Showing papers in "Journal of the American Ceramic Society in 2005"
TL;DR: In this paper, the authors developed a new type of solar cell that is based on a photoelectrochemical process, where the light absorption is performed by a monolayer of dye (i.e., a Ruthenium complex) that is adsorbed chemically at the surface of a semiconductor.
Abstract: During the past five years, the authors have developed in their laboratory a new type of solar cell that is based on a photoelectrochemical process. The light absorption is performed by a monolayer of dye (i.e., a Ruthenium complex) that is adsorbed chemically at the surface of a semiconductor (i.e., titanium oxide (TiO{sub 2})). When excited by a photon, the dye has the ability to transfer an electron to the semiconductor. The electric field that is inside the material allows extraction of the electron, and the positive charge is transferred from the dye to a redox mediator that is present in solution. A respectable photovoltaic efficiency (i.e., 10%) is obtained by the use of mesoporous, nanostructured films of anatase particles. The authors show how the TiO{sub 2} electrode microstructure influences the photovoltaic response of the cell. More specifically, they focus on how processing parameters such as precursor chemistry, temperature for hydrothermal growth, binder addition, and sintering conditions influence the film porosity, pore-size distribution, light scattering, and electron percolation and consequently affect the solar-cell efficiency.
1,552 citations
TL;DR: The available literature on the crystal structure of the metastable alumina polymorphs and their associated transitions is critically reviewed and summarized in this article, where possible mechanisms for the phase transitions between the different alumina morphological polymorphs also discussed.
Abstract: The available literature on the crystal structure of the metastable alumina polymorphs and their associated transitions is critically reviewed and summarized. All the metastable alumina structures have been identified as ordered or partially ordered cation arrays on the interstitial sites of an approximately close-packed oxygen sublattice (either face-centered cubic or hexagonal close packed). The analysis of the symmetry relations between reported alumina polymorphs having an approximately face-centered cubic packing of the oxygen anions allows for an exact interpretation of all the complex domain structures that have been observed experimentally. Possible mechanisms for the phase transitions between the different alumina polymorphs also are discussed.
1,188 citations
TL;DR: In this paper, the authors investigated the properties of lead zirconate titanate (PZT) ceramics over the grain-size range of 0.1-10 μm.
Abstract: The processing, electromechanical properties, and microstructure of lead zirconate titanate (PZT) ceramics over the grain-size range of 0.1-10 μm were studied. Using measurements over a large temperature range (15-600 K), the relative role of extrinsic contribution (i.e., domain-wall motion) was deduced to be influenced strongly by the grain size, particularly for donor-doped PZT. Analytical transmission electron microscopy studies were conducted to investigate the trend in domain configurations with the reduction of grain size. The correlations between domain density, domain variants, domain configurations (before and after poling), spontaneous deformation, and the elastodielectric properties were qualitatively discussed, leading to new insights into the intrinsic and extrinsic effects and relevant size effects in ferroelectric polycrystalline materials.
918 citations
TL;DR: The real part of the permittivity and the tan δ of sintered polycrystalline alumina at about 9 GHz have been measured in this article, where the dielectric properties have been examined as a function of purity, pore volume, and sintering grain size.
Abstract: The real part of the permittivity (E') and the tan δ of sintered alumina (Al 2 O 3 ) at about 9 GHz have been measured. The dielectric properties have been examined as a function of purity, pore volume, and sintered grain size. The tan δ is found to depend very strongly on the pore volume, purity, and grain size. e' is far less sensitive to impurities and grain size. The dependence of e' on porosity can be described by simple mixture models as expected. A model of losses in single crystals cannot be extended easily to these materials where extrinsic factors such as porosity, random crystal orientation, grain boundaries, microcracks, and impurities dominate. These factors have been studied in an attempt to describe the tan δ and e' of sintered polycrystalline alumina. In this work, the tan δ for alumina has been studied in near-theoretical density ranges between 9.1 x 10 -5 and 2.4 x 10 -5 depending on grain size.
815 citations
TL;DR: In this article, a set of material property data is established based on published physical, mechanical, and thermal properties of alumina specimens that conform to the constraints of the material specification.
