全文获取类型
收费全文 | 89563篇 |
免费 | 4887篇 |
国内免费 | 11722篇 |
专业分类
化学 | 70390篇 |
晶体学 | 1151篇 |
力学 | 2338篇 |
综合类 | 841篇 |
数学 | 12687篇 |
物理学 | 18765篇 |
出版年
2024年 | 82篇 |
2023年 | 628篇 |
2022年 | 1549篇 |
2021年 | 1777篇 |
2020年 | 2177篇 |
2019年 | 2139篇 |
2018年 | 1833篇 |
2017年 | 2680篇 |
2016年 | 2819篇 |
2015年 | 2351篇 |
2014年 | 3268篇 |
2013年 | 6557篇 |
2012年 | 6185篇 |
2011年 | 4946篇 |
2010年 | 4169篇 |
2009年 | 5734篇 |
2008年 | 5928篇 |
2007年 | 6140篇 |
2006年 | 5573篇 |
2005年 | 4710篇 |
2004年 | 4372篇 |
2003年 | 3622篇 |
2002年 | 4855篇 |
2001年 | 2787篇 |
2000年 | 2584篇 |
1999年 | 2429篇 |
1998年 | 2111篇 |
1997年 | 1663篇 |
1996年 | 1405篇 |
1995年 | 1345篇 |
1994年 | 1195篇 |
1993年 | 993篇 |
1992年 | 945篇 |
1991年 | 631篇 |
1990年 | 533篇 |
1989年 | 515篇 |
1988年 | 374篇 |
1987年 | 296篇 |
1986年 | 276篇 |
1985年 | 226篇 |
1984年 | 237篇 |
1983年 | 137篇 |
1982年 | 211篇 |
1981年 | 174篇 |
1980年 | 194篇 |
1979年 | 182篇 |
1978年 | 169篇 |
1977年 | 122篇 |
1976年 | 108篇 |
1973年 | 67篇 |
排序方式: 共有10000条查询结果,搜索用时 875 毫秒
951.
几种改进的CoMFA方法比较研究血小板活化因子拮抗剂 总被引:6,自引:1,他引:6
由于传统的比较分子场分析(CoMFA)方法本身存在一些缺陷,使得分子的叠合 规则以及叠合分子的空间取向和空间位置等因素对q~2的影响很大,因此相继提出 了几种改进的CoMFA方法。为了优化CoMFA结果,应用传统的CoMFA方法和交叉验证 的R~2引导的区域选择法(q~2-GRS)、全取向搜索法(AOS)、全空间搜索法(APS) 以及比较分子相似性指数(CoMSIA)等四种改进的CoMFA方法,对18个pinusolide类 衍生物这类新发现的血小板活化因子(PAF)拮抗剂进行了比较研究。结果表明四 种改进的CoMFA方法得到的q~2值均比传统CoMFA的高。q~2-GRS方法得到的q~2值有 所提高,但综合结果并不理想,AOS与APS得到的q~2较为理想,而在CoMSIA中, q~2几乎不受空间取向或空间位置的影响。同时我们引人基于样本的偏最小二乘法 (SAMPLS)取代原AOS/APS程序中的传统PLS进行统计分析,明显提高了其运行速 度。最后,根据q~2最高的CoMFA模型和CoMSIA模型设计了几个预测活性更高的 pinusolide类似物。 相似文献
952.
953.
环境水样及食品中亚硝酸根的分析进展 总被引:5,自引:1,他引:5
评述了自1997年至2002年国内外环境水样及食品中亚硝酸根(NO2^-)的分析进展。包括分子光谱法、电化学分析法和色谱分析法,引用文献96篇。 相似文献
954.
955.
STUDY ON THE CONCENTRATION DEPENDENCE OF ORIENTATION OF POLYSTYRENE ON SILVER BY THE SERS TECHNIQUE*
Dong-hui Zhang Jin-gui Qin Jing-shu Shen You Wang Wan-jun Liu Department of Chemistry Wuhan University Wuhan China Polymer Physics Laboratory Institute of Chemistry Chinese Academy of Sciences Beijing China 《高分子科学》2000,(2):177-180
Polystyrene films were adsorbed onto a silver surface from dichloromethane solutions with concentrations of 0.1 to12 wt%, and then studied by the surface enhanced Raman technique. A critical concentration for coil interpenetration wasobserved. 相似文献
956.
