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   275645篇
  免费   15861篇
  国内免费   659篇
电工技术   4481篇
综合类   180篇
化学工业   52223篇
金属工艺   9056篇
机械仪表   7408篇
建筑科学   7694篇
矿业工程   1382篇
能源动力   6713篇
轻工业   28487篇
水利工程   2873篇
石油天然气   4242篇
武器工业   14篇
无线电   31952篇
一般工业技术   57538篇
冶金工业   46743篇
原子能技术   5281篇
自动化技术   25898篇
  2021年   2705篇
  2020年   3305篇
  2019年   5346篇
  2018年   6847篇
  2017年   7133篇
  2016年   7802篇
  2015年   6425篇
  2014年   7979篇
  2013年   16250篇
  2012年   9836篇
  2011年   11384篇
  2010年   9713篇
  2009年   10155篇
  2008年   10119篇
  2007年   9793篇
  2006年   8353篇
  2005年   7499篇
  2004年   7204篇
  2003年   6932篇
  2002年   6477篇
  2001年   6157篇
  2000年   6026篇
  1999年   5731篇
  1998年   14223篇
  1997年   9857篇
  1996年   7573篇
  1995年   5377篇
  1994年   4888篇
  1993年   4656篇
  1992年   3456篇
  1991年   3305篇
  1990年   3127篇
  1989年   3169篇
  1988年   3023篇
  1987年   2578篇
  1986年   2494篇
  1985年   2946篇
  1984年   2703篇
  1983年   2492篇
  1982年   2271篇
  1981年   2376篇
  1980年   2175篇
  1979年   2268篇
  1978年   2240篇
  1977年   2552篇
  1976年   3573篇
  1975年   1920篇
  1974年   1841篇
  1973年   1861篇
  1972年   1494篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
Aspects of the analysis of an elderly patient with early disintegrative experiences are described: separation anxiety, an internalized attack upon vitality, and the fear of dependency were prominent features of the transference/countertransference relationship, while the search for meaning as death drew nearer continued to be the central theme. The patient's many dreams reveal a wealth of images whose complex interlinkings provided evidence of a heightened and energized individuation process aiming at the integrity of personality.  相似文献   
993.
The direct electrooxidation of methanol in acid medium was studied on electrodes made of a perfluorinated membrane with small amounts of metal catalysts incorporated by chemical reduction. Platinum is a good electrocatalyst for this reaction, but needs to be modified by other metals in order to obtain oxidation potentials much more compatible with a working anode in a direct methanol fuel cell. Ruthenium and tin appear to give encouraging results, leading to a negative shift of more than 250 mV as compared to the potential obtained with pure platinum. Other parameters were investigated in this work, such as the working temperature, and the manner of introduction of the methanol into the cell. By gaseous supply, it was possible to carry out measurements at higher temperatures than with methanol in solution, and consequently to greatly improve the performance of the catalytic electrode.  相似文献   
994.
In patients with progressive systemic sclerosis (PSS) suffering from chronic dyspepsia the stomach may be affected by this disease. The objective of this study was to investigate both antral myoelectrical activity and gastric emptying in PSS patients. Electrogastrography (EGG) was performed in 17 PSS patients (16 female, one male, median age 58 years, range 32-74 years) with chronic dyspepsia. After an overnight fast during one hour in the fasting and one hour in the fed state after ingestion of a liquid-solid test meal (370 kcal; liquid phase labeled with 0.5 mCi 99mTc-colloid) antral electrical activity was measured by one pair of electrodes sonographically placed on the skin overlying the gastric antrum. Several EGG parameters including dominant frequency (DF), percentages of DF in the normal range (2-4 cycles per minute [cpm]), bradygastria (< 2 cpm) and tachygastria (4-10 cpm), dominant frequency instability coefficient (DFIC), and postprandial to preprandial power ratio (PR) were calculated. The data were correlated to results obtained in 20 age- and gender-matched healthy subjects. In addition, the data were compared to gastric retention of the radionuclide at 60 min measured by simultaneous scintigraphy. The PSS patients did not reveal electrical disturbances. They even exhibited a significant postprandial decrease in DFIC, bradygastria, and tachygastria (ns) compared to healthy subjects. Over 50% of the PSS patients showed a delayed gastric emptying. However, EGG did not correlate to radioscintigraphy significantly. Our results reflect an absent relationship between antral myoelectrical activity in EGG and gastric emptying. Therefore, electrogastrography is unsuitable to assess gastric involvement in PSS.  相似文献   
995.
