Effect of Silicon Activity on Liquid-Phase Sintering of Nitrogen Ceramics

Volatilization resulting from the thermal decomposition of Si₃N₄ causes the large weight loss and desintering phenomenon observed during pressureless sintering of Si₃N₄-5% MgO and sialon (z =2)-5% MgO. The addition of a few weight percent of Si to the powder suppresses this volatilazation and helps to achieve fully dense Si₃N₄ components.     

Combustion Synthesis of AlON–BN Composites under Low Nitrogen Pressure

Hexagonal boron nitride (hBN) and aluminum oxinitride (AlON) composites were synthesized by combustion reaction of powder mixtures of Al–B₂O₃–AlN systems under a low pressure of nitrogen gas (0.5 MPa). Explosive combustion reaction of Al–B₂O₃ systems under the same nitrogen pressure produced alumina, aluminum borate, AlN, and AlON depending on the binary mixing ratio, but no trace of BN phases could be identified. Most of the elemental boron product remained unreacted and amorphous. On the other hand, AlN addition as a diluent in the range of 15–30 wt% was effective in producing hBN phase and forming AlON–BN composites. In the composition range of the ternary mixture of Al, B₂O₃, and AlN, where significant BN formation was identified, the primary role of AlN was to react with B₂O₃ to produce BN and α-Al₂O₃. The temperature profile obtained during the combustion reaction by a thermocouple imbedded in the middle of the powder bed revealed that the initial nitridation reaction of aluminum metal provides the heat required for the combustion reaction, creating a state of a "chemical oven." The reaction product, α-Al₂O₃, reacted subsequently with AlN to produce AlON phases to give final AlON–BN composites. The combustion reaction was highly unstable and followed a mixed mode with a regularly reversing spinning mode for aluminum nitridation reaction in the surface region and an oscillatory mode for the BN formation reaction in the subsurface region.     

Abnormal Grain Growth of Alumina

Abnormal grain growth (AGG) is not one of the intrinsic properties of alumina but rather is an extrinsic property that is controlled by certain impurities that are introduced during powder synthesis, processing, or sintering. When small amounts of glass-forming impurities are introduced, some portion beyond their solubility limits will accumulate at grain boundaries at the final stage of densification, form thin intergranular glass films of thermodynamically stable thickness, and induce the sudden appearance of abnormal grains by increasing the rate of grain-boundary migration abruptly. The proposition has been tested experimentally with small, but varying, amounts of silica in ultrapure alumina (99.999%) that has been sintered in a contamination-free condition. Average grain sizes for the appearance of AGG are inversely related to the doping concentration of silica. The thickness of intergranular silicate glass films at the onset of AGG in alumina is constant and estimated to be }3.7 nm.     

Combined effect of ultrasound and organic acids to reduce Escherichia coli O157:H7, Salmonella Typhimurium, and Listeria monocytogenes on organic fresh lettuce

This study was performed to compare the effectiveness of individual treatments (ultrasound and organic acids) and their combination on reducing foodborne pathogens on organic fresh lettuce. Lettuce leaves were inoculated with a cocktail of three strains each of Escherichia coli O157:H7, Salmonella Typhimurium, and Listeria monocytogenes and treated with ultrasound (40 kHz) alone, organic acids (0.3, 0.5, 0.7, 1.0, and 2.0% — malic acid, lactic acid, and citric acid) alone and combined with ultrasound and organic acids for 5 min. For all 3 pathogens, the combined treatment of ultrasound and organic acids resulted in additional 0.8 to 1.0 log reduction compared to individual treatments, without causing significant quality change (color and texture) on lettuce during 7 day storage. The maximum reductions of E. coli O157:H7, S. Typhimurium, and L. monocytogenes were 2.75, 3.18, and 2.87 log CFU/g observed after combined treatment with ultrasound and 2% organic acid for 5 min, respectively. Our results suggest that the combined treatment of ultrasound with organic acids was effective at increasing pathogen reduction compared to individual treatments without significantly affecting quality, and demonstrates its potential as a novel method to increase the microbial safety on organic fresh lettuce.     

Growth Characteristics and Biofilm Formation of Various Spoilage Bacteria Isolated from Fresh Produce

This study investigated the characteristics of spoilage bacteria isolated from fresh produce including growth at various temperatures, biofilm formation, cell hydrophobicity, and colony spreading. The number of spoilage bacteria present when stored at 35 °C was significantly greater than when stored at lower temperatures, and maximum population size was achieved after 10 h. However, Bacillus pumilus, Dickeya zeae, Pectobacterium carotovorum subsp. Carotovorum Pcc21, and Bacillus pumilus (RDA‐R) did not grow at the storage temperature of 5 °C. The biofilm formation by Clavibacter michiganensis, Acinetobacter calcoaceticus, and A. calcoaceticus (RDA‐R) are higher than other spoilage bacteria. Biofilm formation showed low correlation between hydrophobicity, and no significant correlation with colony spreading. These results might be used for developing safe storage guidelines for fresh produce at various storage temperatures, and could be basic information on the growth characteristics and biofilm formation properties of spoilage bacteria from fresh produce.     

