Showing papers in "Journal of The European Ceramic Society in 2010"
TL;DR: In this paper, the dielectric properties of A2+B6+O4 (A2+: Ca, Pb, Ba; B6+: Mo, W) ceramics were investigated as a function of packing fraction and bond valence.
Abstract: Microwave dielectric properties of A2+B6+O4 (A2+: Ca, Pb, Ba; B6+: Mo, W) ceramics were investigated as a function of packing fraction and bond valence. For A2+B6+O4 specimens sintered at 800–1100 °C for 3 h, a single phase with a tetragonal scheelite structure was detected, and the theoretical density was higher than 93% throughout the composition. Although the ionic polarizability of Ba2+ ion was larger than that of Ca2+ ion, the dielectric constant (K) of BaB6+O4 showed a smaller value than that of CaB6+O4. These results could be attributed to changes of the packing fraction due to the effective ionic size. The Q·f value was largely dependent on the packing fraction, as well as the percentages of theoretical density. The temperature coefficients of the resonant frequencies (TCFs) of the specimens were affected by the bond valence of oxygen. The specimens of CaMoO4 sintered at 1000 °C for 3 h showed the K of 10.8, Q·f of 76,990 GHz and TCF of −22.8 ppm/°C, respectively.
362 citations
TL;DR: In this paper, the authors analyze the thermal and mechanical properties of UH-Ceramic components, such as nose tips and wing leading edges for hypersonic applications, to predict component performance in particular flight environments.
Abstract: Analytical modeling of thermal and mechanical response is a fundamental step in the design process for ultra-high-temperature ceramic components, such as nose tips and wing leading edges for hypersonic applications. The purpose of the analyses is to understand the response of test articles to high-enthalpy flows in ground tests and to predict component performance in particular flight environments. Performing these analyses and evaluating the results require comprehensive and accurate physical, thermal, and mechanical properties. In this paper, we explain the nature of the analyses, highlight the essential material properties that are required and why they are important, and describe the impact of property accuracy and uncertainty on the design process.
291 citations
TL;DR: In this article, a composite of β-tricalcium phosphate (β-TCP) and a bioactive glass similar to the 45S5 Henchglass® is used to manufacture customized implants via 3D-printing process.
Abstract: Customized implants for bone replacement are a great help for a surgeon to remodel maxillofacial or craniofacial defects in an esthetical way, and to significantly reduce operation times. The hypothesis of this study was that a composite of β-tricalcium phosphate (β-TCP) and a bioactive glass similar to the 45S5 Henchglass® is suitable to manufacture customized implants via 3D-printing process. The composite was chosen because of the bioresorption properties of the β-TCP, its capability to react as bone cement, and because of the adjustability of the bioactive glass from inert to bioresorbable. Customized implants were manufactured using the 3D-printing technique. The four point bending strength of the printed specimens was 14.9 MPa after sintering. XRD analysis revealed the occurrence of two other phases, CaNaPO4 and CaSiO3, both biocompatible and with the potential of biodegradation. We conclude that it is possible to print tailored bone substitute implants using a bioactive TCP/glass composite. The glass is not involved as reactive substance in the printing process. This offers the opportunity to alter the glass composition and therefore to vary the composition of the implant.
274 citations
TL;DR: The colloidal approach has been studied as an essential step in the tailoring of ceramic nanostructures, but most colloidal processes are limited by the complexity of preparation of highly concentrated and stable suspensions of nanoparticles and their fast ageing as mentioned in this paper.
Abstract: The colloidal approach has been studied as an essential step in the tailoring of ceramic nanostructures, but most colloidal processes are limited by the complexity of preparation of highly concentrated and stable suspensions of nanoparticles and their fast ageing. Electrophoretic deposition (EPD) stands out as the most appropriate colloidal process to produce ceramic structures using low solid content sols and suspensions (
233 citations
TL;DR: In this article, the synthesis of geopolymers based on alkaline polysialate was achieved at low temperature (∼25-80°C) by the alkaline activation of raw minerals and silica fume.
