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   655263篇
  免费   6402篇
  国内免费   2167篇
化学   312149篇
晶体学   8986篇
力学   33991篇
综合类   18篇
数学   109389篇
物理学   199299篇
  2021年   4545篇
  2020年   4857篇
  2019年   5498篇
  2018年   20865篇
  2017年   21136篇
  2016年   17858篇
  2015年   7680篇
  2014年   10520篇
  2013年   25382篇
  2012年   24637篇
  2011年   38942篇
  2010年   25377篇
  2009年   25611篇
  2008年   32653篇
  2007年   36494篇
  2006年   19356篇
  2005年   23108篇
  2004年   19667篇
  2003年   18020篇
  2002年   15545篇
  2001年   16121篇
  2000年   12433篇
  1999年   9462篇
  1998年   7956篇
  1997年   7839篇
  1996年   7581篇
  1995年   6713篇
  1994年   6507篇
  1993年   6542篇
  1992年   7019篇
  1991年   7149篇
  1990年   6824篇
  1989年   6704篇
  1988年   6522篇
  1987年   6554篇
  1986年   6233篇
  1985年   8119篇
  1984年   8290篇
  1983年   6857篇
  1982年   7244篇
  1981年   6780篇
  1980年   6671篇
  1979年   6929篇
  1978年   7198篇
  1977年   6942篇
  1976年   7127篇
  1975年   6522篇
  1974年   6610篇
  1973年   6888篇
  1972年   4615篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
Theoretical and Mathematical Physics - This paper is devoted to tetrahedron maps, which are set-theoretical solutions of the Zamolodchikov tetrahedron equation. We construct a family of tetrahedron...  相似文献   
52.
The health monitoring has been studied to ensure integrity of design of engine structure by detection, quantification, and prediction of damages. Early detection of faults may allow the downtime of maintenance to be rescheduled, thus preventing sudden shutdown of machines. In cylinder pressure developed, vibrations and noise emissions data provide a rich source of information about condition of engines. Monitoring of vibrations and noise emissions are novel non-intrusive methodologies for which positioning of various transducers are important issue. The presented work shows applicability of these diagnosis methodologies adopted in case of diesel engines. The effects of changing various fuel injection parameters was analyzed. Scope of using non-intrusive technique has been analyzed by changing locations of microphone. Novelty of this worklies in exploring signal processing methods for various locations around the engine test set up. Various frequency ranges of contributing noise and vibration sources were identified. Time-Frequency analysis showed the onset of various cyclic. Based on the identification of various frequency bands, it is possible to device suitable filters in order to extract more information.  相似文献   
53.
Crystallography Reports - Two LiInSe2 single crystals, grown under different conditions, have been studied. Characteristics of these crystals for neutron detection have been compared using...  相似文献   
54.
55.
56.
Svetlakova  A. V.  Mendez  M. Sanchez  Tuchin  E. S.  Khodan  A. N.  Traore  M.  Azouani  R.  Kanaev  A.  Tuchin  V. V. 《Optics and Spectroscopy》2021,129(8):846-850
Optics and Spectroscopy - We study the photocatalytic activity of nanocomposites based on θ-phase alumina (Al2O3) with different TiO2 concentrations for heterogeneous photocatalysis and...  相似文献   
57.
Journal of Structural Chemistry - Structural parameters and relative Gibbs free energies are calculated by density functional theory methods for four tetrafluoro derivatives of zinc(II)...  相似文献   
58.
Crystallography Reports - Electron backscattered diffraction (EBSD) has been used to analyze structures of natural minerals and artificial compounds for almost three decades. In recent years, it is...  相似文献   
59.
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   
60.
Theoretical and Mathematical Physics - We present a one-line closed-form expression for the three-parameter breather of the nonlinear Schrödinger equation. This provides an analytic proof of...  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