全文获取类型
收费全文 | 33339篇 |
免费 | 2149篇 |
国内免费 | 28篇 |
专业分类
电工技术 | 239篇 |
综合类 | 17篇 |
化学工业 | 6773篇 |
金属工艺 | 631篇 |
机械仪表 | 710篇 |
建筑科学 | 942篇 |
矿业工程 | 71篇 |
能源动力 | 563篇 |
轻工业 | 5439篇 |
水利工程 | 309篇 |
石油天然气 | 116篇 |
武器工业 | 6篇 |
无线电 | 1266篇 |
一般工业技术 | 5687篇 |
冶金工业 | 8024篇 |
原子能技术 | 123篇 |
自动化技术 | 4600篇 |
出版年
2024年 | 60篇 |
2023年 | 294篇 |
2022年 | 341篇 |
2021年 | 816篇 |
2020年 | 682篇 |
2019年 | 776篇 |
2018年 | 1359篇 |
2017年 | 1324篇 |
2016年 | 1380篇 |
2015年 | 1064篇 |
2014年 | 1253篇 |
2013年 | 2618篇 |
2012年 | 1959篇 |
2011年 | 1759篇 |
2010年 | 1475篇 |
2009年 | 1306篇 |
2008年 | 1265篇 |
2007年 | 1205篇 |
2006年 | 802篇 |
2005年 | 680篇 |
2004年 | 649篇 |
2003年 | 575篇 |
2002年 | 564篇 |
2001年 | 408篇 |
2000年 | 390篇 |
1999年 | 434篇 |
1998年 | 2370篇 |
1997年 | 1617篇 |
1996年 | 1032篇 |
1995年 | 619篇 |
1994年 | 477篇 |
1993年 | 567篇 |
1992年 | 207篇 |
1991年 | 208篇 |
1990年 | 154篇 |
1989年 | 160篇 |
1988年 | 164篇 |
1987年 | 139篇 |
1986年 | 121篇 |
1985年 | 141篇 |
1984年 | 131篇 |
1983年 | 100篇 |
1982年 | 130篇 |
1981年 | 152篇 |
1980年 | 159篇 |
1979年 | 81篇 |
1978年 | 81篇 |
1977年 | 279篇 |
1976年 | 626篇 |
1973年 | 63篇 |
排序方式: 共有10000条查询结果,搜索用时 171 毫秒
171.
172.
The Role of Network Topologies in the Optical Core of IP-over-WDM Networks with Static Wavelength Routing 总被引:3,自引:0,他引:3
Freire Mário M. Rodrigues Joel J.P.C. Coelho Rui M.F. 《Telecommunication Systems》2003,24(2-4):111-122
In this paper, we present a performance analysis of network topologies for the optical core of IP-over-WDM networks with static wavelength routing. The performance analysis is focused on regular degree four topologies, and, for comparison purposes, degree three topologies are also considered. It is shown that the increase of the nodal degree from three (degree three topology with smallest diameter) to four (degree four topology with smallest diameter) improves the network performance if a larger number of wavelengths per link is available. However, the influence of wavelength interchange on the nodal degree gain is small. The performance of regular degree four topologies with smallest diameter is also compared with the performance of mesh–torus topologies (which are also degree four topologies), and it is shown that the blocking probability of degree four topologies with smallest diameter is about two orders of magnitude lower than the blocking probability of mesh–torus topologies. It is also presented a performance comparison of WDM-based networks with nodal degrees ranging from two to five and it is shown that the increase of the nodal degree from two to three leads to high nodal degree gains, while de increase of the nodal degree from four to five leads to low nodal degree gains. These results show that degree three and degree four topologies are very attractive for use in the optical core of IP-over-WDM networks. 相似文献
173.
P. Cañizares J. García-Gómez C. Sáez M.A. Rodrigo 《Journal of Applied Electrochemistry》2003,33(10):917-927
The electrochemical oxidation of 4-chlorophenol, 2,4-dichlorophenol and 2,4,6-trichlorophenol aqueous wastes using boron-doped diamond electrodes was studied. This treatment led to complete mineralization of the wastes regardless of the operating conditions. A simple mechanistic model is consistent with the voltammetric and electrolysis results. According to this model, the electrochemical treatment of chlorophenol aqueous wastes involves the anodic and cathodic release of chlorine followed by the formation of non-chlorinated aromatic intermediates. Subsequent cleavage of the aromatic ring gives rise to non-chlorinated carboxylic acids. Chlorine atoms arising from the hydrodehalogenation of the chlorophenols are converted into more oxidized molecules at the anode. These molecules react with unsaturated C4 carboxylic acid to finally yield trichloroacetic acid through a haloform reaction. The non-chlorinated organic acids are ultimately oxidized to carbon dioxide and the trichloroacetic acid into carbon dioxide and volatile organo-chlorinated molecules. Both direct and mediated electrochemical oxidation processes are involved in the electrochemical treatment of chlorophenols. 相似文献
174.
