Ceramic Pellets Research Articles

Study on Sintering Kinetics of (Th x Ce1-x )O2 Ceramic Pellets Prepared via Sol-Gel Method

(ThxCe1-x)O2 microspheres (x = 0.50, 0.75, and 0.95) prepared by sol-gel microsphere technique were compacted to pellets. The sintering kinetics, diffusion mechanism, and activation energy of the (ThxCe1-x)O2 pellets were investigated by dilatometry for 1100°C, 1200°C, and 1300°C. The rate controlling sintering method, one of the most sensitive methods, was chosen to investigate the sintering kinetics. The pellets were heated with a rate of 10°C/min and were held for 10 h at the above mentioned temperatures under isothermal conditions. The activation energies for the (Th0.50Ce0.50)O2, (Th0.75Ce0.25)O2, and (Th0.95Ce0.05)O2 pellets were calculated as 305, 315, and 419 kJ·mol−1, respectively. In the experiments, green densities of the mixed-oxide pellets were determined as 45% to 47% of the theoretical density for all of the studied ratios. Sintering densities reached up to 94% of theoretical density after sintering at 1300°C. Scanning electron microscopy images of the pellets were taken.

Aluminum-Assisted Densification of Cosputtered Lithium Garnet Electrolyte Films for Solid-State Batteries

Garnet Li7La3Zr2O12 (LLZO) is a promising solid-state electrolyte due to its wide electrochemical stability window and high Li-ion conductivity. This electrolyte has potential to be employed in the form of thin films for solid-state batteries, a promising approach in the quest for safer batteries with higher energy densities at lower fabrication costs. In this study, we use a scalable cosputtering process to fabricate LLZO thin films with subsequent postannealing at a temperature of 700 °C, significantly below the sintering temperatures employed in ceramic pellet processing. We investigate the roles that Li excess and incorporation of Al play in the film’s crystalline phase, microstructure, phase stability, and, ultimately, ionic conductivity. Our results reveal that improving the conductivity of LLZO thin films requires not only the stabilization of the cubic phase but especially the densification of the film and the minimization of the proton exchange degradation mechanism in the presence of moisture and CO2. These issues can be mitigated by effectively controlling the amount of Li and incorporating Al as sintering agent. An ionic conductivity at room temperature of 1.9 × 10–5 S cm–1 was achieved with a 400 nm Al-substituted LLZO thin film. Finally, we prove that these LLZO thin films can be successfully deposited and crystallized on a LiCoO2 cathode.

Preparation and scintillating study of ZnO: Hf ceramic to gamma rays

In this work pure and Hf-doped ZnO nanopowders were synthesized by cost-effective sol-gel technique to prepare ceramic pellets by isostatic pressing. Structural, optical and morphological properties of prepared samples were examined by XRD, FESEM, TEM, EDX, UV–vis, FTIR, Raman and PL measurements. Also the scintillation sensitivity of ceramics was studied by a 60Co radiation source. X-ray diffraction pattern analysis confirmed the hexagonal wurtzite structure of samples; Rietveld refinement for crystallite structure determined the lattice parameters of the nanopowders. FTIR and Raman results confirmed the presence of functional group and chemical bonding attributed to ZnO. FESEM image revealed that the size of HZO nanoparticles was obtained a bout 62 nm. For the first time prepared HZO ceramic showed suitable scintillation sensitivity to gamma ray and exhibited strong violet, blue and red emissions at room temperature. Fabricated ceramic showed good characteristics that can be applied for future photonic applications.

Fission gas release in the micro-cell fuel pellet under normal operating conditions: A simplified approach based on UO2 pellet experience

