全文获取类型
收费全文 | 12112篇 |
免费 | 632篇 |
国内免费 | 293篇 |
专业分类
化学 | 8746篇 |
晶体学 | 128篇 |
力学 | 570篇 |
综合类 | 16篇 |
数学 | 1413篇 |
物理学 | 2164篇 |
出版年
2024年 | 35篇 |
2023年 | 127篇 |
2022年 | 579篇 |
2021年 | 542篇 |
2020年 | 405篇 |
2019年 | 509篇 |
2018年 | 494篇 |
2017年 | 432篇 |
2016年 | 699篇 |
2015年 | 492篇 |
2014年 | 666篇 |
2013年 | 1189篇 |
2012年 | 963篇 |
2011年 | 920篇 |
2010年 | 645篇 |
2009年 | 524篇 |
2008年 | 593篇 |
2007年 | 534篇 |
2006年 | 391篇 |
2005年 | 414篇 |
2004年 | 265篇 |
2003年 | 247篇 |
2002年 | 191篇 |
2001年 | 102篇 |
2000年 | 74篇 |
1999年 | 76篇 |
1998年 | 48篇 |
1997年 | 59篇 |
1996年 | 58篇 |
1995年 | 41篇 |
1994年 | 48篇 |
1993年 | 27篇 |
1992年 | 44篇 |
1991年 | 41篇 |
1990年 | 34篇 |
1989年 | 43篇 |
1988年 | 39篇 |
1987年 | 39篇 |
1986年 | 30篇 |
1985年 | 40篇 |
1984年 | 42篇 |
1983年 | 27篇 |
1982年 | 31篇 |
1981年 | 30篇 |
1980年 | 27篇 |
1979年 | 23篇 |
1978年 | 20篇 |
1976年 | 16篇 |
1974年 | 16篇 |
1972年 | 13篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
141.
Acta Mathematica Sinica, English Series - In this research article, we shall give some reverse Arithmetic-Geometric mean inequalities for unital positive linear maps on Hilbert space operators... 相似文献
142.
Summary
Bis(cyclopentadienyl)titanium(IV) diisothiocyanate [(Cp)2-Ti(NCS)2] reacts with MCl2 (M = Cu, Pd or Pt), [CuCl(PPh3)3], [RuCl2(PPh3)3] and [RuCl2(DMSO)4] (DMSO = dimethylsulphoxide) giving solid compounds of stochiometry [(Cp)2Ti(-NCS)2MCl2] (M = Cu, Pd or Pt), [(Cp)2Ti(-NCS)2CuCl(PPh3)], [(Cp)2Ti(-NCS)2-RuCl(PPh3)2]Cl and [(Cp)2Ti(-NCS)2RuCl2(DMSO)2]. These products have been characterized by physicochemical and spectroscopic methods. 相似文献
143.
144.
In this paper estimation of the probabilities of a multinomial distribution has been studied. The five estimators considered are: unrestricted estimator (UE), restricted estimator (RE) (under model ), preliminary test estimator (PTE) based on a test of the model , shrinkage estimator (SE) and the positive-rule shrinkage estimator (PRSE). Asymptotic distributions of these estimators are given under Pitman alternatives and the asymptotic risk under a quadratic loss has been evaluated. The relative performance of the five estimators is then studied with respect to their asymptotic distributional risks (ADR). It is seen that neither of the preliminary test and shrinkage estimators dominates the other, though each fares well relative to the other estimators. However, the positive rule estimator is recommended for use for dimension 3 or more while the PTE is recommended for dimension less than 3. 相似文献
145.
We use the stochastic rotation dynamics algorithm to investigate the packaging of flexible and semiflexible polymers into a capsid that is permeable to solvent molecules. The model takes into account hydrodynamic interactions arising due to local flow. The flexible chain maintains a random configuration as it is being fed into the capsid, in contrast to the semiflexible chain, whose configuration is initially spool-like, becoming more random at high packing. We measure the packing rate, which is found to decrease with the percentage of the chain packed and highlight the difference between the flexible and semiflexible chains. Reflecting experiments, we find pauses in the packing process for individual chains as the motor loses grip of the fluctuating beads. We also find that hydrodynamics is important, in that the packaging rate is faster when flow is included. 相似文献
146.
