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1.
C. G. Gwie R. J. Griffiths D. T. Cooney M. L. Johns D. I. Wilson 《Journal of the American Oil Chemists' Society》2006,83(12):1053-1062
The spray-freezing of two food fats, tripalmitin (PPP) and cocoa butter (CB) and mixtures thereof, has been modeled experimentally
using a novel single droplet freezing apparatus configured so that temperature profiles or samples for microstructure analysis
can be obtained. For 2 mm diameter droplets suspended in a cold air flow at temperatures around 2–15°C, initial cooling rates
were on the order of 10 K s−1 and the temperature profiles could be correlated directly to DSC data collected at 20 K min−1, indicating that minimal supercooling of the materials occurred in the droplet form. Microstructure analysis confirmed that
PPP crystallized preferentially in mixtures, and that the surface structure was very sensitive to storage conditions. The
bulk structure was much less sensitive, and the internal microstructure of the PPP droplets revealed distinct nucleation sites,
which were absent from the CB: These persisted in the mixtures up to 50 wt%. X-ray analysis indicated that the fats crystallized
in their more stable forms, namely, β for PPP and Form V/V1 in CB. 相似文献
2.
The polymorph method, which provides phase analysis from a small number of integrated intensities in a powder diffraction scan, is adapted for the determination of monoclinic zirconia in a mixture with cubic, tetragonal. and orthorhombic zirconias and the γ-phase (Mg2 Zr5 O12 ). Such a mixture is representative of Mg-PSZ after subeutectoid aging. The quantitative determination of the monoclinic depends in principle on a knowledge of the relative amounts of the other phases present in the mixture. It is demonstrated, however, that without this knowledge, even in complex mixtures, the traditional polymorph method analysis gives an acceptable estimate of the monoclinic fraction in the sample. 相似文献
3.
R. Terki G. Bertrand H. Aourag C. Coddet 《Materials Science in Semiconductor Processing》2006,9(6):1006
The structural and electronic properties of ZrO2 polymorphs were investigated using density functional theory (DFT). The Kohn–Sham equations were solved by applying the full-potential linearized augmented plane wave (FP-LAPW) method. We used the generalized gradient approximation (GGA) in the Perdew–Wang formalism to the exchange and correlation energy functional. The ground state properties such as lattice parameter, transition pressures, bulk modulus and its pressure derivative as well as the structural phase stability were calculated. The results were compared with previous calculations and experimental data when available. The FP-LAPW method correctly orders the zero temperature energies of all zirconia polymorphs. We have also studied the effect of distortion from the cubic to the tetragonal structure on the basis of charge density calculations. On the other hand, band structure and density of states (DOS), which allow us to discuss the features of orbital mixing, are also given. Our results suggest that the cotunnite structure should be better than the other zirconia phases as gate dielectric material. 相似文献
4.
Quantum mechanical calculations based on density functional theory and the generalised gradient approximation show that the equilibrium lattice parameter of a carbon polymorph with space group symmetry
would be 3.08 Å, and hence very different from that of diamond. No local minimum in the dependence of the total energy on the lattice parameter has been found. Such a polymorph would be approximately 5 eV atom−1 less stable than diamond, suggesting that its synthesis is very unlikely. This finding is consistent with older density functional calculations and contradicts recent results of LCAO calculations, which have been used to support the interpretation of experimental data as showing the existence of a metallic carbon polymorph with space group symmetry
. 相似文献
5.
6.