Abstract: The synthesis of geopolymers based on alkaline polysialate was achieved at low temperature (∼25–80 °C) by the alkaline activation of raw minerals and silica fume. The materials were prepared from a solution containing dehydroxylated kaolinite and alkaline hydroxide pellets dissolved in potassium silicate. Then the mixture was transferred to a polyethylene mold sealed with a top and placed in an oven at 70 °C for 24 h. For all geopolymer materials, following dissolution of the raw materials, a polycondensation reaction was used to form the amorphous solid, which was studied by FTIR-ATR spectroscopy. The in situ inorganic foam based on silica fume was synthesized from the in situ gaseous production of dihydrogen due to oxidation of free silicon (content in the silica fume) by water in alkaline medium, which was confirmed via TGA-MS experiments. This foam has potential as an insulating material for applications in building materials since the thermal measurement has a value of 0.22 W m−1 K−1.
201 citations
TL;DR: In this article, the authors describe the mechanical properties of carbon nanotube-reinforced Al2O3 nanocomposites fabricated by hot-pressing and show that the fracture toughness, hardness and flexural strength of the nanocomposition were improved by 94%, 13% and 6.4% respectively, at 4-vol.% CNT additions.
Abstract: This paper describes the mechanical properties of carbon nanotube-reinforced Al2O3 nanocomposites fabricated by hot-pressing. The results showed that compared with monolithic Al2O3 the fracture toughness, hardness and flexural strength of the nanocomposites were improved by 94%, 13% and 6.4% respectively, at 4 vol.% CNT additions. For 10 vol.% CNT additions, with the exception of the fracture toughness, which was improved by 66%, a decrease in mechanical properties was observed when compared with those for monolithic Al2O3. The toughening mechanism is discussed, which is due to the uniform dispersion of CNTs within the matrix, adequate densification, and proper CNT/matrix interfacial connections.
192 citations
TL;DR: In order to improve the fracture toughness, SiC whiskers or SiC chopped fibers were added to a ZrB 2 matrix in volumetric fraction of 10 and 20 vol.%. The composites were hot-pressed between 1650 and 1730°C and their final relative densities were higher than 95 as mentioned in this paper.
Abstract: In order to improve the fracture toughness, SiC whiskers or SiC chopped fibers were added to a ZrB 2 matrix in volumetric fraction of 10 and 20 vol.%. The composites were hot-pressed between 1650 and 1730 °C and their final relative densities were higher than 95%. Even at the lowest sintering temperature, the whiskers showed an evident degradation. On the other hand, the fibers maintained their initial shape and a strong interface formed between matrix and reinforcement. The fracture toughness of the composites increased from 30 to 50% compared to the baseline material, with the fibers showing a slightly higher toughening effect. In the whiskers-reinforced composites, the room-temperature strength increased when 10 vol.% whiskers were added. In the fibers-reinforced composites, the room-temperature strength decreased regardless the amount of fibers added. The high-temperature strength of the composites was higher than that of the baseline material for both types of reinforcement.
171 citations
TL;DR: In this paper, the phase diagram of (1−−x)(Bi0.5Na 0.5)-SrTiO3 was completed and investigations on polarization and strain in this system were carried out.
Abstract: The phase diagram of (1 − x)(Bi0.5Na0.5)TiO3–xSrTiO3 was completed and investigations on polarization and strain in this system were carried out. (1 − x)(Bi0.5Na0.5)TiO3–xSrTiO3-ceramics were prepared by conventional mixed oxide processing. The depolarization temperature (Td), the temperature of the rhombohedral–tetragonal phase transition (Tr–t) and the Curie temperature (Tm) were determined by measuring the temperature dependence of the relative permittivity. All solid solutions of (1 − x)(Bi0.5Na0.5)TiO3–xSrTiO3 show relaxor behavior (A-site relaxor). From XRD-measurements a broad maximum of the lattice parameter can be observed around x = 0.5 but no structural evidence for a morphotropic phase boundary was found. SEM-analysis revealed a decrease of the grain size for increasing SrTiO3-content. At room temperature a maximum of strain of about 0.29% was found at x = 0.25 which coincides with a transition from a ferroelectric to an antiferroelectric phase. The temperature dependence of the displacement indicates an additional contribution from a structural transition (rhombohedral–tetragonal), which would be of certain relevance for the existence of a morphotropic phase boundary.
164 citations
TL;DR: A comprehensive overview of the development of carbon nanotube (CNT)-ceramic nanocomposites by electrophoretic deposition (EPD) can be found in this article.
