全文获取类型
收费全文 | 5797篇 |
免费 | 571篇 |
国内免费 | 452篇 |
专业分类
化学 | 1616篇 |
晶体学 | 56篇 |
力学 | 208篇 |
综合类 | 132篇 |
数学 | 2289篇 |
物理学 | 2519篇 |
出版年
2024年 | 16篇 |
2023年 | 49篇 |
2022年 | 112篇 |
2021年 | 116篇 |
2020年 | 116篇 |
2019年 | 140篇 |
2018年 | 145篇 |
2017年 | 177篇 |
2016年 | 197篇 |
2015年 | 161篇 |
2014年 | 338篇 |
2013年 | 437篇 |
2012年 | 309篇 |
2011年 | 351篇 |
2010年 | 294篇 |
2009年 | 358篇 |
2008年 | 417篇 |
2007年 | 399篇 |
2006年 | 334篇 |
2005年 | 283篇 |
2004年 | 260篇 |
2003年 | 271篇 |
2002年 | 210篇 |
2001年 | 174篇 |
2000年 | 174篇 |
1999年 | 151篇 |
1998年 | 133篇 |
1997年 | 132篇 |
1996年 | 123篇 |
1995年 | 79篇 |
1994年 | 53篇 |
1993年 | 48篇 |
1992年 | 39篇 |
1991年 | 32篇 |
1990年 | 29篇 |
1989年 | 17篇 |
1988年 | 20篇 |
1987年 | 15篇 |
1986年 | 20篇 |
1985年 | 15篇 |
1984年 | 15篇 |
1983年 | 6篇 |
1982年 | 21篇 |
1981年 | 9篇 |
1980年 | 3篇 |
1979年 | 10篇 |
1978年 | 8篇 |
1977年 | 1篇 |
1976年 | 2篇 |
1974年 | 1篇 |
排序方式: 共有6820条查询结果,搜索用时 250 毫秒
11.
12.
本文给出了一个关联图为圈的非负、半正定矩阵A为完全正的一个充要条件.我们还证明了这样的矩阵A(当A为完全正时)的分解指数即为A的阶数. 相似文献
13.
中国证券市场股指波动的条件异方差特性分析 总被引:6,自引:0,他引:6
股指的波动具有持续性、集聚性 ,如何进行判别 ?本文用 Garch模型理论探讨沪深股指的这种条件异方差特征 ,进一步分析波动是否影响股指未来变化 ,以及股市对利好、利空的消息是否存在不对称的反映。同时 ,比较不同类型的股指的共性及差异 ,并对上述现象作了解释和说明。 相似文献
14.
15.
为了准确计算出镀膜过程中每层膜的折射率,介绍了实时监控过程中确定膜层折射率的2种方法:一种是由实测的透射比光谱直接反算出膜层的折射率;另一种是用最小二乘法的优化算法实时拟合折射率。试验结果表明:在线反算适合单点监控,所得折射率误差小于2%。然而在实际镀膜过程中,由于宽带内膜层参数误差较大,一般大于25%。为此,采用最小二乘法拟合,即在整个宽光谱范围内采集每个波长点的信息,所得结果误差很小,一般都在2%~5%之间,有时可达到10%,在很大程度上提高了实际镀膜时膜厚监控的精度。 相似文献
16.
17.
Let {Xk}k?1 be a strictly stationary time series. For a strictly increasing sampling function g:?→? define Yk=Xg(k) as the deterministic sub‐sampled time series. In this paper, the extreme value theory of {Yk} is studied when Xk has representation as a moving average driven by heavy‐tailed innovations. Under mild conditions, convergence results for a sequence of point processes based on {Yk} are proved and extremal properties of the deterministic sub‐sampled time series are derived. In particular, we obtain the limiting distribution of the maximum and the corresponding extremal index. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
18.
We consider the optimal service control of a multiclass M/G/1 queueing system in which customers are served nonpreemptively and the system cost rate is additive across classes and increasing convex in the numbers present in each class. Following Whittle's approach to a class of restless bandit problems, we develop a Langrangian relaxation of the service control problem which serves to motivate the development of a class of index heuristics. The index for a particular customer class is characterised as a fair charge for service of that class. The paper develops these indices and reports an extensive numerical investigation which exhibits strong performance of the index heuristics for both discounted and average costs. 相似文献
19.
Anuj Mittal Swaminathan Sivaram 《Journal of polymer science. Part A, Polymer chemistry》2005,43(21):4996-5008
A tridentate ligand, BPIEP: 2,6‐bis[1‐(2,6‐diisopropyl phenylimino) ethyl] pyridine, having central pyridine unit and two peripheral imine coordination sites was effectively employed in controlled/“living” radical polymerization of MMA at 90°C in toluene as solvent, CuIBr as catalyst, and ethyl‐2‐bromoisobutyrate (EBiB) as initiator resulting in well‐defined polymers with polydispersities Mw/Mn ≤ 1.23. The rate of polymerization follows first‐order kinetics, kapp = 3.4 × 10?5 s?1, indicating the presence of low radical concentration ([P*] ≤ 10?8) throughout the reaction. The polymerization rate attains a maximum at a ligand‐to‐metal ratio of 2:1 in toluene at 90°C. The solvent concentration (v/v, with respect to monomer) has a significant effect on the polymerization kinetics. The polymerization is faster in polar solvents like, diphenylether, and anisole, as compared to toluene. Increasing the monomer concentration in toluene resulted in a better control of polymerization. The molecular weights (Mn,SEC) increased linearly with conversion and were found to be higher than predicted molecular (Mn,Cal). However, the polydispersity remained narrow, i.e., ≤1.23. The initiator efficiency at lower monomer concentration approaches a value of 0.7 in 110 min as compared to 0.5 in 330 min at higher monomer concentration. The aging of the copper salt complexed with BPIEP had a beneficial effect and resulted in polymers with narrow polydispersitities and higher conversion. PMMA obtained at room temperature in toluene (33%, v/v) gave PDI of 1.22 (Mn = 8500) in 48 h whereas, at 50°C the PDI is 1.18 (Mn = 10,300), which is achieved in 23 h. The plot of lnkapp versus 1/T gave an apparent activation energy of polymerization as (ΔE≠app) 58.29 KJ/mol and enthalpy of equilibrium (ΔH0eq) to 28.8 KJ/mol. Reverse ATRP of MMA was successfully performed using AIBN in bulk as well as solution. The controlled nature of the polymerization reaction was established through kinetic studies and chain extension experiments. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 4996–5008, 2005 相似文献
20.
Hiroto Kudo Nobuhito Ito Keiji Nakajima Masami Ochiai Tadatomi Nishikubo 《Journal of polymer science. Part A, Polymer chemistry》2007,45(19):4421-4429
The polyaddition of fluorine‐containing bis(epoxide)s and fluorine‐containing triazine di(aryl ether)s were examined to give the corresponding fluorine‐containing poly(cyanurate)s. It was observed that the synthesized fluoropolymers had good thermal stabilities and good film‐forming properties. The glass transition temperatures (Tg's) and refractive‐indices (nD's) of synthesized polymers were determined by differential scanning calorimetry and ellipsometry, respectively, and it was found that the values of Tg's and nD's were supported by their fluorine containing ratios and skeletons. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 4421–4429, 2007 相似文献