George Fern - Academia.edu (original) (raw)
Papers by George Fern
Proceedings of the International Display Workshops
Catalysts, 2020
The present work involves the development of a novel method for the fabrication of zirconium nick... more The present work involves the development of a novel method for the fabrication of zirconium nickel (Zr(x)Ni(y)) alloy used as a nanocatalyst to improve the hydrogen storage properties of the Mg/MgH2 system. The catalyst was fabricated through the high-pressure reactor and activated under hydrogen prior to being mechanically milled with the MgH2 for 5 h under argon. The microstructure characterisation of the samples was determined via SEM-EDX (scanning electron microscope analysis–energy dispersive X-ray spectroscopy), XRD (X-ray diffraction) and FE-HRTEM (field emission high resolution transmission electron microscopy), and the desorption characteristic of the nanocomposite (10 wt.% Zr(x)Ni(y)–MgH2) was determined via TPD (temperature-programmed desorption). The nanostructured MgH2 powder milled with 10 wt.% of the activated Zr(x)Ni(y) based nanocatalyst resulted in a faster hydrogen release—5.9 H2-wt.% at onset temperature 210 °C/peak temperature 232 °C. The observed significant i...
SID Symposium Digest of Technical Papers, 2002
Phosphor particles of regular shape and size have been synthesised by a new method. Depending on ... more Phosphor particles of regular shape and size have been synthesised by a new method. Depending on conditions the shape of the particles can be controlled and varied. The particles orient when deposited on screens and display greater luminance at low voltages than commercial and spherical phosphor samples.
ECS Journal of Solid State Science and Technology
Y 3 Al 5 O 12 (YAG) and Lu 3 Al 5 O 12 (LuAG) doped with rare earth elements are widely used as s... more Y 3 Al 5 O 12 (YAG) and Lu 3 Al 5 O 12 (LuAG) doped with rare earth elements are widely used as scintillators, solid state lasers, and phosphors in white light emitting diodes (LEDs). Additionally, (Gd,La)B 3 O 6 :Bi has found application as a UV-B emitting phosphor in fluorsecent lamps a long time ago. This paper reports both the photoluminescence (PL) and cathodoluminescence (CL) of Pr 3+ or Gd 3+ substituted YAG and LuAG in comparison to the widely applied UV-B phosphor (Gd,La)B 3 O 6 :Bi under vacuum ultra violet (VUV) and cathode ray excitation, respectively. The powder diffraction pattern, emission, excitation, and reflection spectra and a comparison of the luminescence efficiency are shown and discussed. It turned out that all investigated phosphors show efficient luminescence in the UV-B spectral range. Moreover, it is found that LuAG:Gd has the highest efficiency upon VUV and upon CL excitation too.
SID Symposium Digest of Technical Papers
Spherical, sub-micrometer Y2O3: Eu3+ phosphor particles have been synthesized by a homogenous sol... more Spherical, sub-micrometer Y2O3: Eu3+ phosphor particles have been synthesized by a homogenous solution method. These cubic Y2O3: Eu spherical phosphor particles (ca. 300 nm) will be ideal for FED devices as they can densely close-pack in very small pixels on screens (as shown herein.). The pixels are approximately hexagonal in shape where the length of the hexagonal side is about 5μm. The particles manifest high luminous efficiency (better than a commercial sample) under low voltage electron beam excitation. It is also shown herein that these particles are useful in voltage ranges up to 5000 V and perform well at such voltages.
SID Symposium Digest of Technical Papers
ABSTRACT Phosphors having sub-micrometer primary particle sizes and narrow size distributions wer... more ABSTRACT Phosphors having sub-micrometer primary particle sizes and narrow size distributions were synthesized. They were then incorporated into screen printable, aqueous ink formulations. Aggregation and settlement problems in the inks were addressed and the rheological properties of the inks were adjusted in order to facilitate the printing of phosphors in high resolution features. The relative cathodoluminescence of adjacent pixels, all containing the same red emitting cubic-Y2O3: Eu3+ phosphor powder have been compared to assess the repeatability of the screen-printing of the inks.
