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Transition from source- to stress-controlled plasticity in nanotwinned materials below a softening temperature (低于软化温度时纳米孪晶材料塑性的源-控向应力-控转变)
Seyedeh Mohadeseh, Taheri Mousavi, Haofei Zhou, Guijin Zou & Huajian Gao 
npj Computational Materials 5:2 (2019)
doi:s41524-018-0140-5
Published online:04 January 2019
Abstract| Full Text | PDF OPEN

摘要:纳米孪晶材料是一种具有高强度、良好延展性、断裂韧性大、抗疲劳性能好、蠕变稳定性好等优异性能的纳米结构材料。最近出现了一个明显的争议,即关于纳米孪晶材料的强度如何随着孪晶厚度的减小而变化。当孪晶厚度降低到临界值以下时,纳米孪晶Cu发生了从硬化到软化的转变,而在陶瓷和金刚石中则发生了连续硬化。本研究通过原子模拟和纳米孪晶PdCu系统的理论模型构建,发现存在一个软化温度,当低于该软化温度时,材料随孪晶厚度减小而不断硬化(如纳米孪晶陶瓷和金刚石),而高于该软化温度时,其强度先增加后降低,在临界孪晶厚度下,材料强度达到最大值,材料由硬化过渡到软化(如纳米孪晶Cu)。这一重要现象归因于在软化温度以下,塑性从“源-控”向“应力-控”的转变。同时,这一现象表明,即使在相同的纳米孪晶材料中,也可能存在不同的硬化行为,且在一定的温度下,不同的材料在不同的软化温度下也会表现出不同的硬化行为   

Abstract:Nanotwinned materials have been widely studied as a promising class of nanostructured materials that exhibit an exceptional combination of high strength, good ductility, large fracture toughness, remarkable fatigue resistance, and creep stability. Recently, an apparent controversy has emerged with respect to how the strength of nanotwinned materials varies as the twin thickness is reduced. While a transition from hardening to softening was observed in nanotwinned Cu when the twin thickness is reduced below a critical value, continuous hardening was reported in nanotwinned ceramics and nanotwinned diamond. Here, by conducting atomistic simulations and developing a theoretical modeling of nanotwinned Pd and Cu systems, we discovered that there exists a softening temperature, below which the material hardens continuously as the twin thickness is reduced (as in nanotwinned ceramics and diamond), while above which the strength first increases and then decreases, exhibiting a maximum strength and a hardening to softening transition at a critical twin thickness (as in nanotwinned Cu). This important phenomenon has been attributed to a transition from source- to stress-controlled plasticity below the softening temperature, and suggests that different hardening behaviors may exist even in the same nanotwinned material depending on the temperature and that at a given temperature, different materials could exhibit different hardening behaviors depending on their softening temperature. 

Editorial Summary

Nanotwinned materials: Plasticity below a softening temperature 纳米孪晶材料:软化温度下的塑性转变(自拟) 

该研究证明了纳米孪晶(nanotwined materials, nt)材料存在一个软化温度Ts,温度低于Ts时材料随着孪晶厚度的减小而持续硬化,而温度高于Ts时,强度先增加后减小,在临界孪晶厚度下,材料强度达到最大值,材料由硬化过渡到软化。来自美国 Brown UniversityHuajian Gao教授领导的团队,使用分子动力学(MD)对多晶nt-Pdnt-Cu样品进行了模拟,并建立了不受MD尺寸和时间尺度限制的基本理论模型,研究了孪晶厚度降低到临界值以下时硬度的变化。研究结果表明,在非常小的孪晶厚度(<λcrit)下,变形受孪晶晶界(twin boundaries, TBs)的迁移控制,这些TB与在TB-晶界(grain boundaries, GBs)交叉点成核的孪晶部分位错有关。虽然孪晶部分的成核受限于高于Ts的位错源的数量,但相同的成核过程在低于Ts时,则会受到TB-GB交叉点局部应力集中的限制,其峰值应力水平随着TB间距的减小而减小,导致连续硬化。因此,软化温度Ts划分了从位错源数-控制(源-控)向位错应力值-控制(应力-控)的TB迁移转变。该理论模型提示,原子键合越强,软化温度越高。他们所观察到的规律可适用于所有nt材料。

There exists a softening temperature, Ts, for nano-twinned (nt) materials, below which the material hardens continuously as the twin thickness is reduced, while above which the strength first increases and then decreases, exhibiting a maximum strength and a hardening to softening transition at a critical twin thickness. A team led by Prof. Huajian Gao from Brown University in the United States established the basic phenomenon thorough molecular dynamics (MD) simulations of polycrystalline nt-Pd and nt-Cu samples, and by theoretical modeling that is not subjected to the usual limitations of MD in size and time scale. The change in hardness when the thickness of the nt-materials is reduced below the critical value. Their simulation and modeling results reveal that at very small twin thicknesses (<λcrit), the deformation is governed by the migration of twin boundaries (TBs) associated with twinning partial dislocations nucleated at TB–grain boundaries (TBs) intersections. While the nucleation of twinning partials is limited by the number of dislocation sources above Ts, below Ts the same nucleation process becomes limited by local stress concentration at the TB–GB intersections, whose peak stress level, decreases with reduced TB spacing, leading to continuous hardening. Thus, the softening temperature Ts demarcates a transition from source- to stress-controlled TBs migration. The theoretical model suggests that the stronger the atomic bonding, the higher the softening temperature, and that the observed behavior could be generic to all nt-materials.

