation of Varistor Characteristics by the Grain-Boundary Penetration of ZnO-PrOx Liquid into ZnO Ceramics

Penetration of a liquid (ZnO-PrO _x ) into the grain boundaries of sintered, cobalt-doped ZnO pellets resulted in varistors with breakdown voltages per grain boundary in the 1-2 V range and nonlinearity coefficients of 22-37. The varistors were fabricated by spreading a thin layer of Pr₆O₁₁ powder paste on the surface of ZnO pellets and heating to various temperatures (1200°-1400°C) and times (0-60 min). Comparing the varistor properties per grain boundary (e.g., threshold voltage, donor concentration, and barrier height) of liquid penetration to those of conventional method indicated the individual grain boundaries were electrically activated when the samples were heat-treated above liquid-phase formation temperature.     

空调压缩机用ZrO2密封保护陶瓷垫片的制备

对空调压缩机ZrO2密封保护绝缘垫片的配方和工艺进行了探索研究。制品经压制、烧结、抛光等工艺制成。经配方优化后,所得的产品表面光洁度高,致密光滑,维氏硬度（HV10）达到1200以上。产品断面的SEM分析表明,改进后的产品弥散在晶界之间的玻璃相较多,能起到有效的封孔作用。     

Fabrication of polycrystalline (Y0.7Gd0.3)2O3:Eu ceramics: The influence of initial pressure and sintering temperature on its morphology and photoluminescence activity

Nanocrystalline (Y_0.7Gd_0.3)₂O₃ powder, synthetised via polymer complex solution method, was compacted into 25 pellets applying high pressures (173-867 MPa) for 30 s that were subsequently sintered at different temperatures (800-1400 °C) for 18 h. The morphology and optical characteristics of the starting powder and prepared ceramic samples were monitored and discussed in order to identify the changes induced with the variations of initial compacting pressure, which influence is often neglected, and with sintering temperature. The grain size tends to decrease significantly with increasing pressure, even when elevated temperatures are used for annealing, while low compacting pressure resulted in grain coarsening and, in some cases, even in anomalous morphology of ceramic samples. Luminescence emission in ceramic samples decays faster than in nanopowders, that is in complete agreement with the grain formation and gradual transformation to the bulk material. Judd-Ofelt intensity parameters and branching ratios were calculated taking into account the difference in effective refractive index for nanopowder and ceramic samples.     

Effect of External Lubricant on Mechanical Properties of Dry-Pressed Green Bodies

Tensile strength and fracture toughness of uniaxially compacted spray-dried ferrite powders, with and without external lubricant were measured. The addition of external lubricant significantly increased the fracture strength and decreased the strength variability. The external lubricant also led to an increase in fracture toughness and a decrease in the critical flaw size. Scanning electron microscopy examination of fracture surfaces showed an increase in the amount of transgranular fracture for the pellets compacted from the externally lubricated powder. It was concluded that the external lubricant strengthened the bonds between the spray-dried granules, thereby increasing the fracture toughness and reducing the critical flaw size. X-ray tomography of pellets showed that there was no appreciable difference in density gradients between the samples.     

Low Breakdown Voltage Varistors by Grain Boundary Diffusion of Molten Bi2O3 in ZnO

Diffusion of molten Bi₂O₃ into the grain boundaries of sintered, alumina-doped (0.23 and 0.7 mol%) ZnO pellets resulted in varistors with breakdown voltages in the 3–5 V range and nonlinearity coefficients of 10–24. The varistors were fabricated by spreading a thin layer of Bi₂O₃ powder on the surface of ZnO pellets and heating the combination to various temperatures (860–1155°C) and different times. The highest nonlinearity coefficients (20–24) and lowest breakdown voltages (3–5 V) were recorded in samples annealed at 860°C for 35 min. Longer annealing times and/or higher temperatures resulted in progressively higher breakdown voltages. Eventually the devices became insulating, which was attributed to the formation of an insulating Bi₂O₃ layer between the grains. Separate wetting experiments have shown that the penetration of Bi₂O₃ into ZnO grain boundaries was a strong function of alumina doping —the penetration rate was decreased by a factor of 5–7 as the ZnO was doped with as little as 0.2 mol% alumina. It is this slowing down of the penetration of the ZnO grain boundaries that is believed to be critical in the development of the low breakdown voltages observed.     

Grain size-dependent magnetic and electric properties in nanosized YMnO<Subscript>3</Subscript> multiferroic ceramics

Magnetic and electric properties are investigated for the nanosized YMnO₃ samples with different grain sizes (25 nm to 200 nm) synthesized by a modified Pechini method. It shows that magnetic and electric properties are strongly dependent on the grain size. The magnetic characterization indicates that with increasing grain size, the antiferromagnetic (AFM) transition temperature increases from 52 to 74 K. A corresponding shift of the dielectric anomaly is observed, indicating a strong correlation between the electric polarization and the magnetic ordering. Further analysis suggests that the rising of AFM transition temperature with increasing grain size should be from the structural origin, in which the strength of AFM interaction as well as the electrical polarization is dependent on the in-plane lattice parameters. Furthermore, among all samples, the sample with grain size of 95 nm is found to have the smallest leakage current density (< 1 μA/cm²).     

