Β-phase Fraction Research Articles

Effect of electrospinning parameters on piezoelectric properties of electrospun PVDF nanofibrous mats under cyclic compression

The high piezoelectric response capacity of electrospun PVDF nanofibers is currently a challenging area of research, and this study attempts to optimize the piezoelectric properties of electrospun PVDF nanofibrous mat under controlled electrospinning process. Some experiments were designed to study the relationship between electrospinning processing parameters and the piezoelectric properties of PVDF mats under cyclic compression, and this relationship was compared with the change of crystalline structure features with the electrospinning parameters. The excitation method is based on low velocity compression test by cyclically pressing flat contactor on samples. The impact of each of processing parameters has been investigated. It was found that three electrospinning parameters including: applied voltage, feeding volume flow rate and syringe needle tip diameter, are significant influential factors to the piezoelectric properties of the electrospun PVDF nanofibrous mats. However, the feeding volume flow rate has a greater effect on the piezoelectric responses to cyclic compression than both applied voltage and volume flow rate does, and its effect on the piezoelectric responses is different from that of β-phase fraction of crystalline structure within PVDF nanofiber. It was also observed that the relationship between the piezoelectric peak intensity and the applied voltage as well as the feeding volume flow rate is nonlinear.

Development of ductile γ+α2 titanium aluminide through ingot metallurgy route, thermomechanical processing and characterization

γ+α2 Titanium aluminide (Ti-28Al-10Nb-3Cr-0.02Bwt%) was processed through vacuum arc remelting (VAR) route using Ti sponge and pure alloying elements. Electrode for VAR melting which was designed to obtain uniform chemistry across the cross-section and throughout the height of the billet was realized through cold compaction. The alloy was hot worked in the temperature range of 1200–1100°C. Tensile properties of the alloy at ambient temperature were evaluated at different strain rates ranging from 10−4s−1 and 10−1s−1. The alloy exhibited ultimate tensile strength in the range of (386–553MPa) when tested at ambient temperature and strain rates of 10−4s−1 and 10−1s−1, whereas it varied from 630–673MPa at intermediate strain rates of 10−3s−1 and 10−2s−1. Percentage elongation is found to be 4.87–5.49% at strain rates of 10−4s−1 and at 10−1s−1 whereas it showed a relatively higher % elongation (5.42–7.19%) at intermediate strain rates of 10−3s−1 and 10−2s−1, indicating intermediate strain rates are better for optimum strength as well as for ductility.The alloy is found to have predominantly two phases γ (TiAl) and α2 (Ti3Al) with moderately random texture and presence of small volume fraction of β-phase. Microstructure showed presence of varying width of α2+γ lamellar structure. Transmission electron microscopy (TEM) confirmed presence of extensive twinning, dislocations and stacking faults with two phase structure indicating twinning has played a significant role in deformation. Presence of minor amount of β phase has been found to be contributing factor for improvement in ductility. Fracture analysis shows failure was mainly by rupture of lamellar interfaces. Occasional deformation of lamellae is also noticed in the uniaxial tensile tested samples at intermediate strain rates of 10−3s−1 and 10−2s−1 wherever small increase in ductility is observed. Marginally lower ductility at lower strain rate is due to growth of microcracks at interlamellar sites of dissimilar orientation and at higher strain rate due to faster growth of microcracks at lamellae of similar orientation.

Mixed effect of main electrospinning parameters on the β-phase crystallinity of electrospun PVDF nanofibers

