全文获取类型
收费全文 | 311038篇 |
免费 | 4423篇 |
国内免费 | 878篇 |
专业分类
化学 | 165026篇 |
晶体学 | 4382篇 |
力学 | 12872篇 |
综合类 | 2篇 |
数学 | 40035篇 |
物理学 | 94022篇 |
出版年
2021年 | 2433篇 |
2020年 | 2920篇 |
2019年 | 3141篇 |
2018年 | 4334篇 |
2017年 | 4074篇 |
2016年 | 6551篇 |
2015年 | 4664篇 |
2014年 | 6233篇 |
2013年 | 15032篇 |
2012年 | 12670篇 |
2011年 | 14534篇 |
2010年 | 9891篇 |
2009年 | 9277篇 |
2008年 | 13037篇 |
2007年 | 12913篇 |
2006年 | 12028篇 |
2005年 | 11138篇 |
2004年 | 10025篇 |
2003年 | 8660篇 |
2002年 | 8255篇 |
2001年 | 8423篇 |
2000年 | 6607篇 |
1999年 | 5106篇 |
1998年 | 4284篇 |
1997年 | 4077篇 |
1996年 | 4127篇 |
1995年 | 3604篇 |
1994年 | 3638篇 |
1993年 | 3688篇 |
1992年 | 3847篇 |
1991年 | 3902篇 |
1990年 | 3692篇 |
1989年 | 3574篇 |
1988年 | 3567篇 |
1987年 | 3421篇 |
1986年 | 3233篇 |
1985年 | 4447篇 |
1984年 | 4595篇 |
1983年 | 3654篇 |
1982年 | 3947篇 |
1981年 | 3844篇 |
1980年 | 3683篇 |
1979年 | 3769篇 |
1978年 | 3871篇 |
1977年 | 3751篇 |
1976年 | 3888篇 |
1975年 | 3517篇 |
1974年 | 3556篇 |
1973年 | 3599篇 |
1972年 | 2457篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
981.
A digital autoradiographic-thin-layer chromatographic method involving simple steps is described for thorough separation of eight major androgens (testosterone, androstenedione, dihydrotestosterone, androsterone, epiandrosterone, androstanedione, 3 alpha-androstanediol and 3 beta-androstanediol) derived from testosterone metabolism. Their direct quantification is performed by radioscanning, which avoids tedious plate-scraping and liquid scintillation counting. Satisfactory accuracy is obtained by both external standardization of plates by calibrated amounts of radiolabelled standard steroids and internal standardization of bioassays by radiolabelled squalane. Coefficients of variation are below 8.5% in the range 100-500 dpm. Some analytical criteria related to chromatographic conditions and quantification parameters depending on position-sensitive proportional counter are discussed. 相似文献
982.
Proton magnetic resonance imaging was performed on rats before induction of diabetes with streptozotocin (STZ) and at 2 and 12 days postinduction. Images revealed an increase in maximal longitudinal and axial dimensions of the kidneys at 2 days and a further increase at 12 days. Similarly, an increase in the size of the remaining kidney was seen in a rat which underwent uninephrectomy as a positive control. Two major differences were observed between the kidney undergoing compensatory hypertrophy and those developing diabetic nephropathy: (i) Expansion of the renal vasculature was seen only in images of the diabetic rat; (ii) A loss in conspicuity of the normal corticomedullary junction was seen in the T2-weighted images of the diabetic rat but not in the uninephrectomized rat. Histologic examination revealed that the medulla increased to a size greater than the cortex during diabetic nephropathy whereas the medullary volume was less than that of the cortex during compensatory hypertrophy. In vitro T1 relaxation times in cortex, outer medulla and inner medulla of kidneys from control rats were measured and compared with the same respective regions in diabetic rats. When these values were correlated with tissue water content, a linear increase in relaxation rate versus percent water content from cortex to inner medulla was found in the control kidneys, but this correlation was absent in diabetic nephropathy. These studies demonstrate that MRI is an effective noninvasive tool for studying the course of renal hypertrophy and hydration changes in the development of renal disease in STZ-induced diabetes in the rat. 相似文献
983.
