全文获取类型
收费全文 | 14195篇 |
免费 | 1073篇 |
国内免费 | 395篇 |
专业分类
电工技术 | 658篇 |
综合类 | 479篇 |
化学工业 | 1964篇 |
金属工艺 | 487篇 |
机械仪表 | 548篇 |
建筑科学 | 677篇 |
矿业工程 | 149篇 |
能源动力 | 378篇 |
轻工业 | 1151篇 |
水利工程 | 204篇 |
石油天然气 | 265篇 |
武器工业 | 75篇 |
无线电 | 1711篇 |
一般工业技术 | 1620篇 |
冶金工业 | 3393篇 |
原子能技术 | 153篇 |
自动化技术 | 1751篇 |
出版年
2024年 | 61篇 |
2023年 | 214篇 |
2022年 | 423篇 |
2021年 | 623篇 |
2020年 | 454篇 |
2019年 | 388篇 |
2018年 | 437篇 |
2017年 | 410篇 |
2016年 | 425篇 |
2015年 | 476篇 |
2014年 | 606篇 |
2013年 | 733篇 |
2012年 | 766篇 |
2011年 | 855篇 |
2010年 | 633篇 |
2009年 | 578篇 |
2008年 | 588篇 |
2007年 | 513篇 |
2006年 | 424篇 |
2005年 | 339篇 |
2004年 | 289篇 |
2003年 | 252篇 |
2002年 | 278篇 |
2001年 | 221篇 |
2000年 | 183篇 |
1999年 | 250篇 |
1998年 | 1118篇 |
1997年 | 630篇 |
1996年 | 447篇 |
1995年 | 273篇 |
1994年 | 266篇 |
1993年 | 254篇 |
1992年 | 100篇 |
1991年 | 104篇 |
1990年 | 79篇 |
1989年 | 96篇 |
1988年 | 63篇 |
1987年 | 59篇 |
1986年 | 61篇 |
1985年 | 65篇 |
1984年 | 38篇 |
1983年 | 42篇 |
1982年 | 47篇 |
1981年 | 37篇 |
1980年 | 57篇 |
1979年 | 25篇 |
1978年 | 21篇 |
1977年 | 85篇 |
1976年 | 161篇 |
1975年 | 21篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
F Goffinet B Langer B Carbonne N Berkane D Tardif F Le Goueff M Laville F Maillard 《Canadian Metallurgical Quarterly》1997,177(5):1238-1246
OBJECTIVE: Our purpose was to evaluate the feasibility of intrapartum fetal pulse oximetry, the distribution of fetal oxygen saturation values, and the relationship with the neonatal outcome in a population with an abnormal fetal heart rate. STUDY DESIGN: A prospective multicenter observational study was performed from June 1994 to November 1995. Fetal oxygen saturation was continuously recorded with use of a Nellcor N-400 fetal pulse oximeter in case of an abnormal fetal heart rate during labor. Simultaneous readings of fetal oxygen saturation and fetal blood analysis were obtained at inclusion and before birth. Feasibility, adverse effects, distribution of fetal oxygen saturation values, and relationship with neonatal outcome were assessed. RESULTS: One hundred seventy-four patients were included. From 172 attempted sensor placements, the procedure was impossible in three cases and fetal oxygen saturation values were obtained in 164 cases (95.3%). Physicians considered sensor placement an easier task than an attempt at fetal blood analysis (easy in 87.5% vs 78.9% for fetal blood analysis, p = 0.03). The mean reliable signal time (+/- SD) was 64.7% +/- 32% during the first stage. There were no serious adverse effects in the study population. The mean fetal oxygen saturation during the first stage of labor was 42.2% +/- 8.0% (10th to 90th percentile range 30% to 53%). Fetal oxygen saturation was significantly correlated with scalp pH (r = 0.29, p = 0.01) but not with neonatal umbilical artery pH or gas values. There was a significant association between low fetal oxygen saturation (< 30%) and poor neonatal condition. CONCLUSION: The feasibility of fetal pulse oximetry is satisfactory in clinical practice. It is easy to use and provides a fair rate of recorded values, even in a population with suspicion of fetal distress. A low fetal oxygen saturation is significantly associated with an abnormal neonatal outcome. 相似文献
52.
