Positron lif etimes and Do ppler broadening of the annihi l atio n line mea - surements were perf... more Positron lif etimes and Do ppler broadening of the annihi l atio n line mea - surements were perf ormed to study the vacancy typ e defects in polycrystallin e GeT e and (GeT e) ( 1 À x ) (A gBiT e2 ) x ( = 0 0 0 3 0 05 0 1 0 15 0 2, and 1) solid solutio ns. The values of lif etimes obtained are explaine d as due to positron and p ositronium saturated trapping at v acancies and vacancy clusters. T he interdep endence betw een mean positron lif etime and hole concentration is discussed.
The present work considers the formation of an appropriate Cu-Co spinel structure, supported on p... more The present work considers the formation of an appropriate Cu-Co spinel structure, supported on pure Al 2 O 3 and Al 2 O 3 modified with MgO. The prepared catalyst samples have been characterized by X-ray photoelectron spectra (XPS), X-ray diffraction analysis (XRD) and AAS. The specific surface area has been determined by the BET method. The catalytic activity of the samples has been studied in the reaction of NO reduction by CO. The results show that the best Cu-Co spinel structure is formed in the case of catalyst based on alumina support in the presence of 8 % MgO.
Zn(1-x)NixAl2O4 (x = 0.0-1.0) spinels were prepared at 800 °C by co-precipitation method and char... more Zn(1-x)NixAl2O4 (x = 0.0-1.0) spinels were prepared at 800 °C by co-precipitation method and characterized by infrared spectroscopy, X-ray diffraction, scanning electron microscopy and X-ray photoelectron spectroscopy. The specific surface area was determined by BET. SEM image showed nano sized spherical particles. XPS confirmed the valence states of the metals, showing moderate Lewis character for the surface of materials. The powders were successfully used as new heterogeneous catalysts of Biginelli’s reaction, a one-pot three- component reaction, leading to some dihydropyrimidinones (DHPMs). These new catalysts that produced good yields of DHPMs, were easily recovered by simple filtration and subsequently reused with persistent activity, and they are non-toxic and environmentally friendly. The optimum amount of catalyst is 20% by weight of benzaldehyde derivatives, while the doping amount has been found optimal for x = 0.1.
The influence of CuBr vapor (Cu-Br) and CO2 laser radiation on polypropylene corona electrets is ... more The influence of CuBr vapor (Cu-Br) and CO2 laser radiation on polypropylene corona electrets is investigated in the present paper. The surface potential was measured before (V0) and after (Virr) irradiation using the method of the vibrating electrode with compensation. The sample surfaces were characterized by XPS.
ABSTRACT The thermoelectric parameters (thermoelectric power S, electrical conductivity σand ther... more ABSTRACT The thermoelectric parameters (thermoelectric power S, electrical conductivity σand thermal conductivity κ) from the system (GeTe)1—x(AgBiTe2)x are measured in the temperature range from 300 to 800 K on hot-pressed samples with x = 0.03, 0.10, 0.15, 0.20. The thermoelectric figure of merit Z and the Ioffe criterion ZTaverage are calculated. The maximum value ZT = 1.32 at T = 700 K is found for the composition containing 3 mol% AgBiTe2. This value is practically the same as for TAGS 85, a material widely applied in USA space devices, and determines the utility of the new thermoelectric material.
ABSTRACT We report on the study of deposition of carbon films on different substrates: (001) Si, ... more ABSTRACT We report on the study of deposition of carbon films on different substrates: (001) Si, (001) Si covered with 300 nm \(\hbox {SiO}_{2} (\hbox {Si/SiO}_{2})\) film as well as on (001) Si covered with \(\hbox {SiO}_{2}\) and diamond-like carbon layers (DLC) carried out by sublimation of pyrolytic carbon layers. It is established by Raman as well as by X-ray photoelectron spectroscopy (XPS) that the layers deposited on Si as well as on \(\hbox {Si/SiO}_{2}\) substrates consist of amorphous \(\hbox {sp}^{2}\) —bonded carbon ( \(\upalpha \) -C) and the longer deposition time leads to formation of micro-sized \(\upalpha \) -C islands. The Raman studies of the films deposited on the \(\hbox {SiO}_{2}\) covered areas in the third type substrates have requisites of defected graphene (presence of clear D and weak broadening of the 2D band while the intensity ratio of 2D to G and D bands remains high: more than 2.5 and 3.1, respectively) and the films are predominantly single-layered. These films are determined as “polygraphene” (mainly single-layered graphene film consisting of mutually misoriented areas). The films deposited on the DLC paths in the third type of substrates most probably consist of few layers of mixed few-layered polygraphene and \((\hbox {sp}^{2}\hbox {C})\) - and \((\hbox {sp}^{3}\hbox {C})\) - H phases. The thorough XPS study indirectly confirms the above conclusions. The formation of polygraphene and mixed phases is explained with nucleation of many stable carbon aggregates which, once formed, are not sufficiently mobile to mutually re-orientate until forming defect-free graphene.
