全文获取类型
收费全文 | 29748篇 |
免费 | 2702篇 |
国内免费 | 946篇 |
专业分类
电工技术 | 1255篇 |
综合类 | 1602篇 |
化学工业 | 6216篇 |
金属工艺 | 1143篇 |
机械仪表 | 1252篇 |
建筑科学 | 1674篇 |
矿业工程 | 448篇 |
能源动力 | 844篇 |
轻工业 | 4439篇 |
水利工程 | 560篇 |
石油天然气 | 748篇 |
武器工业 | 136篇 |
无线电 | 2516篇 |
一般工业技术 | 3646篇 |
冶金工业 | 1336篇 |
原子能技术 | 257篇 |
自动化技术 | 5324篇 |
出版年
2024年 | 99篇 |
2023年 | 384篇 |
2022年 | 967篇 |
2021年 | 1316篇 |
2020年 | 1007篇 |
2019年 | 896篇 |
2018年 | 996篇 |
2017年 | 1063篇 |
2016年 | 1003篇 |
2015年 | 1308篇 |
2014年 | 1745篇 |
2013年 | 2320篇 |
2012年 | 2458篇 |
2011年 | 2705篇 |
2010年 | 2210篇 |
2009年 | 2162篇 |
2008年 | 1975篇 |
2007年 | 1861篇 |
2006年 | 1486篇 |
2005年 | 1107篇 |
2004年 | 757篇 |
2003年 | 613篇 |
2002年 | 507篇 |
2001年 | 350篇 |
2000年 | 288篇 |
1999年 | 235篇 |
1998年 | 278篇 |
1997年 | 210篇 |
1996年 | 169篇 |
1995年 | 128篇 |
1994年 | 97篇 |
1993年 | 90篇 |
1992年 | 66篇 |
1991年 | 51篇 |
1990年 | 47篇 |
1989年 | 49篇 |
1988年 | 35篇 |
1987年 | 37篇 |
1986年 | 33篇 |
1985年 | 34篇 |
1984年 | 21篇 |
1983年 | 20篇 |
1982年 | 24篇 |
1981年 | 27篇 |
1980年 | 19篇 |
1979年 | 19篇 |
1978年 | 25篇 |
1977年 | 15篇 |
1976年 | 25篇 |
1973年 | 12篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
Algorithm for cosine transform of Toeplitz matrices 总被引:1,自引:0,他引:1
An algorithm for calculating the 2D cosine transform of a Toeplitz matrix is presented. The algorithm is based on the application of 1D cosine transforms. More specifically, four 1D cosine transforms of size N are needed to obtain the transform of a Toeplitz matrix of size N×N. This is an improvement over previously published algorithms. The algorithm is also simple and regular 相似文献
72.
Abstract: Maintainability problems associated with traditional software systems are exacerbated in rule-based systems. The very nature of that approach — separation of control knowledge and data-driven execution — hampers maintenance. While there are widely accepted techniques for maintaining conventional software, the same is not true for rule-based systems. In most situations, both a knowledge engineer and a domain expert are necessary to update the rules of a rule-based system. This paper presents, first, an overview of the software engineering techniques and object-oriented methods used in maintaining rule-based systems. It then discusses alternate paradigms for expert system development. The benefits of using case-based reasoning (from the maintenance point of view) are illustrated through the implementation of a case-based scheduler. The main value of the scheduler is that its knowledge base can be modified by the expert without the assistance of a knowledge engineer. Since changes in application requirements can be given directly to the system by the expert, the effort of maintaining the knowledge base is greatly reduced. 相似文献
73.
74.
Disassembly sequence planning in a disassembly cell context 总被引:2,自引:0,他引:2
In this paper a two-phase approach is proposed for determining the optimal disassembly sequence when the disassembly system has a cellular configuration. Operations are first grouped into cells based on the resources they require with the goal of minimizing machine acquisition costs. The aim is to group together those operations that use similar equipment in order to achieve good utilization levels of such equipment. A maximum cell size may be imposed. Once the cells have been formed and the operations have been assigned to them, a metaheuristic algorithm (namely GRASP) is used to search for the disassembly sequence for each product that leads to the minimum number of intercellular movements. To account for uncertainty regarding the condition in which the product may arrive, each disassembly task is assumed to be required with a certain probability, regardless of the other tasks. AND/OR precedence relations among the disassembly tasks are also considered. The proposed approach is illustrated on a randomly generated disassembly problem. 相似文献
75.
