全文获取类型
收费全文 | 221545篇 |
免费 | 1929篇 |
国内免费 | 629篇 |
专业分类
化学 | 121421篇 |
晶体学 | 3382篇 |
力学 | 8492篇 |
综合类 | 6篇 |
数学 | 24367篇 |
物理学 | 66435篇 |
出版年
2020年 | 2040篇 |
2019年 | 2377篇 |
2018年 | 3337篇 |
2017年 | 3242篇 |
2016年 | 4482篇 |
2015年 | 2546篇 |
2014年 | 4059篇 |
2013年 | 9830篇 |
2012年 | 7236篇 |
2011年 | 8814篇 |
2010年 | 6378篇 |
2009年 | 6181篇 |
2008年 | 8430篇 |
2007年 | 8493篇 |
2006年 | 7861篇 |
2005年 | 7147篇 |
2004年 | 6351篇 |
2003年 | 5834篇 |
2002年 | 5648篇 |
2001年 | 6053篇 |
2000年 | 4787篇 |
1999年 | 3548篇 |
1998年 | 3093篇 |
1997年 | 3095篇 |
1996年 | 2882篇 |
1995年 | 2401篇 |
1994年 | 2459篇 |
1993年 | 2485篇 |
1992年 | 2651篇 |
1991年 | 2694篇 |
1990年 | 2610篇 |
1989年 | 2577篇 |
1988年 | 2519篇 |
1987年 | 2477篇 |
1986年 | 2404篇 |
1985年 | 3115篇 |
1984年 | 3287篇 |
1983年 | 2609篇 |
1982年 | 2878篇 |
1981年 | 2765篇 |
1980年 | 2578篇 |
1979年 | 2768篇 |
1978年 | 2888篇 |
1977年 | 2910篇 |
1976年 | 2890篇 |
1975年 | 2732篇 |
1974年 | 2707篇 |
1973年 | 2848篇 |
1972年 | 1894篇 |
1967年 | 1835篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Improved quantitation of 13-cis- and all-trans-acitretin in human plasma by normal-phase high-performance liquid chromatography. 总被引:1,自引:0,他引:1
E Meyer W E Lambert A P De Leenheer J P Bersaques A H Kint 《Journal of chromatography. A》1991,570(1):149-156
A reliable analytical method has been developed for measurement of 13-cis- and all-trans-acitretin (Neotigason) in human plasma by normal-phase high-performance liquid chromatography, with ultraviolet detection. Human plasma was obtained after centrifugation of whole blood samples and deproteinized by ethanolic denaturation. After liquid-liquid extraction with water-n-hexane, and aliquot ws chromatographed on a silica column using isocratic elution with n-hexane-methylsalicylate-acetic acid (200:18:0.6, v/v). The wavelength was set at 360 nm, and for plasma samples a limit of quantification of 3-4 ng/ml was obtained. All manipulations were carried out under dim light conditions to prevent photoisomerization. 相似文献
992.
Withanolides and related ergostane-type steroids. 总被引:1,自引:0,他引:1
E Glotter 《Natural product reports》1991,8(4):415-440
993.
Non-Markovian Brownian motion in a periodic potential is studied by means of an electronic analogue simulator. Velocity spectra, the Fourier transforms of velocity autocorrelation functions, are obtained for three types of random force, that is, a white noise, an Ornstein—Uhlenbeck process, and a quasimonochromatic noise. The analogue results are in good agreement both with theoretical ones calculated with the use of a matrix-continued-fraction method, and with the results of digital simulations. An unexpected extra peak in the velocity spectrum is observed for Ornstein-Uhlenbeck noise with large correlation time. The peak is attributed to a slow oscillatory motion of the Brownian particle as it moves back and forth over several lattice spaces. Its relationship to an approximate Langevin equation is discussed. 相似文献
994.
995.
Shigenori Okada Shigemitsu Yoshida Shahid H Ashrafi Dean E Schraufnagel 《Microscopy and microanalysis》2002,8(2):104-115
Osteocytes communicate through a canalicular system that maintains the vitality and mineral metabolism of bone. Casting the vascular canals and canaliculi of compact bone with methacrylate and viewing them with scanning electron microscopy shows their extent and relationships. Confocal laser scanning microscopy of the same specimen before corrosion establishes the degree of calcification of the different tissue components. These methods were used to compare basal with alveolar compact bone in the rat mandible at different ages. Sections of the mandibular molar region were placed in a methacrylate resin. After polymerization and study with confocal microscopy, the organic matrix was removed. Juvenile rats had large irregular central vascular canals and lacunae that were more concentric in the basal than the alveolar bone. Cast lacunae were round, and the canaliculi from these lacunae were short and thick in both bones. Adult rats had regular concentrically arranged lacunae in the basal bone. Cast lacunae were ellipsoid and flatter in the basal bone than in the alveolar bone. The intercommunicating canaliculi were increased and canaliculi had more branching than the juvenile rats. The aged rats had fewer vascular canals, lacunae, and canaliculi and had osteoporotic changes. The cast lacunae were slender and flat especially in the basal bone. The porosity of the mandible became more pronounced in the alveolar than in the basal bone with aging. The canaliculi of mandibular compact bone thinned and developed extensive branching with adulthood but decreased in size and number with advanced age. Lacunae proceed from the large circular structures of youth to the flat forms of the aged. These studies show that the internal structure of compact bone changes with age and mirrors its functional state. 相似文献
996.
Scott P. Lewis Cengiz Alacaci George E. O'Brien Zhonghong Jiang 《School science and mathematics》2002,102(4):172-180
The use of a project‐based science (PBS) approach to teaching encourages students to integrate mathematics and science in meaningful ways as they create projects. As a beginning study of how students use mathematics in such an approach, an analysis of 23 projects developed by preservice elementary teachers enrolled in an elementary science course was conducted. Findings showed that students made a number of different types of mathematical errors and underutilized data representation and summary forms. Implications included the importance of developing methods for supporting the use of mathematical tools in utilizing a project‐based approach and considering ways that such tools mediate scientific thinking. 相似文献
997.
V. A. Donchenko M. V. Kabanov V. T. Kalaida B. I. Kulakov E. V. Ovcharenko I. G. Sokovets 《Russian Physics Journal》1998,41(5):476-484
The problems of determining and investigating the informative parameters of atmospheric electricity are discussed. It is shown
that short-period variations in atmospheric electricity are ecologically significant. An automated system for monitoring atmospheric
electricity and a method of processing the results of measurements are described.
V. D. Kuznetsov Siberian Physicotechnical Institute, Tomsk State University. Translated from Izvestiya Vysshikh Uchebnykh
Zavedenii, Fizika, No. 5, pp. 88–98, May, 1998. 相似文献
998.
L. A. Chetkina V. E. Zavodnik A. N. Sobolev V. K. Bel'skii B. P. Bespalov A. G. Abolin 《Journal of Structural Chemistry》1991,32(3):442-445
L. Ya. Karpov Physical Chemical Research Institute. Translated from Zhurnal Strukturnoi Khimii, Vol. 32, No. 3, pp. 141–144, May–June, 1991. 相似文献
999.
1000.