The effect of processing conditions on the morphology, thermomechanical, dielectric, and piezoelectric properties of P(VDF-TrFE)/BaTiO3 composites

In this study (0–3) P(VDF-TrFE)/BaTiO₃ composites containing up to 60 vol% of ceramic phase were prepared by solvent casting or compression molding. Their thermomechanical, dielectric, and piezoelectric properties were investigated, and discussed in the light of the properties of the basic components, the processing route and the resulting morphology. The crystalline structure of the P(VDF-TrFE) matrix was found to be highly dependent on the processing route, while the structure of BaTiO₃ was not affected by any of the processing steps. The mechanical properties of the solvent cast materials showed a maximum at 30 vol% BaTiO₃, while they increased monotonically with BaTiO₃ content for compression molded materials. This difference was attributed to a higher amount of porosity and inhomogeneities in the solvent cast composites. Permittivity as high as 120 and piezoelectric coefficient d ₃₃ up to 32 pC/N were obtained for compression molded composites, and the observed decrease in d ₃₃ with aging time was attributed to the effect of mechanical stress release in the polymer matrix.     

New approach to MoSi2/SiC intermetallic-ceramic composite with B4C

The effects of SiC and B₄C additives in the MoSi₂ matrix on the microstructures and mechanical properties at room temperature were investigated. Their coefficients of thermal expansion (CTE) were also evaluated up to 1200°C by a thermal mechanical analysis (TMA). The experimental results show that the Mo₂B₅ reinforced phase was formed in situ in the hot-pressed MoSi₂/SiC/B₄C composites. Both the Mo₂B₅ phase and the SiC phase significantly improved the mechanical behavior of MoSi₂. Besides, the SiC with a high content up to 40 vol% could be added into the MoSi₂ composite with the B₄C additive. As a result, a dense and homogenous MoSi₂/SiC/B₄C composite was obtained, which possessed a relatively high bending strength and fracture toughness. Meanwhile, the CTE of the MoSi₂/SiC/B₄C composites linearly decreased with the increasing SiC content, which dropped to 21% at 1200°C in comparison with the pure MoSi₂ when adding 40 vol% SiC. This MoSi₂/SiC/B₄C composite system is very important for developing new applications at elevated temperature, particularly for high-temperature coating applications.     

Mechanical properties of Al2O3 and Al2O3 + ZrO2 ceramics reinforced by SiC whiskers

The effect of the SiC whisker content on the mechanical properties of Al₂O₃ and Al₂O₃ + 20 vol% ZrO₂ (2 mol% Y₂O₃) ceramic composites has been investigated. It is shown that the strength and fracture toughness of the composites are increased by the addition of 0–30 vol% SiC whiskers with only one exception that 30 vol% SiC whisker leads to a decrease in the flexure strength. The addition of 20 vol% ZrO₂ (2 mol% Y₂O₃) significantly improves the mechanical properties of the Al₂O₃ + SiC whisker (SiC_w) composites and the t-m phase transformation of ZrO₂ is enhanced by the residual stresses caused by the thermal incompatibility between the SiC_w and the matrix. The toughening effect of both SiC whiskers and the t-m phase transformation of ZrO₂ (2 mol% Y₂O₃) is shown to be additive, but the addition of ZrO₂ decreases the strengthening effect of the SiC whiskers.     

Solidification and microstructure of ZA27/SiCp composite fabricated by mechanical–electromagnetic combination stirring process

