Thermal expansion, gas permeability, and conductivity of Ni-YSZ anodes produced by different techniques

The supported Ni-YSZ (50 wt.% Ni?+?50 wt.% Zr_0.84Y_0.16O_1.92) anodes were produced of powders, obtained by the ceramic method, combustion synthesis, deposition of nickel oxide onto the YSZ ceramics, and deposition of 28 wt.% of nickel oxide onto the 39 wt.% NiO?+?61 wt.% YSZ powders. The influence of the NiO-YSZ powder production technique, the amount of pore former and sintering temperature on the porosity, gas permeability, thermal expansion, and anode conductivity were studied. The porosity of anodes made of powders obtained by the ceramic method is always lower than the porosity of the anodes made of powders produced by combustion synthesis under otherwise equal conditions. The anode electrical conductivity greatly depends on the powder production techniques, while the anode thermal expansion is only slightly influenced by them.     

Characterization of niobium(v) oxide received from different sources

The characterization of three commercial powders of niobium(V) oxide received from two producers was made. The thermal behavior of Nb₂O₅ up to melting point and its microstructure were studied using X-ray powder diffraction, thermoanalytical methods (DSC/TG), infrared spectroscopy (IR) and scanning microscopy. Analysis of the obtained results revealed that the starting structure of niobium(V) oxide and its thermal behavior depend on the origin of niobia. Depending on the origin of the powder and of its thermal treatment, three polymorphs of Nb₂O₅ can be observed. Sintering of powders above 1200 °C results in the formation of single phase, H-Nb₂O₅.     

Sintering studies on Ni–Cu–YSZ SOFC anode cermet processed by mechanical alloying

New 40 vol%[(Cu)–Ni]–YSZ cermet materials processed by mechanical alloying (MA) of the row powders are prepared. The powder compacts are sintered in air, hydrogen and inert (argon) atmospheres at a dilatometer and tubular furnace up to 1,350 °C. Sintering by activated surface concept (SAS) can anticipate and enhance the densification in such powders. Stepwise isothermal dilatometry (SID) sintering kinetics study is performed allowing determining kinetic parameters for Ni–YSZ and Ni–Cu–YSZ pellets. Two-steps sintering processes is indicated while Cu-bearing material features the smallest activation energy for sintering. The allied MA–SAS method is a promising route to prepare SOFC fuel cell anode materials.     

Calorimetric analysis of dysprosia and dysprosia-doped zirconia ceramics

Dysprosia (Dy₂O₃) and dysprosia-doped ZrO₂ (~10?wt% Dy₂O₃) samples were subjected to calorimetric and thermal analyses to understand the effect of dysprosia doping on the thermal properties of ZrO₂ and to compare the thermal conductivity of dysprosia-doped ZrO₂ (DySZ) to standard 7YSZ (7?wt% Y₂O₃ in ZrO₂). All doped samples were plasma sprayed and subsequently sintered to ensure material densification (reduced porosity for bulk property analysis) and sufficient diffusion of constituents throughout the samples. Differential scanning calorimetry was used to measure the specific heat capacity values for powder and sintered samples as a function of temperature. The thermal conductivities of sintered samples were measured using laser flash techniques. The results showed that the addition of dysprosia to ZrO₂ has lowered both the specific heat capacity and thermal diffusivity when compared to the standard 7YSZ. The resulting thermal conductivity of DySZ was 75% lower than that of 7YSZ under the sintered condition.     

Preparation and characterization of alumina-doped powders for the design of multi-phasic nano-microcomposites

The composite powders 90 vol.% Al₂O₃–5 vol.% YAG–5 vol.% ZrO₂ were produced by doping commercial alumina powders with zirconium and yttrium chloride aqueous solutions. Both a nanocrystalline transition alumina and a pure α-phase powder were used as starting materials. The obtained materials were characterized by DTA-TG, XRD and dilatometric analyses and compared to the respective biphasic systems developed by the same procedure. Pressureless sintering at 1500 °C for 3 h was able to consolidate the doped powders in fully dense bodies, characterized by a very fine and homogeneous dispersion of the second phases into the micronic alumina matrix.     

