全文获取类型
收费全文 | 26855篇 |
免费 | 688篇 |
国内免费 | 34篇 |
专业分类
电工技术 | 329篇 |
综合类 | 14篇 |
化学工业 | 5934篇 |
金属工艺 | 519篇 |
机械仪表 | 466篇 |
建筑科学 | 1161篇 |
矿业工程 | 332篇 |
能源动力 | 677篇 |
轻工业 | 1726篇 |
水利工程 | 229篇 |
石油天然气 | 245篇 |
无线电 | 1714篇 |
一般工业技术 | 4089篇 |
冶金工业 | 6149篇 |
原子能技术 | 266篇 |
自动化技术 | 3727篇 |
出版年
2022年 | 248篇 |
2021年 | 415篇 |
2020年 | 268篇 |
2019年 | 346篇 |
2018年 | 423篇 |
2017年 | 370篇 |
2016年 | 491篇 |
2015年 | 391篇 |
2014年 | 617篇 |
2013年 | 1658篇 |
2012年 | 942篇 |
2011年 | 1209篇 |
2010年 | 932篇 |
2009年 | 975篇 |
2008年 | 1154篇 |
2007年 | 1066篇 |
2006年 | 974篇 |
2005年 | 886篇 |
2004年 | 745篇 |
2003年 | 713篇 |
2002年 | 706篇 |
2001年 | 487篇 |
2000年 | 394篇 |
1999年 | 447篇 |
1998年 | 441篇 |
1997年 | 430篇 |
1996年 | 473篇 |
1995年 | 443篇 |
1994年 | 451篇 |
1993年 | 437篇 |
1992年 | 431篇 |
1991年 | 271篇 |
1990年 | 410篇 |
1989年 | 387篇 |
1988年 | 336篇 |
1987年 | 404篇 |
1986年 | 348篇 |
1985年 | 441篇 |
1984年 | 463篇 |
1983年 | 398篇 |
1982年 | 375篇 |
1981年 | 298篇 |
1980年 | 298篇 |
1979年 | 357篇 |
1978年 | 299篇 |
1977年 | 238篇 |
1976年 | 217篇 |
1975年 | 245篇 |
1974年 | 217篇 |
1973年 | 224篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
This paper reviews the brittle fracture behavior of dielectric ceramics such as barium titanate, and describes some of the relationships between defects such as cracks and electrical degradation and failure of multilayer capacitors. Stresses arising from the ferroelectric phase transformation in these dielectric materials are shown to play a part as a driving force for crack growth. In addition, possible contributions to failure from stresses arising from thermal excursions in the capacitor are discussed. Low-voltage failures arising from a short between the electrodes in multilayer capacitors are shown to be related to the growth of cracks in the dielectric. A technique for predicting the onset of these types of failures based upon fracture mechanics techniques is described. Possible effects of the electric field itself in promoting or retarding the growth of cracks are discussed. 相似文献
992.
Microstructural Coarsening During Sintering of Boron Carbide 总被引:8,自引:0,他引:8
Stephen L. Dole Svante Prochazka Robert H. Doremus 《Journal of the American Ceramic Society》1989,72(6):958-966
The sintering behavior of boron carbide was investigated with particular attention given to microstructure development at various stages in the sintering process. Hot-pressing and pressureless sintering techniques were employed and the effects of heating rate, firing atmosphere, and composition were used to characterize the sintering behavior. Pressureless sintering at temperatures up to 2300°C produces only limited densification. Microstructural coarsening is responsible for this since it leads to conditions where densification is slow. Hot-pressing and carbon additions suppressed coarsening and permitted densification to >95% of theoretical density. 相似文献
993.
Current trends in spark ignition engines are toward multivalve engines capable of operating at relatively high speeds to maximize performance and fuel economy at acceptable emissions levels. Structural ceramics, with their low density and good high-temperature properties, appear to offer potential as an alternative valve material to the steels, superalloys, and titanium alloys currently in use. Progress to date on materials development, component design, component performance, and engine testing indicates that silicon nitride valves will operate satisfactorily. 相似文献
994.
A test system based around a thin‐walled extruded solid electrolyte tubular reactor has been developed, which enables the
fuel reforming catalysis and surface chemistry occurring within solid oxide fuel cells and the electrochemical performance
of the fuel cell to be studied under genuine operating conditions. It permits simultaneous monitoring of the catalytic chemistry
and the cell performance, allowing direct correlation between the fuel cell performance and the reforming characteristics
of the anode, as well as enabling the influence of drawing current on the catalysis and surface reaction pathways to be studied.
