全文获取类型
收费全文 | 21071篇 |
免费 | 1017篇 |
国内免费 | 148篇 |
专业分类
电工技术 | 375篇 |
综合类 | 38篇 |
化学工业 | 4712篇 |
金属工艺 | 647篇 |
机械仪表 | 665篇 |
建筑科学 | 400篇 |
矿业工程 | 53篇 |
能源动力 | 1472篇 |
轻工业 | 1242篇 |
水利工程 | 164篇 |
石油天然气 | 104篇 |
武器工业 | 1篇 |
无线电 | 2649篇 |
一般工业技术 | 4810篇 |
冶金工业 | 1667篇 |
原子能技术 | 178篇 |
自动化技术 | 3059篇 |
出版年
2024年 | 80篇 |
2023年 | 428篇 |
2022年 | 996篇 |
2021年 | 1213篇 |
2020年 | 945篇 |
2019年 | 982篇 |
2018年 | 1280篇 |
2017年 | 1023篇 |
2016年 | 988篇 |
2015年 | 640篇 |
2014年 | 919篇 |
2013年 | 1701篇 |
2012年 | 994篇 |
2011年 | 1188篇 |
2010年 | 945篇 |
2009年 | 880篇 |
2008年 | 800篇 |
2007年 | 675篇 |
2006年 | 531篇 |
2005年 | 427篇 |
2004年 | 322篇 |
2003年 | 308篇 |
2002年 | 240篇 |
2001年 | 213篇 |
2000年 | 207篇 |
1999年 | 198篇 |
1998年 | 363篇 |
1997年 | 293篇 |
1996年 | 256篇 |
1995年 | 202篇 |
1994年 | 186篇 |
1993年 | 190篇 |
1992年 | 128篇 |
1991年 | 162篇 |
1990年 | 134篇 |
1989年 | 112篇 |
1988年 | 93篇 |
1987年 | 108篇 |
1986年 | 93篇 |
1985年 | 102篇 |
1984年 | 95篇 |
1983年 | 89篇 |
1982年 | 75篇 |
1981年 | 89篇 |
1980年 | 66篇 |
1979年 | 46篇 |
1978年 | 37篇 |
1977年 | 35篇 |
1976年 | 46篇 |
1975年 | 17篇 |
排序方式: 共有10000条查询结果,搜索用时 125 毫秒
141.
In this communication, the stability of the double-diffusive solar ponds with non-uniform temperature and salinity gradients has been investigated. This is a further generalization of our approach to this problem initiated in Ref. [7]. Using a stochastic approach, the linearized system of basic equations of motion is reduced to a single integro-differential equation. For convective motion, this equation reduces to a time-independent Schrödinger equation for a particle moving in a potential field ƒ(Z) characterized by the non-uniform temperature and salinity gradients. This equation can, in general, be solved (exactly or approximately depending on the form of the gradient profile) by methods commonly used in quantum mechanics.
In the Appendix, we show that, for a quadratic gradient profile, the above equation has an analytical solution similar to that obtained by Zangrando using numerical computations. 相似文献
142.
J. Siva Kumar K. Vijaya Kumar A. R. Subrahmanyam M. Jaipal Reddy 《Journal of Materials Science》2007,42(14):5752-5755
Ion conducting thin film polymer electrolytes based on polyethylene oxide (PEO) complexed with NaHCO3 salt has been prepared using solution-cast technique. The complexation of NaHCO3 salt with PEO is confirmed by XRD and IR studies. DC conductivity in the temperature range 303–368 K has been evaluated.
The conductivity is found to increase in the PEO complex with the NaHCO3 salt and also with an increase in temperature. Using this polymer electrolyte, an electrochemical cell with the configuration
Na/(PEO + NaHCO3)/(I2 + C + electrolyte) has been fabricated and its discharge characteristics studied. Open Circuit Voltage (OCV) and Short Circuit
Current (SCC) are found to be 2.69 V and 1.28 mA, respectively. Other parameters associated with the cell are evaluated and
presented in this paper. 相似文献
143.
144.
