Al Vacancies Research Articles

Effects of vacancy defects on vibrational properties of Ni–Al

The phonon spectra of Ni, Al, and NiAl, as well as their electronic characteristics, are investigated using density functional theory and density functional perturbation theory. Our research on phonon dispersion has concentrated on every high symmetry point of NiAl, both with and without defects. The phonon gap of NiAl is measured to be 1.78 THz between the lowest value of the optical branch and the top bound of the acoustic branch. The NiAl supercell with a center Al vacancy was built to investigate the effects of defects on its vibrational characteristics. The phonon gap of a NiAl crystal at the center of the Brillouin zone with no defect is calculated to be 1.06 THz. This value of the phonon gap is affected by the Al vacancy defect and decreases by 7.67%. The development of phonon spectrum is investigated using electronic structure and properties. The majority of free electrons accumulated at the Fermi level contribute to promoting the vibration. The importance of a heavier element in a compound has also been explored in terms of gap tuning. Reduced thermal conductivity by narrowing the gap between optical and acoustic modes raises the figure of merit (zT) value, which improves the thermoelectric energy conversion efficiency.

First-Principles Study of Atomic Diffusion by Vacancy Defect of the L12-Al3M (M = Sc, Zr, Er, Y) Phase.

Atomic diffusion by the vacancy defect of L12-Al3M (M = Sc, Zr, Er, Y) was investigated based on a first-principles calculation. The point defect formation energies were firstly evaluated. Then, the migration energy for different diffusion paths was obtained by the climbing-image nudged elastic band (CI-NEB) method. The results showed that Al atomic and M atomic diffusions through nearest-neighbor jump (NNJ) mediated by Al vacancy (VAl) were, respectively, the preferred diffusion paths in Al3M phases under both Al-rich and M-rich conditions. The other mechanisms, such as six-jump cycle (6JC) and next-nearest-neighbor jump (NNNJ), were energetically inhibited. The order of activation barriers for NNJ(Al-VAl) was Al3Zr < Al3Y < Al3Er < Al3Sc. The Al3Sc phase had high stability with a high self-diffusion activation barrier, while the Al3Zr and Al3Y phases were relatively unstable with a low self-diffusion activation energy. Moreover, the atomic-diffusion behavior between the core and shell layers of L12-Al3M was also further investigated. Zr atoms were prone to diffusion into the Al3Y core layer, resulting in no stable core-shelled Al3(Y,Zr), which well agreed with experimental observation.

The Promoting Effect of Metal Vacancy on CoAl Hydrotalcite-Derived Oxides for the Catalytic Oxidation of Formaldehyde

Formaldehyde (HCHO) is a major harmful volatile organic compound (VOC) that is particularly detrimental to human health indoors. Therefore, effectively eliminating formaldehyde is of paramount importance to ensure indoor air quality. In this study, CoAl hydrotalcite (LDH) was prepared using the co-precipitation method and transformed into composite metal oxides (LDO) through calcination. Additionally, a metal Al vacancy was constructed on the surface of the composite metal oxides (EX-LDO and EX-LDO/NF) using an alkaline etching technique. SEM demonstrated the successful loading of CoAl-LDO onto nickel foam surfaces (LDO/NF), and an extended etching time resulted in a greater number of porous structures in the samples. XRD confirmed the successful synthesis of the precursor materials, CoAl hydrotalcite (CoAl-LDH) and CoAl layered double oxides (CoAl-LDO). EDS analysis confirmed a reduction in aluminum content after alkaline etching. XPS analysis verified the presence of abundant Co2+ and surface oxygen as crucial factors contributing to the catalyst’s excellent catalytic activity. The experimental results indicated that catalysts containing metal cation vacancies exhibited superior catalytic performance in formaldehyde oxidation compared to conventional hydrotalcite-derived composite oxides. H2-TPR confirmed a significant enhancement in the participation of lattice oxygen in the catalytic oxidation reaction; it was found that the proportion of surface lattice oxygen consumption by the E5-LDO catalyst (30.2%) is higher than that of the LDO catalyst (23.4%), and the proportion of surface lattice oxygen consumption by the E1-LDO/NF catalyst (27.5%) is higher than that of the LDO/NF catalyst (14.6%), suggesting that cation vacancies can activate the surface lattice oxygen of the material, thereby facilitating improved catalytic activity. This study not only reveals the critical role of surface lattice oxygen in catalytic oxidation activity, but also aids in the further development of novel catalysts for efficient room-temperature oxidation of HCHO. Moreover, it provides possibilities for developing high-performance catalysts through surface modification.

