全文获取类型
收费全文 | 2422篇 |
免费 | 357篇 |
国内免费 | 252篇 |
专业分类
化学 | 1925篇 |
晶体学 | 17篇 |
力学 | 125篇 |
综合类 | 28篇 |
数学 | 170篇 |
物理学 | 766篇 |
出版年
2024年 | 12篇 |
2023年 | 38篇 |
2022年 | 85篇 |
2021年 | 97篇 |
2020年 | 103篇 |
2019年 | 100篇 |
2018年 | 57篇 |
2017年 | 66篇 |
2016年 | 77篇 |
2015年 | 102篇 |
2014年 | 135篇 |
2013年 | 166篇 |
2012年 | 196篇 |
2011年 | 202篇 |
2010年 | 153篇 |
2009年 | 138篇 |
2008年 | 154篇 |
2007年 | 144篇 |
2006年 | 114篇 |
2005年 | 92篇 |
2004年 | 83篇 |
2003年 | 75篇 |
2002年 | 70篇 |
2001年 | 77篇 |
2000年 | 79篇 |
1999年 | 57篇 |
1998年 | 61篇 |
1997年 | 47篇 |
1996年 | 46篇 |
1995年 | 34篇 |
1994年 | 20篇 |
1993年 | 25篇 |
1992年 | 15篇 |
1991年 | 24篇 |
1990年 | 9篇 |
1989年 | 11篇 |
1988年 | 11篇 |
1987年 | 12篇 |
1986年 | 5篇 |
1985年 | 4篇 |
1984年 | 2篇 |
1983年 | 6篇 |
1978年 | 4篇 |
1977年 | 3篇 |
1976年 | 2篇 |
1974年 | 2篇 |
1966年 | 2篇 |
1931年 | 1篇 |
1924年 | 1篇 |
1884年 | 1篇 |
排序方式: 共有3031条查询结果,搜索用时 0 毫秒
31.
J. V. Crivello Daoshen Bi 《Journal of polymer science. Part A, Polymer chemistry》1993,31(12):3109-3119
Silicon-containing epoxide compounds bearing Si ? H groups can be readily prepared in high yields by the regioselective rhodium-catalyzed monohydrosilation of α,ω-dihydrogen functional siloxanes and silanes with vinyl epoxides. The remaining Si ? H groups in these compounds can be further selectively hydrosilated with unsaturated epoxides to give a series of unique ambifunctional monomers containing two different epoxide groups in the same molecule. The photopolymerization of these monomers has been studied using analytical techniques including real time infrared spectroscopy and differential scanning photocalorimetry. On photopolymerization, the new monomers yield interesting networks. © 1993 John Wiley & Sons, Inc. 相似文献
32.
Paul De Bièvre 《Accreditation and quality assurance》2000,5(10-11):423-428
Talking about "traceability" means talking about a "property of the result of a measurement", about "the value of a standard", about "stated references" and about an "unbroken chain of comparisons". It describes by which comparison, and to which other value, the result of a measurement has been obtained, i.e. is "traceable to". It is about the underlying structure of the measurement process of the result of a measurement and therefore about the authority of the result. Since values carried by (certified) reference materials have also been obtained by measurement, the definition of traceability equally applies. Traceability in the context of reference materials is also about the authority of the values carried by the (certified) reference materials and is, therefore, of key importance for the authority of the reference materials themselves. Hence, values of results of measurements constitute part of the traceability chain and their uncertainties are an intrinsic accompanying phenomenon. Uncertainties need a traceability chain against which they can be evaluated, and a traceability chain is an a priori requirement for evaluating the uncertainty budget of a measurement result. An attempt has been made to exemplify "traceability" chains in some types of chemical measurement and to identify the degree of international agreement on the key elements of "traceability". It is concluded that there is less than universal agreement on this issue. The debate should continue in order to arrive at the international understanding and agreement needed, as "traceability" is now being incorporated in the International Organization for Standardization (ISO), the International Laboratory Accreditation Co-operation (ILAC) and in other "guiding" or regulatory documents. It is also the reason why the Institute for Reference Materials and Measurements (IRMM) has taken up the study of the concept in its core programme on Metrology in Chemistry, and why it sponsored the Workshop in Bratislava. 相似文献
33.
34.
[reaction: see text] A facile and efficient synthetic route toward highly substituted 2,3-dihydrothiopyran-4-ones 2 has been developed via a formal [5C + 1S] annulation of readily available alpha-alkenoyl ketene-(S,S)-acetals 1 with sodium sulfide nonahydrated salt (Na2S x 9H2O) and utilized in the synthesis of 2-(4-chlorophenyl)-6-(morpholin-4-yl)-4H-thiopyran-4-one 5l, an inhibitor of DNA-dependent protein kinase (DNA-PK). 相似文献
35.
RU Chang Bi Zbigniew Dauter Eleanor Dodson Guy Dodson Federico Gordiano Rod Hubbard Colin Reynolds 《Journal of Chemical Sciences》1983,92(4-5):473-483
Insulin modified by the removal of its 5 B chain C terminal residues is monomeric but remains substantially potent. The crystal
structures of the beef and insulin (dpi) with two molecules in the asymmetric unit has been determined by x-ray analysis. The 3-dimensional structure ofdpi proves to be generally similar to that of native molecule in 2Zn insulin. More detailed comparison reveals that the slight
differences in the two independent molecules of beefdpi are distributed uniformly throughout the structure in contrast to insulin in 2Zn insulin, where the structural changes are
concentrated in specific regions.
