全文获取类型
收费全文 | 36779篇 |
免费 | 1254篇 |
国内免费 | 11篇 |
专业分类
化学 | 20586篇 |
晶体学 | 162篇 |
力学 | 768篇 |
数学 | 6090篇 |
物理学 | 10438篇 |
出版年
2023年 | 198篇 |
2021年 | 279篇 |
2020年 | 471篇 |
2019年 | 306篇 |
2018年 | 421篇 |
2017年 | 405篇 |
2016年 | 945篇 |
2015年 | 806篇 |
2014年 | 899篇 |
2013年 | 2005篇 |
2012年 | 1528篇 |
2011年 | 1874篇 |
2010年 | 948篇 |
2009年 | 777篇 |
2008年 | 1794篇 |
2007年 | 1769篇 |
2006年 | 1558篇 |
2005年 | 1373篇 |
2004年 | 1084篇 |
2003年 | 928篇 |
2002年 | 879篇 |
2001年 | 733篇 |
2000年 | 659篇 |
1999年 | 445篇 |
1998年 | 400篇 |
1997年 | 399篇 |
1996年 | 503篇 |
1995年 | 451篇 |
1994年 | 469篇 |
1993年 | 551篇 |
1992年 | 453篇 |
1991年 | 342篇 |
1990年 | 382篇 |
1989年 | 312篇 |
1988年 | 323篇 |
1987年 | 327篇 |
1986年 | 295篇 |
1985年 | 468篇 |
1984年 | 453篇 |
1983年 | 421篇 |
1982年 | 431篇 |
1981年 | 438篇 |
1980年 | 400篇 |
1979年 | 363篇 |
1978年 | 403篇 |
1977年 | 383篇 |
1976年 | 336篇 |
1975年 | 319篇 |
1974年 | 307篇 |
1973年 | 316篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
Zusammenfassung Das Zündverhalten von mehr als 20 organylsilylsubstituierten Hydrazinen mit conc. Salpetersäure, Fluor, flüssigem Sauerstoff, mit O2 verdünntem gasförmigen Ozon und mit verflüssigtem O3 wurde beobachtet und im Falle HNO3 auch mit einer schnell-laufenden Kamera gemessen. Die wesentlichsten Resultate können aus Tab. 1 und den Abb. 1 bis 4 entnommen werden.
Mit 4 Abbildungen
60. Mitt.:U. Wannagat undF. Höfler, Mh. Chem.97, 976 (1966).
Zugleich 11. Mitt. über Silicium—Hydrazin-Verbindungen; 10. Mitt.:H. Bürger undF. Höfler, Mh. Chem.97, 984 (1966).
Auszüge aus der DiplomarbeitF. Höfler, T. H. Graz 1964. 相似文献
The hypergolic behaviour of more than 20 organosilyl substituted hydrazines in contact with conc. HNO3, with gaseous F2, with liquid O2, with liquid O3 and with gaseous O3 (diluted with O2), was observed and, in the case of conc. HNO3, was registered by a fastrunning camera. The most interesting results are to be seen in table 1 and in pictures 1 to 4.
Mit 4 Abbildungen
60. Mitt.:U. Wannagat undF. Höfler, Mh. Chem.97, 976 (1966).
Zugleich 11. Mitt. über Silicium—Hydrazin-Verbindungen; 10. Mitt.:H. Bürger undF. Höfler, Mh. Chem.97, 984 (1966).
Auszüge aus der DiplomarbeitF. Höfler, T. H. Graz 1964. 相似文献
992.
