全文获取类型
收费全文 | 33104篇 |
免费 | 2863篇 |
国内免费 | 1861篇 |
专业分类
电工技术 | 2164篇 |
综合类 | 2596篇 |
化学工业 | 5113篇 |
金属工艺 | 1862篇 |
机械仪表 | 2106篇 |
建筑科学 | 2633篇 |
矿业工程 | 815篇 |
能源动力 | 908篇 |
轻工业 | 2793篇 |
水利工程 | 657篇 |
石油天然气 | 1604篇 |
武器工业 | 318篇 |
无线电 | 3959篇 |
一般工业技术 | 3814篇 |
冶金工业 | 1524篇 |
原子能技术 | 433篇 |
自动化技术 | 4529篇 |
出版年
2024年 | 123篇 |
2023年 | 582篇 |
2022年 | 866篇 |
2021年 | 1255篇 |
2020年 | 1003篇 |
2019年 | 799篇 |
2018年 | 904篇 |
2017年 | 1074篇 |
2016年 | 933篇 |
2015年 | 1276篇 |
2014年 | 1736篇 |
2013年 | 1961篇 |
2012年 | 2248篇 |
2011年 | 2370篇 |
2010年 | 2195篇 |
2009年 | 2108篇 |
2008年 | 2149篇 |
2007年 | 2017篇 |
2006年 | 1928篇 |
2005年 | 1669篇 |
2004年 | 1242篇 |
2003年 | 1098篇 |
2002年 | 1123篇 |
2001年 | 946篇 |
2000年 | 782篇 |
1999年 | 686篇 |
1998年 | 529篇 |
1997年 | 446篇 |
1996年 | 388篇 |
1995年 | 308篇 |
1994年 | 275篇 |
1993年 | 187篇 |
1992年 | 168篇 |
1991年 | 107篇 |
1990年 | 119篇 |
1989年 | 62篇 |
1988年 | 53篇 |
1987年 | 29篇 |
1986年 | 25篇 |
1985年 | 7篇 |
1984年 | 8篇 |
1983年 | 12篇 |
1982年 | 10篇 |
1981年 | 5篇 |
1980年 | 8篇 |
1979年 | 5篇 |
1977年 | 1篇 |
1959年 | 1篇 |
1951年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 819 毫秒
41.
Mining constrained gradients in large databases 总被引:1,自引:0,他引:1
Dong G. Han J. Lam J.W.M. Pei J. Wangm K Zou W. 《Knowledge and Data Engineering, IEEE Transactions on》2004,16(8):922-938
Many data analysis tasks can be viewed as search or mining in a multidimensional space (MDS). In such MDSs, dimensions capture potentially important factors for given applications, and cells represent combinations of values for the factors. To systematically analyze data in MDS, an interesting notion, called "cubegrade" was recently introduced by Imielinski et al. [2002], which focuses on the notable changes in measures in MDS by comparing a cell (which we refer to as probe cell) with its gradient cells, namely, its ancestors, descendants, and siblings. We call such queries gradient analysis queries (GQs). Since an MDS can contain billions of cells, it is important to answer GQs efficiently. We focus on developing efficient methods for mining GQs constrained by certain (weakly) antimonotone constraints. Instead of conducting an independent gradient-cell search once per probe cell, which is inefficient due to much repeated work, we propose an efficient algorithm, LiveSet-Driven. This algorithm finds all good gradient-probe cell pairs in one search pass. It utilizes measure-value analysis and dimension-match analysis in a set-oriented manner, to achieve bidirectional pruning between the sets of hopeful probe cells and of hopeful gradient cells. Moreover, it adopts a hypertree structure and an H-cubing method to compress data and to maximize sharing of computation. Our performance study shows that this algorithm is efficient and scalable. In addition to data cubes, we extend our study to another important scenario: mining constrained gradients in transactional databases where each item is associated with some measures such as price. Such transactional databases can be viewed as sparse MDSs where items represent dimensions, although they have significantly different characteristics than data cubes. We outline efficient mining methods for this problem. 相似文献
42.