Strain engineering of electro-optic constants in ferroelectric materials (铁电材料电-光常数的应变工程)
Charles PaillardSergei Prokhorenko & Laurent Bellaiche 
npj Computational Materials 5:6 (2019)
doi:s41524-018-0141-4
Published online:08 January 2019
Abstract| Full Text | PDF OPEN

摘要:基于电-光效应可以利用电场调控光强变化成为可能,该效应对于当今信息和通信技术(例如电视显示器和光纤)十分重要。在薄膜中寻找大的电-光常数对于电-光器件的小型化和提高电-光效率十分关键。本研究证明了通过PbTiO3 薄膜的应变工程可以有选择性地改进某个电-光常数。我们预测了高达100pmV-1 的无应力电光常数,该数值既可以源于拉伸应变作用下相变边界处光学声子模式的软化,亦可源于压缩应变产生等效负压导致的大压电常数。具体地,可以通过在硅这一技术上十分重要的衬底材料上生长出PbTiO3,来获得大的r33-光系数,其数值可为常用的LiNbO3  电光材料的两倍   

Abstract:Electro-optic effects allow control of the ow of light using electric fields, and are of utmost importance for today’s information and communication technologies, such as TV displays and fiber optics. The search for large electro-optic constants in films is essential to the miniaturization and increased efficiency of electro-optic devices. In this work, we demonstrate that strain-engineering in PbTiO3 films allows to selectively choose which electro-optic constant to improve. Unclamped electro-optic constants larger than 100pmV-1  are predicted, either by driving the softening of an optical phonon mode at a phase transition boundary under tensile strain, or by generating the equivalent of a negative pressure via compressive strain to obtain extremely large piezoelectric constants. In particular, a r33 electro-optic coefficient twice as large as the one of the commonly used LiNbO3 electro-optic material is found here when growing PbTiO3 on the technologically important Si substrate. 

Editorial Summary

Large electric-optic constant: Strain engineering in PbTiO3 film 

基于铁电薄膜PbTiO3 的应变工程(拉伸或压缩)即可选择性地获得大的电-光常数。在薄膜中寻找大电-光常数对于电-光器件的小型化和提高电-光效率是必不可少的。来自美国阿肯色大学的LaurentBellaiche 领导的团队,采用精确的从头算技术,研究了应变对于PbTiO3这一经典铁电体电-光特性的影响,以企确定导致铁电PbTiO3薄膜中高电光性能的一般机制。计算结果显示,应变工程可以有选择性地改进某个电-光常数,具体增强机制有两种:其一为利用拉伸应变实现相变边界处光学声子模式的软化;其二为利用压缩应变产生等效负压导致大的压电系数。他们通过计算预测出了应变下的大于100pmV-1  的大电-光常数发现若以硅材料作生长PbTiO3薄膜的衬底,所获得的薄膜材料的r33-光系数可达57 pm/V,接近标准-材料LiNbO3的两倍!

Large electric-optic constant can be obtained by strain engineering in ferroelectric film. Films with large electro-optic (EO) constants is essential for miniaturization of EO devices and improving EO efficiency. A team led by Laurent Bellaiche from the University of Arkansas in the United States, using precise ab initio techniques, explored a wide range of epitaxial strain conditions to determine EO effects in ferroelectric PbTiO3 films. They demonstrated that strain engineering of PbTiO3 films can be arbitrarily chosen to improve a certain EO constant. Two enhancing mechanisms were found, as the first driven force is softening of an optical phonon mode at a phase transition boundary under tensile strain, while the other is driven by the extremely large piezoelectric constants resulted from generating the equivalent of a negative pressure via compressive strain. EO constants larger than 100pmV-1 were predicted, and could attain 57 pm/V, almost twice the standard electro-optical material LiNbO3 if PbTiO3 film growing on the technologically important Si substrate.

Multi-loop node line states in ternary MgSrSi-type crystals (三元MgSrSi型晶体中的多环节点线状态)
Jinling LianLixian YuQi-Feng Liang, Jian Zhou, Rui Yu & Hongming Weng 
npj Computational Materials 5:10 (2019)
doi:s41524-019-0150-y
Published online:21 February 2019
Abstract| Full Text | PDF OPEN

摘要:受镜像对称(mirror symmetry, m-NLs)、空间反演和时间反演对称的乘积S = PTs-NLs),或非简单空间群对称等保护的节点线能带交叉是拓扑半金属在布里渊区中的非平凡拓扑对象。本研究使用第一原理计算筛选了一系列的MgSrSi型三元晶体,并发现超过70个成员是节点线半金属。特别是在AsRhTi晶体中发现了一种新颖的多节线结构,其中一个s-NL在某个“链接点”处与一个m-NL稳健地接触,同时该s-NL还与另一个m-NL嵌套形成Hopf环链。与先前提出的由两个s-NL或两个m-NL形成的Hopf环链不同,由一个s-NL和一个m-NL形成的Hopf环链需要最小的三带模型来表征其基本电子结构。本研究还获得了AsRhTi晶体的不同表面上的相关拓扑表面态。在AsFeNbPNiNb中预测了更复杂和奇特的NLs多环结构。我们的研究可能有助于在真实材料中寻找奇异的多环节点线半金属   

Abstract:Node line band-touchings protected by mirror symmetry (named as m-NLs), the product of inversion and time reversal symmetry S=PT (named as s-NLs), or nonsymmorphic symmetry are nontrivial topological objects of topological semimetals in the Brillouin Zone. In this work, we screened a family of MgSrSi-type crystals using first principles calculations, and discovered that more than 70 members are node-line semimetals. A new type of multi-loop structure was found in AsRhTi that a s-NL touches robustly with a m-NL at some “nexus point”, and in the meanwhile a second m-NL crosses with the s-NL to form a Hopf-link. Unlike the previously proposed Hopf-link formed by two s-NLs or two m-NLs, a Hopf-link formed by a s-NL and a m-NL requires a minimal three-band model to characterize its essential electronic structure. The associated topological surface states on different surfaces of AsRhTi crystal were also obtained. Even more complicated and exotic multi-loop structure of NLs were predicted in AsFeNb and PNiNb. Our work may shed light on search for exotic multi-loop node-line semimetals in real materials. 

