Lionel Aranda

Controlling ZIF-8 nano- and microcrystal formation and reactivity through zinc salt variations

Metal organic frameworks (MOFs) are porous crystalline materials composed of metal ions or clusters of metal ions coordinated with organic linkers (such as terephthalic acid, 1,3,5-benzenetricarboxylic acid, or imidazoles). MOFs exhibit tunable structures, low density, ultrahigh surface area and have various potential applications in catalysis, hydrogen storage, and adsorption/separation of liquid or gaseous mixtures. Among the MOFs structures, the zeolitic imidazolate frameworks (ZIFs) have recently attracted considerable attention. In these materials, metal atoms such as Zn2+ are linked through N atoms by the ditopic 2-methylimidazolate ligand to form neutral frameworks. ZIF-8 has a sodalite zeolite-type topology with cages of 11.6 A and pores of 3.4 A in diameter. ZIF-8 are characterized by high thermal stability (550 oC in N2), large surface area (BET: 1630 m2/g) and high resistance to various solvents. Concerning ZIF-8 chemical properties, these materials can successfully be used for hydrogen, carbon dioxide and iodide storage, Knoevenagel condensations, cycloadditions, oxidations, trans-esterification, and Friedel-Crafts alkylations. Variations of synthetic parameters (solvent, concentration, temperature, time, molar ratio of reactants) are commonly used to manipulate the morphology and size of ZIF-8 crystals. In this work, we demonstrate that the reactivity of the Zn(+2) salt in the growth solution can also markedly affect the size and the morphology of ZIF-8 particles. Small ZIF-8 nanocrystals with diameters varying between ca. 50 and 200 nm were obtained with reactive zinc salts like Zn(acac)2, Zn(NO3)2, ZnSO4 or Zn(ClO4)2. The use of ZnCl2, Zn(OAc)2 or ZnI2 afforded crystals with sizes varying between ca. 350 and 650 nm. Finally, the low reactive ZnBr2 was found to generate microsized crystals. These significant changes in particle size induced distinctive changes in adsorption properties as demonstrated by BET measurements but also in the catalytic performances of ZIF-8 crystals in a Knoevenagel condensation used as model. The small sized crystals produced from Zn(NO3)2 exhibit the highest surface area and the best catalytic activity.

Cu2+-doped zeolitic imidazolate frameworks (ZIF-8): efficient and stable catalysts for cycloadditions and condensation reactions

Cu2+-doped zeolitic imidazolate framework (ZIF) crystals were efficiently prepared by reaction of Cu(NO3)2, Zn(NO3)2, and 2-methylimidazole in methanol at room temperature. Scanning electron microscopy, transmission electron microscopy and X-ray diffraction showed that the Cu/ZIF-8 particles were nanosized (between ca. 120 and 170 nm) and that the body-centered cubic crystal lattice of the parent ZIF-8 framework is continuously maintained, regardless of the doping percentage. Moreover, thermogravimetric analyses and specific BET surface area measurements demonstrated that the doping does not alter the high stability of ZIF-8 crystals and that the porosity only decreases at a high doping percentage (25% in Cu2+). The Cu/ZIF-8 material showed excellent catalytic activity in the [3 + 2] cycloaddition of organic azides with alkynes and in Friedlander and Combes condensations due to the high catalyst surface area and the high dispersion of Cu/ZIF-8 particles. Notably, the Cu/ZIF-8 particles not only exhibit excellent performance but also show great stability in the reaction, allowing their reuse up to ten times in condensation reactions. Our findings explored a simple and powerful way to incorporate metal ions into the backbones of open framework materials without losing their properties.