Electrospun polyvinylidene fluoride (PVDF) nanofiber has shown its advantages of producing piezoelectric devices. Although the effect of electrospinning parameters on the formation of β crystalline phase has been investigated, their mixed effect has not been fully understood. In this study, two dependent design of experiments were utilized to systematically control the transformation of crystalline phases by three typical electrospinning parameters, i.e. applied voltage, needle tip diameter and feeding flow rate, and the mixed interaction of these parameters in improving the fraction of β crystalline phase was investigated. The structures and the contents of crystalline phases within PVDF nanofibers were analyzed by Fourier transform infrared spectra and x-ray diffraction patterns. The results of single factorial experiments showed that all of three electrospinning parameters have a significant effect on the formation and fraction of β-phase crystallinity of electrospun PVDF nanofibers. Furthermore, both the fraction and the crystallinity of β phase initially increase and then decrease with an increase of applied voltage, and consistently decrease with the needle-tip diameter. However, the β-phase fraction parabolically increases with the increasing feeding flow rates, whereas its crystallinity linearly deceases. The orthogonal experimental results demonstrated that the mixed effect of three electrospinning parameters is different from that of single parameter on the fraction of β-phase crystalline, and that the feeding flow rates among three parameters have the greatest effect. Taken together, these results suggested a new strategy to improve the β-phase fraction of electrospun PVDF fibers via sequence of priority of electrospinning parameters.

Creep Life Degradation and Microstructure Degeneration in a Low-Pressure Turbine Blade of a Military Aircraft Engine

Military aircraft engines operate under arduous environmental conditions with rapid throttle excursions as a part of its mission requirements. Life of an aircraft engine particularly of a military class is limited by the longevity of hot end components. Total technical life of a gas turbine component is dictated by the bulk properties while time between overhaul is decided by the aero-thermal degradation which in turn is influenced by the surface protective coating. In the present research, stress rupture tests were carried out to evaluate the creep life of blades while swelling of the coating and secondary reaction zone (SRZ) were evaluated to assess the coating damage. Further, an investigation was made to assess the degradation in creep life and microstructure degeneration during the service period when turbine blades made of wrought nickel-based alloy is provided with aluminide coating. Minimum creep rate and time to rupture are used as the characteristic parameters to evaluate the degradation in creep life. Coarsening of gamma prime (γ′) and degeneration of carbides was evaluated for microstructure degeneration in the bulk material. Degradation in coating life was evaluated by volume fraction of β-phase and development of SRZ in the coating.

Significantly enhanced electroactive β phase crystallization and UV-shielding properties in PVDF nanocomposites flexible films through loading of ATO nanoparticles: Synthesis and formation mechanism

In this work, we report on the preparation and characterization of a poly(vinylidene fluoride) (PVDF) film filled with antimony tin oxide nanoparticles (ATO-NPs), films were prepared via a facile solution casting method to investigate the electroactive β-polymorph formation mechanism and the optical properties of the (ATO/PVDF) nanocomposite films. The impact of the ATO loading on the structural and morphological properties of PVDF were investigated by Fourier transform infrared spectroscopy, X-ray diffraction and scanning electron microscopy techniques. Designing nanocomposites with a combination of the advantages of both PVDF and ATO nanofiller leads to a feasible route to a remarkable increase of the electroactive phase enriched PVDF films. The (0.05) ATO/PVDF composite film exhibits a maximum β-phase fraction of 86% compared with other loadings this may be assigned to the presence of interfacial interactions at the interface between NPs surface and CH2/CF2 dipoles of PVDF inducing the electroactive phases. Furthermore, the thermal properties of the composites were investigated by differential scanning calorimetry technique revealed the effect of the ATO loading on the structural and morphological properties of PVDF. Studying the optical properties of the nanocomposite films exhibit a strong absorbance in the entire UV region compared with neat PVDF, which is very appealing for UV blocking properties.

Effect of synthesis parameters on structural and mechanical properties of Ti3Al–Nb–Mo intermetallic compound obtained by powder metallurgy techniques

The influence of microstructure, heat treatment and alloying addition on mechanical and fracture properties of Ti3Al-based intermetallic at room and elevated temperatures was studied. Ti3Al–11Nb–1Mo (mole fraction, %) alloy was consolidated via powder metallurgy processing by mechanical alloying (MA) and hot pressing (HP). MA powders were characterized using XRD and SEM-EDS. Optimum MA duration was 25 h and HP conditions of 1350 °C, 2 h, 35 MPa. After HP, solution treatment at 1050 °C for 1 h and water quenching α2+β Widmanstätten microstructure is present, while subsequent aging at 800 °C during 24 h induces small content of O-phase. High fraction of β-phase is a direct consequence of Mo. Compression tests were performed from room temperature to 750 °C in vacuum. The yield strength of compacts increases with temperature up to 250 °C (pyramidal slip systems activation), after which it decreases. Ductility increases throughout the whole temperature range. The presence of O phase contributed to ductility increase in aged alloys, while negligibly lowering yield strength. Registered drop in the yield strength of aged alloys compared with non-aged ones was mostly influenced by precipitation of α″2 particles. Mixed fracture modes are operative at all temperatures.

