全文获取类型
收费全文 | 330054篇 |
免费 | 3044篇 |
国内免费 | 933篇 |
专业分类
化学 | 164869篇 |
晶体学 | 5381篇 |
力学 | 15532篇 |
综合类 | 7篇 |
数学 | 38464篇 |
物理学 | 109778篇 |
出版年
2021年 | 3132篇 |
2020年 | 3330篇 |
2019年 | 3760篇 |
2018年 | 5198篇 |
2017年 | 5439篇 |
2016年 | 7368篇 |
2015年 | 4139篇 |
2014年 | 7029篇 |
2013年 | 15907篇 |
2012年 | 12378篇 |
2011年 | 14832篇 |
2010年 | 11013篇 |
2009年 | 10920篇 |
2008年 | 13237篇 |
2007年 | 13200篇 |
2006年 | 12145篇 |
2005年 | 10525篇 |
2004年 | 9836篇 |
2003年 | 8688篇 |
2002年 | 8675篇 |
2001年 | 10396篇 |
2000年 | 7657篇 |
1999年 | 5970篇 |
1998年 | 4976篇 |
1997年 | 4814篇 |
1996年 | 4482篇 |
1995年 | 3872篇 |
1994年 | 3823篇 |
1993年 | 3706篇 |
1992年 | 4099篇 |
1991年 | 4310篇 |
1990年 | 4099篇 |
1989年 | 4009篇 |
1988年 | 3782篇 |
1987年 | 3919篇 |
1986年 | 3684篇 |
1985年 | 4637篇 |
1984年 | 4684篇 |
1983年 | 3887篇 |
1982年 | 3976篇 |
1981年 | 3697篇 |
1980年 | 3653篇 |
1979年 | 3918篇 |
1978年 | 3903篇 |
1977年 | 3906篇 |
1976年 | 3880篇 |
1975年 | 3675篇 |
1974年 | 3615篇 |
1973年 | 3650篇 |
1972年 | 2603篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
871.
P. Stumpf W. Barrett N. Popplewell A.H. Shah S. Balakrishnan 《Experimental Techniques》1994,18(4):34-37
It is common practice in power transmission to use bundled overhead electrical conductors because the lower rate of corona discharge, compared with single conductors, makes the bundle attractive for high voltages. However, secondary problems may arise from the vibrations induced by the aerodynamic forces which act on the leeward conductors as a result of their immersion in the wake of a windward conductor. Thus, in addition to possible aeolian and galloping oscillations observed for a single conductor, bundle conductors may also be subjected to wake-induced oscillations. 1,2 Although extensive research has been done on the aeroelastic behavior of single and bundled conductors which are bare, little is known about the galloping or wake oscillation characteristics of given bundle conductors which, depending upon the environmental conditions during the formation of ice, may have five or so representative shaped cross-sections. One important task involves the aerodynamic quasi-static force and moment measurements in a wind tunnel using full scale conductor cross-sections each of which must be rotated through many angles at different spacings. Such measurements are repetitive as well as tediously time consuming and, hence, are beneficially automated. A reliable, accurate and inexpensive automated apparatus is described in this paper which permits the aerodynamic measurements to be performed efficiently and without intervention. Most importantly, it relieves the operator of the usual responsibility for manually controlling the separation and orientation of the sections as well as the need to manually compensate fluctuations in the free stream air speed. For simplicity, the apparatus is detailed in the context of a twin bundle conductor. 相似文献
872.
S. M. Kelly 《Liquid crystals》1991,10(2):273-287
Three separate series of new materials of weak to strongly positive dielectric anisotropy have been prepared. Each series contains four sub-sets of materials each incorporating a different four unit linking group (i.e., C4H8, C4H6, C3H6O and C3H4O) and the same series of end groups (i.e. F, CN and OCF3) in various substitution patterns. The synthesis and liquid crystal transition temperatures of these novel substances are described and compared with those of the corresponding materials incorporating standard central linkages (i.e.-, C2H4, CH2O, COO). The effect of an additional trans carbon-carbon double bond in the terminal alkyl chain and in the central linking unit has also been studied. 相似文献
873.
The kinetics and mechanism of the oxidation of 3,3′-dimethoxybenzidine (oda, o-dianisidine) by potassium bromate in aqueous acidic medium were studied by monitoring the formation rate of the reaction product, 3,3′-dimethoxy 4,4′-diphenoquinone at 447 nm. The reaction is, first order with respect to both the substrate and oxidant, and second order with respect to H+. The oda: bromate stoichiometric ratio is 1:1. Plausible mechanism and rate laws are proposed accounting the experimental findings. Computer simulations were done using the proposed mechanism. 相似文献
874.
875.
V. S. Zhgenti 《International Applied Mechanics》1991,27(5):459-465
Scientific Research Institute of Applied Mathematics, Tbilisi University. Translated from Prikladnaya Mekhanika, Vol. 27, No. 5, pp. 37–44, May, 1991. 相似文献
876.
Phase transitions in the systems cyanoethyl cellulose-DMF, cyanoethyl cellulose-DMAA, and cyanoethyl cellulose-(trifluoroacetic acid + methylene chloride) were studied by means of the cloud-point and polarization microscopy techniques, as well as with a photoelectric polarization unit and a modified plasticorder. It was shown that the LC phase appears at higher concentrations and lower temperatures as the polarity of solvent molecules increases. The shear deformation of cyanoethyl cellulose solutions in DMF and DMAA results in the expansion of the temperature-concentration region of existence of the LC phase. The effect of shear field on phase transitions in cyanoethyl cellulose solutions is nonmonotonic in character. 相似文献
877.
A textured pellet of YBa2Cu3O7?δ was prepared and doped with57Co. After subsequent heat treatments the cobalt was found at the two copper sites. The degree of alignment of individual crystallites (about 40%) was determined from the Mössbauer spectrum by using a method developed by the authors [1]. The same method showed that the most characteristic doublet component of such spectra (δ=0.08 mm/s; |ΔE Q|=1.96 mm/s) is associated with a site, most probably a Cu(1) site, where the EFG is parallel to thec direction of the unit cell. The sign ofV zz turned out to be negative, giving a negative quadrupole splitting (ΔE Q=?1.96 mm/s) for that component. 相似文献
878.
Abdulaziz S. Alidi 《Applied Mathematical Modelling》1992,16(12):645-651
The improper handling and disposal of hazardous wastes cause threats to human health and the environment. One reason for the improper handling and disposal of these wastes is that not much consideration is usually given to the logistical aspects of hazardous waste systems. In this paper an integer goal programming model is developed that takes into consideration the multiple goals and needs of many groups involved in managing and planning hazardous waste systems. The model can easily be implemented and can be used to address many of the issues related to facility location, recycling, treatment, and disposal of hazardous wastes. 相似文献
879.
880.
NIKITIN S.; PR?TZEL-WOLTERS D. 《IMA Journal of Mathematical Control and Information》1991,8(4):431-439
In 1983 Morse proved, for unknown scalar one-dimensional linearsystems, the nonexistence of rational or polynomial universalstabilizers (UAS). In 1983, Nussbaum gave an example of an analyticUAS. In our paper, it is shown that there exist time-invariantpolynomial UAS's with multidimensional gain adaptation. Thedesign procedure is developed for linear, minimum-phase systemsof relative degree one. Convergence of the closed-loop systemis proved. Some numerical simulations are provided. 相似文献