首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   1111篇
  免费   30篇
  国内免费   12篇
电工技术   63篇
综合类   5篇
化学工业   332篇
金属工艺   30篇
机械仪表   41篇
建筑科学   18篇
能源动力   55篇
轻工业   116篇
水利工程   2篇
无线电   57篇
一般工业技术   230篇
冶金工业   115篇
原子能技术   39篇
自动化技术   50篇
  2023年   3篇
  2022年   8篇
  2021年   17篇
  2020年   8篇
  2019年   16篇
  2018年   5篇
  2017年   17篇
  2016年   19篇
  2015年   13篇
  2014年   27篇
  2013年   60篇
  2012年   35篇
  2011年   70篇
  2010年   58篇
  2009年   65篇
  2008年   55篇
  2007年   49篇
  2006年   49篇
  2005年   39篇
  2004年   52篇
  2003年   41篇
  2002年   44篇
  2001年   30篇
  2000年   28篇
  1999年   29篇
  1998年   52篇
  1997年   34篇
  1996年   26篇
  1995年   21篇
  1994年   21篇
  1993年   10篇
  1992年   13篇
  1991年   18篇
  1990年   11篇
  1989年   13篇
  1988年   12篇
  1987年   4篇
  1986年   12篇
  1985年   6篇
  1984年   7篇
  1983年   7篇
  1982年   5篇
  1981年   6篇
  1979年   4篇
  1978年   4篇
  1977年   10篇
  1976年   5篇
  1973年   3篇
  1970年   2篇
  1969年   2篇
排序方式: 共有1153条查询结果,搜索用时 15 毫秒
71.
The off-axis and on-axis spectra in the far zone of an aperture for the case in which a particular class of partially coherent light with a broad spectrum is diffracted by an aperture are studied. It is shown that the spectrum in the far zone is generally different from that at the aperture; i.e., the spectrum is split into two or more peaks. Moreover, the spectrum varies with the diffractive angle. For a fixed diffractive angle, the spectral shift, defined as the difference between the frequencies at which the observed spectrum and the spectrum at the aperture take their maximum, shows a gradual change with the change in the coherence at the aperture. However, as the coherence reaches some critical values, the spectral shift exhibits a rapid transition; i.e., spectral switch occurs. The coherence that causes the spectral switch to take place is different for different diffractive angles. Therefore we propose a new kind of 1 x N spectral switch, where N detectors (output ports) are placed at different diffractive angles in the far zone, and the spectral shifts at different detectors are measured. By adjusting the coherence of the aperture (input port) to the desired values, we obtain a rapid transition of the spectral shift in the desired output ports.  相似文献   
72.
The fermentation product of herbs by lactic acid bacteria (FHL), in which Enterococcus faecalis TH10 predominated, was assayed for antifungal activity against tinea. The antifungal activity of FHL was as high as that of a synthetic fungicide. Autoclaving FHL did not reduce its antifungal activity, whereas neutralizing it did. The results suggested that nonproteinaceous compounds or organic acids in FHL could inhibit the growth of the dermatophyte tinea under low-pH conditions, and that malonic acid and acetic acid could have especially high antifungal activity against tinea.  相似文献   
73.
The existing design guidelines for buried flexible pipes are limited to depth up to 10?m. The increasing use of difficult terrains for infrastructure, landfills, and residential and industrial developments has prompted installation of drainage pipelines under 20–30?m high fills. This paper presents the behavior of an instrumented flexible pipe buried under a 47.1?m deep fill. For filling above 20?m, the measured vertical stress above the pipe exhibited a concave distribution, corresponding to 90 and 110% of the average vertical pressure at the center and edges of the pipe, respectively. The measured results suggest that a triangular lateral pressure distribution can lead to overly conservative and uneconomical results for high fills while Spangler’s analysis is unconservative. Based on the measured results, a revised vertical and lateral earth pressure diagram was proposed for the design of flexible pipe under high fills >20?m. This paper proposes closed-form analyses for estimating the moments and displacements of the pipe subjected to high fill earth pressures. These closed-form solutions agree well with the measured pipe displacements and strains.  相似文献   
74.
Laparoscopic cholecystectomy (LC) and endoscopic sphincterotomy (EST) are widely accepted procedures for cholecysto-choledocholithiasis in adults. However, their use in infants has not been reported. An 8-month-old girl presented with high fever and obstructive jaundice. Ultrasound scan showed acute cholecystitis with stones in the bile duct. After 2-week-long antibiotic therapy the acute cholecystitis and hepatic impairment resolved. An endoscopic retrograde cholangiopancreatography (ERCP) confirmed choledocholithiasis and cholecystolithiasis. Risk factors for the development of biliary calculi were not detected. One month after the restoration of her liver function, she underwent EST using a side-viewing endoscope with a small sphincterotome. A common bile duct stone was extracted using a basket catheter. LC was then carried out. The time interval between the EST and LC was 34 days. No complications have been noted for 6 months.  相似文献   
75.
