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   265篇
  免费   8篇
  国内免费   26篇
化学   224篇
晶体学   5篇
力学   13篇
综合类   2篇
数学   7篇
物理学   48篇
  2024年   1篇
  2023年   5篇
  2022年   22篇
  2021年   37篇
  2020年   18篇
  2019年   7篇
  2018年   3篇
  2017年   10篇
  2016年   6篇
  2015年   7篇
  2014年   4篇
  2013年   22篇
  2012年   11篇
  2011年   9篇
  2010年   14篇
  2009年   11篇
  2008年   11篇
  2007年   14篇
  2006年   25篇
  2005年   10篇
  2004年   9篇
  2003年   9篇
  2002年   5篇
  2001年   1篇
  2000年   6篇
  1999年   4篇
  1998年   3篇
  1997年   2篇
  1996年   4篇
  1995年   4篇
  1994年   2篇
  1993年   1篇
  1990年   1篇
  1986年   1篇
排序方式: 共有299条查询结果,搜索用时 15 毫秒
71.
Abstract

The lipidic fraction compositions of both concentrated vinasses, a by-product of the sugar industry, and a compost made basically from olive oil vegetation waters (alpechin) were studied. The alpechin lipids are composed mainly by series of n-alkanes and lineal and branched fatty acids, whereas the major lipids in vinasses were n-alkanes, n-alkanols and acetals. Concentrations and composition of lipids in both materials do not seem of concern.