ECS Journal of Solid State Science and Technology
Journal of Physics: Conference Series
Cathodoluminescence (CL) spectra have been collected from single nanometer-sized crystals of Y 1.... more Cathodoluminescence (CL) spectra have been collected from single nanometer-sized crystals of Y 1.98 Tb 0.02 O 2 S and Gd 1.98 Tb 0.02 O 2 S using a Gatan Vulcan cathodoluminescence imaging spectrometer. Slight variations observed in the CL spectra taken from the crystals are explained, and discussed in relation to bulk samples.
Journal of Materials Science: Materials in Electronics
AC powder electroluminescent lamps have been known and used for many years, but their mechanism o... more AC powder electroluminescent lamps have been known and used for many years, but their mechanism of operation is still debated. Many thousands of phosphors are known, but the vast majority are not electroluminescent. A number of materials do exhibit the effect. Of these, however, ZnS doped with Cu is absolutely in a class of its own, and is the only material from which viable lamps can be made. In this work studies have been made of the performance of devices under a range of pulsed and continuous excitation conditions and new hypotheses presented which attempt to explain the behavior of this unique material.
Materials
Herein, we describe three advanced techniques for cathodoluminescence (CL) spectroscopy that have... more Herein, we describe three advanced techniques for cathodoluminescence (CL) spectroscopy that have recently been developed in our laboratories. The first is a new method to accurately determine the CL-efficiency of thin layers of phosphor powders. When a wide band phosphor with a band gap (E g > 5 eV) is bombarded with electrons, charging of the phosphor particles will occur, which eventually leads to erroneous results in the determination of the luminous efficacy. To overcome this problem of charging, a comparison method has been developed, which enables accurate measurement of the current density of the electron beam. The study of CL from phosphor specimens in a scanning electron microscope (SEM) is the second subject to be treated. A detailed description of a measuring method to determine the overall decay time of single phosphor crystals in a SEM without beam blanking is presented. The third technique is based on the unique combination of microscopy and spectrometry in the transmission electron microscope (TEM) of Brunel University London (UK). This combination enables the recording of CL-spectra of nanometre-sized specimens and determining spatial variations in CL emission across individual particles by superimposing the scanning TEM and CL-images.
SAE Technical Paper Series, 2017
ECS Journal of Solid State Science and Technology, 2016
In this article we describe the redshift of the charge transfer band of nanosized cubic (Y 1−x Eu... more In this article we describe the redshift of the charge transfer band of nanosized cubic (Y 1−x Eu x) 2 O 3 upon increasing the Eu 3+ concentration. This redshift amounts to 0.43 eV (25 nm) in going from 0.1 Mol % Eu 3+ to 100 Mol % (which is pure Eu 2 O 3). The charge transfer band consists of two broad sub-bands; both shift almost parallel with the Eu 3+ concentration and are related to the two symmetry sites for the cation, C 2 and C 3i , in the bixbyite-type lattice. The area ratio of the bands is 3:1 and is the first direct evidence for the population of the two lattice sites by the Eu 3+ cations being in accord with the crystal structure ratio. A model is presented that quantitatively describes the redshift of the charge transfer band of (Y 1−x Eu x) 2 O 3. This model is based on the Madelung energy of the transferred charge. Other models are briefly discussed, but are discarded.
