Hexagonal Array Of Pores Research Articles

Relations between Pore Structure Parameters and Their Implications for Characterization of MCM-41 Using Gas Adsorption and X-ray Diffraction

Structural parameters for ordered mesoporous materials are shown to be strongly interrelated as a result of their well-defined structures. Equations for mesoporous materials with hexagonal arrays of uniform pores (e.g., MCM-41 and SBA-15) are presented, which can be used to calculate the pore size, pore wall thickness, and specific surface area on the basis of several quantities, which are easily available from X-ray diffraction and gas adsorption data (i.e., the interplanar spacing, primary mesopore volume, and micropore volume). The influence of assumptions about the pore shape, pore wall density, and the presence of microporosity or disordered nonmesostructured domains on the evaluation of structural parameters is examined. It is suggested that because of very large specific surface areas and primary mesopore volumes for many MCM-41 materials, the existence of extensive disordered domains is not a common feature of MCM-41. Examination of X-ray diffraction (XRD) patterns and nitrogen adsorption isotherm...

Hexagonal pore arrays with a 50–420 nm interpore distance formed by self-organization in anodic alumina

Self-organized hexagonal pore arrays with a 50–420 nm interpore distance in anodic alumina have been obtained by anodizing aluminum in oxalic, sulfuric, and phosphoric acid solutions. Hexagonally ordered pore arrays with distances as large as 420 nm were obtained under a constant anodic potential in phosphoric acid. By comparison of the ordered pore formation in the three types of electrolyte, it was found that the ordered pore arrays show a polycrystalline structure of a few micrometers in size. The interpore distance increases linearly with anodic potential, and the relationship obtained from disordered porous anodic alumina also fits for periodic pore arrangements. The best ordered periodic arrangements are observed when the volume expansion of the aluminum during oxidation is about 1.4 which is independent of the electrolyte. The formation mechanism of ordered arrays is consistent with a previously proposed mechanical stress model, i.e., the repulsive forces between neighboring pores at the metal/oxide interface promote the formation of hexagonally ordered pores during the oxidation process.

Self‐Organized Formation of Hexagonal Pore Structures in Anodic Alumina

The morphology and formation conditions of ordered hexagonal pore arrays in anodic alumina are discussed. The ordered arrangement of pores is shown to form by a self‐organized process starting from randomly distributed pore positions at the surface of the alumina. The influence of the pretreatment of the aluminum substrate and the anodizing conditions on the growth kinetics and the tendency to form hexagonal pore structures were investigated. Homogeneous etching conditions are required in order to obtain regular pore arrays. This observation corresponds to the finding that hexagonal pore arrays are related to a smooth etching front and a homogeneous depth of neighboring pores.

Self-organized formation of hexagonal pore arrays in anodic alumina

The conditions for the self-organized formation of ordered hexagonal structures in anodic alumina were investigated for both oxalic and sulfuric acid as an electrolyte. Highly ordered pore arrays were obtained for oxidation in both acids. The size of the ordered domains depends strongly on the anodizing voltage. This effect is correlated with a voltage dependence of the volume expansion of the aluminum during oxidation and the current efficiency for oxide formation. The resulting mechanical stress at the metal/oxide interface is proposed to cause repulsive forces between the neighboring pores which promote the formation of ordered hexagonal pore arrays.

Manganese Oxide Mesoporous Structures: Mixed-Valent Semiconducting Catalysts

Hexagonal and cubic phases of manganese oxide mesoporous structures (MOMS) have been prepared by means of the oxidation of Mn(OH) 2 . The hexagonal MOMS materials form a hexagonal array of pores with an open porous structure, thick walls (1.7 nanometers), and exceptional thermal stability (1000°C). The walls of the mesopores are composed of microcrystallites of dense phases of Mn 2 O 3 and Mn 3 O 4 , with MnO 6 octahedra as the primary building blocks. The calcined hexagonal MOMS have an electrical conductivity of 8.13 × 10 −6 per ohm·centimeter, an average manganese oxidation state of 3.55, and a band gap of 2.46 electron volts. Catalytic oxidations of cyclohexane and n -hexane in aqueous solutions in a batch reactor show conversions of ∼10 and ∼8 percent, respectively. Characterization and catalytic data suggest that MOMS systems show significant enhancement in thermal stability with respect to octahedral molecular sieve materials.