Abstract: Results of a data evaluation exercise are presented for a particular specification of sintered alpha-alumina (mass fraction of Al2O3, ≥0995; relative density (rho/rhotheoretical), ≥098; and nominal grain size, 5 μm) A comprehensive set of material property data is established based on published physical, mechanical, and thermal properties of alumina specimens that conform to the constraints of the material specification The criteria imposed on the properties are that the values should be derived from independent experimental studies, that the values for physically related properties should be mutually self-consistent, and that the sets of values should be compatible with established material property relations The properties assessed in this manner include crystallography, thermal expansion, density, sound velocity, elastic modulus, shear modulus, Poisson's ratio, bulk modulus, compressive strength, flexural strength, Weibull characteristic strength, Weibull modulus, tensile strength, hardness, fracture toughness, creep rate, creep rate stress exponent, creep activation energy, friction coefficient, wear coefficient, melting point, specific heat, thermal conductivity, and thermal diffusivity
669 citations
TL;DR: In this article, the authors review the nature and mechanics of damage induced in ceramics by spherical indenters, from the classical studies of Hertz over a century ago to the present day.
Abstract: In this article we review the nature and mechanics of damage induced in ceramics by spherical indenters, from the classical studies of Hertz over a century ago to the present day. Basic descriptions of continuum elastic and elastic-plastic contact stress fields are first given. Two distinct modes of damage are then identified: Hertzian cone cracks, in relatively hard, homogeneous materials, such as glasses, single crystals, fine-grain ceramics (tensile, brittle mode); and diffuse subsurface damage zones, in relatively tough ceramics with heterogeneous microstructures (shear, quasi-plastic mode). Ceramographic evidence is presented for the two damage types in a broad range of materials, illustrating how an effective brittle-ductile transition can be engineered by coarsening and weakening the grain structure. Continuum analyses for cone fracture and quasi plasticity, using Griffith-Irwin fracture mechanics and yield theory, respectively, are surveyed. Recent micromechanical models of the quasi-plastic mode are also considered, in terms of grain-localized shear faults with extensile wing cracks. The effect of contact-induced damage on the ensuing strength properties of both brittle and quasi-plastic ceramics is examined. Whereas cone cracking causes abrupt losses in strength, the effect of quasi-plastic damage is more gradual-so that more heterogeneous ceramics are more damage tolerant. On the other hand, quasi-plastic ceramics are subject to accelerated strength losses in extreme cyclic conditions (contact fatigue), because of coalescence of attendant microcracks, with implications concerning wear resistance and machinability. Extension of Hertzian contact testing to novel layer structures with hard, brittle outer layers and soft, tough underlayers, designed to impart high toughness while preserving wear resistance, is described.
656 citations
TL;DR: In this paper, three types of materials were investigated: silica for investment casting, alumina and silicon nitride for structural parts, and three-dimensional objects were fabricated from a 0.50 volume fraction silica suspension.
Abstract: Ceramic green bodies can be created using stereolithography methods where a ceramic suspension consisting of 0.40–0.55 volume fraction ceramic powder is dispersed within an ultraviolet-curable solution. Three ceramic materials were investigated: silica for investment casting purposes, and alumina and silicon nitride for structural parts. After mixing the powders in the curable solution, the ceramic suspension is photocured, layer by layer, fabricating a three-dimensional ceramic green body. Subsequent binder removal results in a sintered ceramic part. Three-dimensional objects have been fabricated from a 0.50 volume fraction silica suspension.
620 citations
TL;DR: In this article, it was shown that the enhanced piezoelectric response along nonpolar directions, observed in many perovskite systems, is a consequence of the flattening of the Gibbs free energy profile.
Abstract: The piezoelectric effect in ferroelectric single crystals and ceramics is investigated considering intrinsic (lattice), and extrinsic (originating mainly from displacement of domain walls) contributions. The focus of the study of intrinsic properties is on piezoelectric anisotropy, which was examined using the Landau-Ginsburg-Devonshire phenomenological theory. It is shown that the enhanced piezoelectric response along nonpolar directions, observed in many perovskite systems, is a consequence of the flattening of the Gibbs free energy profile. This flattening is common for temperature-, composition-, and external field-induced enhancement of the piezoelectric properties along nonpolar axes. A brief review of recent advances in understanding the origins of the piezoelectric nonlinearity, hysteresis, and frequency dispersion is also given.
527 citations
TL;DR: The single phase cubic perovskite region of the LaO1.5-SrO-Gao 1.5 MgO phase diagram was determined from room temperature and high-temperature X-ray diffraction.
