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   5366篇
  免费   90篇
  国内免费   4篇
电工技术   45篇
综合类   3篇
化学工业   699篇
金属工艺   64篇
机械仪表   65篇
建筑科学   117篇
矿业工程   13篇
能源动力   74篇
轻工业   363篇
水利工程   22篇
石油天然气   22篇
无线电   477篇
一般工业技术   617篇
冶金工业   2304篇
原子能技术   53篇
自动化技术   522篇
  2022年   30篇
  2021年   35篇
  2020年   30篇
  2018年   42篇
  2017年   33篇
  2016年   35篇
  2015年   31篇
  2014年   82篇
  2013年   189篇
  2012年   129篇
  2011年   125篇
  2010年   113篇
  2009年   116篇
  2008年   135篇
  2007年   146篇
  2006年   115篇
  2005年   122篇
  2004年   111篇
  2003年   110篇
  2002年   98篇
  2001年   89篇
  2000年   93篇
  1999年   134篇
  1998年   647篇
  1997年   375篇
  1996年   278篇
  1995年   183篇
  1994年   170篇
  1993年   180篇
  1992年   89篇
  1991年   63篇
  1990年   84篇
  1989年   79篇
  1988年   70篇
  1987年   66篇
  1986年   61篇
  1985年   78篇
  1984年   59篇
  1983年   75篇
  1982年   61篇
  1981年   54篇
  1980年   56篇
  1979年   48篇
  1978年   45篇
  1977年   89篇
  1976年   124篇
  1975年   39篇
  1974年   24篇
  1973年   32篇
  1972年   25篇
排序方式: 共有5460条查询结果,搜索用时 218 毫秒
91.
The effect of 2 months of treatment with the oral growth hormone (GH) secretagogue MK-677 on markers of bone metabolism was determined in healthy obese male subjects. This was a randomized, double-blind, parallel, placebo-controlled study. Twenty-four healthy obese males, 19-49 years of age, with body mass index > 30 kg/m2 were treated with MK-677 (25 mg/day; n = 12) or placebo (n = 12) for 8 weeks. MK-677 increased markers of bone formation; a 23% increase in the carboxy-terminal propeptide of type I procollagen levels and a 28% increase in procollagen III peptide levels were seen with as little as 2 weeks of MK-677 treatment (p < 0.01 and p = 0.001 vs. placebo, respectively) while a 15% increase in serum levels of osteocalcin was not detected until 8 weeks of treatment (p < 0.01 vs. placebo). Markers of bone resorption were induced within 2 weeks of treatment with MK-677; serum levels of the carboxy-terminal cross-linked telopeptide of type I collagen were increased 26% at 8 weeks (p = 0.001 vs. placebo), and urine hydroxyproline/creatinine and calcium/creatinine ratios at 8 weeks were increased by 23% (p < 0.05 vs. placebo) and 46% (p < 0.05 vs placebo), respectively, MK-677 increased serum insulin-like growth factor binding protein-5 (IGFBP-5) by 43-44% after 2-8 weeks of treatment (p < 0.01 vs. placebo). Serum IGFBP-4 was increased by 25% after 2 weeks of treatment (p < 0.001 vs. placebo) but no significant change from baseline was observed after 8 weeks of treatment. Plasma interleukin-6 was not significantly changed by active treatment. In conclusion, short-term treatment of healthy obese male volunteers with the GH secretagogue MK-677 increases markers of both bone resorption and formation. Large increases in serum levels of IGF-1 and IGFBP-5 and a transient increase in serum IGFBP-4 were found. Future long-term studies are needed to investigate if prolonged treatment with MK-677 increases bone mass.  相似文献   
92.
