全文获取类型
收费全文 | 544篇 |
免费 | 10篇 |
国内免费 | 112篇 |
专业分类
化学 | 379篇 |
力学 | 17篇 |
综合类 | 4篇 |
数学 | 201篇 |
物理学 | 65篇 |
出版年
2023年 | 16篇 |
2022年 | 5篇 |
2021年 | 13篇 |
2020年 | 12篇 |
2019年 | 8篇 |
2018年 | 6篇 |
2017年 | 15篇 |
2016年 | 19篇 |
2015年 | 17篇 |
2014年 | 19篇 |
2013年 | 50篇 |
2012年 | 35篇 |
2011年 | 33篇 |
2010年 | 39篇 |
2009年 | 28篇 |
2008年 | 34篇 |
2007年 | 37篇 |
2006年 | 31篇 |
2005年 | 36篇 |
2004年 | 26篇 |
2003年 | 28篇 |
2002年 | 17篇 |
2001年 | 17篇 |
2000年 | 10篇 |
1999年 | 19篇 |
1998年 | 7篇 |
1997年 | 6篇 |
1996年 | 13篇 |
1995年 | 16篇 |
1994年 | 10篇 |
1993年 | 3篇 |
1992年 | 4篇 |
1991年 | 8篇 |
1990年 | 4篇 |
1989年 | 3篇 |
1988年 | 6篇 |
1987年 | 1篇 |
1986年 | 1篇 |
1985年 | 3篇 |
1984年 | 1篇 |
1982年 | 3篇 |
1981年 | 2篇 |
1980年 | 2篇 |
1978年 | 2篇 |
1975年 | 1篇 |
排序方式: 共有666条查询结果,搜索用时 31 毫秒
51.
以双硫酯为链转移剂的活性自由基聚合 总被引:6,自引:0,他引:6
合成并研究了两种双硫酯链转移剂的纯化方法 ,进行了多种单体以双硫酯为链转移剂的活性自由基聚合及嵌段共聚 .发现以PhC(S)SC(CH3) 2 Ph为链转移剂的效果比PhC(S)SCH(CH3)Ph好 ,聚合产物的多分散性系数较小 .引发剂与链转移剂的摩尔数比为 1∶3 5~ 1∶4 2时 ,得到多分散性系数小 ,实测分子量与理论分子量相近的聚合产物 .聚合物的分子量随时间和转化率的增加而增加 ,加入第二单体形成嵌段共聚物 ,具有活性聚合特征 .聚甲基丙烯酸酯大分子引发剂引发丙烯酸酯单体聚合时 ,聚合速度最快 . 相似文献
52.
Abstract. A novel germanate compound, |[Ni(dien)2]3(H2O)3|[Ge7O13F5]2(designated JU‐85, dien = diethylenetriamine), was solvothermally synthesized. The structure of JU‐85 was determined by single‐crystal X‐ray diffraction and further characterized by powder X‐ray diffraction, inductively coupled plasma, infrared spectroscopy, elemental analysis, and thermogravimetric analysis. JU‐85 has dissymmetric chains constructed from diagonally linked Ge7 building units and various Ni(dien)22+ complexes formed in situ during the synthesis. Compared with its structural analogue, FJ‐6, JU‐85 contains less complex cations and different host‐guest assembly. Besides the diagonal linkage in JU‐85, other dissymmetric linkages of Ge7 building units were enumerated, which could be used as the stereogenic centers for the design of novel chiral germanate compounds. 相似文献
53.
Two soluble isomerized polyimides(PIs)synthesized from 2,2′-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride(6FDA)with either 2,2′-dimethylbenzidine(2,2′-DMB)or 3,3′-DMB were investigated by means... 相似文献
54.
Three octacyanometallate‐based hetero‐bimetallic complexes, [Ln(H2O)4(CH3CN)2][M(CN)8] · CH3CN [Ln = La, M = Mo( 1 ), W( 2 ); Ln = Ce, M = W( 3 )], were synthesized and characterized structurally. Single‐crystal X‐ray analysis reveals that 1 – 3 are isomorphous and consist of infinite one‐dimensional (1D) 3,3 rope‐ladder chains, in which the 12‐membered puckered square Ln2M2(CN)4 is the basic building unit. The 1D chains are further linked through interchain hydrogen bonds, resulting in a three‐dimensional (3D) supramolecular network. 相似文献
55.
Gábor Hetyei 《Graphs and Combinatorics》2002,18(3):533-564
56.
