全文获取类型
收费全文 | 4604篇 |
免费 | 120篇 |
国内免费 | 38篇 |
专业分类
化学 | 3035篇 |
晶体学 | 42篇 |
力学 | 104篇 |
数学 | 854篇 |
物理学 | 727篇 |
出版年
2022年 | 32篇 |
2021年 | 45篇 |
2020年 | 36篇 |
2019年 | 51篇 |
2018年 | 48篇 |
2017年 | 43篇 |
2016年 | 79篇 |
2015年 | 96篇 |
2014年 | 78篇 |
2013年 | 264篇 |
2012年 | 210篇 |
2011年 | 319篇 |
2010年 | 137篇 |
2009年 | 121篇 |
2008年 | 256篇 |
2007年 | 268篇 |
2006年 | 266篇 |
2005年 | 228篇 |
2004年 | 198篇 |
2003年 | 186篇 |
2002年 | 200篇 |
2001年 | 81篇 |
2000年 | 66篇 |
1999年 | 94篇 |
1998年 | 65篇 |
1997年 | 72篇 |
1996年 | 92篇 |
1995年 | 52篇 |
1994年 | 57篇 |
1993年 | 68篇 |
1992年 | 57篇 |
1991年 | 59篇 |
1990年 | 52篇 |
1989年 | 38篇 |
1988年 | 38篇 |
1987年 | 39篇 |
1986年 | 24篇 |
1985年 | 45篇 |
1984年 | 60篇 |
1983年 | 38篇 |
1982年 | 37篇 |
1981年 | 58篇 |
1980年 | 40篇 |
1979年 | 34篇 |
1978年 | 40篇 |
1977年 | 33篇 |
1976年 | 38篇 |
1975年 | 30篇 |
1974年 | 37篇 |
1972年 | 23篇 |
排序方式: 共有4762条查询结果,搜索用时 500 毫秒
51.
Conductance measurements at 25°C have been carried out on Pr4NBr, Bu4NBr,n-Pe4NPr, Pr4NI, Bu4NI,n-Pe4NI, Bu4NClO4, Bu4NBF4, and Bu4PClO4 in isobutyl alcohol and on Bu4NBr and Bu4NClO4 in isopentyl alcohol. The main purpose of the measurements was to investigate the effect of increased branching in the alkyl portion of the alcohol on both ionic mobilities and ionic association. Comparison is made with the two isodielectric solvents butanone andn-butyl alcohol. Branching appears to produce little change in ionic mobilities and approximately a 10% increase in association. 相似文献
52.
Wen-Ping Wang Robert G. Parr Danny R. Murphy George A. Henderson 《Chemical physics letters》1976,43(3):409-412
It is demonstrated that the ground-state atomic kinetic energy functional T[?], where ? is the electron density, can be computed to surprising accuracy from the truncated gradient expansion: T[?] = + T2[?] + T4[?], with To[?] = (3π2) ∫ ? dτ, T2 [?] = ∫ (??)2??1 dτ, and T4 [?] given by the formula of Hodges. Calculations of T0, T2 and T4 are reported for He with ? both the Hartree—Fock and a very accurate density, and for Ne, Ar and Kr with ? the Hartree—Fock density. For Kr, T0 + T2 + T4 is within 0.3% of the exact Hartree—Fock T, with T2/T0 = 0.05, T4/T2 = 0.17. 相似文献
53.
A non-destructive method for the quantification of eleven elements in bone samples is described. The analytical scheme is based on short (30 s) irradiation with thermal neutrons followed immediately (decay time 10 s) by counting fluorine-20 for 30 s and, after a total waiting time of 150 s, by 10-min γ-spectrometry counting, which give data for Ca, Cl, Mg, Mn, Na, and V. Use of a boron carbide shield for a second set of irradiations with epithermal neutrons permits the additional determination of bromine and strontium and calculation of the contribution of aluminium and phosphorus to the total activity of 28Al. 相似文献
54.
A heated injection system for a microprocessor-controlled GC has been developed for the (GC)2 analysis of the volatile phase of whole smoke of a cigarette. Effects of injection port temperature and the presence of a Cambridge filter pad are demonstrated. Chromatograms are shown for smoke samples with and without a Cambridge Filter with the sample valve oven at 25°, 165° and 205°C. The use of a flame ionization and a nitrogen-phosphorous detector is illustrated. 相似文献
55.