Realize low-power artificial photonic synapse based on (Al,Ga)N nanowire/graphene heterojunction for neuromorphic computing

The fast development of brain-inspired neuromorphic computing systems has stimulated urgent requirements for artificial synapses with low-power consumption. In this work, a photonic synaptic device based on (Al,Ga)N nanowire/graphene heterojunction has been proposed and demonstrated successfully. In the artificial synaptic device, the incident light, the nanowire/graphene heterojunction, and the light-generated carriers play the roles of action potential, pre-synaptic/post-synaptic membrane, and neurotransmitter in a biological synapse, respectively. As a key synaptic function, the paired pulse facilitation index of the photonic synapse can reach 202%, which can be modulated by the interval time between two adjacent light pulses. It is found that the graphene defects, the surface band bending, and the Al vacancies on the surface of (Al,Ga)N nanowires can be the key reasons contributing to the synaptic characteristics of artificial photonic devices. Hence, the dynamic “learning–forgetting” performance of the artificial synaptic device can resemble the “learning–forgetting” behavior of the human brain. Furthermore, the hand-written digits are set up to mimic a typical characteristic of human perceptual learning. After only three training epochs, the simulated network can achieve a high recognition rate of over 90% based on the experimental conductance for long-term potentiation and long-term depression. In supervised learning processes, such few training times are beneficial to reduce energy consumption significantly. Therefore, in the area of neuromorphic computing technology and artificial intelligence systems requiring low-power consumption, this work paves a potential way to develop the optoelectronic synapse based on semiconductor nanowires.

Dopant concentration dependent room temperature ferromagnetism in crystalline Sc doped AlN thin films

In this study, high uniformity AlN thin films with varying Sc doping concentration levels were deposited on Ti (120 nm)/Si substrates. The impact of Sc doping ratio and annealing temperature on the magnetic properties of Al1−xScxN were studied. This work suggests that the origin of ferromagnetism of Al1−xScxN is the Al vacancies introduced by Sc ions. The amount of Al vacancies grew as the Sc doping ratio increased. According to the results of spin-polarized density of states (DOS), it is hypothesized that when the concentration of Al vacancies in Al1−xScxN grows, polarized itinerant electrons exist and result a magnetic phase change.

Ab initio study of effects of Al on the defect behaviors of AlxCoCrFeNi high entropy alloys

The influence of Al on the defect behaviors of AlxCoCrFeNi high-entropy alloys (HEAs) is studied by ab initio calculations. The lattice distortion, defect formation energies, and defect migration energies were calculated for different Al concentrations ranging from 0 to 6.25 at. %. First, the increase in the lattice distortion of AlxCoCrFeNi by the addition of Al is demonstrated, but the increment is smaller than that predicted by the atomic size difference of constituent elements. Second, although formation energies of Al vacancies and dumbbells containing Al are higher, the addition of Al decreases the average formation energies of vacancies and interstitials, which is probably due to the Al-induced local strain field and lattice distortion. The migration energies of interstitials and vacancies exhibit opposite variations, the migration energies of interstitials are increased while that of vacancies are decreased with the Al content. Moreover, the distributions of formation and migration energies of both defects are significantly broadening with increasing Al content, which could further enhance the characteristic effects of HEAs and play a more important role in the irradiation resistances of HEAs. Based on the calculation results, the effects of Al on the irradiation responses of AlxCoCrFeNi are discussed.

Mechanical and Electronic Properties of Al(111)/6H-SiC Interfaces: A DFT Study.

