Ceramic powder filled polytetrafluoroethylene (PTFE) has been widely studied in fabrication of microwave devices because of its very low loss tangent and excellent chemical resistance. PTFE is a kind of high performance high performance thermoplastic polymer with excellent electrical properties. It has a low dielectric constant (εr = 2.1) and an extremely low loss tangent (tanδ = 0.0003) which are stable over a wide range of frequencies . However, the disadvantages of PTFE are its high linear coefficient of thermal expansion (CTE = 109 ppm∙˚C−1) and low mechanical strength. One of the methods to control the CTE and mechanical strength of PTFE was to add inorganic ceramic fillers into the PTFE matrix    . In previous work, several researches have investigated the effects of ceramic filler materials, such as TiO2 , MgTiO3 , CaTiO3 , etc.
It is well known that both the microwave dielectric properties and the thermal expansion of substrate composites are very important factors for the fabrication of microwave circuit. The object of this work is to study the effect of CaTiO3 loading on the properties of TiO2 filled PTFE composites, such as density, moisture absorption, thermal expansion, microstructure and microwave dielectric properties. CaTiO3 and TiO2 powders were prepared by conventional solid-state reaction technique. The total content of inorganic ceramic fillers was a fixed value of 61 wt%, and the content of CaTiO3 in PTFE/TiO2 matrix varied from 0 wt% to 16 wt% with a step size of 4 wt%. The effects of CaTiO3 loading on the properties of PTFE/TiO2 composites have been investigated systematically. It is expected that this work will be useful in the applications of TiO2/PTFE substrate composites in practice.
2. Experimental Procedures
The raw materials used were rutile TiO2 powder, CaTiO3 and PTFE aqueous dispersion (TE-3865C, Dupont, USA). Phenyltrimethoxysilane (PTMS, TCI Corporation, Japan) was used as silane coupling agent. The average size of rutile TiO2 and CaTiO3 powders were 5 μm and 2 μm, respectively. The values of D50 were measured by Laser particle size analyzer. Table 1 gives the properties of raw materials.
2.2. Fabrication of the Composite
Phenyltrimethoxysilane (PTMS) was used to coat the surface of filler particles as coupling agent. First, PTMS was hydrolyzed in alcohol and deionized water for 1 h at 55˚C. The amount of PTMS was 1.5% of the weight of ceramic filler and the amount of water was controlled exactly for the hydrolysis of PTMS. Then CaTiO3 and TiO2 were 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.
Table 1. The density, dielectric properties and thermal expansion coefficient of raw materials.
After drying, the ceramic powder previously obtained was weight accurately to prepare the composites according to the weight radio of x CaTiO3 (61-x) TiO2/39PTFE (x = 0, 4, 8, 12, 16). The coupling agent treated ceramic powder was added into the aqueous PTFE matrix and mixed by heavy stirring for 3 h to obtain good slurry. The slurry was then dried at 120˚C for 24 h to remove water. The dried dough was smashed by a high speed milling. The obtained composite powder was pressed into square slices with length of30 mm, width of 20 mm and height of 1 mm by cold pressing under a pressure of 20 MPa. 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.
2.3. Characterization Studies
Archimedes’ principle was used to study the density of the PTFE reinforced with TiO2 and CaTiO3 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 . Netzsch DIL402PC was used to measure the coefficient of thermal expansion (CTE) of the substrate composites.
The dielectric constant (εr) and dielectric loss (tanδ) of CaTiO3/TiO2-filled PTFE composites were measured by stripline resonator method using Agilent E8363A microwave network analyzer according to IPC-TM-650 18.104.22.168 specification. The testing sizes of samples were 30 mm long, 20 mm wide and 1 mm thick. The testing frequencies varied from 7.0 GHz to 13.0 GHz. The εr and tanδ of the CaTiO3/TiO2-filled PTFE composites reported in this paper was at a frequency about 10 GHz.
3. Results and Discussion
3.1. Density Tests
Variation of the theoretical density and bulk density of TiO2/CaTiO3 co-filled composites with CaTiO3 loading is shown in Figure 1. A comparison of experimental results with theoretical values is also presented in Figure 1. It is difficult for PTFE to have strong adhesion with ceramic powder as PTFE has a low surface energy. Experimental results are always lesser than theoretical values because of the generating of pores at the interface region of matrix, which would decrease the density of the composite. With increasing CaTiO3 addition, the relative density of the substrate displayed a continuous decrease. Obviously, with the CaTiO3 loading amount increased to 16 wt%, the density reached the minimum value of 2.99 g/cm3. In our previous study , it is true that interface volume fraction increases with decreasing ceramic particle size, which promotes porosity factor of the substrate. This is because the ceramic powders trend to agglomerate with the decrease of particle size, and the bad dispersion in the PTFE matrix which produces more pores in the matrix and decreases the density.
Figure 1. Variation of the theoretical density and bulk density of TiO2/CaTiO3 co-filled composites with CaTiO3 loading.
