scholarly journals Investigation of Thermal Stability and Reactivity of Rh Nanoclusters on an Ultrathin Alumina Film

Catalysts ◽  
2019 ◽  
Vol 9 (11) ◽  
pp. 971 ◽  
Author(s):  
Zhen-He Liao ◽  
Po-Wei Hsu ◽  
Ting-Chieh Hung ◽  
Guan-Jr Liao ◽  
Zhao-Ying Chern ◽  
...  
Keyword(s):  
Morphological Evolution ◽  
Cluster Structure ◽  
Total Surface Area ◽  
Lattice Parameter ◽  
Temperature Tolerance ◽  
High Reactivity ◽  
Alumina Film ◽  
Surface Probe ◽  
Fcc Phase ◽  
Small Clusters

We studied the structural and morphological evolution of Rh clusters on an ordered ultrathin alumina film grown on NiAl(100) in annealing processes, under ultrahigh vacuum conditions and with various surface probe techniques. The Rh clusters, prepared on vapor deposition of Rh onto the alumina film at 300 K, had an fcc phase and grew in the (100) orientation; the annealing altered the cluster structure little—the lattice parameter decreased by a factor <2%—but the cluster morphology significantly. With elevated temperature, small clusters (diameter ≤1.5 nm) decreased little in size; in contrast, large clusters (diameter ≥2.0 nm) varied in a complex manner—their mean diameter decreased to about 1.5 nm on annealing to 450 K, despite their similar height, while it increased to above 2.0 nm at temperature ≥570 K. This atypical decrease in size was governed predominantly by energetics. Such a reduced size enhanced the total surface area as well as the reactivity of the clusters toward methanol decomposition, so increased the production of D2 (H2) and CO from decomposed methanol-d4 (or methanol). The result implies a higher temperature tolerance for Rh clusters on the alumina film and a practical approach to prepare small Rh clusters with high reactivity.

scholarly journals Role of base strength, cluster structure and charge in sulfuric-acid-driven particle formation

2019 ◽  
Vol 19 (15) ◽  
pp. 9753-9768 ◽  
Author(s):  
Nanna Myllys ◽  
Jakub Kubečka ◽  
Vitus Besel ◽  
Dina Alfaouri ◽  
Tinja Olenius ◽  
...  
Keyword(s):  
Sulfuric Acid ◽  
Cluster Structure ◽  
Particle Formation ◽  
Charge Effects ◽  
Low Concentrations ◽  
Base Strength ◽  
Small Clusters ◽  
Ammonia Medium ◽  
Good Agreement

Abstract. In atmospheric sulfuric-acid-driven particle formation, bases are able to stabilize the initial molecular clusters and thus enhance particle formation. The enhancing potential of a stabilizing base is affected by different factors, such as the basicity and abundance. Here we use weak (ammonia), medium strong (dimethylamine) and very strong (guanidine) bases as representative atmospheric base compounds, and we systematically investigate their ability to stabilize sulfuric acid clusters. Using quantum chemistry, we study proton transfer as well as intermolecular interactions and symmetry in clusters, of which the former is directly related to the base strength and the latter to the structural effects. Based on the theoretical cluster stabilities and cluster population kinetics modeling, we provide molecular-level mechanisms of cluster growth and show that in electrically neutral particle formation, guanidine can dominate formation events even at relatively low concentrations. However, when ions are involved, charge effects can also stabilize small clusters for weaker bases. In this case the atmospheric abundance of the bases becomes more important, and thus ammonia is likely to play a key role. The theoretical findings are validated by cluster distribution experiments, as well as comparisons to previously reported particle formation rates, showing a good agreement.

scholarly journals Role of Base Strength, Cluster Structure and Charge in Sulfuric Acid-Driven Particle Formation

2019 ◽  
Author(s):  
Nanna Myllys ◽  
Jakub Kubečka ◽  
Vitus Besel ◽  
Dina Alfaouri ◽  
Tinja Olenius ◽  
...  
Keyword(s):  
Sulfuric Acid ◽  
Cluster Structure ◽  
Particle Formation ◽  
Charge Effects ◽  
Low Concentrations ◽  
Base Strength ◽  
Small Clusters ◽  
Ammonia Medium ◽  
Good Agreement

