全文获取类型
收费全文 | 4873篇 |
免费 | 594篇 |
国内免费 | 473篇 |
专业分类
化学 | 3532篇 |
晶体学 | 52篇 |
力学 | 294篇 |
综合类 | 41篇 |
数学 | 534篇 |
物理学 | 1487篇 |
出版年
2024年 | 17篇 |
2023年 | 76篇 |
2022年 | 132篇 |
2021年 | 174篇 |
2020年 | 157篇 |
2019年 | 174篇 |
2018年 | 158篇 |
2017年 | 132篇 |
2016年 | 226篇 |
2015年 | 197篇 |
2014年 | 241篇 |
2013年 | 340篇 |
2012年 | 335篇 |
2011年 | 386篇 |
2010年 | 261篇 |
2009年 | 260篇 |
2008年 | 287篇 |
2007年 | 260篇 |
2006年 | 259篇 |
2005年 | 184篇 |
2004年 | 172篇 |
2003年 | 149篇 |
2002年 | 188篇 |
2001年 | 145篇 |
2000年 | 109篇 |
1999年 | 122篇 |
1998年 | 103篇 |
1997年 | 95篇 |
1996年 | 57篇 |
1995年 | 80篇 |
1994年 | 65篇 |
1993年 | 52篇 |
1992年 | 56篇 |
1991年 | 33篇 |
1990年 | 38篇 |
1989年 | 17篇 |
1988年 | 20篇 |
1987年 | 13篇 |
1986年 | 27篇 |
1985年 | 24篇 |
1984年 | 19篇 |
1983年 | 15篇 |
1982年 | 12篇 |
1981年 | 12篇 |
1980年 | 6篇 |
1979年 | 10篇 |
1977年 | 4篇 |
1974年 | 7篇 |
1973年 | 7篇 |
1969年 | 4篇 |
排序方式: 共有5940条查询结果,搜索用时 15 毫秒
991.
由于介孔二氧化硅单晶规整的外观形貌和有序的内部孔道结构性能, 使其在微激光器件领域具有十分重要的潜在应用. 试着用高电荷密度的阳离子三头季铵盐表面活性剂(C18-2-3-1)作模板剂, 用正硅酸四乙酯作无机前驱体, 在硫酸介质中通过调变有机-无机物种之间的协同组装作用, 成功地合成了具有简单立方相Pm3n结构的介孔二氧化硅单晶. 扫描电子显微镜(SEM)观察到这种介孔二氧化硅单晶具有菱形十二面体形貌, X射线衍射(XRD)和透射电子显微镜(TEM)测试结果证实这种介孔单晶属于简单立方Pm3n对称结构. 氮气吸附数据测得这种单晶具有较大的BET比表面积(550 cm2/g), 较大的孔体积(BJH 0.20 cm3/g)和孔径(BJH 2.4 nm). 相似文献
992.
An electrochemical sensor was developed for the detection of organophosphate pesticides based on electrodeposition of gold nanoparticles on a multi-walled carbon nanotubes modified glassy carbon electrode. Cyclic voltammetry was employed in the process of electrodeposition. Field emission scanning electron microscope and X-ray diffraction techniques were used for characterization of the composite. Organophosphate pesticides (e.g. parathion) were determined using linear scan voltammetry. A highly linear response to parathion in the concentration range from 6.0?×?10?5 to 5.0?×?10?7 M was observed, with a detection limit of 1.0?×?10?7 M estimated at a signal-to-noise ratio of 3. The method has been applied to the analysis of parathion in real samples. 相似文献
993.
An impedimetric immunosensor was fabricated for rapid and non-labeled detection of sulfate-reducing bacteria, Desulforibrio caledoiensis (SRB) by immobilizing lectin-Concanavalin A using an agglutination assay. The immobilization of lectin was conducted using amine coupling on the surface of a gold (Au) electrode assembled with 11-Mercaptoundecanoic acid. Electrochemical impedance spectroscopy (EIS) was used to verify the stepwise assembly of the sensor system. The work conditions of the impedimetric immunosensor, such as pH of the buffer solutions and the incubation time of lectin, were optimized. Faradic impedance spectra for charge transfer for the redox probe Fe(CN)63−/4−were measured to determine SRB concentrations. The diameter of the Nyquist diagram that is equal to the charge-transfer resistance (Rct) increased with increasing SRB concentration. A linear relationship between Rct and SRB concentration was obtained in SRB concentration range of 1.8 to 1.8 × 107 cfu/ml. The variation of the SRB population during the growth process was also monitored using the impedimetric immunosensor. This approach has great potential for simple, low-cost, and time-saving monitoring of microbial populations. 相似文献
994.
LingShu Wan BeiBei Ke XiaoKai Li XiangLin Meng LuYao Zhang ZhiKang Xu 《中国科学B辑(英文版)》2009,52(7):969-974
Highly ordered honeycomb-patterned polystyrene (PS)/poly(ethylene glycol) (PEG) films were prepared by a water-assisted method
using an improved setup, which facilitated the formation of films with higher regularity, better reproducibility, and larger
area of honeycomb structures. Surface aggregation of hydrophilic PEG and adsorption of bovine serum albumin (BSA) on the honeycomb-patterned
films were investigated. Field emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM) were used to
observe the surface morphologies of the films before and after being rinsed with water. As confirmed by the FESEM images and
the AFM phase images, PEG was enriched in the pores and could be gradually removed by water. The adsorption of fluorescence-labeled
BSA on the films was studied in visual form using laser scanning confocal microscopy. Results clearly demonstrated that the
protein-resistant PEG was selectively enriched in the pores. This water-assisted method may be a latent tool to prepare honeycomb-patterned
biofunctional surfaces.
