最近研究成果 | Recent News

Higher order J-Tz-AT solution for three-dimensional crack border fields in power-law hardening solids

11/06/2019 - 16:08

Higher order J-A2 solution has been developed to improve the HRR singular solution under ideal plane strain conditions in power-law hardening solids with the the second A2 being considered to take into account of the in-plane constraint effect, and the J-Tz singular solution has been obtained for three-dimensional (3D) cracked body by introducing the out-of-plane stress constraint factor Tz. Here a higher order J-Tz-AT solution is developed on the basis of the J-Tz and J-A2 solutions and validated against comprehensive 3D finite element (FE) analyses for specimens with through-thickness, surface, embedded and corner cracks. It is shown that better agreements are obtained between the higher order J-Tz-AT solution and 3D FE results in all simulated conditions than previously available two- or three-parameter solutions. For specimens of high in-plane constraint, such as the single-edge cracked tension specimen, compact specimen and single-edge-notched bending specimen under three-point bending,the J-Tz leading singular solution itself shows sufficient accuracy. This universal characterization of crack border stress fields confirms that the developed J-Tz-AT solution combines the advantages of the J-Tz and J-A2 solutions, which can service as a solid foundation of elastic-plastic fracture mechanics.



Fig. 11. Comparisons of the tensile stress σθθ and out-of-plane stress σzz obtained by the 3D FE results and the J-Tz, J-A2, J-Q, J-Tz-QT and developed higher order J-Tz-AT solutions. (a) z/B = 0, σθθ; (b) z/B = 0.45, σzz; (c) z/B = 0, σθθ; (d) z/B = 0.45, σzz.

论文链接:
https://authors.elsevier.com/a/1Zy7Z38l3Tdn-

美国化学学会会刊(JACS)发表纳米科学研究所团队最新研究成果

05/28/2019 - 20:11

近日,纳米科学研究所团队的一项新的研究成果荣登美国化学学会会刊(JACS)!

这是一项关于二维金属氢化物的研究成果,第一作者是博士生周晓成与杭阳,通讯作者为张助华教授和郭万林院士,共同作者还包括刘立仁博士。

那么二维金属氢化物到底是什么?让我们一起来揭开这神奇的面纱吧!

很多材料具有天然的层状结构——原子在层内以强的化学键结合而层与层之间仅通过范德华力粘合。石墨是最典型的例子,由无数蜂窝结构的碳原子单层堆叠而成。通过机械剥离法分离出的碳原子单层便是石墨烯。

采用类似的方式,研究者陆续得到了单层氮化硼(俗称白石墨烯),单层过渡金属插层化合物,单层黑磷(俗称磷烯)等广泛研究的二维材料。

2018年,基于先进的计算技术与模拟方法,人们从材料晶体结构数据库中鉴定出1800多种层状材料可剥离出稳定的二维材料,形成了数目庞大,功能各异的二维材料家族。

天然的层状结构可以机械分离出单层原子,那原本没有层状晶格的材料是否可以产出二维单层材料呢?
这方面的例子有硅原子单层—硅烯、锗原子单层—锗烯、硼原子单层—硼烯等,而宏观硅、锗和硼材料却不具有层状的晶格结构。这类二维材料的诞生完全归功于人类的创造力,因此被称为人造二维材料。


人造二维材料的晶格结构和化学成分高度可设计,能产生更丰富的性质,最近迅速成为材料领域的研究热点。而现有人造二维材料一大缺陷是易与周围环境发生化学反应,从而改变它们本征的结构和性质,限制了它们的应用潜力。而纳米科学研究所团队通过高通量计算和第一原理方法预测出一大类全新的二维金属氢化物材料。该研究发现了110种二维金属氢化物具有类似范德华粘合的弱层间相互作用以及良好的结构稳定性。而且,这类二维金属氢化物不易与周围环境发生化学反应,有利于实验的制备与转移。这一结果令研究者感到惊讶,因为宏观金属氢化物没有类似石墨的层状晶格结构。这归因于氢在纳米尺度下的强大而灵活的成键能力。

