全文获取类型
收费全文 | 3125篇 |
免费 | 61篇 |
国内免费 | 34篇 |
专业分类
电工技术 | 81篇 |
综合类 | 38篇 |
化学工业 | 114篇 |
金属工艺 | 54篇 |
机械仪表 | 34篇 |
建筑科学 | 70篇 |
矿业工程 | 8篇 |
能源动力 | 21篇 |
轻工业 | 110篇 |
水利工程 | 26篇 |
石油天然气 | 10篇 |
武器工业 | 7篇 |
无线电 | 166篇 |
一般工业技术 | 152篇 |
冶金工业 | 2082篇 |
原子能技术 | 26篇 |
自动化技术 | 221篇 |
出版年
2022年 | 15篇 |
2021年 | 10篇 |
2019年 | 14篇 |
2018年 | 15篇 |
2017年 | 17篇 |
2016年 | 16篇 |
2015年 | 11篇 |
2014年 | 18篇 |
2013年 | 40篇 |
2012年 | 49篇 |
2011年 | 86篇 |
2010年 | 63篇 |
2009年 | 59篇 |
2008年 | 78篇 |
2007年 | 80篇 |
2006年 | 60篇 |
2005年 | 72篇 |
2004年 | 43篇 |
2003年 | 41篇 |
2002年 | 38篇 |
2001年 | 33篇 |
2000年 | 33篇 |
1999年 | 76篇 |
1998年 | 600篇 |
1997年 | 359篇 |
1996年 | 226篇 |
1995年 | 131篇 |
1994年 | 117篇 |
1993年 | 117篇 |
1992年 | 33篇 |
1991年 | 31篇 |
1990年 | 26篇 |
1989年 | 45篇 |
1988年 | 45篇 |
1987年 | 34篇 |
1986年 | 25篇 |
1985年 | 27篇 |
1984年 | 13篇 |
1983年 | 12篇 |
1982年 | 23篇 |
1981年 | 19篇 |
1980年 | 39篇 |
1979年 | 7篇 |
1978年 | 6篇 |
1977年 | 84篇 |
1976年 | 152篇 |
1975年 | 10篇 |
1974年 | 14篇 |
1972年 | 5篇 |
1970年 | 5篇 |
排序方式: 共有3220条查询结果,搜索用时 15 毫秒
21.
OBJECTIVE: To review epidemiological studies dealing with the temporal and geographic variability in the occurrence of rheumatoid arthritis (RA) and clinical studies that address the variability of severity and manifestations among populations. METHODS: An extensive search of the literature, including a Medline search, was completed. Studies addressing the origin, history, and trends in the occurrence of RA were reviewed first. Next, studies of the prevalence and incidence of RA in different populations were reviewed, and occurrence rates compared. Standardization was attempted by tabulating adult prevalence rates of studies using equivalent sets of criteria. Studies comparing RA patients from two populations were sought next. Finally, studies dealing with explanations of the presumed variability were reviewed. RESULTS: Temporal variability is indicated by paleopathological evidence that RA has existed in the New World since 4000 BC, whereas there is no evidence that it occurred in Europe before the 17th century, or in Africa before the 20th century. Epidemiological studies show a possible trend of decreasing incidence of RA in the United States and Western Europe, whereas reports from Africa note a rising incidence. In white populations of Europe and America, prevalence is approximately 1%, and incidence is 0.03%. Significantly higher rates are found in some North American Indians, and significantly lower rates in some Asian and African populations, even when the different population structures are taken into account. In the latter populations, different patterns of occurrence from those observed in whites emerge, such as greater female preponderance and a much younger peak age at onset. Direct standardized comparisons of two diverse populations of RA patients showed some differences in expression, severity, or manifestations of RA between populations. CONCLUSION: The occurrence and manifestations of RA are temporally and geographically variable. 相似文献
22.
23.
ME Goldberg BW Lex NK Mello JH Mendelson TA Bower 《Canadian Metallurgical Quarterly》1996,66(2):228-238
Research on achievement goal orientation in sport has relied primarily on traditional statistical methodology to examine group mean differences. Unfortunately, examination of the measurement model is generally ignored prior to means testing. This study reports an application of structural equation modeling (SEM) in testing measurement invariance and latent mean structure of the Task and Ego Orientation in Sport Questionnaire (TEOSQ; Duda & Nicholls, 1989) using male and female college students. A confirmatory factor analysis for testing invariance revealed invariant measurement properties and factor structures across gender, indicating that task and ego orientation are similarly conceptualized by male and female students. Subsequent testing of latent mean structures, however, showed significant gender differences with respect to ego orientation, but no difference in task orientation. The SEM procedures used in the present study demonstrate additional construct validity and internal consistency reliability for the TEOSQ and, by confirming its factor structure, provide a sound psychometric basis for its continued use in substantive studies focusing on the comparison of achievement goal orientation across gender. 相似文献
24.
