全文获取类型
收费全文 | 33798篇 |
免费 | 5616篇 |
国内免费 | 4737篇 |
专业分类
化学 | 24292篇 |
晶体学 | 507篇 |
力学 | 2369篇 |
综合类 | 412篇 |
数学 | 3965篇 |
物理学 | 12606篇 |
出版年
2024年 | 63篇 |
2023年 | 549篇 |
2022年 | 935篇 |
2021年 | 1101篇 |
2020年 | 1222篇 |
2019年 | 1261篇 |
2018年 | 1085篇 |
2017年 | 1034篇 |
2016年 | 1562篇 |
2015年 | 1612篇 |
2014年 | 1885篇 |
2013年 | 2478篇 |
2012年 | 2934篇 |
2011年 | 3178篇 |
2010年 | 2159篇 |
2009年 | 2161篇 |
2008年 | 2276篇 |
2007年 | 2099篇 |
2006年 | 2053篇 |
2005年 | 1659篇 |
2004年 | 1508篇 |
2003年 | 1175篇 |
2002年 | 1249篇 |
2001年 | 1099篇 |
2000年 | 847篇 |
1999年 | 746篇 |
1998年 | 586篇 |
1997年 | 454篇 |
1996年 | 457篇 |
1995年 | 422篇 |
1994年 | 382篇 |
1993年 | 318篇 |
1992年 | 294篇 |
1991年 | 248篇 |
1990年 | 220篇 |
1989年 | 188篇 |
1988年 | 113篇 |
1987年 | 110篇 |
1986年 | 94篇 |
1985年 | 92篇 |
1984年 | 48篇 |
1983年 | 48篇 |
1982年 | 39篇 |
1981年 | 25篇 |
1980年 | 17篇 |
1979年 | 11篇 |
1977年 | 5篇 |
1975年 | 7篇 |
1957年 | 9篇 |
1936年 | 6篇 |
排序方式: 共有10000条查询结果,搜索用时 171 毫秒
991.
IntroductionDAisoneofessentialparticipantsintheneuro transmissionprocessinmammaliancentralnervoussys tem .AlossofDA containingneuronsmayresultinsomeseriousdiseasesuchasParkinsonism .1Sinceitsdiscov eryinthe 195 0s ,DAhasbeenofinteresttoneuroscien tistsandchem… 相似文献
992.
In this paper, a micellar electrokinetic chromatographic (MEKC) method using ionic liquid as modifier for the quantification of the active components of lignans found in the medicinal herbs Schisandra species was developed for the first time. Preliminary investigations employing sodium dodecyl sulfate (SDS) as surfactant did not lead to the necessary resolution of the studied compounds, the addition of ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate (BMIM-BF4) to the SDS micellar system resulted in the complete separation of all the compounds. The effects on the separation by several parameters such as BMIM-BF4 and SDS concentration, applied voltage, background electrolyte pH and concentration, were evaluated. Under the optimal conditions (5 mM borate-5 mM phosphate buffer in the presence of 20 mM SDS and 10 mM BMIM-BF4, pH 9.2, applied voltage 25 kV and detection at 254 nm), the method successfully applied to the determination of lignans in extracts of Schisandra chinensis (Turcz.) Baill. and Schisandra henryi C.B. Clarke in less than 13 min. The separation mechanism was also discussed. 相似文献
993.
Jian Yan Xiang‐Ming Zhang Zhong‐Rong Li Lin Zhou Jian‐Chao Chen Li‐Rong Sun Ming‐Hua Qiu 《Helvetica chimica acta》2005,88(2):240-244
Three new triterpenoids, (3β,8β,14α,21α)‐26,27‐dinoronocerane‐3,8,14,21‐tetrol ( 1 ), (3β,8β,14α,21β)‐26,27‐dinoronocerane‐3,8,14,21‐tetrol ( 2 ), and lycopodiin A ( 3 ), together with four known compounds, lycoclavanol ( 4 ), lycoclaninol ( 5 ), α‐onocerin ( 6 ), and 3‐epilycoclavanol ( 7 ), were isolated from Lycopodium japonicum Thunb (Lycopodiaceae). Their structures were established by means of spectroscopic analyses. Compounds 3 and 7 showed moderate antitumor activity. Compounds 4 and 6 exhibited acetylcholinesterase inhibition activity. 相似文献
994.
