全文获取类型
收费全文 | 35775篇 |
免费 | 5115篇 |
国内免费 | 3575篇 |
专业分类
化学 | 20701篇 |
晶体学 | 344篇 |
力学 | 1153篇 |
综合类 | 188篇 |
数学 | 2520篇 |
物理学 | 8715篇 |
无线电 | 10844篇 |
出版年
2024年 | 129篇 |
2023年 | 689篇 |
2022年 | 922篇 |
2021年 | 1286篇 |
2020年 | 1238篇 |
2019年 | 1236篇 |
2018年 | 1095篇 |
2017年 | 1058篇 |
2016年 | 1556篇 |
2015年 | 1623篇 |
2014年 | 1903篇 |
2013年 | 2552篇 |
2012年 | 2886篇 |
2011年 | 2973篇 |
2010年 | 2198篇 |
2009年 | 2211篇 |
2008年 | 2486篇 |
2007年 | 2215篇 |
2006年 | 2043篇 |
2005年 | 1814篇 |
2004年 | 1453篇 |
2003年 | 1349篇 |
2002年 | 1445篇 |
2001年 | 1023篇 |
2000年 | 780篇 |
1999年 | 704篇 |
1998年 | 512篇 |
1997年 | 464篇 |
1996年 | 379篇 |
1995年 | 336篇 |
1994年 | 249篇 |
1993年 | 223篇 |
1992年 | 233篇 |
1991年 | 195篇 |
1990年 | 144篇 |
1989年 | 113篇 |
1988年 | 93篇 |
1987年 | 71篇 |
1986年 | 72篇 |
1985年 | 82篇 |
1984年 | 51篇 |
1983年 | 39篇 |
1982年 | 42篇 |
1981年 | 32篇 |
1980年 | 26篇 |
1979年 | 19篇 |
1978年 | 21篇 |
1977年 | 20篇 |
1975年 | 23篇 |
1970年 | 18篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
31.
32.
A (w,r) cover‐free family is a family of subsets of a finite set such that no intersection of w members of the family is covered by a union of r others. A (w,r) superimposed code is the incidence matrix of such a family. Such a family also arises in cryptography as the concept of key distribution pattern. In the present paper, we give some new results on superimposed codes. First we construct superimposed codes from super‐simple designs which give us results better than superimposed codes constructed by other known methods. Next we prove the uniqueness of the (1,2) superimposed code of size 9 × 12, the (2,2) superimposed code of size 14 × 8, and the (2,3) superimposed code of size 30 × 10. Finally, we improve numerical values of upper bounds for the asymptotic rate of some (w,r) superimposed codes. © 2004 Wiley Periodicals, Inc. 相似文献
33.
34.
Neodymium doped bismuth layer structure ferroelectrics (BLSFs) ceramics CaBi4−xNdxTi4O15 (x=0, 0.25, 0.50, 0.75) were prepared by solid-state reaction method. X-ray diffraction pattern showed that single phase was formed when x=0-0.75. The refined lattice parameters showed that a (b) axes decrease at x=0.25 and increase with more Nd3+ dopant. The effects of Nd3+ doping on the dielectric and ferroelectric properties of CaBi4Ti4O15 ceramics are studied. Nd3+ dopant decreased the Curie temperature linearly, and the dielectric loss, tan δ, as well. The remnant polarization of Nd3+ doped CaBi4Ti4O15 ceramics was increased by 80% at x=0.25, while more Nd3+ dopant decreased the remnant polarization. CaBi3.75Nd0.25Ti4O15 ceramics had the largest piezoelectric constant d33. The structure and properties of CaBi4−xNdxTi4O15 ceramics showed that Nd3+ may occupy different crystal locations when Nd3+ content x is less than 0.25 and more than 0.50. 相似文献
35.
We have rigorously calculated for the first time all the inter-electrode and pixel capacitances of Thin-Film Transistor Liquid Crystal Displays using the electrical energy distribution inside a liquid crystal cell. The energy distribution is obtained from the three-dimensional profiles of potential distribution and molecular director. The dynamic equation of continuum theory for liquid crystals is described in a tensorial form in order to maintain the equivalence of n and -n . The effects of lateral fields generated by multiple electrodes of finite sizes are taken into account in the simulation. As a numerical technique, we used a finite difference method which is suitable for the highly non-linear equations. As a result, we confirmed that the pixel capacitance for our pixel structure is about 40% larger than that of the conventional approach. It is also revealed that the gate-common and gate-data inter-electrode capacitances are not negligible. 相似文献
36.
A practical resource management method that can significantly reduce cochannel interference (CCI) and improve spectrum utilisation in FH-OFDMA packet-based cellular networks is presented. The proposed method seeks an effective combination of dynamic resource allocation with fractional coding and bit loading to respectively minimise CCI and maximise system throughput for a desired performance. 相似文献
37.
Design and practical implementation of multifrequency RF front ends using direct RF sampling 总被引:1,自引:0,他引:1
Psiaki M.L. Powell S.P. Hee Jung Kintner P.M. 《Microwave Theory and Techniques》2005,53(10):3082-3089
The use of direct RF sampling has been explored as a means of designing multifrequency RF front ends. Such front ends will be useful to multifrequency RF applications such as global navigation satellite system receivers that use global positioning system (GPS) L1, L2, and L5 signals and Galileo signals. The design of a practical multifrequency direct RF sampling front end is dependent on having an analog-to-digital converter whose input bandwidth accommodates the highest carrier frequency and whose maximum sampling frequency is more than twice the cumulative bandwidth about the multiple carrier signals. The principle of direct RF sampling is used to alias all frequency bands of interest onto portions of the Nyquist bandwidth that do not overlap. This paper presents a new algorithm that finds the minimum sampling frequency that avoids overlap. This design approach requires a multifrequency bandpass filter for the frequency bands of interest. A prototype front end has been designed, built, and tested. It receives a GPS coarse/acquisition code at the L1 frequency and GPS antispoofing precision code at both L1 and L2. Dual-frequency signals with received carrier-to-noise ratios in excess of 52 dB-Hz have been acquired and tracked using this system. 相似文献
38.
39.
Anti-BZ-Structure in Effect Algebras 总被引:1,自引:0,他引:1
The definitions of sharply approximating effect algebras, anti-BZ-effect algebras, central approximating effect algebras, and S-anti-BZ-effect algebras are given, the relationships between sharply approximating effect algebras and anti-BZ-effect algebras, between central approximating effect algebras and anti-BZ-effect algebras are established, and the set of anti-BZ-sharp elements in S-anti-BZ-effect algebras is proved to be an orthomodular lattice. 相似文献
40.
In this paper, by extending Kucera's idea to the function field case, we obtain several determinant formulas involving the real class number and the relative class number of any subfield of cyclotomic function fields. We also provide several examples using these determinant formulas.