Vacancy Sinks Research Articles

Enhanced energy-storage performances in lead-free ceramics via the Co-modulation by conduction effect and domain engineering

The main factors that limit the practical application of bismuth ferrite-based energy storage ceramics are their low breakdown electric field strength and large remnant polarization. Here, we achieve high energy storage behavior in (0.72-x)BiFeO3-0.28BaTiO3-xLa(Mg1/2Zr1/2)O3 (BF-BT-xLMZ) ferroelectric ceramics through directional defect modulation based on a transformation of the conductance mechanisms. The systematic experimental analysis coupled with the vacancy sink model suggests that the introduction of LMZ changes the trap-filled-limit conduction mode dominated by oxygen vacancy, leading to 4 times enhancement of the breakdown electric field in BF-BT-0.2LMZ. Meanwhile, the induced nanodomains create a size effect that effectively reduces the remnant polarization, resulting in an increase in efficiency by 3.5 times. As a result, the recoverable energy storage density of the ceramic reaches an outstanding 4.2 J/cm3, together with a high efficiency of 75.2%. This work provides a feasible strategy for the development of ferroelectric family in the field of high-energy storage.

On Dislocation Climb as an Important Deformation Mechanism for Planetary Interiors

An understanding of the rheological behavior of the solid Earth is fundamental to provide a quantitative description of most geological and geophysical phenomena. The continuum mechanics approach to describing large-scale phenomena needs to be informed by a description of the mechanisms operating at the atomic scale. These involve crystal defects, mainly vacancies and dislocations. This often leads to a binary view of creep reduced to diffusion creep or dislocation creep. However, the interaction between these two types of defects leading to dislocation climb plays an important role, and may even be the main one, in the high-temperature, low strain rate creep mechanisms of interest to the Earth sciences. Here we review the fundamentals of dislocation climb, highlighting the specific problems of minerals. We discuss the importance of computer simulations, informed by experiments, for accurately modeling climb. We show how dislocation climb increasingly appears as a deformation mechanism in its own right. We review the contribution of this mechanism to mineral deformation, particularly in Earth's mantle. Finally, we discuss progress and challenges, and we outline future work directions. ▪Dislocations can be sources or sinks of vacancies, resulting in a displacement out of the glide plane: climb.▪Dislocation climb can be a recovery mechanism during dislocation creep but also a strain-producing mechanism.▪The slow natural strain rates promote the contribution of climb, which is controlled by diffusion.▪In planetary interiors where dislocation glide can be inhibited by pressure, dislocation climb may be the only active mechanism.

Platinum Catalyst Degradation: Unusual Nucleation and Growth of Nanoislands

The longevity of a catalyst plays a crucial role for many industrial scale applications. Platinum is the best candidate to be used in electrochemical energy conversion systems due to its high activity. However, platinum catalysts degrade during fuel cell operation. The origin of this degradation, the loss of material, is very likely rooted in the roughening of the surface: the nucleation and growth of nanoislands [1, 2, 3, 4]. It is known that the extend of this roughening can be tuned by additives, change of the electrolyte pH, or operation within different potential windows [5, 6, 7]. In this work, we study the degradation of different platinum stepped surfaces under potential cycling to oxidative potentials. Interestingly, we quantify significantly less degradation on surfaces with narrower terraces: Ra(Pt(111)) > Ra(Pt(15 15 14)) > Ra(Pt(554)) > Ra(Pt(553)) > Ra(Pt(533)). We present here a model that explains this trend, in which steps act like sinks for both adatoms and vacancies, slowing down the nucleation and growth of the above mentioned nanoislands. We support our results, which were derived until now solely on electrochemical measurements, with in-operando Electrochemical Scanning Tunneling Microscopy (EC-STM) images.