A density functional theory (DFT) calculation is carried out in this work to investigate the effect of vacancies on the behavior of Al(111)/6H SiC composites. Generally, DFT simulations with appropriate interface models can be an acceptable alternative to experimental methods. We developed two modes for Al/SiC superlattices: C-terminated and Si-terminated interface configurations. C and Si vacancies reduce interfacial adhesion near the interface, while Al vacancies have little effect. Supercells are stretched vertically along the z-direction to obtain tensile strength. Stress-strain diagrams illustrate that the tensile properties of the composite can be improved by the presence of a vacancy, particularly on the SiC side, compared to a composite without a vacancy. Determining the interfacial fracture toughness plays a pivotal role in evaluating the resistance of materials to failure. The fracture toughness of Al/SiC is calculated using the first principal calculations in this paper. Young's modulus (E) and surface energy (Ɣ) is calculated to obtain the fracture toughness (KIC). Young's modulus is higher for C-terminated configurations than for Si-terminated configurations. Surface energy plays a dominant role in determining the fracture toughness process. Finally, to better understand the electronic properties of this system, the density of states (DOS) is calculated.

Ab Initio Simulations of Tritium Diffusion in Al-Rich Iron Aluminide Coating Phases

Surface coatings of steels used in extreme conditions and corrosive environments generally aim to provide protection and increased durability. In the case of tritium-producing burnable absorber rods (TPBARs) used in nuclear reactors, a 316 stainless steel has been coated with Al. Scanning transmission electron microscopy (STEM) characterization of the coating found three Al-rich (>60 atom % Al) iron aluminide alloys identified as hexagonal FeNiAl5, monoclinic Fe4Al13, and orthorhombic Fe2Al5. Density functional theory simulations using nudged elastic band have been performed to investigate the diffusion of interstitial tritium in each Al-rich iron aluminide phase. While FeNiAl5 and Fe4Al13 can be viewed as the stacking of two layers, the structural peculiarity of Fe2Al5 is that channels of variable Al vacancy content are present along the c-axis. Therefore, three stoichiometries for Fe2Alx phase, namely, Fe2Al4, Fe2Al5, and Fe2Al6, have been considered to evaluate the impact of Al vacancy concentration on tritium diffusion behavior. Altogether, we found that at 600 K, tritium diffusion decreases from a faster rate in the channels of Fe2Alx phases (DT ≤ 10–11 m2·s–1) to Fe4Al13 (DT ≈ 10–12 m2·s–1), and finally in FeNiAl5 (DT ≈ 10–13 m2·s–1). We also find that interstitial tritium generally diffuses faster in Fe–Al coating phases than in the tritium breeding material γ-LiAlO2 (DT ≈ 10–14 m2·s–1) but slightly slower than in 316 stainless steel (DT ≈ 10–10 m2·s–1).

A first-principles study on the effect of point defects on the magnetism of the AlN:Li/Na/K system

When AIN semiconductors are grown by metal–organic chemical vapor deposition and vacuum-coating methods, interstitial H atoms are unintentionally incorporated into AlN and are often overlooked. To solve this problem, the present study adopted the first-principles hybrid functional HSE06 method under the framework of density functional theory to examine the impact of the point defects (VAl and Hi) on the magnetic properties of the AlN:Li/Na/K system. Results showed that all doped systems had single-ion spin-polarized N2− at different positions to generate magnetism, different from the source of magnetic coupling in conventional doped systems. Compared with the Al34MN36 (M = Li/Na) and Al34N36 systems, the magnetic moments of the Al34MHiN36 (M = Li/Na) and Al34HiN36 systems increased after incorporating the interstitial H. Moreover, the Al34MHiN36 (M = Li/Na) and Al34HiN36 systems unpaired itinerant electrons, and the magnetism was generated mostly by the unpaired itinerant electrons of N2− ions. The itinerant electrons exchanged electrons through Al vacancies to generate magnetism. Furthermore, the total magnetic moment of the Al34KHiN36 and Al35HiN36 systems were zero, and the use of interstitial H regulation as magnetic switches was very advantageous for these systems.

