全文获取类型
收费全文 | 14535篇 |
免费 | 625篇 |
国内免费 | 42篇 |
专业分类
化学 | 9906篇 |
晶体学 | 229篇 |
力学 | 464篇 |
综合类 | 1篇 |
数学 | 1112篇 |
物理学 | 3490篇 |
出版年
2024年 | 48篇 |
2023年 | 164篇 |
2022年 | 374篇 |
2021年 | 427篇 |
2020年 | 466篇 |
2019年 | 522篇 |
2018年 | 487篇 |
2017年 | 462篇 |
2016年 | 667篇 |
2015年 | 474篇 |
2014年 | 759篇 |
2013年 | 1257篇 |
2012年 | 1107篇 |
2011年 | 1147篇 |
2010年 | 760篇 |
2009年 | 587篇 |
2008年 | 721篇 |
2007年 | 721篇 |
2006年 | 560篇 |
2005年 | 490篇 |
2004年 | 377篇 |
2003年 | 311篇 |
2002年 | 251篇 |
2001年 | 155篇 |
2000年 | 130篇 |
1999年 | 99篇 |
1998年 | 71篇 |
1997年 | 99篇 |
1996年 | 94篇 |
1995年 | 73篇 |
1994年 | 68篇 |
1993年 | 93篇 |
1992年 | 97篇 |
1991年 | 77篇 |
1990年 | 68篇 |
1989年 | 71篇 |
1988年 | 49篇 |
1987年 | 45篇 |
1986年 | 42篇 |
1985年 | 63篇 |
1984年 | 62篇 |
1983年 | 53篇 |
1982年 | 47篇 |
1981年 | 37篇 |
1980年 | 45篇 |
1979年 | 62篇 |
1978年 | 48篇 |
1977年 | 53篇 |
1976年 | 41篇 |
1975年 | 34篇 |
排序方式: 共有10000条查询结果,搜索用时 859 毫秒
281.
Garg BS Sharma RK Kumar V Reddy MJ Mittal S 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2002,58(12):2789-2794
Novel complexes of Co(II), Ni(II), Cu(II) and Pd(II) with the new ligand [N,N'-bis(2-carboxy-1-oxo-phenelenyl)ethylenediamine] (H2L) have been synthesized and characterized on the basis of elemental analyses, magnetic susceptibility, thermal, infrared, electronic, 1H NMR and EPR spectral studies. Infrared and 1H NMR spectra show that H2L acts as a binegative tetradentate ligand. Coordination occurs through deprotonated carboxylate oxygens and nondeprotonated amido nitrogens in all the complexes. Electronic spectral studies and magnetic moment values suggest N2O2 coordination around each metal centre with strong field square planar chromophores. The probable structures of the complexes have been assigned on the basis of spectral studies. The complex formation between M(II) [M(II) = Mn(II), Co(II), Ni(II), Cu(II) and Zn(II)] and (L2-) has also been studied potentiometrically in 75% aqueous DMF at 25 degrees C in 0.1 M NaClO4. The stability constants were found to follow the order: Mn(II) < Co(II) < Ni(II) < Cu(II) > Zn(II). 相似文献
282.
283.
284.
Kalyan Kali Sen Gupta Saroj Chandra Kumar Pratik Kumar Sen Amalendu Banerjee 《Reaction Kinetics and Catalysis Letters》1988,36(2):423-428
The kinetics of oxidation of methanol by bromate ion in hydrochloric acid medium has been investigated. A mechanism consistent with the experimental observations is suggested.
. , .相似文献
285.
The adsorptive cathodic stripping voltammetry technique (AdCSV) is used to determine copper(II) using salicylaldehyde thiosemicarbazone (N, S- donor) as a complexing agent on hanging mercury drop electrode at pH 9.3. Variable factors affecting the response, i.e. the concentration of ligand, pH, adsorption potential and adsorption time are assessed and optimized. The adsorbed complex of copper(II) and salicylaldehyde thiosemicarbazone gives a well defined cathodic stripping peak current at −0.35 V, which has been used for the determination of copper in the concentration range of 7.85 × 10−9 to 8.00 × 10−6 M with accumulation time of 360 s at −0.1 V versus Ag/AgCl. This technique has been applied for the determination of copper in various digested samples of whole blood at trace levels. 相似文献
286.
