Negative Formation Enthalpy Research Articles

The phase transformation and electrochemical properties of TiNi alloys with Cu substitution: Experiments and first-principle calculations

Cu substituted TiNi alloys have been investigated as hydrogen storage material for Ni-MH batteries by experiments and first principle calculations. The amount of Cu (x in TiNi1−xCux) is varied from 0.1 to 0.3. All of samples were prepared by mechanical alloying using a planetary high-energy ball mill and subsequent heat treatment at 750 °C for 0.5 h. The structural transformation was characterized by X-ray diffraction method (XRD) and scanning electron microscopy (SEM). It indicated that mechanical alloyed TiNi1−xCux alloys possessed broad diffraction peaks related to BCC structure. After heat treatment, studied TiNiCu materials consisted of Ti(Ni,Cu) B2 phase as main phase. The cell parameter of B2 phase increased linearly with increasing Cu content. The influence of Cu substitution for Ni in TiNi cubic phases was investigated by first principle calculation. It is found that TiNi0.7Cu0.3 possessed the most negative enthalpy of formation. The stability of TiNi1−xCux phase increased with Cu content. The width of valence bands was enlarged by substituting Cu atom. The discharge capacities of annealed samples were tested by electrochemical measurements at galvanostatic conditions. The results showed that the substitution of Cu for Ni seemed to deteriorate the activation properties of TiNi based alloys. Among all of studied materials, unmodified TiNi showed the highest discharge capacity which is equal to 154 mAh/g. Accompanying the substitution of Cu for Ni, the discharge capacity and cycling abilities of Cu substituted TiNi alloys declined and increased, respectively. All of the substituted samples exhibited at least 97% retaining rate after 20 cycles.

Thermodynamic Stability of Low‐k Amorphous SiOCH Dielectric Films

Si–O–C‐based amorphous or nanostructured materials are now relatively common and of interest for numerous electronic, optical, thermal, mechanical, nuclear, and biomedical applications. Using plasma‐enhanced chemical vapor deposition (PECVD), hydrogen atoms are incorporated into the system to form SiOCH dielectric films with very low dielectric constants (k). While these low‐k dielectrics exhibit chemical stability as deposited, they tend to lose hydrogen and carbon (as labile organic groups) and convert to SiO2 during thermal annealing and other fabrication processes. Therefore, knowledge of their thermodynamic properties is essential for understanding the conditions under which they can be stable. High‐temperature oxidative drop solution calorimetry measurement in molten sodium molybdate solvent at 800°C showed that these materials possess negative formation enthalpies from their crystalline constituents (SiC, SiO2, C, Si) and H2. The formation enthalpies at room temperature become less exothermic with increasing carbon content and more exothermic with increasing hydrogen content. Fourier transform infrared spectroscopy (FTIR) spectroscopy examined the structure from a microscopic perspective. Different from polymer‐derived ceramics with similar composition, these low‐k dielectrics are mainly comprised of Si–O(C)–Si networks, and the primary configuration of carbon is methyl groups. The thermodynamic data, together with the structural analysis suggest that the conversion of sp2 carbon in the matrix to surface organic functional groups by incorporating hydrogen increases thermodynamic stability. However, the energetic stabilization by hydrogen incorporation is not enough to offset the large entropy gain upon hydrogen release, so hydrogen loss during processing at higher temperatures must be managed by kinetic rather than thermodynamic strategies.

The study of electronic, elastic, magnetic and optical response of Zn1-xTixY (Y = S, Se) through mBJ potential

