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51.
Effect of Annealing Environment on the Crack Healing and Mechanical Behavior of Silicon Carbide-Reinforced Alumina Nanocomposites 总被引:4,自引:0,他引:4
Irene A. Chou Helen M. Chan Martin P. Harmer 《Journal of the American Ceramic Society》1998,81(5):1203-1208
The crack healing and strength behavior of an alumina-silicon carbide (Al2 O3 -SiC) nanocomposite (Al2 O3 + 5 vol% 0.2 μm SiC particles) has been studied, as a function of the crack size and the annealing environment. Results show that annealing treatments can significantly increase the indentation strength. The annealing atmosphere has a profound influence on the extent of crack healing and the degree of strength recovery. Annealing in argon results in a strength increase of 50%, whereas annealing in air yields a three-fold improvement in the indentation strength. Scanning electron microscopic observation has shown that healing of indentation cracks occurs in both environments, with the greater degree of healing occurring during annealing in air. Implications of the findings to the strengthening mechanism in Al2 O3 (SiC) nanocomposites will be discussed. 相似文献
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54.
John Bruley Junghyun Cho Helen M. Chan Martin P. Harmer Jeffrey M. Rickman 《Journal of the American Ceramic Society》1999,82(10):2865-2870
High-spatial-resolution analytical electron microscopy using energy-dispersive X-ray (EDX) and electron energy-loss spectrometry (EELS) of yttrium- and lanthanum-doped Al2 O3 has been conducted to ascertain the level of segregation of these impurities to grain boundaries. Line profile analyses indicate that the segregation is confined to a layer thickness of <3 nm. Similar amounts of excess solute have been observed in both dopant systems: 4.4 ± 1.5 and 4.5 ± 0.9 at./nm2 for yttrium and lanthanum, respectively. Assuming all the segregant is uniformly distributed within ±0.5 nm of the boundary, this excess corresponds to 9 ± 3 at.% for yttrium-doped Al2 O3 and 10 ± 2 at.% for lanthanum-doped Al2 O3 . For both dopant systems, examination of the spatially resolved electron energy-loss near-edge structures (ELNES) on the Al- L 2,3 edge suggests a loss in octahedral symmetry and a slight Al-O bond-length expansion. No significant change is noted in the O- K edge. 相似文献
55.
The superplastic behavior of YBa2 Cu3 O7− x ceramic superconductors was studied. Large compressive deformation over 100% strain was measured in the temperature range of 775°–875°C, with a strain rate of 1 × 10−5 to 1 × 10−3 /s, and a grain size of 0.5–1.4 μm. The nature of the deformation was investigated in terms of three deformation parameters: the stress exponent ( n ), the grain size exponent ( p ), and the activation energy ( Q ). The measured values of these parameters were n = 2 ± 0.3, p = 2.7 ± 0.7, and Q = 745 ± 100 kJ/mol. With the aid of the deformation map, the deformation mechanism was identified as grain boundary sliding accommodated by grain boundary diffusion. The conclusion is consistent with the microstructural observations made by SEM and TEM: the invariance of equiaxed grain shape, the absence of significant dislocation activity, no grain boundary second phases, and no significant texture development. 相似文献
56.
The dimerization of α-methylstyrene (AMS) 1 over highly dispersed strongly acidic ion exchange resins based on Nafion and
entrapped in a porous silica matrix has been investigated in the continuous liquid phase. Over these new catalysts the dimerization
rate is independent of the choice of solvent, which highlights the increased accessibility of the acid sites. Depending upon
the reaction conditions different dimerization products are obtained. Complete conversion is achieved even at very high space
velocities.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
57.
