Session P20: Focus Session: Engineering Interfaces for New Materials I: Internal Interfaces

Segregation Effects at Internal Interfaces in Alloys: Atom-Probe Tomographic Experiments and Simulations

This talk first focuses on experimental studies of solute segregation effects on an atomic scale of solute segregation at grain boundaries (GBs) and heterophase interfaces employing atom-probe field-ion microscopy and three-dimensional atom-probe tomography; both instruments provide a spatial resolution of ca. 0.2 nm in direct space. It is demonstrated that the Gibbsian interfacial excess of solute at an internal interface depends on its five macroscopic degrees of freedom (DOFs), which is consistent with J. Cahn's local phase rules for GBs and heterophase interfaces. Experimental data is presented for GBs in metallic alloys (e.g. Fe-Si, Al-Sc-Mg, Ni-Al-Cr alloys), and metal silicide/silicon and indium arsenide heterophase interfaces. Secondly, atomic-scale simulations will be presented of GB segregation in binary metallic alloys described by embedded-atom method potentials employing Metropolis algorithm Monte Carlo simulations, which further demonstrate the intimate relationships between GB structure, on an atomic scale, and the Gibbsian interfacial excess of solute. It is also shown how the microscopic DOFs of a GB affect the Gibbsian interfacial excess of solute. Additionally, the results of atom-probe tomographic studies of segregation effects at heterophase interfaces between the gamma (f.c.c.) and gamma prime (L1$_{2}$ structure) heterophase interfaces in Ni-Al-Cr alloys are discussed and compared in detail with the results of lattice kinetic Monte Carlo (LKMC) simulations, which involves a vacancy mediated diffusion mechanism. The LKMC simulation allow us to explain the role of vacancy-solute binding energies on the observed concentration profiles of Ni, Al, and Cr between the gamma and gamma prime phases. These detailed experimental and simulation studies of segregation effects result in a relatively new atomistic picture of segregation at internal interfaces that differs from the conventional wisdoms found in the literature concerning segregation.   

Interface structure and radiation damage resistance in Cu-Nb multilayer nanocomposites

We use atomistic simulations to show that misfit dislocations in Cu-Nb interfaces can shift location between two adjacent planes by forming pairs of extended jogs, a mechanism that involves removal or insertion of atoms. Different jog combinations give rise to interface structures with unlike densities but nearly degenerate energies, making Cu-Nb interfaces virtually inexhaustible sinks for irradiation-induced point defects and catalysts for efficient Frenkel pair recombination.   

Threshold Shear Stresses at Aluminum-Silicon interfaces

The critical shear stress (CSS) was determined using molecular dynamics and MEAM potential for various Al/Si interfaces with different alignments (normal to the interface) and orientations (parallel to the interface). It was found that the primary influence parameter for CSS was the general crystallographic alignment of the interface. For all Al/Si interfaces the fracture under shear is mostly localized within 10 {\AA} in Al close to the interface. The critical shear stress of Al/Si interface is significantly lower than the critical tensile stress due to the partial stick-slip in sliding. In addition, there is not explicit relationship between shear and tensile critical stresses, which is dramatically different from isotropic materials, where the shear stress is about half of the tensile stress. The mis-orientation effects show great contrary in homogenous Al/Al interfaces and heterogeneous Al/Si interface: the mis-orientation can reduce the CSS at Al/Al interfaces by two orders of magnitude; while it has insignificant effect on CSS in Al/Si. Therefore, in general, introducing Si improves the strength of the interface (and the composite material in general) for different grain orientations.   

The strength characterization of Al/Si interfaces with a hybrid nanoindentation/FEM method

The mechanical property characterization of the reinforcement/matrix interface in a metal matrix composite (MMC) is entailed for tailoring the interface in the microstructure design of the composite. In this work we developed a hybrid method to characterize the interface strength of an MMC, combining a nanoindentation experiment and a finite element analysis. The nanoindentation experiment was carried out by indenting individual reinforcement particles on a free surface with a nanoindenter. The dependence of indentation response on the interface properties was systematically studied through the finite element analysis with cohesive zone modeling of the interface failure. The interface strength could then be extracted from the comparison between the experimental and FEM results. With this method, the shear strength of an Al/Si interface was measured approximately 240MPa which compares well with the lower bound of an atomistic simulation with a modified EAM potential. The intrinsic fracture toughness of the interface crack tip surrounded by densely populated dislocations was measured 0.25 J/m2. We also studied the effect of the strontium modification on the interface strength with this hybrid method.   

