全文获取类型
收费全文 | 3122篇 |
免费 | 49篇 |
国内免费 | 7篇 |
专业分类
化学 | 1966篇 |
晶体学 | 13篇 |
力学 | 53篇 |
数学 | 634篇 |
物理学 | 512篇 |
出版年
2020年 | 36篇 |
2019年 | 26篇 |
2017年 | 30篇 |
2016年 | 47篇 |
2015年 | 52篇 |
2014年 | 44篇 |
2013年 | 95篇 |
2012年 | 78篇 |
2011年 | 97篇 |
2010年 | 65篇 |
2009年 | 61篇 |
2008年 | 95篇 |
2007年 | 83篇 |
2006年 | 89篇 |
2005年 | 75篇 |
2004年 | 65篇 |
2003年 | 69篇 |
2002年 | 66篇 |
2001年 | 38篇 |
2000年 | 44篇 |
1999年 | 35篇 |
1998年 | 36篇 |
1997年 | 40篇 |
1996年 | 38篇 |
1995年 | 33篇 |
1994年 | 28篇 |
1993年 | 37篇 |
1992年 | 29篇 |
1991年 | 48篇 |
1990年 | 41篇 |
1989年 | 37篇 |
1988年 | 37篇 |
1987年 | 30篇 |
1986年 | 31篇 |
1985年 | 35篇 |
1984年 | 43篇 |
1983年 | 34篇 |
1982年 | 47篇 |
1981年 | 51篇 |
1980年 | 50篇 |
1979年 | 54篇 |
1978年 | 44篇 |
1977年 | 32篇 |
1976年 | 23篇 |
1975年 | 34篇 |
1973年 | 27篇 |
1970年 | 21篇 |
1933年 | 22篇 |
1929年 | 26篇 |
1928年 | 23篇 |
排序方式: 共有3178条查询结果,搜索用时 0 毫秒
81.
82.
A combined transient and steady state approach for robust lifetime spectroscopy with micrometer resolution 下载免费PDF全文
We present the combination of two complementary micro‐photoluminescence spectroscopic techniques operating in transient and steady state condition, respectively. Introducing the time domain into the well‐established micro‐photoluminescence mapping approach operating under steady state conditions demonstrates a distinct improvement of the robustness and reliability in the determination of charge carrier lifetime measured with micrometer spatial resolution. Lifetimes from 50 ns to above ms are accessible. We elaborate a calibration procedure and apply the combined all‐photoluminescence setup to high‐performance multicrystalline silicon. A lifetime image obtained from the established photoluminescence imaging technique is reconstructed from the microscopic map by considering lateral diffusion and optical blurring, revealing a more detrimental influence of small angle grain boundaries as well as a higher lifetime within grains as may be deduced from the standard imaging technique. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim) 相似文献
83.
Dipl.‐Chem. Hilke Wolf Dr. Dirk Leusser Dr. Mads R. V. Jørgensen Dr. Regine Herbst‐Irmer Dr. Yu‐Sheng Chen Dr. Ernst‐Wilhelm Scheidt Prof. Dr. Wolfgang Scherer Prof. Dr. Bo B. Iversen Prof. Dr. Dietmar Stalke 《Chemistry (Weinheim an der Bergstrasse, Germany)》2014,20(23):7048-7053
In this contribution, the solid‐state low‐temperature phase structure of [2,2]‐paracyclophane is unambiguously characterised by single‐crystal X‐ray analysis. Additionally, a heat capacity measurement was undertaken, which proves the existence of a λ‐type phase transition at 45 K, a transition that is connected with the formation of a secondary Cp/T feature at 60 K. The low‐temperature phase (<45 K) crystallises in the lower symmetry space group P$\bar 4$ n2, whereas the high‐temperature phase (>60 K) crystallises in space group P42/mnm. This proves what has been postulated both by experimental and theoretical chemists but has repeatedly been dismissed as speculation many times. 相似文献
84.
