全文获取类型
收费全文 | 257641篇 |
免费 | 2976篇 |
国内免费 | 887篇 |
专业分类
电工技术 | 4879篇 |
综合类 | 197篇 |
化学工业 | 37015篇 |
金属工艺 | 9609篇 |
机械仪表 | 7652篇 |
建筑科学 | 6130篇 |
矿业工程 | 884篇 |
能源动力 | 7430篇 |
轻工业 | 21313篇 |
水利工程 | 2245篇 |
石油天然气 | 3232篇 |
武器工业 | 15篇 |
无线电 | 35075篇 |
一般工业技术 | 50914篇 |
冶金工业 | 49244篇 |
原子能技术 | 5109篇 |
自动化技术 | 20561篇 |
出版年
2022年 | 1587篇 |
2021年 | 2340篇 |
2020年 | 1707篇 |
2019年 | 2048篇 |
2018年 | 3480篇 |
2017年 | 3471篇 |
2016年 | 3566篇 |
2015年 | 2309篇 |
2014年 | 4124篇 |
2013年 | 11793篇 |
2012年 | 6411篇 |
2011年 | 8796篇 |
2010年 | 7069篇 |
2009年 | 8068篇 |
2008年 | 8561篇 |
2007年 | 8415篇 |
2006年 | 7499篇 |
2005年 | 6604篇 |
2004年 | 6367篇 |
2003年 | 6618篇 |
2002年 | 6108篇 |
2001年 | 6567篇 |
2000年 | 6028篇 |
1999年 | 6454篇 |
1998年 | 16693篇 |
1997年 | 11544篇 |
1996年 | 8906篇 |
1995年 | 6696篇 |
1994年 | 5984篇 |
1993年 | 5930篇 |
1992年 | 4196篇 |
1991年 | 4073篇 |
1990年 | 3904篇 |
1989年 | 3694篇 |
1988年 | 3595篇 |
1987年 | 3054篇 |
1986年 | 2994篇 |
1985年 | 3261篇 |
1984年 | 2976篇 |
1983年 | 2864篇 |
1982年 | 2664篇 |
1981年 | 2572篇 |
1980年 | 2486篇 |
1979年 | 2322篇 |
1978年 | 2128篇 |
1977年 | 2642篇 |
1976年 | 3624篇 |
1975年 | 1787篇 |
1974年 | 1754篇 |
1973年 | 1683篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
161.
N. Balchev D. Kovacheva V. Lovchinov K. Konstantinov K. Petrov 《Journal of Superconductivity》1993,6(1):49-54
The preparation conditions, phase composition, and superconducting properties of Bi-Pb-Sr-Ca-Cu oxide materials from different nominal compositions have been investigated. Nearly single-phase samples from Bi2Pb0.4Sr2Ca3Cu4O
y
, as well as from the proposed compositions Bi1.8Pb0.4Sr2Ca3Cu4O
y
and Bi1.8Pb0.4Sr2Ca2.5Cu3.5O
y
were obtained by a solid-state reaction in air. Samples with the third nominal composition showed the best superconducting properties (T
on=111 K and zero resistance atT
0=103 K). A possible mechanism for the 2223 phase formation in the three investigated compositions has been discussed. 相似文献
162.
This paper discusses changes in the spectrum and distortion of the electron wave function of a GaAs quantum well when a thin
AlGaAs barrier is introduced into it. The potential difference generated across the quantum well by distortion of the electron
wave function is calculated, along with its dependence on the position of the barrier in the quantum well. The photovoltaic
response of the structure to optical intersubband excitations is also calculated, along with the role of wave function and
electronic spectrum distortion as well as intersubband nonradiative transitions in generating this response. The suitability
of a GaAs quantum well with a thin barrier for use as an infrared detector is considered.
Fiz. Tekh. Poluprovodn. 32, 1246–1250 (October 1998) 相似文献
163.
K Fortini 《Canadian Metallurgical Quarterly》1997,117(9):667-669
The use of body mediated therapies at the Clinical geriatric psychiatry became possible due to the convergence between multi, inter and transdisciplinary approaches and the clinical experience in this application of various therapeutic technics. This approach at the aged clinic must take into consideration the specificity of the correlation to the body of the aged person beside the advantages and the needs that resent this approaches to both the patient and his therapist. 相似文献
164.
As shown previously for two-dimensional geometries, anisotropy effects should not be ignored in electrical impedance tomography (EIT) and structural information is important for the reconstruction of anisotropic conductivities. Here, we describe the static reconstruction of an anisotropic conductivity distribution for the more realistic three-dimensional (3-D) case. Boundaries between different conductivity regions are anatomically constrained using magnetic resonance imaging (MRI) data. The values of the conductivities are then determined using gradient-type-algorithms in a nonlinear-indirect approach. At each iteration, the forward problem is solved by the finite element method. The approach is used to reconstruct the 3-D conductivity profile of a canine torso. Both computational performance and simulated reconstruction results are presented together with a detailed study on the sensitivity of the prediction error with respect to different parameters. In particular, the use of an intracavity catheter to better extract interior conductivities is demonstrated 相似文献
165.
