Intermediate Al Content Research Articles

Computational simulation study of the influence of faujasite Si/Al ratio on CO2 capture by temperature swing adsorption

Grand Canonical Monte-Carlo simulations are used to assess ten faujasite structures, the well-known family of zeolites with different Al content in post-combustion CO2 capture via Temperature Swing Adsorption (TSA) processes, at 313–473K and 100kPa. Selectivity, regenerability, purity, isosteric heat and working capacity values, for each structure, have been calculated from simulations, providing a rather complete evaluation of adsorbents’ performance. Additionally, for all the structures the temperature dependence of the heat capacity has been modeled to estimate the thermal regeneration energy. Calculated heat capacities range from 0.78–0.86kJ/kgK at 313K to 0.98–1.15kJ/kgK at 473K, values considerably lower than those corresponding to aqueous amine solutions. Comparison of TSA results with previous Vacuum and Pressure Swing Adsorption (VSA and PSA) ones shows that there is no structure that works well for all three processes. Instead, each process reaches optimum conditions for certain range of Al content. Results indicate that high Al content faujasites, 64-to-96-FAU, are the most effective for TSA with working capacities above 1.7mol/kg, doubling PSA/VSA values. Intermediate Al content 48-,64-FAU perform better at VSA conditions and low Al content 12-,24-FAU structures are more suitable for PSA processes. At moderate operative conditions (i.e., regeneration temperature of 413K), TSA shows the highest purities (above 99% for one-stage process), followed by VSA and PSA. Finally, TSA is more effective in cleaning faujasites with 48 or more Al, compared to PSA/VSA, leading to a higher regenerability (energetic cost index range between 2.3 and 2.4GJ/tCO2).

Optimal Faujasite structures for post combustion CO2 capture and separation in different swing adsorption processes

Grand Canonical Monte–Carlo (GCMC) simulations are used in this work, to assess optimum faujasite structures, the well–known family of zeolites, in CO2 capture processes. Pressure Swing Adsorption (PSA) and Vacuum Swing Adsorption (VSA) procedures have been considered to evaluate purity, working capacity and breakthrough time. To this purpose, ten faujasite structures with different Al content were selected, and the best conditions for CO2 capture maximization have been calculated for each structure. Further results show that zeolites having intermediate Al content are the most effective for VSA processes, whereas low Al content faujasites perform better at PSA conditions. Remarkably, present work best results clearly improve Faujasite 13X VSA–PSA performances, so far considered the industrial reference in absence of water. Moreover, combined VPSA processes, in terms of working capacity and adiabatic work required for compression/expansion, have also been studied, showing that VPSA systems are more efficient than pure PSA/VSA, for structures with intermediate Al content. Finally, an improved methodology has been derived, where GCMC mixture isotherms and energetic cost calculations are combined, and a more accurate way of estimating working capacities and breakthrough times is proposed. This new approach allows more realistic evaluations of adsorbents’ performances, than those found in the literature based on pure adsorption data.

A phase-field study of the aluminizing of nickel

A quantitative phase-field approach for multiphase systems that is based upon CALPHAD free energies is used to model the aluminization of nickel wires, wherein vapour-phase alloying is used to deposit Al on the surface of the Ni wire and then the wire is annealed so that to remove all Al gradients and achieve a homogenous Ni-Al alloy. Both processes are modelled and numerical results are compared with experiments. It is found that the kinetics of both processes is controlled by bulk diffusion. During aluminization at 1273 K, formation and growth of intermetallics, Ni2Al3 NiAl and Ni3Al, are strongly dependent on the Al content in the vapour phase. Ni2Al3 growth is very fast compared with NiAl and Ni3Al. It is also found that an intermediate Al content in the vapour phase is preferable for aluminization, since the Ni2Al3 coating thickness is difficult to control. Ni2Al3 is found to disappear in a few minutes during homogenization at 1373 K. Thereafter, the NiAl phase, in which the composition is highly non-uniform after aluminization, continues growing until the supersaturation in this phase vanishes. Then, NiAl coating disappears concomitantly with the growth of Ni3Al, which disappears thereafter. Finally, the Al concentration profile in Ni(Al) homogenizes.

On the Lewis acidity of protonic zeolites

IR spectra of hydroxyl groups, adsorbed CO, pivalonitrile and pyridine on three H-MFI zeolite samples and on two H-Y faujasites are reported and discussed. Samples richer in Al (H-MFI (Si/Al2=30) and H-Y (Si/Al2=5.1)) show the presence of extraframework species and the presence of Lewis acidity together with Brønsted acidity. H-MFI with lowest Al content (Si/Al2=280) does not show any extraframework species (EF) and only presents Brønsted acidity. H-MFI with intermediate Al content (Si/Al2=50) possess very small amount of EF species and of Lewis acidity. H-Y with low Al content (Si/Al2=30) does not show extraframework species but shows the presence of Lewis acidity together with Brønsted acidity. The role of extraframework material as carrier of Lewis acidity is confirmed. It is proposed that Lewis acidity of low Al-content H-Y can arise from framework tetrahedral Al ions, which can enlarge their coordination to five without any previous dehydroxylation. A support for this hypothesis is given by the reversible shift of the LF OH stretching band, whose extent depends on the strength of the basic molecules: this is certainly not due to a direct interaction of the OH groups responsible for the LF band, which are located in cavities (sodalite cavity and hexagonal prisms) where the molecular probes cannot access.

