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We are conducting research on the mechanical properties and multiphysics of materials with atomistic modeling and ab initio calculation.

Project-driven research topics

Atomistic modeling simulation of anode materials of solid oxide fuel cells (CREST)

Solid oxide fuel cells (SOFCs) are one of the most promising energy sources in the next generation due to their high efficiency. This study targets anode materials for SOFCs and performs classical molecular dynamics simulations and first-principles calculations to reveal the mechanisms of reduction in triple phase boundary (TPB) density by sintering and chemical reaction at TPB.

Molecular dynamics simulation of NiO reduction using ReaxFF.

Multiscale simulation of structural polymer materials and composites (ImPACT)

As the application of plastic composite materials to automobiles and airplanes is getting more and more accelerated these days, it is urged to establish guidelines for designing polymer materials possessing both high strength and toughness. We perform a 'bottom-up type' multi-scale simulation aided by the coarse-grained molecular dynamics method with the aim to obtain constitutive laws of materials by numerical simulations. We also carry out finite-element method (FEM) simulations of crack propagation problems to build a model that reproduce experimental results and to clarify the mechanism of crack propagation.


FEM simulation of crack propagation in rubbers. So-called velocity jump phenomenon is successfully reproduced.


Molecular dynamics simulation of tension of polymer.

Molecular simulation for lubrication mechanism of metal surfaces with hetero-nano structure (Industry-academia liaison for fundamental research)

Experiments have demonstrated peculiar lubrication properties of steel having hetero-nano structures (i.e., ultrafine grain structures). With the aim to eucidate its mechanism, we perform coarse-grained molecular dynamics simulations of the system consisting of hetero-nano structured metal surfaces and lubricant molecules. We are tackling this problem from the viewpoint of polymer brush structures formed on the metal surfaces and their relation to grain structures.

     
Coarse-grained molecular dynamics simulation of lubricants in between metal surfaces mimicking hetero-nano structures.

Analysis of fracture in environmental barrier coatings for ceramics (SIP)

We perform finite element method (FEM) simulations of environmental barrier coating (EBC) for SiC-based ceramics used in aircraft engines. We calculate energy release rates of crack propagation under practical mechanical and thermal conditions in order to evaluate the reliability of EBC.

General research topics

Ideal strength of solids

The ideal strength is defined as the maximum stress that a perfect crystal can attain when the crystal undergoes uniform (ideal) deformation. While macroscopic materials possess far smaller strength than the ideal strength due to defects being sources of fracture, nano-scale materials with few or no defects can be closer the ideal strength. The ideal strength is also a key index to understand elementary process of plastic deformation in crystals. For example, the critical shear stress for dislocation nulcreation in pristine crystals is close to the ideal shear strength. Our group has been evaluating ideal strengths of various crystals by means of density functional method calculations with the aim to reveal fundamental mechanical properties of crystals.

Ideal shear strength of crystals
When dislocations are nucleated, crystal lattices in the vicinity of the nucleation site undergoes shear deformation. The nucleation condition should be closely related with the crystal strength of ideal shear deformation, i.e., the ideal shear strength. We have performed first-principles density functional theory calculations of Si, SiC and GaN to obtain their ideal shear strengths. For SiC and GaN, we also took into account the effect of superimposed normal stress components on the shear strength, which provides useful information to understand crystals' behavior under real situation.
[Materials Science and Engineering: B, 88-1(2002), pp.79-84]
[Modelling and Simulation in Materials Science and Engineering, 15 (2007), pp.27-37]

Effect of normal stress on ideal shear strength
In real situation, crystals undergo various combinations of stress components. For example, shear stress on slip planes together with compression is exerted on crystal lattices in dislocation nucleation in a nano-indentation test. It is known that, in metal, the critical shear stress (ideal shear strength) is usually increased by compression. Our group has examined the effect of normal stress on ideal shear strength of covalent crystals. We have revealed that, in covalent crystals, the response of shear strength to normal stress depends on the type of crystal, meaning that the ideal shear strength is decreased by compression in some covalent crystals, in contrast to the phenomenon in metal.
[Physical Review B, 77 (2008), art. 100101R] [Journal of Physics: Condensed Matter, Vol. 23 (2011), art. 385401]
Tensile strength of Si nanofilms
様々な構造が強度低下にどの程度影響を及ぼすのかを検討するため,シリコンナノ薄膜を対象に第一原理引張りシミュレーションを行い,(100) 理想表面が強度に及ぼす影響を検討しました.[Physical Review B, 72 (2005), art. 165431]
Ideal shear strength under finite temperatures
現実ではモノは有限温度下で変形するため,絶対零度下の理想強度評価とは乖離が生じます.上記のような理論強度解析と実験で測定した臨界応力に乖離があるとき,それが欠陥などに由来するのか温度に由来するのかが不明でした.そこで,「欠陥を含まない理想構造」に対して,有限温度下での理想変形シミュレーションを行うことにより,純粋に温度が強度に及ぼす影響を検討しました.[Physical Review B, Vol. 84 (2011), art. 224118]

