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41.
Transition-metal borides show not only promising physical properties but also a rich variety of crystal structures. In this context, quantum-chemical tools can shed light on important facets of the chemistry within such intermetallic borides. Using density-functional theory (DFT), we analyze in detail two phases of significant structural-chemical importance: the recently synthesized Ti(1+x)Os(2-x)RuB(2) and the isotypical Ti(1+x)Os(3-x)B(2). Starting from the observation of different Ti/Os occupations in X-ray crystal structure analysis, we assess suitable computational models and rationalize how the interplay of Ti-Ti, Ti-Os, and Os-Os bonds drives the site preferences. Then, we move on to a systematic investigation of the metal-boron bonds which embed the characteristic, trigonal-planar B(4) units within their metallic surroundings. Remarkably, the different Ti-B bonds in Ti(1+x)Os(2-x)RuB(2) (and also in its ternary derivative) are of vastly different strength, and the strength of these bonds does not correlate with their length. The tools presented in this work are based on simple and insightful chemical arguments together with DFT, and may subsequently be transferred to other intermetallic phases--transition-metal borides and beyond. 相似文献
42.
John Ogonji Agure David Otieno Ambogo Fredrick Oluoch Nyamwala 《Mathematische Nachrichten》2013,286(4):323-339
Using subspace theory together with appropriate smoothness and decay conditions, we calculated the deficiency indices and absolutely continuous spectrum of fourth order difference equations with unbounded coefficients. In particular, we found the absolutely continuous spectrum to be ${\mathbb {R}}$ with a spectral multiplicity one. 相似文献
43.
Aurélien Habrioux Claudia Morais Teko W. Napporn Boniface Kokoh 《Current Opinion in Electrochemistry》2020
This review aims at presenting recent findings in the understanding of oxygen and hydrogen electrocatalysis in alkaline electrolytes that are key processes for the emergence of sustainable energy storage and conversion devices such as anion exchange membrane fuel cells and electrolyzers. In these systems, the exchange of electrons through electrochemical reactions provides a unique pathway to reversibly convert the electricity vector into chemical one: hydrogen. A concise and critical review of advances made during the last past years in the design of catalysts is provided. Challenges and opportunities for the development of the next catalyst generation are also addressed. 相似文献
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45.
Dr. Pritam Shankhari Nika G. Bakshi Prof. Dr. Yuemei Zhang Dr. Dejan Stekovic Prof. Dr. Mikhail E. Itkis Prof. Dr. Boniface P. T. Fokwa 《Chemistry (Weinheim an der Bergstrasse, Germany)》2020,26(9):1979-1988
Metal-rich borides with the Ti3Co5B2-type structure represent an ideal playground for tuning magnetic interactions through chemical substitutions. In this work, density functional theory (DFT) and experimental studies of Ru-rich quaternary borides with the general composition A2MRu5B2 (A=Zr, Hf, M=Fe, Mn) are presented. Total energy calculations show that the phases Zr2FeRu5B2 and Hf2FeRu5B2 prefer ground states with strong antiferromagnetic (AFM) interactions between ferromagnetic (FM) M-chains. Manganese substitution for iron lowers these antiferromagnetic interchain interactions dramatically and creates a strong competition between FM and AFM states with a slight preference for AFM in Zr2MnRu5B2 and for FM in Hf2MnRu5B2. Magnetic property measurements show a field dependence of the AFM transition (TN): TN is found at 0.1 T for all phases with predicted AFM states whereas for the predicted FM phase it is found at a much lower magnetic field (0.005 T). Furthermore, TN is lowest for a Hf-based phase (20 K) and highest for a Zr-based one (28 K), in accordance with DFT predictions of weaker AFM interactions in the Hf-based phases. Interestingly, the AFM transitions vanish in all compounds at higher fields (>1 T) in favor of FM transitions, indicating metamagnetic behaviors for these Ru-rich phases. 相似文献
46.
Wireless Information Transmission System Powered by an Abiotic Biofuel Cell Implanted in an Orange 下载免费PDF全文
Yaovi Holade Kevin MacVittie Tyler Conlon Nataliia Guz Karine Servat Teko W. Napporn K. Boniface Kokoh Evgeny Katz 《Electroanalysis》2015,27(2):276-280
An “abiotic” biofuel cell composed of catalytic electrodes modified with inorganic nanoparticles (NPs) deposited on carbon black (CB) was used to activate a wireless information transmission system. The cathode and anode were made of carbon paper modified with Pt‐NPs/CB and buckypaper modified with Au80Pt20‐NPs/CB, respectively. The cathode/anode pair was implanted in orange pulp extracting power from its content (glucose and fructose in the juice). The open circuit voltage, Voc, short circuit current density, jsc, and maximum power produced by the biofuel cell, Pmax, were found as 0.36 V, 1.3 mA cm?2 and 182 µW, respectively. The voltage produced by the biofuel cell was amplified with an energy harvesting circuit and applied to a wireless transmitter. The present study continues the research line where different implantable biofuel cells are used for activation of electronic devices. 相似文献
47.
