Controllable Pseudoelasticity in Metallic Nanocrystals by Grain Boundary Engineering.

Materials with pseudoelasticity can recover from large strains exceeding their elastic limits during unloading, making them promising damage-tolerant building blocks for advanced nanodevices. Nevertheless, a practical approach to realize controllable pseudoelastic behavior at nanoscale remains challenging. Here, we proposed a grain boundary (GB) engineering approach to endow metallic nanocrystals with a controllable pseudoelasticity. Both in situ nanomechanical testing and atomistic simulations demonstrate that such controllable pseudoelasticity is governed by the extension and contraction of an inherent stacking fault array at the GB. By precisely tuning GB misorientation and inclination, our simulation results reveal that metallic nanocrystals can exhibit tailored pseudoelastic performance across a broad spectrum of GBs in different face-centered cubic metals. These findings enrich our understanding of the intrinsic pseudoelasticity of GBs and provide a GB engineering approach toward metallic materials with reversible deformability.

Inhibiting Mg Diffusion and Evaporation by Forming Mg-Rich Reservoir at Grain Boundaries Improves the Thermal Stability of N-Type Mg3 Sb2 Thermoelectrics.

N-type Mg3 Sb2 -based thermoelectric materials show great promise in power generation due to their mechanical robustness, low cost of Mg, and high figure of merit (ZT) over a wide range of temperatures. However, their poor thermal stability hinders their practical applications. Here, MgB2 is introduced to improve the thermal stability of n-type Mg3 Sb2 . Enabled by MgB2 decomposition, extra Mg can be released into the matrix for Mg compensation thermodynamically, and secondary phases of Mg─B compounds can kinetically prevent Mg diffusion along grain boundaries. These synergetic effects inhibit the formation of Mg vacancies at elevated temperatures, thereby enhancing the thermal stability of n-type Mg3 Sb2 . Consequently, the Mg3.05 (Sb0.75 Bi0.25 )1.99 Te0.01 (MgB2 )0.03 sample exhibits negligible variation in thermoelectric performance during the 120-hour continuous measurement at 673 K. Moreover, the ZT of n-type Mg3 Sb2 can be maintained by adding MgB2 , reaching a high average ZT of ≈1.1 within 300-723 K. An eight-pair Mg3 Sb2 -GeTe-based thermoelectric device is also fabricated, achieving an energy conversion efficiency of ≈5.7% at a temperature difference of 438 K with good thermal stability. This work paves a new way to enhance the long-term thermal stability of n-type Mg3 Sb2 -based alloys and other thermoelectrics for practical applications.

Grain-Boundary-Rich RuO2 Porous Nanosheet for Efficient and Stable Acidic Water Oxidation.

RuO2 has been considered as the most likely acidic oxygen evolution reaction (OER) catalyst to replace IrO2, but its performance, especially long-term stability under harsh acidic conditions, is still unacceptable. Here, we propose a grain boundary (GB) engineering strategy by fabricating the ultrathin porous RuO2 nanosheet with abundant of grain boundaries (GB-RuO2) as an efficient acid OER catalyst. The involvement of GB induces significant tensile stress and creates an unsaturated coordination environment, effectively optimizing the adsorption of intermediates and stabilizing active site structure during OER process. Notably, the GB-RuO2 not only exhibits a low overpotential (η10=187 mV) with an ultra-low Tafel slope (34.5 mV dec-1), but also steadily operates for over 550 h in 0.1 M HClO4. Quasi in situ/operando methods confirm that the improved stability is attributed to GB preventing Ru dissolution and greatly inhibiting the lattice oxygen oxidation mechanism (LOM). A proton exchange membrane water electrolysis (PEMWE) using the GB-RuO2 catalyst operates a low voltage of 1.669 V at 2 A cm-2 and operates stably for 100 h at 100 mA cm-2.

Exploration of Twin-Modified Grain Boundary Engineering in Metallic Copper Predominated Electromagnetic Wave Absorber.

