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991.
Noise is usually identified as either Johnson or shot noise in its nature. Since both types of noise are produced by nonuniform velocity distributions in charge-carrier plasmas, the average available noise-power spectral density is in both cases determined by the plasma temperature alone, p?n-junction and vacuum diodes are used to demonstrate this theory. 相似文献
992.
Henggeler Scott W.; Sallis James F.; Cooper Peter F. 《Canadian Metallurgical Quarterly》1980,27(2):217
A random and representative sample of 457 undergraduate and graduate students completed the Needs Assessment Questionnaire that required each respondent to rate on a 5-point scale the seriousness of 24 mental health problems among university students and their families. A similar questionnaire was completed by 30 university-employed mental health professionals. A comparison of the ratings given to specific mental health problems by student and professional groups revealed extensive between-groups differences. The students rated problems of substance abuse as most serious, whereas the professionals rated problems of an academic and neurotic nature as most serious. The results have important implications for the validity and utility of needs assessment strategies. (3 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
993.
994.
Loblolly pine plantations are the most important source of forest products in the US and the slash remaining after conventional harvest represents a significant potential source of bioenergy. However, slash removal in intensive harvests might, under some circumstances, reduce site productivity by reducing soil organic matter and associated nutrients. Two complimentary studies in the Gulf Coastal Plain of the southeastern US were designed to test whether harvest intensity (level of biomass removal) could have a negative long-term impact on site productivity. Harvesting tree crowns in addition to the merchantable bole had a negative impact (18%) on pine biomass accumulation by age 7–10 years on 15 of 19 research blocks. Sites at risk of harvest-induced reductions in productivity were relatively unproductive prior to harvest and had low soil phosphorus (P) concentrations. Intensive harvesting, fertilization, and chemical control of non-crop vegetation were all energy efficient; the additional biomass energy gained through these practices was two-orders of magnitude greater than the energy needed to conduct the activities. Harvest of slash for bioenergy in the Gulf Coastal Plain of the southeastern US has the potential to reduce productivity on infertile soils, but fertilization has the potential to restore and even improve productivity on those sites in an energy-efficient way. 相似文献
995.
Wai-Tat Fu Daniel Bothell Scott Douglass Craig Haimson Myeong-Ho Sohn John Anderson 《Interacting with computers》2006,18(6):1215-1241
This article describes the development of a real-time model-based training system that provides adaptive “over-the-shoulder” (OTS) instructions to trainees as they learn to perform an Anti-Air Warfare Coordinator (AAWC) task. The long-term goal is to develop a system that will provide real-time instructional materials based on learners’ actions, so that eventually the initial set of instructions on a task can be strengthened, complemented, or overridden at different stages of training. The training system is based on the ACT-R architecture, which serves as the theoretical background for the cognitive model that monitors the learning process of the trainee. An experiment was designed to study the impact of OTS instructions on learning. Results showed that while OTS instructions facilitated short-term learning, (a) they took time away from the processing of current information, (b) their effects tended to decay rapidly in initial stages of training, and (c) their effects on training diminished when the OTS instructions were proceduralized in later stages of training. A cognitive model that learned from both the upfront and OTS instructions was created and provided good fits to the learning and performance data collected from human participants. Our results suggest that to fully capture the symbiotic performance between humans and intelligent training systems, it is important to closely monitor the learning process of the trainee so that instructional interventions can be delivered effectively at different stages of training. We proposed that such a flexible system can be developed based on an adaptive cognitive model that provides real-time predictions on learning and performance. 相似文献
996.
The efficiency and selectivity of two approaches for introducing alkoxysilane functionality to polyethylene (PE) are examined along with the moisture‐curing performance of the resulting products. Although the peroxide‐initiated grafting of vinyltriethoxysilane to PE is accompanied by undesirable crosslinking, comparable silane contents can be introduced without affecting the melt viscosity through the addition of mercaptopropyltriethoxysilane (MPTES) to the unsaturation within the polymer. Rapid hydrogen atom donation by thiols underlies this unique selectivity for grafting versus molecular weight alteration, and gives rise to a remarkable tolerance of MPTES additions to phenolic antioxidants. Direct comparisons of the moisture‐curing efficiencies provided by the two functionalization techniques reveal few significant differences in crosslink yields or composition distributions. POLYM. ENG. SCI., 46:480–485, 2006. © 2006 Society of Plastics Engineers. 相似文献
997.
