Tape casting of nanocrystalline ceria gadolinia powder

A ceramic ceria gadolinia solid solution membrane for solid oxide fuel cells was fabricated by tape casting using a nanopowder of 37 nm average particle size. A novel combination of solvent and dispersant was used to disperse the nanoparticles. The polymer was added in a dilute stage to guarantee a homogeneous distribution. After casting a remarkable densification of the cast tape suspension from a solid loading of 20 up to 42 vol.% was observed during drying. The green tape was sintered to >92% theoretical density and was dense towards perfusion. The resulting grain size in the sintered specimen still was <200 nm.     

Structure characterization and mechanical properties of CeO2–ZrO2 solid solution system

The measured and calculated lattice parameters, microstructures, and mechanical properties (fracture toughness and microhardness) of CeO₂–ZrO₂ system ceramics are investigated, using CeO₂–ZrO₂ solid solution powder prepared by a microwave-induced combustion process. The CeO₂–ZrO₂ solid solution ceramics were sintered at 1500 °C for 6 h in air; the density of all specimens was greater than 94% of the theoretical density. For Ce_1−xZr_xO₂ (0.00  x  0.50), the measured lattice parameter is in accordance with that of Kim's doped CeO₂ model_. On the other hand, for x  0.50, the measured values fit Kim's doped ZrO₂ model. The fracture toughness and microhardness of CeO₂–ZrO₂ system ceramics with various compositions were investigated with Vickers indentation. The results showed that the crack mode of CeO₂–ZrO₂ solid solution was Palmqvist cracks under loads of 1 kg. Generally, the fracture toughness should increase with grain size at the submicron scale. However, larger grains may lead to spontaneous transformation, which should decrease the potential toughening at room temperature. This behavior was observed in the Ce_0.25Zr_0.75O₂ ceramic, which demonstrated a high fracture toughness that may be ascribed to two causes: (1) fine grain size and (2) transformation toughening.     

Processing and performance of solid oxide fuel cell with Gd-doped ceria electrolyte

Fuel Cell performance was measured at 792-1095 K for Ni-GDC (Gd-doped ceria) anode-supported GDC film (60 μm thickness) with a (La_0.8Sr_0.2)(Co_0.8Fe_0.2)O₃ cathode using H₂ fuel containing 3 vol% H₂O. A maximum power density, 436 mW/cm², was obtained at 1095 K. The electrical conductivity of GDC electrolyte in N₂ atmosphere of 10⁻¹⁵-10⁰ Pa oxygen partial pressures (Po₂) at 773-1073 K was independent of Po₂, which indicated the diffusion of oxide ions. The conductivity of GDC in H₂O/H₂ atmosphere increased because of the further formation of electrons due to the dissociation of hydrogen in GDC (H₂ → 2H⁺ + 2e⁻). The hole conductivity was observed at 873 K in Po₂ = 10⁰-10⁴ Pa. The key factors in increasing power density are the increase of open circuit voltage and the suppression of H₂ fuel dissolution in GDC electrolyte. These are controlled by the cathode material and Gd-dopant composition.     

Hydrogen formation from a real biogas using electrochemical cell with gadolinium-doped ceria porous electrolyte

The electrochemical cell consisting of a gadolinium-doped ceria (GDC, Ce_0.9Gd_0.1O_1.95) porous electrolyte, Ni–GDC cathode and Ru–GDC anode was applied for the dry-reforming (CH₄+CO₂→2H₂+2CO) of a real biogas (CH₄ 60.0%, CO₂ 37.5%, N₂ 2.5%) produced from waste sweet potato. The composition of the supplied gas was adjusted to CH₄/CO₂=1/1 volume ratio. The supplied gas changed continuously into a H₂–CO mixed fuel with H₂/CO=1/0.949–1/1.312 vol ratios at 800 °C for 24 h under the applied voltage of 1–2 V. The yield of the mixed fuel was higher than 80%. This dry-reforming reaction was thermodynamically controlled at 800 °C. The application of external voltage assisted the reduction of NiO and the elimination of solid carbon deposited slightly in the cathode. The decrease of heating temperature to 700 °C reduced gradually the fraction of the H₂–CO fuel (61.3–18.6%) within 24 h. Because the Gibbs free energy change was calculated to be negative values at 700–600 °C, the above result at 700–600 °C originated from the gradual deposition of carbon over Ni catalyst through the competitive parallel reactions (CH₄→C+2H₂, 2CO→C+CO₂). The application of external voltage decreased the formation temperature of carbon by the disproportionation of CO gas. At 600 °C, the H₂–CO fuel based on the Faraday's law was produced continuously by the electrochemical reforming of the biogas.     

