Metals

2653 KiB

Open AccessReview

Catalytic Reactions over Halide Cluster Complexes of Group 5–7 Metals

by Sayoko Nagashima, Satoshi Kamiguchi and Teiji Chihara

Metals 2014, 4(2), 235-313; https://doi.org/10.3390/met4020235 - 23 Jun 2014

Cited by 18 | Viewed by 8139

Halide clusters of Group 5–7 metals develop catalytic activity above 150–250 °C, and the activity is retained up to 350–450 °C by taking advantage of their thermal stability, low vapor pressure, and high melting point. Two types of active site function: the solid [...] Read more.

Halide clusters of Group 5–7 metals develop catalytic activity above 150–250 °C, and the activity is retained up to 350–450 °C by taking advantage of their thermal stability, low vapor pressure, and high melting point. Two types of active site function: the solid Brønsted acid site and a coordinatively unsaturated site that catalyzes like the platinum metals do. Various types of catalytic reactions including new reactions and concerted catalyses have been observed over the clusters: hydrogenation, dehydrogenation, hydrogenolysis, isomerization of alkene and alkyne, and alkylation of toluene, amine, phenol, and thiol. Ring-closure reactions to afford quinoline, benzofuran, indene, and heterocyclic common rings are also catalyzed. Beckmann rearrangement, S-acylation of thiol, and dehydrohalogenation are also catalyzed. Although the majority of the reactions proceed over conventional catalysts, closer inspection shows some conspicuous features, particularly in terms of selectivity. Halide cluster catalysts are characterized by some aspects: cluster counter anion is too large to abstract counter cation from the protonated reactants, cluster catalyst is not poisoned by halogen and sulfur atoms. Among others, cluster catalysts are stable at high temperatures up to 350–450 °C. At high temperatures, apparent activation energy decreases, and hence weak acid can be a catalyst without decomposing reactants. Full article

(This article belongs to the Special Issue Metal and Molecular Clusters)

► Show Figures

Graphical abstract

2331 KiB

Open AccessReview

Non-Equilibrium Solidification of Undercooled Metallic Melts

by Dieter M. Herlach

Metals 2014, 4(2), 196-234; https://doi.org/10.3390/met4020196 - 20 Jun 2014

Cited by 67 | Viewed by 15301

Abstract

If a liquid is undercooled below its equilibrium melting temperature an excess Gibbs free energy is created. This gives access to solidification of metastable solids under non-equilibrium conditions. In the present work, techniques of containerless processing are applied. Electromagnetic and electrostatic levitation enable [...] Read more.

If a liquid is undercooled below its equilibrium melting temperature an excess Gibbs free energy is created. This gives access to solidification of metastable solids under non-equilibrium conditions. In the present work, techniques of containerless processing are applied. Electromagnetic and electrostatic levitation enable to freely suspend a liquid drop of a few millimeters in diameter. Heterogeneous nucleation on container walls is completely avoided leading to large undercoolings. The freely suspended drop is accessible for direct observation of rapid solidification under conditions far away from equilibrium by applying proper diagnostic means. Nucleation of metastable crystalline phases is monitored by X-ray diffraction using synchrotron radiation during non-equilibrium solidification. While nucleation preselects the crystallographic phase, subsequent crystal growth controls the microstructure evolution. Metastable microstructures are obtained from deeply undercooled melts as supersaturated solid solutions, disordered superlattice structures of intermetallics. Nucleation and crystal growth take place by heat and mass transport. Comparative experiments in reduced gravity allow for investigations on how forced convection can be used to alter the transport processes and design materials by using undercooling and convection as process parameters. Full article

(This article belongs to the Special Issue Advances in Solidification Processing)

► Show Figures

Figure 1

3840 KiB

Open AccessArticle

Phase Equilibria of the Ce-Mg-Zn Ternary System at 300 °C

by Ahmad Mostafa and Mamoun Medraj

Metals 2014, 4(2), 168-195; https://doi.org/10.3390/met4020168 - 28 May 2014

Cited by 8 | Viewed by 8980

Abstract

The isothermal section of the Ce-Mg-Zn system at 300 °C was experimentally established in the full composition range via diffusion multiple/couples and key alloys. Annealed key alloys were used to confirm the phase equilibria obtained by diffusion multiple/couples and to determine the solid [...] Read more.

