Superior stability of a bifunctional oxygen electrode for primary, rechargeable and flexible Zn-air batteries.

Central to commercializing metal-air batteries is the development of highly efficient and stable catalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). In this study, a composite catalyst with a unique interpenetrating network (denoted as NiCo2O4@MnO2-CNTs-3) was synthesized and exhibited better bifunctional activity (ΔE = 0.87 V) and durability than both Pt/C and Ir/C catalysts. The improved performance arises from three factors: (i) MnO2 promotes the ORR while NiCo2O4 facilitates the OER; (ii) carbon nanotubes improve the electronic conductivity; and (iii) the highly porous structure enables the adsorption-desorption of O2 and enhances the structural stability. As a result, the primary and rechargeable Zn-air battery affords a high power density and specific capacity (722 mA h g-1), an outstanding discharge stability (255 mW cm-2 after 1000 cycles) and a high cycling stability (over 2280 cycles). Electron microscopy and electrochemical analysis revealed that the degradation of the rechargeable Zn-air battery performance resulted from the damage of the air electrode and the hydrogen evolution reaction on the zinc electrode. A flexible Zn-air battery employing a solid-state electrolyte showed an exciting stability (540 cycles) and high power density (85.9 mW cm-2), suggesting that the anion exchange membrane effectively prevents the migration of Zn2+ ions and the deposition of carbonates.

Mechanisms of goniochromism relevant to restorative dentistry.

OBJECTIVES: This study aims to determine the effects of multiple translucent layers, the alignment of composite structures, and specular reflecting backings as goniochromatic mechanisms relevant to dentistry. METHODS: Rectangular composite specimens were filled with very short E-glass fibers (120 microm length, 16 microm diameter). The fibers were oriented random, perpendicular, and parallel to the surface normal using an electric field. A Minolta CS-100 colorimeter was used to measure the color at various angles of samples with various filler alignments, translucent sublayers, and specular reflecting backings. RESULTS: All three investigated mechanisms were proven to have a goniochromatic effect. Filler alignment perpendicular to the surface made the composites more transmissive and reduced the dependence of lightness on observation angle. Backing composites with a pigmented sublayer caused the color of the sublayer to be more apparent when the observation angle is perpendicular to the surface. The specular reflection of a gold backing was only partially diffused by the specimen. SIGNIFICANCE: The determination of the effects of these goniochromatic mechanisms is relevant to dentistry, because teeth are naturally aligned composites composed of multiple translucent layers. Therefore, understanding these goniochromatic effects is important to recreating them in restorative materials. Also, gold backings were previously found to give restoratives a more vital appearance, and this perceived vitality may be related to goniochromism.

Fraunhofer diffraction of short-fiber-reinforced composites aligned by an electric field.

OBJECTIVES: This study aims to determine if aligned, short-fiber composites are capable of producing Fraunhofer diffraction patterns similar to those of human enamel sections and to validate the mechanism of diffraction by comparing the experimental and theoretical fiber spacings. METHODS: Rectangular composite specimens were filled with short E-glass fibers (120 microm length, 15 microm diameter) to contents of 1, 5, 10, and 25% (vol%). The fibers were oriented perpendicular to the surface normal using an alternating electric field of 0.75 kV/mm. A He-Ne laser was used with thin slices of specimen placed behind a pinhole to produce diffraction patterns. The locations of the diffraction maxima were used to determine the theoretical slit spacings which were compared to the experimental slit spacings determined by microscopy. RESULTS: The specimens produced diffraction patterns analogous to the theoretical Fraunhofer diffraction of light through multiple slits. This was verified by comparing the theoretical spacing of the fibers calculated from the diffraction pattern with the experimental spacing of the fibers determined from the optical micrographs. SIGNIFICANCE: This work has verified that orientation of short-fiber composites using an electric field can yield composites with sufficient order to produce Fraunhofer diffraction patterns that are qualitatively similar to the diffraction patterns of human enamel sections.

Diffuse reflectance of short-fiber-reinforced composites aligned by an electric field.

OBJECTIVES: This study aims to characterize the translucency of aligned, short-fiber composites by determining the effects of filler particle orientation on the Kubelka-Munk absorption and scattering coefficients. METHODS: Rectangular composite specimens (n = 3) were filled with very short E-glass fibers. The fibers were oriented random, perpendicular, and parallel to the surface normal using an alternating electric field of 0.75 kV/mm. Diffuse reflectances of the composites on white and black backings were acquired with a spectrophotometer. The Kubelka-Munk model was then employed to calculate absorption and scattering coefficients. RESULTS: The ordering of the reflectances on a white backing is neither the same nor the reverse of the reflectances on the black backing. The effects of orientation were different for the absorption and scattering coefficients. Orientation parallel to the average light flux notably decreases the amount of absorption, while perpendicular orientation appreciably increases the amount of scattering. SIGNIFICANCE: Since orientation affects the absorption and scattering coefficients differently, the ordering of the reflectances may be different when the backing is varied. This is because the translucency is composed of two parameters (absorption and scattering) and cannot be adequately summarized with one parameter such as transmittance or reflectance. Therefore, the use of a diffuse reflectance model, such as Kubelka-Munk, is essential to a quantitative understanding of the translucency effects of filler orientation. Understanding how the filler orientation affects diffuse reflectance of composites will yield insight as to how the orientation of enamel rods affects the diffuse reflectance in teeth by analogy.

Adsorption of catechol and comparative solutes on hydroxyapatite.

Contemporary medical and dental adhesives often have difficulty sticking to wet surfaces or weaken with long-term exposure to water. Substantial research has been dedicated to finding a means of achieving adhesion in an aqueous environment. A study evaluates the adsorption of catechol relative to other chemical groups as means of gauging how effective they may be as adsorptive groups in adhesives. Contact angle and surface-tension measurements of solutions of catechols and other chemical groups were used to determine their works of adhesion. Adsorption isotherms were also constructed to ascertain Langmuir constants. Solutes containing catechol groups were compared to solutes containing other polar groups to see how well catechol adsorbs to hydroxyapatite, the mineral component of bones and teeth, relative to other chemical groups found in adhesives. The results of this study show that catechol and molecules containing catechol groups have higher rates and energies of adsorption to hydroxyapatite than do groups such as alcohols, amines, and carboxylic acids.