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排序方式: 共有184条查询结果,搜索用时 31 毫秒
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Dongming Hu Radovan Kovacevic 《International Journal of Machine Tools and Manufacture》2003,43(1):419
Laser-Based Additive Manufacturing (LBAM) is a promising manufacturing technology that can be widely applied to part preparation, surface modification, and Solid Freeform Fabrication (SFF). A large number of parameters govern the LBAM process. These parameters are sensitive to the environmental variations, and they also influence each other. This paper introduces the research work in RCAM on improving the performance of the LBAM process. Metal powder delivery real-time sensing and control is studied to achieve a controllable powder delivery for fabrication of functionally graded material. A closed-loop control system based on infrared image sensing is built for control of the heat input and size of the molten pool in the LBAM process. The closed-loop control results show a great improvement in the geometrical accuracy of the built features. A three-dimensional finite element model is also established to explore the thermal behavior of the molten pool in the closed-loop controlled LBAM process. 相似文献
3.
Radovan Novotný Radek Vlach Jaroslav Kadlec Radek Kuchta 《Microsystem Technologies》2014,20(4-5):719-728
Ceramics joints are applied for producing products that should be made in a general shapes and dimensions for more advantageous usage. The article presents the research work related to ceramic joint quality evaluation, the thermal-structural analysis of ceramic joining and ceramic bond design and implementation. The role of ceramic material in the electronics industry and motivation for joining ceramics is described in the introduction. The requirements and methods for improving the quality of joints are summarized. Also, the results of simulations of pressure sensor cooling after removal from the oven during joining are discussed. The experimental results are evaluated by using a t-test before and after process cooling modification. Important directions for future research are summarized, with emphasis on the statistical determination of poor connection, and how the interface of modification of joint technology and process setting affects results and parameters that have been achieved. 相似文献
4.
Discussions about the achievements and limitations of the various approaches to the development of intelligent systems can have an essential impact on empirically based research, and with that also on the future development of computer technologies. However, such discussions are often based on vague concepts and assumptions. In this context, we claim that the proposed `three-world ontology' offers the most appropriate conceptual framework in which the basic problems concerned with cognition and computation can be suitably expressed and discussed, although the solutions of some of these problems seem to lie beyond the horizon of our current understanding. We stress the necessity to differentiate between authentic and functional cognitive abilities; although computation is not a plausible way towards authentic intelligence, we claim that computational systems do offer virtually unlimited possibilities to replicate and surpass human cognitive abilities on the functional level. 相似文献
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Prabu Balu Perry Leggett Radovan Kovacevic 《Journal of Materials Processing Technology》2012,212(7):1598-1610
The manner with which the composite powder particles injected into the laser formed molten pool decides the deposition quality in a typical laser-based powder deposition of composite material. Since, the morphology and physical properties of nickel (Ni) and tungsten carbide (WC) are different their powder flow characteristics such as the powder particles stream structure, maximum concentration at the converging spot, and the powder particles velocity are noticeably different. In the current study, a computational fluid dynamics (CFD) based powder flow model is established to characterize the coaxial powder flow behavior of Ni–WC composite powders. The key powder flow characteristics such as the stand-off distance, the diameter of the powder stream at the stand-off distance, and the velocity of the powder particles are measured using three different vision based techniques. Both the numerical and experimental results reveal the exact stand-off distance where the substrate needs to be placed, the diameter of the concentration spot of powder at the stand-off distance, and a combination of suitable nozzle angle, diameter, and carrier gas flow rate to obtain a maximum powder concentration at the stand-off distance with a stable composite powder flow. 相似文献
7.
Wei Liu Junjie MaGuang Yang Radovan Kovacevic 《Journal of Materials Processing Technology》2014,214(12):2823-2833
This study investigated the synergetic effect between laser beam and electrical arc during hybrid welding by using a spectral diagnostic technique. The synergetic effect increased the energy density in the keyhole and deepened the weld penetration, resulting in a lower plasma electron temperature. The metal transfer mode was a globular one at a small offset distance while a spray mode was achieved with an increase in the offset distance. The decrease in the arc voltage and arc current due to the synergetic effect caused this transition from spray to globular modes. Globular transfer mode destabilized the molten pool and keyhole with the large droplet impingement, leading to the formation of porosity in the corresponding weld bead. The presence of porosity was on-line detected by identifying serious fluctuations in the Fe I electron temperature signals based on the fact that the instability of the molten pool and keyhole is strongly related to the signals coming from the plasma. 相似文献
8.