The isothermal section of the Ce-Mg-Zn system at 300 °C was experimentally established in the full composition range via diffusion multiple/couples and key alloys. Annealed key alloys were used to confirm the phase equilibria obtained by diffusion multiple/couples and to determine the solid solubility ranges. Spot analysis was carried out, using wavelength dispersive X-ray spectroscopy (WDS), to identify the composition of the observed phases. The composition profiles were obtained using WDS line-scans across the diffusion zones. X-ray diffraction (XRD) was performed to identify the phases in the annealed alloys and to confirm the WDS results. Eight ternary compounds, in the Ce-Mg-Zn isothermal section at 300 °C, were observed from 45–80 at.% Zn. These are: τ₁ (Ce₆Mg₃Zn₁₉), τ₂ (CeMg₂₉Zn₂₅), τ₃ (Ce₂Mg₃Zn₃), τ₄ (CeMg₃Zn₅), τ₅ (CeMg₇Zn₁₂), τ₆ (CeMg_2.3_−xZn_12.8+x; 0 ≤ x ≤ 1.1), τ₇ (CeMgZn₄) and τ₈ (Ce(Mg₁_−yZn_y)₁₁; 0.096 ≤ y ≤ 0.43). The ternary solubility of Zn in the Ce-Mg compounds was found to increase with a decrease in Mg concentration. Accordingly, the ternary solid solubility of Zn in CeMg₁₂ and CeMg₃ was measured as 5.6 and 28.4 at.% Zn, respectively. Furthermore, the CeMg and CeZn showed a complete solid solubility. The complete solubility was confirmed by a diffusion couple made from alloys containing CeMg and CeZn compounds. Full article

► Show Figures

Figure 1

291 KiB

Open AccessArticle

The Origins of Spontaneous Grain Refinement in Deeply Undercooled Metallic Melts

by Andrew M. Mullis

Metals 2014, 4(2), 155-167; https://doi.org/10.3390/met4020155 - 22 May 2014

Cited by 8 | Viewed by 5657

Abstract

Phase-field modeling of rapid alloy solidification, in which the rejection of latent heat from the growing solid cannot be ignored, has lagged significantly behind the modeling of conventional casting practices which can be approximated as isothermal. This is in large part due to [...] Read more.

Phase-field modeling of rapid alloy solidification, in which the rejection of latent heat from the growing solid cannot be ignored, has lagged significantly behind the modeling of conventional casting practices which can be approximated as isothermal. This is in large part due to the fact that if realistic materials properties are adopted, the ratio of the thermal to solute diffusivity (the Lewis number) is typically 10³–10⁴, leading to severe multi-scale problems. However, use of state-of-the-art numerical techniques, such as local mesh adaptivity, implicit time-stepping and a non-linear multi-grid solver, allow these difficulties to be overcome. Here we describe how the application of such a model, formulated in the thin-interface limit, can help to explain the long-standing phenomenon of spontaneous grain refinement in deeply undercooled melts. We find that at intermediate undercoolings the operating point parameter, σ*, may collapse to zero, resulting in the growth of non-dendritic morphologies such as doublons and ‘dendritic seaweed’. Further increases in undercooling then lead to the re-establishment of stable dendritic growth. We postulate that remelting of such seaweed structures gives rise to the low undercooling instance of grain refinement observed in alloys. Full article

(This article belongs to the Special Issue Advances in Solidification Processing)

► Show Figures

Figure 1

793 KiB

Open AccessArticle

Optimization of Squeeze Casting Parameters for 2017 A Wrought Al Alloy Using Taguchi Method

by Najib Souissi, Slim Souissi, Christophe Le Niniven, Mohamed Ben Amar, Chedly Bradai and Foued Elhalouani

Metals 2014, 4(2), 141-154; https://doi.org/10.3390/met4020141 - 25 Apr 2014

Cited by 29 | Viewed by 10710

Abstract

This study applies the Taguchi method to investigate the relationship between the ultimate tensile strength, hardness and process variables in a squeeze casting 2017 A wrought aluminium alloy. The effects of various casting parameters including squeeze pressure, melt temperature and die temperature were [...] Read more.

