Matthew J. Olszta

Scanning electron microscopic analysis of the mineralization of type I collagen via a polymer-induced liquid-precursor (PILP) process.

We have put forth the hypothesis that collagen is mineralized during bone formation by means of a polymer-induced liquid-precursor (PILP) process, in which a liquid-phase mineral precursor could be drawn into the gaps and grooves of the collagen fibrils by capillary action, and upon solidification, leave the collagenous matrix embedded with nanoscopic crystallites of hydroxyapatite. This hypothesis is based upon our observations of capillarity seen for liquid-phase mineral precursors generated with calcium carbonate. Here, we demonstrate proof-of-concept of this mechanism by mineralizing Cellagen™ sponges (type I reconstituted bovine collagen) in the presence of a liquid-precursor phase to calcium carbonate. Scanning electron microscopy (SEM) was used to examine the mineralized collagen, which in combination with selective etching studies, revealed the extent to which the mineral phase infiltrated the collagenous matrix. A roughly periodic array of disk-like crystals was found to be embedded within the collagen fibers, demonstrating that the mineral phase spans across the diameter of the fibers. Some of the morphological features of the mineralized fibers in our in vitro model system are similar to those seen in natural bone (albeit of a different mineral phase), lending support to our hypothesis that these non-equilibrium morphologies might be generated by a PILP process. SEM provides a different perspective on the morphology of bone, and has been useful here for examining the extent of mineralization in composite structures generated via the PILP process. However, further investigation is needed to examine the nanostructural arrangement of the crystallites embedded within the collagenous matrix.

Stoichiometry and valence measurements of niobium oxides using electron energy-loss spectroscopy

Qualitative and quantitative electron energy‐loss spectroscopy analyses have been performed on niobium and stable niobium oxides (NbO, NbO2 and Nb2O5). At integration windows (Δ) greater than 75 eV, k‐factor analysis can be used to distinguish between the stoichiometry of the three oxides within 5.7% error. As seen in other metal oxides, with increasing oxidation state the metal ionization edges shift to higher energies relative to the O‐K edge. Normalized M2,3 white‐line intensities show a strong correlation with 4d occupancy for each compound. The data are in correspondence with that observed in the literature for 4d transition metals using normalized L2,3 white lines. Lastly, a distinctive energy‐loss near‐edge, structure of the O‐K edge was observed for each oxide, which could be used as a fingerprint for analysis of unknowns.

Interface stoichiometry and structure in anodic niobium pentoxide.

High-resolution transmission electron microscopy and electron energy loss spectroscopy (EELS) were performed on electrochemically anodized niobium and niobium oxide. Sintered anodes of Nb and NbO powders were anodized in 0.1 wt% H3PO4 at 10, 20, and 65 V to form surface Nb2O5 layers with an average anodization constant of 3.6+/-0.2 nm/V. The anode/dielectric interfaces were continuous and the dielectric layers were amorphous except for occurrences of plate-like, orthorhombic pentoxide crystallites in both anodes formed at 65 V. Using EELS stoichiometry quantification and relative chemical shifts of the Nb M4,5 ionization edge, a suboxide transition layer at the amorphous pentoxide interface on the order of 5 nm was detected in the Nb anodes, whereas no interfacial suboxide layers were detected in the NbO anodes.

Structure and dielectric properties of amorphous tantalum pentoxide thin film capacitors

Amorphous tantalum pentoxide films are currently being studied as a high-k dielectric for high energy-density metal-insulator-metal capacitors. Tantalum pentoxide thin films were prepared through pulsed-dc reactive magnetron sputtering at a high deposition rate (15 Aring/s). The films were amorphous as determined by X-ray and electron diffraction through transmission electron microscopy (TEM) at all sputtering conditions of both low and high ion bombardments unlike other oxides such as zirconium oxide. The structure was also confirmed by electron energy loss spectra using anodized Ta2O5 films as a benchmark. After annealing at 750degC, the films crystallized to the beta-Ta2O5 phase (X-ray analysis). The dielectric constant and loss of the 2 mum-thick films are 21 and 0.3%, respectively, at 1 kHz at room temperature of 25degC. The amorphous films have a Temperature Coefficient of dielectric constant (TCK) of 2.1x10-3ldrC-1, similar to crystalline forms of Ta2O5 namely, alpha-Ta2O5 and beta-Ta2O5. Electrical breakdown field of these amorphous tantalum pentoxide films is as high as 400 MV/m with a corresponding energy density of 14 J/cm3. Electrical breakdown is affected by material crystallinity, which is controlled by annealing. The crystallinity is studied both at bulk level through X-ray diffraction and at the local atomic level through fluctuation electron microscopy (FEM), which is an electron microscopy technique used to study medium range order (MRO) on the length scale of 1-3 nm in apparently diffraction amorphous (TEM and X-ray) materials.