An Easy Approach to Control β-Phase Formation in PFO Films for Optimized Emission Properties.

We demonstrate a novel approach to control β-phase content generated in poly(9,9-dioctylfluorene) (PFO) films. A very small amount of paraffin oil was used as the additive to the PFO solution in toluene. The β-phase fraction in the spin-coated PFO films can be modified from 0% to 20% simply by changing the volume percentage of paraffin oil in the mixed solution. Organic light emitting diodes (OLEDs) and amplified spontaneous emission (ASE) study confirmed low β-phase fraction promise better OLEDs device, while high β-phase fraction benefits ASE performance.

Morphology and dielectric investigations of hydrated-halt P(VDF-HFP) membranes

This study focuses on a preparation and characterization of hydrated-salt membranes based on the blend of poly(vinylidene fluoride-hexafluoropropylene) [P(VDF-HFP)] and magnesium chloride (MgCl2⋅6H2O). The membranes with MgCl2⋅6H2O concentration of 0-4 wt% were prepared by solution casting technique. An elemental composition of the prepared samples was analyzed using energy dispersive x-ray spectroscopy (EDS). The surface morphology and pore size of these membranes were investigated by scanning electron microscopy (SEM). Surface roughness of the as-received membranes was evaluated by atomic force microscopy (AFM). The degree of the crystallinity and phase structures of the P(VDF-HFP) membranes were analyzed by X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). The dielectric and the electrical conductivity of all samples were measured in a range of 1-105 Hz. The SEM and AFM images revealed that the formation of the microporous P(VDF-HFP) membranes was controllable by the adjustment of MgCl2⋅6H2O content. The surface roughness, pore size and ionic conductivity of the as-received membranes increased with MgCl2⋅6H2O concentration. The dielectric constant reached its maximum value of about 25 at 3.5 wt% of MgCl2⋅6H2O. The XRD results showed that the mixture of MgCl2⋅6H2O salt led to a decrease in the degree of crystallinity. However, the salt-loaded P(VDF-HFP) enhanced the fraction of an electroactive β-phase up to 84% and this was confirmed by the FTIR results. This pointed out that the hydrated-salt P(VDF-HFP) membranes had a potential in new applications such as energy harvesting area.

A new approach to fabricate poly(vinylidene fluoride-trifluoroethylene) fibers using a torsion-stretching method and characterization of their piezoelectric properties

A new, simple, and economical approach to fabricate P(VDF-TrFE) fibers using a torsion-stretching method is introduced. The average Young’s modulus, strength, and nominal strain of the fibers were 902 MPa, 258.56 MPa, and 35.23%, respectively. Three conventional treatments (mechanical stretching, electrical poling, and annealing-cooling) were conducted to enhance the β-phase of the P(VDF-TrFE) fibers. All these treatments were effective in enhancing the crystallinity of the β-phase of the fibers. The maximum β-phase fraction (60.78%) was obtained from annealing-cooling, while the control group exhibited a β-phase fraction of only 48.29%. Furthermore, using a carbon-epoxy composite laminate an electro-mechanical test was conducted to estimate the piezoelectric performance of the P(VDF-TrFE) fibers for assessing the relationship between the generated voltage and the compressive force. The maximum generated voltage from the fiber-type sensors treated by annealing-cooling was 0.14 V, which had the highest relative β fraction among the three treatments.

