Silicates as Bilayer

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Intercalated Hexagonal Boron Nitride/ Silicates as Bi-layer Multifunctional Ceramics Rouzbeh Shahsavari ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b15377 • Publication Date (Web): 08 Jan 2018 Downloaded from http://pubs.acs.org on January 8, 2018

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ACS Applied Materials & Interfaces

Intercalated Hexagonal Boron Nitride/Silicates as Bi-layer Multifunctional Ceramics

Rouzbeh Shahsavaria,b,c,* a

Department of Civil and Environmental Engineering, Rice University, Houston, TX 77005

b c

Department of Material Science and NanoEngineering, Rice University, Houston, TX 77005

Smalley Institute for Nanoscale Science and Technology

*Corresponding author email: [email protected]

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Abstract By performing advanced syntheses and an extensive 150+ first principles calculations, this work demonstrates how the exotic properties of emerging 2D hBN nanosheets (e.g. ultrahigh surface area, high mechanical and thermal tolerance) can be coupled strategically (via exfoliation and geometrical compatibility) with the lamellar nanostructure of calcium-silicate crystals to introduce “reinforcement” at the basal plane of materials, i.e. the smallest possible scale. Probing mechanical properties show significant enhancement in strength, toughest, stiffness and strain, providing key guidelines to intercalate a suite of emerging 2D materials in ceramics for the bottom-up design and fabrication of ultra-high performance and multifunctional ceramic composites.

Keywords: Boron Nitride, Hybrid Cement, Intercalation, Multifunctional Composites, Mechanical Properties

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Boron nitride is a synthetic material made from boric acid or boron trioxide.1 Among its various crystalline forms, its hexagonal allotrope, hexagonal boron nitride (h-BN), is similar to graphite in structure and layered form but with alternating B and N atoms. h-BN has remarkable thermodynamic (air stable up to 1000 oC) and chemical stability, exceptional hardness, and great thermal conductivity while electrically insulating. These properties make h-BN suitable for many technological applications. hBN can also exhibit unique features such as superb thermal conductivity, excellent mechanical strength along with remarkable chemical stability.2 Moreover, superhydrophobic nature of hBN is useful in non-wetting surfaces or underwater constructions.1 BN can also be used for corrosion resistant surfaces. Current commercial products of h-BN include various thermal management materials such as thermal pads, thermal grease, t h e r m a l coatings, and various cosmetics (due to their role as solid lubricants). Furthermore, high neutron absorption cross section of Boron and advantages of multilayered nanostructured materials (i.e. numerous interfaces) to sink radiation3, make h-BN a suitable candidate for intercalation in ceramics for nuclear shielding.

From a mechanical perspective, 2D h-BN sheet, is an excellent reinforcing materials, similar graphene or graphene oxide. Recently, Walker et al. demonstrated a giant enhancement in the toughness o f ceramic composites using graphene platelets.4 The key factors to fracture toughness were mechanism of anchoring and wrapping the graphene platelet fillers underneath the silica grains, forming a continuous wall of graphene platelet fillers along the grain boundaries. This arrangement effectively arrests crack propagation in 3D rather than 2D. With relatively similar structures, graphite and BN are also used to synthesize various 3



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forms of carbon nanotubes (CNTs) and boron nitride nanotubes (BNNTs). In spite of the differences in the electronic properties of CNTs and BNNTs, they possess similar mechanical properties specifically in the Young’s modulus, demonstrating their potential applications as mechanical reinforcement 5,6. A key advantage of 2D nanomaterials such as hBN over 1D materials such as BNNT or CNT is that they posses double surface area for an identical mass (imagine an unzipped BNNT). This property lead to extremely high surface area (ideally ~2200 m2/g for an exfoliated single layer h-BN), providing excellent capacity for functionalization and binding to the surrounding matrix

7,8,9,10

sheets can

properties of BN-based

significantly improve the

composites. An addition of o n l y

mechanical

. For instance, ultrathin h-BN polymer

~0.3 wt.% h - B N nanosheets in Poly(methyl

methacrylate) (PMMA) increases the elastic modulus and strength by ~22% and ~11% respectively 11.

