arXiv subjects

Fedwa El-Mellouhi

Publications and source records attributed to Fedwa El-Mellouhi.

14 recordsLinked to original sources

arXiv2026

Multiphysics Modeling of Thermo-Viscoelastic Damage in Functionally Graded Abradable Coatings with Probabilistic Geometric Tolerance Analysis

In aircraft engines, functionally graded abradable coatings are used to control blade-tip clearance, but their durability is governed by effects that are often treated separately in existing models, including temperature-dependent viscoelastic softening, progressive damage, deposition-induced microstructural modulation, and geometric tolerances. This study integrates these effects within a unified multiphysics--probabilistic framework. The solved domain is a local through-thickness coating column driven by prescribed strain and temperature histories that include thermal eigenstrain and coating--substrate expansion mismatch. The results show that periodic property modulation increases end-of-cycle damage relative to the monotonic gradient and concentrates the maximum stress within a localized modulation crest. Propagating symmetric geometric tolerances through the coupled solver produces a strongly right-skewed damage distribution and a higher classification-level exceedance probability for the modulated gradient than for the monotonic gradient. This exceedance behavior cannot be obtained from a deterministic analysis performed at nominal geometry. The implementation is verified through spatial and temporal refinement, closed-form unit tests, nested Monte Carlo convergence with Wilson confidence intervals, and a hold-out-validated polynomial response surface. Numerical verification is clearly distinguished from experimental validation, and the simulated case is presented as a generic benchmark rather than an identified material model. The framework links deposition parameters and tolerance bands to damage-exceedance risk, supporting reliability-oriented screening of graded and multilayer coating systems.

cond-mat.mes-hall↗

arXiv2017

An Efficient Descriptor Model for Designing Materials for Solar Cells

An efficient descriptor model for fast screening of potential materials for solar cell applications is presented. It works for both excitonic and non-excitonic solar cells materials, and in addition to the energy gap it includes the absorption spectrum ($α(E)$) of the material. The charge transport properties of the explored materials are modeled using the characteristic diffusion length ($L_{d}$) determined for the respective family of compounds. The presented model surpasses the widely used Scharber model developed for bulk-heterojunction solar cells [Scharber \textit{et al., Advanced Materials}, 2006, Vol. 18, 789]. Using published experimental data, we show that the presented model is more accurate in predicting the achievable efficiencies. Although the focus of this work is on organic photovoltaics (OPV), for which the original Scharber model was developed, the model presented here is applicable also to other solar cell technologies. To model both excitonic and non-excitonic systems, two different sets of parameters are used to account for the different modes of operation. The analysis of the presented descriptor model clearly shows the benefit of including $α(E)$ and $L_{d}$ in view of improved screening results.

physics.app-ph↗

arXiv2016

Hydrogen Bonding: A Mechanism for Tuning Electronic and Optical Properties of Hybrid Organic-Inorganic Frameworks

Here we demonstrate that significant progress in this area may be achieved by introducing structural elements that form hydrogen bonds with environment. Considering several examples of hybrid framework materials with different structural ordering containing protonated sulfonium cation H3S+ that forms strong hydrogen bonds with electronegative halogen anions (Cl-, F-), we found that hydrogen bonding increases the structural stability of the material and may be used for tuning electronic states near the bandgap. We suggest that such a behavior has a universal character and should be observed in hybrid inorganic-organic framework materials containing protonated cations. This effect may serve as a viable route for optoelectronic and photovoltaic applications.

cond-mat.mtrl-sci↗

arXiv2016

Enhancing Intrinsic Stability of Hybrid Perovskite Solar Cell by Strong, yet Balanced, Electronic Coupling

In the past few years, the meteoric development of hybrid organic--inorganic perovskite solar cells (PSC) astonished the community. The efficiency has already reached the level needed for commercialization; however, the instability hinders its deployment on the market. Here, we report a mechanism to chemically stabilize PSC absorbers. We propose to replace the widely used methylammonium cation (\ce{CH3NH3+}) by alternative molecular cations allowing an enhanced electronic coupling between the cation and the \ce{PbI6} octahedra while maintaining the band gap energy within the suitable range for solar cells. The mechanism exploits establishing a balance between the electronegativity of the materials' constituents and the resulting ionic electrostatic interactions. The calculations demonstrate the concept of enhancing the electronic coupling, and hence the stability, by exploring the stabilizing features of \ce{CH3PH3+}, \ce{CH3SH2+}, and \ce{SH3+} cations, among several other possible candidates. Chemical stability enhancement hence results from a strong, yet balanced, electronic coupling between the cation and the halides in the octahedron. This shall unlock the hindering instability problem for PSCs and allow them to hit the market as a serious low-cost competitor to silicon based solar cell technologies.