The effect of TiO2 additives on the structural stability and thermal properties of yttria fully-stabilized zirconia

TiO₂(0–20 mol%)-8 mol% YSZ (8YSZ) ceramics were synthesized by a traditional solid-state reaction method. A cubic single phase was observed for 8YSZ, 4 mol% TiO₂-8YSZ and 8 mol% TiO₂-8YSZ. Tetragonal and cubic mixed phases were observed for 12–20 mol% TiO₂-8YSZ ceramics. The sintering temperature was 1,700 °C for 8YSZ and 4 mol% TiO₂-8YSZ ceramics, whereas it was 1,500 °C for 8–20 mol% TiO₂-8YSZ. The thermal conductivity at room temperature decreased in proportion to increasing TiO₂ content, from 3.0 to 2.3 W/m K. The specific heat of TiO₂-8YSZ ceramics was unaltered as the TiO₂ content changed.     

Thermal analyses applied to ceramic nanopowders: from synthesis to sintering

The free sintering of ceramic powders into fully dense nanostructured materials is still a challenging process, even more complex when nanostructured transition alumina is used as starting powder. In this paper, biphasic (Alumina–YAG) and triphasic (Alumina–YAG–ZrO₂) composite powders were produced by doping the same nanocrystalline transition alumina with inorganic precursors of the second-phases, which were subsequently yielded under controlled thermal treatments. The added dopants significantly increased both the θ- to α-phase transformation and the sintering temperatures, making even more difficult the retention of the starting nanometric grain size into the final dense materials. Thermal analyses (such as TG–DTA and dilatometry) are here used to support most of the ceramic processing steps involved in a successful elaboration of the desired ultra-fine structures. In fact, the thermal pre-treatments of the doped powders were set up on the ground of the DTA–TG curves whereas the dilatometric analyses were exploited to design optimised sintering cycles, through which the green bodies were successfully consolidated into fully dense materials, characterised by highly homogeneous and tailored micro/nanostructures.     

Oxygen Transport in the SOFC Cathode

The impedance of the La_0.75Sr_0.2MnO₃-cathode/electrolyte interface for cathodes with different porosity is measured. The impedance spectra are fitted using a developed model of the oxygen transport at this interface. After the measurements, the cathode is removed from the electrolyte. The contact area and the three-phase boundary length (TPBL) at the interface are estimated from SEM images of the electrolyte surface. The dependence of the interfacial electrical resistance on the microstructure is discussed. It is shown that the bulk diffusion of oxygen vacancies at the interface at 950°C is high enough to use the whole La_0.75Sr_0.2MnO₃/YSZ contact area F for the oxygen transport into the electrolyte for microstructures with 2F/TPBL 2 m. The impact of the surface diffusion of oxygen species on polarization resistance at operation temperatures <900°C is discussed. The polarization resistance and the morphology of composite cathodes made from La_0.75Sr_0.2MnO₃/YSZ and yttria- or scandia-stabilized zirconia powders (3YSZ, 8YSZ, 10ScSZ) are investigated by impedance spectroscopy at 800–950°C. The polarization (interfacial) resistance decreases gradually with addition of electrolyte powder in the uLSM cathode material independent of the electrolyte powder used. The interfacial resistance of the uLSM/3YSZ, uLSM/8YSZ, and uLSM/10ScSZ composite cathodes is almost the same. The interaction between uLSM and doped zirconia particles is discussed on the basis of the interfacial resistance, activation energies, and high-frequency impedance.     

