Radiation detector made of a high-quality polycrystalline diamond

Radiation detector was made of a high-quality CVD polycrystalline diamond composed of frost column like structure diamond grains, and induced charge distribution spectra and drift velocities were measured by using alpha particles. As a result, the CVD polycrystalline achieved maximum induced charge of 83% of HP/HT type IIa diamond. Moreover, the CVD crystal had lower charge loss on electrons compared with the HP/HT type IIa diamond. Drift velocities of electrons and holes were v_e = 7.7 × 10⁴ and v_h = 7.3 × 10⁴cm/s at an electric field of 20 kV/cm, respectively. In addition, response function measurement for 14 MeV neutrons was carried out.     

Radiation tolerance of type IIa synthetic diamond detector for 14 MeV neutrons

Radiation tolerance of a type IIa synthetic diamond detector was examined from irradiation of mono-energetic 14 MeV neutrons. Measurements of I–V (current–voltage) characteristics and energy spectrum for 5.486 MeV alpha particles were performed after neutron irradiation. In the I–V characteristics measurement, enhancement of rectification was observed after neutron irradiation of up to 2.0 × 10¹² n/cm². Concurrently with the enhancement of rectification, significant decrease in signal amplitude was observed in energy spectrum measurement for alpha particles. It is considered that these changes were due to increase in the concentration of defects acting as shallow energy levels in the forbidden band. For neutron irradiation of higher than 1.6 × 10¹³ n/cm², weakening of the rectification characteristics and recovery of the signal amplitude were observed. These changes imply that deep energy levels, which were also considered to be introduced by defects, were dominant and weakened the effects of the shallow energy levels. Increase in the concentration of the deep trapping levels resulted in gradual decrease of the signal amplitude and degradation in the energy resolution. The peak for the alpha particles was obtained up to 5.5 × 10¹³ n/cm².     

Single-step synthesis and characterization of single-walled carbon nanohorns hybridized with Pd nanoparticles using N2 gas-injected arc-in-water method

Single-walled carbon nanohorns (SWCNHs) hybridized with palladium (Pd) nanoparticles were synthesized by a single-step gas-injected arc-in-water method (GI-AIW) with a Pd wire inserted inside the anode hole. In the arc zone, carbon and Pd were vaporized simultaneously, leading to the formation of hybrid material of SWCNHs and Pd nanoparticles due to effective quenching. Based on TEM and CO chemisorption analyses, Pd nanoparticles were found to be embedded inside SWCNH aggregates. The size of Pd nanoparticles, determined by X-ray diffraction, was in the range of 3–6 nm when Pd wires with diameters of 0.1 and 0.3 mm were used. Using a Pd wire with a diameter larger than 0.5 mm results in larger Pd nanoparticles which tend to be exposed to the outer surface of the hybrid material. According to thermogravimetric analyses, the weight fraction of Pd nanoparticles is increased by increasing the Pd wire diameter although the yield of Pd nanoparticles decreased. SWCNHs hybridized with dispersed Pd nanoparticles, synthesized with 0.1 mm Pd wire, exhibited strong anti-oxidation resistance with a highly graphitic structure.     

Voltammetric study of electrocatalysis for dioxygen reduction by trinuclear rutheniumammine complex confined in a polymer membrane

Electrocatalytic O₂ reduction was studied using a modified electrode coated with a Nafion membrane (Nf) dispersing a trinuclear ruthenium ammine complex ([(NH₃)₅Ru^IIIORu^IV(NH₃)₄ORu^III(NH₃)₅]Cl₆, Ru-red). When measuring cyclic voltammogram under O₂ atmosphere (at 0.5 mV s⁻¹), catalytic currents due to O₂ reduction were found to develop below −0.2 V (vs. Ag/AgCl). Since Ru-red undergoes irreversible decomposition into the mononuclear complexes via the reduced state (Ru^III-Ru^III-Ru^III) (∼−0.1 V), it is suggested that the electrocatalysis originates from the decomposed species (initial active species: Ru^II(NH₃)₅(OH₂) and Ru^II(NH₃)₄(OH₂)₂) rather than from the Ru-red. Although the present electrocatalyst was also applied to H₂O₂ reduction system, the catalytic activity was found to be poor from the voltammetric behavior. It appeared that the kinetics of the electrocatalysis is much faster in the O₂ reduction than in the H₂O₂ one. A selective and direct catalysis for O₂ reduction into H₂O was suggested from a ring-disk voltammogram to take place by an aggregate of the mononuclear ruthenium complexes in the polymer matrix. In addition, it was found that electrocatalytic O₂ reduction involves a slow kinetic process, so that factors affecting the overall kinetics were discussed in terms of the catalysis mechanism.     