Atomic Resolution Study of the Interfacial Bonding at Si$_3$N$_4$/CeO$_{2-\delta}$ Grain Boundaries

Using a combination of atomic resolution Z-contrast imaging and electron energy-loss spectroscopy (EELS) in the scanning transmission electron microscope, we examine the atomic and electronic structures at the interface between Si$_3$N$_4$ $(10 \overline{1}0)$ and CeO$_{2-\delta}$ inter-granular film (IGF). Ce atoms are observed to segregate to the interface in a two-layer periodic arrangement, which is significantly different compared to the structure observed in a previous study. Our EELS experiments show that {\bf (i)} oxygen is present at the interface in direct contact with the terminating Si$_3$N$_4$ open-ring structures, {\bf (ii)} the Ce valence state changes from +3 to +4 in going from the interface into the IGF, and {\bf (iii)} while the N concentration decreases away from the Si$_3$N$_4$ grains into the IGF, the Si concentration remains uniform across the whole width of the IGF. Possible reasons for these observed structural and electronic variations at the interface and their implications for future studies on Si$_3$N$_4$/rare-earth oxide interfaces are briefly discussed.   

The Influences of different cathode materials on Tris-(8-Hydroxyquinoline)- Aluminum Doped with CsNO$_{3}$ in Organic Light emitting Devices

This paper presents the investigations of interfacial interactions and electron-injection mechanisms between cesium nitrate (CsNO$_{3})$ and different cathode materials. By using ultraviolet and x-ray photoemission spectroscopy, the properties of electronic structures and the interfacial chemistry are studied. According to our results, there exists a phenomenon of electron exchange at the interface results in changes of Aluminum 2s core level binding energy by 1 eV when aluminum was deposited on CsNO$_{3.}$ This means electrons transfer from cathode materials to the surface of CsNO$_{3}$, forming a strong dipolar field at the interface and reduction of the electron injection barrier. But, in contract, there exists nearly no reaction between CsNO$_{3}$ and silver cathode. The evidences show that CsNO$_{3}$ is more effective only with aluminum cathode due to a reaction between Aluminum, Cesium and Nitrogen atoms.   

Atomic-resolution study of cobalt valence and spin-state transitions in Ca$_{3}$Co$_{4}$O$_{9}$ using in-situ scanning transmission electron microscopy.

The misfit-layered Ca$_{3}$Co$_{4}$O$_{9 }$(CCO) has been of great interest due to its high thermo-electric power and thermal stability. The CCO structure consists of five layers: three rock salt-type layers Ca$_{2}$CoO$_{3}$ are sandwiched between two CdI$_{2}$-type CoO$_{2}$ layers along the c-direction. The presence of different Co valence states is assumed to account for the thermal stability of CCO, and the abrupt changes of electrical resistivity at 420K is believed to be due to a Co spin-state transition. Here, we combine scanning transmission electron microscopy (STEM) with electron energy loss spectroscopy (EELS) to study the atomic and electronic structure of CCO. Using atomic-column resolved EELS, the Co valence states in the different layers are quantified and significant charge transfer from CoO$_{2}^{ }$to Ca$_{2}$CoO$_{3}$ is measured. The effects of the potential spin-state transition at 420K on the local structure will be studied by in-situ heating experiments. We will show how atomic-resolution Z-contrast imaging in combination with EELS and in-situ experiments can be utilized to understand the effects of interfacial charge transfer and spin-state transitions in complex oxide materials.   

The Classical Size Effect: Impact of Grain Boundaries on Resistivity in Encapsulated Cu Thin Films

Surface and grain boundary electron scattering contribute significantly to resistivity as the dimensions of polycrystalline metallic conductors are reduced to, and below, the electron mean free path. A methodology is developed to independently evaluate surface and grain boundary scattering in encapsulated polycrystalline Cu thin films, with thicknesses in the range of 27-165 nm. The film resistivity, measured at both room temperature and at 4K, is compared for samples having different grain sizes (as determined from 400 to 1,500 grains per sample) and film thicknesses. The experimental data is compared to models of surface and grain boundary scattering in thin films. The resistivity contribution from grain boundary scattering is found to be significantly greater than that of surface scattering in Cu thin films which allows a quantitative measurement of the parameters for the Mayadas-Shatzkes model. It is also found that the Ta barrier layer prohibit grain growth which explains the higher resistivities observed in encapsulated Cu samples with Ta barrier layers.   