This study applies the Taguchi method to investigate the relationship between the ultimate tensile strength, hardness and process variables in a squeeze casting 2017 A wrought aluminium alloy. The effects of various casting parameters including squeeze pressure, melt temperature and die temperature were studied. Therefore, the objectives of the Taguchi method for the squeeze casting process are to establish the optimal combination of process parameters and to reduce the variation in quality between only a few experiments. The experimental results show that the squeeze pressure significantly affects the microstructure and the mechanical properties of 2017 A Al alloy. Full article

(This article belongs to the Special Issue Advances in Solidification Processing)

► Show Figures

Graphical abstract

846 KiB

Open AccessArticle

Phase Transitions in Mechanically Milled Mn-Al-C Permanent Magnets

by Michael J. Lucis, Timothy E. Prost, Xiujuan Jiang, Meiyu Wang and Jeffrey E. Shield

Metals 2014, 4(2), 130-140; https://doi.org/10.3390/met4020130 - 17 Apr 2014

Cited by 29 | Viewed by 7413

Abstract

Mn-Al powders were prepared by rapid solidification followed by high-energy mechanical milling. The rapid solidification resulted in single-phase ε. The milling was performed in both the ε phase and the τ phase, with the τ-phase formation accomplished through a heat treatment at 500 [...] Read more.

Mn-Al powders were prepared by rapid solidification followed by high-energy mechanical milling. The rapid solidification resulted in single-phase ε. The milling was performed in both the ε phase and the τ phase, with the τ-phase formation accomplished through a heat treatment at 500 °C for 10 min. For the ε-milled samples, the conversion of the ε to the τ phase was accomplished after milling via the same heat treatment. Mechanical milling induced a significant increase in coercivity in both cases, reaching 4.5 kOe and 4.1 kOe, respectively, followed by a decrease upon further milling. The increase in coercivity was the result of grain refinement induced by the high-energy mechanical milling. Additionally, in both cases a loss in magnetization was observed. Milling in the ε phase showed a smaller decrease in the magnetization due to a higher content of the τ phase. The loss in magnetization was attributed to a stress-induced transition to the equilibrium phases, as no site disorder or oxidation was observed. Surfactant-assisted milling in oleic acid also improved coercivity, but in this case values reached >4 kOe and remained stable at least through 32 h of milling. Full article

(This article belongs to the Special Issue Manganese-based Permanent Magnets)

► Show Figures

Figure 1

430 KiB

Open AccessArticle

A Novel Research on Behavior of Zinc Ferrite Nanoparticles in Different Concentration of Poly(vinyl pyrrolidone) (PVP)

by Halimah Mohamed Kamari, Mahmoud Goodarz Naseri and Elias B. Saion

Metals 2014, 4(2), 118-129; https://doi.org/10.3390/met4020118 - 4 Apr 2014

Cited by 23 | Viewed by 8597

Abstract

Zinc ferrite nanocrystals were prepared from an aqueous solution containing metal nitrates and various of concentrations of poly(vinyl pyrrolidone) (PVP), i.e., 0, 15, 40, and 55 g/L, as a capping agent. To stabilize the particles, they were thermally treated at 873 K, [...] Read more.

Zinc ferrite nanocrystals were prepared from an aqueous solution containing metal nitrates and various of concentrations of poly(vinyl pyrrolidone) (PVP), i.e., 0, 15, 40, and 55 g/L, as a capping agent. To stabilize the particles, they were thermally treated at 873 K, as an optimum calcination temperature. The behaviors of the polymeric precursor were analyzed by use of simultaneous thermo-gravimetry (TG) and derivative thermo-gravimetry analyses (DTG). The presence of the crystalline phase in each sample was confirmed by X-ray diffraction (XRD) analysis. The average particle size and the morphology of the nanoparticles were determined by transmission electron microscopy (TEM), and these parameters were found to differ at various concentrations of PVP. Fourier transform infrared spectroscopy (FT-IR) confirmed the presence of metal oxide bands for all the PVP concentrations and confirmed the absence of organic bands for PVP concentrations less than 55 g/L. Measurements of the magnetization value of the zinc ferrite nanoparticles were obtained at room temperature by using a vibrating sample magnetometer (VSM), which showed that, in the absence of PVP, the sample exhibited a paramagnetic behavior while, in the presence of PVP, samples have a super-paramagnetic behavior. Full article

