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   3688篇
  免费   40篇
  国内免费   3篇
电工技术   19篇
化学工业   437篇
金属工艺   22篇
机械仪表   36篇
建筑科学   81篇
矿业工程   11篇
能源动力   25篇
轻工业   223篇
水利工程   12篇
石油天然气   4篇
无线电   281篇
一般工业技术   228篇
冶金工业   2091篇
原子能技术   22篇
自动化技术   239篇
  2022年   11篇
  2021年   26篇
  2020年   16篇
  2019年   18篇
  2018年   22篇
  2017年   16篇
  2016年   15篇
  2015年   27篇
  2014年   30篇
  2013年   95篇
  2012年   43篇
  2011年   75篇
  2010年   58篇
  2009年   76篇
  2008年   67篇
  2007年   89篇
  2006年   61篇
  2005年   60篇
  2004年   56篇
  2003年   55篇
  2002年   56篇
  2001年   40篇
  2000年   43篇
  1999年   97篇
  1998年   532篇
  1997年   341篇
  1996年   253篇
  1995年   128篇
  1994年   129篇
  1993年   162篇
  1992年   54篇
  1991年   66篇
  1990年   57篇
  1989年   48篇
  1988年   53篇
  1987年   65篇
  1986年   68篇
  1985年   47篇
  1984年   27篇
  1983年   32篇
  1982年   22篇
  1981年   40篇
  1980年   43篇
  1979年   16篇
  1978年   24篇
  1977年   87篇
  1976年   151篇
  1975年   31篇
  1974年   13篇
  1973年   17篇
排序方式: 共有3731条查询结果,搜索用时 10 毫秒
1.
Intended learning outcomes (ILOs) indicate what learners will be able to achieve after they are taught. Traditionally, ILOs are expressed as plain text or unstructured documents. What if all ILOs of a specific course of study can be conceptualized through a structured diagrammatic technique? It was hypothesized that learners can benefit from this conceptualization in learning, especially in self‐regulated learning. The aim of this study was to investigate whether the ILOs represented in unstructured or structured formats can facilitate learners to identify learning paths. The results revealed that the mean ratings of all learning paths were statistically significantly higher with structured ILOs.  相似文献   
2.
3.
4.
One of the software programs we take for granted is antivirus software. Perhaps part of the reason for not being excited about protecting our computers resides in the fact that most PCs marketed over the past few years are bundled with an antivirus solution. When you initialize the computer you also start your virus protection program, which will often provide you with only a short period of protection before pop‐up messages appear reminding you to renew your antivirus program license to ensure your protection is up to date. What is more than likely never mentioned to consumers is the fact that the antivirus software bundled with your computer was positioned there due to the software vendor paying the computer manufacturer a fee. While most third‐party software placed on new computers must pass a certain level of quality assurance, the computer manufacturer is more interested in the placement fee than whether the software is the best for the purchaser of the computer. Thus, one of the things we should do when our antivirus software license expires, if not sooner, is to look at other possible solutions beyond the software bundled with the computer. One such antivirus product that warrants our attention and which is the focus of this column is ESET's NOD32 for Windows antivirus solution, which was originally developed in Slovakia.  相似文献   
5.
A case of traumatic extracranial vertebral arterial dissection leading to vertebrobasilar thrombosis and respiratory compromise requiring mechanical ventilation was managed with intraarterial thrombolysis and stenting of the vertebral intimal dissection. In contrast to similar, previously reported cases, this critically ill patient made a full recovery, returning to his job as a secondary school teacher.  相似文献   
6.
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   
7.
