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   29289篇
  免费   1591篇
  国内免费   188篇
电工技术   536篇
综合类   286篇
化学工业   4851篇
金属工艺   807篇
机械仪表   702篇
建筑科学   808篇
矿业工程   176篇
能源动力   505篇
轻工业   4123篇
水利工程   233篇
石油天然气   156篇
武器工业   14篇
无线电   1590篇
一般工业技术   5334篇
冶金工业   7263篇
原子能技术   168篇
自动化技术   3516篇
  2023年   323篇
  2022年   350篇
  2021年   713篇
  2020年   552篇
  2019年   459篇
  2018年   896篇
  2017年   795篇
  2016年   898篇
  2015年   799篇
  2014年   926篇
  2013年   1868篇
  2012年   1334篇
  2011年   1329篇
  2010年   1148篇
  2009年   1080篇
  2008年   1211篇
  2007年   1139篇
  2006年   820篇
  2005年   657篇
  2004年   504篇
  2003年   492篇
  2002年   485篇
  2001年   418篇
  2000年   353篇
  1999年   456篇
  1998年   2261篇
  1997年   1369篇
  1996年   951篇
  1995年   580篇
  1994年   467篇
  1993年   422篇
  1992年   170篇
  1991年   174篇
  1990年   153篇
  1989年   159篇
  1988年   145篇
  1987年   159篇
  1986年   139篇
  1985年   219篇
  1984年   181篇
  1983年   165篇
  1982年   129篇
  1981年   174篇
  1980年   171篇
  1979年   122篇
  1978年   105篇
  1977年   216篇
  1976年   474篇
  1975年   86篇
  1973年   111篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
Parameters of haemostasis, endothelial cell markers and lipid peroxide levels were studied in 64 Type 1 (insulin-dependent) and 94 Type 2 (non-insulin-dependent) diabetic patients according to their urinary albumin excretion rate in comparison with age-matched control subjects. We determined plasma levels of fibrinogen (Clauss' method), coagulation factor VII:activity (clotting assay), factor VII antigen, protein C and S antigen, von Willebrand factor antigen, D-dimer concentration (ELISA), and lipid peroxide levels (thiobarbituric acid) in relation to urinary albumin excretion rate (RIA). Significant positive correlations were found between urinary albumin excretion rate and plasma fibrinogen (p < 0.005, p < 0.02), factor VII activity (p < 0.0002, p < 0.002), factor VII antigen (p < 0.0001, p < 0.001), protein C (p < 0.003, p < 0.05), and lipid peroxides (p < 0.02, p < 0.004) in Type 1 as well as in Type 2 diabetes. Von Willebrand factor (p < 0.001) and protein S (p < 0.0005) correlated with albuminuria only in patients with Type 1 diabetes. Although most of the haemostatic abnormalities are already found in normoalbuminuric patients, the significant positive correlations to urinary albumin excretion indicate that endothelial cell damage and coagulation disorders deteriorate with the progression of diabetic nephropathy.  相似文献   
82.
83.
The effects of arginine vasopressin (AVP) and oxytocin (OT) upon thyroid-stimulating hormone (TSH), free thyroxine (FT4) and free triiodothyronine (FT3) release were studied in euthyroid rats. Intracerebroventricular (i.c.v.) infusion of AVP in doses of 0.5 ng or 5 ng led to significant increases in plasma levels of TSH as well as FT4 and FT3. The effects of OT injected i.c.v. in similar doses were not consistent (there was no parallel between the changes of respective hormones plasma levels). It may be concluded that vasopressin modulate the pituitary-thyroid system function; AVP is probably a physiological stimulator of TSH and thyroid hormones secretion.  相似文献   
84.
85.
Pre-pruning and Post-pruning are two standard techniques for handling noise in decision tree learning. Pre-pruning deals with noise during learning, while post-pruning addresses this problem after an overfitting theory has been learned. We first review several adaptations of pre- and post-pruning techniques for separate-and-conquer rule learning algorithms and discuss some fundamental problems. The primary goal of this paper is to show how to solve these problems with two new algorithms that combine and integrate pre- and post-pruning.  相似文献   
86.
To investigate the energetic costs of lactation in a female mammal in relation to previous reproductive history, we compared the performance of adult female Long-Evans rats that had previously bred (multiparous) with young females that had not previously given birth (primiparous). All litters were standardized to 10 +/- 1 young. We compared maternal production (growth of pups), body mass, and energy intake (food consumption) of mothers, as well as their energy expenditure (resting oxygen consumption). The mass of litters at birth and the growth of pups during lactation did not differ according to reproductive history of the mothers. The body mass of primiparous mothers was less than that of multiparous mothers, and primiparous mothers showed an increase in mass during early lactation. To accomplish the essentially identical production of offspring under these circumstances, the primiparous mothers consumed and expended more energy than the multiparous mothers. This remarkable performance of first-time mothers results in an overall efficiency of energy allocation to reproduction amounting to only 25%, compared with 38% in multiparous mothers. The energetic inefficiency of primiparous female lactation results largely from the excessive expenditures associated with physiological and behavioral performances of first-time reproduction, together with a small component of additional expenditure due to further growth by the primiparous mothers. We suggest that this inefficiency probably contributes to the observed low reproductive success of novice breeders; furthermore, active restraint of fecundity may be an evolutionary response to the constraints of the energetic inefficiency of primiparous breeding by female mammals.  相似文献   
87.
