Search

Tandem reactions of Pd‐catalyzed cross‐coupling of 3‐(2‐isocyanoethyl)indoles with diazoacetates and subsequent spirocyclization/Mannich‐type reaction have been developed to assemble polycyclic spiroindoline skeletons. Formation of spiroindolenines has been proven as the crucial step for the following Mannich‐type cyclization reaction. Accordingly, a novel approach on chiral phosphoric acid catalyzed Mannich‐type cyclization toward the formation of diastereomerically and enantiomerically enriched pentacyclic spiroindolines has been established. Moreover, the products of the reaction are versatile building blocks in synthetic chemistry, as demonstrated by the synthesis of the key framework of aspidosperma and kopsia alkaloids. [ABSTRACT FROM AUTHOR]

Electrical properties of Nb-, V-, and W-doped Bi4Ti3O12–MBi4Ti4O15 (BIT-MBTi) (M=Ca,Sr) compounds were investigated. The remanent polarization (2Pr) and piezoelectric coefficient (d33) of BIT-MBTi are greatly increased by such donor doping. W-doped and Nb-doped BIT-SBTi exhibited the greatest enlargement in 2Pr and d33, respectively. Nb doping also increases 2Pr and d33 of BIT-CBTi though not that much as in the BIT-SBTi case. BIT-CBTi thin film shows a high 2Pr value of 39 μC/cm2. The enhanced properties are thought to stem from the reduced concentration and weakened mobility of oxygen vacancies. Increased activation energy of conduction further confirmed the restraint of oxygen vacancies. The different optimal V-doping content for 2Pr and d33 in BIT-SBTi might be related to the fact that V doping could affect the structure and density of domain more apparently than the concentration of oxygen vacancies. The thermal variation of dielectric constant of BIT-CBTi shows a distinctive double anomaly at 658 and 728 °C. The first dielectric anomaly is related to the regulation of BIT parts, and the second one corresponds to the ferroelectric-paraelectric phase transition. [ABSTRACT FROM AUTHOR]

The effect of Nd modification on ferro-, piezo-, and dielectric properties of intergrowth Bi4Ti3O12-SrBi4Ti4O15 ceramics was investigated. X-ray diffraction and Raman scattering were used to identify the crystal phase and to distinguish the doping sites. With increasing Nd content up to 0.50, both remanent polarization (2Pr) and piezoelectric coefficient (d33) were found to increase and reach the maximum value of 33.2 μC/cm2 and 14 pC/N, respectively, which gained an enlargement over 1.7 times in 2Pr and a desirable 75% increment in d33 value. However, further Nd modification starts to deteriorate the ferro- and piezoelectric behavior. Impedance spectroscopy shows the activation energy of conductivity increased with increasing Nd content, which can be regarded as direct proof of the restraint of oxygen vacancies. The thermal variations of dielectric permittivity and loss tan δ with Nd content show the characteristic of diffuse phase transformation while the convincible defect-related relaxation phenomenon was not found. Interestingly, in contrast to La modification, Nd modification does not induce the relaxor behavior even at very high doping content. [ABSTRACT FROM AUTHOR]

The ferroelectric and dielectric properties of vanadium-doped Sr(sub 2)Bi(sub 4)Ti(sub 5)O18 ceramics was investigated and the ferroelectricity of Sr(sub 2)Bi(sub 4-x/3)Ti(5-x)VxO18 samples was improved by vanadium doping. It was suggested with the help of results that the substitution of a small amount of V(super 5+) for Ti(super 4+) reduced the concentration of oxygen vacancies under the restriction of charge neutrality.

We reported a cupreidine catalyzed enantioselective cascade cyclization reaction of 2‐alkynyl‐3,3‐difluoro‐3H‐indole and 2‐mercaptoimidazole under simple and mild conditions. The corresponding chiral gem‐difluorinated C2‐spiro indolines were obtained in moderate to good yields and enantioselectivities, which could be interesting compounds from synthetic and medicinal chemistry points of view. This work represents the first catalytic enantioselective example of utilizing 2‐alkynyl‐3,3‐difluoro‐3H‐indoles as the bis‐electrophiles in a cyclization reaction. [ABSTRACT FROM AUTHOR]

Abstract: In this communication, we report a substrate‐controlled product diversity in the 1,5,7‐triazabicyclo[4.4.0]dec‐5‐ene (TBD)‐catalyzed cascade cyclization of 2‐alkynyl‐3,3‐difluoro‐3H‐indoles with N,N‐ or N,S‐bis‐nucleophiles. The products represent two important heterocyclic compounds, C2‐spiro indoline, and pyrimido[1,2‐a]benzimidazole, featuring a versatile gem‐difluoromethylene group on the framework. In these cascade processes, two new C−N bonds, or one C−S and one C−N bond, are consecutively formed in a single step. The present protocol is characterized with high regioselectivity, high yield, broad substrate scope, good functional group tolerance, facile scalability and mild reaction conditions. [ABSTRACT FROM AUTHOR]

