全文获取类型
收费全文 | 1251篇 |
免费 | 163篇 |
国内免费 | 127篇 |
专业分类
化学 | 883篇 |
晶体学 | 9篇 |
力学 | 70篇 |
综合类 | 9篇 |
数学 | 185篇 |
物理学 | 385篇 |
出版年
2024年 | 7篇 |
2023年 | 26篇 |
2022年 | 38篇 |
2021年 | 39篇 |
2020年 | 41篇 |
2019年 | 45篇 |
2018年 | 40篇 |
2017年 | 31篇 |
2016年 | 69篇 |
2015年 | 49篇 |
2014年 | 57篇 |
2013年 | 74篇 |
2012年 | 95篇 |
2011年 | 116篇 |
2010年 | 75篇 |
2009年 | 81篇 |
2008年 | 80篇 |
2007年 | 66篇 |
2006年 | 77篇 |
2005年 | 46篇 |
2004年 | 48篇 |
2003年 | 41篇 |
2002年 | 24篇 |
2001年 | 18篇 |
2000年 | 31篇 |
1999年 | 39篇 |
1998年 | 22篇 |
1997年 | 25篇 |
1996年 | 20篇 |
1995年 | 10篇 |
1994年 | 19篇 |
1993年 | 16篇 |
1992年 | 15篇 |
1991年 | 11篇 |
1990年 | 9篇 |
1989年 | 9篇 |
1988年 | 7篇 |
1987年 | 1篇 |
1986年 | 2篇 |
1985年 | 6篇 |
1984年 | 2篇 |
1983年 | 1篇 |
1982年 | 3篇 |
1980年 | 2篇 |
1979年 | 1篇 |
1978年 | 1篇 |
1977年 | 1篇 |
1976年 | 1篇 |
1975年 | 1篇 |
1973年 | 3篇 |
排序方式: 共有1541条查询结果,搜索用时 236 毫秒
981.
Jianping Ou 《Journal of mathematical chemistry》2008,43(1):328-337
The total number of matchings of a graph is defined as its Hosoya index. Conjugated and non-conjugated acyclic graphs that
have maximal Hosoya index and short diameter are characterized in this paper, explicit expressions of the Hosoya indices of
these extremal graphs are also presented. 相似文献
982.
Yuwei Fang Boping Liu Kouichi Hasebe Minoru Terano 《Journal of polymer science. Part A, Polymer chemistry》2005,43(19):4632-4641
Phillips catalyst has been contributing to about 40% of world high‐density polyethylene production because of its ability to give products with unique microstructures like broad molecular weight distribution as well as short and long chain branches. Even after 50 years' effort, some crucial problems concerning the nature of active sites, polymerization, and branching mechanisms are still kept mysterious. In this work, ethylene and 1‐hexene copolymerization with Phillips catalyst prereduced by CO was carried out in the presence of triethyl aluminum (TEA) cocatalyst. The microstructures of polymers were investigated by 13C NMR and gel permeation chromatography (GPC) methods. A hybrid‐type kinetics was found for both homo‐ and copolymerization kinetics, which indicated that there existed two types of active sites namely site A and site B. Site A with instant activation, high activity, and fast decay was transformed from a metathesis site, namely Cr(II) site, coordinated with CO or CO2 through desorption of CO or CO2 by TEA, which contributed to the formation of short chain branches, especially methyl branches. Site B with slow activation, low activity, and slow decay was generated from reduction of residual chromate (VI) by TEA. Both 1‐hexene and TEA can decrease the molecular weight of polyethylene as well as enhance short chain branching. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 4632–4641, 2005 相似文献
983.
班主任工作效果的马尔可夫链评估法 总被引:2,自引:0,他引:2
本文研究了班主任工作效果的评估标准及马尔可夫链评估法,并通过实例分析指出了该方法的理论依据及其特点。 相似文献
984.
在实验室中.采用溶胶一凝胶法,在比较低的温度下。合成出Li Gd_(0.958)SiO_4:0.035Eu~(3+),0.007Bi~(3+)发光体。利用红外光谱、X射线粉末衍射谱、热重及差热分析、激发光谱和发光光谱,研究了由凝胶至发光晶体的转变过程,讨论了Eu~(3+)、Bi~(3+)在稀土硅酸盐中的发光行为。 相似文献
985.
