Ceramics — Open Access Journal

Compositions of (ZrO₂)_0.92(Y₂O₃)_0.08 (zirconia: 8 mol % yttria—8YSZ) and (CeO₂)_0.8(Sm₂O₃)_0.2 (ceria: 20 mol % samaria—SDC20) ceramic powders were prepared by attrition milling to form an equimolar powder mixture, followed by uniaxial and isostatic pressing. The pellets were quenched to room temperature from 1200 °C, 1300 °C, 1400 °C and 1500 °C to freeze the defects configuration attained at those temperatures. X-ray diffraction analyses, performed in all quenched pellets, show the evolution of the two (8YSZ and SDC20) cubic fluorite structural phases to a single phase at 1500 °C, identified by Rietveld analysis as a tetragonal phase. Impedance spectroscopy analyses were carried out in pellets either quenched or slowly cooled from 1500 °C. Heating the quenched pellets to 1000 °C decreases the electrical resistivity while it increases in the slowly cooled pellets; the decrease is ascribed to annealing of defects created by lattice micro-tensions during quenching while the increase to partial destabilization of the tetragonal phase. Full article

Shaped porous ceramics have proven to be the most adapted materials for several industrial applications, both at low and high temperatures. Recent research has been focused on developing shaping techniques, allowing for a better control over the total porosity and the pores characteristics. In this study, macroporous alumina foams were fabricated by gel-casting using pre-expanded polymeric microspheres with average sizes of 40 μm, 20 μm, and 12 μm as sacrificial templates. The gel-casting method, as well as the drying, debinding, and presintering conditions were investigated and optimized to process mechanically strong and highly porous alumina scaffolds. Furthermore, a reliable model relating the amount of pre-expanded polymeric microspheres and the total porosity of the presintered foams was developed and validated by mercury intrusion porosimetry measurements. The electron microscopy investigation of the presintered foams revealed that the size distribution and the shape of the pores could be tailored by controlling the particle size distribution and the shape of the wet pre-expanded microspheres. Highly uniform and mechanically stable alumina foams with bimodal porosity ranging from 65.7 to 80.2 vol. % were processed, achieving compressive strengths from 3.3 MPa to 43.6 MPa. Given the relatively open pore structure, the pore size distribution, the presintered mechanical strength, and the high porosity achieved, the produced alumina foams could potentially be used as support structures for separation, catalytic, and filtration applications. Full article

Macroporous composite foams consisting of β-tricalcium phosphate (β-TCP) and titanium nitride (TiN) have been prepared by a facile emulsion route involving sintering at elevated temperatures after shaping. Commercially available hydroxyapatite and titanium particles are used as the starting material; to which, the surface of the particles has been modified by preferential adsorption of hexadecylamine to change from hydrophilic to hydrophobic character in water. This renders stable air-in-water emulsions from the particle-filled suspensions by simple mechanical frothing. Sintered β-TCP/TiN foams with a porosity of 65–70%, pore size of 20–2000 nm, and three-point rupture strength of 25–43 kPa have been obtained. Electrical resistance has been found to reduce pronouncedly when the initial titanium loading exceeds 15 vol.% for the composite foams sintered at 1000 °C under reducing nitrogen-hydrogen atmosphere. Full article

Lead-free piezoelectric 0.95(Na_0.49K_0.49Li_0.02)(Nb_0.8Ta_0.2)O₃–0.05CaZrO₃ with 2 wt % MnO₂ addition was prepared using mechanochemically-assisted solid-state synthesis. Upon mechanochemical activation of the mixture of reagents partial amorphization occurs which contributes to a significantly lower temperature of completion of the solid-state reaction, ~600 °C as opposed to ~700 °C for the conventional solid-state synthesis as determined by thermal analysis. The ceramic specimens prepared by the mechanochemically-assisted route exhibit improved compositional homogeneity and slightly enhanced piezoelectric properties, achieved in a considerably shorter processing time compared to the conventional solid-state synthesis route, which was studied as a reference. Full article

