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   1212篇
  免费   18篇
  国内免费   5篇
化学   127篇
晶体学   1篇
力学   125篇
综合类   2篇
数学   532篇
物理学   448篇
  2024年   9篇
  2023年   31篇
  2022年   39篇
  2021年   42篇
  2020年   54篇
  2019年   25篇
  2018年   25篇
  2017年   65篇
  2016年   89篇
  2015年   69篇
  2014年   94篇
  2013年   38篇
  2012年   62篇
  2011年   43篇
  2010年   36篇
  2009年   53篇
  2008年   26篇
  2007年   37篇
  2006年   41篇
  2005年   29篇
  2004年   15篇
  2003年   10篇
  2002年   23篇
  2001年   21篇
  2000年   10篇
  1999年   25篇
  1998年   13篇
  1997年   9篇
  1996年   11篇
  1995年   7篇
  1994年   3篇
  1989年   4篇
  1988年   15篇
  1987年   31篇
  1986年   25篇
  1985年   5篇
  1984年   4篇
  1981年   3篇
  1978年   3篇
  1975年   4篇
  1972年   8篇
  1971年   11篇
  1970年   7篇
  1969年   4篇
  1968年   7篇
  1967年   10篇
  1966年   5篇
  1965年   7篇
  1963年   5篇
  1956年   2篇
排序方式: 共有1235条查询结果,搜索用时 0 毫秒
1.
Numerical problems that are usually ignored in the dynamic analysis of hydraulic control valves are described, and an analysis of the effects of such problems on the numerical modeling is provided. Previous studies have ignored the effects of changes in the flow coefficient in the orifice, the solenoid force along the spool movement in the valve and an ascending tendency of pressure during reach to the steady state. To eliminate these problems, it was studied a method to substantiate the non-linearity of the pressure loss caused by passing between the orifice and port as well as that caused by interaction with the solenoid. Moreover, the movement of the spool and spring is expressed using the time-delay-element (TDE). The proposed numerical model has been used in the Bond graphs method of a hydraulic control valve and the simulation results have been shown to be accurate. It is known that differences between simulated and experimental results can have a considerable impact on the function of actual systems. The contribution of each parameter is measured separately for the transient state and steady state. Analysis standard observed the first peak value, pressure increase to the steady state and the settling time in the response results.  相似文献   
2.
We report on effective sulfur-based passivation treatments of type-II InAs/GaSb strained layer superlattice detectors (100% cut-off wavelength is 9.8 μm at 77 K). The electrical behavior of detectors passivated by electrochemical sulfur deposition (ECP) and thioacetamide (TAM) was evaluated for devices of various sizes. ECP passivated detectors with a perimeter-to-area ratio of 1600 cm?1 exhibited superior performance with surface resistivity in excess of 104 Ω cm, dark current density of 2.7 × 10?3 A/cm2, and specific detectivity improved by a factor of 5 compared to unpassivated devices (VBias =  ? 0.1 V, 77 K).  相似文献   
3.
In this paper, a blind dual color image watermarking scheme based on Schur decomposition is introduced. This is the first time to use Schur decomposition to embed color image watermark in color host image, which is different from using the binary image as watermark. By analyzing the 4 × 4 unitary matrix U via Schur decomposition, we can find that there is a strong correlation between the second row first column element and the third row first column element. This property can be explored for embedding watermark and extracting watermark in the blind manner. Since Schur decomposition is an intermediate step in SVD decomposition, the proposed method requires less number of computations. Experimental results show that the proposed scheme is robust against most common attacks including JPEG lossy compression, JPEG 2000 compression, low-pass filtering, cropping, noise addition, blurring, rotation, scaling and sharpening et al. Moreover, the proposed algorithm outperforms the closely related SVD-based algorithm and the spatial-domain algorithm.  相似文献   
4.
The effects of HBr/Ar and HBr/Cl2 mixing ratios in the ranges of 0–100% Ar or Cl2 on plasma parameters, densities of active species influencing the dry etch mechanisms were analyzed at fixed total gas flow rate of 40 sccm, total gas pressure of 6 mTorr, input power of 700 W and bias power of 300 W. The investigation combined plasma diagnostics by Langmuir probes and the 0-dimensional plasma modeling. It was found that the dilution of HBr by Ar results in maximum effect on the ion energy flux with expected impact on the etch rate in the ion-flux-limited etch regime, while the addition of Cl2 influences mainly the relative fluxes of Br and Cl atoms on the etched surface with expected impact on the etch rate in the reaction-rate-limited etch regime.  相似文献   
5.
A novel Fe3O4@SiO2@poly-l-alanine peptide brush–magnetic microsphere (PBMMs) was synthesized from amine-functionalized Fe3O4 through the surface-initiated polymerization of N-carboxyanhydrides. Two materials with different peptide lengths were obtained from different amounts of N-carboxyanhydrides. These materials were characterized by Fourier transform infrared, transmission electron microscopy, large-angle powder X-ray diffraction, vibrating sample magnetometer and elemental analysis. Furthermore, the loading and release behavior of ibuprofen and the enrichment of bovine serum albumin on the two materials were investigated and it was shown that the PBMMs have a maximum uptake amount of ibuprofen of 40.3 mg g−1, and an enrichment of bovine serum albumin of 20.9 mg g−1. These materials are promising candidates for targeted drug delivery and protein enrichment.  相似文献   
6.