Electronic properties and passivation mechanism of AlGaN/GaN heterojunction with vacancies: a DFT study

A significant issue for GaN-based high-electron-mobility transistors (HEMTs) in high power devices is the material defect, particularly the defect states generated by the defects, which has a negative impact on the device carrier concentration and carrier transport. Based on density functional theory (DFT), we investigate the microscopic properties of different type point vacancies in the AlGaN/GaN heterojunction. It is found that N vacancy introduces defect states near the conduction band minimum (CBM) of the GaN layer and AlGaN/GaN interface. Ga and Al vacancies introduce defect states near the valence band maximum (VBM) in bulk and interface of AlGaN/GaN heterojunction. Moreover, Al vacancy is more likely to be an effective candidate for acceptor defect than Ga vacancy. We further study several AlGaN/GaN interface passivation schemes by introducing F, V group element P, and III group element B at the AlGaN/GaN heterojunction interface to analyze the passivation mechanism. According to the results of the passivation models, B passivation of Ga and Al vacancies is an effective method to completely remove the defect states from Ga and Al vacancy defects. Combining the III and V groups elements into the passivated process may be effective in achieving high-quality AlGaN/GaN heterojunction interface for the future GaN-based HEMTs fabrication.

Cathodoluminescence studies of point defects in aluminum nitride

There is near-ultraviolet and yellow light absorption in AlN crystal. The defect of this absorption hampers the transmittance of ultraviolet light, which, in turn, limits the application of aluminum nitride substrate in deep ultraviolet electronic devices. In this paper, the sources of absorption defects are determined through the first principle simulation method and the cathodoluminescence test of four samples with different growth modes on two substrates. The peak positions of the two AlN grown by metal–organic chemical vapor deposition were observed at 370 and 480 nm; two AlN samples grown by hydride vapor phase epitaxy only observed luminescence near 370 nm. Through the characterization of sample components by x-ray photoelectron spectroscopy, it is speculated that O impurity component and Al vacancy in AlN crystal form donor–acceptor pairs, which are the source of 370 nm luminescence; with the increase in O concentration, a composite defect with Al vacancy is formed, which is the source of 480 nm luminescence. This is very important for further research on eliminating impurity absorption in an AlN epitaxial layer.

Promoting Reversibility of Co‐Free Layered Cathodes by Al and Cation Vacancy (Adv. Energy Mater. 12/2023)

Promoting Reversibility of Co‐Free Layered Cathodes by Al and Cation Vacancy (Adv. Energy Mater. 12/2023)

Promoting Reversibility of Co‐Free Layered Cathodes by Al and Cation Vacancy

AbstractLi‐ion batteries adopting layered cathodes can effectively alleviate the range limitations of electric vehicles. Unfortunately the scarcity of Co inhibits massive deployment of layered cathodes. Eliminating Co from layered cathodes is necessary to make a breakthrough in global application of electric vehicles. However, Co‐free layered cathodes face challenges in delithiation/lithiation reversibility since Co plays a pivotal role in suppressing Li/Ni mixing. Many metals have been proposed to replace Co in layered cathodes, and sophisticated compositional designs always consider suppressing Li/Ni mixing as the top priority. Here, the authors show that Li/Ni mixing can be suppressed by Al and cation vacancies in different manners; however, irreversible capacities are prominently different given similar Li/Ni mixing. In‐situ X‐ray diffraction monitored potentiostatic‐intermittent‐titration measurements reveal that Al decreases the lattice strain between H2 and H3 phase by 13.6%, but Al has an adverse impact on structure reversibility. While Al and cation vacancies together not only significantly reduce the H2/H3 lattice strain by 78% but also greatly enhance structure reversibility. Furthermore, the first‐principles calculation shows that cation vacancies can remarkably reduce the Li migration energy barrier by an order of magnitude. Reversibility of the newly designed Co‐free cathode is significantly enhanced through the synergic effect of Al and cation vacancies.

