全文获取类型
收费全文 | 7566篇 |
免费 | 1253篇 |
国内免费 | 689篇 |
专业分类
化学 | 1771篇 |
晶体学 | 52篇 |
力学 | 593篇 |
综合类 | 75篇 |
数学 | 887篇 |
物理学 | 2775篇 |
无线电 | 3355篇 |
出版年
2024年 | 27篇 |
2023年 | 95篇 |
2022年 | 196篇 |
2021年 | 202篇 |
2020年 | 222篇 |
2019年 | 192篇 |
2018年 | 211篇 |
2017年 | 297篇 |
2016年 | 322篇 |
2015年 | 342篇 |
2014年 | 451篇 |
2013年 | 659篇 |
2012年 | 562篇 |
2011年 | 591篇 |
2010年 | 448篇 |
2009年 | 471篇 |
2008年 | 494篇 |
2007年 | 538篇 |
2006年 | 456篇 |
2005年 | 363篇 |
2004年 | 361篇 |
2003年 | 317篇 |
2002年 | 238篇 |
2001年 | 205篇 |
2000年 | 139篇 |
1999年 | 155篇 |
1998年 | 120篇 |
1997年 | 146篇 |
1996年 | 121篇 |
1995年 | 84篇 |
1994年 | 65篇 |
1993年 | 62篇 |
1992年 | 56篇 |
1991年 | 54篇 |
1990年 | 31篇 |
1989年 | 37篇 |
1988年 | 32篇 |
1987年 | 14篇 |
1986年 | 14篇 |
1985年 | 17篇 |
1984年 | 12篇 |
1983年 | 9篇 |
1982年 | 12篇 |
1981年 | 10篇 |
1980年 | 14篇 |
1979年 | 21篇 |
1978年 | 8篇 |
1977年 | 3篇 |
1976年 | 4篇 |
1973年 | 3篇 |
排序方式: 共有9508条查询结果,搜索用时 0 毫秒
911.
912.
913.
设计理想的太阳电池正面电极栅线图形,使高注入条件下的聚光太阳电池获得较高转换效率,是聚光太阳电池研制中的一个重要问题。文章从栅线的总相对功率损失理论出发,采用计算机模拟分析,获得聚光倍数与栅线尺寸及总相对功率损耗的关系;并给出了典型聚光倍数(即250倍、500倍、1000倍)条件下的栅线优化设计。研究结果可为不同聚光倍数下太阳电池电极栅线的制作提供理论依据。 相似文献
914.
Beyond petrochemicals: the renewable chemicals industry 总被引:1,自引:0,他引:1
Vennestrøm PN Osmundsen CM Christensen CH Taarning E 《Angewandte Chemie (International ed. in English)》2011,50(45):10502-10509
915.
Ulrich Soupremanien Stephane Le PersonMichel Favre-Marinet Yann Bultel 《Experimental Thermal and Fluid Science》2011,35(5):797-809
This article presents experiments conducted with two single rectangular mini-channels of same hydraulic diameter (1.4 mm) and different aspect ratios for conditions of horizontal boiling flow. The Forane® 365 HX used was subcooled (ΔTsub = 15 °C) for all the boiling curves presented in the paper. Local heat transfer coefficients were measured for heat flux ranging from 25 to 62 kW m−2 and mass flux from 200 kg m−2 s−1 to 400 kg m−2 s−1. The boiling flows were observed with two different cameras (depending on the flow velocity) through a visualization window. The flow patterns in the two channels were compared for similar conditions. The results show that the boiling heat transfer coefficient and the pressure drop values are different for the two single mini-channels. For low heat flux condition, the channel with lowest aspect ratio (H/W = 0.143) has a higher heat transfer coefficient. On the other hand, for high heat flux condition, the opposite situation occurs, namely the heat transfer coefficient becomes higher for the channel with highest aspect ratio (H/W = 0.43). This is probably due to the earlier onset of dryout in the channel with lowest aspect ratio. For the two cases of heating, the pressure drop for the two-phase flow remains lower for the channel with lowest aspect ratio. These results show that the aspect ratio plays a substantial role for boiling flows in rectangular channels. As for single-phase flows, the heat transfer characteristics are significantly influenced (even though the hydraulic diameter remains the same) by this parameter. 相似文献
916.
