Hexagonal Etch Pits Research Articles

The reactivity of graphite surfaces with atoms and molecules of hydrogen, oxygen and nitrogen

The topographical changes resulting from the attack of gaseous hydrogen, oxygen and nitrogen on the basal plane surfaces of natural graphite crystals have been studied optically. Undissociated hydrogen or nitrogen appear unreactive toward heated graphite in the temperature range 300°–1200°C. The hexagonal etch pits produced by atomic hydrogen on graphite at 700–800°C have two sides oriented perpendicular to the {112̄1} twin bands, whereas nitrogen atoms impinging on the crystals at temperatures above 1000°C give rise to hexagonal pits with two sides parallel to the twin bands. These latter pits are similar to those found with molecular oxygen at lower temperatures. By contrast, atomic oxygen causes random unoriented pitting over the whole basal plane surface. These results are discussed in terms of the possible reaction products and estimates of the behavior of the adsorbate species on the graphite surface.

The Effect of Lead Ions on the Dissolution and Deposition Characteristics of a Zinc Single Crystal in 6 N KOH

Dissolution of the basal plane of single crystal zinc and deposition of zinc on the same material in alkaline solutions in the presence of 10−4M lead ion, has been studied using in situ microscopy and the scanning electron microscope (SEM). It was found that in the presence of lead ions, most of the surface was protected against dissolution when polarized as much as 100 mV positive to the rest potential. Dissolution occurs initially in the form of a limited number of hexagonal etch pits that grow in size and number with time. The pits appear to correspond to sites of macroscopic crystalline imperfection.In the absence of Pb ions, deposition of Zn on the basal plane of a zinc crystal tends toward epitaxial growth at low overpotentials. In the presence of Pb ions, at both high and low overpotentials, Zn deposition tends to initiate mainly at the few sites not protected by the microscopically smooth Pb film. At high overpotentials, growth tends to be cylindrical and microcrystalline, rather than dendritic. It is shown that, under the present conditions, no increase in the cathodic current is to be expected with increase of time.

Interaction of Atomic Hydrogen and Nitrogen with Graphite Surfaces

RECENT optical studies of the topographical changes occurring at graphite surfaces during oxidation indicate that the cores of non-basal dislocations are often the preferred sites for attack by O2 molecules1. The orientation of the hexagonal etch pits on the graphite basal plane reveal that below 800° C the oxidation rate is faster in the 〈10&1bar;0〉 than in the 〈11&2bar;0〉 directions, whereas the reverse is true above 1,000° C (ref. 2).

Localized oxidation rates on graphite surfaces by optical microscopy

Previously published data relating to the oxidation of graphite have been obtained, almost without exception, from measurements of mass loss of the solid or by analysis of the products of reaction. An attempt has been made to obtain the overall activation energies associated with the oxidation of graphite single crystals, by oxygen, along the <101̄0> and <112̄0> directions by measuring directly, using optical microscopy, the rate of expansion of hexagonal etch pits and the rate of movement of surface steps. Activation energies which are significantly greater than those obtained using conventional methods have been determined in the temperature range 800–875°C, the values for the <101̄0> and <112̄0> directions being, respectively, 66 ±5 and 62 ±2 kcal. mole −1.