全文获取类型
收费全文 | 10080篇 |
免费 | 288篇 |
国内免费 | 85篇 |
专业分类
化学 | 7179篇 |
晶体学 | 46篇 |
力学 | 238篇 |
数学 | 1440篇 |
物理学 | 1550篇 |
出版年
2022年 | 71篇 |
2021年 | 116篇 |
2020年 | 152篇 |
2019年 | 148篇 |
2018年 | 111篇 |
2017年 | 83篇 |
2016年 | 235篇 |
2015年 | 198篇 |
2014年 | 233篇 |
2013年 | 556篇 |
2012年 | 567篇 |
2011年 | 727篇 |
2010年 | 339篇 |
2009年 | 254篇 |
2008年 | 542篇 |
2007年 | 640篇 |
2006年 | 653篇 |
2005年 | 538篇 |
2004年 | 484篇 |
2003年 | 430篇 |
2002年 | 387篇 |
2001年 | 154篇 |
2000年 | 119篇 |
1999年 | 98篇 |
1998年 | 85篇 |
1997年 | 123篇 |
1996年 | 152篇 |
1995年 | 118篇 |
1994年 | 95篇 |
1993年 | 123篇 |
1992年 | 103篇 |
1991年 | 70篇 |
1990年 | 91篇 |
1989年 | 54篇 |
1988年 | 67篇 |
1987年 | 71篇 |
1986年 | 87篇 |
1985年 | 120篇 |
1984年 | 115篇 |
1983年 | 77篇 |
1982年 | 116篇 |
1981年 | 110篇 |
1980年 | 99篇 |
1979年 | 78篇 |
1978年 | 94篇 |
1977年 | 70篇 |
1976年 | 62篇 |
1975年 | 65篇 |
1974年 | 41篇 |
1973年 | 66篇 |
排序方式: 共有10000条查询结果,搜索用时 10 毫秒
41.
A constraintg(x)0 is said to be a reverse convex constraint if the functiong is continuous and strictly quasi-convex. The feasible regions for linear programs with an additional reverse convex constraint are generally non-convex and disconnected. It is shown that the convex hull of the feasible region is a convex polytope and, as a result, there is an optimal solution on an edge of the polytope defined by only the linear constraints. The only possible edges which can contain such an optimal solution are characterized in relation to the best feasible vertex of the polytope defined by only the linear constraints. This characterization then provides a finite algorithm for finding a globally optimal solution.Research supported by NSF Grant ENG76-12250 and ONR Contract N00014-78-C-0428. 相似文献
42.
Norbert V. Fehringer Dalia M. Gilvydis Stephen M. Walters Colin F. Poole 《Journal of separation science》1992,15(2):124-127
The combination of open tubular column gas chromatography with electrolytic conductivity detection has been evaluated for the determination of nitrogen-containing pesticide residues in food extracts. Optimization of the column position at the column-detector interface was crucial to the successful operation of the detector. The signal-to-noise ratio and response stability of the detector are greatly influenced by the composition of the electrolyte solvent. Large volume splitless injections using retention gaps and optimized detector operating conditions enabled pesticide residues in food extracts to be determined at sub parts-per-million levels. Although the electrolytic conductivity detector is less sensitive than the thermionic ionization detector, its greater nitrogen selectivity can he crucial to the determination of nitrogen-containing contaminants in food extracts, particularly in complex mixtures where phosphorus-containing contaminants or matrix compounds are also present. 相似文献
43.
[reaction: see text] As a hybrid of the N,N-dibenzylammonium and 1,2-bis(pyridinium)ethane axles, various N-benzylanilinium cations were investigated as suitable axles for the formation of [2]pseudorotaxanes with the 24-membered crown ethers 24C8 and DB24C8. The effect of electron-donating OCH(3) and electron-withdrawing CF(3) groups on both the anilinium and benzyl aromatic rings was studied. Formation constants and structural details were compared to the [2]pseudorotaxanes formed by the two aforementioned dibenzylammonium and 1,2-bis(pyridinium)ethane axles. 相似文献
44.
Paul RL Argent SP Jeffery JC Harding LP Lynam JM Ward MD 《Dalton transactions (Cambridge, England : 2003)》2004,(21):3453-3458
Reaction of the bis-bidentate bridging ligand L(3), in which two bidentate chelating 3(2-pyridyl)pyrazole units are separated by a 3,3'-biphenyl spacer, with Co(II) salts affords tetranuclear cage complexes of composition [Co(4)(L(3))(6)]X(8)(X =[BF(4)](-), [ClO(4)](-), [PF(6)](-) or I(-)) in which four 6-coordinate Co(II) ions in an approximately tetrahedral array are connected by six bis-bidentate bridging ligands, one spanning each of the six edges of the Co(4) tetrahedron. In every case, X-ray crystallography reveals that the 'apical' Co(II) ion has a fac tris-chelate geometry, whereas the other three Co(II) ions have mer tris-chelate geometries, resulting in (non-crystallographic)C(3) symmetry for the cages; that this structure is retained in solution is confirmed by (1)H NMR spectroscopy of the paramagnetic cages. In every case one of the anions is located inside the central cavity of the cage, with the remaining seven outside. We found no clear evidence for an anion-based templating effect. The cage superstructure is sufficiently large to leave gaps in the centres of the faces through which the internal and external anions can exchange. Variable-temperature (19)F NMR spectroscopy was used to investigate the dynamic behaviour of the cages with X =[BF(4)](-) and [PF(6)](-) in MeCN solution: in both cases two separate signals, corresponding to external and internal anions, are clear at 233 K which have coalesced to a single signal at room temperature. Analysis of the linewidth of the minor signal (for the internal anion) at various temperatures below coalescence gave an activation energy for anion exchange of ca. 50 kJ mol(-1) in each case, a figure which suggests that anion exchange can occur via a conformational rearrangement of the cage superstructure in solution rather than opening of the cavity by cleavage of metal-ligand bonds. 相似文献
45.