Henrik Eickhoff Christian Sedlmeier Wilhelm Klein Gabriele Raudaschl-Sieber Hubert A. Gasteiger Thomas F. Fässler 《无机化学与普通化学杂志》2020,646(3):95-102
Recently fast lithium ion conductors were discovered in compounds containing tetrahedral SiP48– and GeP48– units. In the context of material development for all solid state batteries the ternary Li/Ge/P phase system has been further investigated and two new lithium phosphidogermanates were discovered on the lithium poor side of the ternary composition diagram. Li2GeP2 crystallizes in space group I41/acd with unit cell parameters of a = 12.3069(1) Å and c = 19.0306(4) Å, consists of a framework of Ge4P10 supratetrahedra, and exhibits an ionic conductivity of 1.5(3)×10–7 S · cm–1 at 27 °C. LiGe3P3 crystallizes in Pbam with a = 9.8459(5) Å, b = 15.7489(7) Å, and c = 3.5995(2) Å. In LiGe3P3 Ge and P atoms form a two dimensional polyanion. The slabs consist of five- and six-membered heteroatomic rings comprising GeP4 and Ge(P3Ge) tetrahedra including homoatomic Ge–Ge bonds. A semiconducting behavior with an electronic conductivity of ∼10–4 S · cm–1 and a remarkable stability vs. air and moisture is observed. 相似文献
85.
Astrid Brandt Wilhelm Schwieger Karl-Heinz Bergk Paul Grabner Martin Porsch 《Crystal Research and Technology》1989,24(1):47-54
The mechanism of the thermal behaviour of Na-magadiite at a temperature range from 293–573 K was developed based on X-ray diffraction, DTA, DTL, Q-TG, and gravimetric measurements. A structure model (BRANDT et al.) previously proposed was additionally used. It is possible to show that there probably is a connection between the typical modifications of basal spacing and significant shiftings of silicate sheets. Different steps of dehydration, decrystallization and structure variations can be ascribed to the several thermal transitions. The results depend on sample treatment such as the existent or not existent solid-water vapour interaction and the establishment of equilibrium. It seems that there is a predominance of the first mentioned influence. 相似文献
86.
Wilhelm P. Neumann 《Nachrichten aus der Chemie》1982,30(3):190-194
87.
Luiz Pereira Ramos Helena Maria Wilhelm 《Applied biochemistry and biotechnology》2005,123(1-3):807-819
In recent years, the concept of producing biodiesel from renewable lipid sources has regained international attention. In
Brazil, a national program was launched in 2002 to evaluate the technical, economic, and environmental competitiveness of
biodiesel in relation to the commercially available diesel oil. Several research projects were initiated nationwide to investigate
and/or optimize biodiesel production from renewable lipid sources and ethanol derived from sugarcane (ethyl esters). Once
implemented, this program will not only decrease our dependence on petroleum derivatives but also create new market opportunities
for agribusiness, opening new jobs in the countryside, improving the sustainability of our energy matrix, and helping the
Brazilian government to support important actions against poverty. This article discusses the efforts to develop the Brazilian
biodiesel program in the context of technical specifications as well as potential oilseed sources. 相似文献
88.
89.
90.
Summary For a square matrixT
n,n
, where (I–T) is possibly singular, we investigate the solution of the linear fixed point problemx=T
x+c by applying semiiterative methods (SIM's) to the basic iterationx
0
n
,x
k
T
c
k–1+c(k1). Such problems arise if one splits the coefficient matrix of a linear systemA
x=b of algebraic equations according toA=M–N (M nonsingular) which leads tox=M
–1
N
x+M
–1
bT
x+c. Even ifx=T
x+c is consistent there are cases where the basic iteration fails to converge, namely ifT possesses eigenvalues 1 with ||1, or if =1 is an eigenvalue ofT with nonlinear elementary divisors. In these cases — and also ifx=T
x+c is incompatible — we derive necessary and sufficient conditions implying that a SIM tends to a vector
which can be described in terms of the Drazin inverse of (I–T). We further give conditions under which
is a solution or a least squares solution of (I–T)x=c.Research supported in part by the Alexander von Humboldt-Stiftung 相似文献