To explore the structural, electronic, magnetic and optical properties of Zn1-xTixY (Y=S, Se) alloys, in the composition range 0 ≤ x ≤ 1, we have applied full-potential linearized augmented plane wave plus local orbital (FP-LAPW + lo) scheme, which is based on the density functional theory (DFT). Structural optimizations have been done in ferromagnetic (FM) and antiferromagnetic (AFM) phase by using Wu–Cohen generalized gradient approximation (WC-GGA), whereas recently developed modified Becke and Johnson (mBJ) potential was employed to study electronic and optical properties. The lower value of ground state energy and negative enthalpy of formation confirm stability in the FM phase. Due to the spin polarization of electrons in the Ti-d orbital, origin of half-metallic ferromagnetism has been expressed by the calculated band structures, electronic density of states (DOS) and magnetic moments. We also have calculated the exchange splitting energy Δx (d), crystal field energy (ΔEcrystal = Et2g−Eeg) and exchange constants (N0α and N0β). The negative value of exchange constant (N0β) and large splitting of 3d-states of Ti show that the down spin potential is more effective than up spin. Finally, the results of optical parameters such as complex dielectric constant ε (ω), refractive index n (ω), normal incident reflectivity R (ω), absorption coefficient ɑ (ω), optical conductivity σ (ω) and optical loss factor L (ω) are discussed in the energy range 0–14 eV. Moreover, we have verified the Penn's model by showing the inverse relation between the static dielectric constant and the optical band gap. The direct relation between static dielectric constant and static refractive index has been observed by increasing the composition of Ti. The calculated parameters provide valuable theoretical information for optical and spintronics device applications.

Multiferroic and optical studies on the effects of Ba2+ ions in BiFeO3 nanoparticles

Single phase Ba2+ ions doped BiFeO3 nanoparticles have been successfully synthesized with varied concentration (0.0 ≤ x ≤ 0.2) using the auto combustion method. The structural properties of Bi1−xBaxFeO3 (x-0.0, 0.1 and 0.2) nanoparticles reveal the formation of highly crystalline rhombohedrally distorted perovskites without undergoing any structural phase transformation analyzed via Rietveld refinement. The Pauling’s equation confirmed that Ba2+ ions doped BiFeO3 ceramics have much more negative enthalpy of formation (ΔHf) than undoped BiFeO3. A reduction of particle size has been observed up to x = 0.1 and then after increases, whereas the band gap shows a reverse trend. Magnetic and ferroelectric transition temperatures are found to be slightly shifted towards the lower temperature up to x = 0.1 and then higher temperature side which could be attributed to the particle size effect. Ba2+ doped BiFeO3 ceramic with concentration x = 0.2 exhibit larger magnetization as compared to others at room temperature, but these magnetization curves were not saturated. Remnant polarization is found to decrease from 7.693 to 5.141 µC/cm2 along with the doping of Ba2+ ions. The on-site d-to-d crystal field excitations of Fe3+ ions associated with 6A1g–4T1g or 6A1g–4T2g electronic energy levels, Fe13d–Fe23d inter-site electron transfer and inter-atomic O 2p–Fe 3d electronic energy levels are observed by the means of absorption spectroscopy.

Quantum Chemical Modeling of Molecule 6-Aminopenicillanic Acid (6-Apa), of Model of 6-Apa-Phosgene and 6-Apa-Acetyl Chloride

For the first time a quantum-chemical method PM3 (Parametric Method 3) in semi empirical approach appreciated geometric and electronic parameters of the new acylated form the molecules 6-APA (6-aminopenicillanic acid), identified patterns of change according to the nature of the acylating agent. In general, the process of acylation of the molecule 6-APA by acylating agents (phosgene and acetyl chloride) leads to much larger changes in new systems of electronic structure than in its spatial structure. In this electronic structure analysis indicates that the acylation of 6-APA of the molecule both acylating agents leads to formation of more stable in the plan hydrolytic cleavage of form of this acid. This fact is a prediction of the characters and is positive for future synthesis of new antibiotics on the basis of 6-APA. Significant absolute value and the sign of the negative enthalpy of formation calculated by PM3, for both systems 6-APA-phosgene 6-APA-acetyl chloride indicate their high thermodynamic stability. The most stable in terms of the thermodynamic system is the 6-APA-acetyl chloride.

