Hybrid silica-porphyrin materials with tailored pore sizes

This study is concerning about optical and morphological properties of novel porphyrin doped silica materials consisting in 5,10,15,20-tetrakis(4-allyloxyphenyl)porphyrin (TAPP) encapsulated in silica matrices, exhibiting intensive absorption of light in the red-near IR region. The silica-porphyrin materials were prepared by the sol-gel process, by using different porphyrin immobilization schemes: in situ and by impregnation. As starting materials tetraethoxysilane and isobuthyltrietoxysilane, as silica precursors, N-buthyl-3-methylpyridinium tetrafluoroborate ionic liquid, as additive, and hydrochloric acid and sodium fluoride, as catalysts, were used. The obtained hybrid porphyrin-silica materials were characterized by using BET measurements (Brunauer-Emmett-Teller analysis), thermal analysis, FT-IR, fluorescence and UV-vis spectroscopy techniques. UV-vis behavior and fluorescence emissions and excitations were evaluated in terms of synthesis stages and immobilization processes. The obtained hybrid porphyrin-silica materials presented increased fluorescence emission with maxima situated at about 655 nm and 715 nm in comparison with the porphyrin base that make these transparent materials candidates for second generation photosensitizers. BET analysis revealed that every introduction of TAPP causes decreasing on surface area of the nanomaterial. Although, when the porphyrin is immobilized by in situ method the reduction is lower than in case of using impregnation method, that is leading to the conclusion that the porphyrin is placed inside on the silica network in both studied cases, independent of the performed method of immobilization. The pore size is narrowly distributed in the range of 1.97-3.81 nm for in situ obtained materials and in the range of 3.07-4.62 for hybrids obtained by impregnation. These materials with tunable pore sizes diameter are promising for building of sensor devices.     

Synthesis of tailored porous alumina with a bimodal pore size distribution

Hierarchical channel or well-connected small and large pore networks show multiple advantages for application in catalysis or adsorbent in aqueous condition. Micro- and mesopores provide size or shape selectivity for a guest molecule, while additional macropores reduce transport limitations. In this study, we proposed a novel method to prepare bimodal porous aluminas, which have meso- and macropores with narrow pore size distribution and well defined pore channels. The framework of the porous alumina is prepared via a chemical templating method using alkyl carboxylates. Polystyrene (PS) beads are employed as a physical template for macropores. We examined polydiallyldimethylammonium chloride (PDDA)-treated aluminas as organic adsorbent in aqueous solution. Above 90% of the anionic dye (acid red 44) is removed within 10 min, and the adsorption rate of PDDA/P4 (supported on the bimodal porous alumina) is faster than that of PDDA/P2 (supported on the unimodal porous alumina) because macropore of P4 have reduced transport limitation and enhanced the accessibility to the active site of cationic charge.     

Magnetic and superconducting properties of niobium oxides

Low temperature magnetic susceptibilities of niobium oxides have been measured. A homogeneous sample of NbO prepared by arc melting and checked by using X-ray and metallographic techniques exhibits no ferromagnetism but becomes superconducting at 1.20°K.     

Mesoporous silica photoluminescence properties in samples with different pore size

Ultraviolet excited photoluminescence of porous silica under vacuum conditions is investigated in samples with different pore diameters. By exciting in the 180–300 nm range, a manifold luminescence activity in the 300–600 nm is found. The reconstructed three-dimensional photoluminescence excitation pattern shows how the distribution of the emissions changes by varying the pore size of the samples. Two different centers are singled out and their relative concentration is suggested to be a function of the pore size of the samples.     

Self-setting particle-stabilized foams with hierarchical pore structures

This study presents a novel method to produce self-setting inorganic foams with unique hierarchical pore structures. The combination of particle-stabilized alumina foams with calcium aluminate cement leads to a macroporous ceramic material which can be shaped and consolidated at room temperature, bypassing the challenging and sometimes extensive drying and sintering steps. Due to the water consuming cement hydration reaction, no macroscopic shrinkage was observed and crack and cavity formation was prevented throughout the entire specimens. The final microstructure features a porosity of 76 vol.% and a unique hierarchical pore structure with interconnecting pores of 200 µm diameter, separated by mesoporous pore walls. The authors believe that this newly developed method opens a door to novel applications where so far the drying and sintering step and its inherent shrinkage of the ceramic foams were the limiting factors.     

Mesoporous titania films with adjustable pore size coated on stainless steel substrates

Mesoporous layers of titania were prepared on stainless steel substrates of defined roughness by dip coating. Ordered arrays of micelles formed from amphiphilic block copolymers served as pore templates during film drying. Coating of the precursors solution on freshly grinded steel resulted in extensively fractured films with severely distorted templated porosity. In contrast, films produced on precalcined steel showed good integrity, high substrate coverage and narrow pore size distribution with pores interconnected and ordered in a short range. This difference in film quality and morphology was ascribed to the reaction between template polymers and metal ions leached from the steel of grinded substrate surfaces. Films were ca. 700 nm thick and composed of nanocrystalline titania. The pore size of titania coatings was varied between 5 and 16 nm employing polymer templates of different structure and molecular weight.     

