全文获取类型
收费全文 | 4274篇 |
免费 | 273篇 |
国内免费 | 154篇 |
专业分类
化学 | 898篇 |
晶体学 | 17篇 |
力学 | 503篇 |
综合类 | 45篇 |
数学 | 2681篇 |
物理学 | 557篇 |
出版年
2024年 | 12篇 |
2023年 | 54篇 |
2022年 | 63篇 |
2021年 | 74篇 |
2020年 | 100篇 |
2019年 | 118篇 |
2018年 | 112篇 |
2017年 | 112篇 |
2016年 | 122篇 |
2015年 | 113篇 |
2014年 | 165篇 |
2013年 | 434篇 |
2012年 | 201篇 |
2011年 | 204篇 |
2010年 | 156篇 |
2009年 | 255篇 |
2008年 | 269篇 |
2007年 | 264篇 |
2006年 | 213篇 |
2005年 | 187篇 |
2004年 | 179篇 |
2003年 | 124篇 |
2002年 | 120篇 |
2001年 | 99篇 |
2000年 | 107篇 |
1999年 | 94篇 |
1998年 | 90篇 |
1997年 | 91篇 |
1996年 | 67篇 |
1995年 | 46篇 |
1994年 | 56篇 |
1993年 | 36篇 |
1992年 | 47篇 |
1991年 | 27篇 |
1990年 | 27篇 |
1989年 | 29篇 |
1988年 | 16篇 |
1987年 | 24篇 |
1986年 | 23篇 |
1985年 | 12篇 |
1984年 | 22篇 |
1983年 | 15篇 |
1982年 | 14篇 |
1981年 | 18篇 |
1980年 | 18篇 |
1979年 | 19篇 |
1978年 | 15篇 |
1977年 | 12篇 |
1976年 | 14篇 |
1974年 | 5篇 |
排序方式: 共有4701条查询结果,搜索用时 15 毫秒
101.
In this paper, we aim to investigate the difference equation
\begin{align*}
\Delta^{2}y(t-1)+|y(t)|=0, \ \ \ \ \ t\in[1,T]_{\mathbb{Z}}
\end{align*}
with different boundary conditions $y(0)=0$ or $\Delta y(0)=0$ and $y(T+1)=B$ or $\Delta y(T)=B$,\ where\ $T\geq 1$ is an integer and $B\in\mathbb{R}$. We will show that how the values of $T$ and $B$ influence the existence and uniqueness of the solutions to the about problem. In details, for the different problems, the $TB$-plane explicitly divided into different parts according to the number of the solutions to the above problems. These parts of $TB$-plane for the value of $T$ and $B$ guarantee the uniqueness, the existence and the nonexistence of solutions respectively. 相似文献
102.
Necessary conditions for a given pointx
0 to be a locally weak solution to the Pareto minimization problem of a vector-valued functionF=(f
1,...,f
m
),F:XR
m,XR
m, are presented. As noted in Ref. 1, the classical necessary condition-conv {Df
1(x
0)|i=1,...,m}T
*(X, x
0) need not hold when the contingent coneT is used. We have proven, however, that a properly adjusted approximate version of this classical condition always holds. Strangely enough, the approximation form>2 must be weaker than form=2.The authors would like to thank the anonymous referee for the suggestions which led to an improved presentation of the paper. 相似文献
103.
L. Montrucchio 《Journal of Optimization Theory and Applications》1994,80(3):385-406
We study the relationship between the dynamical complexity of optimal paths and the discount factor in general infinite-horizon discrete-time concave problems. Given a dynamic systemx
t+1=h(x
t
), defined on the state space, we find two discount factors 0 < * ** < 1 having the following properties. For any fixed discount factor 0 < < *, the dynamic system is the solution to some concave problem. For any discount factor ** < < 1, the dynamic system is not the solution to any strongly concave problem. We prove that the upper bound ** is a decreasing function of the topological entropy of the dynamic system. Different upper bounds are also discussed.This research was partially supported by MURST, National Group on Nonlinear dynamics in Economics and Social Sciences. The author would like to thank two anonymous referees for helpful comments and suggestions. 相似文献
104.
Nitro aromatic compound can be obtained in high yields via nitration of aromatic compound with wet carbon-based solid acid and NaNO_3 under solvent free oxidation at room temperature. 相似文献
105.