High density and skin effect restrict the research progress of metal predominated electromagnetic wave absorbing (EMA) materials. Although some works try to solve it, they do not focus on the metal itself and do not involve the optimization of the active site of the inherent defects of the metal. In this work, the modulation of morphology, composition, interface, defects, and conductivity is achieved by adjusting the ratio of copper salt to reducing agent chitosan. Uniquely, the appearance of twin boundaries (TBs) accelerates the ability of the homogeneous interfaces to transfer charges, resists the oxidation of metal Cu0 , keeps the high electric conductivity of Cu0 nanoparticles, and enhances the conduction loss, which provides a boost for electromagnetic wave dissipation. As a result, the metal Cu0 predominated absorber (Cu-NC (N-doped carbon)-10,) exhibits an ultra-width effective absorption band of 8.28 GHz (9.72-18.00 GHz) at a thickness of 2.47 mm and the minimum reflection loss (RL) value of -63.8 dB with a thickness of 2.01 mm. In short, this work explores the EM regulation mechanism of TBs compared with grain boundaries (GBs), which provides a new insight for the rational design of metal predominated EMA materials.

Regain Strain-Hardening in High-Strength Metals by Nanofiller Incorporation at Grain Boundaries.

Grain refinement to the nano/ultrafine-grained regime can make metals several times stronger, but this process is usually accompanied by a dramatic loss of ductility. Such strength-ductility trade-off originates from a lack of strain-hardening capacity in tiny grains. Here, we present a strategy to regain the strain-hardening ability of high-strength metals by incorporation of extrinsic nanofillers at grain boundaries. We demonstrate that the dislocation storage ability in Cu grains can be considerably improved through this novel grain-boundary engineering approach, leading to a remarkably enhanced strain-hardening capacity and tensile ductility (uniform elongation). Experiments and large-scale atomistic simulations reveal that a key benefit of incorporated nanofillers is a reduction in the grain-boundary energy, enabling concurrent dislocation storage near the boundaries and in the Cu grain interior during straining. The strategy of grain-boundary engineering through nanofillers is easily controllable, generally applicable, and may open new avenues for producing nanostructured metals with extraordinary mechanical properties.

Grain boundary engineering for improved thin silicon photovoltaics.

In photovoltaic devices, the bulk disorder introduced by grain boundaries (GBs) in polycrystalline silicon is generally considered to be detrimental to the physical stability and electronic transport of the bulk material. However, at the extremum of disorder, amorphous silicon is known to have a beneficially increased band gap and enhanced optical absorption. This study is focused on understanding and utilizing the nature of the most commonly encountered Σ3 GBs, in an attempt to balance incorporation of the advantageous properties of amorphous silicon while avoiding the degraded electronic transport of a fully amorphous system. A combination of theoretical methods is employed to understand the impact of ordered Σ3 GBs on the material properties and full-device photovoltaic performance.

Chiral Twist Interface Modulation Enhances Thermoelectric Properties of Tellurium Crystal.

Manipulating the grain boundary and chiral structure of enantiomorphic inorganic thermoelectric materials facilitates a new degree of freedom for enhancing thermoelectric energy conversion. Chiral twist mechanisms evolve by the screw dislocation phenomenon in the nanostructures; however, contributions of such chiral transport have been neglected for bulk crystals. Tellurium (Te) has a chiral trigonal crystal structure, high band degeneracy, and lattice anharmonicity for high thermoelectric performance. Here, Sb-doped Te crystals are grown to minimize the severe grain boundary effects on carrier transport and investigate the interface of chiral Te matrix and embedded achiral Sb2Te3 precipitates, which induce unusual lattice twists. The low grain boundary scattering and conformational grain restructuring provide electrical-favorable semicoherent interfaces. This maintains high electrical conductivity leading to a twofold increase in power factor compared to polycrystal samples. The embedded Sb2Te3 precipitates concurrently enable moderate phonon scattering leading to a remarkable decrease in lattice thermal conductivity and a high dimensionless figure of merit (zT) of 1.1 at 623 K. The crystal growth and chiral atomic reorientation unravel the emerging benefits of interface engineering as a crucial contributor to effectively enhancing carrier transport and minimizing phonon propagation in thermoelectric materials.

Evolution of Grain Boundary Character Distribution in B10 Alloy from Friction Stir Processing to Annealing Treatment.