We evaluated the influence of aging on mechanical properties of 8% yttria-doped zirconia (8YSZ) from room temperature to 1200 K. The temperature dependence of the dynamic Young’s and shear moduli of 8YSZ with and without the aging treatment was investigated by using a resonance method. The dynamic Young’s and shear moduli of 8YSZ without the aging treatment decreased by 33% below 700 K and gradually increased at higher temperatures with increasing temperature. On the other hand, those with the aging treatments decreased by around 20% below 600 K while did not significantly change above 600 K with increasing temperature. These demonstrated the effect of aging on the dynamic Young’s and shear moduli of 8YSZ was most remarkable at intermediate temperatures (600~1000 K). Although it was suggested that the existence ratio of the metastable tetragonal phase was increased during the aging treatment, it is likely that the influence of this phase transition on the dynamic Young’s and shear moduli was not significant. It seemed that the difference in the dynamic Young’s and shear moduli of 8YSZ with and without the aging treatment at intermediate temperatures was due to the local ordering of the oxygen vacancies. Full article

Optical and structural properties of glasses and glass-ceramics (GC) obtained by different heat-treatment of Tb and Tb-Yb doped sol-gel derived 30ZrO₂-70SiO₂ materials were investigated. A glass was formed after treatment at 700 °C whereas devitrification of the media after the treatment at 1000 and 1100 °C, led to the formation of GC containing up to three different crystalline phases, namely, tetragonal ZrO₂, Yb-disilicate and cristobalite. The modification of the optical properties through the heat treatment was caused by redistribution of the rare earth elements (REE) among the different phases: both Tb and Yb entered the t-ZrO₂ lattice, Yb can also be present in the form of a Yb₂Si₂O₇ crystal. Devitrification led to an increase in Tb→Yb energy transfer efficiency as compared to the glass, though it was higher in the samples heat-treated at 1000 °C than in those treated at 1100 °C. The most intensive Yb³⁺ luminescence, induced by the energy transfer from the Tb³⁺ ion, was observed at the interface between t-ZrO₂ and the glassy phases, due to the high concentration of REE in this area caused by the inability of ZrO₂ to accept larger amounts of the REE. The mechanisms of the Tb→Yb energy transfer vary between different phases of the GC. The results obtained in this study are important for the development of spectral down-converters for potential solar energy applications based on Tb-Yb co-doped glass-ceramics. Full article

We investigated copper electrodes deposited onto a BaZr_0.7Ce_0.2Y_0.1O_3-δ (BZCY72) proton-conducting membrane via a novel electroless plating method, which resulted in significantly improved performance when compared to a traditional painted copper electrode. The increased performance was examined with a multiscale multitechnique characterization method including time-of-flight secondary-ion mass spectroscopy (TOF-SIMS), transmission electron microscopy (TEM), scanning spreading-resistance microscopy (SSRM), and atom-probe tomography (APT). Through this method, we observed that a palladium catalyst layer alloys with the copper electrode. We also explored the nature of a non-coking-induced carbon-rich phase that may be involved with the improved performance of the electrode. Full article

In this work, nanostructured (La_0.6Sr_0.4)_0.99CoO₃ (LSC)-Ce_0.8Gd_0.2O_1.9 (CGO) core-shell particles were prepared by precipitating CGO nanoparticles on the surface of LSC particles under hydrothermal conditions. The as-prepared core-shell particles were sintered by spark plasma sintering (SPS) and conventional sintering, and the microstructure evolution and densification behavior were studied. Dense microstructures were reached using both sintering methods at relatively low temperatures. In the case of SPS, the core-shell architecture was partially maintained and nano-structured CGO grains were formed, while conventional sintering led to the formation of larger CGO grains. This work covers a detailed characterization of (a) the individual LSC-CGO core-shell particles and (b) the composites after densification. Full article