Rats were trained to discriminate between ethanol (1.0 g/kg; 10% v/v) and saline under a fixed ratio 10 schedule of sweetened milk reinforcement. Both diazepam [nonselective, full benzodiazepine (BZ) receptors agonist] and bretazenil (nonselective, partial BZ receptor agonist) produced dose-dependent ethanol-appropriate responding (>75%). Neither diazepam nor bretazenil affected the response rate at the doses producing maximal generalisation from ethanol. In contrast, zolpidem (full BZ1 receptor agonist) and abecarnil (full BZ1/full or partial BZ2 receptor agonist) produced only moderate (<50%) ethanol-appropriate responding when tested up to doses that markedly decreased the overall response rate. These results suggest that: 1) there are no major differences between full and partial, nonselective BZ receptor agonists in their ability to substitute for 1.0 g/kg dose of ethanol; 2) stimulation of BZ1 receptors alone is not sufficient to produce ethanol-like discriminative stimulus effects in the rat. 相似文献
166.
Saitoh T. Mattori S. Kinugawa S. Miyagi K. Taniguchi A. Kourogi M. Ohtsu M. 《Lightwave Technology, Journal of》1998,16(5):824-832
We report the modulation characteristics of a waveguide-type optical frequency comb generator (WG-OFCG) with the advantages of compactness, high modulation index and low driving power. The characteristics of the optical frequency comb (OFC) generated when the modulation index exceeds 2π are discussed. The power of the modulation sidebands was measured by the optical heterodyne method with an external-cavity laser as a local oscillator. The dependence of the modulation index of the WG-OFCG on the modulation frequency up to 40 GHz was measured. The generation span of the OFC at the modulation frequency detuned from an integer multiple of the free spectral range (FSR) is discussed 相似文献
167.
Kobayashi K. Kameyama M. Higuchi T. 《Industrial Electronics, IEEE Transactions on》1997,44(3):418-426
A new token-passing mechanism, priority token passing, which features real-time access and fast detection and recovery of transmission errors, is discussed in detail in comparison with standard token-passing protocols, and its large-scale integration (LSI)-oriented design concept is described. Priority token passing includes only a small performance overhead, due to its switching functions, which can change network topology from ring to broadcast medium. A token-holding node passes the token to another node after determining the successor through priority comparison. Errors occurring during token passing can, thus, be detected and corrected simply and promptly. Priority token passing has a simple hardware implementation, requiring only small additions to the frame control circuitry, and has a small implementation overhead. The priority token-passing protocol and two other important network communication functions, dual ring network reconfiguration and high-level data link control (HDLC) normal response mode-based message transmission, are designed as a single finite-state machine, and implemented into a compact LSI chip. This integrated instrument network (IINET) chip provides complete network communication services and requires only three additional external electronic components for operation 相似文献
168.
169.
This paper studies a particular single-stage power-factor-correction (PFC) switching regulator employing a discontinuous-conduction-mode (DCM) boost-input cell and a continuous-current-mode (CCM) forward output cell. Although this single-stage PFC regulator can provide a reasonably high power factor when its PFC stage is operating in discontinuous mode, substantial reduction in line-current harmonics is possible by applying a suitable frequency-modulation scheme. This paper derives a frequency-modulation scheme and proposes a practical implementation using a simple translinear analog circuit. A quantitative analysis on the total harmonic distortion (THD) of the line current when the circuit is subject to a limited range of frequency variations is presented along with some considerations for practical design. Experimental data obtained from a prototype confirms the effectiveness of the proposed frequency-modulation scheme. The proposed analog translinear circuit allows custom integrated circuit implementation, making it a viable low-cost solution to the elimination of line-current harmonics in switching regulators 相似文献
170.
Business process redesign (BPR) has been widely touted as a means for leveraging the power of information technology to change business processes radically, resulting in substantial improvements in organizational effectiveness and efficiency. This study represents an early attempt at examining the strategic nature of the phenomenon. Because BPR is radical change and has the ability to alter both the competitive and operational nature of the corporation, it is critical that BPR be coordinated with corporate strategy. Through a survey of information system executives, the relationships between corporate competitive strategy and three types of BPR – intrafunctional, interfunctional and interorganizational – were examined. It was found that organizations following a cost strategy tended to do more BPR projects on interfunctional processes. Furthermore, the positive effect of a cost strategy on interfunctional BPR was found to be strengthened by the degree of integration of IS and business planning. It was also found that while interorganizational BPR was not related to a specific strategy, it was strongly related directly with IS–business planning integration. 相似文献