Dielectric properties of polymer/ceramic composites based on thermosetting polymers

Summary A series of thermosetting polymer/ceramic composites were prepared. Three kinds of thermosetting polymers, i.e. cyanate resin, bismaleimide resin, and epoxy resin, were used as matrixes, and BaTiO₃ particles were as fillers. The dielectric properties of these composites were investigated. Experimental data of the dielectric constants were fitted to several theoretical equations in order to obtain the best-fitting equations of the dielectric constants of these composites. The result indicates that the dielectric constants of composites all increase with the increase of BaTiO₃ content. Using bismaleimide resin and epoxy resin as matrixes, the dielectric losses both increase obviously as the amount of BaTiO₃ particles is increased, but the dielectric loss of cyanate/BaTiO₃ composite decreases. With the increase of the frequency, the variation ranges of the dielectric constant and dielectric loss of cyanate/BaTiO₃ composite are both the smallest. The predications of the effective dielectric constants by Lichterecker mixing rule are in good agreement with experiment data.     

Fabrication and electrical properties of ceramic superconductor/polymer composites

Superconductor/polymer composites were prepared by mixing powders representing the Y-Ba-Cu-O and Bi(Pb)-Sr-Ca-Cu-O systems with high-density polyethylene. Their electrical resistivities were measured as a function of temperature. By controlling the powder preparation techniques, it was possible to fabricate composites with superconductive transition to zero resistivity in the Bi(Pb)-Sr-Ca-Cu-O system. These composites can be fabricated in certain desired shapes.     

Effects of filler on heat transmission behavior of flowing melt polymer composites

A probe consisting of a cartridge heater was inserted into melted polypropylene composites (PP/mica = 5, 10, and 20 wt%) flowing in a die having a diameter of 10 mm, and the effects of mica on the heat transfer coefficient and the heat transfer behavior from the surface of the probe to the flow compound material were examined experimentally. As a result, the heat transfer coefficient of the PP/mica composites increases 15–20% depending on the flow velocity and filler content. In general, the Prandtl number (Pr) for high‐viscosity (η) melted polymer is greater than one (Pr > 1), and the Prandtl number for polypropylene composites increases as the viscosity increases and decreases depending on the flow velocity and the temperature. The relationship between Pr and η is directly proportional, and the β value, which is given by the slope of the Pr ? η diagram, is a constant that shows the viscosity dependency of the Prandtl number. The β value can be determined by the ratio of the specific heat to the thermal conductivity. The Nusselt number depends on the mica content of the filler and increases gradually. POLYM. ENG. SCI. 46:1387–1393, 2006. © 2006 Society of Plastics Engineers     

Reactive wetting behavior and mechanism of AlN ceramic by CuNi-Xwt%Ti active filler metal

In order to propel the application of the developed CuNi-Xwt%Ti active filler metal in AlN brazing and get the universal reactive wetting mechanism between liquid metal and solid ceramic, the reactive wetting behavior and mechanism of AlN ceramic by CuNi-Xwt%Ti active filler metal were investigated. The results indicate that, with the increasing Ti content, surface tension for liquid CuNi-Xwt%Ti filler metal increases at low-temperature interval, but very similar at high-temperature interval, which influence the wetting behavior on AlN ceramic obviously. CuNi/AlN is the typical non-reactive wetting system, the wetting process including rapid wetting stage and stable stage. The wettability is depended on surface tension of the liquid CuNi filler metal completely. However, the wetting process of CuNi-8wt.%Ti/AlN and CuNi-16 wt%Ti/AlN reactive wetting system is composed by three stages, which are rapid wetting stage decided by surface tension, slow wetting stage caused by interfacial reaction and stable stage. For CuNi-8wt.%Ti/AlN and CuNi-16 wt%Ti/AlN reactive wetting system, although the surface tension of liquid filler metal is the only factor to influence the instant wetting angle θ₀ at rapid wetting stage, the reduced free energy caused by interfacial reaction at slow wetting stage plays the decisive role in influencing the final wettability.     

