全文获取类型
收费全文 | 16909篇 |
免费 | 3176篇 |
国内免费 | 2769篇 |
专业分类
化学 | 10810篇 |
晶体学 | 342篇 |
力学 | 1527篇 |
综合类 | 173篇 |
数学 | 1287篇 |
物理学 | 8715篇 |
出版年
2024年 | 38篇 |
2023年 | 161篇 |
2022年 | 410篇 |
2021年 | 458篇 |
2020年 | 661篇 |
2019年 | 510篇 |
2018年 | 510篇 |
2017年 | 627篇 |
2016年 | 717篇 |
2015年 | 752篇 |
2014年 | 897篇 |
2013年 | 1473篇 |
2012年 | 1046篇 |
2011年 | 1198篇 |
2010年 | 984篇 |
2009年 | 1124篇 |
2008年 | 1181篇 |
2007年 | 1282篇 |
2006年 | 1160篇 |
2005年 | 1054篇 |
2004年 | 885篇 |
2003年 | 879篇 |
2002年 | 770篇 |
2001年 | 595篇 |
2000年 | 583篇 |
1999年 | 511篇 |
1998年 | 374篇 |
1997年 | 348篇 |
1996年 | 289篇 |
1995年 | 213篇 |
1994年 | 213篇 |
1993年 | 154篇 |
1992年 | 144篇 |
1991年 | 118篇 |
1990年 | 72篇 |
1989年 | 87篇 |
1988年 | 56篇 |
1987年 | 48篇 |
1986年 | 26篇 |
1985年 | 57篇 |
1984年 | 31篇 |
1983年 | 19篇 |
1982年 | 36篇 |
1981年 | 20篇 |
1980年 | 13篇 |
1979年 | 15篇 |
1977年 | 7篇 |
1976年 | 12篇 |
1974年 | 4篇 |
1973年 | 12篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
91.
C. Yeung T. Rogers A. Hernandez-Machado David Jasnow 《Journal of statistical physics》1992,66(3-4):1071-1088
We study phase separation dynamics in a driven diffusive system. Our simulations are based on the Cahn-Hilliard equation with an additional flux term due to an external field. We study the dynamical scaling parallel and perpendicular to the field. A crossover is observed from isotropic domains at early times to extremely anisotropic domains at later times. We find that the inverse interfacial density (an isotropic measure of the domain size) increases ast
, with =1/3, from early times independent of the field strength, even though we do not observe dynamical scaling during these times. Our results indicate that a growth exponent =1/3 may be more universal than previously expected. We analyze the dynamics in terms of surface driven instabilities and one-dimensional solitary waves. 相似文献
92.
A. Miele B. P. Mohanty P. Venkataraman Y. M. Kuo 《Journal of Optimization Theory and Applications》1982,38(1):97-109
This paper contains general transformation techniques useful to convert minimax problems of optimal control into the Mayer-Bolza problem of the calculus of variations [Problem (P)]. We consider two types of minimax problems: minimax problems of Type (Q), in which the minimax function depends on the state and does not depend on the control; and minimax problems of Type (R), in which the minimax function depends on both the state and the control. Both Problem (Q) and Problem (R) can be reduced to Problem (P).For Problem (Q), we exploit the analogy with a bounded-state problem in combination with a transformation of the Jacobson type. This requires the proper augmentation of the state vectorx(t), the control vectoru(t), and the parameter vector , as well as the proper augmentation of the constraining relations. As a result of the transformation, the unknown minimax value of the performance index becomes a component of the parameter vector being optimized.For Problem (R), we exploit the analogy with a bounded-control problem in combination with a transformation of the Valentine type. This requires the proper augmentation of the control vectoru(t) and the parameter vector , as well as the proper augmentation of the constraining relations. As a result of the transformation, the unknown minimax value of the performance index becomes a component of the parameter vector being optimized.In a subsequent paper (Part 2), the transformation techniques presented here are employed in conjunction with the sequential gradient-restoration algorithm for solving optimal control problems on a digital computer; both the single-subarc approach and the multiple-subarc approach are discussed.This research was supported by the National Science Foundation, Grant No. ENG-79-18667, and by Wright-Patterson Air Force Base, Contract No. F33615-80-C3000. This paper is a condensation of the investigations reported in Refs. 1–7. The authors are indebted to E. M. Coker and E. M. Sims for analytical and computational assistance. 相似文献
93.
