Hard-to-Soft Transition in Radial Buckling of Multi-Concentric Nanocylinders

We investigate the cross-sectional buckling of multi-concentric tubular nanomaterials, which are called multiwalled carbon nanotubes (MWNTs), using an analysis based on thin-shell theory. MWNTs under hydrostatic pressure experience radial buckling. As a result of this, different buckling modes are obtained depending on the inter-tube separation d as well as the number of constituent tubes N and the innermost tube diameter. All of the buckling modes are classified into two deformation phases. In the first phase, which corresponds to an elliptic deformation, the radial stiffness increases rapidly with increasing N. In contrast, the second phase yields wavy, corrugated structures along the circumference for which the radial stiffness declines with increasing N. The hard-to-soft phase transition in radial buckling is a direct consequence of the core-shell structure of MWNTs. Special attention is devoted to how the variation in d affects the critical tube number Nc, which separates the two deformation phases observed in N -walled nanotubes, i.e., the elliptic phase for N < Nc and the corrugated phase for N > Nc. We demonstrate that a larger d tends to result in a smaller Nc, which is attributed to the primary role of the interatomic forces between concentric tubes in the hard-to-soft transition during the radial buckling of MWNTs.


Introduction
The term "buckling" refers to a deformation through which a pressurized material undergoes a sudden failure and exhibits a large displacement in a direction transverse to the load [1].A typical example of buckling occurs when pressing opposite edges of a long, thin elastic beam toward one another.For small loads, the beam is compressed in the axial direction while keeping its linear shape and the strain energy is proportional to the square of the axial displacement.Beyond a certain critical load, however, it suddenly bends into an arc shape and the strain energy and displacements are no longer related by a quadratic expression.Besides axial compression, bending and torsion give rise to buckling of elastic objects, where the buckled patterns depend strongly on the geometric and material parameters.
An interesting class of elastic buckling can be ob-served in structural pipe-in-pipe cross sections under hydrostatic pressure [2,3].Pipe-in-pipe (i.e., a pipe inserted inside another pipe) applications are commonly used in offshore oil and gas production systems in civil engineering.In subsea pipelines in deep water, for instance, buckling resistance to huge external hydrostatic pressure is a key structural design requirement.Pipe-inpipe systems may be an efficient design solution that meets this strict requirement, because their concentric structures enable the cross section to withstand high pressure without collapsing.The above argument regarding macroscopic objects poses a question as to what buckling behavior may be observed in nanometer-scale (10 -9 m) counterpart objects.In nanomaterial sciences, the buckling of carbon-based hollow cylinders with nanometric diameters (called carbon nanotubes) has drawn great attention [4].Extensive studies on carbon nanotube mechanics have been thus far driven by their exceptional resilience against deformation; that is, the recovery of the original cylindrical shapes of the carbon nanotubes upon unloading, even when subjected to severe loading conditions.In addition to the excellent strain-relaxation reversibility, carbon nanotubes exhibit high fatigue resistance; therefore, they are a promising medium for the storage of mechanical energy with an extremely high energy density [5].Nevertheless, due to their nanometric scales, the similarities and differences in the buckling patterns compared with those of their macroscopic counterparts are not trivial.This complexity has motivated tremendous efforts toward the analysis of the buckling of carbon nanotubes under diverse loading conditions: axial compression [6][7][8][9][10], radial compression [11][12][13][14][15][16][17][18][19][20][21][22], bending [23][24][25][26][27][28], torsion [29][30][31][32], and combinations of these [33].
In this article, we focus our attention on the radial buckling of carbon nanotubes observed under hydrostatic pressure on the order of several hundreds of megapascal.Thin-shell-theory based analysis on the cross-sectional deformation of nanotubes leads us to the conclusion that the buckled patterns strongly depend on the inter-tube separation , the number of constituent tubes , and the innermost tube diameter .In particular, the expansion of from its equilibrium value (0.34 nm) causes a lowering of the critical tube number c that characterizes the hard-to-soft transition in the nanotubes' radial buckling.These results shed light on the possible control of the morphology of carbon nanotubes by experimentally tuning .