Effects of polymer molecular weight and filler particle size on flow behavior of wood polymer composites

The influence of polymer matrix molecular weight and filler particle size on rheological properties and extrudate distortions of metallocene polyethylene (mPE)/wood flour (WF) composites has been investigated by rotational and capillary rheometers. It was found that at low shear rates smaller filler particles provide higher shear viscosity than the larger sized filler. At high shear rates and WF loadings above 30 wt%, the effect of particle size on the melt flow properties becomes negligible. The relative increase of the storage modulus with decreasing particle size is more pronounced in the case of low molecular weight polymer matrix than that in higher molecular weight polyethylene based composites. The wood filled polyethylenes exhibit extrudate surface defects, which are complex function of the shear rate, polymer matrix molecular weight, and filler particle size. Increasing the shear rate results in pressure oscillations and spurt‐flow. It was also observed that the evolution of the extrudate surface tearing is strongly dependent on the pressure during a single pressure oscillation cycle in the spurt flow regime. POLYM. COMPOS., 2008. © 2008 Society of Plastics Engineers     

Temperature effect on the electrical resistivity of metal/polymer composites

In this paper the effect of temperature and filler content ? on electrical resistivity of styrene-acrylonitrile polymer filled with iron or aluminium powders has been investigated. The resistivity of such composites decreases suddenly by several orders of magnitude at a critical volume concentration ?_c, which depends on the size distribution of particles and on thermal history. For filler content lower than ?_c, the resistivity decreases with increasing temperature, whereas for ? > ?_c there is a sudden increase in resistivity at temperatures near the glass transition temperature of the polymeric matrix. This effect is more pronounced than the previously reported data on other composite systems.     

The effect of the applied field on the electrical properties of metal polymer composites

The effect of filler content and applied electrical field on the electrical resistivity of poly(methyl methacrylate) (PMMA) filled with aluminum particles was investigated. The resistivity of such composite suddenly decreases by several orders of magnitude at a critical volume concentration ϕ_c. In addition, for filler contents lower than ϕ_c, the PMMA/Al composite undergoes a transition from a high to a low resistance material in correspondence of a critical value of the external applied electrical field. This value has been related to thermal breakdown occurring in the regions between conductive particles. Moreover, the permittivity spectrum as a function of frequency is reported for composites of several filler contents, and the Lal and Parshad law was used to correlate the experimental data.     

Computer simulation of the electrical conductivity of polymer composites containing metallic fillers

In this study, a theoretical basis for the use of conductive composites was established. A percolation simulation program was used to determine critical area fractions for dispersions of rectangles in two dimensions. Both the aspect ratio and orientations of the rectangles were varied independently, and the simulation results were used to predict the effect of these parameters on the critical concentration of conductive flakes in a filled polymer. Above a certain aspect ratio defined as the “scaling limit,” the critical area fraction for rectangles was inversely proportional to aspect ratio. The scaling limit was smallest for a set of randomly oriented rectangles, and its value became larger as greater degrees of alignment were imposed. The smallest critical area fractions belonged to high aspect ratio, randomly oriented rectangles. The predictions of percolation theory were compared with results for dispersions of nickel-coated mica in fine glass powder and in compression molded polyethylene. The critical volume fraction of mica was inversely proportional to flake aspect ratio over the range of aspect ratios tested. Electromagnetic interference (EMI) shielding effectiveness was examined for composites containing both aligned and randomly oriented flakes. For a given filler loading, the shielding effectiveness of the aligned flake composites was substantially lower than that of the composites containing randomly oriented flakes.     

Mechanical,thermal, and electrical properties of metal fiber-filled polymer composites

The critical concentration at which a metal-filled composite becomes electrically conductive can be dramatically reduced by adding the metal as randomly dispersed fibers. The higher the aspect ratio of the fibers, the lower the concentration needed to induce electrical conductance. Composites exhibiting resistivities below 20 ohm-cm have been produced with less than 8 volume percent aluminum fibers, having an aspect ratio of 24:1. At low fiber loadings the tensile strength of the composites is similar to that of the unfilled polymer. The thermal properties of these composites are shown to increase monotonically in accordance with the theoretical development of Nielsen.     

