全文获取类型
收费全文 | 6878篇 |
免费 | 1097篇 |
国内免费 | 781篇 |
专业分类
化学 | 4712篇 |
晶体学 | 52篇 |
力学 | 467篇 |
综合类 | 33篇 |
数学 | 776篇 |
物理学 | 2716篇 |
出版年
2024年 | 14篇 |
2023年 | 134篇 |
2022年 | 200篇 |
2021年 | 194篇 |
2020年 | 223篇 |
2019年 | 255篇 |
2018年 | 188篇 |
2017年 | 191篇 |
2016年 | 291篇 |
2015年 | 317篇 |
2014年 | 315篇 |
2013年 | 388篇 |
2012年 | 572篇 |
2011年 | 633篇 |
2010年 | 417篇 |
2009年 | 336篇 |
2008年 | 461篇 |
2007年 | 413篇 |
2006年 | 394篇 |
2005年 | 363篇 |
2004年 | 319篇 |
2003年 | 229篇 |
2002年 | 200篇 |
2001年 | 160篇 |
2000年 | 126篇 |
1999年 | 156篇 |
1998年 | 111篇 |
1997年 | 123篇 |
1996年 | 133篇 |
1995年 | 122篇 |
1994年 | 90篇 |
1993年 | 73篇 |
1992年 | 70篇 |
1991年 | 72篇 |
1990年 | 79篇 |
1989年 | 55篇 |
1988年 | 47篇 |
1987年 | 38篇 |
1986年 | 42篇 |
1985年 | 40篇 |
1984年 | 28篇 |
1983年 | 19篇 |
1982年 | 17篇 |
1981年 | 12篇 |
1980年 | 16篇 |
1979年 | 9篇 |
1978年 | 7篇 |
1977年 | 8篇 |
1976年 | 12篇 |
1974年 | 17篇 |
排序方式: 共有8756条查询结果,搜索用时 15 毫秒
101.
Jie Chen Rong Ji Dai Bin Tong Sheng Yuan Xiao Wei Wei Meng 《中国化学快报》2007,18(1):10-12
The reduction of 4-nitrophenol catalyzed by nitroreductase in the presence of NADH was investigated in this paper.4-Amino- phenol and 4-bydroxylamino-phenol were found in the reductive products.The relationship between reaction time and the reductive ratio were studied.The similar reducing ratios of 4-nitrophenol were obtained under aerobic and anaerobic conditions.The results indicated that an oxygen-insensitive reaction was proceeded in the reduction of 4-nitrophenol and nitroreductase was an oxygen- insensitive enzyme.The reductive products of 4-nitrophenol were determined by HPLC and MS. 相似文献
102.
A practical LC-MS method for determination of (24R)-24,25-dihydroxyvitamin D(3) [24,25(OH)(2)D(3)] in human plasma has been developed using derivatization with a Cookson-type reagent, 4-[4-(6-methoxy-2-benzoxazolyl)phenyl]-1,2,4-triazoline-3,5-dione (MBOTAD). The derivatization with MBOTAD significantly improved the ionization efficiency of the analyte with a detection limit of 18 fmol [equivalent to 7.5 pg of 24,25(OH)(2)D(3)] per injection. The method employed protein precipitation with acetonitrile, purification with OASIS HLB cartridge and silica gel column, derivatization with MBOTAD and atmospheric pressure chemical ionization MS detection. The mass spectrometer was operated in the positive-ion mode of mass chromatography and [26,26,26,27,27,27-(2)H(6)]-24,25(OH)(2)D(3) was used as an internal standard. The intra- and inter-assay coefficients of variation were below 3.4 and 2.5%, respectively, and the analytical recovery of 24,25(OH)(2)D(3) was quantitative. Assay linearity was obtained in the range of 0.05-1.2 ng per tube and the limit of quantitation was 0.23 ng/mL for a 0.3 mL plasma aliquot. The developed method was applied to plasma samples obtained from volunteers and gave satisfactory results. 相似文献
103.
Haiying Zhang Jiarou Peng Shiqi Peng Guowang Wu Duenhua Dai Guozheng Zhu 《Journal of mass spectrometry : JMS》1992,27(1):50-52
The reactions of sulphur with phosphine and phosphite were investigated by low-energy electron impact mass spectrometry and fast atom bombardment mass spectrometry. A previously reported mechanism of the Ph3P–S8 reaction was supported and a possible mechanism for the (PhO)3P–S8 reaction is proposed. 相似文献
104.
From the changes in energy band, density of state (DOS) and electronic structure calculated by the DFT/B3LYP method, the conductivity of carbon doped boron‐nitride nanotube (BNNT), which is formed by C‐C bond substituting B‐N bond, was discussed. The results indicate that the substitution will strengthen the conductivity of the tube, and its semi‐conductivity might be adjusted by controlling the substitution. 相似文献
105.
