全文获取类型
收费全文 | 77253篇 |
免费 | 7762篇 |
国内免费 | 5586篇 |
专业分类
化学 | 41614篇 |
晶体学 | 749篇 |
力学 | 2347篇 |
综合类 | 334篇 |
数学 | 5920篇 |
物理学 | 17564篇 |
无线电 | 22073篇 |
出版年
2024年 | 208篇 |
2023年 | 1146篇 |
2022年 | 1567篇 |
2021年 | 2177篇 |
2020年 | 1965篇 |
2019年 | 2106篇 |
2018年 | 1893篇 |
2017年 | 1756篇 |
2016年 | 2764篇 |
2015年 | 2744篇 |
2014年 | 3511篇 |
2013年 | 4971篇 |
2012年 | 5589篇 |
2011年 | 5960篇 |
2010年 | 4325篇 |
2009年 | 4306篇 |
2008年 | 5321篇 |
2007年 | 4831篇 |
2006年 | 4586篇 |
2005年 | 3976篇 |
2004年 | 3433篇 |
2003年 | 3047篇 |
2002年 | 3009篇 |
2001年 | 2367篇 |
2000年 | 1960篇 |
1999年 | 1580篇 |
1998年 | 1132篇 |
1997年 | 1047篇 |
1996年 | 1008篇 |
1995年 | 821篇 |
1994年 | 781篇 |
1993年 | 714篇 |
1992年 | 601篇 |
1991年 | 484篇 |
1990年 | 448篇 |
1989年 | 367篇 |
1988年 | 262篇 |
1987年 | 206篇 |
1986年 | 162篇 |
1985年 | 187篇 |
1984年 | 148篇 |
1983年 | 119篇 |
1982年 | 125篇 |
1981年 | 125篇 |
1980年 | 87篇 |
1979年 | 81篇 |
1978年 | 90篇 |
1976年 | 83篇 |
1975年 | 69篇 |
1974年 | 69篇 |
排序方式: 共有10000条查询结果,搜索用时 6 毫秒
81.
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
82.
Hong Wang Rong Zeng Xiuping Li 《Microwave Theory and Techniques》2005,53(2):564-570
In this paper, RF noise in 0.18-mum NMOSFETs concerning the contribution of carrier heating and hot carrier effect is characterized and analyzed in detail via a novel approach that modulates the channel carrier heating and number of hot carriers using body bias. We confirm qualitatively a negligible role of hot carrier effect on the channel noise in deep-submicrometer MOSFETs. For a device under reverse body bias (Vb), even though the increase in hot carrier population is clearly characterized by dc measurements, the device high-frequency noise is found to be irrelevant to the increase in the channel hot carriers. Experimental results show that the high-frequency noise is slightly reduced with the increase in |Vb|, and can be qualitatively explained by secondary effects such as the suppression of nonequilibrium channel noise and substrate induced noise. The reduction of NFmin and Rn with the increase in |Vb| may provide a possible methodology to finely adjust the device high-frequency noise performance for circuit design 相似文献
83.
本文详细探讨了使用无线网络分析器调试ZigBee协议系统的基本方法,从而便于更好地确保网络的可靠性. 相似文献
84.
A 0.9 V 92 dB Double-Sampled Switched-RC Delta-Sigma Audio ADC 总被引:1,自引:0,他引:1
Min Gyu Kim Gil-Cho Ahn Hanumolu P.K. Sang-Hyeon Lee Sang-Ho Kim Seung-Bin You Jae-Whui Kim Temes G.C. Un-Ku Moon 《Solid-State Circuits, IEEE Journal of》2008,43(5):1195-1206
A 0.9 V third-order double-sampled delta-sigma audio ADC is presented. A new method using a combination of a switched-RC technique and a floating switched-capacitor double-sampling configuration enabled low-voltage operation without clock boosting or bootstrapping. A three-level quantizer with simple dynamic element matching was used to improve linearity. The prototype IC implemented in a 0.13 CMOS process achieves 92 dB DR, 91 dB SNR and 89 dB SNDR in a 24 kHz audio signal bandwidth, while consuming 1.5 mW from a 0.9 V supply. The prototype operates from 0.65 V to 1.5 V supply with minimal performance degradation. 相似文献
85.
Jong Chan Lee Jin Young Park So Young Yoon Yong Hun Bae Seung Jun Lee 《Tetrahedron letters》2004,45(1):191-193
A novel and direct method for the synthesis of α-halocarbonyl compounds using sequential treatment of carbonyl compounds with [hydroxy(tosyloxy)iodo]benzene followed by magnesium halides under solvent-free microwave irradiation conditions is described. 相似文献
86.
Various 2-substituted indoles were prepared by heteroannulation of o-iodoanilines and terminal alkynes in a one-pot reaction with a Pd(II)-NaY zeolite catalyst. The product formation largely depended on the solvent, base, and reaction temperature. The recycled catalyst showed good reusability in the heteroannulation reaction. 相似文献
87.
88.
Han-Kyu Lim Deog Kyoon Jeong KyungTae Kim JunMo Park Han-gyoo Kim 《Communications Magazine, IEEE》2005,43(5):141-148
Network direct attached storage (NDAS) is a network storage architecture that allows direct attachment of existing ATA/ATAPI devices to Ethernet without a separate server. Unlike other architectures such as NAS, SAN, and USB mass storage, no server computer intervenes between the storage and the client hosts. We describe an NDAS disk controller (NDC) amenable to low-cost single-chip implementation that processes a simplified L3/L4 protocol and converts commands between ATA/ATAPI and Ethernet, while the remaining complex tasks are performed by remote hosts. Unlike NAS architectures that use TCP/IP, NDAS uses a TCP-like lean protocol that lends itself well to high-performance hardware realization. Thanks to the simple NDAS architecture and protocol, an NDC implemented on a single 4 mm /spl times/ 4 mm chip in 0.18 /spl mu/m CMOS technology achieves a maximum throughput of 55 Mbytes/s on gigabit Ethernet, which is comparable to that of a high-performance disk locally attached to a host computer. 相似文献
89.