全文获取类型
收费全文 | 344597篇 |
免费 | 3566篇 |
国内免费 | 1297篇 |
专业分类
化学 | 153743篇 |
晶体学 | 4514篇 |
力学 | 12678篇 |
综合类 | 10篇 |
数学 | 31330篇 |
物理学 | 93928篇 |
无线电 | 53257篇 |
出版年
2021年 | 2195篇 |
2020年 | 2404篇 |
2019年 | 2652篇 |
2018年 | 3249篇 |
2017年 | 3064篇 |
2016年 | 4971篇 |
2015年 | 3346篇 |
2014年 | 5188篇 |
2013年 | 14171篇 |
2012年 | 10578篇 |
2011年 | 13123篇 |
2010年 | 9200篇 |
2009年 | 9697篇 |
2008年 | 12934篇 |
2007年 | 13484篇 |
2006年 | 12588篇 |
2005年 | 11805篇 |
2004年 | 10717篇 |
2003年 | 9630篇 |
2002年 | 9463篇 |
2001年 | 11617篇 |
2000年 | 9140篇 |
1999年 | 7308篇 |
1998年 | 6199篇 |
1997年 | 6247篇 |
1996年 | 5710篇 |
1995年 | 5405篇 |
1994年 | 5240篇 |
1993年 | 5232篇 |
1992年 | 5725篇 |
1991年 | 5772篇 |
1990年 | 5399篇 |
1989年 | 5067篇 |
1988年 | 4983篇 |
1987年 | 4393篇 |
1986年 | 4120篇 |
1985年 | 5397篇 |
1984年 | 5501篇 |
1983年 | 4469篇 |
1982年 | 4734篇 |
1981年 | 4562篇 |
1980年 | 4367篇 |
1979年 | 4473篇 |
1978年 | 4695篇 |
1977年 | 4517篇 |
1976年 | 4628篇 |
1975年 | 4102篇 |
1974年 | 4242篇 |
1973年 | 4495篇 |
1972年 | 2760篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
101.
Seghir K. Cristoloveanu S. Jerisian R. Oualid J. Auberton-Herve A.-J. 《Electron Devices, IEEE Transactions on》1993,40(6):1104-1111
The leakage and charge pumping currents were measured in gate-controlled MOS p-i-n diodes fabricated on thin SIMOX substrates. The efficiencies of the techniques as well as their complementary features are analyzed for various experimental conditions. The interface properties of device-grade SIMOX wafers are characterized and shown to be compatible with VLSI requirements. Special interface coupling effects, which occur only in fully depleted SOI devices and modify the conventional signature of charge pumping and leakage current, are thoroughly investigated 相似文献
102.
Various wireless systems and the research preceding their practical use in China are described. The topics discussed include research establishments, research and development projects underway, challenges inherent to digital radio, and expansion opportunities afforded by satellite communications 相似文献
103.
A criterion for determining the maximum spacing between magnetometers for measuring the magnetic field is derived. A two-dimensional (2-D) filter model is employed to determine the maximum spatial frequency component present in the magnetic field that is above the spectral noise level. This maximum frequency component is then sampled at a rate greater than twice per period as indicated by the Nyquist criterion, yielding the required magnetometer spacing. It is shown that the rule-of-thumb employed in current clinical biomagnetic array systems, that the spacing between the coils should be approximately equal to the depth of the source, is adequate when the signal-to-noise power ratio is less than 28.4 (14.5 dB). The analysis also quantitatively demonstrates that reducing the separation between the measurement and source planes has a greater effect on the resolution than decreasing the noise level by the same factor. This result is important for employing high Tc superconductor magnetometers that allow thinner thermal insulating layers at the cost of higher thermal noise 相似文献
104.
A new practical design approach for minimum-phase FIR or IIR filters, setting out from a high dimensionality FIR linear-phase prototype is described. The novelty of this technique lies in overcoming the inherent problem of finding the roots of a high order polynomial with repeated and/or very closely clustered roots 相似文献
105.
