全文获取类型
收费全文 | 1633篇 |
免费 | 18篇 |
国内免费 | 8篇 |
专业分类
化学 | 1357篇 |
晶体学 | 10篇 |
力学 | 20篇 |
数学 | 78篇 |
物理学 | 194篇 |
出版年
2020年 | 10篇 |
2018年 | 11篇 |
2017年 | 10篇 |
2016年 | 12篇 |
2015年 | 8篇 |
2014年 | 16篇 |
2013年 | 70篇 |
2012年 | 46篇 |
2011年 | 73篇 |
2010年 | 42篇 |
2009年 | 41篇 |
2008年 | 73篇 |
2007年 | 81篇 |
2006年 | 94篇 |
2005年 | 89篇 |
2004年 | 94篇 |
2003年 | 63篇 |
2002年 | 73篇 |
2001年 | 19篇 |
2000年 | 31篇 |
1999年 | 24篇 |
1998年 | 22篇 |
1997年 | 18篇 |
1996年 | 18篇 |
1995年 | 19篇 |
1994年 | 20篇 |
1993年 | 20篇 |
1992年 | 15篇 |
1991年 | 20篇 |
1990年 | 12篇 |
1989年 | 11篇 |
1988年 | 28篇 |
1987年 | 19篇 |
1986年 | 18篇 |
1985年 | 34篇 |
1984年 | 36篇 |
1983年 | 14篇 |
1982年 | 38篇 |
1981年 | 51篇 |
1980年 | 34篇 |
1979年 | 26篇 |
1978年 | 28篇 |
1977年 | 23篇 |
1976年 | 31篇 |
1975年 | 20篇 |
1974年 | 28篇 |
1973年 | 19篇 |
1971年 | 7篇 |
1970年 | 8篇 |
1967年 | 8篇 |
排序方式: 共有1659条查询结果,搜索用时 15 毫秒
101.
The regioselective ring-opening reaction of α-methyl-β-propiolactone with 3,3-ethylene-dioxybutylmagnesium bromide in the presence of copper(I) catalyst afforded 2-methyl-6-oxoheptanoic acid, which was easily converted into (E)-3,7-dimethyl-2-2-octene-1,8-diol in good yield. 相似文献
102.
103.
The refractive index of SiO2-P2O5 glass prepared by a modified chemical vapor deposition method is measured using an interference microscope. It is found that the refractive index in bulk form increases linearly at 9.5×10-4 (mol.%)-1 as the P2O5 concentration increases. It is also found that quenching at extremely high speed reduces the refractive index over 2 mol.% P2O5. The wavelength dispersion of the refractive index dn/dλ is constant up to 5 mol.% P2O5 producing a refractive index difference of 5×10-3 compared with fused silica. Consequently, this glass materials is thought to be suitable for wide band-width optical fiber applications. 相似文献
104.
Pressure, temperature, and composition phase equilibrium diagrams of new solid solution systems of the Cd1?xMxS (M = Mg, Ca, Sr) type were investigated using the quenching method. The stable region for the rock-salt-type phase is widely extended toward the high-temperature/low-pressure region by substituting 10–20 mole% of Cd with Ca or Sr. Temperature and composition phase diagrams for each solid solution system were obtained at 2 GPa. The rock-salt-type phase stability is discussed in view of these phase relations. 相似文献
105.
Kunihiko Takabe Suguru Ohkawa Toshio Sato Gia Hien Tang Takao Katagiri 《Tetrahedron letters》1980,21(40):3883-3884
Dimerization of 3-methyl-3-butenenitrile (1) by lithium dialkylamide gave selectively 3,5,5-trimethyl-1-amino-4-cyanocyclohexa-1,3-diene (2), which was easily hydrolyzed by aqueous acid to afford 4-cyanoisophorone (3,5,5-trimethyl-4-cyano-cyclohex-2-enone) (3) quantitatively. 相似文献
106.
107.
108.