Structural stability of characteristic interface for NiTi/Nb Nanowire: First-Principle study

Compared with some other conventional interface models, the interface of NiTi(211)/Nb(220) in NiTiNb metal nanocomposite had been simulated and analyzed carefully. Results show that only several interface models, i.e., NiTi(100)/Nb(100)(NiNb), NiTi(110)/Nb(110) and NiTi(211)/Nb(220), can be formed accordingly with their negative formation enthalpy. Therein the cohesive energy ΔE and Griffith rupture work W of NiTi(211)/Nb(220) interface model are the lowest among them. Density of states shows that there exists only one electronic bonding peak for NiTi(211)/Nb(220) interface model at -2.5 eV. Electron density difference of NiTi(211)/ Nb(220) shows that the Nb-Nb, Nb-Ti and Nb-Ni bonding characters seem like so peaceful as a fabric twisting every atom, which is different from conventional metallic bonding performance. Such appearance can be deduced that the metallic bonding between Nb-Nb, Nb-Ti and Nb-Ni in NiTi(211)/Nb(220) may be affected by its nanostructure called nanometer size effect. Thus, our findings open an avenue for detailed and comprehensive studies of nanocomposite.

P93] Microstructure and properties of Cu-Al-Fe high-temperature shape memory alloys

Fe-Cr system attracts lot of attention in condensed matter physics due to its technological importance and extraordinary physics related to a non-trivial interplay between magnetic and chemical interactions. However, the effect of multicomponent alloying on the properties of Fe-Cr alloys are less studied. We have calculated the mixing enthalpy, magnetic moments, effective chemical, strain-induced and magnetic exchange interactions to investigate the alloying effect of Ni, Mn, Mo on the phase stability of the ferromagnetic bcc Fe−Cr system at zero K. We demonstrate that the alloying reduces the stability of Fe-Cr alloys and expands the region of spinodal decomposition. At the same time, the mixing enthalpy in ternary Fe100-≿-05Cr≿Ni05 alloys indicates a stability of solid solution phase up to 6 at. % Cr. In Fe100-≿-07Cr≿Ni05Mn01Mo01 alloys, we did not find any alloy composition that has negative enthalpy of formation. Analyzing magnetic and electronic properties of the alloys and investigating magnetic, chemical and strain-induced interactions in the studied systems, we provide physically transparent picture of the main factors leading to the destabilization of the Fe-Cr solid solutions by the multicomponent alloying with Ni, Mn, Mo.

Controlling the Activity and Stability of Pt-Based Electrocatalysts By Means of the Lanthanide Contraction

In order to reduce the Pt loading at the cathode of proton exchange membrane fuel cells (PEMFCs) more active and stable catalysts are needed to drive the oxygen reduction reaction. Most research has focussed on achieving this by alloying Pt with Fe, Co, Ni or Cu [1,2]. However, these compounds typically degrade under PEMFC conditions, due to dealloying. Alloys of Pt and lanthanides may be inherently less prone to dealloying under reactions conditions, due to their negative enthalpy of formation [2-4]. Herein we present a systematic study on the trends in activity of seven novel Pt-lanthanide electrodes (Pt5La, Pt5Ce, Pt5Sm, Pt5Gd, Pt5Tb, Pt5Dy and Pt5Tm). The materials are highly active, presenting a 3 to 6-fold activity enhancement over Pt [3-5], amongst the most active polycrystalline Pt-based catalyst ever reported. Moreover, our recent study showed that Pt x Gd is highly active in the nanoparticulate form [6]. On the bulk alloys, a Pt overlayer with a thickness of few Pt layers is formed onto the bulk alloys by acid leaching (Fig. 1A) [3-5]. The ORR activity versus the lattice parameter obtained by X-ray diffraction measurements follows a volcano relationship (Fig. 1B). Furthermore, we explain the trends in stability, and present the lattice parameter as a new descriptor that controls both the activity and stability of these materials. Using the lanthanide contraction we demonstrate that the electrocatalytic performance can be engineered by tuning the Pt-Pt distance.