To understand the role of NMAA in the bonding of composite resin to a dentin surface, we investigated the effects of N-methacryloyl amino acid (NMAA) application on the expansion of aggregated collagen fibers, formation of a hybrid layer, and the tensile bond strength between composite resin and dentin. Four NMAA derivatives--N-methacryloyl-alpha-glycine (NMGly), N-methacryloyl-gamma-amino n-butyric acid (NMBu), N-methacryloyl-alpha-hydroxyproline (NMHPro), and N-methacryloyl-alpha-glutamic acid (NMGlu)--were prepared and applied to dentin surfaces which had been etched with 40% by mass H3PO4 and air-blown. The shrunken collagenous layer expanded by approximately 50% to 70% by volume of the original collagenous layer thickness after application of the NMAA primers. Application of the bonding agent and composite resin after NMAA treatment resulted in the formation of a hybrid layer. The thickness of the hybrid layer was somewhat smaller than the collagenous layer formed by the NMAA treatment only, regardless of the type of NMAA used. The thickness of the hybrid layer was approximately ten times larger than that formed without NMAA treatment. Although all NMAA primers formed hybrid layers of similar thickness, higher tensile bond strengths, from 13 to 15 MPa, were obtained when etched and air-blown dentin was treated with NMBu, NMGly, or NMGlu. NMHPro gave only 6.6 MPa, a value similar to that obtained when no NMAA was used. We concluded, therefore, that formation of the hybrid layer is a necessary but insufficient condition for high bond strength.  相似文献   
76.
One difficulty in analyzing the state of filament in the dry spinning process is that in the boundary conditions required to solve the equations of mass, momentum and energy which are derived on the consideration of balance with respect to infinitesimally small element are not given a priori. The equations which include these boundary conditions in themselves are derived by considering mass, momentum, and energy balances with respect to the entire cross section of filament. These additional macroscopic equations are simplified to a great extent by assuming the flat velocity profile through the cross section of filament. Besides, in the steady state, these macroscopic equations are modified to give the equations of average solvent content, spinning tension, cross-sectional area, and average temperature. When the spinning conditions are given and the physical constants are measured for a given polymer and solvent system, it becomes possible to calculate the state of filament by solving these microscopic and macroscopic equations simultaneously without resorting to actual spinning.  相似文献   
77.
Electrophoretic karyotype studies have shown that clinical isolates of Candida albicans have extensive chromosome length polymorphisms. Chromosome translocation is one of the causes of karyotypic variation. Chromosome translocation events have been shown to occur very frequently at or near the major repeat sequence (MRS) on chromosomes. The MRS consists of the repeated sequences RB2, RPS and HOK, and the repeated sequences are considered to be the template for recombination. To investigate which element of the MRS is important for chromosome translocation, we constructed three cassettes, each containing a URA blaster and sequences homologous to one of the repeats, for insertion into the MRS region on the chromosomes. The ura3 strain STN22u2, which shows a stable, standard karyotype, was transformed with each construct. Insertion events with each cassette occurred at almost all chromosomes. Insertion into the RB2 repeat, but not into the RPS repeat, was accompanied by chromosome translocation in some transformants: chromosome translocations between chromosomes R and 7 and chromosomes 1 and 7 were found, as well as deletions of 7A and 7C from chromosome 7. We conclude that the insertion at the RB2 region may initiate chromosome translocation in C. albicans.  相似文献   
78.
It is shown that when partially coherent polychromatic light is focused by a filter-lens system with chromatic aberration, a spectral shift exists in the focused field, and a spectral switch that is defined as a sharp transition of the spectral shift also takes place at some positions of the focused field. The influence of the chromatic aberration of the lens, the coherence of the partially coherent light in the filter (a circular aperture), the radius of the aperture, and the spectral width of the partially coherent light in the aperture on the spectral shift and the spectral switch are investigated in detail. The numerical results show that these parameters affect the spectral shift and the spectral switch significantly. Potential applications of the spectral shift and the spectral switch of the partially coherent light are discussed.  相似文献   
79.
Diversity control in ant colony optimization   总被引:1,自引:0,他引:1  
Optimization inspired by cooperative food retrieval in ants has been unexpectedly successful and has been known as ant colony optimization (ACO) in recent years. One of the most important factors to improve the performance of the ACO algorithms is the complex trade-off between intensification and diversification. This article investigates the effects of controlling the diversity by adopting a simple mechanism for random selection in ACO. The results of computer experiments have shown that it can generate better solutions stably for the traveling salesmen problem than ASrank which is known as one of the newest and best ACO algorithms by utilizing two types of diversity.  相似文献   
80.
To clarify the effects of microwave roasting on the distribution of tocopherols and FA of phospholipids within soybeans, whole soybeans (Glycine max) were treated by microwave and further evaluted as compared to a raw sample. Tocopherol homologs, measured using HPLC, and phospholipid profiles, quantified with GC, were determined in the seed coat, the embryonic axis, and selections of cotyledons separated from three cultivars. The tocopherols were predominantly detected in the axis, followed by the cotyledons, and then very little in the coat. As much as 25% of the individual tocopherols originally present in the coat were lost at 12 min of roasting, whereas <25% was lost in the cotyledons and the axis after 20 min of roasting. The greatest rate of phospholipid loss (P<0.05) was observed in PE, followed by PC and PI, and their changing patterns were more pronounced in the coat than in the cotyledons or the axis. Thus, tocopherol content and phospholipid profiles change with microwave roasting according to tissue.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号