Fractionation of the venom of the spider Phoneutria nigriventer revealed that it was a mixture of several neurotoxic peptides. The peptides so far characterized either inhibited or induced neurotransmitter release. These effects were mediated by Ca2+ channels or increasing Na+ permeability through voltage sensitive Na(+)-channels, respectively. The pooled toxic components (fraction P4) showed stimulatory effects on acetylcholine release from brain cortical slices. In addition, a component of the observed effects resembling that of alpha-latrotoxin was identified, which was characterized by the ability to provoke release of acetylcholine (ACh) at low temperature and in a manner independent of extracellular Ca2+ and of voltage sensitive Na(+)-channels.  相似文献   
996.
The knowledge-representation and compilation techniques used in a model-based, automatic software synthesis environment are discussed. The environment was used to build Caddmus, a system with more than 250 cooperating processes. The real-time execution environment automatically generates a macro-dataflow computation from declarative models. Central to the approach is the Multigraph Architecture, which provides the framework for model-based synthesis in real-time, parallel-computing environments. Application of Caddmus to analysis of all data related to testing new and redesigned turbine engines is described  相似文献   
997.
Lattice-matched InAlAs-InGaAs HEMTs with dry etched and wet etched gate recesses have been fabricated and both high-frequency and noise measurements have been carried out. The highly selective dry etching process ensures uniform device parameters. The small signal and noise performance shows only minor differences between the two transistor types. There is no evidence of detrimental effects caused by dry etching that reduce the electrical and noise performance of the device at high frequencies. These results show that dry etched InP HEMT's have suitable characteristics for the fabrication of MM-wave integrated circuits  相似文献   
998.
The design problem of a multiproduct dehydration plant involving continuous conveyor-belt dryers utilised for the dehvdration of agricultural products over a fmite tim horizon, is presented. The peculiarities of the problem under demand-varying market patterns and time-variable market prices of raw materials are underlined. Limitations imposed by storage capacity, custormer orders, and constraints dictated by the process itself, are also taken into consideration. The overall problem is cast into a MINLP formulation involving the total annual cost of the plant as its objective function. This was infered by deducing the mathematical model of conveyor-belt dryers. Repeated evaluation of the total annual cost for various structures and operating points, allowed the determination of overall performance characteristic operational and structural curves. approximated by suitable mathematical expressions. A test case is presented in order to demonstrate the effectiveness of the proposed approach.  相似文献   
999.
A prospective assessment of late changes in breast appearance in 559 patients after tumour excision and radiotherapy for early breast cancer noted a strong association with breast size. Only 3/48 (6%) patients with small breasts developed moderate or severe late changes compared with 94/423 (22%) with medium sized breasts and 34/88 (39%) patients with large breasts (p < 0.001). One possibility is that greater radiation changes are related to greater dose inhomogeneity in women with large breasts. To explore this hypothesis, radiation dose distributions were assessed in a separate group of 37 women in whom three-level transverse computer tomographic images of the breast in the treatment position were available. A significant correlation was found between breast size and dose inhomogeneity which may account for the marked changes in breast appearance reported in women with large breasts.  相似文献   
1000.
An in situ NMR study of tert-butanol dehydration in 95 and 85% sulfuric acid did not indicate the formation of tert-butyl cations as active reaction intermediates. Instead, only NMR lines from oxonium ions under the condition of fast exchange with water and the acid were observed. It is most likely that, in the concentrated acid, the active intermediates of this reaction are represented by tert-butyl sulfuric ester which probably is a precursor of invisible tert-butyl carbenium ions representing short-lived excited or transition states.  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