全文获取类型
收费全文 | 16343篇 |
免费 | 264篇 |
国内免费 | 125篇 |
专业分类
化学 | 10049篇 |
晶体学 | 202篇 |
力学 | 438篇 |
数学 | 2725篇 |
物理学 | 3318篇 |
出版年
2020年 | 144篇 |
2019年 | 143篇 |
2018年 | 124篇 |
2017年 | 98篇 |
2016年 | 229篇 |
2015年 | 200篇 |
2014年 | 250篇 |
2013年 | 804篇 |
2012年 | 631篇 |
2011年 | 735篇 |
2010年 | 431篇 |
2009年 | 417篇 |
2008年 | 703篇 |
2007年 | 717篇 |
2006年 | 787篇 |
2005年 | 730篇 |
2004年 | 641篇 |
2003年 | 563篇 |
2002年 | 564篇 |
2001年 | 282篇 |
2000年 | 285篇 |
1999年 | 191篇 |
1998年 | 201篇 |
1997年 | 204篇 |
1996年 | 283篇 |
1995年 | 204篇 |
1994年 | 249篇 |
1993年 | 255篇 |
1992年 | 253篇 |
1991年 | 198篇 |
1990年 | 186篇 |
1989年 | 192篇 |
1988年 | 154篇 |
1987年 | 190篇 |
1986年 | 170篇 |
1985年 | 287篇 |
1984年 | 272篇 |
1983年 | 187篇 |
1982年 | 218篇 |
1981年 | 237篇 |
1980年 | 264篇 |
1979年 | 229篇 |
1978年 | 235篇 |
1977年 | 192篇 |
1976年 | 219篇 |
1975年 | 214篇 |
1974年 | 224篇 |
1973年 | 219篇 |
1972年 | 104篇 |
1971年 | 106篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
161.
The focus of this paper is on understanding the exchange between dissolved fats and fat crystals in dispersions by quantifying the rate of the exchange between the fats in solution and in crystals. In the studies reported here, tripalmitin (PPP) was used as model fat. The dispersions contained PPP crystals and dissolved radio-labelled PPP (14C-PPP, used as the probe) in a medium chain triglyceride oil (MCT). The exchange process between the dissolved 14C-PPP and unlabelled PPP crystals was followed using a radio-detector. The effect of the crystal concentration on the exchange rate was studied. The results suggested that the rate of the exchange is dependent upon both the dissolution of the surface of the crystals and also diffusion and crystallisation of the dissolved compounds. 相似文献
162.
The application of DTA to fluxed melts is discussed, and an apparatus for the determination of the solubility of crystalline materials in molten solutions is described. An exploratory study of the crystallization of nickel ferrite from barium borate fluxes is also described, and solubility curves are presented for fluxes which favour crystal growth. The heat of solution of nickel ferrite was found to be 73 ± 2 Kjoule mole–1.
Zusammenfassung Es wurde über die Anwendung der DTA Methode bei der Untersuchung von Flußmittel enthaltenden Schmelzen berichtet und ein Apparat zur Bestimmung der Löslichkeit von kristallinischen Substanzen in Flüssen beschrieben. Eingehend wurden die Kristallisation von Nickelferrit aus Bariumborat-Flüssen untersucht und Löslichkeitskurven für den Kristallwachstum begünstigende Flüssen gegeben. Die Lösungswärme von Nickelferrit betrug 73 ± 2 Kjoule Mol–1.
Résumé On a décrit l'application de la méthode ATD à l'étude des fusions avec fondant et un appareil adapté à la détermination de la solubilité des substances crystallines dans les fondants. On a étudié la cristallisation de ferrite de nickel des flux de borate de baryum. Des courbes de solubilité pour des flux favorisant les grossissement des cristaux sont présentées. La chaleur de solution du ferrite de nickel a été trouvée 73 ± 2 kjoule mole–1.
A ( ). , . 73±2 –1.相似文献
163.
Reactive 1,6-disubstituted perfluorohexanes have been prepared the reaction of 1,6-(dimethylhydrosilyl)-perfluorohexanes with alkyllithium and Grignard reagents. The resulting species may be derivatized in the same manner as organolithium reagents, but are appreciably more stable than the 1,6-dilithioperfluorohexane which is presumably generated by the halogen-metal exchange between an alkyllithium reagent and Br(CF2)6Br. 1,6-bis(Bromomagnesium)-perfluorohexane was also prepared in good yield by a halogen-metal exchange reaction. 相似文献
164.
