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Detalles de la revista
Formato
Revista
eISSN
2083-134X
ISSN
2083-1331
Publicado por primera vez
16 Apr 2011
Periodo de publicación
4 veces al año
Idiomas
Inglés

Buscar

Volumen 29 (2011): Edición 3 (September 2011)

Detalles de la revista
Formato
Revista
eISSN
2083-134X
ISSN
2083-1331
Publicado por primera vez
16 Apr 2011
Periodo de publicación
4 veces al año
Idiomas
Inglés

Buscar

11 Artículos
Acceso abierto

Role of pH and current density in electrodeposited soft magnetic Co-Ni-Fe alloy thin films

Publicado en línea: 29 Feb 2012
Páginas: 165 - 170

Resumen

Abstract

The influence of pH and current density on the structural and magnetic behavior of soft magnetic Co-Ni-Fe alloy thin films has been studied. The effect of pH and current density on the compositional, structural, and magnetic properties of the as-obtained films was investigated by EDX (energy dispersive analysis by X-rays), XRD (X-ray diffractometer) and VSM (vibrating sample magnetometer). The EDX results revealed that at the optimized deposition conditions, nickel content was low compared with cobalt and ferrous content. X-ray diffraction patterns revealed that the deposited films have polycrystalline nature with mixed (fcc-bcc) cubic structure and small crystallite size (<20 nm). The films prepared in optimized conditions exhibit high saturation magnetization (4πMs value above 2T) and low coercivity (below 160 A/m), which may be due to the reduced crystallite size.

Palabras clave

  • Electrodeposition
  • Co-Ni-Fe alloy
  • structural properties
  • magnetic properties
Acceso abierto

FeS2 quantum dots sensitized nanostructured TiO2 solar cell: photoelectrochemical and photoinduced absorption spectroscopy studies

Publicado en línea: 29 Feb 2012
Páginas: 171 - 176

Resumen

Abstract

Thin films of nanostructured TiO2 have been modified with FeS2 (pyrite) nano-particles by a low temperature chemical reaction of iron pentacarbonyl with sulfur in xylene. Quantum size effects are manifested by the observation of a blue shift in both absorption and photocurrent action spectra. PIA (Photoinduced absorption spectroscopy), where the excitation is provided by a square-wave modulated (on/off) monochromatic light emitting diode, is a multipurpose tool in the study of dye-sensitized solar cells. Here, PIA is used to study quantum-dot modified TiO2 nanostructured electrodes. The PIA spectra obtained give evidence for long-lived photoinduced charge separation: electrons are injected into the metal oxide and holes are left behind in the FeS2 quantum dot. Time-resolved PIA shows that recombination between electrons and holes occurs on a millisecond timescale. The Incident-Photon-to-Current Efficiency of about 23 % was obtained at 400 nm excitation. The performances of TiO2 electrodes modified with FeS2 are relatively low, which is explained by the presence of FeS2 phases other than the photoactive pyrite phase, as follows from the XRD spectrum.

Palabras clave

  • FeS2
  • quantum-dots
  • TiO2
  • nanostructures
  • PIA
  • photoelectrochemistry
Acceso abierto

Influence of thermal treatment on the corrosion resistance of electrolytic Zn-Ni coatings

