Rooban Venkatesh K.G. Thirumalai

Characterization of the crystalline regions of cured urea formaldehyde resin

Urea formaldehyde (UF) resins are widely used thermosetting polymers in adhesives, finishes, molded objects, etc. The existence of crystalline regions has been detected in UF resins. These crystalline regions are believed to be beneficial for the hydrolytic stability or water resistance and advanced mechanical properties of the resin. In this study, characterization was conducted on crystalline regions of a cured UF resin with a formaldehyde to urea (F/U) molar ratio of 1.2. A slow scanned X-ray diffraction (XRD) pattern was obtained to estimate the crystalline percentage, grain sizes, and interplanar spacing (d-spacing) of the resin crystalline structures. The results showed that the crystalline regions accounted for nearly 14.48% of the resin. From the XRD pattern, the estimated grain size of 4.1 nm was accounted for the peak (two theta degrees, 2θ) of 21.55°, 4.1 nm for 24.35°, 4.2 nm for 31.18°, and 4.8 nm for 40.43°, respectively. Furthermore, a selected area electron diffraction (SAD) pattern of the resin was obtained to confirm the results. The calculated d-spacing values were 2.2242 A for the peak (2θ) of 21.55°, 1.2833 A for 24.35°, and 1.0978 A for 31.18°, respectively. The obtained SAD pattern matched the corresponding XRD pattern of a UF resin. This work provides information for studying the mechanism of formation and the application of crystalline regions of UF resins.

Experimental Study of High Velocity Penetration of an Aluminum Target Plate by a Spherical Projectile

A high velocity penetration experimental between an aluminum sphere and a square aluminum plate is carried out to provide data for depicting the penetration process and behavior of both projectile and target at multi-length scales. Residual stress patterns of the plate’s surface before and after penetration are recorded and compared through X-ray diffraction (XRD). Effects of the penetration on microstructure of the Al plate and the material’s microstructural evolution are also illustrated through SEM. Important Physical features of the penetration phenomena are observed from the penetration tests. A good understanding of Al-Al penetration mechanism can therefore be achieved from the experimental study, along which a standard experimental procedure can be established for investigating plasticity mechanisms that govern engineering material penetration.Copyright

Silicon Carbide (SiC) Antennas for High-Temperature and High-Power Applications

The main objective of this letter is to evaluate semi-insulating silicon carbide (SiC) material as a candidate for dielectric substrate for patch antennas, with a long-term potential for monolithic antenna integration on a SiC semiconductor chip, operation in extreme environments, and other applications. First, computer-aided design of microstrip patch antennas operating at 10 GHz was conducted. The antenna designs were implemented using semi-insulating SiC substrates and gold ground planes and patches. A good agreement between the experimental results and simulation was obtained at the band of operation. Return loss and radiation patterns were investigated. As future work, a possibility of utilizing highly conductive (heavily doped) SiC epitaxial layers as the ground planes and radiating patches were investigated using computer simulations.

Use of Vanadium Doping for Compensated and Semi-Insulating SiC Epitaxial Layers for SiC Device Applications

Vanadium doping from SiCl4 source during epitaxial growth with chlorinated C and Si precursors was investigated as a mean of achieving compensated and semi-insulating epitaxial 4H-SiC layers for device applications. Thin epilayers were grown at 1450°C with a growth rate of ~6 μm/h. Experiments at 1600°C resulted in the growth rates ranging from 60 to 90 µm/h producing epilayers with thickness above 30 µm. V concentrations up to about 1017cm-3 were found safe for achieving defect-free epilayer surface morphology, however certain degradation of the crystalline quality was detected by XRD at V concentrations as low as 3-5x1015 cm-3. Controllable compensation of nitrogen donors with V acceptors provided low-doped and semi-insulating epitaxial layers. Mesa isolated PiN diodes with V-acceptor-compensated n- epilayers used as drift regions showed qualitatively normal forward- and reverse-bias behavior.

Orientation, alignment, and polytype control in epitaxial growth of SiC nanowires for electronics application in harsh environments

SiC nanowires (NWs) are attractive building blocks for the next generation electronic devices since silicon carbide is a wide bandgap semiconductor with high electrical breakdown strength, radiation resistance, mechanical strength, thermal conductivity, chemical stability and biocompatibility. Epitaxial growth using metal-catalyst-based vapor-liquid-solid mechanism was employed for SiC NW growth in this work. 4H-SiC substrates having different crystallographic orientations were used in order to control NW alignment and polytype. A new technique based on vapor-phase delivery of the metal catalyst was developed to facilitate control of the NW density. Both 4H and 3C polytypes with a strong stacking disorder were obtained. The 4H and 3C NWs had different orientations with respect to the substrate. 4H NWs grew perpendicular to the c-plane of the substrate. The stacking faults (SFs) in these nanowires were perpendicular to the [0001] nanowire axes. All 3C NWs grew at 20° with respect to the substrate c-plane, and their projections on the c-plane corresponded to one of the six equivalent ⟨101-0⟩ crystallographic directions. All six orientations were obtained simultaneously when growing NWs on the (0001) substrate surface, while only one or two NW orientations were observed when growing NWs on any particular crystallographic plane parallel to the c-axis of the substrate. Growth on {101-0} surfaces resulted in only one NW orientation, thereby producing well-aligned NW arrays. Preliminary measurements of the NW electrical conductivity are reported utilizing two-terminal device geometry.

Vapor-Phase Catalyst Delivery Method for Growing SiC Nanowires

A method was developed for growing SiC nanowires without depositing a metal catalyst on the targeted surfaces prior to the CVD growth. The proposed method utilizes in-situ vapor-phase catalyst delivery via sublimation of the catalyst from a metal source placed in the hot zone of the CVD reactor, followed by condensation of the catalyst-rich vapor on the bare substrate surface to form the catalyst nanoparticles. The vapor-phase catalyst delivery and the resulting nanowire density was found to be influenced by both the gas flow rate and the catalyst diffusion through the boundary layer above the catalyst source. The origin of undesirable bushes of nanowires and the role of the C/Si ratio were established.

Growth of SiC Nanowires on Different Planes of 4H-SiC Substrates

Growth of SiC nanowires (NWs) on monocrystalline 4H-SiC substrates was conducted to investigate a possibility of NW alignment and polytype control. The growth directions of the NWs on the top surfaces and the vertical sidewalls of 4H-SiC mesas having different crystallographic orientations were investigated. The majority of the NWs crystallize in the 3C polytype with the growth axis. Six orientations of the 3C NWs axis with respect to the substrate were obtained simultaneously when growing on the (0001) plane. In contrast, no more than two NW axis orientations coexisted when growing on a particular mesa sidewall. Growth on a particular {10-10} plane resulted in only one NW axis orientation, giving well-aligned NWs.