全文获取类型
收费全文 | 17997篇 |
免费 | 523篇 |
国内免费 | 18篇 |
专业分类
电工技术 | 237篇 |
综合类 | 9篇 |
化学工业 | 3549篇 |
金属工艺 | 280篇 |
机械仪表 | 310篇 |
建筑科学 | 1099篇 |
矿业工程 | 109篇 |
能源动力 | 505篇 |
轻工业 | 1381篇 |
水利工程 | 158篇 |
石油天然气 | 194篇 |
武器工业 | 1篇 |
无线电 | 1064篇 |
一般工业技术 | 2876篇 |
冶金工业 | 4167篇 |
原子能技术 | 162篇 |
自动化技术 | 2437篇 |
出版年
2022年 | 120篇 |
2021年 | 213篇 |
2020年 | 170篇 |
2019年 | 223篇 |
2018年 | 265篇 |
2017年 | 259篇 |
2016年 | 319篇 |
2015年 | 238篇 |
2014年 | 380篇 |
2013年 | 1101篇 |
2012年 | 663篇 |
2011年 | 870篇 |
2010年 | 674篇 |
2009年 | 662篇 |
2008年 | 858篇 |
2007年 | 870篇 |
2006年 | 685篇 |
2005年 | 719篇 |
2004年 | 559篇 |
2003年 | 569篇 |
2002年 | 509篇 |
2001年 | 329篇 |
2000年 | 300篇 |
1999年 | 303篇 |
1998年 | 312篇 |
1997年 | 300篇 |
1996年 | 299篇 |
1995年 | 333篇 |
1994年 | 267篇 |
1993年 | 303篇 |
1992年 | 272篇 |
1991年 | 164篇 |
1990年 | 237篇 |
1989年 | 283篇 |
1988年 | 204篇 |
1987年 | 215篇 |
1986年 | 211篇 |
1985年 | 265篇 |
1984年 | 258篇 |
1983年 | 227篇 |
1982年 | 217篇 |
1981年 | 213篇 |
1980年 | 172篇 |
1979年 | 187篇 |
1978年 | 176篇 |
1977年 | 177篇 |
1976年 | 147篇 |
1975年 | 178篇 |
1974年 | 165篇 |
1973年 | 133篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
191.
AA Bialasiewicz JX Ma U Schandig D von Domarus G Richard 《Canadian Metallurgical Quarterly》1998,213(5):262-270
BACKGROUND: Tumor necrosis may reflect a destructive immune reaction. Systematic and statistically significant comparative clinico-histopathologic studies have not yet been reported. PATIENTS AND METHODS: 113 necrotizing choroidal melanomas (NCM) recruited from 701 enucleated globes 1967-1988 were resectioned, stained and compared to 100 choroidal melanomas without necrosis (CM), and data of 74 patients with a follow-up of more than 10 years were evaluated. RESULTS: Statistically significant characteristics of NCM were: patient age < 60 yrs. for NCM 27.4%, CM 46%; patient age in men for NCM was 64 yrs on average (CM: 58 yrs.), in women for NCM 67 yrs. (CM: 59 yrs.). Time elapsed between first symptoms and enucleation was < 12 months in 15.9% of NCM (89% for CM), and > 12 months in 23.9% of NCM (11% in CM). Mixed or epitheloid cell tumors was seen in 54.9% of NCM and 49% of CM, spindle cell tumors in 36.3% of NCM and 51% in CM. Advanced tumor stages T3 and T4 were present in 45.1% resp. 36.3% of NCM compared to 37% resp. 16% in CM. Scleral invasion was documented in 67.3% of NCM and 37% of CM, extrascleral dissemination in 43% of NCM and 16% of CM. Secondary glaucomas were seen in 62.2% of NCM and 6% CM, a penetration through Bruch's membrane in 61.0% of NCM and 46% of CM. Intratumoral hemorrhage was noted in 68.14% of NCM and 24% of CM, extratumoral bleeding in 23.9% of NCM and 0% CM. Inflammatory reactions in tumors were observed in 96.7% of NCM harboring > 30% necrosis compared to 5% in CM, and extratumoral in 94.5% of NCM and 0% of CM. Intraocular extratumoral necrosis was seen in 23.9% of NCM and 0% of CM. There were no significant differences in the degree of pigmentation of the 90.3% pigmented NCM or of the 94% pigmented CM, neither in the tumor localization, being constantly behind the equator in 87% of cases. Survival of patients with NCM patients was 5 years and 9 months on average (5-year mortality rate 41.9%), and 74.3% were deceased from metastatic spread. CONCLUSIONS: Significant clinical and histopathological differences between necrotizing and non-necrotizing malignant melanomas of the choroid can be identified. The inflammatory reaction of NCM must be further elucidated, particularly with respect to the nature of tumor-infiltrating lymphocytes. 相似文献
192.
