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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.
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.
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•Acid treatment generates oxidized species for all the temperatures investigated.•At high treatment temperatures a contribution of a disorder graphene peak is observed.•The amount of ...the oxidized species varies with the treatment temperature.•There is deterioration in the walls of the carbon nanotubes after acid treatment.•MWCNT treated at 110 °C present greater mobility at acid pH than the others.
We present a study of sulfonitric treatment and its effect on MWCNTs at different temperatures (90, 110, 130 and 150°C) using DRX, XPS, FTIR, Raman spectroscopy, TEM and zeta potential. It was found that oxidation starts with the C-C and C-H bonds generating different oxidized groups from alcohol to carboxylic acid, following a sequential oxidation. Given that heterocoagulation needs a maximum zeta potential gap between the ceramic and the MWCNT surface and it significantly exist a risk of manipulate acids at high temperature it is recommended to use acid treatment of CNT at 110 °C for generating ceramic composites by heterocoagulation.
The addition of sintering additives has always been detrimental to the mechanical properties of sintered ceramics; therefore, methods to reduce or, as in this case, eliminate sintering additives are ...usually relevant. In this paper, dense zircon ceramics were obtained starting from mechanically activated powder compacted by spark plasma sintering without employing sintering additives.
The high energy ball milling (HEBM) of starting powder was effective to enhance the sintering kinetics. The structural changes of the zircon powder introduced by the HEBM were evaluated. The phase composition and the microstructure of bulk zircon material were analyzed by SEM (EDAX) and XRD. The Vickers hardness and the fracture toughness were evaluated as well.
Fully dense materials were obtained at 1400°C with a heating rate of 100°C/min, 10min soaking time and 100MPa uniaxial pressure. The zircon samples sintered at temperatures above 1400°C were dissociated in monoclinic zirconia and amorphous silica. The dissociation was detrimental for the mechanical properties. Unlike conventional sintering methods (hot pressing, pressureless sintering) SPS permitted to overcome the dissociation of the zircon material and to obtain additive free, fully dense zircon ceramic with outstanding mechanical properties.
Lanthanum silicate oxyapatite (LSO) is an encouraging material for its use as an electrolyte in intermediate temperature solid oxide fuel cells. There is a vacancy in knowledge respecting to LSO's ...mechanical properties, and thus they are the scope of research of the present work.
Isostatically pressed bars were sintered at temperatures from 1200 °C to 1500 °C. The dependence of Young's modulus with porosity was fitted using three different modeling curves, and extrapolated for a fully dense material. The fitting of these equations give an E0 between 144 GPa and 169 GPa. This is the first time that LSO's Young's modulus is reported.
Flexural strength,Vickers hardness, fracture toughness (KIC) from microindentation cracks and nanohardness were also measured. Scanning electron micrography was performed on both a sub-sintered and the most sintered sample, and the microstructure and fracture mechanism were analyzed.
In general, the mechanical behaviour is similar to other intermediate temperature solid oxide electrolytes. The fracture mechanism of the most sintered sample makes it well-suited for future SOFC applications.
Highly concentrated zirconia-carbon nanotube (CNT) water suspensions were prepared using an advanced milling technique. The bead-milling operation parameters were optimized for this system and used ...to prepare zirconia-stabilized water-based suspensions with different CNT contents. The effects of different milling conditions were studied. The particle dispersion was evaluated by SEM observations on dried suspension. Green’s density and SEM observations of compacts were used to follow the colloidal dispersability of the composites. Materials of tetragonal zirconia and CNTs were prepared with a high concentration of CNTs (1, 5, and 10 wt pct CNT). The homogeneous dispersion and distribution of the fibers in the bulk material after slip casting of the suspension were examined. The samples were sintered using spark plasma sintering (SPS) at 1473 K (1200 °C) and finally, fully dense materials were obtained. The mechanical properties were evaluated using the Vickers indentation technique.
During a heating–cooling cycle, zirconia (ZrO
2
) undergoes a martensitic transformation from monoclinic to tetragonal structure phases, which presents special hysteresis loop in the dilatometry ...curve at temperatures between 800 and 1100 °C. Monoclinic zirconia (m-ZrO
2
) particles reinforced ceramic matrix composites not always present this behavior. In order to elucidate this fact a series of zircon–zirconia (ZrSiO
4
–ZrO
2
) ceramic composites have been obtained by slip casting and characterized. The final properties were also correlated with the zirconia content (0–30 vol.%). The influence of the martensitic transformation (m–t) in well-dispersed zirconia grains ceramic composite on the thermal behavior was analyzed. Thermal behavior evaluation was carried out; the correlation between the thermal expansion coefficients with the zirconia content showed a deviation from the mixing rule applied. A hysteresis loop was observed in the reversible dilatometric curve of composites with enough zirconia grains (≥10 vol.%). Over this threshold the zirconia content is correlated with the loop area. The transformation temperatures were evaluated and correlated with the zirconia addition. When detected the m–t temperature transformation is slightly influenced by the zirconia content (due to the previously evaluated decrease in the material stiffness) and similar to the temperature reported in literature. The reverse (cooling) transformation temperature is strongly decreased by the ceramic matrix. The DTA results are consistent with the dilatometric analysis, but this technique showed more reliable results. Particularly the endothermic m–t transformation temperature showed to be easily detected even when the only m-ZrO
2
present was the product of the slight thermal dissociation of the zircon during the processing of the pure zircon material.
Mullite–zirconia composites were prepared using two different processing routes such as reaction sintering (RS) of alumina and zircon and direct sintering of mullite–zirconia grains. For the RS ...process, alumina and zircon (AZ) as raw materials were used, while in the other route mullite–zirconia-electrofused grains (MZ) previously mixed with a second ceramic phase as a binder were employed. These both types of ceramics were produced by slip casting in plaster molds and finally sintered at 1600
°C for 2
h. In this work, the microstructure and the resistance to thermal shock for both high-density mullite–zirconia composites were studied. For materials characterization, density, XRD and SEM techniques were employed. Thermal shock behavior was determined by quenching in water with Δ
T of 200–1200
°C and for 1–10 repeated cycles for each Δ
T used. The variation of damage with thermal shock severity was followed by measuring the dynamic elastic modulus
E using the impulse excitation method.
The two processing routes resulted in ceramic composites with similar phase contents, but with microstructural differences. The dependence of elastic modulus on severity of thermal shock was in accordance with literature predictions, where below a critical quenching temperature difference (Δ
T
c) no degradation in the elastic behavior was found.
The thermal shock resistance increased when zircon was used as bonding phase, rather than alumina or alumina–zircon mix.
We have studied the sintering kinetic of 3 and 8mol% of yttria stabilized zirconia under isothermal conditions. Sintering was performed in the temperature range between 1200 and 1450°C. The sintering ...kinetic process was followed by measuring density as a function of sintering time. A model was applied to the first stage of densification. Sintering obeys to the grain boundary diffusion mechanism for both materials. It was possible to calculate the activation energy as well as the diffusion coefficients. 887 and 731kJmol−1 were the activation energies for the initial stage of sintering for 3YTZ and 8YSZ respectively.
Finally the diffusion activation energy was estimated for both materials. The diffusion coefficients were also estimated at 1400°C in 4.05×10−14 and 6.00×10−11cm2s−1 for tetragonal and cubic zirconia respectively. The obtained results support the observations of a faster densification for 8YSZ.
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