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   324107篇
  免费   4922篇
  国内免费   723篇
电工技术   6362篇
综合类   575篇
化学工业   48635篇
金属工艺   10648篇
机械仪表   9141篇
建筑科学   8786篇
矿业工程   931篇
能源动力   9089篇
轻工业   32137篇
水利工程   2662篇
石油天然气   3689篇
武器工业   9篇
无线电   41092篇
一般工业技术   61289篇
冶金工业   59607篇
原子能技术   5627篇
自动化技术   29473篇
  2021年   2684篇
  2019年   2494篇
  2018年   4028篇
  2017年   3821篇
  2016年   4036篇
  2015年   2947篇
  2014年   4949篇
  2013年   15395篇
  2012年   8205篇
  2011年   11220篇
  2010年   8650篇
  2009年   9918篇
  2008年   10461篇
  2007年   10326篇
  2006年   9183篇
  2005年   8475篇
  2004年   8220篇
  2003年   7915篇
  2002年   7743篇
  2001年   7836篇
  2000年   7501篇
  1999年   7869篇
  1998年   18322篇
  1997年   13211篇
  1996年   10018篇
  1995年   8034篇
  1994年   7368篇
  1993年   7078篇
  1992年   5406篇
  1991年   5127篇
  1990年   4996篇
  1989年   4826篇
  1988年   4712篇
  1987年   3929篇
  1986年   4052篇
  1985年   4797篇
  1984年   4343篇
  1983年   4072篇
  1982年   3629篇
  1981年   3817篇
  1980年   3497篇
  1979年   3471篇
  1978年   3264篇
  1977年   3883篇
  1976年   4934篇
  1975年   2821篇
  1974年   2715篇
  1973年   2698篇
  1972年   2250篇
  1971年   1979篇
排序方式: 共有10000条查询结果,搜索用时 36 毫秒
991.
In order to precise the indications and results of this procedure, we assessed 11 cases of transformation of ileorectal anastomosis (IRA) to ileal pouch-anal anastomosis (IPAA) in ulcerative colitis (UC). These 5 men and 6 women had undergone IRA at a mean age of 31 years, 33 months after the diagnosis of UC (range 3-120). Four of these IRA, excluded by an ileostomy, had never been in function: the cause was severe persistent proctitis in 2 cases and anastomotic leakage and peritonitis in 2 cases. The other 7 IRA had been in function during a mean period of 25 months (range 6-45) and were reoperated because of anal sepsis (1 case), low rectal stenosis (1 case), disabling proctitis (4 cases) and rectal dysplasia (1 case). No patient had specific pathologic signs of Crohn's disease. The 11 IPAA were complicated by pelvic sepsis in 3 cases; surgical drainage succeeded in 1 case, but the 2 others needed pouch excision and terminal ileostomy. The diagnosis of Crohn's disease was eventually made in these 2 patients. The 9 patients with functioning IPAA, at a mean follow-up of 40 months (range 12-60), had 5.2 stools per 24 h (range 2-12), 5 patients had no nocturnal stooling, and 6 had a perfect continence. One patient had disabling chronic pouchitis. In conclusion, proctectomy with IPAA is always feasible when a previous IRA for UC had failed or offers poor results, but should be rejected in case of anal involvement, as that may suggest Crohn's disease. This procedure is followed by similar functional results than after primary IPAA.  相似文献   
992.