Tubular oxygen transport membranes (OTMs) that can be directly integrated in high temperature processes have a large potential to reduce CO₂ emissions. However, the challenging processing of these multilayered tubes, combined with strict material stability requirements, has so far hindered such a direct integration. We have investigated if a porous support based on (Y₂O₃)_0.03(ZrO₂)_0.97 (3YSZ) with a dense composite oxygen membrane consisting of (Y₂O₃)_0.01(Sc₂O₃)_0.10(ZrO₂)_0.89 (10Sc1YSZ) as an ionic conductor and LaCr_0.85Cu_0.10Ni_0.05O_3−δ (LCCN) as an electronic conductor could be fabricated as a tubular component, since these materials would provide outstanding chemical and mechanical stability. Tubular components were made by extrusion, dip coating, and co-sintering, and their chemical and mechanical integrity was evaluated. Sufficient gas permeability (≥10⁻¹⁴ m²) and mechanical strength (≥50 MPa) were achieved with extruded 3YSZ porous support tubes. The high co-sintering temperature required to densify the 10ScYSZ/LCCN membrane on the porous support, however, causes challenges related to the evaporation of chromium from the membrane. This chemical degradation caused loss of the LCCN electronic conducting phase and the formation of secondary lanthanum zirconate compounds and fractures. LCCN is therefore not suitable as the electronic conductor in a tubular OTM, unless means to lower the sintering temperature and reduce the chromium evaporation are found that are applicable to the large-scale fabrication of tubular components. Full article

The evaluation of the piezoelectric properties of ferroelectric ceramics generally has a high level of uncertainty, due to incomplete poling, porosity, domain wall clamping and other effects. In addition, the poling process is often difficult and dangerous, due to the risk of breaking or damaging the sample. A method is described for the evaluation of the potential intrinsic piezoelectric response that a ceramic would have after full poling, without poling it. The method relies on the fact that any material undergoes an elastic softening below the ferroelectric transition temperature, whose magnitude can be expressed in terms of the intrinsic piezoelectric and dielectric coefficients of the material. Such a softening is equivalent to an electromechanical coupling factor averaged over all the components, due to the unpoled state of the sample, and can be deduced from a single temperature scan of an elastic modulus of a ceramic sample, spanning the ferroelectric and paraelectric states. The strengths, limits and possible applications of the method are discussed. Full article

Nanoindentation-based fracture toughness measurements of ceramic materials like silicon carbide (SiC) with pyramidal indenters are of significant interest in materials research. A majority of currently used fracture toughness models have been developed for Vickers indenters and are limited to specific crack geometries. The validity of the indentation-cracking method for the fracture toughness measurement of single crystal SiC, the elastic-plastic anisotropy and orientation dependence around the c-axis when indented in the <0001> direction is examined using nanoindentation with different pyramidal indenters. The residual impressions are analyzed using scanning electron microscopy to measure the crack lengths and the validity of existing fracture toughness measurement methods and equations is analyzed. A combination of nanoindentation with different pyramidal indenters to produce a wide range of effective strains and finite element simulation is used to extract flow properties of single crystal SiC in the <0001> direction. It is observed that there is no orientation dependence around the c-axis when SiC-6H is indented in the <0001> direction with a Berkovich indenter, i.e., it is transversely isotropic. It is also found that for a Berkovich indenter, the Jang and Pharr model, which is based on the Lawn model for cone/halfpenny cracks, gives approximately constant values at low loads (<1 N), while at higher loads (>1 N), the Laugier model gives constant fracture toughness values. Finite element analysis using equivalent cones is used along with measured hardness values to estimate the yield strength, the work hardening exponents and the stress–strain curve for single crystal SiC-6H in the <0001> direction. Full article

