3-3 Results and Discussion

Figure 3-1 shows the DTA curves for pure MOD silver 2-ethylhexanoate and pure silver oxalate. The thermal decomposition of silver oxalate occurs at a lower temperature than that of silver 2-ethylhexanoate. It manifests that both reactions are exothermic, which generates a relative large amount of heat. The silver oxalate shows an exothermic peak at 229.39°C and the silver 2-ethylhexanoate at 266.25°C.

The reactions are as follows:

AgC₈H₁₅O₂ +

(

)

(

)

Previous study revealed that the thermal treatment of C8H15O2Ag results in the formation of various organic species including CH3, CO, O2, CH2CO, CO2, C4H9, CH5COOH, C5H10O, etc., depending on the temperature and the atmosphere [9]. On the other hand, thermal decomposition of the silver oxalate is different from most oxalates that usually decompose to form a metal carbonate or a metal oxide. It gives rise to silver as the solid product and CO2 as the gaseous product [10].

The decomposition of the silver oxalate produces nearly 71 wt% of fine silver catalyst, which is certainly a good silver source among various MOD compounds. When the silver 2-ethylhexanoate or silver oxalate was mixed with solvent α-terpineol, the former is soluble in the solvent, but the latter is not. Previous study has shown that the decomposition temperatures of silver 2-ethylhexanoate or silver oxalate in α-terpineol

44

-are reduced to 190.3 and 212.14°C, respectively [5].

Figure 3-2 shows the results of the thermogravimetric analysis (TGA) for the pastes without and with 3 wt% silver oxalate added in air.

For the paste without silver oxalate added, the decompositions of the α-terpineol and silver 2-ethylhexanoate lead to a weight loss of ≈17.26 wt% at temperatures below 190°C. There is a weight loss of ≈0.3%

observed at ≈235°C, which corresponds to the decomposition of the lubricant, fatty acid, coated on the silver flakes. For the paste with 3 wt%

silver oxalate added, there are three weight drops as the temperature increases from room temperature to 300°C. Weight loss of ≈16.76 wt%

occurs at temperatures below 190°C due to the decompositions of α-terpineol and silver 2-ethylhexanoate, ≈0.87 wt% at ≈210°C resulting from the decomposition of silver oxalate, and ≈0.3% at ≈222°C associated with removal of lubricant from the surfaces of silver flakes.

The weight losses observed are relatively consistent with the theoretical values calculated from the paste formulations and the chemical formula of the compounds. The results verify that the addition of the silver oxalate not only produces fresh fine silver particles, but also reduces the decomposition temperature of the lubricant coated on the silver flakes.

Rheological characteristics of the pastes with various amounts of silver oxalate added are shown in Figure 3-3. It indicates that all pastes have pseudoplastic flow (shear-thinning) property [Figure 3-3(a)]. The solid loading of the paste increases with the silver oxalate content since it is not soluble in the α-terpineol. However, the viscosity of the paste only slightly increases with the content of silver oxalate. Figure 3-3(b) indicates that all pastes possess pseudoplastic flow property with an

45

-apparent yield point, which is beneficial to the dimensional control during screen printing process. The apparent yield point does not vary with the content of silver oxalate. Based on the paste formulations shown in Table 3-1, the total silver content in the paste slightly decreases from 82.74 to 81.67 wt% as the content of the silver oxalate increases from 0 to 10 wt%.

Whereas, the ratio of the fresh fine silver particles, decomposed from the silver oxalate and silver 2-ethylhexanoate, would increase from 1.74 to 8.03 wt% of the total paste. This is an effective route to provide fine silver particles to the paste without significantly changing the rheology of the paste. Direct addition of a small quantity of nano-size silver particles in the paste would significantly increase the viscosity of the paste to an un-acceptable value and easily leads to agglomeration of the silver particles.

Figure 3-4 shows the SEM micrographs of the films, prepared from the pastes with 0, 3, and 10wt% of silver oxalate additions, after being cured for 5 min at 225°C. The films generally contain silver grains with a wide size distribution. They indicate that a higher amount of silver oxalate added produces a higher packing density of film. Fine silver particles, produced from the decomposition of silver oxalate and silver 2-ethylhexanoate, occupied the voids among silver flakes. This will enhance the connectivity of the silver particles in the film, as shown in Figures 3-4(b) and 3-4(c).

