全文获取类型
收费全文 | 40883篇 |
免费 | 1429篇 |
国内免费 | 134篇 |
专业分类
化学 | 22032篇 |
晶体学 | 306篇 |
力学 | 1061篇 |
综合类 | 5篇 |
数学 | 6680篇 |
物理学 | 12362篇 |
出版年
2022年 | 283篇 |
2021年 | 433篇 |
2020年 | 531篇 |
2019年 | 403篇 |
2018年 | 554篇 |
2017年 | 542篇 |
2016年 | 1145篇 |
2015年 | 857篇 |
2014年 | 979篇 |
2013年 | 2015篇 |
2012年 | 1880篇 |
2011年 | 2045篇 |
2010年 | 1371篇 |
2009年 | 1127篇 |
2008年 | 1892篇 |
2007年 | 1771篇 |
2006年 | 1676篇 |
2005年 | 1502篇 |
2004年 | 1294篇 |
2003年 | 1061篇 |
2002年 | 929篇 |
2001年 | 854篇 |
2000年 | 695篇 |
1999年 | 494篇 |
1998年 | 430篇 |
1997年 | 420篇 |
1996年 | 527篇 |
1995年 | 466篇 |
1994年 | 500篇 |
1993年 | 511篇 |
1992年 | 563篇 |
1991年 | 387篇 |
1990年 | 401篇 |
1989年 | 357篇 |
1988年 | 335篇 |
1987年 | 350篇 |
1986年 | 331篇 |
1985年 | 512篇 |
1984年 | 486篇 |
1983年 | 409篇 |
1982年 | 434篇 |
1981年 | 433篇 |
1980年 | 430篇 |
1979年 | 414篇 |
1978年 | 403篇 |
1977年 | 403篇 |
1976年 | 376篇 |
1975年 | 361篇 |
1974年 | 351篇 |
1973年 | 294篇 |
排序方式: 共有10000条查询结果,搜索用时 500 毫秒
991.
Moore EG Bernhardt PV Fürstenberg A Riley MJ Vauthey E 《The journal of physical chemistry. A》2005,109(51):11715-11723
The synthesis, structural characterization, and photophysical behavior of a 14-membered tetraazamacrocycle with pendant 4-dimethylaminobenzyl (DMAB) and 9-anthracenylmethyl groups is reported (L3, 6-((9-anthracenylmethyl)amino)-trans-6,13-dimethyl-13-((4-dimethylaminobenzyl)amino)-1,4,8,11-tetraazacyclotetradecane). In its free base form, this compound displays rapid intramolecular photoinduced electron transfer (PET) quenching of the anthracene emission, with both the secondary amines and the DMAB group capable of acting as electron donors. When complexed with Zn(II), the characteristic fluorescence of the anthracene chromophore is restored as the former of these pathways is deactivated by coordination. Importantly, it is shown that the DMAB group, which remains uncoordinated and PET active, acts only very weakly to quench emission, by comparison to the behavior of a model Zn complex lacking the pendant DMAB group, [ZnL2]2+ (Chart 1). By contrast, Stern-Volmer analysis of intermolecular quenching of [ZnL2]2+ by N,N-dimethylaniline (DMA) has shown that this reaction is diffusion limited. Hence, the pivotal role of the bridge in influencing intramolecular PET is highlighted. 相似文献
992.
Oxidation of alkynyl ethers and -amines with iodosylbenzene in presence of Ru-catalysts affords α-keto esters and α-keto amides in 44–85% yield. These conversions can also be effected with RuO4. 相似文献
993.
The applicability of the carbon paste electrode to the determination of trace quantities of mercury has been investigated in a base electrolyte of 0.1 N KSCN + 0.025 N HCl containing 25 ng/ml copper. The detection limit of the mercury determination has a value near 2.5 ng/ml (1.25×10?8 M), if pre-electrolysis is carried out 10 min at ?1.0 V and the current voltage curve is registered with a scan rate of 16.7 mV/sec between ?0.5 and +0.5 V. The peak height is directly proportional to the concentration in the range from 1.25×10?8 to 1.25×10?6 M. With appropriate preconditioning the carbon paste electrode can be used for several measurements without renewing the surface. 相似文献
994.
Time-resolved laser-induced fluorescence (TRLIF) has been used to study the interaction of uranyl ion with sodium dodecyl sulfate (SDS) micelles in H(3)PO(4) 1 M. The titration curve consists of two curved regions with different slopes, one of them more pronounced at low concentration of SDS and the other, with a less pronounced positive slope at larger [SDS] until a plateau is reached. The fluorescence quenching of uranyl ion by para-substituted phenol compounds was studied by TRLIF and steady-state emission intensity measurements. The results were interpreted in terms of binding of phenolic compounds to the micelle. The binding constant (K(Q)) as well as the entrance and exit rate constants were determined for all the quenchers used. 相似文献
995.