A successful application of microwave irradiation, in which phthalocyanines were synthesized under solventless conditions from 1,2-phthalonitrile or phthalic anhydride and urea in the presence of metal templates is described. It was found that in comparison with conventional heating, the microwave process is a very useful alternative for cyclotetramerization processes because of reduction of the reaction time, better yield, and easy-to-perform procedure. 相似文献
106.
Singlet oxygen was generated by reaction of sodium hypochlorite and hydrogen peroxide in a micro-channel. The two reagent solutions were delivered into the micro-channel by a syringe pump, providing a laminar flow liquid-liquid interface. The chemiluminescence from the singlet oxygen was emitted in the collapse of the interface due to molecular diffusion under laminar flow conditions. The chemiluminescence intensity was observed continuously and stably for each combination of reagents fed into the micro-channel; while, in the normal batch-type reactor the chemiluminescence peaks from singlet oxygen were observed within ca. 5 s. The features of the chemiluminescence emitted under laminar flow conditions were examined by changing the concentrations of sodium hypochlorite and hydrogen peroxide; the concentrations of 2.5 mM sodium hypochlorite and 7.5 mM hydrogen peroxide provided highest chemiluminescence intensities without bubble formation. Also, the effects of beverages, such as green tea, coffee, white wine, red wine, and sake (rice wine), on the chemiluminescence intensity as well as the concentrations of sodium hypochlorite and hydrogen peroxide were examined. The chemiluminescence intensities observed with addition of the beverages to the reagents decreased in the following orders; green tea > coffee > red wine > rice wine > white wine (being added to sodium hypochlorite); coffee > white wine > green tea > red wine > rice wine (being added to hydrogen peroxide). It was found that coffee decreased the chemiluminescence intensity (ca. 33% chemiluminescence decrease) without altering the concentrations of sodium hypochlorite or hydrogen peroxide. The cause of the decrease in chemiluminescence with coffee is discussed. 相似文献
107.
Functional-based screening of crude β-galactosidase activities from 42 yeast strains resulted in the selection of a single
enzyme of potential interest as a digestive supplement. β-Galactosidase produced by Kluyveromyces marxianus DSM5418 was purified to homogeneity by a combination of gel filtration, ion-exchange, and hydroxylapatite chromatographies.
The denatured (123 kDa) and native molecular masses (251 kDa) suggest that the enzyme is a homodimer. The optimum pH and temperature
of the purified enzyme were 6.8 and 37°C, respectively. The unpurified β-galactosidase in particular displayed a high level
of stability when exposed to simulated intestinal conditions in vitro for 4 h. Matrix-assisted laser desorption ionization
mass sectrometry analysis revealed that the enzyme's trypsin-generated peptide mass fingerprint shares several peptide fragment
hits with β-galactosidases from Kluyveromyces lactis. This confirms the enzyme's identity and indicates that significant sequence homology exists between these enzymes. 相似文献
108.
沉淀二氧化硅消光剂的制备研究 总被引:1,自引:0,他引:1
涂料中的消光剂是一类能够改变涂膜表面光学性能的助剂。这类助剂能够在涂膜表面产生预期微粗糙度,使涂膜的光泽显著降低,给人一种古朴典雅的感觉。近几年来,我国的涂料工业方兴未艾,涂料总产量也跃居世界的前列。在这期间高光泽的亮光涂料以其色泽鲜艳、明亮等优点深受消费者的喜爱,并在相当长的时间内一统着涂料市场[1]。但是,随着人们生活水平的不断提高,一方面消费者感觉到了高光泽的亮光涂料成膜后反光比较严重,对人的眼睛有害;另一方面消费者的审美观念越来越倾向于休闲、时尚和个性化。这些因素就导致了人们对具有柔和外观的低光泽涂… 相似文献
109.
110.
用十二烷基二甲基苄基氯化铵 (1227)、十六烷基三甲基溴化铵 (1631)、十八烷基三甲基溴化铵 (1831) 3种表面活性剂改性天然累托石 (REC) 粘土,分别得到3种有机改性累托石:1227-REC、1631-REC和1831-REC。用有机改性累托石作吸附剂,对水溶液中的苯酚进行吸附研究,考察了pH值、吸附时间、吸附剂用量、温度等因素对吸附效果的影响。结果发现,在本文研究条件下,3种吸附剂对苯酚达到较理想吸附效果的合理条件是:pH值分别为6、8、8;吸附平衡时间分别需要90、20、40min;吸附剂用量均为20g/L;常温。在确定的合理条件下,去除率分别能达到66.2%、99.0%、99.8%。 相似文献