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   483129篇
  免费   7229篇
  国内免费   1541篇
电工技术   8473篇
综合类   336篇
化学工业   78255篇
金属工艺   20097篇
机械仪表   15636篇
建筑科学   11615篇
矿业工程   2586篇
能源动力   12680篇
轻工业   42853篇
水利工程   4778篇
石油天然气   9531篇
武器工业   19篇
无线电   56360篇
一般工业技术   94020篇
冶金工业   83503篇
原子能技术   11160篇
自动化技术   39997篇
  2021年   4529篇
  2019年   3985篇
  2018年   6672篇
  2017年   6610篇
  2016年   7266篇
  2015年   5043篇
  2014年   8365篇
  2013年   21935篇
  2012年   13553篇
  2011年   18130篇
  2010年   14549篇
  2009年   16409篇
  2008年   16421篇
  2007年   15775篇
  2006年   13873篇
  2005年   12774篇
  2004年   12028篇
  2003年   11468篇
  2002年   11400篇
  2001年   10938篇
  2000年   10393篇
  1999年   10558篇
  1998年   25089篇
  1997年   18142篇
  1996年   14252篇
  1995年   10890篇
  1994年   9498篇
  1993年   9296篇
  1992年   6996篇
  1991年   6901篇
  1990年   6579篇
  1989年   6618篇
  1988年   6440篇
  1987年   5548篇
  1986年   5509篇
  1985年   6273篇
  1984年   5904篇
  1983年   5403篇
  1982年   5073篇
  1981年   5224篇
  1980年   4955篇
  1979年   4890篇
  1978年   4957篇
  1977年   5695篇
  1976年   7334篇
  1975年   4440篇
  1974年   4244篇
  1973年   4318篇
  1972年   3735篇
  1971年   3437篇
排序方式: 共有10000条查询结果,搜索用时 11 毫秒
111.
We examined whether either psychotic features (e.g., delusions and hallucinations) or EEG abnormalities are associated with more rapid progression of Alzheimer's disease (AD). AD patients with psychosis have exhibited more EEG abnormalities than those without psychosis, and both abnormal EEG and psychosis have been noted to be predictors of functional and cognitive decline in AD. Ninety-five probable AD patients participating in a longitudinal study of dementia had an EEG and a semistructured psychiatric interview at baseline. Using EEG spectral analysis, we classified records as normal/abnormal based on the parasagittal mean frequency. Patients with abnormal EEGs were more functionally (e.g., Blessed Rating Scale for activities of daily living) and cognitively (e.g., Mini-Mental State) impaired than patients with normal EEG. AD patients with psychosis were only more functionally impaired than patients without psychosis. A two-factor analysis showed no interaction between abnormal EEG and psychosis. In addition, using a Cox proportional hazard model adjusted for age and education, the presence of an abnormal EEG or psychotic symptom at study entry was associated with higher risk of reaching severe cognitive and functional impairment during follow-up. Neither abnormal EEG nor the presence of psychosis predicted death. These results indicate that both abnormal EEG and psychosis are independent predictors of disease progression but not of physical survival.  相似文献   
112.
Ultra-high-sensitivity, biopolymer sequencing is a goal in many fields of molecular biology, and collisionally activated decomposition electrospray mass spectrometry (CAD ES MS/MS) using a triple quadrupole mass spectrometer has become a method of choice for work in the high- to mid-femtomole range. However, when the detection of ions becomes statistical, as it may in that range, the mass assignment of fragment ions is inaccurate and either sequencing becomes impossible or ambiguities result due, for example, to the closeness in amino acid residue masses (I/L, N or K/Q, E). Some ambiguities may be resolved by synthesizing possible sequences, but this is unsatisfactory. In considering the limitations of triple quadrupole MS/MS with respect to scanning ion detection, resolution, transmission, and mass accuracy, we reasoned that a novel geometry quadrupole orthogonal acceleration time-of-flight (Q-TOF) instrument would have special merit for ultra-high-sensitivity MS/MS sequencing, and suggested its construction for this purpose some three years ago. A prototype Q-TOF has now been built by Micromass [Morris et al. (1996), Rapid Commun. Mass Spectrom. 10, 889-896], and in the first research on the instrument, including MHC antigen and filarial nematode glycoprotein studies, we demonstrate low-femtomole- and attomole-range sequencing with mass accuracy of better than 0.1 Da throughout the daughter-ion spectrum, thus removing sequencing ambiguities in some of the most challenging work demanding the highest sensitivity.  相似文献   
113.
BACKGROUND: The effect of topical corticosteroids after excimer laser photorefractive keratectomy (PRK) remains a matter of some controversy. Refractive effects may be different according to the amount of myopia and timing of instillation. METHODS: Two groups of patients were studied: Study A consisted of 215 eyes (128 patients) with PRK (mean baseline myopia, -6.53 +/- 2.22 D) that received no corticosteroids (No Corticosteroid Group) unless significant regression or corneal haze appeared (Delayed Corticosteroid Group), and in Study B, we randomly assigned eyes to the Initial Corticosteroid Group (mean baseline myopia, -6.39 +/- 1.84 D) or the No/delayed Corticosteroid Group (mean baseline myopia -5.78 +/- 2.02 D). Clinical results after PRK for low-to-moderate and high myopia were compared. RESULTS: In the first group, 70.9% (73 eyes) of moderately myopic eyes (mean, -4.56 +/- 1.10 D) belonged to the No Corticosteroid Group that had a mean refraction of -5.39 +/- 1.77 D. Delayed Corticosteroid Group eyes were more myopic (mean, -7.52 +/- 2.10 D), and showed more severe haze than those in the No Corticosteroid Group. In study B, only in high myopes with more than -6.00 D (mean, -7.76 +/- 1.15 D) did refraction and corneal haze outcomes show significant difference between the Initial Corticosteroid Group and the No/delayed Corticosteroid Group. CONCLUSIONS: The effects of topical corticosteroids after PRK were less in moderate myopes compared to high myopes. Delayed instillation of corticosteroids did not reverse the regression or haze whereas initial instillation showed a beneficial effect on high myopes but not on moderate myopes.  相似文献   
114.
