Naidu V. Seetala

Characterization of Alumina and Silica Sol-Gel Encapsulated Fe/Co/Ru Nanocatalysts in Microchannel Reactors for F-T Synthesis of Higher Alkanes

We have been investigating conversion of syngas (CO: H 2 ) to higher alkanes [Fischer-Tropsch (F-T) Process] in 5 μm and 25 μm channel microreactors coated with sol-gel encapsulated Fe/Co-nanocatalysts. These nano-metal-catalysts were incorporated into the sol-gel matrix by two methods: 1) metal nitrate solutions; 2) metal oxide nanoparticles. Characterization of these catalysts containing Co and Fe in alumina and silica sol-gel has been carried out by several techniques. The surface area measurements by BET method show an average specific surface area of 285 m 2 /g for alumina and 300 m 2 /g for silica sol-gel encapsulated catalysts. In order to optimize the sol-gel preparation and deposition in the microchannels, the elemental composition of sol-gel encapsulated catalyst was examined by EDX. The SEM and AFM images of the reactors before and after deposition of the catalysts have also been studied. Hydrogenation-reduction efficiency of the activated Fe-Co catalysts and the level of poisoning after the reaction were estimated using a vibrating sample magnetometer (VSM). The result suggests more efficient reduction in the case of the nano-particle metal oxides compared to that derived from metal nitrate solutions. In overall, 85% of the catalyst is poisoned after 25 hrs of catalytic reaction. The surface area and the syngas conversion results indicate that silica sol-gel matrix may be a better catalyst support. For alumina sol-gel support, higher conversion of syn-gas is observed with 25 μm microreactor channels. For silica sol-gel, syngas conversion as high as 73% has been achieved by adding Ru as a promoter to the Fe/Co catalyst mixture.

Positron lifetime and SEM studies of porous silica

Porous silica has been used as support for catalysts used to convert renewable and fossil resources into useful energy sources. Understanding the pore structure of these materials is crucial for the design and synthesis of supported metal catalysts. Positron annihilation lifetime spectroscopy (PALS) can be used to characterize the nanoscale pore structure of these materials. Silica powders with two different porous structures were subjected to compressive pressure deformation using a hydraulic press. SEM images at 100K magnification show that the more-porous silica has larger granules compared to low-porous silica. A decrease in granular size due to deformation is clearly observed for more-porous silica and almost unchanged for low-porous silica. The positron third lifetime component in the more-porous silica showed 6.6 ns lifetime (pore size of ~ 11 A) with 3.4% intensity, while these values after 20 kPSI pressure deformation were 2.9 ns (pore size of ~ 6.8 A) with 12.4% intensity. This shows that the la...

A micromachined wide-bandwidth magnetic field sensor based on all-PMMA electron tunneling transducer

All-PMMA-based tunneling magnetic sensors were fabricated by hot embossing replication with silicon templates. The silicon templates had smooth surfaces, positive profiles, and pyramid-like pits with a high aspect ratio. With this fast (20 min), simple (one-step), and repeatable method, the all-PMMA tunneling sensor platform yielded sharp tunneling tips with 75 /spl mu/m in baseline and 50 /spl mu/m in depth. The sensors were assembled and fixed with measurement circuits, after their electrodes were patterned with modified photolithography and Co film was deposited with e-beam evaporation. A natural frequency response of 1.3 kHz was observed, and a tunneling barrier height of 0.713 eV was tested. Due to the quadratic relation between magnetic force and the field, the sensor field response (7.0/spl times/10/sup 6/ V/T/sup 2/) was also quadratic. The noise voltage at 1 kHz is 0.2 mV, corresponding to a magnet field of 0.46/spl times/10/sup -6/ T. The bandwidth of this sensor is 18 kHz. This new type of sensor platform is promising for the next generation of microsensing applications.

Densification and Microhardness of Spark Plasma Sintered ZrB2+SiC Nano-Composites

Ultra-High-Temperature Ceramics (UHTCs) such as ZrB2 and HfB2 with incorporation of SiC nanofiller are useful as structural materials for applications in propulsion and thermal protection systems such as turbine-engine hot section components, leading edge of hypersonic vehicles, where extremely high heat fluxes generate very high temperatures and steep temperature gradients [1]. Spark plasma sintering (SPS) technique is used for densifying the UHTCs under the influence of uniaxial pressure and pulsed direct current [2]. Here, we study the densification, grain growth, and microhardness of ZrB2 nanocomposites with 15% and 20% SiC consolidated using SPS.

Synthesis and characterization of polyimide-carbon nanotube composites

Polyimide nanocomposites were prepared with 0 and 1 wt% single wall-and double wall- CNTs (functionalized and non-functionalized) from BPADA and BAPP by refluxing in NMP. These nanocomposites were characterized using FT-IR, TGA, DSC, tensile strength, and Positron annihilation lifetime spectroscopy (PALS). The FT-IR spectra for all the samples showed the characteristic peaks of polyimide. TGA curves showed weight loss with temperature in two stages. The first stage 180-300 °C showed a weight loss of ~ 15% that may be associated with the release of trapped NMP. The second stage 500-750 °C with a drastic weight loss is associated with decomposition. The residual weight is ~ 40% at 750 °C for both pure polyimide and polyimide nano composites made with functionalized single or double wall CNTs. The non-functionalized CNT dispersed polyimide showed similar two-step behavior, but the weight loss is remarkably less and about 80% weight remained at 750 °C. DSC curves of all polyimide samples showed two distinguis...