Microstructure and superconducting properties of Sn–In and Sn–In–Bi alloys as Pb-free superconducting solders

In this work the microstructure and superconducting properties of various compositions of binary Sn–In and ternary Sn–In-Bi alloys have been studied as potential replacements for the Pb-based superconducting solders currently used for superconducting joints between technological low temperature superconducting wires. The influence of chemistry and microstructure on the superconducting properties are investigated. Both the Sn–In β and γ phases are stable over a fairly wide range of chemical composition, with the superconducting properties of each phase improving with increasing solute concentration, but the results show that the In-rich β phase has better superconducting properties than the Sn-rich γ phase. For this reason, in both the binary Sn–In and the ternary Sn–In–Bi systems, the best superconducting properties are achieved in alloys with a high volume fraction of β-phase. The ternary alloys generally show better superconducting properties than the binary alloys, with the highest values of 6.9 K, 0.18 T and 1.3 × 108 A m−2 measured for TC, BC2 (at 4.2 K) and JC (at 4.2 K, 0.01 T) respectively in the Sn35In50Bi15 alloy that consists of a matrix of β-phase with fine fibres of γ and BiIn2. The dependence of the pinning force on the reduced magnetic field exhibits a scaling law behaviour with the maximum at B/BC2 ∼ 0.2 indicating that the dominant mechanism is surface normal pinning in these solders. Rapid solidification in liquid nitrogen results in manipulation of the phase chemistry by both increasing the solute content in the majority phase, changing the fraction of each phase and reducing the scale of the microstructure, leading to significant improvements in superconducting performance. However, the quenched alloys age considerably, even at room temperature, resulting in a deterioration in BC2 and JC values over a few days.

Characterization of an electroactive polymer (PVDF-TrFE) film-type sensor for health monitoring of composite structures

Poly(vinylidene fluoride) (PVDF) is one of the most promising electroactive polymers with numerous potential applications in many industrial fields. Three different conventional treatments (annealing and cooling, electrical poling, and pressing) were conducted to enhance the β-phase of the P(VDF-TrFE) films. Moreover, sequential treatments involving each process were conducted to elucidate the effect of the sequence of treatments on the β-phase transformation in the P(VDF-TrFE). By primarily using X-ray diffraction, microscopic analysis was conducted to investigate the effect of such treatments on the piezoelectric properties of the P(VDF-TrFE) films. It could be determined from the results that the annealing and cooling, electrical poling, and pressing treatments were all effective and the sequential treatments were also effective in enhancing the crystallinity of the β-phase of the P(VDF-TrFE). The maximum β-phase fraction (66.33%) was obtained from the sequential treatment of “annealing and cooling-pressing-electrical poling” while the control group only exhibited a β-phase fraction of 45.29%.

Electrospinning-induced preferred dipole orientation in PVDF fibers

Polyvinylidene fluoride (PVDF) can be made electroactive by properly mechanical stretching and electric poling treatments of its film, which may be easily realized by single-step electrospinning. This technique is acknowledged as an effective approach to induce rich ferroelectric β-phase in electrospun PVDF fibers; however, the investigation of dipole arrangement during the electrospinning process is still lacking. Here, the piezoelectricity of β-PVDF fibers by electrospinning and forcespinning, a mechanical spinning process without static electric field bias, has been demonstrated. Results show that the electrospun fibers can generate piezoelectric voltage after deformation, while the forcespun fibers nearly show no piezoelectricity for the same condition, revealing that electric field during the electrospinning process can perform in situ poling effect and therefore induces preferred dipole orientation in electrospun PVDF fibers. Further experiments performed in this work show that piezoelectricity of the electrospun fibers increases with increasing fraction of β-phase and/or the applied electric field strength of electrospinning, which provides good guideline for preparing high-performance piezoelectric fibers.

Effect of Li content on the indentation creep characteristics of cast Mg–Li–Zn alloys

Creep behavior of the hcp Mg–4Li–1Zn (LZ41), hcp/bcc Mg–8Li–1Zn (LZ81), and bcc Mg–12Li–1Zn (LZ121) cast alloys was studied by long-term Vickers indentation testing in the temperature range of 423–498 K and under constant load of 5 N. It was established that increasing Li content decreased the creep resistance at all test temperatures. This was stemmed from the effect of Li on the crystal structure of the alloys, the increment of which increased the volume fraction of the bcc β-phase. This phase activated non-basal slip and facilitated the slip-assisted deformation, and thus, resulted in lower strength and creep resistance. The average stress exponents of about 7.0, 4.5, and 4.2 and activation energies of 90.0, 91.7 and 98.2 kJ/mol were obtained for the LZ41, LZ81, and LZ121 alloys, respectively. It is suggested that the operative creep mechanism is dislocation pipe diffusion in the LZ41 alloy, and pipe-diffusion-controlled dislocation viscous glide in the LZ81 and LZ121 alloys.