Abstract: The single-phase, cubic-perovskite region of the LaO1.5-SrO-Gao1.5-MgO phase diagram was determined from room-temperature and high-temperature X-ray diffraction. Two impurity phases were identified, LaSrGaO4 and aSrGa3O7. The conductivity of the oxygen-deficient perovskite phase was shown to be essentially a purely oxide-ion conductivity sigmao over a wide range of oxygen partial pressures 10-22 lessthan equal to PO2 lessthan equal to 1 atm. The highest values of sigmao = 0.17 and 0.08 S/cm were found for La0.8Sr0.2Ga0.83Mg0.17O0.2815 at 800° and 700°C, respectively; they remain stable over a week-long test. The Arrhenius plot of sigmao is curved, dividing into a high-temperature region T > T* similar/congruent 600°C and a low-temperature region T < T*. Above T* all the oxygen vacancies appear to be mobile; below T* they progressively condense into clusters of ordered vacancies.
525 citations
TL;DR: In this paper, the influence of transition metal oxides additives, especially zinc oxide, on the densification and electrical properties of doped barium zirconate have been examined.
Abstract: The influence of transition metal oxides additives, especially zinc oxide, on the densification and electrical properties of doped barium zirconate have been examined. With the use of zinc oxide as a sintering aid, BaZr_(0.85)V_(0.15)O_(3-δ) was readily sintered to above 93% of theoretical density at 1300 degrees C. Scanning electron microscopic investigations showed Zn accumulation in the intergranular regions. Thermogravimetric analysis of the material under flowing CO_2 showed ZnO-modified barium zirconate to exhibit excellent chemical stability. The conductivity, as measured by A.C. impedance spectroscopy under H_2O saturated nitrogen, was slightly lower than that of unmodified barium zirconate. Electromotive force measurements under fuel cell conditions revealed the total ionic transport number to be ~0.9 at 600 degrees C. The combination of electrical and chemical properties and good sinterabifity render ZnO-modified barium zirconate an excellent candidate for reduced temperature solid oxide fuel cell applications.
503 citations
TL;DR: In this article, the electrical conductivity of donor-doped and undoped strontium titanate (SrTiO3) ceramics and, in some cases, single crystals, was investigated in the temperature range of 1000°-1400°C under oxygen partial pressures, PO2, of 10-20-1 bar.
Abstract: The electrical conductivity, sigma, of donor-doped and undoped strontium titanate (SrTiO3) ceramics and, in some cases, single crystals, Sr1-xLaxTiO3 (0 ≤ to x ≤ 0.1), was investigated in the temperature range of 1000°-1400°C under oxygen partial pressures, PO2, of 10-20-1 bar. In conjunction with Hall data and thermopower data from related papers, a set of constants for a defect-chemical model was determined, precisely describing point-defect concentrations and transport properties of these materials. In contrast to former works, temperature-dependent transport parameters and their non-negligible influence on the determination of the constants was considered, as well as the equilibrium restoration phenomena of the cation sublattice, which can be studied only at such high temperatures. It was shown that defects in the cation sublattice completely govern the electrical behavior of donor-doped and undoped SrTiO3. In the latter case, frozen-in strontium vacancies act as intrinsic acceptors, determining the sigma(PO2) curves at lower temperatures. This intrinsic acceptor concentration also can be calculated with this model. The very good agreement between calculation and measurement is shown in many examples.
TL;DR: In this paper, the sinterability and piezoelectric properties of KNN-based ceramics were studied as lead-free, lead free, and non-convex materials.
Abstract: The preparations of (K 0.5 Na 0.5 )NbO 3 (KNN)-based ceramics were studied as lead-free piezoelectric materials. The authors found that the addition of CuO greatly enhanced the sinterability of the KNN-based ceramics. The sinterability and piezoelectric properties of these ceramics were dependent upon the A/B ratio, CuO doping, and the formation of a solid solution with KTaO 3 . Perovskite (K 0.5 Na 0.5 ) x NbO 3 (x = A/B ratio) was synthesized with A/B ratios of 1.00 and 1.05 by CuO doping, while (K 0.5 Na 0.5 )NbO 3 contained K 4 CuNb 8 O 23 as a second phase with A/B ratios below 0.98. Although the A site-rich (K 0.5 Na 0.5 )NbO 3 (x = 1.00 and 1.05) ceramics exhibited deliquescence, the A site-poor (K 0.5 Na 0.5 )NbO 3 (x ≤ 0.98) ceramics with K 4 CuNb 8 O 23 had higher densities without deliquescence. K 4 CuNb 8 O 23 also improved Q m , which reached 1400 for the (K 0.5 Na 0.5 ) 1.0 NbO 3 doped with 0.5 mol% K 4 CuNb 8 O 23 . The formation of a solid solution with KTaO 3 raised the melting point of the system and also improved its sinterability. The k p and Q m of (K 0.5 Na 0.5 ) 0.97 (Nb 0.95 Ta 0.05 )O 3 with CuO were 0.41 and 1400, respectively.