Preliminary results on the effects of the application of both materials over two years on the organic status of an agricultural soil are also reported. No significant changes were observed in total organic carbon and contents in humic fractions and lipids before and after the applications. However, analysis by GC-MS of the lipid compounds present in bound forms in the subsoil layer revealed that some hydrophobic components were accumulated in the soil following the waste applications.  相似文献   
72.
乌榄中矿质元素含量的测定   总被引:1,自引:0,他引:1  
采用原子吸收分光光度法,对新鲜乌榄及腌制乌榄的果实和果仁中的钙、铁、镁、锰、钼、钠、镍、钴8种矿质元素的含量进行了测定,为乌榄的开发利用提供依据。结果表明,乌榄中含有丰富的钠、钙、镁、铁、锰、钼等人体必需的元素,有害元素钴、镍的含量较少。  相似文献   
73.
Olives (Olea europaea L.) are a significant part of the agroindustry in China. Olive leaves, the most abundant by-products of the olive and olive oil industry, contain bioactive compounds that are beneficial to human health. The purpose of this study was to evaluate the phytochemical profiles and antioxidant capacities of olive leaves from 32 cultivars grown in China. A total of 32 phytochemical compounds were identified using high-performance liquid chromatography–electrospray ionization–tandem mass spectrometry, including 17 flavonoids, five iridoids, two hydroxycinnamic acids, six triterpenic acids, one simple phenol, and one coumarin. Specifically, olive leaves were found to be excellent sources of flavonoids (4.92–18.29 mg/g dw), iridoids (5.75–33.73 mg/g dw), and triterpenic acids (15.72–35.75 mg/g dw), and considerable variations in phytochemical content were detected among the different cultivars. All tested cultivars were classified into three categories according to their oil contents for further comparative phytochemicals assessment. Principal component analysis indicated that the investigated olive cultivars could be distinguished based upon their phytochemical profiles and antioxidant capacities. The olive leaves obtained from the low-oil-content (<16%) cultivars exhibited higher levels of glycosylated flavonoids and iridoids, while those obtained from high-oil-content (>20%) cultivars contained mainly triterpenic acids in their compositions. Correspondingly, the low-oil-content cultivars (OL3, Frantoio selection and OL14, Huaou 5) exhibited the highest ABTS antioxidant activities (758.01 ± 16.54 and 710.64 ± 14.58 mg TE/g dw, respectively), and OL9 (Olea europaea subsp. Cuspidata isolate Yunnan) and OL3 exhibited the highest ferric reducing/antioxidant power assay values (1228.29 ± 23.95 mg TE/g dw and 1099.99 ± 14.30 mg TE/g dw, respectively). The results from this study may be beneficial to the comprehensive evaluation and utilization of bioactive compounds in olive leaves.  相似文献   
74.
草酸钙结石研究中的化学基础   总被引:21,自引:0,他引:21  
欧阳健明 《化学通报》2002,65(5):326-332
介绍了草酸钙结石研究中涉及的化学问题,包括结石的化学组分及其性质、草酸钙的存在形式和草酸钙结石中的有机基质。重点讨论了凝血酶原片段1(UPTF1)、酸性粘多糖(GAGs)、TH蛋白和肾钙素(NC)等基质大分子在草酸钙结石形成和抑制中的作用。从基质大分子与草酸钙结石和亲和力、分子尺雨效应、Zeta电位、溶液pH和离子强度等方面讨论了草酸钙结石的形成机制。最后讨论了种晶技术和恒组份法在草酸钙结石体外化学模拟中的应用。  相似文献   
75.
Some agroforestry residues such as orange and olive tree pruning have been extensively evaluated for their valorization due to its high carbohydrates content. However, lignin-enriched residues generated during carbohydrates valorization are normally incinerated to produce energy. In order to find alternative high added-value applications for these lignins, a depth characterization of them is required. In this study, lignins isolated from the black liquors produced during soda/anthraquinone (soda/AQ) pulping of orange and olive tree pruning residues were analyzed by analytical standard methods and Fourier-transform infrared spectroscopy (FTIR), nuclear magnetic resonance (solid state 13C NMR and 2D NMR) and size exclusion chromatography (SEC). Thermal analysis (thermogravimetric analysis (TGA), differential scanning calorimetry (DSC)) and antioxidant capacity (Trolox equivalent antioxidant capacity) were also evaluated. Both lignins showed a high OH phenolic content as consequence of a wide breakdown of β-aryl ether linkages. This extensive degradation yielded lignins with low molecular weights and polydispersity values. Moreover, both lignins exhibited an enrichment of syringyl units together with different native as well as soda/AQ lignin derived units. Based on these chemical properties, orange and olive lignins showed relatively high thermal stability and good antioxidant activities. These results make them potential additives to enhance the thermo-oxidation stability of synthetic polymers.  相似文献   
76.
77.
The consolidation of materials concept, which consists of introducing a chemical substance (consolidant) into degraded stone, has been applied to architectural conservation. Silicon compounds such as tetraethoxysilane (TEOS) are frequently used as a base for commercial consolidant formulations due to their ability to form a siloxane polymer such as SiO2. However, the silica xerogels deposited into the stone show poor performance and the gels obtained are non‐porous and tend to crack during the drying stage. In order to avoid the fractures and to improve gel properties, we propose the synthesis of a hybrid consolidant based on TEOS and fillers such as colloidal silica (200 nm in diameter) and hydroxy‐terminated polydimethylsiloxane (PDMS‐OH). Both additives enhance gel properties such as porosity and elasticity, leading to the formation of non‐fractured and permeable gels. Characterization of the hybrid xerogel was carried out by nitrogen adsorption and 29Si MAS‐NMR. The properties of the hybrid xerogels were compared with those prepared from a formulation based on TEOS (T‐ME) with a composition similar to a commercial product. In order to evaluate the effectiveness of the hybrid consolidant, it was applied to tuff‐stone of historical monuments in the city of Guanajuato, Mexico. The tuff‐stone was also treated with the formulation T‐ME. Both treatments were studied by determining the percentage of consolidant deposited, evaluating changes in porosity and hardness of the treated stone. The applicability of the hybrid consolidant for the decayed tuff‐stone is under study. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   
78.
Urolithiasis is a prevalent, disturbing, and highly recurrent disease. Knowing the composition of a urinary stone is important for prevention purposes. Traditional urinary stone analysis methods need large stone fragments for analysis. However, the advancement of ureteroscopic lithotripsy (URSL) has resulted in micro‐stone fragments and unapparently expelled urinary stone powder. In this study, we developed a micro‐Raman spectroscopy (MRS) based diagnosis method for detecting micro‐stones or stone powders in urine after URSL. In our experiment, urine samples of 10 ml each were collected from 12 patients over the fragmented stone site in the ureter after the URSL procedure. The post‐URSL urine sediments extracted from urine were analyzed by MRS. The small urinary stones caught by grasping forceps were analyzed by both MRS and Fourier‐transform infrared (FTIR) spectroscopy. We have identified common urinary stone compositions: calcium oxalate monohydrate (COM), calcium oxalate dihydrate (COD), dicalcium phosphate dihydrate (DCPD), calcium phosphate hydroxide (hydroxyl apatite or HAP), and uric acid, by using a 632.8 nm He‐Ne laser for excitation, a 100× microscope objective lens for irradiation and collection, and a short photobleaching time for fluorescent background reduction. Thus, we developed an MRS‐based method for analyzing the composition of urinary stone powders directly from the urine samples after the URSL procedure. This approach provides a quick and convenient method for urinary stone analysis. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   
79.
Colour is an organoleptic characteristic of virgin olive oil and an important attribute that affects the consumer perception of quality. Chlorophylls and carotenoids are the main pigments responsible for the colour of virgin olive oil. A simple analytical method for the quantitative determination of chlorophylls and carotenoids in virgin olive oils has been developed. The pigments were isolated from small samples of oil (1.0 g) by solid-phase extraction using diol-phase cartridges (diol-SPE), and the extract was analysed by reverse-phase HPLC with diode-array UV detection. Chromatographic peak resolution, reproducibility (coefficient of variation (C.V.) <4.5%) and recovery (>98.4%) for each component were satisfactory. A comparative study of the proposed method was performed versus classical liquid-liquid extraction (LLE) with N,N′-dimethylformamide and solid-phase extraction using a C18 column (C18-SPE). While 96.4% of the pigments were recovered by LLE, only 51.3% were isolated by C18-SPE in comparison to diol-SPE. Likewise, a higher alteration of pigment composition was observed when such LLE and C18-SPE procedures were used. In this sense, a higher ratio of pheophytin in comparison to that isolated by the diol-SPE procedure was achieved with both extraction procedures, indicating a greater extent of the pheophytinization reaction. Therefore, quantification of pigments from virgin olive oil by diol-SPE followed by RP-HPLC was found to be rapid, simple, required only a small amount of sample, consumed only small amounts of organic solvents, and provided high recoveries, accuracy and precision.  相似文献   
80.

针对组合碎石桩–不排水桩复合地基固结问题,考虑碎石桩径向和竖向组合渗流、附加应力随时间和深度变化等因素对复合地基固结速率的影响,建立组合桩复合地基固结计算模型,采用解析法推导了变附加应力条件下组合桩复合地基固结解析解。进行固结度解退化研究,与已有解对比分析,验证了此解的正确性。通过算例计算,绘制了不同因素影响下复合地基固结度变化曲线。结果表明:增加地基顶部与底部的附加应力比值、碎石桩与土体的径向渗透系数比值,复合地基固结速率加快;忽略碎石桩的径向渗流,会高估复合地基的固结速率;减小桩径比可显著提高复合地基固结速率;增加外围不排水桩的置换率,对复合地基固结速率的加快效果不明显。

  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