In-plane thermal expansion behavior of dense ceramic matrix composites containing SiBC matrix

In order to reveal the effect of matrix cracks resulted from thermal residual stresses (TRS) on the thermal expansion behavior of ceramic matrix composites, SiBC matrix was introduced into C_f/SiC and SiC_f/SiC by liquid silicon infiltration. The TRS in both two composites were enlarged with incorporating SiBC matrix which has higher coefficients of thermal expansion (CTEs) than SiC matrix. Due to the relatively high TRS, matrix cracks and fiber/matrix (f/m) debonding exist in C_f/SiC-SiBC, which would provide the space for the expansion of matrix with higher CTEs. For SiC_f/SiC, no matrix cracking and f/m debonding took place due to the close CTEs between fiber and matrix. Accordingly, with the incorporation of SiBC matrix, the in-plane CTE of C_f/SiC between room temperature to 1100 °C decreases from 3.65 × 10⁻⁶ to 3.19 × 10⁻⁶ K^-1, while the in-plane CTE of SiC_f/SiC between room temperature to 1100 °C increases slightly from 4.97 × 10⁻⁶ to 5.03 × 10⁻⁶ K^-1.     

Automatic quantification of filler dispersion in polymer composites

A facile and objective method is introduced to automatically quantify the filler dispersion in polymer composites through image analysis. This method consists of automatic identification of the fillers in the image and a rigorous measurement of the filler dispersion within the space of functions. Compared with previous methods, this method has the advantages of 1) automatically recognizing the fillers, 2) minimizing the subjectivity induced by the inhomogeneity and noise of the background in the images, 3) a mathematically more rigorous definition of the deviation of the filler dispersion from uniformity, 4) a single performance metric reflecting both the distribution and the size of fillers. Both synthetic and real images from model compounds are used to demonstrate the sensitivity of the proposed method to the dispersion and aggregation of fillers. The computed dispersion index shows good agreement with visual observation of synthetic images and mechanical properties of the model compounds.     

Effect of electrically inert particulate filler on electrical resistivity of polymer/multi-walled carbon nanotube composites

Electrically conductive polypropylene (PP)/multi-walled carbon nanotube (MWCNT) composites containing electrically inert particulate filler calcium carbonate (CaCO₃) have been prepared in a rotational rheometer. A significant reduction in electrical resistivity was found with the addition of CaCO₃. The concept of effective concentration of MWCNTs is proposed to quantitatively evaluate the effect of CaCO₃. A master curve was achieved by plotting electrical conductivity (or resistivity) data of various composite systems, with or without CaCO₃, against their effective volume fraction of MWCNT, validating the concept of effective concentration. Similar results were obtained from investigations on MWCNT composite systems of different inert fillers, including talc and wollastonite, and host polymers, such as polyoxymethylene and polyamide, demonstrating the generality of the present observation.     

Electrical behavior of carbon black-filled polymer composites: Effect of interaction between filler and matrix

The effect of interaction between carbon black and polymer on electrical behavior was studied using the ESR method. The polymer matrices used were HDPE, LDPE, and ethylene/vinyl acetate (EVA). Two kinds of carbon blacks (CB), high structure CSF-III and low structure FEF, were used as a conductive filler. Compared to that of the HDPE/FEF compound, the positive temperature coefficient (PTC) intensity is lower and electrical reproducibility is worse for the HDPE/CSF-III compound; however, it can be improved significantly by radiation cross-linking. On the other hand, the cross-linking has no practical effect on the PTC intensity of the LDPE/CSF-III compound while it can be achieved by mixing the compound for a longer time. The great PTC intensity was obtained in the HDPE/EVA/CSF-III compound, and it is greater than that of HDPE/CSF-III or EVA/CSF-III. We explain these results using the concept of interaction between the filler and matrix. The absorption of the polymer on the carbon black surface may be physical or chemical; the latter is caused by the free-radical reaction between the polymer and carbon black, and it can occur during the radiation or preparation process of the compound. These “bound polymers” are essentially important for materials to have a great PTC intensity and good reproducibility. © 1994 John Wiley & Sons, Inc.     