ECS Journal of Solid State Science and Technology, 2015
Herein the photoluminescence spectra of nanosized cubic Y 2 O 3 :Tb 3+ having Tb 3+ concentration... more Herein the photoluminescence spectra of nanosized cubic Y 2 O 3 :Tb 3+ having Tb 3+ concentrations varying between 0.1 and 10 Mol% are described. Low temperature cathodoluminescence spectra from these materials recorded in a scanning transmission electron microscope are presented and discussed. By studying the photoluminescence-spectra recorded at room temperature and focused on the 5 D 4 → 7 F 5 (C 2) and 5 D 4 → 7 F 5 (C 3i) transitions, at 542.8 and 544.4 nm respectively, it was found that the critical distance for energy transfer from Tb 3+ ions at C 3i lattice sites to Tb 3+ ions at C 2 lattice sites was 1.7 nm; at distances >1.7 nm, which prevail at low Tb 3+ concentration, this energy transfer virtually stops. The gradual change of the excitation spectra upon increasing the Tb 3+ concentration is also explained in terms of energy transfer from Tb 3+ at C 3i sites to Tb 3+ at C 2 sites. Cathodoluminescence spectra recorded at low temperatures with the scanning transmission electron microscope provided additional evidence for this radiationless energy transfer.
SID Symposium Digest of Technical Papers, 2015
In this paper, we describe measurements of the cathodoluminescence of nanosized cubic Y2O3:Tb3+ w... more In this paper, we describe measurements of the cathodoluminescence of nanosized cubic Y2O3:Tb3+ with a transmission electron microscope. From the CL-spectra we determined the energy flow from excited Tb3+ in a S6 lattice site to Tb3+ in a C2 site.
SID Symposium Digest of Technical Papers, 2005
An oxide/oxysulfide based phosphor triad with particle sizes of around 1micrometer (green Y2SiO5:... more An oxide/oxysulfide based phosphor triad with particle sizes of around 1micrometer (green Y2SiO5:Tb or Y2O2S:Tb, and blue Y2SiO5:Ce) down to 250nm for the red Y2O3: Eu is reported. These phosphors perform well in brightness under the conditions necessary for use in FED's and are of suitable size for use in high definition screens. We also report a green emitting Y2O2S:Tb phosphor that has been prepared by a new method involving homogeneous precipitation of the precursors and a new firing regime. The resulting cathodoluminescent phosphor having submicrometer size particles has a luminous efficiency of 34.6 lm/W at an electron beam accelerating voltage of 5000 V.
Proceedings of the International Display Workshops
Catalysts, 2020
The present work involves the development of a novel method for the fabrication of zirconium nick... more The present work involves the development of a novel method for the fabrication of zirconium nickel (Zr(x)Ni(y)) alloy used as a nanocatalyst to improve the hydrogen storage properties of the Mg/MgH2 system. The catalyst was fabricated through the high-pressure reactor and activated under hydrogen prior to being mechanically milled with the MgH2 for 5 h under argon. The microstructure characterisation of the samples was determined via SEM-EDX (scanning electron microscope analysis–energy dispersive X-ray spectroscopy), XRD (X-ray diffraction) and FE-HRTEM (field emission high resolution transmission electron microscopy), and the desorption characteristic of the nanocomposite (10 wt.% Zr(x)Ni(y)–MgH2) was determined via TPD (temperature-programmed desorption). The nanostructured MgH2 powder milled with 10 wt.% of the activated Zr(x)Ni(y) based nanocatalyst resulted in a faster hydrogen release—5.9 H2-wt.% at onset temperature 210 °C/peak temperature 232 °C. The observed significant i...
SID Symposium Digest of Technical Papers, 2002
Phosphor particles of regular shape and size have been synthesised by a new method. Depending on ... more Phosphor particles of regular shape and size have been synthesised by a new method. Depending on conditions the shape of the particles can be controlled and varied. The particles orient when deposited on screens and display greater luminance at low voltages than commercial and spherical phosphor samples.