In this study, the grain boundary character distribution (GBCD) of a B10 alloy was optimized, employing thermomechanical processing consisting of friction stirring processing (FSP) and annealing treatment. Using electron backscatter diffraction, the effects of rotational speed of FSP and annealing time on the evolution of GBCD were systematically investigated. The GBCD evolution was analyzed concerning various parameters, such as the fraction of low-Σ coincidence site lattice (CSL) boundaries, the average number of grains per twin-related domain (TRD), the length of longest chain (LLC), and the triple junction distribution. The experimental results revealed that the processing of a 1400 rpm rotational speed of FSP followed by annealing at 750 °C for 60 min resulted in the optimum grain boundary engineering (GBE) microstructure with the highest fraction of low-Σ CSL boundaries being 82.50% and a significantly fragmented random boundary network, as corroborated by the highest average number of grains per TRD (14.73) with the maximum LLC (2.14) as well as the highest J2/(1 - J3) value (12.76%). As the rotational speed of FSP increased from 600 rpm to 1400 rpm, the fraction of low-Σ CSL boundaries monotonously increased. The fraction of low-Σ CSL boundaries first increased and then decreased with an increase in annealing time. The key to achieving GBE lies in inhibiting the recrystallization phenomenon while stimulating abundant multiple twinning events through strain-induced boundary migration.

Modulating Grain Boundary Networks to Achieve Superior Chemomechanical Coupling Properties in Nickel-Rich Cathode Materials.

To forge ahead with the next generation of power batteries boasting superior energy density, nickel-rich layered oxides are regarded as some of the most promising cathode materials. However, challenges such as microcracks, which are attributed to the elevated nickel content of the materials, have posed impediments to their further development and application. Consequently, this article focuses on the understanding of the materials in the deep delithiation state, dissecting their degradation mechanisms through a dual lens of electrochemical and mechanical properties. The comprehensive analysis reveals that microcracks within the particles exhibit a degree of reversibility. However, with repeated Li+ de-/intercalation, these microcracks progressively propagate and permeate the entire particle, ultimately leading to particle fragmentation. Therefore, this study employs Dy2O3 as an inducer to facilitate the growth of primary crystal grains, reducing the internal porosity of the particles. This effectively enhances the conductivity and lithium-ion diffusion kinetics in deep lithium-ion deintercalation states of nickel-rich cathode materials. The modified material exhibits significant suppression of microcrack formation and growth during cycling, leading to notable improvements in its chemical-mechanical properties. These degradation mechanisms and modification strategies of Ni-rich cathodes offer valuable insights into the development of Ni-rich cathode materials tailored for electric vehicles.

A Review on Controlling Grain Boundary Character Distribution during Twinning-Related Grain Boundary Engineering of Face-Centered Cubic Materials.

Grain boundary engineering (GBE) is considered to be an attractive approach to microstructure control, which significantly enhances the grain-boundary-related properties of face-centered cubic (FCC) metals. During the twinning-related GBE, the microstructures are characterized as abundant special twin boundaries that sufficiently disrupt the connectivity of the random boundary network. However, controlling the grain boundary character distribution (GBCD) is an extremely difficult issue, as it strongly depends on diverse processing parameters. This article provides a comprehensive review of controlling GBCD during the twinning-related GBE of FCC materials. To commence, this review elaborates on the theory of twinning-related GBE, the microscopic mechanisms used in the optimization of GBCD, and the optimization objectives of GBCD. Aiming to achieve control over the GBCD, the influence of the initial microstructure, thermo-mechanical processing (TMP) routes, and thermal deformation parameters on the twinning-related microstructures and associated evolution mechanisms are discussed thoroughly. Especially, the development of twinning-related kinetics models for predicting the evolution of twin density is highlighted. Furthermore, this review addresses the applications of twinning-related GBE in enhancing the mechanical properties and corrosion resistance of FCC materials. Finally, future prospects in terms of controlling the GBCD during twinning-related GBE are proposed. This study will contribute to optimizing the GBCD and designing GBE routes for better grain-boundary-related properties in terms of FCC materials.

Study on Stability of Mechanical Properties for Porous Fe-Cr-Al Alloys after Long-Term Aging.