An unprecedented C,C-linked dimeric norcembranoid (sinulochmodin A, 1), a novel isocembranoid (sinulochmodin B, 2), and a novel yonarane norditerpenoid (sinulochmodin C, 3) were isolated from the soft coral Sinularia lochmodes. The structures of these metabolites were elucidated by extensive spectroscopic analysis and on the basis of the absolute structures of two related norditerpenoids (4 and 5), which were determined for the first time by a modified Mosher method. A plausible pathway for the biosynthesis of 1 and 3-5 from 2 was postulated. [structure: see text] 相似文献
106.
Peng Chen Xiaojun Luo Guanqi Dai Yuchuan Jiang Yue Luo Shuang Peng Hao Wang Penghui Xie Chen Qu Wenyu Lin Jian Hong Xue Ning Aimin Li 《Experimental & molecular medicine》2020,52(7):1062
Dexmedetomidine (DEX) is an anesthetic that is widely used in the clinic, and it has been reported to exhibit paradoxical effects in the progression of multiple solid tumors. In this study, we sought to explore the mechanism by which DEX regulates hepatocellular carcinoma (HCC) progression underlying liver fibrosis. We determined the effects of DEX on tumor progression in an orthotopic HCC mouse model of fibrotic liver. A coculture system and a subcutaneous xenograft model involving coimplantation of mouse hepatoma cells (H22) and primary activated hepatic stellate cells (aHSCs) were used to study the effects of DEX on HCC progression. We found that in the preclinical mouse model of liver fibrosis, DEX treatment significantly shortened median survival time and promoted tumor growth, intrahepatic metastasis and pulmonary metastasis. The DEX receptor (ADRA2A) was mainly expressed in aHSCs but was barely detected in HCC cells. DEX dramatically reinforced HCC malignant behaviors in the presence of aHSCs in both the coculture system and the coimplantation mouse model, but DEX alone exerted no significant effects on the malignancy of HCC. Mechanistically, DEX induced IL-6 secretion from aHSCs and promoted HCC progression via STAT3 activation. Our findings provide evidence that the clinical application of DEX may cause undesirable side effects in HCC patients with liver fibrosis.Subject terms: Cancer microenvironment, Cell growth 相似文献
107.
D Dai 《Journal of solid state chemistry》2003,175(2):341-347
First principles spin-polarized electronic band structure calculations were carried out for three ordered spin states of CdVO3, and the strengths of its corner- and edge-sharing spin exchange interactions were estimated. To gain insight into why CdVO3 exhibits ferromagnetism while CaV2O5 does not despite their apparent structural similarity, the spin exchange interactions of CdVO3 and CaV2O5 were compared in terms of spin dimer analysis using extended Hückel tight binding calculations, and the local geometries of their V4+ ions were examined. 相似文献
108.
T. Takahagi I. Shimada M. Fukuhara K. Morita A. Ishitani 《Journal of polymer science. Part A, Polymer chemistry》1986,24(11):3101-3107
In the carbon fiber production process from polyacrylonitrile (PAN), PAN precursor is heated first in air to secure stabilization in the succeeding carbonization process at higher temperature. The mechanism of the stabilization reaction and chemical structure of the stabilized PAN have been examined by x-ray photoelectron spectroscopy and elemental analysis. The stabilized PAN was determined to have a ladderlike structure consisting of 40% acridone ring, 30% naphtyridine ring, 20% hydronaphtyridine ring, and others. This structure well explains the stability of the polymer in the succeeding carbonization process on carbon fiber production with conjugated π-electron systems over the whole polymer chain and intermolecular hydrogen bonds. A comonomer addition to the precursor was found to accelerate the dehydrogenation reaction in the stabilization process. 相似文献
109.
金属离子掺杂对TiO2光催化性能的影响 总被引:14,自引:1,他引:14
TiO2光催化反应过程涉及光生电荷、电荷迁移、电荷在TiO2表面的反应和溶液体相反应4个顺序相接并相互影响的步骤.在TiO2中掺杂金属离子对以上4个步骤均有重要影响,合理的掺杂可有效地提高其光催化性能.本文综合了国内外此方面的最新研究成果,从提高TiO2光催化性能和优化光催化反应的角度出发,在材料吸光能力、电荷扩散、表面反应、粒径和晶型等方面,全面地分析总结了金属离子掺杂的影响效果和规律性认识,并对TiO2基光催化材料的金属离子掺杂改性研究的未来发展方向提出了建议.文中还简要介绍了相关的掺杂方法和材料表征手段. 相似文献
110.
I. Shimada T. Takahagi M. Fukuhara K. Morita A. Ishitani 《Journal of polymer science. Part A, Polymer chemistry》1986,24(8):1989-1995
The stabilized fibers prepared by heating PAN and a PAN copolymer in air and under reduced pressure have been examined by FT-IR spectroscopy in order to determine their chemical structures. Three kinds of reactions, cyclization, dehydrogenation, and oxygen uptake are found to occur almost simultaneously in the stabilization process in air by digital difference spectrum method. The effect of the comonomer is confirmed to accelate the dehydrogenation reaction and also the formation of acridone ring in the thermal stabilization in air by kinetic study. 相似文献