全文获取类型
收费全文 | 699965篇 |
免费 | 8808篇 |
国内免费 | 1435篇 |
专业分类
电工技术 | 11903篇 |
综合类 | 2790篇 |
化学工业 | 108437篇 |
金属工艺 | 31420篇 |
机械仪表 | 24162篇 |
建筑科学 | 16092篇 |
矿业工程 | 5159篇 |
能源动力 | 16385篇 |
轻工业 | 53275篇 |
水利工程 | 8596篇 |
石油天然气 | 15781篇 |
武器工业 | 43篇 |
无线电 | 72959篇 |
一般工业技术 | 143347篇 |
冶金工业 | 107297篇 |
原子能技术 | 15270篇 |
自动化技术 | 77292篇 |
出版年
2021年 | 6289篇 |
2020年 | 4686篇 |
2019年 | 5875篇 |
2018年 | 25902篇 |
2017年 | 25270篇 |
2016年 | 21443篇 |
2015年 | 7675篇 |
2014年 | 11684篇 |
2013年 | 28525篇 |
2012年 | 21311篇 |
2011年 | 34060篇 |
2010年 | 28492篇 |
2009年 | 27632篇 |
2008年 | 29045篇 |
2007年 | 30330篇 |
2006年 | 17392篇 |
2005年 | 18196篇 |
2004年 | 16689篇 |
2003年 | 16252篇 |
2002年 | 14657篇 |
2001年 | 13324篇 |
2000年 | 12898篇 |
1999年 | 12501篇 |
1998年 | 29903篇 |
1997年 | 21411篇 |
1996年 | 16549篇 |
1995年 | 12571篇 |
1994年 | 11360篇 |
1993年 | 11086篇 |
1992年 | 8501篇 |
1991年 | 8293篇 |
1990年 | 8142篇 |
1989年 | 7910篇 |
1988年 | 7632篇 |
1987年 | 6866篇 |
1986年 | 6675篇 |
1985年 | 7551篇 |
1984年 | 6848篇 |
1983年 | 6543篇 |
1982年 | 5899篇 |
1981年 | 6036篇 |
1980年 | 5747篇 |
1979年 | 5823篇 |
1978年 | 5748篇 |
1977年 | 6331篇 |
1976年 | 8020篇 |
1975年 | 5163篇 |
1974年 | 4948篇 |
1973年 | 5035篇 |
1972年 | 4310篇 |
排序方式: 共有10000条查询结果,搜索用时 12 毫秒
41.
Effect of modified atmosphere packaging on the shelf-life of coated, whole and sliced mushrooms 总被引:2,自引:0,他引:2
Whole and sliced fresh mushrooms (Agaricus bisporus) were packaged with PVC wrap or two polyolefins (PD-941 and PD-961) films after coating with CaCl2 and chitosan. Package gas composition, color, weight loss and maturity were measured during storage at 12 °C and 80%RH. For PD-961, the highest in-package concentration occurred during the first day of storage regardless of treatments, while wrap and PD-941 showed varying degrees in-package concentration with different processes and coatings. The whiteness of whole mushrooms varied significantly with the type of coating, but not with the type of films. The extent of darkening was greater in coated whole mushrooms than in sliced ones. Weight loss occurred in all packages and varied from 3 (g/100 g) to about 7 (g/100 g) after 6 days of storage. Due to a lower permeability, PD-961 packages had the lowest weight loss. The type of packaging films significantly affected the maturity index, where PD-961 most effectively lowered maturity index for both whole and sliced mushrooms, thus extending the shelf-life. The type of coating did not appear to affect maturity index except for the wrap package where chitosan coating markedly lowered the maturity index of sliced mushrooms. 相似文献
42.
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
43.
This work considers the problem of increasing the performance of the ciphers based on Data-Dependent (DD) operations (DDO)
for VLSI implementations. New minimum size primitives are proposed to design DDOs. Using advanced DDOs instead of DD permutations
(DDP) in the DDP-based iterative ciphers Cobra-H64 and Cobra-H128 the number of rounds has been significantly reduced yielding
enhancement of the “performance per cost” value and retaining security at the level of indistinguishability from a random
transformation. To obtain further enhancement of this parameter a new crypto-scheme based on the advanced DDOs is proposed.
The FPGA implementation of the proposed crypto-scheme achieves higher throughput value and minimizes the allocated resources
than the conventional designs. Design of the DDO boxes of different orders is considered and their ASIC implementation is
estimated. 相似文献
44.
Quasi-static stress fields for a crack inclined to the property gradation in functionally graded materials 总被引:3,自引:0,他引:3
Summary. Quasi-static stress fields for a crack inclined to the direction of property gradation in functionally graded materials (FGMs)
are obtained through an asymptotic analysis coupled with Westergaard's stress function approach. The elastic modulus of the
FGM is assumed to vary exponentially along the gradation direction. The mode mixity due to the inclination of the property
gradient is accommodated in the analysis through superposition of opening and shear modes. The first four terms in the expansion
of the stress field are derived to explicitly bring out the influence of nonhomogeneity on the structure of the stress field.
Using these stress field contours of constant maximum shear stress, constant maximum principal stress, constant first stress
invariant and constant out of plane displacement are generated, and the effect of inclination of the property gradation direction
on these contours is discussed.
Received September 22, 2002
Published online: May 20, 2003
The financial support of National Science Foundation (NSF) under grant no. CMS 99000138 is gratefully acknowledged. 相似文献
45.
46.
V. N. Tsygankov V. V. Safonov A. I. Kozlov V. P. Gavrilov 《Inorganic Materials》2003,39(10):1076-1078
The GeO2–NiO system is studied in the stability range of Ni2GeO4 by temperature-dependent resistivity measurements, in combination with x-ray diffraction, differential thermal analysis, and thermogravimetry. The resistivity of the starting mixtures and heat-treated samples is measured in the composition range 55–90 mol % NiO. The effect of air humidity on the resistivity of the samples is examined. 相似文献
47.
A cell vertex finite volume method for the solution of steady compressible turbulent flow problems on unstructured hybrid
meshes of tetrahedra, prisms, pyramids and hexahedra is described. These hybrid meshes are constructed by firstly discretising
the computational domain using tetrahedral elements and then by merging certain tetrahedra. A one equation turbulence model
is employed and the solution of the steady flow equations is obtained by explicit relaxation. The solution process is accelerated
by the addition of a multigrid method, in which the coarse meshes are generated by agglomeration, and by parallelisation.
The approach is shown to be effective for the simulation of a number of 3D flows of current practical interest.
Sponsored by The Research Council of Norway, project number 125676/410
Dedicated to the memory of Prof. Mike Crisfield, a respected colleague 相似文献
48.
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
49.
Two types of photo heterojunction bipolar transistor (HBT) to directly down-convert optical signals to electronic signals have been reported in the literature: a conventional photo-HBT in which light penetrates through the area of the base-collector junction and an HBT where light shines through the base-collector edge for higher conversion efficiency. Although the performance in relation to bias conditions has been published, the detailed analyses for identifying the parameters and bias conditions that provide optimum direct down-conversion have not been examined. This paper provides a full explanation of the operation of the down-conversion for both HBT configurations based on low-frequency analyses. Such information is useful for both understanding the nonlinear mechanisms involved and designing for maximum efficiency. In addition, a new circuit has been developed from the basic HBT down-conversion circuit that provides improved performance. 相似文献
50.
The current Internet and wireless networks are harsh environments for transporting high-bandwidth multimedia data. We examine the technical issues involved, and describe an end-to-end solution to support a Web-based learn-on-demand system that operates in a wireless campus environment. 相似文献