全文获取类型
收费全文 | 7905篇 |
免费 | 701篇 |
国内免费 | 559篇 |
专业分类
电工技术 | 175篇 |
技术理论 | 1篇 |
综合类 | 479篇 |
化学工业 | 2817篇 |
金属工艺 | 738篇 |
机械仪表 | 108篇 |
建筑科学 | 797篇 |
矿业工程 | 374篇 |
能源动力 | 247篇 |
轻工业 | 419篇 |
水利工程 | 36篇 |
石油天然气 | 242篇 |
武器工业 | 48篇 |
无线电 | 317篇 |
一般工业技术 | 847篇 |
冶金工业 | 672篇 |
原子能技术 | 454篇 |
自动化技术 | 394篇 |
出版年
2024年 | 46篇 |
2023年 | 141篇 |
2022年 | 296篇 |
2021年 | 331篇 |
2020年 | 276篇 |
2019年 | 224篇 |
2018年 | 206篇 |
2017年 | 261篇 |
2016年 | 307篇 |
2015年 | 270篇 |
2014年 | 349篇 |
2013年 | 511篇 |
2012年 | 474篇 |
2011年 | 502篇 |
2010年 | 392篇 |
2009年 | 435篇 |
2008年 | 400篇 |
2007年 | 525篇 |
2006年 | 441篇 |
2005年 | 388篇 |
2004年 | 340篇 |
2003年 | 344篇 |
2002年 | 284篇 |
2001年 | 221篇 |
2000年 | 196篇 |
1999年 | 148篇 |
1998年 | 124篇 |
1997年 | 106篇 |
1996年 | 105篇 |
1995年 | 76篇 |
1994年 | 72篇 |
1993年 | 68篇 |
1992年 | 43篇 |
1991年 | 56篇 |
1990年 | 40篇 |
1989年 | 24篇 |
1988年 | 24篇 |
1987年 | 25篇 |
1986年 | 13篇 |
1985年 | 17篇 |
1984年 | 14篇 |
1983年 | 7篇 |
1982年 | 11篇 |
1981年 | 5篇 |
1980年 | 8篇 |
1979年 | 4篇 |
1978年 | 2篇 |
1975年 | 2篇 |
1959年 | 2篇 |
1951年 | 7篇 |
排序方式: 共有9165条查询结果,搜索用时 0 毫秒
91.
针对电子战中各种信号混叠严重难以分离的现象,在盲源分离开关算法基础上提出一种新的盲信源分离拟开关算法.该算法引入单位对称加权滑动向量来加权每次迭代所得的分离信号作为源信号,用峭度取代原算法的峭度符号位作为判断函数来自适应选择加权相应激活函数,以此优化学习算法,结合信号分选的具体应用,给出了迭代结束的评判方法.计算机仿真实验表明,在强噪声背景影响下,该算法能够更加有效地分离空间未知多源线性混叠信号,且在分离效果、稳定性、处理速度和抗噪性能上都比原算法有较大改进. 相似文献
92.
Thermal activation and radiation quenching characteristics and their application to the pre-dose dating of porcelain 总被引:1,自引:1,他引:0
WeiDa Wang 《中国科学E辑(英文版)》2009,52(2):324-331
The pre-dose technique is important for thermoluminescence (TL) dating of porcelain. The principle of pre-dose dating is based on two characteristics: thermal activation and radiation quenching of the porcelain. Based on these principles, two measurement methods, "the activation method" and "the quenching method", respectively, have been developed for evaluation of the porcelain paleodose. The paleodose values obtained are the same for the ages in the range of 100―1000 a BP for porcelains measured by these two methods. But for dating at lower age limits (less than 100 a BP), the activation method is more accurate; conversely, at higher age limits (greater than 1000 a BP) the quenching method is more accurate. In addition, two specific calculating methods are described for a few porce-lains having anomalous activation and quenching characteristics that make these two methods invalid. 相似文献
93.
The ablation in solid-propellant rocket nozzle is a coupling process resulted by chemistry, heat and mass transfer. Based on the heat and mass transfer theory, the aero-thermo-dynamic, and thermo-chemical kinetics, the thermal-chemical ablation model is established. Simulations are completed on the heat flow field and chemical ablation in the nozzle with different concentrations, frequency factors and activation energy of H2. The calculation results show that the concentration and the activation energy of H... 相似文献
94.
SiC‐ and B4C‐filled NBR rubber composites were prepared with various volume fractions of filler by a conventional roll‐mill method. The morphological structures of the NBR–SiC and NBR–B4C composites were analyzed by scanning electron microscopy. The dependence of room‐temperature volume resistivity (ρv) on the concentration of filler in the two systems was studied. In addition, variation in the number of current carriers (n), mobility carriers (μ), dielectric constant (ε), and dielectric loss factor (tan δ) on filler concentration in the two systems were investigated in detail. The applicability of composites as negative temperature coefficient (NTCR) linear thermistors was studied by the dependence of volume resistivity on temperature. The resistivity showed negative temperature dependence and changed linearly with temperature parallel. The conduction mechanism of the conductivity of the two composites was analyzed in terms of the computed activation energy and hopping energy. Change in volume resistivity as a function of frequency for the two systems was also investigated. Finally, the dependence of volume resistivity on applied pressure and possible real applications of these composites as transducers in pressure sensors were also studied. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 2158–2165, 2007 相似文献
95.