Effect of polymorphic phase transformations in Al2O3 film on oxidation kinetics of aluminum powders 总被引:1,自引:0,他引:1
Mikhaylo A. Trunov Mirko Schoenitz Xiaoying Zhu Edward L. Dreizin 《Combustion and Flame》2005,140(4):310-318
Thermogravimetry was used to study the oxidation of aluminum powders at elevated temperatures. Aluminum powders of various particle sizes and surface morphologies were heated in oxygen up to 1500 °C at different heating rates. Partially oxidized samples were recovered from selected intermediate temperatures and the oxide phases present were analyzed by X-ray diffraction. The experimental data were related to current information on stabilities and phase changes of Al2O3 polymorphs. Aluminum powders were observed to oxidize in four distinct stages in the temperature range from 300 to 1500 °C. During stage I, from 300 to about 550 °C, the thickness of the natural amorphous alumina layer on the particle surface increases. The rate of this process is controlled by the outward diffusion of Al cations. At about 550 °C, when the oxide layer thickness exceeds the critical thickness of amorphous alumina of about 4 nm, the oxide transforms into γ-Al2O3. The specific volume of γ-Al2O3 is less than that of amorphous alumina; therefore, the newly formed γ-Al2O3 only partially covers the aluminum surface. The oxidation rate increases rapidly at the onset of stage II, but it decreases when the γ-Al2O3 layer becomes continuous. During stage III oxidation, the γ-Al2O3 layer grows and partially transforms into the structurally similar θ-Al2O3 polymorph. Finally, oxidation stage IV is observed after the transition to stable -Al2O3 results in an abrupt reduction of oxidation rate. Qualitative analysis of the rates of oxidation at the different stages enables one to understand the wide range of aluminum ignition temperatures observed for particles of different sizes. 相似文献
7.
8.
An‐Wu Xu Wen‐Fei Dong Markus Antonietti Helmut Cölfen 《Advanced functional materials》2008,18(8):1307-1313
Polymer‐controlled crystallization of calcium carbonate crystals in solution by a gas diffusion method has been carried out in the presence of poly(sodium 4‐styrene sulfonate‐co‐N‐isopropylacrylamide) (PSS‐co‐PNIPAAM), and for the first time all three anhydrous polymorphs, calcite, vaterite, and aragonite could be selectively produced with a single additive. The selective polymorph synthesis can be nicely adjusted simply by concentration variations of polymer and calcium ions in the present reaction system. The simplicity of the system reveals the influence of Ca2+ and polymer concentration on the nucleation and crystal growth of CaCO3 via the balance between thermodynamic and kinetic reaction control. A single mechanistic framework employing particle mediated as well as ion mediated crystallization for polymorph control is proposed. 相似文献
9.
Haichao Liu Yue Shen Yan Yan Changjiang Zhou Shitong Zhang Bao Li Ling Ye Bing Yang 《Advanced functional materials》2019,29(34)
Commonly, one stimulus only induces one luminescence switching in stimuli‐responsive experiment. Herein, it is reported that one stimulus in situ induces two luminescent switches, resulting from two phase transitions in a solvent‐fuming process. Two phase transitions are in situ composed of a first fast and a subsequent slow process, corresponding to the change of molecular packing from the amorphous state to the π–π dimer crystalline state to the cocrystalline state with the inclusion of solvents, accompanied with luminescent transformation from pure blue to green to deep blue. Theoretical and experimental results reveal that the staggered π–π dimer stacking of anthracenes serves as the intermediate state to bridge the two phase transitions. This finding expands a new horizon in the stimuli‐responsive field and inspires novel applications in information storage and security fields. 相似文献
10.
本文结合孤立的P2O7二聚体和Mg O4四面体,采用高温熔液法合成了α和β-Rb2Mg3(P2O7)2同质多晶.α-Rb2Mg3(P2O7)2结晶于非中心对称P212121空间群,β-Rb2Mg3(P2O7)2结晶于中心对称P21/c空间群.两种结构均含有三维[Mg3P4O14]^2-阴离子骨架,而Rb+离子位于空间中.结构分析表明,孤立的P2O7通过调整其可变构型和取向以适合不同配位环境的阳离子,这有利于同质多晶Rb2Mg3(P2O7)2的形成.另外,从α-到β-Rb2Mg3(P2O7)2的相转变过程也可以通过粉末X射线衍射和热重-差热测试被进一步证明.紫外-可见-近红外漫反射光谱测试表明两种材料都有深紫外截止边(190 nm以下),并且α-Rb2Mg3(P2O7)2具有倍频效应,我们通过倍频密度计算研究了其倍频效应的来源.为了更好地理解上述化合物的结构性能关系,我们还进行了第一性原理计算. 相似文献