全文获取类型
收费全文 | 1362篇 |
免费 | 44篇 |
国内免费 | 46篇 |
专业分类
化学 | 954篇 |
晶体学 | 1篇 |
力学 | 156篇 |
综合类 | 6篇 |
数学 | 119篇 |
物理学 | 216篇 |
出版年
2024年 | 4篇 |
2023年 | 21篇 |
2022年 | 66篇 |
2021年 | 33篇 |
2020年 | 44篇 |
2019年 | 30篇 |
2018年 | 32篇 |
2017年 | 54篇 |
2016年 | 47篇 |
2015年 | 42篇 |
2014年 | 53篇 |
2013年 | 71篇 |
2012年 | 65篇 |
2011年 | 80篇 |
2010年 | 61篇 |
2009年 | 75篇 |
2008年 | 81篇 |
2007年 | 69篇 |
2006年 | 58篇 |
2005年 | 46篇 |
2004年 | 51篇 |
2003年 | 33篇 |
2002年 | 26篇 |
2001年 | 21篇 |
2000年 | 25篇 |
1999年 | 45篇 |
1998年 | 28篇 |
1997年 | 24篇 |
1996年 | 14篇 |
1995年 | 20篇 |
1994年 | 9篇 |
1993年 | 9篇 |
1992年 | 3篇 |
1991年 | 10篇 |
1988年 | 8篇 |
1987年 | 12篇 |
1986年 | 15篇 |
1985年 | 3篇 |
1984年 | 5篇 |
1983年 | 2篇 |
1982年 | 4篇 |
1981年 | 8篇 |
1980年 | 5篇 |
1979年 | 5篇 |
1978年 | 7篇 |
1977年 | 5篇 |
1976年 | 8篇 |
1975年 | 4篇 |
1974年 | 2篇 |
1966年 | 2篇 |
排序方式: 共有1452条查询结果,搜索用时 15 毫秒
71.
采用多参考组态相互作用(MRCI)方法,结合aug-cc-pV6Z(AV6Z)基组,计算了C2+(X4Σg-,14Σu+)的势能曲线,计算过程中考虑了Davidson修正和相对论效应,并将结果外推至完备基组(CBS)的极限.基于得到的单点能量,用最小二乘法方法进行了Murrell-Sorbie函数拟合,得到了势能函数解析式(APEF).基于APEF,计算了C2+(X4Σg-,14Σu+)离子体系的离解能De,平衡核间距Re,光谱常数ωe,ωeχe,Be,αe,结果与实验和其他理论计算值符合... 相似文献
72.
级联掺磷光纤喇曼激光器的解析优化 总被引:1,自引:0,他引:1
讨论了二级级联喇曼光纤激光器的解析解和优化设计.通过引入几何平均功率、增益因子和归一化光纤有效长度,将描述泵浦光和斯托克斯光沿喇曼增益光纤分布的微分方程组简化成代数方程组,在对泵浦光采用线性传播近似后,获得了二级级联喇曼激光器的解析解.所获得的解析解同数值模拟结果吻合得很好.利用该解析解可方便和快速地讨论级联喇曼激光器的优化设计,计算不同泵浦功率下的最佳光纤长度、输出光纤光栅反射率和转换效率.泵谱功率越大,最佳光纤长度越短,最佳输出光纤光栅反射率越小. 相似文献
73.
Multivariate Curve Resolution Alternating Least Squares (MCR-ALS) can analyze three-way data under the assumption of a trilinear model using the trilinearity constraint. However, the rigid application of this constraint can produce unrealistic solutions in practice due to the inadequacy of the analyzed data to the characteristics and requirements of the trilinear model. Different methods for the relaxation of the trilinear model data requirements have been proposed, like in the PARAFAC2 and in the direct non-trilinear decomposition (DNTD) methods. In this work, the trilinearity constraint of MCR-ALS is adapted to different data scenarios where the profiles of all or some of the components of the system are shifted (not equally synchronized) or even change their shape among different slices in one of their data modes. This adaptation is especially useful in gas and liquid chromatography (GC and LC) and in Flow Injection Analysis (FIA) with multivariate spectroscopic detection. In a first data example, a synthetic LC-DAD dataset is built to investigate the possibilities of the proposed method to handle systematic changes (shifts) in the retention times of the elution profiles and the results are compared with those obtained using alternative methods like ATLD, PARAFAC, PARAFAC2 and DNTD. In a second data example, multiple wine samples were simultaneously analyzed by GC-MS where elution profiles presented large deviations (shifts) in their peak retention times, although they still preserve the same peak shape. Different modelling scenarios are tested and the results are also compared. Finally, in the third example, sample mixtures of acid compounds were analyzed by FIA under a pH gradient and monitored by UV spectroscopy and also examined by different chemometric methods using a different number of components. In this case, however, the departure of the trilinear model comes from the acid base speciation of the system depending on the pH more than from the shifting of the FIA diffusion profiles. 相似文献
74.