Following the Fukushima accident in March 2011, Korea Atomic Energy Research Institute (KAERI) has developed micro-cell fuel pellets for the purpose of reducing gas release, both stable and radioactive, not only under accident conditions, but during normal operation as well. The micro-cell pellet is composed of many cells covered with a ceramic or metallic wall material, whose function is either to lower or block the movement of gas atoms out of the cells, thereby reducing gas release out of the pellet. Based on both a Halden irradiation test of the micro-cell pellets up to 16 MWd/kgU and experience of fission gas release (FGR) in the conventional UO2 pellet, a simplified approach is proposed to estimate FGR in the micro-cell pellets when it is irradiated with a constant linear power of 30 kW/m up to 60 MWd/kgU under normal operating conditions. Considering the lack of measured data on diffusivity in the micro-cell pellets, sensitivity studies for FGR were performed for two cases: diffusivity in the micro-cell pellet is the same as and 10 times higher than in the UO2 pellet, respectively. Even for the 10 times higher diffusivity, FGR in the ceramic micro-cell pellet is at least comparable to or lower than in the UO2 pellet, and FGR in the metallic micro-cell pellet is clearly lower than in the UO2 pellet, due to its lower fuel temperature and the impact of its walls on gas movement. In real situations, the degree of reduction of FGR in the micro-cell pellets would be determined by the fraction of perfect walls and the magnitude of gas diffusivity. Until PIE results become available for the irradiated micro-cell pellets, the current approach provides only rough estimates and therefore its results need to be taken as preliminary.

Conduction and relaxation phenomena in barium zirconate ceramic in wet N2 environment

Proton conducting barium zirconate perovskite was prepared by conventional mixed oxide reaction route. The x-ray diffraction patterns proved the single phase cubic perovskite structure of the synthesized ceramic. SEM analysis of the ceramic pellets sintered at 1600 °C reveals the formation of dense microstructure with average grain size of 5–6 μm. Complex impedance spectroscopy study was performed over a wide range of frequency (10 Hz-5MHz) under wet N2 (pH2O = 0.031 atm) and dry N2 environment respectively varying the temperature from 300 °C to 600 °C. Two types of relaxation phenomena were evident from the impedance spectroscopic analysis, indicating the effect of grain and grain boundary to the overall resistance of the compositions. The conductivity due to grain and grain boundary contributions measured for the sample under wet N2 environment was found fairly one order higher in magnitude than that measured in dry N2 atmosphere respectively. The activation energy for conduction under wet N2 condition is found lower than that measured in dry conditions. The comparison of conductivity under two different environments reveals that additional charge species take part in conduction process under wet condition. These are attributed to the protonic defects which are incorporated into barium zirconate through extrinsic oxygen vacancies generated due to sintering at higher temperature.

Enhancement of the Ionic Conductivity in Electric Field-Assisted Pressureless Sintered BITIVOX Solid Electrolytes

Bi4V1.8Ti0.2O11 (BITIVOX) ceramic pellets, prepared with powders obtained by a sol gel technique, were sintered either conventionally at 800 °C/8 h or by applying an AC electric voltage, limiting the electric current through the pellets. Electric voltages were applied isothermally at 700 °C and 800 °C during 5 min in the green pellet positioned in the sample holder of a dilatometer for monitoring thickness variation. The BITIVOX pellets shrank 13.6% after applying 200 V cm−1 at 800 °C and 10.4% heating to 800 °C for 8 h. Thermal analysis and X-ray diffraction of the powders were performed to evaluate the crystallization temperature and the structural phase, respectively. The electrical behavior of the sintered BITIVOX pellets was analyzed by the impedance spectroscopy technique, showing that the sample flash sintered at 800 °C/5 min had lower bulk resistivity than the sample conventionally sintered at 800 °C/8 h. The surfaces of the sintered pellets were observed in a scanning electron microscope showing similar grain sizes and pore content in all sintered samples.

Solution deposition of Pt contacts suited to proton-conducting ceramics

We have developed a simple method for depositing high quality platinum (Pt) metal contacts without the necessity of placing the substrate in vacuum or heating it above 180 °C. This method is particularly beneficial for hydrated oxide ceramics, as it prevents water loss and subsequent conductivity degradation. A mixture of two organic solvents was used: ethylene glycol monoethyl ether (EEE) and acetone. By dispersing the precursor PtCl4 in this mixed non-volatile/volatile solvent, homogeneous drying patterns were achieved. EEE was used as the reducing agent, providing a relatively slow rate of Pt deposition, and thereby avoiding formation and subsequent coagulation of Pt clusters. Drop casting of the solution (25 μL/cm2) on mirror-polished surfaces was performed at 25 °C followed by 5 min partial solvent evaporation. The substrate was then heated to 180 °C for ~30 s, resulting in the formation of a metallic Pt layer. The method was successfully tested on 5 mol% Y doped BaZrO3 and 10 mol% Gd doped ceria ceramic pellets, alumina plates, and Kapton ® polymer sheets. The in-plane sheet resistance was 30–110 Ω at room temperature after a single deposition cycle. Because of its versatility and low cost per deposition, the proposed method is very promising for a variety of substrates where high quality electrical contacts prepared at low temperature are required.

