全文获取类型
收费全文 | 2493篇 |
免费 | 218篇 |
国内免费 | 158篇 |
专业分类
化学 | 488篇 |
晶体学 | 12篇 |
力学 | 394篇 |
综合类 | 49篇 |
数学 | 1428篇 |
物理学 | 498篇 |
出版年
2024年 | 7篇 |
2023年 | 30篇 |
2022年 | 55篇 |
2021年 | 51篇 |
2020年 | 68篇 |
2019年 | 58篇 |
2018年 | 52篇 |
2017年 | 79篇 |
2016年 | 99篇 |
2015年 | 84篇 |
2014年 | 120篇 |
2013年 | 212篇 |
2012年 | 114篇 |
2011年 | 108篇 |
2010年 | 83篇 |
2009年 | 128篇 |
2008年 | 130篇 |
2007年 | 164篇 |
2006年 | 160篇 |
2005年 | 136篇 |
2004年 | 101篇 |
2003年 | 87篇 |
2002年 | 103篇 |
2001年 | 86篇 |
2000年 | 91篇 |
1999年 | 68篇 |
1998年 | 56篇 |
1997年 | 54篇 |
1996年 | 44篇 |
1995年 | 29篇 |
1994年 | 22篇 |
1993年 | 22篇 |
1992年 | 19篇 |
1991年 | 9篇 |
1990年 | 22篇 |
1989年 | 17篇 |
1988年 | 6篇 |
1987年 | 11篇 |
1986年 | 14篇 |
1985年 | 10篇 |
1984年 | 14篇 |
1983年 | 7篇 |
1982年 | 4篇 |
1981年 | 8篇 |
1980年 | 6篇 |
1979年 | 3篇 |
1978年 | 2篇 |
1977年 | 6篇 |
1976年 | 3篇 |
1974年 | 2篇 |
排序方式: 共有2869条查询结果,搜索用时 0 毫秒
61.
62.
Květa Kalíková Petr Kozlík Martin Gilar Eva Tesařová 《Journal of separation science》2013,36(15):2421-2429
Hydrophilic interaction liquid chromatography is a separation technique suitable for the separation of moderately and highly polar compounds. Various stationary phases (SPs) for hydrophilic interaction liquid chromatography are commercially available. While the SPs based on the same type of ligand are available from different providers, they can display a distinct retention characteristics and separation selectivity. The current work is focused on characterization and comparison of the separation systems of two amide‐based HPLC columns from two producers, i.e. XBridge Amide column and TSK gel Amide‐80 column. Several characterization procedures (tests) were used to investigate the differences between these columns. The chromatographic behavior of selected analytes indicates that multimodal interactions are responsible for retention and separation on these columns. Multiple testing approaches were used in order to reveal subtle differences between the SPs. Both amide‐based columns showed certain differences in retention, selectivity, and plate counts. Based on the tests used in this study, we conclude that the investigated columns provide a different degree of H‐bonding interactions. 相似文献
63.
Mahsa Baghban Salehi Ebrahim Vasheghani-Farahani Asefe Mousavi Moghadam 《Journal of Dispersion Science and Technology》2014,35(5):663-671
Among the methods available to reduce water production during oil recovery, injecting a gelling system composed of a polymer and a crosslinker has been widely used. In this study, a Plackett-Burman design was used for screening a large number of factors such as concentrations of polymer, crosslinker, pH, temperature, and presence or absence of NaCl, CaCl2, MgCl2, KCl, thiourea, sodium lactate, and nanoclay on the gelation time of sulfonated polyacrylamide nanocomposite hydrogels by rheological tests. Among these factors, temperature, pH, and CaCl2 concentration were found to have the greatest effect on the gelation time. The effects of these three factors and their interactions on the gelation time were then determined by using central composite design of response surface method. As a result, the interactions of CaCl2 concentration with temperature and pH were considerably more than the interactions of pH and temperature on the gelation time. At low pH (3 < pH < 7), the gelation time decreased by decrease of pH while at CaCl2 concentration of 3750–11250 ppm and at 7 < pH < 11, the gelation time increased with the increase of pH. It was found that temperature was the most effective parameter to control the gelation time. 相似文献
64.
《Comptes Rendus Chimie》2014,17(7-8):752-759
Single-cell and half-cell degradation test procedures were evaluated for carbon-supported Pt/C, PtCo/C and PtNi/C catalysts. Half-cell analyses were employed to understand the effect of the number of cycles and of the scan rate over the cathode catalysts degradation under potential cycling from 0.6 to 1.2 V. The data suggested a time-dependent degradation for all three catalytic systems. Single-cell measurements were used to evaluate the impact of catalyst degradation on fuel cell performance. The measurements in both setups showed similar ECSA and ORR mass activity losses. Specific degradation mechanisms related to Pt dissolution, Pt agglomeration, and transitional metal leaching were quantified and correlated with performance losses. 相似文献
65.
