Permittivity measurement of thermoplastic composites at elevated temperature

[Abstract]: The material properties of greatest importance in microwave processing of a dielectric are the complex relative permittivity Epsilon = Epsilon' - jEpsilon'', and the loss tangent, tan Delta = Epsilon'/Epsilon''. This paper describes two convenient laboratory based methods to obtain Epsilon', Epsilon'' and hence tan Delta of fibre-reinforced thermoplastic (FRTP) composites. One method employs a microwave network analyser in conjunction with a waveguide transmission technique, chosen because it provides the widest possible frequency range with high accuracy. The values of the dielectric constant and dielectric loss of glass fibre reinforced (33%) low density polyethylene, LDPE/GF (33%), polystyrene, PS/GF (33%), and Nylon 66/GF (33%), were obtained. Results are compared with those obtained by another method using a high-temperature dielectric probe

Joining of thirty three percent by weight random glass fibre reinforced polystyrene using variable frequency microwave

[Abstract]: This paper extends the range of applications for Variable Frequency Microwave (VFM) (2 – 18 GHz) facilities to joining thirty three percent by weight glass fibre reinforced polystyrene composite [PS/GF (33%)]. With a given power level, the composite was exposed to various exposure times to microwave irradiation. The primer or coupling agent used was 5-minute two-part adhesive containing 100% liquid epoxy and 8% amine, i.e. Araldite, which was more readily microwave reactive than the composite itself. Bond strengths of the joints were lap shear tested and results were compared with those obtained using fixed frequency (2.45 GHz) microwave processing. The VFMF was operated under software control, which provided automatic data logging facilities. The maximum lap shear bond strength of joint was 430 N/cm2 using variable frequency microwave facility while that obtained by fixed frequency microwave configuration was only 331 N/cm2. The former is nearly 30% stronger than the latter

Enhancement of β-phase in PVDF films embedded with ferromagnetic Gd₅Si₄ nanoparticles for piezoelectric energy harvesting

Self-polarized Gd5Si4-polyvinylidene fluoride (PVDF) nanocomposite films have been synthesized via a facile phase-inversion technique. For the 5 wt% Gd5Si4-PVDF films, the enhancement of the piezoelectric β-phase and crystallinity are confirmed using Fourier transform infrared (FTIR) spectroscopy (phase fraction, Fβ, of 81% as compared to 49% for pristine PVDF) and differential scanning calorimetry (crystallinity, ΔXc, of 58% as compared to 46% for pristine PVDF), respectively. The Gd5Si4 magnetic nanoparticles, prepared using high-energy ball milling were characterized using Dynamic Light Scattering and Vibrating Sample Magnetometry (VSM) to reveal a particle size of ∼470 nm with a high magnetization of 11 emu/g. The VSM analysis of free-standing Gd5Si4-PVDF films revealed that while the pristine PVDF membrane shows weak diamagnetic behavior, the Gd5Si4-PVDF films loaded at 2.5 wt% and 5 wt% Gd5Si4 show enhanced ferromagnetic behavior with paramagnetic contribution from Gd5Si3 phase. The interfacial interactions between Gd5Si4 and PVDF results in the preferential crystallization of the β-phase as confirmed via the shift in the CH2 asymmetric and symmetric stretching vibrations in the FTIR. These results confirm the magnetic Gd5Si4 nanoparticles embedded in the PVDF membrane lead to an increased β-phase fraction, which paves the way for future efficient energy harvesting applications using a combination of magnetic and piezoelectric effects

Application of variable frequency microwave (VFM) to adhesives

[Abstract]: Microwave processing of adhesives is a relatively new technology alternative that provides new approaches for enhancing material properties as well as economic advantages through energy savings and accelerated product development. Alternative in the sense that most adhesives are normally cured in ambient conditions or in ovens. However, the most commonly used facilities for microwave processing of materials operate on fixed frequency microwaves (FFM), e.g., 2.45 GHz. This paper presents a review of microwave technologies, processing methods and industrial applications, using variable frequency microwave (VFM) facilities. The technique offers rapid, uniform and selective heating over a large volume at a high energy coupling efficiency. This is accomplished using a preselected bandwidth sweeping around a central frequency by employing tunable frequency sources. Successful applications of these modern facilities include finding out the optimum cavity conditions of glass or carbon fibre reinforced thermoplastic matrix composites, and of adhesives, e.g., two-part five-minute Araldite, and the joining of the above-mentioned composite materials with, or without, primers. Finding out the optimum cavity conditions of a material has helped identify the best frequency range to process the material using microwave energy and by means of the VFM facility. Microwave energy has been used to rapidly cure several types of two-part epoxy based adhesives, e.g., Araldite. Bond strengths obtained using variable frequency microwave (VFM)techniques are compared with adhesive joints cured in fixed frequency microwave (FFM) conditions

Review - microwave processing of materials: part III

[Abstract]: Part I and part II of the paper in the same series, Review Microwave Processing of Materials: Part I and Review Microwave Processing of Materials: Part II, described the fixed frequency microwave processing of materials and variable frequency processing of materials respectively. This paper, Review Microwave Processing of Materials: Part III, details the simulation of fixed and variable frequency heating and processing of materials. Numerical techniques applied by different researchers include finite-element time-domain (FE-TD), finite-difference time-domain (FD-TD) and finite-volume time-domain (FV-TD). Two successful examples of fixed frequency heating simulation packages developed at two Australia universities are described. One American, one British and one Germany simulation codes are cited. Their advantages and limitations are also discussed. The cost and benefits of using fixed and variable frequency facilities are also described in detail. The cost of a complete system of fixed frequency facility of 2.45 GHz (NRC, 1994; Sheppard, 1988) is approximately 1600-8000 Australian dollars per kW. While the cost of a complete system for a variable frequency microwave (VFM) oven of 750 W is as high as 330,000 Australian dollars

Microwave energy effects on polymer-matrix and fibre reinforcement of composites during bonding process by microwaves

[Abstract]: One important thing to be studied in the bonding of thermoplastic composite material using microwave irradiation is the effects of microwave energy on the matrix and fibre reinforcement of the composite. In this research, thirty three percent by weight random glass fibre reinforced polystyrene [PS/GF (33%)] is chosen for the study. Microscopy study is used to find out the microstructural characteristics along and around the interface of the welds. The separation distance of the reinforcing carbon filaments was varied and heat transfer in the material during joining was studied. The reasons why the thermoplastic matrix composite materials were weakened by prolonged microwave irradiation were also studied and analysed

Lap shear strength comparison between two types of random carbon fibre reinforced thermoplastic matrix composites bonded using variable frequency microwaves (VFM) irradiation

[Abstract]: Fibre reinforced thermoplastic matrix composite materials have very strong development in the last 10-12 years because of their potential advantages and unique characteristics that cannot be found in their thermosett counterparts. This paper compares the lap shear bond strengths of 2 types of random carbon fibre reinforced thermoplastic matrix composites joined by microwave energy. Variable frequency microwave (VFM) (2 – 18 GHz) facilities are used to join thirty three percent by weight random carbon fibre reinforced low density polyethylene [LDPE/CF (33%)] and thirty three percent by weight random carbon fibre reinforced polystyrene [PS/GF (33%)]. With a given power level, the composites were exposed to various exposure times to microwave irradiation. The lap shear strengths of the joints were compared with those obtained using fixed frequency (2.45 GHz) microwave facility configuration. The VFMF was operated under software control, which provided automatic data logging facilitie