Balloon angioplasty as the treatment of first choice in the setting of an acute myocardial infarction (AMI) is gaining widespread acceptance because of favourable results from specialised centres concerning high patency rates and low mortality. This study reports the results of angioplasty for AMI at large community hospitals during 1992-1995. 4625 procedures were performed at 68 centres of the Arbeitsgemeinschaft Leitender Kardiologischer Krankenhaus?rzte (ALKK). The age of the patients was 60.8 +/- 11.3 years, with 75.1% men. The infarct related artery was the left anterior descendent in 43%, the right coronary artery in 37%, the circumflex artery in 16%, a bypass graft in 2.3% and the left main stem in 1.4% of patients. The success rate (residual stenosis < 50%) of the intervention was 86%. There was a wide range of procedures per centre, with a median of 40 AMI angioplasties per year and centre. The amount of angioplasties for AMI in relation to all angioplasties performed during this period rose from 5.2% in 1992 to 5.9% in 1995 (p = 0.01). Local complications at the puncture site occurred in 3.2%, with the need for a surgical intervention in 1.1% of patients. In 273 (5.9%) of the patients a second angioplasty was performed during the hospital stay. Aortocoronary bypass surgery was performed in 3% of the patients. Hospital mortality was 9.5% (438/4625 patients). The mortality rate remained constant during the years investigated (1992: 10.6%; 1993: 8.6%; 1994: 9.7%; 1995: 9.8%; p = ns). Higher mortality was observed in older patients, patients with multiple vessel disease, the left anterior descending artery or a bypass graft as infarct related artery as well as in patients with failed reperfusion (residual stenoses > 50%). Hospitals with a case load of more than 40 angioplasties for AMI per year showed a lower mortality as compared to the others. In clinical practice at large community hospitals results of angioplasty for AMI concerning mortality, complications and technical success rate are comparable to those of highly specialised centres. The absolute numbers of angioplasties for AMI increased constantly over the years.  相似文献   
88.
Dear colleagues     
Reliable Computing -  相似文献   
89.
AIM/BACKGROUND: The pseudoexfoliation (PEX) syndrome is frequently associated with impairment of the blood-aqueous barrier. This study analysed if this might stimulate secondary cataract following cataract extraction. METHODS: This historical cohort study included 197 eyes of 197 patients (99 with and 98 without PEX) that underwent extracapsular cataract extraction with posterior chamber lens implantation (PMMA optic) between 1985 and 1991. Secondary cataract was defined as opacification of the axial posterior capsule and decrease of visual acuity by two or more lines. Mean follow up was 23.8 months. For statistical analysis, the Kaplan-Meier method and multivariate Cox regression analysis were used. RESULTS: Secondary cataract was observed within 24 months in 35% (SD 7%) of all eyes, and was significantly more frequent in eyes with PEX (45 (11)%) than in eyes without PEX (24 (9)%, p < 0.03). Eyes with diabetes mellitus (n = 32) showed a significantly lower frequency of secondary cataract (11 (11)%) than eyes without diabetes mellitus (39 (8)%, p < 0.01). The influences of sex, open angle glaucoma, type of cataract, surgeon, positioning of IOL, and phacoemulsification versus nuclear expression on secondary cataract did not reach statistical significance. CONCLUSION: The higher frequency of secondary cataract could be considered as another potential complication of cataract surgery in eyes with PEX.  相似文献   
90.
BACKGROUND: To assess the relationship between placental inflammation, umbilical artery Doppler waveforms and fetal biophysical profile score, umbilical artery Doppler studies and fetal biophysical evaluations were performed in 24 preterm pregnants with premature rupture of membranes (PPROM). SUBJECTS: After delivery, the placentas were microscopically examined and two subgroups were formed including noninflamed or inflamed placentas. RESULTS: In the first group, which includes 14 cases with no histological signs of placental inflammation, we found increased systolic/diastolic ratio only in one patient, whereas in the second group including ten cases with microscopically proven inflammation, nine were found to have increased systolic/diastolic ratios (p < 0.05). Mean systolic/diastolic ratio in the first and the second groups were 2.74 +/- 0.18 and 4.64 +/- 0.93 respectively (p < 0.001). Mean biophysical profile score was 9 +/- 1.04 in the first group and 7 +/- 1.05 in the second group (p < 0.001). CONCLUSION: Abnormal biophysical profile scores along with increased arterial systolic/diastolic ratios have been shown to be the markers of impending clinical infection.  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