Alteration of substrate specificity of cholesterol oxidase from Streptomyces sp. by site-directed mutagenesis

Despite the structural similarities between cholesterol oxidasefrom Streptomyces and that from Brevibacterium, both enzymesexhibit different characteristics, such as catalytic activity,optimum pH and temperature. In attempts to define the molecularbasis of differences in catalytic activity or stability, substitutionsat six amino acid residues were introduced into cholesteroloxidase using site-directed mutagenesis of its gene. The aminoacid substitutions chosen were based on structural comparisonsof cholesterol oxidases from Streptomyces and Brevibacterium.Seven mutant enzymes were constructed with the following aminoacid substitutions: L117P, L119A, L119F, V145Q, Q286R, P357Nand S379T. All the mutant enzymes exhibited activity with theexception of that with the L117P mutation. The resulting V145Qmutant enzyme has low activities for all substrates examinedand the S379T mutant enzyme showed markedly altered substratespecificity compared with the wild-type enzyme. To evaluatethe role of V145 and S379 residues in the reaction, mutantswith two additional substitutions in V145 and four in S379 wereconstructed. The mutant enzymes created by the replacement ofV145 by Asp and Glu had much lower catalytic efficiency forcholesterol and pregnenolone as substrates than the wild-typeenzyme. From previous studies and this study, the V145 residueseems to be important for the stability and substrate bindingof the cholesterol oxidase. In contrast, the catalytic efficiencies(k_cat/K_m) of the S379T mutant enzyme for cholesterol and pregnenolonewere 1.8- and 6.0-fold higher, respectively, than those of thewild-type enzyme. The enhanced catalytic efficiency of the S379Tmutant enzyme for pregnenolone was due to a slightly high k_catvalue and a low K_m value. These findings will provide severalideas for the design of more powerful enzymes that can be appliedto clinical determination of serum cholesterol levels and assterol probes.     

Characterization of a Silk‐Resinified Compact Fabricated Using a Pulse‐Energizing Sintering Device

Pulse electric current sintering is used to prepare a compact from resinificated hydrous silk powder. Compacts with no remnant silk powders are formed with 20 wt% added water, 20–40 MPa molding pressure, and >353 K molding temperature. The latter two are much lower than those used for conventional hot pressing. No dependence on molding pressure and temperature are found in XRD or FT‐IR analysis, except for a compact molded at 473 K, for which silk fibroin decomposition is confirmed by DSC, EGA‐MS, and molecular weight measurements. The compact's three‐point bending strength depends on the molding temperature, except for the temperature at which silk fibroin decomposes. The maximum three‐point bending strength resembles that of general‐purpose epoxy resin and is much higher than that of PLA.

     

Phase transformation and interface segregation behavior in Si3N4 ceramics sintered with La2O3–Lu2O3 mixed additive

Microstructure and mechanical property of silicon nitride (Si₃N₄) ceramic are strongly dependent on the selection of sintering additives. When rare‐earth (RE) oxide is used as the sintering additive, segregation of RE ions at interface between Si₃N₄ grain and intergranular glassy film (IGF) is believed to play a critical role. Although the ionic radius of RE ion is known to be an empirical parameter to modify the mechanical property, the correlation between the segregated ions and their ionic radii is still under controversy. In order to address this issue, (i) rate of α‐β phase transformation and (ii) segregation behavior at the interface were studied for Si₃N₄ ceramics sintered using mixture of La₂O₃ and Lu₂O₃ as additives in this study. Specimens of Lu content 30% and higher exhibited lower activation energies for the α‐β phase transformation as compared with those of Lu content 20% and lower. In terms of the segregation behavior, La was preferably segregated at one site and Lu at the other site along β‐Si₃N₄/IGF interface in the specimens of Lu content 30% and higher. It is understood from these results that Lu segregation site should be more closely related with grain growth.     

Separation of endocrine disruptors from aqueous solutions by pervaporation: Dioctylphthalate and butylated hydroxytoluene in mineral water

We examined the existence of endocrine disruptors in mineral water, ultrapure water, and tap water. GC/MS analysis revealed that dioctylphthalate (di‐n‐octylphthalate and di(2‐ethylhexyl)phthalate) in the water was found to be on the order of parts per billion. The dioctylphthalate concentration remained consistent for mineral water of the same brand, regardless of whether it was bottled in a PET bottle or a glass bottle. Therefore, the dioctylphthalate contamination in mineral water originated from the manufacturing line of the mineral water and not as a result of leaching from PET bottles. Butylated hydroxytoluene (BHT), an oxidization prevention agent in plastics, was detected at 2.05 ± 0.1 ppb in the mineral water bottled in glass bottles, but was not detected in the mineral water bottled in PET bottles. BHT contamination in the mineral water in the glass bottles is likely to have originated from the cap of the glass bottles, as the caps were sealed with polyethylene. We further investigated the feasibility of separating dioctylphthalate and BHT from the mineral water by pervaporation using hydrophobic polydimethylsiloxane membranes. We found that trace amounts (on the order of parts per billion) of organic chemicals such as dioctylphthalate and BHT in aqueous solutions can be removed and concentrated by the pervaporation using polydimethylsiloxane membranes. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 1737–1742, 2004