全文获取类型
收费全文 | 1565篇 |
免费 | 167篇 |
国内免费 | 156篇 |
专业分类
化学 | 952篇 |
晶体学 | 40篇 |
力学 | 51篇 |
综合类 | 4篇 |
数学 | 147篇 |
物理学 | 267篇 |
综合类 | 427篇 |
出版年
2023年 | 18篇 |
2022年 | 27篇 |
2021年 | 38篇 |
2020年 | 21篇 |
2019年 | 32篇 |
2018年 | 22篇 |
2017年 | 28篇 |
2016年 | 57篇 |
2015年 | 65篇 |
2014年 | 82篇 |
2013年 | 99篇 |
2012年 | 99篇 |
2011年 | 105篇 |
2010年 | 59篇 |
2009年 | 56篇 |
2008年 | 96篇 |
2007年 | 73篇 |
2006年 | 76篇 |
2005年 | 63篇 |
2004年 | 70篇 |
2003年 | 68篇 |
2002年 | 99篇 |
2001年 | 72篇 |
2000年 | 56篇 |
1999年 | 54篇 |
1998年 | 31篇 |
1997年 | 23篇 |
1996年 | 25篇 |
1995年 | 18篇 |
1994年 | 27篇 |
1993年 | 23篇 |
1992年 | 18篇 |
1991年 | 10篇 |
1990年 | 18篇 |
1989年 | 11篇 |
1988年 | 11篇 |
1987年 | 16篇 |
1986年 | 8篇 |
1985年 | 10篇 |
1984年 | 11篇 |
1982年 | 16篇 |
1981年 | 9篇 |
1979年 | 8篇 |
1978年 | 4篇 |
1976年 | 4篇 |
1974年 | 7篇 |
1971年 | 6篇 |
1970年 | 4篇 |
1968年 | 6篇 |
1967年 | 4篇 |
排序方式: 共有1888条查询结果,搜索用时 31 毫秒
41.
The synthesis of derivatives of 3-β-D-xylopyranosyl-1,2,4-oxadiazolesis accomplished by condensing protected β-D-xylopyranosyl amidoxime with acidanhydrides or various substituted benzoyl chlorides in good yield.Thestructures of now derivatives were identified by spectra and elemental analysis.The stability of 1,2,4-oxadiazole ring and mechanism of cyclization were.investigated. 相似文献
42.
Tai Yup Chang 《International journal of quantum chemistry》1971,5(4):469-473
The relativistic long-range intermolecular interaction energy term of Mavroyannis and Stephen is estimated for some amino acids by using optical rotatory dispersion data and for hexahelicene by using theoretical values of excitation energies and rotational strengths. The result shows that the interaction energy may be significant for the interaction between some essentially dissymmetric chromophores such as hexahelicene, but that it is unimportant for other cases. 相似文献
43.
Mixing enhancement has drawn great attention from designers of micromixers, since the flow in a microchannel is usually characterized by a low Reynolds number (Re) which makes the mixing quite a difficult task to accomplish. In this paper, a novel integrated efficient micromixer named serpentine laminating micromixer (SLM) has been designed, simulated, fabricated and fully characterized. In the SLM, a high level of efficient mixing can be achieved by combining two general chaotic mixing mechanisms: splitting/recombination and chaotic advection. The splitting and recombination (in other terms, lamination) mechanism is obtained by the successive arrangement of "F"-shape mixing units in two layers. The advection is induced by the overall three-dimensional serpentine path of the microchannel. The SLM was realized by SU-8 photolithography, nickel electroplating, injection molding and thermal bonding. Mixing performance of the SLM was fully characterized numerically and experimentally. The numerical mixing simulations show that the advection acts favorably to realize the ideal vertical lamination of fluid flow. The mixing experiments based on an average mixing color intensity change of phenolphthalein show a high level of mixing performance was obtained with the SLM. Numerical and experimental results confirm that efficient mixing is successfully achieved from the SLM over the wide range of Re. Due to the simple and mass producible geometry of the efficient micromixer, SLM proposed in this study, the SLM can be easily applied to integrated microfluidic systems, such as micro-total-analysis-systems or lab-on-a-chip systems. 相似文献
44.