Among various materials that can be benefited from the exotic properties of h-BN, infrastructure materials hold a great promise for impactful outcomes. Concrete, as the most widely used material on Earth, is a brittle material with a strong compression strength (>200 MPa), but relatively weak tension, flexural and fracture toughness (that correlates with ductility)12. While reinforcing steel bars can partially overcome these issues, they are not able to prevent local cracking and allowing the material to resist high flexural loads. This has encouraged incorporation of several additives into cement paste, which is the key binder in concrete. Examples include polymer modified cement (for enhancing ductility), fiberreinforced cement (for micro reinforcement), etc. While multifunctional 2D materials (e.g. high mechanical, thermal, radiation, and corrosion resistance)13,14 hold a great promise for 4


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incorporating into cements and ceramics, the difficulty lies in making these 2D nanomaterials as an “integral” part of the system (crystal), as opposed to conventional 1D and 2D filer-type reinforcement.

Here, we introduce the next-generation of bi-layer hBN/cement crystal with superior properties and performance (e.g. concurrent high strength and high toughness) that is unachievable by conventional admixtures. We demonstrate, for the first time, the feasibility (stability) and mechanical enhancement of intercalated h-BN sheets, as small as one atom in thickness, in the inter-layer spaces of Calcium-Silicate-Hydrate (C-S-H), the primary product of cement hydration15,16,17,18. C-S-H is a layered material with an interlayer space (gel pores) of ~1-1.5 nm, akin to the minerals tobermorite19 (Fig. 1a)

20

. This layered structure, when properly tailored to

the ultrathin layered structure of h-BN and its high surface-to-volume ratio, result in multifunctional hBN/cement nanocomposites with superior characteristics such as mechanical, durability, thermal, and radiational properties. Here, we demonstrate the mechanical property enhancements by performing 150+ expensive density functional theory (DFT) calculations.

We used tobermorite 11 Å, Ca6Si6.2H2O, with a monoclinic structure as an structural model for C-S-H21. It has a crystalline structure composed of silica chains (with the repeating units of silica dimers connected by a bridging silicon tetrahedra) connected to calcium atoms (Fig 1). We used single layer exfoliated h-BN sheet with 36 atoms for intercalation in between the tobermorite layers (Fig 1d). Except stated otherwise, Periodic boundary conditions (PBC) are utilized.

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(a)$

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(c)

(b)$

(d)

Typical$ bridging$Silicon$ Tetrahedron$

Interlayer$ space$

b

c$

c$ a

b

b 123°

a

(a) (e)

120°

a

a

(b) (f) A typical O-B bond Wavy BN sheets

c

a

c

b

a

(g) tobermorite

(h) BN-tobermorite

-0.04 e

hBN layer

Tobermorite layer

hBN layer

0.02 e

Fig 1. a-c) Atomistic structure of Tobermorite 11 Å as an structural model for C-S-H. For clarity a supercell of 2 x 2 x1 is shown. (a) and (b) are sideviews and (c) is the top view.. d) a top view of a single layer 2D h-BN. e-f) Relaxed atomistic structure of intercalated BN-Tobermorite crystal. For clarity a supercell of 4 x 4 x1 is shown. Covalent bonds between B and the Oxygen of the bridging silicon tetrahedra in tobermorite are clearly identifiable in side views of (e) and 6


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(f). These bonds change the initially flat form of BH sheets to a wavy surface illustrated in (f). In (a-f) Yellow, red, blue and white colors represent Si, O, Ca and H atoms, respectively while dark blue and pink denote N and B. (g-h) charge density in pure tobermorite and hybrid BNtobermorite where the latter shows orbital mixing between BN and tobermorite (red arrows). All DFT calculations are conducted using GGA exchange correlation functionals22. For energy and stress calculations, we used ultrasoft pseudopotentials with a plane wave basis set and a cutoff energy of 420 eV for the wavefunctions and 50350 eV for the charge density, implemented in Quantum Espresso. As the system sizes were relatively large in our system (72 atoms in Tobermorite unit cell and 108 atoms in composite BN-Tobermorote unit cell), we use γpoint-sampling of the Brillouin zone. First, we performed 0 K energy minimization via variablecell relaxations in PWSCF to fully relax the crystals and avoid any possible metastable state.

