Monosized spherical particles of (Zr,Sn)Ti04 with an average diameter of about 0.3 pm were synthesized by the controlled hydrolysis of metal alkoxides. The as-prepared, amorphous, particles were compacted without any sintering aid, crystallized, and then sintered at 1600°C for 3 h into bodies with >96.0% of theoretical density. The dense sintered bodies of (Zro.80Sno.zo)Ti04 showed good microwave characteristics: E~ = 40.0, Q = 5000, and 71 = 3 ppm/"C at 10 GHz. The dielectric constant was remarkably dependent upon the relative densities of the sintered bodies and the change of the lattice parameters, while Q value was mainly affected by the oxygen deficiency. An increase of the dielectric constant with the substitution of Sn4+ in ZrTi04 was attributable to the enhancement of ionic polarization with the increase of the c-axis length. [
The effect on the ␥-Al 2 O 3 -to-␣-Al 2 O 3 phase transition of adding divalent cations was investigated by differential thermal analysis, X-ray diffractometry, and surface-area measurements. The cations, Cu 2؉ , Mn 2؉ , Co 2؉ , Ni 2؉ , Mg 2؉ , Ca 2؉ , Sr 2؉ , and Ba 2؉ , were added by impregnation, using the appropriate nitrate solution. These additives were classified into three groups, according to their effect: (1) those with an accelerating effect (Cu 2؉ and Mn 2؉ ), (2) those with little or no effect (Co 2؉ , Ni 2؉ , and Mg 2؉ ), and (3) those with a retarding effect (Ca 2؉ , Sr 2؉ , and Ba 2؉ ). The crystalline phase formed by reaction of the additive with ␥-Al 2 O 3 at high temperature was a spinel-type structure in groups (1)
and (2) and a magnetoplumbite-type structure in group (3). In groups (2) and (3), a clear relationship was found between the transition temperature and the difference in ionic radius of Al3؉ and the additive (⌬r): The transition temperature increased as ⌬r increased. This result indicates that additives with larger ionic radii are more effective in suppressing the diffusion of Al 3؉ and O 2؊ in ␥-Al 2 O 3 , suppressing the grain growth of ␥-Al 2 O 3 , and retarding the transformation into ␣-Al 2 O 3 . Fig. 5. Relationship between T EX and E a for ␣-Al 2 O 3 .
were added by an impregnation method, using the appropriate nitrate solution. -Al 2 O 3 was the crystalline aluminate phase that formed by reaction between these additives and Al 2 O 3 in the vicinity of the ␥-to-␣-Al 2 O 3 transition temperature, with the exception of Li ؉ . The transition temperature increased as the ionic radii of the additive increased. The change in specific surface area of these samples after heat treatment showed a trend similar to that of the phasetransition temperature. Thus, Cs ؉ was concluded to be the most effective of the present monovalent additives for enhancing the thermal stability of ␥-Al 2 O 3 . Because the order of the phase-transition temperature coincided with that of the formation temperature of -Al 2 O 3 in these samples, suppression of ionic diffusion in ␥-Al 2 O 3 by the amorphous phase containing the added cations must have played an important role in retarding the transition to ␣-Al 2 O 3 . Larger cations suppressed the diffusion reaction more effectively.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.