全文获取类型
收费全文 | 37179篇 |
免费 | 2412篇 |
国内免费 | 988篇 |
专业分类
电工技术 | 1482篇 |
技术理论 | 4篇 |
综合类 | 1554篇 |
化学工业 | 6075篇 |
金属工艺 | 1536篇 |
机械仪表 | 1670篇 |
建筑科学 | 1689篇 |
矿业工程 | 758篇 |
能源动力 | 1195篇 |
轻工业 | 2234篇 |
水利工程 | 448篇 |
石油天然气 | 1011篇 |
武器工业 | 146篇 |
无线电 | 5408篇 |
一般工业技术 | 5074篇 |
冶金工业 | 4911篇 |
原子能技术 | 388篇 |
自动化技术 | 4996篇 |
出版年
2024年 | 105篇 |
2023年 | 449篇 |
2022年 | 863篇 |
2021年 | 1195篇 |
2020年 | 927篇 |
2019年 | 799篇 |
2018年 | 898篇 |
2017年 | 946篇 |
2016年 | 861篇 |
2015年 | 1069篇 |
2014年 | 1414篇 |
2013年 | 2118篇 |
2012年 | 1986篇 |
2011年 | 2308篇 |
2010年 | 1833篇 |
2009年 | 1878篇 |
2008年 | 1929篇 |
2007年 | 1739篇 |
2006年 | 1567篇 |
2005年 | 1255篇 |
2004年 | 1108篇 |
2003年 | 1193篇 |
2002年 | 1398篇 |
2001年 | 1177篇 |
2000年 | 817篇 |
1999年 | 785篇 |
1998年 | 1721篇 |
1997年 | 1067篇 |
1996年 | 815篇 |
1995年 | 616篇 |
1994年 | 493篇 |
1993年 | 470篇 |
1992年 | 269篇 |
1991年 | 246篇 |
1990年 | 250篇 |
1989年 | 222篇 |
1988年 | 195篇 |
1987年 | 189篇 |
1986年 | 147篇 |
1985年 | 136篇 |
1984年 | 110篇 |
1983年 | 80篇 |
1982年 | 74篇 |
1981年 | 67篇 |
1980年 | 109篇 |
1979年 | 51篇 |
1978年 | 62篇 |
1977年 | 85篇 |
1976年 | 174篇 |
1975年 | 48篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
51.
At present, the preferred tool for parameter estimation in compartmental analysis is an iterative procedure; weighted nonlinear regression. For a large number of applications, observed data can be fitted to sums of exponentials whose parameters are directly related to the rate constants/coefficients of the compartmental models. Since weighted nonlinear regression often has to be repeated for many different data sets, the process of fitting data from compartmental systems can be very time consuming. Furthermore the minimization routine often converges to a local (as opposed to global) minimum. In this paper, we examine the possibility of using artificial neural networks instead of weighted nonlinear regression in order to estimate model parameters. We train simple feed-forward neural networks to produce as outputs the parameter values of a given model when kinetic data are fed to the networks' input layer. The artificial neural networks produce unbiased estimates and are orders of magnitude faster than regression algorithms. At noise levels typical of many real applications, the neural networks are found to produce lower variance estimates than weighted nonlinear regression in the estimation of parameters from mono- and biexponential models. These results are primarily due to the inability of weighted nonlinear regression to converge. These results establish that artificial neural networks are powerful tools for estimating parameters for simple compartmental models. 相似文献
52.
Wen-Chau Liu Lih-Wen Laih Shiou-Ying Cheng Wen-Lung Chang Wei-Chou Wang Jing-Yuh Chen Po-Hung Lin 《Electron Devices, IEEE Transactions on》1998,45(2):373-379
In this paper, a new multiple negative-differential-resistance (MNDR) device based on a metal-insulator-semiconductor-insulator-metal (MISIM)-like structure with step-compositional InxGa1-x As quantum wells has been fabricated and demonstrated. The interesting MNDR phenomena are found in the current-voltage (I-V) characteristics of this device. At room temperature, the triple switching behaviours and quadruple stable operation states are obtained. In addition, the sixfold switching behaviors and a staircase-shaped I-V characteristic are observed at -105°C. A sequential carrier accumulation at InGaAs subwells and the potential lowering process are used to qualitatively explain the interesting MNDR phenomena. From the experimental results, it is shown that the studied device has good potential in multiple-valued logic applications 相似文献
53.
