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   252545篇
  免费   3794篇
  国内免费   1045篇
电工技术   4335篇
综合类   201篇
化学工业   37249篇
金属工艺   8224篇
机械仪表   7279篇
建筑科学   6798篇
矿业工程   541篇
能源动力   7243篇
轻工业   28674篇
水利工程   2122篇
石油天然气   1853篇
武器工业   5篇
无线电   33420篇
一般工业技术   46250篇
冶金工业   46590篇
原子能技术   3374篇
自动化技术   23226篇
  2021年   1815篇
  2020年   1359篇
  2019年   1680篇
  2018年   2585篇
  2017年   2607篇
  2016年   2892篇
  2015年   2171篇
  2014年   3675篇
  2013年   11985篇
  2012年   6776篇
  2011年   9682篇
  2010年   7466篇
  2009年   8415篇
  2008年   8934篇
  2007年   8942篇
  2006年   8298篇
  2005年   7497篇
  2004年   7218篇
  2003年   7103篇
  2002年   6599篇
  2001年   7031篇
  2000年   6331篇
  1999年   6832篇
  1998年   16406篇
  1997年   11396篇
  1996年   8602篇
  1995年   6557篇
  1994年   5718篇
  1993年   5573篇
  1992年   4001篇
  1991年   3782篇
  1990年   3553篇
  1989年   3374篇
  1988年   3281篇
  1987年   2591篇
  1986年   2490篇
  1985年   3160篇
  1984年   2797篇
  1983年   2559篇
  1982年   2347篇
  1981年   2394篇
  1980年   2205篇
  1979年   2048篇
  1978年   1879篇
  1977年   2176篇
  1976年   2728篇
  1975年   1584篇
  1974年   1490篇
  1973年   1559篇
  1972年   1144篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
The disposition behavior of trientine, a selective copper-chelating drug for Wilson's disease, and its metabolites in normal patients with Wilson's disease and rats were studied. A high concentration of metabolites appeared in blood samples of patients and rats in the early stage after administration of trientine. Furthermore, large amount of trientine metabolites were excreted into the urine of patients. These results suggest that trientine is remarkably subjected to a first-pass effect. The drug concentration area under the curve (AUC) of the unchanged form and the metabolites of trientine in patients was not dependent on the administered dosage. It seems that the absorption process is an important factor for the disposition behavior of trientine, we have also investigated the uptake characteristics of trientine by rat intestinal brush-border membrane vesicles. The uptake characteristics of trientine were similar to the physiological polyamines, spermine and spermidine. The uptake rate of trientine was dose-dependently inhibited by spermine and spermidine. Moreover, spermine competitively inhibited the uptake of trientine with a Ki value of 18.6 muM. This value is very close to the Km value for spermine (30.4 muM). These data suggested that the uptake mechanism of trientine in rat small intestinal brush-border membrane vesicles was almost identical to that of spermine and spermidine, and that the physiological polyamines seem to have the ability to inhibit the absorption of trientine from the gastrointestinal tract.  相似文献   
992.
The effects of ionic strength on myofibrils and myosin from rabbit fast-twitch Psoas major (PM) and slow-twitch Semimembranosus proprius (SMp) muscles before and after heating were studied by electron microscopy and thermal scanning rheometry. The direct suspension of proteins in low ionic strength (0.2M KCl; pH 6.0) led to very weak gels, whereas a gradual lowering of the ionic strength (by dialysis against 0.2M KCl; pH 6.0) of 0.6M KCl protein solutions induced strand-type networks at low temperature and strong heat-induced gels. As shown by transmission and scanning electron micrographs, in low ionic strength, SMp myosins formed shorter filaments before heating and thinner and shorter structures in heat-induced gels, as well as a lower gel porosity than PM myosins.  相似文献   
993.