Tei T Sato Y Hagiya K Tai A Okuyama T Sugimura T 《The Journal of organic chemistry》2002,67(19):6593-6598
The stereocontrol mechanism of the 2,4-pentanediol (PD)-tethered reaction was studied in detail using a reaction system consisting of phenyl and rhodium carbenoid moieties. Different tethers were examined to analyze the effects of the methyl groups on the PD tether. Among the reactions with these tethers, the PD tether achieves an unmeasurably high stereoselectivity in a diastereomeric ratio of >500. Another tether showing a high but measurable stereoselectivity in a ratio of 41 is mostly controlled by the entropy term. To clarify the role of the methyl groups on the chiral tethers, which are the origin of the stereocontrol, the "chiral perturbation factor" is introduced. This parameter is defined as the rate of a chiral reaction relative to that of an achiral reference reaction. By analyzing the temperature dependence of the chiral perturbation factors for different chiral-tethered reactions, high potentials of the PD-tethered reaction in its stereocontrol are concluded to be due to the entropy term. 相似文献
45.
46.
47.
We have provided a model to interpret the non-quadratic-intensity dependence behavior commonly observed in the two-photon fluorescence (TPF) experiment excited with high laser intensity. The model also provides one with a different technique to measure the two-photon absorption cross section of an organic chromophore in solution. In contrast to the commonly used low intensity technique that depends on the quadratic-intensity law, the present technique is based on the non-quadratic-intensity dependence of two-photon fluorescence. Auxiliary data such as two-photon quantum efficiency and fluorescence collection efficiency, essential in the low intensity method, are not required in the present technique. TPF measurements of Rhodamine B in methanol are carried out to demonstrate the validity of the present method. The method is used to determine the two-photon absorption cross section of a new chromophore attached with tricyano-derivatized furan as the electron acceptor. The two-photon absorption cross section measured using this method is also compared with that using a conventional transmission technique. 相似文献
48.
Two new standard reference materials (SRM) for drugs of abuse in human hair have been developed. SRM 2379 consists of hair spiked with cocaine, benzoylecgonine, cocaethylene, phencyclidine, amphetamine, and methamphetamine. SRM 2380 consists of hair spiked with codeine, morphine, monoacetylmorphine, and tetrahydrocannabinol (THC). The SRMs were prepared by soaking the hair in a solution of the target analytes in water-dimethylsulfoxide. The concentration of each analyte was determined using two methods, one based upon gas chromatography/mass spectrometry (GC/MS) and one based upon liquid chromatography/mass spectrometry (LC/MS). Both methods used 0.1 M HCl for extraction of all the analytes from the hair, except for THC, which was extracted with 1 M NaOH. For isolation of the analytes from the extracts, the GC/MS-based methods used different clean-up procedures from those used for the LC/MS-based methods. The results from the two methods were in good agreement with mean differences for the analytes ranging from 4% to 16%. These materials will enable laboratories performing analyses of hair for drugs of abuse to test the accuracy of their methods. 相似文献
49.
Yibin Pan Lili Yan Qiaoqiao Chen Cheng Wei Yongdong Dai Xiaomei Tong Haiyan Zhu Meifei Lu Yanling Zhang Xiaoying Jin Tai Zhang Xiaona Lin Feng Zhou Songying Zhang 《Experimental & molecular medicine》2021,53(1):52
In early pregnancy, the placenta anchors the conceptus and supports embryonic development and survival. This study aimed to investigate the underlying functions of Shh signaling in recurrent miscarriage (RM), a serious disorder of pregnancy. In the present study, Shh and Gli2 were mainly observed in cytotrophoblasts (CTBs), Ptch was mainly observed in syncytiotrophoblasts (STBs), and Smo and Gli3 were expressed in both CTBs and STBs. Shh signaling was significantly impaired in human placenta tissue from recurrent miscarriage patients compared to that of gestational age-matched normal controls. VEGF-A and CD31 protein levels were also significantly decreased in recurrent miscarriage patients. Furthermore, inhibition of Shh signaling impaired the motility of JAR cells by regulating the expression of Gli2 and Gli3. Intriguingly, inhibition of Shh signaling also triggered autophagy and autolysosome accumulation. Additionally, knockdown of BECN1 reversed Gant61-induced motility inhibition. In conclusion, our results showed that dysfunction of Shh signaling activated autophagy to inhibit trophoblast motility, which suggests the Shh pathway and autophagy as potential targets for RM therapy.Subject terms: Embryology, Endocrine reproductive disorders, Autophagosomes 相似文献
50.
计算全息半色调编码技术的研究 总被引:1,自引:0,他引:1
论述了计算全息的基本原理及常用的编码方法,在罗曼型和博奇型编码的基础上,运用空间脉冲调制技术,提出了新型的编码技术--半色调位相型和标准型编码,半色调型编码用圆形通光孔代替矩形孔,以圆孔面积及位置来记录全息信息,改变了输出模式,减少了编码参数,为全息图的后期作提供了一种新的方法,同时通过理论计算和结果分析分析证明了半色调位相型编码具有和罗曼型同样好的衍射效率,半色调标准型编码具有和博奇型一样大的带宽,但综们的图形数据减少一倍。 相似文献