Mobile Defects Research Articles

Role of Sintering Aids in Electrical and Material Properties of Yttrium- and Cerium-Doped Barium Zirconate Electrolytes

Ceramic proton conductors have the potential to lower the operating temperature of solid oxide cells (SOCs) to the intermediate temperature range of 400–600 °C. This is attributed to their superior ionic conductivity compared to oxide ion conductors under these conditions. However, prominent proton-conducting materials, such as yttrium-doped barium cerates and zirconates with specified compositions like BaCe1−xYxO3−δ (BCY), BaZr1−xYxO3−δ (BZY), and Ba(Ce,Zr)1−yYyO3−δ (BCZY), face significant challenges in achieving dense electrolyte membranes. It is suggested that the incorporation of transition and alkali metal oxides as sintering additives can induce liquid phase sintering (LPS), offering an efficient method to facilitate the densification of these proton-conducting ceramics. However, current research underscores that incorporating these sintering additives may lead to adverse secondary effects on the ionic transport properties of these materials since the concentration and mobility of protonic defects in a perovskite are highly sensitive to symmetry change. Such a drop in ionic conductivity, specifically proton transference, can adversely affect the overall performance of cells. The extent of variation in the proton conductivity of the perovskite BCZY depends on the type and concentration of the sintering aid, the nature of the sintering aid precursors used, the incorporation technique, and the sintering profile. This review provides a synopsis of various potential sintering techniques, explores the influence of diverse sintering additives, and evaluates their effects on the densification, ionic transport, and electrochemical properties of BCZY. We also report the performance of most of these combinations in an actual test environment (fuel cell or electrolysis mode) and comparison with BCZY.

Effect of irradiation temperature on the mobility of structural and vacancy defects in the damaged layer of Li2ZrO3 ceramics

The paper presents the assessment results of the irradiation temperature effect on the change in the type of structural defects caused by irradiation with helium ions in the near-surface layer of Li2ZrO3 ceramics, as well as determining the nature of structural damage using the electron paramagnetic resonance method in the case of irradiation fluence variation. During characterization of the structural changes caused by helium ion irradiation, the main attention was paid to detailing the type of structural defects and radiolysis products arising during irradiation, alongside alterations in their concentration using the electron paramagnetic resonance method. During the experiments, it was determined that the observed effects of thermally stimulated mobility of vacancy defects in the damaged layer caused by irradiation with helium ions indicate a positive effect of thermal heating of samples during irradiation at low fluences. This effect consists in a decline in the number of oxygen vacancies in the damaged layer at low irradiation fluences (1015 – 1016 cm-2), the concentration of which was determined using the EPR method, and the observed structural changes at low irradiation fluences in the case of an elevation in irradiation temperatures are associated with deformation distortion of the crystalline structure of ceramics caused by transformation energy processes, and thermal expansion effects.

Displacement cascade bombardment of delta-hydrides in alpha-zirconium

One of the factors affecting the in-pile performance of Zr-based alloys is the precipitation of hydrides once H concentrations exceed the terminal solubility limit. H transport and hydride precipitation/dissolution is commonly modeled in codes such as BISON, but most of the experimental data supporting these models has been collected on unirradiated materials. As such, there is considerable uncertainty as to the influence of irradiation effects. In this work, molecular dynamics simulations of displacement cascades were performed on δ-hydrides to elucidate: 1) the extent of H dissolution following cascade impacts and 2) any alterations to defect production characteristics when compared to cascades in bulk Zr. The immediate amount of H dissolved in a high-energy cascade impact is notable, but a considerable fraction of the dissolved H atoms are rapidly re-absorbed into the hydride at reactor-relevant temperatures. The amount of dissolved H also decreases with increasing hydride size. When considering the expected volume fractions of hydrides, it is not expected that the irradiation-induced H dissolution rate will significantly affect the availability of H in the Zr lattice. In terms of defect production, cascades which overlap δ-hydrides produced an order of magnitude more stable defects than equivalent-energy cascades in bulk Zr. Vacancy defects are predominantly contained within the hydride structure while interstitials clusters are found adjacent to the hydride surface. Interstitials are strongly repelled by the hydride structure which may drive the expulsion of cascade-generated interstitials to the hydride surface and impede athermal recombination. Thus, the interatomic potential used in this work predicted a significant alteration to the defect survival efficiency and a stark production bias in the availability of mobile defects in bulk Zr following hydride-overlapped displacement cascades.

