全文获取类型
收费全文 | 112678篇 |
免费 | 9958篇 |
国内免费 | 7377篇 |
专业分类
电工技术 | 7215篇 |
技术理论 | 6篇 |
综合类 | 10650篇 |
化学工业 | 12921篇 |
金属工艺 | 9196篇 |
机械仪表 | 13454篇 |
建筑科学 | 8851篇 |
矿业工程 | 5385篇 |
能源动力 | 2354篇 |
轻工业 | 9676篇 |
水利工程 | 2616篇 |
石油天然气 | 4571篇 |
武器工业 | 1158篇 |
无线电 | 6521篇 |
一般工业技术 | 11406篇 |
冶金工业 | 5781篇 |
原子能技术 | 566篇 |
自动化技术 | 17686篇 |
出版年
2024年 | 462篇 |
2023年 | 2367篇 |
2022年 | 3961篇 |
2021年 | 4383篇 |
2020年 | 3852篇 |
2019年 | 3049篇 |
2018年 | 2758篇 |
2017年 | 3276篇 |
2016年 | 3644篇 |
2015年 | 3786篇 |
2014年 | 6295篇 |
2013年 | 5490篇 |
2012年 | 7718篇 |
2011年 | 8182篇 |
2010年 | 6248篇 |
2009年 | 6637篇 |
2008年 | 5780篇 |
2007年 | 7730篇 |
2006年 | 7337篇 |
2005年 | 6130篇 |
2004年 | 4976篇 |
2003年 | 4563篇 |
2002年 | 3805篇 |
2001年 | 3137篇 |
2000年 | 2706篇 |
1999年 | 2162篇 |
1998年 | 1679篇 |
1997年 | 1372篇 |
1996年 | 1148篇 |
1995年 | 1068篇 |
1994年 | 903篇 |
1993年 | 638篇 |
1992年 | 552篇 |
1991年 | 432篇 |
1990年 | 346篇 |
1989年 | 339篇 |
1988年 | 247篇 |
1987年 | 114篇 |
1986年 | 114篇 |
1985年 | 110篇 |
1984年 | 69篇 |
1983年 | 75篇 |
1982年 | 47篇 |
1981年 | 62篇 |
1980年 | 49篇 |
1979年 | 32篇 |
1978年 | 22篇 |
1977年 | 18篇 |
1976年 | 16篇 |
1975年 | 15篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
51.
Condition monitoring and fault diagnosis of rolling element bearings timely and accurately are very important to ensure the reliability of rotating machinery. This paper presents a novel pattern classification approach for bearings diagnostics, which combines the higher order spectra analysis features and support vector machine classifier. The use of non-linear features motivated by the higher order spectra has been reported to be a promising approach to analyze the non-linear and non-Gaussian characteristics of the mechanical vibration signals. The vibration bi-spectrum (third order spectrum) patterns are extracted as the feature vectors presenting different bearing faults. The extracted bi-spectrum features are subjected to principal component analysis for dimensionality reduction. These principal components were fed to support vector machine to distinguish four kinds of bearing faults covering different levels of severity for each fault type, which were measured in the experimental test bench running under different working conditions. In order to find the optimal parameters for the multi-class support vector machine model, a grid-search method in combination with 10-fold cross-validation has been used. Based on the correct classification of bearing patterns in the test set, in each fold the performance measures are computed. The average of these performance measures is computed to report the overall performance of the support vector machine classifier. In addition, in fault detection problems, the performance of a detection algorithm usually depends on the trade-off between robustness and sensitivity. The sensitivity and robustness of the proposed method are explored by running a series of experiments. A receiver operating characteristic (ROC) curve made the results more convincing. The results indicated that the proposed method can reliably identify different fault patterns of rolling element bearings based on vibration signals. 相似文献
52.
In this study, the crystal structure, thermal, oxygen transport, electrical conductivity and electrochemical properties of the perovskite NdBa0.5Sr0.5Co2O5+δ (NBSC55) are investigated. In the temperature range of 250 °C–350 °C, the weight loss upon heating was due to a partial loss of lattice oxygen and along with a reduction of Co4+ to Co3+. The tend of weight-loss slows down as temperature increased above 350 °C indicating a reduction of Co3+ to Co2+ during this stage. The oxygen migration is dominated by surface exchange process at high temperature range (650-800 °C); however, the bulk diffusion process prevails at low temperature range (500–600 °C). For long-term testing, the polarization resistance of NBSC55 increases gradually form 3.13 Ω cm2 for 2 h to 3.34 Ω cm2 for 96 h at 600 °C and an increasing-rate for polarization resistance is around 0.22% h?1. The power density of the single cell with NBSC55 cathode reached 341 mW cm?2 at 800 °C. 相似文献
53.