To achieve equilibrium, each of the stress tensor components were below 0.5 kbar, and force components were below 0.025 eV/Å. To obtain the stress-strain plots, a series of small increments of ~0.1 Å is imposed; after each increment the atomic configuration is relaxed under fixed volumes and the stress tensor is determined from the virial expression. For non-orthogonal direction such as b-lattice direction (Fig 1), the normal stress is calculated through stress tensor transformations. DFT at its current state of development may not predict accurate van der Waals dispersion forces. However, in Tobermorite structures, the interlayer interactions are dominated by coulombic rather than van der Waals dispersion forces22. Thus, this issue should not affect our results.

The relaxed structure of BN-Tobermorite is shown in Fig 1e-f with cell parameters in Table 1, representing a triclinic structure. For clarity, a supercell of 4 x 4 x 1 is shown. Comparing these 7



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cell parameters, one notices key changes only in the c direction where the h-BN are intercalated. The hexagonal angle of h-BN (120 deg) closely matches with the γ angle of monoclinic tobermorite, i.e. ~123 deg, (Fig 1c-d)22. This interesting geometrical compatibility results in minimal mismatch in the in-plane structures of both tobermorite and h-BN during DFT relaxations, explaining the stability of the intercalated composite as a hybrid bi-layer crystal. Interestingly, along the out-of-plane direction, the initially flat BN sheets are changed to wavy surfaces because of the interactions of the Oxygens of the bridging Silicon tetrahedra, which covalently bond with B atoms of the BN sheets (Fig 1e-f). The typical O-B bond distances is ~1.5 Å. This bonding imposes a geometrical constraint, which make the flat hBN wavy (buckled) and our DFT results shows a significant charge transfer from Tobermorite to BN, confirming the covalent bonding between the two. The calculated Mulliken charges is 0.9 |e| indicating that BN is negatively charged, hence the substrate (tobermorite) is positively charged or in other words, tobermorite is p-doped. The magnitude of this charge transfer along with the buckled structural deformation of BN supports the BN/Tobermorite covalent interactions, explaining the origin of enhanced mechanical properties (to be discussed shortly). In brief, the deformation of BN (buckled structure) facilitates an overlap (donation) between B atoms and Tobermorite that stabilizes the adsorbate. The presence of interaction and charge transferring in BN-Tobemorite is revealed by computing and plotting the charge density, which is the sum of multiple local density of states at every point in space as a function of position and energy (Figure 1g-h). Remarkably, overlapping and mixing of states cause high local density of states (Figure 1h red color) and charge accumulation in BN-tobermorite. Such bonding (waviness) and charge transfer will have interesting influence on the fracture and toughening behavior, which will be discussed shortly. 8


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Table 1. Relaxed lattice dimensions of hybrid bi-layer BN-Tobermorite compared to tobermorite Crystal

a (Å)

b (Å)

c (Å)

α (deg)

β (deg)

γ (deg)

BN-Tobermorite

6.832

7.465

30.709

90.808

89.994

122.85

Tobermorite

6.60

7.40

23.13

90

90

123.62

(a) Tensile Stress (Gpa)

