Effect of Ag on the microstructure and electrical properties of ZnO
Introduction
Zinc oxide (ZnO) is an n-type semiconductor with a wide band gap (3.437 eV at 2 K),1, 2 and has been used as the material for surge suppressors, gas sensors and transducers, etc.3, 4, 5, 6 For many electrical applications, silver and silver alloys are used as the electrode materials. In order to reduce the manufacturing cost, the electrodes are frequently co-fired with ZnO to elevated temperatures. ZnO and Ag may interact with each other during co-firing; however, such interaction has received relatively little attention in the literature.
In a study conducted by Fan and Freer,7 the effects of 1000 ppm of Ag doping on the electrical properties of ZnO varistor compositions (i.e. ZnO mixed with Bi2O3, Sb2O3, Co2O3, Cr2O3, MnO2 and B2O3) were studied. They found that both the grain and grain boundary resistances increase with the addition of Ag, and proposed that Ag+ could substitute Zn2+ and acts as an acceptor in ZnO, expressed as
Due to the formation of Ag acceptors, the grain resistance is increased. Fan and Freer also suggested that Ag+ may behave like many other monovalent dopant ions (e.g. Na+ and K+) which have the ability to occupy both the lattice and interstitial sites (i.e. amphoteric dopants), expressed as
They proposed that may occupy the grain boundary sites, and consequently, the grain boundary resistance is increased. The effects of Ag addition on the microstructural evolution of ZnO during sintering were not addressed in their study.
In a study conducted by Jose and Khadar,8 nano-sized ZnO and Ag (5–30 wt.%) particles were mixed together, and the impedance spectrums of the green compacts were studied. Both the grain and grain boundary resistances of ZnO increase slightly with the addition of nano-sized Ag particles. Jose and Khadar suggested that the increase in the resistances is related to the presence of Ag particles at the grain boundaries and triple junctions of the nanocrystalline ZnO. In their study the specimens (i.e. green compacts) were not co-fired at an elevated temperature. A more intense Ag–ZnO interaction might be observed if a suitable heat treatment was applied.
In the present study, ZnO and Ag powders are mixed together. The Ag-doped ZnO specimens are then co-fired together to elevated temperatures. The effects of Ag on the microstructural evolution and electrical properties of ZnO are investigated.
Section snippets
Experimental procedure
Zinc oxide (>99.95% ZnO, d50 = 1.2 μm) and Ag (>99.9% Ag, d50 = 1.4 μm) powders were ball-milled together in ethyl alcohol for 4 h using zirconia media. The amount of Ag ranged from 0.08 to 7.7 mol% (i.e. 0.1–10 wt.%). The slurry was dried in a rotary evaporator to remove the alcohol, and then sieved through a #150 plastic mesh. The mixed powders were consolidated into discs of 10 mm diameter and 3 mm thickness at a uniaxial pressure of about 25 MPa. These sample discs were sintered at 800–1200 °C in air
Phase analysis
Fig. 1 shows the XRD patterns of the Ag-doped ZnO specimens sintered at 1200 °C for 1 h. The XRD patterns reveal that apart from ZnO and Ag, no other reaction phases are present. The Si peak is resulted from the coated Si paste. From the position of the Si peak, it is possible to calibrate the values of ZnO peaks.
It is evident that with increasing Ag doping level, the (1 0 0), (0 0 2) and (1 0 1) peaks of ZnO shift to the right progressively. Using these characteristic peaks, the lattice parameters a
Solubility of Ag in ZnO
To determine a minute solubility in ceramics by using XRD technique is a challenging task. An external standard, Si powder, is applied to coat onto the specimen before the XRD measurement. The Si peak is then used to calibrate the two values of the ZnO peaks. This technique has been well received by many research groups. For example, Kim et al. used such technique to determine the lattice parameters of HfO2 and ZrO2 crystals.13 Their results had demonstrated that accuracy of the technique could
Conclusion
The effects of Ag addition on the microstructural evolution and electrical properties of ZnO have been investigated. It is found that a small amount of Ag, around 0.08 mol% or slightly higher, can dissolve into the ZnO lattice. The presence of Ag solutes increases the rates of densification and grain growth. The Ag solutes tend to segregate at the grain boundaries, and this segregation of Ag+ ions significantly raises the grain boundary resistance and establishes an electrostatic barrier against
Acknowledgement
Financial support is provided by the National Science Council, Taiwan, under the contract number NSC94-2216-E-002-008.
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