Volume 101, Issue 1

We describe a first-principles-based strategy to predict the macroscopic toughness of a γ-Ni(Al)/α-Al 2 O 3 interface. Density functional theory calculations are used to ascertain energy changes upon displacing the two materials adjacent to the interface, with relaxation conducted over all atoms located within adjoining rows. Traction/displacement curves are obtained from derivatives of the energy. Calculations are performed in mode I (opening), mode II (shear) and at a phase angle of 45°. The shear calculations are conducted for displacements along <110> and <112> of the Ni lattice. A generalized interface potential function is used to characterize the results. Initial fitting to both the shear and normal stress results is required to calibrate the unknowns. Thereafter, consistency is established by using the potential to predict other traction quantities. The potential is incorporated as a traction/displacement function within a cohesive zone model and used to predict the steady-state toughness of the interface. For this purpose, the plasticity of the Ni alloy must be known, including the plasticity length scale. Measurements obtained for a γ-Ni superalloy are used and the toughness predicted over the full range of mode mixity. Additional results for a range of alloys are used to demonstrate the influences of yield strength and length scale.

Magnetic tunnel junctions are nanoscale oxide heterostructures that exhibit the phenomenon of tunnel magnetoresistance. The transport and magnetic behavior of these structures depends critically on parameters such as the layer thickness and the roughness and chemical abruptness of the interfaces between the layers, which are nanometer-scale in thickness. We have used high resolution electron microscopy, transmission electron microscopy chemical mapping and atom probe tomography to understand the microstructural origins of the magnetic and transport properties of magnetoresistive structures. We have also used in-situ transmission electron microscopy nanobiasing experiments to probe the local transport properties of magnetic tunnel junctions as a function of processing conditions.

Aberration correction in the scanning transmission electron microscope allows spatial resolutions of the order of one Ångström to be routinely achieved. When combined with electron energy loss spectroscopy, it is possible to simultaneously map the structure, the chemistry and even the electronic properties of materials in one single experiment. Here we will apply these techniques to the characterization of thin films and interfaces based on complex oxides with the perovskite structure. The relatively large lattice parameter of these materials combined with the fact that most of them have absorption edges within the reach of the spectrometer optics makes these materials ideal for these experiments. We will show how it is possible to map the chemistry of interfaces atomic plane by atomic plane, including light elements such as O. Applications to cobaltite and titanate thin films will be described.

A system's van der Waals–London dispersion interactions are often ignored, poorly understood, or crudely approximated, despite their importance in determining the intrinsic properties and intermolecular forces present in a given system. There are several key barriers that contribute to this issue: 1) lack of the required full spectral optical properties, 2) lack of the proper geometrical formulation to give meaningful results, and 3) a perception that a full van der Waals–London dispersion calculation is somehow unwieldy or difficult to understand conceptually. However, the physical origin of the fundamental interactions for carbon nanotube systems can now be readily understood due to recent developments which have filled in the missing pieces and provided a complete conceptual framework. Specifically, our understanding is enhanced through a combination of a robust, ab-initio method to obtain optically anisotropic properties out to 30 electron Volts, proper extensions to the Lifshitz's formulations to include optical anisotropy with increasingly complex geometries, and a proper methodology for employing optical mixing rules to address multi-body and multi-component structures. Here we review this new framework to help end-users understand these interactions, with the goal of better system design and experimental prediction. Numerous examples are provided to show the impact of a material's intrinsic geometry, including optical anisotropy as a function of that geometry, and the effect of the size of the nanotube core and surfactant material present on its surface. We'll also introduce some new examples of how known trends in optical properties as a function of [n, m] can result in van der Waals interactions as a function of nanotube classification, radius, and other parameters. The concepts and framework presented are not limited to the nanotube community, and can be equally applied to other nanoscale or even biological systems.

The electrostatic potential arising from charge bound at grain boundary cores in ceramics and the accumulation of space charge in their vicinity is in many cases made responsible for the ion blocking or conducting behavior of electroceramics. While interpretation of impedance spectra of nominally undoped and acceptor doped SrTiO 3 ceramics and bicrystals implies that grain boundaries are positively charged and accompanied by fairly wide regions of negative space charge on both sides, a critical analysis of electron holography data available in the literature yields very narrow potential profiles of the opposite sign. This paper attempts to reconcile this apparent discrepancy by showing that within the Mott–Schottky model, impedance data which implies space charge dominated grain boundary resistivity must be interpreted in terms of even wider space charge regions than generally assumed. A review of electron holography results from grain boundaries in SrTiO 3 is extended by new results which were obtained by a novel inline electron holography reconstruction algorithm at a near Σ13 grain boundary in nominally undoped SrTiO 3 . This work is dedicated to the memory of Prof. Rowland M. Cannon.

