全文获取类型
收费全文 | 3406篇 |
免费 | 414篇 |
国内免费 | 333篇 |
专业分类
化学 | 951篇 |
晶体学 | 7篇 |
力学 | 164篇 |
综合类 | 49篇 |
数学 | 1104篇 |
物理学 | 513篇 |
无线电 | 1365篇 |
出版年
2024年 | 10篇 |
2023年 | 32篇 |
2022年 | 87篇 |
2021年 | 109篇 |
2020年 | 106篇 |
2019年 | 95篇 |
2018年 | 97篇 |
2017年 | 121篇 |
2016年 | 152篇 |
2015年 | 163篇 |
2014年 | 281篇 |
2013年 | 249篇 |
2012年 | 264篇 |
2011年 | 266篇 |
2010年 | 208篇 |
2009年 | 240篇 |
2008年 | 231篇 |
2007年 | 217篇 |
2006年 | 188篇 |
2005年 | 186篇 |
2004年 | 136篇 |
2003年 | 127篇 |
2002年 | 101篇 |
2001年 | 62篇 |
2000年 | 57篇 |
1999年 | 63篇 |
1998年 | 44篇 |
1997年 | 52篇 |
1996年 | 34篇 |
1995年 | 27篇 |
1994年 | 36篇 |
1993年 | 14篇 |
1992年 | 13篇 |
1991年 | 10篇 |
1990年 | 12篇 |
1989年 | 5篇 |
1988年 | 10篇 |
1987年 | 5篇 |
1986年 | 1篇 |
1985年 | 9篇 |
1984年 | 5篇 |
1983年 | 6篇 |
1982年 | 6篇 |
1981年 | 6篇 |
1979年 | 6篇 |
1978年 | 2篇 |
1977年 | 1篇 |
1969年 | 1篇 |
排序方式: 共有4153条查询结果,搜索用时 15 毫秒
11.
铁路接触网绝缘子状态检测对铁路行车安全有着 重大的意义,为解决目前人工对绝缘 子图像检测结果的不确定性,提出一种深度学习结合灰度纹理特征的检测方法。首先使用 Faster R-CNN (faster region-based convolutional neural network)目标检测算法对图像中绝缘子精确识别,再通过灰度共生矩阵对绝缘子纹理 特征进行分析提取,之后结合支持向量机将绝缘子分为正常绝缘子和异常绝缘子,实验数 据结果证明使用能量、熵、相关度3种纹理特征进行绝缘子状态分类时对实验数据中的正 常状态绝缘子的分类精度可达100%,异常状态绝缘子的分类精度达97.5%,最后依据绝缘 子图像灰度分布的周期性特点,利用灰度积分投影将异常绝缘子分为破损绝缘子和夹杂异 物绝缘子。实验结果表明所提方法可以有效对绝缘子状态进行检测分类。 相似文献
12.
J. M. Borwein J. D. Vanderwerff 《Proceedings of the American Mathematical Society》1996,124(3):751-755
It is shown that the existence of a closed convex set all of whose points are properly supported in a Banach space is equivalent to the existence of a certain type of uncountable ordered one-sided biorthogonal system. Under the continuum hypothesis, we deduce that this notion is weaker than the existence of an uncountable biorthogonal system.
13.
14.
Xuran Li Simiao Chen Jinghui Zhang Li Yu Weiyan Chen Yuyan Zhang 《Molecules (Basel, Switzerland)》2022,27(10)
Dried roots of Polygala tenuifolia (YuanZhi in Chinese) are widely used in Chinese herbal medicine. These components in YuanZhi have significant anti-oxidation properties owing to high levels of 3,6’-disinapoylsucrose (DISS) and Polygalaxanthone III (PolyIII). In order to efficiently extract natural medicines, response surface methodology (RSM) and least squares support vector machine (LSSVM) were used for the modeling and optimization of ultrasound-assisted extraction of DISS and PolyIII together to determine the antioxidant activity of the extracts obtained from YuanZhi. For the optimal combination of the comprehensive yield of DISS and PolyIII (Y), the Box-Behnken design (BBD) was used to improve extraction time (X1), extraction temperature (X2), liquid–solid ratio (X3), and ethanol concentration (X4). The optimal process parameters were determined to be as follows: extraction time, 93 min; liquid–solid ratio, 40 mL/g; extraction temperature, 48 °C; and ethanol concentration, 67%. With these conditions, the predictive optimal combination comprehensive evaluation value is 13.0217. It was clear that the LS-SVM model had higher accuracy in predictive and optimization capabilities, with higher antioxidant activity and lower relative deviations values, than did RSM. Hence, the LS-SVM model proved to be more effective for the analysis and improvement of the extraction process. 相似文献
15.
