全文获取类型
收费全文 | 934篇 |
免费 | 37篇 |
国内免费 | 3篇 |
专业分类
化学 | 606篇 |
晶体学 | 24篇 |
力学 | 6篇 |
数学 | 37篇 |
物理学 | 301篇 |
出版年
2022年 | 14篇 |
2021年 | 17篇 |
2020年 | 21篇 |
2019年 | 14篇 |
2018年 | 23篇 |
2017年 | 16篇 |
2016年 | 36篇 |
2015年 | 25篇 |
2014年 | 25篇 |
2013年 | 89篇 |
2012年 | 77篇 |
2011年 | 56篇 |
2010年 | 46篇 |
2009年 | 47篇 |
2008年 | 48篇 |
2007年 | 48篇 |
2006年 | 38篇 |
2005年 | 28篇 |
2004年 | 24篇 |
2003年 | 16篇 |
2002年 | 25篇 |
2001年 | 8篇 |
2000年 | 12篇 |
1999年 | 8篇 |
1998年 | 7篇 |
1997年 | 6篇 |
1996年 | 7篇 |
1995年 | 14篇 |
1994年 | 11篇 |
1993年 | 9篇 |
1992年 | 8篇 |
1991年 | 9篇 |
1990年 | 8篇 |
1989年 | 7篇 |
1988年 | 4篇 |
1987年 | 7篇 |
1985年 | 4篇 |
1984年 | 7篇 |
1983年 | 11篇 |
1982年 | 5篇 |
1981年 | 8篇 |
1980年 | 12篇 |
1979年 | 4篇 |
1977年 | 4篇 |
1976年 | 5篇 |
1975年 | 6篇 |
1973年 | 7篇 |
1970年 | 9篇 |
1963年 | 3篇 |
1957年 | 3篇 |
排序方式: 共有974条查询结果,搜索用时 15 毫秒
141.
Synthesis and physical properties of Ca- and Ta-modified (K,Na)NbO3 lead-free piezoelectric ceramics
Indrani Coondoo Neeraj Panwar Radheshyam Rai Harvey Amorín Andrei L. Kholkin 《Phase Transitions》2013,86(11):1130-1140
Polycrystalline samples of lead-free Ca and Ta co-substituted potassium sodium niobate (K0.5Na0.5NbO3, KNN) ceramics have been prepared by solid state reaction technique. X-ray diffraction showed formation of a single-phase perovskite structure with orthorhombic symmetry. Substitution inhibits the grain growth, improves densification and decreases the ferro-paraelectric phase transition temperature. Temperature dependent dielectric permittivity studies demonstrate significant decrease in peak-permittivity values in the substituted samples. Bulk longitudinal piezoelectric coefficient is significantly enhanced, up to ~155 pC/N for (K0.48Na0.48Ca0.02)(Nb0.85Ta0.15O3) as compared to 95 pC/N for pristine KNN ceramic. Local piezoelectric properties have been observed by piezoresponse force microscopy (PFM) technique. Distinct piezocontrast was studied in both vertical and in-plane modes of PFM for all samples. The samples exhibit self-polarization effect in the unpoled state and effective local vertical piezoelectric coefficient was the largest in Ca and Ta co-substituted sample whereas the in-plane piezoelectric coefficient was maximum for Ca-substituted KNN sample. These studies are important for using substituted lead free KNN materials in various piezoelectric applications. 相似文献
142.
The IR spectra have been recorded in the solid state for the parent molecule, 6-(N-pyrrolyl)purine (1) and its N-9-and N-7-substituted derivatives: 9-and 7-(2-hydroxyprop-1-yl)-6-(N-pyrrol-1-yl)purine (2 and 3), 9-and 7-(2-acetoxyprop-1-yl)-6-(N-pyrrol-1-yl)purine (4 and 5), 9-and 7-(2,3-dihydroxyprop-1-yl)-6-(N-pyrrol-1-yl)purine (6 and 7) as well as 9-and 7-(2,3-diacetoxyprop-1-yl)-6-(N-pyrrol-1-yl)purine (8 and 9). Analysis of the characteristic bands has proved to be useful in differentiating between N-9 and N-7 regioisomers. 相似文献
143.
Abstract The modified method of Del Bene and Jaffe' (CNDO-CI) has been applied to study the electronic structure and spectra of the three isomeric xylenes taking a three dimensional geometry for the CH3 group. The spectral transition energies and the corresponding oscillator strengths have been computed and the results are compared with ?-electron calculations and experimental results. Quite satisfactory results have been obtained. Ionization potentials have also been computed by the INDO and extended Hückel methods. 相似文献
144.
