Al(2)O(3(w))-Al(2)O(3(n))-ZrO(2) (TZ-3Y)(n) multi-scale nanocomposite: an alternative for different dental applications?

The influence of the addition of Al(2)O(3) whiskers (2.5wt.% up to 30wt.%) on Vickers hardness and fracture toughness in an Al(2)O(3(n))+ZrO(2) (TZ-3Y)(n) (90, 80 and 70wt.%) composite was investigated. Green compacts were obtained by uniaxial pressing at 50MPa and pressureless sintering at 1500 degrees C in air for 2h. After sintering, relative densities ranging from 75% to 97% were reached. The whiskers resisted particle rearrangement owing to the extensive sliding distances along the whisker boundaries during sintering and the high length/diameter ratios. Sintering becomes more difficult with increasing whisker content, because whiskers come into contact with each other, forming a rigid network which hinders densification. The 2.5wt.% Al(2)O(3) whiskers+27.5wt.% Al(2)O(3) nanoparticles+70wt.% TZ-3Y composite showed a hardness>13GPa and a maximum fracture toughness of 6.9MPam(-1/2), with an average grain size of 0.4+/-0.17microm. The observed crack deflection was an important mechanism in the improved fracture toughness of the composite. In addition, the grain size and residual porosity also seem to be factors in obtaining a wide range of hardness as well as fracture toughness by varying the Al(2)O(3) whiskers and ZrO(2) (TZ-3Y) content. The use of alumina-whisker-reinforced composites in dental applications could be promising for increasing hardness and fracture toughness compared with other materials. The reported values for these composites can compete with those of commercially available materials in different dental applications.

In situ study of the metal-insulator transition in VO2 by EELS and ab initio calculations.

Changes in the dielectric properties during the thermochromic transition of commercial VO(2) powders were determined in situ, by analyzing the low-loss region of the electron energy-loss spectroscopy (EELS) spectra in a transmission electron microscope at room temperature (insulator phase) and 100 degrees C (metallic phase). A comparison of experimental EELS spectra and ab initio density-functional theory calculations (WIEN2k code) within the generalized gradient approximation (GGA) is presented. A characteristic peak around 5.6 eV appears in the energy-loss function in metallic phase, which is absent in insulator phase. The origin of the characteristic peak is analyzed by means of energy-band structure calculations.

Evidence of multi-walled carbon nanotube fragmentation induced by sonication during nanotube encapsulation via bulk-suspension polymerization.

The synthesis of multi-walled carbon nanotube/polystyrene composites, with nanotube concentrations of 0.04, 0.08 and 0.16 wt%, was carried out by in situ bulk-suspension polymerization with the assistance of sonication. By using this method both encapsulation and exfoliation of the nanotubes into the polymer host were achieved. Evidence of significant nanotube fragmentation was found by scanning electron microscopy; the cause of such fragmentation was attributed to the induction of strong cavitation due to the application of ultrasound during the synthesis. Infrared spectroscopy showed no evidence of the formation of covalent bonds between the nanotubes and the polystyrene during the process of synthesis. The thermal stability was not improved by the inclusion of the nanotubes, it was attributed to the low nanotube concentrations; however, composites glass transition temperature showed improvements.

Experimental and theoretical determination of the low-loss electron energy loss spectroscopy of LiMn2O4.

The dielectric properties of cubic spinel-type LiMn(2)O(4), used as cathode material in lithium ion secondary batteries, are studied by analyzing the low-loss region of the electron energy loss spectroscopy (EELS) spectrum in a transmission electron microscope. A comparison of experimental EELS spectra and ab initio density-functional theory calculations (WIEN2k code) within the generalized gradient approximation (GGA) is presented. The origins of interband transitions are identified in the electronic band structure, by calculating the partial imaginary part of the dielectric function and the partial density of states of Li, Mn and O. Good agreement with experimental spectra is observed which allowed interpreting main features of the EELS spectrum.

Phenol sorption on surfactant-modified Mexican zeolitic-rich tuff in batch and continuous systems.

Surfactant modified clinoptilolite-rich tuff was used for the removal of phenol from aqueous solutions. The zeolitic rock from Oaxaca (Mexico) was treated with sodium chloride and then modified with hexadecyltrimethylammonium chloride or bencylcetildimethylammonium chloride in different experimental conditions. Phenol sorption isotherms and column experiments were performed; in both cases, phenol was determined in the aqueous solutions by UV-vis spectroscopy. The results showed that the sorption of phenol depends on the quantities of surfactant in the zeolitic rocks, the kind of surfactant, modification conditions and pH of the solutions. When the initial concentration of phenol increased, the adsorption of phenol in the surfactant-modified zeolite increased and the experimental data was best adjusted to the Langmuir model. The saturation of the columns rapidly reached high percentages.

Ion exchange resins as catalyst for the isomerization of alpha-pinene to camphene.

Camphene is an industrial intermediate compound for commercial chemicals such as isoborneol, isobornyl acetate and camphor. Industrially, the conventional process for camphene production consists of the isomerization of alpha-pinene using acidic TiO2 as catalyst. The use of this catalyst presents problems such as considerable time for preparation, reproducibility and recovery of catalyst from products after the alpha-pinene isomerization. For the first time, a commercial exchange resin was used as catalyst for this reaction. Based on the concentration of product as a function of the reaction time, the path of the alpha-pinene transformation to camphene and byproducts is proposed. Temperature and alpha-pinene/catalyst ratio were studied in order to optimize the yield to camphene production. The obtained results were comparable with those reported for acidic TiO2.