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排序方式: 共有790条查询结果,搜索用时 15 毫秒
81.
Hidden node collision in a contention-based medium access control protocol contributes to poor wireless network performance. This paper extended the Bianchi’s study and introduces a mathematical model that can be used to calculate throughput and delay for the IEEE 802.11 distributed coordination function of a multihop wireless network infrastructure assuming the presence of hidden node collision. This research investigates three essential parameters of multi-hop wireless networks. More specifically, this paper aims to analyze the effect of hidden nodes, network size, and maximum backoff stage on the overall system throughput and packet delay. Results clearly reveal the effect of large wireless network size, maximum backoff stage, and collision probability on throughput and packet delay. On one hand, throughput does not depend on the maximum backoff stage (m) for a small network size (e.g., n \(=\) 10). On the other hand, throughput does not strongly depend on the number of nodes when the backoff stage values are high. Comparing our proposed model in case single-hop with the Bianchi model, the analysis results indicate that the throughput values in our model when the numbers of nodes are 10, 50, and 100 are 0.6031, 0.4172 and 0.3433 respectively; whereas the throughput values are respectively 0.8370, 0.8317 and 0.8255 at the same number of nodes for the Bianchi model. The difference can be attributed to several assumptions made in our proposed model that were not considered in the Bianchi model. 相似文献
82.
H. Dhaou F. Askri M. Ben Salah A. Jemni S. Ben Nasrallah J. Lamloumi 《International Journal of Hydrogen Energy》2007
The hydriding/dehydriding rates and the pressure–composition isotherms were measured for LaNi5, LaNi4.85Al0.15 and LaNi4.75Fe0.25 under quasi-isothermal and variable pressure conditions. Isothermal conditions were obtained by reducing the thermal time constant of the experimental device. Empirical rate equations to describe the sorption reaction kinetics were derived. These rates are expressed as a function of temporal composition, saturated composition, temperature, applied pressure and essentially the initial operating conditions which were not considered in most of all the previous studies related to the reaction kinetics of metal hydrides. Besides, the rate equations presented in this work can be integrated easily in the numerical models that predict dynamic flow and heat and mass transfer within realistic metal–hydrogen devices. This paper also discusses the effects of Fe and Al as substituents for Ni on P–C isotherms and reaction rates of LaNi5 alloy. 相似文献
83.
Raji Soundararajan Kelvin G. Lynn Salah Awadallah Csaba Szeles Su-Huai Wei 《Journal of Electronic Materials》2006,35(6):1333-1340
We have studied the defect levels in as grown and post growth processed cadmium telluride (CdTe) using thermoelectric effect
spectroscopy (TEES) and thermally stimulated current (TSC) techniques. We have extracted the thermal energy (Eth) and trapping cross section (σth) for the defect levels using the initial rise and variable heating rate methods. We have identified 10 different defect levels
in the crystals. Thermal ionization energy values obtained experimentally were compared to theoretical values of the transition-energy
levels of intrinsic and extrinsic defects and defect complexes in CdTe determined by first-principles band-structure calculations.
On the basis of this comparison, we have associated the observed ionization levels with various native defects and impurity
complexes. 相似文献
84.
The Iosipescu shear test specimen geometry has been investigated by a number of research workers in recent years with conflicting
results. The paper describes a numerical study of a compact shear test specimen, based on the Iosipescu geometry, which is
proposed to investigate size effects in shear failure. A range of geometries has been studied and the extreme cases are reported.
Results are presented for the largest absolute principal stresses together with a detailed study of the stresses between and
around the roots of the two notches introduced in the test specimens. The results for the largest absolute principal stresses
show that tensile stresses are created at the roots of the two notches. These tensile stresses may result in Mode I failure
and probably account for the Mode I or mixed mode fracture observed in tests using the Iosipescu geometry. The results for
the distribution of stresses between the roots of the two notches show that deep notches increase the likelihood of shear
fracture prior to tensile failure. Shallow notches give a stress distribution similar to that developed in the indirect tensile
test and hence tensile failure is likely to precede shear failure in such cases. Further numerical and experimental work is
proposed.
