全文获取类型
收费全文 | 12926篇 |
免费 | 954篇 |
国内免费 | 1078篇 |
专业分类
化学 | 5533篇 |
晶体学 | 116篇 |
力学 | 909篇 |
综合类 | 96篇 |
数学 | 2318篇 |
物理学 | 4178篇 |
无线电 | 1808篇 |
出版年
2024年 | 17篇 |
2023年 | 104篇 |
2022年 | 172篇 |
2021年 | 195篇 |
2020年 | 289篇 |
2019年 | 241篇 |
2018年 | 256篇 |
2017年 | 353篇 |
2016年 | 424篇 |
2015年 | 355篇 |
2014年 | 667篇 |
2013年 | 860篇 |
2012年 | 788篇 |
2011年 | 890篇 |
2010年 | 739篇 |
2009年 | 906篇 |
2008年 | 855篇 |
2007年 | 910篇 |
2006年 | 878篇 |
2005年 | 649篇 |
2004年 | 550篇 |
2003年 | 533篇 |
2002年 | 431篇 |
2001年 | 308篇 |
2000年 | 309篇 |
1999年 | 259篇 |
1998年 | 208篇 |
1997年 | 218篇 |
1996年 | 205篇 |
1995年 | 154篇 |
1994年 | 170篇 |
1993年 | 125篇 |
1992年 | 122篇 |
1991年 | 115篇 |
1990年 | 88篇 |
1989年 | 73篇 |
1988年 | 65篇 |
1987年 | 62篇 |
1986年 | 45篇 |
1985年 | 48篇 |
1984年 | 62篇 |
1983年 | 15篇 |
1982年 | 42篇 |
1981年 | 37篇 |
1980年 | 24篇 |
1979年 | 31篇 |
1977年 | 25篇 |
1976年 | 15篇 |
1974年 | 17篇 |
1973年 | 15篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
971.
Darian E Hnizdo V Fedorowicz A Singh H Demchuk E 《Journal of computational chemistry》2005,26(7):651-660
A method of statistical estimation is applied to the problem of evaluating the absolute entropy of internal rotation in a molecule with two torsional degrees of freedom. The configurational part of the entropy is obtained as that of the joint probability density of an arbitrary form represented by a two-dimensional Fourier series, the coefficients of which are statistically estimated using a sample of the torsional angles of the molecule obtained by a stochastic simulation. The internal rotors in the molecule are assumed to be attached to a common frame, and their reduced moments of inertia are initially calculated as functions of the two torsional angles, but averaged over all the remaining internal degrees of freedom using the stochastic-simulation sample of the atomic configurations of the molecule. The torsional-angle dependence of the reduced moments of inertia can be also averaged out, and the absolute internal-rotation entropy of the molecule is obtained in a good approximation as the sum of the configurational entropy and a kinetic contribution fully determined by the averaged reduced moments of inertia. The method is illustrated using Monte Carlo simulations of isomers of stilbene and halogenated derivatives of propane. The two torsional angles in cis-stilbene are found to be much more strongly correlated than those in trans-stilbene, while the degree of the angular correlation in propane increases strongly on substitution of hydrogen atoms with chlorine. 相似文献
972.
采用自制二胺1,2-二氢-2-(4-氨基苯基)-4-[4-(4-氨基苯氧基)-苯基]-二氮杂萘-1-酮、商品二胺4,4′-二氨基二苯甲烷和对苯二胺与对苯二甲酰氯进行低温溶液缩聚反应,改变三种二胺的比例,得到了一系列共聚酰胺树脂,其特性粘数为1·24~2·32dL/g,用FT-IR、1H-NMR手段研究了聚合物的结构;利用DSC和TGA研究了聚合物的耐热性能,结果表明,该类聚合物具有较高玻璃转化温度(301℃以上);氮气气氛中5%热失重温度在438℃以上;用一定比例的二胺制得的聚合物分别能溶于N-甲基吡咯烷酮、N,N-二甲基乙酰胺等非质子极性溶剂中并浇注得到韧性薄膜. 相似文献
973.
