全文获取类型
收费全文 | 6099篇 |
免费 | 339篇 |
国内免费 | 221篇 |
专业分类
化学 | 958篇 |
晶体学 | 19篇 |
力学 | 886篇 |
综合类 | 47篇 |
数学 | 3940篇 |
物理学 | 809篇 |
出版年
2024年 | 14篇 |
2023年 | 59篇 |
2022年 | 82篇 |
2021年 | 95篇 |
2020年 | 130篇 |
2019年 | 152篇 |
2018年 | 149篇 |
2017年 | 135篇 |
2016年 | 151篇 |
2015年 | 161篇 |
2014年 | 245篇 |
2013年 | 623篇 |
2012年 | 271篇 |
2011年 | 319篇 |
2010年 | 273篇 |
2009年 | 392篇 |
2008年 | 404篇 |
2007年 | 409篇 |
2006年 | 309篇 |
2005年 | 252篇 |
2004年 | 246篇 |
2003年 | 185篇 |
2002年 | 166篇 |
2001年 | 145篇 |
2000年 | 133篇 |
1999年 | 131篇 |
1998年 | 117篇 |
1997年 | 123篇 |
1996年 | 101篇 |
1995年 | 70篇 |
1994年 | 82篇 |
1993年 | 49篇 |
1992年 | 64篇 |
1991年 | 43篇 |
1990年 | 39篇 |
1989年 | 37篇 |
1988年 | 30篇 |
1987年 | 27篇 |
1986年 | 28篇 |
1985年 | 24篇 |
1984年 | 36篇 |
1983年 | 20篇 |
1982年 | 20篇 |
1981年 | 20篇 |
1980年 | 21篇 |
1979年 | 21篇 |
1978年 | 17篇 |
1977年 | 13篇 |
1976年 | 14篇 |
1974年 | 5篇 |
排序方式: 共有6659条查询结果,搜索用时 15 毫秒
101.
Summary The advantages and disadvantages of high performance precipitation liquid chromatography have been demonstrated for polystyrene
homopolymers. Depending on the mobile phase composition at the dissolution point of the polymeric sample and surface properties
of the stationary phase, elution is governed either by a solution process or by adsorption. A contribution by adsorption was
noticed on silica as well as on reversed phases based on silica with a normal phase gradient of increasing polarity (heptane
to dichloromethane). Elution was solely governed by solubility of the polymers on both types of stationary phase for polystyrenes
with a molecular weight above 35 000 and reversed phase gradient of decreasing polarity (methanol to dichloromethane). Under
these conditions an identical dependence of elution solvent composition on sample size was found as for turbidity titrations.
Due to differences in the velocity of the eluent front and the polymeric sample with porous stationary phases the polymers
can be eluted as colloidal solutions Non-porous stationary phases are superior in this respect because the velocities of eluent
and solutes are identical. 相似文献
102.
An evaluation of measurements of heat capacities by modulated differential scanning calorimetry, MDSC is presented. Heat capacities were obtained from 130 to 550 K by a non isothermal technique in which a periodic modulation was added to the linear heating rate. Effects of amplitude and period of modulation, sample weight, sample type, pan type, and cell imbalance are described. Results are compared with those obtained using the isothermal technique. Heat capacity could be measured well into the decomposition region and separated from the non reversing signal due to chemical reaction (degradation), thus allowing a precise detection of onsets of the thermal degradation. This additional information will aid in the interpretation of the degradation chemistry, a field vital for the petroleum-industry.Dedicated to Professor Bernhard Wunderlich on the occasion of his 65th birthdayPart of this paper was presented at the 23rd Conference of the North American Thermal Analysis Society, Toronto, Canada, September 25–28, 1994.The author (MVN) acknowledges the experimental assistance provided by J. Balogh of Exxon Research and Engineering Company, Linden. Helpful discussions with A. Boller of the University of Tennessee at Knoxville, Dr. Y. Jin, General Electrical, and Dr. S. Sauerbrunn formerly of TA Instruments are also acknowledged. 相似文献
103.
