全文获取类型
收费全文 | 30437篇 |
免费 | 1516篇 |
国内免费 | 132篇 |
专业分类
电工技术 | 473篇 |
综合类 | 241篇 |
化学工业 | 5537篇 |
金属工艺 | 807篇 |
机械仪表 | 675篇 |
建筑科学 | 1015篇 |
矿业工程 | 148篇 |
能源动力 | 541篇 |
轻工业 | 4065篇 |
水利工程 | 213篇 |
石油天然气 | 135篇 |
武器工业 | 10篇 |
无线电 | 1579篇 |
一般工业技术 | 5461篇 |
冶金工业 | 7447篇 |
原子能技术 | 173篇 |
自动化技术 | 3565篇 |
出版年
2023年 | 302篇 |
2022年 | 300篇 |
2021年 | 636篇 |
2020年 | 517篇 |
2019年 | 456篇 |
2018年 | 885篇 |
2017年 | 791篇 |
2016年 | 909篇 |
2015年 | 799篇 |
2014年 | 927篇 |
2013年 | 1758篇 |
2012年 | 1361篇 |
2011年 | 1410篇 |
2010年 | 1164篇 |
2009年 | 1127篇 |
2008年 | 1265篇 |
2007年 | 1212篇 |
2006年 | 878篇 |
2005年 | 721篇 |
2004年 | 540篇 |
2003年 | 501篇 |
2002年 | 456篇 |
2001年 | 374篇 |
2000年 | 363篇 |
1999年 | 486篇 |
1998年 | 2334篇 |
1997年 | 1410篇 |
1996年 | 990篇 |
1995年 | 621篇 |
1994年 | 504篇 |
1993年 | 461篇 |
1992年 | 209篇 |
1991年 | 225篇 |
1990年 | 190篇 |
1989年 | 180篇 |
1988年 | 173篇 |
1987年 | 194篇 |
1986年 | 165篇 |
1985年 | 254篇 |
1984年 | 202篇 |
1983年 | 193篇 |
1982年 | 153篇 |
1981年 | 203篇 |
1980年 | 199篇 |
1979年 | 147篇 |
1978年 | 130篇 |
1977年 | 239篇 |
1976年 | 491篇 |
1975年 | 108篇 |
1973年 | 123篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
Robert R. Gillies Jayne Brim Box Jürgen Symanzik Eli J. Rodemaker 《Remote sensing of environment》2003,86(3):411-422
Impervious surface area (ISA) was derived for a period from 1979 to 1997 from Landsat MSS and TM data for the Line Creek watershed that lies to the south of the city of Atlanta, GA. The change in ISA is presented as an ecological indicator to examine the cumulative water resource impacts on mussel population in three sub-watersheds of Line Creek—namely, Line, Flat, and Whitewater creeks. The satellite analysis shows that ISA expansion occurred substantially from 1987 to 1997 and is predominantly in industrial, commercial, and shopping center (ICS) complexes but also in smaller lot-size residential development. Evidence of mussel habitat degradation is indicated and loss of species (in the region of 50 to 70%) is present in areas where ISA expansion is observed—specifically in ICS complex development in and around Peachtree City that drains directly into the Line and Flat creeks. This is in marked contrast to Whitewater Creek where overall development of ISA is less and no major loss of mussel species is observed. 相似文献
82.
83.
OBJECTIVE: To investigate the efficacy of the rectus abdominis muscle flap (RAMF) technique for the closure and augmentation of small, non-compliant bladder exstrophies. PATIENTS AND METHODS: The RAMF technique was used in two girls and two boys (mean age at operation, 31.7 months, range 3-72) with bladder exstrophy. The clinical outcome and urodynamics were assessed during a follow-up of 29 months to 6 years (mean 49.2 months) and included imaging, cystoscopy, biochemical and microbiological studies. RESULTS: There were no urinary tract infections, metabolic problems or electrolyte disturbances and kidney function remained normal in all patients. Radiography confirmed intact function and anatomy of the urinary tract and cystoscopy showed complete coverage of the inner peritoneal layer of RAMF with uroepithelium. No stone formation or mucus production were detected. Currently, three patients void using clean intermittent catheterization through the native urethra and the fourth through an appendiceal Mitrofanoff valve. However, the bladder volume was insufficient at the late follow-up and three patients required bladder augmentation. CONCLUSIONS: The RAMF technique is a good alternative for closing bladder exstrophies and achieves an increase in bladder capacity; however, although there is a mild improvement in bladder capacity and compliance. RAMF should not be used as a bladder augmentation procedure. The technique is indicated in the closure of large bladder defects, bladder exstrophies with small, inelastic, non-compliant bladder remnants and failed primary closures. 相似文献
84.
85.
P Kn?bl G Schernthaner C Schnack P Pietschmann A Griesmacher R Prager M Müller 《Canadian Metallurgical Quarterly》1993,36(10):1045-1050
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
86.
87.
E Kievit FB van Gog HM Schlüper GA van Dongen HM Pinedo E Boven 《Canadian Metallurgical Quarterly》1997,38(4):813-823
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
88.
89.
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
90.
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