Study of the Behavior in Oxidation at High Temperature of Ni, Co and Fe-Base Alloys Containing Very High Fractions of Carbides

Nine cast alloys reinforced by very high fractions of carbides, Ni-30Cr-xC, Co-30Cr-xC and Fe-30Cr-xC with x varying from 1.2 to 2.0, were tested in oxidation at high temperature between 1,000 and 1,200°C in air for 50 hours. After oxidation, their surfaces and sub-surfaces were characterized. Even for very high carbon contents, the chromia-forming behaviour of the nickel alloys is kept. The oxidation modes of the cobalt alloys and iron alloys are not changed compared to low carbon alloys of these families. The differences of diffusion easiness of chromium in matrix, between nickel alloys, cobalt alloys and iron alloys are the same as for alloys with lower carbon contents, as suggested by the lower chromium gradients in the nickel alloys compared to the two other alloy types. Sub-surface microstructure transformations due to oxidation were observed in some cases (coarsening of carbides due to an inwards diffusion of carbon, change of the sharing between BCC-FCC of iron matrix due to outwards diffusion of chromium). Catastrophic oxidation never occurred for these alloys during the 50 hours of exposition to air at high temperature.

Influence of Tantalum on the Rates of High Temperature Oxidation and Chromia Volatilization for Cast (Fe and/or Ni)-30Cr-0.4C Alloys

Four cast superalloys, Fe-base and (Fe,Ni)-base alloys, all containing 30%Cr and 0.4%C, were elaborated with addition of 3% and 6% of tantalum. Their oxidation behaviours were studied at 1000, 1100 and 1200°C during 50 hours. The oxidized surfaces of the samples were quantitatively characterized by measurements of thicknesses and surface fractions of internal oxides. The thermogravimetry files were treated according to the {m×(dm/dt) = Kp - Kv×m} equation, to obtain simultaneously the parabolic constant and the chromia volatilization constant. The internal tantalum oxides are more present in the Fe-base alloys and the carbide-free zones are less developed for the (Fe,Ni)-base alloys than for the others. The Fe-base alloys oxidize faster than the (Fe,Ni)-base and Ni-base alloys. The comparison with the corresponding Ta-free ternary alloys shows that the presence of Ta tends to accelerate the oxidation.

Thermal Expansion Behaviour of Ternary Nickel-Based, Cobalt-Based, and Iron-Based Alloys Containing Very High Fractions of Carbides

Some of the wear-resistant pieces or coatings, constituted of a metallic matrix and of carbides present in high fractions, are a mix, in similar quantities, of two materials displaying greatly different levels of hardness but also of thermal expansion coefficient. When temperature increases, the second difference of property may lead to particular geometrical behaviours. To study these differences, nine nickel-based, cobalt-based, and iron-based alloys containing very high quantities of carbides were elaborated by foundry. In their as-cast conditions, the microstructures of these alloys were characterized; their hardness and thermal expansion until 1200°C were measured and analysed, with regard to the evolution of the structures predicted by thermodynamic calculations. The hardness of the alloys is high (nickel alloys) or very high (cobalt and iron alloys, 600 Hv and more) while the thermal expansion is greatly influenced by carbides, notably when temperature has become very high. Some of the variations of thickness at the end of heating or during an isothermal stage at 1200°C, essentially contraction, directly result from the mechanical interaction between matrix and carbides which was accumulated during the heating.

Effect of a Preliminary Aging Treatment on the Oxidation Kinetic at High Temperature for a Cobalt–Based Alloy Strengthened by Tantalum Carbides

The purpose of this work is to examine if the behavior of refractory alloys in oxidation at high temperature can be influenced by the modification of the interdendritic carbides morphology due to a preliminary aging treatment at high temperature. A cobalt-based alloy strengthened by tantalum carbides was first exposed to 1200°C over a period of 100 hours, and characterized in high temperature oxidation, with samples cut in the middle of the alloy. Thermogravimetry runs were performed in air at 1000, 1100 and 1200°C during 50 hours, and the results were analyzed in order to specify the rates of transient oxidation, of parabolic oxidation and of chromia volatilization. The oxidized samples were metallographically examined. The fragmentation of the TaC carbides induced by the aging treatment tends to accelerate the oxidation kinetics in general, with higher values of parabolic and volatilization constants especially at 1200°C.