Hans Zimmer Adel Amer Douglas Ho R. Palmer-Sungail 《Journal of heterocyclic chemistry》1991,28(6):1501-1510
The reaction pathway of 4-aroyl-3-hydroxy-2(5H)-furanones 1 with diamines depends on the nature of the amine as well as on the applied reaction conditions. Thus, the reaction of 1a-d with 5,6-diamino-1,3-dimethyluracil 5 led to the formation of two isomeric Schiff bases 7a-d and 8a-d . Conversely type 1 compounds reacted with 4,5-diaminopyrimidine 9 or 2,3-diaminopyridine 10 to form the mono acid-base adducts 11a and 11b respectively. When type 1 compounds were reacted with aliphatic diamines 13a-d or p-phenylenediamine and p-xylenediamine, respectively also an immediate formation of acid-base adducts 15a-f was observed. The reaction of a number of O-methylated type 1 compounds with 1,2-ethylenediamine afforded the novel seven-membered ring compounds 18a-d in good yields. The analogous reaction of O-alkylated 1a with o-phenylenediamine 2 or 2,3-diaminonaphthalene gave the expected tricyclic ring systems 19 or 20 . 相似文献
56.
Douglas F. Foster Barry S. Nicholls Anthony K. Smith 《Journal of organometallic chemistry》1982,236(3):395-402
Reactions of Rh6(CO)16 with bis(diphenylphosphino)methane (dppm) gave Rh6(CO)14(dppm), Rh6(CO)12(dppm)2, or Rh6(CO)10(dppm)3, depending upon the reaction conditions. Rh4(CO)10(dppm) may be obtained from the reaction of Rh4(CO)12 with dppm, but this derivative rapidly decomposes in solution to give Rh4(CO)8(dppm)2, Rh6(CO)14(dppm), and Rh6(CO)12(dppm)2. Ir4(CO)10(dppm) and Ir4(CO)8(dppm)2 have also been prepared, and their structures are discussed on the basis of infrared and 31P NMR spectroscopic data. 相似文献
57.
Geng J Ducati C Shephard DS Chhowalla M Johnson BF Robertson J 《Chemical communications (Cambridge, England)》2002,(10):1112-1113
A nanoscale ruthenium/gold bimetallic cluster of clusters has been used as a molecular precursor to produce pure ruthenium nanoparticles (seeds) as catalysts for the growth of carbon nanohorns (CNHs). 相似文献
58.
Zhang Z Blom DA Gai Z Thompson JR Shen J Dai S 《Journal of the American Chemical Society》2003,125(25):7528-7529
One-dimensional (1D) magnetic nanomaterials have attracted much attention recently because of their applications in magnetic recording and spintronics. Nevertheless, it remains a challenge to prepare free-standing magnetic nanowires in high yield. This Communication reports the successful high-yield synthesis of an interesting 1D ferromagnetic CoPt alloy by direct decomposition of platinum acetylacetonate and cobalt carbonyl compound in ethylenediamine solvent through a solvothermal reaction. The CoPt alloy nanowires obtained have a tunable diameter of 10-50 nm and a length along the longitudinal axis of up to several microns, depending on crystallization temperature and reaction time. A unique formation mechanism involving coarsening and ripening under solvothermal conditions was discovered. This research opens new opportunities in synthesizing nanomaterials through low-temperature solvothermal processes. 相似文献
59.
THREE EXPONENTIAL FLUORESCENCE DECAY OF HORSE LIVER ALCOHOL DEHYDROGENASE REVEALED BY IODIDE QUENCHING 总被引:1,自引:0,他引:1
David R. Demmer † Douglas R. James † Ronald P. Steer Ronald E. Verrall 《Photochemistry and photobiology》1987,45(1):39-48
Abstract— The time resolved fluorescence decay of horse liver alcohol dehydrogenase was measured using a frequency doubled picosecond dye laser and time-correlated single-photon counting detection. A flow-cell technique is described which eliminates the photodegradation artifacts which commonly occur with laser excitation. A procedure is introduced which uses fluorescence quenching to reveal minor fluorescence lifetime components. The decay of the unquenched tryptophanyl fluorescence could be described by a double exponential decay law, but experiments conducted in the presence of iodide ion showed that the fluorescence decay must be more complex than this. A model is presented in which the fluorescence decay consists of three exponential components, only two of which are susceptible to quenching by iodide ion. Several possibilities are presented for the origin of this minor decay component, the most reasonable of which is that it arises from conformational heterogeneity in the solvent-exposed tryptophanyl residue. 相似文献
60.
Schneider BB Douglas DJ Chen DD 《Rapid communications in mass spectrometry : RCM》2002,16(20):1982-1990
A multiple sprayer electrospray ion source for high-throughput analysis is described. The ion source is comprised of multiple electrospray capillaries, each with an ion lens located near the tip. The electric potentials applied to the ion lenses are used to control the sprayers. The use of ion lenses eliminates the need for mechanical blocking devices to selectively enable or disable the sprayers, and results in a less expensive and more reliable set-up. Sprayers can be enabled or disabled within approximately 50-250 ms when the lens potentials are controlled manually. For simultaneous operation of multiple electrospray capillaries, it is advantageous to orient the capillaries so that the spray from each passes directly in front of the entrance aperture of the mass spectrometer. 相似文献