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We use a first-principles calculation and small-angle neutron scattering (SANS) to investigate the mechanism and the nanosize products of the sol-gel reaction with diphenylsilanediol (DPD) and 3-methacryloxypropyltrimethoxysilane (MEMO) precursors in synthesizing a hybrid waveguide material. It is predicted that switching between a DPD hydroxyl and a MEMO methoxy with a reaction rate of 6.8 x 10(-6) s(-1) at 300 K is the fastest process for the first reaction step, thus generating diphenylmethoxysilanol (DPM) and 3-methacryloxypropyldimethoxysilanol (MEDO) as products. However, we determine that this reaction pathway could be modified by the presence of the H2O released from a catalyst such as Ba(OH)2.H2O. Next, switching between the DPM hydroxyl and the MEDO methoxy is followed to generate diphenyldimethoxysilane (DPDM) and 3-methacryloxypropylmethoxysilanediol (MEMDO). However, condensation between a MEMDO hydroxyl and a DPDM methoxy is found to be most favorable for the third reaction step, which generates the DPDM-MEMDO dimer and CH3OH molecule as products. In a similar fashion, a DPDM methoxy of the DPDM-MEMDO dimer can condense with a MEMDO hydroxyl of the second DPDM-MEMDO dimer to increase the chain, but its reaction rate of 2.8 x 10(-11) s(-1) is predicted to be about 5 times smaller than that between a DPDM methoxy and a MEMDO hydroxyl. This implies that the reaction rate for the larger nanostructures becomes smaller. Additionally, our SANS measurements determine that the final products from our sol-gel reaction are on the nanometer scale, at sizes from 1.76 to 2.36 nm.

We describe the synthesis and structural characterization of new aliovalent cation-substituted n = 3 Aurivillius phases of the formulas, Bi 2 SrNaNb 2 TaO 12 (I), Bi 2 Sr 2 Nb 2 ZrO 12 (II), Bi 2 Sr 2 Nb 2.5 Fe 0.5 O 12 (III) and Bi 2 Sr 2 Nb 2.67 Zn 0.33 O 12 (IV). Energy dispersive X-ray (EDX) investigation of the chemical compositions showed that while cation-stoichiometric materials are formed for I and II, single-phase materials are obtained only for the compositions given for III and IV suggesting that the compositions tend to be oxygen-stoichiometric. The results show that aliovalent cation-substituted n = 3 Aurivillius phases similar to Bi 2 Sr 2 Nb 2 M IV O 12 (M = Ti, Mn) exist for several metal cations, viz., Ta V , Zr IV , Fe III and Zn II . Refinement of the crystal structures from the powder X-ray diffraction (XRD) data for I and III revealed that bismuth is essentially confined to the Bi 2 O 2 layers and the aliovalent cation shows a preference for the middle perovskite sheet. The present work is significant for two reasons: firstly, the presence of cations such as Sr 2+ , Na + (that do not contain lone pair s 2 electrons) in the perovskite slabs seems to render the structure centrosymmetric; secondly, a preferential/partial ordering of octahedral site cations in the perovskite slabs obtains in these materials, that seems to be dictated by a second order Jahn-Teller effect associated with d 0 cations.

An open cell configuration has been employed for the photoacoustic measurement of the thermal diffusivity of undoped Bi 2 Se 3 crystals and Bi 2 Se 3 crystals doped with various concentrations of Te. The amplitude of the photoacoustic signal obtained under heat transmission configuration as a function of chopping frequency is used to evaluate the numerical value of thermal diffusivity, α. Doped samples show a substantial reduction in the value of α compared to undoped samples. The variations in the thermal diffusivity of the doped samples are explained in terms of the phonon assisted heat transfer mechanism. It is seen that α is very sensitive to structural variations arising from doping. The experimentally observed results are correlated with X-ray diffraction studies.

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Experimental and theoretical approaches are used to determine hydrogen storage mechanisms in nanopores of multiwalled carbon nitride nanotubes ͑MWCNNTs͒. First, the authors produce ϳ0.6 nm pores on the stems of MWCNNTs by plasma-enhanced chemical vapor deposition. Next, thermal desorption spectra were measured and obtained two different peaks. This is explained by hydrogen desorption barriers of 0.36-0.50 eV attributed to two different types of ϳ0.6 nm pores. Moreover, H 2 adsorption between complete interlayers is found to be endothermic by 1.27 eV. In this respect, open channels and ϳ0.6 nm pores on MWCNNTs are considered to provide the route for reversible hydrogen storage.

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