Majid Zandian D. Scott J. Garnett D. D. Edwall J. Pasko M. Farris M. Daraselia J. M. Arias J. Bajaj D. N. B. Hall S. Jacobson G. Luppino S. Parker 《Journal of Electronic Materials》2005,34(6):891-897
Growth of Hg1−xCdxTe by molecular beam epitaxy (MBE) has been under development since the early 1980s at Rockwell Scientific Company (RSC),
formerly the Rockwell Science Center; and we have shown that high-performance and highly reproducible MBE HgCdTe double heterostructure
planar p-on-n devices can be produced with high throughput for various single- and multiplecolor infrared applications. In
this paper, we present data on Hg1−xCdxTe epitaxial layers grown in a ten-inch production MBE system. For growth of HgCdTe, standard effusion cells containing CdTe
and Te were used, in addition to a Hg source. The system is equipped with reflection high energy electron diffraction (RHEED)
and spectral ellipsometry in addition to other fully automated electrical and optical monitoring systems. The HgCdTe heterostructures
grown in our large ten-inch Riber 49 MBE system have outstanding structural characteristics with etch-pit densities (EPDs)
in the low 104 cm−2 range, Hall carrier concentration in low 1014 cm−3, and void density <1000 cm2. The epilayers were grown on near lattice-matched (211)B Cd0.96Zn0.04Te substrates. High-performance mid wavelength infrared (MWIR) devices were fabricated with R0A values of 7.2×106 Ω-cm2 at 110 K, and the quantum efficiency without an antireflection coating was 71.5% for cutoff wavelength of 5.21 μm at 37 K.
For short wavelength infrared (SWIR) devices, an R0A value of 9.4×105 Ω-cm2 at 200 K was obtained and quantum efficiency without an antireflection coating was 64% for cutoff wavelength of 2.61 μm at
37 K. These R0A values are comparable to our trend line values in this temperature range. 相似文献
998.
E. Todd Voiles L. Scott Chumbley Yong Han Mufit Akinc 《Journal of the American Ceramic Society》1994,77(12):3273-3276
Monodisperse, spherical, submicrometer-size ZnS particles with nanocrystalline substructure were heated and examined in situ using a Gatan hot stage and a transmission electron microscope (TEM) at temperatures ranging from 500° to 900°C. The goal of this study was to observe the micro-structural changes that occur in these particles during heat treatment. The particles, whose morphology consists of an agglomeration of nanocrystals, showed significant grain growth and sintering at temperatures as low as 600°C, much lower than expected for bulk ZnS crystals. In addition, the particles were seen to develop hollow cores during heat treatment. This hollowing effect appears to be related to the nanocrystalline/nanoporous nature of the particles. 相似文献
999.
Children's understanding of their own cognitive skills, or metacognition, has been hypothesized to play a major role in learning and development. In this study, we examine the developing relation between children's metacognition and reading comprehension. Children in third- and fifth-grade classes were given an experimental curriculum, Informed Strategies for Learning (ISL), designed to increase their awareness and use of effective reading strategies. In both grades, children in experimental classes made significant gains in metacognition and the use of reading strategies compared with children in control classes. The multivariate profiles of reading skills derived from the developmental analyses helped to identify subgroups of children who responded differently to the metacognitive instruction. Although there were specific aptitude-by-treatment interactions, there was a general trend for metacognition and strategic reading to become more congruent from 8 to 10 years of age. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
1000.
A. Ferreira M. A. Meyers N. N. Thadhani 《Metallurgical and Materials Transactions A》1992,23(12):3251-3261
A detailed microstructural analysis and evaluation of the mechanical properties of titanium aluminides consolidated by novel
shock processes[13] are presented. Successful consolidation was obtained and was evidenced by strong bonding between individual particles. Additions
of Nb and Ti and Al elemental powders resulted in enhanced interparticle bonding through intense plastic deformation of Nb
and shock-induced reactions between Ti and Al. Rapid cooling of interparticle molten layers yielded amorphous Ti-Al alloys;
this interparticle melting and rapid cooling are a unique feature of shock processing. Embrittlement of individual particles
of Ti3Al-based alloy after exposure to 550 °C and 750 °C was observed. There is evidence of phase transformation after preheating
the powder, and this fact can explain the high density of cracks obtained with this alloy after high-temperature shock consolidation.
Mechanical properties of the Ti3Al-based alloy were determined at room temperature and the fracture modes were studied. The microstructural observations are
correlated with the mechanical properties. 相似文献