全文获取类型
收费全文 | 21758篇 |
免费 | 1322篇 |
国内免费 | 57篇 |
专业分类
电工技术 | 311篇 |
综合类 | 25篇 |
化学工业 | 4895篇 |
金属工艺 | 880篇 |
机械仪表 | 1299篇 |
建筑科学 | 426篇 |
矿业工程 | 6篇 |
能源动力 | 864篇 |
轻工业 | 1851篇 |
水利工程 | 91篇 |
石油天然气 | 16篇 |
无线电 | 3655篇 |
一般工业技术 | 4627篇 |
冶金工业 | 1541篇 |
原子能技术 | 291篇 |
自动化技术 | 2359篇 |
出版年
2024年 | 22篇 |
2023年 | 275篇 |
2022年 | 399篇 |
2021年 | 684篇 |
2020年 | 470篇 |
2019年 | 497篇 |
2018年 | 699篇 |
2017年 | 661篇 |
2016年 | 768篇 |
2015年 | 603篇 |
2014年 | 938篇 |
2013年 | 1389篇 |
2012年 | 1456篇 |
2011年 | 1790篇 |
2010年 | 1299篇 |
2009年 | 1333篇 |
2008年 | 1211篇 |
2007年 | 937篇 |
2006年 | 813篇 |
2005年 | 708篇 |
2004年 | 638篇 |
2003年 | 586篇 |
2002年 | 593篇 |
2001年 | 517篇 |
2000年 | 437篇 |
1999年 | 423篇 |
1998年 | 709篇 |
1997年 | 420篇 |
1996年 | 395篇 |
1995年 | 257篇 |
1994年 | 169篇 |
1993年 | 151篇 |
1992年 | 112篇 |
1991年 | 100篇 |
1990年 | 75篇 |
1989年 | 88篇 |
1988年 | 74篇 |
1987年 | 63篇 |
1986年 | 51篇 |
1985年 | 43篇 |
1984年 | 36篇 |
1983年 | 28篇 |
1982年 | 31篇 |
1981年 | 26篇 |
1980年 | 29篇 |
1979年 | 13篇 |
1977年 | 22篇 |
1976年 | 33篇 |
1975年 | 17篇 |
1974年 | 13篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
181.
Nucleoside-diphosphate kinase (NDP kinase), a key enzyme in nucleotide metabolism, is also known to be involved in growth and developmental control and tumor metastasis suppression. Interestingly, we find that coexpression of NDP kinase with Taz1, a Tar/EnvZ chimera, in the absence of its native signal, can activate a porin gene ompC-lacZ expression in Escherichia coli. Further studies show that NDP kinase can act as a protein kinase to phosphorylate histidine protein kinases such as EnvZ and CheA which are members of the His-Asp phosphorelay signal transduction systems in E. coli. Instead of ATP, the exclusive phosphodonor for histidine kinases, GTP can be utilized in vitro in the presence of NDP kinase to phosphorylate EnvZ and CheA, which then transfer the phosphoryl group to OmpR and CheY, the respective response regulators. The direct involvement of GTP for the phosphorylation of EnvZ through NDP kinase was further demonstrated by the use of a mutant EnvZ, which lost ability to be autophosphorylated with ATP. Phospho-OmpR thus formed can bind specifically to an ompF promoter sequence. These results suggest that NDP kinase may play a physiological role in signal transduction. 相似文献
182.
183.
Choong Soo Park Rin Chul Kim Sang Uk Lee 《Electronics letters》1996,32(15):1361-1362
A novel method for generating an optimal binary prefix condition code is described. The proposed code generation method generates optimal variable length codes that have short average error spans compared to the existing self-synchronising codes. A technique for calculating the expected error spans of variable length codes are also presented 相似文献
184.
In this paper a discrete-time adaptive sliding mode control method is newly developed and applied to the power system stabilization problem. A controllable canonical form of state space realization is constructed using the parameters identified by the on-line recursive least squares method and the system state is estimated from the input/output measurements and the simple state transformations. The identified parameters and the estimated state are then used by the discrete-time sliding mode control, which is suitable for the digital equipment. The most important advantage of the proposed power system stabilizer (PSS) is that it is able to maintain its regulating performance with a slower sampling period than that of the conventional sliding mode PSS because it is developed in a pure discrete-time domain. Another advantage of the proposed PSS is that it needs neither a mathematical model of the power system nor the full-state measurements because they are identified through on-line identifications. Several computer simulations for the linear power system are performed to verify the performance of the proposed PSS. In the computer simulations for various circumstances which are probable in a power system are considered, such as transitions of the active and reactive powers, change of parameters of the synchronous machine, line-to-ground faults and measurement noise. As a result, a new power system stabilizer which can operate in a wide range of operating conditions and can overcome various disturbances and measurement noises is proposed. 相似文献
185.
