Search

The characteristics and properties of fumed oxides depend strongly on various external actions that is of importance from a practical point of view. Therefore, gelation or high-pressure cryogelation (HPC) of aqueous media pure or with 0.1 M NaCl, and mechanochemical activation (MCA) of dry or wetted powders of individual (silica, alumina, their mechanical blends) and complex (silica/titania, alumina/silica/titania, AST1, AST1/A-300) nanooxides were studied to analyze the influence of the nanooxide composition, particulate morphology, and preparation conditions on changes in the morphological and textural characteristics of treated samples. The temperature-pressure behavior of different phases (silica, alumina, and titania) under HPC can result in destroy of complex core-shell nanoparticles (100-200 nm in size) in contrast to small nonporous nanoparticles, NPNP (5-20 nm). The textural characteristics of nanooxides are sensitive to any external actions due to compaction of such supra-NPNP structures as aggregates of nanoparticles, agglomerates of aggregates, and visible structures in powders. The compaction of supra-NPNP enhances the pore volume but much weakly affects the specific surface area (with one exception of AST1) because small NPNP are relatively stable during any external actions (HPC, MCA). The compacted materials are characterized by enhanced mesoporosity shifted to macroporosity with decreasing specific surface area and increasing sizes of nanoparticles or to mesopores with increasing MCA time or amounts of water in wetted powders. At low hydration of the A-300 powder (h = 0.5 g/g), the value of SBET slightly increases if MCA is provided by stirring or ballmilling. Diminution of the freezing temperature from 208 to 77.4 K during HPC results in enhanced compaction of aggregates and agglomerates but this does not practically affect the primary nanoparticles. The degree of decomposition of core-shell nanoparticles of AST1 does not practically increase with decreasing freezing temperature from 208 to 77.4 K. Decomposition of core-shell AST1 particles is inhibited under HPC by added A-300 (1 : 1) working as a damper. [ABSTRACT FROM AUTHOR]

In this present study, the preference selection index method (PSI) was applied to identify the best formulation among ceramic fillers such as glass ionomer cement (GIC), nanosilica and nanoalumina nanofiller in the dental composites. The comparative assessment of the effect of nanofillers (nanoalumina and nanosilica fillers) and glass ionomer cement has been evaluated. Dental composites with 0, 10, and 20 wt% of nanoalumina, nanosilica and glass ionomer cement were fabricated. These composites were analyzed for physico‐mechanical and wear properties. The PSI multi‐criteria decision making technique was used to rank the composites with various filler content. The performance defining attributes for the comparative assessment were set as void content, hardness, compressive strength, flexural strength, diametral tensile strength, and wear properties against load and speed. The PSI of dental composite reinforced with 20 wt% nanosilica (DCSi‐20) was maximum (0.896) followed by dental composite reinforced with 20 wt% glass ionomer (DCGi‐20) (0.889) and hence DCSi‐20 demonstrated as most preferred dental composite material followed by DCGi‐20. DC‐0 showed the least preference dental composite materials on the basis of desirable physical, mechanical, and wear properties. Finally, it can be concluded that despite of more void content and low flexural strength, overall performance of glass ionomer is comparable with nanosilica and more significant with respect to nanoalumina. [ABSTRACT FROM AUTHOR]

In this study, nanoalumina (Al2O3) and nanosilica (SiO2) particles are evaluated as tougheners for a high glass-transition temperature (Tg) epoxy system in correlation with liquid composite molding (LCM) processability. The aim of this paper is to directly compare the effectiveness of nanoalumina and nanosilica of the same nominal particle size as epoxy tougheners on the same neat resin system. The epoxy resin system used in this study was Dow D.E.R. 330 amine cured epoxy with a Tg of 150℃. Both particle types are observed to be Tg neutral and increase fracture toughness of the base epoxy system. Between the two particle types, nanoalumina is found to be more effective than nanosilica in terms of achievable fracture toughness at a given particle loading. As resin viscosity increases with particle addition, the addition of fewer particles with the use of nanoalumina is also beneficial to LCM processing where a lower viscosity is preferable. [ABSTRACT FROM AUTHOR]

