TiO2 filled polytetrafluoroethylene (PTFE) composite were fabricated for microwave circuit applications. PTFE/TiO2 composites were prepared by cold pressing and hot treating. The particle size of TiO2 varied from 5 μm to 11 μm.TiO2 powders with different sizes were prepared by the solid state ceramic route. The effects of TiO2 particle size on the microstructure, density, moisture absorption, thermal conductivity and microwave dielectric properties of PTFE/TiO2 composites were investigated. The density showed an increasing trend as the TiO2 particle size increased, while the dielectric loss (tanδ) and moisture absorption decreased with the increase of TiO2 particle size. The dielectric constant (εr) and thermal conductivity (λ) decreased up to D50 = 6.5 μm and then sharply increased. Good properties with values of εr = 6.8, tanδ = 0.0012 and λ = 0.533 W?m?1?K?1 were obtained in PTFE/TiO2 composites when the particle size of TiO2 was 11 μm.
Ceramic powder filled polytetrafluoroethylene (PTFE) has been widely used in manufacturing microwave devices because of its very low dielectric constant and excellent chemical resistance. PTFE is a kind of high performance thermoplastic polymer with unique electrical properties. It has a low dielectric constant (εr = 2.1) and an extremely low dielectric loss (tanδ = 0.0003) which are stable over a wide range of frequencies [
The object of this work is to study the effect of TiO2 particle size on the properties of TiO2-filled PTFE composites material, such as microstructure, density, moisture absorption, thermal conductivity and microwave dielectric properties. TiO2 powders were prepared by the solid state ceramic route. The different size of TiO2 powders were obtained by controlling the time of ball milling. It is expected that this work may be useful in the applications of TiO2 filled PTFE composites in practice.
The raw materials used were rutile TiO2 powder and PTFE aqueous dispersion (TE-3865C, Dupont, USA). Phenyltrimethoxysilane (PTMS, TCI Corporation, Japan) was used as coupling agent. The different size of rutile TiO2 powder was ball milled for different hours and sintered at 1350˚C for 2 hours. The values of D50 (5 μm, 6.5 μm, 8 μm, 9.6 μm, 11 μm) were measured by Laser particle size analyzer.
PTMS was used as coupling agent to coat the surface of filler particles. First, PTMS was hydrolyzed in alcohol and ion-free water at 55˚C for 1 h. The amount of PTMS was 1.5% of the weight of TiO2 filler and the amount of water was controlled exactly for the hydrolysis of PTMS. Then TiO2 was added and dispersed into the hydrolyzed coupling agent solution by heavy stirring for 2 h. The product was further dried in an oven at 120˚C for 3 h and silane coupling agent treated ceramic powder was obtained.
The TiO2 powder previously obtained was weight accurately to prepare the composites according to the weight radio TiO2/PTFE. The coupling agent treat- ed TiO2 powder was added into the aqueous PTFE dispersion and mixed by heavy stirring for 3 h to obtain a fine mixture. The mixture was then dried at 120˚C for 24 h to remove water. The dried dough was smashed by a high speed
Properties | PTFE | TiO2 |
---|---|---|
Density/(g/cm3) | 2.2 | 4.0 |
Dielectric constant | 2.1 | 90 |
Dielectric loss | 0.0003 | 0.008 |
Thermal expansion coefficient/(10-6K-1) | 109 | 9 |
milling. The obtained composite powder was pressed into square slices (30 mm * 20 mm * 1 mm) by cold pressing. The slices were hot pressing sintered at 360˚C and 10 Mpa. Hot treatment was performed in a program controlled oven. PTFE was melted and coalesced during hot treating, and then recrystallized while cooling.
Archimedes’ principle was used to study the density of the PTFE/TiO2 composites. The microstructure of the substrate was observed by scanning electron microscopy (SEM, model JEOL JSM-6490). Water absorption of the composite sample was measured as reported earlier by Murali etc [
The dielectric constant (εr) and dielectric loss (tanδ) of TiO2-filled PTFE composites were measured by stripline resonator method using Agilent E8363A microwave network analyzer according to IPC-TM-650 2.5.5.5 specification [
The dielectric property, tensile strength and thermal expansion of the substrate composites were significantly influenced by the density property. The variation of density of substrate composites with respect to TiO2 particle size is shown in
decrease of TiO2 particle size, and the bad dispersion in the PTFE matrix which produces more pores in the matrix. However, as the D50 decreases 11 μm to 5 μm, it is more difficult for PTFE to have a good adhesion with TiO2 fillers so that many pores appeared which results in the decline of density [
Porosity factor of the composite samples was determined by the volume fraction of inner pore using the equation:
where ρ is theoretical density and ρ0 is the experimental density.
Density of the composite samples had been calculated using the rule of mixtures:
where ρi, vi and mi are the density, volume fraction and weight of the filler and matrix, respectively.
Previous studies showed that the electric property of substrate composite was influenced by moisture absorption, significantly. Since water has high loss tangent, the electrical property would deteriorate while water absorption increases. As the weight fraction of TiO2 and PTFE were fixed, the water absorption was general determined by porosity factor. For smaller ceramic particle size, agglomeration of ceramic filler leads to increasing fraction of porosity and water absorption. The variation of porosity factor and water absorption of the composites as a function of TiO2 particle size was shown in
The cross sectional SEM images of TiO2/PTFE composites are shown in Figures 3(a)-(e). The filler particle size changed from 5 μm to 11 μm in successive. It can be seen that all samples exhibit two-phase structure viz. PTFE and ceramic powder. Since PTFE has a low surface energy, it is difficult for ceramic powders to have a good adhesion with PTFE. Many pores can be observed in all images. As particle size increases, a denser microstructure with lesser pores is observed. Sample showed well-dense and homogeneous microstructure when D50 = 11 μm. The typical planar SEM micrograph of D50 = 11 μm is shown in
The variation of thermal conductivity of the composites as a function of D50 is shown in
creasing fraction of porosity and high humidity which would promote the value of λ when D50 = 5 μm.
The dielectric properties of the substrate composites depend on not only the dielectric properties of the components but also other factors such as the size of fillers, the interactions between ceramics and polymers, and the dispersion of fillers in polymer matrix [
Rutile TiO2 powder was modified and filled in PTFE matrix in 50wt% through cold pressing and hot treating process, and to fabricate TiO2/PTFE composites for microwave substrate applications. Microstructure, density, moisture absorption, thermal conductivity and microwave dielectric properties of PTFE/TiO2 composites were studied from D50 = 5 μm to 11 μm filler average particle size. The moisture absorption and porosity factor decrease, whereas the density increases as the D50 of ceramic particle size increase from 5 μm to 11 μm. The loss tangent shows a decreasing trend with the D50 increasing. The dielectric constant sharply drops up to 5.6 when D50 = 6.5 μm and then slightly increases. The changing of thermal conductivity was same to that of dielectric constant in a manner. The composite has a dielectric constant of 6.8, a loss tangent of 0.0012, moisture absorption of 0.04%, density of 2.797 g/cm3, and thermal conductivity of 0.233W(m˚C)−1. Present study shows that TiO2 filled PTFE composites are promising candidates for microwave circuit applications.
Luo, F.C., Yuan, Y., Tang, B., Yang, J. and Zhang, S.R. (2017) The Effects of TiO2 Particle Size on the Properties of PTFE/TiO2 Composites. Journal of Materials Science and Chemical Engineering, 5, 53-60. https://doi.org/10.4236/msce.2017.57007