Sintering Behavior and Electrical Properties of Nanosized Doped‐ZnO Powders Produced by Metallorganic Polymeric Processing

Abstract

Homogeneous and nanosized (28 nm crystallite size) dopedZnO ceramic powders were obtained by a metallorganic polymeric method. Calcining and granulating resulted in green compacts with uniform powder packing and a narrow pore-size distribution (pore size 19 nm). Dense ceramic bodies (>99% of theoretical) were fabricated by normal liquidphase sintering at 850°and 940°C for 1-5 h. Apparently, the low pore-coordination number allowed a uniform filling of the small pores by the liquid formed in the early stages of sintering, and, consequently, high shrinkage and rapid densification occurred in a short temperature interval (825°-850°C). At these sintering temperatures, limited grain growth occurred, and the grain size was maintained at <1 m. Ceramics so-fabricated showed a nonlinear coefficient, ␣, of >70, and a breakdown voltage, V b (1 mA/cm 2 ), of >1500 V/mm. The high electrical performance of the doped-ZnO dense ceramics was attributed to liquid-phase recession on cooling, which enhanced the ZnO-ZnO direct contacts and the potential barrier effect.

The effect of processing conditions on varistors prepared from nanocrystalline ZnO

“…The better sintering in the former case can be explained by the homogeneous addition of dopant ions among the grains. 6 Microstructural and electrical studies. Most metal oxide varistors contain ZnO grains as the major component with several other metal oxides as additives.…”

“…[5][6][7][8][9][10][11][12] Varistors with inhomogeneous microstructure can cause a large spread in current/voltage characteristics due to high local currents and this leads to the degradation of the varistor during electrical operation. 2,6,8,11 It is well documented that electrical and electronic characteristics can be altered by varying the microstructure at the grain boundaries, 2,[11][12][13][14][15] and careful control of the microstructure is required to produce a high performance varistor. Commercial varistors are usually fabricated by solid state mixing of 0.5 to 3 mm ZnO particles with dopant oxides such as Bi 2 O 3 , Sb 2 O 3 , CoO, MnO, NiO and Cr 2 O 3 , 2,5-7,11 the mixed powder then being pressed and sintered at higher temperatures.…”

The effect of vibratory milling on the powder properties of zinc oxide varistors

“…The method of preparation, crystalline size, additive homogeneity and chemical composition are the critical parameters to produce a better varistor material [8,9]. The relationship between powder properties and varistor electrical properties has been given considerable attention.…”

“…It consists predominantly of zinc oxide grains, secondary phases including spinel and pyroclore, surrounded by a three dimensional network of bismuth rich phases [1,5]. Varistors with inhomogeneous microstructure can cause a large spread in current-voltage characteristics due to high local currents and this leads to the degradation of the varistor during electrical operation [2,[6][7][8]. It is well documented that the electrical characteristics can be altered by varying the microstructure at the grain boundaries [2,9,10].…”

Self-assembled arrays of ZnO nanoparticles and their application as varistor materialsElectronic supplementary information (ESI) available: XRD plots and FESEM images. See http://www.rsc.org/suppdata/jm/b4/b400927d/

“…Nanocrystalline ZnO has received considerable attention recently because of its wide range of possible technological applications such as solar cells, 4 photovoltaic devices, 5 varistors 6 and sensors, 7 the increased importance of nano-materials being a result of the unique properties that can be obtained as a result of nanostructuring. 6,8,9 A number of preparative techniques, many based on the reaction of zinc salt with alkali metal hydroxides or on alkoxide based sol-gel methodologies, have been shown to yield spherical well-defined particles in the range of 2 to 7 nm. 10 Many different morphological structures of ZnO microparticles have been reported including stars, 11 rods, 11,12 snow flakes 12 and spheres.…”