首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   280篇
  免费   12篇
  国内免费   1篇
化学   252篇
数学   15篇
物理学   26篇
  2023年   5篇
  2022年   26篇
  2021年   35篇
  2020年   19篇
  2019年   12篇
  2018年   12篇
  2017年   10篇
  2016年   16篇
  2015年   14篇
  2014年   11篇
  2013年   15篇
  2012年   19篇
  2011年   20篇
  2010年   8篇
  2009年   4篇
  2008年   11篇
  2007年   10篇
  2006年   10篇
  2005年   10篇
  2004年   5篇
  2003年   6篇
  2002年   4篇
  2000年   1篇
  1997年   1篇
  1996年   1篇
  1994年   1篇
  1987年   1篇
  1986年   1篇
  1977年   1篇
  1975年   1篇
  1974年   3篇
排序方式: 共有293条查询结果,搜索用时 328 毫秒
51.
This work explores the use of magnetic resonance imaging (MRI) for nondestructive determination of wood characteristics and for 3D wood modeling. In this context, one of the applications under development is the automatic recognition and reconstruction of rings from transversal images obtained from MRI scanners. The algorithm analyzes a set of transversal MRI images, detecting and reconstructing growth ring edges. The information generated is then interpolated in order to obtain an accurate 3D picture of the log and its fundamental constituents (individual rings, knots, defects, etc). Results also show that the technique has potential for defect recognition, providing a powerful tool for future developments in wood analysis. The results are encouraging and further research is needed to develop automatic detection not only of rings, but also of different types of defects that are of paramount importance in the sawmill and plywood industries.  相似文献   
52.
In this study, Eucalyptus nitens species was nano-characterized to determine variability in nanomechanical properties within the cellular ultra-structure between the bark and wood fibers. Three factors, including site (2 levels), family (2 levels) and fiber type (bark and wood) were analyzed using three response variables, including the elastic modulus (E), hardness (H) and ductility ratio (E/H) in the middle lamella (ML) and the cell wall within the S2 layer. The results indicated significant differences for E S2 and H S2 when comparing fiber types: E S2≈12.52 GPa and H S2≈0.31 GPa for wood fiber and E S2≈10.81 GPa and H S2≈0.26 GPa for bark fiber. There is not statistically significant difference in ductility ratio (E/H) in S2 and ML between fiber types. These results indicate that bark and wood fibers can be used together or separately in the development of new composite materials and engineering products.  相似文献   
53.
Numerous members of the Anthemideae tribe are important as cut flowers and ornamental crops, as well as being medicinal and aromatic plants, many of which produce essential oils used in folk and modern medicine and in the cosmetics and pharmaceutical industry. Essential oils generally have a broad spectrum of bioactivity, owing to the presence of several active ingredients that work through various modes of action. Due to their mode of extraction, mostly by distillation from aromatic plants, they contain a variety of volatile molecules such as terpenes, phenol-derived aromatic and aliphatic components. The large genus Artemisia L., from the tribe Anthemideae, comprises important medicinal plants which are currently the subject of phytochemical attention due to their biological and chemical diversity. Artemisia species, widespread throughout the world, are one of the most popular plants in Chinese traditional preparations and are frequently used for the treatment of diseases such as malaria, hepatitis, cancer, inflammation and infections by fungi, bacteria and viruses. Extensive studies of the chemical components of Artemisia have led to the identification of many compounds as well as essentials oils. This review summarizes some of the main reports on the chemistry and anti-infective activities of Artemisia. Li. essential oils from the data in the recent literature (2000-2011).  相似文献   
54.
This paper presents a preliminary study in building discriminant models from solid-state NMR spectrometry data to detect the presence of acetaminophen in over-the-counter pharmaceutical formulations. The dataset, containing 11 spectra of pure substances and 21 spectra of various formulations, was processed by partial least squares discriminant analysis (PLS-DA). The model found coped with the discrimination, and its quality parameters were acceptable. It was found that standard normal variate preprocessing had almost no influence on unsupervised investigation of the dataset. The influence of variable selection with the uninformative variable elimination by PLS method was studied, reducing the dataset from 7601 variables to around 300 informative variables, but not improving the model performance. The results showed the possibility to construct well-working PLS-DA models from such small datasets without a full experimental design.  相似文献   
55.