Structural and Morphological Investigation of Ce0.6Zr0.4O2 Oxides Synthesized by Sol-Gel Method: Influence of Calcination and Redox Treatments

Ceria-zirconia mixed oxides of nominal composition Ce_0.6Zr_0.4O₂ were prepared by sol-gel method. The structural and morphological modifications of these materials due to calcination and redox treatments up to 1000°C were studied by X-ray diffraction (XRD) and Rietveld refinements, specific surface area measurements (BET), oxygen storage capacity (OSC), TEM and SEM analyses. The materials maintain high OSC after several hours of treatments at 1000°C in oxidizing and reducing atmosphere, showing good resistance to the thermal ageing. An improvement of the OSC is manifested after repetitive redox cycles, even though textural characterization shows that reduction/oxidation processes induce sintering of the crystallites.     

Thermal behaviour study of some sol-gel TiO2 based materials

Among the great number of sol-gel materials prepared, TiO₂ holds one of the most important places due to its photocatalytic properties, both in the case of powders and coatings. Impurity doping is one of the typical approaches to extend the spectral response of a wide band gap semiconductor to visible light. This work has studied some un-doped and Pd-doped sol-gel TiO₂ nanopowders, presenting various surface morphologies and structures. The obtained powders have been embedded in vitreous TiO₂ matrices and the corresponding coatings have been prepared by dipping procedure, on glass substrates. The relationship between the synthesis conditions and the properties of titania nanosized materials, such as thermal stability, phase composition, crystallinity, morphology and size of particles, and the influence of dopant was investigated. The influence of Pd on TiO₂ crystallization both for supported and unsupported materials was studied (lattice parameters, crystallite sizes, internal strains). The hydrophilic properties of the films were also connected with their structure, composition and surface morphology. The methods used for the characterization of the materials have been: simultaneous thermogravimetry and differential thermal analysis, powder X-ray diffraction, electron microscopy (TEM, SAED) and AFM.     

Thermal and spectroscopic characterization of nanostructured zirconia–scandia–dysprosia

Zirconia containing 10 mol% scandia and x mol% dysprosia (0 ≤ x ≤ 1.5) gels was synthesized by simultaneous precipitation at room temperature. The aim of this work is to verify the effect of dysprosium on the cubic phase stabilization of the zirconia–scandia solid electrolyte. The gel was characterized by thermogravimetry, differential scanning calorimetry, and differential thermal analyses. The thermally treated powders were analyzed by Fourier transform infrared spectroscopy, thermal analyses, and X-ray diffraction techniques. For comparison purpose, a commercial zirconia–10 mol% scandia powder was subjected to some characterization techniques. The infrared spectrum shows characteristic absorption bands due to residual material from the synthesis on the surface of the powder particles. Nanostructured powders were obtained after thermal treatments at 500 °C for 2 h. Infrared spectroscopy and X-ray diffraction results evidence the stabilization of the cubic phase in zirconia–scandia containing dysprosium. The thermal stability of the cubic phase during thermal cycling was ascertained by thermal analysis.     

Preparation of silane-coated TiO2 nanoparticles in supercritical CO2

Nanometric inorganic pigments are widely used as fillers for hybrid composite materials. However, these nanometric powders are hydrophilic in nature and their surface must be functionalized before use. In this work, titanium dioxide (TiO₂) nanoparticles were coated using silane coupling agents with alkyl functionality. A supercritical carbon dioxide (scCO₂) method was used for surface silanization. Five alkylalkoxysilanes with different alkyl chain length and structure were studied: methyltrimethoxy, isobutyltriethoxy, octyltriethoxy, octyldimethylmethoxy and octadecyltrimethoxysilane. The microstructure and thermal stability of deposited monolayers were characterized using thermogravimetric analysis, ATR–IR spectroscopy, transmission electron microscopy, wettability characterization and low-temperature N₂ adsorption/desorption analysis. The use of scCO₂ as a solvent provided an effective approach to functionalize individual inorganic nanoparticles due to the enhanced diffusivity of the solution molecules in the aggregates interparticle voids. The trifunctional silanes employed here yielded surfaces with better thermal stabilities and greater hydrophobicities than the used monofunctional silane.     