Tobermorite 20

BN-Tobermorite

Unfold S ffening

2

Failure

15

1

10

2

5

1

0

0

0

0.1

0.2

0.3

0.4

strain

(b) Compressive Stress (Gpa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

2

0

1

2

2 1

30 25 20 15 10 5 0

0

0

2

1

Folding of BN sheets

c 0

0.1

0.2

strain

0.3

0.4

1

2

1

2

a

Fig 2. In-plane stress-strain plots of the intercalated BN-Tobermorite along a-lattice direction obtained by DFT. (a) tensile behavior where BN-Tobermorite shows a two-regime behavior: i) unfolding the BN network, and ii) stretching the backbone BN (stiffening). On the right, the atomic snapshots of the unit cells of tobermorite and BN-tobermorite are shown, which correspond to the red/blue points on the stress-strain plots. The “0” labels indicate the relaxed state prior to applying any load. (b) compressive behavior where BN-Tobermorite contributes significantly to materials resilience. Plot legends and relaxed atomistic snapshots labeled by red/blue “0” are identical in (a) and (b). Yellow, red, blue and white colors represent Si, O, Ca and H atoms, respectively while dark blue and pink denote N and B.

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Now we study the elastic, inelastic and fracture patterns for each of the a, b and c lattice directions. Fig 2a shows the calculated stress-strain plots under tension in a-lattice direction. Tobermorite fails mainly due to rotations of silica chains along their axes (snapshots 1&2 in Fig 2a) with a strength of ~7 GPA and yield strain of ~8%. However, in the case of BN-Tobermorite, the h-BN sheets reinforce the structure such that its stress-strain behavior shows an interesting two-regime behavior including unfolding, and stiffening. Upon applying tensile load to the relaxed BN-Tobermorite structure (snapshot 0 in Fig 2a labeled in blue), the first stress-strain regime up to the yielding point 1 is chiefly due to unfolding (straightening) the wavy BN sheets (snapshot 1 in Fig 2a). This regime has low stiffness (i.e. slope of the curve) because it mainly involves geometrical changes. Then, the second regime from point 1 to point 2 corresponds to stretching the backbone of the BN-sheets where the actual backbone of B-N bonds are strained. This stretch of strong ionic B-N bonds results in sharp stiffening behavior with an ultimate strength of 18 GPA, ~3X larger than that of Tobermorite, and strain of ~25% (ultimate strength refer to the highest peak in the stress-strain plots).

In compression (Fig 2b), Tobermorite shows a low yielding strength of ~10 GPA with a yield strain of 7% (point 1) due to silica chains rotations. However, BN-Tobermorite behaves elastically up to 20% strain without any sign of yield or strength retrogression until the yield strength of 25 GPa. These high yield strength and yield strain are particularly important to increase material’s resilience, which indicates the maximum capacity to restore elastic energy. By acting as barriers, the network of wavy BN sheets pinned by Oxygen atoms of Tobermortie (Fig 1), contributes to the compressive resistance of BN-Tobermorite. However, at strains

10


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beyond 20%, the wavy BH sheets start to fold more and the stress drops (blue points on the curve and snapshots 1&2 in Fig 2b).

Along the b-lattice direction, since all silica tetrahedra chains of Tobermorite are in parallel to this direction, Tobermorite is able to resist relatively high strains and stresses in both tension and compression. This capacity in Tobermorite traces back to i) its strong Si-O bonds22, and ii) its silica tetrahedra re-orientations, which allows Tobermorite to accommodate larger strains/loads. For instance, under compression, typical Si-O-Si angles, which are ~140 deg in relaxed configurations22, reduce to ~130 deg at yield stress (point 1 in Fig 3b), followed by further reduction to ~120 deg at ultimate stress (point 2 in Fig 3b). In BN-Tobermorite, the BN sheets act in parallel to the silica chains. Under tensile load, while this joint contribution does not affect the yield strain (~12%), it increases both stiffness (note the higher slope in Fig 3a) and yield strength, thus contributing to resilience. This contribution in stiffness and strength in tensile loading comes from the strong (slightly ionic) B-N bonds, which are much more difficult to deform compared to the Si-O bonds, and rotation of silica tetrahedral in pure tobermorite. This explains the higher performance of BN-Tobermorite in Fig 3a. Similarly, under a compressive load, the BN sheets contribute positively to stiffness, but do not change the yield strength since BN sheets locally buckle at ~7% strain (snapshot in Fig 3b) and afterwards solely Tobrmorite takes any stress and strain. This buckling occurs between successive pining points supported by O-B bonds (Fig 1f). Toughness is also increased slightly in the compression. As expected, during both tension and compression, once the BN sheets fail, the stress values in BN-Tobermorite approach to that of pure tobermorite as if no BN sheets existed.

