Structural intricacies of the orange-red nitride phosphor system La3-xCexSi6N11 (0 < x ≤ 3) have ... more Structural intricacies of the orange-red nitride phosphor system La3-xCexSi6N11 (0 < x ≤ 3) have been elucidated using a combination of state-of-the art tools, in order to understand the origins of the exceptional optical properties of this important solid-state lighting material. In addition, the optical properties of the end-member (x = 3) compound, Ce3Si6N11, are described for the first time. A combination of synchrotron powder X-ray diffraction and neutron scattering is employed to establish site preferences and the rigid nature of the structure, which is characterized by a high Debye temperature. The high Debye temperature is also corroborated from ab initio electronic structure calculations. Solid-state (29)Si nuclear magnetic resonance, including paramagnetic shifts of (29)Si spectra, are employed in conjunction with low-temperature electron spin resonance studies to probes of the local environments of Ce ions. Detailed wavelength-, time-, and temperature-dependent luminescence properties of the solid solution are presented. Temperature-dependent quantum yield measurements demonstrate the remarkable thermal robustness of luminescence of La2.82Ce0.18Si6N11, which shows little sign of thermal quenching, even at temperatures as high as 500 K. This robustness is attributed to the highly rigid lattice. Luminescence decay measurements indicate very short decay times (close to 40 ns). The fast decay is suggested to prevent strong self-quenching of luminescence, allowing even the end-member compound Ce3Si6N11 to display bright luminescence.
The oxide garnet Y3Al5O12 (YAG), when substituted with a few percent of the activator ion Ce3+ to... more The oxide garnet Y3Al5O12 (YAG), when substituted with a few percent of the activator ion Ce3+ to replace Y3+, is a luminescent material that is nearly ideal for phosphor-converted solid-state white lighting. The local environments of the small number of substituted Ce3+ ions are known to critically influence the optical properties of the phosphor. Using a combination of powerful experimental methods, the nature of these local environments is determined and is correlated with the macroscopic luminescent properties of Ce-substituted YAG. The rigidity of the garnet structure is established and is shown to play a key role in the high quantum yield and in the resistance toward thermal quenching of luminescence. Local structural probes reveal compression of the Ce3+ local environments by the rigid YAG structure, which gives rise to the unusually large crystal-field splitting, and hence yellow emission. Effective design rules for finding new phosphor materials inferred from the results establish that efficient phosphors require rigid, highly three-dimensionally connected host structures with simple compositions that manifest a low number of phonon modes, and low activator ion concentrations to avoid quenching.
We present a rapid microwave-assisted approach for the preparation of Eu2+-doped orthosilicate ph... more We present a rapid microwave-assisted approach for the preparation of Eu2+-doped orthosilicate phosphors. The preparation method relies on a citrate based sol–gel reaction with subsequent combustion in a domestic microwave oven, in contrast to more conventional solid-state methods. This sol–gel pathway yields phase pure, high quality orthosilicates, in less than 25 min of final heating time. In addition, superior morphology control is achieved employing the sol–gel method compared to solid-state preparations. In order to understand the formation process of the final products, thermogravimetric analyses and temperature-dependent X-ray diffraction data were acquired and compared to the conventional solid-state preparation. The morphology and elemental composition of the obtained luminescent materials were investigated using scanning electron microscopy and energy-dispersive X-ray spectroscopy. The optical properties were elucidated by measuring room-temperature emission and excitation spectra, and the application and efficiency of the obtained phosphors in LED devices was studied.
We present a rapid microwave-assisted approach for the preparation of Eu2+-doped orthosilicate ph... more We present a rapid microwave-assisted approach for the preparation of Eu2+-doped orthosilicate phosphors. The preparation method relies on a citrate based sol–gel reaction with subsequent combustion in a domestic microwave oven, in contrast to more conventional solid-state methods. This sol–gel pathway yields phase pure, high quality orthosilicates, in less than 25 min of final heating time. In addition, superior morphology control is achieved employing the sol–gel method compared to solid-state preparations. In order to understand the formation process of the final products, thermogravimetric analyses and temperature-dependent X-ray diffraction data were acquired and compared to the conventional solid-state preparation. The morphology and elemental composition of the obtained luminescent materials were investigated using scanning electron microscopy and energy-dispersive X-ray spectroscopy. The optical properties were elucidated by measuring room-temperature emission and excitation spectra, and the application and efficiency of the obtained phosphors in LED devices was studied.
