全文获取类型
收费全文 | 10322篇 |
免费 | 794篇 |
国内免费 | 897篇 |
专业分类
化学 | 3951篇 |
晶体学 | 95篇 |
力学 | 1366篇 |
综合类 | 113篇 |
数学 | 1995篇 |
物理学 | 4493篇 |
出版年
2024年 | 8篇 |
2023年 | 129篇 |
2022年 | 255篇 |
2021年 | 288篇 |
2020年 | 264篇 |
2019年 | 202篇 |
2018年 | 214篇 |
2017年 | 248篇 |
2016年 | 240篇 |
2015年 | 264篇 |
2014年 | 388篇 |
2013年 | 829篇 |
2012年 | 608篇 |
2011年 | 702篇 |
2010年 | 514篇 |
2009年 | 629篇 |
2008年 | 599篇 |
2007年 | 692篇 |
2006年 | 551篇 |
2005年 | 484篇 |
2004年 | 470篇 |
2003年 | 436篇 |
2002年 | 370篇 |
2001年 | 276篇 |
2000年 | 283篇 |
1999年 | 219篇 |
1998年 | 272篇 |
1997年 | 229篇 |
1996年 | 197篇 |
1995年 | 156篇 |
1994年 | 137篇 |
1993年 | 139篇 |
1992年 | 116篇 |
1991年 | 74篇 |
1990年 | 82篇 |
1989年 | 72篇 |
1988年 | 68篇 |
1987年 | 34篇 |
1986年 | 42篇 |
1985年 | 51篇 |
1984年 | 31篇 |
1983年 | 11篇 |
1982年 | 36篇 |
1981年 | 14篇 |
1980年 | 19篇 |
1979年 | 16篇 |
1978年 | 14篇 |
1977年 | 5篇 |
1976年 | 13篇 |
1973年 | 8篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
Sankar Prasad Bhuniya Safikur Rahman Anshul J. Satyanand Mahendrasinh M. Gharia Ashok M. Dave 《Journal of polymer science. Part A, Polymer chemistry》2003,41(11):1650-1658
Maize starch was modified by allyl chloride adopting an interfacial reaction technique with cetyltrimethyl ammonium bromide as a phase‐transfer catalyst and pyridine as an acid acceptor. The degree of substitution was determined from an increasing carbon content of the modified starch. The percentage of carbon and hydrogen of the allyl‐modified starch was estimated by elemental analysis (C, H, and N), and the product characterization was done through 1H NMR and 13C NMR analyses. The allyl‐modified starch was then copolymerized with methacrylic acid and a combination of methacrylic acid and acrylamide at 50 and 70 °C with potassium persulfate as an initiator. The copolymer thus formed swelled in distilled water after neutralization with sodium carbonate. The percentage of absorption capacity of the hydrogels was determined with distilled water and 0.9% NaCl solution. The highest percentage of absorption, 6500%, was achieved for the developed hydrogel containing allyl starch and acrylic monomer in a 1.7:1 w/w ratio and acrylic monomer, namely, methacrylic acid and acrylamide in a 3.2:1 w/w ratio. The study on biodegradability of the developed hydrogel showed that the hydrogel is degradable in the presence of diastase (amylase). © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 1650–1658, 2003 相似文献
92.
93.
该文用顺磁离子对其近邻~(13)C核的NMR吸收增宽效应,系统地研究了抗坏血酸在不同溶剂中对金属离子Mn~(2+)、Gd~(3+)的配合行为.在水溶液中,抗坏血酸可通过两种配合方式与Mn~(2+)作用,但与Gd~(3+)的作用只有一种稳定的结构.在水中抗坏血酸对金属的配位活性部位分别是1位羟基氧,3位离解的羟基氧和6位羟基氧.在DMSO中,3位羟基由于H~+对该基团的静电遮蔽而变得对金属离子Mn~(2+)表现为配位惰性,同时6位羟基配位活性较大增强.对Gd~(3+)来说,此时没有优势配位活性部位存在.文章还讨论了抗坏血酸在H_2O-DMSO混合溶剂中的~(13)C NMR谱及在DMSO中NaOH对其~(13)C NMR谱及其与金属络合结构的影响. 相似文献
94.
J.-L. Garden 《Thermochimica Acta》2007,452(2):85-105
What is really measured in dynamic calorimetric experiments is still an open question. This paper is devoted to this question, which can be usefully envisaged by means of macroscopic non-equilibrium thermodynamics. From the pioneer work of De Donder on chemical reactions and with other authors along the 20th century, the question is tackled under an historical point of view. A special attention is paid about the notions of frequency dependent complex heat capacity and entropy production due to irreversible processes occurring during an experiment. This phenomenological approach based on thermodynamics, not widely spread in the literature of calorimetry, could open significant perspectives on the study of macro-systems undergoing physico-chemical transformations probed by dynamic calorimetry. 相似文献
95.