Editorial Summary

Multi-loop node line state: in ternary MgSrSi-type crystal三元MgSrSi型晶体:多环节点线态 

该研究发现了70多种MgSrSi型晶体化合物在其带结构中存在节点线能带交叉。拓扑半金属中导带和价带的带交叉是布里渊区的有趣拓扑对象,它赋予拓扑半金属独特的电子结构和电学性质。其中节点线(node-line, NL)半金属的带交叉形成闭环,但这仅是依据一些材料提出的概念,在实际材料中的直接证据则几乎没有。来自绍兴文理学院的梁奇锋教授和武汉大学的余睿教授领导的联合团队,使用第一原理计算,对MgSrSi三元晶体族的660种晶体作了筛选,发现有70多种化合物是节点线半金属。由于空间群中反射对称性和时间反演对称性的共存,AsRhTi表现出新颖的多环NL结构,其中由PT对称性保护的s-NL与由镜像对称保护的m-NL在一些“链接点”稳健地接触,同时该s-NL还与另一个m-NL嵌套形成一个Hopf-环链。这个Hopf-环链需要最小的三带k p模型来表征其基本电子结构。研究结果进一步展示了由非平凡NL结构所导致的拓扑表面态。拓扑表面态具有非平凡NL结构。他们的研究可能有助于在真实材料中寻找奇异的多环节点线半金属

More than 70 kinds of MgSrSi-type crystals are node-line semimetals showing a variety of NL structures. The intersection of the conduction band and the valence band in the topological semimetal is an interesting topological object of the Brillouin zone, which endows unique electronic structure and electrical properties to the topological semimetal. Among them, the node-line (NL) semimetal band form a closed loop, but this is only a proposed concept based on some materials, and there is almost no direct evidence in real materials. A joint team led by Professor Qi-feng Liang from Shaoxing University and Professor Rui Yu from Wuhan University used the first principle calculation to screen the ternary crystal family of MgSrSi-type crystals which consists of 660 members, and discovered that more than 70 compounds were node line semimetals. A new type of multi-loop structure was found in AsRhTi that a s-NL touches robustly with a m-NL at some “nexus point”, and in the meanwhile a second m-NL crosses with the s-NL to form a Hopf-link. This Hopf-link requires a minimal three-band k p model to characterize its basic electronic structure. Their study further demonstrates that these non-trivial NL structures induce topological surface states on the crystal surfaces. Their research may help find exotic multi-loop node-line semimetals in real materials.

Local-environment dependence of stacking fault energies in concentrated solid-solution alloys高浓度固溶体合金中堆垛层错能的局域环境依赖性
Shijun Zhao, Yuri Osetsky, G. Malcolm Stocks & Yanwen Zhang 
npj Computational Materials 5:13 (2019)
doi:s41524-019-0150-y
Published online:04 February 2019
Abstract| Full Text | PDF OPEN

摘要:基于3d过渡金属的高浓度固溶体合金(CSA)拥有非凡的机械性能和抗辐照性能,这些优异的性能都与其较低的堆垛层错能(SFE)相关。由于内在的无序原子排布,CSA中的SFE值取决于局部原子的空间分布。本研究基于经验势和第一原理计算,研究了NiCoNiFeNiCoCr的等高浓度CSASFE的分布。我们的研究表明,无序结构的CSASFE的分布取决于计算中采用的堆垛层错区域的大小。通过电子结构分析,我们发现CSASFE的变化与堆垛层错区域中的电荷密度再分布有关。我们进一步提出了一个化学键断裂和再形成的模型来描述这种局部SFE的变化,从而可以基于局部结构来研究和预测CSA中的SFE的分布。我们的结果还表明,对于NiCo,堆垛层错引起的扰动仅局限于层错附近的最近邻密排面,而在NiFeNiCoCr中,由于FeCr的部分填充的d电子的存在,层错的扰动可以延伸到第三近邻的密排面   

Abstract:Concentrated solid-solution alloys (CSAs) based on 3d transition metals have demonstrated extraordinary mechanical properties and radiation resistance associated with their low stacking fault energies (SFEs). Owing to the intrinsic disorder, SFEs in CSAs exhibit distributions depending on local atomic configurations. In this work, the distribution of SFEs in equiatomic CSAs of NiCo, NiFe, and NiCoCr are investigated based on empirical potential and first-principles calculations. We show that the calculated distribution of SFEs in chemically disordered CSAs depends on the stacking fault area using empirical potential calculations. Based on electronic structure calculations, we find that local variations of SFEs in CSAs correlate with the charge density redistribution in the stacking fault region. We further propose a bond breaking and forming model to understand and predict the SFEs in CSAs based on the local structure alone. It is shown that the perturbation induced by a stacking fault is localized in the first-nearest planes for NiCo, but extends up to the third nearest planes for NiFe and NiCoCr because of partially filled d electrons in Fe and Cr. 