In the manganite La1‐xMxMnO3 (M = Ca, Ba, Sr) the doping concentration introduces a mixed valency (Mn3+, Mn4+) which governs the magnetic and electrical properties of the compound. The perovskite oxides La1‐3xCaxBaxSrxMnO3 (x = 0.00, 0.05, 0.10) were prepared by chemical method. Single‐phase formation is confirmed by XRD studies. The electrical behavior of compositions with x = 0.00, 0.05 and 0.10 in the system La1‐3xCaxBaxSrxMnO3 was studied in the temperature range 300‐420 K. It is observed that conductivity decreases with increasing temperature as well as dopants concentration. Metallic behavior of these compositions decreases with increasing dopants concentration (x). The microstructures of these samples have been characterized using scanning electron microscopy (SEM). (© 2007 WILEY ‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
145.
Vijai K. Rai Suhasini Mahata Smita R. Bhardiya Prashant Shukla Ankita Rai Manorama Singh 《Tetrahedron letters》2019,60(7):524-529
Reduced graphene oxide (rGO)–NaBH4 is reported as mild and efficient catalyst-system for chemo-/regioselective reduction of structurally different aliphatic, aromatic as well as α,β-unsaturated aldehydes and ketones in water. The rGO was prepared by reducing graphene oxide using Tulsi leaf extract as bio-reductant. Operational simplicity, ambient reaction condition, high yield of pure products (80–97%), no by-product formation, no use of column chromatography for purification are the salient features of the envisaged protocol. Furthermore, the recovered TRGO was recycled and reused for subsequent reductions up to five times without any loss in activity. 相似文献
146.
ABSTRACTWe study the structural, energetic and electronic properties of the structured water chain clusters within the density functional theory. We refer the structured water chains to those water clusters that have specific geometric patterns stretched along one direction. External electric field required to keep the structures open chain, thereby preventing them to form closed structures, is applied along the length of the chain. The structures are essentially periodic with basic repeating unit consisting of the corner- or edge-sharing 4-, 5- or 6-membered ring water clusters. Our calculations underscore the possible existence of such structured water clusters in the electrostatic environments, which we simulate in its simplicity employing a dipolar, uniform and static electric field. Analysis reveals that the 5-membered ring water chain clusters, i.e. the pentamer chain clusters have the lowest average dipole moment per water molecule while the threshold field, that marks the onset of the field-induced closure of the HOMO (highest occupied molecular orbital)-LUMO (lowest unoccupied molecular orbital) energy gap, is highest, followed by that for the tetramer and hexamer chains. The results suggest that the pentamer chains are the most stable clusters over a wide range of electric fields. 相似文献
147.
JPC – Journal of Planar Chromatography – Modern TLC - Standardization has become mandatory for global acceptance of herbal oriental medicines which lack validated methods of analysis... 相似文献
148.
This study compared eight major ginsenosides (Rg1, Rg2, Rf, Re, Rd, Rc, Rb1 and Rb2) between Panax sokpayensis and Panax bipinnatifidus collected from Sikkim Himalaya, India. High-performance liquid chromatographic analysis revealed that all major ginsenosides were present in the rhizomes of P. sokpayensis except ginsenoside Rc, whereas ginsenoside Rf, Rc and Rb2 were not detected in P. bipinnatifidus. 相似文献
149.
The crystal structure of the title compound PhAs(S2COPri)2 has been determined by single crystal X-ray diffraction and refined by full matrix least squares analysis with anisotropic temperature factors. The crystal is monoclinic, space groupP21/c,a=8.889(6),b=16.174(19),c=13.506(14) Å,=102.02(7)°,V=1899.2 Å3,Z=4,D
c=1.478 g cm–3, monochromated Mo-K radiation, =0.71069 Å,=22.07 cm–1.The geometry around the arsenic atom is square pyramidal. Each ligand is bonded to an arsenic atom through two unequal As-S bonds measuring 2.298 and 3.041 Å. The apical position is occupied by a phenyl group and a lone pair of electrons is presenttrans to the phenyl group. 相似文献
150.
Tripathi G Rai VK Rai A Rai SB 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2008,71(2):486-489
The energy transfer in Er3+: Sm3+ codoped binary TeO2-Li2O (TLO) glass has been studied using 532 nm laser radiation on the basis of fluorescence intensity and the lifetime measurements. It is observed that the trace of erbium ion can be utilized to sensitize the samarium. The mechanism involved in the present case is found to be dipole-dipole. The energy transfer efficiencies, probabilities of energy transfer and the average donor acceptor distance has been evaluated. 相似文献