Search Results

You are looking at 1 - 10 of 13 items for

  • Author or Editor: V. Popa x
Clear All Modify Search

Abstract  

By using m-structures m 1, m 2 on a topological space (X, τ), we define a set D(m 1,m 2) = {A: m 1 Int (A) = m 2 Int (A)} and obtain many decompositions of open sets and weak forms of open sets. Then, the decompositions provide many decompositions of continuity and weak forms of continuity.

Restricted access

Abstract  

We introduce and investigate R-M-continuous functions defined between sets satisfying some minimal conditions. The functions enable us to formulate a unified theory of modifications of R-continuity [22]: R-irresoluteness [6], R-preirresoluteness [7].

Restricted access

Abstract  

DSC endotherms of ice melting in hermetically sealed aluminum pans are analyzed by means of fitting functions derived for spectroscopy and chromatography signal processing. Two sets of experiments are performed: 1) variable heating rate, investigated between 1-5 K min–1 for a small sample of 0.3 mg, and 2) variable sample mass, for different samples within 2-11 mg. Variations of both broadness and shape of the corresponding endotherms are observed and analyzed. Heating rate influence is best described by an intermediate form of the Pearson IV function, derived and widely used in statistics and also in chromatography, and found to accurately account for the instrumental profile. Thermal transfer through the outer (calorimeter/pan) and inner (pan/sample) interfaces of hermetical Al pans is suspected as the main contributor to the shape of the observed endotherms. An important threshold in their shape is noticed between heating rates of 1 and 2 K min–1, where peaks plotted in temperature scale change from fronted to tailed. For time scale plotted peaks the same change occurs between 3 and 4 K min–1. Symmetrical spectroscopic functions are not suitable for the correct description of the observed shape effects. At least in a formal manner, mass influence closely resembles the column overload effects in gas chromatography, where axial diffusion and dispersion take place. The GC derived HVL (Haarhoff-Van der Linde) function offers an excellent fit of endotherms with mass overload. This apparent coincidence is ascribed to an initial partial melting that makes most of the remaining melting process to be controlled by thermal transfer across the liquid interface layer thus formed.

Restricted access

Abstract  

This paper reports a comparative study of the non-isothermal decompositions of the heteropolyacids HPM and HPVM, with structures consisting of Keggin units (KUs). Non-isothermal analysis at low heating rates demonstrated the existence of 4 crystal hydrate species, depending on the temperature. The stability domains of the anhydrous forms of HPM and HPVM were found to be 150–380°C, respectively. Processing of the TG curves obtained at different heating rates by the Ozawa method revealed that the decomposition of anhydrous HPM takes place according to a unitary mechanism, whilst for anhydrous HPVM two mechanisms are observed. Thus, the first part of the constitution water is lost simultaneously with the departure of vanadium from the KU as VO2+, while the second part is lost at higher temperatures as in the case HPM.

Restricted access

Abstract  

The aim of this study is to investigate the influence of some monovalent counter-ions (NH4 +, K+ and Cs+) on thermal behavior of polyoxometalates derived from H3PMo12O40 (HPM) and H4PVMo11O40 (HPVM) by replacing the protons. The IR and UV-VIS-DRS spectra of some acid and neutral NH4 +, K+, Cs+ salts, which derived from HPM and HPVM, confirmed the preservation of Keggin units (KU) structure. The X-ray diffraction spectra clearly showed the presence of a cubic structure. The non-isothermal decomposition of studied polyoxometalates proceeds by a series of processes: the loss of crystallization water; the loss of O2 accompanying with a reduction of V5+→V4+ and Mo6+→Mo5+; the loss of constitution water started at 360C for HPVM salts and 420C for HPM salts; the decomposition of ammonium ion over 420C with NH3, N2 and H2O elimination and simultaneous processes of reduction (V5+→ V4+ and Mo6+→ Mo5+ or Mo4+) associating with endothermic effects; reoxidation of Mo5+, Mo4+ and V4+with a strong exothermic effect; destruction of KU to the oxides: P2O5, MoO3 and V2O5 and the crystallization of MoO3.

Restricted access
Restricted access

Abstract  

Dehydration of irradiated and non-irradiated asparagine monohydrate was investigated by means of a computer interfaced PerkinElmer 1B DSC in isothermal conditions and static atmosphere. Isothermal runs were performed at 358, 363, 368 and 373 K. Samples were γ-irradiated at room temperature, using a 137Cs source with an activity of 3·1013 Bq and a dose rate of 4·102 Gy h−1, with irradiation times between 8–116 h. Isothermal kinetics were analyzed via the common factorized rate equation. Šesták-Berggren conversion function was found to best fit the experimental data. Of the three fitting parameters, only the one associated with the activation energy was found to follow a coherent variation with the exposure time. Even within this simple model, that makes the activation energy a useful stability criterion within a set of similar samples.

Restricted access

Abstract  

This paper deals with the bioaccumulation of some radioactive ions from contaminated waste solutions, on hydrophytic vegetal organisms. In order to follow the distribution of radioactive ions 137Cs+, 60Co2+ and 51Cr3+ in various cell components extracted from Spirulina platensis, Porphiridium cruentum, Scenedesmus quadricauda, Lemna minor,Elodea canadensis, Pistia stratiotes and Riccia fluitans, the plants were cultivated in radioactive solutions. The resulting complexes were extracted with acetone or acetic acid and separated chromatographically. The results show an intense activity of the polysaccharide and lipoid fractions in the bioaccumulation process. The bioaccumulation varies in the series: Spirulina>Scenedesmus>Porphiridium>Riccia>Pistia> Lemna 3 Elodea for 137Cs+ and 60Co2+; Spirulina>Porphiridium>Scenedesmus>Riccia>Pistia>Lemna>Elodea for 51Cr3+.

Restricted access