The Rare Earth Metal Polyselenides Gd8Se15, Tb8Se15?x, Dy8Se15?x, Ho8Se15?x, Er8Se15?x, and Y8Se15?x – Increasing Disorder in Defective Planar Selenium Layers Single crystals of the rare earth metal polyselenides Gd8Se15, Tb8Se15?x, Dy8Se15?x, Ho8Se15?x, Er8Se15?x, and Y8Se15?x (0 < x ≤ 0.3) have been prepared by chemical transport reactions (1120 K→ 970 K, 14 days, I2 as carrier) starting from pre‐annealed powders of nominal compositions between LnSe2 and LnSe1.9. The isostructural title compounds adopt a 3 × 4 × 2 superstructure of the ZrSSi type and can be described in space group Amm2 with lattice parameters of a = 12.161(1) Å, b = 16.212(2) Å and c = 16.631(2) Å (Gd8Se15), a = 12.094(2) Å, b = 16.123(2) Å and c = 16.550(2) Å (Tb8Se15?x), a = 12.036(2) Å, b = 16.060(2) Å and c = 16.475(2) Å (Dy8Se15?x), a = 11.993(2) Å, b = 15.999(2) Å and c = 16.471(2) Å (Ho8Se15?x), a = 11.908(2) Å, b = 15.921(2) Å and c = 16.428(2) Å (Er8Se15?x), and a = 12.045(2) Å, b = 16.072(3) Å and c = 16.626(3) Å (Y8Se15?x), respectively. The structure consists of puckered [LnSe] double slabs and planar Se layers alternating along [001]. The planar Se layers contain a disordered arrangement of dimers, Se2? and vacancies. All compounds are semiconducting and contain trivalent rare earth metals (Ln3+). 相似文献
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Structural‐Distortion‐Driven Magnetic Transformation from Ferro‐ to Ferrimagnetic Iron Chains in B6‐based Nb6FeIr6B8 下载免费PDF全文
Neetika Sharma Mohammed Mbarki Yuemei Zhang Ashfia Huq Boniface P. T. Fokwa 《Angewandte Chemie (International ed. in English)》2018,57(32):10323-10327
We report on a structural distortion of kinetically stable B6‐based ferromagnetic Nb6FeIr6B8 that induces an unprecedented transformation of a ferromagnetic Fe chain into two ferrimagnetic Fe chains through superstructure formation. Density functional theory calculations showed that the ferromagnetic Fe–Fe intrachain interactions found in the undistorted structure become ferrimagnetic in the distorted superstructure, mainly because the two independent iron atoms building each chain interact antiferromagnetically and carry different magnetic moments. High‐temperature SQUID magnetometry confirmed ferrimagnetic ordering at 525 K with a high and negative Weiss constant of ?972 K indicating the presence of strong antiferromagnetic interactions, as predicted. This finding paves the way for the development of low‐dimensional magnetic intermetallic systems based on Heisenberg ferrimagnetic chains, which have previously been studied only in molecular‐based compounds. 相似文献
50.
Cyril Párkányi Christian Boniface Jean-Jacques Aaron Mame Diabou Gaye Ratna Ghosh László von Szentpály Krishnan S. RaghuVeer 《Structural chemistry》1992,3(4):277-289
Electronic absorption and fluorescence emission spectra of several biologically important pyrimidines were measured at room temperature (298 K) in the following solvents: dioxane, ethyl ether, chloroform, ethyl acetate, 1-butanol, 2-propanol, methanol, dimethylformamide, acetonitrile, and dimethyl sulfoxide. The compounds studied were uracil, thymine, cytosine, 5-fluorouracil, 5-chlorouracil, 5-bromouracil, 5-iodouracil, 2-thiouracil, barbituric acid, and orotic acid. In combination with the ground-state dipole moments of the above compounds, these spectral data were used to determine their lowest excited singlet-state dipole moments using the soivatochromic method. The effects of the solvent upon the spectral properties and of the structure upon the ground and excited singlet-state dipole moments are discussed. For most of the compounds, the excited singlet-state dipole moments are higher than their ground-state counterparts.The theoretical dipole moments for all the pyrimidines listed above, as well as for pyrimidine, alloxan, and uracil-5-carboxylic acid, were calculated by two methods. One approach involved a combination of the PPP (-LCI-SCF-MO) method for the -contribution to the overall dipole moment and the -contribution obtained as a vector sum of the -bond moments and group moments. The second set of theoretical values was obtained by the CNDO/2 method. The results were compared with the experimental dipole moments.Presented, in part, at the XIth IUPAC Symposium on Photochemistry, Lisbon, Portugal, July 27–August 1, 1986, and at the 192nd National Meeting of the American Chemical Society, Anaheim, CA, September 7–12, 1986. Part of this work was carried out at the University of Texas at El Paso, El Paso, TX, during the tenure of one of the authors there (C. P.). This paper is dedicated to Dr. Rudolf Zahradník, the teacher of one of the authors (C. P.) and the authors' friend and colleague. 相似文献