全文获取类型
收费全文 | 4354篇 |
免费 | 106篇 |
国内免费 | 5篇 |
专业分类
电工技术 | 44篇 |
综合类 | 1篇 |
化学工业 | 848篇 |
金属工艺 | 79篇 |
机械仪表 | 75篇 |
建筑科学 | 186篇 |
矿业工程 | 23篇 |
能源动力 | 85篇 |
轻工业 | 394篇 |
水利工程 | 25篇 |
石油天然气 | 18篇 |
无线电 | 262篇 |
一般工业技术 | 650篇 |
冶金工业 | 1089篇 |
原子能技术 | 53篇 |
自动化技术 | 633篇 |
出版年
2022年 | 33篇 |
2021年 | 41篇 |
2019年 | 37篇 |
2018年 | 37篇 |
2017年 | 40篇 |
2016年 | 56篇 |
2015年 | 51篇 |
2014年 | 76篇 |
2013年 | 245篇 |
2012年 | 142篇 |
2011年 | 171篇 |
2010年 | 149篇 |
2009年 | 167篇 |
2008年 | 197篇 |
2007年 | 194篇 |
2006年 | 179篇 |
2005年 | 133篇 |
2004年 | 124篇 |
2003年 | 109篇 |
2002年 | 124篇 |
2001年 | 92篇 |
2000年 | 85篇 |
1999年 | 77篇 |
1998年 | 64篇 |
1997年 | 74篇 |
1996年 | 74篇 |
1995年 | 81篇 |
1994年 | 75篇 |
1993年 | 76篇 |
1992年 | 90篇 |
1991年 | 33篇 |
1990年 | 70篇 |
1989年 | 56篇 |
1988年 | 59篇 |
1987年 | 67篇 |
1986年 | 66篇 |
1985年 | 89篇 |
1984年 | 86篇 |
1983年 | 81篇 |
1982年 | 83篇 |
1981年 | 74篇 |
1980年 | 45篇 |
1979年 | 61篇 |
1978年 | 60篇 |
1977年 | 54篇 |
1976年 | 52篇 |
1975年 | 64篇 |
1974年 | 38篇 |
1973年 | 39篇 |
1972年 | 32篇 |
排序方式: 共有4465条查询结果,搜索用时 15 毫秒
21.
The purpose of this study was to examine the relationships between computer use, identity formation, and self-esteem among Black and White emerging adult females. One hundred 56 female college students participated in this study by completing a survey that included the Internet/Computer Use Questionnaire, the Ego Identity Process Questionnaire, and the Rosenberg Self Esteem Scale. Racial differences were found between Black and White females with regard to perceived problematic use of the Internet and use of the computer for communication and use of the Internet for schoolwork. In addition, differences between Black and White females were found between identity formation and problematic use of the Internet and between self-esteem and type of use of the Internet. These findings were discussed in relation to future research and interventions. 相似文献
22.
23.
24.
The objective of this research is to investigate the feasibility of a theoretical development which could lead to advanced manipulator concepts offering fast dynamic and control features. Specifically, the application of “dynamic linearization” to a kinematically redundant planar manipulator is studied. In comparing such a manipulator with a standard non-linear, non-redundant design it is shown that in addition to the expected merits in linearity, dexterity, and computational simplicity, savings in actuator energy consumption can also be realized using a relative simple trajectory optimization scheme. 相似文献
25.
Treatment of exponentially growing cultures ofStreptococcus mutans BHT with growth-inhibitory concentrations (0.2 μg/ml) of benzylpenicillin stimulates the incorporation of [2-14C] acetate into lipids excreted by the cells by as much as 69-fold, but does not change the amount of14C incorporated into intracellular lipids. At this concentration of penicillin cellular lysis does not occur. The radioactive
label is incorporated exclusively into the fatty acid moieties of the glycerolipids. The increase in the radioactive content
of the extracellular lipids reflects an actual net increase in the total fatty acid content as determined by a chemical assay.
During a 4-hr incubation in the presence of penicillin, the extracellular fatty acid ester concentration (per mg cell dry
weight) increases 1.5 fold, even though there is no growth or cellular lysis. No change is observed in the intracellular fatty
acid ester content.
An indication of the relative rate of fatty acid synthesis was most readily obtained by placingS. mutans BHT in a buffer containing14C-acetate. Under these nongrowing conditions free fatty acids are the only lipids labeled, a factor which simplifies the assay.
The addition of glycerol to the buffer causes all of the nonesterified fatty acids to be incorporated into glycerolipid. The
cells excrete much of the lipid whether glycerol is present or not. Addition of penicillin to the nongrowth supporting buffer
system does not stimulate the incorporation of [14C]-acetate into fatty acids. However, if cells are exposed to penicillin in a growth-supporting medium and then are transferred
to the nongrowing buffer system containing no penicillin, the previously exposed cells retain the ability to incorporate [14C]-acetate into fatty acid at a higher rate than untreated cells over a prolonged period of time. The stimulation of [14C]-acetate into fatty acids in this system parallels but is not dependent on the stimulation by penicillin of the incorporation
of [14C]-glycerol into glycerolipid and lipoteichoic acid synthesis previously demonstrated by our laboratory.