Thermal stability and conductivity of hot-pressed Si3N4–graphene composites

This study concerns new Si₃N₄–graphene composites manufactured using the hot-pressing method. Because of future applications of silicon nitride for cutting tools or specific parts of various devices having contact with high temperatures there is a need to find a ceramic composite material with good mechanical and especially thermal properties. Excellent thermal properties in the major directions are characteristic of graphene. In this study, the graphene phase is added to the silicon nitride phase in a quantity of up to 10 mass%, and the materials are sintered under uniaxial pressure. The mixture of AlN and Y₂O₃ is added as sintering activator to the composite matrix. The studies focus on thermal stability of produced composites in argon and air conditions up to the temperature of 1,000 °C. The research also concerns the influence of applied uniaxial pressure during the sintering process on the orientation of graphene nanoparticles in the Si₃N₄ matrix. The study also presents research on anisotropy of thermal diffusivity and following thermal conductivity of ceramic matrix composites versus the increasing graphene quantity. Most of the presented results have not been published in the literature yet.     

Grain-size effects on gas response in nanostructured Gd0.9Ba0.1CoO3

The perovskite Gd_0.9Ba_0.1CoO₃ was prepared via solution and solution-polymerization methods in aqueous media. The thermal decomposition of precursor powders produced well crystallized nanostructured materials possessing an extensive nanoporosity. The average particle size obtained by the solution route was 150 nm, whereas by the solution-polymerization method was 61 nm. The characterization of gas response, performed on thick-films, revealed that the electrical resistance variation increases with reducing grain size. In oxygen, the films prepared with powder of solution-polymerization registered an increment of 1000%, compared to the powder made by the solution route, whereas in carbon dioxide was 1100%.     

Synthesis of high specific surface area YSZ (ZrO<Subscript>2</Subscript>–8Y<Subscript>2</Subscript>O<Subscript>3</Subscript>) nanocrystalline powder by modified polymerized complex method

In this study high specific surface area yttria-stabilized zirconia (ZrO₂–8Y₂O₃) nanocrystalline powder have been synthesized through “modified polymerized complex (MPC) method”. Zirconium chloride, yttrium nitrate, citric acid and ethylene glycol were polymerized at 80 °C to produce a gel-like mass in which metallic ions were uniformly distributed. During the thermal treatment of dried gel, nanocrystalline YSZ powder was formed. Thermal reactions and phase formation of dried gel were investigated through thermal analysis (DTA/TG) and X-ray diffraction (XRD) analysis, respectively. Chemical bonding and thermal decomposition behavior of dried gel was investigated by FTIR analysis. During decomposition, the nature of the bonding between carboxylate groups and the cations changed from unidentate to bridging at 370 °C and carbonate species were detected at 470 °C. Morphology of powder calcined at 650 °C was analyzed by scanning electron microscope (SEM). YSZ powder with high specific surface area was prepared successfully by this method.     

Inductively Coupled Plasma Synthesis of CeO<Subscript>2</Subscript>-based Powders from Liquid Solutions for SOFC Electrolytes

Doped cerium oxide (CeO₂) based electrolytes are attractive alternative materials to replace the existing yttria-stabilized zirconia (YSZ) used as electrolyte for SOFC (solid oxide fuel cells). Cerium oxide electrolytes offer a similar performance to YSZ electrolytes at a lower cell operating temperature (~600--800 ^°C), therefore reducing thermal stresses and solid state reactions among the cell components.Doped Ce_1-xMe_xO_2-x/2(Me = Gd, Sm or Y) fine \hbox{powders} were synthesized from nitrate salts dissolved in water using a radio frequency inductively coupled plasma reactor. It was demonstrated that the relative concentrations of Ce and dopants fed in the solutions were retained in the synthesized powders. The products were all nano-crystalline with the basic crystal structure of CeO₂ and the crystal size of the products was essentially independent of the dopant used. The particle size distributions obtained were multimodal and in most cases trimodal. The results obtained differ from a previously reported mechanism of particle synthesis from liquid precursors.     

