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   103005篇
  免费   1346篇
  国内免费   1230篇
电工技术   1925篇
综合类   138篇
化学工业   10511篇
金属工艺   5462篇
机械仪表   3190篇
建筑科学   2033篇
矿业工程   162篇
能源动力   2961篇
轻工业   6060篇
水利工程   704篇
石油天然气   671篇
武器工业   15篇
无线电   15967篇
一般工业技术   21785篇
冶金工业   25260篇
原子能技术   1383篇
自动化技术   7354篇
  2022年   454篇
  2021年   698篇
  2020年   523篇
  2019年   676篇
  2018年   1131篇
  2017年   1093篇
  2016年   1175篇
  2015年   926篇
  2014年   1491篇
  2013年   4676篇
  2012年   2569篇
  2011年   3846篇
  2010年   3099篇
  2009年   3721篇
  2008年   3898篇
  2007年   4082篇
  2006年   3690篇
  2005年   3328篇
  2004年   3182篇
  2003年   3030篇
  2002年   2676篇
  2001年   2972篇
  2000年   2723篇
  1999年   3080篇
  1998年   9440篇
  1997年   6157篇
  1996年   4762篇
  1995年   3165篇
  1994年   2789篇
  1993年   2725篇
  1992年   1629篇
  1991年   1594篇
  1990年   1519篇
  1989年   1323篇
  1988年   1175篇
  1987年   864篇
  1986年   888篇
  1985年   917篇
  1984年   804篇
  1983年   699篇
  1982年   701篇
  1981年   682篇
  1980年   567篇
  1979年   476篇
  1978年   414篇
  1977年   536篇
  1976年   958篇
  1975年   301篇
  1974年   276篇
  1973年   256篇
排序方式: 共有10000条查询结果,搜索用时 10 毫秒
81.
For the noninvasive diagnosis of heart disease based on the acoustic and elastic characteristics of the heart muscle, it is necessary to transcutaneously measure small vibration signals, including components with an amplitude of less than 100 μm, from various parts of the heart wall continuously for periods of more than several heartbeats in a wide frequency range up to 1 kHz. Such measurement, however, has not been realized by any ultrasonic diagnostic methods or systems to date. By introducing the constraint least-square approach, this paper proposes a new method for accurately tracking the movement of the heart wall based on both the phase and magnitude of the demodulated signal to determine the instantaneous position of the object so that the vibration velocity of the moving object can be accurately estimated. By this method, small vibrations of the heart wall with small amplitudes less than 100 μm on the motion resulting from a heartbeat with large amplitude of 10 mm can be successfully detected with sufficient reproducibility in the frequency range up to several hundred Hertz continuously for periods of about 10 heartbeats. The resultant small vibration is analyzed not only in the time domain, but also in the frequency domain. As confirmed by the preliminary experiments herein reported, the new method offers potential for research in acoustical diagnosis of heart disease  相似文献   
82.
Fuzzy inference, a data processing method based on the fuzzy theory that has found wide use in the control field, is reviewed. Consumer electronics, which accounts for most current applications of this concept, does not require very high speeds. Although software running on a conventional microprocessor can perform these inferences, high-speed control applications require much greater speeds. A fuzzy inference date processor that operates at 200000 fuzzy logic inferences per second and features 12-b input and 16-b output resolution is described  相似文献   
83.
The oxygen absorption process in highly aligned YBa2Cu3Ox samples in air and in pure oxygen was studied at several constant temperatures. The process can be expressed in two steps, a chemical reaction controlled step and a diffusion controlled step. The kinetic equations are as follows respectively:
(1 − at) ln(1 − at) + at = kDt

The activation energy was calculated using an Arrhenius equation. The technical conditions of heat treatment for oxygen absorption of highly aligned YBa2Cu3Ox crystals are discussed.  相似文献   

84.
85.
86.
