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排序方式: 共有87条查询结果,搜索用时 31 毫秒
31.
K. L. Sandhya D. S. Shankar Rao S. Krishna Prasad Corresponding author Uma S. Hiremath C. V. Yelamaggad 《Liquid crystals》2013,40(11):1351-1355
We report high pressure investigations on a homologous series of liquid crystalline dimeric molecules in which the terminal chain length is kept constant but the length of the methylene spacer connecting the two mesogenic units is varied. We find that unlike the nematic–isotropic transition temperature and associated entropy change, there is no alternation in the slope of the nematic–isotropic phase boundary in the pressure–temperature plane as a function of the length of the spacer group in the molecule. By applying the Clausius–Clapeyron equation we conclude that the volume change at the transition should exhibit a strong odd–even effect. Measurements on the shortest homologue of the series, which is non-mesomorphic, show the expected result that the application of pressure induces mesomorphism in non-mesomorphic compounds. 相似文献
32.
The synthesis and evaluation of the liquid crystalline properties of non-conventional liquid crystals, consisting of two non-identical mesogenic segments interconnected via a paraffinic chain spacer, are of considerable current interest. In particular, chiral dimesogens possessing a cholesteryl ester unit as the chiral entity joined to other aromatic mesogens through a polymethylene spacer have shown unique and interesting thermal behaviour. In continuation of our investigations on this topic, here we present the synthesis and characterization of the first examples of cholesterol-based unsymmetrical dimesogenic bidentate ligands and their Cu(II) and Pd(II) metal-organic systems (metallomesogens). Our studies reveal that the dimesogenic bidentate ligands exhibit multiple mesophases, whereas their metal complexes stabilize only the mesophase. 相似文献
33.
Two novel series of optically active dimers comprising cholesterol and biphenyl-4-yl 4-(n-alkoxy)benzoate cores interlinked though either an odd-parity/even-parity spacer have been prepared and characterized. They stabilize an extremely complex, frustrated liquid crystalline state viz., the twist grain boundary (TGB) phase with chiral smectic C structure, denoted as TGBC∗ phase, over a wide (50–110 °C) temperature range. Notably, the dimers with an odd-parity spacer show an additional frustrated liquid crystal phase namely, the blue phase (BP). The presence of such frustrated phases suggests that the synthesized dimers are characterized by high enantiomeric excess and strong molecular chirality. Thus, 12 new optically active, nonsymmetric dimers reported herein constitute new examples of rarely found strongly chiral, optically pure dimers showing frustrated liquid crystal phase over an adequately wide thermal range. 相似文献
34.
R. C. Hiremath R. V. Jagadeesh Puttaswamy S. M. Mayanna 《Journal of Chemical Sciences》2005,117(4):333-336
Chloramphenicol (CAP) is an antibiotic drug having a wide spectrum of activity. The kinetics of oxidation of chloramphenicol
by 1-chlorobenzotriazole (CBT) in HClO4 medium over the temperature range 293–323 K has been investigated. The reaction exhibits first-order kinetics with respect
to [CBT]o and zero-order with respect to [CAP]o. The fractional-order dependence of rate on [H+] suggests complex formation between CBT and H+. It fails to induce polymerization of acrylonitrile under the experimental conditions employed. Activation parameters are
evaluated. The observed solvent isotope effect indicates the absence of hydride transfer during oxidation. Effects of dielectric
constant and ionic strength of the medium on the reaction rate have been studied. Oxidation products are identified. A suitable
reaction scheme is proposed and an appropriate rate law is deduced to account for the observed kinetic data. 相似文献
35.
Rahamatalla M. Mulla Gurubasavaraj C. Hiremath Sharanappa T. Nandibewoor 《Journal of Chemical Sciences》2005,117(1):33-42
Kinetics of ruthenium (III) catalyzed oxidation of atenolol by permanganate in alkaline medium at constant ionic strength
of 0.30 mol dm3 has been studied spectrophotometrically using a rapid kinetic accessory. Reaction between permanganate and atenolol in alkaline
medium exhibits 1 : 8 stoichiometry (atenolol : KMnO4). The reaction shows first-order dependence on [permanganate] and [ruthenium (III)] and apparently less than unit order on
both atenolol and alkali concentrations. Reaction rate decreases with increase in ionic strength and increases with decreasing
dielectric constant of the medium. Initial addition of reaction products does not affect the rate significantly. A mechanism
involving the formation of a complex between catalyst and substrate has been proposed. The active species of ruthenium (III)
is understood as [Ru(H2O)5OH]2+. The reaction constants involved in the different steps of mechanism are calculated. Activation parameters with respect to
the slow step of the mechanism are computed and discussed and thermodynamic quantities are also calculated. 相似文献
36.