The influence of incorporation of micro and nano sized particles such as fly ash, silica fume, nanosilica and nanoalumina on the hydrated cementitious system is studied. The transformation in the hydrated products due to these additions are analyzed using X ray diffraction and scanning electron microscopy. Further, transition in the pore structure owing to the formation of hydrated products are evaluated through nitrogen adsorption/desorption (NAD) technique method and image analysis. The pore structure is characterized by the total pore volume and pore size distribution using NAD method and pore area fraction from image analysis. It is found that the micro and nano additions do not always improve the microstructure by reducing the pores, sometimes (in case of addition of nano materials/silica fume) increase the size of the pores by pore widening phenomena. Neverthless, it is realized that the nano alumina addition helps in filling the pores only at later ages. [ABSTRACT FROM AUTHOR]

High-pressure cryogelation (HPC) of individual (silica, alumina, their mechanical blends) and complex (silica/titania, alumina/silica/titania, AST) nanooxides were studied to reveal the influence of nanooxide composition and gelation conditions on changes in the textural characteristics of the materials. The temperature-pressure behavior of different phases (silica, alumina, and titania) under HPC can result in destroy of complex nanoparticles, especially large (100-200 nm in size) core-shell secondary nanoparticles composed of a number of small primary nanoparticles (~10 nm in size) covered by a shell destroyed under HPC. The main changes in the texture of HPC or gelled nanooxides are due to rearrangement (compaction) of secondary structures such as aggregates of primary nanoparticles and agglomerates of aggregates. This compaction enhances the pore volume but much weakly affect the specific surface area (with one exception of AST) because small primary nanoparticles (10-20 nm in size) are relatively stable during HPC. The HPC materials are characterized by enhanced mesoporosity shifted to macroporosity with decreasing specific surface area and increasing size of primary nanoparticles. Diminution of the freezing temperature from 208 to 77.4 K during HPC results in an increase in pressure (up to 1,050 atm in stainless steel freezing bombs) and enhanced compaction of aggregates and agglomerates but this does not practically affect the primary nanoparticles. The degree of decomposition of secondary core-shell nanoparticles of AST does not practically increase with this decreasing freezing temperature. Graphical Abstract: High-pressure cryogelation can destroy core-shell nanoparticles. [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]

Fumed silica, initial alumina blend, after mechanochemical activation (MCA), and fumed binary silica/alumina (SA) were studied using infrared spectroscopy, X-ray diffraction (XRD), ultrasoft X-ray emission (USXE) spectroscopy (giving emission bands Si L α , Al L α and O K α related to valence electron transfer onto core levels) and quantum chemistry. The MCA influence on nanoparticle characteristics (sizes, electronic structure) increases with increasing alumina content in the blends due to stronger abrasive effect of alumina nanocrystallites (snagging a surface layer of nanoparticles during MCA) than non-crystalline silica nanoparticles. A difference in Si L α , Al L α and O K α affected by MCA increases with increasing alumina content. It is greater for the top peak of the upper valence band (UVB) than for a lower energy peak in the bottom of the UVB. These spectral changes suggest redistribution of electron density between Si, Al and O atoms depending on alumina content, material type and treatment conditions. The main difference in the properties of the SA blends and binary SA is due to distribution of Si atoms in alumina phase and Al atoms in silica phase in fumed SA (which is amorphous at C Al 2 O 3 ≤ 30 wt.%) in contrast to the SA blends with practically separated silica and alumina nanoparticles. [ABSTRACT FROM AUTHOR]

To study changes in the characteristics of gelled/dried nanooxides (fumed silicas PS300 and PS100, alumina, silica/titania ST, alumina/silica/titania AST), high-pressure cryogelation, HPCG (at 208K or 260K) or gelation (293K, 1atm) was carried out using 20wt.% aqueous suspensions pure or with addition of 0.1M NaCl. The nanooxide samples were studied after drying at room temperature. Maximal changes in the textural and crystalline characteristics are observed for cryogels with AST prepared at ~1000atm and 208K due to decomposition of complex nanoparticles. Its specific surface area SBET increases from 83 to 160m²/g, and the crystallinity degree grows by 11%. Mixing of PS300 and AST (1:1, w/w) in the suspension prevents the decomposition of AST particles in the cryogel since SBET decreases in comparison with that of the initial blend powders. Addition of NaCl (2.8wt.% in dried powders) reduces decomposition of AST particles due to changes in HPCG conditions. For binary ST, SBET increases but much smaller than for AST. HPCG of individual silica and alumina leads to a decrease in the SBET value; i.e. nanoparticles are not decomposed. A significant increase in the volume of large mesopores (pore radius 5-10nm

