全文获取类型
收费全文 | 226187篇 |
免费 | 1619篇 |
国内免费 | 543篇 |
专业分类
化学 | 105968篇 |
晶体学 | 3401篇 |
力学 | 10310篇 |
综合类 | 10篇 |
数学 | 21993篇 |
物理学 | 64915篇 |
无线电 | 21752篇 |
出版年
2021年 | 2196篇 |
2020年 | 2275篇 |
2019年 | 2908篇 |
2018年 | 3990篇 |
2017年 | 3982篇 |
2016年 | 5265篇 |
2015年 | 2537篇 |
2014年 | 4747篇 |
2013年 | 9323篇 |
2012年 | 7357篇 |
2011年 | 8639篇 |
2010年 | 7028篇 |
2009年 | 7462篇 |
2008年 | 8610篇 |
2007年 | 8648篇 |
2006年 | 7708篇 |
2005年 | 6925篇 |
2004年 | 6558篇 |
2003年 | 6023篇 |
2002年 | 5972篇 |
2001年 | 6431篇 |
2000年 | 4955篇 |
1999年 | 3895篇 |
1998年 | 3360篇 |
1997年 | 3418篇 |
1996年 | 3088篇 |
1995年 | 2775篇 |
1994年 | 2848篇 |
1993年 | 2746篇 |
1992年 | 3037篇 |
1991年 | 3155篇 |
1990年 | 3081篇 |
1989年 | 3087篇 |
1988年 | 2951篇 |
1987年 | 2791篇 |
1986年 | 2643篇 |
1985年 | 3338篇 |
1984年 | 3350篇 |
1983年 | 2803篇 |
1982年 | 2727篇 |
1981年 | 2666篇 |
1980年 | 2472篇 |
1979年 | 2810篇 |
1978年 | 2778篇 |
1977年 | 2956篇 |
1976年 | 2958篇 |
1975年 | 2743篇 |
1974年 | 2677篇 |
1973年 | 2796篇 |
1972年 | 2138篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
161.
Richards-Kortum R. Mahadevan-Jansen A. Ramanujam N. 《Circuits and Devices Magazine, IEEE》1996,12(4):34-40
A novel non-invasive technique for detecting cancer and precancer offers a viable adjunct to biopsies. Here, we summarize the principles of optical spectroscopy and present some of the highlights obtained with the successful application of fluorescence and Raman spectroscopy for cancer and precancer detection 相似文献
162.
We propose an approximate analytical evaluation technique of self-phase-modulation (SPM) in cascaded optical amplifier systems. The effect of SPM on optical pulse distortion is represented by the increase in the α-parameter of the transmitter. The waveform distortion is approximated by assuming that the pulse is transmitted over a dispersive fiber without nonlinearity. This technique greatly reduces the calculation time, and is especially useful in designing cascaded amplifier systems, evaluating theoretical maximum transmission distance 相似文献
163.
Fukuda H. Ebara M. Kobayashi A. Sugiura N. Yoshikawa M. Saisho H. Kondo F. Yoshino S. Yahagi T. 《IEEE transactions on bio-medical engineering》1998,45(3):396-400
To objectively evaluate the parenchymal echo pattern of cirrhotic liver and chronic hepatitis, the authors applied an image analyzing system (IAS) using a neural network. Autopsy specimens in a water tank (n=13) were used to examine the relationship between the diameter of the regenerative nodule and the coarse score (CS) calculated by IAS. CS was significantly correlated with the diameter of the regenerative nodule (p<0.0001, r=0.966). CS is considered to be useful for evaluating the coarseness of the parenchymal echo pattern 相似文献
164.
G. Carelli N. Ioli J. C. S. Moraes A. Moretti F. Strumia G. E. Annino A. Bertolini 《Journal of Infrared, Millimeter and Terahertz Waves》1996,17(6):1023-1030
The results described in this work are part of a systematic search for long wavelength laser lines to be used in high magnetic field EPR applications and in plasma diagnostic. Four new far-infrared laser lines of CH2 = CF2 (1,1 difluoroethylene), optically pumped by a waveguide CO2 laser, have been discovered and characterized in wavelength, polarization relative to the pumping radiation and offset relative to the CO2 center frequency. New measurements of polarization and offset of 5 already known laser lines are also reported. A table of all of the known CO2 pumped FIR laser lines from this molecule is given. 相似文献
165.
