Preparation of zinc oxide seed grains
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ZnO seed grains are derived from washing ZnO-BaO sintered disks in boiling water. The grain growth of ZnO results from the energy difference across a curved grain boundary. The growth dynamics of ZnO grains and the dissolution of BaO in the boiling water are studied. The classical model shows that the growth of ZnO grains depends on grain boundary diffusion. The dissolution of BaO in boiling water can be considered as the diffusion of BaO in boiling water. The weight percent of BaO dissolved in boiling water varies linearly with the thickness of ZnO-BaO sintered disks and the reciprocal of the square root of the boiling time.
I. INTRODUCTION ZnO varistors are ceramic devices made by sintering ZnO fine powder with small amounts of additives such as Bi 2 O 3 , CoO, and MnO 2 . 1 These devices have nonlinear voltage-current characteristics that find wide applications in electric circuits such as surge protectors. In electric circuits, the threshold voltage of ZnO varistors has to be low. In order to obtain low-voltage ZnO varistors, Eda et al.2 presented a method called the seed grains method. Since added ZnO seed grains promote grain growth, the threshold voltage of ZnO varistors is decreased. The threshold voltage of ZnO varistors is equal to the production of the number of ZnO grains in series within a sample and the threshold voltage per grain boundary which is about 3 V. The number of ZnO grains in series decreases with growing ZnO grains. Therefore, the threshold voltage of ZnO varistors will decrease.
This weight difference is divided by the weight of BaO which disperses uniformly in each sintered disk before boiling; thus, the weight percent can be obtained from 100 timing the quotient. The weight of each sintered disk was weighed by means of a high precision analytical balance. III. RESULTS AND DISCUSSION The preparation of ZnO seed grains can be considered as two distinct procedures, i.e., the growth of ZnO grains and the dissolution of BaO in boiling water. A. The growth of ZnO grains It is known that the growth dynamics of ZnO grains in ZnO-BaO ceramics, like the other ceramic systems, derives from the energy difference across a curved grain boundary,2 which can be deduced from thermodynamics and written as 3
II. EXPERIMENTAL
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ZnO and 1 mol % BaO were mixed in a vibration ball mill for 6 h. After mixing, the mixture was pressed, at 29.4 MPa, into different thickness disks of 50 mm in diameter. The disks were sintered at 1300 °C for 4 h in air and then washed in boiling water for several hours. During washing, the sintered disks decomposed into small particles called seed grains. BaO, segregated to the grain boundaries during sintering, dissolves in the boiling water. The seed grains, dried in an oven and classified by sieves, were usually 60-140 yttm in diameter. The weight percent (wt. %) of BaO dissolved can be obtained by the following method. The weight of BaO dissolved, in fact, is equal to the weight difference of each ZnO-BaO sintered disk before and after boiling. 164
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