cond-mat.mtrl-sci↗

arXiv2015

Domain Walls Conductivity in Hybrid Organometallic Perovskites: The Key of CH3NH3PbI3 Solar Cell High Performance

The past several years has witnessed a surge of interest in organometallic trihalide perovskites, which are at the heart of the new generation of solid-state solar cells. Here, we calculated the static conductivity of charged domain walls in n- and p- doped organometallic uniaxial ferroelectric semiconductor perovskite CH3NH3PbI3 using the Landau-Ginzburg-Devonshire (LGD) theory. We find that due to the charge carrier accumulation, the static conductivity may drastically increase at the domain wall by 3-4 orders of magnitude in comparison with conductivity through the bulk of the material. Also, a two-dimensional degenerated gas of highly mobile charge carriers could be formed at the wall. The high values of conductivity at domain walls and interfaces explain high efficiency in organometallic solution-processed perovskite films which contains lots of different point and extended defects. These results could suggest new routes to enhance the performance of this promising class of novel photovoltaic materials.

cond-mat.mtrl-sci↗

arXiv2013

The role of screened exact exchange in accurately describing properties of transition metal oxides: Modeling defects in LaAlO3

The properties of many intrinsic defects in the wide band gap semiconductor LaAlO3 are studied using the screened hybrid functional of Heyd, Scuseria, and Ernzerhof (HSE). As in pristine structures, exact exchange included in the screened hybrid functional alleviates the band gap underestimation problem, which is common to semilocal functionals; this allows accurate prediction of defect properties. We propose correction-free defect energy levels for bulk LaAlO3 computed using HSE that might serve as guide in the interpretation of photoluminescence experiments.

cond-mat.mtrl-sci↗

arXiv2013

Neutral defects in SrTiO3 studied with screened hybrid density functional theory

The properties of neutral defects in SrTiO3 are calculated using the screened hybrid density functional of Heyd, Scuseria, and Ernzerhof. The formation energies, the crystal field splittings affecting the SrTiO3 band structure, and the relaxation geometries around each defect are discussed. Oxygen vacancies introduced in SrTiO3 are found to cause a small tetragonal elongation of the lattice along the z-axis. The resulting conduction band minimum electron effective masses deviate from the bulk values and support the proposal of enhanced electron mobility along the direction of the compressive strain. The locations of the various defect bands within the SrTiO3 gap are estimated without introducing any post hoc corrections, thus allowing a more reliable comparison with experiment.

cond-mat.mtrl-sci↗

arXiv2012

Structural Phase Transitions of the Metal Oxide Perovskites SrTiO3, LaAlO3 and LaTiO3 Studied with a Screened Hybrid Functional

We have investigated the structural phase transitions of the transition metal oxide perovskites SrTiO$_{3}$, LaAlO$_{3}$ and LaTiO$_{3}$ using the screened hybrid density functional of Heyd, Scuseria and Ernzerhof (HSE06). We show that HSE06-computed lattice parameters, octahedral tilts and rotations, as well as electronic properties, are significantly improved over semilocal functionals. We predict the crystal field splitting ($Δ_{CF}$) resulting from the structural phase transition in SrTiO$_{3}$ and LaAlO$_{3}$ to be 3 meV and 10 meV, respectively, in excellent agreement with experimental results. HSE06 identifies correctly LaTiO$_{3}$ in the magnetic sates as a Mott insulator. Also, it predicts that the GdFeO$_{3}$-type distortion in non-magnetic LaTiO$_{3}$ will induce a large $Δ_{CF}$ of 410 meV. This large crystal-field splitting associated with the large magnetic moment found in the G-type antiferromagnetic state suggest that LaTiO$_{3}$ has an induced orbital order, which is confirmed by the visualisation of the highest occupied orbitals. These results strongly indicate that HSE06 is capable of efficiently and accurately modeling perovskite oxides, and promises to efficiently capture the physics at their heterointerfaces.

