全文获取类型
收费全文 | 23915篇 |
免费 | 372篇 |
国内免费 | 121篇 |
专业分类
电工技术 | 504篇 |
综合类 | 65篇 |
化学工业 | 3253篇 |
金属工艺 | 953篇 |
机械仪表 | 676篇 |
建筑科学 | 576篇 |
矿业工程 | 67篇 |
能源动力 | 612篇 |
轻工业 | 1885篇 |
水利工程 | 222篇 |
石油天然气 | 180篇 |
武器工业 | 1篇 |
无线电 | 2727篇 |
一般工业技术 | 4164篇 |
冶金工业 | 6326篇 |
原子能技术 | 431篇 |
自动化技术 | 1766篇 |
出版年
2021年 | 154篇 |
2020年 | 115篇 |
2019年 | 160篇 |
2018年 | 238篇 |
2017年 | 238篇 |
2016年 | 267篇 |
2015年 | 185篇 |
2014年 | 315篇 |
2013年 | 853篇 |
2012年 | 532篇 |
2011年 | 802篇 |
2010年 | 577篇 |
2009年 | 643篇 |
2008年 | 735篇 |
2007年 | 774篇 |
2006年 | 655篇 |
2005年 | 739篇 |
2004年 | 610篇 |
2003年 | 653篇 |
2002年 | 698篇 |
2001年 | 686篇 |
2000年 | 621篇 |
1999年 | 629篇 |
1998年 | 2207篇 |
1997年 | 1447篇 |
1996年 | 1076篇 |
1995年 | 738篇 |
1994年 | 641篇 |
1993年 | 656篇 |
1992年 | 372篇 |
1991年 | 360篇 |
1990年 | 352篇 |
1989年 | 364篇 |
1988年 | 304篇 |
1987年 | 275篇 |
1986年 | 257篇 |
1985年 | 290篇 |
1984年 | 228篇 |
1983年 | 210篇 |
1982年 | 190篇 |
1981年 | 217篇 |
1980年 | 203篇 |
1979年 | 166篇 |
1978年 | 154篇 |
1977年 | 284篇 |
1976年 | 402篇 |
1975年 | 128篇 |
1974年 | 110篇 |
1973年 | 117篇 |
1972年 | 90篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
Mass transfer and electrocrystallization analyses of nanocrystalline nickel production by pulse plating 总被引:4,自引:0,他引:4
R. T. C. Choo J. M. Toguri A. M. El-Sherik U. Erb 《Journal of Applied Electrochemistry》1995,25(4):384-403
A comparison between the experimental process parameters employed for the pulse plating of nanocrystalline nickel and the solution-side mass transfer and electrokinetic characteristics has been carried out. It was found that the experimental process parameters (on-time, off time and cathodic pulse current density) for cathodic rectangular pulses are consistent and within the physical constraints (limiting pulse current density, transition time, capacitance effects and integrity of the waveform) predicted from theory with the adopted postulates. This theoretical analysis also provides a means of predicting the behaviour of the process subject to a change in the system, kinetic and process parameters. The product constraints (current distribution, nucleation rate and grain size), defined as the experimental conditions under which nanocrystalline grains are produced, were inferred from electrocrystallization theory. High negative overpotential, high adion population and low adion surface mobility are prerequisites for massive nucleation rates and reduced grain growth; conditions ideal for nanograin production. Pulse plating can satisfy the former two requirements but published calculations show that surface mobility is not rate-limiting under high negative overpotentials for nickel. Inhibitors are required to reduce surface mobility and this is consistent with experimental findings. Sensitivity analysis on the conditions which reduce the total overpotential (thereby providing more energy for the formation of new nucleation sites) are also carried out. The following lists the effect on the overpotential in decreasing order: cathodic duty cycle, charge transfer coefficient, Nernst diffusion thickness, diffusion coefficient, kinetic parameter () and exchange current density.Nomenclature
A
constant employed in Fig. 8, (nFi0)/(RT
e
C
a)(s–1)
-
B
constant in Equation 38 (V2)
-
C
cation concentration (molcm–3)
-
C
a
capacitance of double layer (µFcm–2)
-
C
s
cation surface concentration (molcm–3)
-
C
s
*
dimensionless cation surface concentration, C
s/C
(–)
-
C
cation bulk concentration (molcm–3)
-
D
diffusion coefficient of cation (cm2s–1)
- E
total applied potential (V)
- E
0
standard cell potential (V)
-
F
Faraday constant (Cmol–1)
-
function defined in Appendix C(–)
-
Fr
frequency of waveform (Hz)
-
f
i,p
function defined in Appendix C for pth period (–)
-
f
i,
function defined in Appendix C for p period (–)
-
G
j
function defined in Appendix B (–)
-
gi
function defined in Appendix B (–)
-
i
current density (Acm¨)
-
i
ac
unsteady fluctuating a.c. current density (Acm–2)
-
i
c
capacitance current density (Acm–2)
-
i
dc
steady time-averaged d.c. current density (Acm–2)
-
i
F
Faradaic current density (Acm–2)
-
i
lim
limiting d.c. current density (Acm–2)
-
i
0
exchange current density (Acm–2)
-
i
PL
limiting pulse current density, i
1{Cs = 0 at t = (p – 1) T + t
1(Acm–2)
-
i
1
cathodic pulse current density (Acm–2)
-
i
2
relaxed or low current pulse current density (Acm–2)
-
iin
anodic pulse current density (Acm–2)
-
i
*
dimensionless current density, i/|i
lim| (–)
-
i
0
*
dimensionless exchange current density, i
dc/|i
lim| (–)
-
i
dc
*
dimensionless steady time-averaged d.c. current density, i
dc/|i
lim| (–)
-
i
PL
*
dimensionless limiting cathodic pulse current density, i
PL/|i
lim| (–)
-
i
PL,p
*
dimensionless limiting pulse current density at pth period, i
1(C
s = 0)/|i
lim| (–)
-
i
PL,
*
dimensionless limiting pulse current density for p , i
1(C
s = 0)/|i
lim| (–)
-
i
1
*
dimensionless cathodic pulse current density, i
1/|i
lim| (–) 相似文献
993.