Tajiri A. Minakuchi K. Komeda K. Bessho Y. Inoue Y. Yodoshi K. Yamaguchi T. 《Electronics letters》1993,29(4):369-370
A highly reliable 200 mW AlGaAs laser diode with a fundamental transverse mode has been developed, by optimising its structure with a 0.8 mu m thick p-cladding layer, a 1200 mu m long cavity length, and a front facet coating with a low reflectivity of 2%. The maximum output power was 500 mW, and stable fundamental transverse mode operation was obtained up to 350 mW. Stable operation under 200 mW and 50 degrees C was confirmed for more than 1200 h. Optical feedback noise was below 3*10/sup -14/ Hz/sup -1/.<> 相似文献
106.
107.
A mixed mode digital/analog special purpose VLSI hardware implementation of an associative memory with neural architecture is presented. The memory concept is based on a matrix architecture with binary storage elements holding the connection weights. To enhance the processing speed analog circuit techniques are applied to implement the algorithm for the association. To keep the memory density as high as possible two design strategies are considered. First, the number of transistors per storage element is kept to a minimum. In this paper a circuit technique that uses a single 6-transistor cell for weight storage and analog signal processing is proposed. Second, the device precision has been chosen to a moderate level to save area as much as possible. Since device mismatch limits the performance of analog circuits, the impact of device precision on the circuit performance is explicitly discussed. It is shown that the device precision limits the number of rows activated in parallel. Since the input vector as well as the output vector are considered to be sparsely coded it is concluded, that even for large matrices the proposed circuit technique is appropriate and ultra large scale integration with a large number of connection weights is feasible. 相似文献
108.
O. Perat J. M. Dorkel E. Scheid P. Temple Boyer Y. S. Chung A. Peyre-Lavigne M. Zecri P. Tounsi 《Microelectronics Reliability》2002,42(7):1053-1058
Reliability of thermomechanical simulations is critically linked to the accuracy of the mechanical properties that govern the behaviour of structure, like Young's modulus (E) and coefficient of thermal expansion (CTE). For many cases, the values found in literatures are dealing with bulk properties without detailed information on temperature effects. To address such issues, it is necessary to measure the materials parameters as a function of temperature. The measurement of CTE is usually accomplished by evaluating the thermal deflections of a subjected material layer deposited on a substrate, providing that E is known at a specific temperature of experiment. A bilayer method, based on theory of elasticity, is proposed to determine both E and CTE for a given temperature with a good resolution. This paper presents the theoretical analysis, the design and process of the microsystem test structures, and the main calculation results. 相似文献
109.
This paper analyzes probability of bit-error (Pe) performance of asynchronous bandlimited direct-sequence code-division multiple-access systems with binary phase-shift keying spreading. The two present methods of Pe analysis under bandwidth-efficient pulse shaping: the often-cited standard Gaussian approximation and the characteristic function (CF) method suffer from either a low accuracy in regions of low Pe (< 10-3) or a prohibitively large computational complexity. The paper presents an alternate method of Pe analysis with moderate computational complexity and high accuracy based on a key observation. A sequence of chip decision statistics (whose sum yields a bit statistic) forms a stationary, m-dependent sequence when conditioned on the chip delay and phase offset of each interfering signal. This observation permits the generalization of the improved Gaussian approximation previously derived for the rectangular pulse and the derivation of a numerically efficient approximation based on the CF method. Numerical examples of systems using the square-root raised-cosine and IS-95 pulses illustrate THE P e performance, user capacity and the accuracy of the proposed method 相似文献
110.
Two‐dimensional arrangements of molecules can show remarkable cooperative electronic effects. Such effects can serve to achieve direct electronic sensing of chemical and physical processes via electrostatic effects, i.e., without transfer of charge or matter between the locus of sensing and that of detection. 相似文献