Temperature‐programmed reaction measurements can be carried out on anodes in an actual SOFC, and have been used to investigate
the reduction characteristics of different anode formulations, methane activation and methane steam reforming, and to evaluate
the nature and level of carbon deposition on the anode during reforming.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
995.
Marina Villegas Amador C. Caballero Carlos Moure Pedro Durán José F. Fernández Robert E. Newnham 《Journal of the American Ceramic Society》2000,83(1):141-46
Pb(Zn1/3 Nb2/3 )O3 -based ceramics have been prepared by two different processing methods: conventional (PZN-C) and reaction-sintering (PZN-RS). The conventionally prepared PZN-based ceramics densified at lower temperatures (950°C) than the reaction-sintered samples (1100°C), but the perovskite/pyrochlore ratio was always higher in PZN-RS. The presence of a substantial amount of pyrochlore phase in PZN-C ceramics caused a decrease in the electrical properties. The maximum dielectric constant values in PZN-C ceramics were 10%–15% lower than those of PZN-RS, despite a similar average grain size, 7 ± 0.2 μm. The temperature of the maximum of the dielectric constant ( T max ) was lower than that expected from the mixing rule because of the possible formation of Ba–Nb clusters. The higher chemical homogeneity in PZN-RS ceramics is the main reason for the higher dielectric constant, T max and electromechanical response, as well as for the lower difference between T max and the depolarization temperature ( T d ) and the lower diffusiveness parameter (δ). 相似文献
996.
Susan Schonfield Peter Frost Robert Cramb Chris Royle 《Journal of Automated Methods and Management in Chemistry》1989,11(1):22-27
The Nova Stat Profile 1 analyser, a combined sodium, potassium,
ionized calcium, haematocrit and conventional blood gas analyser,
was evaluated over a four month period. In addition to assessing
and demonstrating that the instrument met analytical requirements,
an appreciation of the use of ionised calcium (iCa) was made.
Prospective costs were characterised and practical problems of iCa
measurement addressed. 相似文献
997.
Robert A. Webster Peter Molnar Stephen E. Kahn 《Journal of Automated Methods and Management in Chemistry》1989,11(6):254-259
The negative interference caused by haemolysis in manual bilirubin
assays contrasts with the positive interference reported for some
automated methods utilizing the same basic chemistry. A
comparison was therefore made of the haemolysis interference
associated with several automated bilirubin methodologies: multilayer
film (Kodak Ektachem- total bilirubin (TBil), direct
bilirubin (DBil), conjugated bilirubin (Bc), unconjugated bilirubin
(Bu),; tableted reagents (Baxter Paramax- TBil, DBil);
continuous flow (Technicon SMAC — TBil). Thirty serum pools
were analysed (five concentrations of bilirubin, 2-229 μmol/l; six
concentrations of haemoglobin, 0.00002-0.052 mmol/1). All
methods, except one (Bc), exhibited both positive and negative
interference, depending upon the relative haemoglobin and bilirubin
concentrations. This interference, at any given haemoglobin
concentration, was neither constant nor proportional with increasing
bilirubin concentration. These complex patterns of interference
suggest that the best characterization of interference is obtained
when (1) both percentage-difference and absolute-difference
‘interferographs’ are plotted; and (2) the interference is determined
at multiple analyte concentrations. 相似文献
998.
Controlled synthesis and enhanced field emission characteristics of conical carbon nanotubular arrays 总被引:1,自引:0,他引:1
Santoshrupa Dumpala Abdelilah Safir David Mudd Robert W. Cohn Mahendra K. Sunkara Gamini U. Sumanasekera 《Diamond and Related Materials》2009,18(10):1262-1266
Conical carbon nanotubes (CCNTs) with unique structural characteristics arising from their tapered morphologies compared to uniform diameter carbon nanotubes, have been shown to exhibit enhanced field emission properties and support high current densities. Specifically, several CCNT arrays with different morphological characteristics (tip radius, aspect ratio, density and wall structure) were synthesized by variations in the process parameters using a microwave plasma chemical vapor deposition (MWCVD) reactor. The field emission characteristics for a CCNT array sample with a tip radius of 5 nm, density of 108/cm2 and having the highest aspect ratio exhibited a low turn-on electric field (< 0.7 V/μm) and a high field enhancement factor (β > 7500). Other samples with lower emission characteristics were attributed either to the presence of field screening effect resulting from higher CCNT density or due to the corresponding tip and wall characteristics. 相似文献
999.
