全文获取类型
收费全文 | 252908篇 |
免费 | 15989篇 |
国内免费 | 8148篇 |
专业分类
电工技术 | 11555篇 |
技术理论 | 11篇 |
综合类 | 13712篇 |
化学工业 | 40026篇 |
金属工艺 | 14218篇 |
机械仪表 | 13905篇 |
建筑科学 | 15923篇 |
矿业工程 | 5152篇 |
能源动力 | 6125篇 |
轻工业 | 15089篇 |
水利工程 | 4474篇 |
石油天然气 | 9959篇 |
武器工业 | 1347篇 |
无线电 | 29252篇 |
一般工业技术 | 36919篇 |
冶金工业 | 10091篇 |
原子能技术 | 2233篇 |
自动化技术 | 47054篇 |
出版年
2024年 | 606篇 |
2023年 | 2652篇 |
2022年 | 4719篇 |
2021年 | 6939篇 |
2020年 | 5163篇 |
2019年 | 4433篇 |
2018年 | 18492篇 |
2017年 | 18242篇 |
2016年 | 14404篇 |
2015年 | 7465篇 |
2014年 | 8654篇 |
2013年 | 10321篇 |
2012年 | 14109篇 |
2011年 | 21091篇 |
2010年 | 18560篇 |
2009年 | 15285篇 |
2008年 | 16339篇 |
2007年 | 16780篇 |
2006年 | 9588篇 |
2005年 | 9392篇 |
2004年 | 6849篇 |
2003年 | 6337篇 |
2002年 | 5796篇 |
2001年 | 4725篇 |
2000年 | 4581篇 |
1999年 | 4443篇 |
1998年 | 3710篇 |
1997年 | 3115篇 |
1996年 | 2889篇 |
1995年 | 2479篇 |
1994年 | 2029篇 |
1993年 | 1471篇 |
1992年 | 1162篇 |
1991年 | 925篇 |
1990年 | 666篇 |
1989年 | 571篇 |
1988年 | 472篇 |
1987年 | 303篇 |
1986年 | 200篇 |
1985年 | 126篇 |
1984年 | 82篇 |
1983年 | 52篇 |
1982年 | 81篇 |
1981年 | 55篇 |
1980年 | 46篇 |
1968年 | 43篇 |
1965年 | 47篇 |
1959年 | 43篇 |
1955年 | 63篇 |
1954年 | 68篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
951.
J. Xu 《Acta Informatica》1992,29(2):121-160
This paper presents a new model for studying the concurrency vs. computation time tradeoffs involved in on-line multiversion database concurrency control. The basic problem that is studied in our model is the following: Given:a current database system state which includes information such as which transaction previously read a version from which other transaction; which transaction has written which versions into the database; and the ordering of versions previously written; anda set of read and write requests of requesting transactions. Question: Does there exist a new database system state in which the requesting transactions can be immediately put into execution (their read and write requests satisfied, or in the case of predeclared writeset transactions, write requests are guaranteed to be satisfied) while preserving consistency under a given set of additional constraints? (The amount of concurrency achieved is defined by the set of additional constraints). In this paper we derive “limits” of performance achievable by polynomial time concurrency control algorithms. Each limit is characterized by a minimal set of constraints that allow the on-line scheduling problem to be solved in polynomial time. If any one constraint in that minimal set is omitted, although it could increase the amount of concurrency, it would also have the dramatic negative effect of making the scheduling problem NP-complete; whereas if we do not omit any constraint in the minimal set, then the scheduling problem can be solved in polynomial time. With each of these limits, one can construct an efficient scheduling algorithm that achieves an optimal level of concurrency in polynomial computation time according to the constraints defined in the minimal set. 相似文献
952.
本文介绍了一种用集成电路制作的特技切换器,具有成本低,线路简单,易于制作的特点。可以完成简单,基本的扫换,键控,特技效果和快切等功能。 相似文献
953.
954.
Rong-Jun Xie Mamoru Mitomo Kyota Uheda Fang-Fang Xu Yoshio Akimune 《Journal of the American Ceramic Society》2002,85(5):1229-1234
Rare-earth-doped oxynitride or nitride compounds have been reported to be luminescent and may then serve as new phosphors with good thermal and chemical stabilities. In this work, we report the photoluminescence (PL) spectra of europium-, terbium-, and praseodymium-doped Ca-α-SiAlON ceramics. The highly dense ceramics were prepared by hot pressing at 1750°C for 1 h under 20 MPa in a nitrogen atmosphere. Europium-doped Ca-α-SiAlON displayed a single broad emission band peaking at λ= 550–590 nm depending on the europium concentration. The emission bands in the spectra of europium-doped Ca-α-SiAlONs were assigned to the allowed transition of Eu2+ from the lowest crystal field component of 4 f 6 5 d to 8 S7/2 (4 f 7 ) ground-state level. The emission spectra of terbium- and praseodymium-doped Ca-α-SiAlON ceramics both consisted of several sharp lines, which were attributed to the 5 D4 →7 F j ( j = 3, 4, 5, 6) transitions of Tb3+ and 3 P0 →3 H j ( j = 3, 4, 5) transitions of Pr3+ , respectively. In particular, the terbium-doped α-SiAlON ceramics showed a strong green emission among these phosphors. 相似文献
955.
介绍了利用单片机的串、并行接口和专用显示接口 IC 设计显示接口电路的方法,并提供了4种动态和静态显示的硬件接口电路。 相似文献
956.
MPC系统模块结构中的流场研究 总被引:5,自引:0,他引:5
本研究中,应用Simple数值计算方法开发了求解MPC系统模块结构中二维流场分布的计算软件。为了验证计算结果的精确性,用多普勒测速仪实测了用有机玻璃制成的MPC模块二维模型的流场,模拟计算结果与实测值对比取得了较好的一致性。四种不同流出角的MPC模块平均总压损失计算结果表明,已开发的软件可作为设计低流阻MPC模块的良好工具。 相似文献
957.
在有壁垒膜存在的表面,多孔膜的生长在氧化初期受到延迟阻碍,这一现象在直流恒压多孔阳极氧化电流一时间曲线上反映出来。电流达到稳态氧化的时间愈长,壁垒膜愈厚。文中应用此法判断了0.5MH_3BO_3介质的pH值、温度对壁垒阳极氧化膜厚的影响,并应用记录同一介质环境壁垒恒流阳极氧化的槽压一时间曲线得到验证。 相似文献
958.
阐述了生产子午线轮胎用圆锥式和斜平面式活络模具在制造工艺、使用性能、维修和热传导等方面的差异。认为圆锥式结构更优于斜平面式。 相似文献
959.
GaN buffer and main layers were grown by the conventional hydride vapor phase epitaxy technique using GaCl3 consecutively. The deposited buffer layers were investigated by atomic force microscopy and X-ray analysis. To examine the behavior of the buffer layers at main layer growth temperature, heat treatment was conducted at 900°C. Based on the results of the buffer layer study, GaN thick films were grown at 1050°C. Optimum deposition conditions of buffer layer from the buffer and main layer studies generally coincided. On the φ scanning pattern, the GaN films grown on (0001) Al2 O3 were single-crystalline. Band-edge emission dominated photoluminescence was observed at room temperature. 相似文献
960.