制冷剂热物理性质的快速计算方法

应用origin软件对常用制冷工质(R600a、R134a、R404A)热物性曲线拟合,方程在整个区间内均取得近100%的拟合度。该方法为今后制冷剂热物性仿真提供了一种新思路,因方程都是显式,具有计算快速、稳定的特点,对提高制冷系统仿真快速性、精确性、稳定性具有十分明显的效果。     

Dielectric properties of epoxy nanocomposites

The dielectric properties of epoxy nanocomposites with insulating nano-fillers, viz., TiO₂, ZnO and AI₂O₃ were investigated at low filler concentrations by weight. Epoxy nanocomposite samples with a good dispersion of nanoparticles in the epoxy matrix were prepared and experiments were performed to measure the dielectric permittivity and tan delta (400 Hz-1 MHz), dc volume resistivity and ac dielectric strength. At very low nanoparticle loadings, results demonstrate some interesting dielectric behaviors for nanocomposites and some of the electrical properties are found to be unique and advantageous for use in several existing and potential electrical systems. The nanocomposite dielectric properties are analyzed in detail with respect to different experimental parameters like frequency (for permittivity/tan delta), filler size, filler concentration and filler permittivity. In addition, epoxy microcomposites for the same systems were synthesized and their dielectric properties were compared to the results already obtained for nanocomposites. The interesting dielectric characteristics for epoxy based nanodielectric systems are attributed to the large volume fraction of interfaces in the bulk of the material and the ensuing interactions between the charged nanoparticle surface and the epoxy chains.     

Dielectric nanocomposites with insulating properties

Polymer nanocomposites possess promising high performances as engineering materials, if they are prepared and fabricated properly. Some work has been recently done on such polymer nanocomposites as dielectrics and electrical insulation. This was reviewed in 2004 based on the literatures published up to 2003. New significant findings have been added since then. Furthermore, a multi-core model with the far-distance effect, which is closely related to an "interaction zones", has been proposed from consideration of mesoscopic analysis of electrical and chemical structures of an existing interface with finite thickness. It is speculatively examined in the paper how the model works for various properties and phenomena already found in nanocomposites as dielectrics focusing on electrical characteristics, resistance to high voltage environment, and thermal properties.     

Dielectric properties of holmium oxide films

Results of dielectric studies of electron-beam deposited holmium oxide thin films, examined in metal/insulator/metal (MIM) structures, are reported. The frequency domain characteristics of the capacitance, dielectric loss and complex impedance have been estimated from the time domain measurements of the charging and discharging currents. The influence of the voltage on the dielectric characteristics at 430 K has been tested. Experimental results clearly show that the dielectric response in the frequency range from 100 kHz to 20 Hz comes from the insulating film of Ho₂O₃ as well as from metal-insulator interfaces. The capacitance and resistance of Ho₂ O₃ film and Al-Ho₂O₃ regions have been determined     

Dielectric properties of three liquid crystal polymers

Dielectric properties of three kinds of liquid crystal polymers (LCPs) were investigated by measuring their thermal properties, thermally stimulated polarization and depolarization currents, and complex dielectric permittivity. To make clear the differences among the three LCP samples, complex electric modulus was introduced, which was found to be effective to suppress the effects of large conduction currents at high temperatures. Moreover, the slopes of the frequency dependence of ɛ[_r' and ɛ]_r'' are different below and above the phase transition temperature, indicating that the accumulation of charges occurs in front of the electrodes. The experimental results indicate that the phase transition has a significant influence on both the electrical conduction and dielectric relaxation for all LCPs. © 2015 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.     

Dielectric properties, of metallized paper-film capacitors

A capacitor formed of fluid-impregnated paper metalized on one side and a polymer film results in a relaxation peak caused by the much more rapid polarization of the dielectric fluid impregnated paper relative to the polymer film. This results in the capacitance being a function of frequency, with a substantial increase in capacitance at very low frequencies. A theory for such a capacitor is developed and compared with measurements on actual capacitors with relatively good agreement. The implications of the theory for discharge current and discharge efficiency are discussed.     

Dielectric relaxation properties of filled ethylene propylenerubber

Thermally stimulated discharge currents and time domain dielectric spectroscopy were employed to characterize the behavior of clay-filled ethylene propylene rubber. Measurements were made on samples with different clay concentrations and particle sizes. The main effect of the clay filler on the electrical properties is on interfacial polarization which occurs at the clay polymer interface. The experimental results are sensitive to the shape of the clay particles. A few results indicate that other mechanisms also affect the electrical behavior of this material     

Dielectric mixtures: electrical properties and modeling

A review of the current state of understanding of dielectric mixture properties, and approaches to use numerical calculations for their modeling are presented. It is shown that interfacial polarization can yield different non-Debye dielectric responses depending on the properties of the constituents, their concentrations and geometrical arrangements. Future challenges on the subject are also discussed.     

