全文获取类型
收费全文 | 338005篇 |
免费 | 2732篇 |
国内免费 | 1001篇 |
专业分类
化学 | 184588篇 |
晶体学 | 5289篇 |
力学 | 13882篇 |
综合类 | 9篇 |
数学 | 35657篇 |
物理学 | 102313篇 |
出版年
2021年 | 2599篇 |
2020年 | 2924篇 |
2019年 | 3293篇 |
2018年 | 4427篇 |
2017年 | 4387篇 |
2016年 | 6292篇 |
2015年 | 3691篇 |
2014年 | 5755篇 |
2013年 | 14403篇 |
2012年 | 10925篇 |
2011年 | 13327篇 |
2010年 | 9516篇 |
2009年 | 9446篇 |
2008年 | 12797篇 |
2007年 | 12745篇 |
2006年 | 12078篇 |
2005年 | 10981篇 |
2004年 | 9813篇 |
2003年 | 9021篇 |
2002年 | 8745篇 |
2001年 | 10570篇 |
2000年 | 8013篇 |
1999年 | 6098篇 |
1998年 | 4813篇 |
1997年 | 4916篇 |
1996年 | 4579篇 |
1995年 | 3961篇 |
1994年 | 4053篇 |
1993年 | 3875篇 |
1992年 | 4494篇 |
1991年 | 4599篇 |
1990年 | 4408篇 |
1989年 | 4304篇 |
1988年 | 4224篇 |
1987年 | 4237篇 |
1986年 | 3967篇 |
1985年 | 5090篇 |
1984年 | 5252篇 |
1983年 | 4205篇 |
1982年 | 4439篇 |
1981年 | 4170篇 |
1980年 | 3960篇 |
1979年 | 4410篇 |
1978年 | 4620篇 |
1977年 | 4571篇 |
1976年 | 4529篇 |
1975年 | 4272篇 |
1974年 | 4248篇 |
1973年 | 4381篇 |
1972年 | 2946篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
81.
The known plant growth regulator, Al'den [1-(3,7-dimethyloctyl)-1-allylpiperidinium bromide], and also 1-(3,7-dimethyloctyl)-1-methylpiperidinium iodide were synthesized by treatment with allyl bromide and methyl iodide, respectively, of exhaustively hydrogenated 1-nerylpiperidine which was obtained by anionic telomerization of isoprene with piperidine. Various quaternary ammonium salts having a terpene sub- stituent with unnatural dimethyloctane skeleton, which effectively stimulate florification of Aster Chinensis L., were prepared by the action of alkyl halides on telomerization products derived from isoprene and piperidine or morpholine in the presence of palladium complexes. 相似文献
82.
L. Villa P. D' Agati C. Mansueto C. Pellerito M. Scopelliti T. Fiore L. Nagy L. Pellerito 《应用有机金属化学》2003,17(2):106-112
Ascidia malaca gametes before fertilization incubated in 10?5 or 10?7 M solutions of tributyltin(IV) chloride, TBTCl, for 3 h appear highly damaged under transmission electron microscopy observation. Also, the fertilization process is affected by the compound: the damaged spermatozoa are present in the vitelline coat and the egg does not cleave. An increase of microbodies, structurally similar to peroxisomes, have been detected in the egg peripheral cytoplasm, probably in relation to their role in alleviating damage to some cellular components. The results have shown that the reproduction of ascidians under unfavourable environmental conditions is prevented. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
83.
84.
H. Okumura T. Takahagi N. Nagai S. Shingubara 《Journal of Polymer Science.Polymer Physics》2003,41(17):2071-2078
The structural change in the depth direction of a polyimide (UPILEX‐S) film treated in alkaline solution, which was a representative surface treatment used to form a seed layer for plating and to improve the adhesive strength, was analyzed by means of micro Fourier transform infrared attenuated total reflection (FTIR‐ATR) line analysis with gradient shaving preparation. The polyimide film was treated with KOH. The imide ring opened through the alkaline treatment, and the amide structure and carboxylic acid salt were formed. The attainment depth of this structural change was almost proportional to the treatment time, and it reached about 8 μm after a 30‐min treatment. The degree of structural change through the alkaline treatment was almost constant after it reached a considerably degraded stage, and the chemically changed region penetrated into the inner part of the film from the surface. An intermediate layer before the final degraded stage appeared in the treated layer, and its thickness increased with the treatment time. The region that was changed chemically by the alkaline treatment progressed to the inner part simultaneously and continuously as the treatment time increased. The combined use of gradient shaving preparation and micro FTIR‐ATR line analysis was found to be extremely effective for the depth profiling of organic materials. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2071–2078, 2003 相似文献
85.
C. Vaca‐Garcia G. Gozzelino W. G. Glasser M. E. Borredon 《Journal of Polymer Science.Polymer Physics》2003,41(3):281-288
The main transitions of cellulose fatty esters with different degrees of substitution (DSs) were investigated with dynamic mechanical thermal analysis. Two distinct main relaxations were observed in partially substituted cellulose esters (PSCEs). They were attributed to the glass‐transition temperature and to the chain local motion of the aliphatic substituents. The temperatures of both transitions decreased when DS or the number of carbon atoms (n) of the acyl substituent increased. Conversely, all the transitions of fully substituted cellulose esters occurred within a narrow temperature range, and they did not vary significantly with n. This phenomenon was explained by the formation of a crystalline phase of the fatty substituents. The presence of few residual OH groups in PSCEs was responsible for a large increase in the storage bending modulus, and it eliminated the effect of n on damping. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 281–288, 2003 相似文献
86.
87.
88.
Fix integers m, n such that 1 ≤ m ≤ n ? 3. Let X ? Pn be an integral non-degenerate m-dimensional variety. Assume either char(K) = 0 or char(K) > deg(X). Here we prove that all general 0-dimensional sections of X containing a tangent vector to a smooth point of X are protectively equivalent if and only if n ? m + 1 ≤ deg(X) ≤ n ? m + 2. 相似文献
89.
The process of focusing of an inhomogeneously polarized beam passed through a system consisting of a uniaxial crystal and a polarization filter is considered. It is shown that the action of this system is equivalent to the action of a phase transparency with a complex relief of the refractive index. In particular, if the angle between the axes of the quarter-wave plate and the polarizer is equal to 45°, this transparency has a helicoidal relief. Simultaneously, two singular beams with different curvatures of their wavefronts arise in the transparency. The action of the lens consists in formation of a combined singular beam having three focal waists. The boundary wave arising upon focusing is able not only to form a new spectrum of toroidal vortices, but also to straighten the focusing region, forming a fairly extended range with a weak divergence. 相似文献
90.
A semiclassical theory of electromagnetically induced transparency in a Bose-Einstein condensate is considered. A nonlinear Schrödinger equation that describes the interaction of the Bose-Einstein condensate with an electromagnetic field is derived. An analysis of the obtained solutions to this equation demonstrates a decrease in the group velocity of the light pulse. 相似文献