Abstract

The solidification behaviours and microstructural characteristics of both ZA27/SiC_p composites and monolithic ZA27 alloy were studied by using differential scanning calorimetry, scanning electron microscopy, transmission electron microscopy, electron probe microanalysis, and X-ray diffraction. It was found that there were differences in the transformation temperature and volume fraction of the phases, although the solidification process was almost identical for the composite and the monolithic alloy. The incorporation of SiC particles in the ZA27 alloy led to slight refinement of primary grains and reduced volume fraction of eutectic-like phase. The SiC particles obstructed Zn diffusion in the residual melt during the formation of proeutectic β phase, but promoted Zn diffusion from (Al) to η (Zn) phase during eutectoid transformation. During solidification, Cu was mainly segregated in the final solidification regions; Mg was present not only in the matrix but also on SiC particles; and oxide inclusions were mainly distributed around SiC particles. The matrix microstructure for both materials mainly consisted of primary cores of Al rich +η eutectoid; β′ phase resulting from the eutectoid transformation of the proeutectic β phase; and Zn rich +η eutectoid resulting from the eutectoid transformation of the eutectic-like phase. The SiC particles were mainly distributed around the primary grains. Several new phases based on the Al–Zn–Mg–Cu system and interfacial reaction products, including Al₂₁Fe₃Si, Cu₅Zn₈, Mg₆Cu₃Al₇, MgAl₂O₄, and amorphous oxide inclusions, were identified in the final solidification regions. The nucleation of both primary phase and eutectic-like +η phase at the surface of SiC particles and their crystallographic orientation relationships were investigated theoretically and experimentally. No distinct crystallographic orientation relationship between the matrix and SiC has been identified, although the mismatch between (0001)_SiC and (111)_ was calculated to be as small as 7·6%.     

Fabrication process and thermal properties of SiCp/Al metal matrix composites for electronic packaging applications

The fabrication process and thermal properties of 50–71 vol% SiC_p/Al metal matrix composites (MMCs) for electronic packaging applications have been investigated. The preforms consisted with 50–71 vol% SiC particles were fabricated by the ball milling and pressing method. The SiC particles were mixed with SiO₂ as an inorganic binder, and cationic starch as a organic binder in distilled water. The mixtures were consolidated in a mold by pressing and dried in two step process, followed by calcination at 1100 °C. The SiC_p/Al composites were fabricated by the infiltration of Al melt into SiC preforms using squeeze casting process. The thermal conductivity ranged 120–177 W/mK and coefficient of thermal expansion ranged 6–10 × 10^–6/K were obtained in 50–71 vol% SiC_p/Al MMCs. The thermal conductivity of SiC_p/Al composite decreased with increasing volume fraction of SiC_p and with increasing the amount of inorganic binder. The coefficient of thermal expansion of SiC_p/Al composite decreased with increasing volume fraction of SiC_p, while thermal conductivity was insensitive to the amount of inorganic binder. The experimental values of the coefficient of thermal expansion and thermal conductivity were in good agreement with the calculated coefficient of thermal expansion based on Turner's model and the calculated thermal conductivity based on Maxwell's model.     

Fabrication of SiC particles-reinforced magnesium matrix composite by ultrasonic vibration

Magnesium matrix composites reinforced with two volume fractions (1 and 3%) of SiC particles (1 μm) were successfully fabricated by ultrasonic vibration. Compared with as-cast AZ91 alloy, with the addition of the SiC particles grain size of matrix decreased, while most of the phase Mg₁₇Al₁₂ varied from coarse plates to lamellar precipitates in the SiCp/AZ91 composites. With increasing volume fraction of the SiC particles, grains of matrix in the SiCp/AZ91 composites were gradually refined. The SiC particles were located mainly at grain boundaries in both 1 vol% SiCp/AZ91 composite and 3 vol% SiCp/AZ91 composite. SiC particles inside the particle clusters may be still separated by magnesium. The study of the interface between the SiC particle and the alloy matrix suggested that SiC particles bonded well with the alloy matrix without interfacial reaction. The ultimate tensile strength, yield strength, and elongation to fracture of the SiCp/AZ91 composites were simultaneously improved compared with that of the as-cast AZ91 alloy.     

Processing,microstructure, and properties of Mg–SiC composites synthesised using fluxless casting process

Abstract

In the present study, elemental magnesium and magnesium–silicon carbide composites were synthesised using the methodology of fluxless casting followed by hot extrusion. Microstructural characterisation studies revealed low porosity and a completely recrystallised matrix in every material. The average size of the recrystallised grains was found to decrease with an increasing presence of SiC particulates. For the reinforced magnesium, fairly uniform distribution of SiC particulates and good SiC–Mg interfacial integrity was realised. The results of X-ray diffraction studies indicated the absence of oxide phases and no evidence of interfacial reaction products except in the case of Mg–26.0 wt-%SiC sample. Results of physical and mechanical properties characterisation revealed that an increase in the amount of SiC particulates incorporated leads to an increase in macrohardness and elastic modulus, which does not affect the 0.2% yield strength and reduces the ultimate tensile strength, ductility, and coefficient of thermal expansion. The weight percentage of SiC particulates when plotted against hardness and 0.2% yield strength revealed a linear correlationship. An attempt is made to investigate the effect of increasing amount of SiC particulates on the microstructural features, and physical and mechanical properties of the magnesium matrix.     

