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81.
Fractal Characteristics of Fracture Surfaces 总被引:2,自引:0,他引:2
George R. Baran Claude Roques-Carmes Dohloul Wehbi Michel Degrange 《Journal of the American Ceramic Society》1992,75(10):2687-2691
Quantitative fractography is often used to study material failure mechanisms. During calculation of surface or profile roughness parameters, the magnification used in obtaining fractographic data is found to influence the value of the parameters. Fractal geometry has been developed into a tool capable of defining surface and profile topography without sensitivity to magnification, and several studies have related fractal dimension ( D F ) to other physical or mechanical properties. In this study, we obtained the fractal dimension of profiled fracture surfaces of one glass and three proprietary dental porcelains. The fracture toughness ( K 1c ) of these materials was also measured using the indentation-strength method. Results show the surfaces to be fractal. No quantitative relationship between fractal dimension and toughness was found. Differences in K 1c were demonstrated between some materials. It is postulated that the size range within which fractal dimension can be defined as constant is dependent on the toughening mechanism, and that the relationship between K Ic and D F cannot be identical for all materials. 相似文献
82.
Antoine T Priem B Heyraud A Greffe L Gilbert M Wakarchuk WW Lam JS Samain E 《Chembiochem : a European journal of chemical biology》2003,4(5):406-412
Two metabolically engineered Escherichia coli strains have been constructed to produce the carbohydrate moieties of gangliosides GM2 (GalNAcbeta-4(NeuAcalpha-3)Galbeta-4Glc; Gal = galactose, Glc = glucose, Ac = acetyl) and GM1 (Galbeta-3GalNAcbeta-4(NeuAcalpha-3)Galbeta-4Glc. The GM2 oligosaccharide-producing strain TA02 was devoid of both beta-galactosidase and sialic acid aldolase activities and overexpressed the genes for CMP-NeuAc synthase (CMP = cytidine monophosphate), alpha-2,3-sialyltransferase, UDP-GlcNAc (UDP = uridine diphosphate) C4 epimerase, and beta-1,4-GalNAc transferase. When this strain was cultivated on glycerol, exogenously added lactose and sialic acid were shown to be actively internalized into the cytoplasm and converted into GM2 oligosaccharide. The in vivo synthesis of GM1 oligosaccharide was achieved by taking a similar approach but using strain TA05, which additionally overexpressed the gene for beta-1,3-galactosyltransferase. In high-cell-density cultures, the production yields for the GM2 and GM1 oligosaccharides were 1.25 g L(-1) and 0.89 g L(-1), respectively. 相似文献
83.
We have studied the effect of the amount Φc of a reactive compatibilizer on the rheological properties of a polyethylene/polyamide blend, under steady shear and transient extensional flows. Here, we propose to describe the viscosity η(γ) and the first normal stress coefficient γ1(γ) using a Carreau‐type power‐law model, which is a three‐parameter model. A single model is sufficient to express the behavior of γ1(γ) On the other hand, the complete η(γ) curve is described by the superposition of two Carreau models, in relation to the presence of two relaxation mechanisms. Moreover, the extentional viscosity ηE(?), estimated using the end pressure drop observed in capillary flow experiments, is expressed by a two‐parameter power‐law model. 相似文献
84.
This paper reports experimental and numerical results of an investigation of five identical cogeneration systems using PEM (Polymer Exchange Membrane) fuel cells and running on natural gas. The natural gas is reformed locally to produce hydrogen. The accuracy of numerical results is validated by comparison with experimental data and the system performances are analysed in terms of electrical, thermal and total efficiencies. It appears that the energetic performances are low, particularly at low current. Simple solutions for enhancing the system electrical performances by modifying control laws are proposed. 相似文献
85.
The main contribution of the present article consists of new 3D parallel and symmetric thinning schemes which have the following qualities: They are effective and sound, in the sense that they are guaranteed to preserve topology. This guarantee is obtained thanks to a theorem on critical kernels; They are powerful, in the sense that they remove more points, in one iteration, than any other symmetric parallel thinning scheme; They are versatile, as conditions for the preservation of geometrical features (e.g., curve extremities or surface borders) are independent of those accounting for topology preservation; They are efficient: we provide in this article a small set of masks, acting in the grid ?3, that is sufficient, in addition to the classical simple point test, to straightforwardly implement them. 相似文献
86.
In this paper, deformation and breakup under simple shear of single molten polymer drops in a polymer matrix were investigated. Flow visualization was carried out in a Couette‐Flow apparatus under relatively high shear rates and temperatures up to 230°C. Drop/Matrix combinations were composed of polystyrene drops of 0.5–0.6 mm in diameter in polyethylene matrix, and ethylene–propylene copolymer drops of approximately the same size in polypropylene matrix. The deformation and breakup processes were studied under steady state and time‐dependent shearing conditions. Either for steady state or time‐dependant shearing conditions, drop elasticity generated at relatively high shear rates helped the drops to align perpendicular to the flow direction, i.e., parallel to vorticity axis. Also, the most striking non‐Newtonian effects for the high viscosity ratio systems were the surface erosion and the drop splitting mechanisms. The particles eroded off the main droplet surface were very fine, in the range of 10–50 μm, and led to a significant reduction in main drop size before its final breakup. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 2582–2591, 2006 相似文献
87.
