全文获取类型
收费全文 | 277287篇 |
免费 | 4571篇 |
国内免费 | 992篇 |
专业分类
电工技术 | 5222篇 |
综合类 | 426篇 |
化学工业 | 40547篇 |
金属工艺 | 10212篇 |
机械仪表 | 8121篇 |
建筑科学 | 6815篇 |
矿业工程 | 1023篇 |
能源动力 | 7673篇 |
轻工业 | 24316篇 |
水利工程 | 2404篇 |
石油天然气 | 3357篇 |
武器工业 | 25篇 |
无线电 | 35661篇 |
一般工业技术 | 54497篇 |
冶金工业 | 54286篇 |
原子能技术 | 5233篇 |
自动化技术 | 23032篇 |
出版年
2022年 | 1702篇 |
2021年 | 2755篇 |
2020年 | 2046篇 |
2019年 | 2316篇 |
2018年 | 4086篇 |
2017年 | 4009篇 |
2016年 | 4217篇 |
2015年 | 2948篇 |
2014年 | 4798篇 |
2013年 | 13065篇 |
2012年 | 7439篇 |
2011年 | 9763篇 |
2010年 | 7936篇 |
2009年 | 8866篇 |
2008年 | 9510篇 |
2007年 | 9339篇 |
2006年 | 8112篇 |
2005年 | 7119篇 |
2004年 | 6736篇 |
2003年 | 6954篇 |
2002年 | 6417篇 |
2001年 | 6792篇 |
2000年 | 6248篇 |
1999年 | 6761篇 |
1998年 | 18195篇 |
1997年 | 12495篇 |
1996年 | 9554篇 |
1995年 | 7054篇 |
1994年 | 6283篇 |
1993年 | 6207篇 |
1992年 | 4295篇 |
1991年 | 4144篇 |
1990年 | 3975篇 |
1989年 | 3772篇 |
1988年 | 3662篇 |
1987年 | 3141篇 |
1986年 | 3051篇 |
1985年 | 3384篇 |
1984年 | 3082篇 |
1983年 | 2957篇 |
1982年 | 2731篇 |
1981年 | 2672篇 |
1980年 | 2598篇 |
1979年 | 2391篇 |
1978年 | 2186篇 |
1977年 | 2777篇 |
1976年 | 3951篇 |
1975年 | 1831篇 |
1974年 | 1788篇 |
1973年 | 1754篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
931.
This paper discusses changes in the spectrum and distortion of the electron wave function of a GaAs quantum well when a thin
AlGaAs barrier is introduced into it. The potential difference generated across the quantum well by distortion of the electron
wave function is calculated, along with its dependence on the position of the barrier in the quantum well. The photovoltaic
response of the structure to optical intersubband excitations is also calculated, along with the role of wave function and
electronic spectrum distortion as well as intersubband nonradiative transitions in generating this response. The suitability
of a GaAs quantum well with a thin barrier for use as an infrared detector is considered.
Fiz. Tekh. Poluprovodn. 32, 1246–1250 (October 1998) 相似文献
932.
There has been an increasing interest in the use of code-division multiple access (CDMA) in cellular mobile and wireless personal communications. The choice of such multiaccess technique is attractive because of its potential capacity increases and other technical factors such as privacy and multipath rejection capabilities. However, it is well known that the performance of CDMA can be significantly degraded due to cochannel interference (CI) and the near-far effects. We consider the performance of direct-sequence (DS)-based CDMA over fading channels that are modeled as slowly varying Rayleigh-fading discrete multipath channels. Specifically, we propose and analyze an adaptive multistage interference cancellation strategy for the demodulation of asynchronous DS spread-spectrum multiple-access signals. Numerical results show that the proposed multistage detector, which alleviates the detrimental effects of the near-far problem, can significantly improve the system performance 相似文献
933.