Efficiency of the vacancy pipe diffusion along an edge dislocation in MgO

This study focuses on the mechanisms of pipe diffusion and the kinetic of point defect diffusion along dislocation line in MgO. We developed a numerical approach, based on atomic scale calculations and the use of the elasticity theory, to determine the migration energies of point defects. The kinetic of diffusion along the dislocation is then evaluated according to a on-lattice atomistic kinetic Monte Carlo algorithm informed by atomistic simulations. We show that edge dislocation in MgO behaves as a strong sink for vacancies which, combined with a lower migration energy at dislocation core region, strongly enhances the diffusion of point defect in the vicinity of the dislocation with respect to the bulk material. At low and intermediate temperatures, pipe diffusion results in an increase of the diffusivity of several order of magnitude. Accounting more precisely for the effect of pipe diffusion may therefore be a key to reconcile the experimentally measured scattering of diffusivity in MgO.

Model for non-equilibrium vacancy diffusion applied to study the Kirkendall effect in high-entropy alloys

The effect of the non-equilibrium vacancy on the Kirkendall porosity formation was studied by means of a developed model of the multi-component diffusion with vacancies (MDV) which includes the intrinsic fluxes with vacancy gradient and non-ideal sources and sinks for vacancies. For this study, the diffusion couple experiments in multi-component alloys were chosen. To handle the case of concentration-dependent equilibrium vacancy concentration, we introduced the interaction parameters between the components and vacancies, which can have strong effects on the equilibrium vacancy concentration in alloys and on thermodynamic factors. The diffusion profiles of components and vacancies were simulated by using thermodynamic and kinetic data. The different intensity of the vacancy annihilation/generation and different initial vacancy distributions were considered. Furthermore, we show that the conventional model of diffusion in multi-component systems is a particular case of the MDV with a specific sink/source term. The conventional model was extended by the vacancy diffusion term, similar to the MDV, which significantly reduces the vacancy gradient and the pore formation near the Matano plane. The numerical results demonstrate that the diffusion profiles of substitutional components slightly depend on the sink/source intensity if the none-zero net flux of substitutional components is not significant and the sources and sinks of vacancies are not sparse, whereas the porosity depends very strongly and correlates with the vacancy distribution. For the simulation of variable equilibrium vacancy concentrations using the MDV, the corresponding interaction parameters related to vacancies are necessary to be included in the thermodynamic assessment.

The mechanism behind the high radiation tolerance of Fe–Cr alloys

Fe–Cr alloys are at the forefront for high radiation tolerant materials with long-standing validated performance. Yet, the detailed mechanism behind their high radiation resistance is in question and understanding the effect of varying Cr percentage is a grand challenge limiting further improvements. Here, we applied depth-resolved positron annihilation lifetime spectroscopy and Doppler broadening spectroscopy to study the effect of Cr alloying on the formation and evolution of atomic size clusters induced by ion-irradiation in Fe. We also used atom probe tomography to investigate the possible presence of Cr clusters or α′ phase precipitates with high Cr composition. The study reveals that the well-known resistance to radiation in Fe–Cr alloys may arise from the stabilization of vacancy clusters around Cr atoms, which act as sinks for radiation-induced defects. This implies that Cr atoms do not provide a direct sink for interstitials; rather defect complexes that consist of Cr atoms and vacancies, in turn, act as sinks for irradiation-induced vacancies and interstitials. we also find that lower amounts of Cr create smaller defect clusters that act as efficient sinks for radiation damage, but larger quantities of Cr form a defect structure that is less homogenous and larger in size, resulting in less efficient damage recombination. No evidence of α′ was found before or after irradiation, which indicates that it does not contribute to the observed radiation tolerance.