Publicado en línea: 29 Feb 2012
Páginas: 177 - 183

Resumen

Abstract

This study was undertaken in order to obtain and characterize the corrosion resistance of Zn-Ni coating. The process was carried out under galvanostatic conditions (j = 50 mA·cm−2) chosen on the ground of an analysis of the deposition process in the Hull’s cell. The Zn-Ni coatings were deposited on austenitic (OH18N9) steel substrate from the ammonia bath. Thermal treatment of Zn-Ni coating was carried out in argon atmosphere. Structural investigations were conducted by X-ray diffraction method. Surface morphology of the obtained coatings was determined using a scanning electron microscope (JEOL JSM-6480) with EDS attachment. The electrochemical corrosion resistance of the prepared Zn-Ni coatings, austenitic (OH18N9) and (St3S) steels, was defined. The studies of electrochemical corrosion resistance were carried out in 5 % NaCl, using potentiodynamic and electrochemical impedance spectroscopy (EIS) methods. Examinations of localized corrosion resistance were conducted using scanning vibrating electrode technique (SVET). On the grounds of these investigations it was found that Zn-Ni coating after thermal treatment was more corrosion resistant than the Zn-Ni coating before thermal treatment. The relatively good corrosion resistance of Zn-Ni coatings is not as high as the resistance of (OH18N9) steel substrate, but higher compared to (St3S) steel. Therefore, the Zn-Ni coatings may be regarded as a protective coating for St3S steel.

Palabras clave

  • electrodeposition
  • corrosion
  • Zn-Ni coatings
Acceso abierto

Novel microwave assisted synthesis of highly doped phase pure Nd:YAG nanopowder

Publicado en línea: 29 Feb 2012
Páginas: 184 - 188

Resumen

Abstract

For the first time, the studies on 2 to 10 at.% neodymium (Nd3+) ion doped Yttrium Aluminum Garnet (Nd:YAG) nanopowders obtained by microwave assisted citrate nitrate gel combustion synthesis is described in this work. This paper reports on high doping of Nd3+ ions with retaining the cubic garnet structure of YAG as evidenced from XRD, except the case of 8 at.% doped Nd:YAG. Phase pure YAG formation with 8 at.% Nd3+ doping was explored by using urea and alanine as alternative to citric acid complexing agents. Complete crystallization of YAG as a result of 2 hour thermal treatment at 900 °C under oxygen supply was studied by using Fourier Transform Infra-Red Spectroscopy (FTIR) and X-Ray Diffraction (XRD) techniques. With an increase in the dopant concentration a red shift in the FTIR peaks was observed. Using the XRD data, the cell parameter of Nd3+ (2 to 6 and 10 at.%) YAG was found to increase with an increase in the dopant concentration. The average primary particle size calculated using Scherrer’s equation was ∼25 nm which was additionally supported by Transmission Electron Microscopy (TEM) results yielding particle sizes in the range of ∼25 to 30 nm for all the cases.

Palabras clave

  • high doping
  • Nd:YAG
  • microwave assisted sol-gel process
  • cell parameter
Acceso abierto

TiO2 supported on SiO2 photocatalysts prepared using ultrasonic-assisted sol-gel method

Publicado en línea: 29 Feb 2012
Páginas: 189 - 194

Resumen

Abstract

TiO2-SiO2 (TiO2 supported on SiO2) photocatalysts were prepared using an ultrasonic-assisted sol-gel method. These photocatalysts were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR) and photoluminescence spectra (PL). Their photocatalytic activities were investigated by the method of methyl orange oxidation. It was found that the photocatalytic activity of TiO2-SiO2 was optimal when the molar ratio of hexadecyl trimethyl ammonium bromide to titanium butoxide was 1:10. The average crystallite size of TiO2-SiO2 was smaller than that prepared by the stirring method. Furthermore, for pure anatase phase samples, it was shown that the lower the photoluminescence intensity, the higher the photocatalytic activity.

Palabras clave

  • TiO2-SiO2
  • ultrasonic-assisted
  • X-ray diffraction
  • photocatalytic property
Acceso abierto

The influence of sublayer material on surface properties of electrodeposited nickel with periodical structures

Publicado en línea: 29 Feb 2012
Páginas: 195 - 202

Resumen

Abstract

Surface topography, hardness and microstructure of nickel coatings electrodeposited on Si master with a periodical structure have been studied depending on vacuum deposited sublayer material (Ni, Cu or Ag). It is shown that the quality of replication of silicon master in a nickel shim electrodeposited simultaneously on different sublayers is dependent on the material used. All types of the analyzed coatings enabled transfer of lateral dimensions and showed good replication quality of the tested periodical structures (2 μm period), while the structure replicated using the coatings deposited on the Ni sublayer exhibited the worst roughness. The hardness of the electrodeposited layers was found to be dependent on the sublayer material used as well as on the side of deposit. Despite the fact that the backside of nickel shim had the same hardness for all the sublayers used, it was found that the hardness of the working surface with periodical structure (the side that has been in contact with the vacuum deposited layer) is dependent on the sublayer material: the Ni and Cu sublayers increased the hardness of Ni coating, while for the nickel shim deposited on Ag sublayer the hardness was reduced.