Bernardo Duarte Renato Mamede Irina A. Duarte Isabel Caador Susanne E. Tanner Marisa Silva David Jacinto Teresa Cruz Vanessa F. Fonseca 《Molecules (Basel, Switzerland)》2022,27(4)
The stalked barnacle Pollicipes pollicipes is an abundant species on the very exposed rocky shore habitats of the Spanish and Portuguese coasts, constituting also an important economical resource, as a seafood item with high commercial value. Twenty-four elements were measured by untargeted total reflection X-ray fluorescence spectroscopy (TXRF) in the edible peduncle of stalked barnacles sampled in six sites along the Portuguese western coast, comprising a total of 90 individuals. The elemental profile of 90 individuals originated from several geographical sites (N = 15 per site), were analysed using several chemometric multivariate approaches (variable in importance partial least square discriminant analysis (VIP-PLS-DA), stepwise linear discriminant analysis (S-LDA), linear discriminant analysis (LDA), random forests (RF) and canonical analysis of principal components (CAP)), to evaluate the ability of each approach to trace the geographical origin of the animals collected. As a suspension feeder, this species introduces a high degree of background noise, leading to a comparatively lower classification of the chemometric approaches based on the complete elemental profile of the peduncle (canonical analysis of principal components and linear discriminant analysis). The application of variable selection approaches such as the VIP-PLS-DA and S-LDA significantly increased the classification accuracy (77.8% and 84.4%, respectively) of the samples according to their harvesting area, while reducing the number of elements needed for this classification, and thus the background noise. Moreover, the selected elements are similar to those selected by other random and non-random approaches, reinforcing the reliability of this selection. This untargeted analytical procedure also allowed to depict the degree of risk, in terms of human consumption of these animals, highlighting the geographical areas where these delicacies presented lower values for critical elements compared to the standard thresholds for human consumption. 相似文献
75.
Giulia Festa Claudia Scatigno Francesco Armetta Maria Luisa Saladino Veronica Ciaramitaro Viviana Mollica Nardo Rosina Celeste Ponterio 《Molecules (Basel, Switzerland)》2022,27(1)
Spectral preprocessing data and chemometric tools are analytical methods widely applied in several scientific contexts i.e., in archaeometric applications. A systematic classification of natural powdered pigments of organic and inorganic nature through Principal Component Analysis with a multi-instruments spectroscopic study is presented here. The methodology allows the access to elementary and molecular unique benchmarks to guide and speed up the identification of an unknown pigment and its recipe. This study is conducted on a set of 48 powdered pigments and tested on a real-case sample from the wall painting in S. Maria Delle Palate di Tusa (Messina, Italy). Four spectroscopic techniques (X-ray Fluorescence, Raman, Attenuated Total Reflectance and Total Reflectance Infrared Spectroscopies) and six different spectrometers are tested to evaluate the impact of different setups. The novelty of the work is to use a systematic approach on this initial dataset using the entire spectroscopic energy range without any windows selection to solve problems linked with the manipulation of large analytes/materials to find an indistinct property of one or more spectral bands opening new frontiers in the dataset spectroscopic analyses. 相似文献
76.
高功率半导体激光端面泵浦方形掺Nd3+离子激光晶体热形变研究 总被引:5,自引:4,他引:1
为了解决高功率半导体激光器端面泵浦激光晶体引起的热效应问题,激光晶体泵浦端面的热形变必须进行准确的计算.通过对于全固态激光器中激光晶体的工作特点分析,建立了矩形截面激光晶体热分析模型.基于热传导方程,提出了泊松方程的一种新解,并获得了矩形截面激光晶体端面热形变分布的一般解析表达式.同时讨论了半导体激光器偏心泵浦激光晶体给端面热形变带来的影响.与有限元分析方法以及其他数值分析方法相比,解析分析方法不会给计算引入任何的误差.热形变的解析分析为解决激光晶体的热效应问题以及提高激光器的性能提供了理论的依据. 相似文献
77.
Temperature-dependent near-infrared (NIR) spectroscopy has been developed and taken as a powerful technique for analyzing the structure of water and the interactions in aqueous systems. Due to the overlapping of the peaks in NIR spectra, it is difficult to obtain the spectral features showing the structures and interactions. Chemometrics, therefore, is adopted to improve the spectral resolution and extract spectral information from the temperature-dependent NIR spectra for structural and quantitative analysis. In this review, works on chemometric studies for analyzing temperature-dependent NIR spectra were summarized. The temperature-induced spectral features of water structures can be extracted from the spectra with the help of chemometrics. Using the spectral variation of water with the temperature, the structural changes of small molecules, proteins, thermo-responsive polymers, and their interactions with water in aqueous solutions can be demonstrated. Furthermore, quantitative models between the spectra and the temperature or concentration can be established using the spectral variations of water and applied to determine the compositions in aqueous mixtures. 相似文献
78.
对近十年来化学计量学在生命科学领域相关研究中的应用进行了综述,主要对其在DNA、蛋白质及氨基酸这三大生命物质有关研究中的使用展开了评述和讨论,参考文献59篇。 相似文献
79.
通过固定光谱校准曲线,压缩光谱谱带间隙,可降低测试成本,工作效率提高1.5倍,分析质量准确可靠. 相似文献
80.
《Physics letters. A》2020,384(23):126556
Vibration characteristics of elastic nanostructures embedded in fluid medium have been used for biological and mechanical sensing, and also to investigate materials mechanical properties. An analytical approach based on the exact theory has been developed in this paper, to establish a new accurate and simple generalized frequency equation to predict spheroidal vibration of an elastic nanosphere, in a compressible viscoelastic fluid using linear Maxwell fluid model. To demonstrate the accuracy of the present approach, a comparison is made with the published theoretical results in the literature in some particular cases, which shows a very good agreement. Thus, the obtained frequency equation can be very useful to interpret the experimental measurements of vibrational dynamics of nanospheres and can serve as benchmark solution in design of liquid sensors. 相似文献