全文获取类型
收费全文 | 522篇 |
免费 | 47篇 |
国内免费 | 2篇 |
专业分类
化学 | 480篇 |
晶体学 | 12篇 |
力学 | 6篇 |
数学 | 26篇 |
物理学 | 47篇 |
出版年
2023年 | 5篇 |
2022年 | 12篇 |
2021年 | 21篇 |
2020年 | 24篇 |
2019年 | 27篇 |
2018年 | 15篇 |
2017年 | 10篇 |
2016年 | 21篇 |
2015年 | 20篇 |
2014年 | 19篇 |
2013年 | 38篇 |
2012年 | 32篇 |
2011年 | 41篇 |
2010年 | 24篇 |
2009年 | 18篇 |
2008年 | 22篇 |
2007年 | 28篇 |
2006年 | 27篇 |
2005年 | 23篇 |
2004年 | 26篇 |
2003年 | 16篇 |
2002年 | 9篇 |
2001年 | 3篇 |
2000年 | 5篇 |
1999年 | 2篇 |
1998年 | 3篇 |
1996年 | 8篇 |
1995年 | 2篇 |
1994年 | 5篇 |
1993年 | 6篇 |
1992年 | 4篇 |
1991年 | 6篇 |
1990年 | 5篇 |
1989年 | 2篇 |
1988年 | 4篇 |
1987年 | 2篇 |
1986年 | 2篇 |
1985年 | 5篇 |
1984年 | 3篇 |
1983年 | 2篇 |
1982年 | 3篇 |
1980年 | 3篇 |
1979年 | 3篇 |
1978年 | 2篇 |
1974年 | 1篇 |
1973年 | 3篇 |
1972年 | 2篇 |
1971年 | 1篇 |
1968年 | 2篇 |
1896年 | 1篇 |
排序方式: 共有571条查询结果,搜索用时 15 毫秒
1.
In the classical channel assignment problem, transmitters that are sufficiently close together are assigned transmission frequencies that differ by prescribed amounts, with the goal of minimizing the span of frequencies required. This problem can be modeled through the use of an L(2,1)-labeling, which is a function f from the vertex set of a graph G to the non-negative integers such that |f(x)-f(y)|? 2 if xand y are adjacent vertices and |f(x)-f(y)|?1 if xand y are at distance two. The goal is to determine the λ-number of G, which is defined as the minimum span over all L(2,1)-labelings of G, or equivalently, the smallest number k such that G has an L(2,1)-labeling using integers from {0,1,…,k}. Recent work has focused on determining the λ-number of generalized Petersen graphs (GPGs) of order n. This paper provides exact values for the λ-numbers of GPGs of orders 5, 7, and 8, closing all remaining open cases for orders at most 8. It is also shown that there are no GPGs of order 4, 5, 8, or 11 with λ-number exactly equal to the known lower bound of 5, however, a construction is provided to obtain examples of GPGs with λ-number 5 for all other orders. This paper also provides an upper bound for the number of distinct isomorphism classes for GPGs of any given order. Finally, the exact values for the λ-number of n-stars, a subclass of the GPGs inspired by the classical Petersen graph, are also determined. These generalized stars have a useful representation on Möebius strips, which is fundamental in verifying our results. 相似文献
2.
3.
C.A. Cody R.C. Levitt R.K. Khanna Philip J. Miller 《Journal of solid state chemistry》1978,26(3):293-309
The infrared and Raman spectra of NaH3(SeO3)2 and NaD3(SeO3)2 have been recorded from 24 to 300°K. The interpretation, assignments, and analysis of the spectral studies are presented on the paraelectric α phase (proton disordered), ferroelectric β phase (proton ordered) and ferroelectric γ phase (proton ordered). A discussion of a newly proposed proton-triggered phase transition mechanism and a possible origin of the hydrogen-bonded OH stretching region of KH2PO4-type ferroelectrics is given. 相似文献
4.
Determination of methamphetamine enantiomer ratios in urine by gas chromatography-mass spectrometry. 总被引:2,自引:0,他引:2
J T Cody 《Journal of chromatography. A》1992,580(1-2):77-95
5.
Wu Z Zhang XJ Cody RB Wolfe RR 《European journal of mass spectrometry (Chichester, England)》2004,10(5):619-623
The application of time-of-flight mass spectrometry to isotope ratio measurements has been limited by the relatively low dynamic range of the time-to-digital converter detectors available on commercial LC/ToF-MS systems. Here we report the measurement of phenylalanine isotope ratio enrichment by using a new LC/ToF-MS system with wide dynamic range. Underivatized phenylalanine was injected onto a C18 column directly with 0.1% formic acid/acetonitrile as the mobile phase. The optimal instrument parameters for the time-of-flight mass spectrometer were determined by tuning the instrument with a phenylalanine standard. The accuracy of the isotope enrichment measurement was determined by the injection of standard solutions with known isotope ratios ranging from 0.02% to 9.2%. A plot of the results against the theoretical values gave a linear curve with R2 of 0.9999. The coefficient of variation for the isotope ratio measurement was below 2%. The method is simple, rapid, and accurate and presents an attractive alternative to traditional GC/MS applications. 相似文献
6.
7.
Dickerson CA Fisher MJ Sykora RE Albrecht-Schmitt TE Cody JA 《Inorganic chemistry》2002,41(4):640-642
The compound K(3)PSe(4).2Se(6) was synthesized at 110 degrees C via solventothermal techniques from binary starting materials and Se in acetonitrile. The compound crystallizes in the space group Fd macro 3 of the cubic system with eight formula units in a cell with a dimension of a = 16.415(2) A at T = 193 K. The structure contains an unusual intermixing of ionic and uncharged species. The selenophosphate tetrahedral trianions PSe(4)(3-) are surrounded by potassium cations; other potassium cations in the structure are coordinated to 12 selenium atoms from four Se(6) rings in a tetrahedral arrangement. There are no short contacts between adjacent selenium rings. Heating the same reaction mixture to 160 degrees C results in the formation of only needles of trigonal selenium. 相似文献
8.
Pua Aileen Goh Rui Min Vivian Ee Kim-Huey Huang Yunle Liu Shao Quan Lassabliere Benjamin Yu Bin 《Chromatographia》2021,84(5):507-515
Chromatographia - The separation of isomers is one of the key issues in analysing complex natural matrices, thus commanding a higher separating power from the applied analytical system. In this... 相似文献
9.
10.
Sai Kumar Chakka Zamani E.D. Cele Sphelele C. Sosibo Vivian Francis Per I. Arvidsson Hendrik G. Kruger Glenn E.M. Maguire Thavendran Govender 《Tetrahedron: Asymmetry》2012,23(8):616-622
A series of novel TIQ based N,N′-oxide ligands were synthesised and screened for their catalytic activity in the enantioselective conjugate addition of thioglycolate to chalcones. Bulky groups on the side chain of the TIQ backbone provided the highest enantioselectivity of up to 88% with 10 mol % catalyst loading. It was also observed that these reactions proceeded optimally in the presence of dichloromethane as a solvent. Screening of various metals emphasized La(OTf)3 as the ideal pre-catalyst for this particular reaction. 相似文献