Palabras clave

  • electrodeposition
  • replication
  • periodical structure
  • hardness
  • nickel shim
Acceso abierto

Effect of heat-treatment on grain growth of nanocrystalline tricalcium phosphate powder synthesized via the precipitation method

Publicado en línea: 29 Feb 2012
Páginas: 203 - 208

Resumen

Abstract

Nanocrystalline tricalcium phosphate powder was synthesized via the solution-precipitation method followed by heat treatment in order to achieve phase evolution, which was then studied by XRD and TEM techniques. The crystallites sizes were estimated by the Scherrer method and results were confirmed by TEM micrographs. The experimental observations showed that nanocrystalline tricalcium phosphate can be successfully prepared from raw materials by the precipitation technique. This technique is a competitive method for nanocrystalline tricalcium phosphate synthesis compared to other techniques. Moreover, a simple kinetic growth investigation was performed on the nanocrystalline growth process during heat treatment. Results have shown growth rate to increase exponentially with temperature and the growth rate constants to increase with time. The average activation energies of tricalcium phosphate grain growth obtained by this method were 84.78 and 134.38 KJ/mol.

Palabras clave

  • tricalcium phosphate
  • nanostructures
  • crystal growth
  • electron microscopy
  • X-ray diffraction
Acceso abierto

AC susceptibility study of YBCO

Publicado en línea: 29 Feb 2012
Páginas: 209 - 215

Resumen

Abstract

The temperature dependence of ac susceptibility of YBCO bulk samples was measured as a function of ac field amplitude and frequency. Analysis of the temperature dependence of the ac susceptibility near the transition temperature (T c) has been done employing the simplified Kim model. We have obtained an empirical function for the penetration field H p = H α(1−t)β, t = T=T c. Best fitting to data was obtained with parameters H α ≈ 6:2 × 103 A/m and β ≈ 1.50. The experimental value agrees well with the model calculations. In addition, as the frequency increases, the peak temperature (T p) shifts to higher temperature. This effect can be interpreted in terms of flux creep. The field dependence of activation energy obtained from the Arrhenius plots for the frequency (f) and (T p) can be described as U ∝ (H ac)−β′ with β′ ≈ 0:38 for YBCO.

Palabras clave

  • YBCO
  • ac susceptibility
  • simplified Kim model
  • activation energy
Acceso abierto

Comparative cavitation erosion test on steels produced by ESR and AOD refining

Publicado en línea: 29 Feb 2012
Páginas: 216 - 222

Resumen

Abstract

Cavitation erosion studies of steels produced by Electroslag Refining (ESR) and Argon Oxygen Decarburization (AOD refining) have been carried out. The experiments were conducted using the modified ultrasonically induced cavitation test method. Erosion rates were measured and the morphology of damages under cavitation action was studied by scanning electron microscopy and optical microscopy techniques. The present work is aimed at understanding the cavitation erosion behaviour of electroslag refined steel (ESR) compared with the steel produced by Argon Oxygen Decarburization (AOD refining), commonly used in the production of hydraulic machinery parts (Pelton blades). The results exhibited lower cavitation rate of ESR steel compared with AOD steel, as a consequence of its better mechanical properties and homogeneous and fine-grained microstructure.

