全文获取类型
收费全文 | 2956篇 |
免费 | 25篇 |
国内免费 | 1篇 |
专业分类
电工技术 | 17篇 |
综合类 | 1篇 |
化学工业 | 123篇 |
金属工艺 | 18篇 |
机械仪表 | 14篇 |
建筑科学 | 35篇 |
矿业工程 | 3篇 |
能源动力 | 25篇 |
轻工业 | 88篇 |
水利工程 | 4篇 |
无线电 | 84篇 |
一般工业技术 | 127篇 |
冶金工业 | 2325篇 |
原子能技术 | 13篇 |
自动化技术 | 105篇 |
出版年
2020年 | 6篇 |
2019年 | 5篇 |
2018年 | 8篇 |
2017年 | 16篇 |
2016年 | 13篇 |
2015年 | 8篇 |
2014年 | 10篇 |
2013年 | 24篇 |
2012年 | 20篇 |
2011年 | 30篇 |
2010年 | 11篇 |
2009年 | 20篇 |
2008年 | 39篇 |
2007年 | 28篇 |
2006年 | 19篇 |
2005年 | 38篇 |
2004年 | 26篇 |
2003年 | 20篇 |
2002年 | 19篇 |
2001年 | 17篇 |
2000年 | 19篇 |
1999年 | 98篇 |
1998年 | 807篇 |
1997年 | 441篇 |
1996年 | 273篇 |
1995年 | 154篇 |
1994年 | 142篇 |
1993年 | 144篇 |
1992年 | 25篇 |
1991年 | 31篇 |
1990年 | 25篇 |
1989年 | 31篇 |
1988年 | 29篇 |
1987年 | 32篇 |
1986年 | 28篇 |
1985年 | 30篇 |
1984年 | 20篇 |
1983年 | 10篇 |
1982年 | 13篇 |
1981年 | 18篇 |
1980年 | 15篇 |
1979年 | 8篇 |
1978年 | 12篇 |
1977年 | 43篇 |
1976年 | 84篇 |
1975年 | 8篇 |
1973年 | 7篇 |
1972年 | 7篇 |
1970年 | 6篇 |
1967年 | 4篇 |
排序方式: 共有2982条查询结果,搜索用时 500 毫秒
71.
Assessing obesity: classification and epidemiology 总被引:1,自引:0,他引:1
Obesity is generally defined as a body mass index (BMI) of 30 kg/m2 and higher. Overweight is defined as a BMI between 25 and 30 kg/m2. The prevalence varies considerably between countries, and between regions within countries. It is estimated that more than half of adults aged 35-65 living in Europe are either overweight or obese. Overweight is more common among men than among women but obesity is more common among women. The prevalence of obesity in Europe is probably in the order of 10-20% in men and 15-25% in adult women. In most European countries who have reliable data on time-trends the prevalence of obesity seems to be increasing. In most European countries, obesity is usually inversely associated with socio-economic status, particularly among women. New classifications of overweight may be based on cut-off points for simple anthropometric measures which reflects both total adiposity as well as abdominal fatness. 相似文献
72.
Christopher L Brown Gillian Bushell Michael W Whitehouse DS Agrawal SG Tupe KM Paknikar Edward RT Tiekink 《Gold bulletin》2007,40(3):245-250
Nanosized gold particles (27 +/− 3 nm) have been proven to be effective in ameliorating the symptoms of mycobacterial-, collagen-
and pristane-induced arthritis in rat models. This contrasts with the drug sodium aurothiomalate that was only effective against
mycobacterial-induced arthritis but not to the same extent as Au0. Gold in the traditional Indian Ayurvedic medicine,Swarna bhasma (gold ash), has been characterized as globular particles of gold with an average size of 56–57 nm. 相似文献
73.