Processing and properties of ZrB2-SiC composites obtained by aqueous tape casting and hot pressing

ZrB₂-SiC composites with different SiC content were prepared through aqueous tape casting and hot pressing. The influences of dispersant, SiC content and binder content on the rheological properties of slurries were investigated and the conditions for preparing stable ZrB₂-SiC suspensions were optimized. After tape casting and drying, the green ZrB₂-SiC tapes showed good flexibility, lubricious surface and homogeneous microstructure. The ZrB₂ ceramics could be densified to 97.2% after hot-pressing, while the ZrB₂ containing 20 and 30 vol% SiC ceramics were nearly fully densified (>99%). The sintered ZrB₂-20 vol% SiC ceramic had improved mechanical properties compared with ZrB₂ ceramic. Further increase in SiC content resulted in lower flexural strength and fracture toughness. SEM and TEM showed a fine microstructure with a clear grain boundary. The fracture mode changed from intragranular type for ZrB₂ to both intragranular and intergranular type for ZrB₂-SiC composites.     

Effect of precursor characteristics on zirconia and ceria particle morphology in spray pyrolysis

Ultrasonic spray pyrolysis of acetate-based precursors with precisely measured precursor drop size was employed to produce ZrO₂ and CeO₂ particles. A bimodal size distribution of the product particles indicates a significant influence of the gas-to-particle conversion mechanism in addition to the conventionally accepted one-particle-per-drop mechanism. Due to the differences in solubility of the precursors, ZrO₂ particles are spherical in shape and smooth on their surfaces while the CeO₂ particles are bowl-like in shape with uneven surfaces. Spherical and monodispersed particles with a peak diameter <100 nm can be obtained by reducing the precursor concentrations to 0.01 wt.% in both the different precursor system.     

Growth of 20 mol% Gd-doped ceria thin films by RF reactive sputtering: The O2/Ar flow ratio effect

The search for optimal materials and the utilization of proper manufacturing techniques to replace conventional electrolytes are our research objectives for the operation of solid oxide fuel cells under intermediate temperatures. Furthermore, understanding the effects of process parameters will be helpful for obtaining suitable materials for applications. In this study, we investigate the O₂/Ar flow ratio effect by employing RF reactive sputtering to fabricate 20 mol% Gd-doped ceria (20GDC) films on alumina substrates. The morphology of films was aggregated by nano-scale size of grains which gradually reduced in size from lower to higher O₂/Ar flow ratios. The microstructure of films was transferred from incomplete oxidized materials to well-crystallized cubic fluorite structures using an increased O₂/Ar flow ratio up to 0.30. The oxygen/metal ratio of films was increased gradually and saturated around 2.05 for O₂/Ar flow ratios over 0.25 and remained in uniform composition through whole films for each flow ratios.     

Aqueous tape casting of B4C ceramics

B₄C green tapes are prepared by aqueous tape casting and a spark plasma sintering (SPS) process using polyethylenimine (PEI) as dispersant, hydroxypropyl methyl cellulose (HPMC) as binder and polyethylene glycol (PEG) as plasticiser. The influences of solid content, dispersant content, mass ratio of plasticiser to binder (R value) and milling time on the slurry viscosity are studied. The samples are characterised by means of hardness tester, universal testing machine and scanning electron microscopy. The results indicate that the solid content of B₄C slurry achieves 47.5?wt-% with milling time of 12?h when the content of PEI, HPMC and PEG is 1.5, 5 and 5?wt-%, respectively. The relative density of B₄C ceramics subject to SPS at 1600°C and 50?MPa for 8?min is up to 97.2%. The Vickers hardness, flexural strength and fracture toughness of B₄C ceramics reach 36.5?±?0.7?GPa, 510.3?±?19.4?MPa and 5.04?±?0.29?MPa?m^?1/2, respectively.     

