全文获取类型
收费全文 | 41870篇 |
免费 | 787篇 |
国内免费 | 314篇 |
专业分类
化学 | 20019篇 |
晶体学 | 308篇 |
力学 | 1616篇 |
综合类 | 12篇 |
数学 | 10907篇 |
物理学 | 10109篇 |
出版年
2023年 | 105篇 |
2022年 | 131篇 |
2021年 | 200篇 |
2020年 | 293篇 |
2019年 | 240篇 |
2018年 | 1391篇 |
2017年 | 1586篇 |
2016年 | 1190篇 |
2015年 | 1001篇 |
2014年 | 956篇 |
2013年 | 1288篇 |
2012年 | 3976篇 |
2011年 | 3242篇 |
2010年 | 2399篇 |
2009年 | 2024篇 |
2008年 | 1378篇 |
2007年 | 1375篇 |
2006年 | 1298篇 |
2005年 | 5094篇 |
2004年 | 4481篇 |
2003年 | 2720篇 |
2002年 | 896篇 |
2001年 | 498篇 |
2000年 | 253篇 |
1999年 | 345篇 |
1998年 | 250篇 |
1997年 | 255篇 |
1996年 | 176篇 |
1995年 | 175篇 |
1994年 | 175篇 |
1993年 | 137篇 |
1992年 | 272篇 |
1991年 | 226篇 |
1990年 | 193篇 |
1989年 | 188篇 |
1988年 | 170篇 |
1987年 | 99篇 |
1986年 | 85篇 |
1985年 | 102篇 |
1984年 | 80篇 |
1983年 | 72篇 |
1982年 | 79篇 |
1981年 | 75篇 |
1980年 | 58篇 |
1979年 | 65篇 |
1976年 | 105篇 |
1975年 | 72篇 |
1974年 | 68篇 |
1973年 | 79篇 |
1972年 | 57篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
941.
Miguel Clemente-LeónEugenio Coronado José R. Galán-MascarósCarlos Giménez-Saiz Carlos J. Goémez-Garc?&#x;aJean M. Fabre G.A. MousdisG.C. Papavassiliou 《Journal of solid state chemistry》2002,168(2):616-625
The synthesis, structure and physical characterization of three new radical salts formed by the organic donor bis(ethylenediseleno)tetrathiafulvalene (BEDS-TTF or BEST) and the paramagnetic hexacyanoferrate(III) anion [Fe(CN)6]3− or the photochromic nitroprusside anion [Fe(CN)5NO]2− are reported: (BEST)4[Fe(CN)6] (1), (BEST)3[Fe(CN)6]2·H2O (2) and (BEST)2[Fe(CN)5NO] (3). Salts 1 and 3 show a layered structure with alternating organic (β-type packing) and inorganic slabs. Salt 2 shows an original interpenetrated structure probably due to the unprecedented presence of (BEST)2+ dications. The three salts are semiconductors although salt 1 exhibits a high room temperature conductivity and a semiconducting-semiconducting transition at ca. 150 K which has been attributed to a dimerization in the organic sublattice. 相似文献
942.
Monika Moszner Anna M Trzeciak Józef J Zió?kowski 《Journal of molecular catalysis. A, Chemical》1998,130(3):351-248
Rhodium(II) complexes with dioximes [Rh(Hdmg)2(PPh3)]2 [I] (Hdmg=monoanion of dimethylglyoxime) and [Rh(Hdmg)(ClZndmg)(PPh3)]2 [II] catalyse hydroformylation and hydrogenation reactions of 1-hexene at 1 MPa CO/H2 and 0.5 MPa H2 at 353 K, respectively. Hydroformylation with complex [I] produces 94% of aldehydes (n/iso=2.2) and 6% 2-hexene whereas the second catalyst [II] gives ca. 40% of aldehydes (n/iso=2.1) and 60% of 2-hexene. Corresponding Rh(III) complexes are inactive in hydroformylation except of RhH(Hdmg)2(PPh3) [III], which shows activity similar to [I]. Complexes [Rh(Hdmg)2(PPh3)]2 [I], [Rh(Hdmg)(ClZndmg)(PPh3)]2 [II], RhH(Hdmg)2(PPh3) [III] and [Rh(Hdmg)2(PPh3)2]ClO4 [V] catalyse 1-hexene hydrogenation with an average TON ca. 18 cycles/mol [Rh]×min. Complex [II] has also been found to catalyse hydrogenation of cyclohexene, 1,3-cyclohexadiene and styrene. 相似文献
943.