Abstract. In atmospheric sulfuric acid-driven particle formation, bases are able to stabilize the initial molecular clusters, and thus enhance particle formation. The enhancing potential of a stabilizing base is affected by different factors, such as the basicity and abundance. Here we use weak (ammonia), medium strong (dimethylamine) and very strong (guanidine) bases as representative atmospheric base compounds, and systematically investigate their ability to stabilize sulfuric acid clusters. Using quantum chemistry, we study proton transfer as well as intermolecular interactions and symmetry in clusters, of which the former is directly related to the base strength and the latter to the structural effects. Based on the theoretical cluster stabilities and cluster population kinetics modeling, we provide molecular-level mechanisms of cluster growth and show that in electrically neutral particle formation, guanidine can dominate formation events even at relatively low concentrations. However, when ions are involved, charge effects can stabilize small clusters also for weaker bases. In this case the atmospheric abundance of the bases becomes more important, and thus ammonia is likely to play a key role. The theoretical findings are validated by cluster distribution experiments, as well as comparisons to previously reported particle formation rates, showing a good agreement.

SYNTHESIS AND CHARACTERIZATION OF NANOCRYSTALLINE FeNi AND Ni3Fe ALLOYS

2011 ◽  
Vol 25 (07) ◽  
pp. 1013-1019 ◽  
Author(s):  
S. AZADEHRANJBAR ◽  
F. KARIMZADEH ◽  
M. H. ENAYATI
Keyword(s):  
Ball Mill ◽  
Morphological Evolution ◽  
Lattice Parameter ◽  
Internal Stresses ◽  
Alloy Formation ◽  
X Ray Diffraction ◽  
X Ray ◽  
Powder Particles ◽  
Fe Alloys

Nanocrystalline FeNi and Ni 3 Fe alloys were prepared by mechanical alloying of Fe and Ni elemental powders using a planetary ball mill under protection atmosphere. X-ray diffraction measurements were performed to follow alloy formation process in these alloys. A heat treatment of 1 h at 800°C was carried out to relax the internal stresses of the milled samples. Morphological evolution of powder particles was revealed by scanning electron microscopy. The value of lattice parameter was reached to 0.35762 nm and the hardness was found to be 686 HV at 30 h milled FeNi powder. In the case of Ni 3 Fe the values of 0.3554 nm and 720 HV were obtained for lattice parameter and hardness, respectively.

Magnetic Studies of Ni3Al(Fe) Compounds

1984 ◽  
Vol 39 ◽  
Author(s):  
S. T. Sekula ◽  
H. R. Kerchner ◽  
J. R. Thompson ◽  
Th. Leventouri
Keyword(s):  
Curie Temperature ◽  
Magnetic Measurements ◽  
Lattice Parameter ◽  
Magnetic Studies ◽  
Fe Content ◽  
Magnetization Data ◽  
Ordered Intermetallic ◽  
Material Forming ◽  
Fcc Phase ◽  
Fe Impurities

ABSTRACTSeveral samples of the ordered intermetallic compound with the Ll2 structure and chemical composition NiO.75-xAl0.25-xFe2x (doped with 0.2 at. % B) have been studied over an Fe compositional range 0 ≤ 2x ≤ 0.20. Stoichiometric Ni3Al is known to be a strongly exchange-enhanced-material, forming a giant moment system when Fe impurities are introduced in trace amounts. For the greater Fe concentrations employed in this work, more conventional ferromagnetic systems are obtained. Well below the ferromagnetic Curie temperature Tc the samples exhibit a spontaneous moment in small magnetic fields and at higher fields the isothermal magnetization data show a clear tendency toward saturation. Magnetic investigations using vibrating-sample techniques permitted measurements over the temperature interval from 4.2 K to 800 K in fields up to 5.0 T. The Curie temperature was found to increase from ∼50 K for the Fe-free sample in the series to 410 K for the limiting concentration of 12 at. % Fe in the Ll2 structure. Samples with higher Fe content up to 20 at. % contain a second, disordered fcc phase with Tc ≃ 715 K. Peaks in the temperature dependence of the lattice parameter observed in x-ray studies yield Tc values in reasonable agreement with the magnetic measurements.

