全文获取类型
收费全文 | 4234篇 |
免费 | 1590篇 |
国内免费 | 321篇 |
专业分类
化学 | 549篇 |
晶体学 | 45篇 |
力学 | 382篇 |
综合类 | 129篇 |
数学 | 1508篇 |
物理学 | 3532篇 |
出版年
2024年 | 11篇 |
2023年 | 36篇 |
2022年 | 116篇 |
2021年 | 153篇 |
2020年 | 151篇 |
2019年 | 123篇 |
2018年 | 119篇 |
2017年 | 194篇 |
2016年 | 197篇 |
2015年 | 181篇 |
2014年 | 327篇 |
2013年 | 389篇 |
2012年 | 323篇 |
2011年 | 379篇 |
2010年 | 293篇 |
2009年 | 249篇 |
2008年 | 300篇 |
2007年 | 358篇 |
2006年 | 301篇 |
2005年 | 247篇 |
2004年 | 212篇 |
2003年 | 217篇 |
2002年 | 189篇 |
2001年 | 163篇 |
2000年 | 164篇 |
1999年 | 122篇 |
1998年 | 75篇 |
1997年 | 94篇 |
1996年 | 74篇 |
1995年 | 58篇 |
1994年 | 37篇 |
1993年 | 39篇 |
1992年 | 35篇 |
1991年 | 30篇 |
1990年 | 26篇 |
1989年 | 23篇 |
1988年 | 28篇 |
1987年 | 10篇 |
1986年 | 15篇 |
1985年 | 23篇 |
1984年 | 7篇 |
1982年 | 11篇 |
1981年 | 10篇 |
1980年 | 6篇 |
1979年 | 3篇 |
1978年 | 5篇 |
1977年 | 4篇 |
1976年 | 3篇 |
1974年 | 5篇 |
1936年 | 3篇 |
排序方式: 共有6145条查询结果,搜索用时 312 毫秒
121.
Summary A modified Wald statistic for testing simple hypothesis against fixed as well as local alternatives is proposed. The asymptotic
expansions of the distributions of the proposed statistic as well as the Wald and Rao statistics under both the null and alternative
hypotheses are obtained. The powers of these statistics are compared and its is shown that for special structures of parameters
some statistics have same power in the sence of order
. The results obtained are applied for testing the hypothesis about the covariance matrix of the multivariate normal distribution
and it is shown that none of the tests based on the above statistics is uniformly superior.
Research supported by the National Science Foundation Grant MCS 830149. 相似文献
122.
In the study, a series of C_(30)-C_(32) lanostane homologs have been identified from Eogene source rocks and crude oils of Biyang depression in Henan Province, China. Based upon the geochemical investigations and simulating experiments, the idea that the generation of lanostane may mainly be related to the specific original organic inputs and low maturity has been proposed. 相似文献
123.
采用大功率高重复频率准分子激光溅射热解石墨靶制备了类金刚石碳膜, 研究了实验条件对类金刚石膜光学性能的影响, 发现氢可以提高膜中sp3键的含量和膜的光学透过率. 在实验参数范围内, 膜的光学性能随着氢压的增加而提高. 根据类金刚石膜的反应沉积机理对上述结果进行了分析、解释. 相似文献
124.
125.
本文提出在原子吸收光谱测定中,采用Smith-Hieftje法校正背景时,空心阴极灯供电条件(即最佳宽窄脉冲电流)的选择依据和选择方法;并给出24个常见元素的宽窄脉冲电流的选定结果。 相似文献
126.
Single phase of ammonium tetramolybdate in the micro power form was prepared from polyphase ammonium paramolybdate. Its heat capacity from 14.25℃ to 120.12 ℃ was measured by drop method and the result is
Cp=0.3936+7.4047×10-4T+6.3543×10-3T-2(J•K-1•g-1) 相似文献
Cp=0.3936+7.4047×10-4T+6.3543×10-3T-2(J•K-1•g-1) 相似文献
127.