Measurements of the scattering cross section of a number of bare soils have been made with CO2 laser illumination at 10.59 μm. The primary focus was on absolute calibration of the measurements. First, comparison of emissivity values resulting from the application of Kirchhoff's relation after angular integration of the bidirectional measurements, with emissivity values obtained from the analysis of the emitted radiation show excellent agreement to within less than 0.3%. Second, it was found that a simple formula holds for a relationship between the emissivity and co- and cross-polarized backscattering cross section at an angle of 30°. Third, a clear correlation was observed between emissivity and composition (in this case % Al+Fe oxides; % SiO2) for a homogeneous series of samples from the same area in Niger. These results emphasize the importance of calibrated experimental data. The implications of the research give evidence of the advantage of obtaining emissivity from remote reflectivity measurements and possibly only backscattering measurements, and remotely estimating mineral composition 相似文献
166.
Schubert E.F. Hunt N.E.J. Malik R.J. Micovic M. Miller D.L. 《Lightwave Technology, Journal of》1996,14(7):1721-1729
Resonant-cavity light-emitting diodes (RCLED) are novel, high-efficiency light-emitting diodes which employ optical microcavities. These diodes have higher intensities and higher spectral purity as compared to conventional LEDs. Analytical formulas are derived for the enhancement of the spontaneous emission along the optical axis of the cavity. The design rules for high-efficiency operation of RCLEDs are established. The temperature dependence of the emission intensity is analyzed in the range 20-80° and it is described by an exponential dependence with a characteristic temperature of 112 K. The modulation characteristics of RCLEDs exhibit 3 dB frequencies of 580 MHz. Eye diagrams at transmission rates of 622 Mb/s are wide open indicating the suitability of RCLEDs for high-speed data transmission 相似文献
167.
This paper presents applications of microwaves and millimeter waves for the characterization of teeth. This is done by measuring the complex permittivity over the frequency range from 0.04 to 40 GHz. These measurements have revealed that dental caries are significantly more lossy to microwaves and millimeter waves than the healthy tooth, and this difference can be used for dental diagnosis. The experimental results have been confirmed by using the finite-difference time-domain (FDTD) method. In addition, millimeter-wave heating of the lossy dental caries can be used as a sterilization treatment. It is concluded that millimeter waves can be used for dental medical diagnosis as well as dental medical treatment 相似文献
168.
V. P. Evtikhiev I. V. Kudryashov E. Yu. Kotel’nikov V. E. Tokranov A. N. Titkov I. S. Tarasov Zh. I. Alferov 《Semiconductors》1998,32(12):1323-1327
The electroluminescence and stimulated emission of lasers with one layer of InAs quantum dots (QD’s) grown in a single molecular-beam
epitaxial process on vicinal GaAs(001) surfaces misoriented in the direction [010] by 2, 4 and 6° are investigated. It is
discovered that an increase in the misorientation angle leads to a blue shift and a decrease in the full width at half maximum
(FWHM) of the electroluminescence spectrum. This effect is attributed to a decrease in the size of the quantum dots and improvement
in their size uniformity. A strong dependence of the threshold current density on the width of the spontaneous luminescence
spectrum is discovered. The room-temperature threshold current density of the lasers with one layer of quantum dots and the
spontaneous luminescence spectrum having the smallest FWHM (54 meV) equals 210 A/cm2.
Fiz. Tekh. Poluprovodn. 32, 1482–1486 (December 1998) 相似文献
169.
170.
An experiment to determine the effect of gate electrode resistivity on circuit speed gave unexpected results: circuits with the lowest sheet resistance had the poorest circuit speed. Explanation of this behaviour required development of a new high-frequency method of measuring the impedance of the gate electrode. This method revealed that the circuits with a composite gate electrode had been formed with a partial discontinuity. The measurement technique is described, and the evidence of the discontinuity is shown. The effect of the discontinuity on device and circuit speed is demonstrated 相似文献