全文获取类型
收费全文 | 96510篇 |
免费 | 3937篇 |
国内免费 | 1192篇 |
专业分类
化学 | 55697篇 |
晶体学 | 735篇 |
力学 | 2305篇 |
综合类 | 49篇 |
数学 | 10801篇 |
物理学 | 16325篇 |
无线电 | 15727篇 |
出版年
2023年 | 767篇 |
2022年 | 1050篇 |
2021年 | 1634篇 |
2020年 | 1697篇 |
2019年 | 1700篇 |
2018年 | 1307篇 |
2017年 | 1282篇 |
2016年 | 2616篇 |
2015年 | 2253篇 |
2014年 | 2795篇 |
2013年 | 4859篇 |
2012年 | 5764篇 |
2011年 | 6329篇 |
2010年 | 3867篇 |
2009年 | 3720篇 |
2008年 | 5712篇 |
2007年 | 5454篇 |
2006年 | 5353篇 |
2005年 | 5018篇 |
2004年 | 4269篇 |
2003年 | 3605篇 |
2002年 | 3504篇 |
2001年 | 2036篇 |
2000年 | 1802篇 |
1999年 | 1441篇 |
1998年 | 1266篇 |
1997年 | 1296篇 |
1996年 | 1401篇 |
1995年 | 1126篇 |
1994年 | 1082篇 |
1993年 | 1070篇 |
1992年 | 983篇 |
1991年 | 858篇 |
1990年 | 728篇 |
1989年 | 647篇 |
1988年 | 621篇 |
1987年 | 593篇 |
1986年 | 499篇 |
1985年 | 846篇 |
1984年 | 742篇 |
1983年 | 607篇 |
1982年 | 725篇 |
1981年 | 602篇 |
1980年 | 563篇 |
1979年 | 509篇 |
1978年 | 539篇 |
1977年 | 530篇 |
1976年 | 481篇 |
1975年 | 504篇 |
1974年 | 408篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
Yong‐Jin Kim Jin‐Woong Kim Jung‐Eun Lee Jee‐Hyun Ryu Junoh Kim Ih‐Seop Chang Kyung‐Do Suh 《Journal of polymer science. Part A, Polymer chemistry》2004,42(22):5627-5635
Mesoporous polymer microspheres with gold (Au) nanoparticles inside their pores were prepared considering their surface functionality and porosity. The Au/polymer composite microspheres prepared were characterized by transmission electron microscope (TEM), X‐ray diffraction (XRD), and Brunauer–Emmett–Teller (BET) techniques. The results showed that the adsorption of Au nanoparticles could be increased by imparting the pore structure and surface‐functional groups into the supporting polymer microspheres (in this study, poly (ethylene glycol dimethacrylate‐co‐acrylonitrile) and poly (EGDMA‐co‐AN) system). Above all, from this study, it was established that the porosity of the polymer microspheres is the most important factor that determines the distribution and adsorption amount of face‐centered cubic (fcc) Au nanoparticles in the final products. Our study showed that the continuous adsorption of Au nanoparticles with the aid of the large surface area and surface interaction sites formed more favorably the Au/polymer composite microspheres. The BET measurements of Au/poly(EGDMA‐co‐AN) composite microspheres reveals that the adsorption of Au nanoparticles into the pores kept the pore structure intact and made it more porous. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 5627–5635, 2004 相似文献
82.
For any two points p and q in the Euclidean plane, define LUNpq = { v | v ∈ R2, dpv < dpq and dqv < dpq}, where duv is the Euclidean distance between two points u and v . Given a set of points V in the plane, let LUNpq(V) = V ∩ LUNpq. Toussaint defined the relative neighborhood graph of V, denoted by RNG(V) or simply RNG, to be the undirected graph with vertices V such that for each pair p,q ∈ V, (p,q) is an edge of RNG(V) if and only if LUNpq (V) = ?. The relative neighborhood graph has several applications in pattern recognition that have been studied by Toussaint. We shall generalize the idea of RNG to define the k-relative neighborhood graph of V, denoted by kRNG(V) or simply kRNG, to be the undirected graph with vertices V such that for each pair p,q ∈ V, (p,q) is an edge of kRNG(V) if and only if | LUNpq(V) | < k, for some fixed positive number k. It can be shown that the number of edges of a kRNG is less than O(kn). Also, a kRNG can be constructed in O(kn2) time. Let Ec = {epq| p ∈ V and q ∈ V}. Then Gc = (V,Ec) is a complete graph. For any subset F of Ec, define the maximum distance of F as maxepq∈Fdpq. A Euclidean bottleneck Hamiltonian cycle is a Hamiltonian cycle in graph Gc whose maximum distance is the minimum among all Hamiltonian cycles in graph Gc. We shall prove that there exists a Euclidean bottleneck Hamiltonian cycle which is a subgraph of 20RNG(V). Hence, 20RNGs are Hamiltonian. 相似文献
83.
Mary E. Neubert DAVID G. ABDALLAH Jr Sandra S. Keast Julie M. Kim Soonnam Lee Ryan M. Stayshich Margaret E. Walsh Rolfe G. Petschek Shin-Tson Wu 《Liquid crystals》2003,30(6):711-731
New diphenyldiacetylenes of the type with A, B = H and/or F; m = 0, 1; n = 1-4; and X = C n H 2n + 1 , F, CF 3 or CN were synthesized and their mesomorphic properties determined by hot stage polarizing microscopy and DSC. When m = 0, all of these compounds showed only a nematic phase except when X = CF 3 when both nematic and smectic A phases were seen. Both clearing and melting temperatures were higher than those reported for substitution with the corresponding alkyl chains but the much larger increase in clearing temperatures produced considerably wider nematic phases. Eutectic mixtures of a few of these olefins yielded nematic materials also having much wider temperature ranges and higher clearing temperatures than the eutectic mixtures of the alkyl compounds, while retaining their high birefringence and low viscosities. Such materials are of interest for beam-steering devices.