Editorial Summary

Concentrated solid-solution alloys: stacking fault energy and its local environment dependence高浓度固溶体合金:堆垛层错能与层错局部环境的关系 

该研究发现高浓度固溶合金中的堆垛层错能与堆垛层错所在区域的电荷密度再分布相关。来自美国橡树岭国家实验室的Shijun Zhao(赵仕俊,现为香港城市大学机械工程系助理教授)等人,基于经验势和第一原理计算,研究了NiCoNiFeNiCoCr的等高浓度固溶合金中堆垛层错能的分布。他们发现堆垛层错的影响在NiCo中是局部的,而在NiFeNiCoCr中是相对长程的,证明了高浓度固溶体合金中的局部堆垛层错能可以用化学键临界点处的电荷密度再分布来表征,并进一步提出了一个化学键的断裂和再形成模型。依据该模型仅凭短程效应即可预测特定原子空间构型下的局部堆垛层错能,并可表征堆垛层错能对局部环境的依赖性。这些研究结果使局部原子空间排列及其堆垛层错能之间建立了明确的联系,对理解高浓度固溶体合金的局部性质和预测相关效应(如材料相和结构稳定性、原子转移、位错性质等)都非常重要

Stacking fault energy (SFE) in concentrated solid-solution alloys (CSAs) is related to charge density redistribution in the region where stacking faults are located. A team from Oak Ridge National Laboratory, studied the SFE distribution of NiCo, NiFe and NiCoCr CSAs based on empirical and first-principles calculations. They found that the effect of stacking faults is localized in NiCo and relatively long-ranged in NiFe and NiCoCr, demonstrating that the local SFE can be described by charge density redistribution at the bond critical points. Based on electronic structure results, they further proposed a bond breaking and formation model to understand the SFE distributions in CSAs, by which the short-range effect can be used to predict the local SFE under a specific atomic configuration. These findings establish a clear link between local atomic configuration and SFE, which are important for understanding the local properties of CSAs and predicting the associated effects.

Learning to predict single-wall carbon nanotube-recognition DNA sequences (机器学习预测单壁碳纳米管识别的DNA序列)
Yoona YangMing Zheng & Anand Jagota 
npj Computational Materials 5:3 (2019)
doi:s41524-018-0142-3
Published online:10 January 2019
Abstract| Full Text | PDF OPEN

摘要:DNA 具有分散和筛选不同手性和偏手性的单壁碳纳米管的特性,这将带来诸多应用。为寻找能够筛选任一给定类型碳管的DNA序列,人们已经开展了大量工作,并在理解DNA/碳管复合物结构及热力学方面取得了很大进展。然而,从头预测识别序列的方法尚难以实现,而且通过搜索巨大的单链DNA库来寻找序列的成功率非常低。本研究提出了一种通过机器学习现有实验序列数据集来预测识别序列的有效方法。采用了多种输入特征构造法(位置特定、时期频率、组合或分段术语频率向量,以及基于基序的特征),并进行了比较。利用转换后的特性训练了几种分类算法(逻辑回归、支持向量机器学习和人工神经网络)。采用训练过的模型预测新的识别序列集,并以多个模型之间的一致性成功抵消数据集规模不够的限制。采用水性两相分离法测试了预测的准确性。将实验测试过的新预测序列数据集进一步添加到原始数据集,以此新获取的数据重新训练模型。基于训练模型识别出序列的正确率,相比原始训练数据集训练模型大幅提升,从~10%提高到>50   

Abstract:DNA/single-wall carbon nanotube (SWCNT) hybrids have enabled many applications because of their special ability to disperse and sort SWCNTs by their chirality and handedness. Much work has been done to discover sequences which recognize specific chiralities of SWCNT, and significant progress has been made in understanding the underlying structure and thermodynamics of these hybrids. Nevertheless, de novo prediction of recognition sequences remains essentially impossible and the success rate for their discovery by search of the vast single-stranded DNA library is very low. Here, we report an effective way of predicting recognition sequences based on machine learning analysis of existing experimental sequence data sets. Multiple input feature construction methods (position-specific, term-frequency, combined or segmented term frequency vector, and motif-based feature) were used and compared. The transformed features were used to train several classifier algorithms (logistic regression, support vector machine, and artificial neural network). Trained models were used to predict new sets of recognition sequences, and consensus among a number of models was used successfully to counteract the limited size of the data set. Predictions were tested using aqueous two-phase separation. New data thus acquired were used to retrain the models by adding an experimentally tested new set of predicted sequences to the original set. The frequency of finding correct recognition sequences by the trained model increased to >50% from the ~10% success rate in the original training data set. 

Editorial Summary

Machine Learning: Predicting DNA Sequences for recognizing carbon nanotubes机器学习:预测识别单壁碳纳米管的DNA序列 

该研究采用机器学习系统地预测了可识别特定单壁碳纳米管的DNA序列。来自美国Lehigh大学化学与生物分子工程系的Anand Jagota等,基于现有实验序列数据集,报告了一种用机器学习分析来预测识别DNA序列的有效方法。为便于分析、解释,他们将SWCNT识别DNA序列限制为只有2种碱基组合(CT)的、12个碱基构成的短序列。以已知数据训练机器学习模型,并将实验测试过的新序列数据集添加到原始数据集,重新训练模型。通过交叉验证和新测试集上的预测误差来评估预测性能,并通过特征表示方法改进模型性能。结果显示准确预测识别序列的频率从原始训练集的10%显着提升到> 50%。他们所获得的机器学习模型,有可能为更普遍的序列选择问题提供新的途径