994.
Contribution of legumes towards N economy in cereal-based cropping systems is well-known but there has been a gradual decline
in the cultivation of grain legumes, threatening sustainability of maize (Zea mays)–wheat (Triticum aestivum) cropping system in north-western India. A study was made to evaluate and quantify the effect of different grain legumes
on productivity, profitability, N economy and soil fertility in maize–wheat cropping system at New Delhi during 2002–2004.
Five legumes, viz. blackgram (Vigna mungo), greengram (Vigna radiata), cowpea (Vigna unguiculata), groundnut (Arachis hypogaea) and soybean (Glycine max) were either intercropped with maize or grown in sole cropping, and their residues were incorporated before the following
crop of wheat, which was grown with varying rates of N, viz. 0, 40, 80 and 120 kg N ha−1. Maize-equivalent productivity was significantly more with intercropped greengram (16.1–29.9%), cowpea (24.8%) and groundnut
(11.1–16.6%) than in sole maize. Land equivalent ratio and other competitive functions were favourably influenced with intercropped
maize + greengram and maize + cowpea. Addition of N through legume residues varied from 11.5–38.5 kg ha−1 in intercropped system and 17.5–83.5 kg ha−1 in sole cropping, which improved productivity of following wheat to a variable extent. Nitrogen economy in wheat was 21 kg ha−1 due to residue incorporation of intercropped greengram, cowpea and groundnut; and 49–56 kg N ha−1 of sole cropped greengram and groundnut. Residual soil fertility in terms of organic C and KMnO4-N showed an improvement under maize-based intercropping systems followed by wheat, and the beneficial effect was more pronounced
with sole cropping of legumes due to greater addition of residues. Apparent N balance as well as actual change in KMnO4-N at the end of study was positive in most intercropped legumes as well as sole cropping systems, with greater improvement
noticed under groundnut, soybean and greengram. Net returns were marginal with maize-based intercropping or sole cropping
of legumes, but improved considerably with wheat, particularly when greengram, cowpea and groundnut were grown in the previous
season. The studies suggested that inclusion of grain legumes, particularly greengram, cowpea and groundnut was beneficial
for improving productivity, profitability, N economy and soil fertility in maize–wheat cropping system. 相似文献
995.
Enhancement of Thermoelectric Performance in Hierarchical Mesoscopic Oxide Composites of Ca3Co4O9 and La0.8Sr0.2CoO3
下载免费PDF全文
![点击此处可从《Journal of the American Ceramic Society》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Sajid Butt Wei Xu Muhammad U. Farooq Guang K. Ren Fida Mohmed Yuanhua Lin Ce‐Wen Nan 《Journal of the American Ceramic Society》2015,98(4):1230-1235
The natural contradiction in enhancing electrical conductivity and thermopower in thermoelectric oxides makes it hard to improve the performance of a single thermoelectric oxide material. We report a facile method to construct a unique architecture of thermoelectric oxides that is promising to realize a simultaneous improvement of overall electrical conductivity and thermopower. Here, a series of two‐phase nanocomposites comprising of Ca3Co4O9 (CCO) and La0.8Sr0.2CoO3 (LSCO) has been synthesized through ball milling followed by spark plasma sintering (SPS) method. The electron microscope images reveal that the two constituents form the unique composites while retaining their individual crystalline and morphological identities. Owing to the hierarchical mesoscopic structure with nanoscale particles and submicrometer scale grain boundaries, an external strain is induced into the CCO grains by the LSCO nanoparticles to enhance the thermopower. The mesoscopic structure is also favorable for improving the electrical conductivity. Moreover, the long‐wavelength phonons can be scattered effectively from LSCO nanoparticles and the thermal conductivity is further suppressed. With compromises between power factor and thermal conductivity, the largest ZT achieved is up to 0.41 at 1000 K for the composites with 25 wt% of LSCO. 相似文献
996.