The conversion efficiency of stimulated Raman scattering (SRS) in CH4 is studied by using a single longitudinal mode second-harmonic Nd:YAG laser (532 nm, linewidth 0.003 cm-1, pulse-width (FWHM) 6.5 ns).Due to the heat release from vibrationally excited particles, SRS processes often suffer from the thermal defocusing effect (TDE). In view of 6.5 ns laser pulse width is much shorter than the vibrational relaxation time of CH4 molecules, TDE can only affect the SRS processes afterwards. In the cases of low laser repetition, TDE will be not serious, because it will be removed by the thermal diffusion in Raman medium before the next pulse arrives. At the laser repetition rate 2 Hz, CH4 pressure 1.1 MPa and pump laser energy 95 m J, the quantum conversion efficiency of backward first-Stokes (BS1) has attained 73%. This represents the highest first-stokes conversion efficiency in CH4. Furthermore, due to the relaxation oscillation, the BS1pulses are narrowed to about 1.2 ns. As a result, the BS1 peak power turns out to be 2.7 times that of the pump. Its beam quality is also much better and is only slightly affected by TDE. This reason is that BS1 represents a wave-front-reversed replica of the pump beam, which can compensate the thermal distortions in Raman amplify process. Under the same conditions, but pump laser repetition rate as 10 Hz, the conversion efficiency of BS1 goes down to 36% due to TDE. From this study, we expect that a well-behaved 630 nm Raman laser may be designed by using a closed CH4/He circulating-cooling system, which may have some important applications. 相似文献
974.
M. P. Sánchez Sánchez J. M. Salas Peregrin M. A. Romero Molina 《Monatshefte für Chemie / Chemical Monthly》1987,118(8-9):883-891
Some news thiopyrimidine derivatives and complexes [4-amino-5-nitroso-6-oxo-1,2,3,6-tetrahydro-2-thio-pyrimidine (TANH), its 2-methylthio derivative (MTH), the ammonium salt ofTANH (sTANH) and six new complexes of formulas: Rh(MT)2Cl · 2H2O, Pd(MTH)2Cl2, Pt(MTH)2Cl4, Au(MTH)Cl3 Pd(TANH)2Cl2 and Au(TAN
–)Cl] have been synthesized and characterized by elemental analysis, IR and1H-NMR spectroscopy techniques. The thermal behaviour of all compounds has also been studied.
Rh(III), Pd(II), Pt(IV) und Au(III) Komplexe von 2-Thiopyrimidin Derivaten
Zusammenfassung Es wurden einige neue Thiopyrimidinderivate und deren Komplexe synthetisiert und mittels Elementaranalyse, IR und1H-NMR charakterisiert: 4-Amino-5-nitroso-6-oxo-1,2,3,6-tetrahydro-2-thio-pyrimidin (TANH), dessen 2-Methylthio-Derivat (MTH), das Ammoniumsalz vonTANH (sTANH) und sechs neue Komplexe der Formeln Rh(MT)2Cl · 2H2O, Pd(MTH)2Cl2, Pt(MTH)2Cl4, Au(MTH)Cl3, Pd(TANH)2Cl2 und Au(TAN –)Cl. Das thermische Verhalten der Verbindungen wurde ebenfalls untersucht.相似文献
975.
为提高数字电路的可测性,提出了可实现任意逻辑函数的正极性或-符合网络的易测性实现,并给出了测试网络中所有单固定故障的通用测试集.该实现基于逻辑函数的正极性或-符合展开,网络的同或部分分别采用了串联和树形结构.为提高可测性,同或串的实现结构只需增加1个控制端及1个观察端,同或树的实现结构只需增加3个控制端及1个观察端.对于1个n变量的逻辑函数,两种实现结构下通用测试集的基数分别为(n+4)和(n+5).这样短的通用测试集非常适合用内建自测试实现,从而有效地缩短测试时间. 相似文献
976.
