Brian W. Sheldon

CNT-reinforced ceramics and metals

Recent research on the incorporation of carbon nanotubes (CNTs) into ceramic and metal matrices to form composite structures is briefly reviewed, with an emphasis on processing methods, mechanical performance, and prospects for successful applications.

The impact of diamond nanocrystallinity on osteoblast functions

Nanocrystalline diamond has been proposed as an anti-abrasive film on orthopedic implants. In this study, osteoblast (bone forming cells) functions including adhesion (up to 4h), proliferation (up to 5 days) and differentiation (up to 21 days) on different diamond film topographies were systematically investigated. In order to exclude interferences from changes in surface chemistry and wettability (energy), diamond films with nanometer and micron scale topographies were fabricated through microwave plasma enhanced chemical-vapor-deposition and hydrogen plasma treatment. Scanning electron microscopy (SEM), atomic force microscopy (AFM), Raman spectroscopy and water contact angle measurements verified the similar surface chemistry and wettability but varied topographies for all of the diamond films prepared on silicon in this study. Cytocompatibility assays demonstrated enhanced osteoblast functions (including adhesion, proliferation, intracellular protein synthesis, alkaline phosphatase activity and extracellular calcium deposition) on nanocrystalline diamond compared to submicron diamond grain size films for all time periods tested up to 21 days. An SEM study of osteoblast attachment helped to explain the topographical impact diamond had on osteoblast functions by showing altered filopodia extensions on the different diamond topographies. In summary, these results provided insights into understanding the role diamond nanotopography had on osteoblast interactions and more importantly, the application of diamond films to improve orthopedic implant lifetimes.

Intrinsic stress, island coalescence, and surface roughness during the growth of polycrystalline films

During film growth by a variety of techniques, intrinsic tensile stresses can be created by the coalescence of neighboring islands. Experimental results with diamond films produced by chemical vapor deposition are compared with a relatively simple model to demonstrate that a realistic interpretation of these coalescence stresses must account for effects that are associated with surface roughness. First, the interpretation of curvature measurements during the early stages of film growth must account for this surface roughness. Also, the experiments show that tensile stresses are induced by grain boundary formation during continuing growth after the initial island coalescence event. This understanding differs from the traditional interpretation that continuing intrinsic stress is produced by “templated” growth onto an already strained crystalline lattice. A kinetic model of stress evolution during postcoalescence growth is also presented.

Emission spectroscopy during direct‐current‐biased, microwave‐plasma chemical vapor deposition of diamond

Optical emission spectroscopy was used to investigate dc biasing during diamond film synthesis in a microwave plasma. These measurements show that biasing produces significant changes near the substrate (i.e., close to the sheath region). Increasing the negative bias voltage (Vb) from 0 to −180 V in a CH4/H2/Ar (4/496/30 sccm) mixture increases the intensities of the hydrogen Balmer α and β lines. The relative concentrations of neutral atomic hydrogen were estimated by using an Ar(750.4 nm) emission line as an actinometer. At 38 Torr, increasing Vb from 0 to −150 V increased the concentration of atomic hydrogen by more than 20%. In addition, increasing Vb also increased the electron temperature near the substrate. These effects are likely to play an important role in the enhanced diamond nucleation that has been observed after negative‐biased pretreatment.

High quality factor gigahertz frequencies in nanomechanical diamond resonators

We report actuation and detection of gigahertz-range resonance frequencies in nanocrystalline diamond mechanical resonators. High order transverse vibration modes are measured in coupled-beam resonators exhibiting frequencies up to 1.441GHz. The cantilever-array design of the resonators translates the gigahertz-range resonant motion of micron-long cantilever elements to the displacement of the central supporting structure. Use of nanocrystalline diamond further increases the frequency compared to single crystal silicon by a factor of 3. High clamping losses usually associated with micron-sized straight beams are suppressed in the periodic geometry of our resonators, allowing for high quality factors exceeding 20 000 above 500MHz.

