a. Geometrical design
To investigate the mechanical behavior of one, two and three wall BeONTs all zigzag and armchair samples were designed under the MD framework. Figure 1 presents a schematic of the simulated nanotubes studied in the current article and Table 1 includes all of the structures that were simulated along with their number of atoms. The average Be-O bond length was obtained equal to 1.57Å and 1.58Å for the zigzag and armchair structures respectively which are in good accordance with the results of the earlier studies [33–36].
Table 1
Stoichiometry of SWBeONTs, DWBeONTs and TWBeONTs.
Chirality
|
Number of Atoms
|
(4,4) SWBeONT
|
352
|
(6,6) SWBeONT
|
528
|
(8,8) SWBeONT
|
704
|
(10,10) SWBeONT
|
880
|
(12,12) SWBeONT
|
1056
|
(8,0) SWBeONT
|
384
|
(10,0) SWBeONT
|
480
|
(12,0) SWBeONT
|
576
|
(14,0) SWBeONT
|
672
|
(16,0) SWBeONT
|
768
|
(18,0) SWBeONT
|
864
|
(20,0) SWBeONT
|
960
|
(4,4)@(8,8) DWBeONTs
|
1056
|
(4,4)@(9,9) DWBeONTs
|
1144
|
(4,4)@(10,10) DWBeONTs
|
1232
|
(8,0)@(14,0) DWBeONTs
|
1056
|
(8,0)@(15,0) DWBeONTs
|
1104
|
(8,0)@(16,0) DWBeONTs
|
1152
|
(8,0)@(18,0) DWBeONTs
|
1248
|
(4,4)@(8,8)@(12,12) TWBeONT
|
2112
|
(8,0)@(14,0)@(20,0) TWBeONT
|
2016
|
Table 2
Mechanical properties of armchair SWBeONTs under uniaxial tensile tests at 300°K.
Chirality
|
(4,4)
|
(6,6)
|
(8,8)
|
(10,10)
|
(12,12)
|
Young’s Modulus
|
624.81
|
619.21
|
613.48
|
605.09
|
583.09
|
Failure Stress
|
101.62
|
101.37
|
102.42
|
100.86
|
102.40
|
Failure Strain
|
0.247
|
0.250
|
0.252
|
0.255
|
0.256
|
Table 3
Mechanical properties of zigzag SWBeONTs under uniaxial tensile tests at 300°K.
Chirality
|
(8,0)
|
(10,0)
|
(12,0)
|
(14,0)
|
(16,0)
|
(18,0)
|
(20,0)
|
Young’s Modulus
|
645.71
|
631.92
|
621.02
|
617.13
|
605.82
|
591.83
|
587.86
|
Failure Stress
|
83.60
|
82.55
|
82.58
|
82.25
|
82.13
|
81.55
|
83.12
|
Failure Strain
|
0.192
|
0.189
|
0.186
|
0.186
|
0.185
|
0.182
|
0.182
|
Table 4
Effect of temperature on mechanical properties of zigzag (18,0) and armchair (10,10).
Chirality
|
Mechanical Properties
|
Temperature (°K)
|
300
|
400
|
500
|
600
|
700
|
800
|
900
|
1000
|
(18,0)
|
Young’s Modulus (GPa)
|
591.83
|
586.90
|
579.02
|
563.03
|
554.23
|
546.27
|
526.30
|
507.89
|
Failure Stress (GPa)
|
79.55
|
77.08
|
74.03
|
70.77
|
68.60
|
65.82
|
64.44
|
56.73
|
Failure Strain
|
0.182
|
0.172
|
0.162
|
0.154
|
0.152
|
0.146
|
0.141
|
0.117
|
(10,10)
|
Young’s Modulus (GPa)
|
605.09
|
590.25
|
585.73
|
579.00
|
573.39
|
544.89
|
536.66
|
528.64
|
Failure Stress (GPa)
|
99.86
|
95.51
|
92.85
|
90.74
|
83.70
|
78.97
|
68.85
|
66.67
|
Failure Strain
|
0.255
|
0.229
|
0.220
|
0.214
|
0.189
|
0.180
|
0.153
|
0.149
|
b. Mechanical properties of single-walled BeONTs
Concerning the mechanical properties of single-walled BeO nanotubes, seven zigzag nanotube including structures (8,0), (10,0), (12,0), (14,0), (16,0), (18,0), and (20,0) and five armchair structures including (4,4), (6,6), (8,8), (10,10), and (12,12) were simulated using MD. To calculate Young’s modulus, we put all samples under uniaxial tensile loading and stress-strain curves were plotted accordingly. A schematic of a double-walled nanotube under tensile loading is presented in Fig. 2, and Fig. 3a shows the a stress-strain curve which is plotted for zigzag structure (8,0). As seen, the stress-strain relation is not linear so, to calculate young’s modulus, we fitted a second-order polynomial to the linear part of the stress-strain plot regarding Fig. 3b. The elastic modulus was calculated through the following equation:
1.1. Mechanical properties of double-walled BeONTs
