首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   1856篇
  免费   270篇
  国内免费   174篇
化学   759篇
晶体学   14篇
力学   479篇
综合类   58篇
数学   448篇
物理学   542篇
  2024年   7篇
  2023年   23篇
  2022年   64篇
  2021年   133篇
  2020年   100篇
  2019年   66篇
  2018年   66篇
  2017年   63篇
  2016年   80篇
  2015年   88篇
  2014年   106篇
  2013年   141篇
  2012年   86篇
  2011年   91篇
  2010年   69篇
  2009年   93篇
  2008年   97篇
  2007年   90篇
  2006年   100篇
  2005年   89篇
  2004年   67篇
  2003年   70篇
  2002年   70篇
  2001年   51篇
  2000年   55篇
  1999年   51篇
  1998年   37篇
  1997年   41篇
  1996年   45篇
  1995年   32篇
  1994年   18篇
  1993年   16篇
  1992年   12篇
  1991年   14篇
  1990年   4篇
  1989年   8篇
  1988年   13篇
  1987年   7篇
  1986年   9篇
  1985年   4篇
  1984年   12篇
  1982年   1篇
  1980年   1篇
  1979年   1篇
  1974年   1篇
  1973年   1篇
  1959年   1篇
  1957年   6篇
排序方式: 共有2300条查询结果,搜索用时 15 毫秒
991.
《Analytical letters》2012,45(1):69-80
ABSTRACT

This paper demonstrates the usefulness of near-infrared (NIR) spectra and artificial neural network (ANN) in nondestructive quantitative analysis of pharmaceuticals. Real data sets from near-infrared reflectance spectra of analgini powder pharmaceutical were used to build up an artificial neural network to predict unknown samples. The parameters affecting the network were discussed. A new network evaluation criterion, the degree of approximation, was employed. The overfitting was discussed. Owing to the good nonlinear multivariate calibration nature of ANN, the predicted result was reliable and precise. The relative error of unknown samples was less than 2.5%  相似文献   
992.
《Analytical letters》2012,45(4):439-448
Abstract

Laboratory robots are being used to perform sample preparation and routine analytical determinations 1,2. In addition to these routine applications, laboratory robots should be able to analyze the analytical procedure itself and select the optimal experimental condition for any analytical method. The application of the fixed-sized SIMPLEX algorithm, the Nelder and Mead variable sized SIMPLEX algorithm and response surface fitting methods in analytical robotics is presented 3.  相似文献   
993.
New multi‐modular donor–acceptor conjugates featuring zinc porphyrin (ZnP), catechol‐chelated boron dipyrrin (BDP), triphenylamine (TPA) and fullerene (C60), or naphthalenediimide (NDI) have been newly designed and synthesized as photosynthetic antenna and reaction‐center mimics. The X‐ray structure of triphenylamine‐BDP is also reported. The wide‐band capturing polyad revealed ultrafast energy‐transfer (kENT=1.0×1012 s?1) from the singlet excited BDP to the covalently linked ZnP owing to close proximity and favorable orientation of the entities. Introducing either fullerene or naphthalenediimide electron acceptors to the TPA‐BDP‐ZnP triad through metal–ligand axial coordination resulted in electron donor–acceptor polyads whose structures were revealed by spectroscopic, electrochemical and computational studies. Excitation of the electron donor, zinc porphyrin resulted in rapid electron‐transfer to coordinated fullerene or naphthalenediimide yielding charge separated ion‐pair species. The measured electron transfer rate constants from femtosecond transient spectral technique in non‐polar toluene were in the range of 5.0×109–3.5×1010 s?1. Stabilization of the charge‐separated state in these multi‐modular donor–acceptor polyads is also observed to certain level.  相似文献   
994.
尚吉扬  陈驰  张宇  王文生 《光子学报》2014,40(8):1231-1237
为了解决在联合变换相关器上无法识别复杂背景下畸变目标(目标的平面内旋转或尺寸改变)的问题,对其应用最大平均相关高度滤波算法进行畸变目标识别.首先在频域内优化最大平均相关高度滤波器的三个性能控制参量|然后将优化好的最大平均相关高度频域滤波器映射到物空间,得到最大平均相关高度参考模板.作为实例,本文对复杂背景下的军舰和汽车目标进行了光学识别实验,结果表明,当滤波器的控制参量为γ=1,α=0,β=0.1时,得到的MACH参考模板最为清晰,目标产生的相关峰最为尖锐明亮.通过这种方法,在联合变换相关器上能够得到的比例畸变容差为28%,旋转畸变容差为20°,对于超出上述畸变容差范围的目标,相关峰强度逐步减弱,但部分仍然可以识别.改进后的MACH滤波器在畸变目标的探测领域具有很强的优势和良好的应用前景.  相似文献   
995.