Abstract: The purpose of this paper is to present an up-to-date comprehensive overview of current research progress in the development of carbon nanotube (CNT)–ceramic nanocomposites by electrophoretic deposition (EPD). Micron-sized and nanoscale ceramic particles have been combined with CNTs, both multiwalled and single-walled, using EPD for a variety of functional, structural and biomedical applications. Systems reviewed include SiO2/CNT, TiO2/CNT, MnO2/CNT, Fe3O4/CNT, hydroxyapatite (HA)/CNT and bioactive glass/CNT. EPD has been shown to be a very convenient method to manipulate and arrange CNTs from well dispersed suspensions onto conductive substrates. CNT–ceramic composite layers of thickness in the range <1–50 μm have been produced. Sequential EPD of layered nanocomposites as well as electrophoretic co-deposition from diphasic suspensions have been investigated. A critical step for the success of EPD is the prior functionalization of CNTs, usually by their treatment in acid solutions, in order to create functional groups on CNT surfaces so that they can be dispersed uniformly in solvents, for example water or organic media. The preparation and characterisation of stable CNT and CNT/ceramic particle suspensions as well as relevant EPD mechanisms are discussed. Key processing stages, including functionalization of CNTs, tailoring zeta potential of CNTs and ceramic particles in suspension as well as specific EPD parameters, such as deposition voltage and time, are discussed in terms of their influence on the quality of the developed CNT/ceramic nanocomposites. The analysis of the literature confirms that EPD is the technique of choice for the development of complex CNT–ceramic nanocomposite layers and coatings of high structural homogeneity and reproducible properties. Potential and realised applications of the resulting CNT–ceramic composite coatings are highlighted, including fuel cell and supercapacitor electrodes, field emission devices, bioelectrodes, photocatalytic films, sensors as well as a wide range of functional, structural and bioactive coatings.
157 citations
TL;DR: In this article, the Na 0.5 Bi 0.3 -K 0.25 -TiO 3 (NBT-KBT) system, with its complex perovskite structure, was investigated as a promising material for piezoelectric applications.
Abstract: We have investigated the Na 0.5 Bi 0.5 TiO 3 –K 0.5 Bi 0.5 TiO 3 (NBT–KBT) system, with its complex perovskite structure, as a promising material for piezoelectric applications. The NBT–KBT samples were synthesized using a solid-state reaction method and characterized with XRD and SEM. Room-temperature XRD showed a gradual change in the crystal structure from tetragonal in the KBT to rhombohedral in the NBT, with the presence of an intermediate morphotropic region in the samples with a compositional fraction x between 0.17 and 0.25. The fitted perovskite lattice parameters confirmed an increase in the size of the crystal lattice from NBT towards KBT, which coincides with an increase in the ionic radii. Electrical measurements on the samples showed that the maximum values of the dielectric constant, the remanent polarization and the piezoelectric coefficient are reached at the morphotropic phase boundary (MPB) ( ɛ = 1140 at 1 MHz; P r = 40 μC/cm 2 ; d 33 = 134 pC/N).
153 citations
TL;DR: In this article, a mixture of CoO, ZnO, NiO, Nb 2 O 5 and WO 3 was added to BiFeO 3 polycrystalline ceramics to obtain high density products up to 96% theoretical.
Abstract: BiFeO 3 polycrystalline ceramics were prepared by the mixed oxide route and a chemical route, using additions of Co, ZnO, NiO, Nb 2 O 5 and WO 3 . The powders were calcined at 700 °C and then pressed and sintered at 800–880 °C for 4 h. High density products up to 96% theoretical were obtained by the use of CoO, ZnO or NiO additions. X-ray diffraction, SEM and TEM confirmed the formation of the primary BiFeO 3 and a spinel secondary phase (CoFe 2 O 4 , ZnFe 2 O 4 or NiFe 2 O 4 depending on additive). Minor parasitic phases Bi 2 Fe 4 O 9 and Bi 25 FeO 39 reduced in the presence of CoO, ZnO or NiO. Additions of Nb 2 O 5 and WO 3 did not give rise to any grain boundary phases but dissolved in BiFeO 3 lattice. HRTEM revealed the presence of domain structures with stripe configurations having widths of typically 200 nm. In samples prepared with additives the activation energy for conduction was in the range 0.78–0.95 eV compared to 0.72 eV in the undoped specimens. In co-doped specimens (Co/Nb or Co/W) the room temperature relative permittivity was ∼110 and the high frequency dielectric loss peaks were suppressed. Undoped ceramics were antiferromagnetic but samples prepared with Co or Ni additions were ferromagnetic; for 1% CoO addition the remanent magnetization ( M R ) values were 1.08 and 0.35 emu/g at temperatures of 5 and 300 K, respectively.