What Are "Carbon Nanotubes"?
Carbon nanotubes are one of the most promising nanomaterials, and they consist of layers of graphene sheets that are each a single atom thick (two-dimensional hexagonal lattices of carbon atoms) rolled up into concentric cylinders [34].By convention, they are categorized as single-walled nanotubes (SWNTs) or multi-walled nanotubes (MWNTs): the former is made by wrapping one single layer into one seamless cylinder, while the latter comprise two or more concentric graphitic tubes.The constituent tubes in MWNTs are coupled to one another via the van der Waals (vdW) interaction, wherein the separation between adjacent concentric tubes is approximately 0.34 nm in equilibrium conditions.
The excellent mechanical properties of carbon nanotubes are characterized by the remarkably high Young's modulus, which is on the order of terapascal (i.e., several times stiffer than steel), and the tensile strength, which is as high as tens of gigapascal [33].These properties are proof that carbon nanotubes are the stiffest and strongest materials on earth.In addition to the marked stiffness, carbon nanotubes exhibit astounding flexibility when subjected to external hydrostatic pressure.The radial deformation both of SWNTs and MWNTs is an important consequence of this flexibility; however, the theoretical understanding of the flexibility of MWNTs is still lacking due to their structural complexity.
Emphasis should be placed on the fact that on application of a mechanical deformation, carbon nanotubes show significant changes in their physical and chemical properties [34,35].Precise knowledge of their deformation mechanism and available geometry is, therefore, crucial for understanding their structure-property relations and for developing next generation carbon-nanotube-based applications.

Continuum Approximation
The aim of this section is to deduce the stable cross-sectional shape of a MWNT under a hydrostatic pressure .The continuum elastic approximation [36][37][38][39][40][41] allows the mechanical energy of a MWNT per axial length to be expressed as follows: Here, D U is the deformation energy of all concentric tubes, I U is the interaction energy of all adjacent pairs of tubes, and  is the potential energy of the applied pressure.All these three energy terms are functions of and the deformation amplitudes p  The optimal displacements i and i that minimize under a given are obtained via the calculus of variations to U with respect to i and .To proceed, we derive the explicit forms of

Strain-Displacement Relation
Evaluating the functional form of D U u requires the relation between the displacements, i and i , and the circumferential strain, v  , of a hollow tube driven by cross-sectional deformation.Suppose there is a circumferential line element of length 1  lying at an arbitrary point within the cross section of a tube with thickness .The hydrostatic pressure upon the tube causes an extensional strain d   h p  of the line element, which is de- 1 Throughout this subsection, the tilde attached to variables in-   dicates to take a quantity at arbitrary point within the cross section of the -thickness tube.On the other hand, variables with no tilde indicates the quantity just on the centroidal arc of the -thickness tube.

See the difference between h h
 and  given in Equation (8) for a clear example.
Copyright © 2012 SciRes.WJM fined as follows: Here, d d , and is the length of the line element after deformation (the asterisk symbolizes the quantity after deformation).The coordinates  of the element after deformation are given as follows: where and are the components of the displacement vector in the radial and circumferential directions, respectively.We can then write the following relationships: the following relationship can be obtained: where 6) accounts for the rotation of the line element due to the deformation [16].The formula ( 6) is valid for an arbitrarily large rotation,  .
Hereafter, we assume that  and  are both significantly smaller than unity, because an infinitesimal deflection of the initially circular cross section is assumed to determine the critical buckling pressure.The second term in the right side in Equation ( 6) can therefore be omitted if the possibility that u  or v  is larger than is excluded.We further assume that the normals to the undeformed centroidal circle of the hollow tube's cross section remain straight, normal, and unextended during the deformation [16].The second assumption means that within each cross section, neither shear deformation nor thickness modulation arises in the circumferential direction; this leads to the following expressions: where and denote the displacements of a point that lies on , and is a radial coordinate measured from . By substituting Equation ( 7) into Equation ( 6), we can derive the following strain-displacement relationship: where the following definitions hold true: Here, i  and are the in-plane and bending strains, respectively, of the th tube; is the radius of the undeformed circle .Equations ( 8) and ( 9) state that the circumferential strain at an arbitrary point in the cross section is determined by the displacements

 
v  of a point that lies on the undeformed centroidal circle .C

Deformation Energy
We are now ready to derive the explicit form of the deformation energy D U .Suppose that the th constituent tube has a thickness .A surface element of the crosssection of the hollow tube can then be expressed by i h  .The stiffness of the surface element for stretching along the circumferential direction is given as follows: where and E  are the Young's modulus and Poisson's ratio, respectively, of the tube.Thus, the deformation energy D U per axial length can be written as follows: in which the component ( )   i D U associated with the th tube is written as follows: From Equations ( 8) and ( 12) we obtain the following relationship:  which can also be written as follows: The constant  denotes the in-plane stiffness,  the flexural rigidity, and  the Poisson ratio of each tube.
For quantitative discussions, the values of  and  must be carefully determined.In cases of macroscopic objects, they are defined as Eh   and  .However, for carbon nanotubes, the macroscopic relations for and noted above fail because there is no unique way of defining the thickness of the graphene tube [42]     in a similar fashion as a previous study [43] based on the density functional theory.