Modeling electrical conductivity of polymer nanocomposites with aggregated filler

A mean-field model is developed for the electrical conductivity of microcomposites and nanocomposites with polymer matrices. The model accounts for aggregation of filler into clusters (involving both conducting and nonconducting particles) and rearrangement of these clusters with the growth of volume fraction of filler (which leads to a reduction in tunneling resistivity and an increase in the number of bridging contacts between conducting particles). The governing equations involve five material constants with transparent physical meaning: the depolarization factor of clusters, volume fraction of polymer in clusters of filler, effective conductivity of an individual filler particle, and two coefficients characterizing an increase in the effective electrical conductivity of filler driven by the growth of bridging contacts between neighboring particles in clusters. Good agreement is demonstrated between results of simulation and experimental data on the electrical conductivity of epoxy resin reinforced with carbon black and graphite particles, poly(vinyl chloride) reinforced with copper and nickel particles, polypropylene loaded with spherical and spheroidal tin particles, poly(butylene terephthalate) reinforced with graphene nanosheets, and polypropylene loaded with multiwalled carbon nanotubes.     

Network behavior of thermosetting polyimide/multiwalled carbon nanotube composites

In previous published research, network formation has been used to understand morphology and properties in polymer nanocomposites containing carbon nanotubes (CNTs) through measurements of rheological and electrical percolation thresholds, largely in thermoplastic matrices. In this research, these tools are explored as a means to understand network transport mechanisms and changes in CNT dispersion during curing in a thermosetting matrix. Specifically, rheological and electrical measurements were performed on the uncured nanocomposites, and electrical measurements were performed on the cured nanocomposites. The resulting data were applied to a percolation model. The results showed that the uncured resin played a limited role in mediating rheological transport and that little CNT aggregation occurred during curing. The results of this initial work suggest that such a combination of techniques is applicable to understanding dispersion changes resulting from curing and provides complementary insight to that provided by electron microscopy imaging of the same phenomenon.     

Synthesis and characterization of ceramic - polymer composites containing bioactive synthetic hydroxyapatite for biomedical applications

Presented research involved preparation of hydroxyapatite and synthesis of composites based on gelatin, albumin and polyvinylpyrrolidone (PVP) modified with the obtained compound. Hydroxyapatite was attained as a product of two-stage processing of pig bones. Applied procedure involved hydrolysis of the raw material in acidic environment and double calcination. Molar ratio Ca/P of hydroxyapatite has been determined and its chemical structure has been characterized using X-ray diffraction and FT-IR spectroscopy. Ratio Ca/P calculated on the basis of conducted research was 1.50?±?0.05. Thus prepared material met the ISO requirements, which assume that the Ca/P ratio should be in the range 1.5–2.0, which qualifies the material for further studies. Next, series of polymer matrix on the basis of gelatin, albumin and polyvinylpyrrolidone (PVP) has been synthesized and subjected to some analyzes. On the basis of the conducted studies, matrixes with the most favorable features such as desirable strength, flexibility and crosslinking degree were modified with previously prepared hydroxyapatite. Surface morphology and elemental composition of the composites have been analyzed using SEM-EDS method. Additionally, sorption capacity of modified composites and their behavior in simulated body fluids have been determined. Based on the conducted research it can be concluded that pig bones represent a good material for preparation of hydroxyapatite. Furthermore, composites based on proteins of natural origin modified with attained hydroxyapatite constitute a promising material that can be used for biomedical purposes.     

