全文获取类型
收费全文 | 9496篇 |
免费 | 1106篇 |
国内免费 | 1805篇 |
专业分类
化学 | 7572篇 |
晶体学 | 507篇 |
力学 | 79篇 |
综合类 | 57篇 |
数学 | 1976篇 |
物理学 | 2216篇 |
出版年
2024年 | 35篇 |
2023年 | 174篇 |
2022年 | 240篇 |
2021年 | 259篇 |
2020年 | 331篇 |
2019年 | 367篇 |
2018年 | 294篇 |
2017年 | 400篇 |
2016年 | 401篇 |
2015年 | 350篇 |
2014年 | 451篇 |
2013年 | 927篇 |
2012年 | 562篇 |
2011年 | 764篇 |
2010年 | 570篇 |
2009年 | 627篇 |
2008年 | 648篇 |
2007年 | 626篇 |
2006年 | 543篇 |
2005年 | 458篇 |
2004年 | 436篇 |
2003年 | 422篇 |
2002年 | 435篇 |
2001年 | 288篇 |
2000年 | 234篇 |
1999年 | 219篇 |
1998年 | 166篇 |
1997年 | 135篇 |
1996年 | 141篇 |
1995年 | 132篇 |
1994年 | 105篇 |
1993年 | 86篇 |
1992年 | 78篇 |
1991年 | 65篇 |
1990年 | 53篇 |
1989年 | 23篇 |
1988年 | 28篇 |
1987年 | 19篇 |
1986年 | 28篇 |
1985年 | 39篇 |
1984年 | 22篇 |
1983年 | 22篇 |
1982年 | 25篇 |
1981年 | 25篇 |
1980年 | 16篇 |
1979年 | 23篇 |
1978年 | 23篇 |
1976年 | 14篇 |
1974年 | 16篇 |
1971年 | 16篇 |
排序方式: 共有10000条查询结果,搜索用时 10 毫秒
101.
In this paper we introduce two tree-width-like graph
invariants. The first graph invariant, which we denote by
=(G), is defined in terms of positive
semi-definite matrices and is similar to the graph invariant
(G), introduced by Colin de Verdière in
[J. Comb. Theory, Ser. B., 74:121–146, 1998]. The second graph
invariant, which we denote by (G), is defined in terms of a certain
connected subgraph property and is similar to (G), introduced by van der Holst,
Laurent, and Schrijver in [J. Comb. Theory, Ser. B., 65:291–304,
1995]. We give some theorems on the behaviour of these
invariants under certain transformations. We show that
=(G)=(G)
for any graph G with
=(G)4, and we give minimal forbidden
minor characterizations for the graphs satisfying
=(G)k
for k=1,2,3,4.This paper is extracted from two chapters of [7].
This work was done while the author was at the Centrum voor
Wiskunde en Informatica, Kruislaan 413, 1098 SJ Amsterdam, The
Netherlands. 相似文献
102.
以介孔二氧化硅MCM-41为硅源,采用低温水热反应与后续煅烧相结合的方法合成Sr2-xZnSi2O7∶xEu3+红色荧光粉。实验结果表明,150℃水热处理16 h后的反应产物含Sr2ZnSi2O7相,经后续950℃煅烧3h得到Sr2ZnSi2O7纯相。在392 nm近紫外光的有效激发下,Sr2-xZnSi2O7∶xEu3+荧光粉显示Eu3+离子的特征跃迁发射,以615 nm的5D0→7F2跃迁发射峰为最强。适量Eu3+的掺杂对Sr2ZnSi2O7相结构几乎没有影响,当Eu3+离子的掺杂量x=0.10时,荧光粉发光强度最大。 相似文献
103.
104.
Michael Zech Matthias Saurer Mario Tuthorn Katja Rinne Roland A. Werner Rolf Siegwolf 《Isotopes in environmental and health studies》2013,49(4):492-502
Although the instrumental coupling of gas chromatography-pyrolysis-isotope ratio mass spectrometry (GC-Py-IRMS) for compound-specific δ18O analysis has been commercially available for more than a decade, this method has been hardly applied so far. Here we present the first GC-Py-IRMS δ18O results for trimethylsilyl-derivatives of plant sap-relevant sugars and a polyalcohol (glucose, fructose, sucrose, raffinose and pinitol). Particularly, we focus on sucrose, which is assimilated in leaves and which is the most important transport sugar in plants and hence of utmost relevance in plant physiology and paleoclimate studies. Replication measurements of sucrose standards and concentration series indicate that the GC-Py-IRMS δ18O measurements are not stable over time and that they are amount (area) dependent. We, therefore, suggest running sample batch replication measurements in alternation with standard concentration series of reference material. This allows for carrying out (i) a drift correction, (ii) a calibration against reference material and (iii) an amount (area) correction. Tests with 18O-enriched water do not provide any evidence for oxygen isotope exchange reactions affecting sucrose and raffinose. We present the first application of GC-Py-IRMS δ18O analysis for sucrose from needle extract (soluble carbohydrate) samples. The obtained δ18Osucrose/ Vienna Standard Mean Ocean Water (VSMOW) values are more positive and vary in a wider range (32.1–40.1 ‰) than the δ18Obulk/ VSMOW values (24.6–27.2 ‰). Furthermore, they are shown to depend on the climate parameters maximum day temperature, relative air humidity and cloud cover. These findings suggest that δ18Osucrose of the investigated needles very sensitively reflects the climatically controlled evaporative 18O enrichment of leaf water and thus highlights the great potential of GC-Py-IRMS δ18Osucrose analysis for plant physiology and paleoclimate studies. 相似文献
105.