Unraveling the effect of excessive Al on the growth of high-quality YbAl3 film

Heavy-fermion YbAl3 intermetallic compound has attracted attention due to the distinctive 4f electronic structure and excellent electrical transport properties. However, it is extremely difficult to prepare high-quality YbAl3 film due to the highly saturated vapor pressure of Yb. Herein, the excessive Al strategy has been developed to grow high-quality YbAl3 films during both vacuum evaporating and annealing processes. The crucial role of excessive Al in growing YbAl3 film is revealed by the experimental and theoretical results. It is found that the excessive Al can effectively suppress Yb and Al vacancies and promote the crystallization of YbAl3 perfect crystal. The electrical conductivity and power factor of the as-prepared YbAl3 film can reach 1.71 × 106 S m−1 and 5.37 mW m−1 K−2, respectively. Our work demonstrates that keeping excessive Al is essential for the formation of high-quality YbAl3 films during the evaporating and annealing processes.

Amorphous alumina supercapacitors with voltage-charging performance

The electric storage effect of fast charging by voltage application for amorphous alumina (cAAO) supercapacitors fabricated using a crystalline aluminum alloy was investigated in terms of charging/discharging behaviors and AC impedance. The mechanism for the electrical storage of a cAAO supercapacitor was proposed based on the coupling of electrons on Al atoms and positively charged holes in Al vacancies, using an electric double-layer model containing an electrolyte with an electrical resistivity of 6.3 × 105Ωcm. The supercapacitor demonstrated fast charging at a voltage application of up to 500 V.

Understanding the Effect of Oxygen Content on Ferroelectric Properties of Al-Doped HfO Thin Films

To satisfy the demand of high-speed and low-power memory in the field of integrated circuits, different elements doped HfO2 ferroelectric tunnel junctions have been developed. In this work, we investigate the annealing atmosphere effects on the ferroelectric property of the Al-doped HfO2 (HfAlO) ferroelectric tunnel junctions based on first-principles calculation and experiments. The ferroelectric layer/electrode interface has the obvious effect on ferroelectric property of the devices under different oxygen content annealing atmosphere, including 0%, 50% and 100% O2. The first-principle analysis verified that stable o-phase proportion in HfAlO film owing to the co-existence of the Al and oxygen vacancy is the key element to stable ferroelectric performance of device. This work could promote the development of HfAlO ferroelectric tunnel junctions for next-generation in-memory computing applications.

Strain effect on the defect formation and diffusion in Ti2AlC and Ti3AlC2: A first-principles study

Mn+1AXn phases have the prospect of being the candidate materials as coating and fuel cladding in advanced nuclear systems. It is well known that nuclear materials are subjected to harsh environments such as high temperature, high pressure, and neutron radiation et al. during their application. Consequently, defects and strains inevitably arise in Mn+1AXn phases used as nuclear materials. In this work, by using a first-principles method, the strain effects on the defect formation and diffusion in Ti2AlC and Ti3AlC2 were studied by imposing an equibiaxial in-plane strain along the a and b (denoted as ab) axes. The formation energies of Ti vacancies are substantially larger than those of Al and C vacancies. As the imposed equibiaxial strain goes from compressive to tensile state with strain ranges from −4% to 4%, the formation energies of Ti and C vacancies increase, while the formation energy of Al vacancy decreases. The formation energies of the antisite defect TiAl-AlTi, the substitutional defect AlTi, and the substitutional defect TiAl all increase with the applied biaxial strain. In both Ti2AlC and Ti3AlC2, the process of Al atom diffusing to the Ti vacancy is energetically favorable, conversely, the process of Ti atom diffusing to the Al vacancy is energetically unfavorable. Therefore, the diffusion of Al atom is expected to play an important role in the microstructural evolution under irradiation environments. According to our research about the atomic diffusions at different strains, atomic diffusion across the atomic layer can be slowed down under the compressive strain along the ab axes. As a consequence, the MAX materials show better microstructural stability and improved radiation resistance. That is, the irradiation resistance of the MAX materials can be improved when they are applied under a certain compressive strain in nuclear systems.