从冷却曲线上相变段倾角最小原则出发,研究了新型激冷剂的配比与热分析法检测铸铁成分精度的关系,实验结果表明,能使相变段倾角最小的成分配比,也能提高对铸铁成分的热分析和检测精度。 相似文献
54.
55.
GIS技术与微机制图 总被引:1,自引:0,他引:1
地理信息系统(GIS)随着信息社会的发展愈发显示出强大的生命力。本文简要叙述GIS的重要作用及其发展进程中涌出出来的四个设计模型,总结了运用GIS国产软件平台MAPGIS制作遥感地质构造解译图的工作步骤和技术方法。 相似文献
56.
The purpose of this experiment was to investigate the functional contributions of various limbic structures to locomotion and stereotypy induced by dopaminergic drugs. Female rats were randomly assigned to one of 5 groups (n = 10-14 rats/group) that received either a lesion of the hippocampus (colchicine + kainic acid), basolateral amygdala (quinolinic acid), frontal cortex (aspiration), nucleus accumbens (ibotenic acid), or served as unoperated controls. Beginning at least 2 weeks following surgery locomotion (measured as photocell beam breaks) elicited by D-amphetamine (0.0, 0.32, 1.0 and 3.2 mg/kg), SKF 82958 (0.0, 0.04, 0.08 and 0.16 mg/kg) or quinpirole (0.0, 0.25, 0.1 and 0.5 mg/kg) was determined. In agreement with previous results rats with hippocampal lesions were hyperactive in response to amphetamine. In comparison to these changes in drug-induced locomotion, lesions of the basolateral amygdala, and frontal cortex had only minor effects on drug-induced locomotion. Lesions of the nucleus accumbens produced consistent hyperactivity that was suppressed by doses of amphetamine or quinpirole that elicited behavioral stereotypy. These results provide evidence suggesting that, in comparison to other limbic structures that have substantial inputs to the nucleus accumbens, the hippocampus play a relatively prominent role in the modulation of drug-induced locomotion. 相似文献
57.
We examined the molecular mechanism of metronidazole resistance by constructing a lambda-Zap II phagemid expression library with genomic DNA from a metronidazole-resistance strain of Helicobacter pylori. Twenty-two clones were found to have elevated MTZ resistances in XLOLR strain of E. coli. Phagemids belonging to the twenty two clones were extracted and then retransformed into the XLOLR strain of E. coli. After MTZ selection, five clones could confer metronidazole resistance consistently. According to Southern hybridization and DNA sequencing, the five clones contained a same locus, recA. In addition, transforming the five clones into BL21 strain of E. coli produced a higher resistance to MTZ. Interestingly, electroporation of one of the five phagemid clones into two MTZ sensitive H. pylori yielded MTZ resistant strains. Comparing amino acid sequence in MTZ resistant with sensitive isolates revealed two point mutations at this locus. Above results suggest that mutation in recA may be associated with metronidazole resistance of H. pylori. 相似文献
58.
The response of P. falciparum to chloroquine and pyrimethamine-sulfadoxine in vivo and chloroquine and amodiaquine in vitro was investigated in parasitaemic school children from six locations. Mean parasite sensitivity to chloroquine at day 7 was 74% (range 61-97) with parasite clearance rates between 2-3 days and complete defervescence in 85% of febrile children. Sensitivity declined in the four sites followed up to day 14 to 45% (range 37-53). Parasites were significantly more sensitive to pyrimethamine/sulfadoxine at 5/6 sites (100% day 7) but 5% of subjects became parasitaemic by day 14. In vitro isolates were significantly less sensitive to chloroquine than to amodiaquine with a mean 99% effective concentration of 348 mumol/L compared to 6.44 mumol/L. Clearly the role of chloroquine as the primary therapy for uncomplicated P. falciparum malaria should be reconsidered especially in the light of increasing disease severity and resurgence. Amodiaquine may be suitable alternative with pyrimethamine/sulfadoxine as second line and for more severe malaria prior to referral. The cost of alternative antimalarials and the dynamic and deteriorating pattern of resistance are powerful arguments for more objective slide diagnosis to minimise drug pressure and a regular drug sensitivity surveillance system. We believe that the latter should concentrate on measuring clinical drug efficacy in symptomatic outpatients rather than in asymptomatic children while the former needs more pragmatic and economical strategies possibly centred on seasonality and risk. 相似文献
59.
60.