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排序方式: 共有188条查询结果,搜索用时 11 毫秒
81.
Dorin Dadarlat Mircea Nicolae Pop Mihaela Streza Stephane Longuemart Michael Depriester Abdelhak Hadj Sahraoui Viorica Simon 《International Journal of Thermophysics》2011,32(10):2092-2101
Photopyroelectric calorimetry in the front detection configuration (FPPE) and photothermal radiometry (PTR) were simultaneously
used, together with the thermal-wave resonator cavity method (TWRC), in order to investigate the thermal effusivity of solids
inserted as backing layers in a detection cell. A new combined FPPE–PTR–TWRC setup was designed. It was demonstrated experimentally
that the PTR technique, combined with the TWRC method, is able to provide calorimetric information about the third layer of
a detection cell. Applications on solids with different values of the thermal effusivity (starting from metals, down to thermal
isolators) are presented. The values of the thermal effusivity obtained with the PTR technique are similar to those obtained
with the PPE technique, and in agreement with literature values; the two methods reciprocally support each other. The accuracy
of both methods is higher when the values of the thermal effusivity of the backing layer and coupling fluid are close. 相似文献
82.
Domga Rodrigue K. Stanica Razvan Tchuente Maurice Valois Fabrice 《Wireless Networks》2019,25(8):4947-4962
Wireless Networks - In a multi-hop wireless sensor network with a convergecast communication model, there is a high traffic accumulation in the neighborhood of the sink. This area constitutes the... 相似文献
83.
The academic e-learning practice has to deal with various participation patterns and types of online learners with different support needs. The online instructors are challenged to recognize these and react accordingly. Among the participation patterns, special attention is requested by dropouts, which can perturbate online collaboration. Therefore we are in search of a method of early identification of participation patterns and prediction of dropouts. To do this, we use a quantitative view of participation that takes into account only observable variables. On this background we identify in a field study the participation indicators that are relevant for the course completion, i.e. produce significant differences between the completion and dropout sub-groups. Further we identify through cluster analysis four participation patterns with different support needs. One of them is the dropout cluster that could be predicted with an accuracy of nearly 80%. As a practical consequence, this study recommends a simple, easy-to-implement prediction method for dropouts, which can improve online teaching. As a theoretical consequence, we underline the role of the course didactics for the definition of participation, and call for refining previous attrition models. 相似文献
84.
Nicolae Serban Vasile-Danut Cojocaru Mihai Butu 《JOM Journal of the Minerals, Metals and Materials Society》2012,64(5):607-614
Equal-channel angular pressing (ECAP) is a very interesting method for modifying the microstructure in producing ultrafine-grained materials and nanomaterials. It consists mainly of pressing test samples through a die containing two channels that are equal in cross section and intersect at a certain angle. As a result of pressing, the sample theoretically deforms by simple shear and retains the same cross-sectional area to allow repeat pressing for several cycles. A 6063-T1 aluminum alloy was investigated in this study. The specimens were processed for up to nine passes (one, three, six, and nine ECAP passes, respectively) using a die channel angle of 90°. After ECAP, the samples were cut from each specimen and prepared for metallographic analysis and mechanical testing. The microstructures of the ECAP treated and as-received material were investigated using both optical microscopy and scanning electron microscopy. All samples (ECAP processed and as-received) were mechanically investigated in compression tests. Ultimate compressive strength, yield strength, and compression modulus were obtained. Also, all ECAP processed specimens were investigated for microhardness and compared with the as-received material. 相似文献
85.
Ioana Moleavin Silvia Grama Irina Cârlescu Dan Scutaru Nicolae Hurduc 《Polymer Bulletin》2010,65(1):69-81
Photosensitive micelles based on amphiphilic azo-polysiloxanes were obtained and characterized. The amphiphilic polymers were synthesized in a two-step reaction, starting from a polysiloxane containing chlorobenzyl groups in the side chain. In the first step, the polysiloxane was modified with azo-aromatic groups (35–45% substitution degree) and in the second step the unreacted chlorobenzyl groups were quaternized, using different tertiary amines. The structure of the polysiloxanes and their aggregation/disaggregation capacity were evaluated by 1H-NMR, DSC, fluorescence and UV–VIS spectroscopy, dynamic light scattering and transmission electron microscopy. All the amphiphilic polysiloxanes are capable of generating micelles, the critical concentration of the aggregation values being situated in the range 10−3–10−2 g/L. As a function of the ternary amine structure used in the quarterisation reaction, the micellar aggregation process is different, individual micelles or micellar clusters being obtained. The clusters' dimension cannot be controlled, the polydispersity index having high values. The disaggregation processes of the micelles under UV irradiation reveal that the polymer chemical structure influences the aggregates stability. A total micelles disaggregation was obtained for the polysiloxane modified with azophenol and amine containing a long hydrocarbon segment. In the other cases, only a partially disaggregation process takes place. 相似文献
86.