TL;DR: In this article, porous yttria-stabilized zirconia (YSZ) ceramics with unidirectionally aligned pore channels were prepared by freezing YSZ/tert-butyl alcohol slurry under different freezing temperatures.
Abstract: Porous yttria-stabilized zirconia (YSZ) ceramics with unidirectionally aligned pore channels were prepared by freezing YSZ/tert-butyl alcohol (TBA) slurry under different freezing temperatures of −30, −78 and −196 ◦ C, respectively. After removing the frozen TBA via freeze-drying in vacuum at −50 ◦ C, the green samples were sintered at 1450 ◦ C for 2 h in air. The results showed that the freezing temperature significantly influenced microstructure and properties of the porous YSZ ceramics. Both microstructure observation and pore size distribution indicated that the pore channel size decreased significantly with decreasing freezing temperature, regardless of microstructure variations in the individual sample. Both porosity and room-temperature thermal conductivity of the porous YSZ ceramics varied under different freezing temperatures. Regardless of microstructure variations in the samples under different freezing temperatures, all samples had unidirectional pore channels with increasing pore channel size along the freezing direction. The fabricated samples had remarkably low thermal conductivities both in directions perpendicular and parallel to the channel direction, thus rendering them suitable for applications in thermal insulations. © 2010 Elsevier Ltd. All rights reserved.
TL;DR: In this article, Li-doped NiO films were deposited by radio frequency magnetron sputtering on Corning 1737 glass substrates and the Li concentrations varied from 0 to 1629 at%, as determined by wavelength-dispersive X-ray analysis and inductively coupled plasma mass spectrometry.
Abstract: Lithium-doped NiO films were deposited by radio frequency magnetron sputtering on Corning 1737 glass substrates The Li concentrations in the films varied from 0 to 1629 at%, as determined by wavelength-dispersive X-ray analysis and inductively coupled plasma mass spectrometry The effects of Li content on properties such as microstructure, resistivity, and electrical stability were studied The results show that the doped Li ions tend to occupy crystal defect sites such as vacancies or segregate on the film surface Initially, doped Li occupied the nickel vacancies in the film, decreasing the electrical conductivity When the Li concentration was further increased, some Li segregated on the film surface and formed bulges at high Li concentrations These Li-rich oxides covering the film surface served as partitions between the film and moisture from the atmosphere As a result, the Li-doped NiO films show a relatively high arrestment to electrical resistance aging
TL;DR: In this article, the microstructural development of a carbon fibre reinforced ZrC matrix composite, manufactured by reactive melt infiltration (RMI), was investigated. And the composite matrix was revealed by optical microscopy (OM), X-ray diffraction (XRD), and transmission electron microscopy(TEM).
Abstract: The microstructural development of a carbon fibre reinforced ZrC matrix composite, Cf/ZrC, manufactured by reactive melt infiltration (RMI) was investigated. The microstructural features of the composite were revealed by optical microscopy (OM), X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). It was found that the carbon fibre bundles are surrounded by continuous ZrC layers, while the composite matrix is composed of island-like ZrC particles dispersed within an α-Zr–ZrC eutectic phase. Nanosized inclusions were found inside some ZrC particles and it was demonstrated that they were α-Zr or α-Zr–ZrC. A formation mechanism of the unique matrix microstructure is proposed.
TL;DR: In this article, the resistance to oxidation and optical properties of a hot-pressed ZrB2-SiC composite were studied under aero-thermal heating in a strongly dissociated flow that simulates hypersonic re-entry conditions.
Abstract: The resistance to oxidation and optical properties of a hot-pressed ZrB2–SiC composite were studied under aero-thermal heating in a strongly dissociated flow that simulates hypersonic re-entry conditions. Ultra-high temperature ceramic models with a blunt or sharp profile were exposed to high enthalpy flows of an N2/O2 gas mixture up to 10 MJ/kg for a full duration of 540 s, the surface temperatures approaching 2100 K. Stagnation-point temperatures as well as spectral emissivities were directly determined using an optical pyrometer. Microstructural features of the oxidized layers were correlated to optical properties through computational fluid dynamics calculations which allow for numerical rebuilding of key parameters like surface temperatures, wall heat fluxes, shear stresses or concentrations of the species composing the reacting gas mixture. Gradients of temperature on the surfaces facing the hot gas flow established different boundary conditions that led to the formation and evolution of distinct layered oxide scales.