To decrease the switching loss and the dead-time effect of resonant half-bridge inverter, a novel adaptive dead-time control circuit of resonant half-bridge driver Integrated Circuit (IC) is presented. Without increasing the pin number of IC, this circuit takes a novel strategy to adaptively regulate dead time to a temperate range between high and low thresholds. The high and low thresholds are adaptive to the fall time of output signal in a half-bridge clock cycle. The IC of the designed circuit is suitable for high-voltage applications. The dead-time regulation range of this circuit achieves 0–3.5 µs. The range of temperate dead-time state is 300 ns. The failure signal of this circuit can protect the IC and peripheral power devices by regulating operation in three clock cycles. Both simulation and measurement of the proposed circuit in a half-bridge driver IC with an operating frequency at 50 kHz are presented based on the 0.5 µm 700 V BCD process. The results of simulation and measurement show that the presented circuits’ performance is perfect. [ABSTRACT FROM PUBLISHER]

Bi5Fe1-xCoxTi3O15 (x = 0.0, 0.2, 0.4, 0.5, 0.6, and 0.8) multiferroic ceramics are synthesized in two steps using the solid state reaction technique. X-ray diffraction patterns show that the samples have four-layer Aurivillius phases. At room temperature (RT), the samples each present a remarkable coexistence of ferromagnetism (FM) and ferroelectricity (FE). The remnant polarization (2Pr) reaches its greatest value of 14 µC/cm2 at x = 0.6. Remnant magnetization (2Mr) first increases and then decreases, and the greatest 2Mr is 7.8 menu/g when x = 0.5. The magnetic properties for x = 0.4 are similar to those for x = 0.6, indicating that the magnetic properties originate mainly from the coupling between Fe3+ and Co3+ ions, rather than from their own magnetic moments. [ABSTRACT FROM AUTHOR]

Sr2Bi4Ti5O18 (SBTi) and Sr-deficient SBTi thin films were deposited on Pt/Ti/SiO2/Si(100) substrates using a sol-gel method. Structure, morphology and electric properties were investigated systematically. These films were random-oriented and were composed of rod-like grains. The remanent polarization (2Pr) and coercive field (Ec) of SBTi films were 24 μC/cm2 and 68 kV/cm, respectively. This value of 2Pr was much higher than the reported value of SBTi prepared by pulse laser deposition. The compounds with Sr-deficient and Bi-excess composition showed a very large remanent polarization of 35.5 μC/cm2. More importantly, both films showed high fatigue resistance against continuous switching up to 3 × 1010 cycles and excellent charge-retaining ability up to 3 × 104 s. [ABSTRACT FROM AUTHOR]

Ceramic samples of SrBi4−xNdxTi4O15 (SBNT-x, x = 0.00–1.00) have been prepared by the conventional solid-state reaction method. As the Nd doping content increases from 0.00 to 0.18, the remnant polarization (2Pr) increases steadily and reaches a maximum value of 25.8 µC cm−2, which is almost 56% higher than that of the non-doped sample. However, 2Pr decreases with further substitution. The variation of 2Pr is dominated by the restraint of space charge and the relief of structural distortion. The coercive field (Ec) remains almost unchanged at a value of ∼80 kV cm−1 in the case of x = 0.00–0.18, then decreases with further Nd-doping, which is probably due to the formation of anti-phase boundaries induced by the modification. As doping content exceeds 0.75, the specimens exhibit relaxor behaviour due to a ferroelectric microdomain state caused by the random fields. [ABSTRACT FROM AUTHOR]

Abstract: Several lanthanum-doped intergrowth ferroelectric materials of type Bi4− x La x Ti3O12–SrBi4− y La y Ti4O15 [BLT–SBLT(x+y)] were prepared with the lanthanum content, (x+y), ranging from 0.00 to 1.50. XRD patterns imply that the lattice space of (116) for Bi4Ti3O12–SrBi4Ti4O15 is the average of those of (115) for Bi4Ti3O12 and (117) for SrBi4Ti4O15. By assuming that this relationship is not affected by doping, the La ion substitution site is calculated. The results indicate that La ions show pronounced site selectivity for the BTO blocks when the La content is lower than 1.25. As (x+y)=1.50, more La ions substitute the Bi ions in SBTi blocks. These calculated results are consistent with the estimation from their Curie temperature. [Copyright &y& Elsevier]

The investigations on the ferroelectric and dielectric properties of vanadium-doped Bi4Ti3O12 ceramics are presented. V-doping did not affect the basic crystal structure of B4Ti3O12. However, the degree of the lattice distortion varied with doping, which causes the variation of the internal strain. The sample with the minimized lattice distortion has the largest remnant polarization. The ferroelectric property of Bi4Ti3O12 was significantly improved by doping. The 2Pr, of Bi4Ti3O12 was 16 μC/cm² and it reaches a maximum value of 26 μC/cm² when the vanadium content is 0.03. There were two relaxation peaks (P1, P2) in the dielectric loss (D) curves. The activation of the P1 peak for Bi4Ti3O12 ceramics was 0.66 eV. The P1 and P2 peaks relating to oxygen vacancies reduced with V doping, which give a clue that V-doping restrains the oxygen vacancy concentration. The enlargement 2Pr in V-doped Bi4Ti3O12 is associated with the internal strain and the restraint of oxygen vacancies. [ABSTRACT FROM AUTHOR]