► Show Figures

Figure 1

1167 KiB

Open AccessArticle

Surface Growth and Diffusion Energetics of Ag Monolayers on Cu (001)

by Georgios S.E. Antipas

Metals 2014, 4(2), 108-117; https://doi.org/10.3390/met4020108 - 4 Apr 2014

Cited by 1 | Viewed by 5251

Abstract

The growth of Ag monolayers on Cu (001) was studied by periodic Density Functional Theory (DFT). Despite the limited solid solubility of Ag in Cu, the growth of a single Ag overlayer on Cu (001) was predicted as feasible. In contrast, the growth [...] Read more.

The growth of Ag monolayers on Cu (001) was studied by periodic Density Functional Theory (DFT). Despite the limited solid solubility of Ag in Cu, the growth of a single Ag overlayer on Cu (001) was predicted as feasible. In contrast, the growth of consecutive Ag monolayers was found to be energetically forbidden. Inter-diffusion of Ag monolayers into Cu was raised as a possibility but it was dependent on the sequence in which the Ag monolayers were introduced into the Cu bulk. The Ag layers preferred to be kept neither too far apart nor too close to each other, the optimum spacing between two Ag monolayers determined to be that of two consecutive Cu layers. Ag diffusion mediated tensile stress in the Cu cell by causing an increase of the unit cell constant by as much as 22%. Interactions between the Ag and Cu species also involved a degree of covalency. In general, progression of a surface Ag monolayer into the Cu bulk involved charge depletion over the Ag species and a simultaneous charge concentration over neighboring Cu atoms; this mechanism was found to influence Cu up to a depth of four surface layers. Full article

► Show Figures

Figure 1

1621 KiB

Open AccessReview

Characterization of Catalytically Active Octahedral Metal Halide Cluster Complexes

by Satoshi Kamiguchi, Sayoko Nagashima and Teiji Chihara

Metals 2014, 4(2), 84-107; https://doi.org/10.3390/met4020084 - 1 Apr 2014

Cited by 20 | Viewed by 11325

Abstract

Halide clusters have not been used as catalysts. Hexanuclear molecular halide clusters of niobium, tantalum, molybdenum, and tungsten possessing an octahedral metal framework are chosen as catalyst precursors. The prepared clusters have no metal–metal multiple bonds or coordinatively unsaturated sites and therefore required [...] Read more.

Halide clusters have not been used as catalysts. Hexanuclear molecular halide clusters of niobium, tantalum, molybdenum, and tungsten possessing an octahedral metal framework are chosen as catalyst precursors. The prepared clusters have no metal–metal multiple bonds or coordinatively unsaturated sites and therefore required activation. In a hydrogen or helium stream, the clusters are treated at increasingly higher temperatures. Above 150–250 °C, catalytically active sites develop, and the cluster framework is retained up to 350–450 °C. One of the active sites is a Brønsted acid resulting from a hydroxo ligand that is produced by the elimination of hydrogen halide from the halogen and aqua ligands. The other active site is a coordinatively unsaturated metal, which can be isoelectronic with the platinum group metals by taking two or more electrons from the halogen ligands. In the case of the rhenium chloride cluster Re₃Cl₉, the cluster framework is stable at least up to 300 °C under inert atmosphere; however, it is reduced to metallic rhenium at 250–300 °C under hydrogen. The activated clusters are characterized by X-ray diffraction analyses, Raman spectrometry, extended X-ray absorption fine structure analysis, thermogravimetry–differential thermal analysis, infrared spectrometry, acid titration with Hammett indicators, and elemental analyses. Full article

(This article belongs to the Special Issue Metal and Molecular Clusters)

► Show Figures

Graphical abstract

Journal Menu

Journal Browser

Metals, Volume 4, Issue 2 (June 2014) – 9 articles , Pages 84-313

Further Information

Guidelines

MDPI Initiatives

Follow MDPI