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Dive into the research topics where R. Chelcea is active.

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Featured researches published by R. Chelcea.


Journal of Magnetic Resonance | 2009

Distributions of transverse relaxation times for soft-solids measured in strongly inhomogeneous magnetic fields

R. Chelcea; Radu Fechete; E. Culea; Dan E. Demco; Bernhard Blümich

The single-sided NMR-MOUSE sensor that operates in highly inhomogeneous magnetic fields is used to record a CPMG (1)H transverse relaxation decay by CPMG echo trains for a series of cross-linked natural rubber samples. Effective transverse relaxation rates 1/T(2,short) and 1/T(2,long) were determined by a bi-exponential fit. A linear dependence of transverse relaxation rates on cross-link density is observed for medium to large values of cross-link density. As an alternative to multi-exponential fits the possibility to analyze the dynamics of soft polymer network in terms of multi-exponential decays via the inverse Laplace transformation was studied. The transient regime and the effect of the T(1)/T(2) ratio in inhomogeneous static and radiofrequency magnetic fields on the CPMG decays were studied numerically using a dedicated C++ program to simulate the temporal and spatial dependence of the CPMG response. A correction factor T(2)/T(2,eff) is derived as a function of the T(1)/T(2) ratio from numerical simulations and compared with earlier results from two different well logging devices. High-resolution T(1)-T(2) correlations maps are obtained by two-dimensional Laplace inversion of CPMG detected saturation recovery curves. The T(1)-T(2) experimental correlations maps were corrected for the T(1)/T(2) effect using the derived T(2)/T(2,eff) correction factor.


Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy | 2011

Structural study of ternary iron-lead-germanate glass ceramics

R. Chelcea; S. Rada; E. Culea; M. Rada

Glass ceramics with the composition xFe(2)O(3)·(100-x)[7GeO(2)·3PbO(2)] where 0≤x≤60 mol% were obtained and studied using XRD, FTIR and UV-vis spectroscopy investigations. Heat treatment of glass samples at 400°C for 8 h led to the formation of α, γ-PbGe(4)O(9), Pb(3)Fe(2)Ge(4)O(14) and PbO(1.44) crystalline phases. The content of these crystalline phases depends of Fe(2)O(3) concentration. FTIR spectroscopy data suggest that the lead ions have a pronounced affinity towards [GeO(5)] structural units containing non-bridging oxygens and [FeO(4)] anions producing formation of the Pb(3)Fe(2)Ge(4)O(14) crystalline phase. The introduction of low concentrations of Fe(2)O(3) into the host matrix results in the formation of new absorption UV bands between 320 and 450 nm. These bands arise from to the d-d transitions of the Fe(+3) ions. The light absorption in the range from 250 to 600 nm increases with increasing iron oxide content in matrix network, accompanied with the changes on color from white to brown yellow and darker brown.


Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy | 2013

Effect of aluminum oxide codoping on copper–lead–germanate glasses

M. Rada; R. Chelcea; S. Rada; L. Rus; N. Dura; Tania Ristoiu; T. Rusu; E. Culea

Glasses from xCuO⋅(100-x)[7GeO(2)⋅3PbO(2)⋅0.05Al(2)O(3)] system where x=0, 1, 5, 10, 20 and 30 mol% CuO were studied by FT-IR, UV-VIS and ESR spectroscopy in order to obtain information about the structural correlations and the relationship between structure and optical properties in these materials. The analyses of these IR spectra reveal that the accommodation of the network with the excess of oxygen ions is possible by the depolymerization of the germanate network in shorter chains, especially ortho- and/or pyrogermanate structural units. UV-VIS absorption spectrum of the sample with x=1% CuO begins with a rising absorption band situated at about 250 nm. This band can be assigned to the GeOGe wrong bonds such as the Ge(+2) centers. Optical study is performed to calculate the refractive index and optical band gap using UV-VIS spectra in the wavelength range 250-1,000 nm. The increase in optical band is explained on the basis of the average bond energy of the system and the number of non-bridging oxygen ions. ESR spectra of CuO substituted samples are characterized by broad peaks probably because of the formation of Cu(+2)-Ge(+2) exchange pairs which are weakly coupled though the oxygen atom.


