Extended Interplanar Linking in Graphite Formed from Vacancy Aggregates

Abstract

The mechanical and electrical properties of graphite and related materials such as multilayer graphene depend strongly on the presence of defects in the lattice structure, particularly those which create links between adjacent planes. We present findings which suggest the existence of a new type of defect in the graphite or graphene structure which connects adjacent planes through continuous hexagonal sp 2 bonding alone and can form through the aggregation of individual vacancy defects. The energetics and kinetics of the formation of this type of defect are investigated with atomistic density functional theory calculations. The resultant structures are then employed to simulate high resolution transmission electron microscopy images, which are compared to recent experimental images of electron irradiation damaged graphite. The rich variety of structures and topologies that result from the ways that carbon atoms can bond with each other provides us with a multitude of opportunities and challenges. A particularly important problem concerns the response of graphite to damaging radiation, especially where it forms the neutron moderator and reflector blanket in a nuclear fission reactor. In this application, it is essential for the structural integrity of the material to be maintained for decades in a harsh, inaccessible environment. The mechanical and electrical properties of graphite, as well as other forms of nanostructured carbon and graphene, can be substantially altered through irradiation [1][2][3]. In the case of graphite, irradiation (with neutrons, electrons, or ions) causes changes to electrical resistance and thermal conductance [2,4,5] and anisotropic changes to both elastic properties and the crystal dimensions, together with expansion in the prismatic direction, balanced by shrinkage in the basal plane [6,7]. These changes arise from a complex evolution of a population of point defects in the graphite structure (Frenkel pairs of lattice vacancies and self-interstitials created when impacting energetic particles displace atoms) into prismatic and basal dislocations [8] and other interstitial and vacancy aggregates [9][10][11]. The processes involved occur over many time and length scales, and the fundamental mechanisms behind the observed property changes are still unresolved, even after 70 years of intensive research.One of the most useful experimental insights into the evolution of the graphite structure under irradiation comes from high resolution transmission electron microscopy (HRTEM) images [12]. This method can resolve prismatic edge and basal dislocations in the graphite structure when viewing along the c axis (i.e., perpendicular to the planes) and the topology of individual layers and prismatic edge dislocations when viewing along the planes (side on) [13]. HRTEM images typically show evidence for the rupturing and bending of the planes [14,15] as well as dislocation climb and the growth of new planes under irradiation [16]. However, the atomistic mechanisms driving these processes are not re...

Electron irradiation of nuclear graphite studied by transmission electron microscopy and electron energy loss spectroscopy

“…Damage accumulation at temperatures below 200 °C increases the Wigner energy of the graphite, due to a lack of atomic diffusion [16]. It is widely agreed that single vacancies become mobile at 100 -200 °C whereas interstitial atoms become mobile at temperatures of 500 °C [17]. Thus only at higher temperatures such as those in the Gen IV graphite moderated Very High Temperature Gas Reactors (> 300 °C) does stored energy dissipation occur by diffusion driven atomic re-ordering and the problem is addressed in the short term.…”

Exohedral Physisorption of Ambient Moisture Scales Non-monotonically with Fiber Proximity in Aligned Carbon Nanotube Arrays

“…This is likely a result of the size and morphology of wall defects, which are smaller than the size of the water clusters (∼ 1 nm) that enable the physisorption of water onto the CNT walls. [31,[33][34][35] Recent work on graphite indicates that lattice monovacancies can become mobile at temperatures 200 • C, [59] and that the aggregation of these vacancies can lead to the formation of interlayer bonds, known as interlayer divacancies, [59][60][61] which can become the nucleation site for an extended interlayer defect, where two CNT wall planes become connected via a graphene ribbon.[60] The aggregation of native CNT wall defects likely do not provide continuous throughthickness pathways of sufficient size ( 0.6 nm) to enable the water clusters to enter the inner volume of the CNTs. Further work is necessary to determine the size and morphology of the native wall defects present in the CNTs used here, and the effect of the CNT structure and surface chemistry on the kinetics and energetics of the exohedral physisorption of water.…”

Interlayer vacancy diffusion and coalescence in graphite

“…5(a). This structure comprises two coplanar V 3 complexes in each layer that face each other, with the overlapping edge α atoms forming two bonds between the layers [14]. From this bound interlayer state, three of the atoms in one layer, forming half of the central hexagonal unit (highlighted in red), can move into the adjacent layer in a single concerted transition, releasing 4.4 eV and resulting in the formation of a V 6 ring in one layer.…”

“…The specific processes presented and discussed here deal with only relatively small aggregates. As the number of vacancies is increased, different and more complex interplane aggregate structures and transitions are possible, and we suggest that one possibility is the formation of extended defects that connect neighboring graphene layers in graphite together via a sheet or strip of graphene, similar to a ramp between the floors in a multistory car park [14].…”

“…Resolution in HRTEM images of bulk graphite is limited to the observation of prismatic edges and a cross section of the topology of the individual layers. However, there is evidence for the formation of vacancy loops (holes), as well as extended defects linking adjacent planes as a result of electron irradiation [13,14]. Large prismatic vacancy loops have been observed to form in neutronirradiated graphite at temperatures above 900 • C [15][16][17]; however, very small loops (six or fewer vacancies) would not have been visible in these experiments [11].…”