全文获取类型
收费全文 | 36378篇 |
免费 | 1303篇 |
国内免费 | 63篇 |
专业分类
电工技术 | 371篇 |
综合类 | 29篇 |
化学工业 | 7250篇 |
金属工艺 | 725篇 |
机械仪表 | 739篇 |
建筑科学 | 1964篇 |
矿业工程 | 114篇 |
能源动力 | 1054篇 |
轻工业 | 2909篇 |
水利工程 | 430篇 |
石油天然气 | 118篇 |
武器工业 | 5篇 |
无线电 | 2475篇 |
一般工业技术 | 6118篇 |
冶金工业 | 6601篇 |
原子能技术 | 268篇 |
自动化技术 | 6574篇 |
出版年
2023年 | 214篇 |
2022年 | 440篇 |
2021年 | 683篇 |
2020年 | 465篇 |
2019年 | 617篇 |
2018年 | 781篇 |
2017年 | 695篇 |
2016年 | 835篇 |
2015年 | 759篇 |
2014年 | 1042篇 |
2013年 | 2375篇 |
2012年 | 1680篇 |
2011年 | 2097篇 |
2010年 | 1660篇 |
2009年 | 1549篇 |
2008年 | 1803篇 |
2007年 | 1775篇 |
2006年 | 1592篇 |
2005年 | 1441篇 |
2004年 | 1175篇 |
2003年 | 1127篇 |
2002年 | 1052篇 |
2001年 | 703篇 |
2000年 | 549篇 |
1999年 | 595篇 |
1998年 | 584篇 |
1997年 | 576篇 |
1996年 | 550篇 |
1995年 | 573篇 |
1994年 | 525篇 |
1993年 | 510篇 |
1992年 | 500篇 |
1991年 | 288篇 |
1990年 | 418篇 |
1989年 | 389篇 |
1988年 | 319篇 |
1987年 | 355篇 |
1986年 | 310篇 |
1985年 | 418篇 |
1984年 | 416篇 |
1983年 | 318篇 |
1982年 | 295篇 |
1981年 | 281篇 |
1980年 | 269篇 |
1979年 | 270篇 |
1978年 | 247篇 |
1977年 | 225篇 |
1976年 | 206篇 |
1975年 | 194篇 |
1974年 | 173篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
21.
This investigation tested the hypothesis that social comparison processes mediate the relation between ability-grouping practices in mathematics and students' achievement expectancies in a district-wide sample of 6th graders (N?=?452). Compared with between-classroom ability grouping, within-classroom grouping raises high achievers' achievement expectancies, math grades, and tendency to make downward comparisons (i.e., with a classmate who is worse at math). Within-classroom grouping lowers low achievers' expectancies and math grades and increases their tendency to make upward comparisons. When controls for the direction of students' social comparison choices and for their mathematics grades are introduced, the independent effect of ability grouping on achievement expectancies is consistently and substantially reduced. It is argued that ability-grouping practices constrain the choices available to students and teachers for social comparison of abilities and thereby influence the frame of reference students use for self-assessment and teachers use for assigning grades. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
22.
Why was there a preponderance of women engaged in laboratory-based studies of animal behavior in Britain in the early years of the 20th century? As a new experimental subject with recently neglected potential, animal psychology then represented an opportunity for women to make further inroads into academic science. Because the study of psychology before World War I offered negligible professional opportunities for the application of the subject, the intake of students was restricted. Perhaps this encouraged academic access to it by aspiring female scientists who felt that career prospects, uncertain and socially unexpected of them in any case, were worth chancing for greater long-term rewards. The academic circumstances and contributions of 3 British female pioneers in experimental comparative psychology are discussed. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
23.
A fully-dense Cu-75 vol pct ZrW2O8 metal matrix composite was fabricated by hot isostatic pressing of Cu-coated ZrW2O8 particles. A small amount of the high-pressure γ-ZrW2O8 phase was created during the cooldown and depressurization following densification; near complete transformation to γ-ZrW2O8 was achieved by subsequent cold isostatic pressing. The thermal expansion behavior of the composite between 25°C and 325°C
was altered by the cold isostatic pressing treatment, and also depended on the length of time that had passed between thermal
cycles. The measured thermal expansion coefficients within specific temperature ranges varied from −6·10−6 K−1 to far above the thermal expansion coefficient of the copper matrix. The complex temperature-dependent expansion/contraction
behavior could be justified by considering the evolution of phase transformations taking place in the ZrW2O8 phase, which were observed by in-situ synchrotron X-ray diffraction measurements. 相似文献
24.
