Role Of Rhenium Research Articles

What is the role of rhenium in single crystal superalloys?

Rhenium plays a critical role in single-crystal superalloys –its addition to first generation alloys improves creep life by a factor of at least two, with further benefits for fatigue performance. Its use in alloys such as PWA1484, CMSX-4 and Rene N5 is now widespread, and many in this community regard Re as the “magic dust”. In this paper, the latest thinking concerning the origins of the “rhenium-effect” is presented. We start by reviewing the hypothesis that rhenium clusters represent barriers to dislocation motion. Recent atom probe tomography experiments have shown that Re may instead form a solid solution with Ni at low concentrations ( ′ interfaces, which require non-conservative climb and thus an associated vacancy flux.

Role of Rhenium on Solidification, Microstructure, and Mechanical Properties of Standard Alloy 718

The effect of Rhenium (Re) additions to Standard Alloy 718 on solidification, microstructure, and mechanical properties represented by hardness was studied. Re has a very high strain hardening rate, which is about 3.5 times than that of W or Mo. Re retards γ″ and γ′ coarsening, and also forms clusters within the γ matrix. In addition, Re does not from carbides. However, the microstructure of Re-containing Alloy 718 has a higher volume fraction of δ phase than the standard alloy. Electron microprobe analysis and transmission electron microscopy observations were used to identify the precipitation of hard γ″ and γ′ phases as well as δ phase. Re elevates phase transformation temperatures of γ matrix, eutectic γ/NbC, and eutectic γ/Ni2Nb. The volume fractions of γ dendrites, NbC and Ni2Nb phases, are increased with Re content in Alloy 718. Enhancement of hardness was also evident in the vicinity of a Re-cluster.

Role of Rhenium in Single Crystal Ni-Based Superalloys

The effect of rhenium on the microstructure and mechanical properties of single crystal superalloys with a nominal composition of Ni-3Cr-12Co-1Mo-6W-6Al-8Ta-0.1Hf-(0, 2, 4) Re has been studied. With the rise of rhenium content, the size of as-cast  particles becomes smaller. Rhenium addition elevates the incipient melting temperature and slows down the solid solution process. Even after 2000h prolonged aging both at 950°C and 1050°C, no topologically close-packed phase precipitation is found in the three alloys with Re content up to 4%. Re retards the  coarsening and is beneficial to improving the stress rupture life. The deformation mechanisms together with the dislocation configuration have been studied and discussed.

The role of rhenium in the conversion of glycerol to synthesis gas over carbon supported platinum–rhenium catalysts

Reaction kinetics measurements were carried out to study the conversion of aqueous solutions of glycerol (30 and 80 wt%) over carbon-supported Pt and Pt–Re catalysts at temperatures from 483 to 523 K. The results of these studies show that the turnover frequencies for production of H2, CO, CO2, and light alkanes (primarily methane) all increase upon the addition of Re to Pt/C catalysts. The molar ratio of H2/CO increases, while the CO/CO2 ratio decreases with Re addition, indicating increased rate of water–gas shift. For glycerol conversion over a Pt–Re/C catalyst with an atomic Pt:Re ratio of 1:1, increasing pressure or decreasing temperature leads to an increase in the production of alkanes and light oxygenated hydrocarbons (ethanol, methanol, propanediols, and acetone) at the expense of CO and CO2. Temperature-programmed desorption studies and microcalorimetric measurements indicate that addition of Re to Pt modifies the interaction of CO with surface sites. X-ray absorption spectroscopy and transmission electron microscopy studies provide evidence indicating that Pt–Re/C catalysts consist primarily of bimetallic nanoparticles with sizes below 2 nm, and Re inhibits the sintering of these nanoparticles during reaction conditions for glycerol conversion. The results of these reaction kinetic studies and characterization studies indicate that the performance of Pt–Re/C catalysts for glycerol conversion is related to the formation of Pt–Re nanoparticles, for which Re promotes the overall rate of glycerol reforming by reducing the binding energy of CO to Pt, thereby leading to less extensive blocking of surface sites by reaction intermediates and/or products. In addition, the presence of Re facilitates water–gas shift and CO bond cleavage reactions.

The electrical and optical anisotropy of rhenium-dopedWSe2 single crystals

Single crystals of rhenium-dopedWSe2 with large edge plane have been grown by the chemical vapour transport method usingbromine as the transporting agent. From the x-ray diffraction patterns, boththe doped and undoped crystals are found to crystallize in the 2H structure.The role of rhenium in affecting both the electrical and optical properties of theWSe2 crystals is examined.The thicker Re-doped WSe2 samples enable easier exploration of the electrical and optical anisotropiesof the materials both parallel and perpendicular to the crystalc-axis. The samples are n-type in nature from Hall measurements andthe conductivity anisotropy decreases drastically as a result of doping.Photovoltage measurements revealed that both undoped and Re-dopedWSe2 are indirect semiconductors. The indirect band gap of the doped sample showed considerable redshift compared with the undoped sample and also exhibited anisotropy along and perpendicular tothe c-axis. The anisotropy in the band gap is attributed to crystal anisotropy, which was alsostudied via the polarization-dependent electrolyte electroreflectance measurements takenalong the van der Waals plane (; ) and the as-grown edge plane (, ; ). The strong dependence of excitonic transitions A and B on polarization compared withthat of the undoped samples.

