全文获取类型
收费全文 | 38762篇 |
免费 | 6318篇 |
国内免费 | 4414篇 |
专业分类
化学 | 27647篇 |
晶体学 | 449篇 |
力学 | 2315篇 |
综合类 | 287篇 |
数学 | 4091篇 |
物理学 | 14705篇 |
出版年
2024年 | 85篇 |
2023年 | 777篇 |
2022年 | 859篇 |
2021年 | 1312篇 |
2020年 | 1529篇 |
2019年 | 1486篇 |
2018年 | 1295篇 |
2017年 | 1246篇 |
2016年 | 1805篇 |
2015年 | 1776篇 |
2014年 | 2074篇 |
2013年 | 2763篇 |
2012年 | 3359篇 |
2011年 | 3409篇 |
2010年 | 2311篇 |
2009年 | 2199篇 |
2008年 | 2508篇 |
2007年 | 2318篇 |
2006年 | 2117篇 |
2005年 | 1780篇 |
2004年 | 1358篇 |
2003年 | 1155篇 |
2002年 | 1120篇 |
2001年 | 843篇 |
2000年 | 794篇 |
1999年 | 887篇 |
1998年 | 757篇 |
1997年 | 746篇 |
1996年 | 759篇 |
1995年 | 633篇 |
1994年 | 604篇 |
1993年 | 464篇 |
1992年 | 403篇 |
1991年 | 364篇 |
1990年 | 319篇 |
1989年 | 235篇 |
1988年 | 184篇 |
1987年 | 145篇 |
1986年 | 153篇 |
1985年 | 141篇 |
1984年 | 87篇 |
1983年 | 79篇 |
1982年 | 68篇 |
1981年 | 46篇 |
1980年 | 40篇 |
1979年 | 19篇 |
1978年 | 10篇 |
1977年 | 11篇 |
1975年 | 7篇 |
1973年 | 7篇 |
排序方式: 共有10000条查询结果,搜索用时 280 毫秒
941.
两次双波长分光光度法同时测定钴、镍、铜 总被引:1,自引:0,他引:1
本文用两次双波长分光光度法实现了三组分混合体系的同时测定。结果表明,两次双波长测量信号△A_1和△A_2的线性组合与混合体系中各组分浓度呈线性关系。该法用于钻、镍、铜三组分同时测定,定量结果令人满意。 相似文献
942.
943.
944.
945.
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
946.
Poor sensitivity and low phase ratio are the main drawbacks of open tubular capillary electrochromatography (OTCEC). The poor sensitivity results from the use of narrow bore size capillary, whereas the low phase ratio, which limits the separation capability, is caused by the limited surface area of conventional capillary. Two strategies may be useful to overcome these disadvantages. First, an extended light path (ELP) capillary, which has a bubble cell at the detection point, is used to improve the sensitivity. Secondly, an etched capillary of a 1,000-fold increased surface area is used to enhance the phase ratio. In this work, use of an ELP capillary and an etched capillary in OTCEC was evaluated with a chiral stationary phase of avidin prepared with the physical adsorption method. With a 20 microm I.D. ELP capillary with a 150 microm bubble cell, the peak height was enhanced by 4-10-fold and the corrected peak area was increased by 12-fold relative to a 20 microm I.D. conventional capillary. However, the peak efficiency and resolution decreased noticeably. The phase ratio on the etched capillary was slightly enhanced, by a factor of 1.64 relative to an unetched capillary. Consequently, the separation capability was slightly improved. The increase in the phase ratio was much lower than that expected from the increase in surface area, the reason for which is probably the reduced density of surface silanol group and the generation of nitrogen-containing groups due to the etching process. 相似文献
947.
J. G. Liu M. H. He H. W. Zhou Z. G. Qian F. S. Wang S. Y. Yang 《Journal of polymer science. Part A, Polymer chemistry》2002,40(1):110-119
Organosoluble polyimides were synthesized with the alicyclic dianhydride 1,8‐dimethylbicyclo[2,2,2]oct‐7‐ene‐2,3,5,6‐tetracarboxylic dianhydride and aromatic diamines. The polyimides possessed good solubility both in strong dipolar solvents and in common solvents; the thermal decomposition temperature of the polyimides exceeded 420 °C. Strong and flexible films of the polyimides, with the cutoff of ultraviolet–visible absorption lower than 310–320 nm, exhibited good features as the alignment layers for nematic liquid crystals with pretilt angles of 1.5–2.9°. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 40: 110–119, 2002 相似文献
948.
Development of high-performance liquid chromatographic fingerprints for distinguishing Chinese Angelica from related umbelliferae herbs 总被引:6,自引:0,他引:6
Lu GH Chan K Liang YZ Leung K Chan CL Jiang ZH Zhao ZZ 《Journal of chromatography. A》2005,1073(1-2):383-392
A high-performance liquid chromatographic (HPLC) fingerprint of Chinese Angelica (CA) was developed basing on the consistent chromatograms of 40 CA samples (Angelica sinensis (Oliv.) Diels). The unique properties of this HPLC fingerprints were validated by analyzing 13 related herbs including 4 Japanese Angelicae Root samples (JA, A. acutiloba Kitagawa and A. acutiloba Kitagawa var. sugiyame Hikino), 6 Szechwan Lovage Rhizome samples (SL, Ligusticum chuanxiong Hort.) and 3 Cnidium Rhizome samples (CR, Cnidium officinale Makino). Both correlation coefficients of similarity in chromatograms and relative peak areas of characteristic compounds were calculated for quantitative expression of the HPLC fingerprints. The amount of senkyunolide A in CA was less than 30-fold of that in SL and CR samples, which was used as a chemical marker to distinguish them. JA was easily distinguished from CA, SL and CR based on either chromatographic patterns or the amount of coniferyl ferulate. No obvious difference between SL and CR chromatograms except the relative amount of some compounds, suggesting that SL and CR might have very close relationship in terms of chemotaxonomy. Ferulic acid and Z-ligustilide were unequivocally determined whilst senkyunolide I, senkyunolide H, coniferyl ferulate, senkyunolide A, butylphthalide, E-ligustilide, E-butylidenephthalide, Z-butylidenephthalide and levistolide A were tentatively identified in chromatograms based on their atmospheric pressure chemical ionization (APCI) MS data and the comparison of their UV spectra with those published in literatures. 相似文献
949.
950.
杂氮硅三环是一类具有特殊生理活性的新型有机硅化合物,有关其合成及性质在近20年中已有大量报道。特别是当R为氯甲基时,该化合物的生理活性很高,用它制成的药膏已用来治疗脱发及伤口愈合。因此,1-氯甲基杂氮硅三环(Ⅰ)的分子结构引起了人们的关注,先后有人报道了(Ⅰ)及1-氯甲基-3,7-二甲基,1-氯甲基-3,7,10-三甲基 相似文献