High Temperature Thermal Insulation System for Millimeter Wave Sintering of B4C

Boron carbide (B₄C) is one of advanced materials and is being used in a wide rage of applications. The unique feature of this material is its large neutron-absorbing cross-section. Some of its most prominent applications are controlling rods in nuclear reactors and radiation protection. 24 GHz microwave processing for B₄C ceramics were performed under flowing argon gas using the sintering system. Sintering at the high temperature (up to 2200°C) was achieved using thermal insulation system consists of fiber-board, boron nitride powder, and boron nitride case. The sintered samples were achieved 90 % of theoretical.     

Diversity-oriented synthesis of multisubstituted olefins through the sequential integration of palladium-catalyzed cross-coupling reactions. 2-pyridyldimethyl(vinyl)silane as a versatile platform for olefin synthesis.

A novel strategy for the diversity-oriented synthesis of multisubstituted olefins, where 2-pyridyldimethyl(vinyl)silane functions as a versatile platform for olefin synthesis, is described. The palladium-catalyzed Heck-type coupling of 2-pyridyldimethyl(vinyl)silanes with organic iodides took place in the presence of Pd2(dba)3/tri-2-furylphosphine catalyst to give beta-substituted vinylsilanes in excellent yields. The Heck-type coupling occurred even with alpha- and beta-substituted 2-pyridyldimethyl(vinyl)silanes. The one-pot double Heck coupling of 2-pyridyldimethyl(vinyl)silane took place with two different aryl iodides to afford beta,beta-diarylated vinylsilanes in good yields. The palladium-catalyzed Hiyama-type coupling of 2-pyridyldimethyl(vinyl)silane with organic halides took place in the presence of tetrabutylammonium fluoride to give di- and trisubstituted olefins in high yields. The sequential integration of Heck-type (or double Heck) coupling and Hiyama-type coupling produced the multisubstituted olefins in regioselective, stereoselective, and diversity-oriented fashions. Especially, the one-pot sequential Heck/Hiyama coupling reaction provides an extremely facile entry into a diverse range of stereodefined multisubstituted olefins. Mechanistic considerations of both Heck-type and Hiyama-type coupling reactions are also described.     

Metal-catalyzed hydrosilylation of alkenes and alkynes using dimethyl(pyridyl)silane

Metal-catalyzed hydrosilylation of alkenes and alkynes using dimethyl(pyridyl)silane is described. The hydrosilylation of alkenes using dimethyl(2-pyridyl)silane (2-PyMe(2)SiH) proceeded well in the presence of a catalytic amount of RhCl(PPh(3))(3) with virtually complete regioselectivity. By taking advantage of the phase tag property of the 2-PyMe(2)Si group, hydrosilylation products were isolated in greater than 95% purity by simple acid-base extraction. Strategic catalyst recovery was also demonstrated. The hydrosilylation of alkynes using 2-PyMe(2)SiH proceeded with a Pt(CH(2)=CHSiMe(2))(2)O/P(t-Bu)(3) catalyst to give alkenyldimethyl(2-pyridyl)silanes in good yield with high regioselectivity. A reactivity comparison of 2-PyMe(2)SiH with other related hydrosilanes (3-PyMe(2)SiH, 4-PyMe(2)SiH, and PhMe(2)SiH) was also performed. In the rhodium-catalyzed reaction, the reactivity order of hydrosilane was 2-PyMe(2)SiH > 3-PyMe(2)SiH, 4-PyMe(2)SiH, PhMe(2)SiH, indicating a huge rate acceleration with 2-PyMe(2)SiH. In the platinum-catalyzed reaction, the reactivity order of hydrosilane was PhMe(2)SiH, 3-PyMe(2)SiH > 4-PyMe(2)SiH > 2-PyMe(2)SiH, indicating a rate deceleration with 2-PyMe(2)SiH and 4-PyMe(2)SiH. It seems that these reactivity differences stem primarily from the governance of two different mechanisms (Chalk-Harrod and modified Chalk-Harrod mechanisms). From the observed reactivity order, coordination and electronic effects of dimethyl(pyridyl)silanes have been implicated.     

Synthesis of alkenylene-bridged macrocyclic silsesquioxanes by ruthenium or rhodium-catalyzed ring-closing reactions of bis(allyldimethylsilyl) groups

Novel alkenylene-bridged oligosilsesquioxanes composed of half-caged structures with 16- or 17-membered rings as well as a titanium-containing molecule with a twisted 21-membered macrocycle have been synthesized by ruthenium or rhodium-catalyzed ring-closing reactions of silsesquioxanes with bis(allyldimethylsilyl) groups.     

Design of a CW 1 THz Gyrotron (Gyrotron Fu Cw III) Using a 20 T Superconducting Magnet

Design of a CW 1 THz gyrotron at second harmonic operation using a 20 T superconducting magnet has been described. The mode competition analysis is employed to investigate operation conditions of second harmonic mode, which is being excited at the frequency ranging from 920 GHz to 1014 GHz. The output power up to 250 watt corresponding to the efficiency of 4.16 percent could be achieved by using an electron beam with accelerating voltage 30 kV and current 200 mA. The important advantage of this gyrotron is that the single mode excitation at second harmonic, and extremely high frequency of the radiation, could be maintained even at high currents. It opens possibility to realize a high power radiation source at 1 THz. Such gyrotron is under construction at FIR Center, University of Fukui.