Nowadays, both the ferrite phase and B2-structured intermetallic in the Fe-Cr-Al alloy system are developed as porous materials, which have been further applied as high-temperature filter materials in industry. This work presents a comparative study of the mechanical properties of porous Fe20Cr5Al, Fe10Cr10Al and Fe10Cr20Al aged at 480 °C for 500 h. The changes in tensile strength, elongation and hardness were determined, and the microstructure changes as well as slight oxidation states of the aged samples were investigated. The results show that the precipitated Cr-rich phase in porous Fe20Cr5Al can increase the hardness and decrease the ductility, while intergranular oxidation can degrade the mechanical performance of the three porous Fe-Cr-Al materials. It is noted that porous Fe10Cr20Al exhibits relatively superior mechanical stability during long-term aging. Meanwhile, by introducing boron, the mechanical performance of the aged porous Fe-Cr-Al alloys can be stabilized since the possible internal oxidation of the exposed grain boundaries is inhibited.

Slip-Line-Guided Growth of Graphene.

Manipulating the crystal orientation of emerging 2D materials via chemical vapor deposition (CVD) is a key premise for obtaining single-crystalline films and designing specific grain-boundary (GB) structures. Herein, the controllable crystal orientation of graphene during the CVD process is demonstrated on a single-crystal metal surface with preexisting atomic-scale stair steps resulting from dislocation slip lines. The slip-line-guided growth principle is established to explain and predict the crystal orientation distribution of graphene on a variety of metal facets, especially for the multidirectional growth cases on Cu(hk0) and Cu(hkl) substrates. Not only large-area single-crystal graphene, but also bicrystal graphene with controllable GB misorientations, are successfully synthesized by rationally employing tailored metal substrate facets. As a demonstration, bicrystal graphenes with misorientations of ≈21° and ≈11° are constructed on Cu(410) and Cu(430) foils, respectively. This guideline builds a bridge linking the crystal orientation of graphene and the substrate facet, thereby opening a new avenue for constructing bicrystals with the desired GB structures or manipulating 2D superlattice twist angles in a bottom-up manner.

Effect of Cold-Rolling Reduction on Recrystallization Microstructure, Texture and Corrosion Properties of the X2CrNi12 Ferritic Stainless Steel.

X2CrNi12 ferritic stainless steel has a wide range of application prospects in the railway transportation, construction, and automobile fields due to its excellent properties. The properties of X2CrNi12 ferritic stainless steel can be further improved by cold-rolling and subsequent annealing treatment. The purpose of this work is to investigate the effect of cold-rolling reduction on the microstructure, texture and corrosion properties of the recrystallized X2CrNi12 ferritic stainless steel by using SEM, TEM, EBSD and electrochemical testing technology. The results show that the crystal orientation characteristics of the cold-rolled sheet could be inherited into the annealed sheet. The higher cold-rolling reduction could promote the deformed grains rotating into the {111} orientation, increasing storage energy and driving force for recrystallization, which could reduce the recrystallized grain size. The orientation densities of α-fiber and γ-fiber were low at 50% cold-rolling reduction. After recrystallization annealing, a large number of grains with random orientation could be produced, and the texture strength was weakened. When the cold-rolling reduction rose to 90%, the γ-fiber texture at {111}<110> was strengthened and the α-fibers, particularly the {112}<110> component, were weakened after recrystallisation annealing, which could improve the formability of the steels. The proportions of special boundaries, i.e., low-angle grain boundaries and low-Σ CSL boundaries, among the grain boundary distribution of the recrystallized X2CrNi12 stainless steel were higher when the reduction was 90%, especially when the annealing temperature was 770 °C. Additionally, the proportion of LAGBs and low-Σ CSL boundaries were 53% and 7.43%, respectively, which improves the corrosion resistance of the matrix, showing the best corrosion resistance.

Improved Thermal Stability and Enhanced Thermoelectric Properties of p-Type BaCu2Te2 by Doping of Cl.

