Porosity development in chars from thermal degradation of poly(p-phenylene benzobisoxazole)

The main objective of this work was to investigate porosity development in carbonaceous materials formed by thermal degradation of the high thermal stability polymer, poly(p-phenylene benzobisoxazole) (PBO). Two varieties of PBO, as-spun (AS) and high-modulus (HM) were studied. Chars obtained at various polymer decomposition temperatures (selected on the basis of thermogravimetric and differential thermal analysis results) were characterized by elemental analysis, infrared spectroscopy, X-ray diffraction and CO₂ adsorption at 273 K. The obtained adsorbents essentially contained narrow micropores, and maximum pore development took place at ca. 1000 K, when the polymer was not yet completely decomposed. At higher temperatures, there is a certain pore widening along with a displacement towards smaller pore sizes, and the pore volume significantly decreases.     

Extended Hyperspherical-Harmonic Expansion Bases

 The use of overcomplete expansion basis functions can be useful for reproducing important details of the structure of a few-particle system by a relatively small number of basis functions. The case considered in this paper is the hyperspherical harmonic basis which is complete but gives slow convergence patterns in constructing the wave functions of systems with interactions that change rapidly with the interparticle distances. The problem is investigated here for the three-nucleon ground state and a number of NN potential models. A faster convergence is obtained by enlarging the hyperspherical harmonic basis in a simple way. Such a behavior should also be important for calculations of the bound state of larger systems and of breakup scattering states. Received December 30, 1998; accepted for publication February 19, 1999     

The method of interpolating integral equations for quantum Fluids.—III

Summary A method for interpolating between the (F)HNC and (F)PY approaches in order to take into account elementary contributions has been presented in two preceding papers concerned with the properties of zero-temperature quantum fluids, described by short-range correlated wave functions. In the present paper, both for Bose and for Fermi systems, the technique is extended to the case in which the two-body radial distribution function contains a long-range tail, going asr ⁻⁴. The results obtained for the energy per particle and momentum distribution of liquid⁴He, polarized hydrogen and³He are presented in correspondence to variational wave functions containing only two-particle correlations.

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Riassunto In due precedenti lavori è stata sviluppata una tecnica di interpolazione tra le approssimazioni (F)HNC e (F)PY per tenere conto dei cosiddetti contributi elementari nel calcolo delle proprietà dei fluidi quantistici a temperatura zero e descritti da funzioni d’onda con fattori di correlazione a corto raggio. Nel presente lavoro il metodo è esteso, per sistemi sia bosonici che fermionici, al caso in cui la funzione radiale di distribuzione contenga una coda con andamentor ⁻⁴. Sono presentati i risultati ottenuti per l’energia per particella e la distribuzione di quantità di moto dell’⁴He liquido, dell’idrogeno polarizzato e dell’³He liquido, in corrispondenza a funzioni d’onda variazionali contenenti solo correlazioni di coppia.

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Резюме В двух предыдущих работах был предложен метод интерполяции между (F)HNC и (F)PY подходами. В этих работах рассматривались свойства квантовых жидкостей при нулевой температуре, которые описываются волновыми функциями с короткодействующими корреляциями. В данной работе, для Бозеи Ферми-систем, предложенная тенхика обобщается на случай, когда двух-частичная радиальная функция распределения содержит длиннодействующий хвост, который спадает какr ⁻⁴. Приводятся результаты для энергии на одну частицу и импульсное распределение для жидкого⁴He, поляризованного водорода и³H, причем соответствующие вариационные волновые функции содержат только двух-частичные корреляции.

     

Infinitesimal deformations of restricted simple Lie algebras I

We compute the infinitesimal deformations of two families of restricted simple modular Lie algebras of Cartan-type: the Witt–Jacobson and the Special Lie algebras.     

Nonlinear convective flows in multilayer fluid system

The nonlinear thermocapillary and buoyant-thermocapillary flows in a three-layer system, filling a closed cavity and subjected to a temperature gradient directed along the interfaces, are investigated. The nonlinear simulations of convective regimes are performed by the finite-difference method. The process of transition of unicell structures into multicell structures is studied.     

Optimization of matrix solid-phase dispersion extraction method for the analysis of isoflavones in Trifolium pratense

A method based on matrix solid-phase dispersion (MSPD) has been developed for the determination of 12 isoflavones in Trifolium pratense L. Dried leaf samples were blended with C₁₈, placed in small columns and isoflavones extracted with dichloromethane–methanol. Analyses were performed by high performance liquid chromatography with diode array detection (HPLC-DAD) with 2-methoxyflavone as internal standard. Several dispersants, eluents and clean-up steps were tested during the optimization of the process in order to obtain the best selectivity and yields. Mean recoveries ranged from 70% to 119%, with relative standard deviations <18%. The limits of detection were between 0.006 mg/l for biochanin A and 0.108 mg/l for daidzin. The performance of the optimized method in real samples was compared with a conventional method based in solid–liquid extraction (SLE).