Inter-Tube Coupling Energy
The energy associated with the van der Waals (vdW) interaction between adjacent pairs of tubes, designated by I U in Equation ( 1), can be written as a sum of components as follows: We derive the coefficients , i j in Equation ( 15) through a first order Taylor approximation of the vdW pressure [39,44] associated with the vdW potential as follows: Here,  is the distance between a pair of carbon atoms, nm is the equilibrium distance between two interacting atoms, and •nm is the well depth of the potential [45].The resulting equilibrium spacing between neighboring tubes is 0.3415 nm.The derivative F V     represents the force between two carbon atoms, and its surface integral provides the inter-wall pressure induced by the vdW coupling.
The vdW pressures on the inner and outer tubes of a concentric two-walled tube with radii inn and out are given as follows [44] (with positive signs for compression): .
In Equation (18), , and In the following, we obtain analytical expressions for i j by linearizing the Equation ( 17) for the pressure [22].Note that depends quadratically on the change in spacing between two adjacent tubes.Consider two consecutive tubes with radii i and , where the subscripts and correspond to and , respectively.The vdW energy stored due to a perturbation along the positive direction of pressure is given as follows: where is the mean radius and  15), the following expressions for the vdW coefficients can be found: where the derivatives in Equation ( 20) are defined as follows: Note that i j c  is symmetric.The set of Equations ( 15), (20), and ( 21) allows for the evaluation of I U .

Pressure-Induced Energy
We finally derive an explicit form of , which is the negative of the work done by the external pressure during cross-sectional deformation.Using this definition we can write the following expression: where S  is the area surrounded by the th tube after deformation (the sign of is assumed to be positive inward).N p S  can then be obtained by evaluating the following expression: By substituting Equations ( 3) and (4) into Equation ( 23), and by using the periodicity relation

Critical Pressure Evaluation
This section presents our method for determining the critical pressure c above which the circular cross section of MWNTs is elastically deformed into a non-circular one.To carry out this analysis, we decompose the radial displacement terms according to Here, indicates a uniform radial contraction of the i th tube at   describes a deformed, non-circular cross section observed just above .Similarly, we can write By applying the variational method to U with respect to i and i v , we obtain the following system of 2N linear differential equations: where In deriving Equations ( 25) and ( 26), the quadratic and cubic terms in  are omitted because we only consider elastic deformation with sufficiently small displacements.
In addition, the terms consisting only of and are also omitted; the sum of such terms should be equal to zero 3 because represents an equilibrium circular cross-section under .25) and ( 26) are given by the Fourier series expansions as follows: Substituting these into Equations ( 25) and ( 26) leads to the matrix equation , in which the vector consists of with all possible and , and the matrix involves one variable as well as parameters such as i i n M p  and i  .The matrix can be expressed as a block diagonal matrix of the form 1 due to the orthogonality of n and sin n .Here, is a submatrix that satisfies . As a result, the secular equation that provides nontrivial solutions of Equations ( 25) and ( 26) can be rewritten as follows: By solving Equation ( 27) with respect to , we obtain a sequence of discrete values of .Each of these values is the smallest solution of . The minimum of these values serves as the critical pressure c that is associated with a specific integer p m  .From the definition, the c associated with a specific m allows only    to be finite, however, it also requires Immediately above c , therefore, the circular cross section of MWNTs becomes radially deformed as follows: cos and where the value of is uniquely determined by the one-to-one relation between and ., which is followed by a slow decay when nm (and also for smaller D). 4 The increase in c for small is interpreted as the "hardening" of the MWNTs, i.e., an enhancement of the radial stiffness of the entire MWNT by encapsulation.This hardening effect disappears with a further increase in , which results in the decay of A decay in c implies that a relatively low pressure suffices to produce a radial deformation, which indicates an effective "softening" of the MWNT.These two contrasting effects, i.e., hardening and softening, are both due to the encapsulation of MWNTs.p We emphasize that in Figure 1(a), the softening region (i.e., c -decay region) is enlarged by expanding the inter-tube distance prior to deformation.As will be confirmed later, this tendency agrees with the existing numerical simulations that were based on a coarsegrained model of MWNTs [21].The variation of is thought to be feasible in MWNT synthesis.During synthesis, the interlayer thermal contraction upon cooling and/or the intertube adhesion energy owing to the increased intertube commensuration may result in a deviation in from the equilibrium value [46,47].above c for fixed D and d  .The most striking observation is the successive transformation of the cross section with an increase in N .e see from Figure 2(a) that the deformation mode observed just above c p jum abruptly from 2 n  t 8 at 25 N  is followed by successive emergences of higher corrugation modes with larger n .These transitions in n originate from the two competing effects inherent in MWNTs with N = 1, that is, the relative rigidity of the inner tubes and the mechanical instability of the outer tubes.A large discrepancy in the radial stiffness of the inner and outer tubes gives rise to an uneven distribution of the deformation amplitudes of concentric tubes that interact through the vdW forces, which consequently produces an abrupt change in the observed deformation mode at some N .