Identification and elimination by site-directed mutagenesis of thermolabile aspartyl bonds in Aspergillus awamori glucoamylase 总被引:2,自引:0,他引:2
Chen Hsiu-mei; Ford Clark; Reilly Peter J. 《Protein engineering, design & selection : PEDS》1995,8(6):575-582
Both Dative Aspergillus niger glucoamylase and wild-type Aspergillusawamori glucoamylase expressed in Saccharo-myces cerevisiae,which have identical primary structures, undergo hydrolysisat aspartyl bonds at low pH values and elevated temperatures.In native A.niger enzyme the Aspl26Glyl27 bond was preferentiallycleaved at pH 3.5,while at pH 4.5 cleavage of the Asp257Pro258and Asp293Gly294 bonds was dominant. In wild-type A.awamoriglucoamylase, cleavage of the latter was dominant at both pH3.5 and 4.5. Site-directed mutations Aspl26Glu and Glyl27Alain wild-type enzyme decreased specific activities by 60 and30%, respectively, and increased irreversible thermoinactivationrates 3- to 4-fold at pH 4.5. Replacement of Asp257 with Gluand Asp293 with Glu or Gin decreased specific activities by20%, but greatly reduced cleavage of the Asp257Pro258and Asp293Gly294 bonds. The Asp257Glu mutant was producedvery slowly and was more thermostable than wildtype glucoamylaseat pH 4.5up to 70°C. Replacement of Asp293 with either Gluor Gln significantly raised protein production and slightlyincreased thermostability at pH 3.5 and 4.5, but not at pH5. 相似文献
74.
Effect of amino acid deletions in the O-glycosylated region of Aspergillus awamori glucoamylase 总被引:1,自引:0,他引:1
Libby Carol Baker; Cornett Catherine A.G.; Reilly Peter J.; Ford Clark 《Protein engineering, design & selection : PEDS》1994,7(9):1109-1114
Aspergillus awamori glucoamylase (GA) contains globular catalyticand starch-binding domains (residues 1471 and 509616,respectively). A heavily O-glycosylated sequence comprises twoparts. The first (residues 441471) in the crystal structurewraps around an /-barrel formed by residues 1440. Thesecond (residues 472508) is an extended, semi-rigid linkerbetween the two domains. To investigate the functional roleof this linker, we made internal deletions to remove residues466512 (GA1), 485512 (GA2) and 466483 (GA3).GA2 and GA3 were expressed in Saccharomyces cerevisiae culturesupernatants at 60 and 20% the wild-type level, respectively,while GA1 was almost undetectable. Western blots comparing extracellularand intracellular fractions indicated that the region deletedin GA3 was critical for secretion, while the region deletedin GA2 contributed to the production of a stable enzyme structure.The activities of purified GA2 and GA3 on soluble and insolublestarch were similar to those of wild-type GA, indicating thatfor soluble starch their deletions did not affect the catalyticdomain and for insoluble starch the linker does not coordinatethe activities of the catalytic and starch-binding domains.The deletions had a significant negative effect on GA2 and GA3thermos tabilities. 相似文献
75.
Automated airborne collision‐detection systems are a key enabling technology for facilitating the integration of unmanned aerial vehicles (UAVs) into the national airspace. These safety‐critical systems must be sensitive enough to provide timely warnings of genuine airborne collision threats, but not so sensitive as to cause excessive false alarms. Hence, an accurate characterization of detection and false‐alarm sensitivity is essential for understanding performance tradeoffs, and system designers can exploit this characterization to help achieve a desired balance in system performance. In this paper, we experimentally evaluate a sky‐region, image‐based, aircraft collision‐detection system that is based on morphological and temporal processing techniques. (Note that the examined detection approaches are not suitable for the detection of potential collision threats against a ground clutter background.) A novel collection methodology for collecting realistic airborne collision‐course target footage in both head‐on and tail‐chase engagement geometries is described. Under (hazy) blue sky conditions, our proposed system achieved detection ranges greater than 1540 m in three flight test cases with no false‐alarm events in 14.14 h of nontarget data (under cloudy conditions, the system achieved detection ranges greater than 1170 m in four flight test cases with no false‐alarm events in 6.63 h of nontarget data). Importantly, this paper is the first documented presentation of detection range versus false‐alarm curves generated from airborne target and nontarget image data. © 2012 Wiley Periodicals, Inc. 相似文献
76.
Cameron M. Ford Mark P. Sharfman James W. Dean 《Creativity & Innovation Management》2008,17(3):171-185
The study and practice of business strategy is fundamentally based on employing creative solutions to differentiate a firm from its competitors. Theories used to describe the causes and consequences of strategic differentiation tend to focus on organization‐level characteristics such as resources, capabilities and structures. However, less is known about day‐to‐day processes and practices whereby strategic managers develop create solutions necessary to establish strategic differentiation. This paper presents a preliminary field study of factors suggested by previous strategy process and micro‐strategy research that may lead to, and result from, creative strategic decisions. Findings produced by a longitudinal field study of 52 strategic decisions reveal that creative strategic choices arise in response to managers' perceptions of uncertainty and competition. The findings also suggest that creativity may improve the ultimate effectiveness of strategic choices by 5–10 per cent. 相似文献
77.