Electrical and optical anisotropic properties of rhenium-doped molybdenum disulphide

Single crystals of rhenium (Re)-doped MoS 2 have been grown by chemical vapor transport method using bromine as a transport agent. Single crystalline platelets up to 10×10 mm 2 surface area and 2 mm in thickness are obtained. By analyzing the X-ray diffraction profile, the structure of the single crystals shows rhombohedral symmetry. The Hall measurements indicate that the samples are n-type in nature. The thicker Re-doped MoS 2 samples enable easier exploration of the electrical and optical anisotropies of the materials both parallel and perpendicular to the c axis of the crystals. The anisotropy of electrical conductivity perpendicular to and along the c axis for the Re-doped MoS 2 is about 10 2 and is at least one order smaller than 10 3 for the undoped samples. The optical anisotropic property was studied using electrolyte electroreflectance measurements. The excitonic transitions of A and B features parallel to c axis show different degrees of red shift in comparing with the features perpendicular to c axis and with that of the undoped sample. The role of rhenium in affecting both the electrical and optical properties of the MoS 2 crystals is examined.

Supported Pt and Re–Pt on Alumina Prepared by Sol-Gel Synthesis within SituWater Formation: Role of Rhenium

Sol-gel chemistry offers flexible methods for the preparation of porous metal oxides such as the transition aluminas used as catalyst supports. The physical properties of sol-gel materials depend on the nature of the reactants, the rate of mixing, and the conditions of drying. Sol-gel chemistry has also been investigated for the preparation of supported metal catalysts such as Pt on metal oxides, including Al{sub 2}O{sub 3} and SiO{sub 2}, and Sn-Pt/Al{sub 2}O{sub 3}. In the synthesis of Pt/Al{sub 2}O{sub 3}, the physical properties of the porous material were controlled by the synthesis conditions, but only little control of the Pt dispersion was achieved. The bimetallic catalysts had high surface areas and pore volumes, but a typical Pt dispersion was not high (e.g., a H/Pt ratio of 0.3 was found for a sample containing 1 wt% Pt and 0.9 st% Sn). Because the oxophilic metal Re in Re-Pt/Al{sub 2}O{sub 3} catalysts evidently helps to maintain the dispersion of Pt, the authors investigated the role of Re in influencing the dispersion of Pt in Re-Pt/Al{sub 2}O{sub 3} prepared by a sol-gel synthesis. Alumina-supported samples were prepared in the initial absence of water under conditions allowing uniform reaction of the precursors. The precursorsmore » were Al(O-s-Bu){sub 3}, Pt(acac){sub 2}, and Re{sub 2}(CO){sub 10} in s-butyl alcohol. Acetic acid was included in the reactant mixture because it catalyzes the dehydration of the alcohol, leading to in situ formation of water uniformly in the medium and thereby to control of the rates of the hydrolysis and condensation reactions that lead to formation of a gelatinous precipitate. The gelatinous precipitate was transformed into porous products by calcination in air. 10 refs., 3 figs., 1 tab.« less

Role of rhenium in Pt‐Re‐Al2O3 reforming catalysis—An integrated study

AbstractThe inclusion of Re in Pt‐Re‐Al2O3 reforming catalysts to improve the lifetime is known, but how rhenium functions and how it is disposed in the catalyst remain unclear. These fundamental questions have been attempted in this study by the use of proton‐induced X‐ray emission and Rutherford backscattering spectrometry (together with electron microscopy and chemisorption studies) to characterize a commercial like catalyst for dehydrogenation of methylcyclohexane.

Role of rhenium in bimetallic reforming catalysts

The influence of rhenium on the behavior of a Pt/Al2O3 catalyst was studied in n-hexane reaction by means of dilution of the catalyst bed with Re/Al2O3. Parallel to activity, selectivity and ageing data obtained in slug pulse and continuous flow reactors, the amount of the surface carbon formed during the reaction was determined and its reactivity in hydrogen atmosphere was also studied. It is suggested that in the presence of rhenium due to the increase in the amount of hydrogen available for hydrogen consuming reactions including hydrodepolymerization of the carbonaceous surface overlayer, the routes of ageing processes are changed, resulting in lower carbon content on the catalyst and less pronounced catalyst deactivation.

The role of rhenium and sulfur in platinum-based hydrocarbon-conversion catalysts

Chemisorption, ir, and XPS data indicate that rhenium atoms covered with chemisorbed sulfur atoms divide the metal surface of platinum-containing catalysts into ensembles consisting of a small number of contiguous platinum atoms. Data on the catalytic conversion of n-hexane show that it is the combined action of rhenium and sulfur that converts platinum into a catalyst of very high selectivity for mild dehydrogenation. It thus appears that the beneficial effects on catalyst selectivity and stability which are brought about by adding widely different catalyst modifiers (gold, tin, sulfur, carbon) to platinum catalysts are largely due to one common cause, namely, the division of the platinum surface into very small ensembles of platinum atoms.