Summary This paper is concerned with the study of a newM/G/1 retrial queueing system in which the delays between retrials are exponentially distributed random variables with linear intensityg(n)=α+nμ, when there aren≥1 customers in the retrial group. This new retrial discipline will be calledlinear control policy. We carry out an extensive analysis of the model, including existence of stationary regime, stationary distribution of the
embedded Markov chain at epochs of service completions, joint distribution of the orbit size and the server state in steady
state and busy period. The results agree with known results for special cases. 相似文献
57.
In this paper, we derive a stochastic model for the HIV epidemic in homosexual populations involving age and race. To account for effects of different mixing patterns, a low risk selection rule is introduced. The model is then formulated in terms of chain multinomial distributions by means of which the means are derived. Some simulation studies by computer indicate clearly that age and race have a significant impact on the HIV epidemic. 相似文献
58.
** Email: giorgio.vittadini{at}unimib.it*** Email: simona.minotti{at}unicatt.it In this paper we propose a methodology for measuring the relativeeffectiveness of healthcare services (i.e. the effectof hospital care on patients) under general conditions, in which:) a healthcare outcome underlies qualitative and quantitativeobservable indicators; ß) we are interested in studyingthe simultaneous dependency of multiple outcomes on covariates(where the outcomes can also be correlated to each other); )the relative effectiveness is adjusted for hospital-specificcovariates; ) we hypothesise a general distribution for randomdisturbances and the random parameters of relative effectiveness.For this topic, a generalisation of the SURE (seemingly unrelatedregression equations) multilevel model is proposed. The solutionsare obtained by means of Bayesian inference methods. Since thereis currently no software available to estimate this model, anSAS procedure based on Markov Chain Monte Carlo methods hasbeen developed by the authors, in line with Goldstein &Spiegelhalter (1996, J. R. Stat. Soc. Ser. A, 159, 385443),Spiegelhalter et al. (1996, Bayesian Using Gibbs Sampling Manual.Cambridge: MRC Biostatistic Unit, Institute of Public Health)and Albert & Chib (1997, J. Am. Stat. Assoc., 92, 916925).In addition, a new theoretical result regarding the joint posteriordistribution for the parameters is provided. The model proposedhas been implemented for an effectiveness study of a selectionof Lombard hospitals. 相似文献
59.
The objective is to derive the probability distribution of the frequency of occurrence of a subsequence within a nucleotide
sequence under the hypothesis that the four nucleotides occur at random and with equal probability.
We also consider the Compound Poisson approximation for the same distribution. The exact probability distribution can be obtained
by the finite Markov chain imbedding technique introduced by Fu and Koutras (1994), however we can manage the case as well
if the probabilities are not all equal. The compound Poisson approximation by Stein-Chen's method can be used to develop an
approximate probability distribution with proper setting of the definition of the sets of dependence. Such structure gives
a bound on the total variation distance, which tends to get relatively larger as the frequency goes up.
AMS 2000 Subject Classification: Primary: 60E05; Secondary: 60J10 相似文献
60.
The germanate compound Cu2Sc2Ge4O13 has been synthesized by solid-state ceramic sintering techniques between 1173 and 1423 K. The structure was solved from single-crystal data by Patterson methods. The title compound is monoclinic, a=12.336(2) Å, b=8.7034(9) Å, c=4.8883(8) Å, β=95.74(2), space group P21/m, Z=4. The compound is isotypic with Cu2Fe2Ge4O13, described very recently. The structure consists of crankshaft-like chains of edge-sharing ScO6 octahedra running parallel to the crystallographic b-axis. These chains are linked laterally by [Cu2O6]8− dimers forming a sheet of metal-oxygen-polyhedra within the a-b plane. These sheets are separated along the c-axis by [Ge4O13]10− units. Cooling to 100 K does not alter the crystallographic symmetry of Cu2Sc2Ge4O13. While the b, c lattice parameter and the unit cell volume show a positive linear thermal expansion (α=6.4(2)×10−6, 5.0(2)×10−6 and 8.3(2)×10−6 K−1 respectively), the a lattice parameter exhibits a negative thermal expansion (α=−3.0(2)×10−6 K−1) for the complete T-range investigated. This negative thermal expansion of a is mainly due to the increase of the Cu-Cu interatomic distance, which is along the a-axis. Average bond lengths remain almost constant between 100 and 298 K, whereas individual ones partly show both significant shortages and lengthening. 相似文献