全文获取类型
收费全文 | 397篇 |
免费 | 12篇 |
学科分类
工业技术 | 409篇 |
出版年
2023年 | 3篇 |
2022年 | 4篇 |
2021年 | 5篇 |
2020年 | 3篇 |
2019年 | 4篇 |
2018年 | 11篇 |
2017年 | 6篇 |
2016年 | 7篇 |
2015年 | 9篇 |
2014年 | 12篇 |
2013年 | 26篇 |
2012年 | 21篇 |
2011年 | 18篇 |
2010年 | 13篇 |
2009年 | 17篇 |
2008年 | 25篇 |
2007年 | 20篇 |
2006年 | 25篇 |
2005年 | 14篇 |
2004年 | 17篇 |
2003年 | 17篇 |
2002年 | 9篇 |
2001年 | 10篇 |
2000年 | 5篇 |
1999年 | 8篇 |
1998年 | 12篇 |
1997年 | 15篇 |
1996年 | 6篇 |
1995年 | 4篇 |
1994年 | 8篇 |
1993年 | 5篇 |
1992年 | 2篇 |
1990年 | 2篇 |
1989年 | 2篇 |
1988年 | 2篇 |
1987年 | 2篇 |
1986年 | 3篇 |
1984年 | 2篇 |
1982年 | 7篇 |
1981年 | 4篇 |
1980年 | 3篇 |
1979年 | 2篇 |
1978年 | 2篇 |
1976年 | 2篇 |
1975年 | 4篇 |
1974年 | 1篇 |
1973年 | 3篇 |
1971年 | 1篇 |
1969年 | 1篇 |
1939年 | 1篇 |
排序方式: 共有409条查询结果,搜索用时 15 毫秒
1.
Free-space optical communication between satellites networked together can enable a high data rate between the satellites. Coherence multiplexing (CM) is an attractive technique for satellite networking due to its ability to cope with the asynchronous nature of communication traffic and the dynamic changes taking place in the satellite constellation. The use of optical radiation for intersatellite links creates very narrow beam divergence angles. Due to the narrow beam divergence angle, the vibration of the pointing system, the movement of the satellite, and the large distance between them the pointing from one satellite to another is a complicated task. The vibration of the pointing system is caused by two stochastic fundamental mechanisms: 1) tracking noises created by the electrooptic tracker and 2) vibrations created by internal satellite mechanical mechanisms and external environments. We derive mathematical models of signal, noise, the approximate signal-to-noise ratio, and the approximate bit-error rates of optical communication satellite networks as functions of the system's parameters, the number of satellites, and the vibration amplitude for frequency-shift keying coherence multiplexing (FSK-CM). Based on these models, we can calculate the negative impact of both the number of satellites and the optical terminal vibration on the system's performance. 相似文献
2.
Francesco Sarti Yaacov ArkinJean Chorowicz Arnon KarnieliTeresa Cunha 《Remote sensing of environment》2003,86(2):141-149
Differential radar interferometry, using archived ERS data over the region of the Dead Sea Transform, allows to detect ground movement (subsidence or uplift) in playas within the southern Arava Valley segment of the Dead Sea Rift. These measurements are consistent with a mean displacement rate of about 0.4 cm/month, in the direction of the radar beam, during the 8-month period preceding the Nuweiba earthquake of 22 November 1995. In the 3 years following the earthquake, the measured rate was smaller by a factor of 10. These movements are not related to salt diapirs or water pumping activities in the area. The exact location, along faults, suggests a possible correlation with pre-seismic and post-seismic fault deformation.A simple fault model consistent with the observed phenomena associates the observed subsidence/uplift to right and left stepping en-echelon fault patterns related to inter-seismic tensional accumulation along the faults. Further observations are necessary on this site and similar fault areas to corroborate the correlation between seismic activity and the observed phenomena. Monitoring of these sites should continue with differential Global positioning system (GPS) measurements and radar interferometric analysis using Envisat and Radarsat as well as archived data (including J-ERS). 相似文献
3.
In the last decade there was growing interest in strategic management literature about factors that influence a company's ability to use IT. There is general consensus that knowledge and competency are necessary in developing an IT capability, but there is very little understanding of what the necessary competencies are, and how they influence IS usage in different contexts. The small and medium-sized enterprise context is particularly interesting for two reasons: it constitutes a major part of the economy and it has been relatively unsuccessful in exploiting e-business. 相似文献
4.
Persistence has been identified as a crucial quality of learning. However, it is hard to attain in online game-based environments as the drive to progress in the game may influence the ability to achieve the learning goals. This study aimed to examine the associations between micro-persistence, that is, the tendency to complete an individual task successfully, and task difficulty while acquiring computational thinking (CT). We further explored whether contextual or personal attributes better explain micro-persistence. We analysed data of 111 school students who used the CodeMonkey platform. We took a learning analytics approach for analysing the platform's log files. We found that micro-persistence is associated with task difficulty and that students who demonstrated an aptitude to learn new material are motivated to achieve the best solution. We also found that contextual variables better-explained micro-persistence than personal attributes. Encouraging micro-persistence can improve CT acquisition and the learning processes involved. 相似文献
5.
