全文获取类型
收费全文 | 4968篇 |
免费 | 546篇 |
国内免费 | 404篇 |
专业分类
电工技术 | 504篇 |
综合类 | 604篇 |
化学工业 | 113篇 |
金属工艺 | 102篇 |
机械仪表 | 298篇 |
建筑科学 | 537篇 |
矿业工程 | 151篇 |
能源动力 | 44篇 |
轻工业 | 85篇 |
水利工程 | 110篇 |
石油天然气 | 82篇 |
武器工业 | 69篇 |
无线电 | 1319篇 |
一般工业技术 | 172篇 |
冶金工业 | 115篇 |
原子能技术 | 12篇 |
自动化技术 | 1601篇 |
出版年
2024年 | 33篇 |
2023年 | 71篇 |
2022年 | 126篇 |
2021年 | 123篇 |
2020年 | 163篇 |
2019年 | 119篇 |
2018年 | 102篇 |
2017年 | 140篇 |
2016年 | 153篇 |
2015年 | 198篇 |
2014年 | 350篇 |
2013年 | 273篇 |
2012年 | 361篇 |
2011年 | 394篇 |
2010年 | 289篇 |
2009年 | 335篇 |
2008年 | 344篇 |
2007年 | 412篇 |
2006年 | 302篇 |
2005年 | 306篇 |
2004年 | 234篇 |
2003年 | 225篇 |
2002年 | 195篇 |
2001年 | 140篇 |
2000年 | 133篇 |
1999年 | 86篇 |
1998年 | 56篇 |
1997年 | 55篇 |
1996年 | 35篇 |
1995年 | 36篇 |
1994年 | 24篇 |
1993年 | 17篇 |
1992年 | 20篇 |
1991年 | 10篇 |
1990年 | 10篇 |
1989年 | 6篇 |
1988年 | 13篇 |
1987年 | 6篇 |
1986年 | 3篇 |
1985年 | 3篇 |
1984年 | 2篇 |
1983年 | 2篇 |
1979年 | 1篇 |
1977年 | 1篇 |
1975年 | 1篇 |
1972年 | 1篇 |
1970年 | 2篇 |
1969年 | 2篇 |
1968年 | 2篇 |
1960年 | 1篇 |
排序方式: 共有5918条查询结果,搜索用时 15 毫秒
901.
902.
Mechanism of inactivation of heat-tolerant spores of Bacillus stearothermophilus IFO 12550 by Rapid Decompression 总被引:1,自引:0,他引:1
ISAO HAYAKAWA SOICHI FURUKAWA AKIHIRO MIDZUNAGA HIROSHI HORIUCHI TAICHI NAKASHIMA YSUAKU FUJIO YUKIO YANO TSUYOSHI ISHIKURA KADZUO SASAKI 《Journal of food science》1998,63(3):371-374
The effect of three rapid decompression methods to clear the mechanism of inactivation of heat tolerant spores of Bacillus stearothermophilus IFO 12550 was investigated. Pressurization of the spores at 200 MPa and 75°C for 60 min caused a kill of 104 CFU/mL by the link-motion system but the nonrotational rod valve and E.G. seal methods gave a kill of about 103 CFU/mL. Sterilization was due to the physical breakdown of spore coat, and was induced by its physical permeability of water at high pressure and temperature. Rapid decompression by the link-motion system at 200 MPa decreased the D-value of the spores from 3000 min (100°C, one atmosphere) to 6 min, 11 min, and 17 min at 95, 85, and 75°C, respectively. 相似文献
903.