Grant McClelland Georges Zwingelstein C. Richard Taylor Jean-Michel Weber 《Lipids》1995,30(2):147-153
The goals of this study were to determine: (i) whether mammals mobilize particular nonesterified fatty acids (NEFA) preferentially
during locomotion, (ii) if differences in aerobic capacity or diet can affect the pattern of NEFA mobilization and (iii) which
individual NEFA are most representative of total NEFA concentration changes, to use them as tracers for turnover studies.
Individual NEFA were measured in trained dogs and goats (VO2max dog/VO2max goat=2.2; where VO2max=max-imal oxygen consumption) during treadmill exercise at 40 and 60% VO2max. Important interspecies differences in individual NEFA concentrations could be attributed to differences in aerobic capacity.
The more aerobic species (dog) had much higher plasma NEFA concentrations for all but one NEFA (18∶0), when compared with
the low-aerobic species (goat). In addition, exercise caused a large increase in concentration of individual NEFA in the dogs,
with the largest increases seen in 18∶1 (150% above resting values) and 16∶0 (60% increase), but it had no effect in goats.
Therefore, the aerobic species has a much higher ability for mobilizing and transporting NEFA in plasma than its low-aerobic
counterpart. Two NEFA accounted for more than half total plasma NEFA in both species, 18∶1 (about 35% total NEFA) and 16∶0
(20%). Calculation of variability in percent composition reveals that oleate and palmitate also closely reflect changes in
total NEFA and are therefore the most appropriate tracers forin vivo kinetic studies in exercising mammals. Differences in diet and digestion physiology explained some differences in the plasma
NEFA composition of the two species; this was reflected in the percent contributions of individual fatty acids to total NEFA:
in dogs 18∶1>16∶0>18∶2>18∶0, while in goats 18∶1>16∶0>18∶0>18∶2. Also, only goats had 18∶3 (6% total NEFA), a fatty acid of
plant origin, while only dogs had 16∶1 (7% total NEFA). Overall, however, the plasma NEFA composition of goats did not exactly
reflect that of their diet due to preabsorptive modification of unsaturated fatty acids, while in dogs there was a good correlation
between dietary and plasma NEFA. 相似文献
193.
194.
A method for exploring the effects of attrition in randomized experiments with dichotomous outcomes.
Shadish William R.; Hu Xiangen; Glaser Renita R.; Kownacki Richard; Wong Seok 《Canadian Metallurgical Quarterly》1998,3(1):3
Attrition from conditions in randomized experiments is common. Yet it is difficult to assess the possible effects of attrition because the outcome status of the dropouts is usually unknown. This article develops methods to assess those effects in studies with dichotomous outcomes, illustrating the methods with randomized experiments in drug abuse treatment, smoking cessation treatment, and alcoholism treatment. The methods include computing the lowest and highest possible effect sizes that could have been observed, enumerating the percent of possible study outcomes below a given threshold, estimating the probability that an outcome beyond any given threshold would be observed if all participants were measured, and constructing attrition analysis plots showing the effects of attrition under varied assumptions. For the kind of study to which they apply, these methods should replace the treatment of missing participants as failures in an "intent-to-treat" analysis. A user-friendly personal computer program is available to implement all of these analyses. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
195.