H. Postma J. D. Bowman C. D. Bowman J. E. Bush P. P. J. Delheij C. M. Frankle C. R. Gould D. G. Haase J. Knudson G. E. Mitchell S. Penttilä N. R. Roberson S. J. Seestrom J. J. Szymansky S. H. Yoo V. W. Yuan X. Zhu 《Hyperfine Interactions》1992,75(1-4):153-163
Parity violation effects have been studied at 40 neutron p-wave resonances of the even-even nuclei238U and232Th. Of these 11 show parity violation effects larger than 2 standard deviations, making parity violation a rather common phenomenon.
Parity mixing up to 10% has been found. The root-mean squared matrix elements for parity violation derived from these resonances
are M=0.58 (+0.50/-0.25) meV for238U, respectively 1.39 (+0.35/-0.38) meV for232Th. 相似文献
984.
Principles and recent developments in ultrasound contrast agents 总被引:6,自引:1,他引:5
The behaviour of gas bubbles and gas encapsulated spheres as echographic contrast agents is reviewed. Compared with rigid spheres, gas bubbles are superior scattering agents and they offer a number of useful properties which can be exploited in a variety of ways. The analysis of their velocity of sound, back-scatter intensity, second harmonic emission and resonant frequency opens up new perspectives in the development of contrast agents for echocardiographic research with potential clinical applications. 相似文献
985.
Liquid crystal televisions have been employed as spatial light modulators in a variety of optical image processing applications. We have used such devices to develop techniques in speckle metrology for the electronic addition of speckle patterns and the display of speckle correlation, and also for the real-time display of object motion. Factors which affect the performance in terms of the useful working range and the accuracy of displacement measurement are discussed. Some modifications to the LCTV which should increase the potential of these devices as spatial light modulators, when used in both amplitude and phase modulation, are considered briefly. 相似文献
986.
987.
In this paper we present photoionization cross sections of the Cu and Al dimers and tetramers. The local spin density method is applied to calculate the electronic structure and the ground state potential. The cross sections are calculated using the continuum multiple scattering method, and a basic analysis of the photoionization process is given within the independent particle picture. 相似文献
988.
In the framework of the extended Hubbard model (copper-oxygen and oxygen-oxygen hopping) the energy and effective mass of a mobile hole is expressed in terms of hopping parameters and the correlators between spins of the background SnSn+1. Due to interaction of the mobile hole with localized holes, its effective mass increases about four times. 相似文献
989.
P. J. Skevington M. A. G. Halliwell M. H. Lyons S. J. Amin M. A. Z. Rejman-Greene G. J. Davies 《Journal of Crystal Growth》1992,120(1-4):328-332
InP/InGaAs multiple quantum well structures with up to 200 periods have been grown by CBE. These structures exhibit exceptional lateral uniformity, measured as ±1 Å in period, ±13 ppm in lattice mismatch and ±0.5 nm in wavelength across a 2 inch wafer. Good surface morphology, sharp interfaces and excellent growth control have all been demonstrated. 相似文献
990.
G. K. Hubler C. N. Waddell E. P. Donovan J. M. Zavada 《Optical and Quantum Electronics》1991,23(7):S883-S893
Precision reflection measurements were performed on GaAs/AlAs superlattices of the same composition but different layer spacings. Nonlinear-least-squares fits to the data were performed to a single layer. Measurements were extracted for the superlattice thickness, thickness of a disturbed interface layer between the superlattice and substrate, the uniformity in composition and/or spacing and the composition. It was demonstrated that these nondestructive measurements in the infrared region (3000 to 12 000 cm–1) in conjunction with a simple single layer model are capable of accurately yielding the above quantities with high precision. 相似文献