Formation of RDX nanoparticles by rapid expansion of supercritical solution : in situ characterization by laser scattering

Cyclo-1,3,5-trimethylene-2,4,6-trinitramine (RDX), a well-known energetic material, is highly explosive. Reduction of the sensitivity of RDX is desired for safe handling and storage in military applications. Data on the sensitivity of RDX in the 10-1000 µm crystal size range suggested that the impact sensitivity could be further reduced by reduction of the crystal size to the sub-micron or nano-scale (Armstrong, 1990). Recent research with nano-RDX obtained using Rapid Expansion of Supercritical Solutions (RESS) confirmed these expectations (Stepanov, 2005). RESS process appeared to be one out of a few techniques of production of nanoscale energetic materials free of the risk of explosion. Current research was aimed at the understanding of the fundamentals of the RESS process and the mechanism of nanoparticle formation via an in situ particle monitoring and characterization in the RESS jet combined with the characterization of the final product. In this research, nanoparticles of RDX generated by RESS using supercritical CO2 were characterized in situ by a pulse laser light scattering imaging technique using gated ICCD camera. The sensitivity was determined using Rayleigh scattering from air as well as light scattering from standard polystyrene spheres. The size distribution functions of the particles formed in the RESS jet were determined using the calibrated sensitivity. The final diameter of RDX particles at the pre-expansion pressure of 180 bar was 73 nm at the maximum of the size distribution function. Assuming that the particles near the nozzle consisted mainly of CO2 and the log-normal size distribution, the diameter of the particles near the nozzle (7.5 mm from the nozzle) at the distribution maximum was 3.3 µm at the pre-expansion pressure of 180 bar. The number densities of the particles in the RESS jet were determined by counting individual particles in the light scattering images. Based on the measured particle size distributions and the number density of particles along the RESS jet, the mechanism of particle formation in the RESS is discussed. The homogeneous nucleation mechanism is rejected as it fails to explain the large particle size experimentally observed. Instead, a modified spray-drying mechanism is suggested

撹拌槽解析のための数値計算手法の開発

学位の種別: 課程博士審査委員会委員 : （主査）東京大学教授 越塚 誠一, 東京大学教授 大橋 弘忠, 東京大学教授 鈴木 克幸, 東京大学講師 柴田 和也, 東京大学教授 高木 周, 東京大学教授 奥田 洋司University of Tokyo(東京大学

Concanavalin A-mediated T cell proliferation is regulated by herpes virus entry mediator costimulatory molecule

T cell activation is regulated by two distinct signals, signals one and two. Concanavalin A (ConA) is an antigen-independent mitogen and functions as signal one inducer, leading T cells to polyclonal proliferation. CD28 is known to be one of major costimulatory receptors and to provide signal two in the ConA-induced T cell proliferation. Here, we have studied the implication of other costimulatory pathways in the ConA-mediated T cell proliferation by using soluble recombinant proteins consisting of an extracellular domain of costimulatory receptors and Fc portion of human IgG. We found that T cell proliferation induced by ConA, but not PMA plus ionomycin or anti-CD3 mAb, is significantly inhibited by herpes virus entry mediator (HVEM)-Ig, even in the presence of CD28 signaling. Moreover, the high concentration of HVEM-Ig molecules almost completely suppressed ConA-mediated T cell proliferation. These results suggest that HVEM might play more important roles than CD28 in ConA-mediated T cell proliferation. © 2013 The Society for In Vitro Biology

Rapid proliferation of activated lymph node CD4+ T cells is achieved by greatly curtailing the duration of gap phases in cell cycle progression

