全文获取类型
收费全文 | 8139篇 |
免费 | 1451篇 |
国内免费 | 2182篇 |
专业分类
化学 | 6337篇 |
晶体学 | 179篇 |
力学 | 661篇 |
综合类 | 225篇 |
数学 | 921篇 |
物理学 | 3449篇 |
出版年
2024年 | 21篇 |
2023年 | 118篇 |
2022年 | 269篇 |
2021年 | 302篇 |
2020年 | 316篇 |
2019年 | 309篇 |
2018年 | 275篇 |
2017年 | 323篇 |
2016年 | 316篇 |
2015年 | 392篇 |
2014年 | 442篇 |
2013年 | 650篇 |
2012年 | 645篇 |
2011年 | 700篇 |
2010年 | 576篇 |
2009年 | 556篇 |
2008年 | 645篇 |
2007年 | 531篇 |
2006年 | 541篇 |
2005年 | 474篇 |
2004年 | 418篇 |
2003年 | 310篇 |
2002年 | 318篇 |
2001年 | 317篇 |
2000年 | 321篇 |
1999年 | 266篇 |
1998年 | 150篇 |
1997年 | 135篇 |
1996年 | 140篇 |
1995年 | 117篇 |
1994年 | 109篇 |
1993年 | 108篇 |
1992年 | 85篇 |
1991年 | 80篇 |
1990年 | 74篇 |
1989年 | 74篇 |
1988年 | 61篇 |
1987年 | 47篇 |
1986年 | 28篇 |
1985年 | 34篇 |
1984年 | 26篇 |
1983年 | 21篇 |
1982年 | 25篇 |
1981年 | 20篇 |
1980年 | 15篇 |
1979年 | 9篇 |
1976年 | 8篇 |
1975年 | 6篇 |
1973年 | 7篇 |
1972年 | 5篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
891.
The decomposition of 1,1‐dimethyl‐1‐silacyclobutane (DMSCB) on a heated tungsten filament has been studied using vacuum ultraviolet laser single photon ionization time‐of‐flight mass spectrometry. It is found that the decomposition of DMSCB on the W filament to form ethene and 1,1‐dimethylsilene is a catalytic process. In addition, two other decomposition channels exist to produce methyl radicals via the Si? CH3 bond cleavage and to form propene (or cyclopropane)/dimethylsilylene. It has been demonstrated that both the formation of ethene and that of propene are stepwise processes initiated by the cleavage of a ring C? C bond and a ring Si? C bond, respectively, to form diradical intermediates, followed by the breaking of the remaining central bonds in the diradicals. The formation of ethene via an initial cleavage of a ring C? C bond is dominant over that of propene via an initial cleavage of a ring Si? C bond. When the collision‐free condition is voided, secondary reactions in the gas‐phase produce various methyl‐substituted 1,3‐disilacyclobutane molecules. The dominant of all is found to be 1,1,3,3‐tetramethyl‐1,3‐disilacyclobutane originated from the dimerization of 1,1‐dimethylsilene. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
892.
We present here a study of the phase behavior of mixed component NaNO(3)-Na(2)SO(4) (SNS) droplets with NaNO(3) to Na(2)SO(4) molar ratios of 1:1, 3:1, and 10:1, comparing observations with thermodynamic predictions. Measurements are made by Fourier transform infrared attenuated total reflection and micro-Raman spectroscopy for SNS droplets deposited on ZnSe and quartz substrates, respectively. The conventional deliquescence/efflorescence hysteresis in phase behavior is observed. On drying, heterogeneous crystallization leads to phase behavior that is consistent with bulk solution thermodynamics, with the formation of the mixed salt NaNO(3)·Na(2)SO(4)·H(2)O, Na(2)SO(4) (s), and NaNO(3) (s) all observed to form at relative humidities that coincide with predictions by the aerosol inorganics model. However, conditioning of the droplet at high relative humidity prior to drying is observed to lead to quantitative differences between the fractions of different salts formed. When substrate effects do not influence the crystallization process, supersaturated solutions are formed, and this leads to the observation of contact ion pairs. Such measurements of the phase behavior of mixed component droplets are important for testing the reliability of thermodynamic models. 相似文献
893.
Tong XL Hu TL Zhao JP Wang YK Zhang H Bu XH 《Chemical communications (Cambridge, England)》2010,46(45):8543-8545
The enantiomers of complex 1 (1a and 1b) have been obtained by spontaneous resolution upon crystallization in the absence of a chiral source. The enantiomeric nature of 1a and 1b was confirmed by circular dichroism (CD) spectra and theoretical investigation. 相似文献
894.