针对电子系统中多值测试条件下的测试排序问题,提出了一种诊断策略优化方法。首先,以相关性矩阵模型为基础,将已有的二值测试的优化算法同多值测试问题相结合,提出了适用于多值测试的基于霍夫曼编码的启发式函数;其次,将平均测试代价最小和平均测试步骤最少作为优化目标,采用与或树启发式搜索算法生成诊断树,得到多值测试的诊断策略,并给出了诊断策略优化方法的具体实现步骤;最后,将其应用到航空设备的实例中。结果表明提出的基于霍夫曼编码的与或树启发式搜索算法是可行的,其生成的诊断树是最优的,比基于信息熵的与或树启发式搜索算法具有更小的平均测试代价和更少的平均测试步骤。 相似文献
66.
67.
To optimize the accuracy of near-infrared non-invasive hemoglobin (Hb) clinical detection, high-performance instrument and prepossessing algorithm have been investigated. A near-infrared spectrophotometric system was constructed adopting InGaAs detector array with 16 pixels and plane grating spectrometer to obtain high signal noise ratio (SNR) spectral data. In our experiment, we applied the device independently to collect spectra data from 91 volunteers’ fingertips non-invasively. Two prediction tests were conducted to verify the effects of preprocessing algorithms improving the accuracy of near-infrared Hb detection and exclude the occasionality of satisfactory results in a single trial. Our non-invasive Hb detection methods were based on partial least squares (PLS). In each test, PLS, MSC coupled with PLS, DOSC coupled with PLS, three methods for non-invasive Hb detection, were analyzed respectively. The results of two trials showed that only DOSC & PLS performed excellently in both predictive ability and stability, obviously better than other two methods. Relative RMSEP was 6.16% in predicting test 1, 6.08% in predicting test 2, almost reaching the requirements of clinical application. It indicates that our independent-developed high-performance instrument and the method DOSC coupled with PLS are promising in non-invasive Hb detection clinical application. 相似文献
68.
In this article, boundary characteristic orthogonal polynomials have been implemented in the Rayleigh–Ritz method to investigate free vibration of non-uniform Euler–Bernoulli nanobeams based on nonlocal elasticity theory. Non-uniform cross section of nanobeams has been considered by taking linear as well as quadratic variations of Young's modulus and density along the space coordinate. Detailed analysis has been reported for all the possible cases of such variations. The objective of the present study is to analyze the effects of nonlocal parameter, boundary condition, length-to-diameter ratio and non-uniform parameter on the frequency parameters. It is found that clamped nanobeams are having highest frequency parameters than other types of boundary conditions for a particular set of parameters. It is also observed that frequency parameters decrease with increase in scaling effect parameter. First four deflection shapes of non-uniform nanobeams have also been incorporated. In this analysis, some of the new results in terms of boundary conditions have also been included. 相似文献
69.
Eleonora Bolli Alessandra Fava Saulius Kaciulis Alessio Mezzi Roberto Montanari Alessandra Varone 《Surface and interface analysis : SIA》2020,52(12):1089-1092
Cr martensitic steels are promising materials for structural applications in future nuclear fusion reactors. Because the embrittlement after tempering treatments can be a serious problem, the fracture mode of a steel with 10.5 wt% of Cr treated at 700°C for 18 h has been investigated through Charpy tests in the temperature range from −100°C to +150°C. X-ray photoelectron spectroscopy (XPS) analyses carried out on the fracture surfaces evidenced the segregation of Cr in both ductile and brittle (quasicleavage) fields. The unexpected result indicates that Cr segregation weakens the atomic bonds; thus, the fracture path in both the cases corresponds to the zones with higher Cr content. 相似文献
70.
Joji Tanaka Satu Hkkinen Parker T. Boeck Yidan Cong Sbastien Perrier Sergei S. Sheiko Wei You 《Angewandte Chemie (International ed. in English)》2020,59(18):7203-7208
An orthogonal combination of cationic and radical RAFT polymerizations is used to synthesize bottlebrush polymers using two distinct RAFT agents. Selective consumption of the first RAFT agent is used to control the cationic RAFT polymerization of a vinyl ether monomer bearing a secondary dormant RAFT agent, which subsequently allows side‐chain polymers to be grafted from the pendant RAFT agent by a radical‐mediated RAFT polymerization of a different monomer, thus completing the synthesis of bottlebrush polymers. The high efficiency and selectivity of the cationic and radical RAFT polymerizations allow both polymerizations to be conducted in one‐pot tandem without intermediate purification. 相似文献