Mohammad Ali Zolfigol Khodabakhsh Niknam Mojtaba Bagherzadeh Arash Ghorbani‐Choghamarani Nadiya Koukabi Maryam Hajjami Eskandar Kolvari 《中国化学会会志》2007,54(5):1115-1118
Tribromoisocyanuric acid (TBCA) and Oxone®‐MX systems were used as effective oxidizing agents for the oxidation of thiols to their corresponding disulfides under mild conditions at room temperature with good to excellent yields. 相似文献
104.
Summary New separation procedures for alkaloids of similar polarity and structure or of very different polarity and structure, based upon two-dimensional thin-layer chromatography on unmodified silica gel under mild conditions are described. Separation factors and separation mechanisms based on the structure of the bases and mobile phase composition are discussed for some examples of very efficient procedures.Proportions in solvent mixtures are v/v except where otherwise indicated. 相似文献
105.
Necessary conditions for a given pointx
0 to be a locally weak solution to the Pareto minimization problem of a vector-valued functionF=(f
1,...,f
m
),F:XR
m,XR
m, are presented. As noted in Ref. 1, the classical necessary condition-conv {Df
1(x
0)|i=1,...,m}T
*(X, x
0) need not hold when the contingent coneT is used. We have proven, however, that a properly adjusted approximate version of this classical condition always holds. Strangely enough, the approximation form>2 must be weaker than form=2.The authors would like to thank the anonymous referee for the suggestions which led to an improved presentation of the paper. 相似文献
106.
L. Montrucchio 《Journal of Optimization Theory and Applications》1994,80(3):385-406
We study the relationship between the dynamical complexity of optimal paths and the discount factor in general infinite-horizon discrete-time concave problems. Given a dynamic systemx
t+1=h(x
t
), defined on the state space, we find two discount factors 0 < * ** < 1 having the following properties. For any fixed discount factor 0 < < *, the dynamic system is the solution to some concave problem. For any discount factor ** < < 1, the dynamic system is not the solution to any strongly concave problem. We prove that the upper bound ** is a decreasing function of the topological entropy of the dynamic system. Different upper bounds are also discussed.This research was partially supported by MURST, National Group on Nonlinear dynamics in Economics and Social Sciences. The author would like to thank two anonymous referees for helpful comments and suggestions. 相似文献
107.
Nitro aromatic compound can be obtained in high yields via nitration of aromatic compound with wet carbon-based solid acid and NaNO_3 under solvent free oxidation at room temperature. 相似文献
108.
多相高分子快离子导体阻抗谱的研究 总被引:1,自引:0,他引:1
研究了以LiClO4掺杂的聚乙二醇聚氨酯双离子导体及磺酸型聚乙二醇聚氨酯离聚物单一离子导体的阻抗谱,给出了多相快离子导体交流阻抗测试系统的等效电路.通过阻抗谱计算机解析,计算得出了样品的基本电学参数.通过讨论界面阻抗的色散现象,计算出样品的离子电导率,和阻抗谱解析法得到的结果相一致. 相似文献
109.
A successful application of microwave irradiation, in which phthalocyanines were synthesized under solventless conditions from 1,2-phthalonitrile or phthalic anhydride and urea in the presence of metal templates is described. It was found that in comparison with conventional heating, the microwave process is a very useful alternative for cyclotetramerization processes because of reduction of the reaction time, better yield, and easy-to-perform procedure. 相似文献
110.
Functional-based screening of crude β-galactosidase activities from 42 yeast strains resulted in the selection of a single
enzyme of potential interest as a digestive supplement. β-Galactosidase produced by Kluyveromyces marxianus DSM5418 was purified to homogeneity by a combination of gel filtration, ion-exchange, and hydroxylapatite chromatographies.
The denatured (123 kDa) and native molecular masses (251 kDa) suggest that the enzyme is a homodimer. The optimum pH and temperature
of the purified enzyme were 6.8 and 37°C, respectively. The unpurified β-galactosidase in particular displayed a high level
of stability when exposed to simulated intestinal conditions in vitro for 4 h. Matrix-assisted laser desorption ionization
mass sectrometry analysis revealed that the enzyme's trypsin-generated peptide mass fingerprint shares several peptide fragment
hits with β-galactosidases from Kluyveromyces lactis. This confirms the enzyme's identity and indicates that significant sequence homology exists between these enzymes. 相似文献