本文以3-甲基苯甲酸为配体,合成了与三价稀土镧离子的固体配合物LaL3.H2O,并对该配合物进行了元素分析,红外光谱,UV光谱,TG-DTA,HNMR等的测试与表征。该配合物属螯合双齿配位。 相似文献
96.
林支桂 《数学物理学报(B辑英文版)》1998,(3)
1IntroductionInthispaper,weconsidertheinitial-boundaryvalueproblem,where"o(x)isnon-negativesmoothfunctionsatisfying"o.(0)=0,"o.(1)=1'Whenconsideringtheblow-upofsolution,thefollowingproblemarisenaturally:Doesblow-upoccur?Howdoesthesolutionapproachtheblow-uptime'!Andwilersisthehotspotlocated(blow-upset)?WelookattheheatequationwithanonlillearboundaryconditiollHerefiisaboundeddomaininR",p>1isarealnumber.IthasbeenknownforalongtimethattheDroblelil(1.1).(1.2)withAL(~,0)--'no(x)doesnothaveaglobals… 相似文献
97.
本文将布拉格方程推广应用到高聚物的结构研究中,得到了经不同剂量辐照的聚乙烯单晶的长周期,并辅以示差扫描量热法研究了γ-射线辐照对聚乙烯单晶结构的影响. 相似文献
98.
Manicka Dhanasekar Jianjun Han Qinghua Qin 《Finite Elements in Analysis and Design》2006,42(14-15):1314-1323
Much work on special elements that simplify geometrical modelling of structures containing holes, cracks and/ or inclusions has been reported extensively in the literature. This paper presents a hybrid-Trefftz element containing elliptic hole formulated using Hellinger–Reissner principle by employing trial functions based on the mapping technique and the Cauchy integral method. The element presented in this paper could be regarded as an improved formulation over Piltner [Special finite elements with holes and internal cracks, Int. J. Numer. Methods Eng. 21 (1985) 1471–1485] element because the chosen trail functions in this paper have provided relatively more stable solutions. The use of the element with other ordinary displacement-based finite elements has also yielded very accurate solutions even when very coarse meshes relative to the size of the elliptic hole have been used. 相似文献
99.
Unconventional antiferromagnetic correlations of the doped Haldane gapsystem Y 2 BaNi 1 - x Zn x O 5
V. Villar R. Mélin C. Paulsen J. Souletie E. Janod C. Payen 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,25(1):39-51
We make a new proposal to describe the very low temperature susceptibility of the doped Haldane gap compound Y2BaNi1-xZnxO5. We propose a new mean field model relevant for this compound. The ground state of this mean field model is unconventional
because antiferromagnetism coexists with random dimers. We present new susceptibility experiments at very low temperature.
We obtain a Curie-Weiss susceptibility χ(
T
) ∼
C
/(Θ +
T
) as expected for antiferromagnetic correlations but we do not obtain a direct signature of antiferromagnetic long range order.
We explain how to obtain the “impurity” susceptibility
(
T
) by subtracting the Haldane gap contribution to the total susceptibility. In the temperature range [1 K, 300 K] the experimental
data are well fitted by T
(
T
) =
C
imp
1 +
T
imp
/
T
. In the temperature range [100 mK, 1 K] the experimental data are well fitted by T
(
T
) =
A
ln(
T
/
T
c
), where T
c
increases with x. This fit suggests the existence of a finite Néel temperature which is however too small to be probed directly in our experiments.
We also obtain a maximum in the temperature dependence of the ac-susceptibility
(
T
) which suggests the existence of antiferromagnetic correlations at very low temperature.
Received 17 July 2001 相似文献
100.
A new and rather trivial model is suggested with mechanism that implies simultaneous violation of the zeroth and the second
laws of thermodynamics. Mathematically rigorous quantum theory reduces to a trivial application of the Golden rule formula.
It yields exciton on-energy-shell diffusion caused by bath-nonassisted excitation hopping between tails of different exciton
site levels ε1 < ε2 broadened by bath-assisted finite life-time effects. The elastic character of the hopping implies 1 ↔ 2-symmetric transfer
rate W. Thus the net diffusion exciton flow W(P
1 - P
2) and also, as argued, the net energy flow are possible due to different near-to-equilibrium exciton populations P
1 > P
2. As the sites are provided with two different baths, the population imbalance and the flows survive even for slightly different
local bath temperatures T
1 < T
2 < T
1ε2/ε1. Thus spontaneous exciton and also energy flows against temperature step become possible, in contradiction with the Clausius
form of the second law. Violations of both the laws disappear in the high-temperature, i.e. classical limit
Received 16 May 2001 and Received in final form 20 September 2001 相似文献