ECS Journal of Solid State Science and Technology
Y 3 Al 5 O 12 (YAG) and Lu 3 Al 5 O 12 (LuAG) doped with rare earth elements are widely used as s... more Y 3 Al 5 O 12 (YAG) and Lu 3 Al 5 O 12 (LuAG) doped with rare earth elements are widely used as scintillators, solid state lasers, and phosphors in white light emitting diodes (LEDs). Additionally, (Gd,La)B 3 O 6 :Bi has found application as a UV-B emitting phosphor in fluorsecent lamps a long time ago. This paper reports both the photoluminescence (PL) and cathodoluminescence (CL) of Pr 3+ or Gd 3+ substituted YAG and LuAG in comparison to the widely applied UV-B phosphor (Gd,La)B 3 O 6 :Bi under vacuum ultra violet (VUV) and cathode ray excitation, respectively. The powder diffraction pattern, emission, excitation, and reflection spectra and a comparison of the luminescence efficiency are shown and discussed. It turned out that all investigated phosphors show efficient luminescence in the UV-B spectral range. Moreover, it is found that LuAG:Gd has the highest efficiency upon VUV and upon CL excitation too.
SID Symposium Digest of Technical Papers
Spherical, sub-micrometer Y2O3: Eu3+ phosphor particles have been synthesized by a homogenous sol... more Spherical, sub-micrometer Y2O3: Eu3+ phosphor particles have been synthesized by a homogenous solution method. These cubic Y2O3: Eu spherical phosphor particles (ca. 300 nm) will be ideal for FED devices as they can densely close-pack in very small pixels on screens (as shown herein.). The pixels are approximately hexagonal in shape where the length of the hexagonal side is about 5μm. The particles manifest high luminous efficiency (better than a commercial sample) under low voltage electron beam excitation. It is also shown herein that these particles are useful in voltage ranges up to 5000 V and perform well at such voltages.
SID Symposium Digest of Technical Papers
ABSTRACT Phosphors having sub-micrometer primary particle sizes and narrow size distributions wer... more ABSTRACT Phosphors having sub-micrometer primary particle sizes and narrow size distributions were synthesized. They were then incorporated into screen printable, aqueous ink formulations. Aggregation and settlement problems in the inks were addressed and the rheological properties of the inks were adjusted in order to facilitate the printing of phosphors in high resolution features. The relative cathodoluminescence of adjacent pixels, all containing the same red emitting cubic-Y2O3: Eu3+ phosphor powder have been compared to assess the repeatability of the screen-printing of the inks.
ECS Journal of Solid State Science and Technology
Journal of Physics: Conference Series
Cathodoluminescence (CL) spectra have been collected from single nanometer-sized crystals of Y 1.... more Cathodoluminescence (CL) spectra have been collected from single nanometer-sized crystals of Y 1.98 Tb 0.02 O 2 S and Gd 1.98 Tb 0.02 O 2 S using a Gatan Vulcan cathodoluminescence imaging spectrometer. Slight variations observed in the CL spectra taken from the crystals are explained, and discussed in relation to bulk samples.
Journal of Materials Science: Materials in Electronics
AC powder electroluminescent lamps have been known and used for many years, but their mechanism o... more AC powder electroluminescent lamps have been known and used for many years, but their mechanism of operation is still debated. Many thousands of phosphors are known, but the vast majority are not electroluminescent. A number of materials do exhibit the effect. Of these, however, ZnS doped with Cu is absolutely in a class of its own, and is the only material from which viable lamps can be made. In this work studies have been made of the performance of devices under a range of pulsed and continuous excitation conditions and new hypotheses presented which attempt to explain the behavior of this unique material.
Materials
Herein, we describe three advanced techniques for cathodoluminescence (CL) spectroscopy that have... more Herein, we describe three advanced techniques for cathodoluminescence (CL) spectroscopy that have recently been developed in our laboratories. The first is a new method to accurately determine the CL-efficiency of thin layers of phosphor powders. When a wide band phosphor with a band gap (E g > 5 eV) is bombarded with electrons, charging of the phosphor particles will occur, which eventually leads to erroneous results in the determination of the luminous efficacy. To overcome this problem of charging, a comparison method has been developed, which enables accurate measurement of the current density of the electron beam. The study of CL from phosphor specimens in a scanning electron microscope (SEM) is the second subject to be treated. A detailed description of a measuring method to determine the overall decay time of single phosphor crystals in a SEM without beam blanking is presented. The third technique is based on the unique combination of microscopy and spectrometry in the transmission electron microscope (TEM) of Brunel University London (UK). This combination enables the recording of CL-spectra of nanometre-sized specimens and determining spatial variations in CL emission across individual particles by superimposing the scanning TEM and CL-images.