The stability of aluminum oxynitride (AlON) powder in aqueous system and feasible gel-casting for highly-transparent ceramic

The stability of AlON powder in the aqueous system was studied during the gel-casting procedure of AlON ceramic. The pH value of AlON slurry, the phase assemblage and the morphology of AlON powder all suggested the excellent stability of untreated AlON powder even in hot water. Finally, highly-transparent AlON ceramic pellets (D = 110 mm and D = 22 mm) were fabricated by a feasible and easy aqueous gel-casting method. The polished AlON ceramics with 3.5 mm thickness possesses the highest inline transmittance above 86% at the wavelength of 2000 nm.

Effect of burnup on the mechanical properties of doped UO2 ceramic pellets using the simfuel technique

To determine the mechanical properties varying with burnup, the simulated 10 wt% ZrO2-doped UO2 ceramic pellets with burnups ranging from 0 to 250 GW d/tU were prepared by pressureless sintering in a hydrogen reducing environment. Mechanical properties such as hardness, elastic modulus and fracture toughness, as well as microstructures of the simulated pellet samples with different burnups (0, 75, 150 and 250 GW d/tU, respectively) were characterized by Vickers micro-indentation technique, X-ray diffraction, scanning electron microscopy and electron probe X-ray microanalysis, respectively. Results show that the relative densities of the simulated pellet pellets are between 93.5% and 95% and the measured elemental compositions are similar to the expected values. There are three distinct phases in the simulated 10 wt% ZrO2-doped UO2 fuel pellet samples, i.e., the matrix phase with the fluorite type structure, the oxide phase with perovskite type structure, and the metallic ε-phase. The lattice parameter of the simulated 10 wt% ZrO2-doped UO2 fuels gradually decreases with the increase of burnup. The hardness, elastic modulus and fracture toughness of simulated 10 wt% ZrO2-doped UO2 ceramic fuel pellets are significantly decreased by approximately 38.7%, 29.5% and 13.5% when the burnup increases from 0 to 250 GW d/tU, respectively.

Corrosion resistance and thermal-mechanical properties of ceramic pellets to molten calcium-magnesium-alumina-silicate (CMAS)

Because gas turbine engines must operate under increasingly harsh conditions, the degradation of thermal barrier coatings (TBCs) by calcium-magnesium-alumina-silicate (CMAS) is becoming an urgent issue. Mullite (3Al 2 O 3 ·2SiO 2 ) is considered a potential material for CMAS resistance; however, the performance of mullite in the presence of CMAS is still unclear. In this study, mullite and Al 2 O 3 –SiO 2 were premixed with yttria stabilized zirconia (YSZ) in different proportions, respectively. Porous ceramic pellets were used to conduct CMAS hot corrosion tests, and the penetration of molten CMAS and its mechanism were investigated. The thermal and mechanical properties of the samples were also characterized. It was found that the introduction of mullite and Al 2 O 3 –SiO 2 mitigated the penetration of molten CMAS into the pellets owing to the formation of anorthite, especially at 45 wt% mullite/55 wt% YSZ. Compared with Al 2 O 3 –SiO 2 , mullite possesses a higher chemical activity and undergoes a faster reaction with CMAS, thus forming a sealing layer in a short time. Additionally, the thermal expansion coefficient, thermal conductivity, and fracture toughness of different samples were considered to guide the architectural design. Considering the CMAS corrosion resistance, thermal and mechanical performance of TBCs systematically, a TBC system with a multilayer architecture is proposed to provide a theoretical and practical basis for the design and optimization of the TBC microstructure.

Effects of Bi2O3 Doping on the Mechanical Properties of PbO Ceramic Pellets Used in Lead-Cooled Fast Reactors.