TL;DR: The use of self-reinforcement by larger elongated grains in silicon nitride ceramics requires judicious control of the microstructure to achieve high steady-state toughness and high fracture strength as discussed by the authors.
Abstract: The use of self-reinforcement by larger elongated grains in silicon nitride ceramics requires judicious control of the microstructure to achieve high steady-state toughness and high fracture strength. With a distinct bimodal distribution of grain diameters, such as that achieved by the addition of 2% rodlike seeds, the fracture resistance rapidly rises with crack extension to steady-state values of up to 10 MPa{center_dot}m{sup 1/2} and is accompanied by fracture strengths in excess of 1 GPa. When the generation of elongated reinforcing grains is not regulated, a broad grain diameter distribution is typically generated. While some toughening is achieved, both the plateau (steady-state) toughness and the R-curve response suffer, and the fracture strength undergoes a substantial reduction. Unreinforced equiaxed silicon nitride exhibits the least R-curve response with a steady-state toughness of only 3.5 MPa{center_dot}m{sup 1/2} coupled with a reduced fracture strength.
TL;DR: In this paper, the crystal structure of tobermorite 14 A (plombierite) was solved by means of the application of the order disorder (OD) theory and was refined through synchrotron radiation diffraction data.
Abstract: The crystal structure of tobermorite 14 A (plombierite) was solved by means of the application of the order-disorder (OD) theory and was refined through synchrotron radiation diffraction data. Two polytypes were detected within one very small crystal from Crestmore, together with possibly disordered sequences of layers, giving diffuse streaks along c*. Only one of the two polytypes could be refined: it has B11b space group symmetry and cell parameters a = 6.735(2) A, b = 7.425(2) A, c = 27.987(5) A, γ = 123.25(1)°. The refinement converged to R = 0.152 for 1291 reflections with F o >4σ(F o ). The characteristic reflections of the other polytype, F2dd space group, a 11.2 A, b 7.3 A, c 56 A, were recognized but they were too weak and diffuse to be used in a structure refinement. The structure of tobermorite 14 A is built up of complex layers, formed by sheets of sevenfold coordinated calcium cations, flanked on both sides by wollastonite-like chains. The space between two complex layers contains additional calcium cations and H 2 O molecules; their distribution, as well as the system of hydrogen bonds, are presented and discussed. The crystal chemical formula indicated by the structural results is Ca 5 Si 6 O 16 (OH) ) . 7H 2 O.
TL;DR: In this article, microstructural observations of damage around indentations in Ti 3 SiC 2 are presented, and the following energy absorbing mechanisms have been identified from scanning electron micrographs of areas in the vicinity of the indentation: diffuse microcracking, delamination, crack deflection, grain push-out, grain pull-out and buckling of individual grains.
Abstract: Microstructural observations of damage around indentations in Ti 3 SiC 2 are presented. The Vickers hardness decreased with increasing load and asymptotically approached 4 GPa at the highest loads. No indentation cracks were observed even at loads as high as 300 N. Preliminary strength versus indentation plots indicate that, at least for the large-grained material (100 μm) studied here, Ti 3 SiC 2 is a damage-tolerant material able to contain the extent of microdamage to a small area around the indent. The following multiple energy-absorbing mechanisms have been identified from scanning electron micrographs of areas in the vicinity of the indentation: diffuse microcracking, delamination, crack deflection, grain push-out, grain pull-out, and the buckling of individual grains.
TL;DR: In this paper, the AC impedance response of mixed ionic and electronic conductors (MIECs) is derived from first principles and quantitatively compared with experimental data, and a broad spectrum of electrical and thermodynamic properties is extracted solely from the measurement of impedance spectra over wide oxygen partial pressure and temperature ranges.