Srdja Zekovic Rajeev Dwivedi Radovan Kovacevic 《International Journal of Machine Tools and Manufacture》2007,47(1):112-123
Laser-based direct metal deposition (DMD) is a solid freeform fabrication process capable of fabricating fully dense and metallurgically sound parts. The process has been greatly enhanced toward multi-directional deposition by the use of discontinuous radially symmetrical powder nozzles to supply the build material. Since many operational parameters depend on the gas–powder stream characteristics between the nozzles and the deposition point, an extensive understanding of the gas–powder flow is necessary. Three-dimensional (3D) multi-phase gas–powder flow structures of radially symmetrical nozzles are modeled using computational fluid dynamics methods. The obtained results are in good agreement with the experimental ones, and they provide a good insight into the process phenomena. 相似文献
9.
Toxicity increases in ice containing monochlorophenols upon photolysis: environmental consequences 总被引:4,自引:0,他引:4
Bláha L Klánová J Klán P Janosek J Skarek M Růzicka R 《Environmental science & technology》2004,38(10):2873-2878
The toxic effects of photoproducts formed upon the photolysis of 2- and 4-chlorophenol (CP) frozen solutions in polycrystalline ice phase were determined with a bacterial luminescence test (Vibrio fisheri), and in vitro biomarker assay for dioxin-like effects (inductions of AhR-dependent luciferase in H4IIE-luc cells) and compared to the toxic effects of products of the same photoreaction in aquatic phase. Coupling photoproducts formed in ice samples (3'-chlorobiphenyl-2,4'-diol and 3-chlorobiphenyl-2,2'-diol from 2-CP photolysis and 5-chlorobiphenyl-2,4'-diol from 4-CP photolysis) were found to be more toxic to V. fisheri than parent CPs and elicited significant inductions of dioxin-like effects (the effective concentrations EC50 approximately 3 x 10(-5) mol L(-1) corresponded to known weaker ligands of AhR, such as nonplanar polychlorinated biphenyls or polycyclic aromatic hydrocarbons). To complete the picture, a photoproduct formed from 4-CP (5-chlorobiphenyl-2,4'-diol) was synthesized, and a detailed toxicity assessment with purified compound confirmed the results obtained with irradiated samples. Our findings support a recently proposed model according to which solar radiation can trigger the formation of new types of organic pollutants in polar ice or tropospheric ice cloud particles, presenting possibly greater risk to the environment than the parent compounds. 相似文献
10.
Dysbaric osteonecrosis is a major hazard to compressed air workers and an increasing hazard to divers. Symptomless osteonecrosis is best recognised by radiology and it is of fundamental importance that interpretation be reliable and accurate. There is much controversy concerning the significance of bone islands and cystic areas; several authors report bone islands and cystic areas to be up to eight times more frequent in compressed air workers and divers and believe that these represent positive evidence of osteonecrosis. Skeletal radiographs of 100 Royal Navy divers, all of whom were at risk to osteonecrosis were matched for age and rank with 100 controls, none of whom had been exposed to high pressures. The radiographs were interpreted by three radiologists working independently without knowing whether the radiographs were in the diving or control group. It is concluded that bone islands, cystic areas and sclerotic areas, as described in this text, occur with equal frequency in divers as in the normal population and do not represent positive evidence of osteonecrosis. The report includes autopsy studies of the proximal femur and proximal humerus in six diving fatalities. Five bone islands in the proximal humerus and one in the femoral neck were studied in bones removed from five divers. In the sixth case a cyst was seen to develop radiologically in the femoral neck and then reduce in size over 3 years and 10 months. Slab radiographs and histological section in all cases show no evidence of osteonecrosis. 相似文献