Vascular endothelial growth factor (VEGF) is a major contributor to retinal neovascularization. The possible participation of VEGF and its high-affinity tyrosine kinase receptors, flk-1 and flt-1, in early background diabetic retinopathy was studied in the streptozotocin-induced diabetic rat model of experimental retinopathy using in situ hybridization, blotting techniques, and immunohistochemistry. Diabetic retinopathy was assessed by quantitative morphometry of retinal digest preparations. The number of acellular capillaries increased 2.7-fold in diabetic animals with diabetes' duration of 6 months compared with nondiabetic controls. VEGF expression was not detectable by in situ hybridization in nondiabetic rats but was highly increased in the ganglion cell layer and in the inner and outer nuclear layers of retinas from diabetic animals. VEGF protein was extractable only from diabetic retinas, and a strong immunolabeling was detected in vascular and perivascular structures. Increased flk-1 and flt-1 mRNA levels were also found in the ganglion cell and both nuclear layers of diabetic samples only. Dot blot and Western blot analyses confirmed the increase in flk-1 mRNA and protein in diabetic retinas. Also, flk-1 immunoreactivity was associated with vascular and nonvascular structures of the inner retinas from diabetic animals. These data obtained from a rodent model in which retinal neovascularization does not occur support the concept that the VEGF/VEGF receptor system is upregulated in early diabetic retinopathy. 相似文献
53.
The behavioral disturbances of 16 nursing home patients with a history of sundowning behaviour and dementia were treated with light therapy instead of psychopharmacological medication. After the withdrawal of the before established medication a very individual, heterogenous deterioration of the symptoms was noted. A significant improvement of the behaviour was not found by light therapy in any patient. The problems in practical handling and possible consequences of further investigation are discussed. 相似文献
54.
55.
Two recursive algorithms are presented for calculating the coefficients of a bandpass transfer function, given the lowpass prototype transfer function. In both cases, the method is general, valid for prototypes having both poles and zeros, and provides excellent accuracy. Moreover, it is shown that the coefficients of band-rejection filters can be derived by using either algorithm.<> 相似文献
56.
1. Animal studies have shown that angiotensin II has a biphasic effect on urinary sodium excretion. To examine whether this is also true in man, we studied seven salt-replete male subjects in a single-blind placebo-controlled manner. 2. While undergoing maximum diuresis, subjects were infused with 0, 1, 2, 5 or 10 ng of angiotensin II min-1 kg-1 over 80 min. Subjects were studied while seated, and stood every 20 min for urine collection. 3. Angiotensin II produced a dose-dependent antidiuretic effect. The urine flow rate, in ml/min expressed as the change from baseline with increasing dose of angiotensin, was: +3.4 +/- 1.77, -1.26 +/- 0.49 (P < 0.05), -2.75 +/- 1.23 (P < 0.05), -4.21 +/- 0.82 (P < 0.05) and -6.51 +/- 1.07 (P < 0.01). 4. In contrast, the effect of angiotensin II on sodium excretion showed a flat dose-response curve beyond 5 ng min-1 kg-1. The urinary sodium excretion, in mumol/min expressed as the change from baseline with increasing dose of angiotensin, was: 9.5 +/- 21.2, -18.9 +/- 29.6, -37.0 +/- 11.6 (P < 0.05), -67.7 +/- 19.6 (P < 0.01) and -63.8 +/- 14.3 (P < 0.01). 5. The fractional distal reabsorption of sodium, determined by using the lithium clearance technique, showed a rise with all doses of angiotensin II used and reached statistical significance with the top two doses. 6. Unlike antidiuresis, antinatriuresis after graded doses of angiotensin II in human subjects showed a flat dose-response curve beyond 5 ng min-1 kg-1. Pressor doses of angiotensin II also have a significant effect on the distal tubule in promoting sodium reabsorption. 相似文献
57.
AD Rossi E Ahlbom SO Ogren P Nicotera S Ceccatelli 《Canadian Metallurgical Quarterly》1997,117(3):428-436
In the present study the neurotoxic effects of a low dosage (0.5 mg/kg per day) of methylmercury (MeHg) on the developing nervous system were investigated. Pregnant rats were treated with MeHg from day 7 of pregnancy to day 7 of lactation. Locomotor activity (locomotion, rearing, and motility) and spatial learning ability were tested in the offspring at 6 months of age. The expression of tyrosine hydroxylase (TH) was examined by immunohistochemistry and in situ hybridization. A significant decrease in spontaneous motility and rearing was observed only in the MeHg-treated male rats. After administration of a low dose of d-amphetamine (0.5 mg/kg) no differences could be observed between control and MeHg-treated male rats, suggesting that changes in dopaminergic transmission were involved. However, no change in TH messenger RNA expression was observed. No changes in spatial learning acquisition or memory were shown in MeHg-treated rats. Taken together, these findings show that during development a very low dosage of MeHg exerts neurotoxic effects detectable in adulthood, and that susceptibility is gender-dependent. 相似文献
58.
59.
60.
Cadiou J.F. Guena J. Penard E. Legaud P. Minot C. Palmier J.F. Le Person H. Harmand J.E. 《Electronics letters》1994,30(20):1690-1691
For the first time, a direct optically injection-locked oscillator based on the Esaki-Tsu effect in superlattices is reported. The microwave output power is -7 dBm at 20 GHz with an optical input power of -10 dBm at a wavelength of 1.3 μm 相似文献