Atomistic structural instability analysis

塑性変形の素過程は,転位や双晶変形など原子配列の「ずれ」であるので,材料の変形の性質を明らかにするためには,究極的には原子レベルで考える必要があります.転位のような現象は,局所的な原子構造が不安定になって生じたものと理解することができます.そこで本研究室では,結晶材料の構造不安定性の発現メカニズムを原子レベルから明らかにすることに取り組んでいます.
我々は,任意の原子構造体に対して「厳密に」構造不安定性を評価できる手法(Atomistic Structural Instability Analysis; ASIA)を提案し,様々なナノ構造の構造不安定問題に適用することで,塑性変形のメカニズムの本質に迫ろうとしています.
[Computational Materials Science, Vol.29-4 (2004), pp.499-510]
[Materials Science and Engineering: A, Vol.379/1-2 (2004), pp.229-233]
[Materials Science and Engineering: A, 462/1-2 (2007), pp.450-455]
[Physical Review B, 80 (2010), art. 104108 (11 pp)]
[Key Engineering Materials, Vols.592-593 (2014), pp.39-42]

Ferroelectric capacitors and piezoelectric materials

 強誘電体とは,原子の位置が理想配置から少しずれることによって自発的に(外力や電場の作用なしに)電気分極を持つことができる材料のことです.この性質を利用すれば,微小なメモリデバイスの素子を作ることができます.電極薄膜で強誘電体をサンドイッチすることにより,不揮発性メモリ(電源をオフにしても情報が消えない)として機能することがわかっており,超省電力のメモリデバイスとして応用が期待されています.
 我々は,第一原理計算を用いることで,この薄膜構造を理論的にはどこまで薄くすることができるか,電極材料を違うものに変えるとどうなるのか,といった,設計上欠かせない特性を解析しています.
[Physical Review B, 74 (2006), art. 060101R]
[Physical Review B, 80 (2009), art. 205122 (8 pp)]


Deformation of CNTs

 カーボンナノチューブに変形を加えることで電子のバンド構造が変わり,電気的特性を変化させることで新奇のナノデバイスに応用することが考えられます.そこで,軸方向引張りや半径方向圧縮による変形挙動とバンドギャップの変化を計算し,外力負荷による電気伝導性変化の性質を明らかにしました.
 また,多層ナノチューブに圧力を加えることでしわ状の構造が現れる現象についても原子シミュレーションを行っており,センサなどの新たなデバイスへの応用可能性を検討しています.
Computational Materials Science, Vol.30/3-4 (2004), pp.283-287
Computational Materials Science, Vol.31/1-2 (2004), pp.33-41
Mechanics of Advanced Materials and Structures, to be published


Magnetic materials under deformation

 磁気記憶デバイスやスピントロにクスへの応用が期待される磁性材料は最も注目を集める機能性材料の一つです.微小デバイスでは界面のミスマッチ等により高ひずみ状態となりうるなどの理由から,ひずみ(応力)の磁気的特性への影響を検討することが求められています.本研究室ではこれまで,第一原理解析を用いて,有機質強磁性半導体やハーフメタリック材料を対象に引張り(圧縮)やせん断ひずみ下での磁気的特性変化のメカニズムの検討を行っています.たとえば,有機質ネットワーク構造を持つ材料では,せん断ひずみを与えるとネットワーク要素が回転することでせん断ひずみを吸収し,それぞれの要素構造はほとんど変化しないため磁気的性質はあまり変わらないなど,興味深い結果が得られています.
[Journal of Physics: Condensed Matter 24 (2012), 245501 doi:10.1088/0953-8984/24/24/245501]
[Journal of Materials Research, Vol. 28, No. 12 (2013), pp.1559-1566]


Surface stress response to charging

ナノポーラス金属を電解液中に浸し,帯電させることによって金属の収縮が起こるという現象が報告されています.これは表面帯電が金属の表面応力に影響するためと考えられます.我々は第一原理計算により,帯電が金属表面応力に及ぼす影響を理論的・定量的に評価し,そのメカニズムを解明しました.
[Europhysics letters, Vol. 78 (2007), 13001]
[Europhysics letters, Vol. 84 (2008), 13002 (6 pp)]
[Physical Review B 85 (2012), 125118 (5 pp)]

Stacking fault of SiC

半導体パワーデバイス等への応用が期待されるSiCの製造製造プロセスで,様々な構造欠陥が導入されてしまうことが問題になっています.そのため,構造欠陥を減らすためのエンジニアリングが重要であり,そのためにはSiCの構造欠陥特性(欠陥エネルギー等)を理解することが必要です.我々は,3C-SiCの積層欠陥エネルギー,積層欠陥生成によって発生する応力,窒素元素添加の影響などを第一原理解析により定量的に評価しました.
[Physica Status Solidi (b) 249, No.6 (2012) pp.1229-1234, DOI: 10.1002/pssb.201147487]


Atomistic simulation of solid oxide fuel cells (SOFCs)

[ECS Transactions 57-1 (2013) pp.2799-2809]
[ECS Transactions 57-1 (2013) pp.2811-2819]

Construction of interatomic potentials