全文获取类型
收费全文 | 210篇 |
免费 | 4篇 |
专业分类
化学 | 173篇 |
晶体学 | 12篇 |
数学 | 15篇 |
物理学 | 14篇 |
出版年
2023年 | 2篇 |
2022年 | 10篇 |
2021年 | 15篇 |
2020年 | 1篇 |
2019年 | 8篇 |
2018年 | 1篇 |
2017年 | 3篇 |
2016年 | 7篇 |
2015年 | 4篇 |
2014年 | 7篇 |
2013年 | 14篇 |
2012年 | 16篇 |
2011年 | 14篇 |
2010年 | 14篇 |
2009年 | 8篇 |
2008年 | 12篇 |
2007年 | 14篇 |
2006年 | 15篇 |
2005年 | 14篇 |
2004年 | 1篇 |
2003年 | 6篇 |
2002年 | 6篇 |
2001年 | 1篇 |
1998年 | 3篇 |
1997年 | 2篇 |
1995年 | 1篇 |
1994年 | 2篇 |
1993年 | 1篇 |
1990年 | 2篇 |
1989年 | 1篇 |
1988年 | 1篇 |
1983年 | 3篇 |
1981年 | 1篇 |
1977年 | 1篇 |
1973年 | 1篇 |
1967年 | 2篇 |
排序方式: 共有214条查询结果,搜索用时 0 毫秒
71.
Karla Miriam Reyes Leiva Milagros Jan-Vargas Miguel ngel Cuba Sergio Snchez Lara Jos Javier Serrano Olmedo 《Entropy (Basel, Switzerland)》2021,23(7)
The rehabilitation of a visually impaired person (VIP) is a systematic process where the person is provided with tools that allow them to deal with the impairment to achieve personal autonomy and independence, such as training for the use of the long cane as a tool for orientation and mobility (O&M). This process must be trained personally by specialists, leading to a limitation of human, technological and structural resources in some regions, especially those with economical narrow circumstances. A system to obtain information about the motion of the long cane and the leg using low-cost inertial sensors was developed to provide an overview of quantitative parameters such as sweeping coverage and gait analysis, that are currently visually analyzed during rehabilitation. The system was tested with 10 blindfolded volunteers in laboratory conditions following constant contact, two points touch, and three points touch travel techniques. The results indicate that the quantification system is reliable for measuring grip rotation, safety zone, sweeping amplitude and hand position using orientation angles with an accuracy of around 97.62%. However, a new method or an improvement of hardware must be developed to improve gait parameters’ measurements, since the step length measurement presented a mean accuracy of 94.62%. The system requires further development to be used as an aid in the rehabilitation process of the VIP. Now, it is a simple and low-cost technological aid that has the potential to improve the current practice of O&M. 相似文献
72.
73.
Karla Delgado‐Rodríguez Francisco J. Enríquez‐Medrano Adriana Berenice Espinoza‐Martínez Gastn P. Barreto Gabriel Merino Graciela Morales 《Journal of polymer science. Part A, Polymer chemistry》2019,57(9):997-1007
The decomposition rate constant (kd) of pinacolone diperoxide (PDP, 3,6‐diterbutyl‐3,6‐dimethyl‐1,2,4,5‐tetraoxacyclohexane) in methyl methacrylate (MMA) is determined by the kinetic study of its thermal decomposition at temperatures from 110 °C to 140 °C. The calculated kd values for PDP are higher than the corresponding values previously determined and reported for diethyl ketone triperoxide (DEKTP, 3,3,6,6,9,9‐hexaethyl‐1,2,4,5,7,8‐hexaoxacyclononane), for example, at 140 °C the kd for PDP is 75.4 × 10?5 s?1, while for DEKTP, it is 50.6 × 10?5 s?1. The difference in the kd between 130 °C and 140 °C indicates that the decomposition mechanism, sequential and/or concerted, is a function of temperature. The conformations of both initiators justify the higher kd for PDP in MMA than DEKTP, where one single conformer is found for PDP, whereas 212 conformers are found for DEKTP. Bulk polymerization of MMA using PDP as the initiator reveals also the presence of an induction period, such as in DEKTP case. This work provides mechanistic insights into the interactions among the bifunctional cyclic peroxide PDP and the MMA monomer and their influence on the polymerization kinetics. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019 , 57, 997–1007 相似文献
74.