Since the mid-1990s, phosphors have played a key role in emerging solid-state white-lighting tech... more Since the mid-1990s, phosphors have played a key role in emerging solid-state white-lighting technologies that are based on combining a III-nitride-based near-UV or blue solid-state light source with downconversion to longer wavelengths. Almost all widely used phosphors comprise a crystalline oxide, nitride, or oxynitride host that is appropriately doped with either Ce3+ or Eu2+. These ions, with [Xe] 4f(n)5d(0) configurations (n = 1 for Ce3+ and 7 for Eu2+) have proximal excited states that are [Xe] 4f(n-1)5d(1). Optical excitation into these states and concomitant reemission can be tuned into the appropriate regions of the visible spectrum by the crystal these ions are hosted in. In this article, we review the current needs and key aspects of the conversion process. We describe some currently used families of phosphors and consider why they are suitable for solid-state lighting. Finally, we describe some empirical rules for new and improved host materials.
The title compound is prepared by a rapid and energy efficient microwave assisted solid state rea... more The title compound is prepared by a rapid and energy efficient microwave assisted solid state reaction of stoichiometric amounts of CaCO3, MgO, SiO2, and Eu2O3 using small amounts of H3BO3 as flux (domestic microwave oven).
A near-UV excited, oxyfluoride phosphor solid solution Sr1.975Ce0.025Ba(AlO4F)1−x(SiO5)x has been... more A near-UV excited, oxyfluoride phosphor solid solution Sr1.975Ce0.025Ba(AlO4F)1−x(SiO5)x has been developed for solid state white lighting applications. An examination of the host lattice, and the local structure around the Ce3+ activator ions through a combination of density functional theory, synchrotron X-ray and neutron powder diffraction and total scattering, and electron paramagnetic resonance, points to how chemical substitutions play a crucial role in tuning the optical properties of the phosphor. The maximum emission wavelength can be tuned from green (λem = 523 nm) to yellow (λem = 552 nm) by tuning the composition, x. Photoluminescent quantum yield is determined to be 70 ± 5% for some of the examples in the series. Excellent thermal properties were found for the x = 0.5 sample, with the photoluminescence intensity at 160 °C only decreased to 82% of its room temperature value. Phosphor-converted LED devices fabricated using an InGaN LED (λmax = 400 nm) exhibit high color rendering white light with Ra = 70 and a correlated color temperature near 7000 K. The value of Ra could be raised to 90 by the addition of a red component, and the correlated color temperature lowered to near 4000 K.
Ce 3+ -substituted aluminum garnet compounds of yttrium (Y 3 Al 5 O 12) and lutetium (Lu 3 Al 5 O... more Ce 3+ -substituted aluminum garnet compounds of yttrium (Y 3 Al 5 O 12) and lutetium (Lu 3 Al 5 O 12)both important compounds in the generation of (In,Ga)N-based solid state white lightinghave been prepared using a simple microwave heating technique involving the use of a microwave susceptor to provide the initial heat source. Carbon used as the susceptor additionally creates a reducing atmosphere around the sample that helps stabilize the desired luminescent compound. High quality, phase-pure materials are prepared within a fraction of the time and using a fraction of the energy required in a conventional ceramic preparation; the microwave technique allows for a reduction of about 95% in preparation time, making it possible to obtain phase pure, Ce 3+ -substituted garnet compounds in under 20 min of reaction time. It is estimated that the overall reduction in energy compared with ceramic routes as practised in the lab is close to 99%. Conventionally prepared material is compared with material prepared using microwave heating in terms of structure, morphology, and optical properties, including quantum yield and thermal quenching of luminescence. Finally, the microwave-prepared compounds have been incorporated into light-emitting diode "caps" to test their performance characteristics in a real device, in terms of their photon efficiency and color coordinates.
A rapid and energy efficient microwave assisted solid state preparative route for europium-doped ... more A rapid and energy efficient microwave assisted solid state preparative route for europium-doped Akermanite (Ca2MgSi2O7) has been developed. This method reduces the reaction time and energy needed by more than 90%, compared to the preparation carried out in a conventional furnace. The obtained samples are phase pure as has been determined using synchrotron X-ray powder diffraction data and Rietveld analyses. Scanning electron microscopy was employed to investigate the morphology of the microwave prepared compounds whilst energy dispersive X-ray spectroscopy (EDX) was used to verify the elemental composition of the specimens. A systematic investigation of the influence of the utilized microwave setup is presented. Finally, the microwave prepared materials were subject to temperature dependent photoluminescence measurements in order to investigate the thermal quenching of the luminescence.