Rapid in-situ non-ambient X-ray diffraction represents a powerful tool for characterizing the evolution of crystalline materials in real time. The calcium aluminate system and formation of Ca₁₂Al₁₄O₃₃ (C12A7) is particularly sensitive to processing conditions. This report characterizes the kinetic pathways to thermodynamic equilibrium as a function of atmosphere (ambient, dry, and vacuum) and reactant heterogeneity (as-received, milled, and sol-gel reactants). When reactants are heterogenous (as-received and milled), intermediary phases of Ca₃Al₂O₆ (C3A) and CaAl₂O₄ (CA) are observed as the route to C12A7 formation and Ca₅Al₆O₁₄ (C5A3) is only observed as a decomposition product of C12A7. When reactants are heterogenous, C12A7 is only thermodynamically favorable under ambient conditions due to the stability provided by hydration. When reactants are homogenous (sol-gel), direct crystallization of C12A7 from an amorphous precursor is observed at low temperature regardless of atmosphere defining C12A7 as the kinetic equilibrium. These findings accurately define the heterogenous formation pathways and report for the first time the formation of C12A7 under a carbon-free vacuum environment. Full article

Polymer-derived SiC-polycrystalline fibers show excellent heat-resistance up to 2000 °C, and relatively high strength. Up to now, through our research, the relationship between the strength and residual defects of the fiber, which were formed during the production processes (degradation and sintering), has been clarified. In this paper, we addressed the relationship between the production conditions and the surface smoothness of the obtained SiC-polycrystalline fiber, using three different raw fibers (Elementary ratio: Si₁Al_0.01C_1.5O_0.4~0.5) and three different types of reactors. With increase in the oxygen content in the raw fiber, the degradation during the production process easily proceeded. In this case, the degradation reactions (SiO + 2C = SiC + CO and SiO₂ + 3C = SiC + 2CO) in the inside of each filament become faster, and then the CO partial pressure on the surface of each filament was considered to be increased. As a result, according to Le Chatelier’s principle, the surface degradation reaction and grain growth of formed SiC crystals would be considered to become slower. That is to say, using the raw fiber with higher oxygen content and closed system (highest CO content in the reactor), a much smoother surface of the SiC-polycrystalline fiber could be achieved. Furthermore, the similar effect obtained by simple oxidation of the SiC-polycrystalline fiber was confirmed, and the advantageous points of the aforementioned process were also considered. Full article

The addition of graphene-based nanostructures (GBNs) can improve the inherent fragility of ceramics and provide them with improved electrical and thermal conductivities. However, both the starting material (ceramic matrix and GBNs) and the processing/sintering approach are crucial for the final composite microstructure and properties. This work focuses on the influence of the content and dimensions of the GBN filler (10 and 20 vol%; 3 and ~150 layers), the powder-processing conditions (dry versus wet), and the homogenization method (ultrasound sonication versus high-energy planetary ball milling) on GBN/tetragonal zirconia (3YTZP) composites. The microstructure and electrical properties of the spark plasma sintered (SPS) composites were quantified and analyzed. The highest microstructural homogeneity with an isotropic microstructure was achieved by composites prepared with thicker GBNs milled in dry conditions. A high content (20 vol%) of few-layered graphene as a filler maximizes the electrical conductivity of the composites, although it hinders their densification. Full article

