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Papers by Stefania Stoleriu

Revista Romana de materiale = Romanian journal of materials = RRM

Eu-doped lanthanum orthophosphate samples were prepared by precipitation in aqueous solution of l... more Eu-doped lanthanum orthophosphate samples were prepared by precipitation in aqueous solution of lanthanide ions using different precipitation agents or applying a hydrothermal treatment. The influence of the precipitation parameters on the structure and morphology of La0.95Eu0.05PO4 was studied analyzing the samples by XRD, FM and SEM. La0.95Eu0.05PO4 synthesized by precipitation method presented rhabdophane structure, whereas the hydrothermal method leads to rhabdophane phase when H3PO4 was added in the synthesis and a mixture of rhabdophane and monazite phases when KH2PO4 or Na2HPO4 were used. The annealing step at 550 degrees C/lh led to the monazite phase stabilization. The luminescent properties of La0.95Eu0.05PO4 samples were investigated.

Revista Romana de materiale = Romanian journal of materials = RRM

The paper's aim was to prepare and characterize ceramic powders of fully stabilized zirconia ... more The paper's aim was to prepare and characterize ceramic powders of fully stabilized zirconia (with 8 mol% yttria), and alumina, obtained through co-precipitation starting from the corresponding chlorides. Co-precipitation was taken using water or ethanol as a solvent and considering the pH influence (7.5, 10.5 and 12.5 obtained by using ammonia solution) upon precipitation was analyzed. The obtained powders were dried at 80 degrees C for 24 h and afterwards calcinated at 350 degrees C for 2 hours. The powders were evaluated through thermal analysis, X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The XRD results show that 8YSZ calcinated powders is a tetragonal ZrO(2) and, concerning alumina powder, besides gamma-Al(2)O(3), delta-Al(2)O(3) was identified. The TEM examination shows that the tetragonal ZrO(2) has a spherical like morphology with a size ranging from 5 nm to 12 nm. Also, it was observed that the best condition for nanopowders ob...

Key Engineering Materials, 2004

Key Engineering Materials, 2004

Key Engineering Materials, 2004

ABSTRACT

Hydroxiapatite with stabilised or unstabilised zirconia ceramics were synthesised by solid state-... more Hydroxiapatite with stabilised or unstabilised zirconia ceramics were synthesised by solid state-reaction of oxide powders. Their sinterability as well as phase analysis were studied by X-ray diffraction analysis. The microstructure of sintered materials was investigated by scanning electron microscopy (SEM) and EDAX. The ceramic and some mechanical properties were investigated using a comparative approach. An interaction mechanism between zirconia and its polymorphs with calcium phosphates existing or formed after sintering is discussed.

Polymer International, 2007

Epoxy-clay nanocomposites were synthesized using different types of modified montmorillonite, eit... more Epoxy-clay nanocomposites were synthesized using different types of modified montmorillonite, either with a classic quaternary ammonium salt or with protonated adducts synthesized by reacting resorcinol diglycidyl ether with monoamines (benzylamine or cyclohexylamine). The chemical structure was investigated using Fourier transform infrared and 1 H NMR spectrometry. The nanocomposite structures were confirmed using X-ray diffraction analysis and transmission electron microscopy. The influence of the montmorillonite modifier on the glass transition temperature of the cured composites was studied using dynamic mechanical analysis.

Materials Research Bulletin, 2012

ABSTRACT Dielectric materials with a high permittivity (ɛr), a high quality factor (Q) and a low ... more ABSTRACT Dielectric materials with a high permittivity (ɛr), a high quality factor (Q) and a low temperature coefficient of the resonant frequency (τf) have become very important for the miniaturization of microwave devices, such as filters or antennas. In this work, Ba(Zn1/3Ta2/3)O3 (BZT) ceramics doped with Nb2O5, MnO2 or V2O3 were obtained by the conventional solid-state reaction method. We report on the compositional, structural and morphological characterization of BZT resonators, as well as on the influence of the dopant (type and quantity) and sintering temperature on their dielectric properties. The best microwave dielectric properties (ɛr ∼ 28.4 and Q × f ∼ 236 THz) were achieved in the case of 1% V2O3 doping and 1600 °C sintering temperature.