Now, DNA sequence for recognizing single-walled carbon nanotube (SWCNTs) in DNA/SWCNT hybrid systems can be systematically predicted using machine learning (ML). A team lead by Anand Jagota from the Department of Chemistry and Biomolecular Engineering at Lehigh University in the United States reported an effective method for predicting recognition sequences using machine learning analysis based on existing experimental sequence data sets. For ease of analysis and interpretation, they restricted the SWCNT recognition sequence to a short sequence of 12 bases with only 2-base (C & T) combinations. ML models were trained on available data, and retrained twice based on new experimental data. Cross-validation and prediction error were used to evaluate the new test set and feature representation methods were selected to improve the model performance. The results show that the frequency of accurate prediction recognition sequences is significantly increased from 10% of the original training set to > 50% of the ML prediction sequence set. The ML model may has the potential to provide new avenues for overcoming more general sequence selection problems.

Large piezoelectric response in a family of metal-free perovskite ferroelectric compounds from first-principles calculations (第一性原理计算无金属有机钙钛矿铁电类化合物的大压电响应)
Hui WangHuihui LiuZeyu ZhangZihan LiuZhenlong LvTongwei LiWeiwei JuHaisheng LiXiaowu Cai & Han Han 
npj Computational Materials 5:17 (2019)
doi:s41524-019-0157-4
Published online:06 January 2019
Abstract| Full Text | PDF OPEN

摘要:最近研究表明,不含金属元素的有机钙钛矿铁电材料具有诸多引人注目的特性,包括高自发极化和高压电系数。特别是含有机胺、无机铵和氢卤酸的溶液,经过缓慢蒸发后可产生几种有趣的MDABCO-NH4-I3类材料(MDABCON-甲基-N'-二氮杂双环[2,2,2]八络合阳离子盐)。本研究用第一性原理计算了MDABCO-NH4-X3 (X=Cl, Br, I)铁电极化的起源、电子态密度、压电系数和弹性性质。发现MDABCONH4基团的固有偶极矩非常小,因此MDABCO-NH4-I3 的自发极化主要源自相MDABCONH4相对于I离子发生的偏移。。虽然有机材料的压电响应通常很弱,但我们的计算结果显示MDABCO-NH4-X3具有很大的压电系数MDABCO-NH4-Cl3MDABCO-NH4-Br3MDABCO-NH4-Cl3dx5值分别为119248178 pC / N。这一特性与顺度系数s44密切相关。这些结果表明, MDABCO-NH4-X3是一类具有较大压电系数和弹性顺度系数且不含金属元素的有机钙钛矿材料   

Abstract:Metal-free organic perovskite ferroelectric materials have been shown recently to have a number of attractive properties, including high spontaneous polarization and piezoelectric coefficients. In particular, slow evaporation of solutions containing organic amines, inorganic ammoniums, and dilute hydrohalogen acid has been shown to produce several attractive materials in the MDABCO-NH4-I3 family (MDABCO is N-methyl-N’-diazabicyclo[2,2,2] octonium). In the present work, we study by first-principles calculations the origin of polarizaiton, electronic density of state, piezoelectric response, and elastic properties of MDABCO-NH4-X3 (X=Cl, Br, I). We find that the dipole moments of the MDABCO and NH4 groups are negligible, and the large spontaneous polarization of MDABCO-NH4-I3 mainly results from MDABCO and NH4 being off-center relative to I ions. Although the piezoelectric response of organic materials is usually very weak, we observe large piezoelectric strain components, dx4 and dx5; the calculated dx5 is 119 pC/N for MDABCO-NH4-Cl3, 248 pC/N for MDABCO-NH4-Br3 and 178 pC/N for MDABCO-NH4-I3. The large value of dx5 is found to be closely related with the large value of elastic compliance tensor, s44. These results show that MDABCO-NH4-X3 metal-free organic perovskites have large piezoelectric response with soft elastic properties. 

Editorial Summary

Organic Perovskite: Calculation of Large Piezoelectric Response有机钙钛矿:大压电响应的计算 

该研究计算发现,不含金属元素的有机钙钛矿材料具有较大的压电系数。来自河南科技大学物理与工程学院的王翚副教授和上海科技大学物质科学与技术学院的韩晗副研究员共同领导的团队,采用第一性原理计算了无金属有机钙钛矿MDABCO-NH4-X3的晶体结构、电子态密度、压电系数和弹性性质。该研究发现,MDABCO-NH4-X3表现出的较大的自发极化强度主要源自MDABCONH4基团相对于卤素阴离子中心的偏离。当XI离子时,计算得到的压电应变分量dz3与实验值非常吻合。进一步发现,XClBrI离子时,其压电系数dx5值分别为119248178 pC / N-这一特性与较大的顺度系数s44密切相关,说明MDABCO-NH4-X3 较大的压电应变响应主要源于它们的柔性。 此类有机钙钛矿材料独特的性能使其成为柔软、轻便和生物友好型压电材料的理想选择,适用于各种重要方面的应用,如柔性设备、可植入系统和微型机器人

A family of metal-free organic perovskites (MDABCO-NH4-X3) have greater piezoelectric responsiveness and flexibility. A team co-led by Professor Wang Hui, from the School of Physics and Engineering, Henan University of Science and Technology, and Professor Han Han from the School of Physical Science and Technology, University of Shanghai for Science and Technology, used the first-principles to calculate the structure and electronic density, piezoelectric response and elastic properties of the metal-free organic perovskite. They explained the close correlation between the high ferroelectric Curie temperature and the interruption of this H-X interaction, and found that the large spontaneous polarization of MDABCO-NH4-I3 results mainly from the off-centers of MDABCO and NH4 relative to X ions. Their calculation results show that the piezoelectric strain component dz3 is in good agreement with the experimental value when X is I. Furthermore, the dx5 values, when X is Cl, Br, I, are 119, 248 and 178 pC / N, respectively, the large value of which is closely related to that of s44 showing flexibility. This results indicates that the large piezoelectric strain response of MDABCO-NH4-X3 is mainly due to their softness. All unique properties including soft, low-density and bio-friendly piezoelectricity of this kind of perovskites make it a promise choice for a variety of important applications such as flexible devices, implantable systems and micro-robots.

