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排序方式: 共有7831条查询结果,搜索用时 19 毫秒
21.
ZnO-based varistor samples were prepared by the direct mixing of the constituent phases (DMCP) and sintering at 1100 °C for 2 h. The influence of the starting powder mixture's composition – the amounts of the pre-reacted varistor compounds and their composition – and its preparation, either with or without mechano-chemical activation (MCA), on the microstructure, phase composition and electrical characteristics of the varistor samples was studied. It showed that MCA improved the density and microstructural homogeneity of the varistor samples. MCA strongly affected the grain growth: it enhanced the nucleation of inversion boundaries (IBs) in the ZnO grains and the IBs-induced grain-growth mechanism resulted in uniform grain growth and hence a microstructure with smaller ZnO grains and a narrower grain size distribution. The final phase composition of the samples prepared by the DMCP method mainly depended on the presence of varistor dopants that can prevent the formation of the pyrochlore phase, especially Cr2O3, while MCA can affect it mostly by providing a homogeneous distribution of those dopants. The DMCP varistor samples prepared with MCA had much better current–voltage characteristics than the samples of the same composition prepared from unactivated powders. 相似文献
22.
Ohne Zusammenfassung 相似文献
23.
Mariana Ušáková Jozef Lukáč Rastislav Dosoudil Vladimír Jančárik Anna Grusková Elemír Ušák Jozef Sláma Jan Šubrt 《Journal of Materials Science: Materials in Electronics》2007,18(12):1183-1189
The paper is devoted to the preparation of NiZn ferrite with small substitutions of copper by means of ceramic technology.
The influence of small Cu substitution on the microstructural and magnetic properties of NiZn ferrites have been analysed
by means of various experimental methods and interpreted from the point of view of preparation technology optimisation and
possible applications of such materials. A strong correlation between the substituent content and resulting properties has
been observed, thus allowing preparation of material with the properties tailored for any particular application. 相似文献
24.
Dragan Simić Svetlana Simić 《Soft Computing - A Fusion of Foundations, Methodologies and Applications》2007,11(12):1185-1192
A concept of business intelligent system for financial prediction is considered in this paper. It provides data needed for
fast, precise and good business decision support to all levels of management. The aim of the project is the development of
a new online analytical processing oriented on case-based reasoning (CBR) where a previous experience for every new problem
is taken into account. Methodological aspects have been tested in practice as a part of the management information system
development project of “Novi Sad Fair”. A case study of an improved application of CBR in prediction of future payments is
discussed in the paper.
This paper is originally presented at The International Conference on Hybrid Information Technology 2006, at the special session
on “Intelligent Information Systems for Financial Engineering”, November 2006 in Cheju Island, Korea. 相似文献
25.
Novolac-type polycondensation of benzaldehyde (B) and pyrogallol (P) has been carried out at 60°C, 75°C and 90°C and at B/P mole ratios of 1.5 and 3.0 using phosphoric acid as catalyst. The reaction follows a 2nd order rate law. By using GC consumption data of benzaldehyde and pyrogallol, kinetic parameters such as the overall rate constants, activation energies (Ea) and logA values are investigated. The activation energies for 1.5 and 3.0 B/P mole ratios are found as 62.3 kJ mol?1 and 56.4 kJ mol?1, respectively. The molecular weights of the resins determined by measuring intrinsic viscosities (25°C, THF) are in the range of 0.03 to 0.07 dL g?1 at various temperatures and B/P mole ratios. 相似文献
26.
Wenzhong Zhu John J. Hughes Nenad Bicanic Chris J. Pearce 《Materials Characterization》2007,58(11-12):1189
This paper reports a study to assess nanoindentation mapping of mechanical properties of cement paste and natural rocks. Initial work seems to show that mechanical property mapping by nanoindentation is feasible and can be related to microscopic information. Further work is however required on the effect of indent size and spacing. Such a testing technique can be very useful for materials with different phases to study the intrinsic properties of each component, and also the interaction and properties of the interfacial regions of different phases. The values of Young's modulus and hardness of the individual mineral phases were also determined by statistically analysing a large number of experimental data. 相似文献
27.