Journal of the European Ceramic Society, 2007

TaC and TiC powders were added, respectively to a matrix of (CaO+MgO) totally stabilized zirconia... more TaC and TiC powders were added, respectively to a matrix of (CaO+MgO) totally stabilized zirconia and to a CeO2 stabilized zirconia (PSZ) in order to produce composites by uniaxial pressing. The samples were sintered in vacuum at 1450°C and in argon environment at 1800°C, respectively. The purpose of present paper is to establish influence of the type of secondary phase on the mechanical properties of composites and to correlate the physical – mechanical and structural properties of composites with different matrices as function of the amount of secondary phase by varying the TiC and respectively TaC content in 5–30% weight range. Comparative microstructure investigations were made by SEM on sample surfaces. The X-ray diffraction analysis was in accordance with the determined properties of the studied compositions.

Journal of the European Ceramic Society, 2007

Improvement of mechanical properties of alumina ceramics is investigated by addition of MgO and Z... more Improvement of mechanical properties of alumina ceramics is investigated by addition of MgO and ZrO2 starting from inorganic precursors. The doping of alumina ceramics is performed by infiltration of solutions of magnesium chloride and zirconium oxychloride into preformed samples. Two types of alumina powders were used. The impregnated samples were fired in 1550–1780°C temperature range and characterised by bulk density, bending strengths and their microstructure are analysed. The dispersion and concentration of reinforcing particles is analysed by electron microprobe (energy dispersive analysis).After firing the infiltration method leads the dopants concentrate especially at the surface of samples. The surface enrichment in dopants determines increased strengths with about 17–27% as function of sintering temperature.

Journal of the European Ceramic Society, 2007

Stabilisation in binary systems is a very widely approached process, but in many cases the temper... more Stabilisation in binary systems is a very widely approached process, but in many cases the temperatures needed for its achievement are too high and sometimes, even if tetragonal solid solutions are formed, they are unstable and are changing into a monoclinic structure on cooling. Due to ...

Journal of Applied Physics, 2011

The electric field dependence of the dielectric constant (dc-tunability) as a function of tempera... more The electric field dependence of the dielectric constant (dc-tunability) as a function of temperature for a few compositions of Ba(Sn,Ti)O 3 ceramics was investigated. Dense and homogeneous BaSn x Ti 1Àx O 3 ceramics (ferroelectric for x ¼ 0.05 and x ¼ 0.10, relaxor for x ¼ 0.15 and 0.20) with high dielectric constant and low dielectric losses were prepared by solid state reaction. Their dc-tunability characteristics were investigated at a few temperatures from 20-120 C, including the Curie range. The tunability data were discussed in terms of the Johnson model completed with a Langevin term that describes the "extrinsic" contribution to the nonlinear e(E) dependences.

Journal of Alloys and Compounds, 2011

PVP-BaTiO 3 composite nanofibers were successfully prepared by electrospinning and pure BaTiO 3 f... more PVP-BaTiO 3 composite nanofibers were successfully prepared by electrospinning and pure BaTiO 3 fibers were produced after calcination at 1000 8C. A homogeneous viscous solution of barium acetate + titanium acetate/titanium isopropoxide in poly vinyl pyrrolidone (PVP) was prepared by varying PVP concentration in the range of 8-12%. The above sols were electrospun at 9 kV DC by maintaining tip to collector distance (TCD) of 7 cm. The electrospun fibers were calcined at 1000 8C for 2 h. Thermo gravimetric analysis (TGA) of the fibers indicates the complete decomposition of organics below 700 8C with 45% weight loss. Scanning electron microscopy (SEM) study shows the fibers cylindrical, smooth with diameters in the range of 50-400 nm and the aspect ratio >1000. The average diameter of the fibers increases with the increase in PVP concentration. The calcined BaTiO 3 nanofibers were found to be coarse, brittle and diameter reduced by 12%. FT-IR study confirms the formation of metal oxide bond at higher temperature. #

Cement and Concrete Composites, 2005

The present paper deals with an overall interpretation of information regarding the behaviour of ... more The present paper deals with an overall interpretation of information regarding the behaviour of binders free of Portland cement, containing slag with or without fly ash, activated by sodium and calcium compounds. The information was obtained from experiments regarding the chemically bound water and mechanical strength determination, together with pH measurement, thermal analysis and X-ray diffraction (XRD). The results emphasize the possibility of obtaining alkali-activated binders incorporating slag and with or without fly ash and which have mechanical properties equivalent to normal Portland cement. The binder systems, especially those adequately activated and showing enhanced performance, can incorporate up to 40% fly ash without any major influence upon binder quality.