Hard-to-Soft Transition
Of further interest is that the critical number of tubes c separating the elliptic phase ( ) from the corrugation phase ( ) is identified as the that   sponds to , which implies the occurrence of the corrugation mode of .It should be noted that for can never attain the minimum value at any .This is why the corrugation mode of with cannot be observed for MWNTs with

Summary
A thin-shell-theory based analysis has been employed to detect the mechanical hard-to-soft transition relevant to the radial buckling of MWNTs subject to hydrostatic pressure.Various buckled patterns are found to be available, and the parameters , , and strongly influence which pattern is energetically favored.We have the phase boundary and the critical sure c for several pairs of and , revealing the artificial expansion of results in a decrease in c .studies will shed light on other mechanical properties MWNTs and suggest applications based on their unique cross-sectional deformations.

Acknowledgements
The fruitful discussion with S. Ghosh and M. Arroyo on the vdW-interaction formulas is greatly acknowledged.


displacements, respectively, of the th tube.See Equation(7) below for the precise definitions of and .
can be obtained:


are periodic in  , the general solutions of Equations (

Figures 1 (
Figures 1(a) and (b)show c as a function of for various values of the initial tube-tube separation prior to the application of pressure.For all , we observe a rapid increase in c with , which is followed by a slow decay when nm (and also for smaller D).4The increase in c for small is interpreted as the "hardening" of the MWNTs, i.e., an enhancement of the radial stiffness of the entire MWNT by encapsulation.This hardening effect disappears with a further increase in , which results in the decay of

Figure 2
Figure 2 provides (a) the index of deformation modes and (b)-(g) the cross-sectional views observed just n

Figure 1 .
Figure 1.(Color online) Critical pressure curves showing P c required to produce radial deformation of N-walled nanotubes with fixed D: (a) D = 3.0 nm, and (b) D = 5.0 nm.

Figure 2 .Figure 3 .
Figure 2. [Upper panel] (a) Stepwise increase in the index n of radial buckling modes.The index n indicates the circumferential wave number of the deformed cross-section.[Bottom panel] Cross-sectional views of buckled MWNTs under high hydrostatic pressure: (b)-(d) Elliptic deformation mode (n = 2) for N = 5, 10, 20; (e) Radial corrugation mode with n = 8 for N = 25; (f) n = 9 for N = 35; (g) n = 11 for N = 50.yields a cusp in the curve of [see Figure 1(a)].In contrast, no singularity is observed in the curve of   c p N

Figure 3 depicts
the N-dependence of  p N r several n vaues, where the innermost tube radius is fixed to be nm.For 24 N  , the values of for p 2 n  are less than those for , which implies that the elliptic mode occurs for MWNTs with .However, at 3 n  24 N  25 N  (and 26 27 N   ), the minimum correp

Figure 3 .
Figure 3. Branches of solutions p(N) for the secular equation det(M) = 0 (refer text).The innermost tube diameter is set to be D = 3.0 nm for all curves.For a fixed N, the minimum value of p among the branches takes a role of the critical pressure p c at that N.
also follows from Figure3that the cusps in the curves c occur only at the phase boundary c separating the elliptic phase 2Thus, the values of