The influence of thin layers of Al2O3 and Ce2O3–CeO2, electrodeposited on stainless steel OC4004,... more The influence of thin layers of Al2O3 and Ce2O3–CeO2, electrodeposited on stainless steel OC4004, on the corrosion behaviour of the systems Al2O3/SS, Ce2O3–CeO2/SS and Al2O3–Ce2O3–CeO2/SS has been studied in sulfuric acid medium. A pronounced stabilizing effect on the passive state of steel and enhancement of its corrosion resistance has been established both for the samples as deposited and for the
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2005
ABSTRACT Plasma based Ar+ beam performed in RF (13.56 MHz) low-pressure (200 mTorr) glow discharg... more ABSTRACT Plasma based Ar+ beam performed in RF (13.56 MHz) low-pressure (200 mTorr) glow discharge (at 100 W, 1200 W and 2500 W) with a serial capacitance was employed for surface modification of poly(dimethylsiloxane) (PDMS) aimed at improvement of its interactions with living cells. The presence of a serial capacitance ensures arise of an ion-flow inside the plasma volume directed toward the treated sample and the vary of the discharge power ensures varied density of the ion-flow.XPS analysis was performed to study the changes in the surface chemical composition of the modified samples and the corresponding changes in the surface energy were monitored by contact angle measurements.We found that plasma based Ar+ beam transforms the initially hydrophobic PDMS surface into a hydrophilic one mainly due to a raising of the polar component of the surface tension, this effect being most probably due to an enrichment of the modified surface layer with permanent dipoles of a [SiOx]-based network and elimination of the original methyl groups. The initial adhesion of human fibroblast cells was studied on the described above plasma based Ar+ beam modified and acrylic acid (AA) grafted or not fibronectin (FN) pre-coated or bare surfaces.The cell response seems to be related with the peculiar structure and wettability of the modified PDMS surface layer after plasma based Ar+ beam treatment followed or not by AA grafting.
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2003
A number of recent results indicate that C60 films can superconduct at considerably high temperat... more A number of recent results indicate that C60 films can superconduct at considerably high temperatures. The response of a material to impurities can give an information about its superconducting properties when more conventional methods for spotting superconductivity are difficult to apply. While magnetic impurities act as strong Cooper pair breakers in conventional superconductors, nonmagnetic impurities have none or negligible effect.
ABSTRACT The density-of-states effective masses versus composition are calculated. The comparison... more ABSTRACT The density-of-states effective masses versus composition are calculated. The comparison between density-of-states effective mass and reverse mobility show that the variation of the mobility should be governed by the change in the effective mass.
Positron lif etimes and Do ppler broadening of the annihi l atio n line mea - surements were perf... more Positron lif etimes and Do ppler broadening of the annihi l atio n line mea - surements were perf ormed to study the vacancy typ e defects in polycrystallin e GeT e and (GeT e) ( 1 À x ) (A gBiT e2 ) x ( = 0 0 0 3 0 05 0 1 0 15 0 2, and 1) solid solutio ns. The values of lif etimes obtained are explaine d as due to positron and p ositronium saturated trapping at v acancies and vacancy clusters. T he interdep endence betw een mean positron lif etime and hole concentration is discussed.
The present work considers the formation of an appropriate Cu-Co spinel structure, supported on p... more The present work considers the formation of an appropriate Cu-Co spinel structure, supported on pure Al 2 O 3 and Al 2 O 3 modified with MgO. The prepared catalyst samples have been characterized by X-ray photoelectron spectra (XPS), X-ray diffraction analysis (XRD) and AAS. The specific surface area has been determined by the BET method. The catalytic activity of the samples has been studied in the reaction of NO reduction by CO. The results show that the best Cu-Co spinel structure is formed in the case of catalyst based on alumina support in the presence of 8 % MgO.
Zn(1-x)NixAl2O4 (x = 0.0-1.0) spinels were prepared at 800 °C by co-precipitation method and char... more Zn(1-x)NixAl2O4 (x = 0.0-1.0) spinels were prepared at 800 °C by co-precipitation method and characterized by infrared spectroscopy, X-ray diffraction, scanning electron microscopy and X-ray photoelectron spectroscopy. The specific surface area was determined by BET. SEM image showed nano sized spherical particles. XPS confirmed the valence states of the metals, showing moderate Lewis character for the surface of materials. The powders were successfully used as new heterogeneous catalysts of Biginelli’s reaction, a one-pot three- component reaction, leading to some dihydropyrimidinones (DHPMs). These new catalysts that produced good yields of DHPMs, were easily recovered by simple filtration and subsequently reused with persistent activity, and they are non-toxic and environmentally friendly. The optimum amount of catalyst is 20% by weight of benzaldehyde derivatives, while the doping amount has been found optimal for x = 0.1.
The influence of CuBr vapor (Cu-Br) and CO2 laser radiation on polypropylene corona electrets is ... more The influence of CuBr vapor (Cu-Br) and CO2 laser radiation on polypropylene corona electrets is investigated in the present paper. The surface potential was measured before (V0) and after (Virr) irradiation using the method of the vibrating electrode with compensation. The sample surfaces were characterized by XPS.
