Effect of Porosity and Grain Size on the Microwave Dielectric Properties of Sintered Alumina

Abstract

The real part of the permittivity (E') and the tan 6 of sintered alumina (A1,0,) at about 9 GHz have been measured. The dielectric properties have been examined as a function of purity, pore volume, and sintered grain size. The tan 6 is found to depend very strongly on the pore volume, purity, and grain size. E' is far less sensitive to impurities and grain size. The dependence of E' on porosity can be described by simple mixture models as expected. A model of losses in single crystals cannot be extended easily to these materials where extrinsic factors such as porosity, random crystal orientation, grain boundaries, microcracks, and impurities dominate. These factors have been studied in an attempt to describe the tan 6 and E' of sintered polycrystalline alumina. In this work, the tan 6 for alumina has been studied in near-theoretical density ranges between 9.1 x and 2.4 x 10" depending on grain size.

Enabling Simultaneous Extreme Ultra Low-k in Stiff, Resilient, and Thermally Stable Nano-Architected Materials

“…porous polycrystalline alumina, 25 and poor thermal stability, i.e. Sr-based metal-organic frameworks.…”

“…Their practical 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 2 use has been limited by the strong coupling between mechanical, thermal, and electrical properties of materials and their dielectric constant; low-k is usually attained by materials that are very porous, which results in high compliance, i.e. silica aerogels, 39 high dielectric loss, i.e.porous polycrystalline alumina, 25 and poor thermal stability, i.e. Sr-based metal-organic frameworks.…”

“…22 Other properties also have been shown to degrade with introducing porosity; for example, the dielectric constant of a Sr-based metalorganic framework was reported to increase by 18% over the temperature range of 0°C-350°C, 23 the breakdown voltage field of porous SiCOH deposited using plasma enhanced chemical vapor deposition (PECVD) was reported to decrease from 5.5 MV/cm at 30% porosity to 3.6 MV/cm at 45% porosity, 24 and the dielectric loss, tan δ, of polycrystalline alumina was reported to increase over two orders of magnitude when the porosity increased from 2% to 40%. 25 The deterioration of these properties can be attributed to large variations in aggregated particle and pore sizes, disordered pore distributions, and interconnected pores. 26 The low k of 1.25-1.36 have been attained in aerosols at high frequencies of 0.050-1.3 GHz, and as low as 1.08 in the higher 11-12…”

Engineered Carbon-Nanotubes-Based Composite Material for RF Applications

“…Meanwhile the size of air interstices between alumina nanoparticles decreases, their number has significantly increased. It is reported in [9] that the losses are less sensitive for granular particles with size greater than 3-4 μm in a fully dense material. However, for this case study, the loss dependence is strongly influenced by the porosity of the alumina nanoparticles.…”

Effect of starting PMMA content on microstructure and properties of gel casting BN/Si3N4 ceramics with spherical-shaped pore structures

“…e and tand of pores can be regarded as 1 and 0, separately, and the variation of e with porosity can be considered by using an approximation as Eq. (3) based on mixture law [28]:…”