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991.
Xiangyang Zhou Shankar Mahalingam David Weise 《Proceedings of the Combustion Institute》2007,31(2):2547-2555
This paper presents a combined study of laboratory scale fire spread experiments and a three-dimensional large eddy simulation (LES) to analyze the effect of terrain slope on marginal burning behavior in live chaparral shrub fuel beds. Line fire was initiated in single species fuel beds of four common chaparral plants under various fuel bed configurations and ambient conditions. An LES approach was developed to model fire spreading through a fuel bed with a subgrid scale turbulent combustion model based on a flame surface density concept. By examining two fuel bed slope configurations, it was found that upslope fire spread depends not only on the increased radiant heat transfer but also on the aerodynamic effect created by the interaction of the flame with the inclined surface. Under certain conditions, the convective heat transfer induced by this interaction becomes the dominant mechanism in determining fire spread success. Seventy-three (or 42%) of 173 experimental fires successfully propagated for slopes ranging from −70% to 70%. It was found there exists a critical slope above which fire spread in these live fuel beds was successful, and below which fire spread was unsuccessful. This critical slope for marginal burning varied widely with fuel moisture content and fuel loading. A stepwise logistic regression model was developed from experimental data to predict the probability of successful fire spread. It is expected that this model may be helpful in providing guidelines for prescribed fire application. 相似文献
992.
A method for hiding information is proposed. Firstly, the image to be hidden is encoded by the doubled-random phase-encoding technique. Then, the real part of the encoded data, together with the imaginary part, is embedded into a sufficiently large host image, which has been enlarged according to some rules. The superposition weighting is adjusted to a proper value such that the composed image will not be subject to severe degradation compared to the original host image. When decryption is needed, the reconstructed hidden image can be extracted directly from the composed image without using original host image, with the result being quite satisfactory. The quality of the reconstructed hidden image and of the composed image has been analyzed as the superposition weight varies, allowing the determination of the optimum superposition weight. The optimum superposition weight is related to the average gray level value of the hiding image. 相似文献
993.
Zhang FaQing Li ZhengHong Yang JianLun Ye Fan Wang Zhen Xia GuangXin Ying ChunTong Liu GuangJun 《中国科学G辑(英文版)》2007,50(6):698-706
For a scintillating-fiber array fast-neutron radiography system, a point-spread- function computing model was introduced, and the simulation code was developed. The results of calculation show that fast-neutron radiographs vary with the size of fast neutron sources, the size of fiber cross-section and the imaging geometry. The results suggest that the following qualifications are helpful for a good point spread function: The cross-section of scintillating fibers not greater than 200μm×200μm, the size of neutron source as small as a few millimeters, the distance between the source and the scintillating fiber array greater than 1 m, and inspected samples placed as close as possible to the array. The results give suggestions not only to experiment considerations but also to the estimation of spatial resolution for a specific system. 相似文献
994.
基于空间干涉调制成像光谱仪的成像特点,提出了一种适合于千涉多光谱图像的不等重要性权值率失真斜率提升图像压缩算法.该算法根据退卷积技术对复原光谱信噪比的影响,推导出时域中随着光程差的增大,干涉图像压缩失真对频域中光谱曲线的信噪比影响越大.对各个码块的率失真斜率按空间域中各光程差对恢复光谱信息的贡献重要性不同赋予不同的重要性权值,增大对光谱信息的保护程度,使图像在相同的光谱分辨率下拥有更好的空间分辨率.实验结果表明,该算法比传统算法更好地保护了多光谱图像的光谱信息,在8倍压缩比下,满足该类干涉多光谱图像压缩系统的质量要求. 相似文献
995.
Strategies for reliable automatic onset time picking of acoustic emissions and of ultrasound signals in concrete 总被引:3,自引:0,他引:3
Determining the onset of transient signals like seismograms, acoustic emissions or ultrasound signals is very time consuming if the onset is picked manually. Therefore, different approaches exist, especially in seismology. The concepts of the most popular approaches are summarized. An own approach adapted to ultrasound signals and acoustic emissions, based on the Akaike Information Criterion (AIC), is presented. The AIC-picker is compared to an automatic onset detection algorithm based on the Hinkley criterion and also adapted to acoustic emissions. Manual picks performed by an analyst are used as reference values. Both automatic onset detection algorithms are applied to ultrasound signals which are used to monitor the setting and hardening of concrete. They are also applied to acoustic emissions recorded during a pull-out test. The AIC-picker produces sufficient reliable results for ultrasound signals where the deviation from the manual picks varies between 2% and 4%. Concerning acoustic emissions, only 10% of the events result in a mislocation vector greater than 5mm. It can be shown that our AIC-picker is a reliable tool for automatic onset detection for ultrasound signals and acoustic emissions of varying signal to noise ratio. 相似文献
996.
We prove that a GF(q)-linear Rédei blocking set of size q
t + q
t–1 + ··· + q + 1 of PG(2,q
t) defines a derivable partial spread of PG(2t – 1, q). Using such a relationship, we are able to prove that there are at least two inequivalent Rédei minimal blocking sets of size q
t + q
t–1 + ··· + q + 1 in PG(2,q
t), if t 4. 相似文献
997.
998.
倒谱在混合模糊图像分析中的应用 总被引:2,自引:1,他引:2
降晰参数识别在模糊图像恢复过程中具有很重要的作用。在各种图像捕获系统中,有两种形式的图像模糊比较常见:一种是由光学系统散焦造成的散焦模糊;另一种是物体与照相机之间的相对运动造成的运动模糊。相对单个模糊模型的参数识别来说,混合了散焦和运动模糊的图像,其模糊参数的识别要复杂得多。许多识别方法一般都是用来分析某一特定的模糊模型的,而对两种模糊混合在一起的情况来说是很难区分的。提出了一种倒谱分析方法,在倒谱域同时对这两种模型参数进行识别。在分析过程中,仍需要利用这两种模糊模型在频域的特征,首先应用一些调整性的变换,然后再转换到倒谱域,以便更准确地评估模糊参数。 相似文献
999.
基于光学菲涅耳全息图和相位密码板,结合离散余弦变换,设计了一种新的正实值编码的“盲数字水印”计算方法.相位密码板是多个点光源的菲涅耳衍射光场的相角之和,原始水印图像在其菲涅耳衍射域中与通过相位密码板的参考光作相干叠加,形成菲涅耳全息图;之后将其嵌入到原始宿主图像的离散余弦变换中,同时将此叠加水印信息的原像素值用其邻近的原像素均值来替换;通过作逆离散余弦变换,获得了已嵌入水印信息的正实数值的目标图像;通过对载有水印信息的目标图像作逆运算,从中提取了原始水印图像.数值计算结果表明:该水印计算法对JPEG有损压缩、剪切、噪音污染和重采样等攻击,具有很强的鲁棒性.由于本算法属“盲数字水印”技术,以及水印信息的灵活嵌入和多重密钥(衍射距离、多个点光源位置等)的随意选择,从而使该算法具有很高的安全性和实用价值. 相似文献
1000.
M. Feito F. J. Cao 《The European Physical Journal B - Condensed Matter and Complex Systems》2007,59(1):63-68
Closed-loop or feedback controlled ratchets are
Brownian motors that operate using information about the state of
the system. For these ratchets, we compute the power output and
we investigate its relation with the information used in the
feedback control. We get analytical expressions for one-particle
and few-particle flashing ratchets, and we find that the maximum
power output has an upper bound proportional to the information.
In addition, we show that the increase of the power output that
results from changing the optimal open-loop ratchet to a
closed-loop ratchet also has an upper bound that is linear in the
information. 相似文献