全文获取类型
收费全文 | 3222篇 |
免费 | 17篇 |
国内免费 | 2篇 |
专业分类
电工技术 | 18篇 |
化学工业 | 174篇 |
金属工艺 | 9篇 |
机械仪表 | 20篇 |
建筑科学 | 38篇 |
矿业工程 | 1篇 |
能源动力 | 13篇 |
轻工业 | 112篇 |
水利工程 | 7篇 |
石油天然气 | 3篇 |
无线电 | 123篇 |
一般工业技术 | 130篇 |
冶金工业 | 2450篇 |
原子能技术 | 14篇 |
自动化技术 | 129篇 |
出版年
2021年 | 7篇 |
2019年 | 14篇 |
2018年 | 7篇 |
2017年 | 8篇 |
2015年 | 7篇 |
2014年 | 20篇 |
2013年 | 61篇 |
2012年 | 32篇 |
2011年 | 43篇 |
2010年 | 29篇 |
2009年 | 26篇 |
2008年 | 42篇 |
2007年 | 35篇 |
2006年 | 24篇 |
2005年 | 36篇 |
2004年 | 28篇 |
2003年 | 39篇 |
2002年 | 24篇 |
2001年 | 16篇 |
2000年 | 11篇 |
1999年 | 101篇 |
1998年 | 673篇 |
1997年 | 429篇 |
1996年 | 293篇 |
1995年 | 178篇 |
1994年 | 138篇 |
1993年 | 150篇 |
1992年 | 37篇 |
1991年 | 40篇 |
1990年 | 36篇 |
1989年 | 46篇 |
1988年 | 46篇 |
1987年 | 45篇 |
1986年 | 37篇 |
1985年 | 33篇 |
1984年 | 9篇 |
1983年 | 15篇 |
1982年 | 19篇 |
1981年 | 20篇 |
1980年 | 23篇 |
1979年 | 11篇 |
1978年 | 18篇 |
1977年 | 83篇 |
1976年 | 154篇 |
1975年 | 12篇 |
1972年 | 7篇 |
1968年 | 6篇 |
1967年 | 6篇 |
1966年 | 7篇 |
1955年 | 6篇 |
排序方式: 共有3241条查询结果,搜索用时 15 毫秒
1.
HCI recognizes the importance of visions as both a design and futuring approach and a problem-solving technique. One challenge of visioning work is who can propose and shape IT visions and how they can achieve the widespread buy-in needed to make them efficacious. In this paper, we focus on the potential of making, open design, and open manufacturing for contributing toward, if not fully achieving, the broadening of participation in IT envisioning. We use Taiwan’s MakerPro, a manufacturers and IT R&D community as a case to unpack what collective IT visioning looks like, how it shapes IT agendas concretely, and the implications for open design/open manufacturing research agenda in HCI. Our findings reveal how MakerPro members constructed and developed visions for open design and open manufacturing in Taiwan, obstacles to such visions, and how these obstacles can be collaboratively overcome through a participative and even democratic process. We also show that the collective purposiveness not only defines a regional vision agenda but also embraces an entire nation’s future. 相似文献
2.
