Zinc oxide sintering

Melnick (11) has made the most extensive study of the slow photoconductive response. Using zinc oxide sintered in air, he has studied responses in the time range from 0.3 to 10 sec. 

The extraction of zinc by carbothermic reduction of zinc oxide sinter at 1,100°C can be represented by the reaction... 

Zinc Roasting Sintering Calcining Retorts electric arc Particulates (dust) and SO2 Particulates (dust) and SO2 Zinc oxide fume, particulates, SO2, CO Exhaust system, humidifier, cyclone, scrubber, electrostatic precipitator, and acid plant Exhaust system, humidifier, electrostatic precipitator, and acid plant Exhaust system, baghouse, scrubber or acid plant... 

In the blast furnace reduction slag-making materials are also added together with a small amount of iron, the function of which is to reduce any sulphide which remains, to the product of the roasting operation to produce a sinter. The sinter is then reduced with coke in a vertical shaft blast furnace in which air is blown tluough tuyeres at the bottom of tire shaft. The temperature in the heartlr where metal is produced must be controlled to avoid the vaporization of any zinc oxide in the sinter. The products of tlris process are normally quite complex, and can be separated into four phases. Typical compositions of these are shown in Table 13.1. 

The measures of solid state reactivity to be described include experiments on solid-gas, solid-liquid, and solid-solid chemical reaction, solid-solid structural transitions, and hot pressing-sintering in the solid state. These conditions are achieved in catalytic activity measurements of rutile and zinc oxide, in studies of the dissolution of silicon nitride and rutile, the reaction of lead oxide and zirconia to form lead zirconate, the monoclinic to tetragonal transformation in zirconia, the theta-to-alpha transformation in alumina, and the hot pressing of aluminum nitride and aluminum oxide. 

Two methods are available for the preparation of the powder (Smith, 1969). In one, zinc oxide is ignited at 900 to 1000 °C for 12 to 24 hours until activity is reduced to the desired level. This oxide powder is yellow, presumably because zinc is in excess of that required for stoichiometry. Alternatively, a blend of zinc oxide and magnesium oxide in the ratio of 9 1 is heated for 8 to 12 hours to form a sintered mass. This mass is ground and reheated for another 8 to 12 hours. The powder is white. Altogether the powder is similar to that used in zinc phosphate cements. 

Magnesium oxide is always blended with the zinc oxide prior to ignition. Magnesium oxide promotes densification of the zinc oxide, preserves its whiteness and renders the sintered powder easier to pulverize (Crowell, 1929). The sintered mixed oxide has been shown to contain zinc oxide and a solid solution of zinc oxide in magnesium oxide (Zhuravlev, Volfson Sheveleva, 1950). Specific surface area is reduced compared with that of pure zinc oxide and cements prepared from the mixed oxides are stronger (Crowell, 1929 Zhuravlev, Volfson Sheveleva, 1950). 

Dollimore, D. Spooner, P. (1971). Sintering studies on zinc oxide. 

The presence of water on the oxide surface can enhance the sintering of zinc oxide particles (Dollimore Spooner, 1971). The amount of water reversibly absorbed on zinc oxide surfaces is affected by heat treatment... 

Figure 9.4 The effect of sintering temperature on the morphology of zinc oxide particles. Zinc oxide from zinc oxalate (a) 400 °C, (b) 800 °C. Carmox zinc oxide (c) 400 °C, (d) 800 °C (Prosser Wilson, 1982). 

Lee, V. J. Parravano, G. (1959). Sintering reactions on zinc oxide. Journal of Applied Physics, 30, 1735-40. 

To dissociate molecules in an adsorbed layer of oxide, a spillover (photospillover) phenomenon can be used with prior activation of the surface of zinc oxide by particles (clusters) of Pt, Pd, Ni, etc. In the course of adsorption of molecular gases (especially H2, O2) or more complex molecules these particles emit (generate) active particles on the surface of substrate [12], which are capable, as we have already noted, to affect considerably the impurity conductivity even at minor concentrations. Thus, the semiconductor oxide activated by cluster particles of transition metals plays a double role of both activator and analyzer (sensor). The latter conclusion is proved by a large number of papers discussed in detail in review [13]. The papers cited maintain that the particles formed during the process of activation are fairly active as to their influence on the electrical properties of sensors made of semiconductor oxides in the form of thin sintered films. 