Integration of GaAs on Ge/Si towers by MOVPE

2013 ◽  
Vol 1538 ◽  
pp. 283-289
Author(s):  
A. G. Taboada ◽  
T. Kreiliger ◽  
C. V. Falub ◽  
M. Richter ◽  
F. Isa ◽  
...  
Keyword(s):  
Vapor Deposition ◽  
Morphological Evolution ◽  
Chemical Vapor ◽  
Lattice Parameter ◽  
Low Energy ◽  
X Ray Diffraction ◽  
X Ray ◽  
Si Substrates ◽  
Organic Vapor ◽  
Metal Organic

ABSTRACTWe report on the maskless integration of micron-sized GaAs crystals on patterned Si substrates by metal organic vapor phase epitaxy. In order to adapt the mismatch between the lattice parameter and thermal expansion coefficient of GaAs and Si, 2 μm tall Ge crystals were first grown as virtual substrate by low energy plasma enhanced chemical vapor deposition. We investigate the morphological evolution of the GaAs structures grown on top of the Ge crystals at the transition towards full pyramids with energetically stable {111} facets. A substantial release of strain is shown in GaAs crystals with a height of 2 μm and lateral sizes up to 15×15 μm2 by both X-ray diffraction and photoluminescence.

scholarly journals Effect of Mechanical Milling and Cold Pressing on Co Powder

2012 ◽  
Vol 2012 ◽  
pp. 1-7 ◽  
Author(s):  
A. S. Bolokang ◽  
M. J. Phasha ◽  
D. E. Motaung ◽  
S. Bhero
Keyword(s):  
Compaction Pressure ◽  
Milled Powder ◽  
Xrd Analysis ◽  
High Energy ◽  
Lattice Parameter ◽  
Exothermic Peak ◽  
Cold Pressing ◽  
First Order Phase Transition ◽  
Fcc Phase ◽  
First Time

Cold pressing (CP) of the amorphous-like Co powder suppressed most of the XRD peaks, in particular the peak along (100) plane. The DSC curve of unmilled CP Co powder has shown a distinct sharp exothermic peak at 615C°. Upon annealing at 700C°, only the FCC phase with lattice parameter of 3.51 Å was detected by XRD. Our results implied that the exotherm at 615C° corresponds to compaction-pressure-assisted HCP to FCC first-order phase transition. The XRD analysis of 30 h milled powder revealed for the first time the FCC phase with a=3.80 Å. However, due to presence of (100) and (210) peaks, this phase is thought to be FCT with lattice parameters a=b=3.80 and c=3.07 Å. Consequently, the high-energy milling carried out in the current work induced for the first time HCP to FCT transition in Co. Upon CP of milled powder, the lattice parameter a shrunk from 3.80 to 3.75 Å. However, during annealing of the CP milled Co powder at 750C°, the FCT to FCC transition occurred, yielding the FCC phase with a=3.51 Å.

Mechanochemical Carboaluminothermic Reduction of V2O5 to Produce VC-Al2O3 Nanocomposite

2012 ◽  
Vol 59 (2) ◽  
Author(s):  
Mohsen Hossein-Zadeh ◽  
Fatemeh Tebyani ◽  
Javad Yazdani Cherati ◽  
Omid Mirzaee
Keyword(s):  
Crystallite Size ◽  
Morphological Evolution ◽  
Xrd Analysis ◽  
Lattice Parameter ◽  
Milling Process ◽  
Xrd Study ◽  
Heat Treated ◽  
Nanocomposite Powders

