全文获取类型
收费全文 | 39869篇 |
免费 | 3216篇 |
国内免费 | 2281篇 |
专业分类
化学 | 27639篇 |
晶体学 | 500篇 |
力学 | 1630篇 |
综合类 | 187篇 |
数学 | 3892篇 |
物理学 | 11518篇 |
出版年
2024年 | 52篇 |
2023年 | 401篇 |
2022年 | 906篇 |
2021年 | 1099篇 |
2020年 | 1013篇 |
2019年 | 1017篇 |
2018年 | 850篇 |
2017年 | 828篇 |
2016年 | 1435篇 |
2015年 | 1303篇 |
2014年 | 1684篇 |
2013年 | 2588篇 |
2012年 | 3110篇 |
2011年 | 3334篇 |
2010年 | 2262篇 |
2009年 | 1967篇 |
2008年 | 2638篇 |
2007年 | 2281篇 |
2006年 | 2297篇 |
2005年 | 2030篇 |
2004年 | 1722篇 |
2003年 | 1479篇 |
2002年 | 1493篇 |
2001年 | 991篇 |
2000年 | 892篇 |
1999年 | 707篇 |
1998年 | 495篇 |
1997年 | 468篇 |
1996年 | 518篇 |
1995年 | 417篇 |
1994年 | 385篇 |
1993年 | 373篇 |
1992年 | 310篇 |
1991年 | 260篇 |
1990年 | 247篇 |
1989年 | 202篇 |
1988年 | 155篇 |
1987年 | 130篇 |
1986年 | 112篇 |
1985年 | 137篇 |
1984年 | 90篇 |
1983年 | 75篇 |
1982年 | 84篇 |
1981年 | 86篇 |
1980年 | 43篇 |
1979年 | 54篇 |
1978年 | 49篇 |
1976年 | 45篇 |
1975年 | 37篇 |
1974年 | 38篇 |
排序方式: 共有10000条查询结果,搜索用时 10 毫秒
111.
A non-destructive neutron activation method was developed for the determination of silver and antimony in high-purity bismuth ; γ,γ-coincidence counting of 110mAg or 124Sb was applied. The activity of the matrix or other impurities did not interfere. When 1-g samples were irradiated at a neutron flux of 4·1011 n cm-2 sec-1 for 25 days, the sensitivity of the method was in the p.p.b. region. 相似文献
112.
The conversion of carbamates into ureas using aluminum amide complexes is reported. This reaction is a convenient method to prepare bi-, tri- and tetra-substituted ureas from carbamate-protected primary or secondary amines by reaction with primary or secondary amines in the presence of stoichometric quantities of trimethylaluminum. A reactivity trend of the various carbamates was observed and methyl and benzyl carbamates were reacted selectively in the presence of t-butyl carbamates. 相似文献
113.
A Chelex-100 resin column has been employed for the preliminary concentration of trace elements in water samples before their determination by neutron-activation analysis. The column, filled with a 1:1 mixture of the resin (50-100 mesh) and Pyrex glass powder of the same mesh-size, is shown to maintain a constant flow-rate and give reproducible results. By a combination of preconcentration and neutron-activation analysis it is possible to determine Ba, Ca, Cd, Ce, Co, Cr, Cu, Fe, La, Mg, Mn, Sc, U, V and Zn in sea-water and/or fresh water simultaneously at the parts per milliard level. 相似文献
114.
本文采用XRD,NH_3-TPD,IR以及TEM等方法考察了Pt/BaKL沸石中Ba~(2+)的作用。NH_3-TPD结果表明,交换Ba~(2+)后的L型沸石的酸性,由于阳离子的作用仅在弱酸范围内有些改变,但与载铂的沸石的催化活性无明显的对应关系。IR及XRD谱线表明,BaKL沸石中Ba~(2+)在最佳交换度(26.3%)时,随预培烧温度的提高,使Ba~(2+)处于沸石孔道中B、C、D位置,接近于最佳分布,此时IR谱线上1421cm~(-1)处有一新峰。当沸石负载铂后,因铂与Ba~(2+)的相互作用,新峰消失,导致铂粒在沸石上分布更加均匀;铂粒在1.5—3.0nm之间占有最大比例,使得Pt/BaKL沸石呈现芳构化活性及选择性高于Pt/KL沸石,分别为~98%和~95%。 相似文献
115.
The novel atrane-like six-coordinate (RO)(2)TaL complexes [where R = Me or Et and L = tris(2-oxy-3,5-dimethylbenzyl)amine] containing three six-membered rings have been synthesized and characterized. The R = Me complex is the first group 5 representative of this class of compounds structurally characterized by X-ray means. Somewhat surprisingly, these compounds failed to function as single-site initiators for the polymerization of l-LA to isotactic PLA and rac-LA to atactic PLA, whereas Ta(OEt)(5) and two titanium analogues ROTiL (where R = 2,6-di-i-PrC(6)H(3) and i-Pr) as well as Ti(O-i-Pr)(4) were effective catalysts for both polymerizations. 相似文献
116.