Spark Plasma Heat Treated ZrB2-SiC and HfB2-SiC Composites

Ultra-high-temperature ceramics (UHTC) such as ZrB2 and HfB2 with SiC nanofiller are useful for propulsion and thermal protection systems. ZrB2 and HfB2 with 10-20 wt% SiC were prepared using ultra-sonication, rotary evaporation, and spark plasma heat treatment to high temperatures (~2,000°C) and pressures (50-60 MPa). We used positron annihilation lifetime spectroscopy (PALS) to study the nanoporosity, SEM for particle size distribution, and microhardness tester for Vickers hardness. The PALS studies were performed using a 22Na positron source and the positron lifetime spectra were analyzed to three components using POSFIT program. The first and second components are related to positrons annihilating in bulk and in vacancy clusters, respectively; and the third component to positronium annihilation in nanopores within the granules. The PALS results indicate that HfB2 has larger vacancy clusters and nanopores with lesser concentrations compared to ZrB2 and SiC. The SEM observations showed that HfB2 has larger particles compared to ZrB2 and SiC showed wide range of size distribution. The Vickers-Hardness Number (VHN) is measured for spark plasma heat treated composites using a microhardness tester and the results indicate that 10wt%SiC composite has higher hardness compared to 20wt%SiC in both ZrB2-SiC and HfB2-SiC composites. HfB2-SiC composites seem to be more brittle compared to ZrB2-SiC composites. This may be due to larger size and smoother surface of HfB2 particles (600 nm) compared to ZrB2 particles (240 nm).

Positron Lifetime Studies of Irradiated Ultra-High Molecular Weight Polyethylene and Composites Made of Martian Regolith

Positron Annihilation Lifetime Spectroscopy (PALS) is used to study the nanoporosity and fractional free volume in Ultra High Molecular Weight Polyethylene (UHMWPE) and composites with the addition of Martian Regolith (UHMWPE-MR) as-made and irradiated with 56Fe heavy ions at an energy of 600 MeV/u to three different doses (10, 32, 64 Gy). The positron lifetime spectra were obtained using 22Na positron source and the spectra were analyzed to two lifetime components using POSFIT program. First short lifetime component around 0.28 ns is related to positron annihilation in material including vacancy defects and the second long lived component around 1.7 ns is due to Positronium formation in free volume pores. UHMWPE-MR composites were shown to be less porous with much lower nanopores concentration compared to the UHMWPE polymer. The average size of the nanopores is around 0.5 nm (obtained from a simple model). Larger variations in positron lifetime parameters are observed with increasing irradiation dose for UHMWPE polymer compared to UHMWPE+MR composites. The 3-point bend test results also showed larger variations with increasing irradiation dose for the UHMWPE polymer. The variations in PALS parameters may indicate an increasing competition between two processes at higher irradiation doses: 1) vacancy defects aggregation and 2) escape of vacancy defects as the local temperature increases at higher doses resulting in increased vacancy defects mobility. Present results clearly indicate a qualitative inverse relationship between nanoscale porosity measured by positron life time and mechanical properties of UHMWPE and its composite with MR.

Effect of Argon Gas Purging of Spark Plasma Sintered ZrB2+SiC Nano-Powder Composites

Spark Plasma Sintering (SPS) consolidated ZrB2+SiC composites using nano-powders (around 40 nm) showed smaller grains compared to those using micron size powders and segregation of SiC into islands is minimal but with higher oxidation of ZrB2 to form ZrO2 in nano-composites. Argon-gas purging prior to SPS consolidation at around 2000 °C and 40 MPa of ZrB2+20vol.%SiC nano-powders was used to minimize the oxidation and obtain fine granules with high densification. The densification of the Argon-gas purged nano-composites is higher compared to those consolidated without Argon gas purging. The EDX analysis showed a strong reduction in the oxygen peak for the Argon gas purged composites. The XRD spectra also support this observation with less ZrO2 phase composition in Argon gas purged composites. The Vickers micro-hardness showed slightly lower values for Argon gas purged composites though they have higher densification.

Comparative Study between Vickers and Knoop Micro-hardness of Ultra High Temperature Ceramics

Ultra-high temperature ceramics such as ZrB2 and HfB2 with small percentage of SiC are useful as structural materials for applications in leading edge of hypersonic vehicles [1]. Spark plasma sintering (SPS) technique is used for densifying the UHTCs under the influence of uniaxial pressure and pulsed direct current [2]. Fine grain, low porosity, and high densification yield higher micro-hardness ceramics those can be used for high temperature oxidation resistant materials. Here we made a comparative study between Vickers and Knoop micro-hardness of SPS consolidated UHTC composites starting with micronand nano-powders.

High-energy Ball-milling of ZrB 2 and HfB 2 Powders: Effect on Particle Size and Crystalline Grain Distribution

Introduction: Ultra-High-Temperature Ceramics (UHTCs) such as ZrB2 and HfB2 with incorporation of SiC are being considered as structural materials for applications in propulsion and thermal protection systems such as turbine-engine hot section components, leading edge of hypersonic vehicles, where extremely high heat fluxes generate very high temperatures and steep temperature gradients [1]. We used high energy ball milling of the precursor powders to increase lattice distortion enhanced inter-diffusion, uniform distribution of SiC, and reduce grain growth during Spark plasma sintering (SPS). Here, we study the effect of high energy ball milling of ZrB2 or HfB2 with 20 vol% SiC on the particle size and crystalline-grain distribution.