全文获取类型
收费全文 | 97825篇 |
免费 | 3709篇 |
国内免费 | 2960篇 |
专业分类
化学 | 38693篇 |
晶体学 | 1065篇 |
力学 | 8006篇 |
综合类 | 200篇 |
数学 | 34388篇 |
物理学 | 22142篇 |
出版年
2024年 | 47篇 |
2023年 | 388篇 |
2022年 | 481篇 |
2021年 | 646篇 |
2020年 | 808篇 |
2019年 | 808篇 |
2018年 | 10948篇 |
2017年 | 10793篇 |
2016年 | 6888篇 |
2015年 | 1749篇 |
2014年 | 1394篇 |
2013年 | 1732篇 |
2012年 | 5626篇 |
2011年 | 12283篇 |
2010年 | 6924篇 |
2009年 | 7203篇 |
2008年 | 7891篇 |
2007年 | 9907篇 |
2006年 | 1416篇 |
2005年 | 2266篇 |
2004年 | 2230篇 |
2003年 | 2620篇 |
2002年 | 1717篇 |
2001年 | 840篇 |
2000年 | 725篇 |
1999年 | 593篇 |
1998年 | 524篇 |
1997年 | 446篇 |
1996年 | 531篇 |
1995年 | 378篇 |
1994年 | 313篇 |
1993年 | 281篇 |
1992年 | 246篇 |
1991年 | 262篇 |
1990年 | 203篇 |
1989年 | 189篇 |
1988年 | 150篇 |
1987年 | 136篇 |
1986年 | 148篇 |
1985年 | 114篇 |
1984年 | 106篇 |
1983年 | 89篇 |
1982年 | 82篇 |
1981年 | 70篇 |
1980年 | 77篇 |
1979年 | 63篇 |
1978年 | 59篇 |
1977年 | 48篇 |
1973年 | 56篇 |
1914年 | 45篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
采用示差脉冲伏安法,在乳酸脱氢酶(LDH)酶促体系“丙酮酸盐 + NADH +H+ (?) 乳酸盐 + NAD+”中,通过检测NAD+还原峰电流的变化,测定了不同条件下(不同酶用量、缓冲液pH值以及温度)LDH的活性、酶促体系的米氏常数KmNADH以及最大反应速率vmax。并且在最佳实验条件下,通过检测LDH活性的改变,实验考察了3种纳米物质(ZnS,TiO2(R)和TiO2(A))对乳酸脱氢酶酶促体系的影响。 相似文献
992.
The equilibrium constants, K
2, have been determined for the proton-transfer reactions of 1-phenacylquinolinium ion, PHQ+, with several amines {triethylamine (TEA), N,N,N′,N′-tetramethylethylenediamine (ED), N,N,N′, N′-tetramethylpropanediamine (PD), N,N,N′,N′-tetramethylbutanediamine (BD), and 1,8-bis(dimethylamino-naphthalene (DMAN)} in acetonitrile (AN), AN-tetrahydrofuran (THF)
and AN-ethanol (EtOH) mixtures. The reaction was followed spectrophotometrically using a stopped-flow technique. The K
2 value decreased for DMAN and increased for TEA with increasing vol-% of THF in AN-THF mixtures. The changes in the K
2 value for ED, PD and BD changed in the order: ED, PD and BD from a pattern similar to TEA to a pattern similar to DMAN. The
change in the K
2 value for DMAN with increasing vol-% of THF in AN-THF mixtures was explained by the effect of polarity on the stability of
P−Q+ (the deprotonated product of PHQ+). The effect of THF on the K
2 value is consistent with that of the peak wavelength of the absorption spectrum of P−Q+. The change in the K
2 value for TEA, ED, PD and BD depended on the structures of the protonated bases, one of the products for this reaction. The
effect of EtOH on the K
2 value for DMAN was examined in ternary EtOH-THF-AN mixtures that contain different amounts of EtOH and whose relative permittivities
were adjusted to that of EtOH. The K
2 value increased with increasing vol-% of EtOH because of the stabilization of P−Q+ upon the formation of the hydrogen-bonded complex with EtOH. The absorption spectrum of P−Q+ demonstrated a blue shift as the vol-% of EtOH increased. 相似文献
993.
The total energy of all π-electrons in a conjugated hydrocarbon (within the framework of HMO approximation) is the sum of the absolute value of all
the eigenvalues of its corresponding graph. In this paper, we consider “double hexagonal chains” as benzenoids constructed
by successive fusions of successive naphthalenes along a zig–zag sequence of triples of edges as appear on opposite sides
of each naphthalene unit. It is shown that if the fusions are such as to give a polyaceacene then the total π-electron energy
is the minimum from among all the double hexagonal chains with the same number of naphthalene units.
相似文献
994.
995.
