全文获取类型
收费全文 | 1901篇 |
免费 | 58篇 |
专业分类
电工技术 | 13篇 |
综合类 | 1篇 |
化学工业 | 367篇 |
金属工艺 | 15篇 |
机械仪表 | 32篇 |
建筑科学 | 78篇 |
矿业工程 | 2篇 |
能源动力 | 35篇 |
轻工业 | 214篇 |
水利工程 | 12篇 |
石油天然气 | 3篇 |
无线电 | 76篇 |
一般工业技术 | 370篇 |
冶金工业 | 568篇 |
原子能技术 | 6篇 |
自动化技术 | 167篇 |
出版年
2023年 | 24篇 |
2022年 | 45篇 |
2021年 | 58篇 |
2020年 | 38篇 |
2019年 | 34篇 |
2018年 | 37篇 |
2017年 | 37篇 |
2016年 | 36篇 |
2015年 | 34篇 |
2014年 | 46篇 |
2013年 | 92篇 |
2012年 | 100篇 |
2011年 | 133篇 |
2010年 | 100篇 |
2009年 | 84篇 |
2008年 | 114篇 |
2007年 | 102篇 |
2006年 | 100篇 |
2005年 | 86篇 |
2004年 | 76篇 |
2003年 | 62篇 |
2002年 | 69篇 |
2001年 | 43篇 |
2000年 | 37篇 |
1999年 | 23篇 |
1998年 | 56篇 |
1997年 | 29篇 |
1996年 | 43篇 |
1995年 | 30篇 |
1994年 | 24篇 |
1993年 | 19篇 |
1992年 | 11篇 |
1991年 | 9篇 |
1990年 | 13篇 |
1989年 | 11篇 |
1988年 | 8篇 |
1987年 | 5篇 |
1986年 | 4篇 |
1985年 | 11篇 |
1984年 | 7篇 |
1983年 | 7篇 |
1982年 | 6篇 |
1981年 | 4篇 |
1980年 | 5篇 |
1978年 | 4篇 |
1976年 | 5篇 |
1975年 | 4篇 |
1974年 | 4篇 |
1969年 | 4篇 |
1967年 | 5篇 |
排序方式: 共有1959条查询结果,搜索用时 15 毫秒
71.
The n-6/n-3 fatty acid (FA) ratio has increased in the Western-style diet to ~10–15:1 during the last century, which may have
contributed to the rise in cardiovascular disease (CVD). Prior studies have evaluated the effects on CVD risk factors of manipulating
the levels of n-6 and n-3 FA using food and supplements or investigated the metabolic fate of linoleic acid (LNA) and α-linolenic
acid (ALA) by varying the n-6/n-3 ratios. However, no previous studies have investigated the potential interaction between
diet ratios and supplementation with eicosapentaenoic acid (EPA, 20:5n-3) and docosahexaenoic acid (DHA, 22:6n-3). We used
a factorial design approach with adults (n = 24) in a controlled feeding trial to compare the accretion of EPA and DHA into red blood cell membranes (RBC) by adding
a direct source (algal oil supplement) of EPA and DHA in a diet with a 10:1 versus 2:1 ratio of n-6/n-3 FA. Subjects were
randomized into 8-week crossover diet sequences and each subject consumed three of four diets [10:1, 10:1 plus supplement
(10:1 + S), 2:1 and 2:1 + S]. LNA and ALA intakes were 9.4 and 7.7%, and 1.0 and 3.0% during the low and high ALA diets, respectively.
Compared to the Western-style 10:1 diet, the 2:1 diet increased EPA by 60% (P < 0.0001) in RBC membranes without the direct EPA source and a 34% increase (P = 0.027) was observed with the 10:1 + S diet; however, DHA levels increased in both diet ratios only with a direct DHA source.
Shifting towards a 2:1 diet is a valid alternative to taking EPA-containing supplements. 相似文献
72.