Crystalline and electronic structures of nanoalumina alone and in different mixtures with nanosilica have been analyzed using X-ray diffraction (XRD) and ultrasoft X-ray emission spectroscopy (USXES). Narrowing of the OKα bands of crystalline alumina occurs with decreasing nanoparticle size. Electron transfer from oxygen to aluminum atoms due to Al-O bond breakage under irradiation leads to occupation of high-energy 3d level of Al. In nanoparticles, the Al-O bond strength and O-O interactions enhanced by Laplace (bubble) pressure lead to δ phase stabilizing if coherent-scattering region size (dCSR) reaches 7 nm which is smaller than that for ϑ phase (dCSR = 11-20 nm) of alumina and silica/alumina studied. The dCSR values are in agreement with nanoparticle sizes calculated using self-consistent regularization method applied to nitrogen adsorption data. [ABSTRACT FROM AUTHOR]

Nanoscience and nanotechnology provide numerous opportunities for enhancing the properties of wood composites. Formaldehyde emissions from wood composites are of great importance because of their negative impact on human health. Developing low formaldehyde-emitting particleboard and plywood panels as environmentally friendly composites by nanotechnology was the object of this study. The urea formaldehyde and melamine urea formaldehyde resins that were used to produce particleboard and plywood panels, respectively, were reinforced with various nanomaterials at different loading levels. Formaldehyde emission tests were carried out according to standard TS 4894 EN 120. The results acquired in this work indicated that nanomaterial reinforcement significantly affected the formaldehyde emission properties of the particleboard and plywood panels. Formaldehyde emissions of the composite panels decreased after reinforcement with nanoSiO2, nanoAl2O3, and nanoZnO materials at proper loading levels. Therefore, using nanotechnology, it is possible to produce environmentally friendly wood composite panels with low formaldehyde emissions. [ABSTRACT FROM AUTHOR]

Abstract: Morphological, structural, and adsorptive characteristics of such fumed oxides as alumina, silica, composite silica/alumina and alumina/silica/titania were studied using a variety of experimental methods. Heating of nanooxides caused several processes resulting in the temperature dependent structural changes of the materials because of changes in the amounts of intact and dissociatively adsorbed water not only at the surface of the nanoparticles but also in their volume that lead to changes in the oxide matrix structure. These processes dependent on the composition of nanooxides are much stronger for nanosilica than for alumina or composite oxides. The morphology of primary nanoparticles is more stable on different treatments than the structure of secondary and ternary particles. The amounts of water adsorbed onto the nanooxides from air correspond to the amounts of water strongly bound (unfrozen at T <273K) to the oxide surfaces in aqueous suspensions. [Copyright &y& Elsevier]

In this study polyimide/inorganic silica-alumina nanoparticle hybrid films are first prepared by sol-gel method to find a promising method for enhancing thermal property and other properties. Silica sol and alumina sol starting from tetraethoxysilicane (TEOS) and aluminum isopropylate (ALP) are doped in a solution of poly (amic acid). The hybrid films are obtained by a stepwise heating method. The structure, micro-morphology, and thermal properties of the films are characterized by Fourier transform infrared (FTIR), scanning electron microscopy (SEM), and thermogravimetric analysis (TGA). The micrographs show uniform distribution of club shaped particles in the films containing silica and alumina. The micrographs of polyimide/silica hybrid films revealed sphericity and uniform distribution of particles. Al2O3 particles are poorly dispersed and are prone to agglomerate in the polyimide/alumina hybrid films. Moreover, there are chemical bonds between inorganic and organic phases in the hybrid films. The thermo-decomposition temperature of the hybrid films is also improved markedly after the addition of silica and alumina nanoparticles. [ABSTRACT FROM AUTHOR]