The effect of Fluorine solutions on the surface and bacterial adhesion of lithium disilicate is a concern. The aim was to evaluate the surface roughness and the adhesion of Streptococcus sanguinis on lithium disilicate ceramic, under the influence of different solutions containing Fluorine. Forty lithium disilicate (IPS e.max Press Impulse) discs (2.5 × 5 mm) was divided into 4 groups (n = 10): artificial saliva (Group AS), 0.2% sodium fluoride (Group NaF), 1.23% acidulated phosphate fluoride gel (Group APF), and mouthwash (Group MW). Roughness analyses were performed before and after the immersion. The surface aspect was evaluated by scanning electron microscopy (SEM) and the adhesion of Streptococcus sanguinis were evaluated after immersion in the solutions. The data obtained were submitted to the analysis of variance (ANOVA) and the Tukey test (α = 0.05). The Group APF presented a bigger roughness (3.263), statistically different to the other solutions. The bacterial adhesion in the Group APF (5.85) presented statistical difference to the other solutions. The SEM micrographs showed a rougher surface in Group APF. The 1.23% APF gel promoted major surface roughness and bacterial adhesion and could be inadequate for the use of patients with lithium disilicate ceramic restorations. Clinical significance: The Fluorine solution can affect the lithium disilicate ceramics, generating a rough and non-esthetic surface. This altered surface could be susceptible to bacterial adhesion, being directly related with periodontal health, the longevity of the restoration and the success of the rehabilitation. Full article

Weyl semimetals can be described as the three-dimensional analogue of graphene, showing linear dispersion around nodes (Weyl points). Tantalum arsenide is among the most studied Weyl semimetals. It has been demonstrated that TaAs has a very high value of the real part of the complex refractive index in the infrared region. In this work we show one-dimensional photonic crystals alternating TaAs with SiO₂ or TiO₂ and a microcavity where a layer of TaAs is embedded between two SiO₂-TiO₂ multilayers. Full article

Carbon fibers are outstanding reinforcements for ceramic components due to their excellent creep and long-term thermochemical and thermomechanical stability. Nevertheless, these properties are dramatically downgraded if the unprotected fibers are exposed to an oxidative or corrosive environment. Thin ceramic coatings can improve the corrosion resistance and tailor the fiber/matrix interface in order to achieve optimized stress transfer and damage tolerance. The continuous liquid phase coating (CLPC) technique with subsequent pyrolysis is a promising alternative to chemical vapor deposition (CVD) processes. The possibility to deposit homogenous thin flaw-free coating layers on every filament of high tenacity carbon fiber bundles has been successfully proven in previous studies. In this work, high modulus carbon fibers were coated with different polysiloxane-based resins, and the obtained rovings were implemented in SiOC matrices by the precursor impregnation and pyrolysis (PIP) route. Thermogravimetric analysis shows an increased oxidation resistance of the coated fibers compared with reference samples. Enhanced fiber/matrix interface strength further improved the mechanical performance of the fabricated composites. Full article

A wet chemical method was used to obtain tungsten oxide nanoparticles from tungsten tetrachloride and natural microfibrous inorganic clay (sepiolite) as a starting material. Precipitation of tungsten oxide species onto sepiolite under basic conditions and subsequent thermal treatment was investigated, prompted by the abundance of sepiolite in nature and the useful environmental applications that could be attained. Laser granulometry, X-ray diffraction, field emission scanning electron microscopy (FE-SEM), energy dispersive X-ray spectroscopy (EDX), and high-resolution transmission electron microscopy (HR-TEM) techniques were used to study the particle-size distribution, the morphology, and the composition of the prepared sample. Our findings show the presence of tungsten oxide nanoparticles, which are less than 50 nm, on the needles of the modified sepiolite. Full article

In recent years, the detection of ultratraces of nitroaromatic compounds (NACs), such as 2,4,6-trinitrotoluene (TNT), has gained considerable attention due to associated problems related to environment, security against terrorists and health. The principle of NACs detection is simple since any explosive emits a rather small, but detectable number of molecules. Thus, numerous detection techniques have been developed throughout the years, but their common limitations are rather large sizes and weights, high power consumption, unreliable detection with false alarms, insufficient sensitivity and/or chemical selectivity, and hyper-sensitivity to mechanical influences associated with very high price. Thus, there is a strong need of cheap, rapid, sensitive, and simple analytical methods for the detection and monitoring of these explosives in air. Semiconducting metal oxides (SMOs) allow the preparation of gas sensors able to partially or totally overcome these drawbacks, and this paper aims to shortly review the most recent SMOs nanocomposites able to sense explosives. Full article