全文获取类型
收费全文 | 70724篇 |
免费 | 5844篇 |
国内免费 | 6419篇 |
专业分类
化学 | 25484篇 |
晶体学 | 282篇 |
力学 | 6648篇 |
综合类 | 1284篇 |
数学 | 25745篇 |
物理学 | 23544篇 |
出版年
2023年 | 561篇 |
2022年 | 880篇 |
2021年 | 1158篇 |
2020年 | 1349篇 |
2019年 | 1587篇 |
2018年 | 1348篇 |
2017年 | 1635篇 |
2016年 | 1899篇 |
2015年 | 1693篇 |
2014年 | 2532篇 |
2013年 | 4615篇 |
2012年 | 2838篇 |
2011年 | 3282篇 |
2010年 | 2793篇 |
2009年 | 3987篇 |
2008年 | 4403篇 |
2007年 | 4622篇 |
2006年 | 4277篇 |
2005年 | 3624篇 |
2004年 | 3292篇 |
2003年 | 3357篇 |
2002年 | 2944篇 |
2001年 | 2548篇 |
2000年 | 2459篇 |
1999年 | 2172篇 |
1998年 | 2025篇 |
1997年 | 1684篇 |
1996年 | 1476篇 |
1995年 | 1411篇 |
1994年 | 1311篇 |
1993年 | 1127篇 |
1992年 | 1081篇 |
1991年 | 831篇 |
1990年 | 697篇 |
1989年 | 621篇 |
1988年 | 559篇 |
1987年 | 437篇 |
1986年 | 374篇 |
1985年 | 438篇 |
1984年 | 440篇 |
1983年 | 212篇 |
1982年 | 343篇 |
1981年 | 405篇 |
1980年 | 302篇 |
1979年 | 293篇 |
1978年 | 222篇 |
1977年 | 207篇 |
1976年 | 157篇 |
1974年 | 109篇 |
1973年 | 106篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
101.
We give a sheaf theoretic interpretation of Potts models with external magnetic field, in terms of constructible sheaves and their Euler characteristics. We show that the polynomial countability question for the hypersurfaces defined by the vanishing of the partition function is affected by changes in the magnetic field: elementary examples suffice to see non-polynomially countable cases that become polynomially countable after a perturbation of the magnetic field. The same recursive formula for the Grothendieck classes, under edge-doubling operations, holds as in the case without magnetic field, but the closed formulae for specific examples like banana graphs differ in the presence of magnetic field. We give examples of computation of the Euler characteristic with compact support, for the set of real zeros, and find a similar exponential growth with the size of the graph. This can be viewed as a measure of topological and algorithmic complexity. We also consider the computational complexity question for evaluations of the polynomial, and show both tractable and NP-hard examples, using dynamic programming. 相似文献
102.
《中国物理 B》2021,30(5):57303-057303
A novel super-junction LDMOS with low resistance channel(LRC), named LRC-LDMOS based on the silicon-oninsulator(SOI) technology is proposed. The LRC is highly doped on the surface of the drift region, which can significantly reduce the specific on resistance(Ron,sp) in forward conduction. The charge compensation between the LRC, N-pillar,and P-pillar of the super-junction are adjusted to satisfy the charge balance, which can completely deplete the whole drift,thus the breakdown voltage(BV) is enhanced in reverse blocking. The three-dimensional(3 D) simulation results show that the BV and R_(on,sp) of the device can reach 253 V and 15.5 mΩ·cm~2, respectively, and the Baliga's figure of merit(FOM = BV~2/R_(on,sp)) of 4.1 MW/cm~2 is achieved, breaking through the silicon limit. 相似文献
103.
Palladium‐catalyzed decarboxylative coupling of α,β‐unsaturated carboxylic acids with aryl tosylates
We report a general method for selective cross‐coupling of α,β‐unsaturated carboxylic acids with aryl tosylates enabled by versatile Pd(II) complexes. This method features the general cross‐coupling of ubiquitous α,β‐unsaturated carboxylic acids by decarboxylation. The transformation is characterized by its operational simplicity, the use of inexpensive, air‐stable Pd(II) catalysts, scalability and wide substrate scope. The reaction proceeds with high trans selectivity to furnish valuable (E)‐1,2‐diarylethenes. 相似文献
104.