The loss of symmetry in thedpi crystal appears to be the inability of the A9 serine to pack effectively in the C2 cell. The efficient packing of the sheepdpi molecule whose crystal structure has also been determined and where A9 is glycine supports this conclusion. 相似文献
36.
A new fertilizer of cocrystal type,known as durably efficacious ammonium bicarbonate (DEAB),has been developed by adding a certain amount of dicyandiamide (DCD) as an ammonia-stabilizing agent to ammonium bearbenate(AB)during the process of its production.As compared with AB,DEAB was found to have a reduction of direct volatilization loss by 53%,a fertilizer availability period prolonged from 35-45 to 90-110 d,and an increase in the rate of nitrogen in fertilizer being utilized by 5.9%-10.2%,and a saving of the amount of fertilizer to be ap-phed by 20%-30% for the same level of yield,or an increase of the crop yield by over 10% for the same level of ni-tregen fertilization;in addition,it was found to show usually a function of promoting the crop to early mature.It can be apptied as basal dressing all in one time to soil and thus also used as a labour-saving and crop yield-increasing fertil-izer for is non-mtertillage,plastics film covering and water-saving agriculture 相似文献
37.
Non-negative matrix factorization (NMF), with the constraints of non-negativity, has been recently proposed for multi-variate data analysis. Because it allows only additive, not subtractive, combinations of the original data, NMF is capable of producing region or parts-based representation of objects. It has been used for image analysis and text processing. Unlike PCA, the resolutions of NMF are non-negative and can be easily interpreted and understood directly. Due to multiple solutions, the original algorithm of NMF [D.D. Lee, H.S. Seung, Nature 401 (1999) 788] is not suitable for resolving chemical mixed signals. In reality, NMF has never been applied to resolving chemical mixed signals. It must be modified according to the characteristics of the chemical signals, such as smoothness of spectra, unimodality of chromatograms, sparseness of mass spectra, etc. We have used the modified NMF algorithm to narrow the feasible solution region for resolving chemical signals, and found that it could produce reasonable and acceptable results for certain experimental errors, especially for overlapping chromatograms and sparse mass spectra. Simulated two-dimensional (2-D) data and real GUJINGGONG alcohol liquor GC-MS data have been resolved soundly by NMF technique. Butyl caproate and its isomeric compound (butyric acid, hexyl ester) have been identified from the overlapping spectra. The result of NMF is preferable to that of Heuristic evolving latent projections (HELP). It shows that NMF is a promising chemometric resolution method for complex samples. 相似文献
38.
The lack of reference materials, accurately certified for nitrite, is a problem in view of the importance of this species for environmental and medical reasons. This work outlines a plan for the preparation of nitrite isotopic reference materials (IRMs) in the form of high purity solutions, certified for their nitrite-nitrogen isotopic composition and nitrite concentration. To achieve the desired accuracy (expanded uncertainty U with a coverage factor k=2 of =2%), primary methods of measurement such as isotope dilution mass spectrometry (IDMS), gravimetry, and titrimetry must be used. The main difficulty is the stability of nitrite. Other problems expected in the preparation and certification of nitrite IRMs are described. Results from long term stability studies (up to 1.5 years) and a procedure for the purification of the candidate nitrite IRMs are presented. The purpose is to use these IRMs for high accuracy method calibrations and as anchor points for SI-traceable nitrite concentrations. Reference values linked to the SI system are useful to demonstrate the degree of international comparability of nitrite measurements in intercomparison programmes such as the IRMM-International Measurement Evaluation Programme (IMEP). 相似文献
39.
Chun‐Bo Liu Guang‐Bo Che Chuan‐Bi Li Yun‐Cheng Cui 《Acta Crystallographica. Section C, Structural Chemistry》2006,62(4):m153-m155
An in situ reaction under hydrothermal conditions leads to the formation of the title compound, diaqua(pyridine‐2‐carboxylato)(pyridine‐2,6‐dicarboxylato)indium(II) trihydrate, [In(C6H4NO2)(C7H3NO4)(H2O)2]·3H2O, in which the central InIII atom is seven‐coordinated by one pyridine‐2,6‐dicarboxylate ligand, one pyridine‐2‐carboxylate ligand and two water molecules in a pentagonal–bipyramidal coordination environment. An indium(III)–water chain based on an unusual water pentamer is observed. 相似文献
40.
Yao‐Cheng Shi Bei‐Bei Zhu Xiao‐Bi Jing 《Acta Crystallographica. Section C, Structural Chemistry》2006,62(11):m559-m562
The molecules of N,N′‐bis(2‐pyridylmethyl)ferrocene‐1,1′‐diyldicarboxamide, [Fe(C12H11N2O)2], contain intramolecular N—H⋯N hydrogen bonds and are linked into sheets by three independent C—H⋯O hydrogen bonds. The molecules of the isomeric compound N,N′‐bis(3‐pyridylmethyl)ferrocene‐1,1′‐diyldicarboxamide lie across inversion centres, and the molecules are linked into sheets by a combination of N—H⋯N hydrogen bonds and π–π stacking interactions between pyridyl groups. 相似文献