The grain boundary character distributions of selected doped aluminas were measured from normal and abnormal populations. The relative energies of the A-, C-, and R-planes of undoped alumina were also measured. There is an inverse relationship between the population of grain boundaries and the relative energies of grain boundary planes in undoped alumina. This relationship is found to be qualitative for abnormal grains, whose interfacial anisotropy may be affected by kinetic factors. It is found that the relative grain boundary anisotropy correlates with the temperature dependence of grain boundary complexion transitions in a particular system that is prone to abnormal grain growth. However, there is no direct correlation between the total anisotropy of the grain boundary character distribution and that system's propensity to undergo a particular complexion transition. Therefore, anisotropy will significantly affect the microstructural evolution in systems that are prone to abnormal grain growth, but the magnitude of anisotropy is not a sufficient selection criterion for determining which systems will undergo abnormal grain growth.

Molecular dynamics simulations have been performed to investigate intergranular glassy film in Si 3 N 4 –SiO 2 ceramics. A 90°-twist grain boundary was employed as a representative random grain boundary and an amorphous structure of the intergranular glassy film was introduced in the calculations. It is found that the presence of the intergranular glassy film at the grain boundary significantly decreases the number of dangling bonds at the interface originating from geometric strain due to misorientation of adjacent Si 3 N 4 grains. Calculations with a several of thicknesses of the intergranular glassy film revealed that the excess energy significantly decreased with the increase in the thickness smaller than 7.0Å, while further increase in the thickness did not contribute to a release in the geometric strain energy. Equilibrium thickness of the intergranular glassy film was estimated from the calculations. A new approach to estimating the equilibrium thickness is proposed. A possible explanation of the disparity between intergranular glassy film thicknesses of the intergranular glassy film measured by high-resolution electron microscopy and electron energy-loss spectroscopy is given in terms of ordering of atoms at the interface.

This review outlines the essence of a progressive study on the glassy inter-granular film (IGF) in a model ceramic system, the low-CaO-doped HIPed high-purity Si 3 N 4 . This was initiated from the finding of a systematic variation of equilibrium IGF thickness following the dopant chemistry, manifesting its fundamental important to ceramic processing. By employing analytical transmission electron microscopy to measure the local chemistry in IGF, however, significant discrepancy was found between trends of local IGF chemistry and thickness. A stable IGF composition was revealed in this system, while a bi-level distribution of Ca segregation establishes a correspondence between the IGF structure and the surface crystallography. The detection of similar levels of nitrogen in IGF through the whole series further supports the presence of a rather stable IGF chemistry. After the saturation of dopants in the stable IGF, extra CaO was found to re-distribute in pockets by enrichment at tips, leading to a liquid phase separation with the Ca-rich phase wetting the entrance zone contacting IGF. The perspective for establishing a comprehensive correlation between the inter-granular phases and the bi-modal microstructure induced by faster growth of basal facets is briefly discussed to pave the way for future work.

A sintering process of two fully faceted particles of cubic or prismatic shape contacting each other along a grain boundary is considered. Analytic expressions for sintering kinetics are obtained under the assumption that the sintering process is controlled by self-diffusion along the facets and the grain boundary. It is shown that sintering kinetics are determined by the surface self-diffusion coefficient and exhibit only a weak dependence on the grain boundary diffusivity. The initial rate of sintering is time-independent, except for the case of two prismatic particles brought in contact along their edges, for which the time dependence of shrinkage is, under certain circumstances, similar to that for isotropic spherical particles. It is demonstrated that particle faceting slows down the sintering process.

The aim of this work is the determination of the grain growth kinetic law in 4 mol.% yttria tetragonal zirconia polycrystals. To that purpose, several sets of slices of 4 mol.% yttria tetragonal zirconia polycrystals were annealed for different times at a constant temperature as well as at a constant time for different temperatures. The mean grain size was determined by means of scanning electron microscopy observations of the annealed surfaces, after fitting the grain size distributions to lognormal functions. A power law of the mean grain size versus the annealing time was found to describe the experimental results. Surprisingly, the same power law was shown to account for the dependence of the grain size standard deviations versus time. A correlation of these results with a model reported in the literature on the yttrium segregation to grain boundaries is discussed.

In this paper a method to evaluate monoclinic fraction depth profile in zirconia, based on X-ray diffraction at grazing angles, is proposed. The necessary mathematical developments are exposed. The method is numerically tested in different theoretical configurations, and then on zirconia-toughened alumina and on alumina-toughened zirconia. The results show that it is possible to follow both surface fraction and thickness of the monoclinic layer, within the penetration depth of the X-rays. This is a considerable improvement over simple X-ray diffraction measurements. However, its accuracy is limited for high depth, but considerably improved as compared to previous methods.

Y-doped alumina was sintered at 1500°C for 10 h under ultra-clean experimental conditions without experiencing any abnormal grain growth. The yttrium was fairly homogeneously distributed at the grain boundaries, with a mean value of Y = 5.5 at nm −2 . The Y–Al–O precipitates in the clean, Y 2 O 3 -doped alumina specimen were the YAP (YAlO 3 ) phase, whereas only the YAG (Y 3 Al 5 O 12 ) phase was present in the Y 2 O 3 -doped alumina samples contaminated with SiO 2 . The excess concentrations of Y and Si atoms at the grain boundaries that, at the same time, provoke the formation of structurally complex YAG precipitates and abnormal grain growth were both estimated to be at 4–5 at nm −2 . The compositions of the triple point pocket phases found in the region of the exaggeratedly grown alumina grains indicate the presence of alumino-silicate bulk liquids at the sintering temperature.