Abstract: The AC impedance response of mixed ionic and electronic conductors (MIECs) is derived from first principles and quantitatively compared with experimental data. While the approach is not entirely new, the derivation is provided in a unified and comprehensive manner. Using Sm_(0.15)Ce_(0.85)O_(1.925–δ) with Pt electrodes as a model system, a broad spectrum of electrical and thermodynamic properties is extracted solely from the measurement of impedance spectra over wide oxygen partial pressure and temperature ranges. Here, the oxygen partial pressure was varied from air [po_2=0.21 atm] to H_2 [po_2=10^(−31) atm], and the temperature was varied from 500° to 650°C. It was essential for this analysis that the material under investigation exhibit, under some conditions, purely ionic behavior and, under others, mixed conducting behavior. The transition from ionic to mixed conducting behavior is recognizable not only from the oxygen partial pressure dependence of the total conductivity but also directly from the shape of the impedance spectra. Within the electrolytic regime, the impedance spectra (presented in Nyquist form) take the shape of simple, depressed arcs, whereas within the mixed conducting regime (under reducing conditions), the spectra exhibit the features associated with a half tear-drop-shaped element. Parameters derived from quantitative fitting of the impedance spectra include the concentration of free electron carriers, the mobilities and activation energies for both ion and electron transport, the electrolytic domain boundary, and the entropy and enthalpy of reduction. In addition, the electrochemical behavior of O_2 and H_2 at the Pt|ceria interface has been characterized from these measurements. Under oxidizing conditions, the data suggest an oxygen electrochemical reaction that is rate limited by the dissociated adsorption/diffusion of oxygen species on the Pt electrode, similar to Pt|YSZ (yttria-stabilized zirconia). Under reducing conditions, the inverse of the electrode resistivity obeys aJ_i^(mass)dependence, with an activation energy that is similar to that measured for the electronic conductivity. These results suggest that ceria is electrochemically active for hydrogen electro-oxidation and that the reaction is limited by the rate of removal of electrons from the ceria surface.
TL;DR: A series of low-toxicity gelcasting systems have been developed as discussed by the authors, which have very low acute toxicity and perform at least as well as, and in some cases better than, the original acrylamide-based system.
Abstract: A series of low-toxicity gelcasting systems has been developed. The reagents used in these systems have very low acute toxicity. The new systems perform at least as well as, and in some cases better than, the original acrylamide-based system. The development of these systems is described herein, including the search for new gel compositions, the study of suspensions made with the new gel precursor solutions, and pyrolysis of the dried gels and gelcast parts. Applications of the new gelcasting systems include complex silicon nitride parts, large-diameter rings, rapid prototyping by green machining, and metal-powder gel casting.
TL;DR: In this article, a low-temperature synthesis route of preparing bismuth ferrite nanopowders through soft chemical route using nitrates of Bismuth and Iron is described.
Abstract: The present research describes a simple low-temperature synthesis route of preparing bismuth ferrite nanopowders through soft chemical route using nitrates of Bismuth and Iron. Tartaric acid is used as a template material and nitric acid as an oxidizing agent. The synthesized powders are characterized by X-ray diffractometry, thermogravimetry and differential thermal analysis, infrared spectroscopy, and scanning electron microscopy. The particle size of the powder lies between 3 and 16 nm. In the process, phase pure bismuth ferrite can be obtained at a temperature as low as 400°C, in contrast to 550°C for coprecipitation route. On the other hand, we find that, like solid state reaction route, Pechini's autocombustion method of synthesis generates a lot of impurity phases along with bismuth ferrite.
TL;DR: In this paper, a bending bar made from a single piece of material is introduced, and the application of a voltage then causes the bar to bend due to the differential stresses that are induced.
Abstract: Internal strain amplification can be utilized in piezoelectric ceramics to obtain relatively large displacements. The monomorph described in the present work is a bending bar made from a single piece of material into which a smooth gradient of piezoelectric activity is introduced. The application of a voltage then causes the bar to bend due to the differential stresses that are induced. These are relatively uniformly distributed and do not peak in the center as in a conventional bimorph. Significantly increased life and reliability can then be achieved.