We prepared [U-14C]cellobiose by cultivating Acetobacter pasteurianus in the presence of [U-14C]glucose and hydrolyzing the [U-14C]cellulose formed with beta-glucosidase-free cellulase from Trichoderma reesei. This 14C-labeled cellobiose was used to investigate the presence of an uptake system for cellobiose in T. reesei. Evidence was obtained for the presence of a high affinity (Km for cellobiose 0.3 microM) but low activity (2.5 milliunits/mg fungal dry weight) cellobiose permease. The permease is formed constitutively, but higher levels are formed after addition of sophorose (glucosyl-beta-1,2-diglucoside), a reputed cellulase inducer. The permease appears to be specific for beta-diglucosides, as the uptake of [U-14C]cellobiose is inhibited by sophorose, gentiobiose (glucosyl-beta-1,3-glucoside), and cellobiose. Under these conditions, cellooligodextrines (n, 4-7; final concentration, 1 mM) are not inhibitors. Glucose, but no other monosaccharides, inhibits the permease. The hypersecretory mutant T. reesei RUT C-30 exhibits elevated permease activities, whereas in T. reesei QM 9979, a mutant strain defective in the induction of cellulases by cellulose or sophorose, strongly reduced permease activities were demonstrated. The results stress a hitherto not recognized point of control in the induction of cellulases by T. reesei at the level of uptake of cellulose oligosaccharides.  相似文献   
993.
The morphologic changes and signal intensity of the spinal cord on preoperative magnetic resonance images were correlated with postoperative outcomes in 74 patients undergoing decompressive cervical surgery for compressive myelopathy. The transverse area of the spinal cord on T1-weighted images at the level of maximum compression was closely correlated with the severity of myelopathy, duration of disease, and recovery rate as determined by the Japanese Orthopaedic Association score. In patients with ossification of the posterior longitudinal ligament or cervical spondylotic myelopathy, the increased intramedullary T2-weighted magnetic resonance imaging signal at the site of maximal cord compression and duration of disease significantly influenced the rate of recovery. A multiple regression equation was then developed with these three variables to predict surgical outcomes.  相似文献   
994.
Spontaneous nasal tumors are rare in mice, and only one adenocarcinoma and two more primary neoplasms of the nose have been observed in our files of long-term feeding studies, which are composed of 3,419 male and 3,521 female CD-1 (Crl:CD-1 (ICR)BR) mice. This adenocarcinoma was a 1-cm-diameter mass observed grossly in the right nasal cavity of a 454-day-old, male CD-1 mouse from a treated group in a bioassay study conducted with 340 males and 340 females. The neoplastic epithelial cells affected the normal nasal architecture on the right side of the nose. Roughly, tumor neoplastic cells of the outer, lateral portion occurred as cuboidal to low columnar cells with basilarily located nuclei and eosinophilic cytoplasm. These cells were arranged in cylindrical profiles and frequently entrapped acini of the glands of the maxillary sinus. Neoplastic epithelial cells of the inner, medial portion appeared as serous acinar or ductular structures circumscribed by multiple lagers of myoepithelial-like cells. Staining failed to demonstrate mucous secretion. The site of origin of this neoplasm appeared to be the serous glands of the maxillary sinus. The adenocarcinoma was believed to be spontaneous.  相似文献   
995.
Low concentrations of some neutral dipeptides, such as L-Ala-L-Ala, rapidly disrupt rat liver lysosomes. The phenomenon has been attributed to an osmotic imbalance generated by the production of amino acids in the lysosome by lysosomal dipeptidase activity. This hypothesis is challenged by testing several pairs of dipeptides available in both D- and L-forms and a range of dipeptides whose susceptibility to lysosomal dipeptidase activity is known. A good correlation was found between the lytic ability of dipeptides and their capacity to cross the lysosome membrane and be hydrolysed by lysosomal dipeptidase. The osmotic-imbalance hypothesis is critically evaluated in the light of the results and of recent information concerning the carrier-mediated transport of amino acids and dipeptides across the lysosome membrane. It is concluded that intralysosomal generation of amino acids remains the most plausible explanation of the lytic activity of dipeptides, and that the dipeptide porter(s) in the lysosome membrane must have higher Km than the amino acid porters.  相似文献   
996.
