全文获取类型
收费全文 | 44796篇 |
免费 | 1849篇 |
国内免费 | 970篇 |
专业分类
化学 | 24101篇 |
晶体学 | 684篇 |
力学 | 2076篇 |
综合类 | 49篇 |
数学 | 4583篇 |
物理学 | 16122篇 |
出版年
2022年 | 445篇 |
2021年 | 585篇 |
2020年 | 639篇 |
2019年 | 627篇 |
2018年 | 791篇 |
2017年 | 697篇 |
2016年 | 1072篇 |
2015年 | 799篇 |
2014年 | 1176篇 |
2013年 | 2225篇 |
2012年 | 2254篇 |
2011年 | 2602篇 |
2010年 | 1872篇 |
2009年 | 1830篇 |
2008年 | 2096篇 |
2007年 | 1965篇 |
2006年 | 1865篇 |
2005年 | 1582篇 |
2004年 | 1483篇 |
2003年 | 1249篇 |
2002年 | 1198篇 |
2001年 | 1539篇 |
2000年 | 1102篇 |
1999年 | 949篇 |
1998年 | 748篇 |
1997年 | 730篇 |
1996年 | 656篇 |
1995年 | 579篇 |
1994年 | 553篇 |
1993年 | 477篇 |
1992年 | 517篇 |
1991年 | 519篇 |
1990年 | 516篇 |
1989年 | 461篇 |
1988年 | 461篇 |
1987年 | 440篇 |
1986年 | 399篇 |
1985年 | 458篇 |
1984年 | 481篇 |
1983年 | 356篇 |
1982年 | 393篇 |
1981年 | 384篇 |
1980年 | 327篇 |
1979年 | 404篇 |
1978年 | 397篇 |
1977年 | 402篇 |
1976年 | 403篇 |
1975年 | 347篇 |
1974年 | 329篇 |
1973年 | 341篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
Z. G. Aliev L. O. Atovmyan A. M. Sipyagin V. G. Kartsev O. V. Dobrokhotova 《Chemistry of Heterocyclic Compounds》1987,23(4):390-395
A one-step synthesis of azetidin-3-ones by intramolecular cyclization of 1-diazo-3-arenesulfamoylalkan-2-ones was developed. The yield of cyclic product increases to 70% in the presence of an alkyl or benzyl substituent in the hydrocarbon chain of the diazoketone. The structure of N-tosyl-2-ethylazetidin-3-one was studied by x-ray diffraction analysis and it was shown that the four-membered ring has 15 inflection.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 4, pp. 468–473, April, 1987. 相似文献
992.
A model describing the wetting of structural elements in polycrystals such as grain boundaries (faces), triple lines (edges), and quaternary nodes (vertices) is proposed. It is assumed that the only driving force during wetting is the tendency of a system to the energy minimization due to the substitution of low-energy liquid–solid interfaces for high-energy grain boundaries. In this case, the anisotropy of grain boundary energy caused by the differences in misorientation of adjacent grains is taken into account, whereas the liquid–solid interface energy is assumed to be independent of the orientation. The well-known condition of grain boundary wetting (the Gibbs–Smith condition) is extended to the triple lines and quaternary nodes by introducing parameters determined as characteristic free energy of these elements. 相似文献
993.
Sequential leaching methods have been used for the speciation of Be, Ni, and V in five soil samples from Csepel Island on the Danube river located near an oil-fired power plant. The concentrations of the elements of the extracts were determined by inductively coupled plasma mass spectrometry. The total efficiency (the sum of the extracted metals divided by metal fraction which is soluble in aqua regia) of the five-step method was always higher. The difference for Ni was considerable, because of the high abundance of Ni found in the moderately reducible fraction, which is absent from the three-step method. The sum of the mobile species (exchangeable, carbonatic, and easily reducible) determined by both methods, were in reasonable agreement; this was not so for the individual fractions. There were greater differences between the non-mobile fractions (moderately reducible and oxidizable), because of the presence or absence of the moderately reducible fraction. For both methods there was good correlation between the oxidizable fraction and the organic matter content of the soils. 相似文献
994.
