AVS2013 Session MG+AS+EM+NS+SA+SE+SP+SS+TF-FrM: Novel Synthesis Approaches and Innovative Characterization Techniques Coupled with Theory & Computations

A combination of polarized total reflection x-ray absorption spectroscopy (XAS) and atomic force microscopy (AFM) data have been used to determine the morphological properties of the size-selected micelle-prepared Pt nanoparticles (NPs) supported on TiO₂(110) and SiO₂/Si(111) in an ultrahigh vacuum (UHV) environment. The Pt NPs were cleaned in UHV using an O₂-plasma followed by annealing in O₂ (1x10^-6 mbar) at 600°C after the ex situ sample transfer. Extended X-ray absorption fine-structure (EXAFS) measurements were carried out at Argonne National Laboratory in fluorescence mode. Measurements along different orientations, namely one perpendicular and two parallel to the sample surface were carried out on the Pt NPs supported on TiO₂(110). Our analysis revealed that while the Pt NPs on SiO₂ remained spherical, the NPs on TiO₂ wetted the substrate and adopted an epitaxial relationship with the support. It will be shown that the first shell EXAFS analysis of the data acquired along different directions provides valuable information regarding the size and aspect ratio of the NPs as well as the degree of disorder and strain in their structure. FEFF simulations were carried out on model NP shapes and the theoretical data compared with experimental EXFAS data.

In general, nanotechnology provides the tools for controlling key parameters for thin films performance: chemical composition (and crystalline structure at nano-sized domains), thickness and topography (including nano-scale patterning of thin films’ surface) and controlled interfaces at the nanoscale.

As research in nanotechnology develops, new characterization and production tools, new materials and process models as well as more computational power will become available. Nanotechnology is giving a new boost to thin films’ application development.

Pulsed Laser Deposition (PLD) is one of the attractive research tools for complex materials because it is fast and one can easily investigate a wide range of different materials and compositions. Currently, a major issue in the growth of oxide materials with PLD is the control of the surface morphology. For most materials it is necessary to control the thickness and roughness of the thin films down to an atomic scale. Such well-controlled growth can also be used to manufacture artificially layered structures. In this way it is possible to create a whole new class of materials.

Much effort is put in the deposition of excellent textured layers without grain boundaries. In general the properties of highly oriented films approximate the properties of single crystals. Single or multi-layer structures require a well-conditioned process technique. The deposited layers must have a large homogeneity with well-defined material properties, smooth surfaces, and, in the case of oxides, the correct oxygen stoichiometry. Important is the possibility to combine PLD with standard in-situ diagnostic techniques, like high pressure Reflecting High Energy Electron Diffraction (RHEED).

With our development of pulsed laser deposition with control at atomic level we are able to control the growth of complex materials and to introduce new growth manipulations, like pulsed laser interval deposition. At present, new superlattices can be synthesized that exhibit rare properties, like multiferroics. Extremely sharp and homogeneous interfaces can be realized and this is, for example, yet utilized in SrTiO₃-LaAlO₃ interfaces and artificial ferroelectric structures. In this presentation I like to show this unique technique and their use in obtaining complex materials systems ‘on demand’.

The advantages of replacing metals in aircraft turbine engines with High-Temperature Polymer Matrix Composites (HTPMC’s) include weight savings accompanied by high strength to weight ratio, and lower manufacturing costs. Most of the nacelle of a modern aero gas turbine is made of HTPMC, a component that accounts for approximately 25% of the weight and 20% of the cost of the power plant. Unfortunately, they are limited to applications where they are not exposed to high-temperature oxidizing atmospheres and/or particulates from ingested air. This is because oxidation and erosion occur on the surface, leading to weight loss, and/or cracking on the surface and a consequent decline of mechanical properties over time. Resistance to surface erosion is one rarely reported property of HTPMC’s in engine applications because HTPMC's are generally softer than metals and their erosion resistance suffers.

Although prior research has shown that oxidation can be slowed down when metallic or ceramic coatings are applied onto HTPMC’s, there remains a need for erosion-resistant coatings that protect HTPMC's from high-velocity particulates in the engine flow path. These erosion-resistant coatings could extend the life of polymer composites and also reduce the overall cost of the turbine assembly. Hence in this paper we report the development of coating materials that are compatible with HTPMC’s such as Avimid N and AFR-PE-4 using Cathodic Arc Physical Vapor Deposition (CAPVD). We report the feasibility of applying multilayer PVD TiN coatings onto the HTPMC substrates and characterize their adhesion, microstructure and erosion resistance. We also show the effectiveness of duplex process of thermal spray and PVD to enhance the overall adhesion of the multilayer coating.