Predicting Polymorphism of Electrospun Polyvinylidene Fluoride Membranes by Their Morphologies

Although electrospinning of polyvinylidene fluoride (PVDF) has been studied for more than 10 years, the crystalline phase differentiation of the electrospun mats is still normally through the combination of different characterization techniques, and the relationship between polymorphism and morphology of the fibers in electrospun PVDF membranes has never been reported. Here, we show their close relationships by conducting room-temperature electrospinning experiments on various polymer/solvent systems. The electrospun membranes full of bead-free fibers have a very high fraction of β-phase, F(β), over 90%, and high orientation, whereas the membranes comprising beads and/or a large number of beaded fibers most often result in a low fraction of β-phase (F(β) normally below 50%) and low orientation. On the other hand, electrospun membranes consisting of both bead-free fibers and a very limited number of beaded fibers showed a medium high fraction of β-phase, F(β) more than 70% but less than 90%. These findings suggest the feasibility of intuitively predicting the crystalline phase of electrospun PVDF membranes directly by their morphologies, which is obviously simple, inexpensive and convenient for future investigations.

Effect of fiber size on structural and tensile properties of electrospun polyvinylidene fluoride fibers

We use electrospinning to obtain polyvinylidene fluoride (PVDF) fibers and demonstrate simultaneous improvements in β-crystal microstructure and in tensile properties of fibers with reduction of their diameter. PVDF fibers with average diameters ranging from 70 to 400 nm are obtained by controlling the concentration of the polymer in the electrospinning solution. The amount of β-crystals present is found to be greater for finer diameter fibers, yielding a maximum β-phase fraction of 0.86 in the 70-nm fibers. Moreover, the deformation behavior of the fibers reveals that the tensile modulus and strength improve with reductions in fiber size. Sharp increases in tensile properties are demonstrated when the size of the fibers is reduced below 175 nm. We attribute the enhanced concentration of β-crystals and the tensile behavior of finer diameter fibers to the extensional forces experienced by the material during electrospinning. POLYM. ENG. SCI., 55:1812–1817, 2015. © 2014 Society of Plastics Engineers

Influence of Microstructure and CO2 on the NaCl-Induced Atmospheric Corrosion of Semi-Solid Cast and High Pressure Diecast Mgal Alloys

The NaCl induced atmospheric corrosion of MgAl alloys AM50 and AZ91 was studied in the laboratory at 95.0% relative humidity (RH) and 22oC. The two alloys were investigated both in semi-solid cast (SSC) and in high pressure diecast (HPDC) states. Exposures were conducted in the presence of 400ppm CO2 and in the absence of CO2. Crystalline corrosion products were identified by X-ray diffraction (XRD). X-ray photoelectron spectroscopy (XPS) was employed to examine the surface film formed in the initial stages of corrosion. Magnesium carbonate and magnesium oxide (oxide/hydroxide) were detected in the top and bottom of the surface film, respectively. Alloy microstructure and corrosion morphology were characterized by scanning electron microscopy (SEM). Corrosion was more localized in CO2-free environment compared with air with 400ppm CO2. The same alloy prepared by SSC and HPDC routes showed different corrosion properties. It is suggested that the influence of casting technology on corrosion is caused by differences in the area fraction and distribution of β-phase.

