全文获取类型
收费全文 | 105486篇 |
免费 | 17744篇 |
国内免费 | 8296篇 |
专业分类
化学 | 84994篇 |
晶体学 | 977篇 |
力学 | 5362篇 |
综合类 | 537篇 |
数学 | 10682篇 |
物理学 | 28974篇 |
出版年
2024年 | 249篇 |
2023年 | 1485篇 |
2022年 | 2408篇 |
2021年 | 2715篇 |
2020年 | 3663篇 |
2019年 | 4784篇 |
2018年 | 3194篇 |
2017年 | 2634篇 |
2016年 | 6143篇 |
2015年 | 6347篇 |
2014年 | 6868篇 |
2013年 | 8768篇 |
2012年 | 8813篇 |
2011年 | 8195篇 |
2010年 | 6796篇 |
2009年 | 6719篇 |
2008年 | 6478篇 |
2007年 | 5447篇 |
2006年 | 4967篇 |
2005年 | 4625篇 |
2004年 | 3796篇 |
2003年 | 3113篇 |
2002年 | 3588篇 |
2001年 | 2819篇 |
2000年 | 2659篇 |
1999年 | 1958篇 |
1998年 | 1414篇 |
1997年 | 1253篇 |
1996年 | 1289篇 |
1995年 | 1126篇 |
1994年 | 1053篇 |
1993年 | 906篇 |
1992年 | 793篇 |
1991年 | 699篇 |
1990年 | 585篇 |
1989年 | 503篇 |
1988年 | 377篇 |
1987年 | 362篇 |
1986年 | 308篇 |
1985年 | 318篇 |
1984年 | 212篇 |
1983年 | 186篇 |
1982年 | 152篇 |
1981年 | 116篇 |
1980年 | 83篇 |
1978年 | 56篇 |
1977年 | 52篇 |
1975年 | 58篇 |
1974年 | 51篇 |
1973年 | 58篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
银微电极微分电位溶出分析法研究 总被引:3,自引:1,他引:3
本文首次报道了银微电极微分电位溶出分析法。用化学刻蚀法方便地制作了银微电极,用于DPSA具有背景值低,分辨率好、精密度及灵敏度高,在不搅拌,仅需一定酸度而酸度而无其它介质的条件下就能测试等优点,对人工试样及自来水样分析,结果令人满意。 相似文献
992.
993.
994.
995.
Chen T Barton SC Binyamin G Gao Z Zhang Y Kim HH Heller A 《Journal of the American Chemical Society》2001,123(35):8630-8631
996.
On-line photochemical reaction of reserpine in the presence of acetone was investigated. Acetone was found to speed up the on-line photochemical conversion of reserpine into an intensively fluorescent compound. Not only reaction acidity but also the acetate buffer concentration affected the on-line photochemical induced fluorescence signal. Based on the observation an automated flow injection photochemical fluorimetric approach was developed. An injected sample zone was carried by a water stream to be merged with a acetate buffer (pH 3.4) solution containing 0.02% acetone in a knotted PTFE reactor (KR), which was freely coiled around a 6-W low pressure mercury lamp. While passing the KR, reserpine was transformed into an intensively fluorescent compound. It was on-line detected in a flow-through cell at the emission wavelength of 490 nm and excitation wavelength of 386 nm. At optimized conditions, a detection limit 0.45 mug l(-1) was achieved at a sampling rate of 90 h(-1). Eleven determinations of a 0.5 mg l(-1) reserpine standard solution gave a R.S.D. of 0.3%. The linear dynamic range of reserpine calibration curve was 0.01-0.75 mg l(-1). The proposed method was applied to assay the reserpine content in tablets and to monitor the dissolution profile of reserpine tablets. Satisfactory results were obtained for both the assays and dissolution studies. 相似文献
997.
Golebiowski A Klopfenstein SR Shao X Chen JJ Colson AO Grieb AL Russell AF 《Organic letters》2000,2(17):2615-2617
[structure: see text]The solid-supported synthesis of a bicyclic diketopiperazine, a potential peptide beta-turn mimetic, is described. The Ugi reaction between the resin ester of alpha-N-Boc-diaminopropionic acid (an amine input), alpha-bromo acid, aldehyde, and isocyanide is the key step in the proposed protocol. 相似文献
998.
Transition metal oxide doped lanthanum gallates, La0.9Sr0.1Ga0.8M0.2O3 (where M=Co, Mn, Cr, Fe, or V), are studied as mixed ionic-electronic conductors (MIECs) for electrode applications. The
electrochemical properties of these materials in air and in H2 are characterized using impedance spectroscopy, open cell voltage measurement, and gas permeation measurement. Three single
cells based on La0.9Sr0.1Ga0.8 Mg0.2O3 (LSGM) electrolyte (1.13 to 1.65 mm thick) but with different electrode materials are studied under identical conditions
to characterize the effectiveness of the lanthanum gallate-based MIECs for electrode applications. At 800 °C, a single cell
using La0.9Sr0.1- Ga0.8Co0.2O3 as the cathode and La0.9Sr0.1Ga0.8Mn0.2O3 as the anode shows a maximum power density of 88 mW/cm2, which is better than that of a cell using Pt as both electrodes (20 mW/cm2) and that of a cell using La0.6Sr0.4CoO3 (LSC) as the cathode and CeO2-Ni as the anode (61 mW/cm2) under identical conditions. The performance of LSGM-based fuel cells with MIEC electrodes may be further improved by reducing
the electrolyte thickness and by optimizing the microstructures of the electrodes through processing.
Received: 9 January 1998 / Accepted: 1 May 1998 相似文献
999.
Commercial poly(ethylene terephthalate) (PET) was treated at R. H.>80% and room temperature for a set time. The glass transition
temperature (Tg) decreases with the time of exposure to high humidity. The decrease in Tg is a result of plasticization. Our
data indicate that the Tg of dry PET of 76-78°C may decrease to as low a temperature as 65-67°C when it is wet. Induced crystallization
of PET in the presence of water reduces the cold crystallization temperature (Tc). The structure of water-induced crystals
is imperfect and can be improved in perfection by annealing.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
1000.
实验研究结果表明双核金属酞菁类化合物MPc-PcM(M=V、Cr、Mn、Fe、Co、Ni、Cu)是催化H_2S液相氧化反应的良好催化剂,且其催化活性顺序为Co>Ni>V>Fe>Cu>Cr>Mn.采用量子化学理论计算方法(INDO/S)分析了其前线分子轨道的构成特征,结果表明这几种MPc-PcM的LUMO轨道皆为有金属离子参与形成的π轨道,但HOMO轨道间的差异却很大.M=Co、Ni、Cu、Cr时,其HOMO为π轨道;而M=Fe、Mn时,其HOMO却为σ轨道.正是在前线分子轨道的轨道类型、共轭程度及金属轨道贡献三方面因素的协同作用下,才导致MPc-PcM的催化活性顺序并非按中心金属离子价电子层d电子数的递变而呈现出规律性的变化. 相似文献