首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   280篇
  免费   5篇
  国内免费   1篇
化学   121篇
晶体学   1篇
力学   12篇
数学   32篇
物理学   120篇
  2018年   5篇
  2015年   5篇
  2014年   6篇
  2013年   10篇
  2012年   7篇
  2011年   7篇
  2010年   11篇
  2009年   4篇
  2008年   8篇
  2007年   13篇
  2006年   17篇
  2005年   10篇
  2004年   14篇
  2003年   9篇
  2002年   10篇
  2001年   5篇
  2000年   6篇
  1999年   9篇
  1998年   2篇
  1997年   2篇
  1996年   2篇
  1994年   9篇
  1993年   6篇
  1992年   9篇
  1991年   8篇
  1990年   7篇
  1989年   5篇
  1988年   5篇
  1987年   6篇
  1986年   4篇
  1985年   7篇
  1984年   2篇
  1983年   3篇
  1982年   3篇
  1981年   4篇
  1980年   3篇
  1977年   5篇
  1976年   3篇
  1975年   2篇
  1974年   4篇
  1973年   3篇
  1970年   2篇
  1966年   3篇
  1962年   1篇
  1961年   1篇
  1960年   2篇
  1959年   1篇
  1958年   1篇
  1957年   2篇
  1956年   2篇
排序方式: 共有286条查询结果,搜索用时 15 毫秒
91.
The ambient-pressure synthesis of a new tetragonal alkaline-earth superconducting cuprate, Sr0.8Ba1.2CuO3+δ, from a cupro-oxycarbonate is reported. Magnetic-susceptibility measurements show the presence of a superconducting transition 50 K in a post-annealed sample. The crystal structure, refined from time-of-flight powder neutron-diffraction data was found to have an oxygen-deficient La2CuO4-type tetragonal T structure (a = 3.8988(3) Å and C = 12.815(3) Å) with oxygen vacancies located within the CuO2 planes. Ordering of these oxygen vacancies is responsible for the observation of a superlattice in both neutron- and electron-diffraction measurements. An interpretation of the electron-diffraction patterns suggests that the superlattice in Sr0.8Ba1.2CuO3+δ and also in the isostructural superconductor Sr2CuO3+δ are of an identical nature.  相似文献   
92.
93.
Summary Analogs are proved for sequences in Φ=GF[q, x], the ring of polynomials in x over a finite field of q elements, of results of Niven, Zane, and Cavior concerning uniform distribution of sequences of integers generated by polynomials with integer coefficients. The uniform distribution (mod m), m an integer, in these papers is replaced here by a concept called weak uniform distribution (mod M), M ε Φ. Supported byNSF Research GrantGP 6515. Entrata in Redazione il 21 settembre 1968.  相似文献   
94.
95.
96.
Carbon dioxide (CO2) isotopic ratios on samples of pure CO2 were measured in the 1.6 μm wavelength region using the frequency-stabilized cavity ring-down spectroscopy (FS-CRDS) technique. We present CO2 absorption spectra with peak signal-to-noise ratios as high as 28,000:1. Measured single-spectrum signal-to-noise ratios were as high as 8900:1, 10,000:1, and 1700:1 for 13C/12C, 18O/16O, and 17O/16O, respectively. In addition, we demonstrate the importance of utilizing the Galatry line profile in the spectrum analysis. The use of the Voigt line profile, which neglects the observed collisional narrowing, leads to large systematic errors which are transition-dependent and vary with temperature and pressure. While the relatively low intensities of CO2 transitions near λ=1.6 μm make this spectral region non-optimal, the sensitivity and stability of FS-CRDS enabled measurement precision of pure CO2 samples which are comparable to those of other optical techniques which operate at far more propitious wavelengths. These results indicate that a FS-CRDS spectrometer designed to probe CO2 bands near wavelengths of 2.0 μm or 4.3 μm could achieve significantly improved precision over the present instrument and likely be competitive with mass spectrometric methods.  相似文献   
97.
98.
Abstract

This article introduces a general method for Bayesian computing in richly parameterized models, structured Markov chain Monte Carlo (SMCMC), that is based on a blocked hybrid of the Gibbs sampling and Metropolis—Hastings algorithms. SMCMC speeds algorithm convergence by using the structure that is present in the problem to suggest an appropriate Metropolis—Hastings candidate distribution. Although the approach is easiest to describe for hierarchical normal linear models, we show that its extension to both nonnormal and nonlinear cases is straightforward. After describing the method in detail we compare its performance (in terms of run time and autocorrelation in the samples) to other existing methods, including the single-site updating Gibbs sampler available in the popular BUGS software package. Our results suggest significant improvements in convergence for many problems using SMCMC, as well as broad applicability of the method, including previously intractable hierarchical nonlinear model settings.  相似文献   
99.

Abstract  

Chalcones, 1,3-diphenyl-2-propene-1-ones, exist naturally and synthetically, and are characterized by the linkage of two aromatic rings joining a three carbon α-β-unsaturated carbonyl entity. We report the observation of two new polymorphs of a hydroxy chalcone, C6H5–CO–CH=CH–C6H4 (m-OH), identified as a result of a Claisen–Schmidt synthesis and manual screening technique. One polymorph of this compound has been reported previously and exists in the monoclinic system, space group P2/n, with unit cell parameters a = 13.295(6) ?, b = 5.659(2) ?, c = 16.144(8) ?, β = 109.73(5)°, V = 1143.3(9) ?3, and Z = 4. Two crystalline forms (II and III) reported herein, are polymorphs of the reported monoclinic form (I). Form II exists in the orthorhombic system, space group Pca21, with unit cell constants a = 11.631 (2) ?, b = 13.163 (3) ?, c = 7.3977 (14) ?, V = 1132.6 (4) ?3, and Z = 4. Form III also crystallizes in the orthorhombic system, however in space group Pbca, with unit cell parameters a = 11.8100(8) ?, b = 7.4075(5) ?, c = 25.8729(19) ?, V = 2263.4(3) ?3, and Z = 8. Variations in the hydrogen bonding connectivity help to distinguish these two forms from the monoclinic, whereas crystal packing differentiates the two orthorhombic forms. The conformation of the C=C (C2–C3) is E for both orthorhombic forms.  相似文献   
100.
The unsteady shallow-water equations for barotropic/baroclinic (free-surface/density-stratified) flows with non-linear coupling of density transport and momentum are solved using a family of two-time-level, semi-implicit predictor–corrector methods (PC2). The PC2 methods are a general family that includes the popular TRIM method for hydrostatic flows. PC2 is characterised by four ‘θ’ parameters controlling the time ‘n’ and ‘n + 1’ weighting of (1) free surface gradient, (2) predictor step, (3) baroclinic gradient and (4) density temporal interpolation. Stability of the non-linear coupling between momentum and density transport for PC2 is examined in the inviscid limit. Central difference and quadratic (QUICK) spatial interpolation for density are compared. Second-order temporal accuracy for both barotropic and baroclinic flows is simultaneously obtained with appropriate θ parameters, which has previously been shown to be impractical for TRIM. The 2nd-order PC2 method has near-neutral non-linear stability (slightly positive amplification factor) where linear theory predicts exactly neutral stability. QUICK is shown to be preferable to central difference spatial discretisation to reduce the amplification factor. Adjusting the baroclinic weighting or adding small artificial viscosities can stabilise the model for non-linear internal wave simulations.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号