全文获取类型
收费全文 | 331629篇 |
免费 | 10864篇 |
国内免费 | 5297篇 |
专业分类
电工技术 | 10490篇 |
技术理论 | 2篇 |
综合类 | 6036篇 |
化学工业 | 49133篇 |
金属工艺 | 15707篇 |
机械仪表 | 12211篇 |
建筑科学 | 12906篇 |
矿业工程 | 2331篇 |
能源动力 | 8852篇 |
轻工业 | 31153篇 |
水利工程 | 3630篇 |
石油天然气 | 5208篇 |
武器工业 | 678篇 |
无线电 | 42560篇 |
一般工业技术 | 55928篇 |
冶金工业 | 54528篇 |
原子能技术 | 4862篇 |
自动化技术 | 31575篇 |
出版年
2022年 | 3394篇 |
2021年 | 5112篇 |
2020年 | 3547篇 |
2019年 | 3503篇 |
2018年 | 4645篇 |
2017年 | 4996篇 |
2016年 | 4859篇 |
2015年 | 5280篇 |
2014年 | 7552篇 |
2013年 | 16344篇 |
2012年 | 11414篇 |
2011年 | 14420篇 |
2010年 | 12030篇 |
2009年 | 12507篇 |
2008年 | 13018篇 |
2007年 | 12872篇 |
2006年 | 12066篇 |
2005年 | 10602篇 |
2004年 | 8997篇 |
2003年 | 8537篇 |
2002年 | 8536篇 |
2001年 | 8317篇 |
2000年 | 7656篇 |
1999年 | 8302篇 |
1998年 | 18308篇 |
1997年 | 13116篇 |
1996年 | 10290篇 |
1995年 | 7830篇 |
1994年 | 6786篇 |
1993年 | 6440篇 |
1992年 | 4618篇 |
1991年 | 4429篇 |
1990年 | 3933篇 |
1989年 | 3782篇 |
1988年 | 3672篇 |
1987年 | 3074篇 |
1986年 | 2982篇 |
1985年 | 3542篇 |
1984年 | 3142篇 |
1983年 | 2810篇 |
1982年 | 2610篇 |
1981年 | 2621篇 |
1980年 | 2475篇 |
1979年 | 2300篇 |
1978年 | 2262篇 |
1977年 | 2836篇 |
1976年 | 4216篇 |
1975年 | 1913篇 |
1974年 | 1811篇 |
1973年 | 1799篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
Dominant rock properties affecting the performance of conical picks and the comparison of some experimental and theoretical results 总被引:7,自引:0,他引:7
N. Bilgin M.A. Demircin H. Copur C. Balci H. Tuncdemir N. Akcin 《International Journal of Rock Mechanics and Mining Sciences》2006,43(1):139-156
Conical picks are the essential cutting tools used especially on roadheaders, continuous miners and shearers and their cutting performance affects directly the efficiency and the cost of rock/mineral excavation. In this study, in order to better understand the effects of dominant rock properties on cutter performance, 22 different rock specimens having compressive strength values varying from 10 to 170 MPa are first subjected to a wide range of mechanical tests. Then, laboratory full-scale linear cutting tests with different depth of cut and cutter spacing values are realized on large blocks of rock specimens using one type of conical pick. Specific energy, cutting and normal force values for relieved and unrelieved cutting modes are recorded using a triaxial force dynamometer with capacity of 50 tonnes and a data acquisition system. Cutter force and specific energy values are correlated with rock properties and theoretical force and specific energy values obtained from widely used theoretical approaches.The results indicate that uniaxial compressive strength among the rock properties investigated is best correlated with the measured cutter performance values, which is in good agreement with previous studies. However, it is also emphasized in this study that Brazilian tensile strength, Schmidt hammer rebound values, static and dynamic elasticity modulus are also dominant rock properties affecting cutter performance.Theoretical specific energy defined by different researchers has a meaningful relationship with the experimental specific energy, which is an essential parameter for predicting the instantaneous cutting rates of mechanical excavation systems. It is also demonstrated that the experimental cutter forces obtained for 5 mm depth of cut are in good agreement with theoretical force values, if the friction angle between rock and cutting tool is included in the theoretical formulation. It is emphasized that, to some extend, laboratory tests can help to minimize high cost of a trial–error approach in the field. 相似文献
82.