Arjunan Arulchakkaravarthi Rakesh Kumar Parthasarathy Santhanaraghavan Sivaramakrishnan Muralithar Rengasamy Gopalakrishanan Perumalsamy Ramasamy 《Materials Research Innovations》2002,6(5-6):273-276
Organic molecular scintillating crystals are noted for their good timing and particle discrimination process. Trans-stilbene
is one such candidate noted for its good particle detection characteristics for the past five decades. Progressive strengthening
of detection characteristics of trans-stilbene has been attempted by improving crystal perfection. A series of timing resolution
studies have been carried out for the Bridgman grown trans-stilbene crystals under different experimental conditions. The
results were compared with the previously reported values. Pulse shape discrimination process has been carried out for 241Am and 252Cf sources and good discrimination has been obtained for gamma-alpha and gamma-neutron sources from the grown organic phosphor
crystal.
Electronic Publication 相似文献
145.
146.
Gelatin-g-poly(ethyl acrylate) copolymers were prepared in an aqueous medium, using K2S2O8 initiator. The composition of the graft copolymers was dependent upon temperature and duration of the reaction. The number of grafting sites was small and molecular weight of the grafted poly(ethyl acrylate) branches was high. Three copolymer samples with grafting efficiencies of 33.3%, 61.0%, and 84.0%, were tested for their microbial susceptibility in a synthetic medium employing a mixed inoculum of Bacillus subtilis, Pseudomonas aeruginosa, and Serratia marcescens and the percent weight losses were 12%, 10.1%, and 6.0%, respectively, after 6 weeks of incubation. The extent of degradation seems to decrease with increasing grafting efficiency. There was initial rapid weight loss accompanied by the exponential increase in bacterial population and pH of the culture medium during the first week. The nitrogen analysis also showed the utilization of the polymer. A parallel set of experiments, carried out by employing the samples as the only source of both carbon and nitrogen, showed a marginal but definite increase in the utilization of the polymer. 相似文献
147.
Fraser M.D. Kumar K. Vaishnavi V.K. 《IEEE transactions on pattern analysis and machine intelligence》1991,17(5):454-466
The differences between informal and formal requirements specification languages are noted, and the issue of bridging the gap between them is discussed. Using structured analysis (SA) and the Vienna development method (VDM) as surrogates for informal and formal languages, respectively, two approaches are presented for integrating the two. The first approach uses the SA model of a system to guide the analyst's understanding of the system and the development of the VDM specifications. The second approach proposes a rule-based method for generating VDM specifications from a set of corresponding SA specifications. The two approaches are illustrated through a simplified payroll system case. The issues that emerge from the use of the two approaches are reported 相似文献
148.
Development of nano indium tin oxide (ITO) grains by alkaline hydrolysis of In(III) and Sn(IV) salts
Indium tin oxide (ITO) nano powders of different compositions (In: Sn = 90: 10, 70: 30 and 50: 50) were prepared by heat treatment
(300-450°C) of mixed hydroxides of In(III) and Sn(IV). The hydroxides were obtained by the reaction of aq. NH3 with mixed aq. solutions of In(NO3)3 and SnCl4. FTIR and TG/DTA studies revealed that powders existed as In(OH)3H2O—SnO3H2H2O in the solid state and then they transformed to In2O3—SnO2 via some metastable intermediates after 300°C. Cubic phase of In2O3 was identified by XRD for the oxides up to 30% of Sn. Particle size measurements of the solid dispersed in acetone and SEM
study for microstructure showed that the oxides were in the nano range (55-75 nm) whereas the size range determined from Debye-Scherrer
equation were 11–24 nm. 相似文献
149.
150.
We consider a system comprising a finite number of nodes, with infinite packet buffers, that use unslotted ALOHA with Code
Division Multiple Access (CDMA) to share a channel for transmitting packetised data. We propose a simple model for packet
transmission and retransmission at each node, and show that saturation throughput in this model yields a sufficient condition
for the stability of the packet buffers; we interpret this as the capacity of the access method. We calculate and compare
the capacities of CDMA‐ALOHA (with and without code sharing) and TDMA‐ALOHA; we also consider carrier sensing and collision
detection versions of these protocols. In each case, saturation throughput can be obtained via analysis of a continuous time
Markov chain. Our results show how saturation throughput degrades with code‐sharing. Finally, we also present some simulation
results for mean packet delay. Our work is motivated by optical CDMA in which “chips” can be optically generated, and hence
the achievable chip rate can exceed the achievable TDMA bit rate which is limited by electronics. Code sharing may be useful
in the optical CDMA context as it reduces the number of optical correlators at the receivers. Our throughput results help
to quantify by how much the CDMA chip rate should exceed the TDMA bit rate so that CDMA‐ALOHA yields better capacity than
TDMA‐ALOHA.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献