The material of this paper is part of a thesis to be submitted by J.L.B. in partial fulfillment of the requirements for the
Ph.D. degree from the Department of Biochemistry, Temple University. 相似文献
26.
Douglas H. Taylor Kirk E. Lagory Daniel J. Zaccaro Ronald J. Pfohl R.Dana Laurie 《The Science of the total environment》1985
Trichloroethylene (TCE) is a common contaminant of underground water supplies. To examine the effect of TCE on the developing central nervous system, rats were exposed to TCE throughout gestation until 21 days postpartum via their dams' drinking water. TCE concentrations of 312 mg/l, 625 mg/l and 1250 mg/l were tested. Exploratory behavior was higher in 60- and 90-day old male rats which were exposed to any level of TCE. The effect of TCE-exposure on locomotor activity (running wheel) was also examined in 60-day old males (625 and 1250 ppm exposure groups). Locomotor activity was significantly higher in rats exposed to 1250 ppm TCE. These data suggest that TCE has long-term effects on behaviour. 相似文献
27.
Shmuel Yannai Ronald R. Marquardt 《Journal of the science of food and agriculture》1985,36(11):1161-1168
Rats were fed an adequate or a deficient diet and offered water or buthionine sulphoximine (BSO) solution for 2 weeks, and then the same diets with vicine for another week in experiment 1. BSO in combination with the deficient diet caused a marked decrease in blood glutathione (GSH) and growth retardation but failed to show any effects resulting from supplementation with vicine. In experiment 2 the rats were given an adequate diet and BSO as before, and injected intravenously with divicine (DV). Here again, BSO depressed rat growth, and so did DV. Each of the insults also caused haematological changes, especially a fall in GSH, but the most severe changes appeared in the group treated with both BSO and DV. A decrease in haematocrit and increases in adrenal and spleen weight were also noted. In experiment 3 the rats were injected with different doses of DV, without pretreatment with BSO. The main effect was a drop in blood GSH and haematocrit, and an increase in adrenal and spleen weights, all of which were dose-related. Administration of the higher doses of DV resulted in a severe cyanosis followed by death within a relatively short period of time. 相似文献
28.
We were concerned with the effects of item repetition, list length, and class of item on free recall in elderly as compared with young adults. In Experiment 1, samples of young and elderly adults recalled a list of 27 words and a list of 27 action events (minitasks performed by the subjects). Some items were presented once and some twice. Although the younger subjects showed better recall on both types of lists, the older sample benefited from item repetition as much as did the younger sample. This finding was replicated in Experiment 2. A second finding in Experiment 2 was a significant aging effect in the recall of long but not of short lists of both words and action events. The absence of an Age?×?Repetition Effect interaction was ascribed to the strength nature of the repetition manipulation. The age effects in the recall of the long lists were attributed to possible deficits in retrieval proficiency. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
29.
Ronald Y. S. Pak Feng Ji 《International journal for numerical methods in engineering》1994,37(14):2501-2520
A boundary integral equation method is presented for the analysis of a thin cylindrical shell embedded in an elastic half-space under axisymmetric excitations. By virtue of a set of ring-load Green's functions for the shell and a group of dynamic fundamental solutions for the semi-infinite medium, the structure–medium interaction problem of wave propagation is shown to be reducible to a set of coupled boundary integral equations. Through the analysis of an auxiliary pair of Cauchy integral equations, the singularities of the contact stress distributions arc rendered explicit. With a direct incorporation of such analytical features into the formulation, an effective computational procedure is developed which involves an interpolation of regular functions only. Typical results for the dynamic contact load distributions, displacements, and complex compliance functions are included as illustrations. In addition to furnishing quantities of direct engineering interest, this treatment is apt to be useful as a foundation for further rigorous as well as approximate developments for various related physical problems and boundary integral methods. 相似文献
30.
Teresa A Morrison J Raymond Kessler Ronald D Hatfield Dwayne R Buxton 《Journal of the science of food and agriculture》1994,65(2):133-139
To ascertain the temporal relationships of phcnylpropanoid and lignin pathway enzymes, phenylalanine ammonia-lyase (PAL) and cinnamyl alcohol- NADPH dehydrogenase(CAD), with lignin concentration, PAL and CAD activities and lignin concentrations were assessed during progressive development of the 10th internode in maize (Zea mays L). Enzyme activities were quantified and lignin concentrations were determined by the detergent system of fiber fractiona- tion from lower, middle, and upper sections of the elongating internode harvested 3, 5, 7, 9, 11, 13, and 15 days after the internode had reached 10 mm in length. Over 15 days, a coordinated, sequential, and basipetal pattern of enzyme activities and lignin accumulation evolved through the internode, spanning the stages of rapid elongation, cellular differentiation, and secondary cell wall formation. PAL activity was initiated first, rising to peak activity in elongating tissue, and falling basipetally in tissues as they matured. CAD activity rose in fully elongated, maturing internode tissue where PAL activity was waning following its peak activity. Lignin accumulated in tissues with high CAD activity. CAD activity and lignin deposition patterns were similar: simultaneously increasing in activity and deposition over time while activity and deposition also shifting basipetally through the internode, keeping pace with secondary cell wall formation. Lignin concentration correlated significantly with CAD, but not with PAL. 相似文献