Palabras clave

  • cavitation
  • refining
  • SEM
  • optical microscopy
  • microhardness
Acceso abierto

Determination of the electronic band structure of the rutile polymorph of TiO2: a quantum chemical approach

Publicado en línea: 29 Feb 2012
Páginas: 223 - 232

Resumen

Abstract

The aim of this work is the investigation of the relationship between the electronic band structure of the TiO2 rutile and the dimensionality of the system. For three dimensional system the bulk form of rutile was considered, while a slab model was chosen in order to represent the titanium (IV) dioxide (110) surface. The influence of changing the number of atomic layers on the bandgap value for the (110) surface was also examined. Density of states referring to the bands from the first valence band up to the bottom of the conduction band was projected on the whole set of atomic orbitals as well as on the significant shells of the titanium and oxygen atoms. Ab initio calculations with a B3LYP functional were carried out. Basis sets used were modified Ti_86-411(d31)G_darco_unpub and O 8_411_muscat_1999. The results are compared with experimental and computational data already available in the literature. Surface termination problem was discussed and the application of the obtained results as a starting point to obtain the first model of the rutile titania nanotube was proposed. The surface formation energies for rutile planes with a different surface terminations were compared and the modification to the equation needed for surface energy calculation was introduced.

Palabras clave

  • rutile
  • band structure
  • B3LYP
  • surface termination
  • surface formation energy
Acceso abierto

Mechanisms of carriers transport in Ni/n-SiC, Ti/n-SiC ohmic contacts

Publicado en línea: 29 Feb 2012
Páginas: 233 - 240

Resumen

Abstract

A mechanism of carriers transport through metal-semiconductor interface created by nickel or titanium-based ohmic contacts on Si-face n-type 4H-SiC is presented herein. The mechanism was observed within the temperature range of 20 °C ÷ 300 °C which are typical for devices operating at high current density and at poor cooling conditions. It was found that carriers transport depends strongly on concentration of dopants in the epitaxial layer. The carriers transport has thermionic emission nature for low dopant concentration of 5×1016 cm−3. The thermionic emission was identified for moderate dopant concentration of 5×1017 cm−3 at temperatures higher than 200 °C. Below 200 °C, the field emission dominates (for the same doping level of 5×1017 cm−3). High dopant concentration of 5×1018 cm−3 leads to almost pure field emission transport within the whole investigated temperature range.

Palabras clave

  • SiC
  • ohmic contact
  • carrier transport
  • silicide
  • carbide
11 Artículos
Acceso abierto

Role of pH and current density in electrodeposited soft magnetic Co-Ni-Fe alloy thin films

Publicado en línea: 29 Feb 2012
Páginas: 165 - 170

Resumen

Abstract

The influence of pH and current density on the structural and magnetic behavior of soft magnetic Co-Ni-Fe alloy thin films has been studied. The effect of pH and current density on the compositional, structural, and magnetic properties of the as-obtained films was investigated by EDX (energy dispersive analysis by X-rays), XRD (X-ray diffractometer) and VSM (vibrating sample magnetometer). The EDX results revealed that at the optimized deposition conditions, nickel content was low compared with cobalt and ferrous content. X-ray diffraction patterns revealed that the deposited films have polycrystalline nature with mixed (fcc-bcc) cubic structure and small crystallite size (<20 nm). The films prepared in optimized conditions exhibit high saturation magnetization (4πMs value above 2T) and low coercivity (below 160 A/m), which may be due to the reduced crystallite size.

Palabras clave

  • Electrodeposition
  • Co-Ni-Fe alloy
  • structural properties
  • magnetic properties
Acceso abierto

FeS2 quantum dots sensitized nanostructured TiO2 solar cell: photoelectrochemical and photoinduced absorption spectroscopy studies

