Characterization of Freeze-Dried Al2O3 and Fe2O3
01 Aug 1970-Journal of the American Ceramic Society (Blackwell Publishing Ltd)-Vol. 53, Iss: 8, pp 440-444
TL;DR: In this article, the formation and growth of chain-like aggregates of crystallites were studied for the representative materials Al2O3 and Fe 2O3, and X-ray line broadening data showed that crystallite size is a function of the 1/5 to 1/7 power of time for isothermal treatments.
Abstract: Oxide crystallite formation and growth from freeze-dried sulfates were studied for the representative materials Al2O3 and Fe2O3. Transmission and scanning electron micrographs showed the formation and growth of chainlike aggregates of crystallites. Aggregation occurred as part of the nucleation and growth of the oxide, and discrete oxide particles were never present. Orientation of the chain aggregates was related to the ice structure formed during freezing. X-ray line broadening data showed that crystallite size is a function of the 1/5 to 1/7 power of time for isothermal treatments. A qualitative analysis of material transport favored the surface diffusion mechanism.
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TL;DR: This review addresses recent advances made in studies of hierarchically porous materials and methods to control their structure and morphology and hopes that this review will be helpful for those entering the field and also for those in the field who want quick access to helpful reference information.
Abstract: Owing to their immense potential in energy conversion and storage, catalysis, photocatalysis, adsorption, separation and life science applications, significant interest has been devoted to the design and synthesis of hierarchically porous materials. The hierarchy of materials on porosity, structural, morphological, and component levels is key for high performance in all kinds of applications. Synthesis and applications of hierarchically structured porous materials have become a rapidly evolving field of current interest. A large series of synthesis methods have been developed. This review addresses recent advances made in studies of this topic. After identifying the advantages and problems of natural hierarchically porous materials, synthetic hierarchically porous materials are presented. The synthesis strategies used to prepare hierarchically porous materials are first introduced and the features of synthesis and the resulting structures are presented using a series of examples. These involve templating methods (surfactant templating, nanocasting, macroporous polymer templating, colloidal crystal templating and bioinspired process, i.e. biotemplating), conventional techniques (supercritical fluids, emulsion, freeze-drying, breath figures, selective leaching, phase separation, zeolitization process, and replication) and basic methods (sol–gel controlling and post-treatment), as well as self-formation phenomenon of porous hierarchy. A series of detailed examples are given to show methods for the synthesis of hierarchically porous structures with various chemical compositions (dual porosities: micro–micropores, micro–mesopores, micro–macropores, meso–mesopores, meso–macropores, multiple porosities: micro–meso–macropores and meso–meso–macropores). We hope that this review will be helpful for those entering the field and also for those in the field who want quick access to helpful reference information about the synthesis of new hierarchically porous materials and methods to control their structure and morphology.
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TL;DR: In this article, the authors demonstrate the capability of the ice-segregation-induced self-assembly (ISISA) process for the preparation of materials with highly sophisticated structures (e.g., hierarchical materials exhibiting organization at different scale levels).
Abstract: This review aims to demonstrate the capability of the ice-segregation-induced self-assembly (ISISA) process for the preparation of materials with highly sophisticated structures (e.g., hierarchical materials exhibiting organization at different scale levels). Cryogenic processes (consisting of the freezing, storage in the frozen state for a definite time, and defrosting of low - or high-molecular-weight precursors, as well as colloid systems, as either a water solution or suspension, or forming a hydrogel) have been widely used for the scaffolds preparation. However, the recent success in the control of the morphology (e.g., by unidirectional freezing in nitrogen liquid) and the possibility to extend the compositional nature of the resulting materials has recently attracted much attention to the ISISA process. Besides, this review aims to exemplify how the aqueous nature of the ISISA process allows for the in-situ incorporation of biological entities which provides not only hierarchy but also functionalit...
400 citations
TL;DR: In this paper, the surface area and residual carbon content of the strontium-substituted LaMnO3 have been determined as a function of the decomposition/oxidation temperature.
