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•After the acid treatment highly increase the amount carbonyl and carboxylic groups.•The oxidation of MWCNT generates a high negative charge of it in all the pH range.•It could ...achieve a good dispersion of the MWCNT in water-based suspension.•There is morphological damage on the surfaces of MWCNT after the acid treatment.•Some surface defects but no shortening were observed by TEM images.
Carbon nanotubes are widely used for electronic, mechanical, and optical devices due to their unique structural and quantum characteristics. The species generated by oxidation on the surface of these materials permit binding new reaction chains, which improves the dispersibility, processing and compatibility with other materials.
Even though different acid treatments and applications of these CNT have been reported, relatively few research studies have focused on the relationship between the acid treatment and the formation of nanodefects, specific oxidized species or CNT surface defects.
In this work, multiwall carbon nanotube (MWCNT) oxidation at 90°C was characterized in order to determine the acid treatment effect on the surface.
It was found that oxidized species are already present in MWCNT without an acid treatment, but there are not enough to cause water-based dispersion. The species were identified and quantified by infrared spectroscopy and X-ray photoelectron spectroscopy. Also, transmission electron microscopy observations showed not only modifications of the oxidized species, but also morphological damage on the surfaces of MWCNT after being subjected to the acid treatment. This effect was also confirmed by Raman spectroscopy. The acid treatment generates higher oxidized species, decreasing the zeta potential in the whole pH range.
A Zr-modified bentonite (B-OHZr) obtained by intercalation of OH-Zr species was obtained, and its thermal transformations at > 800 °C were studied. Raw clay and B-OHZr were characterized by X-ray ...diffraction, thermogravimetric and differential thermal analysis, and its sintering behavior was studied by hot-stage microscopy. Two Zr-based phases—tetragonal zirconia and zircon—were observed in thermally treated B-OHZr with a sharp decrease in amorphous glassy phase in comparison with the parental clay. An increase in refractoriness was observed in B-OHZr, enabling the sintering of samples up to 1300 °C, while the parental clay bloated and deformed with thermal treatment. The Zr interlayering enhanced the refractoriness at high temperatures of the parental clay, eliminating thermal expansion (bloating) and enabling sintering at
T
> 1000 °C. The observed features and thermochemical processes enlighten the temperature usage range of these kinds of materials. Finally these results yield promising features for potential use of these clays for the preparation of ceramic materials with zircon content.
Aluminum titanate Al
2
TiO
5
materials were successfully processed from different fine commercial powders and characterized. Particularly, two calcined aluminas were compared through a multitechnique ...approach including differential thermal analysis and dilatometry together with structural, microestructural, and mechanical characterization. This allowed the description of all the thermochemical processes during thermal treatment. Developed phases were established. Relatively dense ceramics were obtained, and complex microstructures were described with interlocked grains and an interconnected microcrack matrix that do not jeopardize the material integrity. Multistep sintering and reaction sintering processes were observed in both samples. The first stage consists of the sintering of the starting powders (alumina and titania). A second sintering stage of the starting powders was observed for both samples as well. Once advanced, the second one is overlapped with Al
2
TiO
5
formation that starts at 1380 °C and finishes at 1440 °C. They affect crack development and, in consequence, the thermal behavior. The lower alumina particle size enhances the sintering and reaction advance processes. In the technological temperature range (room temperature—1000 °C), low or even negative thermal expansion behaviors were observed in the developed materials. This, together with the mechanical behavior, encourages structural applications with high thermomechanical solicitations of Al
2
TiO
5
based materials.
The formation of aluminum borates (Al18B4O33 and Al4B2O9) from alumina and boron oxide occurs between 600 and 800°C. These materials have refractory properties and corrosion resistance. The objective ...of this work is to develop materials from the Al2O3-B2O3 system, employing alumina and boric acid as starting powders, to study the critical processing variables and describe the developed microstructure and properties.
Three formulations (13, 19.5 and 26wt% B2O3) were studied. In order to confirm the formation of borates, the differential thermal analysis and thermogravimetric analysis were carried out. Afterwards, uniaxially pressed disc-shaped specimens were fired at four temperatures above the formation temperature. The textural properties of the ceramics were evaluated by the immersion method, this permit to evaluate the sintering processes. Then the degree of borate formation was confirmed by X-ray diffraction.
Finally, the developed microstructures were characterized by scanning electron microscopy, and the diametral compression behavior was evaluated.
A series of porous (≈50%) refractory materials from the Al2O3-B2O3 system were developed. The processing strategy resulted in materials with Al18B4O33 as the main crystalline phase. Needle grains with diameters between 0.2 and 1µm and an aspect ratio over 20:1 were obtained. Thus, based on the information gathered from our research, aluminum borate ceramic materials can be designed for structural, insulating or filtering applications employing only alumina and boric acid as boron oxide source.
This work reports a gauge-including projected augmented waves (GIPAW) method study of the structural and electronic properties of kaolinite (Al2Si2O5(OH)4). Different equilibrium structures were ...determined, and the corresponding electronic density of states and the electric field gradients (EFG) at each atomic site were calculated. The comparison of those predicted properties with measurements that come from neutron powder diffraction, single crystal synchrotron measurements, and nuclear magnetic resonance spectroscopies allowed a detailed study of the structure. In particular, the first principles calculations considered in this work cover different scales, going from the crystallographic structure to the atomic local environments, and serve as a tool to link both the structural and the hyperfine properties. This methodology predictions successfully solves a kaolinite structure in which the atomic local surroundings are consistent with the EFG measurements, thus providing answers to previous controversies between experimental studies about Al and Si local structures and the orientation of OH groups within the structure. In this sense, the GIPAW calculations support that kaolinite consists of asymmetrically distorted Si tetrahedra and Al octahedra sheets, and the interlayer OH groups are oriented nearly perpendicular to the layer.
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•A detailed first principles study of the bulk structure of kaolinite is performed.•This study focuses on the electric-field-gradients at the atomic sites.•The predicted atomic local structures are compared with available experimental data.•Predictions solve previous controversies around kaolinite bulk structure.•The electronic density of states of kaolinite is also analyzed.
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