Development of Burner Rig Corrosion Tests for Gas Turbine Alloys: Study of Protective Metallic Coatings and Inhibitors for Hot Salt Corrosion

The efficiency of all types of gas turbine engines (aircraft, terrestrial and marine) is proportional to firing or turbine inlet temperature. In most cases, the use of heavy or contaminated fuels (by Na, S, V) in such combustion equipment is the life-limiting factor for the metallic parts constituting the hot gas path. Since several years, a burner rig was developed in the laboratory to evaluate the corrosiveness of the ash deposit formed on the gas turbine alloys in dynamic conditions. It has been designed to easily adapt the ash deposit rate and the level of the contaminants. The first part of the present paper is devoted to the evaluation of the resistances of several types of metallic coatings against sodium-induced hot corrosion. These coatings were deposited on IN738 alloy and have been treated during 1000h of test. For this, a NaCl solution was injected in the hot gas flux to obtain a deposit rate of 1 mg/cm 2 /100h at 850°C. Different types of aluminide coatings obtained by pack cementation were tested and compared to a NiCrAlY coating produced by LPPS process. Among all these coatings, the Pt-modified alumides show the best anti-corrosion behaviour against the Na-induced hot corrosion. In the second part, the burner rig has been used to test additives introduced into crude oil burned in gas turbines. Indeed, the usual and low-cost defence against (Na,V)-induced hot corrosion is the addition to the fuel of some metals as corrosion inhibitors. To simulate this corrosion and its inhibition in the burner rig, an aqueous solution containing adjusted concentrations of sodium, vanadium with or without inhibitor was injected into the combustion gas just after the burner. A Mg/V ratio equal to 3 was tested to evaluate the efficiencies of the inhibition and the deposition rate in the presence of magnesium.

Kinetic and metallographic study of oxidation at high temperature of cast Ni 25Cr alloy in water vapour rich air

Abstract The high temperature oxidation behaviour of a Ni–25 wt-%Cr alloy in air enriched with water vapour (180 mbars H2O) was studied at 1000, 1100, 1200 and 1300°C. The oxidised samples were characterised by X-ray diffraction, electron microscopy and wavelength dispersion spectroscopy. The obtained data were compared to the ones earlier obtained for the same alloy oxidised in dry air. Water vapour globally induced at all temperatures a decrease of the parabolic constant Kp and an increase in the chromia volatilisation constant Kv. The oxide scales do not present morphologic difference between the two atmospheres. After oxidation in humidified air the scale thickness is thinner and the Cr depleted depth is lower than in dry air.

Experimental and thermodynamic study of nickel (30 wt.%Cr) – based alloys containing between 2.5 and 5.0 wt.% carbon

Abstract Six {Ni-30 wt.% Cr}-based alloys containing from 2.5 to 5 wt.% C, were elaborated by casting and heat treated at 1000, 1100, and 1200°C for 50 h to achieve stabilization. Their melting ranges and the phase surface fractions were established. These results were compared to thermodynamic calculations. All microstructures are hyper-eutectic and carbides are either Cr7C3 or Cr3C2. The carbide fraction increases with the carbon content and graphite appears for carbon contents higher than 4 wt.% C. The experimental solidus and liquidus temperatures are several tens of degrees lower than the calculated ones but their evolution versus the carbon content are well represented by calculations. There is a good agreement between experiments and calculations concerning graphite. The hardness is about 300 HV and decreases when the heat-treatment temperature increases.

Comparaison du joint de colle entre les attaches préencollées APC™ PLUS et les attaches APC™ Flash-Free

INTRODUCTION Does the new adhesive-coated APC™ Flash-Free bracket from the 3M Unitek® group simplify the bonding protocol without compromising precision? OBJECTIVES The aim of this study was to compare the morphology of the adhesive joint between the classic APC™ PLUS adhesive-coated brackets and APC™ Flash-Free brackets. MATERIALS AND METHODS In vitro bonding of esthetic brackets in the CLARITY™ ADVANCED range was performed to compare the morphology of the excess flash between APC™ PLUS and APC™ Flash-Free brackets. RESULTS No statistically significant difference was found concerning the morphology of the excess flash between APC™ PLUS and APC™ Flash-Free brackets. A statistically significant difference was found regarding the thickness of the adhesive between the two types of bracket. The adhesive used for the APC™ Flash-Free brackets was significantly thicker than for the APC™ PLUS brackets (P=0.0001). Adhesive thickness was also more homogeneous on the APC™ Flash-Free brackets (P=0.001 for the relative difference). CONCLUSION The adhesive is thicker but adhesive homogeneity is greater with APC™ Flash-Free brackets than with APC™ PLUS brackets.