How does stuff break? An important step in this process is inelastic deformation of the material, the formation of a first disclocation. We report on the for the first experiments which due to the small size and atomic scale control of the indenter allow a direct quantitative comparison with molecular dynamics simulations and state-of-the-art electronic transport theory.

We have studied the formation of the smallest permanent indentation in the Au(111) model surface by a combination of scanning tunneling microscopy (STM) and atomic force microscopy (AFM) in ultrahigh vacuum (UHV). We use field ion microscopy (FIM) to characterize the nanometer scale spherical apex of the W(111) indenter in UHV prior to the indentation experiments [1,2]. Knowledge of the indenter geometry is necessary to extract quantitative parameters such as contact pressures and stresses within the sample during indentation.

Traditional nanoindentation measures depth to high precision, but typically does not possess the force resolution (nN) to detect initial plastic events [3]. Indentation with standard AFM allows for excellent force resolution, but large piezo displacements required to load the contact with a soft cantilever hamper the extraction of true indentation depth because of quantitative optical beam deflection calibration issues (beam placement, sensitivity to mode shape, etc.) and piezo creep. I will describe how our set-up overcomes these limitations and allows us to quantitatively assess elastic and plastic behaviour in an indentation curve.

We report on the transition from elastic to plastic deformation in the indentation of Au(111). This is done by producing arrays of indentations to forces near the plastic yield point and examining the resulting force-displacement curves for both elastic and plastic indentation sites. Plasticity can be identified by features in the force displacement curves, such as the sudden displacement excursions of the tip (pop-ins), the work done by the indenter, and the sink-in depth measured at mild repulsive loads. These indicators of plasticity can also be correlated with the permanent impressions in the surface imaged by STM. The measured forces at the initial yield points correspond to shear stresses lower than those expected for the homogeneous dislocation nucleation. We suggest that heterogeneous nucleation involving surface effects and atomic scale indenter roughness is likely to play a role in the observed plastic behaviour.

[1] W. Paul et al. Nanotechnology 23, 335702 (2012) [2] D. J. Oliver et al., PNAS 109, 19097 (2012) [3] Minor et al., Nature Materials 5, 697 (2006)

Functionalized Shape Memory Biomaterials

Valerie Sheares Ashby

University of North Carolina at Chapel Hill, Department of Chemistry

Chapel, Hill, NC 27599

919-962-3663(Phone); 919-9625467(Fax)

ashby@email.unc.edu

Shape memory polymers (SMPs) are a class of smart materials that can change their shape in a predetermined fashion when exposed to the appropriate external stimulus. Since Langer and Lendlein first demonstrated the tremendous potential of SMPs in biomedical applications there has been a growing interest in utilizing these materials as intelligent medical devices and minimally-invasive implants. Our research focuses on several areas: micro- and nanotopography, reactive functionality and switching stimuli. Recently, we have developed thermally responsive micro-patterned poly(ε-caprolactone) (PCL) shape memory films to examine the interaction of human mesenchymal stem cells (hMSCs) with dynamic surface patterns using PRINT^TM (pattern replication in non-wetting templates). We have also begun research on novel polymer structures containing functionality with the goal of developing SMPs capable of micro- or nanoscopic shape memory and post-functionalization. Finally, there have been several examples of dual (one temporary shape) and triple shape memory (two temporary shapes). Our efforts focus on the synthesis of novel bifunctional monomers and biodegradable polyester prepolymers containing photo-responsive side groups (cinnamamide or cinnamate) using a polycondensation method to allow uniform repeat units each possessing a photo group, followed by end-functionalization and crosslinking to create networks with photo and thermal shape memory switches. The synthesis of these materials, which has led to shape memory polymers with switching temperatures near physiological temperature and unique functionality, in addition to characterization of thermal and shape memory properties, will be presented. Fabrication of the patterned surfaces, initial cytotoxicity and cellular response to dynamic shape switching will also be discussed.