Polaron-assisted dielectric relaxation processes in donor-doped BaTiO3-based ceramics

Ceramic samples based on the Ba1-xGdxTiO3 system, where x = 0.001, 0.002, 0.003, 0.004 and 0.005, were prepared via the Pechini’s chemical synthesis route. Structural properties, analyzed from X-ray diffraction and Rietveld refinement, revealed the formation of the pure ABO3 perovskite structure with tetragonal symmetry (P4mm) for all the studied compositions. Doping with Gd3+ promoted a reduction in the unit-cell volume, confirming the preferential substitution of the rare-earth cation at the A-site. The dielectric properties have been analyzed over a wide temperature and frequency range, revealing a significant contribution of the conduction mechanisms in the dielectric response of the studied ceramics. In fact, by using the Davidson-Cole formalism, the observed electrical behavior was found to be associated with relaxation processes related to intrinsic defects mobility promoted by a thermally-activated polaronic mechanism. The obtained values of the activation energy for the relaxation processes, estimated from the Arrhenius’ law for the mean relaxation time, revealed a decrease from 0.29 up to 0.21 eV as the Gd-doping concentration increases, which suggests the conduction process to be associated with the polaronic effects due to the coexistence of Ti4+ and Ti3+ ions in the structure. Analysis from the conductivity formalism, by using the Jonscher’s universal power-law, confirmed the polaron-type conduction mechanism for the dielectric dispersion, as suggested by the dielectric analysis, being the nature of the hopping mechanism governed by small polaron hopping (SPH) charge transport in the studied Ba1–xGdxTiO3 ceramics.

Dynamic Deformation in Nuclear Graphite and Underlying Mechanisms.

Time-dependent deformation in nuclear graphite is influenced by the creation and migration of radiation-induced defects in the reactor environment. This study investigates the role of pre-existing defects such as point defect clusters and Mrozowski cracks in nuclear graphite IG-110. Separate specimens were irradiated with a 2.8 MeV Au2+ beam with a fluence of 4.38 × 1014 cm-2 and an 8 MeV C2+ beam with a fluence of 1.24 × 1016 cm-2. Microscopic specimens were either mechanically loaded inside a transmission electron microscope (TEM) or subjected to ex situ indentation-based creep loading. In situ TEM tests showed significant plasticity in regions highly localized around the Mrozowski cracks, resembling slip or ripplocation bands. Slip bands were also seen near regions without pre-existing defects but at very high stresses. Ex situ self-ion irradiation embrittled the specimens and decreased the creep displacement and rate, while heavy ion irradiation resulted in the opposite behavior. We hypothesize that the large-sized gold ions (compared to the carbon atoms) induced interplanar swelling as well as cross-plane channels for increased defect mobility. These findings illustrate the role of pre-existing defects in the dynamic relaxation of stresses during irradiation and the need for more studies into the radiation environment's impact on the mechanical response of nuclear graphite.

Heterogeneity in Point Defect Distribution and Mobility in Solid Ion Conductors.