The kinetics of liquid crystalline copolyester synthesis via melt transesterification between poly(butylene terephthalate) (PBT), terephthalic acid (TA) and hydroquinone diacetate (HQDA) is examined. Two different copolyester compositions PBT30/(HQDA+TA) 70 and PBT 50/(HQDA+TA) 50 mol% ratio were synthesized. The ratio of HQDA to TA was kept constant for all the reactions.The copolyesters were synthesized via melt polycondensation route at 265°C, 275°C and 285°C using two different transesterification catalysts, zinc acetate and dibutyl tin oxide. A key postulation assumed in this work is that the reaction originates between TA and HQDA to form a dimer which slices PBT chain. The copolyesterification rate constant for a system containing butylene glycol a more nonpolar moiety compared to ethylene glycol in poly(ethylene terephthalate) has been determined. The activation energy values for the different copolymer systems has also been determined. The rate constants for the uncatalyzed and catalyzed copolyesterification reaction and the activation energy values for the reaction have been determined.  相似文献   
7.
传递对准是舰载武器惯导系统初始对准的一种有效办法,为满足舰载武器要反应快的要求,必须在短时间内对初始不对准角进行估计并加以补偿。文中建立了采用四元数的传递对准的,线性模型,在此基础上进行卡尔曼滤波,解决了舰载垂直发射导弹的初始对准问题,同时又克服了一般四元数对准模型非线性滤波计算量大,对准时间长的缺点,使导弹能在短时间内完成对准,提高其反应速度,增加其命中率。仿真结果表明,该文提出的快速对准方法具有对准速度快、精度高的优点。  相似文献   
8.
    
The paper presents average flow visualizations and measurements, obtained with the Particle Image Velocimetry (PIV) technique, of a submerged rectangular free jet of air in the range of Reynolds numbers from Re = 35,300 to Re = 2200, where the Reynolds number is defined according to the hydraulic diameter of a rectangular slot of height H. According to the literature, just after the exit of the jet there is a zone of flow, called zone of flow establishment, containing the region of mixing fluid, at the border with the stagnant fluid, and the potential core, where velocity on the centerline maintains a value almost equal to the exit one. After this zone is present the zone of established flow or fully developed region. The goal of the paper is to show, with average PIV visualizations and measurements, that, before the zone of flow establishment is present a region of flow, never mentioned by the literature and called undisturbed region of flow, with a length, LU, which decreases with the increase of the Reynolds number. The main characteristics of the undisturbed region is the fact that the velocity profile maintains almost equal to the exit one, and can also be identified by a constant height of the average PIV visualizations, with length, LCH, or by a constant turbulence on the centerline, with length LCT. The average PIV velocity and turbulence measurements are compared to those performed with the Hot Film Anemometry (HFA) technique. The average PIV visualizations show that the region of constant height has a length LCH which increases from LCH = H at Re = 35,300 to LCH = 45H at Re = 2200. The PIV measurements on the centerline of the jet show that turbulence remains constant at the level of the exit for a length, LCT, which increases from LCT = H at Re = 35,300 to LCT = 45H at Re = 2200. The PIV measurements show that velocity remains constant at the exit level for a length, LU, which increases from LU = H at Re = 35,300 to LU = 6H at Re = 2200 and is called undisturbed region of flow. In turbulent flow the length LU is almost equal to the lengths of the regions of constant height, LCH, and constant turbulence, LCT. In laminar flow, Re = 2200, the length of the undisturbed region of flow, LU, is greater than the lengths of the regions of constant height and turbulence, LCT = LCH = 45H. The average PIV and HFA velocity measurements confirm that the length of potential core, LP, increases from LP = 45H at Re = 35,300 to LP = 78H at Re = 2200, and are compared to the previous experimental and theoretical results of the literature in the zone of mixing fluid and in the fully developed region with a good agreement.  相似文献   
9.
We describe a protocol for distilling maximally entangled bipartite states between random pairs of parties from those sharing a tripartite W state |W=(1/sqrt[3])(|100+|010+|001)(ABC), and show that the total distillation rate E(t)(infinity) [the total number of Einstein-Podolsky-Rosen (EPR) pairs distilled per W, irrespective of who shares them] may be done at a higher rate than EPR distillation between specified pairs of parties. Specifically, the optimal rate for distillation to specified parties has been previously shown to be 0.92 EPR pairs per W, while our protocol can asymptotically distill 1 EPR pair per W between random pairs of parties, which we conjecture to be optimal. We thus demonstrate a tradeoff between overall distillation rate and final distribution of EPR pairs. We further show that there exist states with fixed lower-bounded E(t)(infinity), but arbitrarily small distillable entanglement for specified parties.  相似文献   
10.
During laser spectroscopic measurement, a part of laser energy will be converted into heat in the processes of excitation and light emission. Temperature monitoring can help to evaluate such nonradiative process. Upconversion luminescence of phosphor Y2O3:Er,Yb under laser excitation at 980 nm was investigated in this work. Point temperature of the phosphor was monitored using the fluorescent intensity ratio (FIR) technology. Laser induced temperature rising was identified by comparison with a theoretically ideal temperature calibration function: lnR = 3.1738–1167/T. The monitored temperature of laser heating rises monotonically with increasing laser power. Circumstances around heating point will modify the calibration function, but the linear slope of lnR ~ 1/T is constant.  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