Our recent studies identified a novel pathway of nicotinamide metabolism that involves 4-pyridone-3-carboxamide-1-β-D-ribonucleoside (4PYR) and demonstrated its endothelial cytotoxic effect. This study tested the effects of 4PYR and its metabolites in experimental models of breast cancer. Mice were divided into groups: 4T1 (injected with mammary 4T1 cancer cells), 4T1 + 4PYR (4PYR-treated 4T1 mice), and control, maintained for 2 or 21 days. Lung metastasis and endothelial function were analyzed together with blood nucleotides (including 4PYR), plasma amino acids, nicotinamide metabolites, and vascular ectoenzymes of nucleotide catabolism. 4PYR metabolism was also evaluated in cultured 4T1, MDA-MB-231, MCF-7, and T47D cells. An increase in blood 4PYR in 4T1 mice was observed at 2 days. 4PYR and its metabolites were noticed after 21 days in 4T1 only. Higher blood 4PYR was linked with more lung metastases in 4T1 + 4PYR vs. 4T1. Decreased L-arginine, higher asymmetric dimethyl-L-arginine, and higher vascular ecto-adenosine deaminase were observed in 4T1 + 4PYR vs. 4T1 and control. Vascular relaxation caused by flow-dependent endothelial activation in 4PYR-treated mice was significantly lower than in control. The permeability of 4PYR-treated endothelial cells was increased. Decreased nicotinamide but enhanced nicotinamide metabolites were noticed in 4T1 vs. control. Reduced N-methylnicotinamide and a further increase in Met2PY were observed in 4T1 + 4PYR vs. 4T1 and control. In cultured breast cancer cells, estrogen and progesterone receptor antagonists inhibited the production of 4PYR metabolites. 4PYR formation is accelerated in cancer and induces metabolic disturbances that may affect cancer progression and, especially, metastasis, probably through impaired endothelial homeostasis. 4PYR may be considered a new oncometabolite.Subject terms: Mechanisms of disease, Pathogenesis, Breast cancer  相似文献   
56.
Pyrazine and its derivatives are a large group of compounds that exhibit broad biological activity, the changes of which can be easily detected by a substituent effect or a change in the functional group. The present studies combined theoretical research with the density functional theory (DFT) approach (B3LYP/6-311+G**) and experimental (potentiometric and spectrophotometric) analysis for a thorough understanding of the structure of chlorohydrazinopyrazine, its physicochemical and cytotoxic properties, and the site and nature of interaction with DNA. The obtained results indicated that 2-chloro-3-hydrazinopyrazine (2Cl3HP) displayed the highest affinity to DNA. Cytotoxicity studies revealed that the compound did not exhibit toxicity toward human dermal keratinocytes, which supported the potential application of 2Cl3HP in clinical use. The study also attempted to establish the possible equilibria occurring in the aqueous solution and, using both theoretical and experimental methods, clearly showed the hydrophilic nature of the compound. The experimental and theoretical results of the study confirmed the quality of the compound, as well as the appropriateness of the selected set of methods for similar research.  相似文献   
57.
58.
Naringin and limonin are the two main bitter compounds of citrus products such as grapefruit juice. The aim of this investigation was to evaluate the reduction in both bitter components simultaneously using a combined biochemical and physical approach. The proposed strategy was based on the use of heterofunctional supports with glyoxyl groups that allow for the covalent immobilization of naringinase, which hydrolyses naringin and alkyl groups that allow for the adsorption of limonin. The supports were butyl-glyoxyl agarose (BGA) and octyl-glyoxyl agarose (OGA), which were characterized in terms of aldehyde group quantification and FTIR analysis. The optimal pH and temperature of free and immobilized enzymes were assessed. The maximum enzyme loading capacity of supports was analyzed. Debittering of grapefruit juice was evaluated using soluble enzyme, enzyme-free supports, and immobilized catalysts. Enzyme immobilized in BGA reduced naringin and limonin concentrations by 54 and 100%, respectively, while the use of catalyst immobilized in OGA allowed a reduction of 74 and 76%, respectively, obtaining a final concentration of both bitter components under their detection threshold. The use of OGA biocatalyst presented better results than when soluble enzyme or enzyme-free support was utilized. Biocatalyst was successfully applied in juice debittering in five repeated batches.  相似文献   
59.
The aim of the work has been to develop freeze-dried fruit snacks in the form of bars with the use of by-products derived from fruit processing. In effect 14 product designs of fruit gels based on: apple pulp, apple juice, water, sodium alginate and only apple or only chokeberry pomace were prepared. The snacks were freeze-dried. The dry matter content, water activity, structure, colour, mechanical properties, as well as organoleptic evaluation were determined. Freeze-dried bares were obtained according to sustainability production which in this case was relied on application of fruit pomace. The freeze-drying process ensures the microbiological safety of the product without the need to use chemical preservatives. Freeze-dried samples obtained low water activity in the range of 0.099–0.159. The increase in pomace concentration (3–9%) boosted the dry matter content to above 98%, and decreased the brightness of the freeze-dried bars about 6 to 10 units. Mechanical properties varied depending on the product design. With the increase in the amount of pomace, the shear force increased at 23% to 41%. Based on the results, the best variant, that has the most delicate structure and the best organoleptic properties, has proven to contain 1% sodium alginate and 2% pomace.  相似文献   
60.
Strong heterodimeric interactions of phenylboronic acids with l-proline or betaine are evident in the solid state. The interaction energy is over 23 kcal/mol (at MP2/6-31+G).  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号