Göran Gellerstedt Eva-lisa Lindfors Maria Pettersson Danielle Robert 《Research on Chemical Intermediates》1995,21(3-5):441-456
The reactions between chlorine dioxide and the residual lignin in oxygen-bleached softwood kraft pulps have been studied. In a first series, isolated lignin samples have been subjected to chlorine dioxide oxidation at different pH values and subsequently analysed by oxidative degradation and elemental analysis. Different analytical techniques have also been employed to follow the gradual chemical changes in lignins isolated from kraft pulps after each of the bleaching stages in the OD(EOP)DD sequence. The results demonstrate that, in order to minimize chlorination of the lignin, the first chlorine dioxide stage should be carried out at a pH around or above three. At this pH level, a high degree of lignin oxidation is also achieved. A certain (mono)-chlorination of the lignin in the first D stage cannot be avoided, but this chlorine is to a large extent removed in the later bleaching stages. The efficient and non-selective oxidation of the various phenolic lignin end groups by chlorine dioxide is clearly illustrated by the analytical data. Moreover,13C NMR reveals that reduced lignin structures formed during the kraft cook survive the oxidative bleaching stages to a large extent. 相似文献
993.
Zusammenfassung Versuche über die Adsorption von Radioblei an Glas zeigen, daß aus wäßrigen Lösungen aufgenommene Bleiionen in zwei verschiedenen Zuständen am Glas vorliegen — einem lockeren Adsorptions-Zustand, der auf Ionenaustauschadsorption beruht, und einer festen, vorwiegend homöopolaren Bindung an das Glasnetzwerk. Weitere Versuche gaben Aufschluss über den Mechanismus der Aufnahme in die beiden Zustände. Die Messung der Adsorption und Desorption von Radioblei ist zur Untersuchung von Glasoberflächen geeignet: So ist es — wie durch Versuche an Natronglas und Quarzglas gezeigt wird —möglich, charakteristische Unterschiede zwischen den Oberflächen verschiedener Gläser festzustellen. Weiters erhält man für Glasproben gleicher Zusammensetzung Einblick in die chemische und thermische Vorgeschichte der Oberflächen. Schließlich liefert die Ablösung von Radioblei von Gläsern unter gewissen Bedingungen Aussagen über den Angriff von Lösungen auf das Glas. Auch Wismut kann an Glasoberflächen sehr fest gebunden werden.Herrn Prof. Dr.L. Ebert in Verehrung zum 60. Geburtstag gewidmet. 相似文献
994.
Iridium(III or IV) reacts with dithizone on heating at pH 6 to form a 1∶2 complex, which can be concentrated by extraction into chloroform. Based on this reaction, a reproducible, selective determination of iridium (1 ppb) is achieved by means of substoichiometric isotope dilution analysis based on192Ir. 相似文献
995.
996.
A procedure is described for the determination of the total concentration of organophosphorus pesticides in aqueous solution based on conversion of the compounds to orthophosphoric acid by oxygen-flask combustion and spectrophotometric determination as phosphomolybdenum blue. The procedure is particularly sensitive when preceded by efficient and selective accumulation of pesticides. The adsorption of malathion, parathion and phosalone on Amberlite XAD-4 resin from 1 l of 7–36 μM water solutions, followed by combustion of 500 μl of eluate yields of orthophosphoric acid with 90–105% recoveries. The detection limit is ca. 150 ng ml?1 pesticide in an aqueous sample. Inorganic phosphates and most of some commonly present non-pesticide phosphorus compounds are eliminated from the water sample during the adsorption step. In combination with a test for monitoring traces of cholinesterase inhibitors in surface waters, the determination of the total amount of organophosphorus pesticides as a cumulative parameter offers reliable information on the pollution level of the water environment. Application of such a combined procedure to a pond water from an apple orchard showed good correlation between phosphrus content and the cholinesterase inhibition. 相似文献
997.
The reaction behavior of NaN3, AgN3, and Me3SiN3 towards FNO2, CINO2, NO2SbF6, and NO2BF4 was investigated. At -30°C or below in a solvent-free system sodium azide did not react with CINO2, NO2BF4, or NO2SbF6. Below -30°C silver azide did not react either with neat C1NO2. Treatment of Me3SiN3 with pure C1NO2 led to the formation of C1N3, N2O, and Me3SiOSiMe3. A mechanism for this reaction has been proposed. Pure chlorine azide was isolated by fractional condensation and identified by its low-temperature Raman spectrum (liquid state). The reaction of Cp2Ti(N3)2 with C1NO2 also yielded C1N3 as the only azide-containing reaction product. Treatment of FNO2 with NaN3 at temperatures as low as -78°C always ended in an explosion which was probably due to the formation of FN3 as one of the reaction products. The reaction of NO2SbF6 with NaN3 in liquid CO2 (-55°C· T· -35°C) as the solvent afforded a new azide species which was stable at low temperature in solution only and was investigated by means of low-temperature Raman spectroscopy. The obtained vibrational data give strong evidence for the presence of tetranitrogen dioxide, N4O2, which can be regarded as nitryl azide (NO2N3). The structure and vibrational frequencies of N4O2 were computed ab initio at correlated level (MP2/6-31 + G*). In liquid xenon (-100°C· T· -60°C) NaN3 did not react with NO2SbF6. A previous literature report on the preparation of N4O2 could not be established. 相似文献
998.