What are the key elements of a sustainable university? 总被引:1,自引:0,他引:1
Nowadays, the principles of sustainable development are becoming increasingly important and universities are acting as agents
in promoting these principles within society. In the future, universities will inevitably play crucial role in propagating
these principles. This paper highlights important documents and discusses definitions of the term education for sustainable development. Elements, based on continuous feedback loop (Deming spiral) are discussed, in order to incorporate sustainability principles
into university activities. The University of Maribor has been used as a case study to test the effectiveness of the proposed
integration of sustainable development principles. 相似文献
28.
Residual fracture energy of high-performance and normal concrete subject to high temperatures 总被引:2,自引:0,他引:2
Test data on the residual fracture energy of two significantly different concrete types are presented. About 80 beams of high
performance basalt concrete and ordinary gravel concrete have been tested in accordance with the RILEM work of fracture method.
The beams are heated at 1°C per minute up to a certain maximum temperature and kept at this temperature for 8 hours before
cooling them back to room temperature and testing in three-point bending.
The tests show that the two concretes behave almost identifical when the fracture energyG
F is considered as a function of maximum temperature. It is found that the damage introduced by a maximum temperature of 300
to 400°C increases the fracture energy by 50% compared with the reference tests at room temperature. A more tortuous crack
surface is one plausible explanation for the significant increase inG
F.
The article also presents temperature and weight loss recordings from the heating scenarios and finally, the characteristic
length and the cohesive tensile softening curve are shown to depend on the maximum temperature. Basically it is demonstrated
that the temperature exposure makes the concrete significantly more ductile.
Résumé Des données sur l'énergie résiduelle de rupture de deux bétons différents sont présentées. Environ 80 poutres de béton à base de basalte de très haute performance et de béton à base de gravier ordinaire ont été examinées conformément au travaux de la RILEM sur l'énergie de rupture. Les poutres sont chauffées à 1°C par minute jusqu'à une température maximale, puis maintenues à cette température pendant 8 heures avant d'être refroidies de nouveau à la température ambiante. Les poutres sont ensuite testées en flexion (système de flexion en trois points). Les essais prouvent que les deux bétons se comportent d'une manière presque identique quandG F est considéré comme une fonction de la température maximale. On peut aussi constater que le dommage occasionné par une température maximale de 300 à 400°C augmente l'énergie de rupture de 50% par comparaison aux essais de référence réalisés à la température ambiante. Une surface de rupture plus tortueuse semble être une explication plausible pour l'augmentation significative deG F. L'article présente également les évolutions de la température et de la perte de poids pour les scénarios de chauffage utilisés. En conclusion, cette étude montre que la longueur caractéristique et la courbe de post pic dépendent de la température maximale. Fondamentalement cela démontre que l'exposition à la température rend le béton sensiblement plus ductile.相似文献
29.
J. Gajdoš Kljusurić 《Sadhana》2003,28(6):991-998
This work is an experimental study of the differential scanning calorimetry characterisation of polymer materials used in
food packaging materials, such as polypropylene (0.03 mm), polyethylene (0.1 and 0.03 mm), poly(D-(-)-Β-hydroxybutyrate) (powder),
two-layered polypropylene (0.064 mm), and two-layered polypropylene with poly-vinylidene-chloride (0.012/0.021). The polymer
stability was checked by simulation of conditions during food preparation in microwave ovens, sterilisation or rapid freezing.
The materials were tested in the temperature range from 40 to 200‡C at different scan rates from 2 to 30°C min−1 during heating or cooling. The enthalpies show a high correlation coefficient (0.964) with scan rate. All samples undergo
phase change in the temperature range from 107 to 173°C during heating and enthalpies are in the range from 31.8 to 71.1Jg−1. Upon subsequent cooling from 200°C, the temperature range of phase changes is shifted to lower temperatures from 86 to 102°C
with enthalpies ranging from 30.4 to 57.8 J g−1.
Experiments with exposure of polymers to microwave radiation and freezing prove that the phase change considering the temperature
range is very similar in all experiments. 相似文献
30.
Jiři Gregor 《Mathematics of Control, Signals, and Systems (MCSS)》1991,4(2):205-215
A significant part of the theory of one-dimensional linear shift-invariant systems is based on the concept of weighting function
(or impulse response): the output is the convolution of the weighting function with the input. This paper introduces the concept
of linear translation-invariant systems and uses this notion in studying impulse response, z-transforms, and transfer functions
for multidimensional systems. 相似文献