Novel thermoreversible physical hydrogels formed from polymers with linear and star architectures possessing a linear poly(ethyleneimine) (PEI) backbone have been investigated. The hydrogelation occurred simply upon natural cooling of hot aqueous solutions of PEIs to room temperature. The X-ray diffraction and differential scanning calorimetry measurements for the resultant hydrogels unambiguously indicated that the hydrogelation originated from the formation of dihydrate crystalline structures of PEI. These crystalline hydrogels are structurally unique and hierarchical. Microscopic images revealed that the morphologies of the crystalline hydrogels depend on their molecular architectures. The linear PEI resulted in branched fibrous bundles organized by unit crystalline nanofibers with a width of ca. 5-7 nm. The six-armed star with benzene ring core produced fanlike fibrous bundles while the four-armed star with porphyrin core assembled into asterlike aggregates. The critical concentration of gelation (C(G)) was low (about 0.2 approximately 0.3%) and the thermoreversible gel-sol transition temperatures (T(G)) were controllable from approximately 43 to approximately 79 degrees C. The hydrogels formed in the presence of the various aqueous additives including organic solvents, hydrophilic polymers, physical cross-linker, chemical cross-linker, and base enabling modification and functionalization during synthesis. The mechanical properties of the hydrogels could be improved by chemical cross-linking of preformed hydrogels by glutaraldehyde. Physically and physical/chemical cross-linked hydrogels served as excellent template roles in biomimetic silicification, which produced silica-PEI hybrid powder or monolith constructed by nanofibers. 相似文献
117.
In association with microtubules, a variety of kinesins play important roles in cellular functions such as intracellular transport of organelles or vesicles, signal transduction, and cell division. In a previous study we revealed that human kinesin superfamily protein member 4 (KIF4) is a chromokinesin that binds to chromosomes. Since localization of several kinds of kinesin at midzone called central spindle, or midbody that connects two daughter cells, or both, suggests their implication in cell division, we investigated KIF4 localization of during mitosis and cytokinesis in Hela cells. In addition to association with segregating chromosomes through entire mitosis, it also localized to the midzone and to midbody at ana/telophase through cytokinesis. Especially in cells at cytokinesis, KIF4 appeared as a doublet facing each other at the apical ends of two daughter cells. Three- dimensional analysis of architectural relationship between microtubule bundles and KIF4 indicated that KIF4 forms a ring structure wrapping around the microtubule bundles. These results suggest that KIF4 is involved in cytokinesis, although direct evidence was not provided in this study. 相似文献
118.
In contrast to lower phosphorylation states (e.g. the tryptic monophosphopeptide FQpSEEQQQTEDELQDK from bovine beta-casein), the specific detection of multi-phosphorylated peptides (e.g. the tetraphosphopeptide RELEELNVPGEIVEpSLpSpSpSEESITR from tryptic digestion of bovine beta-casein) has often been problematic for liquid chromatographic mass spectrometric (LC/MS) analysis owing to their high affinity for adsorption to exposed surfaces. We observed an enhancement in the overall detection of phosphopeptides on addition of phosphoric acid (0.1-1.0%) to the sample solution; a 10-fold increase in sensitivity was determined for the detection of two tryptic phosphopeptides and also a significant improvement in the detection of the tetraphosphopeptide. Using capillary LC with ion trap tandem MS for detection and identification, the achievable detection limits were 50 fmol and 50 pmol for the monophosphopeptide and the tetraphosphopeptide, respectively. Phosphoric acid is believed to act as a blocking agent to available silanol groups on both the silica capillary surface and the C(18)-bonded stationary phase silica surface. 相似文献
119.
GITR (glucocorticoid-induced TNF receptor) is a recently identified member of the TNF receptor superfamily. The receptor is preferentially expressed on CD4(+)CD25(+) regulatory T cells and GITR signals break the suppressive activity of the subset. In this study, we wanted to reveal the in vivo function of GITR in herpes simplex virus type 1 (HSV-1) infection. A single injection of anti-GITR mAb (DTA-1) immediately after viral infection significantly increased the number of CD4(+) and CD8(+) T cells expressing CD25, an activation surface marker, and secreting IFN-gamma. We confirmed these in vivo observations by showing ex vivo that re-stimulation of CD4(+) or CD8(+) T cells with a CD4(+) or CD8(+) T-cell-specific HSV-1 peptide, respectively, induced a significant elevation in cell proliferation and in IFN-gamma secretion. Our results indicate that GITR signals play a critical role in the T-cell immunity to HSV-1. 相似文献
120.
We report a series of experiments and a theoretical model designed to systematically define and evaluate the relative importance of nanoparticle, oligonucleotide, and environmental variables that contribute to the observed sharp melting transitions associated with DNA-linked nanoparticle structures. These variables include the size of the nanoparticles, the surface density of the oligonucleotides on the nanoparticles, the dielectric constant of the surrounding medium, target concentration, and the position of the nanoparticles with respect to one another within the aggregate. The experimental data may be understood in terms of a thermodynamic model that attributes the sharp melting to a cooperative mechanism that results from two key factors: the presence of multiple DNA linkers between each pair of nanoparticles and a decrease in the melting temperature as DNA strands melt due to a concomitant reduction in local salt concentration. The cooperative melting effect, originating from short-range duplex-to-duplex interactions, is independent of DNA base sequences studied and should be universal for any type of nanostructured probe that is heavily functionalized with oligonucleotides. Understanding the fundamental origins of the melting properties of DNA-linked nanoparticle aggregates (or monolayers) is of paramount importance because these properties directly impact one's ability to formulate high sensitivity and selectivity DNA detection systems and construct materials from these novel nanoparticle materials. 相似文献