全文获取类型
收费全文 | 38312篇 |
免费 | 6394篇 |
国内免费 | 4328篇 |
专业分类
化学 | 26157篇 |
晶体学 | 519篇 |
力学 | 2467篇 |
综合类 | 285篇 |
数学 | 4424篇 |
物理学 | 15182篇 |
出版年
2024年 | 143篇 |
2023年 | 777篇 |
2022年 | 1285篇 |
2021年 | 1367篇 |
2020年 | 1486篇 |
2019年 | 1413篇 |
2018年 | 1255篇 |
2017年 | 1174篇 |
2016年 | 1775篇 |
2015年 | 1760篇 |
2014年 | 2138篇 |
2013年 | 2692篇 |
2012年 | 3263篇 |
2011年 | 3397篇 |
2010年 | 2322篇 |
2009年 | 2287篇 |
2008年 | 2484篇 |
2007年 | 2234篇 |
2006年 | 2138篇 |
2005年 | 1650篇 |
2004年 | 1331篇 |
2003年 | 1121篇 |
2002年 | 1039篇 |
2001年 | 834篇 |
2000年 | 874篇 |
1999年 | 821篇 |
1998年 | 731篇 |
1997年 | 683篇 |
1996年 | 691篇 |
1995年 | 616篇 |
1994年 | 540篇 |
1993年 | 430篇 |
1992年 | 425篇 |
1991年 | 329篇 |
1990年 | 302篇 |
1989年 | 239篇 |
1988年 | 204篇 |
1987年 | 172篇 |
1986年 | 153篇 |
1985年 | 152篇 |
1984年 | 105篇 |
1983年 | 73篇 |
1982年 | 60篇 |
1981年 | 36篇 |
1980年 | 16篇 |
1979年 | 10篇 |
1977年 | 2篇 |
1971年 | 1篇 |
1959年 | 1篇 |
1957年 | 3篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Recently GaN-based high electron mobility transistors (HEMTs) have
revealed the superior properties of a high breakdown field and high
electron saturation velocity. Reduction of the gate leakage current
is one of the key issues to be solved for their further improvement.
This paper reports that an Al layer as thin as 3 nm was inserted
between the conventional Ni/Au Schottky contact and n-GaN epilayers,
and the Schottky behaviour of Al/Ni/Au contact was investigated
under various annealing conditions by current--voltage (I--V)
measurements. A non-linear fitting method was used to extract the
contact parameters from the I--V characteristic curves.
Experimental results indicate that reduction of the gate leakage
current by as much as four orders of magnitude was successfully
recorded by thermal annealing. And high quality Schottky contact
with a barrier height of 0.875 eV and the lowest reverse-bias
leakage current, respectively, can be obtained under 12 min
annealing at 450°C in N2 ambience. 相似文献
992.
Controlled growth and field emission of vertically aligned AlN nanostructures with different morphologies 下载免费PDF全文
The controllable growth of three different morphologies of AlN
nanostructures (nanorod, nanotip and nanocrater) arrays are
successfully realized by using chemical vapour deposition (CVD)
technology. All three nanostructures are of single crystal h-AlN
with a growth orientation of [001]. Their growth is attributed to
the vapour-liquid-solid (VLS) mechanism. To investigate the factors
affecting field emission (FE) properties of AlN nanostructures, we
compare their FE behaviours in several aspects. Experimental results
show that AlN nanocrater arrays possess the best FE properties, such
as a threshold field of 7.2~V/μm and an emission current
fluctuation lower than 4%. Moreover, the three AlN nanostructures
all have good field emission properties compared with a number of
other excellent cathode nanomaterials, which suggests that they are
future promising FE nanomaterials. 相似文献
993.
合成了一种侧链共轭的聚噻吩衍生物聚3-(5′辛基-噻吩乙烯基)噻吩(POTVTh),并通过溶液旋涂制备了聚合物薄膜.吸收光谱显示该聚合物薄膜具有较小的禁带宽度和宽的光谱响应.采用Z扫描技术在800 nm下用飞秒激光器研究了该聚合物薄膜的三阶非线性光学特性,非线性吸收系数为5.63×10-7cm/W,非线性折射率为-6.38×10-11 cm2/W,三阶非线性极化率为4.21×10-9esu,比侧链未共轭的
关键词:
聚噻吩衍生物
侧链共轭
三阶非线性极化率
非线性折射率 相似文献
994.
