全文获取类型
收费全文 | 1636篇 |
免费 | 19篇 |
国内免费 | 3篇 |
专业分类
电工技术 | 15篇 |
化学工业 | 123篇 |
金属工艺 | 19篇 |
机械仪表 | 19篇 |
建筑科学 | 45篇 |
矿业工程 | 3篇 |
能源动力 | 10篇 |
轻工业 | 67篇 |
水利工程 | 4篇 |
石油天然气 | 1篇 |
无线电 | 78篇 |
一般工业技术 | 97篇 |
冶金工业 | 1117篇 |
原子能技术 | 5篇 |
自动化技术 | 55篇 |
出版年
2020年 | 6篇 |
2019年 | 4篇 |
2018年 | 7篇 |
2017年 | 8篇 |
2016年 | 14篇 |
2015年 | 7篇 |
2014年 | 10篇 |
2013年 | 31篇 |
2012年 | 19篇 |
2011年 | 27篇 |
2010年 | 20篇 |
2009年 | 26篇 |
2008年 | 21篇 |
2007年 | 28篇 |
2006年 | 16篇 |
2005年 | 20篇 |
2004年 | 25篇 |
2003年 | 16篇 |
2002年 | 17篇 |
2001年 | 7篇 |
2000年 | 15篇 |
1999年 | 42篇 |
1998年 | 342篇 |
1997年 | 197篇 |
1996年 | 146篇 |
1995年 | 82篇 |
1994年 | 57篇 |
1993年 | 75篇 |
1992年 | 22篇 |
1991年 | 20篇 |
1990年 | 15篇 |
1989年 | 20篇 |
1988年 | 21篇 |
1987年 | 11篇 |
1986年 | 13篇 |
1985年 | 22篇 |
1983年 | 13篇 |
1982年 | 19篇 |
1981年 | 13篇 |
1980年 | 20篇 |
1979年 | 7篇 |
1978年 | 8篇 |
1977年 | 28篇 |
1976年 | 29篇 |
1975年 | 10篇 |
1973年 | 4篇 |
1965年 | 5篇 |
1964年 | 4篇 |
1954年 | 6篇 |
1941年 | 4篇 |
排序方式: 共有1658条查询结果,搜索用时 31 毫秒
81.
The aim of this study was to provide valid data on the demography, medical history and clinical findings among adult women presenting with urinary incontinence to general practitioners. In a rural community in Norway, all women > 20 years who consulted their general practitioner for urinary incontinence during a 3 year period were included in a prospective study. A thorough medical history and both a general and focused clinical examination were undertaken. Gynecological examination, stress provocation test, and 48 h frequency/volume chart and pad weighing test were also performed. 105 women were included (4.4% of women > 20 years in the total population). Mean age was 57 years, 64% were postmenopausal. A lot of comorbidity was reported. Duration of incontinence was > 5 years in 49%. By a severity index, 64% were classified as severe, 28% as moderate and 8% as having slight incontinence. 59% were using protective pads or garments. Mean leakage per 24 h was 31 g. 38% had significant genital prolapse. Contractility of the pelvic floor muscles was weak in 28%. Diagnostic classification revealed 50% stress incontinence, 10% urge and 40% mixed incontinence. 42% of the patients were a great deal or much bothered by their incontinence. Patients with stress incontinence were less bothered than others. Women presenting with urinary incontinence at a primary care level are prevalent, and often have significant incontinence. It is a challenge for the general practitioners to investigate and treat these patients optimally. 相似文献
82.
83.
Constructed the Children's Self-Efficacy for Peer Interaction Scale (CSPI) and administered it to 245 3rd–5th graders along with the Piers-Harris Children's Self-Concept Scale and the Teacher Rating Scale of Social Efficacy. Analyses of reliability and construct validity indicated that the CSPI has psychometric properties that warrant its use. Ss' self-efficacy varied by grade and situation. The usefulness of the CSPI for research on children's social development is discussed. (24 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
84.
The Landsat multispectral scanner data have been analyzed with a view toward crop identification and inventory. The notion of spectral-temporal trajectory as crop signature, introduced in an earlier paper, has been further developed leading to a formulation of the problem similar to one of recognition of hand-written characters. A simple classification rule based on angular features of the trajectory is discussed and classification results are given for Landsat images from several sites. 相似文献
85.