SAE Technical Paper Series, 2017
ECS Journal of Solid State Science and Technology, 2016
In this article we describe the redshift of the charge transfer band of nanosized cubic (Y 1−x Eu... more In this article we describe the redshift of the charge transfer band of nanosized cubic (Y 1−x Eu x) 2 O 3 upon increasing the Eu 3+ concentration. This redshift amounts to 0.43 eV (25 nm) in going from 0.1 Mol % Eu 3+ to 100 Mol % (which is pure Eu 2 O 3). The charge transfer band consists of two broad sub-bands; both shift almost parallel with the Eu 3+ concentration and are related to the two symmetry sites for the cation, C 2 and C 3i , in the bixbyite-type lattice. The area ratio of the bands is 3:1 and is the first direct evidence for the population of the two lattice sites by the Eu 3+ cations being in accord with the crystal structure ratio. A model is presented that quantitatively describes the redshift of the charge transfer band of (Y 1−x Eu x) 2 O 3. This model is based on the Madelung energy of the transferred charge. Other models are briefly discussed, but are discarded.
ECS Journal of Solid State Science and Technology, 2015
Herein the photoluminescence spectra of nanosized cubic Y 2 O 3 :Tb 3+ having Tb 3+ concentration... more Herein the photoluminescence spectra of nanosized cubic Y 2 O 3 :Tb 3+ having Tb 3+ concentrations varying between 0.1 and 10 Mol% are described. Low temperature cathodoluminescence spectra from these materials recorded in a scanning transmission electron microscope are presented and discussed. By studying the photoluminescence-spectra recorded at room temperature and focused on the 5 D 4 → 7 F 5 (C 2) and 5 D 4 → 7 F 5 (C 3i) transitions, at 542.8 and 544.4 nm respectively, it was found that the critical distance for energy transfer from Tb 3+ ions at C 3i lattice sites to Tb 3+ ions at C 2 lattice sites was 1.7 nm; at distances >1.7 nm, which prevail at low Tb 3+ concentration, this energy transfer virtually stops. The gradual change of the excitation spectra upon increasing the Tb 3+ concentration is also explained in terms of energy transfer from Tb 3+ at C 3i sites to Tb 3+ at C 2 sites. Cathodoluminescence spectra recorded at low temperatures with the scanning transmission electron microscope provided additional evidence for this radiationless energy transfer.
SID Symposium Digest of Technical Papers, 2015
In this paper, we describe measurements of the cathodoluminescence of nanosized cubic Y2O3:Tb3+ w... more In this paper, we describe measurements of the cathodoluminescence of nanosized cubic Y2O3:Tb3+ with a transmission electron microscope. From the CL-spectra we determined the energy flow from excited Tb3+ in a S6 lattice site to Tb3+ in a C2 site.
SID Symposium Digest of Technical Papers, 2005
An oxide/oxysulfide based phosphor triad with particle sizes of around 1micrometer (green Y2SiO5:... more An oxide/oxysulfide based phosphor triad with particle sizes of around 1micrometer (green Y2SiO5:Tb or Y2O2S:Tb, and blue Y2SiO5:Ce) down to 250nm for the red Y2O3: Eu is reported. These phosphors perform well in brightness under the conditions necessary for use in FED's and are of suitable size for use in high definition screens. We also report a green emitting Y2O2S:Tb phosphor that has been prepared by a new method involving homogeneous precipitation of the precursors and a new firing regime. The resulting cathodoluminescent phosphor having submicrometer size particles has a luminous efficiency of 34.6 lm/W at an electron beam accelerating voltage of 5000 V.