Sautter A Schmid DG Jung G Würthner F 《Journal of the American Chemical Society》2001,123(23):5424-5430
Tetraaryloxy-substituted diazadibenzoperylene bridging ligands 1a,b were employed in transition metal-directed self-assembly with Pd(II) and Pt(II) phosphane triflates 2a,b which resulted in complex dynamic equilibria between molecular triangles 3a-d and molecular squares 4a-d in solution. Characterization of the equilibria and assignment of the metallacycles was accomplished by (1)H and (31)P[(1)H] NMR spectroscopy in combination with electrospray ionization Fourier transform ion cyclotron mass spectrometry (ESI-FTICR-MS). It was found that the equilibria depend on several factors, such as the metal ion (Pd(2+) or Pt(2+)), the solvent, and the steric demand of the phenoxy substituents of the diazadibenzoperylene ligands 1a,b. Introduction of bulky tert-butyl groups in 1b shifts the equilibrium significantly in the direction of the molecular squares. Molecular dynamics simulations of the triangle and square structures revealed critical steric effects and restricted conformational flexibilities of the phosphane and diazadibenzoperylene ligands that help explain the distinct dynamic behavior observed in variable-temperature NMR studies. Concentration-dependent UV/vis and fluorescence spectroscopy revealed the limited stability of the assemblies and confirmed the reversible nature of the dynamic equilibria. 相似文献
996.
From a Cs doped Fe2O3-V2O5 (Fe:V=1.4) catalyst exhibiting an inhomogeneous composition after calcination, three different fractions have been separated by visual inspection. One fraction has been found to consist of mainly α-Fe2O3, another one of mainly FeVO4. The third fraction (S1 in the text) is the catalytically most active fraction containing besides both of the former components an amorphous phase of FexVyOz and also the dopant. By combining the results from transmission and conversion electron Mössbauer spectra it has been concluded that the amorphous component is enriched in the surface region of the crystallites of the catalyst. In addition, the formation of a thin surface layer of iron sulfide (Fe1?xS) on the third fraction has been identified. This sulfide is assumed to be formed during calcination from Cs2SO4 used for the preparation of the catalyst. 相似文献
997.
Frank C Sottmann T Stubenrauch C Allgaier J Strey R 《Langmuir : the ACS journal of surfaces and colloids》2005,21(20):9058-9067
In ternary water-oil-nonionic alkyl polyglycol ether (C(i)E(j)) microemulsions, an increase in efficiency is always accompanied by the formation of a lamellar (L(alpha)) phase. The addition of an amphiphilic block copolymer to the ternary base system increases the efficiency of the microemulsion drastically while suppressing--at least partly--the formation of the L(alpha) phase. However, amphiphilic block copolymers can be used not only to suppress the formation of lyotropic liquid crystals but also for the opposite effect, namely, to induce their formation. To understand to what extent the increase in efficiency is accompanied by the formation of lyotropic liquid crystals, we studied phase diagrams of water-n-alkane-n-alkyl polyglycol ethers (C(i)E(j))-PEPX-PEOY at a constant volume fraction of oil in the water/oil mixture. Using polymers of the poly(ethylene propylene)-copoly(ethylene oxide) type, with M(PEP) = X kg mol(-1) and M(PEO) = Y kg mol(-1), we determined phase diagrams as a function of the polymer concentration, size, and symmetry. Moreover, the influence of a particular polymer mixture was studied, which turned out to be the best system if both a high efficiency and a low tendency to form an L(alpha) phase are needed. 相似文献
998.
Wolfgang Schwack Bernd Brüger Steven Nyanzi 《Analytical and bioanalytical chemistry》1995,351(2-3):297-300
The differential pulse-polarographic (DPP) determination of both CS2 and COS gases, after their absorption in a methanolic piperidine reagent and the subsequent application of this technique to the residue analysis of dithiocarbamate fungicides is described. Rectilinear calibration curves for both CS2 and COS in the respective regions of 1.5–9.2 and 2.1–12.6 μmol/l were obtained. The DPP method has been successfully applied to the determination of thiram residues on apples after a hot acid hydrolysis of the fruits. 相似文献
999.
The nucleation frequency of isotactic polypropylene shows for certain molecular weight distributions during isothermal crystallization
a “stepwise” development of the nuclei as a function of time. The resulting curve can be fitted by a superposition of exponential
functions assuming a separate nucleation density for each nucleation step. The multimodal nucleation is discussed as a consequence
of molecular fractionation effects during crystallization.
Received: 23 October 1997 Accepted: 12 May 1998 相似文献
1000.
The modification of the triple helical structure in aqueous gelatin solutions by changing pH and adding alkyl sulphates at 298 K and after rechilling the solution to 283 K was investigated by CD-measurement. At 298 K the triple helical content at the IEP of the gelatin has its maximum value. It is only weakly affected by adding sodium dodecyl sulphate (SDDS) at concentrations <10–4 M/dm3. The unfolding of the triple helix affected by pH and SDDS is reversible by rechilling the solution. The triple helical content of gelatin solutions decreases at SDDS concentrations higher than 10–4 M/dm3. In all cases the decrease of the amount of triple helical structure is connected with an increase of the cis-configuration in single chains and leads to chain reversals. At sufficiently high SDDS concentrations-sheets are formed. These changes are thermally irreversible. Sodium decyl sulphate (SDS) has a more minor influence than SDDS except in the range of the c.m.c. of SDS. At sufficiently high SDS concentrations,-turns appear. 相似文献