In order to promote the efficient use of composite materials in civil engineering infrastructure, effort is being directed
at the development of design criteria for composite structures. Insofar as design with regard to buckling of composite shells
is concerned, it is well known that a key step is to investigate the influence of initial geometric imperfection. At present,
imperfection sensitivity study of composite shells has not been explored in detail. Thus, the objective of this paper is to
present the formulation used in developing a composite shell element and to validate the element from the composite curved
panel. The non-linear formulation of the shell element is based on the updated Lagrangian method. The shell element is capable
of small strain and large displacement analysis with finite rotations. In order to remove the rigid body rotation, a co-rotational
method is used. Subsequently the postbuckling analyses from the modeling of the curved panel with initial imperfection damage
are performed to investigate the effect of initial geometric imperfection shape and amplitude. The results are used to estimate
imperfection sensitivity for such panels. 相似文献
186.
187.
Young -Wook Kim Jin -Soo Song Sang -Whan Park June -Gunn Lee 《Journal of Materials Science》1993,28(14):3866-3868
A new, faster process was developed for the fabrication of Nicalon-fibre-reinforced SiC composites by combining polymer solution infiltration (PSI) and chemical vapour infiltration (CVI). The process led to the near-net-shape fabrication of fibre-reinforced ceramic-matrix composites and reduced infiltration time. Typical flexural strength and fracture toughness of these composites were 296 MPa and 10.9 MPa m1/2 at room temperature (RT) and 252 MPa and 9.6 MPa m1/2 at 1000 °C, respectively. The composites exhibited load-carrying capability after crack initiation. 相似文献
188.
Tae Geun Kim Kyung Hyun Park Sung-Min Hwang Yong Kim Eun Kyu Kim Suk-Ki Min Si-Jeong Leem Jong-Il Jeon Jung-Ho Park Chang W.S.C. 《Quantum Electronics, IEEE Journal of》1998,34(8):1461-1468
GaAs-AlGaAs V-grooved inner stripe (VIS) quantum-well wire (QWW) lasers grown by metalorganic chemical vapor deposition with different current blocking configurations, n-blocking on p-substrate (VIPS), p-n-p-n blocking on n-substrate (VI(PN)nS) and p-blocking on n-substrate (VINS) have been fabricated and characterized. The VIPS QWW lasers show the most stable characteristics with effective current confinement: one of the lasers shows fundamental transverse mode, lasing up to 5 mW/facet, typical threshold current of 39.9 mA at 818.5 mm, an external differential quantum efficiency of 24%/facet, and characteristic temperature of 92 K. The current tuning rate was almost linear at 0.031 mm/mA, and the temperature tuning rate was measured to be 0.14 nm/°C. Comparison of the light output versus current characteristics of the lasers with different current blocking configurations is presented here 相似文献
189.
Motion based object tracking with mobile camera 总被引:4,自引:0,他引:4
Kyu Won Lee Seong Won Ryu Soo Jong Lee Kyu Tae Park 《Electronics letters》1998,34(3):256-258
An efficient algorithm which detects and tracks a moving object is proposed. The global motion energy caused by camera movement is eliminated by finding the maximal matching position with the cross-correlation value between two consecutive frames using the spiral scanning technique. The region of a moving object is segmented exactly by combining the results of the temporal derivatives and edge map from one image, and the actual centroid of the moving object is determined as the mid-point of the cumulative distribution of the projection profile of the region 相似文献
190.
Beckwith-Wiedemann syndrome (BWS) comprises of a number of childhood abnormalities, often associated with one or more tumors. Thirty-eight patients were investigated to determine clinical and/or biological signs associated with a tumor presence. Our patients exhibited a higher incidence of tumor development (21%) than that previously reported, underlying the care with which such patients should be followed, when particular clinical features are observed: visceromegaly affecting three organs (liver, kidney, spleen), and also family history with sign of BWS such as macroglossia, omphalocele, hemihypertrophy, embryonic tumor), high body weight at birth (> or = +2 standard deviations and diastasis recti. 相似文献