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1.
Reaction-bonded Si3N4 (RBSN) made from high-purity Si powder is unusually resistant to degradation caused by exposures to air for up to 50 h at temperatures up to 1400°C. The weight gain during oxidation of this SiH4-originating RBSN is approximately 10 times less than conventional RBSN. Contrary to normally observed strength degradations, room-temperature strengths of this high-purity, oxidized RBSN (avg = 435 MPa, max. = 668 MPa) remained at their unusually high, as-processed levels after 1000° and 1400°C oxidizing exposures. Fracture toughness values were unaffected by oxidation ( K IC= 2.3 to 2.4 MPa · m1/2). This superior oxidation resistance results from the high purity and the small diameter pore channels (0.01 to 0.06 μm) achieved in this SiH4-originating RBSN.  相似文献   
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Economic forces, competitive pressures and technological advances have created an environment within which firms have developed new ways of organizing (e.g. virtual work settings) and managing their resources (e.g. knowledge management) in order to maintain and improve firm performance. Extant research has highlighted the challenges associated with managing knowledge in virtual settings. However, researchers are still struggling to provide effective guidance to practitioners in this field. We believe that a better understanding of individual virtual competency is a potential avenue for managing the complexity of knowledge transfer in virtual settings. In particular, we suggest that optimal knowledge transfers can be achieved by individuals armed with the right personal capabilities and skills for virtual work, particularly when those knowledge transfers are emergent, bottom-up and cannot be specified a priori. The virtual competency exhibited by individuals can be the key to overcoming the constraints of knowledge transfers with such characteristics because underlying competency can facilitate effective action in unfamiliar and novel situations. In this conceptual research, we develop a theoretical model of individual virtual competence and describe its role in the communication process, which underpins effective knowledge transfer in virtual settings. Additionally, we consider the antecedent role that prior experience in virtual activity plays in aiding workers to develop virtual competence, which in turn engenders effective knowledge transfer. We conclude with implications for future research and for practicing managers.  相似文献   
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Family risk-related variations in proximal parent and young adolescent outcomes of a universal family-focused preventive intervention were examined using a cumulative index of risk incorporating sociodemographic characteristics and social-emotional adjustment measures. Results of an initial investigation involving 209 families of young adolescents suggested that intervention efficacy was largely unrelated to cumulative family risk. These findings were replicated with a second sample of 428 families. Implications for future intervention applications and outcome research are discussed.  相似文献   
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A major threat to the information economy is denial-of-service (DoS) attacks. These attacks are highly prevalent despite the widespread deployment of perimeter-based countermeasures. Therefore, more effective approaches are required to counter the threat. This requirement has motivated us to propose a novel, distributed, and scalable mechanism for effective early detection and prevention of DoS attacks at the router level within a network infrastructure. This paper presents the design details of the new mechanism. Specifically, this paper shows how the mechanism combines both stateful and stateless signatures to provide early detection of DoS attacks and, therefore, protect the enterprise network. More importantly, this paper discusses how a domain-based approach to an attack response is used by the mechanism to block attack traffic. This novel approach enables the blockage of an attack to be gradually propagated only through affected domains toward the attack sources. As a result, the attack is eventually confined within its source domains, thus avoiding wasteful attack traffic overloading the network infrastructure. This approach also provides a natural way of tracing back the attack sources, without requiring the use of specific trace-back techniques and additional resources for their implementation.  相似文献   
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Directional melting and solidification of Ba2YCu3O6+8 in low oxygen partial pressures were investigated by the laser-heated floating zone (LHFZ) technique. The following reaction was observed on melting at Po2 > 8 Pa: Ba2YCu3O6+8→ BaY2CuO5 + L where BaY2CuO5 phase comprised equiaxed particles. At < 8 Pa < Po2 < 103 Pa, the preceding reaction occurred, and at higher temperatures, the following reaction also occurred to produce a rodlike BaY2O4 phase, oriented in the growth direction: BaY2Cu05 ar BaY2O4 + L At pressures below about 8 Pa, the low temperature reaction observed was Ba2YCu2O6+8 → BaY2O4 + L In this case, the oriented BaY2O4 phase formed directly from Ba2YCu2O6+8 and was platelike in morphology. Directional growth of the BaY2O4 phase from BaY2CuO5 was modeled in a simple way. The BaY2Cu05 particles ahead of the BaY2O4 interface dissolve in the superheated liquid, and solute transport for BaY2O2 growth is assumed to be limited by diffusion along the growth direction over distances equal to half the rod or plate spacing. A relation between supereating at the growth interface and growth rate was derived from the present model. Some possibilities for making use of such aligned structures in the growth of the superconducting compound Ba2YCu3O6+8 are discussed.  相似文献   
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The four subunits (alpha, beta, gamma, delta) of the acetylcholine receptor from Torpedo californica have been isolated by preparative gel electrophoresis in sodium dodecyl sulfate. After removal of the sodium dodecyl sulfate by dialysis of the polypeptides against a cholate-containing buffer, the alpha subunit, but not the other chains, binds 125I-alpha-bungarotoxin in a saturable manner. The binding affinity, 0.1-0.2 microM, is approximately 10(4)-fold lower than that observed for native acetylcholine receptor. For three preparations of alpha subunit, 1 mol of subunit bound 0.87, 0.38, and 0.33 mol of 125I-alpha-bungarotoxin at saturation. The binding was inhibited by cholinergic ligands, although the apparent affinities of these ligands for alpha were 50-100-fold lower than that found for the native receptor. These results indicate that at least part of the alpha-bungarotoxin binding site resides on the alpha subunit.  相似文献   
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