Software Product Line Engineering (SPLE) deals with developing artifacts that capture the common and variable aspects of software product families. Domain models are one kind of such artifacts. Being developed in early stages, domain models need to specify commonality and variability and guide the reuse of the artifacts in particular software products. Although different modeling methods have been proposed to manage and support these activities, the assessment of these methods is still in an inceptive stage. In this work, we examined the comprehensibility of domain models specified in ADOM, a UML-based SPLE method. In particular, we conducted a controlled experiment in which 116 undergraduate students were required to answer comprehension questions regarding a domain model that was equipped with explicit reuse guidance and/or variability specification. We found that explicit specification of reuse guidance within the domain model helped understand the model, whereas explicit specification of variability increased comprehensibility only to a limited extent. Explicit specification of both reuse guidance and variability often provided intermediate results, namely, results that were better than specification of variability without reuse guidance, but worse than specification of reuse guidance without variability. All these results were perceived in different UML diagram types, namely, use case, class, and sequence diagrams and for different commonality-, variability-, and reuse-related aspects. 相似文献
6.
Arnon Shani 《Journal of chemical ecology》1990,16(3):971-980
A cotton wool plug, used as the source for pheromone release, was placed in closed 1-quart Mason jars, either at the mouth or at the rear of the jar. Air sampling of the two components of the sex pheromone (total 2.2 mg at the source) of the almond moth (Ephestia cautella) female showed that the saturation period near the source in still air was 20–22 hr and that far from the source was 40–50 hr, reaching a level of less than 1 ng/ml air. The ratio between the components (Z,E)-9,12-tetradecadienyl acetate, designated D, and (Z)-9-tetradecenyl acetate designated M, in the air was close to the original ratio for both sampling sites, albeit somewhat richer in the more volatile (Z)-9-tetradecenyl acetate (source 77.023.0 DM, air 73.0– 74.327.0–25.7; source 80.119.9, air 77.622.4; source 25.174.9, air 23.976. 1 DM). The total amount of pheromone per milliliter of air was two to three times larger near the source than far from it at the early stages of the evaporation and saturation process. When the amount of pheromone applied to the source was tripled (7 mg), the amount far from the source was almost tripled, or the saturation time was cut by factor of two to three. 相似文献
7.
Kirk Weller Ilana Arnon Ed Dubinsky 《Canadian Journal of Science, Mathematics, & Technology Education》2013,13(2):129-159
Abstract In an earlier study of preservice elementary and middle school teachers’ beliefs about repeating decimals, Weller, Arnon, and Dubinsky (2009) reported on a comparison of the mathematical performance of 77 preservice teachers who completed an APOS-based instructional unit with 127 preservice teachers who completed traditional instruction. The purpose of the current study, based on 47 interviews conducted 4 months after the instruction, during which time there was no further instruction on this topic, is to determine the strength and stability (over time) of the students’ beliefs, to uncover thinking that did not arise in the earlier study, and to see whether the interviews yield similar comparative results. The interviews did uncover changes in student thinking. The students who received the APOS-based instruction developed stronger and more stable (over time) beliefs that a repeating decimal is a number; a repeating decimal has a fraction or integer to which it corresponds; a repeating decimal in general equals its corresponding fraction or integer; and, in particular, In addition, a number of indices and categories were developed that may prove useful in other studies consisting of the comparison of interview and questionnaire data involving a large number of interview subjects. 相似文献
8.
Performance limitations of a free-space optical communication satellite network owing to vibrations: heterodyne detection 总被引:1,自引:0,他引:1
Free-space optical communication between satellites in a distributed network can permit high data rates of communication between different places on Earth. To establish optical communication between any two satellites requires that the line of sight of their optics be aligned during the entire communication time. Because of the large distance between the satellites and the alignment accuracy required, the pointing from one satellite to another is complicated because of vibrations of the pointing system caused by two fundamental stochastic mechanisms: tracking noise created by the electro-optic tracker and vibrations derived from mechanical components. Vibration of the transmitter beam in the receiver plane causes a decrease in the received optical power. Vibrations of the receiver telescope relative to the received beam decrease the heterodyne mixing efficiency. These two factors increase the bit-error rate of a coherent detection network. We derive simple mathematical models of the network bit-error rate versus the system parameters and the transmitter and receiver vibration statistics. An example of a practical optical heterodyne free-space satellite optical communication network is presented. From this research it is clear that even low-amplitude vibration of the satellite-pointing systems dramatically decreases network performance. 相似文献
9.
Björn Regnell Martin Höst Johan Natt och Dag Per Beremark Thomas Hjelm 《Requirements Engineering》2001,6(1):51-62
When developing packaged software, which is sold ‘off-the-shelf’ on a worldwide marketplace, it is essential to collect needs
and opportunities from different market segments and use this information in the prioritisation of requirements for the next
software release. This paper presents an industrial case study where a distributed prioritisation process is proposed, observed
and evaluated. The stakeholders in the requirements prioritisation process include marketing offices distributed around the
world. A major objective of the distributed prioritisation is to gather and highlight the differences and similarities in
the requirement priorities of the different market segments. The evaluation through questionnaires shows that the stakeholders
found the process useful. The paper also presents novel approaches to visualise the priority distribution among stakeholders,
together with measures on disagreement and satisfaction. Product management found the proposed charts valuable as decision
support when selecting requirements for the next release, as they revealed unforeseen differences among stakeholder priorities.
Conclusions on stakeholder tactics are provided and issues of further research are identified, including ways of addressing
identified challenges. 相似文献
10.
A new formalism for predicate logic is introduced, with a non-standard method of binding variables, which allows a compositional formalization of certain anaphoric constructions, including donkey sentences and cross-sentential anaphora. A proof system in natural deduction format is provided, and the formalism is compared with other accounts of this type of anaphora, in particular Dynamic Predicate Logic.Both authors have been supported by grants from the Swedish Council for Research in the Humanities and Social Sciences. 相似文献