全文获取类型
收费全文 | 1894篇 |
免费 | 41篇 |
国内免费 | 6篇 |
专业分类
化学 | 1099篇 |
晶体学 | 73篇 |
力学 | 53篇 |
数学 | 282篇 |
物理学 | 434篇 |
出版年
2022年 | 19篇 |
2021年 | 17篇 |
2020年 | 16篇 |
2019年 | 14篇 |
2018年 | 21篇 |
2017年 | 21篇 |
2016年 | 31篇 |
2015年 | 27篇 |
2014年 | 40篇 |
2013年 | 125篇 |
2012年 | 80篇 |
2011年 | 112篇 |
2010年 | 67篇 |
2009年 | 52篇 |
2008年 | 98篇 |
2007年 | 82篇 |
2006年 | 95篇 |
2005年 | 74篇 |
2004年 | 65篇 |
2003年 | 61篇 |
2002年 | 52篇 |
2001年 | 33篇 |
2000年 | 36篇 |
1999年 | 33篇 |
1998年 | 22篇 |
1997年 | 31篇 |
1996年 | 24篇 |
1995年 | 21篇 |
1994年 | 23篇 |
1993年 | 31篇 |
1992年 | 21篇 |
1991年 | 20篇 |
1990年 | 16篇 |
1989年 | 22篇 |
1988年 | 28篇 |
1987年 | 22篇 |
1986年 | 21篇 |
1985年 | 18篇 |
1984年 | 32篇 |
1983年 | 12篇 |
1982年 | 32篇 |
1981年 | 15篇 |
1980年 | 16篇 |
1979年 | 18篇 |
1978年 | 15篇 |
1977年 | 27篇 |
1976年 | 23篇 |
1975年 | 22篇 |
1974年 | 21篇 |
1973年 | 18篇 |
排序方式: 共有1941条查询结果,搜索用时 31 毫秒
71.
R. Rajagopal S. A. Siddiqui Thomas Daniel R. J. Lahoti K. V. Srinivasan 《Journal of molecular catalysis. A, Chemical》2004,210(1-2):165-169
Regioselective monobromination of aromatic substrates with N-bromosuccinimide has been achieved in excellent isolated yields (84–98%) using phosphotungstic acid supported on zirconia as a novel heterogeneous catalyst. The catalyst has been characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), surface area and acidity measurements. Remarkably, the new catalyst system described brought about the side-chain bromination of aromatics to afford bromomethyl arenes in excellent yields (86–98%) without the need for a radical initiator. Recovery and recylability of the catalyst have been well established. 相似文献
72.
Narayani Ramakrishnan Marian E. Clay Liang-YAN Xue Helen H. Evans Antonio Rodriguez-Antunez Nancy L. Oleinick 《Photochemistry and photobiology》1988,48(3):297-303
Abstract— Chloroaluminum phthalocyanine (CAPC) is an efficient photosensitizer for the inactivation of Chinese hamster V79 cells. In order to investigate possible molecular mechanisms in the photo-dynamic action of CAPC and visible light, the induction and repair rate of two classes of DNA lesions have been determined, i.e. DNA single-strand breaks and DNA-protein cross-links. In cells pretreated with 1 μ.M CAPC, a fluence of 12 kJ/m2 of red light (>600 nm) kills approximately 50% of the cells and induces 3 to 3.5 Gy-equivalents of single-strand breaks. The repair of these breaks was slower than the repair of single-strand breaks induced by -irradiation. The photodynamic action of CAPC also induces a large number of DNA-protein cross-links which, in contrast to -radiation-induced DNA-protein cross-links, do not appear to be repaired during 4 h of post-treatment incubation in fresh medium. These studies suggest that DNA may be an important target for the cytotoxicity of CAPC + red light. 相似文献
73.
74.
Parmeswaran D Pushpan SK Srinivasan A Ravi Kumar M Chandrashekar TK Ganesan S 《Photochemistry and photobiology》2003,78(5):487-495
The core modification of expanded porphyrins has been proved to have better photochemical properties, which are favorable for photodynamic therapy (PDT) applications. In this context, this study was aimed to investigate the in vitro and in vivo photodynamic activity of one such core-modified expanded porphyrin, namely, ammonium salt of 5,10,15,20-tetrakis-(meso-p-sulfonato phenyl)-25,27,29-trithia sapphyrin. For the in vitro studies, human erythrocytes were used as a membrane semimodel system to investigate the partitioning ability and drug-uptake characteristics. The partition studies on the membrane semimodel system revealed that maximum partitioning occurs at 12 microgm/mL concentration, and from the drug-uptake studies it is observed that maximum amount of the sensitizer is bound to the erythrocyte membranes during a 45 min incubation period. Photohemolysis studies at different concentrations of the sensitizer and exposure time showed maximum damage at 5 microgm/mL and 30 min exposure time. In vivo studies were performed on 7,12-dimethylbenz(a)nthracene-induced superficial squamous cell carcinoma on mouse skin. The sensitizer at a concentration of 2.5% in 2.0% dimethyl sulfoxide was applied topically on the tumor spot. After 1 h incubation the tumor spot was exposed to laser irradiation from Nd-YAG laser at its second harmonic wavelength of 532 nm. The photodynamic efficacy was estimated by tumor volume measurements at regular intervals after the treatment. One month after PDT exposure a 3.9-fold decrease in the tumor volume was observed with respect to the tumor volume before treatment. The treatment efficacy was further confirmed by histological and fluorescence spectroscopic evaluations of the tissue biopsy sample from the treated area. The results of our study suggest that the ammonium salt of 5,10,15,20-tetrakis-(meso-p-sulfonato phenyl)-25,27,29-trithia sapphyrin may find possible applications in the new modality of cancer treatment. 相似文献
75.
