全文获取类型
收费全文 | 335700篇 |
免费 | 24312篇 |
国内免费 | 12683篇 |
专业分类
电工技术 | 18156篇 |
技术理论 | 56篇 |
综合类 | 19854篇 |
化学工业 | 56896篇 |
金属工艺 | 18768篇 |
机械仪表 | 21370篇 |
建筑科学 | 25954篇 |
矿业工程 | 11072篇 |
能源动力 | 9817篇 |
轻工业 | 18821篇 |
水利工程 | 5307篇 |
石油天然气 | 23342篇 |
武器工业 | 2619篇 |
无线电 | 36867篇 |
一般工业技术 | 40023篇 |
冶金工业 | 18493篇 |
原子能技术 | 3238篇 |
自动化技术 | 42042篇 |
出版年
2024年 | 1305篇 |
2023年 | 5258篇 |
2022年 | 8967篇 |
2021年 | 12693篇 |
2020年 | 9940篇 |
2019年 | 8078篇 |
2018年 | 9291篇 |
2017年 | 10474篇 |
2016年 | 9362篇 |
2015年 | 12353篇 |
2014年 | 16018篇 |
2013年 | 19323篇 |
2012年 | 20335篇 |
2011年 | 22604篇 |
2010年 | 19367篇 |
2009年 | 18487篇 |
2008年 | 18155篇 |
2007年 | 17551篇 |
2006年 | 18432篇 |
2005年 | 16269篇 |
2004年 | 10441篇 |
2003年 | 9203篇 |
2002年 | 8203篇 |
2001年 | 7542篇 |
2000年 | 8065篇 |
1999年 | 9716篇 |
1998年 | 8192篇 |
1997年 | 6801篇 |
1996年 | 6394篇 |
1995年 | 5371篇 |
1994年 | 4396篇 |
1993年 | 3104篇 |
1992年 | 2500篇 |
1991年 | 1969篇 |
1990年 | 1514篇 |
1989年 | 1234篇 |
1988年 | 1022篇 |
1987年 | 684篇 |
1986年 | 538篇 |
1985年 | 343篇 |
1984年 | 244篇 |
1983年 | 205篇 |
1982年 | 196篇 |
1981年 | 129篇 |
1980年 | 131篇 |
1979年 | 67篇 |
1978年 | 33篇 |
1977年 | 38篇 |
1976年 | 55篇 |
1975年 | 20篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
81.
This is a study of the differences in the risk factors for being either hepatitis B surface antigen positive [HBsAg(+)] or antibody to hepatitis C virus positive [Anti-HCV(+)] in A-Lein, a rural area in southern Taiwan, an area which also has a high hepatoma mortality rate. Three hundred eighty-five patients age > or =40 years participated in hepatoma screening at the A-Lein Community Health Center during 1995. Those who were HBsAg(-) and anti-HCV(-) or had coinfection of HBsAg(+) and anti-HCV(+) were excluded, leaving 293 patients: 109 HBsAg(+) and 184 anti-HCV(+). The anti-HCV(+) patients had a lower socioeconomic status (as defined by level of education and type of occupation) and were older than HBsAg(+) patients (P < 0.05). Those with higher alanine aminotransferase levels (ALT) also had a higher anti-HCV(+) to HBsAg(+) odds ratio (OR), and a dose response relationship was found, P < 0.0001. Anti-HCV(+) patients were more likely than HBsAg(+) patients to have a spouse who shared the infection, OR = 5.11; 95% CI, 2.30-11.28. Anti-HCV(+) patients were more likely than HBsAg(+) patients to have had blood transfusions (OR = 2.66; 95% CI, 1.20-5.89), frequent medical injections (OR = 2.64; 95% CI, 1.62-4.31), or injections by non-licensed medical providers (OR = 1.91; 95% CI, 1.18-3.09). Multiple logistic regression analysis showed that the significant factors for anti-HCV(+) patients vs. HBsAg(+) patients are drinking habit (OR = 3.45; 95% CI, 1.02-11.60), age (OR = 6.33; 95% CI, 2.93-13.68), and frequent medical injections (OR = 2.88; 95% CI, 1.65-5.03). The transmission of hepatitis C in A-Lein is closely related to low socioeconomic status, age, alcohol abuse, spouses being anti-HCV(+), and frequent medical injections, especially from non-licensed medical providers, including both pharmacists and those with no medical licensing whatsoever. These nonlicensed medical providers sometimes reuse needles to save money, which is a likely route of infection. 相似文献
82.
83.
Transient hot-electron effect and its impact on circuit reliability are investigated. The rate of device decay is monitored as a function of the gate pulse transient period. Simulation results reveal that excess charges during a fast turn off time may cause an increase in the maximum substrate current. This, along with our experimental data, identifies that transient excess carrier may cause the enhancement of device degradation under certain stress conditions. The enhancement factor of the degradation is a function of the gate pulse transient time. Correlation between the analysis based upon AC/DC measurement and calculations based upon transient simulation are shown in the paper. Better agreement with experimental data is obtained by using the transient analysis and on chip test/stress structures. The correlation between AC and DC stress data is also shown based on the impact ionization model. A hot-electron design guideline is proposed based on the circuit reliability analysis. This guideline can help improve the circuit reliability without adversely effecting the circuit performance. 相似文献
84.
The relationship between the distance properties of trellis codes and the computational effort and error performance of sequential decoding is studied and optimum distance profile (ODP) and optimum free distance (OFD) trellis codes are constructed for 8-PSK and 16 QAM modulation. A comparison of the performance of both the ODP and the OFD trellis codes reveals that neither class of codes results in the best trade-off between error performance and computational effort when sequential decoding is used. A new algorithm is then proposed to construct robustly good trellis codes for use with sequential decoding. New trellis codes with asymptotic coding gains up to 6.66 dB are obtained using this algorithm, and the new codes achieve nearly the same free distances as the OFD codes and nearly the same distance profiles as the ODP codes 相似文献
85.
介绍了液体通过核孔膜规律方面取得的新认识,包括:(1)纯净液体通过核孔膜的规律;(2)核孔膜测定液体粘滞系数的各种方法;(3)各种物质溶液浓度的核孔膜测定;(4)液相混合物快速分离和化学分离;(5)流体中固体微粒对核孔膜的堵塞及其公式;(6)用核孔膜滤除液中各各斩规律。 相似文献
86.
87.
在讨论MPT1327信令的集群移动通信系统与国内No.7信令的PSTN网间互连,帝现两种信令配合中,研究了集群系统的仿真模型和模拟算法,同时模拟出整个集群系统的工作情况,运行结果证实了所提方案的可行性与准确性。 相似文献
88.
本文着重将目前国外比较活跃的几种新型反垣克武器;软杀伤反坦克弹药,末敏弹,反坦克地雷以及智能反坦克子弹药的基本原理,作战效能,装备情况作一介绍,并对其发展方向进行了预测。 相似文献
89.
用膨胀法对28Cr2Mo1VA钢测定了变速冷却过冷奥氏体转变动力学曲线,结果表明,先慢冷后快冷,珠光体、贝氏体转变滞后;反之,转变超前。并提出用图解计算法求转变点,结果与实验相符合。 相似文献
90.
Yi-Min Wang Hua Lee Dipali V. Apte 《International journal of imaging systems and technology》1992,4(3):201-206
The matrix pencil (MP) method, based on the singular value decomposition (SVD), is applied to quantitative NMR spectroscopy. Its relationship to other SVD-based methods is also presented. Simulations and applications are given to demonstrate the capability of superior performance.©1993 John Wiley & Sons Inc 相似文献