全文获取类型
收费全文 | 3504篇 |
免费 | 486篇 |
国内免费 | 237篇 |
专业分类
化学 | 883篇 |
晶体学 | 57篇 |
力学 | 440篇 |
综合类 | 71篇 |
数学 | 1277篇 |
物理学 | 1499篇 |
出版年
2023年 | 20篇 |
2022年 | 55篇 |
2021年 | 64篇 |
2020年 | 98篇 |
2019年 | 84篇 |
2018年 | 98篇 |
2017年 | 87篇 |
2016年 | 126篇 |
2015年 | 84篇 |
2014年 | 189篇 |
2013年 | 219篇 |
2012年 | 176篇 |
2011年 | 229篇 |
2010年 | 189篇 |
2009年 | 215篇 |
2008年 | 211篇 |
2007年 | 228篇 |
2006年 | 202篇 |
2005年 | 202篇 |
2004年 | 153篇 |
2003年 | 155篇 |
2002年 | 151篇 |
2001年 | 116篇 |
2000年 | 112篇 |
1999年 | 87篇 |
1998年 | 78篇 |
1997年 | 60篇 |
1996年 | 74篇 |
1995年 | 38篇 |
1994年 | 53篇 |
1993年 | 42篇 |
1992年 | 30篇 |
1991年 | 33篇 |
1990年 | 34篇 |
1989年 | 22篇 |
1988年 | 28篇 |
1987年 | 21篇 |
1986年 | 19篇 |
1985年 | 20篇 |
1984年 | 24篇 |
1983年 | 7篇 |
1982年 | 16篇 |
1981年 | 18篇 |
1980年 | 6篇 |
1979年 | 8篇 |
1978年 | 5篇 |
1977年 | 11篇 |
1976年 | 7篇 |
1974年 | 4篇 |
1973年 | 7篇 |
排序方式: 共有4227条查询结果,搜索用时 15 毫秒
81.
Summary The members of the power divergence family of statistics
all have an asymptotically equivalent χ2 distribution (Cressie and Read [1]). An asymptotic expansion for the distribution function is derived which shows that the
speed of convergence to this asymptotic limit is dependent on λ. Known results for Pearson'sX
2 statistic and the log-likelihood ratio statistic then appear as special cases in a continuum rather than as separate (unrelated)
expansions. 相似文献
82.
Two equivalents cf anhydrous m-chloroperbenzoic acid (m-CPBA) cleaved the pyridone ring of 10-(3-chlorophenyl)-6,8,9,10-tetrahydrobenzo[b][1,8]naph forming; the ten-membered lactam α-diketone 12-(3-chlorophenyl)-7,8,9,10-tetrahydropyrido[2,3-b]azecine-5,6,11(12H)-trione. Under aqueous conditions, one equivalent of m-CPBA and the same pyridone formed the lactam α-ketol 12-(3-chlorophenyl)-7,8,9,10-tetrahydro-6-hydroxypyrido[2,3-b]azecine-5,11(6H, 12H)-dio 相似文献
83.
M.G. Marmorino 《Journal of mathematical chemistry》2002,31(2):205-210
The asymptotic behavior of a system's ground-state energy from the t expansion of Horn and Weinstein has been suggested to have the form E
1(t)=E
1+exp(–a
n
t+b
n
). In the limit of very large t, this becomes E
1(t)=E
1+exp(–a
1
t+b
1). A simple analysis shows that the parameters are a
1=E
2–E
1 and b
1=ln[(E
2–E
1)|c
2|2/|c
1|2]. Functions are introduced which allow determination of a
1, b
1 and lower bounds to E
1. 相似文献
84.
Rodová M. Brožek J. Knížek K. Nitsch K. 《Journal of Thermal Analysis and Calorimetry》2003,71(2):667-673
Phase transitions in ternary caesium lead bromide (CsPbBr3) were studied by means of DSC, TMA and high temperature X-ray diffraction. The samples were prepared from the solution by
water evaporation and from the melt. on the DSC curves as well as on the temperature dependence of the lattice constants of
CsPbBr3 only two effects were found belonging to the earlier published phase transitions at 88 and 130°C and no further effects.
Linear thermal expansion coefficient α of individual CsPbBr3 modifications were calculated from both TMA and high temperature X-ray diffraction. The structural parameters of the room
temperature orthorhombic phase were refined and the results are presented. CsPbBr3 prepared from the solution contained about 10% of CsPb2Br5 and so the DSC curve of pure CsPb2Br5 was also measured and an effect at a temperature of 68.5°C was found.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
85.
