全文获取类型
收费全文 | 40442篇 |
免费 | 1788篇 |
国内免费 | 2804篇 |
专业分类
化学 | 26752篇 |
晶体学 | 50篇 |
力学 | 1450篇 |
综合类 | 56篇 |
数学 | 2550篇 |
物理学 | 14176篇 |
出版年
2023年 | 102篇 |
2022年 | 166篇 |
2021年 | 143篇 |
2020年 | 152篇 |
2019年 | 172篇 |
2018年 | 249篇 |
2017年 | 225篇 |
2016年 | 354篇 |
2015年 | 332篇 |
2014年 | 425篇 |
2013年 | 388篇 |
2012年 | 3455篇 |
2011年 | 4613篇 |
2010年 | 1060篇 |
2009年 | 433篇 |
2008年 | 3266篇 |
2007年 | 3443篇 |
2006年 | 3473篇 |
2005年 | 3440篇 |
2004年 | 2732篇 |
2003年 | 2074篇 |
2002年 | 1888篇 |
2001年 | 1544篇 |
2000年 | 1512篇 |
1999年 | 776篇 |
1998年 | 339篇 |
1997年 | 243篇 |
1996年 | 700篇 |
1995年 | 638篇 |
1994年 | 624篇 |
1993年 | 776篇 |
1992年 | 688篇 |
1991年 | 373篇 |
1990年 | 412篇 |
1989年 | 369篇 |
1988年 | 352篇 |
1987年 | 362篇 |
1986年 | 332篇 |
1985年 | 371篇 |
1984年 | 187篇 |
1983年 | 131篇 |
1982年 | 125篇 |
1981年 | 99篇 |
1975年 | 123篇 |
1959年 | 83篇 |
1958年 | 128篇 |
1957年 | 126篇 |
1956年 | 113篇 |
1955年 | 119篇 |
1954年 | 92篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
1.
We give a characterization of the cyclic subgroup separability and weak potency of the fundamental group of a graph of polycyclic-by-finite groups and free-by-finite groups amalgamating edge subgroups of the form × D,where h has infinite order and D is finite. 相似文献
2.
Applied Mathematics and Mechanics - Mechanical models of residually stressed fibre-reinforced solids, which do not resist bending, have been developed in the literature. However, in some residually... 相似文献
3.
多孔有机笼(POCs)是具有一类内在的、客体可及的空腔的离散分子,是一类独特的微孔材料。本文根据席夫碱反应原理,用(1R,2R)-二氨基环己烷和3,3′,5,5′-四醛基-4,4′-联苯二酚缩合成一种孔径均匀、高比表面积、热化学稳定性良好的棱柱形手性多孔有机笼(POCs)。采用核磁共振氢谱仪、红外光谱、热重分析和扫描电子显微镜对该材料进行表征。将该材料溶解在二氯甲烷中,用动态涂覆法将溶液均匀地涂覆在石英毛细管内壁上,制成毛细管电色谱柱。结果表明,该手性电色谱柱不仅能拆分氧氟沙星、特罗格尔碱、2-氨基-1-丁醇和1-苯基-1-戊醇4种手性药物,还能拆分o,m,p-甲苯胺和o,m,p-氯苯胺2种位置异构体,说明该手性柱具有良好的手性分离能力。通过研究氧氟沙星、特罗格尔碱、2-氨基-1-丁醇和1-苯基-1-戊醇的最佳拆分条件,得出电压、缓冲溶液浓度和pH值对组分分离度有显著影响,其中,氧氟沙星、特罗格尔碱、2-氨基-1-丁醇的最优分离电压均为15 kV, 1-苯基-1-戊醇的最佳分离电压为17 kV;氧氟沙星、特罗格尔碱、2-氨基-1-丁醇、1-苯基-1-戊醇的最佳缓冲液浓度均为0.100 ... 相似文献
4.
Dongyoon Shin Sabita Bhandari Marc FTesch Shannon ABonke Frédéric Jaouen Sonia Chabbra Christoph Pratsch Alexander Schnegg Anna K.Mechler 《Journal of Energy Chemistry》2022,(2)
The incorporation of Pt into an iron-nitrogen-carbon(Fe NC)catalyst for the oxygen reduction reaction(ORR)was recently shown to enhance catalyst stability without Pt directly contributing to the ORR activity.However,the mechanistic origin of this stabilisation remained obscure.It is established herein with rotating ring disc experiments that the side product,H2O2,which is known to damage FeNC catalysts,is suppressed by the presence of Pt.The formation of reactive oxygen species is additionally inhibited,independent of intrinsic H2O2 formation,as determined by electron paramagnetic resonance.Transmission electron microscopy identifies an oxidised Fe-rich layer covering the Pt particles,thus explaining the inactivity of the latter towards the ORR.These insights develop understanding of Fe NC degradation mechanisms during ORR catalysis,and crucially establish the required properties of a precious metal free protective catalyst to improve Fe NC stability in acidic media. 相似文献
5.