Henriksen T Juhler RK Svensmark B Cech NB 《Journal of the American Society for Mass Spectrometry》2005,16(4):446-455
The purpose of the work presented here was to evaluate the influence of solution composition and analyte characteristics on responsiveness to analysis with negative ion electrospray ionization mass spectrometry. The responses of a series of structurally diverse acidic molecules were compared in various solvents. Response was generally observed to be higher in methanol than acetonitrile and response for all analytes was poorer when water was mixed with the organic solvent. A positive correlation between negative ion ESI-MS response and log P was observed when either acetonitrile or methanol was used as the electrospray solvent. This result was expected because analytes with significant nonpolar character should be particularly responsive to ESI-MS analysis due to their higher affinity for electrospray droplet surfaces. It was also predicted that highly acidic analytes would be most responsive to analysis with negative ion ESI-MS due to their tendency to form negative ions. However, for the analytes studied here, acidity was found not to have a consistent influence on ESI-MS response. Many of the highly acidic molecules were quite polar and, consequently, were poorly responsive. Furthermore, the deprotonated molecular ion was detected for a number of molecules with very high pKa values, which would not be expected to form negative ions in the bulk solution. Ultimately, these results indicate that acidity is not a conclusive parameter for prediction of the relative magnitudes of negative ion ESI-MS response among a diverse series of analytes. Analyte polarity does; however, appear to be useful for this purpose. 相似文献
995.
Ferreira KQ Cardoso LN Nikolaou S da Rocha ZN da Silva RS Tfouni E 《Inorganic chemistry》2005,44(16):5544-5546
The synthesis of cis-[Ru(II)(cyclen)(L)(x)](n+) (cyclen = 1,4,7,10-tetraazacyclododecane and L = 2,2'-bipyridine (bpy), phenanthroline (phen) or 4-cyanopyridinium (4-NCpyH(+))) is reported. The freshly prepared complexes are stable in aprotic solvents and cyclen undergoes oxidative dehydrogenation reaction at high pH. These compounds also present solvent dependent conformational isomerization. 相似文献
996.
A study of the tautomerism of the N-alkyl-3-methoxycarbonyl-4-piperidinones and their hydrochlorides
B. A. Arbuzov O. A. Erastov A. B. Remizov L. Z. Nikonova 《Chemistry of Heterocyclic Compounds》1969,5(1):62-65
The positions of the equilibria of N-alkyl-3-methoxycarbonyl-4-piperidinones in the liquid state, in water, in alcohols, and in CCl4 have been determined by IR and UV spectroscopy. It has been shown that Meyer's equation is not satisfied for them. This is due to the high steric requirements of the solvating electron pair of the nitrogen atom. On the basis of the fact that on passing from the methyl ester of cyclohexan-1-one-2-carboxylic acid to the N-alkyl-3-methoxycarbonyl-4-piperidones the position of the equilibrium in the liquid state and in CCl4 does not shift in the direction of the enol it is deduced that the spatial requirements of the free electron pair of the nitrogen atom do not appreciably exceed the steric requirements of the hydrogen atom. The IR and UV spectra of the chlorides of the N-alkyl-3-methoxycarbonyl-4-piperidinones in the solid state and in water and in alcohols are given. 相似文献
997.
Zusammenfassung Es wurde die thermische Zersetzung dercis-undtrans-Isomeren des [PtAox
2Cl2] sowie des [PtAox
3Cl]Cl mit Hilfe derTG-undTD-Kurven im Falle des reinen Komplexes sowie im Falle einer Mischung mit Al2O3 (1:3) geklärt. Dietrans-Form ist stabiler als diecis-Form, die Zersetzung wird durch Al2O3 nicht beeinflu\t. [PtAox
3Cl]Cl wird infolge der Erhitzung in die stabileretrans-Form [PtAox
2Cl2] umgewandelt und verhält sich auch weiterhin wie diese.