A high prevalence of epilepsy (up to 1.3%) was observed in an area hyperendemic for onchocerciasis in the province of Bururi, Burundi. Following this observation, epileptics and controls were examined in 2 communes (administrative units) with different endemicity for onchocerciasis. Altogether, onchocerciasis was more frequent in epileptics (81.8%) than in controls from the same households (68.3%; P < 0.05). This difference was more marked in the hyperendemic area. Other possible causes of epilepsy, including cysticercosis, were infrequent (18 possible cases among 110 epileptics). During this survey, 9 epileptics with growth retardation were seen. They showed most characteristics of Nakalanga syndrome, which was described from Uganda. All 9 such cases were suffering from onchocerciasis. These findings give more evidence of a possible association between onchocerciasis and epilepsy, and between onchocerciasis and Nakalanga syndrome.  相似文献   
115.
OBJECTIVE: To evaluate the relationships between patient and physician pretreatment expectations of pain relief and subsequent pain relief reported by chronic pain patients immediately after treatment. DESIGN: Prospective study of consecutive patients undergoing a procedure in a pain clinic for treatment of chronic pain. Patients rated their current pain level and their expectation of pain relief immediately prior to undergoing a procedure (e.g., intravenous drug infusion, nerve block) for the treatment of chronic pain. Simultaneously and independently, the treating physician completed a similar questionnaire. At completion of the procedure, patients rated their current pain level and degree of pain relief. SETTING: University of Washington Multidisciplinary Pain Center procedure suite. PATIENTS: Forty-six consecutive chronic pain patients. INTERVENTION: Intravenous drug infusions and nerve blocks. OUTCOME MEASURES: Current pain and pain relief ratings. RESULTS: Patients' pain relief expectation ratings were not correlated significantly with their postprocedure pain relief ratings or pre-post procedure changes in pain ratings. However, a statistically significant correlation was found between physician expectations of pain relief and patient pain relief ratings and patient pre-post procedure changes in pain. CONCLUSIONS: The results of this study suggest that physicians are better predictors than are patients of patients responses to these procedures and/or that physicians may somehow subtly communicate their expectations to patients during the procedure, and these expectations then influence patient response. Patient pretreatment expectations may not always play a significant role in nonspecific treatment effects.  相似文献   
116.
The ITER magnet system consists of structurally linked sets of toroidal (TF) and poloidal (PF) field coils, central solenoid (CS), and various support structures. The coils are superconducting, force flow Helium cooled with a Kapton-Glass-Epoxy multilayer insulation system. The stored magnetic energy is about 100GJ in the TF system and 20GJ in the PF-CS. Coils and structure are maintained at 4 K by enclosing them in a vacuum cryostat. The cryostat, comprising an outer envelope to the magnets, forms most of the second radioactivity confinement barrier. The inner primary barrier is formed by the vacuum vessel, its ports and their extensions. To keep the machine size within acceptable bounds, it is essential that the magnets are in close proximity to both of the nuclear confinement barriers. The objective of the magnet design is that, although local damage to one of the barriers may occur in very exceptional circumstances, large scale magnet structural or thermal failure leading to simultaneous breaching of both barriers is not credible. Magnet accidents fall into three categories: thermal (which includes arcing arising from insulation failure and local overheating due to discharge failure in the event of a superconductor quench), structural (which includes component mechanical failure arising from material inadequacies, design errors and exceptional force patterns arising from coil shorts or control failures), and fluid (Helium release due to cooling line failure). After a preliminary survey to select initial faults conceivable within the present design, these faults are systematically analyzed to provide an assessment of the damage potential. The results of this damage assessment together with an assessment of the reliability of the monitoring and protective systems, shows that the magnets can operate with the required safety condition.  相似文献   
117.
118.
A study was undertaken to determine fertility in a group of females who as children had been operated on for appendicitis. The 134 women operated on for appendicitis were reviewed. Their ages ranged from 2-18 years at the time of appendectomy. Our data show that perforated appendicitis before puberty has little if any role in the etiology of tubal infertility.  相似文献   
119.
This paper presents a new self-routing packet network called the plane interconnected parallel network (PIPN). In the proposed design, the traffic arriving at the network is shaped and routed through two banyan network based interconnected planes. The interconnections between the planes distribute the incoming load more homogeneously over the network. The throughput of the network under uniform and heterogeneous traffic requirements is studied analytically and by simulation. The results are compared with the results of the baseline network and another banyan network based parallel interconnection network. It is shown that, for the proposed design, a higher degree of heterogeneity results in better performance  相似文献   
120.
We predict that, for wavelength division multiplexing optical-network applications, an asymmetrically dilated configuration of a 2×2 cross-connect is significantly better in terms of overall crosstalk when the levels of the bar-port crosstalk and the cross-port crosstalk are significantly different from each other, as is the case with optical-frequency filters which utilize grating-assisted coupling. As a verification, we present a simulation study with 2×2 polarization-diversified acousto-optic tunable filters. We present a recursive method to extend the principle of asymmetric dilation to larger-size cross-connect switches, and make a recommendation for an asymmetrically dilated 4×4 cross-connect configuration  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