The thermal decomposition behaviors of mixtures containing α-terpineol, Ag2O or silver stearate, characterized using the TGA, are shown Figure 4-1. For the mixture of 70 wt% α-terpineol and 30 wt% Ag2O, a significant weight loss was observed below 189°C and then the sample’s weight levels off at 28.3 wt%
of its original weight, which is close to the theoretical weight percentage taking account of the evaporation of α-terpineol and the reduction of Ag2O to silver. It appears that Ag2O can catalyze the decomposition of the α-terpenol solvent, which is identical with the results indicated in a previous study [13]. For the TGA trace of 70 wt% α-terpineol and 30 wt% silver stearate, there are three regions with significant weight loss, including ≈ 70 wt% weight loss below 198°C due to the evaporation of α-terpineol and then totally ≈ 22 wt% weight loss in the temperature range from 198°C to 380°C resulting from the thermal decomposition of silver stearate. Above 380°C, there is no additional weight loss, which reveals that all silver compounds were transferred to silver. For the mixture of 70 wt% α-terpineol, 20 wt% Ag2O, and 10 wt% silver stearate, ≈ 72.2 wt% weight loss was observed below 189°C, which is corresponding to the combination of the evaporation of 70 wt% α-terpineol, the reduction of 20 wt%
Ag2O, and the decomposition of 1.1 wt% silver stearate. The remaining 8.9 wt%
silver stearate was further decomposed at temperatures above ≈ 240°C. No weight loss was observed at temperatures higher than 260°C. Apparently Ag2O can catalyze the decomposition of silver stearate, including 1.1 wt% below 189°C and the rest between 189 and 260°C. Calculations based on the above result indicates that 100 grams of Ag2O can catalyze the decomposition of ≈ 5.5 gram silver stearate at temperatures below 189°C, however, the weight ratio for the reaction between Ag2O and silver stearate is dependent on the physical
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characteristics of the Ag2O powders, such as particle size.
Figure 4-2 shows the TGA traces of Ag2O, silver stearate, and Ag2O with 5 wt% silver stearate. Without the presence of α-terpineol, Ag2O will not get reduced until the temperature reaches 370°C. Thermal decomposition path of pure silver stearate is similar to that observed in the mixture α-terpineol and silver stearate, as shown in Figure 4-1. It is evident that the presence of α-terpineol promotes the reduction of Ag2O but not the decomposition reaction of silver stearate. For the Ag2O with 5 wt% silver stearate, ≈ 7.8 wt% rapid weight loss was observed at ≈ 160°C, which corresponds to the transfer of silver compounds to pure silver (89.9 % of the original weight theoretically). XRD analysis on the sample after being heat-treated at 160°C indicates that silver and small amount of Ag2O are present in the residue. It manifests that Ag2O not only catalyzes the decomposition of silver stearate but also the decomposed silver induces the reduction of Ag2O concurrently, which confirms the observation in Figure 4-1. Not all Ag2O was reduced to silver. The Ag2O residue was transferred back to silver at temperatures higher than 410°C, which is coincident with that observed in the literature.
In order to determine the optimum ratio of the Ag2O/ silver stearate, TGA studies on the silver oxide with additions of 3, 4, 5, and 6 wt% silver stearate were performed and the results are shown in Figure 4-3. Similar to the decomposition behavior of Ag2O/silver stearate shown in Figure 4-2, the mixtures reduce their weights dramatically at the temperatures ranging from 155 to 160°C, and Ag2O residues reduce back to silver at temperatures higher than 410°C. Increasing the amount of silver steartae addition, the weight loss increases due to the complete decomposition of silver stearate and partial reduction of Ag2O. As more fresh silver decomposed from silver stearate, more Ag2O was catalyzed and reduced back to silver. Comparing the weight loss at 165°C for samples with 5 and 6% stearate, ≈0.2 wt% difference in weight loss
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seen corresponds to the decomposition of silver stearate. No additional reduction of Ag2O was observed. Therefore, it can be concluded that for the silver stearate content exceeding 5 wt%, the weight loss at temperatures below 160°C corresponding to the reduction of Ag2O does not increase.
