全文获取类型
收费全文 | 155020篇 |
免费 | 1978篇 |
国内免费 | 657篇 |
专业分类
电工技术 | 3154篇 |
综合类 | 192篇 |
化学工业 | 24530篇 |
金属工艺 | 5831篇 |
机械仪表 | 4992篇 |
建筑科学 | 4545篇 |
矿业工程 | 399篇 |
能源动力 | 3987篇 |
轻工业 | 17581篇 |
水利工程 | 1163篇 |
石油天然气 | 625篇 |
武器工业 | 5篇 |
无线电 | 20691篇 |
一般工业技术 | 29394篇 |
冶金工业 | 24823篇 |
原子能技术 | 2369篇 |
自动化技术 | 13374篇 |
出版年
2019年 | 854篇 |
2018年 | 1126篇 |
2017年 | 1161篇 |
2016年 | 1320篇 |
2015年 | 1101篇 |
2014年 | 1847篇 |
2013年 | 6747篇 |
2012年 | 3284篇 |
2011年 | 4735篇 |
2010年 | 3690篇 |
2009年 | 4256篇 |
2008年 | 4781篇 |
2007年 | 5047篇 |
2006年 | 4451篇 |
2005年 | 4205篇 |
2004年 | 4075篇 |
2003年 | 3988篇 |
2002年 | 4002篇 |
2001年 | 4045篇 |
2000年 | 3813篇 |
1999年 | 3769篇 |
1998年 | 6844篇 |
1997年 | 5370篇 |
1996年 | 4565篇 |
1995年 | 3770篇 |
1994年 | 3425篇 |
1993年 | 3256篇 |
1992年 | 2812篇 |
1991年 | 2711篇 |
1990年 | 2668篇 |
1989年 | 2651篇 |
1988年 | 2488篇 |
1987年 | 2214篇 |
1986年 | 2145篇 |
1985年 | 2598篇 |
1984年 | 2363篇 |
1983年 | 2229篇 |
1982年 | 2097篇 |
1981年 | 2020篇 |
1980年 | 1895篇 |
1979年 | 1893篇 |
1978年 | 1799篇 |
1977年 | 2133篇 |
1976年 | 2627篇 |
1975年 | 1602篇 |
1974年 | 1449篇 |
1973年 | 1467篇 |
1972年 | 1205篇 |
1971年 | 1128篇 |
1970年 | 959篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Droplet formation on Si surfaces bombarded by 10 keV beams from a liquid metal indium ion source has been observed. It has been verified that excess indium is transported in the ion beam, probably in the form of charged droplets. SIMS, SEM and RBS techniques have been used to study the distribution of droplets at the surface. At the specified ion source operating conditions, equilibrium droplet coverage is of the order of 25%. Individual droplets have an average diameter of 0.35 ωm and an average height of 2.2 nm. At a bombardment current density of 0.1 mA cm?2, equilibrium coverage is reached in about 1 s. 相似文献
992.
S. W. Sloan J. R. Booker 《International journal for numerical methods in engineering》1992,33(1):163-196
This paper considers the problem of integrating the constitutive relations for Tresca and Mohr–Coulomb materials under conditions of plane strain. In the case of a Tresca material, we show that the constitutive law may be integrated exactly by assuming the strain rates d?/dλ to be constant. We also derive a semi-analytic method for integrating both types of constitutive law which assumes that the quantities d?/dλ are constant. This approach is motivated by the fact that the exact variation of the strains during a time interval is unknown and leads to a single non-linear equation in λ which can be solved efficiently to yield the unknown stresses. Finally, we compare the results from the analytic and semi-analytic methods with those from a variety of numerical integration schemes. 相似文献
993.
Insights into soot formation processes are gained from chemical sampling and thermocouple probing of co-flowing inverse diffusion flames (IDFs), with the oxidizer in the center. The transition from near-to slightly sooting flames and the effects of flame temperature, fuel concentration, and fuel structure (using methane, ethene, propene and 1-butene) are investigated. The aromatic content of IDFS scales with the fuel's sooting tendency, and suggests that the formation of the aromatic ring is a controlling step in soot formation. In addition to the relatively well-established reactions involving C4 and C2 species, benzene may form directly from two C3 species for fuels that readily produce C3 species during pyrolysis and/or oxidative pyrolysis. The total concentration of growth species increases almost linearly with fuel concentration, but depends more weakly on flame temperature than would be expected if pure pyrolysis governed the intermediate hydrocarbon behavior. 相似文献
994.
