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   3090篇
  免费   54篇
  国内免费   1篇
电工技术   15篇
综合类   1篇
化学工业   181篇
金属工艺   10篇
机械仪表   26篇
建筑科学   28篇
能源动力   25篇
轻工业   218篇
水利工程   8篇
石油天然气   7篇
无线电   61篇
一般工业技术   138篇
冶金工业   2309篇
原子能技术   2篇
自动化技术   116篇
  2024年   3篇
  2023年   7篇
  2022年   20篇
  2021年   33篇
  2020年   29篇
  2019年   30篇
  2018年   31篇
  2017年   33篇
  2016年   40篇
  2015年   22篇
  2014年   41篇
  2013年   50篇
  2012年   62篇
  2011年   74篇
  2010年   48篇
  2009年   41篇
  2008年   38篇
  2007年   30篇
  2006年   35篇
  2005年   23篇
  2004年   21篇
  2003年   18篇
  2002年   15篇
  2001年   14篇
  2000年   23篇
  1999年   75篇
  1998年   729篇
  1997年   438篇
  1996年   281篇
  1995年   128篇
  1994年   130篇
  1993年   154篇
  1992年   18篇
  1991年   29篇
  1990年   24篇
  1989年   28篇
  1988年   32篇
  1987年   26篇
  1986年   27篇
  1985年   26篇
  1984年   4篇
  1983年   6篇
  1982年   10篇
  1981年   16篇
  1980年   21篇
  1978年   10篇
  1977年   38篇
  1976年   105篇
  1975年   4篇
  1955年   3篇
排序方式: 共有3145条查询结果,搜索用时 15 毫秒
71.
We conducted a multiyear study in two phases to determine preweaning performance traits of Senepol (S x S), Hereford (H x H), and reciprocal (S x H and H x S) F1 crossbred calves and feedlot performance and carcass characteristics of steers. In Phase I, from 1985 to 1989, data from S x S (n = 194), H x H (n = 383), and S x H (n = 120) calves were used. Numbers of S x S cows were increased during Phase I so that data from H x S (n = 74) calves could be included in Phase II (1990 to 1992) in addition to S x S (n = 118), H x H (n = 130), and S x H (n = 56) calves. Also during Phase II, feedlot performance and carcass characteristics were determined for S x S (n = 30), H x H (n = 26), H x S (n = 36), and S x H (n = 26) steers. In Phase I, S x S calves had heavier (P < .01) birth weights and heavier (P < .01) 205-d adjusted weaning weights than H x H calves. Birth weights of S x H calves were heavier (P < .01) than the mean of the purebred calves, but 205-d adjusted weaning weights did not differ (P > .10). In phase II, direct heterosis was 3.5% for birth weight (P < .05) and 5.1% for 205-d adjusted weaning weight (P < .01). Senepol maternal breed effects were 1.9 kg for birth weight (P < .10) and 37.9 kg for 205-d adjusted weaning weight (P < .01). Levels of direct heterosis, Senepol maternal breed effects, and Hereford direct breed effects were significant for most feedlot performance traits of steer calves that were fed to a common end point. Breeds did not differ (P > .10) for USDA yield and quality grades, and direct heterosis was not significant for Warner-Bratzler shear force. These results demonstrate significant levels of heterosis in preweaning performance between S x S and H x H calves and in feedlot performance of steers. Levels of heterosis were smaller and nonsignificant for most carcass traits including meat tenderness, which did not differ between S x S and H x H steers in this study.  相似文献   
72.
A microstructural analysis of licking behavior in nondeprived and 23-hr food-deprived rats (n = 15) presented with various sucrose solutions (0.03-1.0 M) in daily single-bottle, 1-hr sessions was conducted. Food deprivation and concentration interacted to increase total licks. The effects of food deprivation and concentration on burst size (BS), burst number (BN), and other parameters varied as a function of the pause criterion (PC; 0.3-100 s) used to define licking bursts. A rationale for selecting a 1-s PC for further analysis is presented. Despite the lack of correlations between temporally contiguous burst and pause combinations, mean BS decreased and pause duration increased as meals progressed. At the 1-s PC, BS increased linearly with concentration, implying that this microstructural parameter is influenced in part by taste. Food deprivation did not affect BS but rather increased BN and proportionally extended the meal duration.  相似文献   
73.