In this study, the silver flake and binder such as ethyl cellulose were replaced by Ag2O and silver stearate in the paste, to reduce the curing temperature while maintaining the required paste characteristics and electrical properties of the final film. Rheological behaviors of the pastes with solid loading of 70wt% and 80 wt% are characterized and the results are shown in Figures 4-4 (a) and (b), respectively. It was elucidated in such a way that shear stress versus shear rate for various ratios of Ag2O and silver stearate are calculated. The results indicate that all pastes appear to have pseudoplastic flow (shear-thinning) property. The initial shear stress and viscosity increases with increasing silver stearate content and the solid loading. The dissolution of silver stearate in solvent raised the viscosity of the paste significantly. It is similar to that observed in the typical isotropically conductive adhesive (ICA) formulations, in which stearic acids are widely used as the rheology modification agent or lubricant layer on the surfaces of metal powders, to modify the viscosity of conductive adhesive paste [16]. For the paste with 80wt% solid loading, the pastes with 5 and 6 wt% silver stearate show shear-thinning with an apparent yield point. The dissolution of silver steatrate in solvent leads to viscoelasticity properties with a stable viscosity and shear stress versus shear rate, which is acceptable for high shear rate applications, such as roll-to-roll printing and screen printing [3].
Figures 4-5(a) and 4-5(b) show the electrical resistivities of silver films prepared from the pastes with solid loadings of 70, 75 and 80 wt%, after being cured at 160°C for 5min and 10min, respectively. The film resistivity decreases with increasing the ratio of Ag2O/ silver stearate up to 100:5 and then increases again with the content of silver stearate. Increasing the soaking time from 5 min
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to 10 min does not change the resistivity significantly. The electrical resistivity decreases with the solid loading, which is due the higher packing density of the conducting particles after being cured. The electrical resistivity of the films prepared from pastes with the Ag2O/silver stearate ratio of 100:5, at solid loading of 70, 75, and 80wt% are 24.3x10-6 Ω-cm, 16.9x10-6 Ω-cm, and 13.2x10-6 Ω-cm, respectively, after being cured at 160°C for 5min, which meets the requirements of low-temperature and high speed manufacturing for practical applications. The electrical resistivity of the film depends on the connectivity and film density of various species in the film. As the paste is heated, Ag2O catalyzes the decomposition of silver stearate and, simultaneously, undergoes reduction and transforms to silver particles, which may be catalyzed by the fresh fine silver particles formed on their surfaces. Reduction of Ag2O and thermal decomposition of silver stearate proceed with time during curing which decreases the resivitivity of the films. Residual Ag2O was found in XRD results of the films after being cured at 160°C, as shown in Figure 4-6. It is evident enough that the reduction reaction of Ag2O does not proceed to completion, and leads to a small quantity of Ag2O still remaining after thermal curing. The incomplete reduction reaction of Ag2O retards further reduction of resistivity of silver films.
Ag2O catalyzes the evaporation of α-terpineol and the decomposition of silver stearate, which decreases the curing temperature of films to 160°C and shortens the soaking time to 5min. At a lower content of silver stearate (Ag2O/silver stearate = 100:3), the insufficient fine silver particles decomposed from silver stearate leads to a higher resistivity. At a higher content of silver stearate (Ag2O/silver stearate = 100:6), the extra silver stearate could not further induce the reduction of Ag2O during curing and, also, the high viscosity of the paste due to the dissolution of silver stearate in the solvent reduces the packing density (Figures 4-4 and 4-5). Therefore, higher electrical resistivity was
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obtained for Ag2O/silver stearate ratios of 100:3 and 100:6 after being cured.