TL;DR: In this article, a series of these phases with substitution levels ranging from 1% to 5% BaZrO 3 were examined using high-resolution TEM and it was shown that the low losses of the 1:2 ceramics are derived from the stabilization of the ordering-induced domain boundaries via the partial segregation of the Zr cations.
Abstract: Small substitutions of BaZrO 3 into Ba[(Zn,Ni) 1/3 Ta 2/3 ]O 3 are utilized in the commercial preparation of low-loss perovskite microwave dielectrics. The structures of a series of these phases with substitution levels ranging from 1% to 5% BaZrO 3 were examined using high-resolution TEM. For < 2.15% BaZrO 3 the solid solutions retain the ordered 1:2 structure of the Ba[(Zn,Ni) 1/3 Ta 2/3 ]O 3 end-member but are comprised of small ordered domains whose size decreases as the Zr content is raised. The decrease in the size of the domains parallels a decrease in the processing time required to access a low-loss state. Although for pure Ba[(Zn,Ni) 1/3 Ta 2/3 ]O 3 reductions in the degree of cation order produce a large increase in the dielectric loss, the Zr-substituted ceramics retain a very low loss. We believe the low losses of the 1:2 ceramics are derived from the stabilization of the ordering-induced domain boundaries via the partial segregation of the Zr cations. For substitutions between 3% and 5% BaZrO 3 the size of the ordered domains continues to decrease but the system undergoes an abrupt transformation to a cubic 1:1 ordered structure with a doubled perovskite repeat. The structures of these phases have been interpreted using a random layer model in which one site is occupied by Ta and the other by a random distribution of Zn, Zr, and the remaining Ta cations, i.e., Ba{[Zn (2-y)/3 Ta (1-2y)/3 Zr y ] 1/2 [Ta 1/2 ]}O 3 . Although the ordering is confined to nano-sized domains, these ceramics also exhibit low losses, again reflecting the relative stability of the domain boundaries. In this case we believe the low losses reflect the effectiveness of the random layer in stabilizing the anti-phase boundaries.
TL;DR: In this article, transparent aluminum-doped zinc oxide (ZnO) films were prepared via the sol-gel method on silica-glass substrates from 2-methoxyethanol solutions of zinc acetate and aluminum chloride that contained monoethanolamine.
Abstract: Transparent aluminum-doped zinc oxide (ZnO) films were prepared via the sol-gel method on silica-glass substrates from 2-methoxyethanol solutions of zinc acetate and aluminum chloride that contained monoethanolamine. Dip coating was conducted at room temperature, with substrate withdrawal rates of 1.2-7.0 cm/min. After each deposition, the films were heat-treated in air at 200°-450°C for 10 min (pre-heat-treatment). After six to fourteen layers had been deposited, the films were then subjected to annealing in air at 500°-800°C for 1 h (the first post-heat-treatment), followed by annealing in nitrogen at 500°-700°C for 15 min to 4 h (the second post-heat-treatment). All the films obtained were transparent and showed only an extremely sharp ZnO (002) peak in the X-ray diffractometry (XRD) patterns. The effects of the aluminum content, the substrate withdrawal speed, and the heat-treatment conditions on the electrical resistivity of the films were studied. All these factors strongly affected the resistivity. The lowest resistivity value (6.5 10-3 Omegacm) was achieved in a film that contained 0.5 at.% aluminum, prepared with a low substrate withdrawal speed (1.2 cm/min), and a pre-heat-treatment of individual layer at 400°C in air and a post-heat-treatment of the entire film at 600°C in air, followed by a post-heat-treatment at 600°C in nitrogen. These preparation parameters also affected the degree of crystal orientation, which was revealed by the intensity of the ZnO (002) XRD peak. Higher crystal orientation was effective in reducing the film resistivity, whereas the higher grain-packing density and possible aluminum segregation were thought to have positive and negative effects, respectively, in reducing the resistivity.
TL;DR: In this paper, the distribution and orientation of platelet-shaped particles of α-alumina in a fine-grained alumina matrix is shown to template texture development via anisotropic grain growth.
Abstract: The distribution and orientation of platelet-shaped particles of α-alumina in a fine-grained alumina matrix is shown to template texture development via anisotropic grain growth. The textured microstructure ranges from 4 wt% oriented platelet particles in calcined samples to nearly 100% oriented α-Al2O3 grains after sintering at 1400°C. A CaO + SiO2 liquid phase creates favorable thermodynamic and kinetic conditions for anisotropic grain growth and grain reorientation during sintering. Important criteria for templated grain growth include (1) anisotropic crystal structure and growth, (2) high thermodynamic driving force for template grain growth, and (3) modification of diffusion in the system to continuously provide material to the anisotropically growing template grains.
