CaTiO3 induced ferroelectric phase coexistence and low temperature dielectric relaxation in BaTiO3–BaZrO3 ceramics

The series of 0.86BaTiO₃–(0.14?x)BaZrO₃–xCaTiO₃ (abbreviated as BT–BZ–xCT) ceramics with 0.03 ≤ x ≤ 0.11 were studied to obtain high piezoelectric properties. Rietveld refinement analysis indicated that the BT–BZ–CT compositions follow a gradual rhombohedral (R) → orthorhombic (O) + R → O + tetragonal (T) → T phase transformation with increasing x. Clear evidence of the series of ferroelectric phase transitions was also found in the dielectric results. The R‐O and O‐T transition temperature shifted close to ambient temperature, while the Curie temperature slightly increased with increasing x. In addition to the dielectric loss peaks associated with the structural phase transitions, a broad low‐temperature dielectric loss peak was detected in the R phase at T = 90‐150 K. This dielectric relaxation was attributed to the domain wall freezing and fits well to the Vogel‐Fulcher model with activation energy E_a ≈ 60‐300 meV and freezing temperature T_VF ≈ 75‐140 K. High piezoelectric strain coefficient (d₃₃*) of about 1030 pm/V at 10 kV was achieved at x = 0.07, and a high Curie temperature (T_C) was maintained at about 375 K.     

Effects of mixing technique and filler content on physical properties of bovine bone‐based CHA/PLA composites

This study presented influence of mixing technique as well as filler content on physical and thermal properties of bovine bone‐based carbonated hydroxyapatite (CHA)/poly(lactic acid) (PLA) composites. CHA/PLA composites at various contents of CHA were prepared by either melt‐mixing or solution‐mixing techniques. Thermal properties, morphologies, and mechanical properties of the CHA/PLA composites including molecular weight deterioration of PLA matrices were investigated. Average molecular weights of PLA in the composites prepared by both techniques decreased with increasing CHA content, whereas their molecular weight distributions (MWDs) increased. Nonetheless, average molecular weights of PLA in melt‐mixed composites were lower than those of solution‐mixed composites. With increasing CHA content, elongation at break, tensile strength, and impact strength of the composites were decreased, whereas the tensile moduli of the composites were increased. In comparison between two mixing techniques, the melt‐mixing distributed and dispersed CHA into PLA matrix more effectively than the solution‐mixing did. Therefore, tensile moduli, tensile strength, and impact strength of the melt mixed composites were higher than those of the solution‐mixed composites of the corresponding CHA content. Moreover, decomposition temperatures and % crystallinity of the melt‐mixed composites were higher than those of the solution‐mixed composites. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011.     

Quiescent crystallization of natural fibers–polypropylene composites

The effect of natural fibers (vetiver grass and rossells) on quiescent crystallization of polypropylene (PP) composites was analyzed in this study. Also, equilibrium melting temperature (T) of the composites was elucidated. Natural fiber‐PP composites showed lower T when compared to neat PP. Thermal analysis was performed via differential scanning calorimeter to study the crystallization kinetics. Natural fiber‐PP composites exhibited higher rate of crystallization than that of neat PP. Furthermore, spherulitic growth rate and transcrystallinity of the composites were investigated under a polarized light optical microscope. It was found that the growth rates of the composites were lower than that of neat PP. The spherulitic growth rates combined with the crystallization rates were used to calculate number of effective nuclei. It was shown that the number of effective nuclei of the composites was higher than that of neat PP. This suggested that natural fibers could act as a nucleating agent in the composite. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

Effect of silane coupling agent treated bovine bone based carbonated hydroxyapatite on in vitro degradation behavior and bioactivity of PLA composites

In this study, effect of treating bovine bone based carbonated hydroxyapatite (CHA) with silane coupling agent on in vitro degradation and bioactivity of PLA composites were investigated. PLA composite specimens containing CHA and silane-treated CHA were immersed in phosphate-buffered solution at 37 °C for the periods of time up to 8 weeks. The changes in specimen weights and morphologies, pH of PBS solution and PLA molecular weight were examined. The results showed that the strong interfacial bonding between silane-treated CHA and PLA matrix significantly delayed in vitro degradation of the PLA composites. However, the bioactivity of the PLA/silane treated CHA composites, determined by the formation of poorly crystalline calcium phosphate compounds on the specimen surface after immersion in simulated body fluid (SBF), seemed to be lower than that of the PLA/CHA composite.     

