全文获取类型
收费全文 | 48983篇 |
免费 | 4999篇 |
国内免费 | 3718篇 |
专业分类
电工技术 | 1822篇 |
综合类 | 4925篇 |
化学工业 | 6620篇 |
金属工艺 | 5718篇 |
机械仪表 | 3829篇 |
建筑科学 | 6650篇 |
矿业工程 | 2914篇 |
能源动力 | 1378篇 |
轻工业 | 2892篇 |
水利工程 | 2142篇 |
石油天然气 | 3220篇 |
武器工业 | 435篇 |
无线电 | 1894篇 |
一般工业技术 | 7592篇 |
冶金工业 | 3072篇 |
原子能技术 | 468篇 |
自动化技术 | 2129篇 |
出版年
2024年 | 234篇 |
2023年 | 751篇 |
2022年 | 2166篇 |
2021年 | 2157篇 |
2020年 | 1677篇 |
2019年 | 1506篇 |
2018年 | 1377篇 |
2017年 | 1703篇 |
2016年 | 1834篇 |
2015年 | 1858篇 |
2014年 | 2647篇 |
2013年 | 3024篇 |
2012年 | 3208篇 |
2011年 | 3507篇 |
2010年 | 2636篇 |
2009年 | 2754篇 |
2008年 | 2568篇 |
2007年 | 3104篇 |
2006年 | 2744篇 |
2005年 | 2347篇 |
2004年 | 2066篇 |
2003年 | 1757篇 |
2002年 | 1574篇 |
2001年 | 1297篇 |
2000年 | 1206篇 |
1999年 | 983篇 |
1998年 | 817篇 |
1997年 | 743篇 |
1996年 | 624篇 |
1995年 | 512篇 |
1994年 | 436篇 |
1993年 | 329篇 |
1992年 | 313篇 |
1991年 | 222篇 |
1990年 | 195篇 |
1989年 | 199篇 |
1988年 | 128篇 |
1987年 | 89篇 |
1986年 | 58篇 |
1985年 | 66篇 |
1984年 | 64篇 |
1983年 | 38篇 |
1982年 | 50篇 |
1981年 | 12篇 |
1980年 | 23篇 |
1979年 | 24篇 |
1964年 | 10篇 |
1961年 | 6篇 |
1959年 | 10篇 |
1955年 | 11篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
2.
Recommendation systems aim to recommend items or packages of items that are likely to be of interest to users. Previous work on recommendation systems has mostly focused on recommending points of interest (POI), to identify and suggest top-k items or packages that meet selection criteria and satisfy compatibility constraints on items in a package, where the (packages of) items are ranked by their usefulness to the users. As opposed to prior work, this paper investigates two issues beyond POI recommendation that are also important to recommendation systems. When there exist no sufficiently many POI that can be recommended, we propose (1) query relaxation recommendation to help users revise their selection criteria, or (2) adjustment recommendation to guide recommendation systems to modify their item collections, such that the users׳ requirements can be satisfied.We study two related problems, to decide (1) whether the query expressing the selection criteria can be relaxed to a limited extent, and (2) whether we can update a bounded number of items, such that the users can get desired recommendations. We establish the upper and lower bounds of these problems, all matching, for both combined and data complexity, when selection criteria and compatibility constraints are expressed in a variety of query languages, for both item recommendation and package recommendation. To understand where the complexity comes from, we also study the impact of variable sizes of packages, compatibility constraints and selection criteria on the analyses of these problems. Our results indicate that in most cases the complexity bounds of query relaxation and adjustment recommendation are comparable to their counterparts of the basic recommendation problem for testing whether a given set of (resp. packages of) items makes top-k items (resp. packages). In other words, extending recommendation systems with the query relaxation and adjustment recommendation functionalities typically does not incur extra overhead. 相似文献
3.