Half-Metallic Properties in the New Ti2NiB Heusler Alloy

The electronic structure and magnetic properties of the new Ti2NiB Heusler alloy, which has a high-ordered CuHg2Ti structure, were examined using the self-consistent full-potential linearised augmented plane wave method within density functional theory. Analysis of the electronic band structures and state densities of the Ti2NiB full-Heusler compound revealed that whereas the spin-up electrons are metallic, the spin-down bands are characterised by a gap of 0.62 eV, resulting in stable half-metallic ferromagnetism with a magnetic moment of 3 μ B/f.u. The total magnetic moment (M t) and number of valence electrons (Z t) in the Ti2NiB compound were found to obey the Slater-Pauling (SP) rule of M t = Z t − 18. Due to their different surroundings, the magnetic moments of Ti (1) and Ti (2) exhibit distinct differences, with the magnetic contribution of Ti (1) slightly larger than that of Ti (2). A negative formation enthalpy value was also obtained, thus encouraging the experimental manufacture of the alloy. The band gap was elucidated to be mainly determined by the bonding and antibonding states created from the hybridisations of the d states between the Ti (1)-Ti (2) coupling and the Ni atom. Finally, the Ti2NiB compound was found to maintain 100 % polarisation at the Fermi level, with the half-metallic character present for lattice constants ranging from 5.60 to 6.50 A, thus making the alloy favourable for practical applications.

What Is the Optimum Strain for Pt Alloys for Oxygen Electroreduction?

In order to make low-temperature fuel cells commercially viable, it is crucial to develop oxygen reduction catalysts based on more active, stable and abundant materials. A fruitful strategy for enhancing the oxygen reduction reaction (ORR) activity is to alloy Pt with transition metals [1]. However, commercial alloys of Pt and late transition metals such as Ni, Co or Fe are typically unstable under fuel-cell conditions [2]. The very negative enthalpy of formation of alloys of Pt and lanthanides could provide them with greater long term stability than Pt and late transition metals. Herein, we show the trends in activity and stability novel Pt-lanthanide (Pt-Ln) alloys as efficient ORR catalysts. Sputter-cleaned, polycrystalline Pt5Gd shows a 5-fold increase in ORR activity [3], relative to Pt. All the Pt-lanthanide alloys are at least 3 times more active than Pt for the ORR [3-5]. A compressed Pt overlayer is formed onto the bulk alloy. Accordingly, the effect of alloying Pt is to impose strain onto the Pt overlayer [3-5]. It is likely that this strain would be relaxed by defects [6]. The activity of the Pt-based electrocatalysts versus the lattice parameter in the bulk shows a volcano relationship (Fig. 1A). The lattice parameter is presented as a new descriptor that controls both the activity and stability of these materials [5]. The best performance (activity-stability) is achieved by Pt5Gd. Furthermore, mass-selected PtxGd nanoparticles synthesised by the gas aggregation technique present a significant ORR activity enhancement as compared to pure Pt nanoparticles, PtxGd 8 nm showing 3.6 A (mg Pt)-1 mass activity (Fig. 1B) [7], surpassing the highest activity reached with PtxY nanoparticles [8]. The activity of PtxGd nanoparticles also correlates strongly with compressive strain. Our results demonstrate that we can engineer both the activity and stability by tuning the Pt-Pt distance.

Theoretical study of the structural, elastic and thermodynamic properties of chalcopyrite ZnGeP2

The structural, elastic, and thermodynamic properties of ZnGeP2 with chalcopyrite structure are investigated using the pseudo-potentials plane wave method based on the density functional theory with the generalized gradient approximation. The lattice parameters (a, c and u) are directly calculated and agree well with previous experimental and theoretical results. The obtained negative formation enthalpy shows that ZnGeP2 crystal has strong structural stability. We have also calculated the bulk modulus B and the elastic parameters (C11, C12, C13, C33, C44, and C66) which have not been measured yet. The accuracy and reliability of the calculated elastic constants of ZnGeP2 crystal are discussed. In addition, the pressure and temperature dependencies of the lattice parameters, bulk modulus, Debye temperature, Grüneisen parameter, entropy, volume thermal expansion coefficient, and specific heat capacity are obtained in the ranges of 0–20GPa and 0–1200K using the quasi-harmonic Debye model. To our knowledge this is the first quantitative theoretical prediction of the thermodynamic properties for ZnGeP2 compound and still awaits experimental confirmations.

