全文获取类型
收费全文 | 1987篇 |
免费 | 511篇 |
国内免费 | 1475篇 |
专业分类
化学 | 1293篇 |
晶体学 | 8篇 |
力学 | 114篇 |
综合类 | 52篇 |
数学 | 2044篇 |
物理学 | 462篇 |
出版年
2024年 | 9篇 |
2023年 | 17篇 |
2022年 | 42篇 |
2021年 | 64篇 |
2020年 | 35篇 |
2019年 | 35篇 |
2018年 | 22篇 |
2017年 | 37篇 |
2016年 | 42篇 |
2015年 | 46篇 |
2014年 | 183篇 |
2013年 | 197篇 |
2012年 | 229篇 |
2011年 | 150篇 |
2010年 | 365篇 |
2009年 | 292篇 |
2008年 | 399篇 |
2007年 | 313篇 |
2006年 | 186篇 |
2005年 | 123篇 |
2004年 | 92篇 |
2003年 | 114篇 |
2002年 | 77篇 |
2001年 | 159篇 |
2000年 | 198篇 |
1999年 | 122篇 |
1998年 | 59篇 |
1997年 | 33篇 |
1996年 | 41篇 |
1995年 | 67篇 |
1994年 | 66篇 |
1993年 | 51篇 |
1992年 | 27篇 |
1991年 | 14篇 |
1990年 | 25篇 |
1989年 | 19篇 |
1988年 | 8篇 |
1987年 | 3篇 |
1986年 | 3篇 |
1985年 | 3篇 |
1984年 | 2篇 |
1983年 | 3篇 |
1982年 | 1篇 |
排序方式: 共有3973条查询结果,搜索用时 15 毫秒
961.
给出光滑映射芽关于t-P-K-等价与t-S.P-K-等价的有限决定性概念,利用Mather的有限决定性理论,证得光滑映射芽在上述等价关系下的有限决定性定理,为对具有有区别参数的光滑映射芽的分类研究提供了有力工具. 相似文献
962.
In this paper, by using the Nehari manifold and variational methods, we study the existence and multiplicity of positive solutions for a multi-singular quasilinear elliptic problem with critical growth terms in bounded domains. We prove that the equation has at least two positive solutions when the parameters A belongs to a certain subset of JR. 相似文献
963.
Generation of a continuous-wave squeezed vacuum state at 1.3 μm by employing a home-made all-solid-state laser as pump source
下载免费PDF全文
![点击此处可从《中国物理 B》网站下载免费的PDF全文](/ch/ext_images/free.gif)
We present a continuous-wave squeezed vacuum generation system at a telecommunication wavelength of 1.3 μm. By employing a home-made single-frequency Nd:YVO4 laser with dual wavelength outputs as the pump source, via an optical parameter oscillator based on periodically poled KTP, a squeezed vacuum of 6.1 dB±0.1 dB below the shot noise limit at 1342 nm is experimentally measured. This system could be utilized for demonstrating practical quantum information networks. 相似文献
964.
Theseparationofchiralcompoundsisofkeyimportanceindifferentareasofresearch,e,g.,pharmaceutical,biological.,environmentaletc.'Capillaryelectrophoresis(CE)isapowerfultechniqueforchiralreparation.ThegeneralmeritsofCEsuchasimpressivepeakefficiency,rapidityofanalysieismallvolumesandlowcostsarethemostimportantfactorsjcontributingtothesucclessfuldevelopmentofthistechnique"=.Currentlycyclodextrins(CDs)arethemostcommonlyusedchiralresolvingagentsinCE.Enantiomericseparationisbasedonthedifferenceinincl… 相似文献
965.