Sexed semen has been a long-anticipated tool for dairy farmers to obtain more heifer calves, but challenges exist for integrating sexed semen into commercial dairy farm reproduction programs. The decreased conception rates (CR) experienced with sexed semen make virgin heifers better suited for insemination with sexed semen than lactating dairy cows. This research sought to identify when various sexed semen breeding strategies provided higher expected net present value (NPV) than conventional artificial insemination (AI) breeding schemes, indicating which breeding scheme is advisable under various scenarios. Budgets were developed to calculate the expected NPV of various AI breeding strategies incorporating conventional (non-sexed) and sexed semen. In the base budgets, heifer and bull calf values were held constant at $500 and $110, respectively. The percentage of heifers expected to be born after breeding with conventional and sexed semen used was 49.2 and 90%, respectively. Breeding costs per AI were held constant at $15.00 per AI for conventional semen and $45.00 per AI for sexed semen of approximately the same genetic value. Conventional semen CR of 58 and 65% were used, and an AI submission rate was set at 100%. Breeding strategies with sexed semen were assessed for breakeven heifer calf values and sexed semen costs to obtain a NPV equal to that achieved with conventional semen. Breakeven heifer calf values for pure sexed semen strategies with a constant 58 and 65% base CR in which sexed semen achieved 53% of the base CR are $732.11 and $664.26, respectively. Breakeven sexed semen costs per AI of $17.16 and $22.39, compared with $45.00 per AI, were obtained to obtain a NPV equal to that obtained with pure conventional semen for base CR of 58 and 65%, respectively. The strategy employing purely sexed semen, with base CR of both 58 and 65%, yielded a lower NPV than purely conventional semen in all but the best-case scenario in which sexed semen provides 90% of the CR of conventional semen. Other potential advantages of sexed semen that were not quantified in the scenarios include biosecurity-related concerns, decreased dystocia due to increased numbers of heifer calves, and implications for internal herd growth. 相似文献
904.
To improve the robustness and reliability of wireless transmissions, two complementary link adaptation techniques are employed: adaptive modulation and coding (AMC) at the physical layer and hybrid automatic retransmission request (HARQ) at the medium access control layer. Because of their effectiveness in combating errors induced by the wireless channel, AMC and HARQ are now integral components of most emerging broadband wireless system standards, for example, LTE and WiMAX. Spectral efficiency (SE) as measured in bit per second per Hertz is one important parameter used to characterize a wireless system for comparison between different systems or between different configurations of the same system. This work provides a holistic approach of cross‐layer optimizations with the intent of maximizing SE by combining AMC and HARQ. It formulates closed‐form equations for calculating the average SE for wireless systems with the Rayleigh fading channel model. A new online algorithm is developed to optimize SE for both Rayleigh and non‐Rayleigh fading channel. Simulations using proven LTE model are performed to compare SE obtained from closed‐form equations and the developed algorithm for different system configurations. With the developed algorithm to determine how many retransmissions required in addition to the initial transmission in advance depending on the current wireless channel condition, the latency can be reduced up to 24 ms when sending the initial transmission and all of its retransmissions sooner than waiting for retransmission requests as is done previously. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
905.
906.
王强 《中国电机工程学报》2011,31(27)
为提高逆变器的转换效率,提出一种新型零电压开关谐振直流环节逆变器。通过在传统硬开关逆变器的直流环节添加辅助谐振单元,使直流母线电压周期性地归零,实现逆变桥开关器件在零电压条件下完成切换,而且辅助谐振单元中的开关器件也可以实现零电压开通和零电流关断。此外,辅助谐振单元中的开关器件和谐振电感都没被设置在直流母线上,降低了辅助谐振单元的损耗。对其工作原理进行分析,给出不同工作模式下的等效电路图和回路的参数设计原则。制作一个10kW的实验样机,通过实验结果验证该谐振直流环节逆变器的有效性。 相似文献
907.
908.
齿形链链板级进模设计 总被引:2,自引:2,他引:0
分析了齿形链链板的冲压工艺,介绍了零件排样、模具结构与工作过程及模具关键零件设计。实践证明:模具设计合理,操作简便,生产效率高,能满足零件质量和生产要求。 相似文献
909.
910.
针对无人飞行器大动态运动特点,提出利用地空测控窄带扩频链路的信道估计,辅助OFDM传输方式的下行宽带链路进行信道估计的方法。基于OFDM信道估计基带模型,设计了奇偶OFDM符号分组交错导频插入方式,设计了基于窄带链路多普勒频偏、多径时延信道估计的宽带链路信道估计算法。在大多普勒与多径传输信道下对算法误码率性能进行了仿真,验证了下行宽带链路具有较强的抗大多普勒、抗多径衰落的性能。 相似文献