Doped ceria powders prepared by spray pyrolysis for gas sensing applications

In the present study, ceria powder with and without gadolinium (Gd) or zirconium (Zr) dopants were synthesized by a spray pyrolysis (SP) process. The resulting powders (undoped CeO₂, Gd-doped ceria and Zr-doped ceria) were first mixed with organic binders, screen printed on alumina substrates, and then heat treated at 1200 °C for 2 h in air. Experimental results showed that the as-pyrolyzed powders were nanocrystalline and spherical in shape with uneven surfaces. After heat treatment, the powder coatings showed a 3-D network structure with interconnected pores exhibiting a high surface area. The electrical conductivity of CeO₂ was increased by the dope of Zr. The increasing rate of conductivity of ZDC (Zr-doped ceria) with decreasing oxygen pressure was also higher than that of undoped CeO₂. The CeO₂ and ZDC exhibited an n-type semiconductance in all the oxygen pressure regions, showing promise as candidates for sensor applications. The GDC (Gd-doped ceria) revealed a p-type and an n-type semiconductance in high- and low-oxygen pressure regions, respectively. The sensor applications of GDC are thus limited. Meanwhile, the ZDC exhibited a shorter response time due to its smaller grain size, showing a better oxygen sensing behavior.     

Crystallization behavior of Bi1.25Y1.75Fe5O12 prepared by coprecipitation process

A non-isothermal study of the crystallization kinetic of coprecipitation of Bi_1.25Y_1.75Fe₅O₁₂ was carried out by differential scanning calorimetry (DSC). The Avrami exponent n suggesting the dimensionality of crystal growth was determined using the Ozawa equation. From non-isothermal DSC data presented values in range of 775–1023 kJ/mol and 3.28–2.10 for the activation energy of crystallization and the Avrami exponent, respectively. These Avrami exponent values were consistent with surface and internal crystallizations growth simultaneously.     

Preparation of aluminum nitride green sheets by aqueous tape casting

Aluminum nitride green sheets were prepared by aqueous tape casting. The characteristics of a treated AlN were studied in aqueous ball-milling media. The oxygen content picked up with the increase of ball-milling time. It was noted that the oxygen content of AlN powder with the dispersant DP270 was lower than that of AlN powder without the dispersant DP270. The isoelectric points of the treated AlN with and without DP270 were, respectively, at pH 3.35 and pH 3.90. The dispersant DP270 not only efficiently dispersed AlN powder in water to form a stable suspension, but also formed a coat onto AlN surface to limit hydrolysis of the AlN powder. The tape casting slurry exhibited a typical shear-thinning behavior. Aqueous AlN green tape had a smooth surface and a narrow pore size distribution. Its relative density was 52.6%. No other crystalline phase was detected by XRD except for AlN and sintering aid yttria in AlN green sheet.     

Modified solution combustion route for the preparation of plasma sprayable ceria powder

Plasma sprayable grade ceria powder was prepared by the solution combustion method. This is the first report on the application of solution combustion for the synthesis of plasma sprayable grade oxide powders. The fuels and fuel ratios used in the solution combustion were modified to achieve adequate flowability. It was found that when a mixture of fuels like glycine and ammonium acetate was used, the combustion process yielded larger agglomerates. Phase purity of the powders was confirmed by powder XRD. The morphology of the particles was determined by scanning electron microscopy.     

Red ceramic pigments of terbium-doped ceria prepared through classical and non-conventional coprecipitation routes

Reddish ceramic pigments based on cerianite compositions Ce_1−xTb_xO₂ (x = 0.1, 0.2 and 0.3) and Ce_0.9Tb_0.05Y_0.05O₂ have been prepared. Firstly, the optimal composition giving the best red colour was selected (Ce_0.9Tb_0.1O₂) from ceramic and chlorides coprecipitation samples fired at 1100–1500 °C. Secondly, the effect of using different classical and non-conventional coprecipitation routes on the synthesis and colouring performance of CeO₂-Tb red pigments was analyzed for the first time. In addition to classical coprecipitation we tested also the use of hydrothermal treatment and other more homogeneous coprecipitation methods with oxalates and urea. Homogeneous coprecipitation was also combined with ultrasonic or microwave irradiation. Samples were characterized by thermal analysis, XRD, SEM/EDX, optical spectroscopy and colour measurements. Interestingly, the optimization of synthesis methods enabled to obtain more homogeneous (single phase) CeO₂-Tb solid solutions at lower temperatures (400–1100 °C), exhibiting very nice reddish colourations associated to lower energy bandgaps (E_g below 2.30 eV). The obtained optimal compositions could be therefore alternative candidates as environmentally friendly reddish ceramic pigments.     