任兵  马军  蓝岚  王文生 《光子学报》2014,43(9):910004
为解决目标识别技术中混合畸变目标的识别难题,提出了一种面积极坐标变换算法.该算法利用相似图形尺寸与面积的满射函数关系和极坐标变换的旋转不变特性,对目标与模板同时进行面积变换和极坐标变换.将面积极坐标变换算法与基于形态学膨胀的边缘加粗处理算法相结合应用于光电混合联合变换相关器,实现了大尺度混合畸变目标的识别.以空中飞机为例,光学相关实验表明,该方法可显著提高混合畸变目标的相关峰强度,旋转畸变容差为0°~100°,比例畸变容差为100%~200%,具有较大尺度的混合畸变容差,扩大了识别范围.其混合畸变识别能力优于极坐标对数变换算法与综合鉴别函数法其衍生方法,其"时间复杂度"远小于综合鉴别函数及其衍生方法.  相似文献   
996.
The study of operators satisfying
σja(T)=σa(T)σja(T)=σa(T)
is of significant interest. Does
σja(T)=σa(T)σja(T)=σa(T)
for n-perinormal operator
T∈B(H)?TB(H)?
This question was raised by Mecheri and Braha [Oper. Matrices 6 (2012), 725–734]. In the note we construct a counterexample to this question and obtain the following result: if T is a n-perinormal operator in B(H), then
σja(T)\{0}=σa(T)\{0}.σja(T)\{0}=σa(T)\{0}.
We also consider tensor product of n-perinormal operators.  相似文献   
997.
为更好地解决最大二等分问题,提出了一种求解该问题的混合二进制人工蜂群算法。首先,针对传统人工蜂群算法不能解决离散问题的缺陷,根据最大二等分问题的特点,重新设计了蜂群的食物源更新方法,新产生的食物源既继承了先前找到的高质量解的优良结构,又具有良好的多样性。其次,采用填充函数算法对新产生的食物源进行进一步优化,有效提高了人工蜂群算法的局部搜索能力。最后,通过比较混合二进制人工蜂群算法和其他现有算法对不同规模标准测试例子的计算结果,验证了本算法的优越性。  相似文献   
998.
The newest achievements in the application of liposomes in different chromatographic technologies such as high-performance liquid chromatography and electrically driven separation methods was compiled and critically evaluated. The employment of chromatographic methodologies for the determination of molecular parameters of biological importance is also discussed in detail. The future trends of the use of liposomes is also shortly discussed.  相似文献   
999.
The toxicity of amyloid‐forming proteins is correlated with their interactions with cell membranes. Binding events between amyloidogenic proteins and membranes result in mutally disruptive structural perturbations, which are associated with toxicity. Membrane surfaces promote the conversion of amyloid‐forming proteins into toxic aggregates, and amyloidogenic proteins, in turn, compromise the structural integrity of the cell membrane. Recent studies with artificial model membranes have highlighted the striking resemblance of the mechanisms of membrane permeabilization of amyloid‐forming proteins to those of pore‐forming toxins and antimicrobial peptides.  相似文献   
1000.
The r‐Laplacian has played an important role in the development of computationally efficient models for applications, such as numerical simulation of turbulent flows. In this article, we examine two‐level finite element approximation schemes applied to the Navier‐Stokes equations with r‐Laplacian subgridscale viscosity, where r is the order of the power‐law artificial viscosity term. In the two‐level algorithm, the solution to the fully nonlinear coarse mesh problem is utilized in a single‐step linear fine mesh problem. When modeling parameters are chosen appropriately, the error in the two‐level algorithm is comparable to the error in solving the fully nonlinear problem on the fine mesh. We provide rigorous numerical analysis of the two‐level approximation scheme and derive scalings which vary based on the coefficient r, coarse mesh size H, fine mesh size h, and filter radius δ. We also investigate the two‐level algorithm in several computational settings, including the 3D numerical simulation of flow past a backward‐facing step at Reynolds number Re = 5100. In all numerical tests, the two‐level algorithm was proven to achieve the same order of accuracy as the standard one‐level algorithm, at a fraction of the computational cost. © 2011 Wiley Periodicals, Inc. Numer Methods Partial Differential Eq, 2011  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号