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41.
42.
The thermal shock behavior of an alumina monolith and two alumina–iron ceramic-matrix composites has been investigated by superimposing the measured K R -curves of the materials onto the theoretically generated curves of the thermally induced stress intensity factor. Predictions of the critical-temperature differentials and retained strengths after quenching are in good agreement with the experimental data. The inclusion of metallic particles into an alumina matrix improves the thermal shock resistance, although the increase in toughness is not solely responsible for this improvement. There is a decrease in thermal stress-intensity factor that is generated for the composites; this decrease is due to a reduction in the Young's modulus and/or Biot modulus. However, the increased toughness for large crack lengths may offer increased damage resistance for severe thermal shock treatments. 相似文献
43.
A.J. Harris B. Vaughan J.A. Yeomans P.A. Smith S.T. Burnage 《Journal of the European Ceramic Society》2013,33(15-16):2925-2934
Surface treatments of silicon carbide have been investigated with the aim of improving the strength of the bond between the ceramic and an epoxy adhesive. Three surface conditions have been characterised; as-fired, air re-fired and KrF laser processed. A number of characterisation techniques have been used to determine the morphological and chemical changes that have occurred to the surface. Scanning electron microscopy of the re-fired and laser processed samples showed surfaces that appeared glassy, with the laser processed surface showing a different morphology. X-ray photoelectron spectroscopy indicated both treatments had oxidised the surface and the laser processed surface also had a greater concentration of hydroxyl groups. The wettability of both surfaces had improved and the laser processed surface was found to be highly hydrophilic. Mechanical testing of joints prepared with this technique showed them to have the highest strength in tension, with the locus of failure being cohesive. 相似文献
44.
The acoustic startle reflex (ASR) in rats is attenuated by a light paired with food or, in humans, by "pleasant" pictures. Rats were trained to barpress for lateral hypothalamus (LH) stimulation. ASR amplitudes were then measured at 4 intensities, with or without a light. Control rats that did not receive brain-stimulation reward (BSR) showed initially lower ASR amplitudes than did rats exposed to BSR, but both groups responded similarly with or without light. Next, experimental rats were given BSR in the presence of light but not in its absence. After conditioning, ASR amplitudes were reduced, and ASR thresholds were raised by a mean of 2.6 dB in the light but remained at preconditioning levels without light. No such change was found for control rats or rats with placements outside the LH. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
45.
46.
M Kohler BE King NR Stevenson RB Schubank YM Shin RA Ristinen P Amaudruz PP Delheij DC Healey BK Jennings DF Ottewell G Sheffer GR Smith GD Wait JT Brack A Feltham M Hanna RR Johnson FM Rozon V Sossi D Vetterli P Weber N Grion R Rui EL Mathie R Tacik M Yeomans CA Gossett GJ Wagner JM Lee KS Chung 《Canadian Metallurgical Quarterly》1994,49(3):1715-1717
47.
This article presents a study of business education in schools in England and Wales. Its aim is to show three things. First, how, as business education in schools was reconstituted in the 1980s and 1990s, it became imbued with discourses through which syllabus content, teacher perspectives and teacher-student interactions were structured and channelled in particular ways so as to attempt to produce certain kinds of vocational identities congruent with and supportive of an emerging culture of enterprise. Secondly, the article then makes visible through a detailed account of a business studies lesson how this process inverted students' lived relations with the adult world through a problem-solving exercise on 'Retailing and Security' that situated them as business decision-makers. Last, we show that the effects of this process re-defined students' lived connections with the outside world and reproduced a particular kind of identity. In this respect, the ethnography attempts to show how processes of ideological reproduction and identity formation were constituted in the ongoing activities of teachers and students, organised under particular circumstances and, in particular ways, as business education within the 14-18 secondary curriculum. 相似文献
48.
49.
A range of Al2O3-Cr and Al2O3-Cr/Ni composites have been made using either pressureless sintering in the presence of a graphite bed or hot pressing. Examination of the microstructures shows that they are fully dense (typically 98–99% of the theoretical density) and that the micrometre-scale metallic particles remain discrete and homogeneously dispersed in all composites. All of the hot pressed specimens had higher flexural strengths than the sintered materials. Within each processing route, the composites had slightly lower strength values than the equivalent monolithic alumina specimens. This was attributed to weak interfacial bonding. Fracture toughness behaviour was investigated using indentation and double cantilever beam methods. All of the composites were found to be tougher than the parent alumina and to show resistance-curve behaviour. For the composites, maximum fracture toughness values were 5–6 MPa m1/2 (about double the value for alumina) for process zone sizes of a few millimetres, although steady state was not reached in the limited number of specimens tested. Examination of fracture surfaces and indentation cracks showed that the toughening potential of the metal particles was not exploited to any significant extent. This was mainly due to weak metal-Al2O3 interfaces, but also because of carbon embrittlement of the metallic particles in which chromium was the major constituent. 相似文献
50.
Quantitative properties of the neural system mediating the rewarding and priming effects of medial forebrain bundle (MFB) stimulation in the rat have been determined by experiments that trade one parameter of the electrical stimulus against another. The 1st order neurons in this substrate are generally long, thin, myelinated axons, coursing in the MFB and ventral tegmentum, with absolute refractory periods in the range .5–2.2 msec and conduction velocities of 2–8 m/sec. Local potentials in these axons decay with a time constant of about .1 msec. A supernormal period follows the recovery from refractoriness. These axons integrate current over exceptionally long intervals, accommodate slowly, and fire on the break of prolonged anodal pulses. These properties rule out the hypothesis that catecholamine pathways constitute the 1st-order axons. The 2nd-order (postsynaptic) part of the substrate shows surprisingly simple spatial and temporal integrating characteristics. The authors examine the logic that permits conclusions of this sort to be derived from behavioral data and the role of these derivations in establishing neurobehavioral linkage hypotheses. (78 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献