Inner shell ionization cross sections by low-energy positron impact have been measured. Development of an x-ray detector with thin Si(Li) crystals has enabled the first measurements of the absolute cross sections for the positron impacts in the energy range below 30 keV. Threshold behavior of the measured cross sections for the Cu K shell and Ag L shell are compared with the theoretical results of Gryzinski and Kowalski [Phys. Lett. A 183, 196 (1993)]] and Khare and Wadehra [Can. J. Phys. 74, 376 (1996)]]. Good agreement has been found for the Cu K shell, while the experimental values for the Ag L shell were found to be smaller than the corresponding theoretical results. 相似文献
109.
Effect of moisture on impact toughness of sugar-coated tablets manufactured by the dusting method 总被引:1,自引:0,他引:1
The purpose of this study was to clarify the effect of moisture on the impact toughness of sugar-coated tablets manufactured by the dusting method. We demonstrated that moisture plays an important role in the impact toughness of sugar-coated tablets. Moisturizing the sugar-coating layer resulted in enhancement of impact toughness of sugar-coated tablets, while reducing moisture in the sugar-coating layer resulted in weakening of the impact toughness. This was due to the characteristics of sucrose, the main ingredient of the sugar-coating layer, which is a soft and non-fragile material at high moisture levels, but hard and fragile at low moisture levels. We also demonstrated that friability as an indicator of impact toughness changed with time, and friability should be measured at 14 d after manufacture. This is due to moisture movement from outer sugar-coating layer into the inner sugar-coated tablets. Incorporating microcrystalline cellulose (MCC) in the subcoating layer resulted in sugar-coating layers with high resistance against impact even though moisture content of sugar-coated tablets was low. We confirmed the high impact toughness of the sugar-coated tablets with MCC whose moisture content was low from the results of both free fall and friability tests. We suggest that the dusting method using dusting powder containing MCC is a useful method for the production of sugar-coated tablets containing moisture sensitive drugs. 相似文献
110.
Reliability of mean transit time obtained using perfusion-weighted MR imaging; comparison with positron emission tomography 总被引:2,自引:0,他引:2
Mihara F Kuwabara Y Tanaka A Yoshiura T Sasaki M Yoshida T Masuda K Matsushima T 《Magnetic resonance imaging》2003,21(1):33-39
The purpose of this project was to assess the reliability of the cerebral mean transit time (MTT) obtained using perfusion-weighted MR imaging by comparing it with the MTT obtained when performing positron emission tomography (PET). Ten patients with chronic occlusive cerebrovascular disease were investigated. They had either unilateral internal carotid artery occlusion or middle cerebral artery occlusion. The regions-of-interest were placed in non-infarcted areas within the territory of the middle cerebral artery on the affected side. Control regions-of-interest were placed in mirrored regions of the contralateral side. Linear regression analyses were performed using the parameters of the MTT obtained with perfusion-weighted MR imaging and the MTT, cerebral blood flow, vascular reactivity, and oxygen extraction fraction obtained with PET. The respective MTTs of the affected and non-affected sides obtained with perfusion-weighted MR imaging versus those with PET were 7.3 +/- 2.2 s and 6.0 +/- 1.2 s versus 8.2 +/- 3.0 s and 6.4 +/- 1.7 s. The MTT obtained using perfusion-weighted MR imaging and PET demonstrated statistically significant correlation (r = 0.87, p < 0.0001). The MTT obtained with perfusion-weighted MR imaging correlated statistically with cerebral blood flow (r = -0.74, p < 0.001), vascular reactivity (r = -0.73, p < 0.001) and oxygen extraction fraction (r = 0.61, p < 0.01). Similarly, the MTT obtained using PET statistically correlated with cerebral blood flow (r = -0.78, p < 0.0001), vascular reactivity (r = -0.51, p < 0.05) and oxygen extraction fraction (r = 0.68, p < 0.01). The reliability of the MTT obtained using perfusion-weighted MR imaging appears to be approximately equal to that obtained with positron emission tomography. 相似文献