全文获取类型
收费全文 | 17385篇 |
免费 | 1721篇 |
国内免费 | 1446篇 |
专业分类
化学 | 11264篇 |
晶体学 | 93篇 |
力学 | 56篇 |
综合类 | 113篇 |
数学 | 94篇 |
物理学 | 2757篇 |
综合类 | 6175篇 |
出版年
2024年 | 46篇 |
2023年 | 191篇 |
2022年 | 598篇 |
2021年 | 493篇 |
2020年 | 442篇 |
2019年 | 452篇 |
2018年 | 444篇 |
2017年 | 635篇 |
2016年 | 762篇 |
2015年 | 727篇 |
2014年 | 929篇 |
2013年 | 1384篇 |
2012年 | 1293篇 |
2011年 | 1158篇 |
2010年 | 947篇 |
2009年 | 1037篇 |
2008年 | 882篇 |
2007年 | 1194篇 |
2006年 | 1002篇 |
2005年 | 930篇 |
2004年 | 811篇 |
2003年 | 641篇 |
2002年 | 539篇 |
2001年 | 405篇 |
2000年 | 337篇 |
1999年 | 334篇 |
1998年 | 250篇 |
1997年 | 227篇 |
1996年 | 225篇 |
1995年 | 197篇 |
1994年 | 174篇 |
1993年 | 140篇 |
1992年 | 139篇 |
1991年 | 125篇 |
1990年 | 133篇 |
1989年 | 95篇 |
1988年 | 59篇 |
1987年 | 65篇 |
1986年 | 29篇 |
1985年 | 12篇 |
1984年 | 15篇 |
1983年 | 7篇 |
1982年 | 16篇 |
1981年 | 7篇 |
1980年 | 9篇 |
1979年 | 6篇 |
1977年 | 2篇 |
1976年 | 2篇 |
1959年 | 2篇 |
1955年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
82.
Nadeem Saeed Basant K Puri Angela Oatridge Joseph V Hajnal Ian R Young 《Magnetic resonance imaging》1998,16(10):1237-1247
Two semi-automated methods for quantification of ventricular volume change from baseline and follow-up magnetic resonance imaging scans have been developed. Technique 1 employs direct segmentation of the ventricles from both the scans using thresholding and contour extraction. Technique 2 operates on difference images produced by voxel based intensity subtraction of the baseline from the registered follow-up images. Here, all voxels with intensities above a noise threshold and in a restricted area are monitored to compute volumetric changes. In phantom measurements the first technique was accurate to 0.0046%, the second to 0.167% of the phantom volume. Results from normal volunteers was that the average ventricular volume changed by 1.52% and 1.54% for images acquired within 9 months using techniques 1 and 2, respectively. With schizophrenic patients mean change of 10.78% and 9.43% were found employing the first and second procedures, respectively. All measurements agreed with a radiologist’s visual grading of the changes. Robust, objective, fast, easy-to-use, and fairly accurate procedures have been developed and validated to quantify volumetric changes. 相似文献
83.
84.
甜菜皂素是一种天然的非离子型表面活性剂,用无机酸回流甜菜废丝,将所得的滤液蒸发浓缩至粘稠,再加入一定量的无水乙醇反复回流,脱色,纯化后得乳白色甜菜皂素片状晶体。 相似文献
85.
C. N. Carducci S. E. Lucangioli V. G. Rodríguez G. C. Fernndez Otero 《Journal of chromatography. A》1996,730(1-2):313-319
Sample preparation procedures using octadecyl (C18) extraction disks were developed to obtain accurate and reproducible results for determinations of clenbuterol (20 μg per dose) and levothyroxine (100 μg per dose) in dissolution media of solid oral dosage forms. Preconcentration of samples allowed final concentrations of 1.1 μg/ml of clenbuterol and 4.0 μg/ml of levothyroxine to be reached prior to CE analysis. The results obtained by CE were in good agreement with those of HPLC. The precision of the migration time, peak area, peak height and accuracy were determined in both intea-day (n = 6) and inter-day (n =18) assays. Linearity was demonstrated over the ranges 0.5–80.0 μg/ml of clenbuterol and 1.0–30.0 μg/ml of levothyroxine. The mean recoveries were higher than 94.0%, ranging from 50 to 125% levels with respect to dose potencies. The proposed methodology may be generally applied to determine drugs at ng/ml concentrations. 相似文献
86.
87.
从芒果皮中提取果胶的工艺研究 总被引:5,自引:0,他引:5
果胶是一种大分子多糖类物质 ,具有良好的胶凝作用 ,用途广泛 ,在国内外市场上的销量很好。 1 990年世界上果胶总产量约 2 5 0 0 0吨 ,而我国 1 990年仅在食品工业上消耗的果胶就达 1 0 0 0吨 ,但国内市场上销售的果胶大部分为进口货[1] 。果胶在我国的生产仍处于开发阶段 ,所以发掘新的提取资源、提高果胶质量、降低果胶生产成本的意义十分重大。目前 ,已有以柑桔皮、向日葵盘、苹果皮、香蕉皮、马铃薯渣为原料提取果胶的研究报道 ,如何从芒果皮中提取果胶却未见报道。芒果在制造罐头和饮料中副产大量下脚料———芒果皮 ,未能得到充分利用… 相似文献
88.
本文研究了硫酸根阴离子对1-苯基-3-甲基-4-苯甲酰基吡唑酮-5(HPMBP,HL)及三辛胺(TOA)协同萃取镨(Ⅲ)及钕(Ⅲ)的影响。发现在SO_4~(2-)体系中协萃机理为:(LnL_3)_o+(HTOAL)_o(HTOA~+LuL_4~-)_o及(HLnL_4)_o+(TOA)_o(HTOA~+LnL_4~-)_o式中Ln~(3+)=Pr~(3+),下标“o”指示有机相中物种,无下标指示水相中物种。固体萃合物的红外光谱及元素分析数据确证了上述机理。 相似文献
89.
90.
Based on the investigation of a non-ionic surfactant-salt-H2O liquid-liquid extraction system, general rules concerning salt selection are concluded and the mechanism of phase separation is explained. The extraction behavior of chlorpromazine hydrochloride (CPZ) and procaine hydrochloride (PCN) in such a system is studied. Research shows that the extraction efficiency of CPZ can amount to 96% by twice extraction, which means that quantitative extraction is realized, while that of PCN is 77%. This system produces distribution coefficients (KD) of 12.3 and 2.6, respectively, for CPZ and PCN. Extraction mechanism is deduced according to ultraviolet absorbance; and molecular fluorescence spectra change of the drugs in the system studied. 相似文献