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Tobermorite

BN-Tobermorite

1

25 20

2

1

15

2

10 5 0

0

0

0.1

0.2

0.3

strain

0

1

2

1

0

2


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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30 25 20 15 10 5 0

0

1

2

1 2

Local Buckling of BN sheets

c 0

0.05

0.1

strain

0.15

0.2

0.25

1

2

1

2

b

a

Fig 3. In-plane stress-strain plots of the intercalated BN-Tobermorite along b-lattice direction obtained by DFT. (a) tensile behavior. On the right, the atomic snapshots of the unit cells of Tobermorite and BN-Tobermorite are shown, which correspond to the red and blue points on the stress-strain plots. (b) compressive behavior where BN sheets buckle starting at ~7% strain. The snapshots on the right show a locally bucked BN sheet. The relaxed configurations (labeled by “0”) and plot legends are identical in (a) and (b). Yellow, red, blue and white colors represent Si, O, Ca and H atoms, respectively while dark blue and pink denote N and B.

It view of out of plane direction (Figure 4), it appears that BN sheets have almost no positive effects on the stress-strain plots. The charged lamella of Tobermorite cause strong electrostatic interactions between the lamella23. At ~1 nm, which is about the interlayer distance of Tobermorite 11 Å, these long range columbic interactions become comparable to the in-plane (iono)covalent bonds resulting in comparable yield strengths (compare point 1 in Figs 2a and 4a).22 Interestingly, snapshots 1&2 labeled by red color in Fig 4a show that the rupture occurs in the intralayer Calcium sheet of Tobermorite. This is in contrast to the common intuition that the 12


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weakest plane in a layered material is the interlayer distance. This feature of Tobermorite corroborates on their strong interlayer columbic interactions. However, increasing the interlayer distance or addition of water/impurities in the interlayer space diminishes the interlayer columbic interactions22. In BN-Tobermorite, BN sheets act as impurities by shielding the electrostatic interactions as well as increasing the interlayer distance from ~1.156 nm to 1.535 nm. This shielding of electrostatic interactions results in lower ultimate strength in BN-Tobrmorite. However, the out of plane stiffness of BN-Tobermorite is higher than pure Tobrmorite due to the contributing role of the covalent O-B bonds between the silica chains and BN sheets. These sparse B-O bonds do not have the critical mass to chemically link the Tobermorite layers, and thus the interlayer distance - where BN sheets are intercalated - becomes eventually the weakest plane for strength (snapshots 1&2 labeled by blue in Fig 4a). Under compression (Fig 4b), the stress-strain behavior of BN-Tobermorite is improved, specially at higher strains. This improvement is typically expected due to overlapping constraints of atomic radii.

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Tobermorite 8

BN-Tobermorite

2

6

1

4

1 2

2 0

0

0

0.1

0.2

0.3

0.4

0.5

strain

0

1

2

0

1

2


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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2

40

2

30

1

20 10

00

c

1

0

0.1

0.2

strain

0.3

0.4

1

2

1

2

b

a

Fig 4. Out of plane stress-strain plots of the intercalated BN-Tobermorite obtained by DFT. (a) tensile behavior. On the right, the atomic snapshots of the unit cells of Tobermorite and BNTobermorite are shown, which correspond to the red and blue points on the stress-strain plots. (b) compressive behavior. The relaxed configurations (labeled by “0”) and plot legends are identical in (a) and (b). Yellow, red, blue and white colors represent Si, O, Ca and H atoms, respectively while dark blue and pink denote N and B.