Abstract: The amorphous citrate process has been used to produce Mn2O3, Mn3O4, LaMnO3, SrMnO3 and strontium-substituted LaMnO3. The citrate-nitrate gels were dehydrated at 70‡ C to yield solid precursor materials. The decomposition/oxidation of the precursors have been studied using thermogravimetry and evolved gas analysis. The products of decomposition have been characterized by X-ray diffraction analysis, scanning electron microscopy and, in the case of the strontium-substituted LaMnO3, by analytical electron microscopy. The surface area and residual carbon content of the strontium-substituted LaMnO3 have been determined as a function of the decomposition/oxidation temperature. Both the process yield and compositional homogeneity of the strontium-substituted LaMnO3 have been shown to increase as the decomposition temperature increases. The residual carbon content has been shown to decrease as decomposition temperature increases. However, the surface areas of the powders decrease significantly as decomposition temperature increases. Consequently, it is evident that there is a conflict in the experimental conditions required for optimum yield, homogeneity and residual carbon content compared to those required for maximum surface area.
319 citations
TL;DR: Extensive grain growth was observed by scanning electron microscopy in very porous Al2O3 compacts, even at densities <40% of theoretical after ∼7% shrinkage at 1700°C, the grain size increased from ∼03 to 051 μm in a compact having a relative green density of 031 as mentioned in this paper.
Abstract: Extensive grain growth was observed by scanning electron microscopy in very porous Al2O3 compacts, even at densities <40% of theoretical After ∼7% shrinkage at 1700°C, the grain size increased from ∼03 to 051 μm in a compact having a relative green density of 031 During grain growth in highly porous compacts, the grains appear initially to be chainlike, then to be oblong, and finally to be equiaxed The proposed mechanism of initial grain growth involves the filling of necks between adjacent grains followed by the movement of the grain boundary through the smaller grain Although grain growth in very porous compacts is quite different from coalescence and ordinary grain growth, the kinetics are similar
207 citations
TL;DR: In this paper, a quantitative fatigue model is proposed for ferroelectric thin films on the basis of effective one-directional movement of defects due to asymmetric polarization under alternating pulses, defect entrapment at electrode-ferroelectric interface (and/or at grain and domain boundaries), and the resultant polarization loss at these surfaces due to structural damage.
Abstract: A quantitative fatigue model is proposed for ferroelectric thin films on the basis of effective one-directional movement of defects due to asymmetric polarization under alternating pulses, defect entrapment at electrode-ferroelectric interface (and/or at grain and domain boundaries), and the resultant polarization loss at these surfaces due to structural damage. A fatigue equation is derived in accordance with this model and three fatigue parameters such as initial saturation polarization, piling constant, and decay constant are defined. The predicted values from the model show very good agreement with experimental data. Temperature and voltage dependence of fatigue parameters are studied on the basis of the fatigue model. The jump distance of defects is calculated from the voltage dependence of the decay constant which turns out essentially to be the same as the lattice parameter. Based on the fatigue model, an improved method to overcome fatigue property is also presented.
Un modele quantitatif de tenue en fatigue de films ferroelectriques minces est propose sur la base de mouvements effectifs unidirectionnels des defauts cause par polarization asymetrique sous impulsions alternatives, capture des defauts a l'interface electrode-ferroelectrique (et/ou celle des frontieres de grains at de domaines), et la perte de polarisation resultante dans des surfaces a ete obtenue en accord avec ce modele et trois parametres de fatigue tel que la polarisation de saturation initiale, la constante d'empilement et la constante de deterioration ont ete definis. Les valeurs predites par le model montrent un tres bon accord avec les resultats experimentaux. La dependance des parametres de fatigue en fonction de la temperature et du voltage ont ete etudies sur la base du modele de fatigue. La distance de saut (qui apparait en fait ětre pratiquement egale au parametre de lattice) des defauts a ete calcule a partir de la dependance de la constante de deterioration en fonction du voltage. Sur la base de ce modele de fatigue, une methode pour ameliorer les proprietes de tenue en fatigue est presentee.
162 citations