Doping in semiconductors is a widely implemented strategy for manipulation of carrier concentration, which is a critical parameter to regulate the thermoelectric performance. Stoichiometric BaCu2Te2 shows high hole concentration and unstable transport properties owing to the inherent Cu vacancy and dynamic precipitation behavior. In this work, Te has been partially substituted by Cl in BaCu2Te2 to suppress the overhigh hole concentration. Due to the high electronegativity of Cl, strong Cl-Cu bonds can significantly inhibit the Cu migration and the consequent dynamic precipitation. Meanwhile, nano-precipitate BaCl2 distributes in the grain boundary, acting as ionic blocking layers. Therefore, the thermal stability of the samples can be essentially improved via chemical bonding strengthening and grain boundary engineering. In terms of thermal transport, the introduced point defects and second phase strengthen the short-wavelength and medium-wavelength phonon scattering, leading to further reduced thermal conductivity. Eventually, the repeatable ZT value of BaCu2Te1.98Cl0.02 reached 1.22 at 823 K, which is higher by 19.6% compared with 1.02 of pristine BaCu2Te2. The average ZTs of BaCu2Te2-xClx (x = 0, 0.02, 0.04, and 0.06) in the temperature range of 323-823 K are 0.737 for x = 0.02, 0.689 for x = 0.04, and 0.667 for x = 0.06, which are 24.6, 17.2, and 13.4% higher than the average ZT of 0.588 corresponding to the undoped sample, respectively. The study shows that synergetic enhancements of thermal stability and thermoelectric properties can be achieved by strengthening chemical bonding and constructing ionic blocking layers in the grain boundary, which can be applied to other fast-ionic conductor thermoelectric materials.

Grain-Boundary Engineering of Monolayer MoS2 for Energy-Efficient Lateral Synaptic Devices.

Synaptic devices based on 2D-layered materials have emerged as high-efficiency electronic synapses and neurons for neuromorphic computing. Lateral 2D synaptic devices have the advantages of multiple functionalities by responding to diverse stimuli, but they consume large amounts of energy, far more than the human brain. Moreover, current lateral devices employ several mechanisms based on conductive filaments and grain boundaries (GBs), but their formation is random and difficult to control, also hindering their practical applications. Here, four-terminal, lateral synaptic devices with artificially engineered GBs are reported, which are made from monolayer MoS2 . With lithography-free, direct-laser-writing-controlled MoS2 /MoS2- x Oδ GBs, such synaptic devices exhibit short-term and long-term plasticity characteristics that are responsive to electric and light stimulation simultaneously. This enables detailed simulations of biological learning and cognitive processes as well as image perception and processing. In particular, the device exhibits low energy consumption, similar to that of the human brain and much lower than those of other lateral 2D synaptic devices. This work provides an effective way to fabricate lateral synaptic devices for practical application development and sheds light on controllable electrical state switching for neuromorphic computing.

Simultaneously Breaking the Double Schottky Barrier and Phonon Transport in SrTiO3-Based Thermoelectric Ceramics via Two-Step Reduction.

The low powder factor (PF) of polycrystalline oxide perovskites induced by the resistive grain boundaries or known as double Schottky barrier (DSB) greatly restricts their thermoelectric performance in application. Here, a general protocol including (i) powder and (ii) bulk reduction in H2/Ar forming gas is demonstrated to break the DSB in La and Nb codoped SrTiO3. While the powder reduction guarantees a high carrier concentration by fully stimulating the donor doping effect, the bulk reduction effectively lowers the DSB by influencing the point defects at grain boundaries, which is proved by the combination of cathode luminescence spectra and energy-dispersive X-ray spectroscopy in transmission electron microscopy. The Hall mobility can approach 10 cm2 V-1 s-1 after two-step reduction, which is similar to the level of single crystals. However, the Seebeck coefficient is not compromised, giving rise to high PF values up to 1.70 mW m-1 K-1 under proper reduction strength. Meanwhile, the reduction process also promotes mild precipitation of Nb nanoparticles, thus effectively lowering the lattice thermal conductivity by scattering phonons. As a result, a remarkable figure of merit reaching 0.4 at 700 K is obtained, which validates the two-step reduction as a reliable strategy toward "electron crystal-phonon glass" behavior in SrTiO3-based perovskites.

Evaluation of Grain Boundary Network and Improvement of Intergranular Cracking Resistance in 316L Stainless Steel after Grain Boundary Engineering.