986.
以碳氮纳米管(NCNTs)为载体, 利用氮的锚定作用, 采用三种不同的制备方法(等体积浸渍法、胶体法和沉积沉淀法)方便地构建了负载铁物种的Fe/NCNTs催化剂. 系统考察了制备方法对催化剂的结构及费托反应性能的影响. 结果表明, 制备方法影响铁纳米粒子的粒度和分布、催化剂的还原和碳化行为, 使催化剂表现出不同的催化性能. 等体积浸渍法得到分散性较好、粒径小和分布窄[(8±4) nm]、容易还原和碳化的催化剂, 反应中呈现出最高的低碳烯烃选择性、催化活性和稳定性. 胶体法得到了形貌各异的粒子, 反应中活性相易被氧化使得催化剂活性及稳定性较差. 沉积沉淀法得到了粒径过大、难以还原和碳化的粒子, 反应15 h后催化剂活性及稳定性急剧下降. 该研究为利用氮掺杂碳纳米材料作为载体设计和开发高性能的费托合成催化剂提供了有益参考. 相似文献
987.
建立了烟叶中25种酚类物质的高效液相色谱-紫外/质谱(HPLC-UV/MS)测定方法,其中11种主要酚类物质采用HPLC-UV法进行绝对定量分析,其余采用HPLC-MS法进行半定量分析。方法验证结果表明:11种酚类标准物质在0.90~99.00 mg/L(绿原酸及芸香苷为0.95~380.00 mg/L)范围内呈现很好的线性关系,相关系数为0.9993~0.9999。在22.5~24.8、45.0~49.5、67.5~74.3 mg/L 3个水平的加标回收率为91.0%~112.4%。方法重复性的相对标准偏差(RSD)为1.48%~13.40%,日内日间精密度的RSD为0.35%~15.54%。采用本方法对云南、河南、贵州的成熟期新鲜烟叶样品进行了分析,主要酚类物质总量为贵州>云南>河南,贵州与河南两地间存在显著性差异;芸香苷含量为云南>贵州>河南,且相互间有显著性差异。本方法预处理简单,酚类物质的分析覆盖面广,可用于批量烟叶样品的检测。 相似文献
988.
Wenjian Lao Xuejun Sun Jinmao You Qingyu Ou 《Monatshefte für Chemie / Chemical Monthly》2000,131(7):803-811
Summary. A rapid one-pot synthesis of substituted carbazole-9-acetic and -propionic acids under microwave irradiation is described.
N-Alkylation and hydrolysis was carried out in DMF without catalyst. The absorption and fluorescence spectroscopic characteristics of the title compounds are discussed.
Received January 31, 2000. Accepted (revised) March 7, 2000 相似文献
989.
This paper examines an assemble-to-order environment involving a short-life-cycle product that is sold in two different configurations, each requiring a unique component that must be stocked in advance. Both configurations of the product are assembled on the same equipment which has limited capacity. The focus of the analysis is on the determination of the appropriate stocking quantities for each of the configuration specific components. Because the same assembly capacity is often reused for different instances of the component stocking problem, we treat assembly capacity as exogenous. This represents the fact that capacity decisions are often made less frequently than are procurement/production quantity decisions. We first solve for the first-best stocking policy when the components are produced internally. We then consider the case when the components are procured from external suppliers, and investigate how different forms of contract between the assembler and the component suppliers affect coordination of the supply chain as well as each party’s profit. One particularly interesting finding is that it is possible to coordinate the supply chain with a single-price contract between each supplier and the assembler while awarding all parties positive profit. 相似文献
990.
This paper presents a modified ODE-based algorithm for unconstrained optimization problems. It combines the idea of IMPBOT algorithm with nonmonotone and subspace techniques. The main feature of this method is that at each iteration, a lower dimensional system of linear equations is solved to obtain a trial step. Under some standard assumptions, the method is proven to be globally convergent. Numerical results show the efficiency of this proposed method in practical computation. 相似文献