The percolation threshold in a composite depends on the processing conditions used to fabricate it along with the size and shape of the filler and the matrix. In this study, borosilicate glass microspheres were used as the matrix material, and nanosized antimony tin oxide (ATO) particles were used as the filler. The glass microsphere/ATO composites were fabricated by hot pressing at temperatures in the range between the glass transition temperature and the softening temperature of the glass to control the viscosity. The pressure and temperature applied allowed the ATO to be confined to the spaces between certain glass particles, forming percolating networks at low volume fractions of the ATO. The viscous flow of the glass allowed for the composite to have near full densities, while allowing for the nanoparticle segregation to occur. Even though apparently similar microstructures were made using different heating schedules, the percolation behavior and electrical conductivity showed noticeable differences. The percolation threshold ranged from 0.1 to 2.5 phr (parts per hundred glass) and the change in electrical conductivity was around seven to nine orders of magnitude. The differences were attributed to the interaction of the segregated ATO particles with one another. The electrical properties were examined using ac impedance spectroscopy along with current atomic force microscopy (C-AFM), which allowed for valuable insights in the structure-property-processing relationships in these materials.

Preceramic polymers (PCPs) are a group of specialty macromolecules that serve as precursors for generating inorganics, including ceramic carbides, nitrides, and borides. PCPs represent interesting synthetic challenges for chemists due to the elements incorporated into their structure. This group of polymers is also of interest to engineers as PCPs enable the processing of polymer-derived ceramic products including high-performance ceramic fibers and composites. These finished ceramic materials are of growing significance for applications that experience extreme operating environments (e.g., aerospace propulsion and high-speed atmospheric flight). This Review provides an overview of advances in the synthesis and postpolymerization modification of macromolecules forming nonoxide ceramics. These PCPs include polycarbosilanes, polysilanes, polysilazanes, and precursors for ultrahigh-temperature ceramics. Following our review of PCP synthetic chemistry, we provide examples of the application and processing of these polymers, including their use in fiber spinning, composite fabrication, and additive manufacturing. The principal objective of this Review is to provide a resource that bridges the disciplines of synthetic chemistry and ceramic engineering while providing both insights and inspiration for future collaborative work that will ultimately drive the PCP field forward.

Glass nanocomposites, fabricated using borosilicate glass microspheres and antimony tin oxide (ATO) nanoparticles, were previously reported to have formed segregated networks at the boundaries of the glass particles. This resulted in an electrically conducting composite at low volume fractions (~0.5-0.8 vol%) of ATO nanoparticles. The wide range of electrical response in these borosilicate glass composites containing networks of varying concentration of ATO was examined using impedance spectroscopy. The electrical resistance of these composites varied over a range of around 12 orders of magnitude and exhibited several different types of insulator and conductor behavior. The formation of the ATO network was identified and tracked by scanning electron microscopy images and energy dispersive X-ray spectroscopy (EDS) scans. Detailed impedance spectroscopy analysis using all of the dielectric functions (impedance, permittivity, electric modulus, and admittance) was found to be an excellent method for detecting the development of the network and the effect that processing variables can have on its formation and the overall electrical properties of the nanocomposites. F. Zok-contributing editor Manuscript No. 34068.

Phthalonitrile polymers have potential for high-temperature applications in polymer matrix composites as electronic encapsulation compounds. To investigate the effect of inclusion of an organosilicon moiety, a tetraphenylsilane-containing phthalonitrile monomer was synthesized in high yields. The monomer possessed a high melting point of 222–223°C, while no hydrolytic sensitivity was observed. Cured polymers exhibited glass transitions in the range of 290–325°C and coefficients of thermal expansion of 73–77 µm/(m °C). In thermogravimetric analysis (TGA), 5 wt% loss was observed at 482–497°C and 519–526°C, under air and nitrogen, respectively. Infrared (IR)-TGA of evolved gases revealed multiple degradations in both nitrogen and air. The material possessed good thermo-oxidative stability (TOS) when aged in air at 250°C. After aging for 5000 h, oxidative degradation was characterized using Fourier transform IR microscopy, energy dispersive spectroscopy, optical microscopy, and Knoop hardness testing. Four zones were identified in aged samples. The cleavage of Si-phenyl bonds and the formation of Si–O phases and carbonyl groups were observed.

Glass composites containing percolated segregated networks of conducting antimony tin oxide (ATO) nanoparticles were fabricated through the use of a hot-pressing technique, which resulted in glass microspheres deforming into faceted polyhedra with the ATO located at the edges. Once the ATO percolated, it was shown that minor changes in the processing parameters could cause drastic differences in the electrical properties (as much as 4-5 orders of magnitude in some cases). This study aims to investigate how the hot-pressing processing conditions, that is, temperature and pressure, can influence the electrical properties of percolated glass/ATO composites. Glass composites containing 4.8 wt% ATO, which is a concentration higher than the percolation threshold, were hot pressed at several different temperatures (550°C-675°C) and pressures (5.8-23.4 MPa) and were examined using impedance spectroscopy. A comprehensive equivalent circuit model was developed based on the microstructure of the composites and the individual impedance behavior of the materials involved. It was found that the physical arrangement as well as the individual properties of the glass, the ATO nanoparticles, and the different interfaces between ATO (point contact vs partially sintered vs glass coated) all contributed to the measured response in a quantitative way. The equivalent circuit model was successful in fitting all of the impedance behaviors at different temperature and pressures thus revealing the influence of the processing conditions on the electrical properties of percolated ATO/glass composites.