Combined effect of calcium oxide and sonication to reduce foodborne pathogens on fresh produce

This study was conducted to investigate the antibacterial activity of calcium oxide (CaO), sonication, and their combination against foodborne pathogens on several fresh produce. The results showed that combined treatment inhibited Escherichia coli O157:H7, Listeria monocytogenes, and Salmonella Typhimurium on fresh produce. However, the effectiveness of treatments for reducing populations of foodborne pathogens varied depending on the types and condition of the fresh produce. This study demonstrated that the antibacterial potency of CaO and its application with sonication could be one of alternatives for controlling foodborne pathogens on fresh produce.     

Hot Isostatic Pressing of Sintered Silicon Nitride

Pressureless-sintered silicon nitride with varying additives was hot isostatic pressed under 150 MPa of nitrogen at 1800°C. Moderate increases in densities were observed when sintered densities exceeded 90% of theoretical. However, density changes became insignificant as the amount of additives exceeded 12 wt%; moreover, density reduction was occasionally observed. Microstructural analysis, after the silicon nitride was reannealed at 1650°C under 0.1 MPa of nitrogen, revealed that intergranular glass was supersaturated with nitrogen and "bloated" as a result of nitrogen evolution. This result suggested that the effectiveness of container-free hot isostatic pressing of silicon nitride was severely limited by enhanced solubility of nitrogen gas in the glassy phase under high pressure.     

Formation Mechanisms of Tetragonal Barium Titanate Nanoparticles in Alkoxide–Hydroxide Sol-Precipitation Synthesis

The early stage of barium titanate (BaTiO₃) nanoparticle formation is investigated by in situ X-ray diffraction (XRD) and X-ray absorption near-edge structure (XANES) using synchrotron radiation. BaTiO₃ nanoparticles are synthesized via dissolution of barium hydroxide octahydrate and hydrolysis of titanium (IV) isopropoxide in isopropanol. In the course of raising the temperature of the alkoxide–hydroxide mixture solution to 80°C, in situ synchrotron XRD reveals that BaTiO₃ nanocrystals smaller than 6 nm begin to nucleate at 50°C without intermediate TiO₂ anatase formation, and Ti K edge absorption spectra also confirm the formation of corner-sharing TiO₆ octahedra at 60°C. The average size of BaTiO₃ precipitates increases to about 7.5 nm at 80°C. The synthesized nanopowders show an anomalously high tetragonality according to the Rietveld refinement of synchrotron XRD data.     

Fabrication of 3-dimensional PbZr(1-x)Ti(x)O3 nanoscale thin film capacitors for high density ferroelectric random access memory devices

PbZr(x)Ti(1-x)O3 (PZT) thin films were deposited on 3-dimensional (3D) nano-scale trench structures for use in giga-bit density ferroelectric random access memories. PZT thin films were deposited by liquid delivery metalorganic chemical vapor deposition using Pb(thd)2, Zr(MMP)4, and Ti(MMP)4 precursors dissolved in ethyl cyclohexane. Iridium thin films were deposited by atomic layer deposition, and they exhibited excellent properties for capacitor electrodes even at a thickness of 20 nm. The trench capacitor was composed of three layers, viz. Ir/PZT/lr (20/60/20 nm), and had a diameter of 250 nm and a height of 400 nm. Almost 100% step coverage was obtained at a deposition temperature of 530 degrees C. The PZT thin film capacitors with a thickness of 60 nm on a planar structure exhibited a remnant polarization (Pr) of 28 microC/cm2, but the Pr value of the 3D PZT capacitors decreased slightly with decreasing 3D trench pattern size. The transmission electron microscope analysis indicated that the PZT thin films had compositional uniformity in the 3D trench region. Both columnar and granular grains were formed on the sidewalls of the trench capacitors, and their relative proportion exhibited strong size dependence. The trench capacitors with more columnar PZT grains showed good switching behavior under an external bias of 2.1 V and had a remnant polarization of 19-24 microC/cm2.     

Prevalence and genetic diversity of Bacillus cereus in dried red pepper in Korea

Bacillus cereus is a foodborne spore-forming bacterial pathogen that is ubiquitous in the natural environment. Infections with this pathogen manifest as diarrheal or emetic types of food poisoning. In this study, 140 samples of dried red pepper purchased in Korea were assayed for the presence of B. cereus according to the U.S. Food and Drug Administration standard culture method. A multiplex PCR assay was developed for the rapid confirmation of B. cereus as an alternative to conventional biochemical confirmation tests. The genetic diversity of B. cereus isolates was investigated using a random amplified polymorphic DNA (RAPD) assay. B. cereus was found in 84.3% of the dried red pepper samples, with an average concentration of 1.9 x 10(4) CFU/g. B. cereus could be detected and distinguished from B. thuringiensis in the multiplex PCR assay by using the BCFW1 plus BCrevnew and the K3 plus K5 primer sets designed to detect the gyrB gene of B. cereus and B. thuringiensis and the cry gene of B. thuringiensis. A RAPD assay using the OPG 16 and MUP 3 primers was used to successfully distinguish among isolates, thus elucidating the genetic diversity of B. cereus isolates. The discriminating ability of the OPG 16 primer (142 types) was about threefold higher than that of MUP 3 (52 types) in the RAPD assay.