Development of a multicriteria assessment model for ranking biomass feedstock collection and transportation systems 总被引:1,自引:0,他引:1
Amit Kumar Shahab Sokhansanj Peter C. Flynn 《Applied biochemistry and biotechnology》2006,129(1-3):71-87
This study details multicriteria assessment methodology that integrates economic, social, environmental, and technical factors
in order to rank alternatives for biomass collection and transportation systems. Ranking of biomass collection systems is
based on cost of delivered biomass, quality of biomass supplied, emissions during collection, energy input to the chain operations,
and maturity of supply system technologies. The assessment methodology is used to evaluate alternatives for collecting 1.8×106 dry t/yr based on assumptions made on performance of various assemblies of biomass collection systems is based on cost of
delivered biomass, quality of biomass supplied, emissions during collection, energy input to the chain operations, and maturity
of supply system technologies. The assessment methodology is used to evaluate alternatives for collecting 1.8×106 dry t/yr based on assumptions made on performance of various assemblies of biomass collection systems. A proposed collection
option using loafer/stacker was shown to be the best option followed by ensiling and baling. Ranking of biomass transport
systems is based on cost of biomass transport, emissions during transport, traffic congestion, and maturity of different technologies.
At a capacity of 4×106 dry t/yr, rail transport was shown to be the best option, followed by truck transport and pipeline transport, respectively.
These rankings depend highly on assumed maturity of technologies and scale of utilization. These may change if technologies
such as loafing or ensiling (wet storage) methods are proved to be infeasible for large-scale collection systems. 相似文献
287.
Ionic discotic liquid crystals: synthesis and characterization of pyridinium bromides containing a triphenylene core 总被引:1,自引:0,他引:1
Five novel pyridinium salts tethered with hexaalkoxytriphenylene molecules were synthesized by the quaternization of pyridine with ω-bromo-substituted triphenylene derivatives. Their chemical structures were determined by 1H NMR, 13C NMR, IR, UV spectroscopy and elemental analysis. The thermotropic liquid crystalline properties of these salts were investigated by polarizing optical microscopy and differential scanning calorimetry. These triphenylene-based pyridinium salts with bromide as counterion were found to be mesomorphic over a wide temperature range. 相似文献
288.
Efficient and simple colorimetric fluoride ion sensor based on receptors having urea and thiourea binding sites 总被引:6,自引:0,他引:6
[structure: see text] Novel colorimetric receptors for selective fluoride ion sensing containing anthraquinone as chromogenic signaling subunit and urea (N,N' '-(9,10-dihydro-9,10-dioxo-1,2-anthracenediyl)bis[N'-phenyl])/thiourea (N,N' '-(9,10-dihydro-9,10-dihydro-9,10-dioxo-1,2-antrhacenediyl)bis[N-phenyl]) binding sites have been reported. These receptors have shown no affinity for other halide ions (Cl-, Br-, and I- ions). Well-defined color change in the visible region of the spectrum was observed upon addition of fluoride ion in DMSO/CH3CN solution of the receptors 1 and 2. 相似文献
289.
Ashish Surya?Prakash?Singh Ashok?Kumar?Singh Bharat?SinghEmail author 《Transition Metal Chemistry》2005,30(5):610-615
Kinetics and mechanism of the Os(VIII) catalysed oxidation of crotonic acid (CA) by KBrO3 in alkaline medium have been investigated. Zero order dependence in [KBrO3] was observed, while first order with respect to CA in its lower concentration range tends to zero order at its higher concentration range. The order in [Os(VIII)] was found to be unity and a positive effect of [OH−] was observed. Variation of the ionic strength (μ) and dielectric constant of the medium and addition of Hg(OAc)2 (used as Br− scavenger) had an insignificant effect on the rate of reaction. Thermodynamic parameters have also been calculated and reported. A suitable mechanism consistent with the observed kinetic results has been suggested and the related rate law deduced. 相似文献
290.
Nath M Jairath R Eng G Song X Kumar A 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2005,61(13-14):3155-3161
New diorganotin(IV) derivatives of the general formula R2Sn(Umb)2 (where R = n-Bu, n-Oct and Ph; Umb = umbelliferone anion) have been synthesized either by the reaction of R2SnO with umbelliferone under azeotropic removal of water or by the reaction of R2SnCl2 with sodium salt of umbelliferone. Further, the adducts of the general formula R2Sn(Umb)2.phen (where R = n-Bu and n-Oct; phen = 1,10-phenanthroline) have also been synthesized by the interaction of R2Sn(Umb)2 with 1,10-phenanthroline. The bonding and coordination behavior in these derivatives are discussed on the basis of IR and 119Sn M?ssbauer spectroscopic studies in solid state. Their coordination behavior in solution is discussed by the multinuclear (1H, 13C and 119Sn) NMR spectral studies. The M?ssbauer and IR studies indicate that umbelliferone acts as a monoanionic bidentate ligand in R2Sn(Umb)2 coordinating through O(7) and O(1). A distorted octahedral geometry around tin has been proposed for R2Sn(Umb)2 as well as for R2Sn(Umb)2.phen in solid state. The newly synthesized derivatives have been tested for their anti-inflammatory and cardiovascular activities. The average LD50 value >1000 mg kg(-1) of these compounds indicates their safety margin. 相似文献