Zinc oxide beds are limited to operation at temperatures below 430°C probably because of pore plugging during sulfur removal and sintering. Thermodynamics also favors lower temperatures. At the higher temperatures, the H2S cannot be reduced to levels low enough for shift catalyst or to reach fuel cell limits. 

Most of the electrical and optical investigations on zinc oxide have been performed, not on single crystals, but on compressed powders, evaporated layers, and sintered samples, all of which constitute geometries of high potential surface-to-volume ratios. 

A photomicrograph of a typical sintered zinc oxide sample, sintered in air at 1000° is shown in Fig. 2. The specimen was prepared for observation by boiling the sample in a solution of an organic dye, to expel the air in the pores, and allowing the sample to cool in the dye, which was then sucked into the pores. The sample was then cut and polished for observation. The white portions of the photomicrograph show the grain structure, the dark portions are the dye. 

Fig. 2. Photomicrograph of sintered zinc oxide. Magnification 800X.

As evidence that surface effects do control the resistance of sintered zinc oxide (and hence probably the resistance of evaporated layers),... 

Morrison (31) has compared measurements of the Hall effect and of the resistance. The Hall voltage is inversely proportional to the average concentration of carriers in the material (24), and so, for zinc oxide, will be inversely proportional to the concentration of carriers in the large grains (Fig. 2) of the material. Figure 3 shows an example in which the resistance and the inverse of the Hall voltage measured on a sintered sample of zinc oxide are plotted as functions of the time. This illustrates that the number of carriers in the bulk of the sample may remain relatively constant, while the conductance varies widely, all at constant temperature. 

The existence of other deep surface levels, for example Tamm levels (discussed in the preceding section), on the surface of zinc oxide is placed in doubt by an experiment of Bevan and Anderson (32) on sintered zinc oxide. They observed that the activation energy of the conduction electrons (of the order of an electron volt when the sample is subjected to high oxygen pressure) decreases to a few hundredths of an electron volt if the measurements are taken at low pressure (less than 10 mm.) and high temperature (the order of 600°C). Surface traps other than those asso-... 

Bevan and Anderson (32) deduced that adsorbed oxygen controlled the resistance of sintered zinc oxide at temperatures between 500°C and 1000°C. This conclusion was based on the observations that (1) the oxygen pressure had a reversible controlling effect on the resistance down to 500°C, too low a temperature for thermodynamic equilibrium to be... 

Morrison and Miller (33) have made some direct measurements on the adsorption of oxygen on zinc oxide powder (Fig. 4), which seem in accord with the conductivity measurements of Bevan and Anderson on sintered zinc oxide. The reversible region of chemisorption was shown to be above about 45O C, corresponding to the reversible region of conductivity versus oxygen pressure found by Bevan and Anderson to be above about 500°C. 

Considering the method of preparation of these ZnO samples, these results correspond to what one would expect on the basis of the adsorption model. The samples were sintered or evaporated at some high temperature, and then cooled to room temperature in air. As discussed in Section IV, 1, adsorption will occur until the rate of electrons crossing the surface barrier is pinched off to zero at room temperature. If the temperature is now lowered below room temperature, no electron transfer will be possible between the surface level and the bulk of the solid due to this high surface barrier. Thus the surface levels will be isolated and unable to affect the conductivity, which will therefore reflect bulk properties of the zinc oxide. 

Morrison (31), using sintered zinc oxide, applied a different technique to study the conductivity effects in the range between room temperature and 500°C. He studied the variation in conductance as a function of time with the temperature held constant. Figure 3 shows one such conductivity-time experiment. The sample used was a slab of zinc oxide cut from a pill which had been compressed and sintered in air for eighteen hours at 1000°C. The sample was immersed in oil (the oil does not penetrate into the pores of the sample) at the start of the run. The sample container was immersed in boiling water, the temperature reaching 100 C in the order of one-half of a minute. The conductance was recorded as a function of time while the sample was held at 100°C. The results are shown in Fig. 3. The inverse of the Hall voltage is also plotted as a function of time. An interpretation of the Hall measurement is discussed in Section III. 

This evidently does not apply to sintered zinc oxide, since at low temperature its conductivity shows an activation energy of a few hundredths of an electron volt, the same as is shown by the Hall coefficient (26), independent of the previous treatment of the sample. This, then, indicates that the grains of sintered zinc oxide are actually fused, rather than merely touching. 

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