Measuring the fracture strength of brittle materials is often difficult, because alignment of the load with the test specimen is critical. Any off-axis misalignment of the load will produce fracture in an undesired mode. This report describes several micrometer-scale devices, in which the “load cell” is fabricated simultaneously with the fracture mechanics specimen, so that precise alignment is assured. The devices utilize the residual stresses contained within the materials, which are deposited onto Si substrates, to create the loading forces and determine the fracture strength. The devices are passive, meaning that the fracture results are obtained immediately upon fabrication, and no external loading is required. Stress concentrations are generated at notches, and the fracture strength is determined by the critical stress required for failure. A variety of devices have been fabricated from materials of interest for microelectromechanical systems devices, including polysilicon, silicon nitride, and aluminum, with both tensile and compressive residual stresses.

This work explores the sintering by spark plasma sintering of self-doped submicrometre alumina powders following an alcoxide route. It is shown that the self-doping process leads to nucleation of alumina nanoparticles located in the surface of the initial alumina grains, therefore avoiding the presence of phases other than α-Al 2 O 3 in the final microstructure. The presence of such alumina nanocrystals leads to a modification of the sintering process minimizing grain growth compared to un-doped alumina powders under similar sintering conditions.

Graphitic materials obtained at low temperatures are interesting for a wide range of industrial applications including bipolar plates. In this work, graphite based nanocomposites have been obtained starting from carbon nanofibers and a mixture of carbon nanofibers with 20 vol.% of alumina nanopowders. High density carbon components were obtained by using Spark Plasma Sintering at temperatures as low as 1500–1800°C for this kind of materials. The effect of spark plasma sintering parameters on the final density, and the mechanical and electrical properties of resulting nanocomposites have been investigated. Pure carbon nanofibers with around 90 % of theoretical density and fracture strength of 60 MPa have been obtained at temperatures as low as 1500°C applying a pressure of 80 MPa during sintering. It has been proved that attrition milling is a suitable method for preparing homogeneous mixtures of carbon nanofibers and alumina powders.

Calcium phosphate based bioceramics have an important position in the bone substitutions field, because of their excellent biocompatibility, bioactivity and osteoconduction characteristics. To enhance the bone ingrowth between implants and natural bone a hydroxyapatite/ß-tricalcium phosphate (HA/ß-TCP) composite has been chosen. The composites were formed by the robocasting technique for being sintered in conventional furnace under air. The behaviour of the system HA/ß-TCP was studied with the aim of employing the optimal conditions to sinter the samples formed by robocasting. The final composition in each sample was analyzed using X-ray diffraction and quantified by the Rietveld method. A powder mixture with the same HA/ß-TCP composition was formed and sintered by another non-conventional technique: Spark plasma sintering. Finally, the phase compositions and microstructures obtained by both techniques were compared.

Silver-hydroxyapatite nanocomposites containing 1 wt.% of metallic silver have been obtained by a colloidal chemical route and subsequent chemical reduction process. The silver nanoparticles are supported on the hydroxyapatite surface without a high degree of agglomeration. The bactericidal effect against common Gram-positive and Gram-negative bacteria, as well as antifungal activity against yeast have been investigated. The results indicated a high antimicrobial activity for E. coli, M. Luteus and I. Orientalis, so this material can be a promising antimicrobial biomaterial for implant and reconstructive surgery, among other applications.

The work shows some results concerning a new cermet material 40 vol.% [(Cu)–Ni]–YSZ processed by mechanical alloying followed by Sintering by Activated Surface method. The projected cermet microstructure for this application is expected to possess microstructural characteristics that lead to better electric and ionic percolating, higher electrocatalytic activity and fuel reforming. The powder samples prepared by mechanical alloying optimized conditions show a homogeneous mixture. Transmission and scanning electron microscope analysis have demonstrated the powder particles are nanosized after 2 h of milling, showing lamellar internal structure aggregates. Suitable sintered pellets are obtained from these powders, within the required porosity and microstructure. Sintering kinetics studies for pellets of Ni–YSZ and Ni–Cu–YSZ indicate 2-step sintering processes. Copper additive promotes sintering and refines the microstructure.

The vast majority of man-made devices contain joints between similar or dissimilar materials. Ideally, joining is fast, reliable, and inexpensive. In this work we demonstrate that robust refractory joints between Al 2 O 3 bodies can be produced rapidly and consistently by a transient-liquid-phase bonding process exploiting microengineered multilayer Ni/Nb/Ni interlayers. Samples were bonded for times ranging from 5 min to 6 h at 1400°C. Well-bonded assemblies were produced in all cases. Interlayers with remelt temperatures > 2200°C were achieved in as little as 5 min at 1400°C. In room-temperature bend tests, fractures initiated and propagated almost exclusively in the ceramic. Extension of this approach to other advanced ceramics should be possible, providing a rapid and reliable alternative to conventional joining approaches.