Epidermal growth factor (EGF) has been considered to be a candidate for neurotrophic factors on the basis of the results of several in vitro studies. However, the in vivo effect of EGF on ischemic neurons as well as its mechanism of action have not been fully understood. In the present in vivo study using a gerbil ischemia-model, we examined the effects of EGF on ischemia-induced learning disability and hippocampal CA1 neuron damage. Cerebroventricular infusion of EGF (24 or 120 ng/d) for 7 days to gerbils starting 2 hours before or immediately after transient forebrain ischemia caused a significant prolongation of response latency time in a passive avoidance task in comparison with the response latency of vehicle-treated ischemic animals. Subsequent histologic examinations showed that EGF effectively prevented delayed neuronal death of CA1 neurons in the stratum pyramidale and preserved synapses intact within the strata moleculare, radiatum, and oriens of the hippocampal CA1 region. In situ detection of DNA fragmentation (TUNEL staining) revealed that ischemic animals infused with EGF contained fewer TUNEL-positive neurons in the hippocampal CA1 field than those infused with vehicle alone at the seventh day after ischemia. In primary hippocampal cultures, EGF (0.048 to 6.0 ng/mL) extended the survival of cultured neurons, facilitated neurite outgrowth, and prevented neuronal damage caused by the hydroxyl radical-producing agent FeSO4 and by the peroxynitrite-producing agent 3-morpholinosydnonimine in a dose-dependent manner. Moreover, EGF significantly attenuated FeSO4-induced lipid peroxidation of cultured neurons. These findings suggest that EGF has a neuroprotective effect on ischemic hippocampal neurons in vivo possibly through inhibition of free radical neurotoxicity and lipid peroxidation.  相似文献   
74.
BACKGROUND: The main purpose of the reconstruction of the cranium is the protection of the brain. Besides we have to consider important functional and aesthetic necessities in order to achieve satisfactory results. METHODS: Thirty-six clinical cases, operated from November 1991 to June 1996, in which the reconstruction of the cranial vault is carried out by a polymethylmethacrylate acrylic resin are analysed. The causes and locations of the most common bone defects and the main indications for reconstruction are examined. While the repair of the osseous gaps caused by neoplasms is immediate, in the traumatic occurrences, in order to reduce the probability of infectious complications, an average time of 11 months elapsed from the first operation. The surgical technique, with slightest alterations, is the same in all the presented cases, preparing the acrylic resin straight on the operating table. The resin, moulded and adapted to the defect until its complete hardening, presents, thanks to its properties, manifold advantages (and few real disadvantages). RESULTS: The results, in terms of complications, are very satisfactory, with an infectious rate of 2.7%. Besides, in one third of the patients, a considerable clinical improvement after the repair has been observed. CONCLUSIONS: According to personal experience, it is possible to affirm that polymethylmethacrylate, with its remarkable plasticity and stability in time, can always guarantee a satisfactory functional and aesthetic result.  相似文献   
75.
Lactation records (n = 2362) of 1402 crossbred cows in 22 cooperating dairy herds in southeastern Brazil were evaluated. Cows were mixtures of Zebu (Gir, Guzera, and unknown) and European breeding (mostly Holstein). Lactation milk yields were expressed as total, 3050-d, or deviated 305-d yields, either adjusted or unadjusted for days in milk (DIM). Mean DIM was 280. Arithmetic means unadjusted for DIM were 1942, 1666, and 5 kg per record. Milk yields of daughters from sires of 6/8 and 7/8 European breeds were higher than yields of daughters from sires of 5/8 European breeds when data were either adjusted or unadjusted for DIM. The differences associated with breed group of sire were only slightly reduced when records were adjusted for DIM. There was no evidence of a decline in milk yield as the fraction of European breeding of the sire increased from 6/8 to 7/8. For a given breed group of sire, whether the grandsire was purebred or crossbred had no detectable effect. These results should be useful in determining strategies for crossbreeding of dairy cows in tropical areas, particularly when crossbred sires are used.  相似文献   
76.