Physical aging of both thick and thin films of “high free-volume” glassy perfluoropolymers was studied by monitoring changes in pure gas permeability of O2, N2 and CH4. All permeability measurements were done at a fixed temperature of 35 °C for more than 1000 h of aging. Two grades of perfluoropolymers, Teflon AF and Hyflon AD, having different comonomer structures but with similar comonomer ratios were studied to understand the effect of comonomer type and content on the aging behavior. The effect of casting process (solution vs. spin coating) and solvent type (vapor pressure and boiling point) had a significant effect on the absolute permeability of both thick and thin films; however, the aging rates were more affected by thickness and solvent type rather than the casting process for similar thicknesses. After 1000 h of aging, the relative permeability for thin films of Teflon AF 2400 was decreased by 27% compared to only 10% for thick films prepared from Novec 7500 solvent. Teflon AF, which has a higher fractional free volume (FFV) than Hyflon AD, is believed to undergo significant aging well before the initial permeability measurement could be made (after ∼ 1 h of aging) and, therefore, Teflon AF materials showed a lower decrease in relative permeability compared to Hyflon AD for the same aging time. The comonomer type and content has a significant effect on the permeability; the initial absolute oxygen permeability for AF 2400 was an order of magnitude higher compared to AD 60. The physical aging of thin films of the various glassy perfluoropolymers was also tracked by recording changes in the refractive index and thickness with time by ellipsometry. The ellipsometry data also confirmed higher aging rates in Hyflon AD compared to Teflon AF materials. The volumetric aging rate, obtained from the change in the refractive index using the Lorentz–Lorenz equation, and the permeability reduction rate from the (P1000h/P1h) ratio showed an excellent linear correlation. The (P1000h/P1h) ratio also showed a stronger correlation with (Tg−35) °C than with FFV. 相似文献
3.
David F. Sanders Ruilan Guo Zachary P. Smith Qiang Liu Kevin A. Stevens James E. McGrath Donald R. Paul Benny D. Freeman 《Polymer》2014
Thermal rearrangement of polyimides with ortho-position groups to polybenzoxazoles and related structures has been of recent interest for producing gas separation membranes. This study explores the influence of synthesis route and ortho-position functional group on the thermal rearrangement process and the fractional free volume of thermally rearranged (TR) polymers produced from polyimides derived from 3,3′-dihydroxy-4,4′-diamino-biphenyl and 2,2′-bis-(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (HAB–6FDA). Acetate, propanoate, and pivalate ortho-position functional groups were considered. Thermogravimetric analysis (TGA) was used to study thermal rearrangement at temperatures between 350 and 450 °C, and evolved gases from TGA were analyzed via mass spectrometry to characterize the byproducts of thermal rearrangement and thermal degradation. CO2 was the major byproduct of thermal rearrangement for all samples, and its evolution began well before the onset of thermal degradation. When non-hydroxyl ortho-position groups were present in the polymers, several byproducts other than CO2 were also observed due to the loss of these ortho-position groups before thermal rearrangement. Free volume generally increased with increasing extent of thermal rearrangement, but precise values of free volume could not be accurately determined for polymers with propanoate and pivalate ortho-position functional groups due to uncertainties in the chemical structure of partially converted materials. For polymers with acetate and hydroxyl ortho-position groups, free volume could be determined within the uncertainty of density measurements. Thermal rearrangement behavior and free volume results for acetate containing polymers synthesized via different routes were very similar. Based on these results, the chemical structure of the ortho-position functional group has a larger impact on TR polymer properties than the polyimide precursor synthesis route. 相似文献
4.
Claire Freeman 《Landscape Research》2003,28(2):183-203
In New Zealand's lowland production landscapes less than 5% of the land comprises native vegetation. The continuing loss of lowland habitats remains, as the New Zealand Biodiversity Strategy (2000) recognized, one of the most intractable of New Zealand's conservation problems. The reasons behind this loss are explored and possible approaches to arresting this decline are considered. Field research was undertaken in two coastal plains landscapes. The purpose of the research was to examine the extent and character of native habitats remaining on the plains. A database comprising data on 744 sites with predominantly native habitats was created. The majority of these sites were not included on existing conservation databases and as such fall outside the remit of existing conservation planning and management processes. The study indicates that if the decline of native habitats on the plains is to be halted and reversed, a new approach to conservation planning is needed. A strategic planning approach to conservation is required, based on ecological regions where the focus is on co-ordinated planning of natural areas at the landscape scale. 相似文献
5.