Thermal properties of chemical vapour-deposition SiC-C nanocomposites

The relationship between the thermal properties and the microstructure of chemical vapour-deposition (CVD) SiC-C nanocomposites, covering the entire composition range from SiC to C, was investigated after measuring thermal conductivity and thermal expansion. The samples were prepared under deposition temperatures (T _dep) of 1673 and 1773 K and total gas pressure (P_tot) of 40 kPa. The thermal conductivity of CVD SiC-C nanocomposites decreased as C content increased. For the deposits containing 24.3 to 71 mol % C prepared atT _dep = 1773 K, some parts of the C phase formed a layered structure having its plane parallel to the deposition surface. This arrangement reduced the thermal conductivity in the direction perpendicular to the deposition surface to a much lower value. The CVD C and CVD C-SiC containing < 1.5 mol % SiC showed strong anisotropic thermal expansion. However, the thermal expansion of CVD SiC-C nanocomposites having a C content up to about 70 mol % was isotropic and nearly equal to that of CVD SiC. The low preferred orientation and the low modulus of elasticity of the C phase may be reasons for these results.     

Microstructure and mechanical properties of barium aluminosilicate glass-ceramic matrix composites reinforced with SiC whiskers

BAS glass-ceramic composites reinforced with different volume fractions (0, 10, 20, 30, 40 vol%) of SiC whiskers were successfully fabricated by a hot-pressing method. The microstructure, whisker/matrix interface structure, phase constitution and mechanical properties of the composites have been systematically studied by means of SEM, TEM, XRD techniques as well as three-point bending tests. It was demonstrated that the incorporation of SiC whiskers could significantly increase the flexural strength and fracture toughness of BAS glass-ceramic matrixes. The celsian seeds can effectively promote the hexacelsian-to-celsian transformation in BaAl₂Si₂O₈. The active Al₂O₃ added to the BAS matrix obviously reduced the amount of SiO₂ in the matrix and formed needle-like mullite. The high temperature strengths of the composites were also investigated.     

Effect of silane coupling agent on the morphology,structure, and properties of poly(vinylidene fluoride–trifluoroethylene)/BaTiO3 composites

Micron- and submicron-sized barium titanate (BaTiO₃) particles, untreated and surface modified with aminopropyl triethoxy silane, were incorporated in poly(vinylidene fluoride–trifluoroethylene) to fabricate composites with up to 60 vol% of ceramic phase. The morphology and structure of solvent cast and compression-molded films, and their thermal, viscoelastic, and dielectric properties were investigated. When surface-modified BaTiO₃ was used, it was possible to decrease both the viscoelastic and the dielectric losses of highly filled solvent cast films, while their storage modulus and relative permittivity either increased or remained equal, owing to reduced porosity and improved matrix-filler compatibility. The effect of BaTiO₃ surface modification on the morphology of compression-molded films was less marked, leading to unchanged viscoelastic properties, and lower permittivity and dielectric losses. For all composites the frequency dependency of the dielectric properties at low frequencies was suppressed with modified BaTiO₃.     

Thermal stability and mechanical properties of yttria-doped tetragonal zirconia polycrystals with dispersed alumina and silicon carbide particles

Yttria-doped tetragonal zirconia polycrystals in which were dispersed various amounts of Al₂O₃ and SiC particles were sintered at 1500° C for 3 h, and the mechanical properties and the thermal stability of the sintered bodies were evaluated. Dispersion of Al₂O₃ caused no significant effect on sinterability, and increased the hardness and elasticity of the composites. Dispersion of SiC particles decreased the relative density and the grain size of composites. Elasticity and hardness increased by dispersing less than 10 vol% SiC, but decreased above 10 vol% SiC due to the decrease of relative density. Dispersion of both Al₂O₃ and SiC particles slightly increased the fracture toughness of ZrO₂-3 mol% Y₂O₃ ceramics but significantly decreased that of ZrO₂-2 mol% Y₂O₃ ceramics. The rate of the tetragonal-to-monoclinic phase transformation decreased by dispersing both Al₂O₃ and SiC particles. The transformation depth increased rapidly and then slowly with increasing the annealing time. The rate of increase in the transformation depth greatly decreased by dispersing Al₂O₃ particles.     

