全文获取类型
收费全文 | 36108篇 |
免费 | 7020篇 |
国内免费 | 5739篇 |
专业分类
化学 | 26823篇 |
晶体学 | 576篇 |
力学 | 2329篇 |
综合类 | 395篇 |
数学 | 4013篇 |
物理学 | 14731篇 |
出版年
2024年 | 107篇 |
2023年 | 658篇 |
2022年 | 1305篇 |
2021年 | 1364篇 |
2020年 | 1473篇 |
2019年 | 1530篇 |
2018年 | 1324篇 |
2017年 | 1362篇 |
2016年 | 1788篇 |
2015年 | 1978篇 |
2014年 | 2396篇 |
2013年 | 2988篇 |
2012年 | 3487篇 |
2011年 | 3522篇 |
2010年 | 2617篇 |
2009年 | 2678篇 |
2008年 | 2897篇 |
2007年 | 2406篇 |
2006年 | 2233篇 |
2005年 | 1800篇 |
2004年 | 1346篇 |
2003年 | 1129篇 |
2002年 | 1144篇 |
2001年 | 926篇 |
2000年 | 748篇 |
1999年 | 616篇 |
1998年 | 444篇 |
1997年 | 402篇 |
1996年 | 354篇 |
1995年 | 286篇 |
1994年 | 289篇 |
1993年 | 224篇 |
1992年 | 175篇 |
1991年 | 143篇 |
1990年 | 139篇 |
1989年 | 88篇 |
1988年 | 80篇 |
1987年 | 61篇 |
1986年 | 70篇 |
1985年 | 47篇 |
1984年 | 24篇 |
1983年 | 28篇 |
1982年 | 13篇 |
1981年 | 14篇 |
1980年 | 9篇 |
1979年 | 19篇 |
1978年 | 7篇 |
1977年 | 11篇 |
1957年 | 8篇 |
1925年 | 10篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
961.
Speciation of chromium by in-capillary reaction and capillary electrophoresis with chemiluminescence detection 总被引:3,自引:0,他引:3
A sensitive method for the simultaneous determination of chromium(III) (Cr3+) and chromium(VI) (CrO4(2-)) using in-capillary reaction, capillary electrophoresis (CE) separation and chemiluminescence (CL) detection was developed. The chemiluminescence reaction was based on luminol oxidation by hydrogen peroxide in basic aqueous solution catalyzed by Cr3+ ion followed by capillary electrophoresis separation. Based on in-capillary reduction, chromium(VI) can be reduced by acidic sodium hydrogensulfite to form chromium(III) while the sample is running through the capillary. Before the electrophoresis procedure, the sample (Cr3+ and CrO4(2-)), buffer and acidic sodium hydrogensulfite solution segments were injected in that order into the capillary, followed by application of an appropriate running voltage between both ends. As both chromium species have opposite charges, Cr3+ ions migrate to the cathode, while CrO4(2-) ions, moving in the opposite direction toward the anode, react with acidic sodium hydrogensulfite which results in the formation of Cr3+ ions. Because of the migration time difference of both Cr3+ ions, Cr(III) and Cr(VI) could be separated. The running buffer was composed of 0.02 mol l(-1) acetate buffer (pH 4.7) with 1 x 10(-3) mol l(-1) EDTA. Parameters affecting CE-CL separation and detection, such as reductant (sodium hydrogensulfite) concentration, mixing mode of the analytes with CL reagent, CL reaction reagent pH and concentration, were optimized. The limits of detection (LODs) of Cr(III) and Cr(VI) were 6 x 10(-13) and 8 x 10(-12) mol l(-1) (S/N=3), respectively. The mass LODs for Cr(III) and Cr(VI) were 1.2 x 10(-20) mol (12 zmol) and 3.8 x 10(-19) mol (380 zmol), respectively. 相似文献
962.
An electrochemical synthesis of oxazol‐2‐ones and imidazol‐2‐ones has been developed via 5‐exo‐dig cyclization of propargylic carbamates‐ and ureas‐derived amidyl radicals. The electrosynthesis relies on the dual function of 2,2,6,6‐tetramethylpiperidin‐ 1‐yl)oxyl (TEMPO) as a redox mediator for amidyl radical formation and an oxygen atom donor. The reactions are conducted under mild conditions using a simple setup and provide convenient access to functionalized oxazol‐2‐ones and imidazol‐2‐ones from readily available materials. 相似文献
963.