It is common experience that aged surfaces are often difficult to bond to. We report an examination of bonding to thermally-aged epoxy surfaces, using as the adhesive the same epoxy as that of the aged surface. The cured and postcured epoxy was aged at 200 ° C, with the ageing time varying from 2 to 8 h. The fracture energy of the bond line was measured by mode I cleavage under conditions of relatively slow crack growth. The bondline fracture energy was found to decrease logarithmically with ageing time. The fracture energies for bonds to surfaces aged for 2, 4, and 8 h at 200 ° C were 0.077, 0.059, and 0.050 kJ M–2, respectively. These compare to 0.13 kJ M–2 for a bond to an unaged surface and 0.21 kJ m–2 for bulk fracture. Fracture surfaces resulting from both slow and rapid fracture were examined by optical and scanning electron microscopy. Fracture features different from those arising from bulk fracture were found. Areas with good adhesion occurred amidst fields of featureless fracture surface; the frequency and size of these areas decreased with increased ageing time. Evidence of plastic deformation was found, always occurring on the new side of the bond: ridges parallel with crack propagation at high crack speeds and subsurface undulations perpendicular to crack propagation at low speeds. The bond has the effect of channelling the crack along the bondline, but fracture does not always remain exactly at the interface. Fracture often occurred a relatively constant distance away from the interface, suggesting that the presence of the interface was felt for some distance. 相似文献
196.
Conjugated polymers such as polyacetylene are semiconductors, and can be chemically doped to give very high values of conductivity (up to 150 000 S cm–1). The semiconducting properties are due to delocalization of the electron system formed along the polymer chain, and this general class of materials shows many desirable semiconductor properties for electrical and optical applications. There are considerable challenges to find synthetic routes to allow processing of the polymer to the forms required for applications, and many of the recent important advances in the field are due to improvements in this area. 相似文献
197.
Macwan Richard Benezeth Yannick Mansouri Alamin 《Journal of Mathematical Imaging and Vision》2021,63(7):807-820
Journal of Mathematical Imaging and Vision - Remote photoplethysmography (rPPG) has been at the forefront recently, thanks to its capacity in estimating non-contact physiological parameters such as... 相似文献
198.
Alain Denise Marie-Claude Gaudel Sandrine-Dominique Gouraud Richard Lassaigne Johan Oudinet Sylvain Peyronnet 《International Journal on Software Tools for Technology Transfer (STTT)》2012,14(1):73-93
This paper presents several randomised algorithms for generating paths in large models according to a given coverage criterion.
Using methods for counting combinatorial structures, these algorithms can efficiently explore very large models, based on
a graphical representation by an automaton or by a product of several automata. This new approach can be applied to random
exploration in order to optimise path coverage and can be generalised to take into account other coverage criteria, via the
definition of a notion of randomised coverage satisfaction. Our main contributions are a method for drawing paths uniformly
at random in composed models, i.e. models that are given as products of automata, first without and then with synchronisation;
a new efficient approach to draw paths at random taking into account some other coverage criterion. Experimental results show
promising agreement with theoretical predictions and significant improvement over previous randomised approaches. This work
opens new perspectives for future studies of statistical testing and model checking, mainly to fight the combinatorial explosion
problem. 相似文献
199.
Bing Zhu Richard Marciano Reagan Moore Laurin Herr Jurgen Schulze 《International Journal on Digital Libraries》2012,12(1):41-49
Distributed digital repositories can be used to address critical issues of long-term digital preservation and disaster management for large data centers. A policy-driven system provides an ideal solution for managing distributed repositories that require high flexibility and high configurability. Recent studies demonstrate that the integrated Rule-Oriented Data System, a peer-to-peer server middleware, provides the requisite dynamic extensibility needed to manage time-varying policies, automate validation of assessment criteria, manage ingestion processes, manage access policies, and manage preservation policies. The policy management can be implemented underneath existing digital library infrastructure such as Fedora. 相似文献
200.
The influence of stacking fault energy on microvoid coalescence in pure materials has been studied. It was shown that as a material's stacking fault energy (SFE) decreased, the extent of microvoid coalescence that occurred during ductile fracture also decreased. The decrease of microvoid coalescence in low SFE materials was attributed to a hindrance in the development of dislocation cells associated with the restricted motion of dislocations. In pure materials, microvoids are believed to initiate and grow along dislocation cell walls formed during deformation. As such, the absence or scarcity of cells in lower SFE materials limits the formation of these voids during ductile fracture. 相似文献