Journal of Physics: Conference Series | 2009

Immobilization of gadolinium in borate-tellurate glasses

S. Rada; E. Culea; M. Rada; Vistrian Maties; Maria Bosca; Lidia Pop; Radu Fechete; R. Chelcea; Dumitriţa Moldovan

Glasses in the xGd2O3?(100-x)[6TeO2?4B2O3] system, where x = 0, 5, 10, 15, 25 and 30 mol%, have been prepared from melt quenching method. The structural changes were studied by FTIR spectroscopy and DFT calculations. The FTIR spectroscopy data for these systems show that the gadolinium ions have a strong affinity towards the structural units containing non-bridging oxygens, which are negative-charged, because they are readily available for charge compensation. So that, it seems that the content of [BO4] structural units cannot become higher, because the modified [BO3] units containing one or more B-O-Gd bonds are unable to accept a fourth oxygen atom. On the other hand, the gadolinium ions have also an affinity pronounced towards [TeO3] structural units yielding the deformation of the Te-O-Te linkages. The compositional evolution of the network was accommodated with excess of oxygen by the formation of orthoborate structural units, disintegration of some boroxol rings and radical rearrangement of the network formed by the [TeO6] octahedral units.


Analytical Letters | 2018

In Vivo 1H Nuclear Magnetic Resonance Spectroscopy and Relaxometry Maps of the Human Female Pelvis

Réka Emma Dávid; Radu Fechete; Silviu Sfrângeu; Dumitriţa Moldovan; R. Chelcea; Iris Adina Morar; Florin Stamatian; Tunde Kovacs; Paula Popoi

ABSTRACT An essential organ of the female reproductive system, the uterus, is histologically divided into three layers: the perimetrium, myometrium, and endometrium. In vivo nuclear magnetic resonance (NMR) imaging and spectroscopy are modern methods that become important tools used by radiologists to increase the degree in confidence in their diagnosis, to provide noninvasive metabolic information, and to establish a standardized characterization of the nonpathological tissue. We developed a complex protocol based on in vivo magnetic resonance spectroscopy and imaging correlated with data analysis for the assessment of specific parameters, such as metabolite content, 1H spin-density, effective transverse relaxation time, and effective longitudinal relaxation time parameter maps, characteristic of normal endometrium in a group of 11 healthy volunteer women. This study involved: (i) a series of data analysis steps performed by a dedicated software and primary NMR signal processing (water and metabolite signal separation, phase and baseline correction, apodization) and (ii) advanced analysis based on dedicated programs. We observed and investigated three states of water: free water, loosely and strongly bound water characterized by different transverse relaxation times. The four most common metabolites reported in previous studies of malignant uterus were identified for normal endometrium. Qualitative transverse and longitudinal relaxation time parameter maps were obtained. The distributions of transverse relaxation times for various uterine areas were demonstrated by statistical analysis. The good signal-to-noise ratio allows the identification of other metabolites such as N-acetyl aspartate and M1 and M2 macromolecular resonances. A relatively high choline and lipid content and low creatine and lactate content were calculated for the group of healthy women.


Electrochimica Acta | 2013

Electrochemical characterization and structure of tungsten-lead-germanate glasses and glass ceramics

S. Rada; R. Chelcea; M. Rada; A. Bot; N. Aldea; V. Rednic; E. Culea


Ceramics International | 2013

Physical properties and electrochemical performance of molybdenum–lead-germanate glasses and glass ceramics

S. Rada; E. Culea; R. Chelcea; M. Rada; A. Bot; N. Aldea; V. Rednic


Journal of Alloys and Compounds | 2011

Structural studies on iron-tellurite glasses prepared by sol-gel method

S. Rada; A. Dehelean; M. Stan; R. Chelcea; E. Culea


Journal of Molecular Structure | 2010

Experimental and theoretical investigations of the copper-lead-germanate glasses

S. Rada; R. Chelcea; Monica Culea; A. Dehelean; E. Culea


Journal of Molecular Modeling | 2011

Experimental and theoretical investigations on the structure-properties interrelationship of the gadolinium-vanadate-germanate glasses

S. Rada; R. Chelcea; E. Culea

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E. Culea

Technical University of Cluj-Napoca

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S. Rada

Technical University of Cluj-Napoca

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A. Dehelean

Technical University of Cluj-Napoca

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M. Rada

Technical University of Cluj-Napoca

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Radu Fechete

Technical University of Cluj-Napoca

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Dumitriţa Moldovan

Technical University of Cluj-Napoca

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Monica Culea

University of Agricultural Sciences

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I. Coroiu

Technical University of Cluj-Napoca

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L. Rus

Technical University of Cluj-Napoca

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