A comparison of sequential extraction techniques for determining arsenic fractionation in synthetic mineral mixtures 总被引:6,自引:0,他引:6
The sequential extraction methods according to Tessier et al. [1], Borovec et al. [2], Zhang and Moore [3] and Hall et al. [4] have been tested for their suitability for arsenic fractionation in samples of artificially prepared mineral mixtures. Mixtures containing different amounts of As-containing phases were prepared so that their compositions corresponded to weathering products on As-bearing ore deposits. A comparison of different procedures on simple mineral mixtures containing calcium arsenate (CaHAsO4·H2O), As-bearing goethite (FeOOH) and arsenopyrite (FeAsS) showed that only the results of the Hall method satisfactorily correspond to the expected arsenic distribution. A detailed verification of the Hall method was subsequently carried out on most complex synthetic mineral mixtures with varying amounts of As-containing kaolinite and carbonate, calcium arsenate, As-bearing goethite and arsenopyrite. The results confirm that the Hall method cannot be fully employed for an accurate As speciation but may be applied for a route identification of As distribution between "labile", "medium-labile" and "residual" forms in heavily polluted soils. 相似文献
944.
945.
Two new reagents for in situ peptide coupling based on the 2-mercaptopyrimidine core have been developed. The readily prepared thiouronium salts were found to promote both peptide and segment coupling efficiently with low racemization/epimerization levels. 相似文献
946.
M. S. Rahman M. Parre?o R. Bossi A. B. Payá-Pérez B. Larsen 《Fresenius' Journal of Analytical Chemistry》1998,360(5):556-563
Six extraction methods for the analysis of PCBs (CB-28, CB-52, CB-101, CB-118, CB-138, CB-153 and CB-180) in sewage sludge
were tested. A certified reference material (CRM 392) was used for the evaluation of the performance of the methods. Soxhlet-Dean-Starch
with toluene as solvent, Soxhlet with hexane:acetone (2:3), cold digestion/saponification with 2 mol/L KOH in methanol followed
by partition with hexane, and sonicated liquid-solid extraction with hexane:acetone (1:1) produced accurate results (97%,
93%, 104%, and 88%, respectively) with acceptable precisions (6.2%, 6.8%, 15% and 12%, respectively). Results in close agreement
with the certified value for all congeners were obtained by treatment with BF3-methanol prior to partition with dichloromethane. However, this is a tedious procedure and involves the use of hazardous
compounds. Cyclic steam distillation produced results with an accuracy of around 80% and a good precision (5.2%). The very
low consumption of solvents and other expensive chemicals by this technique and the possibility of analyzing the extract directly
without clean-up make it an interesting alternative to the more sophisticated methods. Column elution with dichloromethane
was found to be less efficient (61%), but it is a rapid, direct method with a low consumption of solvents and it may therefore
serve as screening method.
Received: 29 April 1997 / Revised: 30 July 1997 / Accepted: 6 August 1997 相似文献
947.
The use of capillary electrophoresis (CE) with laser-induced fluorescence excited by ultraviolet (UV) lasers in the range 200–300 nm has been restricted by the available wavelengths and expense of UV lasers. The integration of a NeCu deep UV laser operating at 248.6 nm with a single channel CE system with post-column sheath flow detection allows detection limits for serotonin and tryptophan of 3.9×10-8 M and 4.5×10-8 M respectively. Single cell analysis of serotonergic metacerebral cells from the sea slug Aplysia californica yields a value of 800±85 fmol of serotonin in each cell soma. For the first time, serotonin is directly detected in electrically stimulated release from single metacerebral cell soma, with approximately 4% of the serotonin contained in the soma released from a semi-intact preparation with a 2 min electrical stimulation. 相似文献
948.