The kinetics of the acid and alkaline hydrolysis of monoflorophosphorous acid has been studied by P-31 NMR and static pH titration over a wide temperature range. The acid catalyzed hydrolysis has a rate constant at 25°C equal to 0.35 dm3 mol?1 s?1 and an activation energy of 53 kJ while the alkaline hydrolysis has a rate constant of 4.6 dm3 mol?1 s?1 and an activation energy of 42 kJ. When the hydrogen in this compound is replaced by either fluorine or a hydroxyl group, the rates of reaction decrease by two orders of magnitude. © 1994 John Wiley & Sons, Inc. 相似文献
75.
Karla Schenzel Alfred Kolbe Peter Reich 《Monatshefte für Chemie / Chemical Monthly》1990,121(8-9):615-623
Summary Results are reported of IR and Raman investigations on four 1-substituted 2,8,9-tricarbastannatranes (1,X=Cl;2,X=Br;3,X=I;4,X=Me). Group-theoretic investigations confirm the trigonal-bipyramidal configuration of the tin atoms. The vibrational frequencies of the coordination (SnN) are correlated to the distancesd (Sn-N). Besides the results on the vibrations of the coordination polyhedrons other results are reported concerning the enantiomerization of the chiral atran skeleton. 相似文献
76.
Bautista-Ibañez L Ramírez-Gualito K Quiroz-García B Rojas-Aguilar A Cuevas G 《The Journal of organic chemistry》2008,73(3):849-857
Can a benzene molecule differentiate between two isomeric carbohydrates? It is generally accepted that two factors govern molecular recognition: complementarity and preorganization. Preorganization requires the presence of cavities for positioning the host's groups of complementary nature to those of the guest. This study shows that, in fact, groups should be complementary to recognize each other (for the case presented here, it is controlled by the CH/pi interaction) but preorganization is not essential. Since weak interactions have their origin in dispersion forces, they also have impact on the enthalpic term of the free energy, so it was considered that their participation can be demonstrated by measuring the energy involved. For recognition to happen, two conditions must be satisfied: specificity and associated stabilizing energy. In this study we evaluated the heat of dissolution of different carbohydrates such as methyl 2,3,4,6-tetra-O-methyl-alpha-d-mannopyranoside and methyl 2,3,4,6-tetra-O-methyl-beta-d-galactopyranoside using different aromatic solvents. The solvation enthalpies in benzene were -78.8 +/- 3.9 and -88.7 +/- 5.5 kJ mol(-1) for each carbohydrate, respectively; and these values yielded a CH/pi energy of interaction of 9.9 kJ mol(-1). In addition, NMR studies of the effect of the addition of benzene to chloroform solutions of the two carbohydrates showed that benzene specifically interacts with the hydrogen atoms of the pyranose ring at positions 3, 4, and 5 located on the alpha face of the methyl-beta-galactoside, so it is, in fact, able to recognize it. Thus, the interactions between carbohydrates and the aromatic residues of proteins occur in the absence of the confinement generated by the protein structure. By experimentally measuring the energy associated with this interaction and comparing it to theoretical calculations, it was also possible to unequivocally determine the existence of CH/pi interactions between carbohydrates and proteins. 相似文献
77.
Bravo-Altamirano K Abrunhosa-Thomas I Montchamp JL 《The Journal of organic chemistry》2008,73(6):2292-2301
Hypophosphorous compounds (MOP(O)H(2), M = H, R(3)NH) effectively participate in metal-catalyzed C-P bond-forming reactions with allenes, dienes, and activated allylic electrophiles under mild conditions. The catalytic system Pd(2)dba(3)/xantphos is crucial to avoid or minimize the competitive reductive transfer-hydrogenation pathway available to hypophosphorous acid derivatives. Further investigation into the allylation mechanism provided access to the analogy allylic acetate-allylic phosphinate, which then led to the development of a Pd-catalyzed rearrangement of preformed allylic phosphinates esters and, ultimately, to a catalytic dehydrative allylation of hypophosphorous acid with allylic alcohols. The reactions disclosed herein constitute efficient synthetic approaches, not only to prepare allylic H-phosphinic acids but also their esters via one-pot tandem processes. In addition, the potential of H-phosphinates as useful synthons for the preparation of other organophosphorus compounds is demonstrated. 相似文献
78.