应用Nd:YAG激光(波长1.06μm,功率5w,照射时间0.5-3 min)和Ar+激光(波长488nm,功率70mw,照射时间5-15min)对长期保存的不同保存形式的塔胞藻(Pyramidomonassp.)进行辐照处理。研究了不同剂量激光辐照对藻体生长和叶绿素含量的影响。实验结果表明:照射剂量为60s的Nd:YAG激光及剂量为15min的Ar+激光对液体保存形式的藻种有较明显的促长效果,接种后这两种激光处理组的比生长速率分别较对照提高达53.33%和28.89%,叶绿素含量分别增加73.33%和65.69%。两种激光对液体保存藻种的活化效果均好于固体保存种。 相似文献
43.
44.
45.
采用0.8μm标准数字CMOS工艺(VTN0=0.836V,VTP0=0.930V),设计并流片验证了具有宽工作电压范围(3~6V),可作SOC系统动态电源管理芯片内部误差放大器应用的单电源CMOS运算放大器。该误差放大器芯核同时具有适合低电压工作,并对工艺参数变化不敏感的优点。对于相同的负载情况,在3V的工作电压下,开环电压增益AD=83.1dB,单位增益带宽GB=2.4MHz,相位裕量Φ=85.2°,电源抑制比PSRR=154.0dB,转换速率Sr=2.2V/μs;在6V工作电压下,AD=85.1dB,GB=2.4MHz,Φ=85.4°,PSRR=145.3dB,Sr=3.4V/μs。 相似文献
46.
本文从二极管箝位型三电平逆变器的基本结构出发,分析其输出状态,推导出六角形空间电压矢量图。然后从一种改进的,可以减小开关损耗,避免高的dv/dt三电平空间矢量PWM调制策略出发,建立了基于Matlab的详细仿真方案,验证了方案的可行性。文章给出了扇区判断的算法流程图,脉冲生成模块的内部结构图,其中的扇区判断的算法和脉冲生成方法对于用DSP实现三电平SVPWM具有重要的参考价值。 相似文献
47.
48.
Laparoscopic retroperitoneal lymph node dissection is a new surgical procedure used to enhance staging in men with clinical stage I nonseminomatous germ cell tumors of the testis. The procedure has been performed in a limited number of patients at several centers with extensive laparoscopic experience. Laparoscopic retroperitoneal lymphadenectomy is a technically demanding procedure which can be successfully completed in the majority of patients. However, the risk of complications is greater than in patients who undergo standard open retroperitoneal lymph node dissection. The primary advantage of a laparoscopic approach is shortened hospitalization and rapid return to normal activity. The role of laparoscopy in the management of patients with testis malignancy has not been defined. The use of this staging procedure may help minimize the need for surveillance studies following surgery and may be best utilized in men with a lower likelihood of nodal metastases. Ultimately, prospective study in large groups of patients will be necessary to determine the role of laparoscopic retroperitoneal lymph node dissection in patients with testis cancer. 相似文献
49.
In the present study, attempts are made to extend the application of the mechanical model for the fatigue crack initiation (FCI) and the FCI life formula of metallic notched elements in laboratory air to those in the corrosive environment. The test results and analysis of the corrosion FCI (CFCI) life of aluminum alloys and Ti---6A1---4V show that the expression of the CFCI life obtained by modifying the FCI life formula in laboratory air can give a good fit to the test results of the CFCI life. The salt water (3.5% NaCl) environment has no effects on the CFCI resistant coefficient compared with the FCI resistant coefficient in laboratory air. However, 3.5% NaCl environment greatly decreases the CFCI threshold of aluminum alloy, but has little effect on the CFCI threshold of Ti---6A1---4V. The loading frequency ranging from 1 Hz to 10 Hz has no appreciable effect on the CFCI life, and thus, the CFCI threshold of aluminum alloys investigated. Hence, the expression for the CFCI life of metallic notched elements proposed in this study is a better one, which reveals a correlation between the CFCI life and the governing parameters, such as, the geometry of the notched elements, the nominal stress range, the stress ratio, the tensile properties and the CFCI threshold. However, this new expression of the CFCI life needs to be verified by more test results. 相似文献
50.
制作压力传感器时,在二氧化硅层上淀积多晶硅膜,既可利用优良的机械特性,又可保证压敏电阻与衬底间具有良好的绝缘性,由此可大大提高器件的温度特性。介绍了一种多晶硅压力传感器的原理和设计。实验结果表明,这类传感器具有灵敏度好,精度高等特点,电路工作范围为0-250℃,且具有良好的温度稳定性。 相似文献