Turmeric curcuminoids are important natural pigments but their instability limits their use in many foods. This study was conducted to examine light-induced degradation of turmeric curcumin as affected by solvent system and oxygen. The rate constants and half-lives were determined for purified curcumin, demethoxycurcumin and bis-demethoxycurcumin exposed to 1450 lux using reversed-phase HPLC. Rates of degradation of each pigment followed first-order kinetics. Differences in rate constants and half-lives between the individual pigments were minor in an acetic acid-NaCl solution; however, in methanol, large differences in stability to light of the individual pigment were observed. Stability of all the pigments to light was several fold greater in methanol than in acid brine. In methanol sparged with air, stability to photooxidation was curcumin > demethoxycurcumin > bis-demethoxycurcumin. In methanol, curcumin was more stable than the other curcuminoids when sparged with air than when sparged with nitrogen. In methanol sparged with nitrogen, demethoxycurcumin was the most stable pigment. All of the pigments were stable in the dark.  相似文献   
994.
To examine the separate effects of viscosity and sweetness on astringency, aqueous solutions of grape seed tannin (GST) were thickened with carboxymethyl cellulose (CMC) from 2 to 45 cP (experiment 1) or sweetened with 0 to 1.8 g/L aspartame (experiment 2). Trained subjects continuously rated astringency and bitterness in duplicate. Subjects were categorized by the salivary flow induced by citric acid and ability to taste n-propyl thiouracil (PROP). In experiment 1, maximum intensity and total duration of astringency were significantly decreased as viscosity rose, although time to maximum intensity of astringency was not affected. Maximum intensity and total duration of bitterness were not significantly affected by increasing viscosity; however, the onset of bitterness was significantly delayed. In experiment 2, increasing sweetness had no affect on any astringency parameter, although maximum intensity of bitterness was significantly decreased. Neither PROP nor salivary flow-status had any effect on perception of bitterness or astringency in either experiment.  相似文献   
995.
996.
Tomato seed oil was extracted with supercritical carbon dioxide in a semibatch-flow extractor within the temperature range 313–343 K and the pressure range 10.8–24.5MPa. The extraction rates increased with pressure, but decreased with temperature increase because of the variation in solvent density and resultant differences in oil solubility. The fatty acid composition of the extracted oil was similar to that of soybean oil.  相似文献   
997.
OBJECTIVE: Chronic Q fever is seldom recognized; before 1989, only 234 cases had been reported in the literature. The 92 cases of chronic Q fever collected at the French National Reference Center for Rickettsioses from 1982 through 1990 represent the largest series ever reported. PATIENTS: The patients included in the study were diagnosed between July 31, 1982, and August 1, 1990, at the French National Reference Center for Rickettsioses as having chronic Q fever by the following criteria: presence of antibody against Coxiella burnetii phase I antigen at a titer greater than or equal to 800 for IgG and 50 for IgA by the indirect immunofluorescence test. Epidemiologic, clinical, laboratory, and treatment data were collected from 39 different collaborative hospitals throughout France. MAIN OUTCOME MEASURE: For each serologically selected patient, a computerized questionnaire was utilized to record 188 different items of demographic, epidemiologic, clinical, laboratory, and therapeutic data, which were analyzed. RESULTS: Chronic Q fever occurs more frequently in city dwellers than in rural inhabitants, and exposure to domestic ruminants and raw milk is an important feature. Immunocompromising conditions (20.2%) and underlying heart disease (88.4%) or vascular disease are the most important risk factors to consider in potential cases of chronic Q fever. The mortality in these patients with endocarditis was high (23.5%). The clinical spectrum of 84 patients included 57 cases of endocarditis, three cases of vascular prosthesis infection, three cases of aneurysmal infection, three cases of osteoarthritis, four cases with lung localizations, nine asymptomatic cases, three cases of hepatitis, and two cases with cutaneous forms of the disease. CONCLUSIONS: In patients with unexplained fever, negative blood cultures, and a history of underlying vascular or cardiac disease, Q fever should be considered.  相似文献   
998.