A fast,simple and accurate time-varying frequency estimation method for single-phase electric power systems 总被引:4,自引:0,他引:4
This paper presents a simple and accurate method to estimate time-varying frequency for single-phase electric power systems, based on three equally spaced samples. A sinusoidal voltage signal model, without dc offset, with time-varying frequency was assumed. Analytical formulas are derived. The method shows good estimation accuracy over a real world wide range of frequency changes. Simulations have been performed. 相似文献
87.
Radu Banica Paul Barvinschi Nicolae Vaszilcsin Terezia Nyari 《Journal of Alloys and Compounds》2009,483(1-2):402
Molybdenum oxides thin films electrochemical deposition was performed using solutions of peroxo-polymolybdate at pH 2.3 and ammonium molybdate at pH 5.5 as precursors and smooth copper and platinum as supports. The deposition has been carried out at constant potentials in the range of −600 to −800 mV vs. Ag/AgCl (sat. KCl). The thin films deposited on copper were then heated at 350 and 450 °C in argon. In the case of thin films deposited from ammonium molybdate and heated at 450 °C, the XRD spectra reveal, along with MoO2, the presence of Cu6Mo5O18 phase. For the thin films prepared from peroxo-polymolybdate and subjected to the same heat treatment, the only XRD phase present was MoO2. Thermogravimetric (TG) analysis was performed on samples prepared by scraping away the thin films (molybdate precursors) from the copper support. Before heat treatment, the AFM images of the as-deposited thin film reveal a granular morphology, with diameters in the 20–80 nm range. 相似文献
88.
V. Babin M. Popescu M. Ciobanu N. D. Nicolae C. Talianu G. Ciuciu 《Journal of Materials Science: Materials in Electronics》2006,17(4):307-314
It is proposed a quasi-linear algorithm for the solving the kinetic equations system, which describes the excitation and relaxation
processes in the electronic and phononic subsystems of amorphous chalcogenides. The nonlinear interaction between an optical
field and an amorphous chalcogenide medium is analyzed. The nonlinear interaction between an excitation incident optical field
and the non-crystalline chalcogenide materials, evidences the specific nonlinear phenomena in amorphous semiconductors, which
have no correspondent in the case of crystalline semiconductors. The model is limited to phenomena occurring in chalcogenide
semiconductors, simultaneously with the optical field propagation, by neglecting the excitation phenomena. 相似文献
89.
Tamara Slatineanu Mircea Nicolae Palamaru Ovidiu Florin Caltun Liviu Leontie 《Materials Research Bulletin》2011,46(9):1455-1460
Nanocrystalline powders of nickel substituted zinc ferrite with general formula NixZn1−xFe2O4 (x = 0, 0.2, 0.4, 0.6, 0.8, 1) have been synthesized via sol-gel auto-combustion method using tartaric acid as combustion-complexing agent. Samples were sintered at 773 K and 973 K in static air atmosphere. The absence of the organic phase and the spinel formation were monitored by using Fourier transform infrared spectroscopy. The structure and crystallite size were analyzed from X-ray diffraction data revealing spinel mono-phase formation in the range of nanometric crystallite size confirmed also through scanning electron microscopy. Mean size of crystallites lay in the range 20-40 nm. The influence of nickel content on the microstructure was investigated considering the crystallite size, distance between adjacent crystal planes, lattice parameter and porosity. The variation of magnetic properties of the samples was studied by using vibrating samples magnetometer and discussed considering the proposed cation distribution, relative bond angles and canting angles. The highest maximum value of the magnetization (63 emu/g) was found for Ni0.8Zn0.2Fe2O4. 相似文献
90.
Hae‐Ryong Kim Alexander Haensch Il‐Doo Kim Nicolae Barsan Udo Weimar Jong‐Heun Lee 《Advanced functional materials》2011,21(23):4456-4463
The humidity dependence of the gas‐sensing characteristics in SnO2‐based sensors, one of the greatest obstacles in gas‐sensor applications, is reduced to a negligible level by NiO doping. In a dry atmosphere, undoped hierarchical SnO2 nanostructures prepared by the self‐assembly of crystalline nanosheets show a high CO response and a rapid response speed. However, the gas response, response/recovery speeds, and resistance in air are deteriorated or changed significantly in a humid atmosphere. When hierarchical SnO2 nanostructures are doped with 0.64–1.27 wt% NiO, all of the gas‐sensing characteristics remain similar, even after changing the atmosphere from a dry to wet one. According to diffuse‐reflectance Fourier transform IR measurements, it is found that the most of the water‐driven species are predominantly absorbed not by the SnO2 but by the NiO, and thus the electrochemical interaction between the humidity and the SnO2 sensor surface is totally blocked. NiO‐doped hierarchical SnO2 sensors exhibit an exceptionally fast response speed (1.6 s), a fast recovery speed (2.8 s) and a superior gas response (Ra/Rg = 2.8 at 50 ppm CO (Ra: resistance in air, Rg: resistance in gas)) even in a 25% r.h. atmosphere. The doping of hierarchical SnO2 nanostructures with NiO is a very‐promising approach to reduce the dependence of the gas‐sensing characteristics on humidity without sacrificing the high gas response, the ultrafast response and the ultrafast recovery. 相似文献