S. Krishna Prasad Yoji Maeda Shankar D. S. Rao Anitha S. Nagamani Uma S. Hiremath C. V. Yelamaggad 《Liquid crystals》2003,30(11):1277-1283
The phase behaviour of two achiral bent core banana-shaped compounds, the hexyloxy (compound I) and decyloxy (compound II) members of the 1,3-phenylene bis[N-(2-hydroxy-4-n-alkoxybenzylidene)-4'-aminobenzoate] series was investigated under hydrostatic pressures up to 300 MPa using high pressure differential thermal analysis and light transmission methods. The reversible transition sequence crystal (Cr1)-B1 phase-isotropic liquid (I), observed at room pressure for compound I, remains in the pressure region up to c 70 MPa. At higher pressures a pressure-induced crystalline phase (Cri) appears between the Cr1 and B1 phases, its temperature region becoming wider with increasing pressure. The temperature vs. pressure phase diagram shows a triple point of 72.9 MPa and 160.3°C for the Cr1, Cri and B1 phases, indicating the lower limit of pressure for the Cri phase. In compound II the reversible transition sequence crystal (Cr1)-B2 phase-I is seen over the whole pressure region, and the temperature range of the B2 phase remains unaltered. It is concluded that both the B1 and B2 banana phases are stable over the whole pressure region studied. 相似文献
37.
Grain size is important in the barium titanate ceramics used as capacitors and thermistors. Normally this is controlled by chemical additives and firing schedules, but growth twins also lead to increased grain size. Seeding experiments described in this paper demonstrate how the twins can be used to produce large-grained BaTiO3 ceramics. 相似文献
38.
Often the rate of passage of gaseous molecules through model zeolites is too small to be computed directly. An estimate for the rate of passage of CCl4 through the 8-ring window in a model of zeolite A has been obtained by combining a direct evaluation of the free energy profile and an adaptation of the rare events method. First the free energy profile is found from a direct evaluation of the canonical partition function at high dilution and the transition state theory rate constant obtained. The dynamic correction factor is then estimated from molecular dynamics runs and used to compute the actual rate keff. The method is used to estimate the rate of passage through the 8-ring window in a rigid model of zeolite A, and the results are compared with those obtained from rigid models with expanded windows and from the flexible model. Even a small expansion in the 8-ring window diameter increases the rate significantly, but the changes associated with a flexible cage are small. 相似文献
39.
40.
Rahamatalla M. Mulla Gurubasavaraj C. Hiremath Sharanappa T. Nandibewoor 《Monatshefte für Chemie / Chemical Monthly》2004,135(12):1489-1502
Summary. The kinetics of ruthenium(III) catalysed oxidation of sulfanilic acid (p-aminobenzenesulfonic acid) by hexacyanoferrate(III) in alkaline medium at a constant ionic strength of 2.5mol·dm–3 has been studied spectrophotometrically using a rapid kinetic accessory. The reaction exhibits 2:8 stoichiometry (SNA:HCF(III)). The reaction showed first order kinetics in [hexacyanoferrate(III)] and [ruthenium(III)] and apparent less than unit order in both sulfanilic acid and alkali concentrations. The reaction rate increases with increasing ionic strength but the relative permittivity (T) of the medium has a negligible effect on the rate of the reaction. Initial addition of reaction products did not affect the rate significantly. A mechanism involving the formation of a complex between sulfanilic acid and hydroxylated species of ruthenium(III) has been proposed. The active species of HCF(III) and ruthenium(III) are understood as [Fe(CN)63–] and [Ru(H2O)5OH]2+, respectively. The main products were identified by IR, NMR, and mass spectral studies. The reaction constants involved in the different steps of mechanism are calculated. The activation parameters with respect to the slow step of the mechanism are computed and discussed and thermodynamic quantities are also calculated. 相似文献