Ta2C precipitation after low pressure carburizing of tantalum

Low pressure carburizing cycles were carried out on tantalum samples. A Ta2C precipitates layer is present under the TaC and Ta2C surface layers. As the TaC and Ta2C layers, the Ta2C precipitates layer (TPL) grows by carbon diffusion in tantalum matrix. Specific zones with low precipitates content were observed at the bulk surface and in the tantalum grain boundaries. Other zones with lower precipitates content are also present in carburized and annealed samples. All the zones with low precipitates content are induced by the Ta2C precipitation mechanism described by Dahmen et al. [1,2]. Indeed, Ta2C needs vacancies to precipitate. If vacancy sinks are present at the bulk surface, at a grain boundary or in a dislocation, then Ta2C precipitation is highly reduced. Another effect of the annealing treatment is an increase of the Ta2C precipitates layers growth rate.

Bubble Swelling in Ferritic/Martensitic Steels Exposed to Radiation Environment with High Production Rate of Helium.

Reduced-activativon ferritic/martensitic (RAFM) steels are prospective structural materials for fission/fusion nuclear applications because their radiation and swelling resistance outperforms their austenitic counterparts. In radiation environments with a high production rate of helium, such as fusion or spallation applications, these materials suffer from non-negligible swelling due to the inhibited recombination between vacancy and interstitial-type defects. In this work, swelling in helium-implanted Eurofer 97 steel is investigated with a focus on helium production rates in a wide range of helium/dpa ratios. The results show virtually no swelling incubation period preceding a steady-state swelling of about 2 × 10−4%/He-appm/dpa. A saturation of swelling above 5000 He-appm/dpa was observed and attributed to helium bubbles becoming the dominant sinks for new vacancies and helium atoms. Despite a relatively low irradiation temperature (65 ± 5 °C) and a rather high concentration of helium, transmission electron microscope (TEM) results confirmed a microstructure typical of ferritic/martensitic steels exposed to radiation environments with high production rates of helium.

Resolving atomic-scale phase transformation and oxygen loss mechanism in ultrahigh-nickel layered cathodes for cobalt-free lithium-ion batteries

Summary Doped LiNiO2 has recently become one of the most promising cathode materials for its high specific energy, long cycle life, and reduced cobalt content. Despite this, the degradation mechanism of LiNiO2 and its derivatives still remains elusive. Here, by combining in situ electron microscopy and first-principles calculations, we elucidate the atomic-level chemomechanical degradation pathway of LiNiO2-derived cathodes. We uncover that the O1 phase formed at high voltages acts as a preferential site for rock-salt transformation via a two-step pathway involving cation mixing and shear along (003) planes. Moreover, electron tomography reveals that planar cracks nucleated simultaneously from particle interior and surface propagate along the [100] direction on (003) planes, accompanied by concurrent structural degradation in a discrete manner. Our results provide an in-depth understanding of the degradation mechanism of LiNiO2-derived cathodes, pointing out the concept that suppressing the O1 phase and oxygen loss is key to stabilizing LiNiO2 for developing next-generation high-energy cathode materials.

Solubility and vacancy-mediated inter-diffusion in the Zr-Nb-Cr system

To better understand the microstructural evolution of Cr-coated Zr alloy cladding, we investigate, using density functional theory calculations, the vacancy-mediated diffusion behaviour of Zr and Nb solutes in BCC-Cr as well as Zr and Cr solutes in BCC-Nb. The calculated vacancy formation and migration energies are in good agreement with available literature, as are the self-diffusivities. We show that in BCC-Cr both Nb and Zr are faster diffusers than Cr (vacancy-mediated self-diffusion). It was also found that both Zr and Nb segregate towards vacancy sinks in BCC-Cr at normal reactor operating temperatures, but at elevated temperatures their flux is expected to be in opposite directions. In BCC-Nb, we show that Cr is a slower diffuser than Nb self-diffusion, while Zr is faster; and both Zr and Cr are expected to decorate vacancy sinks in BCC-Nb at all temperatures relevant to reactor operation. A similar behaviour is likely to occur in β-Nb phase found in Zr-Nb alloys. The implications of these findings for Cr-coated Zr alloy cladding are discussed.