Edson R. Leite Adam M. Scotch Ajmal Khan Tao Li Helen M. Chan Martin P. Harmer Shi-Fang Liu Seung-Eek Park 《Journal of the American Ceramic Society》2002,85(12):3018-3024
Pb[(Mg1/3 Nb2/3 )0.65 Ti0.35 ]O3 (PMN–35PT) powder was prepared using the columbite precursor method. Fully dense compacts were formed by hot-pressing the powder at 950°C, and then the compacts were annealed at 1150°C for 5 and 10 h, respectively. Dielectric and piezoelectric properties of the as-hot-pressed and annealed samples were measured and correlated with microstructure. The as-hot-pressed material exhibited relaxor–ferroelectric-like behavior, with a relatively low dielectric constant maximum measured at 1 kHz ( K m@1kHz ) of 8160. Annealing resulted in a transition to weak normal-ferroelectric behavior, a shift in the dielectric maximum temperature from 190°C to 169°C, and a dramatic increase of K m@1kHz to a maximum value of 41 720 for the longer anneal. The as-hot-pressed microstructure was chemically heterogeneous, characterized by submicrometer-sized regions of varying magnesium, niobium, and titanium content that likely originated from chemical heterogeneities that were present in the as-prepared PMN-PT powder. The as-hot-pressed properties have been explained as being the integrated response of many discrete ferroelectric responses as dictated for each of these regions by the local chemistry. The transition on annealing has been explained in terms of chemical homogenization to a near-morphotropic phase-boundary composition that is intrinsically weak normal-ferroelectric. Differences in polarization-versus-electric-field and strain-versus-electric-field behavior between the hot-pressed and annealed materials have been discussed in terms of differences in domain mobility. 相似文献
58.
P. V. Jithesh P. Donachy T. Harmer N. Kelly R. Perrott S. Wasnik J. Johnston M. McCurley M. Townsley S. McKee 《Journal of Grid Computing》2006,4(2):209-222
The emergence of Grid computing technology has opened up an unprecedented opportunity for biologists to share and access data, resources and tools in an integrated environment leading to a greater chance of knowledge discovery. GeneGrid is a Grid computing framework that seamlessly integrates a myriad of heterogeneous resources spanning multiple administrative domains and locations. It provides scientists an integrated environment for the streamlined access of a number of bioinformatics programs and databases through a simple and intuitive interface. It acts as a virtual bioinformatics laboratory by allowing scientists to create, execute and manage workflows that represent bioinformatics experiments. A number of cooperating Grid services interact in an orchestrated manner to provide this functionality. This paper gives insight into the details of the architecture, components and implementation of GeneGrid. 相似文献
59.
Chong-Min Wang Junghyun Cho Helen M. Chan Martin P. Harmer Jeffrey M. Rickman 《Journal of the American Ceramic Society》2001,84(5):1010-1016
The microstructural features and tensile creep behavior of Al2 O3 doped with Nd2 O3 at levels ranging from 100 to 1000 ppm (Nd:Al atomic ratio) were systematically investigated. Compositional mapping, using both high-resolution scanning transmission electron microscopy and secondary ion mass spectroscopy revealed that, for all of the compositions studied, the Nd3+ ions were strongly segregated to the Al2 O3 grain boundaries. Microstructural observations revealed that the solubility of Nd2 O3 was between 100 and 350 ppm. Tensile creep tests were conducted over a range of temperatures (1200°–1350°C) and stresses (20–75 MPa). Both the stress and grain-size exponents were analyzed. In selected experiments, controlled grain-growth anneals were used to enable creep testing of samples of the same average grain size but different neodymium concentrations. Independent of dopant level, the neodymium additions decreased the creep rate by 2–3 orders of magnitude, compared with that of undoped Al2 O3 . The value of the apparent creep activation energy increased with increased dopant concentration and then saturated at dopant levels exceeding the solubility limit. Overall, the results of the present study were consistent with a creep-inhibition mechanism whereby oversized segregant ions reduce grain-boundary diffusivity by a site-blocking mechanism. 相似文献
60.
Several motion detection schemes are considered and their responses to noisy signals investigated. The schemes include the Reichardt correlation detector, shunting inhibition and the Horridge template model. These schemes are directionally selective and independent of the direction of change in contrast. They function by using spatial information and comparing it at successive time intervals. A rudimentary noise analysis is performed on the Reichardt and inhibition detectors to compare their natural robustness against noise. Using these detectors, stochastic resonance (SR) is applied, which is characterised by an improvement in response when noise is added to the input signal. It is found that the performance of the detectors degrades with the addition of noise. Employing Stocks' suprathreshold SR, an improvement can be gained when considering a network of detectors. Furthermore, when using an incorrect threshold setting for the template model, SR can be displayed. 相似文献