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   6393篇
  免费   123篇
  国内免费   25篇
电工技术   353篇
综合类   17篇
化学工业   1408篇
金属工艺   213篇
机械仪表   128篇
建筑科学   97篇
矿业工程   1篇
能源动力   205篇
轻工业   414篇
水利工程   18篇
石油天然气   10篇
武器工业   1篇
无线电   622篇
一般工业技术   1052篇
冶金工业   1431篇
原子能技术   170篇
自动化技术   401篇
  2023年   27篇
  2022年   58篇
  2021年   84篇
  2020年   29篇
  2019年   66篇
  2018年   60篇
  2017年   42篇
  2016年   72篇
  2015年   67篇
  2014年   86篇
  2013年   245篇
  2012年   176篇
  2011年   244篇
  2010年   205篇
  2009年   213篇
  2008年   231篇
  2007年   207篇
  2006年   203篇
  2005年   166篇
  2004年   154篇
  2003年   194篇
  2002年   153篇
  2001年   172篇
  2000年   131篇
  1999年   186篇
  1998年   583篇
  1997年   399篇
  1996年   289篇
  1995年   178篇
  1994年   176篇
  1993年   172篇
  1992年   95篇
  1991年   101篇
  1990年   82篇
  1989年   87篇
  1988年   78篇
  1987年   66篇
  1986年   75篇
  1985年   59篇
  1984年   71篇
  1983年   57篇
  1982年   65篇
  1981年   55篇
  1980年   50篇
  1979年   49篇
  1978年   31篇
  1977年   50篇
  1976年   57篇
  1975年   26篇
  1973年   35篇
排序方式: 共有6541条查询结果,搜索用时 15 毫秒
21.
The effects of local anaesthetics, bupivacaine and lidocaine, on Ca2+ flux behaviour of sarcoplasmic reticulum and on sarcolemmal functions were studied in the rabbit masseter muscle. The experiments were performed on sarcoplasmic reticulum and sarcolemmal vesicles prepared at 1 to 10 days after injection of local anaesthetics or saline into masseter muscle as well as on sarcoplasmic reticulum vesicles prepared from non-treated rabbits (for assessment of the effect on in vitro incubation with local anaesthetics). Bupivacaine potently reduced the efficiency of active sarcoplasmic reticulum Ca2+ transport as evaluated by coupling ratio (Ca2+ transported/ATP hydrolyzed, in the presence of oxalate) at 3 days after the injection; there was only a slight degree of uncoupling of Ca2+ transport from ATP hydrolysis with lidocaine injection. Bupivacaine but not lidocaine, at 3 days after injection, decreased both the apparent permeability of sarcoplasmic reticulum vesicles to Ca2+, determined by measuring net efflux of Ca2+ after stopping pump-mediated fluxes, and the steady-state Ca2+ load in sarcoplasmic reticulum, but had no effect on overall turnover of the Ca2+ATPase. The effects of bupivacaine on apparent sarcoplasmic reticulum Ca2+ permeability and steady-state Ca2+ load were inhibited by a Ca2+ antagonist verapamil. The reduction of Ca2+ uptake of sarcoplasmic reticulum and the protective effect of verapamil were reproduced in unfractionated homogenates prepared at 3 days after bupivacaine injection. In vitro exposure of sarcoplasmic reticulum vesicles to bupivacaine (0.5 to 50 mM) reduced steady-state Ca2+ load in a dose-dependent manner. The observed effect elicited by bupivacaine (25 mM) was partially protected by procaine, an inhibitor of Ca2(+)-induced Ca2+ release from sarcoplasmic reticulum, or by specific closure of the sarcoplasmic reticulum Ca2+ release channel by ryanodine, suggesting the possibility that in vitro exposure of sarcoplasmic reticulum vesicles to bupivacaine may produce an increase in apparent permeability of sarcoplasmic reticulum to Ca2+. In sarcolemma, bupivacaine reduced Na+,K(+)-ATPase and Na(+)-Ca2+ exchange activities at 3 days after injection; the effects on sarcolemmal vesicles were prevented by verapamil. These results suggest that although the effects elicited by bupivacaine injection and the in vitro exposure to bupivacaine on steady-state Ca2+ load of sarcoplasmic reticulum vesicles were similar, the membrane properties of the vesicles from bupivacaine-treated masseter muscles and those from normal untreated muscles may not be the same, which indicates that pure bupivacaine effect is due partly by an effect on ryanodine- and procaine-sensitive Ca2+ channels.(ABSTRACT TRUNCATED AT 400 WORDS)  相似文献   
22.
An outbreak of Cryptosporidiosis occurred over three months in a British Columbia community, peaking in December 1990. Results of the case-control study and illness surveys support the hypothesis that transmission occurred in a public children's pool at the local recreation centre. Analysis using lab-confirmed cases revealed a matched odds ratio of 4.5 [95% CI 0.97, 20.83], and using clinical cases an unmatched odds ratio of 12.8 [95% CI 3.68, 46.77], associated with swimming in the children's pool within two weeks prior to onset of illness. Other risk factors were not significant. Attack rates in various groups of children's pool users ranged from 8% to 78%. The children's pool was closed for steam cleaning and disinfection. Unusually frequent defecations including liquid stools had occurred before and during the outbreak. Improvements were instituted for removal of feces and superchlorination of pool water.  相似文献   
23.