55.
Digital technology becomes more powerful, intelligent, pervasive and ubiquitous. Ethical aspects of this development have not yet drawn the appropriate attention of researchers and engineers. This paper presents an instrument that aims at measuring the individual ethical position with regard to the design and development of computer software. The development of the Epos tool was based on two data collections. The data of the first survey (n1 = 147 participants) were used to select items and to determine the factorial structure of the questionnaire. Results show that the Epos instrument reliably assesses peoples’ ethical opinion with respect to five central components: (1) regulation, (2) data privacy, (3) domain specific knowledge, (4) societal responsibility and (5) company responsibility. In the second survey, we determined the stability of the instruments factor structure by assessing a sample of n2?=?196 participants. A confirmatory factor analysis (CFA) supported the initial factor structure. Next steps and further implications are discussed regarding the final version of the questionnaire. 相似文献
56.
目的 以气调包装酱卤鸭肉制品为研究对象,在冷链温度范围内建立一套准确、高效的货架期预测模型。方法 利用选择性培养基测定不同温度下产品各微生物数量,确定4~25℃条件下产品优势腐败菌。对乳酸菌数量与感官评定值进行了回归分析确定最小腐败量Ns。分别采用修正的Gompertz方程和平方根方程建立一、二级模型,并通过预测值与实测值对比验证模型的可靠性。结果 确定了4~25℃条件下产品优势腐败菌为乳酸菌,最小腐败量Ns=6.14(lg(cfu /g))。一、二级模型拟合度均良好,三种温度下模型预测值与实际值间的差异均在30%左右,波动幅度在10%以内。结论 实现了对4~25℃内任何时间点产品剩余货架期的预测,为冷链条件下气调包装酱卤鸭肉制品品质的变化提供了理论指导。 相似文献
57.
针对平面并联机构无奇异位置工作空间求解困难、过程繁琐、计算量大等问题,提出了基于CAD求解平面并联机构工作空间的三维螺旋扫描方法。将[n]自由度平面并联机构分解成[n]条支链进行独立分析,得到每条支链下末端执行器的可达区域,再将所有支链可达区域取交集即为平面并联机构工作空间。应用SolidWorks软件建立平面并联机构模型,进行几何特征处理,通过自动求解器求解,将求解过程图形化,快速得到同轴布局5R机构和平面3-RPR并联机构的无奇异位置工作空间。通过同轴布局5R机构的运动学实验,验证了该求解方法的可行性。 相似文献
58.
针对现有混合入侵检测模型仅定性选取特征而导致检测精度较低的问题,同时为了充分结合误用检测模型和异常检测模型的优势,提出一种采用信息增益率的混合入侵检测模型.首先,利用信息增益率定量地选择特征子集,最大程度地保留样本信息;其次,采用余弦时变粒子群算法确定支持向量机参数构建误用检测模型,使其更好地平衡粒子在全局和局部的搜索能力,然后,选取灰狼算法确定单类支持向量机参数构建异常检测模型,以此来提高对最优参数的搜索效率和精细程度,综合提高混合入侵检测模型对攻击的检测效果;最后,通过两种数据集进行仿真实验,验证了所提混合入侵检测模型具有较好的检测性能. 相似文献
59.
A flower‐shaped ultra‐wideband fractal antenna is presented. It comprises a fourth iterative flower‐shaped radiator, asymmetrical stub‐loaded feeding line, and coplanar quarter elliptical ground planes. A wide operating band of 12.12 GHz (4.58‐16.7 GHz) for S 11 ≤ ? 10 dB is achieved along with an overall antenna footprint of 15.7 × 11.4 mm2. In addition, other desirable characteristics, that is, omnidirectional radiation patterns, peak gain upto 5 dB, and fidelity factor more than 75% are achieved. A good agreement exists between the simulation and measured results. The obtained results illustrate that this antenna has wide operating range and compact dimensions than available structures. 相似文献
60.