Thermal Properties and Transition Temperatures of AlB<sub>2</sub>-Type Diborides

The thermal properties, cohesion and transition temperatures of 14 compounds in the AlB2-type diborides have been calculated by first-principles. The obtained thermal properties, cohesion and transition temperatures Tc were compared with both available experimental data and other theoretical results. The relationship among enthalpy formation, bulk modulus and melting temperature in these diborides were further analyzed. The results illustrate that ZrB2 is the most stable, and AuB2 has the largest c/a ratio and it is also most unstable to a phase separation. It is observed that the diborides including Mg, Sc, and Hf series with a more negative enthalpy of formation have a larger bulk modulus and higher melting temperature. AuB2 has higher electron-phonon coupling constant and hence possesses a higher Tc.

Formation enthalpies of Al–Fe–Zr–Nd system calculated by using geometric and Miedema's models

Formation enthalpy is important for the phase stability and amorphous forming ability of alloys. The formation enthalpies of Fe17RE2 (RE=Ce, Pr, Nd, Gd and Er) obtained by Miedema's theory are in good agreement with those of the experiments. The dependence of formation enthalpy on concentration of Al for intermetallic (AlxFe1−x)17Nd2 have been calculated by Miedema's theory and the geometric model. The solid solubility of Al in (AlxFe1−x)17Nd2 is coincident with the concentration dependence of formation enthalpy. The mixing enthalpies of liquid alloys and formation enthalpies of alloys for Al–Fe–Zr–Nd system have been predicted. The calculated mixing enthalpy indicates that the adding of Fe or Nd decreases monotonously the magnitude of enthalpy. The formation enthalpies of Al–Fe–Zr–Nd system indicate that the shape of the enthalpy contour map changes when the content of Al is less than 50.0at% and then it remains unchanged except the decrease of magnitude. The formation enthalpy of Al–Fe–Zr–Nd increases with the increase of Fe and/or Nd content. The negative formation enthalpy indicates that Al–Fe–Zr–Nd system has higher amorphous forming ability and wide amorphous forming range. The certain contents of Zr and/or Al are beneficial for the formation of Al–Fe–Zr–Nd intermetallics.

First-principles study of electronic structures and phase stabilities of ternary intermetallic compounds in the Mg-Y-Zn alloys

In the paper, the first-principles pseudopotential plane-wave method based on density functional theory is used to investigate the crystal structures, enthalpies of formation and electronic structures of X-Mg12YZn phase and W-Mg3Y2Zn3 phase in Mg-Y-Zn alloys. The obtained lattice constants of two phases are in good agreement with the available experimental values, which can reasonably reflect the accuracy of theoretical calculation. The calculated enthalpies of formation indicate that the W-Mg3Y2Zn3 and X-Mg12YZn phases have negative enthalpies of formation, which are-0.2787 eV/atom and-0.0268 eV/atom respectively. Both phases can form stable structures relative to single crystals Mg, Y and Zn, and the enthalpy of formation of W-Mg3Y2Zn3 phase is lower than that of X-Mg12YZn phase. The results for density of states show that the bonding of W-Mg3Y2Zn3 phase occurs mainly among the valence electrons of Mg 2p, Zn 3p and Y 4d orbits, the bonding peaks between-2.53 and 0 eV are derived from the hybridization of Mg 2p, Zn 3p and Y 4d orbits, the peaks between 5.07 and 7.51 eV predominantly originate from the hybridization of Mg 2p and Y 4d orbits. However, the bonding of X-Mg12YZn phase is mainly among the valence electrons of Mg 3s, Mg 2p, Zn 3p and Y 4d orbits. The bonding peaks between-2.30 and 0 eV originate mainly from 2p, 3p, and 4d orbit hybridization of Mg, Zn and Y, the peaks between 0 and 2.08 eV originate from the hybridization of Mg 3s, Mg 2p, Zn 3p and Y 4d orbits. At the same time, there is a pseudo-gap near each Fermi level of W-Mg3Y2Zn3 and X-Mg12YZn phases, which implies the presence of covalent bonding in the two phases. In addition, the charge densities respectively on (011) plane of W-Mg3Y2Zn3 phase and (0001) plane of X-Mg12YZn phase are analyzed, and the results indicate that the Zn-Y band exhibits covalent features in W-Mg3Y2Zn3 phase and X-Mg12YZn phase, the covalent bonding of W-Mg3Y2Zn3 phase is stronger than that of X-Mg12YZn phase. Compared with X-Mg12YZn phase, W-Mg3Y2Zn3 phase has a good phase stability attributed to its more bonding electron numbers in a low-energy region of the Fermi level.

