Effects of fiber diameter and CO2 activation temperature on the pore characteristics of polyacrylonitrile based activated carbon nanofibers

The effects of fiber diameter and activation temperature on the pore characteristics of polyacrylonitrile based activated carbon nanofibers are investigated. It was found that lower fiber diameters as well as higher activation temperatures lead to a higher weight loss, specific surface area and total pore volume. The nitrogen adsorption capacity of activated carbon nanofibers is almost three times that of activated carbon fiber with a diameter of 10 µm. As far as the size of pores in activated carbon nanofibers is concerned, it is basically the micropores that dominate the scene. Moreover, tailoring the pore characteristics by adjusting the activation temperature and fiber diameter is plausible. Copyright © 2009 John Wiley & Sons, Ltd.     

Complex aggregation patterns in drying nanocolloidal suspensions: size matters when it comes to the thermomechanical stability of nanoparticle-based structures

We report the results of a model study on the interrelation among the occurrence of complex aggregation patterns in drying nanofluids, the size of the constitutive nanoparticles (NPs), and the drying temperature, which is a critical issue in the genesis of complex drying patterns that was never systematically reported before. We show that one can achieve fine control over the occurrence and topological features of these drying-mediated complex structures through the combination of the particle size, the drying temperature, and the substrate surface energy. Most importantly, we show that a transition in the occurrence of the patterns appears with the temperature and the particle size, which accounts for the size dependence of the thermomechanical stability of the aggregates in the nanoscale range. Using simple phenomenological and scaling considerations, we showed that the thermomechanical stability of the aggregates was underpinned by physical quantities that scale with the size of the NPs (R) either as R(-2) or R(-3). These insights into the size-dependent dissipation mechanisms in nanoclusters should help in designing NPs-based structures with tailored thermomechanical and environmental stability and hence with an optimized morphological stability that guarantees their long-term functional properties.     

Thermal analysis of CsH<Subscript>2</Subscript>PO<Subscript>4</Subscript> nanoparticles using surfactants CTAB and F-68

A study concerned to thermogravimetric analysis is performed in cesium dihydrogen phosphate (CsH₂PO₄) that was synthesized, using cetyltrimethylammonium-bromide (CTAB), polyoxyethylene-polyoxypropylene (F-68) and mixture of (F-68:CTAB) with two mole ratio 0.06 and 0.12 as surfactant. The dehydration behavior of particles was studied using thermal gravimetric analysis and differential scanning calorimetric. Subsequently, the experimental results indicated that the first dehydration temperature in the range of 237–239 °C upon heating, the second peaks occur at temperature range 290–295 °C and overlapping in the thermogravimetric events is observed. The mass loss values are obtained in the range of 6.62–6.97 wt% that is less than reported theoretical value 7.8 wt%. These values show well compatibility of reaction CsH₂PO₄ to Cs₂H₂P₂O₇ with 3.92 wt% whereas mass loss value of CsH₂PO₄ to CsPO₃ is less than theoretical value 7.8 wt%. The activation energy of two steps dehydration are calculated using Kissinger equation for the samples synthesized via CTAB and (F-68) with minimum value mass loss 6.62% and maximum value mass loss 6.97%, respectively. The calculation results reveal that the reaction rate in the first step (CsH₂PO₄ → Cs₂H₂P₂O₇) is faster than the second step (CsH₂PO₄ → CsPO₃). The weight loss values of the samples demonstrate that existence of CTAB can be considered as effective factor which prevents more weight loss during the dehydration process.     

Short time synthesis of high quality carbon nanotubes with high rates by CVD of methane on continuously emerged iron nanoparticles

We report the variation of yield and quality of carbon nanotubes (CNTs) grown by chemical vapor deposition (CVD) of methane on iron oxide-MgO at 900-1000 °C for 1-60 min. The catalyst was prepared by impregnation of MgO powder with iron nitrate, dried, and calcined at 300 °C. As calcined and unreduced catalyst in quartz reactor was brought to the synthesis temperature in helium flow in a few minutes, and then the flow was switched to methane. The iron oxide was reduced to iron nanoparticles in methane, while the CNTs were growing.TEM micrographs, in accordance with Raman RBM peaks, indicate the formation of mostly single wall carbon nanotubes of about 1.0 nm size. High quality CNTs with I_G/I_D Raman peak ratio of 14.5 are formed in the first minute of CNTs synthesis with the highest rate. Both the rate and quality of CNTs degrades with increasing CNTs synthesis time. Also CNTs quality sharply declines with temperature in the range of 900-1000 °C, while the CNTs yield passes through a maximum at 950 °C. About the same CNTs lengths are formed for the whole range of the synthesis times. A model of continuous emergence of iron nanoparticle seeds for CNTs synthesis may explain the data. The data can also provide information for continuous production of CNTs in a fluidized bed reactor.     

