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1.
The shape of the impactor with the maximum depth of penetration (DOP) for a given impact velocity is found using a numerical procedure for solving a corresponding non-classical variational problem. It is shown that the optimum shape in a general case is close to a blunt cone. The variation of the optimal shape of the impactor and the dependence of the DOP vs. the initial (impact) velocity and friction coefficient is studied. The analysis is performed also for optimal conical impactors. 相似文献
2.
PURPOSE: Little has been written regarding the ultrasound imaging features that might allow prediction of fetal viability in abdominal pregnancies. Toward this goal, we present our experience with a series of 11 abdominal pregnancies. MATERIALS AND METHODS: From 1981 to 1993, 11 patients presented to Universidad Catolica, Santiago, and Universidad de Austral, Valdivia, Chile, with third trimester abdominal pregnancies. Five had complete ultrasound examinations, and these five patients were managed expectantly. The other six women presented as acute abdominal emergencies and underwent emergent surgery. RESULTS: Four of five fetuses that survived had a complete placental attachment to the uterus, and one surviving neonate had a partial attachment of the placenta to the uterus. Three fetuses died prior to delivery, and all three had a complete mesenteric placental attachment. Two died in the early neonatal period. One had a complete uterine placental attachment, and the other had a partial attachment. CONCLUSIONS: The survival rate of abdominal pregnancies may be better than is generally believed. Placental attachment to the uterus appears to be a factor related to fetal survival and maternal morbidity. More cases are necessary to determine the feasibility of managing women with abdominal pregnancies expectantly. 相似文献
3.
Kirk Weller Ilana Arnon Ed Dubinsky 《Canadian Journal of Science, Mathematics, & Technology Education》2013,13(2):129-159
Abstract In an earlier study of preservice elementary and middle school teachers’ beliefs about repeating decimals, Weller, Arnon, and Dubinsky (2009) reported on a comparison of the mathematical performance of 77 preservice teachers who completed an APOS-based instructional unit with 127 preservice teachers who completed traditional instruction. The purpose of the current study, based on 47 interviews conducted 4 months after the instruction, during which time there was no further instruction on this topic, is to determine the strength and stability (over time) of the students’ beliefs, to uncover thinking that did not arise in the earlier study, and to see whether the interviews yield similar comparative results. The interviews did uncover changes in student thinking. The students who received the APOS-based instruction developed stronger and more stable (over time) beliefs that a repeating decimal is a number; a repeating decimal has a fraction or integer to which it corresponds; a repeating decimal in general equals its corresponding fraction or integer; and, in particular, In addition, a number of indices and categories were developed that may prove useful in other studies consisting of the comparison of interview and questionnaire data involving a large number of interview subjects. 相似文献
4.
5.
Modeling biomass as a renewable energy source poses many challenges with respect to feedstock variability, which are difficult to account for. It is found that at the preliminary stages of energy planning, heating value and moisture content of the feedstock are the most important factors. In addition, the effects of harvesting, transportation and storage are found to be significant even though they are often overlooked. Using the gathered information a biomass module for energy planning is created and integrated to H2RES, a renewable energy planning program. Using this excel based software, a case study for a wood processing factory is performed, using the waste wood as feedstock. Comparing various scenarios, it is concluded that using a combination of solid oxide fuel cells, solar panels and steam turbines can satisfy the factories energy requirements with excess sold to the grid. 相似文献
6.
Alfassi ZB Zlatin T Manor O Dubinsky S German U 《Radiation protection dosimetry》2004,110(1-4):207-211
Most high-efficiency interactions for neutron detection are with thermal neutrons, and the detection of the prompt gamma radiation emitted as a result of thermal neutron absorption in specific materials was sometimes used. These materials are named converters, the most widespread being 10B, 113Cd or 73Ge (the last two materials are detector materials themselves for CdZnTe and HPGe detectors). However, for these materials, the main gamma-ray energies are relatively low (in the region 473-600 keV), and there are interferences from other gamma rays present in background. Different combinations of moderator-converter-detector systems were employed to optimise the system efficiency and signal-to-background ratio, using B, Cd, Gd and Cl converters and HPGe, NaI(Tl) and BGO detectors. When using a Cl converter, the 1164.7 keV peak seems to be a good choice when a HPGe spectrometer is used. A very practical material containing chlorine is PVC. 相似文献
7.