Electrodeposition of CoPt Alloys from the Stable Hexachloroplatinate Solution: Electrochemical Studies

A novel plating solution for CoPt alloys electrodeposition containing CoSO4 as a source of Co and H2PtCl6 as a source of Pt, has been developed. The solution was stable for more than one year. It was proposed that the most abundant six-coordinate PtIVL62− electroactive complex in new plating solution is PtCl5(OH)2−. The electrochemical conversion of PtIVL62− → PtIIL42−, occurring through two electron transfer and two ligand loss, was studied by using several electroanalytical techniques. It was concluded that the mechanism of the electron- transfer-PtL-bond-breaking process in the electrochemical reduction of PtIVL62- complex is a function of electrode materials (Pt, Ru). On a Pt electrode the reduction through stepwise pathway occurs, while on Ru the concerted mechanism prevails. The composition of CoPt alloys is strongly dependent on potential and agitation of the solution. Electrodeposition of CoPt alloys is demonstrated at a potential positive to that required to deposit elemental Co, which is attributed to the negative enthalpy of CoPt formation.

Effects of chemical fuel composition on energy generation from thermopower waves

Thermopower waves, which occur during combustion within hybrid structures formed from nanomaterials and chemical fuels, result in a self-propagating thermal reaction and concomitantly generate electrical energy from the acceleration of charge carriers along the nanostructures. The hybrid structures for thermopower waves are composed of two primary components: the core thermoelectric material and the combustible fuel. So far, most studies have focused on investigating various nanomaterials for improving energy generation. Herein, we report that the composition of the chemical fuel used has a significant effect on the power generated by thermopower waves. Hybrid nanostructures consisting of mixtures of picric acid and picramide with sodium azide were synthesized and used to generate thermopower waves. A maximum voltage of ∼2 V and an average peak specific power as high as 15 kW kg−1 were obtained using the picric acid/sodium azide/multiwalled carbon nanotubes (MWCNTs) array composite. The average reaction velocity and the output voltage in the case of the picric acid/sodium azide were 25 cm s−1 and 157 mV, while they were 2 cm s−1 and 3 mV, in the case of the picramide/sodium azide. These marked differences are attributable to the chemical and structural differences of the mixtures. Mixing picric acid and sodium azide in deionized water resulted in the formation of 2,4,6-trinitro sodium phenoxide and hydrogen azide (H-N3), owing to the exchange of H+ and Na+ ions, as well as the formation of fiber-like structures, because of benzene π stacking. The negative enthalpy of formation of the new compounds and the fiber-like structures accelerate the reaction and increase the output voltage. Elucidating the effects of the composition of the chemical fuel used in the hybrid nanostructures will allow for the control of the combustion process and help optimize the energy generated from thermopower waves, furthering the development of thermopower waves as an energy source.

Structure and bonding features of Ae6Mg7H26 (Ae = Sr, Ba) with complex polyhedrons: First-principles calculations

Structure and bonding features of hydride Ae6Mg7H26 (Ae=Sr, Ba) were investigated by first principles calculations. The Ba–H distance in Ba6Mg7H26 is longer relative to the Sr–H in Sr6Mg7H26, while the Ba-containing polyhedron with higher symmetry and high coordination numbers possesses high stability reflected by the more negative formation enthalpy obtained for Ba6Mg7H26. Hydrogen release from Sr6Mg7H26 would be easier. The electronic structures indicate that the Ae6Mg7H26 (Ae=Sr, Ba) exhibits non-metallic properties and the valence band is primarily dominated by H–s electrons. The chemical bonds in Ae6Mg7H26 (Ae=Sr, Ba) exhibit primary covalent feature with definite ionic component, the covalent feature of Ba–H is more obvious than that of Sr–H. These electronic structures reveal the underlying mechanism for structure and bonding features of the hydride Ae6Mg7H26 (Ae=Sr, Ba).

