Grain growth in a high-purity ZnO and for the same ZnO with Bi2O3 additions from 0.5 to 4 wt% was studied for sintering from 900° to 1400°C in air. The results are discussed and compared with previous studies in terms of the phenomenological kinetic grain growth expression: Gn—Gn0=K0t exp(—Q/RT). For the pure ZnO, the grain growth exponent or n value was observed to be 3 while the apparent activation energy was 224 ± 16 kJ/mol. These parameters substantiate the Gupta and Coble conclusion of a Zn2+ lattice diffusion mechanism. Additions of Bi2O3 to promote liquidphase sintering increased the ZnO grain size and the grain growth exponent to about 5, but reduced the apparent activation energy to about 150 kJ/mol, independent of Bi2O3 content. The preexponential term K0 was also independent of Bi2O3 content. It is concluded that the grain growth of ZnO in liquid-phase-sintered ZnO-Bi2O3 ceramics is controlled by the phase boundary reaction of the solid ZnO grains and the Bi2O3-rich liquid phase.

Grain Growth in Sintered ZnO and ZnO-Bi2O3 Ceramics

The electrical resistivity of ZnO doped with Al2O3 was measured in air and under reduced pressure (∼0.5 mm Hg) in the range from 30° to 680°C as a function of the degree of sintering. The data obtained were explained in terms of the effects of the microstructure of the sintered body and the chemisorbed oxygen. There are two mechanisms of electrical conduction involved, only one of which is affected by the microstructure.

Dependence of Electrical Conductivity of ZnO on Degree of Sintering

A two-sensor array was fabricated to improve gas selectivity, an intrinsic disadvantage of mono-metal oxide semiconductor gas sensor. The sensors comprising the array were made of Ga-doped ZnO (GZO) annealed at different temperatures, and the selectivity over relative humidity (RH 0-70 +/- 10%) was greatly improved in formaldehyde gas sensing. The physical and mathematical principle for high selectivity is based on the different responses of the two sensors to analyte (formaldehyde) and to interferent (RH), that is. one sensor has larger response to formaldehyde than the other but the same response to RH. But under the same fabrication process for ZnO without Ga dopant, one sensor was insensitive either to formaldehyde or to RH due to the reaction of ZnO with the SiO(2) substrate. Consequently, such a sensor array cannot be fabricated using ZnO sensors without Ga dopant. Possible mechanism concerning interactions between GZO and the two gases (formaldehyde and water vapor) was discussed. (C) 2009 Elsevier B.V. All rights reserved.

Improving humidity selectivity in formaldehyde gas sensing by a two-sensor array made of Ga-doped ZnO

Titanium dioxide and zinc oxide films were fabricated by spin-on and dip-coating methods. Both types of films exhibited columnar grains when the single coating was thin, ∼10 nm. The columnar TiO2 films were dense, as confirmed by their density values calculated from the refractive index and TEM results. The addition of Al cations into the ZnO suppressed grain growth, because Zn had a lower diffusivity in the doped films. The doped ZnO films had nearly the same electrical resistivity as that of the undoped films.

Microstructure of TiO2 and ZnO Films Fabricated by the Sol-Gel Method

Grain growth of ZnO during the liquid-phase sintering of binary ZnO–Bi2O3 ceramics has been studied for Bi2O3 contents from 3 to 12 wt% and sintering from 900° to 1400°C. The results are considered in combination with previously published studies of ZnO grain growth in the ZnO–Bi2O3 system. For the Bi2O3 contents of the present study, the rate of ZnO grain growth is found to decrease with increasing Bi2O3. Activation analysis, when combined with the results of similar analyses of the previous studies, reveals a change in the rate-controlling mechanism for ZnO grain growth. Following a low-Bi2O3-content region of nearly constant activation energy values of about 150 kJ/mol, further Bi2O3 additions cause an increase of the activation energy to about 270 kJ/mol. consistent with accepted models of liquid-phase sintering, it is concluded that the rate-controlling mechanism of ZnO grain growth during liquid-phase sintering in the presence of Bi2O3 changes from one of a phase-boundary reaction at low Bi2O3 levels to one of diffusion through the liquid phase at about the 5 to 6 wt% Bi2O3 level and above.

Grain Growth of ZnO during Bi2O3 Liquid-Phase Sintering

The influence of dopents (Li2O, Na2O, Al2O3 and In2O3) on the sintering of ZnO and NiO has been studied in the temperature range 750° to 1200°C in air. The dopents, Li2O and Na2O, were found to promote the sintering of ZnO and the other dopents, Al2O3 and In2O3 to inhibit. The result is explained by the diffusional transfer of interstitial zinc ions.While for the sintering of NiO, the Li2O and Na2O were found to promote the sintering, the Al2O3 and In2O3 to inhibit. This result shows evidently that material transport in NiO does not depend on the concentration of nickel ion vacancies, but on the concentration of electron holes, trivalent nickel ions Ni3+, which have smaller ionic radii than bivalent nickel ions.

Syoji Hujita

Papers

Effects of Dopents on Sintering of Zinc Oxide and Nickel Oxide