Publicado en línea: 29 Feb 2012
Páginas: 171 - 176

Resumen

Abstract

Thin films of nanostructured TiO2 have been modified with FeS2 (pyrite) nano-particles by a low temperature chemical reaction of iron pentacarbonyl with sulfur in xylene. Quantum size effects are manifested by the observation of a blue shift in both absorption and photocurrent action spectra. PIA (Photoinduced absorption spectroscopy), where the excitation is provided by a square-wave modulated (on/off) monochromatic light emitting diode, is a multipurpose tool in the study of dye-sensitized solar cells. Here, PIA is used to study quantum-dot modified TiO2 nanostructured electrodes. The PIA spectra obtained give evidence for long-lived photoinduced charge separation: electrons are injected into the metal oxide and holes are left behind in the FeS2 quantum dot. Time-resolved PIA shows that recombination between electrons and holes occurs on a millisecond timescale. The Incident-Photon-to-Current Efficiency of about 23 % was obtained at 400 nm excitation. The performances of TiO2 electrodes modified with FeS2 are relatively low, which is explained by the presence of FeS2 phases other than the photoactive pyrite phase, as follows from the XRD spectrum.

Palabras clave

  • FeS2
  • quantum-dots
  • TiO2
  • nanostructures
  • PIA
  • photoelectrochemistry
Acceso abierto

Influence of thermal treatment on the corrosion resistance of electrolytic Zn-Ni coatings

Publicado en línea: 29 Feb 2012
Páginas: 177 - 183

Resumen

Abstract

This study was undertaken in order to obtain and characterize the corrosion resistance of Zn-Ni coating. The process was carried out under galvanostatic conditions (j = 50 mA·cm−2) chosen on the ground of an analysis of the deposition process in the Hull’s cell. The Zn-Ni coatings were deposited on austenitic (OH18N9) steel substrate from the ammonia bath. Thermal treatment of Zn-Ni coating was carried out in argon atmosphere. Structural investigations were conducted by X-ray diffraction method. Surface morphology of the obtained coatings was determined using a scanning electron microscope (JEOL JSM-6480) with EDS attachment. The electrochemical corrosion resistance of the prepared Zn-Ni coatings, austenitic (OH18N9) and (St3S) steels, was defined. The studies of electrochemical corrosion resistance were carried out in 5 % NaCl, using potentiodynamic and electrochemical impedance spectroscopy (EIS) methods. Examinations of localized corrosion resistance were conducted using scanning vibrating electrode technique (SVET). On the grounds of these investigations it was found that Zn-Ni coating after thermal treatment was more corrosion resistant than the Zn-Ni coating before thermal treatment. The relatively good corrosion resistance of Zn-Ni coatings is not as high as the resistance of (OH18N9) steel substrate, but higher compared to (St3S) steel. Therefore, the Zn-Ni coatings may be regarded as a protective coating for St3S steel.

Palabras clave

  • electrodeposition
  • corrosion
  • Zn-Ni coatings
Acceso abierto

Novel microwave assisted synthesis of highly doped phase pure Nd:YAG nanopowder

Publicado en línea: 29 Feb 2012
Páginas: 184 - 188

Resumen

Abstract

For the first time, the studies on 2 to 10 at.% neodymium (Nd3+) ion doped Yttrium Aluminum Garnet (Nd:YAG) nanopowders obtained by microwave assisted citrate nitrate gel combustion synthesis is described in this work. This paper reports on high doping of Nd3+ ions with retaining the cubic garnet structure of YAG as evidenced from XRD, except the case of 8 at.% doped Nd:YAG. Phase pure YAG formation with 8 at.% Nd3+ doping was explored by using urea and alanine as alternative to citric acid complexing agents. Complete crystallization of YAG as a result of 2 hour thermal treatment at 900 °C under oxygen supply was studied by using Fourier Transform Infra-Red Spectroscopy (FTIR) and X-Ray Diffraction (XRD) techniques. With an increase in the dopant concentration a red shift in the FTIR peaks was observed. Using the XRD data, the cell parameter of Nd3+ (2 to 6 and 10 at.%) YAG was found to increase with an increase in the dopant concentration. The average primary particle size calculated using Scherrer’s equation was ∼25 nm which was additionally supported by Transmission Electron Microscopy (TEM) results yielding particle sizes in the range of ∼25 to 30 nm for all the cases.