Interactions of fluoride anions and organocations with crystalline silicate frameworks are shown ... more Interactions of fluoride anions and organocations with crystalline silicate frameworks are shown to depend subtly on the architectures of the organic species, which significantly influence the crystalline structures that result. One- and two-dimensional (2D) (1)H, (19)F, and (29)Si nuclear magnetic resonance (NMR) spectroscopy measurements establish distinct intermolecular interactions among F(-) anions, imidazolium structure-directing agents (SDA(+)), and crystalline silicate frameworks for as-synthesized siliceous zeolites ITW and MTT. Different types and positions of hydrophobic alkyl ligands on the imidazolium SDA(+) species under otherwise identical zeolite synthesis compositions and conditions lead to significantly different interactions between the F(-) and SDA(+) ions and the respective silicate frameworks. For as-synthesized zeolite ITW, F(-) anions are established to reside in the double-four-ring (D4R) cages and interact strongly and selectively with D4R silicate framework sites, as manifested by their strong (19)F-(29)Si dipolar couplings. By comparison, for as-synthesized zeolite MTT, F(-) anions reside within the 10-ring channels and interact relatively weakly with the silicate framework as ion pairs with the SDA(+) ions. Such differences manifest the importance of interactions between the imidazolium and F(-) ions, which account for their structure-directing influences on the topologies of the resulting silicate frameworks. Furthermore, 2D (29)Si{(29)Si} double-quantum NMR measurements establish (29)Si-O-(29)Si site connectivities within the as-synthesized zeolites ITW and MTT that, in conjunction with synchrotron X-ray diffraction analyses, establish insights on complicated order and disorder within their framework structures.
A solid solution strategy helps increase the efficiency of Ce{sup 3+} oxyfluoride phosphors for s... more A solid solution strategy helps increase the efficiency of Ce{sup 3+} oxyfluoride phosphors for solid-state white lighting. The use of a phosphor-capping architecture provides additional light extraction. The accompanying image displays electroluminescence spectra from a 434-nm InGaN LED phosphor that has been capped with the oxyfluoride phosphor.
A significant obstacle in the development of YAG:Ce nanoparticles as light converters in white LE... more A significant obstacle in the development of YAG:Ce nanoparticles as light converters in white LEDs and as biological labels is associated with the difficulty of finding preparative conditions that allow simultaneous control of structure, particle size and size distribution, while maintaining the optical properties of bulk samples. Preparation conditions frequently involve high-temperature treatments of precursors (up to 1400 °C), which result in increased particle size and aggregation, and lead to oxidation of Ce(iii) to Ce(iv). We report here a process that we term protected annealing, that allows the thermal treatment of preformed precursor particles at temperatures up to 1000 °C while preserving their small size and state of dispersion. In a first step, pristine nanoparticles are prepared by a glycothermal reaction, leading to a mixture of YAG and boehmite crystalline phases. The preformed nanoparticles are then dispersed in a porous silica. Annealing of the composite material at 1000 °C is followed by dissolution of the amorphous silica by hydrofluoric acid to recover the annealed particles as a colloidal dispersion. This simple process allows completion of YAG crystallization while preserving their small size. The redox state of Ce ions can be controlled through the annealing atmosphere. The obtained particles of YAG:Ce (60 ± 10 nm in size) can be dispersed as nearly transparent aqueous suspensions, with a luminescence quantum yield of 60%. Transparent YAG:Ce nanoparticle-based films of micron thickness can be deposited on glass substrates using aerosol spraying. Films formed from particles prepared by the protected annealing strategy display significantly improved photostability over particles that have not been subject to such annealing.