Liquid viscosity and density of six binary mixtures of HFC-134a with glycols [ethylene glycol, diethylene glycol, triethylene glycol, polyethylene glycol (400), and polypropylene glycol (2000)] have been measured in the temperature range from 273 to 333 K. The viscosity was measured by a rolling-ball viscometer calibrated with standard liquids of viscosities and densities (JS5, JS10, JS20, and JS50). The density was measured with a glass pycnometer. The uncertainties of the measurements were estimated to be less than 3.4 % for viscosity and 0.04 % for density, respectively. An equation is given to represent the obtained viscosity values as a function of weight fraction and temperature.  相似文献   
24.
The carcinogenicity of medium-viscosity liquid paraffin was examined in Fischer 344 rats. Groups of 50 males and 50 females were given the material at dietary doses of 0 (control), 2.5 or 5% for 104 wk. Slight increases in food consumption and body weight were observed in both sexes of the 5% group. However, no significant differences between the control and treated groups were noted with regard to clinical signs, mortality and haematology findings. A variety of tumours developed in all groups, including the control group, but all the neoplastic lesions were histologically similar to those known to occur spontaneously in F344 rats, and no statistically significant increase in the incidence of any tumour type was found for either sex in the treated groups. Granulomatous inflammation in the mesenteric lymph nodes, considered to be a reaction to paraffin absorption, was observed with similar incidence and severity in both sexes of the 2.5 and 5% groups. Thus, it is concluded that under the present experimental conditions, the high dose, about 2000-200,000 times higher than the current temporary acceptable daily intake, does not have any carcinogenic potential in F344 rats. Furthermore, granulomatous inflammation observed in mesenteric lymph nodes were not associated with any development of neoplastic lesions.  相似文献   
25.
Rheumatoid arthritis frequently contributes to instability of the upper cervical spine. Rotational instability of the upper cervical spine was evaluated in rheumatoid arthritis patients using biplanar x-ray photogrammetry. Three-dimensional cervical motion and the instantaneous axis of rotation of the atlas relative to the axis were evaluated in normal and rheumatoid arthritis patients during axial rotation in the horizontal plane. Anterior atlantoaxial subluxation did not increase during axial head rotation in either the atlantoaxial subluxation or the vertical subluxation groups, while the instantaneous axes of rotation were distributed posteriorly in the dens in the RA-normal group, but were widely scattered in the atlantoaxial subluxation group.  相似文献   
26.
Middle latency auditory responses (MLRs) were studied in 55 normal subjects and 49 epileptic patients. We evaluated the function of the auditory system and of the temporal lobe in epileptic patients using MLRs. (1) The patients with epilepsy had significantly prolonged Pa and Nb latencies and the significantly increased amplitude of Pa-Nb component. (2) The prolongation and the increase of MLR components in latency and amplitude were more obvious in patients with intractable epilepsy than patients without intractable epilepsy. (3) The difference between left and right sides was larger in epileptic patients than normal subjects. (4) Epileptic patients with polypharmacy had reduced amplitude of Pa-Nb component and prolonged latency of Pa component than patients with monotherapy. Antiepileptic drugs (AEDs) appeared to decrease the amplitude of Pa-Nb component. It was supposed that the enhancement of amplitude of Pa-Nb component represented the abnormality of central nerve system in epileptic patients, in part.  相似文献   
27.
Simulation of very fast surge phenomena in a three-dimensional (3-D) structure requires a method based on Maxwell's equations, such as the finite-difference time-domain (FDTD) method or the method of moments, because circuit-equation-based methods cannot handle the phenomena. This paper uses a method of thin-wire representation of the vertical conductor system for the FDTD method which is suitable for the 3-D surge simulation. The thin-wire representation is indispensable to simulate electromagnetic surges on wires or steel frames in which the radius is smaller than a discretized space step used in the FDTD simulation. In this paper, a general surge analysis program named the virtual surge test lab based on the Maxwell's equations formulated by the FDTD method, is used to simulate the surge phenomena of a vertical conductor, including the effects of horizontal wave incidence and vertical wave incidence. Experimental results on the reduced scale model have been presented in order to compare among the simulation results by the FDTD method and the results using numerical electromagnetic code based on the MoM.  相似文献   
28.
This paper presents a method of predicting the steady‐state performance of a new hybrid‐excitation synchronous machine (HESM) theoretically. The field pole of this HESM is axially divided into two parts; one is an excitation part and the other a permanent‐magnet (PM) part. A nonlinear equivalent circuit, which can include the saliency of the rotor and the magnetic saturation due to the iron core, is derived. Based on this equivalent circuit, the steady‐state performance of the HESM is calculated, and the results are confirmed through experiments. © 2004 Wiley Periodicals, Inc. Electr Eng Jpn, 150(2): 43–49, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.20033  相似文献   
29.
30.
A modified Monte Carlo (MC) simulation was performed to investigate the hydrogen absorption behavior in Pd and Pd–Ag alloys of the composition PdxAg1−x (x=0.7–0.8) under H2 pressure (0.1 MPa) at different temperatures. The present method employed can consider the dissociative adsorption of hydrogen molecule and the subsequent absorption of hydrogen atom by formalizing the relationship between the pressure of hydrogen molecule and hydrogen atom. The potential parameters were determined to reproduce the solution enthalpy of hydrogen in pure metals. The results are in good agreement with experimental findings as well as previous theoretical studies. We confirmed that our method is useful to simulate the absorption of hydrogen in metals and alloys.  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