全文获取类型
收费全文 | 13002篇 |
免费 | 1138篇 |
国内免费 | 592篇 |
专业分类
电工技术 | 487篇 |
综合类 | 609篇 |
化学工业 | 4377篇 |
金属工艺 | 1056篇 |
机械仪表 | 229篇 |
建筑科学 | 556篇 |
矿业工程 | 203篇 |
能源动力 | 1230篇 |
轻工业 | 482篇 |
水利工程 | 147篇 |
石油天然气 | 411篇 |
武器工业 | 55篇 |
无线电 | 743篇 |
一般工业技术 | 3271篇 |
冶金工业 | 594篇 |
原子能技术 | 138篇 |
自动化技术 | 144篇 |
出版年
2024年 | 61篇 |
2023年 | 402篇 |
2022年 | 443篇 |
2021年 | 530篇 |
2020年 | 551篇 |
2019年 | 551篇 |
2018年 | 505篇 |
2017年 | 543篇 |
2016年 | 446篇 |
2015年 | 438篇 |
2014年 | 668篇 |
2013年 | 810篇 |
2012年 | 774篇 |
2011年 | 962篇 |
2010年 | 663篇 |
2009年 | 725篇 |
2008年 | 584篇 |
2007年 | 749篇 |
2006年 | 666篇 |
2005年 | 552篇 |
2004年 | 452篇 |
2003年 | 404篇 |
2002年 | 321篇 |
2001年 | 265篇 |
2000年 | 230篇 |
1999年 | 205篇 |
1998年 | 197篇 |
1997年 | 143篇 |
1996年 | 115篇 |
1995年 | 114篇 |
1994年 | 89篇 |
1993年 | 65篇 |
1992年 | 71篇 |
1991年 | 76篇 |
1990年 | 59篇 |
1989年 | 58篇 |
1988年 | 42篇 |
1987年 | 31篇 |
1986年 | 34篇 |
1985年 | 24篇 |
1984年 | 25篇 |
1983年 | 20篇 |
1982年 | 23篇 |
1981年 | 15篇 |
1980年 | 13篇 |
1979年 | 8篇 |
1978年 | 2篇 |
1975年 | 2篇 |
1973年 | 2篇 |
1959年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 13 毫秒
991.
J. N. Sweet 《International Journal of Thermophysics》1986,7(4):743-754
New techniques have been developed for reducing thermal conductivity data from thermal comparative measurements. The first of these techniques is based on making a Taylor-series expansion of the stack centerline temperature profile. The result is an expression giving the ratio of sample to reference conductivities at any temperature as a function of measured quantities, the stack thermocouple readings and stack element thicknesses. The conventional formula presently used to reduce comparative conductivity data is shown to be a special result of the general analysis. A second technique involves the use of linear least-squares (LS) techniques to derive both the sample and the reference conductivities from the measured data. The LS technique provides the coefficients for a polynomial temperature expansion of the reference and sample conductivities directly. Use of the new techniques is illustrated in a reduction of some comparative data on the conductivities of Pyrex 7740 and Pyroceram 9606. It is shown that a highly self-consistent pair of conductivity functions can be derived for these two commonly used reference materials if the conductivity vs temperature relation for Pyrex is modified slightly from its recommended value. The Pyroceram conductivity results from the comparative measurements are in good agreement with a conductivity derived from pulse diffusivity and differential scanning calorimetry measurements and also in good agreement with the recommended Pyroceram conductivity function.Paper presented at the Ninth Symposium on Thermophysical Properties, June 24–27, 1985, Boulder, Colorado, U.S.A. 相似文献
992.
Nanostructured porous InP samples were prepared by electrochemical anodic dissolution of InP for various current densities
and etching periods. The samples were characterized by SEM and photoluminescence (PL) where a blue shift was observed in PL.
Thermal properties studied by photoacoustic (PA) spectroscopy revealed one order decrease in thermal conductivity of porous
InP compared to the bulk. Further it is shown that the thermal conductivity of porous InP decreases with decrease in size
of the particles. 相似文献
993.
