Transparent ceramics: Processing, materials and applications

Thermal conductivity of 2D nano-structured boron nitride (BN) and its composites with polymers

Rheology and applications of highly filled polymers: A review of current understanding

To achieve polymer-based composites for electronic packaging with low dielectric constant, low dielectric loss tangent and high thermal conductivity, silane coupling agent KH550 modified hexagonal boron nitride (hBN) platelets were introduced into PTFE matrix via a cold pressing and sintering method. The effect of surface treatment on the morphology, thermal conductivity and dielectric properties of the composites was investigated. The results revealed that after surface treatment, the interfacial adhension between hBN platelets and PTFE matrix was improved and the in-plane orientation degree of hBN platelets in PTFE matrix decreased, which effectively improvd the thermal conductivity of the composites. The thermal conductivity of hBN-KH550/PTFE composite with 30 vol% filler content is 0.722 W/mK, which is 2.7 folds of pure PTFE. Moreover, the enhanced interfacial adhension and reduced surface hydrophilicity of hBN platelets significantly decreased the interfacial polarization, resulting in not only lower dielectric constant and dielectric loss tangent but also weaker frequency-dependence.

Improved thermal conductivity and dielectric properties of hBN/PTFE composites via surface treatment by silane coupling agent

Transparent ceramics have various potential applications such as infrared (IR) windows/domes, lamp envelopes, opto-electric components/devices, composite armors, and screens for smartphones and they can be used as host materials for solid-state lasers. Transparent ceramics were initially developed to replace single crystals because of their simple processing route, variability in composition, high yield productivity, and shape control, among other factors. Optical transparency is one of the most important properties of transparent ceramics. In order to achieve transparency, ceramics must have highly symmetric crystal structures; therefore, the majority of the transparent ceramics have cubic structures, while tetragonal and hexagonal structures have also been reported in the open literature. Moreover, the optical transparency of ceramics is determined by their purity and density; the production of high-purity ceramics requires high-purity starting materials, and the production of high-density ceramics requires sophisticated sintering techniques and optimized sintering aids. Furthermore, specific mechanical properties are required for some applications, such as window materials and composite armor. This review aims to summarize recent progress in the fabrication and application of various transparent ceramics.

Materials development and potential applications of transparent ceramics: A review

Dielectric and thermal properties of isotactic polypropylene/hexagonal boron nitride composites for high-frequency applications

The microwave dielectric properties and crystal structures of MgAl2O4 ceramics synthesized using either molten-salt (MA-M) or solid-state reaction (MA-S) methods were characterized in this study. Raman and 27Al solid-state nuclear magnetic resonance spectra indicated that Al3+ cations primarily occupied in tetrahedral sites of the MA-M ceramic. The degree of inversion x, i.e., degree of cation disorder in tetrahedral and octahedral sites, of the MA-M was higher than that of MA-S; such the preferential site occupation of Al3+ cations enhanced the covalency of the M O bonds in the MO4 tetrahedra (M = Mg and Al), leading to a decrease in the lattice parameters. The Q·f of the MA-M fired at 1600 °C was 201,690 GHz, while the MA-S synthesized at 1600 °C exhibited a Q·f of just 85,100 GHz. Based on these results, an intermediate spinel structure with a greater x evidently has a higher Q·f, and therefore the cation distribution is closely related to the Q·f of the ceramic.

Microwave dielectric properties and crystal structures of spinel-structured MgAl2O4 ceramics synthesized by a molten-salt method

MgO ceramics with a nominal composition of (1-x)MgO–xLiF (x = 0.02–0.10) were synthesized and the effects of LiF addition and sintering conditions on the microwave dielectric properties of MgO ceramics were investigated in this study. With LiF addition, the MgO ceramics were well-sintered at 950 °C in a covered crucible, and the Qf value of the LiF-added MgO ceramic strongly depended on the amount of LiF and sintering time. As a result, the MgO ceramics with a nominal composition of 0.96MgO–0.04LiF (x=0.04) exhibited the highest Qf value of 751,500 GHz (er = 9.6 and TCf= - 56 ppm/°C) when they were sintered at 950 °C for 100 h in a covered crucible. The endothermic reaction of the MgO ceramics was observed with LiF addition by differential thermal analysis and thermograviometry (DTA–TG), and it was considered that the partial melting of MgO with LiF addition to form the liquid phase enhances the sinterability of the MgO ceramics. The crystal structure analysis of the LiF-added MgO ceramics showed a slight increase in the lattice parameters of the MgO ceramics, suggesting the Li substitution for Mg in the MgO structure.

http://iopscience.iop.org/article/10.1143/JJAP.50.09NF02/pdf

Low-Temperature Sintering and Microwave Dielectric Properties of MgO Ceramic with LiF Addition

The spinel-structured Zn 1-3 x Al 2+2 x O 4 ( x  = 0–0.2) ceramics having defective structures were synthesized using the molten salt method, and their microwave dielectric properties and cation distributions were assessed. The 27 Al solid-state nuclear magnetic resonance spectra of these ceramics demonstrate that they have an intermediate spinel structure in which the tetrahedral site occupancy increases from 0.03 to 0.64 as x increases. Moreover, crystal structure refinements suggest that cation vacancies are located at octahedral sites for x  = 0.1 and 0.2. Based on these data, the introduction of cation vacancies at octahedral sites appears to enhance the preferential occupation of tetrahedral sites by Al 3+ . The e r of these ceramics slightly decreased from 8.5 to 8.2 with increasing x , while the Q · f value increased significantly, from 127,532 to 202,468 GHz, upon the introduction of cation vacancies. An intermediate spinel structure with preferential occupancy of tetrahedral sites by trivalent cations exhibits an enhanced Q · f value.

Microwave dielectric properties and cation distributions of Zn1-3xAl2+2xO4 ceramics with defect structures

The electronic state and microwave dielectric properties of MgAl 2 O 4 prepared using solid-state (MA-S) and molten salt (MA-M) methods and those of Mg 0.4 Al 2.4 O 4 (M04A24) were investigated. The λ values, which correspond to the fraction of Al 3+ cations in tetrahedral sites, for MA-S, MA-M, and M04A24 were 0.23, 0.41, and 0.60, respectively. In molecular orbital calculations, a larger overlap was observed between Al-3 s or Al-3 p in tetrahedral sites and O-2 p orbitals for M04A24, and the bond order for Al O at tetrahedral sites of M04A24 (0.241) was higher than those for MA-S (0.178) and MA-M (0.205). The dielectric constant, e r , for M04A24 (7.6) was lower than those for MA-S and MA-M (both 7.9), and the highest quality factor, Q · f , was obtained for M04A24 (235, 800 GHz). It was found that the covalency of the Al O bonds in the M O 4 tetrahedra is closely related to the Q ･ f values of the present ceramics.

Susumu Takahashi

Papers

Dielectric and thermal properties of isotactic polypropylene/hexagonal boron nitride composites for high-frequency applications

Microwave dielectric properties and crystal structures of spinel-structured MgAl2O4 ceramics synthesized by a molten-salt method

Low-Temperature Sintering and Microwave Dielectric Properties of MgO Ceramic with LiF Addition

Microwave dielectric properties and cation distributions of Zn1-3xAl2+2xO4 ceramics with defect structures

Electronic states and cation distributions of MgAl2O4 and Mg0.4Al2.4O4 microwave dielectric ceramics