Experimental results about concrete under sulfate attack are summarized,which include the variation of mass density of samples and velocity of ultrasonic wave propagating in samples.The evolution damage is analyzed in terms of the experimental results,and close attention is paid to the effect of damage evolution on Poisson’s ratio.This study shows that Poisson’s ratio is significantly affected by the concentration of solution and water-cement ratio.Poisson’s ratio of concrete changes very little when the water-cement ratio is selected as 0.6 or 0.8,so that such change may be neglected.If water-cement is 0.4,however,the Poisson’s ratio of the sample significantly changes.When the concrete sample of 0.4 water-cement ratio is immersed in sodium sulfate solution of 8% concentration for 285 days,Poisson’s ratio increase 10.14% compared with its initial value.There exist a sensitive region and a non-sensitive region for the change rate of Poisson’s ratio with respect to corrosion time.The change rate of Poisson’s ratio monotonously decreases with corrosion time in the sensitive region;in the non-sensitive region,the change rate of Poisson’s ratio is almost equal to zero. 相似文献
917.
918.
A numerical study of the three-dimensional fluid flow has been carried out to determine the effects of the transverse aspect ratio, Ay, on the flow structure in two-sided non-facing lid-driven cavities. The flow is complex, unstable and can undergo bifurcation. The numerical method is based on the finite volume method and multigrid acceleration. Computations have been investigated for several Reynolds numbers and various aspect ratio values. At a fixed Reynolds number, Re=500, the three-dimensional flow characteristics are analyzed considering four transverse aspect ratios, Ay=1,0.75,0.5 and 0.25. It is observed that the transition to the unsteady regime follows the classical scheme of a Hopf bifurcation. An analysis of the flow evolution shows that, at Ay=0.75, the flow bifurcates to a periodic regime at (Re=600) with a frequency f=0.093 less than the predicted value in the cubical cavity. A correlation is established when Ay=0.5 and gives the critical Reynolds number value. At Ay=0.25, the periodic regime occurs at high Re value beyond 3500, after which the flow becomes chaotic. It is shown that, when increasing Ay over the unit, the flow in the cavity exhibits a complex behavior. The kinetic energy transmission from the driven walls to the cavity center is reduced at low Ay values. 相似文献
919.
In this paper, we consider the problem of central configurations of the n-body problem with the general homogeneous potential 1/rα. A configuration q=(q1,q2,…,qn) is called a super central configuration if there exists a positive mass vector m=(m1,…,mn) such that q is a central configuration for m with mi attached to qi and q is also a central configuration for m′, where m′≠m and m′ is a permutation of m. The main discovery in this paper is that super central configurations of the n-body problem have surprising connections with the golden ratio φ. Let r be the ratio of the collinear three-body problem with the ordered positions q1, q2, q3 on a line. q is a super central configuration if and only if 1/r1(α)<r<r1(α) and r≠1, where r1(α)>1 is a continuous function such that , the golden ratio. The existence and classification of super central configurations are established in the collinear three-body problem with general homogeneous potential 1/rα. Super central configurations play an important role in counting the number of central configurations for a given mass vector which may decrease the number of central configurations under geometric equivalence. 相似文献
920.
采用传统固相法成功制备了(1-x)(Ba0.85Ca0.15)(Zr0.05Ti0.95)O3-xYMnO3(BCZT-YMn)无铅压电陶瓷,研究了YMnO3掺杂对BCZT陶瓷相结构及电性能的影响。结果表明,陶瓷在相变中出现了铁电性优化的现象,当x=0.060时,获得了较佳的铁电性能,外加电场强度为4kV/mm时,其剩余极化强度为18.03μC/cm2,矫顽电场强度为0.9kV/mm,室温下介电常数为3 375,最大介电常数为3 507。当x>0.060时,铁电性突变消失,介电性出现居里峰展宽的现象,同时居里温度亦随之变化。 相似文献