全文获取类型
收费全文 | 93866篇 |
免费 | 4702篇 |
国内免费 | 12788篇 |
专业分类
化学 | 74881篇 |
晶体学 | 1181篇 |
力学 | 2398篇 |
综合类 | 870篇 |
数学 | 12698篇 |
物理学 | 19328篇 |
出版年
2024年 | 122篇 |
2023年 | 698篇 |
2022年 | 1934篇 |
2021年 | 1876篇 |
2020年 | 2301篇 |
2019年 | 2249篇 |
2018年 | 1926篇 |
2017年 | 2786篇 |
2016年 | 2917篇 |
2015年 | 2456篇 |
2014年 | 3478篇 |
2013年 | 6900篇 |
2012年 | 6377篇 |
2011年 | 5164篇 |
2010年 | 4372篇 |
2009年 | 5909篇 |
2008年 | 6127篇 |
2007年 | 6357篇 |
2006年 | 5802篇 |
2005年 | 4943篇 |
2004年 | 4610篇 |
2003年 | 3812篇 |
2002年 | 5010篇 |
2001年 | 2902篇 |
2000年 | 2693篇 |
1999年 | 2513篇 |
1998年 | 2214篇 |
1997年 | 1742篇 |
1996年 | 1485篇 |
1995年 | 1446篇 |
1994年 | 1290篇 |
1993年 | 1070篇 |
1992年 | 1007篇 |
1991年 | 702篇 |
1990年 | 573篇 |
1989年 | 561篇 |
1988年 | 389篇 |
1987年 | 318篇 |
1986年 | 298篇 |
1985年 | 233篇 |
1984年 | 248篇 |
1983年 | 140篇 |
1982年 | 214篇 |
1981年 | 175篇 |
1980年 | 194篇 |
1979年 | 184篇 |
1978年 | 171篇 |
1977年 | 123篇 |
1976年 | 108篇 |
1973年 | 68篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
901.
Poly(n-vinylpyrrolidone) hydrogels: 2.Hydrogel composites as wound dressing for tropical environment
POLY(N-VINYLPYRROLIDONE) HYDROGELS: 2. HYDROGEL COMPOSITES AS WOUND DRESSING FOR TROPICAL ENVIRONMENT. The effects of irradiation on hydration and other properties of poly(vinylpyrrolidone) (PVP) hydrogel composites have been investigated. The aqueous solution of vinylpyrrolidone (VP) 10 wt % was mixed with several additives such as agar and polyethylen glycol (PEG). The solution was then irradiated with gamma rays from Cobalt-60 source at room temperature. Several parameters such as elongation at break (EB), tensile strength (TS), degree of swelling (DS), water vapor transmission rate (WVTR), equilibrium water content (EWC), microbial growth and penetration test, and water activity (Aw) were analysed at room temperature of 29 ±2°C humidity of 80 ± 10%. Results show that elongation at break of hydrogel membranes with initial composition of VP with agar, VP with agar and PEG were 240 % and 250 % kGy, the equilibrium water content of membranes were 96 to 90%, whereas degree of swelling were 55 to 10. The WVTR of hydrogel membranes with initial composition of VP with agar and PEG was 70 g m-2h-1, while the water activity was 0.9. Such hydrogel membranes exhibits the following properties: They are elastic, transparent, flexible, impermeable for bacteria. They absopt a high capacity of water, attached to healthy skin but not to the wound and they are easy to remove. These properties of the hydrogel membranes allow for applying as a wound dressings in tropical environment. 相似文献
902.