相似文献
85.
Raouf Sh. Mikhail Salah A. Abo-El-Enein Mohamed Abd-El-Khalik 《Journal of chemical technology and biotechnology (Oxford, Oxfordshire : 1986)》1975,25(11):849-857
The total surface areas, total pore volumes, and the distribution of pore surface and pore volume in pores of different sizes were determined on the hardened slag-cement pastes. Adsorption isotherms of four different adsorbates, namely water, methanol, nitrogen and cyclohexane, were used for surface area and pore structure determinations. The results obtained indicated that water can penetrate into the total pore system, namely both the micropores and the wide pores; whereas methanol can measure wide pores and some of the micropores; moreover, nitrogen and cyclohexane molecules tend to measure only wider groups of pores. The indication gained from this result is that the molecular sieve property plays the most significant role in surface area and pore structure determinations of the hardened cement pastes. The polarity of the molecules seems to be important at low relative vapour pressures, and at high pressures it causes expansion of the pastes. 相似文献
86.
The need for water in the Middle East has been a challenging problem for industrial development and expansion. The great abundance of high salinity and brackish water resources, the vast population growth and the increasing need for food in the area have enhanced the development of several desalting projects and have initiated long-term plans for irrigation of arid land. The interest in producing fresh water from the sea using different energy sources in the Middle East dates back to a few decades. Currently there is a trend to actively participate in the progress and development of desalination science and technology utilizing available manpower and other resources in research projects, in experiments, and in the construction of prototype plants to examine the feasibility of novel processes especially those suitable for agriculture water production. Present plans involve enhancement of the transfer of technology and the establishment of centers of research and development and international cooperative project to serve local and global interest. Recent developments and the state-of-the-art of the different aspects of desalination research and technology in the Middle East are reviewed including a discussion of current experimental and theoretical activities. Practice in plant operation and the generated experience are assessed. The highlights of future plans are presented. 相似文献
87.
88.
Astudy is reported examining movement in psychotherapy using the Butler-Haigh Q Sort, comparing before and after therapy self- and ideal-self ratings of 79 high school and college students. Especially important was the examination of congruence (a concept offered by Rogers) in relation to therapeutic change; data do not support Rogers' findings regarding self moving toward ideal during and after therapy; the general notion of congruence is questioned on the basis of relatively unvarying self-ideal correlations and on the basis of item shifts on a before- and after-therapy basis. (16 ref.) (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
89.
Oxotris(dimethyl dithiocarbamato) vanadium(V) [VO(S2CN(CH3)2)3] sensitizes the polymerization of styrene when irradiated by light of λ = 365 nm at 25°C. Under the experimental conditions employed, no retardation occurs, and the rate of initiation is independent of monomer concentration. The mean values of the quantum yield of iniiation (?i) and polymerization (?o) are 2.85 × 10?3 and 6.72 respectively. Spectroscopic analysis shows that initiation occurs predominatly through scission of the N,N-dimethyl dithiocarbamate ligand (—SC(S)N(CH3)2) with reduction of vanadium(V) to (IV), and VO (S2CN(CH3)2)2 is the final photolytic product. A reaction mechanism is proposed based on an intramolecular photoredox reaction which leads to the primary formation of SC(S)N(CH3)2 radicals and a vanadium(IV) chelate complex. The rellevant kinetic parameters are evaluated. The polystyrene produced shows a photoactivity when irradiated with UV-light. 相似文献
90.
When the cost of reducing the duration of activities is convex and nonlinear, it may be advisable (to reduce the computing burden) to seek a “satisficing” answer, in which the project is compressed to a desired completion time with a prespecified tolerable relative error. We treat the problem of constructing the optimal first degree interpolating linear spline that guarantees such maximal error and consider various possible refinements. 相似文献