A study of the transport of Hg(II) ions through a tri-n-octylamine (TOA) - sorbital monooleate (Span 80) -toluene liquid membrane has been performed with varying concentrations of HCl, KCl, TOA, Span 80 and NaOH in the feed, membrane and stripping solutions. Maximum transport was observed with 0.01 M KCl, 2.5 x 10(-2) M HCl, 1.5 x 10(-2) M TOA, 3% (w/v) Span 80 and 0.05 M NaOH. With this system, mercury could be completely separated from Cu, Zn, Fe, Co, Ni, Pb, Mn and Cd. The transport mechanism of this metal ion through the membrane is based on the association of metal anions (HgCl(4)(2)) with protonated TOA molecules at the feedside interface, diffusion through the membrane, decomposition of the complex at the strip-solution-side membrane interface under alkaline conditions, and backdiffusion of TOA molecules. Transport with the membrane is dependent on the concentration gradient but in the surrounding solutions it is inversely related to the concentration gradient. 相似文献
964.
Reactions of [(dtc)2Mo2(S)2(μ-S)2] with one or two equivalents of CuBr in CH2Cl2 afforded two new heterobimetallic sulfide clusters, [(dtc)2Mo2(μ3-S)(μ-S)3(CuBr)] (1) and [(dtc)2Mo2(μ3-S)4(CuBr)2] (2). Both compounds were characterized by elemental analysis, IR, UV-vis and X-ray analysis. Compound 1 contains a butterfly-shaped Mo2S4Cu core in which one CuBr unit is coordinated by one bridging S and two terminal S atoms of the [(dtc)2Mo2(S)2(μ-S)2] moiety. In the structure of 2, one [(dtc)2Mo2(S)2(μ-S)2] moiety and two CuBr units are held together by six Cu-μ3-S bonds, forming a cubane-like Mo2S4Cu2 core. 相似文献
965.
A method for direct de termination of total in organic arsenic (III+V), arsenic (III) and dimethylarsinate (DMA) in sea water was developed by combining continuous‐flow selective hydride generation and inductively coupled plasma mass spectrometry (ICP‐MS) is presented. The principle underlying selective hydride generation is based on proper control of the reaction conditions for achieving separation of the respective arsenic species. The effects of pH and composition of reaction media on mutual interference between the arsenic species were investigated in detail. The results indicate that the appropriate media for the selective determination of total in organic arsenic, DMA and As(III) are 6 M HNO3, acetate buffer at pH = 4.63 and citrate buffer at pH = 6.54, respectively. The concentrations of total inorganic arsenic species, As(III+V), and As(III) were respectively deter mined and that of As(V) was obtained by the difference between them. As to the concentration of DMA, it was obtained after correction from the interference caused by As(III) and As(V). By following the established procedure, the detection lim its (as based on 3‐sigma criterion) for As(III+V), As(III) and DMA were 0.050, 0.009, and 0.002 ng/mL, respectively. There liability of the pro posed method was evaluated in terms of precision and spike addition. The results indicated that the precision of better than 3% and spike recovery of 95 to 105% for all the arsenic species tested in the natural sea water samples can be obtained. 相似文献
966.
J. H. Chao H. P. Lee C. L. Tseng C. Y. Chen Y. Y. Wei 《Journal of Radioanalytical and Nuclear Chemistry》2002,254(3):577-581
Evaluation of minimal detectable amounts (MDA) of iodine in various or specified irradiation and measurement conditions is described. Interfering contributions due to Na and Cl to the spectral background as well as the dead time of detection system were calculated in details based on an experimental approach. The MDA of iodine for some important biological samples with various Na and Cl concentrations can be evaluated and predicted according to the specified irradiation and measurement arrangements to meet the requirement of analytical purposes. 相似文献
967.
Tiantian Liu Xiaojie Chen Juan Zhao Weichun Wei Zhu Mao William Wu Shibo Jiao Yang Liu Zhiyong Yang Zhenguo Chi 《Chemical science》2021,12(14):5171
Pure organic emitters with full utilization of triplet excitons are in high demand for organic light-emitting diodes (OLEDs). Herein, through modulation of electron donors and introduction of phenyl rings as π spacers, we present three pure organic fluorophores (BCz, BTCz and BPTCz) with the hybridized local and charge-transfer (HLCT) excited state feature for OLED fabrication. Importantly, the introduction of π spacers in BPTCz not only enhances locally excited character with a fast radiative decay but also promotes intermolecular interactions to suppress non-radiative decays, contributing to a high solid-state fluorescence efficiency over 90%. Significantly, BPTCz not only endows its doped OLEDs with an external quantum efficiency (EQE) up to 19.5%, but also its non-doped OLED with a high EQE of 17.8%, and these outstanding efficiencies are the state-of-the-art performances of HLCT-based OLEDs.Three purely organic fluorophores with a hybridized local and charge-transfer excited state feature are presented and enabled organic light-emitting diodes with record high external quantum efficiencies close to 20%. 相似文献
968.