Microstructure and Oxide Ion Conductivity in a Dense La9.33(SiO4)6O2 Oxy-Apatite

Oxy-apatites are one of the most promising oxide ion conducting electrolytes for intermediate-temperature solid oxide fuel cells, those operating close to 950 K. La_9.33(SiO₄)₆O₂ has the simplest stoichiometry among the oxy-apatite family of compounds, and the range of oxide ion conductivity reported for dense ceramic pellets is very high, almost two orders of magnitude at a given temperature. The spread in conductivity values including porous pellets is obviously larger. Here, we show that dense pellets of La_9.33(SiO₄)₆O₂ ceramics prepared by three different methods, conventional, spark plasma, and reaction sintering (RS), have different bulk conductivities. Bulk activation energies are equal for all the samples studied. Furthermore, the grain boundary conductivity cannot be described by the simple brick-layer model. These experimental findings suggest segregation at the grain boundary region, with slightly different compositions for different sintering conditions, likely leaving the grain interior with a compositional gradient. A two-step RS led to dense pellets with the highest bulk and grain boundary oxide ion conductivities.     

A Novel Processing Route to Control Grain Growth in Submicrometer Alumina Compacts

A novel processing procedure for significantly suppressing grain growth in submicrometer alumina compacts has been developed and implemented with the intent of ultimately using the same processing route to control grain size in nanophase alumina compacts. In this study, partially sintered alumina pellets made from 0.5 µm starting powders are altered by the chemical infiltration of Si₃N₄. The control and infiltrated pellets are then heated to 1650°C for 4 h. The fully sintered pellets are approximately 97% dense. Suppressed grain growth is observed in the infiltrated pellets. The average grain size in the control pellets after densification is 4.2 and 1.2 µm in the infiltrated pellets. Depth of infiltration is measured using transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Depending on the specific infiltration conditions used, the outer 15-50% of the infiltrated pellets exhibit a graded microstructure consisting of a region of abnormal grain growth and a region of suppressed grain growth. Abnormal grain growth is visible on the outer surfaces of the infiltrated pellets where a relatively high ratio of Si to N is present. Further into the pellet, after some depletion of the Si source gas has occurred, regions of suppressed grain growth are apparent. Based on these results, an infiltration profile is determined. A mathematical model is developed to describe the infiltration process and to determine optimal infiltration conditions. High-resolution electron microscopy (HREM), energy dispersive spectroscopy (EDS), electron energy loss spectroscopy (EELS), and X-ray photoelectron spectroscopy (XPS) are used to study the infiltrated samples.     

Effect of segregation on particle size stability and SPS sintering of Li2O-Doped magnesium aluminate spinel

Magnesium aluminate spinel (MAS) was prepared using the simultaneous precipitation method by varying the concentration of Li₂O from 0 to 5 mol%. No residual chlorine from the LiCl precursor was detected in the final powders while Li achieved the target concentration in all samples and contributed to stabilizing nanoparticles smaller than 10 nm. Li segregation to both interfaces (surfaces and grain boundaries) occurred and tended to be more pronounced at the grain boundaries stabilizing this type of interface during processing rather than surfaces. Spark plasma sintering (SPS) was used to consolidate the nanopowders into fully dense nanostructured pellets. The increase in Li content facilitated the sintering process and pore elimination occurred at 850–900 °C, a much lower temperature range as compared to conventional sintering (1650 °C). Samples containing 5 mol% Li sintered at 850 °C exhibited a medium grain size of ?25 nm, microhardness of ?24 GPa and ?50% in-line optical transmission at the 800 nm.     

Fractal Networks of Inter‐Granular Voids in Pressed TATB

Small‐angle neutron scattering techniques were used to study the evolution of void morphology with pressed density of the insensitive high explosive, TATB. Samples were studied as a loose powder and as pressed pellets, ranging in density from approx. 1 to 1.804 g cm⁻³. Inter‐granular voids in the loose powder were randomly arranged (non‐fractal) and had a surface defined mean size of 0.66 μm. Pressing was found to induce a fractal network of voids with fractally rough interfaces. The surface‐defined mean void size of the pressed samples was between 0.21–0.33 μm over the range of densities studied and was found to increase with pressed density up to 1.720 g cm⁻³, decreasing thereafter. The volume fractal dimension, indicative of the void arrangement, mirrored the changes in the mean void size. No systematic change in the surface fractal dimension was found. Surface area analysis allowed the average TATB grain size within the pressed samples to be quantified. An initial decrease of the mean grain size followed by an increase with pressed density suggests that the TATB grains behave in a brittle fashion at low densities and ductile at higher pressed densities.     