Intrafibrillar Mineralization of Collagen using a Liquid-Phase Mineral Precursor

Intrafibrillar mineralization of type-I collagen with hydroxyapatite (HA) is the basis of the complex biological composite known as bone, which from a material science perspective is a fascinating example of an interpenetrating bioceramic composite. Using a polymer-induced liquid-precursor (PILP) process, collagen substrates were highly infiltrated with a liquid-phase mineral precursor to calcium carbonate (CaCO 3 ). At sections of partially mineralized collagen, banded mineral patterns were observed perpendicular to the collagen fibrils, while other fibrils were completely mineralized. An acid etch, used to preferentially remove superficial mineral, further revealed such banded patterns in fully mineralized samples. Removal of the collagen matrix with a dilute hypochlorite solution showed an interpenetrating mineral phase, with mineral disks that spanned the diameter of the pre-existing collagen fibrils, supporting our hypothesis that intrafibrillar mineralization can be achieved via capillary action applied to a liquid-phase mineral precursor.

Electron Microscopy Analysis of Grain Boundary Corrosion in Ni-Cr Binary Alloys Exposed to High Temperature Hydrogenated Water

Intergranular (IG) corrosion and stress corrosion cracking are important issues in current and advanced nuclear systems, and the understanding the controlling mechanisms are of great concern to the nuclear industry. In order to better elucidate fundamental aspects, this study focuses on exposure of coupons of varying Cr content in Ni-Cr binary alloys. Detailed evaluation of Cr concentration effects on protective film formation and localized corrosion provides underpinning processes controlling behavior of complex Ni-base alloys in light-water reactor service.

Focused Ion Beam (FIB) Preparation and Electron Microscopy Analysis of Individual Microbolometer Pixels

In an ever turbulent political and military landscape, it becomes necessary to develop detection technologies which can cover a wide-range of applications. One such technology, un-cooled infrared (IR) detectors, is being utilized in a variety of national security applications, ranging from night-vision cameras, to missile tracking systems to screening detectors for concealed weaponry. Previous generations of IR detectors required large cooling apparatus in order to maintain sub-zero temperatures that would ensure proper detection. Microbolometer technology, with a free-standing design (Figure 1A, B), is one of the leading candidates to replace the large, cryogenically cooled systems. The free-standing design is comprised of a 1-2 μm thick pixel (Figure 2) which is suspended above a reflector by two metallic legs (Figure 1B). IR detection is measured by a small change in resistance in the ~125 nm thin vanadium oxide (VOx) film, and thus the chemical composition, microstructure, and continuity of this layer is quite important. Due to highly involved and intricate processing techniques, the morphology of each individual pixel is also important to the efficacy of finished microbolometer devices. The spatial geometry of each pixel requires that (each pixel ~50 x 50 μm) the analysis of materials properties of individual pixels as related to processing and failure must be performed by transmission electron microscopy (TEM). Because these detectors are comprised of thousands of pixels in a space no larger than a square inch, conventional sample preparation is not suitable.

Interfaces in Next Generation Ta and NbO Solid Electrolytic Capacitors

On the forefront of capacitor technology sit solid electrolytic capacitors, which are high capacitance capacitors produced from powders of valve metals (e.g., Ta, Nb) or valve metal oxides (e.g., NbO). By utilizing the high CV/g of these materials, smaller capacitors (e.g., surface mount devices) can be produced to accommodate the electronics industries push towards miniaturization, such as for use in cell phones and laptops. The increased charge and miniaturization are achieved by sintering these powders and subsequently anodizing the sintered body to create one half of a capacitor. The thickness of the dielectric layer, and thus the capacitance, is dependent upon the anodization voltage. To complete the metal-insulator-semiconductor structure, a manganese oxide (MnO2) or conductive polymer layer is then coated onto the newly formed dielectric layer. Unfortunately, as the dimensions decrease towards the nanoscale, new problems begin to arise from size effects (e.g., cracking of the oxide at grain boundaries). In addition to physical flaws, there are numerous conductive, amorphous intermediate oxides, which can diminish the capacitance of the part. Lastly, the dielectric oxide can also crystallize and become conductive, which leads to current leakage.