AbstractEngineers use reliability experiments to determine the factors that drive product reliability, build robust products, and predict reliability under use conditions. This article uses recent testing of a howitzer to illustrate the challenges in designing reliability experiments for complex, repairable systems. We review research in complex system reliability models, failure-time experiments, and experimental design principles. We highlight the need for reliability experiments that account for various intended uses and environments. We leverage lessons learned from current research and propose methods for designing an experiment for a complex, repairable system. 相似文献
6.
Jared Freeman Drew Leins Conrad Bell IV The SD Research Consortium 《Theoretical Issues in Ergonomics Science》2019,20(1):27-38
AbstractOrganisations conducting research programs often focus the work of their scientists and technologists on challenge problems (CPs). These challenges are designed to ensure that progress is measurable and relevant to the goals of the program sponsor. Generating and selecting pertinent CPs is difficult, as is assessing their value. We describe a method of generating and selecting CPs and its application in a highly collaborative, multi-organisation research program. Thirty-eight biologists, chemists, mathematicians and computer scientists across academic, commercial and government organisations generated and ranked their top choices from among 12 richly described candidate challenge problems. A ranked-choice voting formula was applied. Five CPs were highly scored; the remaining seven were distributed across a lower range of scores. The program sponsor subsequently directed researchers to address six CPs, including the elected five. Analysis of the rationales that participants offered for their CP rankings revealed four domain-independent dimensions of value: capability, speed, impact and synergy. These dimensions of value can help managers of interdisciplinary research programs systematically select a portfolio of CPs that will efficiently apply utilise resources towards program goals and facilitate measurement of scientific progress. 相似文献
7.
8.
Cora-Costanze Sommerey Tim L. WilliamsIan McCrone Aran Ruiz-FerrerasDan Freeman Joy Archer 《Journal of dairy science》2014
Thromboelastography is a whole blood-based coagulation assay that can be used to investigate hypocoagulability and hypercoagulability, as seen with thromboembolic diseases and disseminated intravascular coagulation. Numerous coagulopathies due to different causes are reported in cows. The objective was to establish reference intervals for thromboelastography using the TEG 5000 (Haemonetics GmbH, Munich, Germany) with citrated whole blood samples and kaolin activation in dairy cows and to investigate possible thromboelastographic changes between cows in different lactation periods. An additional objective was to test the stability of samples for up to 100 h. Sixty blood samples from healthy Holstein-Friesian cows were examined. The samples were allocated to 3 different lactation groups (≤30 d postcalving, 31–99 d postcalving, ≥100 d postcalving). Thromboelastography was performed by using the TEG 5000 analyzer with citrated whole blood samples with kaolin activation. The calculated reference intervals were as follows: reaction time = 2.2 to 6.2 min, coagulation time = 0.8 to 2.0 min, angle α = 58.2 to 81.8°, maximum amplitude = 64.3 to 89.2 mm, and clot rigidity = 9.2 to 41.2 dyn/cm2. The 3 different lactation groups showed no significant differences in TEG parameters. No significant difference was seen in samples stored for up to 48 h at room temperature, which indicates that delays in processing samples, such as those arising during transit, are not an issue. 相似文献
9.
The purpose of this study was to develop regression equations to predict manual arm strength for a wide variety of hand locations within the reach envelope. Maximum voluntary manual arm strength was determined from 71 female participants in six exertion directions (superior, inferior, anterior, posterior, medial and lateral), in a total of 28 hand locations. Forces ranged from 51.3 to 164.4 N, and had a pooled coefficient of variation of 29.9%. Across all 168 combinations of hand locations and exertion directions, the multivariate regression equations explained 92.5% of the variance and had a root mean square error (RMSE) of only 6.4 N, using only the anterior, lateral and vertical location of the hand relative to the active shoulder joint as inputs. These equations provide a proof-of-principle for our novel regression approach, and represent a first step towards a more comprehensive equation to estimate maximum acceptable forces for occupational tasks. 相似文献
10.
Biography of Brigadier John Tiltman, who worked as a cryptologist for the British and American governments from 1920 to 1980. 相似文献