Given a graded ample Hausdorff groupoid, we realise its graded Steinberg algebra as a partial skew inverse semigroup ring. We use this to show that for a partial action of a discrete group on a locally compact Hausdorff topological space which is totally disconnected, the Steinberg algebra of the associated groupoid is graded isomorphic to the corresponding partial skew group ring. We show that there is a one-to-one correspondence between the open invariant subsets of the topological space and the graded ideals of the partial skew group ring. We also consider the algebraic version of the partial -algebra of an abelian group and realise it as a partial skew group ring via a partial action of the group on a topological space. Applications to the theory of Leavitt path algebras are given. 相似文献
106.
H.M. Martínez J. Torres M.E. Rodríguez-García L.D. López Carreño 《Physica B: Condensed Matter》2012,407(16):3199-3202
Thin films of molybdenum trioxide (MoO3) were deposited on common glass using the chemical spray pyrolysis technique. A (NH4)6Mo7O244H20 solution 0.1 M was used as the precursor one. The influence of substrate temperature on the crystallographic structure, surface morphology and electrical behavior of MoO3 thin films was studied. MoO3 can exist in two crystalline forms, the thermodynamically stable orthorhombic α-MoO3 and the metastable monoclinic β-MoO3 phase. XRD-spectra showed a growth of α-MoO3 phase percentage as substrate temperature increases from 420 K up to 670 K. Films deposited in the 500–600 K range have a clearly porous surface structure of nanometer order as can be seen in SEM images. Changes up to six magnitude orders were observed in MoO3 thin films electrical resistance when films temperature varied from 100 K up to 500 K. The sensing property of these MoO3 films was also studied. The sensitivity was investigated in the temperature range 160 and 360 K for H2O and CO gases, respectively. Both of them are of reducing nature. In all studied cases sensitivity decreases slowly as film temperature is raised. At room temperature the sensitivity changes from 12 up to 75% depending on substrate temperature. The sensitivity for CO gas was found to be lower than that of H2O. 相似文献
107.
Monte Carlo study of the universal area distribution of clusters in the honeycomb O(n) loop model 下载免费PDF全文
We investigate the area distribution of clusters (loops) in the honeycomb O(n) loop model by means of the worm algorithm with n = 0.5, 1, 1.5, and 2. At the critical point, the number of clusters, whose enclosed area is greater than A, is proportional to A-1 with a proportionality constant C. We confirm numerically that C is universal, and its value agrees well with the predictions based on the Coulomb gas method. 相似文献
108.
We consider solving eigenvalue problems or model reduction problems for a quadratic matrix polynomial Iλ2 − Aλ − B with large and sparse A and B. We propose new Arnoldi and Lanczos type processes which operate on the same space as A and B live and construct projections of A and B to produce a quadratic matrix polynomial with the coefficient matrices of much smaller size, which is used to approximate the original problem. We shall apply the new processes to solve eigenvalue problems and model reductions of a second order linear input-output system and discuss convergence properties. Our new processes are also extendable to cover a general matrix polynomial of any degree. 相似文献
109.
David Cox 《Journal of Pure and Applied Algebra》2008,212(7):1787-1796
Let a,b,c be linearly independent homogeneous polynomials in the standard Z-graded ring R?k[s,t] with the same degree d and no common divisors. This defines a morphism P1→P2. The Rees algebra of the ideal I=〈a,b,c〉 is the graded R-algebra which can be described as the image of an R-algebra homomorphism h: . This paper discusses one result concerning the structure of the kernel of the map h and its relation to the problem of finding the implicit equation of the image of the map given by a, b, c. In particular, we prove a conjecture of Hong, Simis and Vasconcelos. We also relate our results to the theory of adjoint curves and prove a special case of a conjecture of Cox. 相似文献
110.
Data-extrapolating (extension) technique has important applications in image processing on implicit surfaces and in level set methods. The existing data-extrapolating techniques are inefficient because they are designed without concerning the specialities of the extrapolating equations. Besides, there exists little work on locating the narrow band after data extrapolating—a very important problem in narrow band level set methods. In this paper, we put forward the general Huygens’ principle, and based on the principle we present two efficient data-extrapolating algorithms. The algorithms can easily locate the narrow band in data extrapolating. Furthermore, we propose a prediction–correction version for the data-extrapolating algorithms and the corresponding band locating method for a special case where the direct band locating method is hard to apply. Experiments demonstrate the efficiency of our algorithms and the convenience of the band locating method. 相似文献