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   9725篇
  免费   448篇
  国内免费   127篇
化学   6773篇
晶体学   86篇
力学   506篇
综合类   3篇
数学   1246篇
物理学   1686篇
  2023年   85篇
  2022年   167篇
  2021年   330篇
  2020年   359篇
  2019年   443篇
  2018年   448篇
  2017年   364篇
  2016年   568篇
  2015年   407篇
  2014年   556篇
  2013年   1046篇
  2012年   803篇
  2011年   759篇
  2010年   549篇
  2009年   442篇
  2008年   488篇
  2007年   425篇
  2006年   307篇
  2005年   307篇
  2004年   189篇
  2003年   189篇
  2002年   126篇
  2001年   78篇
  2000年   47篇
  1999年   52篇
  1998年   35篇
  1997年   45篇
  1996年   41篇
  1995年   33篇
  1994年   39篇
  1993年   20篇
  1992年   36篇
  1991年   32篇
  1990年   30篇
  1989年   36篇
  1988年   32篇
  1987年   33篇
  1986年   30篇
  1985年   36篇
  1984年   33篇
  1983年   18篇
  1982年   26篇
  1981年   19篇
  1980年   23篇
  1979年   20篇
  1978年   18篇
  1977年   13篇
  1976年   15篇
  1974年   16篇
  1972年   12篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
The nanofluid and porous medium together are able to fulfill the requirement of high cooling rate in many engineering problems. So, here the impact of various shapes of nanoparticles on unsteady stagnation-point flow of Cu-H2O nanofluid on a flat surface in a porous medium is examined. Moreover, the thermal radiation and viscous dissipation effects are considered. The problem governing partial differential equations are converted into self-similar coupled ordinary differential equations and those are numerically solved by the shooting method. The computed results can reveal many vital findings of practical importance. Firstly, dual solutions exist for decelerating unsteady flow and for accelerating unsteady and steady flows, the solution is unique. The presence of nanoparticles affects the existence of dual solution in decelerating unsteady flow only when the medium of the flow is a porous medium. But different shapes of nanoparticles are not disturbing the dual solution existence range, though it has a considerable impact on thermal conductivity of the mixture. Different shapes of nanoparticles act differently to enhance the heat transfer characteristics of the base fluid, i.e., the water here. On the other hand, the existence range of dual solutions becomes wider for a larger permeability parameter related to the porous medium. Regarding the cooling rate of the heated surface, it rises with the permeability parameter, shape factor (related to various shapes of Cu-nanoparticles), and radiation parameter. The surface drag force becomes stronger with the permeability parameter. Also, with growing values of nanoparticle volume fraction, the boundary layer thickness (BLT) increases and the thermal BLT becomes thicker with larger values of shape factor. For decelerating unsteady flow, the nanofluid velocity rises with permeability parameter in the case of upper branch solution and an opposite trend for the lower branch is witnessed. The thermal BLT is thicker with radiation parameter. Due to the existence of dual solutions, a linear stability analysis is made and it is concluded that the upper branch and unique solutions are stable solutions.  相似文献   
2.
Alamri  Sagr  Rajhi  Ali A.  Wu  Liang 《Structural chemistry》2022,33(3):671-677
Structural Chemistry - In order to evaluate the electrical response of boron nitride nanocones (BNNCs) to phenol gas, density functional theory (DFT) calculations are employed. The sensitivity and...  相似文献   
3.
4.
Controllable tailoring of metal-free/carbon-based nanostructures tends an encouraging way to enhance the bifunctional activity of electrodes, but a great challenge owing to the sluggish kinetics of oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). Herein, a facile tempted-defects assisted fractionation strategy is presented to synthesize N, S, and O tri-doped metal-free catalyst (DE-TDAP). Due to this effective tempted-defects and heteroatoms interlinking in DE-TDAP, it delivers the lowest overpotential toward both the OER (346 mV) and HER (154 mV) at 10 mA cm?2. Remarkably, the DE-TDAP-electrode carries only a cell voltage of 1.81 V at 10 mA cm?2 for overall water splitting and long-term stability. Considerably, the density functional theory (DFT) calculation exposes that the tailored-defects in tri-doped interlinking could enhance bifunctional catalytic performance devising from lower Gibbs free energy of OER/HER intermediates on active sites. This struggle henceforth provides a perceptive understanding of the synergetic principles of heteroatom-interlinking-tailoring nanostructures in water splitting.  相似文献   
5.
Crystallography Reports - Large study of theoretical calculations for the hybrid organic-inorganic heteropolyoxometalate (C6H8N)5[HAs2Mo6O26(H2O)]⋅3H2O (1) is reported. The results of...  相似文献   
6.
Previous hotel performance studies neglected the role of information entropy in feedback processes between input and output management. This paper focuses on this gap by exploring the relationship between hotel performance at the industry level and the capability of learning by doing and adopting best practices using a sample of 153 UK hotels over a 10-year period between 2008–2017. Besides, this research also fills a literature gap by addressing the issues of measuring hotel performance in light of negative outputs. In order to achieve this, we apply a novel Modified slack-based model for the efficiency analysis and Least Absolute Shrinkage and Selection Operator to examine the influence of entropy related variable on efficiency score. The Results indicate that less can be learnt from inputs than from outputs to improve efficiency levels and resource allocation is more balanced than cash flow and liquidity. The findings suggest that market dynamics explains the cash flow generation potential and liquidity. We find that market conditions are increasingly offering the opportunities for learning and improving hotel efficiency. The results report that the distinctive characteristic of superior performance in hotel operations is the capability to match the cash flow generation potential with market opportunities.  相似文献   
7.
Journal of Algebraic Combinatorics - The symbolic powers, in general, are not equal to the ordinary powers. Therefore, one interesting question here is for what classes of ideals ordinary and...  相似文献   
8.

Calixarenes, which have a great place in supramolecular chemistry, have become the most prominent macrocyclic compounds in synthetic organic chemistry due to their easy synthesis and functionalization. In this study, p-tert-butyl calix[4]arene dihydrazide derivative was synthesized and then reacted with 3-oxo-3,4-dihydro-2 H-benzo[b][1,4] thiazin-2-ylideneacetyl chloride to prepare new calixarene based chromophore compound 4. The structure of the synthesized compound was elucidated by spectroscopic methods such as 1H NMR 13C NMR and FT-IR spectroscopy. Chromogenic and fluorescence properties of compound 4 were evaluated. It was observed from both studies that compound 4 was Co2+ selective and shows fluorescence Switched-off behavior. Stoichiometry, binding constant and the detection limit were calculated. The stoichiometry between compound 4 and Co2+ was found to be 1:1. The binding constant value (K) was calculated as 666.67 M??1 using Benesi–Hildebrand equation, while the detection limit for Co2+ ion was calculated as 0.0465 µM.

  相似文献   
9.
Journal of Thermal Analysis and Calorimetry - Nanofluids have recently attracted attention of many researchers due to their growing potential applications in heat transfer devices. They possess...  相似文献   
10.
Journal of Thermal Analysis and Calorimetry - Nanofluids are prepared to enrich thermo-physical and convective heat transfer properties by suspending nanometer particles in a base fluid. For...  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