全文获取类型
收费全文 | 104365篇 |
免费 | 16664篇 |
国内免费 | 11895篇 |
专业分类
化学 | 72890篇 |
晶体学 | 1140篇 |
力学 | 5884篇 |
综合类 | 669篇 |
数学 | 12662篇 |
物理学 | 39679篇 |
出版年
2024年 | 352篇 |
2023年 | 2077篇 |
2022年 | 3677篇 |
2021年 | 3956篇 |
2020年 | 4187篇 |
2019年 | 3979篇 |
2018年 | 3726篇 |
2017年 | 3346篇 |
2016年 | 5031篇 |
2015年 | 4941篇 |
2014年 | 6072篇 |
2013年 | 7798篇 |
2012年 | 9295篇 |
2011年 | 9497篇 |
2010年 | 6646篇 |
2009年 | 6379篇 |
2008年 | 6703篇 |
2007年 | 5988篇 |
2006年 | 5598篇 |
2005年 | 4674篇 |
2004年 | 3667篇 |
2003年 | 2944篇 |
2002年 | 2686篇 |
2001年 | 2245篇 |
2000年 | 1960篇 |
1999年 | 2057篇 |
1998年 | 1739篇 |
1997年 | 1560篇 |
1996年 | 1557篇 |
1995年 | 1366篇 |
1994年 | 1241篇 |
1993年 | 1004篇 |
1992年 | 903篇 |
1991年 | 779篇 |
1990年 | 652篇 |
1989年 | 503篇 |
1988年 | 378篇 |
1987年 | 338篇 |
1986年 | 335篇 |
1985年 | 262篇 |
1984年 | 188篇 |
1983年 | 153篇 |
1982年 | 126篇 |
1981年 | 78篇 |
1980年 | 59篇 |
1979年 | 26篇 |
1978年 | 22篇 |
1976年 | 22篇 |
1975年 | 22篇 |
1957年 | 26篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
941.
Natural logarithm wavelength modulation spectroscopy(ln-WMS) is demonstrated in this Letter. Unlike the conventional wavelength modulation spectroscopy(WMS)-2 f technique, it is a linear method even for large absorbance, which is the core advantage of ln-WMS. The treating method used in ln-WMS is to take the natural logarithm of the transmitted intensity. In order to determine the proper demodulation phase, the η-seeking algorithm is introduced, which minimizes the absolute value of the first harmonic within the non-absorbing region. Subsequently, the second harmonic of the absorption signal is extracted by setting the demodulating phase as 2η. To illustrate the validity of ln-WMS, it was applied to water vapor experimentally. The result shows that even if the absorbance(base-e) is between 1.60 and 6.26, the linearity between ln-WMS-2 f and volume fraction is still established. For comparison, measurement with conventional WMS-2 f was also done, whose response no longer kept linearity. The η values retrieved in continuous measurements and the residuals were shown so as to evaluate the performance of the η-seeking algorithm. Time consumed by this algorithm was roughly 0.28 s per measurement. As an alternative WMS strategy, ln-WMS has a wide range of potential applications, especially where the absorbance is large or varies over a wide area. 相似文献
942.
The d-d transition spectrum of Sr[Co(HEDTA)H2O]2 · 4 H2O crystal was recorded at room temperature and experimental results are discussed quantitatively. Using the ligand field theory and the radial wave function of non-free Co(II), the d-d transition energy levels of the title compound were calculated. And the results are seen to be in good agreement with the experimental values. 相似文献
943.
Haitao Han Liang Kan Ping Li Guoshuai Zhang Kaiyue Li Wuping Liao Yunling Liu Wei Chen Chunhua T.Hu 《中国科学:化学(英文版)》2021,(3):426-431
Three 4.8 nm isostructural{M72}(M=CoⅡ for CIAC-128,NiⅡ for CIAC-129,FeⅡ for CIAC-130) metal-organic polyhedra(MOPs) are constructed by eighteen M4-p-tert-butylthiacalix[4]arene (M4-TC4A) units bridged via sixteen four-connected 5,5′-(1H-1,2,4-triazole-3,5-diyl) diisophthalic acid (H4TADIPA) linkers.These MOPs are featured with a rarely reported concave coordination cage,which can be simplified as a squeezed double-decker hexagonal prism pressed from the top and bottom hexagonal faces.Furthermore,CIAC-128,CIAC-129 and CIAC-130 are the highest nuclearity discrete clusters of Co,Ni and Fe reported so far.CIAC-128 has higher separation selectivity of C3H8 than CH4 under ambient conditions,and also exhibits separation selectivity for C2H6/CH4,C2H2/CH4,and C2H4/CH4.In addition,CIAC-128 can provide the cavity for the confined synthesis of noble metal nanoclusters such as Pd nanoclusters and the resulting Pd@CIAC-128 hybrids show higher electrocatalytic activity than commercial Pt/C towards hydrogen evolution reaction (HER). 相似文献
944.
