全文获取类型
收费全文 | 42164篇 |
免费 | 4922篇 |
国内免费 | 3649篇 |
专业分类
电工技术 | 2915篇 |
技术理论 | 15篇 |
综合类 | 5002篇 |
化学工业 | 1442篇 |
金属工艺 | 537篇 |
机械仪表 | 1116篇 |
建筑科学 | 4782篇 |
矿业工程 | 1460篇 |
能源动力 | 843篇 |
轻工业 | 1506篇 |
水利工程 | 2669篇 |
石油天然气 | 1557篇 |
武器工业 | 216篇 |
无线电 | 6370篇 |
一般工业技术 | 1709篇 |
冶金工业 | 799篇 |
原子能技术 | 205篇 |
自动化技术 | 17592篇 |
出版年
2024年 | 204篇 |
2023年 | 582篇 |
2022年 | 1101篇 |
2021年 | 1320篇 |
2020年 | 1389篇 |
2019年 | 1045篇 |
2018年 | 966篇 |
2017年 | 1239篇 |
2016年 | 1343篇 |
2015年 | 1530篇 |
2014年 | 3303篇 |
2013年 | 2687篇 |
2012年 | 3363篇 |
2011年 | 3874篇 |
2010年 | 2916篇 |
2009年 | 3023篇 |
2008年 | 3049篇 |
2007年 | 3450篇 |
2006年 | 2972篇 |
2005年 | 2420篇 |
2004年 | 1997篇 |
2003年 | 1693篇 |
2002年 | 1186篇 |
2001年 | 886篇 |
2000年 | 655篇 |
1999年 | 485篇 |
1998年 | 360篇 |
1997年 | 257篇 |
1996年 | 231篇 |
1995年 | 209篇 |
1994年 | 196篇 |
1993年 | 132篇 |
1992年 | 109篇 |
1991年 | 74篇 |
1990年 | 63篇 |
1989年 | 71篇 |
1988年 | 31篇 |
1987年 | 25篇 |
1986年 | 25篇 |
1985年 | 55篇 |
1984年 | 45篇 |
1983年 | 35篇 |
1982年 | 30篇 |
1981年 | 19篇 |
1980年 | 14篇 |
1979年 | 15篇 |
1978年 | 9篇 |
1977年 | 10篇 |
1975年 | 5篇 |
1955年 | 4篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
31.
32.
Twenty-one 15- to 17-year-olds attempted to purchase cigarettes in 232 stores in the manner that confederates typically do in access studies, as well as in the manipulative ways (e.g., lying about their ages) that youth smokers do, thereby modeling youth access to tobacco within versus outside of studies, respectively. Youth typical-research versus manipulative behavior was contrasted with clerk behavior (requests for youth ID cards) to examine the relative contributions of both to youth access to tobacco for the 1st time. Results revealed that clerk behavior was the strongest predictor of cigarette sales to youth and hence underscore the need for interventions with merchants. Sales nonetheless were higher under youth-manipulative conditions and thereby highlight the low ecological validity of access research. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
33.
一种适合协同工作系统的安全认证方案 总被引:2,自引:0,他引:2
针对网络化协同工作系统的安全认证需求,综合应用Kerberos协议、X.509证书和口令认证技术,借鉴PKI的思想,设计了一种适合网络化协同工作系统的灵活而易于扩展的安全认证方案。这个方案能够保证证书的安全分发,满足网络化协同工作系统对分散、动态的用户安全身份认证的要求。 相似文献
34.
NCFW系统是适合于花岗岩与变质岩地区的勘探地下水专家系统,系统的人机交互既可用菜单(亦称项目单)方式,亦可用汉语对话方式。这两种方式既可混合使用亦能灵活切换。本文着重描述了汉语接口的设计思想、实现中采用关键技术以及系统运行实例。 相似文献
35.
36.