In this paper we articulate our philosophy and approach to the design and control of high speed data networks. The object is to put into perspective and to explain the coordination of various isolated pieces of detailed technical analyses that have been reported in several recent papers. In the process we summarize what we have learnt in our recent work and, also, we give indications of the direction of our future work. Our scheme integrates feedback and open loop control. The feedback control is exercised by sliding windows; access controllers regulate bursty sources. All our design proposals are rooted in asymptotic analyses; the justification for asymptotics comes from the largeness of the parameters, such as propagation delay, speed, window size, buffer size, and the number of virtual circuits. This analysis makes a strong case for operating in a specific moderate usage regime, and adaptive dynamic windowing algorithms are given that make this happen; moreover, when in this regime, buffers may be sized aggressively small without jeopardizing performance and the simplicity of the retransmission protocol. The topics in the paper are: model of communication, results on the steady-state behavior of the basic model, access control, small buffers and retransmission protocols, dynamic adaptive windows, bursty sources, and contrast with previous work.I. Mitrani's work was done while AT&T Bell Laboratories. 相似文献
76.
77.
The kinetics of the oxidation of methyl phenyl sulfoxide (MPSO) by potassium peroxodiphosphate (PP) in 50% aqueous acetic acid obeys the rate law.-d[PP]T/dt=k[PP] [MPSO] at constant [H+]. The active species in the oxidation has been found to be H3P2O
8
–
. The kinetic results have been explained by a polar mechanism.
() () 50%- (./.) -d[]T/dt=k·[]·[] [H*]. H3P2O 8 – . .相似文献
78.
A hydrothermal reaction of a mixture of ZnCO3, phosphoric acid, 1, 10‐phenanthroline in H2O gave rise to large plates of a new zinc phosphate, [(C12H8N2Zn)2(HPO4)(H2PO4)2], I . The structure consists of ZnO3N2 distorted trigonal‐bipyramidal and PO4 tetrahedral units linked through their vertices to give rise to a zero‐dimensional molecular solid (monomer). The structure of the monomer appears to be similar to the secondary building unit (SBU) 4 = 1, commonly found in many fibrous zeolites. To our knowledge, this is the first time this building unit has been isolated. The structure, with a unique composition, is stabilized by hydrogen bond interactions between the terminal —OH groups forms a one‐dimensional molecular wire and also by strong π…π interactions between the 1, 10‐phenanthroline units. Photoluminescence studies show that there is a ligand‐to‐metal charge transfer (LMCT). Crystal data: orthorhombic, space group = Fdd2 (no. 43), a = 40.4669(1), b = 7.4733(2), c = 17.4425(5)Å, V = 5274.9(2)Å3, Z = 8. 相似文献
79.
Bikshandarkoil R. Srinivasan Christian N?ther Sunder N. Dhuri Wolfgang Bensch 《Monatshefte für Chemie / Chemical Monthly》2006,121(2):397-411
Four new organic ammonium tetrathiotungstates (N–Me–enH2)[WS4] (1), (N,N′-dm-1,3-pnH2)[WS4] (2), (1,4-bnH2)[WS4] (3), and (mipaH)2[WS4] (4), (N–Me–enH2 = N-methylethylenediammonium, N,N′-dm-1,3-pnH2 = N,N′-dimethyl-1,3-propanediammonium, 1,4-bnH2 = 1,4-butanediammonium, and mipaH = monoisopropylammonium) were synthesized by the base promoted cation exchange reaction and characterized by elemental analysis,
infrared, Raman, UV-Vis and 1H NMR spectroscopy as well as single crystal X-ray crystallography. The structures of 1–4 consist of [WS4]2− tetrahedra which are linked to the organic ammonium cations via N–H⋯S hydrogen bonding. The strength and number of the S⋯H
interactions affect the W–S bond lengths as evidenced by distinct short and long W–S bonds. The IR spectra exhibit splitting
of the W–S vibrations, which can be attributed to the distortion of the [WS4]2− tetrahedron. From a comparative study of several known tetrathiotungstates it is observed that a difference of more than
0.033 ? between the longest and shortest W–S bonds in a tetrathiotungstate will result in the splitting of the asymmetric
stretching vibration of the W–S bond. 相似文献
80.
Two polymorphs of zero-dimensional zinc phosphate with the formula, 0∞[Zn(2,2′-bipy)(H2PO4)2], have been synthesized employing hydrothermal technique and their structure determined by single crystal X-ray diffraction. Both the structures consists of ZnO3N2 distorted trigonal-bipyramidal and PO2(OH)2 tetrahedral units linked through their vertices giving rise to a zero-dimensional molecular zinc phosphate. The structures are stabilized by extensive hydrogen bond interactions between zero-dimensional monomers. The structures display subtle differences in their packing created by hydrogen bond interactions. Crystal data: polymorph I, triclinic, space group (No. 2), , , , α=67.32(3)°, β=81.67(3)°, γ=69.29(3)°, , Z=2; polymorph II, triclinic, space group (No. 2), , , , α=97.37(2)°, β=100.54(2)°, γ=100.98(2)°, , Z=2. 相似文献