Dielectric properties of nanostructured polypropylene-polyhedral oligomeric silsesquioxane compounds

This paper presents the results of the dielectric measurements conducted on polymer nanocompounds consisting of polypropylene (PP) and polyhedral oligomeric silsesquioxane (POSS). The material compounds were analyzed with a scanning electron microscope (SEM) and Raman-atomic force microscope (Raman-AFM). Ac and lightning impulse (LI) breakdown strength of the material compounds were measured. Relative permittivity, loss factor and volume resistivity measurements were also conducted on the material samples. Two types of POSS, octamethyl and isooctyl, were used in different quantities. The thickness of the samples was approximately 600 mum. Statistical analysis was applied to the results to determine the effects of the additive type and amount on the breakdown strength of polypropylene. The paper discusses the possibilities and restrictions in order to achieve advantages in high voltage applications using polyhedral oligomeric silsesquioxanes.     

Dielectric properties of the BaTiO3-AlN-additive system

The mixed system of BaTiO₃ and AlN has been investigated in terms of dielectric properties and microstructure. Two different types of additives, bismuth oxide and bismuth borosilicate glass, were used to lower sintering temperature. First, the addition of a fixed content (3 wt.%) of Bi₂O₃ provided densification at 1200^∘C where monotonous decreases of dielectric constant were found with increasing the content of AlN. On the other hand, the bismuth borosilicate glass was effectively used to decrease firing temperature to 850^∘C, which is suitable for thick film capacitor applications. A practical demonstration of thick film capacitors using a Ag electrode on a 96% alumina substrate indicated that the optimum composition of 76BaTiO₃-20AlN-4glass may be adequate for generating k of 79.4 and tan δ of 0.014 at 1 MHz as a result of the low temperature firing of 850^∘C in air atmosphere.     

Dielectric and ferroelectric properties of BTFCO thin films

Journal of Electroceramics - Single phase Bi3.25La0.75Ti2.5Nb0.25Fe0.125Co0.125O12 (BTFCO) thin films were deposited on Pt/TiO2/SiO2/Si substrates by RF-magnetron sputtering. Ferroelectric domain...     

Dielectric and piezoelectric properties in fluoride-doped PMNT ceramics

The effect of fluoride (LiF, MgF₂ and their mixture) on microstructure and electrical properties of 0.68 Pb(Mg_1/3Nb_2/3)O₃–0.32 PbTiO₃ (PMNT) ceramic was investigated. The X-ray study has shown each sample to be a pseudo-cubic perovskite single phase. Microstructure analysis has shown the fluoride-doped PMNT ceramics exhibit a typical liquid phase sintering character, and the fracture of LiF containing PMNT ceramics is transgranular fracture. The dielectric constants of fluoride-doped PMNT ceramics are higher than that of pure PMNT ceramic. The LiF containing PMNT ceramics have more apparent diffusion phase transition (DPT) and frequency dispersion than pure and MgF₂-doped PMNT ceramics. These results may be ascribed to structural disorder and compositional fluctuations in solid solution. The LiF containing PMNT ceramics have better piezoelectric properties than the other two samples. The values of d ₃₃ and K _p for LiF-doped PMNT ceramic arrive to 624 pC/N and 68%, respectively.     

Dielectric material properties investigated through space charge measurements

This paper investigates means of exploitation of space charge measurement results in order to achieve information about chemical, physical and microstructural properties of dielectric materials. The derivation of quantities as mean volume density of space charge, threshold for DC space charge accumulation, apparent-trap controlled mobility is discussed and various applications to dielectric materials are shown. In particular, the differences of trapping behavior of polyethylene-based materials, as low-density polyethylene, cross-linked polyethylene and high-density polyethylene, the effect of additives containing polar groups, the evaluation of cellular polymeric electrets are discussed on the basis of the results of space charge measurements and the relevant extracted quantities.     