作战指挥决策支持系统是指挥自动化系统的重要组成部分,作战方案生成是作战指挥决策支持系统的基本功能.为了能够快速、准确地生成作战方案,为指挥员提供决策依据,建立了一种过程推理系统的改进模型并将其运用于作战方案生成.根据对过程推理系统的介绍和分析,针对其重复劳动和交互不足的问题,对战术意图集进行分类处理、对所选作战规划进行合并处理以及对更新状态规则和评价标准进行改进.通过这些改进可提高其运行效率,实现对战术意图的批处理.最后建立了改进的过程推理系统的体系结构模型,描述了其生成作战方案的运行过程,表明其可以基本满足作战方案生成的要求. 相似文献
16.
Rice blast is a serious threat to rice yield. Breeding disease-resistant varieties is one of the most economical and effective ways to prevent damage from rice blast. The traditional identification of resistant rice seeds has some shortcoming, such as long possession time, high cost and complex operation. The purpose of this study was to develop an optimal prediction model for determining resistant rice seeds using Ranman spectroscopy. First, the support vector machine (SVM), BP neural network (BP) and probabilistic neural network (PNN) models were initially established on the original spectral data. Second, due to the recognition accuracy of the Raw-SVM model, the running time was fast. The support vector machine model was selected for optimization, and four improved support vector machine models (ABC-SVM (artificial bee colony algorithm, ABC), IABC-SVM (improving the artificial bee colony algorithm, IABC), GSA-SVM (gravity search algorithm, GSA) and GWO-SVM (gray wolf algorithm, GWO)) were used to identify resistant rice seeds. The difference in modeling accuracy and running time between the improved support vector machine model established in feature wavelengths and full wavelengths (200–3202 cm−1) was compared. Finally, five spectral preproccessing algorithms, Savitzky–Golay 1-Der (SGD), Savitzky–Golay Smoothing (SGS), baseline (Base), multivariate scatter correction (MSC) and standard normal variable (SNV), were used to preprocess the original spectra. The random forest algorithm (RF) was used to extract the characteristic wavelengths. After different spectral preproccessing algorithms and the RF feature extraction, the improved support vector machine models were established. The results show that the recognition accuracy of the optimal IABC-SVM model based on the original data was 71%. Among the five spectral preproccessing algorithms, the SNV algorithm’s accuracy was the best. The accuracy of the test set in the IABC-SVM model was 100%, and the running time was 13 s. After SNV algorithms and the RF feature extraction, the classification accuracy of the IABC-SVM model did not decrease, and the running time was shortened to 9 s. This demonstrates the feasibility and effectiveness of IABC in SVM parameter optimization, with higher prediction accuracy and better stability. Therefore, the improved support vector machine model based on Ranman spectroscopy can be applied to the fast and non-destructive identification of resistant rice seeds. 相似文献
17.
18.
为降低光纤周界安防信号中噪声对分类结果的影响,提升信号分类的准确率和运行效率,提出一种融合了相关变分模态分解(Correlation Variational Mode Decomposition, CVMD)、蜣螂算法(Dung Beetle Optimizer, DBO)和支持向量机(Support Vector Machine, SVM)的分类方法。利用CVMD去除原始信号中的噪声分量,并提取去噪后信号的能量、能量熵和峭度作为特征向量。采用DBO算法优化SVM,得到最佳惩罚因子和核函数参数,并构建DBO-SVM分类模型。搭建了基于相位敏感光时域反射(Φ-OTDR)技术的周界安防系统,采集了攀爬、敲击、踩踏和无入侵四类信号。实验结果表明,CVMD-DBO-SVM的分类准确率相比CVMD-PSO-SVM和CVMD-GA-SVM更高,达到了98.75%,同时运行时间更短,综合性能最优。 相似文献
19.
20.
基于对图像拼接技术的分析,提出了一种基于马尔科夫模型与Hilbert-Huang变换(HHT)的图像拼接盲检测算法。该算法计算图像DCT域上的马尔科夫转移概率矩阵,同时对图像进行Hilbert-Huang分析,得到两类特征值集,并通过计算相关系数矩阵分析了两者之间的相关性,最后使用支持向量机进行训练与分类。实验结果表明,相对于已有文献,该算法具有较高的检测准确率。 相似文献