416 International ournal of Applied Ceramic Technolog-Sebastian and Jantunen Vol.7,No.4,2010 possibilities of polymers. It is well known that the con- wavelength travelling though the medium is inversely nectivity between the phases in the composite materials proportional to the square root of the permittivity. In is very important in achieving the desired properties. many cases, this leads to products not possible in any The preparation of 0-3 type composite materials by other way, also with a low production cost. Although combining a dielectric or a ferroelectric ceramic and a many ceramic materials with a high Er and low loss are polymer for suitable properties means not only choosing available, they are generally brittle in nature. This leads to a difficulty in the fabrication of complex shapes or also coupling them with the best possible design struc- machining the ceramic substrates during circuit fabri- ture. This concept of connectivity, first reported by cation. These difficulties can be avoided by using 0-3 Newnham et al, describes the interspatial relationships polymer-ceramic composites as an alternative, which in a multiphase material controlling the mechanical and offer excellent material characteristics such as low-tem electrical pro and thermal fluxes between the perature processability, fexibility, machinability, chem- phases. Additionally, the filler particle size, interfacial ical resistance, tailored dielectric properties, etc. Such properties, percolation level, and porosity effects can composites include glass- or ceramic-reinforced epoxy, lso play a role in the composite properties. Interfacial PTFE, and various types of thermoset hydrocarbons effects can occur between the ic grains and the Among these, PTFE is the most preferred host matrix polymer matrix, leading to large dielectric relaxations that exhibits excellent dielectric properties such as low normally at low frequencies( called Maxwell-Wagner permittivity, extremely low loss tangent, and is stable relaxations). However, the response of the composite across a wide range of frequencies. The loss tangent of ay have a property that isplayed in the indi- PTFE at a high frequency maintains the signal integrity vidual phases. The feasibility of these composites for and minimizes radiation effects during data transfer use in piezoelectric and pyroelectric applications, flexible The properties of PTFE such as chemical inertness, low sensors,transducers, thick film dielectrics, embedded moisture absorption, high service temperatures,etc capacitors, and other multilayer RF devices has been are also important for many microwave applications cher gated6-8 The research was mainly focused on Although PTFE-based substrates are well accepted for mose thermoplastic poly and elastomers like microwave circuit applications, their wide usage is PVDF, P(VDF-TrFe), silicon-rubber, stricted because of the processing difficulties. Unlike polyimide, 6 pVC, 7cyanoet lated cellulose polymer, 8 other polymers, PTFE cannot be processed through in- polydimethylsiloxane, polystyrene, benzocycobu- jection molding or melt extrusion because of its very chough polymer-bas olefin copolymers, polyphenylene sulfide, poly- substrates are very common in use, the lite erature re- olefin elastomer(POe), polypropelene,and polyure- garding the preparation and characterization of these thane. The properties of the ceramic-polymer technologically and commercially important materials is composites are determined by the number of phases very limited. Most of the works in this line are focused and the way in which the phases are interconnected. on the developments of commercial products and are The response of a ceramic-polymer composite to an ex- protected by patent laws. Ceramic-filled polymers are ternal excitation(electric field, temperature, stress, etc. used in electronic packaging for device encapsulation depends on the response of individual phases, their Encapsulation of electronic devices protects them from interfaces as well as the connectivity concept. Polymer- an adverse environment and increases their long-term ezoelectric ceramic Fexible ferroelectric composites reliabilit have been extensively investigated.u for piezoelectric With the recent progress in nanoscience and tech- and pyroelectric transducer applications and hence will nology, there is an increasing interest in polyn not be discussed in the present review. composites both in scientific research and in engineering applications. For example, the epoxy-silica nanocom- posite has been used in electronic and optical packag Microwave Substrate ing. The epoxy nanofiller composite has good optical Materials with a high relative permittivity (er) Properties due to the smaller filler particle size. The electrical characteristics of microelectronic devices, such reduce the size of the microwave devices because the propagation velocity, and crosspossibilities of polymers. It is well known that the con￾nectivity between the phases in the composite materials is very important in