Eunyoung Lee Michelle Novais de Paula Sangki Baek Huynh Kim Khanh Ta Minh Tan Nguyen Taeck-Hyun Jeong Chong Jai Kim Yeon Jin Jang Han Choe 《International journal of molecular sciences》2021,22(12)
Human stem-cell factor (hSCF) stimulates the survival, proliferation, and differentiation of hematopoietic cells by binding to the c-Kit receptor. Various applications of hSCF require the efficient and reliable production of hSCF. hSCF exists in three forms: as two membrane-spanning proteins hSCF248 and hSCF229 and truncated soluble N-terminal protein hSCF164. hSCF164 is known to be insoluble when expressed in Escherichia coli cytoplasm, requiring a complex refolding procedure. The activity of hSCF248 has never been studied. Here, we investigated novel production methods for recombinant hSCF164 and hSCF248 without the refolding process. To increase the solubility of hSCF164, maltose-binding protein (MBP) and protein disulfide isomerase b’a’ domain (PDIb’a’) tags were attached to the N-terminus of hSCF164. These fusion proteins were overexpressed in soluble form in the Origami 2(DE3) E. coli strain. These solubilization effects were enhanced at a low temperature. His-hSCF248, the poly-His tagged form of hSCF248, was expressed in a highly soluble form without a solubilization tag protein, which was unexpected because His-hSCF248 contains a transmembrane domain. hSCF164 was purified using affinity and ion-exchange chromatography, and His-hSCF248 was purified by ion-exchange and gel filtration chromatography. The purified proteins stimulated the proliferation of TF-1 cells. Interestingly, the EC50 value of His-hSCF248 was 1 pg/mL, 100-fold lower than 9 ng/mL hSCF164. Additionally, His-hSCF248 decreased the doubling time, increased the proportion of S and G2/M stages in the cell cycle, and increased the c-Myc expression at a 1000-fold lower concentration than hSCF164. In conclusion, His-hSCF248 was expressed in a soluble form in E. coli and had stronger activity than hSCF164. The molecular chaperone, MBP, enabled the soluble overexpression of hSCF164. 相似文献
73.
Michelle Greenough Zeyu Zhao Luiz G. Jacobsohn Jianhua Tong Rajendra K. Bordia 《Journal of the European Ceramic Society》2021,41(12):5882-5889
This study focuses on the chemistry, thermal stability, and electrical conductivity of low/intermediate pyrolysis temperature (700?900 °C) polysiloxane derived ceramics. These ceramics were modified with additional carbon derived from divinylbenzene (DVB) added to the precursor. Their electrical properties were investigated for potential uses in micro-electrical mechanical systems (MEMS) and anodes for lithium batteries. The microstructure and chemical composition was investigated by attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), Raman spectroscopy, and x-ray photoelectron spectroscopy (XPS); thermogravimetric analysis (TGA) provided insight into the thermal stability; and electrochemical impedance spectroscopy (EIS) into the electrical properties of the material. The increase of pyrolysis temperature and carbon content lead to an enhancement of the electrical conductivity, higher than previously reported values for intermediate pyrolysis temperature SiOC polymer derived ceramics. A limit of the amount of DVB that can be added to PHMS to produce a hybrid precursor has also been obtained. 相似文献
74.
Perception of odor can figure prominently in complaints about indoor air,yet identification of the responsible compound(s) is often difficult. For example, paint emissions contain a variety of odorous volatile organic compounds (VOCs) which maytrigger reports of irritation and upper respiratory health effects. Texanol ester alcohol (2,2,4-trimethyl-1,3-pentanediol monoisobutyrate), a paint coalescing agent, is frequently associated with the "persistent, characteristic odor" of water-based paint. To evaluate the sensory impact of Texanol, naive (unfamiliar with paint constituents) and experienced (familiar with paint constituents) subjects evaluated the odor properties of paints with and without Texanol. VOC emissions from neat paint and paint applied to gypsum wallboard were collected via solid-phase microextraction and analyzed by gas chromatography/ mass spectrometry and gas chromatography/olfactometry. Regardless of subjects' prior experience, aromatic hydrocarbons and oxygenated compounds, introduced from other paint additives and not Texanol, were most commonly associated with paint odor. However, quantitative sensory techniques demonstrated that addition of Texanol to paints led to an overall increase in the perceived intensity of the coating. The combined use of these techniques proved to be an effective methodology for analyzing the structure of paint volatiles and their sensory properties and holds promise for solving many odorous indoor air problems. 相似文献
75.
Knowledge of explosives sorption and transformation processes is required to ensure that the proper fate and transport of such contaminants is understood at military ranges and ammunition production sites. Bioremediation of 2,4,6-trinitrotoluene (TNT), hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), and related nitroaromatic compounds has met with mixed success, which is potentially due to the uncertainty of how energetic compounds are bound to different soil types. This study investigated the dissolution and sorption properties of TNT and RDX explosives associated with six different soil types. Understanding the associations that explosives have with a different soil type assists with the development of conceptual models used for the sequestration process, risk analysis guidelines, and site assessment tools. In three-way systems of crystalline explosives, soil, and water, the maximum explosive solubility was not achieved due to the sorption of the explosive onto the soil particles and observed production of transformation byproducts. Significantly different sorption effects were also observed between sterile (gamma-irradiated) and nonsterile (nonirradiated) soils with the introduction of crystalline TNT and RDX into soil-water systems. 相似文献
76.
77.