ABSTRACT The thermoelectric parameters (thermoelectric power S, electrical conductivity σand ther... more ABSTRACT The thermoelectric parameters (thermoelectric power S, electrical conductivity σand thermal conductivity κ) from the system (GeTe)1—x(AgBiTe2)x are measured in the temperature range from 300 to 800 K on hot-pressed samples with x = 0.03, 0.10, 0.15, 0.20. The thermoelectric figure of merit Z and the Ioffe criterion ZTaverage are calculated. The maximum value ZT = 1.32 at T = 700 K is found for the composition containing 3 mol% AgBiTe2. This value is practically the same as for TAGS 85, a material widely applied in USA space devices, and determines the utility of the new thermoelectric material.
ABSTRACT We report on the study of deposition of carbon films on different substrates: (001) Si, ... more ABSTRACT We report on the study of deposition of carbon films on different substrates: (001) Si, (001) Si covered with 300 nm \(\hbox {SiO}_{2} (\hbox {Si/SiO}_{2})\) film as well as on (001) Si covered with \(\hbox {SiO}_{2}\) and diamond-like carbon layers (DLC) carried out by sublimation of pyrolytic carbon layers. It is established by Raman as well as by X-ray photoelectron spectroscopy (XPS) that the layers deposited on Si as well as on \(\hbox {Si/SiO}_{2}\) substrates consist of amorphous \(\hbox {sp}^{2}\) —bonded carbon ( \(\upalpha \) -C) and the longer deposition time leads to formation of micro-sized \(\upalpha \) -C islands. The Raman studies of the films deposited on the \(\hbox {SiO}_{2}\) covered areas in the third type substrates have requisites of defected graphene (presence of clear D and weak broadening of the 2D band while the intensity ratio of 2D to G and D bands remains high: more than 2.5 and 3.1, respectively) and the films are predominantly single-layered. These films are determined as “polygraphene” (mainly single-layered graphene film consisting of mutually misoriented areas). The films deposited on the DLC paths in the third type of substrates most probably consist of few layers of mixed few-layered polygraphene and \((\hbox {sp}^{2}\hbox {C})\) - and \((\hbox {sp}^{3}\hbox {C})\) - H phases. The thorough XPS study indirectly confirms the above conclusions. The formation of polygraphene and mixed phases is explained with nucleation of many stable carbon aggregates which, once formed, are not sufficiently mobile to mutually re-orientate until forming defect-free graphene.
The influence of thin layers of Al2O3 and Ce2O3–CeO2, electrodeposited on stainless steel OC4004,... more The influence of thin layers of Al2O3 and Ce2O3–CeO2, electrodeposited on stainless steel OC4004, on the corrosion behaviour of the systems Al2O3/SS, Ce2O3–CeO2/SS and Al2O3–Ce2O3–CeO2/SS has been studied in sulfuric acid medium. A pronounced stabilizing effect on the passive state of steel and enhancement of its corrosion resistance has been established both for the samples as deposited and for the
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2005
ABSTRACT Plasma based Ar+ beam performed in RF (13.56 MHz) low-pressure (200 mTorr) glow discharg... more ABSTRACT Plasma based Ar+ beam performed in RF (13.56 MHz) low-pressure (200 mTorr) glow discharge (at 100 W, 1200 W and 2500 W) with a serial capacitance was employed for surface modification of poly(dimethylsiloxane) (PDMS) aimed at improvement of its interactions with living cells. The presence of a serial capacitance ensures arise of an ion-flow inside the plasma volume directed toward the treated sample and the vary of the discharge power ensures varied density of the ion-flow.XPS analysis was performed to study the changes in the surface chemical composition of the modified samples and the corresponding changes in the surface energy were monitored by contact angle measurements.We found that plasma based Ar+ beam transforms the initially hydrophobic PDMS surface into a hydrophilic one mainly due to a raising of the polar component of the surface tension, this effect being most probably due to an enrichment of the modified surface layer with permanent dipoles of a [SiOx]-based network and elimination of the original methyl groups. The initial adhesion of human fibroblast cells was studied on the described above plasma based Ar+ beam modified and acrylic acid (AA) grafted or not fibronectin (FN) pre-coated or bare surfaces.The cell response seems to be related with the peculiar structure and wettability of the modified PDMS surface layer after plasma based Ar+ beam treatment followed or not by AA grafting.
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2003
A number of recent results indicate that C60 films can superconduct at considerably high temperat... more A number of recent results indicate that C60 films can superconduct at considerably high temperatures. The response of a material to impurities can give an information about its superconducting properties when more conventional methods for spotting superconductivity are difficult to apply. While magnetic impurities act as strong Cooper pair breakers in conventional superconductors, nonmagnetic impurities have none or negligible effect.
ABSTRACT The density-of-states effective masses versus composition are calculated. The comparison... more ABSTRACT The density-of-states effective masses versus composition are calculated. The comparison between density-of-states effective mass and reverse mobility show that the variation of the mobility should be governed by the change in the effective mass.
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Papers by Ivalina Avramova