Single channel currents were activated by GABA (0.5 to 5 microM) in cell-attached and inside-out patches from cells in the dentate gyrus of rat hippocampal slices. The currents reversed at the chloride equilibrium potential and were blocked by bicuculline (100 microM). Several different kinds of channel were seen: high conductance and low conductance, rectifying and "nonrectifying." Channels had multiple conductance states. The open probability (Po) of channels was greater at depolarized than at hyperpolarized potentials and the relationship between Po and potential could be fitted with a Boltzmann equation with equivalent valency (z) of 1. The combination of outward rectification and potential-dependent open probability gave very little chloride current at hyperpolarized potentials but steeply increasing current with depolarization, useful properties for a tonic inhibitory mechanism. 相似文献
78.
R. A. Olson B. J. Christensen R. T. Coverdale S. J. Ford G. M. Moss H. M. Jennings T. O. Mason E. J. Garboczi 《Journal of Materials Science》1995,30(20):5078-5086
The d.c. conductivity, , and low-frequency relative dielectric constant, k, of Portland cement paste were monitored, using impedance spectroscopy, during cooling from room temperature down to -50 °C. Dramatic decreases in the values of and k, as great as two orders of magnitude, occurred at the initial freezing point of the aqueous phase in the macropores and larger capillary pores. This result provides strong experimental support for the dielectric amplification mechanism, proposed in Part II of this series, to explain the high measured low-frequency relative dielectric constant of hydrating Portland cement paste. Only gradual changes in the electrical properties were observed below this sudden drop, as the temperature continued to decrease. The values of and k of frozen cement paste, at a constant temperature of -40 °C, were dominated by properties of calcium-silicate-hydrate (C-S-H) and so increased with the degree of hydration of the paste, indicating a C-S-H gel percolation threshold at a volume fraction of approximately 15%–20%, in good agreement with previous predictions. Good agreement was found between experimental results and digital-image-based model computations of at -40 °C. Freeze-thaw cycling caused a drop in the dielectric constant of paste in the unfrozen state, indicating that measurements of k could be useful for monitoring microstructural changes during freeze-thaw cycling and other processes that gradually damage parts of the cement paste microstructure. 相似文献
79.
DV Beard PL Molina KE Muller KM Denelsbeck BM Hemminger JR Perry MP Braeuning DH Glueck WD Bidgood M Mauro 《Canadian Metallurgical Quarterly》1995,197(3):753-758
PURPOSE: Interpretation time of serial staging chest CT cases, which each contained current and previous examinations, with a simple prototype workstation called filmstack was experimentally compared with interpretation time with a film alternator. MATERIALS AND METHODS: The filmstack displayed a "stack" of sections for each examination; user controls allowed rapid selection of preset attenuation windows and both synchronized and unsynchronized scrolling. Eight radiologists were timed as they used the filmstack and the film alternator to interpret four ergonomically complex serial CT cases. RESULTS: All reports dictated on the basis of findings with filmstack and film were of acceptable clinical accuracy. The time to examine a case with filmstack was significantly faster than the time with film, including the time to load and unload the alternator (99% confidence [P = .01]). There was no statistically significant difference in interpretation time between filmstack and prehung film. CONCLUSION: Use of a low-cost stacked CT workstation with a single 1,024 x 1,024 monitor is an effective means of interpreting cases that require comparison of multiple CT examinations. 相似文献
80.
A detailed RFLP map was used to determine the chromosomal locations and subgenomic distributions of cotton (Gossypium) genes/QTLs that confer resistance to the bacterial blight pathogen, Xanthomonas campestris pv. malvacearum (Xcm). Genetic mapping generally corroborated classic predictions regarding the number and dosage effects of genes conferring Xcm resistance. One recessive allele (b6) was a noteworthy exception to the genetic dominance of most plant resistance alleles. This recessive allele appeared to uncover additional QTLs from both resistant and ostensibly susceptible genotypes, some of which corresponded in location to resistance (R)-genes effective against other Xcm races. One putatively "defeated" resistance allele (B3) reduced severity of Xcm damage by "virulent" races. Among the six resistance genes derived from tetraploid cottons, five (83%) mapped to D-subgenome chromosomes-if each subgenome were equally likely to evolve new R-gene alleles, this level of bias would occur in only about 1.6% of cases. Possible explanations of this bias include biogeographic factors, differences in evolutionary rates between subgenomes, gene conversion or other intergenomic exchanges that escaped detection by genetic mapping, or other factors. A significant D-subgenome bias of Xcm resistance genes may suggest that polyploid formation has offered novel avenues for phenotypic response to selection. 相似文献