A new series 5-thio aryl pyrazole derivatives were proposed aiming analgesic activity. In this work, 8 new compounds of this class were synthesized using usual synthetic methodology, having as key intermediate the 3-methyl-4-nitro-5-chloropyrazole-1-phenyl derivative and subsequent reaction with several nucleophiles sulfides. Pharmacological evaluation of this series showed analgesic activity in the some extent in especially for 5-(4-bromophenyl)-thio-3-methyl-4-nitro-1-phenylpyrazole which was the most potent in this series, presenting an analgesic action comparable to that show by dipyrone.  相似文献   
77.
78.
The relative risk of death by calendar date of diagnosis was investigated in a population-based incident cohort of 845 (463 males:382 females) IDDM diagnosed in Leicestershire before the age of 17 years between 1940 and 1989. The mortality status of 844 (99.9%) patients was determined as of the 31 December 1991, representing 14,346 person-years of risk. Trends in relative risk of death were investigated using Cox proportional hazards modelling for within cohort comparisons and age/sex and calendar time adjusted standardized mortality ratios (SMR) using generalized linear modelling for external comparisons. Median age at diagnosis was 10 years (range 3 months to 16 years); median duration of diabetes 15 years (range 1-51 years). Forty-four patients had died (5.2%; median age at death 31 years, range 11-51 years). A further four patients died at presentation (within 24 h) from ketoacidosis and are excluded from all analyses. Calendar date of diagnosis was found to be an important predictor of mortality. Adjusting for attained age there was evidence of a decline in relative risk of death with calendar date of diagnosis of 3.4% (95% CI, 0.005-6.9%) per annum, equivalent to a 32% fall per decade (95% CI, 5-51%), or 84% (95% CI, 21-97) from 1940 to 1989. The data are consistent with a large fall in mortality between the 1940s and 1950s representing over 50% of the total reduction in mortality between 1940 and 1991. Neither sex nor age at diagnosis were significant predictors of mortality. Over the study period 1940-89 the SMR (male and female combined) fell from 981 (541-1556) to 238 (60-953) relative to the general population. This population-based study shows that the prognosis for Type 1 (insulin-dependent) diabetes mellitus has improved markedly over the period 1940-1991.  相似文献   
79.
T cells infiltrating the iris/ciliary body of Lewis rats with anterior uveitis (AU) that had been induced by myelin basic protein (MBP) immunization were previously found to share surface markers common to the T cells that cause experimental autoimmune encephalomyelitis (EAE). To determine whether these AU-associated T cells are in fact the same as those that infiltrate the central nervous system to cause EAE, we examined TCR V gene expression in T cells infiltrating the anterior chamber in rats with AU. As with EAE, we found a biased expression of Vbeta8.2 and Valpha2 in the iris/ciliary body and, although one would expect an influx of nonspecific inflammatory T cells, these biases were still evident at the peak of AU. An analysis of the TCR Vbeta8.2 and Valpha2 sequences derived from the iris/ciliary body demonstrated the presence of the same complementarity determining region 3 motifs found in MBP-specific T cells that are pathogenic for EAE and found in T cells derived from the central nervous system of rats with EAE. Finally, T cells isolated from the iris/ciliary body of rats with AU were found to proliferate in a specific fashion to MBP Ags. Thus, it appears that MBP-specific T cells are pathogenic for AU as well as EAE in the Lewis rat. In addition, the long-term presence of this highly restricted MBP response in the iris/ciliary body indicates that distinct immunoregulatory mechanisms exist in the environment of the eye. This provides an interesting model with which to address questions pertaining to the nature of T cells infiltrating the eye and their regulation during EAE and other systemic diseases.  相似文献   
80.
A hyperdynamic circulatory state with elevated cardiac output, decreased peripheral vascular resistance, and sodium retention occurs in patients with portal cirrhosis. Surgical portal-systemic shunts and transjugular intrahepatic portal-systemic shunts (TIPS) have been shown to worsen the high-output state in these patients. However, clinical evidence of high-output congestive heart failure has been reported only rarely to complicate cirrhosis. We describe a patient who developed high-output congestive heart failure with markedly elevated filling pressures after TIPS and had complete resolution of heart failure after liver transplantation.  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