Revista Romana de materiale = Romanian journal of materials = RRM

Eu-doped lanthanum orthophosphate samples were prepared by precipitation in aqueous solution of l... more Eu-doped lanthanum orthophosphate samples were prepared by precipitation in aqueous solution of lanthanide ions using different precipitation agents or applying a hydrothermal treatment. The influence of the precipitation parameters on the structure and morphology of La0.95Eu0.05PO4 was studied analyzing the samples by XRD, FM and SEM. La0.95Eu0.05PO4 synthesized by precipitation method presented rhabdophane structure, whereas the hydrothermal method leads to rhabdophane phase when H3PO4 was added in the synthesis and a mixture of rhabdophane and monazite phases when KH2PO4 or Na2HPO4 were used. The annealing step at 550 degrees C/lh led to the monazite phase stabilization. The luminescent properties of La0.95Eu0.05PO4 samples were investigated.

Revista Romana de materiale = Romanian journal of materials = RRM

The paper's aim was to prepare and characterize ceramic powders of fully stabilized zirconia ... more The paper's aim was to prepare and characterize ceramic powders of fully stabilized zirconia (with 8 mol% yttria), and alumina, obtained through co-precipitation starting from the corresponding chlorides. Co-precipitation was taken using water or ethanol as a solvent and considering the pH influence (7.5, 10.5 and 12.5 obtained by using ammonia solution) upon precipitation was analyzed. The obtained powders were dried at 80 degrees C for 24 h and afterwards calcinated at 350 degrees C for 2 hours. The powders were evaluated through thermal analysis, X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The XRD results show that 8YSZ calcinated powders is a tetragonal ZrO(2) and, concerning alumina powder, besides gamma-Al(2)O(3), delta-Al(2)O(3) was identified. The TEM examination shows that the tetragonal ZrO(2) has a spherical like morphology with a size ranging from 5 nm to 12 nm. Also, it was observed that the best condition for nanopowders ob...

Key Engineering Materials, 2004

Key Engineering Materials, 2004

Key Engineering Materials, 2004

ABSTRACT

Hydroxiapatite with stabilised or unstabilised zirconia ceramics were synthesised by solid state-... more Hydroxiapatite with stabilised or unstabilised zirconia ceramics were synthesised by solid state-reaction of oxide powders. Their sinterability as well as phase analysis were studied by X-ray diffraction analysis. The microstructure of sintered materials was investigated by scanning electron microscopy (SEM) and EDAX. The ceramic and some mechanical properties were investigated using a comparative approach. An interaction mechanism between zirconia and its polymorphs with calcium phosphates existing or formed after sintering is discussed.

Polymer International, 2007

Epoxy-clay nanocomposites were synthesized using different types of modified montmorillonite, eit... more Epoxy-clay nanocomposites were synthesized using different types of modified montmorillonite, either with a classic quaternary ammonium salt or with protonated adducts synthesized by reacting resorcinol diglycidyl ether with monoamines (benzylamine or cyclohexylamine). The chemical structure was investigated using Fourier transform infrared and 1 H NMR spectrometry. The nanocomposite structures were confirmed using X-ray diffraction analysis and transmission electron microscopy. The influence of the montmorillonite modifier on the glass transition temperature of the cured composites was studied using dynamic mechanical analysis.

Materials Research Bulletin, 2012

ABSTRACT Dielectric materials with a high permittivity (ɛr), a high quality factor (Q) and a low ... more ABSTRACT Dielectric materials with a high permittivity (ɛr), a high quality factor (Q) and a low temperature coefficient of the resonant frequency (τf) have become very important for the miniaturization of microwave devices, such as filters or antennas. In this work, Ba(Zn1/3Ta2/3)O3 (BZT) ceramics doped with Nb2O5, MnO2 or V2O3 were obtained by the conventional solid-state reaction method. We report on the compositional, structural and morphological characterization of BZT resonators, as well as on the influence of the dopant (type and quantity) and sintering temperature on their dielectric properties. The best microwave dielectric properties (ɛr ∼ 28.4 and Q × f ∼ 236 THz) were achieved in the case of 1% V2O3 doping and 1600 °C sintering temperature.