Hans-Georg Henning Giedrius Mazunaitis 《Monatshefte für Chemie / Chemical Monthly》1992,123(1-2):93-98
Summary Irradiation of solutions of excess cyclohexene and 2-acetyl-5,5-dimethyl-1,3-cyclohexanedione (1), and 3-acetyl-1,5,5-trimethyl-2,4-pyrrolidinedione (4) results mainly in the formation of 1,5-diones2 and5. These originate from intermediate cycloadducts of cyclohexene and theexo-enols of the cyclic 1,3-diketones. The yields decrease with increasing polarity of the solvent. In solution2 and5 are in equilibrium with the cyclic hemiacetales3 and6. 相似文献
957.
Borohydride exchange resin-transition metal(Co,Ni,Cu,)compoundsystems reduced C=C double to C-C single bond in ethanol. 相似文献
958.
The solid diffusion coefficient of lithium-ion in LiCoO2 cathode material has been investigated by the capacity intermittent titration technique (CITT) at different voltages and at different charge/discharge cycles. By SEM, XRD and FTIR techniques, the structure of LiCoO2 was studied before and after charge-discharge cycles, and the relationship between solid diffusion coefficient and crystal structure was further discussed. CITT results show that the value of Li+ solid diffusion coefficient of LiCoO2 is about 10-12 cm2·s-1. During the whole charge-discharge cycles, the Li+ solid diffusion coefficient decreased within the voltage of 4.0~4.3 V, which is attributed to the change of the structure of LiCoO2. 相似文献
959.
Shkumbatiuk R Bazel YR Andruch V Török M 《Analytical and bioanalytical chemistry》2005,382(6):1431-1437
A new, simple, rapid, and sensitive spectrophotometric method has been developed for the determination of nitrophenols [picric acid (PA); dinitrophenols (DNP)] in wastewater samples. The method is based on the reaction of nitrophenols with 2-[(E)-2-(4-diethylaminophenyl)-1-ethenyl]-1,3,3-trimethyl-3 H-indolium chloride reagent to form the colored ion associates, which are extracted by organic solvents. The molar absorptivity of the ion associates of PA with the investigated reagent ranges from 8.3×104 to 11.3×104 L mol–1 cm–1, depending on the extractant. Because only PA is extracted in an acidic medium with the investigated reagent, but both PA and DNP are extracted in an alkaline medium, it is possible to determine both substances in a mixture. Appropriate reaction conditions have been established. The absorbance of the colored extracts obeys Beers law in the range of 0.04–4.58 mg L–1 PA, 1.0–18.4 mg L–1 2,4-DNP and 1.2–14.7 mg L–1 2,6-DNP, respectively. The limit of detections, calculated from a blank test (n=10; P=0.95), are 0.05 mg L–1 PA, 0.9 mg L–1 (2,4-DNP), and 1.1 mg L–1 (2,6-DNP), respectively. 相似文献
960.
Luciani G. Costantini A. Branda F. Scardi P. Lanotte L. 《Journal of Thermal Analysis and Calorimetry》2003,72(1):105-111
A traditional TG apparatus was modified by placing two permanent magnets producing a controlled magnetic field (TG(M): Magneto
Thermogravimetry). This technique proved to be useful to study both structural relaxation and crystallisation of ferromagnetic
metallic glasses. Results obtained for the amorphous alloys Fe40Ni40P14B6 and Fe62.5Co6Ni7.5Zr6Nb2Cu1B15, are reported in this paper. Structural relaxation can be evaluated by measuring changes in Curie temperature induced by
thermal treatments. Crystallisation in TG(M) is detected through a change in the measured apparent mass (difference between
the sample mass and magnetic force driving it upward). These results were confirmed by DSC analysis. Whether the obtained
crystalline phase is ferromagnetic, it can be identified through its Curie temperature, measured by TG(M). In fact the value
of 770°C measured as Curie temperature of crystallised Fe62.5Co6Ni7.5Zr6Nb2Cu1B15led to conclude that the only ferromagnetic crystalline phase is a-Fe. These hypothesis was confirmed by XRD analysis, showing
that the first crystallisation yields to a-Fe nanocrystals.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献