Interface Science

Characterization of the structure of surfaces is very important in order to develop a fundamental understanding of the electronic, mechanical and chemical properties of a material. While transmission electron microscopy imaging (TEM) and diffraction (TED) techniques are capable of providing surface structural information at the atomic level, such data would be suspect if obtained under conventional vacuum conditions (10-6–10-8 Torr). Ultrahigh vacuum (UHV) conditions are imperative during both preparation and observation of clean surfaces/interfaces. Conventional TEM techniques are very powerful for UHV-TEM investigations; however, the marriage of surface science and conventional TEM to yield an UHV-TEM is a complex task. These complexities and some of the results obtained using UHV-TEM and UHV-TED techniques for surfaces i.e. solid-vacuum interfaces will be illustrated.

The segregation behaviour of a cation (yttrium) with a low solubility in the polycrystalline oxide host (α-Al2O3) has been investigated at temperatures between 1450 and 1650°C using analytical scanning transmission electron microscopy. Three distinct segregation regimes were identified. In the first, the yttrium adsorbs to all grain boundaries with a high partitioning coefficient, and this can be modelled using a simple McLean-Langmuir type absorption isotherm. In the second, a noticeable deviation from this isotherm is observed and the grain boundary excess reaches a maximum of 9 Y-cat/nm2 and precipitates of a second phase (yttrium aluminate garnet, YAG) start to form. In the third regime, the grain boundary excess of the cation settles down to a value of 6–7 Y-cat/nm2 that is in equilibrium with the YAG precipitates. In a material (accidentally) co-doped with Zr, the Zr seems to behave in a similar way to the Y and the Y + Zr grain boundary excess behaves in the same way as the Y grain boundary excess in the pure Y-doped system. In this latter system, Y-stabilised cubic zirconia is precipitated in addition to YAG at higher Y + Zr concentrations.

In this paper we address the problems related to critical misfit and thickness in epilayer-substrate combinations of comparable bond strengths; specifically the case in which a pseudomorphic monolayer (ML) is stable and the critical thickness is about three MLs or less. Of particular interest are the average energies related to misfit strain f KS and misfit dislocations (MDs)—in the latter case the individual contributions of the oscillatory strains 〈V〉 and the epilayer-substrate disregistry 〈V〉MD. The individual energies are of interest because they may play different roles in the realization of specific growth modes. The analytical approach involves the following assumptions: (a) a rigid substrate as source of a periodic epilayer atom-substrate interaction potential which we model in terms of a low order truncated Fourier series; and (b) an epilayer which (i) deforms harmonically with zero strain gradient normal to the film plane, (ii) grows in Kurdjumov-Sachs (KS) orientation due to small misfit. f KS and in the layer-by-layer growth mode. Arguments are presented claiming that this interfacial situation may be approximated by a one-dimensional problem in which epilayer stiffness constants and equilibrium structure, as well as epilayer-substrate interaction depend on epilayer thickness; which poses a complex problem. An approximate solution could be obtained by assuming these quantities to be independent of thickness and proximities of the vacuum and the substrate. The most prominent conclusions are that the equilibrium density of MDs and hence the transition from misfit accommodation by MS to one containing MDs is a catastrophic process and that sustained minimum energy may require the overcoming of an energy barrier. While elementary implementation of the results to equilibrium growth mode theory suggests—independently of the catastrophic nature—that energetically favored misfit strain relief by misfit dislocations may, or may not, effect a transition to Stranski-Krastanov growth, a crude numerical calculation favors the transition. A proper implementation of the results require extensive numerical calculations and is planned for the near future.

Stainless steel and aluminum have been bonded by the surfaceactivated bonding method. Both transmission electron microscopy (TEM)and scanning electron microscopy (SEM) have been used to investigatethe interface microstructure of the as-bonded and annealed joints. Aperfect interface did not show any microcracks or porosity for theas-bonded joints. An 10 nm thick intermediate layer composed ofmainly silicon and certain amounts of oxygen and carbon was foundbetween stainless steel and aluminum by means of high resolutionelectron microscopy (HRTEM). The interface morphology of the jointsvaried gradually as the bonded joints are heated at elevatedtemperature. When heated to 573 K, individual precipitate-likefluctuation at the interface area was detected, with slightmodification of the interface morphology. Bulky intermetalliccompounds finally formed throughout the original interface boundarywhen heated to 873 K and contributed to the weakening of theinterface boundary of the joints.

Amorphous carbon/graphite interface is modeled by molecular dynamic simulation using a Tersoff-type potential function with the Brenner parameters for in-plane interaction combined with the pair potential function for the interplanar bonding. The interface is created by compressing the amorphous carbon produced in a separate simulation with perfect crystalline graphite terminated to expose (1120) planes. The planar structure and weak interplanar bonding allow the graphitic planes to deform in order to accommodate the bonds formed at the interface, which is consistent with the HRTEM study of the interface. The simulation indicates that the generated interface mostly consists of nearly sp2 hybridized bonding connecting the two sides. The bonds across the interface when formed are likely to maintain their equilibrium configurations. Due to the large interplanar spacing, many atoms both on the graphite and a-C sides are left unbonded leaving the interface energetically unfavorable with respect to the bulk. These unbonded radicals probably weaken the structural rigidity of the interface providing a fracture path under stress.

Bulk and grain boundary (GB) diffusion of 14C in Nb has been studied by the radiotracer serial sectioning technique. B and C kinetic regimes were realized for GB diffusion in the temperature range from 800 to 1173 K. The values of P = sδD gb, D gb and s follow the Arrhenius dependencies: P = 5.15 × 10−15 exp[−(83.1 kJ/mol)/RT] m3/s (973–1173 K), D gb = 2.3 × 10−6 exp[−(133.0 kJ/mol)/RT] m2/s (800–950 K), and s = 4.7 exp[(49.9 kJ/mol)/RT]. The increase in the GB diffusion compared with self-diffusion is very large despite the probable retardation effect due to the strong segregation. The results for GB diffusion of C in Nb as well as for other interstitial solutes (P, S) in bcc transition metals (α- Fe, Mo) are discussed in the framework of the transition state theory. It is assumed that GB segregation decreases the energy of the ground state whereas the change in the diffusion mechanism (e.g. from vacancy to interstitial) leads to a strong decrease of the transition state energy. This change in the diffusion mechanism results in a fast GB diffusion of interstitial solutes in spite of their large tendency to segregate to GBs.

The energies of Si(001) twist boundaries have been measured relative to Si(solid)-Sn(liquid) interfacial energies by a dihedral angle method at 1473 K. Shallow and wide width energy cusps were found at non-coincidence site lattice (csl)-misorientations, in addition to shallow and narrow width energy cusps at csl-misorientations with low Σ. These wide width cusps existed at misorientations dividing, into 1:1 or 1:2, misorientation differences between two csl-misorientations. The energy of the general boundaries discontinuously varied with misorientations across the wide cusps. These misorientations divided the whole misorientation region with respect to energies.