Hydrodynamic pressure exerted on a plunge pool slab by jet impingement is of high interest in high dam projects. The present study experimentally investigated the characteristics of pressure induced by a jet through a constant width flip bucket (CFB) and a slit flip bucket (SFB). A pressurized plane pipe was employed in the flume experiments to control the inlet velocities in the flip buckets. A concise method is proposed to predict the mean dynamic pressure field. Its implementation is summarized as follows: First, the position of the pressure field is determined by the trajectories of free jets, and to calculate its trajectories, an equation based on parabolic trajectory theory is used; second, the maximum mean dynamic pressure is obtained through dimensional analysis, and then the pressure field is established by applying the law of Gaussian distribution. Those steps are integrated into a concise computing procedure by using some easy-to-obtain parameters. Some key parameters, such as takeoff velocity coefficient, takeoff angle coefficient, and the parameter , are also investigated in this paper. The formulas of these coefficients are obtained by fitting the experimental data. Using the proposed method, the easy-to-obtain geometric parameters and initial hydraulic conditions can be used to calculate the maximum mean dynamic pressure on the slab. A comparison between experimental data and calculated results confirmed the practicability of this model. These research results provide a reference for hydraulic applications. 相似文献
945.
Yuxing Li Peiyuan Gao Bingzhao Tang Yingmin Yi Jianjun Zhang 《Entropy (Basel, Switzerland)》2022,24(1)
In order to accurately identify various types of ships and develop coastal defenses, a single feature extraction method based on slope entropy (SlEn) and a double feature extraction method based on SlEn combined with permutation entropy (SlEn&PE) are proposed. Firstly, SlEn is used for the feature extraction of ship-radiated noise signal (SNS) compared with permutation entropy (PE), dispersion entropy (DE), fluctuation dispersion entropy (FDE), and reverse dispersion entropy (RDE), so that the effectiveness of SlEn is verified, and SlEn has the highest recognition rate calculated by the k-Nearest Neighbor (KNN) algorithm. Secondly, SlEn is combined with PE, DE, FDE, and RDE, respectively, to extract the feature of SNS for a higher recognition rate, and SlEn&PE has the highest recognition rate after the calculation of the KNN algorithm. Lastly, the recognition rates of SlEn and SlEn&PE are compared, and the recognition rates of SlEn&PE are higher than SlEn by 4.22%. Therefore, the double feature extraction method proposed in this paper is more effective in the application of ship type recognition. 相似文献
946.
The vibration signal of gearboxes contains abundant fault information, which can be used for condition monitoring. However, vibration signal is ineffective for some non-structural failures. In order to resolve this dilemma, infrared thermal images are introduced to combine with vibration signals via fusion domain-adaptation convolutional neural network (FDACNN), which can diagnose both structural and non-structural failures under various working conditions. First, the measured raw signals are converted into frequency and squared envelope spectrum to characterize the health states of the gearbox. Second, the sequences of the frequency and squared envelope spectrum are arranged into two-dimensional format, which are combined with infrared thermal images to form fusion data. Finally, the adversarial network is introduced to realize the state recognition of structural and non-structural faults in the unlabeled target domain. An experiment of gearbox test rigs was used for effectiveness validation by measuring both vibration and infrared thermal images. The results suggest that the proposed FDACNN method performs best in cross-domain fault diagnosis of gearboxes via multi-source heterogeneous data compared with the other four methods. 相似文献
947.
948.
The Existence and Global $p$-Exponential Stability of Periodic Solution for Stochastic BAM Neural Networks with Delays 下载免费PDF全文
In this paper, we consider a class of stochastic BAM neural networks with delays. By establishing new integral inequalities and using the properties of spectral radius of nonnegative matrix, some sufficient conditions for the existence and global $p$-exponential stability of periodic solution for stochastic BAM neural networks with delays are given. An example is provided to show the effectiveness of the theoretical results. 相似文献
949.
The complex crystal of Pd(Tyr)2Cl · H2O was synthesized and its PA spectra (with L-tyrosine) were determined and explained. A method used to resolve the PA amplitude spectrum was suggested. With the phase spectrum of the complex, the PA absorption peaks were resolved by this method, and the non-radiative relaxation time of all absorption bands were calculated. 相似文献
950.
Samples of 5Cr2NiMoVSi die steel which underwent thermal fatigue with different fatigue cycles have been studied by using microhardness and positron lifetime measurements. It is found that the microhardness and the positron lifetime parameters increase and decrease alternatively with increasing the number of the fatigue cycles. The experimental results reveal that the fatigue strain, the dynamical recovery, and recrystallization and the precipitation are main causes of variation of the defects in the process of thermal fatigue. 相似文献