The Change of X‐ray Diffraction Peak Width During in situ Conventional Sintering of Nanoscale Powders

Diffraction peaks of nanoscale particles of 3 mol% yttria‐stabilized zirconia become sharper as the powder sinters. The reduction in the peak width is correlated with the increase in density. The sharpening of the peak agrees reasonably well with the remaining free surface area as the sample sinters. Therefore, high curvature of the free surface of the pores is assumed to lead to peak broadening (the grain boundaries that grow at the expense of the free surfaces of the pores do not have this curvature). The change in the grain size during sintering does not make a significant contribution to peak width.     

Preparation of Y2O3:Eu nanopowders via polymer complex solution method and luminescence properties of the sintered ceramics

Herein we presented polymer complex solution method for production of well crystalline europium doped Y₂O₃ nanopowders. Polyethylene glycol (PEG) of five different molecular weights is used both as a fuel and as a nucleation agent for the crystallization. Powders were cold-pressed and sintered to obtain ceramics. SEM images taken from ceramic pellets indicate formation of a dense structure, with a pronounced grain growth and low pore concentration. Luminescence emission spectra of powders and ceramics are similar, and in good agreement with theoretical data. Lifetimes of Eu³⁺⁵D₀ level in nanocrystalline powders are higher compared to one observed in bulk, confirming in this case theory of lifetime lengthening in nanophosphors due to the change of effective refraction index. As expected, lifetime values in ceramic samples decrease toward the value in bulk Y₂O₃. The optical filling factor is calculated from observed decay times, providing a measure of discrepancy between powder and bulk state regarding their luminescent properties.     

Grain Boundary Oxidation in PTCR Barium Titanate Thermistors

The phenomenon of grain boundary oxidation in PTCR BaTiO₃ thermistors is discussed. In particular, the energy spectra of the surface states were calculated for different samples, and these were related to the nominal composition, the impurity content of the base BaTiO₃ powder used, and the prevalent atmospheric conditions during cooling and/or annealing. It is proposed that the interaction of manganese with oxygen creates deep-lying traps, and, in general, some proof is offered that the majority of the surface states are due to different oxidizing chemisorbed gases. It is believed that the ability of a particular sample to adsorb such gases in adequate amounts, and thus exhibit an appreciable PTCR effect, is related to the presence of acceptor-type dopants perferentially segregated onto the grain surfaces. Notably, the role of 3 d transition metal cations in this process is discussed in some detail.     

Microwave-assisted synthesis and sintering of mullite

Mullitization behaviour of a mixture of clay and alumina as the starting materials was examined by microwave heating of (a) mixed powder and (b) compacted powder samples for different soaking times. X-ray diffraction results showed that in compacted samples mullitization process was completed after 20 min heating with a density of about 87%. Densification and microstructure of samples with different green densities heated in a microwave oven and conventional electric furnace were compared. Results showed that the grain growth of mullite was restricted by microwave heating.     

Sintering and Phase Evolution of Electroless-Nickel-Coated Alumina Powder

Alumina-nickel powders have been prepared via electroless-nickel (EN) coating of submicrometer-sized alumina powder. The EN layers contain 5.1 ± 0.4 wt% phosphorus and are very unevenly distributed on the surfaces of the alumina particles. These layers consist of amorphous and microcrystalline phases. At temperatures greaterthan equal to1300°C, the EN layers de-wet from the alumina surfaces to become discrete, round particles in the alumina matrix. The alumina-EN pellets can be sintered to reach ~95% of the theoretical density at 1500°C for 4 h in graphite crucibles. The phases of the sintered samples consist of alumina, nickel, and nickel phosphides. However, in the exterior region of the sample sintered at 1500°C for 4 h, Ni₃Al will also form.     

Sub-micrometre grained UO2 pellets consolidated from sol gel beads using spark plasma sintering (SPS)

UO₂ beads from the sol supported precipitation method were calcined at a low temperature in order to obtain porous micro-beads, composed of nanometric particles. The sintering behaviour of the beads in spark plasma sintering was investigated. The powder had a good sinterability and the final grain size of the pellets could be tailored by varying the processing conditions, in order to resemble the microstructure of the traditionally fabricated UO₂ pellets (i.e. grains of several µm size), or to achieve sub-micrometre size as observed in the high burnup structure. Dense UO₂ pellets with a grain size as small as 300 nm were obtained by sintering at 835 °C without dwell time, whereas 3 µm grained pellets were obtained at 1000 °C and a 5 min dwell time.