全文获取类型
收费全文 | 505499篇 |
免费 | 10989篇 |
国内免费 | 4159篇 |
专业分类
电工技术 | 11880篇 |
技术理论 | 3篇 |
综合类 | 3123篇 |
化学工业 | 73603篇 |
金属工艺 | 18473篇 |
机械仪表 | 16238篇 |
建筑科学 | 14601篇 |
矿业工程 | 2822篇 |
能源动力 | 14386篇 |
轻工业 | 48891篇 |
水利工程 | 5063篇 |
石油天然气 | 6510篇 |
武器工业 | 504篇 |
无线电 | 66140篇 |
一般工业技术 | 92471篇 |
冶金工业 | 92019篇 |
原子能技术 | 8332篇 |
自动化技术 | 45588篇 |
出版年
2022年 | 4170篇 |
2021年 | 6090篇 |
2020年 | 4527篇 |
2019年 | 4656篇 |
2018年 | 6948篇 |
2017年 | 7345篇 |
2016年 | 7378篇 |
2015年 | 6267篇 |
2014年 | 9659篇 |
2013年 | 24723篇 |
2012年 | 15193篇 |
2011年 | 19982篇 |
2010年 | 15904篇 |
2009年 | 17265篇 |
2008年 | 18076篇 |
2007年 | 17585篇 |
2006年 | 15704篇 |
2005年 | 13933篇 |
2004年 | 13169篇 |
2003年 | 13132篇 |
2002年 | 12135篇 |
2001年 | 12803篇 |
2000年 | 11606篇 |
1999年 | 12434篇 |
1998年 | 31052篇 |
1997年 | 21553篇 |
1996年 | 16351篇 |
1995年 | 12374篇 |
1994年 | 10898篇 |
1993年 | 10735篇 |
1992年 | 7579篇 |
1991年 | 7210篇 |
1990年 | 6926篇 |
1989年 | 6577篇 |
1988年 | 6404篇 |
1987年 | 5278篇 |
1986年 | 5140篇 |
1985年 | 5985篇 |
1984年 | 5371篇 |
1983年 | 5113篇 |
1982年 | 4687篇 |
1981年 | 4688篇 |
1980年 | 4446篇 |
1979年 | 4112篇 |
1978年 | 3798篇 |
1977年 | 4558篇 |
1976年 | 6079篇 |
1975年 | 3177篇 |
1974年 | 3060篇 |
1973年 | 3078篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
131.
K Senior 《Canadian Metallurgical Quarterly》1997,3(12):528-532
As we uncover the physiological and molecular mechanisms behind human reproduction, we gain the potential to exert more control over our reproductive capabilities. In the past two decades, the prospects for 'infertile' women to bear children, or 'sterile' men to father them, have improved dramatically; recently, women have given birth in their sixties, well beyond their natural menopause. Thanks to developments in contraception, couples can now enjoy an active sex-life without a significant risk of pregnancy, and more accurately control the size and timing of their families. But these new freedoms have a price: they also have the potential to cause demographic distortions, medical harm to individuals and abuse of human rights. 相似文献
132.
This paper is concerned with partially observed stochastic optimal control problems when nonlinearities enter the dynamics of the unobservable state and the observations as gradients of potential functions. Explicit representations for the information state are derived in terms of a finite number of sufficient statistics. Consequently, the partially observed problem is recast as one of complete information with a new state generated by a modified version of the Kalman filter. When the terminal cost is quadratic in the unobservable state and includes the integral of the nonlinearities, the optimal control laws are explicitly computed, similar to linear-exponential-quadratic-Gaussian (LEQG) and linear-quadratic-Gaussian (LQG) tracking problems. The results are applicable to filtering and control of Hamiltonian systems 相似文献
133.
B Amblard C Assaiante JC Fabre L Mouchnino J Massion 《Canadian Metallurgical Quarterly》1997,114(2):214-225
The ability voluntarily to stabilize the head in space during lateral rhythmic oscillations (0.59+/-0.09 Hz) of the trunk has been investigated during microgravity (microG) and normal gravity (nG) conditions (parabolic flights). Five healthy young subjects, who gave informed consent, were examined. The movements were performed with eyes open or eyes closed, during phases of either microG or nG. The main result was that head orientation with respect to vertical may be stabilized about the roll axis under microG with, as well as without vision, despite the reduction in vestibular afferent and muscle proprioceptive inputs. Moreover, the absence of head stabilization about the yaw axis confirms that the degrees of freedom of the neck can be independently controlled, as was previously reported. These results seem to indicate that voluntary head stabilization does not depend crucially upon static vestibular afferents. Head stabilization in space may in fact be organized on the basis of either dynamic vestibular afferents or a short-term memorized postural body schema. 相似文献
134.
135.
