“…The classic Wagner theory and previous studies associated with the high-temperature oxidation behaviour of the NiAl alloys demonstrated that in the initial oxidation stage, external oxygen molecules first physically adsorbed on the alloy surface to form a two-dimensional oxygen molecule layer, and then the oxygen atom and the metal atom in the alloy surface layer changed places with each other to induce the reconstruction of the alloy surface layer and the formation of pseudo-oxides. With the increase of oxidation time, base metal oxides began to nucleate and gradually grew into the oxide film, and subsequent growth mechanism of the oxide film was simultaneous reverse diffusion of Al and O, and the oxidation kinetics of the alloy was mainly controlled by the outward diffusion of Al through the alloy substrate and the external oxide film and inward diffusion of O through the external oxide film [26,38,40,41]. Figure 3 FE-SEM secondary electron fracture images of the NiAl alloys after 50 h cyclic oxidation at 1473 K: (a) RE-free NiAl single-crystal alloy, (b) NiAl-0.05 at.-%Hf-0.05 at.-%Zr single-crystal alloy and (c) RE-free NiAl poly-crystal alloy.…”
Section: Resultsmentioning
confidence: 99%
“…The results showed that minor RE doping produced a significant effect on the increase of alumina film adhesion and the decrease of alumina film growth rate. Many mechanisms have been put forward to clarify this so-called reactive element effect (REE) and a summary of representative theories associated with film adhesion improvement is as follows: (1) inhibiting the interfacial void growth beneath the film through suppressing sulphur segregation at the film/alloy interface and eliminating the 'sulphur effect' [20,21], (2) relieving the film shrinkage through dredging internal stress by altering the microstructure of the film or forming a buffer layer between the film and the substrate and increasing their compatibility [22,23], (3) mechanically pinning the film on the substrate through forming uniformly-distributed 'pegs' comprised of RE-rich oxide core and alumina shell [24,25], and (4) directly enhancing the interfacial adhesion of the film by participating in chemical bonding across the interface [26,27]. For the reduction of film growth rate, the widely accepted mechanism is the 'modification of cation transport process' which was observed by Pint et al through a combined characterisation of O 18 isotopes and secondary ion mass spectroscopy and recognised as dynamic segregation theory [28][29][30][31].…”
The microstructure and cyclic oxidation behaviour of Hf and Zr co-doped B2 NiAl single-crystal alloys at 1473 K were investigated. Hf and Zr co-precipitated into one phase during alloy manufacturing. The RE-free single-crystal alloy obtained a double-layered alumina film comprised of outer equiaxed grains and inner columnar grains and exhibited lower oxidation rate than the corresponding poly-crystal alloy, while the co-doped single-crystal alloy displayed higher oxidation rate than the RE-free single-crystal alloy and no synergistic effect was discovered. The effect of Hf and Zr co-doping on the growth behaviour of the alumina film was discussed. It is proposed that the existence of grain boundaries is an important precondition for activating the reactive element effect. The absence of grain boundaries significantly weakened the positive effect of Hf and Zr codoping and distinctly magnified its negative effect on the cyclic oxidation resistance of the alloy in the meantime.
Highlights. Cyclic oxidation behaviour of Hf and Zr co-doped NiAl single-crystals is compared.. The undoped alloy obtains a double-layered alumina film during oxidation.. The co-doped alloy shows higher alumina film growth rate than the undoped alloy.. The absence of alloy grain boundaries suppresses the reactive element effect.
