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排序方式: 共有882条查询结果,搜索用时 15 毫秒
51.
The effect of the quenching medium and of varying the microstructure on low cycle fatigue behaviour of aluminium alloy RR58 has been investigated at 423K. It is observed that fatigue resistance is significantly impaired if, following solutionizing, the alloy is quenched in water instead of in oil. Thermal as well as thermomechanical treatments were employed to produce different microstructures. The overaged microstructure displayed the highest fatigue life. It is shown with the help of fractographic observations that differences in fatigue resistance due to differences in severity in quenching or in microstructure arise mainly due to the influence of these variables on crack initiation and early crack propagation. It has also been demonstrated that as the rate of hardening or softening is increased, cyclic strength coefficient K′ and fatigue hardening exponent n′ increase; the fatigue ductility coefficient ?′f decreases as a result of varying microstructures, and the fatigue resistance at 423K of the alloy is lowered. When compared with the behaviour at ambient temperature, the test temperature of 423K has been found to have no significant effect on the fatigue life/strain plot. 相似文献
52.
Mass transfer and biochemical reaction in enzyme membrane reactor systems—I. Single enzyme reactions
W.R. Vieth A.K. Mendiratta A.O. Mogensen R. Saini K. Venkatasubramanian 《Chemical engineering science》1973,28(4):1013-1020
The effective use of an enzyme as a biological catalyst for process scale conversion is greatly enhanced by immobilizing the enzyme within the matrix of a membrane whose walls are permeable to the substrate and product. The enzyme is thus stabilized for reuse, but the apparent activity is reduced compared to that obtained with the free enzyme because of transport resistances in the bulk fluid near the membrane and within the membrane itself. To account for this reduction, solutions to the differential equations describing the mass transport-kinetic models can be obtained for certain important cases. The reaction is considered to be first-order and isothermal, and to occur in one of two modes of operation: fixed-bed or batch (limited volume). Irreversible and reversible reactions and plane sheet and spherical geometries of the membrane are considered. An example of the application of these solutions to actual data obtained with a laboratory scale immobilized enzyme reactor is presented. 相似文献
53.
In this paper we present and evaluate algorithms to address combined path and server selection (CPSS) problems in highly dynamic multimedia environments. Our goal is to ensure effective utilization of network and server resources while tolerating imprecision in system state information. Components within the framework implement the optimized scheduling policies as well as collect/update the network and server parameters using a directory service. We present and analyze multiple policies to solve the CPSS problem. In addition, we study multiple techniques for updating the directory service with system state information. We further evaluate the performance of the CPSS policies under different update mechanisms and study the implications of the CPSS policies on directory service management. 相似文献
54.
K.V.Rama Rao 《International Journal of Engineering Science》1980,18(5):741-750
In the present paper we have considered thermal instability in a heat conducting micropolar fluid layer under the influence of a transverse magnetic field. Assuming the bounding surfaces to be rigid the eigenvalue problem is solved using finite-difference and Wilkinson's iteration techniques. Here it is seen that the instability sets in not only for adverse temperature gradient but also for positive temperature gradient. Both the microtation and the magnetic field are seen to stabilize the fluid layer. However, the stabilizing effect of microrotation becomes less significant when the strength of the magnetic field is large. In the case of heating from below, the critical wave number is seen to be insensitive to increase in the strength of the magnetic field, while it increases significantly when the fluid is heated from above. 相似文献
55.
Chebrolu Rama Mohan Rao Pratap N. Mehrotra 《Journal of chemical technology and biotechnology (Oxford, Oxfordshire : 1986)》1979,29(9):578-580
The object of the present work was to illustrate the effect of impurities on the kinetics and energetics of the thermal decomposition of pure cobalt oxalate. Cationic impurities like Mg2+, Mn2+, Fe2+, Ni2+, Cu2+, Zn2+, Cd2+, Pb2+, Zr4+ have been introduced into pure cobalt oxalate dihydrate and differential thermal analysis has been used to determine the enthalpy and energy of activation for the decomposition reaction of pure and doped samples. 相似文献
56.
S. Pandian V. Chandrasekaran K. J. L. Iyer K. V. S. Rama Rao 《Journal of Materials Science》2001,36(24):5903-5907
Sintered samples of (Nd14.9 Dy1.9) (Fe65 Co8 Cu1.0 Ga1.0 Nb0.7)B7.5 were prepared and subjected to stepwise annealing in the temperature range 875 K–675 K. The XRD and metallographic (optical and electron microscopy) studies reveal a multi-phase microstructure with each phase showing different solubility of the alloying additions. This alloy has T
C of 705 K with an intrinsic coercivity of 1000 kA/m and energy product of 250 kJ/m3 at RT. Solubility of Co into the matrix phase and that of Ga and Cu into the Nd-rich grain boundary phase are considered to be the main contributing factors for the significant enhancement in T
C and H
ci respectively of the multi-component alloy when compared to those of ternary NdFeB, wherein H
ci = 720 kA/m and T
C = 585 K. 相似文献
57.
58.
We report the fluorescence properties of Sm3+-doped lead telluroborate glasses of composition PbF2.TeO2.H3BO3.Sm2O3 as a function of Sm3+ concentration. A Judd-Ofelt scheme was used to determine the intensity parameters and radiative properties of Sm3+ ion. The emission and decay measurements were carried out at 402 nm excitation. Beyond 1.0 mol% Sm3+ concentration, the luminescence quenching is observed. The decay curves of 4G5/2 level are well fitted to a single exponential function. The evaluated radiative properties suggest that the 4G5/2 → 6H7/2 transition is responsible for reddish-orange luminescence which might be used in the development of visible lasers. 相似文献
59.
Mahesha B. Herath Stephen E. Creager Rama V. Rajagopal Olt E. Geiculescu Darryl D. DesMarteau 《Electrochimica acta》2009,54(24):5877-5883
We report synthesis, characterization and ion transport in polyether-based ionic melt electrolytes consisting of Li salts of low-basicity anions covalently attached to polyether oligomers. Purity of the materials was investigated by HPLC analysis and electrospray ionization mass spectrometry. The highest ionic conductivity of 7.1 × 10−6 S/cm at 30 °C was obtained for the sample consisting of a lithium salt of an arylfluorosulfonimide anion attached to a polyether oligomer with an ethyleneoxide (EO) to lithium ratio of 12. The conductivity order of various ionic melts having different polyether chain lengths suggests that at higher EO:Li ratios the conductivity of the electrolytes at room temperature is determined in part by the amount of crystallization of the polyether portion of the ionic melt. 相似文献
60.
Low-stress creep behaviour of a two-phase Zr–2.5%Nb alloy, differently heat treated, has been investigated using helical test
specimens. The phase diagram of the α (hcp) + β (bcc) alloy is characterized by the monotectoid reaction at 893 ± 10 K: β
1 → (α + β
2) where β
1 (Zr–20Nb) and β
2 (Zr–80Nb) have widely differing compositions. At the creep testing temperature, 818 K, which is close to but below the monotectoid
temperature, the creep rate for samples with the equilibrium α + β
2 structure has been found to be considerably higher, over an order of magnitude, than that in samples with the metastable
α + β
1 structure. Microstructural changes accompanying the markedly enhanced creep rate for the α + β
2 structure at stresses as low as 1–4 MPa have been explained in terms of the relative stability of the β
1 and the β
2 phases during the creep process. An attempt has been made to elicit the likely mechanism underlying the observed enhancement
of creep rate and the changes in morphology, composition and volume fraction of the β phase. 相似文献