Alkali metal anodes paired with solid ion conductors offer promising avenues for enhancing battery energy density and safety. To facilitate rapid ion transport crucial for fast charging and discharging of batteries, it is essential to understand the behavior of point defects in these conductors. In this study, we investigate the heterogeneity of defect distribution in two prototypical solid ion conductors, Li3OCl and Li2PO2N (LiPON), by quantifying the defect formation energy (DFE) as a function of distance from the surface and interface through first-principles simulations. To simulate defects at the electrode-electrolyte interface, we perform calculations of Li+ vacancy in Li3OCl near its interface with lithium metal. Our results reveal a significant difference between the bulk and surface/interface DFE which could lead to defect aggregation/depletion near the surface/interface. Interestingly, while Li3OCl has a lower surface DFE than the bulk in most cases, LiPON follows the opposite trend with a higher surface DFE compared to the bulk. Due to this difference between bulk and surface DFE, the defect density can be up to 14 orders of magnitude higher at surfaces compared to the bulk. Further, we reveal that the DFE transition from surface/interface to bulk is precisely characterized by an exponentially decaying function. By incorporating this exponential trend, we develop a revised model for the average behavior of defects in solid ion conductors that offers a more accurate description of the influence of grain sizes. Surface effects dominate for grain sizes ≲1 μm, highlighting the importance of surface defect engineering and the DFE function for accurately capturing ion transport in devices. We further explore the kinetics of defect redistribution by calculating the migration barriers for defect movement between bulk and surfaces. We find a highly asymmetric energy landscape for the lithium vacancies, exhibiting lower migration barriers for movement toward the surface compared to the bulk, while interstitial defects exhibit comparable kinetics between surface and bulk regions. These insights highlight the importance of considering both thermodynamic and kinetic factors in designing solid ion conductors for improved ion transport at surfaces and interfaces.

Mobile intrinsic point defects for conductive neutral domain walls in LiNbO3.

Conductive ferroelectric domain walls (DWs) hold great promise for neuromorphic nanoelectronics as they can contribute to realize multi-level diodes and nanoscale memristors. Point defects accumulating at DWs will change the local electrical transport properties. Hence, local, inter-switchable n- and p-type conductivity at DWs can be achieved through point defect population control. Here, we study the impact of point defects on the electronic structure at neutral domain walls in LiNbO3 by density functional theory (DFT). Segregation of Li and O vacancies was found to be energetically favourable at neutral DWs, implying that charge-compensating electrons or holes can give rise to n- or p-type conductivity. Changes in the electronic band gap and defect transition levels are discussed with respect to local property engineering, opening the pathway for reversible tuning between n- and p-type conduction at neutral ferroelectric DWs. Specifically, the high Curie temperature of LiNbO3 and the significant calculated mobility of O and Li vacancies suggest that thermal annealing and applied electric fields can be used experimentally to control point defect populations, and thus enable rewritable pn-junctions.

Temperature-dependent electron–phonon coupling changes the damage cascades in neutron-irradiation molecular dynamics simulation in W

We present a first-principles-based electron-temperature model that can be used in atomistic calculations. The electron–phonon coupling coefficient in the model is derived from the density of states as a function of electron temperature, and the thermal conductivity of tungsten from our model shows significant improvement over the baseline atomistic calculations in which only ion-thermal contribution to the thermal conductivity is available. The correction to the thermal conductivity also changes damage cascades as cascades cool down more rapidly within our model. The mobility of defects is consequently reduced, leaving more residual damage than the predictions without an electron-temperature model.

Pt nanoparticles in cooperation with Mn-P composite drive base-free selective oxidation of 5-hydroxymethylfurfural

Selective oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA) is a key approach to sustainable upgrading of furfural based biomass resources. In this work, small Pt nanoparticles (about 2.1 nm) loaded on MnPnOx composite were shown to effectively drive the base-free aerobic oxidation of HMF to FDCA in water. Regulating the P content in carrier enabled to tune HMF conversion and product distribution. The optimal Pt/MnP0.5Ox catalyst can reach 98% yield of FDCA at 110 °C under 10 bar of O2 after 24 h. It also showed the highest initial conversion rate of HMF (13 mmol molPt−1 s−1) and productivity of FDCA (4.1 mmol molPt−1 h−1) among all the Pt/MnPnOx catalysts. The carrier effect of P addition on promoting Pt oxidation catalysis was elucidated by using rigorous kinetic investigations and comprehensive characterizations. The initial conversion rate, rate constant and apparent activation energy were independently measured on oxidation of HMF and its derived intermediates. ICP-MS, XRD, TEM, H2-TPR, O2-TPD and XPS were used to acquire catalytic properties. It was demonstrated that P addition led to changes in carrier structure and metal-support interaction, which eventually promoted the formation of highly active electron-rich Pt0 sites and mobile reactive defect oxygen species. These features allowed improving the selective oxidation catalysis of supported-Pt nanoparticles for the base-free conversion of HMF to FDCA.

