全文获取类型
收费全文 | 465380篇 |
免费 | 4720篇 |
国内免费 | 1529篇 |
专业分类
化学 | 233025篇 |
晶体学 | 6882篇 |
力学 | 22300篇 |
综合类 | 8篇 |
数学 | 65080篇 |
物理学 | 144334篇 |
出版年
2020年 | 2695篇 |
2019年 | 2996篇 |
2018年 | 11545篇 |
2017年 | 12337篇 |
2016年 | 9460篇 |
2015年 | 4979篇 |
2014年 | 6003篇 |
2013年 | 17539篇 |
2012年 | 17022篇 |
2011年 | 27700篇 |
2010年 | 17134篇 |
2009年 | 17203篇 |
2008年 | 24438篇 |
2007年 | 27688篇 |
2006年 | 14055篇 |
2005年 | 19516篇 |
2004年 | 14879篇 |
2003年 | 13287篇 |
2002年 | 11283篇 |
2001年 | 11751篇 |
2000年 | 9095篇 |
1999年 | 6901篇 |
1998年 | 5650篇 |
1997年 | 5569篇 |
1996年 | 5455篇 |
1995年 | 4994篇 |
1994年 | 4700篇 |
1993年 | 4659篇 |
1992年 | 5114篇 |
1991年 | 5143篇 |
1990年 | 4732篇 |
1989年 | 4706篇 |
1988年 | 4758篇 |
1987年 | 4599篇 |
1986年 | 4365篇 |
1985年 | 6213篇 |
1984年 | 6325篇 |
1983年 | 5186篇 |
1982年 | 5631篇 |
1981年 | 5422篇 |
1980年 | 5254篇 |
1979年 | 5304篇 |
1978年 | 5562篇 |
1977年 | 5287篇 |
1976年 | 5461篇 |
1975年 | 5039篇 |
1974年 | 5080篇 |
1973年 | 5418篇 |
1972年 | 3283篇 |
1971年 | 2441篇 |
排序方式: 共有10000条查询结果,搜索用时 546 毫秒
21.
Ibotov Sh. Kh. Yuldasheva N. K. Mukarramov N. I. Zakirova R. P. Gusakova S. D. 《Chemistry of Natural Compounds》2022,58(4):728-731
Chemistry of Natural Compounds - 相似文献
22.
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
23.
Abiola O. Ajayeoba Adewoye A. Olanipekun Wasiu A. Raheem Oluwaseun O. Ojo Ayowumi R. Soji–Adekunle 《声与振动》2021,55(1):69-85
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
24.
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
25.
Restricted testing conditions were considered recently. For the maximal operator, Hytönen, Li and Sawyer [8] first obtained parental testing condition. Later, they [9] showed that it suffices to restrict testing to doubling cubes. Chen and Lacey [3] gave a similar restricted testing condition. In our paper, we discuss a version of the latter in the multilinear setting.
相似文献26.
Physics of Particles and Nuclei Letters - When creating accelerators and storage-ring installations in a compact space, there is not always enough space for all the necessary magnet elements. In... 相似文献
27.
Paired single residue‐transposed Lys‐N and Lys‐C digestions for label‐free identification of N‐terminal and C‐terminal MS/MS peptide product ions: ultrahigh resolution Fourier transform ion cyclotron resonance mass spectrometry and tandem mass spectrometry for peptide de novo sequencing 下载免费PDF全文
28.
The influence of different referencing methods on the accuracy of δ13C value measurement of ethanol fuel by gas chromatography/combustion/isotope ratio mass spectrometry 下载免费PDF全文
29.
The growth-fragmentation equation describes a system of growing and dividing particles, and arises in models of cell division, protein polymerisation and even telecommunications protocols. Several important questions about the equation concern the asymptotic behaviour of solutions at large times: at what rate do they converge to zero or infinity, and what does the asymptotic profile of the solutions look like? Does the rescaled solution converge to its asymptotic profile at an exponential speed? These questions have traditionally been studied using analytic techniques such as entropy methods or splitting of operators. In this work, we present a probabilistic approach: we use a Feynman–Kac formula to relate the solution of the growth-fragmentation equation to the semigroup of a Markov process, and characterise the rate of decay or growth in terms of this process. We then identify the Malthus exponent and the asymptotic profile in terms of a related Markov process, and give a spectral interpretation in terms of the growth-fragmentation operator and its dual. 相似文献
30.