Bacterial infection is a major threat to human health, and can cause several diseases including gastroenteritis, influenza, tetanus, and tuberculosis. As conventional antibiotic treatment may cause various undesirable effects such as stomach disorder and bacterial resistance, it is necessary to improve the antibacterial efficiency of antibiotics. Here, we synthesized a peptide-based copolymer, poly(ε-caprolactone)-block-poly(glutamic acid)-block-poly(lysine-stat-phenylalanine)[PCL34-b-PGA30-b-P(Lys16-stat-Phe12)] by ring-opening polymerization (ROP) of ε-caprolactone and amino acid N-carboxyanhydride (NCA). Successful synthesis of the copolymer was verified by proton nuclear magnetic resonance and size exclusion chromatography. This copolymer can self-assemble into negatively charged micelles (-26.7 mV) under alkaline conditions by solvent switch method. The micelle structure was confirmed by transmission electron microscopy and dynamic light scattering, and revealed to have a diameter of ~42 nm. Antibiotics were loaded into micelles during the self-assembly process, and cell viability assay was conducted to evaluate its cytotoxicity with and without tobramycin. No obvious cytotoxicity was observed for both micelles when the concentration was lower than 300 μg·mL-1. The antibiotic-loaded micelles demonstrated very low minimum inhibitory concentrations (MICs) against both Gram-negative Escherichia coli (E. coli) (7.8 μg·mL-1) and Gram-positive Staphylococcus aureus (S. aureus) (18.2 μg·mL-1), while the MICs of free tobramycin were 3.9 and 1.0 μg·mL-1, respectively. The drug-loading content and efficiency of the micelles were 5.2% and 24.3%, respectively. Therefore, the MICs of the loaded tobramycin against E. coli and S. aureus were 0.4 and 0.9 μg·mL-1, respectively, suggesting that the micelle could enhance the antibacterial activity of antibiotics. Tobramycin-loaded micelles demonstrated a sustained release characteristic, with 85% of the antibiotics released after 8 h. In bacteria-induced acidic microenvironment, the coil conformation of PGA blocks transforms and PGA blocks shrink toward the micelle core. Concomitantly, the carboxyl side chains are protonated in an acidic environment, increasing the hydrophobicity of this micelle. Antibiotics will be captured when reaching the outer core to slow down the releasing process. Furthermore, the poly(lysine-stat-phenylalanine) [P(Lys-stat-Phe)] coronas with broad spectrum intrinsic antibacterial activity can penetrate the bacterial cell membrane, leading to leakage of the cellular contents of the bacteria and ultimately their death. Due to the sustained release property of micelle and the intrinsic activity of the antibacterial peptide segments, this micelle can greatly enhance the antibacterial activity of antibiotics. Overall, this antibiotic-loaded micelle provides a novel approach for significantly reducing the antibiotics dosage and avoiding the associated health risks. 相似文献
966.
作物病害类型的快速无损检测对提高作物品质和产量至关重要。传统的病害分类方法费时费力且不能实时检测。为此,利用高光谱进行大豆病害分类。以健康大豆为对照,灰斑病和细菌性斑点病两种病害为研究对象,获取三种类别叶片高光谱数据。基于高光谱曲线分析病害与健康叶片反射率的变化规律。采用主成分分析(PCA)和光谱指数(SI)两种单一方法进行病害有效信息提取,共使用30个SI。在此基础上,提出一种PCA与SI相结合的组合方法(PCA-SI),通过提取有效主成分(PC)及有效SI,将有效SI按得分情况分为两组(9SIs和18SIs),再分别对应每一个有效PC进行分组,形成病害光谱有效信息的变量集。采用三种方法分别进行病害有效信息的提取,基于提取后的光谱变量,采用最小二乘支持向量机(LSSVM)和支持向量机(SVM)两种分类器建立病害分类模型。以原始高光谱为基准,以病害分类正确率为指标,评价模型的病害分类性能及不同病害有效信息提取方法和分类器的有效性。结果表明:高光谱反射率具有可见光450~700 nm波段范围病害叶片高于健康叶片而近红外760~1 000 nm波段范围其特征完全相反的规律。采用单一PCA方... 相似文献
967.
968.
在对称平方损失函数下, 利用逐步增加首失效截尾样本,
研究两参数Pareto分布族参数的一致最小方差无偏估计(UMVUE),
Bayes估计和参数型经验Bayes(PEB)估计. 按照均方误差(MSE)准则,
比较UMVUE与PEB估计的优良性. 根据风险函数导出Bayes估计与PEB估计的渐近性,
并获得它们的收敛速度. 在相同的置信水平下,
研究参数分别在经典统计和Bayes统计中的区间估计,
并利用数值模拟说明Bayes区间估计的精度高于经典统计区间估计. 相似文献
969.
970.
首先简要介绍自1977年左右开始的寻找连续时间马尔可夫跳过程实用的唯一性充分条件的故事.对于一般状态空间的一般性结果是1985年得到的.为展示这些充分条件的精确性,于1991年找到非唯一性的对偶判别准则.本文主要限于离散空间(此时的跳过程也称为Q过程或马尔可夫链).在这种情况下,我们将在文末证明上述充分条件也是必要的.我们还将举例说明不论对于唯一性或非唯一性,我们的充分条件不仅强有力,而且精确. 相似文献