Ethanol and (−)-α-Pinene: Attractant Kairomones for Bark and Ambrosia Beetles in the Southeastern US
In 2002–2004, we examined the flight responses of 49 species of native and exotic bark and ambrosia beetles (Coleoptera: Scolytidae
and Platypodidae) to traps baited with ethanol and/or (−)-α-pinene in the southeastern US. Eight field trials were conducted
in mature pine stands in Alabama, Florida, Georgia, North Carolina, and South Carolina. Funnel traps baited with ethanol lures
(release rate, about 0.6 g/day at 25–28°C) were attractive to ten species of ambrosia beetles (Ambrosiodmus tachygraphus, Anisandrus sayi, Dryoxylon onoharaensum, Monarthrum mali, Xyleborinus saxesenii, Xyleborus affinis, Xyleborus ferrugineus, Xylosandrus compactus, Xylosandrus crassiusculus, and Xylosandrus germanus) and two species of bark beetles (Cryptocarenus heveae and Hypothenemus sp.). Traps baited with (−)-α-pinene lures (release rate, 2–6 g/day at 25–28°C) were attractive to five bark beetle species
(Dendroctonus terebrans, Hylastes porculus, Hylastes salebrosus, Hylastes tenuis, and Ips grandicollis) and one platypodid ambrosia beetle species (Myoplatypus flavicornis). Ethanol enhanced responses of some species (Xyleborus pubescens, H. porculus, H. salebrosus, H. tenuis, and Pityophthorus cariniceps) to traps baited with (−)-α-pinene in some locations. (−)-α-Pinene interrupted the response of some ambrosia beetle species
to traps baited with ethanol, but only the response of D. onoharaensum was interrupted consistently at most locations. Of 23 species of ambrosia beetles captured in our field trials, nine were
exotic and accounted for 70–97% of total catches of ambrosia beetles. Our results provide support for the continued use of
separate traps baited with ethanol alone and ethanol with (−)-α-pinene to detect and monitor common bark and ambrosia beetles
from the southeastern region of the US. 相似文献
1000.
Catalytic efficiency, stability and environmental applicability of five iron(III) oxide nanopowders differing in surface area and crystallinity were tested in degradation of concentrated phenolic aqueous solutions (100 g/L) at mild temperature (30 °C), initially almost neutral pH and equimolar ratio of hydrogen peroxide and phenol. The catalyst properties were easily controlled by varying in reaction time during isothermal treatment of ferrous oxalate dihydrate in air at 175 °C. Although the catalytic efficiency clearly increases with the surface area of the nanopowders, it is not due to the solely heterogeneous catalytic mechanism as would be expected. The amorphous Fe2O3 nanopowders possessing the largest surface areas (401 m2 g−1, 386 m2 g−1) are the most efficient catalysts evidently due to their highest susceptibility to leaching in acidic environment arising as a consequence of phenol degradation products. Thus, these amorphous samples act partially as homogeneous catalysts, which was confirmed by a high concentration of leached Fe(III) ions in the solution (19 ppm). The crystalline hematite (α-Fe2O3) samples, varying in surface area between 337 m2 g−1 and 245 m2 g−1, are generally less efficient when compared to the amorphous powders, however their catalytic action is almost exclusively heterogeneous as only 3 ppm of leached Fe(III) was found in the reaction systems catalyzed by nanohematite samples. A significant difference in relative contributions of heterogeneous and homogenous catalysis was definitely established in buffered reaction systems catalyzed by amorphous Fe2O3 and nanocrystalline hematite. The nanohematite sample exhibiting the highest heterogeneous action was tested at decreased initial phenol concentration (10 g/L), which is closer to the real contents of phenol in waste waters, and at different hydrogen peroxide/phenol molar ratios to consider its environmental applicability. At the hydrogen peroxide/phenol ratio equal to 5, no traces of the leached iron were detected and the phenol conversion of 84% was reached. Moreover, such a high degree of conversion is accompanied by a decrease of the chemical oxygen demand (COD) from the initial value of 11.23 g/L to 4.22 g/L after 125 min. This fact indicates that the considerable fraction of primary reaction products was totally degraded. 相似文献