High-throughput 3D reconstruction of stochastic heterogeneous microstructures in energy storage materials (储能材料中随机异质微结构的高通量三维重建)
Yanxiang Zhang, Mufu Yan, Yanhong Wan, Zhenjun Jiao, Yu Chen, Fanglin Chen, Changrong Xia & Meng Ni 
npj Computational Materials 5:11 (2019)
doi:s41524-019-0149-4
Published online:31 January 2019
Abstract| Full Text | PDF OPEN

摘要:随机异质微结构广泛应用于结构和功能材料,在决定其性能方面发挥着至关重要的作用。X射线断层摄影和聚焦离子束连续切片是3D微结构重建的常用方法,但不仅技术要求高而且分辨率有限。本研究开发了一种基于距离相关函数(DCFs)的高通量多阶段3D重建方法,仅用单个具有代表性的大尺寸2D显微照片即可重建材料的三维结构,并适用于各向异/同性材料,且通过固体氧化物燃料电池电极的X射线微断层扫描数据证实了结果。该方法为能量转换与存储材料中的随机异质微结构3D重建提供了一种经济、易用、高通量技术,可拓展应用于其他材料   

Abstract:Stochastic heterogeneous microstructures are widely applied in structural and functional materials, playing a crucial role in determining their performance. X-ray tomography and focused ion beam serial sectioning are frequently used methods to reconstruct three-dimensional (3D) microstructures, yet are demanding techniques and are resolution-limited. Here, a high-throughput multi-stage 3D reconstruction method via distance correlation functions is developed using a single representatively large-sized 2D micrograph for stochastic microstructures, and verified by X-ray micro-tomography datasets of isotropic and anisotropic solid oxide fuel cell electrodes. This method provides an economic, easy-to-use and high-throughput approach for reconstructing stochastic heterogeneous microstructures for energy conversion and storage devices, and can readily be extended to other materials. 

Editorial Summary

Stochastic Heterogeneous Microstructures: High-throughput 3D Reconstruction储能材料随机异质微结构:高通量三维重建 

该研究仅以2D显微照片即可重建代表性大尺寸3D微结构。随机异质微结构广泛应用于结构和功能材料,调控了材料的性能。为高效重建随机异质微结构,被人们寄予厚望的距离相关函数,却难以收敛到真实值,因而能否准确捕获多尺度微观结构特征,成了微结构重建的关键问题。来自哈尔滨工业大学材料科学与工程学院金属精密热加工国家级重点实验室的张雁祥副教授和闫牧夫教授等,使用单个代表性大尺寸2D显微照片开发了基于距离相关函数的高通量多阶段3D重建方法,并使用固体氧化物燃料电池电极的X射线微断层扫描数据集证实了结果。他们的方法也适用于各种各向异/同性微结构,并可进一步扩展用以研究材料加工、微结构和性能之间的关系

Representative large-size 3D microstructures was reconstructed using only 2D micrographs. Stochastic heterogeneous microstructures are widely used in structural and functional materials to regulate the properties of materials. To efficiently reconstruct stochastic heterostructures, high-order distance correlation functions (DCFs) have been used, but it difficult to converge to real values. Therefore, it is a big problem to fulfill the reconstruction. A team led by Yanxiang Zhang and Mufu Yan from National Key Laboratory for Precision Hot Processing of Metals, Harbin Institute of Technology, developed a high-throughput multi-stage 3D reconstruction method via distance correlation functions using a single representatively large-sized 2D micrograph for stochastic microstructures, and verified by X-ray micro-tomography data sets of isotropic and anisotropic solid oxide fuel cell electrodes. Their method can be further extended to reveal relations among material processing, microstructure, and performance.

First-principles-based prediction of yield strength in the RhIrPdPtNiCu high-entropy alloy (基于第一性原理的RhIrPdPtNiCu高熵合金屈服强度预测)
Binglun Yin & William A. Curtin 
npj Computational Materials 5:14 (2019)
doi:s41524-019-0151-x
Published online:05 February 2019
Abstract| Full Text | PDF OPEN

摘要:高熵合金是具有5种或5种以上组分的无规合金,各元素的比例接近于等组分, 通常能表现出优异的力学性能。在广泛的组分空间中指导新材料的设计,需要通过从头算方法计算必要的基础材料参数。本研究将密度泛函理论用于计算fcc贵金属RhIrPdPtNiCu中的元素错配体积、合金晶格常数、弹性常数和稳定的堆垛层错能。然后将这些计算结果应用于新近的温度和应变率依赖性屈服强度理论。发现无参数预测得到的583 MPa屈服强度与测量值527 MPa非常一致。在没有任何实验输入的情况下,合金成分和屈服强度之间的定量联系,使得这种基于密度泛函理论的方法路径,可与第一原理化学精度法结合,探索新的有潜力的高强度高熵合金,科研是是本研究中的合金体系,也可以是其他合金体系   

Abstract:High-entropy alloys are random alloys with five or more components, often near equi-composition, that often exhibit excellent mechanical properties. Guiding the design of new materials across the wide composition space requires an ability to compute necessary underlying material parameters via ab initio methods. Here, density functional theory is used to compute the elemental misfit volumes, alloy lattice constant, elastic constants, and stable stacking fault energy in the fcc noble metal RhIrPdPtNiCu. These properties are then used in a recent theory for the temperature and strain-rate dependent yield strength. The parameter-free prediction of 583MPa is in excellent agreement with the measured value of 527MPa. This quantitative connection between alloy composition and yield strength, without any experimental input, motivates this general density functional theory-based methodological path for exploring new potential high-strength high-entropy alloys, in this and other alloy classes, with the chemical accuracy of first-principles methods. 