K Stubbs LA Martin DC Dimmitt N Pready WF Hahne 《Canadian Metallurgical Quarterly》1997,37(10):926-936
In previous studies, dolasetron was shown to have both renal and hepatic elimination mechanisms. This study was conducted to determine the impact of varying degrees of hepatic dysfunction on the pharmacokinetics and safety of dolasetron and its reduced metabolites. Seventeen adults were studied: six healthy volunteers (group I), seven patients with mild hepatic impairment (Child-Pugh class A; group II), and four patients with moderate to severe hepatic impairment (Child-Pugh class B or C1; group III). Single 150-mg doses of dolasetron mesylate were administered intravenously and orally, with a 7-day washout period separating treatments. After intravenous administration, no differences were observed between healthy volunteers and patients with hepatic impairment in maximum plasma concentration (Cmax), areas under the plasma concentration-time curve (AUC), or elimination half-life (t1/2) of intact dolasetron. No significant differences were found in Cmax, AUC, or apparent clearance (C(lapp)) of hydrodolasetron, the primary metabolite of dolasetron. The mean t1/2 increased from 6.87 hours in group I to 11.69 hours in group III. After oral administration, C(lapp) of hydrodolasetron decreased by 42%, and Cmax increased by 18% in patients with moderate to severe hepatic impairment. There were less changes in patients with mildly hepatic impairment. Total percentage of dose excreted as metabolites was similar for healthy volunteers and patients with hepatic impairment, although urinary metabolite profiles differed slightly. Dolasetron was well tolerated and there were no apparent differences in adverse effects between groups or treatments. Because hepatic impairment did not influence Cl(app) of hydrodolasetron after intravenous administration, and the range of plasma concentrations of hydrodolasetron after oral administration was not different from those observed in healthy volunteers, dosage adjustments are not recommended for patients with hepatic disease and normal renal function. 相似文献
934.
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
935.
This paper formulates the pickup and delivery problem, also known as the dial-a-ride problem, as an integer program. Its polyhedral structure is explored and four classes of valid inequalities developed. The results of a branch-and-cut algorithm based on these constraints are presented. 相似文献
936.
937.
The incidence of Campylobacter jejuni and Campylobacter coli in broiler farms was 33.9% (19/56). C. jejuni-positive flocks accounted for 20.0% (17/85) and C. coli-positive ones was 4.7% (4/85). There were 14 patterns (fla type) of restriction fragment length polymorphism (RFLP) of flagellin A gene among these 22 strains of C. jejuni and C. coli including the standard strain C. jejuni ATCC 33560. Different fla types of Campylobacter were isolated from broilers in different growing cycles on the same farms. Four strains of C. jejuni were isolated from four breeder farms and four fla types of C. jejuni were detected from their progenies reared on growing farms. Three fla types of C. jejuni detected from the progenies were different from those of each breeder. Also, the other three fla types of C. jejuni were detected from different progenies of each growing farm during the next growing cycle. These findings indicate that the RFLP analysis may contribute to epidemiological studies of C. jejuni and C. coli contamination of broilers and suggest the risk of contamination with different types of Campylobacter in every growing cycle of broilers on the farm even on the same farm. They also supported that there was little likeliness of the vertical transmission of C. jejuni and C. coli from breeders to broilers. 相似文献
938.
939.
Baciu G. Kesavan H.K. 《IEEE transactions on systems, man, and cybernetics. Part A, Systems and humans : a publication of the IEEE Systems, Man, and Cybernetics Society》1997,27(2):244-250
The distinction between geometry and dynamic interactions is fundamental for the consistent dynamic analysis of physical systems. A unified treatment of such systems is possible when we adopt a hierarchical mathematical model with a consistent set of embedded abstractions. This new view is adopted in the general formulation strategy for obtaining a simplified dynamics model of mechanical systems. We show that there exists a consistent general extension from the model of constrained particle-mass systems (PMS) to the model of multibody systems (MBS) based entirely on graph-theoretic concepts 相似文献
940.
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