Peripheral T cells are in G0 phase and do not proliferate. When they encounter an antigen, they enter the cell cycle and proliferate in order to initiate an active immune response. Here, we have determined the first two cell cycle times of a leading population of CD4+ T cells stimulated by PMA plus ionomycin in vitro. The first cell cycle began around 10 h after stimulation and took approximately 16 h. Surprisingly, the second cell cycle was extremely rapid and required only 6 h. T cells might have a unique regulatory mechanism to compensate for the shortage of the gap phases in cell cycle progression. This unique feature might be a basis for a quick immune response against pathogens, as it maximizes the rate of proliferation.In Pres

TPR5 is involved in directional cell division and is essential for the maintenance of meristem cell organization in Arabidopsis thaliana

Root growth in plants is achieved through the co-ordination of cell division and expansion. In higher plants, the radial structure of the roots is formed during embryogenesis and maintained thereafter throughout development. Here we show that the tetratricopeptide repeat domain protein TPR5 is necessary for maintaining radial structure and growth rates in Arabidopsis thaliana roots. We isolated an A. thaliana mutant with reduced root growth and determined that TPR5 was the gene responsible for the phenotype. The root growth rate of the tpr5-1 mutant was reduced to ~60% of that in wild-type plants. The radial structure was disturbed by the occurrence of occasional extra periclinal cell divisions. While the number of meristematic cells was reduced in the tpr5 mutants, the cell length in the mature portion of the root did not differ from that of the wild type, suggesting that TPR5 is required for proper cell division but dispensable for cell elongation. Expression of the TPR5–GFP fusion protein driven by the TPR5 promoter displayed fluorescence in the cytoplasm of root meristems, but not in mature root regions. DNA staining revealed that frequencies of micronuclei were increased in root meristems of tpr5 mutants. From this study, we concluded that TPR5 is involved in preventing the formation of micronuclei and is necessary for both the activity and directionality of cell division in root meristems

Proteasomal degradation of BRAHMA promotes Boron tolerance in Arabidopsis

High levels of boron (B) induce DNA double-strand breaks (DSBs) in eukaryotes, including plants. Here we show a molecular pathway of high B-induced DSBs by characterizing Arabidopsis thaliana hypersensitive to excess boron mutants. Molecular analysis of the mutants revealed that degradation of a SWItch/Sucrose Non-Fermentable subunit, BRAHMA (BRM), by a 26S proteasome (26SP) with specific subunits is a key process for ameliorating high-B-induced DSBs. We also found that high-B treatment induces histone hyperacetylation, which increases susceptibility to DSBs. BRM binds to acetylated histone residues and opens chromatin. Accordingly, we propose that the 26SP limits chromatin opening by BRM in conjunction with histone hyperacetylation to maintain chromatin stability and avoid DSB formation under high-B conditions. Interestingly, a positive correlation between the extent of histone acetylation and DSB formation is evident in human cultured cells, suggesting that the mechanism of DSB induction is also valid in animals

Dynamics of light-induced anomalous Hall effect in the three-dimensional Dirac semimetal Cd3_3As2_2

We experimentally study the dynamical behavior of the light-induced anomalous Hall effect in a three-dimensional Dirac semimetal, Cd$_3$As$_2$. An ultrashort, circularly polarized, multi-terahertz pump pulse breaks the time-reversal symmetry of a thin film sample. The resulting anomalous Hall effect is clearly observed through the polarization rotation of a single-cycle terahertz probe pulse. Comparing the experimental result with theory, we find that the field-induced injection current dominates the anomalous Hall effect during pump irradiation, while the Berry curvature of the Floquet-Weyl semimetal state does not appreciably contribute. Remarkably, even after pump irradiation, we observe an anomalous Hall effect that lasts for more than 10 ps. A model fit to the Hall conductivity spectrum reveals a relatively long scattering time over 400 fs. This result shows that circularly polarized light creates a polarization of the isospin degree of freedom in the Dirac semimetal, which labels the crystallographic point group representation of the overlapping Weyl semimetal bands. Our observation paves the way for conversion of a robust isospin flow into an electric current at room temperature, being a new analogue of the inverse spin Hall effect.Comment: 44 pages, 14 figure