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   355571篇
  免费   3977篇
  国内免费   1193篇
化学   192148篇
晶体学   4412篇
力学   14928篇
综合类   5篇
数学   38881篇
物理学   110367篇
  2020年   2749篇
  2019年   3083篇
  2018年   3854篇
  2017年   3829篇
  2016年   5946篇
  2015年   3901篇
  2014年   5869篇
  2013年   16138篇
  2012年   12004篇
  2011年   14766篇
  2010年   10025篇
  2009年   9947篇
  2008年   13590篇
  2007年   13471篇
  2006年   12689篇
  2005年   11720篇
  2004年   10493篇
  2003年   9313篇
  2002年   8536篇
  2001年   11169篇
  2000年   8033篇
  1999年   6232篇
  1998年   4747篇
  1997年   4967篇
  1996年   4539篇
  1995年   4528篇
  1994年   4402篇
  1993年   4264篇
  1992年   5015篇
  1991年   5089篇
  1990年   4798篇
  1989年   4751篇
  1988年   4770篇
  1987年   4670篇
  1986年   4430篇
  1985年   5912篇
  1984年   6045篇
  1983年   4937篇
  1982年   5226篇
  1981年   5008篇
  1980年   4838篇
  1979年   5159篇
  1978年   5449篇
  1977年   5288篇
  1976年   5398篇
  1975年   4896篇
  1974年   4979篇
  1973年   5297篇
  1972年   3336篇
  1971年   2595篇
排序方式: 共有10000条查询结果,搜索用时 218 毫秒
1.
2.
Economic wood processing employs the use of industrial machines for cutting, shaping, milling, and sawing timber, thereby leading to the generation of high levels of noise. Published data from empirical studies have categorized noise as an environmental hazard of global significance. Furthermore, noise exposure limits for different industries and all the industrial machines available has not been formally established as it presently exists in developed nations around the world. Therefore, this study assessed the daily exposure of sawmills workers to noise in Southwestern Nigeria. Reconnaissance surveys were first carried out in Osun, Oyo, Ondo, Ekiti, Lagos, and Ogun States to select sawmills that were fully operational and fit for the study. Two fully functional sawmills in two cities of each State were eventually selected for data collection, making a total of 24 sawmills, while the Circular Machines (CM), Planer Machines (PM), and Band-saw Machines (BM) were the machines in each sawmill considered. Two machines each of CM, PM, and BM were considered in each sawmill, making a total of forty-eight (48) machines each of CM, PM, and BM. Sound data were collected between 7 am and 7 pm each day for six days (between Monday and Saturday) using Extech 407732 sound level meter and all stabilized measurements were taken three times at different intervals. The data collected were in three different periods: Machine No-work Period (NPm), Machine Idle Period (IPm), and Machine Working Period (WPm). A two–way Analysis of Variance (ANOVA) was carried out at P < 0.05 to determine whether there is a significant difference in the sound level average before and after the break, for both the idle and working periods of the three machines considered. This was also done to determine whether there is a significant difference between the sound level average of the results collected during idle and working periods of the three machines. Noise Pollution Levels (Lnp) ranged from 83.20 dB (PM) to 107.65 (BM) and 93.42 (CM and PM) – 116.00 (BM) respectively, while IPm also gave the least noise pollution level of 95.79 dB and WPm gave the highest level of 102.88 dB. The results revealed that all the machines’ Lnp values in the working period are more than the 90 dB acceptable limit the recommendation value of 90 dB while 89.6% of CMs, 75% of PMs, and 89.6% of BM had their Lnp above 90 dB in the idle period respectively. The minimum and the maximum noise dose levels for IPm, WPm and overall are 0.09 (BM) and 2.37 (CM), 0.50 (CM), and 4.77 (PM) and 0.69 (BM) and 6.64 (PM) respectively. The study found out that the fundamental contributing factors to the high noise levels in sawmills are poor machine maintenance, use of old and obsolete machines, poor housekeeping strategy, limited space, workers’ negligence, lack of PPE, and lack of occupational safety training. The study recommends that proper workplace practices such as use of personal protective equipment, new and modern machines, training, and occupational safety programmes be implemented in the considered sawmills.  相似文献   
3.
Evropeitsev  E. A.  Serov  Yu. M.  Nechaev  D. V.  Jmerik  V. N.  Shubina  T. V.  Toropov  A. A. 《JETP Letters》2021,113(8):504-509
JETP Letters - The decay kinetics of low-temperature exciton photoluminescence in a heterostructure with multiple GaN/AlN monolayer quantum wells, which is prepared by molecular beam epitaxy, is...  相似文献   
4.
Crystallography Reports - Crystals of mutants of uridine phosphorylase from Shewanella oneidensis MR-1 at the active-site threonine residue were obtained, and the three-dimensional structures of...  相似文献   
5.
Journal of Applied Mechanics and Technical Physics - The possibility of phase transitions (martensitic transformations) in&nbsp;shape-memory alloys is evaluated using the concept of eigenmoduli...  相似文献   
6.
Acoustical Physics - The paper studies the evolution of a pulsed signal initiated by the pushing of a cylindrical piston and propagating through a stationary fluid filling a buried pipeline with an...  相似文献   
7.
Crystallography Reports - Reactions of nitrilotriacetic acid with 2-aminoethanol, 2-amino-2-(hydroxymethyl)propane-1,3-diol, and 2,2'-(ethylenedioxy)bis(ethylamine) afford the following salts...  相似文献   
8.
Crystallography Reports - The first representative of the eudialyte group with a primitive unit cell has been investigated by X-ray diffraction analysis, electron probe microanalysis, and IR...  相似文献   
9.
Gohl  J. A.  Thiele-Sardina  T. C.  Rencheck  M. L.  Erk  K. A.  Davis  C. S. 《Experimental Mechanics》2021,61(7):1209-1213
Background

Peel tests are frequently used to perform measurements of adhesive strength for pressure sensitive adhesive (PSA) tapes. Current lab methodologies for 90° peel tests translate the model substrate orthogonally to the peel direction in order to maintain the peel angle, precluding testing from immovable substrates.

Objective

It was our objective to develop a peel fixture capable of testing temporary pavement marking (TPM) tapes and other PSA tapes from immovable substrates such as roadways surfaces.

Methods

We present a modular peel fixture for conducting peel experiments directly on immovable substrates. The fixture was validated through a series of peel tests on consumer tapes to reproduce the linear width dependence and viscoelastic rate dependence found in traditional peeling setups. To test the capabilities of the fixture, a series of peel tests were conducted with various tapes on controlled surfaces, and a commercial tape on various immovable substrates.

Results

We demonstrate the ability of our fixture to reproduce results reported for traditional peel tests from literature. In addition, we were able to conduct peel tests directly on immovable substrates such as the benchtop.

Conclusions

This fixture shows potential for both traditional peeling tests, and for use in in-situ peel experiments from substrates relevant to the end application of the PSA tape.

  相似文献   
10.
Theoretical and Mathematical Physics - We consider three one-dimensional superconducting structures: 1) the one with $$p$$ -wave superconductivity; 2) the main experimental model of a nanowire with...  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