The η-Al5Fe2 phase in the Al–Fe system: The issue with the sublattice model

The thermodynamic descriptions of a non-stoichiometric crystalline phase used within the framework of the CALPHAD approach has to include an appropriate model to estimate the configurational entropy, e.g. within the compound energy formalism. Thereby, typically a sublattice model based on the known crystal structure data of the considered phase is employed. In the present work a revised thermodynamic description of the Al−Fe system is presented with focus on the sublattice model for the η-Al5Fe2 phase. In view of its channel structure containing Al+Fe as channel atoms with largely composition-independent atomic density (and not Al atoms and vacancies as believed previously), a new sublattice model Al4Fe2(Fe,Al)m with m = 1.5 is proposed. In contrast to previously proposed sublattice models, the new model is well able to reproduce the experimentally observed homogeneity range of the η-Al5Fe2 phase in the course of thermodynamic modelling. Thereby, in order to derive the new model, it was necessary to transfer the crystallographic model used during structure refinement specifically to a sublattice model for the thermodynamic modelling, in order to naturally represent the otherwise easily overestimated the configurational entropy.

First-principle study on the effect of point defects on the mechanical properties, thermal conductivity, and optical properties of wurtzite AlN

Defects (VAl/VN) and unintentional impurities (C/O/Si/Hi) will inevitably exist during the preparation of AlN by metal-organic chemical vapor deposition and physical vapor transport. In particular, Hi is very difficult to remove in a vacuum environment. First-principle calculations were used to research the mechanical properties, thermal conductivity, and optical properties of AlN and its defect-containing systems. Results showed that the deformation resistance of Al36SiN35 and Al35ON36 systems was relatively poor, and the hardness was relatively small. When the impurity C or O located in the N position, the effect on the anisotropy of AlN was relatively large. O impurities and Al vacancies had relatively large effects on the thermal conductivity of AlN lattice. When the O impurity was located at the N position and the Si impurity was located at the Al position, the influence on the optical properties of AlN was relatively large, which would help improve the polarization ability, refractive index, reflectivity, and absorption of the visible light of AlN.

First-principles insight of hydrogen dissolution and diffusion properties in γ-Al2O3

First-principles theory is applied to identify the structure of γ-Al2O3 first. Then, hydrogen dissolution and diffusion behavior was further studied to reveal the reason that permeation reduction factor (PRF) of γ-Al2O3 is lower than that of α-Al2O3. Our calculations show that the formation energy for Al vacancy at the octahedral interstitial site (VAl−OIS−3) is lower than that at the tetrahedral interstitial site (VAl−TIS−3), and are far lower than those of other defects in both defective spinel and non-spinel γ-Al2O3. Thus, the stability of VAl−OIS−3 is higher than that of VAl−TIS−3 in both Al-rich and O-rich environments. However, when hydrogen atoms are introduced into γ-Al2O3, the stability of [VAl-TIS-H]−2 is higher than that of [VAl-OIS-H]−2 in the defective spinel γ-Al2O3 structure, while the opposite is true for the non-spinel γ-Al2O3 structure. The vibrational frequencies for OH− in [VAl-TIS-H]−2 and [VAl-OIS-H]−2 for defective γ-Al2O3 are calculated to be 3608 cm−1 and 3374 cm−1, respectively, which is in excellent agreement with the infrared (IR) absorption peaks observed at ∼3500 and ∼3300 cm−1. The defective spinel model is more suitable for describing the γ-Al2O3 structure. In defective spinel γ-Al2O3, there are many native Al vacancies, which are arranged in a straight line along the [21¯1¯0] direction. The migration barrier for H diffusion along these native Al vacancies is so low that these Al vacancies can provide a rapid diffusion channel for H. Thus, the permeability of H in γ-Al2O3 is much higher than that in α-Al2O3 leading to a lower permeation reduction factor (PRF). Our results can provide not only a sound theoretical explanation for the low PRF of γ-Al2O3 but also a direction to improve the efficiency in preventing H permeation through FeAl/Al2O3 tritium permeation barriers (TPBs).