全文获取类型
收费全文 | 147099篇 |
免费 | 1461篇 |
国内免费 | 395篇 |
专业分类
化学 | 80391篇 |
晶体学 | 2390篇 |
力学 | 6063篇 |
综合类 | 3篇 |
数学 | 13584篇 |
物理学 | 46524篇 |
出版年
2021年 | 1035篇 |
2020年 | 1182篇 |
2019年 | 1253篇 |
2018年 | 1639篇 |
2017年 | 1690篇 |
2016年 | 2542篇 |
2015年 | 1562篇 |
2014年 | 2350篇 |
2013年 | 6050篇 |
2012年 | 4859篇 |
2011年 | 5952篇 |
2010年 | 4190篇 |
2009年 | 4217篇 |
2008年 | 5742篇 |
2007年 | 5509篇 |
2006年 | 5398篇 |
2005年 | 4959篇 |
2004年 | 4458篇 |
2003年 | 4134篇 |
2002年 | 4015篇 |
2001年 | 5544篇 |
2000年 | 3976篇 |
1999年 | 3074篇 |
1998年 | 2109篇 |
1997年 | 2209篇 |
1996年 | 2072篇 |
1995年 | 1918篇 |
1994年 | 1920篇 |
1993年 | 1728篇 |
1992年 | 2213篇 |
1991年 | 2305篇 |
1990年 | 2135篇 |
1989年 | 2104篇 |
1988年 | 2084篇 |
1987年 | 2087篇 |
1986年 | 1917篇 |
1985年 | 2378篇 |
1984年 | 2398篇 |
1983年 | 1910篇 |
1982年 | 1918篇 |
1981年 | 1760篇 |
1980年 | 1744篇 |
1979年 | 2029篇 |
1978年 | 2102篇 |
1977年 | 2047篇 |
1976年 | 1976篇 |
1975年 | 1859篇 |
1974年 | 1844篇 |
1973年 | 1877篇 |
1972年 | 1235篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
951.
952.
G. Sreenivasulu Reddy J. Hanumanthachari T. Vasanthi 《Southeast Asian Bulletin of Mathematics》2003,26(2):331-335
This paper is mainly dealt with the structure of totally ordered rings (t.o. rings) whose positive cones are finitely generated as multiplicative right ideals.AMS Subject Classification (1991): 16W80, 13J25 相似文献
953.
Abhimanyu O. Patil Stephen Zushma Robert T. Stibrany Steven P. Rucker Louise M. Wheeler 《Journal of polymer science. Part A, Polymer chemistry》2003,41(13):2095-2106
Novel nickel(II) bisbenzimidazole complexes were prepared via a three‐step synthetic procedure consisting of aniline/diacid condensation, ligand N‐alkylation, and metal complexation. The complexes were characterized by X‐ray crystallography and found to possess a pseudotetrahedral geometry. Upon activation with methylaluminoxane, these nickel bisbenzimidazoles did not polymerize simple olefins (e.g., ethylene, propylene, and 1‐butene) but were found to carry out the rapid and efficient polymerization of norbornene. The polynorbornene products were characterized by gel permeation chromatography/light scattering, 13C NMR, and IR, and their Mark–Houwink and dn/dc parameters were determined. The molecular weights of the polynorbornenes were very high (weight‐average molecular weight = 587,000–797,000 g/mol). 13C NMR suggested that the polymerization occurred via vinyl addition (i.e., a 2,3‐linked polymer); no ring‐opened product was observed. Thermogravimetric analysis indicated that the polynorbornenes were stable up to 400 °C under nitrogen. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 2095–2106, 2003 相似文献
954.
955.
D. V. Isakov T. R. Volk L. I. Ivleva K. Betzler C. David A. Tunyagi M. Wöhlecke 《JETP Letters》2004,80(4):258-262
Measurements of 90°-scattering of weak laser light are used to investigate pulsed domain switching in ferroelectrics. The studies were performed on strontium-barium niobate (SBN) single crystals. A good agreement of the switching parameters estimated from the optical measurements with those obtained by means of conventional electrical methods proves the validity of the optical method for switching studies. Due to the limited scattering volume in all three spatial dimensions, the method facilitates local probing of the switching within the crystal bulk. In particular, local specialities of the domain density can be detected. Furthermore, the excellent time resolution inherent in optical probing techniques allows for a comprehensive study of the dynamics. 相似文献
956.
An exfoliated poly(4,4′-oxybis(benzene)disulfide) (POBDS)/vermiculite (VMT) nanocomposite was prepared by using cyclo(4,4′-oxybis(benzene)disulfide) (COBDS) oligomers and cetyltrimethyl ammonium bromide exchanged VMT. The POBDS/VMT nanocomposites were fabricated in two steps. First, the COBDS oligomers were used to swell and exfoliate organo VMT to afford COBDS-VMT nanocomposite precursor. Subsequently, the exfoliated POBDS-VMT nanocomposite can be made via in situ and instant melt ring-opening polymerization of the COBDS-VMT nanocomposite precursor. High molecular weight POBDS polymer can be formed in a few minutes. The nano scale dispersion of VMT layers within POBDS polymer was confirmed by both the X-ray diffraction patterns and TEM examinations. This methodology provides a potential approach to synthesize high performance polymer nanocomposite. 相似文献
957.
V. T. Dolgopolov E. V. Deviatov A. A. Shashkin U. Wieser U. Kunze G. Abstreiter K. Brunner 《Superlattices and Microstructures》2003,33(5-6):271
The small, about 30% magnetoresistance at the onset of full spin polarization in the 2D electron system in a modulation-doped Si/SiGe quantum well gives evidence that it is the remote doping that determines the transport scattering time. Measurements of the mobility in this strongly-interacting electron system with remote-doping scattering allow us to arrive at a conclusion that the Hubbard form underestimates the local field corrections by about a factor of 2. 相似文献
958.
T W Sloan 《The Journal of the Operational Research Society》2004,55(6):647-656
In many environments, product yield is heavily influenced by equipment condition. Despite this fact, previous research has either focused on the issue of maintenance, ignoring the effect of equipment condition on yield, or has focused on the issue of production, omitting the possibility of actively changing the machine state. We formulate a Markov decision process model of a single-stage production system in which demand is random. The product yield has a binomial distribution that depends on the equipment condition, which deteriorates over time. The objective is to choose simultaneously the equipment maintenance schedule as well as the quantity to produce in a way that minimizes the sum of expected production, backorder, and holding costs. After proving some results about the structural properties of the optimal policy, numerical problems are used to compare this method to the typical approach of solving the maintenance and production problems sequentially. The results show that the simultaneous solution provides substantial gains over the sequential approach. In the cases studied, the proposed method resulted in an average cost savings of approximately 18%. 相似文献
959.
960.