The heterogeneous reaction of SO2 on TiO2 particles was studied using in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The influences of the oxygen concentration, relative humidity (RH), and ultraviolet (UV) light illumination (λ ≈ 365 nm) intensity on the reaction were investigated. The main product of the heterogeneous reaction of SO2 on TiO2 particles was sulfate with UV illumination and sulfite without it. The production of sulfate was promoted significantly with UV illumination or water, and there was a synergistic effect when both were present. In the dry system without UV, the heterogeneous reaction of SO2 on TiO2 particles was found to be second-order for SO2 and the initial uptake coefficient, γBET, was determined to be 1.94 × 10?6. With UV and RH = 40%, the reaction order was first-order and the initial uptake coefficient was 1.35 × 10?5. 相似文献
895.
An ionic liquid (IL) based on lactate, 1-butyl-3-methylimidazolium lactate ([C4mim][Lact]), has been prepared and characterized by 1H nuclear magnetic resonance (NMR) spectroscopy and differential scanning calorimetry (DSC). Since the IL can form strong hydrogen bonds with water, trace water is a problematic impurity in the IL. Using the standard addition method (SAM), the density, refractive index and surface tension of [C4mim][Lact] were measured in the temperature range (308.15 to 343.15 ± 0.05) K. On the basis of the experimental data, the parachor and molar volume for [C4mim][Lact], and the molecular volume V m, surface tension γ, molar enthalpy of vaporization Δl g H m 0, refractive index n D, and the thermal expansion coefficients α, for the homologues [C n mim][Lact] (n = 2, 3, 4, 5, and 6) were estimated using semi-empirical methods. The estimated values are in good agreement with the experimental data. 相似文献
896.
Yu‐Bo Wang Tong‐Hen Pan Qian Liang Dong‐Sheng Liu Chang‐Cang Huang 《Acta Crystallographica. Section C, Structural Chemistry》2010,66(5):m127-m129
The asymmetric unit of the title complex, [CdCl2(C14H12N4O2S)2]n, consists of one CdII ion located on the crystallographic inversion centre, one 4‐benzoyl‐1‐isonicotinoylthiosemicarbazide ligand and one chloride ligand. The central CdII ion adopts a distorted octahedral coordination geometry formed by two pyridyl N atoms of two ligands, two S atoms of two other ligands and two chloride ligands. The thiosemicarbazide ligands act as bridges, linking the metal ions into a two‐dimensional layered structure parallel to the bc plane. Intermolecular N—H...O hydrogen bonds and C—H...π interactions exist between adjacent layers. 相似文献
897.
898.
血糖近红外光谱分析的Savitzky-Golay平滑模式与偏最小二乘法因子数的联合优选 总被引:8,自引:1,他引:7
利用偏最小二乘法(PLS)和光谱Savitzky-Golay(SG)平滑方法,建立血清葡萄糖近红外光谱分析的优化模型。基于最优单波数模型的预测效果,提出划分校正集和验证集的一种新方法。采用10000~5300cm-1和4920~4160cm-1的组合波段,光谱经过SG平滑处理,利用PLS方法建立定标预测模型。将平滑点数扩充为5,7,…,87(奇数),多项式次数扩充为n=2,3,4,5,6,得到包含582个平滑模式的14个平滑系数表。对所有平滑模式和PLS因子数(1~40)分别建立PLS模型。按照预测效果进行优选,得到最优SG平滑模式为1阶导数平滑,3、4次多项式类型,SG平滑点数为53,最优PLS因子数为7,最优RMSEP达到0.376mmol/L。所采用的划分校正集和验证集的方法、SG平滑模式的扩充、SG平滑模式和PLS因子数的联合大范围筛选能够有效地应用于近红外光谱分析的模型优化。 相似文献
899.
A new molecularly imprinted polymer (MIP) targeting to quinolones (Qs) and tetracyclines (TCs) was synthesized using itaconic acid (ITA) and ciprofloxacin (CIP) as a functional monomer and template molecule, respectively. Factors affecting the overall performance of MIP were investigated, and the results showed that Fe(3+) ion play a vital role in the formation of MIP with high molecular imprinting effect. Meanwhile, the chelating ability of monomer, species of template molecule, as well as the molar ratio of monomer and template also contribute to the performance of the obtained MIP. Cyclic voltammetry verified that, with the participation of Fe(3+) ions, a ternary complex of ITA-Fe(3+)-CIP could be formed before polymerization. Compared with conventional MIP prepared from commonly used monomer, methacrylic acid (MAA), the new MIP show significantly enhanced molecular imprinting effect and higher capacity for specific adsorption of target compounds as revealed by static and dynamic binding experiments. The MIP was successfully used as solid-phase extraction (SPE) adsorbent for enriching a broad spectrum of antibiotics containing beta-diketone structure from surface water sample. HPLC detection showed that high recovery rate (78.6-113.6%) was found in these spiked antibiotics, whereas recovery rate for the non structurally related drugs, epinephrine (EP) and dopamine (DOPA), was very low (4.7-7.6%) on the MIP cartridges. The results demonstrate that the MIP prepared by the strategy proposed in this work, could specifically target to a series of structurally related antibiotics containing beta-diketone structure. 相似文献
900.