Microstructure,stability and biocompatibility of hydroxyapatite – titania nanocomposites formed by two step sintering process

In the present work, we report the characterization of TiO₂-hydroxyapatite (HA) nanocomposites obtained by a two-step sintering (TSS) process of a mixture of HA and titanium hydride (TiH₂) powders. The reactions underwent by TiH₂ in the presence of HA and hydrogen release, and subsequently, titanium oxidation was examined by thermal analysis. A longer holding time in the second sintering stage enabled obtaining a homogenous TiO₂-HA (36% rutile) composite with a thermal expansion coefficient of 11.46 · 10⁻⁶ C⁻¹ in the 40–1000 °C range. Unconventional TSS process hinders HA decomposition to detrimental tricalcium phosphate (TCP). Wear rate of ceramics was determined by tribological measurements and the material biocompatibility was evaluated using MTT assay. Overall, cell viability results correlated with morphological observations indicated a good biocompatibility of HA-based composites at all tested concentrations. Incorporation of the TiO₂ phase in HA by TSS process was found to be an efficient way to prepare bioceramics with improved performances.     

Phase,crystal struture and sintering behavior of shock-synthesized Pb(Zr0.95Ti0.05)O3 powders

With a cylindrical shock-wave-loading technique, the single perovskite-phase Pb(Zr_0.95Ti_0.05)O₃ powders (PZT 95/5) were synthesized by shock-induced chemical reactions in heterogeneous multi-material powder mixtures of Pb₃O₄, ZrO₂ and TiO₂. The phase and crystal structure of as-synthesized powders were characterized by X-ray diffraction (XRD) and fourier transform infrared (FT-IR) analysis. And the microstructure and electrical properties of PZT 95/5 ceramics prepared with as-synthesized PZT powders at different sintering temperature were analyzed. The results showed that the shock-wave-induced a large quantity of lattice defects and distortion of the crystal structure in the shock-synthesized PZT powders, which could enhance the sintering activity. Thus, the optimal density and electrical properties of PZT ceramics prepared with as-synthesized powders could be obtained at a sintering temperature of 1200–1225 °C for 3 h, significantly lower than the sintering temperature of PZT 95/5 ceramics prepared by conventional solid-state reaction.     

Programmed Pore-Shape Fixing in a Soft Porous Molecular Crystal—Navigating the Phase Interconversion Landscape

Pore-shape fixing effects (PSFEs) in soft porous crystals are a relatively unexplored area of materials chemistry. We report the PSFE in the prototypical dynamic van der Waals solid p-tert-butylcalix[4]arene (TBC4). Starting with the high-density guest-free phase, two porous shape-fixed phases were programmed using the stimuli of CO₂ pressure and temperature. A suite of complementary in situ techniques, including variable-pressure (VP) single-crystal X-ray diffraction, VP powder X-ray diffraction, VP differential scanning calorimetry, volumetric sorption analysis, and attenuated total reflectance Fourier-transform infrared spectroscopy was used to track dynamic guest-induced transformations, providing molecular-level insight into the PSFE. The interconversion between the two metastable phases is particle size dependent, making this the second example of the PSFE by crystal downsizing, and the first example involving a porous molecular crystal: larger particles undergo reversible transitions while smaller particles remain fixed in the metastable phase. A complete phase interconversion scheme was constructed for the material, thus allowing navigation of the phase interconversion landscape of TBC4 using the easily applied stimuli of CO₂ pressure and thermal treatment.     

Synthesis of Y<Subscript>3−x</Subscript>Lu<Subscript>x</Subscript>Al<Subscript>3</Subscript>MgSiO<Subscript>12</Subscript> garnet powders by sol–gel method

Y_3−xLu_xAl₃MgSiO₁₂ (x = 0–3) garnet powders were synthesized by an aqueous sol–gel method based on metal chelates with 1,2-ethanediol in aqueous media. Target samples were characterized by powder X-ray diffraction (XRD), scanning electron microscopy and reflection spectra. XRD analysis revealed that sintering of polycrystalline Y_3−xLu_xAl₃MgSiO₁₂ powders at 1,600 °C results in single-phase garnet materials.