TL;DR: In this paper, a CNT/Al 2 O 3 composites can be densified by pressureless sintering without damage to the CNTs, achieving relative densities as high as 99%.
Abstract: Alumina ceramics reinforced with 1, 3, or 5 vol.% multi-walled carbon nanotubes (CNTs) were densified by pressureless sintering. Commercial CNTs were purified by acid treatment and then dispersed in water at pH 12. The dispersed CNTs were mixed with Al 2 O 3 powder, which was also dispersed in water at pH 12. The mixture was freeze dried to prevent segregation by differential sedimentation during solvent evaporation. Cylindrical pellets were formed by uniaxial pressing and then densified by heating in flowing argon. The resulting pellets had relative densities as high as ∼99% after sintering at 1500 °C for 2 h. Higher temperatures or longer times resulted in lower densities and weight loss due to degradation of the CNTs by reaction with the Al 2 O 3 . A CNT/Al 2 O 3 composite containing 1 vol.% CNT had a higher flexure strength (∼540 MPa) than pure Al 2 O 3 densified under similar conditions (∼400 MPa). Improved fracture toughness of CNT–Al 2 O 3 composites was attributed to CNT pullout. This study has shown, for the first time, that CNT/Al 2 O 3 composites can be densified by pressureless sintering without damage to the CNTs.
TL;DR: The structural evolution and microwave dielectric properties of (1 - x)Li2TiO3 + xMgO system (0 <= x <= 0.5) have been investigated in this article.
Abstract: The structural evolution and microwave dielectric properties of (1 - x)Li2TiO3 + xMgO system (0 <= x <= 0.5) have been investigated in this paper. The ordering degree decreased with the increase of MgO content. The microcracks and cleavage on (0 0 1) due to the weak Li-O bonds disappeared with the increase of MgO content. The dielectric constant and temperature coefficient of resonant frequency decreased with the increase of MgO content. The Q x f value increased with X LIP to X = 0.2 and then decreases with the further increase of x. An excellent combined microwave dielectric properties could be obtained when x = 0.24, epsilon(r) = 19.2, Q x f = 106,226 GHz and tau(f) = 3.56 ppm/degrees C. (C) 2009 Elsevier Ltd. All rights reserved.
TL;DR: In this paper, Zirconium diboride and boron carbide particles were used to improve the ablation resistance of carbon-carbon composites at high temperature (1500-°C).
Abstract: Zirconium diboride and boron carbide particles were used to improve the ablation resistance of carbon–carbon (C–C) composites at high temperature (1500 °C). Our approach combines using a precursor to ZrB 2 and processing them with B 4 C particles as filler material within the C–C composite. An oxyacetylene torch test facility was used to determine ablation rates for carbon black, B 4 C, and ZrB 2 –B 4 C filled C–C composites from 800 to 1500 °C. Ablation rates decreased by 30% when C–C composites were filled with a combination of ZrB 2 –B 4 C particles over carbon black and B 4 C filled C–C composites. We also investigated using a sol–gel precursor method as an alternative processing route to incorporate ZrB 2 particles within C–C composites. We successfully converted ZrB 2 particles within C–C composites at relatively low temperatures (1200 °C). Our ablation results suggest that a combination of ZrB 2 –B 4 C particles is effective in inhibiting the oxidation of C–C composites at temperatures greater than 1500 °C.
TL;DR: In this paper, the effects of ZrO 2 addition on the transparency, grain size and lattice parameter of Y 2 O 3 ceramics were studied and it was shown that with the addition of ZRO 2 the transparency increased markedly and the grain size decreased markedly by cation diffusivity mechanism.
Abstract: Commercial Y 2 O 3 powder was used to fabricate highly transparent Y 2 O 3 ceramics with the addition of ZrO 2 via slip casting and vacuum sintering. The effects of ZrO 2 addition on the transparency, grain size and lattice parameter of Y 2 O 3 ceramics were studied. With addition of ZrO 2 the transparency of Y 2 O 3 ceramics increased markedly and the grain size of Y 2 O 3 ceramics decreased markedly by cation diffusivity mechanism and the lattice parameter of Y 2 O 3 ceramics slightly decreased. The highest transmittance (at wavelength 1100 nm) of the 5.0 mol% ZrO 2 –Y 2 O 3 ceramic (1.0 mm thick) sintered at 1860 °C for 8 h reached 81.7%, very close to the theoretical value of Y 2 O 3 .