Editorial Summary

High entropy alloy: yield strength prediction高熵合金:屈服强度预测 

该研究提出了一种新的通用方法,不依赖于任何拟合参数,实现了屈服强度的预测来自瑞士洛桑联邦理工学院多尺度力学模型实验室及国家计算设计与发现新材料中心的尹冰轮博士等,提出了一种通用的方法。根据新近提出的溶质强化模型理论,利用密度泛函理论(DFT)计算出模型所需要的材料基本属性,然后通过模型理论准确地预测出任何随机合金(包括高熵合金)的屈服强度。DFT计算的物理量包括随机合金中各元素的平均错配体积、合金晶格常数、弹性常数和稳定的堆垛层错能。该方法预测的贵金属高熵合金RhIrPdPtNiCu屈服强度583 MPa与实验测得的强度527 MPa相当接近。同时该工作还评估了其方法中的不确定性,也评估了多种扫描组分空间的方案,以优化获得材料最高屈服强度的组分。该方法建立了合金成分与屈服强度之间的定量联系,无需实验输入,可用于探索新的、有潜力的、高强度高熵合金,并为高强度合金的计算指导设计提供了重要途径

A new general method that achieves a prediction of yield strength independent of any fitting parameters. Dr. Binglun Yin, et al. from the Federal Institute of Technology in Lausanne, Switzerland, proposed a general method for performing DFT-level calculations using a newly established solute strengthening model theory to compute properties required, so that they can accurately predict the yield strength of any random alloy (including high-entropy alloy). The physical quantities calculated by DFT include the elemental misfit volumes, alloy lattice constant, elastic constants, and stable stacking fault energy. The predicted yield strength of the noble metal high-entropy alloy RhIrPdPtNiCu is 583 MPa compare to the experimentally measured 527 MPa. Meanwhile, the work also evaluated the uncertainty of the method, and a variety of scanning component space schemes to optimize the components for preparing materials with the highest yield strength. This method establishes a quantitative relationship between alloy composition and yield strength without experimental input, which can be used to explore new promising high-strength high-entropy alloys, and provides an important approach to “computation-oriented design” of high-strength alloys.

Excitation to defect-bound band edge states in two-dimensional semiconductors and its effect on carrier transport (二维半导体中缺陷束缚带边态的激发及其对载流子传输的影响)
Dan WangDong Han, Damien WestNian-Ke Chen, Sheng-Yi Xie, Wei Quan Tian, Vincent Meunier, Shengbai Zhang & Xian-Bin Li 
npj Computational Materials 5:8 (2019)
doi:s41524-018-0145-0
Published online:15 January 2019
Abstract| Full Text | PDF OPEN

摘要:杂质离化向半导体带边提供载流子是实现电子器件功能化的关键过程。在二维材料中,由于维度降低和电荷屏蔽减弱,使得杂质电离这一过程变得更加困难。本研究利用第一性原理计算方法,提出了二维半导体掺杂杂质的独特电离过程:载流子只被激发到缺陷束缚带边态,而不是传统三维半导体的自由带边态。这些缺陷束缚带边态的电离能较小,波函数空间分布范围较大。在缺陷密度合适的条件下,载流子可通过这些缺陷束缚带边态传输   

Abstract:The ionization of dopants is a crucial process for electronics, yet it can be unexpectedly difficult in two-dimensional materials due to reduced screening and dimensionality. Using first-principles calculations, here we propose a dopant ionization process for two-dimensional semiconductors where charge carriers are only excited to a set of defect-bound band edge states, rather than to the true band edge states, as is the case in three-dimensions. These defect-bound states have small enough ionization energies but large enough spatial delocalization. With a modest defect density, carriers can transport through band by such states. 

Editorial Summary

2D semiconductor: defect bound band edge state二维半导体:缺陷束缚带边态 

本研究报道了二维半导体存在一种缺陷束缚带边态 (defect-bound band edge, DBBE),对缺陷电离和载流子输运影响显著。来自吉林大学集成光电子国家重点实验室的李贤斌教授领导的团队,以单层MoS2为例,使用第一性原理计算证实了缺陷束缚带边态的存在及其对载流子传输的潜在影响。他们发现形成缺陷束缚带边态的原因是带电缺陷和激发载流子之间存在强束缚作用。该强束缚作用是维度降低的结果,也是二维材料中电荷屏蔽减弱的结果。载流子激发到缺陷束缚带边态所需能量低于形成自由载流子的所需能量,并且位于DBBE的载流子可以在适当浓度下通过波函数重叠提供传输通道。该传输机制可能是实验观测的二维材料电导的潜在机制。他们的研究揭示了二维半导体中载流子的杂质电离和传输的全新物理图像,对设计新兴纳米电子器件具有重要影响