Overall, when averaged over all directions and over both tensile and compressive behaviors, BN sheets improve (in some cases marginally) all mechanical properties including yield strength, stiffness, etc (Table 2). This “averaging” is relevant because the loading directionality of such tiny hybrid nanomaterials in real setting is random and an average method is best to quantify the response, akin to the concept of bulk and shear moduli for low symmetry tobermorite crystals22,23. However, BN sheets serves best as strengthening components in the interlayer direction. Along the a-direction, the BN sheets can improve the ultimate tensile strength by 14


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stiffening the structure. Similarly, increased compressive yield strain and strength lead to enhanced resilience in compression. Along the in-plane b-direction, the changes in mechanical properties are less pronounced because Tobermorite has already strong silica chains parallel to this direction. In the out-of-plane direction, there are neither enough covalent nor electrostatic and van Der Waals interactions between BN and Tobermorite. Therefore, addition of BN sheets leads to small improvement in mechanical properties. Functionalization of BN sheets should change this picture, which we study in the future. There are also generally other toughening mechanisms associated with hBN sheets dispersed in the matrix1 including crack- deflection, bifurcation, and pull-out24.

a-direction T

Compression

Tension

Yield Strain (%) Yiled Strength (Gpa) Ultimate strain (%)

6.5 10 19

b-direction

BN-T Diff (%) 19 26.5 26.5

T

BN-T Diff (%)

192 165 39

13 18.5 22

24.5 19.5 1.2 0.63 142 257.1 12.5 12.5 12 22 22.7 22.9

Ultimate strength (Gpa) Resilience (Gpa) Stiffness (Gpa) Yield Strain (%) Yiled Strength (Gpa) Ultimate strain (%)

26 16.5 0.33 2.518 154 139.5 8 8.1 7.5 5 20 22

-37 675 -9 1 -33 10

Ultimate strength (Gpa) Resilience (Gpa) Stiffness (Gpa)

5 17 0.3 0.203 93.8 61.73

240 -33 -34

15 0.75 96

c-direction

7 18 17

15 1.375 176

T

Overall Improvement BN-T Diff (%) (%) -36 -54 9

37 36 8

38 38.5 1.58 0.46 97.2 69.57 9 6 4.7 3.5 21 14.8

1 -71 -28 -33 -26 -30

-19 185 14 -11 8 -6

6.2 2.8 0.21 0.105 52.2 58.33

-55 -50 12

62 0 20

-46 -3 -23

18 17.5 32

-20 -48 81 0 83 1 0 83 83

11.5 8 34.8

Table 2. Comparison of the mechanical properties of the intercalated BN-Tobermorite versus pure Tobermorite. We demonstrated the feasibility of intercalating ultra-thin exfoliated sheets of emerging 2D materials (hBN) in the nanometer interlayer spaces (gel pores) of C-S-H crystal, the smallest building blocks of concrete. This fundamental approach illustrates reinforcement of C-S-H at the smallest possible scale (the basal plane), resulting in a hybrid bi-layer hBN/cement triclinic 15



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crystal where hBN is an integral part of the system (vs conventional 1D/2D fillers). The energetic stability of this bi-layer crystal is due to the close proximity of the hexagonal angle of h-BN (120 deg) with the γ angle of monoclinic C-S-H (~123 deg), leading to minimal crystal mismatch. Furthermore, the hBN sheets in the hybrid structure exhibit a wavy pattern, significant enhancement in mechanical properties, owing to covalent bonding, charge transfer and orbital mixing between tobermorite and BN. Propelled by h-BN and along the in-plane direction, a tworegime deformation phenomena, i.e. unfolding and stiffening, confer enhanced strength and toughness concurrently, properties that are mutually exclusive in typical engineered materials25,26,27,28,29. Compared to 1D nanomaterials such as CNT, 2D materials such as hBN exhibit double surface area per unit mass (ideally 2200 m2/g) that is attainable by exfoliation (and further enhanced by functionalization), thus perfect for bridging and reinforcement at smallest possible scales. This strategy in intercalating ultrathin emerging 2D materials in cement holds a great promise for designing next generation multifunctional bi-layer cementitious materials. Broadly, this work can impact other systems such as stacked heterostructures30 and ceramics and opens up a new space to intercalate a suite of other emerging multifunctional 2D mono- and multi-layer atomic sheets (e.g., MoS2 and WS2, layered double hydroxides) in ceramic-based materials for a bottom-up fabrication of ultra-high performance composites.