Theoretical Study of the Influence of Microalloying on Sensitization of AA5083 and Moderation of Sensitization of a Model Al-Mg-Mn Alloy via Sr Additions

The calculated effect of a range of quaternary alloying additions to aluminum alloy (AA)5083 (Al-4.4Mg-0.5Mn [UNS A95083]) was investigated. Attention was given to the influence of composition on the volume fraction of β-phase (Mg2Al3), and the possible formation of any additional phases (which we term γ-phase herein). Alloying additions of silver, cerium, copper, lithium, neodymium, nickel, scandium, silicon, strontium, yttrium, zinc, and zirconium were studied in hypothetical additions of 0.1 wt% and 0.5 wt%, revealing that there is potential to modify the fraction of β-phase, and hence sensitization, in AA5083. Calculations indicated strontium to be the most effective addition in decreasing β-phase fraction during thermal exposure (sensitization); therefore, the influence of Sr additions were studied empirically via production of custom Al-Mg-Mn alloys. Sensitization was investigated via the nitric acid mass loss test (NAMLT), revealing that mass loss from intergranular corrosion decreased by more than...

Tool wear mechanisms in machining of three titanium alloys with increasing β-phase fraction

Mechanical and microstructural properties of titanium alloys change with β-phase fraction. This in turn influences the dominant tool wear mechanisms in their machining. This work therefore involves turning experimentation on three titanium alloys with varying β-phase fraction, namely, α, α+ β and β-rich alloys using coated carbide tools, to identify dominant wear mechanisms, which hitherto have not been adequately investigated. The dominant wear mechanisms were investigated using scanning electron microscopy and energy-dispersive X-ray analysis of worn tool surfaces and were also correlated with the cutting forces during machining. Abrasion, abrasion with built-up edge and plastic deformation of cutting edge appeared to be the dominant tool wear mechanisms in α, α+ β and β-rich alloy, respectively. At the same time, diffusion of Sn, V and Mo from α, α+ β and β-rich alloys to tool face, respectively, was observed. The chip–tool contact length predicted using the analytical models from the literature matched closely with the experimental values.

Effects of Substrate on Piezoelectricity of Electrospun Poly(vinylidene fluoride)-Nanofiber-Based Energy Generators

We report the effects of various substrates and substrate thicknesses on electrospun poly(vinylidene fluoride) (PVDF)-nanofiber-based energy harvesters. The electrospun PVDF nanofibers showed an average diameter of 84.6 ± 23.5 nm. A high relative β-phase fraction (85.2%) was achieved by applying high voltage during electrospinning. The prepared PVDF nanofibers thus generated considerable piezoelectric potential in accordance with the sound-driven mechanical vibrations of the substrates. Slide glass, poly(ethylene terephthalate), poly(ethylene naphthalate), and paper substrates were used to investigate the effects of the intrinsic and extrinsic substrate properties on the piezoelectricity of the energy harvesters. The thinnest paper substrate (66 μm) with a moderate Young's modulus showed the highest voltage output (0.4885 V). We used high-performance 76, 66, and 33 μm thick papers to determine the effect of paper thickness on the output voltage. The thinnest paper substrate resulted in the highest voltage output (0.7781 V), and the numerical analyses of the sound-driven mechanical deformation strongly support the hypothesis that substrate thickness has a considerable effect on piezoelectric performance.

An enthalpy method for modeling eutectic solidification

This paper presents a new micro-scale model for solidification of eutectic alloys. The model is based on the enthalpy method and simulates the growth of adjacent α and β phases from a melt of eutectic composition in a two-dimensional Eulerian framework. The evolution of the two phases is obtained from the solution of volume averaged energy and species transport equations which are formulated using the nodal enthalpy and concentration potential values. The three phases are tracked using the β-phase fraction and the liquid fraction values in all the computational nodes. Solutal convection flow field in the domain is obtained from the solution of volume-averaged momentum and continuity equations. The governing equations are solved using a coupled explicit–implicit scheme.The model is qualitatively validated with Jackson–Hunt theory. Results show expected eutectic growth pattern and proper species transfer and diffusion field ahead of the interface. Capabilities of the model such as lamella width selection, division of lamella into thinner lamellae and the presence of solutal convection are successfully demonstrated. The present model can potentially be incorporated into the existing framework of enthalpy based micro-scale dendritic solidification models thus leading to an efficient generalized microstructure evolution model.