Fabrication of carbon capsules with hierarchical pore structures

Carbon capsules with hierarchical pore structures were fabricated by using core-shell silica nanoparticles as templates and phenolic resin as a carbon precursor. Carbon capsules with hierarchical pore structures were obtained via in-situ polymerization of the phenolic resin on the surface of the silica nanoparticles followed by the carbonization and removal of the silica templates. The hierarchically pored carbon capsules exhibited multimodal porosity with a high specific surface area (approximately 1834 m2/g) and a large pore volume (approximately 1.83 cm3/g).     

Photoluminescence of layered perovskite oxides with triple- octahedra slabs containing titanium and niobium

Abstracts are not published in this journal This revised version was published online in November 2006 with corrections to the Cover Date.     

Preparation of tin oxide gels with versatile pore structures

Hydrous tin oxide gels were subjected to consecutive solvent-evaporation (SE) and CO₂ supercritical (CS) drying steps, followed by re-hydration at low (30%) humidity, and the effects on pore structure of the SE drying duration and particle surface potential of the gel were investigated. The difference in surface potential has been found to impose significant variations in initial pore size distribution, while the SE drying step tends to narrow such differences and simultaneously reduce the mean pore size. It is demonstrated that varieties of pore structures have thus been obtained by simply varying these two parameters.     

Response of spatially tailored structures to thermal loading

The paper presents the formulation and analysis of composite plates serving as STATs, i.e., spatially tailored advanced thermal structures where the distribution of the constituent phases varies throughout the surface as well as through the thickness. This is an extension of the well-known concept of functionally graded materials (FGM) and structures with the constituent phases varying only in the latter direction. As a result of two- or three-dimensional grading it is possible to optimize the response and properties of the structure providing multitask and multi-scale optimization. The response of plates with two- or three-dimensional grading to an arbitrary thermal loading is elucidated, including the conditions that result in thermal bending versus thermal instability.     

Bioresorbable and bioactive polymer/Bioglass® composites with tailored pore structure for tissue engineering applications

An overview about the development of porous bioresorbable composite materials for applications as scaffolds in tissue engineering is presented. A thermally induced phase separation method was developed to fabricate porous foam-like structures of poly(lactide-co-glycolide) (PLGA) containing bioactive glass particle additions (up to 50 wt.%) and exhibiting well-defined, oriented and interconnected porosity. The in vitro bioactivity and the degradability of the composite foams were investigated in contact with phosphate buffer saline (PBS). Weight loss, water absorption and molecular weight measurements were used to monitor the polymer degradation after incubation periods of up to 7 weeks in PBS. It was found that the presence of bioactive glass retards the polymer degradation rate for the time period investigated. The present results show a way of controlling the in vitro degradation behaviour of PLGA porous composite scaffolds by tailoring the concentration of bioactive glass.     

不同结构活性炭对甲苯的吸附性能

考察了不同结构的活性炭样品对高浓度和低浓度甲苯蒸汽的吸附行为,采用低温(77 K)氮气吸附和129Xe-核磁共振方法对所用活性炭的结构进行了表征.并将活性炭对甲苯的吸附性能与其结构进行了关联.结果表明孔容积大的活性炭对高浓度甲苯蒸汽吸附容量大,而具有丰富微孔和较小平均孔径的活性炭对低浓度(2×10-5)甲苯蒸汽具有高的吸附容量.沥青基活性炭纤维对低浓度(2×10-5)甲苯蒸汽表现出较好的吸附能力.随着比表面积的增大,活性炭纤维对低浓度(2×10-5)甲苯蒸汽的吸附容量略有增加.OG5A,OG10A,OG15A和OG20A在30 ℃下对2×10-5甲苯蒸汽的饱和吸附容量分别为202 mg/g,219 mg/g,221 mg/g和235 mg/g.     

Development of superconducting niobium accelerating structures for heavy ions

Results of tests of two new designs for superconducting niobium resonant cavities are presented. Both types resonate at 145.5 MHz and accelerate most efficiently for particle velocities β= v/c = 0.16. One resonator is of the split-ring type, but of a simpler design than a previously reported β=.16 unit. Although the surface fields are higher, the performance is somewhat better than for the earlier design: an accelerating field Ea= 4.3 MV/m has been obtained at 4.2K with 4w of rf input, where Eais defined as the energy gain per unit charge for a synchronous particle averaged over the interior resonator length. The other resonator is an 8-inch OD tapered coaxial quarter-wave line terminated with a drift tube of 1.50 inch aperture. At 4.2K, this resonator exhibits a low-level Q of 2×10⁹, and has achieved Ea= 4.7 MV/m with 2.8w of rf input.     

