全文获取类型
收费全文 | 9141篇 |
免费 | 300篇 |
国内免费 | 64篇 |
专业分类
化学 | 5789篇 |
晶体学 | 84篇 |
力学 | 250篇 |
综合类 | 1篇 |
数学 | 1314篇 |
物理学 | 1402篇 |
无线电 | 665篇 |
出版年
2022年 | 72篇 |
2021年 | 100篇 |
2020年 | 175篇 |
2019年 | 131篇 |
2018年 | 94篇 |
2017年 | 106篇 |
2016年 | 198篇 |
2015年 | 165篇 |
2014年 | 213篇 |
2013年 | 414篇 |
2012年 | 483篇 |
2011年 | 586篇 |
2010年 | 285篇 |
2009年 | 245篇 |
2008年 | 521篇 |
2007年 | 501篇 |
2006年 | 470篇 |
2005年 | 470篇 |
2004年 | 449篇 |
2003年 | 308篇 |
2002年 | 328篇 |
2001年 | 99篇 |
2000年 | 90篇 |
1999年 | 72篇 |
1998年 | 85篇 |
1997年 | 112篇 |
1996年 | 153篇 |
1995年 | 90篇 |
1994年 | 78篇 |
1993年 | 107篇 |
1992年 | 75篇 |
1991年 | 88篇 |
1990年 | 87篇 |
1989年 | 63篇 |
1988年 | 79篇 |
1987年 | 68篇 |
1986年 | 67篇 |
1985年 | 121篇 |
1984年 | 144篇 |
1983年 | 100篇 |
1982年 | 123篇 |
1981年 | 117篇 |
1980年 | 97篇 |
1979年 | 86篇 |
1978年 | 112篇 |
1977年 | 101篇 |
1976年 | 85篇 |
1975年 | 74篇 |
1974年 | 69篇 |
1973年 | 60篇 |
排序方式: 共有9505条查询结果,搜索用时 205 毫秒
881.
G. Cynthia Jemima SwarnavalliV. Joseph D. RoopsinghV. Kannappan 《Journal of Molecular Liquids》2011,164(3):243-249
Nanocrystallite silver and silver sol were prepared and characterized by UV-visible spectra, XRD, TEM and HRTEM. The crystallite silver is re-dispersed in two different media, namely, water and alcohol and sonicated before ultrasonic investigation. The silver sol was used as such. Three different models for the propagation of ultrasound through two phase media are compared in these three different types of nano suspensions. Effect of particle size and medium on ultrasonic velocity (U), compressibility (κ) impedance (Z) and viscous relaxation time (τ) is studied. The particle concentration range was 0.2-1 v/v. Density and viscosity of the dispersion and sol are measured at different particle volume fractions. Effective density and ultrasonic velocity are computed by Urick, Kuster and Toksöz and Urick and Ament models and compared with experimental velocities. Values of effective density obtained by using Urick and Urick and Ament equations closely agree with experimental results of density while Urick's equation prediction of velocity is in close agreement with the experimental velocities. This comes as a surprise in view of the large density difference between the medium and suspended particle and suggests the possibility of the balancing effect of the inertial and viscous forces operating in the suspension. 相似文献
882.
883.
We consider project scheduling where the project manager’s objective is to minimize the time from when an adversary discovers the project until the completion of the project. We analyze the complexity of the problem identifying both polynomially solvable and NP-hard versions of the problem. The complexity of the problem is seen to be dependent on the nature of renewable resource constraints, precedence constraints, and the ability to crash activities in the project. 相似文献
884.
Joseph Hundley 《The Ramanujan Journal》2014,34(2):283-306
This paper treats the problem of determining conditions for the Fourier coefficients of a Maass–Hecke newform at cusps other than infinity to be multiplicative. To be precise, the Fourier coefficients are defined using a choice of matrix in $\mathit{SL}(2, \mathbb{Z})$ which maps infinity to the cusp in question. Let c and d be the entries in the bottom row of this matrix, and let N be the minimal level. In earlier work with Dorian Goldfeld and Min Lee, we proved that the coefficients will be multiplicative whenever N divides 2cd. This paper proves that they will not be multiplicative unless N divides 576cd. Further, if one assumes that the Hecke eigenvalue vanishes less than half the time, then this number drops to 4cd, and a precise condition governing the case where N divides 4cd and not 2cd is obtained. 相似文献
885.
We consider the problem of estimating regression models of two-dimensional random fields. Asymptotic properties of the least squares estimator of the linear regression coefficients are studied for the case where the disturbance is a homogeneous random field with an absolutely continuous spectral distribution and a positive and piecewise continuous spectral density. We obtain necessary and sufficient conditions on the regression sequences such that a linear estimator of the regression coefficients is asymptotically unbiased and mean square consistent. For such regression sequences the asymptotic covariance matrix of the linear least squares estimator of the regression coefficients is derived. 相似文献
886.