At this stage we modeled four zigzag double-walled structures including (8,0)@(14,0), (8,0)@(15,0,), (8,0)@(16,0), and (8,0)@(17,0) and three armchair structures including (4,4)@(8,8), (4,4)@(9,9), and (4,4)@(10,10) as shown in Fig. 7. Interlayer distance has undoubtedly increased with an increase in the based nanotubes’ radius. As well as the previous section, all double-walled structures were put under uniaxial tensile loading and the obtained results are illustrated in Fig. 8, Table 5 and Table 6. Regarding Fig. 8, we can see that armchair and zigzag structures revealed contradictory behavior via interlayer distance. Similar to single-walled structures Young’s modulus of zigzag DWBeONTs generally reduced (except one point) via an increase in the interlayer distance while this property rose constantly via the increase in interlayer distance of the armchair structures. The highest calculated values of Young’s modulus belonged to (8,0)@(15,0,) zigzag and (4,4)@(10,10) armchair DWBeONTs with the magnitudes of 659.75GPa and 640.67GPa respectively. In an MD theoretical approach Fereidoon et l. found a reduction in Young’s modulus of both armchair and zigzag DWBNNTs via interlayer distance rising [15]. Furthermore, the failure properties of the zigzag double-walled structures did not show a significant trend as well as single-walled ones while the failure stress and failure strain of the armchair DWBeONTs increased slightly similar to what we had already observed for Young’s modulus of this chirality. Besides, a snapshot of the failure process of zigzag (8,0)@(14,0) structure is presented in Fig. 9. This figure demonstrates that the failure of the double-walled structure which started from the inner layer occurred at the same length increment compared to single-walled one (19% against 19%) and the inner layer failed sooner than the outer one.
Table 5
Mechanical properties of zigzag DWBeONTs under uniaxial tensile tests at 300°K.
Chirality
|
(8,0)@(14,0)
|
(8,0)@(15,0)
|
(8,0)@(16,0)
|
(8,0)@(18,0)
|
Young’s Modulus (GPa)
|
649.69
|
659.75
|
643.97
|
629.19
|
Failure Stress (GPa)
|
83.36
|
83.96
|
83.74
|
85.31
|
Failure Strain
|
0.191
|
0.188
|
0.189
|
0.194
|
Table 6
Mechanical properties of armchair DWBeONTs under uniaxial tensile tests at 300°K.
Chirality
|
(4,4)@(8,8)
|
(4,4)@(9,9)
|
(4,4)@(10,10)
|
Young’s Modulus (GPa)
|
636.12
|
638.67
|
640.67
|
Failure Stress (GPa)
|
101.59
|
102.98
|
103.46
|
Failure Strain
|
0.250
|
0.250
|
0.251
|
1.2. Mechanical properties of triple-walled BeONTs
To study more about the effect of adding walls on the properties of nanotubes, a triple-walled (8,0)@(14,0)@(20,0) zigzag BeONT and a triple-walled (4,4)@(8,8)@(12,12) armchair BeONT were simulated using MD as seen in Fig. 10. By putting both samples under uniaxial tensile loading we obtained the values of Young’s modulus equal to 668.04GPa and 647.57GPa for zigzag and armchair TWBeONTs respectively shown in Table 7. These results are in agreement with those of single-walled and double-walled results which had demonstrated higher modulus for the zigzag BeONTs compared to armchair ones. Similarly, failure properties of the armchair TWBeONTs were higher than those of zigzag ones similar to what we had already found in SWBeONTs (see Fig. 4). The variation of Young’s modulus with adding walls is displayed in Fig. 11. This figure confirms that with raising the number of walls from one to two and then three Young’s modulus of both chirality’s increased slightly so adding walls could result in nanotubes with better mechanical properties.
Table 7
Mechanical properties of zigzag and armchair TWBeONTs under uniaxial tensile tests at 300°K.
Chirality
|
(8,0)@(14,0)@(20,0)
|
(4,4)@(8,8)@(12,12)
|
Young’s Modulus (GPa)
|
668.04
|
647.57
|
Failure Stress (GPa)
|
96.26
|
102.41
|
Failure Strain
|
0.22
|
0.24
|