Temperature history can have a significant effect on the strength of water-saturated chalk.In this study,hydrostatic stress cycles are applied to understand the mechanical response of chalk samples exposed to temperature cycling between each stress cycle,compared to the samples tested at a constant temperature.The total accumulated strain during a stress cycle and the irreversible strain are reported.Chalk samples from Kansas(USA)and Mons(Belgium),with different degrees of induration(i.e.amount of contact cementation),were used.The samples were saturated with equilibrated water(polar)and nonpolar Isopar H oil to quantify water weakening.All samples tested during 10 stress cycles with varying temperature(i.e.temperature cycled in between each stress cycle)accumulated more strain than those tested at constant temperatures.All the stress cycles were performed at 30℃.The two chalk types behaved similarly when saturated with Isopar H oil,but differently when saturated with water.When saturated with water,the stronger Kansas chalk accumulated more total strain and more irreversible strain within each stress cycle than the weaker Mons chalk. 相似文献
4.
A new constitutive equation of thermoelasticity for crystals is presented based on the interatomic potential and solid mechanics at finite temperature. Using the new constitutive equation, the calculations for crystal copper and graphene are carried out under different loading paths at different temperatures. The calculated results are in good agreement with those of the previous thermoelasticity constitutive equation based on quantum mechanics, which clearly indicates that our new constitutive equation of thermoelasticity is correct. A lot of comparisons also show that the present theory is more concise and efficient than the previous thermal stress theory in the practical application. 相似文献
5.
Role stress has received a lot of research attention in psychological, sociological, and organizational studies over the last several decades. Based on a literature review of about 300 journal articles, this article examines prominent consequences of role stress. Specific focus is on researching differences in relationships between facets of role stress (i.e., role ambiguity, role conflict, and role overload) and frequently cited consequences using techniques of meta-analysis. Findings indicate that each role stress facet has a different relationship with the eight consequences studied. Role stress research can benefit from looking at each facet individually in addition to role stress generally. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
6.
Rheological properties of MR fluids under large step strain shear are presented in this paper. The experiments were carried out using a rheometer with parallel-plate geometry. Under the large step strain shear, MR fluids behave as nonlinear viscoelastic properties, where the stress relaxation modulus, G(t, γ), shows a decreasing trend with step strain. The experimental results indicate that G(t, γ) obeys time-strain separability. Thus, a mathematical form based on finite exponential serials is proposed to predict MR behavior. In this model, G(t, γ) is represented as the product of a linear stress relaxation, G(t), and the damping function, h(γ), i.e. G(t, γ)=G(t) h(γ). G(t) is simply represented as a three-parameter exponential serial and h(γ) has a sigmoidal form with two parameters. The parameters are identified by adopting an efficient optimization method proposed by Stango et al. The comparison between the experimental results and the model-predicted values indicates that this mathematical model can accurately predict MR behavior. 相似文献
7.
The tensile elastic modulus (E), yield stress (σY) and microhardness (MH) of neat and binary and ternary blends of glassy semicrystalline ethylene–vinyl alcohol copolymer (EVOH), a glassy amorphous polyamide and a semicrystalline nylon‐containing ionomer covering a broad range of properties were examined. The tests were carried out on dry and water‐equilibrated samples to produce stiffer and softer materials, respectively. From the results, more accurate linear correlations were found to describe adequately the microhardness, modulus and yield stress of these strongly self‐associated polymers through hydrogen bonding. Copyright © 2003 Society of Chemical Industry 相似文献
8.
9.
Kruk Menno R.; Halász József; Meelis Wout; Haller József 《Canadian Metallurgical Quarterly》2004,118(5):1062
Aggressive behavior induces an adrenocortical stress response, and sudden stressors often precipitate violent behavior. Experiments in rats revealed a fast, mutual, positive feedback between the adrenocortical stress response and a brain mechanism controlling aggression. Stimulation of the aggressive area in the hypothalamus rapidly activated the adrenocortical response, even in the absence of an opponent and fighting. Hypothalamic aggression, in turn, was rapidly facilitated by a corticosterone injection in rats in which the natural adrenocortical stress response was prevented by adrenalectomy. The rapidity of both effects points to a fast, mutual, positive feedback of the controlling mechanisms within the time frame of a single conflict. Such a mutual facilitation may contribute to the precipitation and escalation of violent behavior under stressful conditions. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
10.