全文获取类型
收费全文 | 12772篇 |
免费 | 830篇 |
国内免费 | 1297篇 |
专业分类
化学 | 5499篇 |
晶体学 | 151篇 |
力学 | 579篇 |
综合类 | 141篇 |
数学 | 4633篇 |
物理学 | 3896篇 |
出版年
2024年 | 17篇 |
2023年 | 188篇 |
2022年 | 303篇 |
2021年 | 277篇 |
2020年 | 383篇 |
2019年 | 361篇 |
2018年 | 329篇 |
2017年 | 340篇 |
2016年 | 441篇 |
2015年 | 382篇 |
2014年 | 574篇 |
2013年 | 897篇 |
2012年 | 549篇 |
2011年 | 791篇 |
2010年 | 669篇 |
2009年 | 802篇 |
2008年 | 812篇 |
2007年 | 897篇 |
2006年 | 678篇 |
2005年 | 574篇 |
2004年 | 558篇 |
2003年 | 554篇 |
2002年 | 423篇 |
2001年 | 367篇 |
2000年 | 345篇 |
1999年 | 301篇 |
1998年 | 287篇 |
1997年 | 228篇 |
1996年 | 222篇 |
1995年 | 175篇 |
1994年 | 169篇 |
1993年 | 113篇 |
1992年 | 106篇 |
1991年 | 104篇 |
1990年 | 68篇 |
1989年 | 59篇 |
1988年 | 68篇 |
1987年 | 42篇 |
1986年 | 45篇 |
1985年 | 55篇 |
1984年 | 45篇 |
1983年 | 28篇 |
1982年 | 45篇 |
1981年 | 40篇 |
1980年 | 24篇 |
1979年 | 35篇 |
1978年 | 30篇 |
1977年 | 29篇 |
1976年 | 29篇 |
1973年 | 13篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
941.
Debdas Roy 《Journal of Physics and Chemistry of Solids》2007,68(12):2321-2325
A variety of bioactive glasses have been investigated over the last two decades as substitute material for diseased or damaged tissues in a human body. In this investigation, three different melt derived bioactive glasses, each having 55% by mole SiO2 and ratio of MgO to Na2O varying from 1:8 to 8:1, were prepared by melting various oxides at temperature >1250 °C. After microstructure evolution, vitro reactivity of these glasses was examined by keeping them in simulated body fluid (trans buffered pH 7.25 at 25 cc). The surface reactivity of these glasses gradually increased with increasing Na2O/MgO ratio. 相似文献
942.
Aaron Dodd Allan McKinley Martin Saunders 《Journal of Physics and Chemistry of Solids》2007,68(12):2341-2348
Nanoparticulate TiO2 is of interest for a variety of technological applications, including optically transparent UV-filters and photocatalysts for the destruction of chemical waste. The successful use of nanoparticulate TiO2 in such applications requires an understanding of how the synthesis conditions effect the optical and photocatalytic properties. In this study, we have investigated the effect of heat treatment temperature on the properties of nanoparticulate TiO2 powders that were synthesised by solid-state chemical reaction of anhydrous TiOSO4 with Na2CO3. It was found that the photocatalytic activity increased with the heat treatment temperature up to a maximum at 600 °C and thereafter declined. In contrast, the optical transparency decreased monotonically with the heat treatment temperature. These results indicate that solid-state chemical reaction can be used to prepare powders of nanoparticulate TiO2 with properties that are optimised for use as either optically transparent UV-filters or photocatalysts. 相似文献
943.
S. Pisana M. Cantoro A. Parvez S. Hofmann A.C. Ferrari J. Robertson 《Physica E: Low-dimensional Systems and Nanostructures》2007,37(1-2):1
Catalyst films undergo considerable surface morphology restructuring prior to carbon nanotube nucleation, deeply influencing the nanostructures obtained. Here we study the influence of different gaseous atmospheres on the structure of thin Fe films. The morphology is influenced by process temperature and substrate interactions and varying the gas type and pressure can control the average catalyst island height. 相似文献
944.
The one-dimensional totally asymmetric simple exclusion process (TASEP) is considered. We study the time evolution property
of a tagged particle in the TASEP with the step initial condition. Calculated is the multi-time joint distribution function
of its position. Using the relation of the dynamics of the TASEP to the Schur process, we show that the function is represented
as the Fredholm determinant. We also study the scaling limit. The universality of the largest eigenvalue in the random matrix
theory is realized in the limit. When the hopping rates of all particles are the same, it is found that the joint distribution
function converges to that of the Airy process after the time at which the particle begins to move. On the other hand, when
there are several particles with small hopping rate in front of a tagged particle, the limiting process changes at a certain
time from the Airy process to the process of the largest eigenvalue in the Hermitian multi-matrix model with external sources. 相似文献
945.