Thin film laminates composed of sputtered indium zinc oxide and silver, optimized for conductance... more Thin film laminates composed of sputtered indium zinc oxide and silver, optimized for conductance and transparency, were tested for water vapor permeation as well as mechanical durability in tension. The ~ 82 nm thick optimized indium–zinc-oxide/silver/indium–zinc-oxide (IZO/Ag/IZO) films were > 80% transparent in the visible range (400 nm–700 nm) with measured sheet resistances less than 5 Ω/sq. The water vapor permeation measurements using Ca test methods at several temperature/ humidity conditions indicated that the addition of the thin Ag layer provided little improvement relative to a single indium–zinc-oxide (IZO) layer of similar thickness. However, the critical strain in bending tests for IZO/Ag/IZO films was improved compared to IZO films. The modulus (E ~ 113 GPa), hardness (H ~ 7 GPa), fracture toughness (KIC ~ 1.1 MPa⋅m0.5), and interfacial shear (“adhesion”) (τc ~ 16 MPa) of/related to IZO, and measured by nanoindention are consistent with other brittle ceramic thin film materials
Cyclopentane (CyC5) hydrate interparticle adhesion force measurements in the presence of small am... more Cyclopentane (CyC5) hydrate interparticle adhesion force measurements in the presence of small amounts of crude oil (up to 8 wt % in cyclopentane) were performed at 3.2 °C, under atmospheric pressure, using a micromechanical force apparatus. The adhesion forces obtained for cyclopentane hydrate in small amounts of crude oil in CyC5 bulk fluid were lower than those measured for CyC5 hydrate in pure CyC5 bulk fluid. CyC5 hydrate-normalized adhesive forces were measured to be on the order of 0.5 mN/m for samples containing approximately 5−8 wt % of Caratinga and Troika crude. Hydrate-normalized adhesive forces were found to increase when the surface-active components (including acids and asphaltenes) were removed from the crude oil. These results suggest that crude oils with high contents of acids and asphaltenes may be more likely to exhibit nonplugging tendencies in oil and gas flowlines.
The structural and electronic properties of MnB4 were studied by high-temperature powder X-ray di... more The structural and electronic properties of MnB4 were studied by high-temperature powder X-ray diffraction and measurements of the conductivity and Seebeck coefficient on spark-plasma-sintered samples. A transition from the room-temperature monoclinic structure (space group P21 /c) to a high-temperature orthorhombic structure (space group Pnnm) was observed at about 650 K. The material remained semiconducting after the transition, but its behavior changed from p-type to n-type. (55) Mn NMR measurements revealed an isotropic chemical shift of -1315 ppm, confirming an oxidation state of Mn close to I. Solid solutions of Cr1-x Mnx B4 (two phases in space groups Pnnm and P21 /c) were synthesized for the first time. In addition, nanoindentation studies yielded values of (496±26) and (25.3±1.7) GPa for the Young's modulus and hardness, respectively, compared to values of 530 and 37 GPa obtained by DFT calculations.
We present a rapid microwave-assisted approach for the preparation of Eu 2þ-doped orthosilicate p... more We present a rapid microwave-assisted approach for the preparation of Eu 2þ-doped orthosilicate phosphors. The preparation method relies on a citrate based solegel reaction with subsequent combustion in a domestic microwave oven, in contrast to more conventional solid-state methods. This solegel pathway yields phase pure, high quality orthosilicates, in less than 25 min of final heating time. In addition, superior morphology control is achieved employing the solegel method compared to solid-state preparations. In order to understand the formation process of the final products, thermogravimetric analyses and temperature-dependent X-ray diffraction data were acquired and compared to the conventional solid-state preparation. The morphology and elemental composition of the obtained lumi-nescent materials were investigated using scanning electron microscopy and energy-dispersive X-ray spectroscopy. The optical properties were elucidated by measuring room-temperature emission and excitation spectra, and the application and efficiency of the obtained phosphors in LED devices was studied.
A rapid and energy efficient microwave assisted solid state preparative route for europium-doped ... more A rapid and energy efficient microwave assisted solid state preparative route for europium-doped Åkermanite (Ca 2 MgSi 2 O 7) has been developed. This method reduces the reaction time and energy needed by more than 90%, compared to the preparation carried out in a conventional furnace. The obtained samples are phase pure as has been determined using synchrotron X-ray powder diffraction data and Rietveld analyses. Scanning electron microscopy was employed to investigate the morphology of the microwave prepared compounds whilst energy dispersive X-ray spectroscopy (EDX) was used to verify the elemental composition of the specimens. A systematic investigation of the influence of the utilized microwave setup is presented. Finally, the microwave prepared materials were subject to temperature dependent photoluminescence measurements in order to investigate the thermal quenching of the luminescence.