In this paper, the effects of Bi2O3 doping on the mechanical properties of PbO ceramic pellets were studied. Different ratios of Bi2O3/PbO (i.e., xBi2O3-(1−x) PbO, where x is 0, 1, 3, 5, or 7 wt.%) were fabricated and sintered at 570, 620, and 670 °C. Mechanical properties including density, hardness, flexural strength, and sintering of PbO were studied for each of the aforementioned compositions. Phase composition, microstructure, and the worn surfaces of the composites were characterized by scanning electron microscopy and X-ray diffraction (XRD). The XRD analysis revealed that a solid solution formed in the composite ceramic. The best suited conditions of temperature and doping of Bi2O3 for optimal sintering were found to be 620 °C and 3 wt.%, respectively. The hardness of the 3 wt.% Bi2O3-97 wt.% PbO ceramic was found to be 717 MPa, which is about four times higher than the hardness of pure PbO. In addition, the strength of the composites was found to be 43 MPa, which is two times higher than that of pure PbO. The integrity of the composites was verified using the lead–bismuth eutectic alloy flushing experiment. The results of this research paper are important for future studies of oxygen control in the lead–bismuth eutectic alloy of lead-cooled fast reactors.

Rubbing induced strain-glass phase on ceramic BaTiO3 surface

Abstract A comparative microstructural study of as sintered and rubbed surfaces of BaTiO3 (BTO) ceramic pellet has been carried out using lab (8 keV Cu-Kα) as well as synchrotron (8–19 keV) based powder x-ray diffraction (XRD) at room temperature. These studies show a drastic change in the surface-microstructure of the BTO ceramics. Just a minutes rubbing of the BTO ceramics on a rough surface causes broadening of diffraction peaks. Nearly 3-times increase in the peak width and ∼ 3-times increase in integrated peak intensity of (00l) type peaks has been observed. The changes were analyzed as rubbing induced (00l) texturing and occurrence of anisotropic strain with maxima along [00l]. High temperature XRD studies reveal that for the strained BTO grains, the tetragonal to cubic transformation does not initiates at the proper TC ∼120 °C and does not complete even at temperatures ∼ 350 °C, which is ∼ 3 times the normal TC. About 20% of the tetragonal phase still remains untransformed. These features have been attributed to the formation of strain-glass phase at the rubbed surface of the BTO ceramics.

Electrical Behavior of Electric Field-Assisted Pressureless Sintered Ceria-20 mol% Samaria

CeO2:20 mol% Sm2O3 green ceramic pellets were sintered conventionally at 1500 °C/2 h and flash sintered by applying a 200 V cm−1 electric field at 800 °C, 1000 °C and 1200 °C. The thickness shrinkage of the pellets was followed bythe specimen being positioned inside a dilatometer adapted with platinum electrodes and terminal leads connected to a power supply for application of the electric voltage. The microstructure of the surfaces of the sintered samples were observed in a scanning electron microscope. The electrical properties were evaluated by the impedance spectroscopy technique in the 5 Hz–13 MHz frequency range from 210 °C to 280 °C. The main results show that (i) the final shrinkage level is nearly independent of the temperature when the electric field is applied and slightly better than that of the 1500 °C sintered pellet, and (ii) the bulk conductivity of the sample flash sintered at 1200 °C is similar to that of the sample sintered at 1500 °C. The availability of a pathway for the electric current pulse derived from the applied electric field is proposed as the reason for the achieved shrinkages. Scavenging of the grain boundaries by Joule heating is proposed as the reason for the improved oxide ion bulk conductivity.