Figure 2. Variation of water absorption with respect to CaTiO3 loading.
3.2. Water Absorption
It is well known that water absorption is an important parameter to control for the practical use of substrate composites. Previous studies showed that the moisture absorption of substrate composite was strongly influenced by the hydrophilic nature of ceramic filler and porosity factor of substrate. Figure 2 shows the variation of water absorption of CaTiO3/TiO2-filled PTFE composites with respect to CaTiO3 filler loading. Since modified CaTiO3, TiO2 powder and PTFE are hydrophilic, the moisture of the composite was mainly determined by the porosity factor of the matrix. As shown in Figure 2, the water absorption displayed a continuous increase with the addition of CaTiO3 loading amount. This finding probably was related to more pores generating at the interface region, leading to the deterioration of moisture absorption.
3.3. Thermal Expansion Property
In addition to dielectric properties, the coefficient of thermal expansion of the substrate is also very important for the application of the composite. The composite must have a low value of CTE in order to match with the CTE of copper conductor layer. Variation of CTE of CaTiO3/TiO2 co-filled composites with respect to CaTiO3 loading amount is shown in Figure 3. As CaTiO3 loading amount increased from 0 wt% to 16 wt% with step size of 4 wt%, the thermal expansion initially displayed a sharp increase , later a slight climb when the content of CaTiO3 was 12 wt%.
3.4. Morphology Aspects
Morphology and the dispersion of ceramic filler in PTFE matrix were observed using SEM. The cross sectional SEM images of pure TiO2 filled PTFE sample and 16 wt%CaTiO3/TiO2 co-filled sample are shown in Figure 4. It can be seen that all samples exhibit two-phase structure including PTFE and ceramic powder. It is difficult for ceramic powders to have a good adhesion with PTFE because of the low surface energy of PTFE has a low surface energy. Large amounts of pores can be observed in the images. Obviously, as the addition of CaTiO3 loading amount the agglomeration of ceramic filler appeared which deteriorates the porosity factor of samples.
The typical planar SEM micrographs of pure TiO2 filled PTFE sample and 16 wt%CaTiO3/TiO2 co-filled sample are shown in Figure 5. As shown in Figure 5(a), PTFE has a better adhesion with pure TiO2 powder. However, Figure 5(b) shows a terrible bond between ceramic powder and PTFE.
3.5. Dielectric Properties
The dielectric properties of the substrate composites depend on not only the dielectric properties of the components but also other factors such as the interactions between ceramics and polymers, the size of fillers, and the dispersion of
Figure 3. Coefficient of thermal expansion of TiO2/CaTiO3 co-filled composites.
Figure 4. Cross section SEM images of TiO2/CaTiO3 co-filled composites (a) pureTiO2 filled PTFE (b) content of CaTiO3 was 16 wt%.
Figure 5. Surface SEM images of TiO2/CaTiO3 co-filled composites (a) pureTiO2 filled PTFE (b) content of CaTiO3 was 16 wt%.
fillers in polymer matrix   . Figure 6 shows the dielectric constant and dielectric loss of the composites as a function of CaTiO3 loading percent. The composite has low loss tangent values (<0.002). As the addition of CaTiO3 loading amount increased, the loss tangent increases sharply at first, and then go up to the maximum values when the loading content of CaTiO3 exceeds 12 wt%. Since water has high loss tangent (tanδ = 35), the dielectric loss of the substrate composites would deteriorate while moisture absorption increases . With increasing CaTiO3 content, the value of dielectric has a tendency of persistent increase. The main reason for this phenomenon is that the dielectric constants of composites are strongly dependent on the relative permittivity of the ceramic components.
Rutile TiO2 and CaTiO3 powder were modified and filled in PTFE matrix in fixed ratio of xCaTiO3(61 − x)TiO2/39PTFE (x = 0, 4, 8, 12, 16) through cold pressing and hot treating process to fabricate TiO2/CaTiO3 co-filled PTFE composites for microwave substrate applications. The effects of CaTiO3 loading amount on the density, moisture absorption, thermal expansion, microstructure and microwave dielectric properties were studied. The density experienced a continuous decrease with the addition of CaTiO3. The moisture absorption displayed a steady increase with the increase of CaTiO3 loading amount. As CaTiO3
Figure 6. Variation of dielectric constant and loss tangent at 10 GHz of TiO2/CaTiO3 co-filled composites with respect to the content of CaTiO3.
loading amount increased from 0 wt% to 16 wt%, the thermal expansion displayed a sharp increase at first and then a slight climb when the content of CaTiO3 was 12 wt%. The changing of dielectric constant (εr) and loss tangent (tanδ) were similar to that of the moisture absorption in a manner. Good dielectric properties with values of εr = 11.60, tanδ = 0.002 were obtained in the PTFE matrix with 16 wt% CaTiO3 and 45 wt% TiO2. At last, TiO2/CaTiO3 co-filled PTFE composites are promising candidates for microwave circuit applications.