Mit 4 Abbildung 相似文献
A contribution to the study of the compounds of Pt(II) with acetoxime
The thermal decomposition ofcis- andtrans-[PtAox 2Cl2] and of [PtAox 3Cl]Cl was studied by means ofTGA andTDA on pure substances as well as on mixtures with Al2O3 (1:3). Thetrans form is more stable then thecis form, the presence of Al2O3 has no influence on its decomposition. The [PtAox 3Cl]Cl is transformed by heating into the more stabletrans-isomer and its further behaviour is identical with that of thetrans isomer.
Mit 4 Abbildung 相似文献
998.
Z. F. Chai Q. F. Qian X. Q. Feng P. Q. Zhang N. Q. Liu W. Y. Feng M. X. Kuang H. Y. Wang Y. Z. Zhang 《Journal of Radioanalytical and Nuclear Chemistry》2004,259(1):153-156
The occupational health impact of atmospheric pollution on exposed workers at one iron and steel complex was studied by instrumental neutron activation analysis of workers' hair samples and medical examination. The experimental results indicate that there is a positive correlation between the high inhalation amounts of iron and other trace elements by the exposed workers and the symptom of their high blood pressure and hypoglycemia, which implies that the atmospheric environment polluted by iron and steel industry has an adverse health impact on the exposed workers. The measures to relieve and abate the occupational diseases caused by air-borne particulate matter should be taken 相似文献
999.
J. Kučera J. Kučera A. R. Byrne A. R. Byrne J. Mizera J. Mizera M. Lučaníková M. Lučaníková Z. Řanda Z. Řanda 《Journal of Radioanalytical and Nuclear Chemistry》2006,269(2):251-257
Summary Radiochemical neutron activation procedures using liquid-liquid extraction with tetraphenylarsonium chloride in chloroform
from 1M HCl and solid extraction with ALIQUAT 336 incorporated in a polyacrylonitrile binding matrix from 0.1M HCl were developed
for accurate determination of rhenium in biological and environmental samples at the sub-ng . g-1 level. Concentrations of Re in the range of 0.1 to 2.4 ng . g-1 were determined in several botanical reference materials (RM), while in a RM of road dust a value of ~10 ng . g-1 was found. Significantly elevated values of Re, up to 90 ng . g-1 were found in seaweed (brown algae). Results for Re in the brown algae Fucus vesiculosus in which elevated 99Tc values had previously been determined suggested possible competition between Re and Tc in the accumulation process. 相似文献
1000.
H.?Hussain H.?U.?Rehman Z.?AhmadEmail author 《Journal of Sol-Gel Science and Technology》2005,36(3):239-248
Nano-composites from aramid-silica system have been prepared via sol-gel process. Poly (phenyleneterephthalamide) copolymer
chains were prepared by reacting a mixture of p- and m-phenylenediamines with terephthaloyl chloride in dimethylacetamide used as solvent. The sol-gel process in the polymer
matrix was carried out through hydrolysis and condensation of a mixture of tetraethoxysilane and (3-glycidoxypropyl) trimethoxysilane.
The latter was used to develop linkage, on one hand with silica network structure using alkoxy groups and on the other hand
with aramid chains at its secondary amine groups through glycidal groups of silane. Mutual interaction between the two disparate
phases aramid and silica network was thus created. Thin films of the composites containing different proportions of silica
ranging from 5.0 to 25.0-wt% were cast by the solvent elution technique. The α-relaxation temperature associated with the
glass transition was measured by the dynamic mechanical thermal analysis. The results showed an increase in the glass transition
temperature from 328°C for the pure aramid to 352°C for the hybrid materials containing 25-wt% silica, an indicative of the
increased interfacial interaction between the two phases. Films having relatively low silica content were flexible and transparent,
but those with high silica content were opaque and brittle. These films were tested for their tensile strength, modulus and
toughness. The mechanical strength of the composites as compared to the pure aramid increased initially but with further addition
of silica the strength decreased. The initial increase can be explained due to increased interfacial interaction between the
two phases, however agglomeration of silica particles was responsible for decreasing strength at higher silica contents. 相似文献