Resistivity induced by electron-grain boundary scattering on thin gold films deposited on mica substrates under high vacuum

We report measurements of the room temperature resistivity and of the grain size distribution on a family of gold films deposited onto mica substrates under high vacuum. The films are of the same thickness (100 nm $\pm$ 10{\%}), deposited at the same rate (3 nm/min), varying both the temperature of the substrate and the annealing temperature (if any) between --170 $^{\circ}$C and +270 $^{\circ}$C. The grain distribution was measured with a Scanning Tunneling Microscope. The average grain size decreases from approximately 200 nm to a few nm, when the temperature of the substrate decreases from +270 $^{\circ}$C to --170 $^{\circ}$C during evaporation. The monotonic decrease in grain size leads to a monotonic increase in resistivity of almost one order of magnitude. The resistivity of the film evaporated with the substrate held at +270 $^{\circ}$C and annealed for one hour at +270 $^{\circ}$C after evaporation, exhibits a room temperature resistivity only a few percent larger than crystalline gold. Research funded by FONDECYT 1040723.   

The effect of hydrogen content on intrinsic stress in nanocrystalline diamond (NCD) coatings

The stress control is critical to ensure the reliability of nanocrystalline diamond (NCD) coatings. We found the intrinsic stress in NCD is tensile at deposition temperature above 700$^{\circ}$C. Decreasing the deposition temperature decreases this tensile stress, and eventually leads to compressive stresses. The stress evolution appears to be largely dictated by grain boundary formation and hydrogen incorporation, which involves absorption, desorption, and recombination kinetics on diamond surfaces. The competition between these reactions indicates that the hydrogen coverage at interfaces should increase with decreasing growth temperature. This is consistent with Raman spectra and elastic recoil detection. To understand hydrogen effects, density functional theory (DFT) is used to model the coalescence of two diamond grains that approach each other to form a grain boundary. The two surfaces exhibit attractive forces when hydrogen coverage is less than 75{\%}, and repulsive forces when all the surface bonds are hydrogen terminated (100{\%} hydrogen coverage). In this way, differences in the hydrogen coverage can explain the observed transition from tensile to compressive intrinsic stress as the growth temperature decreases.   

Super Hard Cubic Phases of Period VI Transition Metal Nitrides: A First Principles Investigation

We report a systematic study of mechanical and electronic properties of 32 cubic phases of nitrides of the transition metals M (M = Hf, Ta, W, Re, Os, Ir, Pt, Au), in zinc-blende, rocksalt, pyrite, and fluorite structure using \textit{ab initio} computations. Our results reveal that MN$_{2 }$(M = W, Re, Os, Ir, Pt, Au) in pyrite phase, have a bulk moduli greater than 330 GPa, MN$_{2 }$(M = Re, Os, Ir) in fluorite phase have a bulk moduli greater than 350 GPa and TaN in rocksalt phase has a bulk modulus of 380 GPa making them candidates for super hardness. Based on the bulk and shear modulus for stable phases, potential hard coating materials for cutting tools have been identified. The local density of states of all phases has been obtained and linked to mechanical stability. The high values of bulk moduli are attributed to strong bonding of transition metal d-orbitals with nitrogen p-orbitals. The trend in the bulk modulus is related to the valence electron density of these materials.   

Coherent atomic motion in nano-crystal film

We report a theoretical study of the structural dynamics in metallic film in response to ultrafast laser heating. A two-dimensional model using a harmonic approximation is used to simulate the lattice thermal expansion dynamics in thin films. The results show that the surface shape and the orientations of nano-crystal grains are essential to determine the modes of lattice motions. Moreover, a large projection of coherent lattice oscillation in the in-plane direction is found, which was previously thought to be very small and was neglected in one-dimensional models. The simulation agrees well with our femtosecond electron diffraction measurements   

Tuning physical properties by assembling subnanometer inorganic and organic units

Designing inorganic-organic hybrid materials in a nanoscopic scale allows taking the full advantage of the two worlds, which has recently been demonstrated in a new family of hybrid crystalline materials that are the fully ordered assemblies of sub-nanometer scale inorganic units (e.g., few monolayer-thick slab, single atomic chain) and organic molecules[1].They have been shown to exhibit a number of unique properties that are not readily available in either of the components or their nanostructures: for instance, strongly enhanced exciton-polariton absorption and exciton binding energy[2], a massive bandgap blue shift ($\sim $ 2 eV) from that of the bulk inorganic semiconductor[3], and fine-tuning of thermal expansion and achieving zero-thermal expansio[4]. They have great potential for applications in areas including transparent conducting materials, thermoelectric materials, UV optoelectronic devices, because of their unusual electronic, vibrational and optical properties and the flexibility in tailoring the material properties adapting to the specific application requirements. .[1] X. H. Huang et al., JACS \textbf{125}, 7049 (03). [2] Y. Zhang et al., PRL \textbf{96}, 26405 (06). [3] B. Fluegel et al., PRB \textbf{70}, 205308 (04). [4] Y. Zhang et al., PRL \textbf{99}, 215901 (07).