Structural intricacies of the orange-red nitride phosphor system La3-xCexSi6N11 (0 < x ≤ 3) have ... more Structural intricacies of the orange-red nitride phosphor system La3-xCexSi6N11 (0 < x ≤ 3) have been elucidated using a combination of state-of-the art tools, in order to understand the origins of the exceptional optical properties of this important solid-state lighting material. In addition, the optical properties of the end-member (x = 3) compound, Ce3Si6N11, are described for the first time. A combination of synchrotron powder X-ray diffraction and neutron scattering is employed to establish site preferences and the rigid nature of the structure, which is characterized by a high Debye temperature. The high Debye temperature is also corroborated from ab initio electronic structure calculations. Solid-state (29)Si nuclear magnetic resonance, including paramagnetic shifts of (29)Si spectra, are employed in conjunction with low-temperature electron spin resonance studies to probes of the local environments of Ce ions. Detailed wavelength-, time-, and temperature-dependent luminescence properties of the solid solution are presented. Temperature-dependent quantum yield measurements demonstrate the remarkable thermal robustness of luminescence of La2.82Ce0.18Si6N11, which shows little sign of thermal quenching, even at temperatures as high as 500 K. This robustness is attributed to the highly rigid lattice. Luminescence decay measurements indicate very short decay times (close to 40 ns). The fast decay is suggested to prevent strong self-quenching of luminescence, allowing even the end-member compound Ce3Si6N11 to display bright luminescence.
The oxide garnet Y3Al5O12 (YAG), when substituted with a few percent of the activator ion Ce3+ to... more The oxide garnet Y3Al5O12 (YAG), when substituted with a few percent of the activator ion Ce3+ to replace Y3+, is a luminescent material that is nearly ideal for phosphor-converted solid-state white lighting. The local environments of the small number of substituted Ce3+ ions are known to critically influence the optical properties of the phosphor. Using a combination of powerful experimental methods, the nature of these local environments is determined and is correlated with the macroscopic luminescent properties of Ce-substituted YAG. The rigidity of the garnet structure is established and is shown to play a key role in the high quantum yield and in the resistance toward thermal quenching of luminescence. Local structural probes reveal compression of the Ce3+ local environments by the rigid YAG structure, which gives rise to the unusually large crystal-field splitting, and hence yellow emission. Effective design rules for finding new phosphor materials inferred from the results establish that efficient phosphors require rigid, highly three-dimensionally connected host structures with simple compositions that manifest a low number of phonon modes, and low activator ion concentrations to avoid quenching.
We present a rapid microwave-assisted approach for the preparation of Eu2+-doped orthosilicate ph... more We present a rapid microwave-assisted approach for the preparation of Eu2+-doped orthosilicate phosphors. The preparation method relies on a citrate based sol–gel reaction with subsequent combustion in a domestic microwave oven, in contrast to more conventional solid-state methods. This sol–gel pathway yields phase pure, high quality orthosilicates, in less than 25 min of final heating time. In addition, superior morphology control is achieved employing the sol–gel method compared to solid-state preparations. In order to understand the formation process of the final products, thermogravimetric analyses and temperature-dependent X-ray diffraction data were acquired and compared to the conventional solid-state preparation. The morphology and elemental composition of the obtained luminescent materials were investigated using scanning electron microscopy and energy-dispersive X-ray spectroscopy. The optical properties were elucidated by measuring room-temperature emission and excitation spectra, and the application and efficiency of the obtained phosphors in LED devices was studied.
We present a rapid microwave-assisted approach for the preparation of Eu2+-doped orthosilicate ph... more We present a rapid microwave-assisted approach for the preparation of Eu2+-doped orthosilicate phosphors. The preparation method relies on a citrate based sol–gel reaction with subsequent combustion in a domestic microwave oven, in contrast to more conventional solid-state methods. This sol–gel pathway yields phase pure, high quality orthosilicates, in less than 25 min of final heating time. In addition, superior morphology control is achieved employing the sol–gel method compared to solid-state preparations. In order to understand the formation process of the final products, thermogravimetric analyses and temperature-dependent X-ray diffraction data were acquired and compared to the conventional solid-state preparation. The morphology and elemental composition of the obtained luminescent materials were investigated using scanning electron microscopy and energy-dispersive X-ray spectroscopy. The optical properties were elucidated by measuring room-temperature emission and excitation spectra, and the application and efficiency of the obtained phosphors in LED devices was studied.