TL;DR: In this paper, the influence of stabilizer type on the phase stability of thermal barrier coatings (TBCs) produced by air plasma spraying was explored, and it was shown that decomposition of the non-transformable tetragonal phase occurs to a greater or lesser extent for all dopants at these temperatures, depending on the dopant kind, amount and on the presence of Al2O3 and SiO2 impurities.
Abstract: The influence of stabilizer type on the phase stability of thermal barrier coatings (TBCs) produced by air plasma spraying was explored. Together with the widely used zirconia-stabilized with yttria, other novel compositions, such as dysprosia-stabilized zirconia, yttria–lanthana-stabilized zirconia and ceria-stabilized zirconia were also investigated. The effect of isothermal heat treatment on the phase stability was explored. Results suggest that decomposition of the “non-transformable” tetragonal phase occurs to a greater or lesser extent for all dopants at these temperatures. The effect of Al2O3 and SiO2 content was also explored. The rate of decomposition depends on the dopant kind, amount and on the presence of Al2O3 and SiO2 impurities.
TL;DR: In this paper, different mechanisms for spark plasma sintered (SPSed) ceramics are presented, discussing the electric field effect on the liquid phase behaviour and how it may influence the liquid-phase sintering of Si3N4-based materials.
Abstract: Several key topics on the current assisted sintering of Si3N4-based materials are reviewed. First, different proposed mechanisms for spark plasma sintered (SPSed) ceramics are presented, discussing the electric field effect on the liquid phase behaviour and how it may influence the liquid phase sintering of Si3N4 ceramics. Next, we show that the SPS is a powerful tool to develop new Si3N4-based materials with tailored microstructures, such as functionally graded materials (FGMs) and carbon nanotubes (CNTs) containing Si3N4 matrix composites. Si3N4 FGMs are fabricated from a sole homogenous Si3N4 mixture just modifying the SPS system punches set-up, thus creating a temperature gradient through the specimen. Finally, the capability of SPS to get dense Si3N4/CNTs composites overcoming both constraint densification and nanotubes degradation is proved.
TL;DR: In this article, a systematic experimental design is used to study the influence of Mg, Y and La single or co-doping (75-450ppm) as well as the SPS sintering pressure and temperature on the RIT and grain size of transparent polycrystalline alumina (PCA).
Abstract: Transparent polycrystalline alumina (PCA) is a promising replacement for sapphire. Its optical properties however are highly dependent on the grain size and residual porosity which need to be controlled for real inline transmittances (RIT), that are high enough for possible applications. To achieve high RITs, doping as well as pressure assisted sintering is often used. In this study spark plasma sintering (SPS) and doping are investigated. A systematic experimental design is used to study the influence of Mg, Y and La single or co-doping (75–450 ppm) as well as the SPS sintering pressure and temperature on the RIT and grain size of PCA. Using optimized sintering parameters, RITs of >50% were attained in the visible wavelength (640 nm) for 0.8 mm thick samples for almost all doping strategies. The best RIT of 57% was for triple-doped samples at a total dopant level of 450 ppm. These results are significantly better than previously published SPS studies and illustrate that SPS sintered alumina can attain high and reproducible optical transmittances under various doping and sintering conditions.
TL;DR: In this paper, an advanced preparation technique was proposed to optimize sample geometries for X-ray nano-CT, utilizing a focused ion beam (FIB) system to shape the sample according to the Xray field of view at the required magnification.
Abstract: High-resolution tomography techniques have facilitated an improved understanding of solid oxide fuel cell (SOFC) electrode microstructures. The use of X-ray nano computerised tomography (nano-CT) imposes some geometrical constraints on the sample under investigation; in this paper, we present the development of an advanced preparation technique to optimise sample geometries for X-ray nano-CT, utilizing a focused ion beam (FIB) system to shape the sample according to the X-ray field of view at the required magnification. The technique has been successfully applied to a Ni-YSZ electrode material: X-ray nano-CT has been conducted at varying length scales and is shown to provide good agreement; comparison of results from X-ray and more conventional FIB tomography is also demonstrated to be favourable. Tomographic reconstructions of SOFC electrodes with volumes spanning two orders of magnitude are presented.