J. M. Ginés M. J. Arias J. R. Moyano Cs. Novak G. Pokol P. J. Sánchez-Soto 《Journal of Thermal Analysis and Calorimetry》1996,47(6):1743-1753
A thermal study using DSC and Hot Stage Microscopy (HSM) was carried out to investigate the interaction in solid state of the binary system PEG 4000 — oxazepam, and to establish their phase diagram. The eutectic composition, which melting occurs at lower temperature as compared with the pure components, has been determined. The results obtained by DSC and HSM have indicated that PEG 4000 — oxazepam mixtures displays no obvious incompatibilities, and that the system shows a typical eutectic behaviour. However because of the closeness of the melting of PEG 4000 to the eutectic temperature, it was difficult to determine precisely the eutectic composition and temperature on the basis of DSC measurements alone. The use of heats of fusion corresponding to physical mixtures allowed an estimation of the eutectic composition at 6% w/w oxazepam. Additional information of temperature (57.6C) and composition (5–10% w/w oxazepam) of the eutectic was obtained by HSM using the contact method. This low melting temperature in this range of compositions offers advantages in terms of drug stability and easy manufacture. 相似文献
94.
J. A. Navio M. Macias A. Justo C. Real 《Journal of Thermal Analysis and Calorimetry》1992,38(4):673-682
The thermal decomposition of sodium nitrite or nitrate pre-adsorbed upon TiO2 surfaces has been investigated by employing several techniques as infrared spectroscopy (IR) and temperature programmed desorption in conjunction with mass spectrometry analysis (TPD-MS) to study the features observed during these thermal decompositions. Differential thermal analysis (DTA) in combination with X-ray diffraction analysis (XRD) were used to investigate the possibility of a solid state chemical reaction between the solid products originated from the thermal decomposition of the pre-adsorbed species and the TiO2. On the basis of our results, various characteristic features of these thermal decomposition reactions will be discussed.This work was supported by JUNTA DE ANDALUCIA (financial support for research groups/1990). 相似文献
95.
L. F. Brum Malta Marta Eloisa Medeiros 《Journal of Thermal Analysis and Calorimetry》2005,81(1):149-152
Summary This work reports the room-temperature stabilization of the Bi4V2-xFexIIO11-1.5x γ ‘ phase, a promising ionic conductive material that finds application in solid oxide fuel cell and oxygen sensor devices. The Fe(II) cation proved to be a better stabilizer than Fe(III), which was previously used, since a lower substitution degree of V5+ is needed for the former. Powder X-ray diffraction, Fourier-transform infrared spectroscopy and differential scanning calorimetry were used in these experiments. 相似文献
96.
Solid solutions NaCl—CdCl2 were studied in an interval of CdCl2 concentrations of 0.05—3 mol.% by Raman spectroscopy. The molecular form Na2CdCl4 decomposes: Na2CdCl4 2Na+ + Cd2+ + 2Cl2
– + 2e–; 2Na + 2e– 2Na0. Free sodium atoms form color centers of crystal (F centers) in the region of jumpwise changing the partition coefficient of CdCl2 in NaCl (K jump). 相似文献
97.
The Studies of the Heterogeneous Properties of Solid Surfaces by Means of the Derivatograph Q-1500 D
A simple method has been adopted to obtain an important information on the heterogeneous properties of materials studied,
adsorbed liquid films and liquid-solid interactions. The method utilizes Q-TG mass loss and the first derivative Q-DTG mass
loss curves with respect to temperature and time obtained during programmed liquid thermodesorption in quasi-isothermal conditions.
The values of the adsorption capacity, total porosity of material, the value of active centers, desorption energy distribution
and mesopore-size distribution functions obtained by this method are in good agreement with those estimated on the basis of
independent methods. The theoretical and experimental results provided novel and unique data on the heterogeneity of solid
surfaces, properties of liquid adsorbed films and thermal stability of the liquid/solid interfaces.
This paper presents the more important results obtained so far in the studies the liquid/solid systems by means of the Paulik's
classical and quasi-isothermal techniques.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
98.
The analysis of samples contaminated by organic compounds is an important aspect of environmental monitoring. Because of the complex nature of these samples, isolating target organic compounds from their matrices is a major challenge. A new isolation technique, solid phase microextraction, or SPME, has recently been developed in our laboratory. This technique combines the extraction and concentration processes into one step; a fused silica fiber coated with a polymer is used to extract analytes and transfer them into a GC injector for thermal desorption and analysis. It is simple, rapid, inexpensive, completely solvent-free, and easily automated. To minimize matrix interferences in environmental samples, SPME can be used to extract analytes from the headspace above the sample. The combination of headspace sampling with SPME separates volatile and semi-volatile analytes from non-volatile compounds, thus greatly reducing the interferences from non-target compounds. This paper reports the use of headspace SPME to isolate volatile organic compounds from various matrices such as water, sand, clay, and sludge. By use of the technique, benzene, toluene, ethyl-benzene, and xylene isomers (commonly known as BTEX), and volatile chlorinated compounds can be efficiently isolated from various matrices with good precision and low limits of detection. This study has found that the sensitivity of the method can be greatly improved by the addition of salt to water samples, water to soil samples, or by heating. Headspace SPME can also be used to sample semi-volatile compounds, such as PAHs, from complex matrices. 相似文献
99.
100.