Endoglucanase production was measured in culture filtrates of four species of Saccobolus growing in media containing glucose or crystalline cellulose as the only carbon sources. Enzyme activity was four to seven times higher in the presence of cellulose than glucose. S. saccoboloides showed maximal growth and enzyme production. The extracellular proteins secreted during growth on cellulose were separated by polyacrylamide gel electrophoresis and stained for proteins. A zymogram technique was used to visualize bands with endoglucanase activity. The four species showed different protein and isoenzyme patterns.  相似文献   
87.
The effects of anticancer drugs on cell growth have been investigated by MTT colorimetric assay using mouse and rat cultured cells transformed by various oncogenes and tumor viruses. Aclarubicin, mitomycin C and 1-hexylcarbamoyl-5-fluorouracil showed higher growth-inhibitory activities toward transformed cells than those toward the normal counterparts. Nimustine, bleomycin and 5-fluorouracil also showed selective growth-suppressive activities toward transformed cells except for a few cell lines. ras-oncogene-transformed cells were more sensitive toward 5-fluorouracil and 1-hexylcarbamoyl-5-fluorouracil than the normal parent cells and other transformed cells. These drugs would thus be effective in the chemotherapy of ras-induced cancer.  相似文献   
88.
AIM: The aim of the study is to analyse the benefits and risks of PEE in patients, cared for by a team with many years experience. PATIENTS AND METHODS: From 16. 2. 1988 until 31. 12. 1993 246 PEEs (229 gastrostomies, 6 duodenostomies, 7 jejunostomies, 4 attempts) were performed on 234 patients (56% male, 44% female, mean age 68.3 years). 117 patients had tumorous and 117 neurological diseases. We used the pull technique with the Fresenius Freka PEG-system. Analysis was performed using a standardised documentation sheet which was filled out until the end of tube feeding. In total, we registered 39,678 days of tube feeding, 4513 of which were in hospitalized patients. RESULTS: The mean intubation time was 192.6 days (maximum 1496). In 8 cases, the tube could be explanted before the patient was discharged; 68 patients were discharged to a nursing home and 71 patients were allowed to go home. The tube-independent hospital lethality was 36.64%. A PEE-specific lethality had not been registered. Complications arose in a total of 37 patients (15.04%), 4 of which were severe (1.63%) Fifty-nine patients (25.43%) reported short-term feelings of ill health (vomiting, diarrhoea, pain). CONCLUSION: PEE is an effective and low-risk method of long-term nutrition. The advantages are simple insertion, safe handling by patients and relatives/nursing staff and the low cost.  相似文献   
89.
The purpose of this study is to determine whether left ventricular dysfunction following coronary artery spasm by 123I-BMIPP myocardial imaging. To reveal the clinical efficacy of 123I-BMIPP SPECT, 20 patients with vasospastic angina were studied using resting, 3-hour delayed image with 123I-BMIPP and exercise, 3-hour delayed image with 201Tl SPECT. 123I-BMIPP uptake was decreased compared to 201Tl (discordant) in 12 patients (60%) and in 49/100 myocardial segments (49%). The extent and severity score in resting image with 123I-BMIPP were significantly larger than that in delayed image with 201Tl (p < 0.01). In 123I-BMIPP SPECT, the severity score in the latest ischemia were significantly larger than that in others. The incidence of a complete agreement of decreased 123I-BMIPP uptake and coronary artery spasm was significantly higher (75%) than that in 201Tl (28%, p < 0.01). Furthermore, compared to 201Tl uptake, decreased 123I-BMIPP uptake much more corresponded to reduced wall motion in 9 of patients with mismatching. The severity of regional wall motion abnormality was significantly correlated with severity score of 123I-BMIPP. Late redistribution in delayed image with 123I-BMIPP was seen in 6 patients. The regional washout rate and the severity of regional wall motion abnormality in 6 patients was significantly lower than that in others (p < 0.05). Thus, metabolic abnormality assessed by 123I-BMIPP is well associated with left ventricular asynergy and spastic region in patients with vasospastic angina. In conclusion, 123I-BMIPP SPECT may sensitively delineate the impaired myocardium following coronary artery spasm, and it is very useful in diagnosing and estimating the severity of vasospastic angina.  相似文献   
90.
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