Effects of ultra-high-pressure annealing on characteristics of vacancies in Mg-implanted GaN studied using a monoenergetic positron beam

Vacancy-type defects in Mg-implanted GaN were probed by using a monoenergetic positron beam. Mg ions were implanted into GaN to obtain 0.3-μm-deep box profiles with Mg concentrations of 1 × 1019 cm−3. The major defect species in an as-implanted sample was determined to be Ga-vacancy related defects such as a complex between Ga and N vacancies. The sample was annealed under a nitrogen pressure of 1 GPa in a temperature range of 1000–1480 °C without a protective capping layer. Compared with the results for Mg-implanted GaN annealed with an AlN capping layer, the defect concentration was decreased by the cap-less annealing, suggesting that the surface of the sample was an effective sink for vacancies migrating toward the surface. Depth distributions of Mg after annealing above 1300 °C were influenced by the presence of residual vacancies at this temperature. Hydrogen atoms were unintentionally incorporated into the sample during annealing, and their diffusion properties were also affected by both vacancies and Mg.

Natural ageing clustering under different quenching conditions in an Al-Mg-Si alloy

During quenching of aluminium alloys from the solutionising temperature vacancies are partially conserved as excess vacancies, partially lost to vacancy sinks, the exact fractions depending on the cooling rate. Positron lifetime measurements in samples from interrupted quenching experiments reveal that vacancies are lost during cooling down to 200 °C, after which solute atoms start to form clusters down to 20 °C. Slow cooling leads to 1 to 2 orders of magnitude lower excess vacancies than fast cooling. Since quenched-in vacancies are crucial for natural ageing (NA) in Al-Mg-Si alloys it is surprising to find just small differences between the NA hardening kinetics after different quenches. Specifically, hardening rates differ only in the initial stage (<100 min), after which they are almost identical for NA up to ~1 year. This suggests that interactions between vacancies and early-stage solute clusters help equalising the free vacancy fractions in differently quenched samples.

Defect cluster and nonequilibrium gas bubble associated growth in irradiated UMo fuels – A cluster dynamics and phase field model

Irradiation examination shows that gas bubble swelling kinetics are much faster after irradiation-induced recrystallization than that prior to recrystallization in U-10 wt% Mo alloy (UMo) fuels. These kinetics imply that gas bubbles in coarse grains and small recrystallized grains have different growth behavior. For the first time, researchers developed a phase-field model of gas bubble evolution integrating microstructure-dependent cluster dynamics to study the gas bubble swelling behavior in the recrystallization zone of UMo fuels. Generation, diffusion, reaction, sink, emission, and clustering of vacancies and interstitials are described by the cluster dynamics model while a phase-field model is used to describe the evolution of nonequilibrium gas bubbles including nucleation and growth. With the coupled model, the effect of defect generation rate, clustering rate, interstitial emission, and sink rates on grain boundaries on the gas bubble evolution are systematically simulated. A set of model parameters (defect generation rate, clustering rate, interstitial emission, and sink rates) is determined by comparing measured and simulated gas bubble swelling kinetics. The results demonstrate that interstitial clustering is one of the important physical mechanisms that results in fast gas bubble swelling kinetics in the recrystallization zone. The developed model can also be extended to study the associated growth of defect and second-phase precipitates often observed in irradiated materials.

Helium effect on the sink strength of grain boundaries in F82H

Abstract F82H were irradiated by Kr2+ ions with and without helium up to 1 displacement per atom at 500 °C, and cavity distribution around grain boundary was investigated by transmission electron microscopy. In F82H irradiated by Kr2+ ions, the cavity number density and the cavity size tended to be decreased and increased as a function of the distance from grain boundary, respectively. This gradient of cavity distribution seemed to be depending on the character of grain boundary. While, the size distribution of cavity seemed to be homogeneous in F82H irradiated by Kr2++He+ ions. This indicates that even prior austenite grain boundary hardly acts as sinks for vacancies if a high number density of helium atoms exists, which is probably due to the decrease in vacancy mobility by helium trapping.