Kiarash Amiri Joseph R. Cavallaro Chris Dick Raghu Mysore Rao 《Journal of Signal Processing Systems》2011,62(2):233-245
Spatial division multiplexing (SDM) in MIMO technology significantly increases the spectral efficiency, and hence capacity,
of a wireless communication system: it is a core component of the next generation wireless systems, e.g. WiMAX, 3GPP LTE and
other OFDM-based communication schemes. Moreover, spatial division multiple access (SDMA) is one of the widely used techniques
for sharing the wireless medium between different mobile devices. Sphere detection is a prominent method of simplifying the
detection complexity in both SDM and SDMA systems while maintaining BER performance comparable with the optimum maximum-likelihood
(ML) detection. On the other hand, with different standards supporting different system parameters, it is crucial for both
base station and handset devices to be configurable and seamlessly switch between different modes without the need for separate
dedicated hardware units. This challenge emphasizes the need for SDR designs that target the handset devices. In this paper,
we propose the architecture and FPGA realization of a configurable sort-free sphere detector, Flex-Sphere, that supports 4, 16, 64-QAM modulations as well as a combination of 2, 3 and 4 antenna/user configuration for handsets.
The detector provides a data rate of up to 857.1 Mbps that fits well within the requirements of any of the next generation
wireless standards. The algorithmic optimizations employed to produce an FPGA friendly realization are discussed. 相似文献
887.
Moon HR Siddiqui MA Sun G Filippov IV Landsman NA Lee YC Adams KM Barchi JJ Deschamps JR Nicklaus MC Kelley JA Marquez VE 《Tetrahedron》2010,66(34):6707-7430
Steric and electronic parameters, such as the anomeric effect (AE) and gauche effect play significant roles in steering the North⇆South equilibrium of nucleosides in solution.Two isomeric oxa-bicyclo[3.1.0]hexane nucleosides that are conformationally locked in either the North or the South conformation of the pseudorotational cycle were designed to study the consequences of having the AE operational or not, independent of other parameters. The rigidity of the system allowed the orientation of the orbitals involved to be set in ‘fixed’ relationships, either antiperiplanar where the AE is permanently ‘on’, or gauche where the AE is impaired. The consequences of these two alternatives were subjected to high-level calculations and measured experimentally by X-ray crystallography, hydrolytic stability of the glycosyl bond, and pKa values. 相似文献
888.
A new method for solving a system of generalized nonlinear variational inequalities in Banach spaces
The purpose of this paper is by using the generalized projection approach to introduce an iterative scheme for finding a solution to a system of generalized nonlinear variational inequality problem. Under suitable conditions, some existence and strong convergence theorems are established in uniformly smooth and strictly convex Banach spaces. The results presented in the paper improve and extend some recent results. 相似文献
889.
Michael Theuer Joseph S. Melinger 《Journal of Infrared, Millimeter and Terahertz Waves》2011,32(11):1267-1284
Terahertz time-domain spectroscopy accesses the frequency range between 100 GHz and 5 THz by using the coherent generation
and detection based on femtosecond laser sources. On the way to obtain fingerprint absorption spectra of molecular solids,
terahertz waveguides have proven to be a valuable tool to extend the results to narrow and high resolution linewidths of crystalline
solids. We will discuss the development, properties and applications of terahertz waveguide geometries for spectroscopic applications,
in particular high-resolution measurements using parallel-plate waveguides. 相似文献
890.
Michael Wu Yang Sun Guohui Wang Joseph R. Cavallaro 《Journal of Signal Processing Systems》2011,65(2):171-183
Turbo code is a computationally intensive channel code that is widely used in current and upcoming wireless standards. General-purpose
graphics processor unit (GPGPU) is a programmable commodity processor that achieves high performance computation power by
using many simple cores. In this paper, we present a 3GPP LTE compliant Turbo decoder accelerator that takes advantage of
the processing power of GPU to offer fast Turbo decoding throughput. Several techniques are used to improve the performance
of the decoder. To fully utilize the computational resources on GPU, our decoder can decode multiple codewords simultaneously,
divide the workload for a single codeword across multiple cores, and pack multiple codewords to fit the single instruction
multiple data (SIMD) instruction width. In addition, we use shared memory judiciously to enable hundreds of concurrent multiple
threads while keeping frequently used data local to keep memory access fast. To improve efficiency of the decoder in the high
SNR regime, we also present a low complexity early termination scheme based on average extrinsic LLR statistics. Finally,
we examine how different workload partitioning choices affect the error correction performance and the decoder throughput. 相似文献