Ti-based ceramic composites derived from polymer pyrolysis

The formation and microstructure of the Ti-based ceramic composite derived from polymer pyrolysis in various atmosphere were investigated. Methylpolysiloxane was mixed with TiH₂ as a filler and pyrolyzed in nitrogen, argon and oxygen atmosphere at 1400–1600 °C. TiH₂ as active filler were reacted with atmospheric gas phase and pyrolytic products such as C and SiO₂. TiN, TiC, TiO₂ and Ti₅Si₃ were formed respectively depending on the atmospheric conditions used. Consequently, microcrystalline composites with the filler reaction products embedded in a silicon oxycarbide glass matrix were formed. Depending on the pyrolysis conditions, ceramic composites with a density of 85–88 TD% were obtained.     

Electrical conductivity and rheological behavior of multiphase polymer composites containing conducting carbon black

Multiphase polymer composites of carbon black (CB), polypropylene (PP) and low density polyethylene (LDPE) were prepared by melt‐mixing method to reduce the amount of CB in the conductive composites. SEM images showed that CB preferably located in LDPE phase and formed electrically conductive path. The measurement of conductive properties showed that the ternary materials possessed lower percolation than binary composites of CB/PP or CB/LDPE, the former was ～6 wt% and the latter was 9–10 wt%. Positive temperature coefficient (PTC) effects of the binary and ternary composites were investigated, indicating that the latter exhibited a relatively high PTC intensity. A rheological percolation estimated by a power law function is 2.66 wt% of CB loading, suggesting an onset of solid‐like behavior at low frequencies. This difference between the electrical and rheological percolation thresholds may be understood in terms of the smaller CB–CB distance required for electrical conductivity as compared with that required to impede polymer mobility. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers     

Tape casting of piezo ceramic/polymer composites

1-3 Piezoelectric composites are used as ultrasonic transducers for naval sonar devices, medical diagnostic systems, and non-destructive materials testing. Their excellent coupling behaviour to ultrasonic signals in water is due to the high fraction of polymer as the matrix and a very fine structured piezoelectric ceramic, which allows an effective transfer of the hydrostatic waves from the polymer to the ceramic. The aim is to obtain a composite of maximum hydrostatic performance characterised by d_h x g_h (d_h: hydrostatic charge coefficient; g_h: hydrostatic voltage coefficient), which allows a detection of signals even with low intensity. Different processing routes are known to manufacture such 1-3 composites, but there is a need for a flexible, low cost process for medium quantities. Tape casting is a very adaptable method to produce such composites. Strips were cut into PZT green tapes line by line. Subsequently, the sheets were sintered and stacked, by using spacers between the sheets. The free spaces are filled with polymer, and finally the edges of the composite are cut off to remove the frame of the PZT-sheets. Different techniques were investigated to structure the green tapes without damaging them and to preserve the fine structure of the tapes during firing by avoiding warpage.     

Characterization and transport properties of ceramic filler incorporated natural rubber composites

Crystalline glass–ceramic fillers were prepared from calcium carbonate, silica, alumina, and calcium fluoride by heating and subsequent quenching in cold water. The fillers were incorporated into natural rubber (1,4-cis-polyisoprene) and the filled rubber composites were crosslinked with sulfur in the presence of different rubber additives. The unfilled and filled rubber composites were characterized. The transport properties of benzene, toluene, and p-xylene (BTX) through the rubber composites were studied in terms of sorption, diffusion, permeation, and mass transfer coefficients. The effect of the ceramic fillers on the mechanical, thermal and transport properties were studied. The sorption data at different temperatures were used for calculating activation energy of diffusion, permeation, free energy, and enthalpy of sorption. The BTX remained in the liquid state within the composite matrix as evident from negative ΔS. The diffusion coefficient (D) and mass transfer coefficient (k_mtc) of BTX decreased with the increase in filler loading. Accordingly, for the transport of BTX the unfilled rubber showed a D (D × 10⁷ cm²/s) and mass transfer coefficient (k_mtc × 10⁴ cm/s) of 5.67/3.97/2.96 and 7.71/7.08/7.04, respectively which decreased to 5.06/2.95/2.57 and 7.53/6.95/6.90, respectively for the composite containing 50 wt.% ceramic filler.