995.
ShuXiaCAO JianChenZHANG MingYuNIU KuiLU XinChengLIAO YuFenZHAO 《中国化学快报》2004,15(6):652-654
N-Phosphoryl peptide libraries were constructed by transformation from homo-oligopeptide libraries, which was synthesized by self-assembly of amino acids with the assistance of phosphorus oxychloride. Electrospray ionization mass spectrometry (ESI-MS) was used to monitor the reaction. 相似文献
996.
Qunying Jiang Min Pan Jialing Hu Junlin Sun Lei Fan Zhiqiao Zou Jianshuang Wei Xiaoquan Yang Xiaoqing Liu 《Chemical science》2021,12(1):148
Many cancer treatments including photodynamic therapy (PDT) utilize reactive oxygen species (ROS) to kill tumor cells. However, elevated antioxidant defense systems in cancer cells result in resistance to the therapy involving ROS. Here we describe a highly effective phototherapy through regulation of redox homeostasis with a biocompatible and versatile nanotherapeutic to inhibit tumor growth and metastasis. We systematically explore and exploit methylene blue adsorbed polydopamine nanoparticles as a targeted and precise nanocarrier, oxidative stress amplifier, photodynamic/photothermal agent, and multimodal probe for fluorescence, photothermal and photoacoustic imaging to enhance anti-tumor efficacy. Remarkably, following the glutathione-stimulated photosensitizer release to generate exogenous ROS, polydopamine eliminates the endogenous ROS scavenging system through depleting the primary antioxidant, thus amplifying the phototherapy and effectively suppressing tumor growth in vitro and in vivo. Furthermore, this approach enables a robust inhibition against breast cancer metastasis, as oxidative stress is a vital impediment to distant metastasis in tumor cells. Innovative, safe and effective nanotherapeutics via regulation of redox balance may provide a clinically relevant approach for cancer treatment.Amplified oxidative stress achieved by modulating redox homeostasis with PDA–MB for highly effective synergistic phototherapy to inhibit primary tumors and metastases. 相似文献
997.
荧光探针法研究可聚合有机分子(BMDM)/二苯醚的凝胶化过程 总被引:3,自引:0,他引:3
用荧光素作荧光探针 ,研究了可聚合凝胶因子 4,4′ 二 (α 甲基丙烯酰氧基 1 ,3 亚乙氧基羰基丙酰氨基 )二苯甲烷 (BMDM)在二苯醚的凝胶化过程中的聚集状态 .研究结果发现 ,与荧光素在二苯醚溶液中的荧光光谱相比 ,在凝胶中出现了 1 0~2 5nm的红移 ;而且发现分子凝胶的凝胶化时间随着BMDM浓度的增加而减小 .偏振荧光法研究分子凝胶的取向发现 ,该分子凝胶的形成过程是从各向同性的溶液(P0 =0 )向各向异性的溶胶 (P>0 )转变 ,再向各向同性的凝胶 (Pgel≈ 0 )转变的过程 相似文献
998.