Jerkiewicz G DeBlois M Radovic-Hrapovic Z Tessier JP Perreault F Lessard J 《Langmuir : the ACS journal of surfaces and colloids》2005,21(8):3511-3520
The Pt(111) electrode is modified by an overlayer of C6H6 (ads) upon its cycling in the 0.05-0.80 V range in aq H2SO4 + 1 mM C6H6. The C6H6 (ads) overlayer significantly changes the underpotential-deposited H (H(UPD)) and anion adsorption, and cyclic-voltammetry (CV) profiles show a sharp cathodic peak and an asymmetric anodic one in the 0.05-0.80 V potential range. The C6H6 (ads) layer blocks the (bi)sulfate adsorption but facilitates the adsorption of one monolayer of H(UPD). Cycling of the benzene-modified Pt(111) in benzene-free aq 0.05 H2SO4 from 0.05 to 0.80 V results in a partial desorption of C6H6 (ads) and in a partial recovery of the CV profile characteristic of an unmodified Pt(111). The peak potential of the cathodic and anodic feature is independent of the scan rate, s (10 < or = s < or = 100 mV s(-1)), and the peak current density increases linearly with an increase of the scan rate. Temperature variation modifies the peak potential and current density but does not affect the charge density of the cathodic or anodic feature. Temperature-dependent studies allow us to determine the thermodynamic state function for the H(UPD) adsorption and desorption. Delta G degrees(ads)(H(UPD))assumes values from -4 to -12 kJ mol(-1), while has values from 9 to 14 kJ mol(-1). The values of delta Delta G degrees (delta Delta G degrees = delat Delta G degrees(ads) + delta Delta D degrees(des)) decrease almost linearly from 6 kJ mol(-1) at theta(H(UPD) --> 0 to 0 kJ mol(-1) at theta(H(UPD) --> 1. The nonzero values of delta Delta G degrees testify that the adsorbing and desorbing H(UPD) adatoms interact with an energetically different substrate. The lateral interactions changed from repulsive (omega = 29 kJ mol(-1) at theta(H(UPD) --> 0) to attractive (omega = -28 kJ mol(-1) at theta(H(UPD) --> 1) as the H(UPD) coverage increases. The values of delta S degrees(ads)(H(UPD)) increase from 19 to 56 J K(-1) mol(-1), while those of delta S degrees(des)(H(UPD)) decrease from 45 to -30 J K(-1) mol(-1) with an increase of H(UPD) coverage. The values of delta H degrees(des)(H(UPD)) and delta H degrees(des)(H(UPD)) vary from 0 to 27 kJ mol(-1). The Pt(111)-H(UPD) surface bond energy at the benzene-modified Pt(111) electrode falls in the 191-218 kJ mol(-1) range and is weaker than in the case of the unmodified Pt(111) electrode in the same electrolyte. 相似文献
949.
The peculiarities of the equilibrium swelling ratio and swelling-shrinking kinetics of polyelectrolyte copolymeric hydrogels consisting of acrylamide and itaconic acid (AAm/IA) have been studied in water/nonsolvent (acetone, methanol, ethanol and 1-butanol) mixtures as a function of solvent composition and IA content in the hydrogel. The phase transition of these hydrogels was generated by changing the solvent composition by progressive substitution of water by the nonsolvent. For all P(AAm/IA) hydrogels, the form of the shrinking curves was determined to be strongly dependent on the type of the nonsolvent used. The rate of shrinking of these hydrogels increased in the order 1-butanol < ethanol < methanol < acetone. 相似文献
950.
An electrochemically stable monolayer of tris(2,2'-bipyridyl)ruthenium(II) was obtained for the first time. It was based on the electrostatic attachment of Ru(bpy)(3)(2+) to the benzene sulfonic acid monolayer film, which was covalently bound onto glassy carbon electrode by the electrochemical reduction of diazobenzene sulfonic acid. The surface-confined Ru(bpy)(3)(2+) underwent reversible surface process, and reacted with the coreactant, tripropylamine, to produce electrochemiluminescence. In view of the stability of the electrode, the results strongly suggested that light was emitted from the surface-confined Ru(bpy)(3)(2+), not from the detached Ru(bpy)(3)(2+). The Ru(bpy)(3)(2+) modified electrode was used to the determination of tripropylamine. It showed good linearity in the concentration range from 5 muM to 1 mM with a detection limit of 1 muM (S/N=4). The good stability of the Ru(bpy)(3)(2+) modified electrode also showed that the benzene sulfonic acid monolayer film prepared can be served as an excellent support to construct multilayers. 相似文献