NH3-NO/NO2 SCR for diesel exhausts after treatment: mechanism and modelling of a catalytic converter
Nova Isabella Ciardelli Cristian Tronconi Enrico Chatterjee Daniel Weibel Michel 《Topics in Catalysis》2007,42(1-4):43-46
We review herein the key mechanistic and kinetic features of the reactions involved in the NH3-NO/NO2 SCR system investigated by dynamic reactive experiments over a V-based commercial powdered catalyst, eventually leading to
the proposal of an original redox scheme which accounts for stoichiometry, selectivity and intrinsic kinetics of the global
SCR process. 相似文献
88.
L. Thimon F. Peyoux R. Maget-Dana G. Michel 《Journal of the American Oil Chemists' Society》1992,69(1):92-93
The interfacial behavior of antifungal lipopetides of the iturinic group was studied in comparison with that of surfactin,
an anionic lipopeptide. All these lipopeptides were isolated from various strains ofBacillus subtilis; each strain produced surfactin and one antifungal compound. The iturinic compounds differ from surfactin by their lower
surfactant properties. The critical micelle concentration (CMC) values were dependent on the nature of the peptide moiety
in the iturinic compounds. The highest values were observed for anionic antibiotics. The arrangement of lipopeptides at the
air-water interface was largely dependent on the size of the lipid moiety; surfactin, which has a C14 or C15 β-hydroxy fatty acid, iturins A, C, and bacillomycins D, L, which have a C14 or C15 β-amino fatty acid, occupied a smaller area than mycosubtilin and bacillomycin F, which have a C16 or C17 β-amino fatty acid. These data can be related to bioactivity of these lipopeptides. 相似文献
89.
Michel Keddam Christiane Rakotomavo Hisasi Takenouti 《Journal of Applied Electrochemistry》1984,14(4):437-448
When the impedance is measured on a battery, an inductive impedance is often observed in a high frequency range. This inductance is frequently related to the cell geometry and electrical leads. However, certain authors claimed that this inductance is due to the concentration distribution of reacting species through the pores of battery electrodes. Their argument is based on a paper in which a fundamental error was committed. Hence, the impedance is re-calculated on the basis of the same principle. The model shows that though the diffusion process plays an outstanding role, the overall reaction rate is never completely limited by this process. The faradaic impedance due to the concentration distribution is capacitive. Therefore, the inductive impedance observed on battery systems cannot be, by any means, attributed to the concentration distribution inside the pores. Little frequency distribution is found and the impedance is close to a semi-circle. Therefore depressed impedance diagrams in porous electrodes without forced convection cannot be ascribed to either a Warburg nor a Warburg-de Levie behaviour.Nomenclature
A
D¦C¦ (mole cm s–1)
-
B
j+K¦C¦ (mole cm s–1)
-
b
Tafel coefficient (V–1)
-
C(x)
Concentration ofS in a pore at depthx (mole cm–3)
-
C
0
Concentration ofS in the solution bulk (mole cm–3)
- C C(x)
change under a voltage perturbation (mole cm–3)
- ¦C¦
Amplitude of C (mole cm–3)
-
D
Diffusion coefficient (cm2 s–1)
-
E
Electrode potential (V)
- E
Small perturbation inE namely a sine-wave signal (V)
- ¦E¦
Amplitude of E(V)
-
F
Faraday constant (96500 A s mol–1)
-
F(x)
Space separate variable forC
-
f
Frequency in Hz (s–1)
-
g(x)
KC(x)¦E¦(mole cm s–1)
-
I
Apparent current density (A cm–2)
-
I
st
Steady-state current per unit surface of pore aperture (A cm–2)
- j
Imaginary unit [(–1)1/2]
-
K
Pseudo-homogeneous rate constant (s–1)
-
K
Potential derivative ofK, dK/dE (s–1 V–1)
-
K
*
Heterogeneous reaction rate constant (cm s–1)
-
L
Pore depth (cm)
-
n
Reaction order
- P
Reaction product
-
p
Parameter forF(x), see Equation 13
-
q
Parameter forF(x), see Equation 13
-
R
e
Electrolyte resistance (ohm cm)
-
R
p
Polarization resistance per unit surface of pore aperture (ohm cm2)
-
R
t
Charge transfer resistance per unit surface of pore aperture (ohm cm2)
- S
Reacting species
-
S
a
Total surface of pore apertures (cm2)
-
S
0
Geometrical surface area
-
S
p
Developed surface area of porous electrode per unit volume (cm2 cm–3)
-
s
Concentration gradient (mole cm–3 cm–1)
-
t
Time
-
U
Ohmic drop
-
x
Distance from pore aperture (cm)
-
Z
Faradaic impedance per unit surface of pore aperture (ohm cm2)
-
Z
x
Local impedance per unit pore length (ohm cm3)
-
z
Charge transfer number
-
Porosity
-
Thickness of Nernst diffusion layer
-
Penetration depth of reacting species (cm)
-
Penetration depth of a.c. signal determined by the potential distribution (cm)
-
Electrolyte (solution) resistivity (ohm cm)
-
0
Flow of S at the pore aperture (mole cm2 s–1)
-
Angular freqeuncy of a.c. signal, 2f(s–1)
-
Integration constant 相似文献
90.
The microactivity test (MAT) for cracking catalyst test works with an approximately plug flow isothermal reactor. The yields at the outlet of such a reactor is numerically computed in the case of lumped kinetic with large molecular expansion and rapid catalyst deactivation expressed versus poison concentration. The poison (coke in cracking reaction) is considered as a product formed by several reaction routes. The results are applied to catalytic cracking kinetic with a four lump model (feedstock, gasoline, gas, coke). The comparison between experimental and computed yields, permits the adjustment of kinetic constants with a set of experimental results obtained from a laboratory scale reactor derived from the MAT. The simulation of concentration profile in the reactor permits a better understanding of the reaction courses. 相似文献