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   577759篇
  免费   5923篇
  国内免费   1536篇
电工技术   10390篇
综合类   390篇
化学工业   87656篇
金属工艺   27137篇
机械仪表   19874篇
建筑科学   11605篇
矿业工程   4939篇
能源动力   14287篇
轻工业   35677篇
水利工程   7255篇
石油天然气   16913篇
武器工业   36篇
无线电   62496篇
一般工业技术   126368篇
冶金工业   97985篇
原子能技术   16418篇
自动化技术   45792篇
  2021年   5543篇
  2020年   4275篇
  2019年   5431篇
  2018年   9773篇
  2017年   9968篇
  2016年   10637篇
  2015年   6254篇
  2014年   10575篇
  2013年   26877篇
  2012年   16203篇
  2011年   21261篇
  2010年   17067篇
  2009年   19210篇
  2008年   19652篇
  2007年   19223篇
  2006年   16719篇
  2005年   14881篇
  2004年   14302篇
  2003年   14089篇
  2002年   13389篇
  2001年   13215篇
  2000年   12597篇
  1999年   12582篇
  1998年   29748篇
  1997年   21097篇
  1996年   16108篇
  1995年   12225篇
  1994年   10904篇
  1993年   11031篇
  1992年   8365篇
  1991年   8043篇
  1990年   8128篇
  1989年   7685篇
  1988年   7365篇
  1987年   6729篇
  1986年   6520篇
  1985年   7222篇
  1984年   6781篇
  1983年   6241篇
  1982年   5788篇
  1981年   5906篇
  1980年   5646篇
  1979年   5769篇
  1978年   5878篇
  1977年   6354篇
  1976年   7791篇
  1975年   5205篇
  1974年   5116篇
  1973年   5216篇
  1972年   4546篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
Intracranial tumours, often presenting with progressive hydrocephalus, are rare congenital diseases accounting for 0.5-1.5 per cent of all cases of brain tumours diagnosed during childhood. The differential diagnosis includes vascular malformations, infarctions, and haemorrhages. Sonographic signs suggestive of glioblastoma, teratoma, and astrocytoma do not establish the histological diagnosis, however. We report a case of an undifferentiated fetal glioma detected at 29 weeks' gestation. The diagnosis of an undifferentiated brain tumour was suspected by sonography because of the lack of normal brain structures in conjunction with a diffuse echogenic central lesion and an external hydrocephalus. Because of the very poor prognosis, we induced labour by intravaginal and intravenous administration of prostaglandin E2 and achieved the vaginal delivery of a stillborn child whose head circumference corresponded to 38 weeks of pregnancy. Histological and immunochemical features of this undifferentiated congenital glioma (glioblastoma) are presented.  相似文献   
992.
OBJECTIVE: The purpose of our study was to test the hypothesis that surfactant dosing through a proximal sideport adapter on an endotracheal tube leads to more dosing-associated hypoxemia compared with a method of dosing that uses a double-lumen endotracheal tube. STUDY DESIGN: Using adequate sample size to compare significant changes in O2 saturation (power > 0.8, alpha < 0.05) we enrolled 36 infants with respiratory distress syndrome in this randomized trial. A 10% change in O2 saturation was considered clinically significant. Nineteen infants received 38 doses of surfactant through the sideport adapter. Seventeen infants received 31 doses of surfactant through the nonventilation lumen of a double-lumen endotracheal tube. Two main outcome measures were assessed: time-averaged O2 saturation values 30 minutes after dosing and the largest absolute fall in O2 saturation for each patient. RESULTS AND CONCLUSIONS: Time-averaged O2 saturation measures were higher in the proximal sideport group (p = 0.02), but the magnitude of difference was probably not clinically significant. No significant difference was detected between groups when we compared largest absolute drop in O2 saturation. Secondary analyses found no effect of birth weight or dose number (second vs third dose) on either outcome measure.  相似文献   
993.
The targeting of karyophilic proteins to nuclear pores is mediated via the formation of a nuclear pore-targeting complex, through the interaction of nuclear localization signal (NLS) with its NLS receptor. Recently, a novel human protein, Qip1, was identified from a yeast two-hybrid system with DNA helicase Q1. This study demonstrates that Qip1 is a novel third class of NLS receptor that efficiently recognizes the NLS of the helicase Q1. Moreover, the data obtained in this study show that the specific interaction between Qip1 and the NLS of the helicase Q1 requires its upstream sequence of the minimal essential NLS. By using purified recombinant proteins alone in the digitonin-permeabilized cell-free transport system, it was demonstrated that the two known human NLS receptors, Rch1 and NPI-1, are able to transport all the tested NLS substrates into the nucleus, while Qip1 most efficiently transports the helicase Q1-NLS substrates, which contain its upstream sequence in so far as we have examined the system. Furthermore, in HeLa cell crude cytosol, it was found that endogenous Rch1 binds to all the tested NLS substrates, while the binding of endogenous NPI-1 is restricted to only some NLSs, despite the fact that NPI-1 itself shows binding activity to a variety of NLSs. These results indicate that at least three structurally and functionally distinct NLS receptors exist in the human single cell population, and suggest that the nuclear import of karyophilic proteins may be controlled in a complex manner at the NLS recognition step by the existence of a variety of NLS receptors with various specificities to each NLS.  相似文献   
994.