The SSC GEM silicon Central Tracker design incorporated 18-cm long single-sided AC-coupled silicon microstrip ladders. Compared to the 12-cm long ladders considered in the preliminary stages of the tracker design, the 18-cm long ladders have the advantage of reduced cost, channel count and overall power consumption, and led to a simplified tracker assembly. However, such long ships also present the challenge of maintaining satisfactory performance. The increased capacitance and series resistance contribute to lower signal-to-noise ratios, longer time walk, higher power consumption per channel and increased probability of crosstalk to neighboring channels. In this paper, an accurate method to calculate the geometric capacitance of the AC-coupled microstrips is presented and the calculated results are compared with measurements, SPICE simulations are performed to predict the noise, the extent of interstrip capacitive coupling and the dispersion of the detector signal due to the finite series resistance of the metal strips and the long length of the detector. The influence of the preamplifier current and the shaping time on the signal and noise levels is also presented. The study concludes that the 18-cm long ladders can successfully satisfy the performance goals of the GEM silicon Central Tracker  相似文献   
997.
Acamprosate (calcium-acetyl homotaurinate) is a new compound in the treatment of alcoholism. Its efficacy has been proven in several clinical trials and registration is now pending in most European countries. The basic mechanisms by which acamprosate elicits its anti-craving action, thereby leading to reduced relapse rates, is not known at the moment. In the present study we describe a rat model of long-term alcohol-drinking which mimics relapse behavior in human alcoholics. The effect of acamprosate was studied in this model. Wistar rats had a free choice between water and alcohol solutions of different concentrations (5, 10, 20% v/v). After two months of continuous alcohol access, rats were deprived of alcohol for three days. Following this deprivation phase, all alcohol solutions were presented again. This procedure was repeated monthly for the following six months. The rats consumed 3.5 +/- 0.3 g/kg alcohol a day. After alcohol deprivation, alcohol intake rose to 5.2 +/- 0.3 g/kg per day resulting in blood alcohol levels of 30 +/- 6 mg/dl. Interestingly, the addition of quinine to the alcohol solutions or the additional presentation of a 5% sucrose solution did not affect the alcohol-deprivation effect after eight months of this intermittent alcohol exposure. However, when acamprosate (50-200 mg/kg i.p.) was administered twice daily, alcohol-drinking following an alcohol-deprivation phase was decreased dose dependently. Given at the highest dose alcohol intake even dropped significantly below baseline drinking. Together, these results show that acamprosate effectively diminishes the alcohol-deprivation effect. Furthermore, the described model seems to be a suitable animal model to screen compounds for their anti-relapse properties and subsequently for their anti-craving action.  相似文献   
998.
999.
The constant modulus (CM) array is a blind adaptive beamformer capable of recovering a narrowband signal among several cochannel sources without using a pilot or training signal. It is a conventional weight-and-sum adaptive beamformer whose weights are updated by the constant modulus algorithm. An adaptive signal canceller follows the beamformer to remove the captured signal from the array input and to provide an estimate of its direction vector. Based on a Wiener model, we investigate the steady-state properties of the CM array and the signal canceller. For mutually uncorrelated sources and noise, it is shown that the signal canceller exactly removes the source captured by the array. Thus, identical stages of the CM array and signal canceller may be used in a multistage system to recover several cochannel sources. Computer simulations are presented to verify the analytical results and to illustrate the transient behavior of the system  相似文献   
1000.
Exchange and oxidation of C16O were investigated at 450°C on 18O-predosed Rh and Pt catalysts supported on A12O3, CeO2 and CeO2-Al2O3. In all cases, a rapid exchange of C16O with the surface can be observed. CO oxidation leads to C16O2, C16O18O and C18O2. Significant formation of C16O2 is due to the relatively high 16O coverage in reaction resulting from the C16O exchange and from an exchange between O surface species and 16O internal atoms. Hydrogen is also formed via a water-gas shift reaction (CO + surface OH) in higher proportion on CeO2-containing catalysts than on A12O3. Chlorine inhibits all the reactions (exchange, oxidation and WGS) and particularly the internal exchange.  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