Vacancy loops in Breakaway Irradiation Growth of zirconium: Insight from atomistic simulations

Irradiation of zirconium alloys by neutrons causes dimensional changes associated with the formation of dislocation loops. In undeformed single crystals of this hexagonal close-packed material, an expansion in the crystallographic <a>-direction and a contraction in the <c>-direction are observed as a function of neutron fluence consisting of three stages, namely a rapid initial change, followed by a plateau, and then an accelerated “breakaway” growth whilst the volume remains constant. Molecular dynamics simulations suggest an atomic-level explanation: the initial dimensional changes are related to the formation of <a>-type nano-clusters of self-interstitial atoms (SIAs) while vacancies remain mostly isolated. In the plateau-region, formation of <a>-type vacancy loops compensates for the effect of growing SIA clusters and SIA <a>-loops. Upon further irradiation, vacancy <c> loops grow while the anisotropically diffusing SIAs anneal preferentially vacancy <a>-loops. By having a preferential vacancy sink on the basal plane and a net flow of interstitials to <a>-loops, the dimensional changes accelerate, thus leading to breakaway growth. The present simulations yield values for the thermodynamic stability, strain fields, and their effect on dimensional changes for vacancy <a> and <c>-loops as a function of their size, thus providing critical input for continuum models of radiation-induced growth of zirconium.

In-situ irradiation tolerance investigation of high strength ultrafine tungsten-titanium carbide alloy

Refining grain size and adding alloying elements are two complementary approaches for enhancing the radiation tolerance of existing nuclear materials. Here, we present detailed in-situ irradiation research on defect evolution behavior and irradiation tolerance of ultrafine W-TiC alloys (thin foils) irradiated with 1 MeV Kr+2 at RT and 1073 K, and compare their overall performance to pure coarse grained tungsten. Loop Burgers vector was studied confirming the presence of <100> loops whose population increased at high temperature. Loop density, average loop area, and overall damage are reported as a function of irradiation dose revealing distinct defect evolution behavior from pure materials. The overall damage generally followed the average loop size trend, which decreased with time for both temperatures, but was higher at 1073 K and attributed to biased vacancy sink behavior of the TiC dispersoids evidenced by large vacancy clusters on their interfaces. By comparison, the overall loop and void damage in pure tungsten was larger by a factor of six and two, respectively. The improved irradiation damage resistance in the alloys is thus attributed to the effect of dispersoids in 1) the enhancement in annihilating defects and mutual defect recombination due to both dispersoids and a higher grain boundary density; 2) decreasing the loop mobility, causing shrinkage and annihilation of loop density, which was confirmed via in-situ video. Several mechanisms are illustrated to describe the performance of the complex alloy system. The results motivate further experimental and modeling research that aims to understand the many different phenomena occurring at different time scales.

Tailoring Ferroelectric Properties of 0.37BiScO3–0.63PbTiO3Thin Films Using a Multifunctional LaNiO3Interlayer

Functional oxide thin films integrated into CMOS technology often exhibit degraded properties with respect to bulk materials because of complex interfacial phenomena. In this contribution, we demonstrate that a sol-gel LaNiO3 (LNO) interlayer deposited onto the surface of a Pt/TiO2/SiO2/Si substrate prior to growth of sol-gel BiScO3-PbTiO3 (BSPT) films acts: i) to seed nucleation of perovskite structured; ii) to template growth to give controlled orientation and enhanced crystallinity and iii) as a sink for oxygen vacancies (VO). The LNO interlayer therefore not only improves the ferroelectric, piezoelectric and dielectric properties but also reduces leakage current and prevents degradation of the remanent polarization during fatigue tests. We propose that the use of a LNO interfacial layer may offer a generic solution to interfacial degradation in functional oxide films.