Palabras clave

  • high doping
  • Nd:YAG
  • microwave assisted sol-gel process
  • cell parameter
Acceso abierto

TiO2 supported on SiO2 photocatalysts prepared using ultrasonic-assisted sol-gel method

Publicado en línea: 29 Feb 2012
Páginas: 189 - 194

Resumen

Abstract

TiO2-SiO2 (TiO2 supported on SiO2) photocatalysts were prepared using an ultrasonic-assisted sol-gel method. These photocatalysts were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR) and photoluminescence spectra (PL). Their photocatalytic activities were investigated by the method of methyl orange oxidation. It was found that the photocatalytic activity of TiO2-SiO2 was optimal when the molar ratio of hexadecyl trimethyl ammonium bromide to titanium butoxide was 1:10. The average crystallite size of TiO2-SiO2 was smaller than that prepared by the stirring method. Furthermore, for pure anatase phase samples, it was shown that the lower the photoluminescence intensity, the higher the photocatalytic activity.

Palabras clave

  • TiO2-SiO2
  • ultrasonic-assisted
  • X-ray diffraction
  • photocatalytic property
Acceso abierto

The influence of sublayer material on surface properties of electrodeposited nickel with periodical structures

Publicado en línea: 29 Feb 2012
Páginas: 195 - 202

Resumen

Abstract

Surface topography, hardness and microstructure of nickel coatings electrodeposited on Si master with a periodical structure have been studied depending on vacuum deposited sublayer material (Ni, Cu or Ag). It is shown that the quality of replication of silicon master in a nickel shim electrodeposited simultaneously on different sublayers is dependent on the material used. All types of the analyzed coatings enabled transfer of lateral dimensions and showed good replication quality of the tested periodical structures (2 μm period), while the structure replicated using the coatings deposited on the Ni sublayer exhibited the worst roughness. The hardness of the electrodeposited layers was found to be dependent on the sublayer material used as well as on the side of deposit. Despite the fact that the backside of nickel shim had the same hardness for all the sublayers used, it was found that the hardness of the working surface with periodical structure (the side that has been in contact with the vacuum deposited layer) is dependent on the sublayer material: the Ni and Cu sublayers increased the hardness of Ni coating, while for the nickel shim deposited on Ag sublayer the hardness was reduced.

Palabras clave

  • electrodeposition
  • replication
  • periodical structure
  • hardness
  • nickel shim
Acceso abierto

Effect of heat-treatment on grain growth of nanocrystalline tricalcium phosphate powder synthesized via the precipitation method

Publicado en línea: 29 Feb 2012
Páginas: 203 - 208

Resumen

Abstract

Nanocrystalline tricalcium phosphate powder was synthesized via the solution-precipitation method followed by heat treatment in order to achieve phase evolution, which was then studied by XRD and TEM techniques. The crystallites sizes were estimated by the Scherrer method and results were confirmed by TEM micrographs. The experimental observations showed that nanocrystalline tricalcium phosphate can be successfully prepared from raw materials by the precipitation technique. This technique is a competitive method for nanocrystalline tricalcium phosphate synthesis compared to other techniques. Moreover, a simple kinetic growth investigation was performed on the nanocrystalline growth process during heat treatment. Results have shown growth rate to increase exponentially with temperature and the growth rate constants to increase with time. The average activation energies of tricalcium phosphate grain growth obtained by this method were 84.78 and 134.38 KJ/mol.

Palabras clave

  • tricalcium phosphate
  • nanostructures
  • crystal growth
  • electron microscopy
  • X-ray diffraction
Acceso abierto

AC susceptibility study of YBCO

Publicado en línea: 29 Feb 2012
Páginas: 209 - 215

Resumen

Abstract

The temperature dependence of ac susceptibility of YBCO bulk samples was measured as a function of ac field amplitude and frequency. Analysis of the temperature dependence of the ac susceptibility near the transition temperature (T c) has been done employing the simplified Kim model. We have obtained an empirical function for the penetration field H p = H α(1−t)β, t = T=T c. Best fitting to data was obtained with parameters H α ≈ 6:2 × 103 A/m and β ≈ 1.50. The experimental value agrees well with the model calculations. In addition, as the frequency increases, the peak temperature (T p) shifts to higher temperature. This effect can be interpreted in terms of flux creep. The field dependence of activation energy obtained from the Arrhenius plots for the frequency (f) and (T p) can be described as U ∝ (H ac)−β′ with β′ ≈ 0:38 for YBCO.