Superplastic forming using conventional alloys and product forms may be a cost effective route for component manufacture. In this paper superplastic deformation of Ti-3Al-4V extruded tube is examined in terms of the strain anisotropy, microstructural changes, the texture and the post-formed tensile properties. Superplastic deformation with low flow stresses over a wide range of strain rates was observed. Strain anisotropy and surface roughening occurred during deformation and was associated with the two-phase, aligned and banded microstructure. A small reduction in the tensile properties was found and changes in the texture were noted after superplastic deformation. 相似文献
995.
Summary Styrene polymerization was initiated by methylaluminoxane in the presence of haloalkane. The propagating species are shown by trapping experiments and electron paramagnetic resonance to be both radicals and cations. 相似文献
996.
An averaging circuit model is developed for the class E resonant DC-DC converter regulated at a fixed switching frequency. The regulation is achieved by use of an auxiliary switch. The model is obtained based on the circuit analysis using the Fourier series expansion. Steady-state and small-signal dynamic analysis is presented, which reveals that the DC output is well controlled by the control angle of the auxiliary switch and that there exists a right-half-plane zero in the control-to-output transfer function. The analysis results are verified by the experiments 相似文献
997.
G.W.A.D. 《Microelectronics Reliability》1991,31(1)
998.
Schilling D.L. Milstein L.B. Pickholtz R.L. Bruno F. Kanterakis E. Kullback M. Erceg V. Biederman W. Fishman D. Salerno D. 《Communications Magazine, IEEE》1991,29(11):86-93
Multipath propagation in a broadband CDMA environment is described. A propagation model for broadband spread-spectrum signals is presented. Experimental results relating to the sharing of the band by fixed service microwave users and mobile personal communications network (PCN) users are discussed. Field tests indicate that PCN systems can provide high-quality communications when sharing the spectrum with fixed-service microwave systems in suburban and urban areas 相似文献
999.
Previous research has shown that a combination of feed restriction and dietary 1,3-butanediol starting at 14 d post-partum resulted in fatty liver and ketosis. Sixteen multiparous Holstein cows were used to determine effects of feed restriction or 1,3-butanediol as separate treatments. Treatments during d 14 to 42 postpartum were 1) control (ad libitum intake), 2) 20% feed restriction, or 3) control plus dietary 1,3-butanediol (5.5% of DM). From d 43 to 56, cows assigned to treatments 2 and 3 received a combination of feed restriction and butanediol. One cow on treatment 2 developed ketosis, but not fatty liver, after only 4 d of feed restriction. No other cows developed fatty liver or ketosis. Both treatments decreased milk production compared with controls. Feed restriction increased the extent of negative energy balance and caused transient increases in concentrations of NEFA, acetate, and beta-hydroxybutyrate in plasma. Concentrations of beta-hydroxybutyrate and insulin in plasma were increased by butanediol, which is a potent ketone body precursor. Concentration of glycogen in liver was less in feed-restricted cows, whereas glycogen and total lipid were greater in cows given butanediol separately. Gluconeogenic capacity of liver slices was not different among groups. Addition of 1,3-butanediol to in vitro incubation media decreased oxidation of propionate to CO2. Neither feed restriction nor dietary 1,3-butanediol as separate treatments induced the fatty liver and ketosis observed in earlier experiments in which the two treatments were given together. 相似文献
1000.
J. H. Marsh S. A. Bradshaw A. C. Bryce R. Gwilliam R. W. Glew 《Journal of Electronic Materials》1991,20(12):973-978
Impurity induced disordering of GaInAs quantum well structures with barriers of AlGaInAs and of GaInAsP has been investigated
using boron and fluorine. The impurities were introduced by ion implantation followed by thermal annealing. Annealing unimplanted
P-based quaternary material at temperatures greater than 500° C caused a blue shift of the exciton peak. At annealing temperatures
greater than 650° C red shifts in the exciton peak of unimplanted Al-based quaternary material were observed. Boron implantation
caused small blue shifts of the exciton peak in both material systems at low annealing temperatures. Much larger blue shifts
were observed in the fluorine implanted samples. 相似文献