The present research describes the synthesis of the citrate of Pr by the amorphous citrate method and the study of the thermal decomposition of the complex in air, nitrogen and oxygen, in order to identify the final compounds and the intermediates obtained during the decomposition. A special study was dedicated to the thermal decomposition of the Pr-carbonate octahydrate used in the synthesis as starting material. The thermal decomposition of the lanthanide citrate was investigated mainly by thermogravimetry, differential thermal analysis, X-ray diffraction and infrared spectroscopy. The results showed that the complex decomposes to PrO1.83 with formation of an intermediate oxycarbonate compound in air or directly to the oxide in oxygen; in nitrogen atmosphere, amorphous products were obtained after thermal decomposition.
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.
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.
Authors:M. Popa, J. Calderon-Moreno, D. Crisan, and M. Zaharescu
This work introduces results obtained during the preparation of a Bi-based material with superconducting properties by oxalate
The influence of Fe presence on the precursors thermal stability and on the superconducting phases formation mechanism are
presented. The thermal decomposition and the stability in air of FeC2O42H2O and also of the components mixture were studied by DTA/TG. It was evidenced that iron oxalate decomposes at the lowest temperature
compared to the decomposition temperatures of the individual oxalates.
XRD, IR and TEM/ED studies were approached to investigate the individual oxalates and the mixture coprecipitates for the high-Tc superconducting material synthesis.
Authors:Mihaela Frunza, Gabriela Lisa, M. Popa, N. Miron, and Denisa Nistor
The thermal behavior of the new inorganic-organic nanohybrid materials obtained by intercalation of chloramphenicol and salicylate
into layered double hydroxides (LDHs) by direct synthesis method, anion exchange reaction and the reconstruction method was
evaluated by dynamic thermogravimetric analysis in air, at heating rates of 5°C min−1. The XRD patterns of the samples are characteristic for those of well crystallized solids with layered double hydroxide structures.
The FTIR spectroscopy results show the presence of the organic compound within the network structure of the synthesized LDHs.
The kinetic parameters (reaction order (n) apparent activation energy (Ea) and pre-exponential factor (lnA) were calculated by the Coats-Redfern method. The compensation effects were determined.
Authors:Ana Neacsu, I. Contineanu, V. Popa, and M. Contineanu
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.
Authors:Al. Cecal, K. Popa, M. Craus, S. Patachia, and R. Moraru
The present paper deals with an experimental study on the bioleaching of a poor uranium ore by means of hydrophytic plants Lemna minor and Riccia fluitans, under various operating conditions. The maximum degree of bioleaching (42%) of the reduced uranium species to U(VI) has been attained for the ore-Lemna minor-alkaline carbonate solution system. The UO22+ ions amount accumulated in the plants is negligible as compared to the dissolved quantity, owing to the ionic competition between uranyl ions and the cations necessary to the mineral nutrition. The X-ray diffraction patterns prove that the uranium species in pyrochlore mineral are completely oxidized to U(VI), while thucolite is only partially turned into UO22+ ions, in the presence of living plants.
Authors:T. Lupascu, I. Dranca, V. Popa, and M. Vass
TG, DTG and DTA methods were used for the investigation of some waste agricultural products, such as grape seeds, walnut shells,
plum and peach stones, which can serve as raw materials for the production of active carbons. It was demonstrated that thermo
analytical methods are appropriate to study the thermal characteristics of the above wastes and the data obtained can be applied
to the technological processes of active carbon preparation.
Authors:M. Ştefănescu, M. Stoia, O. Ştefănescu, A. Popa, M. Simon, and C. Ionescu
Hybrid organic-inorganic materials,
silica – polyols (ethylene-glycol – EG; 1,2 propane diol –
1,2PG; 1,3 propane diol – 1,3PG and glycerol – GL), were prepared
by a sol-gel process starting from tetraethylorthosilicate (TEOS) and polyols,
in acid catalysis. The resulting materials were studied by thermal analysis
(in air and nitrogen), FTIR and solid state 29Si-NMR
spectroscopy. These techniques evidenced the presence of polyols in the silica
matrix both hydrogen bounded and chemically bounded in the silica network.
The thermal analysis proves to be the most appropriate technique to evidence
the organic chains linked in the matrix network and to follow the thermal
evolution of the gels to the SiO2 matrix.