Phenotypic continuum and poor intracytoplasmic sperm injection intracytoplasmic sperm injection prognosis in patients harboring HENMT1 variants.

Small RNAs interacting with PIWI (piRNAs) play a crucial role in regulating transposable elements and translation during spermatogenesis and are essential in male germ cell development. Disruptions in the piRNA pathway typically lead to severe spermatogenic defects and thus male infertility. The HENMT1 gene is a key player in piRNAs primary biogenesis and dysfunction of HENMT1 protein in meiotic and haploid germ cells resulted in the loss of piRNA methylation, piRNA instability, and TE de-repression. Henmt1-knockout mice exhibit a severe oligo-astheno-teratozoospermia (OAT) phenotype, whereas patients with HENMT1 variants display more severe azoospermia phenotypes, ranging from meiotic arrest to hypospermatogenesis. Through whole-exome sequencing (WES) of infertile patient cohorts, we identified two new patients with variants in the HENMT1 gene presenting spermatozoa in their ejcaulate, providing us the opportunity to study spermatozoa from these patients. Investigate the spermatozoa of two patients harboring an HENMT1 variant to determine whether or not these scarce spermatozoa could be used with assisted reproductive technologies. HENMT1 variants identified by WES were validated through Sanger sequencing. Comprehensive semen analysis was conducted, and sperm cells were subjected to transmission electron microscopy for structural examination, in situ hybridization for aneuploidy assessment, and aniline blue staining for DNA compaction status. Subsequently, we assessed their suitability for in vitro fertilization using intracytoplasmic sperm injection (IVF-ICSI). Our investigations revealed a severe OAT phenotype similar to knockout mice, revealing altered sperm concentration, mobility, morphology, aneuploidy and nuclear compaction defects. Multiple IVF-ICSI attempts were also performed, but no live births were achieved. We confirm the crucial role of HENMT1 in spermatogenesis and highlight a phenotypic continuum associated with HENMT1 variants. Unfortunately, the clinical outcome of these genetic predispositions remains unfavorable, regardless of the patient's phenotype. The presence of spermatozoa is insufficient to anticipate ICSI pregnancy success in HENMT1 patients.

Phase and defect structures of MnO2‐doped 75BiFeO3–25BaTiO3 ceramics and their effects on electrical properties

AbstractBismuth ferrite (BiFeO3)‐based high‐temperature piezoelectric ceramics have high Curie temperatures and excellent thermal stability; however, their applications are limited by inadequate piezoelectric performance due to large coercive fields and high leakage conduction. This study investigates the impact of MnO2 doping on the phase transition, defect structure, domain morphology, as well as the dielectric, ferroelectric, and electrical conduction behaviors of 75BiFeO3–25BaTiO3 ceramics sintered under an oxygen atmosphere. A structural transformation from distorted rhombohedral to pseudo‐cubic symmetry was observed with increasing MnO2 content, accompanied by domain fragmentation. MnO2 doping at an appropriate level inhibited defects such as and . Regarding electrical properties, the reduction in rhombohedral distortion and enhancement in the pseudo‐cubic phase after MnO2 doping resulted in decreased remanent polarization and electrostrain, which could potentially hinder piezoelectric response. However, appropriate MnO2 doping effectively reduced leakage conduction, significantly enhancing the poling efficiency of the ceramics. The maximum d33 value of 107 pC/N and kp of 0.36 were achieved at an MnO2 doping content of 0.1 wt.%. Excessive MnO2 addition may generate defect dipoles, as indicated by increased coercive field (Ec) and dielectric loss. The observed increase in high‐temperature resistivity with MnO2 content is primarily attributed to reduced grain size and increased concentration of mobile defects. This study provides valuable insights for further research on the application of 75BiFeO3–25BaTiO3 systems in high‐temperature piezoelectric devices.