Journal of the European Ceramic Society, 2007

TaC and TiC powders were added, respectively to a matrix of (CaO+MgO) totally stabilized zirconia... more TaC and TiC powders were added, respectively to a matrix of (CaO+MgO) totally stabilized zirconia and to a CeO2 stabilized zirconia (PSZ) in order to produce composites by uniaxial pressing. The samples were sintered in vacuum at 1450°C and in argon environment at 1800°C, respectively. The purpose of present paper is to establish influence of the type of secondary phase on the mechanical properties of composites and to correlate the physical – mechanical and structural properties of composites with different matrices as function of the amount of secondary phase by varying the TiC and respectively TaC content in 5–30% weight range. Comparative microstructure investigations were made by SEM on sample surfaces. The X-ray diffraction analysis was in accordance with the determined properties of the studied compositions.

Journal of the European Ceramic Society, 2007

Improvement of mechanical properties of alumina ceramics is investigated by addition of MgO and Z... more Improvement of mechanical properties of alumina ceramics is investigated by addition of MgO and ZrO2 starting from inorganic precursors. The doping of alumina ceramics is performed by infiltration of solutions of magnesium chloride and zirconium oxychloride into preformed samples. Two types of alumina powders were used. The impregnated samples were fired in 1550–1780°C temperature range and characterised by bulk density, bending strengths and their microstructure are analysed. The dispersion and concentration of reinforcing particles is analysed by electron microprobe (energy dispersive analysis).After firing the infiltration method leads the dopants concentrate especially at the surface of samples. The surface enrichment in dopants determines increased strengths with about 17–27% as function of sintering temperature.

Journal of the European Ceramic Society, 2007

Stabilisation in binary systems is a very widely approached process, but in many cases the temper... more Stabilisation in binary systems is a very widely approached process, but in many cases the temperatures needed for its achievement are too high and sometimes, even if tetragonal solid solutions are formed, they are unstable and are changing into a monoclinic structure on cooling. Due to ...

Journal of Applied Physics, 2011

The electric field dependence of the dielectric constant (dc-tunability) as a function of tempera... more The electric field dependence of the dielectric constant (dc-tunability) as a function of temperature for a few compositions of Ba(Sn,Ti)O 3 ceramics was investigated. Dense and homogeneous BaSn x Ti 1Àx O 3 ceramics (ferroelectric for x ¼ 0.05 and x ¼ 0.10, relaxor for x ¼ 0.15 and 0.20) with high dielectric constant and low dielectric losses were prepared by solid state reaction. Their dc-tunability characteristics were investigated at a few temperatures from 20-120 C, including the Curie range. The tunability data were discussed in terms of the Johnson model completed with a Langevin term that describes the "extrinsic" contribution to the nonlinear e(E) dependences.

Journal of Alloys and Compounds, 2011

PVP-BaTiO 3 composite nanofibers were successfully prepared by electrospinning and pure BaTiO 3 f... more PVP-BaTiO 3 composite nanofibers were successfully prepared by electrospinning and pure BaTiO 3 fibers were produced after calcination at 1000 8C. A homogeneous viscous solution of barium acetate + titanium acetate/titanium isopropoxide in poly vinyl pyrrolidone (PVP) was prepared by varying PVP concentration in the range of 8-12%. The above sols were electrospun at 9 kV DC by maintaining tip to collector distance (TCD) of 7 cm. The electrospun fibers were calcined at 1000 8C for 2 h. Thermo gravimetric analysis (TGA) of the fibers indicates the complete decomposition of organics below 700 8C with 45% weight loss. Scanning electron microscopy (SEM) study shows the fibers cylindrical, smooth with diameters in the range of 50-400 nm and the aspect ratio >1000. The average diameter of the fibers increases with the increase in PVP concentration. The calcined BaTiO 3 nanofibers were found to be coarse, brittle and diameter reduced by 12%. FT-IR study confirms the formation of metal oxide bond at higher temperature. #

Cement and Concrete Composites, 2005

The present paper deals with an overall interpretation of information regarding the behaviour of ... more The present paper deals with an overall interpretation of information regarding the behaviour of binders free of Portland cement, containing slag with or without fly ash, activated by sodium and calcium compounds. The information was obtained from experiments regarding the chemically bound water and mechanical strength determination, together with pH measurement, thermal analysis and X-ray diffraction (XRD). The results emphasize the possibility of obtaining alkali-activated binders incorporating slag and with or without fly ash and which have mechanical properties equivalent to normal Portland cement. The binder systems, especially those adequately activated and showing enhanced performance, can incorporate up to 40% fly ash without any major influence upon binder quality.