Effect of microstructure and phase constitution on mechanical properties of Ti1−xAlxN coatings

Monolithic TiAlN coatings with varying Al content in the range 0–65 at.% were deposited by cathodic arc evaporation. The variation in mechanical properties was studied by nanoindentation and scratch testing, and correlated with the phase constitution, grain size and residual stress. The hardness was found to be nearly stable up to Al content of 53% followed by a large drop at 65%. Depending on the stoichiometry, phase constitution and microstructure of the Ti1−xAlxN coatings, the mechanical property measurements were observed to reveal distinct trends at particular Al contents-ranging from a large scatter to clustering of data around specific values. Focused Ion Beam milling and Transmission Electron Microscopy studies showed a gradual change in microstructure, from large columnar grains in TiN to finer columns at intermediate Al content and near equiaxed, ultrafine grains with a nanocomposite structure in case of Ti0.35Al0.65N. Scratch studies revealed the deformation modes to vary with Al content, with the ductile failure modes at low Al content giving way to brittle failure at the highest Al content. Toughness studies showed a gradual increase in toughness with Al%, with the maximum seen at 53% and a moderate drop seen at 65%. The toughness shows a close dependence on the mechanical properties, phase constitution and microstructure. The study outlines the role of Al content on the microstructure of PVD TiAlN coatings and highlights the advantage of a cubic, nanocomposite structure for enhancing the toughness of these coatings.

Electronic structures and transformations of intermediate phases and intermetallic compounds of binary Ti–Al alloys

Effects of interstitial impurities on valence electron structures and phase transformation of Ti–Al alloys are analyzed using the average lattice and atom models of the empirical electron theory of solids and molecules (EET). The descendant degree of bond energy, melting point, and liquidus temperatures affected by interstitial impurities are calculated based on the bond energy formula of the EET. It is demonstrated that because of the effects of interstitial impurities, the atom states of the intermediate phases increase, and bond structures of intermetallic compounds become seriously anisotropic, which results in very complex phase transformations in intermediate Al content. It is also shown that the melting point and liquidus temperatures decrease due to interstitial impurities, and the average decreased degree can be estimated very well using the EET.

An empirical theoretical investigation into phase and structural behaviour in Ti–Al alloys

According to the average lattice and atom models of the empirical electron theory of solids and molecules (EET), effects of interstitial impurities on valence electron structures and phase transformation of Ti–Al alloys are analysed. Furthermore, the descendant degree of bond energy, melting point, and liquidus temperatures affected by interstitial impurities are calculated based on the bond energy formula of the EET. Moreover, the main controversial experimental results on the phase transformation in Ti–Al alloys are explained well. It is demonstrated that because of the effects of interstitial impurities, atom states increase, bond structures are seriously anisotropic, which results in very complex phase transformations in intermediate Al content. It is also shown that the melting point and liquidus temperatures decrease due to interstitial impurities, and the average decreased degree can be estimated very well using the EET.

Al concentration dependence of electrical and photoluminescent properties of co-doped ZnO films

This study reports on the electrical and photoluminescent properties of Al–N co-doped ZnO films with different Al contents. The results suggest that N solubility in ZnO is limited even by the co-doping method. Intermediate Al contents were found to be most important for achieving high-hole-concentration p-type ZnO. Carrier mobility was found to decrease with increasing Al content. Low temperature photoluminescence spectra revealed a dominant emission band at 3.13 eV, which was attributed to a DAP transition. The observed broadened and red-shifted DAP band in the high-Al-containing co-doped samples could relate to rather potential fluctuations.

Effect of composition on the microstructure and mechanical properties of Mg–Zn–Al alloys

Magnesium is receiving great attention for transport applications, particularly its cast alloys. This investigation focuses on the as-cast microstructure and mechanical properties of permanent-mould cast Mg–Zn–Al alloys with typical compositions within the high zinc castable domain. Three types of alloys were identified and characterized by Mg 32(Al, Zn) 49, also known as the τ phase; MgZn phase, also known as the ɛ phase; and a ternary icosahedral quasi-crystalline phase, denoted as the Q phase, respectively. A schematic phase diagram is proposed to show the change of microstructral constituents with element content and the Zn/Al ratio. The diagram reveals that the microstructral constituent is dominated by both the content of Zn or Al and the Zn/Al mass ratio; alloys with a high Zn/Al ratio and a low Al content fall into the ɛ-type; alloys with an intermediate Zn/Al ratio and an intermediate Al content favour the τ-type; and those with a low Zn/Al ratio and a high Al are dominated by the icosahedral quasi-crystalline phase. No Mg 17Al 12 (γ) phase was found in those ZA series alloys. The solidification process and its effects on the phase constituents were discussed. Preliminary mechanical property testing showed that all the experimental alloys possess comparable ultimate strength and yield strength with the AZ91 alloy at ambient temperature, but show far superior creep resistance at elevated temperatures. Moreover, while ambient-temperature properties solely depend on the total element contents, the τ- and the Q-type alloys show greater potential than the ɛ-type alloys on the improvement of elevated temperature properties.

