全文获取类型
收费全文 | 99374篇 |
免费 | 9036篇 |
国内免费 | 9648篇 |
专业分类
化学 | 33579篇 |
晶体学 | 988篇 |
力学 | 14138篇 |
综合类 | 1590篇 |
数学 | 37115篇 |
物理学 | 30648篇 |
出版年
2023年 | 901篇 |
2022年 | 1406篇 |
2021年 | 1767篇 |
2020年 | 2313篇 |
2019年 | 2345篇 |
2018年 | 2023篇 |
2017年 | 2535篇 |
2016年 | 2826篇 |
2015年 | 2262篇 |
2014年 | 3928篇 |
2013年 | 6646篇 |
2012年 | 4693篇 |
2011年 | 5440篇 |
2010年 | 4630篇 |
2009年 | 5971篇 |
2008年 | 6306篇 |
2007年 | 6535篇 |
2006年 | 6029篇 |
2005年 | 5317篇 |
2004年 | 4710篇 |
2003年 | 4625篇 |
2002年 | 4069篇 |
2001年 | 3420篇 |
2000年 | 3237篇 |
1999年 | 2886篇 |
1998年 | 2664篇 |
1997年 | 2257篇 |
1996年 | 1958篇 |
1995年 | 1816篇 |
1994年 | 1718篇 |
1993年 | 1446篇 |
1992年 | 1401篇 |
1991年 | 1093篇 |
1990年 | 847篇 |
1989年 | 747篇 |
1988年 | 633篇 |
1987年 | 497篇 |
1986年 | 406篇 |
1985年 | 477篇 |
1984年 | 481篇 |
1983年 | 239篇 |
1982年 | 379篇 |
1981年 | 419篇 |
1980年 | 314篇 |
1979年 | 324篇 |
1978年 | 243篇 |
1977年 | 233篇 |
1976年 | 165篇 |
1974年 | 116篇 |
1973年 | 108篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
利用hirota双线性法,得到(3+1)维孤子方程、(3+1)维KP-Boussinesq方程、(2+1)维修正Caudrey-Dodd-Gibbon-Kotera-S awada方程、Hirota-Satsuma浅水波方程的精确解,并做出一部分解的图形,进一步研究解的结构和性质. 相似文献
2.
This paper proposes a method for solving optimisation problems involving piecewise quadratic functions. The method provides a solution in a finite number of iterations, and the computational complexity of the proposed method is locally polynomial of the problem dimension, i.e., if the initial point belongs to the sufficiently small neighbourhood of the solution set. Proposed method could be applied for solving large systems of linear inequalities. 相似文献
3.
In the assessment of most complex socioeconomic phenomena with the use of multicriteria methods, continuous data are used, the source of which are most often public statistics. However, there are complex phenomena such as quality of life and quality of services in the assessment, for which questionnaire surveys and ordinal measurement scales are used. In this case, the use of classic multicriteria methods is very difficult, taking into account the way of presenting this type of data by official statistics, as well as their permissible transformations and arithmetic operations. Therefore, the main purpose of this study was the presentation of a novel framework which can be applied for assessing socioeconomic phenomena on the basis of survey data. It was assumed that the object assessments may contain positive or negative opinions and an element of uncertainty expressed in the form a “no”, “difficult to say”, or “no opinion” answers. For this reason, the intuitionistic fuzzy TOPSIS (IF-TOPSIS) method is proposed. To demonstrate the potential of this solution, the results of measuring the subjective quality of life of the inhabitants of 83 cities in EU countries, EFTA countries, the UK, the Western Balkans, and Turkey are presented. For most cities, a high level of subjective quality of life was observed using the proposed approach. The highest level of quality of life was observed in Zurich, whereas the lowest was observed in Palermo. 相似文献
4.
Sol-gel method has been used for the synthesis of biocompatible superparamagnetic nanoferrites of AFeO2 (A = Li, Na, K, Ca). Structural study of the nanoferrites reveals that LiFeO2 exhibits cubic phase on the other hand NaFeO2, KFeO2, CaFeO2 nanoparticles possess orthorhombic phase. Transmission electron microscopy (TEM) suggests that synthesized nanoferrites are nano-sized with spherical morphology. Optical properties confirm that nanoferrites emit and absorb light in a visible range of the electromagnetic spectrum. International Commission on Illumination (CIE) study discloses that the nanoparticles can be used to produce light of various colors. Magnetic study reveals that the nanoferrites exhibit superparamagnetic nature with high values of saturation magnetization 40.26 emu/g, 41.69 emu/g, 57.16 emu/g, and 43.66 emu/g, respectively for LiFeO2, NaFeO2, KFeO2, and CaFeO2. Biocompatibility study of the nanoferrites has been performed using Sulforhodamine B (SRB) and 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. The reason behind the observed properties and behavior has been discussed. 相似文献
5.
In this paper, we consider a scenario where the base station (BS) collects time-sensitive data from multiple sensors through time-varying and error-prone channels. We characterize the data freshness at the terminal end through a class of monotone increasing functions related to Age of information (AoI). Our goal is to design an optimal policy to minimize the average age penalty of all sensors in infinite horizon under bandwidth and power constraint. By formulating the scheduling problem into a constrained Markov decision process (CMDP), we reveal the threshold structure for the optimal policy and approximate the optimal decision by solving a truncated linear programming (LP). Finally, a bandwidth-truncated policy is proposed to satisfy both power and bandwidth constraint. Through theoretical analysis and numerical simulations, we prove the proposed policy is asymptotic optimal in the large sensor regime. 相似文献
6.
Compared with the traditional magnetic bubble, a skyrmion has a smaller size, and better stability and therefore is considered as a very promising candidate for future memory devices. When skyrmions are manipulated, erased and created, the density of skyrmions can be varied, however the relationship between the radii and the densities of skyrmions needs more exploration. In this paper, we study this problem both theoretically and by using the lattice simulation. The average radius of skyrmions as a function of material parameters, the strength of the external magnetic field and the density of skyrmions is obtained and verified. With this explicit function, the skyrmion radius can be easily predicted, which is helpful for the future study of skyrmion memory devices. 相似文献
7.
8.
数学均匀化方法是计算周期复合材料结构的有效方法之一,单胞边界条件施加的合理性直接决定了影响函数控制方程的计算效率和精度,进而影响均匀化弹性参数和摄动位移的计算精度.本文首先将单胞影响函数作为虚拟位移处理,给出了单胞在结构中真实的边界条件,结果表明,四边固支适合作为二维结构单胞边界条件;其次,针对二维结构提出了超单胞周期边界条件,有效提高了影响函数的计算精度,并使用与虚拟位移相对应的虚拟势能泛函验证超单胞周期边界条件的有效性;最后,利用数值分析验证多尺度渐进展开方法的计算精度,强调了二阶摄动的必要性. 相似文献
9.
10.
给出了求解多自由度动力学系统响应的M atlab程序,这些程序基于振型叠加法可用于求解由质量矩阵M和刚度矩阵K以及常见阻尼矩阵描述的线性离散系统的时域和频域解.对于无阻尼系统,用户可以选择数值解或符号解析解(以时间或频率表示),并利用复模态叠加法计算了阻尼系统的数值解.总结了模态叠加方法下动力学响应的求解,并在简短的M atlab程序中实现.以三自由度系统和悬臂梁模型为例说明了程序的应用.这些程序也可用于工程应用中,通过对商用有限元软件包产生的质量和刚度进行后处理,产生感兴趣的时域和频域响应. 相似文献