In this paper we design an experiment which can be depicted as a simple scenario, a very limited 'world'. In this world, there are an actor that can pursue a project and an observer that is keeping its eyes on the actor. We try to show in the experiment that the observer can to some degree understand the actor based on its knowledge and some metaphors, i.e. understand what the actor is doing and why. As the conclusion of this experiment, we try to show some features of 'understanding'. These are (1) that 'understanding' has to be based on some preliminary knowledge; (2) that 'understanding' is a process of incremental learning; (3) that, as for symbolic systems, some metaphors are necessary for mapping real entities into concepts in mind. 相似文献
25.
David L. Hoover 《Computers and the Humanities》2003,37(2):151-178
This article examines the usefulness ofvocabulary richness for authorship attributionand tests the assumption that appropriatemeasures of vocabulary richness can capture anauthor's distinctive style or identity. Afterbriefly discussing perceived and actualvocabulary richness, I show that doubling andcombining texts affects some measures incomputationally predictable but conceptuallysurprising ways. I discuss some theoretical andempirical problems with some measures anddevelop simple methods to test how wellvocabulary richness distinguishes texts bydifferent authors. These methods show thatvocabulary richness is ineffective for largegroups of texts because of the extremevariability within and among them. I concludethat vocabulary richness is of marginal valuein stylistic and authorship studies because thebasic assumption that it constitutes awordprint for authors is false. 相似文献
26.
The new Editor in Chief of IEEE Micro introduces himself and the first issue of 2007. He thanks outgoing Editor in Chief Pradip Bose for his outstanding work on Micro during his tenure. He assesses the current state of the microarchitecture field, speculates on the future, and asks readers for their suggestions on topics the magazine should cover in coming issues. 相似文献
27.
This paper investigates the effect of chelating agents on the bioavailability of Fe and Cu during anaerobic digestion. The results on metal speciation and methane production in anaerobic serum bottles showed that biomass was able to grow in the presence of citrate 1?mM and nitrilotriacetic acid (NTA) 1?mM, suggesting that the binding sites at the cell surface competed efficiently for the metals with the chelating agents added. The presence of free ethylenediaminetetraacetic acid 1?mM inhibited methanogenesis, and this seemed to be related to a loss in metal uptake capacity. Although the addition of soluble microbial products (SMP) did not change metal distribution in anaerobic systems, it caused an increase in the rate of methane production, and it is believed that direct uptake of Cu-SMP complexes was responsible for this increase. The best protection against Cu toxicity occurred when stoichiometric amounts of NTA, which should complex and solubilize most of the Cu, was added, and it is likely that NTA prevented lethal concentrations of Cu from being adsorbed onto the cell and hence internalized. 相似文献
28.
Sinéad C. Mac Namara Maria M. Garlock David P. Billington 《Canadian Metallurgical Quarterly》2007,21(2):152-156
Steam generator replacement at nuclear power plants requires large construction openings in nuclear containment structures. This study examines the effects of such openings placed in a 61?cm (2?ft) thick dome of a shield building. The dome is cast in two layers, and the limiting cases of the two layers acting as one and the lower layer carrying the entire load are considered. Without openings, the stresses due to dead load in the structure are very low when compared to the material strength. With openings, the loads are easily redistributed around the opening, and the maximum stresses for the conservative case are between three and four and one-half times larger than in the original structure with no openings. The extent of the affected area around the holes is relatively small. Smooth corners in the openings are necessary to avoid high local stress concentrations. 相似文献
29.
30.
Replies to R. E. Roughton's (see record 1994-27310-001) response to D. K. Flaks's (see record 1993-23357-001) article on homophobia and the psychologists's role in psychoanalytic training institutes. Flaks commends attempts by the American Psychoanalytic Association to combat homophobia within the organization but notes potential problems that might limit its success. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献