A new analytic technique is applied to solve the unsteady viscous flow due to an infinite rotating disk, governed by a set
of two fully coupled nonlinear partial differential equations deduced directly from the exact Navier-Stokes equations. The
system of coupled nonlinear partial differential equations is replaced by a sequence of uncoupled systems of linear ordinary
differential equations. Different from all other previous analytic results, our series solution is accurate and valid for
all time in the whole spatial region. Accurate expressions for skin friction coefficients are given, which are valid for all
time. Such kind of series solutions have not been reported, to the best of our knowledge.
相似文献
995.
Existence of Positive Solutions for Operator Equations and Applications to Semipositone Problems 总被引:2,自引:0,他引:2
In this paper we study the existence of positive solutions of the following operator equation in a Banach space E:
where G(x, λ) = λKFx+e0, K: E↦ E is a linear completely continuous operator, F: P↦ E is a nonlinear continuous , bounded operator, e0∈ E, λ is
a parameter and P is a cone of Banach space E. Since F is not assumed to be positive and e0 may be a negative element, the operator equation is a so-called semipositone problem. We prove that under certain super-linear
conditions on the operator F the operator equation has at least one positive solution for λ > 0 sufficiently small, and that under certain sub-linear
conditions on the operator F the operator equation has at least one positive solution for λ > 0 sufficiently large. In addition, we briefly outline an application of our results which simplify previous theorems in
the literature. 相似文献
996.
Mǒsbauer study of the field induced uniaxial anisotropy in electro-deposited FeCo alloy films 下载免费PDF全文
Thin ferromagnetic films with in-plane magnetic anisotropy are promising materials for obtaining high microwave permeability.The paper reports a Mo¨ssbauer study of the field induced in-plane uniaxial anisotropy in electro-deposited FeCo alloy films.The FeCo alloy films were prepared by the electro-deposition method with and without an external magnetic field applied parallel to the film plane during deposition.Vibrating sample magnetometry and Mo¨ssbauer spectroscopy measurements at room temperature indicate that the film deposited in external field shows an in-plane uniaxial anisotropy with an easy direction coinciding with the external field direction and a hard direction perpendicular to the field direction,whereas the film deposited without external field does not show any in-plane anisotropy.Mo¨ssbauer spectra taken in three geometric arrangements show that the magnetic moments are almost constrained in the film plane for the film deposited with applied magnetic field.Also,the magnetic moments tend to align in the direction of the applied external magnetic field during deposition,indicating that the observed anisotropy should be attributed to directional ordering of atomic pairs. 相似文献
997.
Tian Qin WANG Guang Shan XU 《数学学报(英文版)》2007,23(1):83-88
In this paper, we give the p-adic measures of algebraic independence for the values of Ramanujan functions and Klein modular functions at algebraic points. 相似文献
998.
In this work, Gd-oxide dielectric films were deposited on Si by pulse laser deposition method (PLD), moreover, the micro-structures and electrical properties were reported. High-resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD) indicated that Gd-oxide was polycrystalline Gd2O3 structure, and no Gd metal phase was detected. In addition, both interface at Si and Ni fully silicide (FUSI) gate were smooth without the formation of Si-oxide. X-ray photoelectron spectroscopy (XPS) confirmed the formation of Gd2O3 and gave an atom ratio of 1:1 for Gd:O, indicating O vacancies existed in Gd2O3 polycrystal matrix even at O2 partial pressure of 20 mTorr. Electrical measurements indicated that the dielectric constant of Gd-oxide film was 6 and the leakage current was 0.1 A/cm2 at gate bias of 1 V. 相似文献
999.
In this paper, we discuss the controllability of a nonlinear degenerate parabolic system with bilinear control. Based on the shrinking property of the solutions, we prove that the system is not globally approximately controllable. Furthermore, we give an approximate null controllability result. We also prove that the system is not globally exactly null controllable by a comparison principle. 相似文献
1000.
The paper is concerned with a two-delay singular differential system with a twin parameter. Applying fixed-point index theory, we show the relationship between the asymptotic behaviors of nonlinearities (at zero and infinity) and the open regions (eigenvalue regions) of parameters, which are correlated with delays, such that the system has zero, one and two positive solution(s). 相似文献