Nutrient availability has long been considered one of the most important factors regulating production of benthic algae in oligotrophic rivers; yet, empirical relationships do not have as wide an application as similar models derived for lentic systems. The aim of this research was to derive empirical relationships between nutrient concentrations and benthic algal abundance and to identify commonalities with other studies to improve our understanding of constraints on algae in oligotrophic rivers. Surveys of physical, chemical and biological attributes of oligotrophic mountain rivers in spring, summer and autumn for 2 years confirmed that small amounts of anthropogenic phosphorus (0.1–5.6 µg/L total phosphorus (TP)) resulted in 4‐ to 30‐fold increases in abundance of benthic algae and benthic macroinvertebrates (BMIs). Algal accrual along a gradient in nutrient availability was not masked by grazing pressure but was positively correlated with abundance of scrapers. Epilithic abundance was highest downstream of anthropogenic nutrient sources in autumn. We concluded that benthic algal abundance in these mountain rivers was weakly correlated with phosphorus availability if light was not limiting but ultimately controlled by temperature and river discharge. Therefore, we recommend more direct measures of nutrient limitation be used to predict changes in ecological integrity at the lower end of the resource gradient. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
78.
Douglas Bessette Michelle Rutty Grant Gunn Volodymyr Tarabara Robert Richardson 《Journal of Great Lakes research》2021,47(1):226-235
Using the psychometric paradigm of risk in conjunction with surveys of the Michigan public (n = 638) and a regional planning organization (n = 65), we examine the perceived risk and concerns associated with underwater oil pipelines, the Enbridge Line 5 pipeline in particular, and oil spills under ice. The fate of Line 5 is heavily debated in Michigan, specifically the portion that traverses the Straits of Mackinac, which can be ice-covered for months. Scant literature examines how individuals perceive the risk associated with Line 5, its alternatives, or potential spills in open water or under ice. Here we identify considerable concern regarding both the pipeline and the potential for spills under ice on behalf of the public, and increased concern about spills under ice on behalf of the planning organization. Organization members' concerns are significantly predicted by beliefs about the difficulty in remediating spills, however not by beliefs about spills' likelihood, difficulty in detection, noticeability, or consequences. Our results identify the need to better examine and communicate the risks associated with underwater pipelines and spills, both in open water and under ice, as well as options for remediating oil captured under ice. Furthermore, we recommend the adoption of decision-making and risk governance processes that explicitly expand analysis of the social, economic and environmental tradeoffs of underwater pipelines such as Line 5. 相似文献
79.
Riparian and riverine systems around the world are subject to human mediated degradation. It is increasingly recognized that rehabilitation of these systems is important to retain biodiversity and important ecosystem services. Plant functional traits represent a potentially useful but underutilized tool in the restoration or rehabilitation of ecosystems such as riparian systems. This study uses a functional trait approach to provide insight into the changes in ecosystem function that have occurred with the loss of native plant species and their replacement by exotics. We examined the plant functional traits associated with extinct native, extant native and extant exotic species in the riparian margin of the Hunter River, New South Wales, Australia. Comparisons of 11 traits were conducted using univariate (analysis of variance and χ 2) and multivariate (principal component analysis and analysis of similarity) methods. Both extant native and extant exotic species had traits associated with a short lived, ruderal strategy: high specific leaf area (SLA), soft leaves, herbaceous growth forms and therophyte life form. In contrast, extinct native species had traits such as low SLA, tough leaves, tree and shrub growth form and phanerophyte life form. This shift in plant trait assemblages associated with the transition from a native to predominately exotic flora has important implications for ecosystem processes in this riverine system. We suggest that such a functional trait approach can be integrated with knowledge of other ecosystem components to provide an understanding of ecosystem function that can be used to guide rehabilitation initiatives. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
80.
This paper examines the optimal land allocation for two perennial crops, switchgrass and miscanthus that can be co-fired with coal for electricity generation. Detailed spatial data at county level is used to determine the costs of producing and transporting biomass to power plants in Illinois over a 15-year period. A supply curve for bioenergy is generated at various levels of bioenergy subsidies and the implications of production for farm income and greenhouse gas (GHG) emissions are analyzed. GHG emissions are estimated using lifecycle analysis and include the soil carbon sequestered by perennial grasses and the carbon emissions displaced by these grasses due to both conversion of land from row crops and co-firing the grasses with coal. We find that the conversion of less than 2% of the cropland to bioenergy crops could produce 5.5% of the electricity generated by coal-fired power plants in Illinois and reduce carbon emissions by 11% over the 15-year period. However, the cost of energy from biomass in Illinois is more than twice as high as that of coal. Costly government subsidies for bioenergy or mandates in the form of Renewable Portfolio Standards would be needed to induce the production and use of bioenergy for electricity generation. Alternatively, a modest price for GHG emissions under a cap-and-trade policy could make bioenergy competitive with coal without imposing a fiscal burden on the government. 相似文献