Osamu Sangen Singo Masuda Hidehiko Nakano Kazusige Yamana Kenichi Sasaki Naoki Ikeda 《Journal of polymer science. Part A, Polymer chemistry》1992,30(6):1171-1175
A new synthetic pathway of A–B–A tri-block copolyether which is composed of a hydrophilic poly(oxyethylene) unit as an A part and a hydrophobic poly(oxy-2-methyl-trimethylene) unit as a B part is proposed. Telechelic α-tosyl-ω-tosyloxypoly(oxy-2-methyl-trimethylene) derived from tosylation of poly(oxy-2-methyl-trimethylene glycol) (PMTG) was allowed to react with poly(ethylene glycol) (PEG) in the presence of sodium hydroxide. Tg of the resulting A–B–A tri-block copolyether (PEMG) (M?n = 1600) was ?72°C and its specific gravity [D415] was 1.055. 相似文献
903.
A new series of group 6 carbonyl complexes of N-[(2-pyridyl)methyliden]-α (or β)-aminonaphthalene (α/β-NaiPy) are described in this work. The complexes are formulated as cis-[M(CO)4(α/β-NaiPy)] by elemental, mass and other spectroscopic data. The complexes show emission spectra at room temperature and their quantum yield lies between 0.4 and 0.5. All the complexes exhibit negative solvatochromism. Cyclic voltammetry shows metal centred oxidation and ligand reductions. 相似文献
904.
R.-L. Yun Y.-M. Chang C.-H. Lin K.-H. Hu C.-M. Shu 《Journal of Thermal Analysis and Calorimetry》2006,85(1):107-113
In industrial processes, information
on the safety property of chemicals is essentially crucial for safe handling
during unit operations. Ensuring the safe use of combustible or flammable
substances in processes is unlikely without detailed investigations of their
flammability characteristics and related hazards. We studied 3-methyl pyridine
(3-picoline), e.g., flammability limits (LFL/UFL), maximum explosion pressure
(P
max), maximum
explosion pressure rise (dP/dt)max, minimum oxygen concentration
(MOC), vapor deflagration index (K
g),
and characterized the influence of inert steam (H2O)
on critical parameters for 3-picoline/water mixtures at 270°C, 1 atm,
various oxygen concentrations, and vapor mixing ratios (100/0, 30/70, 10/90
and 5/95 vol.%) with a 20-L-Apparatus in simulated conditions, respectively.
The results showed that the flammability characteristics
of 3-picoline(aq) all increased with the oxygen concentration.
However, as the composition of inert steam increased, the flammability parameters
and the degree of fire and explosion hazards were significantly reduced, instead.
This study elucidated the flammability properties of 3-picoline mixed with
inert steam. The conclusions could be applied to proactively prevent the relevant
processes from incurring fire and explosion accidents. 相似文献
905.
N. G. Komissarova N. G. Belenkova L. V. Spirikhin O. V. Shitikova M. S. Yunusov 《Chemistry of Natural Compounds》2002,38(1):58-61
Oxidation of betulin by pyridinium dichromate, pyridinium chlorochromate, and K2Cr2O7 -H2SO4 in the presence of tetrabutylammonium bromide was studied. Products of regioselective C-3, C-28-, and exhaustive oxidation, 28-hydroxylup-20(29)-3-one, 3-hydroxy- and 3-oxolup-20(29)-en-28-al were obtained. 相似文献
906.
Petr Zuman 《Microchemical Journal》2002,72(3):241-250
Applications of chromatographic and atomic absorption methods are compared with those of electroanalytical techniques. The limitations of electroanalytical procedures and their advantages, such as speed, sensitivity and speciation are discussed. Among applications of polarography and voltammetry to a solution of basic problems, attention has been paid to the possibility to distinguish the degree of conjugation (e.g. of compounds containing the grouping CNNC), to a contribution to understanding of solution chemistry of mitomycin C and to processes involved in the reduction of cephalosporins. Among practical applications, the possibility to analyze suspensions, which can be carried out without separation offers advantages. Examples are interactions in suspensions of bile salts in the presence of metal (II) ions, adsorptions of various pesticides and other toxic materials on lignin and the possibility to follow alkaline cleavage of lignin in buffers pH 8-12 at 25 °C. Most of these studies would not be possible to carry out using other techniques. 相似文献
907.