Dielectric properties of an improved magnet wire for inverter-fedmotors

The ability to predict accurately the long-term pulse endurance performance of wire is essential both to the wire manufacturer and to the motor manufacturer. This paper provides information about the new magnet wire that can help design engineers employ it effectively in their applications. Accelerated pulse endurance test results of this new wire with and without varnishes at different pulse voltage levels, different pulse frequencies, different oven temperatures, and different rise times are presented     

Dielectric properties of mica-alumina combined insulation at high temperatures

Development of sodium-immersed-type Electromagnetic Pumps (EMP) is expected to improve reliability, safety, maintainability, and construction cost of the Fast Breeder Reactor (FBR) compared with the conventional mechanical pumps. To realize the EMP, establishment of a high-temperature-resistant electrical insulating system is indispensable. A mica-alumina combination is selected for the insulating system at temperatures higher than 500°C. Using this insulating system, an aging pretest was performed. The aging condition was to apply 1.5-kV ac voltage in the nitrogen atmosphere at 550°C for 4000 hours. The dielectric properties before and after the aging were measured to understand the tendency of temporal change in the aging. In this result, the dielectric performance after aging was improved compared with the initial performance. The curious phenomenon was clarified to be caused by a decrease of organic component in the insulation, which remained due to incomplete burning. Also, the possibility of thermal runaway was considered to be higher as a dielectric breakdown mechanism.     

Dielectric properties and scaling behavior of lithium tungsten phosphate glasses

In the present study a series of ternary (30 Li₂O, (70-x) P₂O₅, xWO3) glasses were prepared and their dielectric properties and ac conductivity were investigated. The measurements have been taken in the frequency range from 100?Hz to 100?kHz and over the temperature range from 296?K to 578?K. The temperature dependence of ac conductivity can be adequately explained by considering the contributions from mixed ionic and electronic mechanisms. In the studied glasses it is found that the ac conductivity increases with increasing frequency. By investigating the relation between temperature and the frequency exponent “s” of the power law σ_ac?=?Aω^s, it is found that the Correlated Barrier Hopping model (CBH) is appropriate for describing the conduction mechanism in the samples. In an attempt to investigate the universality of ac conductivity in these glasses, it is found that the data obtained follow Rolling scaling model. When considering the dielectric properties, it is found that the M″vs. M′ plots give master Cole-Cole curves at all temperatures. These results can be considered as an indication of the presence of space charge or accumulation of charges in some regions inside the samples. The relation between M″/ M_max″ and f/f _p represent a master plot at different temperatures. These scaling suggest the existence of a distribution of potential wells, in which the carriers are trapped.     

Dielectric and electrical properties of polycrystalline rocks atvarious hydration levels

The dielectric properties of low porosity natural polycrystalline rock material (marble) were investigated using broadband 10^-2 to 10⁶ Hz) dielectric relaxation spectroscopy at several hydration levels, from dry to fully hydrated samples, with the water content varying from 0 to 0.08% g water/g dry material. The measured dielectric response is characterized by large low frequency dispersions, which depend strongly on the water content. Upon varying the sample thickness, dielectric and water sorption measurements reveal that bulk rather than electrode/sample interface effects are responsible for the main part of the high values obtained for the complex permittivity ε*. Comparative analysis of the experimental data within the complex permittivity ε*, impedance Z* and modulus M*, allows a detailed investigation of the AC conductivity mechanisms existing in the rocks and of water content dependency. Concerning the rock-water interaction, a critical water content h_c, (h_c~0.04%, the water content for ambient humidity conditions) has been found     

Dielectric properties of ZnNb2O6/epoxy composites

The dielectric properties of epoxy composites with various contents of ZnNb₂O₆ filler were investigated at microwave frequencies. Increasing frequency from 1 GHz to 9 GHz did not affect the composites’ dielectric constants but increased their dielectric losses; both of which depended on ZnNb₂O₆ content and showed typical values of 9.63 and 0.019, respectively, at 9 GHz in the composite with 0.6 volume fraction ZnNb₂O₆. The results were compared with theoretical models and shown to be well described by the EMT model.     

Dielectric and piezoelectric properties of PZT ceramics with anisotropic porosity

Pb(Zr,Ti)O₃ ceramics with homogenous, anisotropic porosity were manufactured and the influence of the anisotropic porosity on the dielectric and piezoelectric properties was studied. Image analysis allowed the quantification of the preferential orientation and the calculation of the distribution of the aspect ratio of the pores. The different dependence of the permittivities e₃₃^T\varepsilon _{33}^T and e₁₁^T\varepsilon _{11}^T on the porosity content demonstrates that the population of pores is constituted of a high volume of pores with low aspect ratio and a low volume of pores with high aspect ratio. In view of the quantitative results obtained on the microstructure, an explanation of the frequency dependence of the dielectric and piezoelectric behaviour is suggested. Furthermore, the higher d₃₁ piezoelectric coefficient in the material with anisotropic pores than in material with isotropic pores is related to higher effective fields which are a consequence of the enhanced longitudinal mobility of the material under transverse stress.