Grażyna Janowska Teresa Mikołajczyk Dorota Wołowska-Czapnik M. Boguń 《Journal of Thermal Analysis and Calorimetry》2005,82(1):129-135
The paper discusses the thermal properties of alginate fibres made from alginic acid or sodium alginate and from alginates
substituted with divalent metal ions during the fibre-forming stage. Alginate fibres with an addition of silica nanoparticles
have also been examined. The selection of fibre-forming parameters was intended to obtain the best either sorption or strength
properties depending on the specific fibre application. Thermal curves of the fibres under investigations obtained by under
air atmosphere and differential scanning calorimetry (DSC) under neutral gas atmosphere have been interpreted from the view
of physical and chemical changes in the fibre-forming material. Based on thermogravimetric curves, the fibre thermal stability
indices have been determined. It has been found that the addition of silica nanoparticles exerts a positive influence on the
thermal properties of the examined fibres. 相似文献
996.
Pyrazoles as molecular probes to study the properties of co-crystals by solid state NMR spectroscopy
Concepción López Rosa M. Claramunt M. Ángeles García José Elguero 《Central European Journal of Chemistry》2004,2(4):660-671
Equimolar mixtures of 3,5-dimethylpyrazole (1) with four NH-imidazoles (2–5) have been studied by13C and 15N CPMAS NMR and by DSC. In three cases, the solid mixture behaves as the sum of the individual components [imidazole (2), 2-methylimidazole (3) and 2,4(5)-dimethylimidazole (5)]. In one case [4,5-dimethylimidazole (4)], the mixture corresponds to a new species in which the dynamic behavior of1 no longer exists. 相似文献
997.
Wei Shuo FANG* Ying LIU Qi Cheng FANG Institute of Materia Medica Chinese Academy of Medical Sciences Peking Union Medical College Beijing 《中国化学快报》2002,(8)
The chemistry of anticancer drug paclitaxel (=Taxol, 1a) has been extensively explored in recent years. A variety of its analogs with modified taxane frame work or substitutions to its core structure, as well as C-13 substituted isoserine side chains have been synthesized and biologically evaluated1. The marketing of docetaxel (=Taxotere, 1b) is one of the fruitful results from these research efforts. The beginning of clinical trial for analog 2 developed by Indena is the new example. Ba… 相似文献
998.
Summary. Phenoxyacetic acid distribution in two-phase systems n-aliphatic hydrocarbon (C5–C8) – water and its dimerization in organic phase were investigated. The values of distribution coefficient (D
HR), distribution constant (K
D), and dimerization constant (K
dim) of acid were obtained. The empirical correlations of these quantities with Hildebrand solubility parameter of organic solvents were established. The influence of pH of the aqueous phase as well as the polarity of the applied organic solvents on phenoxyacetic acid physical chemistry in
the two-phase systems was described. 相似文献
999.
Ronaldo Santos da Silva Maria Inês Basso Bernardi Antonio Carlos Hernandes 《Journal of Sol-Gel Science and Technology》2007,42(2):173-179
In this work, we have studied the influence of the pH on the synthesis and structural properties of the Ba0.77Ca0.23TiO3 nanopowders synthesized by a modified polymeric precursor method, in order to achieve non-agglomerated powders. Synthesis,
morphology, thermal reactions, crystallite and average particle size of the synthesized powders were investigated through
thermal analysis (DTA/TG), X-ray diffraction (XRD), field emission scanning electron microscope (FE-SEM) and Infrared spectroscopy.
In summary, Ba0.77Ca0.23TiO3 nanopowders were synthesized for the first time at a relative low temperature (500 °C). It was also found that the alkalinity
and acidity of the solution presented a great influence on the powder properties. The best results were obtained from solutions
with pH = 8.5 and 11 whose nanopowders presented weakly agglomerate, with homogeneous particle size and a narrow size distribution
(30–40 nm). This behavior could be explained based on the FT-IR results in which it was possible to see the increased of the
chelation in higher pHs. 相似文献
1000.
Raquel Peña-Alonso Gian Domenico Sorarù 《Journal of Sol-Gel Science and Technology》2007,43(3):313-319
The sol–gel method has been used for the synthesis of borosilicate gels from mixtures of methyltriethoxysilane (MTES) and
dimethyldiethoxysilane (DMDES) and boric acid. The use of boric acid, B(OH)3 allows the hydrolysis and condensation of hybrid silicon alkoxides without further addition of water or catalyst. The use
of difunctional silicon units, –(CH3)2SiO– promote the formation, during the sol–gel process, of linear oligomers which facilitate fiber drawing before gelation.
Gel characterization performed by FT-IR, XRD, TG-DTA and DCS analysis indicates the formation of a mixed network with incorporation
of the boron units via =B-O-Si≡ bridges. The formation of borosiloxane bonds seems favored by the presence of DMDES. SiBOC
glasses were obtained after pyrolysis of the borosilicate gels in argon atmosphere at 1000 °C. TG-DTA study indicates that
the ceramic yield decreases by increasing the amount of DMDES. Gel fibers were successfully prepared from convenient partially-aged
solutions by hand drawing. Pyrolysis of the obtained gel fibers under argon atmosphere at 1000 °C open the possibility to
produce SiBOC homogeneous glass fibers with diameter as low as 10 μm. 相似文献