The ratio of peak 3 to the sum of peaks 4 + 5 in TLD-100 was measured for various pre-irradiation and post-irradiation time periods, under conditions characteristic of routine personal dosimetry. It was confirmed that the value of this ratio depends only on the elapsed time between the prior readout and the present one, independent of the moment when the irradiation took place during the total time interval (storage time). This effect indicates that fading of peak 3 seems to be due mainly to changes in the unoccupied traps, and not to decay of trapped charges, being almost independent of the presence of electrons or holes in the traps. This observation leads to the conclusion that the suggestions in the past to use the decay of peak 3 in TLD-100 for the measurement of the elapsed time between irradiation and readout may have been wrong. On the other hand, the decay of peak 2 can be used to measure the elapsed time from irradiation, since the rate of decay is different when related to pre-irradiation and post-irradiation times, indicating a much higher decay rate of the trapped charges (Randall-Wilkins decay). However, because of the fast decay rate of peak 2, its use for determination of the elapsed time since irradiation is of little practical significance. 相似文献
8.
Birgit J.M. Hausmann Mughees Khan Yinan Zhang Tom M. Babinec Katie Martinick Murray McCutcheon Phil R. Hemmer Marko Lončar 《Diamond and Related Materials》2010,19(5-6):621-629
We present a design and a top-down fabrication method for realizing diamond nanowires in both bulk single crystal and polycrystalline diamond. Numerical modeling was used to study coupling between a Nitrogen Vacancy (NV) color center and optical modes of a nanowire, and to find an optimal range of nanowire diameters that allows for large collection efficiency of emitted photons. Inductively coupled plasma (ICP) reactive ion etching (RIE) with oxygen is used to fabricate the nanowires. Drop-casted nanoparticles (including Au, SiO2 and Al2O3) as well as electron beam lithography defined spin-on glass and evaporated Au have been used as an etch mask. We found Al2O3 nanoparticles to be the most etch resistant. At the same time FOx e-beam resist (spin-on glass) proved to be a suitable etch mask for fabrication of ordered arrays of diamond nanowires. We were able to obtain nanowires with near-vertical sidewalls in both polycrystalline and single crystal diamond. The heights and diameters of the polycrystalline nanowires presented in this paper are ≈ 1 μm and 120–340 nm, respectively, having a 200 nm/min etch rate. In the case of single crystal diamond (types Ib and IIa) nanowires the height and diameter for different diamonds and masks shown in this paper were 1–2.4 μm and 120–490 nm with etch rates between 190 and 240 nm/min. 相似文献
9.
The Liar, Context and Logical Form 总被引:1,自引:0,他引:1
Lon A. Berk 《Journal of Logic, Language and Information》2004,13(3):267-286
This essay attempts to give substance to the claim that the liar'sparadox shows the truth predicate to be context sensitive. The aim ismodest: to provide an account of the truth predicate's contextsensitivity (1) that derives from a more general understanding ofcontext sensitivity, (2) that does not depend upon a hierarchy ofpredicates and (3) that is able to address the liar's paradox. Theconsequences of achieving this goal are not modest, though. Perhapssurprisingly, for reasons that will be discussed in the last section ofthis essay, a natural account of the truth predicate's contextsensitivity appears to lead naturally to a version of the correspondencetheory of truth according to which the truth predicate can be understoodas a relation holding between a sentence and a salient set of contexts.The plan of this essay is as follows. Section 1 contains a generalaccount of context sensitivity. The purpose of this section is toisolate certain features of context sensitivity and formal methods oftreating them, which we will then apply to the truth predicate. Section 2then outlines two minimal conditions to be satisfied by a truthpredicate. In Section 3, I present a version of the liar paradoxthat results from these conditions and the assumption that the truthpredicate is not context sensitive in the sense described in sectionone. Finally, in section four, I provide what appear to be naturalconsequences of a truth predicate's context sensitivity. Section 4 isadmittedly speculative and points in the direction for future research. 相似文献
10.
This article offers a research update on a 3-year programme initiated by the Kamloops Art Gallery and the University College
of the Cariboo in Kamloops, British Columbia. The programme is supported by a ‘Community–University Research Alliance’ grant
from the Social Sciences and Humanities Research Council of Canada, and the collaboration focuses on the cultural future of
small cities – on how cultural and arts organisations work together (or fail to work together) in a small city setting. If
not by definition, then certainly by default, ‘culture’ is associated with big city life: big cities are equated commonly
with ‘big culture’; small cities with something less. The Cultural Future of Small Cities research group seeks to provide
a more nuanced view of what constitutes culture in a small Canadian city. In particular, the researchers are exploring notions
of social capital and community asset building: in this context, ‘visual and verbal representation’, ‘home’, ‘community’ and
the need to define a local ‘sense of place’ have emerged as important themes. As the Small Cities programme begins its second
year, a unique but key aspect has become the artist-as-researcher.
Correspondence and offprint requests to: L. Dubinsky, Kamloops Art Gallery, 101–465 Victoria Street, Kamloops, BC V2C 2A9 Canada. Tel.: 250-828-3543; Email: ldubinsky@museums.ca 相似文献