Ultralow loss polycrystalline alumina

Abstract

Polycrystalline alumina with extremely low microwave dielectric loss is reported with properties analogous to a theoretical ensemble of randomly oriented, single crystal sapphire grains. By avoiding deleterious impurities and by careful control of microstructure, we show that grain boundaries in aluminum oxide have only a limited influence on the dielectric loss. A method of measuring the electric permittivity and loss tangent of low-loss microwave ceramic dielectrics is reported. The electrical parameters such as relative permittivity and loss tangent are extracted using the radial mode matching technique. Ϫ5 at 10 GHz. 1 Polycrystalline analogs of these materials usually have similar permittivity, but losses that are generally a factor of at least two higher. The measurement of low loss, low permittivity microwave dielectric ceramics using the TE 01␦ is limited in accuracy by the electric filling factor and geometric factor of the conducting shield. By carefully choosing the dimensions of the conducting shield for optimum electric filling factor P e and geometric factor G, it is possible to measure the tan ␦ with 10% accuracy for tan ␦Ͼ10 Ϫ5 .2 The preferred method of measurement for tan ␦Ͻ10Ϫ6 utilizes high order whispering gallery modes, 3,4 which have electric filling factors approaching unity and very high geometric factors. Unfortunately, the modal number density is usually very high in the frequency range in which whispering gallery modes exist, and careful choice of cavity dimensions is necessary to avoid spurious modes.5 This letter seeks to extend the usefulness of the TE 01␦ mode measurement method by eliminating a great source of error: the support. The relative permittivity of the dielectric resonator can be calculated using the radial mode matching method. 6 Instead of solving for frequency, as is usually done, one can instead solve for r . After calculation of the electric filling factor and geometric factors, the tan ␦ can be obtained from the expressionwhere P e is the electric filling factor, Q 0 is the unloaded quality factor, R s is the surface resistivity, and G is the geometric factor. An alumina dielectric resonator was constructed as shown in Fig. 1 7 This material is high purity and fine grained, both of which are necessary in order to achieve very low loss. 8,9 The dimensions of the conducting shield are diameter 36.00 mm, height 23.48 mm. The alumina puck has diameter 10.69 mm and height 4.34 mm. The alumina spacer has diameter 4.13 mm and height 6.77 mm.The resonant frequency of the quasi-TE 011 (TE 01␦ ) mode was measured at room temperature ͑300 K͒ and found to be 8.95 557 GHz. The measurement was performed in transmission (S 21 ) using an Agilent network analyzer ͑HP8722͒. Input and output coupling to the resonator was achieved using coaxial transmission lines with small loops formed by soldering the central conductors to the outer shield of the coaxial cable. The coupling was Ϫ40.5 dB, measured from the insertion loss. The loaded quality factor Q L of the mode was measured and f...

Dielectric properties of pure alumina from 8 GHz to 73 GHz

“…Grain boundaries in polycrystalline microwave dielectric ceramics have long been suspected of increasing dielectric loss. However, Alford et al show recently that grain boundaries is problematic to quantify in practice and their influence on dielectric loss [13,14], as suggested in this work.…”

Do Grain Boundaries Affect Microwave Dielectric Loss in Oxides?

“…A single grain boundary in MgO has no effect on the microwave dielectric loss at room temperature and a very slight effect at low temperature (around 45 K). Further, at room temperature, the difference in microwave loss between very pure single crystals and their polycrystalline counterparts is small and this has been shown to be the case in a number of ceramic dielectric materials including Al 2 O 3 , TiO 2 , MgO, and LaAlO 3 4,24–26 . In these studies it was found that impurities and porosity were particularly deleterious to microwave loss.…”

Modification of ZnAl2O4‐Based Low‐Permittivity Microwave Dielectric Ceramics by Adding 2MO–TiO2 (M=Co, Mg, and Mn)

“…Several low‐permittivity microwave dielectric ceramics, like Al 2 O 3 ‐based, 4,5 silicates, 2,6,7 R 2 BaMO 5 (R=Y, Sm, Nd, and Yb, M=Cu, Zn, and Ni), 8–11 MCu 2 Nb 2 O 8 (M=Zn, Co, Ni, Mg, and Ca), 12 Re 3 Ga 5 O 12 (Re=Nd, Sm, Eu, Dy, Yb, and Y), 3 etc., have been developed for microwave substrate and antenna application. To exploit new low‐permittivity material systems, spinel‐based materials such as M 2 TiO 4 (M=Mg and Co) 1 and MAl 2 O 4 (M=Zn and Mg) 13–16 have attracted much scientific attention of late.…”