PL Pirie BL Rooney TF Pechacek HA Lando LA Schmid 《Canadian Metallurgical Quarterly》1997,22(1):131-137
Exposure to foreign particles sometimes causes inflammatory reactions through production of cytokines and chemoattractants by phagocytic cells. In this study, we focused on macrophage migration inhibitory factor (MIF) to evaluate its pathophysiological role in the phagocytic process. Immunohistochemical analysis of human pseudosynovial tissues retrieved at revision of total hip arthroplasty showed that infiltrating mononuclear and multinuclear cells were positively stained by both an anti-CD68 antibody and anti-human MIF antibody. For in vitro study, MIF was released from murine macrophage-like cells (RAW 264.7) in response to phagocytosis of fluorescent-latex beads in a particle dose-dependent manner. Northern blot analysis showed marked elevation of the MIF mRNA level in the phagocytic macrophage-like cells. Moreover, pretreatment of RAW 264.7 cells with rat recombinant MIF increased the extent of phagocytosis by 1.6-fold compared with the control. Taken together, these results suggest that MIF plays an important role by activating macrophages in autocrine and paracrine fashion to phagocytose foreign particles. 相似文献
25.
26.
27.
PAC-learnability of probabilistic deterministic finite state automata in terms of variation distance
We consider the problem of PAC-learning distributions over strings, represented by probabilistic deterministic finite automata (PDFAs). PDFAs are a probabilistic model for the generation of strings of symbols, that have been used in the context of speech and handwriting recognition, and bioinformatics. Recent work on learning PDFAs from random examples has used the KL-divergence as the error measure; here we use the variation distance. We build on recent work by Clark and Thollard, and show that the use of the variation distance allows simplifications to be made to the algorithms, and also a strengthening of the results; in particular that using the variation distance, we obtain polynomial sample size bounds that are independent of the expected length of strings. 相似文献
28.
R Albrecht TC Awes C Baktash P Beckmann F Berger R Bock G Claesson G Clewing L Dragon A Eklund RL Ferguson A Franz S Garpman R Glasow HA Gustafsson HH Gutbrod J Idh P Jacobs K Kampert BW Kolb P Kristiansson IY Lee H Loehner I Lund FE Obenshain A Oskarsson I Otterlund T Peitzmann S Persson F Plasil AM Poskanzer M Purschke H Ritter S Saini R Santo HR Schmidt T Siemiarczuk SP Sorensen E Stenlund ML Tincknell GR Young 《Canadian Metallurgical Quarterly》1991,44(6):2736-2752
29.
Narratives are gaining recognition as important ways occupational therapists and other clinicians can think about the life stories of clients. The purpose of this article is to examine a conceptualization of how changes from one chapter to another occur in life stories, using the metaphor of an adaptive repertoire, and to consider how this notion can be useful in helping clients maintain continuity and a coherent life story in times of change. Three premises based on the concept of adaptation address (a) configurations of occupational forms embedded in particular local worlds, (b) cumulative development of an adaptive repertoire that allows one to perform both competently and appropriately, and (c) adaptive transitions and application of one's repertoire to new circumstances. Implications for research and clinical practice in occupational therapy also are examined. 相似文献
30.
The lateral packing of a collagen-like molecule, CH3CO-(Gly-L-Pro-L-Pro)4-NHCH3, has been examined by energy minimization with the ECEPP/3 force field. Two current packing models, the Smith collagen microfibril twisted equilateral pentagonal model and the quasi-hexagonal packing model, have been extensively investigated. In treating the Smith microfibril model, energy minimization was carried out on various conformations including those with the symmetry of equivalent packing, i.e., in which the triple helices were arranged equivalently with respect to each other. Both models are based on the experimental observation of the characteristic axial periodicity, D = 67 nm, of light and dark bands, indicating that, if any superstructure exists, it should consist of five triple helices. The quasi-hexagonal packing structure is found to be energetically more favorable than the Smith microfibril model by as much as 31.2 kcal/mol of five triple helices. This is because the quasi-hexagonal packing geometry provides more nonbonded interaction possibilities between triple helices than does the Smith microfibril geometry. Our results are consistent with recent x-ray studies with synthetic collagen-like molecules and rat tail tendon, in which the data were interpreted as being consistent with either a quasi-hexagonal or a square-triangular structure. 相似文献