Andrey A. Dobrynin 《Discrete Mathematics》2019,342(1):74-77
Distance between two vertices is the number of edges in a shortest path connecting them in a connected graph . The transmission of a vertex is the sum of distances from to all the other vertices of . If transmissions of all vertices are mutually distinct, then is a transmission irregular graph. It is known that almost no graphs are transmission irregular. Infinite families of transmission irregular trees of odd order were presented in Alizadeh and Klav?ar (2018). The following problem was posed in Alizadeh and Klav?ar (2018): do there exist infinite families of transmission irregular trees of even order? In this article, such a family is constructed. 相似文献
105.
Robin van der Veer 《Discrete Mathematics》2019,342(9):2680-2693
106.
107.
108.
Crisólogo Dolores-Flores Martha Iris Rivera-López Javier García-García 《International Journal of Mathematical Education in Science & Technology》2019,50(3):369-389
This paper reports the results of a research exploring the mathematical connections of pre-university students while they solving tasks which involving rates of change. We assume mathematical connections as a cognitive process through which a person finds real relationships between two or more ideas, concepts, definitions, theorems, procedures, representations or meanings or with other disciplines or the real-world. Four tasks were proposed to the 33 pre-university students that participated in this research; the central concept of the first task is the slope, the last three tasks contain concepts like velocity, speed and acceleration. Task-based interviews were conducted to collect data and later analysed with thematic analysis. Results showed most of the students made mathematical connections of the procedural type, the mathematical connections of the common features type are made in smaller quantities and the mathematical connection of the generalization type is scarcely made. Furthermore, students considered slope as a concept disconnected from velocity, speed and acceleration. 相似文献
109.
Given a graph we are interested in studying the symmetric matrices associated to with a fixed number of negative eigenvalues. For this class of matrices we focus on the maximum possible nullity. For trees this parameter has already been studied and plenty of applications are known. In this work we derive a formula for the maximum nullity and completely describe its behavior as a function of the number of negative eigenvalues. In addition, we also carefully describe the matrices associated with trees that attain this maximum nullity. The analysis is then extended to the more general class of unicyclic graphs. Further our work is applied to re-describing all possible partial inertias associated with trees, and is employed to study an instance of the inverse eigenvalue problem for certain trees. 相似文献
110.
Nan Zhang Dr. Tohru Kawamoto Yong Jiang Dr. Akira Takahashi Dr. Manabu Ishizaki Miyuki Asai Prof. Masato Kurihara Prof. Zhenya Zhang Dr. Zhongfang Lei Dr. Durga Parajuli 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(23):5950-5958
Cobalt hexacyanoferrate of various compositions was prepared in flow mode and the role of the vacancy on the structure, thermogravimetric (TG) properties, and the adsorption efficiency was studied. The material, NayCo[Fe(CN)6]1−x ⋅ z H2O, with a minimum vacancy of x=0.014 to the highest x=0.47, was obtained. The TG-differential scanning calorimetry (DSC) profile showed a distinct influence of the vacancy on the water release temperature. Materials with x>0.35 showed a smooth release of water at a relatively lower temperature. However, for the materials with x<0.35, water release took place in multiple steps, suggesting the existence of various forms of water. The FTIR profiles supported the existence of free and bonded water molecules. However, the materials with multiple water peaks in the FTIR spectra showed a shift of the major XRD peaks when heated at 285 °C in N2 atmosphere. Regarding the effect of the vacancy on the adsorption behavior, for NH4, the adsorption was found to be proportional to the number of Na atoms in the material, confirming the ion-exchange process. On the contrary, the materials with low vacancy and high Na content showed nominal Cs adsorption capacity. Interestingly, the K adsorption capacity was found to be in between that of the other two ions. This means the ionic size decides the rate of placement into the interstitial sites. For larger ions like Cs, the ease of percolation via the vacancy decides the overall adsorption efficiency. 相似文献