Strength modeling of brittle materials with two- and three-dimensional pore structures

Strength under compression of highly porous aerated autoclaved concrete was modeled by means of Finite Element Analysis of the porous microstructure. The complex microstructure of aerated autoclaved concrete is characterized by three hierarchical levels of pores. Strength and failure behaviour is controlled by large artificial air pores (AAP) with a mean diameter of 0.5–3 mm. Stress distribution in the brittle matrix material under external load was calculated by FEA for different pore arrangements. Based on the stress distribution multiaxial Weibull Theory was used to predict failure probability with regard to the porous microstructure. Two- and three-dimensional ordered pore arrangements show an exponential decrease of strength with porosity in the range of 0–0.4, depending on the Weibull parameter m of the matrix material. Strength vs. porosity relation differs significantly for pore structures with simple cubic and body centered cubic pore arrangements. The compatibility of two-dimensional with three-dimensional models is examined.     

不同结构中孔炭材料的制备和热稳定性研究

利用溶剂挥发自组装的方法制备了2种不同孔结构中孔炭材料。低温N2吸脱附、SAXS、HRSEM和TEM测试表明,所制备的材料分别具有柱状结构和墨水瓶状的中孔结构。考察了不同炭化温度对2种结构中孔炭的影响,结果表明柱状结构的中孔炭材料具有较好的热稳定性。     

Highly ordered porous alumina with tailor-made pore structures fabricated by pulse anodization

A new anodization method for the preparation of nanoporous anodic aluminum oxide (AAO) with pattern-addressed pore structure was developed. The approach is based on pulse anodization of aluminum employing a series of potential waves that consist of two or more different pulses with designated periods and amplitudes, and provides unique tailoring capability of the internal pore structure of anodic alumina. Pores of the resulting AAOs exhibit a high degree of directional coherency along the pore axes without branching, and thus are suitable for fabricating novel nanowires or nanotubes, whose diameter modulation patterns are predefined by the internal pore geometry of AAO. It is found from microscopic analysis on pulse anodized AAOs that the effective electric field strength at the pore base is a key controlling parameter, governing not only the size of pores, but also the detailed geometry of the barrier oxide layer.     

Dependency of mechanochemical reactions forming complex oxides on the crystal structures of starting oxides

Mechanochemical reactions applicable to syntheses of perovskite-type compounds, LaMO₃, where M is trivalent metals, are taken as examples to investigate effects of crystal structures of starting materials, M ₂O₃, on the reactions. The investigated oxides M ₂O₃ are categorized into two groups: one is the corundum structure and the other the non-corundum structure. Under the experimental conditions, it has been found that the oxides with non-corundum structure react easily with La₂O₃ by their co-grinding at room temperature to form the corresponding compounds. On the contrary, none of such reactions can be occurred for the oxides with corundum structure. The difference has been discussed based on the grinding-induced phase transformation from loose packing of atoms to close packing of atoms. Based on the results, the extension to the synthesis of other compounds such as CrO₂ is presented.     

Single-walled carbon nanotube network with bimodal pore structures of uniform microporosity and mesoporosity

We characterized single-walled carbon nanotubes before and after HNO3/H2SO4 treatments for different times by scanning electron microscopy, Raman spectroscopy, and N2 adsorption at 77 K. Single-walled carbon nanotube assembly revealed a bimodal pore structure of microporosity (surface area of 476 m2 g(-1)) and mesoporosity (surface area of 476 m2 g(-1)) with a high total surface area of 1048 m2g(-1). The microporosity increased prominently after HNO3/H2SO4 treatments, whereas the mesoporosity decreased progressively with the treatment time. The HNO3/H2SO4 treatment of nanotubes induced an aggregation and alignment that should transform larger mesopores of nanotube assemblies into smaller ones, and smaller mesopores into micropores, resulting in the decrease of external surface area. This effect was attributed to the presence of abundant defects on the tube wall that were saturated by functional groups during the acid treatment of the single-walled carbon nanotubes.     

Synthesis, structure, and properties of mixed niobium(IV,V) oxides

This review paper summarizes data on mixed oxides containing niobium in an intermediate valence state, between 4+ and 5+. The major preparation procedures are considered, and the effects of different process parameters—composition of the starting reagents, temperature, pressure, duration, and environment—are analyzed from the viewpoint of the preparation of bulk, single-phase materials with niobium in a controlled formal oxidation state. The optimal synthesis conditions are established for reduced niobates with different structures. The oxidation behavior of the niobates(IV,V) is examined as a function of the structure type. The structural and electrical properties of the reduced niobates are shown to correlate with the carrier concentration (formal oxidation state of niobium). The major factors determining the carrier concentration in the niobates are identified. The results are used to establish the conditions under which a semiconductor—metal transition is possible in reduced niobates