Acknowledgement R.S. acknowledges the financial support from the National Science Foundation (grant number 1538312) and the Research Computing Support Group at Rice University for providing supercomputing facilities. The supercomputer machines utilized in this work were supported in part by NIH award NCRR S10RR02950 and an IBM Shared University Research (SUR) Award 16


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in partnership with CISCO, Qlogic and Adaptive Computing, and in part by the Data Analysis and Visualization Cyber infrastructure funded by NSF under grant OCI-0959097.

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Shayeganfar, F.; Shahsavari, R. Oxygen- and Lithium-Doped Hybrid Boron-Nitride/Carbon Networks for Hydrogen Storage. Langmuir 2016, 32 (50), 13313−13321.

7

Shahsavari, R.; Sakhavand, N., Hybrid Cementitious Materials: Nanoscale Modeling and Characterizations Innovative Development of Advanced Multifunctional Nanocomposites in Civil and Environmental Engineering, 1st ed.; Woodhead Publishing: Cambridge, U.K., 2016; p 7910.1016/B978-1-78242-326-3.00005-1.

8

Tao, L.; Theruvakkattil Sreenivasan, S.; Shahsavari, R. Interlaced, Nanostructured Interface with Graphene Buffer Layer Reduces Thermal Boundary Resistance in Nano/Microelectronic Systems. ACS Appl. Mater. Interfaces 2017, 9 (1), 989−998.

9

Sakhavand, N.; Shahsavari, R. Asymmetric junctions boost in-plane thermal transport in pillared graphene, ACS Appl. Mater. Interfaces 2016, 9, 39122-39126.

10

Shayeganfar, F.; Beheshtiyan, J.; Neek-Amal, M.; Shahsavari, R. Electro- and opto-mutable properties of MgO nanoclusters adsorbed on mono- and double-layer graphene. Nanoscale 2017, 9 (12), 4205−4218. 17



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Zhi, C.; Bando, Y.; Tang, C.; Kuwahara, H.; Golberg, D. Large-Scale Fabrication of Boron Nitride Nanosheets and Their Utilization in Polymeric Composites with Improved Thermal and Mechanical Properties, Adv. Mater. 2009, 21, 2889.

12

Shahsavari R., Hierarchical Modeling of Structure and Mechanics of Cement Hydrate, Ph.D Thesis, Cambridge, MIT, Cambridge, MA, 2010. 13

Kim, N. D.; Metzger, A.; Hejazi, V.; Li, Y.; Kovalchuk, A.; Lee, S.-K.; Ye, R.; Mann, J. A.; Kittrell, C.; Shahsavari, R.; Tour, J. M. Microwave Heating of Functionalized Graphene Nanoribbons in Thermoset Polymers for Wellbore Reinforcement. ACS Appl. Mater. Interfaces 2016, 8 (20), 12985−12991. 14

Shahsavari, R.; Sakhavand, N. Junction configuration-induced mechanisms govern elastic and inelastic deformations in hybrid carbon nanomaterials. Carbon 2015, 95, 699−709.

15

Shahsavari, R.; Tao, L.; Chen, L. Structure, Energetics, and Impact of Screw Dislocations in Tricalcium Silicates. J. Am. Ceram. Soc. 2016, 99 (7), 2512−2520.