Carl M. Bender Fred Cooper Gerald Guralnik Harvey A. Rose David H. Sharp 《Journal of statistical physics》1980,22(6):647-660
We discuss the simple, randomly driven systemdx/dt = –x –x3 +f(t), wheref(t) is a Gaussian random function or stirring force with f(t)f(t) = (t – t). We show how to obtain approximately the coefficients of the expansion of the equal-time Green's functions as power series in (1/R)n, whereR is the internal Reynolds number ()1/2/, by using a new expansion for the path integral representation of the generating functional for the correlation functions. Exploiting the fact that the action for the randomly driven system is related to that of a quantum mechanical anharmonic oscillator with Hamiltonianp
2/2 +m
2
x
2/2 +vx
4 +x
6/2, we evaluate the path integral on a lattice by assuming that thex
6 term dominates the action. This gives an expansion of the lattice theory Green's functions as power series in 1/(a)1/3, wherea is the lattice spacing. Using Padé approximants to extrapolate toa = 0, we obtain the desired large-Reynolds-number expansion of the two-point function.Supported financially by the National Science Foundation and the U.S. Department of Energy. 相似文献
86.
Giorgio Parisi 《Journal of statistical physics》1980,23(1):49-82
We review the physical principles which are at the basis of recent field-theoretic computations of the critical exponents in two- and three-dimensional systems. We concentrate on those points that do not show up explicitly in the more standard-expansion: they must be discussed with care if one uses a perturbative approach at fixed space dimensions (the loop expansion). We present in detail simple computations of the critical exponents, while we summarize the results of longer and more accurate computations. 相似文献
87.
Exact renormalization group equations are derived for a position-space renormalization of spin systems with weak long-range forces. It is shown how an apparent dependence of the critical exponents on the choice of the renormalization group can be resolved via the mechanism of dangerous irrelevant variables and that this same mechanism is responsible for the breakdown of hyperscaling. The dimensiond=4 can be seen to be a borderline dimension above which classical critical exponents are expected. 相似文献
88.
OUH体系的结构和分析势能函数 总被引:3,自引:0,他引:3
采用密度泛涵B3LYP方法优化出了OUH分子的各种结构,确定了最稳定构型和离解能,以及它们的谐性力常数,并导出双原子分子UH,UO的Murrell-Sorbie势能函数及其光谱数据。采用多体项展式方法,导出OUH(X^4A')基态分子的分析势能函数,获得OUH(X^4A')体系的势能面,考察了这个势能函数的基本性质,正确地复现出OUH分子的平衡结构特征,结果表明:U+OH,O+UH,H+UO的反应均为无阈能的放热能反应。为进一步探讨OUH体系的反应动力学过程打下了基础。 相似文献
89.
R.K.B GoverN.D. Withers S. AllenR.L. Withers J.S.O. Evans 《Journal of solid state chemistry》2002,166(1):42-48
SnP2O7 is a member of the ZrP2O7 family of materials, several of which show unusual thermal expansion behavior over certain temperature ranges and which show a number of displacive phase transitions on cooling from high temperature. Here we describe the structural properties of SnP2O7 from 100 to 1243 K as determined by X-ray and neutron powder diffraction. These studies reveal that SnP2O7 shows two phase transitions in this temperature range. At room temperature the material has a pseudo-cubic 3×3×3× superstructure. Electron diffraction studies show that the symmetry of this structure is P213 or lower. On warming to ∼560 K it undergoes a phase transition to a structure in which the subcell reflections show a triclinic distortion; above 830 K the subcell reflections show a rhombohedral distortion. Significant hysteresis in cell parameters is observed between heating and cooling. The structure of SnP2O7 is discussed with references to other members of the AM2O7 family of materials. 相似文献
90.
Diego Ardura 《Tetrahedron letters》2004,45(47):8691-8694
UB3LYP/6-311++G(d,p) and ROMP2/6-311++G(d,p)//UB3LYP/6-311++G(d,p) calculations including the effect of benzene solvent through the PCM-UAHF method render a concerted mechanism without fragmentation as the most favourable one for the Dowd-Beckwith radical ring expansion of the bromomethyl adduct of methyl cyclopentanone-2-carboxylate to yield methyl cyclohexanone-3-carboxylate. The corresponding concerted TS is a bicyclic alcoxy radical. 相似文献