As writing teachers, we seek to validate the knowledge that students bring to the classroom while introducing and cultivating new methods, strategies, techniques and technologies that may serve them now and in the future. In this article we describe an approach we take that integrates curricular and extracurricular co-apprenticeship strategies using public writing models in conjunction with social networking tools. With publication as a final goal, we urge students to move from expressing personal problems to addressing social issues, using a private, non-commercial learning network NoDiff <http://NoDiff.com> as a safe zone for skills development, knowledge production and social inquiry. In taking this approach, we resist dichotomies that place professional skills training in strict opposition to critical pedagogy. We argue that many of the so-called “extracurricular” composing activities that most of today's students engage in lay the groundwork for both professional and civic participation. Acknowledging that we as composition teachers are also beginners in the world of 21st century communication, we emphasize both (1) the continual transference of skills and knowledge among teachers and learners; and (2) the importance of providing what is often lacking in skills-based ‘new literacy’ training, namely a critical pedagogical approach to learning as social engagement and critical intervention.  相似文献   
93.
A new method is presented for calculating the effective thermal conductivity of a composite material containing spherical inclusions. The surface of a large body is assumed kept at a uniform temperature. This body is in contact with a composite material of infinite extent having a lower temperature far from the heated body. Green's theorem is then used to calculate the rate of heat transfer from the heated body to the composite material, yielding $$k_e /k = 1 + \frac{{3(\alpha - 1)}}{{[\alpha + 2 - (\alpha - 1)\phi ]}}\{ \phi + f(\alpha )\phi ^2 + 0(\phi ^3 )\} $$ where k e is the effective thermal conductivity, k is the thermal conductivity of the continuous phase, α is the ratio of the thermal conductivity of the spherical inclusions to k, and φ is the volume fraction occupied by the dispersed phase. The function f(α) is presented in this work. Although a similar result has been found previously by renormalization techniques, the method presented in this paper has merit in that a decaying temperature field is used. As a result, only convergent integrals are encountered, and a renormalization factor is not needed. This method is more straightforward than its predecessors and sheds additional light on the basic properties of two-phase materials.  相似文献   
94.
The adsorption of selected organic hydration inhibitors onto Forest Products Laboratory (FPL)-etched aluminium surfaces and the subsequent hydration of the treated surfaces have been studied using X-ray photoelectron spectroscopy (XPS) and surface behaviour diagrams (SBDs) supplemented by Fourier Transform Infra-red Spectroscopy (FTIR). Wedge tests were used to evaluate performance of these inhibitors in improving bond durability and the locus of failure was identified by XPS and high resolution scanning electron microscopy (X-SEM). The results indicate that nitrilotris methylene phosphonic acid (NTMP) and related compounds adsorb to the alumina surface via the POH bonds of the phosphonic acid groups, resulting in a displacement of water normally adsorbed onto the surface. A model of adsorption was developed which suggests that after treatment with very low concentrations of inhibitor (1 ppm), only one leg of the NTMP molecule adsorbs onto the surface although at higher concentrations (100 ppm) all three legs adsorb. Hydration is a three-step process: (i) reversible physisorption of water; (ii) slow dissolution of the inhibitor followed by rapid hydration of the freshly exposed Al2O3 to boehmite (AlOOH); and (iii) further hydration of the AlOOH to bayerite [Al(OH)3]. Analysis of the adsorption, hydration, and wedge test results using different inhibitors suggests the following five inhibitor characteristics that promote good bond performance: (i) displacement of water and occupation of all active sites on the Al2O3 surface; (ii) formation of strong inhibitor surface bonds; (iii) insolubility of the resulting inhibitor-aluminium complex in aqueous solutions; (iv) compatibility with the adhesive or primer; (v) coupling of the inhibitor to the adhesive.  相似文献   
95.
