全文获取类型
收费全文 | 51868篇 |
免费 | 5378篇 |
国内免费 | 4125篇 |
专业分类
化学 | 26150篇 |
晶体学 | 1321篇 |
力学 | 4302篇 |
综合类 | 816篇 |
数学 | 10388篇 |
物理学 | 18394篇 |
出版年
2023年 | 474篇 |
2022年 | 819篇 |
2021年 | 1897篇 |
2020年 | 1443篇 |
2019年 | 1494篇 |
2018年 | 1142篇 |
2017年 | 1283篇 |
2016年 | 1754篇 |
2015年 | 1687篇 |
2014年 | 2236篇 |
2013年 | 3913篇 |
2012年 | 2574篇 |
2011年 | 2928篇 |
2010年 | 2668篇 |
2009年 | 3285篇 |
2008年 | 3410篇 |
2007年 | 3583篇 |
2006年 | 2915篇 |
2005年 | 2080篇 |
2004年 | 1883篇 |
2003年 | 1774篇 |
2002年 | 1662篇 |
2001年 | 1471篇 |
2000年 | 1162篇 |
1999年 | 952篇 |
1998年 | 940篇 |
1997年 | 736篇 |
1996年 | 722篇 |
1995年 | 662篇 |
1994年 | 676篇 |
1993年 | 632篇 |
1992年 | 582篇 |
1991年 | 441篇 |
1990年 | 357篇 |
1989年 | 301篇 |
1988年 | 310篇 |
1987年 | 273篇 |
1986年 | 253篇 |
1985年 | 374篇 |
1984年 | 275篇 |
1983年 | 166篇 |
1982年 | 336篇 |
1981年 | 495篇 |
1980年 | 436篇 |
1979年 | 486篇 |
1978年 | 397篇 |
1977年 | 290篇 |
1976年 | 248篇 |
1974年 | 76篇 |
1973年 | 160篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
31.
《Physics letters. A》2019,383(20):2370-2375
We consider a bosonic Josephson junction in the Bose-Hubbard two-mode approximation where some of the parameters are corrupted by physically meaningful noise processes and study the corresponding relaxation dynamics towards its equilibrium state. We show with numerical simulations that this model can essentially capture all the important features observed in a recent experiment regarding the relaxation dynamics in one-dimensional bosonic Josephson junctions, namely the damped oscillations of the population imbalance and the relative phase, as well as the large final coherence factor. We expect that this work will further motivate research about the origin of relaxation mechanism in these systems. 相似文献
32.
Oxidative dehydrogenation (ODH) of n-octane was carried out over a vanadium–magnesium oxide catalyst in a continuous flow fixed bed reactor. The catalyst was characterized by ICP–OES, powder XRD and SEM. The catalytic tests were carried out at different gas hourly space velocities (GHSVs), viz. 4000, 6000, 8000, and 10,000 h?1. The best selectivity for octenes was obtained at the GHSV of 8000 h?1, while that for C8 aromatics was attained at the GHSV of 6000 h?1 at high temperatures (500 and 550 °C). The catalytic testing at the GHSV of 10,000 h?1 showed the lowest activity, while that at the GHSV of 4000 h?1 consistently showed the lowest ODH selectivity. Generally, the best ODH performance was obtained by the catalytic testing at the GHSVs of 6000 and 8000 h?1. No phasic changes were observed after the catalytic testing. 相似文献
33.
A priori bounds and multiplicity for fully nonlinear equations with quadratic growth in the gradient
We consider fully nonlinear uniformly elliptic equations with quadratic growth in the gradient, such asin a bounded domain with a Dirichlet boundary condition; here , , , and the matrix M satisfies . Recently this problem was studied in the “coercive” case , where uniqueness of solutions can be expected; and it was conjectured that the solution set is more complex for noncoercive equations. This conjecture was verified in 2015 by Arcoya, de Coster, Jeanjean and Tanaka for equations in divergence form, by exploiting the integral formulation of the problem. Here we show that similar phenomena occur for general, even fully nonlinear, equations in nondivergence form. We use different techniques based on the maximum principle.We develop a new method to obtain the crucial uniform a priori bounds, which permit to us to use degree theory. This method is based on basic regularity estimates such as half-Harnack inequalities, and on a Vázquez type strong maximum principle for our kind of equations. 相似文献
34.
