Note to Reader

Portions of this chapter, including figures have been submitted to peer-review in the

Journal of American Ceramic Society [76].

3.2 Introduction

One of the key goals of this chapter is to evaluate the impact of a high-temperature sintering

process as an effective enhancing strategy on the resultant dielectric and loss properties (i.e.,

relative permittivity εr and dielectric loss tangent tan δd) of EM composite materials composed of

Polydimethylsiloxane (PDMS) loaded with sintered MgCaTiO2 and TiO2 ceramic fillers. Five

types of flexible high-permittivity and low-loss polymer-ceramic EM composite materials based

on PDMS elastomer host-matrix reinforced by MgCaTiO2 and TiO2 microfillers are rigorously

investigated and presented. The ceramic fillers are analyzed with x-ray diffraction (XRD), energy

dispersive spectroscopy (EDS) and scanning electron microscope (SEM) for morphology and

crystallinity before and after a high-temperature sintering process steps. All the EM composites

were loaded with ceramic fillers sintered at temperatures up to 1500C with concentrations as high

as 49% by volume. The TiO2 and MgCaTiO2 based composites were characterized up to 17 GHz

by the cavity resonator method and at 5 GHz using a custom-built near-field microwave

microscopy (NFMM) system. For frequencies up to 17 GHz, the 36 vol. % PDMS-MgCaTiO2

composites with 1100C sintered fillers have exhibited a stable r of 10.27 and tan δd lower than 0.021, which correspond to an enhancement of 20% in relative permittivity and 29% in dielectric

26

loss tangent when compared to the specimen loaded with unsintered raw ceramic powders.

Furthermore the 36 vol. % PDMS-MgCaTiO2 composites with 1500C sintered fillers have

exhibited a stable r of 9.93 and loss tangent (tan δd) lower than 0.021 for frequencies up to 17 GHz, which correspond to an enhancement of 16.41% in relative permittivity and 28% in dielectric

loss tangent. Evidently, there is an optimum sintering temperature for improving the effective

dielectric and loss properties of MgCaTiO2 microfillers. The 49 vol. % PDMS-MgCaTiO2

composites with 1100C sintered fillers have exhibited a r of 16.33 and loss tangent (tan δd) lower than 0.021 at 19 GHz. Similarly, 38 vol. % PDMS-TiO2 sample with particles sintered at 1100C

has exhibited a dielectric permittivity of r of 9.73 and slightly a higher loss tangent (tan δd lower than 0.031) at frequencies up to 17 GHz, which suggests an enhancement of 9.8% in relative

permittivity but a 27% increase of the dielectric loss tangent as compared to those of specimen

loaded with unsintered raw TiO2 powders. This indicates that an optimal sintering

condition/temperature exists.

Meanwhile, 38 vol. % PDMS-TiO2 composites with fillers sintered at 1500C have shown

a stable r of 8.3 and loss tangent (tan δd) lower than 0.025 at frequencies up to 17 GHz, which suggests a slight decrease of 7% in relative permittivity and a 3% increase of the dielectric loss

tangent. The uniform dispersion of sintered ceramic high-k microfillers has effectively increased

the effective permittivity for all the samples compared to that of pure PDMS host matrix.

Meanwhile, the inclusion of the ceramic powders after the sintering process, in particular, has

significantly decreased the dielectric losses of the MgCaTiO2 based composite samples up to 30%.

The near-field microwave microscopy (NFMM) analysis revealed that the MgCaTiO2 ceramic

particles are more uniformly distributed than the TiO2 particles over an area of 50 m  50 m,

27 3.3 Experimental Procedure

In this chapter three sintering temperatures up to the ceramics’ melting points reported in

Table 3.1 are evaluated to find the best sintering conditions to enhance the effective dielectric and

loss properties at microwave frequencies in composite materials based on PDMS. For this purpose,

polymer-ceramic composite samples were prepared with MgCaTiO2 or TiO2 fillers under “as-is”

unsintered condition or sintered at 1100C and 1500C for 3 hours. The high-temperature sintering

process is a critical step for enhancing the dielectric and loss properties of the ceramic powders.

The assessment of dielectric properties was carried out via cavity resonators based on the cavity

perturbation theory[57]-[59]. A 0.5 mm-thick molded thin-sheet specimen composed of different

volume concentrations of MgCaTiO2 or TiO2 ceramic fillers (sintered at 1100C or 1500ºC for

three hours) loaded PDMS composites were prepared and characterized. The 36 vol. % PDMS-

MgCaTiO2 composite molded specimen has exhibited a measured r of 10.27 and loss tangent tan δd lower than 0.021 between 0.4 GHz and 17 GHz. Furthermore, a PDMS-MgCaTiO2 composite sample loaded with 49 vol. % MgCaTiO2 has exhibited a measured r of 16.33 and a loss tangent tan δd lower than 0.021 between 0.4 GHz and 20 GHz. Similarly 38 vol. % PDMS-TiO2 sample

with particles sintered at 1100C has exhibited a dielectric permittivity of r of 9.73 and a loss tangent (tan δd lower than 0.031) at frequencies up to 17 GHz. Meanwhile, a 38 vol. % PDMS-

TiO2 composite sample with TiO2 microfillers sintered at 1500C has shown r of 8.3 and tan δd lower than 0.025 between 0.4 GHz and 17 GHz. Moreover, the flexibility of the high-k composite

substrates seems to be well suited for future RF/microwave device prototypes that conform to

uneven or curved surfaces for applications up to the Ku-band and K-band frequencies.

In this study, the best EM characteristics were achieved for composite specimens filled

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20% and 10% increases of the relative permittivities, respectively, and a 29% reduction of the

dielectric losses for MgCaTiO2 loaded samples. However, a 27% increase in the dielectric losses

was observed for the TiO2 loaded composites (as seen in Table 3.6). In my prior study of PDMS-

MgCaTiO2 using a different batch of MgCaTiO2 ceramic powders, we have demonstrated a 51%

increases of the relative permittivity and 38% decrease in the relative dielectric loss by employing

a three hours sintering process at 1100ºC in the air [49], [50].

3.4 Fabrication Process of Polymer Matrix Composites