We report 34 cases of tsutsugamushi disease seen from 1989 to 1993 at Yagi Clinic, northern Osumi, Kagoshima Prefecture. Nineteen patients (55.9%) showed the highest antibody titers against the Kawasaki strain Orientia tsutsugamushi (Ot) and 13 (38.2%) against the Kuroki strain Ot. It is suggested that two antigenic types (Kawasaki and Kuroki) of Ot were distributed in Kagoshima Prefecture, and the Kawasaki type Ot more or less dominates Kuroki type Ot. There was no difference in clinical features between the two groups of patients.  相似文献   
995.
OBJECTIVE: To investigate the efficacy of the rectus abdominis muscle flap (RAMF) technique for the closure and augmentation of small, non-compliant bladder exstrophies. PATIENTS AND METHODS: The RAMF technique was used in two girls and two boys (mean age at operation, 31.7 months, range 3-72) with bladder exstrophy. The clinical outcome and urodynamics were assessed during a follow-up of 29 months to 6 years (mean 49.2 months) and included imaging, cystoscopy, biochemical and microbiological studies. RESULTS: There were no urinary tract infections, metabolic problems or electrolyte disturbances and kidney function remained normal in all patients. Radiography confirmed intact function and anatomy of the urinary tract and cystoscopy showed complete coverage of the inner peritoneal layer of RAMF with uroepithelium. No stone formation or mucus production were detected. Currently, three patients void using clean intermittent catheterization through the native urethra and the fourth through an appendiceal Mitrofanoff valve. However, the bladder volume was insufficient at the late follow-up and three patients required bladder augmentation. CONCLUSIONS: The RAMF technique is a good alternative for closing bladder exstrophies and achieves an increase in bladder capacity; however, although there is a mild improvement in bladder capacity and compliance. RAMF should not be used as a bladder augmentation procedure. The technique is indicated in the closure of large bladder defects, bladder exstrophies with small, inelastic, non-compliant bladder remnants and failed primary closures.  相似文献   
996.
Karode  S.L. Fusco  V.F. 《Electronics letters》1997,33(16):1350-1351
A new architecture for the Van Atta self-steering array is presented. Active antenna techniques are used to realise a compact self-steering array with enhanced retrodirective properties. The experimental performance of the array is demonstrated and a technique is described which compensates for the beam pointing error induced by antenna array element directivity  相似文献   
997.
Using an enhanced multiple output domino logic (EMODL) implementation of a carry lookahead adder (CLA), sums of several consecutive bits can be built in one nFET tree with a single carry-in. Based on this result, a new sparse carry chain architecture is proposed for the CLA adder. We demonstrate the design approach using a 32-b adder, and show that only four carries are sufficient for generating all sums, with a consequent reduction in the number of stage delays. Using a 1.2-μm CMOS technology, we verify our simulation procedures by fabrication and measurement of a 2.7 ns critical path  相似文献   
998.
Bufferless distributed circuit (BDC) broadcasting is proposed as a technique for broadcasting high-speed chip input signals to a series of on-chip destination cells as needed in crosspoint switch, parallel multiplier, distributed amplifier, etc., chip designs. In contrast with conventional techniques that use an on-chip buffer to assist broadcasting, BDC broadcasting offers the advantage of lower signal delay and power dissipation. In an experimental GaAs heterojunction bipolar transistor (HBT) 8×4 crosspoint switch assembly, BDC broadcasting was found to achieve a 40% power savings with little or no penalty in jitter or bit error rate performance at a 10-Gb/s data rate  相似文献   
999.
Presents a model suitable for computing images of absorption cross sections of thick tissue structures illuminated at near infrared (NIR) wavelengths from tomographic projection data. Image reconstruction is accomplished by solving a system of linear equations derived from transport theory. Reconstruction results using different algebraic solvers are shown for anatomical maps of the breast, derived from magnetic resonance imaging data, containing two simulated pathologies, in which case qualitatively good reconstructions were obtained. Evaluation of magnetic resonance (MR) data to optimize NIR optical tomographic imaging methods and to assess the feasibility of a combined MR-optical measurement scheme is discussed  相似文献   
1000.
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