Anyun Zhang Yuezhou Wei Harutaka Hoshi Mikio Kumagai Masayoshi Kamiya Tomozo Koyama 《Radiation Physics and Chemistry》2005,72(6):669-678
The resistance of a novel silica-based N,N,N′,N′-tetraoctyl-3-oxapentane-1,5-diamide (TODGA) polymeric adsorption material (TODGA/SiO2-P) against nitric acid, temperature and γ-irradiation had been investigated. The adsorption property of the treated TODGA/SiO2-P was evaluated by a 3 M HNO3 solution containing 0.01 M Nd(III). It was found that both 3 and 0.01 M HNO3 concentrations did not decrease the stability of TODGA/SiO2-P at 25°C. The quantity of TODGA leaked from TODGA/SiO2-P was equivalent to its solubility in the corresponding HNO3 aqueous solution. The effect of 3 M HNO3 on the leakage of TODGA at 80°C was significantly higher than that in 0.01 M HNO3 as well as in all cases at 25°C. The amount of Nd(III) adsorbed towards the treated TODGA/SiO2-P was determined in the range of 0.143–0.148 mmol/g for the HNO3 concentration effect and 0.142–0.0506 mmol/g for the temperature effect. γ-Irradiation showed a more noticeable destruction effect on TODGA/SiO2-P. The content of TODGA leaked increased with an increase in the γ-irradiation dose (ID) from 1.06 to 3.72 MGy in terms of the linear equation [TODGA]=794.5ID+84.0. The amount of Nd(III) adsorbed onto the irradiated TODGA/SiO2-P decreased rapidly from 0.134 to 0.0438 mmol/g, which was lower than 0.153 mmol/g, the adsorption of fresh TODGA/SiO2-P for Nd(III), according to the equation QNd(III)=−0.0301ID+0.160, showing that a large quantity of TODGA leaked from TODGA/SiO2-P. The adsorbed amount of Nd(III) decreased obviously in this order: the HNO3 concentration effect, temperature effect and γ-irradiation. 相似文献
969.
Wei Li 《Tetrahedron》2007,63(45):11016-11020
We report herein stereoselective additions of a nitrene derived from PhI(OAc)2 and 2,4-dinitrophenylsulfenamide with methylenecyclopropanes (MCPs) and a methylenecyclobutane (MCB) to give the corresponding ring enlargement products, (cyclobutylidene)amide or (cyclopentylidene)amide derivatives, in moderate to excellent yields. The reaction mechanism has been discussed on the basis of control experiments and previous investigation. 相似文献
970.
Yibin Pan Lili Yan Qiaoqiao Chen Cheng Wei Yongdong Dai Xiaomei Tong Haiyan Zhu Meifei Lu Yanling Zhang Xiaoying Jin Tai Zhang Xiaona Lin Feng Zhou Songying Zhang 《Experimental & molecular medicine》2021,53(1):52
In early pregnancy, the placenta anchors the conceptus and supports embryonic development and survival. This study aimed to investigate the underlying functions of Shh signaling in recurrent miscarriage (RM), a serious disorder of pregnancy. In the present study, Shh and Gli2 were mainly observed in cytotrophoblasts (CTBs), Ptch was mainly observed in syncytiotrophoblasts (STBs), and Smo and Gli3 were expressed in both CTBs and STBs. Shh signaling was significantly impaired in human placenta tissue from recurrent miscarriage patients compared to that of gestational age-matched normal controls. VEGF-A and CD31 protein levels were also significantly decreased in recurrent miscarriage patients. Furthermore, inhibition of Shh signaling impaired the motility of JAR cells by regulating the expression of Gli2 and Gli3. Intriguingly, inhibition of Shh signaling also triggered autophagy and autolysosome accumulation. Additionally, knockdown of BECN1 reversed Gant61-induced motility inhibition. In conclusion, our results showed that dysfunction of Shh signaling activated autophagy to inhibit trophoblast motility, which suggests the Shh pathway and autophagy as potential targets for RM therapy.Subject terms: Embryology, Endocrine reproductive disorders, Autophagosomes 相似文献