Stacking due to ionic transport number mismatch during sample sweeping on microchips 总被引:1,自引:0,他引:1
Sample stacking can occur in isoconductive buffer systems as a result of ion transport mismatches that cause changes in buffer conductivity during electrophoresis. Fluorescence imaging was used to examine this effect in the sweeping of hydrophobic dyes with sodium dodecyl sulfate (SDS) on microchips. Imaging revealed the occurrence of a stacking effect in a sodium borate buffer system in which the sample buffer and SDS-containing run buffer had the same initial conductivity. Injected sample plugs were first swept by SDS micelles and the swept band was then stacked at the trailing end of the sample zone. This effect is due to changes in conductivity at both the front and back interfaces of the injected sample plug and can be modeled by moving boundary equations. Maximum signal enhancements of 86-, 160- and 560-fold were obtained for Rhodamine 560, Rhodamine B and Rhodamine 6G, respectively, by the combination of sweeping and stacking within a 1 cm section of microchannel. Based on sample sweeping/stacking and manipulation of the electric field polarity, a method of trapping and concentrating analyte from multiple injections was also demonstrated. 相似文献
46.
Stephen E. Ullrich 《Photochemistry and photobiology》1996,64(2):254-258
In addition to being the primary cause of skin cancer, UV radiation is immune suppressive and there appears to be a link between the ability of UV to suppress the immune response and induce skin cancer. Cytokines made by UV-irradlated keratinocytes play an essential role in activating immune suppression. In particular, we have found that keratinocyte-derlved interleukin (IL)-10 is responsible for the systemic impairment of antigenpresenting cell function and the UV-induced suppression of delayed-type hypersensitivity (DTH). Antigen presentation by splenic adherent cells isolated from UV-irradiated mice to T helper-1 type T (Th1) cells is suppressed, whereas antigen presentation to T helper-2 type T (Th2) cells is enhanced. The enhanced antigen presentation to Th2 cells and the impaired presentation to Th1 cells can be reversed in vivo by injecting the UV-irradiated mice with monoclonal anti-IL-10 antibody. Furthermore, immune suppression can be transferred from UV-irradiated mice to normal recipients by adoptive transfer of T cells. Injecting the recipient mice with anti-IL-4 or anti-IL-10 prevents the transfer of immune suppression, suggesting the suppressor cells are Th2 cells. In addition, injecting UV-irradiated mice with IL-12, a cytokine that has been shown to be the primary inducer of Th1 cells, and one that prevents the differentiation of Th2 cells in vivo, reverses UV-induced immune suppression. These findings support the hypothesis that UV exposure activates IL-10 secretion, which depresses the function of Th1 cells, while enhancing the activity of Th2 cells. 相似文献
47.
The atomic correlation terms necessary to lead to anaccurate 4s23d8-4st 3d9 separation for the Ni atom have been incorporated into all-electron MC SCF/Cl calculations for the X2 Δ state of NiH. The calculated potential curve properties are significantly improved compared to calculations which dissociate to Hartree-Fock atoms. 相似文献
48.
49.
A new spectrophotometric determination of the sulphate ion is described which uses 2-aminoperimidine hydrochloride as a precipitating reagent. The excess of the reagent is measured at 305 nm. The procedure deals with sulphate in the range 4–120 p.p.m., and relies on simple calibration. The relative standard deviation for 50 p.p.m. of sulphate is 3.7%. The interference of 100 p.p.m. of phosphate, fluoride and nitrate is only slight in the determination of 50 p.p.m. of sulphate. The method is recommended for the determination of sulphate in rain and surface waters. 相似文献
50.
Huang F Haydock SF Mironenko T Spiteller D Li Y Spencer JB 《Organic & biomolecular chemistry》2005,3(8):1410-1418
The biosynthetic gene cluster of the 2-deoxystreptamine (DOS)-containing aminoglycoside antibiotic neomycin has been cloned for the first time by screening of a cosmid library of Streptomyces fradiae NCIMB 8233. Sequence analysis has identified 21 putative open reading frames (ORFs) in the neomycin gene cluster (neo) with significant protein sequence similarity to gene products involved in the biosynthesis of other DOS-containing aminoglycosides, namely butirosin (btr), gentamycin (gnt), tobramycin (tbm) and kanamycin (kan). Located at the 5'-end of the neo gene cluster is the previously-characterised neomycin phosphotransferase gene (apH). Three genes unique to the neo and btr clusters have been revealed by comparison of the neo cluster to btr, gnt, tbm and kan clusters. This suggests that these three genes may be involved in the transfer of a ribose moiety to the DOS ring during the antibiotic biosynthesis. The product of the neo-6 gene is characterised here as the L-glutamine : 2-deoxy-scyllo-inosose aminotransferase responsible for the first transamination in DOS biosynthesis, which supports the assignment of the gene cluster. 相似文献