The clay catalyzed dimer of linoleic acid has been examined by mass spectrometry of the unhydrogenated, the partially hydrogenated
and completely hydrogenated dimer. The results show that monocyclic, bicyclic and tricyclic structures are present. Monocyclic
structures predominate, bicyclic structures are also prominent, and tricyclic structures are relatively minor. The monocyclic
structure is believed to arise from a Diels-Alder type addition reaction. The bicyclic structure may result from a free radical
coupling followed by intramolecular ring closure. The monocyclic structure in the unhydrogenated dimer appears to be mostly
a benzene ring with saturated and unsaturated side-chains. It probably is formed by hydrogen transfer from the Diels-Alder
cyclohexene structure first formed. Little, if any, of the Diels-Alder dimer structure as such is present. The catalytic linoleate
dimer has a higher ratio of monocyclic to bicyclic dimer than does the noncatalytic (thermal) dimer made from normal (nonconjugated)
linoleate, while the thermal dimer of a conjugatedtrans-trans linoleate is exclusively monocyclic. It is suggested that the clay catalyzes conjugation and hence favors the Diels-Alder
reaction, and then catalyzes hydrogen transfer to aromatize the cyclohexene ring. 相似文献
86.
Characteristics of the calcareous deposits on several hundred IUDs of various designs and materials following in vivo exposures were examined, and an in vitro method of study was devised. There were 650 copper-wound IUDs and 18 plastic IUDs. The copper-wound IUDs had been in the uteri for 6-27 months; the plastic IUDs for 3 months to over 8 years. Microscopic examinations, X-ray diffraction, weight changes, and chemical analyses were used. X-ray diffraction showed that calcite (CaCO3) was the major crystalline constituent of the calcareous deposits. A large fraction of the deposited material was organic. Surfaces in contact with the uterine wall were essentially free of depos its. Microscopic inspection of copper-wound IUDs revealed the amount of deposit. Length of exposure increased the amount of deposit, but there were marked variations in amount. Accumulation of deposits was mostly at the fundal end. More marked local erosion of copper was noted where there was little or no carbonate deposit. Deposits occurred in patches on Lippes loops and Margulies spirals. The in vitro carbonate deposits did not reproduce the rates or morphology of the in vivo deposits. Variations in uterine carbonate formation are attributed to differences in uterine chemistry of individuals. 相似文献
87.
Collis VJ Reid CA Hutchison ML Davies MH Wheeler KP Small A Buncic S 《Journal of food protection》2004,67(11):2397-2402
The spread of microbial contamination on the hides of beef was investigated at two stages in the meat chain: (i) in a simulated livestock market ("the market") using 33 animals, and (ii) in the unloading-to-skinning area of a commercial abattoir using 18 animals. At both stages, harmless bacterial markers (nalidixic acid-resistant Escherichia coli K-12; rifampicin- and nalidixic acid-resistant Pseudomonas fluorescens; and a tetracycline-resistant E. coli) were inoculated on the hides of a small number of selected animals, and their transfer to other animals and the environment was examined. At the market, the initial prevalence of animals positive for the hide markers (9.1% in each phase) introduced in the presale pen, sale ring, and postsale pen changed to 39.4, 15.1, and 54.5%, respectively, by the end of the market process. In addition, widespread contamination of the market environment with the hide markers was observed. At the abattoir, the initial prevalence of animals positive for the hide marker (11.1%) inoculated at unloading increased to 100% (hide before skinning) and 88.8% (skinned carcass). In addition, another marker inoculated on environmental surfaces in lairage pens, races, and stunning box was detected on 83.3% (hide before skinning) and 88.8% (skinned carcass). These results, although obtained with a relatively small number of animals, demonstrate that both the livestock market process and the unloading-to-skinning process at abattoirs can facilitate the extensive spread of microbial contamination on hides not just within, but also between, batches of animals. 相似文献
88.
Wheeler EK Benett W Stratton P Richards J Chen A Christian A Ness KD Ortega J Li LG Weisgraber TH Goodson K Milanovich F 《Analytical chemistry》2004,76(14):4011-4016
We have fabricated a low-cost disposable polymerase chain reaction thermal chamber that uses buoyancy forces to move the sample solution between the different temperatures necessary for amplification. Three-dimensional, unsteady finite element modeling and a simpler 1-D steady-state model are used together with digital particle image velocimetry data to characterize the flow within the device. Biological samples have been amplified using this novel thermal chamber. Time for amplification is less than 30 min. More importantly, an analysis of the energy consumption shows significant improvements over current technology. 相似文献
89.
In an infinite planar array of elements with periodic spacing, the element active impedance varies with phasing for beam steering. This impedance variation may be expressed as the sum of a double Fourier series. This series is identified with the periodic grating-lobe pattern on the "sin theta plane" which is also the plane of two-dimensional phasing coordinates. An "impedance crater," with contours peculiar to the kind of element, is placed on every grating-lobe center. The inside of the central crater, which coincides with the unit circle of real space on this plane, determines the resistance variation with scan angle of the main lobe. The central crater and the skirts of the surrounding craters overlap in this circle; their sum determines the accompanying reactance variation. All craters together form the "grating-lobe series," which gives a picture of the entire impedance variation with scan angle. In a simple example, the reactance variation associated with half-wave spacing of the elements is found to be nearly equal to the resistance variation associated with the kind of element. 相似文献
90.