achieving the desired properties.2,3 The preparation of 0–3 type composite materials by combining a dielectric or a ferroelectric ceramic and a polymer for suitable properties means not only choosing the right materials, processed in a particular way, but also coupling them with the best possible design struc￾ture. This concept of connectivity, first reported by Newnham et al.,2 describes the interspatial relationships in a multiphase material controlling the mechanical and electrical properties and thermal fluxes between the phases. Additionally, the filler particle size, interfacial properties, percolation level, and porosity effects can also play a role in the composite properties.4 Interfacial effects can occur between the ceramic grains and the polymer matrix, leading to large dielectric relaxations normally at low frequencies (called Maxwell–Wagner relaxations). However, the response of the composite may have a property that is not displayed in the indi￾vidual phases.5 The feasibility of these composites for use in piezoelectric and pyroelectric applications, flexible sensors, transducers, thick film dielectrics, embedded capacitors, and other multilayer RF devices has been investigated.6–8 The research was mainly focused on thermoset, thermoplastic polymers and elastomers like epoxy,9–12 PVDF,13 P(VDF-TrFe),14 silicon-rubber,15 polyimide,16 PVC,17 cyanoethylated cellulose polymer,18 polydimethylsiloxane,19 polystyrene,20 benzocyclobu￾tene,21 polymethyl methocrylate,22 metallocene cyclic olefin copolymers,23,24 polyphenylene sulfide,25,26 poly￾olefin elastomer (POE),27 polypropelene,28 and polyure￾thane.29 The properties of the ceramic–polymer composites are determined by the number of phases and the way in which the phases are interconnected. The response of a ceramic–polymer composite to an ex￾ternal excitation (electric field, temperature, stress, etc.) depends on the response of individual phases, their interfaces as well as the connectivity concept. Polymer– piezoelectric ceramic flexible ferroelectric composites have been extensively investigated6,30 for piezoelectric and pyroelectric transducer applications and hence will not be discussed in the present review. Microwave Substrates Materials with a high relative permittivity (er) reduce the size of the microwave devices because the wavelength travelling though the medium is inversely proportional to the square root of the permittivity. In many cases, this leads to products not possible in any other way, also with a low production cost. Although many ceramic materials with a high er and low loss are available, they are generally brittle in nature. This leads to a difficulty in the fabrication of complex shapes or machining the ceramic substrates during circuit fabri￾cation. These difficulties can be avoided by using 0–3 polymer–ceramic composites as an alternative, which offer excellent material characteristics such as low-tem￾perature processability, flexibility, machinability, chem￾ical resistance, tailored dielectric properties, etc.31 Such composites include glass- or ceramic-reinforced epoxy, PTFE, and various types of thermoset hydrocarbons. Among these, PTFE is the most preferred host matrix that exhibits excellent dielectric properties such as low permittivity, extremely low loss tangent, and is stable across a wide range of frequencies. The loss tangent of PTFE at a high frequency maintains the signal integrity and minimizes radiation effects during data transfer. The properties of PTFE such as chemical inertness, low moisture absorption, high service temperatures, etc. are also important for many microwave applications. Although PTFE-based substrates are well accepted for microwave circuit applications, their wide usage is re￾stricted because of the processing difficulties. Unlike other polymers, PTFE cannot be processed through in￾jection molding or melt extrusion because of its very high melt viscosity. Although polymer-based composite substrates are very common in use, the literature re￾garding the preparation and characterization of these technologically and commercially important materials is very limited. Most of the works in this line are focused on the developments of commercial products and are protected by patent laws. Ceramic-filled polymers are used in electronic packaging for device encapsulation. Encapsulation of electronic devices protects them from an adverse environment and increases their long-term reliability. With the recent progress in nanoscience and tech￾nology, there is an increasing interest in polymer nano￾composites both in scientific research and in engineering applications. For example, the epoxy–silica nanocom￾posite has been used in electronic and optical packag￾ing.32 The epoxy nanofiller composite has good optical properties due to the smaller filler particle size. The electrical characteristics of microelectronic devices, such as signal attenuation, propagation velocity, and cross 416 International Journal of Applied Ceramic Technology—Sebastian and Jantunen Vol. 7, No. 4, 2010