Adsorption Study of Surface and Structural Properties of MCM-41 Materials of Different Pore Sizes

Nitrogen adsorption measurements were performed over a wide range of relative pressures (10-6−0.995) for a series of siliceous MCM-41 samples obtained using alkyltrimethylammonium surfactants with different chain length. Both high- and low-pressure adsorption data were analyzed. The pore size was shown to increase in a regular way with the chain length of the surfactant used. Moreover, a very good correlation between the pore size and the interplanar spacing of the MCM-41 samples was observed. Methods used to calculate the pore diameter were critically compared, and a new procedure to estimate the pore size of MCM-41 materials was proposed. This new procedure is based on geometrical considerations of the ratio of the pore volume to the pore wall volume for an infinite hexagonal array of cylindrical pores. Adsorption measurements showed that the amount adsorbed in the low-pressure region increases with a decrease in the pore size of the samples probably because of the enhancement of the gas−surface interac...

"Tubules-Within-a-Tubule" Hierarchical Order of Mesoporous Molecular Sieves in MCM-41

The recently discovered mesoporous aluminosilicate MCM-41 consists of hexagonal arrays of nanometer-sized cylindrical pores. It is shown that this material can be synthesized by cooperative condensation of silicate and cylindrical cationic micelles. Careful control of the surfactant-water content and the rate of condensation of silica at high alkalinity resulted in hollow tubules 0.3 to 3 micrometers in diameter. The wall of the tubules consisted of coaxial cylindrical pores, nanometers in size, that are characteristic of those of MCM-41. The formation of this higher order structure may take place through a liquid crystal phase transformation mechanism involving an anisotropic membrane-to-tubule phase change. The hierarchical organization of this "tubules-within-a-tubule" particle texture is similar to that of the frustules of marine diatoms.

Synthesis and Characterization of Hexagonally Packed Mesoporous Tantalum Oxide Molecular Sieves

The synthesis and characterization of Ta-TMS1, a new member of a growing family of hexagonally packed transition-metal oxide mesoporous molecular sieves (termed TMS1) is described. Ta-TMS1 exhibits a hexagonal array of pores which can be varied in size from 20 to 40 A and surface areas of over 500 m2/g. The thermal and hydrothermal stabilities of Ta-TMS1 are 500 and 450 °C, respectively, making this system the most stable transition-metal oxide molecular sieve yet isolated. The high hydrocarbon adsorption capacities of this material make it a promising candidate as a catalyst support for hydrocarbon re-forming processes. The synthesis of this material was achieved by a novel approach involving the careful hydrolysis of long-chain primary amine complexes of Ta(OEt)5. This ligand-assisted templating mechanism represents a new approach to the synthesis of porous materials in that the inorganic precursor is covalently bonded to the template throughout synthesis. The high thermal stability, ease of synthesis a...

On the Formation of Voids in Anodic Oxide Films on Aluminum

A mechanism is proposed for the formation of voids at triple points between grains in the anodic oxide formed on aluminum and for the growth of porous films. The proposed mechanism involves the condensation of cation and/or metal vacancies below regions in the barrier film (triple points) that are characterized by high cation vacancy diffusivity, due to the high degree of lattice disorder. Vacancy condensation causes local decohesion of the barrier layer from the substrate, thereby inhibiting the growth of the layer into the metal in these regions. The lower film growth rate, relative to the surrounding areas, accounts for the observation that the voids subtend protrusions of the metal into the film. Periodic detachment of the voids, followed by nucleation and growth of new voids at the apex of the protrusions, accounts for decoration of the barrier layer with voids as well as for the formation of the hexagonal array of pores, when aluminum is anodized in some electrolytes.

Dependence of surface coverage on pore geometry in dispenser cathodes

This paper shows that the surface coverage of Ba BaO on tungsten is more complete when the activator is supplied via slotted pores rather than circular pores. Both theoretical and experimental evidence is given to support this contention. The effect is primarily a geometrical one, since the surface diffusion in the case of circular pores is two-dimensional, whereas the surface diffusion for slotted pores is linear. The contrast becomes less pronounced as the circular pore size decreases. For dimensions of the order of those found on cathode surfaces (e.g., 10 μ m diameter pores), a hexagonal array of circular pores can be optimized to produce an emitting area of 88% of the total, with a pore open area of about 11%. For slotted pores, the slot widths can be made arbitrarily narrow, consequently, the emitting area approaches 100% while pore evaporation losses are minimized. A slotted-pore cathode should, therefore, be capable of higher and more uniform current density with less barium dispensation. When the pore geometry is controlled, either for round holes or slots, the cathode should be less prone to space-charge-limited slump than those based on random sintered pores

Nuclear pores in the spermatozoon of the rat.

The paper describes a hexagonal array of nuclear pores in a non-redundant region of the nuclear envelope underlying the basal surface of the rat spermatozoon head. It is concluded that intranuclear material protruding through these pores is the cause of the characteristic rows of circular 'bumps' found in surface replicas of this region.