TL;DR: In this paper, a model was developed to describe the adsorption behavior of these superplasticizers, which consisted of grafted polyethylene oxide (PEO) chains on a poly(carboxylic acid)-type (PC) backbone on cement surfaces.
Abstract: Interparticle potential energy calculations were performed to investigate the mechanisms by which a new class of concrete admixtures, generally referenced as poly(carboxylic acid)-type (PC) superplasticizers, which aid in dispersing cement particles, are formed. These calculations consisted of long-range Van der Waals, electrostatic, and steric interactions. The repulsive potential that resulted from electrostatic interactions was negligible, which would allow cement particles to flocculate in the absence of steric contributions. A model was developed to describe the adsorption behavior of these superplasticizers, which consisted of grafted polyethylene oxide (PEO) chains on a PC backbone on cement surfaces. Using this adsorption model, the influence of the length of the PEO molecular chain and the density per unit area on the steric contribution was quantified. Steric hindrance effects were the dominant stabilizing mechanism in this system. As expected, enhanced stability was observed with increasing adlayer thickness (and/or density). The results of this study may be useful in designing the molecular structure of this new and important class of dispersion aids for cement-based systems.
TL;DR: In this article, the synthesis of technologically important ferrites such as ZnFe 2 O 4, NiFe 2O 4, MnFe 2 o 4, and CoFe O 4 by using novel microwave-hydrothermal processing was reported.
Abstract: This paper reports the synthesis of technologically important ferrites such as ZnFe 2 O 4 , NiFe 2 O 4 , MnFe 2 O 4 , and CoFe 2 O 4 by using novel microwave-hydrothermal processing. Nanophase ferrites with high surface areas, in the range of 72-247m 2 /g, have been synthesized in a matter of a few minutes at temperatures as low as 164°C. The rapid synthesis of nanophase ferrites via an acceleration of reaction rates under microwave-hydrothermal conditions is expected to lead to energy savings.
TL;DR: In this paper, it was shown that the surface of the alumina gel films immersed in boiling water consisted of boehmite crystals, and the existence of air in the hydrophobic pores on the surface caused the superwater-repellency.
Abstract: Alumina thin films with a roughness of 20 to 50 nm were formed by immersing porous alumina gel films prepared by the sol-gel method in boiling water. When hydrolyzed fluoroalkyltrimethoxysilane was coated on the alumina films, the contact angle for water was increased with an increase of immersion time, and immersion for 30 s was long enough to give super-water-repellency and high transparency; the contact angle for water of the film was 165° and the transmittance for the visible light was higher than 92%. It was found that the surface of the alumina gel films immersed in boiling water consisted of boehmite crystals. It was shown that the existence of air in the hydrophobic pores on the surface caused the super-water-repellency.
TL;DR: In this article, a barrier-layer model with dielectric response based on the Maxwell-Wagner type of relaxation for ceramic CCTO is proposed, and two kinds of morphologies, i.e., terraces with ledges and bump domains, were discovered inside the grains under thermal etching conditions at 960°C for 72 min.
Abstract: CaCu 3 Ti 4 O 12 (CCTO) was prepared by solid-state reaction and identified by X-ray diffractometry. The evolution of the microstructure was observed by scanning electron microscopy (SEM). It was found that discontinuous grain growth developed during sintering, and large abnormal grains played a very significant role in the dielectric behavior. Cu ions segregated to the boundaries and CuO located at the triple-point sites of the abnormal large grains were observed by electron energy loss and energy-dispersive X-ray spectroscopy, respectively. Moreover, two kinds of morphologies, i.e., terraces with ledges and bump domains, were discovered inside the grains under the thermal etching conditions at 960°C for 72 min, which solves the contradiction related to the interpretation of the giant dielectric response between ceramic and single-crystal CCTO. Complex impedance spectroscopy was used to analyze the conductivity of ceramic CCTO, which suggests that it consists of conducting domains with two kinds of insulating barrier layers of domain and grain boundaries. The insulating domain and grain boundaries were attributed to orderly arranged dislocations and segregation of Cu ions, respectively. The conduction of CCTO was found to be related to the porosity, the grain size, and the thickness of the insulating boundary layers. For a sample sintered at 1065°C for 3 h, the anomalously low resistivity and temperature-dependent dielectric constant were due to the fact that domain boundaries were not substantially formed. The possible reasons for the development of barrier layers and the variation of the dielectric constant with the sintering time are discussed. A barrier-layer model with dielectric response based on the Maxwell-Wagner type of relaxation for ceramic CCTO is proposed. Two kinds of dielectric responses occur, depending on the microstructure: they are dominated by the domain and domain boundary and by the grain and grain boundary for large grains and fine grains, respectively.