Gas by-products of CF3I under AC partial discharge

This paper presents the decomposition by-products of trifluro-iodo-methane and their relative proportions in the gas phase under the occurrence of partial discharge. The experiment was performed in the presence of water vapor from 250 to 400 ppm under a non-uniform electric field configuration. The experimental results reveal that the by-products of C₂F₆, C₂F₄, C₂F₅I with the amount of 1300, 200, and 55 (CH₃I) ppm, respectively, were produced for a cumulative charge of 161 mC. Other by-products, such as C₃F₈, CHF₃, C₃F₆ CH₃I were obtained at less than 30 ppm C₂F₆ was the dominant gas by-product of trifluro-iodo-methane suffering partial discharge.     

Spin compensation temperatures induced by longitudinal fields in a mixed spin-3/2 and spin-5/2 Ising ferrimagnet

I studied the ferrimagnetic Ising model with nearest neighbour interactions for a square lattice and simple cubic one, using mean field theory. The free energy of a mixed spin Ising ferrimagnetic model was calculated from a mean field approximation of the Hamiltonian. By minimizing the free energy, I obtained the equilibrium magnetizations and the compensation temperatures. Clear indications of the single-ion anisotropies on the compensation points of the mixed spin-3/2 and spin-5/2 ferrimagnetic lattices are found. Some interesting behaviors of these systems are obtained depending not only on the values of magnetic anisotropies for both sublattice sites but also on the lattice structure. The longitudinal magnetic fields dependence of the spin compensation temperature is the main focus of research. The possibility of many compensation temperatures is indicated.     

Co-crystal structure of mixed molecules of methyl 2-(3-chloro-4-methyl-2-oxo-2H-chromen-7-yloxy)acetate and 2-(2-aminophenyl)benzothiazole

A co-crystal is obtained in a methanolic solution from methyl 2-(3-chloro-4-methyl-2-oxo-2H-chromen-7-yloxy)acetate and 2-(2-aminophenyl)benzothiazole. In the crystal these molecules are connected via usual N-H…O and weak C-H…O H-bonds. The co-crystals are very stable.     

n-Type GaAs/AlGaAs heterostructure detector with a 3.2 THz threshold frequency

Terahertz detection using the free-carrier absorption requires a small internal work function of the order of a few millielectron volts. A threshold frequency of 3.2 THz (93 microm or approximately 13 meV work function) is demonstrated by using a 1 x 10(18) cm(-3) Si-doped GaAs emitter and an undoped Al(0.04)Ga(0.96)As barrier structure. The peak responsivity of 6.5 A/W, detectivity of 5.5 x 10(8) Jones, and quantum efficiency of 19% were obtained at 7.1 THz under a bias field of 0.7 kV/cm at 6 K, while the detector spectral response range spans from 3.2 to 30 THz.     

Energetic 2,2‐Dimethyltriazanium Salts: A New Family of Nitrogen‐Rich Hydrazine Derivatives

Amination of 1,1‐dimethylhydrazine with NH₂Cl or hydroxylamine‐O‐sulfonic acid yields 2,2‐dimethyltriazanium (DMTZ) chloride ( 3 ) and sulphate ( 4 ), respectively. The DMTZ cation was paired with the nitrogen‐rich anions 5‐aminotetrazolate ( 5 ), 5‐nitrotetrazolate ( 6 ), 5,5′‐azobistetrazolate ( 7 ), and azide ( 8 ), yielding a new family of energetic salts. The synthesis was carried out by metathesis reactions of salts 3 or 4 and a suitable silver or barium salt. To minimize the risks involved when using heavy metal salts, we used electrodialysis for the synthesis of azide 8 , which avoids the use of highly sensitive species. The DMTZ derivatives were characterized by IR and multinuclear NMR spectroscopy, elemental analysis, and X‐ray diffraction. Thermal stabilities were measured using DSC analysis and their sensitivities towards classical stimuli were determined using standard tests. Lastly, the relationship between hydrogen bonding in the solid state and sensitivity is discussed.