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81.
A fully-dense Cu-75 vol pct ZrW2O8 metal matrix composite was fabricated by hot isostatic pressing of Cu-coated ZrW2O8 particles. A small amount of the high-pressure γ-ZrW2O8 phase was created during the cooldown and depressurization following densification; near complete transformation to γ-ZrW2O8 was achieved by subsequent cold isostatic pressing. The thermal expansion behavior of the composite between 25°C and 325°C
was altered by the cold isostatic pressing treatment, and also depended on the length of time that had passed between thermal
cycles. The measured thermal expansion coefficients within specific temperature ranges varied from −6·10−6 K−1 to far above the thermal expansion coefficient of the copper matrix. The complex temperature-dependent expansion/contraction
behavior could be justified by considering the evolution of phase transformations taking place in the ZrW2O8 phase, which were observed by in-situ synchrotron X-ray diffraction measurements. 相似文献
82.
The author tested the hypothesis that a history of drug-induced antagonism of alcohol impairment would enhance alcohol tolerance in humans. Groups of participants (N = 21) repeatedly performed a psychomotor task under different drug treatments: 0.65 g/kg alcohol, 4 mg/kg caffeine, or both drugs combined. Tolerance to a 0.65 g/kg alcohol dose challenge was then tested. Results showed that a history of combined alcohol and caffeine administrations increased alcohol tolerance compared with an exposure history to either drug alone. The findings contribute to the understanding of the complexities of polydrug use history and provide a useful model to examine how alcohol tolerance might be affected by a history of coadministration with other drugs (e.g., cocaine and nicotine). (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
83.
Nicolas Clauvelin Basile Audoly Sébastien Neukirch 《Scientific Modeling and Simulation》2007,14(1):95-101
The DNA molecule is modeled as an elastic rod with bending and twisting rigidities, subjected to external tension and twist applied at one end, the other end being clamped. We study the plectonemic equilibrium of such a rod, taking into account the impenetrability constraint. Numerical solutions of this boundary value problem have previously shown that purely elastic models can reproduce the supercoiling response of the DNA molecule. Using a variational approach, we derive analytical formulae for the elastic response of the filament, and extend former numerical results. 相似文献
84.
85.
The current study replicated, in a sample of 2,300 outpatients seeking psychiatric treatment, a previous study (R. F. Krueger & M. S. Finger, 2001) that implemented an item response theory approach for modeling the comorbidity of common mood and anxiety disorders as indicators along the continuum of a shared latent factor (internalizing). The 5 disorders examined were major depressive disorder, social phobia, panic disorder/agoraphobia, specific phobia, and generalized anxiety disorder. The findings were consistent with the prior research. First, a confirmatory factor analysis yielded sufficient evidence for a nonspecific factor underlying the 5 diagnostic indicators. Second, a 2-parameter logistic item response model showed that the diagnoses were represented in the upper half of the internalizing continuum, and each was a strongly discriminating indicator of the factor. Third, the internalizing factor was significantly associated with 3 indexes of social burden: poorer social functioning, time missed from work, and lifetime hospitalizations. Rather than the categorical system of presumably discrete disorders presented in DSM-IV, these 5 mood and anxiety disorders may be alternatively viewed as higher end indicators of a common factor associated with social cost. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
86.
87.
We show new upper bounds for problems in the W-hierarchy of fixed-parameter complexity. A crucial ingredient of our proofs
is an extension to the W-RAM model, which permits more-powerful operations but remains equivalent to the original. We use
the extended model to give new upper bounds for Subsetsum, Maximum Irredundant Set, and various problems concerning intersection
of finite-state machines. 相似文献
88.
Daniel Meister 《Theory of Computing Systems》2007,41(2):257-289
A finite recurrent system over sets of natural numbers of dimension n is a pair composed of n n-ary functions over sets of
natural numbers and an n-tuple of singleton sets of natural numbers. Every function is applied to the entries of the tuple
and computes a set of natural numbers, that may also be empty. The results are composed into another tuple, and the process
is started anew. Thus, a finite recurrent system defines an infinite sequence of n-tuples containing sets of natural numbers.
The last entry of a generated n-tuple is called the output of a step, and the union of the output sets of all steps is the
set defined by the finite recurrent system. Membership problems ask whether a given number is in a specified output set or
in some output set. We study membership problems for special finite recurrent systems, whose functions are built from the
set operations union, intersection and complementation and the arithmetical operations addition and multiplication. Sum and
product of two sets of natural numbers are defined elementwise. We restrict the set of operations from which functions are
built and determine the impact on the complexity of the membership problems. We focus on PSPACE-decidable membership problems
and show completeness results for the complexity classes NL, NP and PSPACE. 相似文献
89.
Dynamic complexity investigates the required effort to maintain knowledge about a property of a structure under changing operations.
This article introduces a refined notion of dynamic problems which takes the initial structure into account. It develops the
basic structural complexity notions accordingly. It also shows that the dynamic version of the LOGCFL-complete problem D2LREACH(acyclic) can be maintained with first-order updates. 相似文献
90.
We consider the problem of scheduling jobs on related machines owned by selfish agents. We provide a 5-approximation deterministic
truthful mechanism, the first deterministic truthful result for the problem. Previously, Archer and Tardos showed a 2-approximation
randomized mechanism which is truthful in expectation only (a weaker notion of truthfulness). In case the number of machines
is constant, we provide a deterministic Fully Polynomial-Time Approximation Scheme (FPTAS) and a suitable payment scheme that
yields a truthful mechanism for the problem. This result, which is based on converting FPTAS to monotone FPTAS, improves a
previous result of Auletta et al., who showed a (4 + ε)-approximation truthful mechanism. 相似文献