Huilan C Deyan H Tian L Hong Y Wenxia T Jian Chen J Peiju Zheng P Chenggang Chen C 《Inorganic chemistry》1996,35(6):1502-1508
Alkylcobalt(III) Schiff base B(12) model complexes with secondary alkyls or a bulky diamine in the equatorial position were synthesized and characterized. Structures have been first determined by X-ray diffraction analysis for i-C(4)H(9)Co(salen)(gamma-pic) (I), n-C(3)H(7)Co(salen)(gamma-pic) (II) and C(2)H(5)Co(SB) (III), where salen = N,N'-ethylenebis(salicylideneamine) dianion; SB = 1,1,2,2-tetramethyl-N,N'-ethylenebis(salicylideneamine) dianion, gamma-pic = gamma-picoline. Crystal data for I (CoC(26)N(3)O(2)H(30)): space group P2(1)/c with a = 6.661(5) ?, b = 18.612(2) ?, c = 19.533(3) ?, beta = 98.93(1) degrees, V = 2392.10 ?(3), D(calcd) = 1.320 g.cm(-3), Z = 4, and R = 0.048 for 4469 measured reflections. Crystal data for II (CoC(25)N(3)O(2)H(28)): space group P2(1)/c, a = 9.609(6) ?, b = 19.169(8) ?, c = 12.995(9) ?, beta = 106.9(7) degrees, V = 2290.4 ?(3), D(calcd) = 1.332 g.cm(-1), Z = 4, and R = 0.048 for 4358 measured reflections. Crystal data for III (CoC(22)N(2)O(2)H(27)): space group P2(1)/c, a = 8.318(3) ?, b = 21.579(2) ?, c = 11.572(2) ?, beta = 93.35(1) degrees, V = 2073.7 ?(3), D(calcd) = 1.314 g.cm(-1), Z = 4, and R = 0.060 for 3954 measured reflections. The crystal structure data reveal that complexes I and II display six-coordinate octahedral geometry; their Co-C, Co-N bond lengths, as well as the Co-C-C angles, are very close to those in 5'-deoxyadenosylcobalamin. Complex III is one of the very few compounds having five-coordinate square pyramidal geometry and observed instability of the Co-C bond. 相似文献
999.
Wang C Zhu G Li J Cai X Wei Y Zhang D Qiu S 《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,11(17):4975-4982
We demonstrate a novel strategy for the preparation of mesoporous silica-supported, highly dispersed, stable metal and bimetal nanoparticles with both size and site control. The supporting mesoporous silica, functionalized by polyaminoamine (PAMAM) dendrimers, is prepared by repeated Michael addition with methyl acrylates (MA) and amidation reaction with ethylenediamine (EDA), by using aminopropyl-functionalized mesoporous silica as the starting material. The encapsulation of metal nanoparticles within the dendrimer-propagated mesoporous silica is achieved by the chemical reduction of metal-salt-impregnated dendrimer-mesoporous silica by using aqueous hydrazine. The site control of the metal or bimetal nanoparticles is accomplished by the localization of inter- or intradendrimeric nanoparticles within the mesoporous silica tunnels. The size of the encapsulated nanoparticles is controlled by their confinement to the nanocavity of the dendrimer and the mesopore. For Cu and Pd, particles locate at the lining of mesoporous tunnels, and have diameters of less than 2.0 nm. For Pd/Pt, particles locate at the middle of mesoporous tunnels and have diameters in the range of 2.0-4.2 nm. The Pd and Pd/Pt nanoparticles are very stable in air, whereas the Cu nanoparticles are stable only in an inert atmosphere. 相似文献
1000.
Density functional theory calculations have been used to investigate the chemisorption of H, S, SH, and H(2)S as well as the hydrogenation reactions S+H and SH+H on a Rh surface with steps, Rh(211), aiming to explain sulfur poisoning effect. In the S hydrogenation from S to H(2)S, the transition state of the first step S+H-->SH is reached when the S moves to the step-bridge and H is on the off-top site. In the second step, SH+H-->H(2)S, the transition state is reached when SH moves to the top site and H is close to another top site nearby. Our results show that it is difficult to hydrogenate S and they poison defects such as steps. In order to address why S is poisoning, hydrogenation of C, N, and O on Rh(211) has also been calculated and has been found that the reverse and forward reactions possess similar barriers in contrast to the S hydrogenation. The physical origin of these differences has been analyzed and discussed. 相似文献