The investigations on the ferroelectric and dielectric properties of vanadium-doped Bi4Ti3O12 ceramics are presented. V-doping did not affect the basic crystal structure of B4Ti3O12. However, the degree of the lattice distortion varied with doping, which causes the variation of the internal strain. The sample with the minimized lattice distortion has the largest remnant polarization. The ferroelectric property of Bi4Ti3O12 was significantly improved by doping. The 2Pr, of Bi4Ti3O12 was 16 μC/cm² and it reaches a maximum value of 26 μC/cm² when the vanadium content is 0.03. There were two relaxation peaks (P1, P2) in the dielectric loss (D) curves. The activation of the P1 peak for Bi4Ti3O12 ceramics was 0.66 eV. The P1 and P2 peaks relating to oxygen vacancies reduced with V doping, which give a clue that V-doping restrains the oxygen vacancy concentration. The enlargement 2Pr in V-doped Bi4Ti3O12 is associated with the internal strain and the restraint of oxygen vacancies. [ABSTRACT FROM AUTHOR]

Using the standard solid-state reaction method, several vanadium-doped ferroelectric ceramics of type SrBi4−x/3Ti4−xVxO15 (SBTV−x) were synthesized. The vanadium doping content, x, rangs from 0.00 to 0.06. The crystal structure of SrBi4Ti4O15 is not affected by V-doping. The electric breakdown voltage of the samples increases with V content. Meanwhile, V-doping results in a notable enlargement of remnant polarization (2Pr). The 2Pr of STBV−0.03 reaches a very large value, which is over 50 μC/cm2 and is nearly twice greater than that at zero doping. The Curie temperatures of V-doped samples decrease slightly in comparison with that of SrBi4Ti4O15. V-doping can improve the electric properties of SrBi4Ti4O15 without sacrificing its thermal stableness. [Copyright &y& Elsevier]

The effect of Nd modification on ferro-, piezo-, and dielectric properties of intergrowth Bi4Ti3O12-SrBi4Ti4O15 ceramics was investigated. X-ray diffraction and Raman scattering were used to identify the crystal phase and to distinguish the doping sites. With increasing Nd content up to 0.50, both remanent polarization (2Pr) and piezoelectric coefficient (d33) were found to increase and reach the maximum value of 33.2 μC/cm2 and 14 pC/N, respectively, which gained an enlargement over 1.7 times in 2Pr and a desirable 75% increment in d33 value. However, further Nd modification starts to deteriorate the ferro- and piezoelectric behavior. Impedance spectroscopy shows the activation energy of conductivity increased with increasing Nd content, which can be regarded as direct proof of the restraint of oxygen vacancies. The thermal variations of dielectric permittivity and loss tan δ with Nd content show the characteristic of diffuse phase transformation while the convincible defect-related relaxation phenomenon was not found. Interestingly, in contrast to La modification, Nd modification does not induce the relaxor behavior even at very high doping content. [ABSTRACT FROM AUTHOR]

Sr2Bi4Ti5O18 (SBTi) and Sr2Bi4−x/3Ti5−xVxO18 (x = 0.018, SBTV) films were deposited on Pt/Ti/SiO2/Si(100) substrates using a sol–gel method. Structure, morphology and electric properties were investigated systematically. These films were randomly oriented and composed of rod-like grains. The remanent polarization (2Pr) and coercive field (Ec) of SBTi films were 23 µC cm−2 and 71 kV cm−1, respectively. This value of 2Pr was much higher than the reported value of the SBTi thin film prepared by pulse laser deposition. A small content substitution of V5+ for Ti4+ was also confirmed to be effective for further improvement in the ferroelectric properties of SBTi films. The V-modified SBTi films showed a very large remanent polarization (2Pr) of 34 µC cm−2. More importantly, both films showed high fatigue resistance against continuous switching up to 4 × 1010 cycles and excellent charge-retaining ability up to 4 × 104 s. [ABSTRACT FROM AUTHOR]

Abstract: Ferroelectric and dielectric properties of tungsten-doped Bi4Ti3O12-SrBi4Ti4O15 ceramics were investigated. A pure phase of all the samples is confirmed by X-ray diffraction patterns. The remanent polarization (2P r) of the samples increases initially and reaches its maximum value of 43.2μC/cm2 when W content is 0.03, which is over twice as large as that of non-doped one, then decreases with further doping. The coercive field (E c) shows a weak tungsten content dependency. It is considered that the enlarged 2P r could be mainly attributed to the restraint of oxygen vacancies as well as the weakening of their mobility. The Curie temperature decreases a little with increasing doping content, which indicates the good thermal stability is not deteriorated by tungsten doping. [Copyright &y& Elsevier]