The effect of doping process on microstructure and dielectric properties of BaTiO3-based X7R materials

The effect of doping process on the dielectric properties, sintering behavior and microstructure were investigated on the BaTiO₃–Nb₂O₅–Co₃O₄ ternary system ceramic. Temperature stable dielectric ceramics were obtained by different doping processes if only appropriate Nb⁵⁺+Co³⁺ amount and Nb⁵⁺/Co³⁺ ratio were adopted. The dielectric constant was enhanced to the largest extent by nanometer oxide doping and the temperature characteristic satisfied the X7R specification. Two kinds of grains were observed in all the samples: matrix grains (BaTiO₃) and the secondary phase grains (Ba₆Ti₁₇O₄₀) formed by the incorporation of Nb⁵⁺ and Co³⁺ into BaTiO₃ lattice and Ti⁴⁺ segregation. The matrix grains were about 1 m in diameter and showed little grain growth with increasing temperature in all the doped samples, whereas the sizes of the secondary phase grains were strongly dependent on the doping process. The secondary phase formed liquid phase during firing, but the liquid phase contributed little to the densification of ceramics.     

Effects of attrition milling and post-sintering heat treatment on fabrication,microstructure and properties of transformation toughened ZrO2

The effect of attrition milling and post-sintering heat treatment on the fabrication, phase relations, microstructure and properties of ZrO₂ (+2.3vol% Y₂O₃) powder used to produce a transformation toughened material was examined. Powder used to fabricate the unmilled material was treated and consolidated by a colloidal method. The same powder, treated and consolidated by the same method, but ball milled in a commercial alumina mill before consolidation, was used to fabricate the milled material. Both materials were sintered at 1400° C for 1 h and then heat treated at higher temperatures. Milling introduced Al₂O₃ inclusions (< 1 vol%) and a glass phase (7 to 10 vol%). The milled powder was more difficult to sinter and exhibited more bloating (density decrease) during subsequent heat treatment. Transmission electron microscopy observations indicated that the larger glass content of the milled material beneficially reduced residual stresses that arose due to thermal contraction anistropy. Post-sintering heat treatment at temperatures > 1450° C produced detectable amounts of cubic ZrO₂ consistent with previously reported phase studies of the ZrO₂-Y₂O₃ system. The development of a bimodal grain structure was concurrent with the formation of detectable cubic phase. The larger grains in this bimodal distribution were primarily observed on the external surface and co-ordinating pores produced during the post-sintering heat treatments which were responsible for the bloating phenomenon. It is hypothesized that the pores were produced by the release of high pressure oxygen during cubic phase formation. Both fracture toughness (K _c) and hardness of the as-sintered materials were unaffected by milling. Hardness decreased with bloating and the decrease was more pronounced for the milled material which exhibited more bloating.     

Thermal Conductivity of ZrB2?SiC?B4C from 25 to 2000 °C

The thermal diffusivities of ZrB₂–SiC (10.7, 21.9, or 48.7 vol% SiC) with B₄C sintering aid were measured over 25–2000 °C using laser flash. The composition with the highest SiC showed the highest thermal conductivity (k) at 25 °C, but the lowest above ≈400 °C, because of the greater k temperature sensitivity of the SiC phase. Finite difference calculations of k, using selected literature data for the individual phases, and the concentration of phases from microstructures, correctly predicted temperature and phase concentration dependencies, but were lower than experimental results. The k of pure ZrB₂ and SiC as a function of temperature were back‐calculated from the experimental results for the multi‐phase materials; they were in good agreement with specific literature values.     

Dynamic mechanical analysis of prestrained Al2O3/Al metal-matrix composite

The effect of prestraining on the elastic modulus,E, and damping capacity, tan, of 10 and 20 vol% Al₂O₃ particle-reinforced composites has been investigated as function of temperature using dynamic mechanical analysis. Both elastic modulus and damping capacity were found to increase with volume fraction. At 10 vol% the modulus and damping were relatively insensitive to prestrain. However, at 20 vol% it was observed that the modulus decreased with increasing prestrain while damping increased significantly. These results are discussed in terms of fraction of broken particles, particle size, and differential in thermal expansion between the matrix and Al₂O₃ particulate.     