A hybrid carbon system of graphite powder (GP) and continuous carbon fibre fabric (CFF) is used for an epoxy composite to improve the electrical conductivity, mechanical properties and mouldability of a composite bipolar plate. These improvements are achieved simultaneously by inserting several layers of CFF into the GP/epoxy composite to enhance the mechanical properties and in-plane conductivity. The electrical properties, flexural strength and mouldability of the composite plates are measured as a function of conducting filler content and number of CFF layers. The composites show improved electrical conductivity, flexural properties and mouldability. Composites with 70-75 vol.% carbon fillers have the highest electrical conductivity with reasonable flexural properties. These results suggest that the poor mouldability and low through-plane electrical conductivity of the continuous fibre composite bipolar plate, as well as the weak flexural properties of GP composites, can be overcome by incorporating a GP/CFF hybrid system. 相似文献
994.
Kitiya Hongsirikarn James G. Goodwin Jr. Scott Greenway Stephen Creager 《Journal of power sources》2010,195(1):30-38
The effect of NH3 and NH4+ poisoning on the conductivity of Nafion membranes was investigated via electrochemical impedance spectroscopy. The conductivities of membranes prepared with different NH4+ compositions were measured in deionized water at room temperature and compared to those at 80 °C in a gas phase for various relative humidities. The liquid-phase conductivity decreased linearly with an increase in the NH4+ composition in the membrane (yNH4+), with that of the NH4+-form having a conductivity 25% that of the H+-form. The gas-phase conductivity of the NH4+-form, on the other hand, declined by 66–98% relative to the H+-form depending on humidity. The conductivities of fresh membranes in the presence of gas-phase NH3 at different humidities were also studied. The conductivity decreased with time-on-stream and reached the same conductivity at a given humidity regardless of the NH3 concentration, but the time to reach steady-state varied with NH3 concentration. The yNH4+ at steady-state conductivity was equivalent for all the NH3 concentrations studied. The kinetics of conductivity decrease was slower at higher humidities. The humidity and yNH4+ appear to have a concerted effect on the conductivity. The quantitative conductivity data under practical fuel cell conditions should be useful for future fuel cell modeling. 相似文献
995.
Thermodynamic and Kinetic Effects of Oxygen Removal on the Thermal Conductivity of Aluminum Nitride 总被引:12,自引:0,他引:12
Anil V. Virkar T. Barrett Jackson Raymond A. Cutler 《Journal of the American Ceramic Society》1989,72(11):2031-2042
High thermal conductivity, low dielectric constant, high electrical resistivity, low density, and a thermal expansion coefficient that matches well with that of silicon are the principal attributes of AIN that have attracted much attention over the past decade. It is also now well established that oxygen as an impurity lowers the thermal conductivity of AIN. Processing techniques have been developed which not only facilitate pressureless densification of AIN but also enhance its thermal conductivity. The present work explores the thermodynamics and the kinetics of oxygen removal and the resultant enhancement of thermal conductivity. Polycrystalline AIN ceramics were fabricated with Y2O3, Dy2O3, Yb2O3, CaO, BaO, or MgO as additives. Samples were sinter/annealed at 1850°C for up to 1000 min. The AIN grain size of sintered samples ranged between 2 and 9 μm. The samples typically contained two or three phases with the predominant phase being AIN. Secondary phases in Y2O3-doped AIN consisted of yttrium aluminates which were along three grain junctions and along grain facets. The presence of Y3Al5O12, YAIO3, and Y4Al2O9, as well as Y2O3, depending upon the Y2O3/Al2O3 ratio, was revealed by X-ray diffraction. Thermal conductivity increased with the amount of additive and annealing time. Thermal conductivity also depended on the type of additive. Samples with thermal conductivity up to 200 W/(m · K) were fabricated. The variation in thermal conductivity with the type and the amount of the additive is explained on the basis of the thermodynamics of oxygen removal. In particular, the higher thermal conductivity of CaO-doped, in comparison with MgO-doped, samples is rationalized on the basis that the free energy of formation, ΔG°, of CaAl2O4 is less than that of MgAl2O4. It is proposed that the higher the |ΔG°|, with ΔG° < 0, the higher is the resultant thermal conductivity. An increase in the thermal conductivity with annealing time is attributed to the kinetics of oxygen removal from AIN grains. 相似文献
996.