“…The classic Wagner theory and previous studies associated with the high-temperature oxidation behaviour of the NiAl alloys demonstrated that in the initial oxidation stage, external oxygen molecules first physically adsorbed on the alloy surface to form a two-dimensional oxygen molecule layer, and then the oxygen atom and the metal atom in the alloy surface layer changed places with each other to induce the reconstruction of the alloy surface layer and the formation of pseudo-oxides. With the increase of oxidation time, base metal oxides began to nucleate and gradually grew into the oxide film, and subsequent growth mechanism of the oxide film was simultaneous reverse diffusion of Al and O, and the oxidation kinetics of the alloy was mainly controlled by the outward diffusion of Al through the alloy substrate and the external oxide film and inward diffusion of O through the external oxide film [26,38,40,41]. Figure 3 FE-SEM secondary electron fracture images of the NiAl alloys after 50 h cyclic oxidation at 1473 K: (a) RE-free NiAl single-crystal alloy, (b) NiAl-0.05 at.-%Hf-0.05 at.-%Zr single-crystal alloy and (c) RE-free NiAl poly-crystal alloy.…”
Section: Resultsmentioning
confidence: 99%
“…The results showed that minor RE doping produced a significant effect on the increase of alumina film adhesion and the decrease of alumina film growth rate. Many mechanisms have been put forward to clarify this so-called reactive element effect (REE) and a summary of representative theories associated with film adhesion improvement is as follows: (1) inhibiting the interfacial void growth beneath the film through suppressing sulphur segregation at the film/alloy interface and eliminating the 'sulphur effect' [20,21], (2) relieving the film shrinkage through dredging internal stress by altering the microstructure of the film or forming a buffer layer between the film and the substrate and increasing their compatibility [22,23], (3) mechanically pinning the film on the substrate through forming uniformly-distributed 'pegs' comprised of RE-rich oxide core and alumina shell [24,25], and (4) directly enhancing the interfacial adhesion of the film by participating in chemical bonding across the interface [26,27]. For the reduction of film growth rate, the widely accepted mechanism is the 'modification of cation transport process' which was observed by Pint et al through a combined characterisation of O 18 isotopes and secondary ion mass spectroscopy and recognised as dynamic segregation theory [28][29][30][31].…”
The microstructure and cyclic oxidation behaviour of Hf and Zr co-doped B2 NiAl single-crystal alloys at 1473 K were investigated. Hf and Zr co-precipitated into one phase during alloy manufacturing. The RE-free single-crystal alloy obtained a double-layered alumina film comprised of outer equiaxed grains and inner columnar grains and exhibited lower oxidation rate than the corresponding poly-crystal alloy, while the co-doped single-crystal alloy displayed higher oxidation rate than the RE-free single-crystal alloy and no synergistic effect was discovered. The effect of Hf and Zr co-doping on the growth behaviour of the alumina film was discussed. It is proposed that the existence of grain boundaries is an important precondition for activating the reactive element effect. The absence of grain boundaries significantly weakened the positive effect of Hf and Zr codoping and distinctly magnified its negative effect on the cyclic oxidation resistance of the alloy in the meantime.
Highlights. Cyclic oxidation behaviour of Hf and Zr co-doped NiAl single-crystals is compared.. The undoped alloy obtains a double-layered alumina film during oxidation.. The co-doped alloy shows higher alumina film growth rate than the undoped alloy.. The absence of alloy grain boundaries suppresses the reactive element effect.
“…It has been extensively used for all kinds of phosphor materials via some technologies including sol‐gel, precipitation, atomic layer deposition and so on 13‐15 . And the common coating materials are oxides, such as Al 2 O 3 , SiO 2 , MgO and so on, which might accelerate the oxidation of the activator in the long duration of the run 16‐19 . To strengthen the oxidation resistance, the reductive material, such as carbon, could be a good choice to be the coating material.…”
Section: Introductionmentioning
confidence: 99%
“…[13][14][15] And the common coating materials are oxides, such as Al 2 O 3 , SiO 2 , MgO and so on, which might accelerate the oxidation of the activator in the long duration of the run. [16][17][18][19] To strengthen the oxidation resistance, the reductive material, such as carbon, could be a good choice to be the coating material. Yin et al reported that phosphor materials were coated with a carbon layer via chemical vapor deposition and heat-treated subsequently, which contributes to the better luminescence properties.…”
Nitride phosphors have been widely used in phosphor-converted white-light-emitting diodes (pc-wLEDs) since they exhibit broad-band excitation and emission, tunable spectrum, high luminescence efficiency, outstanding stability, and low pollution. 1,2 Rare-earth ions activated nitride phosphors with characteristic highly dense (Al/Si)N 4 tetrahedron show long wavelength emissions due to high covalence of the host crystal and large crystal field splitting of the Eu 2+ 5d level. 3 A series of Eu 2+-activated nitride red-emitting phosphors, which provide a high color rendering index for wLEDs, 4,5 have been developed recently, such as (Ca,Sr) AlSiN 3 :Eu 2+ , 6 Sr 2 Si 5 N 8 :Eu 2+ , 7 Sr[LiAl 3 N 4 ]:Eu 2+ , 8 et al So far, CaAlSiN 3 :Eu 2+ (CASN:Eu 2+) phosphor has gained much more attention as down-conversion luminescent material due to its high quantum efficiency and chemical stability. However, the wider applications in high power white-lightemitting diodes (HP-wLEDs), whose operated current density becomes more than 10 times with respect to ordinary wLEDs, have raised more demanding requirements for excellent thermal stability of phosphors. 9 Therefore, enhancing thermal stability of CASN:Eu 2+ is an important materials challenge. At present, some methods have been developed to enhance thermal stability of CASN:Eu 2+ phosphor. Li et al synthesized a series of rare-earth doped Ca 1-x Al 1-x Si 1+x N 3-x O x samples with the inspiration of the solid solution strategy, and the
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