Modeling fixed-bed reactors from metal-foam pellets with detailed CFD

Open-cell metal foams are a promising alternative to conventional solid pellets for the application as structured catalyst support. Pellets made from metal foams can be used in fixed-bed reactors instead of ceramic pellets. In this contribution, a comprehensive and detailed computational fluid dynamics (CFD) model is presented for fixed-bed reactors consisting of metal foams. The inner structure of the foams, i.e. pores, struts, and nodes, is not resolved. Instead, a pseudo-homogeneous model is applied to describe pressure drop and heat transfer inside the foam pellets. The fixed-bed structure is particle-resolved. The bed is generated synthetically with the discrete element method (DEM) and the results are in great agreement with µCT data. The experimental pressure drop for two different porous pellet shapes can be reproduced reasonably with CFD simulations. In an illustrative heat-transfer study, the performance of a state-of-technology solid-pellet bed and a porous cube bed is compared. Whereas the heat transfer is similar in both cases, the pressure drop is lower in the bed of porous cubes. A residence-time distribution analysis reveals that the porous pellet shape should be optimized to obtain a better mixing behavior. The presented fixed-bed model can be applied for such tasks, since it is valid for almost any pellet shape.

Synergistic effect of cation ordered structure and grain boundary engineering on long-term cycling of Li0.35La0.55TiO3-based solid batteries

In this study, Li0.35La0.55TiO3 (LLTO) was coupled with Al-doped lithium lanthanum zirconate (LLZO) to improve the grain boundary and total conductivity. The obtained ceramic pellets (LLTZO) demonstrated a recordable grain boundary and total conductivity of 3.41 × 10−4 and 3.03 × 10−4 S/cm, respectively. The obtained results establish that the heteroatoms can perturb the cation ordered structure and improve the 3D conductivity in grain bulk. In addition, the residual Al-doped LLZO on the grain boundary led to a decline in the boundary resistance. An LiFeCoPO4 |Li cell was adopted to demonstrate the enhanced conductivity of LLTO. The solid state battery rendered a specific capacity of over 101.2 mAhg−1 after 300 cycles at a relatively high rate of 0.5C. It is established from the experiments that manufacturing a solid battery using the all-coating technique provides a promising approach to achieve a practical application.

Microstructure and physical characterization of alumina-sintered body via hot isostatic pressing

Hot isostatic pressing technique is one of the alternative fabrication processes that have been used to enhance the densification rate in ceramic processing. The aim of this paper is to observe the microstructure and determine the physical and mechanical characterization of alumina ceramic pellets sintered via conventional technique compared to hot isostatic pressing. The semidry alumina ceramic pellets obtained from uniaxially pressing were dried in an oven earlier before sintering via two different techniques. The microstructure of alumina ceramic pellets analyzed by scanning electron microscopy that showed the pellet with particle size (25 μm) and sintered via hot isostatic pressing had larger uniform grain shape and size (16.25 μm), lowest porosity (20.76%), highest bulk density (3.60 g cm−3), low top (2.87%) and bottom (2.68%) shrinkage, and highest hardness (871.56 Hv). The optimum particle size of alumina and sintering technique (HIP) improved the microstructure and physical characterization of ceramic pellets.

Structural, optical and electrical characterizations of Ln6WO12 (Ln=La, Nd, Sm, Gd) nanoceramics

Nanoparticles of Ln6WO12 (Ln=La, Nd, Sm, Gd) abbreviated as LWO, NWO, SWO, and GWO are synthesised by combustion method. The synthesised materials are annealed at 900 °C and then subjected to different characterization techniques. XRD studies show that LWO crystallizes in cubic structure and NWO, SWO, and GWO crystallize in tetragonal structure. The size and nature of the nanoparticles are characterized through the TEM analysis. The Raman and FTIR studies conducted on the samples support the structural data obtained from the XRD data. The UV–visible absorbance is recorded and optical bandgap is calculated using the Tauc’s equation. The photoluminescence spectra of samples show that intense emission in green, blue, orange, and red regions and transitions responsible for the emissions are identified. Bulk ceramic pellets of LWO, NWO, SWO, and GWO prepared from their nanopowders are sintered and impedance studies are carried. The surface morphology of the sintered pellets of LWO, NWO, SWO, and GWO is studied by scanning electron microscopy. The frequency and temperature-dependent dielectric studies of prepared pellets are also carried out.