Since the mid-1990s, phosphors have played a key role in emerging solid-state white-lighting tech... more Since the mid-1990s, phosphors have played a key role in emerging solid-state white-lighting technologies that are based on combining a III-nitride-based near-UV or blue solid-state light source with downconversion to longer wavelengths. Almost all widely used phosphors comprise a crystalline oxide, nitride, or oxynitride host that is appropriately doped with either Ce3+ or Eu2+. These ions, with [Xe] 4f(n)5d(0) configurations (n = 1 for Ce3+ and 7 for Eu2+) have proximal excited states that are [Xe] 4f(n-1)5d(1). Optical excitation into these states and concomitant reemission can be tuned into the appropriate regions of the visible spectrum by the crystal these ions are hosted in. In this article, we review the current needs and key aspects of the conversion process. We describe some currently used families of phosphors and consider why they are suitable for solid-state lighting. Finally, we describe some empirical rules for new and improved host materials.
The title compound is prepared by a rapid and energy efficient microwave assisted solid state rea... more The title compound is prepared by a rapid and energy efficient microwave assisted solid state reaction of stoichiometric amounts of CaCO3, MgO, SiO2, and Eu2O3 using small amounts of H3BO3 as flux (domestic microwave oven).
A near-UV excited, oxyfluoride phosphor solid solution Sr1.975Ce0.025Ba(AlO4F)1−x(SiO5)x has been... more A near-UV excited, oxyfluoride phosphor solid solution Sr1.975Ce0.025Ba(AlO4F)1−x(SiO5)x has been developed for solid state white lighting applications. An examination of the host lattice, and the local structure around the Ce3+ activator ions through a combination of density functional theory, synchrotron X-ray and neutron powder diffraction and total scattering, and electron paramagnetic resonance, points to how chemical substitutions play a crucial role in tuning the optical properties of the phosphor. The maximum emission wavelength can be tuned from green (λem = 523 nm) to yellow (λem = 552 nm) by tuning the composition, x. Photoluminescent quantum yield is determined to be 70 ± 5% for some of the examples in the series. Excellent thermal properties were found for the x = 0.5 sample, with the photoluminescence intensity at 160 °C only decreased to 82% of its room temperature value. Phosphor-converted LED devices fabricated using an InGaN LED (λmax = 400 nm) exhibit high color rendering white light with Ra = 70 and a correlated color temperature near 7000 K. The value of Ra could be raised to 90 by the addition of a red component, and the correlated color temperature lowered to near 4000 K.
Ce 3+ -substituted aluminum garnet compounds of yttrium (Y 3 Al 5 O 12) and lutetium (Lu 3 Al 5 O... more Ce 3+ -substituted aluminum garnet compounds of yttrium (Y 3 Al 5 O 12) and lutetium (Lu 3 Al 5 O 12)both important compounds in the generation of (In,Ga)N-based solid state white lightinghave been prepared using a simple microwave heating technique involving the use of a microwave susceptor to provide the initial heat source. Carbon used as the susceptor additionally creates a reducing atmosphere around the sample that helps stabilize the desired luminescent compound. High quality, phase-pure materials are prepared within a fraction of the time and using a fraction of the energy required in a conventional ceramic preparation; the microwave technique allows for a reduction of about 95% in preparation time, making it possible to obtain phase pure, Ce 3+ -substituted garnet compounds in under 20 min of reaction time. It is estimated that the overall reduction in energy compared with ceramic routes as practised in the lab is close to 99%. Conventionally prepared material is compared with material prepared using microwave heating in terms of structure, morphology, and optical properties, including quantum yield and thermal quenching of luminescence. Finally, the microwave-prepared compounds have been incorporated into light-emitting diode "caps" to test their performance characteristics in a real device, in terms of their photon efficiency and color coordinates.
A rapid and energy efficient microwave assisted solid state preparative route for europium-doped ... more A rapid and energy efficient microwave assisted solid state preparative route for europium-doped Akermanite (Ca2MgSi2O7) has been developed. This method reduces the reaction time and energy needed by more than 90%, compared to the preparation carried out in a conventional furnace. The obtained samples are phase pure as has been determined using synchrotron X-ray powder diffraction data and Rietveld analyses. Scanning electron microscopy was employed to investigate the morphology of the microwave prepared compounds whilst energy dispersive X-ray spectroscopy (EDX) was used to verify the elemental composition of the specimens. A systematic investigation of the influence of the utilized microwave setup is presented. Finally, the microwave prepared materials were subject to temperature dependent photoluminescence measurements in order to investigate the thermal quenching of the luminescence.