There exists a defect-bound band edge (DBBE) state in 2D semiconductors, which has a significant effect on defectionization and carrier transport. A team led by Professor Li Xianbin from the State Key Laboratory of Integrated Optoelectronics of Jilin University used first-principles calculations to verify the existence of DBBE state in single-layer MoS2, an example for 2D semiconductor, and its potential impact on carrier transport. They found that the reason for the formation of DBBE states is strong binding between charged defect and excited carrier. The strong binding is a result of the spatial confinement as well as the reduced screening in 2D materials. The energy required to excite the carrier to DBBE state is lower than the energy required to form the free carrier, and the carriers in DBBE state can provide a transmission channel by wave function overlap at an appropriate concentration. This transport may be a potential mechanism for experimentally observed 2D material conductance. Their research reveals a new physical image of ion ionization and carrier transport in 2D semiconductors, which has an important impact on the design of emerging nanoelectronic devices.

Distortion-stabilized ordered structures in A2BB O7 mixed pyrochlores (A2BB O7 混合烧绿石中畸变稳定的有序结构)
Ghanshyam Pilania, Brian Puchala & Blas P. Uberuaga 
npj Computational Materials 5:7 (2019)
doi:s41524-018-0144-1
Published online:14 January 2019
Abstract| Full Text | PDF OPEN

摘要:烧绿石氧化物(A2BBO7 )在许多技术应用中都很有趣,包括耐辐射损伤和作为离子导体。混合烧绿石(包含两个A/或两个B位阳离子)由于在该化学空间中可获得的化学和构型自由度不同,为定制性能提供了更大的灵活性。本研究以Zr/Ti的含量作为变量,考察一种双烧绿石Gd2(ZrxTi1x)2O7模型中不同阳离子排序的相对稳定性。我们的结果表明,在某些特定的局部阳离子排列下,由于氧弛豫位移大,该系统中的某些阳离子有序成分高度稳定,从而形成有序的“双”烧绿石结构。这些异常氧弛豫的起源可追溯到局部阳离子对称性和Zr原子对采用7配位环境的强化学偏好,而不是常规烧绿石结构中的6配位构型。随后,我们在131个其它烧绿石成分中考察了这类有序结构的稳定性。我们的研究意义与系统中所观察到的组成离子电导率有关,并与之前报道的实验结果有关   

Abstract:Pyrochlore oxides (A2B2O7) are interesting for a number of technological applications, including radiation damage tolerance and as ionic conductors. Mixed pyrochlores—containing two A and/or two B site cations—provide even more flexibility for tailoring properties owing to the diverse chemical and configurational degrees of freedom accessible within this chemical space. Here, we examine relative stability of different cation orderings in one model double pyrochlore Gd2(ZrxTi1x)2O7, as a function of Zr content x. Our results show that, in the presence of some very specific local cation arrangements, certain cation-ordered compositions in this system are highly stabilized as a result of large oxygen relaxation displacements, leading to the formation of an ordered ‘double’ pyrochlore structure. The origins of these anomalous oxygen relaxations are traced back to both the local cation symmetry and a strong chemical preference of Zr atoms towards adopting a 7-fold coordination environment, as opposed to a 6-fold coordination available in a regular pyrochlore structure. Subsequently, we examine the stability of this type of ordering in 131 other pyrochlore compositions. Implications of our findings are discussed in relation to the observed composition-dependent ionic conductivity in these systems and connections with previously reported experimental findings are made. 

Editorial Summary

Mixed pyrochlore: an ordered structure with stable distortion混合烧绿石:畸变稳定的有序结构 

该研究在混合Gd2Ti2O7–Gd2Zr2O7 烧绿石结构中发现了新型低能有序结构。来自美国洛斯阿拉莫斯国家实验室的Ghanshyam PilaniaBlas P. Uberuaga等,使用密度泛函理论(DFT),Zr含量x的作为变量,研究了一种双烧绿石Gd2(ZrxTi1x)2O7中不同阳离子排序的相对稳定性。他们的结果表明,在某些特定的局部阳离子排列下,由于氧弛豫位移大,该系统中的某些阳离子有序成分高度稳定,从而形成有序的“双”烧绿石结构。这些异常氧弛豫的起源可追溯到局部阳离子对称性和Zr原子对采用7配位环境的强化学偏好,而不是常规烧绿石结构中的6配位构型。混合烧绿石中潜在的有序结构开辟了定制这些系统性质的新途径,扩展了这类化合物的功能空间。双烧绿石这一研究可能与其组成离子的电导率有关,并与之前报道的实验结果有关。

A novel low-energy ordered structure of mixed Gd2Ti2O7–Gd2Zr2O7 pyrochlore was reported. Ghanshyam Pilania and Blas P. Uberuaga from Los Alamos National Laboratory and Brian Puchala used density functional theory (DFT) to examine relative stability of different cation orderings in one model double pyrochlore Gd2(ZrxTi1x)2O7, as a function of Zr content x. Their results show that, in the presence of some very specific local cation arrangements, certain cation-ordered compositions in this system are highly stabilized as a result of large oxygen relaxation displacements, leading to the formation of an ordered ‘double’ pyrochlore structure. The lowest energy structures are stabilized by an oxygen relaxation that allows for 7-fold coordination of some of the Zr cations in these structures, rather than 6-fold in the conventional pyrochlore structure. The potentially ordered structure of mixed pyrochlore opens up new avenues for customizing the properties of these systems, extending the functional space of such compounds. The study of double pyrochlore may be related to the conductivity of its constituent ions and is related to the previously reported experimental results.

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