Bilateral Chemical Linking at NiOx Buried Interface Enables Efficient and Stable Inverted Perovskite Solar Cells and Modules

AbstractInverted NiOx‐based perovskite solar cells (PSCs) exhibit considerable potential because of their low‐temperature processing and outstanding excellent stability, while is challenged by the carriers transfer at buried interface owing to the inherent low carrier mobility and abundant surface defects that directly deteriorates the overall device fill factor. Present work demonstrates a chemical linker with the capability of simultaneously grasping NiOx and perovskite crystals by forming a Ni−S−Pb bridge at buried interface to significantly boost the carriers transfer, based on a rationally selected molecule of 1,3‐dimethyl‐benzoimidazol‐2‐thione (NCS). The constructed buried interface not only reduces the pinholes and needle‐like residual PbI2 at the buried interface, but also deepens the work function and valence band maximum positions of NiOx, resulting in a smaller VBM offset between NiOx and perovskite film. Consequently, the modulated PSCs achieved a high fill factor up to 86.24 %, which is as far as we know the highest value in records of NiOx‐based inverted PSCs. The NCS custom‐tailored PSCs and minimodules (active area of 18 cm2) exhibited a champion efficiency of 25.05 % and 21.16 %, respectively. The unencapsulated devices remains over 90 % of their initial efficiency at maximum power point under continuous illumination for 1700 hours.

Bilateral Chemical Linking at NiOx Buried Interface Enables Efficient and Stable Inverted Perovskite Solar Cells and Modules.

Inverted NiOx-based perovskite solar cells (PSCs) exhibit considerable potential because of their low-temperature processing and outstanding excellent stability, while is challenged by the carriers transfer at buried interface owing to the inherent low carrier mobility and abundant surface defects that directly deteriorates the overall device fill factor. Present work demonstrates a chemical linker with the capability of simultaneously grasping NiOx and perovskite crystals by forming a Ni-S-Pb bridge at buried interface to significantly boost the carriers transfer, based on a rationally selected molecule of 1,3-dimethyl-benzoimidazol-2-thione (NCS). The constructed buried interface not only reduces the pinholes and needle-like residual PbI2 at the buried interface, but also deepens the work function and valence band maximum positions of NiOx, resulting in a smaller VBM offset between NiOx and perovskite film. Consequently, the modulated PSCs achieved a high fill factor up to 86.24 %, which is as far as we know the highest value in records of NiOx-based inverted PSCs. The NCS custom-tailored PSCs and minimodules (active area of 18 cm2) exhibited a champion efficiency of 25.05 % and 21.16 %, respectively. The unencapsulated devices remains over 90 % of their initial efficiency at maximum power point under continuous illumination for 1700 hours.

Displacement cascade and defect-driven simulations in V-Ti-Ta-Nb high-entropy alloy

High-entropy alloys (HEA) have attracted considerable attention in the development of nuclear materials due to their excellent properties. In this study, the primary irradiation damage and long-term defect evolution of pure V, V-5Ti-5Ta conventional alloy, V-Ti-Ta MEA and V-Ti-Ta-Nb HEA were simulated by molecular dynamics (MD) to understand the irradiation resistance mechanism of these four systems. The primary irradiation damage simulation results indicate that the V-Ti-Ta MEA and V-Ti-Ta-Nb HEA exhibit a delayed thermal peak, a longer defect recombination time and a slightly higher number of final Frenkel pairs (FPs) than pure V and V-5Ti-5Ta alloy. Both primary irradiation damage and long-time defect evolution results show that V-Ti-Ta MEA and V-Ti-Ta-Nb HEA have lower defect clustering fraction, cluster size and dislocation loop size than pure V and V-5Ti-5Ta. This is because V-Ti-Ta MEA and V-Ti-Ta-Nb HEA exhibit lower dislocation loop binding energy and defect mobility compared to pure V and V-5Ti-5Ta alloy. This study investigates the reasons for the better radiation resistance of V-Ti-Ta MEA and V-Ti-Ta-Nb HEA than pure V and V-5Ti-5Ta conventional alloys.