TL;DR: In this article, the authors investigated the effect of Ca substitution on the structure, microstructure and microwave dielectric properties of Li 2 A 1− x Ca x Ti 3 O 8 (A = Mg, Zn and x ǫ = 0-0.2) ceramics.
Abstract: The Li 2 Mg 1− x Zn x Ti 3 O 8 ( x = 0–1) and Li 2 A 1− x Ca x Ti 3 O 8 (A = Mg, Zn and x = 0–0.2) ceramics are synthesized by solid-state ceramic route and the microwave dielectric properties are investigated. The Li 2 MgTi 3 O 8 ceramic shows ɛ r = 27.2, Q u × f = 42,000 GHz, and τ f = (+)3.2 ppm/°C and Li 2 ZnTi 3 O 8 has ɛ r = 25.6, Q u × f = 72,000 GHz, and τ f = (−)11.2 ppm/°C respectively when sintered at 1075 °C/4 h. The Li 2 Mg 0.9 Zn 0.1 Ti 3 O 8 dielectric ceramic composition shows the best dielectric properties with ɛ r = 27, Q u × f = 62,000 GHz, and τ f = (+)1.1 ppm/°C. The effect of Ca substitution on the structure, microstructure and microwave dielectric properties of Li 2 A 1− x Ca x Ti 3 O 8 (A = Mg, Zn and x = 0–0.2) has also been investigated. The materials reported in this paper are excellent in terms of dielectric properties and cost of production compared to commercially available high Q dielectric resonators.
TL;DR: In this article, a novel gelation-freezing (GF) method was used to obtain ultra high porosity and unidirectionally oriented micrometer-sized cylindrical pores.
Abstract: Silicon carbide (SiC) with ultra high porosity and unidirectionally oriented micrometer-sized cylindrical pores was prepared using a novel gelation–freezing (GF) method. Gelatin, water and silicon carbide powder were mixed and cooled at 7 °C. The obtained gels were frozen from −10 to −70 °C, dried using a vacuum freeze drier, degreased at 600 °C and then sintered at 1800 °C for 2 h. The gels could be easily formed into various shapes, such as cylinders, large pipes and honeycombs using molds. Scanning electron microscopy (SEM) observations of the sintered bodies showed a microstructure composed of ordered micrometer-sized cylindrical cells with unidirectional orientation. The cell size ranging from 34 to 147 μm could be modulated by changing the freezing temperatures. The numbers of cells for the samples frozen at −10 and −70 °C were 47 and 900 cells/mm 2 , respectively, as determined from cross-sections of the sintered bodies. The resulting porous SiC with a total porosity of 86%, exhibited air permeability from 2.3 × 10 −11 to 1.0 × 10 −10 m 2 , which was the same as the calculated ideal permeability, and high compressive strength of 16.6 MPa. The porosity, number of cells, air permeability and strength of the present porous SiC were significantly higher than that reported for other porous SiC ceramics.
TL;DR: In this paper, microstructural evolution on heating was investigated in a reference industrial composition (50% kaolinitic clay, 40% feldspar and 10% quartz) of porcelain stoneware, fast fired at different temperatures (500-1400°C).
Abstract: Microstructural evolution on heating was investigated in a reference industrial composition (50% kaolinitic clay, 40% feldspar and 10% quartz) of porcelain stoneware, fast fired at different temperatures (500–1400 °C). The evolution of mullite crystals, regarding shape and size progress, was examined by scanning electron microscopy (SEM). The proportion of Type I mullite crystals decreases with firing temperature and simultaneously, the size of crystals increases, reaching the maximum value of aspect ratio (3:1) at 1400 °C. Type II and Type III secondary mullite needles increase with temperature in both number and length, which leads to an increase in the aspect ratio from 5:1 to ∼20:1 in Type II crystals and from ∼33:1 to 50:1 in Type III mullite needles. Finally, clusters of Type III mullite fibres are observed in porcelain stoneware samples fast fired in the 1250–1280 °C interval.
TL;DR: ZrB 2 and SiC composites with smaller starting particle sizes demonstrated marked plasticity and significant reduction in the flexural strength at 1800-°C as mentioned in this paper, and their corresponding strength retentions are 13% and 7%, respectively.