Figure 3-5 shows the resistivities of silver films prepared from the pastes with various amounts of silver oxalate added and cured at different temperatures for 5 min. The film resistivity generally decreases with increasing curing temperature, due to a better connectivity of the metal

46

-particles associated with the decomposition of organics. The resistivity decreases with increasing silver oxalate content for the films cured at temperatures less than 250°C. At the curing temperature of 225°C, the resistivity decreases from 180.1 to 31.9 μΩ-cm, as the silver oxalate content increases from 0 to 10 wt%. There are two contributions which are responsible for lowering the resistivity of the films. One is that the fresh fine silver particles in the cured silver film increases from 2.10 to 10.17% wt% of the total silver, due to the decomposition of MOD compounds, as the silver oxalate content raises from 0 to 10 wt%. (Table 3-1). The other contribution is from the addition of the silver oxalate in the paste that reduces the thermal-decomposition-temperature of the lubricant, fatty acid, coated on the silver flakes. The thermal decomposition of silver oxalate and silver 2-ethylhexanoate produces active silver catalysts, which increase the packing density and assist the necking of silver flakes in the films. The removal of the lubricant from the surfaces of silver flakes would increase their connectivity. These would be beneficial to the electron conduction and thus decrease the resistivity of the film. For the curing temperature above 250°C, the resistivities of the films are very similar and range from 10.18 to 15.03 μΩ-cm, since there are no organic residues left and the connectivity of the silver particles gets to a saturation limit. The resistivity is insensitive to the increasing curing temperature, unless it reaches the sintering temperature of silver particles.

47

-3-4 Summary

In this study, MOD silver screen-printable pastes with various amounts of silver oxalate added were prepared. With the addition of silver oxalate, the decompositions of α-terpineol and silver 2-ethylhexanoate occur at temperatures below 190°C, the silver oxalate at

≈210°C, and the lubricant on the surfaces of silver flakes at ≈222°C. The resistivity of the film prepared from the pastes decreases with increasing curing temperature, due to a better connectivity of the metal particles associated with the decomposition of organics. A resistivity of 31.9 μΩ-cm was obtained for the film prepared from the paste with 10 wt% silver oxalate added and curried at 225°C.

48

-References

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Electron. Devices. 51 (2004) No. 12, 1978.

2. V. Subramanian, J. M. J. Frechet, P. C. Chang, D. C. Huang, J. B. Lee, S. E. Molesa, A. R. Murphy, D. R. Redinger, S. K. Volkman: Proc.

IEEE. 93 (2005) No. 7, 1330.

3. P. H. Kydd: United States Patent, No. 6,036,889 (2000).

4. P. H. Kydd: PCT Patent, WO 98/37133 (1998).

5. C. A. Lu, P. Lin, H. C. Lin and S. F. Wang: Jpn. J. Appl. Phys. 45 (2006), 6987.

6. A.L. Dearden, P.J. Smith, D.Y. Shin, N. Reis, B. Derby, and P. O’Brien:

Marcromol. Rapid Commun. 26 (2005) 315.

7. D. Kim, and J. Moon: Electrochemical and Solid State Letters, 8 (2005) J30.

8. K. F. Teng and R. W. Vest: IEEE. Trans. Components. Hybrid. and Manufacturing. V CHMT-12 (1987) No. 4, 545.

9. C. A. Lu, P. Lin, H. C. Lin and S. F. Wang: Jpn. J. Appl. Phys. 46 (2007), 251.

10. V. V. Boldyrev: Thermochimica Acta. 388 (2002), 63.

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-150 200 250 300 350

Silver 2-ethylhexanoate 266.25⁰C

Detla T (endot hermic dow n)

Temperature (

C )

229.39⁰C

Silver Oxalate

Figure 3-1. DTA curves for pure silver 2-ethylhexanoate and pure silver oxalate.

50

-160 180 200 220 240

80 82 84 86 88 90 92

0wt% Silver Oxalate 3wt% Silver Oxalate

We ight Los s (w t% )

Temperature (

C )

Figure 3-2. TGA curves for the pastes with 0 and 3wt% silver oxalate additions.

51

-0 50 100 150 200 250 300 350 400

10³ 10⁴ 10⁵

Visco sity ( P a .S )