全文获取类型
收费全文 | 99286篇 |
免费 | 7579篇 |
国内免费 | 12271篇 |
专业分类
化学 | 56856篇 |
晶体学 | 710篇 |
力学 | 6526篇 |
综合类 | 1520篇 |
数学 | 26163篇 |
物理学 | 27361篇 |
出版年
2023年 | 815篇 |
2022年 | 1275篇 |
2021年 | 2078篇 |
2020年 | 2155篇 |
2019年 | 2432篇 |
2018年 | 2093篇 |
2017年 | 2569篇 |
2016年 | 2948篇 |
2015年 | 2735篇 |
2014年 | 3706篇 |
2013年 | 7196篇 |
2012年 | 4869篇 |
2011年 | 4840篇 |
2010年 | 4234篇 |
2009年 | 5646篇 |
2008年 | 6231篇 |
2007年 | 6577篇 |
2006年 | 6124篇 |
2005年 | 5412篇 |
2004年 | 4911篇 |
2003年 | 4691篇 |
2002年 | 4380篇 |
2001年 | 3447篇 |
2000年 | 3359篇 |
1999年 | 2965篇 |
1998年 | 2669篇 |
1997年 | 2331篇 |
1996年 | 1956篇 |
1995年 | 1869篇 |
1994年 | 1683篇 |
1993年 | 1440篇 |
1992年 | 1393篇 |
1991年 | 1052篇 |
1990年 | 867篇 |
1989年 | 794篇 |
1988年 | 687篇 |
1987年 | 517篇 |
1986年 | 432篇 |
1985年 | 483篇 |
1984年 | 481篇 |
1983年 | 235篇 |
1982年 | 387篇 |
1981年 | 428篇 |
1980年 | 325篇 |
1979年 | 308篇 |
1978年 | 232篇 |
1977年 | 210篇 |
1976年 | 157篇 |
1974年 | 114篇 |
1973年 | 111篇 |
排序方式: 共有10000条查询结果,搜索用时 869 毫秒
81.
A. Negr n-Mendoza G. Albarran S. Ramos-Bernal 《Radiation Physics and Chemistry》1993,42(4-6):1003-1006
The effect of clay on the radiolysis of malonic acid solutions was studied. The results showed an enhanced of decarboxylation rather than condensation reactions. 相似文献
82.
We discuss an error estimation procedure for the global error of collocation schemes applied to solve singular boundary value problems with a singularity of the first kind. This a posteriori estimate of the global error was proposed by Stetter in 1978 and is based on the idea of Defect Correction, originally due to Zadunaisky. Here, we present a new, carefully designed modification of this error estimate which not only results in less computational work but also appears to perform satisfactorily for singular problems. We give a full analytical justification for the asymptotical correctness of the error estimate when it is applied to a general nonlinear regular problem. For the singular case, we are presently only able to provide computational evidence for the full convergence order, the related analysis is still work in progress. This global estimate is the basis for a grid selection routine in which the grid is modified with the aim to equidistribute the global error. This procedure yields meshes suitable for an efficient numerical solution. Most importantly, we observe that the grid is refined in a way reflecting only the behavior of the solution and remains unaffected by the unsmooth direction field close to the singular point. 相似文献
83.
Lu Chuanrong 《数学年刊B辑(英文版)》1997,18(1):71-78
Let $\{\xi_{\bold t}, {\bold t}
\in {\bold Z}^d\}$ be a nonuniform $\varphi$-mixing strictly stationary
real random field with
$E\xi_{\bold 0}=0, E|\xi_{\bold 0}|^{2+\delta}<\infty$ for some
$0<\delta<1$. A sufficient
condition is given for the sequence of partial sum set-indexed process
$\{Z_n(A),\ A\in \Cal A\}$ to converge to Brownian motion. By a direct
calculation, the author shows
that the result holds for a more general class of set index ${\Cal A}$, where
${\Cal A}$ is assumed only to have the metric entropy exponent $r, 0相似文献
84.
This paper deals mainly with generalizations of results in finitary combinatorics to infinite ordinals. It is well-known that for finite ordinals ∑bT<αβ is the number of 2-element subsets of an α-element set. It is shown here that for any well-ordered set of arbitrary infinite order type α, ∑bT<αβ is the ordinal of the set M of 2-element subsets, where M is ordered in some natural way. The result is then extended to evaluating the ordinal of the set of all n-element subsets for each natural number n ≥ 2. Moreover, series ∑β<αf(β) are investigated and evaluated, where α is a limit ordinal and the function f belongs to a certain class of functions containing polynomials with natural number coefficients. The tools developed for this result can be extended to cover all infinite α, but the case of finite α appears to be quite problematic. 相似文献
85.
Levoglucosenone reacts with α-aminoazoles to yield azolo[1,5-a]pyrimidine systems fused with a carbohydrate fragment. The reaction oocurs much more smoothly than in the case of other α,β-unsaturated
ketones. The reactions of levoglucosenone with β-dicarbonyl compounds (dimedone, barbituric acid) in the presence of a base
results in the pyran ring closure, which has never been observed earlier in reactions of β-dicarbonyl compounds with α,β-unsaturated
ketones under the conditions of basic catalysis. The structures of products were established by IR and NMR spectroscopy.
For Part 3, see Ref. 1.
Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 553–558, March, 1997. 相似文献
86.
M. M. Moustafa 《Journal of Thermal Analysis and Calorimetry》1997,50(3):463-471
The oxime and Schiff's base of N-furoylphenylhydroxylamine form complexes with Co2+, Cu2+, Zn2+ and Fe3+. These complexes were studied by means of the UV — Vis. spectra of nujol mulls, electron spin resonance (ESR) spectra and magnetic susceptibility to determine their stereochemistry. The complexes were characterized via elemental analyses, molar conductivities and thermogravimetric analyses. Organic reagents were used for the gravimetric determination of Co2+, Cu2+ and Fe3+ through the precipitation of their complexes. The compounds were also used for separation of a binary mixture of Cu2+ and Cu2+. Interferences were studied. 相似文献
87.
Separation of 23 dansylated amino acids by micellar electrokinetic chromatography at low temperature
Summary The baseline separation of 23 dansylated amino acids (including 3 didansylated species) is reported. Lowering of the analysis
temperature and fine control of the surfactant concentration were essential in obtaining this result. Calibration graphs with
good linearity were obtained. 相似文献
88.
89.
杂多酸催化合成乙酸乙酯 总被引:14,自引:0,他引:14
回流吸附法制备的负载型12-钨磷酸(HPW/C)是合成乙酸乙酯的高效催化剂。最佳反应条件下乙醇转化率达96%,酯化选择性100%。催化剂的高活性是由于HPW在活性炭上高度分散,使活性位增多,有效酸量增大。HPW/C还具有热稳定性高和不易流失等优点。 相似文献
90.
采用基于第一性原理的密度泛函理论研究了四角晶相二氧化铪(t-HfO2)体相及 其(001)表面的原子几何与电子结构.理论计算结果表明,t-HfO2(001)表面不会 产生重构现象.与体相电子结构相比, t-HfO2(001)表面态密度明显高于体相态 密度.其次,表面原子的态密度更靠近费米能级(EF),价带往低能量处移动,并 有表面态产生.计算结果表明了t-HfO2表面禁带宽度明显低于体相的禁带宽度. t-HfO2(001)的表面态产生以及表面禁带宽度减小是由于Hf原子与O原子的配位 数减少,表面原子周围的环境发生变化而引起的.
关键词:
密度泛函理论
2(001)')" href="#">t-HfO2(001)
表面电子结构 相似文献