Soluble epoxide hydrolase (sEH) is abundant in the brain, is upregulated in type 2 diabetes mellitus (DM2), and is possible mediator of ischemic injury via the breakdown of neuroprotective epoxyeicosatrienoic acids (EETs). Prophylactic, pre-ischemic sEH blockade with 4-[[trans-4-[[(tricyclo[3.3.1.13,7]dec-1-ylamino)carbonyl]amino]cyclohexyl]oxy]-benzoic acid (tAUCB) reduces stroke-induced infarct in normal and diabetic mice, with larger neuroprotection in DM2. The present study tested whether benefit occurs in normal and DM2 mice if tAUCB is administered after stroke onset. We performed 60 min middle cerebral artery occlusion in young adult male C57BL mice divided into four groups: normal or DM2, with t-AUCB 2 mg/kg or vehicle 30 min before reperfusion. Endpoints were (1) cerebral blood flow (CBF) by laser Doppler, and (2) brain infarct at 24 h. In nondiabetic mice, t-AUCB reduced infarct size by 30% compared to vehicle-treated mice in the cortex (31.4 ± 4 vs. 43.8 ± 3 (SEM)%, respectively) and 26% in the whole hemisphere (26.3 ± 3 vs. 35.2 ± 2%, both p < 0.05). In contrast, in DM2 mice, tAUCB failed to ameliorate either cortical or hemispheric injury. No differences were seen in CBF. We conclude that tAUCB administered after ischemic stroke onset exerts brain protection in nondiabetic but not DM2 mice, that the neuroprotection appears independent of changes in gross CBF, and that DM2-induced hyperglycemia abolishes t-AUCB-mediated neuroprotection after stroke onset.  相似文献   
96.
Metallurgical and Materials Transactions A - Banding in commercial dual-phase steels, such as banded ferrite and pearlite or ferrite and martensite microstructures, is inherited from segregation...  相似文献   
97.
A flotation circuit operating on a continuous basis has been designed and constructed for removal of Sr ions from simple aqueous solutions. Under steady-state conditions, a continuous extraction rate of 97.3% has so far been achieved. The effects of changing the gas rate and flow rates of both the Sr and collector solutions were also examined. Finally a discussion is included where the results of these experiments are compared with those from similar continuous-scale experiments where Sr ions are removed by the technique of foam separation.  相似文献   
98.
Diet and fatty acid metabolism interact in yet unknown ways to modulate membrane fatty acid composition and certain cellular functions. For example, dietary precursors or metabolic products of n-3 fatty acid metabolism differ in their ability to modify specific membrane components. In the present study, the effect of dietary 22∶6n−3 or its metabolic precursor, 18∶3n−3, on the selective accumulation of 22∶6n−3 by heart was investigated. The mass and fatty acid compositions of individual phospholipids (PL) in heart and liver were quantified in mice fed either 22∶6n−3 (from crocodile oil) or 18∶3n−3 (from soybean oil) for 13 wk. This study was conducted to determine if the selective accumulation of 22∶6n−3 in heart was due to the incorporation of 22∶6n−3 into cardiolipin (CL), a PL most prevalent in heart and known to accumulate 22∶6n−3. Although heart was significantly enriched with 22∶6n−3 relative to liver, the accumulation of 22∶6n−3 by CL in heart could not quantitatively account for this difference. CL from heart did accumulate 22∶6n−3, but only in mice fed preformed 22∶6n−3. Diets rich in non-22∶6n−3 fatty acids result in a fatty acid composition of phosphatidylcholine (PC) in heart that is unusually enriched with 22∶6n−3. In this study, the mass of PC in heart was positively correlated with the enrichment of 22∶6n−3 into PC. The increased mass of PC was coincident with a decrease in the mass of phosphatidylethanolamine, suggesting that 22∶6n−3 induced PC synthesis by increasing phosphatidylethanolamine-N-methyltransferase activity in the heart.  相似文献   
99.
萧条的1992年促使轮胎生产者调整BruceDavis著曾泽新译吴秀兰校预期1992年的经济复苏并未实现,恢复的进程拖得比预期的更长。结果,大多数轮胎生产者被迫再次采取保守的经营,裁减人员,让股东对利润降低,甚至亏损作好思想准备。据报道,在大轮胎公司...  相似文献   
100.
We present several novel techniques to summarize the high-level behavior in surveillance video. Our proposed methods can employ either optical flow or trajectories as input, and incorporate spatial and temporal information together, which improve upon existing approaches for summarization. To begin, we extract common pathway regions by performing graph-based clustering on similarity matrices describing the relationships between location/orientation states. We then employ the activities along the pathway regions to extract the aggregate behavioral patterns throughout scenes. We show how our summarization methods can be applied to detect anomalies, retrieve video clips of interest, and generate adaptive-speed summary videos. We examine our approaches on multiple complex urban scenes and present experimental results.  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