Interactions of fluoride anions and organocations with crystalline silicate frameworks are shown ... more Interactions of fluoride anions and organocations with crystalline silicate frameworks are shown to depend subtly on the architectures of the organic species, which significantly influence the crystalline structures that result. One- and two-dimensional (2D) (1)H, (19)F, and (29)Si nuclear magnetic resonance (NMR) spectroscopy measurements establish distinct intermolecular interactions among F(-) anions, imidazolium structure-directing agents (SDA(+)), and crystalline silicate frameworks for as-synthesized siliceous zeolites ITW and MTT. Different types and positions of hydrophobic alkyl ligands on the imidazolium SDA(+) species under otherwise identical zeolite synthesis compositions and conditions lead to significantly different interactions between the F(-) and SDA(+) ions and the respective silicate frameworks. For as-synthesized zeolite ITW, F(-) anions are established to reside in the double-four-ring (D4R) cages and interact strongly and selectively with D4R silicate framework sites, as manifested by their strong (19)F-(29)Si dipolar couplings. By comparison, for as-synthesized zeolite MTT, F(-) anions reside within the 10-ring channels and interact relatively weakly with the silicate framework as ion pairs with the SDA(+) ions. Such differences manifest the importance of interactions between the imidazolium and F(-) ions, which account for their structure-directing influences on the topologies of the resulting silicate frameworks. Furthermore, 2D (29)Si{(29)Si} double-quantum NMR measurements establish (29)Si-O-(29)Si site connectivities within the as-synthesized zeolites ITW and MTT that, in conjunction with synchrotron X-ray diffraction analyses, establish insights on complicated order and disorder within their framework structures.
A solid solution strategy helps increase the efficiency of Ce{sup 3+} oxyfluoride phosphors for s... more A solid solution strategy helps increase the efficiency of Ce{sup 3+} oxyfluoride phosphors for solid-state white lighting. The use of a phosphor-capping architecture provides additional light extraction. The accompanying image displays electroluminescence spectra from a 434-nm InGaN LED phosphor that has been capped with the oxyfluoride phosphor.
A significant obstacle in the development of YAG:Ce nanoparticles as light converters in white LE... more A significant obstacle in the development of YAG:Ce nanoparticles as light converters in white LEDs and as biological labels is associated with the difficulty of finding preparative conditions that allow simultaneous control of structure, particle size and size distribution, while maintaining the optical properties of bulk samples. Preparation conditions frequently involve high-temperature treatments of precursors (up to 1400 °C), which result in increased particle size and aggregation, and lead to oxidation of Ce(iii) to Ce(iv). We report here a process that we term protected annealing, that allows the thermal treatment of preformed precursor particles at temperatures up to 1000 °C while preserving their small size and state of dispersion. In a first step, pristine nanoparticles are prepared by a glycothermal reaction, leading to a mixture of YAG and boehmite crystalline phases. The preformed nanoparticles are then dispersed in a porous silica. Annealing of the composite material at 1000 °C is followed by dissolution of the amorphous silica by hydrofluoric acid to recover the annealed particles as a colloidal dispersion. This simple process allows completion of YAG crystallization while preserving their small size. The redox state of Ce ions can be controlled through the annealing atmosphere. The obtained particles of YAG:Ce (60 ± 10 nm in size) can be dispersed as nearly transparent aqueous suspensions, with a luminescence quantum yield of 60%. Transparent YAG:Ce nanoparticle-based films of micron thickness can be deposited on glass substrates using aerosol spraying. Films formed from particles prepared by the protected annealing strategy display significantly improved photostability over particles that have not been subject to such annealing.
