全文获取类型
收费全文 | 2381篇 |
免费 | 54篇 |
国内免费 | 8篇 |
专业分类
化学 | 1379篇 |
晶体学 | 32篇 |
力学 | 71篇 |
数学 | 291篇 |
物理学 | 670篇 |
出版年
2022年 | 34篇 |
2021年 | 22篇 |
2020年 | 46篇 |
2019年 | 57篇 |
2018年 | 46篇 |
2017年 | 41篇 |
2016年 | 72篇 |
2015年 | 46篇 |
2014年 | 52篇 |
2013年 | 169篇 |
2012年 | 120篇 |
2011年 | 144篇 |
2010年 | 85篇 |
2009年 | 72篇 |
2008年 | 92篇 |
2007年 | 86篇 |
2006年 | 80篇 |
2005年 | 62篇 |
2004年 | 63篇 |
2003年 | 53篇 |
2002年 | 40篇 |
2001年 | 46篇 |
2000年 | 39篇 |
1999年 | 23篇 |
1998年 | 25篇 |
1997年 | 18篇 |
1996年 | 23篇 |
1995年 | 21篇 |
1994年 | 45篇 |
1993年 | 32篇 |
1992年 | 46篇 |
1991年 | 28篇 |
1990年 | 42篇 |
1989年 | 29篇 |
1988年 | 25篇 |
1987年 | 21篇 |
1986年 | 27篇 |
1985年 | 26篇 |
1984年 | 35篇 |
1983年 | 27篇 |
1982年 | 30篇 |
1981年 | 25篇 |
1980年 | 19篇 |
1979年 | 36篇 |
1978年 | 36篇 |
1977年 | 27篇 |
1976年 | 19篇 |
1975年 | 21篇 |
1974年 | 18篇 |
1973年 | 23篇 |
排序方式: 共有2443条查询结果,搜索用时 31 毫秒
31.
The inelastic interactions of 150 GeV positive muons produced at Fermi Laboratory have been studied in nuclear emulsion. Secondary particles produced in these interactions have been identified, the energy spectra and angular distributions for pions and protons are given. The inclusive pionproduction reaction is studied and the relative shape ofp t 2 distribution is discussed. The average charged multiplicity relations are determined as a function of different parameters and are compared with the multiplicities produced in pure hadronic interactions. The integral cross sections have been measured and are compared with the first and second powers of 1/(1+q 2/m p 2 ) forσ exp(q 2,v), the quantity commonly called the “virtualphoton-nucleon total cross section”. The values of the structure function νW 2 are calculated for 0.02≤q 2≤0.5. The present data are compared with other muon data at the same energy. 相似文献
32.
The temperature dependence of the electric quadrupole interaction frequency
l
o
, experienced by the181Ta probe occupying Hf site in the compounds Hf1-xTaxV2 has been measured for x=0.06, 0.10 and 0.20 in the temperature region between 8.5 K to 296 K. The measurements show lattice transformation in all the three compounds below 120 K. 相似文献
33.
Garraffo HM Spande TF Jain P Kaneko T Jones TH Blum MS Ali TM Snelling RR Isbell LA Robertson HG Daly JW 《Rapid communications in mass spectrometry : RCM》2001,15(16):1409-1415
Chemical ionization tandem mass spectrometry (CI-MS/MS) of alkaloids with ammonia reagent gas and collision-activated dissociation as well as EI-MS/MS were applied to the tetraponerine alkaloids in extracts from six pseudomyrmecine ants of the genus Tetraponera. The MS/MS techniques along with gas chromatography Fourier transform infrared (GC/FTIR) spectra allowed identification in two extracts of seven of the eight known tetraponerines. The EI-MS/MS fragmentations proved diagnostic for the ring system and the CI-MS/MS patterns for the C-8 or C-9 substitution, while the Bohlmann bands in FTIR spectra were diagnostic for the C-8 or C-9 configurations. An Indian ant (T. allaborans) had T-2, T-4 and T-8, while a Chinese ant (T. binghami) had T-5, T-6, T-7 and T-8. Four other ants, T. rufonigra (India), T. penzigi (Africa), T. clypeata (Africa) and T. sp. cf. emeryi (Africa), had no tetraponerines. 相似文献
34.
Incorporation of tyrosine, dopa, dopamine, 4-hydroxyphenylpyruvic acid, (±)-, norcoclaurine-1-carboxylic acid, -norcoclaurine, -coclaurine, and -N-methylcoclaurine into N-methylcrotsparine, N-methylcrotsparinine and N-methylsparsiflorine in Croton sparsiflorus Morong has been studied. The evidence supports the direct oxidative coupling of (+)-, and (-)-N-methylcoclaurines to give N-methylcrotsparine and N-methylcrotsparinine respectively. Tracer experiment show that N-methylcrotsparine undergoes dienone-phenol rearrangement to give N-methylsparsiflorine. A double labelling experiment with (±)-N[14C]methyl[1-3H]coclaurine demonstrated that the H atom at the asymmetric centre in the 1-benzylisoquinoline precursor is retained in the bioconversion. The intermediacy of norcoclaurine-1-carboxylic acid and specific incorporation of dehydro-N-methylcoclaurinium salt into the bases have been demonstrated. 相似文献
35.