Effects of Micro and Macroalloying on the Formation and Thermal Stability of Nanocrystalline L12-Al3V

Structural modification of nanocrystalline trialuminde Al3V by microalloying with Ti and macroalloying with Zr was studied. For micro and macroalloying, the Al75(VxTi1−x) x = 0-25 and Al5ZrV2 were synthesized by mechanical alloying of elemental blends of the nominal composition. The structural changes of powder particles during mechanical alloying were investigated by x-ray diffraction patterns. Microstructure of powders was characterized by scanning electron microscopy and the hardness of powder particles was determined by Vickers microhardness measurement. It was found that microalloying of trialuminides by Ti led to the formation of Al3(V,Ti) with stable tetragonal structure (DO22) in different amounts of Ti after 40 h milling. Thermodynamic calculations, which were based on Miedema model, showed that the higher negative formation enthalpy of intermetallics in Al-V-Ti system induced refinement of trialuminides. In addition, it was found that the hardness of compounds was increased by enhancement of Ti in trialuminide composition. For Al5V2Zr compound, the metastable L12 phase was formed after 20 h milling. Thermal analysis of the synthesized compound showed an exothermic peak around 550 °C, which was related to the partial transition of L12 structure to DO22. Moreover, ordering of L12 phase took place due to increasing temperature.

Thermodynamic stability of oxide phases of Fe–Cr based ODS steels via quantum mechanical calculations

Using density functional theory-based calculated formation and reaction enthalpies, we have examined the relative stability of a large number of likely oxide phases of typical ODS steels based on the Fe–Cr–Al–Ti–Y–O system. Calculated formation and reaction enthalpies are in good agreement with available measured values. The relatively more negative formation enthalpies of Y2O3, Y2TiO5, Y4Al2O9, Y2Ti2O7 and YAlO3 oxides show them to be more stable corroborating with their observation in the microstructure of ODS steels. Analysis of reaction enthalpies, guided by convex-hull consideration, gives a mechanism by which reaction of Y2O3 with Al2O3 for the formation of YAlO3, Y4Al2O9, and Y3Al5O12 and reaction of Y2O3 with TiO2 for the formation of Y2TiO5 and Y2Ti2O7 are favored over other likely oxides, such as, those based on Y–Fe–O, Y–Cr–O and Fe–Cr–O.

Van der Waals density functional study of the energetics of alkali metal intercalation in graphite

We report on the energetics of intercalation of lithium, sodium and potassium in graphite by density functional theory using recently developed van der Waals (vdW) density functionals. First stage intercalation compounds are well described by conventional functionals like GGA, but van der Waals functionals are crucial for higher stage intercalation compounds and graphite, where van der Waals interactions are important. The vdW-optPBE functional gave the best agreement with reported structure and energetics for graphite and LiC6 and was further applied for intercalation of Na and K. The enthalpy of formation of LiC6 and KC8 were found to be −16.4 and −27.5 kJ mol−1 respectively. NaC6 and NaC8 were unstable with positive enthalpies of formation (+20.8 and +19.9 kJ mol−1). The energetics of stacking of graphene and intercalant layers was investigated from first to fifth stage intercalation compounds. Higher stage compounds of Li and K were stable, but with less negative enthalpy of formation with increasing stage order. The higher stage Na compounds possessed positive enthalpy of formation, but lower in magnitude than the energy difference of 0.6 kJ mol−1 between graphite with AB and AA stacking. The abnormal behaviour of the lower stage Na intercalation compounds was rationalized by the lower energy involved in the formation of the chemical bond between carbon Na relative to the corresponding bond with Li or K. The chemical bond between alkali metal and carbon is characterized by charge transfer from the alkali-metal to carbon resulting in ionized alkali-metals. The intercalation induces only a subtle increase in the in-plane C–C bond lengths, with longer C–C bonds in the vicinity of the alkali metals but without breaking the hexagonal symmetry.