Orthopedic nano diamond coatings: Control of surface properties and their impact on osteoblast adhesion and proliferation

The superior mechanical and tribological properties of diamond coatings suggest their promise for improving current orthopedic implants. Therefore, understanding and controlling biological responses on diamond coatings are important and necessary for their advancement in orthopedics. For this reason, the objective of the present study was to correlate surface properties of diamond coatings with osteoblast (OB) adhesion and proliferation. Diamond coatings on silicon of variable surface features (specifically, grain size, surface roughness and surface chemistry) were fabricated by microwave plasma enhanced chemical-vapor-deposition (MPCVD). Scanning electron microscopy (SEM) as well as atomic force microscopy (AFM) was applied for topographical characterization and contact angles were measured to assess surface wettability. Results revealed that the grain size, surface roughness and wettability of diamond coatings can be controlled by adjusting H(2) plasma in the MPCVD process. Further, results showed enhanced OB adhesion on nanocrystalline diamond (ND) with grain sizes less than 100 nm whereas nanostructured diamond/amorphous carbon coatings (NDp) and microcrystalline diamond (MD) inhibited OB adhesion. H(2) plasma treated ND (NDH) also promoted OB adhesion. Similarly, OB proliferated to a greater extent on ND and NDH compared with MD and uncoated silicon controls. In summary, surface properties (including topography and chemistry) of diamond coatings can be controlled to either promote or inhibit OB functions, which implies that various forms of diamond coatings can be used to either support or inhibit bone growth in different regions of an orthopedic implant.

Competition between tensile and compressive stress mechanisms during volmer-weber growth of aluminum nitride films

Stress evolution during molecular-beam epitaxy of AIN films was monitored with in situ curvature measurements. Changes in the growth rate produced large stress variations, with more tensile stress observed at higher growth rates. For example, at a growth temperature of 750°C the instantaneous steady-state stress in films with similar grain sizes varied from −0.15GPa at a growth rate of 90nm∕h, to approximately 1.0GPa at a growth rate of 300nm∕h. To explain these results, we develop a kinetic model of stress evolution that describes both tensile and compressive mechanisms. The tensile component is based on a mechanism which is proposed here as an inherent feature of grain-boundary formation. The compressive component is based on our recent model of atom insertion, driven by the excess chemical potential of surface adatoms that is created by the growth flux. The combined model predicts that the stress is largely governed by the competition between tensile and compressive mechanisms, which can be convenientl...

In Situ Atomic Force Microscopy Study of Initial Solid Electrolyte Interphase Formation on Silicon Electrodes for Li-Ion Batteries

Precise in situ atomic force microscopy (AFM) is used to monitor the formation of the solid electrolyte interphase (SEI) on Si electrodes. The stability of these passivation films on negative electrodes is critically important in rechargeable Li-ion batteries, and high capacity materials such as Si present substantial challenges because of the large volume changes that occur with Li insertion and removal. The results reported here show that the initial rapid SEI formation can be stabilized before significant Li insertion into the Si begins and that the rate at which this occurs varies significantly with the nature of the surface. The initial cycling conditions also have a substantial impact on the SEI that forms, with faster rates leading to a smoother, thinner SEI film. To quantitatively interpret the SEI measurements, irreversible expansion of the Si during the first cycle was also monitored in situ with specifically designed specimen configurations. On the basis of the experimental results, relatively simple models were also used to describe the initial formation and stabilization of the SEI and to describe the relationship between the SEI thickness and expected SEI degradation mechanisms.

Synergetic Effects of Inorganic Components in Solid Electrolyte Interphase on High Cycle Efficiency of Lithium Ion Batteries.

The solid electrolyte interphase (SEI), a passivation layer formed on electrodes, is critical to battery performance and durability. The inorganic components in SEI, including lithium carbonate (Li2CO3) and lithium fluoride (LiF), provide both mechanical and chemical protection, meanwhile control lithium ion transport. Although both Li2CO3 and LiF have relatively low ionic conductivity, we found, surprisingly, that the contact between Li2CO3 and LiF can promote space charge accumulation along their interfaces, which generates a higher ionic carrier concentration and significantly improves lithium ion transport and reduces electron leakage. The synergetic effect of the two inorganic components leads to high current efficiency and long cycle stability.

Monitoring stress in thin films during processing

Abstract Wafer curvature measurements are a simple yet sensitive way to measure stress in thin films. A multibeam optical system designed for in situ monitoring makes it possible to monitor the evolution of thin film stress in real time in a variety of deposition and processing environments. Examples of stress measurements in epitaxial systems, polycrystalline films and hard coatings are discussed.