O. Bastian 《Drying Technology》1997,15(9):2145-2164
This paper presents a synthesis of our scientific activity in the area of heat and moisture transfers in capillary-porous bodies. The materials which have been under investigation are essentially in the field of Civil Engineering but our ways of experimentation may be generalized.
Transfers may be characterized by two independent variables : temperature and water content. We have been concerned by the achievement of measurement technics of these quantities and of perturbation technics of materials at equilibrium.
When comparing a fitting model and the temperature or water content response to such perturbations, it is possible to infer the values of parameters which are significant. Then, the experimental results must be compared with results obtained by other authors or by means of different methods.
In the future, the assumption of an unstrained porous skeleton will not be kept any longer and the coupling of transfers with mechanical phenomena will be considered. 相似文献
Transfers may be characterized by two independent variables : temperature and water content. We have been concerned by the achievement of measurement technics of these quantities and of perturbation technics of materials at equilibrium.
When comparing a fitting model and the temperature or water content response to such perturbations, it is possible to infer the values of parameters which are significant. Then, the experimental results must be compared with results obtained by other authors or by means of different methods.
In the future, the assumption of an unstrained porous skeleton will not be kept any longer and the coupling of transfers with mechanical phenomena will be considered. 相似文献
997.
998.
We investigate the thermoelectric transport properties of carbon atomic clusters connected with two Al electrodes using the first-principles density functional analysis combined with the non-equilibrium Green’s function approach. We find that due to the quantum interference the thermopower S changes sign for even-odd number of carbon atoms. The thermal conductance κ is always positive with a similar oscillation between even- and odd-number of carbon atoms. Due to the orthogonalization of the wave functions of the atomic electrodes and the carbon clusters, the electric conductance G is very small for certain range of chemical potentials. Giant thermopower is found in this region. Our results show that the thermopower and thermal conductance are sensitively related to the distance d between the carbon atomic cluster and the Al electrodes. Oscillatory behaviors of S and κ are found as d is varied. 相似文献
999.
A new interesting category of organometallic polyketones and copolyketones was synthesized via Friedel–Crafts reactions through the polymerization of 2,7‐bis[(2‐ferrocenyl)methylene] cycloheptanone ( II ) with different diacid chlorides. The model compound was synthesized by reacting monomer II with benzoyl chloride and characterized by 1H‐NMR, IR, and elemental analyses. The polyketones and copolyketones were insoluble in most organic solvents but easily soluble in concentrated H2SO4. The thermal properties of these polyketones and copolyketones were evaluated and correlated to their structural units by TGA and DTG measurements and had inherent viscosity of 0.32–0.65 dI g?1. Moreover, the electrical conductivity of polyketone Va and copolyketone VI was investigated above the temperature range (300–500 K) and followed an Arrhenius equation with activation energy 2.09 eV. Also, the morphological properties of selected example of polyketones were detected by scanning electron microscopy. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 2394–2401, 2005 相似文献
1000.
利用溶液聚合方法合成了聚醚聚氨酯,并以聚氨酯、高氯酸锂和增塑剂为组分,制备了一系列新型聚合物固体电解质。运用差示扫描量热分析、动态力学分析、交流复阻抗谱、扫描电镜和原子力显微镜对体系性能和形态进行了研究。结果表明,在聚氨酯/高氯酸锂复合物中,增塑剂的加人会导致体系玻璃化转变温度和力学性能有所下降,离子导电性能显著增加。在所研究的6种增塑剂碳酸丙烯酯、碳酸二乙酯、二乙二醇二甲醚、N,N-二甲基甲酰胺、聚乙二醇400和丙三醇中,聚乙二醇400对聚氨酯/高氯酸锂复合物的增塑效果最好,该体系室温电导率达到10-4S/cm。 相似文献