Engineering the Hole Transport Layer with a Conductive Donor-Acceptor Covalent Organic Framework for Stable and Efficient Perovskite Solar Cells.

Spiro-OMeTAD doped with lithium-bis(trifluoromethylsulfonyl)-imide (Li-TFSI) and tertbutyl-pyridine (t-BP) is widely used as a hole transport layer (HTL) in n-i-p perovskite solar cells (PSCs). Spiro-OMeTAD based PSCs typically show poor stability owing to the agglomeration of Li-TFSI, the migration of lithium ions (Li+), and the existence of potential mobile defects originating from the perovskite layer. Thus, it is necessary to search for a strategy that suppresses the degradation of PSCs and overcomes the Shockley Queisser efficiency limit via harvesting excess energy from hot charge carrier. Herein, two covalent organic frameworks (COFs) including BPTA-TAPD-COF and a well-defined donor-acceptor COF (BPTA-TAPD-COF@TCNQ) were developed and incorporated into Spiro-OMeTAD HTL. BPTA-TAPD-COF and BPTA-TAPD-COF@TCNQ could act as multifunctional additives of Spiro-OMeTAD HTL, which improve the photovoltaic performance and stability of the PSC device by accelerating charge-carrier extraction, suppressing the Li+ migration and Li-TFSI agglomeration, and capturing mobile defects. Benefiting from the increased conductivity, the addition of BPTA-TAPD-COF@TCNQ in the device led to the highest power conversion efficiency of 24.68% with long-term stability in harsh conditions. This work provides an example of using COFs as additives of HTL to enable improvements of both efficiency and stability for PSCs.

Molecular packing regulation of dopant-free hole transport polymers for efficient perovskite solar cells

Spiro-OMeTAD is a primary hole transport material (HTM) employed in most state-of-the-art regular perovskite solar cells (PSCs). The essential reliance on hygroscopic ionic dopants to enhance the conductivity and mobility of Spiro-OMeTAD has dramatically compromised the stability of PSCs. Here, we demonstrated excellent photovoltaic performance of PSCs by developing two dopant-free polymers, namely L1 and L2, using thieno[3,2-b]thiophene as a building block. It is found that the n-hexyl-modified thiophene side chains endow the polymer L2 with favorable crystallinity, unique self-assembly behavior, and a preferable face-on stacking orientation. After the addition of a small amount (10 %) of PM6 to create a polymer alloy named LPA, the above properties were further improved, and the resulting film exhibited a distinct fibrous morphology, resulting in increased hole mobility and effective defect passivation. Consequently, PSCs employing LPA as a dopant-free HTM afforded a high efficiency of 23.81 %. Importantly, LPA-based PSCs exhibit significantly enhanced operational stability with a T80 lifetime of 1572 h at 55 °C. This work provides a crucial guideline for the design of dopant-free polymers, thereby advancing the practical application of PSCs.

In-situ impedance spectroscopy on copper oxide scales under potential controlled oxidation at high temperatures

In recent years, copper oxide (Cu2O) has gained significant attention for its multifaceted role as a catalyst, photocatalyst and semiconductor. Possessing a p-type conductivity and a bandgap of 2.17 eV, Cu2O emerges as an ideal candidate for applications such as water splitting and photovoltaic cells. However, to utilize its full potential, an in-depth understanding of the electrical properties of Cu2O in terms of defect density and defect mobility is required. This knowledge forms the basis for the targeted production of oxides with precisely tuned electrical properties.Extensive research on thermal oxidation under varied conditions has been performed to clarify the mechanisms governing oxide formation and growth. However, the in-situ investigation of the electrical properties of the oxide layers during their formation process is challenging.This work introduces a new method for the controlled oxidation of copper, while concurrently offering an in-depth investigation of the electrical properties of these nanometer-thick oxide layers. The implemented arrangement of potentiostat and frequency response analyzer enables their artifact-free combination. Performing HT-CV measurements in a solid electrolyte cell-setup allows to control the (potential-dependent) oxygen pressure at the copper surface. Using HT-CV, the current represents the kinetics of the redox reactions and the electric charge corresponds to the amount of substance converted. In-situ electrochemical impedance data are used to determine the electrical properties of the controlled grown layers. As a result, the defect density is obtained as a function of the oxygen partial pressure.