Thermal conduction in Cr3C2/SiC composite

Thermal conduction behaviour of Cr₃C₂/SiC composite is investigated in terms of temperature and SiC content. Experimental results showed that thermal diffusivity of the composite increases with SiC content up to 20 vol%, corresponding to a conductivity maximum, then decreases with further increase of SiC. The reduction in diffusivity and conductivity at higher SiC content may due to formation of small amounts of solid solution at the interface and/or of interfacial gaps due to lack of perfect contact among SiC aggregates leading to increased phonon scattering. The thermal conductivity demonstrates a positive temperature dependence but becomes temperature-independent when SiC content is above 30 vol%. A correlation with composite theory is present.     

Structural and chemical characterization of BaTiO3 nanorods

An electron-microscopy investigation was performed on BaTiO₃ nanorods that were processed by sol-gel electrophoretic deposition (EPD) into anodic aluminium oxide (AAO) membranes. The BaTiO₃ nanorods grown within the template membranes had diameters ranging from 150 to 200 nm, with an average length of 10-50 μm. By using various electron-microscopy techniques we showed that the processed BaTiO₃ nanorods were homogeneous in their chemical composition. The BaTiO₃ nanorods were always polycrystalline and were composed of well-crystallized, defect-free, pseudo-cubic BaTiO₃ grains, ranging from 10 to 30 nm. No intergranular phases were observed between the BaTiO₃ grains. A low-temperature hexagonal polymorph that is coherently intergrown with the BaTiO₃ perovskite matrix was also observed as a minor phase. When annealing the AAO templates containing the BaTiO₃ sol in an oxygen atmosphere the presence of the hexagonal polymorph was diminished.     

Grain boundary pinning of polycrystalline Al2O3 by Mo inclusions

Nano- to submicron-sized Mo particulates were deposited on porous Al₂O₃ disks in gradient concentration. The disks were laminated and sintered in inert atmosphere, revealing Mo inclusions in various contents and pining phenomena of matrix Al₂O₃ grains. Detail quantification of the microstructure of Mo and Al₂O₃ grains was conducted with special emphasis on the effects of the ultrafine Mo grains. The results show that the contents (5 vol% and 16 vol%) and the size of Mo inclusion are the critical quantities for the formation of intergranular/intragranular Mo inclusions. The microstructural development of the Al₂O₃ composites is greatly influenced by the states of the inclusions. A discussion on the topological and two other models of microstructural development is presented.     

On the thermal conductivity of bimodal SiC/A356 composites fabricated via powder metallurgy route

The present study investigates the thermal conductivity of bimodal SiC particulate distribution in aluminum matrix composites fabricated via powder metallurgy route. The effects of the SiC_p reinforcement size distribution and processing parameters such as sintering time and temperature on the thermal conductivity have been examined. The Box–Behnken experimental array was employed to identify the effects of selected variables on the thermal conductivity of the composite. A reasonable augmentation in the thermal conductivity was observed with an increase in sintering time and %volume fraction of fine SiC particulates. It has been demonstrated that the matrix doped with fine SiC particulates (37?µm) occupied interstitial positions and formed continuous SiC–matrix network resulting in minimizing the micropores that contributed for good thermal conductivity, that is, 235?W/mK. Scanning electron microscopy (SEM) and x-ray diffraction (XRD) were conducted to evaluate the microstructure architecture and interfacial phase formation.     

Interface structure and mechanical properties of Al2O3-20 vol% ZrO2-20 vol% SiCW ceramic composite

The microstructure, mechanical properties, fracture behaviour and toughening mechanisms of Al₂O₃-20 vol% ZrO₂ (2 mol% Y₂O₃)-20 vol% SiC_W ceramic matrix composite were investigated by X-ray diffraction, scanning and transmission electron microscopies, energy dispersive analysis of X-rays, high-resolution electron microscopy techniques and three-point bending tests. The results show that the Al₂O₃ matrix is simultaneously strengthened and toughened by both ZrO₂ particles and SiC whiskers. The interfacial amorphous layers between SiC whiskers and ZrO₂, and Al₂O₃ grains were observed by both TEM dark-field and high-resolution electron microscopy techniques.