TL;DR: In this paper, a transparent, super-water-repellent coating films of alumina on glass plates was prepared by a combination of geometric and chemical approaches. The contact angle for water in the films was 165° and the transmittance for visible light was higher than 92%.
Abstract: We have prepared transparent, super-water-repellent coating films of alumina on glass plates by a combination of geometric and chemical approaches. The contact angle for water in the films was 165° and the transmittance for visible light was higher than 92%. A roughness of 20 to 50 nm was obtained, which is too small to scatter visible light, while the degree of roughness was great enough to enhance the water-repellent properties together with the chemical effect of a fluorine-containing agent and gave a super-water-repellent surface. The coatings have great potential for practical applications such as eyeglasses, cover glasses for solar cells, windshields of automobiles, and so on.
TL;DR: The optical spectra of zinc aluminate (ZnAl2O4), zinc gallate, and zinc aluminogallate spinel powders were studied at wavelengths in the range of 250-900 nm using reflectance spectroscopy as discussed by the authors.
Abstract: The optical spectra of zinc aluminate (ZnAl2O4), zinc gallate (ZnGa2O4), and zinc aluminogallate (ZnAlGaO4) spinel powders were studied at wavelengths in the range of 250-900 nm using reflectance spectroscopy. The ZnAl2O4 and ZnGa2O4 powders were synthesized by using conventional ceramic processing techniques and had systematic variations in the molar ratio of ZnO to M2O3 (M = Al or Ga). The cubic spinel crystal structure of each composition was confirmed via powder X-ray diffractometry. The ZnAl2O4 powders showed optical properties in the ultraviolet wavelength region and had combined characteristics that were similar to that of ZnO (wurtzite structure) and Al2O3 (corundum structure), which result from the similar local environments of the zinc and aluminum cations within the cubic spinel crystal structure. A mechanically induced optical absorption (optomechanical effect) in the ultraviolet wavelength region was also observed in ZnAl2O4. The ZnGa2O4 powder followed a similar behavior, with the exception that the optomechanical effect did not occur in the gallate. The ZnAlGaO4 showed optical spectra that were intermediate to that of the endpoint compositions.
TL;DR: In this article, small, brittle particulate inclusions have been shown to cause crack-tip bridging at short distances behind the crack tip, and it is the latter that produces the very high fracture strength of the ceramic nanocomposite.
Abstract: Crack-tip bridging by particles is considered to be one of the primary strengthening mechanisms of ceramic nanocomposites. Small, brittle particulate inclusions have been shown to cause crack-tip bridging at short distances behind the crack tip. This mechanism leads to modest toughness but a very steep R-curve, and it is the latter that produces the very high fracture strength of the ceramic nanocomposite. Localized high residual stress around the particles (particularly in the case of silicon carbide-alumina material) causes the strengthening mechanism to operate effectively, even at a small volume fraction of 5%. The present study predicts the magnitude of the toughness increase and the extent of R-curve behavior for the nanocomposite.
TL;DR: In this article, a porphyrias with interconnected pore channels were fabricated by a novel freeze casting technique using camphene-based slurries, and the pore volume fraction and channel size were controllable by the solid content in the slurry.
Abstract: Porous ceramic bodies with interconnected pore channels were fabricated by a novel freeze casting technique using camphene-based slurries. The pore channels are surrounded by almost fully dense walls and have nearly circular cross-sections. The pore volume fraction and the channel size were controllable by the solid content in the slurry. The channels are replicas of entangled dendrites of frozen camphene, which sublimed during the freeze-drying process. This porous structure with entangled pore channels is considered potentially useful in many applications such as implantable bone scaffolds.