Thin film laminates composed of sputtered indium zinc oxide and silver, optimized for conductance... more Thin film laminates composed of sputtered indium zinc oxide and silver, optimized for conductance and transparency, were tested for water vapor permeation as well as mechanical durability in tension. The ~ 82 nm thick optimized indium–zinc-oxide/silver/indium–zinc-oxide (IZO/Ag/IZO) films were > 80% transparent in the visible range (400 nm–700 nm) with measured sheet resistances less than 5 Ω/sq. The water vapor permeation measurements using Ca test methods at several temperature/ humidity conditions indicated that the addition of the thin Ag layer provided little improvement relative to a single indium–zinc-oxide (IZO) layer of similar thickness. However, the critical strain in bending tests for IZO/Ag/IZO films was improved compared to IZO films. The modulus (E ~ 113 GPa), hardness (H ~ 7 GPa), fracture toughness (KIC ~ 1.1 MPa⋅m0.5), and interfacial shear (“adhesion”) (τc ~ 16 MPa) of/related to IZO, and measured by nanoindention are consistent with other brittle ceramic thin film materials
Cyclopentane (CyC5) hydrate interparticle adhesion force measurements in the presence of small am... more Cyclopentane (CyC5) hydrate interparticle adhesion force measurements in the presence of small amounts of crude oil (up to 8 wt % in cyclopentane) were performed at 3.2 °C, under atmospheric pressure, using a micromechanical force apparatus. The adhesion forces obtained for cyclopentane hydrate in small amounts of crude oil in CyC5 bulk fluid were lower than those measured for CyC5 hydrate in pure CyC5 bulk fluid. CyC5 hydrate-normalized adhesive forces were measured to be on the order of 0.5 mN/m for samples containing approximately 5−8 wt % of Caratinga and Troika crude. Hydrate-normalized adhesive forces were found to increase when the surface-active components (including acids and asphaltenes) were removed from the crude oil. These results suggest that crude oils with high contents of acids and asphaltenes may be more likely to exhibit nonplugging tendencies in oil and gas flowlines.
The structural and electronic properties of MnB4 were studied by high-temperature powder X-ray di... more The structural and electronic properties of MnB4 were studied by high-temperature powder X-ray diffraction and measurements of the conductivity and Seebeck coefficient on spark-plasma-sintered samples. A transition from the room-temperature monoclinic structure (space group P21 /c) to a high-temperature orthorhombic structure (space group Pnnm) was observed at about 650 K. The material remained semiconducting after the transition, but its behavior changed from p-type to n-type. (55) Mn NMR measurements revealed an isotropic chemical shift of -1315 ppm, confirming an oxidation state of Mn close to I. Solid solutions of Cr1-x Mnx B4 (two phases in space groups Pnnm and P21 /c) were synthesized for the first time. In addition, nanoindentation studies yielded values of (496±26) and (25.3±1.7) GPa for the Young's modulus and hardness, respectively, compared to values of 530 and 37 GPa obtained by DFT calculations.
We present a rapid microwave-assisted approach for the preparation of Eu 2þ-doped orthosilicate p... more We present a rapid microwave-assisted approach for the preparation of Eu 2þ-doped orthosilicate phosphors. The preparation method relies on a citrate based solegel reaction with subsequent combustion in a domestic microwave oven, in contrast to more conventional solid-state methods. This solegel pathway yields phase pure, high quality orthosilicates, in less than 25 min of final heating time. In addition, superior morphology control is achieved employing the solegel method compared to solid-state preparations. In order to understand the formation process of the final products, thermogravimetric analyses and temperature-dependent X-ray diffraction data were acquired and compared to the conventional solid-state preparation. The morphology and elemental composition of the obtained lumi-nescent materials were investigated using scanning electron microscopy and energy-dispersive X-ray spectroscopy. The optical properties were elucidated by measuring room-temperature emission and excitation spectra, and the application and efficiency of the obtained phosphors in LED devices was studied.
A rapid and energy efficient microwave assisted solid state preparative route for europium-doped ... more A rapid and energy efficient microwave assisted solid state preparative route for europium-doped Åkermanite (Ca 2 MgSi 2 O 7) has been developed. This method reduces the reaction time and energy needed by more than 90%, compared to the preparation carried out in a conventional furnace. The obtained samples are phase pure as has been determined using synchrotron X-ray powder diffraction data and Rietveld analyses. Scanning electron microscopy was employed to investigate the morphology of the microwave prepared compounds whilst energy dispersive X-ray spectroscopy (EDX) was used to verify the elemental composition of the specimens. A systematic investigation of the influence of the utilized microwave setup is presented. Finally, the microwave prepared materials were subject to temperature dependent photoluminescence measurements in order to investigate the thermal quenching of the luminescence.
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