16

Shahsavari, R.; Chen, L.; Tao, L. Edge dislocations in dicalcium silicates: Experimental observations and atomistic analysis. Cem. Concr. Res. 2016, 90, 80−88.

17

Shahsavari, R.; Chen, L. Screw Dislocations in Complex, Low Symmetry Oxides: Core Structures, Energetics, and Impact on Crystal Growth. ACS Appl. Mater. Interfaces 2015, 7 (4), 2223−2234.

18

Moghaddam, S. E.; Hejazi, V.; Hwang, S. H.; Sreenivasan, S.; Miller, J.; Shi, B. H.; Zhao, S.; Rusakova, I.; Alizadeh, A. R.; Whitmire, K. H.; Shahsavari, R. Morphogenesis of cement hydrate. J. Mater. Chem. A 2017, 5 (8), 3798−3811.

19

Tao, L.; Shahsavari, R. Diffusive, Displacive Deformations and Local Phase Transformation Govern the Mechanics of Layered Crystals: The Case Study of Tobermorite. Sci. Rep. 2017, 7 (1), 5907.

20

Zhang, N.; Carrez, P.; Shahsavari, R. Screw-Dislocation-Induced Strengthening−Toughening Mechanisms in Complex Layered Materi- als: The Case Study of Tobermorite. ACS Appl. Mater. Interfaces 2017, 9 (2), 1496−1506.

21

Jalilvand, S.; Shahsavari, R. Molecular Mechanistic Origin of Nanoscale Contact, Friction, and Scratch in Complex Particulate Systems. ACS Appl. Mater. Interfaces 2015, 7 (5), 3362−3372. 22

Shahsavari, R.; Buehler, M. J.; Pellenq, R. J. M.; Ulm, F. J. First- Principles Study of Elastic Constants and Interlayer Interactions of Complex Hydrated Oxides: Case Study of Tobermorite and Jennite. J. Am. Ceram. Soc. 2009, 92 (10), 2323−2330. 18


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Shahsavari, R.; Pellenq, R. J.-M.; Ulm, F.-J. Empirical Force Fields for Complex Hydrated Calcio-Silicate Layered Materials. Phys. Chem. Chem. Phys. 2011, 13 (3), 1002−1011.

24

Farzanian, S.; Shahsavari R. Mapping the coupled role of structure and materials in mechanics of platelet-matrix composites. J. Mech. Phys. Solids 2017, 112, 169-186.

25

Zhang, N.; Shahsavari, R. Balancing strength and toughness of calcium-silicate-hydrate via random nanovoids and particle inclusions: Atomistic modeling and statistical analysis. J. Mech. Phys. Solids 2016, 96, 204−222.

26

Hwang, S. H.; Miller, J. B.; Shahsavari, R. Biomimetic, Strong, Tough, and Self-Healing Composites Using Universal Sealant-Loaded, Porous Building Blocks. ACS Appl. Mater. Interfaces 2017, 9, 37055. 27

Hwang S, Shahsavari R., (2017), Intrinsic size-effect in scaffolded porous nanoparticles and mechanical behavior of their self-assembled ensembles, ACS Appl. Mater. Interfaces, In-Press.

28

Sakhavand, N.; Muthuramalingam, P.; Shahsavari, R. Toughness Governs the Rupture of Interfacial H-bond Assemblies at a Critical Length Scale in Hybrid Materials, Langmuir, 2013, 29, 8154–8163.

29

Sakhavand, N.; Shahsavari, R., Synergistic Behavior of Tubes, Junctions and Sheets Imparts Mechano-Mutable Functionality in 3D Porous Multifunctional Boron Nitride Nanostructure, J. Phys. Chem C, 2 0 1 4 , 18 (39), 22730–22738. 30

Sakhavand, N.; Shahsavari, R. Universal composition− structure−property maps for natural and biomimetic platelet−matrix composites and stacked heterostructures. Nat. Commun. 2015, 6.652310.1038/ncomms7523.

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Silicates as Bilayer

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