The corrosion behavior of Fe-Al alloys in H2/H2S/H2O atmospheres at 700–900°C

The corrosion behavior of five Fe-Al binary alloys containing up to 40 at. % Al was studied over the temperature range of 700–900°C in a H2/H2S/H2O mixture with varying sulfur partial pressures of 10−7−10−5 atm. and oxygen partial pressures of 10−24−10−2° atm. The corrosion kinetics followed the parabolic rate law in all cases, regardless of temperature and alloy composition. The parabolic rate constants decreased with increasing Al content. The scales formed on Fe-5 and −10 at.% Al were duplex, consisting of an outer layer of iron sulfide (FeS or Fe1−xS) and an inner complex scale of FeAl2S4 and FeS. Alloys having intermediate Al contents (Fe-18 and −28 at.% Al) formed scales that consisted of mostly iron sulfide and Al2O3 as well as minor a amount of FeAl2S4. The amount of Al2O3 increased with increasing Al content. The Fe 40 at.% Al formed only Al2O3 at 700°C, while most Al2O3 and some FeS were detected at T≥800°C. The formation of Al2O3 was responsible for the reduction of the corrosion rates.

The high-temperature corrosion behavior of Fe-Mo-Al alloys in H2/H2O/H2S mixed-gas environments

The corrosion behavior of 11 Fe-Mo-Al ternary alloys was studied over the temperature range 700–980°C in H2/H2O/H2S mixed-gas environments. With the exception of Fe-10Mo-7Al, for which breakaway kinetics were observed at higher temperatures, all alloys followed the parabolic rate law, despite two-stage kinetics which were observed in some cases. A kinetics inversion was observed for alloys containing 7 wt.% Al between 700–800°C. The corrosion rates of Fe-20Mo and Fe-30Mo were found to be reduced by five orders of magnitude at all temperatures by the addition of 9.1 or higher wt.% aluminum. The scales formed on low-Al alloys (≤5 wt.% Al) were duplex, consisting of an outer layer of iron sulfide (with some dissolved Al) and a complex inner of Al0.55Mo2S4, FeMo2S4, Fe1.25Mo6S7.7, FeS, and uncorroded FeAl and Fe3Mo2. Platinum markers were always located at the interface between the inner and outer scales for the low-Al alloys, indicating that outer-scale growth was due mainly to outward diffusion of cations (Fe and Al), while the inner scale was formed primarily by the inward flux of sulfur anions. Alloys having intermediate Al contents (7 wt.%) formed scales that consisted of FeS and Al2O3. The amount of Al2O3 increased with increasing reaction temperature. The high-Al-content alloys (9.1 and 10 wt.%) formed only Al2O3 which was responsible for the reduction of the corrosion rates.

Hydration of n-butenes using zeolite catalysts. Influence of the aluminium content on activity

The hydration of n-butene over various protonic zeolites has been investigated in a batch reactor. At 523 K and 65 atm, and with a water/olefin ratio of 4.4, the alcohol yields per gram of catalyst followed the pattern mordenite > ZSM-5 > offretite ≅ omega ≅ Y. In particular, mordenites with intermediate Al contents ( 7 < Si Al < 9 ) enabled 75% of the thermodynamic equilibrium to be reached with 100% selectivity. Overdealumination of mordenite as well as more severe experimental conditions led to the formation of dimer by-products. Zeolites exhibiting a hydrophobic character had a very good activity but hydrophobicity does not appear to play a determining role in the process. It is found that the activities per protonic site are correlated with the aluminium content of the catalyst regardless of its framework structure. It is proposed that, as in aqueous acid media, the hydration activity of zeolites is a direct and single function of their protonic acid strength.

Transport properties of Cr−Al solid solutions

The electrical resistivity between 4·2° and 400°K and the Hall coefficient at 78° and 300°K of the b.c.c. Cr−Al solid solutions indicate that below 12% and above 27% Al these alloys are metallic conductors with predominantly hole carriers. The alloys between 15 and 25% Al are narrow gap semiconductors and the carriers are electrons. Low temperature specific heat results are consistent with these conclusions and they suggest that the semiconducting state results from a gap in the d band, which opens at intermediate Al contents and closes again at Al contents of 27 per cent or more.