Carbon dioxide fixation by microalgae photosynthesis using actual flue gas discharged from a boiler 总被引:2,自引:0,他引:2
Matsumoto Hiroyo Shioji Norio Hamasaki Akihiro Ikuta Yoshiaki Fukuda Yoshinori Sato Minoru Endo Noriyoshi Tsukamoto Toshiaki 《Applied biochemistry and biotechnology》1995,(1):681-692
To mitigate CO2 discharged from thermal power plants, studies on CO2 fixation by the photosynthesis of microalgae using actual exhaust gas have been carried out. The results are as follows.
相似文献
1. | A method is proposed for evaluating the maximum photosynthesis rate in the raceway cultivator using only the algal physical properties; |
2. | Outdoor cultivation tests taking actual flue gas were performed with no trouble or break throughout 1 yr using the strain collected in the test; |
3. | The produced microalgae is effective as solid fuel; and |
4. | The feasibility studies of this system were performed. The system required large land area, but the area is smaller than that required for other biomass systems, such as tree farms. |
128.
采用非线性透过率法测定了多枝[1,3,4]-噁二唑衍生物的双光子吸收性质. 测定了化合物的单光子荧光光谱和双光子荧光光谱, 在800 nm波长的激光激发下, 9-乙基-3,6-双{5-(4-叔丁基苯基)-[1,3,4] 噁二唑-2-苯乙烯基}-咔唑(3)和三-{5-(4-叔丁基苯基)-[1,3,4] 噁二唑-2-苯乙烯基-4-苯基}-胺(4)能够发出很强的蓝色和黄绿色双光子上转换荧光, 荧光峰分别位于485和547 nm. 这些多枝结构化合物的双光子吸收截面较大(数值超过104 GM), 并具有很强的光限幅效应. 多枝分子中重复单元的推拉电子结构和协同效应有效地增强了分子的双光子吸收性质. 相似文献
129.
Vashchenko V. V. Kutulya L. A. Pivnenko M. N. Shkolnikova N. I. 《Russian Chemical Bulletin》2003,52(11):2406-2418
(1R,4R)-2-(4-Hydroxybenzylidene)- and (1R,4R)-2-(4"-hydroxybiphenyl-4-yl)methylene-p-menthan-3-ones were synthesized by condensation of (–)-menthone with O-tetrahydropyran-2-yl derivatives of 4-hydroxybenzaldehyde and 4"-hydroxy-4-formylbiphenyl, respectively, in a DMSO—base medium followed by the removal of the protective group. The reactions of these hydroxy derivatives with 4-alkylbenzoic, 4-alkyloxybenzoic, trans-4-alkylcyclohexane-4-carboxylic, and 4"-alkylbiphenyl-4-carboxylic acids afforded three series of new chiral esters. Compounds containing the arylidene moiety with three benzene rings were found to exhibit liquid-crystalline properties. The characteristic features of these compounds are discussed based on the results of studies by polarizing microscopy, differential scanning calorimetry, and small-angle X-ray scattering. It was found that the mesomorphic compounds under study can form a smectic A mesophase, twist grain boundary mesophases (TGBA), and blue phases in a wide temperature range. Upon dissolution of certain of chiral compounds in 4"-cyano-4-pentylbiphenyl, a rather high twisting power and the thermal stabilizing effect on mesophases were observed. 相似文献
130.
The gradient pathways of the reactions of nucleophilic addition of H2O and HF molecules to formaldehyde in the gas phase and in the XH…H2CO…HC(O)OH complex (X=OH, F) were calculated by theab initio RHF/6-31G**, MP2(fc)/6-31G**, and MP2(full)/6-311++G** methods. Both reactions proceed concertedly. The formation of H-bonded bimolecular pre-reaction complexes is the initial
stage of the gas-phase reactions; at the same time, no indications of the formation of stable π-complexes were found on the
potential energy surfaces of systems under study. The calculated energy barriers to the gasphase reactions exceed 40 kcal
mol−1, while those to reactions in the complex XH…H2CO…HC(O)OH (X=OH, F) become more than halved.
Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2146–2154, November, 1998. 相似文献