Synthesis of oleyl oleate as a jojoba oil analog 总被引:2,自引:0,他引:2
N. Sánchez M. Martinez J. Aracil A. Corma 《Journal of the American Oil Chemists' Society》1992,69(11):1150-1153
Synthesis of a wax ester analog of jojoba oil was accomplished from oleic acid and oleyl alcohol with a zeolite as catalyst.
A full 23 factorial design at two levels has been used in the synthesis. The variables selected were temperature, reduced pressure
and initial catalyst concentration. The most important variable within the range studied was temperature. Reduced pressure
had a negative influence, and initial catalyst concentration showed a positive influence on the process. A response equation
has been determined for the yield of ester. The properties of the synthesized product are similar to those of natural jojoba
oil. 相似文献
175.
To identify possible sources of the observed differences in the career choices of women and men, three facets of career preferences were examined: the relative importance attributed to career-related aspects, the within-aspect preferences (i.e., desirable characteristics of occupations), and the structure of aspects derived from these within-aspect preferences. The career-related preferences of 2,000 young adults who were in the process of making their career decisions were analyzed. These career-related preferences were elicited during their dialogues with a computer-assisted career guidance system. The analyses revealed (a) only small gender differences in the relative importance of the aspects, (b) considerable gender differences in the within-aspect preferences, and (c) certain gender differences in the structure of aspects. The findings and their possible theoretical and practical implications are discussed. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
176.
Francisco J. Collado Eduardo Sevilla Carlos Valcárcel 《American Institute of Chemical Engineers》2002,48(5):1100-1108
New thermodynamic fundamentals for vertical pneumatic conueying are proposed with derivation based strictly on mass conservation and continuum concept. For a 1-D, vertical, steady gas-solid flow, these new mass and energy balances allowed the derivation of a new nondimensional energy factor containing the pressure drop. This energy factor correlated quite well with the difference between the inlet gas velocity and solids terminal velocity, when tested against high-pressure, 15-m-high, vertical pneumatic transport data from Institute of Gas Technology. The energy factor was also checked against atmospheric-pressure data of the 5-m lift line of the Pennsylvania State Uniuersity. The new energy factor covers quite well both the atmospheric- and high-pressure sets of data (923 tests) including two heights, lean- and dense-phase transports, several pipe diameters, broad and narrow particle-size distributions, and different materials. The dissipation terms in gas-solid flow were also clearly identified. 相似文献
177.
178.
179.
The authors suggest that academic institutions build business-oriented policies and practices into the development of any nurse-run clinic to set the stage for financial independence when special or development funding ends. One university-affiliated program that provides 4,000 to 5,000 annual visits drastically changed its strategies when threatened with closure after free rent and other subsidies were withdrawn. The growing emphasis on ambulatory care roles for nurses at all levels makes such clinics critical to the success of the broad-based curricula of nursing education programs, as well as the clinic's value to communities they serve. Funding difficulties frequently threaten the existence of such nurse-run clinics once the initial grant funding is no longer available. This has caused a new emphasis on running such clinics in a business-wise manner. Among the strategies initiated were: direct full-pay at the time of service; a realistic business management plan; aggressive planned marketing; contracts and agreements with other agencies; obtaining provider status with selected HMOs. 相似文献
180.
A Lubics D Regl?di S Slezák M Szelier I Lengvári 《Canadian Metallurgical Quarterly》1997,99(4):459-467
In addition to receptor-type pinealocytes, the mammalian pineal organ contains small and large neurons and ependymal/glial cells as well. Axons of pinealocytes form synaptic ribbon-containing axo-dendritic synapses on large secondary pineal neurons and/or terminate as neurohormonal endings on the basal lamina of the vascular surface of the organ. The small pineal neurons were found to be gamma-aminobutyric acid (GABA)-immunoreactive, while large secondary neurons and pinealocytes contained immunoreactive amino acids (glutamate and aspartate). Glutamate accumulated presynaptically in pinealocytic axon terminals on large secondary neurons and in the axons of these neurons. Glutamate immunoreactive axons of pineal neurons were traced via the pineal tract to the habenular nucleus. Axons containing granular vesicles and coming from extrapineal perikarya are glutamate immunoreactive as well. Aspartate and GABA are also present in some of the myelinated axons, supposedly pinealopetal in the pineal tract. 相似文献