Complex damage distribution behaviour in cobalt implanted rutile TiO 2 (1 1 0) lattice

Abstract The present work investigates the radiation damage, amorphization and structural modifications that are produced by ion–solid interactions in TiO2 crystals during 200 keV Cobalt ion implantation. RBS/C and GIXRD have been utilized to evaluate the damage in the host lattice as a function of ion fluence. Multiple scattering formalism has been applied to extract the depth dependent damage distributions in TiO2(1 1 0). The results have been compared with the MC simulations performed using SRIM-2013. RBS/C results delineate a buried amorphous layer at a low fluence. Surprisingly, ion induced dynamic activation produces a recovery in this damage at higher fluences. This improvement interestingly occurs only in deep regions (60–300 nm) where a systematic lowering in damage with fluence is observed. Formation of Co-Ti-O phases and generation of stress in TiO2 lattice can also be responsible for this improvement in deep regions. In contrast, surface region (0–60 nm) indicates a gradual increase in damage with fluence. Such a switch in the damage behavior creates a cross point in damage profiles at 60 nm. Surface region is a sink of vacancies whereas deep layers are interstitial rich. However, these regions are far separated from each other resulting in an intermediate (100–150 nm) region with a significant dip (valley) in damage which can be characterized by enhanced recombination of point defects. The damage profiles thus indicate a very complex behavior. MC simulations, however, present very different results. They depict a damage profile that extends to a depth of only 150 nm, which is only about half of the damage- width observed here via RBS/C. Moreover, MC simulations do not indicate presence of any valley like structure in the damage profile. The complex nature of damage distribution observed here via RBS/C may be related to the high ionic nature of the chemical bonds in the TiO2 lattice.

Radiation instability of equal channel angular extruded T91 at ultra-high damage levels

Although several studies have shown that nano-grained alloys have higher radiation resistance than coarse-grained variants, with dense grain boundary structures acting as defect sinks to remove damage, the present study shows the failure of severe deformation to suppress void swelling in T91 alloy at ultra-high damage levels. Fe self-ion irradiation up to 1000 peak dpa produced significantly more swelling in extruded T91 than conventional T91 at 475 °C. Dramatic radiation-enhanced grain growth was observed in the extruded T91, manifested in two rapid grain growth stages. One stage occurs at a relatively low damage level, and the other at a higher damage level beyond 750 peak dpa. The second rapid growth stage is correlated with void swelling and starts at the end of the swelling incubation period. The study suggests that the defect sink properties of the growing grain boundaries under a coarsening process are different from that of stable boundary configurations. When void swelling becomes significant and voids act as a biased sink for vacancies, the growing grain boundaries act as a biased sink for interstitials, leading to rapid growth at higher damage levels.

Phosphorous Diffusion in N2+-Implanted Germanium during Flash Lamp Annealing: Influence of Nitrogen on Ge Substrate Damage and Capping Layer Engineering

In this work we present a systematic study on post-implantation phosphorous diffusion control in Ge by co-implanted nitrogen in combination with various surface capping layers (Al2O3, SiO2 and Si3N4). Phosphorous has been implanted at low energy (11 keV) and high dose (1015 cm−2) in p-Ge (100) already implanted or not with low energy (10 keV−5 × 1014 cm−2) N2+. Flash Lamp Annealing (FLA) at 800–850°C for 20 ms in inert ambient has been used as post-implantation annealing scheme. In the absence of nitrogen, significant substrate damage and capping layer deterioration prevents a reliable comparison among the three capping materials. The presence of nitrogen in the Ge substrate, effectively suppresses the damage observed after the FLA. In this case, P diffusion is additionally retarded in the presence of Al2O3 as compared to SiO2 and Si3N4. The experimental results constitute a direct evidence of the action of the three capping layers as sinks for Ge vacancies with different interface recombination velocities. On the contrary, the nitrogen diffusion data suggest that interface recombination velocities of Ge interstitials are almost independent of the capping layer choice.