更为重要的是这类二维材料具有丰富的物性,它们几乎可以提供现有材料所能企及的全部电子性质,从金属跨越到绝缘体,从铁磁体跨越到反铁磁体。研究者还在这类材料中发现了多类对称性保护的拓扑电子态,包括狄拉克锥半金属、节环半金属以及拓扑绝缘体;这些材料中的电子运动速率可趋近光速,具有制造高频电子器件的应用潜力。

二维金属氢化物是二维材料家族崭新的成员,将带给我们更多的惊喜,有待实验与理论研究的进一步挖掘,让我们拭目以待。

论文链接:
https://pubs.acs.org.ccindex.cn/doi/full/10.1021/jacs.9b02279

Waving potential at volt level by a pair of graphene sheets

04/12/2019 - 20:14

Highlights
• Employing a pair of graphene sheets generates waving potential at volt level.

• Such waving potential can be utilized as power supply for external circuit.

• Deep insight of underlying mechanism is given by comprehensive experiments and molecular dynamic simulations.

Electricity generated from liquid flow on graphene has triggered great interests and shown its prospect in harvesting mechanical water energy. However, most attention so far has just been paid to voltage induced in graphene, little is paid for the energy induced by ions movement on liquid side. Here, by employing a pair of graphene sheets, we report the electricity generation in liquid. An open circuit voltage up to 1 V can be obtained as graphene moves across electrolyte solution. Molecular dynamics simulations indicate that the underlying mechanism is due to potential difference generated between solutions near graphene surface and that in the bulk. Experimental results show that the generated voltage has a close relation with moving speed of graphene, ion concentration and species. This work of considering ion motion in solution should provide new ideas for the development in harvesting water energy with graphene.

Illustration of experimental setup and voltage signals at the load resistor

Velocity dependence on voltage signals.

Novel nonlinear coarse-grained potentials of carbon nanotubes

04/12/2019 - 19:58

Centimetres-long carbon nanotube (CNT) bundles with tensile strength over 80 GPa have been fabricated and tested recently [Nat. Nanotechnol. 13, 589–595 (2018)], but it is still a tremendous challenge to predict their nonlinear mechanical behaviors by full-atom molecular dynamics (MD) due to the huge computational cost, particularly for carbon nanotube networks. We completely established here the explicit expressions of the chirality-dependent higher-order nonlinear coarse-grained stretching and bending potentials based on the full-atom Reactive Empirical Bond-Order interatomic potential of second generation (REBO potential). In particular, the coarse-grained non-bonded potentials are improved by using the 18–24 Lennard-Jones potential. By comparison with available experimental results and full-atom MD simulations as well as our analytical results, the present nonlinear coarse-grained potentials have high accuracy. The obtained nonlinear coarse-grained potentials can be used to efficiently characterize the nonlinear mechanical behaviors and understand the failure mechanism of the CNT bundles and networks with 2∼5 orders of magnitude reduction in computing time, which should be of great help for designing and assembling CNT-based flexible microdevices.

The two parallel SWCNTs and their CG model. (a) The CG model for a SWCNT and (b) Full-atom atomic structure and CG model for two parallel SWCNTs

The mechanical properties of different chiral SWCNTs under tension and compression (before buckling) based on the REBO potential using full-atom MD simulations and the nonlinear SSM.

Graphynes for Water Desalination and Gas Separation

02/01/2019 - 14:55

Selective transport of mass through membranes, so-called separation, is fundamental to many industrial applications, e.g., water desalination and gas separation. Graphynes, graphene analogs yet containing intrinsic uniformly distributed pores, are excellent candidates for highly permeable and selective membranes owing to their extreme thinness and high porosity. Graphynes exhibit computationally determined separation performance far beyond experimentally measured values of commercial state-of-the-art polyamide membranes; they also offer advantages over other atomically thin membranes like porous graphene in terms of controllability in pore geometry. Here, recent progress in proof-of-concept computational research into various graphynes for water desalination and gas separation is discussed, and their theoretically predicted outstanding permeability and selectivity are highlighted. Challenges associated with the future development of graphyne-based membranes are further analyzed, concentrating on controlled synthesis of graphyne, maintenance of high structural stability to withstand loading pressures, as well as the demand for accurate computational characterization of separation performance. Finally, possible directions are discussed to align future efforts in order to push graphynes and other 2D material membranes toward practical
separation applications.