Mechanical property and deformation mechanism of gold nanowire with non-uniform distribution of twinned boundaries:A molecular dynamics simulation study 下载免费PDF全文
《中国物理 B》2021,30(5):56101-056101
The mechanical property and deformation mechanism of twinned gold nanowire with non-uniform distribution of twinned boundaries(TBs) are studied by the molecular dynamics(MD) method. It is found that the twin boundary spacing(TBS) has a great effect on the strength and plasticity of the nanowires with uniform distribution of TBs. And the strength enhances with the decrease of TBS, while its plasticity declines. For the nanowires with non-uniform distribution of TBs, the differences in distribution among different TBSs have little effect on the Young's modulus or strength, and the compromise in strength appears. But the differences have a remarkable effect on the plasticity of twinned gold nanowire. The twinned gold nanowire with higher local symmetry ratio has better plasticity. The initial dislocations always form in the largest TBS and the fracture always appears at or near the twin boundaries adjacent to the smallest TBS. Some simulation results are consistent with the experimental results. 相似文献
35.
Cycloparaphenylene (CPP) shows modulated photophysical and electronic properties due to its strained structure and radially oriented π-electron system. Incorporation of CPP into metal-organic frameworks (MOFs) could transfer its extensive properties in solution to porous solids. Moreover, with the unique arrangement of the macrocycles and their interactions with the framework, emerging characteristics are anticipated. As an example of “robust dynamics”, we synthesized the first MOF structure (FDM-1001) with CPP precisely anchored to the ordered framework by employing a [8]CPP-containing linear dicarboxylate linker. Metric relationship between the dynamic macrocycles and the robust backbone creates ideal π-π interactions between them, which leads to an essentially directional arrangement of [8]CPP in the three-dimensional space. Furthermore, the MOF with [8]CPP could be successfully oxidized to generate an infinite array of radicals that show enhanced air stability compared to its molecular analogue. 相似文献
36.
大尺寸低缺陷碳化硅(SiC)单晶体是功率器件和射频(RF)器件的重要基础材料,物理气相传输(physical vapor transport, PVT)法是目前生长大尺寸SiC单晶体的主要方法。获得大尺寸高品质晶体的核心是通过调节组分、温度、压力实现气相组分在晶体生长界面均匀定向结晶,同时尽可能减小晶体的热应力。本文对电阻加热式8英寸(1英寸=2.54 cm)碳化硅大尺寸晶体生长系统展开热场设计研究。首先建立描述碳化硅原料受热分解热质输运及其多孔结构演变、系统热输运的物理和数学模型,进而使用数值模拟方法研究加热器位置、加热器功率和辐射孔径对温度分布的影响及其规律,并优化热场结构。数值模拟结果显示,通过优化散热孔形状、保温棉的结构等设计参数,电阻加热式大尺寸晶体生长系统在晶锭厚度变化、多孔介质原料消耗的情况下均能达到较低的晶体横向温度梯度和较高的纵向温度梯度。 相似文献
37.
针对水化硅酸钙纳米压痕模型忽视了压头与基底之间相互作用的问题,由尺寸差异引起的金刚石压头难以计算的问题,以及Wittmann模型无法得到实际接触面积的问题,提出了新的模型与计算方法.结合分子动力学方法,采用金刚石压头-Wittmann模型基底的组合方式构建无定形态水化硅酸钙纳米压痕试验模型.在建模阶段,考虑到压头模型与基底模型粒子间尺寸差异,提出了等比例替换模型,通过公式推导并就不同尺寸模拟结果验证了等比例替换模型的可行性.在计算阶段,提出了局部前处理的弛豫方法进行模拟.确定最大荷载位置处的接触面积为546 nm2,进而求出水化硅酸钙模型硬度H为0.84 GPa、折合模量Er为30.52 GPa.并通过纳米压痕试验,验证了模拟结果的准确性,证明了模型的科学性,对今后水化硅酸钙(C-S-H)纳米层面的模拟具有重要借鉴意义. 相似文献
38.
《Discrete Mathematics》2022,345(3):112711
We investigate certain nonassociative binary operations that satisfy a four-parameter generalization of the associative law. From this we obtain variations of the ubiquitous Catalan numbers and connections to many interesting combinatorial objects such as binary trees, plane trees, lattice paths, and permutations. 相似文献
39.
We look for positive solutions for the singular equation where , , is a parameter, and has some summability properties. By using a perturbation method and critical point theory, we obtain two solutions when and the parameter is small. 相似文献
40.
The radius of spatial analyticity for solutions of the KdV equation is studied. It is shown that the analyticity radius does not decay faster than as time t goes to infinity. This improves the works of Selberg and da Silva (2017) [30] and Tesfahun (2017) [34]. Our strategy mainly relies on a higher order almost conservation law in Gevrey spaces, which is inspired by the I-method. 相似文献