Abstract: ZrB 2 –15 vol%SiC and ZrB 2 –30 vol%SiC composites with smaller starting particle sizes in which the particle sizes of ZrB 2 and SiC are 2 μm and 05 μm, respectively, demonstrated marked plasticity and significant reduction in the flexural strength at 1800 °C The flexural strengths of these two composites are 112 ± 12 MPa and 48 ± 10 MPa, respectively, and their corresponding strength retentions are 13% and 7%, respectively Large ZrB 2 grains were commonly observed in the samples containing 15 vol%SiC, which are always the sites for the crack initiation Cavities were found in the samples containing 30 vol%SiC and the grain boundaries are the main sites for the crack and cavity nucleation To improve ultra-high temperature strength, larger starting particle sizes (ZrB 2 and SiC are 5 μm and 2 μm, respectively) were used for the preparation of ZrB 2 –15 vol%SiC This sample fractured in an elastic manner up to 1800 °C and showed a very high strength with a value of 217 ± 16 MPa
TL;DR: In this paper, the photocatalytic activity of a series of TiO 2 powders was assessed in gas phase and the results were explained by taking into account the anatase and rutile relative amounts in the samples, their crystallite size, the surface hydroxyl groups adsorbed on the photocats, and the surface area of the mixtures.
Abstract: A series of TiO 2 –SiO 2 mixtures – having the following stoichiometry Ti 1− x Si x O 2 , with x = 0, 0.1, 0.3 and 0.5 atoms per formula unit – were prepared by using precursor oxides and fired at three temperatures (900, 1000 and 1200 °C). The modifications in the structure and, consequently, on the photocatalytic activity, induced by the addition of SiO 2 into the TiO 2 powder, were thoroughly investigated by using various analytical techniques: X-ray powder diffraction, electron microscopy (FE-SEM and TEM), XPS, FT-IR, DRS and BET analysis. The results underlined as essentially no solid solution occurs between the two crystalline end-members. Nevertheless, silica addition caused a retarding effect on anatase-to-rutile phase transformation and on the crystallite growth. The photocatalytic activity of the powders was assessed in gas phase and the results were explained by taking into account the anatase and rutile relative amounts in the samples, their crystallite size, the surface hydroxyl groups adsorbed on the photocatalysts and the surface area of the mixtures.
TL;DR: In this article, the electrical conductivity of carbon nanotubes (CNTs) and carbon black were added to the alumina to convert it into a good electrical conductor.
Abstract: Carbon nanotubes (CNTs) and carbon black were added to alumina to convert it into a good electrical conductor. Alumina–CNT and alumina–carbon black nanocomposites were fabricated by Spark Plasma Sintering (SPS). The electrical conductivity of alumina–CNT nanocomposites was found to be four times higher as compared to alumina–carbon black nanocomposites due to the fibrous nature and high aspect ratio of CNTs. The electrical conductivity of alumina–CNT nanocomposite increased with increasing grain size due to increasing density of CNTs at the grain boundaries. This effect was not observed for alumina–carbon black nanocomposite due to the particulate geometry of the carbon black.
TL;DR: In this paper, the effect of oxidation on the room-temperature flexural strength was investigated for ZrB 2 -10 -vol% SiC (ZB10S) and Zb 2 -30 -vol%) SiC in air at 1500°C with times ranging from 0.5h to 10h.
Abstract: Oxidation behavior and effect of oxidation on the room-temperature flexural strength were investigated for ZrB 2 –10 vol% SiC (ZB10S) and ZrB 2 –30 vol% SiC (ZB30S) in air at 1500 °C with times ranging from 0.5 h to 10 h. The oxide scale of both ZB10S and ZB30S was composed of an outer glassy layer and an inner extended SiC-depleted layer. The changes in weight gain, glass layer thickness, and extended SiC-depleted layer thickness with oxidation were measured. Analysis suggested that the extended SiC-depleted layer was most indicative for evaluating the oxidation resistance. Compared to the ZB10S, the improved oxidation resistance in ZB30S was attributed to the viscosity increase of glassy layer and the lower number of ZrO 2 inclusions in the glassy layer. Because of the healing of surface flaws by the glassy layer, the strength increased significantly by ∼110% for ZB10S and by ∼130% for ZB30S after oxidation for 0.5 h.