Toward a more scientific science

09/27/2018 - 12:59

Climb atop shoulders and wait for funerals. That, suggested Newton and then Planck, is how science advances (more or less). We've come far since then, but many notions about how people and practices, policies, and resources influence the course of science are still more rooted in traditions and intuitions than in evidence. We can and must do better, lest we resign ourselves to “intuition-based policy” when making decisions and investments aimed at driving scientific progress. Science invited experts to highlight key aspects of the scientific enterprise that are steadily yielding to empirical investigation—and to explain how Newton and Planck got it right (and Einstein got it wrong). —Brad Wible

郭老师寄语:希望我们能忠实于事实,质疑教科书式的经验和教条,做出重要科学发现!

Link to the article

4 Water-evaporation-induced electricity with nanostructured carbon materials

01/30/2017 - 11:26

Water evaporation is a ubiquitous natural process that harvests thermal energy from the ambient environment. It has previously been utilized in a number of applications including the synthesis of nanostructures and the creation of energyharvesting
devices. Here, we show that water evaporation from the surface of a variety of nanostructured carbon materials can be used to generate electricity. We find that evaporation from centimetre-sized carbon black sheets can reliably generate
sustained voltages of up to 1 V under ambient conditions. The interaction between the water molecules and the carbon layers and moreover evaporation-induced water flow within the porous carbon sheets are thought to be key to the voltage
generation. This approach to electricity generation is related to the traditional streaming potential, which relies on driving ionic solutions through narrow gaps, and the recently reported method of moving ionic solutions across graphene surfaces, but as it exploits the natural process of evaporation and uses cheap carbon black it could offer advantages in the development of practical devices.

Hydroelectric generator from transparent flexible zinc oxide nanofilms

11/25/2016 - 11:06

Harvesting wave energy based on waving potential, a newly found electrokinetic effect, is attractive but limited mainly to monolayer graphene. Here we demonstrate that moving a transparent flexible ZnO nanofilm across the surface of ionic solutions can generate electricity. The generated electricity increases linearly with the
moving velocity with an open-circuit voltage up to tens of millivolt and a short-circuit current at the order of microampere. The harvested electricity can be efficiently scaled up through series and parallel connections. Theoretical simulations show that it is the proper electrical property that endows the ZnO nanofilm with the outstanding capacity in harvesting the wave energy.

Tunable electronic and magneticproperties of two-dimensionalmaterials and theirone-dimensional derivatives

03/15/2016 - 11:16

Low-dimensional materials exhibit many exceptional properties and functional-ities which can be efficiently tuned by externally applied force or fields. Here wereview the current status of research on tuning the electronic and magnetic prop-erties of low-dimensional carbon, boron nitride, metal-dichalcogenides, phos-phorene nanomaterials by applied engineering strain, external electric field andinteraction with substrates, etc, with particular focus on the progress of computa-tional methods and studies. We highlight the similarities and differences of theproperty modulation among one- and two-dimensional nanomaterials. Recent breakthroughs in experimental demonstration of the tunable functionalities intypical nanostructures are also presented. Finally, prospective and challenges forapplying the tunable properties into functional devices are discussed.

Large-Area, Periodic, Hexagonal Wrinkles on Nanocrystalline Graphitic Film

08/25/2015 - 15:28

Sinusoidal wrinkles develop in compressively stressed film as a means to release stored elastic energy. Here, a simple way to fabricate large-area, periodic, hexagonal wrinkled pattern on nanocrystalline graphitic films grown on c-plane sapphire (<50 nm thick) by the spontaneous delamination–buckling of the as-grown film during cooling is reported. According to the continuum mechanics calculation, strain-relief pattern adopting the hexagonal wrinkled pattern has a lower elastic energy than that of the telephone cord wrinkle at thickness regime below 50 nm. A high-fidelity transfer method is developed to transfer the hexagonal wrinkled films onto arbitrary substrates. Nanoindentation studies show that hexagonal wrinkle film engineered this way may act as shock absorber. The hexagonal wrinkled carbon film is able to selectively promote the differentiation of human mesenchymal stem cell toward the osteogenic lineage in the absence of osteogenic inducing medium.