H. Nava C. Ornelas A. Aguilar G. Berhault S. Fuentes G. Alonso 《Catalysis Letters》2003,86(4):257-265
Unsupported cobalt-molybdenum sulfide catalysts were prepared from bimetallic CoMo alkyl precursors by in situ activation during the hydrodesulfurization (HDS) of dibenzothiophene (DBT). The bimetallic CoMo precursors were prepared by reaction of tetraalkylammonium thiomolybdate salts, (R4N)2MoS4 (where R = H, methyl, butyl, pentyl or hexyl), with CoCl2 in water at a Co/Mo molar ratio of 0.3. These catalysts exhibit a Swiss-cheese-like morphology, high surface areas (from 52 up to 320 m2/g), high content of carbon (C/Mo = 2.2-3.3) and type IV adsorption-desorption isotherms of nitrogen. The in situ activation of these tetraalkylammonium thiobimetalate precursors leads to a mesoporous structure with pore size ranging from 2 to 4.5 nm. X-ray diffraction showed that the structure of unsupported cobalt-molybdenum sulfide catalysts corresponds to a poorly crystalline structure characteristic of 2H-MoS2 with low-stacked layers. The nature of the alkyl group strongly affects both the surface area and the HDS activity. The catalytic activity is strongly enhanced when using carbon-containing precursors; the CoMo catalysts prepared by in situ activation of Co/[N(C4H9)4]2MoS4 presents the highest HDS activity. The highest surface area of the catalysts was observed for the CoMo catalyst formed from Co/[N(C6H13)4N]2MoS4. 相似文献
96.
CORRELATION OF DIFFUSION COEFFICIENTS FOR LIQUIDS AND DENSE FLUIDS WITH TEMPERATURE AND PRESSURE 下载免费PDF全文
On the basis of the free volume theory and activation energy concept,a fundamental equation whichtakes into account the effects of temperature and pressure has been developed.By introducing differentexpressions for the free volume and activation energy,several equations for fluid diffusion coefficients were derivedaccordingly.With the van der Waals free volume and intermal energy formula,a three-parameter model for fluiddiffusion coeffficients at moderate pressure was obtained.The grand average absolute deviation percent of 345data points (44 systems)for self-and infinite dilute inter-diffusivities is 2.32,against the results of the model ofCohen and Turnbull,4.13.In particular,by means of the modified Carnahan-Starling free volume equation,afour-parameter model with average abosolute deviation percent 2.64(30 systems,644 data points)for theestimation of dense fluid inter-and self-diffusivities at high pressures and in supercritical conditions was derived.The derived model is superior to the method of L 相似文献
97.
SiC coatings on the surface of C-C were produced by either silicone resin impregnation/ pyrolysis or reaction sintering. Cycles of resin impregnation/pyrolysis produced an SiC coating, on the walls of fine open pores, which was effective in reducing the oxidation rate of C-C and in shifting the transition temperature to a higher value. Unless it is pre-coated with a pyrocarbon layer before sintering, plain reaction sintered SiC has oxidation behaviors similar to those of the above-mentioned SiC. The dense pyrocarbon film deposition on the surface of C-C could form a better SiC film than others. The carbon film homogenized the surface of C-C and a dense SiC film was established. The oxidation of the coated C-C can be modelled by a set of “oxidation resistors” in series and/or in parallel, with each resistor corresponding to an oxidation element. The controlling mechanism can be resolved from the activation energy. A combined resistant layer, consisting of resin impregnation, pyrocarbon film and reaction sintering SiC, showed the best oxidation resistance of any single-layer coated C-C composite. 相似文献
98.
99.
Viscosities for six binary mixtures of n-butylamine, di-n-butylamine, tri-n-butylamine, n-octylamine, di-n-octylamine, and tri-n-octylamine with cyclohexane have been measured at 303.15 K with an Ubbelohde suspendedlevel viscometer. Deviations of viscosities from a rectilinear dependence on mole fraction are attributed to H-bonding and to the size of alkylamine compounds. The application of the Eyring's theory of activation energy is examined. The free volume theory of Prigogine-Flory-Patterson (PFP) and the experimental excess enthalpy have been used to estimate excess viscosity ln = (ln /
1
0
– x
2 ln
2
0
/
1
0
) and corresponding free volume, enthalpy, and entropy contributions for five binary mixtures of tri-n-alkylamine: triethyl, tripropyl, tributyl, trihexyl, and trioctylamine with cyclohexane. A comparison of experimental and theoretical excess viscosities indicates a failure of the PFP theory when two components of the mixture differ considerably in size. The size difference contribution to excess viscosity is related to (V
2
*1/2
– V
1
*1/2
), where V
1
*
and V
2
*
are hard-core volumes of two components of the mixture. 相似文献
100.
页岩储层大规模体积压裂对储层实施改造,是实现非常规油气资源工业化开发的核心技术,但目前该技术的机理研究和应用都处于起步发展阶段。大规模体积压裂可能带来潜在的地质灾害风险,主要包括淡水资源的消耗、浅水层的污染、断层活化诱发地质灾害和地表侵蚀、甲烷泄露致空气污染、植被破坏等问题。从体积压裂机理入手,研究了上述存在的地质灾变风险及灾害发生机制,并从勘探调研合理的页岩气开采区块、设计科学的钻井完井方式、压裂液注入速率方案、实时监测断层破裂带应力及滑移状态以及废水废气回收处理等角度提出对应的科学指导和防范措施,对国内页岩气顺利开展工业化开采、避免潜在的地质环境灾变对人类产生危害具有重要的指导意义。 相似文献