The aim of this study was to investigate the effect of the absence of elongate spermatids (ES) from the rat seminiferous epithelium on the quantitative secretion and synthesis of the three major Sertoli cell secretory proteins--SGP-1, SGP-2 and CP-2. Seminiferous tubules (ST) were isolated (a) from normal 28-day-old rats, in which the most mature germ cell type is the round spermatid, (b) from normal adult rats at stages IX-XIV of the spermatogenic cycle, i.e. after spermiation, or at stages I-V and VI-VIII, when ES are still attached to the Sertoli cell, and (c) at stages VI-VIII from normal adult rats and from rats treated with methoxyacetic acid (MAA) in order to specifically deplete ES at these stages. Two-dimensional SDS PAGE combined with computerized image analysis was used to analyse 35S-methionine-labelled intracellular and secreted proteins. In the case of SGP-1 and SGP-2, almost all of the protein synthesized by ST was secreted. The total amount of both SGP-1 and CP-2 secreted by unstaged ST from immature rats was significantly lower than that secreted by unstaged ST from adult rats. The total amount of SGP-1 and CP-2 secreted by adult ST at stages IX-XIV of the spermatogenic cycle also declined dramatically compared to ST at earlier stages. The proportion of the total CP-2 synthesized by ST which was secreted also declined in all situations in which ES were absent from the seminiferous epithelium. The synthesis of only SGP-2 was changed by ES depletion from ST at stages VI-VIII, which was almost doubled compared to synthesis of this protein by ST from control rats. Our results suggest strongly that the secretion of SGP-1 and SGP-2 is via the constitutive pathway, and that regulation of these two proteins by ES is at the level of protein synthesis. In contrast, the regulation of CP-2 by ES is predominantly at the level of secretion, suggesting that this protein is secreted via a regulated pathway. Our findings add to the evidence showing that ES play a major role in the regulation of Sertoli cell function.  相似文献   
8.
PURPOSE: The activity of Al3+, Ga3+, and Be2+ ions in the presence of NaF to directly activate G-proteins was investigated by their potentiative effect on forskolin (FSK)-activated adenylyl cyclase in rabbit ciliary process membranes and their effects on aqueous humor dynamics in vivo. METHODS: Adenylyl cyclase (AC) was determined by radiometric conversion of ATP to cAMP by the particulate fraction of rabbit ciliary processes. Intravitreal injections of sterile solutions of analytical grade salts were made into the center of the vitreous in a volume of 20 microliters. Intraocular pressure, aqueous humor flow, and uveoscleral outflow measurements were made by pneumatonometry, fluorophotometry, and fluorescein-dextran method, respectively. Outflow facility was determined by tonography in the intact eyes and by two-level constant pressure perfusion in cannulated eyes. RESULTS: Both Al3+ (EC50, 40 mumol/l) and Be2+ (EC50, 11 mumol/l) in the presence of 0.5-2 mM NaF activated the stimulatory G-protein Gs. Ga3+ was ineffective and did not antagonize the activation by Al3+. Intravitreal injections of Al3+ (1 mumol/eye) or Be2+ (0.5 or 1 mumol/eye) had no significant intraocular pressure (IOP) effect, nor did 1.5 or 3 mumol/eye of NaF, but when either cation was injected together with NaF, IOP decreased by up to 40% for up to 140 hr. At the time of maximum IOP effect (72 hr) aqueous humor flow determined by fluorophotometry was decreased in BeCl2+ NaF-treated eyes by 40% relative to BeCl2-treated eyes; however, tonographic facility of outflow was unaffected. Uveoscleral flow was also decreased by 38% in BeCl2+ NaF treated eyes. CONCLUSIONS: These findings support the hypothesis that Gs activation of ciliary process adenylyl cyclase decreases aqueous humor formation rate in rabbit eyes, and that activation of G-proteins mediates contraction of ciliary muscles causing a decrease of aqueous humor outflow via the uveoscleral route. The results suggest that G-proteins putatively involved in trabecular facility changes are less sensitive to activation by BeF3- than are other parameters of aqueous humor dynamics.  相似文献   
9.
Lactobacillus helveticus ITGL1 is able to hydrolyse many amino-acyl and dipeptidyl-p-nitroanilides. Analysis of heat inactivation kinetics, metal ion and protease inhibitor effects, and the subcellular location of aminopeptidase activities in both the parental strain and mutants deficient in lysyl-p-nitroanilide hydrolysis, led to the characterization of two cell-wall-associated aminopeptidases, APII and APIV. APII, which catalysed L-lysine p-nitroanilide hydrolysis, was purified about 28-fold to homogeneity from cell-wall extracts of L. helveticus ITGL1 and characterized. The purified enzyme appeared to be monomeric, with a molecular mass of 97 kDa. Aminopeptidase activity was greatest at pH 6.5 and 50 degrees C. APII was completely inhibited by bestatin, chelating agents such as EDTA or 1,10-phenanthroline and the divalent cations Zn2+ and Cu2+. The activity of the EDTA-treated enzyme was restored by Co2+, Ca2+ or Mn2+. Although APII was able to degrade several dipeptides and tripeptides with hydrophobic N-terminal amino acid (Leu, Ala), it was inactive on peptides containing Pro or Gly, and may thus contribute to the development of cheese flavour by processing bitter peptides.  相似文献   
10.
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