cond-mat.str-el↗

arXiv2011

Kinetic Activation Relaxation Technique

We present a detailed description of the kinetic Activation-Relaxation Technique (k-ART), an off-lattice, self-learning kinetic Monte Carlo algorithm with on-the-fly event search. Combining a topological classification for local environments and event generation with ART nouveau, an efficient unbiased sampling method for finding transition states, k-ART can be applied to complex materials with atoms in off-lattice positions or with elastic deformations that cannot be handled with standard KMC approaches. In addition to presenting the various elements of the algorithm, we demonstrate the general character of k-ART by applying the algorithm to three challenging systems: self-defect annihilation in c-Si (crystalline silicon), self-interstitial diffusion in Fe and structural relaxation in a-Si (amorphous silicon).

physics.comp-ph↗

arXiv2007

Gallium self-interstitial relaxation in Gallium Arsenide: an {ab initio} characterization

Ga interstitials in GaAs ($I_{Ga}$) are studied using the local-orbital {ab-initio} code SIESTA in a supercell of {216+1} atoms. Starting from eight different initial configurations, we find five metastable structures: the two tetrahedral sites in addition to the 110-split$\mathrm{_{[Ga-As]}}$, 111-split$\mathrm{_{[Ga-As]}}$, and 100-split$\mathrm{_{[Ga-Ga]}}$. Studying the competition between various configuration and charges of $I_{Ga}$, we find that predominant gallium interstitials in GaAs are charged +1, neutral or at most -1 depending on doping conditions and prefer to occupy the tetrahedral configuration where it is surrounded by Ga atoms. Our results are in excellent agreement with recent experimental results concerning the dominant charge of $I_{Ga}$, underlining the importance of finite size effects in the calculation of defects.

cond-mat.mtrl-sci↗

arXiv2006

Charge-dependent migration pathways for the Ga vacancy in GaAs

Using SIEST-ART, a combination of the local-basis \textit{ab-initio} program SIESTA and the activation-relaxation technique (ART nouveau) we study the diffusion mechanisms of the gallium vacancy in GaAs. Vacancies are found to diffuse to the second neighbor using two different mechanisms, as well as to the first and fourth neighbors following various mechanisms. We find that the height of the energy barrier is sensitive to the Fermi-level and generally increases with the charge state. Migration pathways themselves can be strongly charge-dependent and may appear or disappear as a function of the charge state. These differences in transition state and migration barrier are explained by the charge transfer that takes place during the vacancy migration.

cond-mat.mtrl-sci↗

arXiv2006

Thermally-activated charge reversibility of gallium vacancies in GaAs

The dominant charge state for the Ga vacancy in GaAs has been the subject of a long debate, with experiments proposing $-$1, $-$2 or $-$3 as the best answer. We revisit this problem using {\it ab initio} calculations to compute the effects of temperature on the Gibbs free energy of formation, and we find that the thermal dependence of the Fermi level and of the ionization levels lead to a reversal of the preferred charge state as the temperature increases. Calculating the concentrations of gallium vacancies based on these results, we reproduce two conflicting experimental measurements, showing that these can be understood from a single set of coherent LDA results when thermal effects are included.

cond-mat.mtrl-sci↗

arXiv2004

Self-vacancies in Gallium Arsenide: an ab initio calculation

We report here a reexamination of the static properties of vacancies in GaAs by means of first-principles density-functional calculations using localized basis sets. Our calculated formation energies yields results that are in good agreement with recent experimental and {\it ab-initio} calculation and provide a complete description of the relaxation geometry and energetic for various charge state of vacancies from both sublattices. Gallium vacancies are stable in the 0, -, -2, -3 charge state, but V_Ga^-3 remains the dominant charge state for intrinsic and n-type GaAs, confirming results from positron annihilation. Interestingly, Arsenic vacancies show two successive negative-U transitions making only +1, -1 and -3 charge states stable, while the intermediate defects are metastable. The second transition (-/-3) brings a resonant bond relaxation for V_As^-3 similar to the one identified for silicon and GaAs divacancies.

cond-mat.mtrl-sci↗

arXiv2004

Sampling the diffusion paths of a neutral vacancy in Silicon with quantum mechanical calculations

We report a first-principles study of vacancy-induced self-diffusion in crystalline silicon. Starting form a fully relaxed configuration with a neutral vacancy, we proceed to search for local diffusion paths. The diffusion of the vacancy proceeds by hops to first nearest neighbor with an energy barrier of 0.40 eV in agreement with experimental results. Competing mechanisms are identified, like the reorientation, and the recombination of dangling bonds by Wooten-Winer-Weaire process.

cond-mat.mtrl-sci↗