Juan Carlos Benítez Carlos A. Giúdice Vicente J. D. Rascio 《Journal of chemical technology and biotechnology (Oxford, Oxfordshire : 1986)》1985,35(8):387-394
This paper discusses the bioactivity of antifouling paints based on organotin toxicants, such as tributyltin fluoride (TBTF) and triphenyltin fluoride (TPTF). These compounds were used alone or mixed as the main toxicant in formulations. The use of TPTF or mixtures of TPTF and TBTF, with vinyl binders, led to paints with satisfactory bioactivity during a 24 months immersion period. With the same binders, TBTF alone provided only 12 months of antifouling protection. The correlation coefficients calculated from the fouling attachment values and the rosin content in the binder showed that the bioactivity of the tested formulations depended on the solubility of the toxicant in sea water and on the composition and solubility of the binders. 相似文献
76.
Transport problems typically involve at least two types of constraints, on income and on time. Therefore, the indirect utility
function depends either on the income available after having subtracted the cost of the discrete alternative and on the free
time left after having worked and travelled by each competing option. In the typical linear-in-the-attributes and in-the-parameters
specification, that represents the first grade approximation of the indirect utility function, the effect of income and time
constraints cancel out and only the cost and time of the alternatives matter in the comparison between them. From a microeconomic
point of view this is equivalent to assume that income and time effects could be disregarded; which is not always the case.
To account for these effects the utility function should include second order attributes; however, in non-linear utility functions
it may not be easy to distinguish among several effects that could be relevant: direct preferences for good and leisure, and
simple interactions between attributes other than income and time effects. This paper analyses these effects from a theoretical
point of view focusing on the possible confounding problem in detecting income and time effects. We use a dataset collected
for a modal choice context and containing both revealed and stated preference data, and estimate several NL models examining
the effect of the different second-order terms on detecting income and time effects. We compared specifications including
square cost and time attributes, interactions between time and cost, cost divided by the income available to be spent on free
time, and time multiplied by free time. Our results confirm the strong effect of direct preferences for goods and leisure
time on choice, and the potential confounding effect between quadratic attributes and other non-linear omitted terms. Finally,
we also found that care should be taken in highlighting income and time effects using mixed data sources, since confounding
effects can occur when non-linearities are accounted for in both data sets. 相似文献
77.
Eva M. Valero Juan L. Nieves Sérgio M. C. Nascimento Kinjiro Amano David H. Foster 《Color research and application》2007,32(5):352-360
Many spectral‐recovery methods using RGB digital cameras assume the underlying smoothness of illuminant and reflectance spectra, and apply low‐dimensional linear models. The aim of the present work was to test whether a direct‐mapping method could be used instead of a linear‐models approach to recover spectral radiances and reflectances from natural scenes with an RGB digital camera and colored filters. In computer simulations, a conventional RGB digital camera with up to three colored filters was used to image scenes drawn from a hyperspectral image database. Three measures were used to evaluate recovery with the direct‐mapping method: goodness‐of‐fit, root‐mean‐square error, and a color‐difference metric. It was found that with two and three filters both spectral radiances and reflectances could be recovered sufficiently accurately for many practical applications. With little increase in computational complexity, an RGB camera and a few colored filters can provide significantly better recovery of natural scenes than an RGB camera alone. © 2007 Wiley Periodicals, Inc. Col Res Appl, 32, 352–360, 2007 相似文献
78.
本文指出了中餐企业形象建设存在的问题和不足,提出中餐企业可通过树立科学的发展观和正确的经营观念,积极导入CI系统建立个性化的鲜明的企业形象,重点论述了理念识别、行为识别、视觉识别、听觉识别、味觉识别等中餐企业特有的五大识别系统的策划、设计的原则和有效途径。 相似文献
79.
Michael Fox Richard Hastings Scott Lovald Juan Heinrich 《Journal of Failure Analysis and Prevention》2007,7(3):165-174
A failure analysis case study is presented for a two-piece aerosol containing tetrafluoroethane, commonly referred to as Refrigerant
134a. A gentleman was preparing to recharge the air conditioning system of an automobile when the bottom exploded off the
aerosol container, propelling the body of the aerosol container like a rocket, which hit the man in the eye and blinded him
in that eye. The aerosol was never connected to the air conditioner, therefore backpressure from the air conditioner (AC)
compressor was ruled out as a cause for the explosion. The objective of the study was to determine why the aerosol exploded.
Several recently developed test methods were used, including two types of heat-to-burst tests and a puncture chamber to measure
the pressure-versus-temperature behavior of aerosols. More common test methods were also used, such as water bath pressure
tests, hydro pressure burst tests, pneumatic pressure burst tests, hardness measurements, weight measurements, metallography,
scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and an accident scenario recreation. A semi-empirical
correlation between the hardness and weights of the container bottoms was used to determine the explosion temperature and/or
pressure. This semi-empirical correlation agrees in principle with an analysis of the explosion pressures using finite-element
analysis (FEA). The root cause for the explosion was determined to be a lack of strength of the bottom of the two-piece aerosol
coupled with heating the aerosol to temperatures significantly above room temperature. 相似文献
80.