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   5963篇
  免费   139篇
  国内免费   35篇
电工技术   52篇
综合类   59篇
化学工业   570篇
金属工艺   109篇
机械仪表   100篇
建筑科学   91篇
矿业工程   10篇
能源动力   146篇
轻工业   216篇
水利工程   15篇
石油天然气   17篇
武器工业   2篇
无线电   473篇
一般工业技术   717篇
冶金工业   3082篇
原子能技术   15篇
自动化技术   463篇
  2023年   23篇
  2022年   54篇
  2021年   71篇
  2020年   39篇
  2019年   43篇
  2018年   52篇
  2017年   58篇
  2016年   66篇
  2015年   50篇
  2014年   91篇
  2013年   259篇
  2012年   178篇
  2011年   184篇
  2010年   149篇
  2009年   173篇
  2008年   150篇
  2007年   139篇
  2006年   152篇
  2005年   114篇
  2004年   100篇
  2003年   86篇
  2002年   72篇
  2001年   71篇
  2000年   71篇
  1999年   154篇
  1998年   929篇
  1997年   572篇
  1996年   420篇
  1995年   238篇
  1994年   198篇
  1993年   224篇
  1992年   78篇
  1991年   75篇
  1990年   60篇
  1989年   73篇
  1988年   85篇
  1987年   59篇
  1986年   60篇
  1985年   43篇
  1984年   14篇
  1983年   15篇
  1982年   28篇
  1981年   37篇
  1980年   32篇
  1979年   16篇
  1978年   7篇
  1977年   62篇
  1976年   170篇
  1975年   15篇
  1972年   6篇
排序方式: 共有6137条查询结果,搜索用时 17 毫秒
71.
72.
73.
Risks always exist in construction projects and often cause schedule delay or cost overrun. Risk management is a key issue in project management. The first step of risk management is risk identification. It includes the recognition of potential risk event conditions in the project and the clarification of risk responsibilities. We conducted multiple-case studies using a systematic analytical procedure to identify risks in highway projects in Taiwan, to recognize risk allocation by contract clauses, and to analyze the influence of risk allocation on the contractor’s risk handling strategies. The results show that the owner allocates risks by stipulating specific contract clauses into five kinds of risk allocation conditions. If a risk is more controllable by the contractor, the owner has a greater tendency to allocate the risk to the contractor. Risk allocation determines which kinds of risks the contractor would take and influences the contractor’s risk handling decisions. The analysis furthermore indicates that, if the probability of a certain risk event condition is uncontrollable, then with the increasing possibility of taking the risk, the contractor’s tendency of risk handling changes from actively transferring the risk to passively retaining the risk. In contrast, if a risk is controllable and certainly allocated to the contractor, the contractor tends to take the initiative to reduce the impact caused by the risk event rather than retain the risk.  相似文献   
74.
This study tried to show the contributory role of ions (cations and anions) to great contrast in the goitre prevalence between Bassa and Jos Local Government Areas, both of Plateau State, Nigeria. In pursuance of this, the concentration of cations (Ca++, Fe++, K+, Mg++, Na+, & Zn++) and anions (C1-, F, I-, & NO-3) in soil and drinking water in the two LGAs were determined and their results compared and correlated with the goitre prevalence of these areas. It was observed that both Bassa and Jos LGAs have very low but similar amounts of iodide ions in their soils. Besides, Bassa LGA contained more ions in both soil and drinking water than Jos LGA. In conclusion, the results appear to suggest that the higher ion contents of both soil and drinking water in Bassa LGA exacerbated the coexisting low iodide condition of the LGA, thus resulting in the higher goitre prevalence found in the LGA.  相似文献   
75.
76.
BACKGROUND AND PURPOSE: Epidemiological studies indicate a higher incidence of intracerebral (but not subarachnoid) hemorrhagic stroke among persons with low total serum cholesterol levels. This report further examines the prospective relationship of total serum cholesterol with subsequent intracerebral hemorrhage in a large, well-defined population. METHODS: The cohort included 61756 enrollees in a health plan from the San Francisco-Oakland metropolitan area (46% men, 63% white), aged 40 to 89 years and free of cardiovascular disease at baseline. Sixteen-year incidence of combined nonfatal and fatal intracerebral hemorrhagic stroke (International Classification of Diseases [ICD], 8th revision, code 431, or ICD, 9th revision, codes 431 and 432) was investigated in relation to serum cholesterol measured in multiphasic health checkups made in 1977 through 1985. Intracerebral hemorrhagic events were ascertained using hospital discharge records and as underlying cause of death by the California Mortality Linkage Information System. RESULTS: From 1978 through 1993 (average of 10.7 years), there were 386 events (201 in men, 29% fatal; 185 in women, 42% fatal). By multivariate proportional hazards life-table regression analysis, serum cholesterol level below the sex-specific 10th percentile (< 4.62 mmol/L [178 mg/dL] in men), compared with higher cholesterol level, was associated with a significantly increased risk of intracerebral hemorrhage in men aged 65 years or older (relative risk, 2.7; 95% confidence interval, 1.4 to 5.0). An excess risk was also observed among elderly women at the lowest cholesterol range, but a chance finding could not be ruled out. No relationship was seen among men or women aged 40 to 64, and no statistical interaction of low serum cholesterol with hypertension was found in either sex. CONCLUSIONS: In these data, the association between low serum cholesterol level and intracerebral hemorrhage was confined to elderly men.  相似文献   
77.
78.
Giardia lamblia is a frequent cause of diarrhea throughout the United States and the world. Advances in basic biology indicate that different strains of Giardia exist, that mammals can be infected with G. lamblia-type organisms, and that secretory IgA is important to host protection. Although water remains the most common mode of transmission of Giardia, there has been an increase in the number of person-to-person cases, especially related to children in day care, as well as an increase in food-borne cases. New antigen detection assays have improved the ability to diagnose Giardia in the stool and make it unlikely that duodenal sampling will be necessary. Metronidazole has become the drug of choice for most cases of giardiasis because of its efficacy, favorable tolerance, and availability. For pregnant women who require treatment, a non-absorbable aminoglycoside, paromomycin, may be tried first and metronidazole used if initial treatment fails.  相似文献   
79.
80.
In this paper we analyze the algebraic formulations of certain geometry statements appearing in recent literature related to mechanical geometry theorem proving and give several examples to show that one of these formulations can cause serious problems. We clarify a formulation which is essentially due to W. T. Wu and, in our opinion, is the most satisfactory.This author was supported by NSF Grants DCR-8503498 and CCR-8702108.  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