Designing the morphology of ceria particles by precursor complexes

Ceria-based materials are widely used as catalysts, catalyst supports and electrolytes in many industrial applications. The morphological requirements of ceria particles vary depending on their applications. Here we show that complex morphologies of ceria particles can be achieved by using precursor complexes in the spray pyrolysis (SP) method. Three precursor complexes have been investigated: the complex of cerium acetate hydrate (CeA) and cerium nitrate hydrate (CeN); CeA and cerium ammonium nitrate (CeAN); and CeN and CeAN. Our results suggest that the morphological formation mechanism is highly correlated with the factors of precursor solubilities, solvent evaporation rates and precursor melting temperatures.     

Improvement on the mechanical properties of zinc oxide green sheets by aqueous acrylamide gel tape casting

Various organic constituents were used to improve the mechanical properties of green sheets prepared by a novel aqueous gel tape casting. Two deadly problems of green sheets: brittle and surface exfoliation were settled and mechanical properties were estimated. Compared with poly(ethylene glycol) (PEG400 (Mw ≈ 400)), poly(propylene glycol) (PPG400 (Mw ≈ 400)) and di-ethylic phthalate (DEP), glycerol was the most efficient plasticizer to improve the flexibility of the green sheet. The addition of PEG2000 eliminated the surface-exfoliation phenomenon of green sheets in air and had no distinct deterioration in mechanical properties. The flexible and lubricous green sheets were obtained. The solid loading of the suspension reached 73.5 wt.% and the relative green density was 58%.     

Fabrication of short carbon fibre reinforced SiC multilayer composites by tape casting

Silicon carbide multilayer composites containing short carbon fibres (C_sf/SiC) were prepared by tape casting and pressureless sintering. C fibres were dispersed in solvents and then mixed with SiC slurry to make green C_sf/SiC tape. Triton X-100 was found to be the best one for Toho Tenax HTC124 fibres (with water soluble coating) among BYK-163, BYK-410, BYK-2150, BYK-9076, BYK-9077 and Triton X-100 dispersants. C_sf/SiC multilayer composites containing 5 vol.% fibre (mean fibre length of 3, 4.5, and 6 mm) were obtained. Addition of short C fibres seems to worsen the densification process in the C_sf/SiC multilayer composites, whereas anisotropy shrinkage in C_sf/SiC was also observed. Open pores size was increased slightly after the addition of C fibre but it decreased with the mean fibre length. Mechanical properties were affected by high residual porosity. The addition of short C fibre has not changed the crack deflection at weak interfaces. C_sf/SiC multilayer composites containing longer fibres (4.5 and 6 mm) presented higher elastic modulus, bending strength and Vickers hardness as compared to shorter fibres (3 mm). Improved sintering performance and fibre content are necessary to improve mechanical properties.     

Effect of the fuel type on the synthesis of yttria stabilized zirconia by combustion method

Nano-sized 8 mol% yttria stabilized zirconia (YSZ) powders were synthesized by the combustion method using two different fuels (urea and glycine). The effect of the nature and amount of the fuel was investigated on the phase structure, particle size and microstructure of the resulted YSZ ceramics. The results showed that YSZ powders synthesized using urea presented larger crystallite size and lower specific surface area than those derived from glycine route. This behavior is closely related to the combustion flame temperature. The elevated temperature during combustion synthesis with urea favored the formation of large aggregates, instead of loose and porous particles as observed for glycine route. As a consequence, the best result in terms of densification was obtained for the pellets prepared by sintering of powders synthesized through glycine route.     

Formation enthalpy and magnetic properties of Bi-YIG powders

The formation enthalpy and magnetic properties of bismuth-doped yttrium iron garnet powders were investigated. The formation enthalpy and the crystallization temperature both decreased with increasing bismuth substitution for Bi_xY_3−xFe₅O₁₂ (0.25 ≦ x ≦ 1.25) powders prepared by the coprecipitation process. Bi substitution for Y can significantly reduce crystallization temperature for bismuth-doped yttrium iron garnet powders, and the magnetic properties (saturation magnetization, remanence, and coercive force) are independent of Bi substitution amounts. The average particle size has been determined by the specific surface area. As Bi substitution for Y increased, the average particle size also increased, while the specific surface area decreased.     

Influence of erbia or europia doping on crystal structure and microstructure of ceria–zirconia (CZ) solid solutions

In this study we have prepared three zirconia–ceria compositions, namely 12, 50 and 80 mole% CeO₂. Along with each pure ceria–zirconia composition we have prepared two parallel erbia- or europia-doped materials in which 0.5 mole% of each of the two starting oxides is replaced by 1 mole erbia or europia.