V. V. Nikitenko A. I. Martynenko S. A. Ostrovsky V. V. Rusak V. V. Kryuchkov D. A. Topchiev 《Russian Chemical Bulletin》1993,42(2):378-380
3-Methyl-l-vinylpyrazole (M3VP) and 5-methyl-1-vinylpyrazole (M5VP) were isolated as individual substances by vacuum rectification of their mixture (M3VP: M5VP 60 40). For each of them the kinetics of free-radical polymerization in MeOH were measured at low conversions. In both cases the rate of polymerization is proportional to 0.5 order with respect to the initiator (AIBN) concentration. On the other hand, a first order of reaction with respect to monomer concentration is observed only when the latter is relatively low (3M). At higher initial concentrations of monomers the order of reaction becomes less than unity. The overall rate of polymerization for M5VP was higher than for M3VP, whereas the initiation rate remained constant in the whole range of monomer concentrations and did not depend on the exact structure of the monomer. The difference in the rates of polymerization observed for M3VP and M5VP is probably connected with the difference in the key parameterk
p/k
t
1/2
for each of the two isomers. It is concluded that the correct kinetic information about homo- and co-polymerization of M3VP and M5VP cannot be obtained without their adequate separation.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 413–415, February, 1993. 相似文献
908.
Ballesteros I. Ballesteros M. CabaÑas A. Carrasco J. MartÍn C. Negro M. J. Saez F. Saez R. 《Applied biochemistry and biotechnology》1991,(1):307-315
A total of 27 yeast strains belonging to the groupsCandida, Saccharomyces, andKluyveromyces were screened for their ability to grow and ferment glucose at temperatures ranging 32-45°C.
K. marxianus andK. fragilis were found to be the best ethanol producing organisms at the higher temperature tested and, so, were selected for subsequent
simultaneous saccharification and fermentation (SSF) studies. 相似文献
909.
对用调制差热分析(MDTA)准恒温法测样品热容量的情形进行了讨论。通过结合最基本的热传导定律和MDTA模型,指出了目前国际上测量样品热容量的准恒温法只能得到在所测温度范围内的物质热容量平均值,调制温度的幅度越大或调制频率越高,所得到的热容量数据越平滑。在所测温度范围内样品热容量基本不变时,用MDTA准恒温法较好;但当样品热容量在所测温度范围内有明显变化时,用传统差热分析法(DTA)更好一些。 相似文献
910.
Yannis Dotsikas 《Analytica chimica acta》2004,509(1):103-109
This study describes the employment of a novel imidazole-substituted phenol [4-(1-imidazolyl)phenol] as a highly potent signal enhancer in a horseradish peroxidase (HRP)-luminol chemiluminescence (CL) immunoassay. This competitive-type immunoassay for the model antigen fentanyl is based on the use of fentanyl polyclonal antibody immobilized on white microtiter plates and a biotinylated bovine serum albumin (BSA)-fentanyl derivative as a tracer. The latter was detected by means of streptavidin labeled with HRP, resulting in the generation of a high-intensity and relatively stable chemiluminescent signal, immediately after the addition of the substrate solution (NOAS). The developed method fulfilled the requirements of accuracy (percentage recovery ranged from 93.8 to 107%) and precision (intra- and inter-assay CVs were 2.5-5.2 and 4.5-11.9%, respectively). Its plasma detection limit (1.05 pg ml−1) was lower than those of previous immunoassays. The novel assay was compared in terms of sensitivity and concentration range with other common HRP substrate systems: luminol-p-iodophenol-H2O2 and TMB-H2O2. Finally, the described method was compared with an HRP-fentanyl conjugate-based assay, similar to commercially available kits (SKIT), employing the novel substrate solution for both assays and the differences observed were explained by applying previously described models. The detection limit was 4.82 pg ml−1 for SKIT, recovery values were 94.2-105% and intra- and inter-assay CVs were 2.5-5.2 and 4.5-11.9%, respectively. In conclusion, the proposed assay could be utilized for a wide range of molecules and replace the existing enzyme-labeled antigen-based kits. 相似文献