L.H. Nguyen T.V. Phi P.Q. Phan H.N. Vu C. Nguyen-Duc F. Fossard 《Physica E: Low-dimensional Systems and Nanostructures》2007,37(1-2):54
It has been recently demonstrated that carbon nanotubes (CNTs) represent a new type of chemical sensor capable of detecting a small concentration of molecules such as CO, NO2, NH3.In this work, CNTs were synthesized by chemical vapor deposition (CVD) on the SiO2/Si substrate by decomposition of acetylene (C2H2) on sputtered Ni catalyst nanoparticles. Their structural properties are studied by atomic force microscopy, high-resolution scanning electron microscopy (HRSEM) and Raman spectroscopy. The CNTs grown at 700 °C exhibit a low dispersion in size, are about 1 μm long and their average diameter varies in the range 25–60 nm as a function of the deposition time. We have shown that their diameter can be reduced either by annealing in oxygen environment or by growing at lower temperature (less than 600 °C).We developed a test device with interdigital Pt electrodes on an Al2O3 substrate in order to evaluate the CNTs-based gas sensor capabilities. We performed room temperature current–voltage measurements for various gas concentrations. The CNT films are found to exhibit a fast response and a high sensitivity to NH3 gas. 相似文献
946.
Solid solutions of cadmium and lead fluoroapatite [Pb(10−x)Cdx(PO4)6F2 (0?x?5)] were synthesized by a wet process in a basic medium. Replacement of lead by cadmium induces a linear variation of the crystallographic parameters “a” and “c” according to Végard's law. The cadmium content, as obtained from the refinement, is in agreement with the chemical analysis. The distribution of the lead and cadmium ions between two non-equivalent crystallographic sites, M(1) and M(2), was determined by the Rietveld method. The site-occupancy factors of atoms clearly indicate a preference of cadmium for site M(1) in the apatite structure in agreement with its smaller ionic radii. A progressive shift of the F− ion toward the center of the triangles formed by the site M(2) metals has been observed with increasing cadmium content. 相似文献
947.
Naturally occurring saponins are attractive candidates or lead compounds for new drug design and development and thus have become important synthetic targets. This short account reviews the recent progress in the chemical synthesis and structure-activity relationship (SAR) studies of naturally occurring saponins and their derivatives. 相似文献
948.
The Carbon-13 chemical shifts of N-Cinamylidenehexyl, t-butyl and sixteen substituted aryl amines were analysed. The lanthanide induced carbon-13 chemical shifts of the cis and trans of four N-cinamylidene substituted anilines were analysed. The spectra N-cinamylidene hexylamine showed only the trans form. The cis/trans ratio of N-cinamylidene aryl amines increased as the LSR concentration increased. 相似文献
949.
Koel Adhikary 《哲学杂志》2013,93(33):4075-4087
We report on the successful fabrication of polycrystalline silicon films by aluminium-induced crystallisation (AIC) of Radio frequency (rf) plasma-enhanced chemical vapour deposited (PECVD) a-Si films. The effects of annealing at different temperatures (300 and 400°C), below the eutectic temperature of the Si–Al binary system, on the crystallisation process have been studied. This work emphasises the important role of the position of the Al layer with respect to the Si layer on the crystallisation process. The properties of the crystallised films were characterised using X-ray diffraction, Raman spectroscopy, ellipsometry, field-emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM). With an increase in the annealing temperature, it was found that the degree of crystallisation of annealed a-Si/Al and Al/a-Si films increased. The results showed that the arrangement where the Al was on top of the a-Si had a more prominent effect on crystallisation enhancement than when Al was below the a-Si. The interfacial layer between the Al and a-Si film is crucial because it influences the layer-exchange process during annealing. The oxide layer formed between the Al and the a-Si layers greatly retards the crystallisation process in the case of the Al/Si arrangement. Our investigations suggest that polycrystalline Si films formed by AIC can be used as a seed layer in solar cell fabrication. 相似文献
950.
《Journal of computational and graphical statistics》2013,22(2):479-491
Total Variation-based regularization, well established for image processing applications such as denoising, was recently introduced for Maximum Penalized Likelihood Estimation (MPLE) as an effective way to estimate nonsmooth probability densities. While the estimates show promise for a variety of applications, the nonlinearity of the regularization leads to computational challenges, especially in multidimensions. In this article we present a numerical methodology, based upon the Split Bregman L1 minimization technique, that overcomes these challenges, allowing for the fast and accurate computation of 2D TV-based MPLE. We test the methodology with several examples, including V-fold cross-validation with large 2D datasets, and highlight the application of TV-based MPLE to point process crime modeling. The proposed algorithm is implemented as the Matlab function TVMPLE. The Matlab (mex) code and datasets for examples and simulations are available as online supplements. 相似文献