The O-phthalimidomethyl trichloroacetimidate (1), as a latent aminomethylating agent, exhibits high electrophilicity towards a variety of C-nucleophiles in the presence of catalytic amounts of TMSOTf and mild reaction conditions. The nucleophiles include aromatics, alkenes and active methylene compounds 2-11 whereby a phthalimidomethyl group could be introduced to give compounds 12-22. Removal of the phthaloyl group gave the respective amines, β-amino ketones, and β-amino acids. The O-(trichloroacetamido)methyl trichloroacetimidate (35) was also found to be a good amidomethylating agent. 相似文献
147.
148.
Electroactive conducting copolymers of aniline (ANI) and o-aminophenol (OAP) and two-layered poly(o-aminophenol) (POAP)/polyaniline (PANI) composites were prepared in aqueous acidic solution by electrode potential cycling.
Copolymerization was carried out at different feed concentrations of OAP and ANI on a gold electrode. A strong inhibition
of electropolymerization was found at a high molar fraction of OAP in the feed. The copolymers showed good adherence on the
electrode surface and gave a redox response up to pH=10.0. Two transitions were observed in the in situ conductivities of
the copolymers (as with PANI), but the conductivities were lower by 2.5–3 orders of magnitude as compared to PANI. Electrosynthesis
of PANI on POAP modified electrodes showed copolymer formation after reaction initiation and finally formation of a PANI layer
at the copolymer/solution interface. The ‘memory effect’ of the bilayer structures of both polymers was discussed in terms
of protonation/deprotonation and anion consumption taking place during redox processes of both polymers. 相似文献
149.
X-Ray Structural Analyses of Cyclododecasulfur (S12) and Cyclododecasulfur-1-Carbon-disulfide (S12 · CS2) S12 · CS2 crystallizes in space group R&3macr;m–D with hexagonal lattice constants a = 1066.8(3), c = 1155.1(4) pm, Z = 3, dcalc. = 2.04 g · cm?3. The S12 molecules occupy sites of D3d symmetry with bond distance (dss) of 205.4(1) pm, bond angles (α) of 105.80(5) and 106.65(6)º and torsional angle (τ) of 87.20(7)º. The CS2 molecule interacts only very weakly with the S12 units. S12 crystallizes in space group Pnnm–D with lattice constants a = 472.5(2), b = 910.4(3), c = 1453.2(3) pm, Z = 2, dcalc = 2.045 g · cm?3. The molecules with mean parameters d = 205.2 pm, α 106.6º, τ 88.0º occupy sites of C2h symmetry. 相似文献
150.
In this work, two toxic compound, sulfide and thiocyanate were determined simultaneously using kinetic spectrophotometry. These anions have shown the catalytic effects on the reaction between iodine and azide. Since the system was nonlinear, a nonlinear model, principal component-wavelet neural network (PC-WNN) was used as the multivariate calibration method. The principal component analysis was used to decrease the dimension of the original matrix. In other words, the scores of the PCs, 5, instead of the original variables, 301, were used as the input for the model. Two methods were used to select the most relevant principal components: eigenvalue ranking and correlation ranking. In this work, eigenvalue and correlation ranking methods have shown better results for thiocyanate and sulfide, respectively, and it can be concluded that these methods are complementary. The WNN has several advantages relative to other types of neural network such as better convergence ability. The data set was divided to calibration, prediction and validation sets. Each set was selected so that the concentrations of the analytes were approximately covered the entire ranges of the analytes. Mean relative error for thiocyanate and sulfide in validation set were 8.5 and 10.6, respectively. Thiocyanate and sulfide can be determined in the range of 60–700 ng ml−1 and 20–400 ng ml−1, respectively. The proposed method was applied for the determination of sulfide and thiocyanate in real samples such as tap, waste and river waters with satisfactory results. 相似文献