3-Mercapto-2(1H)-pyridinone (1) can be synthesized in three simple high-yielding steps from readily available 2-tert-butylthiazolo[4,5-b]pyridine (2). Its disodium salt condenses with o-chloronitrobenzene, 2-chloro-3-nitropyridine, and 3-chloro-4-nitropyridine 1-oxide to give respectively 4-azaphenoxathiine (10), 1,6-diazaphenoxathiine (12), and 2,6-diazaphenoxathiine 2-oxide (14) which reduces to 2,6-diazaphenoxathiine (15). The structures of these previously unreported azaphenoxathiine systems were confirmed by assignment of their respective (13)C NMR spectra. 相似文献
165.
James Grimshaw Charles S. Sell Reginald J. Haslett 《Journal of mass spectrometry : JMS》1974,8(1):381-386
The mass spectra of some methoxy and methyl derivatives of 2-methylbenzophenone have been examined. Loss of a substituents from 3′-and 4′-positions as well as the previously known loss from 2′-positions are important fragmentation processes. Thus, these fragmentations are of little use in locating substituents on benzophenones of unknown structure. Deuterium labelling shows the [M - 1]+ ion from 3′,4,4′,5,5′-pentamethoxy-2-methyl benzophenone to be due largely to loss of hydrogen from 2′- and 6′-positions. 相似文献
166.
Esahak Ali Ajit K. Chakravarty Satyesh C. Pakrashi Klaus Biemann Charles E. Hignite 《Tetrahedron》1977,33(11):1371-1377
The structure of solanoforthine, m.p. 208–10°, [α]D- 26.6°, a new steroidal alkaloid isolated from Solanum seaforthianum Andr., has been established as 3β-amino-22,26-epimino-16α,23-epoxy-22αH,25βH-cholest- 5- en -23β - ol 3, based on chemical and spectroscopic evidence and by its reduction to solanocapsine 2, another steroidal alkaloid encountered in the same species. The mass spectra of2 and 3 are discussed. 相似文献
167.
To facilitate the use of225Ra as a yield tracer in radiochemical assays of226Ra and228Ra, growth and decay tables for225Ra, and its daughter225Ac, have been computed. 相似文献
168.
William B. Smith 《Journal of heterocyclic chemistry》1987,24(3):745-748
The reduction of aryl nitro groups by the palladium-sodium borohydride system can be controlled in some instances to give products differing from those of catalytic hydrogenation. With six equivalents of borohydride added to the catalyst 2,2′-dinitrobiphenyl formed only 2,2′-diaminobiphenyl. With added sodium hydroxide and varying amounts of borohydride and catalyst, the reduction can be controlled to give benzo[c]cinnoline, benzo[c]cinnoline 5-oxide or benzo[c]cinnoline 5,6-dioxide. In a closed system, the reagents reduce both the nitro and olefin functional groups in 6-nitro-5,8-dimethoxy-1,4-dihydro-1,4-ethano-naphthalene. In an open flask flushed with argon, the reduction is confirmed to the nitro group. Reduction of 2-chloro and 4-chloronitrobenzene with palladium-borohydride gives substantial yields of the appropriate chloroanilines. In contrast, hydrogen and palladium give aniline as the major product from each of these. These results suggest that the addition of sodium borohydride to palladium on carbon produces a reductive entity differing from that of catalytic hydrogenation. 相似文献
169.
Niu W O'Sullivan C Rambo BM Smith MD Lavigne JJ 《Chemical communications (Cambridge, England)》2005,(34):4342-4344
The molecular weight of poly(dioxaborolane)s can be controlled during the polymerization reaction or through post-polymerization processing in such a manner that hydrolytic damage to these materials may be repaired, thereby regenerating the polymer. 相似文献
170.
SCF-MO studies as FeS, CrS and ZnS illustrate the changing role ofd-orbitals along a transition series, where they become at first good acceptors playing an important role in the bonding, then so stable that their electrons behave essentially as “core” electrons in agreement with the conjecture of Phillips and Williams [1]. 相似文献