Beginning with precise data on the ratio of structure functions in deep inelastic scattering(DIS) from 3 He and 3 H, collected on the domain 0.19 ≤ xB≤ 0.83, where xBis the Bjorken scaling variable, we employ a robust method for extrapolating such data to arrive at a model-independent result for the xB= 1 value of the ratio of neutron and proton structure functions. Combining this with information obtained in analyses of DIS from nuclei, corrected for target-structure depen... 相似文献
6.
Kaiya Wang Xueqi Tian Jacobs H.Jordan Krishnasamy Velmurugan Leyong Wang Xiao-Yu Hu 《中国化学快报》2022,(1):89-96
Over the last decade, numerous research efforts have been devoted to pillar[n]arenes since their debut.The popularity of pillararenes is a reflection of current research trend in supramolecular and macrocyclic chemistry in general. Among the vast applications(such as chemosensors, drug delivery, transmembrance channels, and separation) of pillararenes, their utilization in catalysis is a relatively less explored area.However, soaring attention has been paid by researchers in recent years and thi... 相似文献
7.
The characteristics of heat transfer in the three-dimensional stagnationpoint flow past a stretching/shrinking surface of the Al_2O_3-Cu/H_2O hybrid nanofluid with anisotropic slip are investigated. The partial differential equations are converted into a system of ordinary differential equations by valid similarity transformations. The simplified mathematical model is solved computationally by the bvp4c approach in the MATLAB operating system. This solving method is capable of generating more than one solutions when suitable initial guesses are proposed. The results are proven to have dual solutions, which consequently lead to the application of a stability analysis that verifies the achievability of the first solution. The findings reveal infinite values of the dual solutions at several measured parameters causing the non-appearance of the turning points and the critical values. The skin friction increases with the addition of nanoparticles, while the escalation of the anisotropic slip effect causes a reduction in the heat transfer rate. 相似文献
8.
This paper investigates the unsteady stagnation-point flow and heat transfer over a moving plate with mass transfer,which is also an exact solution to the unsteady Navier-Stokes(NS)equations.The boundary layer energy equation is solved with the closed form solutions for prescribed wall temperature and prescribed wall heat flux conditions.The wall temperature and heat flux have power dependence on both time and spatial distance.The solution domain,the velocity distribution,the flow field,and the temperature distribution in the fluids are studied for different controlling parameters.These parameters include the Prandtl number,the mass transfer parameter at the wall,the wall moving parameter,the time power index,and the spatial power index.It is found that two solution branches exist for certain combinations of the controlling parameters for the flow and heat transfer problems.The heat transfer solutions are given by the confluent hypergeometric function of the first kind,which can be simplified into the incomplete gamma functions for special conditions.The wall heat flux and temperature profiles show very complicated variation behaviors.The wall heat flux can have multiple poles under certain given controlling parameters,and the temperature can have significant oscillations with overshoot and negative values in the boundary layers.The relationship between the number of poles in the wall heat flux and the number of zero-crossing points is identified.The difference in the results of the prescribed wall temperature case and the prescribed wall heat flux case is analyzed.Results given in this paper provide a rare closed form analytical solution to the entire unsteady NS equations,which can be used as a benchmark problem for numerical code validation. 相似文献
9.
This work deals with a study of the dynamic and buckling analysis of polymer hybrid composite(PHC) beam. The beam has variable thickness and is reinforced by carbon nanotubes(CNTs) and nanoclay(NC) simultaneously. The governing equations are derived based on the first shear deformation theory(FSDT). A three-phase HalpinTsai approach is used to predict the mechanical properties of the PHC. We focus our attention on the effect of the simultaneous addition of NC and CNT on the vibration and buckling analysis of the PHC beam with variable thickness. Also a comparison study is done on the sensation of three impressive parameters including CNT, NC weight fractions, and the shape factor of fillers on the mechanical properties of PHC beams,as well as fundamental frequencies of free vibrations and critical buckling load. The results show that the increase of shape factor value, NC, and CNT weight fractions leads to considerable reinforcement in mechanical properties as well as increase of the dimensionless fundamental frequency and buckling load. The variation of CNT weight fraction on elastic modulus is more sensitive rather than shear modulus but the effect of NC weight fraction on elastic and shear moduli is fairly the same. The shape factor values more than the medium level do not affect the mechanical properties. 相似文献
10.