全文获取类型
收费全文 | 2876篇 |
免费 | 79篇 |
国内免费 | 20篇 |
专业分类
化学 | 2084篇 |
晶体学 | 96篇 |
力学 | 76篇 |
数学 | 188篇 |
物理学 | 531篇 |
出版年
2023年 | 20篇 |
2022年 | 52篇 |
2021年 | 64篇 |
2020年 | 51篇 |
2019年 | 78篇 |
2018年 | 74篇 |
2017年 | 78篇 |
2016年 | 111篇 |
2015年 | 93篇 |
2014年 | 153篇 |
2013年 | 242篇 |
2012年 | 189篇 |
2011年 | 232篇 |
2010年 | 126篇 |
2009年 | 103篇 |
2008年 | 171篇 |
2007年 | 152篇 |
2006年 | 128篇 |
2005年 | 114篇 |
2004年 | 81篇 |
2003年 | 70篇 |
2002年 | 69篇 |
2001年 | 29篇 |
2000年 | 33篇 |
1999年 | 23篇 |
1998年 | 24篇 |
1997年 | 23篇 |
1996年 | 28篇 |
1995年 | 23篇 |
1994年 | 20篇 |
1993年 | 17篇 |
1992年 | 23篇 |
1991年 | 27篇 |
1990年 | 18篇 |
1989年 | 10篇 |
1988年 | 11篇 |
1987年 | 16篇 |
1986年 | 11篇 |
1985年 | 23篇 |
1984年 | 30篇 |
1983年 | 20篇 |
1982年 | 14篇 |
1981年 | 12篇 |
1980年 | 12篇 |
1979年 | 8篇 |
1978年 | 13篇 |
1977年 | 7篇 |
1976年 | 12篇 |
1975年 | 7篇 |
1974年 | 6篇 |
排序方式: 共有2975条查询结果,搜索用时 0 毫秒
51.
The condensation reaction involving an aldehyde and diketone was efficiently promoted by the Ionic liquid, [Hbim]BF(4) (IL) as a reaction medium with methanol as co-solvent at ambient temperature under ultrasonic irradiation to afford the corresponding 1,8-dioxo-octahydro-xanthene (xanthene) derivatives in excellent yields. The advantages of this method include among others the use of a recyclable, non-volatile ionic liquid, which promotes this protocol under ambient temperature without the requirement of any added catalyst. The reaction times and yields are compared with p-TSA catalyzed synthesis of xanthenes under thermal conditions, which is also reported for the first time under our reaction conditions. 相似文献
52.
K. K. Abdullah K. Karunakaran Nair N. Ramachandran K. M. Varier B. R. S. Babu Antony Joseph Rajive Thomas P. Magudapathy K. G. M. Nair 《Pramana》2010,75(3):459-469
Mass attenuation coefficients (μ/ρ) for Zr, Nb, Mo and Pd elements around their K-edges are measured at 14 energies in the range 15.744–28.564 keV using secondary excitation from thin Zr, Nb, Mo, Rh, Pd,
Cd and Sn foils. The measurements were carried out at the K
α
and K
β
energy values of the target elements by two techniques: (1) Proton-induced X-ray emission (PIXE) and (2) 241Am (300 mCi) source. In PIXE, 2 MeV proton-excited X-rays were detected by a Si(Li) detector. In the second case, X-rays excited
by 59.54 keV photons from the targets were counted by an HPGe detector under a narrow beam good geometry set-up with sufficient
shielding. The results are consistent with theoretical values derived from the XCOM package and indicate that the PIXE data
have better statistical accuracy. 相似文献
53.
Thermal, structural and optical properties of Nd3+ ions in tellurite glass (TeO2-ZnO-Na2O-Li2O-Nb2O5) have been investigated. Differential thermal analysis revealed reasonably good forming tendency of the glass composition. FTIR spectra were used to analyze the functional groups present in the glass. Judd-Ofelt intensity parameters were derived from the absorption spectrum and used to calculate the radiative lifetime, branching ratio and stimulated emission cross-section of the 4F3/2→4I9/2, 11/2, 13/2 transitions. The quantum efficiency of the 4F3/2 level is comparable as well as higher than the typical value of the other tellurite based glasses. The decay from the 4F3/2 level is found to be single exponential for different concentrations of Nd3+ ions with a shortening of lifetime with increasing concentration. The experimental values of branching ratio and saturation intensity of 4F3/2→4I11/2 transition indicate the favourable lasing action with low threshold power. 相似文献
54.
55.
56.
57.
58.
Bhogi Jai Rama Satyanarayana Swamy Rangu Vijay Pamidi Ramesh Babu Bhaskar Sanyal Yerramreddy Gandhi Nalluri Veeraiah 《Ionics》2016,22(9):1625-1634
γ-Ray-induced dielectric dispersion in CaF2–B2O3–P2O5 glasses doped with different concentrations of CuO was investigated. The glass samples were exposed to γ-rays with dose varying within the range 0–10 kGy. The dielectric dispersion and spectroscopic properties were measured before and after γ-ray treatment. Additionally, thermoluminescence studies were performed on post-irradiated glass samples. The results of dielectric properties and dielectric breakdown strength indicated a substantial increase in the insulating strength of CuO containing glasses due to γ-ray irradiation. The analysis of these results together with UV-vis optical absorption, IR spectra, and thermoluminescence studies have indicated a gradual increase in the concentration of mono-valent copper ions due to γ-ray treatment of the glass network. The additional studies have confirmed that these Cu+ ions occupy network-forming positions, increase the polymerization of the borophosphate glass network, and facilitates for the increase of insulating strength of the titled glass. 相似文献
59.
Hu X Jiao X Narayanan S Jiang Z K Sinha S Lurio LB Lal J 《The European physical journal. E, Soft matter》2005,17(3):353-359
We have used measurements of the absolute intensity of diffuse X-ray scattering to extract the interfacial tension of a buried polymer/polymer interface. Diffuse scattering was excited by an X-ray standing wave whose phase was adjusted to have a high intensity at the polymer/polymer interface and simultaneously a node at the polymer/air interface. This method permits the capillary-wave-induced roughness of the interface, and hence the interfacial tension, to be measured independently of the polymer/polymer interdiffusion. 相似文献
60.
Suresh D. Bagul Jamatsing D. Rajput Chitra Srivastava Ratnamala S. Bendre 《Molecular diversity》2018,22(3):647-655
Abstract
In designing of novel insect growth regulators (IGRs), biologically occurring carvacrol has been structurally modified to thiadiazole and oxadiazole moieties. Two series of carvacrol analogs containing 1,3,4-thiadiazole (VIIIa–e) and 1,3,4-oxadiazole (IXa–e) derivatives are designed and synthesized. Their structures are confirmed by FT-IR, \(^{1}\text {H}\) NMR, \(^{13}\)C NMR and LC-MS. IGR activity is tested against Spodoptera litura. Several analogs displayed IGR activity against this insect pest. Compounds VIIIe and IXe displayed relatively good IGR activity with \(\text {GI}_{50 }\)values 117.43 and 108.83 ppm against Spodoptera litura, respectively.Graphical Abstract
Synthesis, characterization and evaluation of carvacrol-based 1,3,4-thiadiazole and 1,3,4-oxadiazole derivatives as potent insect growth regulators (IGRs).