Multilayer membranes prepared via microlayer coextrusion have attracted wide attention due to their unique properties and broad applications. In present study, the foam/film alternating multilayer sheets based on ethylene-vinyl acetate copolymer(EVA) and high-density polyethylene are successfully prepared via microlayer coextrusion. The cells in the sheets are single-cell-array along the foamed EVA layers with uniform cell size. In addition, the effects of layer number and foam relative thickness on morphology, mechanical properties, damping and heat insulation properties are investigated. The cell size decreases significantly with increasing layer number due to the enhanced confine effects. The tensile strength, elongation at break, and heat insulation also increase significantly. However, the mechanical damping properties change little in the observed frequency. Meanwhile, with higher relative thickness of EVA foam, the sheets have lower tensile strength and lower thermal conductivity, while the damping properties are enhanced in a specific frequency scope. The elongation at break of the optimized sample comes to 800% and the thermal conductivity decreases to 61 mW·m~(-1)·K~(-1), which shows high toughness and low thermal conductivity, indicating a possible method for preparing materials with high toughness and heat-insulating properties. 相似文献
977.
Pet'kov V. I. Orlova A. I. Kazantsev G. N. Samoilov S. G. Spiridonova M. L. 《Journal of Thermal Analysis and Calorimetry》2001,66(2):623-632
A5–4xZrxZr(PO4)3 (A=Na, K;0≤x≤1.25), Na1-xCd0.5xZr2(PO4)3 (0≤x≤1), Na5–xCd0.5xZr(PO4)3 (0≤x≤4) compositions which belong to the NZP structural family were synthesized using the sol-gel method. The lattice thermal
expansion of members of these rows were determined up to 600°C by high-temperature X-ray diffractometry. The axial thermal
expansion coefficients change from -5.8·10-6to 7.5·10-6 °C-1 (αa) and from 2.6·10–6 to 22·10–6 °C-1 (αc). These results, in addition to those for other NZP compounds allow us to explain their low thermal expansion. The mechanism
can be attributed to strongly bonded three-dimensional network structure, the existence of structural holes capable to damp
some of the thermal vibrations and anisotropyin the thermal expansion of the lattice.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
978.
红外热像隐身的评价原则 总被引:1,自引:1,他引:1
从热成像系统的静态性能评估公式出发,对实际使用条件下热成隐系统的温度分辨力和空间分辨力进行了修正。以人眼通过热成像系统不能识别目标为条件,得到了红外热像隐身评价原则的关系表达式,并提出了红外热像隐身目标与背景温差阈值的确定方法。 相似文献
979.
Liangliang DuXionghua Wu 《Applied mathematics and computation》2011,218(4):1379-1388
A rational differential quadrature method in irregular domains (RDQMID) is investigated to deal with a kind of singularly perturbed problems with boundary layers. Through a transformation, the boundary layer, which may be not straight, is transformed into a segment of a line parallel to one of the Cartesian axes. The rational differential quadrature method (RDQM) is applied to discretize the governing equation. Finally, a direct expansion method of the boundary conditions (DEMBC) is raised to deal with the boundary conditions. Numerical experiments show that RDQMID is of high accuracy, efficiency and easy to programme. 相似文献
980.
MeiJiao Wang 《Applied mathematics and computation》2012,218(9):5259-5264
By means of the Hermite transformation, a new general ansätz and the symbolic computation system Maple, we apply the Riccati equation rational expansion method [24] to uniformly construct a series of stochastic non-traveling wave solutions for stochastic differential equations. To illustrate the effectiveness of our method, we take the stochastic mKdV equation as an example, and successfully construct some new and more general solutions. The method can also be applied to solve other nonlinear stochastic differential equation or equations. 相似文献