Four of the diacetylenes with m = 1 ( A, B = H) were also prepared ( X = C 6 H 13 , F, n = 2, 3). When X was C 6 H 13 ( n = 2), the nematic range was smaller in the 2- than in the 1-olefin but wider than in the alkyl series. When X = F, either no nematic phase or a monotropic one was observed, whereas the 1-olefins gave a much wider nematic phase. Both transition temperatures were lower than those for the corresponding 1-olefin and alkyl analogues. The compound with X = C 6 H 13 and n = 2 had a melting temperature below room temperature. 相似文献
Four of the diacetylenes with m = 1 ( A, B = H) were also prepared ( X = C 6 H 13 , F, n = 2, 3). When X was C 6 H 13 ( n = 2), the nematic range was smaller in the 2- than in the 1-olefin but wider than in the alkyl series. When X = F, either no nematic phase or a monotropic one was observed, whereas the 1-olefins gave a much wider nematic phase. Both transition temperatures were lower than those for the corresponding 1-olefin and alkyl analogues. The compound with X = C 6 H 13 and n = 2 had a melting temperature below room temperature. 相似文献
84.
Frank W. Schmidt 《Linear and Multilinear Algebra》1991,30(3):229-230
This section appears from time to time. Contributions are invited, and should be submitted to R. C. Thompson, Mathematics Department, University of California, Santa Barbara, CA 93106. 相似文献
85.
Yan R.-H. Lee K.F. Jeon D.Y. Kim Y.O. Park B.G. Pinto M.R. Rafferty C.S. Tennant D.M. Westerwick E.H. Chin G.M. Morris M.D. Early K. Mulgrew P. Mansfield W.M. Watts R.K. Voshchenkov A.M. Bokor J. Swartz R.G. Ourmazd A. 《Electron Device Letters, IEEE》1992,13(5):256-258
The authors report the implementation of deep-submicrometer Si MOSFETs that at room temperature have a unity-current-gain cutoff frequency (f T) of 89 GHz, for a drain-to-source bias of 1.5 V, a gate-to-source bias of 1 V, a gate oxide thickness of 40 Å, and a channel length of 0.15 μm. The fabrication procedure is mostly conventional, except for the e-beam defined gates. The speed performance is achieved through an intrinsic transit time of only 1.8 ps across the active device region 相似文献
86.
本文介绍了宽低温STN-LCD加热装置的设计和制备方法,并对低温加热的实验结果进行了讨论和分析,相应的研究结果表明此法有利于拓宽STN液晶显示器的负温工作范围。 相似文献
87.
A multiserver queueing model of access control strategies for a wideband integrated services digital network (ISDN) is considered. There are two types of service requests (SRs): Type 1 SR requires b servers (basic bandwidth units, BBUs) of the c available servers, while Type 2 SR requires a single server. Both types of SRs are queuable in two separate infinite-size buffers. A Type 1 SR is allowed to seize d (⩽b ) servers at first and then acquire additional servers as they become available to meet the original requirement of b servers. The arrival processes of both types of SRs are Poisson and the service times are exponentially distributed with mean b /d μ1 and 1/μ2 for Type 1 SR and Type 2 SR, respectively. Preemptive and nonpreemptive priority disciplines with movable boundary are analyzed using the Neuts' matrix-analytic approach. Numerical examples of the queue length distribution and the mean waiting time of the SRs are presented 相似文献
88.
89.
K''Andrea C. Bickerstaff Michael J. Schulte Earl E. Swartzlander 《The Journal of VLSI Signal Processing》1995,9(3):181-191
As developed by Wallace and Dadda, a method for high-speed, parallel multiplication is to generate a matrix of partial products
and then reduce the partial products to two numbers whose sum is equal to the final product. The resulting two numbers are
then summed using a fast carry-propagate adder. This paper presents Reduced Area multipliers, which employ a modified reduction
scheme that results in fewer components and less interconnect overhead than either Wallace or Dadda multipliers. This reduction
scheme is especially useful for pipelined multipliers, because it minimizes the number of latches required in the reduction
of the partial products. The reduction scheme can be applied to either unsigned (sign-magnitude) or two's complement numbers.
Equations are given for determining the number of components and a method is presented for estimating the interconnect overhead
for Wallace, Dadda, and Reduced Area multipliers. Area estimates indicate that for non-pipelined multipliers, the reduction
in area achieved with Reduced Area multipliers ranges from 3.7 to 6.6 percent relative to Dadda multipliers, and from 3.8
to 8.4 percent relative to Wallace multipliers. For fully pipelined multipliers, the reduction in area ranges from 15.1 to
33.6 percent relative to Dadda multipliers, and from 2.9 to 9.0 percent relative to Wallace multipliers. 相似文献
90.
Sabate J.A. Jovanovic M.M. Lee F.C. Gean R.T. 《Industrial Electronics, IEEE Transactions on》1995,42(1):63-71
The analysis and design of an LCC resonant inverter for a 20 kHz AC distributed power system are presented. Several resonant converter topologies are assessed to determine their suitability for high efficiency power conversion, under resistive and reactive loads. Two LCC-resonant inverter designs were implemented. One with all switches operating with zero voltage switching (ZVS), and another with two switches operating with ZVS and two switches with zero current switching (ZCS). The experimental results are presented along with a performance comparison of the two versions 相似文献