H Neudeck M Joncic C Schuster S Bisson R Hildebrandt T Oney B Stiemer H Hopp R Graf 《Canadian Metallurgical Quarterly》1997,37(6):449-458
Nitric oxide (NO) acts as a modulator of neuronal transmission in mature neuronal systems, including the retina. Recently, NO has also been suggested to have a trophic function during development. We examined immunocytochemically the distribution of NO-producing cells in developing and transplanted rabbit retinas. An antibody detecting the neuronal isoform of its biosynthetic enzyme, nitric oxide synthase (NOS), was used on normal developing retinas [starting at embryonic day (E) 15] and on rabbit retinal transplants after various survival times (1-139 days after surgery). Weakly stained cell bodies were first observed in the proximal margin of the neuroblastic layer at E 29. Stained processes projecting towards a developing inner plexiform layer were also visible at this time point. Immunoreactive cells were located at later stages in the innermost part of the inner nuclear layer and in the ganglion cell layer, and are likely to correspond mainly to amacrine cells. NOS-labelled cells were also found in retinal transplants. The first NOS-labelled cells appeared, as in normal developing retinas, in ages corresponding to E 29 and were still detected in transplants corresponding to postnatal day 123. NOS-labelled cells were seen in areas between rosettes, where amacrine cells are located. NOS-labelled processes were at times seen to project for long distances, forming very distinct plexuses. NOS-containing amacrine cells thus appear both in the transplants and in developing retinas in the embryonic stages, long before synaptic function involving these cells can be expected, suggesting a role for NO not only in neuromodulation but also in retinal development. 相似文献
136.
K Giesen T Hummel A Stollewerk S Harrison A Travers C Kl?mbt 《Canadian Metallurgical Quarterly》1997,124(12):2307-2316
Two classes of glial cells are found in the embryonic Drosophila CNS, midline glial cells and lateral glial cells. Midline glial development is triggered by EGF-receptor signalling, whereas lateral glial development is controlled by the gcm gene. Subsequent glial cell differentiation depends partly on the pointed gene. Here we describe a novel component required for all CNS glia development. The tramtrack gene encodes two zinc-finger proteins, one of which, ttkp69, is expressed in all non-neuronal CNS cells. We show that ttkp69 is downstream of gcm and can repress neuronal differentiation. Double mutant analysis and coexpression experiments indicate that glial cell differentiation may depend on a dual process, requiring the activation of glial differentiation by pointed and the concomitant repression of neuronal development by tramtrack. 相似文献
137.
Neviani A. Meneghesso G. Zanoni E. Hafizi M. Canali C. 《Electron Device Letters, IEEE》1997,18(12):619-621
Impact ionization is a major limiting factor to the maximum operating voltage of InGaAs-based, high-speed transistors. In this work, data on the positive temperature dependence of the electron impact ionization coefficient αn in In0.53Ga0.47As at medium-low electric fields are reported for the first time. The increase of αn with temperature is opposite to the behavior normally observed in most semiconductors. This anomalous behavior implies the onset of a positive feedback between power dissipation and avalanche generation which may adversely affect the power handling capability of In0.53Ga 0.47As-based devices, and which should be taken into account in device thermal modeling. In the experimental procedure, based on the measurement of the multiplication factor M-1 in npn In0.53Ga 0.47As/InP Heterojunction Bipolar Transistors (HBT), particular care has been taken in order to rule out possible spurious, temperature-dependent contributions to the measured multiplication current 相似文献
138.
The effect of high oxide field stress is studied using capacitance-time (C-t) measurements of MOS capacitors. The stress results in parallel shifts of the C-t curve along the time axis. The flatband voltage shift ΔVFB obtained from the initial deep depletion capacitance C(t=0+) follows the same trend as that from the high-frequency C-V characteristics. However, the discrepancy between the two flatband voltages becomes larger as the stress increases due to the effect of interface charges on C-t characteristics. The flatband voltage difference is converted to interface trap density, showing a steady increase of interface trap density with stress, similar to that from low-frequency C-V measurements 相似文献
139.
This paper presents the design criteria, procedure, and implementation of a soft-switched power-factor-correction (PFC) circuit based on the extended-period quasi-resonant (EPQR) principles. All power electronic devices including switches and diodes in the circuit are fully soft switched. The design method is demonstrated in a prototype circuit. The operating principles are confirmed with computer simulation and experimental results. A comparison of the EP-QR operation and zero-voltage-transition (ZVT) pulse-width modulation (PWM) method 相似文献
140.
Fahmy H.I. Develekos G. Douligeris C. 《Selected Areas in Communications, IEEE Journal on》1997,15(2):226-237
Communication network design is becoming increasingly complex, involving making networks more usable, affordable, and reliable. To help with this, we have proposed an expert network designer (END) for configuring, modeling, simulating, and evaluating large structured computer networks, employing artificial intelligence, knowledge representation, and network simulation tools. We present a neural network/knowledge acquisition machine-learning approach to improve the END's efficiency in solving the network design problem and to extend its scope to acquire new networking technologies, learn new network design techniques, and update the specifications of existing technologies 相似文献