VIP is an established prolactin-releasing factor. VIP gene expression at the anterior pituitary level and the central nervous system is regulated by thyroid hormones. On the other hand, primary hypothyroidism leads in many cases to amenorrhea, galactorrhea and hyperprolactinemia. In this study we assessed prolactin responses to VIP (75 micrograms iv infusion over 12 min) in a group of six hypothyroid women (mean age +/- SE, 38.8 +/- 3.3 yr; serum TSH levels, mU/L, 116.3 +/- 23.9), before treatment and after normalization of thyroid hormone levels during thyroxine (T4) replacement therapy (100-150 micrograms/day over 12-16 weeks). Furthermore, we assessed if VIP infusion had any effects on serum GH levels in these patients. In hypothyroid women, VIP infusion increased serum prolactin concentrations with peak levels being attained at 15 min (28.8 +/- 3.4 micrograms/L). The Area Under the Curve (AUC) was 1921 +/- 103 micrograms/L/2h. PRL responses to VIP were unchanged after T4 therapy, both in terms of peak levels (28.7 +/- 2.2 micrograms/L, NS) and of AUC (2079 +/- 261 micrograms/L/2h, NS). Serum GH levels were unaffected by VIP administration. In conclusion our study shows that, in hypothyroid patients, restoration of normal thyroid hormone levels by thyroxine replacement therapy does not affect lactotroph responsiveness to VIP. Therefore, our data do not support the hypothesis that VIP might contribute to the hypothyroid-induced hyperprolactinemia seen in man.  相似文献   
999.
The effects of celebrity self-disclosure of HIV seropositivity on perceptions of HIV and AIDS were investigated. AIDS-related interest and knowledge measures were collected from 468 men before and after basketball star Earvin "Magic" Johnson's self-disclosure of HIV seropositivity. Increased interest in AIDS paralleled media coverage of the announcement, with the most substantial effects occurring within 2 wks. Perceived impact of the disclosure was greatest among African-American men and men who had not previously known someone with HIV/AIDS. Celebrity self-disclosure appears to affect perceptions through mechanisms similar to those involved in personally knowing someone infected with HIV. (PsycINFO Database Record (c) 2010 APA, all rights reserved)  相似文献   
1000.
The potency of a series of anticholinesterase (anti-ChE) agents and serotonin-related amines as inhibitors of the aryl acylamidase (AAA) activity associated with electric eel acetylcholinesterase (AChE) (EC 3.1.1.7) and horse serum butyrylcholinesterase (BuChE) (EC 3.1.1.8) was examined and compared with the potency of the same compounds as ChE inhibitors. Neostigmine, physostigmine, BW 284C51, (+/-)-huperzine A, E2020, tacrine, edrophonium and heptyl-physostigmine were, in that order, the most potent in inhibiting eel AChE-associated AAA activity, their inhibitor constant (Ki) values being in the range 0.02-0.37 microM. The rank order of the same compounds as AChE inhibitors basically paralleled that of AAA, although they were in general stronger on AChE (Ki = 0.001-0.05). The peripheral anionic site inhibitors propidium and gallamine were inactive on AChE-associated AAA. Serotonin and its derivatives were slightly stronger on AAA (Ki = 7.5-30 microM) than on AChE (Ki = 20-140 microM). Tacrine (IC50 = 0.03 microM), diisopropylfluorophosphate (IC50 = 0.04 microM), heptyl-physostigmine (IC50 = 0.11 microM), physostigmine (IC50 = 0.15 microM) and tetra-iso-propylpyrophosphoramide (iso-OMPA) (IC50 = 0.75 microM) were the most potent in inhibiting horse serum BuChE-associated AAA activity. Serotonin and related amines were very weak on BuChE-associated AAA activity. These results indicate that the inhibitory potencies of the active site anti-ChE agents on the AAA activity associated with eel AChE and horse serum BuChE are closely correlated with their action on the respective ChE. In addition, the efficacy of tacrine, E2020, heptyl-physostigmine and (+/-)-huperzine A in the treatment of Alzheimer's disease is unlikely to be related to the action of these drugs on ChE-associated AAA.  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