Differences in coupling between nuclear and electronic energy losses in UO2 with irradiation temperature: An in situ TEM study

To investigate the coupling between nuclear and electronic energy losses in UO2, we irradiated thin foils with 0.39 MeV Xe and/or 6 MeV Si ions at 93 K using single or simultaneous dual beam ion irradiations. The evolution of perfect dislocation loops was characterized by in situ transmission electron microscopy (TEM). Additional ex situ TEM characterizations at room temperature revealed for the first time in UO2 the presence of faulted Frank loops too small to be measured during in situ experiments and conventional bright field kinematical imaging conditions.For the single Xe irradiation, which favor dominant ballistic energy losses, we observed a continuous nucleation of small perfect dislocation loops, which increase in size for our last fluences by growing through mainly coalescence effect. Both the single Si and dual Xe & Si irradiations showed a coupling between nuclear and electronic energy losses, resulting in a significant loop density increase and a tangled line network formation, respectively. These phenomena occur at lower dpa levels, compared to the single Xe irradiation, likely resulting from the thermal spike effect of Si ions. The present results were compared to our previous work at 293 K to investigate the role of irradiation temperature on the energy losses coupling. For the Xe irradiation, the density increases and the loops are smaller at 93 K compared to 293 K, resulting from the uranium interstitials mobility being prevented or allowed. For the Si irradiation, the dislocation evolution kinetics are similar at both temperatures. The electronic excitations effect seems greater than the irradiation temperature effect in this temperature range. For the Xe & Si irradiation, the loop kinetics change resulting in a tangled line network formation is faster and thus the loop transformation into lines occurs at lower dpa levels at 93 K compared to 293 K. It appears that the irradiation temperature affecting the mobility of some small point defects reduces the electronic excitation effect in this case.

Controlling Chaos: Periodic Defect Braiding in Active Nematics Confined to a Cardioid.

This work examines self-mixing in active nematics, a class of fluids in which mobile topological defects drive chaotic flows in a system comprised of biological filaments and molecular motors. We present experiments that demonstrate how geometrical confinement can influence the braiding dynamics of the defects. Notably, we show that confinement in cardioid-shaped wells leads to realization of the golden braid, a maximally efficient mixing state of exactly three defects with no defect creation or annihilation. We characterize the golden braid state using different measures of topological entropy and the Lyapunov exponent. In particular, topological entropy measured from the stretching rate of material lines agrees well with an analytical computation from braid theory. Increasing the size of the confining cardioid produces a transition from the golden braid, to the fully chaotic active turbulent state.

Electrochemical grand potential-based phase-field simulation of electric field-assisted sintering

An electrochemical grand potential functional was proposed to describe the sintering of an ionic ceramic green body. The resultant phase-field description enables simulation of the consolidation of an arbitrary number of granular particles and their interactions with the surrounding void phase. The model includes the effects of charged vacancies and the associated interactions between internal and applied electric fields. Defect segregation to grain boundaries is also accounted for, as well as enhanced interfacial defect mobilities. The model was parameterized for Y2O3. Simulations of two-particle systems showed that the applied electric field had an increasingly important impact on neck growth as particle size increased. A sudden rapid increase in temperature occurred for larger field strengths, which has been reported to be correlated to the onset of a flash event in flash sintering. Simulations of many particles showed that internal heat generation by Joule heating was localized at particle–particle contacts (grain boundaries), even though their conductivities were lower than nearby internal particle-void interfaces. A percolative path for ionic charge across the green body and the ceramic sintered solid was thus defined, accelerating the Joule heating process as the porosity of the green body is removed.