SEM micrographs of the films, prepared from the pastes with various Ag2O/silver stearate ratios at solid loading of 80wt%, after being cured at 160°C for 10 min are shown in Figure 4-7. They indicate that the linkage of silver particles from the thermal decomposition of silver stearate and the reduction of Ag2O results in high electrical conductivity of films after being cured. For the Ag2O/silver stearate ratio of 100/5, the microstructure seems to have the highest film density, which is coincident with the lowest electrical resistivity of films shown in Figure 4-5. SEM cross-section micrograph of the films shown in Figure 4-8 shows the interconnection of the silver grains that contributes to the low electrical resistivity. Upon carefully examining the microstructure of the films, it appears that the three-dimensional interconnection network has resulted from the coalescence of fine and fresh silver particles decomposed from silver stearate and neckgrowth of the coarse silver grains reduced from Ag2O. TEM micrograph (Figure 4-9) shows the nano silver particles coated on the surfaces of silver grains, which were reduced from Ag2O. Left-top (a) diffraction pattern corresponds to the silver from the reduction of Ag2O and the right-down ring diffraction pattern is due to the silver from the decomposition of silver stearate.
Small silver particles are formed from decomposition of silver stearate catalyzed by Ag2O, which are then adhered onto the surfaces of silver grains that are also simultaneously reduced from Ag2O. Neckgrowth and sintering of silver particles shown in the TEM micrograph illustrates the low electrical resistivity of films after being cured. The low resistivity of the film is facilitated by the combination of the Ag2O and silver stearate contained in the paste used. Ag2O consists of high silver constituent (100 grams of Ag2O produces 93.1 grams silver), which produces a high density of silver matrix after reduction at low temperatures. The presence of a small amount of silver stearate contributes to the rheological behavior of the paste due to its dissolution in the solvent. More importantly, the co-existence of the Ag2O and silver stearate induces their transformation to the
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silver form simultaneously at temperatures below 160°C.
4-4 Summary
In this study, the silver flake and binder in the paste were replaced by Ag2O and silver stearate, to reduce the curing temperature while maintaining the required paste characteristics and electrical properties of the final film. Results indicate that all pastes appear to have pseudoplastic flow (shear-thinning) property. The initial shear stress and viscosity increases with increasing the silver stearate content and the solid loading. As the paste is heated, Ag2O catalyzes the decomposition of silver stearate and, simultaneously, undergoes reduction and transforms to silver particles. The lowest electrical resistivity of 13.2x10-6 Ω-cm was obtained for the film prepared from paste with the Ag2O/silver stearate ratio of 100:5 at a solid loading of 80wt% in solvent α-terpineol, after being cured at 160°C for 5min. The three-dimensional interconnection network was resulted from the coalescence of fine silver particles decomposed from silver stearate and neckgrowth of the coarse silver grains reduced from Ag2O. Ag2O catalyzes the evaporation of α-terpineol and the decomposition of silver stearate, which decreases the curing temperature of films to 160°C and shortens the soaking time to 5min.
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Figure 4-1. TGA results of pastes prepared from various alpha-terpineol/Ag2O/silver stearate ratios.
100 200 300 400 500
0 20 40 60 80 100
α-terpineol/silver stearate (70wt%:30wt%)
α-terpineol/Ag2O/silver stearate (70wt%:20wt%:10wt%)
Weight Loss (wt%)
Temperature (0C)
α-terpineol/Ag2O(70wt%:30wt%)
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Figure 4-2. TGA results of the pure silver oxide, pure silver stearate, and silver-stearate-coated Ag2O (100:5).
100 200 300 400 500
30 40 50 60 70 80 90 100 110
Pure Silver Stearate
Ag2O/silver Stearate = 100/ 5
Temperature (o C)
Weight Loss (wt%)
Pure Ag2O
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Figure 4-3. TGA results of the coated powder of Ag2O with silver stearate at various ratios.
100 200 300 400 500
88 90 92 94 96 98 100
Ag2O/silver stearate=100:6 Ag2O/silver stearate=100:5
Ag2O/silver stearate=100:4 Ag2O/silver stearate=100:3
Weight Loss (wt%)
Temperature(0C)
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