N. Vajda J. Larosa T. Pintér G. Keömley D. Bódizs Zs. Molnár 《Journal of Radioanalytical and Nuclear Chemistry》1993,170(2):399-409
A simple and effective method has been developed for the determination of the transuranium isotopes in primary coolant samples of WWER-440 type reactors. Membrane filters containing undissolved particles from about one liter coolant were destroyed by mineral acids, then americium-curium isotopes and neptunium-plutonium isotopes were coprecipitated sequentially with NdF3 based on their different redox behavior. Alpha-spectra were determined by silicon semiconductor detectors connected to multichannel analyzers. Activity ratios measured in a reactor unit with a failed fuel element were compared with calculated values of the same type of reactor under similar conditions. Parameters of the defective fuel (burnup, original enrichment) could be estimated. 相似文献
999.
The four epimeric azido alcohols of estra-1,3,5(10)-trien-3-methyl ether with nitrogen at C-16 and oxygen at C-17 were prepared by the following reactions: cleavage of the 16α,17α-epoxide 1 with sodium azide affords the 16β,17α-azido alcohol 2a. The analogous reaction of the 16β,17β-epoxide 4 gives the 17α,16β-azido alcohol 5a and the desired 16α,17β-azido alcohol 6a in low yield. 6a is obtained in a smooth reaction by substitution of the 16β,17β-bromohydrine 8 with sodium azide. Sodium borohydride reduction of the 16β-azido-17-ketone 9 yields the 16β,17β-azido alcohol 10a, reduction of 16α-azido-17-ketone 13 with lithium borohydride gives the 16α,17α-azido alcohol 14a. From the azido alcohols the corresponding amino alcohols 3a, 7a, 11a and 15a are prepared with hydrazine hydrate/Raney nickel. The amino alcohols give the acetic anhydride the corresponding acetylamino alcohols. The cis-amino alcohols 11a and 15a react with acetone to the corresponding oxazolidines 12 and 16. 相似文献
1000.
T. Lóránd F. Aradi Á. Szöllösy G. Tóth T. Kónya 《Monatshefte für Chemie / Chemical Monthly》1996,127(8-9):971-977
Summary The acid and base catalyzed isomerization of some tricyclic 2-pyrazolines with N-Carbamoyl-,N-thiocarbamoyl-and N-phenyl substituents was investigated. Starting fromcis ortrans 3-H, 3a-H diastereomers, equilibrium mixtures ofcis andtrans diastereomers were prepared which were separated and subsequently studied by1H NMR and13C NMR spectroscopy. A mechanism for the isomerization of the pyrazolines is suggested, supported by a deuterium exchange at C-3a.
Isomerisierung von einigen substituierten 4,5-Dihydropyrazolen
Zusammenfassung Die Isomerisierung einiger tricyclischer 2-Pyrazoline mit N-Carbamoyl-, N-Thiocarbamoyl-und N-Phenyl-substituenten unter saurer und basischer Katalyse wurde untersucht. Ausgehend von dencis odertrans 3-H,3a-H-Diastereomeren wurdencis- undtrans Gleichgewichtsgemische gewonnen, die getrennt und durch1H- und13C-NMR-Spektroskopie untersucht wurden. Ein Mechanismus für die Isomerisierung von Pyrazolinen wird vorgeschlagen, der durch den Deuteriumaustausch in Position 3a-C unterstützt wird.相似文献