从做大到做强的主体是PCB企业(集团) 中国PCB工业如何从做大到做强!(6) 总被引:2,自引:2,他引:0
从做大到做强是中国PCB工业的唯一出路.而做强中国PCB工业的主体是中国的PCB企业(集团).只有大多数的中国PCB企业做大并做强了,中国PCB工业才能称得上是PCB强国. 相似文献
83.
84.
85.
The catalytic activities of alumina prepared from an Al alkoxide-amine adduct monomer for the reaction of cyclopentene oxide with piperidine was determined after various pretreatments, including calcination and exposure to moisture. They were compared with the activity of alumina prepared by the conventional hydrolysis method. It was found that the as-prepared sample from the alkoxide-amine monomer preparation was five times more active than a conventional preparation, suggesting that it has a higher density of surface Lewis acid sites. However, its activity was much more severely suppressed by exposure to moisture. 相似文献
86.
H. Sakata A. Katagiri M. Yokoi T. Kato H. Morimoto 《Journal of Low Temperature Physics》2003,131(3-4):275-279
We have measured the tunneling spectra in Bi2Sr2CaCu2O8 with a scanning tunneling microscope(STM) at various tip-sample distances by changing the tunneling conductance in a controlled manner. When the tunneling conductance is increased from 1×10–9 to 1×10–5 S, the spectra do not show changes in particular. However, the gap value decreases steeply and the asymmetric back ground density of states turns inverted V-shaped one above 6×10–4 S. The changes in the tunneling spectra at the high tunneling conductances are explained by the enhancement of the local carrier density induced by the pressure that the STM tip applied to the sample. 相似文献
87.
Prediction of brittle-to-ductile transitions in polystyrene 总被引:1,自引:0,他引:1
In this study it is attempted to predict brittle-to-ductile transitions (BDTs) in polystyrene blends, induced either by an increase in temperature or by a decrease in inter-particle distance. A representative, two-dimensional volume element (RVE) of a polystyrene matrix with 20% circular voids, is deformed in tension. During deformation a hydrostatic-stress based craze-nucleation criterion [1] is evaluated. The simulations demonstrate that crazes initiate at low temperatures while a transition from crazing to shear yielding (BDT) is found around 75 °C. The numerical results correlate well with tensile tests on similar heterogeneous polystyrene. The presence of an absolute length, as experimentally found, is more difficult to explain. Near a free surface a Tg-depression is measured for polystyrene and also the resistance to indentation in polystyrene is lower than expected from bulk properties. Both observations are rationalised by an enhanced segmental mobility of chains near a free surface. As a consequence of these findings, an absolute length-scale could be incorporated in the numerical simulations. For simplicity, the length-scale is modelled by taking a temperature gradient over a thin layer near the internal free surfaces of the RVE. Deformation of the RVE with different absolute length-scales shows that indeed also the experimentally found brittle-to-ductile transition can be predicted if the ligament thickness between the inclusions (‘voids’) in polystyrene is below a critical value of ca. 15 nm. 相似文献
88.
This paper describes the methodology for simulating a reprographic ink with a ceramic ink based on a commercially available zirconia powder for direct ceramic ink-jet printing. Of over-riding importance was matching viscosity and this was tested systematically by using a mineral oil–hexane binary system. Of secondary importance was adjustment of the pressure defect behind the nozzle to compensate for small differences in surface tension. The inks tested in the wide array print-head were based on low electrical conductivity liquids to avoid damage to the electroding system. The organic binder for the zirconia ink was paraffin wax and the dispersant was a hydroxystearic acid based polyester. 相似文献
89.
90.
The extended quadratic residue code is the only (48,24,12) self-dual doubly-even code 总被引:3,自引:0,他引:3
Houghten S.K. Lam C.W.H. Thiel L.H. Parker J.A. 《IEEE transactions on information theory / Professional Technical Group on Information Theory》2003,49(1):53-59
An extremal self-dual doubly-even binary (n,k,d) code has a minimum weight d=4/spl lfloor/n/24/spl rfloor/+4. Of such codes with length divisible by 24, the Golay code is the only (24,12,8) code, the extended quadratic residue code is the only known (48,24,12) code, and there is no known (72,36,16) code. One may partition the search for a (48,24,12) self-dual doubly-even code into three cases. A previous search assuming one of the cases found only the extended quadratic residue code. We examine the remaining two cases. Separate searches assuming each of the remaining cases found no codes and thus the extended quadratic residue code is the only doubly-even self-dual (48,24,12) code. 相似文献