Ceramic conversion and densification of zirconium phosphonate sorbent materials

The simple conversion of zirconium phosphonate sorbent materials, with known affinity for lanthanide elements, to durable ceramic waste forms via thermal treatment has been demonstrated. The use of zirconium phosphonate enables both removal of targeted elements from spent nuclear fuel and immobilisation into leach resistant solid products to be achieved using a single material. Thermal conversion was performed on the zirconium phosphonate both before and after loading with europium, which acted as a surrogate for the chemically similar minor actinides. Without europium loaded, the zirconium phosphonate sorbent formed predominantly KZr2(PO4)3 upon heating, independent of the processing conditions used. A maximum relative density of 87% was achieved with cold isostatic pressing (200 MPa) and sintering at 1200 °C for 12 h. When the zirconium phosphonate sorbent was loaded with europium, the phase composition formed upon thermal treatment was more complex. Specifically, mixtures of ZrP2O7, Eu0.33Zr2(PO4)3, EuPO4 and Zr2O(PO4)2 were formed, with phase compositions depending on the temperatures and pressures used. The simplest phase composition for the europium loaded material was achieved via uniaxial pressing (120 MPa) and sintering at 1300 °C for 1 h, although the ceramic pellet produced under these conditions had a relative density of only 53%. The loaded europium deported primarily to a EuPO4 phase, which is known to be highly stable and leach resistant. As such, these zirconium phosphonate materials have potential utility for treatment of nuclear wastes.

Flash Sintering Samaria-Doped Ceria–Carbon Nanotube Composites

Composite ceramic green pellets were prepared by attrition milling a mixture of (CeO2)0.8(Sm2O3)0.2 (samaria-doped ceria, SDC) ceramic powder and carbon nanotubes (CNTs), followed by uniaxial and isostatic pressing. The pellets were sintered inside a dilatometer by applying AC electric fields at 850 °C and limiting the electric current to 1 A, achieving 20.2% final shrinkage. The SDC samples reached 13.3% shrinkage under the same conditions. Higher average grain sizes were measured in specimens flash sintered with CNTs. Impedance spectroscopy analyses show that the specimens flash sintered with addition of CNTs have higher electrical conductivity. Higher delivered Joule heating at the interfaces due to the presence of the electronic conductors (CNTs) are proposed as the main reason for that improvement of the electrical behavior.

Effect of sintering temperature on dosimetric properties of BeO ceramic pellets synthesized using precipitation method

BeO material has been extensively explored owing to its excellent performance in the electronics industry and for interest in ionizing radiation dosimetry. In this study, BeO samples were prepared using precipitation method, pelletized and then sintered at temperatures 1200, 1400 and 1600 °C to afford dried pellets with better crystallinity and luminescence characteristics. The X-ray diffraction (XRD) and Scanning Electron Microscope (SEM) results showed that the prepared pellets were polycrystalline in nature and crystallinity improved with the increase of sintering temperature. The X-ray (XL) luminescence spectra of BeO pellets sintered at 1200 and 1400 °C were obtained as a narrow peak in the region 200–350 nm with the maximum around 250 nm. The emission spectrum of the XL signal from 1600 °C sintered sample was obtained as a broad peak in the region 200–450 nm with two maximums around 250 and 300 nm. The optically stimulated luminescence (OSL) signals were observed for the preheating temperatures from 50 up to 500 °C. The OSL signal was found to originate from the TL traps near 170 and 275 °C for 1200 °C sintering temperature. While, the TL peaks at around 275 °C and 170 °C contribute to the OSL signals at the sintering temperatures 1400 and 1600 °C, respectively. It was investigated that the OSL signal exhibits strong thermal quenching. The thermal quenching energies were estimated as 0.61, 0.55 and 0.53 eV using the OSL signals obtained at various readout temperatures ranging from 50 °C to 110 °C and 0.57, 0.54 and 0.51 eV using the TOSL method, for the samples sintered at 1200, 1400 and 1600 °C, respectively. The dose responses of the total OSL signals are linear from 0.1 to 50 Gy for the sintering temperatures of 1200 and 1400 °C and 1600 °C. After the 24 h in which the shallow traps are responsible from the observed fading of the material, the fading of the total OSL signals was found as −3.1% and −4.3% associated with the 1200 and 1400 °C sintering temperatures, respectively. There was no fading with respect to the first reading for the samples sintered at 1600 °C.The present study may promote the application of BeO luminophore synthesized using precipitation technique and sintered at 1600 °C, in medical dosimetry.