The incorporation of (±)-norcoclaurine, (±)-coclaurine, (±)-N-methylcoclaurine and dehydro-N-methylcoclaurine into nortiliacorinine A in Tiliacora racemosa colebr has been studied and specific utilisation of the (±)-coclaurine demonstrated. The evidence supports oxidative dimerization of two coclaurine units to give nortiliacorinine A. Experiments with (±)-N-methylcoclaurine and (±)-[1-3H, N-14CH3]N-methylcoclaurine established that only one N-methylcoclaurine unit is specifically utilised to constitute that “half” of the base which had phenolic OH group in the benzylic portion and further demonstrated that the H atom at the asymmetric centre in the 1-benzylisoquinoline precursor is retained in the bioconversion into nortiliacorinine A. Double labelling experiment with (±)-[1-3H, 6,0-14CH3]N-methylcoclaurine showed that O-Me function of the precursor is lost in the bioconversion into nortiliacorinine A. Parallel feedings of (+)-(S)- and (-)-(R)-N-methyl-coclaurines and (-)-(S)-, and ( + )-(R)-coclaurines revealed that the stereo-specificity is maintained in the biosynthesis of nortiliacorinine A from 1-benzylisoquinoline precursors and established ‘S,S’-configuration at the two asymmetric centres in nortiliacorinine A. 相似文献
36.
Sham M. Sondhi Shefali Rajvanshi Nirupma Singh Shubhi Jain Anand M. Lahoti 《Central European Journal of Chemistry》2004,2(1):141-187
Non steroidal anti-inflammatory drugs are the most widely used medicines for relief of pain. These drugs have some side effects,
particularly toxicity in the gastrointestinal tract and kidneys. Various approaches have been used for obtaining safer anti-inflammatory
drugs. In this review we have summarized the recent developments in the following areas; (i) mode of action of NSAIDs (ii)
Role of COX-1 & COX-2 in inflammation, (iii) Different approaches used to improve gastric tolerance i.e. chemical manipulation,
formulation & co-administration, development of non specific (COX-1 & COX-2 inhibitors) and specific (COX-2 inhibitors) inflammation
inhibitors, and development of inflammation inhibitors having a mode of action other than COX-1 & COX-2 inhibition. We have
also focused on the safety of COX-2 inhibitors and the synthesis of heterocyclic compounds and their role as inflammation
inhibitors. 相似文献
37.
pH-metric studies show that one mole of Eu(III) interacts with three molecules of each of juglone, plumbagin, lawsone and lapachol in solution. The stability and thermodynamics of these systems (50% aqueous acetone, 0.1 M KNO3 ionic strength) are discussed and explained. 相似文献
38.
Reinvestigation of the reaction of 3-aroylmethyleneindol-2-ones with hydrogen peroxide has revealed that a 1:1 isomeric mixture of spiro[3H-indole-3,2′-oxiran]-2(1H)-ones is formed. The structures of the two isomers have been confirmed by their ir, pmr and mass spectral studies. Mass fragmentation patterns of these compounds are discussed. 相似文献
39.
A comparative adsorption study with different industrial wastes as adsorbents for the removal of cationic dyes from water 总被引:7,自引:0,他引:7
Four adsorbents have been prepared from industrial wastes obtained from the steel and fertilizer industries and investigated for their utility to remove cationic dyes. Studies have shown that the adsorbents prepared from blast furnace sludge, dust, and slag have poor porosity and low surface area, resulting in very low efficiency for the adsorption of dyes. On the other hand, carbonaceous adsorbent prepared from carbon slurry waste obtained from the fertilizer industry was found to show good porosity and appreciable surface area and consequently adsorbs dyes to an appreciable extent. The adsorption of two cationic dyes, viz., rhodamine B and Bismark Brown R on carbonaceous adsorbent conforms to Langmuir equation, is a first-order process and pore diffusion controlled. As the adsorption of dyes investigated was appreciable on carbonaceous adsorbent, its efficiency was evaluated by comparing the results with those obtained on a standard activated charcoal sample. It was found that prepared carbonaceous adsorbent exhibits dye removal efficiency that is about 80-90% of that observed with standard activated charcoal samples. Thus, it can be fruitfully used for the removal of dyes and is a suitable alternative to standard activated charcoal in view of its cheaper cost. 相似文献
40.
Summary Pyridoin has been used as a reagent for the gravimetric determination of molybdenum and tungsten. The reagent quantitatively precipitates molybdenum from ammonium molybdate solution at pH 2.0–9.5 and tungsten from sodium tungstate solution at pH 2.0–4.5. The precipitate in each case is ignited to the oxide and weighed.
Zusammenfassung Zur gravimetrischen Bestimmung von Molybdän und Wolfram wird Pyridoin als Reagens empfohlen. Molybdän wurde aus Ammoniummolybdatlösung bei pH 2,0–9,5 quantitativ ausgefällt, Wolfram aus Natriumwolframatlösung bei pH 2,0–4,5. Die Niederschläge wurden jeweils zum Oxid verglüht und dieses ausgewogen.相似文献