全文获取类型
收费全文 | 42545篇 |
免费 | 3063篇 |
国内免费 | 2144篇 |
专业分类
化学 | 22805篇 |
晶体学 | 265篇 |
力学 | 4134篇 |
综合类 | 571篇 |
数学 | 8871篇 |
物理学 | 11106篇 |
出版年
2024年 | 71篇 |
2023年 | 413篇 |
2022年 | 1123篇 |
2021年 | 1101篇 |
2020年 | 1107篇 |
2019年 | 1052篇 |
2018年 | 939篇 |
2017年 | 1180篇 |
2016年 | 1589篇 |
2015年 | 1209篇 |
2014年 | 1730篇 |
2013年 | 2982篇 |
2012年 | 2423篇 |
2011年 | 2264篇 |
2010年 | 1950篇 |
2009年 | 2488篇 |
2008年 | 2419篇 |
2007年 | 2728篇 |
2006年 | 2263篇 |
2005年 | 2035篇 |
2004年 | 2015篇 |
2003年 | 1646篇 |
2002年 | 1321篇 |
2001年 | 1053篇 |
2000年 | 1031篇 |
1999年 | 920篇 |
1998年 | 867篇 |
1997年 | 758篇 |
1996年 | 676篇 |
1995年 | 652篇 |
1994年 | 583篇 |
1993年 | 482篇 |
1992年 | 442篇 |
1991年 | 371篇 |
1990年 | 342篇 |
1989年 | 229篇 |
1988年 | 229篇 |
1987年 | 146篇 |
1986年 | 129篇 |
1985年 | 161篇 |
1984年 | 125篇 |
1983年 | 64篇 |
1982年 | 99篇 |
1981年 | 68篇 |
1980年 | 50篇 |
1979年 | 83篇 |
1978年 | 40篇 |
1977年 | 21篇 |
1976年 | 26篇 |
1975年 | 10篇 |
排序方式: 共有10000条查询结果,搜索用时 123 毫秒
51.
As borders between different entities, lines are an important element of natural images. Indeed, the neurons of the mammalian
visual cortex are tuned to respond best to lines of a given orientation. This preferred orientation varies continuously across
most of the cortex, but also has vortex-like singularities known as pinwheels. In attempting to describe such patterns of
orientation preference, we are led to consider underlying rotation symmetries: Oriented segments in natural images tend to
be collinear; neurons are more likely to be connected if their preferred orientations are aligned to their topographic separation.
These are indications of a reduced symmetry requiring joint rotations of both orientation preference and the underlying topography.
This is verified by direct statistical tests in both natural images and in cortical maps. Using the statistics of natural
scenes we construct filters that are best suited to extracting information from such images, and find qualitative similarities
to mammalian vision.
PACS84.35+i 89.70.+c 87.57.Nk 相似文献
52.
53.
54.
The high-temperature cubic phase of non-stoichiometric strontium ferrite SrFeOx (2.5≤x≤3.0) has been studied by in situ neutron powder diffraction in air over the temperature range 300-1273 K. The composition of SrFeOx changes within the range 2.56≤x≤2.81 from 1273 to 673 K, respectively.Rietveld refinements of the diffraction patterns show that the high-temperature cubic phase of SrFeOx is consistent with a face-centred Fm3c structure. This structure leads to agreement with previous density measurements. This cell allows the high-temperature structure of SrFeOx to be described in terms of a solid solution of the composition end members. Cubic SrFeOx at high temperature is found to closely obey Vegard's law. The density of cubic SrFeOx is also found to exhibit a linear relationship with composition. 相似文献
55.
56.
The diffusion of the anti-microbial chlorhexidine digluconate (CHG) has been studied in C. albicans biofilms by time-of-flight secondary-ion mass spectrometry (TOF-SIMS). C. albicans has been shown to become resistant to common anti-microbial agents, including CHG, when growing as a biofilm. Mass transport resistance within biofilms has commonly been suggested as a resistance mechanism, but measurement of transport for most anti-microbial agents in biofilms has proven extremely difficult because of the heterogeneity of the biofilms and the difficulty in detecting these agents within an intact biofilm. In this study, TOF-SIMS has been used to study the transport of CHG and glucose in a frozen hydrated biofilm. The TOF-SIMS images reveal a progression of CHG from the top of the biofilm to its base with time. Images suggest that there are channels within the biofilm and show preferential binding of CHG to cellular components of the biofilm. Additionally, both living and dead cells can be identified in the TOF-SIMS images by the sequestration of K+ and the presence of cell markers. This study demonstrates that TOF-SIMS has the unique potential to simultaneously observe the presence of an antimicrobial agent, concentration of nutrients, and the viability of the cell population. 相似文献
57.
58.
59.
60.
The MPEG-1 Layer 3 compression schema of audio signal, commonly known as mp3, has caused a great impact in recent years as it has reached high compression rates while conserving a high sound quality. Music and speech samples compressed at high bitrates are perceptually indistinguishable from the original samples, but very little was known about how compression acoustically affects the voice signal. A previous work with normal voices showed a high fidelity at high-bitrate compressions both in voice parameters and the amplitude-frequency spectrum. In the present work, dysphonic voices were tested through two studies. In the first study, spectrograms, long-term average spectra (LTAS), and fast Fourier transform (FFT) spectra of compressed and original samples of running speech were compared. In the second study, intensities, formant frequencies, formant bandwidths, and a multidimensional set of voice parameters were tested in a set of sustained phonations. Results showed that compression at high bitrates (96 and 128 kbps) preserved the relevant acoustic properties of the pathological voices. With compressions at lower bitrates, fidelity decreases, introducing some important alterations. Results from both works, Gonzalez and Cervera and this paper, open up the possibility of using MPEG-compression at high bitrates to store or transmit high-quality speech recordings, without altering their acoustic properties. 相似文献