Supported by the National Natural Science Foundation of China (Grant No. 50803053), the National Natural Science Foundation
of China for Distinguished Young Scholars (Grant No. 50625309), the National Postdoctoral Science Foundation of China (Grant
Nos. 20070421172 & 20081466) and the National Undergraduate Innovative Test Program 相似文献
995.
Ionic liquid, 1‐butyl‐3‐methylimidazolium chloride ([BMIM]Cl), was used as solvent for the polymerization of acrylonitrile (AN) and the resulting polymer solution was directly applied for spinning to produce polyacrylonitrile (PAN) fiber for the first time. Acrylic polymers with high content of AN, large molecular weight, and low polydispersity index (PDI) were prepared. The PAN macromolecules have little degradation during dissolution of PAN in [BMIM]Cl, which is completely different from the dissolution of biopolymers such as cellulose in ionic liquids. The PAN fiber with round profile and good mechanical properties was successfully spun from the PAN/[BMIM]Cl solution using dry–jet wet spinning technology. It suggests that an environmental friendly process for the acrylic fiber would be developed and the high performance PAN fiber would be obtained when the spinning conditions are optimized further. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
996.
Cuihong Wan Meng Cui Fengrui Song Zhiqiang Liu Shuying Liu 《Journal of the American Society for Mass Spectrometry》2009,20(7):1281-1286
The GGA triplet repeats are widely dispersed throughout eukaryotic genomes. (GGA)n or (GGT)n oligonucleotides can interact
with double-stranded DNA containing (GGA:CCT)n to form triple-stranded DNA. The effects of 8 divalent metal ions (3 alkaline-earth
metals and 5 transition metals) on formation of these purine-rich triple-helix DNA were investigated by electrospray ionization
Fourier transform ion cyclotron resonance mass spectrometry (ESI-FT-MS). In the absence of metal ions, no triplex but single-strand,
duplex, and purine homodimer ions were observed in mass spectra. The triple-helix DNA complexes were observed only in the
presence of certain divalent ions. The effects of different divalent cations on the formation of purine-rich triplexes were
compared. Transition-metal ions, especially Co2+ and Ni2+, significantly boost the formation of triple-helix DNA, whereas alkaline-earth metal ions have no positive effects on triplex
formation. In addition, Ba2+ is notably beneficial to the formation of homodimer instead of triplex. 相似文献
997.
Yanuardi Raharjo Mohd Marsin Sanagi Wan Aini Wan Ibrahim Ahmedy Abu Naim Hassan Y. Aboul‐Enein 《Journal of separation science》2009,32(4):623-629
A liquid‐phase microextraction coupled with LC method has been developed for the determination of organophosphorus pesticides (methidation, quinalphos and profenofos) in drinking water samples. In this method, a small amount (3 μL) of isooctane as the acceptor phase was introduced continually to fill‐up the channel of a 1.5 cm polypropylene hollow fiber using a microsyringe while the hollow fiber was immersed in an aqueous donor solution. A portion of the acceptor phase (ca. 0.4 μL) was first introduced into the hollow fiber and additional amounts (ca. 0.2 μL) of the acceptor phase were introduced to replenish at intervals of 3 min until set end of extraction (40 min). After extraction, the acceptor phase was withdrawn and transferred into a 2 mL vial for a drying step prior to injection into a LC system. Parameters that affect the extraction efficiency were studied including the organic solvent, length of fiber, volume of acceptor and donor phase, stirring rate, extraction time, and effect of salting out. The proposed method provided good enrichment factors of up to 189.50, with RSD ranging from 0.10 to 0.29%, analyte recoveries of over 79.80% and good linearity ranging from 10.0 to 1.25 mg/L. The LOD ranged from 2.86 to 82.66 μg/L. This method was applied successfully to the determination of organophosphorus pesticides in selected drinking water samples. 相似文献
998.
Motivated by just-in-time manufacturing, we consider a single machine scheduling problem with dual criteria, i.e., the minimization of the total weighted earliness subject to minimum number of tardy jobs. We discuss several dominance properties of optimal solutions. We then develop a heuristic algorithm with time complexity O(n3) and a branch and bound algorithm to solve the problem. The computational experiments show that the heuristic algorithm is effective in terms of solution quality in many instances while the branch and bound algorithm is efficient for medium-size problems. 相似文献
999.
1000.
The Legendre spectral Galerkin method for the Volterra integral equations of the second kind is proposed in this paper. We
provide a rigorous error analysis for the proposed method, which indicates that the numerical errors (in the L
2 norm) will decay exponentially provided that the kernel function and the source function are sufficiently smooth. Numerical
examples are given to illustrate the theoretical results.
相似文献