W. Siebourg H. Schmid F. M. Rateike S. Anders U. Grigo H. Lwer 《Polymer Engineering and Science》1990,30(18):1133-1139
Birefringence is an important substrate property for optical data storage media. The basic definitions of birefringence and its relation to the chemical nature of polycarbonate are outlined. The polarizability of the monomer unit, expressed in the rheo-optical constant and the degree of remaining polymer chain orientation, determines the level of birefringence in polycarbonate disk substrates. Based on this theory, the theory of blending materials with positive and negative rheo-optical constants Is developed, and the current work with polycarbonate/polystyrene blends is reviewed. The biggest disadvantage of the reviewed system Js that only binary systems with low LCST (lower critical solution temperature) (<240°C) can be formulated. Finally, we report on successful development work of a single-phase blend of a modified polycarbonate with a special styrene-acrylonitrile copolymer. The experimental results are compared with the theoretical expectations, and the implications on the injection molding of disk substrates are discussed. 相似文献
997.
Cu K-absorption edge and EXAFS measurements on binary Cu/ZnO and ternary Cu/ ZnO-Al2O3 catalysts of varying compositions on reduction with hydrogen at 523 K, show the presence of Cu microclusters and a species of Cu1+ dissolved in ZnO apart from metallic Cu and Cu2O. The proportions of different phases critically depend on the heating rate especially for catalysts of higher Cu content. Accordingly, hydrogen reduction with a heating rate of 10 K/min predominantly yields the metal species (>50%), while a slower heating rate of 0.8 K/min enhances the proportion of the Cu1+ species ( 60%). Reduced Cu/ZnO-Al2O3 catalysts show the presence of metallic Cu (upto 20%) mostly in the form of microclusters and Cu1+ in ZnO as the major phase ( 60%). The addition of alumina to the Cu/ZnO catalyst seems to favour the formation of Cu1+/ZnO species. 相似文献
998.
Dense, single-phase β-sialon ceramics were sintered at 1700°C and 200 MPa using the glass-encapsulated hot isostatic pressing technique. The materials were very hard, 1500 to 1700 kg / mm2 (98 N load), but were fairly brittle, with an indention fracture toughness of about 3 MPa · m1/2 . The addition of 1 wt% Y2 O3 before sintering had a positive effect on the toughness, especially at the low x compositions of Si3-x Alx Ox N4-x , where KIC ∼4 MPa · m1/2 . 相似文献
999.
Pt black was exposed to n>-hexane/H2 mixtures between 483 and 663 K followed by O2 and H2 treatments at 603 K. XP and UP spectra were measured without exposing the samples to air. 20–30% carbon accumulated after hydrocarbon exposures. O2 removed most carbon. The surface C content increased after a subsequent contact with H2, C 1s showing more atomic carbon as opposed to graphite after n-hexane exposure. Anisotropic recrystallization of Pt black favoring (220) and (311) lattice planes occurred under hydrogen-rich conditions. Both findings were attributed to a H2-induced solid-state rearrangement; H atoms penetrating into the crystal lattice, force subsurface carbon and oxygen atoms to the surface and a concomitant restructuring would occur. Thus another hydrogen effect has been recognized, leading to structures favorable for skeletal reactions of alkanes. 相似文献
1000.
The corrosion resistance of conventional and high alloy stainless steels (specifically 316L, 317L, 904L, duplex 2205, 3127hMO, 1925hMO, 254SMO, 654SMO and Remanit-4565) was determined electrochemically in chlorinated and unchlorinated Arabian Gulf Seawater at 25 and 50 °C. The stainless steels 316L and 317L were used as reference alloys. The electrochemical potentiodynamic cyclic polarization method was used to determine the passive film break down potential (E
b), protection potential (E
prot) and maximum current attained on scan reversal (I
max). It was found that at 25 °C in chlorinated and unchlorinated seawater and at 50 °C in unchlorinated seawater, stainless steels 316L and 317L have poor resistance to corrosion, Stainless steels 904L and duplex 2205 at 25 °C in chlorinated and unchlorinated seawater showed good resistance to corrosion but at 50 °C these steels failed to resist. The high alloy stainless steels such as 3127hMO, 1925hMO, 254SMO, 654SMO and Remanit-4565 showed better corrosion resistance under all the test conditions. 相似文献