Sintering of Zinc Oxide

Abstract

Sintering of Zinc Oxide 449 oxygen enters the lattice substitutionally. According to a theory by Lidiard,6 substitutional impurities diffuse in ionic crystals by forming complexes with vacancies on the same sublattice. One may therefore expect oxygen to diffuse into CaF2 by forming complexes with fluorine vacancies.One may further question whether the penetration is limited by the extrinsic or intrinsic diffusion rate of oxygen into CaF2 On the basis of Lidiard's theory, the activation energy for intrinsic diffusion, Qz, in CaF2 should be given by QX = U + (4) in which U is the activation energy for the mobility of complcxes and F is the formation energy of an anti-Frenkel defect. The intrinsic activation energy would be obtained when the oxygen concentration a t the penetration interface is much smaller than the intrinsic vacancy concentration. In the extrinsic limit, the diffusion activation energy, Qe, should be given by( e ) A. R Lidiard, "Impurity Diffusion in Polar Crystals,"Lidiard, "Impurity Diffusion in Crystals (Mainly Ionic Crystals with the Sodium Chloride Structure)," Phil. Mag.The extrinsic diffusion activation energy would be expected to control oxygen penetration if the oxygen concentration at the interface is much greater than the intrinsic fluorine vacancy concentration.Ure4 has determined that F = 65 5 kcal per mole in CaF2, so it remains to estimate U. If the mobility of complexes is limited by the ability of fluorine vacancies to circulate about the oxygen ion, one may expect U to be about equal to the 12 kcal per mole activation energy for fluorine vacancy mobility in CaF2.4 If this hypothesis is valid, Qt G 45 kcal per mole and QeThe experimental activation energy for oxygen penetration is 46 kcal per mole, consistent with the value estimated for the intrinsic-diffusion-limited case. The intrinsic vacancy concentration in the temperature range studied varies from 0.02 to 1.330/0.4 The oxygen concentration was 0.2y0 behind and about 0,0020/o in front of the penetration interface. At the highest temperature, even the oxygen concentration behind the interface satisfies the condition for intrinsic diffusion. Since no break from an Arrhenius relation was observed in the temperature range studied, the writers concluded that the penetration was intrinsic-diffusionlimited throughout. This conclusion was further supported by the fact that the penetration rate was the same in air and in oxygen in spite of the fact that the oxygen concentration behind the diffusion front was an order of magnitude greater for oxygen. 12 kcal per mole.Sintering studies have been conducted with single crystal spheres of zinc oxide in air at total pressures of lop3 to 1.0 atm over the range 1050' to 1250°C. Quantitative observations were made on the rate of growth of a neck between the spheres. No change in the distance between the geometrical centers of the spheres was observed. An analysis of the kinetic data shows that sintering was predominantly achieved by distillation through the surrounding gas phase.At 1150' th...

Grain growth kinetics for B2O3-doped ZnO ceramics

“…3a -d) and also reported in the literature [27,30,31], the addition of B 2 O 3 significantly promoted the grain growth of ZnO and resulted in the uniform microstructure due to the low melting temperature of B 2 O 3 [32]. As mentioned previously, the grain growth exponent value (n) of 3 was consistently observed for pure ZnO [2,[27][28][29]. According to Hng and Halim, the small value of n obtained for ZnO -1 mol %V 2 O 5 system indicated that the grain growth was much faster than for pure ZnO [33].…”

“…Senda and Bradt calculated the activation energy as 224±16 kJ/mol for sintering a pure ZnO system [2]. On the other hand, Norris and Parravano measured the early stages of neck growth of ZnO spheres from 1050 to 1250°C and found much larger activation energy of 440 to 461 kJ/mol as well as smaller grain sizes [28]. This difference can probably be attributed to the processing conditions and to the raw material properties, such as powder particle morphology, particle size distribution or impurities contained within the powder, creating grainboundary drag, decreasing grain-boundary mobility, thus, the grain size, by slowing the rate of the grain growth [2].…”

“…There have been many studies concerning the grain growth of pure ZnO. Although the grain growth exponent value (n) of 3 was consistently observed in these studies, the activation energies for the grain growth process of pure ZnO have been reported in the range of 224 to 461 kJ/mol [2,[27][28][29]. Senda and Bradt calculated the activation energy as 224±16 kJ/mol for sintering a pure ZnO system [2].…”

Synthesis and Sintering of ZnO Nanopowders

“…ZnO sintering mechanisms were extensively studied in the 1960s [12][13][14]; different groups conducted sintering experiments in air and concluded that the densification process of pure submicronic ZnO powders is essentially completed in about 1 h even at a low temperature of 900 • C, with a relative density of around 98% [15]. However, grain growth was important, leading to final grain sizes between 3-50 µm, depending on sintering time and temperature.…”

Role of Sintering Atmosphere and Synthesis Parameters on Electrical Conductivity of ZnO

“…The grain size of ZnO sintered under nitrogen was smaller (about 10 μm) than that sintered under air (above 20 μm). Several previous studies have revealed that the zinc diffusion is responsible for the solid-state sintering (Norris et al 1963;Gupta et al 1968;Lee et al 1959) since the oxygen diffusion coefficient is several orders of magnitude slower than that of zinc (Gupta et al 1968). The oxygen from the atmosphere can diffuse faster along the grain boundaries at high temperatures and react with zinc during sintering (Gupta et al 1968).…”