The aim of this investigation is to produce VC–Al2O3 nanocomposite by reducing V2O5 with Aluminum and black carbon powders via mechanochemical process. The effect of milling time on this process was investigated. Milling process was done powder mixture at a rotation speed of 250 rpm for different times. Results showed that VCx has been synthesized after 1 hour of milling. The characterization of phase formation, crystallite size, strain percentage and lattice parameter was done by XRD analysis. To study the morphological evolution and determination of particle size of nanocomposite powders, Field Emission Scanning Electron Microscope (FESEM) was used. The crystallite size and lattice strain were determined by Williamson-Hall method. XRD study showed that for 6 h milling, the width of V4C3 peaks increased while the crystallite size of these phases decreased to about 27nm. In order to formVC–Al2O3 nanocomposite, the mixture was heat treated with the aid of microwave oven. The composite revealed good microwave absorption and heated up to 1150°C.

scholarly journals Formation Process of Long-Period Stacking-Ordered Structures in Mg97Zn1Y2 Alloy Comprising HCP and Cubic Phases Fabricated by High-Pressure High-Temperature Annealing

Metals ◽  
2021 ◽  
Vol 11 (7) ◽  
pp. 1031
Author(s):  
Atsuki Yokota ◽  
Masafumi Matsushita ◽  
Naruhito Geshi ◽  
Daiki Yamasaki ◽  
Toru Shinmei ◽  
...  
Keyword(s):  
High Pressure ◽  
High Temperature ◽  
Formation Process ◽  
Ambient Pressure ◽  
Lattice Parameter ◽  
Intermediate Structure ◽  
Long Period ◽  
High Pressure High Temperature ◽  
Lpso Structure ◽  
Fcc Phase

As-cast Mg97Zn1Y2 alloy consists of α-Mg matrix and 18R-type long-period stacking-ordered (LPSO) structures. We observed that the alloy undergoes a phase transformation to D03 superlattices and α-Mg matrix due to high-pressure high-temperature (HPHT) annealing at 3 GPa and above 773 K. Further, the alloy recovered after HPHT annealing, consisting of the α-Mg matrix and D03 superlattices, transformed into 18R-type LPSO structures during further annealing at ambient pressure. An fcc structure with a lattice parameter of 1.42 nm, which was twice that of D03, emerged in both the collapse process of the 18R-type LPSO structure under high-pressure, and the formation process of the 18R-type LPSO structure at ambient pressure. This fcc phase was an intermediate structure between 18R-type LPSO and D03. From the electron diffraction results, it is considered that 18R-type LPSO is continuously present with 2H including stacking faults, which almost corresponded with previous studies.

Nanoscale Phase Separation Induced by Mechanical Alloying in the Iron-Erbium-Nitrogen System

1990 ◽  
Vol 186 ◽  
Author(s):  
Z. Fu ◽  
H. J. Fecht ◽  
W. L. Johnson
Keyword(s):  
Phase Separation ◽  
Ball Milling ◽  
High Energy ◽  
Lattice Parameter ◽  
Milling Process ◽  
Parameter Analysis ◽  
Nitride Phase ◽  
Nanoscale Phase Separation ◽  
Amorphous Structures ◽  
Fcc Phase

AbstractMetastable phases, including nanocrystalline and amorphous structures, can be prepared by high energy cyclic deformation processes. In the present study, we compare the behavior of a stable congruent melting compound (Fe2Er Laves phase) with a mixture of pure elemental Fe and Er powders subjected to high energy ball milling. X-ray diffraction and transmission electron microscopy reveal similar results in both cases. In the early stages, a nanocrystalline fcc phase with lattice parameter a = 0.484 nm and a grain size of 6 nm is formed together with a bcc Fe-rich phase. Extended milling results in a nanoscale phase separation into Fe-rich and Er-rich crystallites with average grain sizes of 1.8-4 nm. Based on a lattice parameter analysis, the fcc phase was initially thought to be a metastable FeEr3 phase. Further studies revealed nitrogen gas in the milling vial had reacted with the powder during ball milling to produce the cubic ErN phase (“NaCl” structure with a lattice parameter of 0.4836 nm). Our experiments demonstrate that the steel vials for ball milling do not remain hermetically sealed during the milling process and a nitride phase can be formed easily if a catalyst for the dissociation of nitrogen molecules (such as Fe) exists in the system.

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