Luis A. Puentes-Vara Karla M. Gregorio-Jauregui Ana M. Bolarín Ma. E. Navarro-Clemente Héctor J. Dorantes Mónica Corea 《Journal of nanoparticle research》2016,18(7):212
The addition of superparamagnetic iron nanoparticles into polystyrene matrix allows for the modification of the physical properties as well as the implementation of new features in the hybrid nanomaterials. These materials have excellent potential for biomedical and bioengineering applications. Nevertheless, it is necessary to achieve a good dispersion of magnetic nanoparticles for its successful incorporation into polymer particles. This can be obtained through the use of a stabilizer, which provides stability against aggregation. In this work, magnetic nanoparticles were dispersed using different stabilizers. Subsequently, ferrofluids stabilized using the mixture of ABEX/IGEPAL and acrylic acid (AA) were used to synthesize PS-Fe3O4 nanocomposites, through miniemulsion and emulsion polymerization conventional techniques. Semicontinuous and batch processes were compared, by varying surfactants and their concentrations. The PS-Fe3O4 nanoparticles were characterized by dynamic light scattering, scanning electron microscopy, Raman spectroscopy, and vibrating sample magnetometer. Magnetic nanoparticle dispersions show better results when the anionic and nonionic surfactants are used as a mixture rather than when used alone. Results of DLS showed that the semicontinuous process allowed obtaining monodisperse materials, whereas polidisperse systems are generated in batch process. Raman spectroscopy confirmed the presence of magnetite and polystyrene in the nanocomposites. PS-Fe3O4 nanoparticles showed superparamagnetic behavior with final magnetization of around 0.01 emu/g and low coercivity, properties that make them suitable for applications in wide fields of technology. Particle size (Dz), was lower than 300 nm in all cases. Moreover, the use of AA as stabilizer allows enhancing the PS-Fe3O4 composite properties. These findings showed that particle size, morphology, and agglomeration are directly influenced by the concentration and the type of surfactant employed. 相似文献
79.
Lejsa Jakupovi Marko Kalvarein Karla Bukovina Valentina Poljak Lovorka Vuji Marijana Zovko Kon
i 《Molecules (Basel, Switzerland)》2021,26(6)
Medicago lupulina is an ancient edible plant from the Fabaceae family. In this work, two eco-friendly methods for extraction of bioactive phenolics from M. lupulina were developed using mixtures of water with two non-toxic, skin- and environmentally-friendly polyol solvents: glycerol and polypropylene glycol. Ultrasound-assisted extractions were optimized using a Box–Behnken design. The independent variables were the concentration of organic solvent in water (X1), extraction temperature (X2) and time (X3), while the response was phenolic content. The optimum conditions for extraction of polyphenols were (X1, X2, X3): (45%, 70 °C, 60 min) and (10%, 80 °C, 60 min) for glycerol and polypropylene glycol extraction, respectively. The extracts prepared at optimum conditions were rich in phenolic compounds, mainly derivatives of apigenin, kaempferol, luteolin, quercetin, caffeic and ferulic acid, as well as coumestrol. Their cosmeceutical and antidiabetic activity was tested. Both extracts demonstrated notable antioxidant, anti-lipoxygenase and anti-α-amylase activity. In addition to those activities, the glycerol extract efficiently inhibited protein coagulation, elastase and α-glucosidase activity. Glycerol present in the extract displayed enzyme-inhibiting activity in several assays and supported the action of the bioactive constituents. Thus, the optimized glycerol extract is a desirable candidate for direct incorporation in antidiabetic food supplements and cosmeceutical products. 相似文献
80.
Claudia Morilla Elianis Perdomo Ana Karla Hernndez Ramcy Regalado Amisel Almirall Gastn Fuentes Yaima Campos Mora Timo Schomann Alan Chan Luis J. Cruz 《Molecules (Basel, Switzerland)》2021,26(11)
Calcium phosphate cements have the advantage that they can be prepared as a paste that sets in a few minutes and can be easily adapted to the shape of the bone defect, which facilitates its clinical application. In this research, six formulations of brushite (dicalcium phosphate dihydrated) cement were obtained and the effect of the addition of sodium alginate was analyzed, such as its capacity as a tetracycline release system. The samples that contain sodium alginate set in 4 or 5 min and showed a high percentage of injectability (93%). The cements exhibit compression resistance values between 1.6 and 2.6 MPa. The drug was released in a range between 12.6 and 13.2% after 7 days. The antimicrobial activity of all the cements containing antibiotics was proven. All samples reached values of cell viability above 70 percent. We also observed that the addition of the sodium alginate and tetracycline improved the cell viability. 相似文献