Applications of shape memory alloys in optics

The application of shape memory alloy (SMA) thin films in optical devices is introduced and explored for the first time. Physical and optical properties of titanium-nickel (TiNi) SMA thin films change as these films undergo phase transformation on heating. An optical beam can be modulated either mechanically with a TiNi actuator or by the changes that occur in TiNi's optical properties upon heating and phase transformation. Reflection coefficients of TiNi films were measured in their so-called martensitic (room-temperature) and austenitic (elevated-temperature) phases. The reflection coefficients of the austenitic phase were higher than those of the martensitic phase by more than 45% in the wavelength range between 550 and 850 nm. Also, a microfabricated TiNi diaphragm with a 0.26-mm-diameter hole was used as a prototype light valve. The intensity of the transmitted light through the hole was reduced by 10%-17% when the diaphragm was heated. A novel TiNi light valve fabricated by using silicon micromachining techniques is also proposed and discussed. We present both optical data and structural data obtained by using transmission electron microscopies.     

Shear‐induced crystallization of injection molded vetiver grass‐polypropylene composites

Vetiver grass was used as an alternative filler in polypropylene (PP) composites in this study. Chemical treatment of vetiver grass by alkalization was carried out to obtain alkali‐treated vetiver grass. It was shown that alkali‐treated vetiver grass exhibited higher thermal stability than untreated vetiver grass. Injection molding was used to prepare the composites. The microstructure of injection molded samples showed a distinct skin layer due to shear‐induced crystallization. It was found that normalized thickness of shear‐induced crystallization layer of the composite was lower than that of neat PP. The effect of vetiver particle sizes on shear‐induced crystallization and physical properties of the composites were elucidated. Furthermore, the effect of processing conditions on shear‐induced crystallization, degree of crystallinity, gapwise crystallinity distribution, and mechanical properties of the composite were investigated. It was shown that injection speed and mold temperature affected the normalized thickness of shear‐induced crystallization layer and degree of crystallinity of the composites. However, processing conditions showed insignificant effect on the mechanical properties of vetiver fiber‐PP composites. The degree of crystallinity showed no distribution throughout the thickness direction of the composites. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

High piezoelectric activity in lead-free BaTiO3-BaZrO3-CaTiO3 ceramics with near polymorphic phase boundary

Abstract

Lead-free 0.88BaTiO₃–(0.12-x)BaZrO₃–xCaTiO₃ (BT-BZ-xCT) ceramics were fabricated via solid state reaction. The effect of CaTiO₃ content on crystal structure, phase transition, and electrical properties was investigated systematically. The crystal structure and phase transition of ceramics were characterized by X-ray diffraction (XRD), Raman spectra and dielectric measurement. Results show that ceramic in the composition, x?=?0.02, exhibits a rhombohedral structure. Ceramics with increasing CT content transformed from a rhombohedral to orthorhombic structure in the composition, x?=?0.04, and eventually became a tetragonal structure at the composition, x?≥?0.08. The polymorphic phase boundary (PPB) was observed at the composition, x?=?0.06, with coexistence of orthorhombic and tetragonal phases showing at almost room temperature. This PPB composition exhibited a high piezoelectric response (d₃₃*) of 1,150?pm/V at 10?kV/cm as an electric field was applied. These results indicate that the materials studied have potential as candidates for lead-free piezoelectric ceramics.     

Improvement of impact property of natural fiber–polypropylene composite by using natural rubber and EPDM rubber

In this research, vetiver grass was used as a filler in polypropylene (PP) composite. Chemical treatment was done to modify fiber surface. Natural rubber (NR) and Ethylene Propylene Diene Monomer (EPDM) rubber at various contents were used as an impact modifier for the composites. The composites were prepared by using an injection molding. Rheological, morphological and mechanical properties of PP and PP composites with and without NR or EPDM were studied. Adding NR or EPDM to PP composites, a significant increase in the impact strength and elongation at break is observed in the PP composite with rubber content more than 20% by weight. However, the tensile strength and Young’s modulus of the PP composites decrease with increasing rubber contents. Nevertheless, the tensile strength and Young’s modulus of the composites with rubber contents up to 10% are still higher than those of PP. Moreover, comparisons between NR and EPDM rubber on the mechanical properties of the PP composites were elucidated. The PP composites with EPDM rubber show slightly higher tensile strength and impact strength than the PP composites with NR.