Palabras clave

  • YBCO
  • ac susceptibility
  • simplified Kim model
  • activation energy
Acceso abierto

Comparative cavitation erosion test on steels produced by ESR and AOD refining

Publicado en línea: 29 Feb 2012
Páginas: 216 - 222

Resumen

Abstract

Cavitation erosion studies of steels produced by Electroslag Refining (ESR) and Argon Oxygen Decarburization (AOD refining) have been carried out. The experiments were conducted using the modified ultrasonically induced cavitation test method. Erosion rates were measured and the morphology of damages under cavitation action was studied by scanning electron microscopy and optical microscopy techniques. The present work is aimed at understanding the cavitation erosion behaviour of electroslag refined steel (ESR) compared with the steel produced by Argon Oxygen Decarburization (AOD refining), commonly used in the production of hydraulic machinery parts (Pelton blades). The results exhibited lower cavitation rate of ESR steel compared with AOD steel, as a consequence of its better mechanical properties and homogeneous and fine-grained microstructure.

Palabras clave

  • cavitation
  • refining
  • SEM
  • optical microscopy
  • microhardness
Acceso abierto

Determination of the electronic band structure of the rutile polymorph of TiO2: a quantum chemical approach

Publicado en línea: 29 Feb 2012
Páginas: 223 - 232

Resumen

Abstract

The aim of this work is the investigation of the relationship between the electronic band structure of the TiO2 rutile and the dimensionality of the system. For three dimensional system the bulk form of rutile was considered, while a slab model was chosen in order to represent the titanium (IV) dioxide (110) surface. The influence of changing the number of atomic layers on the bandgap value for the (110) surface was also examined. Density of states referring to the bands from the first valence band up to the bottom of the conduction band was projected on the whole set of atomic orbitals as well as on the significant shells of the titanium and oxygen atoms. Ab initio calculations with a B3LYP functional were carried out. Basis sets used were modified Ti_86-411(d31)G_darco_unpub and O 8_411_muscat_1999. The results are compared with experimental and computational data already available in the literature. Surface termination problem was discussed and the application of the obtained results as a starting point to obtain the first model of the rutile titania nanotube was proposed. The surface formation energies for rutile planes with a different surface terminations were compared and the modification to the equation needed for surface energy calculation was introduced.

Palabras clave

  • rutile
  • band structure
  • B3LYP
  • surface termination
  • surface formation energy
Acceso abierto

Mechanisms of carriers transport in Ni/n-SiC, Ti/n-SiC ohmic contacts

Publicado en línea: 29 Feb 2012
Páginas: 233 - 240

Resumen

Abstract

A mechanism of carriers transport through metal-semiconductor interface created by nickel or titanium-based ohmic contacts on Si-face n-type 4H-SiC is presented herein. The mechanism was observed within the temperature range of 20 °C ÷ 300 °C which are typical for devices operating at high current density and at poor cooling conditions. It was found that carriers transport depends strongly on concentration of dopants in the epitaxial layer. The carriers transport has thermionic emission nature for low dopant concentration of 5×1016 cm−3. The thermionic emission was identified for moderate dopant concentration of 5×1017 cm−3 at temperatures higher than 200 °C. Below 200 °C, the field emission dominates (for the same doping level of 5×1017 cm−3). High dopant concentration of 5×1018 cm−3 leads to almost pure field emission transport within the whole investigated temperature range.

Palabras clave

  • SiC
  • ohmic contact
  • carrier transport
  • silicide
  • carbide

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