For understanding the improvement of intergranular stress corrosion cracking (IGSCC) propagation in grain boundary engineering (GBE)-processed metals exposed to a simulated pressurized water reactor (PWR) environment, characteristics of the grain boundary network of 316L stainless steel before and after GBE were investigated and compared, including proportions both in length and in number of ∑3n boundaries, sizes, and topology of grain clusters (or twin-related domains), and connectivity of random boundaries. The term through-view random boundary path (TRBP) was proposed to evaluate the random boundary connectivity. A TRBP is a chain of end-to-end connected crack-susceptible boundaries that passes through the entire mapped microstructure. The work provides the following key findings: (I) the length fraction of ∑3n boundaries was increased to approximately 75% after GBE, but the number fraction was only approximately 50%; (II) a connected non-twin boundary network still existed in the GBE sample due to the formation of grain clusters; (III) the GBE sample exhibited a higher resistance to IGSCC; and (IV) as the twin boundary fraction increased, the number of TRBPs decreased and the normalized length of the minimum TRBP increased monotonically, leading to a higher resistance to IGSCC.

Impurity Segregation and Nanoparticle Reorganization of Indium Doped MgO Cubes.

Metal oxide nanocomposites are non-equilibrium solids and promising precursors for functional materials. Annealing of such materials can provide control over impurity segregation and, depending on the level of consolidation, represents a versatile approach to engineer free surfaces, particle-particle interfaces and grain boundaries. Starting with indium-magnesium-oxide nanoparticle powders obtained via injection of an indium organic precursor into the magnesium combustion flame and subsequent particle quenching in argon, we investigated the stability of the trivalent In3+ ions in the host lattice of MgO nanoparticles by determining grain growth, morphology evolution and impurity segregation. The latter process is initiated by vacuum annealing at 873 K and can be tracked at 1173 K on a time scale of minutes. In the first instance the surface segregated indium wets the nanoparticle interfaces. After prolonged annealing indium evaporates and leaves the powder via the gas phase. Resulting MgO nanocubes are devoid of residual indium, regain their high morphological definition and show spectroscopic fingerprints (UV Diffuse Reflectance and Photoluminescence emission) that are characteristic of electronically unperturbed MgO cube corner and edge features. The results of this combined XRD, TEM, and spectroscopy study reveal the parameter window within which control over indium segregation is used to introduce a semiconducting metal oxide component into the intergranular region between insulating MgO nanograins.

Grain-Boundary "Patches" by In Situ Conversion to Enhance Perovskite Solar Cells Stability.

The power conversion efficiency of organic-inorganic hybrid perovskite solar cells has increased rapidly, but the device stability remains a big challenge. Previous studies show the grain boundary (GB) can facilitate ion migration and initiate device degradation. Herein, methimazole (MMI) is employed for the first time to construct a surface "patch" by in situ converting residual PbI2 at GBs. The resultant MMI-PbI2 complex can effectively suppress ion migration and inhibit diffusion of the metal electrodes. The origin of the surface "patch" effect and their working mechanisms are investigated experimentally and theoretically at the microscopic level. It hence demonstrates a simple and effective method to prolong the device stability in the context of GB engineering, which could be extensively applied to perovskite-based optoelectronics.

Three-dimensional geometrical and topological characteristics of grains in conventional and grain boundary engineered 316L stainless steel.

The three-dimensional microstructures of a conventional 316L stainless steel and the same material after grain boundary (GB) engineering have been measured by serial sectioning coupled with electron backscatter diffraction mapping. While it is well known that GB engineered materials are differentiated from conventional materials because of the proportion of coincidence site lattice boundaries, the size of their twin-related domains, and their reduced random boundary connectivity, this work provides a quantitative comparison of the geometrical and topological characteristics of grains in 316L stainless steel before and after GB engineering. Specifically, the numbers of grain faces, triple lines, and quadruple unions per grain have been measured and compared. In addition, the distributions of grain sizes, surface areas, and grain boundary areas have been measured and compared. The results show that, in many ways, the three-dimensional geometrical and topological characteristics of the grains in the GB engineered and conventional materials are similar. In both materials, the distributions of the geometrical parameters are well represented by a log-normal distribution. Comparatively, the GB engineered microstructure has grains that, on average, have both fewer faces and higher (specific) surface areas that deviate more from an ideal equiaxed shape, but there are several eccentric or non-compact shaped grains that have a huge number of faces and extremely large surface area in the GB engineered material. All of these characteristics are likely to be a result of the increased number of twins in the GB engineered microstructure. These eccentric grains would have a positive influence on increasing the resistance to intergranular degradation.