Different NMR techniques were combined to obtain the structure and velocity information for a systematic investigation of fixed beds with low aspect ratio (tube diameter to particle diamter, dt/dp) in the range 1.4 to 32. The structure of the void space was determined for a variety of packed beds of glass beads or regular and irregular porous pellets by magnetic resonance imaging (MRI). Based on the images the radial distribution of the voids within the bed was obtained. Ordering effects were found even for non‐spherical and polydisperse particles, and a maximum of the fluid density near the tube wall was confirmed for all pellet geometries and sizes. By combining MRI with velocity encoding, velocity profiles and distributions of flow velocity components of a single fluid phase through packed beds have been acquired. The radial velocity distribution follows an oscillatory pattern which largely reflects the ordering of the particles, which can be accessed from the density distribution of the interparticle fluid. Maximum velocities of up to four times the average value were found to occur near the tube wall. This wall effect was observed for all but the smallest particles, where the aspect ratio was dt/dp = 32. Moreover, a visualisation of flow pattern in the presence of packed particles was achieved by using a tagging technique, and the stationary flow field could be identified for an experimental time of several hours. 相似文献
37.
This work discusses the issue of approximation in point set matching. In general, one may have two classes of approximations when tackling a matching problem: (1) an algorithmic approximation which consists in using suboptimal procedures to infer the assignment, and (2), a representational approximation which involves a simplified and suboptimal model for the original data. Matching techniques have typically relied on the first approach by retaining the complete model and using suboptimal techniques to solve it. In this paper, we show how a technique based on using exact inference in simple Graphical Models, an instance of the second class, can significantly outperform instances of techniques from the first class. We experimentally compare this method with well-known Spectral and Relaxation methods, which are exemplars of the first class. We have performed experiments with synthetic and real-world data sets which reveal significant performance improvement in a wide operating range. 相似文献
38.
A two‐phase flow CFD model using the volume of fluid (VOF) method is presented for predicting the hydrodynamics of falling film flow on inclined plates, corresponding to the surface texture of structured packing. Using the proposed CFD model the influence of the solid surface microstructure, liquid properties and gas flow rate on the flow behavior was investigated. From the simulated results it was shown that under the condition of no gas flow the liquid flow patterns are dependent on the microstructure of the plates, and proper microstructuring of the solid surface will improve the formation of a continuous liquid film. It was also found that liquid properties, especially surface tension, play an important role in determining the thin‐film pattern. However, there are very different liquid film patterns under the action of gas flow. Thinner liquid films break easily, but thicker liquid films can remain continuous even at higher gas flow rates, which demonstrates that all factors affecting the liquid film thickness will affect the liquid film patterns under conditions of counter‐current two‐phase flow. 相似文献
39.
Rowan Paul J.; Cofta-Woerpel Ludmila; Mazas Carlos A.; Vidrine Jennifer Irvin; Reitzel Lorraine R.; Cinciripini Paul M.; Wetter David W. 《Canadian Metallurgical Quarterly》2007,15(4):382
Ecological momentary assessment (EMA) consists of assessing phenomena in real time in the natural environment. EMA allows for more fine-grained analyses of addictive behavior and minimizes threats to internal validity, such as recall biases and errors. However, because of the intensive monitoring involved in EMA, measurement reactivity is a concern. To test whether EMA with palmtop personal computers induces reactivity, the authors compared smoking-related outcomes between smokers using EMA and those not using EMA during a quit attempt. The use of no-EMA control groups has been rare in reactivity investigations to date. The EMA protocol included event-contingent assessments (smoking episodes, urge episodes) and random assessments. Outcomes included biologically confirmed abstinence and self-report measures of withdrawal, self-efficacy, motivation, affect, and temptations. Participants were smokers motivated to quit (N = 96). They were randomized to 1 of 3 groups: EMA for the week preceding a planned quit date, EMA for the week following the quit date, and no EMA. Abstinence rates did not differ between the groups at Day 7 or at Day 28 postcessation. For the 20 subscales assessed at each of 3 assessment times, there were significant differences between participants with and without EMA experience for 3 subscales at the 1st of 3 assessment times, and significant differences for 3 different subscales at the 3rd assessment time. These differences suggest some reactivity to EMA, although the inconsistent pattern across time indicates that further research is needed to definitively conclude that EMA induces reactivity. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
40.
Failure analysis of hollow glasses The failure analysis is very important to clarify damages of hollow glasses. Damages of glass bottle through over pressure are very often linked with damages to persons. This is the reason why the causal research is from particular importance. Different surface characteristics of glass fragments as well as the fracture pattern give hints to the direction of the crack, the stress just before breakage and the reason of stress. A reconstruction of the bottle is a further helpful instrument for cause studies. The difference of the various characteristics of the surface will be explained. Further on there will be discussed different reasons and mechanism of fracture. 相似文献